The contribution of calpains in the down-regulation of Mdm2 and p53 proteolysis in reconstructed human epidermis in response to solar irradiation.

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.

Assessment of the skin photoprotective capacities of an organo-mineral broad-spectrum sunblock on two ex vivo skin models.

UV irradiation can cause cutaneous damage that may be specific according to the wavelength of UV rays. For example, damage from UVB irradiation manifests itself in the form of sunburn cells and enhancement of the expression of p53, while damage from UVA exposure results in an increase in the expression of vimentin. These reactions to UV irradiation were used in this work to evaluate the photoprotective capacities of two sunblock preparations that were applied to the surface of the skin. One sunblock preparation is a UVB absorber containing zinc oxide (ZnO) and titanium oxide (TiO2) exclusively. The other sunblock preparation is a new organo-mineral sunblock containing Tinosorb M, OCM, ZnO and TiO2. Evaluation of the photoprotective capacities of both preparations on hairless rat skin and on in vitro reconstructed human epidermis revealed that they were effective in preventing UVB-induced damage. In contrast, only the organo-mineral sunblock was effective in the prevention of UVA-specific damage such as dermal alterations characterized by the expression of vimentin. Furthermore, our data support the fact that hairless rat skin and in vitro reconstructed human epidermis are a reliable basis for the evaluation of the photoprotective capacities of various sunscreens against UVB and UVA damage.

Modifications of in vitro skin penetration under solar irradiation: evaluation on flow-through diffusion cells.

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.