FC-GAMMA RECEPTOR ACTIVATION INDUCES THE TYROSINE PHOSPHORYLATION OF BOTH PHOSPHOLIPASE-C (PLC)-GAMMA-1 AND PLC-GAMMA-2 IN NATURAL-KILLER-CELLS

Crosslinking of the low affinity immunoglobulin G (IgG) Fc receptor (FcgammaR type III) on natural killer (NK) cells initiates antibody-dependent cellular cytotoxicity. During this process, FcgammaR stimulation results in the rapid activation of phospholipase C (PLC), which hydrolyzes membrane phosphoinositides, generating inositol-1,4,5-trisphosphate and sn-1,2-diacylglycerol as second messengers. We have recently reported that PLC activation after FcgammaR stimulation can be inhibited by a protein tyrosine kinase (PTK) inhibitor. Based on the paradigm provided by the receptor tyrosine kinases, we investigated whether PLC-gamma1 and/or PLC-gamma2 are expressed in NK cells, and whether the PLC-gamma isoforms are tyrosine phosphorylated in response to FcgammaR stimulation. Immunoblotting analyses with PLC-gamma1- and PLC-gamma2-specific antisera demonstrate that both isoforms are expressed in human NK cells. Furthermore, FcgammaR crosslinking triggers the tyrosine phosphorylation of both PLC-gamma1 and PLC-gamma2 in these cells. Phosphorylation of both isoforms is detectable within 1 min, and returns to basal level within 30 min. Pretreatment with herbimycin A, a PTK inhibitor, blocked the FcgammaR-induced tyrosine phosphorylation of PLC-gamma1 and PLC-gamma2, and the subsequent release of inositol phosphates. These results suggest that FcgammaR-initiated phosphoinositide turnover in human NK cells is regulated by the tyrosine phosphorylation of PLC-gamma. More broadly, these observations demonstrate that nonreceptor PTK(s) activated by crosslinkage of a multisubunit receptor can phosphorylate both PLC-gamma isoforms.