Diets high in sugar, fat, and energy induce muscle type-specific adaptations in mitochondrial functions in rats

Differential effects of Western diet and traumatic muscle injury on skeletal muscle metabolic regulation in male and female mice

BACKGROUND

This study was designed to develop an understanding of the pathophysiology of traumatic muscle injury in the context of Western diet (WD; high fat and high sugar) and obesity. The objective was to interrogate the combination of WD and injury on skeletal muscle mass and contractile and metabolic function.

METHODS

Male and female C57BL/6J mice were randomized into four groups based on a two-factor study design: (1) injury (uninjured vs. volumetric muscle loss [VML]) and (2) diet (WD vs. normal chow [NC]). Electrophysiology was used to test muscle strength and metabolic function in cohorts of uninjured + NC, uninjured + WD, VML + NC and VML + WD at 8 weeks of intervention.

RESULTS

VML-injured male and female mice both exhibited decrements in muscle mass (-17%, P < 0.001) and muscle strength (-28%, P < 0.001); however, VML + WD females had a 28% greater muscle mass compared to VML + NC females (P = 0.034), a compensatory response not detected in males. VML-injured male and female mice both had lower carbohydrate- and fat-supported muscle mitochondrial respiration (JO2 ) and less electron conductance through the electron transport system (ETS); however, male VML-WD had 48% lower carbohydrate-supported JO2 (P = 0.014) and 47% less carbohydrate-supported electron conductance (P = 0.026) compared to male VML + NC, and this diet-injury phenotype was not present in females. ETS electron conductance starts with complex I and complex II dehydrogenase enzymes at the inner mitochondrial membrane, and male VML + WD had 31% less complex I activity (P = 0.004) and 43% less complex II activity (P = 0.005) compared to male VML + NC. This was a diet-injury phenotype not present in females. Pyruvate dehydrogenase (PDH), β-hydroxyacyl-CoA dehydrogenase, citrate synthase, α-ketoglutarate dehydrogenase and malate dehydrogenase metabolic enzyme activities were evaluated as potential drivers of impaired JO2 in the context of diet and injury. There were notable male and female differential effects in the enzyme activity and post-translational regulation of PDH. PDH enzyme activity was 24% less in VML-injured males, independent of diet (P < 0.001), but PDH enzyme activity was not influenced by injury in females. PDH enzyme activity is inhibited by phosphorylation at serine-293 by PDH kinase 4 (PDK4). In males, there was greater total PDH, phospho-PDHser293 and phospho-PDH-to-total PDH ratio in WD mice compared to NC, independent of injury (P ≤ 0.041). In females, PDK4 was 51% greater in WD compared to NC, independent of injury (P = 0.025), and was complemented by greater phospho-PDHser293 (P = 0.001).

CONCLUSIONS

Males are more susceptible to muscle metabolic dysfunction in the context of combined WD and traumatic injury compared to females, and this may be due to impaired metabolic enzyme functions.

Different Effects of High-Fat/High-Sucrose and High-Fructose Diets on Advanced Glycation End-Product Accumulation and on Mitochondrial Involvement in Heart and Skeletal Muscle in Mice

Diets with an elevated content of fat, sucrose, or fructose are recognized models of diet-induced metabolic alterations, since they induce metabolic derangements, oxidative stress, and chronic low-grade inflammation associated with local and systemic accumulation of advanced glycation end-products (AGEs). This study used four-week-old C57BL/6 male mice, randomly assigned to three experimental dietary regimens: standard diet (SD), high-fat high-sucrose diet (HFHS), or high fructose diet (HFr), administered for 12 weeks. Plasma, heart, and tibialis anterior (TA) skeletal muscle were assayed for markers of metabolic conditions, inflammation, presence of AGEs, and mitochondrial involvement. The HFHS diet induced a tissue-specific differential response featuring (1) a remarkable adaptation of the heart to HFHS-induced heavy oxidative stress, demonstrated by an increased presence of AGEs and reduced mitochondrial biogenesis, and efficaciously counteracted by a conspicuous increase in mitochondrial fission and PRXIII expression; (2) the absence of TA adaptation to HFHS, revealed by a heavy reduction in mitochondrial biogenesis, not counteracted by an increase in fission and PRXIII expression. HFr-induced mild oxidative stress elicited tissue-specific responses, featuring (1) a decrease in mitochondrial biogenesis in the heart, likely counteracted by a tendency for increased fission and (2) a mild reduction in mitochondrial biogenesis in TA, likely counteracted by a tendency for increased fusion, showing the adaptability of both tissues to the diet.

Resveratrol Blunts Mitochondrial Loss in Slow and Mixed Skeletal Muscle Phenotypes of Non-Human Primates following a Long-Term High Fat/Sugar Diet

Mitochondrial biogenesis and destruction in skeletal muscle are coordinated by distinct signaling pathways that are influenced by internal and exogenous variables including, but not limited to, muscle phenotype, physical activity, dietary composition, or drug administration. Previously we found that long-term resveratrol administration (up to 480 mg/day) ameliorates the slow-to-fast phenotypic shift in soleus muscles and promotes the expression in slow myosin heavy chain in the mixed plantaris muscle of non-human primates consuming a high fat/sugar (HFS) diet. Here, we expand on these earlier findings by examining whether mitochondrial content and the markers that dictate their biogenesis and mitophagy/autophagy are similarly affected by HFS and/or influenced by resveratrol while consuming this diet (HFSR). Compared to controls (n = 9), there was a ∼20-25% decrease in mitochondrial content in HFS (n = 8) muscles as reflected in the COX2- and CYTB-to-GAPDH ratios using PCR analysis, which was blunted by resveratrol in HFSR (n = 7) soleus and, to a lesser degree, in plantaris muscles. A ∼1.5 and 3-fold increase in Rev-erb-α protein was detected in HFSR soleus and plantaris muscles compared to controls, respectively. Unlike in HFSR animals, HFS soleus and plantaris muscles exhibited a ∼2-fold elevation in phosphor-AMPKα (Thr172). HFS soleus muscles had elevated phosphorylated-to-total TANK binding protein-1 (TBK1) ratio suggesting an enhancement in mito/autophagic events. Taken together, resveratrol appears to blunt mitochondrial losses with a high fat/sugar diet by tempering mito/autophagy rather than promoting mitochondrial biogenesis, suggesting that the quantity of daily resveratrol supplement ingested and/or its long-term consumption are important considerations.Supplemental data for this article is available online at http://dx.doi.org/ .

Maintenance of contractile force and increased fatigue resistance in slow-twitch skeletal muscle of mice fed a high-fat diet

Consumption of a high-fat diet (HFD) significantly increases exercise endurance performance during treadmill running. However, whether HFD consumption increases endurance capacity via enhanced muscle fatigue resistance has not been clarified. In this study, we investigated the effects of HFDs on contractile force and fatigue resistance of slow-twitch dominant muscles. The soleus (SOL) muscle of male C57BL/6J mice fed an HFD (60% kcal from fat) or a low-fat diet (LFD) for 12 wk was analyzed. Muscle contractile force was measured under resting conditions and during fatigue induced by repeated tetanic contractions (100 Hz, 50 contractions, and 2-s intervals). Differences in muscle twitch or tetanic force were not evident between HFD and LFD groups, whereas fatigue resistance was higher in the HFD groups. The SOL muscle of HFD-fed mice showed increased levels of markers related to oxidative capacity such as succinate dehydrogenase (SDH) and citrate synthase (CS) activity. In addition, electron microscopy analyses indicated that the total number of mitochondria and mitochondrial volume density increased in the SOL muscle of the HFD groups. These findings suggest that HFD consumption induces increased muscle fatigue resistance in slow-twitch dominant muscle fibers. This effect of HFD may be related to elevated oxidative enzyme activity, high mitochondrial content, or both.NEW & NOTEWORTHY In this study, we examined the effects of HFDs on muscle contractile force and fatigue resistance of slow-twitch dominant muscles ex vivo. We found that contractile function was comparable between the HFD groups and the LFD group, whereas fatigue resistance was higher in the HFD groups. This effect of HFD may be related to elevated oxidative enzyme activity, high mitochondrial content, or both.

