GnRH regulates secretion of the gonadotropins, LH and FSH, in a sexually dimorphic manner. In the present study, we examined GnRH regulation of the gonadotropin α-subunit promoter to assess whether sex-dependent hormonal effects are manifest at the transcriptional level. Primary cultures of male or female rat pituitary cells were transfected with a reporter gene containing the α-promoter linked to luciferase (-420α-LUC) and then subjected to treatment with GnRH for 24 h. Basal α-LUC expression was 4.2- fold greater in pituitary cells from males than in those from females. α- LUC activity was stimulated 5.3-fold by GnRH in males, whereas GnRH induced a 148-fold increase in α-promoter activity in females. The GnRH responsiveness of the transfected α-promoter did not vary in pituitary cells isolated at different stages of the female reproductive cycle, suggesting that acute changes in the hormonal milieu are not sufficient to alter transcriptional responses to GnRH. In males, orchidectomy minimally influenced α-LUC activity, indicating that testosterone does not exert a suppressive effect on GnRH responsiveness. In ovariectomized females, basal expression of α-LUC increased 3.7-fold, and GnRH stimulation was reduced from 165- to 11-fold, suggesting that an ovarian factor suppresses basal activity and enhances GnRH stimulation. Treatment of ovariectomized females with estrogen suppressed basal activity and restored GnRH stimulation of α-LUC, but the estrogen effects required long term treatment (10 days). Addition of progesterone to estrogen or treatment with the progesterone antagonist, RU486, had little effect on GnRH responsiveness. We conclude that estrogen exerts dual effects to suppress basal expression and to dramatically enhance GnRH responsiveness of the α-promoter. This model reveals potent actions of estrogen at the level of transcription and should provide new insights into the mechanisms that control estrogen priming of gonadotrope cells.
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