Potential Role for a Regulator of G Protein Signaling (RGS3) in Gonadotropin-Releasing Hormone (GnRH) Stimulated Desensitization

Abstract

The cellular and molecular mechanisms of gonadotrope desensitization are unknown but transduction of the GnRH signal is known to involve sequentially the GnRH receptor, Gq alpha protein, phospholipase 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 known as regulators of G protein signaling (RGS) that recently have been implicated in desensitization of several ligand induced processes. Using DNA-mediated transfection, we co-expressed the GnRH receptor and RGS1,2,3, 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 protein in a model in vitro system: recombinant RGS3-glutathione-S-transferase (GST) fusion protein bound five-fold more 35S-met labeled Gq alpha protein than did with GST alone, suggesting that the mechanism of RGS3 action is attenuation of Gq alpha protein activation of phospholipase C. RGS3 mRNA and protein were observed to be expressed endogenously 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.

Attenuation of G i - and G q -mediated signaling by expression of RGS4 or GAIP in mammalian cells

“…One explanation for the lack of effect for RGS4 in the GnRH study may be an inadequate level of expression of RGS4. Western and Northern blot data were shown to verify the expression of RGS3 but not the other three RGS proteins (18). Based on the results reported by Neill et al (18) and our own, it appears likely that both RGS3 and RGS4 have the capacity to regulate negatively G q -mediated pathways in vivo.…”

“…Western and Northern blot data were shown to verify the expression of RGS3 but not the other three RGS proteins (18). Based on the results reported by Neill et al (18) and our own, it appears likely that both RGS3 and RGS4 have the capacity to regulate negatively G q -mediated pathways in vivo. The broad selectivity of RGS proteins for G proteins that has been reported thus far will be further refined as more is learned about the endogenous expression of these proteins within cells.…”

“…In another study, Neill et al (18) cotransfected COS cells with cDNAs for the gonadotropin-releasing hormone (GnRH) receptor and RGS1, RGS2, RGS3, or RGS4 (18). In contrast to the capacity of RGS4 to inhibit ␣ q -mediated signaling demonstrated herein, Neill et al (18) found that GnRH-induced increases in inositol trisphosphate were suppressed only in cells transfected with the receptor and RGS3. One explanation for the lack of effect for RGS4 in the GnRH study may be an inadequate level of expression of RGS4.…”

“…The activity of extracellular signal-regulated kinase, which can be stimulated by platelet activating factor by a G protein, was attenuated by RGS1 (introduced by cotransfection of cells with cDNAs for the RGS protein and the kinase) (17). In another study, Neill et al (18) cotransfected COS cells with cDNAs for the gonadotropin-releasing hormone (GnRH) receptor and RGS1, RGS2, RGS3, or RGS4 (18). In contrast to the capacity of RGS4 to inhibit ␣ q -mediated signaling demonstrated herein, Neill et al (18) found that GnRH-induced increases in inositol trisphosphate were suppressed only in cells transfected with the receptor and RGS3.…”

Interaction between RGS7 and polycystin

“…Biological effects of RGS proteins on cellular signaling arise from their capacity to inactivate G␣ i or G ␣ q subunits. Consequently, RGS proteins interfere with multiple G proteincoupled programs including calcium mobilization (14,35), activation of phospholipase C (10, 36), synthesis of inositol triphosphate (36)(37)(38)(39), mGluR1a-and mGluR5a-mediated activation of chloride and potassium currents (40), and G protein-coupled inwardly rectifying K ϩ channels (41) , and EGL-10. Depicted in detail is the region unique to RGS7, containing two PEST sequences and a potential coiled-coil structure.…”