Oleate prevents saturated-fatty-acid-induced ER stress, inflammation and insulin resistance in skeletal muscle cells through an AMPK-dependent mechanism

Extract of Phyllanthus emblica L. fruit stimulates basal glucose uptake and ameliorates palmitate-induced insulin resistance through AMPK activation in C2C12 myotubes

BACKGROUND

The fruit of Phyllanthus emblica L., a traditional medicine in China and India, is used to treat diabetes mellitus. Its water extract (WEPE) has demonstrated hypoglycemic effects in diabetic rats, but its mechanisms on glucose utilization and insulin resistance in skeletal muscle remain unclear. Therefore, this study aims to investigate the effects and underlying mechanisms of WEPE on glucose utilization and insulin resistance using C2C12 myotubes.

METHODS

Effects of WEPE on glucose uptake, GLUT4 translocation, and AMPK and AKT phosphorylation were investigated in C2C12 myotubes and palmitate-treated myotubes. An AMPK inhibitor and siRNA were used to explore the mechanisms of WEPE. Glucose uptake was determined using a 2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) amino)-2-deoxyglucose (2-NBDG) uptake assay, and protein expression and GLUT4 translocation were assessed via western blotting.

RESULTS

In normal myotubes, WEPE significantly stimulated glucose uptake and GLUT4 translocation to the plasma membrane at concentrations of 125 and 250 µg/mL. This was accompanied by an increase in the phosphorylation of AMPK and its downstream targets. However, both compound C and AMPK siRNA blocked the WEPE-induced GLUT4 translocation and glucose uptake. Moreover, pretreatment with STO-609, a calcium/calmodulin-dependent protein kinase kinase β (CaMKKβ) inhibitor, inhibited WEPE-induced AMPK phosphorylation and attenuated the WEPE-stimulated glucose uptake and GLUT4 translocation. In myotubes treated with palmitate, WEPE prevented palmitate-induced insulin resistance by enhancing insulin-mediated glucose uptake and AKT phosphorylation. It also restored the insulin-mediated translocation of GLUT4 from cytoplasm to membrane. However, these effects of WEPE on glucose uptake and GLUT4 translocation were blocked by pretreatment with compound C.

CONCLUSIONS

WEPE significantly stimulated basal glucose uptake though CaMKKβ/AMPK pathway and markedly ameliorated palmitate-induced insulin resistance by activating the AMPK pathway in C2C12 myotubes.

The Lipophilic Extract from Ginkgo biloba L. Leaves Promotes Glucose Uptake and Alleviates Palmitate-Induced Insulin Resistance in C2C12 Myotubes

Ginkgo biloba L. (ginkgo) is a widely used medicinal plant around the world. Its leaves, which have been used as a traditional Chinese medicine, are rich in various bioactive components. However, most of the research and applications of ginkgo leaves have focused on terpene trilactones and flavonol glycosides, thereby overlooking the other active components. In this study, a lipophilic extract (GL) was isolated from ginkgo leaves. This extract is abundant in lipids and lipid-like molecules. Then, its effect and potential mechanism on glucose uptake and insulin resistance in C2C12 myotubes were investigated. The results showed that GL significantly enhanced the translocation of GLUT4 to the plasma membrane, which subsequently promoted glucose uptake. Meanwhile, it increased the phosphorylation of AMP-activated protein kinase (AMPK) and its downstream targets. Both knockdown of AMPK with siRNA and inhibition with AMPK inhibitor compound C reversed these effects. Additionally, GL ameliorated palmitate-induced insulin resistance by enhancing insulin-stimulated glucose uptake, increasing the phosphorylation of protein kinase B (PKB/AKT), and restoring the translocation of GLUT4 from the cytoplasm to the membrane. However, pretreatment with compound C abolished these beneficial effects of GL. In conclusion, GL enhances basal glucose uptake in C2C12 myotubes and improves insulin sensitivity in palmitate-induced insulin resistant myotubes through the AMPK pathway.

A mathematical model of obesity-induced type 2 diabetes and efficacy of anti-diabetic weight reducing drug

The dominant paradigm for modeling the obesity-induced T2DM (type 2 diabetes mellitus) today focuses on glucose and insulin regulatory systems, diabetes pathways, and diagnostic test evaluations. The problem with this approach is that it is not possible to explicitly account for the glucose transport mechanism from the blood to the liver, where the glucose is stored, and from the liver to the blood. This makes it inaccurate, if not incorrect, to properly model the concentration of glucose in the blood in comparison to actual glycated hemoglobin (A1C) test results. In this paper, we develop a mathematical model of glucose dynamics by a system of ODEs. The model includes the mechanism of glucose transport from the blood to the liver, and from the liver to the blood, and explains how obesity is likely to lead to T2DM. We use the model to evaluate the efficacy of an anti-T2DM drug that also reduces weight.

Oleic acid stimulation of amino acid uptake in primary human trophoblast cells is mediated by phosphatidic acid and mTOR signaling

Normal fetal development is critically dependent on optimal nutrient supply by the placenta, and placental amino acid transport has been demonstrated to be positively associated with fetal growth. Mechanistic target of rapamycin (mTOR) is a positive regulator of placental amino acid transporters, such as System A. Oleic acid (OA) has been previously shown to have a stimulatory role on placental mTOR signaling and System A amino acid uptake in primary human trophoblast (PHT) cells. We investigated the mechanistic link between OA and System A activity in PHT. We found that inhibition of mTOR complex 1 or 2, using small interfering RNA to knock down raptor or rictor, prevented OA-stimulated System A amino acid transport indicating the interaction of OA with mTOR. Phosphatidic acid (PA) is a key intermediary for phospholipid biosynthesis and a known regulator of the mTOR pathway; however, phospholipid biosynthetic pathways have not been extensively studied in placenta. We identified placental isoforms of acyl transferase enzymes involved in de novo phospholipid synthesis. Silencing of 1-acylglycerol-3-phosphate-O-acyltransferase-4, an enzyme in this pathway, prevented OA mediated stimulation of mTOR and System A amino acid transport. These data indicate that OA stimulates mTOR and amino acid transport in PHT cells mediated through de novo synthesis of PA. We speculate that fatty acids in the maternal circulation, such as OA, regulate placental functions critical for fetal growth by interaction with mTOR and that late pregnancy hyperlipidemia may be critical for increasing nutrient transfer to the fetus.

BAO-Ag-NPs as Promising Suppressor of ET-1/ICAM-1/VCAM-1 Signaling Pathway in ISO-induced AMI in Rats

OBJECTIVES

Acute myocardial infarction (AMI) is the most prevalent cause of myocardial fibrosis and the leading cause of mortality from cardiovascular disease. The goal of this work was to synthesize Balanites aegyptiaca oil-silver nanoparticles (BAO-Ag-NPs) and evaluate their cardioprotective effect against ISO-induced myocardial infarction in rats, as well as their mechanism.

MATERIALS AND METHODS

BAO was isolated, and the unsaturated fatty acids were estimated. BAO-Ag-NPs was prepared, LD50 was calculated to evaluate its cardioprotective activity against ISO (85 mg/kg)-induced AMI. Different doses of BAO-Ag-NPs (1/50 LD50; 46.6 mg/kg.b.w and 1/20 LD50; 116.5 mg) were received to the rats.

RESULTS

The total fatty acids and unsaturated fatty acids generated by BAO were 909.63 and 653.47 mg/100 g oil, respectively. Oleic acid methyl ester, 9-octadecenoic acid methyl ester, and 9, 12-Octadecadienoic acid methyl ester were the predominant ingredients, with concentrations of 107.6, 243.42, and 256.77 mg/100 g oil, respectively. According to TEM and DLS examinations, BAO-Ag-NPs have a size of 38.20 ± 2.5 nm and a negative zeta potential of -19.82 ± 0.30 mV, respectively. The LD50 of synthesized BAO-Ag-NPs is 2330 mg. On the other hand, BAOAg- NPs reduce myocardial necrosis by lowering increased BNP, cTnI, CK-MB, TC, TG, MDA, MMP2, TGF-β1, PGE2, and IL-6 levels. Furthermore, BAO-Ag-NPs inhibit the expression of ET-1, ICAM-1, and VCAM-1 genes as well as enhance HDL-C, CAT, and GSH levels when compared to the ISO-treated group of rats. Histopathological findings suggested that BAO-Ag- NPs enhance cardiac function by increasing posterior wall thickness in heart tissues.

CONCLUSION

BAO-Ag-NPs protect against AMI in vivo by regulating inflammation, excessive autophagy, and oxidative stress, as well as lowering apoptosis via suppression of the ET-1, ICAM-1, and VCAM-1 signaling pathways.

Integrated mass spectrometry imaging reveals spatial-metabolic alteration in diabetic cardiomyopathy and the intervention effects of ferulic acid

Diabetic cardiomyopathy (DCM) is a metabolic disease and a leading cause of heart failure among people with diabetes. Mass spectrometry imaging (MSI) is a versatile technique capable of combining the molecular specificity of mass spectrometry (MS) with the spatial information of imaging. In this study, we used MSI to visualize metabolites in the rat heart with high spatial resolution and sensitivity. We optimized the air flow-assisted desorption electrospray ionization (AFADESI)-MSI platform to detect a wide range of metabolites, and then used matrix-assisted laser desorption ionization (MALDI)-MSI for increasing metabolic coverage and improving localization resolution. AFADESI-MSI detected 214 and 149 metabolites in positive and negative analyses of rat heart sections, respectively, while MALDI-MSI detected 61 metabolites in negative analysis. Our study revealed the heterogenous metabolic profile of the heart in a DCM model, with over 105 region-specific changes in the levels of a wide range of metabolite classes, including carbohydrates, amino acids, nucleotides, and their derivatives, fatty acids, glycerol phospholipids, carnitines, and metal ions. The repeated oral administration of ferulic acid during 20 weeks significantly improved most of the metabolic disorders in the DCM model. Our findings provide novel insights into the molecular mechanisms underlying DCM and the potential of ferulic acid as a therapeutic agent for treating this condition.

