Estradiol Suppresses Phosphorylation of Cyclic Adenosine 3',5'-monophosphate Response Element Binding Protein (CREB) in the Pituitary: Evidence for Indirect Action Via Gonadotropin-releasing Hormone

Estradiol acts on the hypothalamus and pituitary gland to modulate the synthesis and secretion of gonadotropins. We recently reported that GnRH-induced transcription of the human gonadotropin alpha-gene promoter is increased markedly in transfected pituitary cells derived from animals treated with estradiol. Because the cAMP response element binding (CREB) protein plays an important role in the transcriptional regulation of this promoter and is highly regulated by posttranslational phosphorylation, we hypothesized that it might serve as a target for estradiol-induced sensitivity to GnRH. In this study, we assessed the roles of estradiol and GnRH in the regulation of CREB phosphorylation in the rat pituitary. Using an antibody that specifically recognizes phosphorylated CREB (pCREB), we found that the pituitary content of pCREB was inversely related to the level of estradiol during the estrous cycle. Ovariectomy increased the level of pCREB, and treatment with estradiol for 10 days decreased the content of pCREB dramatically (93% inhibition). A similar reduction of pCREB was seen when ovariectomized rats were treated with a GnRH receptor antagonist for 10 days. This result indicates that the ovariectomy-induced increase in pCREB is GnRH-dependent. In alphaT3 gonadotrope cells, estradiol had no direct effect on CREB phosphorylation, whereas GnRH increased CREB phosphorylation 4- to 5-fold within 5 min. We conclude that estradiol inhibits CREB phosphorylation in the gonadotrope, probably by inhibiting GnRH production. The estradiol-induced decrease in CREB phosphorylation is proposed to lower basal alpha-promoter activity and increase its responsiveness to GnRH.