A phospholipase A2-activating protein (PLAP) stimulates human neutrophil aggregation and release of lysosomal enzymes, superoxide, and eicosanoids.

We have recently isolated a human phospholipase A2-activating protein (PLAP) that shares antigenic and biochemical similarities with melittin, a well characterized bee venom phospholipase-stimulatory peptide. To explore the potential mechanisms of action of PLAP that extend beyond its effects on eicosanoid synthesis, we examined its effects on the release of human neutrophil lysosomal enzymes and superoxide, and on RBC hemolysis. These results were compared to the effects of melittin, which has been reported to induce enzyme release and hemolysis. We also examined the effects of PLAP on neutrophil aggregation and chemotaxis. PLAP induced neutrophils to release beta-glucuronidase and metalloproteinase enzyme activities as well as produce superoxide ion in both a dose- and time-dependent manner. Eicosanoid synthesis inhibitors did not abrogate these responses. PLAP induced release of arachidonic acid metabolites, but this response could be abrogated by eicosanoid synthesis inhibitors. PLAP also induced neutrophil aggregation and chemokinesis, but not chemotaxis. Concentrations of PLAP that induced these responses did not induce cellular toxicity as determined by light and electron microscopy, lactic dehydrogenase release, trypan blue dye exclusion, and RBC hemolysis. In contrast, prolonged incubation with higher concentrations of PLAP induced cell death that was similar to that observed with melittin. These findings suggest that the mechanisms of action of PLAP extend beyond the eicosanoid synthetic pathway, and that disordered regulation of PLAP may be responsible, at least in part, for chronic immune and inflammatory states.

Rheumatoid arthritis synovial fluid phospholipase A2 activating protein (PLAP) stimulates human neutrophil degranulation and superoxide ion production.

Rheumatoid arthritis is characterized by excessive eicosanoid production, and phospholipase enzymes are the rate limiting step in eicosanoid synthesis. We have shown previously that cells from patients with rheumatoid arthritis express enhanced phospholipase A2 enzyme activities. Recently, we have isolated a phospholipase A2 activating protein termed PLAP from rheumatoid synovial fluid. This novel human protein shares biochemical and antigenic similarities with melittin, a bee venom phospholipase activating protein. Because melittin has been shown to induce neutrophil degranulation and superoxide formation, and because exuberent release of lysosomal enzymes and superoxide have been implicated in the pathogenesis of rheumatoid arthritis, we examined the role of PLAP on inducing these neutrophil functions.