Acylcarnitines: potential implications for skeletal muscle insulin resistance

Leucine Supplementation Differently Modulates Branched-Chain Amino Acid Catabolism, Mitochondrial Function and Metabolic Profiles at the Different Stage of Insulin Resistance in Rats on High-Fat Diet

The available findings concerning the association between branched-chain amino acids (BCAAs)—particularly leucine—and insulin resistance are conflicting. BCAAs have been proposed to elicit different or even opposite effects, depending on the prevalence of catabolic and anabolic states. We tested the hypothesis that leucine supplementation may exert different effects at different stages of insulin resistance, to provide mechanistic insights into the role of leucine in the progression of insulin resistance. Male Sprague-Dawley rats were fed a normal chow diet, high-fat diet (HFD), HFD supplemented with 1.5% leucine, or HFD with a 20% calorie restriction for 24 or 32 weeks. Leucine supplementation led to abnormal catabolism of BCAA and the incompletely oxidized lipid species that contributed to mitochondrial dysfunction in skeletal muscle in HFD-fed rats in the early stage of insulin resistance (24 weeks). However, leucine supplementation induced no remarkable alternations in BCAA catabolism, but did enhance mitochondrial biogenesis with a concomitant improvement in lipid oxidation and mitochondrial function during the hyperglycaemia stage (32 weeks). These findings suggest that leucine trigger different effects on metabolic signatures at different stages of insulin resistance, and the overall metabolic status of the organisms should be carefully considered to potentiate the benefits of leucine.

Skeletal Muscle Nucleo-Mitochondrial Crosstalk in Obesity and Type 2 Diabetes

Skeletal muscle mitochondrial dysfunction, evidenced by incomplete beta oxidation and accumulation of fatty acid intermediates in the form of long and medium chain acylcarnitines, may contribute to ectopic lipid deposition and insulin resistance during high fat diet (HFD)-induced obesity. The present review discusses the roles of anterograde and retrograde communication in nucleo-mitochondrial crosstalk that determines skeletal muscle mitochondrial adaptations, specifically alterations in mitochondrial number and function in relation to obesity and insulin resistance. Special emphasis is placed on the effects of high fat diet (HFD) feeding on expression of nuclear-encoded mitochondrial genes (NEMGs) nuclear receptor factor 1 (NRF-1) and 2 (NRF-2) and peroxisome proliferator receptor gamma coactivator 1 alpha (PGC-1α) in the onset and progression of insulin resistance during obesity and how HFD-induced alterations in NEMG expression affect skeletal muscle mitochondrial adaptations in relation to beta oxidation of fatty acids. Finally, the potential ability of acylcarnitines or fatty acid intermediates resulting from mitochondrial beta oxidation to act as retrograde signals in nucleo-mitochondrial crosstalk is reviewed and discussed.

Acylcarnitines as markers of exercise-associated fuel partitioning, xenometabolism, and potential signals to muscle afferent neurons

NEW FINDINGS

What is the central question of this study? Does improved metabolic health and insulin sensitivity following a weight-loss and fitness intervention in sedentary, obese women alter exercise-associated fuel metabolism and incomplete mitochondrial fatty acid oxidation (FAO), as tracked by blood acylcarnitine patterns? What is the main finding and its importance? Despite improved fitness and blood sugar control, indices of incomplete mitochondrial FAO increased in a similar manner in response to a fixed load acute exercise bout; this indicates that intramitochondrial muscle FAO is inherently inefficient and is tethered directly to ATP turnover. With insulin resistance or type 2 diabetes mellitus, mismatches between mitochondrial fatty acid fuel delivery and oxidative phosphorylation/tricarboxylic acid cycle activity may contribute to inordinate accumulation of short- or medium-chain acylcarnitine fatty acid derivatives [markers of incomplete long-chain fatty acid oxidation (FAO)]. We reasoned that incomplete FAO in muscle would be ameliorated concurrent with improved insulin sensitivity and fitness following a ∼14 week training and weight-loss intervention in obese, sedentary, insulin-resistant women. Contrary to this hypothesis, overnight-fasted and exercise-induced plasma C4-C14 acylcarnitines did not differ between pre- and postintervention phases. These metabolites all increased robustly with exercise (∼45% of pre-intervention peak oxygen consumption) and decreased during a 20 min cool-down. This supports the idea that, regardless of insulin sensitivity and fitness, intramitochondrial muscle β-oxidation and attendant incomplete FAO are closely tethered to absolute ATP turnover rate. Acute exercise also led to branched-chain amino acid acylcarnitine derivative patterns suggestive of rapid and transient diminution of branched-chain amino acid flux through the mitochondrial branched-chain ketoacid dehydrogenase complex. We confirmed our prior novel observation that a weight-loss/fitness intervention alters plasma xenometabolites [i.e. cis-3,4-methylene-heptanoylcarnitine and γ-butyrobetaine (a co-metabolite possibly derived in part from gut bacteria)], suggesting that host metabolic health regulated gut microbe metabolism. Finally, we considered whether acylcarnitine metabolites signal to muscle-innervating afferents; palmitoylcarnitine at concentrations as low as 1-10 μm activated a subset (∼2.5-5%) of these neurons ex vivo. This supports the hypothesis that in addition to tracking exercise-associated shifts in fuel metabolism, muscle acylcarnitines act as signals of exertion to short-loop somatosensory-motor circuits or to the brain.

