Metabolic consequences of pregnancy-associated plasma protein-A

599
Metabolic consequences of pregnancy-associated plasma protein-A
deficiency in mice: exploring possible relationship to the longevity
phenotype
Cheryl A Conover, Megan A Mason, James A Levine and Colleen M Novak
Division of Endocrinology, Metabolism and Nutrition, Mayo Clinic, 200 First Street SW, 5-194 Joseph, Rochester, Minnesota 55905, USA
(Correspondence should be addressed to C A Conover; Email: [email protected])
Abstract
Mice born with the deletion of the gene for pregnancyassociated plasma protein-A (PAPP-A), a model of reduced
local IGF activity, live w30% longer than their wild-type
(WT) littermates. In this study, we investigated metabolic
consequences of PAPP-A gene deletion and possible
relationship to lifespan extension. Specifically, we determined
whether 18-month-old PAPP-A knockout (KO) mice when
compared with their WT littermates have reduced energy
expenditure and/or altered glucose–insulin sensitivity. Food
intake, and total energy expenditure and resting energy
expenditure as measured by calorimetry were not different
between PAPP-A KO and WT mice when subjected to the
analysis of covariance with body weight as the covariate.
Introduction
Pregnancy-associated plasma protein-A (PAPP-A) is a
recently discovered metalloproteinase whose major physiological function, as so far identified, is to increase local
insulin-like growth factor (IGF) bioavailability through the
cleavage of inhibitory IGF binding proteins (reviewed in
Boldt & Conover 2007). Reduced IGF signaling is associated
with a prolonged lifespan in a variety of species (Kenyon
2001, Barbieri et al. 2003, Holzenberger et al. 2004,
Richardson et al. 2004). Indeed, mice born with the deletion
of the PAPP-A gene live w30% longer than their wild-type
(WT) littermates (Conover & Bale 2007). However, the
mechanism(s) underlying this enhanced longevity have yet to
be determined. With this study, we initiated investigation into
key factors involved in lifespan extension in a mammalian
model of reduced local IGF action.
Together, with insulin, IGFs are important regulators of
cell metabolism and growth. The ‘rate of living’ theory of
aging posits that a decrease in overall metabolic rate decreases
the production of reactive oxygen species (ROS) during
respiration, thereby slowing the aging process (reviewed in
Masoro 2005). If so, then reduced IGF signaling might result
in decreases in metabolism that could explain the increase in
Journal of Endocrinology (2008) 198, 599–605
0022–0795/08/0198–599 q 2008 Society for Endocrinology
However, there was an increase in spontaneous physical
activity in PAPP-A KO mice. Both WT and PAPP-A KO
mice exhibited mild insulin resistance with age, as assessed by
fasting glucose/insulin ratios. Oral glucose tolerance and
insulin sensitivity were not significantly different between the
two groups of mice, although there appeared to be a decrease
in the average size of the pancreatic islets in PAPP-A KO
mice. Thus, neither reduced ‘rate of living’ nor altered
glucose–insulin homeostasis can be considered key
determinants of the enhanced longevity of PAPP-A KO
mice. These findings are discussed in the context of those
from other long-lived mouse models.
Journal of Endocrinology (2008) 198, 599–605
lifespan in PAPP-A knockout (KO) mice. Decreased body
temperature and lowered metabolism in the Ames and Snell
dwarf mice, long-lived growth hormone (GH)- deficient
mouse models, would support this theory (Bartke et al. 2001).
It is of note that these mice have low circulating IGF-I due to
the loss of GH-dependent hepatic IGF-I expression. Studies
of caloric restriction in rats provide evidence for an
association between lifespan, circulating IGFs, and metabolic
rate (Krystal & Yu 1994). Caloric restriction in rhesus
monkeys also results in decreased core temperature and 24 h
energy expenditure (Lane et al. 1996). On the other hand,
there are several examples of reduced insulin and/or IGF-I
signaling where the organism does not have to be
metabolically compromised to age slowly (Barbieri et al.
