TRAF2 phosphorylation modulates tumor necrosis factor alpha-induced gene expression and cell resistance to apoptosis

TRAF2 is an adaptor protein that regulates the activation of the c-Jun N-terminal kinase (JNK) and IkappaB kinase (IKK) signaling cascades in response to tumor necrosis factor alpha (TNF-alpha) stimulation. Although the downstream events in TNF-alpha signaling are better understood, the membrane-proximal events are still elusive. Here, we demonstrate that TNF-alpha and cellular stresses induce TRAF2 phosphorylation at serine 11 and that this phosphorylation is required for the expression of a subset of NF-kappaB target genes. Although TRAF2 phosphorylation had a minimal effect on the TNF-alpha-induced rapid and transient IKK activation, it was essential for secondary and prolonged IKK activation. Consistent with this, TRAF2 phosphorylation is not required for its recruitment to the TNFR1 complex in response to TNF-alpha stimulation but is required for its association with a cytoplasmic complex containing RIP1 and IKK. In addition, TRAF2 phosphorylation was essential for the full TNF-alpha-induced activation of JNK. Notably, TRAF2 phosphorylation increased both basal and inducible c-Jun and NF-kappaB activities and rendered cells resistant to stress-induced apoptosis. Moreover, TRAF2 was found to be constitutively phosphorylated in some lymphomas. These results unveil a new, finely tuned mechanism for TNF-alpha-induced IKK activation modulated by TRAF2 phosphorylation and suggest that TRAF2 phosphorylation contributes to elevated levels of basal NF-kappaB activity in certain human cancers.     

Autocrine tumor necrosis factor alpha links endoplasmic reticulum stress to the membrane death receptor pathway through IRE1alpha-mediated NF-kappaB activation and down-regulation of TRAF2 expression

NF-kappaB is critical for determining cellular sensitivity to apoptotic stimuli by regulating both mitochondrial and death receptor apoptotic pathways. The endoplasmic reticulum (ER) emerges as a new apoptotic signaling initiator. However, the mechanism by which ER stress activates NF-kappaB and its role in regulation of ER stress-induced cell death are largely unclear. Here, we report that, in response to ER stress, IKK forms a complex with IRE1alpha through the adapter protein TRAF2. ER stress-induced NF-kappaB activation is impaired in IRE1alpha knockdown cells and IRE1alpha(-/-) MEFs. We found, however, that inhibiting NF-kappaB significantly decreased ER stress-induced cell death in a caspase-8-dependent manner. Gene expression analysis revealed that ER stress-induced expression of tumor necrosis factor alpha (TNF-alpha) was IRE1alpha and NF-kappaB dependent. Blocking TNF receptor 1 signaling significantly inhibited ER stress-induced cell death. Further studies suggest that ER stress induces down-regulation of TRAF2 expression, which impairs TNF-alpha-induced activation of NF-kappaB and c-Jun N-terminal kinase and turns TNF-alpha from a weak to a powerful apoptosis inducer. Thus, ER stress induces two signals, namely TNF-alpha induction and TRAF2 down-regulation. They work in concert to amplify ER-initiated apoptotic signaling through the membrane death receptor.     

High-fat diet induces increased tissue expression of TNF-alpha

In several strains of genetically obese and insulin resistant rodents, adipose tissue over expresses mRNA for tumor necrosis factor alpha (TNF-alpha). Our purpose was to determine whether tissue expression of TNF-alpha protein is elevated in rats that are made obese and insulin resistant by administration of a high-fat diet. Young Wistar rats weighing approximately 50 g were fed for 39 days with either normal rat chow (12.4% fat) or a high-fat diet (50% fat). After 33 days, glucose tolerance was assessed and after 39 days, insulin-stimulated transport of [3H]-2-deoxyglucose was assessed in isolated strips of soleus muscle. Rats on the high-fat diet consumed slightly fewer calories but became obese, displaying significant approximately 2-fold increases in the mass of both visceral and subcutaneous fat depots. High-fat feeding also caused a moderate degree of insulin resistance. Fasting serum insulin was significantly increased, as were insulin and glucose concentrations following glucose loading. In isolated strips of soleus muscle, the high-fat diet produced a trend toward a 33% decrease in the insulin-stimulated component of glucose transport (p=0.064). Western analysis of muscle, liver and fat revealed two forms of TNF-alpha, a soluble 17 Kd form (sTNF-alpha) and a 26 Kd membrane form (mTNF-alpha). Both sTNF-alpha and mTNF-alpha were relatively abundant in fat; whereas sTNF-alpha was the predominant form present in muscle and liver. High-fat feeding caused a significant 2-fold increase in muscle sTNF-alpha, along with a trend toward a 54% increase in visceral fat sTNF-alpha (p=0.055). TNF-alpha was undetectable in serum. We conclude that muscle over expression of TNF-alpha occurs during the development of diet-induced obesity and may, in part cause insulin resistance by an autocrine mechanism.     

