Scaffolding Adaptor Protein Gab1 Promotes TLRs　and RIG-I-triggered Innate Immune Responses By Amplifying PI3K Pathway Activation

Toll-like receptors(TLRs) and retinoic acid-inducible gene I(RIG-I)-like helicases are critical in the induction of type I interferon(IFNs) 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 remains unknown.Here we demonstrated that Gab1 significantly enhanced TLR4-,TLR3- and RIG-I-triggered interleukin-6(IL-6),IL-1βand IFN-α/βproduction in macrophages.Gab1 knockdown in primary macrophages or Gabl deficiency in mouse embryonic fibroblasts significantly suppressed TLR4-,TLR3- and RIG-I-triggered production of IL-6,IL-1βand IFN-α/β.Consistently,Gab1 deficiency impaired vesicular stomatitis virus(VSV) infection-induced IFN-α/βproduction,thus leading to the increased VSV replication.In addition to promoting both MyD88- and TRIF-dependent MAPKs and NF-κB activation,Gab1 enhanced PI3K/Akt activation by directly binding p85 in TLR signaling and VSV infection.Blockade of PBK/Akt pathway attenuated Gab1-induced IL-6 and IFN-βproduction.Therefore,Gabl is needed for full activation of TLR4, TLR3 and RIG-I-triggered innate responses by promoting activation of PI3K/Akt, MAPKs and NF-κB pathways.Gab1 is constitutively highly expressed in macrophages and its expression is rapidly down-regulated by TLR4 activation,and such downregulation of Gab1 might be a protective mechanism to avoid excess inflammatory response.