Dominant negative mutants of TRAF3 reveal an important role for the coiled coil domains in cell death signaling by the lymphotoxin-beta receptor

Dominant negative TRAF3 variant with recurrent Mycobacterium abscessus infection and bronchiectasis

Host factors leading to pulmonary NTM (PNTM) disease is poorly understood compared to disseminated NTM disease which is linked to IL12-IFNγ signaling pathway. We elucidated TNF receptor–associated factor 3 (TRAF3) R338W variant in patient with recurrent PNTM infection: demonstrating TRAF3-and TNF-α deficient phenotype, via ex-vivo immune and cloning-transfection cellular studies.

The prevalence and characterisation of TRAF3 and POT1 mutations in canine B-cell lymphoma

The genetic and mutational basis of canine lymphoma remains poorly understood. Several genes, including TRAF3 and POT1, are mutated in canine B-cell lymphoma (cBCL), and are likely involved in the pathogenesis of this disease. The purpose of this study was to assess the prevalence of TRAF3 and POT1 mutations in a cohort of dogs with cBCL, compared to dogs with non-cBCL diseases (including four dogs with T-cell lymphoma [cTCL]). Forty-nine dogs were included (n = 24 cBCL; n = 25 non-cBCL). Eleven dogs had matched non-tumour DNA assessed to determine if mutations were germline or somatic. All dogs had TRAF3 and POT1 assessed by Sanger sequencing. The prevalence of deleterious TRAF3 and POT1 mutations in cBCL was 36% and 17%, respectively. A deleterious TRAF3 mutation was suspected to be germline in 1/5 cases with matched non-tumour DNA available for comparison. Deleterious mutations were not found in specimens from the non-cBCL group. Several synonymous variants were identified in both genes in cBCL and non-cBCL samples, which likely represent polymorphisms. These results indicate TRAF3 and POT1 mutations are common in cBCL. Deleterious TRAF3 and POT1 mutations were only identified in dogs with cBCL, and not in dogs with non-cBCL diseases, suggesting they are important in the pathogenesis of cBCL. Future studies to investigate the prognostic and therapeutic implications of these mutations are required.

IL-17A modulates osteoclast precursors' apoptosis through autophagy-TRAF3 signaling during osteoclastogenesis

Osteoclasts play an important role in bone remodeling. The inflammatory cytokine IL-17A could modulate the RANKL-induced osteoclastogenesis by regulating the autophagic activity. It is well accepted that protective autophagy has an anti-apoptotic effect. It is necessary to elucidate whether IL-17A can influence the apoptosis of osteoclast precursors (OCPs) through autophagy responses during osteoclastogenesis. The results showed that apoptosis of RAW264.7-derived OCPs was promoted by high levels of IL-17A, but the opposite anti-apoptotic function was shown by low levels of IL-17A. Furthermore, the enhanced apoptosis by high levels of IL-17A was reversed by overexpression of autophagy protein Beclin1; conversely, the inhibited apoptosis by low levels of IL-17A was restored by knockdown of Beclin1. It was also found that Beclin1 suppression with Beclin1 inhibitor (spautin1) could block the reduced apoptosis by low levels of IL-17A, which was recovered by TRAF3 knockdown. Moreover, the enhanced apoptosis by high levels of IL-17A decreased following the downregulation of TRAF3. Importantly, overexpression of caspase3 further attenuated osteoclastogenesis treated by high levels of IL-17A, without significantly affecting osteoclastogenesis stimulated by low levels of IL-17A. In conclusion, IL-17A modulates apoptosis of OCPs through Beclin1-autophagy-TRAF3 signaling pathway, thereby influencing osteoclastogenesis. Therefore, our study sheds lights on the improvement of clinical strategies of dental implantation or orthodontic treatment by revealing the novel targets in the bone remodeling.

Molecular characterization and expression analysis of tumor necrosis factor receptor-associated factors 3 and 6 in large yellow croaker (Larimichthys crocea)

The large yellow croaker (Larimichthys crocea) has a well-developed innate immune system. To gain a better understanding of the defense mechanisms involved in this system, we studied tumor necrosis factor receptor-associated factors (TRAFs), which play important roles in the Toll-like receptor (TLR) pathway. We characterized the full-length open reading frames and protein structures of TRAF3 and TRAF6 to determine their identities, and conducted phylogenetic analysis to determine their evolutionary relationships. To assess the roles of TRAFs in innate immune responses in the large yellow croaker, we performed quantitative reverse-transcription PCR (qRT-PCR) to characterize expression profiles in a range of tissues at different stages after challenge with polyinosinic polycytidylic acid (poly I:C) and Vibrio anguillarum. Following poly I:C challenge, the expression levels of TRAF3 and TRAF6 were highest in the kidneys and lowest in the spleen, whereas after infection with V. anguillarum, TRAF6 expression was the highest in the kidneys and lowest in the liver.

Negative Regulation of Interferon-β Production by Alternative Splicing of Tumor Necrosis Factor Receptor-Associated Factor 3 in Ducks

Tumor necrosis factor receptor-associated factor 3 (TRAF3), an intracellular signal transducer, is identified as an important component of Toll-like receptors and RIG-I-like receptors induced type I interferon (IFN) signaling pathways. Previous studies have clarified TRAF3 function in mammals, but little is known about the role of TRAF3 in ducks. Here, we cloned and characterized the full-length duck TRAF3 (duTRAF3) gene and an alternatively spliced isoform of duTRAF3 (duTRAF3-S) lacking the fragment encoding amino acids 217–319, from duck embryo fibroblasts (DEFs). We found that duTRAF3 and duTRAF3-S played different roles in regulating IFN-β production in DEFs. duTRAF3 through its TRAF domain interacted with duMAVS or duTRIF, leading to the production of IFN-β. However, duTRAF3-S, containing the TRAF domain, was unable to bind duMAVS or duTRIF due to the intramolecular binding between the N- and C-terminal of duTRAF3-S that blocked the function of its TRAF domain. Further analysis identified that duTRAF3-S competed with duTRAF3 itself for binding to duTRAF3, perturbing duTRAF3 self-association, which impaired the assembly of duTRAF3-duMAVS/duTRIF complex, ultimately resulted in a reduced production of IFN-β. These findings suggest that duTRAF3 is an important regulator of duck innate immune signaling and reveal a novel mechanism for the negative regulation of IFN-β production via changing the formation of the homo-oligomerization of wild molecules, implying a novel regulatory role of truncated proteins.

