Is skeletal muscle mitochondrial dysfunction a cause or an indirect consequence of insulin resistance in humans?

Insulin sensitivity and skeletal muscle mitochondrial respiration in Black and White women with obesity

OBJECTIVES

Non-Hispanic black women (BW) have a greater risk of type 2 diabetes (T2D) and insulin resistance (IR) compared to non-Hispanic white women (WW). The mechanisms leading to these differences are not understood, and it is unclear whether synergistic effects of race and obesity impact disease risk. To understand the interaction of race and weight, hepatic and peripheral IR were compared in WW and BW with and without obesity.

METHODS

Hepatic and peripheral IR was measured by a labeled, hyperinsulinemic-euglycemic clamp in BW (n=32) and WW (n=32) with and without obesity. Measurements of body composition, cardiorespiratory fitness, and skeletal muscle (SM) respiration were completed. Data were analyzed by mixed model ANOVA.

RESULTS

Subjects with obesity had greater hepatic and peripheral IR and lower SM respiration (P<0.001). Despite 14% greater insulin (P=0.066), BW tended to have lower peripheral glucose disposal (Rd; P=0.062), which was driven by women without obesity (P=0.002). BW had significantly lower glucose production (P=0.005), hepatic IR (P=0.024), and maximal coupled and uncoupled respiration (P<0.001) than WW. Maximal coupled and uncoupled SM mitochondrial respiration was strongly correlated with peripheral and hepatic IR (P<0.01).

CONCLUSION

While BW without obesity had lower Rd than WW, race and obesity did not synergistically impact peripheral IR. Paradoxically, WW with obesity had greater hepatic IR compared to BW. Relationships between SM respiration and IR persisted across a range of body weight. These data provide support for therapies in BW, like exercise, that improve SM mitochondrial respiration to reduce IR and T2D risk.

Creatine Kinase Equilibration and ΔGATP over an Extended Range of Physiological Conditions: Implications for Cellular Energetics, Signaling, and Muscle Performance

In this report, we establish a straightforward method for estimating the equilibrium constant for the creatine kinase reaction (CK Keq″) over wide but physiologically and experimentally relevant ranges of pH, Mg2+ and temperature. Our empirical formula for CK Keq″ is based on experimental measurements. It can be used to estimate [ADP] when [ADP] is below the resolution of experimental measurements, a typical situation because [ADP] is on the order of micromolar concentrations in living cells and may be much lower in many in vitro experiments. Accurate prediction of [ADP] is essential for in vivo studies of cellular energetics and metabolism and for in vitro studies of ATP-dependent enzyme function under near-physiological conditions. With [ADP], we were able to obtain improved estimates of ΔGATP, necessitating the reinvestigation of previously reported ADP- and ΔGATP-dependent processes. Application to actomyosin force generation in muscle provides support for the hypothesis that, when [Pi] varies and pH is not altered, the maximum Ca2+-activated isometric force depends on ΔGATP in both living and permeabilized muscle preparations. Further analysis of the pH studies introduces a novel hypothesis around the role of submicromolar ADP in force generation.

Contribution of skeletal muscle to cancer immunotherapy: A focus on muscle function, inflammation, and microbiota

Sarcopenia, characterized by degenerative and systemic loss of skeletal muscle mass and function, is a multifactorial syndrome commonly observed in individuals with cancer. Additionally, it represents a poor nutritional status and indicates possible presence of cancer cachexia. Recently, with the extensive application of cancer immunotherapy, the effects of sarcopenia/cachexia on cancer immunotherapy, have gained attention. The aim of this review was to summarize the influence of low muscle mass (sarcopenia/cachexia) on the response and immune-related adverse events to immunotherapy from the latest literature. It was revealed that low muscle mass (sarcopenia/cachexia) has detrimental effects on cancer immunotherapy in most cases, although there were results that were not consistent with this finding. This review also discussed potential causes of the paradox, such as different measure methods, research types, muscle indicators, time point, and cancer type. Mechanically, chronic inflammation, immune cells, and microbiota may be critically involved in regulating the efficacy of immunotherapy under the condition of low muscle mass (sarcopenia/cachexia). Thus, nutritional interventions will likely be promising ways for individuals with cancer to increase the efficacy of immunotherapy in the future, for low muscle mass (sarcopenia/cachexia) is an important prognostic factor for cancer immunotherapy.

Bile Acids Induce Alterations in Mitochondrial Function in Skeletal Muscle Fibers

Cholestatic chronic liver disease is characterized by developing sarcopenia and elevated serum levels of bile acids. Sarcopenia is a skeletal muscle disorder with the hallmarks of muscle weakness, muscle mass loss, and muscle strength decline. Our previous report demonstrated that deoxycholic acid (DCA) and cholic acid (CA), through the membrane receptor TGR5, induce a sarcopenia-like phenotype in myotubes and muscle fibers. The present study aimed to evaluate the impact of DCA and CA on mitochondrial mass and function in muscle fibers and the role of the TGR5 receptor. To this end, muscle fibers obtained from wild-type and TGR5^−/− mice were incubated with DCA and CA. Our results indicated that DCA and CA decreased mitochondrial mass, DNA, and potential in a TGR5-dependent fashion. Furthermore, with TGR5 participation, DCA and CA also reduced the oxygen consumption rate and complexes I and II from the mitochondrial electron transport chain. In addition, DCA and CA generated more mitochondrial reactive oxygen species than the control, which were abolished in TGR5^−/− mice muscle fibers. Our results indicate that DCA and CA induce mitochondrial dysfunction in muscle fibers through a TGR5-dependent mechanism.

Kearns–Sayre Syndrome Minus: Two Cases of Identical Large-Scale Mitochondrial DNA Deletions with Presentations outside the Classical Triad

A curious triad of retinitis pigmentosa, external ophthalmoplegia, and complete heart block was presented by Sayre et al. in 1958. Since then, the disorder named Kearns–Sayre syndrome (KSS) has come to represent patients with mitochondrial DNA deletions presenting before adulthood, primarily with chronic progressive external ophthalmoplegia (CPEO) and pigmentary retinopathy. However, it is increasingly noted that the presentations can well be variable despite similar genetic deletions. Here, we present two cases with identical large-scale mitochondrial DNA deletions but very dissimilar outlook.

