Joris Hoeks

Lower intrinsic ADP-stimulated mitochondrial respiration underlies in vivo mitochondrial dysfunction in muscle of male type 2 diabetic patients

OBJECTIVE—A lower in vivo mitochondrial function has been reported in both type 2 diabetic patients and first-degree relatives of type 2 diabetic patients. The nature of this reduction is unknown. Here, we tested the hypothesis that a lower intrinsic mitochondrial respiratory capacity may underlie lower in vivo mitochondrial function observed in diabetic patients. RESEARCH DESIGN AND METHODS—Ten overweight diabetic patients, 12 first-degree relatives, and 16 control subjects, all men, matched for age and BMI, participated in this study. Insulin sensitivity was measured with a hyperinsulinemic-euglycemic clamp. Ex vivo intrinsic mitochondrial respiratory capacity was determined in permeabilized skinned muscle fibers using high-resolution respirometry and normalized for mitochondrial content. In vivo mitochondrial function was determined by measuring phosphocreatine recovery half-time after exercise using 31P-magnetic resonance spectroscopy. RESULTS—Insulin-stimulated glucose disposal was lower in diabetic patients compared with control subjects (11.2 ± 2.8 vs. 28.9 ± 3.7 μmol · kg−1 fat-free mass · min−1, respectively; P = 0.003), with intermediate values for first-degree relatives (22.1 ± 3.4 μmol · kg−1 fat-free mass · min−1). In vivo mitochondrial function was 25% lower in diabetic patients (P = 0.034) and 23% lower in first-degree relatives, but the latter did not reach statistical significance (P = 0.08). Interestingly, ADP-stimulated basal respiration was 35% lower in diabetic patients (P = 0.031), and fluoro-carbonyl cyanide phenylhydrazone–driven maximal mitochondrial respiratory capacity was 31% lower in diabetic patients (P = 0.05) compared with control subjects with intermediate values for first-degree relatives. CONCLUSIONS—A reduced basal ADP-stimulated and maximal mitochondrial respiratory capacity underlies the reduction in in vivo mitochondrial function, independent of mitochondrial content. A reduced capacity at both the level of the electron transport chain and phosphorylation system underlies this impaired mitochondrial capacity.

Mitochondrial dysfunction and lipotoxicity.

Mitochondrial dysfunction in skeletal muscle has been suggested to underlie the development of insulin resistance and type 2 diabetes mellitus. Reduced mitochondrial capacity will contribute to the accumulation of lipid intermediates, desensitizing insulin signaling and leading to insulin resistance. Why mitochondrial function is reduced in the (pre-)diabetic state is, however, so far unknown. Although it is tempting to suggest that skeletal muscle insulin resistance may result from an inherited or acquired reduction in mitochondrial function in the pre-diabetic state, it cannot be excluded that mitochondrial dysfunction may in fact be the consequence of the insulin-resistant/diabetic state. Lipotoxicity, the deleterious effects of accumulating fatty acids in skeletal muscle cells, may lie at the basis of mitochondrial dysfunction: next to producing energy, mitochondria are also the major source of reactive oxygen species (ROS). Fatty acids accumulating in the vicinity of mitochondria are vulnerable to ROS-induced lipid peroxidation. Subsequently, these lipid peroxides could have lipotoxic effects on mtDNA, RNA and proteins of the mitochondrial machinery, leading to mitochondrial dysfunction. Indeed, increased lipid peroxidation has been reported in insulin resistant skeletal muscle and the mitochondrial uncoupling protein-3, which has been suggested to prevent lipid-induced mitochondrial damage, is reduced in subjects with an impaired glucose tolerance and in type 2 diabetic patients. These findings support the hypothesis that fat accumulation in skeletal muscle may precede the reduction in mitochondrial function that is observed in type 2 diabetes mellitus.

Short-term cold acclimation improves insulin sensitivity in patients with type 2 diabetes mellitus

Cold exposure may be a potential therapy for diabetes by increasing brown adipose tissue (BAT) mass and activity. Here we report that 10 d of cold acclimation (14–15 °C) increased peripheral insulin sensitivity by ∼43% in eight type 2 diabetes subjects. Basal skeletal muscle GLUT4 translocation markedly increased, without effects on insulin signaling or AMP-activated protein kinase (AMPK) activation and only a minor increase in BAT glucose uptake.

