Skeletal Muscle Resistance Research Articles

Metabolic effects of insulin in a human model of ketoacidosis combining exposure to lipopolysaccharide and insulin deficiency: a randomised, controlled, crossover study in individuals with type 1 diabetes.

Diabetic ketoacidosis (DKA) is often caused by concomitant systemic inflammation and lack of insulin. Here we used an experimental human model to test whether and how metabolic responses to insulin are impaired in the early phases of DKA with a specific focus on skeletal muscle metabolism. Nine individuals with type 1 diabetes from a previously published cohort were investigated twice at Aarhus University Hospital using a 120min infusion of insulin (3.0/1.5mUkg-1min-1) after an overnight fast under: (1) euglycaemic conditions (CTR) or (2) hyperglycaemic ketotic conditions (KET) induced by an i.v. bolus of lipopolysaccharide and 85% reduction in insulin dosage. The primary outcome was insulin resistance in skeletal muscle. Participants were randomly assigned to one of the two arms at the time of screening using www.randomizer.org . The study was not blinded. All nine volunteers completed the 2days and are included in the analysis. Circulating concentrations of glucose and 3-hydroxybutyrate increased during KET (mean±SEM 17.7±0.6mmol/l and 1.6±0.2mmol/l, respectively), then decreased after insulin treatment (6.6±0.7mmol/l and 0.1±0.07mmol/l, respectively). Prior to insulin infusion (KET vs CTR) isotopically determined endogenous glucose production rates were 17±1.7μmolkg-1min-1 vs 8±1.3μmolkg-1min-1 (p=0.003), whole body phenylalanine fluxes were 2.9±0.5μmolkg-1min-1 vs 3.1±0.4μmolkg-1min-1 (p=0.77) and urea excretion rates were 16.9±2.4g/day vs 7.3±1.7g/day (p=0.01). Insulin failed to stimulate forearm glucose uptake and glucose oxidation in KET compared with CTR (p<0.05). Glycogen synthase phosphorylation was impaired in skeletal muscle. In KET, hyperglycaemia is primarily driven by increased endogenous glucose production. Insulin stimulation during early phases of DKA is associated with reduced glucose disposal in skeletal muscle, impaired glycogen synthase function and lower glucose oxidation. This underscores the presence of muscle insulin resistance in the pathogenesis of DKA. Trial registration www.clinicaltrials.gov (ID number: NCT02157155). Funding This work was funded by the Danish Council for Strategic Research (grant no. 0603-00479B).

High Fat Diet Impairs Mitochondrial ADP Sensitivity and Attenuates Respiration in the Presence of Submaximal, but Not Maximal, ADP Concentrations

The consumption of a high fat (HF) diet is a major contributor of ectopic lipid accumulation in skeletal muscle and insulin resistance. While the underlying mechanisms are not well‐defined, mitochondrial dysfunction, including lower ADP sensitivity through the voltage dependent anion channel (VDAC)/adenine nucleotide transport (ANT) axis, has been implicated in this process. It is currently unknown how a HF diet impairs mitochondrial function, however, lipid moieties (palmitoyl‐CoA; P‐CoA) have been shown to inhibit ANT and reduce ADP sensitivity in situ. Therefore, lipid accumulation following a HF diet could promote inhibition on ANT, explaining these mitochondrial abnormalities and possibly insulin resistance. Since we have previously shown mitochondrial creatine kinase (Mi‐CK) knockout (KO) mice have greater ADP sensitivity through the VDAC/ANT axis when challenged by exercise, a situation that also increases intramuscular lipid supply, we determined whether this would be protective against HF‐induced mitochondrial dysfunction and insulin resistance. Mi‐CK wildtype (WT) and KO mice were fed a control (CON) or HF diet (60% fat) for 4 weeks. High‐resolution respirometry was used to assess ADP sensitivity in the presence or absence of P‐CoA in permeabilized muscle fibres, glucose‐tolerance tests were performed, and western blotting was used to determine VDAC and ANT protein content. Our results show on a CON diet that ADP sensitivity in the absence of P‐CoA was not affected by ablating Mi‐CK, while in contrast these mice displayed attenuated P‐CoA‐mediated inhibition of ADP sensitivity. Therefore, we determined if KO mice would be protected from HF diet impairments in ADP sensitivity, and insulin resistance, through improved ADP transport. However, regardless of genotype, consumption of a HF diet resulted in modest reductions in ADP sensitivity in the absence of P‐CoA, which corresponded with reduced insulin sensitivity. This occurred in the absence of altered VDAC or ANT protein content, highlighting external regulation of these proteins following a HF diet. While Mi‐CK KO mice were not protected from HF diet‐induced insulin resistance, this may occur because the improved ADP sensitivity in the presence of P‐CoA was lost during the HF diet. Altogether, these data demonstrate that reductions in ADP sensitivity may be an early event contributing to insulin resistance, while an increase in intramuscular P‐CoA concentrations and an improved sensitivity to P‐CoA inhibition may exacerbate this response.Support or Funding InformationNatural Sciences and Engineering Research Council of Canada; Canadian Foundation for Innovation/Ontario Research Fund.

