The role of mitochondria in the pathophysiology of skeletal muscle insulin resistance

Adaptive gene expression of alternative splicing variants of PGC-1α regulates whole-body energy metabolism

The transcriptional coactivator PGC-1α has been implicated in the regulation of multiple metabolic processes. However, the previously reported metabolic phenotypes of mice deficient in PGC-1α have been inconsistent. PGC-1α exists as multiple isoforms, including variants transcribed from an alternative first exon. We show here that alternative PGC-1α variants are the main entity that increases PGC-1α during exercise. These variants, unlike the canonical isoform of PGC-1α, are robustly upregulated in human skeletal muscle after exercise. Furthermore, the extent of this upregulation correlates with oxygen consumption. Mice lacking these variants manifest impaired energy expenditure during exercise, leading to the development of obesity and hyperinsulinemia. The alternative variants are also upregulated in brown adipose tissue in response to cold exposure, and mice lacking these variants are intolerant of a cold environment. Our findings thus indicate that an increase in PGC-1α expression, attributable mostly to upregulation of alternative variants, is pivotal for adaptive enhancement of energy expenditure and heat production and thereby essential for the regulation of whole-body energy metabolism.

The PI3K/Akt signaling axis and type 2 diabetes mellitus (T2DM): From mechanistic insights into possible therapeutic targets

Type 2 diabetes mellitus (T2DM) is an immensely debilitating chronic disease that progressively undermines the well-being of various bodily organs and, indeed, most patients succumb to the disease due to post-T2DM complications. Although there is evidence supporting the activation of the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway by insulin, which is essential in regulating glucose metabolism and insulin resistance, the significance of this pathway in T2DM has only been explored in a few studies. The current review aims to unravel the mechanisms by which different classes of PI3Ks control the metabolism of glucose; and also to discuss the original data obtained from international research laboratories on this topic. We also summarized the role of the PI3K/Akt signaling axis in target tissues spanning from the skeletal muscle to the adipose tissue and liver. Furthermore, inquiries regarding the impact of disrupting this axis on insulin function and the development of insulin resistance have been addressed. We also provide a general overview of the association of impaired PI3K/Akt signaling pathways in the pathogenesis of the most prevalent diabetes-related complications. The last section provides a special focus on the therapeutic potential of this axis by outlining the latest advances in active compounds that alleviate diabetes via modulation of the PI3K/Akt pathway. Finally, we comment on the future research aspects in which the field of T2DM therapies using PI3K modulators might be developed.

Dapagliflozin improves skeletal muscle insulin sensitivity through SIRT1 activation induced by nutrient deprivation state

Lipid peroxidation and mitochondrial damage impair insulin sensitivity in skeletal muscle. Sirtuin-1 (SIRT1) protects mitochondria and activates under energy restriction. Dapagliflozin (Dapa) is an antihyperglycaemic agent that belongs to the sodium-glucose cotransporter-2 (SGLT2) inhibitors. Evidence shows that Dapa can induce nutrient deprivation effects, providing additional metabolic benefits. This study investigates whether Dapa can trigger nutrient deprivation to activate SIRT1 and enhance insulin sensitivity in skeletal muscle. We treated diet-induced obese (DIO) mice with Dapa and measured metabolic parameters, lipid accumulation, oxidative stress, mitochondrial function, and glucose utilization in skeletal muscle. β-hydroxybutyric acid (β-HB) was intervened in C2C12 myotubes. The role of SIRT1 was verified by RNA interference. We found that Dapa treatment induced nutrient deprivation state and reduced lipid deposition and oxidative stress, improved mitochondrial function and glucose tolerance in skeletal muscle. The same positive effects were observed after β-HB intervening for C2C12 myotubes, and the promoting effects on glucose utilization were diminished by SIRT1 RNA interference. Thus, Dapa promotes a nutrient deprivation state and enhances skeletal muscle insulin sensitivity via SIRT1 activation. In this study, we identified a novel hypoglycemic mechanism of Dapa and the potential mechanistic targets.

Mitochondrial Dynamics, Diabetes, and Cardiovascular Disease

Mitochondria undergo repeated cycles of fusion and fission that regulate their size and shape by a process known as mitochondrial dynamics. Numerous studies have revealed the importance of this process in maintaining mitochondrial health and cellular homeostasis, particularly in highly metabolically active tissues such as skeletal muscle and the heart. Here, we review the literature on the relationship between mitochondrial dynamics and the pathophysiology of type 2 diabetes and cardiovascular disease (CVD). Importantly, we emphasize divergent outcomes resulting from downregulating distinct mitochondrial dynamics proteins in various tissues. This review underscores compensatory mechanisms and adaptive pathways that offset potentially detrimental effects, resulting instead in improved metabolic health. Finally, we offer a perspective on potential therapeutic implications of modulating mitochondrial dynamics proteins for treatment of diabetes and CVD.

ARTICLE HIGHLIGHTS

Inflammation o'clock: interactions of circadian rhythms with inflammation-induced skeletal muscle atrophy

Circadian rhythms are ∼24 h cycles evident in behaviour, physiology and metabolism. The molecular mechanism directing circadian rhythms is the circadian clock, which is composed of an interactive network of transcription-translation feedback loops. The core clock genes include Bmal1, Clock, Rev-erbα/β, Per and Cry. In addition to keeping time, the core clock regulates a daily programme of gene expression that is important for overall cell homeostasis. The circadian clock mechanism is present in all cells, including skeletal muscle fibres, and disruption of the muscle clock is associated with changes in muscle phenotype and function. Skeletal muscle atrophy is largely associated with a lower quality of life, frailty and reduced lifespan. Physiological and genetic modification of the core clock mechanism yields immune dysfunction, alters inflammatory factor expression and secretion and is associated with skeletal muscle atrophy in multiple conditions, such as ageing and cancer cachexia. Here, we summarize the possible interplay between the circadian clock modulation of immune cells, systemic inflammatory status and skeletal muscle atrophy in chronic inflammatory conditions. Although there is a clear disruption of circadian clocks in various models of atrophy, the mechanism behind such alterations remains unknown. Understanding the modulatory potential of muscle and immune circadian clocks in inflammation and skeletal muscle health is essential for the development of therapeutic strategies to protect skeletal muscle mass and function of patients with chronic inflammation.

Improving Mitochondrial Function in Skeletal Muscle Contributes to the Amelioration of Insulin Resistance by Nicotinamide Riboside

High-fat diet (HFD)-induced insulin resistance (IR) in skeletal muscle is often accompanied by mitochondrial dysfunction and oxidative stress. Boosting nicotinamide adenine dinucleotide (NAD) using nicotinamide riboside (NR) can effectively decrease oxidative stress and increase mitochondrial function. However, whether NR can ameliorate IR in skeletal muscle is still inconclusive. We fed male C57BL/6J mice with an HFD (60% fat) ± 400 mg/kg·bw NR for 24 weeks. C2C12 myotube cells were treated with 0.25 mM palmitic acid (PA) ± 0.5 mM NR for 24 h. Indicators for IR and mitochondrial dysfunction were analyzed. NR treatment alleviated IR in HFD-fed mice with regard to improved glucose tolerance and a remarkable decrease in the levels of fasting blood glucose, fasting insulin and HOMA-IR index. NR-treated HFD-fed mice also showed improved metabolic status regarding a significant reduction in body weight and lipid contents in serum and the liver. NR activated AMPK in the skeletal muscle of HFD-fed mice and PA-treated C2C12 myotube cells and upregulated the expression of mitochondria-related transcriptional factors and coactivators, thereby improving mitochondrial function and alleviating oxidative stress. Upon inhibiting AMPK using Compound C, NR lost its ability in enhancing mitochondrial function and protection against IR induced by PA. In summary, improving mitochondrial function through the activation of AMPK pathway in skeletal muscle may play an important role in the amelioration of IR using NR.

