Pro-apoptotic functions of TRAF2 in p53-mediated apoptosis induced by cisplatin.

Tumor necrosis factor receptor-associated factor 2 (TRAF2) is an essential component of tumor necrosis factor-α (TNF-α) signaling that regulates nuclear factor-κB (NF-κB) and c-Jun N-terminal kinase (JNK) pathways, and compelling evidence has demonstrated that TRAF2 suppresses TNF-α-induced cytotoxicity. On the other hand, it has been reported that oxidative stress-induced cytotoxicity is potentiated by TRAF2, indicating that TRAF2 both positively and negatively regulates stress-induced cytotoxicity in a context-specific manner. However, the causal role of TRAF2 in DNA damage response (DDR) remains to be explored. In this study, we assessed the function of TRAF2 in DDR induced by cisplatin, a representative DNA-damaging agent, and found that TRAF2 exerts pro-apoptotic activity through p53-dependent mechanisms at least in human fibrosarcoma cell line HT1080. TRAF2 deficient cells exhibit significant resistance to cell death induced by cisplatin, accompanied by the reduction of both p53 protein level and caspase-3 activation. Moreover, cisplatin-induced JNK activation was attenuated in TRAF2-deficient cells, and pharmacological inhibition of JNK signaling suppressed p53 stabilization. These results suggest that TRAF2 promotes p53-dependent apoptosis by activating the JNK signaling cascade in HT1080 cells. Thus, our data demonstrate a novel function of TRAF2 in cisplatin-induced DDR as a pro-apoptotic protein.

PSMD1 and PSMD2 regulate HepG2 cell proliferation and apoptosis via modulating cellular lipid droplet metabolism.

BACKGROUND: Obesity and nonalcoholic steatohepatitis (NASH) are well-known risk factors of hepatocellular carcinoma (HCC). The lipid-rich environment enhances the proliferation and metastasis abilities of tumor cells. Previous studies showed the effect of the ubiquitin-proteasome system (UPS) on tumor cell proliferation. However, the underlying mechanism of UPS in regulating the proliferation of lipid-rich tumor cells is not totally clear. RESULTS: Here, we identify two proteasome 26S subunits, non-ATPase 1 and 2 (PSMD1 and PSMD2), which regulate HepG2 cells proliferation via modulating cellular lipid metabolism. Briefly, the knockdown of PSMD1 and/or PSMD2 decreases the formation of cellular lipid droplets, the provider of the energy and membrane components for tumor cell proliferation. Mechanically, PSMD1 and PSMD2 regulate the expression of genes related to de novo lipid synthesis via p38-JNK and AKT signaling. Moreover, the high expression of PSMD1 and PSMD2 is significantly correlated with poor prognosis of HCC. CONCLUSION: We demonstrate that PSMD1 and PSMD2 promote the proliferation of HepG2 cells via facilitating cellular lipid droplet accumulation. This study provides a potential therapeutic strategy for the treatment of lipid-rich tumors.

CD70/CD27 signaling promotes blast stemness and is a viable therapeutic target in acute myeloid leukemia.

Aberrant proliferation, symmetric self-renewal, increased survival, and defective differentiation of malignant blasts are key oncogenic drivers in acute myeloid leukemia (AML). Stem cell gene signatures predict poor prognosis in AML patients; however, with few exceptions, these deregulated molecular pathways cannot be targeted therapeutically. In this study, we demonstrate that the TNF superfamily ligand-receptor pair CD70/CD27 is expressed on AML blasts and AML stem/progenitor cells. CD70/CD27 signaling in AML cells activates stem cell gene expression programs, including the Wnt pathway, and promotes symmetric cell divisions and proliferation. Soluble CD27, reflecting the extent of CD70/CD27 interactions in vivo, was significantly elevated in the sera of newly diagnosed AML patients and is a strong independent negative prognostic biomarker for overall survival. Blocking the CD70/CD27 interaction by mAb induced asymmetric cell divisions and differentiation in AML blasts and AML stem/progenitor cells, inhibited cell growth and colony formation, and significantly prolonged survival in murine AML xenografts. Importantly, hematopoietic stem/progenitor cells from healthy BM donors express neither CD70 nor CD27 and were unaffected by blocking mAb treatment. Therefore, targeting CD70/CD27 signaling represents a promising therapeutic strategy for AML.