The effect of succinic acid on the metabolic profile in high-fat diet-induced obesity and insulin resistance

Obesity, insulin resistance, and poor metabolic profile are hallmarks of a high-fat diet (HFD), highlighting the need to understand underlying mechanisms. Therefore, we sought to determine the effect of succinic acid (SA) on metabolism in high-fat diet (HFD)-induced obesity. Animals were randomly assigned to either low-fat diet (LFD) or a high-fat diet (HFD). Mice consumed their respective diets for 4.5 months and then assigned to the following groups: (LFD)+vehicle, LFD + SA (0.75 mg/ml), HFD + vehicle, or HFD + SA. Body weight (BW), food, and water intake, were tracked weekly. After 6 weeks, insulin, glucose, and pyruvate tolerance tests were completed, and spontaneous physical activity was assessed. Epididymal white adipose tissue (EWAT) mass and in vitro measurements of oxidative skeletal muscle (soleus) respiration were obtained. Expectedly, the HFD increased BW and EWAT mass, and reduced glucose and insulin tolerance. SA significantly reduced EWAT mass, more so in HFD (p < .05), but had no effect on any in vivo measurements (BW, insulin, glucose, or pyruvate tolerance, nor physical activity, all p > .05). A significant (p < .05) interaction was observed between mitochondrial respiration and treatment, where SA increased respiration, likely owed to greater mitochondrial content, as assessed by complex IV activity in both LFD and HFD. In HFD-induced obesity, coupled with insulin desensitization, we found no favorable effect of succinic acid on glucose regulation, though adiposity was attenuated. In oxidative skeletal muscle, there was a tendency for increased respiratory capacity, likely owed to greater mitochondrial content, suggestive of a succinic acid-induced mitochondrial biogenesis.

Catalpol Ameliorates Insulin Sensitivity and Mitochondrial Respiration in Skeletal Muscle of Type-2 Diabetic Mice Through Insulin Signaling Pathway and AMPK/SIRT1/PGC-1α/PPAR-γ Activation

Catalpol was tested for various disorders including diabetes mellitus. Numerous molecular mechanisms have emerged supporting its biological effects but with little information towards its insulin sensitizing effect. In this study, we have investigated its effect on skeletal muscle mitochondrial respiration and insulin signaling pathway. Type-2 diabetes (T2DM) was induced in male C57BL/6 by a high fat diet (60% Kcal) and streptozotocin (50 mg/kg, i.p.). Diabetic mice were orally administered with catalpol (100 and 200 mg/kg), metformin (200 mg/kg), and saline for four weeks. Fasting blood glucose (FBG), HbA1c, plasma insulin, oral glucose tolerance test (OGTT), insulin tolerance test (ITT), oxygen consumption rate, gene (IRS-1, Akt, PI3k, AMPK, GLUT4, and PGC-1α) and protein (AMPK, GLUT4, and PPAR-γ) expression in muscle were measured. Catalpol (200 mg/kg) significantly (p < 0.05) reduced the FBG, HbA1C, HOMA_IR index, and AUC of OGTT whereas, improved the ITT slope. Gene (IRS-1, Akt, PI3k, GLUT4, AMPK, and PGC-1α) and protein (AMPK, p-AMPK, PPAR-γ and GLUT4) expressions, as well as augmented state-3 respiration, oxygen consumption rate, and citrate synthase activity in muscle was observed in catalpol treated mice. The antidiabetic activity of catalpol is credited with a marked improvement in insulin sensitivity and mitochondrial respiration through the insulin signaling pathway and AMPK/SIRT1/PGC-1α/PPAR-γ activation in the skeletal muscle of T2DM mice.

Mitochondrial (Dys)function and Insulin Resistance: From Pathophysiological Molecular Mechanisms to the Impact of Diet

Mitochondrial dysfunction has been implicated in the pathogenesis of insulin resistance, the hallmark of type 2 diabetes mellitus (T2DM). However, the cause-effect relationship remains to be fully elucidated. Compelling evidence suggests that boosting mitochondrial function may represent a valuable therapeutic tool to improve insulin sensitivity. Mitochondria are highly dynamic organelles, which adapt to short- and long-term metabolic perturbations by undergoing fusion and fission cycles, spatial rearrangement of the electron transport chain complexes into supercomplexes and biogenesis governed by peroxisome proliferator-activated receptor γ co-activator 1α (PGC 1α). However, these processes appear to be dysregulated in type 2 diabetic individuals. Herein, we describe the mechanistic link between mitochondrial dysfunction and insulin resistance in skeletal muscle alongside the intracellular pathways orchestrating mitochondrial bioenergetics. We then review current evidence on nutritional tools, including fatty acids, amino acids, caloric restriction and food bioactive derivatives, which may enhance insulin sensitivity by therapeutically targeting mitochondrial function and biogenesis.

High Fat With High Sucrose Diet Leads to Obesity and Induces Myodegeneration

Skeletal muscle utilizes both free fatty acids (FFAs) and glucose that circulate in the blood stream. When blood glucose levels acutely increase, insulin stimulates muscle glucose uptake, oxidation, and glycogen synthesis. Under these conditions, skeletal muscle preferentially oxidizes glucose while the oxidation of fatty acids (FAs) oxidation is reciprocally decreased. In metabolic disorders associated with insulin resistance, such as diabetes and obesity, both glucose uptake, and utilization muscle are significantly reduced causing FA oxidation to provide the majority of ATP for metabolic processes and contraction. Although the causes of this metabolic inflexibility or disrupted "glucose-fatty acid cycle" are largely unknown, a diet high in fat and sugar (HFS) may be a contributing factor. This metabolic inflexibility observed in models of obesity or with HFS feeding is detrimental because high rates of FA oxidation in skeletal muscle can lead to the buildup of toxic metabolites of fat metabolism and the accumulation of pro-inflammatory cytokines, which further exacerbate the insulin resistance. Further, HFS leads to skeletal muscle atrophy with a decrease in myofibrillar proteins and phenotypically characterized by loss of muscle mass and strength. Overactivation of ubiquitin proteasome pathway, oxidative stress, myonuclear apoptosis, and mitochondrial dysfunction are some of the mechanisms involved in muscle atrophy induced by obesity or in mice fed with HFS. In this review, we will discuss how HFS diet negatively impacts the various physiological and metabolic mechanisms in skeletal muscle.

Acute and chronic changes in rat soleus muscle after high-fat high-sucrose diet

The effects of obesity on different musculoskeletal tissues are not well understood. The glycolytic quadriceps muscles are compromised with obesity, but due to its high oxidative capacity, the soleus muscle may be protected against obesity‐induced muscle damage. To determine the time–course relationship between a high‐fat/high‐sucrose (HFS) metabolic challenge and soleus muscle integrity, defined as intramuscular fat invasion, fibrosis and molecular alterations over six time points. Male Sprague‐Dawley rats were fed a HFS diet (n = 64) and killed at serial short‐term (3 days, 1 week, 2 weeks, 4 weeks) and long‐term (12 weeks, 28 weeks) time points. Chow‐fed controls (n = 21) were killed at 4, 12, and 28 weeks. At sacrifice, animals were weighed, body composition was calculated (DXA), and soleus muscles were harvested and flash‐frozen. Cytokine and adipokine mRNA levels for soleus muscles were assessed, using RT‐qPCR. Histological assessment of muscle fibrosis and intramuscular fat was conducted, CD68⁺ cell number was determined using immunohistochemistry, and fiber typing was assessed using myosin heavy chain protein analysis. HFS animals demonstrated significant increases in body fat by 1 week, and this increase in body fat was sustained through 28 weeks on the HFS diet. Short‐term time‐point soleus muscles demonstrated up‐regulated mRNA levels for inflammation, atrophy, and oxidative stress molecules. However, intramuscular fat, fibrosis, and CD68⁺ cell number were similar to their respective control group at all time points evaluated. Therefore, the oxidative capacity of the soleus may be protective against diet‐induced alterations to muscle integrity. Increasing oxidative capacity of muscles using aerobic exercise may be a beneficial strategy for mitigating obesity‐induced muscle damage, and its consequences.

Mitochondria and endoplasmic reticulum: Targets for a better insulin sensitivity in skeletal muscle?