Relationship between monounsaturated fatty acids and sarcopenia: a systematic review and meta-analysis of observational studies

Accumulating evidence suggests that fatty acids (FAs) play an essential role in regulating skeletal muscle mass and function throughout life. This systematic review and meta-analysis aimed to examine the relationship between dietary or circulatory levels of monounsaturated FAs (MUFAs) and sarcopenia in observational studies. A comprehensive literature search was performed in three databases (PubMed, Scopus, and Web of Science) from inception until August 2022. Of 414 records, a total of 12 observational studies were identified for this review. Ten studies were meta-analysed, comprising a total of 3704 participants. The results revealed that MUFA intake is inversely associated with sarcopenia (standardized mean difference = - 0.28, 95% CI - 0.46 to - 0.11; p < 0.01). Despite the limited number of studies, our results suggest that lower MUFA intake is associated with a higher risk of sarcopenia. However, the available evidence is still insufficient and further investigations are needed to demonstrate this relationship.

Effect of lard plus soybean oil on blood pressure and other cardiometabolic risk factors in healthy subjects: a randomized controlled-feeding trial

Lard has been consumed by humans for thousands of years, but its consumption has declined substantially in the last few decades, because of negative publicity about the consumption of animal-derived saturated fats. Emerging evidence highlights that lard plus soybean oil (blend oil) could be more beneficial for body weight and liver function than the individual use of the two oils. This study aimed to evaluate the effects of blend oil on cardiometabolic risk factors in healthy subjects. This was a parallel, three-arm, randomized controlled-feeding trial. 334 healthy subjects (mean age: 33.1 years, 60% women) were randomized into three isoenergetic diet groups with three different edible oils (30 g day^-1) (soybean oil, lard, and blend oil [50% lard and 50% soybean oil]) for 12 weeks. 245 (73.4%) participants completed the study. After the 12-week intervention, reductions in both systolic blood pressure (SBP) and diastolic blood pressure (DBP) were greater in the blend oil group than in the other two groups (P = 0.023 and 0.008 for the interaction between the diet group and time, respectively). Reductions of SBP and DBP in the blend oil group were more significant than those in the soybean oil group with P = 0.008 and P = 0.026 and the lard group with P < 0.001 and P < 0.001. Changes in SBP/DBP at 12 weeks were -6.0 (95% CI: -8.6 to -3.4)/0.8 (95% CI: -1.7 to 3.2) mmHg in the blend oil group, -3.3 (95% CI: -5.7 to -0.9)/1.5 (95% CI: -1.0 to 4.0) mmHg in the soybean oil group and -1.2 (95% CI: -3.7 to 1.4)/3.3 (95% CI: 0.9 to 5.8) mmHg in the lard group. Subgroup analyses showed that blend oil significantly decreased SBP and DBP compared with the other two groups in participants with BP ≥ 130/80 mmHg and body mass index ≥25. There were no significant differences in the changes in body weight, waist circumference, serum lipids, or glucose between groups. In conclusion, our findings suggest that blend oil (lard plus soybean oil) reduces BP compared with soybean oil and lard in healthy subjects.

Interleukin-18 Is a Potential Biomarker Linking Dietary Fatty Acid Quality and Insulin Resistance: Results from a Cross-Sectional Study in Northern Italy

Dietary lipids are pivotal in modulating metabolic inflammation. Among the inflammatory mediators characterizing metabolic inflammation, interleukin 18 (IL-18) has been consistently associated with obesity and insulin resistance. This study aims to evaluate whether the quality of lipid intake impacts upon IL-18 plasma levels and the implications on insulin resistance computed by the homeostatic model assessment for insulin resistance (HOMA-IR). Using a cross-sectional design, this study confirmed that IL-18 correlated positively with insulin resistance and individuals with a HOMA-IR ≥ 2.5 displayed higher circulating IL-18 levels compared with their insulin-sensitive counterparts. In terms of the effect of the quality of dietary lipids on IL-18 circulating levels, the ratio between monounsaturated, omega-3, polyunsaturated and saturated fatty acids as well as the intake of eicosapentaenoic and docosahexaenoic acids correlated negatively with IL-18. Despite this, IL-18 circulating levels, but not dietary fatty acid quality, predicted insulin resistance. Nevertheless, the ratio between omega 3 and saturated fatty acids was a predictor of IL-18 plasma levels. Thus, the downregulation of IL-18 may underpin, at least partially, the beneficial metabolic effects of substituting omega 3 for saturated fatty acids with this cytokine potentially representing a biomarker linking dietary lipids and metabolic outcomes.

Serum concentration impacts myosin heavy chain expression but not cellular respiration in human LHCN-M2 myoblasts undergoing differentiation

NEW FINDINGS

What is the central question of this study? Does the concentration of human serum affect skeletal muscle differentiation and cellular respiration of LHCN-M2 myoblasts? What is the main finding and its importance? The concentration of serum used to differentiate LHCN-M2 skeletal muscle cells impacts the coverage of myosin heavy chain, a marker of terminally differentiated myotubes. Normalisation of mitochondrial function data to total protein negates the differences observed in absolute values, which differ as a result of increased protein content when differentiation occurs with increasing concentration of serum.

ABSTRACT

The human LHCN-M2 myoblast cell line has the potential to be used to investigate skeletal muscle development and metabolism. Experiments were performed to determine how different concentrations of human serum affect myogenic differentiation and mitochondrial function of LHCN-M2 cells. LHCN-M2 myoblasts were differentiated in serum-free medium, 0.5% or 2% human serum for 5 and 10 days. Myotube formation was assessed by immunofluorescence staining of myosin heavy chain (MHC) and molecularly by mRNA expression of Myogenic differentiation 1 (MYOD1) and Myoregulatory factor 5 (MYF5). Following differentiation, mitochondrial function was assessed to establish the impact of serum concentration on mitochondrial function. Time in differentiation increased mRNA expression of MYOD1 (day 5, 6.58 ± 1.33-fold; and day 10, 4.28 ± 1.71-fold) (P = 0.012), while suppressing the expression of MYF5 (day 5, 0.21 ± 0.11-fold; and day 10, 0.06 ± 0.03-fold) (P = 0.001), regardless of the serum concentration. Higher serum concentrations increased MHC area (serum free, 11.92 ± 0.85%; 0.5%, 23.10 ± 5.82%; 2%, 43.94 ± 8.92%) (P = 0.001). Absolute basal respiration approached significance (P = 0.06) with significant differences in baseline oxygen consumption rate (P = 0.025) and proton leak (P = 0.006) when differentiated in 2% human serum, but these were not different between conditions when normalised to total protein. Our findings show that increasing concentrations of serum of LHCN-M2 skeletal muscle cells into multinucleated myotubes, but this does not affect relative mitochondrial function.

Targeted and untargeted metabolomic approach for GDM diagnosis

Gestational diabetes mellitus (GDM) is a disorder which manifests itself for the first time during pregnancy and is mainly connected with glucose metabolism. It is also known that fatty acid profile changes in erythrocyte membranes and plasma could be associated with obesity and insulin resistance. These factors can lead to the development of diabetes. In the reported study, we applied the untargeted analysis of plasma in GDM against standard glucose-tolerant (NGT) women to identify the differences in metabolomic profiles between those groups. We found higher levels of 2-hydroxybutyric and 3-hydroxybutyric acids. Both secondary metabolites are associated with impaired glucose metabolism. However, they are products of different metabolic pathways. Additionally, we applied lipidomic profiling using gas chromatography to examine the fatty acid composition of cholesteryl esters in the plasma of GDM patients. Among the 14 measured fatty acids characterizing the representative plasma lipidomic cluster, myristic, oleic, arachidonic, and α-linoleic acids revealed statistically significant changes. Concentrations of both myristic acid, one of the saturated fatty acids (SFAs), and oleic acid, which belong to monounsaturated fatty acids (MUFAs), tend to decrease in GDM patients. In the case of polyunsaturated fatty acids (PUFAs), some of them tend to increase (e.g., arachidonic), and some of them tend to decrease (e.g., α-linolenic). Based on our results, we postulate the importance of hydroxybutyric acid derivatives, cholesteryl ester composition, and the oleic acid diminution in the pathophysiology of GDM. There are some evidence suggests that the oleic acid can have the protective role in diabetes onset. However, metabolic alterations that lead to the onset of GDM are complex; therefore, further studies are needed to confirm our observations.

