Mitochondrial Oxidative Capacity Research Articles

Lysine-specific demethylase 1 promotes brown adipose tissue thermogenesis via repressing glucocorticoid activation.

Brown adipocytes display phenotypic plasticity, as they can switch between the active states of fatty acid oxidation and energy dissipation versus a more dormant state. Cold exposure or β-adrenergic stimulation favors the active thermogenic state, whereas sympathetic denervation or glucocorticoid administration promotes more lipid accumulation. Our understanding of the molecular mechanisms underlying these switches is incomplete. Here we found that LSD1 (lysine-specific demethylase 1), a histone demethylase, regulates brown adipocyte metabolism in two ways. On the one hand, LSD1 associates with PRDM16 to repress expression of white fat-selective genes. On the other hand, LSD1 represses HSD11B1 (hydroxysteroid 11-β-dehydrogenase isozyme 1), a key glucocorticoid-activating enzyme, independently from PRDM16. Adipose-specific ablation of LSD1 impaired mitochondrial fatty acid oxidation capacity of the brown adipose tissue, reduced whole-body energy expenditure, and increased fat deposition, which can be significantly alleviated by simultaneously deleting HSD11B1. These findings establish a novel regulatory pathway connecting histone modification and hormone activation with mitochondrial oxidative capacity and whole-body energy homeostasis.

Mitochondrial function assessed by 31P MRS and BOLD MRI in non-obese type 2 diabetic rats.

The study aims to characterize age‐associated changes in skeletal muscle bioenergetics by evaluating the response to ischemia‐reperfusion in the skeletal muscle of the Goto‐Kakizaki (GK) rats, a rat model of non‐obese type 2 diabetes (T2D). 31P magnetic resonance spectroscopy (MRS) and blood oxygen level‐dependent (BOLD) MRI was performed on the hindlimb of young (12 weeks) and adult (20 weeks) GK and Wistar (control) rats. 31P‐MRS and BOLD‐MRI data were acquired continuously during an ischemia and reperfusion protocol to quantify changes in phosphate metabolites and muscle oxygenation. The time constant of phosphocreatine recovery, an index of mitochondrial oxidative capacity, was not statistically different between GK rats (60.8 ± 13.9 sec in young group, 83.7 ± 13.0 sec in adult group) and their age‐matched controls (62.4 ± 11.6 sec in young group, 77.5 ± 7.1 sec in adult group). During ischemia, baseline‐normalized BOLD‐MRI signal was significantly lower in GK rats than in their age‐matched controls. These results suggest that insulin resistance leads to alterations in tissue metabolism without impaired mitochondrial oxidative capacity in GK rats.

Hepatic mitochondrial oxidative phosphorylation is normal in obese patients with and without type 2 diabetes.

Hepatic insulin resistance in patients with obesity or type 2 diabetes has been suggested to result from hepatic mitochondrial dysfunction. High-resolution respirometry (HRR) can be used to assess oxidative phosphorylation by measuring the mitochondrial oxygen consumption rate in the individual complexes of the mitochondria. By using HRR, the present study demonstrates no difference in hepatic mitochondrial oxidative phosphorylation among subjects with obesity with or without type 2 diabetes and non-obese controls. Furthermore, the amount of mitochondria, assessed by the citrate synthase activity, is not different between the three groups. Together the present findings indicate that hepatic mitochondrial oxidative phosphorylation capacity is not impaired in patients with obesity or type 2 diabetes. Obese patients with type 2 diabetes (T2DM) and without type 2 diabetes (OB) are characterized by high hepatic lipid content and hepatic insulin resistance. This may be linked to impaired hepatic mitochondrial oxidative phosphorylation (OXPHOS) capacity. The aim of the present study was to investigate and compare hepatic mitochondrial OXPHOS capacity in T2DM, OB and non-obese controls (CON). Seventeen obese patients (nine OB and eight T2DM) and six CON patients had perioperative liver biopsies taken. Samples were divided into three parts to measure (1) complex I, II and IV linked respiration, (2) citrate synthase (CS) activity and (3) lipid droplet (LD) size and area (% of total tissue area filled by LDs). State 3 respiration of complex I, II and IV and the CS activity did not differ in OB, T2DM and CON. LD size was significantly higher in T2DM compared with CON, and LD area tended (P=0.10) to be higher in T2DM and OB compared with CON. The present findings indicate that hepatic OXPHOS capacity is not different in patients with markedly different weight and glycaemic control. Furthermore, the results do not support impaired hepatic mitochondrial respiratory capacity playing a major role in the development of obesity-induced type 2 diabetes.

