Activation of Three Classes of Nonreceptor Tyrosine Kinases Following Fcγ Receptor Crosslinking in Human Monocytes

Fcγ receptors on monocytes/macrophages play an important role in both host defense and autoimmune disorders. Fcγ receptor signaling can lead to such downstream events as phagocytosis and the release of intracellular cytokines and reactive oxygen species. Freshly isolated human monocytes express two major classes of Fcγ receptor proteins, FcγRI (CD64) and FcγRII (CD32). Crosslinking of FcγRI and FcγRII gives rise to rapid and transient phosphorylation of multiple monocyte intracellular proteins including proteins of 40, 68–72, 75–85, 95, and 115–165 kDa. A 72-kDa protein was earlier identified as the tyrosine kinase Syk. Here we identify one of the proteins in the 115- to 165-kDa cluster as FAK, a protein tyrosine kinase localized to focal adhesions. A 68-kDa phosphoprotein was identified as paxillin, a cytoskeleton associated substrate for tyrosine kinases, and a 95-kDa protein was found to be the proto-oncogene product Vav. The Src family protein tyrosine kinase Fgr (p58) also displayed enhanced tyrosine phosphorylation after FcγRI and FcγRII crosslinking. Although FcγRIIA utilizes tyrosines within its own cytoplasmic domain for signaling while FcγRI utilizes the cytoplasmic tyrosines of its associated γ subunit, our results indicate sharing of several proteins for signaling in monocytes by these Fc receptors. These molecules include three distinct classes of tyrosine kinases, Syk, FAK, and Fgr, and the functionally diverse proteins Vav and paxillin.