Obesity and its associated metabolic disorders represent a major health burden, with economic and social consequences. Although adapted lifestyle and bariatric surgery are effective in reducing body weight, obesity prevalence is still rising. Obese individuals often become insulin-resistant. Obesity impacts on insulin responsive organs, such as the liver, adipose tissue and skeletal muscle, and increases the risk of cardiovascular diseases, type 2 diabetes and cancer. In this review, we discuss the effects of obesity and insulin resistance on skeletal muscle, an important organ for the control of postprandial glucose. The roles of mitochondria and the endoplasmic reticulum in insulin signaling are highlighted and potential innovative research and treatment perspectives are proposed.

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.

MECHANISMS IN ENDOCRINOLOGY: Skeletal muscle lipotoxicity in insulin resistance and type 2 diabetes: a causal mechanism or an innocent bystander?

Dysfunctional adipose tissue is associated with an increased risk of developing type 2 diabetes (T2D). One characteristic of a dysfunctional adipose tissue is the reduced expandability of the subcutaneous adipose tissue leading to ectopic storage of fat in organs and/or tissues involved in the pathogenesis of T2D that can cause lipotoxicity. Accumulation of lipids in the skeletal muscle is associated with insulin resistance, but the majority of previous studies do not prove any causality. Most studies agree that it is not the intramuscular lipids per se that causes insulin resistance, but rather lipid intermediates such as diacylglycerols, fatty acyl-CoAs and ceramides and that it is the localization, composition and turnover of these intermediates that play an important role in the development of insulin resistance and T2D. Adipose tissue is a more active tissue than previously thought, and future research should thus aim at examining the exact role of lipid composition, cellular localization and the dynamics of lipid turnover on the development of insulin resistance. In addition, ectopic storage of fat has differential impact on various organs in different phenotypes at risk of developing T2D; thus, understanding how adipogenesis is regulated, the interference with metabolic outcomes and what determines the capacity of adipose tissue expandability in distinct population groups is necessary. This study is a review of the current literature on the adipose tissue expandability hypothesis and how the following ectopic lipid accumulation as a consequence of a limited adipose tissue expandability may be associated with insulin resistance in muscle and liver.

Uric acid and transforming growth factor in fructose-induced production of reactive oxygen species in skeletal muscle

The consumption of fructose, a major constituent of the modern diet, has raised increasing concern about the effects of fructose on health. Research suggests that excessive intake of fructose (>50 g/d) causes hyperuricemia, insulin resistance, mitochondrial dysfunction, de novo lipogenesis by the liver, and increased production of reactive oxygen species (ROS) in muscle. In a number of tissues, uric acid has been shown to stimulate the production of ROS via activation of transforming growth factor β1 and NADPH (nicotinamide adenine dinucleotide phosphate) oxidase 4. The role of uric acid in fructose-induced production of ROS in skeletal muscle, however, has not been investigated. This review examines the evidence for fructose-induced production of ROS in skeletal muscle, highlights proposed mechanisms, and identifies gaps in current knowledge.

Short-term, high-fat diet accelerates disuse atrophy and protein degradation in a muscle-specific manner in mice

BACKGROUND

A short-term high-fat diet impairs mitochondrial function and the ability of skeletal muscle to respond to growth stimuli, but it is unknown whether such a diet alters the ability to respond to atrophy signals. The purpose of this study was to determine whether rapid weigh gain induced by a high-fat (HF) diet accelerates denervation-induced muscle atrophy.

METHODS

Adult, male mice (C57BL/6) were fed a control or HF (60 % calories as fat) diet for 3 weeks (3wHF). Sciatic nerve was sectioned unilaterally for the final 5 or 14 days of the diet. Soleus and extensor digitorum longus (EDL) muscles were removed and incubated in vitro to determine rates of protein degradation and subsequently homogenized for determination of protein levels of LC3, ubiquitination, myosin heavy chain (MHC) distribution, and mitochondrial subunits.

RESULTS

When mice were fed the 3wHF diet, whole-body fat mass more than doubled, but basal (innervated) muscle weights, rates of protein degradation, LC3 content, mitochondrial protein content, and myosin isoform distribution were not significantly different than with the control diet in either soleus or EDL. However in the 14 day denervated soleus, the 3wHF diet significantly augmented loss of mass, proteolysis rate, amount of the autophagosome marker LC3 II, and the amount of overall ubiquitination as compared to the control fed mice. On the contrary, the 3wHF diet had no significant effect in the EDL on amount of mass loss, proteolysis rate, LC3 levels, or ubiquitination. Fourteen days denervation also induced a loss of mitochondrial proteins in the soleus but not the EDL, regardless of the diet.

CONCLUSIONS

Taken together, a short-term, high-fat diet augments denervation muscle atrophy by induction of protein degradation in the mitochondria-rich soleus but not in the glycolytic EDL. These findings suggest that the denervation-induced loss of mitochondria and HF diet-induced impairment of mitochondrial function may combine to promote skeletal muscle atrophy.

One-year high fat diet affects muscle-but not brain mitochondria

It is well known that few weeks of high fat (HF) diet may induce metabolic disturbances and mitochondrial dysfunction in skeletal muscle. However, little is known about the effects of long-term HF exposure and effects on brain mitochondria are unknown. Wistar rats were fed either chow (13E% fat) or HF diet (60E% fat) for 1 year. The HF animals developed obesity, dyslipidemia, insulin resistance, and dysfunction of isolated skeletal muscle mitochondria: state 3 and state 4 were 30% to 50% increased (P<0.058) with palmitoyl carnitine (PC), while there was no effect with pyruvate as substrate. Adding also succinate in state 3 resulted in a higher substrate control ratio (SCR) with PC, but a lower SCR with pyruvate (P<0.05). The P/O2 ratio was lower with PC (P<0.004). However, similar tests on isolated brain mitochondria from the same animal showed no changes with the substrates relevant for brain (pyruvate and 3-hydroxybutyrate). Thus, long-term HF diet was associated with obesity, dyslipidemia, insulin resistance, and significantly altered mitochondrial function in skeletal muscle. Yet, brain mitochondria were unaffected. We suggest that the relative isolation of the brain due to the blood-brain barrier may play a role in this strikingly different phenotype of mitochondria from the two tissues of the same animal.

Silencing miR-106b improves palmitic acid-induced mitochondrial dysfunction and insulin resistance in skeletal myocytes

MicroRNA‑106b (miR‑106b) is reported to correlate closely with skeletal muscle insulin resistance. In the current study the effect of miR‑106b on palmitic acid (PA)‑induced mitochondrial dysfunction and insulin resistance was investigated in C2C12 myotubes via the silencing of miR‑106b. MiR‑106b expression was increased under PA treatment, while miR‑106b loss of function improved insulin sensitivity by upregulating its target mitofusin‑2 (Mfn2) in C2C12 myocytes. Furthermore, miR‑106b loss of function partly improved mitochondrial morphological lesions and increased the levels of mitochondial DNA and intracellular adenosine triphosphate that had been impaired by PA exposure in C2C12 myocytes. MiR‑106b loss of function attenuated the levels of intracellular reactive oxygen species (ROS), and upregulated the expression levels of the estrogen‑related receptor (ERR)‑α/peroxisome proliferative activated receptor γ coactivator (PGC)‑1α/Mfn2 axis under PA exposure. In addition, miR‑106b negatively regulated skeletal muscle mitochondrial function and insulin sensitivity under PA‑induced insulin resistance by targeting Mfn2, which may be associated with reduced ROS and upregulation of the ERR‑α/PGC‑1α/Mfn2 axis.

High-fat feeding does not induce an autophagic or apoptotic phenotype in female rat skeletal muscle

Apoptosis and autophagy are critical in normal skeletal muscle homeostasis; however, dysregulation can lead to muscle atrophy and dysfunction. Lipotoxicity and/or lipid accumulation may promote apoptosis, as well as directly or indirectly influence autophagic signaling. Therefore, the purpose of this study was to examine the effect of a 16-week high-fat diet on morphological, apoptotic, and autophagic indices in oxidative and glycolytic skeletal muscle of female rats. High-fat feeding resulted in increased fat pad mass, altered glucose tolerance, and lower muscle pAKT levels, as well as lipid accumulation and reactive oxygen species generation in soleus muscle; however, muscle weights, fiber type-specific cross-sectional area, and fiber type distribution were not affected. Moreover, DNA fragmentation and LC3 lipidation as well as several apoptotic (ARC, Bax, Bid, tBid, Hsp70, pBcl-2) and autophagic (ATG7, ATG4B, Beclin 1, BNIP3, p70 s6k, cathepsin activity) indices were not altered in soleus or plantaris following high-fat diet. Interestingly, soleus muscle displayed small increases in caspase-3, caspase-8, and caspase-9 activity, as well as higher ATG12-5 and p62 protein, while both soleus and plantaris muscle showed dramatically reduced Bcl-2 and X-linked inhibitor of apoptosis protein (XIAP) levels. In conclusion, this work demonstrates that 16 weeks of high-fat feeding does not affect tissue morphology or induce a global autophagic or apoptotic phenotype in skeletal muscle of female rats. However, high-fat feeding selectively influenced a number of apoptotic and autophagic indices which could have implications during periods of enhanced muscle stress.