Maximal oxygen uptake is proportional to muscle fiber oxidative capacity, from chronic heart failure patients to professional cyclists.

V̇o2 max during whole body exercise is presumably constrained by oxygen delivery to mitochondria rather than by mitochondria's ability to consume oxygen. Humans and animals have been reported to exploit only 60-80% of their mitochondrial oxidative capacity at maximal oxygen uptake (V̇o2 max). However, ex vivo quantification of mitochondrial overcapacity is complicated by isolation or permeabilization procedures. An alternative method for estimating mitochondrial oxidative capacity is via enzyme histochemical quantification of succinate dehydrogenase (SDH) activity. We determined to what extent V̇o2 max attained during cycling exercise differs from mitochondrial oxidative capacity predicted from SDH activity of vastus lateralis muscle in chronic heart failure patients, healthy controls, and cyclists. V̇o2 max was assessed in 20 healthy subjects and 28 cyclists, and SDH activity was determined from biopsy cryosections of vastus lateralis using quantitative histochemistry. Similar data from our laboratory of 14 chronic heart failure patients and 6 controls were included. Mitochondrial oxidative capacity was predicted from SDH activity using estimated skeletal muscle mass and the relationship between ex vivo fiber V̇o2 max and SDH activity of isolated single muscle fibers and myocardial trabecula under hyperoxic conditions. Mitochondrial oxidative capacity predicted from SDH activity was related (r(2) = 0.89, P < 0.001) to V̇o2 max measured during cycling in subjects with V̇o2 max ranging from 9.8 to 79.0 ml·kg(-1)·min(-1) V̇o2 max measured during cycling was on average 90 ± 14% of mitochondrial oxidative capacity. We conclude that human V̇o2 max is related to mitochondrial oxidative capacity predicted from skeletal muscle SDH activity. Mitochondrial oxidative capacity is likely marginally limited by oxygen supply to mitochondria.

Dipeptidyl peptidase-4 inhibitor improved exercise capacity and mitochondrial biogenesis in mice with heart failure via activation of glucagon-like peptide-1 receptor signalling.

Exercise capacity is reduced in heart failure (HF) patients, due mostly to skeletal muscle abnormalities including impaired energy metabolism, mitochondrial dysfunction, fibre type transition, and atrophy. Glucagon-like peptide-1 (GLP-1) has been shown to improve exercise capacity in HF patients. We investigated the effects of the administration of a dipeptidyl peptidase (DPP)-4 inhibitor on the exercise capacity and skeletal muscle abnormalities in an HF mouse model after myocardial infarction (MI). MI was created in male C57BL/6J mice by ligating the left coronary artery, and a sham operation was performed in other mice. The mice were then divided into two groups according to the treatment with or without a DPP-4 inhibitor, MK-0626 [1 mg/kg body weight (BW)/day] provided in the diet. Four weeks later, the exercise capacity evaluated by treadmill test was revealed to be limited in the MI mice, and it was ameliorated in the MI + MK-0626 group without affecting the infarct size or cardiac function. The citrate synthase activity, mitochondrial oxidative phosphorylation capacity, supercomplex formation, and their quantity were reduced in the skeletal muscle from the MI mice, and these decreases were normalized in the MI + MK-0626 group, in association with the improvement of mitochondrial biogenesis. Immunohistochemical staining also revealed that a shift toward the fast-twitch fibre type in the MI mice was also reversed by MK-0626. Favourable effects of MK-0626 were significantly inhibited by treatment of GLP-1 antagonist, Exendin-(9-39) (150 pmol/kg BW/min, subcutaneous osmotic pumps) in MI + MK-0626 mice. Similarly, exercise capacity and mitochondrial function were significantly improved by treatment of GLP-1 agonist, Exendin-4 (1 nmol/kg/BW/h, subcutaneous osmotic pumps). A DPP-4 inhibitor may be a novel therapeutic agent against the exercise intolerance seen in HF patients by improving the mitochondrial biogenesis in their skeletal muscle.

