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Nuclear Factor kappaB
Nuclear Factor kappaB Thomas D. Gilmore Boston University, Boston, Massachusetts, USA
Nuclear factor kappaB (NFkB) is a family of structurally related DNA-binding proteins that act as transcription factors to regulate the expression of a wide variety of genes involved in many key cellular and organismal processes. The discovery of mammalian NFkB is generally attributed to Ranjan Sen and David Baltimore, who were studying the regulation of expression of a specific immunoglobulin gene; however, it is now clear that earlier researchers studying a viral oncoprotein (v-Rel) and a protein (Dorsal) required for the establishment of Drosophila embryonic polarity were also studying an NFkB activity. This entry focuses primarily on the vertebrate NFkB system, although most of the discussion is also applicable to the NFkB system in insects. NFkB activity is controlled by a multicomponent signal transduction pathway, such that in most cells. NFkB is inactive until any of several inducers initiates the signaling pathway that ultimately activates NFkB as a nuclear, DNA-binding protein. Misregulation of NFkB activity has been implicated in a number of human diseases, including several chronic inflammatory diseases and cancer, and the NFkB pathway is modulated by or used by several viruses as part of their pathology or replication. As such, the NFkB pathway is the subject of intense focus for pharmacological intervention.
NFkB Proteins and Protein Structure
p52/p100, and Relish) contains C-terminal sequences that fold back to inhibit the RHD, and thus these C-terminal sequences must be removed to release the active DNA-binding protein (i.e., p50 or p52 from the precursors p105 and p100, respectively). A second group (c-Rel, RelA, RelB, Dorsal, Dif) contains C-terminal sequences that are not cleaved and that activate transcription. NFkB proteins bind to 9– 10 bp DNA sites, usually called kB sites, located in the regulatory regions of many cellular and viral promoters. To bind DNA, NFkB proteins must form dimers. These dimers can, with few exceptions, be any combination of homodimer or heterodimer, although the most common NFkB dimer in many cells is a p50 – RelA dimer (Figure 1B). The affinity of the various NFkB subunits for one another and the affinity of the various dimers for specific target DNA sites can vary greatly. These considerations provide an array of diversity in the NFkB dimers that are found in different cells, in the DNA sites that can be recognized by different NFkB dimers, and in the genes (and consequently cellular responses) that these dimers regulate.
Regulation of NFkB Activity
The nuclear factor kappaB (NFkB) family is comprised of several evolutionarily conserved proteins that are related through a domain, usually called the Rel homology domain (RHD), which has sequences required for DNA binding, dimerization, and nuclear localization (Figure 1A). In Drosophila, there are three known NFkB proteins (Dorsal, Dif, and Relish), whereas in vertebrates, the NFkB protein family is comprised of five proteins: p50/p105, p52/p100, c-Rel, RelA (also called p65), and RelB. There is also an avian retroviral oncoprotein, v-Rel, that is a member of this family. All NFkB proteins contain the , 300 amino acid RHD towards their N termini; however, NFkB proteins can be subdivided into two distinct groups based on sequences C terminal to the RHD. One group (including p50/p105,
In most cells, NFkB is present in the cytoplasm in an inactive state, due to interaction with a family of inhibitor proteins, termed inhibitor of kB (IkB) proteins. IkB proteins include IkBa, IkBb, IkBe, Bcl-3, and the C-terminal sequences of the NFkB precursor proteins p105 and p100. Through a series of , 30 amino acid repeats (called ankyrin repeats), a single IkB protein binds directly to sequences in the RHD of an NFkB dimer. Binding of an IkB to NFkB inhibits the activity of the dimer in two ways: it causes NFkB to be in the cytoplasm and it blocks NFkB DNA binding. A variety of inducers (Table I), including many cytokines, cell stress, and viral or bacterial infections, can activate NFkB. In almost all cases, NFkB activity is induced through activation of a cellular kinase complex called IkB kinase (IKK). IKK is composed of three subunits: two related kinases (IKKa and IKKb) and a
Encyclopedia of Biological Chemistry, Volume 3. q 2004, Elsevier Inc. All Rights Reserved.