Mitochondria: a possible nexus for the regulation of energy homeostasis by the endocannabinoid system?

The endocannabinoid system (ECS) regulates numerous cellular and physiological processes through the activation of receptors targeted by endogenously produced ligands called endocannabinoids. Importantly, this signaling system is known to play an important role in modulating energy balance and glucose homeostasis. For example, current evidence indicates that the ECS becomes overactive during obesity whereby its central and peripheral stimulation drives metabolic processes that mimic the metabolic syndrome. Herein, we examine the role of the ECS in modulating the function of mitochondria, which play a pivotal role in maintaining cellular and systemic energy homeostasis, in large part due to their ability to tightly coordinate glucose and lipid utilization. Because of this, mitochondrial dysfunction is often associated with peripheral insulin resistance and glucose intolerance as well as the manifestation of excess lipid accumulation in the obese state. This review aims to highlight the different ways through which the ECS may impact upon mitochondrial abundance and/or oxidative capacity and, where possible, relate these findings to obesity-induced perturbations in metabolic function. Furthermore, we explore the potential implications of these findings in terms of the pathogenesis of metabolic disorders and how these may be used to strategically develop therapies targeting the ECS.

Mitochondrial protein synthesis is increased in oxidative skeletal muscles of rats with cardiac cachexia

Since cardiac cachexia could be associated with alterations in muscular mitochondrial metabolism, we hypothesized that the expected alterations in the activities of mitochondrial oxidative enzymes could be associated with changes in mitochondrial protein synthesis in oxidative skeletal muscles. Cardiac cachexia was provoked in male rats by the ligation of the left coronary artery. Six cachectic and 6 control rats were age-paired, and their food intake was observed. The synthesis of mitochondrial proteins was measured by [1-13C]-valine infusion in soleus, tibilais, myocardium, and liver. Muscles (soleus, gastrocnemius, and tibialis anterior), heart, kidneys, liver, and visceral adipose tissue were weighed. Mitochondrial cytochrome c oxydase IV as well as citrate synthase and myosin ATPase activities were measured. As expected, decreased food intake was observed in the cachectic group. Heart, kidney, and liver weights were higher in the cachectic group, while the visceral adipose tissue weight was lower (P < .01). No changes in muscle weights were observed. Soleus mitochondrial proteins fractional synthesis rate was higher in the cachectic group (P = .054). Cytochrome c oxydase IV activity was reduced (P = .009) and increased (P = .038) in the soleus and liver of the cachectic rats, respectively. No change in citrate synthase activity was observed. Myosin ATPase activity was reduced in the gastrocnemius of the cachectic group (P < .01). Mitochondrial protein synthesis is increased in the soleus of rats with cardiac cachexia, suggesting a compensatory mechanism of the impaired oxidative mitochondrial function. Further work should assess whether the mitochondrial protein synthesis is altered in chronic heart failure patients with cardiac cachexia, and whether this is the cause or the consequence of cachexia.

Nutritional influences on age-related skeletal muscle loss

Age-related muscle loss impacts on whole-body metabolism and leads to frailty and sarcopenia, which are risk factors for fractures and mortality. Although nutrients are integral to muscle metabolism the relationship between nutrition and muscle loss has only been extensively investigated for protein and amino acids. The objective of the present paper is to describe other aspects of nutrition and their association with skeletal muscle mass. Mechanisms for muscle loss relate to imbalance in protein turnover with a number of anabolic pathways of which the mechanistic TOR pathway and the IGF-1–Akt–FoxO pathways are the most characterised. In terms of catabolism the ubiquitin proteasome system, apoptosis, autophagy, inflammation, oxidation and insulin resistance are among the major mechanisms proposed. The limited research associating vitamin D, alcohol, dietary acid–base load, dietary fat and anti-oxidant nutrients with age-related muscle loss is described. Vitamin D may be protective for muscle loss; a more alkalinogenic diet and diets higher in the anti-oxidant nutrients vitamin C and vitamin E may also prevent muscle loss. Although present recommendations for prevention of sarcopenia focus on protein, and to some extent on vitamin D, other aspects of the diet including fruits and vegetables should be considered. Clearly, more research into other aspects of nutrition and their role in prevention of muscle loss is required.

The Korean Mistletoe (Viscum album coloratum) Extract Has an Antiobesity Effect and Protects against Hepatic Steatosis in Mice with High-Fat Diet-Induced Obesity

This study investigates the inhibitory effects of Korean mistletoe extract (KME) on adipogenic factors in 3T3-L1 cells and obesity and nonalcoholic fatty liver disease (NAFLD) in mice fed a high-fat diet. Male C57Bl/6 mice fed a high-fat diet were treated with KME (3 g/kg/day) for 15 weeks for the antiobesity and NAFLD experiments. Body weight and daily food intake were measured regularly during the experimental period. The epididymal pad was measured and liver histology was observed. The effects of KME on thermogenesis and endurance capacity were measured. The effects of KME on adipogenic factors were examined in 3T3-L1 cells. Body and epididymal fat pad weights were reduced in KME-treated mice, and histological examination showed an amelioration of fatty liver in KME-treated mice, without an effect on food consumption. KME potently induces mitochondrial activity by activating thermogenesis and improving endurance capacity. KME also inhibited adipogenic factors in vitro. These results demonstrate the inhibitory effects of KME on obesity and NAFLD in mice fed a high-fat diet. The effects appear to be mediated through an enhanced mitochondrial activity. Therefore, KME may be an effective therapeutic candidate for treating obesity and fatty liver caused by a high-fat diet.

Effect of ethyl pyruvate on skeletal muscle metabolism in rats fed on a high fat diet

Impaired mitochondrial capacity may be implicated in the pathology of chronic metabolic diseases. To elucidate the effect of ethyl pyruvate supplementation on skeletal muscles metabolism we examined changes in activities of mitochondrial and antioxidant enzymes, as well as sulfhydryl groups oxidation (an indirect marker of oxidative stress) during the development of obesity. After 6 weeks feeding of control or high fat diet, Wistar rats were divided into four groups: control diet, control diet and ethyl pyruvate, high fat diet, and high fat diet and ethyl pyruvate. Ethyl pyruvate was administered as 0.3% solution in drinking water, for the following 6 weeks. High fat diet feeding induced the increase of activities 3-hydroxyacylCoA dehydrogenase, citrate synthase, and fumarase. Moreover, higher catalase and superoxide dismutase activities, as well as sulfhydryl groups oxidation, were noted. Ethyl pyruvate supplementation did not affect the mitochondrial enzymes’ activities, but induced superoxide dismutase activity and sulfhydryl groups oxidation. All of the changes were observed in soleus muscle, but not in extensor digitorum longus muscle. Additionally, positive correlations between fasting blood insulin concentration and activities of catalase (p = 0.04), and superoxide dismutase (p = 0.01) in soleus muscle were noticed. Prolonged ethyl pyruvate consumption elevated insulin concentration, which may cause modifications in oxidative type skeletal muscles.

Differential effects of thyroid status on regional H₂O₂ production in slow- and fast-twitch muscle of ducklings

Birds seem to employ powerful physiological strategies to curb the harmful effects of reactive oxygen species (ROS) because they generally live longer than predicted by the free radical theory of aging. However, little is known about the physiological mechanisms that confer protection to birds against excessive ROS generation. Hence, we investigated the ability of birds to control mitochondrial ROS generation during physiologically stressful periods. In our study, we analyzed the relationship between the thyroid status and the function of intermyofibrillar and subsarcolemmal mitochondria located in glycolytic and oxidative muscles of ducklings. We found that the intermyofibrillar mitochondria of both glycolytic and oxidative muscles down regulate ROS production when plasma T₃ levels rise. The intermyofibrillar mitochondria of the gastrocnemius muscle (an oxidative muscle) produced less ROS and were more sensitive than the pectoralis muscle (a glycolytic muscle) to changes in plasma T₃. Such differences in the ROS production by glycolytic and oxidative muscles were associated with differences in the membrane proton permeability and in the rate of free radical leakage within the respiratory chain. This is the first evidence which shows that in birds, the amount of ROS that the mitochondria release is dependent on: (1) their location within the muscle; (2) the type of muscle (glycolytic or oxidative) and (3) on the thyroid status. Reducing muscle mitochondrial ROS generation might be an important mechanism in birds to limit oxidative damage during periods of physiological stress.