AICAR Improves Outcomes of Metabolic Syndrome and Type 2 Diabetes Induced by High-Fat Diet in C57Bl/6 Male Mice

The aim of the study was to investigate the effect of AMP-activated protein kinase activator 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) on the consequences of metabolic syndrome and type 2 diabetes induced by the consumption of a high-fat diet (HFD) in male C57Bl/6 mice. Additionally, the animals from group 6 were administered Methotrexate (MTX) at a dose of 1 mg/kg in parallel with AICAR, which slows down the metabolism of AICAR. The animals were recorded with signs of metabolic syndrome and type 2 diabetes mellitus by recording their body weights, glucose and insulin levels, and the calculating HOMA-IRs. At the end of the study, at the end of the 13th week, during necropsy, the internal organs were assessed, the masses of the organs were recorded, and special attention was paid to visceral fat, assessing its amount and the mass of the fat surrounding epididymis. The biochemical parameters and histology of the internal organs and tissues were assessed. The animals showed signs of metabolic syndrome and type 2 diabetes, namely, weight gain, hyperglycemia, hyperinsulinemia, an increase in the amount and mass of abdominal fat, and metabolic disorders, all expressed in a pathological change in biochemical parameters and pathological changes in internal organs. The AICAR treatment led to a decrease in body weight, a decrease in the amount and mass of abdominal fat, and an improvement in the pathomorphological picture of internal organs. However, some hepatotoxic effects were observed when the animals, on a received standard diet (STD), were treated with AICAR starting from the first day of the study. The additional administration of MTX, an AICAR metabolic inhibitor, did not improve its efficacy. Thus, AICAR has therapeutic potential for the treatment of metabolic syndrome and type 2 diabetes.

Metformin alleviates neurocognitive impairment in aging via activation of AMPK/BDNF/PI3K pathway

Slowing down age-related neurocognitive impairment has been a challenge. We evaluated the therapeutic effects of metformin in D-galactose-induced aging. Additionally, we studied the potential molecular mechanisms that could be responsible for metformin's anti-aging effects. Thirty male rats were equally divided into: 1-control group, which received saline solution, 2-D-galactose (D-gal) group, which received D-galactose (100 mg/kg/day) by gastric lavage for eight weeks, and 3-D-galactose + Metformin (D-gal + Met) treated group, which received D-galactose + metformin (200 mg/kg/day) by gastric lavage for eight weeks. Neurocognitive assessment was done. Measurement of inflammatory, oxidative stress, and BDNF biomarkers was performed. AMPK and PI3K genes expression were assessed. Hippocampal tissues were dissected for histopathological and immunohistochemical studies. D-gal resulted in neurocognitive impairments, elevation of inflammatory biomarkers, altered oxidative stress markers, decreased BDNF, decreased expression of synaptophysin and Bcl2 with increased expression of Caspase-3, and down-regulation of AMPK and PI3K genes. Neurodegenerative changes were present in the hippocampus. Metformin restored significantly D-gal induced neurodegenerative changes. We concluded that metformin could alleviate age-induced neurocognitive deficit via amelioration of neuroinflammation, attenuation of oxidative stress, reduction of apoptosis, as well as promotion of synaptic plasticity. These mechanisms could be mediated via the activation of the AMPK/BDNF/PI3K pathway.

Potential Therapeutic Properties of the Leaf of Cydonia Oblonga Mill. Based on Mineral and Organic Profiles

Leaf extract of Cydonia Oblonga Mill. is interesting for further exploration of the potential of its substrates for therapeutic supplements. Quantitative and qualitative analyses were conducted on samples of green (October), yellow (November), and brown (December) quince leaves collected in the region of Pinhel, Portugal. Mineral analysis determined the measurements of the levels of several macro- and micro-elements. Organic analysis assessed the moisture content, total phenolic content (TPC), vitamin E, and fatty acid (FA) profiles. Mineral analysis was based on ICP-MS techniques, while the profiles of vitamin E and FA relied on HPLC-DAD-FLD and GC-FID techniques, respectively. Moisture content was determined through infrared hygrometry and TPC was determined by spectrophotometric methods. Regarding the mineral content, calcium, magnesium, and iron were the most abundant minerals. Concerning organic analysis, all leaf samples showed similar moisture content, while the TPC of gallic acid equivalents (GAE) and total vitamin E content, the most predominant of which was the α-tocopherol isomer, showed significant variations between green-brown and yellow leaves. FA composition in all leaf samples exhibited higher contents of SFA and PUFA than MUFA, with a predominance of palmitic and linolenic acids. Organic and inorganic analysis of quince leaves allow for the prediction of adequate physiological properties, mainly cardiovascular, pulmonary, and immunological defenses, which with our preliminary in silico studies suggest an excellent supplement to complementary therapy, including drastic pandemic situations.

Oleic Acid and Palmitic Acid from Bacteroides thetaiotaomicron and Lactobacillus johnsonii Exhibit Anti-Inflammatory and Antifungal Properties

A decrease in populations of Bacteroides thetaiotaomicron and Lactobacillus johnsonii is observed during the development of colitis and fungal overgrowth, while restoration of these populations reduces inflammatory parameters and fungal overgrowth in mice. This study investigated the effect of two fatty acids from B. thetaiotaomicron and L. johnsonii on macrophages and Caco-2 cells, as well as their impact on the inflammatory immune response and on Candida glabrata overgrowth in a murine model of dextran sulfate sodium (DSS)-induced colitis. Oleic acid (OA) and palmitic acid (PA) from L. johnsonii and B. thetaiotaomicron were detected during their interaction with epithelial cells from colon samples. OA alone or OA combined with PA (FAs) reduced the expression of proinflammatory mediators in intestinal epithelial Caco-2 cells challenged with DSS. OA alone or FAs increased FFAR1, FFAR2, AMPK, and IL-10 expression in macrophages. Additionally, OA alone or FAs decreased COX-2, TNFα, IL-6, and IL-12 expression in LPS-stimulated macrophages. In the DSS murine model, oral administration of FAs reduced inflammatory parameters, decreased Escherichia coli and Enterococcus faecalis populations, and eliminated C. glabrata from the gut. Overall, these findings provide evidence that OA combined with PA exhibits anti-inflammatory and antifungal properties.

Reduction in Insulin Mediated ERK Phosphorylation by Palmitate in Liver Cells Is Independent of Fatty Acid Induced ER Stress

Saturated free fatty acids (FFAs) such as palmitate in the circulation are known to cause endoplasmic reticulum (ER) stress and insulin resistance in peripheral tissues. In addition to protein kinase B (AKT) signaling, extracellular signal-regulated kinase (ERK) has been implicated in the development of insulin resistance. However, there are conflicting data regarding role of ERK signaling in ER stress-induced insulin resistance. In this study, we investigated the effects of ER stress on insulin resistance and ERK phosphorylation in Huh-7 cells and evaluated how oleate prevents palmitate-mediated ER stress. Treatment with insulin resulted in an increase of 38–45% in the uptake of glucose in control cells compared to non-insulin-treated control cells, along with an increase in the phosphorylation of AKT and ERK. We found that treatment with palmitate increased the expression of ER stress genes, including the splicing of X box binding protein 1 (XBP1) mRNA. At the same time, we observed a decrease in insulin-mediated uptake of glucose and ERK phosphorylation in Huh-7 cells, without any change in AKT phosphorylation. Supplementation of oleate along with palmitate mitigated the palmitate-induced ER stress but did not affect insulin-mediated glucose uptake or ERK phosphorylation. The findings of this study suggest that palmitate reduces insulin-mediated ERK phosphorylation in liver cells and this effect is independent of fatty-acid-induced ER stress.

mTOR Modulation of IKr through hERG1b-Dependent Mechanisms in Lipotoxic Heart

In the atria, the rapid delayed rectifier channel (IKr) is a critical contributor to repolarization. In lipotoxic atria, increased activity of the serine/threonine mammalian target of rapamycin (mTOR) may remodel IKr and predispose patients to arrhythmias. To investigate whether mTOR produced defects in IKr channel function (protein expression and gating mechanisms), electrophysiology and biochemical assays in HEK293 cells stably expressing hERG1a/1b, and adult guinea pig atrial myocytes were used. Feeding with the saturated fatty acid palmitic acid high-fat diet (HFD) was used to induce lipotoxicity. Lipotoxicity-challenged HEK293 cells displayed an increased density of hERG1a/1b currents due to a targeted and significant increase in hERG1b protein expression. Furthermore, lipotoxicity significantly slowed the hERG1a/1b inactivation kinetics, while the activation and deactivation remained essentially unchanged. mTOR complex 1 (mTORC1) inhibition with rapamycin (RAP) reversed the increase in hERG1a/1b density and inactivation. Compared to lipotoxic myocytes, RAP-treated cells displayed action potential durations (APDs) and IKr densities similar to those of controls. HFD feeding triggered arrhythmogenic changes (increased the IKr density and shortened the APD) in the atria, but this was not observed in low-fat-fed controls. The data are the first to show the modulation of IKr by mTORC1, possibly through the remodeling of hERG1b, in lipotoxic atrial myocytes. These results offer mechanistic insights with implications for targeted therapeutic options for the therapy of acquired supraventricular arrhythmias in obesity and associated pathologies.

Inhibition of stearoyl-CoA desaturase 1 (SCD1) enhances the antitumor T cell response through regulating β-catenin signaling in cancer cells and ER stress in T cells and synergizes with anti-PD-1 antibody

Background

Understanding the mechanisms of non-T cell inflamed tumor microenvironment (TME) and their modulation are important to improve cancer immunotherapies such as immune checkpoint inhibitors. The involvement of various immunometabolisms has recently been indicated in the formation of immunosuppressive TME. In this study, we investigated the immunological roles of stearoyl-CoA desaturase 1 (SCD1), which is essential for fatty acid metabolism, in the cancer immune response.