Endothelium protects smooth muscle from oxidative stress in skeletal muscle resistance arteries

Vascular dysfunction associated with oxidative stress is characteristic of cardiovascular disease. Advanced age can protect microvascular endothelium from oxidative stress (PMC4422569) however it is unknown whether endothelial cells (ECs) or smooth muscle cells (SMCs) of intact microvessels are more susceptible to damage during oxidative stress. Using the superior epigastric artery freshly dissected from anesthetized male C57BL/6 mice (3–4 months old), we tested the hypothesis that, compared to SMCs, ECs are more susceptible to damage during acute oxidative stress. Individual vessels (length, ~2 mm; internal diameter, ~150 μm) were cannulated at both ends and pressurized to 100 cmH2O with physiological salt solution (PSS), then superfused (2 ml/min) with PSS containing 200 μM H2O2 at 37°C (pH=7.36). At defined durations, the lumen was perfused (0.1 ml/min) with PSS containing Hoechst 33342 (turquoise, 1 μM) and propidium iodide (red, 2 μM) to stain nuclei of all cells and nuclei of cells with damaged membranes, respectively. Staining was quantified in the vessel mid‐region (~500 μm × 150 μm; n=5) which contained nuclei of 93±1 ECs (parallel to vessel axis) and 114±2 SMCs (perpendicular to vessel axis). Within 20–30 min of H2O2 superfusion, membrane disruption (red nuclei) occurred in ECs but not SMCs. Following 50 min of H2O2, 60±2% of SMC and 37±2% of EC nuclei stained red (n=5; P&lt;0.05). Neither EC nor SMC nuclei stained red following equivalent superfusion with control PSS. In vessels denuded of endothelium (50 μm diameter tungsten wire), 50 min superfusion with H2O2 stained 90±2% of SMC nuclei red (n=5; P&lt;0.05, denuded vs intact endothelium). In contrast, SMC nuclei of denuded vessels did not stain red following equivalent superfusion with control PSS. Thus, with H2O2 delivered abluminally, membrane damage occurred sooner for ECs than for SMCs and denuding the endothelium exacerbated SMC damage. We suggest that the endothelium of resistance arteries is more vulnerable to oxidative stress while protecting its surrounding smooth muscle layer.Support or Funding InformationNIH R37HL041026

Calcium and electrical signaling in arterial endothelial tubes: New insights into cellular physiology and cardiovascular function.

The integral role of the endothelium during the coordination of blood flow throughout vascular resistance networks has been recognized for several decades now. Early examination of the distinct anatomy and physiology of the endothelium as a signaling conduit along the vascular wall has prompted development and application of an intact endothelial "tube" study model isolated from rodent skeletal muscle resistance arteries. Vasodilatory signals such as increased endothelial cell (EC) Ca2+ ([Ca2+ ]i ) and hyperpolarization take place in single ECs while shared between electrically coupled ECs through gap junctions up to distances of millimeters (≥2mm). The small- and intermediate-conductance Ca2+ activated K+ (SKCa /IKCa or KCa 2.3/KCa 3.1) channels function at the interface of Ca2+ signaling and hyperpolarization; a bidirectional relationship whereby increases in [Ca2+ ]i activate SKCa /IKCa channels to produce hyperpolarization and vice versa. Further, the spatial domain of hyperpolarization among electrically coupled ECs can be finely tuned via incremental modulation of SKCa /IKCa channels to balance the strength of local and conducted electrical signals underlying vasomotor activity. Multifunctional properties of the voltage-insensitive SKCa /IKCa channels of resistance artery endothelium may be employed for therapy during the aging process and development of vascular disease.