RIPK1 Suppresses a TRAF2-Dependent Pathway to Liver Cancer.

Receptor-interacting protein kinase 1 (RIPK1) represents an essential signaling node in cell death and inflammation. Ablation of Ripk1 in liver parenchymal cells (LPC) did not cause a spontaneous phenotype, but led to tumor necrosis factor (TNF)-dependent hepatocyte apoptosis and liver injury without affecting inducible nuclear factor κB (NF-κB) activation. Loss of Ripk1 induced the TNF-dependent proteasomal degradation of the E3-ligase, TNF receptor-associated factor 2 (TRAF2), in a kinase-independent manner, thereby activating caspase-8. Moreover, loss of both Ripk1 and Traf2 in LPC not only resulted in caspase-8 hyperactivation but also impaired NF-κB activation, promoting the spontaneous development of hepatocellular carcinoma. In line, low RIPK1 and TRAF2 expression in human HCCs was associated with an unfavorable prognosis, suggesting that RIPK1 collaborates with TRAF2 to inhibit murine and human hepatocarcinogenesis.

MicroRNA-17 Suppresses TNF-α Signaling by Interfering with TRAF2 and cIAP2 Association in Rheumatoid Arthritis Synovial Fibroblasts.

TNF-α is a major cytokine implicated in rheumatoid arthritis (RA), and its expression is regulated at the transcriptional and posttranscriptional levels. However, the impact of changes in microRNA expression on posttranslational processes involved in TNF-α signaling networks is not well defined in RA. In this study, we evaluated the effect of miR-17, a member of the miR-17-92 cluster, on the TNF-α signaling pathway in human RA synovial fibroblasts (SFs). We demonstrated that miR-17 expression was significantly low in RA serum, SFs, and synovial tissues, as well as in the serum and joints of adjuvant-induced arthritis rats. RNA-sequencing analysis showed modulation of 664 genes by pre-miR-17 in human RA SFs. Ingenuity pathway analysis of RNA-sequencing data identified the ubiquitin proteasome system in the TNF-α signaling pathway as a primary target of miR-17. Western blot analysis confirmed the reduction in TRAF2, cIAP1, cIAP2, USP2, and PSMD13 expression by miR-17 in TNF-α-stimulated RA SFs. Immunoprecipitation assays showed that miR-17 restoration increased the K48-linked polyubiquitination of TRAF2, cIAP1, and cIAP2 in TNF-α-stimulated RA SFs. Thus, destabilization of TRAF2 by miR-17 reduced the ability of TRAF2 to associate with cIAP2, resulting in the downregulation of TNF-α-induced NF-κBp65, c-Jun, and STAT3 nuclear translocation and the production of IL-6, IL-8, MMP-1, and MMP-13 in human RA SFs. In conclusion, this study provides evidence for the role of miR-17 as a negative regulator of TNF-α signaling by modulating the protein ubiquitin processes in RA SFs.

TRAF2 functions as an activator switch in the reactive oxygen species-induced stimulation of MST1.

Reactive oxygen species (ROS) have many physiological and pathological effects on diverse cellular events. In particular, excessive ROS causes oxidative stress that leads to cell death. The mammalian STE20-like kinase-1 (MST1), a multifunctional serine-threonine kinase, plays a pivotal role in oxidative stress-induced cellular signaling events. Tumor necrosis factor receptor (TNFR)-associated factor 2 (TRAF2) is also known to be essential for oxidative stress-induced cell death. Here, we showed that H2O2 induced the physical interaction between TRAF2 and MST1, and that this interaction promoted the homodimerization as well as the activation of MST1. Furthermore, TRAF2 was required for MST1 to mediate the H2O2-induced stimulation of c-Jun N-terminal kinase and p38 kinase as well as apoptosis. Taken together, our results suggest that TRAF2 functions as a key activator of MST1 in oxidative stress-induced intracellular signaling processes.

Leukocyte-specific protein 1 links TNF receptor-associated factor 1 to survival signaling downstream of 4-1BB in T cells.

