Skeletal Muscle Insulin Resistance Research Articles

Adipocyte-specific inactivation of NAMPT, a key NAD+ biosynthetic enzyme, causes a metabolically-unhealthy lean phenotype in female mice during aging.

Nicotinamide adenine dinucleotide (NAD+) is a universal coenzyme regulating cellular energy metabolism in many cell types. Recent studies have demonstrated the close relationships between defective NAD+ metabolism and aging and age-associated metabolic diseases. The major purpose of the present study was to test the hypothesis that NAD+ biosynthesis, mediated by a rate-limiting NAD+ biosynthetic enzyme, nicotinamide phosphoribosyltransferase (NAMPT), is essential for maintaining normal adipose tissue function and whole-body metabolic health during the aging process. To this end, we provided in-depth and comprehensive metabolic assessments for female adipocyte-specific Nampt knockout (ANKO) mice during aging. We first evaluated body fat mass in young (≤ 4-month-old), middle aged (10 to 14-month-old), and old (≥ 18-month-old) mice. Intriguingly, adipocyte-specific Nampt deletion protected against age-induced obesity without changing energy balance. However, data obtained from the hyperinsulinemic euglycemic clamp procedure demonstrated that, despite the lean phenotype, old ANKO mice had severe insulin resistance in skeletal muscle, heart, and white adipose tissue (WAT). Old ANKO mice also exhibited hyperinsulinemia and hypoadiponectinemia. Mechanistically, loss of Nampt caused marked decreases in WAT gene expression of lipogenic targets of peroxisome proliferator-activated receptor gamma (PPARγ) in an age-dependent manner. In addition, administration of a PPARγ agonist rosiglitazone restored fat mass and improved metabolic abnormalities in old ANKO mice. In conclusion, these findings highlight the importance of the NAMPT-NAD+-PPARγ axis in maintaining functional integrity and quantity of adipose tissue, and whole-body metabolic function in female mice during aging.

LECT2 deletion exacerbates liver steatosis and macrophage infiltration in a male mouse model of LPS-mediated NASH.

Leukocyte cell-derived chemotaxin 2 (LECT2) is a protein initially isolated as a neutrophil chemotactic factor. We previously found that LECT2 is an obesity-associated hepatokine that senses liver fat and induces skeletal muscle insulin resistance. In addition, hepatocyte-derived LECT2 activates macrophage pro-inflammatory activity by reinforcing the LPS-induced c-Jun N-terminal kinase (JNK) signaling. Based on these findings, we examined the impact of LECT2 deletion on NAFLD/NASH caused by bacterial translocation. We created the bacterial translocation-mediated NAFLD/NASH model using LECT2 knockout mice (LECT2 KO) with 28 times a low-dose LPS injection under high-fat diet feeding conditions. LECT2 deletion exacerbated steatosis and significantly reduced p38 phosphorylation in the liver. In addition, LECT2 deletion increased macrophage infiltration with decreased M1/M2 ratios. LECT2 might contribute to protecting against lipid accumulation and macrophage activation in the liver under pathological conditions, which might be accomplished via p38 phosphorylation. This study provides novel aspects of LECT2 in the bacterial translocation-mediated NAFLD/NASH model.

Skeletal muscle cystathionine γ-lyase deficiency promotes obesity and insulin resistance and results in hyperglycemia and skeletal muscle injury upon HFD in mice

ABSTRACT Objectives The objective of this study was to investigate whether skeletal muscle cystathionine γ-lyase (CTH) contributes to high-fat diet (HFD)-induced metabolic disorders using skeletal muscle Cth knockout (CthΔskm ) mice. Methods The Cth Δskm mice and littermate Cth-floxed (Cthf/f ) mice were fed with either HFD or chow diet for 13 weeks. Metabolomics and transcriptome analysis were used to assess the impact of CTH deficiency in skeletal muscle. Results Metabolomics coupled with transcriptome showed that CthΔskm mice displayed impaired energy metabolism and some signaling pathways linked to insulin resistance (IR) in skeletal muscle although the mice had normal insulin sensitivity. HFD led to reduced CTH expression and impaired energy metabolism in skeletal muscle in Cthf/f mice. CTH deficiency and HFD had some common pathways enriched in the aspects of amino acid metabolism, carbon metabolism, and fatty acid metabolism. CthΔskm +HFD mice exhibited increased body weight gain, fasting blood glucose, plasma insulin, and IR, and reduced glucose transporter 4 and CD36 expression in skeletal muscle compared to Cthf/f +HFD mice. Impaired mitochondria and irregular arrangement in myofilament occurred in CthΔskm +HFD mice. Omics analysis showed differential pathways enriched between CthΔskm mice and Cthf/f mice upon HFD. More severity in impaired energy metabolism, reduced AMPK signaling, and increased oxidative stress and ferroptosis occurred in CthΔskm +HFD mice compared to Cthf/f +HFD mice. Discussion Our results indicate that skeletal muscle CTH expression dysregulation contributes to metabolism disorders upon HFD.

