CD40 regulates the processing of NF-kappaB2 p100 to p52

Differential B-lymphocyte regulation by CD40 and its viral mimic, latent membrane protein 1

CD40 plays a vital role in humoral immunity, via its potent and multifaceted function as an activating receptor of various immune cells, most notably B lymphocytes. The Epstein-Barr virus-encoded transforming protein latent membrane protein 1 (LMP1) serves as a functional mimic of CD40 signals to B cells but lacks key regulatory controls that restrain CD40 signaling. This allows LMP1 to activate B cells in an abnormal manner that can contribute to the pathogenesis of human B-cell lymphoma and autoimmune disease. This review focuses upon a comparative analysis of CD40 versus LMP1 functions and mechanisms of action in B lymphocytes, discussing how this comparison can provide valuable information on both how CD40 signaling is normally regulated and how LMP1 disrupts the normal CD40 pathways, which can provide information of value to therapeutic design.

IL-23-dependent and -independent enhancement pathways of IL-17A production by lactic acid

Interleukin-17A (IL-17A) is a cytokine produced by T(h)17 cells that plays an important role in inflammatory and autoimmune diseases and cancer. Stimulation with IL-6, transforming growth factor-β , IL-21, IL-1β and IL-23 is required for differentiation of T(h)17 cells and the production of IL-17A. Recently, we reported that tumor-derived lactic acid enhances the toll-like receptor (TLR) ligand-mediated expression of IL-23, leading to increased IL-17A production. Tumor cells secrete large amounts of lactic acid due to the up-regulation of glycolysis, which is known as the Warburg effect. Even without TLR ligand stimulation, lactic acid enhanced antigen-dependent IL-17A production from splenocytes in an IL-23-dependent manner. Here, we show that macrophages and effector/memory CD4(+) T cells are the primary cell types involved in the ability of lactic acid to boost IL-17A production. Although lactic acid suppressed the proliferation of T(h)1 and T(h)17 cells, T(h)17 cells still secreted large amounts of IL-17A. CD40 ligand-CD40 interactions were involved in the up-regulation of IL-17A by lactic acid through IL-12/23p40 production. A new cytokine containing the IL-12/23p40 subunit, but not IL-23, IL-12 or the IL-12p40 homodimer, is a candidate for involvement in the up-regulation of IL-17A. IL-1β also increased IL-17A expression; however, IL-1β, CARD9 and MyD88 signaling pathways activated by known intrinsic inflammatory mediators were hardly required for the enhanced activity induced by lactic acid. Our results show that lactic acid functions as an intrinsic inflammatory mediator that activates IL-23-dependent and -independent pathways, resulting in the promotion of chronic inflammation in tumor microenvironments.

Deubiquitinases in the regulation of NF-κB signaling

Nuclear factor-kappa B (NF-κB) is a critical regulator of multiple biological functions including innate and adaptive immunity and cell survival. Activation of NF-κB is tightly regulated to preclude chronic signaling that may lead to persistent inflammation and cancer. Ubiquitination of key signaling molecules by E3 ubiquitin ligases has emerged as an important regulatory mechanism for NF-κB signaling. Deubiquitinases (DUBs) counteract E3 ligases and therefore play a prominent role in the downregulation of NF-κB signaling and homeostasis. Understanding the mechanisms of NF-κB downregulation by specific DUBs such as A20 and CYLD may provide therapeutic opportunities for the treatment of chronic inflammatory diseases and cancer.

Non-canonical NF-κB activation and abnormal B cell accumulation in mice expressing ubiquitin protein ligase-inactive c-IAP2

Chromosomal translocations between loci encoding MALT1 and c-IAP2 are common in MALT lymphomas. The resulting fusion proteins lack the c-IAP2 RING domain, the region responsible for its ubiquitin protein ligase (E3) activity. Ectopic expression of the fusion protein activates the canonical NF-κB signaling cascade, but how it does so is controversial and how it promotes MALT lymphoma is unknown. Considering recent reports implicating c-IAP1 and c-IAP2 E3 activity in repression of non-canonical NF-κB signaling, we asked if the c-IAP2/MALT fusion protein can initiate non-canonical NF-κB activation. Here we show that in addition to canonical activation, the fusion protein stabilizes NIK and activates non-canonical NF-κB. Canonical but not non-canonical activation depended on MALT1 paracaspase activity, and expression of E3-inactive c-IAP2 activated non-canonical NF-κB. Mice in which endogenous c-IAP2 was replaced with an E3-inactive mutant accumulated abnormal B cells with elevated non-canonical NF-κB and had increased numbers of B cells with a marginal zone phenotype, gut-associated lymphoid hyperplasia, and other features of MALT lymphoma. Thus, the c-IAP2/MALT1 fusion protein activates NF-κB by two distinct mechanisms, and loss of c-IAP2 E3 activity in vivo is sufficient to induce abnormalities common to MALT lymphoma.

NFkappaB: a promising target for natural products in cancer chemoprevention

The transcription factor nuclear factor kappa B (NFkappaB) is found in nearly all animal cell types. It is involved in cellular responses to stimuli such as stress, cytokines, free radicals, ultraviolet irradiation, oxidized LDL and microbial antigens, and has been shown to regulate the expression of a number of genes including bcl-2, bcl-xl, cIAP, suvivin, TRAF, COX-2, MMP-9, iNOS and cell cycle-regulatory components. Many carcinogens, inflammatory agents and tumor promoters have been shown to activate NFkappaB, and resulting tumors demonstrate misregulated NFkappaB. Incorrect regulation of NFkappaB has been linked to inflammatory and autoimmune diseases, septic shock, viral infection and improper immune development. Aberrant regulation of NFkappaB is involved in cancer development and progression as well as in drug resistance. Inhibitors of NFkappaB mediate effects potentially leading to antitumor responses or greater sensitivity to the action of antitumor agents. Tools have been developed for the rapid assessment of NFkappaB activity, so in concert with a better understanding of NFkappaB activation mechanisms, many agents capable of suppressing NFkappaB activation have been identified. The present article focuses on the functions of NFkappaB, its role in human cancer and the therapeutic potential and benefit of targeting NFkappaB by natural products in cancer chemoprevention.

Zinc-finger protein 91 plays a key role in LIGHT-induced activation of non-canonical NF-κB pathway

LIGHT is a member of tumor necrosis factor (TNF) superfamily, and its function is mediated through lymphotoxin-β receptor (LTβR), which is known to play important roles in inflammatory and immune responses through activation of NF-κB signaling pathways. However, molecular mechanism of LTβR ligation-induced NF-κB signaling remains incompletely understood. In this report we demonstrate that a novel zinc-finger protein 91 (ZFP91) is a critical regulator in LIGHT-induced activation of non-canonical NF-κB pathway. ZFP91 appears to be required for NF-κB2 (p100) processing to p52, nuclear translocation of p52 and RelB, and DNA-binding activity of NF-κB in LIGHT-induced activation of non-canonical NF-κB pathway. Furthermore, ZFP91 knock-down by RNA interference blocks the LIGHT-induced accumulation of NIK and p100 processing, as well as the expression of non-canonical NF-κB target genes. These data clearly indicate that ZFP91 is a key regulator in LIGHT-induced activation of non-canonical NF-κB pathway in LTβR signaling.

