Regulation by phosphatidylinositol of rat pituitary plasma membrane and endoplasmic reticulum phosphatidylinositol synthase activities. A mechanism for activation of phosphoinositide resynthesis during cell stimulation.

The mechanism of signal transduction used by a large number of extracellular regulatory molecules involves hydrolysis and resynthesis of phosphoinositides. We recently demonstrated that during stimulation by thyrotropin-releasing hormone of rat pituitary (GH3) cells phosphatidylinositol (PtdIns) resynthesis occurs within the plasma membrane as well as the endoplasmic reticulum (Imai, A., and Gershengorn, M. C. (1987) Nature, 325, 726-728). In this report, we have studied regulation of PtdIns synthase (CDP-diglyceride-inositol phosphatidyltransferase, EC 2.7.8.11) activities associated with plasma membranes and endoplasmic reticulum isolated from GH3 cells. Exogenously added PtdIns noncompetitively inhibited membrane-associated and solubilized PtdIns synthase activities by up to 84 to 91%; half-maximal inhibition occurred between 0.03 and 0.1 mM PtdIns. Similar inhibition of PtdIns synthase activities were observed when PtdIns content of both membrane fractions was increased in vivo in intact GH3 cells prior to assay in vitro. These findings demonstrate that PtdIns synthase activities associated with plasma membrane and endoplasmic reticulum fractions isolated from GH3 cells are inhibited by the product, PtdIns. Because PtdIns levels decrease and PtdIns resynthesis is activated in both membrane fractions during stimulation of GH3 cells by thyrotropin-releasing hormone, it seems likely that activation of PtdIns synthase(s) during cell stimulation occurs by release of this enzyme(s) from inhibition by its product.