Mitochondrial Reactive Oxygen Species Are Obligatory Signals for Glucose-Induced Insulin Secretion

Abstract

OBJECTIVE-Insulin secretion involves complex events in which the mitochondria play a pivotal role in the generation of signals that couple glucose detection to insulin secretion. Studies on the mitochondrial generation of reactive oxygen species (ROS) generally focus on chronic nutrient exposure. Here, we investigate whether transient mitochondrial ROS production linked to glucose-induced increased respiration might act as a signal for monitoring insulin secretion.RESEARCH DESIGN AND METHODS-ROS production in response to glucose was investigated in freshly isolated rat islets. ROS effects were studied using a pharmacological approach and calcium imaging.RESULTS-Transient glucose increase from 5.5 to 16.7 mmol/l stimulated ROS generation, which was reversed by antioxidants. Insulin secretion was dose dependently blunted by antioxidants and highly correlated with ROS levels. The incapacity of ␤-cells to secrete insulin in response to glucose with antioxidants was associated with a decrease in ROS production and in contrast to the maintenance of high levels of ATP and NADH. Then, we investigated the mitochondrial origin of ROS (mROS) as the triggering signal. Insulin release was mimicked by the mitochondrial-complex blockers, antimycin and rotenone, that generate mROS. The adding of antioxidants to mitochondrial blockers or to glucose was used to lower mROS reversed insulin secretion. Finally, calcium imaging on perifused islets using glucose stimulation or mitochondrial blockers revealed that calcium mobilization was completely reversed using the antioxidant trolox and that it was of extracellular origin. No toxic effects were present using these pharmacological approaches. CONCLUSIONS-Altogether, these complementary results demonstrate that mROS production is a necessary stimulus for glucose-induced insulin secretion. Diabetes 58:673-681, 2009 E lucidating the mechanisms by which pancreatic ␤-cells couple glucose sensing to insulin secretion, a vital process in energy homeostasis, is of prime importance. Although ATP production is considered the main mitochondrial signal, detailed studies show that insulin secretion cannot be restricted to ATP synthesis, and numerous experimental clues show that additional mitochondrial factors involved in glucose-secretion coupling are necessary, although not yet identified (1).Transient increases in glucose metabolism generate NADH and FADH 2 , leading rapidly to increased superoxide anion (O 2 ⅐ ) production; obligatorily associated with the respiratory chain function, superoxide anion will be converted into H 2 O 2 (2). This production of mitochondrial reactive oxygen species (mROS)-transient because H 2 O 2 -inactivating enzymes rapidly quench it before a damage to the physiological conditions of the cell occurs-is now recognized as an intracellular messenger (3,4). These features make mROS a good candidate for rapidly regulating pathways that depend directly on metabolic fluxes. Based on such a view, we recently demonstrated that mROS production is required for hypothalamic...

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