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The p53 protein accumulates in human skin cells in vitro and in vivo when UV-irradiated. The transient stability of p53 requires a decrease in the activity of the ubiquitin ligase murine double minute 2 (Mdm2). Solar light irradiation (52.5, 105 and 405 mJ/cm2) of reconstructed human epidermis caused cutaneous damage. Specifically, UV-B induced the formation of sunburn cells and at first, an increase in the accumulation of p53 protein. Unexpectedly, 24 h after irradiation, a specific proteolytic cleavage of p53 resulted in the formation of a 40 kDa fragment. Both the accumulation of p53 and the proteolytic cleavage increased, commensurate with the UV dose. In contrast to p53, the level of expression of Mdm2 decreased drastically with the UV dose. It is important to note that calpastatin (20 microM), a specific inhibitor of calpains, decreased the formation of sunburn cells, inhibited the cleavage of p53 and induced an accumulation of Mdm2. The apoptotic process is strongly repressed. This demonstrates for the first time that calpains can participate in the down-regulation of Mdm2 in the epidermis very rapidly after UV irradiation, and that they contribute to a specific cleavage of p53 protein. All of these processes may be involved in the apoptotic response of the skin to UV stimulation.

The effect of solar irradiation on ex vivo dermatomed hairless rat skin samples maintained in culture on flow-through diffusion cells for at least 24 h was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay and by histological observations. Transepidermal water loss (TEWL) measurements and kinetic analysis of the permeation of both tritiated water and 14C caffeine through the skin were performed after full-spectrum solar exposure involving the use of a xenon arc solar simulator. After a UV exposure of less than 420 mJ/cm2, skin integrity and permeation of both water and caffeine did not change significantly. In contrast, after a 420 mJ/cm2 UV exposure, the epidermis appeared more contracted, associated with an increase of 55% of TEWL and 220% of the skin permeation of tritiated water after 6 h. The data suggested a dramatic alteration of the skin barrier integrity. Moreover, the flux of 14C caffeine increased rapidly by 338% of the absorption of water 12 h after irradiation. These results reveal the presence of a threshold UV exposure that would not modify skin penetration.