Differential activation of the tyrosine kinases ZAP-70 and Syk after FcγRI stimulation

Engagement of the high-affinity IgG Fc receptor (FcγRI) activates a signal transduction pathway involving tyrosine phosphorylation of associated kinases. We compared the activation of the related protein tyrosine kinases (PTKs), Syk and ZAP-70, in FcγRI-mediated signaling. Cross-linking of the FcγRI multimeric receptor in monocytic cells results in tyrosine phosphorylation of the FcεRIγ subunit and association of Syk with this complex. We stably introduced ZAP-70 via a retroviral vector into two monocytic cell lines, U937 and THP-1, which normally do not express ZAP-70. Neither Syk nor MAP kinase activation was affected by the presence of ZAP- 70. Although transduced ZAP-70 had in vitro kinase activity and associated with FcεRIγ after receptor aggregation, it was not tyrosine phosphorylated. In contrast, both ZAP-70 and Syk were phosphorylated in a T-cell line in which their respective levels of expression were similar to those detected in U937/ZAP-70 cells. Therefore, these results suggest that requirements for Syk and ZAP-70 phosphorylation are distinct in a monocytic cell context.