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NUCLEAR FACTOR kappaB
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by the ubiquitin– proteasome pathway. The liberated NFkB complex can then enter the nucleus, bind to DNA, and activate specific target gene expression. In many cases, activation of NFkB is transient (perhaps lasting no more than 30 min) primarily because one of the target genes for NFkB is the gene encoding IkB. Therefore, newly synthesized IkB can cause NFkB to be resequestered in an inactive form in the cytoplasm, thus returning the pathway to its original state. Although the overall pathway of activation is similar (as in Figure 2), there are actually two distinct pathways to activation of NFkB, depending on the nature of the IkB protein and IKK involved. In one pathway, a separate IkB molecule, such as IkBa bound to p50 –RelA, is phosphorylated by the IKKb subunit, which leads to the complete proteolysis of IkBa. In a second activation pathway involving a complex such as p100 – RelB, a C-terminal site in p100 is phosphorylated by IKKa, which then leads to the partial proteolysis of the C-terminal ankyrin repeat sequences of p100, ultimately yielding an active p52 – RelB dimer, which can also translocate to the nucleus to regulate gene expression. However, a variety of recent evidence suggests that the NFkB pathway is regulated by mechanisms in addition to interaction with IkB proteins. For example, NFkB proteins can undergo posttranslational modifications FIGURE 1 NFkB proteins. (A) Shown are the generalized structures, protein names, and genes encoding the two classes of NFkB proteins and the IkB proteins. RH, Rel Homology domain; N, nuclear localization signal; vertical bars, ankyrin repeats. (B) X-ray crystal structure of the NFkB p50–RelA (p65) dimer on DNA. N, N terminus; C, C terminus. Figure provided courtesy of Gourisankar Ghosh.
sensing or scaffold protein called IKKg. Activation of the IKK complex generally requires phosphorylation of residues in the activation loop of the IKKa or IKKb kinase, and probably also phosphorylation of IKKg. Activated IKK then phosphorylates the IkB protein, which in turn signals the IkB sequences for degradation
TABLE I Some Inducers of NFkB Activity General category Bacteria Viruses
Examples Salmonella, Staphylococcus, Lipopolysaccharide HTLV-1, Epstein–Barr virus, hepatitis B virus
Cytokines
Interleukins, tumor necrosis factor
Oxidative stress
Hydrogen peroxide, reperfusion ischemia
Growth factors/ hormones
Platelet-derived growth factor, epidermal growth factor, insulin, transforming growth factor
Drugs
Various chemotherapeutic agents
FIGURE 2 The NFkB signal transduction pathway. Under most circumstances, NFkB dimers (e.g., p50– RelA) are located in the cytoplasm of cells bound to inhibitory IkB proteins. Upon receiving an inducing signal, an IkB kinase (IKK) is activated. Active IKK phosphorylates IkB, and phosphorylated IkB then serves as a substrate for the ubiquitin–proteasome degradation pathway. Free NFkB can then enter the nucleus to increase or decrease specific target gene transcription. The system is eventually returned to its resting state through an auto-regulatory system, in that NFkB turns on expression of the gene encoding IkB, and newly synthesized IkB then resequesters NFkB in the cytoplasm.
98
NUCLEAR FACTOR kappaB
(such as phosphorylation and acetylation) that affect their activity, they can interact with a variety of other proteins (including many other transcription factors), and the IKK proteins may also have effects on chromatin-binding proteins to affect gene expression from kB site-containing promoters.
Genes and Biological Processes Regulated by NFkB
Drosophila embryo. In mammals, NFk B activity appears to be involved in proper limb development, due to the regulation of a gene called Twist. NFkB transcription factors also regulate cell survival in many contexts, either by promoting or, more often, by inhibiting apoptosis. For example, RelA is required to protect liver cells from apoptosis induced by tumor necrosis factor, and RelA knockout mice die embryonically due to massive liver degeneration. Thus, the genes encoding several inhibitors of apoptosis are activated by NFkB.
