Topical retinoic acid enhances, and a dark tan protects, from subedemal solar-simulated photocarcinogenesis.

Studies into the effects of topical retinoic acid on photocarcinogenesis have yielded ambiguous findings. This may be due to different Experimental protocols and ultraviolet spectra. Retinoic acid is commonly used for a range of dermatologic conditions, and therefore it is important to resolve whether it affects skin tumor formation. To address this issue we used a protocol to mimic as closely as possible human use of retinoic acid. Two mouse strains were used: Skh:HR-1 (albino) and Skh:HR-2 (lightly pigmented). The pigmented mice more closely resemble Caucasian skin as they develop a light tan in response to ultraviolet radiation. This tan is greatly augmented by retinoic acid. As these are congenic mice, any differences can be attributed to the development of a tan. Mice were exposed to solar-simulated ultraviolet radiation, followed by treatment with 0.05% retinoic acid. This modeled exposure to sunlight during the day followed by retinoic acid treatment and a night-time period in the absence of sunlight. As it is recommended that ultraviolet exposure is minimized when using topical retinoic acid, the mice were only exposed to one-third of minimal edemal dose of ultraviolet radiation per day. This retinoic acid protocol augmented photocarcinogenesis. Retinoic acid decreased the latency period, reduced the probability that a mouse would survive without a tumor, and increased the number of tumors per mouse. All tumors induced were squamous cell carcinomas, and the skin between the tumors on mice treated with retinoic acid was found to contain carcinoma in situ upon histologic diagnosis. The light tan of the solvent-treated pigmented mice did not provide any protection, whereas the dark tan, which developed in Skh:HR-2 mice in response to retinoic acid, reduced photocarcinogenesis but did not overcome the augmenting effect of retinoic acid. Thus, using this experimental design, topical retinoic acid augmented photocarcinogenesis, and the ability to develop a dark but not light tan provided some, but limited, protection.