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.

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.

SOCS-1 ameliorates smoke inhalation-induced acute lung injury through inhibition of ASK-1 activity and DISC formation.

Smoke inhalation leads to acute lung injury (ALI), a devastating clinical problem associated with high mortality. Suppressor of cytokine signaling-1 (SOCS-1) is a negative regulator of apoptosis and pro-inflammatory cytokine signaling, two major contributors to the pathogenesis of ALI. We have found that SOCS-1 protects lung epithelial cells from smoke-induced apoptosis through two mechanisms. One is that SOCS-1 enhances degradation of ASK-1 and diminishes cleavage of pro-caspase-3 to repress smoke-triggered apoptosis in lung epithelial cells. The other is that SOCS-1 represses smoke-triggered DISC formation through altering TRADD-caspase-8 interaction rather than TNFR-1-TRADD interaction or TNFR-1-TRAF-2 interaction. In conclusion, SOCS-1 relieves smoke inhalation-induced lung injury by repressing ASK-1 and DISC-mediated epithelium apoptosis.

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.

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.

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.

Crucial role for TNF receptor-associated factor 2 (TRAF2) in regulating NFκB2 signaling that contributes to autoimmunity.

TNF receptor-associated factor 2 (TRAF2) is a key intracellular signaling mediator that acts downstream of not only TNFα but also various members of the TNFα superfamily. Here, we report that, despite their lack of TNFα signaling, TRAF2(-/-)TNFα(-/-) mice develop an inflammatory disorder characterized by autoantibody accumulation and organ infiltration by T cells with the phenotypes of activated, effector, and memory cells. RAG1(-/-) mice reconstituted with TRAF2(-/-)TNFα(-/-) bone marrow cells showed increased numbers of hyperactive T cells and rapidly developed progressive and eventually lethal inflammation. No inflammation was observed in RAG1(-/-) mice reconstituted with TRAF2(-/-)TNFα(-/-)T-cell receptor ß(-/-) or TRAF2(-/-)TNFα(-/-)NFκB-induced kinase(+/-) bone marrow cells. The pathogenic TRAF2(-/-)TNFα(-/-) T cells showed constitutive NFκB2p52 activation and produced elevated levels of T-helper 1 and T-helper 17 cytokines. Our results suggest that a regulatory circuit consisting of TRAF2-NFκB-induced kinase-NFκB2p52 is essential for the proper control of effector T-cell polarization and that loss of T-cell TRAF2 function induces constitutive NFκB2p52 activity that drives fatal autoimmune inflammation independently of TNFα signaling. The involvement of this regulatory circuit in controlling autoimmune responses highlights the delicate balance required to avoid paradoxical adverse events when implementing new targeted anti-inflammatory therapies.

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.

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.

IRF7 activation by Epstein-Barr virus latent membrane protein 1 requires localization at activation sites and TRAF6, but not TRAF2 or TRAF3.

Epstein-Barr virus (EBV) latent infection membrane protein 1 (LMP1), a constitutively aggregated and activated pseudoreceptor, activates IFN regulatory factor 7 (IRF7) through RIP1. We now report that the LMP1 cytoplasmic carboxyl terminal amino acids 379-386 bound IRF7 and activated IRF7. IRF7 activation required TRAF6 and RIP1, but not TRAF2 or TRAF3. LMP1 Y(384)YD(386), which are required for TRADD and RIP1 binding and for NF-kappaB activation, were not required for IRF7 binding, but were required for IRF7 activation, implicating signaling through TRADD and RIP1 in IRF7 activation. Association with active LMP1 signaling complexes was also critical for IRF7 activation because (i) a dominant-negative IRF7 bound to LMP1, blocked IRF7 association and activation, but did not inhibit LMP1 induced NF-kappaB or TBK1 or Sendai virus-mediated IFN stimulated response element activation; and (ii) two different LMP1 transmembrane domain mutants, which fail to aggregate, each bound IRF7 and prevented LMP1 from binding and activating IRF7 in the same cell, but did not prevent NF-kappaB activation. Thus, efficient IRF7 activation required association with LMP1 CTAR2 in proximity to LMP1 CTAR2 mediated kinase activation sites.

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.