Muscle Fiber Heterogeneity of Insulin Sensitivity in Syntaxin-4 Enriched Skeletal Muscle

Skeletal muscle insulin resistance (IR) is a primary cause for prediabetes, an intermediary condition of higher-than-normal blood glucose levels that left untreated progresses into type 2 diabetes (T2D). A number of contributing factors associated with skeletal muscle IR include aberrant mitochondrial function, elevated reactive oxygen species, inflammation, and reduction in oxidative slow-twitch type 1 skeletal muscle fibers. One leading candidate for reversing skeletal muscle IR is Syntaxin 4 (STX4), traditionally a cell surface transmembrane SNARE protein for GLUT4 vesicle exocytosis that our lab has recently discovered is also localized in the skeletal muscle outer mitochondrial membrane. Importantly, we previously demonstrated that doxycycline-induced skeletal muscle specific STX4 overexpression in obese, prediabetes male transgenic mice (skmSTX4-iOE) reversed IR and mitochondrial dysfunction associated with diabetes muscle. However, the precise mechanisms of how STX4 restores skeletal muscle insulin sensitivity remains to be determined. To test glucose uptake and insulin sensitivity we used the “gold standard” method of hyper-insulinemic-euglycemic clamp in chow-fed skmSTX4-iOE STX4 enriched mice against non-induced controls, we identified improved glucose clearance was only associated with increased insulin sensitivity in type 1 fiber rich soleus muscle. On the other hand, insulin tolerance test studies in doxycycline-induced skeletal muscle STX4 knockout (skmSTX4-iKO) male mice revealed the opposite phenotype, suggesting that STX4 activity in slow-twitch skeletal muscle may be a contributing factor for the reversal of IR. To determine the heterogeneity of STX4 activity between different skeletal muscle fibers, myogenic precursors and intercalating non-muscular tissue in skeletal muscle STX4 enriched skmSTX4-iOE mice, we aim to use our established single nuclear RNA-Sequencing platform (snRNA-Seq) to identify pathways in distinct fiber types associated with STX4 enrichment. Overall, our work aims to determine the functional requirements of STX4 in slow-and-fast twitch skeletal muscle fiber types and identify novel molecular pathways that may serve as useful targets for the reversal/protection against skeletal muscle IR in prediabetes and T2D. This study was supported by grants from the National Institutes of Health (DK067912, DK1129712 and DK102233) and City of Hope Shared Resources Pilot Grant. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Metformin Blunts Bed Rest-induced Skeletal Muscle Ceramide Accumulation in Older Adults

Skeletal muscle insulin resistance is an undesirable outcome during periods of physical inactivity in older adults, which can lead to a variety of metabolic diseases. Insulin sensitive tissue (e.g., liver, and skeletal muscle) derived ceramides are well known to promote insulin resistance. Evidence suggests that metformin can regulate tissue ceramide accumulation. Therefore, we hypothesized that metformin treatment during a short-term period of bed rest in male and female older adults would reduce skeletal muscle ceramide accumulation and prevent insulin resistance. Our study design consisted of older adults (66 ± 4.5 y/o placebo, n=10; 71.7 ± 5.1 y/o metformin, n=10) who were subjected to 5 days of bed rest. Participants were randomly assigned to placebo or metformin treatment groups. Drug or placebo treatments started two weeks prior to bed rest and continued throughout the 5 days of bed rest. Prior to and after bed rest, participants underwent a euglycemic-hyperinsulinemic clamp. Muscle biopsies (from the vastus lateralis) were taken prior to and after each clamp both before and after bed rest. We found that insulin-stimulated muscle ceramides were increased after bed rest but were blunted in older adults who took metformin. In addition, muscle insulin resistance correlated with the increase in ceramides. We conclude that metformin was effective to reduce ceramide accumulation in muscle during a period of bed rest in older adults. R21AG064576 awarded to M.J.D. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Genomic and epigenomic changes by low force electrically induced exercise of human paralyzed muscle

