Anatomy of TRAF2. Distinct domains for nuclear factor-kappaB activation and association with tumor necrosis factor signaling proteins

Herpesvirus saimiri STP-A oncoprotein utilizes Src family protein tyrosine kinase and tumor necrosis factor receptor-associated factors to elicit cellular signal transduction

The saimiri transforming protein oncogene, called STP-A, of herpesvirus saimiri (HVS) subgroup A is not required for viral replication but is required for lymphoid cell immortalization in culture and lymphoma induction in primates. Here we report that STP-A interacts with cellular tumor necrosis factor receptor-associated factors (TRAF2 and TRAF6) and Src family protein tyrosine kinases (SF-PTKs) in a genetically and functionally separable manner and that each interaction constitutively elicits independent cellular signal transduction. The amino-terminal and central proline-rich motifs of STP-A were responsible for TRAF6 and TRAF2 interactions, respectively, and STP-A and TRAF6 interaction contributed to the majority of NF-kappaB activation, whereas STP-A and TRAF2 interaction played a minor role in NF-kappaB activation. On the other hand, interaction of STP-A with SF-PTKs through its SH2 binding motif effectively elicited AP-1 and NF-AT transcription factor activity. One cellular gene targeted by STP-A is intercellular adhesion molecule 1 (ICAM-1), which participates in a wide range of inflammatory and immune responses. Both TRAF and SF-PTK signal transductions induced by STP-A were required for the marked increase of ICAM-1 expression. These results demonstrate that the viral oncogene STP-A independently targets two vital cellular signaling molecules and that these activities likely contribute to HVS-mediated lymphoid cell immortalization in culture and lymphoma induction in primates.

Function of tumor necrosis factor receptor family members on regulatory T-cells

Effector cells play a crucial role in the immune system of higher vertebrates in eliminating invading pathogens and transformed cells that could cause disease or death of the individual. To be effective and specific, immune responses have to distinguish between self and nonself. Mechanisms of central and peripheral tolerance have evolved to control effector cells that could respond to autoantigens. Regulatory T-cells (Treg cells) are critical modulators of effector cells in the periphery that suppress autoreactive T-cells but are also involved in modulating immune responses against invading pathogens. Identification of surface markers of Treg cells and the development of in vitro systems to study the suppressive function of Treg cells have revealed distinct phenotypic and functional subsets of Treg cells. Several tumor necrosis factor receptor (TNFR) family members have been shown to play a role in the development, homeostasis, and suppressor function of Treg cells. Recent findings suggest that TNFRs and other cell-surface molecules of Treg cells can be explored for therapeutic strategies targeting autoimmune disorders, cancer, and immune responses against pathogens.

TRUSS, a tumor necrosis factor receptor-1-interacting protein, activates c-Jun NH(2)-terminal kinase and transcription factor AP-1

Tumor necrosis factor-alpha (TNF-alpha) induces the transcriptional activation of numerous genes involved in the inflammatory response. The recently identified protein TRUSS was investigated for its role in TNF-alpha-induced activation of c-Jun-NH(2) terminal kinase (JNK) and transcription factor, AP-1. Ectopic expression of TRUSS activated JNK and AP-1 in the absence and presence of TNF-alpha stimulation. The C-terminal region of TRUSS interacted with TNF receptor-associated factor-2 (TRAF2) and co-expression of dominant-inhibitory TRAF2 with TRUSS inhibited AP-1 activation, suggesting that TRUSS signaling occurs upstream of TRAF2. Further, a dominant-inhibitory mutant of TRUSS inhibited TNF-alpha-induced AP-1 activation. These findings suggest that TRUSS activates JNK in a TRAF2-dependent fashion and is involved in TNF-alpha-induced AP-1 activation via JNK kinases.

Interference with nuclear factor kappa B and c-Jun NH2-terminal kinase signaling by TRAF6C small interfering RNA inhibits myeloma cell proliferation and enhances apoptosis

The tumor necrosis factor receptor (TNFR)-associated factor (TRAF) family of six adaptor proteins (TRAF1-6) links the TNFR superfamily to the nuclear factor kappa B (NF-kappaB) and activator protein-1 (AP-1) transcriptional activators. Unlike other TRAFs, TRAF6 is also involved in Toll-like/interleukin (IL)-1 receptor (TIR) signal transduction. Thus, inhibition of TRAF6 function could interrupt both CD40 (TNFR family) and IL-1 growth signals, pathways critical to myeloma proliferation. To block TRAF6-mediated IL-1 signaling, we constructed small interfering RNA (siRNA) against TRAF6. We found that siRNA targeting the TRAF6 C-terminal (siTRAF6C) receptor interaction domain specifically reduced only TRAF6 protein expression, without affecting TRAF2 or 5 levels, and substantially interfered with IL-1-induced NF-kappaB and c-Jun/AP-1 activation. Inhibition by siTRAF6C was concentration-dependent. SiTRAF6C also significantly reduced myeloma proliferation and enhanced apoptosis in a similar dose-dependent fashion in vitro. More importantly, marked siTRAF6C growth inhibition was detected in vivo when these cells were implanted into the bone marrow of irradiated normal mice. In contrast, introduction of siRNA derived from the TRAF6 Zn-finger domain or an irrelevant siRNA construct failed to alter cell growth or cell death. These studies suggest that TRAF6 may be a new molecular target to block cell signal transduction important for the survival and proliferation of multiple myeloma cells.

Human glutathione S-transferase P1-1 interacts with TRAF2 and regulates TRAF2-ASK1 signals

Human glutathione S-transferase P1-1 (GSTP1-1) is an ubiquitously expressed protein that plays an important role in the detoxification and xenobiotics metabolism. It has been shown that GSTP1-1 interacts with c-Jun NH(2)-terminal kinase (JNK) and suppresses its activity. Here, we report a novel function of GSTP1-1 in regulating tumor necrosis factor-alpha (TNF-alpha)-triggered signaling. The present experiments showed that GSTP1-1 physically associated with tumor necrosis factor receptor-associated factor 2 (TRAF2) in vivo and in vitro. Overexpression of GSTP1-1 inhibited TRAF2-induced activation of both JNK and p38 but not of nuclear factor-kappaB (NF-kappaB). Glutathione S-transferase P1-1 also attenuated TRAF2-enhanced apoptosis signal-regulating kinase 1 (ASK1) autophosphorylation and inhibited TRAF2-ASK1-induced cell apoptosis by suppressing the interaction of TRAF2 and ASK1. Conversely, silencing of GSTP1-1 expression through RNA interference (RNAi) resulted in increase of TNF-alpha-dependent TRAF2-ASK1 association followed by hyper-activation of ASK1 and JNK. A mutant GSTP1-1 lacking TRAF domain-binding motif exhibited a significant decline of capacity to bind TRAF2 and block TRAF2-ASK1 signaling compared with the wild type of GSTP1-1. Moreover, the glutathione-conjugating activity of GSTP1-1 was not involved in the regulation of TRAF2 signaling. These findings indicate that GSTP1-1 plays an important regulatory role in TNF-alpha-induced signaling by forming ligand-binding interactions with TRAF2, which provides a new insight for analysing the protective effects of GSTP1-1 in tumor cells.

TRAF3: a new component of the TLR-signaling apparatus

Toll-like receptors (TLRs) are key sensors of microbially derived molecules that, upon activation, provide a pathogen-specific inflammatory response, leading to an efficient eradication of microbial pathogens. An important question has been how TLRs can provide signaling diversity when challenged with genotypic and functionally distinct pathogens. Recently, two studies have shown that tumor-necrosis factor receptor-associated factor 3 (TRAF3) is an essential component of the TLR-signaling pathway, being a crucial regulator in the induction of TLR-specific inflammatory responses.

