Protection of pancreatic β-cells by group VIA phospholipase A ₂-mediated repair of mitochondrial membrane peroxidation

Mitochondrial production of reactive oxygen species and oxidation of cardiolipin are key events in initiating apoptosis. We reported that group VIA Ca²⁺-independent phospholipase A₂ (iPLA2 β) localizes in and protects β-cell mitochondria from oxidative damage during staurosporine-induced apoptosis. Here, we used iPLA₂β-null (iPLA₂β^-/-) mice to investigate the role of iPLA ₂βin the repair of mitochondrial membranes. We show that islets isolated from iPLA₂β^-/- mice are more sensitive to staurosporine-induced apoptosis than those from wild-type littermates and that 2 wk of daily ip administration of staurosporine to iPLA₂β ^-/- mice impairs both the animals' glucose tolerance and glucose-stimulated insulin secretion by their pancreatic islets. Moreover, the iPLA₂β inhibitor bromoenol lactone caused mitochondrial membrane peroxidation and cytochrome c release, and these effects were reversed by N-acetyl cysteine. The mitochondrial antioxidant N-t-butyl hydroxylamine blocked staurosporine-induced cytochrome c release and caspase-3 activation in iPLA₂β^-/- islets. Furthermore, the collapse of mitochondrial membrane potential in INS-1 insulinoma cells caused by high glucose and fatty acid levels was attenuated by overexpressing iPLA ₂β. Interestingly, iPLA₂β was expressed only at low levels in islet β-cells from obesity- and diabetes-prone db/db mice. These findings support the hypothesis that iPLA₂β is important in repairing oxidized mitochondrial membrane components (e.g. cardiolipin) and that this prevents cytochrome c release in response to stimuli that otherwise induce apoptosis. The low iPLA₂β expression level in db/db mouse β-cells may render them vulnerable to injury by reactive oxygen species.