Musculoskeletal deterioration following complete paralysis, such as after a spinal cord injury, results in osteoporotic bone and atrophic, glycolytic, and insulin-resistant skeletal muscle. Decreased bone integrity can be overloaded with very little force; therefore, high force contractions should be avoided in people with long-term spinal cord injury. Together, these changes compromise systemic metabolism for people with SCI and contribute to the development of secondary non-communicable diseases. Electrically induced exercise training offers a unique strategy to ensure full muscle recruitment without high force levels by using high current intensity and a low stimulation frequency (<5Hz) to prevent force summation of stimulus pulses. While acute epigenomic and genomic changes have been found with this low force exercise method; how long-term training affects skeletal muscle and systemic biomarkers of health remains unknown. Purpose: We seek to determine the long-term genomic and epigenomic response of low-force electrically induced exercise in paralyzed skeletal muscle. Methods: 10 males with a complete SCI completed at least 6-months of unilateral electrically induced exercise training using low-force (3Hz) stimulation. Participants completed at least 5 exercise training sessions per week for at least 6-months. A muscle fatigue assessment and fasting venous blood draw were performed before and after training. Percutaneous muscle biopsies were obtained from a trained and untrained vastus lateralis. Gene expression was measured using the Affymetrix HTA 2.0 array and epigenomic expression was measured using Illumina’s MethylEPIC array. Parametric tests were used to compare pre and post training. All participants provided written consent approved by the University of Iowa Institutional Review Board in compliance with the Declaration of Helsinki. Results: Participants maintained a high compliance by completing 119±19% of the training sessions for 7.2±1.2 months. There was a 115±34% change improvement of the fatigue index after training (p=0.002). Serum concentrations of insulin, hsCRP, and myostatin were decreased by 43±8% (p=0.155), 26±8%(p=0.047), 35±9%(p=0.098) after exercise training, respectively. Serum BDNF increased by 68±14%(p=0.093) after exercise training. There was a strong correlation between the primary principal component and the trained limb (R2=0.20, p=0.046). There were 693 and 671 genes to be significantly up or down regulated after exercise training (FDR < 0.05). Of the genes most responsive to exercise training, MYH6, MYH7, and MYL3 were increased in expression and decreased in median methylation. MSTN and ACTN3 were decreased in genomic expression with an increase in median methylation after exercise training. Conclusions: Low-force electrically induced exercise training improved fatigability, a muscle oxidative phenotype transition, and improved serum biomarkers associated with metabolism and inflammation. Together, these data highlight the impact low-force electrically induced exercise can have on paralyzed muscle and is suggestive that this rehabilitation strategy may provide benefits for people living with paralysis. Research Support: This study was funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development: R01HD084645, R01HD082109. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Bed-rest and exercise remobilisation: concurrent adaptations in muscle glucose and protein metabolism

Bed rest (BR) of only a few days duration reduces muscle protein synthesis and induces skeletal muscle atrophy and insulin resistance, but the scale and juxtaposition of these events has not been investigated concurrently in the same individuals. Moreover, the impact of short-term exercise-supplemented remobilisation (ESR) on muscle volume, protein turnover and leg glucose uptake (LGU) in humans is unknown. Ten healthy males (24±1 years, body mass index (BMI) 22.7±0.6) underwent 3 days of BR, followed immediately by 3 days of ESR consisting of 5x30 maximal voluntary single leg isokinetic knee-extensions at 90o/sec each day. An isoenergetic diet was maintained throughout the study (30% fat,15% protein, 55% carbohydrate). Resting LGU was calculated from arteriovenous-venous difference across the leg and leg blood flow during the steady-state of a 3 hr hyperinsulinaemic euglycaemic clamp (60 mU/m2/min) measured pre BR, post BR and after remobilisation. Glycogen content was measured in Vastus Lateralis muscle biopsy samples obtained before and after each clamp. Leg muscle volume (LMV) was measured using magnetic resonance imaging pre BR, post BR and after remobilisation. Cumulative myofibrillar protein fractional synthesis rate (FSR), and whole-body muscle protein breakdown (MPB) were measured over the course of BR and remobilisation using deuterium oxide and 3-methylhistidine stable isotope tracers that were administered orally. Compared with pre BR, there was a 45% decline in insulin-stimulated LGU (p<0.05) after BR, which was paralleled by a reduction in insulin stimulated leg blood flow (p<0.01) and removal of insulin stimulated muscle glycogen storage. These events were accompanied by a 43% reduction in myofibrillar protein FSR (p<0.05) and a 2.5% decrease in LMV (p<0.01) during BR, along with a 30% decline in whole-body muscle myofibrillar protein breakdown after 2 days BR (p<0.05). Myofibrillar protein FSR and LMV were restored by 3 days of ERS (p<0.01, p<0.01; respectively), but not by ambulation alone. However, insulin stimulated LGU and muscle glycogen storage were not restored by ESR. Three days of BR caused concurrent reductions in LMV, myofibrillar protein FSR, myofibrillar protein breakdown and insulin stimulated LGU, leg blood flow and muscle glycogen storage in healthy, young volunteers. Resistance ESR restored LMV and myofibrillar protein FSR, but LGU and muscle glycogen storage remained depressed, highlighting divergences in muscle fuel and protein metabolism. Furthermore, ambulation alone did not restore LMV and myofibrillar protein FSR in the non-exercised contralateral limb, emphasising the importance of exercise rehabilitation following even short-term BR. This study was supported by the Biotechnology and Biological Sciences Research Council (BB/P005004/1), the Centre for Sport, Exercise and Osteoarthritis Research Versus Arthritis (grant no. 21595), and the National Institute of Health Research (NIHR) Nottingham Biomedical Research Centre. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Captopril treatment does not affect inflammatory and autophagy signaling in skeletal muscle of Zucker diabetic fatty rat