Exercise‐like mechanical stimulation improves myogenic responses in aged skeletal muscle resistance arteries

ObjectiveWe tested the hypothesis that exposure to a short‐duration (1 h) increase in intraluminal pressure, to mimic pressure associated with a bout of exercise, would attenuate or reverse the age‐related decline in myogenic constrictor responses in skeletal muscle resistance arteries.MethodsSoleus muscle feed arteries (SFA) from young (4 mo) and old (24 mo) Fischer 344 rats were cannulated and pressurized to 90 (P90) or 130 cmH2O (P130) for 1 h. Following the 1 h treatment period, pressure in the P130 arteries was reduced to 90 cmH2O and SFA were required to develop stable spontaneous tone for 10 min. After stable tone was achieved, myogenic contractile responses were assessed using step‐increases in intraluminal pressure up to 135 cm H2O in 15 cm H2O increments followed by step‐decreases down to 60 cm H2O.ResultsMyogenic vasoconstrictor responses were significantly impaired in old SFA compared to young SFA. Mechanical stimulation with increased pressure for 1 h improved myogenic constriction in old SFA.ConclusionThese results indicate that stimulation with a short‐duration increase in intraluminal pressure, within a range of pressure believed to be present in these arteries during exercise, improves myogenic constrictor responses in aged skeletal muscle resistance arteries.Support or Funding InformationResearch support: AHA grant 0765043Y, AHA grant 4150031, The National Space Biomedical Research Institute through NCC 9–58 and a Sydney and J.L. Huffines Institute of Sports Medicine Graduate Student Research Grant.

Effect of aging on vascular smooth muscle contractility in skeletal muscle resistance arteries

ObjectiveWe tested the hypothesis that vascular smooth muscle contractility declines with age in skeletal muscle resistance arteries.MethodsSoleus muscle feed arteries (SFA) from young (4 mo) and old (24 mo) Fischer 344 rats were cannulated with glass micropipettes and pressurized to 90 cmH2O for 1 h. To isolate the role of vascular smooth muscle, SFA were denuded (endothelium removed) by passing 5 ml of air through the lumen of the artery. At the end of the 1 h equilibration period, contractile responses were assessed using increasing whole log doses of norepinephrine (NE; 10−9–10−4 M), angiotensin II (Ang II; 10−11–10−7 M), and phenylephrine (PE; 10−9–10−4 M).ResultsVasoconstrictor responses NE, Ang II, and PE were significantly impaired in endothelium intact old SFA compared to young SFA. In contrast, vasoconstrictor responses to NE, Ang II, and PE were similar in young and old denuded SFA.ConclusionThese results indicate that the contractile properties of vascular smooth muscle cells are preserved with age and that impaired constrictor responses are manifest only when the endothelium is intact.Support or Funding InformationResearch support: AHA grant 0765043Y, AHA grant 4150031, The National Space Biomedical Research Institute through NCC 9–58 and a Sydney and J.L. Huffines Institute of Sports Medicine Graduate Student Research Grant.

A controlled‐release mitochondrial protonophore reverses hypertriglyceridemia, nonalcoholic steatohepatitis, and diabetes in lipodystrophic mice

Lipodystrophy is a rare disorder characterized by complete or partial loss of adipose tissue. Patients with lipodystrophy exhibit hypertriglyceridemia, severe insulin resistance, type 2 diabetes, and nonalcoholic steatohepatitis (NASH). Efforts to ameliorate NASH in lipodystrophies with pharmacologic agents have met with limited success. We examined whether a controlled-release mitochondrial protonophore (CRMP) that produces mild liver-targeted mitochondrial uncoupling could decrease hypertriglyceridemia and reverse NASH and diabetes in a mouse model (fatless AZIP/F-1 mice) of severe lipodystrophy and diabetes. After 4 wk of oral CRMP (2 mg/kg body weight per day) or vehicle treatment, mice underwent hyperinsulinemic-euglycemic clamps combined with radiolabeled glucose to assess liver and muscle insulin responsiveness and tissue lipid measurements. CRMP treatment reversed hypertriglyceridemia and insulin resistance in liver and skeletal muscle. Reversal of insulin resistance could be attributed to reductions in diacylglycerol content and reduced PKC-ε and PKC-θ activity in liver and muscle respectively. CRMP treatment also reversed NASH as reflected by reductions in plasma aspartate aminotransferase and alanine aminotransferase concentrations; hepatic steatosis; and hepatic expression of IL-1α, -β, -2, -4, -6, -10, -12, CD69, and caspase 3 and attenuated activation of the IRE-1α branch of the unfolded protein response. Taken together, these results provide proof of concept for the development of liver-targeted mitochondrial uncoupling agents as a potential novel therapy for lipodystrophy-associated hypertriglyceridemia, NASH and diabetes.-Abulizi, A., Perry, R. J., Camporez, J. P. G., Jurczak, M. J., Petersen, K. F., Aspichueta, P., Shulman, G. I. A controlled-release mitochondrial protonophore reverses hypertriglyceridemia, nonalcoholic steatohepatitis, and diabetes in lipodystrophic mice.