Molecular and functional characterization of pigeon (Columba livia) tumor necrosis factor receptor-associated factor 3

Tumor necrosis factor receptor-associated factor 3 (TRAF3) plays a key antiviral role by promoting type I interferon production. We cloned the pigeon TRAF3 gene (PiTRAF3) according to its predicted mRNA sequence to investigate its function. The 1704-bp full-length open reading frame encodes a 567-amino acid protein. One Ring finger, two TRAF-type Zinc fingers, one Coiled coil, and one MATH domain were inferred. RT-PCR showed that PiTRAF3 was expressed in all tissues, with relatively weak expression in the heart and liver. In HEK293T cells, over-expression of wild-type, △Ring, △Zinc finger, and △Coiled coil PiTRAF3, but not a △MATH form, significantly increased IFN-β promoter activity. Zinc finger and Coiled coil domains were essential for NF-κB activation. In chicken HD11 cells, PiTRAF3 increased IFN-β promoter activity and four domains were all contributing. R848 stimulation of pigeon peripheral blood mononuclear cells and splenocytes significantly increased expression of PiTRAF3 and the inflammatory cytokine genes CCL5, IL-8, and IL-10. These data demonstrate TRAF3's innate immune function and improve understanding of its involvement in poultry antiviral defense.

DDX3 directly regulates TRAF3 ubiquitination and acts as a scaffold to co-ordinate assembly of signalling complexes downstream from MAVS

The human DEAD-box helicase 3 (DDX3) has been shown to contribute to type I interferon (IFN) induction downstream from antiviral pattern recognition receptors. It binds to TANK-binding kinase 1 and IκB-kinase-ε (IKKε), the two key kinases mediating activation of IFN regulatory factor (IRF) 3 and IRF7. We previously demonstrated that DDX3 facilitates IKKε activation downstream from RIG-I and then links the activated kinase to IRF3. In the present study, we probed the interactions between DDX3 and other key signalling molecules in the RIG-I pathway and identified a novel direct interaction between DDX3 and TNF receptor-associated factor 3 (TRAF3) mediated by a TRAF-interaction motif in the N-terminus of DDX3, which was required for TRAF3 ubiquitination. Interestingly, we observed two waves of K63-linked TRAF3 ubiquitination following RIG-I activation by Sendai virus (SeV) infection, both of which were suppressed by DDX3 knockdown. We also investigated the spatiotemporal formation of endogenous downstream signalling complexes containing the mitochondrial antiviral signalling (MAVS) adaptor, DDX3, IκB-kinase-ε (IKKε), TRAF3 and IRF3. DDX3 was recruited to MAVS early after SeV infection, suggesting that it might mediate subsequent recruitment of other molecules. Indeed, knockdown of DDX3 prevented the formation of TRAF3-MAVS and TRAF3-IKKε complexes. Based on our data, we propose that early TRAF3 ubiquitination is required for the formation of a stable MAVS-TRAF3 complex, while the second wave of TRAF3 ubiquitination mediates IRF3 recruitment and activation. Our study characterises DDX3 as a multifunctional adaptor molecule that co-ordinates assembly of different TRAF3, IKKε and IRF3-containing signalling complexes downstream from MAVS. Additionally, it provides novel insights into the role of TRAF3 in RIG-I signalling.

Identification and characterization of tumor necrosis factor receptor (TNFR)-associated factor 3 from humphead snapper, Lutjanus sanguineus

Tumor necrosis factor receptor (TNFR)-associated factor 3(TRAF3) is a key regulator in TNFR and Toll-like receptor (TLRs)/RIG-I-like receptors (RLRs) signal pathway. Here, a TRAF3 gene (Ls-TRAF3, GenBank Accession No: KJ789921) is cloned from humphead snapper (Lutjanus sanguineus). The Ls-TRAF3 cDNA contains an open reading frame of 1788 bp, which encodes a polypeptide of 595 amino acids. The deduced amino acid of Ls-TRAF3 possesses a RING finger, two TRAF-type zinc fingers, a coiled-coil and a MATH domain. Ls-TRAF3 protein shares high identities with other known TRAF3 proteins. In healthy fish, Ls-TRAF3 transcripts were broadly expressed in all examined tissues with highest expression levels in spleen, liver and head kidney. Quantitative real-time PCR (qRT-PCR) analysis revealed that Ls-TRAF3 could be induced by bacteria or viral PAMP poly I:C stimulation in vivo. Here, we also showed Ls-TRAF3 that, positively regulated IRF3 and Mx upon poly I:C stimuli, whereas prevented production of proinflammatory cytokine IL-6 after LPS injection. Moreover, over-expression of wide type (WT) Ls-TRAF3 and truncated forms, including ΔZinc finger 1, ΔZinc finger 2 and Δcoiled-coil suppressed NF-κB activity significantly, whereas the inhibitory effect of NF-κB was partially impaired when the RING finger or MATH domain deletion, suggesting the latter was more important for downstream signal transduction. Taken together, these results implicated that Ls-TRAF3 might play regulatory roles in immune response to pathogen invasion.

TRAF molecules in cell signaling and in human diseases

The tumor necrosis factor receptor (TNF-R)-associated factor (TRAF) family of intracellular proteins were originally identified as signaling adaptors that bind directly to the cytoplasmic regions of receptors of the TNF-R superfamily. The past decade has witnessed rapid expansion of receptor families identified to employ TRAFs for signaling. These include Toll-like receptors (TLRs), NOD-like receptors (NLRs), RIG-I-like receptors (RLRs), T cell receptor, IL-1 receptor family, IL-17 receptors, IFN receptors and TGFβ receptors. In addition to their role as adaptor proteins, most TRAFs also act as E3 ubiquitin ligases to activate downstream signaling events. TRAF-dependent signaling pathways typically lead to the activation of nuclear factor-κBs (NF-κBs), mitogen-activated protein kinases (MAPKs), or interferon-regulatory factors (IRFs). Compelling evidence obtained from germ-line and cell-specific TRAF-deficient mice demonstrates that each TRAF plays indispensable and non-redundant physiological roles, regulating innate and adaptive immunity, embryonic development, tissue homeostasis, stress response, and bone metabolism. Notably, mounting evidence implicates TRAFs in the pathogenesis of human diseases such as cancers and autoimmune diseases, which has sparked new appreciation and interest in TRAF research. This review presents an overview of the current knowledge of TRAFs, with an emphasis on recent findings concerning TRAF molecules in signaling and in human diseases.

