Solar UV radiation is the most important environmental factor involved in the pathogenesis of skin cancers. The well known genotoxic properties of UVB radiation (290 -320 nm) mostly involve bipyrimidine DNA photoproducts. In contrast, the contribution of more-abundant UVA radiation (320 -400 nm) that are not directly absorbed by DNA remains poorly understood in skin. Using a highly accurate and quantitative assay based on HPLC coupled with tandem mass spectrometry, we determined the type and the yield of formation of DNA damage in whole human skin exposed to UVB or UVA. Cyclobutane pyrimidine dimers, a typical UVBinduced DNA damage, were found to be produced in significant yield also in whole human skin exposed to UVA through a mechanism different from that triggered by UVB. Moreover, the latter class of photoproducts is produced in a larger amount than 8-oxo-7,8-dihydro-2-deoxyguanosine, the most common oxidatively generated lesion, in human skin. Strikingly, the rate of removal of UVA-generated cyclobutane pyrimidine dimers was lower than those produced by UVB irradiation of skin. Finally, we compared the formation yields of DNA damage in whole skin with those determined in primary cultures of keratinocytes isolated from the same donors. We thus showed that human skin efficiently protects against UVB-induced DNA lesions, whereas very weak protection is afforded against UVA. These observations emphasize the likely role played by the UVA-induced DNA damage in skin carcinogenesis and should have consequences for photoprotection strategies.carcinogenesis ͉ DNA damage ͉ mutagenesis ͉ oxidative stress ͉ DNA repair O ccurrence of skin cancers, which mostly arise from exposure to solar UV radiation, has constantly increased in the recent years due to changes in life habits. Harmful effects of UV radiation are mostly associated with both direct and indirect photoinduced damage to DNA (1) that can lead to the induction of mutations. The chemical nature and the formation efficiency of the DNA lesions greatly depend on the wavelength of the incident photons. UVB radiation (290-320 nm), the most energetic mutagenic and carcinogenic component of solar radiation, is directly absorbed by DNA, giving rise to dimeric photoproducts between adjacent pyrimidine bases. Two types of these bulky modifications are produced, namely cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (1). Evidence for the involvement of these lesions in photocarcinogenesis is provided by the high proportion of p53 mutations (TC to TT or CC to TT transitions) detected at bipyrimidine sites in skin tumors (2-4).UVA radiation (320-400 nm), the other component of terrestrial UV radiation, is mutagenic in cultured cells (5, 6) and induces skin tumors in mice (7,8). In addition, UVA has been shown to be involved in immunosuppression (9) and is suspected to play a major role in the induction of melanoma (10, 11), the most severe type of skin cancers. Consequently, the carcinogenic properties of UVA have become a matter of concern...

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