Down-regulation of interferon γ-activated STAT1 by UV light

STAT1 is a cytoplasmic transcription factor that is phosphorylated by Janus kinases (Jak) in response to interferon-γ (IFNγ). Phosphorylated STAT1 translocates to the nucleus, where it turns on specific sets of IFNγ-inducible genes. Here, we show that UV light interferes with tyrosine phosphorylation of STAT1, thereby hindering IFNγ from exerting its biological effects. This effect is not due to a down-regulation of the IFNγ receptor because phosphorylation of upstream-located Jak1 and Jak2 was not suppressed by UV light. In contrast, UV light had no effect on the phosphorylation of STAT3, which is activated by the proinflammatory cytokine interleukin 6. The UV light effect on STAT1 phosphorylation could be antagonized by vanadate, indicating at least partial involvement of a protein tyrosine phosphatase. Therefore, this study indicates a mechanism by which UV light can inhibit gene activation and suggests STAT1 as a new extranuclear UV target closely located to the membrane.