Mitochondrial fat oxidation is essential for lipid-induced inflammation in skeletal muscle in mice

Inflammation, lipotoxicity and mitochondrial dysfunction have been implicated in the pathogenesis of obesity-induced insulin resistance and type 2 diabetes. However, how these factors are intertwined in the development of obesity/insulin resistance remains unclear. Here, we examine the role of mitochondrial fat oxidation on lipid-induced inflammation in skeletal muscle. We used skeletal muscle-specific Cpt1b knockout mouse model where the inhibition of mitochondrial fatty acid oxidation results in accumulation of lipid metabolites in muscle and elevated circulating free fatty acids. Gene expression of pro-inflammatory cytokines, chemokines, and cytokine- and members of TLR-signalling pathways were decreased in Cpt1b^m−/− muscle. Inflammatory signalling pathways were not activated when evaluated by multiplex and immunoblot analysis. In addition, the inflammatory response to fatty acids was reduced in primary muscle cells derived from Cpt1b^m−/− mice. Gene expression of Cd11c, the M1 macrophage marker, was decreased; while Cd206, the M2 macrophage marker, was increased in skeletal muscle of Cpt1b^m−/− mice. Finally, expression of pro-inflammatory markers was decreased in white adipose tissue of Cpt1b^m−/− mice. We show that the inflammatory response elicited by elevated intracellular lipids in skeletal muscle is repressed in Cpt1b^m−/− mice, strongly supporting the hypothesis that mitochondrial processing of fatty acids is essential for the lipid-induction of inflammation in muscle.

Oleate dose-dependently regulates palmitate metabolism and insulin signaling in C2C12 myotubes

Because the protective effect of oleate against palmitate-induced insulin resistance may be lessened in skeletal muscle once cell metabolism is overloaded by fatty acids (FAs), we examined the impact of varying amounts of oleate on palmitate metabolic channeling and insulin signaling in C2C12 myotubes. Cells were exposed to 0.5mM of palmitate and to increasing doses of oleate (0.05, 0.25 and 0.5mM). Impacts of FA treatments on radio-labelled FA fluxes, on cellular content in diacylglycerols (DAG), triacylglycerols (TAG), ceramides, acylcarnitines, on PKCθ, MAPKs (ERK1/2, p38) and NF-ΚB activation, and on insulin-dependent Akt phosphorylation were examined. Low dose of oleate (0.05mM) was sufficient to improve palmitate complete oxidation to CO₂ (+29%, P<0.05) and to alter the cellular acylcarnitine profile. Insulin-induced Akt phosphorylation was 48% higher in that condition vs. palmitate alone (p<0.01). Although DAG and ceramide contents were significantly decreased with 0.05mM of oleate vs. palmitate alone (-47 and -28%, respectively, p<0.01), 0.25mM of oleate was required to decrease p38 MAPK and PKCθ phosphorylation, thus further improving the insulin signaling (+32%, p<0.05). By contrast, increasing oleate concentration from 0.25 to 0.5mM, thus increasing total amount of FA from 0.75 to 1mM, deteriorated the insulin signaling pathway (-30%, p<0.01). This was observed despite low contents in DAG and ceramides, and enhanced palmitate incorporation into TAG (+27%, p<0.05). This was associated with increased incomplete FA β-oxidation and impairment of acylcarnitine profile. In conclusion, these combined data place mitochondrial β-oxidation at the center of the regulation of muscle insulin sensitivity, besides p38 MAPK and PKCθ.

