Effects of obesity and acute resistance exercise on skeletal muscle angiogenic communication pathways.

NEW FINDINGS: What is the central question of this study? Do obesity and acute resistance exercise alter the regulation of muscle intercellular communication pathways consistent with inadequate compensatory angiogenesis in response to muscle loading present in individuals with obesity? What is the main finding and its importance? Obesity is associated with differences in both pro- and anti-angiogenic signalling consistent with lower muscle capillarization. Acute resistance exercise increases the release of skeletal muscle small extracellular vesicles independent of body mass. These results identify new cellular factors associated with impaired angiogenesis in obesity and the positive effects of acute resistance exercise in lean and obese skeletal muscle. ABSTRACT: Obesity (OB) impairs cell-to-cell communication signalling. Small extracellular vesicles (EVs), which include exosomes, are released by skeletal muscle and participate in cell-to-cell communication, including the regulation of angiogenesis. Resistance exercise (REx) increases muscle fibre size and capillarization. Although obesity increases muscle fibre size, there is an inadequate increase in capillarization such that capillary density is reduced. It was hypothesized that REx-induced angiogenic signalling and EV biogenesis would be lower with obesity. Sedentary lean (LN) and OB subjects (n = 8 per group) performed three sets of single-leg knee-extension REx at 80% of maximum. Muscle biopsies were obtained at rest, 15 min and 3 h postexercise and analysed for angiogenic and EV biogenesis mRNA and protein. In OB subjects, muscle fibre size was ∼20% greater and capillary density with type II fibres ∼25% lower compared with LN subjects (P < 0.001). In response to REx, the increase in VEGF mRNA (pro-angiogenic) was similar (3-fold) between groups, while thrombospondin-1 (TSP-1) mRNA (anti-angiogenic) increased ∼2.5-fold in OB subjects only (P = 0.010). miR-130a (pro-angiogenic) was ∼1.4-fold (P = 0.011) and miR-503 (anti-angiogenic) ∼1.8-fold (P = 0.017) greater in OB compared with LN subjects at all time points. In both groups, acute REx decreased the EV surface protein Alix by ∼50%, consistent with the release of exosomes (P = 0.016). Acute REx appears to induce the release of skeletal muscle small EVs independent of body mass. However, with obesity there is predominantly impaired angiogenic signalling, consistent with inadequate angiogenesis in response to basal muscle hypertrophy.

Obesity and exercise training alter inflammatory pathway skeletal muscle small extracellular vesicle microRNAs.

NEW FINDINGS: What is the central question of this study? Is 1 week of exercise training sufficient to reduce local and systemic inflammation? Do obesity and short-term concurrent aerobic and resistance exercise training alter skeletal muscle extracellular vesicle (EV) contents? What is the main finding and its importance? Obesity alters skeletal muscle small EV microRNAs targeting inflammatory and growth pathways. Exercise training alters skeletal muscle small EV microRNAs targeting inflammatory pathways, indicative of reduced inflammation. Our findings provide support for the hypotheses that EVs play a vital role in intercellular communication during health and disease and that EVs mediate many of the beneficial effects of exercise. ABSTRACT: Obesity is associated with chronic inflammation characterized by increased levels of inflammatory cytokines, whereas exercise training reduces inflammation. Small extracellular vesicles (EVs; 30-150 nm) participate in cell-to-cell communication in part through microRNA (miRNA) post-transcriptional regulation of mRNA. We examined whether obesity and concurrent aerobic and resistance exercise training alter skeletal muscle EV miRNA content and inflammatory signalling. Vastus lateralis biopsies were obtained from sedentary individuals with (OB) and without obesity (LN). Before and after 7 days of concurrent aerobic and resistance training, muscle-derived small EV miRNAs and whole-muscle mRNAs were measured. Pathway analysis revealed that obesity alters small EV miRNAs that target inflammatory (SERPINF1, death receptor and Gαi ) and growth pathways (Wnt/ß-catenin, PTEN, PI3K/AKT and IGF-1). In addition, exercise training alters small EV miRNAs in an anti-inflammatory manner, targeting the IL-10, IL-8, Toll-like receptor and nuclear factor-κB signalling pathways. In whole muscle, IL-8 mRNA was reduced by 50% and Jun mRNA by 25% after exercise training, consistent with the anti-inflammatory effects of exercise on skeletal muscle. Obesity and 7 days of concurrent exercise training differentially alter skeletal muscle-derived small EV miRNA contents targeting inflammatory and anabolic pathways.

