Maintenance of Hormone-sensitive Phosphoinositide Pools in the Plasma Membrane Requires Phosphatidylinositol 4-Kinase IIIα

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Abstract
Type III phosphatidylinositol (PtdIns) 4-kinases (PI4Ks) have been previously shown to support plasma membrane phosphoinositide synthesis during phospholipase C activation and Ca2+ signaling. Here, we use biochemical and imaging tools to monitor phosphoinositide changes in the plasma membrane in combination with pharmacological and genetic approaches to determine which of the type III PI4Ks (alpha or beta) is responsible for supplying phosphoinositides during agonist-induced Ca2+ signaling. Using inhibitors that discriminate between the alpha- and beta-isoforms of type III PI4Ks, PI4KIII alpha was found indispensable for the production of phosphatidylinositol 4-phosphate (PtdIns4P), phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P-2], and Ca2+ signaling in angiotensin II (AngII)-stimulated cells. Downregulation of either the type II or type III PI4K enzymes by small interfering RNA ( siRNA) had small but significant effects on basal PtdIns4P and PtdIns( 4,5) P2 levels in P-32-labeled cells, but only PI4KIII alpha down-regulation caused a slight impairment of PtdIns4P and PtdIns(4,5)P-2 resynthesis in AngII-stimulated cells. None of the PI4K siRNA treatments had a measurable effect on AngII-induced Ca2+ signaling. These results indicate that a small fraction of the cellular PI4K activity is sufficient to maintain plasma membrane phosphoinositide pools, and they demonstrate the value of the pharmacological approach in revealing the pivotal role of PI4KIII alpha enzyme in maintaining plasma membrane phosphoinositides.