NF-kappaB inducing kinase (NIK) inhibitors: identification of new scaffolds using virtual screening

As a wide variety of pro-inflammatory cytokines are involved in the development of rheumatoid arthritis (RA), there is an urgent need for the discovery of novel therapeutic strategies. Among these, the inhibition of the NF-kappaB inducing kinase (NIK), a key enzyme of the NF-kappaB alternative pathway activation, represents a potential interesting approach. In fact, NIK is involved downstream of many tumor necrosis factor receptors (TNFR) like CD40, RANK or LTbetaR, implicated in the pathogenesis of RA. But, up to now, the number of reported putative NIK inhibitors is extremely limited. In this work, we report a virtual screening (VS) study combining various filters including high-throughput docking using a 3D-homology model and ranking by using different scoring functions. This work led to the identification of two molecular fragments, 4H-isoquinoline-1,3-dione (5) and 2,7-naphthydrine-1,3,6,8-tetrone (6) which inhibit NIK with an IC(50) value of 51 and 90 microM, respectively. This study opens new perspectives in the field of the NF-kappaB alternative pathway inhibition.

The unexpected role of lymphotoxin beta receptor signaling in carcinogenesis: from lymphoid tissue formation to liver and prostate cancer development

The cytokines lymphotoxin (LT) alpha, beta and their receptor (LTbetaR) belong to the tumor necrosis factor (TNF) superfamily, whose founder-TNFalpha-was initially discovered due to its tumor necrotizing activity. LTbetaR signaling serves pleiotropic functions including the control of lymphoid organ development, support of efficient immune responses against pathogens due to maintenance of intact lymphoid structures, induction of tertiary lymphoid organs, liver regeneration or control of lipid homeostasis. Signaling through LTbetaR comprises the noncanonical/canonical nuclear factor-kappaB (NF-kappaB) pathways thus inducing chemokine, cytokine or adhesion molecule expression, cell proliferation and cell survival. Blocking LTbetaR signaling or Fcgamma-receptor mediated immunoablation of LT-expressing cells was demonstrated to be beneficial in various infectious or noninfectious inflammatory or autoimmune disorders. Only recently, LTbetaR signaling was shown to initiate inflammation-induced carcinogenesis, to influence primary tumorigenesis and to control reemergence of carcinoma in various cancer models through distinct mechanisms. Indeed, LTbetaR signaling inhibition has already been used as efficient anti-inflammatory, anti-cancer therapy in some experimental models. Here, we review the pleiotropic functions attributed to LT, the effects of its deregulation and extensively discuss the recent literature on LT's link to carcinogenesis.

Oncogenic activation of NF-kappaB

Recent genetic evidence has established a pathogenetic role for NF-kappaB signaling in cancer. NF-kappaB signaling is engaged transiently when normal B lymphocytes respond to antigens, but lymphomas derived from these cells accumulate genetic lesions that constitutively activate NF-kappaB signaling. Many genetic aberrations in lymphomas alter CARD11, MALT1, or BCL10, which constitute a signaling complex that is intermediate between the B-cell receptor and IkappaB kinase. The activated B-cell-like subtype of diffuse large B-cell lymphoma activates NF-kappaB by a variety of mechanisms including oncogenic mutations in CARD11 and a chronic active form of B-cell receptor signaling. Normal plasma cells activate NF-kappaB in response to ligands in the bone marrow microenvironment, but their malignant counterpart, multiple myeloma, sustains a variety of genetic hits that stabilize the kinase NIK, leading to constitutive activation of the classical and alternative NF-kappaB pathways. Various oncogenic abnormalities in epithelial cancers, including mutant K-ras, engage unconventional IkappaB kinases to activate NF-kappaB. Inhibition of constitutive NF-kappaB signaling in each of these cancer types induces apoptosis, providing a rationale for the development of NF-kappaB pathway inhibitors for the treatment of cancer.

Constitutive CD40L expression on B cells prematurely terminates germinal center response and leads to augmented plasma cell production in T cell areas

CD40/CD40L engagement is essential to T cell-dependent B cell proliferation and differentiation. However, the precise role of CD40 signaling through cognate T-B interaction in the generation of germinal center and memory B cells is still incompletely understood. To address this issue, a B cell-specific CD40L transgene (CD40LBTg) was introduced into mice with B cell-restricted MHC class II deficiency. Using this mouse model, we show that constitutive CD40L expression on B cells alone could not induce germinal center differentiation of MHC class II-deficient B cells after immunization with T cell-dependent Ag. Thus, some other MHC class II-dependent T cell-derived signals are essential for the generation of germinal center B cells in response to T cell-dependent Ag. In fact, CD40LBTg mice generated a complex Ag-specific IgG1 response, which was greatly enhanced in early, but reduced in late, primary response compared with control mice. We also found that the frequency of Ag-specific germinal center B cells in CD40LBTg mice was abruptly reduced 1 wk after immunization. As a result, the numbers of Ag-specific IgG1 long-lived plasma cells and memory B cells were reduced. By histology, large numbers of Ag-specific plasma cells were found in T cell areas adjacent to Ag-specific germinal centers of CD40LBTg mice, temporarily during the second week of primary response. These results indicate that CD40L expression on B cells prematurely terminated their ongoing germinal center response and produced plasma cells. Our results support the notion that CD40 signaling is an active termination signal for germinal center reaction.

Classical and/or alternative NF-kappaB pathway activation in multiple myeloma

Mutations involving the nuclear factor-kappaB (NF-kappaB) pathway are present in at least 17% of multiple myeloma (MM) tumors and 40% of MM cell lines (MMCLs). These mutations, which are apparent progression events, enable MM tumors to become less dependent on bone marrow signals that activate NF-kappaB. Studies on a panel of 51 MMCLs provide some clarification of the mechanisms through which these mutations act and the significance of classical versus alternative activation of NF-kappaB. First, only one mutation (NFKB2) selectively activates the alternative pathway, whereas several mutations (CYLD, NFKB1, and TACI) selectively activate the classical pathway. However, most mutations affecting NF-kappaB-inducing kinase (NIK) levels (NIK, TRAF2, TRAF3, cIAP1&2, and CD40) activate the alternative but often both pathways. Second, we confirm the critical role of TRAF2 in regulating NIK degradation, whereas TRAF3 enhances but is not essential for cIAP1/2-mediated proteasomal degradation of NIK in MM. Third, using transfection to selectively activate the classical or alternative NF-kappaB pathways, we show virtually identical changes in gene expression in one MMCL, whereas the changes are similar albeit nonidentical in a second MMCL. Our results suggest that MM tumors can achieve increased autonomy from the bone marrow microenvironment by mutations that activate either NF-kappaB pathway.

Aberrant activation of the androgen receptor by NF-kappaB2/p52 in prostate cancer cells

Prostate cancer initiation and progression are uniquely dependent on the androgen receptor (AR). Even when the cancer progresses to a castration-resistant stage, AR signaling remains active via a variety of mechanisms. In the present study, we showed that NF-kappaB/p52 can activate the AR, resulting in increased transactivation of AR-responsive genes, such as PSA and NKX3.1, in a ligand-independent manner. NF-kappaB2/p52 enhances nuclear translocation and activation of AR by interacting with its NH(2)-terminal domain and enhances the recruitment of coactivators such as p300 to the promoters of AR-dependent genes. These results were confirmed in three different prostate cancer cell lines: LAPC-4 (wild-type AR), LNCaP (mutant AR), and C4-2 (castration resistant). Transfection of p52 into LAPC-4 and LNCaP cells (which express low levels of p52) showed increased activation of the endogenous AR. Downregulation of endogenous p52 in C4-2 cells resulted in abrogation of AR constitutive activation. Comparison of the relative effects of p52 and p65 (RelA) showed that p52, but not p65, could activate the AR. Collectively, these findings, together with previous reports that the levels of NF-kappaB2/p52 are elevated in prostate cancer cells and that active NF-kappaB2/p52 promotes prostate cancer cell growth in vitro and in vivo, suggest that NF-kappaB2/p52 may play a critical role in the progression of castration-resistant prostate cancer.