Astaxanthin inhibits nitric oxide production and inflammatory gene expression by suppressing I(kappa)B kinase-dependent NF-kappaB activation

Astaxanthin, a carotenoid without vitamin A activity, has shown anti-oxidant and anti-inflammatory activities; however, its molecular action and mechanism have not been elucidated. We examined in vitro and in vivo regulatory function of astaxanthin on production of nitric oxide (NO) and prostaglandin E2 (PGE2) as well as expression of inducible NO synthase (iNOS), cyclooxygenase-2, tumor necrosis factor-alpha (TNF-alpha), and interleukin-1beta (IL-1beta). Astaxanthin inhibited the expression or formation production of these proinflammatory mediators and cytokines in both lipopolysaccharide (LPS)-stimulated RAW264.7 cells and primary macrophages. Astaxanthin also suppressed the serum levels of NO, PGE2, TNF-alpha, and IL-1beta in LPS-administrated mice, and inhibited NF-kappaB activation as well as iNOS promoter activity in RAW264.7 cells stimulated with LPS. This compound directly inhibited the intracellular accumulation of reactive oxygen species in LPS-stimulated RAW264.7 cells as well as H2O2-induced NF-kappaB activation and iNOS expression. Moreover, astaxanthin blocked nuclear translocation of NF-kappaB p65 subunit and I(kappa)B(alpha) degradation, which correlated with its inhibitory effect on I(kappa)B kinase (IKK) activity. These results suggest that astaxanthin, probably due to its antioxidant activity, inhibits the production of inflammatory mediators by blocking NF-kappaB activation and as a consequent suppression of IKK activity and I(kappa)B-alpha degradation.     

TWEAK induces NF-kappaB2 p100 processing and long lasting NF-kappaB activation

Tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) is a member of the TNF superfamily that has been shown to induce angiogenesis, apoptosis in tumor cells, and NF-kappaB activation through binding to its receptor, fibroblast growth factor-inducible 14. We have identified TWEAK as an inducer of constitutive NF-kappaB activation by expression cloning, and we report here sequential regulation by TWEAK of two separate signaling cascades for NF-kappaB activation, the NF-kappaB essential modulator-dependent and -independent signaling pathways. Upon TWEAK stimulation, IkappaBalpha is rapidly phosphorylated, generating NF-kappaB DNA-binding complexes containing p50 and RelA in a manner dependent on the canonical IkappaB kinase complex. Unlike TNF-alpha, TWEAK stimulation results in prolonged NF-kappaB activation with a transition of the DNA-binding NF-kappaB components from RelA- to RelB-containing complexes by 8 h, and the latter remained active in binding at least until 24 h post-stimulation. This long lasting activation is accompanied by the proteasome-mediated processing of NF-kappaB2/p100, which does not depend on the NF-kappaB essential modulator but requires IkappaB kinase 1 and functional NF-kappaB-inducing kinase activity. Finally, we show that fibroblast growth factor-inducible 14 with a mutation at its TNF receptor-associated factor (TRAF)-binding site cannot activate NF-kappaB and that TWEAK fails to induce the p100 processing and IkappaBalpha phosphorylation in cells deficient for TRAF2 and TRAF5. Our results thus identify TWEAK as a novel physiological regulator of the non-canonical pathway for NF-kappaB activation.     

