A. Imai et al., PROLACTIN RECEPTOR-LINKED TYROSINE-PHOSPHORYLATION OF MEMBRANE-PROTEINS IS MEDIATED BY GTP-BINDING PROTEIN IN ENDOMETRIAL CARCINOMA AND ENDOMETRIUM, International journal of oncology, 5(6), 1994, pp. 1379-1383
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.