Plant derived inhibitors of NF-κB Avi Golan
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Plant derived inhibitors of NF-κB Avi Golan
Plant derived inhibitors of NF-κB Avi Golan-Goldhirsh & Jacob Gopas Phytochemistry Reviews Fundamentals and Perspectives of Natural Products Research ISSN 1568-7767 Volume 13 Number 1 Phytochem Rev (2014) 13:107-121 DOI 10.1007/s11101-013-9293-5 1 23 Your article is protected by copyright and all rights are held exclusively by Springer Science +Business Media Dordrecht. This e-offprint is for personal use only and shall not be selfarchived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com”. 1 23 Author's personal copy Phytochem Rev (2014) 13:107–121 DOI 10.1007/s11101-013-9293-5 Plant derived inhibitors of NF-jB Avi Golan-Goldhirsh • Jacob Gopas Received: 3 January 2013 / Accepted: 11 April 2013 / Published online: 19 April 2013 Ó Springer Science+Business Media Dordrecht 2013 Abstract Plant secondary metabolites (natural products) have been a source for many of our medicines. Their functions in plants remain often unknown, but in recent years there are more and more new compounds isolated and identified and their medicinal potential investigated. The major classes of plant natural products and various derivatives thereof are: phenolics, terpenoids, alkaloids and lignans. The major transcription factor, nuclear factor-jB (NF-jB) is a central downstream regulator of inflammation, cell proliferation and apoptosis that controls the expression of more than 500 genes. It plays an essential role in several aspects of human health including the development of innate and adaptive immunity. The deregulation of NF-jB is associated with many ailments including cancer and chronic inflammatory diseases. In spite of a vast literature describing NF-jB inhibitors A. Golan-Goldhirsh (&) French Associates Institute for Agriculture and Biotechnology of Drylands, Albert Katz Department of Dryland Biotechnologies, The Jacob Blaustein Institutes for Desert Research, Ben Gurion University of the Negev, Sede Boqer Campus, 84990 Midreshet Ben Gurion, Israel e-mail: [email protected] J. Gopas (&) The Shraga Segal Department of Microbiology and Immunology and Genetics, Faculty of Health Sciences and Laboratory of Oncology, Soroka University Medical Center, Ben Gurion University of the Negev, 84105 Beer Sheva, Israel e-mail: [email protected] from many natural or synthetic sources, such modulators have not been fully tapped for therapeutic purposes and the search for effective and specific inhibitors for therapeutic use and minimal side effects is still relevant and ongoing. Plant-derived phytochemicals are promising lead compounds to develop potent and safe inhibitors for cancer and inflammatory disorders driven by NF-jB. We briefly review the recent knowledge on plant derived phytochemicals and their major NF-jB molecular targets. Keywords Cancer Inflammation NF-jB Phytochemicals Abbreviations AMPK Adenosine monophosphate- activated protein kinase AKT Protein Kinase B CBP CREB binding protein COX Cyclooxygenase CXCR CXC Chemokine receptor EGF Epidermal growth factor ERK Extracellular signal-regulated kinase FAK Focal adhesion kinase IAP Inhibitor of apoptosis ICAM Intracellular adhesion molecule IjB Inhibitor of NF-jB IKK Inhibitor of NF-jB Kinase IL-1 Interleukin-1 LPS Lipopolysacharide MAPK Mitogen-activated protein kinase 123 Author's personal copy 108 MMP PI3K RHD ROS c-T3 TBP TLR TNF TNFR VEGF XIAP Phytochem Rev (2014) 13:107–121 Matrix metalloproteinase Phosphoinositide 3-kinase REL homology domain Reactive oxygen species c-tocotrienol TATA binding protein Toll-like receptor Tumor necrosis factor Tumor necrosis factor receptor Vascular endothelial factor X-linked inhibitor of apoptosis protein Targeting NF-jB pathways by phytochemicals The nuclear factor-jB (NF-jB) family of transcription factors regulates the expression of hundreds of genes that are associated with diverse cellular processes, such as proliferation, differentiation and death, as well as innate and adaptive immune responses. The area of research involving NF-jB has grown tremendously in the past decade. This is evident from the fact that although NF-jB was discovered only 25 years ago (Sen and Baltimore 1986) and is one of approximately 2,000 estimated transcription factors in humans (GuhaThakurta 2006), approximately 10 % of research