The Biology Of Cancer

Transcription

The Biology of Cancer
the biology of
S E CO N D ED I T I O N
the
biology
of
Second
Edition
CANCER
CANCER
Media Guide
Robert A. Weinberg
Thoroughly updated and incorporating the most important advances
in the fast-growing field of cancer biology, The Biology of Cancer,
Second Edition, maintains all of its hallmark features admired by
students, instructors, researchers, and clinicians around the world.
The Biology of Cancer is a textbook for students studying the molecular and
cellular bases of cancer at the undergraduate, graduate, and medical school
levels. The principles of cancer biology are presented in an organized, cogent,
and in-depth manner. The clarity of writing, supported by an extensive
full-color art program and numerous pedagogical features, makes the book
accessible and engaging. The information unfolds through the presentation
of key experiments that give readers a sense of discovery and provide
insights into the conceptual foundation underlying modern cancer biology.
DVD-ROM AND POSTER INCLUDED
• The enclosed DVD-ROM includes the book’s art program, a
selection of movies with narration, audio files of mini-lectures
by the author, Supplementary Sidebars, and a Media Guide.
• The revised “Pathways in Human Cancer” poster summarizes
some of the key signaling pathways implicated in
tumorigenesis and tumor progression in humans.
Rooberrt A. Weinb
berrg
Robert A. Weinberg is a founding member of the Whitehead Institute
for Biomedical Research. He is the Daniel K. Ludwig Professor for
Cancer Research and the American Cancer Society Research Professor
at the Massachusetts Institute of Technology (MIT). Dr. Weinberg is an
internationally recognized authority on the genetic basis of human cancer
and was awarded the U.S. National Medal of Science in 1997.
SECOND
EDI T I O N
Besides its value as a textbook, The Biology of Cancer is a useful reference for
individuals working in biomedical laboratories and for clinical professionals.
ISBN 9780815342199
9 780815 342199
www.garlandscience.com
T
his document contains an overview of the contents of the DVD-ROM packaged
with The Biology of Cancer, 2nd Edition by Robert A. Weinberg. It also contains
transcripts of the movies and audio files on the DVD.
The DVD contains the following media for students and instructors:
• Figures from the book (PowerPoint® and JPEG formats)
• Supplementary Sidebars (PDF)
• Movies (QuickTime® and WMV formats)
• Mini-lectures from the author (MP3 format)
The media on this DVD, as well as future media updates, are also available to students
and instructors at www.garlandscience.com.
S E CO N D EDI T I O N
2
Media Guide for The Biology of Cancer, Second Edition
Figures from the Book—The Art of The Biology of Cancer
F
igures, tables, and micrographs from the book are located in the “Art of TBoC2”
folder. They have been pre-loaded into PowerPoint presentations, one for each
chapter of the book. A separate folder contains individual versions of each figure, table,
and micrograph in JPEG format. The folders are called “PowerPoint” and “JPEGs.”
In addition to serving as a tool for creating visual presentations for lectures, the PowerPoint files are also useful for printing the figures. If you wish to print all the figures
from a particular chapter, open the PowerPoint presentation for the chapter, select the
“file/print” menu option, and print “All” the figures. If you wish to print only a select
number or just one figure, choose the appropriate slide number in the PowerPoint
printer options window, accessed through the menu described above. This process is
roughly the same on both Mac OS and Windows operating systems.
You may also print the individual JPEGs from the JPEG archive using Adobe® Photoshop® or similar image editing programs. Both Mac OS and Windows have built-in
image viewers that also allow printing. Please consult the user manual for these programs for further instructions.
The figures can also be imported into Microsoft Word® and other text editing programs through the “insert/picture” menu option or by cutting-and-pasting the figure
from one program to another. In Microsoft Word you can re-size the figure to match
your document.
3
Supplementary Sidebars
T
he Biology of Cancer contains a special feature called
“Sidebars,” which consist of commentary that detours
slightly from the main thrust of the textual discussion. Often
these Sidebars contain anecdotes or elaborate on ideas presented in the main text. In addition to the Sidebars that appear
in the text, the author has written the following additional
Sidebars, which can be accessed from the “Supplementary
Sidebars” folder on the DVD or from the Garland Science
website. These Supplementary Sidebars are cross-referenced
throughout the text. They are available in PDF format for optimal viewing and printing.
Supplementary Sidebar 1.1 Each female cell can access information only from a single X chromosome
Supplementary Sidebar 1.2 Reproductive cloning demonstrates the extraordinary efficiency of the DNA repair apparatus
Supplementary Sidebar 1.3 The network of miRNA-controlled genes
Supplementary Sidebar 1.4 Knocking down gene expression with shRNAs and siRNAs
Supplementary Sidebar 1.5 Gene cloning strategies
Supplementary Sidebar 2.1 Commonly used histopathological techniques
Supplementary Sidebar 2.2 The complicated conventions for classifying and naming tumors
Supplementary Sidebar 3.1 Is SV40 responsible for the mesothelioma plague?
Supplementary Sidebar 3.2 Maintenance of KSHV and HPV genomes in episomal and chromosomal form
Supplementary Sidebar 3.3 Re-engineering the retrovirus genome for gene therapy
Supplementary Sidebar 3.4 Classic Kaposi’s sarcoma appears to be a familial disease
Supplementary Sidebar 3.5 Viruses like RSV have very short lives
Supplementary Sidebar 4.1 Endogenous viruses can explain tumor development in the absence of infectious viral spread
Supplementary Sidebar 4.2 Boveri and Hansemann independently hypothesized genetic abnormality as the cause of
cancer cells’ malignant behavior
Supplementary Sidebar 4.3 Southern and Northern blotting
Supplementary Sidebar 4.4 Genes undergo amplification for a variety of reasons
Supplementary Sidebar 5.1 Making anti-Src antibodies presented a major challenge
Supplementary Sidebar 5.2 The protozoan roots of metazoan signaling
Supplementary Sidebar 5.3 Lateral interactions of cell surface receptors
Supplementary Sidebar 6.1 Systematic surveys of phosphotyrosine and SH2 interactions
Supplementary Sidebar 6.2 The complexities of understanding RTK signaling
Supplementary Sidebar 6.3 The rationale for multi-kinase signaling cascades
Supplementary Sidebar 6.4 Non-canonical Wnt signaling
Supplementary Sidebar 6.5 The Hippo pathway and control of stem cell proliferation
Supplementary Sidebar 7.1 Heterozygosity in the human gene pool
Supplementary Sidebar 7.2 Which is more likely—LOH or secondary, independent mutations?
Supplementary Sidebar 7.3 The polymerase chain reaction makes it possible to genetically map tumor suppressor genes
rapidly
Supplementary Sidebar 7.4 The MSP reaction makes it possible to gauge methylation status of promoters
Supplementary Sidebar 7.5 Ubiquitylation tags cellular proteins for destruction in proteasomes
Supplementary Sidebar 7.6 Krebs cycle enzymes and cancer development
Supplementary Sidebar 7.7 Homologous recombination allows restructuring of the mouse germ line
Supplementary Sidebar 8.1 The origins of embryonic stem cells
Supplementary Sidebar 8.2 Plasticity of the cell cycle clock
Supplementary Sidebar 8.3 Chromatin immunoprecipitation (ChIP)
Supplementary Sidebar 8.4 Some tumors increase Id concentrations by de-ubiquitylating them
Supplementary Sidebar 8.5 Specific targeting of cell cycle regulators by E3 ubiquitin ligases
Supplementary Sidebar 8.6 The major puzzle surrounding the RB gene: retinoblastomas
Supplementary Sidebar 9.1 UV-B radiation, HPV, and cutaneous squamous cell carcinomas
4
Supplementary Sidebar 9.2 The TUNEL assay
Supplementary Sidebar 9.3 Dominant-negative functions of mutant p53 alleles: functional interactions between p53 and its
p63 and p73 cousins
Supplementary Sidebar 9.4 Some mutant p53 alleles cause highly specific tumors
Supplementary Sidebar 9.5 Autophagy is critical to post-fertilization development
Supplementary Sidebar 10.1 The use of the TRAP assay and adaptations thereof permits the rapid and quantitative
assessment of the levels of the catalytic activity of telomerase enzyme in eukaryotic cells
Supplementary Sidebar 11.1 Monoclonal antibodies and fluorescence-activated cell sorting (FACS)
Supplementary Sidebar 11.2 How does multi-step tumor progression actually take place?
Supplementary Sidebar 11.3 Symbiosis between distinct subpopulations within a tumor
Supplementary Sidebar 11.4 Comparative genomic hybridization
Supplementary Sidebar 11.5 Are rodent carcinogen tests reliable indicators of danger to humans?
Supplementary Sidebar 11.6 Does saccharin cause cancer?
Supplementary Sidebar 11.7 How does diet affect colon cancer incidence?
Supplementary Sidebar 12.1 Hematopoiesis as a model for the organization of many kinds of tissues
Supplementary Sidebar 12.2 Stem cell pools may explain the protective effects of pregnancy
Supplementary Sidebar 12.3 The conserved-strand mechanism and protection of the stem cell genome
Supplementary Sidebar 12.4 Oxidation products in urine provide an estimate of the rate of ongoing damage to the cellular
genome
Supplementary Sidebar 12.5 How does red meat cause colon cancer?
Supplementary Sidebar 12.6 A convergence of bacterial, yeast, and human genetics led to the discovery of hereditary nonpolyposis colon cancer genes
Supplementary Sidebar 12.7 Homology-directed repair
Supplementary Sidebar 13.1 Localization of growth factors is important for proper heterotypic interactions
Supplementary Sidebar 13.2 Ongoing heterotypic signaling in carcinomas
Supplementary Sidebar 13.3 Certain highly advanced tumors provide exceptions to the generally observed dependence of
carcinoma cells on stroma
Supplementary Sidebar 13.4 Myofibroblasts predict clinical progression of cancer
Supplementary Sidebar 13.5 A technique for separating stromal from epithelial cells
Supplementary Sidebar 13.6 Microvessel leakiness dooms many forms of anti-cancer therapy: optimizing anti-angiogenic
treatments
Supplementary Sidebar 13.7 The temporary nature of vessel regression created by anti-angiogenesis therapy
Supplementary Sidebar 13.8 Kaposi’s sarcoma cells hold the record for the number of documented heterotypic signals they
receive
Supplementary Sidebar 13.9 Effects of an anti-VEGF-R monoclonal antibody on the growth of a human tumor xenograft
Supplementary Sidebar 13.10 Fibroblasts are heterogeneous and can change dynamically in response to signals
Supplementary Sidebar 14.1 Visualization of the dynamics of pathfinding fibroblasts followed by squamous cell carcinoma
cells
Supplementary Sidebar 14.2 Metastasizing cancer cells often take on hitchhikers while traveling through the blood
Supplementary Sidebar 14.3 Instruments for detecting circulating tumor cells
Supplementary Sidebar 14.4 Hidden micrometastases are revealed through organ transplantation
Supplementary Sidebar 14.5 Wolves in sheep’s clothing: when carcinoma cells invade the stroma
Supplementary Sidebar 14.6 TGF-β works in conflicting ways during tumor progression
Supplementary Sidebar 14.7 Dynamics of EMT induction: the EMT may be controlled in some cancer cells exclusively by
their own genomes
Supplementary Sidebar 14.8 An example of an EMT relatively late in embryonic development
Supplementary Sidebar 14.9 Relatively rapid metastatic dissemination of advanced primary tumor cells
Supplementary Sidebar 14.10 Our cells devote an enormous number of genes to regulating protein degradation
Supplementary Sidebar 14.11 Peritoneovenous shunts provide dramatic support for the seed and soil hypothesis
Supplementary Sidebar 14.12 Tooth extractions may occasionally become exceedingly painful
Supplementary Sidebar 14.13 Tumor stem cells further complicate our understanding of the metastatic process
Supplementary Sidebar 14.14 Does Darwinian evolution accommodate metastasis-specific alleles?
Supplementary Sidebar 15.1 Rearrangements of chromosomal DNA segments generate a vast array of antigen-binding
domains in antibodies and T-cell receptors
Supplementary Sidebar 15.2 Virus-infected cells may not always be recognized by the immune system
Supplementary Sidebar 15.3 Bizarre tumors reveal how cancer cells can become infectious agents
Supplementary Sidebar 15.4 Mice have proven to be far more useful for tumor biologists than chickens
Supplementary Sidebar 15.5 An HPV vaccine protects against many cervical carcinomas
Supplementary Sidebar 15.6 An unexpected type of anti-p53 reactivity is often found in cancer patients
Supplementary Sidebar 15.7 Immune recognition of tumors may be delayed until relatively late in tumor progression
Supplementary Sidebar 15.8 Some paraneoplastic syndromes reveal defective tolerance and overly successful immune
responses to tumors
Supplementary Sidebar 15.9 TSTAs can arise as by-products of chemical and physical carcinogenesis
Supplementary Sidebar 15.10 Are melanomas more antigenic than other tumors?
Supplementary Sidebar 15.11 Strategies for cloning genes encoding melanoma TATAs
Supplementary Sidebar 15.12 Anti-CD47 therapies hold promise in treating lymphomas and other hematopoietic
malignancies
Supplementary Sidebar 15.13 Cancer cells may thwart extravasation by circulating
T cells
Supplementary Sidebar 15.14 Herceptin can be modified to potentiate cancer cell killing
Supplementary Sidebar 15.15 Bone marrow transplantation and the treatment of hematopoietic malignancies
Supplementary Sidebar 15.16 Whole genome sequencing allows a new attack on tumor cells
Supplementary Sidebar 16.1 Modern cancer therapies have had only a minor effect on the overall death rate from the
disease
Supplementary Sidebar 16.2 Prostate cancers usually do not require aggressive intervention—a tale of two countries
Supplementary Sidebar 16.3 Clinical practice and our understanding of disease pathogenesis have often been poorly
aligned, leading to sub-optimal, often tragic outcomes
Supplementary Sidebar 16.4 The ability to assign tumors to specific disease subtypes is critical to the success of drug
development
Supplementary Sidebar 16.5 p53 germ-line polymorphisms and somatic mutations can complicate the induction of
apoptosis by drugs
Supplementary Sidebar 16.6 Ras function can be inhibited by interfering with the enzymes responsible for the maturation of
the Ras protein
Supplementary Sidebar 16.7 Chemical synthesis, compound libraries, and high-throughput screening
Supplementary Sidebar 16.8 Evolution can generate huge collections of structurally similar proteins
Supplementary Sidebar 16.9 Large-scale screen of the inhibitory effects of a drug on various kinases
Supplementary Sidebar 16.10 Epidermal growth factor receptor expression levels predict little about a tumor’s susceptibility
to receptor antagonists
Supplementary Sidebar 16.11 Akt/PKB function is controlled by multiple upstream signals
5
6
Movies
T
he “Movies” folder contains thirty-one movies, available in both QuickTime and
WMV formats, that will aid in understanding some of the proteins and processes
described in the book. The QuickTime movies will provide the optimal viewing experience. The WMV format is included because QuickTime movies will not work in PowerPoint for Windows, but the WMV formatted movies will work seamlessly. Each movie
is accompanied by a voice-over narration. The movie table of contents, followed by the
full text of each movie narration, is below:
Chapter 1: The Biology and Genetics of Cells and Organisms
1.1
1.2
1.3
1.4
Replication I
Replication II
Translation
Transcription
Chapter 2: The Nature of Cancer
2.1
2.2
2.3
Embryonic Origins of Tissues
Mammary Cancer Cells
Visualization of Cancer I: Lymphoma
Chapter 3: Tumor Viruses
3.1
Contact Inhibition
Chapter 4: Cellular Oncogenes
No Movies
Chapter 5: Growth Factors, Receptors, and Cancer
5.1
5.2 5.3 5.4 Cellular Effects of EGF vs. HGF
Activation of Kit Receptor Signaling by SCF EGF Receptor Family IGF Receptors and Monoclonal Antibodies Chapter 6: Cytoplasmic Signaling Circuitry Programs Many of the
Traits of Cancer
6.1
6.2 6.3
Regulation of Signaling by the Src Protein
Signaling by the Ras Protein
EGF Receptors and Signaling
Chapter 7: Tumor Suppressor Genes
7.1
Intestinal Crypt
Chapter 8: pRb and Control of the Cell Cycle Clock
8.1
8.2
Animal Cell Division
CDK2
Chapter 9: p53 and Apoptosis: Master Guardian and Executioner
9.1
9.2
p53 Structure
Apoptosis
Chapter 10: Eternal Life: Cell Immortalization and Tumorigenesis
10.1 Telomere Replication
Chapter 11: Multi-Step Tumorigenesis
No Movies
Chapter 12: Maintenance of Genomic Integrity and the Development
of Cancer
12.1 DNA Repair Mechanisms Media Guide for The Biology of Cancer, Second Edition
Chapter 13: Dialogue Replaces Monologue: Heterotypic Interactions
and the Biology of Angiogenesis
13.1 Mechanisms Enabling Angiogenesis 13.2 Interactions of Innate Immune Cells with a Mammary Tumor
Chapter 14: Moving Out: Invasion and Metastasis
14.1 Adhesion Junctions
14.2 Mechanisms of Brain Metastasis Formation 14.3 Visualization of Cancer II: Metastasis
Chapter 15: Crowd Control: Tumor Immunology and Immunotherapy
15.1 The Immune Response
15.2 Antigen Display and T-Cell Attack
Chapter 16: The Rational Treatment of Cancer
16.1 Drug Export by the Multi-Drug Resistance Pump
16.2 PI3K
7
8
Movie 1.1 Replication I
U
sing computer animation based on molecular research, we are able to picture
how DNA is replicated in living cells. You are looking at an assembly line of amazing miniature biochemical machines that are pulling apart the DNA double helix and
cranking out a copy of each strand. The DNA to be copied enters the production line
from bottom left. The whirling blue molecular machine is called a helicase. It spins
the DNA as fast as a jet engine as it unwinds the double helix into two strands. One
strand is copied continuously and can be seen spooling off to the right. Things are not
so simple for the other strand because it must be copied backwards. It is drawn out
repeatedly in loops and copied one section at a time. The end result is two new DNA
molecules.