Increase in Brown Adipose Tissue Activity after Weight Loss in Morbidly Obese Subjects

CONTEXT Stimulation of thermogenesis in brown adipose tissue (BAT) is a potential target to treat obesity. We earlier demonstrated that BAT activity is relatively low in obese subjects. It is unknown whether BAT can be recruited in adult humans. OBJECTIVE To study the dynamics of BAT, we observed BAT activity in morbidly obese subjects before and after weight loss induced by bariatric surgery. DESIGN This was an observational prospective cohort study. SETTING The study was conducted at a referral center. PATIENTS Ten morbidly obese subjects eligible for laparoscopic adjustable gastric banding surgery were studied before and 1 yr after bariatric surgery. MAIN OUTCOME MEASURE The main outcome measure was BAT activity, as determined after acute cold stimulation using (18)F-fluorodeoxyglucose positron emission tomography and computed tomography. RESULTS Before surgery, only two of 10 subjects showed active BAT. One year after surgery, the number of subjects with active BAT was increased to five. After weight loss, BAT-positive subjects had significantly higher nonshivering thermogenesis compared with BAT-negative subjects (P < 0.05). CONCLUSIONS The results show that in humans BAT can be recruited in the regions in which it was also reported in lean subjects before. These results for the first time show recruitment of BAT in humans and may open the door for BAT-targeted treatments of obesity.

The hypoxia-inducible microRNA cluster miR-199a∼214 targets myocardial PPARδ and impairs mitochondrial fatty acid oxidation.

Peroxisome proliferator-activated receptor δ (PPARδ) is a critical regulator of energy metabolism in the heart. Here, we propose a mechanism that integrates two deleterious characteristics of heart failure, hypoxia and a metabolic shift toward glycolysis, involving the microRNA cluster miR-199a∼214 and PPARδ. We demonstrate that under hemodynamic stress, cardiac hypoxia activates DNM3os, a noncoding transcript that harbors the microRNA cluster miR-199a∼214, which shares PPARδ as common target. To address the significance of miR-199a∼214 induction and concomitant PPARδ repression, we performed antagomir-based silencing of both microRNAs and subjected mice to biomechanical stress to induce heart failure. Remarkably, antagomir-treated animals displayed improved cardiac function and restored mitochondrial fatty acid oxidation. Taken together, our data suggest a mechanism whereby miR-199a∼214 actively represses cardiac PPARδ expression, facilitating a metabolic shift from predominant reliance on fatty acid utilization in the healthy myocardium toward increased reliance on glucose metabolism at the onset of heart failure.

Prolonged Fasting Identifies Skeletal Muscle Mitochondrial Dysfunction as Consequence Rather Than Cause of Human Insulin Resistance

OBJECTIVE Type 2 diabetes and insulin resistance have been associated with mitochondrial dysfunction, but it is debated whether this is a primary factor in the pathogenesis of the disease. To test the concept that mitochondrial dysfunction is secondary to the development of insulin resistance, we employed the unique model of prolonged fasting in humans. Prolonged fasting is a physiologic condition in which muscular insulin resistance develops in the presence of increased free fatty acid (FFA) levels, increased fat oxidation and low glucose and insulin levels. It is therefore anticipated that skeletal muscle mitochondrial function is maintained to accommodate increased fat oxidation unless factors secondary to insulin resistance exert negative effects on mitochondrial function. RESEARCH DESIGN AND METHODS While in a respiration chamber, twelve healthy males were subjected to a 60 h fast and a 60 h normal fed condition in a randomized crossover design. Afterward, insulin sensitivity was assessed using a hyperinsulinemic-euglycemic clamp, and mitochondrial function was quantified ex vivo in permeabilized muscle fibers using high-resolution respirometry. RESULTS Indeed, FFA levels were increased approximately ninefold after 60 h of fasting in healthy male subjects, leading to elevated intramuscular lipid levels and decreased muscular insulin sensitivity. Despite an increase in whole-body fat oxidation, we observed an overall reduction in both coupled state 3 respiration and maximally uncoupled respiration in permeabilized skeletal muscle fibers, which could not be explained by changes in mitochondrial density. CONCLUSIONS These findings confirm that the insulin-resistant state has secondary negative effects on mitochondrial function. Given the low insulin and glucose levels after prolonged fasting, hyperglycemia and insulin action per se can be excluded as underlying mechanisms, pointing toward elevated plasma FFA and/or intramuscular fat accumulation as possible causes for the observed reduction in mitochondrial capacity.

Short-term Cold Acclimation Recruits Brown Adipose Tissue in Obese Humans

Recruitment of brown adipose tissue (BAT) has emerged as a potential tool to combat obesity and associated metabolic complications. Short-term cold acclimation has been shown not only to enhance the presence and activity of BAT in lean humans but also to improve the metabolic profile of skeletal muscle to benefit glucose uptake in patients with type 2 diabetes. Here we examined whether short-term cold acclimation also induced such adaptations in 10 metabolically healthy obese male subjects. A 10-day cold acclimation period resulted in increased cold-induced glucose uptake in BAT, as assessed by [18F]fluorodeoxyglucose positron emission tomography/computed tomography. BAT activity was negatively related to age, with a similar trend for body fat percentage. In addition, cold-induced glucose uptake in BAT was positively related to glucose uptake in visceral white adipose tissue, although glucose uptake in visceral and subcutaneous white adipose tissue depots was unchanged upon cold acclimation. Cold-induced skeletal muscle glucose uptake tended to increase upon cold acclimation, which was paralleled by increased basal GLUT4 localization in the sarcolemma, as assessed through muscle biopsies. Proximal skin temperature was increased and subjective responses to cold were slightly improved at the end of the acclimation period. These metabolic adaptations to prolonged exposure to mild cold may lead to improved glucose metabolism or prevent the development of obesity-associated insulin resistance and hyperglycemia.

