Inhibition by prolactin of membrane-associated phosphatidylinositol kinase of human endometrial fibroblast.

Certain malignant tumors synthesize and secrete a putative peptide mitogen, which elicits a potent proliferative response in their supporting stromal cells. We recently demonstrated that prolactin (PRL) binds to human endometrial fibroblasts and inhibits mitogenicity of an endometrial carcinoma extract (Imai A, et al. Proc Soc Exp Biol Med 203:117-122, 1993). In this report, we have studied inhibitory regulation by PRL of phosphatidylinositol (PtdIns) kinase activity associated with plasma membranes isolated from human endometrial fibroblasts. Incubation of the isolated plasma membrane with [gamma-32P]ATP and exogenous PtdIns caused [32P]phosphate incorporation into PtdIns phosphate (PtdInsP); 95% of the 32P-labeled PtnInsP was accounted for by PtdIns 4-P. The PtdIns phosphorylation by membrane preparations was selectively stimulated in a dose-dependent manner by vanadate, in parallel with an elevated autophosphorylation of endogenous membrane proteins. Concomitant exposure of the membrane preparations to PRL led to a remarkable inhibition of the vanadate-responsive PtdIns phosphorylation and protein autophosphorylation. This inhibition was dependent on PRL dose, and half-maximal effect occurred at a concentration 1-10 nM of PRL. Degradation of the produced PtdInsP in the plasma membranes was not affected by PRL. Similar inhibition of PtdIns kinase activities were observed in membranes prepared from cells that had been pretreated in vivo with PRL prior to assay in vitro. These findings demonstrate that PtdIns kinase activity associated with protein autophosphorylation is suppressed by PRL in plasma membrane isolated from endometrial fibroblasts. The inhibition of vanadate-responsive PtdIns kinase by PRL suggests an involvement of this enzyme in the antimitogenic action of the hormone on human endometrial fibroblasts.