2003, Holzenberger et al. 2003, Hulbert et al. 2004).
Furthermore, a progressive rise in insulin resistance is
associated with aging (Ferrannini et al. 1996, Facchini et al.
2001), and a direct cause and effect relationship has been
suggested. The ‘altered glucose–insulin system’ hypothesis
suggests that the lifetime maintenance of low levels of glucose
and insulin can explain life extension (Masoro 2005).
Invertebrate models have been particularly informative
(Broughton et al. 2005). However, animal models of extended
lifespan are somewhat controversial in this regard. In support
Printed in Great Britain
DOI: 10.1677/JOE-08-0179
Online version via http://www.endocrinology-journals.org
600
C A CONOVER
and others . Metabolic profile of PAPP-A knockout mice
of this hypothesis, GH-deficient and GH-resistant dwarf mice
with reduced circulating IGF-I show decreased fasting levels
of glucose and insulin and increased insulin sensitivity
(Dominici et al. 2002, Coschigano et al. 2003, Bonkowski
et al. 2006). There is a similarly reduced glucose–insulin
profile with caloric restriction (Richardson et al. 2004).
Seemingly against a direct relationship between aging and
insulin resistance are the long-lived Klotho transgenic mice.
These mice have an increase in lifespan and are insulin and
IGF-I resistant (Kurosu et al. 2005).
In this study, we focused on the basic metabolic and activity
profile of PAPP-A KO mice, a model of reduced local IGF-I
action in the context of normal circulating IGF-I levels
(Conover et al. 2003, Conover & Bale 2007). Specifically, we
tested the hypothesis that aged PAPP-A KO mice have
reduced energy expenditure and/or altered glucose–insulin
sensitivity when compared with WT littermates.
Materials and Methods
PAPP-A KO mice
Mice with the targeted deletion of the PAPP-A gene were
generated through homologous recombination in embryonic
stem cells (Conover et al. 2003). These mice were on a mixed
C57BL/6 and 129 background. WT and PAPP-A KO
littermates from heterozygous breeding were used in studies.
Genotyping was performed by PCR as described previously
(Bale & Conover 2005). For all experimental animals,
genotypes were reconfirmed on tail DNA at time of sacrifice.
Data from mice with obvious tumors (one 18-month-old
WT mouse) were eliminated from analyses. All mice were
kept on a 12 h light:12 h darkness cycle with the light phase
beginning at 0600 h.
Food intake, energy expenditure, and spontaneous activity
For food intake, mice were housed for 7 days in a controlled
environment within individual plexiglas cages that matched
the chamber used for energy expenditure and activity
measurements. These cages have ceramic bowls designed not
to tip allowing daily measurement of ad libitum food intake,
which was performed for 5 consecutive days. Energy
expenditure and physical activity were measured as previously
reported (Novak et al. 2006, Barbosa et al. 2007). At least
24 h before the time of measurement, each mouse was allowed
to acclimate to the testing conditions in a cylindrical chamber
(7 l; 30 cm diameter!10 cm high) in the testing area. To
measure energy expenditure, custom made small animal
calorimeters with a 7 l cylindrical chamber (Columbus
Instruments; Columbus, OH, USA) were used. For
calorimetry, the sample flow rate was set at 0.4 l per min
(lpm), and the chamber flow rate at which room air pumped
through the chamber was set at 0.45–0.7 lpm, depending
on the size of the mouse. Samples were collected once every
Journal of Endocrinology (2008) 198, 599–605
60 s for 25 h, except for the reference samples collected for
5 min after every 30 samples throughout the 24-h measurement period. Data for each calorimeter were monitored and
compared with ensure that the data collected and reference
values were comparable between sets of equipment. Data
collected included VO2 and VCO2 (both in ml/kg per h),
respiratory exchange ratio (RER, VCO2/VO2), and energy
expenditure ((3.815C1.232!RER)!VO2) in kcal/h).