Skeletal muscle IGF-1 is lower at rest and after resistance exercise in humans with obesity.

PURPOSE: Obesity is associated with numerous changes in skeletal muscle including greater muscle mass and muscle fiber cross sectional area (FCSA), yet fasted muscle protein synthesis is lower. Activation of the IGF-1/Akt/mTOR pathway is critical for muscle mass maintenance, muscle hypertrophy, and muscle protein regulation. Resistance exercise (RE) increases muscle mass, FCSA, and IGF-1. Persons with obesity have greater skeletal muscle mass and larger skeletal muscle fiber cross sectional area. The IGF-1/Akt/mTOR pathway is critical for the regulation of skeletal muscle mass. Our study found men and women with obesity have lower skeletal muscle IGF-1 mRNA and protein and higher expression of miR-206 an epigenetic regulator of IGF-1, at rest and following an acute bout of resistance exercise. Despite this, Akt mediated signaling was maintained and maintenance of phosphorylation does not appear to be accounted for by compensatory pathways. Our findings suggest a possible negative feedback mechanism via increased miR-206 and in turn decreased IGF-1 to limit further skeletal muscle hypertrophy in persons with obesity. The current work investigated if: (1) obesity dysregulates basal skeletal muscle IGF-1 pathways; and (2) obesity augments the muscle IGF-1 pathway responses to acute RE. METHODS: Eight sedentary (no self-reported physical activity), lean (LN) and eight sedentary subjects with obesity (OB) had vastus lateralis biopsies taken at rest, and 15 min and 3 h post-acute RE for the measurement of the IGF-1 pathway and muscle FCSA. RESULTS: Type II FCSA was larger in OB vs. LN. Skeletal muscle IGF-1 mRNA and IGF-1 protein were lower in OB vs. LN at rest and post-exercise. Acute RE increased IGF-1 protein similarly in both groups. The expression of miR-206, a post-transcriptional inhibitor of IGF-1 expression, was higher in OB vs. LN and linked with lower IGF-1 mRNA (r = - 0.54). CONCLUSION: In spite of greater muscle FCSA, muscle IGF-1 expression was lower in obesity suggesting negative feedback may be limiting muscle mass expansion in obesity.

Multivesicular body and exosome pathway responses to acute exercise.

NEW FINDINGS: What is the central question of this study? What is the impact of acute aerobic and aerobic + resistance (concurrent) exercise on the regulation of multivesicular body formation in human skeletal muscle? What is the main finding and its importance? Gene expression for proteins associated with multivesicular body biogenesis was increased in response to concurrent exercise, and gene expression of microRNA processing (genetic information) was increased in response to aerobic and concurrent exercise. A greater understanding of the processing of multivesicular bodies in response to acute exercise may lead to novel treatments focused on intercellular communication pathways. ABSTRACT: Regular aerobic exercise (AEx) and resistance exercise (REx) promote many beneficial adaptations. Skeletal muscle participates in intercellular communication in part through the release of myokines and extracellular vesicles including exosomes (EXOs), the latter containing mRNA, microRNA (miRNA), lipids and proteins. Exercise-induced regulation of skeletal muscle multivesicular body (MVB) biogenesis leading to EXO formation and release is poorly understood. We hypothesized that acute exercise would increase skeletal muscle MVB biogenesis and EXO release pathways with a greater response to aerobic + resistance exercise (A+REx) than to AEx alone. Twelve sedentary, healthy male subjects exercised on a cycle ergometer for 45 min (AEx) followed by single leg, knee extensor, resistance exercise (A+REx). Vastus lateralis biopsies were obtained at rest and 1 h post-exercise. Key components of the MVB biogenesis, EXO biogenesis and release, and miRNA processing pathways were analysed. Clathrin and Alix mRNA (MVB biogenesis) were increased by A+REx, while DICER and exportin mRNA (miRNA processing) were increased by AEx and A+REx. There were positive relationships between MVBs and miRNA processing genes following both AEx and A+REx consistent with coordinated regulation of these interrelated processes (Alix mRNA increased with Drosha, exportin and Dicer mRNA). Acute exercise increases the regulation of components of MVB and EXO pathways as well as miRNA processing components. A greater understanding of the production and packaging of skeletal muscle MVBs, EXOs and mature miRNA could lead to novel treatments focused on intercellular communication.

