Exercise Protects Against Defective Insulin Signaling and Insulin Resistance of Glucose Transport in Skeletal Muscle of Angiotensin II-Infused Rat

Post-translational dysregulation of glucose uptake during exhaustive cycling exercise in vastus lateralis muscle of healthy homozygous carriers of the ACE deletion allele

Homozygous carriers of the deletion allele in the gene for angiotensin-converting enzyme (ACE-DD) demonstrate an elevated risk to develop inactivity-related type II diabetes and show an overshoot of blood glucose concentration with enduring exercise compared to insertion allele carriers. We hypothesized that ACE-DD genotypes exhibit a perturbed activity of signaling processes governing capillary-dependent glucose uptake in vastus lateralis muscle during exhaustive cycling exercise, which is associated with the aerobic fitness state. 27 healthy, male white Caucasian subjects (26.8 ± 1.1 years; BMI 23.6 +/− 0.6 kg m⁻²) were characterized for their aerobic fitness based on a threshold of 50 ml O₂ min⁻¹ kg⁻¹ and the ACE-I/D genotype. Subjects completed a session of exhaustive one-legged exercise in the fasted state under concomitant measurement of cardiorespiratory function. Capillary blood and biopsies were collected before, and ½ and 8 h after exercise to quantify glucose and lipid metabolism-related compounds (lipoproteins, total cholesterol, ketones) in blood, the phosphorylation of 45 signaling proteins, muscle glycogen and capillaries. Effects of aerobic fitness, ACE-I/D genotype, and exercise were assessed with analysis of variance (ANOVA) under the hypothesis of a dominant effect of the insertion allele. Exertion with one-legged exercise manifested in a reduction of glycogen concentration ½ h after exercise (−0.046 mg glycogen mg⁻¹ protein). Blood glucose concentration rose immediately after exercise in association with the ACE-I/D genotype (ACE-DD: +26%, ACE-ID/II: +6%) and independent of the fitness state (p = 0.452). Variability in total cholesterol was associated with exercise and fitness. In fit subjects, the phosphorylation levels of glucose uptake-regulating kinases [AKT-pT308 (+156%), SRC-pY419, p38α-pT180/T182, HCK-pY411], as well as cytokine/angiotensin 1-7 signaling factors [(STAT5A-pY694, STAT5B-pY699, FYN-pY420, EGFR-pY1086] were higher in angiotensin converting enzyme I-allele carriers than ACE-DD genotypes after exercise. Conversely, the AKT-S473 phosphorylation level (+117%) and angiotensin 2’s blood concentration (+191%) were higher in ACE-DD genotypes. AKT-S473 phosphorylation levels post-exercise correlated to anatomical parameters of muscle performance and metabolic parameters (p < 0.05 and │r│>0.70). The observations identify reciprocal alterations of S473 and T308 phosphorylation of AKT as gatekeeper of a post-translational dysregulation of transcapillary glucose uptake in ACE-DD genotypes which may be targeted in personalized approaches to mitigate type II diabetes.

Alterations in renin-angiotensin receptors are not responsible for exercise preconditioning of skeletal muscle fibers

Endurance exercise training promotes a protective phenotype in skeletal muscle known as exercise preconditioning. Exercise preconditioning protects muscle fibers against a variety of threats including inactivity-induced muscle atrophy. The mechanism(s) responsible for exercise preconditioning remain unknown and are explored in these experiments. Specifically, we investigated the impact of endurance exercise training on key components of the renin-angiotensin system (RAS). The RAS was targeted because activation of the classical axis of the RAS pathway via angiotensin II type I receptors (AT1Rs) promotes muscle atrophy whereas activation of the non-classical RAS axis via Mas receptors (MasRs) inhibits the atrophic signaling of the classical RAS pathway. Guided by prior studies, we hypothesized that an exercise-induced decrease in AT1Rs and/or increases in MasRs in skeletal muscle fibers is a potential mechanism responsible for exercise preconditioning. Following endurance exercise training in rats, we examined the abundance of AT1Rs and MasRs in both locomotor and respiratory muscles. Our results indicate that endurance exercise training does not alter the protein abundance of AT1Rs or MasRs in muscle fibers from the diaphragm, plantaris, and soleus muscles compared to sedentary controls (p ​> ​0.05). Furthermore, fluorescent angiotensin II (AngII) binding analyses confirm our results that exercise preconditioning does not alter the protein abundance of AT1Rs in the diaphragm, plantaris, and soleus (p ​> ​0.05). This study confirms that exercise-induced changes in RAS receptors are not a key mechanism that contributes to the beneficial effects of exercise preconditioning in skeletal muscle fibers.

Aberrant insulin receptor expression is associated with insulin resistance and skeletal muscle atrophy in myotonic dystrophies

Myotonic dystrophy type 1 (DM1) and type 2 (DM2) are autosomal dominant multisystemic disorders linked to two different genetic loci and characterized by several features including myotonia, muscle atrophy and insulin resistance. The aberrant alternative splicing of insulin receptor (IR) gene and post-receptor signalling abnormalities have been associated with insulin resistance, however the precise molecular defects that cause metabolic dysfunctions are still unknown. Thus, the aims of this study were to investigate in DM skeletal muscle biopsies if beyond INSR missplicing, altered IR protein expression could play a role in insulin resistance and to verify if the lack of insulin pathway activation could contribute to skeletal muscle wasting. Our analysis showed that DM skeletal muscle exhibits a lower expression of the insulin receptor in type 1 fibers which can contribute to the defective activation of the insulin pathway. Moreover, the aberrant insulin signalling activation leads to a lower activation of mTOR and to an increase in MuRF1 and Atrogin-1/MAFbx expression, possible explaining DM skeletal muscle fiber atrophy. Taken together our data indicate that the defective insulin signalling activation can contribute to skeletal muscle features in DM patients and are probably linked to an aberrant specific-fiber type expression of the insulin receptor.