TRAF1 regulates Th2 differentiation, allergic inflammation and nuclear localization of the Th2 transcription factor, NIP45

We have previously reported that tumor necrosis factor receptor-associated factor 1 (TRAF1), an intracellular protein, which binds to a range of molecules, including tumor necrosis factor (TNF) receptor family members, regulates TNF-induced NF-kappaB and AP-1 signaling as well as TCR-triggered proliferative responses in T cells. In order to define the role of TRAF1 in Th cell differentiation, we analyzed the responses of TRAF1-/- T cells following TCR activation. Stimulation of TRAF1-/- T cells by antigen resulted in significantly increased expression of the Th2 cytokines (IL-4, IL-5 and IL-13) compared with wild-type (WT) controls. The Th2 bias of TRAF1-/- T cells is T lymphocyte intrinsic, since naive CD4+CD62L+ TRAF1-/- T cells activated with CD3/CD28 produced elevated levels of Th2 cytokines. Consistent with these observations in cultured T cells, TRAF1-/- T cells induced enhanced Th2 responses in vivo. Transfer of ovalbumin (OVA)-immune TRAF1-/- T cells into naive WT recipients conferred significantly more intense pulmonary inflammation and higher airway hyperresponsiveness following inhaled OVA challenge than did transfer of OVA-immune WT T cells. Biochemical analysis of TRAF1-/- T cells revealed that they have elevated nuclear expression of NFAT-interacting protein (NIP45), a Th2 cell-associated transcription factor known to potentiate NFATp-driven IL-4 expression. In further experiments, we demonstrated that TRAF1 associates with a fraction of NIP45 in the cytoplasm and prevents its translocation to the nucleus. Taken together these results suggest that TRAF1 may limit the induction of Th2 responses by decreasing NIP45 concentration to the nucleus and thereby down-regulating the expression of NIP45-dependent IL-4 gene transcription.

Differential regulation of CD40-mediated TNF receptor-associated factor degradation in B lymphocytes

Engagement of CD40 on murine B cells by its ligand CD154 induces the binding of TNFR-associated factors (TRAFs) 1, 2, 3, and 6, followed by the rapid degradation of TRAFs 2 and 3. TRAF degradation occurs in response to signaling by other TNFR superfamily members, and is likely to be a normal regulatory component of signaling by this receptor family. In this study, we found that receptor-induced TRAF degradation limits TRAF2-dependent CD40 signals to murine B cells. However, TRAFs 1 and 6 are not degraded in response to CD40 engagement, despite their association with CD40. To better understand the mechanisms underlying differential TRAF degradation, mixed protein domain TRAF chimeras were analyzed in murine B cells. Chimeras containing the TRAF2 zinc (Zn) domains induced effective degradation, if attached to a TRAF domain that binds to the PXQXT motif of CD40. However, the Zn domains of TRAF3 and TRAF6 could not induce degradation in response to CD40, regardless of the TRAF domains to which they were attached. Our data indicate that TRAF2 serves as the master regulator of TRAF degradation in response to CD40 signaling, and this function is dependent upon both the TRAF Zn domains and receptor binding position.

TRAF7 sequesters c-Myb to the cytoplasm by stimulating its sumoylation

Small ubiquitin-related modifiers (SUMOs) are proteins that are posttranslationally conjugated to diverse proteins. The c-myb proto-oncogene product (c-Myb) regulates proliferation and differentiation of hematopoietic cells. PIASy is the only known SUMO E3 ligase for c-Myb. Here, we report that TRAF7 binds to c-Myb and stimulates its sumoylation. TRAF7 bound to the DNA-binding domain of c-Myb via its WD40 repeats. TRAF7 has an E3 ubiquitin ligase activity for self-ubiquitination, but TRAF7 also stimulated the sumoylation of c-Myb at Lys-523 and Lys-499, which are the same sites as those used for PIASy-induced sumoylation. TRAF7 inhibited trans-activation induced by wild-type c-Myb, but not by the sumoylation site mutant of c-Myb. The expression of both c-myb and TRAF7 was down-regulated during differentiation of M1 cells. Endogenous TRAF7 localized to both the cytoplasm and nucleus of M1 cells. Consistent with this, significant amounts of sumoylated c-Myb were found in the cytoplasm of M1 cells, whereas nonsumoylated c-Myb was found predominantly in the nucleus. Overexpressed TRAF7 was localized in the cytoplasm of CV-1 cells, and sequestered c-Myb and SUMO1 in the cytosol, whereas PIASy was localized in the nucleus. Thus, TRAF7 negatively regulates c-Myb activity by sequestering c-Myb to the cytosol via sumoylation.

Dissecting role of regulatory factors in NF-kappaB pathway with siRNA

NF-kappaB, a family of related transcription factors, has been a focus of intense scientific research during the past decade. Multiple stimuli, both extracellular and intracellular, lead to its activation. The NF-kappaB pathway regulates expression of a diverse array of genes involved in different biological processes. Various pathological states are characterized by the dysregulated NF-kappaB pathway. Recently, NF-kappaB activation has been connected with multiple aspects of oncogenesis and serves as an important mechanism to regulate cell survival in response to chemotherapy by activating different genes that inhibit apoptosis. Several methods of inhibiting NF-kappaB activation, such as antisense oligonucleotides, proteosome inhibitors and RNA interference (RNAi) are currently under investigation. RNAi represents a powerful tool to better define the role of specific genes in different signal transduction pathways and has recently been used to define the function of genes that regulate the NF-kappaB pathway. This review discusses the emerging role of RNAi to dissect the function of regulatory factors in the NF-kappaB pathway and its potential use as a targeted therapy.

Glucocorticoid-Induced TNF Receptor, a Costimulatory Receptor on Naive and Activated T Cells, Uses TNF Receptor-Associated Factor 2 in a Novel Fashion as an Inhibitor of NF-κB Activation

Glucocorticoid-induced TNFR (GITR) has been implicated as an essential regulator of immune responses to self tissues and pathogens. We have recently shown that GITR-induced cellular events promote survival of naive T cells, but are insufficient to protect against activation-induced cell death. However, the molecular mechanisms of GITR-induced signal transduction that influence physiologic and pathologic immune responses are not well understood. TNFR-associated factors (TRAFs) are pivotal adapter proteins involved in signal transduction pathways of TNFR-related proteins. Yeast two-hybrid assays and studies in HEK293 cells and primary lymphocytes indicated interactions between TRAF2 and GITR mediated by acidic residues in the cytoplasmic domain of the receptor. GITR-induced activation of NF-kappaB is blocked by A20, an NF-kappaB-inducible protein that interacts with TRAFs and functions in a negative feedback mechanism downstream of other TNFRs. Interestingly, in contrast with its effects on signaling triggered by other TNFRs, our functional studies revealed that TRAF2 plays a novel inhibitory role in GITR-triggered NF-kappaB activation.

Mass spectrometric analysis of tumor necrosis factor receptor-associated factor 1 ubiquitination mediated by cellular inhibitor of apoptosis 2

Signaling complexes formed on tumor necrosis factor receptor 2 (TNF-R2) contain adaptor proteins TNF-R-associated factors (TRAFs) 1 and 2, and cellular inhibitors of apoptosis (cIAPs) 1 and 2 which function as regulators of programmed cell death. TRAF2, cIAP1 and cIAP2 all have RING finger domains known to possess E3 ubiquitin ligase activity, implying that ubiquitination may play an important role in the TNF signaling pathway. In this report, we have shown that cIAP2 specifically mediated ubiquitination and proteasome-dependent degradation of TRAF1. To identify the sites for cIAP2-mediated ubiquitination of TRAF1, we used high pressure liquid chromatography coupled with tandem mass spectrometry. Lys185 and Lys193 of TRAF1 were found to be modified with ubiquitin chains. Mutation of Lys185 and Lys193 to Arg almost completely blocked cIAP2-mediated ubiquitination of TRAF1, indicating that they are the major, if not the only, sites of TRAF1 ubiquitination. Our data suggest that cIAP2 may regulate the turnover of TRAF1 by adding polyubiquitin chains on Lys185 or Lys193 following its recruitment to TNF-R signaling complexes.

