Excess lipid availability increases mitochondrial fatty acid oxidative capacity in muscle: evidence against a role for reduced fatty acid oxidation in lipid-induced insulin resistance in rodents

Time Course of Decrease in Skeletal Muscle Mitochondrial Biogenesis after Discontinuation of High-Fat Diet

It is known that a high-fat diet induces an increase in mitochondrial biogenesis in skeletal muscle. To examine the time course of decrease in mitochondrial biogenesis in skeletal muscle after discontinuing a high-fat diet feeding, C57BL/6 mice were fed a high-fat diet for 4 wk and then switched to the control diet for another 3 or 7 d. During the high-fat diet withdrawal period, the protein content of the mitochondrial respiratory chain decreased faster than the fatty acid oxidation enzymes. The mitochondrial DNA copy number remained high for at least 1 wk after withdrawing the high-fat diet. These results suggested that after switching to the control diet following a period of high-fat diet, the increased mitochondrial biogenesis levels are maintained for a few days, and the rate of decline is divergent between the different mitochondrial components.

Exercise ameliorates insulin resistance via regulating TGFβ-activated kinase 1 (TAK1)-mediated insulin signaling in liver of high-fat diet-induced obese rats

Exercise is an effective therapy for insulin resistance. However, the underlying mechanism remains to be elucidated. Previous research demonstrated that TGFβ-activated kinase 1 (TAK1)-dependent signaling plays a crucial character in hepatic insulin resistance. Hepatic ubiquitin specific protease 4 (USP4), USP18, and dual-specificity phosphatases 14 (DUSP14) can suppress TAK1 phosphorylation, besides tumor necrosis factor receptor-associated factor 3 (TRAF3) and tripartite motif 8 (TRIM8) promote its phosphorylation. In this study, we tried to verify our hypothesis that exercise improves insulin resistance in high-fat diet (HFD)-induced obese (DIO) rats via regulating the TAK1 dependent signaling and TAK1 regulators in liver. Forty male Sprague-Dawley rats were randomized into four groups (n = 10): standard diet and sedentary as normal control; fed on HFD and DIO-sedentary; fed on HFD and DIO-chronic exercise; and fed on HFD and DIO-acute exercise. HFD feeding resulted in increased body weight, visceral fat mass, serum FFAs and hepatic lipid deposition, but decreased hepatic glycogen content and insulin sensitivity. Moreover, hepatic TRAF3 and TRIM8 protein levels increased, whereas USP4, USP18, and DUSP14 protein levels were decreased under obese status, which resulted in enhanced TAK1 phosphorylation and impaired insulin signaling. Exercise training, containing chronic and acute mode, both ameliorated insulin resistance. Meanwhile, decreased TAK1, c-Jun N-terminal kinase 1 (JNK1), and insulin receptor substrate 1 (IRS1) phosphorylation enhanced Akt phosphorylation in liver. Moreover, exercise enhanced USP4 and DUSP14 protein levels, whereas decreased TRIM8 protein levels in obese rats' liver. These results showed that exercise triggered a crucial modulation in TAK1-dependent signaling and its regulators in obese rats' liver, and distinct improvement in insulin sensitivity, which provide new insights into the mechanism by which physical exercise improves insulin resistance.

Acylcarnitine profiles in serum and muscle of dairy cows receiving conjugated linoleic acids or a control fat supplement during early lactation

Acylcarnitines (ACC) are formed when fatty acid (FA)-coenzyme A enters the mitochondria for β-oxidation and the tricarboxylic acid cycle through the carnitine shuttle. Concentrations of ACC may vary depending on the metabolic conditions, but can accumulate when rates of β-oxidation exceed those of tricarboxylic acid. This study aimed to characterize muscle and blood serum acylcarnitine profiles, to determine the mRNA abundance of muscle carnitine acyltransferases, and to test whether dietary supplementation (from d 1 in milk) with conjugated linoleic acids (CLA; 100 g/d; each 12% of trans-10,cis-12 and cis-9,trans-11 CLA; n = 11) altered these compared with control fat-supplemented cows (CTR; n = 10). Blood samples and biopsies from the semitendinosus musclewere collected on d -21, 1, 21, and 70 relative to parturition. Serum and muscle ACC profiles were quantified using a targeted metabolomics approach. The CLA supplement did not affect the variables examined. The serum concentration of free carnitine decreased with the onset of lactation. The concentrations of acetylcarnitine, hydroxybutyrylcarnitine, and the sum of short-chain ACC in serum were greater from d -21 to 21 than thereafter. The serum concentrations of long-chain ACC tetradecenoylcarnitine (C14:1) and octadecenoylcarnitine (C18:1) concentrations were greater on d 1 and 21 compared with d -21. Muscle carnitine remained unchanged, whereas short- and medium-chain ACC, including propenoylcarnitine (C3:1), hydroxybutyrylcarnitine, hydroxyhexanoylcarnitine, hexenoylcarnitine (C6:1), and pimelylcarnitine were increased on d 21 compared with d -21 and decreased thereafter. In muscle, the concentrations of long-chain ACC (from C14 to C18) were elevated on d 1. The mRNA abundance of carnitine palmitoyltransferase 1, muscle isoform (CPT1B) increased 2.8-fold from d -21 to 1, followed by a decline to nearly prepartum values by d 70, whereas that of CPT2 did not change over time. The majority of serum and muscle short- and long-chain ACC were positively correlated with the FA concentrations in serum, whereas serum carnitine and C5 were negatively correlated with FA. Time-related changes in the serum and muscle ACC profiles were demonstrated that were not affected by the CLA supplement at the dosage used in the present study. The elevated concentrations of long-chain ACC species in muscle and of serum acetylcarnitine around parturition point to incomplete FA oxidation were likely due to insufficient metabolic adaptation in response to the load of FA around parturition.

Remodeling of skeletal muscle mitochondrial proteome with high-fat diet involves greater changes to β-oxidation than electron transfer proteins in mice

Excess fat intake can increase lipid oxidation and expression of mitochondrial proteins, indicating remodeling of the mitochondrial proteome. Yet intermediates of lipid oxidation also accumulate, indicating a relative insufficiency to completely oxidize lipids. We investigated remodeling of the mitochondrial proteome to determine mechanisms of changes in lipid oxidation following high-fat feeding. C57BL/6J mice consumed a high-fat diet (HFD, 60% fat from lard) or a low-fat diet (LFD, 10% fat) for 12 wk. Mice were fasted for 4 h and then anesthetized by pentobarbital sodium overdose for tissue collection. A mitochondrial-enriched fraction was prepared from gastrocnemius muscles and underwent proteomic analysis by high-resolution mass spectrometry. Mitochondrial respiratory efficiency was measured as the ratio of ATP production to O₂ consumption. Intramuscular acylcarnitines were measured by liquid chromatography-mass spectrometry. A total of 658 mitochondrial proteins were identified: 40 had higher abundance and 14 had lower abundance in mice consuming the HFD than in mice consuming the LFD. Individual proteins that changed with the HFD were primarily related to β-oxidation; there were fewer changes to the electron transfer system. Gene set enrichment analysis indicated that the HFD increased pathways of lipid metabolism and β-oxidation. Intramuscular concentrations of select acylcarnitines (C18:0) were greater in the HFD mice and reflected dietary lipid composition. Mitochondrial respiratory ATP production-to-O₂ consumption ratio for lipids was not different between LFD and HFD mice. After the 60% fat diet, remodeling of the mitochondrial proteome revealed upregulation of proteins regulating lipid oxidation that was not evident for all mitochondrial pathways. The accumulation of lipid metabolites with obesity may occur without intrinsic dysfunction to mitochondrial lipid oxidation.

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

Linoleic (LA) and linolenic (ALA) acid concentrations in follicular fluid of prepubertal goats and their effect on oocyte in vitro maturation and embryo development

In this study we assessed the concentration of linoleic acid (LA) and linolenic acid (ALA) in follicular fluid of prepubertal goats according to follicle size (<3mm or ≥3mm) by gas chromatography and tested the addition of different LA and ALA (LA:ALA) concentration ratios (50:50, 100:50 and 200:50µM) to the IVM medium on embryo development, mitochondrial activity, ATP concentration and relative gene expression (RPL19, ribosomal protein L19; SLC2A1, facilitated glucose transporter 1; ATF4, activating transcription factor 4; GPX1, glutathione peroxidase 1; HSPA5, heat-shock protein family A 70 kDa; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; DNMT1, DNA methyltransferase 1; GCLC, glutamate-cysteine ligase catalytic subunit; SOD1, superoxide dismutase 1). Oocytes were in vitro matured, fertilised or parthenogenetically activated and zygotes were cultured following conventional protocols. LA concentration ranged from 247 to 319µM and ALA concentration from 8.39 to 41.19µM without any effect of follicle size. Blastocyst production from the different groups was: control FCS (22.33%) and BSA (19.63%), treatments 50:50 (22.58%), 100:50 (21.01%) and 200:50 (9.60%). Oocytes from the 200:50 group presented higher polyspermy and mitochondrial activity compared with controls and the rest of the treatment groups. No differences were observed in ATP concentration or relative expression of the genes measured between treatment groups. In conclusion, the low number of blastocysts obtained in the 200:50 group was caused by a high number of polyspermic zygotes, which could suggest that high LA concentration impairs oocyte membranes.

