SIRT6 Suppresses Pancreatic Cancer through Control of Lin28b.

Chromatin remodeling proteins are frequently dysregulated in human cancer, yet little is known about how they control tumorigenesis. Here, we uncover an epigenetic program mediated by the NAD(+)-dependent histone deacetylase Sirtuin 6 (SIRT6) that is critical for suppression of pancreatic ductal adenocarcinoma (PDAC), one of the most lethal malignancies. SIRT6 inactivation accelerates PDAC progression and metastasis via upregulation of Lin28b, a negative regulator of the let-7 microRNA. SIRT6 loss results in histone hyperacetylation at the Lin28b promoter, Myc recruitment, and pronounced induction of Lin28b and downstream let-7 target genes, HMGA2, IGF2BP1, and IGF2BP3. This epigenetic program defines a distinct subset with a poor prognosis, representing 30%-40% of human PDAC, characterized by reduced SIRT6 expression and an exquisite dependence on Lin28b for tumor growth. Thus, we identify SIRT6 as an important PDAC tumor suppressor and uncover the Lin28b pathway as a potential therapeutic target in a molecularly defined PDAC subset. PAPERCLIP.

Mutant IDH inhibits HNF-4α to block hepatocyte differentiation and promote biliary cancer.

Mutations in isocitrate dehydrogenase 1 (IDH1) and IDH2 are among the most common genetic alterations in intrahepatic cholangiocarcinoma (IHCC), a deadly liver cancer. Mutant IDH proteins in IHCC and other malignancies acquire an abnormal enzymatic activity allowing them to convert α-ketoglutarate (αKG) to 2-hydroxyglutarate (2HG), which inhibits the activity of multiple αKG-dependent dioxygenases, and results in alterations in cell differentiation, survival, and extracellular matrix maturation. However, the molecular pathways by which IDH mutations lead to tumour formation remain unclear. Here we show that mutant IDH blocks liver progenitor cells from undergoing hepatocyte differentiation through the production of 2HG and suppression of HNF-4α, a master regulator of hepatocyte identity and quiescence. Correspondingly, genetically engineered mouse models expressing mutant IDH in the adult liver show an aberrant response to hepatic injury, characterized by HNF-4α silencing, impaired hepatocyte differentiation, and markedly elevated levels of cell proliferation. Moreover, IDH and Kras mutations, genetic alterations that co-exist in a subset of human IHCCs, cooperate to drive the expansion of liver progenitor cells, development of premalignant biliary lesions, and progression to metastatic IHCC. These studies provide a functional link between IDH mutations, hepatic cell fate, and IHCC pathogenesis, and present a novel genetically engineered mouse model of IDH-driven malignancy.

Forty-Year Trends in Cholangiocarcinoma Incidence in the U.S.: Intrahepatic Disease on the Rise.

BACKGROUND: Challenges in the diagnosis and classification of cholangiocarcinoma have made it difficult to quantify the true incidence of this highly aggressive malignancy. METHODS: We analyzed the Surveillance, Epidemiology, and End Results data to assess long-term trends in the age-standardized incidence of intrahepatic and extrahepatic cholangiocarcinoma between 1973 and 2012, correcting for systematic coding errors. Because intrahepatic cholangiocarcinoma (ICC) may frequently be misdiagnosed as cancer of unknown primary (CUP), we also analyzed trends in the incidence of CUP. RESULTS: Between 1973 and 2012, the reported U.S. incidence of ICC increased from 0.44 to 1.18 cases per 100,000, representing an annual percentage change (APC) of 2.30%; this trend has accelerated during the past decade to an APC of 4.36%. The incidence of extrahepatic cholangiocarcinoma increased modestly from 0.95 to 1.02 per 100,000 during the 40-year period (APC, 0.14%). The incidence of CUP with histologic features potentially consistent with cholangiocarcinoma decreased by 51% between 1973 and 2012 (APC, -1.87%), whereas the incidence of CUP with squamous or nonepithelial histologic features increased modestly (APC, 0.42%). CONCLUSION: The recognized incidence of ICC in the U.S. continues to rise, whereas the incidence of ECC is stable. The incidence of CUP has fallen dramatically during the same time period. IMPLICATIONS FOR PRACTICE: Clinical distinctions between cholangiocarcinoma (particularly intrahepatic cholangiocarcinoma [ICC]) and cancer of unknown primary (CUP) can be challenging. Recent discoveries have identified recurrent and potentially targetable genomic abnormalities in ICC, highlighting the importance of improving diagnosis. This study demonstrates that the incidence of ICC is increasing in the U.S., whereas the incidence of extrahepatic cholangiocarcinoma is stable. Concomitantly, the incidence of CUP has declined dramatically, suggesting that improved distinction between ICC and CUP may be a major driver of the increasing recognized incidence of ICC. The increasing incidence of ICC warrants further study of prevention and treatment approaches.

