Excess reactive oxygen species (ROS) production is thought to play a key role in the loss of pancreatic β-cell number and/or function, in response to high glucose and/or fatty acids. However, contradictory findings have been reported showing that in pancreatic β cells or insulin-secreting cell lines, ROS are produced under conditions of either high or low glucose. Superoxide production was measured in attached INS1E cells as a function of glucose concentration, by following in real time the

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... ation of dihydroethidine. Minimal values of superoxide production were measured at glucose concentrations of 5-20 mM, whereas superoxide generation was maximal at 0-1 mM glucose. Superoxide generation started rapidly (15-30 min) after exposure to low glucose and was suppressed by its addition within minutes. Superoxide was totally suppressed by rotenone, but not myxothiazol, suggesting a role for complex I in this process. Indirect evidence for mitochondrial ROS generation was also provided by a decrease in aconitase activity. Activation of AMPK, a cellular metabolic sensor, and its downstream target ACC by low glucose concentration was largely inhibited by addition of MnTBAP, a MnSOD and catalase mimetic that also totally suppressed superoxide production. Taken together, the data show that low glucose activates AMPK in a superoxide-dependent, AMP-independent way. Excessive reactive oxygen species (ROS) produced by the mitochondria are considered to play a central role in cellular dysfunction as observed in several pathophysiological states, including obesity, diabetes and its complications, ischemic reperfusion injury, degenerative diseases, and aging [1, 2] . Alternatively, accumulating evidence suggests that ROS not only are deleterious by-products of metabolism but also participate to cell signaling and regulation (reviewed in [3]). In diabetes, these dual effects of ROS are well illustrated by their long-term deleterious effects on insulin-secreting and insulin target cells, as well as on endothelial cells [4] , and by the recent demonstration of their contribution to the beneficial effects of exercise on insulin resistance [5] . In pancreatic β cells, a key role has been attributed to ROS produced by excess glucose and/or lipids (glucolipotoxicity) or by proinflammatory cytokines, in the loss of β-cell mass and/or function [6, 7] , in particular because these cells express low levels of ROS-scavenging enzymes [8] [9] [10] . Evidence that this cellular damage is the consequence of a sustained increase in ROS comes from the observation that it is prevented by the normalization of mitochondrial ROS production after the addition of inhibitors of mitochondrial metabolism or by the overexpression of uncoupling protein-1 or manganese superoxide dismutase (MnSOD) [11] . However, the exact relationship between glucose level and reactive oxygen species generation in pancreatic β cells is still debated. Although a positive correlation between glucose level and ROS production has been established in several cell types, contradictory findings have been reported in pancreatic β cells. A positive relationship between glucose level and ROS production has been demonstrated in rat islets [12] , and other studies provided evidence that these ROS are a metabolic signal participating in glucose-stimulated insulin secretion [13, 14] . By contrast, studies measuring either superoxide or hydrogen peroxide generation in isolated rat β cells show that the production of these two compounds is increased at low glucose, in particular in β cells with higher metabolic responsiveness to glucose [15] . A similar increase in ROS production at low glucose concentration has been reported in several insulin-secreting cell lines [16, 17] . AMP-activated protein kinase (AMPK) is a metabolic sensor activated by an increase in the AMP/ATP ratio, i.e., during metabolic stress, exercise, or hypoxia. This activation turns off anabolic pathways consuming ATP and turns on catabolic pathways producing ATP, such as fatty acid oxidation. In addition to the classical pathway of activation by AMP, recent studies have identified in various cell types other physiological activators of this kinase, among which are ROS [18] .