Skeletal muscle plays a key role in the development of Type 2 Diabetes Mellitus (T2DM) as its large capacity to sequester glucose and contribute to the regulation of energy metabolism is compromised. Previous studies indicate that in skeletal muscle, T2DM drives inflammatory signaling and impairs autophagy. Captopril (Cap), an angiotensin-converting enzyme (ACE) inhibitor, is a common treatment for hypertension in T2DM patients; however, the impact of Cap on inflammatory signaling and autophagy in insulin resistant skeletal muscle remains ill-defined. We hypothesized that inflammatory signaling would be elevated and degradation of autophagosomes would be impaired in obese Zucker diabetic fatty (ZDF) rats, which are hyperphagic as they lack the leptin receptor. We further hypothesized that daily Cap treatment would attenuate these disturbances. At 14 weeks of age, the absolute weights of gastrocnemius (p<0.0001), tibialis anterior (p<0.0001), soleus (p=0.002), and extensor digitorum longus (EDL, p<0.0001) muscles were decreased by 23-34% in ZDF compared to lean rats but were not altered by 8 weeks of Cap treatment (0.5g/kg/day). Immunoblotting revealed that in soleus and EDL, inflammatory signaling proteins TLR4, phosphorylated (p)IKK α/β (ser176/180), pNFkB p65 (Ser 536), IkBα, TNFα, and AP1 were similar amongst all groups, whereas MyD88 was elevated in ZDF soleus (p=0.03) but was not corrected by Cap. The autophagy protein, LC3A/B I was increased 32% (p=0.01) in EDL from ZDF rats compared to Con; however, PI3K III, pBeclin1 (ser 93), pULK1 (ser555), LAMP2, LC3A/B II, and the LC3A/B II/I ratio were similar amongst groups in the soleus and EDL and were not altered by Cap. Compared to control, P62, a key autophagy adapter protein, was similar in soleus and decreased 27% in EDL (p<0.01) from ZDF rats and was not altered by Cap. Counter to our expectations, although muscle mass was lower in ZDF groups, obesity and insulin resistance in young ZDF rats were not suffcient to drive inflammatory signaling or disrupt autophagy in glycolytic or oxidative skeletal muscle, and these processes appeared to be resistant to Cap treatment in ZDF rats. USDA-NIFA awarded to MJR (2021-67017-33937). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Inhibitory regulation of FoxO1 in PPARδ expression drives mitochondria dysfunction and insulin resistance.

Forkhead box protein O1 (FoxO1) regulates muscle growth, but the metabolic role of FoxO1 in skeletal muscle and its mechanisms remain unclear. To explore the metabolic role of FoxO1 in skeletal muscle, we generated skeletal muscle-specific FoxO1 inducible knockout (mFoxO1 iKO) mice and fed them a high-fat diet to induce obesity. We measured insulin sensitivity, fatty acid oxidation, mitochondrial function, and exercise capacity in obese mFoxO1 iKO mice, and assessed the correlation between FoxO1 and mitochondrial-related protein in the skeletal muscle of diabetic patients. Obese mFoxO1 iKO mice exhibited improved mitochondrial respiratory capacity, which was followed by attenuated insulin resistance, enhanced fatty acid oxidation, and improved skeletal muscle exercise capacity. Transcriptional inhibition of FoxO1 in peroxisome proliferator-activated receptor δ (PPARδ) expression was confirmed in skeletal muscle and deletion of PPARδ abolished the beneficial effects of FoxO1 deficiency. FoxO1 protein levels were higher in the skeletal muscle of diabetic patients and negatively correlated with PPARδ and electron transport chain protein levels. These findings highlight FoxO1 as a new repressor in PPARδ gene expression in skeletal muscle and suggest that FoxO1 links insulin resistance and mitochondrial dysfunction in skeletal muscle via PPARδ.