Posttranscriptional downregulation of c-IAP2 by the ubiquitin protein ligase c-IAP1 in vivo

Inhibitor of apoptosis proteins (IAPs) c-IAP1 and c-IAP2 were identified as part of the tumor necrosis factor receptor 2 (TNFR2) signaling complex and have been implicated as intermediaries in tumor necrosis factor alpha signaling. Like all RING domain-containing IAPs, c-IAP1 and c-IAP2 have ubiquitin protein ligase (E3) activity. To explore the function of c-IAP1 in a physiologic setting, c-IAP1-deficient mice were generated by homologous gene recombination. These animals are viable and have no obvious sensitization to proapoptotic stimuli. Cells from c-IAP1(-/-) mice do, however, express markedly elevated levels of c-IAP2 protein in the absence of increased c-IAP2 mRNA. In contrast to reports implicating c-IAPs in the activation of NF-kappaB, resting and cytokine-induced NF-kappaB activation was not impaired in c-IAP1-deficient cells. Transient transfection studies with wild-type and E3-defective c-IAP1 revealed that c-IAP2 is a direct target for c-IAP1-mediated ubiquitination and subsequent degradation, which are potentiated by the adaptor function of TRAF2. Thus, the c-IAPs represent a pair of TNFR-associated ubiquitin protein ligases in which one regulates the expression of the other by a posttranscriptional and E3-dependent mechanism.

TRAF3 forms heterotrimers with TRAF2 and modulates its ability to mediate NF-{kappa}B activation

FRET experiments utilizing confocal microscopy or flow cytometry assessed homo- and heterotrimeric association of human tumor necrosis factor receptor-associated factors (TRAF) in living cells. Following transfection of HeLa cells with plasmids expressing CFP- or YFP-TRAF fusion proteins, constitutive homotypic association of TRAF2, -3, and -5 was observed, as well as heterotypic association of TRAF1-TRAF2 and TRAF3-TRAF5. A novel heterotypic association between TRAF2 and -3 was detected and confirmed by immunoprecipitation in Ramos B cells that constitutively express both TRAF2 and -3. Experiments employing deletion mutants of TRAF2 and TRAF3 revealed that this heterotypic interaction minimally involved the TRAF-C domain of TRAF3 as well as the TRAF-N domain and zinc fingers 4 and 5 of TRAF2. A novel flow cytometric FRET analysis utilizing a two-step approach to achieve linked FRET from CFP to YFP to HcRed established that TRAF2 and -3 constitutively form homo- and heterotrimers. The functional importance of TRAF2-TRAF3 heterotrimerization was demonstrated by the finding that TRAF3 inhibited spontaneous NF-kappaB, but not AP-1, activation induced by TRAF2. Ligation of CD40 on Ramos B cells by recombinant CD154 caused TRAF2 and TRAF3 to dissociate, whereas overexpression of TRAF3 in Ramos B cells inhibited CD154-induced TRAF2-mediated activation of NF-kappaB. Together, these results reveal a novel association between TRAF2 and TRAF3 that is mediated by unique portions of each protein and that specifically regulates activation of NF-kappaB, but not AP-1.

TNF activates Syk protein tyrosine kinase leading to TNF-induced MAPK activation, NF-kappaB activation, and apoptosis

Spleen tyrosine kinase (Syk), a nonreceptor protein kinase initially found to be expressed only in hemopoietic cells, has now been shown to be expressed in nonhemopoietic cells and to mediate signaling of various cytokines. Whether Syk plays any role in TNF signaling was investigated. Treatment of Jurkat T cells with TNF activated Syk kinase but not ZAP70, another member of Syk kinase family, and the optimum activation occurred at 10 s and with 1 nM TNF. TNF also activated Syk in myeloid and epithelial cells. TNF-induced Syk activation was abolished by piceatannol (Syk-selective inhibitor), which led to the suppression of TNF-induced activation of c- JNK, p38 MAPK, and p44/p42 MAPK. Jurkat cells that did not express Syk (JCaM1, JCaM1/lck) showed lack of TNF-induced Syk, JNK, p38 MAPK, and p44/p42 MAPK activation, as well as TNF-induced IkappaBalpha phosphorylation, IkappaBalpha degradation, and NF-kappaB activation. TNF-induced NF-kappaB activation was enhanced by overexpression of Syk by Syk-cDNA and suppressed when Syk expression was down-regulated by expression of Syk-small interfering RNA (siRNA-Syk). The apoptotic effects of TNF were reduced by up-regulation of NF-kappaB by Syk-cDNA, and enhanced by down-regulation of NF-kappaB by siRNA-Syk. Immunoprecipitation of cells with Syk Abs showed TNF-dependent association of Syk with both TNFR1 and TNFR2; this association was enhanced by up-regulation of Syk expression with Syk-cDNA and suppressed by down-regulation of Syk using siRNA-Syk. Overall, our results demonstrate that Syk activation plays an essential role in TNF-induced activation of JNK, p38 MAPK, p44/p42 MAPK, NF-kappaB, and apoptosis.

AIP1/DAB2IP, a novel member of the Ras-GAP family, transduces TRAF2-induced ASK1-JNK activation

Previously we have shown that ASK-interacting protein 1 (AIP1, also known as DAB2IP), a novel member of the Ras-GAP protein family, mediates TNF-induced activation of ASK1-JNK signaling pathway. However, the mechanism by which TNF signaling is coupled to AIP1 is not known. Here we show that AIP1 is localized on the plasma membrane in resting endothelial cells (EC) in a complex with TNFR1. TNF binding induces release of AIP1 from TNFR1, resulting in cytoplasmic translocation and concomitant formation of an intracellular signaling complex comprised of TRADD, RIP1, TRAF2, and AIPl. A proline-rich region (amino acids 796-807) is critical for maintaining AIP1 in a closed form, which associates with a region of TNFR1 distinct from the death domain, the site of TNFR1 association with TRADD. An AIP1 mutant with deletion of this proline-rich region constitutively binds to TRAF2 and ASK1. A PERIOD-like domain (amino acids 591-719) of AIP1 binds to the intact RING finger of TRAF2, and specifically enhances TRAF2-induced ASK1 activation. At the same time, the binding of AIP1 to TRAF2 inhibits TNF-induced IKK-NF-kappaB signaling. Taken together, our data suggest that AIP1 is a novel transducer in TNF-induced TRAF2-dependent activation of ASK1 that mediates a balance between JNK versus NF-kappaB signaling.

TRAF family proteins link PKR with NF-kappa B activation

The double-stranded RNA (dsRNA)-dependent protein kinase PKR activates NF-kappa B via the I kappa B kinase (IKK) complex, but little is known about additional molecules that may be involved in this pathway. Analysis of the PKR sequence enabled us to identify two putative TRAF-interacting motifs. The viability of such an interaction was further suggested by computer modeling. Here, we present evidence of the colocalization and physical interaction between PKR and TRAF family proteins in vivo, as shown by immunoprecipitation and confocal microscopy experiments. This interaction is induced upon PKR dimerization. Most importantly, we show that the binding between PKR and TRAFs is functionally relevant, as observed by the absence of NF-kappa B activity upon PKR expression in cells genetically deficient in TRAF2 and TRAF5 or after expression of TRAF dominant negative molecules. On the basis of sequence information and mutational and computer docking analyses, we favored a TRAF-PKR interaction model in which the C-terminal domain of TRAF binds to a predicted TRAF interaction motif present in the PKR kinase domain. Altogether, our data suggest that TRAF family proteins are key components located downstream of PKR that have an important role in mediating activation of NF-kappa B by the dsRNA-dependent protein kinase.

A Functional Interaction between the p75 Neurotrophin Receptor Interacting Factors, TRAF6 and NRIF

Neurotrophin signaling through the p75 receptor regulates apoptosis within the nervous system both during development and in response to injury. Whereas a number of p75 interacting factors have been identified, how these upstream factors function in a coordinated manner to mediate receptor signaling is still unclear. Here, we report a functional interaction between TRAF6 and the neurotrophin receptor interacting factor (NRIF), two proteins known to associate with the intracellular domain of the p75 neurotrophin receptor. The association between NRIF and TRAF6 was direct and occurred with both endogenous and ectopically expressed proteins. A KRAB repressor domain of NRIF and the carboxyl-terminal, receptor-binding region of TRAF6 were required for the interaction. Co-expression of TRAF6 increased the levels of NRIF protein and induced its nuclear translocation. Reciprocally, NRIF enhanced TRAF6-mediated activation of the c-Jun NH2-terminal kinase (JNK) by 3-fold, while only modestly increasing the stimulation of NF-kappaB. The expression of both NRIF and TRAF6 was required for reconstituting p75 activation of JNK in HEK293 cells, whereas NRIF mutants lacking the TRAF6 interaction domain were unable to substitute for the full-length protein in facilitating activation of the kinase. These results suggest that NRIF and TRAF6 functionally interact to facilitate neurotrophin signaling through the p75 receptor.