4-1BB is a member of the TNFR superfamily, which contributes to the activation of signaling pathways required for the survival of activated and memory T cells. We have shown previously that TRAF1, an adaptor protein recruited to 4-1BB, is required for 4-1BB-mediated CD8 T cell survival in vivo. With the use of a proteomics approach in primary T cells, we have identified LSP1 as a novel protein recruited to the 4-1BB signaling complex in a TRAF1-dependent manner. Further characterization of the interaction between TRAF1 and LSP1 revealed that LSP1 requires the TRAF-N domain of TRAF1 for direct association. Similarly to TRAF1(-/-) T cells, LSP1(-/-) T cells exhibit impaired ERK activation following stimulation through 4-1BB and consequently, are unable to down-modulate expression of the proapoptotic Bcl-2 family member Bim. Moreover, we demonstrate that the absence of LSP1 expression leads to defective expansion and survival of T cells in response to 4-1BB stimulation. Thus, we have identified LSP1 as a new mediator involved in 4-1BB signaling and T cell survival. Collectively, our work shows that TRAF1 and LSP1 cooperate downstream of 4-1BB to activate ERK signaling and down-modulate the levels of Bim leading to enhanced T cell survival.

Tumor necrosis factor receptor-associated factor 2 signaling provokes adverse cardiac remodeling in the adult mammalian heart.

BACKGROUND: Tumor necrosis factor superfamily ligands provoke a dilated cardiac phenotype signal through a common scaffolding protein termed tumor necrosis factor receptor-associated factor 2 (TRAF2); however, virtually nothing is known about TRAF2 signaling in the adult mammalian heart. METHODS AND RESULTS: We generated multiple founder lines of mice with cardiac-restricted overexpression of TRAF2 and characterized the phenotype of mice with higher expression levels of TRAF2 (myosin heavy chain [MHC]-TRAF2(HC)). MHC-TRAF2(HC) transgenic mice developed a time-dependent increase in cardiac hypertrophy, left ventricular dilation, and adverse left ventricular remodeling, and a significant decrease in LV+dP/dt and LV-dP/dt when compared with littermate controls (P<0.05 compared with littermate). During the early phases of left ventricular remodeling, there was a significant increase in total matrix metalloproteinase activity that corresponded with a decrease in total myocardial fibrillar collagen content. As the MHC-TRAF2(HC) mice aged, there was a significant decrease in total matrix metalloproteinase activity accompanied by an increase in total fibrillar collagen content and an increase in myocardial tissue inhibitor of metalloproteinase-1 levels. There was a significant increase in nuclear factor-κB activation at 4 to 12 weeks and jun N-terminal kinases activation at 4 weeks in the MHC-TRAF2(HC) mice. Transciptional profiling revealed that >95% of the hypertrophic/dilated cardiomyopathy-related genes that were significantly upregulated genes in the MHC-TRAF2(HC) hearts contained κB elements in their promoters. CONCLUSIONS: These results show for the first time that targeted overexpression of TRAF2 is sufficient to mediate adverse cardiac remodeling in the heart.

Dynamic change of TNIK in response to tumor necrosis factor alpha in a TRAF2-dependent manner.

TNIK (TRAF2- and NCK-interacting kinase) was named because of its association with TRAF2 (tumor necrosis factor receptor-associated factor 2). But the relationship between TNIK and TRAF2 is still elusive, in addition to which the involvement of TNIK in JNK activation by TNFα hints that there maybe a linkage between TNIK and TRAF2. In this work, we illustrated that TNIK protein levels were dynamic in response to TNFα stimulation in an ubiquitin-dependent manner. Further study showed that TRAF2 negatively modulated the levels of TNIK by regulating the ubiquitin conjugation. In conclusion, our data may give evidence that dynamic change of TNIK offers a way to protect cells from outside stimulus.

TRAF2 Sets a threshold for extrinsic apoptosis by tagging caspase-8 with a ubiquitin shutoff timer.

