ESTROGEN RESPONSE ELEMENT DEPENDENT AND INDEPENDENT ESTROGEN RECEPTOR ALPHA SIGNALING PATHWAYS DIFFERENTIALLY REGULATE PITUITARY GONADOTROPIN TRANSCRIPTION AND SECRETION IN THE FEMALE MOUSE

Abstract

Pituitary gonadotropins luteinizing hormone (LH) and follicle stimulating hormone (FSH) are both secreted by pituitary gonadotropes in response to gonadotropin-releasing hormone (GnRH). However, these gonadotropins are independently regulated throughout the estrous cycle to coordinate the timing of follicle development and ovulation. GnRH release is suppressed by estrogen negative feedback until, on the afternoon of proestrous, when estrogen positive feedback actions result in a GnRH and LH surge and the induction of ovulation. Subsequently, a FSH surge occurs to stimulate the development of a new cohort of follicles until when inhibin, produced by follicles, feeds back to the pituitary to suppress further FSH production and release. In ovariectomized females, estrogen treatment is capable of suppressing LH and FSH. While known to require the estrogen receptor alpha (ERalpha), the molecular mechanisms of this suppression remain unclear. Classical ERalpha signaling occurs through binding to an estrogen response element (ERE) in DNA, whereas in nonclassical signaling, ERalpha participates in protein-protein interactions to alter the activity of transcription factors at non-ERE sites (i.e. NF-kB, AP-1, or Sp1). Previously, our lab generated a Nonclassical Estrogen Receptor Knock In (NERKI) mouse model using a mutant ERalpha that is unable to bind to ERE but retains EREindependent activity. Breeding the NERKI mouse with the ERalpha knock out model (ERKO), we are able to selectively restore only EREindependent signaling to the ERalpha null background (NERKI/ERKO) and genetically dissociate in vivo roles for the ERE-dependent and independent ERalpha signaling pathways. Using this model, we have recently shown the ERE-independent ERalpha signaling pathway as capable of mediating partial estrogen negative feedback of serum LH. In this study, the ERE-dependent and independent estrogen effects on serum FSH and pituitary gonadotropin LHbeta and FSHbeta subunit transcription are examined following ovariectomy. We find that estrogen mediated suppression of pituitary LHbeta mRNA and serum FSH require the EREdependent ERalpha pathway. In contrast, the ERE-independent ERalpha signaling pathway is capable of mediating partial estrogen suppression of FSHbeta mRNA transcription and serum LH. These results support a model where, in the absence of other ovarian factors, estrogen acting through the ERE-dependent ERalpha pathway suppresses both LH and FSH transcription and secretion, whereas estrogen acting through the ERE-independent pathway partially suppresses LH secretion and FSHbeta transcription but is permissive to LHbeta transcription and FSH secretion. These data suggest that the balance of ERE-dependent and independent ERalpha signaling pathways may enable estrogen to dynamically regulate the transcription and secretion of both pituitary gonadotropins LH and FSH across the estrous cycle. Supported by NIH/NICHD grant PO1 HD21921, CMBD Training Grant T32 GM008061, UVA Ligand Core- NICHD (SCCPRR) Grant U54-HD28934 (platform)