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 to identify the guanosine triphosphate binding protein (G protein) subtypes linked to gonadotropin-releasing hormone receptor in the tumors. STUDY DESIGN: Endometrial carcinomas surgically removed had been screened for gonadotropin-releasing hormone receptor expression before plasma membrane isolation. The phosphoprotein level was observed in the phosphorus 32-labeled incorporation from [gamma-P-32]adenosine triphosphate into the isolated plasma membranes. The G(i) (alpha subunit) protein was detected by immunoblotting and pertussis toxin-catalyzed adenosine diphosphate ribosylation. RESULTS: Incubation of phosphorus 32-labeled membranes with a gonadotropin-releasing hormone analog in the presence of guanosine thiotriphosphate caused a remarkable loss of phosphoprotein from 35 kd protein. This dephosphorylation action was dose dependent of the gonadotropin-releasing hormone analog, and the maximal effect (90% loss) occurred at 100 nmol/L. Pertussis toxin brought about adenosine diphosphate ribosylation of an immunodetected G alpha(i). Gonadotropin-releasing hormone analog alone or guanosine thiotriphosphate alone had no effect. Pretreatment of the membrane with the pertussis toxin completely inhibited gonadotropin-releasing hormone-mediated dephosphorylation of the 35 kd protein. CONCLUSION: These data demonstrate the coupling of gonadotropin-releasing hormone receptor to protein dephosphorylation through G(i), raising the possibility that the antimitogenic action of gonadotropin-releasing hormone may occur by release of the action of protein phosphorylation to promote cell growth.