Insulin resistance protects the heart from fuel overload in dysregulated metabolic states

Changes in Cells Associated with Insulin Resistance

Insulin is a polypeptide hormone synthesized and secreted by pancreatic β-cells. It plays an important role as a metabolic hormone. Insulin influences the metabolism of glucose, regulating plasma glucose levels and stimulating glucose storage in organs such as the liver, muscles and adipose tissue. It is involved in fat metabolism, increasing the storage of triglycerides and decreasing lipolysis. Ketone body metabolism also depends on insulin action, as insulin reduces ketone body concentrations and influences protein metabolism. It increases nitrogen retention, facilitates the transport of amino acids into cells and increases the synthesis of proteins. Insulin also inhibits protein breakdown and is involved in cellular growth and proliferation. On the other hand, defects in the intracellular signaling pathways of insulin may cause several disturbances in human metabolism, resulting in several chronic diseases. Insulin resistance, also known as impaired insulin sensitivity, is due to the decreased reaction of insulin signaling for glucose levels, seen when glucose use in response to an adequate concentration of insulin is impaired. Insulin resistance may cause, for example, increased plasma insulin levels. That state, called hyperinsulinemia, impairs metabolic processes and is observed in patients with type 2 diabetes mellitus and obesity. Hyperinsulinemia may increase the risk of initiation, progression and metastasis of several cancers and may cause poor cancer outcomes. Insulin resistance is a health problem worldwide; therefore, mechanisms of insulin resistance, causes and types of insulin resistance and strategies against insulin resistance are described in this review. Attention is also paid to factors that are associated with the development of insulin resistance, the main and characteristic symptoms of particular syndromes, plus other aspects of severe insulin resistance. This review mainly focuses on the description and analysis of changes in cells due to insulin resistance.

Class IIa HDACs inhibit cell death pathways and protect muscle integrity in response to lipotoxicity

Lipotoxicity, the accumulation of lipids in non-adipose tissues, alters the metabolic transcriptome and mitochondrial metabolism in skeletal muscle. The mechanisms involved remain poorly understood. Here we show that lipotoxicity increased histone deacetylase 4 (HDAC4) and histone deacetylase 5 (HDAC5), which reduced the expression of metabolic genes and oxidative metabolism in skeletal muscle, resulting in increased non-oxidative glucose metabolism. This metabolic reprogramming was also associated with impaired apoptosis and ferroptosis responses, and preserved muscle cell viability in response to lipotoxicity. Mechanistically, increased HDAC4 and 5 decreased acetylation of p53 at K120, a modification required for transcriptional activation of apoptosis. Redox drivers of ferroptosis derived from oxidative metabolism were also reduced. The relevance of this pathway was demonstrated by overexpression of loss-of-function HDAC4 and HDAC5 mutants in skeletal muscle of obese db/db mice, which enhanced oxidative metabolic capacity, increased apoptosis and ferroptosis and reduced muscle mass. This study identifies HDAC4 and HDAC5 as repressors of skeletal muscle oxidative metabolism, which is linked to inhibition of cell death pathways and preservation of muscle integrity in response to lipotoxicity.

Driving force of deteriorated cellular environment in heart failure: Metabolic remodeling

Heart Failure (HF) has been one of the leading causes of death worldwide. Though its latent mechanism and therapeutic manipulation are updated and developed ceaselessly, there remain great gaps in the cognition of heart failure. High morbidity and readmission rates among HF patients are waiting to be addressed. Recent studies have found that myocardial energy metabolism was closely related to heart failure, in which substrate utilization, as well as intermediate metabolism disorders, insulin resistance, oxidative stress, and mitochondrial dysfunction, might underlie systolic dysfunction and progression of HF. This article centers on the changes and counteraction of cardiac energy metabolism in the failing heart. Therefore, targeting impaired energy provision is of great potential in the treatment of HF. And shifting the objective from traditional neurohormones to improving the cellular environment is expected to further optimize the management of HF.

The Lure of Cardiac Metabolism in the Diagnosis, Prevention, and Treatment of Heart Failure

Energy substrate metabolism and contractile function are tightly coupled in the heart. Within this framework, heart failure may be viewed as a state of impaired energy transfer. The metabolic changes in the failing heart are linked to functional and structural changes. A worthwhile goal is to measure metabolic flux and its regulation quantitatively, and to do this in a manner that leads to targeted interventions. For several good reasons, this goal has been elusive until now. The development of new analytical and imaging techniques offers the potential of exploring the landscape of metabolic changes across the different stages of heart failure. In this Review Topic of the Month, we focus on concepts and brevity to provide a strategic overview of cardiac metabolism in the diagnosis, prevention, and treatment of nonischemic heart failure.

