Role of cytosolic calcium-independent plasmalogen-selective phospholipase A₂ in hypoxic injury to rabbit proximal tubules

Although the activation of calcium-independent phospholipase A₂ (PLA₂) enzymes has been described in the heart, the pathogenetic role of this enzyme(s) in hypoxic cell injury has not been previously examined in any tissue. Therefore, we characterized the time course of activation of calcium- independent PLA₂ using both plasmalogen and diacylglycerophospholipid substrates during hypoxia in rabbit proximal tubules and examined whether inhibition of calcium-independent PLA₂ activity is associated with a cytoprotective effect. Subjecting rabbit proximal tubules to hypoxia for 5 min resulted in at least a threefold increase in cytosolic calcium- independent PLA₂, which was selective for plasmalogen substrates (control 444±69 vs hypoxia 1,675±194 pmol · mg protein^-1 · min^-1, n = 5). In contrast, no changes in PLA₂ activity were observed in the presence of 4 mM EGTA in the membrane fraction using plasmenylcholine substrates, 20 min of hypoxia resulted in an increase in arachidonate from 3±1 to 28±4 ng/mg protein and lactate dehydrogenase release from 7.5±2% to 38±5%, n = 4. Pretreatment of proximal tubules with 10 μM Compound I, a specific inhibitor of calcium-independent PLA₂, resulted in reduction in the magnitude of both hypoxia-induced arachidonic acid release (11±3 ng/mg protein) and lactate dehydrogenase release (18±4%). Our data indicate that a significant fraction of PLA₂ activity in the proximal tubule is calcium-independent and selective for plasmalogen substrates. Furthermore, the activation of this enzyme plays an important role in the pathogenesis of membrane injury during hypoxia in the proximal tubule.