Altered mitochondrial function in insulin-deficient and insulin-resistant states

Diabetes profoundly alters fuel metabolism; both insulin deficiency and insulin resistance are characterized by inefficient mitochondrial coupling and excessive production of reactive oxygen species (ROS) despite their association with normal to high oxygen consumption. Altered mitochondrial function in diabetes can be traced to insulin's pivotal role in maintaining mitochondrial proteome abundance and quality by enhancing mitochondrial biogenesis and preventing proteome damage and degradation, respectively. Although insulin enhances gene transcription, it also induces decreases in amino acids. Thus, if amino acid depletion is not corrected, increased transcription will not result in enhanced translation of transcripts to proteins. Mitochondrial biology varies among tissues, and although most studies in humans are performed in skeletal muscle, abnormalities have been reported in multiple organs in preclinical models of diabetes. Nutrient excess, especially fat excess, alters mitochondrial physiology by driving excess ROS emission that impairs insulin action. Excessive ROS irreversibly damages DNA and proteome with adverse effects on cellular functions. In insulin-resistant people, aerobic exercise stimulates both mitochondrial biogenesis and efficiency concurrent with enhancement of insulin action. This Review discusses the association between both insulin-deficient and insulin-resistant diabetes and alterations in mitochondrial proteome homeostasis and function that adversely affect cellular functions, likely contributing to many diabetic complications.

Impact of Obesity and Hyperglycemia on Placental Mitochondria

A lipotoxic placental environment is recognized in maternal obesity, with increased inflammation and oxidative stress. These changes might alter mitochondrial function, with excessive production of reactive oxygen species, in a vicious cycle leading to placental dysfunction and impaired pregnancy outcomes. Here, we hypothesize that maternal pregestational body mass index (BMI) and glycemic levels can alter placental mitochondria. We measured mitochondrial DNA (mtDNA, real-time PCR) and morphology (electron microscopy) in placentas of forty-seven singleton pregnancies at elective cesarean section. Thirty-seven women were normoglycemic: twenty-one normal-weight women, NW, and sixteen obese women, OB/GDM(−). Ten obese women had gestational diabetes mellitus, OB/GDM(+). OB/GDM(−) presented higher mtDNA levels versus NW, suggesting increased mitochondrial biogenesis in the normoglycemic obese group. These mitochondria showed similar morphology to NW. On the contrary, in OB/GDM(+), mtDNA was not significantly increased versus NW. Nevertheless, mitochondria showed morphological abnormalities, indicating impaired functionality. The metabolic response of the placenta to impairment in obese pregnancies can possibly vary depending on several parameters, resulting in opposite strains acting when insulin resistance of GDM occurs in the obese environment, characterized by inflammation and oxidative stress. Therefore, mitochondrial alterations represent a feature of obese pregnancies with changes in placental energetics that possibly can affect pregnancy outcomes.

Acute activation of pyruvate dehydrogenase increases glucose oxidation in muscle without changing glucose uptake

Pyruvate dehydrogenase (PDH) activity is a key component of the glucose/fatty acid cycle hypothesis for the regulation of glucose uptake and metabolism. We have investigated whether acute activation of PDH in muscle can alleviate the insulin resistance caused by feeding animals a high-fat diet (HFD). The importance of PDH activity in muscle glucose disposal under insulin-stimulated conditions was determined by infusing the PDH kinase inhibitor dichloroacetate (DCA) into HFD-fed Wistar rats during a hyperinsulinemic-euglycemic clamp. Acute DCA infusion did not alter glucose infusion rate, glucose disappearance, or hepatic glucose production but did decrease plasma lactate levels. DCA substantially increased muscle PDH activity; however, this did not improve insulin-stimulated glucose uptake in insulin-resistant muscle of HFD rats. DCA infusion increased the flux of pyruvate to acetyl-CoA and reduced glucose incorporation into glycogen and alanine in muscle. Similarly, in isolated muscle, DCA treatment increased glucose oxidation and decreased glycogen synthesis without changing glucose uptake. These results suggest that, although PDH activity controls the conversion of pyruvate to acetyl-CoA for oxidation, this has little effect on glucose uptake into muscle under insulin-stimulated conditions.

Defining lipid mediators of insulin resistance - controversies and challenges

Essential elements of all cells, lipids play important roles in energy production, signalling and as structural components. Despite these critical functions, excessive availability and intracellular accumulation of lipid is now recognised as a major factor contributing to many human diseases, including obesity and diabetes. In the context of these metabolic disorders, ectopic deposition of lipid has been proposed to have deleterious effects of insulin action. While this relationship has been recognised for some time now, there is currently no unifying mechanism to explain how lipids precipitate the development of insulin resistance. This review summarises the evidence linking specific lipid molecules to the induction of insulin resistance, describing some of the current controversies and challenges for future studies in this field.

ACC2 Deletion Enhances IMCL Reduction Along With Acetyl-CoA Metabolism and Improves Insulin Sensitivity in Male Mice

Intramyocellular lipid (IMCL) accumulation in skeletal muscle greatly contributes to lipid-induced insulin resistance. Because acetyl-coenzyme A (CoA) carboxylase (ACC) 2 negatively modulates mitochondrial fatty acid oxidation (FAO) in skeletal muscle, ACC2 inhibition is expected to reduce IMCL via elevation of FAO and to attenuate insulin resistance. However, the concept of substrate competition suggests that enhanced FAO results in reduced glucose use because of an excessive acetyl-CoA pool in mitochondria. To identify how ACC2-regulated FAO affects IMCL accumulation and glucose metabolism, we generated ACC2 knockout (ACC2-/-) mice and investigated skeletal muscle metabolites associated with fatty acid and glucose metabolism, as well as whole-body glucose metabolism. ACC2-/- mice displayed higher capacity of glucose disposal at the whole-body levels. In skeletal muscle, ACC2-/- mice exhibited enhanced acylcarnitine formation and reduced IMCL levels without alteration in glycolytic intermediate levels. Notably, these changes were accompanied by decreased acetyl-CoA content and enhanced mitochondrial pathways related to acetyl-CoA metabolism, such as the acetylcarnitine production and tricarboxylic acid cycle. Furthermore, ACC2-/- mice exhibited lower levels of IMCL and acetyl-CoA even under HFD conditions and showed protection against HFD-induced insulin resistance. Our findings suggest that ACC2 deletion leads to IMCL reduction without suppressing glucose use via an elevation in acetyl-CoA metabolism even under HFD conditions and offer new mechanistic insight into the therapeutic potential of ACC2 inhibition on insulin resistance.

Protein hypoacylation induced by Sirt5 overexpression has minimal metabolic effect in mice

Sirtuins are a family of evolutionary conserved enzymes that dynamically regulate cellular physiology. Mammals have 7 sirtuins, which are located in different cellular compartments. Sirt5, a sirtuin isoform located in multiple subcellular sites, is involved in regulating a diverse range of cellular and metabolic processes through the removal of a range of acyl-lysine modifications on target proteins. Loss of Sirt5 leads to hyper-malonylation and hyper-succinylation of both mitochondrial and extra-mitochondrial proteins, influencing oxidative phosphorylation, the TCA cycle and glycolysis. However despite these findings, the effect of Sirt5 overexpression on metabolism remains poorly investigated. Here we report that overexpression of Sirt5 has minimal effect on mitochondrial metabolism and overall physiology in mice, despite inducing widespread decreases in protein acylation. Our data confirms the role of Sirt5 as an important demalonylase and desuccinylase enzyme in vivo, but questions the relevance of physiological changes in protein acylation levels in the regulation of cellular metabolism.