Resveratrol ameliorates the chemical and microbial induction of inflammation and insulin resistance in human placenta, adipose tissue and skeletal muscle.

Gestational diabetes mellitus (GDM), which complicates up to 20% of all pregnancies, is associated with low-grade maternal inflammation and peripheral insulin resistance. Sterile inflammation and infection are key mediators of this inflammation and peripheral insulin resistance. Resveratrol, a stilbene-type phytophenol, has been implicated to exert beneficial properties including potent anti-inflammatory and antidiabetic effects in non-pregnant humans and experimental animal models of GDM. However, studies showing the effects of resveratrol on inflammation and insulin resistance associated with GDM in human tissues have been limited. In this study, human placenta, adipose (omental and subcutaneous) tissue and skeletal muscle were stimulated with pro-inflammatory cytokines TNF-α and IL-1β, the bacterial product lipopolysaccharide (LPS) and the synthetic viral dsRNA analogue polyinosinic:polycytidylic acid (poly(I:C)) to induce a GDM-like model. Treatment with resveratrol significantly reduced the expression and secretion of pro-inflammatory cytokines IL-6, IL-1α, IL-1β and pro-inflammatory chemokines IL-8 and MCP-1 in human placenta and omental and subcutaneous adipose tissue. Resveratrol also significantly restored the defects in the insulin signalling pathway and glucose uptake induced by TNF-α, LPS and poly(I:C). Collectively, these findings suggest that resveratrol reduces inflammation and insulin resistance induced by chemical and microbial products. Resveratrol may be a useful preventative therapeutic for pregnancies complicated by inflammation and insulin resistance, like GDM.

Your mitochondria are what you eat: a high-fat or a high-sucrose diet eliminates metabolic flexibility in isolated mitochondria from rat skeletal muscle.

Extreme diets consisting of either high fat (HF) or high sucrose (HS) may lead to insulin resistance in skeletal muscle, often associated with mitochondrial dysfunction. However, it is not known if these diets alter normal interactions of pyruvate and fatty acid oxidation at the level of the mitochondria. Here, we report that rat muscle mitochondria does show the normal Randle‐type fat‐carbohydrate interaction seen in vivo. The mechanism behind this metabolic flexibility at the level of the isolated mitochondria is a regulation of the flux‐ratio: pyruvate dehydrogenase (PDH)/β‐oxidation to suit the actual substrate availability, with the PDH flux as the major point of regulation. We further report that this regulatory mechanism of carbohydrate‐fat metabolic interaction surprisingly is lost in mitochondria obtained from animals exposed for 12 weeks to a HF‐ or a HS diet as compared to rats given a normal chow diet. The mechanism seems to be a loss of the PDH flux decrease seen in controls, when fatty acid is supplied as substrate in addition to pyruvate, and vice versa for the supply of pyruvate as substrate to mitochondria oxidizing fatty acid. Finally, we report that the calculated TCA flux in the isolated mitochondria under these circumstances shows a significant reduction (~50%) after the HF diet and an even larger reduction (~75%) after the HS diet, compared with the chow group. Thus, it appears that obesogenic diets as those applied here have major influence on key metabolic performance of skeletal muscle mitochondria.

Correcting Postprandial Hyperglycemia in Zucker Diabetic Fatty Rats With an SGLT2 Inhibitor Restores Glucose Effectiveness in the Liver and Reduces Insulin Resistance in Skeletal Muscle.

