Metabolic activation of lumisterol to biologically active metabolites and their mechanism of action.

Lumisterol(3) (L(3)) is produced photochemically from 7-dehydrocholesterol (7-DHC) in response to high doses of ultraviolet B (UVB). We previously demonstrated that it can be hydroxylated to mono- and dihydroxy-lumisterols by CYP11A1 and CYP27A1. In the present study, we demonstrate the presence of CYP27A1-derived 25(OH)L(3), (25R)-27(OH)L(3) and (25S)-27(OH)L(3) in the epidermis and human serum. Human immortalized epidermal keratinocytes and human colonic Caco-2 and murine hepatoma (Hepa 1-6) metabolized L(3) to these biologically active hydroxyderivatives. These metabolites stimulated keratinocyte differentiation and inhibited keratinocyte proliferation. We also demonstrate that L(3) and L(3)-hydroxymetabolites protect human epidermal melanocytes against UVB irradiation by inhibiting reactive oxygen species production and DNA damage. Addition of 20S(OH)L(3) to murine dermal fibroblasts induced significant changes in their gene expression profiles that were different from those induced by structurally related 20S(OH)cholesterol, 20S(OH)7-DHC and 20S(OH)D(3). The L(3) hydroxymetabolites interacted with the aryl hydrocarbon receptor (AhR) and the peroxisome proliferator-activated receptor γ (PPARγ), as demonstrated in functional assays and molecular modeling. These findings indicate that these hydroxylumisterols can regulate biological functions of epidermal cells by acting on AhR and PPARγ. Thus, L(3) generated in the skin by UVB radiation can act as a pro-hormone that is metabolized intracellularly by CYP11A1 and CYP27A1 into biologically active metabolites. These metabolites can regulate skin functions, and possibly have other biological functions after they enter the systemic circulation. These findings open previously unexpected, exciting new areas of research on the physiological role of lumisterols through their action on defined nuclear receptors.