Loss of negative feedback control of nuclear factor-kappaB2 activity in lymphocytes leads to fatal lung inflammation

Proteolytic processing of the nuclear factor (NF)-kappaB2 precursor protein p100 generates the active NF-kappaB2 subunit p52, which in turn transcriptionally up-regulates p100 expression. p100 also functions as an IkappaB molecule capable of repressing p52 activity. The biological significance of this negative feedback control loop has yet to be demonstrated in vivo. Here we show that mice deficient in p100 but with constitutive expression of p52 in lymphocytes developed fatal lung inflammation characterized by diffuse alveolar damage with marked peribronchial fibrosis. In contrast, their littermates with only p100 deficiency or constitutive expression of p52 in lymphocytes developed mild lung inflammation with perivascular lymphocyte infiltration and had a normal life span. The fatal lung inflammation is associated with high-level induction of interferon-gamma and its inducible inflammatory chemokines, suggesting the involvement of a T-helper-1 immune response. These findings demonstrate the physiological relevance of the NF-kappaB2 p100 precursor protein in limiting the potentially detrimental effects of constitutive NF-kappaB2 signaling in lymphocytes.

NF-kappaB and cancer: how intimate is this relationship

NF-kappaB, a transcription factor first discovered in 1986, is now known to be closely connected to the process of tumorogenesis based on a multiplicity of evidence. (1) NF-kappaB is activated in response to tobacco, stress, dietary agents, obesity, alcohol, infectious agents, irradiation, and environmental stimuli that account for as much as 95% of all cancers. (2) The transcription factor has been linked with transformation of cells. (3) It is constitutively active in most tumor cells. (4) It has also been linked with the survival of cancer stem cells, an early progenitor cell that has acquired self-renewal potential. (5) NF-kappaB regulates the expression of most anti-apoptotic gene products associated with the survival of the tumor. (6) It also regulates the gene products linked with proliferation of tumors. (7) The transcription factor controls the expression of gene products linked with invasion, angiogenesis, and metastasis of cancer. (8) While most carcinogens activate NF-kappaB, most chemopreventive agents suppress its activation. These observations suggest that NF-kappaB is intimately intertwined with cancer growth and metastasis. The mechanism that leads to constitutive activation of NF-kappaB in hematological, gastrointestinal, genitourinary, gynecological, thoracic head and neck, breast, and skin cancers, and the ways NF-kappaB is activated are the topics of discussion in this review.

Microarray analysis reveals potential target genes of NF-kappaB2/p52 in LNCaP prostate cancer cells

PURPOSE

Our previous studies showed that NF-kappaB2/p52 is involved in the castration-resistant growth of the androgen-sensitive LNCaP prostate cancer cells. The role of NF-kappaB2/p52 in lymphomagenesis has been studied extensively, but its target genes in other cancers remain unknown. In order to identify genes potentially regulated by p52 in prostate cancer cells, we performed a genome-wide microarray analysis of genes differentially up- or down-regulated by the overexpression of p52 by adenoviral-mediated gene delivery in LNCaP cells.

EXPERIMENTAL DESIGN

Total RNAs from vector control-infected and Adeno-p52-infected LNCaP cells were used to prepare cDNAs, which were hybridized to the Whole Genome Human 44k Microarray chips (Agilent Technologies). Data analysis was performed using GeneSpring and Ingenuity Pathway Analysis software. Validation of microarray results was performed by real-time quantitative RT-PCR and Western blot analyses.

RESULTS

Expression of approximately 130 genes was differentially upregulated by >5-fold, whereas approximately 60 genes were differentially downregulated by >2-fold in p52-expressing LNCaP cells. Pathway analysis revealed that the upregulated genes belong to functional categories like cell growth and proliferation, cellular movement, cell-to-cell signaling and interaction, cancer, cell cycle, etc., whereas the downregulated genes were represented by functional categories like cell movement, antigen presentation, and cell death. Six of the top upregulated genes including annexin A2, PLAU, RND3, Twist2, VEGFC, and CXCL1 were validated by real-time PCR and Western blot analysis.

CONCLUSIONS

This study provides a comprehensive analysis of genes potentially regulated by NF-kappaB2/p52 in the LNCaP prostate cancer cell line and provides a rationale for the induction of castration-resistant growth by p52 in LNCaP cells.

TWEAK activates the non-canonical NFkappaB pathway in murine renal tubular cells: modulation of CCL21

TWEAK is a member of the TNF superfamily of cytokines that contribute to kidney tubulointerstitial injury. It has previously been reported that TWEAK induces transient nuclear translocation of RelA and expression of RelA-dependent cytokines in renal tubular cells. Additionally, TWEAK induced long-lasting NFκB activation suggestive of engagement of the non-canonical NFκB pathway. We now explore TWEAK-induced activation of NFκB2 and RelB, as well as expression of CCL21, a T-cell chemotactic factor, in cultured murine tubular epithelial cells and in healthy kidneys in vivo. In cultured tubular cells, TWEAK and TNFα activated different DNA-binding NFκB complexes. TWEAK-induced sustained NFκB activation was associated with NFκB2 p100 processing to p52 via proteasome and nuclear translocation and DNA-binding of p52 and RelB. TWEAK, but not TNFα used as control), induced a delayed increase in CCL21a mRNA (3.5±1.22-fold over control) and CCL21 protein (2.5±0.8-fold over control), which was prevented by inhibition of the proteasome, or siRNA targeting of NIK or RelB, but not by RelA inhibition with parthenolide. A second NFκB2-dependent chemokine, CCL19, was upregulates by TWEAK, but not by TNFα. However, both cytokines promoted chemokine RANTES expression (3-fold mRNA at 24 h). In vivo, TWEAK induced nuclear NFκB2 and RelB translocation and CCL21a mRNA (1.5±0.3-fold over control) and CCL21 protein (1.6±0.5-fold over control) expression in normal kidney. Increased tubular nuclear RelB and tubular CCL21 expression in acute kidney injury were decreased by neutralization (2±0.9 vs 1.3±0.6-fold over healthy control) or deficiency of TWEAK (2±0.9 vs 0.8±0.6-fold over healthy control). Moreover, anti-TWEAK treatment prevented the recruitment of T cells to the kidney in this model (4.1±1.4 vs 1.8±1-fold over healthy control). Our results thus identify TWEAK as a regulator of non-canonical NFκB activation and CCL21 expression in tubular cells thus promoting lymphocyte recruitment to the kidney during acute injury.

Regulation of TNFRSF and innate immune signalling complexes by TRAFs and cIAPs

There have been a number of recent discoveries relating to the functions of inhibitors of apoptosis (IAPs) and TNF receptor-associated factors (TRAFs) in regulating signalling from TNF receptor superfamily (TNFRSF) members and some tantalizing glimpses into a wider area of influence, that of innate immune signalling. Discoveries relating to the function of these ubiquitin E3 ligases in regulating signalling from the eponymous member of the family, TNF-R1, are dealt with superbly in a separate review by Wertz and Dixit and so we will confine our discussion to the subset of the TNFRSF that does not contain a death domain (DD). In line with the available data we will divide the review into two parts, the first is restricted to the role of TRAFs 2 and 3 and cIAPs in regulating TNFRSF signalling, whereas the second will be more speculative, asking what role IAPs and TRAFs have in innate immune signalling.

