Role of an endoplasmic reticulum Ca²⁺-independent phospholipase A₂ in oxidant-induced renal cell death

Phospholipase A₂ (PLA₂) hydrolyzes the sn-2 ester bond in phospholipids, releasing a fatty acid and a lysophospholipid. Recently, a novel 85-kDa membrane-bound-Ca²⁺-independent PLA₂ (iPLA₂) was identified in insect and bacterial cells transfected with candidate PLA₂ sequences. However, few data exist demonstrating a membrane-bound-iPLA₂ in mammalian cells, its subcellular localization, or its physiological role. Herein, we demonstrate the expression of an 85-kDa endoplasmic reticulum (ER)-Ca²⁺-iPLA₂ (ER-iPLA₂) in rabbit renal proximal tubule cells (RPTC) that is plasmalogen selective and is inhibited by the specific Ca²⁺-iPLA₂ inhibitor bromoenol lactone (BEL). RPTC exposed to tert-butylhydroperoxide for 24 h exhibited 20% oncosis compared with 2% in controls. Inhibition of ER-iPLA₂ with BEL before tert-butylhydroperoxide exposure resulted in 50% oncosis. To determine whether this effect was common to oxidants, we tested the ability of BEL to potentiate oncosis induced by cumene hydroperoxide, menadione, duraquinone, cisplatin, and the nonoxidant antimycin A. All oxidants tested produced oncosis after 24 h, and prior inhibition of ER-iPLA₂ potentiated oncosis at least twofold. In contrast, inhibition of ER-iPLA₂ did not alter antimycin A-induced oncosis. Lipid peroxidation increased from 1.4- to 5.2-fold in RPTC treated with BEL before oxidant exposure, whereas no change was seen in antimycin A-treated RPTC. These results are the first to demonstrate the expression and subcellular localization of an ER-iPLA₂. These results also suggest that ER-iPLA₂ functions to protect against oxidant-induced lipid peroxidation and oncosis.