NORMAL BIOLOGICAL ROLES FOR NFkB There are over 300 genes whose transcription is known to be regulated by NFkB activity, and these genes are involved in a multitude of biological processes (Table II). However, a variety of genetic and cellular studies have established that one of the key roles of NFkB is in regulating the innate immune response. In Drosophila, the activities of Dorsal, Dif, and Relish are induced to enter the nucleus by bacterial and fungal pathogens. These fly NFkB proteins then bind upstream and increase the expression of a family of genes called cecropins, which encode antifungal and antibacterial peptides. In mammals, it is also clear that many genes that encode antibacterial peptides, cytokines, and interferons are rapidly induced after infection with various pathogens. Moreover, mice in which individual genes for c-Rel, RelB, p50, and p52 are disrupted (“knockout mice”) have severe defects in T- and B-cellmediated immune responses. For example, c-rel knockout mice have B cells with greatly reduced survival and proliferation in response to antigen stimulation. NFkB proteins are, however, also involved in the control of several other genes and processes that are not directly related to immunity. These include certain developmental processes, apoptosis, cell growth, and cell stress. In terms of development, NFkB activity (as directed by Dorsal) is required for the proper establishment of dorsal-ventral polarity in the early TABLE II Some Biological Processes and Relevant Genes Regulated by NFkB Biological process
NFk B
AND
DISEASE
Aberrant NFkB activity has been implicated in several human diseases (Table III), most notably certain cancers and chronic inflammatory diseases, such as inflammatory bowel disease, asthma, and arthritis. Many cancer cells have constitutively active, nuclear NFkB activity. In certain B-cell lymphomas, notably diffuse large B-cell lymphomas and Hodgkin’s lymphomas, NFkB activity is dysregulated due to either amplification of the c-rel gene or mutations that inactivate the gene encoding IkB. Furthermore, several oncogenic human viruses, such as Epstein-Barr virus (lymphomas, nasopharyngeal carcinomas), HTLV-1 (T-cell leukemias), and Hepatitis B virus (liver cancer), encode proteins that activate NFkB. However, in many other cancers one finds chronic nuclear NFkB activity, which is usually made up of p50 – RelA dimers, in the apparent absence of mutations in the NFkB signaling pathway or associated viral infection. That increased NFkB activity is important for the growth and survival of several cancers is emphasized by studies showing that inhibition of NFkB activity often either causes the tumor cells to die (undergo apoptosis) or to become more sensitive to apoptosis-inducing agents. Because NFkB controls the expression of many cytokines, it is also consistently active in many situations involving acute or chronic inflammation. Moreover, in several animal model systems inhibition of NFkB activity can reduce inflammation. Furthermore, mice with knockouts in the genes encoding p50 or c-Rel are resistant to experimentally induced arthritis.
Relevant NFkB target genes
Angiogenesis
Vascular endothelial growth factor (VEGF)
Apoptosis
Bcl-X1, Bfl/A1, Bcl-2, IAP, TRAF, Fas
Cell adhesion
ICAM-1, VCAM-1
Cell growth Stress
Cyclin D1, c-Myc Cyclooxygenase, superoxide dismutase, nitric oxide synthetase
Innate immunity
Interleukins, interferon, tumor necrosis factor
Viral promoters
HIV-1, cytomegalovirus
TABLE III Some Diseases Associated with Chronic NF kB Activation Arthritis Asthma Cancer Diabetes Inflammatory bowel disease Ischemia/reperfusion damage Sepsis
NUCLEAR FACTOR kappaB
Pharmacological Regulation of NFkB Activity
SEE ALSO
