STAT4 Positively Regulates Innate Antiviral Responses And The Underlying Mechanism

RIG-I-like receptors(RLRs),including RIG-I and MDA5,are essential cytoplasmic innate sensors that detect viral RNA during virus infection.Upon activation by sensing viral RNA,RLRs form a signaling complex to transduce signal and lead to the production of type I interferon(IFN)and proinflammatory cytokines,then eliminate the invading virus.So,much attention has been paid to the regulation of RLR innate signaling in the antiviral immune response.As the most important initiator of RLR signaling for sensing RNA virus,retinoic acid-inducible gene I(RIG-I)is extensively investigated in host innate defense to RNA viral infection and maintenance of immune homeostasis.Numerous factors have been reported to regulate RIG-I function at the protein level through different mechanisms.Here we report that the signal transducer and activator of transcription(STAT)family member STAT4 promotes RIG-I-triggered IFN production in antiviral innate immunity.Silencing of STAT4 impaired RNA virus-induced IFN-?,IL6 and TNFa production in macrophages,but did not affect DNA virus-induced,LPS-induced or poly(I:C)-induced such cytokines.Overexpression of STAT4 enhanced RIG-I-induced IFN-P promoter activation,IFN-stimulated response element(ISRE)activity and NF-?B promoter activation,whereas knock down of STAT4 increased degradation of RIG-I.STAT4,which belongs to transcription factor,translocates to the nucleus when it is activated by phosphorylation.Tyr693 phosphorylation is required for STAT4's classical activation and nuclear translocation.Interestingly,during RNA virus infection STAT4 was found to be constantly present in the cytoplasm of macrophages without Tyr693 phosphorylation.Mechanistically,cytoplasmic STAT4 could interact with E3 ligase CHIP and block RIG-I and CHIP association,preventing CHIP-mediated proteasomal degradation of RIG-I via K48-linked ubiquitination.In summary,our study provide new insights into post-translational regulation of RIG-I signaling and identified novel functions of cytoplasmic STAT4 in innate antiviral immune responses.