Epidermal keratinocytes: a source of 5 alpha-dihydrotestosterone production in human skin.

The major products of testosterone, androstenedione, and progesterone metabolism by human epidermal keratinocytes are 5 alpha-reduced steroids, viz. 5 alpha-dihydrotestosterone, 5 alpha-androstanedione, and 5 alpha-dihydroprogesterone, respectively. The rates of metabolite formation by these cells were linear with incubation time up to 3 h. The apparent Km of keratinocyte 5 alpha-reductase was 1.3 microM for androstenedione and 1.5 microM for progesterone. 5 alpha-Reductase activity was found only in particulate subcellular fractions of a homogenate of epidermal keratinocytes when assayed with tritium-labeled progesterone as the substrate and NADPH as the cofactor. In addition to 5 alpha-reductase activity, other enzymatic activities found in epidermal keratinocytes were 17 beta-hydroxysteroid oxidoreductase and 3 beta-hydroxysteroid oxidoreductase. These enzymes were expressed in the formation of androstenedione from testosterone, testosterone from androstenedione, isoandrosterone from androstenedione, and 3 beta-hydroxy-5 alpha-pregnan-20-one from progesterone. The apparent Km of 17 beta-hydroxysteroid oxidoreductase for androstenedione in epidermal keratinocytes was 10 microM. When measured at weekly intervals, the rates of product formation from testosterone, androstenedione, or progesterone by cultured epidermal keratinocytes increased several-fold with advancing time in culture up to 3 weeks. The results of these studies suggest that epidermal keratinocytes are a major site of synthesis of biologically potent androgens in human skin, viz. testosterone from androstenedione and 5 alpha-dihydrotestosterone from testosterone. Skin is a target organ for 5 alpha-dihydrotestosterone action, and thus, the local formation of 5 alpha-dihydrotestosterone may play an important role in the regulation of proliferation and differentiation of keratinocytes.