Synergistic Effect of Type II Phospholipase A2 and Platelet-Activating Factor on Mac-1 Surface Expression and Exocytosis of Gelatinase Granules in Human Neutrophils: Evidence for the 5-Lipoxygenase-Dependent Mechanism

Stimulation of human neutrophils with inflammatory mediators such as TNF-α or platelet-activating factor (PAF) induces translocation of adhesion molecule Mac-1 (CD11b/CD18) from secretory vesicles to the plasma membrane. Type II phospholipase A 2 (PLA 2 -II) also induces translocation of Mac-1 from secretory vesicles. However, there are more Mac-1 molecules in gelatinase granules and specific granules than in secretory vesicles. Therefore, different combinations of PLA 2 -II and other mediators were examined for their ability to induce gelatinase granules and specific granules to induce Mac-1 surface expression. The combination of PLA 2 -II and PAF synergistically increased Mac-1 surface expression, and the effect was greater than the combinations of PLA 2 -II with TNF-α, IL-8, or FMLP. Additionally, the combination of PLA 2 -II and PAF induced exocytosis of both secretory vesicles and gelatinase granules, which did not occur with either PLA 2 -II alone or PAF alone. The induction was accompanied by marked production of leukotriene B 4 . AA861, an inhibitor of 5-lipoxygenase, did not inhibit exocytosis of secretory vesicles but did inhibit exocytosis of gelatinase granules and decrease Mac-1 surface expression. It was also found that Ca 2+ influx is essential for 5-lipoxygenase activation, because Ni 2+ , which blocks the influx of extracellular Ca 2+ , inhibited the production of leukotriene B 4 . These results suggest that stimulation by the combination of PLA 2 -II and PAF, unlike stimulation by each mediator alone, causes exocytosis of gelatinase granules via the 5-lipoxygenase pathway, resulting in a synergistic increase in neutrophil Mac-1 surface expression during inflammatory processes.