Low carbohydrate high fat ketogenic diets on the exercise crossover point and glucose homeostasis

In exercise science, the crossover effect denotes that fat oxidation is the primary fuel at rest and during low-intensity exercise with a shift towards an increased reliance on carbohydrate oxidation at moderate to high exercise intensities. This model makes four predictions: First, >50% of energy comes from carbohydrate oxidation at ≥60% of maximum oxygen consumption (VO2max), termed the crossover point. Second, each individual has a maximum fat oxidation capacity (FATMAX) at an exercise intensity lower than the crossover point. FATMAX values are typically 0.3–0.6 g/min. Third, fat oxidation is minimized during exercise ≥85%VO2max, making carbohydrates the predominant energetic substrate during high-intensity exercise, especially at >85%VO2max. Fourth, high-carbohydrate low-fat (HCLF) diets will produce superior exercise performances via maximizing pre-exercise storage of this predominant exercise substrate. In a series of recent publications evaluating the metabolic and performance effects of low-carbohydrate high-fat (LCHF/ketogenic) diet adaptations during exercise of different intensities, we provide findings that challenge this model and these four predictions. First, we show that adaptation to the LCHF diet shifts the crossover point to a higher %VO2max (>80%VO2max) than previously reported. Second, substantially higher FATMAX values (>1.5 g/min) can be measured in athletes adapted to the LCHF diet. Third, endurance athletes exercising at >85%VO2max, whilst performing 6 × 800 m running intervals, measured the highest rates of fat oxidation yet reported in humans. Peak fat oxidation rates measured at 86.4 ± 6.2%VO2max were 1.58 ± 0.33 g/min with 30% of subjects achieving >1.85 g/min. These studies challenge the prevailing doctrine that carbohydrates are the predominant oxidized fuel during high-intensity exercise. We recently found that 30% of middle-aged competitive athletes presented with pre-diabetic glycemic values while on an HCLF diet, which was reversed on LCHF. We speculate that these rapid changes between diet, insulin, glucose homeostasis, and fat oxidation might be linked by diet-induced changes in mitochondrial function and insulin action. Together, we demonstrate evidence that challenges the current crossover concept and demonstrate evidence that a LCHF diet may also reverse features of pre-diabetes and future metabolic disease risk, demonstrating the impact of dietary choice has extended beyond physical performance even in athletic populations.

Pathophysiology of type 2 diabetes and the impact of altered metabolic interorgan crosstalk

Diabetes is a complex and multifactorial disease that affects millions of people worldwide, reducing the quality of life significantly, and results in grave consequences for our health care system. In type 2 diabetes (T2D), the lack of β-cell compensatory mechanisms overcoming peripherally developed insulin resistance is a paramount factor leading to disturbed blood glucose levels and lipid metabolism. Impaired β-cell functions and insulin resistance have been studied extensively resulting in a good understanding of these pathways but much less is known about interorgan crosstalk, which we define as signaling between tissues by secreted factors. Besides hormones and organokines, dysregulated blood glucose and long-lasting hyperglycemia in T2D is associated with changes in metabolism with metabolites from different tissues contributing to the development of this disease. Recent data suggest that metabolites, such as lipids including free fatty acids and amino acids, play important roles in the interorgan crosstalk during the development of T2D. In general, metabolic remodeling affects physiological homeostasis and impacts the development of T2D. Hence, we highlight the importance of metabolic interorgan crosstalk in this review to gain enhanced knowledge of the pathophysiology of T2D, which may lead to new therapeutic approaches to treat this disease.

Bavachin and Corylifol A Improve Muscle Atrophy by Enhancing Mitochondria Quality Control in Type 2 Diabetic Mice

Type 2 diabetes reduces muscle mass and function. Chronic inflammation and mitochondrial dysfunction play critical roles in muscle atrophy pathogenesis. Here, we investigated the effects of bavachin and corylifol A from Psoralea corylifolia L. seeds on muscle atrophy in dexamethasone-treated mice and in db/db mice. Bavachin and corylifol A enhanced muscle strength and muscle mass in dexamethasone-treated mice. In diabetic mice, they enhanced muscle strength and cross-sectional areas. Bavachin and corylifol A suppressed inflammatory cytokine (interleukin-6 and tumor necrosis factor-α) expression levels by downregulating nuclear factor-κB phosphorylation. They decreased the muscle atrophic factor (myostatin, atrogin-1, and muscle RING finger-1) expression levels. They activated the AKT synthetic signaling pathway and induced a switch from fast-type glycolytic fibers (type 2B) to slow-type oxidative fibers (types I and 2A). They increased mitochondrial biogenesis and dynamic factor (optic atrophy-1, mitofusin-1/2, fission, mitochondrial 1, and dynamin 1-like) expression levels via the AMP-activated protein kinase-peroxisome proliferator-activated receptor gamma coactivator 1-alpha signaling pathway. They also improved mitochondrial quality by upregulating the mitophagy factor (p62, parkin, PTEN-induced kinase-1, and BCL2-interacting protein-3) expression levels. Therefore, bavachin and corylifol A exert potential therapeutic effects on muscle atrophy by suppressing inflammation and improving mitochondrial function.

Impact of dyslipidemia in the development of cardiovascular complications: Delineating the potential therapeutic role of coenzyme Q10

Dyslipidemia is one of the major risk factors for the development of cardiovascular disease (CVD) in patients with type 2 diabetes (T2D). This metabolic anomality is implicated in the generation of oxidative stress, an inevitable process involved in destructive mechanisms leading to myocardial damage. Fortunately, commonly used drugs like statins can counteract the detrimental effects of dyslipidemia by lowering cholesterol to reduce CVD-risk in patients with T2D. Statins mainly function by blocking the production of cholesterol by targeting the mevalonate pathway. However, by blocking cholesterol synthesis, statins coincidently inhibit the synthesis of other essential isoprenoid intermediates of the mevalonate pathway like farnesyl pyrophosphate and coenzyme Q₁₀ (CoQ₁₀). The latter is by far the most important co-factor and co-enzyme required for efficient mitochondrial oxidative capacity, in addition to its robust antioxidant properties. In fact, supplementation with CoQ₁₀ has been found to be beneficial in ameliorating oxidative stress and improving blood flow in subjects with mild dyslipidemia.. Beyond discussing the destructive effects of oxidative stress in dyslipidemia-induced CVD-related complications, the current review brings a unique perspective in exploring the mevalonate pathway to block cholesterol synthesis while enhancing or maintaining CoQ₁₀ levels in conditions of dyslipidemia. Furthermore, this review disscusses the therapeutic potential of bioactive compounds in targeting the downstream of the mevalonate pathway, more importantly, their ability to block cholesterol while maintaining CoQ₁₀ biosynthesis to protect against the destructive complications of dyslipidemia.

Mitochondrial DNA Copy Number: Linking Diabetes and Cancer

Recent Advances: Various studies have suggested that mitochondrial DNA copy number (mtDNA-CN), a surrogate biomarker of mitochondrial dysfunction, is an easily quantifiable biomarker for chronic diseases, including diabetes and cancer. However, current knowledge is limited, and the results are controversial. This has been attributed mainly to methodology and study design. Critical Issues: The incidence of diabetes and cancer has increased significantly in recent years. Moreover, type 2 diabetes (T2D) has been shown to be a risk factor for cancer. mtDNA-CN has been associated with both T2D and cancer. However, it is not known whether mtDNA-CN plays any role in the association between T2D and cancer. Significance: In this review, we have discussed mtDNA-CN in diabetes and cancer, and reviewed the literature and methodology used in published studies so far. Based on the literature review, we have speculated how mtDNA-CN may act as a link between diabetes and cancer. Furthermore, we have provided some recommendations for reliable translation of mtDNA-CN as a biomarker. Future Directions: Further research is required to elucidate the role of mtDNA-CN in the association between T2D and cancer. If established, early lifestyle interventions, such as physical activity and diet control that improve mitochondrial function, may help preventing cancer in patients with T2D. Antioxid. Redox Signal. 37, 1168-1190.

Mitochondrial and metabolic dysfunction in ageing and age-related diseases

Organismal ageing is accompanied by progressive loss of cellular function and systemic deterioration of multiple tissues, leading to impaired function and increased vulnerability to death. Mitochondria have become recognized not merely as being energy suppliers but also as having an essential role in the development of diseases associated with ageing, such as neurodegenerative and cardiovascular diseases. A growing body of evidence suggests that ageing and age-related diseases are tightly related to an energy supply and demand imbalance, which might be alleviated by a variety of interventions, including physical activity and calorie restriction, as well as naturally occurring molecules targeting conserved longevity pathways. Here, we review key historical advances and progress from the past few years in our understanding of the role of mitochondria in ageing and age-related metabolic diseases. We also highlight emerging scientific innovations using mitochondria-targeted therapeutic approaches.

