Nox2 mediates skeletal muscle insulin resistance induced by a high fat diet

Activation of the ROS/TXNIP/NLRP3 pathway disrupts insulin-dependent glucose uptake in skeletal muscle of insulin-resistant obese mice

Oxidative stress and the activation of the nucleotide-binding domain, leucine-rich-containing family, pyrin domain containing 3 (NLRP3) inflammasome have been linked to insulin resistance in skeletal muscle. In immune cells, the exacerbated generation of reactive oxygen species (ROS) activates the NLRP3 inflammasome, by facilitating the interaction between thioredoxin interacting protein (TXNIP) and NLRP3. However, the precise role of ROS/TXNIP-dependent NLRP3 inflammasome activation in skeletal muscle during obesity-induced insulin resistance remains undefined. Here, we induced insulin resistance in C57BL/6J mice by feeding them for 8 weeks with a high-fat diet (HFD) and explored whether the ROS/TXNIP/NLRP3 pathway was involved in the induction of insulin resistance in skeletal muscle. Skeletal muscle fibers from insulin-resistant mice exhibited increased oxidative stress, as evidenced by elevated malondialdehyde levels, and altered peroxiredoxin 2 dimerization. Additionally, these fibers displayed augmented activation of the NLRP3 inflammasome, accompanied by heightened ROS-dependent proximity between TXNIP and NLRP3, which was abolished by the antioxidant N-acetylcysteine (NAC). Inhibition of the NLRP3 inflammasome with MCC950 or suppressing the ROS/TXNIP/NLRP3 pathway with NAC restored insulin-dependent glucose uptake in muscle fibers from insulin-resistant mice. These findings provide insights into the mechanistic link between oxidative stress, NLRP3 inflammasome activation, and obesity-induced insulin resistance in skeletal muscle.

Unresolved questions in the regulation of skeletal muscle insulin action by reactive oxygen species

Reactive oxygen species (ROS) are well-established signaling molecules implicated in a wide range of cellular processes, including both oxidative stress and intracellular redox signaling. In the context of insulin action within its target tissues, ROS have been reported to exert both positive and negative regulatory effects. However, the precise molecular mechanisms underlying this duality remain unclear. This Review examines the complex role of ROS in insulin action, with a particular focus on skeletal muscle. We aim to address three critical aspects: (a) the proposed intracellular pro-oxidative redox shift elicited by insulin, (b) the evidence supporting that redox-sensitive cysteine modifications impact insulin signaling and action, and (c) cellular mechanisms underlying how ROS can paradoxically act as both enhancers and inhibitors of insulin action. This Review underscores the urgent need for more systematic research to identify specific reactive species, redox targets, and the physiological significance of redox signaling in maintaining insulin action and metabolic health, with a particular emphasis on human skeletal muscle.

NADPH Dynamics: Linking Insulin Resistance and β-Cells Ferroptosis in Diabetes Mellitus

This review offers an in-depth exploration of Nicotinamide Adenine Dinucleotide Phosphate (NADPH) in metabolic health. It delves into how NADPH affects insulin secretion, influences insulin resistance, and plays a role in ferroptosis. NADPH, a critical cofactor in cellular antioxidant systems and lipid synthesis, plays a central role in maintaining metabolic homeostasis. In adipocytes and skeletal muscle, NADPH influences the pathophysiology of insulin resistance, a hallmark of metabolic disorders such as type 2 diabetes and obesity. The review explores the mechanisms by which NADPH contributes to or mitigates insulin resistance, including its role in lipid and reactive oxygen species (ROS) metabolism. Parallelly, the paper investigates the dual nature of NADPH in the context of pancreatic β-cell health, particularly in its relation to ferroptosis, an iron-dependent form of programmed cell death. While NADPH's antioxidative properties are crucial for preventing oxidative damage in β-cells, its involvement in lipid metabolism can potentiate ferroptotic pathways under certain pathological conditions. This complex relationship underscores the delicate balance of NADPH homeostasis in pancreatic health and diabetes pathogenesis. By integrating findings from recent studies, this review aims to illuminate the nuanced roles of NADPH in different tissues and its potential as a therapeutic target. Understanding these dynamics offers vital insights into the development of more effective strategies for managing insulin resistance and preserving pancreatic β-cell function, thereby advancing the treatment of metabolic diseases.

Epstein-Barr virus-acquired immunodeficiency in myalgic encephalomyelitis-Is it present in long COVID?

Both myalgic encephalomyelitis or chronic fatigue syndrome (ME/CFS) and long COVID (LC) are characterized by similar immunological alterations, persistence of chronic viral infection, autoimmunity, chronic inflammatory state, viral reactivation, hypocortisolism, and microclot formation. They also present with similar symptoms such as asthenia, exercise intolerance, sleep disorders, cognitive dysfunction, and neurological and gastrointestinal complaints. In addition, both pathologies present Epstein-Barr virus (EBV) reactivation, indicating the possibility of this virus being the link between both pathologies. Therefore, we propose that latency and recurrent EBV reactivation could generate an acquired immunodeficiency syndrome in three steps: first, an acquired EBV immunodeficiency develops in individuals with "weak" EBV HLA-II haplotypes, which prevents the control of latency I cells. Second, ectopic lymphoid structures with EBV latency form in different tissues (including the CNS), promoting inflammatory responses and further impairment of cell-mediated immunity. Finally, immune exhaustion occurs due to chronic exposure to viral antigens, with consolidation of the disease. In the case of LC, prior to the first step, there is the possibility of previous SARS-CoV-2 infection in individuals with "weak" HLA-II haplotypes against this virus and/or EBV.

NOX2 deficiency exacerbates diet-induced obesity and impairs molecular training adaptations in skeletal muscle

The production of reactive oxygen species (ROS) by NADPH oxidase (NOX) 2 has been linked to both insulin resistance and exercise training adaptations in skeletal muscle. This study explores the previously unexamined role of NOX2 in the interplay between diet-induced insulin resistance and exercise training (ET). Using a mouse model that harbors a point mutation in the essential NOX2 regulatory subunit, p47phox (Ncf1*), we investigated the impact of this mutation on various metabolic adaptations. Wild-type (WT) and Ncf1* mice were assigned to three groups: chow diet, 60% energy fat diet (HFD), and HFD with access to running wheels (HFD + E). After a 16-week intervention, a comprehensive phenotypic assessment was performed, including body composition, glucose tolerance, energy intake, muscle insulin signaling, redox-related proteins, and mitochondrial adaptations. The results revealed that NOX2 deficiency exacerbated the impact of HFD on body weight, body composition, and glucose intolerance. Moreover, in Ncf1* mice, ET did not improve glucose tolerance or increase muscle cross-sectional area. ET normalized body fat independently of genotype. The lack of NOX2 activity during ET reduced several metabolic adaptations in skeletal muscle, including insulin signaling and expression of Hexokinase II and oxidative phosphorylation complexes. In conclusion, these findings suggest that NOX2 mediates key beneficial effects of exercise training in the context of diet-induced obesity.

Skeletal-Muscle-Specific Overexpression of Chrono Leads to Disruption of Glucose Metabolism and Exercise Capacity

Disruption of circadian rhythms is related to disorders of glucose metabolism, and the molecular clock also exists in skeletal muscle. The ChIP-derived repressor of network oscillator (Chrono) and brain and muscle ARNT-like 1 (Bmal1) are core circadian components. Chrono is considered to be the repressor of Bmal1, and the Chrono–Bmal1 pathway is important in regulating the circadian rhythm; it has been speculated that this pathway could be a new mechanism for regulating glucose metabolism. The purpose of this study was to investigate the effects of Chrono on glucose metabolism in skeletal muscle and exercise capacity by using mice with skeletal-muscle-specific overexpression of Chrono (Chrono TG) and wild-type (WT) mice as the animal models. The results of this cross-sectional study indicated that the Chrono TG mice had an impaired glucose tolerance, lower exercise capacity, and higher levels of nonfasted blood glucose and glycogen content in skeletal muscle compared to WT mice. In addition, the Chrono TG mice also showed a significant increase in the amount of Chrono bound to Bmal1 according to a co-IP analysis; a remarkable decrease in mRNA expression of Tbc1d1, Glut4, Hk2, Pfkm, Pdp1, Gbe1, and Phka1, as well as in activity of Hk and protein expression of Ldhb; but higher mRNA expression of Pdk4 and protein expression of Ldha compared with those of WT mice. These data suggested the skeletal-muscle-specific overexpression of Chrono led to a greater amount of Chrono bound to Bmal1, which then could affect the glucose transporter, glucose oxidation, and glycogen utilization in skeletal muscle, as well as exercise capacity.

