Prevention of bacterial infection and removal of tissue debris from areas of traumatised tissue are dependent on the influx of neutrophils and their activation by chemotactic factors. Recently, platelet-derived growth factor (PDGF), which is released from platelets during blood coagulation, was found to be an extremely potent inducer of neutrophil complement receptor (CR) expression. Neutrophil activation by PDGF is unique, since in contrast to other chemoattractants, PDGF does not itself stimulate neutrophil superoxide production or aggregation. This unique effect of PDGF suggests that it activates neutrophils by a mechanism which is different from that of other chemoattractants. This proposal is designed to test the hypothesis that PDGF induces CR expression by binding to a specific membrane receptor which in turn activates intracellular messengers via a mechanism which, in contrast to other chemoattractants, does not involve G proteins. This project will address two specific aims: 1) Determine the mechanism by which PDGF induces neutrophil CR expression. The effect of PDGF on G protein activity, and the effect of G protein inhibition on PDGF CR-inducing activity will be determined. In addition, the involvement of phophoinositides, protein kinase C, intracellular Ca2+, and cAMP metabolism in mediating the CR-inducing effect of PDGF will be defined. 2) Identify the neutrophil receptor for PDGF. The effect of PDGF on neutrophils strongly indicates that PDGF binds to a specific receptor on the neutrophil membrane. However, this receptor has not been identified. We will first determine whether the neutrophil receptor is a tyrosine kinase similar to the 180kD fibroblast receptor by detecting tyrosine phosphorylation of membrane proteins following exposure to PDGF. If the neutrophil receptor is different from the mitogenic receptor, it will be purified by affinity chromatography. Completion of this project will significantly enhance our knowledge of the mechanisms of action of PDGF and the cell biology of neutrophil activation in injured tissue.
