H2O2-Activated Mitochondrial Phospholipase iPLA2 gamma Prevents Lipotoxic Oxidative Stress in Synergy with UCP2, Amplifies Signaling via G-Protein-Coupled Receptor GPR40, and Regulates Insulin Secretion in Pancreatic beta-Cells

摘要

Aims: Pancreatic -cell chronic lipotoxicity evolves from acute free fatty acid (FA)-mediated oxidative stress, unprotected by antioxidant mechanisms. Since mitochondrial uncoupling protein-2 (UCP2) plays antioxidant and insulin-regulating roles in pancreatic -cells, we tested our hypothesis, that UCP2-mediated uncoupling attenuating mitochondrial superoxide production is initiated by FA release due to a direct H2O2-induced activation of mitochondrial phospholipase iPLA(2). Results: Pro-oxidant tert-butylhydroperoxide increased respiration, decreased membrane potential and mitochondrial matrix superoxide release rates of control but not UCP2- or iPLA(2)-silenced INS-1E cells. iPLA(2)/UCP2-mediated uncoupling was alternatively activated by an H2O2 burst, resulting from palmitic acid (PA) -oxidation, and it was prevented by antioxidants or catalase overexpression. Exclusively, nascent FAs that cleaved off phospholipids by iPLA(2) were capable of activating UCP2, indicating that the previously reported direct redox UCP2 activation is actually indirect. Glucose-stimulated insulin release was not affected by UCP2 or iPLA(2) silencing, unless pro-oxidant activation had taken place. PA augmented insulin secretion via G-protein-coupled receptor 40 (GPR40), stimulated by iPLA(2)-cleaved FAs (absent after GPR40 silencing). Innovation and Conclusion: The iPLA(2)/UCP2 synergy provides a feedback antioxidant mechanism preventing oxidative stress by physiological FA intake in pancreatic -cells, regulating glucose-, FA-, and redox-stimulated insulin secretion. iPLA(2) is regulated by exogenous FA via -oxidation causing H2O2 signaling, while FAs are cleaved off phospholipids, subsequently acting as amplifying messengers for GPR40. Hence, iPLA(2) acts in eminent physiological redox signaling, the impairment of which results in the lack of antilipotoxic defense and contributes to chronic lipotoxicity.