Apoptotic caspase activation mechanisms are well defined, yet inactivation modes remain unclear. The death receptors (DRs), DR4, DR5, and Fas, transduce cell-extrinsic apoptotic signals by recruiting caspase-8 into a death-inducing signaling complex (DISC). At the DISC, Cullin3-dependent polyubiquitination on the small catalytic subunit of caspase-8 augments stimulation. Here we report that tumor necrosis factor receptor-associated factor 2 (TRAF2) interacts with caspase-8 at the DISC, downstream of Cullin3. TRAF2 directly mediates RING-dependent, K48-linked polyubiquitination on the large catalytic domain of caspase-8. This modification destines activated caspase-8 molecules to rapid proteasomal degradation upon autoprocessing and cytoplasmic translocation. TRAF2 depletion lowers the signal threshold for DR-mediated apoptosis, altering cell life versus death decisions in vitro and in vivo. Thus, TRAF2 sets a critical barrier for cell-extrinsic apoptosis commitment by tagging activated caspase-8 with a K48-ubiquitin shutoff timer. These results may have important implications for caspase regulation mechanisms.

Scavenger receptor in fish is a lipopolysaccharide recognition molecule involved in negative regulation of NF-κB activation by competing with TNF receptor-associated factor 2 recruitment into the TNF-α signaling pathway.

Scavenger receptors (SRs) play crucial roles in innate immunity by acting as pattern recognition receptors. Although SRs are widely documented in mammals, data on their occurrence and functions in ancient vertebrates are limited. In this study, we report, to our knowledge, the first cloning and functional characterization of an SR molecule from teleost fish (Tetraodon nigroviridis). This SR (TnSR) was identified as a homolog to mammalian scavenger receptor class A member 5 with the conserved structure of a class A SR. TnSR contained multidomains in a type II transmembrane receptor, including an SR cysteine-rich domain, two coiled-coil collagenous domains, a transmmebrane domain, and a short N-terminal intracellular region with an unexpected TNFR-associated factor 2-binding consensus motif similar to that in human MSR molecules. Phylogenetic analysis suggested that TnSR may be an ancient member of class A SRs resulting from the close relationship between scavenger receptor class A member 5 and macrophage SR in vertebrates associated with the subtle differences in TnSR structure. Subcellular localization analysis showed that TnSR was a cell membrane receptor with homotrimer forms involved in the recognition and internalization of LPS from surface membranes into lysosomes. Functionally, TnSR expression was dramatically induced by LPS stimulation. TnSR served as a negative regulator in LPS-induced NF-κB activation by the competitive recruitment of TNFR-associated factor 2 from the TNF-α signaling pathway. To our knowledge, this is the first report showing that SR plays an inhibitory role in LPS-elicited inflammation by cross-talking with the TNF-α inflammatory pathway. These findings contribute to a better understanding of the biological and evolutionary history of the SR family.

Hepatic TRAF2 regulates glucose metabolism through enhancing glucagon responses.

Obesity is associated with intrahepatic inflammation that promotes insulin resistance and type 2 diabetes. Tumor necrosis factor receptor-associated factor (TRAF)2 is a key adaptor molecule that is known to mediate proinflammatory cytokine signaling in immune cells; however, its metabolic function remains unclear. We examined the role of hepatic TRAF2 in the regulation of insulin sensitivity and glucose metabolism. TRAF2 was deleted specifically in hepatocytes using the Cre/loxP system. The mutant mice were fed a high-fat diet (HFD) to induce insulin resistance and hyperglycemia. Hepatic glucose production (HGP) was examined using pyruvate tolerance tests, (2)H nuclear magnetic resonance spectroscopy, and in vitro HGP assays. The expression of gluconeogenic genes was measured by quantitative real-time PCR. Insulin sensitivity was analyzed using insulin tolerance tests and insulin-stimulated phosphorylation of insulin receptors and Akt. Glucagon action was examined using glucagon tolerance tests and glucagon-stimulated HGP, cAMP-responsive element-binding (CREB) phosphorylation, and expression of gluconeogenic genes in the liver and primary hepatocytes. Hepatocyte-specific TRAF2 knockout (HKO) mice exhibited normal body weight, blood glucose levels, and insulin sensitivity. Under HFD conditions, blood glucose levels were significantly lower (by >30%) in HKO than in control mice. Both insulin signaling and the hypoglycemic response to insulin were similar between HKO and control mice. In contrast, glucagon signaling and the hyperglycemic response to glucagon were severely impaired in HKO mice. In addition, TRAF2 overexpression significantly increased the ability of glucagon or a cAMP analog to stimulate CREB phosphorylation, gluconeogenic gene expression, and HGP in primary hepatocytes. These results suggest that the hepatic TRAF2 cell autonomously promotes hepatic gluconeogenesis by enhancing the hyperglycemic response to glucagon and other factors that increase cAMP levels, thus contributing to hyperglycemia in obesity.