Prognosis and Clinicopathologic Features of Patients With Advanced Stage Isocitrate Dehydrogenase (IDH) Mutant and IDH Wild-Type Intrahepatic Cholangiocarcinoma.

BACKGROUND: Conflicting data exist regarding the prognostic impact of the isocitrate dehydrogenase (IDH) mutation in intrahepatic cholangiocarcinoma (ICC), and limited data exist in patients with advanced-stage disease. Similarly, the clinical phenotype of patients with advanced IDH mutant (IDHm) ICC has not been characterized. In this study, we report the correlation of IDH mutation status with prognosis and clinicopathologic features in patients with advanced ICC. METHODS: Patients with histologically confirmed advanced ICC who underwent tumor mutational profiling as a routine part of their care between 2009 and 2014 were evaluated. Clinical and pathological data were collected by retrospective chart review for patients with IDHm versus IDH wild-type (IDHwt) ICC. Pretreatment tumor volume was calculated on computed tomography or magnetic resonance imaging. RESULTS: Of the 104 patients with ICC who were evaluated, 30 (28.8%) had an IDH mutation (25.0% IDH1, 3.8% IDH2). The median overall survival did not differ significantly between IDHm and IDHwt patients (15.0 vs. 20.1 months, respectively; p = .17). The pretreatment serum carbohydrate antigen 19-9 (CA19-9) level in IDHm and IDHwt patients was 34.5 and 118.0 U/mL, respectively (p = .04). Age at diagnosis, sex, histologic grade, and pattern of metastasis did not differ significantly by IDH mutation status. CONCLUSION: The IDH mutation was not associated with prognosis in patients with advanced ICC. The clinical phenotypes of advanced IDHm and IDHwt ICC were similar, but patients with IDHm ICC had a lower median serum CA19-9 level at presentation. IMPLICATIONS FOR PRACTICE: Previous studies assessing the prognostic impact of the isocitrate dehydrogenase (IDH) gene mutation in intrahepatic cholangiocarcinoma (ICC) mainly focused on patients with early-stage disease who have undergone resection. These studies offer conflicting results. The target population for clinical trials of IDH inhibitors is patients with unresectable or metastatic disease, and the current study is the first to focus on the prognosis and clinical phenotype of this population and reports on the largest cohort of patients with advanced IDH mutant ICC to date. The finding that the IDH mutation lacks prognostic significance in advanced ICC is preliminary and needs to be confirmed prospectively in a larger study.

SRC inhibition enables formation of a growth suppressive MAGI1-PP2A complex in isocitrate dehydrogenase-mutant cholangiocarcinoma.