Insulin Metabolism in Polycystic Ovary Syndrome: Secretion, Signaling, and Clearance

Polycystic ovary syndrome (PCOS) is the most common endocrine and metabolic disorder in women of reproductive age. Its heterogeneous clinical presentation is characterized by hyperandrogenemia, reproductive changes, polycystic ovary morphology, and insulin resistance (IR). The primary pathophysiological process in its multifactorial etiology has not yet been identified. However, the two most proposed core etiologies are the disruption of insulin metabolism and hyperandrogenemia, both of which begin to intertwine and propagate each other in the later stages of the disease. Insulin metabolism can be viewed as the interconnectedness of beta cell function, IR or insulin sensitivity, and insulin clearance. Previous studies of insulin metabolism in PCOS patients have yielded conflicting results, and literature reviews have focused mainly on the molecular mechanisms and clinical implications of IR. In this narrative review, we comprehensively explored the role of insulin secretion, clearance, and decreased sensitivity in target cells as a potential primary insult in PCOS pathogenesis, along with the molecular mechanism behind IR in PCOS.

Progression from prediabetes to type 2 diabetes mellitus induced by overnutrition

Prediabetes has developed into a global pandemic, its prevalence increasing year by year. Although lifestyle changes are advocated as the basis for prediabetes treatment, some patients fail to choose or adhere to appropriate interventions. The basis for selecting an appropriate intervention is determining the stage and cause of the disease. In this review, we aimed to examine the various types and disease processes of prediabetes caused by overnutrition, the present review supporting the hypothesis that overnutrition-induced hyperinsulinemia precedes insulin resistance (IR) and independently causes β-cell dysfunction. Tissue insulin resistance is the main feature of prediabetes with the crosstalk between tissues promoting the formation of systemic insulin resistance. Finally, both β-cell dysfunction induced by hyperinsulinemia or IR and reduced β-cell mass can lead to abnormal insulin secretion and contribute to development of type 2 diabetes mellitus (T2DM). Hence, overnutrition can cause multiple prediabetes phenotypes resulting in development of T2DM through different trajectories. Future diagnosis and treatment should therefore more carefully consider the disease phenotype and stage of development in patients with prediabetes to reduce the incidence of T2DM.

Energy substrate metabolism and oxidative stress in metabolic cardiomyopathy

Metabolic cardiomyopathy is an emerging cause of heart failure in patients with obesity, insulin resistance, and diabetes. It is characterized by impaired myocardial metabolic flexibility, intramyocardial triglyceride accumulation, and lipotoxic damage in association with structural and functional alterations of the heart, unrelated to hypertension, coronary artery disease, and other cardiovascular diseases. Oxidative stress plays an important role in the development and progression of metabolic cardiomyopathy. Mitochondria are the most significant sources of reactive oxygen species (ROS) in cardiomyocytes. Disturbances in myocardial substrate metabolism induce mitochondrial adaptation and dysfunction, manifested as a mismatch between mitochondrial fatty acid oxidation and the electron transport chain (ETC) activity, which facilitates ROS production within the ETC components. In addition, non-ETC sources of mitochondrial ROS, such as β-oxidation of fatty acids, may also produce a considerable quantity of ROS in metabolic cardiomyopathy. Augmented ROS production in cardiomyocytes can induce a variety of effects, including the programming of myocardial energy substrate metabolism, modulation of metabolic inflammation, redox modification of ion channels and transporters, and cardiomyocyte apoptosis, ultimately leading to the structural and functional alterations of the heart. Based on the above mechanistic views, the present review summarizes the current understanding of the mechanisms underlying metabolic cardiomyopathy, focusing on the role of oxidative stress.

Investigating the roles of hyperglycaemia, hyperinsulinaemia and elevated free fatty acids in cardiac function in patients with type 2 diabetes via treatment with insulin compared with empagliflozin: protocol for the HyperCarD2 randomised, crossover trial

INTRODUCTION

Type 2 diabetes (T2D) is characterised by elevated plasma glucose, free fatty acid (FFA) and insulin concentrations, and this metabolic profile is linked to diabetic cardiomyopathy, a diastolic dysfunction at first and increased cardiovascular disease (CVD) risk. Shifting cardiac metabolism towards glucose utilisation has been suggested to improve cardiovascular function and CVD risk, but insulin treatment increases overall glucose oxidation and lowers lipid oxidation, without reducing CVD risk, whereas SGLT2 inhibitors (SGLT2i) increase FFA, ketone body concentrations and lipid oxidation, while decreasing insulin concentrations and CVD risk. The aim of the present study is to elucidate the importance of different metabolic profiles obtained during treatment with a SGLT2i versus insulin for myocardial function in patients with T2D.

