Regulation of IL-33 Expression by IFN-γ and Tumor Necrosis Factor-α in Normal Human Epidermal Keratinocytes

IL-33, a member of the IL-1 family, is implicated in type 2 T helper cell immune reactions and acts as an alarmin to induce activation of dendritic cells in response to external stimuli. We investigated the effect of inflammatory cytokines on IL-33 expression in normal human epidermal keratinocytes. IFN-gamma dose- and time-dependently induced IL-33 expression in protein and mRNA; this was dependent on extracellular signal-regulated kinase, p38, EGFR, and JAK phosphorylation. Combined IFN-gamma and tumor necrosis factor (TNF)-alpha treatment induced expression of a 20-kDa band corresponding to mature IL-33, which was abolished by the addition of a calpain inhibitor. The addition of the inhibitor to IFN-gamma and TNF-alpha-stimulated cells also induced strong expression of a 25-kDa band. Small interference (si) RNA for IL-33 abolished expression of the smaller bands and the 30-kDa IL-33 band, suggesting that these IL-33 forms were IL-33 transcription products. Recombinant IL-33 added in the medium induced IL-8 production, and RNA knockdown by siRNA enhanced IL-8 expression, suggesting its dual role as a cytokine and a nuclear factor. These results indicate that IL-33 has a role in inflammatory skin diseases, in which IFN-gamma and TNF-alpha are present in high levels.