Activation and regulation of the IkappaB kinase in human B cells by CD40 signaling

Characterizing Serpinb2 as a Modulator of TCDD-Induced Suppression of the B Cell

2,3,7,8-Tetrachlordibenzo- p-dioxin (TCDD) is an environmental pollutant that can cause various toxic effects, including chloracne, metabolic syndrome, and immune suppression. Most of the toxicity associated with TCDD is mediated through activation of the aryl hydrocarbon receptor (AHR). Recent research has suggested the presence of a wide-range of interindividual variability in TCDD-mediated suppression of the Immunoglobulin-M (IgM) response across the human population. In an attempt to identify putative modifiers of AHR-mediated immunosuppression beyond the AHR, B cells were isolated from a panel of genetically diverse mouse strain to scan for modulators that drive interstrain differences in TCDD-mediated suppression of the IgM response. Results implicated a region of mouse Chromosome 1 near a gene encoding serine peptidase inhibitor, clade B, member 2 ( Serpinb2) whose human ortholog is plasminogen activator inhibitor 2 (PAI2). Further downstream analyses indicated that Serpinb2 is dysregulated by TCDD and, furthermore, that B cells from Serpinb2 ^-/- mice are significantly more sensitive to TCDD-mediated suppression as compared to littermate controls. This study suggests a protective role of Serpinb2 within TCDD-mediated immunosuppression and, furthermore, a novel function of Serpinb2-related activity in the IgM response.

The ligand-bound thyroid hormone receptor in macrophages ameliorates kidney injury via inhibition of nuclear factor-κB activities

In chronic kidney disease (CKD) patients, inflammation plays a pivotal role in the progression of renal fibrosis. Hypothyroidism is associated with an increased occurrence of atherosclerosis and inflammation, suggesting protective roles of thyroid hormones and their receptors against inflammatory processes. The contribution of thyroid hormone receptors to macrophage differentiation has not been well documented. Here, we focused on the endogenous thyroid hormone receptor α (TRα) in macrophages and examined the role of ligand-bound TRα in macrophage polarization-mediated anti-inflammatory effects. TRα-deficient irradiated chimeric mice showed exacerbated tubulointerstitial injury in a unilateral ureteral obstruction model. Compared with wild-type macrophages, macrophages isolated from the obstructed kidneys of mice lacking TRα displayed increased expression of proinflammatory cytokines that was accompanied by enhanced nuclear translocation of p65. Comparison of TRα-deficient bone marrow-derived macrophages with wild-type macrophages confirmed the propensity of the former cells to produce excessive IL-1β levels. Co-culture of these macrophages with renal epithelial cells induced more severe damage to the epithelial cells via the IL-1 receptor. Our findings indicate that ligand-bound TRα on macrophages plays a protective role in kidney inflammation through the inhibition of NF-κB pathways, possibly by affecting the pro- and anti-inflammatory balance that controls the development of CKD.

Interplay of Murine Gammaherpesvirus 68 with NF-kappaB Signaling of the Host

Herpesviruses establish a chronic infection in the host characterized by intervals of lytic replication, quiescent latency, and reactivation from latency. Murine gammaherpesvirus 68 (MHV68) naturally infects small rodents and has genetic and biologic parallels with the human gammaherpesviruses (gHVs), Kaposi's sarcoma-associated herpesvirus and Epstein-Barr virus. The murine gammaherpesvirus model pathogen system provides a platform to apply cutting-edge approaches to dissect the interplay of gammaherpesvirus and host determinants that enable colonization of the host, and that shape the latent or lytic fate of an infected cell. This knowledge is critical for the development of novel therapeutic interventions against the oncogenic gHVs. The nuclear factor kappa B (NF-κB) signaling pathway is well-known for its role in the promotion of inflammation and many aspects of B cell biology. Here, we review key aspects of the virus lifecycle in the host, with an emphasis on the route that the virus takes to gain access to the B cell latency reservoir. We highlight how the murine gammaherpesvirus requires components of the NF-κB signaling pathway to promote replication, latency establishment, and maintenance of latency. These studies emphasize the complexity of gammaherpesvirus interactions with NF-κB signaling components that direct innate and adaptive immune responses of the host. Importantly, multiple facets of NF-κB signaling have been identified that might be targeted to reduce the burden of gammaherpesvirus-associated diseases.