Skeletal muscle autophagy remains responsive to hyperinsulinemia and hyperglycemia at higher plasma insulin concentrations in insulin-resistant mice

Skeletal muscle autophagy is suppressed by insulin, but it is not clear if such suppression is altered with insulin resistance. We investigated if the inhibitory action of insulin on autophagy remains intact despite insulin resistance to glucose metabolism. C57BL/6J mice consumed either a low-fat (10% fat) diet as control or high-fat (60% fat) diet for 12 weeks to induce insulin resistance. Following a 5-hour fast, mice underwent either hyperinsulinemic-euglycemic, hyperinsulinemic-hyperglycemic, or saline infusion to test the effect of insulin on autophagy markers in the quadriceps muscle (n = 8-10 per diet and clamp condition). Mice were anesthetized by sodium pentobarbital for tissue collection after 2 h of infusion. Despite the high-fat group having lower insulin-stimulated glucose uptake, both low-fat and high-fat groups had similar autophagosome abundance during hyperinsulinemic conditions. The lipidation of microtubule-associated proteins 1A/1B light chain 3B (LC3II/LC3I) was decreased in hyperinsulinemia versus saline control (P < 0.01) in low-fat (-54%) and high-fat groups (-47%), demonstrating similar suppression of autophagy between diet groups. Mitochondrial-associated LC3II was greater in the high-fat compared to the low-fat group (P = 0.045) across clamp conditions, suggesting a greater localization of autophagosomes with mitochondria. L6 myotubes were treated with insulin and rapamycin to determine the role of mechanistic target of rapamycin complex-1 (mTORC1) in insulin-mediated suppression of autophagy. Inhibition of mTORC1 blunted the decline of LC3II/LC3I with insulin by 40%, suggesting mTORC1 partially mediates the insulin action to suppress autophagy. Collectively, autophagy remained responsive to the suppressive effects of insulin in otherwise insulin-resistant and obese mice.

Exercise induced improvements in insulin sensitivity are concurrent with reduced NFE2/miR-432-5p and increased FAM3A

AIMS

Little is known regarding whether the NFE2/miR-423-5p and FAM3A-ATP-Akt pathway in liver mediates exercise allured alleviation of insulin resistance connected with diet-induced obesity. This research inquired the influence of exercise on liver insulin sensitivity and whole body insulin resistance in high-fat diet fed rats.

MATERIALS AND METHODS

Forty male Sprague-Dawley rats at seven-week-old were assigned to four groups at random: standard diet as normal control group (NC, n = 10), high-fat diet group (HFD, n = 10), high-fat diet with chronic exercise intervention group (HFD-CE, n = 10) and high-fat diet with acute exercise intervention group (HFD-AE, n = 10).

KEY FINDINGS

Compared with rats fed with a standard diet, eight-week high-fat diet feeding lead to elevated body weight, visceral fat content and serum FFAs, and decreased insulin sensitivity index. Moreover, high-fat diet enhanced NFE2 protein expression and miR-423-5p level, decreased FAM3A mRNA and protein expression, ATP level and Akt phosphorylation in liver. In contrast, physical exercise, both chronic and acute exercise alleviated whole body insulin resistance, reduced hepatic NFE2 and miR-423-5p expression, and serum FFAs level, meanwhile enhanced FAM3A mRNA and protein expression, ATP level and Akt phosphorylation in liver. The current findings indicated that exercise in diet-induced obesity, both chronic and acute, induce a momentous regulation in NFE2/miR-423-5p and FAM3A-ATP-Akt pathway in liver, and improve hepatic insulin sensitivity and whole body insulin resistance.

SIGNIFICANCE

All these results supply crucial evidence in our comprehending of the molecular mechanism that connected exercise to an alleviation of insulin resistance.

Nutrition and Training Influences on the Regulation of Mitochondrial Adenosine Diphosphate Sensitivity and Bioenergetics

Since the seminal finding almost 50 years ago that exercise training increases mitochondrial content in skeletal muscle, a considerable amount of research has been dedicated to elucidate the mechanisms inducing mitochondrial biogenesis. The discovery of peroxisome proliferator-activated receptor γ co-activator 1α as a major regulator of exercise-induced gene transcription was instrumental in beginning to understand the signals regulating this process. However, almost two decades after its discovery, our understanding of the signals inducing mitochondrial biogenesis remain poorly defined, limiting our insights into possible novel training modalities in elite athletes that can increase the oxidative potential of muscle. In particular, the role of mitochondrial reactive oxygen species has received very little attention; however, several lifestyle interventions associated with an increase in mitochondrial reactive oxygen species coincide with the induction of mitochondrial biogenesis. Furthermore, the diminishing returns of exercise training are associated with reductions in exercise-induced, mitochondrial-derived reactive oxygen species. Therefore, research focused on altering redox signaling in elite athletes may prove to be effective at inducing mitochondrial biogenesis and augmenting training regimes. In the context of exercise performance, the biological effect of increasing mitochondrial content is an attenuated rise in free cytosolic adenosine diphosphate (ADP), and subsequently decreased carbohydrate flux at a given power output. Recent evidence has shown that mitochondrial ADP sensitivity is a regulated process influenced by nutritional interventions, acute exercise, and exercise training. This knowledge raises the potential to improve mitochondrial bioenergetics in the absence of changes in mitochondrial content. Elucidating the mechanisms influencing the acute regulation of mitochondrial ADP sensitivity could have performance benefits in athletes, especially as these individuals display high levels of mitochondria, and therefore are subjects in whom it is notoriously difficult to further induce mitochondrial adaptations. In addition to changes in ADP sensitivity, an increase in mitochondrial coupling would have a similar bioenergetic response, namely a reduction in free cytosolic ADP. While classically the stoichiometry of the electron transport chain has been considered rigid, recent evidence suggests that sodium nitrate can improve the efficiency of this process, creating the potential for dietary sources of nitrate (e.g., beetroot juice) to display similar improvements in exercise performance. The current review focuses on these processes, while also discussing the biological relevance in the context of exercise performance.

Consumption of Alcopops During Brain Maturation Period: Higher Impact of Fructose Than Ethanol on Brain Metabolism

Alcopops are flavored alcoholic beverages sweetened by sodas, known to contain fructose. These drinks have the goal of democratizing alcohol among young consumers (12-17 years old) and in the past few years have been considered as fashionable amongst teenagers. Adolescence, however, is a key period for brain maturation, occurring in the prefrontal cortex and limbic system until 21 years old. Therefore, this drinking behavior has become a public health concern. Despite the extensive literature concerning the respective impacts of either fructose or ethanol on brain, the effects following joint consumption of these substrates remains unknown. Our objective was to study the early brain modifications induced by a combined diet of high fructose (20%) and moderate amount of alcohol in young rats by ¹³C Nuclear Magnetic Resonance (NMR) spectroscopy. Wistar rats had isocaloric pair-fed diets containing fructose (HF, 20%), ethanol (Et, 0.5 g/day/kg) or both substrates at the same time (HFEt). After 6 weeks of diet, the rats were infused with ¹³C-glucose and brain perchloric acid extracts were analyzed by NMR spectroscopy (¹H and ¹³C). Surprisingly, the most important modifications of brain metabolism were observed under fructose diet. Alterations, observed after only 6 weeks of diet, show that the brain is vulnerable at the metabolic level to fructose consumption during late-adolescence throughout adulthood in rats. The main result was an increase in oxidative metabolism compared to glycolysis, which may impact lactate levels in the brain and may, at least partially, explain memory impairment in teenagers consuming alcopops.

Effects of high-fat diet and AMP-activated protein kinase modulation on the regulation of whole-body lipid metabolism

Metabolic flexibility, the capacity to adapt to fuel availability for energy production, is crucial for maintaining whole-body energy homeostasis. An inability to adequately promote FA utilization is associated with lipid accumulation in peripheral tissues and contributes to the development of insulin resistance. In vivo assays to quantify whole-body lipid oxidation in mouse models of insulin resistance are lacking. We describe a method for assessing whole-body FA oxidation in vivo, as well as tissue-specific lipid uptake in conscious mice. The method relies on intravenous administration of [9,10-³H(N)]palmitic acid combined with a non-β-oxidizable palmitate analog, [1-¹⁴C]2-bromopalmitic acid. Pretreatment with etomoxir, a CPT1 inhibitor that prevents the shuttling of FAs into mitochondria, markedly reduced the appearance of the β-oxidation product ³H₂O in circulation and reduced lipid uptake by oxidative tissues including heart and soleus muscle. Whole-body fatty oxidation was unaltered between chow- or high-fat-fed WT and transgenic mice expressing a mutant form of the AMPK γ3 subunit (AMPKγ3^R225Q) in skeletal muscle. High-fat feeding increased lipid oxidation in WT and AMPKγ3^R225Q transgenic mice. In conclusion, this technique allows for the assessment of the effect of pharmaceutical agents, as well as gene mutations, on whole-body FA oxidation in mice.

