Hypoxia inducible factor-1α contributes to UV radiation-induced inflammation, epidermal hyperplasia and immunosuppression in mice

Hypoxia inducible factor-1 alpha (HIF-1 alpha), a ubiquitous inducible oxygen-sensing transcription factor, promotes cell survival under hypoxic conditions, including the early pre-angiogenic period of tumorigenesis, and is known to contribute to many malignancies. However HIF-1 alpha can also be activated by inflammatory mediators, and can activate inflammation-modulating proteins itself, including heme oxygenase-1 (HO-1) and the cytokine IL-6. Recently HIF-1 alpha was reported to be induced by UVB (290-320 nm) radiation in cultured human keratinocytes, acting as a stress protein associated with the release of reactive oxygen species. In this in vivo murine study we demonstrate that HIF-1 alpha protein is an early responder to UV radiation in the skin, and its activation can be attenuated by treating mice with its post-translational inhibitor, YC-1. Treatment with YC-1 following UV-irradiation of mice has revealed the involvement of HIF-1 alpha in UV-induced inflammation, IL-6 production, and epidermal hyperplasia. In addition, upregulated cutaneous HIF-1 alpha was found to be an important factor in the UV-suppression of T cell-mediated immunity, measured by contact hypersensitivity (CHS). The mechanism remains unclear, however it did not appear to involve the immunosuppressive cutaneous photoproduct cis-urocanic acid, but HIF-1 alpha induction was inhibited by irradiation with photoimmune protective UVA (320-400 nm), implicating a negative correlation between the two stress proteins, HIF-1 alpha and the photoimmune protective UVA responder HO-1.