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Expression of the Mouse Gonadotropin-Releasing Hormone Receptor Gene in T3-1 Gonadotrope Cells Is Stimulated by Cyclic 3',5'-Adenosine Monophosphate and Protein Kinase A, and Is Modulated by Steroidogenic Factor-1 and Nur77

Department of Biochemistry, University of Stellenbosch, Stellenbosch 7600, South Africa

Address all correspondence and requests for reprints to: Dr. Janet Hapgood, Department of Biochemistry, University of Stellenbosch, Stellenbosch 7600, South Africa. E-mail: jhap{at}sun.ac.za.

Regulation of GnRH receptor (GnRHR) expression levels in the pituitary is a crucial control point in reproduction. The promoter of the mouse GnRHR gene contains nuclear receptor half-sites (NRS) at –244/-236 and -15/-7 relative to the translation start site. Although binding of steroidogenic factor-1 (SF-1) to the –244/-236NRS is implicated in mediating basal and gonadotrope-specific expression, no function or protein-DNA interactions have previously been described for the –15/-7NRS. We report that levels of the endogenous GnRHR mRNA in T3-1 cells are stimulated by forskolin and 8-bromo-cAMP. We also show that the orphan nuclear receptor Nur77 is expressed in T3-1 cells, and that both SF-1 and Nur77 bind to the –15/-7NRS and –244/-236NRS in vitro. We show that the activity of the proximal (-579/+1) mouse GnRHR promoter is up-regulated by protein kinase A, via a mechanism that is modulated by SF-1, both positively and negatively, through binding to the –244/-236NRS or the –15/-7NRS, respectively. Nur77 appears to be capable of acting as a negative regulator of this response, via the –15/-7NRS. Furthermore, we show that forskolin up-regulates SF-1 mRNA levels in T3-1 cells, indicating that the levels of SF-1 play a role in modulating the protein kinase A response.

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