Novel Molecular Interactions of Acylcarnitines and Fatty Acids with Myoglobin

Previous research has indicated that long-chain fatty acids can bind myoglobin (Mb) in an oxygen-dependent manner. This suggests that oxy-Mb may play an important role in fuel delivery in Mb-rich muscle fibers (e.g. type I fibers and cardiomyocytes), and raises the possibility that Mb also serves as an acylcarnitine-binding protein. We report for the first time the putative interaction and affinity characteristics for different chain lengths of both fatty acids and acylcarnitines with oxy-Mb using molecular dynamic simulations and isothermal titration calorimetry experiments. We found that short- to medium-chain fatty acids or acylcarnitines (ranging from C2:0 to C10:0) fail to achieve a stable conformation with oxy-Mb. Furthermore, our results indicate that C12:0 is the minimum chain length essential for stable binding of either fatty acids or acylcarnitines with oxy-Mb. Importantly, the empirical lipid binding studies were consistent with structural modeling. These results reveal that: (i) the lipid binding affinity for oxy-Mb increases as the chain length increases (i.e. C12:0 to C18:1), (ii) the binding affinities of acylcarnitines are higher when compared with their respective fatty acid counterparts, and (iii) both fatty acids and acylcarnitines bind to oxy-Mb in 1:1 stoichiometry. Taken together, our results support a model in which oxy-Mb is a novel regulator of long-chain acylcarnitine and fatty acid pools in Mb-rich tissues. This has important implications for physiological fuel management during exercise, and relevance to pathophysiological conditions (e.g. fatty acid oxidation disorders and cardiac ischemia) where long-chain acylcarnitine accumulation is evident.

Defects in muscle branched-chain amino acid oxidation contribute to impaired lipid metabolism

OBJECTIVE

Plasma levels of branched-chain amino acids (BCAA) are consistently elevated in obesity and type 2 diabetes (T2D) and can also prospectively predict T2D. However, the role of BCAA in the pathogenesis of insulin resistance and T2D remains unclear.

METHODS

To identify pathways related to insulin resistance, we performed comprehensive gene expression and metabolomics analyses in skeletal muscle from 41 humans with normal glucose tolerance and 11 with T2D across a range of insulin sensitivity (SI, 0.49 to 14.28). We studied both cultured cells and mice heterozygous for the BCAA enzyme methylmalonyl-CoA mutase (Mut) and assessed the effects of altered BCAA flux on lipid and glucose homeostasis.

RESULTS

Our data demonstrate perturbed BCAA metabolism and fatty acid oxidation in muscle from insulin resistant humans. Experimental alterations in BCAA flux in cultured cells similarly modulate fatty acid oxidation. Mut heterozygosity in mice alters muscle lipid metabolism in vivo, resulting in increased muscle triglyceride accumulation, increased plasma glucose, hyperinsulinemia, and increased body weight after high-fat feeding.

CONCLUSIONS

Our data indicate that impaired muscle BCAA catabolism may contribute to the development of insulin resistance by perturbing both amino acid and fatty acid metabolism and suggest that targeting BCAA metabolism may hold promise for prevention or treatment of T2D.

Plasma metabolomics identified novel metabolites associated with risk of type 2 diabetes in two prospective cohorts of Chinese adults

BACKGROUND

Metabolomics studies in Caucasians have identified a number of novel metabolites in association with the risk of type 2 diabetes (T2D). However, few prospective metabolomic studies are available in Chinese populations. In the present study, we sought to identify novel metabolites consistently associated with incident T2D in two independent cohorts of Chinese adults.

METHODS

We performed targeted metabolomics (52 metabolites) of fasting plasma samples by liquid chromatography-mass spectrometry in two prospective case-control studies nested within the Dongfeng-Tongji (DFTJ) cohort and Jiangsu Non-communicable Disease (JSNCD) cohort. After following for 4.61 ± 0.15 and 7.57 ± 1.13 years, respectively, 1039 and 520 eligible participants developed incident T2D in these two cohorts, and controls were 1:1 matched with cases by age (± 5 years) and sex. Multivariate conditional logistic regression models were constructed to identify metabolites associated with future T2D risk in both cohorts.