Serum and cecal metabolic profile of the insulin resistant and dyslipidemic p47phox knockout mice

Involvement of NOX-dependent oxidative stress in the pathophysiology of metabolic disorders as well as in the maintenance of metabolic homeostasis has been demonstrated previously. In the present study, the metabolic profile in p47^phox-/- and WT mice fed on a chow diet was evaluated to assess the role of metabolites in glucose intolerance and dyslipidemia under altered oxidative stress conditions. p47^phox-/- mice displayed glucose intolerance, dyslipidemia, hyperglycemia, insulin resistance (IR), hyperinsulinemia, and altered energy homeostasis without any significant change in gluconeogenesis. The expression of genes involved in lipid synthesis and uptake was enhanced in the liver, adipose tissue, and intestine tissues. Similarly, the expression of genes associated with lipid efflux in the liver and intestine was also enhanced. Enhanced gut permeability, inflammation, and shortening of the gut was evident in p47^phox-/- mice. Circulating levels of pyrimidines, phosphatidylglycerol lipids, and 3-methyl-2-oxindole were augmented, while level of purine was reduced in the serum. Moreover, the cecal metabolome was also altered, as was evident with the increase in indole-3-acetamide, N-acetyl galactosamine, glycocholate, and a decrease in hippurate, indoxyl sulfate, and indigestible sugars (raffinose and melezitose). Treatment of p47^phox-/- mice with pioglitazone, marginally improved glucose intolerance, and dyslipidemia, with an increase in PUFAs (linoleate, docosahexaenoic acid, and arachidonic acid). Overall, the results obtained in p47^phox-/- mice indicate an association of IR and dyslipidemia with altered serum and cecal metabolites (both host and bacterial-derived), implying a critical role of NOX-derived ROS in metabolic homeostasis.

Inhibiting NADPH Oxidases to Target Vascular and Other Pathologies: An Update on Recent Experimental and Clinical Studies

Reactive oxygen species (ROS) can be beneficial or harmful in health and disease. While low levels of ROS serve as signaling molecules to regulate vascular tone and the growth and proliferation of endothelial cells, elevated levels of ROS contribute to numerous pathologies, such as endothelial dysfunctions, colon cancer, and fibrosis. ROS and their cellular sources have been extensively studied as potential targets for clinical intervention. Whereas various ROS sources are important for different pathologies, four NADPH oxidases (NOX1, NOX2, NOX4, and NOX5) play a prominent role in homeostasis and disease. NOX1-generated ROS have been implicated in hypertension, suggesting that inhibition of NOX1 may be a promising therapeutic approach. NOX2 and NOX4 oxidases are of specific interest due to their role in producing extra- and intracellular hydrogen peroxide (H₂O₂). NOX4-released hydrogen peroxide activates NOX2, which in turn stimulates the release of mitochondrial ROS resulting in ROS-induced ROS release (RIRR) signaling. Increased ROS production from NOX5 contributes to atherosclerosis. This review aims to summarize recent findings on NOX enzymes and clinical trials inhibiting NADPH oxidases to target pathologies including diabetes, idiopathic pulmonary fibrosis (IPF), and primary biliary cholangitis (PBC).

Resistance Exercise Training Improves Metabolic and Inflammatory Control in Adipose and Muscle Tissues in Mice Fed a High-Fat Diet

This study investigates whether ladder climbing (LC), as a model of resistance exercise, can reverse whole-body and skeletal muscle deleterious metabolic and inflammatory effects of high-fat (HF) diet-induced obesity in mice. To accomplish this, Swiss mice were fed for 17 weeks either standard chow (SC) or an HF diet and then randomly assigned to remain sedentary or to undergo 8 weeks of LC training with progressive increases in resistance weight. Prior to beginning the exercise intervention, HF-fed animals displayed a 47% increase in body weight (BW) and impaired ability to clear blood glucose during an insulin tolerance test (ITT) when compared to SC animals. However, 8 weeks of LC significantly reduced BW, adipocyte size, as well as glycemia under fasting and during the ITT in HF-fed rats. LC also increased the phosphorylation of Akt_Ser473 and AMPK_Thr172 and reduced tumor necrosis factor-alpha (TNF-α) and interleukin 1 beta (IL1-β) contents in the quadriceps muscles of HF-fed mice. Additionally, LC reduced the gene expression of inflammatory markers and attenuated HF-diet-induced NADPH oxidase subunit gp91phox in skeletal muscles. LC training was effective in reducing adiposity and the content of inflammatory mediators in skeletal muscle and improved whole-body glycemic control in mice fed an HF diet.

Apocynin Attenuates Diabetes-Induced Skeletal Muscle Dysfunction by Mitigating ROS Generation and Boosting Antioxidant Defenses in Fast-Twitch and Slow-Twitch Muscles

In response to diabetes mellitus, skeletal muscle is negatively affected, as is evident by reduced contractile force production, increased muscle fatigability, and increased levels of oxidative stress biomarkers. Apocynin is a widely used NADPH oxidase inhibitor, with antioxidant and anti-inflammatory potential. It has been effective for amelioration of a variety of disorders, including diabetic complications. Therefore, the present study was conducted to evaluate the effects and action mechanisms of apocynin in slow- and fast-twitch diabetic rat muscles. Male Wistar rats were rendered diabetic by applying intraperitoneally a single dose of streptozotocin (45 mg/kg). Apocynin treatment (3 mg/kg/day) was administered over 8 weeks. Fasting blood glucose (FBG), insulin tolerance and body weight gain were measured. Both slow (soleus) and fast (extensor digitorum longus, EDL) skeletal muscles were used for muscle function evaluation, oxidative stress markers, and evaluating gene expression using qRT-PCR. Treatment with apocynin significantly reduced FBG levels and enhanced insulin tolerance. Apocynin also prevented muscle contractile dysfunction in EDL muscle but had no significant effect on this parameter in soleus muscles. However, in both types of muscles, apocynin mitigated the oxidative stress by decreasing ROS levels and increasing total glutathione levels and redox state. Concomitantly, apocynin also statistically enhanced Nrf-2 and GLU4 mRNA expression and downregulated NOX2, NOX4, and NF-κB mRNA. Collectively, apocynin exhibits properties myoprotective in diabetic animals. These findings indicate that apocynin predominantly acts as an antioxidant in fast-twitch and slow-twitch muscles but has differential impact on contractile function.

Molecular Mechanism of Lipotoxicity as an Interesting Aspect in the Development of Pathological States-Current View of Knowledge

Free fatty acids (FFAs) play numerous vital roles in the organism, such as contribution to energy generation and reserve, serving as an essential component of the cell membrane, or as ligands for nuclear receptors. However, the disturbance in fatty acid homeostasis, such as inefficient metabolism or intensified release from the site of storage, may result in increased serum FFA levels and eventually result in ectopic fat deposition, which is unfavorable for the organism. The cells are adjusted for the accumulation of FFA to a limited extent and so prolonged exposure to elevated FFA levels results in deleterious effects referred to as lipotoxicity. Lipotoxicity contributes to the development of diseases such as insulin resistance, diabetes, cardiovascular diseases, metabolic syndrome, and inflammation. The nonobvious organs recognized as the main lipotoxic goal of action are the pancreas, liver, skeletal muscles, cardiac muscle, and kidneys. However, lipotoxic effects to a significant extent are not organ-specific but affect fundamental cellular processes occurring in most cells. Therefore, the wider perception of cellular lipotoxic mechanisms and their interrelation may be beneficial for a better understanding of various diseases’ pathogenesis and seeking new pharmacological treatment approaches.

