Effect of ß2-agonist treatment on insulin-stimulated peripheral glucose disposal in healthy men in a randomised placebo-controlled trial.

ß2-agonist treatment improves skeletal muscle glucose uptake and whole-body glucose homeostasis in rodents, likely via mTORC2-mediated signalling. However, human data on this topic is virtually absent. We here investigate the effects of two-weeks treatment with the ß2-agonist clenbuterol (40 µg/day) on glucose control as well as energy- and substrate metabolism in healthy young men (age: 18-30 years, BMI: 20-25 kg/m2) in a randomised, placebo-controlled, double-blinded, cross-over study (ClinicalTrials.gov-identifier: NCT03800290). Randomisation occurred by controlled randomisation and the final allocation sequence was seven (period 1: clenbuterol, period 2: placebo) to four (period 1: placebo, period 2: clenbuterol). The primary and secondary outcome were peripheral insulin-stimulated glucose disposal and skeletal muscle GLUT4 translocation, respectively. Primary analyses were performed on eleven participants. No serious adverse events were reported. The study was performed at Maastricht University, Maastricht, The Netherlands, between August 2019 and April 2021. Clenbuterol treatment improved peripheral insulin-stimulated glucose disposal by 13% (46.6 ± 3.5 versus 41.2 ± 2.7 µmol/kg/min, p = 0.032), whereas skeletal muscle GLUT4 translocation assessed in overnight fasted muscle biopsies remained unaffected. These results highlight the potential of ß2-agonist treatment in improving skeletal muscle glucose uptake and underscore the therapeutic value of this pathway for the treatment of type 2 diabetes. However, given the well-known (cardiovascular) side-effects of systemic ß2-agonist treatment, further exploration on the underlying mechanisms is needed to identify viable therapeutic targets.

Three weeks of time-restricted eating improves glucose homeostasis in adults with type 2 diabetes but does not improve insulin sensitivity: a randomised crossover trial.

AIMS/HYPOTHESIS: Time-restricted eating (TRE) is suggested to improve metabolic health by limiting food intake to a defined time window, thereby prolonging the overnight fast. This prolonged fast is expected to lead to a more pronounced depletion of hepatic glycogen stores overnight and might improve insulin sensitivity due to an increased need to replenish nutrient storage. Previous studies showed beneficial metabolic effects of 6-8 h TRE regimens in healthy, overweight adults under controlled conditions. However, the effects of TRE on glucose homeostasis in individuals with type 2 diabetes are unclear. Here, we extensively investigated the effects of TRE on hepatic glycogen levels and insulin sensitivity in individuals with type 2 diabetes. METHODS: Fourteen adults with type 2 diabetes (BMI 30.5±4.2 kg/m2, HbA1c 46.1±7.2 mmol/mol [6.4±0.7%]) participated in a 3 week TRE (daily food intake within 10 h) vs control (spreading food intake over ≥14 h) regimen in a randomised, crossover trial design. The study was performed at Maastricht University, the Netherlands. Eligibility criteria included diagnosis of type 2 diabetes, intermediate chronotype and absence of medical conditions that could interfere with the study execution and/or outcome. Randomisation was performed by a study-independent investigator, ensuring that an equal amount of participants started with TRE and CON. Due to the nature of the study, neither volunteers nor investigators were blinded to the study interventions. The quality of the data was checked without knowledge on intervention allocation. Hepatic glycogen levels were assessed with 13C-MRS and insulin sensitivity was assessed using a hyperinsulinaemic-euglycaemic two-step clamp. Furthermore, glucose homeostasis was assessed with 24 h continuous glucose monitoring devices. Secondary outcomes included 24 h energy expenditure and substrate oxidation, hepatic lipid content and skeletal muscle mitochondrial capacity. RESULTS: Results are depicted as mean ± SEM. Hepatic glycogen content was similar between TRE and control condition (0.15±0.01 vs 0.15±0.01 AU, p=0.88). M value was not significantly affected by TRE (19.6±1.8 vs 17.7±1.8 µmol kg-1 min-1 in TRE vs control, respectively, p=0.10). Hepatic and peripheral insulin sensitivity also remained unaffected by TRE (p=0.67 and p=0.25, respectively). Yet, insulin-induced non-oxidative glucose disposal was increased with TRE (non-oxidative glucose disposal 4.3±1.1 vs 1.5±1.7 µmol kg-1 min-1, p=0.04). TRE increased the time spent in the normoglycaemic range (15.1±0.8 vs 12.2±1.1 h per day, p=0.01), and decreased fasting glucose (7.6±0.4 vs 8.6±0.4 mmol/l, p=0.03) and 24 h glucose levels (6.8±0.2 vs 7.6±0.3 mmol/l, p<0.01). Energy expenditure over 24 h was unaffected; nevertheless, TRE decreased 24 h glucose oxidation (260.2±7.6 vs 277.8±10.7 g/day, p=0.04). No adverse events were reported that were related to the interventions. CONCLUSIONS/INTERPRETATION: We show that a 10 h TRE regimen is a feasible, safe and effective means to improve 24 h glucose homeostasis in free-living adults with type 2 diabetes. However, these changes were not accompanied by changes in insulin sensitivity or hepatic glycogen. TRIAL REGISTRATION: ClinicalTrials.gov NCT03992248 FUNDING: ZonMW, 459001013.

