Raf Kinase Inhibitor Protein Preferentially Promotes TLR3-triggered Inflammation

Raf kinase inhibitor protein (RKIP) has been demonstrated as a metastasis suppressor, and reduced RKIP expression is related to a number of metastatic cancers. But it remains elusive whether this protein has function in inflammation. Here we find that RKIP preferentially regulates TLR3-mediated immune responses in macrophage. RKIP deficiency or knockdown decreases Poly(I:C)-induced TBK1, IRF3, MKK3/6, P38 activation and IFN-β, TNF-a and IL-6 production without affecting the counterpart induced by LPS or CpG. RKIP-deficient mice produces less IFN-β, TNF-a and IL-6 in serum, and displayed decreased lethality than their wild type counterpart upon peritoneal Poly(I:C) plus D-galactosamine injection, demonstrating RKIP was crucial in amplifying IFN-β and pro-inflammatory cytokine production in TLR3 mediated inflammatory responses. Consistently, the overexpression of RKIP increases TLR3-induced the production of type Ⅰ interferon and inflammatory cytokine and activation of TBK1, IRF3, MKK3/6, P38. We further demonstrate that RKIP promotes TLR3 triggered signaling in Serine 109 phosphorylation-dependent manner. Poly(I:C) treatment induces RKIP phosphorylation at Ser109, which is required for the RKIP interacting with TBK1 and promoting TBK1 activation. On the other hand, phosphorylated RKIP promotes the interaction of TAK1 with MKK3, which leads to enhanced MKK3 and downstream P38 activation. Thus, RKIP functions as a positive feedback regulator of TLR3 induced-inflamation.