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INVITED REVIEW
B activation as a pathological mechanism of septic shock and inflammation
Division of Pulmonary and Critical Care Medicine, Long Island Jewish Medical Center, The Long Island Campus for the Albert Einstein College of Medicine, New Hyde Park, New York; and Department of Pharmacology, University of Illinois at Chicago, Chicago, Illinois
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 TOP ABSTRACT DESCRIPTIONREGULATION OF NF-{kappa}B...BIOLOGICAL FUNCTIONSROLE OF NF-{kappa}B IN...SUMMARY AND CONCLUDING REMARKSREFERENCES |
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B activation is a central event leading to the activation of these networks. The role of NF-B in septic pathophysiology and the signal transduction pathways leading to NF-B activation during sepsis have been an area of intensive investigation. NF-B is activated by a variety of pathogens known to cause septic shock syndrome. NF-B activity is markedly increased in every organ studied, both in animal models of septic shock and in human subjects with sepsis. Greater levels of NF-B activity are associated with a higher rate of mortality and worse clinical outcome. NF-B mediates the transcription of exceptional large number of genes, the products of which are known to play important roles in septic pathophysiology. Mice deficient in those NF-B-dependent genes are resistant to the development of septic shock and to septic lethality. More importantly, blockade of NF-B pathway corrects septic abnormalities. Inhibition of NF-B activation restores systemic hypotension, ameliorates septic myocardial dysfunction and vascular derangement, inhibits multiple proinflammatory gene expression, diminishes intravascular coagulation, reduces tissue neutrophil influx, and prevents microvascular endothelial leakage. Inhibition of NF-B activation prevents multiple organ injury and improves survival in rodent models of septic shock. Thus NF-B activation plays a central role in the pathophysiology of septic shock.
nuclear factor-B; septic pathophysiology; cytokines; signal transduction
Septic shock is a systemic inflammatory response syndrome (SIRS) to infection. It is now generally accepted that it is not the infection itself but, rather, the host response to infection that determines the outcome of sepsis (48). Bacterial pathogens and their products trigger the inflammatory response by transcriptional activation of inflammatory genes, leading to the release of large number of inflammatory mediators, including cytokines, chemokines, adhesion molecules, reactive oxygen species (ROS), and reactive nitrogen species (RNS). Although these mediators are important for host defense against invading bacteria, they frequently cause SIRS and septic shock when their production is uncontrolled and excessive. Simultaneously, anti-inflammatory pathways are also activated, leading to the release of anti-inflammatory cytokines, including interleukin (IL)-4, IL-10, IL-13, IL-1 receptor antagonist, transforming growth factor (TGF)-
, and suppressor of cytokine signal inhibitor-1, that serve as counterregulatory mechanisms to dampen the inflammatory response. However, the predominance of the inflammatory response, in combination with a repressed anti-inflammatory mechanisms (262), results in an imbalance between the two sets of counter mechanisms, leading to the development of septic shock and septic MOD/I. Thus septic shock is a result of exaggerated systemic inflammatory response to infection, and inflammatory mediators are major determinants of septic phenotype.
Bacterial pathogens activate cytokine networks by induction of multiple proinflammatory genes. This process is mediated by the activation of inducible transcription factors, such as activating protein-1 (AP-1) and nuclear factor (NF)-B. NF-B proteins play pivotal roles in immune and inflammatory responses. Recent studies on animal models of septic shock have demonstrated a critical role of NF-B in the pathophysiology of sepsis.
In this review, we will summarize the current research on NF-B and its regulatory mechanisms with emphasis on their pertinence to inflammation and septic shock. We will review the current knowledge about the role of NF-B and NF-B pathways in septic pathophysiology. To better understand the role of NF-B activation in septic MOD/I, we will also discuss the biological functions of NF-B in a wide spectrum of physiological and pathophysiological processes.
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TOPABSTRACTDESCRIPTIONREGULATION OF NF-{kappa}B...BIOLOGICAL FUNCTIONSROLE OF NF-{kappa}B IN...SUMMARY AND CONCLUDING REMARKSREFERENCES |
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B family of proteins.
