Involvement of Lipid Rafts and Caveolins in UVA Signaling

This review provides an overview over the burden solar radiation confers to human skin. Individual exposure doses vary not only due to different ambient UV doses depending on season, time of day, geographical position and weather conditions, but just as importantly to seasonal variation in behaviour. The general photobiological mechanisms underlying UVA, UVB and infrared A signaling are marked. Rafts are tightly packed, ordered and dynamic membrane microdomains rich in sphingolipids and cholesterol. They contribute to signaling events by trapping signaling molecules e.g. receptors or enzymes in order to render them active or inactive. A special subtype of rafts are caveolae also found in basal keratinocytes representing a flask-shaped invagination of the cytoplasmic membrane which are stabilized by caveolins serving as a scaffolding protein to organize lipids and signal transducing proteins. Within UVA signaling these membrane domains have been identified to be a target and a source for formation of reactive oxygen species in keratinocytes. UVA responsiveness with regards to gene expression depends on the ratio of cholesterol vs ceramide in rafts and on the presence of caveolin-1. Cholesterol, phytosterols or several triterpenoids can stabilize these raft structure leading to inhibition of UVA signaling, whereas increased levels of 7-dehydrocholesterol found in Smith-Lemli-Opitz patients suffering from enhanced photosensitivity mainly towards UVA destabilize rafts.