RESULTS

We identified four metabolites consistently associated with an increased risk of developing T2D in the two cohorts, including alanine, phenylalanine, tyrosine and palmitoylcarnitine. In the meta-analysis of two cohorts, the odds ratios (95% confidence intervals, CIs) comparing extreme quartiles were 1.79 (1.32-2.42) for alanine, 1.91 (1.41-2.60) for phenylalanine, 1.85 (1.37-2.48) for tyrosine and 1.63 (1.21-2.20) for palmitoylcarnitine (all P_trend ≤ 0.01).

CONCLUSIONS

We confirmed the association of alanine, phenylalanine and tyrosine with future T2D risk and further identified palmitoylcarnitine as a novel metabolic marker of incident T2D in two prospective cohorts of Chinese adults. Our findings might provide new aetiological insight into the development of T2D.

Macrophage fatty acid oxidation and its roles in macrophage polarization and fatty acid-induced inflammation

Recent research considerably changed our knowledge how cellular metabolism affects the immune system. We appreciate that metabolism not only provides energy to immune cells, but also actively influences diverse immune cell phenotypes. Fatty acid metabolism, particularly mitochondrial fatty acid oxidation (FAO) emerges as an important regulator of innate and adaptive immunity. Catabolism of fatty acids also modulates the progression of disease, such as the development of obesity-driven insulin resistance and type II diabetes. Here, we summarize (i) recent developments in research how FAO modulates inflammatory signatures in macrophages in response to saturated fatty acids, and (ii) the role of FAO in regulating anti-inflammatory macrophage polarization. In addition, we define the contribution of AMP-activated protein kinase (AMPK) and peroxisome proliferator-activated receptors (PPARs), in controlling macrophage biology towards fatty acid metabolism and inflammation.

Palmitoyl-carnitine production by blood cells associates with the concentration of circulating acyl-carnitines in healthy overweight women

BACKGROUND

Circulating acyl-carnitines (acyl-CNTs) are associated with insulin resistance (IR) and type 2 diabetes (T2D) in both rodents and humans. However, the mechanisms whereby circulating acyl-CNTs are increased in these conditions and their role in whole-body metabolism remains unknown. The purpose of this study was to determine if, in humans, blood cells contribute in production of circulating acyl-CNTs and associate with whole-body fat metabolism.

METHODS AND RESULTS

Eight non-diabetic healthy women (age: 47 ± 19 y; BMI: 26 ± 1 kg·m^-2) underwent stable isotope tracer infusion and hyperinsulinemic-euglycemic clamp study to determine in vivo whole-body fatty acid flux and insulin sensitivity. Blood samples collected at baseline (0 min) and after 3 h of clamp were used to determine the synthesis rate of palmitoyl-carnitine (palmitoyl-CNT) in vitro. The fractional synthesis rate of palmitoyl-CNT was significantly higher during hyperinsulinemia (0.788 ± 0.084 vs. 0.318 ± 0.012%·hr^-1, p = 0.001); however, the absolute synthesis rate (ASR) did not differ between the periods (p = 0.809) due to ∼30% decrease in blood palmitoyl-CNT concentration (p = 0.189) during hyperinsulinemia. The ASR of palmitoyl-CNT significantly correlated with the concentration of acyl-CNTs in basal (r = 0.992, p < 0.001) and insulin (r = 0.919, p = 0.001) periods; and the basal ASR significantly correlated with plasma palmitate oxidation (r = 0.764, p = 0.027).

CONCLUSION

In women, blood cells contribute to plasma acyl-CNT levels and the acyl-CNT production is linked to plasma palmitate oxidation, a marker of whole-body fat metabolism. Future studies are needed to confirm the role of blood cells in acyl-CNT and lipid metabolism under different physiological (i.e., in response to meal) and pathological (i.e., hyperlipidemia, IR and T2D) conditions.

Muscle expression of a malonyl-CoA-insensitive carnitine palmitoyltransferase-1 protects mice against high-fat/high-sucrose diet-induced insulin resistance

