Caspases mediate tumor necrosis factor-α-induced neutrophil apoptosis and downregulation of reactive oxygen production

Tumor necrosis factor-α (TNF-α) 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-α-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-α-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-α-induced caspase activation was accompanied by a decrease in the ability of neutrophils to release superoxide anion. Conversely, TNF-α treatment in the presence of zVAD-fmk caused a prolonged augmentation of superoxide release. Granulocyte-macrophage colony- stimulating factor inhibited TNF-α-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-α-treated neutrophils, but also raise the possibility that suppression of caspase activation with enhanced proinflammatory actions of TNF-α may underlie the pathogenesis of inflammatory diseases.