Ten-week-old Zucker diabetic fatty (ZDF) rats at an early stage of diabetes embody metabolic characteristics of obese human patients with type 2 diabetes, such as severe insulin and glucose intolerance in muscle and the liver, excessive postprandial excursion of plasma glucose and insulin, and a loss of metabolic flexibility with decreased lipid oxidation. Metabolic flexibility and glucose flux were examined in ZDF rats during fasting and near-normal postprandial insulinemia and glycemia after correcting excessive postprandial hyperglycemia using treatment with a sodium–glucose cotransporter 2 inhibitor (SGLT2-I) for 7 days. Preprandial lipid oxidation was normalized, and with fasting, endogenous glucose production (EGP) increased by 30% and endogenous glucose disposal (E-Rd) decreased by 40%. During a postprandial hyperglycemic-hyperinsulinemic clamp after SGLT2-I treatment, E-Rd increased by normalizing glucose effectiveness to suppress EGP and stimulate hepatic glucose uptake; activation of glucokinase was restored and insulin action was improved, stimulating muscle glucose uptake in association with decreased intracellular triglyceride content. In conclusion, SGLT2-I treatment improves impaired glucose effectiveness in the liver and insulin sensitivity in muscle by eliminating glucotoxicity, which reinstates metabolic flexibility with restored preprandial lipid oxidation and postprandial glucose flux in ZDF rats.

Effects of aging and exercise training on the dynamics of vasoconstriction in skeletal muscle resistance vessels

We hypothesized that old age will slow, and exercise training will speed, the dynamics of skeletal muscle arteriolar vasoconstriction in resistance vessels of aged rats. Young (6month old) and aged (24month old) male Fischer-344 rats were assigned to sedentary (Sed: n = 6/age group) or exercise-trained (ET: n = 5 aged and 6 young; via treadmill running for 10-12weeks) groups. After completion of training, arterioles from the red portion of the gastrocnemius muscle were removed, cannulated, and exposed to 10-4M norepinephrine (NE) or 20mM caffeine. Changes in luminal diameter were recorded for analysis of constrictor dynamics. Old age blunted all kinetic parameters (i.e., time delay, time constant) resulting in vasoconstriction taking ~3 times as long to reach a steady state (SS) versus younger counterparts for NE (aged-sed: 15.6 ± 6.0 versus young-sed: 4.6 ± 0.5s; P < 0.05) with a similar time course to caffeine. Exercise training resulted in a similar time to SS between age groups for NE (aged-ET: 6.8 ± 1.6 versus young-ET: 7.0 ± 0.6s) and caffeine (aged-ET: 7.8 ± 0.6 versus young-ET: 8.6 ± 1.0s). The results of this study demonstrate that aging blunts the rate of vasoconstriction in skeletal muscle resistance vessels to the sympathetic neurotransmitter NE due, in part, to an attenuated rate of contraction from intracellular calcium release. Further, exercise training speeds the dynamics of constriction to both NE and caffeine with old age.

Effect of exendin-4 on lipid deposition in skeletal muscle of diet-induced obese mice and its underlying mechanism

Objective: To investigate the effect of exendin-4, a glucagon-like peptide-1 (GLP-1) receptor agonist, on reducing lipid deposition and improving insulin resistance in skeletal muscle and the underlying mechanisms in high-fat diet (HFD)-induced obese mice. Methods: Twelve male C57BL/6J mice were challenged with HFD for 12 weeks to induce obesity and then randomly divided into two groups: exendin-4 group (intraperitoneal injection of 24 nmol·kg-1·d-1 exendin-4 for 4 weeks) and HFD group (intraperitoneal injection of normal saline for 4 weeks), with 6 mice in each group. Additional 6 mice were also selected as control group. Body weight, fasting blood glucose were recorded. Serum triglyceride (TG), total cholesterol (TC), insulin and skeletal muscle triglyceride levels were measured by enzyme-linked immunosobent assay (ELISA). Oil red O staining was used for morphologic changes of frozen sections from skeletal muscle. The protein levels of lipid metabolic pathway mediated by AMP-activated protein kinase (AMPK) and insulin signailing pathway were determined by Western blot. Results: Compared with mice in HFD group, exendin-4 significantly decreased body weight[(37.68±1.80) vs (46.03±5.00) g, P<0.025], fasting blood glucose[(5.40±0.33) vs (7.65±1.92) mmol/L, P<0.025], serum TG[(37.78±7.14) vs (80.76±34.22) mg/dl, P<0.025], TC[(180.13±18.75) vs (217.57±22.52) mg/dl, P<0.025], insulin[(0.58±0.01) vs (1.67±1.23) ng/ml, P<0.025]and skeletal muscle TG levels[(9.84±1.08) vs (19.35±7.44) mg/g, P<0.025]of obese mice. Oil red O staining revealed that exendin-4 alleviated the accumulation of larger lipid droplets in skeletal muscle. The protein expressions of lipolysis and lipid oxidation mediated by AMPK and insulin signailing pathway were up-regulated, and the protein expressions of lipogenesis mediated by AMPK were down-regulated after intervention of exendin-4. Conclusion: Exendin-4 reduces lipid deposition and insulin resistance in skeletal muscle of HFD-induced obese mice via activating AMPK and up-regulating insulin signailing pathway.