TRAF6, a molecular bridge spanning adaptive immunity, innate immunity and osteoimmunology

Tumor necrosis factor (TNF) receptor associated factor 6 (TRAF6) is a crucial signaling molecule regulating a diverse array of physiological processes, including adaptive immunity, innate immunity, bone metabolism and the development of several tissues including lymph nodes, mammary glands, skin and the central nervous system. It is a member of a group of six closely related TRAF proteins, which serve as adapter molecules, coupling the TNF receptor (TNFR) superfamily to intracellular signaling events. Among the TRAF proteins, TRAF6 is unique in that, in addition to mediating TNFR family signaling, it is also essential for signaling downstream of an unrelated family of receptors, the interleukin-1 (IL-1) receptor/Toll-like receptor (IL-1R/TLR) superfamily. Gene targeting experiments have identified several indispensable physiological functions of TRAF6, and structural and biochemical studies have revealed the potential mechanisms of its action. By virtue of its many signaling roles, TRAF6 represents an important target in the regulation of many disease processes, including immunity, inflammation and osteoporosis.

Protein kinase PKN1 associates with TRAF2 and is involved in TRAF2-NF-κB signaling pathway

PKN1 is a fatty acid and Rho-activated serine/threonine protein kinase whose catalytic domain is highly homologous to protein kinase C (PKC) family. In yeast two-hybrid screening for PKN1 binding proteins, we identified tumor necrosis factor alpha (TNFalpha) receptor-associated factor 2 (TRAF2). TRAF2 is one of the major mediators of TNF receptor superfamily transducing TNF signal to various functional targets, including activation of NF-kappaB, JNK, and apoptosis. FLAG-tagged PKN1 was co-immunoprecipitated with endogenous TRAF2 from HEK293 cell lysate, and in vitro binding assay using the deletion mutants of TRAF2 showed that PKN1 directly binds to the TRAF domain of TRAF2. PKN1 has the TRAF2-binding consensus sequences PXQX (S/T) at amino acid residues 580-584 (PIQES), and P580AQ582A mutant was not co-immunoprecipitated with TRAF2. Furthermore, the reduced expression of PKN1 by RNA interference (RNAi) down-regulated TRAF2-induced NF-kappaB activation in HEK293T cells. These results suggest that PKN1 is involved in TRAF2-NF-kappaB signaling pathway.

Cloning of a novel signaling molecule, AMSH-2, that potentiates transforming growth factor beta signaling

BACKGROUND

Transforming growth factor-betas (TGF-betas), bone morphogenetic proteins (BMPs) and activins are important regulators of developmental cell growth and differentiation. Signaling by these factors is mediated chiefly by the Smad family of latent transcription factors.

RESULTS

There are a large number of uncharacterized cDNA clones that code for novel proteins with homology to known signaling molecules. We have identified a novel molecule from the HUGE database that is related to a previously known molecule, AMSH (associated molecule with the SH3 domain of STAM), an adapter shown to be involved in BMP signaling. Both of these molecules contain a coiled-coil domain located within the amino-terminus region and a JAB (Domain in Jun kinase activation domain binding protein and proteasomal subunits) domain at the carboxy-terminus. We show that this novel molecule, which we have designated AMSH-2, is widely expressed and its overexpression potentiates activation of TGF-beta-dependent promoters. Coimmunoprecipitation studies indicated that Smad7 and Smad2, but not Smad3 or 4, interact with AMSH-2. We show that overexpression of AMSH-2 decreases the inhibitory effect of Smad7 on TGF-beta signaling. Finally, we demonstrate that knocking down AMSH-2 expression by RNA interference decreases the activation of 3TP-lux reporter in response to TGF-beta.

CONCLUSIONS

This report implicates AMSH and AMSH-2 as a novel family of molecules that positively regulate the TGF-beta signaling pathway. Our results suggest that this effect could be partially explained by AMSH-2 mediated decrease of the action of Smad7 on TGF-beta signaling pathway.

Ubiquitination and translocation of TRAF2 is required for activation of JNK but not of p38 or NF-kappaB

TRAF2 is a RING finger protein that regulates the cellular response to stress and cytokines by controlling JNK, p38 and NF-kappaB signaling cascades. Here, we demonstrate that TRAF2 ubiquitination is required for TNFalpha-induced activation of JNK but not of p38 or NF-kappaB. Intact RING and zinc finger domains are required for TNFalpha-induced TRAF2 ubiquitination, which is also dependent on Ubc13. TRAF2 ubiquitination coincides with its translocation to the insoluble cellular fraction, resulting in selective activation of JNK. Inhibition of Ubc13 expression by RNAi resulted in inhibition of TNFalpha-induced TRAF2 translocation and impaired activation of JNK but not of IKK or p38. TRAF2 aggregates in the cytoplasm, as seen in Hodgkin-Reed-Sternberg lymphoma cells, resulting in constitutive NF-kappaB activity but failure to activate JNK. These findings demonstrate that the TRAF2 RING is required for Ubc13-dependent ubiquitination, resulting in translocation of TRAF2 to an insoluble fraction and activation of JNK, but not of p38 or NF-kappaB. Altogether, our findings highlight a novel mechanism of TRAF2-dependent activation of diverse signaling cascades that is impaired in Hodgkin-Reed-Sternberg cells.

Tumor necrosis factor receptor-associated factor 2 (TRAF2)-deficient B lymphocytes reveal novel roles for TRAF2 in CD40 signaling

CD40 function is initiated by tumor necrosis factor (TNF) receptor-associated factor (TRAF) adapter proteins, which play important roles in signaling by numerous receptors. Characterizing roles of individual TRAFs has been hampered by limitations of available experimental models and the poor viability of most TRAF-deficient mice. Here, B cell lines made deficient in TRAF2 using a novel homologous recombination system reveal new roles for TRAF2. We demonstrate that TRAF2 participates in synergy between CD40 and B cell antigen receptor signals, and in CD40-mediated, TNF-dependent IgM production. We also find that TRAF2 participates in the degradation of TRAF3 associated with CD40 signaling, a role that may limit inhibitory actions of TRAF3. Finally, we show that TRAF2 and TRAF6 have overlapping functions in CD40-mediated NF-kappaB activation and CD80 up-regulation. These findings demonstrate previously unappreciated roles for TRAF2 in signaling by TNF receptor family members, using an approach that facilitates the analysis of genes critical to the viability of whole organisms.

Hydrogen peroxide signaling through tumor necrosis factor receptor 1 leads to selective activation of c-Jun N-terminal kinase

Binding of tumor necrosis factor-alpha (TNFalpha) to its receptor, TNF-R1, results in the activation of inhibitor of kappaB kinase (IKK) and c-Jun N-terminal kinase (JNK) pathways that are coordinately regulated and important in survival and death. We demonstrated previously that in response to hydrogen peroxide (H2O2), the ability of TNFalpha to activate IKK in mouse lung epithelial cells (C10) was inhibited and that H2O2 alone was sufficient to activate JNK and induce cell death. In the current study, we investigated the involvement of TNF-R1 in H2O2-induced JNK activation. In lung fibroblasts from TNF-R1-deficient mice the ability of H2O2 to activate JNK was inhibited compared with fibroblasts from control mice. Additionally, in C10 cells expressing a mutant form of TNF-R1, H2O2-induced JNK activation was also inhibited. Immunoprecipitation of TNF-R1 revealed that in response to H2O2, the adapter proteins, TRADD and TRAF2, and JNK were recruited to the receptor. However, expression of the adaptor protein RIP, which is essential for IKK activation by TNFalpha, was decreased in cells exposed to H2O2, and its chaperone Hsp90 was cleaved. Furthermore, data demonstrating that expression of TRAF2 was not affected by H2O2 and that overexpression of TRAF2 was sufficient to activate JNK provide an explanation for the inability of H2O2 to activate IKK and for the selective activation of JNK by H2O2. Our data demonstrate that oxidative stress interferes with IKK activation while promoting JNK signaling, creating a signaling imbalance that may favor apoptosis.

