Activation of phospholipase D involved in both injury and survival in A549 alveolar epithelial cells exposed to H2O2.

To determine the role of the phospholipase D (PLD) pathway in injury and survival of alveolar epithelial cells, A549 cells were exposed to H2O2 (500μM) which resulted in time-dependent injury and bi-phasic increase of PLD activity at 5min and at 3h, respectively. n-Butanol (0.5%) inhibited PLD activation, attenuated cell injury at 5min of H2O2 exposure, but enhanced injury at 3h of exposure. This activation was inhibited by treatment with catalase (500units/ml). Exogenous phosphatidic acid mimicked the effects of PLD activation, and diphenyliodonium (NADPH oxidase inhibitor) reversed the decline in cell viability induced by H2O2 exposure. Propranolol (phosphatidic acid phospholydrolase inhibitor) and quinacrine (phospholipase A2 inhibitor) had weak effects on H2O2-induced PLD activation but reversed H2O2-induced injury. We speculate that PLD activation at the initiation of H2O2 exposure predominantly results in NAPDH oxidase activation, which mediates A549 cell injury, but turns to mediating cell survival as the H2O2 attack continues, which might be mainly due to the accumulation of intracellular phosphatidic acid.