Glucose and insulin synergistically activate phosphatidylinositol 3-kinase to trigger oscillations of phosphatidylinositol 3,4,5-trisphosphate in β-cells

In insulin-secreting beta-cells, activation of phosphatidylinositol 3'-OH-kinase with resulting formation of phosphatidylinositol 3,4,5-trisphosphate (PIP3) has been implicated in the regulation of ion channels, insulin secretion, and gene transcription as well as in cell growth and survival, but the kinetics of PIP3 signals following physiological stimulation of insulin secretion is unknown. Using evanescent wave microscopy and a green fluorescent protein-tagged PIP3-binding protein domain for real-time monitoring of plasma membrane PIP3 concentration in single MIN6 beta-cells, we now demonstrate that glucose stimulation of insulin secretion results in pronounced PIP3 oscillations via autocrine stimulation of insulin receptors. Glucose lacked effect when insulin secretion was prevented with the hyperpolarizing agent diazoxide, but the sugar dose dependently enhanced the PIP3 response to maximal insulin stimulation without affecting the rate of PIP3 degradation. We conclude that glucose is an important co-activator of phosphatidylinositol-3'-OH-kinase and that the plasma membrane PIP3 concentration in beta-cells undergoes oscillations due to pulsatile release of insulin.