Role of an Endoplasmic Reticulum Ca²⁺-Independent Phospholipase A₂ in Cisplatin-Induced Renal Cell Apoptosis

It has been demonstrated recently that rabbit renal proximal tubule cells (RPTC) express a novel Ca²⁺-independent phospholipase A₂ (iPLA₂) whose activity localizes to the endoplasmic reticulum (ER-iPLA₂) and is similar to group VIB PLA₂, In this study, the expression of group VIB PLA₂ was examined and the role of ER-iPLA₂ in cisplatin-induced apoptosis was determined. Cisplatin induced both time- and concentration-dependent RPTC apoptosis as determined by p53 nuclear localization, annexin V staining, caspase 3 activity, and chromatin condensation. Inhibition of ER-iPLA₂ with bromoenol lactone (5 μM) reduced cisplatin-induced annexin V binding 40%, chromatin condensation 55%, and caspase 3 activity 42%, but had no effect on p53 nuclear localization. Treatment of RPTC with the protein kinase C stimulator phorbol 12-myristate 13-acetate increased the activity of ER-iPLA₂ 2-fold and increased cisplatin-induced RPTC apoptosis. These studies demonstrate that group VIB PLA₂ is expressed in RPTC and suggest that RPTC ER-iPLA₂ is the rabbit homolog of group VIB PLA₂. These data also demonstrate that ER-iPLA₂ acts downstream of p53 and upstream of caspase 3 to mediate cisplatin-induced RPTC apoptosis. Finally, ER-iPLA₂ seems to be regulated by protein kinase C.