Tumor necrosis factor (TNF)-alpha persistently activates nuclear factor-kappaB signaling through the type 2 TNF receptor in chromaffin cells: implications for long-term regulation of neuropeptide gene expression in inflammation.

Chromaffin cells of the adrenal medulla elaborate and secrete catecholamines and neuropeptides for hormonal and paracrine signaling in stress and during inflammation. We have recently documented the action of the cytokine TNF-alpha on neuropeptide secretion and biosynthesis in isolated bovine chromaffin cells. Here, we demonstrate that the type 2 TNF-alpha receptor (TNF-R2) mediates TNF-alpha signaling in chromaffin cells via activation of nuclear factor (NF)-kappaB. Microarray and suppression subtractive hybridization have been used to identify TNF-alpha target genes in addition to those encoding the neuropeptides galanin, vasoactive intestinal polypeptide, and secretogranin II in chromaffin cells. TNF-alpha, acting through the TNF-R2, causes an early up-regulation of NF-kappaB, long-lasting induction of the NF-kappaB target gene inhibitor kappaB (IkappaB), and persistent stimulation of other NF-kappaB-associated genes including mitogen-inducible gene-6 (MIG-6), which acts as an IkappaB signaling antagonist, and butyrate-induced transcript 1. Consistent with long-term activation of the NF-kappaB signaling pathway, delayed induction of neuropeptide gene transcription by TNF-alpha in chromaffin cells is blocked by an antagonist of NF-kappaB signaling. TNF-alpha-dependent signaling in neuroendocrine cells thus leads to a unique, persistent mode of NF-kappaB activation that features long-lasting transcription of both IkappaB and MIG-6, which may play a role in the long-lasting effects of TNF-alpha in regulating neuropeptide output from the adrenal, a potentially important feedback station for modulating long-term cytokine effects in inflammation.

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.

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.

TRAF6 is required for TRAF2-dependent CD40 signal transduction in nonhemopoietic cells.

The emerging role of CD40, a tumor necrosis factor (TNF) receptor family member, in immune regulation, disease pathogenesis, and cancer therapy necessitates the analysis of CD40 signal transduction in a wide range of tissue types. In this study we present evidence that the CD40-interacting proteins TRAF2 and TRAF6 play an important physiological role in CD40 signaling in nonhemopoietic cells. Using mutational analysis of the CD40 cytoplasmic tail, we demonstrate that the specific binding of TRAF2 to CD40 is required for efficient signaling on the NF-kappaB, Jun N-terminal protein kinase (JNK), and p38 axis. In fibroblasts lacking TRAF2 or in carcinoma cells in which TRAF2 has been depleted by RNA interference, the CD40-mediated activation of NF-kappaB and JNK is significantly reduced, and the activation of p38 and Akt is severely impaired. Interestingly, whereas the TRAF6-interacting membrane-proximal domain of CD40 has a minor role in signal transduction, studies utilizing TRAF6 knockout fibroblasts and RNA interference in epithelial cells reveal that the CD40-induced activation of NF-kappaB, JNK, p38, and Akt requires the integrity of TRAF6. Furthermore, we provide evidence that TRAF6 regulates CD40 signal transduction not only through its direct binding to CD40 but also indirectly via its association with TRAF2. These observations provide novel insight into the mechanisms of CD40 signaling and the multiple roles played by TRAF6 in signal transduction.

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.

Structural revelations of TRAF2 function in TNF receptor signaling pathway.

The tumor necrosis factor (TNF) receptor (TNFR) superfamily consists of over 20 type-I transmembrane proteins with conserved N-terminal cysteine-rich domains (CRDs) in the extracellular ligand binding region, which are specifically activated by the corresponding superfamily of TNF-like ligands. Members of this receptor superfamily have wide tissue distribution and play important roles in biological processes such as lymphoid and neuronal development, innate and adaptive immune response, and cellular homeostasis. A remarkable feature of the TNFR superfamily is the ability of these receptors to induce effects either for cell survival or apoptotic cell death. The downstream intracellular mediators of cell survival signal are a group of proteins known as TNFR associated factors (TRAFs). There are currently six canonical mammalian TRAFs. This review will focus on the unique structural features of TRAF2 protein and its role in cell survival signaling.

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.

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.