Long-term ultraviolet A irradiation of the eye induces photoaging of the skin in mice

Irradiation by long-term ultraviolet (UV) A initiates the induction of photoaging. However, the mechanisms responsible for the structural changes of skin induced by UVA irradiation of the eye are still unknown. Male hairless mice were used in this study. The eye or dorsal skin was locally exposed to UVA after covering the remaining body surface with aluminum foil at a dose of 110 kJ/m(2) using a FL20SBLB-A lamp for 60 days. The plasma a-melanocyte stimulating hormone (alpha-MSH), nitrogen oxides (NO2/NO3), tumor necrosis factor-alpha (TNF-alpha), and the prostaglandin E-2 (PGE(2)) content all increased after UVA irradiation. The levels of NO2/NO3, TNF-alpha, and PGE(2) also increased more after UVA skin irradiation than after UVA eye irradiation. However, the level of a-MSH increased more by eye irradiation than skin irradiation. In addition, UVA irradiation of the eye and dorsal skin increased the number of mast cells and fibroblasts. Furthermore, the expression of the melanocortin-1 receptor (MC1R) was increased on the fibroblast surface by UVA irradiation of the eye. These results indicate that the signal evoked by UVA irradiation of the eye, through the hypothalamo-pituitary proopiomelanocortin system, up-regulated the production of alpha-MSH. This hormone controls the collagen generation from fibroblasts, thus suggesting that photoaging was induced by UVA irradiation of the eye.