A common haplotype of the TNF receptor 2 gene modulates endotoxin tolerance.

Endotoxin tolerance is characterized by the suppression of further TNF release upon recurrent exposure to LPS. This phenomenon is proposed to act as a homeostatic mechanism preventing uncontrolled cytokine release such as that observed in bacterial sepsis. The regulatory mechanisms and interindividual variation of endotoxin tolerance induction in man remain poorly characterized. In this paper, we describe a genetic association study of variation in endotoxin tolerance among healthy individuals. We identify a common promoter haplotype in TNFRSF1B (encoding TNFR2) to be strongly associated with reduced tolerance to LPS (p = 5.82 × 10(-6)). This identified haplotype is associated with increased expression of TNFR2 (p = 4.9 × 10(-5)), and we find basal expression of TNFR2, irrespective of genotype and unlike TNFR1, is associated with secondary TNF release (p < 0.0001). Functional studies demonstrate a positive-feedback loop via TNFR2 of LPS-induced TNF release, confirming this previously unrecognized role for TNFR2 in the modulation of LPS response.

Tumor necrosis factor receptor cross-talk.

Extensive research has been performed to unravel the mechanistic signaling pathways mediated by tumor necrosis factor receptor 1 (TNFR1), by contrast there is limited knowledge on cellular signaling upon activation of TNFR2. Recently published data have revealed that these two receptors not only function independently, but also can influence each other via cross-talk between the different signaling pathways initiated by TNFR1 and TNFR2 stimulation. Furthermore, the complexity of this cross-talk is also dependent on the different signaling kinetics between TNFR1 and TNFR2, by which a delicate balance between cell survival and apoptosis can be maintained. Some known signaling factors and the kinetics that are involved in the receptor cross-talk between TNFR1 and TNFR2 are the topic of this review.

RIP1 comes back to life as a cell death regulator in TNFR1 signaling.

Cell death induction by tumor necrosis factor has been an intensively studied area for the last two decades. Although it may appear that the skeleton should have been picked clean by now, new secrets about tumor necrosis factor death signaling are still being uncovered. In particular, the recent evidence that ubiquitination of the death kinase receptor-interacting protein 1 regulates its participation in apoptotic and necrotic cell death is opening up unexplored avenues in the catacombs of tumor necrosis factor death signaling. In this minireview, we focus on two major cell-death checkpoints that determine whether receptor-interacting protein 1 functions as a pro-survival or pro-death molecule.

Recruitment of the linear ubiquitin chain assembly complex stabilizes the TNF-R1 signaling complex and is required for TNF-mediated gene induction.

TNF is a key inflammatory cytokine. Using a modified tandem affinity purification approach, we identified HOIL-1 and HOIP as functional components of the native TNF-R1 signaling complex (TNF-RSC). Together, they were shown to form a linear ubiquitin chain assembly complex (LUBAC) and to ubiquitylate NEMO. We show that LUBAC binds to ubiquitin chains of different linkage types and that its recruitment to the TNF-RSC is impaired in TRADD-, TRAF2-, and cIAP1/2- but not in RIP1- or NEMO-deficient MEFs. Furthermore, the E3 ligase activity of cIAPs, but not TRAF2, is required for HOIL-1 recruitment to the TNF-RSC. LUBAC enhances NEMO interaction with the TNF-RSC, stabilizes this protein complex, and is required for efficient TNF-induced activation of NF-kappaB and JNK, resulting in apoptosis inhibition. Finally, we demonstrate that sustained stability of the TNF-RSC requires LUBAC's enzymatic activity, thereby adding a third form of ubiquitin linkage to the triggering of TNF signaling by the TNF-RSC.