NF-B is a group of structurally related transcriptional proteins that form dimers composed of various combinations of members of the NF-B/Rel family proteins. NF-B proteins in mammalian cells include NF-B1 (p50/p105), NF-B2 (p52/p100), RelA (p65), RelB, and C-Rel (83, 119, 226). Except for a truncated form of RelA, RelA (p37), seen in cells overexpressing proto-oncogene c-Myc (37), no new NF-B protein has been reported since the first description by Sen and Baltimore (213) of NF-B as a B cell nuclear factor binding to a site in the immunoglobin enhancer. The NF-B family of proteins is characterized by the presence of a highly conserved 300-amino acid Rel homology domain (RHD) composed of two immunoglobulin-like structures. RHD is responsible for dimerization, DNA binding, and association with their inhibitory proteins, IBs (119, 225). One structural difference between RelB and RelA (or C-Rel) is that RelB protein contains an NH2-terminus-activating domain (225), which may explain the difference in its regulation. NF-B1 (p50) and NF-B2 (p52) are synthesized as their precursors, p105 and p100, which contain multiple copies of the ankyrin repeat at their COOH termini, a structural characteristic of all IB proteins (1, 119, 225). p105 and p100 can serve as p50 and p52 precursors as well as regulatory proteins. Limited proteolysis of p105 or p100 protein at its COOH terminus yields p50 or p52 protein. This proteolytic degradation of p105 and p100 is accelerated under inflammatory conditions, which is one mechanism regulating the inducible NF-B activation. Interaction between members of NF-B family of proteins forms an NF-B dimer of distinct composition, which reflects variation in stimuli, cell types, or signal transduction pathways (12, 119, 226). Those NF-B dimers can be homodimers or heterodimers, although the most predominant form of NF-B is the p50/p65 heterodimer. Different forms of the NF-B dimer exhibit distinct properties in term of DNA binding preference, selectivity of interaction with IB isoforms, and transcriptional capability. The RelA/C-Rel dimer binds to the sequence of 5'-HGGARNYYCC-3', whereas the p50/p65 dimer preferentially binds to the sequence of 5'-GGGRNNYYCC-3' (12). The RelB/p52 dimer preferentially recognizes a novel NF-B-binding sequence of 5'-GGGAGATTTG-3, which is not recognized by the RelA/p50 dimer (24). RelA-containing NF-B dimers preferentially interact with IB
and IB (12, 226), whereas p50-containing dimers have a preference for IB
and IB
(169, 259). NF-B proteins are constitutively expressed in all cell types with the exception of RelB, the expression of which is restricted to lymphoid tissues (34). Although most NF-B dimers are activators of transcription, p52/p52 (83), p50/p50, and p65/p65 homodimers are transcriptional repressors. The homodimer of p50 or p65 forms a complex with histone deacetylase (HDAC)-1, binds to DNA, and suppresses NF-B-dependent gene expression (267). The RelB/p50 or RelB/p52 dimer acts as a transcriptional activator, whereas the RelA/RelB heterodimer represses the transcription of those genes (155). Distinctive properties of different NF-B dimers increase the ability of NF-B dimer to differentially regulate gene expression.
IB family of proteins.
NF-B activities are regulated by the IB family of proteins, which include IB, IB, IB, IB
, IB, Bcl-3, p105 (NF-B1), p100 (NF-B2), and MAIL (molecule possessing ankyrin-repeats induced by lipopolysaccharide). p105 and p100 have similar structural organization, containing p50 or p52 structure at their NH2 termini and IB structure at their COOH termini (119, 169, 225). The central portion of both proteins contains a glycine-rich region that plays a critical role in processing of the precursors (169). In resting cells, p105 and p100 are partially processed, generating p50 and p52, although the exact mechanisms of this limited processing event is unknown. It is also unclear how and why the proteasome-mediated proteolysis selectively degrades the COOH-terminal portion of p105 and p100 but leaves an intact NH2 terminus (p50 and p52). IB is structurally the COOH-terminal half of p105 protein but is translated from a separately initiated mRNA (226). The two newly discovered IBs, IB and MAIL, functionally differ from other IB proteins (see later discussion), although the COOH-terminal portion of these two proteins share high sequence homology with other members of IB proteins (124, 259). A common structure for all IBs is the six to eight copies of ankyrin repeats, called ankyrin repeat domain (ARD), which mediate IB binding to the NF-B dimers, masking the nuclear localization sequence on NF-B proteins.
IB proteins are different in their structures, preference for binding of NF-B dimers, biological functions, and modes of activation. IB, IB, and IB, but not other IBs, have an NH2-terminal regulatory domain, which is required for stimulation-induced IB degradation (226). Whereas IB and IB preferentially interact with dimeric complexes containing the transactivating subunits (RelA, RelB, and C-Rel), particularly those containing RelA (12, 226), IB and IB have a preference for p50-containing dimers (169, 259). IB effectively dissociates any prebound NF-B complex containing ReA, RelB, or C-Rel from their cognate DNA sites, but it is ineffective in promoting the dissociation of DNA-bound p50/p52 homodimer (226). IB interacts stably with p50/p65 dimer but not p65/p65 or C-Rel/C-Rel dimer (169). IB preferentially associates with p50 rather than p65 and inhibits the DNA binding of the p50/p65 heterodimer as well as the p50/p50 homodimer (259). IB is exclusively found to be associated with RelA and C-Rel (11). Whereas p105 is preferentially associated with p50-containing dimers (169), p100 is associated with RelB-containing dimers (25). The cytoplasmic retention of RelB-containing NF-B dimers is mediated exclusively by p100. There are also differences in the mechanism of regulating IB gene expression. IB, IB, and IB are constitutively expressed (119), but IB and MAIL are induced by lipopolysaccharide (LPS) and proinflammatory cytokines (67, 259). IB, IB, and IB are ubiquitously expressed, whereas IB is expressed only in certain cell types, such as pre-B cells (108). IB, IB, and MAIL are NF-B-regulated genes (67, 110, 119), but the gene expression of IB is not regulated by NF-B (119). IB proteins respond differently to NF-B activators. IB only responds to a subgroup of NF-B activators. IB protein degradation is observed following LPS or IL-1 stimulation, but it is not seen following TNF- or phorbol myristate acetate (PMA) stimulation (239). In contrast, all of these stimuli cause IB degradation (239). Studies using IB knock-in mice, in which the IB gene is replaced by the IB gene, have confirmed the divergent properties of IB and IB. These studies show that LPS causes both IB and IB degradation, but ischemia-reperfusion (I/R) only causes IB degradation (71). Unlike IB, IB is only partially degraded in response to most extracellular signals (44). IB forms an IB/NF-B/Ras complex, which blocks the induced IB phosphorylation and degradation (44). Although both IB and MAIL are inducible proteins, IB is induced by LPS and IL-1 but not by TNF- (259), but MAIL is induced by all three stimuli (12). In contrast to most IB proteins that are constitutively expressed cytoplasmic proteins, IB and MAIL are inducible nuclear proteins (124, 259). Contrasting sharply with the function of other IBs, MAIL serves as a transcriptional enhancer, enhancing LPS-induced IL-6 expression by >20-fold (124). IB, IB, and IB have different biological functions. IB appears to function as a strong negative feedback mechanism that allows a fast turn-off of the NF-B response, whereas IB and IB function to reduce systemic oscillation and stabilize the NF-B response during longer stimulation (97).
