Role of Type I  Phosphatidylinositol-4-Phosphate 5-Kinase in Insulin Secretion, Glucose Metabolism, and Membrane Potential in INS-1  -Cells

Insulin secretion from beta-cells is regulated by a complex signaling network. Our earlier study has reported that Rac1 participates in glucose- and cAMP-induced insulin secretion probably via maintaining a functional actin structure for recruitment of insulin granules. Type I alpha phosphatidylinositol-4-phosphate 5-kinase (PIP5K-I alpha) is a downstream effector of Rac1 and a critical enzyme for synthesis of phosphatidylinositol-4,5-bisphosphate (PIP2). By using an RNA interference technique, PIP5K-I alpha in INS-1 beta-cells could be specifically knocked down by 70-75%. PIP5K-I alpha knockdown disrupted filamentous actin structure and caused changes in cell morphology. In addition, PIP2 content in the plasma membrane was reduced and the glucose effect on PIP2 was abolished but without affecting glucose- induced formation of inositol 1,4,5-trisphosphate. At basal conditions (2.8 mM glucose), PIP5K-I alpha knockdown doubled insulin secretion, elevated glucose metabolic rate, depolarized resting membrane potential, and raised cytoplasmic free Ca2+ levels ([Ca2+](i)). The total insulin release at high glucose was increased upon PIP5K-I alpha knockdown. However, the percent increment of insulin secretion by high glucose and forskolin over the basal release was significantly reduced, an effect more apparent on the late phase of insulin secretion. Metabolism and [Ca2+](i) rises at high glucose were also attenuated in cells after PIP5K-I alpha knockdown. In contrast, PIP5K-I alpha knockdown had no effect on cell growth and viability. Taken together, our data suggest that PIP5K-I alpha may play an important role in both the proximal and distal steps of signaling cascade for insulin secretion in beta-cells. (Endocrinology 150: 2127-2135, 2009)