Inflammatory cardiac valvulitis in TAX1BP1-deficient mice through selective NF-kappaB activation

Nuclear factor kappa B (NF-kappaB) is a key mediator of inflammation. Unchecked NF-kappaB signalling can engender autoimmune pathologies and cancers. Here, we show that Tax1-binding protein 1 (TAX1BP1) is a negative regulator of TNF-alpha- and IL-1beta-induced NF-kappaB activation and that binding to mono- and polyubiquitin by a ubiquitin-binding Zn finger domain in TAX1BP1 is needed for TRAF6 association and NF-kappaB inhibition. Mice genetically knocked out for TAX1BP1 are born normal, but develop age-dependent inflammatory cardiac valvulitis, die prematurely, and are hypersensitive to low doses of TNF-alpha and IL-1beta. TAX1BP1-/- cells are more highly activated for NF-kappaB than control cells when stimulated with TNF-alpha or IL-1beta. Mechanistically, TAX1BP1 acts in NF-kappaB signalling as an essential adaptor between A20 and its targets.     

Pretreatment with PKC inhibitor triggers TNF-alpha induced apoptosis in TNF-alpha-resistant B16 melanoma BL6 cells

Tumor necrosis factor alpha (TNF-alpha) modulates various events through several different pathways. Many tumor cells are resistant to this cytokine. Pretreatment of these cells with actinomycin D enhances TNF-alpha-induced apoptosis. In the present study, we investigated the mechanism of this enhancement and whether or not the apoptosis of TNF-alpha-resistant cancer cells can be induced by the inhibition of Protein kinase C (PKC). When TNF-alpha was added after inhibition of PKC by H7, apoptosis was observed, and companied with the activation of nuclear factor kappa B (NF-kappaB). After the inhibition of protein kinase B (Akt) by LY294002 or p38 mitogen-activated protein kinase (p38MAPK) by SB203580, the addition of TNF-alpha did not cause apoptosis. However, after the inhibition of MAPK/extracellular signal-regulated kinase kinase 1/2 (MEK1/2) with U0126, apoptosis was observed when TNF-alpha was added. In the Western blotting analysis, phosphorylation of MEK1/2 occurred at 60 minutes after the addition of TNF-alpha. However, it was noted that after pretreatment with H7, a significant decrease in phosphorylated MEK1/2 was observed. The present findings suggest that MEK1/2 plays an important role in TNF-alpha-resistance in TNF-alpha-resistant B16 melanoma BL6 cells. Furthermore, it was found that MEK1/2 is more important than NF-kappaB, Akt, and p38MAPK in anti-apoptotic PKC signaling and that TNF-alpha-resistance can be overcome by inhibiting MEK1/2. These results suggest the possibility of development of a new anticancer drug treatment.     

FLASH coordinates NF-kappa B activity via TRAF2

FLASH is a protein recently shown to interact with the death effector domain of caspase-8 and is likely to be a component of the death-inducing signaling complex in receptor-mediated apoptosis. Here we show that antisense oligonucleotide-induced inhibition of FLASH expression abolished TNF-alpha-induced activation of NF-kappaB in HEK293 cells, as determined by luciferase reporter gene expression driven by a NF-kappaB responsive promoter. Conversely, overexpression of FLASH dose-dependently activated NF-kappaB, an effect suppressed by dominant negative mutants of TRAF2, NIK, and IKKalpha, and partially by those of TRAF5 and TRAF6. TRAF2 was co-immunoprecipitated with FLASH from the cell extracts of HEK293 cells or HeLa cells stably expressing exogenous FLASH (HeLa/HA-FLASH). Furthermore, serial deletion mapping demonstrated that a domain spanning the residues 856-1191 of FLASH activated NF-kappaB as efficiently as the full-length and could directly bind to TRAF2 in vitro and in the transfected cells. Taken together, these results suggest that FLASH coordinates downstream NF-kappaB activity via a TRAF2-dependent pathway in the TNF-alpha signaling.     