Membrane tumor necrosis factor (TNF) induces p100 processing via TNF receptor-2 (TNFR2)

Tumor necrosis factor (TNF) elicits its biological activities by stimulation of two receptors, TNFR1 and TNFR2, both belonging to the TNF receptor superfamily. Whereas TNFR1-mediated signal transduction has been intensively studied and is understood in detail, especially with respect to activation of the classical NFkappaB pathway, cell death induction, and MAP kinase signaling, TNFR2-associated signal transduction is poorly defined. Here, we demonstrate in various tumor cell lines and primary T-cells that TNFR2, but not TNFR1, induces activation of the alternative NFkappaB pathway. In accord with earlier findings demonstrating that only membrane TNF, but not soluble TNF, properly activates TNFR2, we further show by use of TNFR1- and TNFR2-specific mutants of soluble TNF and membrane TNF that soluble ligand trimers fail to activate the alternative NFkappaB pathway. In accord with the known inhibitory role of TRAF2 in the alternative NFkappaB pathway, TNFR2-, but not TNFR1-specific TNF induced depletion of cytosolic TRAF2. Thus, we identified activation of the alternative NFkappaB pathway as a TNF signaling effect that can be specifically assigned to TNFR2 and membrane TNF.

Signalling crosstalk in B cells: managing worth and need

The B cell receptor (BCR) and the receptor for B cell-activating factor (BAFFR) have complementary roles in B cells: BCR signals provide a cell-intrinsic measure of suitability for negative or positive selection, whereas BAFFR responds to homeostatic demands based on a cell-extrinsic measure of the size of the mature B cell pool. Because continuous signals from both receptors are required for B cell survival, it is probable that there are mechanisms to integrate the selective and homeostatic signals from these receptors. In this Opinion article, I describe recent evidence to indicate that crosstalk between the downstream biochemical pathways of these receptors mediates this interdependence, such that BCR signals generate a limiting substrate for BAFFR signal propagation.

Constitutive non-canonical NFkappaB signaling in pancreatic cancer cells

Constitutive classical NFkappaB activation has been implicated in the development of pancreatic cancer, and inhibition of classical NFkappaB signaling sensitizes pancreatic cancer cells to apoptosis. However, the role of the more recently described non-canonical NFkappaB pathway has not been specifically addressed in pancreatic cancer. The non-canonical pathway requires stabilization of NIK and IKKalpha-dependent phosphorylation and processing of NFkappaB2/p100 to p52. This leads to the activation of p52-RelB heterodimers that regulate genes encoding lymphoid-specific chemokines and cytokines. We performed qRT-PCR to detect gene expression in a panel of pancreatic ductal adenocarcinoma cell lines (BxPC-3, PCA-2, PANC-1, Capan-1, Hs-766T, AsPC-1, MiaPACA-2) and found only modest elevation of classical NFkappaB-dependent genes. In contrast, each of the tumor cell lines displayed dramatically elevated levels of subsets of the non-canonical NFkappaB target genes CCL19, CCL21, CXCL12, CXCL13 and BAFF. Consistent with activation of the non-canonical pathway, p52 and RelB co-localized in adenocarcinoma cells in sections of pancreatic tumor tissue, and each of the tumor cell lines displayed elevated p52 levels. Furthermore, p52 and RelB co-immunoprecipitated from pancreatic cancer cells and immunoblotting revealed that NIK was stabilized and p100 was constitutively phosphorylated in a subset of the cell lines. Finally, stable overexpression of dominant negative IKKalpha significantly inhibited non-canonical target gene expression in BxPC-3 cells. These findings therefore demonstrate that the non-canonical NFkappaB pathway is constitutively active and functional in pancreatic cancer cells.

NEMO-binding domains of both IKKalpha and IKKbeta regulate IkappaB kinase complex assembly and classical NF-kappaB activation

Proinflammatory NF-kappaB activation requires the IkappaB (inhibitor of NF-kappaB) kinase (IKK) complex that contains two catalytic subunits named IKKalpha and IKKbeta and a regulatory subunit named NF-kappaB essential modulator (NEMO). NEMO and IKKbeta are essential for tumor necrosis factor (TNF)-induced NF-kappaB activation, and we recently demonstrated that NEMO and IKKalpha are sufficient for interleukin (IL)-1-induced signaling. IKKalpha and IKKbeta both contain a functional NEMO-binding domain (NBD); however, the role of NEMO association with each kinase in NF-kappaB signaling and IKK complex formation remains unclear. To address this question, we stably reconstituted IKKalpha(-/-) and IKKbeta(-/-) murine embryonic fibroblasts (MEFs) with wild-type (WT) or NBD-deficient (DeltaNBD) versions of IKKalpha and IKKbeta, respectively. TNF-induced classical NF-kappaB activation in IKKbeta(-/-) MEFs was rescued by IKKbeta(WT) but not IKKbeta(DeltaNBD), whereas neither IKKbeta(WT) nor IKKbeta(DeltaNBD) affected IL-1-induced NF-kappaB signaling. As previously described, classical NF-kappaB transcriptional activity was absent in IKKalpha(-/-) cells. Reconstitution with either IKKalpha(WT) or IKKalpha(DeltaNBD) rescued both IL-1 and TNF-induced transcription, demonstrating that NEMO association is not required for IKKalpha-dependent regulation of NF-kappaB-dependent transcription. Stably expressed IKKalpha(WT) or IKKbeta(WT) associated with endogenous IKKs and NEMO in IKKalpha(-/-) or IKKbeta(-/-) MEFs, respectively, resulting in formation of the heterotrimeric IKKalpha-IKKbeta-NEMO complex. In contrast, although the IKKalpha(DeltaNBD) and IKKbeta(DeltaNBD) mutants associated with endogenous IKKs containing an NBD, these dimeric endogenous IKK-IKK(DeltaNBD) complexes did not associate with NEMO. These findings therefore demonstrate that formation of the heterotrimeric IKKalpha-IKKbeta-NEMO holocomplex absolutely requires two intact NEMO-binding domains.

Periodic, partial inhibition of IkappaB Kinase beta-mediated signaling yields therapeutic benefit in preclinical models of rheumatoid arthritis

We have previously shown that inhibitors of IkappaB kinase beta (IKKbeta), including 4(2'-aminoethyl)amino-1,8-dimethylimidazo(1,2-a)quinoxaline (BMS-345541), are efficacious against experimental arthritis in rodents. In our efforts to identify an analog as a clinical candidate for the treatment of autoimmune and inflammatory disorders, we have discovered the potent and highly selective IKKbeta inhibitor 2-methoxy-N-((6-(1-methyl-4-(methylamino)-1,6-dihydroimidazo[4,5-d]pyrrolo[2,3-b]pyridin-7-yl)pyridin-2-yl)methyl)acetamide (BMS-066). Investigations of its pharmacology in rodent models of experimental arthritis showed that BMS-066 at doses of 5 and 10 mg/kg once daily was effective at protecting rats against adjuvant-induced arthritis, despite showing only weak inhibition at 10 mg/kg against a pharmacodymanic model of tumor necrosis factor alpha production in rats challenged with lipopolysaccharide. The duration of exposure in rats indicated that just 6 to 9 h of coverage per day of the concentration necessary to inhibit IKKbeta by 50% in vivo was necessary for protection against arthritis. Similar findings were observed in the mouse collagen-induced arthritis model, with efficacy observed at a dose providing only 6 h of coverage per day of the concentration necessary to inhibit IKKbeta by 50%. This finding probably results from the cumulative effect on multiple cellular mechanisms that contribute to autoimmunity and joint destruction, because BMS-066 was shown to inhibit a broad spectrum of activities such as T cell proliferation, B cell function, cytokine and interleukin secretion from monocytes, T(H)17 cell function and regulation, and osteoclastogenesis. Thus, only partial and transient inhibition of IKKbeta is sufficient to yield dramatic benefit in vivo, and this understanding will be important in the clinical development of IKKbeta inhibitors.