Because of the role played by NFkB in inflammatory diseases and cancer, it has received much attention as a molecular target for pharmacological intervention. As a multicomponent pathway, NFkB activation can be inhibited at a variety of levels, including activation of IKK, IKK-mediated phosphorylation of IkB, ubiquitination and proteasome-mediated degradation of IkB, nuclear translocation of NFkB, and DNA-binding and transactivation by NFkB. Indeed, NFkB inhibitors that act at each of these steps have been described (partial list in Table IV). Nevertheless, to date, most research effort has focused on identifying NFkB inhibitors that target the IKK complex, due to its central role in controlling the NFkB pathway and to successes in identifying specific kinase inhibitors in other systems. Several natural products with anticancer or antiinflammatory activities have been shown to inhibit activation of NFk B. Among many others, these natural NFkB inhibitors include gold compounds (antiarthritis), green tea (anticancer), curcumin (anticancer, anti-inflammation), and various plant extracts (antiinflammation). In addition, some commonly used antiinflammatory compounds, such as glucocorticoids, ibuprofen, and aspirin, have been reported to have anti-NFkB activity in experimental model systems. However, in these cases, it is not clear if their biological effects in people are exerted through inhibition of NFkB, due to the fact that doses lower than those required to inhibit NFkB are often still effective in reducing inflammation in humans. Because NFkB is constitutively active in many cancers, clinical trials are underway with several compounds known to inhibit NFkB. Early clinical trials have reported promising results in the use of a small-molecule inhibitor of the proteasome for the treatment of multiple myeloma, an NFkB-dependent B-cell cancer. Such compounds inhibit activation of NF kB by blocking proteasome-mediated degradation of IkB. TABLE IV
THE
99
FOLLOWING ARTICLES
Cell Death by Apoptosis and Necrosis † Cytokines † Proteasomes, Overview † Ubiquitin System † UbiquitinLike Proteins
GLOSSARY apoptosis A programmed cell death pathway that is used by an organism to eliminate unwanted cells or maintain organ size. cytokines Factors that regulate the growth and activity of blood cells and lymphocytes. proteasome A cellular complex composed of several proteases that degrades ubiquitinated proteins. protein kinase An enzyme that can add a phosphate group onto specific residues in a target protein, which then affects the activity of the target protein. signal transduction A molecular process by which a signal (often an extracellular compound) effects a cellular response. Often this is a multicomponent step-wise intracellular pathway that involves binding of a ligand to a cell-surface receptor, which initiates a cascade of cytoplasmic enzymatic activities that ultimately leads to the activation of a transcription factor that then regulates the expression of a biologically-relevant set of target genes. transcription factor A protein or protein complex that binds to specific DNA sequences to regulate (increase or decrease) the rate of transcription of a gene. ubiquitin A small (, 8.6 kDa) polypeptide chain that can be conjugated to substrate proteins, which often targets them for degradation by the proteasome.
FURTHER READING Beyaert, R. (ed.) (2003). Nuclear Factor-kB: Regulation and Role in Disease. Kluwer Academic, The Netherlands. Ghosh, S., May, M. J., and Kopp, E. B. (1998). NF-kB and rel proteins: Evolutionarily conserved mediators of immune responses. Annu. Rev. Immunol. 16, 225–260. Gilmore, T. D. (ed.) (1999). NF-kB. Oncogene. 18, 6841–6964. Gilmore, T. D (2003). Rel/NF-kB transcription factors. http://www. nf-kb.org. Gilmore, T. D., and Ip, Y. T. (2003). Signal transduction pathways in development and immunity: Rel pathways. In Nature Encyclopedia of Life Sciences, Nature Publishing Group, London, ([doi: 10.1038/ npg.els.0002332] http://www.els.net). Karin, M., Cao, Y., Greten, F. R., and Li, Z-W. (2002). NF-kB in cancer: From innocent bystander to major culprit. Nat. Rev. Cancer 2, 301–310. Silverman, N., and Maniatis, T. (2001). NF-kB signaling pathways in mammalian and insect innate immunity. Genes and Dev. 15, 2321–2342.
Some Inhibitors of NFkB Signaling Inhibitor Aspirin (high doses) Curcumin Glucocorticoids Green tea compounds PS-341 Sulindac
Molecular target IKK IKK Re1A IKK Proteasome IKK
BIOGRAPHY Thomas D. Gilmore is a Professor in the Department of Biology at Boston University. His general research area is in the molecular basis of cancer. He holds a Ph.D. from the University of California (Berkeley) and received postdoctoral training at the University of Wisconsin (Madison). He has authored many research papers and review articles on the role of Rel/NF-kB transcription factors in leukemia and lymphoma, and recently his laboratory was the first to show that human c-Rel has oncogenic activity.