High temperature ameliorates high-fat diet-induced obesity by promoting ceramide breakdown in skeletal muscle tissue

Abstract Obesity is considered an epidemic often accompanied by insulin resistance (IR). Heat treatment (HT) has been shown to prevent high-fat diet-induced IR in skeletal muscle, but the underlying mechanisms are poorly understood. In this study, we discovered that high temperature alleviated the hallmarks of obesity by promoting glycogen synthesis and lowering blood glucose levels in skeletal muscle tissue (SMT). Additionally, HT maintained the decay phase of heat shock factor 1 (HSF1), leading to the activation of gene expression of heat shock proteins (HSPs), which contributed to the alleviation of IR in SMT of diet-induced obese (DIO) mice. Metabolomics and lipidomics analyses showed that HT promoted ceramide (Cer) breakdown, resulting in an elevation of both sphingomyelin (SM) and sphingosine, which further contributed to the amelioration of IR in SMT of DIO mice. Importantly, the increase in sphingosine was attributed to the heightened expression of the acid ceramidase N-acylsphingosine amidohydrolase 1 (ASAH1), and the inhibition of ASAH1 attenuated HT-relieved IR in SMT of DIO mice. Surprisingly, high temperature increased the composition of Cer and cholesteryl ester in lipid droplets of skeletal muscle cells. This not only helped alleviate IR but also prevented lipotoxicity in SMT of DIO mice. These findings revealed a previously unknown connection between a high-temperature environment and sphingolipid metabolism in obesity, suggesting that high temperature can improve IR by promoting Cer catabolism in SMT of obese mice.

Salubrinal promotes phospho-eIF2α-dependent activation of UPR leading to autophagy-mediated attenuation of iron-induced insulin resistance

Identification of new mechanisms mediating insulin sensitivity is important to allow validation of corresponding therapeutic targets. In this study, we first used a cellular model of skeletal muscle cell iron overload and found that endoplasmic reticulum (ER) stress and insulin resistance occurred after iron treatment. Insulin sensitivity was assessed using cells engineered to express an Akt biosensor, based on nuclear FoxO localization, as well as western blotting for insulin signaling proteins. Use of salubrinal to elevate eIF2α phosphorylation and promote the unfolded protein response (UPR) attenuated iron-induced insulin resistance. Salubrinal induced autophagy flux and its beneficial effects on insulin sensitivity were not observed in autophagy-deficient cells generated by overexpressing a dominant-negative ATG5 mutant or via knockout of ATG7. This indicated the beneficial effect of salubrinal-induced UPR activation was autophagy-dependent. We translated these observations to an animal model of systemic iron overload-induced skeletal muscle insulin resistance where administration of salubrinal as pretreatment promoted eIF2α phosphorylation, enhanced autophagic flux in skeletal muscle and improved insulin responsiveness. Together, our results show that salubrinal elicited an eIF2α-autophagy axis leading to improved skeletal muscle insulin sensitivity both in vitro and in mice.

Syrah Grape Polyphenol Extracts Protect Human Skeletal Muscle Cells from Oxidative and Metabolic Stress Induced by Excess of Palmitic Acid: Effect of Skin/Seed Ripening Stage.

Metabolic skeletal muscle (SM) dysfunction, triggered by increased oxidative stress and mitochondrial impairment, is a pivotal contributor to obesity-associated insulin resistance (IR). Addressing obesity and SM IR demands substantial lifestyle changes including regular exercise and dietary adjustments that are difficult to follow over time. This prompted exploration of alternative approaches. Grape polyphenols (GPPs) have demonstrated a positive impact on metabolism, although few studies have focused on SM. Since grape polyphenolic content and composition depend on tissue and ripening, we explored the antioxidant potential of GPPs from skin (Sk) and seeds (Sd) extracted before veraison (Bv) and at mature (M) stages, on palmitate-induced IR in primary human SM cells. Despite their important difference in polyphenol (PP) content: Sd-BvPP > Sd-MPP/Sk-BvPP > Sk-MPP, all extracts reduced lipid peroxidation by 44-60%, up-regulated the heme-oxygenase 1 protein level by 75-132% and mitochondrial activity by 47-68%. Contrary to the other extracts, which improved insulin response by 50%, Sd-BvPP did not. Our findings suggest that compounds other than stilbenoids or anthocyanin-type molecules, present only in grape Sk, could play an active role in regulating SM oxidative and metabolic stress and insulin sensitivity, paving the way for further exploration of novel bioactive compounds.

The Effects of Resistance Exercise Training on Skeletal Muscle Metabolism and Insulin Resistance Development in Female Rodents with Type 1 Diabetes.