Roles of tumor necrosis factor receptor associated factor 3 (TRAF3) and TRAF5 in immune cell functions

A large and diverse group of receptors utilizes the family of cytoplasmic signaling proteins known as tumor necrosis factor receptor (TNFR)-associated factors (TRAFs). In recent years, there has been a resurgence of interest and exploration of the roles played by TRAF3 and TRAF5 in cellular regulation, particularly in cells of the immune system, the cell types of focus in this review. This work has revealed that TRAF3 and TRAF5 can play diverse roles for different receptors even in the same cell type, as well as distinct roles in different cell types. Evidence indicates that TRAF3 and TRAF5 play important roles beyond the TNFR-superfamily (SF) and viral mimics of its members, mediating certain innate immune receptor and cytokine receptor signals, and most recently, signals delivered by the T-cell receptor (TCR) signaling complex. Additionally, much research has demonstrated the importance of TRAF3-mediated cellular regulation via its cytoplasmic interactions with additional signaling proteins. In particular, we discuss below evidence for the participation by TRAF3 in a number of the regulatory post-translational modifications involving ubiquitin that are important in various signaling pathways.

Enhanced Toll-like receptor (TLR) responses of TNFR-associated factor 3 (TRAF3)-deficient B lymphocytes

The key role of TRAF6 in TLR signaling pathways is well known. More recent evidence has implicated TRAF3 as another TRAF family member important to certain TLR responses of myeloid cells. Previous studies demonstrate that TRAF3 functions are highly context-dependent, displaying receptor and cell-type specificity. We thus examined the TLR responses of TRAF3(-/-)mouse B lymphocytes to test the hypothesis that TRAF3 plays distinct roles in such responses, depending on cell type. TRAF3(-/-) DC are known to have a defect in type 1 IFN production and here, showed diminished production of TNF and IL-10 and unaltered IL-6. In marked contrast, TRAF3(-/-) B cells made elevated amounts of TNF and IL-6 protein, as well as IL-10 and IP-10 mRNA, in response to TLR ligands. Also, in contrast to TRAF3(-/-) DC, the type 1 IFN pathway was elevated in TRAF3(-/-) B cells. Increased early responses of TRAF3(-/-) B cells to TLR signals were independent of cell survival or proliferation but associated with elevated canonical NF-κB activation. Additionally, TRAF3(-/-) B cells displayed enhanced TLR-mediated expression of AID and Ig isotype switching. Thus, TRAF3 plays varied and cell type-specific, biological roles in TLR responses.

Allosteric regulation of the ubiquitin:NIK and ubiquitin:TRAF3 E3 ligases by the lymphotoxin-beta receptor

The lymphotoxin-beta receptor (LTbetaR) activates the NF-kappaB2 transcription factors, p100 and RelB, by regulating the NF-kappaB-inducing kinase (NIK). Constitutive proteosomal degradation of NIK limits NF-kappaB activation in unstimulated cells by the ubiquitin:NIK E3 ligase comprised of subunits TNFR-associated factors (TRAF)3, TRAF2, and cellular inhibitor of apoptosis (cIAP). However, the mechanism releasing NIK from constitutive degradation remains unclear. We found that insertion of a charge-repulsion mutation in the receptor-binding crevice of TRAF3 ablated binding of both LTbetaR and NIK suggesting a common recognition site. A homologous mutation in TRAF2 inhibited cIAP interaction and blocked NIK degradation. Furthermore, the recruitment of TRAF3 and TRAF2 to the ligated LTbetaR competitively displaced NIK from TRAF3. Ligated LTbetaR complexed with TRAF3 and TRAF2 redirected the specificity of the ubiquitin ligase reaction to polyubiquitinate TRAF3 and TRAF2, leading to their proteosomal degradation. Stimulus-dependent degradation of TRAF3 required the RING domain of TRAF2, but not of TRAF3, implicating TRAF2 as a key E3 ligase in TRAF turnover. The combined action of competitive displacement of NIK and TRAF degradation halted NIK turnover, and promoted its association with IKKalpha and signal transmission. These results indicate the LTbetaR modifies the ubiquitin:NIK E3 ligase, and also acts as an allosteric regulator of the ubiquitin:TRAF E3 ligase.

TRAF3 controls activation of the canonical and alternative NFkappaB by the lymphotoxin beta receptor

Components of lymphotoxin beta receptor (LTBR)-associated signaling complexes, including TRAF2, TRAF3, NIK, IKK1, and IKK2 have been shown to participate in the coupling of LTBR to NFkappaB. Here, we report that TRAF3 functions as a negative regulator of LTBR signaling via both canonical and non-canonical NFkappaB pathways by two distinct mechanisms. Analysis of NFkappaB signaling in cell lines with functionally intact NFkappaB pathway but lacking LTBR-mediated induction of NFkappaB target genes revealed an inverse association of cellular TRAF3 levels with LTBR-specific defect in canonical NFkappaB activation. Increased expression of TRAF3 correlated with its increased recruitment to LTBR-induced signaling complexes, decreased recruitment of TRAF2, and attenuated phosphorylation of IkappaB alpha and RelA. In contrast, activation of NFkappaB by TNF did not depend on TRAF3 levels. siRNA-mediated depletion of TRAF3 promoted recruitment of TRAF2 and IKK1 to activated LTBR, enabling LTBR-inducible canonical NFkappaB signaling and NFkappaB target gene expression. TRAF3 knock-down also increased mRNA and protein expression of several non-canonical NFkappaB components, including NFkappaB2/p100, RelB, and NIK, accompanied by processing of NFkappaB2/p100 into p52. These effects of TRAF3 depletion did not require LTBR signaling and were consistent with autonomous activation of the non-canonical NFkappaB pathway. Our data illustrate the function of TRAF3 as a dual-mode repressor of LTBR signaling that controls activation of canonical NFkappaB, and de-repression of the intrinsic activity of non-canonical NFkappaB. Modulation of cellular TRAF3 levels may thus contribute to regulation of NFkappaB-dependent gene expression by LTBR by affecting the balance of LTBR-dependent activation of canonical and non-canonical NFkappaB pathways.