Nuclear factor kappaB (NFkB) is a family of structurally related DNA-binding proteins that act as transcription factors to regulate the expression of a wide variety of genes involved in many key cellular and organismal processes. The discovery of mammalian NFkB is generally attributed to Ranjan Sen and David Baltimore, who were studying the regulation of expression of a specific immunoglobulin gene; however, it is now clear that earlier researchers studying a viral oncoprotein (v-Rel) and a protein (Dorsal) required for the establishment of Drosophila embryonic polarity were also studying an NFkB activity. This entry focuses primarily on the vertebrate NFkB system, although most of the discussion is also applicable to the NFkB system in insects. NFkB activity is controlled by a multicomponent signal transduction pathway, such that in most cells. NFkB is inactive until any of several inducers initiates the signaling pathway that ultimately activates NFkB as a nuclear, DNA-binding protein. Misregulation of NFkB activity has been implicated in a number of human diseases, including several chronic inflammatory diseases and cancer, and the NFkB pathway is modulated by or used by several viruses as part of their pathology or replication. As such, the NFkB pathway is the subject of intense focus for pharmacological intervention.
NFkB Proteins and Protein Structure
p52/p100, and Relish) contains C-terminal sequences that fold back to inhibit the RHD, and thus these C-terminal sequences must be removed to release the active DNA-binding protein (i.e., p50 or p52 from the precursors p105 and p100, respectively). A second group (c-Rel, RelA, RelB, Dorsal, Dif) contains C-terminal sequences that are not cleaved and that activate transcription. NFkB proteins bind to 9– 10 bp DNA sites, usually called kB sites, located in the regulatory regions of many cellular and viral promoters. To bind DNA, NFkB proteins must form dimers. These dimers can, with few exceptions, be any combination of homodimer or heterodimer, although the most common NFkB dimer in many cells is a p50 – RelA dimer (Figure 1B). The affinity of the various NFkB subunits for one another and the affinity of the various dimers for specific target DNA sites can vary greatly. These considerations provide an array of diversity in the NFkB dimers that are found in different cells, in the DNA sites that can be recognized by different NFkB dimers, and in the genes (and consequently cellular responses) that these dimers regulate.
Regulation of NFkB Activity
The nuclear factor kappaB (NFkB) family is comprised of several evolutionarily conserved proteins that are related through a domain, usually called the Rel homology domain (RHD), which has sequences required for DNA binding, dimerization, and nuclear localization (Figure 1A). In Drosophila, there are three known NFkB proteins (Dorsal, Dif, and Relish), whereas in vertebrates, the NFkB protein family is comprised of five proteins: p50/p105, p52/p100, c-Rel, RelA (also called p65), and RelB. There is also an avian retroviral oncoprotein, v-Rel, that is a member of this family. All NFkB proteins contain the , 300 amino acid RHD towards their N termini; however, NFkB proteins can be subdivided into two distinct groups based on sequences C terminal to the RHD. One group (including p50/p105,
In most cells, NFkB is present in the cytoplasm in an inactive state, due to interaction with a family of inhibitor proteins, termed inhibitor of kB (IkB) proteins. IkB proteins include IkBa, IkBb, IkBe, Bcl-3, and the C-terminal sequences of the NFkB precursor proteins p105 and p100. Through a series of , 30 amino acid repeats (called ankyrin repeats), a single IkB protein binds directly to sequences in the RHD of an NFkB dimer. Binding of an IkB to NFkB inhibits the activity of the dimer in two ways: it causes NFkB to be in the cytoplasm and it blocks NFkB DNA binding. A variety of inducers (Table I), including many cytokines, cell stress, and viral or bacterial infections, can activate NFkB. In almost all cases, NFkB activity is induced through activation of a cellular kinase complex called IkB kinase (IKK). IKK is composed of three subunits: two related kinases (IKKa and IKKb) and a
Encyclopedia of Biological Chemistry, Volume 3. q 2004, Elsevier Inc. All Rights Reserved.