Leptin receptor defect with diabetes causes skeletal muscle atrophy in female obese Zucker rats where peculiar depots networked with mitochondrial damages

Tibialis anterior muscles of 45-week-old female obese Zucker rats with defective leptin receptor and non-insulin dependent diabetes mellitus (NIDDM) showed a significative atrophy compared to lean muscles, based on histochemical-stained section's measurements in the sequence: oxidative slow twitch (SO, type I) < oxidative fast twitch (FOG, type IIa) < fast glycolytic (FG, type IIb). Both oxidative fiber's outskirts resembled 'ragged' fibers and, in these zones, ultrastructure revealed small clusters of endoplasm-like reticulum filled with unidentified electron contrasted compounds, contiguous and continuous with adjacent mitochondria envelope. The linings appeared crenated stabbed by circular patterns resembling those found of ceramides. The same fibers contained scattered degraded mitochondria that tethered electron contrasted droplets favoring larger depots while mitoptosis were widespread in FG fibers. Based on other interdisciplinary investigations on the lipid depots of diabetes 2 muscles made us to propose these accumulated contrasted contents to be made of peculiar lipids, including acyl-ceramides, as those were only found while diabetes 2 progresses in aging obese rats. These could interfere in NIDDM with mitochondrial oxidative energetic demands and muscle functions.

Pathophysiological mechanisms explaining poor clinical outcome of older cancer patients with low skeletal muscle mass

Low skeletal muscle mass is highly prevalent in older cancer patients and affects 5% to 89% depending on the type and stage of cancer. Low skeletal muscle mass is associated with poor clinical outcomes such as post-operative complications, chemotherapy toxicity and mortality in older cancer patients. Little is known about the mediating pathophysiological mechanisms. In this review, we summarize proposed pathophysiological mechanisms underlying the association between low skeletal muscle mass and poor clinical outcomes in older cancer patients including a) systemic inflammation; b) insulin-dependent glucose handling; c) mitochondrial function; d) protein status and; e) pharmacokinetics of anticancer drugs. The mechanisms of altered myokine balance negatively affecting the innate and adaptive immune system, and altered pharmacokinetics of anticancer drugs leading to a relative overdosage of anticancer drugs are best-substantiated. The effects of glucose intolerance and circulating mitochondrial DNA as a consequence of low skeletal muscle mass are topics of interest for future research. Restoring myokine balance through physical exercise, exercise mimetics, neuro-muscular activation and adapting anticancer drug dosing on skeletal muscle mass could be targeted approaches to improve clinical outcomes in older cancer patients with low skeletal muscle mass.

Differences in Muscle O2 Dynamics During Treadmill Exercise Between Aerobic Capacity-Matched Overweight and Normal-Weight Adults

The aim of this study was to compare muscle O₂ dynamics during exercise between aerobic capacity-matched overweight and normal-weight adults. Overweight women (OW, n = 9) and normal-weight women (NW, n = 14) performed graded treadmill exercise until exhaustion. Muscle O₂ saturation (SmO₂) and relative changes from rest in deoxygenated hemoglobin concentration (∆deoxy-Hb) and total hemoglobin concentration (∆total-Hb) were monitored continuously at gastrocnemius medialis muscle by near infrared spatial resolved spectroscopy. Significantly higher SmO₂ and lower ∆deoxy-Hb and ∆total-Hb were observed in OW compared with NW. Pulmonary O₂ uptake (VO₂) normalized by fat-free mass was matched between groups. In both groups, peak VO₂ was significantly correlated with change in SmO₂ and ∆deoxy-Hb. Our findings suggest that both muscle blood volume and deoxygenation were lower in overweight adults, compared to aerobic capacity-matched normal-weight adults. Moreover, lowered muscle O₂ extraction was related to peak VO₂ in overweight adults, as well as in normal-weight adults.

Astaxanthin: A Potential Mitochondrial-Targeted Antioxidant Treatment in Diseases and with Aging

Oxidative stress is characterized by an imbalance between prooxidant and antioxidant species, leading to macromolecular damage and disruption of redox signaling and cellular control. It is a hallmark of various diseases including metabolic syndrome, chronic fatigue syndrome, neurodegenerative, cardiovascular, inflammatory, and age-related diseases. Several mitochondrial defects have been considered to contribute to the development of oxidative stress and known as the major mediators of the aging process and subsequent age-associated diseases. Thus, mitochondrial-targeted antioxidants should prevent or slow down these processes and prolong longevity. This is the reason why antioxidant treatments are extensively studied and newer and newer compounds with such an effect appear. Astaxanthin, a xanthophyll carotenoid, is the most abundant carotenoid in marine organisms and is one of the most powerful natural compounds with remarkable antioxidant activity. Here, we summarize its antioxidant targets, effects, and benefits in diseases and with aging.

Mitochondrial Dysfunction in Skeletal Muscle Pathologies

Several molecular mechanisms are involved in the regulation of skeletal muscle function. Among them, mitochondrial activity can be identified. The mitochondria is an important and essential organelle in the skeletal muscle that is involved in metabolic regulation and ATP production, which are two key elements of muscle contractibility and plasticity. Thus, in this review, we present the critical and recent antecedents regarding the mechanisms through which mitochondrial dysfunction can be involved in the generation and development of skeletal muscle pathologies, its contribution to detrimental functioning in skeletal muscle and its crosstalk with other typical signaling pathways related to muscle diseases. In addition, an update on the development of new strategies with therapeutic potential to inhibit the deleterious impact of mitochondrial dysfunction in skeletal muscle is discussed.

A preliminary transcriptome analysis suggests a transitory effect of vitamin D on mitochondrial function in obese young Finnish subjects

OBJECTIVE

The effect of vitamin D at the transcriptome level is poorly understood, and furthermore, it is unclear if it differs between obese and normal-weight subjects. The objective of the study was to explore the transcriptome effects of vitamin D supplementation.

DESIGN AND METHODS

We analysed peripheral blood gene expression using GlobinLock oligonucleotides followed by RNA sequencing in individuals participating in a 12-week randomised double-blinded placebo-controlled vitamin D intervention study. The study involved 18 obese and 18 normal-weight subjects (of which 20 males) with mean (±s.d.) age 20.4 (±2.5) years and BMIs 36 (±10) and 23 (±4) kg/m2, respectively. The supplemental daily vitamin D dose was 50 µg (2000 IU). Data were available at baseline, 6- and 12-week time points and comparisons were performed between the vitamin D and placebo groups separately in obese and normal-weight subjects.

RESULTS

Significant transcriptomic changes were observed at 6 weeks, and only in the obese subjects: 1724 genes were significantly upregulated and 186 genes were downregulated in the vitamin D group compared with placebo. Further analyses showed several enriched gene categories connected to mitochondrial function and metabolism, and the most significantly enriched pathway was related to oxidative phosphorylation (adjusted P value 3.08 × 10-14). Taken together, our data suggest an effect of vitamin D supplementation on mitochondrial function in obese subjects.