Food and water were supplied ad libitum. The calorimetry
and physical activity data were calculated every minute, and
the energy expenditure data were 60 s behind the activity data,
due to the time needed to move and process the air from
the calorimetry chamber. To calculate resting energy expenditure (REE), we calculated the total daily energy expenditure
(TDEE) for the minutes where there were no activity counts
for the current and previous 5 min (see physical activity, below)
and averaged them. When the REE is subtracted from the
TDEE, this yields energy expenditure of activity (EEA).
Attached to the calorimeters were Opto-M Varimex
Minor activity monitors. These devices contain 45 collimated
infrared activity sensors that gather data regarding physical
activity in three axes (horizontal x and y axes, plus vertical
activity) as well as ambulatory activity (which excluded
repetitive signals from a single beam). Data for physical
activity and energy expenditure were collected simultaneously every minute, allowing correlation of the two
variables in each animal tested.
Oral glucose tolerance and insulin sensitivity testing
For oral glucose tolerance testing, food was removed from
cages at w2100 h, and glucose (0.01 ml of a 10% glucose
solution per g body weight) administered by gavage at
w0900 h the next morning. Blood was collected from the tail
vein at 0, 5, 10, 15, 30, 45, 60, 90, and 120 min. Glucose was
measured using a One Touch Ultra glucose meter (LifeScan,
Milpitas, CA, USA). Insulin was measured at 0, 5, 10, 15, and
30 min using an Ultra Sensitive Rat Insulin ELISA Kit from
Crystal Chem Inc. (Downers Grove, IL, USA). Area under
the curve was measured using the trapezoid method. For
insulin sensitivity testing, overnight-fasted mice were given
food at 0830 h for 30 min. At 1200 h, mice then received an
i.p. injection of regular insulin (75 mU/kg body weight) with
blood collections at 0, 20, 40, and 60 min for glucose
measurements. Insulin sensitivity was assessed by comparing
rate-of-decay curves.
Pancreas immunohistochemistry
The pancreas was carefully dissected, fixed in formalin,
embedded, and processed as described by Devedjian et al.
(2000). Briefly, the entire pancreas was cut in 5 mm sections
every 150 mm and every other section stained for insulin by the
Tissue and Cell Molecular Analysis Core Lab using guinea
pig anti-swine insulin antibody from Dako (Carpinteria, CA,
USA). The Nikon Microphot-FXA and Nikon ACT-1 Version
www.endocrinology-journals.org
Metabolic profile of PAPP-A knockout mice .
2.6.2 (Melville, NY, USA) were used to photo-capture the
slides. The number of islets and islet cell mass were measured
using the polygonal lasso tool in Adobe PhotoShop 6.0.1.
Results
Food intake, energy expenditure, and physical activity
Food intake, energy expenditure, and spontaneous physical
activity were measured in 18-month-old WTand PAPP-A KO
mice. Both male and female mice were studied, and the results
are presented in Table 1. Food intake, expressed as g/day, and
TDEE and REE, expressed as kcal/day, were significantly
decreased in male and female PAPP-A KO when compared
with WT mice. However, these calculations do not take into
account the significantly smaller size of the PAPP-A KO
mouse (Conover et al. 2003, Bale & Conover 2005). The
normalization of the data for body weight indicated
significantly increased food intake (males only), TDEE, and
REE in PAPP-A KO mice when compared with WT
Table 1 Food intake, energy expenditure, and physical activity. See
Materials and Methods for descriptions and calculations. Results are
meanGS.E.M. of (n) mice
Males
Food intake
TDEE
REE
BW
Food intake/BW
REE/BW
TDEE/BW
EEA
Horiz
Vert
Ambul
WT (7)
PAPP-A KO (12)
4.0G0.16
19.4G0.80
17.7G0.94
36G2.3
0.11G0.005
0.49G0.016
0.54G0.019
1.7G0.14
24.6G3.30
4.6G0.92
5.8G0.93
3.2G0.16a
16.2G0.41a
14.4G0.29a
20G0.8a
0.15G0.004b
0.72G0.031b
0.82G0.037b
1.8G0.17
26.0G3.19
10.0G1.31b
5.3G0.77
Females
Food intake
TDEE
REE
BW
Food intake/BW
TDEE/BW
REE/BW
EEA
Horiz
Vert
Ambul
WT (13)
PAPP-A KO (18)
3.3G0.17
12.7G0.53
11.4G0.55
30G1.7
0.11G0.009
0.43G0.017
0.38G0.014
1.1G0.12
24.7G2.90
7.3G1.25
6.3G0.94
2.6G0.24a
10.5G0.24a
9.3G0.29a
21G0.8a
0.12G0.009
0.50G0.021b
0.44G0.020b
1.2G0.07
26.4G1.92
9.7G1.16
7.2G0.79
Food intake (g/d). TDEE, total daily energy expenditure (kcal/d); REE, resting
energy expenditure (kcal/d); BW, body weight (g); EEA, energy expenditure
activity (kcal/d); Horiz, horizontal activity; Vert, vertical activity; Ambul,
ambulatory activity.
