Tyrosine Kinase Phosphorylation in Human Neutrophil Activation by PAF and other Agonists

Abstract

Protein kinases specific for tyrosine (PTK) were initially detected in oncogenic products of viral tumors (pp60^v-src) and in some growth factor receptors (1). Until recently, protein phosphorylation on tyrosine residues was only linked to cellular growth and transformation (1). Over the past few years, however, a number of laboratories have reported the presence of tyrosine kinase activities in non-proliferative tissues and cells (e.g. neurons, platelets and leukocytes). Tyrosine-specific kinases, such as the products of the proto-oncogenes c-src, chck, c fes/fps and c-fgr, are known to be present in white blood cells, including neutrophils (2–5), and some are expressed at a high level. Both particulate extracts and cytosolic fractions from neutrophils contain tyrosine kinase and phosphotyrosine phosphatase activities (6,7). Moreover, the CD45 antigen, a glycoprotein of the plasma membrane of all leukocytes, possesses tyrosine phosphatase activity that might regulate signal transduction in those cells (8). Using antibodies to phosphotyrosine (aPTyr) in Western blots, it has been demonstrated that tyrosine-specific protein phosphorylati on occurs when human or rabbit neutrophils are stimulated with a variety of agonists, including platelet-activating factor (PAF), human cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF) and tumor necrosis factor (TNF), the chemotactic peptide fMet-Leu-Phe, phorbol esters (PMA) and the calcium ionophore A23187 (9–17). A summary of the potency of those agents, as it relates to phosphorylation, is given in Table I. It has also been reported that the addition of GTPγS or vanadate to electropermeabilized neutrophils triggers rapid phosphorylation of tyrosine residues of several protein substrates (18,19).