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NUCLEAR FACTOR kappaB
97
by the ubiquitin– proteasome pathway. The liberated NFkB complex can then enter the nucleus, bind to DNA, and activate specific target gene expression. In many cases, activation of NFkB is transient (perhaps lasting no more than 30 min) primarily because one of the target genes for NFkB is the gene encoding IkB. Therefore, newly synthesized IkB can cause NFkB to be resequestered in an inactive form in the cytoplasm, thus returning the pathway to its original state. Although the overall pathway of activation is similar (as in Figure 2), there are actually two distinct pathways to activation of NFkB, depending on the nature of the IkB protein and IKK involved. In one pathway, a separate IkB molecule, such as IkBa bound to p50 –RelA, is phosphorylated by the IKKb subunit, which leads to the complete proteolysis of IkBa. In a second activation pathway involving a complex such as p100 – RelB, a C-terminal site in p100 is phosphorylated by IKKa, which then leads to the partial proteolysis of the C-terminal ankyrin repeat sequences of p100, ultimately yielding an active p52 – RelB dimer, which can also translocate to the nucleus to regulate gene expression. However, a variety of recent evidence suggests that the NFkB pathway is regulated by mechanisms in addition to interaction with IkB proteins. For example, NFkB proteins can undergo posttranslational modifications FIGURE 1 NFkB proteins. (A) Shown are the generalized structures, protein names, and genes encoding the two classes of NFkB proteins and the IkB proteins. RH, Rel Homology domain; N, nuclear localization signal; vertical bars, ankyrin repeats. (B) X-ray crystal structure of the NFkB p50–RelA (p65) dimer on DNA. N, N terminus; C, C terminus. Figure provided courtesy of Gourisankar Ghosh.
sensing or scaffold protein called IKKg. Activation of the IKK complex generally requires phosphorylation of residues in the activation loop of the IKKa or IKKb kinase, and probably also phosphorylation of IKKg. Activated IKK then phosphorylates the IkB protein, which in turn signals the IkB sequences for degradation
TABLE I Some Inducers of NFkB Activity General category Bacteria Viruses
Examples Salmonella, Staphylococcus, Lipopolysaccharide HTLV-1, Epstein–Barr virus, hepatitis B virus
Cytokines
Interleukins, tumor necrosis factor
Oxidative stress
Hydrogen peroxide, reperfusion ischemia
Growth factors/ hormones
Platelet-derived growth factor, epidermal growth factor, insulin, transforming growth factor
Drugs
Various chemotherapeutic agents
FIGURE 2 The NFkB signal transduction pathway. Under most circumstances, NFkB dimers (e.g., p50– RelA) are located in the cytoplasm of cells bound to inhibitory IkB proteins. Upon receiving an inducing signal, an IkB kinase (IKK) is activated. Active IKK phosphorylates IkB, and phosphorylated IkB then serves as a substrate for the ubiquitin–proteasome degradation pathway. Free NFkB can then enter the nucleus to increase or decrease specific target gene transcription. The system is eventually returned to its resting state through an auto-regulatory system, in that NFkB turns on expression of the gene encoding IkB, and newly synthesized IkB then resequesters NFkB in the cytoplasm.
98
NUCLEAR FACTOR kappaB
(such as phosphorylation and acetylation) that affect their activity, they can interact with a variety of other proteins (including many other transcription factors), and the IKK proteins may also have effects on chromatin-binding proteins to affect gene expression from kB site-containing promoters.
Genes and Biological Processes Regulated by NFkB
Drosophila embryo. In mammals, NFk B activity appears to be involved in proper limb development, due to the regulation of a gene called Twist. NFkB transcription factors also regulate cell survival in many contexts, either by promoting or, more often, by inhibiting apoptosis. For example, RelA is required to protect liver cells from apoptosis induced by tumor necrosis factor, and RelA knockout mice die embryonically due to massive liver degeneration. Thus, the genes encoding several inhibitors of apoptosis are activated by NFkB.
