7 Regulation of androgen receptor and 5α-reductase in the skin of normal and hirsute women

The hormonal activity of androgens is mediated in target cells, particularly in human skin, by two kinds of proteins: the androgen receptor and the enzyme 5α-reductase. In well differentiated androgen target cells, 5α-reductase achieves the transformation of testosterone (T) into dihydrotestosterone (DHT), a more active androgen than T, because of its higher affinity for the receptor. In other words, 5α-reductase acts as an amplifier of the androgen signal but is not absolutely required for androgen action. Regarding the regulation of the androgen receptor, minimal information is available. However, in genital skin, the receptor seems to be predominantly localized in the cytosolic compartment before puberty in males and in the nuclear compartment after puberty. In hirsute patients, recent data on genital skin fibroblasts do not show significant differences between the binding capacity of fibroblasts from normal and hirsute women whereas there is no difference between normal men and women.

5α-Reductase activity seems to be a very important step in the processes involved in androgen action. While 5α-reductase activity present in the skin of external genitalia does not seem to be androgen dependent, this is not the case for the enzyme located in pubic skin. In this area, a sex difference between males and females may be observed both in skin homogenates and in cultured fibroblasts. In addition DHT added to a medium of pubic skin fibroblasts is capable of increasing 5α-reductase activity. This increase is not observed when cyproterone acetate is added to the medium and in patients with testicular feminization syndrome without receptors. Pubic 5α-reductase activity is an androgen receptor mediated phenomenon. In patients with hirsutism, and particularly idiopathic hirsutism, 5α-reductase activity is high without an increase in circulating androgens. This may be observed both in pubic skin homogenates and in cultured fibroblasts. Thus, an excess of skin 5α-reductase activity may be considered as a cause of hirsutism but both the exact level of the abnormality in the regulation of the enzyme and its genetic control remain to be elucidated.