Hypoxia-reoxygenation differentially alters the thermal sensitivity of complex I basal and maximal mitochondrial oxidative capacity

Hypoxia-reoxygenation (H-R) transitions and temperature fluctuations occur frequently in biological systems and likely interact to alter cell function. To test how H-R modulates mitochondrial function at different temperatures we measured the effects of H-R on isolated fish liver mitochondrial oxidation rates over a wide temperature range (5–25°C). Subsequently, the mechanisms underlying H-R induced mitochondrial responses were examined. H-R inhibited the complex I (CI) maximal (state 3) and stimulated the basal (state 4) mitochondrial oxidation rates with temperature enhancing both effects. As a result, the thermal sensitivity (Q10) for CI maximal respiration was reduced while that for basal respiration was increased by H-R. H-R reduced both the coupling and phosphorylation efficiencies more profoundly at high temperature suggesting that mitochondria were more resistant to H-R at low temperature. The H-R induced mitochondrial impairments were associated with increased reactive oxygen species (ROS) production and proton leak, dissipation of membrane potential, and loss of structural integrity of the organelles. Overall, our study provides insight into the mechanisms of H-R induced mitochondrial morphofunctional disruption and shows that the moderation of effects of H-R on oxidative phosphorylation by temperature depends on the functional state.

Demonstration of a day-night rhythm in human skeletal muscle oxidative capacity

ObjectiveA disturbed day-night rhythm is associated with metabolic perturbations that can lead to obesity and type 2 diabetes mellitus (T2DM). In skeletal muscle, a reduced oxidative capacity is also associated with the development of T2DM. However, whether oxidative capacity in skeletal muscle displays a day-night rhythm in humans has so far not been investigated. MethodsLean, healthy subjects were enrolled in a standardized living protocol with regular meals, physical activity and sleep to reflect our everyday lifestyle. Mitochondrial oxidative capacity was examined in skeletal muscle biopsies taken at five time points within a 24-hour period. ResultsCore-body temperature was lower during the early night, confirming a normal day-night rhythm. Skeletal muscle oxidative capacity demonstrated a robust day-night rhythm, with a significant time effect in ADP-stimulated respiration (state 3 MO, state 3 MOG and state 3 MOGS, p < 0.05). Respiration was lowest at 1 PM and highest at 11 PM (state 3 MOGS: 80.6 ± 4.0 vs. 95.8 ± 4.7 pmol/mg/s). Interestingly, the fluctuation in mitochondrial function was also observed in whole-body energy expenditure, with peak energy expenditure at 11 PM and lowest energy expenditure at 4 AM (p < 0.001). In addition, we demonstrate rhythmicity in mRNA expression of molecular clock genes in human skeletal muscle. ConclusionsOur results suggest that the biological clock drives robust rhythms in human skeletal muscle oxidative metabolism. It is tempting to speculate that disruption of these rhythms contribute to the deterioration of metabolic health associated with circadian misalignment.

Thermal acclimation in American alligators: Effects of temperature regime on growth rate, mitochondrial function, and membrane composition

We investigated the ability of juvenile American alligators (Alligator mississippiensis) to acclimate to temperature with respect to growth rate. We hypothesized that alligators would acclimate to cold temperature by increasing the metabolic capacity of skeletal muscles and the heart. Additionally, we hypothesized that lipid membranes in the thigh muscle and liver would respond to low temperature, either to maintain fluidity (via increased unsaturation) or to maintain enzyme reaction rates (via increased docosahexaenoic acid). Alligators were assigned to one of 3 temperature regimes beginning at 9 mo of age: constant warm (30°C), constant cold (20°C), and daily cycling for 12h at each temperature. Growth rate over the following 7 mo was highest in the cycling group, which we suggest occurred via high digestive function or feeding activity during warm periods and energy-saving during cold periods. The warm group also grew faster than the cold group. Heart and liver masses were proportional to body mass, while kidney was proportionately larger in the cold group compared to the warm animals. Whole-animal metabolic rate was higher in the warm and cycling groups compared to the cold group – even when controlling for body mass – when assayed at 30°C, but not at 20°C. Mitochondrial oxidative phosphorylation capacity in permeabilized fibers of thigh muscle and heart did not differ among treatments. Membrane fatty acid composition of the brain was largely unaffected by temperature treatment, but adjustments were made in the phospholipid headgroup composition that are consistent with homeoviscous adaptation. Thigh muscle cell membranes had elevated polyunsaturated fatty acids in the cold group relative to the cycling group, but this was not the case for thigh muscle mitochondrial membranes. Liver mitochondria from cold alligators had elevated docosahexaenoic acid, which might be important for maintenance of reaction rates of membrane-bound enzymes.

