TRAF6-dependent Act1 phosphorylation by the IκB kinase-related kinases suppresses interleukin-17-induced NF-κB activation.

Interleukin-17 (IL-17) is critically involved in the pathogenesis of various inflammatory disorders. IL-17 receptor (IL-17R)-proximal signaling complex (IL-17R-Act1-TRAF6) is essential for IL-17-mediated NF-κB activation, while IL-17-mediated mRNA stability is TRAF6 independent. Recently, inducible IκB kinase (IKKi) has been shown to phosphorylate Act1 on Ser 311 to mediate IL-17-induced mRNA stability. Here we show that TANK binding kinase 1 (TBK1), the other IKK-related kinase, directly phosphorylated Act1 on three other Ser sites to suppress IL-17R-mediated NF-κB activation. IL-17 stimulation activated TBK1 and induced its association with Act1. IKKi also phosphorylated Act1 on the three serine sites and played a redundant role with TBK1 in suppressing IL-17-induced NF-κB activation. Act1 phosphorylation on the three sites inhibited its association with TRAF6 and consequently NF-κB activation in IL-17R signaling. Interestingly, TRAF6, but not TRAF3, which is the upstream adaptor of the IKK-related kinases in antiviral signaling, was critical for IL-17-induced Act1 phosphorylation. TRAF6 was essential for IL-17-induced TBK1 activation, its association with Act1, and consequent Act1 phosphorylation. Our findings define a new role for the IKK-related kinases in suppressing IL-17-mediated NF-κB activation through TRAF6-dependent Act1 phosphorylation.