PROLACTIN RECEPTOR-LINKED TYROSINE-PHOSPHORYLATION OF MEMBRANE-PROTEINS IS MEDIATED BY GTP-BINDING PROTEIN IN ENDOMETRIAL CARCINOMA AND ENDOMETRIUM

Prolactin (PRL) blocks the mitogenic activity of the endometrium via a PRL receptor (PRL-R)-mediated mechanism (Proc Soc Exp Biol Med 203: 1 17, 1993, ibid 205: 140, 1994). To elucidate the molecular mechanism o f the antimitogenic action of PRL, we examined the PRL receptor-linked tyrosine kinase and phosphotyrosine phosphatase (PTP), known to const itute the signaling of cell growth, within isolated plasma membranes f rom endometrium and endometrial carcinoma. Surgically removed human en dometrial carcinomas and normal endometrium were examined. PRL recepto r mRNA was determined by reverse transcription-polymerase chain reacti on using oligonucleotide primers synthesized according to the publishe d human PRL receptor sequence. Phosphotyrosine-containing membrane pro teins were analyzed using antiphosphotyrosine antibody in Western blot s. Plasma membrane-associated PTP activity was examined using the synt hetic substrate para-nitrophenyl phosphates (p Npp) in a spectrophotom etric assay, PRL receptor mRNA was detected in all endometria tested, 7 of 9 samples of well-differentiated adenocarcinoma, 7 of 8 of poorly -differentiated adenocarcinoma. In plasma membrane isolated from PRL r eceptor mRNA-positive specimens, PRL induced dose-dependent (i) decrea se in the level of tyrosine-phosphorylated protein and (ii) inhibition of PTP activity in the presence of nonhydrolyzable GTP. PRL alone sho wed no hormonal action. PRL may decrease the phosphotyrosine level in protein substrates through block of tyrosine kinase. The PRL receptor- linked tyrosine phosphorylation may be mediated by a GTP-binding prote in. The inhibition of the tyrosine kinase suggests an involvement of t his enzyme in the growth-inhibiting action of PRL.