Dual specificity phosphatase 1 expression inversely correlates with NF-κB activity and expression in prostate cancer and promotes apoptosis through a p38 MAPK dependent mechanism

Dual specificity phosphatase 1 (DUSP1) and the transcription factor NF-κB are implicated in prostate cancer since their expression levels are altered along this disease, although there are no evidences up to date demonstrating a crosstalk between them. In this report, we show for the first time that DUSP1 over-expression in DU145 cells promotes apoptosis and decreases NF-κB activity by blocking p65/NF-κB nuclear translocation. Moreover, although DUSP1 impairs TNF-α-induced p38 MAPK and JNK activation, only the specific inhibition of p38 MAPK exerts the same effects than DUSP1 over-expression on both apoptosis and NF-κB activity. Consistently, DUSP1 promotes apoptosis and decreases NF-κB activity in cells in which p38 MAPK is induced by TNF-α treatment. These results demonstrate that p38 MAPK is specifically involved in DUSP1-mediated effects on both apoptosis and NF-κB activity. Interestingly, we show an inverse correlation between DUSP1 expression and activation of both p65/NF-κB and p38 MAPK in human prostate tissue specimens. Thus, most of apparently normal glands, benign prostatic hyperplasia and low-grade prostatic intraepithelial neoplasia samples show high DUSP1 expression and low levels of both nuclear p65/NF-κB and activated p38 MAPK. By contrast, DUSP1 expression levels are low or even absent in high-grade prostatic intraepithelial neoplasia and prostatic adenocarcinoma samples, whereas nuclear p65/NF-κB and activated p38 MAPK are highly expressed in the same samples. Overall, our results provide evidence for a role of DUSP1 in the apoptosis of prostate cancer cells, through a mechanism involving the inhibition of p38 MAPK and NF-κB. Furthermore, our findings suggest that the ratio between DUSP1 and p65/NF-κB expression levels, rather than the individual expression of both molecules, is a better marker for diagnostic purposes in prostate cancer.

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.

Naïve and memory B cells in the rhesus macaque can be differentiated by surface expression of CD27 and have differential responses to CD40 ligation

The rhesus macaque (RM) model has the potential to be an invaluable tool for studying B cell populations during pathogenic infections, however, to date, there has been no definitive delineation of naïve and memory B cell populations in the RM. This has precluded a rigorous analysis of the generation, persistence and resolution of a pathogen-specific memory B cell response. The present study utilized multiple analyses to demonstrate that CD27 expression on B cells is consistent with a memory phenotype. Compared to CD20+CD27- B cells, CD20+CD27+ B cells were larger in size, and preferentially accumulated at effector sites. Direct sequence analysis revealed that CD20+CD27+ B cells had an increased frequency of point mutations that were consistent with somatic hypermutation and at a functional level, CD40 ligation improved CD20+CD27- but not CD20+CD27+ B cell survival in vitro. These data provide definitive evidence that the naïve and memory B cell populations of the RM can be differentiated using surface expression of CD27.

