Regulation of phospholipase C isozymes: Activation of phospholipase C-γ in the absence of tyrosine-phosphorylation

Activation of PLC-γ isozymes in response to various agonists involves tyrosine phosphorylation of the effector enzymes. Recent evidence indicates that PLC-γ isozymes are additionally activated by phosphatidic acid, phosphatidylinositol 3,4,5-trisphosphate and arachidonic acid in the absence of PLC-γ tyrosine phosphorylation. These lipid-derived messengers are the immediate products of phospholipase D, phosphatidylinositol 3-kinase, and phospholipase A₂, enzymes which are often stimulated along with PLC-γ in response to an agonist. Furthermore, phosphatidylinositol 4,5-bisphosphate acts as a substrate for both PLC-γ and phosphatidylinositol 3-kinase and as an activator for phospholipase D and phospholipase A₂. These results reveal an elaborate mechanism of cross-talk and mutual regulation between four effector enzymes that participate in receptor signaling by acting on phospholipids. Copyright (C) 1999 Elsevier Science Ireland Ltd.