Is increased 5 alpha-reductase activity a primary phenomenon in androgen-dependent skin disorders?

Testosterone metabolism was investigated in fractions of human skin, enriched in epidermis, dermis, sebaceous glands, and sweat glands, by histologic sectioning of skin punch biopsies, and the results were compared with two culturable skin cells, i.e., keratinocytes and fibroblasts. Since sebocytes could not be brought in culture, metabolism was also investigated in the hamster flank model. In the epidermal tissue of the skin biopsies the predominant metabolite was androstenedione, formed by the enzyme 17 beta-hydroxysteroid dehydrogenase. The same was true for cultured hair follicle keratinocytes. In the deeper skin layers the formation of androstenedione was markedly reduced, whereas the formation of 5 alpha-reduced metabolites was highly increased, with a maximum in the skin fractions containing large sebaceous glands. Cultured shoulder skin fibroblasts showed a markedly different testosterone metabolism compared with the sectioned skin biopsies, suggesting that dermal fibroblasts play a less important role in the overall skin testosterone metabolism. The present approach, allowing the comparison of testosterone metabolism in different substructures of the same skin biopsy provides new evidence that the high 5 alpha-reductase activity in the specific skin fractions must be mainly ascribed to the sebaceous glands. These results render a previous hypothesis, stating that the elevated level of 5 alpha-reductase and subsequent formation of dihydrotestosterone in androgenetic alopecia and acne (usually accompanied by seborrhea) could therefore simply be the consequence of sebaceous gland enlargement, much stronger. This hypothesis is further evaluated by quantitative correlation of sebaceous gland size with enzyme activity in the hamster flank model.