Thyroid hormone antagonizes tumor necrosis factor-alpha signaling in pituitary cells through the induction of dual specificity phosphatase 1

Pituitary function has been shown to be regulated by an increasing number of factors, including cytokines and hormones, such as TNFalpha and T(3). Both the proinflammatory cytokine TNFalpha and T(3) have been suggested to be involved in the maintenance of tissue homeostasis in the anterior pituitary gland. In this report we show that T(3) negatively interferes with MAPK p38 and nuclear factor-kappaB (NF-kappaB) activation by TNFalpha in GH4C1 cells. Our data demonstrate that MAPK p38 is specifically activated upon exposure to TNFalpha and that T(3) abolishes this activation in a time-dependent manner by a mechanism that involves the induction of the MAPK phosphatase, DUSP1. Our data show that the pool of up-regulated DUSP1 by T(3) is mainly localized to the cytosol, and that TNFalpha does not affect this localization. On the other hand, we show that T(3) impairs the activation of the NF-kappaB pathway induced by TNFalpha, producing a significant decrease in NF-kappaB-dependent transcription, phosphorylation of IkappaBalpha, translocation of p65/NF-kappaB to the nucleus, and p65/NF-kappaB transactivation potential. Interestingly, the overexpression of DUSP1 inhibits the NF-kappaB activation achieved by either TNFalpha or ectopic expression of the upstream inducer of MAPK p38. Conversely, DUSP1 depletion abrogates the inhibitory effect of T(3) on the induction of NF-kappaB-dependent transcription by TNFalpha. Overall, our results indicate that T(3) antagonizes TNFalpha signaling in rat pituitary tumor cells through the induction of DUSP1.

Thyroid hormone-mediated activation of the ERK/dual specificity phosphatase 1 pathway augments the apoptosis of GH4C1 cells by down-regulating nuclear factor-kappaB activity

Thyroid hormone (T3) plays a crucial role in processes such as cell proliferation and differentiation, whereas its implication on cellular apoptosis has not been well documented. Here we examined the effect of T3 on the apoptosis of GH4C1 pituitary cells and the mechanisms underlying this effect. We show that T3 produced a significant increase in apoptosis in serum-depleted conditions. This effect was accompanied by a decrease in nuclear factor-kappaB (NF-kappaB)-dependent transcription, IkappaBalpha phosphorylation, translocation of p65/NF-kappaB to the nucleus, phosphorylation, and transactivation. Moreover, these effects were correlated with a T3-induced decrease in the expression of antiapoptotic gene products, such as members of the inhibitor of apoptosis protein and Bcl-2 families. On the other hand, ERK but not c-Jun N-terminal kinase or MAPK p38, was activated upon exposure to T3, and inhibition of ERK alone abrogated T3-mediated apoptosis. In addition, T3 increased the expression of the MAPK phosphatase, dual specificity phosphatase 1 (DUSP1), in an ERK-dependent manner. Interestingly, the suppression of DUSP1 expression abrogated T3-induced inhibition of NF-kappaB-dependent transcription and p65/NF-kappaB translocation to the nucleus, as well as T3-mediated apoptosis. Overall, our results indicate that T3 induces apoptosis in rat pituitary tumor cells by down-regulating NF-kappaB activity through a mechanism dependent on the ERK/DUSP1 pathway.

Nifedipine inhibited angiotensin II-induced monocyte chemoattractant protein 1 expression: involvement of inhibitor of nuclear factor kappa B kinase and nuclear factor kappa B-inducing kinase

OBJECTIVE

The effect of nifedipine, a 1,4-dihydropyridine calcium antagonist, on the expression of monocyte chemoattractant protein 1 (MCP-1) induced by angiotensin II (Ang II) was examined using vascular smooth muscle cells (VSMC) isolated from rat thoracic aorta.

METHODS AND RESULTS

Ang II increased the expression of MCP-1 messenger RNA accompanied by an increase in nuclear factor kappa B (NF-kappaB) binding activity to the cis DNA element in the promoter region of MCP-1. Ang II also decreased the cytosolic level of the inhibitor of NF-kappaB (IkappaB) and increased the phosphorylation of IkappaB subunits, IkappaBalpha and IkappaBbeta, as well as the phosphorylation of IkappaB kinase (IKK) subunits, IKKalpha and IKKbeta, suggesting that Ang II enhanced the breakdown of IkappaB. Nifedipine decreased MCP-1 mRNA expression, together with NF-kappaB binding activity to the promoter region of MCP-1 induced by Ang II. Nifedipine also attenuated the decrease in the cytosolic level of IkappaB, and the phosphorylation of IkappaB and IKK subunits induced by Ang II. Moreover, Ang II increased the phosphorylation of NF-kappaB-inducing kinase (NIK), and this increase was significantly inhibited by nifedipine.