Animation produced for DNA Interactive (www.dnai.org) © 2003 Howard Hughes
Medical Institute, (www.hhmi.org). All rights reserved.
Movie 1.2 Replication II
I
n a replication fork, two DNA polymerases collaborate to copy the leading-strand
template and the lagging-strand template DNA. In this picture, the DNA polymerase that produces the lagging strand has just finished an Okazaki fragment.
The clamp that keeps the lower DNA polymerase attached to the lagging strand dissociates, and the DNA polymerase temporarily releases the lagging strand template
DNA.
As the DNA helicase continues to unwind the parental DNA, the primase becomes
activated and synthesizes a short RNA primer on the growing lagging strand. The DNA
polymerase binds to the DNA again and becomes locked in by the clamp.
The polymerase uses the RNA primer to begin a short copy of the lagging-strand template DNA.The polymerase stalls when it reaches the RNA primer of the preceding
Okazaki fragment, and the entire cycle repeats.
Animation: Sumanas, Inc. (www.sumanasinc.com)
Music: Christopher Thorpe
Movie 1.3 Translation
W
hen the mRNA is complete, it snakes out of the nucleus into the cytosol. Then in
a dazzling display of choreography, all the components of a molecular machine
lock together around the RNA to form a miniature factory called a ribosome. It translates the genetic information in the RNA into a string of amino acids that will become
a protein. tRNA molecules‚ the green triangles‚ bring each amino acid to the ribosome.
The amino acids are the small red tips attached to the tRNAs. There are different tRNAs
for each of the twenty amino acids, each of them carrying a three-letter nucleotide
code that is matched to the mRNA in the machine. Now we come to the heart of the
process. Inside the ribosome, the mRNA is pulled through like a tape. The code for
each amino acid is read off, three letters at a time, and matched to three corresponding letters on the tRNAs. When the right tRNA plugs in, the amino acid it carries is
added to the growing protein chain. You are watching the process in real time. After
a few seconds the assembled protein starts to emerge from the ribosome. Ribosomes
can make any kind of protein. It just depends on what genetic message you feed in on
the mRNA. In this case, the end product is hemoglobin. The cells in our bone marrow
churn out a hundred trillion molecules of it per second! And as a result, our muscles,
brain, and all the vital organs in our body receive the oxygen they need.
Medical Institute (www.hhmi.org). All rights reserved.
Movie 1.4 Transcription
T
ranscription is the process by which DNA is copied into RNA in the first step of
gene expression. It begins with a bundle of factors assembling at the start of a
gene, that is, a linear sequence of DNA instructions, shown here stretching away to the
left. The assembled factors include an RNA polymerase, the blue molecule. Suddenly,
RNA polymerase is let go, racing along the DNA to read the gene. As it unzips the double helix, it copies one of the two strands. The yellow chain snaking out of the top is
the RNA, a copy of the genetic message. The nucleotide building blocks that are used
to make the RNA enter through an intake hole in the polymerase. In the active site of
the enzyme, they are then matched to the DNA, nucleotide by nucleotide, to copy the
A’s, C’s, T’s and G’s of the gene. The only difference is that in the RNA copy, thymine
is replaced with the closely related base uracil, commonly abbreviated “U.” You are
watching this process, called transcription, in real time.
Medical Institute (www.hhmi.org). All rights reserved.
9
10 Media Guide for The Biology of Cancer, Second Edition
Movie 2.1 Embryonic Origins of Tissues
C
ells of this developing frog embryo rearrange in a dramatic ballet of orchestrated
cell movements. In a continuous motion, cells from the outer layer of the embryo
sweep towards the vegetal pole and start invaginating, forming a deep cavity in the
interior. The paths of the cells and the topology of these rearrangements are best seen
in this animation of an embryo that has been sliced open. The different cell layers that
are formed in this way have very different fates. Cells that line the newly formed cavity,
called the endoderm, develop into the lining of the gut and many internal organs such
as liver, pancreas, and lung. Cells in the middle layer, called the mesoderm, give rise to
muscle and connective tissue. Cells remaining on the outside, called the ectoderm, go
on to form the outer layer of the skin, as well as the nervous system.
Jeremy Pickett-Heaps
University of Melbourne
From “From Egg to Tadpole” Jeremy Pickett-Heaps and Julianne Pickett-Heaps
Cytographics (www.cytographics.com)
Movie 2.2 Mammary Cancer Cells
N
ormal human mammary epithelial cells can be suspended and grown in a gellike medium. Under these conditions, they form structures that resemble the little sacs of cells in the mammary gland, called alveoli, in which milk is made. Cells
assemble into a well-organized, polarized epithelium that forms a closed sphere with
an internal lumen.
In the mammary gland, this space would be connected to ducts, and cells would
secrete milk into it.
By contrast, these human breast cancer cells grown under the same conditions divide
aggressively and in an uncontrolled fashion. They are also more migratory and grow
into disorganized clumps, which would form tumors in the body.
Original script: Peter Walter, Howard Hughes Medical Institute, University of
California at San Francisco
Mina J. Bissell, Karen Schmeichel,
Hong Liu, and Tony Hansen
Lawrence Berkeley Laboratories
11
Movie 2.3 Visualization of Cancer I: Lymphoma
M
agnetic resonance imaging tomography enables the visualization of tumors. In
this 3-D reconstruction, we see a lymphoma in living tissue that has been artificially colored red. The lymphoma was generated by cells that initially lacked p53 function; in the absense of p53 expression, the lymphoma grew to be quite large.
In this experiment, p53 expression was reactivated in the lymphoma at Day 0.
18 days after p53 reactivation, we can observe significant regression of the tumor. This
regression illustrates that p53-activating signals were present in the lymphoma cells
during its initial growth, but failed, in the absence of p53 expression, to halt tumor
growth. That is, without p53 expression, these signals alone could not trigger apoptosis
in the tumor cells.
However, once p53 was expressed in the tumor cells, it functioned powerfully to
induce tumor regression, presumably by causing widespread apoptosis in the lymphoma cells.
Andrea Ventura, Tyler Jacks, and
David G. Kirsch
The David H. Koch Institute for
Integrative Cancer Research at MIT
Jan Grimm and Ralph Weissleder
Massachusetts General Hospital
By the 28th day of p53 reactivation, the tumor cannot be seen at all.
3.1 Contact Inhibition
W
hen normal cells are introduced into a Petri dish at low numbers, they begin to
grow and divide. As the cells begin to touch one another, they stop dividing. This
response is called contact inhibition. Once the cells fill up the bottom of the dish, all
cell division ends, creating a state called confluence.
Contact inhibition ensures that the cells create a layer only one cell thick—a monolayer.
The behavior of cancer cells is quite different. If a cancer cell is seeded among normal
cells, all of the cells will proliferate as before. However, once confluence is reached, the
normal cells will stop multiplying while the cancer cells continue to divide, yielding a
clump of cells often called a focus.
Contact inhibition can be demonstrated in vitro by removing cells from a confluent
monolayer. In this experiment, cells are removed by scratching the monolayer with
a needle. The surviving cells at the edge of the wound now do two things. First, they
begin to proliferate again, since they are no longer fully contact-inhibited. And second, they migrate into the empty area of the wound, attempting to fill it up.
Sheryl Denker and Diane Barber
University of California at
San Francisco
Movie 5.1 Cellular Effects of EGF vs. HGF
E
pidermal Growth Factor, or EGF, has been added to these liver epithelial progenitor cells in culture. EGF binds specifically to the Epidermal Growth Factor Receptor on the surface of the cells. This stimulates its tyrosine-kinase activity and a downstream signaling cascade that promotes DNA synthesis and cell proliferation. Note
how the cells cling to one another and form a clump of cells as they divide.
In this movie, another growth factor, Hepatocyte Growth Factor, or HGF, has been
added to liver epithelial progenitor cells in culture. HGF binds to the c-Met receptor on the cell surface, and similar to EGF, it stimulates tyrosine-kinase activity and a
downstream signaling cascade that promotes cell proliferation. However, unlike EGF,
HGF has an additional effect on cells by promoting cell motility. This causes the dividing cells to scatter. Because of this effect, HGF is also known as Scatter Factor or SF.
Andrea Bertotti and Paolo M.
Comiglio
Institute for Cancer Research and
Treatment, University of Torino,
School of Medicine
The differences in the effects of EGF and HGF illustrate the quite different responses
that these two growth factors elicit from cells.
Movie 5.2 Activation of Kit Receptor Signaling by SCF
I
n the absence of ligand, a growth factor receptor exists in a monomeric (single-subunit) form embedded in the plasma membrane. Many growth factor receptors, such
as the Kit receptor shown here, have lateral mobility in the plane of the plasma membrane and are relatively free to wander back and forth across the surface of the cell.
Like many growth factor receptors, the Kit protein can be divided into 3 major
domains. The ectodomain, which sticks out of the surface of the cell and plays a key
role in ligand binding; the transmembrane domain, which is extremely hydrophobic
and spans the plasma membrane; and the cytoplasmic domain, which can initiate
signaling inside the cell.
In the example of Kit, the signaling process begins when Kit encounters its ligand,
called stem cell factor, or SCF. SCF is composed of two identical protein subunits.
When presented with SCF, Kit binds to one of the two identical subunits.
When the SCF/Kit complex encounters an unbound Kit monomer in the plasma
membrane, the unbound Kit monomer can bind to the other subunit of the SCF ligand, and form a dimeric receptor.
Upon dimerization of the ectodomains driven by SCF, the cytoplasmic domains of the
two Kit proteins are brought into close proximity.
The kinase domain of each Kit receptor then phosphorylates tyrosine residues present
in the cytoplasmic domain of the other Kit receptor, a process called transphosphorylation. Transphosphorylation triggers a cascade of signaling activity inside the cell
that can affect cell growth and proliferation.
Joseph Schlessinger
Yale University School of Medicine
13
Movie 5.3 EGF Receptor Family
E
pidermal growth factor receptors, also called EGF receptors, constitute a family of
four similarly structured receptor tyrosine kinases that interact with one another
to promote general cell growth and proliferation.
When EGF receptors bind with their ligands—such as EGF, TGF-α, or NRG—they form
homodimers or heterodimers, and activate their cytoplasmic domains. Once activated, the cytoplasmic domains emit signals inside the cell that activate cytoplasmic
signaling cascades that function, in turn, to promote cell growth and proliferation,
inhibit apoptosis, and increase cell motility.
In cancer cells, signal emission by EGF receptors becomes deregulated and enables
them to promote tumor growth, resistance to chemotherapy, and tumor metastasis.
In the absence of ligand, normal EGF receptors exist in a monomeric form, and move
laterally in the plane of the plasma membrane. Even if they collide, they cannot form
stable dimers.
When an EGF receptor binds with its ligand, it undergoes a dramatic conformational
change in its extracellular domain. This enables the altered extracellular domain of
this receptor to bind to the extracellular domain of a second receptor molecule.
Dimerization brings the intracellular kinase domains of the two receptor molecules
close together. The tyrosine kinases become activated, and each tyrosine kinase phosphorylates the cytoplasmic tail of the other receptor molecule. This process is referred
to as transphosphorylation.
The resulting phosphorylated tails then serve to recruit and activate a series of signal
transducing proteins that in turn activate multiple downstream signaling cascades.
Mark Sliwkowski
Genentech, Inc.
Movie 5.4 IGF Receptors and Monoclonal Antibodies
T
he surface of all cells is studded with a wide variety of receptor tyrosine kinases,
among them the receptors for insulin and insulin-like growth factors, called IGF-1
and IGF-2. Upon binding their insulin or IGF ligands, these two receptors transduce
signals into the cell, including those that encourage cell growth and prevent apoptosis.
The key molecules involved in the IGF system are: the soluble ligands IGF 1 and IGF 2,
a series of IGF-binding proteins, the cell surface IGF receptors, type 1 and type 2, and
the insulin receptor.
Both of the ligands, IGF1 and IGF2, exert their biological activities through binding to
type-1 IGF receptors. They can also bind to the insulin receptor, but with about 1000
times lower affinity.
The type-2 receptor is believed to function as a decoy receptor, to draw IGF2 away
from the type-1 receptor. It is considered to act as a tumor suppressor, and its expression is frequently lost during tumor progression.
In addition, a series of six binding proteins associate with IGFs in plasma to regulate
the amount of free IGFs that are available to bind and activate the type-1 IGF receptor.
The type-1 IGF receptor tyrosine kinase can bind either IGF1 or IGF2 ligands. This
receptor exists on the cell surface as a pre-formed dimer. It is composed of an extracellular alpha subunit, which contains the IGF binding site, and a beta subunit, which
extends through the lipid bilayer of the cell membrane to the inside of the cell, where
the tyrosine kinase enzyme is located.
The insulin receptor is structurally and functionally very similar to the type-1 IGF
receptor. As a result, the type-1 IGF receptor and insulin receptor can form hybrid,
heterodimeric receptors containing an IGF receptor subunit and an insulin receptor
subunit. Studies show that these hybrid receptors preferentially act like type-1 IGF
receptors and bind IGFs with high affinity.
When IGFs bind, the receptor tyrosine kinase activity on the beta subunit is activated,
causing tyrosine residues to become phosphorylated. As is the case with all other tyrosine kinase receptors, these phosphotyrosine residues then recruit signaling proteins
to turn on a variety of downstream signaling pathways, such as the MAP kinase proliferation pathway and the AKT survival pathway. Therefore, binding of IGFs to the
type 1 receptor provides signals to tumor cells that may confer resistance to killing by
cytotoxic drugs.
