Role of Syk in Fc gamma receptor-coupled tyrosine phosphorylation of Cbl in a manner susceptible to inhibition by protein kinase C

Fc gamma receptors (Fc gamma R) of guinea pig neutrophils were ligated and anti-Cbl immunoprecipitates prepared therefrom were assayed for the associa ted protein tyrosine kinase activity, which increased upon ligation of Fc g amma R. The increases were overcome upon activation of cellular protein kin ase C by simultaneous addition of phorbol 12-myristate 13-acetate (PMA) to the ligated cells. Syk proved to be the most important tyrosine kinase boun d to Cbl that served as the major substrate; essentially no tyrosine phosph orylation occurred in the anti-Cbl immunoprecipitates prepared from the cel l lysate that had been depleted of Syk by prior immunoprecipitation with an ti-Syk antibodies. Exposure of the P-32-labeled cells to PMA resulted in ph osphorylation of cellular Cbl on serine residues. Thus, protein kinase C-in duced serine phosphorylation of Cbl suppressed its tyrosine phosphorylation by Syk as a result of tyrosine kinase inhibition by unknown mechanisms, le ading to inhibition of Cbl-mediated signaling such as phosphatidylinositol 3-kinase activation.