CONCLUSION

As NIK is reported to activate IKK, our results suggest that nifedipine attenuates the effect of Ang II on MCP-1 expression in VSMC by regulating the activity of NF-kappaB through NIK, IKK and IkappaB.

TRAF-dependent association of protein kinase Tpl2/COT1 (MAP3K8) with CD40

Signaling by TNF-family receptor CD40 involves TRAF-family adaptor proteins, leading to activation of protein kinases that induce NFkappaB-family transcription factors. We report here that mitogen activated protein kinase kinase kinase-8 (MAP3K8), Tpl2/COT1, is recruited to the CD40 complex via a mechanism dependent on TRAF-binding sites in CD40. Tpl2/COT1 was shown to participate in CD40 signaling based on the ability of a catalytically inactive mutant to suppress CD40-mediated IkappaB kinase activation and induction of NFkappaB-responsive promoters, without affecting signaling by TNF. Tpl2 (-/-) fibroblasts were also deficient in CD40 but not TNF signaling, further supporting a unique role for Tpl2 in CD40 signaling. Experiments using dominant-negative Tpl2 suggest this kinase functions distal to TRAFs but proximal to the TAK1/TAB1 signaling complex, within the IKK/NFkappaB activation pathway. These results indicate a distinction between TNF Receptor family members CD40 and TNFR1 in their utilization of MAP3Ks, and demonstrate TRAF-dependence of Tpl2 association with the CD40 receptor complex.

Nuclear transcription factor-kappaB in Hodgkin's disease

Nuclear factor-kappaB (NF-kappaB) is a transcriptional factor that was originally discovered in the nucleus of B cells that bind to the kappa light chain of the immunoglobulins. Research during 15 years, however, has revealed that NF-kappaB is present in its inactive state in the cytoplasm of almost every cell type. When activated, NF-kappaB translocates to the nucleus, binds the DNA and regulates the expression of over 200 different genes. The product of these genes regulate the immune system, cell proliferation, tumor metastasis, inflammation and viral replication. Several tumor cell types express constitutively activated form of NF-kappaB and it is required for the proliferation of the tumor cells. Numerous studies have shown that Hodgkin's disease cells exhibit constitutive active NF-kappaB. The present review examines the mechanism how NF-kappaB is activated and its relevance to Hodgkin's disease.

CD40 regulates the processing of NF-kappaB2 p100 to p52

The nf-kb2 gene encodes the cytoplasmic NF-kappaB inhibitory protein p100 from which the active p52 NF-kappaB subunit is derived by proteasome-mediated proteolysis. Ligands which stimulate p100 processing to p52 have not been defined. Here, ligation of CD40 on transfected 293 cells is shown to trigger p52 production by stimulating p100 ubiquitylation and subsequent proteasome-mediated proteolysis. CD40-mediated p52 accumulation is dependent on de novo protein synthesis and triggers p52 translocation into the nucleus to generate active NF-kappaB dimers. Endogenous CD40 ligation on primary murine splenic B cells also stimulates p100 processing, which results in the delayed nuclear translocation of p52-RelB dimers. In both 293 cells and primary splenic B cells, the ability of CD40 to trigger p100 processing requires functional NF-kappaB-inducing kinase (NIK). In contrast, NIK activity is not required for CD40 to stimulate the degradation of IkappaBalpha in either cell type. The regulation of p100 processing by CD40 is likely to be important for the transcriptional regulation of CD40 target genes in adaptive immune responses.

