Solar-simulated skin adaptation and its effect on subsequent UV-induced epidermal DNA damage

Repeated skin exposure to ultraviolet radiation leads to increased toleranc e for erythema. Whether this tolerance is accompanied by a significant prot ection against epidermal DNA injury has never been thoroughly investigated. In a first set of experiments we irradiated 25 healthy volunteers three ti mes a week for 3 wk using solar-simulating tanning lamps. In addition, all individuals were exposed to a (challenge) dose of three times the initial m inimal erythema dose on a small area of skin before the first and after the final exposure. On both occasions, cyclobutane pyrimidine dimers were quan tified in biopsies. As expected, repeated ultraviolet exposures resulted in increased epidermal pigmentation and thickness. The ultraviolet sensitivit y for erythema decreased on average by 75%. The cyclobutane pyrimidine dime r formation was reduced on average by 60%. In a second set of experiments, with a group of 13 subjects, DNA repair kinetics were assessed. Within a pe riod of 5 d after a single, slightly erythemal dose (1.2 minimal erythema d ose), levels of cyclobutane pyrimidine dimer and p53-expressing cells were determined in skin biopsies. Both markers of DNA damage were elevated upon the single ultraviolet exposure and returned to background levels after 3-4 d. This information is important when trying to minimize the risk of DNA d amage accumulation after repeated exposures during a tanning course.