CONCLUSIONS

Vitamin D supplementation affects gene expression in obese, but not in normal-weight subjects. The altered genes are enriched in pathways related to mitochondrial function. The present study increases the understanding of the effects of vitamin D at the transcriptome level.

20-Week follow-up of hepatic steatosis installation and liver mitochondrial structure and activity and their interrelation in rats fed a high-fat-high-fructose diet

The incidence of obesity and its metabolic complications are rapidly increasing and become a major public health issue. This trend is associated with an increase in the prevalence of non-alcoholic fatty liver disease (NAFLD), insulin resistance and diabetes. The sequence of events leading to NAFLD progression and mitochondrial dysfunction and their interrelation remains to be elucidated. This study aimed to explore the installation and progression of NAFLD and its association with the liver mitochondrial structure and activity changes in rats fed an obesogenic diet up to 20 weeks. Male Wistar rats were fed either a standard or high-fat-high-fructose (HFHFR) diet and killed on 4, 8, 12, 16 and 20 weeks of diet intake. Rats fed the HFHFR diet developed mildly overweight, associated with increased adipose tissue weight, hepatic steatosis, hyperglycaemia and hyperinsulinaemia after 8 weeks of HFHFR diet. Hepatic steatosis and many biochemical modifications plateaued at 8-12 weeks of HFHFR diet with slight amelioration afterwards. Interestingly, several biochemical and physiological parameters of mitochondrial function, as well as its phospholipid composition, in particular cardiolipin content, were tightly related to hepatic steatosis installation. These results showed once again the interrelation between hepatic steatosis development and mitochondrial activity alterations without being able to say whether the mitochondrial alterations preceded or followed the installation/progression of hepatic steatosis. Because both hepatic steatosis and mitochondrial alterations occurred as early as 4 weeks of diet, future studies should consider these four 1st weeks to reveal the exact interconnection between these major consequences of obesogenic diet intake.

Impact of Endurance Exercise Training in the Fasted State on Muscle Biochemistry and Metabolism in Healthy Subjects: Can These Effects be of Particular Clinical Benefit to Type 2 Diabetes Mellitus and Insulin-Resistant Patients?

Exercise training intervention is a cornerstone in the care of type 2 diabetes mellitus (T2DM) and insulin resistance (IR), and it is pursued in order to optimize exercise interventions for these patients. In this regard, the nutritional state of patients during exercise (being in the fed or fasted state) can be of particular interest. The aim of the present review is to describe the impact of endurance exercise (training) in the fasted versus fed state on parameters of muscle biochemistry and metabolism linked to glycemic control or insulin sensitivity in healthy subjects. From these data it can then be deduced whether exercise training in the fasted state may be relevant to patients with T2DM or IR. In healthy subjects, acute endurance exercise in the fasted state is accompanied by lower blood insulin and elevated blood free fatty acid concentrations, stable blood glucose concentrations (in the first 60-90 min), superior intramyocellular triacylglycerol oxidation and whole-body lipolysis, and muscle glycogen preservation. Long-term exercise training in the fasted state in healthy subjects is associated with greater improvements in insulin sensitivity, basal muscle fat uptake capacity, and oxidation. Therefore, promising results of exercise (training) in the fasted state have been found in healthy subjects on parameters of muscle biochemistry and metabolism linked to insulin sensitivity and glycemic control. Whether exercise training intervention in which exercise sessions are organized in the fasted state may be more effective in improving insulin sensitivity or glycemic control in T2DM patients and insulin-resistant individuals warrants investigation.

Pravastatin improves risk factors but not ischaemic tolerance in obese rats

Statins are effective in management of dyslipidaemia, and a cornerstone of CVD prevention strategies. However, the impacts of their pleiotropic effects on other cardiovascular risk factors and myocardial responses to infarction are not well characterised. We hypothesised that pravastatin treatment in obesity improves lipid profiles, insulin-resistance and myocardial resistance to ischaemia/reperfusion (I/R) injury. Wistar rats were fed a control (C) chow or high carbohydrate and fat diet (HCFD) for 16 weeks with vehicle or pravastatin (prava 7.5 mg/kg/day) treatment for 8 weeks. At 16 weeks HOMAs were performed, blood samples collected and hearts excised for Langendorff perfusions/biochemical analyses. Anti-oxidant activity and proteins regulating mitochondrial fission/fusion and apoptosis were assessed. The HCFD increased body weight (736±15 vs. 655±12 g for C; P<0.001), serum triglycerides (2.91±0.52 vs. 1.64±0.26 mmol/L for C; P<0.001) and insulin-resistance (HOMA- 6.9±0.8 vs. 4.2±0.5 for C; P<0.05) while prava prevented diet induced changes and paradoxically increased lipid peroxidation. The HCFD increased infarct size (34.1±3.1% vs. 18.8±3.0% of AAR for C; P<0.05), which was unchanged by prava in C and HCFD animals. The HCFD decreased cardiac TxR activity and mitochondrial MFN-1 and increased mitochondrial DRP-1 (reducing MFN-1:DRP-1 ratio) and Bax expression, with the latter changes prevented by prava. While unaltered by diet, cytosolic levels of Bax and caspase-3 were reduced by prava in C and HCFD hearts (without changes in cleaved caspase-3). We conclude that obesity, hyper-triglyceridemia and impaired glycemic control in HCFD rats are countered by prava. Despite improved risk factors, prava did not reduce myocardial infarct size, potentially reflecting its complex pleiotropic impacts on cardiac GPX activity and MFN-1, DRP-1, caspase-3 and Bcl-2 proteins.

Implication of SPARC in the modulation of the extracellular matrix and mitochondrial function in muscle cells

Secreted protein, acidic and rich in cysteine (SPARC) is differentially associated with cell proliferation and extracellular matrix (ECM) assembly. We show here the effect of exogenous SPARC inhibition/induction on ECM and mitochondrial proteins expression and on the differentiation of C2C12 cells. The cells were cultured in growth medium (GM) supplemented with different experimental conditions. The differentiation of myoblasts was studied for 5 days, the expressions of ECM and mitochondrial proteins were measured and the formation of the myotubes was quantified after exogenous induction/inhibition of SPARC. The results indicate that the addition of recombinant SPARC protein (rSPARC) in cell culture medium increased the differentiation of C2C12 myoblasts and myogenin expression during the myotube formation. However, the treatment with antibody specific for SPARC (anti-SPARC) prevented the differentiation and decreased myogenin expression. The induction of SPARC in the proliferating and differentiating C2C12 cells increased collagen 1a1 protein expression, whereas the inhibition decreased it. The effects on fibronectin protein expression were opposite. Furthermore, the addition of rSPARC in C2C12 myoblast increased the expression of mitochondrial proteins, ubiquinol-cytochrome c reductase core protein II (UQCRC2) and succinate dehydrogenase iron-sulfur subunit (SDHB), whereas the anti-SPARC decreased them. During the differentiation, only the anti-SPARC had the effects on mitochondrial proteins, NADH dehydrogenase ubiquinone 1 beta subcomplex subunit 8 (NADHB8), SDHB and cytochrome c oxidase 1 (MTCO1). Thus, SPARC plays a crucial role in the proliferation and differentiation of C2C12 and may be involved in the link between the ECM remodeling and mitochondrial function.