Cellular responses to murine CD40 in a mouse B cell line may be TRAF dependent or independent

Engagement of CD40 by its ligand induces IKK and mitogen-activated protein kinase (MAPK) phosphorylation and transcriptional activation, leading to activation and differentiation of B cells. These events are most likely transduced by adaptor molecules that are recruited to the CD40 cytoplasmic domain, called TNF receptor-associated factors (TRAF). We have engineered a chimeric CD40 molecule using the human extracellular sequence and the murine cytoplasmic domain to assess the contribution that specific TRAF binding domains provide to the cytoplasmic signaling functions of CD40. The data presented here show that the shared binding site for TRAF2 and TRAF3 accounts for receptor internalization, and the majority of signaling through CD40, but is redundant with the TRAF6 binding site for activation of p38 and NFkappaB signaling pathways. Disruption of the TRAF2/3 binding site results in a delayed and diminished kinase pathway induction, but complete preclusion of all signals requires the disruption of more than the two known TRAF binding sites. The specific TRAF dependency of CD40-induced growth arrest, TNF-alpha production, and phosphorylation of signaling molecules are shown, while p38 MAPK activation and cell surface antigen modulation suggest TRAF independent CD40 signaling in B cells.

CD40 activates NF-kappa B and c-Jun N-terminal kinase and enhances chemokine secretion on activated human hepatic stellate cells

Activated hepatic stellate cells (HSCs) are the main producers of extracellular matrix in the fibrotic liver and contribute to hepatic inflammation through the secretion of chemokines and the recruitment of leukocytes. This study assesses the function of CD40 on human HSCS: Activated human HSCs express CD40 in culture and in fibrotic liver, as determined by flow cytometry, RT-PCR, and immunohistochemistry. CD40 expression is strongly enhanced by IFN-gamma. Stimulation of CD40 with CD40 ligand (CD40L)-transfected baby hamster kidney cells induces NF-kappaB, as demonstrated by the activation of I-kappaB kinase (IKK), increased NF-kappaB DNA binding, and p65 nuclear translocation. CD40-activated IKK also phosphorylates a GST-p65 substrate at serine 536 in the transactivation domain 1. Concomitant with the activation of IKK, CD40L-transfected baby hamster kidney cell treatment strongly activates c-Jun N-terminal kinase. CD40 activation increases the secretion of IL-8 and monocyte chemoattractant protein-1 by HSCs 10- and 2-fold, respectively. Adenovirally delivered dominant negative (dn) IKK2 and TNFR-associated factor 2dn inhibit IKK-mediated GST-I-kappaB and GST-p65 phosphorylation, NF-kappaB binding, and IL-8 secretion, whereas IKK1dn and NF-kappaB-inducing kinase dominant negative do not have inhibitory effects. We conclude that the CD40-CD40L receptor-ligand pair is involved in a cross-talk between HSCs and immune effector cells that contributes to the perpetuation of HSC activation in liver fibrosis through TNFR-associated factor 2- and IKK2-dependent pathways.

Lack of CD40-dependent B-cell proliferation in B lymphocytes isolated from patients with persistent polyclonal B-cell lymphocytosis

Persistent B-cell lymphocytosis (PPBL) is a haematological disorder diagnosed primarily in adult female smokers that is characterized by a polyclonal increase in peripheral blood B lymphocytes and a moderate elevation of serum IgM. B lymphocyte-associated cellular abnormalities, such as the occurrence of multi-lobed nuclei, increased bcl2/Ig gene rearrangements and the identification of an extra long-arm chromosome (i3)(q10) in the B-cell population, indicate that PPBL could be part of a multi-step process leading to the emergence of a malignant B lymphoproliferation. However, the resulting impact on cellular functional properties remains to be elucidated. Our goal was to address that aspect via the study of B-cell activity following stimulation through CD40, a key molecule of the tumour necrosis factor receptor superfamily involved in B lymphocyte development. In contrast to normal B cells, PPBL B lymphocytes were unable to respond to the proliferative signal delivered in vitro by CD40, indicating a defect in the CD40 activation pathway. Polymerase chain reaction amplification and sequencing of the receptor as well as FACScan analysis of patient B lymphocytes dismissed the possibility of a defect in either CD40 structure or expression. Moreover, Western blot analysis of tyrosine phosphorylation, an early event in the CD40-signalling cascade, was similar in patients and controls, leading to the conclusion that the defect affecting B lymphocytes in PPBL patients is probably located downstream of that signalling cascade.