Regulation and function of NF-kappaB transcription factors in the immune system

The mammalian Rel/NF-kappaB family of transcription factors, including RelA, c-Rel, RelB, NF-kappaB1 (p50 and its precursor p105), and NF-kappaB2 (p52 and its precursor p100), plays a central role in the immune system by regulating several processes ranging from the development and survival of lymphocytes and lymphoid organs to the control of immune responses and malignant transformation. The five members of the NF-kappaB family are normally kept inactive in the cytoplasm by interaction with inhibitors called IkappaBs or the unprocessed forms of NF-kappaB1 and NF-kappaB2. A wide variety of signals emanating from antigen receptors, pattern-recognition receptors, receptors for the members of TNF and IL-1 cytokine families, and others induce differential activation of NF-kappaB heterodimers. Although work over the past two decades has shed significant light on the regulation of NF-kappaB transcription factors and their functions, much progress has been made in the past two years revealing new insights into the regulation and functions of NF-kappaB. This recent progress is covered in this review.

Mutations of multiple genes cause deregulation of NF-kappaB in diffuse large B-cell lymphoma

Diffuse large B-cell lymphoma (DLBCL), the most common form of lymphoma in adulthood, comprises multiple biologically and clinically distinct subtypes including germinal centre B-cell-like (GCB) and activated B-cell-like (ABC) DLBCL. Gene expression profile studies have shown that its most aggressive subtype, ABC-DLBCL, is associated with constitutive activation of the NF-kappaB transcription complex. However, except for a small fraction of cases, it remains unclear whether NF-kappaB activation in these tumours represents an intrinsic program of the tumour cell of origin or a pathogenetic event. Here we show that >50% of ABC-DLBCL and a smaller fraction of GCB-DLBCL carry somatic mutations in multiple genes, including negative (TNFAIP3, also called A20) and positive (CARD11, TRAF2, TRAF5, MAP3K7 (TAK1) and TNFRSF11A (RANK)) regulators of NF-kappaB. Of these, the A20 gene, which encodes a ubiquitin-modifying enzyme involved in termination of NF-kappaB responses, is most commonly affected, with approximately 30% of patients displaying biallelic inactivation by mutations and/or deletions. When reintroduced in cell lines carrying biallelic inactivation of the gene, A20 induced apoptosis and cell growth arrest, indicating a tumour suppressor role. Less frequently, missense mutations of TRAF2 and CARD11 produce molecules with significantly enhanced ability to activate NF-kappaB. Thus, our results demonstrate that NF-kappaB activation in DLBCL is caused by genetic lesions affecting multiple genes, the loss or activation of which may promote lymphomagenesis by leading to abnormally prolonged NF-kappaB responses.

Development of an insect-cell-based assay for detection of kinase inhibition using NF-kappaB-inducing kinase as a paradigm

Identification of small-molecule inhibitors by high-throughput screening necessitates the development of robust, reproducible and cost-effective assays. The assay approach adopted may utilize isolated proteins or whole cells containing the target of interest. To enable protein-based assays, the baculovirus expression system is commonly used for generation and isolation of recombinant proteins. We have applied the baculovirus system into a cell-based assay format using NIK [NF-kappaB (nuclear factor kappaB)-inducing kinase] as a paradigm. We illustrate the use of the insect-cell-based assay in monitoring the activity of NIK against its physiological downstream substrate IkappaB (inhibitor of NF-kappaB) kinase-1. The assay was robust, yielding a signal/background ratio of 2:1 and an average Z' value of >0.65 when used to screen a focused compound set. Using secondary assays to validate a selection of the hits, we identified a compound that (i) was non-cytotoxic, (ii) interacted directly with NIK, and (iii) inhibited lymphotoxin-induced NF-kappaB p52 translocation to the nucleus. The insect cell assay represents a novel approach to monitoring kinase inhibition, with major advantages over other cell-based systems including ease of use, amenability to scale-up, protein expression levels and the flexibility to express a number of proteins by infecting with numerous baculoviruses.

The IkappaB kinase complex: master regulator of NF-kappaB signaling

The Nuclear Factor-kappa B (NF-kappaB) family of transcription factors regulates the expression of a wide range of genes critical for immune and inflammatory responses, cell survival, immune development, and cell proliferation. Dysregulated NF-kappaB activity occurs in a number of chronic inflammatory diseases and certain types of cancers making NF-kappaB signaling an attractive target for the development of anti-inflammatory and anti-cancer drugs. A pivotal regulator of all inducible NF-kappaB signaling pathways is the IkappaB kinase (IKK) complex that consists of two kinases (IKKalpha and IKKbeta) and a regulatory subunit named NF-kappaB essential modulator (NEMO). Genetic analysis of the IKK complex has identified two separate pathways named the classical and non-canonical mechanisms that are dependent on either NEMO and IKKbeta (classical) or IKKalpha alone (non-canonical). To better understand the mechanisms that regulate IKK complex activity and to address the differential functions of IKKalpha and IKKbeta we have molecularly dissected the IKKs. We describe here how these studies have identified a unique inhibitor of pro-inflammatory NF-kappaB signaling, an unforeseen role for IKKalpha in the classical NF-kappaB pathway, and a novel functional domain in IKKbeta that is not present in IKKalpha.

NF-kappaB p50 plays distinct roles in the establishment and control of murine gammaherpesvirus 68 latency

NF-kappaB signaling is critical to the survival and transformation of cells infected by the human gammaherpesviruses Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus. Here we have examined how elimination of the NF-kappaB transcription factor p50 from mice affects the life cycle of murine gammaherpesvirus 68 (MHV68). Notably, mice lacking p50 in every cell type were unable to establish a sufficiently robust immune response to control MHV68 infection, leading to high levels of latently infected B cells detected in the spleen and persistent virus replication in the lungs. The latter correlated with very low levels of virus-specific immunoglobulin G (IgG) in the infected p50(-/-) mice at day 48 postinfection. Because the confounding impact of the loss of p50 on the host response to MHV68 infection prevented a direct analysis of the role of this NF-kappaB family member on MHV68 latency in B cells, we generated and infected mixed p50(+/+)/p50(-/-) bone marrow chimeric mice. We show that the chimeric mice were able to control acute virus replication and exhibited normal levels of virus-specific IgG at 3 months postinfection, indicating the induction of a normal host immune response to MHV68 infection. However, in p50(+/+)/p50(-/-) chimeric mice the p50(-/-) B cells exhibited a significant defect compared to p50(+/+) B cells in supporting MHV68 latency. In addition to identifying a role for p50 in the establishment of latency, we determined that the absence of p50 in a subset of the hematopoietic compartment led to persistent virus replication in the lungs of the chimeric mice, providing evidence that p50 is required for controlling virus reactivation. Taken together, these data demonstrate that p50 is required for immune control by the host and has distinct tissue-dependent roles in the regulation of murine gammaherpesvirus latency during chronic infection.

TRAF2 and TRAF3 independently mediate Ig class switching driven by CD40

The isotype switch defect in CD40(-/-) mice is corrected by wild-type (WT) CD40 transgene, but not by a mutant CD40 transgene that does not bind tumor necrosis factor receptor-associated factors (TRAF) 2 and 3. To define the individual roles of TRAF2 and TRAF3 in CD40 activation of B cells, we introduced mutant CD40 transgenes that selectively lack the ability to bind TRAF2 (DeltaTR2), TRAF3 (DeltaTR3) or both (DeltaTR2,3) into B cells of CD40(-/-) mice. Serum IgG1 and IgE levels, IgG1 antibody response to sub-optimal doses of the T cell-dependent antigen keyhole limpet hemocyanin, germinal center formation, CD40-mediated proliferation, isotype switching and activation of the non-canonical NF-kappaB pathway were partially diminished in DeltaTR2 and DeltaTR3 mice and virtually absent in DeltaTR2,3 mice. These results suggest that TRAF2 and TRAF3 can each independently mediate class switch recombination (CSR) driven by CD40, but both are required for optimal CD40-driven isotype switching.