NORMAL BIOLOGICAL ROLES FOR NFkB There are over 300 genes whose transcription is known to be regulated by NFkB activity, and these genes are involved in a multitude of biological processes (Table II). However, a variety of genetic and cellular studies have established that one of the key roles of NFkB is in regulating the innate immune response. In Drosophila, the activities of Dorsal, Dif, and Relish are induced to enter the nucleus by bacterial and fungal pathogens. These fly NFkB proteins then bind upstream and increase the expression of a family of genes called cecropins, which encode antifungal and antibacterial peptides. In mammals, it is also clear that many genes that encode antibacterial peptides, cytokines, and interferons are rapidly induced after infection with various pathogens. Moreover, mice in which individual genes for c-Rel, RelB, p50, and p52 are disrupted (“knockout mice”) have severe defects in T- and B-cellmediated immune responses. For example, c-rel knockout mice have B cells with greatly reduced survival and proliferation in response to antigen stimulation. NFkB proteins are, however, also involved in the control of several other genes and processes that are not directly related to immunity. These include certain developmental processes, apoptosis, cell growth, and cell stress. In terms of development, NFkB activity (as directed by Dorsal) is required for the proper establishment of dorsal-ventral polarity in the early TABLE II Some Biological Processes and Relevant Genes Regulated by NFkB Biological process
NFk B
AND
DISEASE
Aberrant NFkB activity has been implicated in several human diseases (Table III), most notably certain cancers and chronic inflammatory diseases, such as inflammatory bowel disease, asthma, and arthritis. Many cancer cells have constitutively active, nuclear NFkB activity. In certain B-cell lymphomas, notably diffuse large B-cell lymphomas and Hodgkin’s lymphomas, NFkB activity is dysregulated due to either amplification of the c-rel gene or mutations that inactivate the gene encoding IkB. Furthermore, several oncogenic human viruses, such as Epstein-Barr virus (lymphomas, nasopharyngeal carcinomas), HTLV-1 (T-cell leukemias), and Hepatitis B virus (liver cancer), encode proteins that activate NFkB. However, in many other cancers one finds chronic nuclear NFkB activity, which is usually made up of p50 – RelA dimers, in the apparent absence of mutations in the NFkB signaling pathway or associated viral infection. That increased NFkB activity is important for the growth and survival of several cancers is emphasized by studies showing that inhibition of NFkB activity often either causes the tumor cells to die (undergo apoptosis) or to become more sensitive to apoptosis-inducing agents. Because NFkB controls the expression of many cytokines, it is also consistently active in many situations involving acute or chronic inflammation. Moreover, in several animal model systems inhibition of NFkB activity can reduce inflammation. Furthermore, mice with knockouts in the genes encoding p50 or c-Rel are resistant to experimentally induced arthritis.
Relevant NFkB target genes
Angiogenesis
Vascular endothelial growth factor (VEGF)
Apoptosis
Bcl-X1, Bfl/A1, Bcl-2, IAP, TRAF, Fas
Cell adhesion
ICAM-1, VCAM-1
Cell growth Stress
Cyclin D1, c-Myc Cyclooxygenase, superoxide dismutase, nitric oxide synthetase
Innate immunity
Interleukins, interferon, tumor necrosis factor
Viral promoters
HIV-1, cytomegalovirus
TABLE III Some Diseases Associated with Chronic NF kB Activation Arthritis Asthma Cancer Diabetes Inflammatory bowel disease Ischemia/reperfusion damage Sepsis
NUCLEAR FACTOR kappaB
Pharmacological Regulation of NFkB Activity
SEE ALSO