Effects of Liver Pathology on Sleeve Gastrectomy Outcomes

Introduction: Sleeve gastrectomy engenders weight loss and improves comorbidities at 1 year postoperatively. A relationship has not been established between liver pathology and diabetic outcomes and weight loss following a sleeve gastrectomy. This study evaluates the association between liver pathology and both diabetic remission and weight loss in morbidly obese veterans. Methods: A prospective database of all patients undergoing sleeve gastrectomy with simultaneous liver biopsy at a Veterans Affairs Medical Center was analyzed from 2018 through 2020. The database included patient demographics, liver biopsy pathology, laboratory values, and antihyperglycemic medications. Patient outcomes at 12 months postoperatively were analyzed specifically for diabetic resolution and weight loss. Chi-square test and Fisher's exact test were used for categorical comparisons, and one-way analysis of variance test and two-tailed t-test were used for continuous variable comparisons. Multivariate linear regression models were created to assess the association between liver pathology and changes in body mass index (BMI) and diabetic status. A two-sided P-value of 0.05 indicated significance. Results: Of the 77 patients included in the study, 70.1% of patients achieved diabetic remission at 12 months. After condensing steatosis and fibrosis scores into low- and high-grade categories, patients with no hepatic disease had significantly lower BMI at 12 months postoperatively than patients with low- or high-grade hepatic disease (29.2 ± 3.6 kg/m² versus 35.1 ± 4.0 kg/m² versus 34.5 ± 3.7 kg/m², respectively, P = .009). On multivariate linear regression model, low-grade overall hepatic disease (β = 3.1 ± 1.5; P = .043) and preoperative oral glycemic medications (β = 2.4 ± 1.0; P = .026) were associated with a significantly increased 12-month BMI. Also, Black or African American race compared with White race was associated with a significant decrease in postoperative BMI (β = -1.9 ± 0.8; P = .023). Conclusions: Regardless of preexisting liver disease, most diabetic patients who undergo sleeve gastrectomy experience diabetic remission at 12 months postoperatively. Additionally, patients with no underlying liver disease lose more weight than those with low- or high-grade liver disease.

Mechanism of insulin resistance in obesity: a role of ATP

Obesity increases the risk of type 2 diabetes through the induction of insulin resistance. The mechanism of insulin resistance has been extensively investigated for more than 60 years, but the essential pathogenic signal remains missing. Existing hypotheses include inflammation, mitochondrial dysfunction, hyperinsulinemia, hyperglucagonemia, glucotoxicity, and lipotoxicity. Drug discoveries based on these hypotheses are unsuccessful in the development of new medicines. In this review, multidisciplinary literature is integrated to evaluate ATP as a primary signal for insulin resistance. The ATP production is elevated in insulin-sensitive cells under obese conditions independent of energy demand, which we have named "mitochondrial overheating." Overheating occurs because of substrate oversupply to mitochondria, leading to extra ATP production. The ATP overproduction contributes to the systemic insulin resistance through several mechanisms, such as inhibition of AMPK, induction of mTOR, hyperinsulinemia, hyperglucagonemia, and mitochondrial dysfunction. Insulin resistance represents a feedback regulation of energy oversupply in cells to control mitochondrial overloading by substrates. Insulin resistance cuts down the substrate uptake to attenuate mitochondrial overloading. The downregulation of the mitochondrial overloading by medicines, bypass surgeries, calorie restriction, and physical exercise leads to insulin sensitization in patients. Therefore, ATP may represent the primary signal of insulin resistance in the cellular protective response to the substrate oversupply. The prevention of ATP overproduction represents a key strategy for insulin sensitization.

Umbilical Cord-Mesenchymal Stem Cell-Conditioned Medium Improves Insulin Resistance in C2C12 Cell

BACKGROUND

Umbilical cord-mesenchymal stem cell-conditioned medium (UC-MSC-CM) has emerged as a promising cell-free therapy. The aim of this study was to explore the therapeutic effects of UC-MSC-CM on insulin resistance in C2C12 cell.

METHODS

Insulin resistance was induced by palmitate. Effects of UC-MSC-CM on insulin resistance were evaluated using glucose uptake, glucose transporter type 4 (GLUT4) translocation, the insulin-signaling pathway, and mitochondrial contents and functions in C2C12 cell.

RESULTS

Glucose uptake was improved by UC-MSC-CM. UC-MSC-CM treatment increased only in membranous GLUT4 expression, not in cytosolic GLUT4 expression. It restored the insulin-signaling pathway in insulin receptor substrate 1 and protein kinase B. Mitochondrial contents evaluated by mitochondrial transcription factor A, mitochondrial DNA copy number, and peroxisome proliferator-activated receptor gamma coactivator 1-alpha were increased by UC-MSC-CM. In addition, UC-MSC-CM significantly decreased mitochondrial reactive oxygen species and increased fatty acid oxidation and mitochondrial membrane potential. There was no improvement in adenosine triphosphate (ATP) contents, but ATP synthesis was improved by UC-MSC-CM. Cytokine and active factor analysis of UC-MSC-CM showed that it contained many regulators inhibiting insulin resistance.

CONCLUSION

UC-MSC-CM improves insulin resistance with multiple mechanisms in C2C12 cell.

Berberine attenuates the abnormal ectopic lipid deposition in skeletal muscle

BACKGROUND

Lipid deposition in non-adipose tissue is associated with a propensity to obesity. Skeletal muscle mitochondrial dysfunction, evidenced by incomplete beta oxidation may contribute to ectopic lipid deposition during high fat diet-induced obesity. Berberine (BBR) has been proved to possess the properties of improving metabolic disorders in patients with obesity or type 2 diabetes mellitus. However, the precise mechanism remains obscure.

METHODS

Mice were treated with berberine and metabolic profile were analyzed. Mitochondrial number and function were detected after berberine treatment in vitro and in vivo. The role of Adenosine 5'-monophosphate-activated protein kinase (AMPK)/peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α) was verified after RNA interference or adenovirus infection.

RESULTS

In the current study, we investigated the influence of berberine on the lipid deposition of skeletal muscle and found that berberine could increase the mitochondrial number and function both in vivo and in vitro. Furthermore, berberine promoted the expression of PGC-1α, the crucial transcriptional coactivator related to mitochondrial biogenesis and function, through AMPK pathway. Berberine reduced the basal oxygen consumption rates (OCR) but increased the maximal OCR in C2C12 myocytes, which indicated that berberine could increase the potential function of mitochondria.

CONCLUSION

Our results proved that berberine can protect the lean body mass from excessive lipid accumulation, by promoting the mitochondrial biogenesis and improving fatty acid oxidation in an AMPK/PGC-1α dependent manner.

Mitochondrial destiny in type 2 diabetes: the effects of oxidative stress on the dynamics and biogenesis of mitochondria

Background

One reason for the development of insulin resistance is the chronic inflammation in obesity.

Materials & Methods

Scientific articles in the field of knowledge on the involvement of mitochondria and mitochondrial DNA (mtDNA) in obesity and type 2 diabetes were analyzed.

Results

Oxidative stress developed during obesity contributes to the formation of peroxynitrite, which causes cytochrome C-related damage in the mitochondrial electron transfer chain and increases the production of reactive oxygen species (ROS), which is associated with the development of type 2 diabetes. Oxidative stress contributes to the nuclease activity of the mitochondrial matrix, which leads to the accumulation of cleaved fragments and an increase in heteroplasmy. Mitochondrial dysfunction and mtDNA variations during insulin resistance may be connected with a change in ATP levels, generation of ROS, mitochondrial division/fusion and mitophagy. This review discusses the main role of mitochondria in the development of insulin resistance, which leads to pathological processes in insulin-dependent tissues, and considers potential therapeutic directions based on the modulation of mitochondrial biogenesis. In this regard, the development of drugs aimed at the regulation of these processes is gaining attention.

Conclusion

Changes in the mtDNA copy number can help to protect mitochondria from severe damage during conditions of increased oxidative stress. Mitochondrial proteome studies are conducted to search for potential therapeutic targets. The use of mitochondrial peptides encoded by mtDNA also represents a promising new approach to therapy.

Mitochondrial DNA copy number and diabetes: the Atherosclerosis Risk in Communities (ARIC) study

INTRODUCTION

Mitochondrial DNA copy number (mtDNA-CN) is a measure of mitochondrial dysfunction and is associated with diabetes in experimental models. To explore the temporality of mitochondrial dysfunction and diabetes, we estimated the prevalent and incident association of mtDNA-CN and diabetes.