Skeletal muscle mitochondrial network dynamics in metabolic disorders and aging.

With global demographics trending towards an aging population, the numbers of individuals with an age-associated loss of independence is increasing. A key contributing factor is loss of skeletal muscle mitochondrial, metabolic, and contractile function. Recent advances in imaging technologies have demonstrated the importance of mitochondrial morphology and dynamics in the pathogenesis of disease. In this review, we examine the evidence for altered mitochondrial dynamics as a mechanism in age and obesity-associated loss of skeletal muscle function, with a particular focus on the available human data. We highlight some of the areas where more data are needed to identify the specific mechanisms connecting mitochondrial morphology and skeletal muscle dysfunction.

Lipids activate skeletal muscle mitochondrial fission and quality control networks to induce insulin resistance in humans.

BACKGROUND AND AIMS: A diminution in skeletal muscle mitochondrial function due to ectopic lipid accumulation and excess nutrient intake is thought to contribute to insulin resistance and the development of type 2 diabetes. However, the functional integrity of mitochondria in insulin-resistant skeletal muscle remains highly controversial. METHODS: 19 healthy adults (age:28.4 ±â€¯1.7 years; BMI:22.7 ±â€¯0.3 kg/m2) received an overnight intravenous infusion of lipid (20% Intralipid) or saline followed by a hyperinsulinemic-euglycemic clamp to assess insulin sensitivity using a randomized crossover design. Skeletal muscle biopsies were obtained after the overnight lipid infusion to evaluate activation of mitochondrial dynamics proteins, ex-vivo mitochondrial membrane potential, ex-vivo oxidative phosphorylation and electron transfer capacity, and mitochondrial ultrastructure. RESULTS: Overnight lipid infusion increased dynamin related protein 1 (DRP1) phosphorylation at serine 616 and PTEN-induced kinase 1 (PINK1) expression (P = 0.003 and P = 0.008, respectively) in skeletal muscle while reducing mitochondrial membrane potential (P = 0.042). The lipid infusion also increased mitochondrial-associated lipid droplet formation (P = 0.011), the number of dilated cristae, and the presence of autophagic vesicles without altering mitochondrial number or respiratory capacity. Additionally, lipid infusion suppressed peripheral glucose disposal (P = 0.004) and hepatic insulin sensitivity (P = 0.014). CONCLUSIONS: These findings indicate that activation of mitochondrial fission and quality control occur early in the onset of insulin resistance in human skeletal muscle. Targeting mitochondrial dynamics and quality control represents a promising new pharmacological approach for treating insulin resistance and type 2 diabetes. CLINICAL TRIAL REGISTRATION: NCT02697201, ClinicalTrials.gov.

Human skeletal muscle mitochondrial dynamics in relation to oxidative capacity and insulin sensitivity.

AIMS/HYPOTHESIS: Mitochondria operate in networks, adapting to external stresses and changes in cellular metabolic demand and are subject to various quality control mechanisms. On the basis of these traits, we here hypothesise that the regulation of mitochondrial networks in skeletal muscle is hampered in humans with compromised oxidative capacity and insulin sensitivity. METHODS: In a cross-sectional design, we compared four groups of participants (selected from previous studies) ranging in aerobic capacity and insulin sensitivity, i.e. participants with type 2 diabetes (n = 11), obese participants without diabetes (n = 12), lean individuals (n = 10) and endurance-trained athletes (n = 12); basal, overnight fasted muscle biopsies were newly analysed for the current study and we compared the levels of essential mitochondrial dynamics and quality control regulatory proteins in skeletal muscle tissue. RESULTS: Type 2 diabetes patients and obese participants were older than lean participants and athletes (58.6 ± 4.0 and 56.7 ± 7.2 vs 21.8 ± 2.5 and 25.1 ± 4.3 years, p < 0.001, respectively) and displayed a higher BMI (32.4 ± 3.7 and 31.0 ± 3.7 vs 22.1 ± 1.8 and 21.0 ± 1.5 kg/m2, p < 0.001, respectively) than lean individuals and endurance-trained athletes. Fission protein 1 (FIS1) and optic atrophy protein 1 (OPA1) protein content was highest in muscle from athletes and lowest in participants with type 2 diabetes and obesity, respectively (FIS1: 1.86 ± 0.79 vs 0.79 ± 0.51 AU, p = 0.002; and OPA1: 1.55 ± 0.64 vs 0.76 ± 0.52 AU, p = 0.014), which coincided with mitochondrial network fragmentation in individuals with type 2 diabetes, as assessed by confocal microscopy in a subset of type 2 diabetes patients vs endurance-trained athletes (n = 6). Furthermore, lean individuals and athletes displayed a mitonuclear protein balance that was different from obese participants and those with type 2 diabetes. Mitonuclear protein balance also associated with heat shock protein 60 (HSP60) protein levels, which were higher in athletes when compared with participants with obesity (p = 0.048) and type 2 diabetes (p = 0.002), indicative for activation of the mitochondrial unfolded protein response. Finally, OPA1, FIS1 and HSP60 correlated positively with aerobic capacity (r = 0.48, p = 0.0001; r = 0.55, p < 0.001 and r = 0.61, p < 0.0001, respectively) and insulin sensitivity (r = 0.40, p = 0.008; r = 0.44, p = 0.003 and r = 0.48, p = 0.001, respectively). CONCLUSIONS/INTERPRETATION: Collectively, our data suggest that mitochondrial dynamics and quality control in skeletal muscle are linked to oxidative capacity in humans, which may play a role in the maintenance of muscle insulin sensitivity. CLINICAL TRIAL REGISTRY: numbers NCT00943059, NCT01298375 and NL1888 Graphical abstract.