Association between heat stress and oxidative stress in poultry; mitochondrial dysfunction and dietary interventions with phytochemicals.

Heat as a stressor of poultry has been studied extensively for many decades; it affects poultry production on a worldwide basis and has significant impact on well-being and production. More recently, the involvement of heat stress in inducing oxidative stress has received much interest. Oxidative stress is defined as the presence of reactive species in excess of the available antioxidant capacity of animal cells. Reactive species can modify several biologically cellular macromolecules and can interfere with cell signaling pathways. Furthermore, during the last decade, there has been an ever-increasing interest in the use of a wide array of natural feed-delivered phytochemicals that have potential antioxidant properties for poultry. In light of this, the current review aims to (1) summarize the mechanisms through which heat stress triggers excessive superoxide radical production in the mitochondrion and progresses into oxidative stress, (2) illustrate that this pathophysiology is dependent on the intensity and duration of heat stress, (3) present different nutritional strategies for mitigation of mitochondrial dysfunction, with particular focus on antioxidant phytochemicals.Oxidative stress that occurs with heat exposure can be manifest in all parts of the body; however, mitochondrial dysfunction underlies oxidative stress. In the initial phase of acute heat stress, mitochondrial substrate oxidation and electron transport chain activity are increased resulting in excessive superoxide production. During the later stage of acute heat stress, down-regulation of avian uncoupling protein worsens the oxidative stress situation causing mitochondrial dysfunction and tissue damage. Typically, antioxidant enzyme activities are upregulated. Chronic heat stress, however, leads to downsizing of mitochondrial metabolic oxidative capacity, up-regulation of avian uncoupling protein, a clear alteration in the pattern of antioxidant enzyme activities, and depletion of antioxidant reserves.Some phytochemicals, such as various types of flavonoids and related compounds, were shown to be beneficial in chronic heat-stressed poultry, but were less or not effective in non-heat-stressed counterparts. This supports the contention that antioxidant phytochemicals have potential under challenging conditions. Though substantial progress has been made in our understanding of the association between heat stress and oxidative stress, the means by which phytochemicals can alleviate oxidative stress have been sparsely explored.Electronic supplementary materialThe online version of this article (doi:10.1186/s40104-016-0097-5) contains supplementary material, which is available to authorized users.

Hepatic mTORC1 Opposes Impaired Insulin Action to Control Mitochondrial Metabolism in Obesity

Dysregulated mitochondrial metabolism during hepatic insulin resistance may contribute to pathophysiologies ranging from elevated glucose production to hepatocellular oxidative stress and inflammation. Given that obesity impairs insulin action but paradoxically activates mTORC1, we tested whether insulin action and mammalian target of rapamycin complex 1 (mTORC1) contribute to altered invivo hepatic mitochondrial metabolism. Loss of hepatic insulin action for 2weeks caused increased gluconeogenesis, mitochondrial anaplerosis, tricarboxylic acid (TCA) cycle oxidation, and ketogenesis. However, activation of mTORC1, induced by the loss of hepatic Tsc1, suppressed these fluxes. Only glycogen synthesis was impaired by both loss of insulin receptor and mTORC1 activation. Mice with a double knockout of the insulin receptor and Tsc1 had larger livers, hyperglycemia, severely impaired glycogen storage, and suppressed ketogenesis, as compared to those with loss of the liver insulin receptor alone. Thus, activation of hepatic mTORC1 opposes the catabolic effects of impaired insulin action under some nutritional states.

