Tyrosine Phosphorylation of Phospholipase C-γ2 Is Involved in the Activation of Phosphoinositide Hydrolysis by Fc Receptors in Human Neutrophils

Abstract The stimulation of phosphoinositide hydrolysis by a number of agonists (phosphoinositide response) is a ubiquitous transmembrane signalling process for the regulation of several cell functions. Two mechanisms of activation have been identified that involve different phospholipases C: one regulated by G-proteins and another regulated by receptors having an intrinsic tyrosine kinase domain or that stimulate intracellular tyrosine kinase activity. This last mechanism is activated in several immunological cells, including lymphocytes, mastocytes, NK cells and monocytes, in response to agonists that bind antigen receptors, and receptors for IgE and IgG. In the present study, we have investigated the role of tyrosine phosphorylation in the stimulation of phosphoinositide hydrolysis mediated by FcγRs in human neutrophils. The results demonstrated that: 1) the activation of FcγRs with insoluble immune complexes (IIC) induced a tyrosine phosphorylation of several proteins that was dose-dependently inhibited by the tyrosine kinase inhibitor, genistein; 2) the activation of FcγRs caused a stimulation of phosphoinositide hydrolysis measured as [ 3 H]inositol phosphates formation; 3) genistein depressed the activation of phosphoinositide hydrolysis; 4) among the several proteins that became tyrosine phosphorylated upon FcγRs activation by IIC, one 145 kDa protein was identified as PLC-γ2, using a specific antiserum. The phosphorylation of PLC-γ2 was completely inhibited by genistein. These results demonstrate that the phosphoinositide response to activation of FcγRs involves the tyrosine phosphorylation of PLC-γ2.