New Intracellular Peptide Derived from Hemoglobin Alpha Chain Induces Glucose Uptake and Reduces Blood Glycemia

Intracellular peptides were shown to derive from proteasomal degradation of proteins from mammalian and yeast cells, being suggested to play distinctive roles both inside and outside these cells. Here, the role of intracellular peptides previously identified from skeletal muscle and adipose tissues of C57BL6/N wild type (WT) and neurolysin knockout mice were investigated. In differentiated C2C12 mouse skeletal muscle cells, some of these intracellular peptides like insulin activated the expression of several genes related to muscle contraction and gluconeogenesis. One of these peptides, LASVSTVLTSKYR (Ric4; 600 µg/kg), administrated either intraperitoneally or orally in WT mice, decreased glycemia. Neither insulin (10 nM) nor Ric4 (100 µM) induced glucose uptake in adipose tissue explants obtained from conditional knockout mice depleted of insulin receptor. Ric4 (100 µM) similarly to insulin (100 nM) induced Glut4 translocation to the plasma membrane of C2C12 differentiated cells, and increased GLUT4 mRNA levels in epididymal adipose tissue of WT mice. Ric4 (100 µM) increased both Erk and Akt phosphorylation in C2C12, as well as in epididymal adipose tissue from WT mice; Erk, but not Akt phosphorylation was activated by Ric4 in tibial skeletal muscle from WT mice. Ric4 is rapidly degraded in vitro by WT liver and kidney crude extracts, such a response that is largely reduced by structural modifications such as N-terminal acetylation, C-terminal amidation, and substitution of Leu8 for DLeu8 (Ac-LASVSTV[DLeu]TSKYR-NH2; Ric4-16). Ric4-16, among several Ric4 derivatives, efficiently induced glucose uptake in differentiated C2C12 cells. Among six Ric4-derivatives evaluated in vivo, Ac-LASVSTVLTSKYR-NH2 (Ric4-2; 600 µg/kg) and Ac-LASVSTV[DLeu]TSKYR (Ric4-15; 600 µg/kg) administrated orally efficiently reduced glycemia in a glucose tolerance test in WT mice. The potential clinical application of Ric4 and Ric4-derivatives deserves further attention.

Nicotinamide Adenine Dinucleotide Phosphate Oxidases in Glucose Homeostasis and Diabetes-Related Endothelial Cell Dysfunction

Oxidative stress within the vascular endothelium, due to excess generation of reactive oxygen species (ROS), is thought to be fundamental to the initiation and progression of the cardiovascular complications of type 2 diabetes mellitus. The term ROS encompasses a variety of chemical species including superoxide anion (O₂^•-), hydroxyl radical (OH^-) and hydrogen peroxide (H₂O₂). While constitutive generation of low concentrations of ROS are indispensable for normal cellular function, excess O₂^•- can result in irreversible tissue damage. Excess ROS generation is catalysed by xanthine oxidase, uncoupled nitric oxide synthases, the mitochondrial electron transport chain and the nicotinamide adenine dinucleotide phosphate (NADPH) oxidases. Amongst enzymatic sources of O₂^•- the Nox2 isoform of NADPH oxidase is thought to be critical to the oxidative stress found in type 2 diabetes mellitus. In contrast, the transcriptionally regulated Nox4 isoform, which generates H₂O₂, may fulfil a protective role and contribute to normal glucose homeostasis. This review describes the key roles of Nox2 and Nox4, as well as Nox1 and Nox5, in glucose homeostasis, endothelial function and oxidative stress, with a key focus on how they are regulated in health, and dysregulated in type 2 diabetes mellitus.

Exercise-Stimulated ROS Sensitive Signaling Pathways in Skeletal Muscle

Physical exercise represents a major challenge to whole-body homeostasis, provoking acute and adaptative responses at the cellular and systemic levels. Different sources of reactive oxygen species (ROS) have been described in skeletal muscle (e.g., NADPH oxidases, xanthine oxidase, and mitochondria) and are closely related to the physiological changes induced by physical exercise through the modulation of several signaling pathways. Many signaling pathways that are regulated by exercise-induced ROS generation, such as adenosine monophosphate-activated protein kinase (AMPK), mitogen activated protein kinase (MAPK), nuclear respiratory factor2 (NRF2), and PGC-1α are involved in skeletal muscle responses to physical exercise, such as increased glucose uptake, mitochondriogenesis, and hypertrophy, among others. Most of these adaptations are blunted by antioxidants, revealing the crucial role played by ROS during and after physical exercise. When ROS generation is either insufficient or exacerbated, ROS-mediated signaling is disrupted, as well as physical exercise adaptations. Thus, an understanding the limit between "ROS that can promote beneficial effects" and "ROS that can promote harmful effects" is a challenging question in exercise biology. The identification of new mediators that cause reductive stress and thereby disrupt exercise-stimulated ROS signaling is a trending on this topic and are covered in this current review.

Nox2 Inhibition Regulates Stress Response and Mitigates Skeletal Muscle Fiber Atrophy during Simulated Microgravity

Insufficient stress response and elevated oxidative stress can contribute to skeletal muscle atrophy during mechanical unloading (e.g., spaceflight and bedrest). Perturbations in heat shock proteins (e.g., HSP70), antioxidant enzymes, and sarcolemmal neuronal nitric oxidase synthase (nNOS) have been linked to unloading-induced atrophy. We recently discovered that the sarcolemmal NADPH oxidase-2 complex (Nox2) is elevated during unloading, downstream of angiotensin II receptor 1, and concomitant with atrophy. Here, we hypothesized that peptidyl inhibition of Nox2 would attenuate disruption of HSP70, MnSOD, and sarcolemmal nNOS during unloading, and thus muscle fiber atrophy. F344 rats were divided into control (CON), hindlimb unloaded (HU), and hindlimb unloaded +7.5 mg/kg/day gp91ds-tat (HUG) groups. Unloading-induced elevation of the Nox2 subunit p67phox-positive staining was mitigated by gp91ds-tat. HSP70 protein abundance was significantly lower in HU muscles, but not HUG. MnSOD decreased with unloading; however, MnSOD was not rescued by gp91ds-tat. In contrast, Nox2 inhibition protected against unloading suppression of the antioxidant transcription factor Nrf2. nNOS bioactivity was reduced by HU, an effect abrogated by Nox2 inhibition. Unloading-induced soleus fiber atrophy was significantly attenuated by gp91ds-tat. These data establish a causal role for Nox2 in unloading-induced muscle atrophy, linked to preservation of HSP70, Nrf2, and sarcolemmal nNOS.

Ginsenoside Rc Ameliorates Endothelial Insulin Resistance via Upregulation of Angiotensin-Converting Enzyme 2

Type 2 diabetes mellitus (T2DM) is a major health concern which may cause cardiovascular complications. Insulin resistance (IR), regarded as a hallmark of T2DM, is characterized by endothelial dysfunction. Ginsenoside Rc is one of the main protopanaxadiol-type saponins with relatively less research on it. Despite researches confirming the potent anti-inflammatory and antioxidant activities of ginsenoside Rc, the potential benefits of ginsenoside Rc against vascular complications have not been explored. In the present study, we investigated the effects of ginsenoside Rc on endothelial IR and endothelial dysfunction with its underlying mechanisms using high glucose- (HG-) cultured human umbilical vein endothelial cells (HUVECs) in vitro and a type 2 diabetic model of db/db mice in vivo. The results showed that ginsenoside Rc corrected the imbalance of vasomotor factors, reduced the production of Ang (angiotensin) II, and activated angiotensin-converting enzyme 2 (ACE2)/Ang-(1–7)/Mas axis in HG-treated HUVECs. Besides, ginsenoside Rc improved the impaired insulin signaling pathway and repressed oxidative stress and inflammatory pathways which constitute key factors leading to IR. Interestingly, the effects of ginsenoside Rc on HG-induced HUVECs were abolished by the selective ACE2 inhibitor MLN-4760. Furthermore, ginsenoside Rc exhibited anti-inflammatory as well as antioxidant properties and ameliorated endothelial dysfunction via upregulation of ACE2 in db/db mice, which were confirmed by the application of MLN-4760. In conclusion, our findings reveal a novel action of ginsenoside Rc and demonstrate that ginsenoside Rc ameliorated endothelial IR and endothelial dysfunction, at least in part, via upregulation of ACE2 and holds promise for the treatment of diabetic vascular complications.

Early high-fat feeding improves histone modifications of skeletal muscle at middle-age in mice

The purpose of the present study was to investigate how the effects of high-fat diet feeding on the skeletal muscle persisted during aging using mice. Post-weaned male mice were fed a high-fat diet between 1- and 3-mo-old followed by return to supply a normal diet until 13-mo-old. Monthly physical tests demonstrated that age-related glucose intolerance that was generally developed after 10-mo-old in the control mice was significantly improved in mice fed a high-fat diet. Interestingly, mRNA expressions of Pdk4, Ucp3, and Zmynd17 were up-regulated by high-fat feeding and persisted in the tibialis anterior muscle until 13-mo-old. At Pdk4 and Ucp3 loci, enhanced distributions of active histone modifications were noted in the high-fat-fed mice at 13-mo-old. In contrast, age-related accumulation of histone variant H3.3 at these loci was suppressed. These results indicated that epigenetic modifications caused by early nutrition mediated the changes in skeletal muscle gene expression during aging.

