Molecular mechanisms underlying hormonal regulation of the gonadotropin releasing hormone receptor gene

Abstract

Interaction of gonadotropin-releasing hormone (GnRH) with its receptor on the surface of gonadotropes represents a central point for regulation of reproductive function. Consequently, considerable effort has been devoted toward understanding the regulation of this hormone and its receptor. Toward this end, I have found that transcriptional activity of the murine GnRH receptor (GnRHR) gene is mediated by three elements: a binding site for steroidogenic factor-1 (SF-1), an AP-1 element and the GnRH receptor activating sequence (GRAS). Each of these elements contributes approximately equally to basal activity of the promoter (1). Activin, a member of the TGF-β family of growth and differentiation factors, stimulates expression of the murine GnRHR gene. I have established that 600 bp of 5' flanking sequence from this gene are sufficient to confer activin responsiveness in the gonadotrope-derived αT3-1 cell line. GRAS was both necessary and sufficient to confer activin responsiveness. At issue, then, was the identity of the DNA binding proteins necessary to mediate functional activity at GRAS. I have found that Smad4 interacts at the 5' end of GRAS. Smad3 appears to interact at GRAS based on overexpression and yeast one-hybrid assays. Interestingly, GRAS also mediates a synergistic response to activin and GnRH. Consistent with GnRH regulation at GRAS, I have shown that Jun and Fos bind to the center of GRAS. Furthermore, a recently identified member of the forkhead family of transcription factors, FoxL2, can interact at the 3' end of GRAS. Thus, GRAS represents a composite regulatory element that is bound by a multi-factoral complex. The AP-1 element has been shown to mediate responsiveness to GnRH (2;3). I have found that GnRH responsiveness of the GnRHR gene is greatly attenuated by estradiol. Replacement of the AP-1 element with a cAMP response element (CRE) does not affect GnRH-responsiveness of the promoter, but does eliminate the effect of estradiol.