Intraovarian Sex Steroid Hormone Interactions and the Regulation of Follicular Maturation: Aromatization of Androgens by Human Granulosa Cells in Vitro

Abstract

The present study was undertaken to determine if alterations in the ability of granulosa cells to metabolize biologically active androgens to estrogens could constitute a physiologically significant mechanism for regulating the fate of developing follicles in the human ovary. This work was prompted by earlier studies with experimental animals which showed that estrogen stimulated growth and prevented atresia of preantral follicles, whereas androgen antagonized these effects and promoted atresia. Whole ovaries were obtained from seven young women undergoing unilateral ovariectomy during surgical correction of tubal infertility. The aromatase activities of granulosa cells collected from individual or pooled follicles were assessed by measuring the production of immunoreactive estrogen (17β-estradiol and estrone) during 3-h incubations of cells suspended in medium containing testosterone or androstenedione at a concentration of 10^-7 M. Highest granulosa cell aromatase activities were associated with large, presumptive preovulatory follicles. The concentration of 17β-estradiol in antral fluid collected from such a follicle (20 mm in diameter) exceeded 3 μg⁄ml and was more than 1300 times greater than the average level present in antral fluid collected from a group of smaller follicles (≤l0 mm in diameter) present in the same ovary. The aromatase activity of cells pooled from the latter follicles was approximately 2000 times lower than that of cells from the large follicle, suggesting that the estrogenic composition of the intrafollicular hormonal milieu may be regulated by the prevailing activity of granulosa cell aromatase enzymes. The restricted aromatase activities of granulosa cells collected from these small follicles and those from similar follicles in five other ovaries were significantly stimulated (up to 8-fold) in the presence of highly purified human FSH but not in the presence of hCG in vitro. However, regardless of follicular maturity, basal and human FSH-stimulated aromatization of testosterone in vitro was inhibited in the presence of various nonaromatizable androgen metabolites, including 17β-hydroxy-5α-androstan-3-one and 5α-androstane- 3,17-dione. These biologically active androgens are naturally occurring testosterone metabolites which have been identified in the human ovary. The present observations are consistent with a role for the granulosa cell aromatase enzyme system in the regulation of intrafollicular estrogen⁄androgen levels and indicate that the regulation of granulosa cell aromatase activity in the developing follicle is under the control of FSH. They also raise the possibility that local alterations in C₁₉ steroid 5α-reductase enzymic activity could participate in the control of preovulatory foil icular development and function in the human ovary.