Matrix Metalloproteinase-9 gene induction by a truncated oncogenic NF-kappaB2 protein involves the recruitment of MLL1 and MLL2 H3K4 histone methyltransferase complexes

Constitutive nuclear factor (NF)-kappaB activation in haematological malignancies is caused in several cases by loss of function mutations within the coding sequence of NF-kappaB inhibitory molecules such as IkappaBalpha or p100. Hut-78, a truncated form of p100, constitutively generates p52 and contributes to the development of T-cell lymphomas but the molecular mechanism underlying this oncogenic potential remains unclear. We show here that MMP9 gene expression is induced through the alternative NF-kappaB-activating pathway in fibroblasts and also on Hut-78 or p52 overexpression in fibroblasts as well as in lymphoma cells. p52 is critical for Hut-78-mediated MMP9 gene induction as a Hut-78 mutant as well as other truncated NF-kappaB2 proteins that are not processed into p52 failed to induce the expression of this metalloproteinase. Conversely, MMP9 gene expression is impaired in p52-depleted HUT-78 cells. Interestingly, MLL1 and MLL2 H3K4 methyltransferase complexes are tethered by p52 on the MMP9 but not on the IkappaBalpha promoter, and the H3K4 trimethyltransferase activity recruited on the MMP9 promoter is impaired in p52-depleted HUT-78 cells. Moreover, MLL1 and MLL2 are associated with Hut-78 in a native chromatin-enriched extract. Thus, we identified a molecular mechanism by which the recruitment of a H3K4 histone methyltransferase complex on the promoter of a NF-kappaB-dependent gene induces its expression and potentially the invasive potential of lymphoma cells harbouring constitutive activity of the alternative NF-kappaB-activating pathway.

Bioinformatic analysis reveals cRel as a regulator of a subset of interferon-stimulated genes

Interferons (IFNs) are critical to the host innate immune response by inducing the expression of a family of early response genes, denoted as IFN-stimulated genes (ISGs). The role of tyrosine phosphorylation of STAT proteins in the transcription activation of ISGs is well-documented. Recent studies have indicated that other transcription factors (TFs) are likely to play a role in regulating ISG expression. Here, we describe a novel integrative approach that combines gene expression profiling, promoter sequence analysis, and literature mining to screen candidate regulatory factors in the IFN signal transduction pathway. Application of this method identified the nuclear factor kappaB (NFkappaB) protein, cRel, as a candidate regulatory factor for a subset of ISGs in mouse embryo fibroblasts. Chromatin immunoprecipitation (ChIP) and real-time PCR assays confirmed that cRel directly binds to the promoters of several ISGs, including Cxcl10, Isg15, Gbp2, Ifit3, and Ifi203, and regulates their expression. Thus, our studies identify cRel as an important TF for ISGs, and validate the approach of using Latent Semantic Indexing (LSI)-based methods to identify regulatory factors from microarray data.

Is NF-kappaB a good target for cancer therapy? Hopes and pitfalls

Nuclear factor kappaB (NF-kappaB) transcription factors have a key role in many physiological processes such as innate and adaptive immune responses, cell proliferation, cell death, and inflammation. It has become clear that aberrant regulation of NF-kappaB and the signalling pathways that control its activity are involved in cancer development and progression, as well as in resistance to chemotherapy and radiotherapy. This article discusses recent evidence from cancer genetics and cancer genome studies that support the involvement of NF-kappaB in human cancer, particularly in multiple myeloma. The therapeutic potential and benefit of targeting NF-kappaB in cancer, and the possible complications and pitfalls of such an approach, are explored.

Compensatory IKKalpha activation of classical NF-kappaB signaling during IKKbeta inhibition identified by an RNA interference sensitization screen

A subtype of diffuse large B-cell lymphoma (DLBCL), termed activated B-cell-like (ABC) DLBCL, depends on constitutive nuclear factor-kappaB (NF-kappaB) signaling for survival. Small molecule inhibitors of IkappaB kinase beta (IKKbeta), a key regulator of the NF-kappaB pathway, kill ABC DLBCL cells and hold promise for the treatment of this lymphoma type. We conducted an RNA interference genetic screen to investigate potential mechanisms of resistance of ABC DLBCL cells to IKKbeta inhibitors. We screened a library of small hairpin RNAs (shRNAs) targeting 500 protein kinases for shRNAs that would increase the killing of an ABC DLBCL cell line in the presence of a small molecule IKKbeta inhibitor. Two independent shRNAs targeting IKKalpha synergized with the IKKbeta inhibitor to kill three different ABC DLBCL cell lines but were not toxic by themselves. Surprisingly, IKKalpha shRNAs blocked the classical rather than the alternative NF-kappaB pathway in ABC DLBCL cells, as judged by inhibition of IkappaBalpha phosphorylation. IKKalpha shRNA toxicity was reversed by coexpression of wild-type but not kinase inactive forms of IKKalpha, suggesting that IKKalpha may directly phosphorylate IkappaBalpha under conditions of IKKbeta inhibition. In models of physiologic NF-kappaB pathway activation by CARD11 or tumor necrosis factor-alpha, compensatory IKKalpha activity was also observed with IKKbeta inhibition. These results suggest that therapy for ABC DLBCL may be improved by targeting both IKKalpha and IKKbeta, possibly through CARD11 inhibition.

HIV-1 Tat protein induces IL-10 production in monocytes by classical and alternative NF-kappaB pathways

The human immunodeficiency virus (HIV) transactivating Tat protein is not only critical for viral replication but also affects the host immune system by inducing the production of cytokines such as IL-10. This anti-inflammatory cytokine is upregulated during the course of HIV infection, representing an important pathway by which HIV may induce immunodeficiency. Here, we show that, by acting at the membrane, Tat induces IL-10 expression in primary monocytes and promonocytic U937 cells by NF-kappaB-dependent pathways. The trans-dominant negative mutants of NF-kappaB-inducing kinase (NIK), IKKalpha and IKKbeta expressed in our transactivation model, in accordance with the nuclear binding of p65 and p52 NF-kappaB subunits to the IL-10 promoter, suggest the involvement of both classical and alternative NF-kappaB pathways. In inactivated cells, IKKalpha is localized predominantly in the cytoplasm. Interestingly, Tat stimulates IKKalpha translocation from the cytoplasm to the nucleus in monocytes. Chromatin immunoprecipitation (ChIP) assay experiments, after Tat treatment, revealed IKKalpha and CBP/p300 recruitment to the IL-10 promoter and histone H3 phosphorylation (Ser 10) and acetylation (Lys 14) in this region, presumably leading to chromatin remodeling. We demonstrate that, upstream of NF-kappaB, PKC, ERK1/2 and p38 MAP kinases are involved in Tat-induced IKKalpha nuclear translocation and histone H3 modifications on the IL-10 promoter in accordance with the role of these three kinases in IL-10 production. As a whole, the study demonstrates that Tat activates at least three signaling pathways concurrently, including the classical, alternative and IKKalpha pathways, to promote production of IL-10.