Interleukin 1 activates STAT3/nuclear factor-kappaB cross-talk via a unique TRAF6- and p65-dependent mechanism

Interleukins (IL) 1 and 6 are important cytokines that function via the activation, respectively, of the transcription factors NF-kappaB and STAT3. We have observed that a specific type of kappa B DNA sequence motif supports both NF-kappaB p65 homodimer binding and cooperativity with non-tyrosine-phosphorylated STAT3. This activity, in contrast to that mediated by kappaB DNA motifs that do not efficiently bind p65 homodimers, is shown to be uniquely dependent upon signal transduction through the carboxyl terminus of TRAF6. Furthermore, STAT3 and p65 are shown to physically interact, in vivo, and this interaction appears to inhibit the function of "classical" STAT3 GAS-like binding sites. The distinct p50 form of NF-kappaB is also shown to interact with STAT3. However, in contrast to p65, p50 cooperates with STAT3 bound to GAS sites. These data argue for a novel transcription factor cross-talk mechanism that may help resolve inconsistencies previously reported regarding the mechanism of IL-1 inhibition of IL-6 activity during the acute-phase response.

Epstein-Barr virus latent membrane protein 1: structure and functions

The Epstein-Barr virus latent membrane protein (LMP) 1 is a versatile protein that has profound effects on target cells through its effect on constitutive cellular proteins, e.g. TRAFs, TRADD, RIP, JAK3, BRAM1, and p85. LMP1 can stimulate or inhibit signaling pathways, resulting in transformation of rodent fibroblast cell lines, blockade of differentiation in epithelial cells, upregulation of anti-apoptotic proteins, production of cytokines, upregulation of cell surface markers, upregulation of DNA methyltransferase activity, and downregulation of cell adhesion molecules and cyclin-dependent kinases. Overall, this results in greater transformation and survival in LMP1-expressing cells. Within nasopharyngeal carcinoma biopsy tissues, a naturally occurring LMP1 variant has been identified as having a 10-amino acid deletion in the C-terminus that seems to confer greater transformation potential than non-deleted LMP1. The role of LMP1 as a viral oncogene and its interaction with cellular factors are discussed.

Tumor necrosis factor-induced nuclear factor kappaB activation is impaired in focal adhesion kinase-deficient fibroblasts

Focal adhesion kinase (FAK) is widely involved in important cellular functions such as proliferation, migration, and survival, although its roles in immune and inflammatory responses have yet to be explored. We demonstrate a critical role for FAK in the tumor necrosis factor (TNF)-induced activation of nuclear factor (NF)-kappaB, using FAK-deficient (FAK-/-) embryonic fibroblasts. Interestingly, TNF-induced interleukin (IL)-6 production was nearly abolished in FAK-/- fibroblasts, whereas a normal level of production was obtained in FAK+/- or FAK+/+ fibroblasts. FAK deficiency did not affect the three types of mitogen-activated protein kinases, ERK, JNK, and p38. Similarly, TNF-induced activation of activator protein 1 or NF-IL-6 was not impaired in FAK-/- cells. Of note, TNF-induced NF-kappaB DNA binding activity and activation of IkappaB kinases (IKKs) were markedly impaired in FAK-/- cells, whereas the expression of TNF receptor I or other signaling molecules such as receptor-interacting protein (RIP), tumor necrosis factor receptor-associated factor 2 (TRAF2), IKKalpha, IKKbeta, and IKKgamma was unchanged. Also, TNF-induced association of FAK with RIP and subsequent association of RIP with TRAF2 were not observed, resulting in a failure of RIP to recruit the IKK complex in FAK-/- cells. The reintroduction of wild type FAK into FAK-/- cells restored the interaction of RIP with TRAF2 and the IKK complex and allowed recovery of NF-kappaB activation and subsequent IL-6 production. Thus, we propose a novel role for FAK in the NF-kappaB activation pathway leading to the production of cytokines.

TRAF2 exerts its antiapoptotic effect by regulating the expression of Krüppel-like factor LKLF

Tumor necrosis factor receptor (TNFR)-associated factor 2 (TRAF2) is one of the key factors that mediate TNF signaling. The deletion of TRAF2 renders cells more sensitive to TNF-induced apoptosis. Although TRAF2 is known to be required for TNF-induced JNK and NF-kappaB activation, the underlying mechanism of the increased sensitivity of TRAF2 null cells (TRAF2(-/-)) to TNF-induced apoptosis is not fully understood. To study the underlying mechanism, we examined the difference in gene expression between TRAF2(-/-) and wild-type fibroblast cells by using microarray technology. We found that one of the genes whose expression was dramatically decreased in TRAF2(-/-) cells was the lung Krüppel-like factor (LKLF). Our results indicate that the expression of LKLF requires TRAF2 but is independent of TNF signaling. Although it appears that TRAF2 regulates the expression of the LKLF gene at the transcription level, TRAF2 does not function as a transcription factor itself. Our results suggest that TRAF2 regulates LKLF expression through the mitogen-activated protein kinase p38 pathway. More importantly, ectopic expression of LKLF in TRAF2(-/-) cells protected cells against TNF-induced apoptosis. These results reveal a novel aspect of TRAF2 function: by regulating the expression of genes, such as LKLF, TRAF2 controls cell sensitivity to apoptosis.

S-nitrosylation of thioredoxin mediates activation of apoptosis signal-regulating kinase 1

Apoptosis signal-regulating kinase 1 (ASK1) was recently discovered as a typical member of the mitogen-activated protein (MAP) kinase kinase kinase family, which induces apoptosis by activation of c-Jun-N-terminal kinase/p38 MAP kinase pathways. In normal cells ASK1 is directly inhibited by thioredoxin (Trx), a 12-kDa protein ubiquitously expressed in all living cells, which has a variety of biological functions related to cell proliferation and apoptosis. Here we found that purified Trx is sensitive to S-nitrosylation. Stimulation of HEK-293 cells with S-nitrosoglutathione (GSNO) for 2, 4, 8, and 16h also caused Trx S-nitrosylation, which showed straight correlation with ASK1 activation based on Western blot detection of the enzyme, immunoprecipitation assay, and measurement of its catalytic activity. These results suggest that S-nitrosylation of Trx induces ASK1 activation. Treatment of cells with N-acetyl-cysteine for 2h after 8h of pretreatment with GSNO caused an increase in glutathione and nullified ASK1 activation.

Genetic deletion of the tumor necrosis factor receptor p60 or p80 sensitizes macrophages to lipopolysaccharide-induced nuclear factor-kappa B, mitogen-activated protein kinases, and apoptosis

Whether deletion of tumor necrosis factor (TNF) receptor 1 or 2 affects lipopolysaccharide (LPS)-mediated signaling is not understood. In this report, we used macrophages derived from wild type (wt) mice and from mice null for the type 1 receptor (p60-/-), the type 2 receptor (p80-/-), or both (p60-/- p80-/-) to investigate the effect of these receptors on LPS-mediated activation of NF-kappaB, mitogen-activated protein kinases, and apoptosis. LPS activated NF-kappaB by 3-4-fold in wt cells but by 9-10-fold in p60-/-, p80-/-, and p60-/- p80-/- macrophages. These results correlated with the IkappaBalpha kinase activation, which is needed for NF-kappaB activation. LPS-induced cyclooxygenase-2 and inducible NO synthase proteins and NO production were maximum in p60-/- p80-/- macrophages and minimum in wt cells. LPS activated C-Jun N-terminal kinase, p38MAPK, and extracellular signal-regulated kinase in wt cells, but the levels were much higher in p60-/-, p80-/-, and p60-/- p80-/- cells. LPS-induced cytotoxicity, poly(ADP-ribose) polymerase cleavage, and annexin V staining were also highest in p60-/- p80-/- cells and lowest in wt cells. The difference in LPS signaling was unrelated to the expression of LPS receptors, CD14, or toll-like receptor 4. Overall, our studies indicate that deletion of either of the TNF receptors sensitizes the macrophages to LPS and provide evidence for cross-talk between TNF and LPS signaling.