Impaired skeletal muscle mitochondrial fatty acid oxidation (mFAO) has been implicated in the etiology of insulin resistance. Carnitine palmitoyltransferase-1 (CPT1) is a key regulatory enzyme of mFAO whose activity is inhibited by malonyl-CoA, a lipogenic intermediate. Whereas increasing CPT1 activity in vitro has been shown to exert a protective effect against lipid-induced insulin resistance in skeletal muscle cells, only a few studies have addressed this issue in vivo. We thus examined whether a direct modulation of muscle CPT1/malonyl-CoA partnership is detrimental or beneficial for insulin sensitivity in the context of diet-induced obesity. By using a Cre-LoxP recombination approach, we generated mice with skeletal muscle-specific and inducible expression of a mutated CPT1 form (CPT1mt) that is active but insensitive to malonyl-CoA inhibition. When fed control chow, homozygous CPT1mt transgenic (dbTg) mice exhibited decreased CPT1 sensitivity to malonyl-CoA inhibition in isolated muscle mitochondria, which was sufficient to substantially increase ex vivo muscle mFAO capacity and whole body fatty acid utilization in vivo. Moreover, dbTg mice were less prone to high-fat/high-sucrose (HFHS) diet-induced insulin resistance and muscle lipotoxicity despite similar body weight gain, adiposity, and muscle malonyl-CoA content. Interestingly, these CPT1mt-protective effects in dbTg-HFHS mice were associated with preserved muscle insulin signaling, increased muscle glycogen content, and upregulation of key genes involved in muscle glucose metabolism. These beneficial effects of muscle CPT1mt expression suggest that a direct modulation of the malonyl-CoA/CPT1 partnership in skeletal muscle could represent a potential strategy to prevent obesity-induced insulin resistance.

Early Prediction of Developing Type 2 Diabetes by Plasma Acylcarnitines: A Population-Based Study

OBJECTIVE

Acylcarnitines were suggested as early biomarkers even prior to insulin resistance in animal studies, but their roles in predicting type 2 diabetes were unknown. Therefore, we aimed to determine whether acylcarnitines could independently predict type 2 diabetes by using a targeted metabolic profiling approach.

RESEARCH DESIGN AND METHODS

A population-based prospective study was conducted among 2,103 community-living Chinese individuals aged 50-70 years from Beijing and Shanghai with a mean follow-up duration of 6 years. Fasting glucose, glycohemoglobin, and insulin were determined at baseline and in a follow-up survey. Baseline plasma acylcarnitines were profiled by liquid chromatography-tandem mass spectrometry.

RESULTS

Over the 6-year period, 507 participants developed diabetes. A panel of acylcanitines, especially with long chain, was significantly associated with increased risk of type 2 diabetes. The relative risks of type 2 diabetes per SD increase of the predictive model score were 2.48 (95% CI 2.20-2.78) for the conventional and 9.41 (95% CI 7.62-11.62) for the full model including acylcarnitines, respectively. Moreover, adding selected acylcarnitines substantially improved predictive ability for incident diabetes, as area under the receiver operator characteristic curve improved to 0.89 in the full model compared with 0.73 in the conventional model. Similar associations were obtained when the predictive models were established separately among Beijing or Shanghai residents.

CONCLUSIONS

A panel of acylcarnitines, mainly involving mitochondrial lipid dysregulation, significantly improved predictive ability for type 2 diabetes beyond conventional risk factors. These findings need to be replicated in other populations, and the underlying mechanisms should be elucidated.

Metabolomic Profiling Identifies Novel Circulating Biomarkers of Mitochondrial Dysfunction Differentially Elevated in Heart Failure With Preserved Versus Reduced Ejection Fraction: Evidence for Shared Metabolic Impairments in Clinical Heart Failure

BACKGROUND

Metabolic impairment is an important contributor to heart failure (HF) pathogenesis and progression. Dysregulated metabolic pathways remain poorly characterized in patients with HF and preserved ejection fraction (HFpEF). We sought to determine metabolic abnormalities in HFpEF and identify pathways differentially altered in HFpEF versus HF with reduced ejection fraction (HFrEF).

METHODS AND RESULTS

We identified HFpEF cases, HFrEF controls, and no-HF controls from the CATHGEN study of sequential patients undergoing cardiac catheterization. HFpEF cases (N=282) were defined by left ventricular ejection fraction (LVEF) ≥45%, diastolic dysfunction grade ≥1, and history of HF; HFrEF controls (N=279) were defined similarly, except for having LVEF <45%. No-HF controls (N=191) had LVEF ≥45%, normal diastolic function, and no HF diagnosis. Targeted mass spectrometry and enzymatic assays were used to quantify 63 metabolites in fasting plasma. Principal components analysis reduced the 63 metabolites to uncorrelated factors, which were compared across groups using ANCOVA. In basic and fully adjusted models, long-chain acylcarnitine factor levels differed significantly across groups (P<0.0001) and were greater in HFrEF than HFpEF (P=0.0004), both of which were greater than no-HF controls. We confirmed these findings in sensitivity analyses using stricter inclusion criteria, alternative LVEF thresholds, and adjustment for insulin resistance.

