Phosphatidylinositol 4-kinase serves as a metabolic sensor and regulates priming of secretory granules in pancreatic β cells.

Insulin secretion is controlled by the β cell's metabolic state, and the ability of the secretory granules to undergo exocytosis increases during glucose stimulation in a membrane potential-independent fashion. Here, we demonstrate that exocytosis of insulincontaining secretory granules depends on phosphatidylinositol 4-kinase (PI 4-kinase) activity and that inhibition of this enzyme suppresses glucose-stimulated insulin secretion. Intracellular application of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate [PI(4,5)P[sub 2]] stimulated exocytosis by promoting the priming of secretory granules for release and increasing the number of granules residing in a readily releasable pool. Reducing the cytoplasmic ADP concentration in a way mimicking the effects of glucose stimulation activated PI 4-kinase and increased exocytosis whereas changes of the ATP concentration in the physiological range had little effect. The PI(4,5)P[sub 2]-binding protein Ca[sup 2+]-dependent activator protein for secretion (CAPS) is present in β cells, and neutralization of the protein abolished both Ca[sup 2+]- and PI(4,5)P[sub 2]-induced exocytosis. We conclude that ADPinduced changes in PI 4-kinase activity, via generation of PI(4,5)P[sub 2], represents a metabolic sensor in the β cell by virtue of its capacity to regulate the release competence of the secretory granules. [ABSTRACT FROM AUTHOR]