Malaria Pathogenesis

In the mosquito-human life cycle, the six species of malaria parasites infecting humans (Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale wallickeri, Plasmodium ovale curtisi, Plasmodium malariae, and Plasmodium knowlesi) undergo 10 or more morphological states, replicate from single to 10,000+ cells, and vary in total population from one to many more than 10⁶ organisms. In the human host, only a small number of these morphological stages lead to clinical disease and the vast majority of all malaria-infected patients in the world produce few (if any) symptoms in the human. Human clinical disease (e.g., fever, anemia, coma) is the result of the parasite preprogrammed biology in concert with the human pathophysiological response. Caveats and corollaries that add variation to this host-parasite interaction include parasite genetic diversity of key proteins, coinfections, comorbidities, delays in treatment, human polymorphisms, and environmental determinants.

The risks of sarcopenia, falls and fractures in patients with type 2 diabetes mellitus

Fracture risk in patients with type 2 diabetes mellitus (T2DM) is increased, and the mechanism is multifactorial. Recent research on T2DM-induced bone fragility shows that bone mineral density (BMD) is often normal or even slightly elevated. However, bone turnover may be decreased and bone material and microstructural properties are altered, especially when microvascular complications are present. Besides bone fragility, extra-skeletal factors leading to an increased propensity to experience falls may also contribute to the increased fracture risk in T2DM, such as peripheral neuropathy, retinopathy and diabetes medication (e.g. insulin use). One of the probable additional contributing factors to the increased fall and fracture risks in T2DM is sarcopenia, the age-related decline in skeletal muscle mass, quality and function. Although the association between sarcopenia, fall risk, and fracture risk has been studied in the general population, few studies have examined the association between T2DM and muscle tissue and the risks of falls and fractures. This narrative review provides an overview of the literature regarding the multifactorial mechanisms leading to increased fracture risk in patients with T2DM, with a focus on sarcopenia and falls.

Growth differentiation factor 15 is a myomitokine governing systemic energy homeostasis

Reduced mitochondrial electron transport chain activity promotes longevity and improves energy homeostasis via cell-autonomous and -non-autonomous factors in multiple model systems. This mitohormetic effect is thought to involve the mitochondrial unfolded protein response (UPRmt), an adaptive stress-response pathway activated by mitochondrial proteotoxic stress. Using mice with skeletal muscle-specific deficiency of Crif1 (muscle-specific knockout [MKO]), an integral protein of the large mitoribosomal subunit (39S), we identified growth differentiation factor 15 (GDF15) as a UPRmt-associated cell-non-autonomous myomitokine that regulates systemic energy homeostasis. MKO mice were protected against obesity and sensitized to insulin, an effect associated with elevated GDF15 secretion after UPRmt activation. In ob/ob mice, administration of recombinant GDF15 decreased body weight and improved insulin sensitivity, which was attributed to elevated oxidative metabolism and lipid mobilization in the liver, muscle, and adipose tissue. Thus, GDF15 is a potent mitohormetic signal that safeguards against the onset of obesity and insulin resistance.

Skeletal muscle mitochondria as a target to prevent or treat type 2 diabetes mellitus

Low levels of physical activity and the presence of obesity are associated with mitochondrial dysfunction. In addition, mitochondrial dysfunction has been associated with the development of insulin resistance and type 2 diabetes mellitus (T2DM). Although the evidence for a causal relationship between mitochondrial function and insulin resistance is still weak, emerging evidence indicates that boosting mitochondrial function might be beneficial to patient health. Exercise training is probably the most recognized promoter of mitochondrial function and insulin sensitivity and hence is still regarded as the best strategy to prevent and treat T2DM. Animal data, however, have revealed several new insights into the regulation of mitochondrial metabolism, and novel targets for interventions to boost mitochondrial function have emerged. Importantly, many of these targets seem to be regulated by factors such as nutrition, ambient temperature and circadian rhythms, which provides a basis for nonpharmacological strategies to prevent or treat T2DM in humans. Here, we will review the current evidence that mitochondrial function can be targeted therapeutically to improve insulin sensitivity and to prevent T2DM, focusing mainly on human intervention studies.

Skeletal muscle insulin resistance in hamsters with diabetes developed from obesity is involved in abnormal skeletal muscle LXR, PPAR and SREBP expression

Diabetic ‘lipotoxicity’ theory suggests that fat-induced skeletal muscle insulin resistance (FISMIR) in obesity induced by a high-fat diet (HFD), which leads to ectopic lipid accumulation in insulin-sensitive tissues, may play a pivotal role in the pathogenesis of type 2 diabetes. However, the changes in gene expression and the molecular mechanisms associated with the pathogenesis of FISMIR have not yet been fully elucidated. In the present study the changes in skeletal muscle gene expression were examined in FISMIR in obese insulin-resistant and diabetic hamster models induced by HFD with or without low-dose streptozotocin-treatment. Microarray technology and reverse transcription-quantitative polymerase chain reaction (RT-qPCR) were used to explore the potential underlying molecular mechanisms. The pathophysiological and metabolic features of obesity and type 2 diabetes in humans are closely resembled by these hamster models. The results of microarray analysis showed that the differentially expressed genes associated with metabolism were mostly related to the abnormal regulation and changes in the gene expression of liver X receptor (LXR), peroxisome proliferator-activated receptor (PPAR) and sterol regulatory element-binding protein (SREBP) transcriptional programs in the skeletal muscle from insulin-resistant and diabetic hamsters. The microarray findings confirmed by RT-qPCR indicated that the increased expression of SREBPs and LXRβ and the decreased expression of LXRα and PPARs were involved in the molecular mechanisms of FISMIR pathogenesis in insulin-resistant and diabetic hamsters. A significant difference in the abnormal expression of skeletal muscle LXRs, PPARs and SREBPs was found between insulin-resistant and diabetic hamsters. It may be concluded that the combined abnormal expression of LXR, PPAR and SREBP transcriptional programs may contribute to the development of FISMIR mediated by skeletal muscle lipid accumulation resulting from abnormal skeletal muscle glucose and lipid metabolism in these HFD- and streptozotocin injection-induced insulin-resistant and diabetic hamsters.