Mechanisms underlying skeletal muscle insulin resistance induced by fatty acids: importance of the mitochondrial function

Insulin resistance condition is associated to the development of several syndromes, such as obesity, type 2 diabetes mellitus and metabolic syndrome. Although the factors linking insulin resistance to these syndromes are not precisely defined yet, evidence suggests that the elevated plasma free fatty acid (FFA) level plays an important role in the development of skeletal muscle insulin resistance. Accordantly, in vivo and in vitro exposure of skeletal muscle and myocytes to physiological concentrations of saturated fatty acids is associated with insulin resistance condition. Several mechanisms have been postulated to account for fatty acids-induced muscle insulin resistance, including Randle cycle, oxidative stress, inflammation and mitochondrial dysfunction. Here we reviewed experimental evidence supporting the involvement of each of these propositions in the development of skeletal muscle insulin resistance induced by saturated fatty acids and propose an integrative model placing mitochondrial dysfunction as an important and common factor to the other mechanisms.

Effects of high-fat overfeeding on mitochondrial function, glucose and fat metabolism, and adipokine levels in low-birth-weight subjects

Low birth weight (LBW) is associated with an increased risk of insulin resistance and downregulation of oxidative phosphorylation (OXPHOS) genes when exposed to a metabolic challenge of high-fat overfeeding (HFO). To elaborate further on the differential effects of HFO in LBW subjects, we measured in vivo mitochondrial function, insulin secretion, hepatic glucose production, and plasma levels of key regulatory hormones before and after 5 days of HFO in 20 young LBW and 26 normal-birth-weight (NBW) men. The LBW subjects developed peripheral insulin resistance after HFO due to impaired endogenous glucose storage (9.42 ± 4.19 vs. 5.91 ± 4.42 mg·kg FFM(-1)·min(-1), P = 0.01). Resting muscle phosphorcreatine and total ATP in muscle increased significantly after HFO in LBW subjects only, whereas additional measurements of mitochondrial function remained unaffected. Despite similar plasma FFA levels, LBW subjects displayed increased fat oxidation during insulin infusion compared with normal-birth-weight (NBW) subjects after HFO (0.37 ± 0.35 vs. 0.17 ± 0.33 mg·kg FFM(-1)·min(-1), P = 0.02). In contrast to NBW subjects, the plasma leptin levels of LBW subjects did not increase, and the plasma gastric inhibitory polypeptide (GIP) as well as pancreatic polypeptide (PP) levels increased less in LBW compared with NBW subjects during HFO. In conclusion, HFO unmasks dissociation between insulin resistance and mitochondrial dysfunction in LBW subjects, suggesting that insulin resistance may be a cause, rather than an effect, of impaired muscle OXPHOS gene expression and mitochondrial dysfunction. Reduced increments in response to HFO of fasting plasma leptin, PP, and GIP levels may contribute to insulin resistance, lower satiety, and impaired insulin secretion in LBW subjects.

Cumulative 3-nitrotyrosine in specific muscle proteins is associated with muscle loss during aging

Post-translational oxidative protein modifications which are more marked during aging and/or high-calorie (HC) diets affect protein function and metabolism. Protein function and metabolism are different according to the type of muscle proteins. Oxidative muscle protein modifications may thus be associated with age-related sarcopenia, and HC may be implicated in the development of sarcopenia by emphasizing protein modifications. Understanding the role of protein modifications in the process of sarcopenia and metabolism associated with a high fat diet may be elucidated by investigations with skeletal muscle protein subfractionations. To study this hypothesis, carbonylated protein (CP) and 3-nitrotyrosine (3-NT) levels were measured in mixed, sarcoplasmic, myofibrillar and mitochondrial protein fractions of quadriceps in rats aged 6months (A) and 25months (O) fed a normal calorie (NC) or HC diet for 3months (AN, AH, ON, OH n=7-8). Muscle weight was lower in the older rats (AN: 0.79±0.03g, ON: 0.43±0.12g, P<0.05), but no HC effect was observed. CP did not differ between groups while 3-NT accumulated significantly in ON compared with AN, especially in mitochondria (2.4±0.5, 1.3±0.1, 1.9±0.4, 2.9±1.2 -fold in mixed, sarcoplasmic, myofibrillar and mitochondrial fractions respectively, P<0.05). 3-NT in mixed protein was negatively correlated with muscle mass (r(2)=-0.812). 3-NT accumulation during HC was observed only in specific proteins of mitochondria (100kDa) (1.0±0.6, 1.7±0.9, 3.3±1.4 and 7.0±2.5 -fold in AN, AH, ON and OH, respectively, P<0.05). Hence cumulative 3-NT in skeletal muscle protein appears associated with the development of age-related muscle loss. Mitochondrial proteins are more prone to nitration during aging and nutritional stress.

Improvement of mitochondrial function in muscle of genetically obese rats after chronic supplementation with proanthocyanidins

The aim of this study was to determine the effect of chronic dietary supplementation of a grape seed proanthocyanidin extract (GSPE) at a dose of 35 mg/kg body weight on energy metabolism and mitochondrial function in the skeletal muscle of Zucker obese rats. Three groups of 10 animals each were used: lean Fa/fa lean group (LG) rats, a control fa/fa obese group (OG) of rats, and an obese supplemented fa/fa proanthocyanidins obese group (POG) of rats, which were supplemented with a dose of 35 mg GSPE/kg of body weight/day during the 68 days of experimentation. Skeletal muscle energy metabolism was evaluated by determining enzyme activities, key metabolic gene expression, and immunoblotting of oxidative phosphorylation complexes. Mitochondrial function was analyzed by high-resolution respirometry using both a glycosidic and a lipid substrate. In muscle, chronic GSPE administration decreased citrate synthase activity, the amount of oxidative phosphorylation complexes I and II, and Nrf1 gene expression, without any effects on the mitochondrial oxidative capacity. This situation was associated with lower reactive oxygen species (ROS) generation. Additionally, GSPE administration enhanced the ability to oxidize pyruvate, and it also increased the activity of enzymes involved in oxidative phosphorylation including cytochrome c oxidase. There is strong evidence to suggest that GSPE administration stimulates mitochondrial function in skeletal muscle specifically by increasing the capacity to oxidize pyruvate and contributes to reduced muscle ROS generation in obese Zucker rats.

Differential effects of saturated versus unsaturated dietary fatty acids on weight gain and myocellular lipid profiles in mice

Objective:

In conditions of continuous high-fat (HF) intake, the degree of saturation of the fatty acids (FAs) in the diet might have a crucial role in the onset of obesity and its metabolic complications. In particular, the FA composition of the diet might influence the storage form of lipids inside skeletal muscle. The aim of the present study was to examine whether the FA composition of HF diets differentially affects weight gain and accumulation of myocellular triacylglycerol (TAG) and diacylglycerol (DAG). Furthermore, we examined whether the FA composition of the diet was reflected in the composition of the myocellular lipid intermediates.

Design:

C57Bl6 mice were fed HF diets (45% energy) mainly containing palm oil (PO), cocoa butter (CB), olive oil (OO) or safflower oil (SO; n=6 per group) for 8 weeks. A low-fat diet (10% energy, PO) was used as control. Body weight was monitored weekly. At the end of the dietary intervention, myocellular TAG and DAG content and profiles were measured.

Results:

We here show that HF_CB prevented weight gain after 8 weeks of HF feeding. Furthermore, the HF diet rich in SO prevented the accumulation of both myocellular TAG and DAG. Interestingly, the FA composition of DAG and TAG in skeletal muscle was a reflection of the dietary FA composition.