Skeletal muscle-derived exosomes regulate endothelial cell functions via reactive oxygen species-activated nuclear factor-κB signalling.

NEW FINDINGS: What is the central question of this study? Capillary rarefaction is found in diabetic and aged muscle, whereas exercise increases skeletal muscle angiogenesis. The association implies a crosstalk between muscle cells and endothelial cells. The underlying mechanisms mediating the crosstalk between these cells remains to be elucidated fully. What is the main finding and its importance? Endothelial cell functions are regulated by skeletal muscle cell-derived exosomes via a vascular endothelial growth factor-independent pathway. This study reveals a new mechanism mediating the crosstalk between skeletal muscle cells and endothelial cells. ABSTRACT: Loss of skeletal muscle capillarization, known as capillary rarefaction, is found in type 2 diabetes, chronic heart failure and healthy ageing and is associated with impaired delivery of substrates to the muscle. However, the interaction and communication of skeletal muscle with endothelial cells in the regulation of capillaries surrounding the muscle remains elusive. Exosomes are a type of secreted extracellular vesicle containing mRNAs, proteins and, especially, microRNAs that exert paracrine and endocrine effects. In this study, we investigated whether skeletal muscle-derived exosomes (SkM-Exo) regulate the endothelial cell functions of angiogenesis. We demonstrated that C2C12 myotube-derived exosomes improved endothelial cell functions, assessed by the proliferation, migration and tube formation of human umbilical vein endothelial cells (HUVECs), which were increased by 20, 23 and 40%, respectively, after SkM-Exo exposure. The SkM-Exo failed to activate HUVEC vascular endothelial growth factor (VEGF) signalling. The SkM-Exo increased HUVEC reactive oxygen species and activated the nuclear factor-κB pathway, suggesting that SkM-Exo-induced angiogenesis was mediated by a VEGF-independent pathway. In addition, several angiogenic microRNAs were packaged in SkM-Exo, with miR-130a being particularly enriched and successfully transferred from SkM-Exo to HUVECs. Delivery of miRNAs into endothelial cells might explain the enhancement of reactive oxygen species production and angiogenesis by SkM-Exo. The potential angiogenic effect of SkM-Exo could provide an effective therapy for promoting skeletal muscle angiogenesis in diseases characterized by capillary rarefaction or inadequate angiogenesis.

High Incomplete Skeletal Muscle Fatty Acid Oxidation Explains Low Muscle Insulin Sensitivity in Poorly Controlled T2D.

Context: Almost 50% of type 2 diabetic (T2D) patients are poorly controlled [glycated hemoglobin (HbA1c) ≥ 7%]; however, the mechanisms responsible for progressively worsening glycemic control are poorly understood. Lower skeletal muscle mitochondrial respiratory capacity is associated with low insulin sensitivity and the development of T2D. Objective: We investigated if skeletal muscle insulin sensitivity (SI) was different between well-controlled T2D (WCD) and poorly controlled T2D (PCD) and if the difference was associated with differences resulting from mitochondrial respiratory function. Design: Vastus lateralis muscle mitochondrial respiration, mitochondrial content, mitochondrial enzyme activity, and fatty acid oxidation (FAO) were measured. SI and the acute response to glucose (AIRg) were calculated by MINMOD analysis from glucose and insulin obtained during a modified, frequently sampled, intravenous glucose tolerance test. Results: SI and AIRg were lower in PCD than WCD. Muscle incomplete FAO was greater in PCD than WCD and greater incomplete FAO was associated with lower SI and higher HbA1c. Hydroxyacyl-coenzyme A dehydrogenase expression and activity were greater in PCD than WCD. There was no difference in maximal mitochondrial respiration or content between WCD and PCD. Conclusion: The current results suggest that greater skeletal muscle incomplete FAO in poorly controlled T2D is due to elevated ß oxidation and is associated with worsening muscle SI.