High dietary fat intake leads to insulin resistance in skeletal muscle, and this represents a major risk factor for type 2 diabetes and cardiovascular disease. Mitochondrial dysfunction and oxidative stress have been implicated in the disease process, but the underlying mechanisms are still unknown. Here we show that in skeletal muscle of both rodents and humans, a diet high in fat increases the H(2)O(2)-emitting potential of mitochondria, shifts the cellular redox environment to a more oxidized state, and decreases the redox-buffering capacity in the absence of any change in mitochondrial respiratory function. Furthermore, we show that attenuating mitochondrial H(2)O(2) emission, either by treating rats with a mitochondrial-targeted antioxidant or by genetically engineering the overexpression of catalase in mitochondria of muscle in mice, completely preserves insulin sensitivity despite a high-fat diet. These findings place the etiology of insulin resistance in the context of mitochondrial bioenergetics by demonstrating that mitochondrial H(2)O(2) emission serves as both a gauge of energy balance and a regulator of cellular redox environment, linking intracellular metabolic balance to the control of insulin sensitivity.

Objective This aim of this study was to determine the impact of diabetes on oxidant balance and mitochondrial metabolism of carbohydrate- and lipid-based substrates in myocardium of type 2 diabetic patients. Background Heart failure represents a major cause of death among diabetics, and it has been proposed that derangements in cardiac metabolism and oxidative stress may underlie the progression of this co-morbidity, but scarce evidence exists in support of this mechanism in humans. Methods Mitochondrial O2 consumption and H2O2 emission were measured in permeabilized myofibers prepared from samples of right atrial appendage obtained from non-diabetic (n=13) and diabetic (n=11) patients undergoing non-emergent coronary artery bypass graft surgery. Results Mitochondria in atrial tissue of type 2 diabetic individuals display a sharply decreased capacity for glutamate and fatty acid-supported respiration, in addition to an increased content of myocardial triglycerides, as compared to non-diabetics. Furthermore, diabetics display an increased mitochondrial H2O2 emission during oxidation of carbohydrate- and lipid-based substrates, depleted glutathione, and evidence of persistent oxidative stress in their atrial tissue. Conclusions These findings are the first to directly investigate the effects of type 2 diabetes on a panoply of mitochondrial functions in the human myocardium using cellular and molecular approaches, and they demonstrate that mitochondria in diabetic human heart have specific impairments in maximal capacity to oxidize fatty acids and glutamate, yet increased mitochondrial H2O2 emission, providing insight into the role of mitochondrial dysfunction and oxidative stress in the pathogenesis of heart failure in diabetic patients.

Anderson, Ethan J., and P. Darrell Neufer. Type II skeletal myofibers possess unique properties that potentiate mitochondrial H 2O2 generation. Am J Physiol Cell Physiol 290: C844 -C851, 2006. First published October 26, 2005 doi:10.1152/ajpcell.00402.2005.-Mitochondrial dysfunction is implicated in a number of skeletal muscle pathologies, most notably aging-induced atrophy and loss of type II myofibers. Although oxygen-derived free radicals are thought to be a primary cause of mitochondrial dysfunction, the underlying factors governing mitochondrial superoxide production in different skeletal myofiber types is unknown. Using a novel in situ approach to measure H 2O2 production (indicator of superoxide formation) in permeabilized rat skeletal muscle fiber bundles, we found that mitochondrial free radical leak (H 2O2 produced/O2 consumed) is two-to threefold higher (P Ͻ 0.05) in white (WG, primarily type IIB fibers) than in red (RG, type IIA) gastrocnemius or soleus (type I) myofibers during basal respiration supported by complex I (pyruvate ϩ malate) or complex II (succinate) substrates. In the presence of respiratory inhibitors, maximal rates of superoxide produced at both complex I and complex III are markedly higher in RG and WG than in soleus muscle despite ϳ50% less mitochondrial content in WG myofibers. Duplicate experiments conducted with Ϯexogenous superoxide dismutase revealed striking differences in the topology and/or dismutation of superoxide in WG vs. soleus and RG muscle. When normalized for mitochondrial content, overall H 2O2 scavenging capacity is lower in RG and WG fibers, whereas glutathione peroxidase activity, which is largely responsible for H 2O2 removal in mitochondria, is similar in all three muscle types. These findings suggest that type II myofibers, particularly type IIB, possess unique properties that potentiate mitochondrial superoxide production and/or release, providing a potential mechanism for the heterogeneous development of mitochondrial dysfunction in skeletal muscle.superoxide; reactive oxygen species; skeletal muscle; respiration; fiber type MANY OF THE REDOX REACTIONS within the mitochondrial electron transport chain involve single electron transfers with redox potentials sufficient to reduce oxygen to superoxide (O 2 Ϫ ⅐), a highly reactive free radical and parent molecule of reactive oxygen species (ROS) (47). There is increasing evidence that mitochondrial ROS production is a primary component of the aging process (34,37,46) and plays an important role in the pathogenesis of late-stage diabetes (11) and neurodegenerative diseases (3, 12). In skeletal muscle, aging is associated with a profound loss of muscle mass (i.e., sarcopenia) that is caused by progressive myofiber atrophy culminating in complete fiber loss. Interestingly, sarcopenia occurs almost exclusively in type II myofibers, developing focally along the length of the fiber in regions that are characterized by compromised respiratory function, mitochondrial DNA deletions, oxidative damage, and atrophy (7, 27,...

Mitochondrial dynamics is a conserved process by which mitochondria undergo repeated cycles of fusion and fission, leading to exchange of mitochondrial genetic content, ions, metabolites, and proteins. Here, we examine the role of the mitochondrial fusion protein optic atrophy 1 (OPA1) in differentiated skeletal muscle by reducing OPA1 gene expression in an inducible manner. OPA1 deficiency in young mice results in non-lethal progressive mitochondrial dysfunction and loss of muscle mass. Mutant mice are resistant to age- and diet-induced weight gain and insulin resistance, by mechanisms that involve activation of ER stress and secretion of fibroblast growth factor 21 (FGF21) from skeletal muscle, resulting in increased metabolic rates and improved whole-body insulin sensitivity. OPA1-elicited mitochondrial dysfunction activates an integrated stress response that locally induces muscle atrophy, but via secretion of FGF21 acts distally to modulate whole-body metabolism.