Evaluation of acute effect of light-emitting diode (LED) phototherapy on muscle deoxygenation and pulmonary oxygen uptake kinetics in patients with diabetes mellitus: study protocol for a randomized controlled trial

BACKGROUND

Type 2 diabetes mellitus (DM) is responsible for a significant reduction in the quality of life due to its negative impact on functional capacity. Cardiopulmonary fitness impairment in DM patients has been associated with limited tissue oxygenation. Phototherapy is widely utilized to treat several disorders due to expected light-tissue interaction. This type of therapy may help to improve muscular oxygenation, thereby increasing aerobic fitness and functional capacity.

METHODS/DESIGN

This study is a randomized, double-blind, placebo-controlled crossover trial approved by the Ethics Committee of the Federal University of São Carlos and registered at ClinicalTrials.gov. Four separate tests will be performed to evaluate the acute effect of phototherapy. All participants will receive both interventions in random order: light-emitting diode therapy (LEDT) and placebo, with a minimum 14-day interval between sessions (washout period). Immediately after the intervention, participants will perform moderate constant workload cycling exercise corresponding to 80 % of the pulmonary oxygen uptake [Formula: see text] during the gas exchange threshold (GET). LEDT will be administered with a multidiode cluster probe (50 GaAIA LEDs, 850 ηm, 75 mW each diode, and 3 J per point) before each exercise session. Pulmonary oxygen uptake, muscle oxygenation, heart rate, and arterial pressure will be measured using a computerized metabolic cart, a near-infrared spectrometer, an electrocardiogram, and a photoplethysmography system, respectively.

DISCUSSION

The main objective of this study is to evaluate the acute effects of muscular pre-conditioning using LED phototherapy on pulmonary oxygen uptake, muscle oxygenation, heart rate, and arterial pressure dynamics during dynamic moderate exercise. We hypothesize that phototherapy may be beneficial to optimize aerobic fitness in the DM population. Data will be published after the study is completed.

TRIAL REGISTRATION

Registered at ClinicalTrials.gov under trial number NCT01889784 (date of registration 5 June 2013).

Obesity and metabolomics: metallothioneins protect against high-fat diet-induced consequences in metallothionein knockout mice

Obesity continues to rise as an alarming global epidemic. System level mechanisms, diagnostics, and therapeutics are sorely needed so as to identify at risk individuals and design appropriate population scale interventions. The present study evaluated the protective role of metallothioneins (MTs) against obesity and high-fat diet-induced effects such as insulin resistance in both male and female MT-1+2 knockout and MT-3 knockout mice. As the metabolome is closest to the functional phenotype, changes in metabolite levels were also evaluated, and the direct or indirect involvement of MTs in metabolism examined. MT-1+2-, MT-3 knockout, and wild-type mice were given a high-fat diet for 2 months. Variation in body weight gain, tissue weight, and response to oral glucose tolerance test and insulin tolerance test were determined and compared to mice that received the control diet. Effect of the high-fat diet on the knockout mice were investigated on the metabolome level in specific tissues using metabolomics. Both knockout mice strains were more susceptible to high-fat diet-induced effects, such as weight gain and moderate insulin resistance, with the MT-3 knockout mice most susceptible. Brain tissue of the knockout mice showed most metabolic variation and pointed to possible impairment of mitochondrial function. The protective effect of MTs against high-fat diet and obesity-induced effects such as insulin resistance was evident from our observations. The putative role MTs play in mitochondrial function is possibly the main contributor to the lack of these effects in wild-type mice. Considering the expression profiles of the MT isoforms and similarity in brain metabolic variation in the knockout strains, it appears that they promote mitochondrial function in the hypothalamus, thereby limiting weight gain and insulin resistance. Furthermore, metabolomics research in preclinical models of obesity and in the clinic is warranted in the near future.

Pioglitazone treatment restores in vivo muscle oxidative capacity in a rat model of diabetes

AIM

To determine the effect of pioglitazone treatment on in vivo and ex vivo muscle mitochondrial function in a rat model of diabetes.

METHODS

Both the lean, healthy rats and the obese, diabetic rats are Zucker Diabetic Fatty (ZDF) rats. The homozygous fa/fa ZDF rats are obese and diabetic. The heterozygous fa/+ ZDF rats are lean and healthy. Diabetic Zucker Diabetic Fatty rats were treated with either pioglitazone (30 mg/kg/day) or water as a control (n = 6 per group), for 2 weeks. In vivo ¹H and ³¹P magnetic resonance spectroscopy was performed on skeletal muscle to assess intramyocellular lipid (IMCL) content and muscle oxidative capacity, respectively. Ex vivo muscle mitochondrial respiratory capacity was evaluated using high-resolution respirometry. In addition, several markers of mitochondrial content were determined.

RESULTS

IMCL content was 14-fold higher and in vivo muscle oxidative capacity was 26% lower in diabetic rats compared with lean rats, which was, however, not caused by impairments of ex vivo mitochondrial respiratory capacity or a lower mitochondrial content. Pioglitazone treatment restored in vivo muscle oxidative capacity in diabetic rats to the level of lean controls. This amelioration was not accompanied by an increase in mitochondrial content or ex vivo mitochondrial respiratory capacity, but rather was paralleled by an improvement in lipid homeostasis, that is lowering of plasma triglycerides and muscle lipid and long-chain acylcarnitine content.

CONCLUSION

Diminished in vivo muscle oxidative capacity in diabetic rats results from mitochondrial lipid overload and can be alleviated by redirecting the lipids from the muscle into adipose tissue using pioglitazone treatment.