METHODS AND ANALYSES

Randomised, crossover study, where 20 patients with T2D and body mass index>28 kg/m² receive 25 mg empagliflozin daily or NPH insulin two times per day first for 5 weeks followed by a 3-week washout before crossing over to the remaining treatment. Insulin treatment is titrated to achieve similar glycaemic control as with empagliflozin. In those randomised to insulin first, glycaemia during an initial empagliflozin run-in period prior to randomisation serves as target glucose. Metabolic and cardiac evaluation is performed before and at the end of each treatment period.The primary endpoint is change (treatment-washout) in left ventricular peak filling rate, as assessed by cardiac MRI with and without acute lowering of plasma FFAs with acipimox. Secondary and explorative endpoints are changes in left atrial passive emptying fraction, left ventricular ejection fraction, central blood volume and metabolic parameters.

ETHICS AND DISSEMINATION

This study is approved by the Danish Medicines Agency (ref. nr.: 2017061587), the Danish Data Protection Agency (ref. nr.: AHH-2017-093) and the Capital Region Ethics Committee (ref. nr.: H-17018846). The trial will be conducted in accordance with ICH-GCP guidelines and the Declaration of Helsinki and all participants will provide oral and written informed consent. Our results, regardless of outcome, will be published in relevant scientific journals and we also will seek to disseminate results through presentations at scientific meetings.

TRIAL REGISTRATION NUMBER

EudraCT: 2017-002101.

Could SGLT2 Inhibitors Improve Exercise Intolerance in Chronic Heart Failure?

Despite the constant improvement of therapeutical options, heart failure (HF) remains associated with high mortality and morbidity. While new developments in guideline-recommended therapies can prolong survival and postpone HF hospitalizations, impaired exercise capacity remains one of the most debilitating symptoms of HF. Exercise intolerance in HF is multifactorial in origin, as the underlying cardiovascular pathology and reactive changes in skeletal muscle composition and metabolism both contribute. Recently, sodium-related glucose transporter 2 (SGLT2) inhibitors were found to improve cardiovascular outcomes significantly. Whilst much effort has been devoted to untangling the mechanisms responsible for these cardiovascular benefits of SGLT2 inhibitors, little is known about the effect of SGLT2 inhibitors on exercise performance in HF. This review provides an overview of the pathophysiological mechanisms that are responsible for exercise intolerance in HF, elaborates on the potential SGLT2-inhibitor-mediated effects on these phenomena, and provides an up-to-date overview of existing studies on the effect of SGLT2 inhibitors on clinical outcome parameters that are relevant to the assessment of exercise capacity. Finally, current gaps in the evidence and potential future perspectives on the effects of SGLT2 inhibitors on exercise intolerance in chronic HF are discussed.

Understanding Myocardial Metabolism in the Context of Cardio-Oncology

Cardiovascular events, ranging from arrhythmias to decompensated heart failure, are common during and after cancer therapy. Cardiovascular complications can be life-threatening, and from the oncologist's perspective, could limit the use of first-line cancer therapeutics. Moreover, an aging population increases the risk for comorbidities and medical complexity among patients who undergo cancer therapy. Many have established cardiovascular diagnoses or risk factors before starting these therapies. Therefore, it is essential to understand the molecular mechanisms that drive cardiovascular events in patients with cancer and to identify new therapeutic targets that may prevent and treat these 2 diseases. This review will discuss the metabolic interaction between cancer and the heart and will highlight current strategies of targeting metabolic pathways for cancer treatment. Finally, this review highlights opportunities and challenges in advancing our understanding of myocardial metabolism in the context of cancer and cancer treatment.

Diet-induced obese mice are resistant to improvements in cardiac function resulting from short-term adropin treatment

Previous studies have shown that treatment with recombinant adropin, a circulating peptide secreted by the liver and brain, restores glucose utilization in the hearts of diet-induced obese mice. This restoration of fuel substrate flexibility, which is lost in obese and diabetic animals, has the potential to improve contractile function in the diabetic heart. Using an ex vivo approach, we examined whether short-term adropin treatment could enhance cardiac function in a mouse model of diet-induced obesity. Our study showed that acute adropin treatment reduces inhibitory phosphorylation of pyruvate dehydrogenase in primary neonatal cardiomyocytes, and leads to moderate improvements in ex vivo cardiac function in mice fed a low fat diet. Conversely, short-term exposure to adropin led to a small decrease in cardiac function in mice fed a long-term high fat diet. Insulin treatment did not significantly alter cardiac function in adropin treated hearts from either low or high fat diet mice, however acute adropin treatment did moderately restore some aspects of downstream insulin signaling in high fat diet fed mice. Overall, these data suggest that in an ex vivo setting, acute adropin treatment alone is not sufficient to promote improved cardiac function in obese animals.

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Adropin promotes cardiac glucose utilization in hearts from diet-induced obese mice.

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In this study, we examined whether adropin treatment improves ex vivo cardiac function in obesity.

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While adropin improves contractile function in lean mice, its impact is lost after exposure to a long-term high fat diet.

