In vitro skin models for the evaluation of sunscreen-based skin photoprotection: molecular methodologies and opportunities.

Identifying and understanding the biological events that occur following ultraviolet (UV) exposure are mandatory to elucidate the biological and clinical consequences of sun exposure, and to provide efficient and adequate photoprotection strategies. The main UV-induced biological features (markers related to sunburn, cancer, photoaging immunosuppression, pigmentation), characterized in human skin in vivo, could be reproduced in adapted models of reconstructed skin in vitro, attesting their high relevance in the field of photobiology. In turn, 3D skin models were useful to discover precise biological pathways involved in UV response and were predictive of in vivo situation. Although they did not follow a strict validation process for the determination of protection factors, they enabled to evidence important concepts in photoprotection. Indeed, the use of reconstructed skin model highlighted the the importance of broad spectrum sunscreen use to protect essential cellular functions, and biologically proved that SPF value was not predictive of the level of protection in the UVA wavelength domain. New biological approaches, such as transcriptomic or proteomic studies as well as quantitative and qualitative determination of DNA damage, will indisputably increase the added value of such systems for sunscreen efficiency evaluation.