TRAF2 protects against cerebral ischemia-induced brain injury by suppressing necroptosis.

Necroptosis contributes to ischemia-induced brain injury. Tumor necrosis factor (TNF) receptor associated factor 2 (TRAF2) has been reported to suppress necroptotic cell death under several pathological conditions. In this study, we investigated the role of TRAF2 in experimental stroke using a mouse middle cerebral artery occlusion (MCAO) model and in vitro cellular models. TRAF2 expression in the ischemic brain was assessed with western blot and real-time RT-PCR. Gene knockdown of TRAF2 by lentivirus was utilized to investigate the role of TRAF2 in stroke outcomes. The expression of TRAF2 was significantly induced in the ischemic brain at 24 h after reperfusion, and neurons and microglia were two of the cellular sources of TRAF2 induction. Striatal knockdown of TRAF2 increased infarction size, cell death, microglial activation and the expression of pro-inflammatory markers at 24 h after reperfusion. TRAF2 expression and necroptosis were induced in mouse primary microglia treated with conditioned medium collected from neurons subject to oxygen and glucose deprivation (OGD) and in TNFα-treated mouse hippocampal neuronal HT-22 cells in the presence of the pan-caspase inhibitor Z-VAD. In addition, TRAF2 knockdown exacerbated microglial cell death and neuronal cell death under these conditions. Moreover, pre-treatment with a specific necroptosis inhibitor necrostatin-1 (nec-1) suppressed the cell death exacerbated by TRAF2 knockdown in the brain following MCAO, indicating that TRAF2 impacted ischemic brain damage through necroptosis mechanism. Taken together, our results demonstrate that TRAF2 is a novel regulator of cerebral ischemic injury.

Allantopyrone A interferes with multiple components of the TNF receptor 1 complex and blocks RIP1 modifications in the TNF-α-induced signaling pathway.

Allantopyrone A is a fungal metabolite that uniquely possesses two α,ß-unsaturated carbonyl moieties. We recently reported that allantopyrone A inhibited the nuclear factor-κB (NF-κB) signaling pathway induced by tumor necrosis factor (TNF)-α in human lung carcinoma A549 cells. In the present study, the mechanism by which allantopyrone A inhibits the TNF-α-induced signaling pathway was investigated in more detail. Allantopyrone A blocked extensive modifications to receptor-interacting protein 1 (RIP1) in the TNF receptor 1 (TNF-R1) complex. Allantopyrone A augmented the high-MW bands of TNF-R1, TNF receptor-associated factor 2, RIP1, the NF-κB subunit RelA and inhibitor of NF-κB kinase ß in A549 cells, suggesting that it binds to and promotes the crosslinking of these proteins. The extracellular cysteine-rich domains of TNF-R1 were crosslinked by allantopyrone A more preferentially than its intracellular portion. The present results demonstrate that allantopyrone A interferes with multiple components of the TNF-R1 complex and blocks RIP1 modifications in the TNF-α-induced NF-κB signaling pathway.

Brefeldin A-Inhibited Guanine Nucleotide-Exchange Factor 1 (BIG1) Governs the Recruitment of Tumor Necrosis Factor Receptor-Associated Factor 2 (TRAF2) to Tumor Necrosis Factor Receptor 1 (TNFR1) Signaling Complexes.