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Adropin fails to fully restore cardiac insulin signaling in obese mouse hearts.

Role of Adropin in Cardiometabolic Disorders: From Pathophysiological Mechanisms to Therapeutic Target

Although a large amount of data supports the crucial role of endothelial dysfunction (ED) in cardiovascular diseases (CVDs), there is a large bench-to-bedside chasm between basic and clinical research of ED, limiting the implementation of these findings in everyday clinical settings. Hence, it is important to further investigate the pathophysiological mechanisms underlying ED and find modalities that will alleviate its clinical implementation. Adropin, a highly conserved peptide hormone secreted primarily by the liver, recently emerged as an important regulatory component of the vascular endothelium. Specifically, the vasoprotective role of adropin is achieved mainly by affecting endothelial NO synthesis. Thus, in this review, we aimed to summarize the current knowledge regarding the role of adropin in physiological processes and address the protective role of adropin in endothelium with consequent implications to CV pathologies. We focused on data regarding the role of adropin in the clinical setting, with concurrent implications to future clinical use of adropin. Studies suggest that plasma levels of adropin correlate with indices of ED in various pathologies and enhanced disease progression, implying that adropin may serve as a useful biomarker of ED in the upcoming future. On the other hand, despite notable results with respect to therapeutic potential of adropin in preliminary experiments, further well-designed studies are warranted in order to establish if adropin might be beneficial in this setting.

Homeostasis Disrupted and Restored-A Fresh Look at the Mechanism and Treatment of Obesity During COVID-19

The prevalence of obesity in the United States approaches half of the adult population. The COVID-19 pandemic endangers the health of obese individuals. In addition, the metabolic syndrome poses a challenge to the health of obese adults. Bariatric surgery and diet restore metabolic homeostasis in obese individuals; however, it is still unclear which strategy is most effective. For example, intermittent fasting improves insulin sensitivity and diet alone decreases visceral adipose tissue at a disproportionately high rate compared to weight loss. Bariatric surgery causes rapid remission of type 2 diabetes and increases incretins for long-term remission of insulin resistance before meaningful weight loss has occurred. Malabsorptive surgeries have provided insight into the mechanism of altering metabolic parameters, but strong evidence to determine the duration of their effects is yet to be established. When determining the best method of weight loss, metabolic parameters, target weight loss, and risk-benefit analysis must be considered carefully. In this review, we address the pros and cons for the optimal way to restore metabolic homeostasis.

Insulin signaling in the heart

Insulin receptors are highly expressed in the heart and vasculature. Insulin signaling regulates cardiac growth, survival, substrate uptake, utilization, and mitochondrial metabolism. Insulin signaling modulates the cardiac responses to physiological and pathological stressors. Altered insulin signaling in the heart may contribute to the pathophysiology of ventricular remodeling and heart failure progression. Myocardial insulin signaling adapts rapidly to changes in the systemic metabolic milieu. What may initially represent an adaptation to protect the heart from carbotoxicity may contribute to amplifying the risk of heart failure in obesity and diabetes. This review article presents the multiple roles of insulin signaling in cardiac physiology and pathology and discusses the potential therapeutic consequences of modulating myocardial insulin signaling.

The Diabetic Cardiomyopathy: The Contributing Pathophysiological Mechanisms

Individuals with diabetes mellitus (DM) disclose a higher incidence and a poorer prognosis of heart failure (HF) than non-diabetic people, even in the absence of other HF risk factors. The adverse impact of diabetes on HF likely reflects an underlying “diabetic cardiomyopathy” (DM–CMP), which may by exacerbated by left ventricular hypertrophy and coronary artery disease (CAD). The pathogenesis of DM-CMP has been a hot topic of research since its first description and is still under active investigation, as a complex interplay among multiple mechanisms may play a role at systemic, myocardial, and cellular/molecular levels. Among these, metabolic abnormalities such as lipotoxicity and glucotoxicity, mitochondrial damage and dysfunction, oxidative stress, abnormal calcium signaling, inflammation, epigenetic factors, and others. These disturbances predispose the diabetic heart to extracellular remodeling and hypertrophy, thus leading to left ventricular diastolic and systolic dysfunction. This Review aims to outline the major pathophysiological changes and the underlying mechanisms leading to myocardial remodeling and cardiac functional derangement in DM-CMP.

Run for your life: can exercise be used to effectively target GLUT4 in diabetic cardiac disease?

The global incidence, associated mortality rates and economic burden of diabetes are now such that it is considered one of the most pressing worldwide public health challenges. Considerable research is now devoted to better understanding the mechanisms underlying the onset and progression of this disease, with an ultimate aim of improving the array of available preventive and therapeutic interventions. One area of particular unmet clinical need is the significantly elevated rate of cardiomyopathy in diabetic patients, which in part contributes to cardiovascular disease being the primary cause of premature death in this population. This review will first consider the role of metabolism and more specifically the insulin sensitive glucose transporter GLUT4 in diabetic cardiac disease, before addressing how we may use exercise to intervene in order to beneficially impact key functional clinical outcomes.