articles listed in PubMed on the subject of transcription factors are associated with NF-jB. NF-jB is constitutively active in many cancers, and many of the signaling pathways implicated, are likely to be networked to its activation (Chaturvedi et al. 2011). NF-jB can be modulated by diverse stimuli and by highly networked pathways, therefore suggesting a multi-targeted approach in anti-inflammation and anticancer therapies. Plant derived molecules that are in focus of this review, target multiple steps in the NF-jB pathways are emerging as promising agents for the prevention and treatment of cancers. NF-jB plays a major role in inflammation, immune reactions and carcinogenesis (Baker et al. 2011; Perkins 2012) as well as in related diseases that are not yet well defined in molecular terms such as fatigue, depression, sleepdisorders etc., (Gupta et al. 2011b). Therefore, NF-jB is considered an important target for therapeutic modulation by synthetic and natural new drugs (Gilmore and Herscovitch 2006; Gupta et al. 2010a). The diversity of molecular targets for the same phytochemical complicates the understanding of the 123 cellular mechanism of action and yet may be effective in therapy. Several major common drugs used in the clinic are originally phytochemicals that have been discovered from plant extracts, like aspirin from willow bark, taxol from Pacific yew and metformin from French lilac. The potential of plants as a source of medicines is at its infancy and there are thousands of compounds that have not been identified yet that can be used directly or as lead structures for modification and organic synthesis of new drugs. There have been a few comprehensive recent reviews covering natural products at large, which inhibit/modulate NF-jB by Salminen et al. (2012), Chen (2011) and Gupta et al. (2010b). In this article we review most of the compounds derived solely from plants, since these reviews were published. A summary of the recent plant derived inhibitors of NF-jB is presented in Table 1. In several cases like with Kahweol, compounds that seemed to have a potentially important effect, but were not cited again since they were first published in the context of NF-jB were added. Also extract mixtures of e.g., phenolics (ADEE) or of sesquiterpenoid lactones, which provide either new or better understanding of the mechanism of action of these groups of compounds, were included. The table is arranged according to the major chemical classes of plant secondary metabolites and alphabetically within each group. The largest group of inhibitors is the phenolic with 22 out 59 (1–22, Table 1), 5 quinones (23-27, Table 1) 21 isoprenoids and derviatives (28–48, Table 1), 7 alkaloids (49–55, Table 1), and 4 others (56–59, Table 1). The table provides information about plant species and family origin of the compound, plant tissue from which the compound was isolated, a brief note about the reported mode of action and a recent reference (not all recent references could be included because of size limitations of the review). Many of the major secondary metabolites of plants (phytochemicals), phenolic, isoprenoids and alkaloids possess significant therapeutic properties including anti-inflammatory and anti-cancer effects. They are differentially distributed among limited taxonomic groups in the plant kingdom. Among the 59 compounds reviewed in this paper only the Asteraceae family had 5 compounds in various species, 21 families had one compound, 3 families had 2 compounds, 6 families had 3 compounds and 2 families had 4 compounds. Most of these molecules showed an inhibitory effect on the expression of NF-jB. More Active compounds ADEE Anacardic acid Butein Cardamonin Carnosic acid Carnosol Catechin Cudraflavone B Curcumin (Curcuminoids) No. 1 2 3 4 5 6 7 8 9 Phenolics (mixture) Phenolic, prenylated flavone Phenolic, flavan-3-ol Phenolic diterpenoid Phenolic, benzenediol abietane diterpenoid Phenolic, chalconoid Phenolic, mixture of cumarins, phellopterin, isoimperatorin, imperatorin, alloimperatorin, byakangelicin, isooxypeucedanin, and pimpinellin (Ethanolic extract) Phenolic acid, derivative of salicylic acid Phenolic, chalconoid Chemical class Table 1 Plant derived NF-jB inhibitors Curcuma domestica and Curcuma longa (Zingiberaceae) Camellia sinensis (Theaceae), other fruits and vegetables Morus alba (Moraceae) Rosmarinus officinalis (Lamiaceae) Anacardium occidentale (Anacardiaceae) Toxicodendron vernicifluum (Anacardiaceae) Alpinia rafflesiana (Zingiberaceae) and other species in the family Rosmarinus officinalis (Lamiaceae) Angelica dahurica (Apiaceae) Plant species (Family) Rhizome Root Leaves and other plant organs Leaves Leaves Fruit Shell of cashew nut Stem Root Plant tissue Inhibition of the translocation of NF-jB to the nucleus in macrophages Inhibition of IjBa phosphorylation and p65 phosphorylation and acetylation and nuclear translocation Blocked the nuclear translocation of NF-jB and its upstream signaling including Syk/Src, phosphoinositide 3-kinase (PI3 K), Akt, IKK and IjBa Pretreatment abolishes NF-jB translocation and transcriptional activity Down-regulation the expression of NF-jB Down-regulation of NF-jB. Synergistic to Lunasin peptide Suppress NF-jB, attenuates VEGF, MMP-9 activities Attenuated NF-jB DNA binding activity Inhibited NF-jB translocation to the nucleus by IjBa degradation Mechanism of action Buhrmann et al. (2011) Hosˇek et al. (2011) Bharrhan et al. (2011) Lian et al. (2010) Oh et al. (2012) Hsieh, et al. (2011) Moon et al. (2010a) Chow et al. (2012) Lee et al. (2011) Reference Author's personal copy Phytochem Rev (2014) 13:107–121 109 123 123 DEDC Epigallocatechin gallate Hesperitin metabolites 10 11 Isoliquiritigenin Luteolin Naringenin Piceatannol Quercetin Quince polyphenols Resveratrol 13 14 15 16 17 18 19 12 Active compounds No. Table 1 continued Phenolic, stilbenoid Phenolics Phenolic, flavonol Camellia sinensis (Theaceae), other fruits and vegetables Cydonia oblonga Miller (Rosaceae) Vitis vinifera (Vitaceae) Picea abies (Pinaceae) Citrus 9 paradisi (Rutaceae) Apium graveolens (Apiaceae) and other species Glycyrrhiza glabra (Fabaceae) Camellia sinensis (Theaceae) Citrus genus (Rutaceae) Macrothelypteris torresiana (Gaud.) (Thelypteridaceae) Plant species (Family) Fruit skin Fruit peel Leaves and other plant organs Root Fruit Leaves Root Fruit Leaves Root Plant tissue Inhibited the LPS-mediated activation of NF-jB Decreased the expression of p65 and IjB-a in treated rats Activated NF-jB. Induced apoptosis via ROS. Suppressing NF-jB pathway by pyrrolidine dithiocarbamate significantly increased neuroblastoma cell sensitivity to the pro-apoptotic effect of DEDC Down-regulation of NF-kB, c-Jun and caspase-3 Down-regulated LPS-induced NF-kB activation followed by suppression of IjB degradation and phosphorylation of c-Jun N-terminal kinase1/2 (JNK1/ 2) and p38 MAPKs Inhibited LPS-induced TLR4 dimerization resulting in inhibition of NF-jB Produced intracellular ROS in turn mediate AMPK-NF-jB signaling in HepG2 hepatocarcinoma cells Down-regulated ROS production and inhibited NFjB activity via EGFR-PI3KAkt/ERK MAPKinase signaling pathway Supressed TNFa shedding, leading to inhibition of TNFa/ NFjB pathway Modulated the NF-jB p65 nuclear translocation Mechanism of action Essafi-Benkhadir et al. (2012) Kumar and Sharma (2010) Bhaskar et al. (2011) Liu and Chang (2012) Yang et al. (2011) Hwang et al. (2011) Park and Youn (2010) Giakoustidis et al. (2008) Yang et al. (2012) Liu et al. (2012) Reference 110 Phenolic, stilbenoid Phenolic, flavanone Phenolic, flavonoid Phenolic, licorice chalconoid Phenolic, catechin esterified to gallic acid Phenolic, flavanones Phenolic, flavonoid Chemical class Author's personal copy Phytochem Rev (2014) 13:107–121 Active compounds Rosmarinic acid Wedelolactone Xanthohumol Embelin Citreorosein Isoeleutherin Plumbagin Thymoquinone Artemisinin Bharangin No. 20 21 22 23 24 25 26 27 28 29 Table 1 continued Diterpenoid quinonemethide Sesquiterpenoid lactone Quinone Naphtoquinone Naphtoquinone Anthroquinone Benzoquinone Phenolic, prenylated chalconoid Phenolic, coumestan Phenolic, flavonoid Chemical class Premna herbacea (Lamiaceae) Eleutherine bulbosa (Miller)(Iridaceae) Plumbago zeylanica (Plumbaginaceae) also in Droseraceae, Ancestrocladaceae, and Dioncophyllaceae families Nigella sativa (Ranunculaceae) Artemisia annua (Asteraceae) Polygoni cuspidati (Polygonaceae) Embelia ribes Burm., (Myrsinaceae) Eclipta alba and Wedelia calendulacea (Asteraceae) Humulus lupulus (Cannabaceae) Rosmarinus officinalis (Lamiaceae) Plant species (Family) Root Above ground organs Inhibited phosphorylation and degradation of IjBa and the nuclear translocation of the NF-jB p65 subunit Abolished constitutive and