The etiology of insulin resistance (IR) development in type 1 diabetes mellitus (T1DM) remains unclear; however, impaired skeletal muscle metabolism may play a role. While IR development has been established in male T1DM rodents, female rodents have yet to be examined in this context. Resistance exercise training (RT) has been shown to improve IR and is associated with a lower risk of hypoglycemia onset in T1DM compared to aerobic exercise. The purpose of this study was to investigate the effects of RT on IR development in female T1DM rodents. Forty Sprague Dawley eight-week-old female rats were divided into four groups: control sedentary (CS; n = 10), control trained (CT; n = 10), T1DM sedentary (DS; n = 10), and T1DM trained (DT; n = 10). Multiple low-dose streptozotocin injections were used to induce T1DM. Blood glucose levels were maintained in the 4-9 mmol/l range with intensive insulin therapy. CT and DT underwent weighted ladder climbing 5 days/week for six weeks. Intravenous glucose tolerance tests (IVGTT) were conducted on all animals following the six-week period. Results demonstrate that DS animals exhibited significantly increased weekly blood glucose measures compared to all groups including DT (p < 0.0001), despite similar insulin dosage levels. This was concomitant with a significant increase in insulin-adjusted area under the curve following IVGTT in DS (p < 0.05), indicative of a reduction in insulin sensitivity. Both DT and DS exhibited greater serum insulin concentrations compared to CT and CS (p < 0.05). DS animals also exhibited significantly greater glycogen content in white gastrocnemius muscle compared to CS and DT (p < 0.05), whereas DT and DS animals exhibited greater p-Akt: Akt ratio in the white vastus lateralis muscle and citrate synthase activity in the red vastus lateralis muscle compared to CS and CT (p < 0.05). These results indicate that female rodents with T1DM develop poor glycemic control and IR which can be attenuated with RT, possibly related to differences in intramyocellular glycogen content.

Bed-rest and exercise remobilization: Concurrent adaptations in muscle glucose and protein metabolism.

Bed-rest (BR) of only a few days duration reduces muscle protein synthesis and induces skeletal muscle atrophy and insulin resistance, but the scale and juxtaposition of these events have not been investigated concurrently in the same individuals. Moreover, the impact of short-term exercise-supplemented remobilization (ESR) on muscle volume, protein turnover and leg glucose uptake (LGU) in humans is unknown. Ten healthy males (24±1years, body mass index 22.7±0.6kg/m2) underwent 3days of BR, followed immediately by 3days of ESR consisting of 5×30 maximal voluntary single-leg isokinetic knee extensions at 90°/s each day. An isoenergetic diet was maintained throughout the study (30% fat, 15% protein and 55% carbohydrate). Resting LGU was calculated from arterialized-venous versus venous difference across the leg and leg blood flow during the steady-state of a 3-h hyperinsulinaemic-euglycaemic clamp (60mU/m2/min) measured before BR, after BR and after remobilization. Glycogen content was measured in vastus lateralis muscle biopsy samples obtained before and after each clamp. Leg muscle volume (LMV) was measured using magnetic resonance imaging before BR, after BR and after remobilization. Cumulative myofibrillar protein fractional synthetic rate (FSR) and whole-body muscle protein breakdown (MPB) were measured over the course of BR and remobilization using deuterium oxide and 3-methylhistidine stable isotope tracers that were administered orally. Compared with before BR, there was a 45% decline in insulin-stimulated LGU (P<0.05) after BR, which was paralleled by a reduction in insulin-stimulated leg blood flow (P<0.01) and removal of insulin-stimulated muscle glycogen storage. These events were accompanied by a 43% reduction in myofibrillar protein FSR (P<0.05) and a 2.5% decrease in LMV (P<0.01) during BR, along with a 30% decline in whole-body MPB after 2days of BR (P<0.05). Myofibrillar protein FSR and LMV were restored by 3days of ESR (P<0.01 and P<0.01, respectively) but not by ambulation alone. However, insulin-stimulated LGU and muscle glycogen storage were not restored by ESR. Three days of BR caused concurrent reductions in LMV, myofibrillar protein FSR, myofibrillar protein breakdown and insulin-stimulated LGU, leg blood flow and muscle glycogen storage in healthy, young volunteers. Resistance ESR restored LMV and myofibrillar protein FSR, but LGU and muscle glycogen storage remained depressed, highlighting divergences in muscle fuel and protein metabolism. Furthermore, ambulation alone did not restore LMV and myofibrillar protein FSR in the non-exercised contralateral limb, emphasizing the importance of exercise rehabilitation following even short-term BR.

Effect of Supplementation with Omega-3 Polyunsaturated Fatty Acids on Metabolic Modulators in Skeletal Muscle of Rats with an Obesogenic High-Fat Diet.

