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

Fcγ receptors (FcγR) of guinea pig neutrophils were ligated and anti-Cbl immunoprecipitates prepared therefrom were assayed for the associated protein tyrosine kinase activity, which increased upon ligation of FcγR. The increases were overcome upon activation of cellular protein kinase C by simultaneous addition of phorbol 12-myristate 13-acetate (PMA) to the ligated cells. Syk proved to be the most important tyrosine kinase bound to Cbl that served as the major substrate; essentially no tyrosine phosphorylation occurred in the anti-Cbl immunoprecipitates prepared from the cell lysate that had been depleted of Syk by prior immunoprecipitation with anti-Syk antibodies. Exposure of the ³²P-labeled cells to PMA resulted in phosphorylation of cellular Cbl on serine residues. Thus, protein kinase C-induced serine phosphorylation of Cbl suppressed its tyrosine phosphorylation by Syk as a result of tyrosine kinase inhibition by unknown mechanisms, leading to inhibition of Cbl-mediated signaling such as phosphatidylinositol 3-kinase activation.