The tyrosine kinase c-Src enhances RIG-I (retinoic acid-inducible gene I)-elicited antiviral signaling

Antiviral immune responses are initiated through Toll-like receptors (TLRs) and RIG-I (retinoic acid-inducible gene-I)-like RNA helicases that recognize nucleic acids from distinct viruses. In this study, we show that the tyrosine kinase c-Src participates in antiviral responses induced by the cytoplasmic RNA helicase RIG-I. Sendai virus (SV), which is recognized by RIG-I, induced c-Src phosphorylation. Functional impairment of c-Src through chemical inhibition or transient expression of a c-Src kinase-inactive mutant attenuated production of endogenous antiviral proteins after SV infection or after expression of RIG-I or its adapter protein MAVS. Importantly, SV-stimulated synthesis of antiviral proteins was significantly impaired in cells treated with c-Src small interfering RNA and in cells from c-Src-deficient mice. In addition, we found that c-Src interacted with components of the RIG-I pathway: RIG-I, MAVS, and TRAF3 (tumor necrosis factor receptor-associated factor-3). The interaction between c-Src and TRAF3 was found to occur within the RING domain of TRAF3. Taken together, our results suggest that c-Src enhances RIG-I-mediated signaling, acting at the level of TRAF3.

Protein-protein interactions in TRAF3

TNF-receptor-associated factors (TRAFs) are intracellular proteins that bind to the cytoplasmic portion of TNF receptors and mediate downstream signaling. The six known TRAF proteins play overlapping yet distinct roles in controlling immune responses as well as cellular processes such as activation of NF-kappaB and JNK signaling pathways. For example, CD40 binds to TRAF2, TRAF3 and TRAF6 to control B cell differentiation, proliferation and growth. In contrast, binding of lymphotoxin-beta receptor (LTbetaR) to TRAF2 and TRAF5 propagates signals leading to activation of NF-kappaB, while binding to TRAF3 induces negative regulation of this pathway and leads to apoptosis in tumor cells. Binding recognition is mediated by specific contacts of a consensus recognition sequence in the partner with residues in a hydrophobic crevice on the TRAF molecule. Since each of these protein-protein interactions occurs within this same binding crevice, it appears that TRAF-mediated cellular mechanisms may be regulated, in part, by the level of expression or recruitment of the adaptor proteins or receptors that are competing for the crevice. The specific contacts of CD40, LTbetaR and BAFF-R have been defined in crystal structures of the complex with TRAF3. In addition, the downstream regulator TANK and the viral oncogenic protein LMP1 from the Epstein Barr virus also bind to the same TRAF crevice and these contacts have also been described crystallographically. Comparison of these five crystal structures has revealed that the recognition motifs in each of these proteins are accommodated in one TRAF3 binding crevice and that the binding interface is structurally and functionally adaptive. In this chapter, the molecular details of the interactions will be described and correlated with the functional implications for multiple TRAF3 roles in cellular regulation.

LMP1 TRAFficking Activates Growth and Survival pathways

Epstein-Barr Virus (EBV) Latent Infection Membrane Protein 1 (LMP1) is expressed in all the EBV related malignancies. LMP1 expression is critical for transformation of human B-cells by EBV. LMP1 expression in human B cells induces activation and adhesion molecule expression and cell dumping, which are characteristic of CD40 activated B lymphocytes. In immortalized fibroblasts, LMP1 mimics aspects of activated ras in enabling serum, contact, and anchorage independent growth. Reverse genetic analyses implicate six transmembrane domains (TM), TM1-6, and two C-terminal cytosolic domains, transformation effector sites 1 and 2 (TES1 and 2) or C-terminal activation regions 1 and 2 (CTAR1 and 2) as the essential domains for LMP1 effects. The 6 transmembrane domains cause intermolecular interaction, whereas the C-terminal domains signal through tumor necrosis factor receptor (TNFR) associated factors (TRAFs) or TNFR associated death domain proteins (TRADD) and activate NF-kappaB, JNK, and p38. LMP1 TES1/CTAR1 directly recruits TRAFs 1, 2, 3 and 5 whereas LMP1 TES2/CTAR2 indirectly recruits TRAF6 via BS69. LMP1 TES1/CTAR1 activates TRAF2, NIK, IKKalpha and p52 mediated noncanonical NF-KB pathway and LMP1 TES2/CTAR2 activates TRAF6, TAB1, TAK1, IKKalpha/ IKKbeta/ IKKgamma mediated canonical NF-KB pathway. Interestingly, TRAF3 is a negative regulator of noncanonical NF-kappaB activation, although a positive role in LMP1 signaling has also been described. LMP1 mediated JNK activation is predominantly TES2/CTAR2 dependent and requires TRAF6. LMP1 specifically increases TRAF3 partitioning into lipid rafts and interestingly does not induce degradation of any of the TRAFs upon NF-kappaB activation. Studies of the chemistry and biology of LMP1-TRAF interaction mediated activation of signaling pathways are important for controlling EBV infected cell survival and growth.