Because of the role played by NFkB in inflammatory diseases and cancer, it has received much attention as a molecular target for pharmacological intervention. As a multicomponent pathway, NFkB activation can be inhibited at a variety of levels, including activation of IKK, IKK-mediated phosphorylation of IkB, ubiquitination and proteasome-mediated degradation of IkB, nuclear translocation of NFkB, and DNA-binding and transactivation by NFkB. Indeed, NFkB inhibitors that act at each of these steps have been described (partial list in Table IV). Nevertheless, to date, most research effort has focused on identifying NFkB inhibitors that target the IKK complex, due to its central role in controlling the NFkB pathway and to successes in identifying specific kinase inhibitors in other systems. Several natural products with anticancer or antiinflammatory activities have been shown to inhibit activation of NFk B. Among many others, these natural NFkB inhibitors include gold compounds (antiarthritis), green tea (anticancer), curcumin (anticancer, anti-inflammation), and various plant extracts (antiinflammation). In addition, some commonly used antiinflammatory compounds, such as glucocorticoids, ibuprofen, and aspirin, have been reported to have anti-NFkB activity in experimental model systems. However, in these cases, it is not clear if their biological effects in people are exerted through inhibition of NFkB, due to the fact that doses lower than those required to inhibit NFkB are often still effective in reducing inflammation in humans. Because NFkB is constitutively active in many cancers, clinical trials are underway with several compounds known to inhibit NFkB. Early clinical trials have reported promising results in the use of a small-molecule inhibitor of the proteasome for the treatment of multiple myeloma, an NFkB-dependent B-cell cancer. Such compounds inhibit activation of NF kB by blocking proteasome-mediated degradation of IkB. TABLE IV
THE
99
FOLLOWING ARTICLES
Cell Death by Apoptosis and Necrosis † Cytokines † Proteasomes, Overview † Ubiquitin System † UbiquitinLike Proteins
GLOSSARY apoptosis A programmed cell death pathway that is used by an organism to eliminate unwanted cells or maintain organ size. cytokines Factors that regulate the growth and activity of blood cells and lymphocytes. proteasome A cellular complex composed of several proteases that degrades ubiquitinated proteins. protein kinase An enzyme that can add a phosphate group onto specific residues in a target protein, which then affects the activity of the target protein. signal transduction A molecular process by which a signal (often an extracellular compound) effects a cellular response. Often this is a multicomponent step-wise intracellular pathway that involves binding of a ligand to a cell-surface receptor, which initiates a cascade of cytoplasmic enzymatic activities that ultimately leads to the activation of a transcription factor that then regulates the expression of a biologically-relevant set of target genes. transcription factor A protein or protein complex that binds to specific DNA sequences to regulate (increase or decrease) the rate of transcription of a gene. ubiquitin A small (, 8.6 kDa) polypeptide chain that can be conjugated to substrate proteins, which often targets them for degradation by the proteasome.
FURTHER READING Beyaert, R. (ed.) (2003). Nuclear Factor-kB: Regulation and Role in Disease. Kluwer Academic, The Netherlands. Ghosh, S., May, M. J., and Kopp, E. B. (1998). NF-kB and rel proteins: Evolutionarily conserved mediators of immune responses. Annu. Rev. Immunol. 16, 225–260. Gilmore, T. D. (ed.) (1999). NF-kB. Oncogene. 18, 6841–6964. Gilmore, T. D (2003). Rel/NF-kB transcription factors. http://www. nf-kb.org. Gilmore, T. D., and Ip, Y. T. (2003). Signal transduction pathways in development and immunity: Rel pathways. In Nature Encyclopedia of Life Sciences, Nature Publishing Group, London, ([doi: 10.1038/ npg.els.0002332] http://www.els.net). Karin, M., Cao, Y., Greten, F. R., and Li, Z-W. (2002). NF-kB in cancer: From innocent bystander to major culprit. Nat. Rev. Cancer 2, 301–310. Silverman, N., and Maniatis, T. (2001). NF-kB signaling pathways in mammalian and insect innate immunity. Genes and Dev. 15, 2321–2342.
Some Inhibitors of NFkB Signaling Inhibitor Aspirin (high doses) Curcumin Glucocorticoids Green tea compounds PS-341 Sulindac
Molecular target IKK IKK Re1A IKK Proteasome IKK
BIOGRAPHY Thomas D. Gilmore is a Professor in the Department of Biology at Boston University. His general research area is in the molecular basis of cancer. He holds a Ph.D. from the University of California (Berkeley) and received postdoctoral training at the University of Wisconsin (Madison). He has authored many research papers and review articles on the role of Rel/NF-kB transcription factors in leukemia and lymphoma, and recently his laboratory was the first to show that human c-Rel has oncogenic activity.