Conclusion:

Already after a relatively short period, the dietary FA intake relates to the FA composition of the lipid metabolites in the muscle. A diet rich in polyunsaturated FAs seems to prevent myocellular lipid accumulation.

The embryonic genes Dkk3, Hoxd8, Hoxd9 and Tbx1 identify muscle types in a diet-independent and fiber-type unrelated way

BACKGROUND

The mouse skeletal muscle is composed of four distinct fiber types that differ in contractile function, number of mitochondria and metabolism. Every muscle type has a specific composition and distribution of the four fiber types. To find novel genes involved in specifying muscle types, we used microarray analysis to compare the gastrocnemius with the quadriceps from mice fed a low fat diet (LFD) or high fat diet (HFD) for 8 weeks. Additional qPCR analysis were performed in the gastrocnemius, quadriceps and soleus muscle from mice fed an LFD or HFD for 20 weeks.

RESULTS

In mice fed the 8-week LFD 162 genes were differentially expressed in the gastrocnemius vs. the quadriceps. Genes with the strongest differences in expression were markers for oxidative fiber types (e.g. Tnni1) and genes which are known to be involved in embryogenesis (Dkk3, Hoxd8,Hoxd9 and Tbx1). Also Dkk2, Hoxa5, Hoxa10, Hoxc9, Hoxc10, Hoxc6 and Tbx15 were detectably, but not differentially expressed in adult muscle tissue. Expression of differentially expressed genes was not influenced by an 8-week or 20-week HFD. Comparing gastrocnemius, quadriceps and soleus, expression of Hoxd8 and Hoxd9 was not related with expression of markers for the four different fiber types. We found that the expression of both Hoxd8 and Hoxd9 was much higher in the gastrocnemius than in the quadriceps or soleus, whereas the expression of Dkk3 was high in quadriceps, but low in both gastrocnemius and soleus. Finally, expression of Tbx1 was high in quadriceps, intermediate in soleus and low in gastrocnemius.

CONCLUSIONS

We found that genes from the Dkk family, Hox family and Tbx family are detectably expressed in adult mouse muscle. Interestingly, expression of Dkk3, Hoxd8, Hoxd9 and Tbx1 was highly different between gastrocnemius, quadriceps and soleus. In fact, every muscle type showed a unique combination of expression of these four genes which was not influenced by diet. Altogether, we conclude that genes important for embryogenesis identify mouse muscle types in a diet-independent and fiber type-unrelated manner.

Adipophilin protein expression in muscle--a possible protective role against insulin resistance

Adipophilin is a 50 kDa protein that belongs to the PAT family (perilipin, adipophilin, TIP47, S3-12 and OXPAT), which comprises proteins involved in the coating of lipid droplets. Little is known about the functional role of adipophilin in muscle. Using the C2C12 cell line as a model, we demonstrate that palmitic acid-treated cells highly express the adipophilin protein in a dose-dependent way. Next, we show that oleic acid is a more potent inducer of adipophilin protein levels than palmitic acid. Cells treated with oleic acid have a higher adipophilin protein expression and higher triglyceride levels but less impairment of insulin signaling than cells treated with palmitic acid. Additionally, we show that peroxisome proliferator-activated receptor (PPAR)alpha, PPARbeta/delta and PPARgamma agonists all increase the expression of the adipophilin protein in C2C12 cells. This effect was most pronounced for the PPARalpha agonist GW7647. Furthermore, the expression of adipophilin as a 37 kDa N-terminally truncated protein is higher in the gastrocnemius than in the quadriceps of C57BL/6J mice, especially after an 8-week high-fat diet. The expression of adipophilin was higher in the muscle of mice fed a 4-week high-fat diet based on olive oil or safflower oil than in mice fed a 4-week high-fat diet based on palm oil. After 2 weeks of intervention, plasma glucose, plasma insulin and the homeostasis model assessment of insulin resistance index were lower in mice fed a 4-week high-fat diet based on olive oil or safflower oil than in mice fed a 4-week high-fat diet based on palm oil. Taken together, the results obtained in the present study indicate that adipophilin protein expression in muscle is involved in maintaining insulin sensitivity.

Saturated fatty acid-induced insulin resistance is associated with mitochondrial dysfunction in skeletal muscle cells

Increased plasma levels of free fatty acids (FFA) occur in states of insulin resistance such as obesity and type 2 diabetes mellitus. These high levels of plasma FFA are proposed to play an important role for the development of insulin resistance but the mechanisms involved are still unclear. This study investigated the effects of saturated and unsaturated FFA on insulin sensitivity in parallel with mitochondrial function. C2C12 myotubes were treated for 24 h with 0.1 mM of saturated (palmitic and stearic) and unsaturated (oleic, linoleic, eicosapentaenoic, and docosahexaenoic) FFA. After this period, basal and insulin-stimulated glucose metabolism and mitochondrial function were evaluated. Saturated palmitic and stearic acids decreased insulin-induced glycogen synthesis, glucose oxidation, and lactate production. Basal glucose oxidation was also reduced. Palmitic and stearic acids impaired mitochondrial function as demonstrated by decrease of both mitochondrial hyperpolarization and ATP generation. These FFA also decreased Akt activation by insulin. As opposed to saturated FFA, unsaturated FFA did not impair glucose metabolism and mitochondrial function. Primary cultures of rat skeletal muscle cells exhibited similar responses to saturated FFA as compared to C2C12 cells. These results show that in muscle cells saturated FFA-induced mitochondrial dysfunction associated with impaired insulin-induced glucose metabolism.

Muscle-specific adaptations, impaired oxidative capacity and maintenance of contractile function characterize diet-induced obese mouse skeletal muscle

Background

The effects of diet-induced obesity on skeletal muscle function are largely unknown, particularly as it relates to changes in oxidative metabolism and morphology.

Principal Findings

Compared to control fed mice, mice fed a high fat diet (HFD; 60% kcal: fat) for 8 weeks displayed increased body mass and insulin resistance without overt fasting hyperglycemia (i.e. pre-diabetic). Histological analysis revealed a greater oxidative potential in the HFD gastrocnemius/plantaris (increased IIA, reduced IIB fiber-type percentages) and soleus (increased I, IIA cross-sectional areas) muscles, but no change in fiber type percentages in tibialis anterior muscles compared to controls. Intramyocellular lipid levels were significantly increased relative to control in HFD gastrocnemius/plantaris, but were similar to control values in the HFD soleus. Using a novel, single muscle fiber approach, impairments in complete palmitate and glucose oxidation (72.8±6.6% and 61.8±9.1% of control, respectively; p<0.05) with HFD were detected. These reductions were consistent with measures made using intact extensor digitorum longus and soleus muscles. Compared to controls, no difference in succinate dehydrogenase or citrate synthase enzyme activities were observed between groups in any muscle studied, however, short-chain fatty acyl CoA dehydrogenase (SCHAD) activity was elevated in the HFD soleus, but not tibialis anterior muscles. Despite these morphological and metabolic alterations, no significant difference in peak tetanic force or low-frequency fatigue rates were observed between groups.

Conclusions

These findings indicate that HFD induces early adaptive responses that occur in a muscle-specific pattern, but are insufficient to prevent impairments in oxidative metabolism with continued high-fat feeding. Moreover, the morphological and metabolic changes which occur with 8 weeks of HFD do not significantly impact muscle contractile properties.

Impact of short-term high-fat feeding on glucose and insulin metabolism in young healthy men

A high-fat, high-calorie diet is associated with obesity and type 2 diabetes. However, the relative contribution of metabolic defects to the development of hyperglycaemia and type 2 diabetes is controversial. Accumulation of excess fat in muscle and adipose tissue in insulin resistance and type 2 diabetes may be linked with defective mitochondrial oxidative phosphorylation. The aim of the current study was to investigate acute effects of short-term fat overfeeding on glucose and insulin metabolism in young men. We studied the effects of 5 days' high-fat (60% energy) overfeeding (+50%) versus a control diet on hepatic and peripheral insulin action by a hyperinsulinaemic euglycaemic clamp, muscle mitochondrial function by (31)P magnetic resonance spectroscopy, and gene expression by qrt-PCR and microarray in 26 young men. Hepatic glucose production and fasting glucose levels increased significantly in response to overfeeding. However, peripheral insulin action, muscle mitochondrial function, and general and specific oxidative phosphorylation gene expression were unaffected by high-fat feeding. Insulin secretion increased appropriately to compensate for hepatic, and not for peripheral, insulin resistance. High-fat feeding increased fasting levels of plasma adiponectin, leptin and gastric inhibitory peptide (GIP). High-fat overfeeding increases fasting glucose levels due to increased hepatic glucose production. The increased insulin secretion may compensate for hepatic insulin resistance possibly mediated by elevated GIP secretion. Increased insulin secretion precedes the development of peripheral insulin resistance, mitochondrial dysfunction and obesity in response to overfeeding, suggesting a role for insulin per se as well GIP, in the development of peripheral insulin resistance and obesity.