Role of Skeletal Muscle in Insulin Resistance and Glucose Uptake

The skeletal muscle is the largest organ in the body, by mass. It is also the regulator of glucose homeostasis, responsible for 80% of postprandial glucose uptake from the circulation. Skeletal muscle is essential for metabolism, both for its role in glucose uptake and its importance in exercise and metabolic disease. In this article, we give an overview of the importance of skeletal muscle in metabolism, describing its role in glucose uptake and the diseases that are associated with skeletal muscle metabolic dysregulation. We focus on the role of skeletal muscle in peripheral insulin resistance and the potential for skeletal muscle-targeted therapeutics to combat insulin resistance and diabetes, as well as other metabolic diseases like aging and obesity. In particular, we outline the possibilities and pitfalls of the quest for exercise mimetics, which are intended to target the molecular mechanisms underlying the beneficial effects of exercise on metabolic disease. We also provide a description of the molecular mechanisms that regulate skeletal muscle glucose uptake, including a focus on the SNARE proteins, which are essential regulators of glucose transport into the skeletal muscle. © 2020 American Physiological Society. Compr Physiol 10:785-809, 2020.

Nox2 dependent redox-regulation of Akt and ERK1/2 to promote left ventricular hypertrophy in dietary obesity of mice

BACKGROUND

A Nox2 containing NADPH oxidase (Nox2) is involved in the global oxidative stress found in dietary obesity and metabolic disorders. However, the effects of high fat diet (HFD) on cardiac Nox2 activation and signaling in left ventricular hypertrophy (LVH) remain unknown.

METHODS

Left ventricular (LV) tissues isolated from C57BL/6J wild-type (WT) and Nox2 knockout (Nox2KO) mice (11 months old, n = 6 per group) after 4 months of HFD treatment were used. Cardiomyocyte sizes were measured digitally on LV cross-sections. The levels of cardiac reactive oxygen species (ROS) production was determined using lucigenin-chemiluminescence and in situ dihydroethidium (DHE) fluorescence. The levels of Nox subunit expression and redox signaling were examined by immunoblotting and immunofluorescence.

RESULTS

In comparison to WT normal chow diet control hearts, WT HFD hearts had 1.8-fold increases in cardiomyocyte size, a sign of cardiac hypertrophy, and this was accompanied with ≥2-fold increase in the levels of ROS production, Nox2 expression and the phosphorylation of Akt and ERK1/2. Increased ROS production measured in HFD heart homogenates was inhibited to control levels by Tiron (a cell membrane permeable O₂^•-scavenger), diphenyleneiodonium (DPI, a flavohaemoprotein inhibitor) and Nox2 ds-tat (a Nox2 assembly inhibitor). However, all of these abnormalities were significantly reduced or absent in Nox2KO hearts under the same HFD.

CONCLUSIONS

Nox2 activation in response to dietary obesity and metabolic disorders plays a key role in cardiac oxidative stress, aberrant redox signaling and cardiomyocyte hypertrophy. Knockout of Nox2 protects hearts from oxidative damage associated with obesity and metabolic disorders.

NADPH Oxidase 2 Mediates Myocardial Oxygen Wasting in Obesity

Obesity and diabetes are independent risk factors for cardiovascular diseases, and they are associated with the development of a specific cardiomyopathy with elevated myocardial oxygen consumption (MVO₂) and impaired cardiac efficiency. Although the pathophysiology of this cardiomyopathy is multifactorial and complex, reactive oxygen species (ROS) may play an important role. One of the major ROS-generating enzymes in the cardiomyocytes is nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 (NOX2), and many potential systemic activators of NOX2 are elevated in obesity and diabetes. We hypothesized that NOX2 activity would influence cardiac energetics and/or the progression of ventricular dysfunction following obesity. Myocardial ROS content and mechanoenergetics were measured in the hearts from diet-induced-obese wild type (DIO_WT) and global NOK2 knock-out mice (DIO_KO) and in diet-induced obese C57BL/6J mice given normal water (DIO) or water supplemented with the NOX2-inhibitor apocynin (DIO_APO). Mitochondrial function and ROS production were also assessed in DIO and DIO_APO mice. This study demonstrated that ablation and pharmacological inhibition of NOX2 both improved mechanical efficiency and reduced MVO₂ for non-mechanical cardiac work. Mitochondrial ROS production was also reduced following NOX2 inhibition, while cardiac mitochondrial function was not markedly altered by apocynin-treatment. Therefore, these results indicate a link between obesity-induced myocardial oxygen wasting, NOX2 activation, and mitochondrial ROS.

Role of neutrophils in ischemic heart failure

Amplified innate leukocytes (neutrophils and monocytes/macrophages) are associated with advanced ischemic and non-ischemic heart failure (HF). Intensified neutrophilic leukocytosis (neutrophilia) and sustained activation of neutrophils is the predominant factor that determines over activated inflammation in acute HF and the outcome of long-term chronic HF. After heart attack, the first wave of innate responsive and short-lived neutrophils is essential for the initiation of inflammation, resolution of inflammation, and cardiac repair, however uncontrolled and long-term activation of neutrophils leads to collateral damage of myocardium. In the presented review, we highlighted the interactive and integrative role of neutrophil phenotypes in cellular and molecular events of ischemic HF. In addition, we discussed the current, nonimmune, immune, and novel paradigms of neutrophils in HF associated with differential factors with a specific interest in non-resolving inflammation and resolution physiology.

The Emerging Roles of Nicotinamide Adenine Dinucleotide Phosphate Oxidase 2 in Skeletal Muscle Redox Signaling and Metabolism

Significance: Skeletal muscle is a crucial tissue to whole-body locomotion and metabolic health. Reactive oxygen species (ROS) have emerged as intracellular messengers participating in both physiological and pathological adaptations in skeletal muscle. A complex interplay between ROS-producing enzymes and antioxidant networks exists in different subcellular compartments of mature skeletal muscle. Recent evidence suggests that nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs) are a major source of contraction- and insulin-stimulated oxidants production, but they may paradoxically also contribute to muscle insulin resistance and atrophy. Recent Advances: Pharmacological and molecular biological tools, including redox-sensitive probes and transgenic mouse models, have generated novel insights into compartmentalized redox signaling and suggested that NOX2 contributes to redox control of skeletal muscle metabolism. Critical Issues: Major outstanding questions in skeletal muscle include where NOX2 activation occurs under different conditions in health and disease, how NOX2 activation is regulated, how superoxide/hydrogen peroxide generated by NOX2 reaches the cytosol, what the signaling mediators are downstream of NOX2, and the role of NOX2 for different physiological and pathophysiological processes. Future Directions: Future research should utilize and expand the current redox-signaling toolbox to clarify the NOX2-dependent mechanisms in skeletal muscle and determine whether the proposed functions of NOX2 in cells and animal models are conserved into humans.

Mechanistic Links Between Obesity, Diabetes, and Blood Pressure: Role of Perivascular Adipose Tissue

Obesity is increasingly prevalent and is associated with substantial cardiovascular risk. Adipose tissue distribution and morphology play a key role in determining the degree of adverse effects, and a key factor in the disease process appears to be the inflammatory cell population in adipose tissue. Healthy adipose tissue secretes a number of vasoactive adipokines and anti-inflammatory cytokines, and changes to this secretory profile will contribute to pathogenesis in obesity. In this review, we discuss the links between adipokine dysregulation and the development of hypertension and diabetes and explore the potential for manipulating adipose tissue morphology and its immune cell population to improve cardiovascular health in obesity.

The Role of NADPH Oxidases in the Etiology of Obesity and Metabolic Syndrome: Contribution of Individual Isoforms and Cell Biology

Significance: Highly prevalent in Western cultures, obesity, metabolic syndrome, and diabetes increase the risk of cardiovascular morbidity and mortality and cost health care systems billions of dollars annually. At the cellular level, obesity, metabolic syndrome, and diabetes are associated with increased production of reactive oxygen species (ROS). Increased levels of ROS production in key organ systems such as adipose tissue, skeletal muscle, and the vasculature cause disruption of tissue homeostasis, leading to increased morbidity and risk of mortality. More specifically, growing evidence implicates the nicotinamide adenine dinucleotide phosphate oxidase (NOX) enzymes in these pathologies through impairment of insulin signaling, inflammation, and vascular dysfunction. The NOX family of enzymes is a major driver of redox signaling through its production of superoxide anion, hydrogen peroxide, and attendant downstream metabolites acting on redox-sensitive signaling molecules. Recent Advances: The primary goal of this review is to highlight recent advances and survey our present understanding of cell-specific NOX enzyme contributions to metabolic diseases. Critical Issues: However, due to the short half-lives of individual ROS and/or cellular defense systems, radii of ROS diffusion are commonly short, often restricting redox signaling and oxidant stress to localized events. Thus, special emphasis should be placed on cell type and subcellular location of NOX enzymes to better understand their role in the pathophysiology of metabolic diseases. Future Directions: We discuss the targeting of NOX enzymes as potential therapy and bring to light potential emerging areas of NOX research, microparticles and epigenetics, in the context of metabolic disease.