Inducible NF-jB activation by modifying p65 on cysteine 38 and reduced IjBa kinase activation, inhibited binding of p65 to DNA NF-jB inhibitor Seed Root Bulb Root Basal levels of FAK, AKT and NF-jB signaling pathways were down-regulated Suppressed NF-jB activation and inhibition of IjBa phosphorylation and IjBa degradation Inhibited NF-jB p65 subunit and its cognate DNA-binding activity Inhibits transcriptional activation by LPS Inhibited phosphorylation and DNA-binding activity of NFjB. Binds cys38 in p65. Activation was also reported Inhibited TNF-a-induced ROS generation and NF-jB activation, and enhances TNFa induced apoptosis Selective IjB kinase inhibitor Mechanism of action Female inflorescence (Hops) Fruit Stem Leaves Plant tissue Gupta et al. (2011a) Connelly et al. (2011) Wang et al. (2011) Hafeez et al. (2012) Subramaniya et al. (2011) Song et al. (2009) Lu et al. (2012) Reuter et al. (2010a) Zhang et al. (2012) Benelli et al. (2012) Moon et al. (2010b) Reference Author's personal copy Phytochem Rev (2014) 13:107–121 111 123 123 Sesquiterpenoid b-Caryophyllene Celastrol Diosgenin Escin Ginsenoside Rg3 Ginsenoside Rh2 Impressic acid Kahweol Lupeol Lycopene 30 31 32 33 34 35 36 37 38 39 Tetraterpenoid, carotenoid Triterpenoid Solanum lycopersicum (Solananceae) Mangifera indica (Anacardiaceae) and other species Acanthopanax koreanum (Araliaceae) Coffea arabica (Rubiaceae) Panax ginseng (Araliaceae) Panax ginseng (Araliaceae) Dioscorea althaeoides Knuth (Dioscoreaceae) and other species Aesculus hippocastanum (Sapindaceae) Tripterygium wilfordii (Celastraceae) Piper nigrum (Piperaceae) and other species Plant species (Family) Fruit Fruit Seed Leaves Root Root Seed Tuber Root Fruit Plant tissue Inhibited tumor TNF-a induced NF-jB activity Inhibited NF-jB-dependent transcriptional activity Inhibited cell survival by inactivation of NF-jB through upregulation of its inhibitor Ijba Inhibited activation of NF-jB Inhibited activation of NF-jB through inhibition of IKK, leading to down-regulation of NF-jB regulated cell survival and metastatic gene products, resulting in sensitization of cells to cytokines and chemotherapeutic agents Enhanced susceptibility of colon cancer cells to Docetaxel Decreased p65 Inhibited the activation of extracellular signal-regulated kinase 1/2, NF-jB, IjBkinase a/b, cAMP response element binding and the expression of caspase-3 and Ki-67 Induced ROS. Inhibited IjBa kinase activation, phosphorylation of IjBa and p65 Decreased induced NF-jB Mechanism of action Bae and Bae (2011) Prasad et al. (2009) Kim et al. (2006) Kim et al. (2011) Bi et al. (2012) Kim et al. (2009) Harikumar et al. (2010a) Jung et al. (2010) Kannaiyan et al. (2011) Bento et al. (2011) Reference 112 Diterpenoid Triterpene glycoside, Steroidal saponin Lupane-type triterpenoid Triterpene glycoside Pentacyclic tritepenoid Terpenoid, Steroidal saponin Triterpenoid acid, Quinone methide Chemical class Active compounds No. Table 1 continued Author's personal copy Phytochem Rev (2014) 13:107–121 Active compounds Maslinic acid Nimbolide NUP Parthenolide Picroliv Santamarin Sesquiterpene lactones c-Tocotrienol No. 40 41 42 43 44 45 46 47 Table 1 continued Germacranolides, heliangolides, guaianolides, pseudoguaianolides, hypocretenolides, eudesmanolides Isoprenoid, Vitamin E derivative Sesquiterpene lactone Terpenoid, iridoid glycoside Thio alkaloid-sesqui terpenelactones. A mixture of nupharidines and other unidentified compounds Sesquiterpene lactone Tetranortriterpenoid Pentacyclic triterpenoidic acid Chemical class Found in several food plants, found in rice, barley, oats, and palm Picrorhiza kurrooa (Scrophulariaceae) Saussurea lappa C.B. Clarke (Asteraceae) Various species of Asteraceae Tanacetum parthenium (Asteraceae) Azadirachta indica A. Juss (Meliaceae) Nuphar lutea (Nymphaceae) Olea europaea (Oleaceae) Plant species (Family) Fruit Various organs Root Root Leaves Leaves and flowers Leaf and rhizome Fruit skin Plant tissue Inhibited NF-jB activation by c-T3 and a suppression of key cellular regulators. Inhibited the growth of human pancreatic tumors and sensitized them to gemcitabine by suppressing NF-jB–mediated inflammatory pathways linked to tumorigenesis Inhibited phosphorylation of IjB Alkylating cysteine38 in the DNA binding domain of the p65. 