Platelet-activating factor modulates endotoxin-induced macrophage procoagulant activity by a protein kinase C-dependent mechanism.

Macrophage procoagulant activity is an important mediator of extravascular fibrin deposition at sites of infection and appears to contribute to the pathogenesis of several infectious disease processes. Previous studies have shown that the inflammatory mediator platelet-activating factor was able to prime macrophages for induction of procoagulant activity by bacterial lipopolysaccharide. The present studies were designed to examine the mechanism of this priming effect. Platelet-activating factor (100 nM) primed macrophages for procoagulant activity generation in response to endotoxin at concentrations as low as 100 ng/ml and also following exposure to Escherichia coli, Bacteroides fragilis, and Staphylococcus aureus. The priming effect occurred following a pretreatment with platelet-activating factor for as short as 1 min, suggesting a rapid activation event. Two different doses of the calcium ionophore ionomycin were used to mimic the peak and sustained effects of platelet-activating factor on cytoplasmic calcium levels (1 microM and 100 nM, respectively). Neither dose was able to mimic the priming effect. However, extracellular calcium was necessary for induction of procoagulant activity and the priming effect. By contrast, the protein kinase C agonist phorbol myristate acetate reproduced the priming phenomenon observed for platelet-activating factor. In further support of the concept that protein kinase C activation mediated the effect of platelet-activating factor, the specific protein kinase C inhibitor staurosporine reversed the ability of platelet-activating factor to augment induction of macrophage procoagulant activity by endotoxin. These data suggest mechanisms by which inflammatory mediators within the microenvironment of infection might modulate the host response to bacterial pathogens.