Ultraviolet light suppresses IFN-γ-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 cytokines. Recently, UV light was shown to inhibit keratinocyte release of 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 light, we addressed the molecular mechanisms involved. IFN-γ treatment of the murine keratinocyte cell line Pam 212 resulted in an up-regulation of IL-7 mRNA, while IL-7 transcripts were suppressed in cells exposed to UV before IFN-γ. Because IFN-γ induces IL-7 via activation of an IFN-stimulated response element (ISRE) located in the 5′ upstream region of the IL-7 gene, bandshift assays were performed using the ISRE sequence from the IL-7 gene. Nuclear extracts from untreated cells revealed two bands, a slower migrating band identified by supershift analysis as IFN regulatory factor-2 (IRF-2), a transcriptional repressor, and a more rapidly migrating band identified as IRF-1, a transcriptional activator. IFN-γ significantly induced IRF-1 binding, whereas UV treatment plus IFN-γ decreased IRF-1 binding, suggesting that UV light suppresses IFN-γ-induced expression of IL-7 by interfering with IRF-1. Chloramphenicol transferase assay confirmed functional relevance, showing that the minimal promoter sequence for the ISRE explicitly responded to IFN-γ, which was suppressed by UV irradiation. Northern blot analysis using an IRF-1 cDNA probe revealed that UV light reduced IFN-γ-induced IRF-1 mRNA. This study demonstrates that UV light can inhibit cytokine artivities by interference with transcriptional activators. This newly described ability of UV light may contribute to its immunosuppressive properties.