Tumor necrosis factor receptor-associated factor 2 (TRAF2) is a critical mediator of tumor necrosis factor-α (TNF-α) signaling. However, the regulatory mechanisms of TRAF2 are not fully understood. Here we show evidence that TRAF2 requires brefeldin A-inhibited guanine nucleotide-exchange factor 1 (BIG1) to be recruited into TNF receptor 1 (TNFR1) signaling complexes. In BIG1 knockdown cells, TNF-α-induced c-Jun N-terminal kinase (JNK) activation was attenuated and the sensitivity to TNF-α-induced apoptosis was increased. Since these trends correlated well with those of TRAF2 deficient cells as previously demonstrated, we tested whether BIG1 functions as an upstream regulator of TRAF2 in TNFR1 signaling. As expected, we found that knockdown of BIG1 suppressed TNF-α-dependent ubiquitination of TRAF2 that is required for JNK activation, and impaired the recruitment of TRAF2 to the TNFR1 signaling complex (complex I). Moreover, we found that the recruitment of TRAF2 to the death-inducing signaling complex termed complex II was also impaired in BIG1 knockdown cells. These results suggest that BIG1 is a key component of the machinery that drives TRAF2 to the signaling complexes formed after TNFR1 activation. Thus, our data demonstrate a novel and unexpected function of BIG1 that regulates TNFR1 signaling by targeting TRAF2.

Therapeutic targets in the Wnt signaling pathway: Feasibility of targeting TNIK in colorectal cancer.

The genetic and epigenetic alterations occurring during the course of multistage colorectal carcinogenesis have been extensively studied in the last few decades. One of the most notable findings is that the great majority of colorectal cancers (>80%) have mutations in the adenomatous polyposis coli (APC) tumor suppressor gene. Loss of functional APC protein results in activation of canonical Wnt/ß-catanin signaling and initiates intestinal carcinogenesis. Mutational inactivation of APC is the first genetic event, but colorectal cancer cells retain their dependency on constitutive Wnt signal activation even after accumulation of other genetic events. Accordingly, pharmacological blocking of Wnt signaling has been considered an attractive therapeutic approach for colorectal cancer. Several therapeutics targeting various molecular components of the Wnt signaling pathway, including porcupine, frizzled receptors and co-receptor, tankyrases, and cAMP response element binding protein (CREB)-binding protein (CBP), have been developed, and some of those are currently being evaluated in early-phase clinical trials. Traf2- and Nck-interacting protein kinase (TNIK) has been identified as a regulatory component of the T-cell factor-4 and ß-catenin transcriptional complex independently by two research groups. TNIK regulates Wnt signaling in the most downstream part of the pathway, and its inhibition is expected to block the signal even in colorectal cancer cells with APC gene mutation. Here we discuss some of the TNIK inhibitors under preclinical development.

TRAF1 Coordinates Polyubiquitin Signaling to Enhance Epstein-Barr Virus LMP1-Mediated Growth and Survival Pathway Activation.

The Epstein-Barr virus (EBV) encoded oncoprotein Latent Membrane Protein 1 (LMP1) signals through two C-terminal tail domains to drive cell growth, survival and transformation. The LMP1 membrane-proximal TES1/CTAR1 domain recruits TRAFs to activate MAP kinase, non-canonical and canonical NF-kB pathways, and is critical for EBV-mediated B-cell transformation. TRAF1 is amongst the most highly TES1-induced target genes and is abundantly expressed in EBV-associated lymphoproliferative disorders. We found that TRAF1 expression enhanced LMP1 TES1 domain-mediated activation of the p38, JNK, ERK and canonical NF-kB pathways, but not non-canonical NF-kB pathway activity. To gain insights into how TRAF1 amplifies LMP1 TES1 MAP kinase and canonical NF-kB pathways, we performed proteomic analysis of TRAF1 complexes immuno-purified from cells uninduced or induced for LMP1 TES1 signaling. Unexpectedly, we found that LMP1 TES1 domain signaling induced an association between TRAF1 and the linear ubiquitin chain assembly complex (LUBAC), and stimulated linear (M1)-linked polyubiquitin chain attachment to TRAF1 complexes. LMP1 or TRAF1 complexes isolated from EBV-transformed lymphoblastoid B cell lines (LCLs) were highly modified by M1-linked polyubiqutin chains. The M1-ubiquitin binding proteins IKK-gamma/NEMO, A20 and ABIN1 each associate with TRAF1 in cells that express LMP1. TRAF2, but not the cIAP1 or cIAP2 ubiquitin ligases, plays a key role in LUBAC recruitment and M1-chain attachment to TRAF1 complexes, implicating the TRAF1:TRAF2 heterotrimer in LMP1 TES1-dependent LUBAC activation. Depletion of either TRAF1, or the LUBAC ubiquitin E3 ligase subunit HOIP, markedly impaired LCL growth. Likewise, LMP1 or TRAF1 complexes purified from LCLs were decorated by lysine 63 (K63)-linked polyubiqutin chains. LMP1 TES1 signaling induced K63-polyubiquitin chain attachment to TRAF1 complexes, and TRAF2 was identified as K63-Ub chain target. Co-localization of M1- and K63-linked polyubiquitin chains on LMP1 complexes may facilitate downstream canonical NF-kB pathway activation. Our results highlight LUBAC as a novel potential therapeutic target in EBV-associated lymphoproliferative disorders.

