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Scaffolding Adaptor Protein Gab1 Is Required for TLR3/4- and RIG-I–Mediated Production of Proinflammatory Cytokines and Type I IFN in Macrophages

RIG-I–like helicases and TLRs are critical sensors in the induction of type I IFN and proinflammatory cytokines to initiate innate immunity against invading pathogens. However, the mechanisms for the full activation of TLR and RIG-I–triggered innate response remain to be fully investigated. Grb2-associated binder 1 (Gab1), a member of scaffolding/adaptor proteins, can mediate signal transduction from many receptors, however, whether and how Gab1 is required for TLR and RIG-I–triggered innate responses remain unknown. In this study, we demonstrated that Gab1 significantly enhances TLR4-, TLR3-, and RIG-I–triggered IL-6, IL-1β, and IFN-α/β production in macrophages. Gab1 knockdown in primary macrophages or Gab1 deficiency in mouse embryonic fibroblasts significantly suppresses TLR3/4- and RIG-I–triggered production of IL-6, IL-1β, and IFN-α/β. Consistently, Gab1 deficiency impairs vesicular stomatitis virus (VSV) infection-induced IFN-α/β production. In addition to promoting both MyD88- and TLR/IL-1 receptor domain-containing adaptor protein inducing IFN-β–dependent MAPKs and NF-κB activation, Gab1 enhances PI3K/Akt activation by directly binding p85 in TLR signaling and VSV infection. Accordingly, Gab1 inhibits VSV replication and VSV infection-induced cell damage by inducing type I IFNs and IFN-inducible gene expression via PI3K/Akt pathway. Therefore, Gab1 is needed for full activation of TLR3/4- and RIG-I–triggered innate responses by promoting activation of PI3K/Akt, MAPKs, and NF-κB pathways.