IBs inhibit NF-B activation through three mechanisms, by sequestrating NF-B dimers in the cytoplasm, facilitating dissociation of DNA-bound NF-B dimers from their DNA binding sites, and exporting NF-B dimers from nucleus. NF-B activation increases the expression and synthesis of IB and IB, which bind to NF-B dimers in the nucleus, thereby enhancing their dissociation from DNA and causing their export to cytoplasm by means of nuclear exportation mechanisms (83). Each member IB protein appears to employ different mechanisms for their cytoplasmic retention of NF-B dimers. Formation of IB/NF-B complex masks the nuclear localization sequences (NLS) on p65, not that on p50, resulting in a partial blockage of nuclear entry of NF-B dimers. Some of the IB/NF-B complexes are able to enter the nucleus due to the unmasked NLS on p50. However, those IB/NF-B complexes entering the nucleus are actively transported back to cytoplasm by means of the nuclear export sequences (NES) on IB (83). The nuclear import and export of IB/NF-B complexes are continuous processes. The default location for NF-B dimers is the cytoplasm because the effect of the NES is dominant over that of the NLS (83). This nuclear import and export mechanism is also responsible for the newly synthesized IB to dissociate activated NF-B dimers from DNA and export them from nucleus (83). IB causes cytoplasmic distribution of NF-B dimers through mechanisms similar to that of IB (94). Nuclear import of the p50/p65 heterodimer (or p50/p50 homodimer) is mediated by importin-3 and, to a lesser extent, by importin-4 (69). NLS on p65 or p50 binds directly to the NLS binding site of importin-3, leading to nuclear translocation of the NF-B dimers (69). Binding of IB to NF-B dimers masks the NLS of one NF-B subunit with the NLS on the other subunit being exposed (169). However, the IB/NF-B complex interacts with a Ras-like G protein, B-Ras, to form a new IB/NF-B/B-Ras complex, which effectively masks the NLS on the other NF-B subunit (44). Consequently, the IB/NF-B complexes are unable to shuttle between nucleus and cytoplasm and stay in the cytoplasm (83). Moreover, IB in the ternary complex is resistant to phosphorylation and degradation by most signals, preventing NF-B activation (44). IB/NF-B or p105/NF-B complexes are also cytoplasmic, although only the NLS on p50 is masked (169). One explanation for its cytoplasmic retention is that IB/NF-B (or p105/NF-B) complex interacts with a cytoplasmic protein, leading to its docking or masking of the NLS on p65 (169). IB is located in nucleus, where it negatively regulates NF-B DNA binding and inhibits NF-B-mediated gene transcription (259).
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REGULATION OF NF-B ACTIVITIES
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TOPABSTRACTDESCRIPTIONREGULATION OF NF-{kappa}B...BIOLOGICAL FUNCTIONSROLE OF NF-{kappa}B IN...SUMMARY AND CONCLUDING REMARKSREFERENCES |
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B activity.
NF-B is known to be activated by 
460 activators (with more becoming known every week), including physical stress, chemical stress, oxidant stress, environmental stress, physiological stress, mitogens, modified proteins, receptor ligands, physiological and pathological mediators, apoptotic mediators, bacteria and their products, fungi and their products, viruses and their products, parasites and their products, proinflammatory cytokines, and a variety of pathological conditions. The signal transduction pathways leading to NF-B activation are multiple and complex. Some signaling pathways appear to link to a particular stimulus, whereas other pathways are shared by multiple stimuli. One complexity in understanding the NF-B signaling pathways is that a single stimulus can activate NF-B through multiple signaling pathways, and multiple signaling cascades that lead to NF-B activation can utilize a given signaling component. One example for the former is LPS, which causes NF-B activation by activating multiple signaling pathways (Fig. 2). Examples for the latter are mitogen-activated protein (MAP) kinase and protein kinase C (PKC) pathways, both of which are involved in multiple signaling processes that lead to NF-B activation. The signaling pathways that lead to NF-B activation involve an extremely large number of signaling molecules, particularly kinases, and a detailed description of those molecules is beyond the scope of this review.
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B activation are presented to be complex, those signaling pathways converge at certain nodal points. There are both canonical and noncanonical pathways of NF-B activation. Some NF-B activators act principally through the canonical pathways, whereas others act mainly through noncanonical pathways (193). There are also NF-B stimulators that act through both pathways. For the canonical pathways, the converging point is IB kinase (IKK) complex. NF-B activators activate various signal transduction pathways that ultimately result in the activation of IKK, which in turn causes the rapid phosphorylation of IB proteins, on Ser32 and 36 for IB (119) and Ser19 and 23 on IB (94). Upon phosphorylation by IKKs, IB proteins are recognized by E3RSIKB/-TrCP, the receptor subunit of the Skp1-Cullin1-Roc1-F-box (SCF) ubiquitin ligase complex, which consequently results in polyubiquitination of IBs (at Lys21 and 22 for IB) by the SCF family of ubiquitin ligase (119). The ubiquitinized IB protein is subsequently degraded by 26S proteasome system. IB degradation exposes the NLS on NF-B protein, leading to the translocation of the NF-B dimer into nucleus, where it binds to its consensus sequence on the promoter or enhancer regions of NF-B-regulated genes, resulting in gene transcription (Fig. 1).
