STIMULATION OF FC-GAMMA-RIIIA RESULTS IN PHOSPHOLIPASE C-GAMMA-1 TYROSINE PHOSPHORYLATION AND P56(LCK) ACTIVATION

Binding of ligand to the alpha subunit of FcgammaRIIIA(CD16), expressed at the natural killer (NK) cell membrane in association with homo or heterodimers of proteins of the zeta family, results in phosphorylation of several proteins on tyrosine residues. We have analyzed the role of protein tyrosine phosphorylation in the regulation of molecular events induced upon stimulation of FcgammaRIIIA in NK cells and in T cells expressing the FcgammaRIIIalpha chain in association with endogenous zeta2 homodimers and devoid of other (CD3, CD2) transducing molecules. Our data indicate that treatment of these cells with protein tyrosine kinase inhibitors prevents not only FcgammaRIIIA-induced protein tyrosine phosphorylation but also pbosphatidylinositol 4,5 diphosphate hydrolysis and increased intracellular Ca2+ concentration, indicating a primary role of tyrosine kinase(s) in the induction of these early activation events. Occupancy of FcgammaRIIIA by ligand results in phospholipase C (PLC)-gamma1 tyrosine phosphorylation in NK cells and in FcgammaRIIIA-transfected CD3-/CD2- T cells, and induces functional activation of p56lck in FcgammaRIIIAalpha/zeta2-transfected T cells, suggesting the possibility that the receptor-induced PLC-gamma1 activation occurs upon phosphorylation of its tyrosine residues mediated by this kinase and is, at least in part, responsible for the signal transduction mediated via CD16 upon ligand binding.