Diabetic phenotype and prognosis of patients with heart failure and preserved ejection fraction in a real life cohort

Background

Heart failure with preserved ejection fraction (HFpEF) is a heterogeneous syndrome, with several underlying etiologic and pathophysiologic factors. The presence of diabetes might identify an important phenotype, with implications for therapeutic strategies. While diabetes is associated with worse prognosis in HFpEF, the prognostic impact of glycemic control is yet unknown. Hence, we investigated phenotypic differences between diabetic and non-diabetic HFpEF patients (pts), and the prognostic impact of glycated hemoglobin (HbA1C).

Methods

We prospectively enrolled 183 pts with HFpEF (78 ± 9 years, 38% men), including 70 (38%) diabetics (type 2 diabetes only). They underwent 2D echocardiography (n = 183), cardiac magnetic resonance (CMR) (n = 150), and were followed for a combined outcome of all-cause mortality and first HF hospitalization. The prognostic impact of diabetes and glycemic control were determined with Cox proportional hazard models, and illustrated by adjusted Kaplan Meier curves.

Results

Diabetic HFpEF pts were younger (76 ± 9 vs 80 ± 8 years, p = 0.002), more obese (BMI 31 ± 6 vs 27 ± 6 kg/m², p = 0.001) and suffered more frequently from sleep apnea (18% vs 7%, p = 0.032). Atrial fibrillation, however, was more frequent in non-diabetic pts (69% vs 53%, p = 0.028). Although no echocardiographic difference could be detected, CMR analysis revealed a trend towards higher LV mass (66 ± 18 vs 71 ± 14 g/m², p = 0.07) and higher levels of fibrosis (53% vs 36% of patients had ECV by T1 mapping > 33%, p = 0.05) in diabetic patients.

Over 25 ± 12 months, 111 HFpEF pts (63%) reached the combined outcome (24 deaths and 87 HF hospitalizations). Diabetes was a significant predictor of mortality and hospitalization for heart failure (HR: 1.72 [1.1–2.6], p = 0.011, adjusted for age, BMI, NYHA class and renal function). In diabetic patients, lower levels of glycated hemoglobin (HbA1C < 7%) were associated with worse prognosis (HR: 2.07 [1.1–4.0], p = 0.028 adjusted for age, BMI, hemoglobin and NT-proBNP levels).

Conclusion

Our study highlights phenotypic features characterizing diabetic patients with HFpEF. Notably, they are younger and more obese than their non-diabetic counterpart, but suffer less from atrial fibrillation. Although diabetes is a predictor of poor outcome in HFpEF, intensive glycemic control (HbA1C < 7%) in diabetic patients is associated with worse prognosis.

Metabolic Aspects of Anthracycline Cardiotoxicity

OPINION STATEMENT

Heart failure (HF) is increasingly recognized as the major complication of chemotherapy regimens. Despite the development of modern targeted therapies such as monoclonal antibodies, doxorubicin (DOXO), one of the most cardiotoxic anticancer agents, still remains the treatment of choice for several solid and hematological tumors. The insurgence of cardiotoxicity represents the major limitation to the clinical use of this potent anticancer drug. At the molecular level, cardiac side effects of DOXO have been associated to mitochondrial dysfunction, DNA damage, impairment of iron metabolism, apoptosis, and autophagy dysregulation. On these bases, the antioxidant and iron chelator molecule, dexrazoxane, currently represents the unique FDA-approved cardioprotectant for patients treated with anthracyclines.A less explored area of research concerns the impact of DOXO on cardiac metabolism. Recent metabolomic studies highlight the possibility that cardiac metabolic alterations may critically contribute to the development of DOXO cardiotoxicity. Among these, the impairment of oxidative phosphorylation and the persistent activation of glycolysis, which are commonly observed in response to DOXO treatment, may undermine the ability of cardiomyocytes to meet the energy demand, eventually leading to energetic failure. Moreover, increasing evidence links DOXO cardiotoxicity to imbalanced insulin signaling and to cardiac insulin resistance. Although anti-diabetic drugs, such as empagliflozin and metformin, have shown interesting cardioprotective effects in vitro and in vivo in different models of heart failure, their mechanism of action is unclear, and their use for the treatment of DOXO cardiotoxicity is still unexplored.This review article aims at summarizing current evidence of the metabolic derangements induced by DOXO and at providing speculations on how key players of cardiac metabolism could be pharmacologically targeted to prevent or cure DOXO cardiomyopathy.