RESEARCH DESIGN AND METHODS

We assessed the associations of mtDNA-CN measured from buffy coat with prevalent and incident diabetes, stratified by race, in 8954 white and 2444 black participants in the Atherosclerosis Risk in Communities (ARIC) study, an observational cohort study. Follow-up for incident analyses was complete through visit 6, 2016.

RESULTS

Mean age at mtDNA-CN measurement was 57 years and 59% were female. Prevalence of diabetes at time of mtDNA-CN measurement was higher in blacks (563/2444, 23%) than whites (855/8954, 10%). The fully adjusted odds of prevalent diabetes for the 10th vs 90th percentile of mtDNA-CN was 1.05 (95% CI 0.74 to 1.49) among black and 1.49 (95% CI 1.20 to 1.85) among white participants. Over a median follow-up time of 19 years (Q1, Q3: 11, 24 years), we observed 617 incident diabetes cases among 1744 black and 2121 cases among 7713 white participants free of diabetes at baseline. The fully adjusted hazard of incident diabetes for the 10th vs 90th percentile of mtDNA-CN was 1.07 (95% CI 0.84 to 1.38) among black and 0.97 (95% CI 0.86 to 1.10) among white participants.

CONCLUSIONS

Lower mtDNA-CN in buffy coat was associated with prevalent diabetes in white but not black ARIC participants. Lower mtDNA-CN was not associated with incident diabetes over 20 years of follow-up in whites or blacks.

The Mitochondrial Proteomic Signatures of Human Skeletal Muscle Linked to Insulin Resistance

Introduction: Mitochondria are essential in energy metabolism and cellular survival, and there is growing evidence that insulin resistance in chronic metabolic disorders, such as obesity, type 2 diabetes (T2D), and aging, is linked to mitochondrial dysfunction in skeletal muscle. Protein profiling by proteomics is a powerful tool to investigate mechanisms underlying complex disorders. However, despite significant advances in proteomics within the past two decades, the technologies have not yet been fully exploited in the field of skeletal muscle proteome. Area covered: Here, we review the currently available studies characterizing the mitochondrial proteome in human skeletal muscle in insulin-resistant conditions, such as obesity, T2D, and aging, as well as exercise-mediated changes in the mitochondrial proteome. Furthermore, we outline technical challenges and limitations and methodological aspects that should be considered when planning future large-scale proteomics studies of mitochondria from human skeletal muscle. Authors’ view: At present, most proteomic studies of skeletal muscle or isolated muscle mitochondria have demonstrated a reduced abundance of proteins in several mitochondrial biological processes in obesity, T2D, and aging, whereas the beneficial effects of exercise involve an increased content of muscle proteins involved in mitochondrial metabolism. Powerful mass-spectrometry-based proteomics now provides unprecedented opportunities to perform in-depth proteomics of muscle mitochondria, which in the near future is expected to increase our understanding of the complex molecular mechanisms underlying the link between mitochondrial dysfunction and insulin resistance in chronic metabolic disorders.

Sarcopenic obesity: Myokines as potential diagnostic biomarkers and therapeutic targets?

Sarcopenic obesity (SO) is a condition characterized by the occurrence of both sarcopenia and obesity and imposes a heavy burden on the health of the elderly. Controversies and challenges regarding the definition, diagnosis and treatment of SO still remain because of its complex pathogenesis and limitations. Over the past few decades, numerous studies have revealed that myokines secreted from skeletal muscle play significant roles in the regulation of muscle mass and function as well as metabolic homeostasis. Abnormalities in myokines may trigger and promote the pathogenesis underlying age-related and metabolic diseases, including obesity, sarcopenia, type 2 diabetes (T2D), and SO. This review mainly focuses on the role of myokines as potential biomarkers for the early diagnosis and therapeutic targets in SO.

Mitochondrial dysfunction in metabolic syndrome

Metabolic syndrome is co-occurrence of obesity, insulin resistance, atherogenic dyslipidemia (high triglyceride, low high density lipoprotein cholesterol), and hypertension. It is a global health problem. An estimated 20%-30% of adults of the world have metabolic syndrome. Metabolic syndrome is associated with increased risk of type 2 diabetes mellitus, nonalcoholic fatty liver disease, myocardial infarction, and stroke. Thus, it is a major cause of morbidity and mortality worldwide. However, molecular pathogenesis of metabolic syndrome is not well known. Recently, there has been interest in the role of mitochondria in pathogenesis of metabolic problems such as obesity, metabolic syndrome, and type 2 diabetes mellitus. Mitochondrial dysfunction contributes to the oxidative stress and systemic inflammation seen in metabolic syndrome. Role of mitochondria in the pathogenesis of metabolic syndrome is intriguing but far from completely understood. However, a better understanding will be very rewarding as it may lead to novel approaches to control this major public health problem. This brief review explores pathogenesis of metabolic syndrome from a mitochondrial perspective.

FGF19 alleviates palmitate-induced atrophy in C2C12 cells by inhibiting mitochondrial overload and insulin resistance

Fibroblast growth factor 19 (FGF19) acts as a novel factor in the regulation of skeletal muscle mass in animal models by regulating energy expenditure. People with obesity have a lower content of FGF19 and lose muscle mass easily. However, as the main energy metabolism organelles, the involvement of mitochondria in the protective effect of FGF19 is still unknown. In this study, the protective effects of FGF19 on palmitate-induced damages in differentiated mouse myoblast cells (C2C12) were studied, including myotube morphology, mitochondrial function and the regulation of pathways and genes. Excessive palmitate resulted in myotube atrophy and activation of the mitochondria-mediated apoptosis pathway in C2C12 cells. Palmitate also inhibited glucose uptake and induced insulin resistance. FGF19 addition during the differentiation of C2C12 cells, returned the palmitate-induced mitochondrial respiration and apoptosis to the control levels and improved the insulin sensitivity. The palmitate-induced upregulation of genes involved in β-oxidation (PPARβ/δ, PPARγ, UCP-1, MCAD) and the downregulation of genes related to myotube atrophy (PPARα, PGC-1α and PGC-1β) were also alleviated by FGF19. In summary, FGF19 prevented excessive palmitate-induced dysfunction of C2C12 cells by protecting mitochondrial overload and apoptosis and maintaining normal insulin signaling.

Improved skeletal muscle energy metabolism relates to the recovery of β cell function by intensive insulin therapy in drug naïve type 2 diabetes

AIMS

Diminished energy turnover of skeletal muscle is involved in the development of type 2 diabetes. Intensive insulin therapy has been reported to maintain glycaemic control in newly diagnosed type 2 diabetes, while the underlying mechanism remains unclear. Herein, we aimed to characterize the contribution of muscular mitochondrial oxidative phosphorylation (OxPhos) activity to insulin-induced glycaemic control.

MATERIALS AND METHODS

There were 21 drug naïve patients with type 2 diabetes receiving continuous subcutaneous insulin infusion for 7 days. Nine nondiabetics matched for age, body mass index, and physical activity were recruited as controls. We applied ³¹ P magnetic resonance spectroscopy to record in vivo muscular phosphocreatine (PCr) flux in controls and diabetics before and after insulin therapy. The mitochondrial OxPhos rate was calculated as ΔPCr / Δtime during the first 50 seconds after cessation of exercise.

RESULTS

In drug naïve type 2 diabetes, muscular mitochondrial OxPhos rate was restored after insulin therapy. Notably, this alteration was positively associated with the improvements of 1,5-anhydroglucitol, a serum marker for glucose control over the last 1 week, as well as homeostasis model assessment of β cell function and C-peptide/glucose ratio t₀ , two indices for basal insulin secretion. Furthermore, patients with diabetes family history and more severe glucotoxicity tend to achieve greater improvement in mitochondrial function by insulin.

CONCLUSIONS

This study provides evidence that intensive insulin therapy facilitates muscular energy metabolism in drug naïve type 2 diabetes. It correlates to the recovery of β cell function, contributing to insulin-induced glucose control.