Mitochondrial respiratory capacity remains stable despite a comprehensive and sustained increase in insulin sensitivity in obese patients undergoing gastric bypass surgery

AIM

It has been proposed, but not yet demonstrated by convincing evidence in published articles, that insulin resistance and mitochondrial respiratory function are causally related physiological phenomena. Here, we tested the prediction that weight loss-induced increase in insulin sensitivity will correlate with a corresponding change in mitochondrial respiratory capacity over the same time period.

METHODS

Insulin sensitivity was evaluated using the hyperinsulinaemic-euglycaemic clamp technique, and skeletal muscle mitochondrial respiratory capacity was evaluated by high-resolution respirometry in 26 patients with obesity. Each experiment was performed ~2 months and 1-2 weeks before, and ~4 and ~19 months after Roux-en-Y gastric bypass (RYGB) surgery.

RESULTS

A substantial weight loss was observed in all patients, and insulin sensitivity increased in all patients over the 21-months time period of the study. In contrast, skeletal muscle mitochondrial respiratory capacity, intrinsic mitochondrial respiratory capacity and mitochondrial content remained unchanged over the same time period.

CONCLUSION

Among obese patients with and without type 2 diabetes undergoing RYGB surgery, intrinsic mitochondrial respiratory capacity in skeletal muscle is not correlated with insulin sensitivity before or after the surgical intervention. Mitochondrial respiratory function may not be germane to the pathophysiology and/or aetiology of obesity and/or type 2 diabetes.

Gastric inhibitory polypeptide receptor antagonist, SKL-14959, suppressed body weight gain on diet-induced obesity mice

Objective

Gastric inhibitory polypeptide plays a role in glucose and lipid metabolism and is associated with obesity and insulin resistance. The objective of this study is to confirm the anti-obesity effects of the gastric inhibitory polypeptide receptor antagonist, SKL-14959, on diet-induced obesity mice.

Method

Diet-induced obesity mice at 20 weeks of age were administered with or without SKL-14959 for 96 d. Body weight and food intake were monitored throughout the experiment. Mice were sacrificed, and physiological and biochemical markers were measured, and then histochemical and gene expression analyses were also performed. In further studies, mice were orally gavaged with [14C]-oleic acid to investigate the excursion of digested lipids.

Results

SKL-14959 significantly suppressed weight gain without affecting food intake, decreased triacylglycerol contents in the liver and the muscle and the intensity stained with oil-red in the liver. It also improved plasma glutamic pyruvic transaminase and 3-hydroxybutyrate levels in addition to notably down-regulated relative gene expression of srebf1 and dgat1 in the liver despite not altering in the adipose tissue. Furthermore, SKL-14959 showed remarkable inhibition of lipid uptake in the adipose tissue after the oil challenge.

Conclusion

SKL-14959 inhibited lipids uptake and improved lipids metabolism, results in suppression of body-weight gain.

Region-specific differences in bioenergetic proteins and protein response to acute high fat diet in brains of low and high capacity runner rats

Aerobic capacity is a strong predictor of mortality. Low capacity runner (LCR) rats exhibit reduced mitochondrial function in peripheral organs. A high fat diet (HFD) can worsen metabolic phenotype in LCR rats. Little is known about metabolic changes in the brains of these rats, however. This study examined protein markers of mitochondrial function and metabolism as a function of aerobic running capacity and an acute HFD in four brain regions: the striatum, hippocampus, hypothalamus, and substantia nigra. After 3 days HFD or chow diets, we measured peroxisome proliferator-activated receptor-γ coactivator 1α (PGC1-α), nuclear respiratory factors 1 (Nrf-1), mitochondrial transcription factor A (TFAM), and phosphorylated (activated) AMP-activated protein kinase (p-AMPK) protein levels in the four brain regions. LCR rats exhibited lower levels of mitochondrial proteins (PGC1-α, Nrf-1, TFAM), and greater p-AMPK, in striatum, but not in the other brain regions. Mitochondrial protein levels were greater in HFD LCR striatum, while p-AMPK was lower in this group. Markers of lower mitochondrial biogenesis and increased metabolic demand were limited to the LCR striatum, which nevertheless maintained the capacity to respond to an acute HFD challenge.

Modeling insulin resistance in rodents by alterations in diet: what have high-fat and high-calorie diets revealed?

For over half a century, researchers have been feeding different diets to rodents to examine the effects of macronutrients on whole body and tissue insulin action. During this period, the number of different diets and the source of macronutrients employed have grown dramatically. Because of the large heterogeneity in both the source and percentage of different macronutrients used for studies, it is not surprising that different high-calorie diets do not produce the same changes in insulin action. Despite this, diverse high-calorie diets continue to be employed in an attempt to generate a "generic" insulin resistance. The high-fat diet in particular varies greatly between studies with regard to the source, complexity, and ratio of dietary fat, carbohydrate, and protein. This review examines the range of rodent dietary models and methods for assessing insulin action. In almost all studies reviewed, rodents fed diets that had more than 45% of dietary energy as fat or simple carbohydrates had reduced whole body insulin action compared with chow. However, different high-calorie diets produced significantly different effects in liver, muscle, and whole body insulin action when insulin action was measured by the hyperinsulinemic-euglycemic clamp method. Rodent dietary models remain an important tool for exploring potential mechanisms of insulin resistance, but more attention needs to be given to the total macronutrient content and composition when interpreting dietary effects on insulin action.

Proteomics Analysis of Skeletal Muscle from Leptin-Deficient ob/ob Mice Reveals Adaptive Remodeling of Metabolic Characteristics and Fiber Type Composition

Skeletal muscle insulin resistance, an early metabolic defect in the pathogenesis of type 2 diabetes (T2D), may be a cause or consequence of altered protein expression profiles. Proteomics technology offers enormous promise to investigate molecular mechanisms underlying pathologies, however, the analysis of skeletal muscle is challenging. Using state-of-the-art multienzyme digestion and filter-aided sample preparation (MED-FASP) and a mass spectrometry (MS)-based workflow, we performed a global proteomics analysis of skeletal muscle from leptin-deficient, obese, insulin resistant (ob/ob) and lean mice in mere two fractions in a short time (8 h per sample). We identified more than 6000 proteins with 118 proteins differentially regulated in obesity. This included protein kinases, phosphatases, and secreted and fiber type associated proteins. Enzymes involved in lipid metabolism in skeletal muscle from ob/ob mice were increased, providing evidence against reduced fatty acid oxidation in lipid-induced insulin resistance. Mitochondrial and peroxisomal proteins, as well as components of pyruvate and lactate metabolism, were increased. Finally, the skeletal muscle proteome from ob/ob mice displayed a shift toward the "slow fiber type." This detailed characterization of an obese rodent model of T2D demonstrates an efficient workflow for skeletal muscle proteomics, which may easily be adapted to other complex tissues.

Vinyl chloride dysregulates metabolic homeostasis and enhances diet-induced liver injury in mice

Vinyl chloride (VC), a common industrial organochlorine and environmental pollutant, has been shown to directly cause hepatic angiosarcoma and toxicant‐associated steatohepatitis at high exposure levels. However, the impact of lower concentrations of VC on the progression of underlying liver diseases (e.g., nonalcoholic fatty liver disease [NAFLD]) is unclear. Given the high prevalence of NAFLD in the United States (and worldwide) population, this is an important concern. Recent studies by our group with VC metabolites suggest a potential interaction between VC exposure and underlying liver disease to cause enhanced damage. Here, a novel mouse model determined the effects of VC inhalation at levels below the current Occupational Safety and Health Administration limit (<1 ppm) in the context of NAFLD to better mimic human exposure and identify potential mechanisms of VC‐induced liver injury. VC exposure caused no overt liver injury in mice fed a low‐fat diet. However, in mice fed a high‐fat diet (HFD), VC significantly increased liver damage, steatosis, and increased neutrophil infiltration. Moreover, VC further enhanced HFD‐induced oxidative and endoplasmic reticulum stress. Importantly, VC exposure dysregulated energy homeostasis and impaired mitochondrial function, even in mice fed a low‐fat diet. In toto, the results indicate that VC exposure causes metabolic stress that sensitizes the liver to steatohepatitis caused by HFD. Conclusion: The hypothesis that low‐level (below the Occupational Safety and Health Administration limit) chronic exposure to VC by inhalation enhances liver injury caused by an HFD is supported. Importantly, our data raise concerns about the potential for overlap between fatty diets (i.e., Western diet) and exposure to VC and the health implications of this co‐exposure for humans. It also emphasizes that current safety restrictions may be insufficient to account for other factors that can influence hepatotoxicity. (Hepatology Communications 2018;2:270‐284)

Impact of prolonged overfeeding on skeletal muscle mitochondria in healthy individuals

AIMS/HYPOTHESES

Reduced mitochondrial capacity in skeletal muscle has been observed in obesity and type 2 diabetes. In humans, the aetiology of this abnormality is not well understood but the possibility that it is secondary to the stress of nutrient overload has been suggested. To test this hypothesis, we examined whether sustained overfeeding decreases skeletal muscle mitochondrial content or impairs function.