Inhibition of the nuclear factor kappa B (NF-kappa B) pathway by tetracyclic kaurene diterpenes in macrophages. Specific effects on NF-kappa B-inducing kinase activity and on the coordinate activation of ERK and p38 MAPK

The anti-inflammatory action of most terpenes has been explained in terms of the inhibition of nuclear factor kappaB (NF-kappaB) activity. Ent-kaurene diterpenes are intermediates of the synthesis of gibberellins and inhibit the expression of NO synthase-2 and the release of tumor necrosis factor-alpha in J774 macrophages challenged with lipopolysaccharide. These diterpenes inhibit NF-kappaB and IkappaB kinase (IKK) activation in vivo but failed to affect in vitro the function of NF-kappaB, the phosphorylation and targeting of IkappaBalpha, and the activity of IKK-2. Transient expression of NF-kappaB-inducing kinase (NIK) activated the IKK complex and NF-kappaB, a process that was inhibited by kaurenes, indicating that the inhibition of NIK was one of the targets of these diterpenes. These results show that kaurenes impair the inflammatory signaling by inhibiting NIK, a member of the MAPK kinase superfamily that interacts with tumor necrosis factor receptor-associated factors, and mediate the activation of NF-kappaB by these receptors. Moreover, kaurenes delayed the phosphorylation of p38, ERK1, and ERK2 MAPKs, but not that of JNK, in response to lipopolysaccharide treatment of J774 cells. The absence of a coordinate activation of MAPK and IKK might contribute to a deficient activation of NF-kappaB that is involved in the anti-inflammatory activity of these molecules.

Inhibition of IgG1 and IgE production by stimulation of the B cell CTLA-4 receptor

Although a large amount of information is available on the activity of CTLA-4 in T cells, the role of this receptor in B cells has not been previously characterized. Our results show that CD40 or LPS stimulation in the presence of IL-4 induces CTLA-4 expression in purified B cells; the maximum level is reached in both membrane and intracellular compartments after 48-72 h. Engagement of the B cell CTLA-4 by immobilized mAb inhibits IgG1 and IgE production and reduces the frequency of IgG1- and IgE-expressing B cells. Cepsilon and Cgamma(1) germline mRNA expression as well as NF-kappaB and STAT6 activation, events required for isotype switching, are also inhibited by CTLA-4 engagement. Together these findings show the critical role of CTLA-4 in the control of IL-4-driven isotype switching and suggest new approaches for modulating immediate-type hypersensitivity responses.

Identification and characterization of two CD40-inducible enhancers in the mouse TRAF1 gene locus

We have shown that CD40 engagement induces TRAF1 gene expression in B lymphocytes. Here we report that CD40-dependent TRAF1 gene transcription in murine B cells is controlled by two enhancer regions. One region is located approximately 2 kb upstream of the transcription start site and the other lies in the intron between exons 5 and 6. The upstream enhancer contains a single NF-kappaB site in addition to sites that bind constitutive transcription factors. Mutation of this NF-kappaB site completely abrogates CD40-driven TRAFl transcription. The intronic enhancer contains two sites that strongly bind the CD40-inducible factors NF-kappaB and AP-1. Simultaneous mutation of the AP-1 site and of the NF-kappaB site abolishes transcription driven by this enhancer. When cloned together into reporter constructs, the two TRAF1 enhancers do not synergize, suggesting that each enhancer may separately participate in the induction of TRAF1 transcription in B cells following CD40 activation.