Mitochondrial Utilization of Competing Fuels Is Altered in Insulin Resistant Skeletal Muscle of Non-obese Rats (Goto-Kakizaki).

AIM: Insulin-resistant skeletal muscle is characterized by metabolic inflexibility with associated alterations in substrate selection, mediated by peroxisome-proliferator activated receptor δ (PPARδ). Although it is established that PPARδ contributes to the alteration of energy metabolism, it is not clear whether it plays a role in mitochondrial fuel competition. While nutrient overload may impair metabolic flexibility by fuel congestion within mitochondria, in absence of obesity defects at a mitochondrial level have not yet been excluded. We sought to determine whether reduced PPARδ content in insulin-resistant rat skeletal muscle of a non-obese rat model of T2DM (Goto-Kakizaki, GK) ameliorate the inhibitory effect of fatty acid (i.e., palmitoylcarnitine) on mitochondrial carbohydrate oxidization (i.e., pyruvate) in muscle fibers. METHODS: Bioenergetic function was characterized in oxidative soleus (S) and glycolytic white gastrocnemius (WG) muscles with measurement of respiration rates in permeabilized fibers in the presence of complex I, II, IV, and fatty acid substrates. Mitochondrial content was measured by citrate synthase (CS) and succinate dehydrogenase activity (SDH). Western blot was used to determine protein expression of PPARδ, PDK isoform 2 and 4. RESULTS: CS and SDH activity, key markers of mitochondrial content, were reduced by ∼10-30% in diabetic vs. control, and the effect was evident in both oxidative and glycolytic muscles. PPARδ (p < 0.01), PDK2 (p < 0.01), and PDK4 (p = 0.06) protein content was reduced in GK animals compared to Wistar rats (N = 6 per group). Ex vivo respiration rates in permeabilized muscle fibers determined in the presence of complex I, II, IV, and fatty acid substrates, suggested unaltered mitochondrial bioenergetic function in T2DM muscle. Respiration in the presence of pyruvate was higher compared to palmitoylcarnitine in both animal groups and fiber types. Moreover, respiration rates in the presence of both palmitoylcarnitine and pyruvate were reduced by 25 ± 6% (S), 37 ± 6% (WG) and 63 ± 6% (S), 57 ± 8% (WG) compared to pyruvate for both controls and GK, respectively. The inhibitory effect of palmitoylcarnitine on respiration was significantly greater in GK than controls (p < 10-3). CONCLUSION: With competing fuels, the presence of fatty acids diminishes mitochondria ability to utilize carbohydrate derived substrates in insulin-resistant muscle despite reduced PPARδ content.

Day-night rhythm of skeletal muscle metabolism is disturbed in older, metabolically compromised individuals.

OBJECTIVE: Skeletal muscle mitochondrial function and energy metabolism displays day-night rhythmicity in healthy, young individuals. Twenty-four-hour rhythmicity of metabolism has been implicated in the etiology of age-related metabolic disorders. Whether day-night rhythmicity in skeletal muscle mitochondrial function and energy metabolism is altered in older, metabolically comprised humans remains unknown. METHODS: Twelve male overweight volunteers with impaired glucose tolerance and insulin sensitivity stayed in a metabolic research unit for 2 days under free living conditions with regular meals. Indirect calorimetry was performed at 5 time points (8 AM, 1 PM, 6 PM, 11 PM, 4 AM), followed by a muscle biopsy. Mitochondrial oxidative capacity was measured in permeabilized muscle fibers using high-resolution respirometry. RESULTS: Mitochondrial oxidative capacity did not display rhythmicity. The expression of circadian core clock genes BMAL1 and REV-ERBα showed a clear day-night rhythm (p < 0.001), peaking at the end of the waking period. Remarkably, the repressor clock gene PER2 did not show rhythmicity, whereas PER1 and PER3 were strongly rhythmic (p < 0.001). On the whole-body level, resting energy expenditure was highest in the late evening (p < 0.001). Respiratory exchange ratio did not decrease during the night, indicating metabolic inflexibility. CONCLUSIONS: Mitochondrial oxidative capacity does not show a day-night rhythm in older, overweight participants with impaired glucose tolerance and insulin sensitivity. In addition, gene expression of PER2 in skeletal muscle indicates that rhythmicity of the negative feedback loop of the molecular clock is disturbed. CLINICALTRIALS. GOV ID: NCT03733743.

