Skin, sun, and vitamin A: from aging to cancer.

Human epidermis contains significant amounts of Vitamin A, the enzymes responsible for its metabolism toward either storage or activation, the binding proteins for its protection and specific transport, and the nuclear receptors involved in the vitamin A-induced gene-activity modulation. This complex system may be drastically altered upon ultraviolet light exposure because vitamin A absorbs in the UVB range. We have conducted a series of experiments in order to analyse the effects of UV exposure on the epidermal stores of endogenous vitamin A (retinol and retinyl esters), the activity of enzymes and binding proteins, and some biological parameters such as apoptosis transcription factors expression (cJun) and thymine dimers. Current data indicate that the vitamin A system is a direct target of both UVB and UVA and participates in an adaptive response to UV exposure. The physiological role of this adaptive response to acute and chronic sun exposure should be further analysed. Interfering with this UV-induced vitamin A deficiency is a new concept for the prevention of skin cancer and aging.