Epidermis and serum protect retinol but not retinyl esters from sunlight-induced photodegradation

Sunlight-induced photodegradation of retinyl esters and retinol in human skin, blood and cultured keratinocytes was investigated. Using high-performance liquid chromatography with an extraction method that avoided saponification, the analysis of human foreskin (Caucasian) showed that levels of retinyl esters and retinol were approximately 3.5 and 5.0 times higher, respectively, in the epidermis than in the dermis. Upon irradiation by sunlight, a significant reduction in epidermal retinyl esters was observed in both summer and winter. However, epidermal retinol, dermal retinol and dermal retinyl esters did not show statistically significant reductions. When serum from volunteers who had taken a large dose of retinyl palmitate to elevate serum retinyl esters was exposed to sunlight, the retinyl esters in the serum rapidly disappeared after 10 min of exposure--similar to the photodegradation seen for retinyl palmitate in an organic solvent. While retinol in an organic solvent rapidly photodegraded similar to serum retinyl palmitate, serum retinol slowly declined upon sunlight irradiation. When cultured keratinocytes that took-up 3H-retinol and thereafter contained 3H-retinyl esters and 3H-retinol were exposed to sunlight, 80% of the 3H-retinyl esters disappeared upon sunlight irradiation whereas only about 20% of the 3H-retinol did so. These results suggest that the epidermis, serum and keratinocytes selectively protect retinol from sunlight-induced photodegradation. It is most likely that serum retinol-binding protein and cellular retinol-binding protein protect retinol, a vital epithelial growth factor, from photodegradation.