POTENTIAL ROLE FOR A REGULATOR OF G-PROTEIN SIGNALING (RGS3) IN GONADOTROPIN-RELEASING-HORMONE (GNRH) STIMULATED DESENSITIZATION

The cellular and molecular mechanisms of gonadotrope desensitization a re unknown but transduction of the GnRH signal is known to involve seq uentially the GnRH receptor, Gq alpha protein, phospllolipase C beta-1 , inositol-1,4,5-trisphosphate (IP3), and intracellular Ca+2 release. Here, we report the results of studies of a new family of proteins kno wn as regulators of G protein signaling (RGS) that recently have been implicated in desensitization of several ligand induced processes. Usi ng DNA-mediated transfection, we coexpressed the GnRH receptor and RGS 1, 2, 3, or 4 in COS-1 cells. Control cells and those expressing RGS1, 2; and 4 produced five fold increases in IP3 levels during the 30 sec after treatment with GnRH. In contrast, RGS3 expression suppressed by 75% the GnRH-induced IP3 responses. RGS3 was shown to bind Gq alpha p rotein in a model in vitro system: recombinant RGS3-glutathione-S-tran sferase (GST) fusion protein bound five-fold more S-35-met labeled Gq alpha protein than did with GST alone, suggesting that the mechanism o f RGS3 action is attenuation of Gq alpha protein activation of phospho lipase C. RGS3 mRNA and protein were observed to be expressed endogeno usly in the gonadotropic alpha T3-1 cell line. These results suggest a potential role for RGS3 in modulating the LH secretory responsiveness of the pituitary gonadotrope to GnRH.