Gonadotropin-releasing hormone receptor couples to multiple G proteins in rat gonadotrophs and in GGH3 cells: evidence from palmitoylation and overexpression of G proteins

Abstract

There is evidence in several cell systems suggesting that the GnRH receptor couples to multiple G proteins. Presently there are no published studies showing GnRH receptor coupling to Gialpha, Gsalpha, and Gq/11alpha in a single cell type. To examine this possibility we measured palmitoylation of G proteins in response to GnRH receptor occupancy, since this event is a measure of G-protein activation by cognate receptors. GnRH stimulated time (0-120 min)- and dose (10(-12)-10(-6) g/ml)-dependent palmitoylation of both Gialpha and Gsalpha. Palmitoylation is G-protein activation dependent; accordingly, pertussis toxin (100 ng/ml; PTX), phorbol myristic acid (100 ng/ml), and Antide (50 nM; a GnRH antagonist) did not stimulate palmitoylation of Gialpha or Gsalpha above basal levels. However, cholera toxin (5 microgram/ml), an activator of Gsalpha, stimulated palmitoylation of Gsalpha but not Gialpha. We used a lactotrope-derived cell line expressing the GnRH receptor (GGH3) to examine whether the ability of the receptor to couple multiple G proteins is gonadotroph specific. GGH3 cells were transfected with specific cDNA coding for different G proteins, and agonist-stimulated second messenger production was assessed. Buserelin (a GnRH agonist) stimulated increased cAMP release in Gsalpha cDNA-transfected GGH3 cells, whereas in Gialpha cDNA-transfected cells, both inositol phosphate (IP) production and cAMP release were decreased in response to buserelin. Transfection of Gqalpha, G11alpha, G14alpha, and G15alpha cDNA into GGH3 cells resulted in an increased IP production in response to buserelin, indicating that GnRH receptor couples to this PTX-insensitive G-protein family. The observations presented in this study provide evidence for GnRH receptor coupling to multiple G proteins in a single cell type.