Anti-CD40, anti-CD3, and IL-2 stimulation induce contrasting changes in CB1 mRNA expression in mouse splenocytes

The expression and function of cannabinoid receptor 1 (CB1) in mouse immune cells is unclear. Here we show that splenic B cells express more CB1 mRNA than T cells. Furthermore, splenocytes stimulated with the T cell mitogens, PMA/Io and anti-CD3, showed a decrease in CB1 message while cultures stimulated with the B cell mitogen, anti-CD40 antibody, showed an increase in message. In addition, co-treatment with mitogens and IL-2 uniformly caused an increase in CB1 mRNA. It is suggested that signaling pathways activated by T cell mitogens lead to decreased CB1 gene activation while pathways activated by B cell mitogens and IL-2 lead to increased CB1.

Bruton's tyrosine kinase links the B cell receptor to nuclear factor kappaB activation

The recognition of antigen by membrane immunoglobulin M (mIgM) results in a complex series of signaling events in the cytoplasm leading to gene activation. Bruton's tyrosine kinase (BTK), a member of the Tec family of tyrosine kinases, is essential for the full repertoire of IgM signals to be transduced. We examined the ability of BTK to regulate the nuclear factor (NF)-kappaB/Rel family of transcription factors, as the activation of these factors is required for a B cell response to mIgM. We found greatly diminished IgM- but not CD40-mediated NF-kappaB/Rel nuclear translocation and DNA binding in B cells from X-linked immunodeficient (xid) mice that harbor an R28C mutation in btk, a mutation that produces a functionally inactive kinase. The defect was due, in part, to a failure to fully degrade the inhibitory protein of NF-kappaB, IkappaBalpha. Using a BTK-deficient variant of DT40 chicken B cells, we found that expression of wild-type or gain-of-function mutant BTK, but not the R28C mutant, reconstituted NF-kappaB activity. Thus, BTK is essential for activation of NF-kappaB via the B cell receptor.

Arsenic inhibits NF-kappaB-mediated gene transcription by blocking IkappaB kinase activity and IkappaBalpha phosphorylation and degradation

The inflammatory cytokine, TNF-alpha, induces IL-8 gene transcription via a mechanism involving proteasome-mediated IkappaBalpha degradation and NF-kappaB activation. Here, we investigated whether arsenic, which has been shown to inhibit the ubiquitin-proteasome pathway, could inhibit TNF-alpha-mediated increases in IL-8 expression. Using RT-PCR, we show that the addition of TNF-alpha to human bronchial epithelial (BEAS 2B) or embryonic kidney (HEK293) cells resulted in increased steady-state levels of IL-8 mRNA. This was preceded by a rapid decrease in cellular IkappaBalpha levels, as demonstrated by Western analysis, and an increase in nuclear levels of NF-kappaB, as demonstrated by gel shift analysis. Further demonstrating the activation of NF-kappaB, TNF-alpha induced the transcription of a NF-kappaB-dependent reporter gene. Exposing the cells to 500 microM arsenite, prior to adding TNF-alpha, completely inhibited IkappaBalpha degradation, NF-kappaB translocation, NF-kappaB-dependent gene transcription, and transcription of the endogenous gene for IL-8. In comparison with the proteasome inhibitor MG-132, which does not affect the phosphorylation and ubiquitination of IkappaBalpha, arsenite inhibited the phosphorylation of IkappaBalpha. Furthermore, arsenite directly blocked the activity of IKK, the kinase responsible for IkappaBalpha phosphorylation. These studies demonstrate that high levels of arsenic may inhibit NF-kappaB-mediated gene transcription by specifically blocking IKK activity, thereby limiting the phosphorylation and subsequent degradation of the NF-kappaB inhibitor, IkappaBalpha.