NDUFAB1 protects against obesity and insulin resistance by enhancing mitochondrial metabolism

Mitochondria are fundamental organelles for cellular and systemic metabolism, and their dysfunction has been implicated in the development of diverse metabolic diseases. Boosted mitochondrial metabolism might be able to protect against metabolic stress and prevent metabolic disorders. Here we show that NADH:ubiquinone oxidoreductase (NDU)-FAB1, also known as mitochondrial acyl carrier protein, acts as a novel enhancer of mitochondrial metabolism and protects against obesity and insulin resistance. Mechanistically, NDUFAB1 coordinately enhances lipoylation and activation of pyruvate dehydrogenase mediated by the mitochondrial fatty acid synthesis pathway and increases the assembly of respiratory complexes and supercomplexes. Skeletal muscle-specific ablation of NDUFAB1 causes systemic disruption of glucose homeostasis and defective insulin signaling, leading to growth arrest and early death within 5 postnatal days. In contrast, NDUFAB1 overexpression effectively protects mice against obesity and insulin resistance when the animals are challenged with a high-fat diet. Our findings indicate that NDUFAB1 could be a novel mitochondrial target to prevent obesity and insulin resistance by enhancing mitochondrial metabolism.-Zhang, R., Hou, T., Cheng, H., Wang, X. NDUFAB1 protects against obesity and insulin resistance by enhancing mitochondrial metabolism.

Diagnostic accuracy of triglyceride/glucose and triglyceride/HDL index as predictors for insulin resistance in children with and without obesity

OBJECTIVE

To investigate which of two indexes (TyG or TG/HDL) are the best predictors for insulin resistance (IR) and to evaluate the magnitude of each cardiometabolic risk factor in Mexican schoolchildren of 5-9 years with overweight-obesity and normal weight.

MATERIAL AND METHODS

We realized a comparative cross-sectional prospective study in accordance of STARD guidelines. Setting was Family Medicine Unit (FMU) No. 80 of Mexican Institute of Social Security(IMSS) of Morelia, Michoacán, Mexico. Children between 5 and 9 years, both genders, 104 with normal weight(NW), 97 with Overweight-Obesity(OO Group) were included. Once the informed consent was signed we obtained the BMI, waist circumference, blood pressure(BP) and 5 mL of blood collected for glucose, cholesterol, triglycerides, HDL cholesterol, LDL cholesterol, uric acid and insulin. As main outcome measures TyG or TG/HDL, HOMA-IR, and Receiving Operating Curves(ROC), sensitivity, specificity by ROC were obtained.

RESULTS

Cutoff point TyG: 8.5 by ROC had an area under curve (AUC):0.802 IC95% 0.77to0.893, P = 0.0001; diagnostic accuracy of 73%.

TG/HDL

2.22; AUC:0.729 IC95% 0.622to0.837, P = 0.014; diagnostic accuracy of 52%. TyG can identify cardiometabolic alterations more than HOMA and TG/HDL. Cardiometabolic alterations in the OO group were hypertriglyceridemia:49.5%, low HDL:63.9%, IR:39.2% and in NW group were hypertriglyceridemia:30.8%, low HDL:60.6%, IR:9.6%.

CONCLUSIONS

We reported high frequency of hypertriglyceridemia and low HDL in Mexican children. TyG and TG/HDL are good predictors for IR. TyG has a better diagnostic accuracy. We need implementing TyG for identifying alterations and intervening in a timely manner to delay the onset of chronic diseases in children.

Interrelation of chemerin and TNF-α with mtDNA copy number in adipose tissues and blood cells in obese patients with and without type 2 diabetes

BACKGROUND

Inflammatory response plays a key role in the development of insulin resistance (IR) in obesity. Oxidative stress triggers the replication of the mitochondrial genome and division of the organelle. The purpose of this study was to identify the relationship of chemerin and TNF-α with mitochondrial DNA (mtDNA) copy number in subcutaneous adipose tissue (SAT) and visceral adipose tissue (mesentery of the small intestine (Mes), greater omentum (GO) and blood mononuclear cells (MNCs)) in patients with obesity with/without type 2 diabetes mellitus (T2DM).

METHODS

The study included 142 obese patients and 34 healthy donors. The samples used for the study were peripheral venous blood (MNCs) and ATs (SAT, Mes and GO). The measurement of mtDNA copy number was done by droplet digital PCR. Quantitative determination of insulin, adiponectin, TNF-α and chemerin in serum/plasma was performed by flow-through fluorometry and commercial ELISA kit. Statistical analysis and graphs were obtained in R Statistical Software (version 3.3.1).

RESULTS

The increase in body mass index (BMI) was accompanied by an increase in TNF-α production, an increase in mtDNA copy number in SAT and a decrease in mtDNA copy number in MNCs. The negative association between plasma chemerin and mtDNA copy number in the Mes, as well as between mtDNA copy number and chemerin expression in the Mes, in the group with BMI > 40 kg/m2 without T2DM demonstrates the protective effect of chemerin against the development of IR via the regulation of mtDNA copy number in adipose tissues.

CONCLUSIONS

We thus speculated the existence of a compensatory mechanism in which leads to the increased number of mtDNA copies under the influence of proinflammatory factors. Based on the obtained data, we propose that reducing mtDNA copy number in cases of morbid obesity without T2DM has a positive effect on carbohydrate metabolism, which may help maintain glucose levels within reference values. Obesity, type 2 diabetes, mtDNA, cytokines, TNF-a, chemerin.

Mitochondria, the NLRP3 Inflammasome, and Sirtuins in Type 2 Diabetes: New Therapeutic Targets

SIGNIFICANCE

Type 2 diabetes mellitus and hyperglycemia can lead to the development of comorbidities such as atherosclerosis and microvascular/macrovascular complications. Both type 2 diabetes and its complications are related to mitochondrial dysfunction and oxidative stress. Type 2 diabetes is also a chronic inflammatory condition that leads to inflammasome activation and the release of proinflammatory mediators, including interleukins (ILs) IL-1β and IL-18. Moreover, sirtuins are energetic sensors that respond to metabolic load, which highlights their relevance in metabolic diseases, such as type 2 diabetes. Recent Advances: Over the past decade, great progress has been made in clarifying the signaling events regulated by mitochondria, inflammasomes, and sirtuins. Nod-like receptor family pyrin domain containing 3 (NLRP3) is the best characterized inflammasome, and the generation of oxidant species seems to be critical for its activation. NLRP3 inflammasome activation and altered sirtuin levels have been observed in type 2 diabetes. Critical Issue: Despite increasing evidence of the relationship between the NLRP3 inflammasome, mitochondrial dysfunction, and oxidative stress and of their participation in type 2 diabetes physiopathology, therapeutic strategies to combat type 2 diabetes that target NLRP3 inflammasome and sirtuins are yet to be consolidated.

FUTURE DIRECTIONS

In this review article, we attempt to provide an overview of the existing literature concerning the crosstalk between mitochondrial impairment and the inflammasome, with particular attention to cellular and mitochondrial redox metabolism and the potential role of the NLRP3 inflammasome and sirtuins in the pathogenesis of type 2 diabetes. In addition, we discuss potential targets for therapeutic intervention based on these molecular interactions. Antioxid. Redox Signal. 29, 749-791.

The PI3K/AKT pathway in obesity and type 2 diabetes

Obesity and type 2 diabetes mellitus are complicated metabolic diseases that affect multiple organs and are characterized by hyperglycaemia. Currently, stable and effective treatments for obesity and type 2 diabetes mellitus are not available. Therefore, the mechanisms leading to obesity and diabetes and more effective ways to treat obesity and diabetes should be identified. Based on accumulated evidences, the PI3K/AKT signalling pathway is required for normal metabolism due to its characteristics, and its imbalance leads to the development of obesity and type 2 diabetes mellitus. This review focuses on the role of PI3K/AKT signalling in the skeletal muscle, adipose tissue, liver, brain and pancreas, and discusses how this signalling pathway affects the development of the aforementioned diseases. We also summarize evidences for recently identified therapeutic targets of the PI3K/AKT pathway as treatments for obesity and type 2 diabetes mellitus. PI3K/AKT pathway damaged in various tissues of the body leads to obesity and type 2 diabetes as the result of insulin resistance, and in turn, insulin resistance exacerbates the PI3K/AKT pathway, forming a vicious circle.