Exercise plus caloric restriction lowers soluble RAGE in adults with chronic kidney disease.

INTRODUCTION: The incidence of chronic kidney disease (CKD) has increased in recent years. CKD is associated with obesity, type 2 diabetes, and cardiovascular disease, although the mechanism remains unclear. Elevated soluble form of the receptor for advanced glycation end products ( RAGE) is related to proinflammatory signaling pathways that may promote diabetic nephropathy and vascular dysfunction. Because lifestyle modification reduces systematic inflammation in adults with obesity and hyperglycaemia, the hypothesis that exercise plus caloric restriction would lower soluble RAGE in adults with CKD was tested in this study. METHODS: Eight adults (n = 6 females; age: 56.3 ± 2.8 y; BMI: 43.7 ± 2.2 kg/m2; 2-h OGTT glucose: 215 ± 9.8 mg/dL; eGFR: 49.6 ± 3.3 mL/min/1.73 m2) were enrolled in a 12-week pilot lifestyle intervention (supervised aerobic exercise [5 d/wk, up to 60 min/d at approximately 65%-85% HRmax] plus low-fat dietary counseling). Body composition (DXA), aerobic fitness (VO2max), insulin sensitivity (120 min 75 g OGTT; Matsuda Index), plasma levels of soluble RAGE and fetuin-A were measured before and after the intervention. RESULTS: Exercise reduced body weight, fasting glucose, and fetuin-A as well as increased VO2max, glucose tolerance, and insulin sensitivity (all P < .05). Lifestyle intervention decreased plasma soluble RAGE (pre: 1018.1 ± 163 vs post: 810.6 ± 119.6 ng/mL; P = .02), and the decrease was associated with a lower 2-hour blood glucose (r = 0.76, P = .03) and with increased insulin sensitivity (r = -0.90, P < .01). CONCLUSIONS: Exercise and caloric restriction are effective at lowering soluble RAGE in relation to glucose regulation in patients with CKD.

Lipids and ketones dominate metabolism at the expense of glucose control in pulmonary arterial hypertension: a hyperglycaemic clamp and metabolomics study.

Individuals with idiopathic pulmonary arterial hypertension (PAH) display reduced oral glucose tolerance. This may involve defects in pancreatic function or insulin sensitivity but this hypothesis has not been tested; moreover, fasting nutrient metabolism remains poorly described in PAH. Thus, we aimed to characterise fasting nutrient metabolism and investigated the metabolic response to hyperglycaemia in PAH.12 participants (six PAH, six controls) were administered a hyperglycaemic clamp, while 52 (21 PAH, 31 controls) underwent plasma metabolomic analysis. Glucose, insulin, C-peptide, free fatty acids and acylcarnitines were assessed from the clamp. Plasma metabolomics was conducted on fasting plasma samples.The clamp verified a reduced insulin response to hyperglycaemia in PAH (-53% versus control), but with similar pancreatic insulin secretion. Skeletal muscle insulin sensitivity was unexpectedly greater in PAH. Hepatic insulin extraction was elevated in PAH (+11% versus control). Plasma metabolomics identified 862 metabolites: 213 elevated, 145 reduced in PAH (p<0.05). In both clamp and metabolomic cohorts, lipid oxidation and ketones were elevated in PAH. Insulin sensitivity, fatty acids, acylcarnitines and ketones correlated with PAH severity, while hepatic extraction and fatty acid:ketone ratio correlated with longer six-min walk distance.Poor glucose control in PAH could not be explained by pancreatic ß-cell function or skeletal muscle insulin sensitivity. Instead, elevated hepatic insulin extraction emerged as an underlying factor. In agreement, nutrient metabolism in PAH favours lipid and ketone metabolism at the expense of glucose control. Future research should investigate the therapeutic potential of reinforcing lipid and ketone metabolism on clinical outcomes in PAH.

Exercise Training Rapidly Increases Hepatic Insulin Extraction in NAFLD.