Diabetogenic effect of pravastatin is associated with insulin resistance and myotoxicity in hypercholesterolemic mice

Background

HMG-CoA reductase inhibitors (statins) are cholesterol-lowering drugs widely used to treat hypercholesterolemia and prevent cardiovascular disease. Statins are generally well tolerated, but adverse reactions may occur, particularly myopathy and new onset of diabetes. The exact mechanism of statin-induced myopathy and diabetes has not been fully elucidated. We have previously shown that treatment of hypercholesterolemic (LDLr^−/−) mice with pravastatin for 2 months decreased pancreatic islet insulin secretion and increased oxidative stress and cell death, but no glucose intolerance was observed. The purpose of the current work was to study long-term pravastatin effects on glucose homeostasis, insulin sensitivity, muscle protein turnover and cell viability.

Methods

LDLr^−/− mice were treated with pravastatin for 3, 6 and 10 months. Glucose tolerance, insulin resistance and glucose-stimulated insulin secretion were evaluated. The rates of protein synthesis and degradation were determined in gastrocnemius muscle after 10 months of treatment. Insulin signalling, oxidative stress and cell death were analysed in vitro using C2C12 myotubes.

Results

After 6 and 10 months of treatment, these mice became glucose intolerant, and after 10 months, they exhibited marked insulin resistance. Reduced islet glucose-stimulated insulin secretion was observed after the 3rd month of treatment. Mice treated for 10 months showed significantly decreased body weight and increased muscle protein degradation. In addition, muscle chymotrypsin-like proteasomal activity and lysosomal cathepsin were markedly elevated. C2C12 myotubes exposed to increasing concentrations of pravastatin presented dose-dependent impairment of insulin-induced Akt phosphorylation, increased apoptotic markers (Bax protein and cleaved caspase-3) and augmented superoxide anion production.

Conclusions

In addition to reduced insulin secretion, long-term pravastatin treatment induces insulin resistance and muscle wasting. These results suggest that the diabetogenic effect of statins is linked to the appearance of myotoxicity induced by oxidative stress, impaired insulin signalling, proteolysis and apoptosis.

Oxidative stress induced by palmitic acid modulates KCa2.3 channels in vascular endothelium

Elevated plasma free fatty acids level has been implicated in the development of insulin resistance, inflammation, and endothelial dysfunction in diabetic and nondiabetic individuals. However, the underlying mechanisms still remain to be defined. Herein, we investigated the effect of palmitic acid (PA), the most abundant saturated fatty acid in the human body, on small-conductance Ca²⁺-activated potassium channels (K_Ca2.3)-mediated relaxation in rodent resistance arteries and the underlying molecular mechanism. The effect of PA on K_Ca2.3 in endothelium was evaluated using real-time PCR, Western blotting, whole-cell patch voltage-clamp, wire and pressure myograph system, and reactive oxygen species (ROS) were measured by using dihydroethidium and 2', 7'-dichlorofluorescein diacetate. K_Ca2.3-mediated vasodilatation responses to acetylcholine and NS309 (agonist of K_Ca2.3 and K_Ca3.1) were impaired by incubation of normal mesenteric arteries with 100 μM PA for 24 h. In cultured human umbilical vein endothelial cells (HUVECs), PA decreased K_Ca2.3 current and expression at mRNA and protein levels. Incubation with the NADPH oxidase (Nox) inhibitor dibenziodolium (DPI) partly inhibited the PA-induced ROS production and restored K_Ca2.3 expression. Inhibition of either p38-MAPK or NF-κB using specific inhibitors (SB203580, SB202190 or Bay11-7082, pyrrolidinedithiocarbamate) attenuated PA-induced downregulation of K_Ca2.3 and inhibition of p38-MAPK also attenuated PA-induced phosphorylation of NF-κB p65. Furthermore, DPI reversed the increment of phospho-p38-MAPK by PA. These results demonstrated that PA downregulated K_Ca2.3 expressions via Nox/ROS/p38-MAPK/NF-κB signaling leading to endothelial vasodilatory dysfunction.

Endothelial Dysfunction: Is There a Hyperglycemia-Induced Imbalance of NOX and NOS?

NADPH oxidases (NOX) are enzyme complexes that have received much attention as key molecules in the development of vascular dysfunction. NOX have the primary function of generating reactive oxygen species (ROS), and are considered the main source of ROS production in endothelial cells. The endothelium is a thin monolayer that lines the inner surface of blood vessels, acting as a secretory organ to maintain homeostasis of blood flow. The enzymatic production of nitric oxide (NO) by endothelial NO synthase (eNOS) is critical in mediating endothelial function, and oxidative stress can cause dysregulation of eNOS and endothelial dysfunction. Insulin is a stimulus for increases in blood flow and endothelium-dependent vasodilation. However, cardiovascular disease and type 2 diabetes are characterized by poor control of the endothelial cell redox environment, with a shift toward overproduction of ROS by NOX. Studies in models of type 2 diabetes demonstrate that aberrant NOX activation contributes to uncoupling of eNOS and endothelial dysfunction. It is well-established that endothelial dysfunction precedes the onset of cardiovascular disease, therefore NOX are important molecular links between type 2 diabetes and vascular complications. The aim of the current review is to describe the normal, healthy physiological mechanisms involved in endothelial function, and highlight the central role of NOX in mediating endothelial dysfunction when glucose homeostasis is impaired.

TFAM Enhances Fat Oxidation and Attenuates High-Fat Diet-Induced Insulin Resistance in Skeletal Muscle

Diet-induced insulin resistance (IR) adversely affects human health and life span. We show that muscle-specific overexpression of human mitochondrial transcription factor A (TFAM) attenuates high-fat diet (HFD)-induced fat gain and IR in mice in conjunction with increased energy expenditure and reduced oxidative stress. These TFAM effects on muscle are shown to be exerted by molecular changes that are beyond its direct effect on mitochondrial DNA replication and transcription. TFAM augmented the muscle tricarboxylic acid cycle and citrate synthase facilitating energy expenditure. TFAM enhanced muscle glucose uptake despite increased fatty acid (FA) oxidation in concert with higher β-oxidation capacity to reduce the accumulation of IR-related carnitines and ceramides. TFAM also increased pAMPK expression, explaining enhanced PGC1α and PPARβ, and reversing HFD-induced GLUT4 and pAKT reductions. TFAM-induced mild uncoupling is shown to protect mitochondrial membrane potential against FA-induced uncontrolled depolarization. These coordinated changes conferred protection to TFAM mice against HFD-induced obesity and IR while reducing oxidative stress with potential translational opportunities.

Regulation of mitochondrial quality following repeated bouts of hindlimb unloading

Muscle disuse impairs muscle quality and is associated with increased mortality. Little is known regarding additive effects of multiple bouts of disuse, which is a common occurrence in patients experiencing multiple surgeries. Mitochondrial quality is vital to muscle health and quality; however, to date mitochondrial quality control has not been investigated following multiple bouts of disuse. Therefore, the purpose of this study was to investigate mitochondrial quality controllers during multiple bouts of disuse by hindlimb unloading. Male rats (n ∼ 8/group) were assigned to the following groups: hindlimb unloading for 28 days, hindlimb unloading with 56 days of reloading, 2 bouts of hindlimb unloading separated by a recovery phase of 56 days of reloading, 2 bouts of hindlimb unloading and recovery after each disuse, or control animals with no unloading. At designated time points, tissues were collected for messenger RNA and protein analysis of mitochondrial quality. Measures of mitochondrial biogenesis, such as proliferator-activated receptor gamma coactivator 1 alpha, decreased 30%-40% with unloading with no differences noted between unloading conditions. Measures of mitochondrial translation were 40%-50% lower in unloading conditions, with no differences noted between bouts of unloading. Measures of mitophagy were 40%-50% lower with reloading, with no differences noted between reloading conditions. In conclusion, disuse causes alterations in measures of mitochondrial quality; however, multiple bouts of disuse does not appear to have additive effects. Novelty Disuse atrophy causes multiple alterations to mitochondrial quality control. With sufficient recovery most detriments to mitochondrial quality control are fixed. In general, multiple bouts of disuse do not produce additive effects.

