Effect of photoreactivating light on UV radiation-induced alterations in human skin

Background/Aims: Photoreactivating light (PRL) after ultraviolet radiation (UVR) exposure causes photoreversal of cyclobutane pyrimidine dimers through the activation of photolyase. Although photoreversal has been demonstrated in the ‘three kingdoms of life,’ its existence in man remains controversial. We sought evidence for photoreversal in man. Methods and Results: Seven subjects were spot-irradiated at two sites with 4 minimal erythema doses (MED) of solar-simulating UVR. Of the two sites, one was then immediately exposed to a PRL source. Epidermal biopsies were taken immediately after exposure. No significant difference in the quantity of pyrimidine dimers was detected comparing the ‘UVR only’ site to the ‘UVR, PRL-exposed’ site. Biopsies were repeated 24 h later and no significant difference in p53 protein expression or dendritic cell number was detected. However, the ‘UVR, PRL-exposed’ site showed a greater reduction in pyrimidine dimer quantity. Conclusions: We found no evidence for a direct effect of PRL causing photoreversal of UVR-induced pyrimidine dimers in man. Our results do, however, suggest that some indirect effect of PRL may enhance pyrimidine dimer repair in the 24-h period following UVR exposure.