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B activity is regulated by p100 and p105. Upon stimulation with cytokines such as TNF-, p105 is rapidly phosphorylated by IKKs on Ser927 and 932 in its proline, glutamic acid, serine, and threonine rich region (94). Both IKK and IKK play critical roles in p105 phosphorylation (94). The death domain of p105 protein acts as a docking site for IKK and facilitates an efficient Ser927 phosphorylation (15). p105 phosphorylation generates a binding site for -TrCP, the receptor subunit of the SCF ubiquitin ligase complex, leading to ubiquitination and subsequent degradation of p105 protein by a proteasome system (12, 119). This releases the associated NF-B dimer, which translocates into the nucleus and regulates its target gene transcription (Fig. 1). The p105 phosphorylation and degradation processes are facilitated by molecular cross talk with glycogen synthase kinase-3 (GSK-3) (57) and are inhibited by docking of p50 subunit to the ARD (49).
Upon stimulation, p100 is also subjected to the processes of phosphorylation, ubiquitination, and proteasomal degradation, resulting in the release and nuclear translocation of p52-containing NF-B dimer (25). This p100 processing is tightly regulated and involves multiple functional regions of p100 protein. With the help of the COOH-terminal death domain, the COOH-terminal ARD interacts with its NH2-terminal dimerization domain and NLS, thereby bringing the COOH- and NH2-terminal sequences together to form a three-dimensional domain, which is required for the inducible processing of p100 protein (198). The inducible p100 processing requires NF-B-inducing kinase (NIK) and its downstream kinase IKK activity but does not require IKK and IKK, two key components of the classic IKK complex (25). This suggests that the IKK mediating the activation of the p100 NF-B pathway is an IKK/IKK homodimer complexed directly with NIK. Consistently, the p100-processing based NF-B activation pathway is not activated by typical NF-B inducers such as LPS, TNF-, IL-1, and dsRNA, but it is rather activated by signals involved in B cell maturation and lymphoid organogenesis, including lymphotoxin--receptor activation, engagement of BAFF-R (B cell-activating factor belonging to the TNF family receptor) or CD40 ligand (25). Activation of the IKK/p100/NF-B pathway causes the expression of a subset of NF-B-dependent genes such as organogenic chemokine genes that regulate B cell maturation, B cell function, and lymphoid organogenesis and maintenance of secondary lymphoid organs (25). This p100 processing-based NF-B pathway has been referred to as noncanonical or alternative NF-B pathway in the literature to distinguish it from the classic IKK/IB/NF-B pathway (Fig. 1). However, the cascade of events leading to the activation of the p100/NF-B pathway are identical to that responsible for the activation of the IKK/IB/NF-B pathway. Activation of both pathways is subjected to the process of phosphorylation, ubiquitination, and proteasomal degradation. It is argued that the p100 NF-B pathway should be classified as part of the canonical pathway to distinguish it from true noncanonical pathways that will be discussed below.
Noncanonical pathways are signaling pathways leading to NF-B activation without involving molecular events such as IKK activation, IB serine phosphorylation, or IB degradation by the ubiquitin proteasome system (193). Noncanonical pathways are divergent signaling pathways without a clear converging point. Hypoxia induces NF-B activation without causing IB serine phosphorylation and IB degradation, but rather it causes IB phosphorylation at Tyr42 (119). The subsequent release of NF-B dimer from the tyrosine-phosphorylated IB is suggested to be mediated by interaction with phosphoinositide-3 kinase (119). Hypoxia-reoxygenation or pervanadate treatment-induced NF-B activation is mediated by c-Src-dependent tyrosine phosphorylation of IB, but it is independent of IKK activation (70). Hypoxia-reoxygenation also activates p56/Lck tyrosine kinase, which causes NF-B activation by tyrosine phosphorylation of IB (152). Hydrogen peroxide (H2O2) activates NF-B through a combination of Syk tyrosine kinase-mediated tyrosine phosphorylation of IB and serine phosphorylation of p65 (235). UV radiation-induced NF-B activation requires 26S proteasome-mediated IB degradation (119) but is independent of IKK activity and IB Ser32, Ser36, or Tyr42 phosphorylation (94, 119). The UV radiation-induced NF-B activation depends on IB phosphorylation at a cluster of COOH-terminal sites by casein kinase II (CKII) (94). H2O2 induces NF-B activation without causing IB degradation (30). Studies on mouse embryo fibroblasts deficient in IKK and IKK genes demonstrated that the chemotherapeutic agent doxorubicin induces proteasome-dependent IB degradation and subsequent NF-B activation in the absence of IKK and IKK activities and IB Ser32 and Ser36 phosphorylation (94). Hepatitis C virus protein 5A induces NF-B activation through tyrosine phosphorylation of IB at Tyr42 and Tyr305 and IB degradation. However, IB degradation is not mediated by a proteasome, but rather by the protease calpain (248). Mitochondrial stress-induced NF-B activation involves the inactivation of IB through calcineurin-mediated dephosphorylation, which is independent of IKK and IKK (21). NF-B activity is regulated by posttranslational modification of IB through mechanisms other than IB phosphorylation. Taurine chloramine inhibits NF-B activation by oxidizing IB protein at Met45 (116). Transglutaminase 2 induces NF-B activation without stimulating IB phosphorylation and degradation, but by inducing IB polymerization. The polymerization results in NF-B dissociation and translocation into nucleus where it regulates gene expression (136).