Oxidative stress in rats fed a high-fat high-sucrose diet and preventive effect of polyphenols: Involvement of mitochondrial and NAD(P)H oxidase systems

Mitochondrial and NADPH oxidase systems and oxidative stress were investigated in 12 week high-fat high-sucrose (HFHS) diet-fed rats. A protective effect of wine polyphenol (PP) extract was also examined. In liver, maximal activities of CII and CII+III mitochondrial complexes were decreased but NADPH oxidase expression (p22(phox) and p47(phox)) and NADPH oxidase-dependent superoxide anion production were not modified, whereas oxidative stress (lipid and protein oxidation products and antioxidant systems) was increased with HFHS diet. In muscle, anion superoxide production was slightly increased while mitochondrial complex activities and lipid and protein oxidation products were not modified with HFHS diet. In heart, NADPH oxidase expression and superoxide anion production were increased, and maximal activity of mitochondrial respiratory chain complexes or oxidative stress parameters were not modified. Wine polyphenol extract had an inhibiting effect on liver oxidative stress and on heart NADPH oxidase expression and superoxide anion production, and on induction of hepatic steatosis with HFHS diet. Induction of mitochondrial dysfunction could be a primary event in the development of oxidative stress in liver, while in skeletal muscle and in heart the NADPH oxidase system seems to be mainly involved in oxidative stress. Wine polyphenol extract was shown to partially prevent oxidative stress in liver and heart tissues and to nearly completely prevent steatosis development in liver.

Potential mechanisms of muscle mitochondrial dysfunction in aging and obesity and cellular consequences

Mitochondria play a key role in the energy metabolism in skeletal muscle. A new concept has emerged suggesting that impaired mitochondrial oxidative capacity in skeletal muscle may be the underlying defect that causes insulin resistance. According to current knowledge, the causes and the underlying molecular mechanisms at the origin of decreased mitochondrial oxidative capacity in skeletal muscle still remain to be elucidated. The present review focuses on recent data investigating these issues in the area of metabolic disorders and describes the potential causes, mechanisms and consequences of mitochondrial dysfunction in the skeletal muscle.

Influence of increased adiposity on mitochondrial-associated proteins of the rat colon: a proteomic and transcriptomic analysis

Epidemiological studies report obesity to be an important risk factor influencing colon pathologies, yet mechanism(s) are unknown. Recent studies have shown significant elevation of insulin, leptin and triglycerides associated with increased adipose tissue. In situ hybridisation studies have located insulin, leptin and adiponectin receptor expression in the colon epithelia. The influence of increased adiposity and associated deregulation of insulin and adipokines on regulation of the colon epithelium is unknown. Altered adipokine and insulin signalling associated with obesity has an impact on mitochondrial function and mitochondrial dysfunction is increasingly recognised as a contributing factor in many diseases. Proteomics and transcriptomics are potentially powerful methods useful in elucidating the mechanisms whereby obesity increases risk of colon diseases as observed epidemiologically. This study investigated colon mitochondrial-associated protein profiles and corresponding gene expression in colon in response to increased adiposity in a rat model of diet induced obesity. Increased adiposity in diet-induced obese sensitive rats was found to be associated with altered protein expression of 69 mitochondrial-associated proteins involved in processes associated with calcium binding, protein folding, energy metabolism, electron transport chain, structural proteins, protein synthesis and degradation, redox regulation, and transport. The changes in these mitochondrial protein profiles were not correlated with changes at the gene expression level assessed using real-time PCR arrays.

Effects of trans MUFA from dairy and industrial sources on muscle mitochondrial function and insulin sensitivity

Epidemiological studies suggest that chronic consumption of trans MUFA may alter muscle insulin sensitivity. The major sources of dietary trans MUFA (dairy fat vs. industrially hydrogenated oils) have different isomeric profiles and thus probably different metabolic consequences. These effects may involve alterations in muscle mitochondrial oxidative capacity, which may in turn promote insulin resistance if fatty acid oxidation is reduced. We report that in Wistar rats, an 8 week diet enriched (4% of energy intake) in either dairy, industrial, or control MUFA did not alter insulin and glucose responses to an intraperitoneal glucose tolerance test (1g/kg). In C2C12 myotubes, vaccenic and elaidic acids did not modify insulin sensitivity compared with oleic acid. Furthermore, the ex vivo total, mitochondrial and peroxisomal oxidation rates of [1-(14)C]oleic, vaccenic, and elaidic acids were similar in soleus and tibialis anterior rat muscle. Finally, an 8 week diet enriched in either dairy or industrial trans MUFA did not alter mitochondrial oxidative capacity in these two muscles compared with control MUFA but did induce a specific reduction in soleus mitochondrial ATP and superoxide anion production (P<0.01 vs. control). In conclusion, dietary trans MUFA of dairy or industrial origin have similar effects and do not impair muscle mitochondrial capacity and insulin sensitivity.

Impaired resting muscle energetics studied by (31)P-NMR in diet-induced obese rats

OBJECTIVE

Mitochondrial activity is altered in skeletal muscle of obese, insulin-resistant or type 2 diabetic patients. We hypothesized that this situation was associated with profound adaptations in resting muscle energetics. For that purpose, we used in vivo (31)P-nuclear magnetic resonance ((31)P-NMR) in male sedentary Wistar rats fed with obesogenic diets known to induce alterations in muscle mitochondrial activity.

METHODS AND PROCEDURES

Two experimental diets (high sucrose and high fat) were provided for 6 weeks at two levels of energy (standard, N and high, H) and compared to control diet. The rates of the adenosine triphosphate (ATP) exchange between phosphocreatine (PCr) and gamma-ATP (k(a)) and beta-adenosine diphosphate (beta-ADP) to beta-ATP (k(b)) were evaluated using (31)P-NMR in resting gastrocnemius muscle. Muscle contents in phosphorylated compounds as well as creatine, were assessed using (31)P-NMR and biochemical assays, respectively.

RESULTS

ATP content increased by 6.7-8.5% in standard-energy high-sucrose (NSU), high-energy high-fat (HF) and high-energy high-sucrose (HSU) groups compared to control (P < 0.05), whereas PCr content decreased by 4.2-6.4% (P < 0.01). Consequently, PCr to ATP ratio decreased in NSU, HF, and HSU groups, compared to control (P < 0.01). Furthermore in high-energy groups (HF and HSU) compared to control, creatine contents were decreased by 14-19% (P < 0.001), whereas k(a) and k(b) fluxes were increased by 89-133% (P < 0.001) and 243-277% (P < 0.01), respectively.

DISCUSSION

Our in vivo data showed adaptations of resting skeletal muscle energetics in response to high-energy diets. Increased activity of enzymes catalyzing ATP production may reflect a compensatory mechanism to face impaired mitochondrial ATP synthesis in order to preserve intracellular energy homeostasis.

Liver and muscle metabolic changes induced by dietary energy content and genetic selection in rainbow trout (Oncorhynchus mykiss)

We combined genetic selection and dietary treatment to produce a model to study metabolic pathways involved in genetic and nutritional control of fat deposition in fish muscle. Two experimental lines of rainbow trout, selected for a lean (L) or fat (F) muscle, were fed with diets containing either 10 or 23% lipids from the first feeding, up to 6 mo. At the end of the feeding trial, trout were distinguished by very different muscle fat content (from 4.2 to 10% wet weight), and line x diet interactions were observed for parameters related to fat storage. We analyzed the activity and gene expression of key enzymes involved in lipid metabolism (fatty acid synthase, hydroxyacyl-CoA dehydrogenase, carnitine palmitoyltransferase 1 isoforms, and peroxisome proliferator-activated receptor alpha) and glycolysis (hexokinase 1 and pyruvate kinase) as well as energy production (isocitrate dehydrogenase, citrate synthase, and cytochrome oxidase) in the liver and the white muscle of rainbow trout. The lipid-rich diet repressed the activity of the lipogenic enzymes and stimulated enzymes involved in fatty acid oxidation and glycolysis in liver but had little effect on muscle enzymes assessed in this study. Regarding the selection effect, enzyme activity and expression suggest that compared with the L line, the F line presented reduced hepatic fatty acid oxidation as well as reduced mitochondrial oxidative capacities and enhanced glucose utilization in both liver and muscle. Very few line x diet interactions were found, suggesting that the two factors (i.e., dietary energy content and selection) used in this study to modify muscle lipid content exerted some additive but mostly independent effects on these metabolic actors.

