G(I) PROTEIN-ACTIVATION OF GONADOTROPIN-RELEASING HORMONE-MEDIATED PROTEIN DEPHOSPHORYLATION IN HUMAN ENDOMETRIAL CARCINOMA

OBJECTIVE: Gonadotropin-releasing hormone receptor is demonstrated in uterine endometrial carcinomas. This study was performed to determine gonadotropin-releasing hormone receptor-mediated membrane events and t o identify the guanosine triphosphate binding protein (G protein) subt ypes linked to gonadotropin-releasing hormone receptor in the tumors. STUDY DESIGN: Endometrial carcinomas surgically removed had been scree ned for gonadotropin-releasing hormone receptor expression before plas ma membrane isolation. The phosphoprotein level was observed in the ph osphorus 32-labeled incorporation from [gamma-P-32]adenosine triphosph ate into the isolated plasma membranes. The G(i) (alpha subunit) prote in was detected by immunoblotting and pertussis toxin-catalyzed adenos ine diphosphate ribosylation. RESULTS: Incubation of phosphorus 32-lab eled membranes with a gonadotropin-releasing hormone analog in the pre sence of guanosine thiotriphosphate caused a remarkable loss of phosph oprotein from 35 kd protein. This dephosphorylation action was dose de pendent of the gonadotropin-releasing hormone analog, and the maximal effect (90% loss) occurred at 100 nmol/L. Pertussis toxin brought abou t adenosine diphosphate ribosylation of an immunodetected G alpha(i). Gonadotropin-releasing hormone analog alone or guanosine thiotriphosph ate alone had no effect. Pretreatment of the membrane with the pertuss is toxin completely inhibited gonadotropin-releasing hormone-mediated dephosphorylation of the 35 kd protein. CONCLUSION: These data demonst rate the coupling of gonadotropin-releasing hormone receptor to protei n dephosphorylation through G(i), raising the possibility that the ant imitogenic action of gonadotropin-releasing hormone may occur by relea se of the action of protein phosphorylation to promote cell growth.