Downregulation of lipin-1 induces insulin resistance by increasing intracellular ceramide accumulation in C2C12 myotubes.

Dysregulation of lipid metabolism in skeletal muscle is involved in the development of insulin resistance. Mutations in lipin-1, a key lipid metabolism regulator leads to significant systemic insulin resistance in fld mice. However, the function of lipin-1 on lipid metabolism and insulin sensitivity in skeletal muscle is still unclear. Herein we demonstrated that downregulation of lipin-1 in C2C12 myotubes by siRNA transfection suppressed insulin action, characterized by reduced insulin stimulated Akt phosphorylation and glucose uptake. Correspondingly, decreased lipin-1 expression was observed in palmitate-induced insulin resistance in C2C12 myotubes, suggested that lipin-1 might play a role in the etiology of insulin resistance in skeletal muscle. The insulin resistance induced by lipin-1 downregulation was related to the disturbance of lipid homeostasis. Lipin-1 silencing reduced intracellular DAG and TAG levels, but elevated ceramide accumulation in C2C12 myotubes. Moreover, the impaired insulin stimulated Akt phosphorylation and glucose uptake caused by lipin-1 silencing could be blocked by the pretreatment with SPT inhibitor myriocin, ceramide synthase inhibitor FB1, or PP2A inhibitor okadaic acid, suggested that the increased ceramide accumulation might be responsible for the development of insulin resistance induced by lipin-1 silencing in C2C12 myotubes. Meanwhile, decreased lipin-1 expression also impaired mitochondrial function in C2C12 myotubes. Therefore, our study suggests that lipin-1 plays an important role in lipid metabolism and downregulation of lipin-1 induces insulin resistance by increasing intracellular ceramide accumulation in C2C12 myotubes. These results offer a molecular insight into the role of lipin-1 in the development of insulin resistance in skeletal muscle.

Oleuropein improves insulin resistance in skeletal muscle by promoting the translocation of GLUT4.

As the beneficial effects of the Mediterranean diet on human health are well established, the phenolic compounds in olive oil have been gaining interest. Oleuropein, a major phenolic compound in olives, is known to reduce the blood glucose levels in alloxan-induced diabetic rats and rabbits, however, its effect on type 2 diabetes caused by obesity is not clear. The purpose of this study is clarifying the effect of oleuropein on the glucose tolerance in skeletal muscle under the condition of lipotoxicity caused by type 2 diabetes. Oleuropein enhanced glucose uptake in C2C12 cells without insulin. Translocation of glucose transporter 4 (GLUT4) into the cell membrane was promoted by activation of adenosine monophosphate-activated protein kinase (AMPK) but not protein kinase B (Akt). Physiological concentration of oleuropein (10 µM) was sufficient to express beneficial effects on C2C12 cells. Oleuropein prevented palmitic acid-induced myocellular insulin resistance. Furthermore, in gastrocnemius muscles of mice fed a high fat diet, oleuropein also induced the GLUT4 localization into cell membrane. These results suggest the possibility of oleuropein to be effective for type 2 diabetes by reducing insulin resistance in skeletal muscles.

Linking Cancer Cachexia-Induced Anabolic Resistance to Skeletal Muscle Oxidative Metabolism.

Cancer cachexia, a wasting syndrome characterized by skeletal muscle depletion, contributes to increased patient morbidity and mortality. While the intricate balance between protein synthesis and breakdown regulates skeletal muscle mass, the suppression of basal protein synthesis may not account for the severe wasting induced by cancer. Therefore, recent research has shifted to the regulation of “anabolic resistance,” which is the impaired ability of nutrition and exercise to stimulate protein synthesis. Emerging evidence suggests that oxidative metabolism can regulate both basal and induced muscle protein synthesis. While disrupted protein turnover and oxidative metabolism in cachectic muscle have been examined independently, evidence suggests a linkage between these processes for the regulation of cancer-induced wasting. The primary objective of this review is to highlight the connection between dysfunctional oxidative metabolism and cancer-induced anabolic resistance in skeletal muscle. First, we review oxidative metabolism regulation of muscle protein synthesis. Second, we describe cancer-induced alterations in the response to an anabolic stimulus. Finally, we review a role for exercise to inhibit cancer-induced anabolic suppression and mitochondrial dysfunction.

