Is gonadotrope expression of the gonadotropin releasing hormone receptor gene mediated by autocrine/paracrine stimulation of an activin response element?

Expression of the FSH beta subunit and GnRH receptor (GnRHR) genes in gonad otropes is stimulated by activin. We sought to identify the cis-acting elem ent(s) in the murine GnRHR gene promoter which confer activin responsivenes s. We established that 600 bp of 5' flanking sequence from the murine GnRHR gene were sufficient to confer activin responsiveness in the gonadotrope-d erived alpha T3-1 cell line. Since alpha T3-1 cells, like gonadotropes, sec rete activin, we examined the ability of follistatin, an activin binding pr otein, to block the activin response. Increasing concentrations of follista tin from 0 to 100 ng/ml resulted in a dose dependent decrease in activity o f the -600 promoter. Contained within this region are three elements import ant for expression in alpha T3-1 cells: a Steroidogenic Factor-1 binding si te (SF-1), an Activator Protein-1 (AP-1) element, and an element termed the GnRH receptor activating sequence or GRAS. A block mutation of GRAS inhibi ted the ability of the promoter to respond to follistatin. A more refined a nalysis using a series of two-bp mutations which scan GRAS and flanking seq uence revealed exact convergence of GRAS with activin/follistatin responsiv eness. Finally, a construct consisting of 3 copies of GRAS placed upstream of a heterologous minimal promoter (3xGRAS-PRL-LUC) was responsive to both activin stimulation and follistatin inhibition in alpha T3-1 cells. Thus, a utocrine/paracrine stimulation of gonadotropes by activin illustrates a uni que mechanism for cell-specific gene expression.