Nonredundant and complementary functions of TRAF2 and TRAF3 in a ubiquitination cascade that activates NIK-dependent alternative NF-kappaB signaling

The adaptor and signaling proteins TRAF2, TRAF3, cIAP1 and cIAP2 may inhibit alternative nuclear factor-kappaB (NF-kappaB) signaling in resting cells by targeting NF-kappaB-inducing kinase (NIK) for ubiquitin-dependent degradation, thus preventing processing of the NF-kappaB2 precursor protein p100 to release p52. However, the respective functions of TRAF2 and TRAF3 in NIK degradation and activation of alternative NF-kappaB signaling have remained elusive. We now show that CD40 or BAFF receptor activation result in TRAF3 degradation in a cIAP1-cIAP2- and TRAF2-dependent way owing to enhanced cIAP1, cIAP2 TRAF3-directed ubiquitin ligase activity. Receptor-induced activation of cIAP1 and cIAP2 correlated with their K63-linked ubiquitination by TRAF2. Degradation of TRAF3 prevented association of NIK with the cIAP1-cIAP2-TRAF2 ubiquitin ligase complex, which resulted in NIK stabilization and NF-kappaB2-p100 processing. Constitutive activation of this pathway causes perinatal lethality and lymphoid defects.

NF-kappaB2/p52 enhances androgen-independent growth of human LNCaP cells via protection from apoptotic cell death and cell cycle arrest induced by androgen-deprivation

PURPOSE

Androgen-deprivation therapy only causes a temporary regression of prostate cancer, as all tumors will eventually progress to refractory to hormonal therapy after 1-3 years of treatment. The underlying mechanisms of prostate cancer androgen refractory progression are incompletely understood. In this study, we employed in vitro as well as in vivo models to examine the role of NF-kappaB2/p52 in prostate cancer growth and androgen independent progression.

EXPERIMENTAL DESIGN

The effects of NF-kappaB2/p52 on cell growth, androgen responsiveness, cell cycle and apoptosis were examined in androgen sensitive LNCaP cells. The effect of NF-kappaB2/p52 on tumor growth was examined in intact and castrated male mice.

RESULTS

Overexpression of NF-kappaB2/p52 enhances androgen-sensitive LNCaP human prostate cancer cell growth and clonogenic ability in androgen-deprived condition in vitro. NF-kappaB2/p52 induced androgen-independent growth is through protecting LNCaP cells from apoptotic cell death and cell cycle arrest induced by androgen-deprivation. In addition, NF-kappaB2/p52 stimulates Cyclin D1 expression and knock down of Cyclin D1 expression by siRNA abolished NF-kappaB2/p52-induced cell growth in vitro. Adenoviral mediated NF-kappaB2/p52 expression in LNCaP cells enhances tumor growth in intact male nude mice and induces tumor growth in castrated male nude mice, suggesting that overexpression of NF-kappaB2/p52 induces androgen-independent growth of androgen-sensitive LNCaP cells.

CONCLUSIONS

Overexpression of NF-kappaB2/p52 protects androgen sensitive LNCaP cells from apoptotic cell death and cell cycle arrest induced by androgen-deprivation. NF-kappaB2/p52 activation induces androgen-independent growth in vitro and in vivo.

Noncanonical NF-kappaB activation requires coordinated assembly of a regulatory complex of the adaptors cIAP1, cIAP2, TRAF2 and TRAF3 and the kinase NIK

Recent studies suggest that nuclear factor kappaB-inducing kinase (NIK) is suppressed through constitutive proteasome-mediated degradation regulated by TRAF2, TRAF3 and cIAP1 or cIAP2. Here we demonstrated that the degradation of NIK occurs upon assembly of a regulatory complex through TRAF3 recruitment of NIK and TRAF2 recruitment of cIAP1 and cIAP2. In contrast to TRAF2 and TRAF3, cIAP1 and cIAP2 seem to play redundant roles in the degradation of NIK, as inhibition of both cIAPs was required for noncanonical NF-kappaB activation and increased survival and proliferation of primary B lymphocytes. Furthermore, the lethality of TRAF3 deficiency in mice could be rescued by a single NIK gene, highlighting the importance of tightly regulated NIK.

Cell penetrating peptide inhibitors of nuclear factor-kappa B

The nuclear factor kappa B (NF-kappaB) transcription factors are activated by a range of stimuli including pro-inflammatory cytokines. Active NF-kappaB regulates the expression of genes involved in inflammation and cell survival and aberrant NF-kappaB activity plays pathological roles in certain types of cancer and diseases characterized by chronic inflammation. NF-kappaB signaling is an attractive target for the development of novel anti-inflammatory or anti-cancer drugs and we discuss here how the method of peptide transduction has been used to specifically target NF-kappaB. Peptide transduction relies on the ability of certain small cell-penetrating peptides (CPPs) to enter cells, and a panel of CPP-linked inhibitors (CPP-Is) has been developed to directly inhibit NF-kappaB signaling. Remarkably, several of these NF-kappaB-targeting CPP-Is are effective in vivo and therefore offer exciting potential in the clinical setting.

The IKK-neutralizing compound Bay11 kills supereffector CD8 T cells by altering caspase-dependent activation-induced cell death

Antigen with dual costimulation through CD137 and CD134 induces powerful CD8 T cell responses. These effector T cells are endowed with an intrinsic survival program resulting in their accumulation in vivo, but the signaling components required for survival are unknown. We tested a cadre of pathway inhibitors and found one preclinical compound, Bay11-7082 (Bay11), which prevented survival. Even the gammac cytokine family members IL-2, -4, -7, and -15 could not block death, nor could pretreatment with IL-7. We found that dual costimulation caused loading of phosphorylated IkappaBalpha (p-IkappaBalpha) and high basal levels of NF-kappaB activity in the effector CD8 T cells. Bay11 trumped both events by reducing the presence of p-IkappaBalpha and ensuing NF-kappaB activity. Not all pathways were impacted to this degree, however, as mitogen-mediated ERK phosphorylation was evident during NF-kappaB inhibition. Nonetheless, Bay11 blocked TCR-stimulated cytokine synthesis by rapidly accentuating activation-induced cell death through elicitation of a caspase-independent pathway. Thus, in effector CD8 T cells, Bay11 forces a dominant caspase-independent death signal that cannot be overcome by an intrinsic survival program nor by survival-inducing cytokines. Therefore, Bay11 may be a useful tool to deliberately kill death-resistant effector T cells for therapeutic benefit.

Lymphotoxin-alpha 1 beta 2 and LIGHT induce classical and noncanonical NF-kappa B-dependent proinflammatory gene expression in vascular endothelial cells

Activation of the classical and noncanonical NF-kappaB pathways by ligation of the lymphotoxin (LT)-beta receptor (LTbetaR) plays a crucial role in lymphoid organogenesis and in the generation of ectopic lymphoid tissue at sites of chronic inflammation. Within these microenvironments, LTbetaR signaling regulates the phenotype of the specialized high endothelial cells. However, the direct effects of LTbetaR ligation on endothelial cells remain unclear. We therefore questioned whether LTbetaR ligation could directly activate endothelial cells and regulate classical and noncanonical NF-kappaB-dependent gene expression. We demonstrate that the LTbetaR ligands LIGHT and LTalpha1beta2 activate both NF-kappaB pathways in HUVECs and human dermal microvascular endothelial cells (HDMEC). Classical pathway activation was less robust than TNF-induced signaling; however, only LIGHT and LTalpha1beta2 and not TNF activated the noncanonical pathway. LIGHT and LTalpha1beta2 induced the expression of classical NF-kappaB-dependent genes in HUVEC, including those encoding the adhesion molecules E-selectin, ICAM-1, and VCAM-1. Consistent with this stimulation, LTbetaR ligation up-regulated T cell adhesion to HUVEC. Furthermore, the homeostatic chemokine CXCL12 was up-regulated by LIGHT and LTalpha1beta2 but not TNF in both HUVEC and HDMEC. Using HUVEC retrovirally transduced with dominant negative IkappaB kinase alpha, we demonstrate that CXCL12 expression is regulated by the noncanonical pathway in endothelial cells. Our findings therefore demonstrate that LTbetaR ligation regulates gene expression in endothelial cells via both NF-kappaB pathways and we identify CXCL12 as a bona fide noncanonical NF-kappaB-regulated gene in these cells.