Contaminating cells alter gene signatures in whole organ versus laser capture microdissected tumors: a comparison of experimental breast cancers and their lymph node metastases

Genome-wide expression profiling has expedited our molecular understanding of the different subtypes of breast cancers, as well as defined the differences among genes expressed in primary tumors and their metastases. Laser-capture microdissection (LCM) coupled to gene expression analysis allows us to understand how specific cell types contribute to the total cancer gene expression signature. Expression profiling was used to define genes that contribute to breast cancer spread into and/or growth within draining lymph nodes (LN). Whole tumor xenografts and their matched whole LN metastases were compared to LCM captured cancer cells from the same tumors and matched LN metastases. One-thousand nine-hundred thirty genes were identified by the whole organ method alone, and 1,281 genes by the LCM method alone. However, less than 1% (30 genes) of genes that changed between tumors and LN metastases were common to both methods. Several of these genes have previously been implicated in cancer aggressiveness. Our data show that whole-organ and LCM based gene expression profiling yield distinctly different lists of metastasis-promoting genes. Contamination of the tumor cells, and cross reactivity of mouse RNA to human-specific chips may explain these differences, and suggests that LCM-derived data may be more accurate.

Physiological roles and mechanisms of signaling by TRAF2 and TRAF5

RAF2 and TRAF5 are closely related members of the TRAF family of proteins. They are important signal transducers for a wide range of TNF receptor superfamily members, including TNFR1, TNFR2, CD40 and other lymphocyte costimulatory receptors, RANK/TRANCE-R, EDAR, LTbetaR, LMP-1 and IRE1. TRAF2 andTRAF5 therefore regulate diverse physiological roles, ranging from T and B cell signaling and inflammatory responses to organogenesis and cell survival. The major pathways mediated by TRAF2 and TRAF5 are the classical and alternative pathways of NF-kappaB activation, and MAPK and JNK activation. TRAF2 is heavily regulated by ubiquitin signals, and many of the signaling functions of TRAF2 are mediated through its RING domain and likely its own role as an E3 ubiquitin ligase.

Caspase inhibition sensitizes inhibitor of NF-kappaB kinase beta-deficient fibroblasts to caspase-independent cell death via the generation of reactive oxygen species

Cells lacking functional NF-kappaB die after ligation of some tumor necrosis factor (TNF) receptor family members through failure to express NF-kappaB-dependent anti-apoptotic genes. NF-kappaB activation requires the IkappaB kinase (IKK) complex containing two catalytic subunits named IKKalpha and IKKbeta that regulate distinct NF-kappaB pathways. IKKbeta is critical for classical signaling that induces pro-inflammatory and anti-apoptotic gene profiles, whereas IKKalpha regulates the non-canonical pathway involved in lymphoid organogenesis and B-cell development. To determine whether IKKalpha and IKKbeta differentially function in rescuing cells from death induced by activators of the classical and non-canonical pathways, we analyzed death after ligation of the TNF and lymphotoxin-beta receptors, respectively. Using murine embryonic fibroblasts (MEFs) lacking each of the IKKs, the caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone, and dominant negative Fas-associated death domain protein, we found that deletion of these kinases sensitized MEFs to distinct cell death pathways. MEFs lacking IKKalpha were sensitized to death in response to both cytokines that was entirely caspase-dependent, demonstrating that IKKalpha functions in this process. Surprisingly, death of IKKbeta-/- MEFs was not blocked by caspase inhibition, demonstrating that IKKbeta negatively regulates caspase-independent cell death (CICD). CICD was strongly activated by both TNF and lymphotoxin-beta receptor ligation in IKKbeta-/- MEFs and was accompanied by loss of mitochondrial membrane potential and the generation of reactive oxygen species. CICD was inhibited by the anti-oxidant butylated hydroxyanosole and overexpression of Bcl-2, neither of which blocked caspase-dependent apoptosis. Our findings, therefore, demonstrate that both IKKalpha and IKKbeta regulate cytokine-induced apoptosis, and IKKbeta additionally represses reactive oxygen species- and mitochondrial-dependent CICD.

Complementary action of the PGC-1 coactivators in mitochondrial biogenesis and brown fat differentiation

Mitochondria play an essential role in the ability of brown fat to generate heat, and the PGC-1 coactivators control several aspects of mitochondrial biogenesis. To investigate their specific roles in brown fat cells, we generated immortal preadipocyte lines from the brown adipose tissue of mice lacking PGC-1alpha. We could then efficiently knockdown PGC-1beta expression by shRNA expression. Loss of PGC-1alpha did not alter brown fat differentiation but severely reduced the induction of thermogenic genes. Cells deficient in either PGC-1alpha or PGC-1beta coactivators showed a small decrease in the differentiation-dependant program of mitochondrial biogenesis and respiration; however, this increase in mitochondrial number and function was totally abolished during brown fat differentiation when both PGC-1alpha and PGC-1beta were deficient. These data show that PGC-1alpha is essential for brown fat thermogenesis but not brown fat differentiation, and the PGC-1 coactivators play an absolutely essential but complementary function in differentiation-induced mitochondrial biogenesis.

The TRAF3-binding site of human molluscipox virus FLIP molecule MC159 is critical for its capacity to inhibit Fas-induced apoptosis

Members of the viral Flice/caspase-8 inhibitory protein (v-FLIP) family prevent induction of apoptosis by death receptors through inhibition of the processing and activation of procaspase-8 and -10 at the level of the receptor-associated death-inducing signaling complex (DISC). Here, we have addressed the molecular function of the v-FLIP member MC159 of the human molluscum contagiosum virus. MC159 FLIP powerfully inhibited both caspase-dependent and caspase-independent cell death induced by Fas. The C-terminal region of MC159 bound TNF receptor-associated factor (TRAF)3, was necessary for optimal TRAF2 binding, and mediated the recruitment of both TRAFs into the Fas DISC. TRAF-binding-deficient mutants of MC159 showed impaired inhibition of FasL-induced caspase-8 processing and Fas internalization, and had reduced antiapoptotic activity. Our findings provide evidence that a MC159/TRAF2/TRAF3 complex regulates a new aspect of Fas signaling, and identify MC159 FLIP as a molecule that targets multiple features of Fas-induced cell death.