PURPOSE: We aimed to determine the immediacy of exercise intervention on liver-specific metabolic processes in nonalcoholic fatty liver disease. METHODS: We undertook a short-term (7-d) exercise training study (60 min·d treadmill walking at 80%-85% of maximal heart rate) in obese adults (N = 13, 58 ± 3 yr, 34.3 ± 1.1 kg·m, >5% hepatic lipid by H-magnetic resonance spectroscopy). Insulin sensitivity index was estimated by oral glucose tolerance test using the Soonthorpun model. Hepatic insulin extraction (HIE) was calculated as the molar difference in area under the curve (AUC) for insulin and C-peptide (HIE = 1 - (AUCInsulin/AUCC-Pep)). RESULTS: The increases in HIE, V˙O2max, and insulin sensitivity index after the intervention were 9.8%, 9.8%, and 34%, respectively (all, P < 0.05). Basal fat oxidation increased (pre: 47 ± 6 mg·min vs post: 65 ± 6 mg·min, P < 0.05) and carbohydrate oxidation decreased (pre: 160 ± 20 mg·min vs post: 112 ± 15 mg·min, P < 0.05) with exercise training. After the intervention, HIE correlated positively with adiponectin (r = 0.56, P < 0.05) and negatively with TNF-α (r = -0.78, P < 0.001). CONCLUSIONS: By increasing HIE along with peripheral insulin sensitivity, aerobic exercise training rapidly reverses some of the underlying physiological mechanisms associated with nonalcoholic fatty liver disease, in a weight loss-independent manner. This reversal could potentially act through adipokine-related pathways.

UCC118 supplementation reduces exercise-induced gastrointestinal permeability and remodels the gut microbiome in healthy humans.

Dysregulation of gut microbiota and intestinal barrier function has emerged as potential mechanisms underlying digestive diseases, yet targeted therapies are lacking The purpose of this investigation was to assess the efficacy of UCC118, a characterized probiotic strain, on exercise-induced GI permeability in healthy humans. In a randomized, double-blind, placebo-controlled crossover study, seven healthy adults received 4 weeks of daily UCC118 or placebo supplementation. GI hyperpermeability was induced by strenuous treadmill running performed before and after each supplementation period. While running, participants ingested 5 g of lactulose, rhamnose, and sucrose. Urine was collected before, immediately after, and every hour for 5 h after exercise to assess GI permeability. Metagenomic sequencing was performed on fecal homogenates collected prior to exercise to identify changes in microbial diversity and taxon abundances. Inflammatory biomarkers were assessed from blood and fecal homogenates collected prior to and immediately following the cessation of exercise. Exercise significantly induced intestinal permeability of lactulose, rhamnose, and sucrose (P < 0.001). UCC118 significantly reduced sucrose (Δ = -0.38 ± 0.13 vs. 1.69 ± 0.79; P < 0.05) recovery, with no substantial change in lactulose (Δ = -0.07 ± 0.23 vs. 0.35 ± 0.15; P = 0.16) or rhamnose (Δ = -0.06 ± 0.22 vs. 0.48 ± 0.28; P = 0.22). Taxonomic sequencing revealed 99 differentially regulated bacteria spanning 6 taxonomic ranks (P < 0.05) after UCC118 supplementation. No differences in plasma IL-6 or fecal zonulin were observed after UCC118 supplementation. The results described herein provide proof of principle that 4 weeks of UCC118 supplementation attenuated exercise-induced intestinal hyperpermeability. Further research is warranted to investigate the as-yet-to-be defined molecular processes of intestinal hyperpermeability and the effects of probiotic supplementation.

Exercise training promotes a GDF15-associated reduction in fat mass in older adults with obesity.

Obesity is a major risk factor for metabolic disease. Growth differentiation factor 15 (GDF15) has shown promise as a weight loss agent for obesity in animal studies. In healthy lean humans, fasting plasma GDF15 increases after acute exercise. However, the role of GDF15 in human obesity and the response of plasma GDF15 to exercise training in patients with obesity is unknown. Here, 24 sedentary volunteers with obesity [age: 65 ± 1 yr; body mass index (BMI): 35.3 ± 0.9 kg/m2] participated in a supervised 12-wk aerobic exercise intervention: 1 h/day, 5 days/wk at ~85% maximum heart rate with controlled isocaloric diet. As a result, plasma GDF15 was significantly increased (PRE: 644.1 ± 42.6 pg/ml, POST: 704.4 ± 47.2 pg/ml, P < 0.01) after the exercise intervention. Inconsistent with animal models, ΔGDF15 was not correlated with change in weight, BMI, or resting energy expenditure. However, ΔGDF15 was correlated with a reduction in total fat mass (P < 0.05), abdominal fat mass (P < 0.05), and android fat mass (P ≤ 0.05). Participants with a positive GDF15 response to exercise had increased total fat oxidation (PRE: 0.25 ± 0.05 mg·kg-1·min-1, POST: 0.43 ± 0.07 mg·kg-1·min-1, P ≤ 0.05), metabolic flexibility [PRE: -0.01 ± 0.01 delta respiratory quotient (RQ), POST: 0.06 ± 0.01 delta RQ, P < 0.001], and insulin sensitivity (PRE: 0.33 ± 0.01 QUICKI index, POST: 0.34 ± 0.01 QUICKI index, P < 0.01), suggesting a link between GDF15 and fat mass loss as well as exercise-induced metabolic improvement in humans with obesity. We conclude that the exercise-induced increase in plasma GDF15 and the association with reduced fat mass may indicate a role for GDF15 as a therapeutic target for human obesity.