Failure of measles virus to activate nuclear factor-kappa B in neuronal cells: implications on the immune response to viral infections in the central nervous system

Neurons are postmitotic cells that foster virus persistence. These cells lack the HLA class I molecules required for clearance of infected cells. Previously, we showed that HLA class I is induced by measles virus (MV) on glial cells, which is primarily mediated by IFN-beta. In contrast, MV was unable to induce HLA class I or IFN-beta in neuronal cells. This failure was associated with lack of NF-kappa B binding to the positive regulatory domain II element of the IFN-beta promoter, which is essential for virus-induced IFN-beta gene activity. In this study, we demonstrate that the failure to activate NF-kappa B in neuronal cells is due to the inability of MV to induce phosphorylation and degradation of I kappa B, the inhibitor of NF-kappa B. In contrast, TNF-alpha induced degradation of I kappa B alpha in the neuronal cells, suggesting that failure to induce I kappa B alpha degradation is likely due to a defect in virus-mediated signaling rather than to a defect involving neuronal I kappa B alpha. Like MV, mumps virus and dsRNA failed to induce I kappa B alpha degradation in the neuronal cells, suggesting that this defect may be specific to viruses. Autophosphorylation of the dsRNA-dependent protein kinase, a kinase possibly involved in virus-mediated I kappa B alpha phosphorylation, was intact in both cell types. The failure of virus to induce I kappa B alpha phosphorylation and consequently to activate NF-kappa B in neuronal cells could explain the repression of IFN-beta and class I gene expression in virus-infected cells. These findings provide a potential mechanism for the ability of virus to persist in neurons and to escape immune surveillance.     

NF-kappaB activation by double-stranded-RNA-activated protein kinase (PKR) is mediated through NF-kappaB-inducing kinase and IkappaB kinase

The interferon (IFN)-inducible double-stranded-RNA (dsRNA)-activated serine-threonine protein kinase (PKR) is a major mediator of the antiviral and antiproliferative activities of IFNs. PKR has been implicated in different stress-induced signaling pathways including dsRNA signaling to nuclear factor kappa B (NF-kappaB). The mechanism by which PKR mediates activation of NF-kappaB is unknown. Here we show that in response to poly(rI). poly(rC) (pIC), PKR activates IkappaB kinase (IKK), leading to the degradation of the inhibitors IkappaBalpha and IkappaBbeta and the concomitant release of NF-kappaB. The results of kinetic studies revealed that pIC induced a slow and prolonged activation of IKK, which was preceded by PKR activation. In PKR null cell lines, pIC failed to stimulate IKK activity compared to cells from an isogenic background wild type for PKR in accord with the inability of PKR null cells to induce NF-kappaB in response to pIC. Moreover, PKR was required to establish a sustained response to tumor necrosis factor alpha (TNF-alpha) and to potentiate activation of NF-kappaB by cotreatment with TNF-alpha and IFN-gamma. By coimmunoprecipitation, PKR was shown to be physically associated with the IKK complex. Transient expression of a dominant negative mutant of IKKbeta or the NF-kappaB-inducing kinase (NIK) inhibited pIC-induced gene expression from an NF-kappaB-dependent reporter construct. Taken together, these results demonstrate that PKR-dependent dsRNA induction of NF-kappaB is mediated by NIK and IKK activation.     

NF-kappaB activation but not PI3K/Akt is required for dexamethasone dependent protection against TNF-alpha cytotoxicity in L929 cells

Tumor necrosis factor alpha (TNF-alpha) is one of the best-described cell death promoters. In murine L929 fibroblasts, dexamethasone inhibits TNF-alpha-induced cytotoxicity. Since phosphatidyl inositol 3 kinase (PI3K) and nuclear factor kappa B (NF-kappaB) proteins regulate several survival pathways, we evaluated their participation in dexamethasone protection against TNF-alpha cell death. We interfered with these pathways by overexpressing a negative dominant mutant of PI3K or a non-degradable mutant of inhibitor of NF-kappaB alpha (IkappaBalpha) (the cytoplasmic inhibitor of NF-kappaB) in L929 cells. The mutant IkappaB, but not the mutant PI3K, abrogated dexamethasone-mediated protection. The loss of dexamethasone protection was associated with a diminished accumulation in XIAP and c-IAP proteins.