METHODS

Twenty-six healthy volunteers (21 men, 5 women, age 25.3 ± 4.5 years, BMI 25.5 ± 2.4 kg/m²) underwent a supervised protocol consisting of 8 weeks of high-fat overfeeding (40% over baseline energy requirements). Before and after overfeeding, we measured systemic fuel oxidation by indirect calorimetry and performed skeletal muscle biopsies to measure mitochondrial gene expression, content and function in vitro. Mitochondrial function in vivo was measured by ³¹P NMR spectroscopy.

RESULTS

With overfeeding, volunteers gained 7.7 ± 1.8 kg (% change 9.8 ± 2.3). Overfeeding increased fasting NEFA, LDL-cholesterol and insulin concentrations. Indirect calorimetry showed a shift towards greater reliance on lipid oxidation. In skeletal muscle tissue, overfeeding increased ceramide content, lipid droplet content and perilipin-2 mRNA expression. Phosphorylation of AMP-activated protein kinase was decreased. Overfeeding increased mRNA expression of certain genes coding for mitochondrial proteins (CS, OGDH, CPT1B, UCP3, ANT1). Despite the stress of nutrient overload, mitochondrial content and mitochondrial respiration in muscle did not change after overfeeding. Similarly, overfeeding had no effect on either the emission of reactive oxygen species or on mitochondrial function in vivo.

CONCLUSIONS/INTERPRETATION

Skeletal muscle mitochondria are significantly resilient to nutrient overload. The lower skeletal muscle mitochondrial oxidative capacity in human obesity is likely to be caused by reasons other than nutrient overload per se.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01672632.

The influence of Shc proteins and high-fat diet on energy metabolism of mice

The purpose of this study was to determine if Shc proteins influence the metabolic response to acute (7 days) feeding of a high-fat diet (HFD). To this end, whole animal energy expenditure (EE) and substrate oxidation were measured in the Shc knockout (ShcKO) and wild-type (WT) mice fed a control or HFD. The activities of enzymes of glycolysis, the citric acid cycle, electron transport chain (ETC), and β-oxidation were also investigated in liver and skeletal muscle of ShcKO and WT animals. The study showed that ShcKO increases (P < .05) EE adjusted for either total body weight or lean mass. This change in EE could contribute to decreases in weight gain in ShcKO versus WT mice fed an HFD. Thus, our results indicate that Shc proteins should be considered as potential targets for developing interventions to mitigate weight gain on HFD by stimulating EE. Although decreased levels of Shc proteins influenced the activity of some enzymes in response to high-fat feeding (eg, increasing the activity of acyl-CoA dehydrogenase), it did not produce concerted changes in enzymes of glycolysis, citric acid cycle, or the ETC. The physiological significance of observed changes in select enzyme activities remains to be determined.

SIGNIFICANCE OF THE STUDY

We report higher EE in ShcKO versus WT mice when consuming the HFD. Although decreased levels of Shc proteins influenced the activity of a central enzyme of β-oxidation in response to high-fat feeding, it did not produce concerted changes in enzymes of glycolysis, citric acid cycle, or the ETC. Thus, an increase in EE in response to consumption of an HFD may be a mechanism that leads to decreased weight gain previously reported in ShcKO mice with long-term consumption of an HFD.

Associations of Mitochondrial Fatty Acid Oxidation with Body Fat in Premenopausal Women

Higher in vivo fatty acid (FA) oxidation rates have been reported in obese individuals compared to lean counterparts; however whether this reflects a shift in substrate-specific oxidative capacity at the level of the skeletal muscle mitochondria has not been examined. The purpose of this study was to test the hypothesis that in situ measures of skeletal muscle mitochondria FA oxidation would be positively associated with total body fat. Participants were 38 premenopausal women (BMI = 26.5 ± 4.3 kg/m²). Total and regional fat were assessed by dual-energy X-ray absorptiometry (DXA). Mitochondrial FA oxidation was assessed in permeabilized myofibers using high-resolution respirometry and a palmitoyl carnitine substrate. We found positive associations of total fat mass with State 3 (ADP-stimulated respiration) (r = 0.379, p < 0.05) and the respiratory control ratio (RCR, measure of mitochondrial coupling) (r = 0.348, p < 0.05). When participants were dichotomized by high or low body fat percent, participants with high total body fat displayed a higher RCR compared to those with low body fat (p < 0.05). There were no associations between any measure of regional fat and mitochondrial FA oxidation independent of total fat mass. In conclusion, greater FA oxidation in obesity may reflect molecular processes that enhance FA oxidation capacity at the mitochondrial level.

Effect of Combined Exercise Versus Aerobic-Only Training on Skeletal Muscle Lipid Metabolism in a Rodent Model of Type 1 Diabetes

OBJECTIVES

Abnormal skeletal muscle lipid metabolism is associated with insulin resistance in people with type 1 diabetes. Although lipid metabolism is restored with aerobic exercise training, the risk for postexercise hypoglycemia is increased with this modality. Integrating resistance and aerobic exercise is associated with reduced hypoglycemic risk; however, the effects of this exercise modality on lipid metabolism and insulin resistance remain unknown. We compared the effects of combined (aerobic + resistance) versus aerobic exercise training on oxidative capacity and muscle lipid metabolism in a rat model of type 1 diabetes.

METHODS

Male Sprague-Dawley rats were divided into 4 groups: sedentary control (C), sedentary control + diabetes (CD), diabetes + high-intensity aerobic exercise (DAE) and diabetes + combined aerobic and resistance exercise (DARE). Following diabetes induction (20 mg/kg streptozotocin over five days), DAE rats ran for 12 weeks (5 days/week for 1 hour) on a motorized treadmill (27 m/min at a 6-degree grade), and DARE rats alternated daily between running and incremental weighted ladder climbing.

RESULTS

After training, DAE showed reduced muscle CD36 protein content and lipid content compared to CD (p≤0.05). DAE rats also had significantly increased citrate synthase (CS) activity compared to CD (p≤0.05). DARE rats showed reduced CD36 protein content compared to CD and increased CS activity compared to CD and DAE rats (p≤0.05). DARE rats demonstrated increased skeletal muscle lipid staining, elevated lipin-1 protein content and insulin sensitivity (p≤0.05).

CONCLUSIONS

Integration of aerobic and resistance exercise may exert a synergistic effect, producing adaptations characteristic of the "athlete's paradox," including increased capacity to store and oxidize lipids.

Long-term rates of mitochondrial protein synthesis are increased in mouse skeletal muscle with high-fat feeding regardless of insulin-sensitizing treatment

Skeletal muscle mitochondrial protein synthesis is regulated in part by insulin. The development of insulin resistance with diet-induced obesity may therefore contribute to impairments to protein synthesis and decreased mitochondrial respiration. Yet the impact of diet-induced obesity and insulin resistance on mitochondrial energetics is controversial, with reports varying from decreases to increases in mitochondrial respiration. We investigated the impact of changes in insulin sensitivity on long-term rates of mitochondrial protein synthesis as a mechanism for changes to mitochondrial respiration in skeletal muscle. Insulin resistance was induced in C57BL/6J mice using 4 wk of a high-fat compared with a low-fat diet. For 8 additional weeks, diets were enriched with pioglitazone to restore insulin sensitivity compared with nonenriched control low-fat or high-fat diets. Skeletal muscle mitochondrial protein synthesis was measured using deuterium oxide labeling during weeks 10-12 High-resolution respirometry was performed using palmitoyl-l-carnitine, glutamate+malate, and glutamate+malate+succinate as substrates for mitochondria isolated from quadriceps. Mitochondrial protein synthesis and palmitoyl- l-carnitine oxidation were increased in mice consuming a high-fat diet, regardless of differences in insulin sensitivity with pioglitazone treatment. There was no effect of diet or pioglitazone treatment on ADP-stimulated respiration or H₂O₂ emission using glutamate+malate or glutamate+malate+succinate. The results demonstrate no impairments to mitochondrial protein synthesis or respiration following induction of insulin resistance. Instead, mitochondrial protein synthesis was increased with a high-fat diet and may contribute to remodeling of the mitochondria to increase lipid oxidation capacity. Mitochondrial adaptations with a high-fat diet appear driven by nutrient availability, not intrinsic defects that contribute to insulin resistance.