Intrahepatic cholangiocarcinoma (ICC) is an aggressive bile duct malignancy that frequently exhibits isocitrate dehydrogenase (IDH1/IDH2) mutations. Mutant IDH (IDHm) ICC is dependent on SRC kinase for growth and survival and is hypersensitive to inhibition by dasatinib, but the molecular mechanism underlying this sensitivity is unclear. We found that dasatinib reduced p70 S6 kinase (S6K) and ribosomal protein S6 (S6), leading to substantial reductions in cell size and de novo protein synthesis. Using an unbiased phosphoproteomic screen, we identified membrane-associated guanylate kinase, WW, and PDZ domain containing 1 (MAGI1) as an SRC substrate in IDHm ICC. Biochemical and functional assays further showed that SRC inhibits a latent tumor-suppressing function of the MAGI1-protein phosphatase 2A (PP2A) complex to activate S6K/S6 signaling in IDHm ICC. Inhibiting SRC led to activation and increased access of PP2A to dephosphorylate S6K, resulting in cell death. Evidence from patient tissue and cell line models revealed that both intrinsic and extrinsic resistance to dasatinib is due to increased phospho-S6 (pS6). To block pS6, we paired dasatinib with the S6K/AKT inhibitor M2698, which led to a marked reduction in pS6 in IDHm ICC cell lines and patient-derived organoids in vitro and substantial growth inhibition in ICC patient-derived xenografts in vivo. Together, these results elucidated the mechanism of action of dasatinib in IDHm ICC, revealed a signaling complex regulating S6K phosphorylation independent of mTOR, suggested markers for dasatinib sensitivity, and described a combination therapy for IDHm ICC that may be actionable in the clinic.

SULT1A1-dependent sulfonation of alkylators is a lineage-dependent vulnerability of liver cancers.

Adult liver malignancies, including intrahepatic cholangiocarcinoma and hepatocellular carcinoma, are the second leading cause of cancer-related deaths worldwide. Most individuals are treated with either combination chemotherapy or immunotherapy, respectively, without specific biomarkers for selection. Here using high-throughput screens, proteomics and in vitro resistance models, we identify the small molecule YC-1 as selectively active against a defined subset of cell lines derived from both liver cancer types. We demonstrate that selectivity is determined by expression of the liver-resident cytosolic sulfotransferase enzyme SULT1A1, which sulfonates YC-1. Sulfonation stimulates covalent binding of YC-1 to lysine residues in protein targets, enriching for RNA-binding factors. Computational analysis defined a wider group of structurally related SULT1A1-activated small molecules with distinct target profiles, which together constitute an untapped small-molecule class. These studies provide a foundation for preclinical development of these agents and point to the broader potential of exploiting SULT1A1 activity for selective targeting strategies.

Rescue of TRAF3-null mice by p100 NF-kappa B deficiency.

Proper activation of nuclear factor (NF)-kappaB transcription factors is critical in regulating fundamental biological processes such as cell survival and proliferation, as well as in inflammatory and immune responses. Recently, the NF-kappaB signaling pathways have been categorized into the canonical pathway, which results in the nuclear translocation of NF-kappaB complexes containing p50, and the noncanonical pathway, which involves the induced processing of p100 to p52 and the formation of NF-kappaB complexes containing p52 (Bonizzi, G., and M. Karin. 2004. Trends Immunol. 25:280-288). We demonstrate that loss of tumor necrosis factor (TNF) receptor-associated factor 3 (TRAF3) results in constitutive noncanonical NF-kappaB activity. Importantly, TRAF3-/- B cells show ligand-independent up-regulation of intracellular adhesion molecule 1 and protection from spontaneous apoptosis during in vitro culture. In addition, we demonstrate that loss of TRAF3 results in profound accumulation of NF-kappaB-inducing kinase in TRAF3-/- cells. Finally, we show that the early postnatal lethality observed in TRAF3-deficient mice is rescued by compound loss of the noncanonical NF-kappaB p100 gene. Thus, these genetic data clearly demonstrate that TRAF3 is a critical negative modulator of the noncanonical NF-kappaB pathway and that constitutive activation of the noncanonical NF-kappaB pathway causes the lethal phenotype of TRAF3-deficient mice.

Critical role of TRAF3 in the Toll-like receptor-dependent and -independent antiviral response.