Proposal open for discussion: defining agreed diagnostic procedures in experimental diabetes research

BACKGROUND

Animal experimentation has a long tradition in diabetes research and has provided invaluable benefits with regard to insulin discovery and treatment assessment.

METHODS

The review focuses on chemical-induced diabetes in rats and surveys the protocols of diabetes induction, diabetes diagnosis, and glucose tolerance evaluation in a selection of recent research.

RESULTS

This brief review of techniques in experimental diabetes highlights that there is no uniformity, whereas standardisation of procedures is desirable so that comparability will exist among experiments carried out in different settings.

CONCLUSIONS

On this basis, questions are put and standards are proposed. It would be a platform to promote the exchange of ideas through expert consultation about practical issues related to animal research and a basis on which standards can be set according to user requirements and animal respect.

Transgenerational effects of fetal and neonatal exposure to nicotine

A wide variety of in utero insults are associated with an increased incidence of metabolic disorders in the offspring and in subsequent generations. We have shown that fetal and neonatal exposure to nicotine results in endocrine and metabolic changes in the offspring that are consistent with those observed in type 2 diabetes. This study examines whether fetal and neonatal exposure to nicotine has transgenerational effects in the F2 offspring. Female Wistar rats were given either saline or nicotine (1 mg/kg/d) during pregnancy and lactation to create saline- and nicotine-exposed female F1 progeny. These F1 females were then bred to produce F2 offspring. We examined glucose homeostasis, serum lipids and fat pad weights, mitochondrial enzyme activity in skeletal muscle and blood pressure in these F2 offspring between 13 and 15 weeks of age. Offspring of nicotine- versus saline-exposed mothers had elevated fasting serum insulin concentrations and an enhanced total insulin response to the glucose challenge. This apparent insulin resistance was unrelated to changes in skeletal muscle mitochondrial volume or activity. The offspring of nicotine-exposed mothers also had elevated blood pressure. These data demonstrate that adverse effects of fetal and neonatal exposure to nicotine can influence aspects of metabolic risk in subsequent generations.

Enhanced muscle mixed and mitochondrial protein synthesis rates after a high-fat or high-sucrose diet

OBJECTIVE

Obesity and insulin resistance are associated with muscle mitochondrial dysfunction, which might be related to impairment of mitochondrial protein synthesis. This study aimed at investigating mixed and mitochondrial protein synthesis in skeletal muscle in response to dietary manipulations.

RESEARCH METHODS AND PROCEDURES

High-sucrose (SU) and high-fat, high-sucrose (F) diets were provided for 6 weeks to Wistar rats at standard (N) and high (H) energy intakes and compared with controls. Fractional synthesis rates of mixed (FSRPT) and mitochondrial (FSRm) proteins within the oxidative (soleus) and glycolytic (tibialis) muscles were measured using stable isotope flooding dose technique using L-[13C]-valine. Carbonyl content, citrate synthase, and cytochrome c oxidase activities were assayed spectrophotometrically on isolated mitochondria.

RESULTS

In the soleus, FSRPT was increased by 40% in the NSU and NF groups and by 65% in the HSU and HF groups (p<0.001 vs. control). FSRm was increased with high-fat diets (NF, +16%; HF, +32%; p<0.01). In the tibialis, FSR(PT) was enhanced in all experimental groups (+31% to 37%, p<0.05 vs. control). FSRm was augmented in the NSU, NF, and HF groups (+28% to 32%, p<0.01). Cytochrome c oxidase activity was significantly decreased in all experimental groups in the soleus (p<0.001).

DISCUSSION

Muscle mixed and mitochondrial protein FSR are enhanced after short-term dietary intervention known to induce insulin resistance and obesity. Adaptations are muscle type specific and may not explain alterations in mitochondrial oxidative capacity but might contribute to maintain mitochondrial functioning.

A reciprocal interaction between food-motivated behavior and diet-induced obesity

OBJECTIVES

One of the main causes of obesity is overconsumption of diets high in fat and sugar. We studied the metabolic changes and food-motivated behavior when rats were subjected to a choice diet with chow, lard and a 30% sucrose solution (high fat high sugar (HFHS)-choice diet). Because rats showed considerable variations in the feeding response to HFHS-choice diet and in food-motivated behavior, we investigated whether the motivation to obtain a sucrose reward correlated with the development of obesity when rats were subsequently subjected to HFHS-choice diet.

METHOD

We first studied feeding, locomotor activity and body temperature, fat weights and hormonal concentrations when male Wistar rats were subjected to HFHS-choice diet for 1 week. Second, we studied sucrose-motivated behavior, using a progressive ratio (PR) schedule of reinforcement in rats that were subjected to the HFHS-choice diet for at least 2 weeks, compared to control rats on a chow diet. Third, we measured motivation for sucrose under a PR schedule of reinforcement in rats that were subsequently subjected to HFHS-choice diet or a chow diet for 4 weeks. Fat weights were measured and correlated with the motivation to obtain sucrose pellets.

RESULTS

One week on the HFHS-choice diet increased plasma concentrations of glucose and leptin, increased fat stores, but did not alter body temperature or locomotor activity. Moreover, consuming the HFHS-choice diet for several weeks increased the motivation to work for sucrose pellets. Furthermore, the motivation to obtain sucrose pellets correlated positively with abdominal fat stores in rats subsequently subjected to the HFHS-choice diet, whereas this correlation was not found in rats fed on a chow diet.

CONCLUSION

Our data suggest that the motivation to respond for palatable food correlates with obesity due to an obesogenic environment. Conversely, the HFHS-choice diet, which results in obesity, also increased the motivation to work for sucrose. Thus, being motivated to work for sucrose results in obesity, which, in turn, increases food-motivated behavior, resulting in a vicious circle of food motivation and obesity.

Chronological approach of diet-induced alterations in muscle mitochondrial functions in rats

OBJECTIVE

Mitochondrial dysfunction might predispose individuals to develop insulin resistance. Our objective was to determine whether mitochondrial dysfunction or insulin resistance was the primary event during high-fat (HF) diet.

RESEARCH METHODS AND PROCEDURES

Rats were fed an HF diet for 0, 3, 6, 9, 14, 20, or 40 days and compared with control. Soleus and tibialis muscle mitochondrial activity were assessed using permeabilized fiber technique. Insulin [area under the curve for insulin (AUC(I))] and glucose [area under the curve for glucose (AUC(G))] responses to intraperitoneal glucose tolerance test as well as fasting plasma non-esterified fatty acids (NEFAs), triglyceride, and glycerol concentrations were determined.

RESULTS

AUC(I) and AUC(G) were altered from Day 6 (p < 0.01 vs. Day 0). In soleus, oxidative phosphorylation (OXPHOS) activity was transiently enhanced by 26% after 14 days of HF diet (p < 0.05 vs. Day 0) conjointly with 62% increase in NEFA concentration (p < 0.05 vs. Day 0). This was associated with normalized AUC(G) at Day 14 and with a decline of plasma NEFA concentration together with stabilization of intra-abdominal adiposity at Day 20. Prolongation of HF diet again caused an increase in plasma NEFA concentration, intra-abdominal adiposity, AUC(I), and AUC(G). At Day 40, significant decrease in OXPHOS activity was observed in soleus.

DISCUSSION

Mitochondria first adapt to overfeeding in oxidative muscle limiting excess fat deposition. This potentially contributes to maintain glucose homeostasis. Persistent overfeeding causes insulin resistance and results in a slow decline in oxidative muscle OXPHOS activity. This shows that the involvement of mitochondria in the predisposition to insulin resistance is mainly due to an inability to face prolonged excess fat delivery.