CONCLUSIONS

We identified novel circulating metabolites reflecting impaired or dysregulated fatty acid oxidation that are independently associated with HF and differentially elevated in HFpEF and HFrEF. These results elucidate a specific metabolic pathway in HF and suggest a shared metabolic mechanism in HF along the LVEF spectrum.

Story behind meldonium-from pharmacology to performance enhancement: a narrative review

Recent reports from the World Anti-Doping Agency (WADA) indicate an alarming prevalence in the use of meldonium among elite athletes. Therefore, in January 2016, meldonium was added to WADA's prohibited list after being monitored since 2015. Meldonium has been shown to have beneficial effects in cardiovascular, neurological and metabolic diseases due to its anti-ischaemic and cardioprotective properties, which are ascribed mainly to its inhibition of ß-oxidation and its activation of glycolysis. Despite its widespread use, there are only a few clinical studies or clinical trials available. Meldonium is registered in most Baltic countries and is easily accessible through the internet with no serious adverse effects reported by the manufacturer so far. Among athletes, meldonium is used with the purpose of increasing recovery rate or exercise performance. The benefit of taking meldonium in view of performance enhancement in athletes is quite speculative and is discussed without sound scientific evidence. This narrative review provides a detailed overview of the drug meldonium, focusing on the main topics pharmacology and biochemical actions, clinical applications, pharmacokinetics, methods of detection and potential for performance enhancement in athletes.

Mitochondrial function is altered in horse atypical myopathy

Equine atypical myopathy in Europe is a fatal rhabdomyolysis syndrome that results from the ingestion of hypoglycin A contained in seeds and seedlings of Acer pseudoplatanus (sycamore maple). Acylcarnitine concentrations in serum and muscle OXPHOS capacity were determined in 15 atypical myopathy cases. All but one acylcarnitine were out of reference range and mitochondrial respiratory capacity was severely decreased up to 49% as compared to 10 healthy controls. The hallmark of atypical myopathy thus consists of a severe alteration in the energy metabolism including a severe impairment in muscle mitochondrial respiration that could contribute to its high death rate.

Sexual dimorphism in myocardial acylcarnitine and triglyceride metabolism

Background

Cardiovascular disease is the leading cause of death among diabetic patients. Importantly, recent data highlight the apparent sexual dimorphism in the pathogenesis of cardiovascular disease in diabetics with respect to both frequency- and age-related risk factors. The disposition to cardiovascular disease among diabetic patients has been attributed, at least in part, to excess lipid supply to the heart culminating in lipotoxicity of the heart and downstream derangements. A confounding factor in obese animal models of diabetes is that increased peripheral lipid availability to the heart can induce cardio-metabolic remodeling independent of the underlying pathophysiology of diabetes, thus masking the diabetic phenotype. To that end, we hypothesized that the use of non-obese diabetic (NOD) animal models will reveal metabolic signatures of diabetes in a sex-specific manner.

Methods

To test this hypothesis, male and female NOD Goto-Kakizaki (GK) rats were used to assess the expression profile of 84 genes involved in lipid metabolism. In parallel, targeted lipidomics analysis was performed to characterize sex differences in homeostasis of non-esterified fatty acids (NEFA), acylcarnitines (AC), and triglycerides (TG).

Results

Our analysis revealed significant sex differences in the expression of a broad range of genes involved in transport, activation, and utilization of lipids. Furthermore, NOD male rats exhibited enhanced oxidative metabolism and accumulation of TG, whereas female NOD rats exhibited reduced TG content coupled with accumulation of AC species. Multi-dimensional statistical analysis identified saturated AC16:0, AC18:0, and AC20:0 as dominant metabolites in mediating sex differences in AC metabolism. Confocal microscopy of rat cardiomyocytes exposed to AC14:0, AC16:0, and AC18:0 confirmed induction of ROS with AC18:0 being more potent followed by AC14:0.

Conclusion

Overall, we demonstrate sex differences in myocardial AC and TG metabolism with implications for therapy and diagnosis of diabetic cardiovascular disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13293-016-0077-7) contains supplementary material, which is available to authorized users.