a
Significantly decreased compared with WT.
b
Significantly increased compared with WT.
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C A CONOVER
and others 601
littermates. However, in this case dividing by body weight
may overcorrect energy expenditure values of the larger
animals (Packard & Boardman 1999, Arch et al. 2006).
Therefore, the data were subjected to the analysis of covariance
which uses the group (genotype) comparison of regression
lines to determine whether the groups differ when body
weight is a covariate. The results of these analyses are presented
in Table 2. Differences in TDEE and REE were accounted for
by body weight and not by genotype. EEA was not significantly
different in WT and PAPP-A KO mice and could not be
accounted for by body weight. However, vertical activity was
increased 30–110% in PAPP-A KO mice, reaching significance in males (Table 1). Thus, there was no decrease in
energy expenditure in 18-month-old PAPP-A KO mice
when compared with WT mice which would suggest that a
compensatory slowing of metabolism was the direct link to
longevity. On the contrary, there may be an increase in physical
activity in the PAPP-A KO mice.
Glucose and insulin sensitivity
Fasting glucose and insulin levels were not significantly different
between WTand PAPP-A KO mice at 4 months or 18 months
of age (Table 3). In general, female mice had lower fasting insulin
levels than male mice, which were independent of genotype.
There was an increase in insulin levels in the older animals to
maintain the same glucose levels, suggesting mild insulin
resistance. This increase tended to be less in the PAPP-A KO
mice but the data did not reach statistical significance. Oral
glucose tolerance testing was performed on 12 h fasted WTand
PAPP-A KO mice, both males and females at 18 months of age.
There was no significant difference in the endogenous glucose
or insulin response to the glucose challenge between PAPP-A
KO and WT mice, either male (Fig. 1A) or female (Fig. 1B).
Total pancreatic islet area and number were not significantly
different in 18-month-old WTand PAPP-A KO mice (Table 4).
However, there was a significantly smaller, by w30%, average
size of islets in PAPP-A KO when compared with WT mice.
Sensitivity to exogenously administered insulin was not
Table 2 Analysis of covariance (ANCOVA) for energy expenditure
measurements. The data in Table 1 were subjected to ANCOVA
with body weight (BW) as the covariate. Group P value indicates
whether groups are different in the dependent variable (food intake
or EE) after BW is factored out. BW indicates whether the covariate
had a significant effect on the measure. Group*BW interaction
indicates whether the effect of BW on the food intake or EE of each
group is the same. If this is significant, then the interpretation of a
group effect would be hampered
P value
Group
BW
Group*BW
Food intake
TDEE
REE
EEA
0.640
0.069
0.546
0.508
0.016
0.104
0.547
0.006
0.104
0.432
0.781
0.444
Journal of Endocrinology (2008) 198, 599–605
602
C A CONOVER
and others . Metabolic profile of PAPP-A knockout mice
Table 3 Fasting glucose and insulin levels
WT
4 months
Males
Females
18 months
Males
Females
PAPP-A KO
n
Glucose
Insulin
n
Glucose
Insulin
(9)
(8)
81G3
88G8
0.69G0.137
0.38G0.080
(9)
(8)
89G2
91G8
0.46G0.124
0.36G0.088
(11)
(7)
82G6
89G7
1.23G0.194a
0.88G0.294a
(10)
(9)
79G6
78G6
0.94G0.250a
0.59G0.100a
Glucose: mg/dl. Insulin: ng/ml. Results are meanGS.E.M. of n mice.