Oleate Prevents Palmitate-Induced Atrophy via Modulation of Mitochondrial ROS Production in Skeletal Myotubes.

Accumulation of saturated fatty acids contributes to lipotoxicity-related insulin resistance and atrophy in skeletal muscle. Conversely, unsaturated fatty acids like docosahexaenoic acid were proven to preserve muscle mass. However, it is not known if the most common unsaturated oleate will protect skeletal myotubes against palmitate-mediated atrophy, and its specific mechanism remains to be elucidated. Therefore, we investigated the effects of oleate on atrophy-related factors in palmitate-conditioned myotubes. Exposure of myotubes to palmitate, but not to oleate, led to an induction of fragmented nuclei, myotube loss, atrophy, and mitochondrial superoxide in a dose-dependent manner. Treatment of oleate to myotubes attenuated production of palmitate-induced mitochondrial superoxide in a dose-dependent manner. The treatment of oleate or MitoTEMPO to palmitate-conditioned myotubes led to inhibition of palmitate-induced mRNA expression of proinflammatory (TNF-α and IL6), mitochondrial fission (Drp1 and Fis1), and atrophy markers (myostatin and atrogin1). In accordance with the gene expression data, our immunocytochemistry experiment demonstrated that oleate and MitoTEMPO prevented or attenuated palmitate-mediated myotube shrinkage. These results provide a mechanism indicating that oleate prevents palmitate-mediated atrophy via at least partial modulation of mitochondrial superoxide production.

Maternal butyrate supplementation induces insulin resistance associated with enhanced intramuscular fat deposition in the offspring.

Maternal nutrition is important for the risk of the offspring to develop insulin resistance and adiposity later in life. The study was undertaken to determine effects of maternal butyrate supplementation on lipid metabolism and insulin sensitivity in the offspring skeletal muscle. The offspring of rats, fed a control diet or a butyrate diet (1% sodium butyrate) throughout gestation and lactation, was studied at weaning and at 60 days of age. The offspring of dams fed a butyrate diet had higher HOMA-insulin resistance and impaired glucose tolerance. This was associated with elevated mRNA and protein expressions of lipogenic genes and decreased amounts of lipolytic enzyme. Simultaneously, enhanced acetylation of histone H3 lysine 9 and histone H3 lysine 27 modification on the lipogenic genes in skeletal muscle of adult offspring was observed. Higher concentration of serum insulin and intramuscular triglyceride in skeletal muscle of offspring from the butyrate group at weaning were accompanied by increasing levels of lipogenic genes and enrichment of acetylation of histone H3 lysine 27. Maternal butyrate supplementation leads to insulin resistance and ectopic lipid accumulation in skeletal muscle of offspring, indicating the importance of short chain fatty acids in the maternal diet on lipid metabolism.

The influence of angiotensin-converting enzyme gene ID polymorphism on human physical fitness performance in European and other populations

Angiotensin-converting enzyme (ACE) is a component of the circulating renin–angiotensin system, which influences circulatory homeostasis through the degradation of vasodilator kinins and the generation of vasopressor angiotensin II. Various phenotypic characteristics such as diseases and human performances could be associated with genetic polymorphisms within the ACE gene. To date, one of the most well-studied genetic polymorphisms that has been shown to be associated with athletic performance is that of the ACE gene. Previous studies investigating the influences of polymorphisms and various phenotypic characteristics have produced inconsistent findings due to inter-ethnic variations in the distribution of the different ACE alleles. For example, some studies showed that the I allele was associated with fatigue resistance in skeletal muscle and endurance performance while the D allele had been associated with power or sprint performance. Nevertheless, controversy still exists regarding the above conclusion as related studies reported that the I allele was associated with a better power or sprint performance rather than with athletic endurance abilities. This article discusses the inter-ethnic variations of the distribution of the different ACE alleles in several ethnic groups such as in European, African, American, and Asian populations. Additionally, the influences of the ACE ID polymorphism on human physical fitness performances in European and other populations are discussed.