Redox Regulation of Metabolic Syndrome: Recent Developments in Skeletal Muscle Insulin Resistance and Non-alcoholic Fatty Liver Disease (NAFLD)

Several new discoveries over the past decade have shown that metabolic syndrome, a cluster of metabolic disorders, including increased visceral obesity, hyperglycemia, hypertension, dyslipidemia and low HDL-cholesterol, is commonly associated with skeletal muscle insulin resistance. More recently, non-alcoholic fatty liver disease (NAFLD) was recognized as an additional condition that is strongly associated with features of metabolic syndrome. While the pathogenesis of skeletal muscle insulin resistance and fatty liver is multifactorial, the role of dysregulated redox signaling has been clearly demonstrated in the regulation of skeletal muscle insulin resistance and NAFLD. In this review, we aim to provide recent updates on redox regulation with respect to (a) pro-oxidant enzymes (e.g. NAPDH oxidase and xanthine oxidase); (b) mitochondrial dysfunction; (c) endoplasmic reticulum (ER) stress; (d) iron metabolism derangements; and (e) gut-skeletal muscle or gut-liver connection in the development of skeletal muscle insulin resistance and NAFLD. Furthermore, we discuss promising new therapeutic strategies targeting redox regulation currently under investigation for the treatment of skeletal muscle insulin resistance and NAFLD.

Metabolic Regulation of Methionine Restriction in Diabetes

Although the effects of dietary methionine restriction have been investigated in the physiology of aging and diseases related to oxidative stress, the relationship between methionine restriction (MR) and the development of metabolic disorders has not been explored extensively. This review summarizes studies of the possible involvement of dietary methionine restriction in improving insulin resistance, glucose homeostasis, oxidative stress, lipid metabolism, the pentose phosphate pathway (PPP), and inflammation, with an emphasis on the fibroblast growth factor 21 and protein phosphatase 2A signals and autophagy in diabetes. Diets deficient in methionine may be a useful nutritional strategy in patients with diabetes.

Attenuation of oxidative stress-induced lesions in skeletal muscle in a mouse model of obesity-independent hyperlipidaemia and atherosclerosis through the inhibition of Nox2 activity

Obesity leading to hyperlipidaemia and atherosclerosis is recognised to induce morphological and metabolic changes in many tissues. However, hyperlipidaemia can occur in the absence of obesity. The impact of the latter scenario on skeletal muscle and liver is not understood sufficiently. In this regard, we used the Apolipoprotein E-deficient (ApoE^-/-) mouse model, an established model of hyperlipidaemia and atherosclerosis, that does not become obese when subjected to a high-fat diet, to determine the impact of Western-type diet (WD) and ApoE deficiency on skeletal muscle morphological, metabolic and biochemical properties. To establish the potential of therapeutic targets, we further examined the impact of Nox2 pharmacological inhibition on skeletal muscle redox biology. We found ectopic lipid accumulation in skeletal muscle and the liver, and altered skeletal muscle morphology and intramuscular triacylglycerol fatty acid composition. WD and ApoE deficiency had a detrimental impact in muscle metabolome, followed by perturbed gene expression for fatty acid uptake and oxidation. Importantly, there was enhanced oxidative stress in the skeletal muscle and development of liver steatosis, inflammation and oxidative protein modifications. Pharmacological inhibition of Nox2 decreased reactive oxygen species production and protein oxidative modifications in the muscle of ApoE^-/- mice subjected to a Western-type diet. This study provides key evidence to better understand the pathophysiology of skeletal muscle in the context of hyperlipidaemia and atherosclerosis and identifies Nox2 as a potential target for attenuating oxidative stress in skeletal muscle in a mouse model of obesity-independent hyperlipidaemia.

Loss of peroxiredoxin-2 exacerbates eccentric contraction-induced force loss in dystrophin-deficient muscle

Force loss in skeletal muscle exposed to eccentric contraction is often attributed to injury. We show that EDL muscles from dystrophin-deficient mdx mice recover 65% of lost force within 120 min of eccentric contraction and exhibit minimal force loss when the interval between contractions is increased from 3 to 30 min. A proteomic screen of mdx muscle identified an 80% reduction in the antioxidant peroxiredoxin-2, likely due to proteolytic degradation following hyperoxidation by NADPH Oxidase 2. Eccentric contraction-induced force loss in mdx muscle was exacerbated by peroxiredoxin-2 ablation, and improved by peroxiredoxin-2 overexpression or myoglobin knockout. Finally, overexpression of γcyto- or βcyto-actin protects mdx muscle from eccentric contraction-induced force loss by blocking NADPH Oxidase 2 through a mechanism dependent on cysteine 272 unique to cytoplasmic actins. Our data suggest that eccentric contraction-induced force loss may function as an adaptive circuit breaker that protects mdx muscle from injurious contractions.

Role of ABCA1 on membrane cholesterol content, insulin-dependent Akt phosphorylation and glucose uptake in adult skeletal muscle fibers from mice

The ATP-binding cassette transporter A1 (ABCA1) promotes cellular cholesterol efflux, leading to cholesterol binding to the extracellular lipid-free apolipoprotein A-I. ABCA1 regulates lipid content, glucose tolerance and insulin sensitivity in adipose tissue. In skeletal muscle, most GLUT4-mediated glucose transport occurs in the transverse tubule, a system composed by specialized cholesterol-enriched invaginations of the plasma membrane. We have reported that insulin resistant mice have higher cholesterol levels in transverse tubule from adult skeletal muscle. These high levels correlate with decreased GLUT4 trafficking and glucose uptake; however, the role of ABCA1 on skeletal muscle insulin-dependent glucose metabolism remains largely unexplored. Here, we evaluated the functional role of the ABCA1 on insulin-dependent signaling pathways, glucose uptake and cellular cholesterol content in adult skeletal muscle. Male mice were fed for 8 weeks with normal chow diet (NCD) or high fat diet (HFD). Compared to NCD-fed mice, ABCA1 mRNA levels and protein content were lower in muscle homogenates from HFD-fed mice. In Flexor digitorum brevis muscle from NCD-fed mice, shABCA1-RFP in vivo electroporation resulted in 65% reduction of ABCA1 protein content, 1.6-fold increased fiber cholesterol levels, 74% reduction in insulin-dependent Akt (Ser⁴⁷³) phosphorylation, total suppression of insulin-dependent GLUT4 translocation and decreased 2-NBDG uptake compared to fibers electroporated with the scrambled plasmid. Pre-incubation with methyl-β cyclodextrin reestablished both GLUT4 translocation and 2-NBDG transport. Based on the present results, we suggest that decreased ABCA1 contributes to the anomalous cholesterol accumulation and decreased glucose transport displayed by skeletal muscle membranes in the insulin resistant condition.

Cellular Stresses and Stress Responses in the Pathogenesis of Insulin Resistance

Insulin resistance (IR), a key component of the metabolic syndrome, precedes the development of diabetes, cardiovascular disease, and Alzheimer's disease. Its etiological pathways are not well defined, although many contributory mechanisms have been established. This article summarizes such mechanisms into the hypothesis that factors like nutrient overload, physical inactivity, hypoxia, psychological stress, and environmental pollutants induce a network of cellular stresses, stress responses, and stress response dysregulations that jointly inhibit insulin signaling in insulin target cells including endothelial cells, hepatocytes, myocytes, hypothalamic neurons, and adipocytes. The insulin resistance-inducing cellular stresses include oxidative, nitrosative, carbonyl/electrophilic, genotoxic, and endoplasmic reticulum stresses; the stress responses include the ubiquitin-proteasome pathway, the DNA damage response, the unfolded protein response, apoptosis, inflammasome activation, and pyroptosis, while the dysregulated responses include the heat shock response, autophagy, and nuclear factor erythroid-2-related factor 2 signaling. Insulin target cells also produce metabolites that exacerbate cellular stress generation both locally and systemically, partly through recruitment and activation of myeloid cells which sustain a state of chronic inflammation. Thus, insulin resistance may be prevented or attenuated by multiple approaches targeting the different cellular stresses and stress responses.

