EFFECT OF AGING ON UNSCHEDULED DNA SYNTHESIS INDUCTION BY UV-B IRRADIATION IN HAIRLESS MOUSE EPIDERMIS

Age-related induction of unscheduled DNA synthesis(UDS)by ultraviolet B(UV-B)irradiation was investigated in the epidermis of female hairless mice by means of an in vivo-in vitro assay using a liquid scintillation counting method. Skin samples were taken and cultured in a medium containing[3H]thymidine with or without hydroxyurea(HU)for 2 hr. DNA of the epidermis was extracted, and the incorporation of[3H]thymidine into DNA and the DNA content were determined with a liquid scintillation counter and a fluorescence spectrophotometer, respectively. Induction of UDS by UV-B was judged in terms of the value of the UDS index calculated as a percentage of the respective unexposed control value taken as 100%[the UDS index is given by(the ratio of DNA synthesis in the presence of HU to that in its absence)×100]. DNA synthesis both in the presence and absence of HU decreased with age[12 months old(M)