Inhibition of apoptosis in pulmonary endothelial cells by altered pH, mitochondrial function, and ATP supply.

We investigated the effect of altered extracellular pH, mitochondrial function, and ATP content on development of apoptosis in human pulmonary artery endothelial cells after treatment with staurosporine (STS). STS produced a concentration- and time-dependent increase in caspase-3 activity in pH 7.4 medium that reached a peak at 6 h. The increase in caspase activity was associated with significant DNA fragmentation. Fluorescent imaging of treated monolayers in pH 7.4 medium with Hoechst-33342-propidium iodide demonstrated a large percentage of apoptotic cells ( approximately 40%) with no evidence of necrosis. Caspase activity, DNA fragmentation, and percentage of apoptotic cells were reduced after STS treatment in acidic media (pH 7.0 and 6.6). The Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM inhibited STS-induced apoptosis, whereas the rise in intracellular Ca2+concentration in STS-treated cells in pH 7.4 medium was reduced in pH 7.0 medium. These results suggest that one mechanism for inhibitory effects of acidosis may be a pH-induced alteration in Ca2+ signaling. Treatment with STS in the presence of oligomycin (10 microM), an inhibitor of the mitochondrial F(0)F(1)-ATPase, in glucose-free media abolished caspase activation and DNA fragmentation in association with severe ATP depletion ( approximately 2% of control cells). Imaging demonstrated a change in the mode of cell death from apoptosis to necrosis under these conditions. This change was linked to the level of ATP depletion, because STS treatment in the absence of glucose or the presence of oligomycin in media with glucose still leads to apoptosis in the presence of only moderate ATP depletion. These results demonstrate that pH, mitochondrial function, and ATP supply are important variables that regulate STS-induced apoptosis in human pulmonary artery endothelial cells.

pH-dependent oxidant production following inhibition of the mitochondrial electron transport chain in pulmonary endothelial cells.

We investigated the effect of changes in intracellular pH (pHi) and Na/H antiport activity on intracellular oxidant production in human pulmonary artery endothelial cells (HPAEC) following disruption of cellular metabolism. Oxidant production was measured with oxidant-sensitive probes (2',7'-dichlorofluorescein diacetate [H2DCF], dihydroethidium [DHE]) following treatment with inhibitors of mitochondrial electron transport and glycolysis (antimycin/2-deoxyglucose, A/D). A/D treatment increased oxidant production in a dose-dependent fashion over 2 hours. Omission of 2-deoxyglucose did not alter the magnitude of oxidant production. Inhibition at more proximal sites in the mitochondrial electron transport chain inhibited oxidant production. These data suggested that the mitochondrial electron transport chain was the source of oxidant production. Fluorescent imaging experiments confirmed the mitochondrial origin of the increased oxidant production under these conditions. Maneuvers that reduced pHi and inhibited Na/H exchange (acidosis, specific Na/H exchange inhibitors) attenuated oxidant production, whereas maneuvers that raised pHi (monensin) potentiated oxidant production. The results with the pH-insensitive probe (DHE) confirmed that oxidant production was pH-dependent. Oxidant production preceded significant loss of cell viability at 6 h following A/D treatment. These results demonstrate that oxidant production following inhibition of mitochondrial electron transport in HPAEC is pH-dependent and may contribute to endothelial cell injury by increasing endogenous oxidative stress.

Role of the Na/H antiport in pH-dependent cell death in pulmonary artery endothelial cells.

We investigated the role of intracellular pH (pH(i)) and Na/H exchange in cell death in human pulmonary artery endothelial cells (HPAEC) following a metabolic insult (inhibition-oxidative phosphorylation, glycolysis). Metabolic inhibition in medium at pH 7. 4 decreased viability (0-15% live cells) over 6 h. Cell death was attenuated by maneuvers that decreased pH(i) and inhibited Na/H exchange (acidosis, Na/H antiport inhibitors). In contrast, cell death was potentiated by maneuvers that elevated pH(i) or increased Na/H exchange (monensin, phorbol ester treatment) before the insult. HPAEC demonstrated a biphasic pH(i) response following a metabolic insult. An initial decrease in pH(i) was followed by a return to baseline over 60 min. Maneuvers that protected HPAEC and inhibited Na/H exchange (acidosis, Na(+)-free medium, antiport inhibitors) altered this pattern. pH(i) decreased, but no recovery was observed, suggesting that the return of pH(i) to normal was mediated by antiport activation. Although Na/H antiport activity was reduced (55-60% of control) following a metabolic insult, the cells still demonstrated active Na/H exchange despite significant ATP depletion. Phorbol ester pretreatment, which potentiated cell death, increased Na/H antiport activity above the level observed in monolayers subjected to a metabolic insult alone. These results demonstrate that HPAEC undergo a pH-dependent loss of viability linked to active Na/H exchange following a metabolic insult. Potentiation of cell death with phorbol ester treatment suggests that this cell death pathway involves protein kinase C-mediated phosphorylation events.