Caspases Mediate Tumor Necrosis Factor-–Induced Neutrophil Apoptosis and Downregulation of Reactive Oxygen Production

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Abstract
Tumor necrosis factor- (TNF-) exerts two separate effects on neutrophils, stimulating effector functions while simultaneously inducing apoptosis. We examined here the involvement of caspases in neutrophil apoptosis and the effect of TNF-–induced apoptosis on reactive oxygen production. Immunoblotting and affinity labeling showed activation of caspase-8, caspase-3, and a caspase with a large subunit of 18 kD (T18) in TNF-–treated neutrophils. Active caspase-6 and -7 were not detectable in this cell type. Caspase-8 activated caspase-3 and T18 in neutrophil cytoplasmic extracts. zVAD-fmk blocked neutrophil apoptosis, in parallel with the inhibition of caspase activation. TNF-–induced caspase activation was accompanied by a decrease in the ability of neutrophils to release superoxide anion. Conversely, TNF- treatment in the presence of zVAD-fmk caused a prolonged augmentation of superoxide release. Granulocyte-macrophage colony-stimulating factor inhibited TNF-–induced caspase activation and apoptosis, while reversing the diminution in superoxide release. These observations not only suggest that a caspase cascade mediates apoptotic events and downregulates oxygen radical production in TNF-–treated neutrophils, but also raise the possibility that suppression of caspase activation with enhanced proinflammatory actions of TNF- may underlie the pathogenesis of inflammatory diseases.