Role of SODD in regulation of tumor necrosis factor responses

Signaling from tumor necrosis factor receptor type 1 (TNFR1) can elicit potent inflammatory and cytotoxic responses that need to be properly regulated. It was suggested that the silencer of death domains (SODD) protein constitutively associates intracellularly with TNFR1 and inhibits the recruitment of cytoplasmic signaling proteins to TNFR1 to prevent spontaneous aggregation of the cytoplasmic death domains of TNFR1 molecules that are juxtaposed in the absence of ligand stimulation. In this study, we demonstrate that mice lacking SODD produce larger amounts of cytokines in response to in vivo TNF challenge. SODD-deficient macrophages and embryonic fibroblasts also show altered responses to TNF. TNF-induced activation of NF-kappaB is accelerated in SODD-deficient cells, but TNF-induced c-Jun N-terminal kinase activity is slightly repressed. Interestingly, the apoptotic arm of TNF signaling is not hyperresponsive in the SODD-deficient cells. Together, these results suggest that SODD is critical for the regulation of TNF signaling.

The signaling adaptors and pathways activated by TNF superfamily

Members of the TNF receptor superfamily play pivotal roles in numerous biological events in metazoan organisms. Ligand-mediated trimerization by corresponding homo- or heterotrimeric ligands, the TNF family ligands, causes recruitment of several intracellular adaptors, which activate multiple signal transduction pathways. While recruitment of death domain (DD) containing adaptors such as Fas associated death domain (FADD) and TNFR associated DD (TRADD) can lead to the activation of a signal transduction pathway that induces apoptosis, recruitment of TRAF family proteins can lead to the activation of transcription factors such as, NF-kappaB and JNK thereby promoting cell survival and differentiation as well as immune and inflammatory responses. Individual TNF receptors are expressed in different cell types and have a range of affinities for various intracellular adaptors, which provide tremendous signaling and biological specificities. In addition, numerous signaling modulators are involved in regulating activities of signal transduction pathways downstream of receptors in this superfamily. Most of the TNF receptor superfamily members as well as many of their signaling mediators, have been uncovered in the last two decades. However, much remains unknown about how individual signal transduction pathways are regulated upon activation by any particular TNF receptor, under physiological conditions.

CD40-dependent activation of phosphatidylinositol 3-kinase/Akt pathway mediates endothelial cell survival and in vitro angiogenesis

CD40 has been involved in tumor and inflammatory neoangiogenesis. In this study we determined that stimulation of endothelial CD40 with sCD154 induced resistance to apoptosis and in vitro vessel-like formation by human microvascular endothelial cells (HMEC). These effects were determined to be mediated by CD40-dependent signaling because they were inhibited by a soluble CD40-muIg fusion protein. Moreover, apoptosis of HMEC was associated with an impairment of Akt phosphorylation, which was restored by stimulation with sCD154. The anti-apoptotic effect as well as in vitro vessel-like formation and Akt phosphorylation were inhibited by treatment of HMEC with two unrelated pharmacological inhibitors of phosphatidylinositol 3-kinase (PI3K), wortmannin and LY294002. CD40 stimulation induced a rapid increase in Akt enzymatic activity that was not prevented by cycloheximide, an inhibitor of protein synthesis. The enhanced Akt activity induced by stimulation of endothelial CD40 was temporarily correlated with the association of CD40 with TRAF6, c-Cbl, and the p85 subunit of PI3K. Expression of negative-dominant Akt inhibited the activation of endogenous Akt through CD40 stimulation, despite the observation that association of CD40 with TRAF6, c-Cbl, and PI3K was intact. The defective activation of Akt abrogated not only the anti-apoptotic effect of CD40 stimulation but also the proliferative response, the enhanced motility, and the in vitro formation of vessel-like tubular structures by CD40-stimulated HMEC. In conclusion, these results suggest that endothelial CD40, through activation of the PI3K/Akt signaling pathway, regulates cell survival, proliferation, migration, and vessel-like structure formation, all steps considered critical for angiogenesis.

Hodgkin's lymphoma and CD30 signal transduction

Advances in molecular biology have shed light on the biological basis of Hodgkin's lymphoma (HL). Knowledge of the biological basis has enabled us to understand that most Hodgkin and Reed-Sternberg (H-RS) cells are derived from germinal center B-cells and constitutive nuclear factor kappaB (NF-kappaB) activation is a common molecular feature. Molecular mechanisms responsible for constitutive NF-kappaB activation, Epstein Barr virus latent membrane protein 1, and defective IkappaBalpha and IkappaB kinase activation have been clarified in the past several years. A recent study revealed the biological link between 2 characteristic features of H-RS cells: CD30 overexpression and constitutive NF-kappaB activation. Ligand-independent signaling by overexpressed CD3O was shown to be a common mechanism that induced constitutive NF-kappaB activation in these cells. These results suggest the self-growth-promoting potential of H-RS cells and redefine the biology of HL composed of H-RS cells and lymphocytes.

Regulation of the subcellular localization of tumor necrosis factor receptor-associated factor (TRAF)2 by TRAF1 reveals mechanisms of TRAF2 signaling

Tumor necrosis factor receptor-associated factor (TRAF)2 is a critical adaptor molecule for tumor necrosis factor (TNF) receptors in inflammatory and immune signaling. Upon receptor engagement, TRAF2 is recruited to CD40 and translocates to lipid rafts in a RING finger-dependent process, which enables the activation of downstream signaling cascades including c-Jun NH(2)-terminal kinase (JNK) and nuclear factor (NF)-kappaB. Although TRAF1 can displace TRAF2 and CD40 from raft fractions, it promotes the ability of TRAF2 activate signaling over a sustained period of time. Removal of the RING finger of TRAF2 prevents its translocation into detergent-insoluble complexes and renders it dominant negative for signaling. TRAF1(-/-) dendritic cells show attenuated responses to secondary stimulation by TRAF2-dependent factors and increased stimulus-dependent TRAF2 degradation. Replacement of the RING finger of TRAF2 with a raft-targeting signal restores JNK activation and association with the cyto-skeletal protein Filamin, but not NF-kappaB activation. These findings offer insights into the mechanism of TRAF2 signaling and identify a physiological role for TRAF1 as a regulator of the subcellular localization of TRAF2.

A20 inhibits tumor necrosis factor (TNF) alpha-induced apoptosis by disrupting recruitment of TRADD and RIP to the TNF receptor 1 complex in Jurkat T cells

Tumor necrosis factor receptor 1 (TNFR1) can trigger distinct signaling pathways leading to either the activation of NF-kappaB transcription factors or apoptosis. NF-kappaB activation results in the expression of antiapoptotic genes that inhibit the apoptosis pathway that is activated in parallel. However, the molecular mechanism of this inhibition remains poorly characterized. We have isolated a Jurkat T-cell mutant that exhibits enhanced sensitivity to TNF-induced apoptosis as a result of a deficiency in I-kappaB kinase gamma (IKKgamma)/NEMO, an essential component of the IKK complex and NF-kappaB pathway. We show here that the zinc finger protein A20 is an NF-kappaB-inducible gene that can protect the IKKgamma-deficient cells from TNF-induced apoptosis by disrupting the recruitment of the death domain signaling molecules TRADD and RIP to the receptor signaling complex. Our study, together with reports on the role of other antiapoptotic proteins such as c-FLIP and c-IAP, suggests that, in order to ensure an effective shutdown of the apoptotic pathway, TNF induces multiple NF-kappaB-dependent genes that inhibit successive steps in the TNFR1 death signaling pathway.

Involvement of TRAF4 in oxidative activation of c-Jun N-terminal kinase

We previously found that the angiogenic factors TNFalpha and HIV-1 Tat activate an NAD(P)H oxidase in endothelial cells, which operates upstream of c-Jun N-terminal kinase (JNK), a MAPK involved in the determination of cell fate. To further understand oxidant-related signaling pathways, we screened lung and endothelial cell libraries for interaction partners of p47(phox) and recovered the orphan adapter TNF receptor-associated factor 4 (TRAF4). Domain analysis suggested a tail-to-tail interaction between the C terminus of p47(phox) and the conserved TRAF domain of TRAF4. In addition, TRAF4, like p47(phox), was recovered largely in the cytoskeleton/membrane fraction. Coexpression of p47(phox) and TRAF4 increased oxidant production and JNK activation, whereas each alone had minimal effect. In addition, a fusion between p47(phox) and the TRAF4 C terminus constitutively activated JNK, and this activation was decreased by the antioxidant N-acetyl cysteine. In contrast, overexpression of the p47(phox) binding domain of TRAF4 blocked endothelial cell JNK activation by TNFalpha and HIV-1 Tat, suggesting an uncoupling of p47(phox) from upstream signaling events. A secondary screen of endothelial cell proteins for TRAF4-interacting partners yielded a number of proteins known to control cell fate. We conclude that endothelial cell agonists such as TNFalpha and HIV-1 Tat initiate signals that enter basic signaling cassettes at the level of TRAF4 and an NAD(P)H oxidase. We speculate that endothelial cells may target endogenous oxidant production to specific sites critical to cytokine signaling as a mechanism for increasing signal specificity and decreasing toxicity of these reactive species.

