Syk Mediates IL−17-Induced CCL20 Expression by Targeting Act1-Dependent K63-Linked Ubiquitination of TRAF6

IL−17 has an important role in the immunopathogenesis of autoimmune diseases, and spleen tyrosine kinase (Syk) has been implicated as a critical molecule in the signaling pathways of various immunoreceptors. Chemokine (C-C motif) ligand 20 (CCL20) interacts with chemokine (C-C motif) receptor 6 to recruit IL−17-producing cells into the skin to promote progression of psoriasis. Herein we investigate how Syk regulates IL−17 signaling to affect CCL20 expression in primary human epidermal keratinocytes. We found that IL−17 can induce CCL20 expression and activate TAK, IKK, NF-κB, c-Jun N-terminal kinase, and Syk. Data of TAK inhibitor and Syk small interfering RNA (siRNA) indicate Syk being an upstream molecule of TAK in IL−17-elicited signaling. The promoter activity assay combined with site-directed mutagenesis showed that IL−17-elicited CCL20 upregulation is depending on the Syk-mediated NF-κB pathway. Immunoprecipitation also indicated the interaction of Syk with signal molecules of IL−17R, such as TRAF6 and Act1, under IL−17A stimulation. However, the essential signaling events including TRAF6 interaction with Act1 and TRAF6 polyubiquitination under IL−17A stimulation were diminished by Syk siRNA and pharmacologically inhibiting Syk. Taken together, we identify Syk as an upstream signaling molecule in IL−17A-induced Act1-TRAF6 interaction in keratinocytes, and inhibition of Syk can attenuate CCL20 production, which highlights Syk as a potential therapeutic target for inflammatory skin diseases such as psoriasis.