UCP1 and defense against oxidative stress : 4-hydroxy-2-nonenal effects on brown fat mitochondria are uncoupling protein 1-independent

Uncoupling proteins have been ascribed a role in defense against oxidative stress, particularly by being activated by products of oxidative stress such as 4-hydroxy-2-nonenal (HNE). We have investigated here the ability of HNE to activate UCP1. Using brown fat mitochondria from UCP1+/+ and UCP1–/– mice to allow for identification of UCP1-dependent effects, we found that HNE could neither (re)activate purine nucleotide-inhibited UCP1, nor induce additional activation of innately active UCP1. The aldehyde nonenal had a (re)activating effect only if converted to the corresponding fatty acid by aldehyde dehydrogenase; the presence of a carboxyl group was thus an absolute requirement for (re)activation. The UCP1-dependent proton leak was not increased by HNE but HNE changed basal proton leak characteristics in a UCP1-independent manner. In agreement with the in vitro results, we found, as compared with UCP1+/+ mice, no increase in HNE/protein adducts in brown fat mitochondria isolated from UCP1–/– mice, irrespective of whether they were adapted to thermoneutral temperature (30 °C) or to the cold (4 °C). The absence of oxidative damage in UCP1–/– mitochondria was not due to enhanced activity of antioxidant enzymes. Thus, HNE did not affect UCP1 activity, and UCP1 would appear not to be physiologically involved in defense against oxidative stress. Additionally, it was concluded that at least in brown adipose tissue, conditions of high mitochondrial membrane potential, high oxygen tension, and high substrate supply do not necessarily lead to increased oxidative damage.

Low brown adipose tissue activity in endurance-trained compared with lean sedentary men

Background/objectives:It has now been unequivocally demonstrated that humans possess functional brown adipose tissue (BAT) and that human BAT can be recruited upon chronic cold stimulation. Recruitment of BAT has been postulated as a potential strategy to counteract the current global obesity epidemic. Recently, it was shown in rodents that endurance exercise training could stimulate the recruitment of brown-like adipocytes within white adipose tissue (WAT) via exercise-induced myokines such as irisin (the cleaved circulating product of the type 1 membrane protein FNDC5) and interleukin-6 (IL-6). Our objective was to test whether endurance-trained athletes had increased cold-stimulated BAT activity and browning of subcutaneous WAT compared with lean sedentary males.Subjects/methods:Twelve endurance-trained athletes and 12 lean sedentary males were measured during 2 h of mild cold exposure to determine cold-induced BAT activity via [18F]fluorodeoxyglucose-positron emission tomography-computed tomography ([18F]FDG-PET-CT) scanning. Skeletal muscle FNDC5 expression, as well as plasma irisin and IL-6 levels were determined. In addition, a subcutaneous abdominal WAT biopsy was taken to measure gene expression of several markers for browning of WAT.Results:Cold-induced BAT activity was significantly lower in athletes, and no differences in gene expression of classical brown and beige adipocyte markers were detected in subcutaneous WAT between the groups. As expected, mRNA expression of FNDC5 in skeletal muscle was significantly higher in endurance athletes but plasma irisin and Il-6 levels were similar in both groups.Conclusions:These results indicate that chronic endurance exercise is not associated with brown and beige adipocyte recruitment; in fact endurance training appears to be linked to lower the metabolic activity of BAT in humans.

Muscle mitochondria and insulin resistance: a human perspective

Reduced mitochondrial capacity in skeletal muscle has been suggested to underlie the development of insulin resistance and type 2 diabetes mellitus (T2DM). However, data obtained from human subjects concerning this putative relation indicate that the mitochondrial defect observed in diabetic muscle might be secondary to the insulin-resistant state instead of being a causal factor. Nonetheless, diminished mitochondrial function, even secondary to insulin resistance, may accelerate lipid deposition in non-adipose tissues and aggravate insulin resistance. Indeed, improving mitochondrial capacity via exercise training and calorie restriction is associated with positive metabolic health effects. Here we review muscle mitochondrial dysfunction in humans and propose that targeting muscle mitochondria to improve muscle oxidative capacity should be considered as a strategy for improving metabolic health.