Mechanisms Underlying Fructose-Induced Oxidative Stress in the Cytosol and Mitochondria

Abstract

High levels of dietary fructose are increasingly recognized as an important nutritional factor in the development of nonalcoholic fatty liver disease and intestinal inflammation in both humans and animal models. The signals linking excessive fructose intake on one hand and the onset of these pathologies are not known. We have employed genetically encoded biosensors sensitive to changes in superoxide anion (O2- ) or hydrogen peroxide (H2O2) concentrations to test the hypothesis that fructose metabolism increases the production of reactive oxygen species (ROS) in hepatocytes or intestinal cells. The data indicate that acute treatment with physiological concentrations of fructose (1 - 10 mM) significantly increased the formation of mitochondrial O2- as well as mitochondrial and cytosolic H2O2. The xanthine oxidase inhibitor, allopurinol, inhibited fructose-induced increases in cytosolic H2O2, but was unaffected by apocynin, a NADPH oxidase inhibitor. These data are consistent with the well known effects of acute fructose treatment to reduce ATP levels and stimulate the breakdown of purines. Fructose-induced ROS production in the mitochondria was not altered by allopurinol treatment, whereas apocynin strongly suppressed mitochondrial derived ROS. Fructose administration transiently increased ROS formation in all mitochondria regardless of the subcellular localization and this was paralleled by a sustained rise in mitochondrial membrane potential and an increase in pyridine nucleotide fluorescence. The activation of mitochondrial metabolism was followed by large amplitude O2- bursts in a subset of mitochondria. The addition of mitochondrial respiration inhibitors blocked the effects of fructose on mitochondrial, but not cytosolic ROS production. Taken together, these data indicate that fructose treatment stimulates mitochondrial metabolism leading to an increase in ROS production through an apocynin sensitive pathway. Finally, we show that chronic consumption of fructose results in markedly higher baseline levels of mitochondrial ROS in hepatocytes.