A novel mechanism of CD40-induced apoptosis of carcinoma cells involving TRAF3 and JNK/AP-1 activation

Membrane-presented CD40 agonists can induce apoptosis in carcinoma, but not normal homologous epithelial cells, whereas soluble agonists are growth inhibitory but not proapoptotic unless protein synthesis is blocked. Here we demonstrate that membrane-presented CD40 ligand (CD154) (mCD40L), but not soluble agonists, triggers cell death in malignant human urothelial cells via a direct mechanism involving rapid upregulation of TNFR-associated factor (TRAF)3 protein, without concomitant upregulation of TRAF3 mRNA, followed by activation of the c-Jun N-terminal kinase (JNK)/activator protein-1 (AP-1) pathway and induction of the caspase-9/caspase-3-associated intrinsic apoptotic machinery. TRAF3 knockdown abrogated JNK/AP-1 activation and prevented CD40-mediated apoptosis, whereas restoration of CD40 expression in CD40-negative carcinoma cells restored apoptotic susceptibility via the TRAF3/AP-1-dependent mechanism. In normal human urothelial cells, mCD40L did not trigger apoptosis, but induced rapid downregulation of TRAF2 and 3, thereby paralleling the situation in B-lymphocytes. Thus, TRAF3 stabilization, JNK activation and caspase-9 induction define a novel pathway of CD40-mediated apoptosis in carcinoma cells.

Modulation of life and death by the tumor necrosis factor receptor-associated factors (TRAFs)

The TNF receptor-associated factor (TRAF) family is a group of adapter proteins that link a wide variety of cell surface receptors. Including the TNF and IL-1 receptor superfamily to diverse signaling cascades, which lead to the activation of NF-kappaB and mitogen-activated protein kinases. In addition, TRAFs interact with a variety of proteins that regulate receptor-induced cell death or survival. Thus, TRAF-mediated signals may directly induce cell survival or interfere with the death receptor-induced apoptosis.

Network communications: lymphotoxins, LIGHT, and TNF

Lymphotoxins (LT) provide essential communication links between lymphocytes and the surrounding stromal and parenchymal cells and together with the two related cytokines, tumor necrosis factor (TNF) and LIGHT (LT-related inducible ligand that competes for glycoprotein D binding to herpesvirus entry mediator on T cells), form an integrated signaling network necessary for efficient innate and adaptive immune responses. Recent studies have identified signaling pathways that regulate several genes, including chemokines and interferons, which participate in the development and function of microenvironments in lymphoid tissue and host defense. Disruption of the LT/TNF/LIGHT network alleviates inflammation in certain autoimmune disease models, but decreases resistance to selected pathogens. Pharmacological disruption of this network in human autoimmune diseases such as rheumatoid arthritis alleviates inflammation in a significant number of patients, but not in other diseases, a finding that challenges our molecular paradigms of autoimmunity and perhaps will reveal novel roles for this network in pathogenesis.

TRAF2 plays a key, nonredundant role in LIGHT-lymphotoxin beta receptor signaling

LIGHT is a member of the tumor necrosis factor (TNF) superfamily, and its function is mediated by at least two receptors, including lymphotoxin beta receptor (LTbetaR) and herpes simplex virus entry mediator. However, the molecular mechanism of LIGHT signaling mediated by LTbetaR has not been clearly defined. In this report, we demonstrate that TRAF2 is critical for LIGHT- and LTbetaR-mediated activation of both the transcription factor NF-kappaB and the mitogen-activated protein kinase JNK. In HeLa cells, LIGHT induces NF-kappaB and JNK activation, which can be blocked by the dominant negative mutant of TRAF2. In these cells, LIGHT causes the recruitment of TRAF2, TRAF3, and IkappaB kinase into the LTbetaR complex. Importantly, while both NF-kappaB and JNK are activated by LIGHT in wild-type mouse embryonic fibroblasts, no activation of either of these two pathways is observed in TRAF2 null fibroblasts. However, LIGHT-induced NF-kappaB and JNK activation can be restored by ectopic expression of TRAF2 in TRAF2-/- cells. Interestingly, in contrast to TNF signaling, the activation of both NF-kappaB and JNK by LIGHT was normal in RIP-/- and TRAF5-/- cells. Taken together, our data demonstrate that TRAF2, an important effector molecule of TNF signaling, plays a critical, nonredundant role in LIGHT-LTbetaR signaling.

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.

Mechanism of LIGHT/interferon-?-induced cell death in HT-29 cells

LIGHT is a member of tumor necrosis factor (TNF) superfamily, and previous studies have indicated that in the presence of interferon-gamma (IFN-gamma), LIGHT through LTbetaR signaling can induce cell death with features unlike classic apoptosis. In present study, we investigated the mechanism of LIGHT/IFN-gamma-induced cell death in HT-29 cells, where the cell death was profoundly induced when sub-toxic concentrations of LIGHT and IFN-gamma were co-treated. LIGHT/IFN-gamma-induced cell death was accompanied by DNA fragmentation and slight LDH release. This effect was not affected by caspase, JNK nor cathepsin B inhibitors, but was partially prevented by p38 mitogen-activated protein kinase (MAPK) and poly (ADP-ribose) polymerase (PARP) inhibitors, and abolished by aurintricarboxylic acid (ATA), which is an inhibitor of endonuclease and STATs signaling of IFN-gamma. Immunobloting reveals that LIGHT/IFN-gamma could induce p38 MAPK activity, Bak and Fas expression, but down-regulate Mcl-1. Besides, LIGHT/IFN-gamma could not activate caspase-3 and -9, but decreased mitochondrial membrane potential. Although LIGHT could not affect IFN-gamma-induced STAT1 phosphorylation and transactivation activity, which was required for the sensitization of cell death, survival NF-kappaB signaling of LIGHT was inhibited by IFN-gamma. These data suggest that co-presence of LIGHT and IFN-gamma can induce an integrated interaction in signaling pathways, which lead to mitochondrial dysfunction and mix-type cell death, not involving caspase activation.

T3JAM, a novel protein that specifically interacts with TRAF3 and promotes the activation of JNK(1)

Previous studies suggest that localization of tumor necrosis factor receptor (TNFR)-associated factor (TRAF) family members is important for regulating their signal transduction. During a screen for TRAF3-associated proteins that potentially alter TRAF3 subcellular localization and enable signal transduction, we identified a novel protein, T3JAM (TRAF3-interacting Jun N-terminal kinase (JNK)-activating modulator). This protein associates specifically with TRAF3 but not other TRAF family members. Coexpression of T3JAM with TRAF3 recruits TRAF3 to the detergent-insoluble fraction. More importantly, T3JAM and TRAF3 synergistically activate JNK but not nuclear factor (NF)-kappaB. Our studies indicate that T3JAM may function as an adapter molecule that specifically regulates TRAF3-mediated JNK activation.