a
Significant difference between 4 months and 18 months.
significantly different in 18-month-old WT and PAPP-A KO
mice, either male (Fig. 2A) or female (Fig. 2B).
Discussion
Mice lacking PAPP-A have relatively normal energy
metabolism and glucose/insulin sensitivity compared with
WT, age-matched littermates. Therefore, the findings of this
study do not support reduced ‘rate of living’ or an ‘altered
glucose–insulin system’ as key determinants of the enhanced
longevity of PAPP-A KO mice. Although the underlying
mechanism was not identified in this series of experiments
designed to test specific hypotheses, the data are relevant in
the context of current controversies in the biology of aging.
There was no decrease in total or REE in 18-month-old
PAPP-A KO mice when compared with WT mice when the
significantly smaller size of the PAPP-A KO mice was taken into
account (Packard & Boardman 1999, Arch et al. 2006). Methods
of normalization for body weight can lead to different
conclusions and may explain some of the studies with caloric
restriction and apparent decreases in metabolic rate. Furthermore, recent studies have suggested that, after an initial
reduction, caloric restriction leads to equal or higher metabolic
rates than ad libitum fed animals (Faulks et al. 2006). It has been
assumed that a decrease in metabolic rate would prolong lifespan
by deceasing exposure to ROS produced during respiration.
Alternatively, under normal metabolic conditions, tissues from
PAPP-A KO mice may be less vulnerable to oxidative damage
by the virtue of increased ability to eliminate ROS or show
increased mitochondrial efficiency thereby reducing ROS
production. These have been suggested as contributors to
longevity in other model systems (Balaban et al. 2005).
Furthermore, the PAPP-A KO mice had increased physical
activity in the vertical axis. This reached statistical significance
in males, although a similar trend for increased activity was seen
in the females as well. What this behavior might mean can only
be speculated upon at this time. Mice were monitored
individually so it is not likely to be aggressive posturing, but
rather spontaneous physical activity. There have been few
studies monitoring physical activity in the various mouse
Table 4 Islet cell immunohistochemistry. Sections of pancreata
were stained for insulin as described in Materials and Methods.
Results are meanGS.E.M. of (n) mice
Figure 1 Oral glucose tolerance curves from (A) male and (B) female
PAPP-A KO and WT mice. Eighteen-month-old PAPP-A KO (closed
symbols, solid lines) and WT (open symbols, dashed lines) mice
underwent an oral glucose tolerance test as described in Materials
and Methods. Thicker lines represent the glucose response curve
and thinner lines represent the insulin response curve. Values are
meanGS.E.M., nZ10. There were no significant differences in AUC
for glucose or insulin between WT and PAPP-A KO groups.
Journal of Endocrinology (2008) 198, 599–605
WT (7)
Islet
Area (Pixels)
Number
Average size
(Pixels)
PAPP-A KO (8)
49 786G25 531 39 492G8696
147G36
136G25
390G50
278G20
P value
0.441
0.802
0.048
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Metabolic profile of PAPP-A knockout mice .
Figure 2 Insulin sensitivity in (A) male and (B) female PAPP-A KO
and WT mice. Eighteen-month-old PAPP-A KO (closed symbols,
solid lines) and WT (open symbols, dashed lines) mice underwent
insulin sensitivity testing as described in Materials and Methods.