Type I interferon (IFN) production is a critical component of the innate defence against viral infections. Viral products induce strong type I IFN responses through the activation of Toll-like receptors (TLRs) and intracellular cytoplasmic receptors such as protein kinase R (PKR). Here we demonstrate that cells lacking TRAF3, a member of the TNF receptor-associated factor family, are defective in type I IFN responses activated by several different TLRs. Furthermore, we show that TRAF3 associates with the TLR adaptors TRIF and IRAK1, as well as downstream IRF3/7 kinases TBK1 and IKK-epsilon, suggesting that TRAF3 serves as a critical link between TLR adaptors and downstream regulatory kinases important for IRF activation. In addition to TLR stimulation, we also show that TRAF3-deficient fibroblasts are defective in their type I IFN response to direct infection with vesicular stomatitis virus, indicating that TRAF3 is also an important component of TLR-independent viral recognition pathways. Our data demonstrate that TRAF3 is a major regulator of type I IFN production and the innate antiviral response.

Type I interferon production enhances susceptibility to Listeria monocytogenes infection.

Numerous bacterial products such as lipopolysaccharide potently induce type I interferons (IFNs); however, the contribution of this innate response to host defense against bacterial infection remains unclear. Although mice deficient in either IFN regulatory factor (IRF)3 or the type I IFN receptor (IFNAR)1 are highly susceptible to viral infection, we show that these mice exhibit a profound resistance to infection caused by the Gram-positive intracellular bacterium Listeria monocytogenes compared with wild-type controls. Furthermore, this enhanced bacterial clearance is accompanied by a block in L. monocytogenes-induced splenic apoptosis in IRF3- and IFNAR1-deficient mice. Thus, our results highlight the disparate roles of type I IFNs during bacterial versus viral infections and stress the importance of proper IFN modulation in host defense.

TAS-120 Overcomes Resistance to ATP-Competitive FGFR Inhibitors in Patients with FGFR2 Fusion-Positive Intrahepatic Cholangiocarcinoma.

ATP-competitive fibroblast growth factor receptor (FGFR) kinase inhibitors, including BGJ398 and Debio 1347, show antitumor activity in patients with intrahepatic cholangiocarcinoma (ICC) harboring activating FGFR2 gene fusions. Unfortunately, acquired resistance develops and is often associated with the emergence of secondary FGFR2 kinase domain mutations. Here, we report that the irreversible pan-FGFR inhibitor TAS-120 demonstrated efficacy in 4 patients with FGFR2 fusion-positive ICC who developed resistance to BGJ398 or Debio 1347. Examination of serial biopsies, circulating tumor DNA (ctDNA), and patient-derived ICC cells revealed that TAS-120 was active against multiple FGFR2 mutations conferring resistance to BGJ398 or Debio 1347. Functional assessment and modeling the clonal outgrowth of individual resistance mutations from polyclonal cell pools mirrored the resistance profiles observed clinically for each inhibitor. Our findings suggest that strategic sequencing of FGFR inhibitors, guided by serial biopsy and ctDNA analysis, may prolong the duration of benefit from FGFR inhibition in patients with FGFR2 fusion-positive ICC. SIGNIFICANCE: ATP-competitive FGFR inhibitors (BGJ398, Debio 1347) show efficacy in FGFR2-altered ICC; however, acquired FGFR2 kinase domain mutations cause drug resistance and tumor progression. We demonstrate that the irreversible FGFR inhibitor TAS-120 provides clinical benefit in patients with resistance to BGJ398 or Debio 1347 and overcomes several FGFR2 mutations in ICC models.This article is highlighted in the In This Issue feature, p. 983.

TRAF3 and its biological function.