103 compounds that showed anti-NF-jB activity are detailed Suppressed TNFa-induced NFjB activation, inhibited TNFa-induced IjBa degradation, p65 phosphorylation, and nuclear translocation. Decreased the expression NF-jB regulated genes Abrogated canonical NF-jB. Inhibitd binding to DNA Down-regulated NF-jB and induced apoptosis. Activated NF-jB in Leishmania infected cells NF-jB- and STAT-inhibitionmediated transcriptional suppression of pro-apoptotic genes. Acted at the transcriptional level and by direct inhibition of IKK-b Binds cys38 in p65 Mechanism of action Kunnumakkara et al. (2010) Siedle et al. (2004) Anand et al. (2008) Choi et al. (2012) Mathema et al. (2011) Kavitha et al. (2012) Ozer et al. (2009, 2010) Li et al. (2010) Reference Author's personal copy Phytochem Rev (2014) 13:107–121 113 123 Active compounds Triptolide Berbamine Berberine Cepharanthine Cryptopleurine Dauricine Noscapine Sinomenine Butylidene-phthalide (BP) Crotepoxide Thiocolchicoside Sesamin No. 48 49 50 51 52 53 54 55 56 57 58 59 Table 1 continued 123 Lignan Glucoside Cyclohexane diepoxide Angelica sinensis (Apiaceae) Kaempferia pulchra (Zingiberaceae) Gloriosa superba (Colchicaceae) Sesamum indicum (Pedaliaceae) Papaver somniferum (Papaveraceae) Sinomenium Acutum (Menispermaceae) Berberis amurensis (Berberidaceae) Berberis aquifolium (Berberidaceae) and other species Stephania cepharantha Hayata (Menispermaceae) Boehmeria pannosa (Urticaceae) Menispermum dauricum DC. (Menispermaceae) Tripterygium wilfordii Hook.f (Celastraceae) Plant species (Family) Root, seed and other plant parts Seed Rhizome Root Root Seed Rhizome Root Leaves Roots, rhizome, stems and bark Roots, rhizome, stems and bark Various organs Plant tissue Inhibited NF-jB and NF-jB– regulated gene products Suppressed pathways linked to the NF-jB signaling Inhibited NF-jB activation by inhibiting the IKK pathway Inhibited NF-jB activation pathway Suppressed NF-jB activity and the expression profile of its down-stream genes Suppressed the NF-jB signaling pathway Decrease the mRNA expression of TNF-a by inhibiting the NF-jB) binding activity Suppressed NF-jB-dependent pathways Suppressed NF-jB expression Suppressed constitutive and inducible NF-jB activation, but did not directly inhibit binding of p65 to the DNA. It did block TNF-induced ubiquitination, phosphorylation, and degradation of IjBa, and inhibited acetylation of p65 through suppression of binding of p65 to CBP/p300. It also inhibited IKK and phosphorylation of p65 at serine 276, 536. Inhibited NF-jB and phosphorylation of IjBa Inhibition of IjB phosphorylation Mechanism of action Prasad et al. (2010) Reuter et al. (2010b) Harikumar et al. (2010b) Fu et al. (2011) Chai et al. (2012) Sung et al. (2010) Yang et al. (2010) Kudo et al. (2011) Jin et al. (2012) Liang et al. (2009) Goto et al. (2012) Park et al. (2011) Reference 114 Phtalide derivative Alkaloid Benzylisoquinoline alkaloid Phenanthroquinolizidine alkaloid Alkaloid Alkaloid Isoquinoline alkaloid Isoquinoline alkaloid Diterpenoid triepoxide Chemical class Author's personal copy Phytochem Rev (2014) 13:107–121 Author's personal copy Phytochem Rev (2014) 13:107–121 115 than 700 natural products affecting NF-jB activation pathways were recently reported by Gupta et al. (2010b), including antioxidants, peptides, small RNA/DNA, microbial and viral proteins, small molecules, and engineered dominant-negative or constitutively active polypeptides. The underlying hypothesis in this review is that natural plant derived molecules have a better potential than synthetic ones to fit and interact with macromolecules in humans and animals, since they underwent the evolutionary screening to fit metabolic targets in living organisms, because of molecular conservation and commonality among them. The centrality of NF-jB in cell cycle and metabolism makes its regulation quite complex. One can envision NF-jB in the focus of several pathways of regulation that affect its expression (DiDonato et al. 2012). NF-jB modulators in the cell include protein kinases, protein phosphatases, ubiquitination and protein degradation, acetylation, methylation, nuclear translocation and DNA binding (Fig. 1). Thus it is clear that phytochemicals may inhibit NF-jB by direct binding and/or indirectly through modulating enzymes. Ideally, the search for natural inhibitors of NF-jB, should seek specificity for the isoenzymes directly involved in its metabolic processing. This Fig. 1 Schematic