ULTRAVIOLET-LIGHT SUPPRESSES IFN-GAMMA-INDUCED IL-7 GENE-EXPRESSION IN MURINE KERATINOCYTES BY INTERFERING WITH IFN REGULATORY FACTORS

UV light is a potent stimulus for keratinocytes to release several cyt okines. Recently, UV light was shown to inhibit keratinocyte release o f IL-7, a growth factor for dendritic epidermal T cells. Since to date IL-7 is the only keratinocyte-derived cytokine down-regulated by UV l ight, we addressed the molecular mechanisms involved. IFN-gamma treatm ent of the murine keratinocyte cell line Pam 212 resulted in an up-reg ulation of IL-7 mRNA, while IL-7 transcripts were suppressed in cells exposed to UV before IFN-gamma. Because lFN-gamma induces IL-7 via act ivation of an IFN-stimulated response element (ISRE) located in the 5' upstream region of the IL-7 gene, bandshift assays were performed usi ng the ISRE sequence from the IL-7 gene. Nuclear extracts from untreat ed cells revealed two bands, a slower migrating band identified by sup ershift analysis as IFN regulatory factor-2 (IRF-2), a transcriptional repressor, and a more rapidly migrating band identified as IRF-1, a t ranscriptional activator. IFN-gamma significantly induced IRF-1 bindin g, whereas UV treatment plus IFN-gamma decreased IRF-1 binding, sugges ting that UV light suppresses IFN-gamma-induced expression of IL-7 by interfering with IRF-1. Chloramphenicol transferase assay confirmed fu nctional relevance, showing that the minimal promoter sequence for the ISRE explicitly responded to IFN-gamma, which was suppressed by UV ir radiation. Northern blot analysis using an IRF-1 cDNA probe revealed t hat UV light reduced IFN-gamma-induced IRF-1 mRNA. This study demonstr ates that UV light can inhibit cytokine activities by interference wit h transcriptional activators. This newly described ability of UV light may contribute to its immunosuppressive properties.