Mitochondria and Reactive Oxygen Species in Aging and Age-Related Diseases

Aging has been linked to several degenerative processes that, through the accumulation of molecular and cellular damage, can progressively lead to cell dysfunction and organ failure. Human aging is linked with a higher risk for individuals to develop cancer, neurodegenerative, cardiovascular, and metabolic disorders. The understanding of the molecular basis of aging and associated diseases has been one major challenge of scientific research over the last decades. Mitochondria, the center of oxidative metabolism and principal site of reactive oxygen species (ROS) production, are crucial both in health and in pathogenesis of many diseases. Redox signaling is important for the modulation of cell functions and several studies indicate a dual role for ROS in cell physiology. In fact, high concentrations of ROS are pathogenic and can cause severe damage to cell and organelle membranes, DNA, and proteins. On the other hand, moderate amounts of ROS are essential for the maintenance of several biological processes, including gene expression. In this review, we provide an update regarding the key roles of ROS-mitochondria cross talk in different fundamental physiological or pathological situations accompanying aging and highlighting that mitochondrial ROS may be a decisive target in clinical practice.

Time-controlled fasting prevents aging-like mitochondrial changes induced by persistent dietary fat overload in skeletal muscle

A large body of evidence suggests that persistent dietary fat overload causes mitochondrial dysfunction and systemic metabolic gridlock. Mitochondrial and lipid metabolism in skeletal muscle (SkM) are severely affected upon persistent high fat diet (HFD) leading to premature tissue aging. Here, we designed weekly cycles of fasting (called as time-controlled fasting, TCF) and showed that they were effective in limiting mitochondrial damage and metabolic disturbances induced by HFD. Specifically, TCF was able to prevent the decline of adipose triglyceride lipase (Atgl), maintain efficient mitochondrial respiration in SkM as well as improve blood glucose and lipid profile. Atgl was found to be the mediator of such preventive effects as its downregulation or up-regulation in C2C12 myotubes triggers mitochondrial alteration or protects against the deleterious effects of high fat levels respectively. In conclusion, TCF could represent an effective strategy to limit mitochondrial impairment and metabolic inflexibility that are typically induced by modern western diets or during aging.

MITOCHONDRIAL DYNAMICS AND METABOLIC REGULATION IN CARDIAC AND SKELETAL MUSCLE

Mitochondria are the metabolic powerhouses of cells. In addition to generating adenosine triphosphate (ATP), they play important roles in cell survival pathways such as apoptosis and necrosis. Mitochondrial size and shape are dynamically regulated by a process known as mitochondrial dynamics. The significance of this process in metabolically active cells such as skeletal and cardiac muscle are only now beginning to be elucidated. In cardiac muscle, mitochondrial dynamics plays an important role in mitochondrial quality control and defects in regulatory pathways that govern these processes and leads to heart failure. In response to nutrient excess such as lipid overload, as occurs in diabetes, mitochondrial shape and morphology are altered by effects of nutrient stress on mitochondrial dynamics signaling pathways, which have important implications for understanding mitochondrial dysfunction in diabetic cardiomyopathy. Moreover, crosstalk between mitochondria and other organelles such as the endoplasmic reticulum can regulate generation of hormones such as fibroblast growth factor 21, with potent anti-diabetic and anti-obesity effects.

Evolving Role of Natriuretic Peptides from Diagnostic Tool to Therapeutic Modality

Natriuretic peptides (NP) are widely recognized as key regulators of blood pressure, water and salt homeostasis. In addition, they play a critical role in physiological cardiac growth and mediate a variety of biological effects including antiproliferative and anti-inflammatory effects in other organs and tissues. The cardiac release of NPs ANP and BNP represents an important compensatory mechanism during acute and chronic cardiac overload and during the pathogenesis of heart failure where their actions counteract the sustained activation of renin-angiotensin-aldosterone and other neurohormonal systems. Elevated circulating plasma NP levels correlate with the severity of heart failure and particularly BNP and the pro-peptide, NT-proBNP have been established as biomarkers for the diagnosis of heart failure as well as prognostic markers for cardiovascular risk. Despite activation of the NP system in heart failure it is inadequate to prevent progressive fluid and sodium retention and cardiac remodeling. Therapeutic approaches included administration of synthetic peptide analogs and the inhibition of NP-degrading enzyme neutral endopeptidase (NEP). Of all strategies only the combined NEP/ARB inhibition with sacubitril/valsartan had shown clinical success in reducing cardiovascular mortality and morbidity in patients with heart failure.

Effects of Calorie Restriction and Fiber Type on Glucose Uptake and Abundance of Electron Transport Chain and Oxidative Phosphorylation Proteins in Single Fibers from Old Rats

Calorie restriction (CR; reducing calorie intake by ~40% below ad libitum) can increase glucose uptake by insulin-stimulated muscle. Because skeletal muscle is comprised of multiple, heterogeneous fiber types, our primary aim was to determine the effects of CR (initiated at 14 weeks old) and fiber type on insulin-stimulated glucose uptake by single fibers of diverse fiber types in 23-26-month-old rats. Isolated epitrochlearis muscles from AL and CR rats were incubated with [3H]-2-deoxyglucose ± insulin. Glucose uptake and fiber type were determined for single fibers dissected from the muscles. We also determined CR-effects on abundance of several key metabolic proteins in single fibers. CR resulted in: (a) significantly (p < .05 to .001) greater glucose uptake by insulin-stimulated type I, IIA, IIB, IIBX, and IIX fibers; (b) significantly (p < .05 to .001) reduced abundance of several mitochondrial electron transport chain (ETC) and oxidative phosphorylation (OxPhos) proteins in type I, IIA, and IIBX but not IIB and IIX fibers; and (c) unaltered hexokinase II abundance in each fiber type. These results demonstrate that CR can enhance glucose uptake in each fiber type of rat skeletal muscle in the absence of upregulation of the abundance of hexokinase II or key mitochondrial ETC and OxPhos proteins.

Insulin restores UCP3 activity and decreases energy surfeit to alleviate lipotoxicity in skeletal muscle

An early insulin regimen ameliorates glucotoxicity but also lipotoxicity in type 2 diabetes; however, the underlying mechanism remains elusive. In the present study, we investigated the role of mitochondria in lipid regulation following early insulin administration in insulin-resistant skeletal muscle cells. Male C57BL/6 mice, fed a high-fat diet (HFD) for 8 weeks, were treated with insulin for 3 weeks, and L6 myotubes cultured with palmitate (PA) for 24 h were incubated with insulin for another 12 h. The results showed that insulin facilitated systemic glucose disposal and attenuated muscular triglyceride accumulation in vivo. Recovery of AMP-activated protein kinase (AMPK) phosphorylation, inhibition of sterol-regulated element binding protein-1c (SREBP-1c) and increased carnitine palmitoyltransferase‑1B (CPT1B) expression were observed after insulin administration. Moreover, increased ATP concentration and cellular energy charge elicited by over-nutrition were suppressed by insulin. Despite maintaining respiratory complex activities, insulin restored muscular uncoupling protein 3 (UCP3) protein expression in vitro and in vivo. By contrast, knockdown of UCP3 abrogated insulin-induced restoration of AMPK phosphorylation in vitro. Importantly, the PA-induced decrease in UCP3 was blocked by the proteasome inhibitor MG132, and insulin reduced UCP3 ubiquitination, thereby prohibiting its degradation. Our findings, focusing on energy balance, provide a mechanistic understanding of the promising effect of early insulin initiation on lipotoxicity. Insulin, by recovering UCP3 activity, alleviated energy surfeit and potentiated AMPK-mediated lipid homeostasis in skeletal muscle cells following exposure to PA and in gastrocnemius of mice fed HFD.

Amino Acid Mixture Acts as a Potent VEGF Lowering Agent in CHO-K1 Cells Exposed to High Glucose

BACKGROUND

Though the role of amino acids in Diabetes Mellitus is controversial, the beneficial effect of amino acids in Diabetes Mellitus has been reported based on its anti-glycating property and insulin potentiating effects. In the current study, we evaluated the ROS generation and VEGF expression in CHO-K1 cells induced by high glucose concentration. The effect of amino acids treatment was studied under this condition to evaluate the VEGF lowering effect.

METHOD

CHO-K1 cells were treated various concentration of glucose (7 mmol, 17 mmol and 27 mmol) with and without free amino acids (5 mmol) or the amino acids mixture (AAM). Intracellular reactive oxygen species (ROS) was estimated by fluorescein dye (DCFDA), nitric oxide (NO) by Griess reaction, hydrogen peroxide (H₂O₂) by fluorimetry using Amplex red dye, super oxide dismutase (SOD) by spectrophotometry and VEGF by immunoblotting.