presentation of the major NF-jB activation pathways. The regulatory targets, at which the different classes of the compounds act are shown, based on the following categories: upstream signaling; IKK modulation; IkB modulation and translocation; NF-jB nuclear translocation; NF-jB modulation; NF-jB transcription activity (for compounds in each category see text and Table 1). In the classical pathway, binding to cell membrane receptors triggers the sequential recruitment of adaptor proteins according to the type of stimuli and cell type. The adaptor proteins recruit and activate the IKK complex which in turn leads to the phosphorylation and ubiquitination of IjB, followed by its degradation via the proteasome pathway. The heterodimer p50-p65 is then released and migrates to the nucleus where it undergoes a series of posttranslational modifications and binding to jB sites which enable transcription of target genes. The alternative pathway activates the IKKa dimer inducing the processing of p100 to the release of p52 which together with RELB translocate to the nucleus, triggering transcription of NF-jB target genes. Individual plant derived molecules can inhibit one or more of the main NF-jB activation crossroads 123 Author's personal copy 116 opens-up potentially many targets to modulate the expression of NF-jB with therapeutic application. The NF-jB signaling pathway Detailed descriptions of the NF-jB pathway can be found in a series of excellent recent reviews (DiDonato et al. 2012; Ghosh and Hayden 2012; Perkins 2012,), it is worth highlighting a few key principles to reiterate the concept of regulatory complexity and the effect of phytochemicals on components of this pathway. There are five subunits that make-up the mammalian NF-jB complex family, which all share a related DNA-binding and dimerization domain, termed the REL homology domain (RHD). The carboxy termini of Rel A (p65), REL B, and REL (c-Rel) all contain transactivation domains, which are capable of mediating interactions with basal transcription factors and cofactors such as TATA binding protein (TBP), TFIIB, E1A binding protein 300KD (p300) and CREB binding protein (CBP). The other two family members, NF-jB1 (p105) and NF-jB2 (p100) encode longer precursor proteins to the active DNA-binding forms p50 and p52, respectively (Fig. 1). NF-jB activation constitutes a rapidly inducible first line of defense against infection and stress. Before exposure to an inducing stimulus, NF-jB is kept in the inactive state in the cytosol. To keep NF-jB in this inactive state there exist a family of inhibitors of NF-jB (IjBs). Typically, IjBs bind to NF-jB complexes, inhibiting their translocation to the nucleus and binding to DNA. Stimulation is achieved through the phosphorylation of the IjBs by the IjB kinase (IKK) complex, promoting their ubiquitination and proteosome-mediated degradation with consequent NF-jB translocation to the nucleus and localization at selected promoters on DNA, binding to accessory proteins and enabling gene transcription. NF-jB activity is modulated through p65 phosphorylation, acetylation and methylation. In addition, IjB can localize to the nucleus, bind to and remove NF-jB complexes from selected promoter DNA and enable NF-jB to be recaptured in the cytosol. NF-jB signaling is generally considered to occur through either the canonical (classical) or non-canonical (alternative pathway) pathways. Numerous and diverse stimuli can induce the classical NF-jB 123 Phytochem Rev (2014) 13:107–121 activity. Typical inducers of NF-jB include cytokines such as tumor necrosis factor (TNF), interleukin-1 (IL-1), viral and bacterial products such as lipopolysaccharide (LPS), which can induce Toll-like receptor (TLR) signaling and cellular stress, such as DNA damage, reactive oxygen species (ROS) and hypoxia. Most of these inducers converge on IKKa, b dimers (Fig. 1). The non-canonical pathway is activated through receptor signaling and IKKa, a dimer activation enabling the processing of protein precursors to form the active p50/p52 dimers. This pathway is essential for the proper development of secondary lymphoid organs. Most inhibitors affect the canonical pathway and some both pathways. Thus, the NF-jB response is highly pleiotropic and the consequences of its activation can be context dependent. In the physiological state, an NF-jB response is automatically