Exercise training remodels human skeletal muscle mitochondrial fission and fusion machinery towards a pro-elongation phenotype.

AIMS: Mitochondria exist as a morphologically plastic network driven by cellular bioenergetic demand. Induction of fusion and fission machinery allows the organelle to regulate quality control and substrate flux. Physiological stressors promote fragmentation of the mitochondrial network, a process implicated in the onset of metabolic disease, including type 2 diabetes and obesity. It is well-known that exercise training improves skeletal muscle mitochondrial volume, number, and density. However, the effect of exercise training on muscle mitochondrial dynamics remains unclear. METHODS: Ten sedentary adults (65.8 ± 4.6 years; 34.3 ± 2.4 kg/m2 ) underwent 12 weeks of supervised aerobic exercise training (5 day/wk, 85% of HRMAX ). Body composition, cardio-metabolic testing, hyperinsulinaemic-euglycaemic clamps, and skeletal muscle biopsies were performed before and after training. MFN1, MFN2, OPA1, OMA1, FIS1, Parkin, PGC-1α, and HSC70 protein expression was assessed via Western blot. RESULTS: Exercise training led to improvements in insulin sensitivity, aerobic capacity, and fat oxidation (all P < 0.01), as well as reductions in body weight, BMI, fat mass and fasting glucose (all P < 0.001). When normalized for changes in mitochondrial content, exercise reduced skeletal muscle FIS1 and Parkin (P < 0.05), while having no significant effect on MFN1, MFN2, OPA1, and OMA1 expression. Exercise also improved the ratio of fusion to fission proteins (P < 0.05), which positively correlated with improvements in glucose disposal (r2  = 0.59, P < 0.05). CONCLUSIONS: Exercise training alters the expression of mitochondrial fusion and fission proteins, promoting a more fused, tubular network. These changes may contribute to the improvements in insulin sensitivity and substrate utilization that are observed after exercise training.

Mitochondrial dynamics in skeletal muscle insulin resistance and type 2 diabetes.

The traditional view of mitochondria as isolated, spherical, energy producing organelles, is undergoing a revolutionary change. Emerging data show that mitochondria form a dynamic reticulum that is regulated by cycles of fission and fusion. The discovery of proteins that modulate these activities has led to important advances in understanding human disease. Here, we review the latest evidence that connects the emerging field of mitochondrial dynamics to skeletal muscle insulin resistance and propose some potential mechanisms that may explain the long debated link between mitochondria and the development of type 2 diabetes.

Functional high-intensity exercise training ameliorates insulin resistance and cardiometabolic risk factors in type 2 diabetes.

NEW FINDINGS: What is the central question of this study? Does short-duration, high-intensity exercise training that combines functional aerobic and resistance exercises into training sessions lasting 8-20 min benefit individuals with type 2 diabetes? What is the main finding and its importance? Functional high-intensity training improves insulin sensitivity and reduces cardiometabolic risk in individuals with type 2 diabetes. This type of exercise training may be an effective exercise mode for managing type 2 diabetes. The increase in insulin sensitivity addresses a key defect in type 2 diabetes. ABSTRACT: Functional high-intensity training (F-HIT) is a novel fitness paradigm that integrates simultaneous aerobic and resistance training in sets of constantly varied movements, based on real-world situational exercises, performed at high-intensity in workouts that range from ∼8 to 20 min per session. We hypothesized that F-HIT would be an effective exercise mode for reducing insulin resistance in type 2 diabetes (T2D). We recruited 13 overweight/obese adults (5 males, 8 females; 53 ± 7 years; BMI 34.5 ± 3.6 kg m-2 , means ± SD) with T2D to participate in a 6-week (3 days week-1 ) supervised F-HIT programme. An oral glucose tolerance test was used to derive measures of insulin sensitivity. F-HIT significantly reduced fat mass (43.8 ± 83.8 vs. 41.6 ± 7.9 kg; P < 0.01), diastolic blood pressure (80.2 ± 7.1 vs. 74.5 ± 5.8; P < 0.01), blood lipids (triglyceride and VLDL, both P < 0.05) and metabolic syndrome z-score (6.4 ± 4.5 vs. -0.2 ± 5.2 AU; P < 0.001), and increased basal fat oxidation (0.08 ± 0.03 vs. 0.10 ± 0.04 g min-1 ; P = 0.05), and high molecular mass adiponectin (214.4 ± 88.9 vs. 288.8 ± 127.4 ng mL-1 ; P < 0.01). Importantly, F-HIT also increased insulin sensitivity (0.037 ± 0.010 vs. 0.042 ± 0.010 AU; P < 0.05). Increases in high molecular mass adiponectin and basal fat oxidation correlated with the change in insulin sensitivity (ρ, 0.75, P < 0.05 and ρ, 0.81, P < 0.01, respectively). Compliance with the training programme was >95% and no injuries or adverse events were reported. These data suggest that F-HIT may be an effective exercise mode for managing T2D. The increase in insulin sensitivity addresses a key defect in T2D and is consistent with improvements observed after more traditional aerobic exercise programmes in overweight/obese adults with T2D.