Blockage of binding to the type-1 receptor represents one therapeutic strategy to stop
cell proliferation and affect tumor cell survival.
Anti-type-1 IGF receptor monoclonal antibodies can be generated that are highly specific for binding to the alpha subunit of the type-1-IGF receptor, thereby blocking its
function.
Monoclonal antibodies have been developed that have high selectivity and affinity
for the type-1 IGF receptor, and do not recognize or bind to insulin receptors. Due to
the selectivity for the type-1 IGF receptors, they can also recognize and block hybrid
receptors formed between type-1 IGF receptors and insulin receptors. Treatment of
tumor cells with such an antibody creates a direct blockade of IGF binding to type-1
receptors and inhibits downstream signaling. It also causes type-1 receptor molecules
to be removed from the surface of tumor cells. Ultimately, it is hoped that clinical use
of IGF receptor-blocking antibodies can reduce tumor cell growth and increase apoptosis in tumors.
ImClone Systems
Movie 6.1 Regulation of Signaling by the Src Protein
T
he Src protein kinase acts as a molecular signal integration device. Src consists of
two clearly demarcated regulatory domains: an SH3 domain and an SH2 domain.
The catalytic domain is structured much like other tyrosine kinases. In addition, a
linker and the short C-terminal tail play regulatory roles.
To be an active protein kinase, the active site of the Src kinase domain must first
become accessible to substrate proteins. In the inactive state, the active site is blocked
by an activation loop.
The activation loop must be phosphorylated, most likely through the actions of
another SRC kinase molecule, not shown here. Phosphorylation causes the activation
loop to swing out of the way and rearrange so that other substrate proteins can bind
and become phosphorylated.
Src is kept inactive by an interaction of its SH2 domain with the C-terminal tail peptide.
A phosphorylated tyrosine on its C-terminal tail is buried in a deep binding pocket in
the SH2 domain. To activate Src, this phosphate group is removed by specific phosphatases. Upon dephosphorylation of the tyrosine, the C-terminal tail is released from
the SH2 domain.
In this cartoon view, Src is represented in its inactive state. The linker segment, shown
in red, connects the SH2 and SH3 domains on the one hand, and the kinase domain
on the other. The activation loop, shown in dark green, drapes across the catalytic sites
and, in this configuration, blocks the catalytic domains access to the substrate of the
kinase.
Several other intramolecular interactions hold the Src kinase in an inactive configuration. The SH2 domain, in blue, binds a phosphotyrosine at residue position 527. The
SH3 domain, in light green, binds the linker segment.
When a PDGF receptor becomes activated by ligand binding, its C-terminal tail
becomes phosphorylated on tyrosine residues. One of these phosphotryrosines can
be recognized and bound by the SH2 domain of Src.
The SH2 domain, which until now participated in an intramolecular binding, switches
and forms an intermolecular bridge by binding the phosphotryrosine on the C-terminal tail of the PDGF receptor.
The SH3 domain follows suit. It too breaks its intramolecular binding and binds to a
proline-rich segment on the PDGF receptor.
These shifts liberate the kinase domain of the Src protein. In a series of concerted reactions, the tyrosine residue at position 527 of Src is dephosphorylated, and the activation loop becomes phosphorylated on a tyrosine residue.
This last alteration forces the activation loop to move out of the way so that it no longer
obstructs the catalytic cleft of the Src enzyme. This allows Src to phosphorylate a
diverse array of protein substrates on their tyrosine residues.
Original Storyboard by Peter Walter, Howard Hughes Medical Institute, University of
15
Movie 6.2 Signaling by the Ras Protein
T
he Ras protein is a representative example of the large family of GTPases that can
function as molecular switches. The nucleotide-binding site of Ras is formed by
several conserved protein loops that cluster at one end of the protein. In its inactive
state, Ras is bound tightly to GDP.
As a molecular switch, Ras can toggle between two different conformational states
depending on whether GDP or GTP is bound. Two regions, called switch 1 and switch
2, change conformation dramatically. The change in conformational state allows other
proteins to distinguish active Ras from inactive Ras. Active, GTP-bound Ras binds to,
and activates, downstream target proteins in the cell signaling pathways, such as RAF,
PI3K, and GDS.
A space-filling model shows that the conformational changes between the GDP and
GTP bound forms of Ras spread over the whole surface of the protein. The two switch
regions move the most.
Ras hydrolyzes GTP to switch itself off; that is, to convert from the GTP-bound state
to the GDP-bound state. This hydrolysis reaction requires the action of a Ras GTPaseactivating protein, or Ras GAP for short. RasGAP binds tightly to Ras burying the bound
GTP. It inserts an arginine side chain, sometimes called an Argenine finger, directly
into the active site. The arginine, together with threonine and glutamine side chains
of Ras itself, promotes the hydrolysis of GTP. Ras then sits in its inactive GDP-bound
state, awaiting a stimulatory signal that will cause it to evict GDP and acquire GTP.
Molecular modeling and animation: Timothy Driscoll
Movie 6.3 EGF Receptors and Signaling
W
hen epidermal growth factor (or EGF) binds to receptors on the plasma membrane, the receptor molecules structurally rearrange and dimerize. EGF receptors are in a class of receptor tyrosine kinases, and the dimerization of these receptors
results in the reciprocal activation of the two kinase domains. Once a tyrosine kinase
is activated, it phosphorylates the C-terminal cytoplasmic tail of the other receptor at
multiple tyrosine residues. For many receptors, this transphosphorylation is reciprocal, with each receptor molecule phosphorylating the other. As we will see, the phosphate groups decorating the c-terminal tail enable the receptor to activate a series of
downstream signaling events.
Through a series of intermediary signaling proteins, the activated receptor turns on
a protein called Ras. Ras acts like a binary switch, and EGF signaling triggers Ras to
convert from its GDP-bound inactive state to its GTP-bound, actively signaling state. If
mutated into a constitutively active form, Ras becomes oncogenic.
It has long been known that the activation of the EGF receptor has very similar effects
on the cell as the activation of a Ras protein. This suggested the possibility of signaling
between them.
The discovery of SH2 domains provided important clues of how this signaling proceeds. These domains, which are parts of larger proteins, enable proteins to recognize
and bind phosphotryrosine residues displayed by yet other proteins.
The SH2 group allows recognition of two chemical structures: first, the phosphotyrosine itself; and second, the adjacently located amino acid residues. There are many
phosphotyrosines attached to an activated growth factor receptor, and each of these
receives its unique identity from its neighboring amino acid residues.
Such phosphate groups decorate the C-terminal tail of a growth factor receptor.
Accordingly, a molecule like Shc, which has an SH2 group, can recognize and bind a
phosphotyrosine group on the C-terminal tail of the EGF receptor. In the case of Shc,
the phosphotyrosine is followed by three other amino acids, yielding the sequence:
tyrosine (Y), leucine (L), isoleucine (I), and proline (P).
Like Shc, this theme recurs in a number of other proteins, which also contain their own
specialized SH2 domains. Each recognizes a particular phosphotyrosine followed by
three distinct amino acid residues.
Because certain phosphotyrosines attract multiple SH2 groups, the entire array of SH2
containing proteins that can become associated with a receptor is quite elaborate and
can be depicted like this.
Each of these associated proteins has its own distinct biochemical function, which is
carried out by other domains linked to its SH2 domain. For example, JAK2 is a tyrosine kinase that becomes activated after it binds to the receptor tail via its SH2 group.
PLC-g is an enzyme that cleaves phospholipids in the plasma membrane. And Grb2 is
a member of an important class of adaptor proteins whose sole function is to build a
bridge between the receptor and a third protein.
Focusing on Grb2, we can see that SH2 groups are not the only way by which proteins
can recognize and bind to one another. For example, SH3 groups recognize and bind
proline-rich sequences on other proteins. In the case of Grb2, its two SH3 groups recognize a third protein called SOS. Once SOS becomes bound indirectly to the receptor,
it is brought in close proximity to Ras proteins tethered to the plasma membrane. SOS
acts as a guanine nucleotide exchange factor. Such proteins are capable of inducing
RAS to release its bound GDP, allowing GTP to jump aboard.
These two forms of Ras represent different states of activity. The GDP-bound form is
inactive and the GTP-bound form is active in signaling. The period of active signaling is quite short since other proteins interact with Ras and induce it to hydrolize its
bound GTP.
17
While Ras is in its active state, it is able to interact with a variety of partner proteins that
serve, in turn, to activate downstream signaling pathways. For example, Ras can bind
to the serine threonine kinase called Raf. When Raf interacts with Ras, it becomes an
active signal-emitting kinase.
An immediate downstream target of Raf is called MEK. Once MEK becomes activated
through phosphorylation, it also becomes an active serine threonine kinase. Its most
important substrate is called Erk. Once activated, Erk proceeds to phosphorylate a
diverse group of proteins that favor cell proliferation.
Another important event initiated by Ras is the PI3 kinase cascade. Once again, a
kinase is being activated, but in this case the object of phosphorylation is not a protein, but instead a phospholipid embedded in the plasma membrane.
The phospholipid molecule is constructed from two long hydrocarbon tails embedded
in the plasma membrane, a glycerol, and an inositol sugar-like molecule. In the case of
PI3K, its immediate substrate is a phosphotidyl inositol to which two phosphates have
been previously attached. PI3K extends this process by adding yet another phosphate
to the inositol ring.
This now creates a site to which a number of cytoplasmic proteins can attach. These
proteins use another specialized domain to recognize and bind the triply phosphorylated inositol. The domain is called a PH domain. One of the most important molecules that contains a PH domain is a kinase called Akt or PkB.
Once Akt binds, it becomes phosphorylated and then becomes functionally activated.
Once activated, Akt is released and can travel through the cytoplasm to activate multiple substrates.
The ability of Akt to phosphorylate these other proteins, thereby altering them, has
multiple effects on the cell. For example, by phosphorylating Bad, it reduces the likelihood of apoptosis. By phosphorylating mTOR, it facilitates the physical growth of the
cell. And by phosphorylating GSK-3beta, it stimulates cell proliferation.
Storyboard and Animation: Sumanas, Inc. (www.sumanasinc.com)
19
Movie 7.1 Intestinal Crypt
T
he lining of the small intestine, like the lining of most of the gut, is a single-layered
epithelium.
Depending upon location in the gut, the epithelial cells, usually called enterocytes,
help absorb nutrients from the lumen of the gut or absorb water from the intestinal
contents. In the small intestine, the surface of the lining of the gut is increased enormously by thousands of villi that protrude into the lumen.
Here we see a single villus and its internal architecture. The surface of the villus is covered by a single layer of enterocytes, which extend down into the crypt below.
Stem cells at the bottom of the crypt, located between paneth cells, divide and make
copies of themselves, and also make transit-amplifying cells. The transit amplifying cells proliferate rapidly and move up the walls of the crypt. As the cells migrate
upwards they begin to differentiate into goblet cells and enterocytes. While the cells
are moving up the sides of the villus, they carry out the essential functions of the small
intestine, notably absorption of nutrients.
When the differentiated cells reach the tip of the villus, they undergo apoptosis and
are shed into the lumen of the small intestine. The entire process of out-migration and
cell death is completed in just three to four days.
The process of out-migration and rapid cell replacement is a defense mechanism
against the development of colon cancer, since almost all epithelial cells, including
those that have accidentally sustained mutations, are shed within days of their formation. Therefore, the only mutations that can lead to the development of a cancer are
those that are retained in the crypt. This dictates that such mutations must block the
outmigration of mutant cells from the crypt.
The outmigration of transit amplifying cells from the bottom of the crypt depends on
the protein called adenomatous polyposis coli, or simply APC. In the absence of functional APC, this continuous outmigration is blocked, leading to the accumulation of
transit amplifying cells in the crypt.
In this animation of an experiment, APC loss is achieved through an induced gene
inactivation, which appears to mimic the mutation that initiates most gastrointestinal
tumors.
APC is usually required to inactivate the intracellular protein called β-catenin; the
inactivation of β-catenin permits the differentiation of the transit amplifying cells
and their continued outmigration from the base of the crypt. In the absence of APC
function, β-catenin accumulates within the transit amplifying cells, which blocks both
their outmigration and differentiation.
The accumulated transit amplifying cells do not themselves form a carcinoma. However, they and their descendants can now accumulate additional mutations that will
drive such cells progressively to become full-fledged carcinoma cells.
Animation by Digizyme, Inc. (www.digizyme.com)
Models, Animation, Surfacing, Composite: Eric Keller
Storyboard and Art Direction: Gael McGill
© 2009 by Hans Clevers
Hans Clevers
Hubrecht Institute
Movie 8.1 Animal Cell Division
D
ifferential interference contrast microscopy is used here to visualize mitotic
events in a lung cell grown in tissue culture. Individual chromosomes become
visible as the replicated chromatin starts to condense. The two chromatids in each
chromosome remain paired as the chromosomes become aligned on the metaphase
plate. The chromatids then separate and get pulled by the mitotic spindle into the
two nascent daughter cells. The chromatin decondenses as the two new nuclei form
and cytokinesis continues to constrict the remaining cytoplasmic bridge until the two
daughter cells become separated.
Video reproduced from: The Journal of Cell Biology 122:859–875, 1993.
© The Rockefeller University Press
Edward D. (Ted) Salmon and
Victoria Skeen
University of North Carolina at
Chapel Hill
Robert Skibbens
Lehigh University
Movie 8.2 CDK2
L
ike other kinases, Cyclin-dependent kinases, or Cdks for short, are crucial regulatory proteins in the cell cycle. When activated, Cdks transfer phosphate groups
from ATP to serine and threonine side chains on targeted substrate proteins. When
inactive, the active site of Cdks is sterically obstructed by a loop, often referred to as
the activation loop.
As their name suggests, cyclin-dependent kinases are activated by cyclins. Cyclin
binding to Cdk pulls the activation loop away from the active site and exposes the
bound ATP, allowing it access to target proteins. Thus, a Cdk can phosphorylate target
proteins only when it is in a cyclin–Cdk complex.
A third protein called a Cdk-activating kinase is required for full activation of a Cdk.
This activating kinase adds a phosphate group to a crucial threonine in the activation
loop, thereby completing the activation of the cyclin–CDK complex.
Oligopeptide domains of substrate proteins bind to the active site of the cyclin–Cdk
complex so that the target serine or threonine side chains of the substrate are precisely
positioned with respect to the gamma phosphate of the bound ATP.
Cdk inhibitor proteins, or CKIs, help regulate the rise and fall of cyclin–Cdk activity.
Some inhibitors—like the one shown here—bind directly at the kinase active site and
block kinase activity by interfering with ATP binding. Other inhibitors—which are not
shown here—bind near the active site and interfere with substrate binding.
Movie 9.1 p53 Structure
p53 is a tumor suppressor protein that prevents cells from dividing inappropriately.
Loss of p53 function is associated with many forms of cancer. In this image, p53
polypeptide is shown binding to DNA, reflecting p53’s ability to act as a transcription
factor. In fact, while not shown here, p53 is normally a tetramer and acts to induce
expression of some genes and repress expression of others. p53 has a beta barrel adjacent to its DNA-binding domain.
The p53–DNA interface is complex. It involves several loops and a helix that extends
from the β barrel core. Residues from one loop and the helix bind in the DNA major
groove. Arginine 248 from another loop makes extensive contacts with the DNA backbone and, indirectly through water molecules, with bases in the minor groove. Alterations in arginine 248 and other residues involved in DNA binding are commonly found
in the p53 proteins present in human tumors.
Loop 2 does not bind to DNA directly but is essential for correctly positioning arginine
248 on the DNA. Three cysteines and a histidine from both loop 2 and loop 3 cooperate
to sequester a zinc ion, forming the rigid heart of a zinc-finger motif. Mutations that
affect the zinc finger represent other examples of how DNA binding by p53 is compromised in human tumor cells.