self-limiting, through the induction of negative feedback loops including the transcription of IjBs together with the expression of proteins that negatively regulate the signaling pathways leading to IKK activation, such as A20. However, NF-jB activity becomes deregulated in cancer and chronic inflammatory diseases. This can occur either through mutations leading to high levels of IKK- NF-jB signaling or through continuous exposure to NF-jBactivating external stimuli, such as systemic or tissue microenvironment cytokine release. Crosstalk with other signaling pathways such as PI3K, may result in activation of AKT and MAPK signaling converging in IKK. Moreover, tumor-supressor proteins, such as p53, provide an important mechanism for regulating NF-jB activity. These pathways combine to determine the normal physiological role of NF-jB as well as in disease, in tumorigenesis, resistance to apoptosis, determining response to chemotherapy and in chronic inflammation. Inhibitors of the NF-jB activation pathways Most of the compounds described in table 1 are either phenolics or terpenoids (each approximately 37 %). Quinones and alkaloids are also represented among the inhibitors of NF-jB via various pathways. One or two representatives of other chemical families are also present such as phtalides, cyclohexanes, glucosides and lignans. Active compounds have been reported in Author's personal copy Phytochem Rev (2014) 13:107–121 different organs of the plant: root, tuber, rhizome, leaf, stem, bark fruit and seed. The same activity may be found in some organs in the same plant but not in all of them (Table 1). The season when the plant was harvested may also affect the level of biological activity. Some compounds have been thoroughly investigated and several targets and mechanisms of action have been extensively described (i.e. curcumin, lycopene, parthenolide etc.). In contrast, the NF-jBrelated mode of action of other phytochemicals is starting to be examined (i.e., Cudraflavone B, Plumbagin, Nimbolide etc.). Thorough investigation of the mode of action and specificity of a compound showing effect on NF-jB will determine if a molecule has multiple targets or has limited specificity. Both types of compounds should be therapeutically useful as single agents or in combination, together with other lines of treatment. In addition to a direct effect on NF-jB, inhibitors may be given together with standard anti-cancer drugs or radiotherapy, acting as ‘‘sensitizers’’ or as inhibitors of multidrug resistance (Nakanishi and Toi 2005). It is worth noting that sometimes in purification of an active compound from a plant extract the specific activity declines as purification progresses. This may indicate loss of activity by losing compounds acting synergistically or complementarily. Therefore, we have included in this review, recently reported mixtures of cumarins (ADEE), hesperitin metabolites, quince polyphenols extract, sesquiterpenoid lactones and a partially purified nuphridines extract (NUP) from Nuphar lutea. It could well be that a mixture of derivatives of a given compound is more active than any one specific compound in the mixture. Furthermore, the naturally occurring derivatives can serve as lead compounds for organic synthesis and search for new more potent compounds. In addition, it seems that different secondary metabolite classes can modulate the same targets, for example, quinones, as well as a variety of terpenoids have been demonstrated to inhibit p65 through its cystein 38 and IKK through cystein 179 (Table 1). Upstream regulation of NF-jB depends on many different molecules, which are triggered by an external stimulus unique for the cell type involved. Thus, upstream targets present an opportunity to detect specific inhibitory substances of varied chemical structures, for example, carnosic acid a phenolic diterpenoid, blocks signaling through Syc/Src, PI3K 117 and Akt. Interestingly, citreorosein an anthroquinone and naringenin a phenolic flavanone, both affect PI3K and AKT signaling as well. Gupta et al. (2010b) have thoroughly described the different levels of regulation of NF-jB signaling by a large number of natural products. In this review we have focused only on recently published secondary metabolites of plants (Table 1). In Fig. 1, we show schematically the regulatory targets at which the different classes of the