Fenofibrate reverses changes induced by high-fat diet on metabolism in mice muscle and visceral adipocytes

The effect of fenofibrate on the metabolism of skeletal muscle and visceral white adipose tissue of diet-induced obese (DIO) mice was investigated. C57BL/6J male mice were fed either a control or high-fat diet for 8 weeks. Fenofibrate (50 mg/Kg BW, daily) was administered by oral gavage during the last two weeks of the experimental period. Insulin-stimulated glucose metabolism in soleus muscles, glucose tolerance test, insulin tolerance test, indirect calorimetry, lipolysis of visceral white adipose tissue, expression of miR-103-3p in adipose tissue, and miR-1a, miR-133a/b, miR-206, let7b-5p, miR-23b-3p, miR-29-3p, miR-143-3p in soleus muscle, genes related to glucose and fatty acid metabolism in adipose tissue and soleus muscle, and proteins (phospho-AMPKα2, Pgc1α, Cpt1b), intramuscular lipid staining, and activities of fatty acid oxidation enzymes in skeletal muscle were investigated. In DIO mice, fenofibrate prevented weight gain induced by HFD feeding by increasing energy expenditure; improved whole body glucose homeostasis, and in skeletal muscle, increased insulin dependent glucose uptake, miR-1a levels, reduced intramuscular lipid accumulation, and phospho-AMPKα2 levels. In visceral adipose tissue of obese mice, fenofibrate decreased basal lipolysis rate and visceral adipocytes hypertrophy, and induced the expression of Glut-4, Irs1, and Cav-1 mRNA and miR-103-3p suggesting a higher insulin sensitivity of the adipocytes. The evidence is presented herein that beneficial effects of fenofibrate on body weight, glucose homeostasis, and muscle metabolism might be related to its action in adipose tissue. Moreover, fenofibrate regulates miR-1a-3p in soleus and miR-103-3p in adipose tissue, suggesting these microRNAs might contribute to fenofibrate beneficial effects on metabolism.

Plasma acylcarnitine profiling indicates increased fatty acid oxidation relative to tricarboxylic acid cycle capacity in young, healthy low birth weight men

We hypothesized that an increased, incomplete fatty acid beta‐oxidation in mitochondria could be part of the metabolic events leading to insulin resistance and thereby an increased type 2 diabetes risk in low birth weight (LBW) compared with normal birth weight (NBW) individuals. Therefore, we measured fasting plasma levels of 45 acylcarnitine species in 18 LBW and 25 NBW men after an isocaloric control diet and a 5‐day high‐fat, high‐calorie diet. We demonstrated that LBW men had higher C2 and C4‐OH levels after the control diet compared with NBW men, indicating an increased fatty acid beta‐oxidation relative to the tricarboxylic acid cycle flux. Also, they had higher C6‐DC, C10‐OH/C8‐DC, and total hydroxyl‐/dicarboxyl‐acylcarnitine levels, which may suggest an increased fatty acid omega‐oxidation in the liver. Furthermore, LBW and NBW men decreased several acylcarnitine levels in response to overfeeding, which is likely a result of an upregulation of fatty acid oxidation due to the dietary challenge. Moreover, C10‐OH/C8‐DC and total hydroxyl‐/dicarboxyl‐acylcarnitine levels tended to be negatively associated with the serum insulin level, and the total hydroxyl‐/dicarboxyl‐acylcarnitine level additionally tended to be negatively associated with the hepatic insulin resistance index. This indicates that an increased fatty acid omega‐oxidation could be a compensatory mechanism to prevent an accumulation of lipid species that impair insulin signaling.

Wheat germ supplementation alleviates insulin resistance and cardiac mitochondrial dysfunction in an animal model of diet-induced obesity

Obesity is strongly associated with insulin resistance (IR), along with mitochondrial dysfunction to metabolically active tissues and increased production of reactive O2 species (ROS). Foods rich in antioxidants such as wheat germ (WG), protect tissues from damage due to ROS and modulate some negative effects of obesity. This study examined the effects of WG supplementation on markers of IR, mitochondrial substrate metabolism and innate antioxidant markers in two metabolically active tissues (i.e. liver and heart) of C57BL/6 mice fed a high-fat–high-sucrose (HFS) diet. Male C57BL/6 mice, 6-week-old, were randomised into four dietary treatment groups (n 12 mice/group): control (C, 10 % fat kcal), C+10 % WG, HFS (60 % fat kcal) or HFS+10 % WG (HFS+WG). After 12 weeks of treatment, HFS+WG mice had significantly less visceral fat (−16 %, P=0·006) compared with the HFS group. WG significantly reduced serum insulin (P=0·009), the insulinotropic hormone, gastric inhibitory peptide (P=0·0003), and the surrogate measure of IR, homoeostatic model assessment of IR (P=0·006). HFS diet significantly elevated (45 %, P=0·02) cardiac complex 2 mitochondrial VO2, suggesting increased metabolic stress, whereas WG stabilised this effect to the level of control. Consequently, genes which mediate antioxidant defense and mitochondrial biogenesis (superoxide dismutase 2 (Sod2) and PPARγ coactivator 1-α (Pgc1a), respectively) were significantly reduced (P<0·05) in the heart of the HFS group, whereas WG supplementation tended to up-regulate both genes. WG significantly increased hepatic gene expression of Sod2 (P=0·048) but not Pgc1a. Together, these results showed that WG supplementation in HFS diet, reduced IR and improved cardiac mitochondrial metabolic functions.

Dietary Fat Quantity and Type Induce Transcriptome-Wide Effects on Alternative Splicing of Pre-mRNA in Rat Skeletal Muscle

Background: Fat-enriched diets produce metabolic changes in skeletal muscle, which in turn can mediate changes in gene regulation.Objective: We examined the high-fat-diet-induced changes in skeletal muscle gene expression by characterizing variations in pre-mRNA alternative splicing.Methods: Affymetrix Exon Array analysis was performed on the transcriptome of the gastrocnemius/plantaris complex of male obesity-prone Sprague-Dawley rats fed a 10% or 60% fat (lard) diet for 2 or 8 wk. The validation of exon array results was focused on troponin T (Tnnt3). Tnnt3 splice form analyses were extended in studies of rats fed 10% or 30% fat diets across 1- to 8-wk treatment periods and rats fed 10% or 45% fat diets with fat sources from lard or mono- or polyunsaturated fats for 2 wk. Nuclear magnetic resonance (NMR) was used to measure body composition.Results: Consumption of a 60% fat diet for 2 or 8 wk resulted in alternative splicing of 668 and 726 pre-mRNAs, respectively, compared with rats fed a 10% fat diet. Tnnt3 transcripts were alternatively spliced in rats fed a 60% fat diet for either 2 or 8 wk. The high-fat-diet-induced changes in Tnnt3 alternative splicing were observed in rats fed a 30% fat diet across 1- to 8-wk treatment periods. Moreover, this effect depended on fat type, because Tnnt3 alternative splicing occurred in response to 45% fat diets enriched with lard but not in response to diets enriched with mono- or polyunsaturated fatty acids. Fat mass (a proxy for obesity as measured by NMR) did not differ between groups in any study.Conclusions: Rat skeletal muscle responds to overconsumption of dietary fat by modifying gene expression through pre-mRNA alternative splicing. Variations in Tnnt3 alternative splicing occur independently of obesity and are dependent on dietary fat quantity and suggest a role for saturated fatty acids in the high-fat-diet-induced modifications in Tnnt3 alternative splicing.

Defects in skeletal muscle subsarcolemmal mitochondria in a non-obese model of type 2 diabetes mellitus

Skeletal muscle resistance to insulin is related to accumulation of lipid-derived products, but it is not clear whether this accumulation is caused by skeletal muscle mitochondrial dysfunction. Diabetes and obesity are reported to have a selective effect on the function of subsarcolemmal and interfibrillar mitochondria in insulin-resistant skeletal muscle. The current study investigated the role of the subpopulations of mitochondria in the pathogenesis of insulin resistance in the absence of obesity. A non-obese spontaneous rat model of type 2 diabetes mellitus, (Goto-Kakizaki), was used to evaluate function and biochemical properties in both populations of skeletal muscle mitochondria. In subsarcolemmal mitochondria, minor defects are observed whereas in interfibrillar mitochondria function is preserved. Subsarcolemmal mitochondria defects characterized by a mild decline of oxidative phosphorylation efficiency are related to ATP synthase and structural alterations of inner mitochondria membrane but are considered unimportant because of the absence of defects upstream as shown with polarographic and spectrophometric assays. Fatty acid transport and oxidation is preserved in both population of mitochondria, whereas palmitoyl-CoA increased 25% in interfibrillar mitochondria of diabetic rats. Contrary to popular belief, these data provide compelling evidence that mitochondrial function is unaffected in insulin-resistant skeletal muscle from T2DM non-obese rats.