Protective effect of diosgenin on chondrocytes mediated by JAK2/STAT3 signaling pathway in mice with osteoarthritis

Objective: To investigate the effect of diosgenin (Dgn) on chondrocytes and its relation to JAK2/STAT3 signaling pathway in mice with osteoarthritis (OA).Methods: Fifteen male C57BL/6 mice were randomly divided into three groups:control group, OA group and OA+Dgn group. After 4 weeks of treatment, the histopathological changes of cartilage tissue were observed by toluidine blue staining under light microscopy and the ultrastructure of chondrocytes was observed under electron microscopy. The primarily cultured chondrocytes of OA mice were randomly divided into 4 groups:(1) OA group, (2) Dgn group, (3) Dgn+AG490 group, (4) AG490 group. The expression of p-JAK2, p-STAT3, Bax, succinate dehydrogenase (SDH) and cytochrome c oxidase (COX) were detected by Western blotting, and superoxide dismutase (SOD) was detected using colorimetric method. Results: The morphological observation showed that the chondrocytes of OA group presented considerable pathological changes, while the chondrocytes in OA+Dgn group maintained intact membrane. Electron microscopy observation found obvious injury in cartilage tissues of OA group, while that in OA+Dgn group remained smooth. Compared with OA group, the expressions of p-JAK2 and p-STAT3 in chondrocytes of Dgn group were increased (all P<0.05), and the expressions of Bax protein, SDH, COX and SOD were decreased (all P<0.05). While compared with Dgn group, the expressions of p-JAK2, p-STAT3, SDH, COX and SOD in chondrocytes of Dgn+AG490 group were decreased (all P<0.05), and the expression of Bax protein was increased (P<0.05). Conclusion: Diosgenin can inhibit apoptosis and increase mitochondrial oxidative stress capacity of chondrocytes in mice with osteoarthritis, which is closely related to the activation of JAK2/STAT3 signaling pathway.

Pseudouridine synthase 1 deficient mice, a model for Mitochondrial Myopathy with Sideroblastic Anemia, exhibit muscle morphology and physiology alterations.

Mitochondrial myopathy with lactic acidosis and sideroblastic anemia (MLASA) is an oxidative phosphorylation disorder, with primary clinical manifestations of myopathic exercise intolerance and a macrocytic sideroblastic anemia. One cause of MLASA is recessive mutations in PUS1, which encodes pseudouridine (Ψ) synthase 1 (Pus1p). Here we describe a mouse model of MLASA due to mutations in PUS1. As expected, certain Ψ modifications were missing in cytoplasmic and mitochondrial tRNAs from Pus1−/− animals. Pus1−/− mice were born at the expected Mendelian frequency and were non-dysmorphic. At 14 weeks the mutants displayed reduced exercise capacity. Examination of tibialis anterior (TA) muscle morphology and histochemistry demonstrated an increase in the cross sectional area and proportion of myosin heavy chain (MHC) IIB and low succinate dehydrogenase (SDH) expressing myofibers, without a change in the size of MHC IIA positive or high SDH myofibers. Cytochrome c oxidase activity was significantly reduced in extracts from red gastrocnemius muscle from Pus1−/− mice. Transmission electron microscopy on red gastrocnemius muscle demonstrated that Pus1−/− mice also had lower intermyofibrillar mitochondrial density and smaller mitochondria. Collectively, these results suggest that alterations in muscle metabolism related to mitochondrial content and oxidative capacity may account for the reduced exercise capacity in Pus1−/− mice.

