Aged human skin removes UVB-induced pyrimidine dimers from the epidermis more slowly than younger adult skin in vivo

Although many studies have been reported on the repair of ultraviolet light (UV)-induced cyclobutane-type pyrimidine dimers (CPDs) in DNA, the effects of aging on the removal of UV-induced CPDs from the human skin epidermis in vivo remains uncertain. Therefore, we employed immunoblotting and immunohistochemical methods using monoclonal antibodies (TDM-2) to CPDs to study age-related differences in the time required for the in vivo removal of UVB-induced CPDs. The flexure surfaces of the upper arms of five young men were exposed to UVB light at a fluence of 35 and 700 mJ/cm², and four older men were also irradiated with the same doses of UVB mentioned above. Each area of skin was biopsied before and immediately after irradiation, and at 4, 24 h, 2 and 4 days after irradiation in the younger group; and before and immediately after irradiation, and at 24 h, 4, 7, and 14 days after irradiation in the older group. A total of 108 DNA samples were taken from the epidermis of 108 biopsied specimens. These samples were immunoblotted using TDM-2 and the intensities of the immunoprecipitates were measured by photodensitometer. Our results show that the CPDs had been removed from the epidermis at 4 days after irradiation at either dose in the younger group, and between 7–14 days after irradiation in the aged group. The results of our immunohistochemical studies were consistent with those of our immunoblotting studies, and indicated that basal cells repair CPDs more quickly than prickle cells in the epidermis except the amounts at 24 h after UVB irradiation, and that the CPDs were removed by epidermal turnover after the nucleotide excision repair (NER). Our results showed age-associated decline in the NER in vivo, indicating high risk of UV-associated skin cancer.