Phosphatidylinositol 4-kinases and the role of polyphosphoinositides in cellular regulation

Phosphoinositides play a central role in the transduction of signals for a variety of hormone and growth factor receptors. Multiple derivatives of phosphatidylinositol are present within the cell including phosphatidylinositol 4,5-bisphosphate, the phosphorylated derivative that is hydrolyzed by phospholipase C to produce the two intracellular second messengers, diacylglycerol and inositol 1,4,5-trisphosphate. The synthesis, degradation, and subsequent resynthesis of the phosphoinositides form a metabolic cycle known as the phosphoinositide cycle. The phosphoinositide cycle begins with the phosphorylation of phosphatidylinositol to form phosphatidylinositol 4-phosphate, a reaction catalyzed by phosphatidylinositol 4-kinase. Phosphatidylinositol kinase activity has been reported to be present in a variety of cellular membranes, and multiple isozymes of phosphatidylinositol 4-kinase are present within the cell, suggesting that the product of this reaction may have more than one biological function. The activity of phosphatidylinositol 4-kinase is regulated by growth factors, further underscoring the importance of this enzyme in cellular regulation. Recent data suggest that in addition to serving as substrates for phospholipase C, the polyphosphoinositides may themselves function as intracellular mediators of hormone action. For example, polyphosphoinositides have marked effects on the activity of certain actin binding proteins that may allow these lipids to participate in the regulation of actin polymerization. This review focuses on the properties of the phosphatidylinositol 4-kinases and the potential role of polyphosphoinositides in the regulation of cellular processes.