Skin can control solar UVR-induced mutations through the epidermis-specific response of mutation induction suppression

Skin exposure to solar ultraviolet radiation (UVR) has been a major public concern because of its genotoxicity. We established recently three action spectra of UVR biological effects using inflammation, mutagenicity, and mutation induction suppression (MIS) as indicators to evaluate UVR risk for mammalian skin. MIS is an antigenotoxic epidermis-specific response by which the increase of the mutant frequency (MF) levels off above a certain UVR dose. Here, based on these spectra, the mutation load of the skin after sunlight exposure was evaluated utilizing the spectral solar-UVR intensity data which had been measured at Tsukuba, Japan by the Japan Meteorological Agency. We estimated the daily variation of the solar-UVR effectiveness (effect per second) for the three indicators, and revealed that the effectiveness efficiency (effect per dose) of midday sunlight is 3-4-fold higher than those in the early morning and late afternoon. Based on the daily variations of mutagenicity and MIS effectiveness, we further estimated MFs induced after every one-hour sunlight exposure and reached a remarkable prediction that MFs should be suppressed to a constant level during 9:00-15:00 by MIS. The estimates agreed well with the equivalent values directly determined at Sendai, a site close to Tsukuba, although a small difference was detected for the epidermis at the dose range where the suppressed MFs were predicted. We propose the use of observed minimum inflammation/ erythema doses to improve the difference. Our method could provide reliable estimates of sunlight genotoxicity to evaluate skin cancer probabilities.