21
9.2 Apoptosis
A
poptosis, a form of programmed cell death, has been induced in these cultured
cells. Cell death is characterized by blebbing of the plasma membrane and fragmentation of the nuclei. Suddenly, cells weaken attachment to the substratum that
they have been growing on and shrivel up without lysing.
In the following movies we observe the process at higher magnification. The mechanism of apoptosis involves many tightly controlled steps, three of which are demonstrated here by different visualization techniques.
One initial event is the sudden release of cytochrome c from mitochondria into the
cytosol. This event has been visualized here using fluorescently labeled cytochrome
c. Initially the greenish/yellow staining is restricted to a reticular pattern, which then
suddenly disperses as the mitochondria release their content proteins into the cytosol.
At a later step, the lipid asymmetry of the plasma membrane breaks down. In normal
cells, phosphatidyl serine is found only on the cytosolic side of the plasma membrane;
but when cells undergo apoptosis, it becomes exposed on the outside of the cell. This
event has been visualized here by adding a red fluorescent protein to the media, which
specifically binds phosphatidyl serine head groups as they become exposed. In an
intact organism, exposure of phosphatidyl serine on the cell surface labels the dead
cell and its remnants so that they are rapidly consumed by other cells, such as macrophages.
Finally—although apoptosing cells don’t lyse—their plasma membranes do become
permeable to small molecules. This event has been visualized here by adding a dye to
the media that fluoresces blue when it can enter cells and bind to DNA.
All three of these events can be observed in the same group of cells.
These epithelial cells express green fluorescent cadherin. They are grown at low density, so that isolated cells can be observed. Initially, labeled cadherin is diffusely distributed over the whole cell surface.
As cells crawl around and touch each other, cadherin becomes concentrated as it
forms the adhesion junctions that link adjacent cells. Eventually, as the cell density increases further, the cells become completely surrounded by neighbors and form a tightly packed sheet of epithelial cells.
Part I:
© 2007 The Sakura Motion Picture Company. All rights reserved. Used with permission.
Part II:
© 2001 J.C. Goldstein and D.R. Green. All rights reserved. Used with permission.
Part I:
Shigekazu Nagata
Kyoto University
Part II:
Joshua C. Goldstein
The Genomics Institute of the
Novartis Research Foundation
Douglas R. Green
St. Jude Children’s Research Institute
Movie 10.1 Telomere Replication
T
he ends of linear chromosomes pose unique problems during DNA replication.
Because DNA polymerases can only elongate from a free 3ʹ hydroxyl group, the
replication machinery builds the lagging strand by a backstitching mechanism. RNA
primers provide 3ʹ-hydroxyl groups at regular intervals along the lagging strand template.
Whereas the leading strand elongates continuously in the 5ʹ-to-3ʹ direction all the way
to the end of the template, the lagging strand stops short of the end.
Even if a final RNA primer were built at the very end of the chromosome, the lagging
strand would not be complete.
The final primer would provide a 3ʹ-OH group to synthesize DNA, but the primers
would later need to be removed. The 3ʹ-hydroxyl groups on adjacent DNA fragments
provide starting places for replacing the RNA with DNA. However, at the end of the
chromosome there is no 3ʹ-OH group available to prime DNA synthesis.
Because of this inability to replicate the ends, chromosomes would progressively
shorten during each replication cycle. This “end-replication” problem is solved by
the enzyme telomerase. The ends of chromosomes contain a G-rich series of repeats
called a telomere. Telomerase recognizes the tip of an existing repeat sequence. Using
an RNA template within the enzyme, telomerase elongates the parental strand in the
5ʹ-to-3ʹ direction, and adds additional repeats as it moves down the parental strand.
The lagging strand is then completed by DNA polymerase alpha, which carries a DNA
primase as one of its subunits. In this way, the original information at the ends of linear chromosomes is completely copied in the new DNA.
23
Movie 12.1 DNA Repair Mechanisms
F
aults in DNA molecules can be harmful; so all organisms have DNA repair
mechanisms that can correct most of them. Mutations (that is, changes in DNA
sequence) can appear in DNA either as mistakes in DNA replication, which results
in a mismatched nucleotide pair, or by the chemical or physical action of a mutagen.
Other agents, known as clastogens, can cause breaks in DNA molecules that may, in
turn, lead to mutations.
Most cells possess four different types of DNA repair systems. Direct repair and excision repair systems fix DNA molecules carrying nucleotides damaged by mutagens.
Mismatch repair corrects mismatched, but otherwise normal, nucleotides that result
from errors in replication. Nonhomologous end-joining (NHEJ) is used to mend double-strand breaks in DNA. There is also a fifth repair system called homology-directed
repair, but it will not be covered here.
Direct repair is the simplest repair process and acts directly on damaged nucleotides,
converting them back to their original structure. Only a few types of mutagen damage
can be repaired directly. In this example, guanine acquires an alkyl group, which disrupts its normal pairing with cytosine.
In a future round of DNA replication, the alkylated guanine would pair with thymine, thereby propagating a DNA mutation. To fix this chemical damage, an enzyme
transfers the alkyl group to itself, restoring the base to normal. Many organisms have
enzymes that perform this function, such as the Ada enzyme in E.coli and MGMT in
humans.
In contrast to this simple, single-step repair, most types of DNA damage require several steps to fix. They can be repaired only by removing the damaged nucleotides and
then filling in the gap. This process is called excision repair. Base excision repair is a
type of excision repair mechanism and is used to repair relatively minor damage. The
damaged base is excised from a nucleotide by a specific DNA glycosylase, which first
flips the damaged base out of the helix and then cuts the β-N-glycosidic bond between
the base and sugar, leaving a baseless site, called an AP site. An AP endonuclease cuts
the phosphodiester bond on the 5ʹ side of the AP site, and then the other 3ʹ side is cut,
either by the endonuclease itself or by a phosphodiesterase. The resulting gap is filled
in by a DNA polymerase and sealed by a DNA ligase.
To correct more extensive types of damage, such as those that cause helix distortions, cells use another type of excision repair, called nucleotide excision repair. The
process is different from base excision repair in that it begins with the removal of an
entire block of nucleotides rather than a base from a single nucleotide. A characteristic
example is the short-patch process used by E.coli.
A complex of proteins, called the UvrAB trimer, scans DNA for damage. UvrA dissociates once the site has been found and plays no further part in the repair process. UvrC
now binds, forming an UvrBC dimer that cuts the polynucleotide on either side of the
damaged site, resulting in a 12-nucleotide excision. UvrB cuts a phosphodiester bond
(often the 5th downstream of the damaged site) and then UvrC cuts another bond
(often the 8th upstream of the damaged site). The excised segment is then removed by
DNA helicase II, which breaks the hydrogen bonding holding the damaged segment in
place. UvrC also detaches at this stage, but UvrB remains in place and bridges the gap
produced by the excision. The gap is filled by DNA polymerase I. DNA ligase forms the
last phosphodiester bond. In humans, a similar but more complex repair machinery
fixes this type of damage.
The direct, base excision, and nucleotide excision repair mechanisms all act by
searching for and correcting abnormal chemical structures in DNA. However, they
cannot correct mismatches resulting from errors in DNA replication, because the mismatched nucleotides are not abnormal in any way. A special type of excision repair,
called mismatch repair, is used instead. Rather than detect an abnormal nucleotide,
the mismatch repair system detects the absence of proper base pairing between the
parent and daughter strands. Before the mismatch repair system can excise part of the
daughter strand and fill in the gap, it must first distinguish between the parent and
daughter strands.
In E. coli, the two strands are distinguished by their differing levels of methylation.
The newly synthesized daughter strand is not methylated, but the older parent strand
is methylated at GATC, CCAGG, and CCTGG sequences. In the long-patch mismatch
repair system of E. coli, a protein called MutS recognizes the mismatched nucleotides while another protein, called MutH, distinguishes the two strands by binding
to nearby unmethylated 5ʹ-GATC-3ʹ sequences—that is, by binding to the daughter
strand. MutH cuts the unmethylated DNA at the methylation site. Starting at this cut
site, a DNA helicase detaches a segment of the single strand. The detached singlestranded region is degraded by an exonuclease that follows the helicase and continues
beyond the mismatch site. The gap is then filled in by DNA polymerase I and DNA
ligase. Note that eukaryotes don’t have heavily methylated DNA and likely use a different mechanism to distinguish between the parent and daughter strands.
A double-stranded break in DNA is potentially devastating to a cell. Such breaks can
be generated by exposure to ionizing radiation and some mutagens, and occasionally
during DNA replication. They are repaired either by a process called nonhomologous
end joining or homologous recombination, not illustrated here. In nonhomologous
end-joining, a pair of proteins called Ku bind to broken DNA ends. The individual Ku
proteins also have an affinity for one another, which brings them and the two broken ends of the DNA molecule into proximity. The DNA fragments are joined back
together by a DNA ligase.
Experimental studies indicate that restoring normal DNA structure is difficult to
achieve. Often, the nucleotides flanking the double-stranded break are lost, resulting
in loss of normal DNA sequence. Alternatively, if two chromosomes happen to be broken, a misrepair resulting in hybrid structures occurs relatively frequently. Hence, this
type of repair is often prone to errors, in contrast to base excision repair, or mismatch
repair, which restore normal DNA sequence with far greater fidelity.
25
13.1 Mechanisms Enabling Angiogenesis
A
s a normal part of growth and development, the body must generate new blood
vessels to oxygenate the tissues. In a process called angiogenesis, new vessels
sprout from existing ones. In this movie, we see endothelial cells sprouting to form
new branches from the aorta of a zebrafish embryo. Each sprout is initially formed by
one or a few endothelial cells.
Angiogenesis can be initiated in a number of different ways, such as when cells do not
receive enough oxygen, often due to inadequate access to the circulation. In response,
the cells emit molecules that stimulate angiogenesis, notably vascular endothelial
growth factor, or VEGF.
When VEGF molecules reach a nearby blood vessel, the vessel’s endothelial cells
become motile and form a “tip cell” that produces long extensions called filopodia,
which guide the development of a new vessel toward the cells emitting VEGF.
As the tip cell moves toward its angiogenic stimulus, other endothelial cells behind it
form a stalk. These stalk cells then start to hollow-out and form a tube.
When tip cells emerging from different blood vessels meet, they merge, and blood can
now begin to flow through the new vessel. As the young vessel matures, its endothelial
cells recruit pericytes, which closely resemble smooth muscle cells, to the vessel walls.
The pericytes help to stabilize vessel structure.
Angiogenesis is also critical to the development and proliferation of tumors. From
hypoxia or the actions of oncogenes, tumor cells often emit elevated levels of VEGF.
Increased VEGF causes nearby blood vessels to produce excessive numbers of tip
cells, often resulting in malformed capillaries. The capillaries found in tumors are ten
times more permeable than normal capillaries, a condition directly attributable to the
excessive levels of VEGF.
Because blood cannot flow efficiently through malformed vessels, the tumor cells may
remain hypoxic and continue to emit VEGF, perpetuating the formation of additional
malformed vessels.
Significantly, it is believed that the poorly assembled vessel walls permit escaped
tumor cells to enter the bloodstream more readily, where they can travel to other locations in the body and start secondary tumors.
Peter Carmeliet
Vesalius Research Center, Catholic
University of Leuven
27
13.2 Interactions of Innate Immune Cells with a Mammary
Tumor
T
his time-lapse microscopic video of the tissue of a live mouse shows cells of the
immune system patrolling groups of mammary carcinoma cells. This tumor was
caused by the mouse mammary tumor virus polyoma middle T transgene. The tumor
cells are stained in blue, while cells that have properties of macrophages and dendritic cells are labeled in green. Macrophages are known to play diverse, if not opposing, roles in tumor pathogenesis. One class of macrophages, called M1 macrophages,
facilitate the attack on the tumor by the immune system. A second major class of macrophages, often called M2 macrophages, facilitate inflammatory responses and can
actually accelerate tumorigenesis and malignant progression. As is apparent here,
the cells of the immune system actively patrol the clusters of mammary tumor cells,
ostensibly to acquire antigens for presentation to the immune system, and perhaps to
release cytokines that may affect subsequent responses of the immune system to the
presence of the tumor.
Mikaela Egeblad and Zena Werb
University of California,
San Francisco
Movie 14.1 Adhesion Junctions
T
hese epithelial cells express green fluorescent cadherin. They are grown at low
density, so that isolated cells can be observed.
Initially, labeled cadherin is diffusely distributed over the whole cell surface. As cells
crawl around and touch each other, cadherin becomes concentrated as it forms the
adhesion junctions that link adjacent cells.
Eventually, as the cell density increases further, the cells become completely surrounded by neighbors and form a tightly packed sheet of epithelial cells.
Music: Christopher Thorpe
Stephen J. Smith
Stanford University School of
Medicine
Cynthia Adams
Finch University of Health Sciences
and Chicago Medical School
Yih-Tai Chen
Cellomics, Inc.
W. James Nelson
Stanford University School of
Medicine
Movie 14.2 Mechanisms of Brain Metastasis Formation
M
etastasis, or the formation of malignant growth at a site of the body far removed
from the location of the primary tumor, is responsible for 90% of deaths from
cancer. In this image we see multiple breast cancer metastases to the brain.
Multiphoton laser scanning microscopy, together with image processing software,
can be used to image metastases to the brain of a live mouse. A transparent window
permits visualization of the various steps of the brain metastatic cascade that we will
observe in this movie.
The brain metastatic cascade can be broken down into four steps: (1) Initial arrest of
circulating tumor cells in narrow microvessels; (2) escape of arrested cells from the
microvessel into the brain parenchyma, the process of extravasation; (3) the establishment of the recently extravasated cancer cell in a location close to a blood vessel, referred to as the perivascular position; (4) and finally, two alternative routes of
metastasis formation—either cooptive growth, in which cancer cells grow along existing blood vessels, or angiogenic growth, in which cancer cells actively induce the formation of new blood vessels to nourish them. In the examples that follow, melanoma
cells took on cooptive growth, whereas lung cancer cells were found to take on angiogenic growth.
Here we see the first step involving the initial arrest of a cancer cell, labeled red, in a
capillary, labeled green. A second cell is later seen trapped above in an even smaller
vessel.
The second step in the brain metastatic cascade is the extravasation of cancer cells
from the lumen of the capillary. Cancer cells use several alternative strategies to
extravasate. In this movie, we see a cluster of cancer cells extravasating, one after the
other, out of vessel number one into the brain parenchyma.
The third step involves the establishment of perivascular positions, which seem to
ensure the initial survival of the extravasated cells. This movie shows a tomographic
image of cancer cells in perivascular positions.
Nonetheless, assumption of a perivascular position does not on its own guarantee
long-term survival, as seen in these images. Although this micrometastasis grows in a
perivascular position for the first nine days, it is completely gone by Day 14.
The fourth and final step in the brain metastatic cascade involves two alternative strategies—cooptive or angiogenic growth—depending on the type of cancer cell that has
metastasized. As mentioned previously, lung cancer cells initiate angiogenic growth
in order to nourish themselves.
In this series of images, we see how the potent angiogenic powers of lung carcinoma
cells are able to provoke the florid outgrowth of microvessels that sustain the active
growth of the tumor.
An alternative strategy for growth is exhibited by these melanoma cells, which have
disseminated to the brain and assumed a perivascular position. Here we see how their
expansion has depended on the cooptive strategy involving growth along the outside
of blood vessels.
The multiphoton microscopy used to create these movies demonstrates the extraordinary advances in the powers of modern imaging technology to study cancer in living
tissue.
Frank Winkler
Neurology Clinic and National
Center for Tumor Diseases &
German Cancer Research Center
(DKFZ), University of Heidelberg
29
Movie 14.3 Visualization of Cancer II: Metastasis
M
etastasis has generally been a difficult process to visualize. In this model, highresolution magnetic resonance imaging, or MRI, has been combined with tomography techniques to visualize the metastasis of breast cancer cells to the brain of a
mouse.
