Quantitative detection of ultraviolet light-induced photoproducts in mouse skin by immunohistochemistry

UVB-induced cyclobutane pyrimidine dimers (CPDs) and pyrimidine-pyrimidone (6-4)photoproducts [(6-4)photoproducts] in mouse skin DNA were quantitatively measured using an immunohistochemical approach with a computer-aided color image analyzer. The skins of the C3H/HeN mice were irradiated with ultraviolet B (UV-B, 280-320 nm), and processed to give conventional formalin-fixed, paraffin-embedded histologic sections. Routine immunohistochemistry clearly demonstrated a dose-dependent induction of both photoproducts. CPDs were detectable at doses > or = 125 J/m2, while for (6-4)photoproducts, the minimal dose at which they were detectable was 250 J/m2 in the present study. A time course study showed that the repair of (6-4)photoproducts was more rapid than that of CPDs, and that epidermal cells had a higher capacity for their removal than dermal cells. About half of the (6-4)photoproducts were excised within the first 24 h after the irradiation, and the process was essentially complete by 72 h. In contrast, there was no apparent removal (less than 10%) of CPDs in the first 24 h and they only completely disappeared from the epidermal cells at 120 h after irradiation. The effect of DNA dilution due to increased turnover of epidermal cells after UV-B irradiation was evaluated by quantitative immunohistochemical measurement of the time course of bromodeoxy-uridine (BrdUrd) incorporated into nuclei at 2 days post irradiation when the proliferation reaches a peak. The removal of photoproducts was more marked than the decrease in BrdUrd staining. Our results suggest that mouse skin cells can repair both (6-4)photoproducts and CPDs, but with considerably lower efficiency, especially in the latter case, then human or monkey skin cells.