Hydrogen peroxide alters mitochondrial activation and insulin secretion in pancreatic beta cells

Abstract

The effects of a transient exposure to hydrogen peroxide (10 min at 200 μM H2O2) on pancreatic beta cell signal transduction and insulin secretion have been evaluated. In rat islets, insulin secretion evoked by glucose (16.7 mM) or by the mitochondrial substrate methyl succinate (5 mM) was markedly blunted following exposure to H2O2. In contrast, the secretory response induced by plasma membrane depolarization (20 mM KCl) was not significantly affected. Similar results were obtained in insulinoma INS-1 cells using glucose (12.8 mM) as secretagogne. After H2O2 treatment, glucose no longer depolarized the membrane potential (ΔΨ) of INS-1 cells or increased cytosolic CA2+. Both Δψ and Ca2+ responses were still observed with 30 mM KCl despite an elevated baseline of cytosolic Ca2+ appearing ~10 min after exposure to H2O2. The mitochondrial ΔΨ of INS-1 cells was depolarized by H2O2 abolishing the hyperpolarizing action of glucose. These ΔΨ changes correlated with altered mitochondrial morphology; the latter was not preserved by the overexpression of the antiapoptotic protein Bcl-2. Mitochondrial Ca2+ was increased following exposure to H2O2 up to the micromolar range. No further augmentation occurred after glucose addition, which normally raises this parameter. Nevertheless, KCl was still efficient in enhancing mitochondrial Ca2+. Cytosolic ATP was markedly reduced by H2O2 treatment, probably explaining the decreased endoplasmic reticulum Ca2+. Taken together, these data point to the mitochondria as primary targets for H2O2 damage, which will eventually interrupt the transduction of signals normally coupling glucose metabolism to insulin secretion.