Genomic and functional uniqueness of the TNF receptor-associated factor gene family in amphioxus, the basal chordate.

The TNF-associated factor (TRAF) family, the crucial adaptor group in innate immune signaling, increased to 24 in amphioxus, the oldest lineage of the Chordata. To address how these expanded molecules evolved to adapt to the changing TRAF mediated signaling pathways, here we conducted genomic and functional comparisons of four distinct amphioxus TRAF groups with their human counterparts. We showed that lineage-specific duplication and rearrangement were responsible for the expansion of amphioxus TRAF1/2 and 3 lineages, whereas TRAF4 and 6 maintained a relatively stable genome and protein structure. Amphioxus TRAF1/2 and 3 molecules displayed various expression patterns in response to microbial infection, and some of them can attenuate the NF-kappaB activation mediated by human TRAF2 and 6. Amphioxus TRAF4 presented two unique functions: activation of the NF-kappaB pathway and involvement in somite formation. Although amphioxus TRAF6 was conserved in activating NF-kappaB pathway for antibacterial defense, the mechanism was not the same as that observed in humans. In summary, our findings reveal the evolutionary uniqueness of the TRAF family in this basal chordate, and suggest that genomic duplication and functional divergence of the TRAF family are important for the current form of the TRAF-mediated signaling pathways in humans.

Roles of the TRAF2/3 binding site in differential B cell signaling by CD40 and its viral oncogenic mimic, LMP1.

The EBV protein, latent membrane protein 1 (LMP1), is a functional mimic of the cellular receptor CD40, but signals to B lymphocytes in an amplified and sustained manner compared with CD40. LMP1 contributes to the development of B cell lymphoma in immunosuppressed patients, and may exacerbate flares of certain autoimmune diseases. The cytoplasmic domain of LMP1 binds the signaling adaptor TRAF2 with lower avidity than the cytoplasmic domain of CD40, and TRAF2 is needed for CD40-mediated degradation of TRAFs 2 and 3. LMP1 doesn't induce TRAF degradation, and employs TRAF3 as a positive mediator of cell signaling, whereas CD40 signals are inhibited by TRAF3. We thus tested the hypothesis that relative affinity for TRAF2, and/or distinct sequence differences in the TRAF2/3 binding sites of CD40 vs LMP1, controls the disparate ways in which CD40 and LMP1 use TRAFs 2 and 3, and their distinct signaling characteristics. CD40 and LMP1 mutants in which the TRAF binding site sequences were swapped were examined, testing TRAF binding and degradation, and induction of B cell activation. Results revealed that TRAF binding affinity and TRAF binding site sequence dictate a distinct subset of CD40 vs LMP1 signaling properties. Examination of TRAF binding, degradation, cytokine production, IgM secretion, and the activation of c-Jun kinase and NF-kappaB revealed that some events are dictated by TRAF binding site sequences, others are partially regulated, and still others are independent of the TRAF binding site sequence.

Tumor necrosis factor receptor-associated factor-1 enhances proinflammatory TNF receptor-2 signaling and modifies TNFR1-TNFR2 cooperation.

It has been shown that tumor necrosis factor receptor-2 (TNFR2) stimulation leads to degradation of TNF receptor associated factor-2 (TRAF2) and inhibition of TNFR1-induced activation of NFkappaB and JNK. Here, we show that TRAF1 inhibits TNFR2-induced proteasomal degradation of TRAF2 and relieves TNFR1-induced activation of NFkappaB from the inhibitory effect of TNFR2. TRAF1 co-recruited with TRAF2 to both TNF receptors. Despite lacking an amino-terminal RING/zinc-finger domain, TRAF1 did not interfere with TNFR1-induced activation of JNK and NFkappaB. It is noted that physiological expression levels of TRAF1 enhanced NFkappaB activation and interleukin-8 (IL8) production induced by TNFR2. Thus, TRAF1 shifts the quality of integrated TNFR1-TNFR2 signaling from apoptosis induction to proinflammatory NFkappaB signaling.