Methods

We investigated the roles of SCD1 by inhibition with the chemical inhibitor or genetic manipulation in antitumor T cell responses and the therapeutic effect of anti-programmed cell death protein 1 (anti-PD-1) antibody using various mouse tumor models, and their cellular and molecular mechanisms. The roles of SCD1 in human cancers were also investigated by gene expression analyses of colon cancer tissues and by evaluating the related free fatty acids in sera obtained from patients with non-small cell lung cancer who were treated with anti-PD-1 antibody.

Results

Systemic administration of a SCD1 inhibitor in mouse tumor models enhanced production of CCL4 by cancer cells through reduction of Wnt/β-catenin signaling and by CD8⁺ effector T cells through reduction of endoplasmic reticulum stress. It in turn promoted recruitment of dendritic cells (DCs) into the tumors and enhanced the subsequent induction and tumor accumulation of antitumor CD8⁺ T cells. SCD1 inhibitor was also found to directly stimulate DCs and CD8⁺ T cells. Administration of SCD1 inhibitor or SCD1 knockout in mice synergized with an anti-PD-1 antibody for its antitumor effects in mouse tumor models. High SCD1 expression was observed in one of the non-T cell-inflamed subtypes in human colon cancer, and serum SCD1 related fatty acids were correlated with response rates and prognosis of patients with non-small lung cancer following anti-PD-1 antibody treatment.

Conclusions

SCD1 expressed in cancer cells and immune cells causes immunoresistant conditions, and its inhibition augments antitumor T cells and therapeutic effects of anti-PD-1 antibody. Therefore, SCD1 is an attractive target for the development of new diagnostic and therapeutic strategies to improve current cancer immunotherapies including immune checkpoint inhibitors.

Improved skeletal muscle fatigue resistance in experimental autoimmune myositis mice following high-intensity interval training

Background

Muscle weakness and decreased fatigue resistance are key manifestations of systemic autoimmune myopathies (SAMs). We here examined whether high-intensity interval training (HIIT) improves fatigue resistance in the skeletal muscle of experimental autoimmune myositis (EAM) mice, a widely used animal model for SAM.

Methods

Female BALB/c mice were randomly assigned to control (CNT) or EAM groups (n = 28 in each group). EAM was induced by immunization with three injections of myosin emulsified in complete Freund’s adjuvant. The plantar flexor (PF) muscles of mice with EAM were exposed to either an acute bout or 4 weeks of HIIT (a total of 14 sessions).

Results

The fatigue resistance of PF muscles was lower in the EAM than in the CNT group (P < 0.05). These changes were associated with decreased activities of citrate synthase and cytochrome c oxidase and increased expression levels of the endoplasmic reticulum stress proteins (glucose-regulated protein 78 and 94, and PKR-like ER kinase) (P < 0.05). HIIT restored all these alterations and increased the peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) and the mitochondrial electron transport chain complexes (I, III, and IV) in the muscles of EAM mice (P < 0.05).

Conclusions

HIIT improves fatigue resistance in a SAM mouse model, and this can be explained by the restoration of mitochondria oxidative capacity via inhibition of the ER stress pathway and PGC-1α-mediated mitochondrial biogenesis.

Metabolic and Microbiome Alterations Following the Enrichment of a High-Fat Diet With High Oleic Acid Peanuts Versus the Traditional Peanuts Cultivar in Mice

A new Israeli-developed peanut cultivar, “Hanoch-Oleic” (HO), uniquely contains enlarged oleic acid contents and was designed to confer additional beneficial effects over the traditional cultivar, “Hanoch” (HN). This work elucidates metabolic changes and microbiota adaptations elicited by HO addition to a high-fat diet (HFD). Male C57BL/6 mice were fed for 18 weeks with a normal diet or a HFD with/without the addition of HN (HFDh) or HO (HFDo). Body-weight did not differ between HFD-fed mice groups, while liver and adipose weight were elevated in the HFDh and HFD groups, respectively. Insulin-sensitivity (IS) was also decreased in these groups, though to a much greater extent in the traditional peanuts-fed group. Modifications in lipids metabolism were evident by the addition of peanuts to a HFD. Liver inflammation seems to return to normal only in HFDh. Peanuts promoted an increase in α-diversity, with HFDo exhibiting changes in the abundance of microbiota that is primarily associated with ameliorated gut health and barrier capacity. In conclusion, the HO cultivar appears to be metabolically superior to the traditional peanut cultivar and was associated with an improved inflammatory state and microbial profile. Nevertheless, IS-negative effects reinforced by peanuts addition, predominantly NH, need to be comprehensively defined.

Exercise activates autophagy and regulates endoplasmic reticulum stress in muscle of high-fat diet mice to alleviate insulin resistance

Exercise training has been demonstrated as an effective therapy for insulin resistance (IR) to relieve skeletal muscle metabolic disorders. Physiologic protective autophagy was found blocked by IR induced severe endoplasmic reticulum (ER) stress, which may lead to progression of IR. However, the mechanisms are not fully understood. Therefore, this study sought to investigate that how exercise training act on IR through an exploration into the mechanism of ER stress and high-fat diet (HFD)-blocked autophagy. A rodent model was adopted in mice via a HFD and an 8-week swim exercise training intervention. The mouse IR model was successfully established through HFD treatment: body mass, adipose weight, HOMA-IR index increased, as well as IRS1, inhibited in the muscle of HFD mice. Moreover, the AMPK/PGC1α pathway was depressed and IRE1, PERK, ATF6-related ER stress signaling was activated in the muscle of HFD mice. While autophagy factors, including BNIP3, LC3II/LC3I, and PINK1, decreased. However, additional 8-week exercise training during HFD was found to reverse all these changes and alleviate IR. Thus, exercise training could facilitate HFD-blocked protective autophagy via the activation of the AMPK/PGC1α pathway to relieve insulin resistance in mice. Our study provides experimental data for the prevention of T2DM and insulin resistance.

Endoplasmic reticulum stress downregulates PGC-1α in skeletal muscle through ATF4 and an mTOR-mediated reduction of CRTC2

Background

Peroxisome proliferator-activated receptor γ (PPARγ) coactivator 1α (PGC-1α) downregulation in skeletal muscle contributes to insulin resistance and type 2 diabetes mellitus. Here, we examined the effects of endoplasmic reticulum (ER) stress on PGC-1α levels in muscle and the potential mechanisms involved.

Methods

The human skeletal muscle cell line LHCN-M2 and mice exposed to different inducers of ER stress were used.

Results

Palmitate- or tunicamycin-induced ER stress resulted in PGC-1α downregulation and enhanced expression of activating transcription factor 4 (ATF4) in human myotubes and mouse skeletal muscle. Overexpression of ATF4 decreased basal PCG-1α expression, whereas ATF4 knockdown abrogated the reduction of PCG-1α caused by tunicamycin in myotubes. ER stress induction also activated mammalian target of rapamycin (mTOR) in myotubes and reduced the nuclear levels of cAMP response element-binding protein (CREB)-regulated transcription co-activator 2 (CRTC2), a positive modulator of PGC-1α transcription. The mTOR inhibitor torin 1 restored PCG-1α and CRTC2 protein levels. Moreover, siRNA against S6 kinase, an mTORC1 downstream target, prevented the reduction in the expression of CRTC2 and PGC-1α caused by the ER stressor tunicamycin.

Conclusions

Collectively, these findings demonstrate that ATF4 and the mTOR-CRTC2 axis regulates PGC-1α transcription under ER stress conditions in skeletal muscle, suggesting that its inhibition might be a therapeutic target for insulin resistant states.

EVOO Promotes a Less Atherogenic Profile Than Sunflower Oil in Smooth Muscle Cells Through the Extracellular Vesicles Secreted by Endothelial Cells

Background

Little is known about the effect of extra virgin olive (EVOO) and sunflower oil (SO) on the composition of extracellular vesicles (EVs) secreted by endothelial cells and the effects of these EVs on smooth muscle cells (SMCs). These cells play an important role in the development of atherosclerosis.

Methods

We evaluated the effects of endothelial cells-derived EVs incubated with triglyceride-rich lipoproteins obtained after a high-fat meal with EVOO (EVOO-EVs) and SO (SO-EVs), on the transcriptomic profile of SMCs.

Results

We found 41 upregulated and 19 downregulated differentially expressed (DE)-miRNAs in EVOO-EVs. Afterwards, SMCs were incubated with EVOO-EVs and SO-EVs. SMCs incubated with SO-EVs showed a greater number of DE-mRNA involved in pathways related to cancer, focal adhesion, regulation of actin cytoskeleton, and MAPK, toll-like receptor, chemokine and Wnt signaling pathways than in SMCs incubated with EVOO-EVs. These DE-mRNAs were involved in biological processes related to the response to endogenous stimulus, cell motility, regulation of intracellular signal transduction and cell population proliferation.

Conclusion

EVOO and SO can differently modify the miRNA composition of HUVEC-derived EVs. These EVs can regulate the SMCs transcriptomic profile, with SO-EVs promoting a profile more closely linked to the development of atherosclerosis than EVOO-EVs.

