(PHEO)MELANIN PHOTOSENSITIZES UVA-INDUCED DNA-DAMAGE IN CULTURED HUMAN MELANOCYTES

The question of whether melanins are photoprotecting and/or photosensi tizing in human skin cells continues to be debated. To evaluate the ro le of melanin upon UVA irradiation, DNA single-strand breaks (ssb) wer e measured in human melanocytes differing only in the amount of pigmen t produced by culturing at two different concentrations, basic (0.01 m M) or high (0.2 mM), of L-tyrosine, the main precursor of melanin. In parallel, pheo- and total melanin contents of the cells were determine d. Identical experiments were performed with two melanocyte cultures d erived from a skin type I and a skin type VI individual. For the first time the correlation between UVA-induced genotoxicity and pheo-/total melanin content has been investigated. We observed that cultured in b asic medium, the skin type VI melanocytes contained 10 times more tota l melanin and about seven times more pheomelanin than the skin type I melanocytes, Elevation of tyrosine level in the culture medium resulte d in an increase of both pheo- and total melanin levels in both melano cyte cultures; however, the melanin composition of skin type I melanoc ytes became more pheomelanogenic, whereas that of skin type VI melanoc ytes remained the same, The skin type VI melanocytes cultured in basic medium demonstrated a very high sensitivity (1.18 ssb per 10(10) Da p er kJ per m(2)) toward WA that is probably related to their high pheo- and total melanin content, Their WA sensitivity, however, did not cha nge after increasing their melanin content by culturing at high tyrosi ne concentration. In contrast, the skin type I melanocytes demonstrate d a low sensitivity (0.04 ssb per 10(10) Da per kJ per m(2)) toward UV A when cultured in basic medium, but increasing their melanin content resulted in a 3-fold increase in their WA sensitivity (0.13 ssb per 10 (10) Da per kJ per m(2)). These results demonstrate that UVA-irradiate d cultured human melanocytes are photosensitized by their own synthesi zed chromophores, most likely pheomelanin and/or melanin intermediates .