IκB-ζ Expression Requires Both TYK2/STAT3 Activity and IL-17–Regulated mRNA Stabilization

Cytokine IL-17A (IL-17) acts on various cell types, including epidermal keratinocytes, and induces antimicrobial peptide and chemokine production to elicit antibacterial and antifungal defense responses. Excess IL-17 leads to inflammatory skin diseases such as psoriasis. The IκB family protein IκB-ζ mediates IL-17–induced responses. However, the mechanism controlling IκB-ζ expression in IL-17–stimulated cells remains elusive. In this study, we showed that JAK kinase TYK2 positively regulates IL-17–induced IκB-ζ expression. TYK2-deficient mice showed reduced inflammation and concomitant reduction of IκB-ζ mRNA compared with wild-type mice in imiquimod-induced skin inflammation. The analysis of the IκB-ζ promoter activity using human cell lines (HaCaT and HeLa) revealed that catalytic activity of TYK2 and its substrate transcription factor STAT3, but not IL-17, is required for IκB-ζ promoter activity. In contrast, IL-17–induced signaling, which did not activate STAT3, posttranscriptionally stabilized IκB-ζ mRNA via its 3′-untranslated region. IL-17 signaling protein ACT1 was required to counteract constitutive IκB-ζ mRNA degradation by RNase Regnase-1. These results suggested that transcriptional activation by TYK2–STAT3 pathway and mRNA stabilization by IL-17–mediated signals act separately from each other but complementarily to achieve IκB-ζ induction. Therefore, JAK/TYK2 inhibition might be of significance in regulation of IL-17–induced inflammatory reactions.