Inhibition of cyclobutane pyrimidine dimer formation in epidermal p53 gene of UV‐irradiated mice by α‐tocopherol

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Abstract
Mutations or alterations in the p53 gene have been observed in 50–100% of ultraviolet light (UV)‐induced squamous cell carcinoma in humans and animals. Most of the mutations occurred at dipyrimidine sequences, suggesting that pyrimidine dimers in the p53 gene play a role in the pathogenesis of cutaneous squamous cell carcinoma. We previously showed that topical α‐tocopherol prevents UV‐induced skin carcinogenesis in the mouse. In the present study we asked whether topical α‐tocopherol reduces the level of UV‐induced cyclobutane pyrimidine dimers in the murine epidermal p53 gene. Mice received six dorsal applications of 25 mg each of α‐tocopherol, on alternate days, before exposure to 500 J/m2 of UV‐B irradiation. Mice were killed at selected times after irradiation. The level of dimers in the epidermal p53 gene was measured using the T4 endonu‐clease V assay with quantitative Southern hybridization. Topical α‐tocopherol caused a 55% reduction in the formation of cyclobutane pyrimidine dimers in the epidermal p53 gene. The rate of reduction of pyrimidine dimers between 1 and 10 hours after irradiation was similar in UV‐irradiated mice, regardless of α‐tocopherol treatment. Therefore, the lower level of cyclobutane pyrimidine dimers in UV‐irradiated mice treated with α‐tocopherol than in control UV‐irradiated mice resulted from the prevention of formation of the dimers, and not from enhanced repair of these lesions. Our results indicate that α‐tocopherol acts as an effective sunscreen in vivo, preventing the formation of premutagenic DNA lesions in a gene known to be important in skin carcinogenesis.