The minor inositol-containing membrane phospholipids, the phosphoinositides, play a central role in cell signal transduction. Activation of a hormone-sensitive phospholipase C (phosphoinositidase C) at the plasma membrane results in the rapid catabolism of the polyphosphoinositides to form the two second messengers inositol 1,4,5-trisphosphate (Ins(1,4,5)P 3 ), a water soluble phosphoinositol that promotes the release of Ca 2+ from intracellular stores, and diacylglycerol (DG), which remains in the plasma membrane and activates protein kinase C ( 1 – 3 ). The metabolic pathways involved in the synthesis of phosphatidylinositol 4,5-bisphosphate, and the metabolic fate of the DG and Ins(1,4,5)P 3 formed on activation of phosphoinositidase C, are summarized in Fig 1 . Fig. 1. Metabolic pathways activated as a consequence of phosphoinositidase C action. (A) Major metabolic pathways activated by phosphoinositidase C action on PtdIns(4,5)P 2 are shown with solid arrows. Some of the additional pathways that may be activated are shown by broken arrows. Abbreviations: PtdIns, phosphatidylinositol; PtdIns4P, phosphatidylinositol 4-phosphate; PtdIns(4,5)P 2 , phosphatidylinositol 4,5-bisphosphate; DG, diacylglycerol; PtdOH, phosphatidic acid; CDP-DG, CDP-diacylglycerol; Ins, Inositol. For phosphoinositols, abbreviations are in the form Ins(x,y,z)Pn, wherex, y, andzrefer to the positions of the phosphate groups on the myo-inositol ring andnrefers to the total number of phosphates. (B) A simplified outline of the metabolic pathways in (A) also showing alternative abbreviations. PI, phosphatidylinositol; PIP, phosphatidylinositol phosphate; PIP 2 , phosphatidylinositolbis-phosphate; DG, PtdOH, CDP-DG and Ins as above. Phosphoinositols are referred to as IPn, wherenrefers to the number of phosphates on the inositol ring. In both panels, sites of Li + inhibition are also shown.
