OXIDATIVE DNA-DAMAGE INDUCED BY VISIBLE-LIGHT IN MAMMALIAN-CELLS - EXTENT, INHIBITION BY ANTIOXIDANTS AND GENOTOXIC EFFECTS

The extent of the indirect DNA damage generated in mammalian cells by visible light because of the presence of endogenous photosensitizers w as studied by means of repair endonucleases. In immortalized human ker atinocytes (HaCaT cells) exposed to low doses of natural sunlight, the yield of oxidative DNA base modifications sensitive to the repair end onuclease formamidopyrimidine-DNA glycosylase (Fpg protein) generated by this indirect mechanism was 10% of that of pyrimidine dimers (gener ated by direct DNA excitation). A similar yield of Fpg-sensitive modif ications, which include 8-hydroxyguanine, was observed in primary kera tinocytes. The relative yield of oxidative base modifications decrease d at higher light doses, probably as a result of photodecomposition of the endogenous chromophore involved. For the three cell lines tested, viz. HaCaT cells, L1210 mouse leukemia cells and AS52 Chinese hamster cells, the yield of oxidative base modifications generated by a low d ose of visible light appeared to be correlated with the basal concentr ations of porphyrins in the cells. Induction of cellular porphyrin syn thesis by pretreatment with 5-aminolaevulinic acid increased the light -induced oxidative damage in L1210 cells several-fold. In both induced and uninduced cells, the damage was inhibited by more than 50% in the presence of ascorbic acid (100 mu M), while alpha-tocopherol and the iron chelator o-phenanthroline had no effect and beta-carotene even in creased the damage. Even high doses of visible light did not significa ntly increase the numbers of micronuclei in L1210 cells or of gpt muta tions in AS52 cells, The negative outcome can be fully explained by th e photobleaching of the endogenous photosensitizers, which prevents th e generation of sufficiently high levels of oxidative DNA damage. Ther efore, the mutagenic risk arising from the indirectly generated oxidat ive DNA modifications induced by sunlight may be underestimated when r esults obtained at high doses are extrapolated to low doses or low dos e rates. (C) 1998 Elsevier Science B.V. All rights reserved.