Effect of high-fat diet on rat myometrium during pregnancy-isolated myometrial mitochondria are not affected

Laboring women with elevated body mass index (BMI) have an increased risk of inefficient uterine labor contractions, and despite the significance of mitochondria in the production of energy to drive uterine contractions, mitochondrial function in the myometrium with reference to the BMI has not been explored. The objective of this study was to determine whether obesity prior to and during gestation affects oxidative capacity and/or morphology of mitochondria in the myometrium at term in an animal model. Rat dams were fed for 47 days prior to impregnation and during gestation with either (1) a regular chow diet, (2) a low-fat high-carbohydrate diet, or (3) a high-fat low-carbohydrate diet (n = 10 in each group). On day 20 of gestation, corresponding to term pregnancy, total hysterectomy was performed with subsequent examination of the function and morphology of myometrial mitochondria. Body composition was regularly assessed by quantitative magnetic resonance imaging, and blood sampling was done prior to diet assignment, impregnation, and hysterectomy. Dams on the high-fat low-carbohydrate diet achieved higher fat percentage compared to rats on the regular chow diet (p < 0.05). Maximal oxygen consumption, phosphate/oxygen ratio, or the amount of mitochondria per gram of myometrium did not differ between the three feeding groups. Electron microscopic examinations did not reveal any morphological differences in mitochondria between groups; however, a previously undescribed subsarcolemmal localization of the mitochondria in the myocyte was identified. We did not find evidence of altered myometrial mitochondrial function or morphology in this animal model of obesity prior to and during pregnancy.

Targeting mitochondria as therapeutic strategy for metabolic disorders

Mitochondria are critical regulator of cell metabolism; thus, mitochondrial dysfunction is associated with many metabolic disorders. Defects in oxidative phosphorylation, ROS production, or mtDNA mutations are the main causes of mitochondrial dysfunction in many pathological conditions such as IR/diabetes, metabolic syndrome, cardiovascular diseases, and cancer. Thus, targeting mitochondria has been proposed as therapeutic approach for these conditions, leading to the development of small molecules to be tested in the clinical scenario. Here we discuss therapeutic interventions to treat mitochondrial dysfunction associated with two major metabolic disorders, metabolic syndrome, and cancer. Finally, novel mechanisms of regulation of mitochondrial function are discussed, which open new scenarios for mitochondria targeting.

Diabetes medication use and blood lactate level among participants with type 2 diabetes: the atherosclerosis risk in communities carotid MRI study

BACKGROUND

The objective of this study is to compare lactate levels between users and non-users of diabetes medications under the hypothesis that the level of lactate is a marker of oxidative capacity.

METHODS

The cross-sectional data of 493 participants aged 61-84 with type 2 diabetes who participated in the Atherosclerosis Risk in Communities Carotid MRI study were analyzed using survey weighted linear regression.

RESULTS

Median plasma lactate level was 8.58 (95% CI: 8.23, 8.87) mg/dl. Comparing users of diabetic medications with non-users, thiazolidinedione use was significantly associated with lower lactate level (7.57 (6.95-8.25) mg/dL vs. 8.78 (8.43-9.14) mg/dL), metformin use with a slightly higher lactate level (9.02 (8.51-9.58) mg/dL vs. 8.36 (7.96-8.77) mg/dL), and sulfonylurea and insulin use were not associated with lactate level. After adjustment for demographic and lifestyle factors, the plasma lactate level for thiazolidinedione users was 15.78% lower than that for non-users (p<0.001). Considering use of each medication separately and in combination did not change the results.

CONCLUSION

In conclusion, thiazolidinedione use was associated with lower plasma lactate level compared to non-use and metformin use was only marginally associated with a slightly higher lactate level. These results are consistent with the previously demonstrated effects of diabetes medications on oxidative metabolism. Further investigation of the role that diabetes medications play in improvement of oxidative metabolism is warranted.

Cancer cachexia is associated with a decrease in skeletal muscle mitochondrial oxidative capacities without alteration of ATP production efficiency

BACKGROUND

Cancer cachexia is a complex syndrome related to a negative energy balance resulting in muscle wasting. Implication of muscle mitochondrial bioenergetics alterations during cancer cachexia was suggested. Therefore, the aim of this study was to explore the efficiency of oxidative phosphorylation in skeletal muscle mitochondria in a preclinical model of cancer cachexia.

METHODS

Berlin-Druckrey IX rats with peritoneal carcinosis (PC) were used as a model of cancer cachexia with healthy pair-fed rats (PF) as control. Hindlimb muscle morphology and fibre type composition were analysed in parallel with ubiquitin ligases and UCP gene expression. Oxidative phosphorylation was investigated in isolated muscle mitochondria by measuring oxygen consumption and ATP synthesis rate.

RESULTS

PC rats underwent significant muscle wasting affecting fast glycolytic muscles due to a reduction in fibre cross-sectional area. MuRF1 and MAFbx gene expression were significantly increased (9- and 3.5-fold, respectively) in the muscle of PC compared to PF rats. Oxygen consumption in non-phosphorylating state and the ATP/O were similar in both groups. Muscle UCP2 gene was overexpressed in PC rats. State III and the uncoupled state were significantly lower in muscle mitochondria from PC rats with a parallel reduction in complex IV activity (-30 %).

CONCLUSION

This study demonstrated that there was neither alteration in ATP synthesis efficiency nor mitochondrial uncoupling in skeletal muscle of cachectic rats despite UCP2 gene overexpression. Muscle mitochondrial oxidative capacities were reduced due to a decrease in complex IV activity. This mitochondrial bioenergetics alteration could participate to insulin resistance, lipid droplet accumulation and lactate production.

Activity-based protein profiling reveals mitochondrial oxidative enzyme impairment and restoration in diet-induced obese mice

High-fat diet (HFD) induced obesity and concomitant development of insulin resistance (IR) and type 2 diabetes mellitus have been linked to mitochondrial dysfunction. However, it is not clear whether mitochondrial dysfunction is a direct effect of a HFD, or if mitochondrial function is reduced with increased HFD duration. We hypothesized that the function of mitochondrial oxidative and lipid metabolism functions in skeletal muscle mitochondria for HFD mice are similar, or elevated, relative to standard diet (SD) mice; thereby, IR is neither cause nor consequence of mitochondrial dysfunction. We applied a chemical probe approach to identify functionally reactive ATPases and nucleotide-binding proteins in mitochondria isolated from skeletal muscle of C57Bl/6J mice fed HFD or SD chow for 2-, 8-, or 16-weeks; feeding time points known to induce IR. A total of 293 probe-labeled proteins were identified by mass spectrometry-based proteomics, of which 54 differed in abundance between HFD and SD mice. We found proteins associated with the TCA cycle, oxidative phosphorylation (OXPHOS), and lipid metabolism were altered in function when comparing SD to HFD fed mice at 2-weeks, however by 16-weeks HFD mice had TCA cycle, β-oxidation, and respiratory chain function at levels similar to or higher than SD mice.

Cardiac response to exercise in normal-weight and obese, Hispanic men and women: implications for exercise prescription

AIM

The effects of obesity on cardiac function during incremental exercise to peak oxygen consumption (VO(2peak)) have not been previously described. The purpose of this study was to compare submaximal and maximal cardiac function during exercise in normal-weight and obese adults.