Tumor necrosis factor receptor associated factor 3 (TRAF3) is one of the most enigmatic members in the TRAF family that consists of six members, TRAF1 to 6. Despite its similarities with other TRAFs in terms of structure and protein-protein association, overexpression of TRAF3 does not induce activation of the commonly known TRAF-inducible signaling pathways, namely NF-kappaB and JNK. This lack of a simple functional assay in combination with the mysterious early lethality of the TRAF3-deficient mice has made the study of the biological function of TRAF3 challenging for almost ten years. Excitingly, TRAF3 has been identified recently to perform two seemingly distinct roles. Namely, TRAF3 functions as a negative regulator of the NF-kappaB pathway and separately, as a positive regulator of type I IFN production, placing itself as a critical regulator of both innate and adaptive immune responses.

Polyclonal Secondary FGFR2 Mutations Drive Acquired Resistance to FGFR Inhibition in Patients with FGFR2 Fusion-Positive Cholangiocarcinoma.

Genetic alterations in the fibroblast growth factor receptor (FGFR) pathway are promising therapeutic targets in many cancers, including intrahepatic cholangiocarcinoma (ICC). The FGFR inhibitor BGJ398 displayed encouraging efficacy in patients with FGFR2 fusion-positive ICC in a phase II trial, but the durability of response was limited in some patients. Here, we report the molecular basis for acquired resistance to BGJ398 in three patients via integrative genomic characterization of cell-free circulating tumor DNA (cfDNA), primary tumors, and metastases. Serial analysis of cfDNA demonstrated multiple recurrent point mutations in the FGFR2 kinase domain at progression. Accordingly, biopsy of post-progression lesions and rapid autopsy revealed marked inter- and intralesional heterogeneity, with different FGFR2 mutations in individual resistant clones. Molecular modeling and in vitro studies indicated that each mutation led to BGJ398 resistance and was surmountable by structurally distinct FGFR inhibitors. Thus, polyclonal secondary FGFR2 mutations represent an important clinical resistance mechanism that may guide the development of future therapeutic strategies.Significance: We report the first genetic mechanisms of clinical acquired resistance to FGFR inhibition in patients with FGFR2 fusion-positive ICC. Our findings can inform future strategies for detecting resistance mechanisms and inducing more durable remissions in ICC and in the wide variety of cancers where the FGFR pathway is being explored as a therapeutic target. Cancer Discov; 7(3); 252-63. ©2016 AACR.See related commentary by Smyth et al., p. 248This article is highlighted in the In This Issue feature, p. 235.

The interferon response to bacterial and viral infections.

Type I interferons (IFNs) were first described several decades ago as soluble factors that were capable of 'interfering' with viral replication when added to infected cells. Type I IFNs have been shown to be induced by recognition of viral DNA and RNA via three distinct pathways: (i) a TRIF-dependent pathway in macrophages via TLRs 3 and 4; (ii) a MyD88-dependent pathway in plasmacytoid dendritic cells (pDCs) via TLRs 7/8 and 9; and (iii) an intracellular recognition pathway utilizing the cytoplasmic receptors RIG-I/MDA5. Interestingly, these viral recognition pathways converge on TRAF3, which induces interferon through the activation of IRF3 or IRF7 by the TBK-1 and IKKi complexes. While type I IFN has been traditionally associated with antiviral responses, recent studies have demonstrated that many bacteria also induce type I interferon responses. The mechanisms of type I IFN induction and its role in host defense, however, are largely unclear. Studies with the Gram-positive intracellular bacterium Listeria monocytogenes indicated that it may trigger type I IFN induction through novel TLR-independent intracellular receptors and type I IFN may play a detrimental role to host response against listerial infection. In this article, we summarize some of these findings and discuss the functional differences of type I IFNs in bacterial and viral infections.

Isocitrate Dehydrogenase Mutations Confer Dasatinib Hypersensitivity and SRC Dependence in Intrahepatic Cholangiocarcinoma.