Long-chain ceramides are cell non-autonomous signals linking lipotoxicity to endoplasmic reticulum stress in skeletal muscle

The endoplasmic reticulum (ER) regulates cellular protein and lipid biosynthesis. ER dysfunction leads to protein misfolding and the unfolded protein response (UPR), which limits protein synthesis to prevent cytotoxicity. Chronic ER stress in skeletal muscle is a unifying mechanism linking lipotoxicity to metabolic disease. Unidentified signals from cells undergoing ER stress propagate paracrine and systemic UPR activation. Here, we induce ER stress and lipotoxicity in myotubes. We observe ER stress-inducing lipid cell non-autonomous signal(s). Lipidomics identifies that palmitate-induced cell stress induces long-chain ceramide 40:1 and 42:1 secretion. Ceramide synthesis through the ceramide synthase 2 de novo pathway is regulated by UPR kinase Perk. Inactivation of CerS2 in mice reduces systemic and muscle ceramide signals and muscle UPR activation. The ceramides are packaged into extracellular vesicles, secreted and induce UPR activation in naïve myotubes through dihydroceramide accumulation. This study furthers our understanding of ER stress by identifying UPR-inducing cell non-autonomous signals.

Targeting stearoyl-CoA desaturase in solid tumors

Cancer cells are demarcated from normal cells by distinct biological hallmarks, including the reprogramming of metabolic processes. One of the key players involved in metabolic reprogramming is stearoyl-CoA desaturase (SCD), which converts saturated fatty acids to monounsaturated fatty acids in an oxygen-dependent reaction that is crucial for maintaining fatty acid homeostasis. As such, SCD has been identified as a potential therapeutic target in numerous types of cancers, and its inhibition suppresses cancer cell growth in vitro and in vivo. This review summarizes the evidence implicating SCD in cancer progression and proposes novel therapeutic strategies for targeting SCD in solid tumors.

An extra virgin olive oil-enriched chocolate spread positively modulates insulin-resistance markers compared with a palm oil-enriched one in healthy young adults: A double-blind, cross-over, randomised controlled trial

AIMS

To investigate if extra virgin olive oil (EVOO) or palm oil enriched chocolate spreads consumption leads to different results in terms of plasma ceramides concentration, glucose and lipid metabolism, inflammatory markers and appetite regulation in young healthy subjects.

METHODS

In a 2-week, double-blind, cross-over, randomised controlled trial, 20 healthy, normal-weight subjects with a mean age of 24.2 years (SD: 1.2), consumed chocolate spread snacks (73% of energy [%E] from fat, 20% from carbohydrates and 7% from proteins), providing 570 Kcal/day added to an isocaloric diet. The chocolate spreads were identical, except for the type of fat: EVOO oil, rich in monounsaturated fatty acids (MUFAs), or palm oil, rich in Saturated Fatty Acids (SFAs).

RESULTS

EVOO-enriched chocolate spread consumption led to better circulating sphingolipids and glucose profile, with reduced plasma ceramide C16:0, ceramide C16:0/ceramide C22:0-ceramide C24:0 ratio and sphingomyelin C18:0 (P = 0.030, P= 0.032 and P = 0.042, respectively) compared to the palm oil-enriched chocolate spread diet. HOMA-IR and plasma insulin were lower, while the Quicki and the McAuley Index were higher after the EVOO diet compared to the palm oil diet (P = 0.046, P = 0.045, P = 0.018 and P = 0.039 respectively). Subjects maintained a stable weight throughout the study. No major significant changes in total cholesterol, triglycerides, HDL, inflammatory markers, and appetite-regulating hormones/visual analogue scale were observed between the groups.

CONCLUSIONS

Partially replacing SFAs with MUFAs in a chocolate-based snack as part of a short-term isocaloric diet in healthy individuals may limit SFAs detrimental effects on insulin sensitivity and decrease circulating harmful sphingolipids in young adults.

Metabolic Signatures of Type 2 Diabetes Mellitus and Hypertension in COVID-19 Patients With Different Disease Severity

Introduction: Increased COVID-19 disease severity is higher among patients with type 2 diabetes mellitus and hypertension. However, the metabolic pathways underlying this association are not fully characterized. This study aims to identify the metabolic signature associated with increased COVID-19 severity in patients with diabetes mellitus and hypertension. Methods: One hundred and fifteen COVID-19 patients were divided based on disease severity, diabetes status, and hypertension status. Targeted metabolomics of serum samples from all patients was performed using tandem mass spectrometry followed by multivariate and univariate models. Results: Reduced levels of various triacylglycerols were observed with increased disease severity in the diabetic patients, including those containing palmitic (C16:0), docosapentaenoic (C22:5, DPA), and docosahexaenoic (C22:6, DHA) acids (FDR < 0.01). Functional enrichment analysis revealed triacylglycerols as the pathway exhibiting the most significant changes in severe COVID-19 in diabetic patients (FDR = 7.1 × 10^-27). Similarly, reduced levels of various triacylglycerols were also observed in hypertensive patients corresponding with increased disease severity, including those containing palmitic, oleic (C18:1), and docosahexaenoic acids. Functional enrichment analysis revealed long-chain polyunsaturated fatty acids (n-3 and n-6) as the pathway exhibiting the most significant changes with increased disease severity in hypertensive patients (FDR = 0.07). Conclusions: Reduced levels of triacylglycerols containing specific long-chain unsaturated, monounsaturated, and polyunsaturated fatty acids are associated with increased COVID-19 severity in diabetic and hypertensive patients, offering potential novel diagnostic and therapeutic targets.

Dimethyl itaconate attenuates palmitate-induced insulin resistance in skeletal muscle cells through the AMPK/FGF21/PPARδ-mediated suppression of inflammation

AIM

Itaconate (ITA), a derivative of the tricarboxylic acid cycle, has been documented to have a direct antimicrobial effect by inhibiting isocitrate lyase and suppressing proinflammatory cytokines in LPS-treated macrophages. However, the effects of dimethyl ITA (DITA), a membrane-permeable derivative of ITA, on insulin signaling and inflammation in skeletal muscle in an obese state remain to be elucidated. Thus, this study was designed to investigate the effects of DITA on the impairment of insulin signaling and inflammation in palmitate-treated C2C12 myocytes.

MATERIALS AND METHODS

Western blotting was used to determine the expression of insulin signaling associated genes, inflammatory markers, fibroblast growth factor 21 (FGF21), and PPARδ expression, as well as AMPK phosphorylation in mouse skeletal muscle cells. Secreted proinflammatory cytokine levels were detected by enzyme-linked immunosorbent assay. Insulin signaling was assessed by glucose uptake assay.

KEY FINDINGS

Treating C2C12 myocytes with DITA attenuated palmitate-induced aggravation of insulin signaling markers, such as insulin receptor substrate-1 (IRS-1) and Akt phosphorylation and inflammatory markers, such as NFκB and IκB phosphorylation. AMPK phosphorylation, as well as PPARδ and myokine FGF21 expression, were enhanced in C2C12 myocytes by DITA treatment. siRNA-mediated suppression of AMPK or FGF21 expression abolished the effects of DITA on insulin resistance and inflammation in palmitate-treated C2C12 myocytes.

SIGNIFICANCE

In sum, DITA suppresses inflammation through the AMPK/FGF21/PPARδ signaling, thereby alleviating insulin resistance in palmitate-treated C2C12 myocytes. The current study appears to be an essential basis for performing animal experiments to develop insulin resistance therapeutics.

Exposure to Stearate Activates the IRE1α/XBP-1 Pathway in 3T3-L1 Adipocytes

In the endoplasmic reticulum (ER), accumulation of abnormal proteins with malformed higher-order structures activates signaling pathways (inositol-requiring enzyme 1α (IRE1α)/X-box binding protein 1 (XBP-1) pathway, protein kinase RNA-activated-like endoplasmic reticulum kinase (PERK)/CCAAT/enhancer binding protein-homologous protein (CHOP) pathway and activating transcription factor 6α (ATF6α) pathway) that result in a cellular response suppressing the production of abnormal proteins or inducing apoptosis. These responses are collectively known as the unfolded protein response (UPR). Recently, it has been suggested that the UPR induced by saturated fatty acids in hepatocytes and pancreatic β cells is involved in the development of metabolic diseases such as diabetes. The effect of palmitate, a saturated fatty acid, on the UPR has also been investigated in adipocytes, which are associated with the development of metabolic disorders, but the results were inconclusive. Therefore, as the major saturated fatty acids present in the daily diet are palmitate and stearate, we examined the effects of these saturated fatty acids on UPR in adipocytes. Here, we show that saturated fatty acids caused limited activation of the UPR in adipocytes. Exposure to stearate for several hours elevated the ratio of spliced XBP-1 mRNA, and this effect was stronger than that of palmitate. Moreover, the phosphorylation level of IRE1α, upstream of XBP-1 and expression levels of its downstream targets such as DNAJB9 and Pdia6 were elevated in 3T3-L1 adipocytes exposed to stearate. On the other hand, stearate did not affect the phosphorylation of PERK, its activation of CHOP, or the cleavage of ATF6α. Thus, in adipocytes, exposure to stearate activates the UPR via the IRE1α/XBP-1 pathway, but not the PERK/CHOP and ATF6α pathway.

Mammalian/mechanistic target of rapamycin (mTOR) complexes in neurodegeneration

Novel targets to arrest neurodegeneration in several dementing conditions involving misfolded protein accumulations may be found in the diverse signaling pathways of the Mammalian/mechanistic target of rapamycin (mTOR). As a nutrient sensor, mTOR has important homeostatic functions to regulate energy metabolism and support neuronal growth and plasticity. However, in Alzheimer's disease (AD), mTOR alternately plays important pathogenic roles by inhibiting both insulin signaling and autophagic removal of β-amyloid (Aβ) and phospho-tau (ptau) aggregates. It also plays a role in the cerebrovascular dysfunction of AD. mTOR is a serine/threonine kinase residing at the core in either of two multiprotein complexes termed mTORC1 and mTORC2. Recent data suggest that their balanced actions also have implications for Parkinson's disease (PD) and Huntington's disease (HD), Frontotemporal dementia (FTD) and Amyotrophic Lateral Sclerosis (ALS). Beyond rapamycin; an mTOR inhibitor, there are rapalogs having greater tolerability and micro delivery modes, that hold promise in arresting these age dependent conditions.