METHODS

Normal-weight (n = 20; means ± SE: age = 21.9 ± 0.5 years; BMI = 21.8 ± 0.4 kg m(-2)) and obese (n = 15; means ± SE: age = 25.1 ± 5.2 years; BMI = 34.1 ± 01.0 kg m(-2)) participants were assessed for body composition, VO(2peak) and cardiac variables (thoracic bioimpedance analysis) at rest and at heart rates (HR) of 110, 130, 150 and 170 beats min(-1) and maximal HR during incremental cycling exercise to exhaustion. Differences between groups were assessed with mixed-model ancova with repeated measures. Cardiac variables were statistically indexed for body surface area and resting HR. VO(2) and arteriovenous oxygen difference (a-vO(2)) were statistically indexed for fat-free mass and resting HR.

RESULTS

Significant main effects for group indicated obese participants had higher cardiac output (Q) index and stroke volume (SV) index but lower ejection fraction (EF) and a-vO(2) index during incremental exercise to exhaustion compared with their normal-weight peers, despite similar submaximal and maximal VO(2) and absolute power outputs (P < 0.05).

CONCLUSIONS

Our findings suggest that although Q index and SV index were higher in obese, young adults, EF and a-vO(2) index were significantly lower when compared to matched, normal-weight adults.

Pathobiochemical changes in diabetic skeletal muscle as revealed by mass-spectrometry-based proteomics

Insulin resistance in skeletal muscle tissues and diabetes-related muscle weakness are serious pathophysiological problems of increasing medical importance. In order to determine global changes in the protein complement of contractile tissues due to diabetes mellitus, mass-spectrometry-based proteomics has been applied to the investigation of diabetic muscle. This review summarizes the findings from recent proteomic surveys of muscle preparations from patients and established animal models of type 2 diabetes. The potential impact of novel biomarkers of diabetes, such as metabolic enzymes and molecular chaperones, is critically examined. Disease-specific signature molecules may be useful for increasing our understanding of the molecular and cellular mechanisms of insulin resistance and possibly identify new therapeutic options that counteract diabetic abnormalities in peripheral organ systems. Importantly, the biomedical establishment of biomarkers promises to accelerate the development of improved diagnostic procedures for characterizing individual stages of diabetic disease progression, including the early detection of prediabetic complications.

Discovering pathways of sarcopenia in older adults: a role for insulin resistance on mitochondria dysfunction

The precise cause of sarcopenia, skeletal muscle loss and strength, in older persons is unknown. However, there is a strong evidence for muscle loss due to insulin resistance as well as mitochondrial dysfunction over aging. Considering that epidemiological studies have underlined that insulin resistance may have a specific role on skeletal muscle fibre atrophy and mitochondrial dysfunction has also been extensively shown to have a pivotal role on muscle loss in older persons, a combined pathway may not be ruled out. Considering that there is growing evidence for an insulin-related pathway on mitochondrial signaling, we hypothesize that a high degree of insulin resistance will be associated with the development of sarcopenia through specific alterations on mitochondrial functioning. This paper will highlight recent reviews regarding the link between skeletal muscle mitochondrial dysfunction and insulin resistance. We will specifically emphasize possible steps involved in sarcopenia over aging, including potential biomolecular mechanisms of insulin resistance on mitochondrial functioning.

Short-term increase of plasma free fatty acids does not interfere with intrinsic mitochondrial function in healthy young men

Free fatty acid (FFA)- and obesity-induced insulin resistance has been associated with disturbed mitochondrial function. Elevated plasma FFA can impair insulin-induced increase of adenosine triphosphate synthesis and downregulate the expression of genes important in the biogenesis of mitochondria in human skeletal muscle. Whether FAs have a direct effect on intrinsic mitochondrial capacity remains to be established. Therefore, we measured ex vivo mitochondrial respiratory capacity in human skeletal muscle after exposure to hyperinsulinemia and high levels of plasma FFA. Nine healthy lean men were studied during a 6-hour hyperinsulinemic (600 pmol/L) euglycemic clamp with concomitant infusion of Intralipid (Fresensius Kabi Nederland, Den Bosch, the Netherlands) (FFA clamped at 0.5 mmol/L) or saline. Mitochondrial respiratory capacity was measured by high-resolution respirometry in permeabilized muscle fibers using an Oxygraph (OROBOROS Instruments, Innsbruck, Austria). Each participant served as his own control. Peripheral glucose uptake (rate of disappearance) was significantly lower during infusion of the lipid emulsion compared with the control saline infusion (68 μmol/kg·min [saline] vs 40 μmol/kg·min [lipid], P = .008). However, adenosine diphosphate-stimulated and maximal carbonylcyanide-4-(trifluoromethoxy)-phenylhydrazone-stimulated uncoupled respiration rates were not different in permeabilized skeletal muscle fibers after exposure to high levels of FFA compared with the control condition. We conclude that short-term elevation of FFA within the physiological range induces insulin resistance but does not affect intrinsic mitochondrial capacity in skeletal muscle in humans.

Mitochondrial stress: a bridge between mitochondrial dysfunction and metabolic diseases?

Under pathophysiological conditions such as obesity, excessive oxidation of nutrients may induce mitochondrial stress, leading to mitochondrial unfolded protein response (UPR(mt)) and initiation of a retrograde stress signaling pathway. Defects in the UPR(mt) and the retrograde signaling pathways may disrupt the integrity and homeostasis of the mitochondria, resulting in endoplasmic reticulum stress and insulin resistance. Improving the capacity of mitochondria to reduce stress may be an effective approach to improve mitochondria function and to suppress obesity-induced metabolic disorders such as insulin resistance and type 2 diabetes.

Mitochondria and PGC-1α in Aging and Age-Associated Diseases

Aging is the most significant risk factor for a range of degenerative disease such as cardiovascular, neurodegenerative and metabolic disorders. While the cause of aging and its associated diseases is multifactorial, mitochondrial dysfunction has been implicated in the aging process and the onset and progression of age-associated disorders. Recent studies indicate that maintenance of mitochondrial function is beneficial in the prevention or delay of age-associated diseases. A central molecule seems to be the peroxisome proliferator-activated receptor γ coactivator α (PGC-1α), which is the key regulator of mitochondrial biogenesis. Besides regulating mitochondrial function, PGC-1α targets several other cellular processes and thereby influences cell fate on multiple levels. This paper discusses how mitochondrial function and PGC-1α are affected in age-associated diseases and how modulation of PGC-1α might offer a therapeutic potential for age-related pathology.