Role of Myofibrillar Protein Catabolism in Development of Glucocorticoid Myopathy: Aging and Functional Activity Aspects

Muscle weakness in corticosteroid myopathy is mainly the result of the destruction and atrophy of the myofibrillar compartment of fast-twitch muscle fibers. Decrease of titin and myosin, and the ratio of nebulin and MyHC in myopathic muscle, shows that these changes of contractile and elastic proteins are the result of increased catabolism of the abovementioned proteins in skeletal muscle. Slow regeneration of skeletal muscle is in good correlation with a decreased number of satellite cells under the basal lamina of muscle fibers. Aging causes a reduction of AMP-activated protein kinase (AMPK) activity as the result of the reduced function of the mitochondrial compartment. AMPK activity increases as a result of increased functional activity. Resistance exercise causes anabolic and anticatabolic effects in skeletal muscle: muscle fibers experience hypertrophy while higher myofibrillar proteins turn over. These changes are leading to the qualitative remodeling of muscle fibers. As a result of these changes, possible maximal muscle strength is increasing. Endurance exercise improves capillary blood supply, increases mitochondrial biogenesis and muscle oxidative capacity, and causes a faster turnover rate of sarcoplasmic proteins as well as qualitative remodeling of type I and IIA muscle fibers. The combination of resistance and endurance exercise may be the fastest way to prevent or decelerate muscle atrophy due to the anabolic and anticatabolic effects of exercise combined with an increase in oxidative capacity. The aim of the present short review is to assess the role of myofibrillar protein catabolism in the development of glucocorticoid-caused myopathy from aging and physical activity aspects.

Skeletal Muscle Mitochondrial Content, Oxidative Capacity, and Mfn2 Expression Are Reduced in Older Patients With Heart Failure and Preserved Ejection Fraction and Are Related to Exercise Intolerance

ObjectivesThe aim of this study was to examine skeletal muscle mitochondria content, oxidative capacity, and the expression of key mitochondrial dynamics proteins in patients with heart failure with preserved ejection fraction (HFpEF), as well as to determine potential relationships with measures of exercise performance. BackgroundMultiple lines of evidence indicate that severely reduced peak exercise oxygen uptake (peak VO2) in older patients with HFpEF is related to abnormal skeletal muscle oxygen utilization. Mitochondria are key regulators of skeletal muscle metabolism; however, little is known about how these organelles are affected in HFpEF. MethodsBoth vastus lateralis skeletal muscle citrate synthase activity and the expression of porin and regulators of mitochondrial fusion were examined in older patients with HFpEF (n = 20) and healthy, age-matched control subjects (n = 17). ResultsCompared with age-matched healthy control subjects, mitochondrial content assessed by porin expression was 46% lower (p = 0.01), citrate synthase activity was 29% lower (p = 0.01), and Mfn2 (mitofusin 2) expression was 54% lower (p <0.001) in patients with HFpEF. Expression of porin was significantly positively correlated with both peak VO2 and 6-min walk distance (r = 0.48, p = 0.003 and r = 0.33, p = 0.05, respectively). Expression of Mfn2 was also significantly positively correlated with both peak VO2 and 6-min walk distance (r = 0.40, p = 0.02 and r = 0.37, p = 0.03 respectively). ConclusionsThese findings suggest that skeletal muscle oxidative capacity, mitochondrial content, and mitochondrial fusion are abnormal in older patients with HFpEF and might contribute to their severe exercise intolerance.

Dehydroepiandrosterone protects male and female hippocampal neurons and neuroblastoma cells from glucose deprivation

Dehydroepiandrosterone (DHEA) modulates neurogenesis, neuronal function, neuronal survival and metabolism, enhancing mitochondrial oxidative capacity. Glucose deprivation and hypometabolism have been implicated in the mechanisms that mediate neuronal damage in neurological disorders, and some studies have shown that these mechanisms are sexually dimorphic. It was also demonstrated that DHEA is able to attenuate the hypometabolism that is related to some neurodegenerative diseases, eliciting neuroprotective effects in different experimental models of neurodegeneration. The aim of this study was to evaluate the effect of DHEA on the viability of male and female hippocampal neurons and SH-SY5Y neuroblastoma cells exposed to glucose deprivation. It was observed that after 12h of pre-treatment, DHEA was able to protect SH-SY5Y cells from glucose deprivation for 6h (DHEA 10−12, 10−8 and 10−6M) and 8h (DHEA 10−8M). In contrast, DHEA was not neuroprotective against glucose deprivation for 12 or 24h. DHEA (10−8M) also protected SH-SY5Y cells when added together or even 1h after the beginning of glucose deprivation (6h). Furthermore, DHEA (10−8M) also protected primary neurons from both sexes against glucose deprivation. In summary, our findings indicate that DHEA is neuroprotective against glucose deprivation in human neuroblastoma cells and in male and female mouse hippocampal neurons. These results suggest that DHEA could be a promising candidate to be used in clinical studies aiming to reduce neuronal damage in people from both sexes.