RESULTS

High glucose condition significantly induced the expression of VEGF and this was reduced significantly by AAM treatment (p = 0.004). AAM also significantly decreased the cellular levels of ROS, NO, H₂O₂ as well as the SOD activity in CHO-K1 cells exposed to high glucose condition (p <0.05).

CONCLUSION

The present study identified AAM as a potential VEGF lowering agent that intervenes at the level of oxidative stress in high glucose conditions as evaluated in CHO-K1 cells.

Correlation between Oxidative Stress, Nutrition, and Cancer Initiation

Inadequate or excessive nutrient consumption leads to oxidative stress, which may disrupt oxidative homeostasis, activate a cascade of molecular pathways, and alter the metabolic status of various tissues. Several foods and consumption patterns have been associated with various cancers and approximately 30-35% of the cancer cases are correlated with overnutrition or malnutrition. However, several contradictory studies are available regarding the association between diet and cancer risk, which remains to be elucidated. Concurrently, oxidative stress is a crucial factor for cancer progression and therapy. Nutritional oxidative stress may be induced by an imbalance between antioxidant defense and pro-oxidant load due to inadequate or excess nutrient supply. Oxidative stress is a physiological state where high levels of reactive oxygen species (ROS) and free radicals are generated. Several signaling pathways associated with carcinogenesis can additionally control ROS generation and regulate ROS downstream mechanisms, which could have potential implications in anticancer research. Cancer initiation may be modulated by the nutrition-mediated elevation in ROS levels, which can stimulate cancer initiation by triggering DNA mutations, damage, and pro-oncogenic signaling. Therefore, in this review, we have provided an overview of the relationship between nutrition, oxidative stress, and cancer initiation, and evaluated the impact of nutrient-mediated regulation of antioxidant capability against cancer therapy.

OPA1 deficiency promotes secretion of FGF21 from muscle that prevents obesity and insulin resistance

Mitochondrial dynamics is a conserved process by which mitochondria undergo repeated cycles of fusion and fission, leading to exchange of mitochondrial genetic content, ions, metabolites, and proteins. Here, we examine the role of the mitochondrial fusion protein optic atrophy 1 (OPA1) in differentiated skeletal muscle by reducing OPA1 gene expression in an inducible manner. OPA1 deficiency in young mice results in non-lethal progressive mitochondrial dysfunction and loss of muscle mass. Mutant mice are resistant to age- and diet-induced weight gain and insulin resistance, by mechanisms that involve activation of ER stress and secretion of fibroblast growth factor 21 (FGF21) from skeletal muscle, resulting in increased metabolic rates and improved whole-body insulin sensitivity. OPA1-elicited mitochondrial dysfunction activates an integrated stress response that locally induces muscle atrophy, but via secretion of FGF21 acts distally to modulate whole-body metabolism.

The role of caloric load and mitochondrial homeostasis in the regulation of the NLRP3 inflammasome

Sterile inflammation is a cornerstone of immune activation in obesity and type 2 Diabetes Mellitus. The molecular underpinnings of this inflammation include nutrient excess-mediated activation of the innate immune NLRP3 inflammasome. At the same time, disruption of mitochondrial integrity is emerging as an integral control node in NLRP3 inflammasome activation and is also associated with caloric overload conditions including obesity and diabetes. Conversely, caloric restriction and fasting mimetic interventions alleviate these caloric excess-linked diseases and reduce inflammation and the NLRP3 inflammasome. The objective of this review is to integrate the findings linking mitochondrial integrity to the activation of the NLRP3 inflammasome and to evaluate how caloric restriction or caloric restriction mimetic compounds may play a role in attenuating the NLRP3 inflammasome and sterile inflammation.

Extrapancreatic effects of incretin hormones: evidence for weight-independent changes in morphological aspects and oxidative status in insulin-sensitive organs of the obese nondiabetic Zucker rat (ZFR)

Incretin‐based therapies are widely used to treat type 2 diabetes. Although hypoglycemic actions of incretins are mostly due to their insulinotropic/glucagonostatic effects, they may also influence extrapancreatic metabolism. We administered exendin‐4 (Ex‐4), a long‐acting glucagon‐like peptide receptor agonist, at low dose (0.1 nmol/kg/day) for a short period (10 days), in obese nondiabetic fa/fa Zucker rats (ZFRs). Ex‐4‐treated ZFRs were compared to vehicle (saline)‐treated ZFRs and vehicle‐ and Ex‐4‐treated lean rats (LRs). Blood glucose levels were measured at days 0, 9, and 10. Ingested food and animal weight were recorded daily. On the day of sacrifice (d10), blood was sampled along with liver, epididymal, subcutaneous, brown adipose, and skeletal muscle tissues from animals fasted for 24 h. Plasma insulin and blood glucose levels, food intake, and body and epididymal fat weight were unchanged, but gross morphological changes were observed in insulin‐sensitive tissues. The average size of hepatocytes was significantly lower in Ex‐4‐treated ZFRs, associated with decreased number and size of lipid droplets and 4‐hydroxy‐2‐nonenal (HNE) staining, a marker of oxidative stress (OS). Myocytes, which were smaller in ZFRs than in LRs, were significantly enlarged and depleted of lipid droplets in Ex‐4‐treated ZFRs. Weak HNE staining was increased by Ex‐4. A similar observation was made in brown adipose tissue, whereas the elevated HNE staining observed in epididymal adipocytes of ZFRs, suggestive of strong OS, was decreased by Ex‐4. These results suggest that incretins by acting on OS in insulin‐sensitive tissues may contribute to weight‐independent improvement in insulin sensitivity.

Dissociation of muscle insulin sensitivity from exercise endurance in mice by HDAC3 depletion

Type 2 diabetes and insulin resistance are associated with reduced glucose utilization in the muscle and poor exercise performance. Here we find that depletion of the epigenome modifier histone deacetylase 3 (HDAC3) specifically in skeletal muscle causes severe systemic insulin resistance in mice but markedly enhances endurance and resistance to muscle fatigue, despite reducing muscle force. This seemingly paradoxical phenotype is due to lower glucose utilization and greater lipid oxidation in HDAC3-depleted muscles, a fuel switch caused by the activation of anaplerotic reactions driven by AMP deaminase 3 (Ampd3) and catabolism of branched-chain amino acids. These findings highlight the pivotal role of amino acid catabolism in muscle fatigue and type 2 diabetes pathogenesis. Further, as genome occupancy of HDAC3 in skeletal muscle is controlled by the circadian clock, these results delineate an epigenomic regulatory mechanism through which the circadian clock governs skeletal muscle bioenergetics. These findings suggest that physical exercise at certain times of the day or pharmacological targeting of HDAC3 could potentially be harnessed to alter systemic fuel metabolism and exercise performance.

MECHANISMS IN ENDOCRINOLOGY: Nutrition as a mediator of oxidative stress in metabolic and reproductive disorders in women

Nutrition can generate oxidative stress and trigger a cascade of molecular events that can disrupt oxidative and hormonal balance. Nutrient ingestion promotes a major inflammatory and oxidative response at the cellular level in the postprandial state, altering the metabolic state of tissues. A domino of unfavorable metabolic changes is orchestrated in the main metabolic organs, including adipose tissue, skeletal muscle, liver and pancreas, where subclinical inflammation, endothelial dysfunction, mitochondrial deregulation and impaired insulin response and secretion take place. Simultaneously, in reproductive tissues, nutrition-induced oxidative stress can potentially violate delicate oxidative balance that is mandatory to secure normal reproductive function. Taken all the above into account, nutrition and its accompanying postprandial oxidative stress, in the unique context of female hormonal background, can potentially compromise normal metabolic and reproductive functions in women and may act as an active mediator of various metabolic and reproductive disorders.