Previous studies provided evidence of the benefits of omega-3 polyunsaturated fatty acids (ω-3 PUFA) on the cardiovascular system and inflammation. However, its possible effect on skeletal muscle is unknown. This study aimed to evaluate whether ω-3 PUFA reverses the dysregulation of metabolic modulators in the skeletal muscle of rats on a high-fat obesogenic diet. For this purpose, an animal model was developed using male Wistar rats with a high-fat diet (HFD) and subsequently supplemented with ω-3 PUFA. Insulin resistance was assessed, and gene and protein expression of metabolism modulators in skeletal muscle was also calculated using PCR-RT and Western blot. Our results confirmed that in HFD rats, zoometric parameters and insulin resistance were increased compared to SD rats. Furthermore, we demonstrate reduced gene and protein expression of peroxisome proliferator-activated receptors (PPARs) and insulin signaling molecules. After ω-3 PUFA supplementation, we observed that glucose (24.34%), triglycerides (35.78%), and HOMA-IR (40.10%) were reduced, and QUICKI (12.16%) increased compared to HFD rats. Furthermore, in skeletal muscle, we detected increased gene and protein expression of PPAR-α, PPAR-γ, insulin receptor (INSR), insulin receptor substrate 1 (ISR-1), phosphatidylinositol-3-kinase (PI3K), and glucose transporter 4 (GLUT-4). These findings suggest that ω-3 PUFAs decrease insulin resistance of obese skeletal muscle.

Ketogenic diet ameliorates high-fat diet-induced insulin resistance in mouse skeletal muscle by alleviating endoplasmic reticulum stress

ObjectiveKetogenic diets (KD) have been shown to alleviate insulin resistance (IR) by exerting anti-lipogenic and insulin sensitizing effects in the liver through a variety of pathways. The present study sought to investigate whether a ketogenic diet also improves insulin sensitization in skeletal muscle cells through alleviating endoplasmic reticulum stress. MethodsHigh-fat diet-induced IR mice were allowed to a 2-week ketogenic diet. Insulin resistance and glucose tolerance were evaluated through GTT, ITT, and HOMA-IR. The C2C12 myoblasts exposed to palmitic acid were used to evaluate the insulin sensitization effects of β-hydroxybutyric acid (β-OHB). Molecular mechanisms concerning ER stress signaling activation and glucose uptake were assessed. ResultsThe AKT/GSK3β pathway was inhibited, ER stress signaling associated with IRE1, PERK, and BIP was activated, and the number of Glut4 proteins translocated to membrane decreased in the muscle of HFD mice. However, all these changes were reversed after 2 weeks of feeding on a ketogenic diet. Consistently in C2C12 myoblasts, the AKT/GSK3β pathway was inhibited by palmitic acid (PA) treatment. The endoplasmic reticulum stress-related proteins, IRE1, and BIP were increased, and the number of Glut4 proteins on the cell membrane decreased. However, β-OHB treatment alleviated ER stress and improved the glucose uptake of C2C12 cells. ConclusionOur data reveal that KD ameliorated HFD-induced insulin resistance in skeletal muscle, which was partially mediated by inhibiting endoplasmic reticulum stress. The insulin sensitization effect of β-OHB is associated with up regulation of AKT/GSK3β pathway and the increase in the number of Glut4 proteins on the cell membrane.

Free radical scavenging polyphenols isolated from Phyllanthus niruri L. ameliorates hyperglycemia via SIRT1 induction and GLUT4 translocation in in vitro and in vivo models

Type 2 diabetes milletus (T2DM) is a complex multifaceted disorder characterized by insulin resistance in skeletal muscle. Phyllanthus niruri L. is well reported sub-tropical therapeutically beneficial ayurvedic medicinal plant from Euphorbiaceae family used in various body ailments such as metabolic disorder including diabetes. The present study emphasizes on the therapeutic potential of Phyllanthus niruri L. and its phytochemical(s) against insulin resistance conditions and impaired antioxidant activity thereby aiding as an anti-hyperglycemic agent in targeting T2DM. Three compounds were isolated from the most active ethyl acetate fraction namely compound 1 as 1-O-galloyl-6-O-luteoyl-β-D-glucoside, compound 2 as brevifolincarboxylic acid and compound 3 as ricinoleic acid. Compounds 1 and 2, the two polyphenols enhanced the uptake of glucose and inhibited ROS levels in palmitate induced C2C12 myotubes. PNEAF showed the potent enhancement of glucose uptake in palmitate-induced insulin resistance condition in C2C12 myotubes and significant ROS inhibition was observed in skeletal muscle cell line. PNEAF treated IR C2C12 myotubes and STZ induced Wistar rats elevated SIRT1, PGC1-α signaling cascade through phosphorylation of AMPK and GLUT4 translocation resulting in insulin sensitization. Our study revealed an insight into the efficacy of marker compounds isolated from P. niruri and its enriched ethyl acetate fraction as ROS scavenging agent and helps in attenuating insulin resistance condition in C2C12 myotubes as well as in STZ induced Wistar rat by restoring glucose metabolism. Overall, this study can provide prospects for the marker-assisted development of P. niruri as a phytopharmaceutical drug for the insulin resistance related diabetic complications.