Alterations of Mitochondrial Function and Insulin Sensitivity in Human Obesity and Diabetes Mellitus.

Mitochondrial function refers to a broad spectrum of features such as resting mitochondrial activity, (sub)maximal oxidative phosphorylation capacity (OXPHOS), and mitochondrial dynamics, turnover, and plasticity. The interaction between mitochondria and insulin sensitivity is bidirectional and varies depending on tissue, experimental model, methodological approach, and features of mitochondrial function tested. In human skeletal muscle, mitochondrial abnormalities may be inherited (e.g., lower mitochondrial content) or acquired (e.g., impaired OXPHOS capacity and plasticity). Abnormalities ultimately lead to lower mitochondrial functionality due to or resulting in insulin resistance and type 2 diabetes mellitus. Similar mechanisms can also operate in adipose tissue and heart muscle. In contrast, mitochondrial oxidative capacity is transiently upregulated in the liver of obese insulin-resistant humans with or without fatty liver, giving rise to oxidative stress and declines in advanced fatty liver disease. These data suggest a highly tissue-specific interaction between insulin sensitivity and oxidative metabolism during the course of metabolic diseases in humans.

Palmitate-induced changes in energy demand cause reallocation of ATP supply in rat and human skeletal muscle cells

Mitochondrial dysfunction has been associated with obesity-related muscle insulin resistance, but the causality of this association is controversial. The notion that mitochondrial oxidative capacity may be insufficient to deal appropriately with excessive nutrient loads is for example disputed. Effective mitochondrial capacity is indirectly, but largely determined by ATP-consuming processes because skeletal muscle energy metabolism is mostly controlled by ATP demand. Probing the bioenergetics of rat and human myoblasts in real time we show here that the saturated fatty acid palmitate lowers the rate and coupling efficiency of oxidative phosphorylation under conditions it causes insulin resistance. Stearate affects the bioenergetic parameters similarly, whereas oleate and linoleate tend to decrease the rate but not the efficiency of ATP synthesis. Importantly, we reveal that palmitate influences how oxidative ATP supply is used to fuel ATP-consuming processes. Direct measurement of newly made protein demonstrates that palmitate lowers the rate of de novo protein synthesis by more than 30%. The anticipated decrease of energy demand linked to protein synthesis is confirmed by attenuated cycloheximide-sensitivity of mitochondrial respiratory activity used to make ATP. This indirect measure of ATP turnover indicates that palmitate lowers ATP supply reserved for protein synthesis by at least 40%. This decrease is also provoked by stearate, oleate and linoleate, albeit to a lesser extent. Moreover, palmitate lowers ATP supply for sodium pump activity by 60–70% and, in human cells, decreases ATP supply for DNA/RNA synthesis by almost three-quarters. These novel fatty acid effects on energy expenditure inform the ‘mitochondrial insufficiency’ debate.

Effects of Mitochondria-Targeted Antioxidant Supplementation on Mitochondrial Adaptations to Endurance Training in Healthy Men

The production of reactive oxygen species (ROS) during exercise is one signal for training-induced mitochondrial biogenesis. Previous studies have shown that antioxidant supplementation blunts mitochondrial adaptations to exercise training via suppression of ROS production. PURPOSE: To determine the effects of mitochondria-targeted antioxidant (mtAO) supplementation on mitochondrial adaptations to chronic exercise training. METHODS: Young (22 ± 3 yr), healthy men (n = 19) were randomized to receive either mtAO (n = 9) or placebo (n = 10). Participants performed cycle training for 3-weeks at 50-70% of maximal oxygen uptake (VO2max). VO2max and an in vivo assessment of mitochondrial oxidative capacity were assessed before and after training. Mitochondrial oxidative capacity was assessed via post-exercise recovery kinetics of muscle oxygen consumption using near infrared spectroscopy (NIRS) in the vastus lateralis (VL) muscle. RESULTS: Exercise training increased VO2max (baseline: 40.1 ± 4.8 ml/kg/min; after training: 42.8 ± 3.9 ml/kg/min, main effect: p < 0.001), but no differences were detected between treatment groups (training × treatment: p = 0.29). Similarly, exercise training increased mitochondrial oxidative capacity rate constants (baseline: 1.8 ± 0.3 min-1; after training: 2.2 ± 0.5 min-1, main effect: p < 0.002), with no differences between treatment groups (training × treatment: p = 0.39). CONCLUSIONS: Mitochondria-targeted antioxidant supplementation does not alter mitochondrial adaptations to endurance exercise training. Further, our data indicate that three weeks of cycle training enhances mitochondrial oxidative capacity in the VL as measured by NIRS.