KLF5 Is Induced by FOXO1 and Causes Oxidative Stress and Diabetic Cardiomyopathy

RATIONALE

Diabetic cardiomyopathy (DbCM) is a major complication in type-1 diabetes, accompanied by altered cardiac energetics, impaired mitochondrial function, and oxidative stress. Previous studies indicate that type-1 diabetes is associated with increased cardiac expression of KLF5 (Krüppel-like factor-5) and PPARα (peroxisome proliferator-activated receptor) that regulate cardiac lipid metabolism.

OBJECTIVE

In this study, we investigated the involvement of KLF5 in DbCM and its transcriptional regulation.

METHODS AND RESULTS

KLF5 mRNA levels were assessed in isolated cardiomyocytes from cardiovascular patients with diabetes and were higher compared with nondiabetic individuals. Analyses in human cells and diabetic mice with cardiomyocyte-specific FOXO1 (Forkhead box protein O1) deletion showed that FOXO1 bound directly on the KLF5 promoter and increased KLF5 expression. Diabetic mice with cardiomyocyte-specific FOXO1 deletion had lower cardiac KLF5 expression and were protected from DbCM. Genetic, pharmacological gain and loss of KLF5 function approaches and AAV (adeno-associated virus)-mediated Klf5 delivery in mice showed that KLF5 induces DbCM. Accordingly, the protective effect of cardiomyocyte FOXO1 ablation in DbCM was abolished when KLF5 expression was rescued. Similarly, constitutive cardiomyocyte-specific KLF5 overexpression caused cardiac dysfunction. KLF5 caused oxidative stress via direct binding on NADPH oxidase (NOX)4 promoter and induction of NOX4 (NADPH oxidase 4) expression. This was accompanied by accumulation of cardiac ceramides. Pharmacological or genetic KLF5 inhibition alleviated superoxide formation, prevented ceramide accumulation, and improved cardiac function in diabetic mice.

CONCLUSIONS

Diabetes-mediated activation of cardiomyocyte FOXO1 increases KLF5 expression, which stimulates NOX4 expression, ceramide accumulation, and causes DbCM.

A Role of Glucose Overload in Diabetic Cardiomyopathy in Nonhuman Primates

Type 2 diabetes (T2D) plays a major role in the development of heart failure. Patients with T2D have an increased risk to develop HF than healthy subjects, and they always have very poor outcomes and survival rates. However, the underlying mechanisms for this are still unclear. To help develop new therapeutic interventions, well-characterized animal models for preclinical and translational investigations in T2D and HF are urgently needed. Although studies in rodents are more often used, the research findings in rodents have often failed to be translated into humans due to the significant metabolic differences between rodents and humans. Nonhuman primates (NHPs) serve as valuable translational models between basic studies in rodent models and clinical studies in humans. NHPs can recapitulate the natural progress of these diseases in humans and study the underlying mechanism due to their genetic similarity and comparable spontaneous T2D rates to humans. In this review, we discuss the importance of using NHPs models in understanding diabetic cardiomyopathy (DCM) in humans with aspects of correlations between hyperglycemia and cardiac dysfunction progression, glucose overload, and altered glucose metabolism promoting cardiac oxidative stress and mitochondria dysfunction, glucose, and its effect on cardiac resynchronization therapy with defibrillator (CRT-d), the currently available diabetic NHPs models and the limitations involved in the use of NHP models.

Adropin: a hepatokine modulator of vascular function and cardiac fuel metabolism

Adropin is a nutritionally regulated peptide hormone, secreted primarily by the liver, which modulates metabolic homeostasis in a number of tissues. Growing evidence suggests that adropin is an important regulatory component in a number of cardiovascular pathologies, and may be central to the control of cardiac fuel metabolism and vascular function. In this mini-review, we examine the known facets of adropin biology, discuss open questions in the field, and speculate on the therapeutic potential of targeting adropin-related signaling pathways in cardiovascular diseases.

Myocardial glucotoxicity: Mechanisms and potential therapeutic targets

Besides coronary artery disease, which remains the main cause of heart failure in patients with diabetes, factors independent of coronary artery disease are involved in the development of heart failure in the onset of what is called diabetic cardiomyopathy. Among them, hyperglycaemia - a hallmark of type 2 diabetes - has both acute and chronic deleterious effects on myocardial function, and clearly participates in the establishment of diabetic cardiomyopathy. In the present review, we summarize the cellular and tissular events that occur in a heart exposed to hyperglycaemia, and depict the complex molecular mechanisms proposed to be involved in glucotoxicity. Finally, from a more translational perspective, different therapeutic strategies targeting hyperglycaemia-mediated molecular mechanisms will be detailed.

