Factors influencing follicle-stimulating hormone-responsive steroidogenesis in marmoset granulosa cells: effects of androgens and the stage of follicular maturity.

Preovulatory changes in the steroidogenic function of primate granulosa cells were studied using the cyclic marmoset (Callithrix jacchus) as a model. Antral follicles (greater than or equal to 0.5 mm diameter) were dissected from mid-late follicular phase ovaries (7 days after prostaglandin-induced luteolysis) and classified by diameter as small (0.5-1.0 mm), medium (1.1-1.9 mm) or large (greater than or equal to 2.0 mm). Granulosa cells from follicles in each size category were isolated and pooled to assess steroid biosynthesis. The aromatase activity of freshly isolated granulosa cells from large follicles was 200 times greater than that of small follicles, confirming their relatively advanced preovulatory status. Granulosa cells were cultured for 48 h in the presence and absence of human (h) FSH (0.1 ng/ml), with and without 0.1 microM androgen (testosterone or 5 alpha-dihydrotestosterone), to assess basal and hormone-responsive steroidogenesis (progesterone accumulation in culture medium and aromatase activity in washed granulosa cell monolayers). Basal granulosa cell steroidogenesis increased with follicular size, and there was a development-related pattern of response to hFSH and androgen. hFSH responsiveness (maximum fold-stimulation induced by hFSH) declined with follicular size, being 2-6 times greater for granulosa cells from small vs. large follicles. On the other hand, hFSH sensitivity increased with follicular size; the dose of hFSH giving 50% of the maximum response (ED50) for cells from large follicles being 10-20 times less than that of cells from small follicles. For granulosa cells from small follicles, treatment with 0.1 microM androgen in the presence of hFSH led to dramatic (up to 16-fold) enhancement of steroidogenic responses to hFSH. In contrast, for granulosa cells from large follicles, the presence of androgen substantially inhibited aromatase activity stimulated by hFSH and had weak inhibitory effects on progesterone accumulation. These results show that granulosa cell steroidogenesis becomes increasingly sensitive to hFSH during preovulatory follicular development in marmosets. The marked ability of androgen to directly augment hFSH-responsive steroidogenesis in vitro is lost during preovulatory development, such that androgen acts in mature granulosa cells to suppress hFSH-stimulated aromatase activity. These observations are evidence of development-dependent changes in granulosa cell responses to FSH and androgens which may contribute to the control of preovulatory follicular development in primates.