Blockade of CD40-TRAF2,3 or CD40-TRAF6 is sufficient to inhibit pro-inflammatory responses in non-haematopoietic cells.

Inhibition of the CD40-CD154 pathway controls inflammatory disorders. Unfortunately, administration of anti-CD154 monoclonal antibodies causes thromboembolism. Blockade of signalling downstream of CD40 may represent an approach to treat CD40-driven inflammatory disorders. Blocking tumour necrosis factor receptor-associated factor 6 (TRAF6) signalling downstream of CD40 in MHC II(+) cells diminishes inflammation. However, CD40-TRAF6 blockade may cause immunosuppression. We examined the role of CD40-TRAF2,3 and CD40-TRAF6 signalling in the development of pro-inflammatory responses in human non-haematopoietic and monocytic cells. Human aortic endothelial cells, aortic smooth muscle cells, renal proximal tubule epithelial cells, renal mesangial cells and monocytic cells were transduced with retroviral vectors that encode wild-type CD40, CD40 with a mutation that prevents TRAF2,3 recruitment (ΔT2,3), TRAF6 recruitment (ΔT6) or both TRAF2,3 plus TRAF6 recruitment (ΔT2,3,6). Non-haematopoietic cells that expressed CD40 ΔT2,3 exhibited marked inhibition in CD154-induced up-regulation of vascular cell adhesion molecule 1, intercellular adhesion molecule 1 (ICAM-1), monocyte chemotactic protein 1 (MCP-1), tissue factor and matrix metalloproteinase 9. Similar results were obtained with cells that expressed CD40 ΔT6. Although both mutations impaired ICAM-1 up-regulation in monocytic cells, only expression of CD40 ΔT6 reduced MCP-1 and tissue factor up-regulation in these cells. Treatment of endothelial and smooth muscle cells with cell-permeable peptides that block CD40-TRAF2,3 or CD40-TRAF6 signalling impaired pro-inflammatory responses. In contrast, while the CD40-TRAF2,3 blocking peptide did not reduce CD154-induced dendritic cell maturation, the CD40-TRAF6 blocking peptide impaired this response. Hence, preventing CD40-TRAF2,3 or CD40-TRAF6 interaction inhibits pro-inflammatory responses in human non-haematopoietic cells. In contrast to inhibition of CD40-TRAF6 signalling, inhibition of CD40-TRAF2,3 signalling did not impair dendritic cell maturation. Blocking CD40-TRAF2,3 signalling may control CD40-CD154-dependent inflammatory disorders.

The E3 ubiquitin ligase TRAF2 can contribute to TNF-α resistance in FLT3-ITD-positive AML cells.

TNF-α has pleiotropic effects on cell survival and apoptosis. The E3 ubiquitin ligase TRAF2 plays a crucial role for TNF-α mediated signaling since NF-κB activation by TNF-α is at least partially mediated by TRAF2. The objective of this study was to investigate whether TNF-α can induce apoptosis in FLT3-ITD-positive AML cells and to elucidate the influence of TRAF2. Stable lentiviral mediated down-regulation of TRAF2 resulted in a decrease of phosphorylation of the anti-apoptotic protein AKT and its downstream target GSK-3ß. Induction of apoptosis and impaired proliferation after TNF-α exposure were observed. Co-treatment of FLT3-ITD-positive cells with the specific FLT3 inhibitor AC220 was able to overcome TNF-α resistance. Taken together, we conclude that TRAF2 plays an important role in signal transduction and survival of AML cells.

