No abstract

Linear ubiquitination is a critical regulator of inflammatory signaling pathways. However, linearly ubiquitinated substrates and the biological significance of linear ubiquitination is incompletely understood. Here, we show that STAT1 has linear ubiquitination at Lys511 and Lys652 residues in intact cells, which inhibits STAT1 binding to the type-I interferon receptor IFNAR2, thereby restricting STAT1 activation and resulting in type-I interferon signaling homeostasis. Linear ubiquitination of STAT1 is removed rapidly by OTULIN upon type-I interferon stimulation, which facilitates activation of interferon-STAT1 signaling. Furthermore, viruses induce HOIP expression through the NF-κB pathway, which in turn increases linear ubiquitination of STAT1 and thereby inhibits interferon antiviral response. Consequently, HOIL-1L heterozygous mice have active STAT1 signaling and enhanced responses to type-I interferons. These findings demonstrate a linear ubiquitination-mediated switch between homeostasis and activation of type-I interferon signaling, and suggest potential strategies for clinical antiviral therapy.

During RNA virus infection, the adaptor protein MAVS recruits TRAF3 and TRAF6 to form a signalosome, which is critical to induce the production of type I interferons (IFNs) and proinflammatory cytokines. While activation of the MAVS/TRAF3/TRAF6 signalosome is well studied, the negative regulation of the signalosome remains largely unknown. Here we report that RNA viruses specifically promote the deubiquitinase OTUD1 expression by NF-κB-dependent mechanisms at the early stage of viral infection. Furthermore, OTUD1 upregulates protein levels of intracellular Smurf1 by removing Smurf1 ubiquitination. Importantly, RNA virus infection promotes the binding of Smurf1 to MAVS, TRAF3 and TRAF6, which leads to ubiquitination-dependent degradation of every component of the MAVS/TRAF3/TRAF6 signalosome and subsequent potent inhibition of IFNs production. Consistently, OTUD1-deficient mice produce more antiviral cytokines and are more resistant to RNA virus infection. Our findings reveal a novel immune evasion mechanism exploited by RNA viruses, and elucidate a negative feedback loop of MAVS/TRAF3/TRAF6 signaling mediated by the OTUD1-Smurf1 axis during RNA virus infection.

Pancreatic cancer is lethal in over 90% of cases since it is resistant to current therapeutic strategies. The key role of STAT3 in promoting pancreatic cancer progression has been proven, but effective interventions that suppress STAT3 activities are limited. The development of novel anticancer agents that directly target STAT3 may have potential clinical benefits for pancreatic cancer treatment. Here, we report a new small-molecule inhibitor (N4) with potent antitumor bioactivity, which inhibits multiple oncogenic processes in pancreatic cancer. N4 blocked STAT3 and phospho-tyrosine (pTyr) peptide interactions in fluorescence polarization (FP) assay, specifically abolished phosphor-STAT3 (Tyr705), and suppressed expression of STAT3 downstream genes. The mechanism involved the direct binding of N4 to the STAT3 SH2 domain, thereby, the STAT3 dimerization, STAT3-EGFR, and STAT3-NF-κB cross-talk were efficiently inhibited. In animal models of pancreatic cancer, N4 was well tolerated, suppressed tumor growth and metastasis, and significantly prolonged survival of tumor-bearing mice. Our results offer a preclinical proof of concept for N4 as a candidate therapeutic compound for pancreatic cancer.

Highlights d Viperin protein induction in epithelium is defective in response to viruses or IFNs d Viruses and IFNs utilize HAT1 to induce Viperin acetylation that recruits UBE4A d UBE4A induces Viperin K6 ubiquitination and degrades Viperin protein in epithelium d VIP-IP3 rescues Viperin protein in epithelium and enhances host antiviral defense