In addition to nuclear translocation as discussed above, NF-B activity is controlled by additional regulatory mechanisms. These include regulation of nuclear import and export of NF-B dimers, regulation of the recruitment of NF-B dimer to the promoter or enhancer sites of NF-B target genes, regulation of NF-B transcriptional activity after recruitment, and positive or negative feedback mechanisms. It is reported that low intracellular zinc reduces nuclear import of activated NF-B dimers and inhibits the transcription of NF-B-driven genes in human neuroblastoma cells (151). Increase in intracellular [Ca2+] accelerates NF-B dimer nuclear translocation and promotes NF-B-mediated transcription (125). Commensal anaerobic gut bacteria, Bacteroides thetaiotaomicron, selectively antagonize virulent salmonella-induced NF-B activation by enhancing nuclear export of NF-B dimers (121). This nuclear export does not utilize the traditional chromosomal region maintenance-1 (CRM-1)-dependent nuclear export mechanism but relies on a novel mechanism involving a Bacteroides-induced association between p65 and the nuclear hormone receptor peroxisome proliferator-activated receptor (PPAR)-. Subsequently, the p65/PPAR complex is exported from nucleus, resulting in the attenuation of NF-B activation (121). This mechanism provides a valid explanation how gut commensal bacteria regulate inflammation.
Regulation of the recruitment of NF-B dimer to its target genes and regulation of NF-B-mediated transcriptional activity are primarily mediated by two mechanisms, posttranslational modification of NF-B proteins and synergistic (or antagonistic) interactions between NF-B and other transcription proteins, as well as transcriptional coactivators or co-repressors. NF-B and chromatin interaction is also a critical determinant of NF-B-mediated transcription (171). NF-B proteins, particularly p65, are subjected to a variety of posttranslational modifications, including phosphorylation (83), acetylation (41), S-nitrosylation (201), and S-glutathionylation (190). NF-B protein phosphorylation has emerged as an important mechanism regulating NF-B activity and NF-B-mediated gene transcription. Both p50 and p65 are phosphorylated by protein kinase A (PKA), resulting in increased DNA binding activity (83, 101). However, p65 phosphorylation is a better-characterized event. A variety of stimuli cause p65 phosphorylation (83). Phosphorylation of p65/p50 heterodimer by PKA enhances its DNA binding activity (83). Cells from mice deficient in serine/threonine protein kinase, GSK-3, and NF-B-activating kinase (NAK, also known as TBK or T2K) showed normal NF-B activation in response to a variety of NF-B inducers when measured by IB degradation, NF-B nuclear translocation, and binding to DNA. However, in both cells, NF-B was unable to drive gene transcription, suggesting that NF-B protein phosphorylation by these two kinases plays a critical role in regulating NF-B transactivating activity (83). The phosphatidylinositol 3-kinase (PI3K)/Akt-mediated RelA phosphorylation plays an important role in gram-negative enteric bacteria-induced NF-B activation (91). Numerous kinases cause p65 protein phosphorylation and enhance NF-B transactivation activity. Some of them such as PKA (83), PKC (94), CKII (36, 83), GSK-3 (27), and TNF receptor-associated factor (TRAF) family member-associated NF-B activator-binding kinase 1 (TBK1) (28) act directly on p65 protein, whereas others including NIK (83) and PI3K/Akt (83, 232) act indirectly by activating IKK, which in turn phosphorylates p65. All three IKKs, IKK, IKK, and IKK, phosphorylate p65 at Ser536 (28). Phosphatase 2A (260) and phosphatase 4 (263) dephosphorylate p65 protein. Regardless of the phosphoacceptor sites, p65 serine phosphorylation, in majority of the cases, enhances NF-B transcriptional activity. GSK-3 phosphorylates p65 at Ser468, resulting in reduced basal p65 activity (27). However, GSK-3-mediated p65 serine phosphorylation appears to affect inducible NF-B activity differently. Cells from mice deficient in the GSK-3 gene showed impaired NF-B-mediated transcription in response to a variety of NF-B inducers (83). p65 threonine dephosphorylation increases NF-B activity (263), implying that p65 threonine phosphorylation decreases NF-B activity. The mechanisms underlying the enhancement of NF-B transcriptional activity by p65 serine phosphorylation have also been investigated. It is believed that the COOH-terminal region of nonphosphorylated p65 interacts with RHD, thereby interfering with both DNA and cAMP response element binding protein (CREB) binding protein (CBP)/p300 binding. Phosphorylation of p65 at Ser276 by PKA prevents this intramolecular association, thereby facilitating its DNA binding and interaction with the transcriptional coactivator CBP/p300, enhancing NF-B transcriptional activity (83). However, it remains unknown whether the same mechanism is applicable to other kinases-induced p65 phosphorylation. Kinases causing p65 protein phosphorylation are multiple and diverse. These kinases do not act on the same phosphor-acceptor sites. PKA and CKII phosphorylate p65 at Ser276 and Ser529, respectively (36, 83), whereas PKC phosphorylates p65 at Ser311 (94). IKK, which mediates the effects of NIK and PI3K/Akt, phosphorylates p65 at Ser536 (83, 113), whereas IKK, which does not mediate the effects of NIK and PI3K, also phosphorylates p65 at Ser536 (94). It has been reported that differential p65 phosphorylation modulates NF-B transcriptional activity in a cis-acting element and promoter-specific context, thus leading to a phosphorylation state-dependent gene expression profile (7).