Stearoyl CoA Desaturase 1 and Inositol-Requiring Protein 1α Determine the Efficiency of Oleic Acid in Alleviating Silica Nanoparticle-Induced Insulin Resistance

Despite the widespread use of silica nanoparticles (SiNPs), their metabolic impact and mechanisms of action have not been well studied. Exposure to SiNPs induces insulin resistance (IR) in hepatocytes by endoplasmic reticulum (ER) stress via inositol-requiring protein 1α (IRE1α) activation of c-Jun N-terminal kinases (JNK). It has been well established that stearoyl CoA desaturase (SCD1) and its major product oleic acid elicited beneficial effects in restoring ER homeostasis. However, the potential coordination of SCD1 and IRE1α in determining SiNP regulation of insulin signaling is unclear. Herein, we investigated the effects of SCD1 and oleic acid on IR induced by SiNPs or thapsigargin in hepatocytes. SCD1 overexpression or oleic acid efficiently reversed SiNP-induced ER stress and IR, whereas the effects of thapsigargin treatment could not be restored. Thapsigargin diminished SCD1 protein levels, leading to the accumulation of IRE1α and sustained activation of the IRE1α/JNK pathway. Moreover, knockdown of activating transcription factor 4 (ATF4) upstream of SCD1 suppressed SiNP-induced SCD1 expression, rescued the activated IRE1α, and inhibited insulin signaling but was not able to restore the effects of thapsigargin. Collectively, downregulation of SCD1 and excess accumulation of IRE1α protein prevented the beneficial effects of exogenous oleic acid on IR induced by ER stress. Our results provide valuable mechanistic insights into the synergic regulation of IR by SiNPs and ER stress and suggest a combinational strategy to restore ER homeostasis by targeting SCD1 and IRE1α proteins, as well as supplementation of unsaturated fatty acids.

The Inhibition of Metabolic Inflammation by EPA Is Associated with Enhanced Mitochondrial Fusion and Insulin Signaling in Human Primary Myotubes

BACKGROUND

Sustained fuel excess triggers low-grade inflammation that can drive mitochondrial dysfunction, a pivotal defect in the pathogenesis of insulin resistance in skeletal muscle.

OBJECTIVES

This study aimed to investigate whether inflammation in skeletal muscle can be prevented by EPA, and if this is associated with an improvement in mitochondrial fusion, membrane potential, and insulin signaling.

METHODS

Human primary myotubes were treated for 24 h with palmitic acid (PA, 500 μM) under hyperglycemic conditions (13 mM glucose), which represents nutrient overload, and in the presence or absence of EPA (100 μM). After the treatments, the expression of peroxisome proliferator-activated receptor γ coactivator 1-α (PPARGC1A) and IL6 was assessed by q-PCR. Western blot was used to measure the abundance of the inhibitor of NF-κB (IKBA), mitofusin-2 (MFN2), mitochondrial electron transport chain complex proteins, and insulin-dependent AKT (Ser473) and AKT substrate 160 (AS 160; Thr642) phosphorylation. Mitochondrial dynamics and membrane potential were evaluated using immunocytochemistry and the JC-1 (tetraethylbenzimidazolylcarbocyanine iodide) dye, respectively. Data were analyzed using 1-factor ANOVA followed by Tukey post hoc test.

RESULTS

Nutrient excess activated the proinflammatory NFκB signaling marked by a decrease in IKBA (40%; P < 0.05) and the upregulation of IL6 mRNA (12-fold; P < 0.001). It also promoted mitochondrial fragmentation (53%; P < 0.001). All these effects were counteracted by EPA. Furthermore, nutrient overload-induced drop in mitochondrial membrane potential (6%; P < 0.05) was prevented by EPA. Finally, EPA inhibited fuel surplus-induced impairment in insulin-mediated phosphorylation of AKT (235%; P < 0.01) and AS160 (49%; P < 0.05).

CONCLUSIONS

EPA inhibited NFκB signaling, which was associated with an attenuation of the deleterious effects of PA and hyperglycemia on both mitochondrial health and insulin signaling in human primary myotubes. Thus, EPA might preserve skeletal muscle metabolic health during sustained fuel excess but this requires confirmation in human clinical trials.

Pentadecanoic acid promotes basal and insulin-stimulated glucose uptake in C2C12 myotubes

Background

Saturated fatty acids (SFAs) generally have been thought to worsen insulin-resistance and increase the risk of developing type 2 diabetes mellitus (T2DM). Recently, accumulating evidence has revealed that SFAs are not a single homogeneous group, instead different SFAs are associated with T2DM in opposing directions. Pentadecanoic acid (C15:0, PA) is directly correlated with dairy products, and a negative association between circulating PA and metabolic disease risk was observed in epidemiological studies. Therefore, the role of PA in human health needs to be reinforced. Whether PA has a direct benefit on glucose metabolism and insulin sensitivity needs further investigation.

Objective

The present study aimed to investigate the effect and potential mechanism of action of PA on basal and insulin stimulated glucose uptake in C2C12 myotubes.

Methods

Glucose uptake was determined using a 2-(N-[7-nitrobenz-2-oxa-1,3-diazol-4-yl] amino)-2-deoxyglucose (2-NBDG) uptake assay. Cell membrane proteins were isolated and glucose transporter 4 (GLUT4) protein was detected by western blotting to examine the translocation of GLUT4 to the plasma membrane. The phosphorylation levels of proteins involved in the insulin and 5’-adenosine monophosphate-activated protein kinase (AMPK) pathways were examined by western blotting.

Results

We found that PA significantly promoted glucose uptake and GLUT4 translocation to the plasma membrane. PA had no effect on the insulin-dependent pathway involving insulin receptor substrate 1 (Tyr632) and protein kinase B (PKB/Akt), but increased phosphorylation of AMPK and Akt substrate of 160 kDa (AS160). Compound C (an AMPK inhibitor) blocked PA-induced AMPK activation and reversed PA-induced GLUT4 translocation, indicating that PA promotes glucose uptake via the AMPK pathway in vitro. Moreover, PA significantly promoted insulin-stimulated glucose uptake in myotubes. Under insulin stimulation, PA did not affect the insulin-dependent pathway, but still activated AMPK.

Conclusion

PA, an odd-chain SFA, significantly stimulates glucose uptake via the AMPK-AS160 pathway and exhibits an insulin-sensitizing effect in myotubes.

Oleate Ameliorates Palmitate-Induced Impairment of Differentiative Capacity in C2C12 Myoblast Cells

A common observation in metabolic disorders and aging is the elevation of free fatty acids (FFAs), which can form ectopic fat deposition and result in lipotoxicity. Ectopic fat deposition of skeletal muscle has been recognized as an important component of aging, frailty, and sarcopenia. Previous studies have suggested that lipotoxicity caused by FFAs mainly stemmed from saturated fatty acids and decreased unsaturated/saturated fatty acid ratio in serum are also observed among metabolic disorder patients. However, the different effects of saturated fatty acids and unsaturated fatty acids on skeletal muscle are not fully elucidated. In this study, we verified that palmitate (PA), a saturated fatty acid, could lead to impaired differentiative capacity of C2C12 myoblasts by affecting Pax7, MyoD, and myogenin (MyoG), which are master regulators of lineage specification and the myogenic program. Then, oleate (OA), a monounsaturated fatty acid, were added to culture medium together with PA. Results showed that OA could ameliorate the impairment of differentiative capacity in C2C12 myoblast cells. In addition, we found PI3K/Akt signaling pathway played an important role during the process by RNA sequencing and bioinformatics analysis. The positive effect of OA on myoblast differentiative capacity disappeared if PI3K inhibitor LY294002 was added. In conclusion, our study showed that PA could destroy differentiative capacity of C2C12 myoblasts by affecting the expression of Pax7, MyoD, and MyoG, and OA could improve this impairment through PI3K/Akt signaling pathway.

Adipose tissue and insulin resistance in obese

Currently, obesity has become a global health issue and is referred to as an epidemic. Dysfunctional obese adipose tissue plays a pivotal role in the development of insulin resistance. However, the mechanism of how dysfunctional obese-adipose tissue develops insulin-resistant circumstances remains poorly understood. Therefore, this review attempts to highlight the potential mechanisms behind obesity-associated insulin resistance. Multiple risk factors are directly or indirectly associated with the increased risk of obesity; among them, environmental factors, genetics, aging, gut microbiota, and diets are prominent. Once an individual becomes obese, adipocytes increase in their size; therefore, adipose tissues become larger and dysfunctional, recruit macrophages, and then these polarize to pro-inflammatory states. Enlarged adipose tissues release excess free fatty acids (FFAs), reactive oxygen species (ROS), and pro-inflammatory cytokines. Excess systemic FFAs and dietary lipids enter inside the cells of non-adipose organs such as the liver, muscle, and pancreas, and are deposited as ectopic fat, generating lipotoxicity. Toxic lipids dysregulate cellular organelles, e.g., mitochondria, endoplasmic reticulum, and lysosomes. Dysregulated organelles release excess ROS and pro-inflammation, resulting in systemic inflammation. Long term low-grade systemic inflammation prevents insulin from its action in the insulin signaling pathway, disrupts glucose homeostasis, and results in systemic dysregulation. Overall, long-term obesity and overnutrition develop into insulin resistance and chronic low-grade systemic inflammation through lipotoxicity, creating the circumstances to develop clinical conditions. This review also shows that the liver is the most sensitive organ undergoing insulin impairment faster than other organs, and thus, hepatic insulin resistance is the primary event that leads to the subsequent development of peripheral tissue insulin resistance.