Quercetin and Quercetin-Rich Red Onion Extract Alter Pgc-1α Promoter Methylation and Splice Variant Expression

Pgc-1α and its various isoforms may play a role in determining skeletal muscle mitochondrial adaptations in response to diet. 8 wks of dietary supplementation with the flavonoid quercetin (Q) or red onion extract (ROE) in a high fat diet (HFD) ameliorates HFD-induced obesity and insulin resistance in C57BL/J mice while upregulating Pgc-1α and increasing skeletal muscle mitochondrial number and function. Here, mice were fed a low fat (LF), high fat (HF), high fat plus quercetin (HF + Q), or high fat plus red onion extract (HF + RO) diet for 9 wks and skeletal muscle Pgc-1α isoform expression and DNA methylation were determined. Quantification of various Pgc-1α isoforms, including isoforms Pgc-1α-a, Pgc-1α-b, Pgc-1α-c, Pgc-1α4, total NT-Pgc-1α, and FL-Pgc-1α, showed that only total NT-Pgc-1α expression was increased in LF, HF + Q, and HF + RO compared to HF. Furthermore, Q supplementation decreased Pgc-1α-a expression compared to LF and HF, and ROE decreased Pgc-1α-a expression compared to LF. FL-Pgc-1α was decreased in HF + Q and HF + RO compared to LF and HF. HF exhibited hypermethylation at the −260 nucleotide (nt) in the Pgc-1α promoter. Q and ROE prevented HFD-induced hypermethylation. −260 nt methylation levels were associated with NT-Pgc-1α expression only. Pgc-1α isoform expression may be epigenetically regulated by Q and ROE through DNA methylation.

Growth differentiation factor 15 is a myomitokine governing systemic energy homeostasis

Chung et al. show that the myomitokine GDF15 can act to modulate oxidative and lipolytic function in a non–cell-autonomous manner, thereby regulating systemic energy homeostasis in skeletal muscle-specific Crif1-deficient mice. This pathway may be a potential therapeutic target for preventing the onset of obesity and insulin resistance.

Reduced mitochondrial electron transport chain activity promotes longevity and improves energy homeostasis via cell-autonomous and –non-autonomous factors in multiple model systems. This mitohormetic effect is thought to involve the mitochondrial unfolded protein response (UPR^mt), an adaptive stress-response pathway activated by mitochondrial proteotoxic stress. Using mice with skeletal muscle–specific deficiency of Crif1 (muscle-specific knockout [MKO]), an integral protein of the large mitoribosomal subunit (39S), we identified growth differentiation factor 15 (GDF15) as a UPR^mt-associated cell–non-autonomous myomitokine that regulates systemic energy homeostasis. MKO mice were protected against obesity and sensitized to insulin, an effect associated with elevated GDF15 secretion after UPR^mt activation. In ob/ob mice, administration of recombinant GDF15 decreased body weight and improved insulin sensitivity, which was attributed to elevated oxidative metabolism and lipid mobilization in the liver, muscle, and adipose tissue. Thus, GDF15 is a potent mitohormetic signal that safeguards against the onset of obesity and insulin resistance.

Adult expression of PGC-1α and -1β in skeletal muscle is not required for endurance exercise-induced enhancement of exercise capacity

Exercise has been shown to be the best intervention in the treatment of many diseases. Many of the benefits of exercise are mediated by adaptions induced in skeletal muscle. The peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) family of transcriptional coactivators has emerged as being key mediators of the exercise response and is considered to be essential for many of the adaptions seen in skeletal muscle. However, the contribution of the PGC-1s in skeletal muscle has been evaluated by the use of either whole body or congenital skeletal muscle-specific deletion. In these models, PGC-1s were never present, thereby opening the possibility to developmental compensation. Therefore, we generated an inducible muscle-specific deletion of PGC-1α and -1β (iMyo-PGC-1DKO), in which both PGC-1α and -β can be deleted specifically in adult skeletal muscle. These iMyo-PGC-1DKO animals were used to assess the role of both PGC-1α and -1β in adult skeletal muscle and their contribution to the exercise training response. Untrained iMyo-PGC-1DKO animals exhibited a time-dependent decrease in exercise performance 8 wk postdeletion, similar to what was observed in the congenital muscle-specific PGC-1DKOs. However, after 4 wk of voluntary training, the iMyo-PGC-1DKOs exhibited an increase in exercise performance with a similar adaptive response compared with control animals. This increase was associated with an increase in electron transport complex (ETC) expression and activity in the absence of PGC-1α and -1β expression. Taken together these data suggest that PGC-1α and -1β expression are not required for training-induced exercise performance, highlighting the contribution of PGC-1-independent mechanisms.

Mitochondrial phosphoproteomics of mammalian tissues

Mitochondria are essential for several biological processes including energy metabolism and cell survival. Accordingly, impaired mitochondrial function is involved in a wide range of human pathologies including diabetes, cancer, cardiovascular, and neurodegenerative diseases. Within the past decade a growing body of evidence indicates that reversible phosphorylation plays an important role in the regulation of a variety of mitochondrial processes as well as tissue-specific mitochondrial functions in mammals. The rapidly increasing number of mitochondrial phosphorylation sites and phosphoproteins identified is largely ascribed to recent advances in phosphoproteomic technologies such as fractionation, phosphopeptide enrichment, and high-sensitivity mass spectrometry. However, the functional importance and the specific kinases and phosphatases involved have yet to be determined for the majority of these mitochondrial phosphorylation sites. This review summarizes the progress in establishing the mammalian mitochondrial phosphoproteome and the technical challenges encountered while characterizing it, with a particular focus on large-scale phosphoproteomic studies of mitochondria from human skeletal muscle.

Mitochondrial biogenesis is decreased in skeletal muscle of pig fetuses exposed to maternal high-energy diets

Mitochondria plays an important role in the regulation of energy homeostasis. Moreover, mitochondrial biogenesis accompanies skeletal myogenesis, and we previously reported that maternal high-energy diet repressed skeletal myogenesis in pig fetuses. Therefore, the aim of this study was to evaluate the effects of moderately increased maternal energy intake on skeletal muscle mitochondrial biogenesis and function of the pig fetuses. Primiparous purebred Large White sows were allocated to a normal energy intake group (NE) as recommended by the National Research Council (NRC) and a high energy intake group (HE, 110% of NRC recommendations). On day 90 of gestation, fetal umbilical vein blood and longissimus (LM) muscle were collected. Results showed that the weight gain of sows fed HE diet was higher than NE sows on day 90 of gestation (P<0.05). Maternal HE diet increased fetal umbilical vein serum triglyceride and insulin concentrations (P<0.05), and tended to increase the homeostasis model assessment index (P=0.08). Furthermore, HE fetuses exhibited increased malondialdehyde concentration (P<0.05), and decreased activities of antioxidative enzymes (P<0.05) and intracellular NAD+ level (P<0.05) in LM muscle. These alterations in metabolic traits of HE fetuses were accompanied by reduced mitochondrial DNA amount (P<0.05) and down-regulated messenger RNA expression levels of genes responsible for mitochondrial biogenesis and function (P<0.05). Our results suggest that moderately increased energy supply during gestation decreases mitochondrial biogenesis, function and antioxidative capacity in skeletal muscle of pig fetuses.

Roux-en-Y Gastric Bypass Acutely Decreases Protein Carbonylation and Increases Expression of Mitochondrial Biogenesis Genes in Subcutaneous Adipose Tissue

BACKGROUND

Mitochondrial dysfunction in adipose tissue has been implicated as a pathogenic step in the development of type 2 diabetes mellitus (T2DM). In adipose tissue, chronic nutrient overload results in mitochondria driven increased reactive oxygen species (ROS) leading to carbonylation of proteins that impair mitochondrial function and downregulation of key genes linked to mitochondrial biogenesis. In patients with T2DM, Roux-en-Y gastric bypass (RYGB) surgery leads to improvements in glycemic profile prior to significant weight loss. Consequently, we hypothesized that improved glycemia early after RYGB would be paralleled by decreased protein carbonylation and increased expression of genes related to mitochondrial biogenesis in adipose tissue.

METHODS

To evaluate this hypothesis, 16 obese individuals were studied before and 7-8 days following RYGB and adjustable gastric banding (AGB). Subcutaneous adipose tissue was obtained pre- and post-bariatric surgery as well as from eight healthy, non-obese individual controls.

RESULTS

Prior to surgery, adipose tissue expression of PGC1α, NRF1, Cyt C, and eNOS (but not Tfam) showed significantly lower expression in the obese bariatric surgery group when compared to lean controls (p < 0.05). Following RYGB, but not after AGB, patients showed significant decrease in HOMA-IR, reduction in adipose protein carbonylation, and increased expression of genes linked to mitochondrial biogenesis.

CONCLUSIONS

These results suggest that rapid reduction in protein carbonylation and increased mitochondrial biogenesis may explain postoperative metabolic improvements following RYGB.