Overexpression of protein kinase STK25 in mice exacerbates ectopic lipid accumulation, mitochondrial dysfunction and insulin resistance in skeletal muscle

Aims/hypothesisUnderstanding the molecular networks controlling ectopic lipid deposition and insulin responsiveness in skeletal muscle is essential for developing new strategies to treat type 2 diabetes. We recently identified serine/threonine protein kinase 25 (STK25) as a critical regulator of liver steatosis, hepatic lipid metabolism and whole body glucose and insulin homeostasis. Here, we assessed the role of STK25 in control of ectopic fat storage and insulin responsiveness in skeletal muscle.MethodsSkeletal muscle morphology was studied by histological examination, exercise performance and insulin sensitivity were assessed by treadmill running and euglycaemic–hyperinsulinaemic clamp, respectively, and muscle lipid metabolism was analysed by ex vivo assays in Stk25 transgenic and wild-type mice fed a high-fat diet. Lipid accumulation and mitochondrial function were also studied in rodent myoblasts overexpressing STK25. Global quantitative phosphoproteomics was performed in skeletal muscle of Stk25 transgenic and wild-type mice fed a high-fat diet to identify potential downstream mediators of STK25 action.ResultsWe found that overexpression of STK25 in transgenic mice fed a high-fat diet increases intramyocellular lipid accumulation, impairs skeletal muscle mitochondrial function and sarcomeric ultrastructure, and induces perimysial and endomysial fibrosis, thereby reducing endurance exercise capacity and muscle insulin sensitivity. Furthermore, we observed enhanced lipid accumulation and impaired mitochondrial function in rodent myoblasts overexpressing STK25, demonstrating an autonomous action for STK25 within cells. Global phosphoproteomic analysis revealed alterations in the total abundance and phosphorylation status of different target proteins located predominantly to mitochondria and sarcomeric contractile elements in Stk25 transgenic vs wild-type muscle, respectively, providing a possible molecular mechanism for the observed phenotype.Conclusions/interpretationSTK25 emerges as a new regulator of the complex interplay between lipid storage, mitochondrial energetics and insulin action in skeletal muscle, highlighting the potential of STK25 antagonists for type 2 diabetes treatment.

Intact initiation of autophagy and mitochondrial fission by acute exercise in skeletal muscle of patients with Type 2 diabetes

Type 2 diabetes (T2D) is characterized by insulin resistance, mitochondrial dysregulation and, in some studies, exercise resistance in skeletal muscle. Regulation of autophagy and mitochondrial dynamics during exercise and recovery is important for skeletal muscle homoeostasis, and these responses may be altered in T2D. We examined the effect of acute exercise on markers of autophagy and mitochondrial fusion and fission in skeletal muscle biopsies from patients with T2D (n=13) and weight-matched controls (n=14) before, immediately after and 3h after an acute bout of exercise. Although mRNA levels of most markers of autophagy [PIK3C, MAP1LC3B, sequestosome 1 (SQSTM1), BCL-2/adenovirus E1B 19-kDa-interacting protein 3 (BNIP3), BNIP3-like (BNIP3L)] and mitochondrial dynamics [optic atrophy 1 (OPA1), fission protein 1 (FIS1)] remained unchanged, some either increased during and after exercise (GABARAPL1), decreased in the recovery period [BECN1, autophagy-related (ATG) 7, DNM1L] or both [mitofusin (MFN) 2, mitochondrial E3 ubiquitin ligase 1 (MUL1)]. Protein levels of ATG7, p62/SQSTM1, forkhead box O3A (FOXO3A) and MFN2 (only controls) as well as dynamin-related protein 1 (DRP1) Ser616 phosphorylation increased in response to exercise and/or recovery, whereas microtubule-associated protein 1 light chain 3B (LC3B)-II content was reduced immediately after exercise. Exercise increased the activating Ser555 phosphorylation and reduced the inhibitory Ser757 phosphorylation of Unc-51-like kinase-1 (ULK1). The LC3B-II content and phosphorylation of ULK1 and DRP1 returned towards pre-exercise levels in the recovery period. Insulin sensitivity was reduced in T2D, but with no differences in the autophagic response to exercise. Our results demonstrate that initiation of autophagy and mitochondrial fission is activated by exercise in human skeletal muscle, and that these responses are intact in T2D. The exercise-induced decrease in LC3B-II could be due to increased autophagic turnover.