Cyclooxygenase isozymes and their gene structures and expression

The cyclooxygenase (COX, prostaglandin endoperoxide synthase) is a key enzyme in prostaglandin biosynthesis. Two isoforms of COX, COX-1 and COX-2, have been identified by molecular biological methods. The amino acid sequence homology between COX-1 and COX-2 is about 60% for the human enzymes. COX-1 is constitutively expressed in most tissues and cells in animal species. The COX-1 promoter region lacks a canonical TATA or CAAT box and is GC-rich. These features are consistent with those of a housekeeping gene. On the other hand, COX-2 is an inducible enzyme and is induced by various cytokines and mitogenic factors. The induction of COX-2 is suppressed by dexamethasone and PGJ2. There are many consensus cis-elements in the 5'-flanking region to regulate the expression of COX-2. Among them, a CRE, an NF-kappaB site, a NF-IL6 motif and an E-box, regulate transcription independently or synergistically. Most of the transcriptional signaling pathways require activation of the mitogen-activated protein kinase (MAPK) cascade. Moreover, MAPK signaling pathways are involved in regulating COX-2 gene expression at the post-transcriptional level.

TNF receptor subtype signalling: differences and cellular consequences

Tumour necrosis factor-alpha (TNF alpha) is a multifunctional cytokine belonging to a family of ligands with an associated family of receptor proteins. The pleiotropic actions of TNF range from proliferative responses such as cell growth and differentiation, to inflammatory effects and the mediation of immune responses, to destructive cellular outcomes such as apoptotic and necrotic cell death mechanisms. Activated TNF receptors mediate the association of distinct adaptor proteins that regulate a variety of signalling processes including kinase or phosphatase activation, lipase stimulation, and protease induction. Moreover, the cytokine regulates the activities of transcription factors, heterotrimeric or monomeric G-proteins and calcium ion homeostasis in order to orchestrate its cellular functions. This review addresses the structural basis of TNF signalling, the pathways employed with their cellular consequences, and focuses on the specific role played by each of the two TNF receptor isotypes, TNFR1 and TNFR2.

CD40 ligation stimulates MCP-1 and IL-8 production, TRAF6 recruitment, and MAPK activation in proximal tubule cells

The mechanism of CD40/CD154-induced chemokine production and its potential role in renal inflammatory disease were explored. Human proximal tubule cells maintained in primary culture were used as the experimental model. With the use of immunocytochemistry, confocal microscopy, and a cell fractionation assay, the CD40 receptor was found to be expressed in the cell membrane of the epithelial cell, and, on engagement by CD154, its cognate ligand, translocated to the cytoplasmic compartment. Engagement of CD40 by CD154 stimulated interleukin-8 (IL-8) and monocyte chemoattractant protein-1 (MCP-1) production, which proceeded via receptor activation of the extracellular signal-regulated kinase (ERK)1/2, stress-activated protein kinase (SAPK)/c-Jun NH(2)-terminal kinase (JNK), and p38 mitogen-activated protein kinase (MAPK) pathways. CD40 ligation also engaged tumor necrosis factor receptor-activating factor 6 (TRAF6), as evidenced by colocalization of the activated receptor with TRAF6 in the cytoplasmic compartment, translocation of both proteins from the insoluble to the soluble cell fraction, and coimmunoprecipitation of the two proteins only under ligand-stimulated conditions. Furthermore, an antisense oligodeoxyribonucleotide targeted against TRAF6 mRNA blunted p38 and SAPK/JNK but not ERK1/2 MAPK activities, as well as IL-8 and MCP-1 production, arguing that TRAF6 is an upstream activator. The zinc chelator TPEN, but not the calcium chelator BAPTA, obliterated CD154-evoked MAPK activity and chemokine production, providing indirect evidence for protein-protein interactions playing a critical role in CD40 signaling in these cells. We conclude that in human proximal tubule cells, CD40 and TRAF6 reside in separate low-density, detergent-insoluble membrane microdomains, or rafts, and on activation translocate and associate with one another probably via zinc-finger domains in the soluble or cytoplasmic compartment. TRAF6, in turn, activates SAPK/JNK and p38 MAPK phosphorylation, which in turn stimulates IL-8 and MCP-1 production in these cells.

Nonstructural 5A protein of hepatitis C virus modulates tumor necrosis factor alpha-stimulated nuclear factor kappa B activation

The hepatitis C virus nonstructural protein 5A (NS5A) is a multifunctional phosphoprotein that leads to pleiotropic responses, in part by regulating cell growth and cellular signaling pathways. Here we show that overexpression of NS5A inhibits tumor necrosis factor (TNF)-alpha-induced nuclear factor kappaB (NF-kappaB) activation in HEK293 cells, as determined by luciferase reporter gene expression and by electrophoretic mobility shift assay. When overexpressed, NS5A cannot inhibit the recruitment of TNF receptor-associated factor 2 (TRAF2) and IkappaB kinase (IKK)beta into the TNF receptor 1-TNF receptor-associated death domain complex. In contrast, NS5A is a part of the TNF receptor 1 signaling complex. NF-kappaB activation by TNF receptor-associated death domain and TRAF2 was inhibited by NS5A, whereas MEKK1 and IKKbeta-dependent NF-kappaB activation was not affected, suggesting that NS5A may inhibit NF-kappaB activation signaled by TRAF2. Coimmunoprecipitation and colocalization of NS5A and TRAF2 expressed in vivo provide compelling evidence that NS5A directly interacts with TRAF2. This interaction was mapped to the middle one-third (amino acids 148-301) of NS5A and the TRAF domain of TRAF2. Our findings suggest a possible molecular mechanism that could explain the ability of NS5A to negatively regulate TNF-alpha-induced NF-kappaB activation.

Signaling molecules of the NF-kappa B pathway shuttle constitutively between cytoplasm and nucleus

We aimed to investigate the dynamics of the NF-kappaB signaling pathway in living cells using GFP variants of p65-NF-kappaB, IkappaBalpha, tumor necrosis factor-receptor associated factor 2 (TRAF2), the NF-kappaB inducing kinase (NIK) and IkappaB kinases (IKK1 and IKK2). Detailed kinetic analysis of constitutive nucleocytoplasmic shuttling processes revealed that IkappaBalpha enters the nucleus faster than p65. Examination of signaling molecules upstream of NF-kappaB and IkappaBalpha revealed a predominant cytoplasmic localization at steady state. However, after addition of leptomycin B, NIK rapidly accumulated in the nucleus, whereas we could not detect any significant effect on TRAF2 or IKK2. Using various truncation mutants of NIK, we identified a functional nuclear export signal within the COOH-terminal region 795-805, which counteracts the inherent NLS at amino acids 143-149. Prolonged incubation in the presence of LMB also leads to nuclear accumulation of IKK1, which was dependent on a lysine residue at position 44, which is also essential for kinase activity. Investigation of endogenous protein levels by immunofluorescence staining and Western blots verified the results obtained with GFP chimeras. We conclude that NF-kappaB.IkappaB complexes and the upstream signaling kinases NIK and IKK1 shuttle between cytoplasm and nucleus of nonactivated cells and that this process leads to a basal transcriptional activity of NF-kappaB.