β-Actin shows limited mobility and is required only for supraphysiological insulin-stimulated glucose transport in young adult soleus muscle

Studies in skeletal muscle cell cultures suggest that the cortical actin cytoskeleton is a major requirement for insulin-stimulated glucose transport, implicating the β-actin isoform, which in many cell types is the main actin isoform. However, it is not clear that β-actin plays such a role in mature skeletal muscle. Neither dependency of glucose transport on β-actin nor actin reorganization upon glucose transport have been tested in mature muscle. To investigate the role of β-actin in fully differentiated muscle, we performed a detailed characterization of wild type and muscle-specific β-actin knockout (KO) mice. The effects of the β-actin KO were subtle; however, we confirmed the previously reported decline in running performance of β-actin KO mice compared with wild type during repeated maximal running tests. We also found insulin-stimulated glucose transport into incubated muscles reduced in soleus but not in extensor digitorum longus muscle of young adult mice. Contraction-stimulated glucose transport trended toward the same pattern, but the glucose transport phenotype disappeared in soleus muscles from mature adult mice. No genotype-related differences were found in body composition or glucose tolerance or by indirect calorimetry measurements. To evaluate β-actin mobility in mature muscle, we electroporated green fluorescent protein (GFP)-β-actin into flexor digitorum brevis muscle fibers and measured fluorescence recovery after photobleaching. GFP-β-actin showed limited unstimulated mobility and no changes after insulin stimulation. In conclusion, β-actin is not required for glucose transport regulation in mature mouse muscle under the majority of the tested conditions. Thus, our work reveals fundamental differences in the role of the cortical β-actin cytoskeleton in mature muscle compared with cell culture.

Argirein alleviates vascular endothelial insulin resistance through suppressing the activation of Nox4-dependent O2- production in diabetic rats

BACKGROUND

Insulin resistance in endothelial cells contributes to the development of cardiovascular disease in type 2 diabetes mellitus (T2DM). Therefore, there are great potential clinical implications in developing pharmacological interventions targeting endothelial insulin resistance. Our previous studies indicated that argirein which was developed by combining rhein with L-arginine by a hydrogen bond, could substantially relieved stress related exacerbation of cardiac failure and alleviated cardiac dysfunction in T2DM, which was associated with suppressing NADPH oxidase activity. However, it is unclear whether argirein treatment attenuates the vascular lesion and dysfunction in T2DM and its underlying mechanisms.

METHODS AND RESULTS

The rat aortic endothelial cells (RAECs) were used to treat with palmitic acid (PA), a most common saturated free fatty acid, which could induce insulin resistance. It was showed that argirein increased glucose uptake and glucose transporter-4 (Glut4) expression and reversed the phosphorylation of IRS-1-ser307 and AKT-ser473, consequently resulting in the increase of the production of eNOS and NO in PA-induced RAECs. We further found that argirein blocked the Nox4-dependent superoxide (O₂^-.) generation, which regulated glucose metabolism in RAECs during PA stimulation. In vitro, argirein increased the release of endothelial NO to relieve the vasodilatory response to acetylcholine and insulin, and restored the expression of Nox4 and pIRS-1-ser307 in the aorta endothelium of high-fat diet (HFD)-fed rats following an injection of streptozocin (STZ).

CONCLUSION

These results suggested that argirein could improve endothelial insulin resistance which was attributed to inhibiting Nox4-dependent redox signaling in RAECs. These studies thus revealed the novel effect of argirein to prevent the vascular complication in T2DM.

Nicotinamide adenine dinucleotide phosphate (reduced) oxidase 2 modulates inflammatory vigor during nonalcoholic fatty liver disease progression in mice

Nonalcoholic fatty liver disease (NAFLD) represents a disease spectrum ranging from benign steatosis to life-threatening cirrhosis and hepatocellular carcinoma. Elevated levels of reactive oxygen species (ROS) and exacerbated inflammatory responses have been implicated in NAFLD progression. Nicotinamide adenine dinucleotide phosphate (reduced) oxidase 2 (NOX2; also known as gp91Phox), the main catalytic subunit of the nicotinamide adenine dinucleotide phosphate (reduced) oxidase complex, modulates ROS production, immune responsiveness, and pathogenesis of obesity-associated metabolic derangements. However, the role of NOX2 in the regulation of immune cell function and inflammatory vigor in NAFLD remains underdefined. Here, we demonstrate that obesogenic diet feeding promoted ROS production by bone marrow, white adipose tissue, and liver immune cells. Genetic ablation of NOX2 impeded immune cell ROS synthesis and was sufficient to uncouple obesity from glucose dysmetabolism and NAFLD pathogenesis. Protection from hepatocellular damage in NOX2-deficient mice correlated with reduced hepatic neutrophil, macrophage, and T-cell infiltration, diminished production of key NAFLD-driving proinflammatory cytokines, and an inherent reduction in T-cell polarization toward Th17 phenotype. Conclusion: Current findings demonstrate a crucial role of the NOX2-ROS axis in immune cell effector function and polarization and consequent NAFLD progression in obesity. Pharmacologic targeting of NOX2 function in immune cells may represent a viable approach for reducing morbidity of obesity-associated NAFLD pathogenesis. (Hepatology Communications 2018;2:546-560).

Glucose and lipid metabolism alterations in liver and adipose tissue pre-dispose p47phox knockout mice to systemic insulin resistance

Oxidative stress due to enhanced production or reduced scavenging of reactive oxygen species (ROS) has been associated with diet (dyslipidemia) induced obesity and insulin resistance (IR). The present study was undertaken to assess the role of p47^phox in IR using wild type (WT) and p47^phox-/- mice, fed with different diets (HFD, LFD or Chow). Augmented body weight, glucose intolerance and reduced insulin sensitivity were observed in p47^phox-/- mice fed with 45% HFD and 10% LFD. Further, body fat and circulating lipids were increased significantly with 5 weeks LFD feeding in p47^phox-/- mice, while parameters of energy homeostasis were reduced as compared with WT mice. LFD fed knockout (KO) mice showed an enhanced hepatic glycogenolysis, and reduced insulin signalling in liver and adipose tissue, while skeletal muscle tissue remained unaffected. A significant increase in hepatic lipids, adiposity, as well as expression of genes regulating lipid synthesis, breakdown and efflux were observed in LFD fed p47^phox-/- mice after 5 weeks. On the other hand, mice lacking p47^phox demonstrated altered glucose tolerance and tissue insulin sensitivity after 5 weeks chow feeding, while changes in body weight, respiratory exchange ratio (RER) and heat production are non-significant. Our data demonstrate that lack of p47^phox is sufficient to induce IR through altered glucose and lipid utilization by the liver and adipose tissue.

Peroxiredoxin 2 mediates insulin sensitivity of skeletal muscles through regulation of protein tyrosine phosphatase oxidation

Insulin signaling is essential for regulating glucose homeostasis. Numerous studies have demonstrated that reactive oxygen species (ROS) affect insulin signaling, and low ROS levels can act as a signal to regulate cellular function. Peroxiredoxins (Prxs) are highly abundant and widely expressed antioxidant enzymes. However, it is unclear whether antioxidant enzymes, such as Prx2, mediate insulin signaling. The aim of our study was to investigate the influence of Prx2 deficiency on insulin signaling. Our western blot results showed that Prx2 deficiency enhanced insulin signaling and increased oxidation of protein tyrosine phosphatase 1B (PTP1B) and phosphatase and tensin homologue (PTEN) in mouse embryonic fibroblasts (MEFs) treated with insulin. In addition, we assessed ROS levels with a Cytosol-HyPer H₂O₂ sensor. As a result, increased ROS levels and Akt activation were decreased by N-acetyl-cysteine (Nac), which acted as an antioxidant in Prx2-deficient MEFs. Body weight measurements and glucose tolerance test (GTT) revealed significant body weight reduction and increase in glucose clearance in Prx2^-/- mice fed a high-fat diet. Interestingly, glucose transporter type 4 (GLUT4) was significantly higher in Prx2^-/- mice than in wild-type mice according to western blotting results. Western blotting also revealed that Akt phosphorylation was higher in Prx2^-/- MEFs and muscle tissue than in wild-type. Together, our findings indicate that increased ROS due to Prx2 deficiency promotes insulin sensitivity and glucose clearance in skeletal muscles by increasing protein tyrosine phosphatase (PTPs) oxidation. These results provide novel insights into the fundamental mechanisms of insulin signaling induced by Prx2 deficiency and suggest that ROS-based therapeutic strategies can be used to suppress insulin resistance.

