The data reported herein confirm that during anestrus, estradiol can regulate secretion of gonadotropins by a direct action on the anterior pituitary gland in the absence of any hypothalamic factors. These effects of estradiol appear to be mediated, at least in part, by intrapituitary activin. Inhibition of ovine pituitary pB gene expression by estradiol is consistent with previous findings in rats in which the ovariectomy-induced increase in pB mRNA was prevented by treatment with estradiol. There is a potential estrogen response element (AGGTAAnnnTGACCT) in the 3′-untranslated region of the published sequence of the human activin pB gene that differs from the consensus estrogen response element (AGGTCAnnnTGACCT) by a single base. Thus, the potential for direct estrogenic regulation of activin production exists. Alternatively, it is possible that estradiol may alter the transcription of other pituitary factors that reduce expression of activin pB.
In addition to decreased secretion of FSH, we observed a specific reduction in expression of the FSHp subunit in response to treatment with estradiol in vitro. These results are in good agreement with previous observations made in vivo. During both breeding and nonbreeding seasons, estradiol reduced secretion of FSH and decreased expression of mRNA for the FSHp subunit.
Increased secretion of LH stimulated by estradiol in the present study might have arisen from a direct effect on the gene for LHp because it harbors an estrogen response element (ERE) in the 5′-flanking region. However, there does not appear to be an ERE in the 5′-flanking region of the FSHp gene. Thus, it is likely that estradiol acts at the pituitary level by stimulating other intracellular factors that modulate synthesis and secretion of FSH.
Analysis by RT-PCR failed to demonstrate a correlation between expression of FSHp and follistatin in response to treatment with estradiol. No increase in expression of fol-listatin was noted in cells from any of the five animals following treatment with estradiol even though there was a consistent decrease in expression of FSHp. The lack of increase in expression of follistatin is not surprising in light of the fact that activin induces expression of follistatin in the rat pituitary gland. Because exposure of our cultures to estradiol led to a dose-dependent decrease in expression of pB, there may not have been a stimulus for production of follistatin. These data lead us to believe that estradiol reduces expression of activin at the level of the pituitary gland, independent of GnRH. The effect of estradiol on expression of activin may only partially explain the reduction observed in expression of FSHp.