Palmitate exacerbates bisphenol A toxicity via induction of ER stress and mitochondrial dysfunction

Combined exposure to dietary nutrients and environmental chemicals may elicit significantly different physiological effects than single exposures. Exposure to dietary saturated fats and environmental toxins is a physiologically-significant dual exposure that is particularly associated with lower socioeconomic status, potentially placing these individuals at heightened risk of xenobiotic toxicities. However, no prior studies have examined interactions between specific lipids and environmental xenobiotics in modulating cellular health. Using primary mouse embryonic fibroblasts, we have discovered that prior exposure to the saturated fatty acid, palmitate, exacerbates cellular toxicity associated with the industrial plasticizer, bisphenol A (BPA). Cell death upon BPA exposure following palmitate pre-treatment was greater than that occurring with either exposure alone. Mechanistically, cell death was preceded by increased endoplasmic reticulum stress and loss of mitochondrial membrane potential in palmitate plus BPA exposed cells, leading to increased caspase-3 cleavage and subsequent apoptosis. Interestingly, inclusion of the unsaturated fatty acid, oleate, along with palmitate during the pre-treatment period completely abrogated the ER stress, mitochondrial toxicity, and cell death induced by subsequent exposure to BPA. Thus, our data identify for the first time an important interaction between a fatty acid and an environmental toxin and have implications for developing nutritional interventions to mitigate the deleterious effects of such xenobiotic exposures.

Benzyl Isothiocyanate Ameliorates High-Fat Diet-Induced Hyperglycemia by Enhancing Nrf2-Dependent Antioxidant Defense-Mediated IRS-1/AKT/TBC1D1 Signaling and GLUT4 Expression in Skeletal Muscle

Obesity caused lipotoxicity, which results in insulin resistance. We studied whether benzyl isothiocyanate (BITC) improved insulin resistance in muscle. BITC was studied in vivo in mice fed a high-fat diet (HFD) and in vitro in C2C12 myotubes treated with palmitic acid (PA). In C2C12 cells, BITC mitigated PA inhibition of glucose uptake and phosphorylation of IRS-1, AKT, and TBC1D1 in response to insulin. BITC upregulated the expression of HO-1, GSTP, and GCLM mRNA and protein as well as GSH contents, which suppressed oxidative damage. Knockdown of Nrf2 abrogated BITC enhancement of antioxidant defense and subsequently reversed BITC protection against PA-induced insulin resistance. Moreover, BITC upregulated the expression of GLUT4, PPARγ, and C/EBPα. In HFD-fed mice, plasma total cholesterol, nonesterified fatty acid, and glucose levels and HOMA-IR were dose-dependently decreased with 0.05 or 0.1% BITC administration. In gastrocnemius muscle, compared with the HFD group, BITC increased the phosphorylation of AKT and TBC1D1, GSH contents, and the expression of antioxidant enzymes as well as GLUT4. These results indicate that BITC ameliorates obesity-induced hyperglycemia by enhancing insulin sensitivity in muscle. This is partly attributed to its inhibition of lipotoxicity-induced oxidative insult and upregulation of GLUT4 expression.

Endogenous metabolite, kynurenic acid, attenuates nonalcoholic fatty liver disease via AMPK/autophagy- and AMPK/ORP150-mediated signaling

Endoplasmic reticulum (ER) stress plays a causative role in the development of nonalcoholic fatty liver disease (NAFLD). Kynurenic acid (KA) is a tryptophan metabolite that has been shown to exert anti-inflammatory effects in macrophages and endothelial cells. However, the role of KA in ER stress-associated development of NAFLD has not been fully explored. In the current study, we observed decreased KA levels in the serum of obese subjects. Treated hepatocytes with KA attenuated palmitate-induced lipid accumulation and downregulated lipogenesis-associated genes as well as ER stress markers in a dose-dependent manner. Furthermore, KA augmented AMP-activated protein kinase (AMPK) phosphorylation, oxygen-regulated protein 150 (ORP150) expression, and autophagy markers. The small interfering RNA-mediated suppression of AMPK and ORP150, or 3-methyladenine also abrogated the effects of KA on ER stress and lipid accumulation in hepatocytes. In accordance with in vitro observations, KA administration to mice fed a high-fat diet ameliorated hepatic lipid accumulation and decreased the expression of lipogenic genes as well as ER stress. Moreover, KA treatment increased hepatic AMPK phosphorylation, ORP150 expression, and autophagy related markers in mouse livers. Knockdown of AMPK using in vivo transfection mitigated the effects of KA on hepatic steatosis and ER stress as well as autophagy and ORP150 expression. These results suggest that KA ameliorates hepatic steatosis via the AMPK/autophagy- and AMPK/ORP150-mediated suppression of ER stress. In sum, KA might be used as a promising therapeutic agent for treatment of NAFLD.

Glued in lipids: Lipointoxication in cystic fibrosis

Cystic Fibrosis (CF) is an autosomal recessive disease caused by mutations in the CF transmembrane regulator (CFTR) gene, which encodes a chloride channel located at the apical surface of epithelial cells. Unsaturated Fatty Acid (UFA) deficiency has been a persistent observation in tissues from patients with CF. However, the impacts of such deficiencies on the etiology of the disease have been the object of intense debates. The aim of the present review is first to highlight the general consensus on fatty acid dysregulations that emerges from, sometimes apparently contradictory, studies. In a second step, a unifying mechanism for the potential impacts of these fatty acid dysregulations in CF cells, based on alterations of membrane biophysical properties (known as lipointoxication), is proposed. Finally, the contribution of lipointoxication to the progression of the CF disease and how it could affect the efficacy of current treatments is also discussed.

Palmitate but Not Oleate Exerts a Negative Effect on Oxygen Utilization in Myoblasts of Patients with the m.3243A>G Mutation: A Pilot Study

It is known that exposure to excess saturated fatty acids, especially palmitate, can trigger cellular stress responses interpreted as lipotoxicity. The effect of excessive free fatty acids on oxidative phosphorylation capacity in myoblasts of patients with the m.3243A>G mutation was evaluated with the mitochondrial (Mito) stress test using a Seahorse XF96 analyzer. ß-oxidation, measured with the Seahorse XF96 analyzer, was similar in patients and controls, and reduced in both patients and controls at 40 °C compared to 37 °C. Mito stress test in the absence of fatty acids showed lower values in patients compared to controls. The mitochondrial activity and ATP production rates were significantly reduced in presence of palmitate, but not of oleate in patients, showing a negative effect of excessive palmitate on mitochondrial function in patients. Diabetes mellitus is a frequent symptom in patients with m.3243A>G mutation. It can be speculated that the negative effect of palmitate on mitochondrial function might be related to diacylglycerols (DAG) and ceramides (CER) mediated insulin resistance. This might contribute to the elevated risk for diabetes mellitus in m.3243A>G patients.

Urolithin A Prevents Focal Cerebral Ischemic Injury via Attenuating Apoptosis and Neuroinflammation in Mice

Neuroinflammation contributes to neuronal death in cerebral ischemia. Urolithin A (UA), a gut microbial metabolite of ellagic acid, has emerged as a potential anti-inflammatory agent. However, its roles and precise mechanisms in stroke remain unknown. Here we found that UA treatment ameliorated infarction, neurological deficit scores, and spatial memory deficits after cerebral ischemia. Furthermore, UA significantly reduced neuron loss and promoted neurogenesis after ischemic stroke. We also found that UA attenuated apoptosis by regulating apoptotic-related proteins. Meanwhile, UA treatment inhibited glial activation via affecting inflammatory signaling pathways, specifically by enhancing cerebral AMPK and IκBa activation while decreasing the activation of Akt, P65NFκB, ERK, JNK, and P38MAPK. Our findings reveal a key role of UA against ischemic stroke through modulating apoptosis and neuroinflammation in mice.

C2C12 cell model: its role in understanding of insulin resistance at the molecular level and pharmaceutical development at the preclinical stage

OBJECTIVES

The myoblast cell line, C2C12, has been utilised extensively in vitro as an examination model in understanding metabolic disease progression. Although it is indispensable in both preclinical and pharmaceutical research, a comprehensive review of its use in the investigation of insulin resistance progression and pharmaceutical development is not available.

KEY FINDINGS

C2C12 is a well-documented model, which can facilitate our understanding in glucose metabolism, insulin signalling mechanism, insulin resistance, oxidative stress, reactive oxygen species and glucose transporters at cellular and molecular levels. With the aid of the C2C12 model, recent studies revealed that insulin resistance has close relationship with various metabolic diseases in terms of disease progression, pathogenesis and therapeutic management. A holistic, safe and effective disease management is highly of interest. Therefore, significant efforts have been paid to explore novel drug compounds and natural herbs that can elicit therapeutic effects in the targeted sites at both cellular (e.g. mitochondria, glucose transporter) and molecular level (e.g. genes, signalling pathway).