Before the injection of cancer cells into the circulation of the mouse, the cancer cells
were labeled with a “dilutable” label, that is, one that loses half of its intensity each
time a cell divides. This dilutable label is seen here in red.
Following initial injection at Day 1, many hundreds of labeled cells are apparent in
the brain. In the succeeding days, the number of these cells that retain full label, and
therefore have not divided, is diminished.
At the beginning of the third week, small metastatic growths suddenly begin to appear
in green. These green signals represent actively growing cancer cells. These green
metastases continue to grow in size while the individual nongrowing, single-cell
metastases continue to disappear.
Among other lessons, this experiment illustrates that hundreds if not thousands of
micrometastatic single cells may be seeded initially in a target organ by cells arriving
from the primary tumor via the circulation. However, only a minute fraction of the
initially seeded cells succeed in spawning metastatic colonies.
Movie 15.1 The Immune Response
A
n immune response involves events that unfold both locally, at the site of an
infection, and at more distant sites, such as nearby lymph nodes. We can see the
integration of the different parts of the immune response if we follow the course of a
typical infection. Most pathogens are kept outside of the body by epithelial barriers,
such as the epidermis, and are crossed only when there is an injury or tissue damage.
After an injury, bacteria cross the epidermis and establish an infection in the underlying tissue. Phagocytic cells in the tissues, such as macrophages and neutrophils,
engulf the pathogen. Dendritic cells are also phagocytic and are activated by binding
pathogens to leave the site of infection and migrate to a lymph node. The migrating
dendritic cells enter the lymphatic vessels and are collected in a draining lymph node.
In the lymph node, T cells are activated by antigen presented by the dendritic cells,
and in turn activate B cells to secrete antibody. Effector T cells and antibody molecules
return to the circulation. They leave the circulation again at the site of infection, where
inflammatory mediators have induced changes in the blood vessel endothelium.
CD4 T cells activate macrophages to become more cytotoxic, while antibody recruits
complement to lyse bacteria directly and to opsonize them, enhancing their uptake
by phagocytes. In the case of a viral infection, activated CD8 T cells would kill any
infected cells present.
Ann Chambers
London Health Sciences Centre
Movie 15.2 Antigen Display and T-Cell Attack
W
hen human cells are infected by viruses, the host cell ribosomes are exploited by
the virus to translate their viral mRNA into viral proteins. In the normal course
of protein turnover in the cell, the viral protein, like many endogenous proteins, can
be tagged by the attachment of a chain of Ubiquitin molecules, shown here in blue.
The poly-ubiquitin tagged protein is then led to a proteasome. The protein chain is
then fed into the proteasome, which cleaves it into oligopeptides.
Peptidase enzymes present in the cytosol then digest them further.
On the cytoplasmic surface of the endoplasmic reticulum, some of these peptide fragments will interact with TAP proteins, which are specialized peptide transporters that
pump them into the lumen of the endoplasmic reticulum.
Upon entering the lumen of the endoplasmic reticulum, the peptides encounter MHC
class I molecules. The peptides may then bind tightly to the peptide-binding groove of
the MHC molecules, a specialized part of the MHC molecule, shaped like the palm of
a hand.
A membranous vesicle containing the MHC class I–peptide complex is then pinched
off the ER and dispatched to the inner surface of the plasma membrane. The vesicle
then fuses with the plasma membrane, and thereby exposes the contents of its lumen
at the cell surface. In this way, the MHC class I and bound peptide are displayed on the
cell surface, making them available for surveillance by the immune system.
Cytotoxic T Lymphocytes, or CTLs, are an important arm of the immune system. A
CTL, shown here in pink, may display on its surface an antigen-recognizing protein,
specifically, a T cell receptor. The T cell receptor may recognize the oligopeptide being
displayed by the MHC class I receptor. The CTL also depends on a second cell surface
molecule, called CD8, which recognizes and binds all MHC class I molecules.
This recognition and binding activates the cytotoxic T cell. The activated T cell first
uses perforin to punch holes in the surface of the targeted cell, and then it injects proapoptotic enzymes, called granzymes, into the cytoplasm. Activation of the apoptotic
cascade by granzymes eventually results in the death of the targeted, virus-infected
cell.
Biointeractive, Howard Hughes Medical Institute
www.hhmi.org/biointeractive ©2013 HHMI
Bruce D. Walker
Howard Hughes Medical Institute,
Harvard Medical School
31
Movie 16.1 Drug Export by the Multi-Drug Resistance
Pump
T
ransmembrane drug efflux pumps can remove chemotherapeutic drugs from
inside a cancer cell, thereby reducing intracellular drug concentration, which
generates resistance to the cytotoxic effects of the drug. Certain drug efflux pumps can
extrude multiple types of drug molecules creating the state of multi-drug resistance,
or MDR.
P-glycoprotein (Pgp) is the most prevalent MDR transporter and it is expressed at
elevated levels in many kinds of cancer cells, especially those that have survived a
chemotherapeutic treatment.
This space-filling model of P-glycoprotein shows a drug molecule (colored in pink)
bound to the pump’s drug binding pocket (colored in silver). Such a drug molecule
will have entered the pump and its drug-binding pocket from the cytosol through the
open portal seen at the bottom.
Energy is required to pump drugs out of the cell. P-glycoprotein is an ATP-dependent transmembrane protein, and ATP must bind to the interior nucleotide-binding
domains in order to pump a drug or other toxins out of the cell.
Switching to a ribbon representation of the pump, we can observe the dramatic conformational change in the pump caused by ATP binding. Drugs or other toxins enter
the pump, and attach to the drug-binding pocket, which results in an ATP-dependent
shift in the conformation of the entire pump. The drug is then extruded into to the
extracellular space.
Rotating the structure 90 degrees in the same plane, we can observe the pumping
action, and the opening and closing of the cytosolic and extracellular ends of the
pump.
In a third view, we observe the pump through its channel, looking down from the
extracellular side.
Geoffrey Chang
The Scripps Research Institute
Movie 16.2 PI3K
P
hosphatidylinositol-3 kinase, or PI3K, is a multidomain enzyme. The structure of
PI3K, as determined by X-ray crystallography, is shown here as a ribbon diagram.
Each domain of PI3K is associated with distinct functions and colored differently. The
Ras-binding domain is shown in red; this domain enables Ras to directly activate the
PI3K catalytic domain. The catalytic domain is colored purple.
This space-filling model shows the same protein but more closely resembles the actual
3-dimensional structure.
Zooming-in to the catalytic domain, we enter the catalytic cleft and observe a drug
molecule that inhibits normal PI3K enzyme function. The inhibitor molecule blocks
the catalytic site and prevents PI3K from accessing its usual ATP substrate. This subsequently prevents PI3K from phosphorylating phosphatidylinositol diphosphate and
disrupts downstream signaling.
This drug molecule, called GDC-001, is shown here as a stick figure . . . and now as a
space-filling model. In this view, we can see the complementary three-dimensional
structures of the drug molecule and the walls of the surrounding catalytic cleft.
The specificity of binding between the drug molecule and the catalytic cleft is achieved,
in part, through the formation of hydrogen bonds between the drug molecule and the
side-chains of amino acid residues lining the wall of the cleft.
Paul Workman and Rob L.M. van
Montfort
Cancer Research UK
Mini-Lectures by Robert A. Weinberg
T
he author has recorded sixteen mini-lectures for students and instructors on a
range of topics covered in the book. These are available in MP3 format and can be
transferred to a mobile device, as well as enjoyed on your computer. They are located
in the “Mini-Lectures” folder on the DVD or can be found on the Garland Science
website. Below is a list of the Mini-Lectures, followed by transcripts of the recordings.
Mini-Lecture Table of Contents:
01. Mini-Lecture: Mutations and the Origin of Cancer
02. Mini-Lecture: Epidemiology and Cancer
03. Mini-Lecture: Cancer and Reproduction
04. Mini-Lecture: Growth Factors
05. Mini-Lecture: Tumor Suppressor Genes
06. Mini-Lecture: p53 and Apoptosis
07. Mini-Lecture: Cell Senescence
08. Mini-Lecture: Cancer Diagnosis
09. Mini-Lecture: Cancer Stem Cells
10. Mini-Lecture: Inflammation and Cancer
11. Mini-Lecture: Heterotypic Cells
12. Mini-Lecture: Metastasis I
13. Mini-Lecture: Metastasis II
14. Mini-Lecture: Immunology and Cancer
15. Mini-Lecture: Cancer Therapies
16. Mini-Lecture: The Coming Cancer Epidemic
33
01. Mini-Lecture: Mutations and the Origin of Cancer
A
n abiding theme in much of modern cancer research is the notion that many cancer-causing agents, carcinogens, act through their ability to enter into the body’s
tissues, and to damage specific genes inside previously normal cells. In other words,
that carcinogens can act as mutagens to mutate genes. And, in fact, we do know that
a large number of carcinogenic agents are responsible for creating mutations inside
cells. And through these mutations that they create, these carcinogens are able to elicit
the disease of cancer. Stated differently, we know, without any doubt, that cancer cells
invariably have mutated genomes.
This raises the question of: what are the agents that provoke many human cancers?
In fact, in the case of lung cancer there is a clear chain of causality. Cigarette smoke
contains a large number of combustion products that are inhaled into the lungs, and
these agents, these chemicals, are then converted via various metabolic enzymes into
chemical compounds that are capable of interacting with and forming covalent bonds
with the DNA. Such structurally altered DNA molecules then may be replicated and
yield ultimately altered DNA sequences, which we would call mutant genes. This raises
the question of whether this can be generalized, or whether there are other sources of
human cancer.
The fact of the matter is that lung cancer may be leading us astray because it may be
the case that the great majority of human cancers are not traceable to specific mutagenic chemicals that enter into the body. Possibly it’s the case that many of the agents
that are carcinogens don’t act as mutagens but rather act through other mechanisms
to provoke cancer. For example, we know that certain kinds of chronic viral infections
are able to cause cancer in certain specific tissues. In East Asia for example, we know
that life-long chronic Hepatitis B virus can lead to an almost 100-fold increased risk of
liver cancer. We know that the Hepatitis B virus is not directly mutating genes inside
the liver; instead it’s creating a chronic inflammatory state that, in turn, seems to be
responsible for the pathogenesis for the development of the liver cancers.
In a variety of other tissues we realize as well that areas of chronic inflammation have
a greatly increased risk of ultimately spawning cancers. Then there is the striking and
provocative discovery that certain anti-inflammatory drugs, including even simple
household aspirin, can be very effective if taken on a daily basis in reducing by 30 or
40 % the incidence of certain commonly occurring cancers, including for example,
colon cancer and perhaps even breast cancer. These effects of inflammation might
be integrated into our rapidly evolving understanding of what the real agents are
that are responsible for triggering many kinds of human cancers. Simply focusing on
mutagenic chemicals blinds us to these other agents, which may be more commonly
involved in the etiology that is the causation of human cancers.
02. Mini-Lecture: Epidemiology and Cancer
T
he search for the origins of cancer has gone on for several centuries. Our first clues
to what triggered cancer came in the late eighteenth century, with the discovery
by the London physician Percival Pott, that men who, in their youths, had worked as
chimney sweeps came down with an unusually high rate of cancer of the scrotum,
a disease that was otherwise relatively rare. He speculated at the time that, in fact,
this unusual cancer came from the fact that these individuals were exposed to a large
amount of creosote and tars that were present in the floes of London chimneys. In fact,
his discovery soon went to the Continent, where, within a decade, chimney sweeps
made sure that after they swept chimneys they made sure they washed themselves
within a day or two, unlike in England, where people washed themselves every week or
so, whether or not they needed it. And so by the beginning decades of the nineteenth
century there was a dramatic drop in the rate of scrotal cancer among the chimney
sweeps of the Continent. This interesting tale represents the first documented instance
where we can actually point to a mechanism of cancer that can be traceable ultimately
to an external source, that is, some type of carcinogen entering into the body from the
outside and provoking a disease, in this case a tumor, at a greatly elevated rate.
Today, two centuries later, we take for granted that many kinds of cancers are actually
caused by external sources, external agents, which enter into the body, damage our
tissues in one way or another, and provoke elevated risks of cancer. In the end, much
of our conviction about these external agents for causing cancer comes from the science of epidemiology that surveys tumor incidence and tumor mortality in a variety
of populations and attempts to correlate the risk of developing one or another kind of
cancer with a specific lifestyle. Of course, such correlations do not prove causality, but
sometimes correlations are so striking that they virtually represent a proof of causality.
For example, already in 1950 there were indications in the United States and Britain
that people who smoked heavily, specifically men, had as much as a 20-fold increased
risk of lung cancer. Of course, this did not prove that smoking actually causes lung cancer, but it already sounded an alarm; and in the decades that followed there became
experimental proofs of the notion that some of the substances in tobacco smoke are
actually carcinogenic, that is to say, they are actually cancer-causing.
Today the science of epidemiology has developed in a number of different directions,
but it is clear already from the science of epidemiology that a number of the cancers
that we experience in the Western world are due in no small part to differences in
lifestyle: differences in smoking habits, in exercise, in body mass, and, very importantly, in diet. Again, these are all correlations, but in some cases the correlations are
so strong as to remove much doubt about causation. For example, in many parts of
Africa the rate of colon cancer is 1/20th as much as it is in Europe and in the Western Hemisphere, specifically in the United States and Canada. These staggering differences cannot be attributed to genetic differences in the populations. That is to say,
American blacks that derive largely from western Africa and have therefore largely a
African genetic heritage have rates of colon cancer that are comparable to the white
population, and this proves to us that these staggering differences in colon cancer
incidence and mortality are not due to genetic susceptibility. Similarly, until recently
the rate of different kinds of diet-induced cancers was drastically different between
Japan and the United States. However, when Japanese migrated to Hawaii, within a
generation their cancer rates closely approximated the rates of cancer that people of
European origin developed in the Hawaiian Islands. Again, this provides fuel for the
argument that these staggering differences in cancer rates are not due to genetic susceptibility but rather to differences in lifestyle, in this case, largely one imagines, diet.
As a consequence, one can begin to assess what fraction of cancer in Western societies is due to differences in lifestyle and what differences are due to the surrounding environment. When dealing with environmental pollution and contamination of
the food chain, most epidemiologists estimate that less than one percent of cancer
rates are due to these sources. Instead, the great majority of the cancers in the Western world are due to either tobacco consumption or alternatively to various types of
35
diets. Tobacco consumption is responsible in the United States, for example, for about
30–40 % of cancer incidence. At the same time, one imagines an almost equal amount
of causation to be attributable to various kinds of diet. In 2007, for example, the American Cancer Society estimated that as many as 90,000 of the more than 500,000 deaths
due to cancer in that year were ascribable to the fact that individuals had a high body
mass index, that is to say, they had a high weight ratio compared with the physical
dimensions of their body. In that sense, body mass index, or obesity, is the second
most important cause of cancer causation after tobacco usage. If we look at various
types of commonly occurring cancers in the West, including, for example, colon cancer, prostate cancer, and breast cancer, we see that diet appears to play a very important role in the causation of these diseases. And if we were to add up all these various
causes, one might imagine that as much as 70 % of cancer in the West could be avoided
if people were to alter their lifestyle. This in fact is a stunning revelation because it indicates that to the extent that we anticipate massive reductions in cancer mortality over
the next generation, the bulk of those reductions will come not from the development
of dramatic new cures to treat existing tumors, but rather from changes in lifestyle that
prevent the appearance of cancer in the first place.