UNLABELLED: Intrahepatic cholangiocarcinoma (ICC) is an aggressive liver bile duct malignancy exhibiting frequent isocitrate dehydrogenase (IDH1/IDH2) mutations. Through a high-throughput drug screen of a large panel of cancer cell lines, including 17 biliary tract cancers, we found that IDH mutant (IDHm) ICC cells demonstrate a striking response to the multikinase inhibitor dasatinib, with the highest sensitivity among 682 solid tumor cell lines. Using unbiased proteomics to capture the activated kinome and CRISPR/Cas9-based genome editing to introduce dasatinib-resistant "gatekeeper" mutant kinases, we identified SRC as a critical dasatinib target in IDHm ICC. Importantly, dasatinib-treated IDHm xenografts exhibited pronounced apoptosis and tumor regression. Our results show that IDHm ICC cells have a unique dependency on SRC and suggest that dasatinib may have therapeutic benefit against IDHm ICC. Moreover, these proteomic and genome-editing strategies provide a systematic and broadly applicable approach to define targets of kinase inhibitors underlying drug responsiveness. SIGNIFICANCE: IDH mutations define a distinct subtype of ICC, a malignancy that is largely refractory to current therapies. Our work demonstrates that IDHm ICC cells are hypersensitive to dasatinib and critically dependent on SRC activity for survival and proliferation, pointing to new therapeutic strategies against these cancers. Cancer Discov; 6(7); 727-39. ©2016 AACR.This article is highlighted in the In This Issue feature, p. 681.

IDH mutations in liver cell plasticity and biliary cancer.

Intrahepatic cholangiocarcinoma (ICC) is an aggressive cancer associated with the bile ducts within the liver. These tumors are characterized by frequent gain-of-function mutations in the isocitrate dehydrogenase 1 and 2 (IDH1 and IDH2) genes-that are also common in subsets of neural, haematopoietic and bone tumors, but rare or absent in the other types of gastrointestinal malignancy. Mutant IDH acts through a novel mechanism of oncogenesis, producing high levels of the metabolite 2-hydroxyglutarate, which interferes with the function of α-ketoglutarate-dependent enzymes that regulate diverse cellular processes including histone demethylation and DNA modification. Recently, we used in vitro stem cell systems and genetically engineered mouse models (GEMMs) to demonstrate that mutant IDH promotes ICC formation by blocking hepatocyte differentiation and increasing pools of hepatic progenitors that are susceptible to additional oncogenic hits leading to ICC. We found that silencing of HNF4A-encoding a master transcriptional regulator of hepatocyte identity and quiescence-was critical to mutant IDH-mediated inhibition of liver differentiation. In line with these findings, human ICC with IDH mutations are characterized by a hepatic progenitor cell transcriptional signature suggesting that they are a distinct ICC subtype as compared to IDH wild type tumors. The role of mutant IDH in controlling hepatic differentiation state suggests the potential of newly developed inhibitors of the mutant enzyme as a form of differentiation therapy in a solid tumor.

Circulating oncometabolite 2-hydroxyglutarate is a potential surrogate biomarker in patients with isocitrate dehydrogenase-mutant intrahepatic cholangiocarcinoma.

PURPOSE: Mutations in the IDH1 and IDH2 (IDH1/2) genes occur in approximately 20% of intrahepatic cholangiocarcinoma and lead to accumulation of 2-hydroxyglutarate (2HG) in the tumor tissue. However, it remains unknown whether IDH1/2 mutations can lead to high levels of 2HG circulating in the blood and whether serum 2HG can be used as a biomarker for IDH1/2 mutational status and tumor burden in intrahepatic cholangiocarcinoma. EXPERIMENTAL DESIGN: We initially measured serum 2HG concentration in blood samples collected from 31 patients with intrahepatic cholangiocarcinoma in a screening cohort. Findings were validated across 38 resected patients with intrahepatic cholangiocarcinoma from a second cohort with tumor volume measures. Circulating levels of 2HG were evaluated relative to IDH1/2 mutational status, tumor burden, and a number of clinical variables. RESULTS: Circulating levels of 2HG in the screening cohort were significantly elevated in patients with IDH1/2-mutant (median, 478 ng/mL) versus IDH1/2-wild-type (median, 118 ng/mL) tumors (P < 0.001). This significance was maintained in the validation cohort (343 ng/mL vs. 55 ng/mL, P < 0.0001) and levels of 2HG directly correlated with tumor burden in IDH1/2-mutant cases (P < 0.05). Serum 2HG levels ≥170 ng/mL could predict the presence of an IDH1/2 mutation with a sensitivity of 83% and a specificity of 90%. No differences were noted between the allelic variants IDH1 or IDH2 in regard to the levels of circulating 2HG. CONCLUSIONS: This study indicates that circulating 2HG may be a surrogate biomarker of IDH1 or IDH2 mutation status in intrahepatic cholangiocarcinoma and that circulating 2HG levels may correlate directly with tumor burden. Clin Cancer Res; 20(7); 1884-90. ©2014 AACR.

