Follistatin suppresses steroid-enhanced follicle-stimulating hormone release in vitro in rats

Previous in vitro and in vivo studies from our laboratory showed that proge sterone (P-4), corticosterone (B), and testosterone (T) increase intracellu lar content and release of FSH in the anterior pituitary. Activin (Act) and inhibin (Inh) are structurally related proteins with antagonistic actions, as Act stimulates and Inh inhibits FSH secretion from the anterior pituita ry. Together with follistatin (FS), a protein that bioneutralizes Act, they form an autocrine-paracrine loop in the anterior pituitary that tightly re gulates FSH secretion. The objective of the present study was to test the h ypothesis that P-4, B, and T modulate this autocrine-paracrine loop to favo r increased FSH secretion. If Act were to mediate steroid-induced FSH relea se, FS would be expected to block these effects. To test this interaction, cell cultures were prepared from anterior pituitaries of male and female ra ts, and treated with Act, B, P-4, or T in the absence or presence of FS. Ac t, B, P-4, and T increased FSH release; FS suppressed both basal and Act- a nd steroid-stimulated FSH release to approximately 50% below basal levels. Cell cultures from anterior pituitary of female rats were used to compare t he interaction of incremental concentrations of FS on dose-related Act- and P-4-stimulated FSH release. With increasing concentrations of Act, the FS- induced suppression of FSH release was attenuated and eventually abolished; in contrast, maximally stimulatory concentrations of P-4 did not fully ove rcome the FS-induced suppression of FSH release. The effects of P-4, B, and Act in the presence and absence of estradiol on steady-state mRNA levels o f FSH beta, Act beta(B) and FS were determined in primary pituitary cell cu ltures from metestrous female rats by reverse transcription-polymerase chai n reaction. Whereas Act, P-4, B increased FSH beta mRNA levels, only Act ra ised the level of FS mRNA, and neither steroid increased Act beta(B), mRNA. The results support the hypothesis that endogenous Act is a common mediato r of the action of P-4, B, and T in the rat primary anterior pituitary cell culture. We conclude that the stimulation of FSH release and intracellular content in the gonadotroph by P-4, B, and T is mediated, in part, by Act a nd involves modulation of a tightly regulated Act/FS autocrine-paracrine lo op.