Detection of thymine dimers in suprabasal and basal cells of chronically UV-B exposed hairless mice.

An immunocytochemical method was developed to study induction and removal of DNA damage in specific cell populations in the epidermis of hairless mice during chronic ultraviolet (UV) exposure. Identification of mouse suprabasal cells was performed with an immunoperoxidase stain. This stain was shown not to affect the fluorescent nuclear stains, used to reveal DNA and DNA damage. In skin cells from hairless mice irradiated daily with 1500 J/m2 UV-B for 11 consecutive days, cyclobutane thymine dimers accumulated in epidermal cells and reached a maximum level after 3 d. Thereafter dimer levels dropped to a lower, more constant level. So epidermal cells in vivo, both suprabasal and basal cells, remove dimers effectively, in contrast to cultured rodent cells, which display hardly any repair in genomic DNA. Dimer content in suprabasal cells was higher than that in basal cells, but initially the patterns of induction and removal of dimers in both cell types were rather similar. At days 4-11, however, after the drop in dimer content, the amount of dimers in basal cells prior to UV exposure was almost as low as that in non-exposed cells. The results presented here suggest important roles for both UV-induced DNA repair and cell proliferation in protecting epidermal cells against the mutagenic and carcinogenic effects of UV.