Values are meanGS.E.M., nZ6–12. There were no significant
differences in rate-of-decay curves for WT and PAPP-A KO mice.
models associated with aging research. In caloric-restricted
mice and IGF-I receptor heterozygous mutant mice, there was
no difference in total physical activity when compared to
controls, although dimensional aspects were not recorded
(Holzenberger et al. 2003, Faulks et al. 2006). Interestingly,
spontaneously wiggling Caenorhabditis elegans were longerlived than their counterparts that needed to be prodded to
move (Herndon et al. 2002). Apfeld et al. (2004) suggest that in
this model system there is a direct link between activity levels,
insulin-like signals and lifespan. Thus, this aspect of the
longevity profile warrants further study.
C A CONOVER
and others 603
Fasting insulin/glucose ratios increased significantly with age
in both WT and PAPP-A KO mice, suggesting mild insulin
resistance. Notably, for the purpose of this study, it occurred in
both groups and in both sexes. Thus, PAPP-A deficiency does
not necessarily translate into a ‘youthful’ insulin–glucose profile,
and there was no evidence for sexual dimorphism in insulin
resistance as was seen in liver-specific IGF-I deficient mice with
reduced circulating IGF-I levels (Tang et al. 2005). Eighteenmonth-old WTand PAPP-A KO mice challenged with glucose
had similar glucose tolerance curves. Insulin secretory capacity
was assessed by immunohistochemistry of insulin-producing
b-cells in pancreatic islets. Multiple sections of the total pancreas
were stained for insulin to try to account for heterogeneity of
islet distribution and size. Interestingly, the average size but not
the number or total mass of pancreatic islets was significantly
decreased in PAPP-A KO mice. b-Cell-specific IGF-I receptor
KO mice exhibit normal development of b-cells but defective
glucose-stimulated insulin secretion and impaired glucose
tolerance (Kulkarni et al. 2002). Thus, local IGF signaling
does not appear to be essential for normal growth of pancreatic
islets, which may partially explain the normal islet mass and
number in PAPP-A KO mice. However, secretion of insulin
from pancreatic b-cells is dependent on IGF-I, and a diminished
(but not complete) loss of local IGF-I action in PAPP-A KO
mice may affect optimal secretory response without major
impact on overall glucose homeostasis. Comparable studies in
other mouse models of aging indicated decreased glucose
tolerance in Ames and GH-resistant dwarf mice that was due to a
decrease in total volume of the islets (Parsons et al. 1995, Liu et al.
2004), perhaps reflecting reduced GH and/or circulating IGF-I.
Circulating IGF-I, which is not decreased in PAPP-A KO mice,
is an important component of overall insulin action in peripheral
tissues (Yakar et al. 2001). This may account for unaltered insulin
sensitivity in PAPP-A KO mice. Caloric-restricted mice
maintain glucose homeostasis with markedly less insulin,
which may be associated with increased insulin sensitivity
(Richardson et al. 2004). The co-existence of insulin resistance
and enhanced longevity in Klotho male transgenic mice
(Kurosu et al. 2005) makes sense, as discussed below, if insulin
Table 5 Metabolic parameters of long-lived mouse models. Data from the present study and other published studies were use to compile this
table comparing PAPP-A KO, Ames dwarf, Snell dwarf, GH receptor KO, Klotho transgenic, calorically restricted (CR) and heterozygous IGF-I
receptor mutant (IGFRC/K) mice. References can be found in the text
Lifespan
Metabolic rate
Activity
Fasting insulin/glucose
Glucose tolerance
Insulin sensitivity
GH
Circulating IGF-I
Local IGF-I action
PAPP-A KO
Ames
Snell
GHRKO
Klotho
CR
IGFRC/Ka
[
Z
[
Z
Z
Z
Z
Z
Y
[
Y
?
Y
Y
[
Y
Y
?
[
Y
?
Y
?
?
Y
Y
?
[
?
?
Y
Y
[
[
Y
?
[
Z
?
[Z
?
Y
?
Z
Y
[
ZY[
Z
Y
?
