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Gi protein activation of gonadotropin-releasing hormone-mediated protein dephosphorylation in human endometrial
1Department of Obstetrics and Gynecology, Gifu University School of Medicine, Japan.
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-32P]adenosine triphosphate into the isolated plasma membranes. The Gi (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 Gi, 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.
Insights
This study shows gonadotropin-releasing hormone receptor (GnRHR) in endometrial cancer cells couples to Gi protein, causing dephosphorylation and potentially inhibiting cell growth. This reveals a new mechanism for GnRHR action in tumors.
Area of Science:
- Endocrinology
- Molecular Biology
- Gynecologic Oncology
Background:
- Gonadotropin-releasing hormone receptor (GnRHR) is present in uterine endometrial carcinomas.
- Understanding GnRHR-mediated signaling pathways in these tumors is crucial.
Purpose of the Study:
- To investigate GnRHR-mediated membrane events in endometrial cancer.
- To identify the specific guanosine triphosphate (GTP) binding protein (G protein) subtypes linked to GnRHR in these tumors.
Main Methods:
- Endometrial carcinomas were screened for GnRHR expression.
- Plasma membranes were isolated for analysis of phosphoprotein levels using [gamma-32P]adenosine triphosphate.
- Gi (alpha subunit) protein was detected via immunoblotting and pertussis toxin-catalyzed adenosine diphosphate ribosylation.
Main Results:
- GnRH analog induced dose-dependent dephosphorylation of a 35 kd protein in isolated membranes.
- Pertussis toxin treatment ribosylated and inhibited GnRH-mediated dephosphorylation, indicating Gi protein involvement.
- GnRH analog and GTP alone did not affect phosphoprotein levels.
Conclusions:
- GnRHR is coupled to Gi protein, mediating protein dephosphorylation in endometrial carcinomas.
- This signaling pathway suggests a potential mechanism for the antimitogenic effects of GnRH in cancer.
- GnRH may inhibit tumor cell growth by reducing protein phosphorylation essential for proliferation.