Dynamic regulation of mitochondrial network and oxidative functions during 3T3-L1 fat cell differentiation

Mitochondria have been shown to be impaired in insulin resistance-related diseases but have not been extensively studied during the first steps of adipose cell development. This study was designed to determine the sequence of changes of the mitochondrial network and function during the first days of adipogenesis. 3T3-L1 preadipocytes were differentiated into adipocytes without using glitazone compounds. At days 0, 3, 6, 9, and 12, mitochondrial network imaging, mitochondrial oxygen consumption, membrane potential, and oxidative phosphorylation efficiency were assessed in permeabilized cells. Gene and protein expressions related to fatty acid metabolism and mitochondrial network were also determined. Compared to preadipocytes (day 0), new adipocytes (days 6 and 9) displayed profound changes of their mitochondrial network that underwent fragmentation and redistribution around lipid droplets. Drp1 and mitofusin 2 displayed a progressive increase in their gene expression and protein content during the first 9 days of differentiation. In parallel with the mitochondrial network redistribution, mitochondria switched to uncoupled respiration with a tendency towards decreased membrane potential, with no variation of mtTFA and NRF1 gene expression. The expression of PGC1α and NRF2 genes and genes involved in lipid oxidation (UCP2, CD36, and CPT1) was increased. Reactive oxygen species (ROS) production displayed a nadir at day 6 with a concomitant increase in antioxidant enzyme gene expression. This 3T3-L1-based in vitro model of adipogenesis showed that mitochondria adapted to the increased number of lipid droplets by network redistribution and uncoupling respiration. The timing and regulation of lipid oxidation-associated ROS production appeared to play an important role in these changes.

Is there a metabolic program in the skeletal muscle of obese individuals?

Severe obesity (BMI ≥ 40 kg/m(2)) is associated with multiple defects in skeletal muscle which contribute to insulin resistance and a reduction in fatty acid oxidation (FAO) in this tissue. These metabolic derangements are retained in human skeletal muscle cells raised in culture. Together, these findings are indicative of a dysfunctional global metabolic program with severe obesity which is of an epigenetic or genetic origin. Weight loss via gastric bypass surgery can "turn off" and/or correct components of this metabolic program as insulin sensitivity is restored; however, the impairment in FAO in skeletal muscle remains evident. Physical activity can improve FAO and insulin action, indicating that this patient population is not exercise resistant and that exercise offers a pathway to circumvent the abnormal program. Findings presented in this review will hopefully increase the understanding of and aid in preventing and/or treating the severely obese condition.

Common inherited variation in mitochondrial genes is not enriched for associations with type 2 diabetes or related glycemic traits

Mitochondrial dysfunction has been observed in skeletal muscle of people with diabetes and insulin-resistant individuals. Furthermore, inherited mutations in mitochondrial DNA can cause a rare form of diabetes. However, it is unclear whether mitochondrial dysfunction is a primary cause of the common form of diabetes. To date, common genetic variants robustly associated with type 2 diabetes (T2D) are not known to affect mitochondrial function. One possibility is that multiple mitochondrial genes contain modest genetic effects that collectively influence T2D risk. To test this hypothesis we developed a method named Meta-Analysis Gene-set Enrichment of variaNT Associations (MAGENTA; http://www.broadinstitute.org/mpg/magenta). MAGENTA, in analogy to Gene Set Enrichment Analysis, tests whether sets of functionally related genes are enriched for associations with a polygenic disease or trait. MAGENTA was specifically designed to exploit the statistical power of large genome-wide association (GWA) study meta-analyses whose individual genotypes are not available. This is achieved by combining variant association p-values into gene scores and then correcting for confounders, such as gene size, variant number, and linkage disequilibrium properties. Using simulations, we determined the range of parameters for which MAGENTA can detect associations likely missed by single-marker analysis. We verified MAGENTA's performance on empirical data by identifying known relevant pathways in lipid and lipoprotein GWA meta-analyses. We then tested our mitochondrial hypothesis by applying MAGENTA to three gene sets: nuclear regulators of mitochondrial genes, oxidative phosphorylation genes, and ∼1,000 nuclear-encoded mitochondrial genes. The analysis was performed using the most recent T2D GWA meta-analysis of 47,117 people and meta-analyses of seven diabetes-related glycemic traits (up to 46,186 non-diabetic individuals). This well-powered analysis found no significant enrichment of associations to T2D or any of the glycemic traits in any of the gene sets tested. These results suggest that common variants affecting nuclear-encoded mitochondrial genes have at most a small genetic contribution to T2D susceptibility.

Mitochondria play a crucial role in metabolic homeostasis, and alteration of mitochondrial function is a hallmark of diabetes. While mitochondrial activity is reduced in people with diabetes, it is unclear whether mitochondrial dysfunction is a cause or effect of type 2 diabetes. Genome-wide association studies for type 2 diabetes have explained ≈10% of the heritability of the disease, but none of the loci are known to affect mitochondrial activity. It is possible though that a mitochondrial contribution is hidden in the remaining 90%. Hence, we tested the hypothesis that multiple mitochondria-related genes encoded in the nucleus, each having a weak effect (hard to detect individually), can collectively influence type 2 diabetes. To address this, we developed a computational method (MAGENTA) that allowed us to adequately analyze large collective datasets of human genetic variation obtained from collaborative studies of type 2 diabetes and related glycemic traits. Despite the increased sensitivity of MAGENTA compared to single-DNA variant analysis, we found no support for a causal relationship between mitochondrial dysfunction and type 2 diabetes. These results may help steer future efforts in understanding the pathogenesis of the disease. MAGENTA is broadly applicable to testing associations between other biological pathways and common diseases or traits.

Lipid oversupply, selective insulin resistance, and lipotoxicity: molecular mechanisms

The accumulation of fat in tissues not suited for lipid storage has deleterious consequences on organ function, leading to cellular damage that underlies diabetes, heart disease, and hypertension. To combat these lipotoxic events, several therapeutics improve insulin sensitivity and/or ameliorate features of metabolic disease by limiting the inappropriate deposition of fat in peripheral tissues (i.e. thiazolidinediones, metformin, and statins). Recent advances in genomics and lipidomics have accelerated progress towards understanding the pathogenic events associated with the excessive production, underutilization, or inefficient storage of fat. Herein we review studies applying pharmacological or genetic strategies to manipulate the expression or activity of enzymes controlling lipid deposition, in order to gain a clearer understanding of the molecular mechanisms by which fatty acids contribute to metabolic disease.