Dendritic cells require the NF-kappaB2 pathway for cross-presentation of soluble antigens

NF-kappaB-inducing kinase (NIK) is responsible for activation of the non-canonical p100 processing pathway of NF-kappaB activation. This kinase has been shown to be critical for activation of this pathway after signaling through several TNF family members including CD40. The functional importance of this pathway in CD40 and TLR-induced dendritic cell (DC) differentiation was studied in vivo in the alymphoplasia (Aly) mouse. The Aly mouse expresses a mutant NIK molecule that prohibits the induction of the non-canonical pathway. We show that while MHC class II presentation and in vivo migration of Aly DCs is intact, these cells are unable to cross-prime CD8+ T cells to exogenous Ag. Gene expression array analysis of DCs matured in vivo indicates multiple defects in Ag processing pathways after maturation and provide a global view of the genes that are regulated by the NF-kappaB2 pathway in DCs. These experiments indicate a possible role for NIK in mediating cross-priming of soluble Ag. In addition, our findings explain the profound immune unresponsiveness of the Aly mouse.

SUMO1 modification of NF-kappaB2/p100 is essential for stimuli-induced p100 phosphorylation and processing

A primary step in activating the alternative nuclear factor-kappaB (NF-kappaB) pathway requires NF-kappaB2/p100 processing to generate p52. In most cases, stimuli-induced p100 processing is dependent on NF-kappaB-inducing kinase/IkappaB kinase alpha-mediated phosphorylation and ubiquitination. Here, we report that post-translational modification of p100 at specific sites by the small ubiquitin-like modifier (SUMO) is another determining factor for stimuli-induced p100 processing. The results show that basal SUMO modification is required for stimuli-induced p100 phosphorylation and that blocking SUMOylation of p100, either by site-directed mutation or by short interfering RNA-targeted diminution of E2 SUMO-conjugating enzyme Ubc9, inhibits various physiological stimuli-induced p100 processing and ultimate activation of the alternative NF-kappaB pathway. Together, these findings show the crucial role of SUMO1 modification in p100 processing and provide mechanistic insights into the participation of SUMO1 modification in the regulation of signal transduction.

ChlaDub1 of Chlamydia trachomatis suppresses NF-kappaB activation and inhibits IkappaBalpha ubiquitination and degradation

Chlamydia trachomatis is an obligate intracellular bacterial pathogen that causes various human diseases, including blindness caused by ocular infection and sexually transmitted diseases resulting from urogenital infection. After infecting host cells, Chlamydiae avoid alarming the host's immune system. Among the immune evasion mechanisms, Chlamydiae can inhibit NF-kappaB activation, a crucial pathway for host inflammatory responses. In this study, we show that ChlaDub1, a deubiquitinating and deNeddylating protease from C. trachomatis, is expressed in infected cells. In transfection experiments, ChlaDub1 suppresses NF-kappaB activation induced by several pro-inflammatory stimuli and binds the NF-kappaB inhibitory subunit IkappaBalpha, impairing its ubiquitination and degradation. Thus, we provide further insight into the mechanism by which C. trachomatis may evade the host inflammatory response by demonstrating that ChlaDub1, a protease produced by this microorganism, is capable of inhibiting IkappaBalpha degradation and blocking NF-kappaB activation.

Lack of nuclear factor-kappa B2/p100 causes a RelB-dependent block in early B lymphopoiesis

Nuclear factor-kappaB (NF-kappaB) transcription factors regulate B-cell development and survival. However, whether they also have a role during early steps of B-cell differentiation is largely unclear. Here, we show that constitutive activation of the alternative NF-kappaB pathway in p100(-/-) knockin mice resulted in a block of early B-cell development at the transition from the pre-pro-B to the pro-B-cell stage due to enhanced RelB activity. Expression of the essential B-cell transcription factors EBF and in particular Pax5 was reduced in p100(-/-) B-cell precursors in a RelB-dependent manner, resulting in reduced mRNA levels of B lineage-specific genes. Moreover, enhanced RelB function in p100(-/-) B-cell precursors was accompanied by increased expression of B lineage-inappropriate genes, such as C/EBP alpha, correlating with a markedly increased myeloid differentiation potential of p100(-/-) progenitor B cells. Ectopic expression of Pax5 in hematopoietic progenitors restored early B-cell development in p100(-/-) bone marrow, suggesting that impaired early B lymphopoiesis in mice lacking the p100 inhibitor may be due to down-regulation of Pax5 expression. Thus, tightly controlled p100 processing and RelB activation is essential for normal B lymphopoiesis and lymphoid/myeloid lineage decision in bone marrow.

Constitutive CD40 signaling in B cells selectively activates the noncanonical NF-kappaB pathway and promotes lymphomagenesis

CD40, a member of the tumor necrosis factor (TNF) receptor family, plays an essential role in T cell–dependent immune responses. Because CD40 is widely expressed on the surface of tumor cells in various B cell malignancies, deregulated CD40 signaling has been suggested to contribute to lymphomagenesis. In this study, we show that B cell-specific expression of a constitutively active CD40 receptor, in the form of a latent membrane protein 1 (LMP1)/CD40 chimeric protein, promoted an increase in the number of follicular and marginal zone B cells in secondary lymphoid organs in transgenic mice. The B cells displayed an activated phenotype, prolonged survival and increased proliferation, but were significantly impaired in T cell-dependent immune responses. Constitutive CD40 signaling in B cells induced selective and constitutive activation of the noncanonical NF-κB pathway and the mitogen-activated protein kinases Jnk and extracellular signal–regulated kinase. LMP1/CD40-expressing mice older than 12 mo developed B cell lymphomas of mono- or oligoclonal origin at high incidence, thus showing that the interplay of the signaling pathways induced by constitutive CD40 signaling is sufficient to initiate a tumorigenic process, ultimately leading to the development of B cell lymphomas.

Overexpressed NF-kappaB-inducing kinase contributes to the tumorigenesis of adult T-cell leukemia and Hodgkin Reed-Sternberg cells

The nuclear factor-kappaB (NF-kappaB) transcription factors play important roles in cancer development by preventing apoptosis and facilitating the tumor cell growth. However, the precise mechanisms by which NF-kappaB is constitutively activated in specific cancer cells remain largely unknown. In our current study, we now report that NF-kappaB-inducing kinase (NIK) is overexpressed at the pretranslational level in adult T-cell leukemia (ATL) and Hodgkin Reed-Sternberg cells (H-RS) that do not express viral regulatory proteins. The overexpression of NIK causes cell transformation in rat fibroblasts, which is abolished by a super-repressor form of IkappaBalpha. Notably, depletion of NIK in ATL cells by RNA interference reduces the DNA-binding activity of NF-kappaB and NF-kappaB-dependent transcriptional activity, and efficiently suppresses tumor growth in NOD/SCID/gammac(null) mice. These results indicate that the deregulated expression of NIK plays a critical role in constitutive NF-kappaB activation in ATL and H-RS cells, and suggest also that NIK is an attractive molecular target for cancer therapy.

Inflammation and cancer: role of nuclear factor-kappaB activation

It has been thought that there is a strong relationship between inflammation and carcinogenesis so that the development of cancer occurs with chronic inflammation in many organs. An in-depth understanding of the mechanism by which inflammation can lead to carcinogenesis may enable the development of drugs targeted at important molecules, providing a powerful tool for preventing cancer development. In this review, we focused on a signal transduction system, the nuclear factor-kappaB (NF-kappaB) pathway, which is thought to play a role in the process leading from inflammation to carcinogenesis, and may thus serve as a candidate for targeted intervention.