Evidence for coupling of phosphotyrosine phosphatase to gonadotropin-releasing hormone receptor in ovarian carcinoma membrane.
BACKGROUND
Gonadotropin-releasing hormone (Gn-RH) receptor (Gn-RHR) has been demonstrated in epithelial ovarian carcinoma (Imai et al., Cancer 1994; 74:2555-61). To examine whether Gn-RHR mediates direct antiproliferative effects, we attempted to determine stimulatory regulation by Gn-RH of phosphotyrosine phosphatase (PTP) activity in plasma membranes isolated from ovarian carcinoma samples.


METHODS
Surgically removed ovarian carcinomas were screened for Gn-RHR expression prior to plasma membrane isolation. The phosphotyrosine level was observed by: (1) immunoblotting of membrane extracts with antiphosphotyrosine antibodies, and (2) dephosphorylation from 32P-labeled membrane protein. Membrane PTP activity was determined using the synthetic substrate p-nitrophenyl in a spectrophotometric assay.


RESULTS
A Gn-RH analog alone, or guanosine thiotriphosphate (GTP-gamma-S) alone, caused a remarkable loss of phosphotyrosine from a 35-kD protein of the membranes; incubation with a Gn-RH analog and GTP-gamma-S produced a further dephosphorylation of this endogenous protein. The Gn-RH analog buserelin stimulated the PTP activity of the membranes in a dose-dependent manner (P < 0.01). GTP-gamma-S enhanced the stimulatory action of Gn-RH on PTP; GDP-gamma-S reversed the Gn-RH action. A similar stimulation of PTP was observed (P < 0.01) when carcinoma tissue slices were exposed to Gn-RH analog in vivo prior to assay in vitro.


CONCLUSIONS
Activation of PTP by Gn-RH stimulated the loss of phosphotyrosine from endogenous proteins through GTP-binding protein within plasma membrane isolated from Gn-RHR-expressing ovarian carcinoma. The antimitogenic action of the hormone may occur by counteracting tyrosine phosphorylation to promote cell growth.
Imai
A
A
Department of Obstetrics and Gynecology, Gifu University School of Medicine, Japan.
Takagi
H
H
Furui
T
T
Horibe
S
S
Fuseya
T
T
Tamaya
T
T
D002462:Cell Membrane / Q000201:enzymology*; D005260:Female; D019204:GTP-Binding Proteins / Q000378:metabolism; D007987:Gonadotropin-Releasing Hormone / Q000378:metabolism* / Q000502:physiology*; D006801:Humans; D010051:Ovarian Neoplasms / Q000201:enzymology* / Q000648:ultrastructure; D017027:Protein Tyrosine Phosphatases / Q000378:metabolism*; D014407:Tumor Cells, Cultured
D007987:Gonadotropin-Releasing Hormone; D017027:Protein Tyrosine Phosphatases; D019204:GTP-Binding Proteins