Effect of Roux-en-Y gastric bypass on liver mitochondrial dynamics in a rat model of obesity.

Bariatric surgery provides significant and durable improvements in glycemic control and hepatic steatosis, but the underlying mechanisms that drive improvements in these metabolic parameters remain to be fully elucidated. Recently, alterations in mitochondrial morphology have shown a direct link to nutrient adaptations in obesity. Here, we evaluate the effects of Roux-en-Y gastric bypass (RYGB) surgery on markers of liver mitochondrial dynamics in a diet-induced obesity Sprague-Dawley (SD) rat model. Livers were harvested from adult male SD rats 90-days after either Sham or RYGB surgery and continuous high-fat feeding. We assessed expression of mitochondrial proteins involved in fusion, fission, mitochondrial autophagy (mitophagy) and biogenesis, as well as differences in citrate synthase activity and markers of oxidative stress. Gene expression for mitochondrial fusion genes, mitofusin 1 (Mfn1; P < 0.05), mitofusin 2 (Mfn2; P < 0.01), and optic atrophy 1 (OPA1; P < 0.05) increased following RYGB surgery. Biogenesis regulators, peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α; P < 0.01) and nuclear respiratory factor 1 (Nrf1; P < 0.05), also increased in the RYGB group, as well as mitophagy marker, BCL-2 interacting protein 3 (Bnip3; P < 0.01). Protein expression for Mfn1 (P < 0.001), PGC1α (P < 0.05), BNIP3 (P < 0.0001), and mitochondrial complexes I-V (P < 0.01) was also increased by RYGB, and Mfn1 expression negatively correlated with body weight, insulin resistance, and fasting plasma insulin. In the RYGB group, citrate synthase activity was increased (P < 0.02) and reactive oxygen species (ROS) was decreased compared to the Sham control group (P < 0.05), although total antioxidant capacity was unchanged between groups. These data are the first to show an association between RYGB surgery and improved markers of liver mitochondrial dynamics. These observed improvements may be related to weight loss and reduced energetic demand on the liver, which could facilitate normalization of glucose homeostasis and protect against hepatic steatosis.

A conserved KLF-autophagy pathway modulates nematode lifespan and mammalian age-associated vascular dysfunction.

Loss of protein and organelle quality control secondary to reduced autophagy is a hallmark of aging. However, the physiologic and molecular regulation of autophagy in long-lived organisms remains incompletely understood. Here we show that the Kruppel-like family of transcription factors are important regulators of autophagy and healthspan in C. elegans, and also modulate mammalian vascular age-associated phenotypes. Kruppel-like family of transcription factor deficiency attenuates autophagy and lifespan extension across mechanistically distinct longevity nematode models. Conversely, Kruppel-like family of transcription factor overexpression extends nematode lifespan in an autophagy-dependent manner. Furthermore, we show the mammalian vascular factor Kruppel-like family of transcription factor 4 has a conserved role in augmenting autophagy and improving vessel function in aged mice. Kruppel-like family of transcription factor 4 expression also decreases with age in human vascular endothelium. Thus, Kruppel-like family of transcription factors constitute a transcriptional regulatory point for the modulation of autophagy and longevity in C. elegans with conserved effects in the murine vasculature and potential implications for mammalian vascular aging.KLF family transcription factors (KLFs) regulate many cellular processes, including proliferation, survival and stress responses. Here, the authors position KLFs as important regulators of autophagy and lifespan in C. elegans, a role that may extend to the modulation of age-associated vascular phenotypes in mammals.

Circulating soluble RAGE isoforms are attenuated in obese, impaired-glucose-tolerant individuals and are associated with the development of type 2 diabetes.