Reversible acetylation of NF-B protein serves as another mechanism regulating NF-B activity. Both p50 and p65 subunits can be acetylated at multiple lysine residues, and this acetylation plays an important role in the regulation of NF-B activity in vivo (41, 80). The transcriptional coactivator/acetyltransferase, CBP/p300, is the major acetyltransferase, and HDAC3 is the major deacetylase, mediating NF-B acetylation/deacetylation (41). Acetylation of p50 enhances its activity (80). The effect of p65 acetylation on its activity depends on the site of acetylation. Acetylation at Lys221 enhances p65 DNA binding, impairs its assembly with I-B, and reduces NF-B dimer nuclear exportation, resulting increased NF-B activity (42). Deacetylation of p65 protein by HDAC3 promotes its binding to IB, leading to rapid nuclear exportation of the deacetylated NF-B complex through a CRM-1-dependent mechanism (41). This p65 deacetylation is believed to be an important mechanism in terminating NF-B response. Consequently, the cytoplasmic pool of latent IB/NF-B complexes is replenished. This readies the cells for the next NF-B-mediated response. This reversible p65 acetylation also acts as a molecular switch that controls the duration of NF-B transcriptional activity. Acetylation and deacetylation are key mechanisms regulating chromatin remodeling, which can alter NF-B transcription activity by affecting their recruitment to target promoters.
NF-B activity is also regulated by S-nitrosylation (56, 201) and glutathionylation (174, 190). S-nitrosylation inhibits NF-B activation at several steps in the NF-B activation cascade. S-nitrosylation inhibits IKK activation (201), stabilizes IB protein, protecting it from degradation, and inhibits NF-B binding and transcriptional activities (56). S-glutathionylation of p50 protein is responsible for redox-mediated regulation of NF-B activity (190). The Cys62 of p50 is highly oxidized in the cytoplasm and strongly reduced in the nucleus. The reduced Cys62 is essential for the DNA binding of p50-containing NF-B dimer (174).
NF-B proteins interact with large number of transcriptional proteins either through a direct physical interaction or through interaction with a third protein, resulting in an altered NF-B activity and NF-B-mediated transcription. This interaction can be either synergistic or antagonistic and can be either reciprocal or nonreciprocal. Interaction between NF-B and JunD (90), CREB (223), CCAAT/enhancer-binding protein- (C/EBP, also called NF-IL6) (147, 186), interferon (IFN) regulatory factor-1 (IRF-1) (251), Kruppel-like factor 5 (4), POU-domain transcription factor-2 (Oct-2) (214), and nuclear factor of activated T cell (147) is synergistic and reciprocal, leading to augmented transcription activities mediated by both NF-B and other transcription factors. Interaction between NF-B and activating transcription factor-2 (20), breast cancer gene 1 (17), high mobility group box 1 (3), Notch (66), really interesting new gene (RING) finger protein, AO7 (10), promoter selective transcription factor 1 (142), signal transducer and activator of transcription 6 (STAT6) (222), and transcription factor IIB (254) is synergistic but unreciprocal, leading to the augmentation or facilitation of NF-B-mediated transcription without affecting the transcriptional activity mediated by the partner proteins. NF-B binds to positive transcription elongation factor-b to stimulate transcriptional elongation by RNA polymerase II (14). Interactions between NF-B and STAT1 (81), E2F transcription factor 1 (43), mammalian transcriptional repressor RBP-J (CBF1) (178), IFN-inducible p202a protein (150), forkhead box P3 (Foxp3) (19), and zinc-finger protein ZAS3 (99) are antagonistic and unreciprocal, inhibiting NF-B-mediated transcription without affecting the transcriptional activity of the partner proteins. On other hand, interaction between NF-B and PTEN (phosphatase and tensin homolog deleted on chromosome ten) or estrogen receptor is antagonistic and reciprocal, resulting in a mutual inhibition of the transcriptional activity mediated by both proteins (68, 88, 242). Reciprocally negative cross talk between NF-B and AP-1 has also been reported (123), although positive cross talk between those two proteins is more likely (90). NF-B activity and NF-B-mediated transcription are inhibited by interactions with its negative regulators, protein inhibitor of activated STAT1 (143) and zinc finger protein A20 (94). NF-B interacts with various transcriptional coactivators and co-repressors, and a dynamic balance between these coactivators and co-repressors regulates NF-B-mediated transcription (82).
NF-B activity is influenced by intranuclear p65 protein abundance and stability, which are also actively regulated. The p65 protein is subjected to a Pin1-dependent prolyl isomerization and ubiquitination-mediated proteasomal degradation. Prolyl isomerization enhances NF-B activity by inhibiting p65 binding to IB, resulting in an increased nuclear accumulation and stability of p65 protein (205). Proteasome-mediated proteolysis of p65 protein reduces its nuclear concentration and, thus, NF-B activity (205). Intranuclear proteasome can degrade DNA-bound p65 protein, which not only promotes the termination of NF-B-mediated transcription and response but also reduces intranuclear p65 abundance (206). This accelerated p65 degradation is believed to be a mechanism of inflammation termination and resolution (133).