Sexual differentiation of the gonadotropin surge release mechanism: a new role for the canonical NfκB signaling pathway.

Sex differences in luteinizing hormone (LH) release patterns are controlled by the hypothalamus, established during the perinatal period and required for fertility. Female mammals exhibit a cyclic surge pattern of LH release, while males show a tonic release pattern. In rodents, the LH surge pattern is dictated by the anteroventral periventricular nucleus (AVPV), an estrogen receptor-rich structure that is larger and more cell-dense in females. Sex differences result from mitochondrial cell death triggered in perinatal males by estradiol derived from aromatization of testosterone. Herein we provide an historical perspective and an update describing evidence that molecules important for cell survival and cell death in the immune system also control these processes in the developing AVPV. We conclude with a new model proposing that development of the female AVPV requires constitutive activation of the Tnfα, Tnf receptor 2, NfκB and Bcl2 pathway that is blocked by induction of Tnf receptor-associated factor 2-inhibiting protein (Traip) in the male.

Constitutive BR3 receptor signaling in diffuse, large B-cell lymphomas stabilizes nuclear factor-κB-inducing kinase while activating both canonical and alternative nuclear factor-κB pathways.

Aberrant nuclear factor κB (NF-κB) signaling has been found to be of particular importance in diffuse, large B-cell lymphoma (DLBCL) cell survival and proliferation. Although the canonical NF-κB signaling pathway has been studied in some detail, activation of the alternative NF-κB pathway in DLBCL is not well characterized. Important insights into the regulation of the alternative NF-κB pathway in B lymphocytes has recently revealed the regulatory importance of the survival kinase NIK (NF-κB-inducing kinase) in genetically engineered murine models. Our studies demonstrate that both the canonical and alternative NF-κB pathways are constitutively activated in DLBCL. We also demonstrate that NIK kinase aberrantly accumulates in DLBCL cells due to constitutive activation of B-cell activation factor (BAFF)-R (BR3) through interaction with autochthonous B-lymphocyte stimulator (BLyS) ligand in DLBCL cells. Activation of BR3 in DLBCL induces recruitment and degradation of tumor necrosis factor receptor-associated factor 3, which results in NIK kinase accumulation, IκBα phosphorylation, and NF-κB p100 processing, thereby resulting in continuous activation of both NF-κB pathways in DLBCL cells, leading to autonomous lymphoma cell growth and survival. These results further elucidate mechanisms involved in abnormal NF-κB activation in DLBCL, and should contribute to better future therapeutic approaches for patients with DLBCL.

Soluble and transmembrane TNF-like weak inducer of apoptosis differentially activate the classical and noncanonical NF-kappa B pathway.

TNF-like weak inducer of apoptosis, TWEAK, is a typical member of the TNF ligand family. Thus, it is initially expressed as a type II transmembrane protein from which a soluble variant can be released by proteolytic processing. In this study, we show that membrane TWEAK is superior to soluble variant of TWEAK (sTWEAK) with respect to the activation of the classical NF-kappaB pathway, whereas both TWEAK variants are potent inducers of TNFR-associated factor-2 depletion, NF-kappaB-inducing kinase accumulation and p100 processing, hallmarks of activation of the noncanonical NF-kappaB pathway. Like other soluble TNF ligands with a poor capability to activate their corresponding receptor, sTWEAK acquires an activity resembling those of the transmembrane ligand by oligomerization or cell surface-immobilization. Blockade of the Fn14 receptor inhibited NF-kappaB signaling irrespective of the TWEAK form used for stimulation, indicating that the differential activities of the two TWEAK variants on classical and noncanonical NF-kappaB signaling is not related to the use of different receptors.