Pharmacological effects of meldonium: Biochemical mechanisms and biomarkers of cardiometabolic activity

Meldonium (mildronate; 3-(2,2,2-trimethylhydrazinium)propionate; THP; MET-88) is a clinically used cardioprotective drug, which mechanism of action is based on the regulation of energy metabolism pathways through l-carnitine lowering effect. l-Carnitine biosynthesis enzyme γ-butyrobetaine hydroxylase and carnitine/organic cation transporter type 2 (OCTN2) are the main known drug targets of meldonium, and through inhibition of these activities meldonium induces adaptive changes in the cellular energy homeostasis. Since l-carnitine is involved in the metabolism of fatty acids, the decline in its levels stimulates glucose metabolism and decreases concentrations of l-carnitine related metabolites, such as long-chain acylcarnitines and trimethylamine-N-oxide. Here, we briefly reviewed the pharmacological effects and mechanisms of meldonium in treatment of heart failure, myocardial infarction, arrhythmia, atherosclerosis and diabetes.

Mitochondrial Function and Diabetes: Consequences for Skeletal and Cardiac Muscle Metabolism

SIGNIFICANCE

An early hallmark in the development of type 2 diabetes is the resistance to the effect of insulin in skeletal muscle and in the heart. Since mitochondrial function was found to be diminished in patients with type 2 diabetes, it was suggested that this defect might be involved in the etiology of insulin resistance. Although several hypotheses were suggested, yet unclear is the mechanistic link between these two phenomena.

RECENT ADVANCES

Herein, we review the evidence for disturbances in mitochondrial function in skeletal muscle and the heart in the diabetic state. Also the mechanisms involved in improving mitochondrial function are considered and, whenever possible, human data is cited.

CRITICAL ISSUES

Reported evidence shows that interventions that improve skeletal muscle mitochondrial function also improve insulin sensitivity in humans. In the heart, available data from animal studies suggests that enhancement of mitochondrial function can reverse aging-induced changes in heart function, and can be protective against cardiomyopathy and heart failure.

FUTURE DIRECTIONS

Mitochondria and their functions can be targeted with the aim of improving skeletal muscle insulin sensitivity and cardiac function. However, human clinical intervention studies are needed to fully substantiate the potential of mitochondria as a target to prevent cardiometabolic disease.

Acylcarnitines--old actors auditioning for new roles in metabolic physiology

Perturbations in metabolic pathways can cause substantial increases in plasma and tissue concentrations of long-chain acylcarnitines (LCACs). For example, the levels of LCACs and other acylcarnitines rise in the blood and muscle during exercise, as changes in tissue pools of acyl-coenzyme A reflect accelerated fuel flux that is incompletely coupled to mitochondrial energy demand and capacity of the tricarboxylic acid cycle. This natural ebb and flow of acylcarnitine generation and accumulation contrasts with that of inherited fatty acid oxidation disorders (FAODs), cardiac ischaemia or type 2 diabetes mellitus. These conditions are characterized by very high (FAODs, ischaemia) or modestly increased (type 2 diabetes mellitus) tissue and blood levels of LCACs. Although specific plasma concentrations of LCACs and chain-lengths are widely used as diagnostic markers of FAODs, research into the potential effects of excessive LCAC accumulation or the roles of acylcarnitines as physiological modulators of cell metabolism is lacking. Nevertheless, a growing body of evidence has highlighted possible effects of LCACs on disparate aspects of pathophysiology, such as cardiac ischaemia outcomes, insulin sensitivity and inflammation. This Review, therefore, aims to provide a theoretical framework for the potential consequences of tissue build-up of LCACs among individuals with metabolic disorders.

Increased palmitate intake: higher acylcarnitine concentrations without impaired progression of β-oxidation

Palmitic acid (PA) is associated with higher blood concentrations of medium-chain acylcarnitines (MCACs), and we hypothesized that PA may inhibit progression of FA β-oxidation. Using a cross-over design, 17 adults were fed high PA (HPA) and low PA/high oleic acid (HOA) diets, each for 3 weeks. The [1-(13)C]PA and [13-(13)C]PA tracers were administered with food in random order with each diet, and we assessed PA oxidation (PA OX) and serum AC concentration to determine whether a higher PA intake promoted incomplete PA OX. Dietary PA was completely oxidized during the HOA diet, but only about 40% was oxidized during the HPA diet. The [13-(13)C]PA/[1-(13)C]PA ratio of PA OX had an approximate value of 1.0 for either diet, but the ratio of the serum concentrations of MCACs to long-chain ACs (LCACs) was significantly higher during the HPA diet. Thus, direct measurement of PA OX did not confirm that the HPA diet caused incomplete PA OX, despite the modest, but statistically significant, increase in the ratio of MCACs to LCACs in blood.