Maintaining Myocardial Glucose Utilization in Diabetic Cardiomyopathy Accelerates Mitochondrial Dysfunction

Cardiac glucose uptake and oxidation are reduced in diabetes despite hyperglycemia. Mitochondrial dysfunction contributes to heart failure in diabetes. It is unclear whether these changes are adaptive or maladaptive. To directly evaluate the relationship between glucose delivery and mitochondrial dysfunction in diabetic cardiomyopathy, we generated transgenic mice with inducible cardiomyocyte-specific expression of the GLUT4. We examined mice rendered hyperglycemic following low-dose streptozotocin prior to increasing cardiomyocyte glucose uptake by transgene induction. Enhanced myocardial glucose in nondiabetic mice decreased mitochondrial ATP generation and was associated with echocardiographic evidence of diastolic dysfunction. Increasing myocardial glucose delivery after short-term diabetes onset exacerbated mitochondrial oxidative dysfunction. Transcriptomic analysis revealed that the largest changes, driven by glucose and diabetes, were in genes involved in mitochondrial function. This glucose-dependent transcriptional repression was in part mediated by O-GlcNAcylation of the transcription factor Sp1. Increased glucose uptake induced direct O-GlcNAcylation of many electron transport chain subunits and other mitochondrial proteins. These findings identify mitochondria as a major target of glucotoxicity. They also suggest that reduced glucose utilization in diabetic cardiomyopathy might defend against glucotoxicity and caution that restoring glucose delivery to the heart in the context of diabetes could accelerate mitochondrial dysfunction by disrupting protective metabolic adaptations.

Nonalcoholic fatty liver disease and type 2 diabetes: pathophysiological mechanisms shared between the two faces of the same coin

The pathophysiological mechanisms underlying the close relationship between nonalcoholic fatty liver disease (NAFLD) and type 2 diabetes mellitus (T2DM) are multiple, complex and only partially known. The purpose of this paper was to review the current knowledge of these mechanisms in a unified manner. Subjects with NAFLD and T2DM have established insulin resistance (IR), which exacerbates the two comorbidities. IR worsens NAFLD by increasing the accumulation of free fatty acids (FFAs) in the liver. This occurs due to an increase in the influx of FFAs from peripheral adipose tissue by the activation of hormone-sensitive lipase. In addition, there is de novo increased lipogenesis, a transcription factor, the sterols regulatory element-binding transcription factor 1c (SREBP-1c), which activates the expression of several genes strongly promotes lipogenesis by the liver and facilitate storage of triglycerides. Lipids accumulation in the liver induces a chronic stress in the endoplasmic reticulum of the hepatocytes. Genome-wide association studies have identified genetic variants associated with NAFLD severity, but unrelated to IR. In particular, the alteration of patatin-like phospholipase domain-containing protein 3 contributes to the susceptibility to NAFLD. Furthermore, the lipotoxicity of ceramides and diacylglycerol, well known in T2DM, triggers a chronic inflammatory process favoring the progression from hepatic steatosis to steatohepatitis. Reactive oxygen species produced by mitochondrial dysfunction trigger both liver inflammation and beta-cells damage, promoting the progression of both NAFLD and T2DM. The close association between NAFLD and T2DM is bidirectional, as T2DM may trigger both NAFLD onset and its progression, but NAFLD itself may contribute to the development of IR and T2DM. Future studies on the mechanisms will have to deepen the knowledge of the interaction between the two pathologies and should allow the identification of new therapeutic targets for the treatment of NAFLD, currently substantially absent.

Insulin: too much of a good thing is bad

BACKGROUND

Insulin shares a limited physiological concentration range with other endocrine hormones. Not only too low, but also too high systemic insulin levels are detrimental for body functions.

MAIN BODY

The physiological function and clinical relevance of insulin are usually seen in association with its role in maintaining glucose homeostasis. However, insulin is an anabolic hormone which stimulates a large number of cellular responses. Not only too low, but also excess insulin concentrations are detrimental to the physiological balance. Although the glucoregulatory activity of insulin is mitigated during hyperinsulinemia by dampening the efficiency of insulin signaling ("insulin resistance"), this is not the case for most other hormonal actions of insulin, including the promotion of protein synthesis, de novo lipogenesis, and cell proliferation; the inhibition of lipolysis, of autophagy-dependent cellular turnover, and of nuclear factor E2-related factor-2 (Nrf2)-dependent antioxidative; and other defense mechanisms. Hence, there is no general insulin resistance but selective impairment of insulin signaling which causes less glucose uptake from the blood and reduced activation of endothelial NO synthase (eNOS). Because of the largely unrestricted insulin signaling, hyperinsulinemia increases the risk of obesity, type 2 diabetes, and cardiovascular disease and decreases health span and life expectancy. In epidemiological studies, high-dose insulin therapy is associated with an increased risk of cardiovascular disease. Randomized controlled trials of insulin treatment did not observe any effect on disease risk, but these trials only studied low insulin doses up to 40 IU/day. Proof for a causal link between elevated insulin levels and cardiovascular disease risk comes from Mendelian randomization studies comparing individuals with genetically controlled low or high insulin production.

