Hyperoxia inhibits oxidant-induced apoptosis in lung epithelial cells

It has previously been shown that hyperoxia induces non-apoptotic cell death in cultured lung epithelial cells, whereas hydrogen peroxide (H2O2) and paraquat cause apoptosis, To test whether pathways leading to oxidative apoptosis in epithelial cells are sensitive to molecular O-2, A549 cells were exposed to 95% O-2, prior to exposure to lethal concentrations of H2O2. The extent of H2O2-induced apoptosis was significantly reduced in cells preexposed to hyperoxia compared with room-air controls. Preexposure of the hyperoxia-resistant HeLa-80 cell line to 80% O-2 also inhibited oxidant-induced apoptosis, suggesting that this inhibition is not due to O-2, toxicity. Because hyperoxia generates reactive oxygen species and activates the redox-sensitive transcription factor nuclear factor kappaB (NF-kappaB), the role of antioxidant enzymes and NF-kappaB were examined in this inhibitory process. The onset of inhibition appeared to be directly related to the degradation of I kappaB and subsequent activation of NF-kappaB (either by hyperoxia or TNF-alpha), whereas no significant up-regulation of endogenous antioxidant enzyme activities was found. In addition, suppression of NF-kappaB activities by transfecting A549 cells with a dominant-negative mutant construct of I kappaB significantly augmented the extent of H2O2-induced apoptosis. These data suggest that hyperoxia inhibits oxidant-induced apoptosis and that this inhibition is mediated by NF-kappaB.