The soluble receptor for advanced glycation end products (sRAGE) may be protective against inflammation associated with obesity and type 2 diabetes (T2DM). The aim of this study was to determine the distribution of sRAGE isoforms and whether sRAGE isoforms are associated with risk of T2DM development in subjects spanning the glucose tolerance continuum. In this retrospective analysis, circulating total sRAGE and endogenous secretory RAGE (esRAGE) were quantified via ELISA, and cleaved RAGE (cRAGE) was calculated in 274 individuals stratified by glucose tolerance status (GTS) and obesity. Group differences were probed by ANOVA, and multivariate ordinal logistic regression was used to test the association between sRAGE isoform concentrations and the proportional odds of developing diabetes, vs. normal glucose tolerance (NGT) or impaired glucose tolerance (IGT). When stratified by GTS, total sRAGE, cRAGE, and esRAGE were all lower with IGT and T2DM, while the ratio of cRAGE to esRAGE (cRAGE:esRAGE) was only lower (P < 0.01) with T2DM compared with NGT. When stratified by GTS and obesity, cRAGE:esRAGE was higher with obesity and lower with IGT (P < 0.0001) compared with lean, NGT. In ordinal logistic regression models, greater total sRAGE (odds ratio, 0.91; P < 0.01) and cRAGE (odds ratio, 0.84; P < 0.01) were associated with lower proportional odds of developing T2DM. Reduced values of sRAGE isoforms observed with both obesity and IGT are independently associated with greater proportional odds of developing T2DM. The mechanisms by which each respective isoform contributes to obesity and insulin resistance may reveal novel treatment strategies for diabetes.

In vitro contraction protects against palmitate-induced insulin resistance in C2C12 myotubes.

We are interested in understanding mechanisms that govern the protective role of exercise against lipid-induced insulin resistance, a key driver of type 2 diabetes. In this context, cell culture models provide a level of abstraction that aid in our understanding of cellular physiology. Here we describe the development of an in vitro myotube contraction system that provides this protective effect, and which we have harnessed to investigate lipid-induced insulin resistance. C2C12 myocytes were differentiated into contractile myotubes. A custom manufactured platinum electrode system and pulse stimulator, with polarity switching, provided an electrical pulse stimulus (EPS) (1 Hz, 6-ms pulse width, 1.5 V/mm, 16 h). Contractility was assessed by optical flow flied spot noise mapping and inhibited by application of ammonium acetate. Following EPS, myotubes were challenged with 0.5 mM palmitate for 4 h. Cells were then treated with or without insulin for glucose uptake (30 min), secondary insulin signaling activation (10 min), and phosphoinositide 3-kinase-α (PI3Kα) activity (5 min). Prolonged EPS increased non-insulin-stimulated glucose uptake (83%, P = 0.002), Akt (Thr308) phosphorylation (P = 0.005), and insulin receptor substrate-1 (IRS-1)-associated PI3Kα activity (P = 0.048). Palmitate reduced insulin-specific action on glucose uptake (-49%, P < 0.001) and inhibited insulin-stimulated Akt phosphorylation (P = 0.049) and whole cell PI3Kα activity (P = 0.009). The inhibitory effects of palmitate were completely absent with EPS pretreatment at the levels of glucose uptake, insulin responsiveness, Akt phosphorylation, and whole cell PI3Kα activity. This model suggests that muscle contraction alone is a sufficient stimulus to protect against lipid-induced insulin resistance as evidenced by changes in the proximal canonical insulin-signaling pathway.

Functional high-intensity training improves pancreatic ß-cell function in adults with type 2 diabetes.

Type 2 diabetes (T2D) is characterized by reductions in ß-cell function and insulin secretion on the background of elevated insulin resistance. Aerobic exercise has been shown to improve ß-cell function, despite a subset of T2D patients displaying "exercise resistance." Further investigations into the effectiveness of alternate forms of exercise on ß-cell function in the T2D patient population are needed. We examined the effect of a novel, 6-wk CrossFit functional high-intensity training (F-HIT) intervention on ß-cell function in 12 sedentary adults with clinically diagnosed T2D (54 ± 2 yr, 166 ± 16 mg/dl fasting glucose). Supervised training was completed 3 days/wk, comprising functional movements performed at a high intensity in a variety of 10- to 20-min sessions. All subjects completed an oral glucose tolerance test and anthropometric measures at baseline and following the intervention. The mean disposition index, a validated measure of ß-cell function, was significantly increased (PRE: 8.4 ± 3.1, POST: 11.5 ± 3.5, P = 0.02) after the intervention. Insulin processing inefficiency in the ß-cell, expressed as the fasting proinsulin-to-insulin ratio, was also reduced (PRE: 2.40 ± 0.37, POST: 1.78 ± 0.30, P = 0.04). Increased ß-cell function during the early-phase response to glucose correlated significantly with reductions in abdominal body fat (R2 = 0.56, P = 0.005) and fasting plasma alkaline phosphatase (R2 = 0.55, P = 0.006). Mean total body-fat percentage decreased significantly (Δ: -1.17 0.30%, P = 0.003), whereas lean body mass was preserved (Δ: +0.05 ± 0.68 kg, P = 0.94). We conclude that F-HIT is an effective exercise strategy for improving ß-cell function in adults with T2D.