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

Cardiovascular Research Institute (J.Z., R.L., H.W., G.L.), National University Medical Institutes, Yong Loo Lin School of Medicine, National University of Singapore, Singapore 117597; and Singapore Bioimaging Consortium (S.W., W.H.), Agency for Science, Technology, and Research (A*STAR), Singapore 138667

Address all correspondence and requests for reprints to: GuoDong Li, National University, Medical Institutes, Blk MD11, 02-01, 10 Medical Drive, Singapore 117597. E-mail: nmiligd{at}nus.edu.sg.

Insulin secretion from β-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 phosphatidylinositol-4-phosphate 5-kinase (PIP5K-I) 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 in INS-1 β-cells could be specifically knocked down by 70–75%. PIP5K-I 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 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 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 knockdown. In contrast, PIP5K-I knockdown had no effect on cell growth and viability. Taken together, our data suggest that PIP5K-I may play an important role in both the proximal and distal steps of signaling cascade for insulin secretion in β-cells.