8-oxoguanine DNA glycosylase (OGG1) deficiency elicits coordinated changes in lipid and mitochondrial metabolism in muscle

Oxidative stress resulting from endogenous and exogenous sources causes damage to cellular components, including genomic and mitochondrial DNA. Oxidative DNA damage is primarily repaired via the base excision repair pathway that is initiated by DNA glycosylases. 8-oxoguanine DNA glycosylase (OGG1) recognizes and cleaves oxidized and ring-fragmented purines, including 8-oxoguanine, the most commonly formed oxidative DNA lesion. Mice lacking the OGG1 gene product are prone to multiple features of the metabolic syndrome, including high-fat diet-induced obesity, hepatic steatosis, and insulin resistance. Here, we report that OGG1-deficient mice also display skeletal muscle pathologies, including increased muscle lipid deposition and alterations in genes regulating lipid uptake and mitochondrial fission in skeletal muscle. In addition, expression of genes of the TCA cycle and of carbohydrate and lipid metabolism are also significantly altered in muscle of OGG1-deficient mice. These tissue changes are accompanied by marked reductions in markers of muscle function in OGG1-deficient animals, including decreased grip strength and treadmill endurance. Collectively, these data indicate a role for skeletal muscle OGG1 in the maintenance of optimal tissue function.

Intramuscular triglyceride content precedes impaired glucose metabolism without evidence for mitochondrial dysfunction during early development of a diabetic phenotype

The incidence of type 2 diabetes is highly correlated with obesity; however, there is a lack of research elucidating the temporal progression. Transgenic FVB/N UCP-dta mice, which develop a diabetic phenotype, and their nontransgenic littermates were fed either a high-fat or normal-chow diet and were studied at 6, 9, 12, 15, 18, 21, and 24 weeks of age to test the hypothesis that increased lipid accumulation in skeletal muscle causes mitochondrial dysfunction, leading to the development of insulin resistance. Body composition, intramuscular triglyceride (IMTG) content, glucose metabolism, and mitochondrial function were measured to determine if IMTG drove mitochondrial dysfunction, leading to the development of type 2 diabetes. High-fat-fed transgenic mice had a significantly greater body mass, lipid mass, and IMTG content beginning early in the experiment. Glucose tolerance tests revealed that high-fat-fed transgenic mice developed a significantly insulin resistant response compared with the other 3 groups toward the end of the time course while plasma insulin was elevated very early in the time course. There was no significant difference in several measures of metabolic function throughout the time course. Long-term high-fat feeding in transgenic mice produced increases in IMTG, adiposity, body mass, and plasma insulin accompanied by decreases in glucose metabolism, but did not reveal any deficits in mitochondrial function or regulation during the early stage of the development of type 2 diabetes. It does not appear that lipotoxicity is driving defects in mitochondrial function prior to the onset of insulin resistance.

Obesity-induced discrepancy between contractile and metabolic phenotypes in slow- and fast-twitch skeletal muscles of female obese Zucker rats

A clear picture of skeletal muscle adaptations to obesity and related comorbidities remains elusive. This study describes fiber-type characteristics (size, proportions, and oxidative enzyme activity) in two typical hindlimb muscles with opposite structure and function in an animal model of genetic obesity. Lesser fiber diameter, fiber-type composition, and histochemical succinic dehydrogenase activity (an oxidative marker) of muscle fiber types were assessed in slow (soleus)- and fast (tibialis cranialis)-twitch muscles of obese Zucker rats and compared with age (16 wk)- and sex (females)-matched lean Zucker rats (n = 16/group). Muscle mass and lesser fiber diameter were lower in both muscle types of obese compared with lean animals even though body weights were increased in the obese cohort. A faster fiber-type phenotype also occurred in slow- and fast-twitch muscles of obese rats compared with lean rats. These adaptations were accompanied by a significant increment in histochemical succinic dehydrogenase activity of slow-twitch fibers in the soleus muscle and fast-twitch fiber types in the tibialis cranialis muscle. Obesity significantly increased plasma levels of proinflammatory cytokines but did not significantly affect protein levels of peroxisome proliferator-activated receptors PPARγ or PGC1α in either muscle. These data demonstrate that, in female Zucker rats, obesity induces a reduction of muscle mass in which skeletal muscles show a diminished fiber size and a faster and more oxidative phenotype. It was noteworthy that this discrepancy in muscle's contractile and metabolic features was of comparable nature and extent in muscles with different fiber-type composition and antagonist functions.NEW & NOTEWORTHY This study demonstrates a discrepancy between morphological (reduced muscle mass), contractile (shift toward a faster phenotype), and metabolic (increased mitochondrial oxidative enzyme activity) characteristics in skeletal muscles of female Zucker fatty rats. It is noteworthy that this inconsistency was comparable (in nature and extent) in muscles with different structure and function.

Neuregulin 1 improves complex 2-mediated mitochondrial respiration in skeletal muscle of healthy and diabetic mice

It has been reported that neuregulin1 (NRG1) improves glucose tolerance in healthy and diabetic rodents. In vitro studies also suggest that NRG1 regulates myocyte oxidative capacity. To confirm this observation in vivo, we evaluated the effect on mitochondrial function of an 8-week treatment with NRG1 in db/db diabetic mice and C57BL/6JRJ healthy controls. NRG1 treatment improved complex 2-mediated mitochondrial respiration in the gastrocnemius of both control and diabetic mice and increased mitochondrial complex 2 subunit content by 2-fold. This effect was not associated with an increase in mitochondrial biogenesis markers. Enhanced ERBB4 phosphorylation could mediate NRG1 effects on mitochondrial function through signalling pathways, independently of ERK1/2, AKT or AMPK.

Long-term, but not short-term high-fat diet induces fiber composition changes and impaired contractile force in mouse fast-twitch skeletal muscle

In this study, we investigated the effects of a short-term and long-term high-fat diet (HFD) on morphological and functional features of fast-twitch skeletal muscle. Male C57BL/6J mice were fed a HFD (60% fat) for 4 weeks (4-week HFD) or 12 weeks (12-week HFD). Subsequently, the fast-twitch extensor digitorum longus muscle was isolated, and the composition of muscle fiber type, expression levels of proteins involved in muscle contraction, and force production on electrical stimulation were analyzed. The 12-week HFD, but not the 4-week HFD, resulted in a decreased muscle tetanic force on 100 Hz stimulation compared with control (5.1 ± 1.4 N/g in the 12-week HFD vs. 7.5 ± 1.7 N/g in the control group; P < 0.05), whereas muscle weight and cross-sectional area were not altered after both HFD protocols. Morphological analysis indicated that the percentage of type IIx myosin heavy chain fibers, mitochondrial oxidative enzyme activity, and intramyocellular lipid levels increased in the 12-week HFD group, but not in the 4-week HFD group, compared with controls (P < 0.05). No changes in the expression levels of calcium handling-related proteins and myofibrillar proteins (myosin heavy chain and actin) were detected in the HFD models, whereas fast-troponin T-protein expression was decreased in the 12-week HFD group, but not in the 4-week HFD group (P < 0.05). These findings indicate that a long-term HFD, but not a short-term HFD, impairs contractile force in fast-twitch muscle fibers. Given that skeletal muscle strength largely depends on muscle fiber type, the impaired muscle contractile force by a HFD might result from morphological changes of fiber type composition.

Epigenetic regulation of skeletal muscle metabolism

Normal skeletal muscle metabolism is essential for whole body metabolic homoeostasis and disruptions in muscle metabolism are associated with a number of chronic diseases. Transcriptional control of metabolic enzyme expression is a major regulatory mechanism for muscle metabolic processes. Substantial evidence is emerging that highlights the importance of epigenetic mechanisms in this process. This review will examine the importance of epigenetics in the regulation of muscle metabolism, with a particular emphasis on DNA methylation and histone acetylation as epigenetic control points. The emerging cross-talk between metabolism and epigenetics in the context of health and disease will also be examined. The concept of inheritance of skeletal muscle metabolic phenotypes will be discussed, in addition to emerging epigenetic therapies that could be used to alter muscle metabolism in chronic disease states.