Differential activation of nuclear factor-kappaB by tumour necrosis factor receptor subtypes. TNFR1 predominates whereas TNFR2 activates transcription poorly

Tumour necrosis factor-alpha (TNF-alpha) signals though two receptors, TNFR1 and TNFR2. TNFR1 has a role in cytotoxicity, whereas TNFR2 regulates death responses or proliferation. TNF activates pro-inflammatory transcription factor nuclear factor-kappaB (NF-kappaB) by uncertain signalling mechanisms. Here we report the contribution of each TNFR towards the NF-kappaB activation processes. In human cells expressing endogenous or exogenous TNFR2, in addition to TNFR1, we found both TNFRs capable of activating NF-kappaB, as measured by IkappaBalpha (inhibitor of NF-kappaB) degradation, electrophoretic mobility shift assay and NF-kappaB gene reporter assays. TNFR2 activation did not degrade IkappaBbeta. However, TNF-effects on NF-kappaB activation occurred predominantly through TNFR1, with TNFR2 activating the transcription factor poorly.

All TRAFs are not created equal: common and distinct molecular mechanisms of TRAF-mediated signal transduction

The tumor necrosis factor (TNF) receptor associated factors (TRAFs) have emerged as the major signal transducers for the TNF receptor superfamily and the interleukin-1 receptor/Toll-like receptor (IL-1R/TLR) superfamily. TRAFs collectively play important functions in both adaptive and innate immunity. Recent functional and structural studies have revealed the individuality of each of the mammalian TRAFs and advanced our understanding of the underlying molecular mechanisms. Here, we examine this functional divergence among TRAFs from a perspective of both upstream and downstream TRAF signal transduction pathways and of signaling-dependent regulation of TRAF trafficking. We raise additional questions and propose hypotheses regarding the molecular basis of TRAF signaling specificity.

T2BP, a novel TRAF2 binding protein, can activate NF-kappaB and AP-1 without TNF stimulation

TRAF2 is a key molecule involved in TNF signaling, which is crucial for the regulation of inflammatory processes. We have identified a novel TRAF2 binding protein, designated as T2BP (TRAF2 binding protein), by a mammalian two-hybrid screening approach. T2BP is a relatively small protein of 184 amino acids, which includes a forkhead-associated domain, the phosphopeptide binding motif. The interaction domain search showed that the TRAF domain in TRAF2 is required for the binding to T2BP whereas almost the entire protein in T2BP binds to TRAF2. The interaction was further confirmed by co-immunoprecipitation. Expression profiling for T2BP and TRAF2 revealed an ubiquitous expression in adult mouse tissues. Overexpression of T2BP in HEK293 cells activated NF-kappaB and AP-1 in a dose dependent manner as well as seen in the TNF-treated control cells. Our results suggest that T2BP is involved in the TNF-mediated signaling by its interaction with TRAF2.

Activation of JNK and transcriptional repressor ATF3/LRF1 through the IRE1/TRAF2 pathway is implicated in human vascular endothelial cell death by homocysteine

Endothelial cell injury underlies an increased occurrence of thromboembolic vascular disease in hereditary hyperhomocysteinemia. We have previously shown that homocysteine causes activation of c-Jun NH(2)-terminal kinase (JNK) and activating transcription factor 3/liver regenerating factor 1 (ATF3/LRF1) and induces apoptosis in human umbilical vein endothelial cells (HUVECs). In this study, the activation of JNK and ATF3 in HUVECs was mediated by the endoplasmic reticulum (ER) resident transmembrane kinase IRE1alpha and beta, which sense and transduce signal of the accumulationj of unfolded proteins in the ER. Moreover, dominant negative mutants of tumor necrosis factor receptor-associated factor 2 and mitogen-activated kinase kinase 4 and 7, as well as antisense ATF3 cDNA, inhibited cell death by homocysteine. These results indicate that the activation of JNK and ATF3 through the ER stress of homocysteine plays a role in the homocysteine-induced cell death. The JNK-ATF3 pathway may be implicated in endothelial cell injury associated with hereditary hyperhomocysteinemia.

Molecular characterization of CD40 signaling intermediates

Signal transduction through the CD40 receptor is initiated by binding of its trimeric ligand and propagated by interactions of tumor necrosis factor receptor-associated factor (TRAF) proteins with the multimerized CD40 cytoplasmic domain. Using defined multimeric constructs of the CD40 cytoplasmic domain expressed as either soluble or myristoylated proteins, we have addressed the extent of receptor multimerization needed to initiate signal transduction and identified components of CD40 signaling complexes. Signal transduction in human embryonic kidney 293 cells, measured by nuclear factor kappaB activation, was observed in cells expressing soluble trimeric CD40 cytoplasmic domain and to a lesser extent in cells expressing dimeric CD40 cytoplasmic domain. Nuclear factor kappaB activation was strongest in cells expressing myristoylated trimeric CD40 cytoplasmic domain. Signal transduction through trimeric CD40 cytoplasmic domains with various point mutations in the TRAF binding sites was similar to signal transduction through analogous full-length receptors. Transiently expressed soluble trimeric CD40 cytoplasmic domain was isolated as complexes that contained TRAF2, TRAF3, TRAF5, TRAF6, and the inhibitor of apoptosis protein (c-IAP1). Association of c-IAP1 with the CD40 cytoplasmic domain complex was indirect and dependent on the presence of an intact TRAF1/2/3 binding site. These results suggest that extracellular ligation of CD40 can be bypassed and that soluble trimerized CD40 complexes can be isolated and used to identify components that link CD40 with signaling pathways.

Type II tumour necrosis factor-alpha receptor (TNFR2) activates c-Jun N-terminal kinase (JNK) but not mitogen-activated protein kinase (MAPK) or p38 MAPK pathways

The pleitropic actions of tumour necrosis factor-alpha (TNF) are transmitted by the type I 55 kDa TNF receptor (TNFR1) and type II 75 kDa TNF receptor (TNFR2), but the signalling mechanisms elicited by these two receptors are not fully understood. In the present study, we report for the first time subtype-specific differential kinase activation in cell models that respond to TNF by undergoing apoptotic cell death. KYM-1 human rhabdomyosarcoma cells and HeLa human cervical epithelial cells, engineered to overexpress TNFR2, displayed c-Jun N-terminal kinase (JNK) activation by wild-type TNF, a TNFR1-specific TNF mutant and a TNFR2-specific mutant TNF in combination with an agonistic TNFR2-specific monoclonal antiserum. A combination of the TNFR2-specific mutant and agonistic antiserum elicited maximal endogenous or exogenous TNFR2 responsiveness. Moreover, alternative expression of a TNFR2 deletion mutant lacking its cytoplasmic domain rendered the cells unable to activate JNK activity through this receptor subtype. The profile of JNK activation by TNFR1 was more transient than that of TNFR2, with TNFR2-induced JNK activity also being more sensitive to the caspase inhibitor, benzyloxycarbonyl-Val-Ala-DL-Asp-fluoromethylketone. Conversely, only activation of the TNFR1 could stimulate mitogen-activated protein kinase (MAPK) or p38 MAPK activities in a time-dependent manner. The role of TNFR2 activation in enhanced apoptotic cell death was confirmed with agonistic monoclonal antisera in cells expressing high levels of TNFR2. Activation of TNFR2 alone elicited cell death, but full TNF-induced death required stimulation of both receptor types. These findings indicate that efficient activation of TNFR2 by soluble TNFs is achievable with co-stimulation by antisera, and that both receptors differentially modulate extracellular signal-regulated kinases contributing to the cytokine's cytotoxic response.

Tumor necrosis factor receptor-associated factors (TRAFs)

Tumor necrosis factor receptor-associated factors (TRAFS) were initially discovered as adaptor proteins that couple the tumor necrosis factor receptor family to signaling pathways. More recently they have also been shown to be signal transducers of Toll/interleukin-1 family members. Six members of the TRAF family have been identified. All TRAF proteins share a C-terminal homology region termed the TRAF domain that is capable of binding to the cytoplasmic domain of receptors, and to other TRAF proteins. In addition, TRAFs 2-6 have RING and zinc finger motifs that are important for signaling downstream events. TRAF proteins are thought to be important regulators of cell death and cellular responses to stress, and TRAF2, TRAF5 and TRAF6 have been demonstrated to mediate activation of NF-kappaB and JNK. TRAF proteins are expressed in normal and diseased tissue in a regulated fashion, suggesting that they play an important role in physiological and pathological processes.