Inhibition of NF-kappaB signaling by A20 through disruption of ubiquitin enzyme complexes.

A20 negatively regulates inflammation by inhibiting the nuclear factor kappaB (NF-kappaB) transcription factor in the tumor necrosis factor-receptor (TNFR) and Toll-like receptor (TLR) pathways. A20 contains deubiquitinase and E3 ligase domains and thus has been proposed to function as a ubiquitin-editing enzyme downstream of TNFR1 by inactivating ubiquitinated RIP1. However, it remains unclear how A20 terminates NF-kappaB signaling downstream of TLRs. We have shown that A20 inhibited the E3 ligase activities of TRAF6, TRAF2, and cIAP1 by antagonizing interactions with the E2 ubiquitin conjugating enzymes Ubc13 and UbcH5c. A20, together with the regulatory molecule TAX1BP1, interacted with Ubc13 and UbcH5c and triggered their ubiquitination and proteasome-dependent degradation. These findings suggest mechanism of A20 action in the inhibition of inflammatory signaling pathways.

LIGHT enhances the bactericidal activity of human monocytes and neutrophils via HVEM.

Human monocytes and neutrophils play major roles in clearing bacteria from human blood and tissues. We found that the herpes virus entry mediator (HVEM) was highly expressed in monocytes and neutrophils, and its interaction with "homologous to lymphotoxins, shows inducible expression, and competes with herpes simplex virus glycoprotein D for HVEM/tumor necrosis factor (TNF)-related 2" (LIGHT) enhanced bactericidal activity against Listeria monocytogenes and Staphylococcus aureus. The LIGHT-HVEM interaction increased levels of phagocytosis, interleukin (IL)-8, TNF-alpha, nitric oxide (NO), and reactive oxygen species (ROS) in monocytes and neutrophils. Anti-HVEM monoclonal antibody was able to block LIGHT-induced bactericidal activity, cytokine production (IL-8 and TNF-alpha), and ROS generation. Moreover, inhibition of ROS and NO production blocked LIGHT-induced bactericidal activity. Our results indicate that the LIGHT/HVEM interaction in monocytes and neutrophils contributes to antibacterial activity.

TNF receptor (TNFR)-associated factor (TRAF) 3 serves as an inhibitor of TRAF2/5-mediated activation of the noncanonical NF-kappaB pathway by TRAF-binding TNFRs.

TNF family members and their receptors contribute to increased gene expression for inflammatory processes and intracellular cascades leading to programmed cell death, both via activation of NF-kappaB. TNF receptor (TNFR)-associated factors (TRAFs) are cytoplasmic adaptor proteins binding to various receptors of the TNFR family. In an attempt to delineate the role of individual TRAFs, we compared NF-kappaB activation by CD40(wt) and CD40 mutants with different TRAF recruitment patterns. Recognized only recently, NF-kappaB signaling occurs at least via two different pathways. Each pathway results in nuclear translocation of two different Reldimers, the canonical p50/RelA and the noncanonical p52/RelB. Here, we show that via TRAF6, CD40 mediates only the activation of the canonical NF-kappaB pathway. Via TRAF2/5, CD40 activates both the canonical and the noncanonical NF-kappaB pathways. We observed that TRAF3 specifically blocked the NF-kappaB activation via TRAF2/5. This inhibitory effect of TRAF3 depends on the presence of an intact zinc finger domain. Paradoxically, suppression of TRAF2/5-mediated NF-kappaB activation by TRAF3 resulted in enhanced transcriptional activity of TRAF6-mediated canonical NF-kappaB emanating from CD40. We also observed that 12 TNFR family members (p75TNFR, LTbetaR, RANK, HVEM, CD40, CD30, CD27, 4-1BB, GITR, BCMA, OX40, and TACI) are each capable of activating the alternative NF-kappaB pathway and conclude that TRAF3 serves as a negative regulator of this pathway for all tested receptors.