compounds act, based on the following categories: Inhibition of upstream signaling Through TLR-4 (LPS induced) (isoliquiritigenin, butylidenephtalide); modulation through reactive oxygen species (ROS) (carnosic acid, celastrol, citreorosein, luteolin, naringenin, rosmarinic acid, sesamin); through inhibition of TNF-a signaling (artemisinin, impressic acid, maslinic acid, triptolide, sinomenine, NUP); through ERK1/2 (b-caryophyllene). IKK modulation usually by direct binding to Cys 179: (piceatannol, wedelolactone, xanthohumol, embelin, artemisinin, escin, impressic acid, maslinic acid, parthenolid, triptolide, sinomenine, cepharantine). IjB modulation mainly by phosphorylation and degradation, as well as IjB nuclear translocation (ADEE, carnosic acid, resveratrol, embelin, artemisinin, bharangin, celastrol, santamarin, triptolide, berbamine, berberine). NF-jB modulation anacardic acid, cardamonin, hesperitin, isoliquiritigenin, quercetin, quince polyphenols, resveratrol, diosgenine, ginsenoside Rh2, lupeol, nimbolide, NUP-(both classic and the alternative pathways), cryptopleurine, crotepoxide); through Cys 38: (plumbagin, thymokinone, bharangin, parthenolide, picroliv, sesquiterpene lactones); through NFjB phosphorylation and acetylation: (carnosic acid, curcumin, celastrol, diosgenin, triptolide). NF-jB nuclear translocation (ADEE, carnosic acid, carnosol, cudraflavone B, berbamine). NF-jB DNA-binding activity (cardamonine, citreorosein, bharangin, lycopene, nimbolide, triptolide, sinomenine). NF-jB transcriptional activity (carnosol, cudraflavone B, kahweol, parthenolide, dauricine, c-tocotrienol, noscapine, thiocolchicoside). NF-jB inhibition limiting cell survival (lupeol); induction of apoptosis and caspase activation: (rosmarinic acid, NUP); sensitization of malignant cells to anti-cancer drugs: (DEDC, NUP). 123 Author's personal copy 118 Conclusions The last 25 years have seen remarkable progress in understanding of NF-jB structure–function relationship. Yet NF-jB as a therapeutic target has remained largely unexploited in drug development for clinical use. Therefore, one of the major challenges facing researchers is to develop NF-jB inhibitors aimed at treating different diseases based on their ability to target specific pathways or cells. Development of efficient NF-jB inhibitors which selectively target core components of this pathway will also require the careful establishment of a therapeutic correlation between dose and target inhibition. Key points for therapeutic intervention include targeting upstream activators of IKK, IKK activation, IjB degradation, NF-jB modifications, NF-jB DNA binding and modulation of its transcriptional activity. Full realization of the therapeutic potential of the NF-jB pathway lies within better understanding of its regulatory complexity and the cell type and stimulus dependent selective manipulation of its components. A comprehensive list of diseases where NF-kB is known to be deregulated is presented in Kumar et al. (2004). Preclinical established models for each disease should be used to test hypotheses and pave the way to clinical drug development. In theory, an arsenal of therapeutic agents, natural and synthetic, directed against well defined targets is readily available and many more are continuously added to the list. Phytochemicals represent an essential source for potential drugs. Plants are the source for complex molecules which can be on one hand exquisitely target selective or pleiotropic, multitargets in their activity. Both, selective or pleiotropic agents affecting NF-jB can also modulate other pathways such as PI3K, AKT, MAPK, p53 etc., contributing an additional level of complexity and therapeutic potential. The challenge ahead of us is to choose the right combination of existing and newly discovered molecules and determine their optimal activity, time and concentration wise. Optimal protocols should be determined initially in vitro followed by well established preclinical in vivo studies. Acknowledgments The authors gratefully acknowledge the support of the Israel Ministry of Health through the Winkselbaum fund, the Israel Cancer Association through the Miriam and Shlomo Hasid Memorial fund, ICA in Israel, Jacobs foundation through BG-Negev and The Richard H. Holzer 123 Phytochem Rev (2014) 13:107–121 Foundation. 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