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: 117, 1993, ibid 205: 140, 1994). To elucidate the molecular mechanism of the antimitogenic action of PRL, we examined the PRL receptor-linked tyrosine kinase and phosphotyrosine phosphatase (PTP), known to constitute the signaling of cell growth, within isolated plasma membranes from endometrium and endometrial carcinoma. Surgically removed human endometrial carcinomas and normal endometrium were examined. PRL receptor mRNA was determined by reverse transcription-polymerase chain reaction using oligonucleotide primers synthesized according to the published human PRL receptor sequence. Phosphotyrosine-containing membrane proteins were analyzed using antiphosphotyrosine antibody in Western blots. Plasma membrane-associated PTP activity was examined using the synthetic substrate para-nitrophenyl phosphates (p Npp) in a spectrophotometric 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 receptor mRNA-positive specimens, PRL induced dose-dependent (i) decrease in the level of tyrosine-phosphorylated protein and (ii) inhibition of PTP activity in the presence of nonhydrolyzable GTP. PRL alone showed 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 protein. The inhibition of the tyrosine kinase suggests an involvement of this enzyme in the growth-inhibiting action of PRL.