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 dosedependent induction of both photoproducts. CPDs were detectable at doses ġ 125 J/m(2), while for (6‐4)photoproducts, the minimal dose at which they were detectable was 250 J/m(1) 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 bad 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 bromodeoxyuridine (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, than human or monkey skin cells.