Structurally distinct recognition motifs in lymphotoxin-beta receptor and CD40 for tumor necrosis factor receptor-associated factor (TRAF)-mediated signaling

Lymphotoxin-beta receptor (LTbetaR) and CD40 are members of the tumor necrosis factor family of signaling receptors that regulate cell survival or death through activation of NF-kappaB. These receptors transmit signals through downstream adaptor proteins called tumor necrosis factor receptor-associated factors (TRAFs). In this study, the crystal structure of a region of the cytoplasmic domain of LTbetaR bound to TRAF3 has revealed an unexpected new recognition motif, 388IPEEGD393, for TRAF3 binding. Although this motif is distinct in sequence and structure from the PVQET motif in CD40 and PIQCT in the regulator TRAF-associated NF-kappaB activator (TANK), recognition is mediated in the same binding crevice on the surface of TRAF3. The results reveal structurally adaptive "hot spots" in the TRAF3-binding crevice that promote molecular interactions driving specific signaling after contact with LTbetaR, CD40, or the downstream regulator TANK.

TNF-receptor-associated factors as targets for drug development

TNF-receptor-associated factors (TRAFs) are the bottleneck of the TNF-receptor (TNF-R) family signal transduction. They integrate the signalling from many members of the TNF-R family and initiate intracellular signalling cascades aimed at the activation of NF-kappaB and c-jun, the reprogramming of gene expression and the control of cell death. Deregulation of these pathways is the cause of several autoimmune and inflammatory diseases. The specificity and interaction of the members of the TRAF family with the TNF-R entails the recognition of just a 4 - 6 amino acid motif in the cytosolic region of the receptor, suitable as an attractive target for drug discovery. This review summarises the current knowledge on TRAFs and discusses the pros and cons of their application as targets for drug discovery.

The role of apoptosis signal-regulating kinase 1 in lymphotoxin-beta receptor-mediated cell death

LIGHT (homologous to lymphotoxins, shows inducible expression, and competes with herpes simplex virus glycoprotein D for herpesvirus entry mediator, a receptor expressed by T lymphocytes) is a member of the tumor necrosis factor superfamily that can interact with lymphotoxin-beta receptor (LTbetaR), herpes virus entry mediator, and decoy receptor (DcR3). In our previous study, we showed that LIGHT is able to induce cell death via the non-death domain containing receptor LTbetaR to activate both caspase-dependent and caspase-independent pathway. In this study, a LIGHT mutein, LIGHT-R228E, was shown to exhibit similar binding specificity as wild type LIGHT to LTbetaR, but lose the ability to interact with herpes virus entry mediator. By using both LIGHT-R228E and agonistic anti-LTbetaR monoclonal antibody, we found that signaling triggered by LTbetaR alone is sufficient to activate both caspase-dependent and caspase-independent pathways. Cross-linking of LTbetaR is able to recruit TRAF3 and TRAF5 to activate ASK1, whereas its activity is inhibited by free radical scavenger carboxyfullerenes. The activation of ASK1 is independent of caspase-3 activation, and kinase-inactive ASK1-KE mutant can inhibit LTbetaR-mediated cell death. This suggests that ASK1 is one of the factors involved in the caspase-independent pathway of LTbetaR-induced cell death.

Endogenous association of TRAF2, TRAF3, cIAP1, and Smac with lymphotoxin beta receptor reveals a novel mechanism of apoptosis

Lymphotoxin-beta receptor (LT beta R) is a member of tumor necrosis factor receptor family and plays essential roles in the embryonic development and organization of secondary lymphoid tissues. It binds two types of tumor necrosis factor family cytokines, heterotrimer LT alpha 1 beta 2 and homotrimer LIGHT, and activates multiple signaling pathways including transcriptional factor NF kappa B, c-Jun N-terminal kinase, and cell death. However, the molecular mechanism of the activation of these signaling pathways by LT beta R is not clear. Because there is no enzymatic activity associated with the receptor itself, the signal transduction of LT beta R is mediated by cytoplasmic proteins recruited to receptors. To identify these proteins, we took a proteomic approach. The endogenous LIGHT.LT beta R complex was affinity-purified from U937 cells, and proteins associated with the complex were identified by mass spectrometry. Four of five proteins identified, TRAF2, TRAF3, cIAP1, and Smac, are reported here. Their association with LT beta R was further confirmed by coimmunoprecipitation in U937 cells and HEK293 cells. The presence of cIAP1 and Smac in LIGHT.LT beta R complex revealed a novel mechanism of LIGHT.LT beta R-induced apoptosis.

Structurally adaptive hot spots at a protein interaction interface on TRAF3

Tumor necrosis factor (TNF) signaling is controlled by receptors and intracellular signaling pathways that activate the NF-kappaB transcription factor. The resulting signals elicit immune responses and have important implications for disorders such as autoimmunity or allergic reactions. TNF-receptor-associated factors (TRAFs) bind to the cytoplasmic portion of TNFRs as well as downstream regulators and thus are co-inducers of the signal transduction. TRAF3 binds to diverse receptors and regulators by accomodating a conserved motif that is embedded in completely different structural frameworks. Thus, the protein-protein contact region on TRAF3 represents a binding interface that is structurally and functionally adaptive. In this report, three 'hot spots' at the TRAF3 protein-interaction interface are defined that provide the principal contact regions for different binding partners. The side-chains of residues at these 'hot spots' are flexible and undergo movements on binding the different partners. These side chain rearrangements provide a structural adaptability that promotes interaction with a variety of distinct proteins. It is proposed that similar adaptive 'hot spots' are also present on the binding surfaces of TRAF1, TRAF2 and TRAF5.

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.