Sedentary Behavior Inversely Relates to Insulin Sensitivity and Mitochondrial Oxidative Capacity in Inactive Obese Adults

Sedentary time (ST) is associated with increased risk for insulin resistance, metabolic syndrome, and type 2 diabetes. The impact of ST on insulin sensitivity and muscle mitochondrial oxidative capacity, key mediators of the aforementioned metabolic diseases, is not fully understood. PURPOSE: We sought to determine the impact of ST on insulin sensitivity and muscle oxidative capacity in obese, physically inactive adults. METHODS: We studied 19 inactive, obese (35+1 kg/m2), middle aged (54+1 y) adults. We measured insulin sensitivity using a standardized two-stage hyperinsulinemic-eugIycemic clamp. In isolated vastus lateralis muscle mitochondria, we measured state-3 oxygen flux rates (JO2) by high-resolution respirometry using substrates for complex I, I+II, and II. Finally, we measured ST (<100 counts per min) and time spent in moderate to vigorous physical activity (MVPA, >2020 counts per min) using accelerometry. We used multivariate regression analysis to test for associations between ST and insulin sensitivity and muscle oxidative capacity adjusted for age and MVPA. RESULTS: Data are presented as mean + SEM. The average steady-state glucose infusion rate to maintain euglycemia during the high-dose insulin infusion was 8.4+1 mg/kgFFM/min. The average JO2 were 232 + 16 pmols/s/ml (complex I) and 307+16 pmols/s/ml (complex II). The average wear-time adjusted times spent in ST and MVPA were 570+30 min/d (9.5+0.5 h/d) and 20.4+0.5 min/d. The partial correlations between ST and the average glucose infusion rate and JO2 rates adjusted for age and MVPA were -0.62 (p=0.007), -0.51 (p=0.027, for complex I), and -0.32 (p=0.21, for complex II). CONCLUSION: ST inversely correlates with insulin sensitivity and complex I mediated muscle oxidative capacity in inactive, middle-aged, obese adults when adjusted for age and MVPA. We propose that reducing ST will improve insulin sensitivity and muscle oxidative capacity.

The Female mdx Mouse: An Unexpected Vascular Story

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disease, caused by absence of functional dystrophin and inevitably leading to death. A variable proportion of women carriers (2.5 to 19%) can also manifest symptoms ranging from myalgia to cardiomyopathy, and pathophysiological mechanisms are still not completely understood. Our study focused on 12 month-old female mdx mice, displaying marked chronic muscle lesions, similar to the lesions observed in human DMD. Our aim was to focus on the alterations of the vascular network organisation, and functional repercussions using a combination of histology/morphometry techniques and totally non-invasive functional approach (multiparametric and functional nuclear magnetic resonance), clearly relevant for clinical diagnosis and research, combining arterial spin labeling imaging of perfusion, and 31P-spectroscopy of phosphocreatine kinetics. Collectively, our results demonstrate that the vasculature, both in its steady state organisation and dynamic behaviour after an ischemia-reperfusion stress, is altered in the 12 month-old female mdx mouse: increased density of vascular sections in histology, modification of the post-ischemic hyperemia profile, increase in mitochondrial oxidative rephosphorylation capacity, in striking opposition to what was observed in age-matched male mdx mice. We believe the apparent discordance between vascular and muscular features in the female mdx mouse make it an interesting tool to decipher further dystrophinopathy pathophysiological mechanisms.