Long-chain acylcarnitines activate cell stress and myokine release in C2C12 myotubes: calcium-dependent and -independent effects

Acylcarnitines, important lipid biomarkers reflective of acyl-CoA status, are metabolites that possess bioactive and inflammatory properties. This study examined the potential for long-chain acylcarnitines to activate cellular inflammatory, stress, and death pathways in a skeletal muscle model. Differentiated C2C12 myotubes treated with l-C14, C16, C18, and C18:1 carnitine displayed dose-dependent increases in IL-6 production with a concomitant rise in markers of cell permeability and death, which was not observed for shorter chain lengths. l-C16 carnitine, used as a representative long-chain acylcarnitine at initial extracellular concentrations ≥25 μM, increased IL-6 production 4.1-, 14.9-, and 31.4-fold over vehicle at 25, 50, and 100 μM. Additionally, l-C16 carnitine activated c-Jun NH2-terminal kinase, extracellular signal-regulated kinase, and p38 mitogen-activated protein kinase between 2.5- and 11-fold and induced cell injury and death within 6 h with modest activation of the apoptotic caspase-3 protein. l-C16 carnitine rapidly increased intracellular calcium, most clearly by 10 μM, implicating calcium as a potential mechanism for some activities of long-chain acylcarnitines. The intracellular calcium chelator BAPTA-AM blunted l-C16 carnitine-mediated IL-6 production by >65%. However, BAPTA-AM did not attenuate cell permeability and death responses, indicating that these outcomes are calcium independent. The 16-carbon zwitterionic compound amidosulfobetaine-16 qualitatively mimicked the l-C16 carnitine-associated cell stress outcomes, suggesting that the effects of high experimental concentrations of long-chain acylcarnitines are through membrane disruption. Herein, a model is proposed in which acylcarnitine cell membrane interactions take place along a spectrum of cellular concentrations encountered in physiological-to-pathophysiological conditions, thus regulating function of membrane-based systems and impacting cell biology.

Targeting skeletal muscle mitochondria to prevent type 2 diabetes in youth

The prevalence of type 2 diabetes (T2D) has increased dramatically over the past two decades, not only among adults but also among adolescents. T2D is a systemic disorder affecting every organ system and is especially damaging to the cardiovascular system, predisposing individuals to severe cardiac and vascular complications. The precise mechanisms that cause T2D are an area of active research. Most current theories suggest that the process begins with peripheral insulin resistance that precedes failure of the pancreatic β-cells to secrete sufficient insulin to maintain normoglycemia. A growing body of literature has highlighted multiple aspects of mitochondrial function, including oxidative phosphorylation, lipid homeostasis, and mitochondrial quality control in the regulation of peripheral insulin sensitivity. Whether the cellular mechanisms of insulin resistance in adults are comparable to that in adolescents remains unclear. This review will summarize both clinical and basic studies that shed light on how alterations in skeletal muscle mitochondrial function contribute to whole body insulin resistance and will discuss the evidence supporting high-intensity exercise training as a therapy to circumvent skeletal muscle mitochondrial dysfunction to restore insulin sensitivity in both adults and adolescents.

Undernutrition during pregnancy in mice leads to dysfunctional cardiac muscle respiration in adult offspring

We show that in utero undernutrition is associated with impaired cardiac muscle energetics and increased plasma short-chain acylcarnitines in adult mice. Findings suggest that in utero undernutrition is associated with maladaptive programming processes that have negative effects on the heart.

Intrauterine growth restriction (IUGR) is associated with an increased risk of developing obesity, insulin resistance and cardiovascular disease. However, its effect on energetics in heart remains unknown. In the present study, we examined respiration in cardiac muscle and liver from adult mice that were undernourished in utero. We report that in utero undernutrition is associated with impaired cardiac muscle energetics, including decreased fatty acid oxidative capacity, decreased maximum oxidative phosphorylation rate and decreased proton leak respiration. No differences in oxidative characteristics were detected in liver. We also measured plasma acylcarnitine levels and found that short-chain acylcarnitines are increased with in utero undernutrition. Results reveal the negative impact of suboptimal maternal nutrition on adult offspring cardiac energy metabolism, which may have life-long implications for cardiovascular function and disease risk.