Discussions like these often provoke the question of how a person might be able to
avoid the increased risk of cancer, and here one focuses on the assignable causes that
have been identified to date. Tobacco smoke, either directly inhaled or secondhand
smoke, is an obvious cause, being responsible for about 35 % of cancer and obviously avoidable in most cases. Diet seems to be an extraordinarily important cause
of cancer. Increased body mass index, that is, increased obesity, is correlated directly
with elevated incidence of a variety of different kinds of cancer, as many as a dozen in
men and in women. And this obviously suggests the notion that it is good to stay slim
and physically in good shape throughout one’s life, even though we don’t fully realize
how slim physique actually reduces the risk of cancer incidence. Also, it is becoming
increasingly apparent that certain elements of Western diet, such as large amounts of
red meat, of high fat, and of meat cooked at high temperatures is also deleterious and
is almost certainly responsible for the increased risks of colon cancer, and likely prostate cancer, that are observed in the West. Breast cancer incidence is also influenced
by diet, but here one must also take into account the fact that the reproductive history of a woman is also an important risk factor. For example, a woman who has had
children starting at a relatively early age, say the age of 20, is at a much lower risk of
eventually developing breast cancer than a woman who has her first child at the age of
30 or 35, or a woman who never has had any children. In the end, another important
and unavoidable factor comes from the genes that we inherit from our parents. This
obviously does not fall under the purview of epidemiology, because here the playing
field is tilted from the moment we are born, and here there is little we can do to avoid
certain susceptibilities that we have inherited from our parents.
03. Mini-Lecture: Cancer and Reproduction
B
reast cancer is of great concern to many women. Last year in the United States,
for example, about 40,000 women died of the disease and perhaps five times as
many were actually diagnosed with the disease. With that said, this raises the question of: what are the causes of breast cancer? We know, for example, that the internal
hormonal environment of a woman is a strong determinant of risk of breast cancer; it
is important to realize that because of nutrition and reproductive practices, the internal hormonal environment of women in the United States, for example, has changed
dramatically over the last hundred and fifty years.
We know, for example, that the number of menstrual cycles through which a woman
goes is approximately proportional to her ultimate cancer risk, breast cancer risk
specifically. We know, for example, that the breast cancer risk of a woman is roughly
proportional to the number of menstrual cycles she goes through in a lifetime. These
cycles have changed dramatically over the past 150 years and therefore so has the
internal hormonal environment of women. In the mid-nineteenth century, for example, a woman might at the age of sixteen begin cycling, at the age of eighteen become
married and begin pregnancy, and between subsequent pregnancies, lactation, and
breast feeding, might rarely if ever go through a full menstrual cycle for the next twenty
or thirty years, and then she might go into menopause at the age of forty. These days
a girl will begin menstrual cycling, in the United States at least, at the age of as early
as eleven or twelve because of increased nutrition, and she may not become pregnant until the age of twenty-five or thirty and then have only one or two children. And
because of improved nutrition she may continue to cycle until she’s in her late forties
before she goes into menopause.
What this means is something quite astounding. An average eighteen year old American girl may already have gone through as many menstrual cycles as her great, great
grandmother went through in an entire lifetime, to give you one example of how the
internal hormonal environment has changed dramatically.
In addition we know that once a woman goes through a pregnancy, part of the breast
tissue is converted to a state where it is no longer susceptible to generating breast cancers. And therefore, the earlier in life one goes through this state of bearing a child,
sometimes called parody, the more protective pregnancy is. The longer one postpones
parody, the greater the breast cancer risk; women who never have children have a correspondingly increased risk of breast cancer.
In saying all these things one can see how reproductive practices and nutrition have
had a dramatic affect on the risk of breast cancer and that these effects dwarf any that
might arise from the environment, that is, for example, from environmental pollution,
which seem miniscule in comparison to the profound changes in the hormonal milieu
existing inside a woman’s body.
37
04. Mini-Lecture: Growth Factors
O
ne of the key insights into the peculiar biology of cancer cells has come over the
past 20 years as we’ve realized that there is an intimate connection between the
actions of oncogene proteins, sometimes called oncoproteins, and growth factor signaling. One can trace much of this logic back to a simple notion. In the context of
a complex animal like ourselves, one cannot allow individual cells in one tissue or
another to begin to proliferate on the basis of their own internally generated signals.
Instead, in a well-structured tissue, each cell must consult its neighbors before it converges on the conclusion that it is time for it to grow. In the absence of such continuous
consultation, individual cells here and there in a tissue would spontaneously begin to
proliferate, multiply, maybe even yield large flocks of descendants, and very soon the
architectural integrity of the tissue would suffer enormously. Consequently, each of
the cells in a tissue lives in a condominium where all the neighbors are continuingly
chatting with one another, discussing the appropriateness or the inappropriateness of
one another beginning to proliferate.
We now realize therefore that growth factor signals are transferred from one cell to the
other. These signals more often than not encourage proliferation, but there are also
signals that discourage proliferation. And in all cases these signals are sensed on the
surface of cells that receive these signals by specific, highly specialized receptors (we
call them growth factor receptors), which sense the presence of growth factors in the
extra cellular medium and then transfer the information of this growth factor and its
presence into the cell interior, thereby informing the signaling molecules in the cell
interior that there are signals in the extracellular space that suggest, and induce, and
urge the cell to proliferate.
We now realize that this general scheme of growth factor signaling is often hijacked in
cancer cells. In contrast to normal cells, which never undertake proliferation unless
induced to do so by external signals, cancer cells generate their own internal signals
that stimulate their proliferation and in that way become independent of their environments. We now realize that cancer cells are able to induce these growth stimulatory signals through a variety of different signaling mechanisms.
For example, normally growth factors are released by one cell and impinge on a second cell, making the second cell dependent on the first. In the case of many cancer
cells, in contrast, the cancer cells acquires the ability to release growth factors into its
immediate extracellular environment, and once present there, these growth factors
can then rebind to the surface of the cell that has just produced them.
This thereby allows the cell to stimulate its own proliferation, creating what is sometimes called an autocrine signaling route. Clearly the fact that the cancer cell is now
making its own growth factors makes it independent of growth factors that might originate elsewhere in the tissue or in the body. Similarly, the growth factor receptors that
are present on the surface of cells and enable cells to sense the presence of growth
factor molecules in the extracellular space can also be subverted in cancer cells. Thus,
some cancer cells may express far too many copies of a growth factor receptor molecule on their surface, and in that way induce its inappropriate signaling activation
even in the absence of any of the growth factors that are normally required to induce
this receptor to fire.
Sometimes the receptors displayed on the surface of a cancer cell are structurally
altered in a way that causes the receptor to transmit signals into the cell interior, into
the cytoplasm, that induce the cell to proliferate once again, even in the absence of the
growth factor being present in the extracellular space. A third alternative strategy by
which cancer cells can become growth factor independent derives from the numerous molecules that operate in the cell cytoplasm, and that are normally dedicated to
receiving and processing growth stimulatory signals that are released by receptors. For
example, the ras protein in normal cells is normally in a quiet state in which it does not
signal, and it may persist in that state for an extended period of time.
However, in the event that a growth factor receptor becomes activated, that growth
factor receptor will emit signals, which, through a series of intermediaries, are able
to activate the ras protein, converting it from its quiescent state to an active signalemitting state. And the ras protein thereafter is able to release yet other signals further into the cell that are capable of inducing the cell to commit itself to proliferate.
However, in cancer cells, once again this signaling pathway is subverted because the
structurally altered ras proteins that one can find in as many as one quarter of human
cancers have undergone a profound functional change. Instead of releasing carefully
measured parcels of growth stimulatory signals, which is the behavior of the normal
ras protein in normal cells, the structurally abnormal ras oncoprotein found in cancer cells releases a steady, unabated stream of growth stimulatory signals into the cell
interior, thereby diluting the cell into thinking that it has experienced growth factors
in its extracellular space (when, in reality, no growth factors are present there at all).
In all of these cases, we see examples of how an individual cell becomes cutoff from
communication with its neighbors and begins to generate instead its own agenda of
growth and proliferation-precisely the biological attributes that we ascribe to cancer
cells.
39
05. Mini-Lecture: Tumor Suppressor Genes
T
umor suppressor genes have been intensively studied over the past quarter-century because they provide half of the genetic explanation for the origins of cancer. Thus one might imagine that in any well-balanced control system the signals that
favor, for example, cell proliferation, must be counterbalanced by negative signals that
act against it. Or, to put it in an automotive metaphor, the accelerator pedal needs to
be counterbalanced by the braking system.
The fact is, there are multiple ways in which genes and their encoded proteins can
prevent or block the outgrowth of cancer cells. One group of tumor suppressor genes
blocks cell proliferation, or induces apoptosis, and thereby eliminates incipient cancer cells. Another group of similarly acting genes minimizes the outgrowth of cancer
cells by protecting the genome. This latter class of genes is sometimes called caretaker
genes, to reflect the fact that these genes are responsible for maintaining the integrity of the genome, and in that way reducing the effective mutation rate and thus the
progression of cancers through the multiple steps that lead ultimately to the formation of a malignant growth. In general, these DNA repair genes, these guardians of
the genome, are not thought to be classic tumor suppressor genes, and therefore are
discussed in a quite different context, that is, the issues surrounding the maintenance
of the genome.
If we focus on the genes that are widely agreed to be tumor suppressor genes, we realize
that they have multiple alternative mechanisms of action. The classically studied retinoblastoma gene makes a protein that prevents the progression of the cell through the
G1 phase of the growth and division cycle. The retinoblastoma protein normally acts
to integrate a variety of different signals that converge on the decision as to whether
it is appropriate for a cell to proceed to grow and to divide, or whether the cell should
alternatively halt progression through the cell cycle and enter into the non-growing
G0 state of the cell cycle. In the absence of the Rb protein, the retinoblastoma protein,
the cell proceeds willy-nilly through the G1 phase of the growth cycle, independent of
whether the conditions are suitable or propitious for cell proliferation.
The other frequently studied tumor suppressor gene is the p53 gene, which happens to
be inactivated in almost 50 % of all cancers. The p53 protein is responsible for receiving and integrating a variety of signals that arise from the various systems implanted
throughout the cell that are designed to monitor the cell’s well being, including the
integrity of its genome, the availability of nucleotides, the state of the cellular metabolism, and so forth. In response to some signals, the p53 protein will call a halt to cell
proliferation, possibly a reversible halt. In the presence of other signals, p53 protein
will trigger programmed cell death, or apoptosis. Through either mechanism, the p53
protein reduces the chance that an aberrantly functioning cell will be able to proliferate and spawn the large number of descendants that together constitute a tumor mass.
Yet another frequently studied tumor suppressor protein is p10, which acts as a phosphatase and cleaves PIP3, phosphatidylinositol triphosphate. In so doing, p10 prevents the activation of the AKT anti-apoptotic kinase. And thus, by acting to prevent
this activation, p10 in fact prevents cells from growing and surviving under conditions
where by all rights they should be induced to enter into apoptosis. In citing these three
examples, I mean to indicate that there are a diverse array of biochemical mechanisms
by which tumor suppresser genes and their encoded proteins are able to call a halt to
cell proliferation, indeed, are able to weed out and actively eliminate aberrant cells. In
the absence of these quality control mechanisms, such cells will be able to proliferate
and ultimately evolve towards the highly malignant state that one encounters in the
oncology clinic.
06. Mini-Lecture: p53 and Apoptosis
A
s we’ve learned over the past 20 years, a key defender against cancer that is
implanted in cells throughout the body is the p53 protein. It’s classified as a tumor
suppressor protein because if and when p53 function becomes compromised, there’s
a much greater likelihood that the cell bearing this compromised p53 protein may
begin to proliferate in an abnormal fashion, leading, in the end to a large flock of progeny that grow and form a tumor.
The p53 protein in normal cells is rapidly made in large amounts and rapidly degraded,
leading in the end to a very low steady state level of p53 proteins. However, in cells
that have suffered various types of stress, including anoxia (the absence of oxygen),
DNA damage, and imbalance in the signaling pathway within the cell, p53 levels rise
quickly simply because p53 protein, while it continues to be made in large amounts, is
no longer degraded at equal rates. The resulting p53 protein, once it accumulates, then
acts in the nucleus as a transcription factor, activating the expression of a large bank
of genes that have various effects on cells depending on the nature of the insults that a
cell may be suffering. For example, if a cell has suffered a minimal amount of genetic
damage, p53 levels may increase and induce the expression of cell cycle inhibitors that
prevent subsequent proliferation of a cell until the cell has succeeded in repairing the
damage that was initially inflicted on its genome.
Once this damage has been repaired, then p53 levels decline to normal and the cell
proceeds to go through its normal growth and division cycle. Alternatively, the cell
may suffer devastating damage to its genome, indeed damage that far exceeds its
ability to repair all this damage. That leads, in principle, to the possibility of a highly
mutated genome and once again the outgrowth of an aberrant cancer cell. In response
to this situation, the p53 protein, rather than temporarily halting cell proliferation,
may decide through complex biochemical mechanisms instead to provoke the program of cell suicide, which is called apoptosis.
Apoptosis insures that once activated, it is able to eliminate all traces of a cell within
an hour. The nucleus shrinks up, the plasma membrane in apoptosing cells undergoes
profound remodeling, looking as if it’s rupturing, the DNA in the chromosomes is fragmented and soon, the apoptotic cell breaks up into small clumps that are rapidly consumed either by macrophages or by its neighbors, thereby removing all traces of this
cell. This serves as an extremely effective mechanism for tissues to remove potentially
dangerous cells from their midst. We can realize situations where this may not operate
properly. For example, in more than half of all human tumors, perhaps all of them, the
p53 pathway is not operating properly and under these conditions, even though a cell
may suffer certain kinds of insults, including genomic damage, which leads to mutant
genes, that cell may continue to survive. In effect, once the p53 protein becomes damaged and no longer able to signal, a cell becomes blind to many of its defects and its
apoptotic machinery can no longer be activated, allowing this cell, and by extension,
its descendants, to survive and spawn a large cohort of descendants that could eventually form a tumor. The apoptotic program is interesting in its own right. We might
expect, on the basis of our experience with other signal transduction circuits, that this
pathway is also composed of a series of kinases, for example. But in fact it operates on
very different principles.
Much of the decision as to whether or not apoptosis should be activated is localized to
the surface of the mitochondria and the cytoplasm. We’ve always thought of the mitochondria as being the factories of energy production in the cell, but it now appears
clear that in early evolution of eukaryotic cells, a totally unrelated function became
inserted into the mitochondria; this unrelated function is the program of apoptosis.
We now know that when apoptosis is initiated, one of the first events in many cases is
that the mitochondrial membrane opens, releasing cytochrome c molecules into the
soluble part of the cytoplasm that we call the cytosol.
Cytochrome c molecules have traditionally been studied in the context of cytochrome
c transport inside the mitochondria, but now we learn of a totally unrelated function,
41
because when these molecules are leaked out into the cytoplasm, they associate with
other molecules and activate the functioning of the whole series of intracellular proteases called caspases. These caspases cleave one another in a long sequence, and in
the end the action of the caspases can be used to explain many aspects of the apoptotic program.
07. Mini-Lecture: Cell Senescence
T
he process, or the cellular state, termed senescence was first described more
than 40 years ago when it was observed that when cells are propagated in culture,
that is in petri dishes and in incubators, they have a finite, apparently pre-ordained
number of successive cell generations through which they can pass before they stop
growing. After this number of cell generations, cells seem to enter into an essentially
irreversible state of senescence, as it was termed. They become physically very large,
they develop certain anomalies in the appearance of their nuclei, and can they can
remain viable for many weeks thereafter, once again without ever again entering into
the cell’s growth and division cycle.
The sources of cellular senescence have been strongly debated in recent decades. In
some cases, there has been evidence that the shortening of telomeres at the end of
chromosomes were responsible; but the weight of evidence increasingly suggests that
cellular senescence comes from sub-optimal conditions of cell culture in the incubator and from unbalanced signaling from oncogenes that might be active within cancer
cells. For example, one has routinely propagated cells in the incubator at 20 or 21 %
oxygen, which is the oxygen tension of the surrounding air. But in truth, the oxygen
experienced by cells within our tissue is much closer to three or four percent, rather
than the 20 % that one experiences routinely in the incubator. If one indeed lowers
the oxygen tension to which cells are exposed in the petri dish to a more physiological level, then cells will pass through far more growth and division cycles before they
enter into senescence. Similarly, in the case of certain epithelial cells, if they are propagated on their own in pure culture, they may senesce after six to eight generations.
