The removal of UV-induced pyrimidine dimers from DNA of rat skin cells in vitro and in vivo in relation to aging

Young and old rats were compared with respect to the capacity of their skin fibroblasts and epidermal keratinocytes to remove low levels of ultraviolet light (UV) induced UV-endonuclease sensitive sites (pyrimidine dimers) from their DNA, in vitro and in vivo, respectively. In vitro, over a 24-h time period, fibroblasts from both young and old rats were found to remove about 20% of the pyrimidine dimers originally induced by 4.6 J/m² of UV-C. In vivo, after 2.6 kJ/m² of UV-B hardly any UV lesions were found to be present in fibroblasts, as demonstrated by immunohistochemistry using an anti-thymine dimer antibody. As reported earlier (Mullaart et a., J. Invest. Dermatol., 90 (1988) 346-349) cultured epidermal keratinocytes do not differ from cultured fibroblasts in UV repair kinetics, whereas in vivo they remove at least 50% of the pyrimidine dimers induced by 4 kJ/m² of UV-B within 3 h. We now show that epidermal keratinocytes from old rats are not deficient in their in vivo repair characteristics upon this low UV-B dose. However, since a considerable fraction of the pyrimidine dimers appeared to be persistent in fibroblasts and keratinocytes, demonstrated by both enzymatic and immunochemical assays, the possibility is discussed that long-term exposure of skin cells to UV may lead to an accumulation of DNA damage with age.