The polyphosphoinositide PtdIns(4,5)P # , best known as a substrate for phospholipase C isozymes, has recently been recognized to be involved in a variety of other cellular processes. The aim of this study was to examine whether the cellular levels of this versatile phospholipid are controlled by tyrosine phosphorylation. The studies were performed in human embryonic kidney (HEK)-293 cells stably expressing the M $ muscarinic acetylcholine receptor. Inhibition of tyrosine phosphatases by pervanadate induced an up-to-approx.-2.5-fold increase in the total cellular level of PtdIns(4,5)P # , which was both time-and concentration-dependent. In contrast, the tyrosine kinase inhibitors, genistein and tyrphostin 23, caused a rapid and specific fall in the cellular PtdIns(4,5)P # level and prevented the stimulatory effect of pervanadate on PtdIns(4,5)P # formation. Inactivation of Rho proteins by Clostridium difficile toxin B caused a similar fall in
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