However, if they are provided with a stromal feeder layer beneath them, which sends
certain kinds of growth-stimulatory and survival signals to the epithelial cells, then
epithelial cells in culture can often double for 30, 40, or even 50 cell generations, again
an indication that the number of successive cell generations through which cells pass
in the tissue culture dish is often a reflection of sub-optimal conditions of culture.
Related to these processes are the actions of certain oncogenes. For example, the
oncogene acting through the offices of its encoded oncoprotein, the Ras protein or
Ras p21, is able to induce senescence in primary cells if it is expressed at high levels.
On the one hand, this might suggest that it is very difficult to transform cells with a ras
oncogene, and that this senescence response is a reflection of a built-in or hard-wired
defense that mammalian cells have to the overexpression or the excessive signaling
released or emitted by oncoproteins. In fact, when the Ras oncoprotein is expressed
at more physiologic levels, then cells may begin to exhibit some of the phenotypes of
cell transformation without entering into senescence. Interestingly, the type of senescence into which cells enter in response to excessive ras oncogene signaling is very
similar to the replicative senescence that is observed after extensive culture in vitro.
These various forms of senescence begin to converge on the notion that this senescence state is a reflection of cumulative damage the cells have sustained, either in
culture or being exposed to the signals released by oncogene proteins.
Senescence can also be observed in vivo, where pre-malignant legions, for example,
the nevi that are precursors of melanomas, often show high numbers of senescent
cells, indicating that senescence may be a physiological mechanism that is designed
and hard-wired into our cells to prevent the outgrowth of premalignant cells, in other
words, an anti-cancer defense mechanism. Even in this case, it appears likely that the
senescence exhibited by these premalignant cells is also a reflection of excessive oncogene signaling and cumulative damage sustained by cells that have experienced such
signaling over a period of many days’ and many weeks’ time. Much of the damage
seems to be focused on the cell’s genome, where there seem to be irreparable damage
foci in the chromosomal DNA, whose presence somehow stabilizes the senescence
state and somehow precludes such cells from ever reentering into the active growth
and division cycle.
43
08. Mini-Lecture: Cancer Diagnosis
O
ver the past several decades we’ve developed increasingly powerful mechanisms
and technologies for diagnosing human tumors. Indeed, now we can diagnose
tumors that are so small that in fact in previous years they would have eluded detection by the human eye or most commonly used imaging technologies. This increased
ability to detect small tumors is a double-edged sword, however. In principle, one
might imagine that by detecting a small tumor, one can remove it and thereby prevent
it from growing much larger and becoming life threatening. But there is a complication because many small tumors will actually remain small for the life of the individual
who is carrying them, and therefore with increased powers of diagnoses, one begins to
pick up growths that quite possibly would have never become life threatening during
the life of the patient who happens to carry these small tumors.
One says that these days if one does autopsies on 80-year-old men, independent of the
cause of their death, as many as 80 % of them have demonstrable prostate cancer, and
yet we know that only 3 and 4 % percent of men will actually die from prostate cancer.
This indicates that in a number of tissues as one gets older one accumulates growths
which by certain criteria would be called cancer, but whose future growth, whose
future properties, are really quite ambiguous. And therefore we are confronted with
the dilemma of not knowing precisely how to respond to these recently diagnosed
tumors.
A similar situation, perhaps not as dramatic, operates in the case of breast cancer.
These days we are detecting very small breast tumors that previously would have gone
undiagnosed, but because of various considerations, including malpractice concerns,
these tumors are often treated quite aggressively even though it is clear from the epidemiology of cancer that the majority of them would never become life threatening.
The problem, of course, is then to discern how many actual cases of cancer there are
in the population and how many of these are in part diagnostic bias, that is, we count
them even though we imagine that many of them will never actually become clinically
apparent.
The solution to this, ostensibly, is to develop new ways of stratifying different kinds of
tumors. By that I mean classifying recently diagnosed tumors into those that are likely
to become malignant and aggressive and life threatening, and those tumors that are
likely to remain indolent, that is, sitting there without any active growth.
The use of gene expression arrays may make this possible. Indeed, already it has had
some effects in allowing recently diagnosed breast cancers, for example in the Netherlands, to be treated as indolent growths that will never become life threatening and
never require, for example, surgery and aggressive chemotherapy. Women who have
such small growths are therefore not treated in any aggressive clinical way, in contrast
to those whose gene expression array analyses of their tumors indicates the possibility
that some tumors might, within five or ten or fifteen years, eventually become malignant and perhaps metastatic, and therefore require active intervention.
All this says that with our increased powers of detecting small tumors, we’re now faced
with a flood of growths whose actual optimal treatment remains quite ambiguous. We
don’t know what to do with them in all cases, and in many cases, simply because of a
doubt of their future growth, they’re treated aggressively, even though they probably
don’t need to be.
09. Mini-Lecture: Cancer Stem Cells
T
he discovery of cancer stem cells, which has happened over the last five years or
so, has really revolutionized our thinking about how tumors grow. If you would
have asked me ten, fifteen, or twenty years ago how can we understand the growth
of a tumor, I would say that a tumor is simply a collection of cancer cells which grows
exponentially. To be sure, one had to include in that description the notion of tumorassociated stroma, that is to say, the non-cancerous cells that are recruited into a
tumor like a carcinoma, and are important for supplying the tumor with nutrients via
the vasculature, and for providing other kinds of biological support. But in this case, in
the instance of cancer stem cells, I’m talking about another dimension of complexity.
Work in recent years indicates that both in hematopoietic tumors such as leukemias,
as well as in solid tumors such as carcinomas, not all the cancer cells in each of these
tumors is equivalent.
The fact of the matter is that if one plucks cancer cells out of, for example, a breast
cancer, then divides them on the basis of cell surface markers that they happen to
have, one can identify minority populations through the display of certain cell surface markers that can be introduced into a mouse, for example, a host mouse that is
immuno-compromised, and these human breast cancer cells, in very low numbers, as
few as two hundred, will seed a new tumor. In contrast to the behavior of this minority
population of cells there may be a majority population that includes more than 95 %
of the total cancer cells of the tumor, and when one implants, for example, 20,000 of
those in a mouse, no tumor arises. This indicates that the minority population, far less
than 1 %, has a very high percentage of tumor initiating cells, that is cells that when
introduced into a mouse can trigger the growth of a new tumor, whereas the bulk population of cancer cells contains very few of any of these tumor initiating cells.
We have to keep in mind here that the minority and the majority population are genetically identical. They’re all cancer cells and they’re all part of one genetic clone, however biologically, they’re behaving very differently, and this type of behavior suggests
the operations of a stem cell, that is to say a cell which upon division is able to generate one daughter cell that once again becomes a stem cell thereby perpetuating the
number of stem cells in a tissue, and a second daughter cell that undertakes a program
of growth and division, ultimately spawning fifty or a hundred or even more descendants that acquire the differentiated characteristics of a tissue, but at the same time
give up the option of ever dividing again. In this situation, the great bulk of the cancer
cells in the tumor may actually be post-mitotic. They may lack self-renewal capability,
unlike the stem cell which in principle is able to divide indefinitely each time generating at least one daughter cell that itself becomes a stem cell.
If this kind of hierarchical organization is ultimately proven for a large number of cancers, this has important implications for how we diagnose and treat this disease. For
example, in the instance of developing new kinds of anti-cancer therapeutics, we need
now to begin to direct these drugs towards eliminating the cancer stem cells. Once one
is able to eliminate the cancer stem cells, the tumor has lost its ability to regenerate
itself, and therefore in principle, will ultimately wither away. In contrast, if one eliminates the bulk population of the cells within a tumor, that is the non-stem cells, the
tumor as a whole may shrink, albeit temporarily, because the moment that the therapeutic is removed, the surviving cancer stem cells, which may have been unaffected
by the chemotherapeutic treatment, may then begin the job of regenerating the tumor,
and soon thereafter the tumor will have re-grown and become just as dangerous as it
was prior to the chemo-therapeutic treatment.
We’ve only begun to appreciate the conceptual fallout from the discovery of these cancer stem cells, initially in hematopoietic tumors such as leukemias, and most recently
solid tumors. And it remains to be proven that such cancer stem cells inhabit virtually
all human solid tumors, but this now becomes a possibility.
45
10. Mini-Lecture: Inflammation and Cancer
O
ne of the most astounding discoveries in modern cancer research has been an
epidemiological study of the rates of cancer among individuals who take nonsteroidal anti-inflammatories, often called NSAIDs, such as, for example, a baby aspirin, every year. Such individuals taking a baby aspirin over a period of ten to fifteen
years on a daily basis have significantly reduced rates of breast cancer, colon cancer,
pancreatic cancer, often as much as twenty or thirty of forty percent below the rates
that are typically associated with individuals of their age, their background, and their
exposure to various environmental factors.
This suggests that inflammation in certain tissues throughout the body is actually
conspiring, or contributing at least, to the formation of tumors. And indeed there
is increasing evidence that chronically inflamed tissues represent sites of greatly
increased risk of various kinds of tumors. Possibly the most dramatic example of this
is in the liver, where long term infection with hepatitis B virus or hepatitis C virus creates a state of chronic inflammation, a reflection of the immune system’s attempts to
rid the liver of these viruses, unsuccessful as they may be, and the resulting greatly
increased risk of liver cancer, patocellular carcinoma. But similar situations seem to
operate in a variety of other organs. People who have chronic gallstone irritation of the
gall bladder are at greatly increased risk for gallbladder cancer. Individuals who have
chronic acid reflux in the esophagus are at greatly increased risk of esophageal cancer.
Individuals who have chronic inflammation of the colon because of colitis and related
conditions are at greatly increased risk of colon cancer.
Precisely how these inflammatory states actually predispose to the formation of
tumors is still not worked out in great detail. However, it is clear that one component of
the inflammatory state is the release by various kinds of immune cells of compounds
called prostoglandins that are able to stimulate the growth of epithelial cells, often
conferring on epithelial cells many of the phenotypes of transformed cells. Somehow
this chronic flux of irritating prostaglandins, along with other factors that are released
in a chronically inflamed tissue, creates a biological microenvironment that greatly
favors the expansion of clones of mutated initiated cells that can ultimately evolve into
high grade malignancies.
We are just beginning to figure out how chronic inflammation contributes importantly
to cancer risk, but it is clear that it does in profound ways. This also suggests, by the
way, that if we could figure out ways of lowering the degree of inflammation in certain
tissues, there would be a correspondingly greatly decreased incidence of disease in
those tissues. And since a lowering of incidents is in the end far more effective than
treating disease in terms of reducing mortality in a population, the notion of treating
with anti-inflammatories in certain organs becomes a very attractive one.
11. Mini-Lecture: Heterotypic Cells
T
he most simplistic view of a tumor, such as a carcinoma, which is a tumor of epithelial cells, is that the tumor itself is only composed of a large number of these
cancer cells, a mass of a billion or ten billion cells that together represents the tumor.
But in fact, to the extent that one examines carcinomas under the microscope, one
discovers that they are complex tissues, indeed, as complex as the normal epithelial
tissues from which they arise.
Indeed, the driving forces in carcinomas are the cancerous epithelial cells. However,
surrounding these epithelial cells are a large number of mesenchymal cells, cells of
connected tissue origin, and from the immune system, which constitute together
the stroma and are recruited into the tumor-associated stroma from various sources
throughout the body. Many of the fibroblasts and myofibroblasts that form the bulk
of the stroma are recruited from adjacent normal tissue. Many of the inflammatory
cells that constitute arms of the immune system, and are involved not only in immune
rejection but also inflammation, are recruited ultimately from the bone marrow via
the circulation.
Together, as many as 7 or 8 distinct cell types constitute the tumor-associated stroma.
Included among these are the aforementioned fibroblasts and myofibroblasts, lymphocytes, neutrophils, macrophages, endothelial cells, and pericytes. Together, they
represent an important source of functional support for the carcinoma cells. They cannot grow in a vacuum, and instead require various types of signals from the stroma
for their support. The most obvious type of support comes from the endothelial cells
and pericytes that form the tumor- associated neovasculature, that is to say, the blood
vessels that are formed in the tumor’s stroma and that supply cancer cells with much
needed nutrients and oxygen and that serve to evacuate metabolites, as well as carbon
dioxide. But in addition to these cells, which form the tumor-associated microvasculature, there are a variety of other signals that come from the fibroblasts and myofibroblasts and that provide to the carcinoma cells mitogenic signals, as well as signals that
prevent the carcinoma from entering apoptosis. As a consequence, we realize that this
complex tumor-associated stroma has not been implanted there to confuse cancer
biologists, but rather is recruited purposefully by the carcinoma cells because they
absolutely require this stroma in order to survive and to prosper in the body of a cancer patient.
As tumors become more and more malignant, the associated carcinoma cells may
lessen their dependency on recruited stromal cells from the host. However, in general,
even the most aggressive tumors still show a dependence on these recruited stromal
cells for various types of cell physiologic support. Indeed, the inflammatory cells that
are brought into the tumor-associated stroma and are often active transiently during
the process of wound healing provide many of the signals that goad the cancer cells
into ever-more rapid proliferation, and ultimately enable the cancer cells to evolve
to higher states of malignancy. Accordingly, if we wish to understand the biology of
the tumor as a whole, we must expand our gaze from just focusing on the carcinoma
cells and their intracellular signaling pathways, and must instead now include in our
vision the variety of mesenchymal cells that are present in the tumor stroma and that
exchange signals with the carcinoma cells. This exchange of signals between one cell
type and another is often called heterotypic signaling, and therefore heterotypic signals are increasingly being factored into our view of the biochemistry and signaling
physiology of cancer cells.
47
12. Mini-Lecuture: Metastasis I
I
used to think that metastasis and invasiveness by cancer cells was an impossibly complex thing to study. After all, if you look at a highly malignant cell that has
acquired all these capabilities, not only can it move, not only can it degrade adjacent
extra cellular matrix, but it is able to survive in the blood, it is able to establish itself in
new tissue environments, and it changes many aspects of cell biology simply in order
to achieve these ends.
Such a complicated repertoire of cell biological behaviors suggests enormous complexity. How could one ever encompass it in a simple and a single conceptual scheme?
The truth of the matter is that over the last five years, from a large number of laboratories, there has come increasing evidence that cancer cells acquire the ability to execute
and to choreograph these complex steps by activating transcription factors that are
normally used to program critical steps in early embryonic development. These transcription factors act in a very pleiotropic fashion. By that I mean that each of these
transcription factors is able to simultaneously up-regulate a lot of genes, and therefore
proteins, that enable a cell to change its shape, to acquire motility, to become invasive,
and to execute many of the steps we believe are required in aggregate in order for a cell
to leave a primary tumor and to take up residence in a distant tissue site.
This sequence of steps is often called the invasion metastasis cascade. It involves altogether: the acquisition of local invasiveness by a cancer cell; the invasion of the cell
into blood vessels, which is often called intravasation; the transport of the cancer cells
through the blood vessels to distant tissue sites; the escape of the cancer cells from
the blood vessels, which is often called extravasation; and finally, the acquisition of
the ability of the cancer cell to adapt to the local tissue environment in which it has
landed and to begin to proliferate to form a growth which ultimately becomes visible
at the macroscopic level, that is, with the naked eye, in other words, to create a lifethreatening metastasis.
We know about many of these steps originally from the study of early embryogenesis.
For example, during the process of gastrulation, cells that initially form in the ectoderm migrate into the embryo to form the mesoderm and the endoderm. This migration entails a large number of changes in the cells that started out as ectodermal cells.