NF-B activity is regulated by positive and negative feedback mechanisms. It is reported that RelA increases IB phosphorylation and degradation, which serves as a positive feedback loop for high-affinity NF-B complexes (261). NF-B activation stimulates its upstream kinases (59), which also form positive feedback. NF-B also activates several negative feedback mechanisms. NF-B activation increases IB, IB, or MAIL (119, 67, 110), which in turn inhibits NF-B activation (172). NF-B activation upregulates the expression of NF-B negative regulators, twist-1, twist-2 (231), and p65-interacting inhibitor of NF-B, SINK (253), which interact directly with NF-B protein and inhibits its transcription activity. NF-B also activates negative regulators of upstream signal molecules, resulting in an inhibition of NF-B signaling (185).
IKK.
The key step leading to NF-B activation in the canonical pathways is the activation of IKK. IKK is a large protein complex with a kinase core composed of three subunits: IKK (IKK1), IKK (IKK2), and IKK [also called NF-B essential modulator, NEMO or IKK associated protein, IKKAP] (119). IKK and KK are the catalytic subunits, and IKK is the regulatory subunit. IKK associates with IKK/KK dimer formed between the two catalytic subunits via their leucine zipper motifs to assemble a large IKK holocomplex. This complex assembly is essential for stimulus-dependent IKK activation. IKK lacks catalytic domain, but it is essential for IKK activation. It serves an important regulatory function by connecting IKK/KK dimer to upstream signaling molecules. IKK and KK share 52% overall sequence identity and 65% identity in their catalytic domains, and IKK is not structurally related to the catalytic subunits (119). Depending on the isolation procedure, IKK complex has been reported to be 700–900 kDa as revealed by gel filtration analysis. Because the molecular masses of IKK, IKK, and IKK are 85, 87, and 48 kDa, respectively, the large size of the IKK complex indicates the presence of additional components, including IB and NF-B proteins as well as upstream kinases. Because IKK is the converging point for multiple and divergent signal pathways, it is likely that the upstream kinase in the IKK complex varies with signaling pathway involved. Although IKK and IKK display similar activity in vitro (119), studies using IKK or IKK knockout mice or transgenic mice overexpressing the inactivatable variant IKK have revealed distinct functions for the two catalytic subunits (83, 119). IKK mediates IB phosphorylation and degradation, NF-B nuclear translocation, and NF-B-dependent gene transcription in response to inflammatory mediators and cytokines, whereas IKK is largely dispensable for this response (83, 119). In response to cytokines and inflammatory mediators, IKK contributes to NF-B-mediated gene transcription by its nucleosomal function. IKK phosphorylates p65 protein (94) and causes gene-specific phosphorylation (6) and subsequent acetylation of histone H3 (258), promoting the recruitment of NF-B dimer and enhancing the transcription of NF-B-regulated genes. IKK mediates p100 protein processing and the activation of p100-dependent pathway that has been discussed in detail above (25, 94). The IKK-dependent pathway mediates the activation of innate immunity and inflammatory responses (25), whereas the IKK-dependent pathway is involved in the termination and resolution of inflammatory responses (133). The IKK-dependent pathway suppresses inflammatory response by accelerating RelA and c-Rel protein turnover as well as their removal from promoter of proinflammatory genes (133).
Phosphorylation is an essential step toward IKK activation. Phosphorylation at Ser177 and 181 of the IKK activation loop or Ser176 of the IKK activation loop is required for IKK activation (119). This phosphorylation can be achieved either by the action of an upstream kinase or by an autophosphorylation caused by IKK itself. It is proposed that upstream signaling events induce a proximity mechanism in which the activator contacts each IKK/IKK dimer, increasing their proximity within the higher-order IKK complex and thereby facilitating mutual transautophosphorylation of IKK (83). IKK phosphorylation increases (78, 83, 192, 227, 233), and IKK dephosphorylation by protein phosphatase-2C decreases its activity (194). Numerous kinases are known to phosphorylate and activate IKK, but none has proven to be specific IKK kinase. These kinases include NIK (94), NAK (83), NAK-associated protein 1 (78), MAP/ERK kinase kinase-1 (MEKK1) (119), MEKK3 (94), TGF--activating kinase-1 (TAK1) (227), TBK1 (192), and PKC (233). Activation of these kinases by various signaling pathways results in IKK phosphorylation and activation and the subsequent NF-B activation. It is reported that TNF-- and IL-1-induced MEKK3 activation results in the formation of an IKK/IB/NF-B complex, which regulates rapid NF-B activation, whereas activation of MEKK2 results in the assembly of an IKK/IB/NF-B complex, which controls the delayed NF-B activation (210).
IKK has functions other than phosphorylating IBs. Both IKK and IB phosphorylate -catenin. IKK increases, whereas IKK decreases, -catenin-dependent gene transcription (129). IKK has nuclear function and shuttles into the nucleus where it competes with p65 for binding to CBP, leading to a repression of NF-B-mediated transcription (243).
IB kinase- (IKK or IKKi) is a structural homolog of IKK and IKK. LPS, TNF-, IL-1, and IL-6 induce IKK activity. Overexpression of IKK/IKKi causes IB phosphorylation at Ser32 and Ser36 and stimulates NF-B activity (189). However, dominant negative mutant of IKK/IKKi has no effect on TNF-- or IL-1-induced NF-B activation, although it blocks NF-B activation induced by PMA and T-cell receptor activation (189). Cells lacking the IKK/IKKi gene show normal activation of the canonical NF-B pathway, suggesting that it is not essential for this pathway (94). IKK/IKKi plays a pivotal role in integrating inflammatory signals into a coordinated activation of IRF-3 and NF-B (94) as well as coordinated activation of NF-B and C/EBP or C/EBP
(73), augmenting the inflammatory response.
