Photoaging and DNA repair

The incidence of sunlight-induced skin changes (photoaged skin, skin carcinogenesis) increases with increasing age and it is thought to be associated with an accumulation of mutations in skin cells. These mutations are mainly caused by UV exposure. The reactive oxygen species produced in UV-exposed skin can cause various kinds of DNA damages e.g., 8-oxoguanine, which are primarily repaired by the base excision repair (BER) system. In addition, UV can directly cause DNA damages; cyclobutane pyrimidine dimers (CPD) and pyrimidine-pyrimidone (6-4) photoproducts (6-4PP), both of which can be repaired by the nucleotide excision repair (NER) system. There have been several reports showing an age-related reduction in the DNA repair capacity in the NER, BER, and other repair systems, which contributes to the phenotypes of aging. To clarify the mechanism of skin aging, we examined the NER of skin fibroblasts from healthy donors of different ages. In a host cell reactivation assay, the cells from elderly donors exhibited a significant decline in the ability to restore transfected reporter DNA damaged by UV. In contrast, the ability to remove CPD and 6-4PP declined little with age, as assessed by an enzyme-linked immunosorbent assay. The mRNA expression of DNA repair synthesis-related genes was markedly decreased in the cells from elderly subjects as compared with those from young subjects. These results imply that the age-sensitive step took place after the damage excision in the NER, and that there is an impairment of the latter step of the NER in aging. Based on our data, as well as other reports, the reduced post-UV DNA repair capacity in aging resulting in an accumulation of UV-induced DNA damage is thus considered to be associated with the phenotypes of photoaged skin.