These cells are initially epithelial. Cells in an epithelium normally form two-dimensional sheets, they are unable to move, they are tightly attached to their neighbors and
as such, they also express a number of cytoskeleton proteins, such as keratins, that are
characteristically expressed by all cells in an epithelium.
However, when these cells leave the ectoderm to migrate into the embryo, they must
detach themselves from their epithelial neighbors; they must shut down epithelial
gene expression programs, including the expression of keratins, and they become
quite mesenchymal. That is to say, they take on many of the aspects of fibroblasts. They
become migratory; they release proteases into the extra cellular space that enables
them to degrade certain obstacles that may be in their path. As such these cells have
changed from an epithelial differentiation state to one that is quite mesenchymal, and
as such have undergone what is often called the epithelial mesenchymal transition,
or the EMT.
Once cells migrate to the inside of the embryo, they may retain their mesenchymal
traits, in which case they will become, for example, cells of the mesoderm. Alternatively, some of these cells may become endodermal and as such may once again reacquire epithelial characteristics, in which case their initial conversion from an epithelial to a mesenchymal state was only reversible and could be rapidly changed back to
an epithelial state once they reach their intended goal, which in this case is the inside
of the embryo that forms the primitive gut.
There are strong analogies between this sequence of events and those that happen
during the formation of metastases in human patients that bear for example carcinomas. Carcinomas constitute about 80 % of the total tumor burden; and these all arise
from epithelial cells in various epithelial tissues including the lungs, the esophagus,
the stomach, the liver, the uterus, and many other tissues as well.
The cells lining the glandular tissue in the breast are also epithelial, leading to breast
cancer. In many cases we see that during the progression of a benign tumor to a malignant tumor in these various tissue sites, one initially has cancer cells that exhibit
very many epithelial characteristics. These cells are still localized, they still may form
ducts, for example, which are a characteristic of epithelial cells, and they have not yet
become invasive.
However, as tumor progression proceeds, these cells may shed many of their epithelial characteristics and undergo an epithelial-mesenchymal transition, which enables
them to become motile and invasive and begin the whole process that ultimately leads
to their ability to form metastases in distant tissues. Thus they undergo an epithelialmesenchymal transition, which may profoundly change their entire gene expression
program. And the way they accomplish this, in many cases and perhaps all, is to reactivate early embryonic transcription factors, indeed, the same transcription factors that
are responsible during early embryogenesis for programming critical steps such as
gastrulation.
All of this raises the question of what causes carcinoma cells in the context of a primary tumor to begin to express these early embryonic transcription factors. Here we
begin to believe that the environment of these cancer cells provides the signals that
persuade these cells to turn on expression of these transcription factors. More particularly, one can see that signals that are released by the stromal cells in the tumor,
the non-malignant cells, impinge on the cancer cells, induce them to turn on these
transcription factors, which then enable these cancer cells to acquire mesenchymal
phenotypes and begin to invade and metastasize. Interestingly, when these cells land
in a distant tissue site, they no longer experience the same mixture of signals from the
stromal cells in that distant tissue site. Consequently, they no longer are able to maintain the expression of these early embryonic transcription factors. This allows many
of the metastasizing cells to shut down the expression of these transcription factors,
we believe, and thereby revert from a mesenchymal to an epithelial state, in that way
recapitulating the behavior of their ancestors in the core of the primary tumor from
which they originated.
49
13. Mini Lecture: Metastasis II
T
he process of metastasis is actually quite complicated. Cancer cells within a primary tumor must acquire the ability to invade into adjacent tissue, to enter into
blood vessels and lymphatic ducts, to travel through those vessels to distant sites in
the body, to escape from those vessels, to invade into adjacent tissues, and perhaps
most difficult is the last step of colonizing the tissues in which they have landed. That
is, they begin to adapt to the tissue microenvironment, which is quite foreign from the
microenvironment they experienced in the context of the primary tumor.
An interesting question is how cancer cells acquire the phenotypes of invasion and
metastasis. Here one thinks of the processes that preceded invasiveness and metastasis, that is, the processes that led to the formation of the primary tumor. We imagine
that when a primary tumor forms, it forms as a consequence of a series of mutations
that strike the genomes of evolving cancer cell populations, or at least pre-malignant
populations. Each of these mutations, in a Darwinian sense, confers on the cell that
happens to have sustained this mutation an increased survival advantage or proliferative advantage, and consequently cells that have acquired one or another mutation
begin to expand and increase their number within the context of the primary tumor.
This suggests that most of the traits of cancer cells that inhabit a primary tumor have
been accumulated as a consequence of the fact that they were selectively advantageous in a Darwinian sense during the growth of the primary tumor.
How, then, do cancer cells acquire the ability to invade and metastasize? Are these
traits also advantageous as cancer cells grow with the confines of a primary tumor? In
fact, the answer to that question is hardly obvious. Some might argue that the acquisition of metastatic traits has no advantageous consequences for the cell that happens to have acquired this trait while it is living in the primary tumor. As such, it could
be that the acquisition of invasiveness and metastatic potency may only be an unintended side effect of the fact that cancer cells have acquired certain mutations within
the context of primary tumor growth that happen, in fact, to allow them to invade and
metastasize.
Another point of view holds that once primary cancer cells form they recruit a socalled activated stroma, that is a collection of mesenchymal cells that are brought in
from the host and that help the tumor as a whole to grow. Such an activated stroma has
close resemblance to the stroma in a wound-healing site, and the cells that form this
stroma release a series of signals that are known, at least in some biological contexts,
to induce traits like motility and invasiveness in the cancer cells. As a consequence,
the cells within a primary tumor may actually acquire the cell biological phenotypes
that enable them to leave the primary tumor and to move to distant sites. Hence, in
this kind of scenario, the acquisition of invasive and metastatic traits is really the consequence of the adaptation of cells in the primary tumor to the signals they are receiving from the activated inflamed stroma, which provides many types of biological support to the primary tumor, but also may influence in a profound way the biology of the
cancer cells themselves.
14. Mini-Lecture: Immunology and Cancer
T
hroughout this book, we have seen a number of examples of protective mechanisms that evolution has implanted in cells throughout the body that help to
derail the progress of tumor formation. For example, the whole process of cell suicide
is predicated on the notion that if the cell receives too many growth stimulatory signals, the apoptosis or cell suicide program may be triggered, thereby eliminating a
cell that is the potential progenitor of a flock of tumor cells. There are yet many other
mechanisms that are hard-wired in the cells, which are guaranteed to stop cancer cells
in their tracks, should they begin to proliferate. Of course these mechanisms are not
foolproof. They are not fail-safe, because on frequent occasions, one does see cancers
in the human population.
Nonetheless, these kinds of realizations provoke one to ask another kind of question.
And that is, are there yet any other mechanisms that operate in the body in order to
reduce the likelihood of cancer appearing? More specifically, we can pose the question of whether the immune system also plays a role. This in fact is a very complicated
issue, because much of what we know about the normal immune system stems from
our study of the ways in which it protects us against various kinds of infectious agents.
Thus, we become infected by a virus, or by a fungus, or a by a bacterium, and often we
acquire a life long immunity which protects us from re-infection by that agent.
One has learned over the past 40 years how this protection is achieved, and much of
it depends on the ability of the immune system to discriminate between our own normal cells, and the cells and proteins of infectious agents. Thus, we know of many situations where foreign proteins brought into our body by viruses are readily detected. The
virus particles that are expressing these proteins are rapidly eliminated by the immune
system, which can readily distinguish these foreign proteins from the native proteins
in our tissues.
On the other hand, the immune system becomes tolerant of the proteins that are made
by our normal cells. To the extent it ever loses this tolerance, we have autoimmune
diseases. These considerations serve as a useful background to highlight the possibility that the immune system may be able to recognize cancer cells and eliminate them.
That is, this raises the question of whether cancer cells are foreign enough or strange
enough that they can readily be distinguished from the normal cells in our tissues,
identified as such by certain arms of the immune system, attacked, and eliminated.
The truth is that such discrimination is extraordinarily difficult. If you look at the proteins displayed by cancer cells, 99.9% of them are identical to the proteins made by
normal cells. And therefore, these proteins offer the immune system no clue as to
whether cancer cells are worthy targets of destruction or should be ignored. In fact,
in many cases we believe that incipient cancers are able to fly under the immunological radar and escape detection by the immune system, thereby being able to flourish.
Still, there are certain clues exhibited by cancer cells, which are not exhibited by normal cells, and suggest that the cancer cells are, in one way or another, abnormal. For
example, we know of a class of cells that are part of the normal immune system called
natural killer cells that are endowed with the ability to detect certain abnormal proteins present on the surface of cancer cells that are absent from the surfaces of normal
cells. We believe that cells like natural killer, or NK cells as they are often called, are
important components of the immune defenses against cancer. Perhaps the most persuasive evidence that the immune system plays a key role in defending us against cancer comes from the realization that individuals who are immuno-compromised for a
variety of reasons, including the fact that their immune system has been suppressed
following organ transplants, show significantly increased levels of certain kinds of
cancer, suggesting that their suppressed immune systems have allowed them to tolerate engrafted tissues such as engrafted kidneys or livers. At the same time, these
suppressed immune systems have permitted the outgrowth of cancers that otherwise
would have been eliminated by normal healthy immune systems.
51
In fact, we are just beginning to learn how the immune system is able to distinguish
normal cells from cancer cells and how, having made these distinctions, the immune
system is able to specifically home in on cancer cells and to eradicate them, thereby
reducing significantly the incidence of certain kinds of human cancers.
15. Mini-Lecture: Cancer Therapies
M
odern molecular cancer research really began in 1975 and 1976 with the discovery of the Src proto-oncogene, and over the ensuing 30 years, we have learned
an enormous amount about the molecular mechanisms that create human cancers.
For many people, the motivation for learning all these facts is not simply intellectual
curiosity. Instead, they are much more interested in a far more practical outcome,
which is: how can we exploit this recently acquired information to develop new ways
of treating the disease of cancer?
The truth is that we have discovered a large number of proteins inside cells, specifically inside cancer cells, that in theory should serve as useful targets for intervention
when we are intent upon developing new chemicals that can shut down the growth of
cancer cells, indeed even kill these cells.
For example, a low molecular-weight compound might be able to go into the body,
move throughout the circulation, move into specific tissues, enter cells, and, once
inside cells, it might be able to shut down the firing of a certain protein that is responsible for driving the proliferation of cancer cells or may even be responsible for blocking cell death in these cells.
Once such an enzyme is blocked, the cancer cells may then activate their own cell
suicide program, thereby triggering their own death. That, in the end, is the ideal.
The question is, how can one make such compounds, and if one does make them,
how can one ensure that they are selective? By that I mean to say, how can we know
that these compounds, once they enter into the body, will selectively strike down the
cancer cells, while leaving the normal tissues unharmed? The difficulty of many existing anti-cancer therapies, including many chemotherapies, comes from the fact that
while some of them are highly effective in shutting down the growth of cancer cells
and indeed killing them, many of these chemotherapies also wreak havoc on normal
tissues, and create high and often unacceptable levels of side effect toxicities.
Accordingly, one has attempted to craft new kinds of chemical molecules, new kinds
of drugs, that can enter into cells and specifically shut down only those genes and proteins that are involved in the development of cancer without perturbing the metabolism of normal cells.
More often than not, it is much easier to shut down the firing of an existing enzyme
that is active and signaling than it is to activate a previously latent or silent enzyme. All
of these considerations have focused efforts on developing drugs that can, for example, shut down the firing by certain tyrosine kinase enzymes that are responsible for
driving the proliferation of a variety of different kinds of human carcinomas. We know
that the growth factor receptors at the cell surface of many of these carcinomas are firing into the cell interior by using their tyrosine kinase domains. These tyrosine kinase
domains are able to emit a diverse radiating stream of signals that affects a whole
series of distinct signaling pathways that lead, in turn, to the rapid proliferation and
growth of cancer cells, and thus the growth of tumors.
The question is then: can we develop chemical compounds that are able to shut down
the firing of certain growth factor receptors, while leaving normal cells and their
growth factor receptors untouched? And in recent years, one has had some successes
there in developing highly selective drugs that are specific for interfering with one
tyrosine kinase, but do not interfere with the ninety or so other tyrosine kinases that
might be active within a cell.
In the case of chronic myelogenous leukemia, one has a different example of where a
tyrosine kinase that is not part of a growth factor receptor is running amok, signaling
constitutively, which is to say in an unabated fashion, and driving the growth of these
leukemic cells. Here one has had enormous success in developing a compound that is
able to shut down this kinase molecule; which in this case is called ABL (pronounced
able), without affecting virtually all the other kinds of tyrosine kinases that are present
inside normal cells. And thus one achieves enormous selectivity in hitting the cancer cells while having relatively minimal effect on the normal tissues elsewhere in the
body.
53
16. Mini-Lecture: The Coming Cancer Epidemic
W
e’ve all read in the media about the increased numbers of various types of cancer in the population. In fact, the only really accurate way of measuring cancer
incidence is to measure age-adjusted incidence. In other words, what is the risk of a
60-year-old woman dying this year of breast cancer compared to the risk of a 60-yearold woman dying from breast cancer in the year 1930, rather than simply measuring
the absolute number of cases in the population. This is important in no small part
because cancer incidence varies dramatically at different times of life. If one measures cancer incidence by looking at the age-adjusted rate, one sees that many kinds
of cancers that one has imagined are present in epidemic proportion really have not
increased that much. Breast cancer, which was widely stated to be present in epidemic
proportions in our country, had a rather flat rate of incidence from about 1930 to about
1995 in age-adjusted rates. Starting in 1995, there has been an approximately 20 %
decrease in cancer-associated mortality, once again on an age-adjusted basis. Other
kinds of cancers have also gone down. Stomach cancer has gone down by a factor of
four or five, colon cancer perhaps by 20 or 25 % because of colonoscopy. Cervical cancer has gone down dramatically by about 80 % in Western populations because of the
use of the Pap test to detect premalignant growths.
Still, taking all this into account, and changing patterns of tobacco usage, it becomes
clear that the absolute number of cancers in the population is going to increase over
the coming decade. This is because cancer is essentially a disease of the elderly. The
risk of a 70-year-old man developing colon cancer is about 1000 times higher than
that of a ten-year-old boy. This is true for a variety of other cancers with the exception,
obviously, of childhood cancers, and among adults with breast cancer, which begins
rather early in certain women. Taking all these factors together, one begins to realize
that the number of cases in the population will increase simply because people are
living long enough to develop cancer in their old age. Or, to put it another way, the
number of cases of Alzheimer’s is also increasing. A disease like Alzheimer’s or cancer,
which was previously relatively rare 100 years ago, has now become quite common
because people are reaching the age of 80 or 90 when these diseases become very high
risk- when these diseases begin to appear in large numbers.
If we look at the aging, therefore, of the Western population, the fact is that the number
of people over the age of 65, when most adult cancers increase in incidence, is going
to increase dramatically and therefore the absolute number of cases in any Western
society will increase, even though the risk of any individual dying of cancer at a certain
age may actually be going down.
Apple and QuickTime are registered trademarks of Apple, Inc.
Microsoft Windows, Microsoft Word, and PowerPoint are registered trademarks of Microsoft, Inc.
Adobe and Adobe Photoshop are registered trademarks of Adobe Systems Incorporated

The Biology Of Cancer

Transcription

Similar documents

EasyPure Genomic DNA Spin Kit

Exsanguination - Northern Health

AMPULLA OF VATER STAGING FORM

angionesesis emt metastasis

An Introduction to Next-Generation Sequencing for

Evolutionary optimization in (deformable) registration of medical

- East Midlands Business Link

Karen K. Resendes Westminster College New Wilmington, Pa

(Microsoft PowerPoint - 970418\244\362\253a\263\354 cavernous