IRF3-dependent type I interferon response in B cells regulates CpG-mediated antibody production.

Hypomethylated CpG oligonucleotides (CpG) are not only potent adjuvants for enhancing adaptive immune responses but may also play a critical role in the development of autoimmune diseases such as Rheumatoid Arthritis (RA) and Systemic Lupus Erythematosus (SLE). Here we provide evidence that, in addition to dendritic cells, murine B lymphocytes also exhibit a type I IFN response to CpG-B. Unlike dendritic cells, B cell-mediated type I IFN induction depended on the transcription factor IRF3, but similar to dendritic cells this pathway was independent of the IRF3 kinase TBK1. Utilizing type I IFN receptor-deficient mice, we were able to demonstrate that this IFN pathway enhanced Syndecan-1 expression and IgM production and was required for IgG2a production following CpG-B stimulation. Overall, our findings identify a unique IFN pathway in B cells that may play a central role in mediating B cell biology in response to CpG, potentially implicating this pathway in autoantibody production and the pathogenesis of certain autoimmune diseases.

Specificity of TRAF3 in its negative regulation of the noncanonical NF-kappa B pathway.

Tumor necrosis factor (TNF) receptor-associated factors (TRAFs) are critical signaling adaptors downstream of many receptors in the TNF receptor and interleukin-1 receptor/Toll-like receptor superfamilies. Whereas TRAF2, 5, and 6 are activators of the canonical NF-kappaB signaling pathway, TRAF3 is an inhibitor of the noncanonical NF-kappaB pathway. The contribution of the different domains in TRAFs to their respective functions remains unclear. To elucidate the structural and functional specificities of TRAF3, we reconstituted TRAF3-deficient cells with a series of TRAF3 mutants and assessed their abilities to restore TRAF3-mediated inhibition of the noncanonical NF-kappaB pathway as measured by NF-kappaB-inducing kinase (NIK) protein levels and processing of p100 to p52. We found that a structurally intact RING finger domain of TRAF3 is required for inhibition of the noncanonical NF-kappaB pathway. In addition, the three N-terminal domains, but not the C-terminal TRAF domain, of the highly homologous TRAF5 can functionally replace the corresponding domains of TRAF3 in suppression of the noncanonical NF-kappaB pathway. This functional specificity correlates with the specific binding of TRAF3, but not TRAF5, to the previously reported TRAF3 binding motif in NIK. Our studies suggest that both the RING finger domain activity and the specific binding of the TRAF domain to NIK are two critical components of TRAF3 suppression of NIK protein levels and the processing of p100 to p52.

TRAF3: a new regulator of type I interferons.

Type I interferons are a major and essential component of the mammalian antiviral response. While many cell types produce type I interferons following viral infection, how cells detect the virus has remained a mystery for many years. Recently, multiple genome-encoded viral recognition receptors have been identified. Interestingly, all of the major viral recognition pathways characterized thus far require TRAF3 to initiate type I interferon production. In this review, we comment on the mechanistic and biological implications for the new role of TRAF3 in innate antiviral immunity.