Faecalibacterium prausnitzii treatment improves hepatic health and reduces adipose tissue inflammation in high-fat fed mice

Faecalibacterium prausnitzii is considered as one of the most important bacterial indicators of a healthy gut. We studied the effects of oral F. prausnitzii treatment on high-fat fed mice. Compared to the high-fat control mice, F. prausnitzii-treated mice had lower hepatic fat content, aspartate aminotransferase and alanine aminotransferase, and increased fatty acid oxidation and adiponectin signaling in liver. Hepatic lipidomic analyses revealed decreases in several species of triacylglycerols, phospholipids and cholesteryl esters. Adiponectin expression was increased in the visceral adipose tissue, and the subcutaneous and visceral adipose tissues were more insulin sensitive and less inflamed in F. prausnitzii-treated mice. Further, F. prausnitzii treatment increased muscle mass that may be linked to enhanced mitochondrial respiration, modified gut microbiota composition and improved intestinal integrity. Our findings show that F. prausnitzii treatment improves hepatic health, and decreases adipose tissue inflammation in mice and warrant the need for further studies to discover its therapeutic potential.

Skeletal muscle insulin resistance: role of mitochondria and other ROS sources

At present, obesity is one of the most important public health problems in the world because it causes several diseases and reduces life expectancy. Although it is well known that insulin resistance plays a pivotal role in the development of type 2 diabetes mellitus (the more frequent disease in obese people) the link between obesity and insulin resistance is yet a matter of debate. One of the most deleterious effects of obesity is the deposition of lipids in non-adipose tissues when the capacity of adipose tissue is overwhelmed. During the last decade, reduced mitochondrial function has been considered as an important contributor to 'toxic' lipid metabolite accumulation and consequent insulin resistance. More recent reports suggest that mitochondrial dysfunction is not an early event in the development of insulin resistance, but rather a complication of the hyperlipidemia-induced reactive oxygen species (ROS) production in skeletal muscle, which might promote mitochondrial alterations, lipid accumulation and inhibition of insulin action. Here, we review the literature dealing with the mitochondria-centered mechanisms proposed to explain the onset of obesity-linked IR in skeletal muscle. We conclude that the different pathways leading to insulin resistance may act synergistically because ROS production by mitochondria and other sources can result in mitochondrial dysfunction, which in turn can further increase ROS production leading to the establishment of a harmful positive feedback loop.

Metabolic Syndrome as a Multifaceted Risk Factor for Oxidative Stress

SIGNIFICANCE

Metabolic syndrome (MetS) is associated with a greater risk of diabetes and cardiovascular diseases. It is estimated that this multifactorial condition affects 20%-30% of the world's population. A detailed understanding of MetS mechanisms is crucial for the development of effective prevention strategies and adequate intervention tools that could curb its increasing prevalence and limit its comorbidities, particularly in younger age groups. With advances in basic redox biology, oxidative stress (OxS) involvement in the complex pathophysiology of MetS has become widely accepted. Nevertheless, its clear association with and causative effects on MetS require further elucidation. Recent Advances: Although a better understanding of the causes, risks, and effects of MetS is essential, studies suggest that oxidant/antioxidant imbalance is a key contributor to this condition. OxS is now understood to be a major underlying mechanism for mitochondrial dysfunction, ectopic lipid accumulation, and gut microbiota impairment.

CRITICAL ISSUES

Further studies, particularly in the field of translational research, are clearly required to understand and control the production of reactive oxygen species (ROS) levels, especially in the mitochondria, since the various therapeutic trials conducted to date have not targeted this major ROS-generating system, aimed to delay MetS onset, or prevent its progression.

FUTURE DIRECTIONS

Multiple relevant markers need to be identified to clarify the role of ROS in the etiology of MetS. Future clinical trials should provide important proof of concept for the effectiveness of antioxidants as useful therapeutic approaches to simultaneously counteract mitochondrial OxS, alleviate MetS symptoms, and prevent complications. Antioxid. Redox Signal. 26, 445-461.

Dietary supplementation with bovine-derived milk fat globule membrane lipids promotes neuromuscular development in growing rats

Background

The milk fat globule membrane (MFGM) is primarily composed of polar phospho- and sphingolipids, which have established biological effects on neuroplasticity. The present study aimed to investigate the effect of dietary MFGM supplementation on the neuromuscular system during post-natal development.

Methods

Growing rats received dietary supplementation with bovine-derived MFGM mixtures consisting of complex milk lipids (CML), beta serum concentrate (BSC) or a complex milk lipid concentrate (CMLc) (which lacks MFGM proteins) from post-natal day 10 to day 70.

Results

Supplementation with MFGM mixtures enriched in polar lipids (BSC and CMLc, but not CML) increased the plasma phosphatidylcholine (PC) concentration, with no effect on plasma phosphatidylinositol (PI), phosphatidylethanolamine (PE), phosphatidylserine (PS) or sphingomyelin (SM). In contrast, muscle PC was reduced in rats receiving supplementation with both BSC and CMLc, whereas muscle PI, PE, PS and SM remained unchanged. Rats receiving BSC and CMLc (but not CML) displayed a slow-to-fast muscle fibre type profile shift (MyHCI → MyHCIIa) that was associated with elevated expression of genes involved in myogenic differentiation (myogenic regulatory factors) and relatively fast fibre type specialisation (Myh2 and Nfatc4). Expression of neuromuscular development genes, including nerve cell markers, components of the synaptogenic agrin–LRP4 pathway and acetylcholine receptor subunits, was also increased in muscle of rats supplemented with BSC and CMLc (but not CML).

Conclusions

These findings demonstrate that dietary supplementation with bovine-derived MFGM mixtures enriched in polar lipids can promote neuromuscular development during post-natal growth in rats, leading to shifts in adult muscle phenotype.

Electronic supplementary material

The online version of this article (doi:10.1186/s12986-017-0161-y) contains supplementary material, which is available to authorized users.

Muscle Arnt/Hif1β Is Dispensable in Myofiber Type Determination, Vascularization and Insulin Sensitivity

Aryl Hydrocarbon Receptor Nuclear Translocator/ hypoxia-inducible factor 1 beta (ARNT/ HIF1β), a member of bHLH-PAS family of transcriptional factors, plays a critical role in metabolic homeostasis, insulin resistance and glucose intolerance. The contributions of ARNT in pancreas, liver and adipose tissue to energy balance through gene regulation have been described. Surprisingly, the impact of ARNT signaling in the skeletal muscles, one of the major organs involved in glucose disposal, has not been investigated, especially in type II diabetes. Here we report that ARNT is expressed in the skeletal muscles, particularly in the energy-efficient oxidative slow-twitch myofibers, which are characterized by increased oxidative capacity, mitochondrial content, vascular supply and insulin sensitivity. However, muscle-specific deletion of ARNT did not change myofiber type distribution, oxidative capacity, mitochondrial content, capillarity, or the expression of genes associated with these features. Consequently, the lack of ARNT in the skeletal muscle did not affect weight gain, lean/fat mass, insulin sensitivity and glucose tolerance in lean mice, nor did it impact insulin resistance and glucose intolerance in high fat diet-induced obesity. Therefore, skeletal muscle ARNT is dispensable for controlling muscle fiber type and metabolic regulation, as well as diet-induced weight control, insulin sensitivity and glucose tolerance.

Preference for Western diet coadapts in High Runner mice and affects voluntary exercise and spontaneous physical activity in a genotype-dependent manner

Do animals evolve (coadapt) to choose diets that positively affect their performance abilities? We addressed this question from a microevolutionary perspective by examining preference for Western diet (WD: high in fat and sugar, but lower in protein) versus standard rodent chow in adults of both sexes from 4 lines of mice selectively bred for high levels of voluntary wheel running (High Runner or HR lines) and 4 non-selected control (C) lines. We also assessed whether food preference or substitution affects physical activity (wheel running and/or spontaneous physical activity [SPA] in the attached home cages). In experiment 1 (generation 56), mice were given 6days of wheel acclimation (as is used routinely to pick breeders in the selection experiment) prior to a 2-day food choice trial. In experiment 2 (generation 56), 17days of wheel acclimation allowed mice to reach a stable level of daily running, followed by a 7-day food-choice trial. In experiment 3 (generation 58), mice had 6days of wheel acclimation with standard chow, after which half were switched to WD for two days. In experiment 1, WD was highly preferred by all mice, with somewhat greater preference in male C mice. In experiment 2, wheel running increased and SPA decreased continuously for the first 14days of adult wheel testing, followed by 3-day plateaus in both. During the subsequent 7-day food choice trial, HR mice of both sexes preferred WD significantly more than did C mice; moreover, wheel running increased in all groups except males from C lines, with the increase being significantly greater in HR than C, while SPA declined further in all groups. In experiment 3, the effect of being switched to WD depended on both linetype and sex. On standard chow, only HR females showed a significant change in wheel running during nights 7+8, increasing by 10%. In contrast, when switched to WD, C females (+28%), HR females (+33%), and HR males (+10%) all significantly increased their daily wheel-running distances. Our results show for the first time that dietary preferences can coadapt in response to selection on activity levels.