SUMMARY

The use of C2C12 myoblast cell line is meaningful in pharmaceutical and biomedical research due to their expression of GLUT-4 and other features that are representative to human skeletal muscle cells. With the use of the C2C12 cell model, the impact of drug delivery systems (nanoparticles and quantum dots) on skeletal muscle, as well as the relationship between exercise, pancreatic β-cells and endothelial cells, was discovered.

Oleic Acid Protects Against Insulin Resistance by Regulating the Genes Related to the PI3K Signaling Pathway

BACKGROUND

The effects of different types of fatty acids on the gene expression of key players in the IRS1/PI3K signaling pathway have been poorly studied.

MATERIAL AND METHODS

We analyzed IRS1, p85α, and p110β mRNA expression and the fatty acid composition of phospholipids in visceral adipose tissue from patients with morbid obesity and from non-obese patients. Moreover, we analyzed the expression of those genes in visceral adipocytes incubated with oleic, linoleic, palmitic and dosahexaenoic acids.

RESULTS

We found a reduced IRS1 expression in patients with morbid obesity, independent of insulin resistance, and a reduced p110β expression in those with lower insulin resistance. A positive correlation was found between p85α and stearic acid, and between IRS1 and p110β with palmitic and dosahexaenoic acid. In contrast, a negative correlation was found between p85α and oleic acid, and between IRS1 and p110β with linoleic, arachidonic and adrenic acid. Incubation with palmitic acid decreased IRS1 expression. p85α was down-regulated after incubation with oleic and dosahexaenoic acid and up-regulated with palmitic acid. p110β expression was increased and decreased after incubation with oleic and palmitic acid, respectively. The ratio p85α/p110β was decreased by oleic and dosahexaenoic acid and increased by palmitic acid.

CONCLUSIONS

Our in vitro results suggest a detrimental role of palmitic acid on the expression of gene related to insulin signaling pathway, with oleic acid being the one with the higher and more beneficial effects. DHA had a slight beneficial effect. Fatty acid-induced regulation of genes related to the IRS1/PI3K pathway may be a novel mechanism by which fatty acids regulate insulin sensitivity in visceral adipocytes.

The Short Overview on the Relevance of Fatty Acids for Human Cardiovascular Disorders

This review presents existing evidence of the influence of saturated and unsaturated fatty acids on cardiovascular diseases (CVD). Data are discussed regarding the roles of the most relevant fatty acids, such as myristic (C14:0), palmitic (C16:0), stearic (C18:0), palmitoleic (C16:1), oleic (C18:1), linoleic (C18:2), α-linolenic (C18:3, ω-3), γ-linolenic (C18:3, ω-6), arachidonic (C20:4), eicosapentaenoic (C20:5), docosahexaenoic (C22:6), and docosapentaenoic (C22:5) acid. The accumulated knowledge has expanded the understanding of the involvement of fatty acids in metabolic processes, thereby enabling the transition from basic exploratory studies to practical issues of application of these biomolecules to CVD treatment. In the future, these findings are expected to facilitate the interpretation and prognosis of changes in metabolic lipid aberrations in CVD.

Comparison of dietary and plasma phospholipid fatty acids between normal weight and overweight black South Africans according to metabolic health: The PURE study

BACKGROUND

Information regarding circulating fatty acids (FA) in association with metabolic health in black Africans is scarce, while the usefulness of circulating FAs as biomarkers of dietary fat intake and predictors for medical conditions is increasing.

OBJECTIVE

We compared eleven dietary and the levels of 26 plasma phospholipid FAs in metabolically healthy and unhealthy phenotypes in black South African adults.

METHODS

Adults from the South African arm of the Prospective Urban and Rural Epidemiology study baseline (n = 711) were categorised into four groups, namely normal weight without metabolic syndrome (MetS) (MHNW), normal weight with MetS (MUNW), metabolically healthy overweight/obese (MHO) and metabolically unhealthy overweight/obese (MUO). Dietary and plasma phospholipid FAs were measured by a quantitative food frequency questionnaire and gas chromatography-tandem mass spectrometry, respectively. We compared dietary FAs, plasma phospholipid FAs, and estimated desaturase activity between the metabolic status groups using ANCOVA adjusted for age and energy intake.

RESULTS

MetS was diagnosed in 35% of the participants. After adjustment for age and total energy intake, in comparison to the MHNW reference group, saturated dietary FAs (C14:0 to C18:0) and alpha-linolenic acid intakes were higher in both overweight/obese groups (MHO and MUO), while linoleic acid intakes were higher in the MUO group only. Plasma levels of most saturated FAs (C18:0 to C22:0) and PUFAs were higher, whereas selected MUFAs, palmitic acid, and estimated desaturase activities were lower in the overweight/obese groups.

CONCLUSIONS

The overweight groups generally had higher fat intakes than normal-weight groups, but lower plasma levels of palmitic, palmitoleic, oleic, cis-vaccenic and estimated desaturase activities. Therefore, in this population, lower plasma levels of palmitic, palmitoleic, oleic, and cis-vaccenic acids and decreased estimated desaturase activities may be biomarkers of abnormal metabolic health in overweight/obese study participants.

Reduction of stearoyl-CoA desaturase (SCD) contributes muscle atrophy through the excess endoplasmic reticulum stress in chronic kidney disease

Skeletal muscle atrophy is associated with mortality and poor prognosis in patients with chronic kidney disease (CKD). However, underlying mechanism by which CKD causes muscle atrophy has not been completely understood. The quality of lipids (lipoquality), which is defined as the functional features of diverse lipid species, has recently been recognized as the pathology of various diseases. In this study, we investigated the roles of the stearoyl-CoA desaturase (SCD), which catalyzes the conversion of saturated fatty acids into monounsaturated fatty acids, in skeletal muscle on muscle atrophy in CKD model animals. In comparison to control rats, CKD rats decreased the SCD activity and its gene expression in atrophic gastrocnemius muscle. Next, oleic acid blocked the reduction of the thickness of C2C12 myotubes and the increase of the endoplasmic reticulum stress induced by SCD inhibitor. Furthermore, endoplasmic reticulum stress inhibitor ameliorated CKD-induced muscle atrophy (the weakness of grip strength and the decrease of muscle fiber size of gastrocnemius muscle) in mice and the reduction of the thickness of C2C12 myotubes by SCD inhibitor. These results suggest that the repression of SCD activity causes muscle atrophy through excessive endoplasmic reticulum stress in CKD.

Omega-3 Polyunsaturated Fatty Acids Mitigate Palmitate-Induced Impairments in Skeletal Muscle Cell Viability and Differentiation

Accumulation of excess saturated free fatty acids such as palmitate (PAL) in skeletal muscle leads to reductions in mitochondrial integrity, cell viability and differentiation. Omega-3 polyunsaturated fatty acids (n-3 PUFAs) such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) counteract PAL-induced lipid accumulation. EPA and DHA, as well as the n-3 PUFA docosapentaenoic acid (DPA), may therefore mitigate PAL-induced lipotoxicity to promote skeletal muscle cell survival and differentiation. C2C12 myoblasts were treated with 50 μM EPA, DPA, or DHA in the absence or presence of 500 μM PAL for 16 h either prior to myoblast analysis or induction of differentiation. Myoblast viability and markers of apoptosis, endoplasmic reticulum (ER) stress and myotube differentiation capacity were investigated using fluorescence microscopy and immunoblotting. High-resolution respirometry was used to assess mitochondrial function and membrane integrity. PAL induced cell death via apoptosis and increased protein content of ER stress markers BiP and CHOP. EPA, DPA, and DHA co-treatment maintained cell viability, prevented PAL-induced apoptosis and attenuated PAL-induced increases in BiP, whereas only DPA prevented increases in CHOP. PAL subsequently reduced protein content of the differentiation marker myogenin and inhibited myotube formation, and all n-3 PUFAs promoted myotube formation in the presence of PAL. Furthermore, DPA prevented PAL-induced release of cytochrome c and maintained mitochondrial integrity. These findings demonstrate the n-3 PUFAs EPA, DPA and DHA elicit similar protective effects against PAL-induced impairments in muscle cell viability and differentiation. Mechanistically, the protective effects of DPA against PAL lipotoxicity are attributable in part to its ability to maintain mitochondrial respiratory capacity via mitigating PAL-induced loss of mitochondrial membrane integrity.

Skeletal muscle enhancer interactions identify genes controlling whole-body metabolism

Obesity and type 2 diabetes (T2D) are metabolic disorders influenced by lifestyle and genetic factors that are characterized by insulin resistance in skeletal muscle, a prominent site of glucose disposal. Numerous genetic variants have been associated with obesity and T2D, of which the majority are located in non-coding DNA regions. This suggests that most variants mediate their effect by altering the activity of gene-regulatory elements, including enhancers. Here, we map skeletal muscle genomic enhancer elements that are dynamically regulated after exposure to the free fatty acid palmitate or the inflammatory cytokine TNFα. By overlapping enhancer positions with the location of disease-associated genetic variants, and resolving long-range chromatin interactions between enhancers and gene promoters, we identify target genes involved in metabolic dysfunction in skeletal muscle. The majority of these genes also associate with altered whole-body metabolic phenotypes in the murine BXD genetic reference population. Thus, our combined genomic investigations identified genes that are involved in skeletal muscle metabolism.

Obesity and type 2 diabetes (T2D) are metabolic disorders characterized by insulin resistance in skeletal muscle. Here, the authors map skeletal muscle enhancer elements dynamically regulated after exposure to free fatty acid palmitate or inflammatory cytokine TNFα and identify target genes involved in metabolic dysfunction in skeletal muscle.