Innate direct anticancer effector function of human immature dendritic cells. II. Role of TNF, lymphotoxin-alpha(1)beta(2), Fas ligand, and TNF-related apoptosis-inducing ligand

Our recent studies have demonstrated that human immature dendritic cells (DCs) are able to directly and effectively mediate apoptotic killing against a wide array of cultured and freshly-isolated cancer cells without harming normal cells. In the present study, we demonstrate that this tumoricidal activity is mediated by multiple cytotoxic TNF family ligands. We determine that human immature DCs express on their cell surface four different cytotoxic TNF family ligands: TNF, lymphotoxin-alpha(1)beta(2), Fas ligand, and TNF-related apoptosis inducing ligand; while cancer cells express the corresponding death receptors. Disruptions of interactions between the four ligands expressed on DCs and corresponding death-signaling receptors expressed on cancer cells using specific Abs or R:Fc fusion proteins block the cytotoxic activity of DCs directed against cancer cells. The novel findings suggest that DC killing of cancer cells is mediated by the concerted engagement of four TNF family ligands of DCs with corresponding death receptors of cancer cells. Overall, our data demonstrate that DCs are fully equipped for an efficient direct apoptotic killing of cancer cells and suggest that this mechanism may play a critical role in both afferent and efferent anti-tumor immunity.

Upregulation of TRAF-3 by shear stress blocks CD40-mediated endothelial activation

Atherosclerosis is an inflammatory disease of large arteries that is initiated through the activation of endothelium by proinflammatory mediators. CD40 receptor stimulation has been implicated in the pathogenesis of atherosclerosis. One of the most important atheroprotective stimuli is the viscous drag (shear stress) generated by the streaming blood acting on the endothelial monolayer. Here, we demonstrate that shear stress prevents CD40 ligand-induced endothelial cell activation, and we identify upregulation of TNF receptor-associated factor-3 (TRAF-3) as a potent CD40-inhibitory mechanism. Shear stress specifically upregulates TRAF-3 in cultured endothelial cells. Moreover, in the endothelial cells overlying human atherosclerotic plaques, TRAF-3 expression is upregulated in areas with high shear stress. Overexpression of TRAF-3 inhibits endothelial expression of proinflammatory cytokines and tissue factor and blocks DNA-binding activity of the transcription factor AP-1; it thereby prevents CD40-induced endothelial activation. Thus, upregulation of TRAF-3 represents a novel mechanism for preserving the functional integrity of the endothelial monolayer.

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.

Lymphotoxins and cytomegalovirus cooperatively induce interferon-beta, establishing host-virus détente

Tumor necrosis factor (TNF)-related cytokines regulate cell death and survival and provide strong selective pressures for viruses, such as cytomegalovirus (CMV), to evolve counterstrategies in order to persist in immune-competent hosts. Signaling by the lymphotoxin (LT)-beta receptor or TNF receptor-1, but not Fas or TRAIL receptors, inhibits the cytopathicity and replication of human CMV by a nonapoptotic, reversible process that requires nuclear factor kappa B (NF-kappa B)-dependent induction of interferon-beta (IFN-beta). Efficient induction of IFN-beta requires virus infection and LT signaling, demonstrating the need for both host and viral factors in the curtailment of viral replication without cellular elimination. LT alpha-deficient mice and LT beta R-Fc transgenic mice were profoundly susceptible to murine CMV infection. Together, these results reveal an essential and conserved role for LTs in establishing host defense to CMV.

Recombinant small subunit of smooth muscle myosin light chain phosphatase. Molecular properties and interactions with the targeting subunit

We expressed the small subunit of smooth muscle myosin light chain phosphatase (MPs) in Escherichia coli, and have studied its molecular properties as well as its interaction with the targeting subunit (MPt). MPs (M(r) = 18,500) has an anomalously low electrophoretic mobility, running with an apparent M(r) of approximately 21,000 in sodium dodecyl sulfate-gel electrophoresis. CD spectroscopy shows that it is approximately 45% alpha-helix and undergoes a cooperative temperature-induced unfolding with a transition midpoint of 73 degrees C. Limited proteolysis rapidly degrades MPs to a stable C-terminal fragment (M(r) = 10,000) that retains most of the helical content. Rotary shadowing electron microscopy reveals that it is an elongated protein with two domains. Sedimentation velocity measurements show that recombinant MPt (M(r) = 107,000), intact MPs, and the 10-kDa MPs fragment are all dimeric, and that MPs and MPt form a complex with a molar mass consistent with a 1:1 heterodimer. Sequence analysis predicts that regions in the C-terminal portions of both MPs and MPt have high probabilities for coiled coil formation. A synthetic peptide from a region of MPs encompassing residues 77-116 was found to be 100% alpha-helical, dimeric, and formed a complex with MPt with a molecular mass corresponding to a heterodimer. Based on these results, we propose that MPs is an elongated molecule with an N-terminal head and a C-terminal stalk domain. It dimerizes via a coiled coil interaction in the stalk domain, and interacts with MPt via heterodimeric coiled coil formation. Since other proteins with known regulatory function toward MP also have predicted coiled coil regions, our results suggest that these regulatory proteins target MP via the same coiled coil strand exchange mechanism with MPt.

Role of TNF family ligands in antitumor activity of natural killer cells

NK cells have the ability to destroy tumor cells by two main cytotoxic pathways, the well-known perforin/granzyme-mediated secretory/necrotic killing and the newly defined TNF family ligand-mediated apoptotic killing. The former mechanism is operative mainly against a few cultured leukemia cell targets, while the latter mediates substantial activity against most tumor cell targets. It also appears from emerging data that the apoptotic mechanism is the main antitumor pathway in vito. This review is focused on the apoptotic mechanism of killing, the molecules and cell signaling pathways involved in this process, and its potential biologic significance along with its relation to the secretory/necrotic cytolytic pathway.

The TNF-receptor-associated factor family: scaffold molecules for cytokine receptors, kinases and their regulators

The TNF-receptor-associated factor (TRAF) family is a phylogenetically conserved group of scaffold proteins that link receptors of the IL-1R/Toll and TNF receptor family to signalling cascades, leading to the activation of NF-kappaB and mitogen-activated protein kinases. Furthermore, TRAF proteins serve as a docking platform for a variety of regulators of these signalling pathways and are themselves often regulated at the transcriptional and posttranslational level. In this review, we address the structural and molecular basis of TRAF protein functions and highlight their role in cytokine signalling.