Activin A augments GnRH-mediated transcriptional activation of the mouse GnRH receptor gene.

The response of pituitary gonadotropes to GnRH correlates directly with the concentration of GnRH receptors (GnRHRs) on the cell surface, which is mediated in part at the level of GnRHR gene expression. We have previously localized GnRH responsiveness in the mouse GnRHR (mGnRHR) gene promoter to two elements: activating protein-1 and sequence underlying responsiveness to GnRH-1. This study was designed to investigate potential synergy between GnRH and activin A in transcriptional activation of the mGnRHR gene. In functional transfection studies using alphaT3-1 cells, GnRH agonist stimulation of the mGnRHR gene promoter (-765/+62) resulted in a 10.9-fold increase in activity, which was further increased 2-fold (to 21.3-fold) following activin pretreatment. Activin pretreatment alone had no effect. Deletion of region -387/-308 or mutation of a putative SMAD-binding element at -331/-324 (5'-GTCTAG[T]C-3') abrogated the augmented response to GnRH in the presence of activin but not the response to GnRH alone. Overexpression of SMAD2 and SMAD3 along with SMAD4 increased transcriptional activity of the mGnRHR gene, which was further increased by GnRH agonist stimulation. These data demonstrate that activin augments GnRH-mediated transcriptional activation of the mGnRHR gene and suggest that this effect may be mediated through SMAD transcription factors.