Genistein in combination with Chlorogenic acid ameliorates insulin resistance and oxidative stress in skeletal muscle of high-fat diet fed mice

Objectives: Increasing evidence has demonstrated that oxidative stress plays a significant role in the pathogenesis of muscle insulin resistance. The beneficial impacts of genistein (GEN) and chlorogenic acid (CGA) on insulin resistance have already been revealed. However, their combined effects on skeletal muscle oxidative stress and insulin resistance have not been completely understood. The aim of the present study was to determine the effect of GEN in combination with CGA on skeletal muscle insulin resistance in high-fat diet (HFD) fed C57BL/6 mice.&#x0D; Methods: C57BL/6 male mice were fed an HFD for 15 weeks. The mice were then divided into five groups: standard chow diet (SCD), HFD, HFD + GEN, HFD + CGA, and HFD + GEN + CGA for 10 weeks.&#x0D; Results: The findings indicated that single treatment with GEN or CGA, and with a stronger effect of GEN+CGA combined treatment, decreased body weight gain and improved glucose intolerance. Moreover, following treatment with GEN and CGA alone or in combination with further effect, the level of plasma and muscle triglyceride (TG) were reduced. Furthermore, the combination therapy of GEN and CGA with greater efficacy than the single treatments, could decrease several oxidative stress markers in the skeletal muscle. Treatment with GEN and CGA alone or in combination could increase the expression of NF-E2–related factor 2 (Nrf2), heme oxygenase-1 (HO-1), and NAD(P): Quinone Oxidoreductase 1 (NQO1).&#x0D; Conclusion: The data of this study suggest that the combination of GEN and CGA might ameliorate insulin resistance and reduce oxidative stress in the skeletal muscle of mice fed HFD.

Mitochondrial electron transport chain, ceramide, and coenzyme Q are linked in a pathway that drives insulin resistance in skeletal muscle.

Insulin resistance (IR) is a complex metabolic disorder that underlies several human diseases, including type 2 diabetes and cardiovascular disease. Despite extensive research, the precise mechanisms underlying IR development remain poorly understood. Previously we showed that deficiency of coenzyme Q (CoQ) is necessary and sufficient for IR in adipocytes and skeletal muscle (Fazakerley et al., 2018). Here, we provide new insights into the mechanistic connections between cellular alterations associated with IR, including increased ceramides, CoQ deficiency, mitochondrial dysfunction, and oxidative stress. We demonstrate that elevated levels of ceramide in the mitochondria of skeletal muscle cells result in CoQ depletion and loss of mitochondrial respiratory chain components, leading to mitochondrial dysfunction and IR. Further, decreasing mitochondrial ceramide levels in vitro and in animal models (mice, C57BL/6J) (under chow and high-fat diet) increased CoQ levels and was protective against IR. CoQ supplementation also rescued ceramide-associated IR. Examination of the mitochondrial proteome from human muscle biopsies revealed a strong correlation between the respirasome system and mitochondrial ceramide as key determinants of insulin sensitivity. Our findings highlight the mitochondrial ceramide-CoQ-respiratory chain nexus as a potential foundation of an IR pathway that may also play a critical role in other conditions associated with ceramide accumulation and mitochondrial dysfunction, such as heart failure, cancer, and aging. These insights may have important clinical implications for the development of novel therapeutic strategies for the treatment of IR and related metabolic disorders.

Mitochondrial electron transport chain, ceramide, and coenzyme Q are linked in a pathway that drives insulin resistance in skeletal muscle

Insulin resistance (IR) is a complex metabolic disorder that underlies several human diseases, including type 2 diabetes and cardiovascular disease. Despite extensive research, the precise mechanisms underlying IR development remain poorly understood. Previously we showed that deficiency of coenzyme Q (CoQ) is necessary and sufficient for IR in adipocytes and skeletal muscle (Fazakerley et al., 2018). Here, we provide new insights into the mechanistic connections between cellular alterations associated with IR, including increased ceramides, CoQ deficiency, mitochondrial dysfunction, and oxidative stress. We demonstrate that elevated levels of ceramide in the mitochondria of skeletal muscle cells result in CoQ depletion and loss of mitochondrial respiratory chain components, leading to mitochondrial dysfunction and IR. Further, decreasing mitochondrial ceramide levels in vitro and in animal models (mice, C57BL/6J) (under chow and high-fat diet) increased CoQ levels and was protective against IR. CoQ supplementation also rescued ceramide-associated IR. Examination of the mitochondrial proteome from human muscle biopsies revealed a strong correlation between the respirasome system and mitochondrial ceramide as key determinants of insulin sensitivity. Our findings highlight the mitochondrial ceramide–CoQ–respiratory chain nexus as a potential foundation of an IR pathway that may also play a critical role in other conditions associated with ceramide accumulation and mitochondrial dysfunction, such as heart failure, cancer, and aging. These insights may have important clinical implications for the development of novel therapeutic strategies for the treatment of IR and related metabolic disorders.