COMPLEMENT FRAGMENT C3A STIMULATES CA2+ INFLUX IN NEUTROPHILS VIA A PERTUSSIS-TOXIN-SENSITIVE G-PROTEIN

The signal pathways of neutrophils following stimulation with the complement fragment C3a (C3a) were studied in neutrophils and compared to the pathways activated by complement fragment C5a (C5a) . Analysis of polyphosphoinositol lipid turnover showed that C5a, but not C3a, activated phosphatidylinositol-bisphosphate-3-kinase (PtdInsP2 3-kinase) indicating that different signal pathways are activated by the two anaphylatoxins. To examine whether C3a stimulated Ca2+ transients, cytosolic free Ca2+ levels were analyzed in Fluo-3-labelled neutrophils by flow cytometry. C3a stimulated a fast and concentration-dependent increase of cytosolic free Ca2+. Comparison of the C3a response with that of C5a revealed a more pronounced C5a-triggered Ca2+ rise. Addition of EGTA to the extracellular buffer prior to stimulation did not significantly alter the initial Ca2+ rise at low C5a concentrations, but reduced the time course of the Ca2+ transients at high concentrations. In marked contrast, EGTA completely blocked the Ca2+ response stimulated by C3a in neutrophils labeled with either Indo-1/AM or Fluo-3. Preincubation of neutrophils with pertussis toxin inhibited both C3a- and C5a-stimulated Ca2+ transients, indicating the involvement of guanine-nucleotide-binding proteins (G proteins) in these processes. In order to examine whether the C3a receptor is coupled to G proteins, binding of guanosine 5'-O-(3-[S-35]thiotriphosphate) ([S-35]GTP[S]) to purified neutrophil plasma membranes was studied. Both C3a and C5a stimulated high-affinity binding of [S-35]GTP[S] up to 1.5-fold and 3-fold, respectively. These data suggest that the two anaphylatoxins activate pertussis-toxin-sensitive G proteins, which then trigger different signal transduction pathways. C3a specifically stimulated Ca2+ influx from the extracellular medium, whereas C5a additionally activated the PtdInsP2 3-kinase and stimulated Ca2+ mobilization from intracellular stores.