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BIOLOGICAL FUNCTIONS |
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TOPABSTRACTDESCRIPTIONREGULATION OF NF-{kappa}B...BIOLOGICAL FUNCTIONSROLE OF NF-{kappa}B IN...SUMMARY AND CONCLUDING REMARKSREFERENCES |
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B participates in a variety of cellular activities and plays important roles in diverse biological functions. The two most prominent and well-defined functions for NF-B are regulation of immunological and inflammatory responses and regulation of cell proliferation and apoptosis. In normal cells, NF-B proteins are generally antiapoptotic and are regulators of cellular survival pathways. Activation of NF-B mediates the expression of multiple antiapoptotic or cell survival genes, including bfl-1 (Bcl-2-related genes from a human fetal liver) (61), receptor for activated C kinase-1 (46), inhibitors of apoptosis-1 and -2 (IAP-1 and IAP-2) (224), X chromosome-linked inhibitor of apoptosis protein (141), cellular Fas-associated death domain-like IL-1-converting enzyme (FLICE) inhibitory protein (162), inhibitor of caspase 8 (224), and survivin (224). NF-B promotes survival during mitotic cell cycle arrest (165). Inhibition of NF-B pathway by various means promotes apoptosis (249). Activation of the NF-B pathway suppresses putative proapoptotic and tumor repressor genes such as PTEN and p53 (54, 242). This inhibition is reciprocal, because these two proapoptotic proteins also inhibit NF-B activity (54, 88). Tumor suppressors PTEN and p53 and the proapoptotic protein Fas-associated factor-1 exert their antitumor or proapoptotic action by negatively regulating NF-B activity and NF-B-mediated gene transcription (54, 88, 187). The importance of NF-B in cell proliferation and embryonic development is further evidenced by studies on NF-B or IKK knockout mice. Mice deficient in p65 or IKK and mice deficient in both p50 and p65 died at embryonic day 12.5–14.5 due to massive apoptosis (12, 139). IKK-deficient mice lack limbs, tails, and rears and exhibit severe deformities in craniofacial and several other organs (119). In tumor cells, however, NF-B appears to have dual functions, acting as both tumor promoter and tumor suppressor. Numerous lines of evidence support the notion that NF-B promotes tumorigenesis and tumor progression. A constitutive NF-B activity is detected in most tumor cell lines but is rarely detectable in normal cells (224). Increased NF-B activity is also detected in various cancer tissues (224). Inhibition of NF-B activity in those tumor cells suppresses their proliferation, leading to apoptosis (224). NF-B proteins are oncogenic. Several putative oncogenes induce cellular transformation though activation of the NF-B pathway (224). NF-B mediates the expression of multiple genes that are associated with tumor cell growth and survival and plays an essential role in every step of tumorigenesis and tumor progression (224). NF-B activation promotes tumor cell proliferation and migration and mediates tumor invasion and metastasis (224). NF-B activation also plays important role in angiogenesis (224), which is critical for solid tumor growth. NF-B activity is essential for tumor maintenance and for cancer cell resistance to chemotherapies or TNF- therapy (224). In recent years, evidence has also emerged showing that NF-B activation mediates apoptosis (224). NF-B mediates the expression of several proapoptotic genes including Fas ligand and c-Myc (224). NF-B has been reported to mediate p53 tumor repressor gene expression (224) and to stabilize p53 protein (77), although another report has demonstrated that NF-B inhibits p53 transcription (54). NF-B activity is required for the induction of apoptosis in several cell lines in response to various apoptosis inducers (224). Human melanoma cells are protected against UV-induced apoptosis by downregulation of NF-B activity and Fas expression (224). Activators of NF-B induce apoptosis, and inhibitors of NF-B inhibit apoptosis (224). Blockade of NF-B pathway predisposes and triggers tumor formation in the skin (224). These two opposite sets of data are not necessarily contradictory. They may represent a timely switch from one response to another in adaptation to changes in cellular environment. It has been reported that NF-B can be either proapoptotic or antiapoptotic, depending on the timing of NF-B activity being modulated relative to the death insult (224). Elucidation of the molecular mechanisms underlying this timely switch may help to better understand the biological basis of tumorigenesis and to identify better targets for tumor prevention.
Regulation of immunological and inflammatory responses.
NF-B pathway plays a central role in the immune and inflammatory responses. The important roles of NF-B in adaptive immunity are evidenced by demonstrations that mice deficient in NF-B proteins develop a variety of immunological deficiencies. Mice lacking NF-B1 (p50/p105) exhibit an impaired B cell proliferation, antibody secretion, and defects in B cell-mediated immune response (12 139). Mice deficient in NF-B2 (p52/p100) show defects in B cell maturation and T cell activation (12 139). The RelA (p65) knockout mice display embryonic lethality due to massive apoptosis in the liver (12 139). RelB knockout mice show defects in hematopoiesis, reduced antigen-presenting dendritic cells in the thymus, impaired cellular immunity, as well as multifocal and mixed infiltration of inflammatory cells in several tissues (12 139). C-Rel null mice exhibit defective B cell and T cell proliferation, defective immunoglobin production, and T cell-dependent immune response (12 139). NF-B1 and NF-B2 double knockout mice lack mature B cells and osteoblasts (12 139). NF-B1 and RelB double knockout mice died postnatally due to immune deficiencie