[Z
?
Y
?
[
Z
Z
Z
Z
?
?
[
Y
Gluc, Glucose; [, increased compared with control; Y, decreased compared with control; Z, no difference compared with control; ?, not known.
a
Females only.
www.endocrinology-journals.org
Journal of Endocrinology (2008) 198, 599–605
604
C A CONOVER
and others . Metabolic profile of PAPP-A knockout mice
and/or insulin sensitivity per se was not a key determinant of
enhanced lifespan. Indeed, the extended lifespan of fat-specific
insulin receptor KO mice may not be primarily due to reduced
insulin signaling in adipose tissue, but rather a result of decreased
fat mass (Bluher et al. 2003). The percentage fat is similar in WT
and PAPP-A KO mice (data not shown). Thus, although we
cannot completely rule out a role for reduced insulin signaling as
a contributing factor to the longevity of PAPP-A KO mice, the
similar fasting and fed levels of insulin ad libitum in WT and
PAPP-A KO mice and the similar insulin sensitivity curves in
the longer-lived PAPP-A KO mice indicate that this role would
not be a decisive one.
Metabolic and activity data from different mouse models
of longevity are summarized in Table 5. The many
information gaps indicated in the table make it difficult to
come to firm conclusions. However, there is no consistency
across the seven models in metabolic rate, physical activity,
fasting insulin/glucose, glucose tolerance, insulin sensitivity,
GH, or circulating IGF-I levels. Of course this could be due
to the specifics of the models and the testing methods. In
addition, the age of the mice at the time of determination of
metabolic parameters was not always apparent in the
published articles in the different longevity models. This
could be important for antagonist pleiotropic systems such as
the IGF system, which can have beneficial effects early in life
and detrimental effects late in life (Rincon et al. 2004). We
chose 18 months as our ‘aged’ mouse group because after
this time the WT mice tended to develop tumors, which
could impact metabolic measurements. It is possible that
alterations in metabolism occur in younger mice that could
impact lifespan. However, this did not occur in those systems
that were examined across ages (Coschigano et al. 2003).
Uncertainties notwithstanding, one could propose decreased
local IGF-I action as the common denominator in these
long-lived mouse models. PAPP-A KO, Klotho transgenic,
and IGF-I receptor heterozygous mutant mice have
diminished local IGF-I signaling (Conover et al. 2003,
Holzenberger 2004, Kurosu et al. 2005). It may be that
Ames, Snell, and GH receptor-deficient mice have decreased
local IGF-I action due to partial GH-dependence of IGF-I
expression in peripheral tissues, although there is currently
no evidence that these mice exhibit the loss of function of
IGF-I signaling. Caloric restriction likewise has been shown
to decrease IGF-I expression in hepatic and non-hepatic cells
(Masternak et al. 2005, Papaconstantinou et al. 2005). Thus,
a decrease in effective local IGF-I signaling, whether through
decrease in IGF expression (GH deficiency/resistance,
caloric restriction), decrease in IGF receptor signaling
(IGF-I receptor mutation, Klotho overexpression), or
diminished IGF bioavailability (PAPP-A deficiency), can
increase lifespan in mice. Further studies are necessary to
establish underlying mechanism(s) specific to the PAPP-A
KO model that could enhance our overall understanding of
lifespan determinants and suggest therapeutic targets to
promote healthy aging.
Journal of Endocrinology (2008) 198, 599–605
Declaration of interest
There is no conflict of interest that would prejudice impartiality.
Funding
This work was supported by NIH Grant AG028141 and the Ellison Medical
Foundation (to C A C), NINDS 55859, 0635113N from the American Heart
Association, and a grant from the Minnesota Obesity Consortium (to C M N),
and NIH grants DK56650, DK63226, DK66270, and R04-0771 (to J A L).
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Received in final form 10 June 2008
Accepted 17 June 2008
Made available online as an Accepted Preprint
19 June 2008
Journal of Endocrinology (2008) 198, 599–605