Pro-inflammatory hepatic macrophages generate ROS through NADPH oxidase 2 via endocytosis of monomeric TLR4-MD2 complex

Reactive oxygen species (ROS) contribute to the development of non-alcoholic fatty liver disease. ROS generation by infiltrating macrophages involves multiple mechanisms, including Toll-like receptor 4 (TLR4)-mediated NADPH oxidase (NOX) activation. Here, we show that palmitate-stimulated CD11b⁺F4/80^low hepatic infiltrating macrophages, but not CD11b⁺F4/80^high Kupffer cells, generate ROS via dynamin-mediated endocytosis of TLR4 and NOX2, independently from MyD88 and TRIF. We demonstrate that differently from LPS-mediated dimerization of the TLR4–MD2 complex, palmitate binds a monomeric TLR4–MD2 complex that triggers endocytosis, ROS generation and increases pro-interleukin-1β expression in macrophages. Palmitate-induced ROS generation in human CD68^lowCD14^high macrophages is strongly suppressed by inhibition of dynamin. Furthermore, Nox2-deficient mice are protected against high-fat diet-induced hepatic steatosis and insulin resistance. Therefore, endocytosis of TLR4 and NOX2 into macrophages might be a novel therapeutic target for non-alcoholic fatty liver disease.

Reactive species of oxygen promote the development of hepatic steatosis. Here, Kim et al. demonstrate that palmitate stimulates macrophage infiltration and increases oxidative stress during steatosis by binding to the TLR4–MD2 complex, which results in the activation of NOX2.

Insulin-dependent metabolic and inotropic responses in the heart are modulated by hydrogen peroxide from NADPH-oxidase isoforms NOX2 and NOX4

RATIONALE

Hydrogen peroxide (H2O2) is a stable reactive oxygen species (ROS) that has long been implicated in insulin signal transduction in adipocytes. However, H2O2's role in mediating insulin's effects on the heart are unknown.

OBJECTIVE

We investigated the role of H2O2 in activating insulin-dependent changes in cardiac myocyte metabolic and inotropic pathways. The sources of insulin-dependent H2O2 generation were also studied.

METHODS AND RESULTS

In addition to the canonical role of insulin in modulating cardiac metabolic pathways, we found that insulin also inhibited beta adrenergic-induced increases in cardiac contractility. Catalase and NADPH oxidase (NOX) inhibitors blunted activation of insulin-responsive kinases Akt and mTOR and attenuated beta adrenergic receptor-mediated responses. These insulin responses were lost in a mouse model of type 2 diabetes, suggesting a role for these H2O2-dependent pathways in the diabetic heart. The H2O2-sensitive fluorescent biosensor HyPer revealed rapid increases in cytosolic and caveolar H2O2 concentrations in response to insulin treatment, which were blocked by NOX inhibitors and attenuated in NOX2 KO and NOX4 KO mice. In NOX2 KO cardiac myocytes, insulin-mediated phosphorylation of Akt and mTOR was blocked, while these responses were unaffected in cardiac myocytes from NOX4 KO mice. In contrast, insulin's effects on contractility were lost in cardiac myocytes from NOX4 KO animals but were retained in NOX2 KO mice.

CONCLUSIONS

These studies identify a proximal point of bifurcation in cardiac insulin signaling through the simultaneous activation of both NOX2 and NOX4. Each NOX isoform generates H2O2 in cardiac myocytes with distinct time courses, with H2O2 derived from NOX2 augmenting Akt-dependent metabolic effects of insulin, while H2O2 from NOX4 blocks beta adrenergic increases in inotropy. These findings suggest that insulin resistance in the diabetic heart may lead to potentially deleterious potentiation of beta adrenergic responses.

The Contribution of Singlet Oxygen to Insulin Resistance

Insulin resistance contributes to the development of diabetes and cardiovascular dysfunctions. Recent studies showed that elevated singlet oxygen-mediated lipid peroxidation precedes and predicts diet-induced insulin resistance (IR), and neutrophils were suggested to be responsible for such singlet oxygen production. This review highlights literature suggesting that insulin-responsive cells such as endothelial cells, hepatocytes, adipocytes, and myocytes also produce singlet oxygen, which contributes to insulin resistance, for example, by generating bioactive aldehydes, inducing endoplasmic reticulum (ER) stress, and modifying mitochondrial DNA. In these cells, nutrient overload leads to the activation of Toll-like receptor 4 and other receptors, leading to the production of both peroxynitrite and hydrogen peroxide, which react to produce singlet oxygen. Cytochrome P450 2E1 and cytochrome c also contribute to singlet oxygen formation in the ER and mitochondria, respectively. Endothelial cell-derived singlet oxygen is suggested to mediate the formation of oxidized low-density lipoprotein which perpetuates IR, partly through neutrophil recruitment to adipose tissue. New singlet oxygen-involving pathways for the formation of IR-inducing bioactive aldehydes such as 4-hydroperoxy-(or hydroxy or oxo)-2-nonenal, malondialdehyde, and cholesterol secosterol A are proposed. Strategies against IR should target the singlet oxygen-producing pathways, singlet oxygen quenching, and singlet oxygen-induced cellular responses.

Crossroads between peripheral atherosclerosis, western-type diet and skeletal muscle pathophysiology: emphasis on apolipoprotein E deficiency and peripheral arterial disease

Atherosclerosis is a chronic inflammatory process that, in the presence of hyperlipidaemia, promotes the formation of atheromatous plaques in large vessels of the cardiovascular system. It also affects peripheral arteries with major implications for a number of other non-vascular tissues such as the skeletal muscle, the liver and the kidney. The aim of this review is to critically discuss and assimilate current knowledge on the impact of peripheral atherosclerosis and its implications on skeletal muscle homeostasis. Accumulating data suggests that manifestations of peripheral atherosclerosis in skeletal muscle originates in a combination of increased i)-oxidative stress, ii)-inflammation, iii)-mitochondrial deficits, iv)-altered myofibre morphology and fibrosis, v)-chronic ischemia followed by impaired oxygen supply, vi)-reduced capillary density, vii)- proteolysis and viii)-apoptosis. These structural, biochemical and pathophysiological alterations impact on skeletal muscle metabolic and physiologic homeostasis and its capacity to generate force, which further affects the individual's quality of life. Particular emphasis is given on two major areas representing basic and applied science respectively: a)-the abundant evidence from a well-recognised atherogenic model; the Apolipoprotein E deficient mouse and the role of a western-type diet and b)-on skeletal myopathy and oxidative stress-induced myofibre damage from human studies on peripheral arterial disease. A significant source of reactive oxygen species production and oxidative stress in cardiovascular disease is the family of NADPH oxidases that contribute to several pathologies. Finally, strategies targeting NADPH oxidases in skeletal muscle in an attempt to attenuate cellular oxidative stress are highlighted, providing a better understanding of the crossroads between peripheral atherosclerosis and skeletal muscle pathophysiology.

A high sucrose and high fat diet induced the development of insulin resistance in the skeletal muscle of Bama miniature pigs through the Akt/GLUT4 pathway

A high sucrose and high fat (HSHF) diet induces insulin resistance (IR) and increased susceptibility to type 2 diabetes mellitus (T2DM), but the underlying mechanisms are poorly characterized. This study aimed to investigate the molecular mechanisms by which the HSHF diet impairs insulin sensitivity in Bama miniature pigs (sus scrofa domesticus). Twelve Bama miniature pigs were randomly assigned to the control diet (CD) group (n=6) or the HSHF group (n=6) for 6 months. Biochemical parameters were measured. Western blot, RT-qPCR and immunohistochemistry were used to profile the changes of protein expression, mRNA expression and glucose transporter 4 (GLUT4) expression in skeletal muscle tissues, respectively. In comparison to the CD group, the homeostasis model assessment-insulin resistance (HOMA-IR) index of the HSHF group demonstrated a 2.9-fold increase, and the insulin sensitivity showed a 24.8% decrease. Compared with the CD group, p-Akt S473 decreased by approximately 59% and GLUT4 decreased by 43.8% in the skeletal muscle of the HSHF group. However, the expression of p-mTOR S2448 between the 2 groups was not significantly different (P=0.309). This study demonstrates that a 6-month HSHF diet caused IR, decreased insulin sensitivity, and reduced the expression of p-Akt S473 and GLUT4 in the skeletal muscle of Bama miniature pigs.