CONCLUSIONS

The detrimental actions of prolonged high insulin concentrations, seen also in cell culture, argue in favor of a lifestyle that limits circadian insulin levels. The health risks associated with hyperinsulinemia may have implications for treatment regimens used in type 2 diabetes.

Glucose transporters in cardiovascular system in health and disease

Glucose transporters are essential for the heart to sustain its function. Due to its nature as a high energy-consuming organ, the heart needs to catabolize a huge quantity of metabolic substrates. For optimized energy production, the healthy heart constantly switches between various metabolites in accordance with substrate availability and hormonal status. This metabolic flexibility is essential for the maintenance of cardiac function. Glucose is part of the main substrates catabolized by the heart and its use is fine-tuned via complex molecular mechanisms that include the regulation of the glucose transporters GLUTs, mainly GLUT4 and GLUT1. Besides GLUTs, glucose can also be transported by cotransporters of the sodium-glucose cotransporter (SGLT) (SLC5 gene) family, in which SGLT1 and SMIT1 were shown to be expressed in the heart. This SGLT-mediated uptake does not seem to be directly linked to energy production but is rather associated with intracellular signalling triggering important processes such as the production of reactive oxygen species. Glucose transport is markedly affected in cardiac diseases such as cardiac hypertrophy, diabetic cardiomyopathy and heart failure. These alterations are not only fingerprints of these diseases but are involved in their onset and progression. The present review will depict the importance of glucose transport in healthy and diseased heart, as well as proposed therapies targeting glucose transporters.

Mechanisms underlying diabetic cardiomyopathy: From pathophysiology to novel therapeutic targets

Diabetic cardiomyopathy (DC) is defined as a clinical condition of cardiac dysfunction that occurs in the absence of coronary atherosclerosis, valvular disease, and hypertension in patients with diabetes mellitus (DM). Despite the increasing worldwide prevalence of DC, due to the global epidemic of DM, the underlying pathophysiology of DC has not been fully elucidated. In addition, the clinical criteria for diagnosing DC have not been established, and specific therapeutic options are not currently available. The current paradigm suggests the impaired cardiomyocyte function arises due to a number of DM-related metabolic disturbances including hyperglycemia, hyperinsulinemia, and hyperlipidemia, which lead to diastolic dysfunction and signs and symptoms of heart failure. Other factors, which have been implicated in the progression of DC, include mitochondrial dysfunction, increased oxidative stress, impaired calcium handling, inflammation, and cardiomyocyte apoptosis. Herein, we review the current theories surrounding the occurrence and progression of DC, and discuss the recent advances in diagnostic methodologies and therapeutic strategies. Moreover, apart from conventional animal DC models, we highlight alternative disease models for studying DC such as the use of patient-derived human induced pluripotent stem cells (hiPSCs) for studying the mechanisms underlying DC. The ability to obtain hiPSC-derived cardiomyocytes from DM patients with a DC phenotype could help identify novel therapeutic targets for preventing and delaying the progression of DC, and for improving clinical outcomes in DM patients.

Metabolic remodelling in heart failure

The heart consumes large amounts of energy in the form of ATP that is continuously replenished by oxidative phosphorylation in mitochondria and, to a lesser extent, by glycolysis. To adapt the ATP supply efficiently to the constantly varying demand of cardiac myocytes, a complex network of enzymatic and signalling pathways controls the metabolic flux of substrates towards their oxidation in mitochondria. In patients with heart failure, derangements of substrate utilization and intermediate metabolism, an energetic deficit, and oxidative stress are thought to underlie contractile dysfunction and the progression of the disease. In this Review, we give an overview of the physiological processes of cardiac energy metabolism and their pathological alterations in heart failure and diabetes mellitus. Although the energetic deficit in failing hearts - discovered >2 decades ago - might account for contractile dysfunction during maximal exertion, we suggest that the alterations of intermediate substrate metabolism and oxidative stress rather than an ATP deficit per se account for maladaptive cardiac remodelling and dysfunction under resting conditions. Treatments targeting substrate utilization and/or oxidative stress in mitochondria are currently being tested in patients with heart failure and might be promising tools to improve cardiac function beyond that achieved with neuroendocrine inhibition.

Insulin therapy in patients with type 2 diabetes and high insulin resistance is associated with increased risk of complications and mortality

Objective: To investigate the relationship between insulin use and clinical outcomes in patients with type 2 diabetes stratified by level of insulin resistance (IR).Methods: Cross sectional analysis of the NHANES database from 2001 to 2010. Sample was comprised of 3,124 individuals with diabetes, representing a US population of 16,713,593. Insulin use was self-reported. Fasting glucose and insulin levels were used to assess IR by HOMA-IR determination. Subjects were allocated within High or Low HOMA-IR groups based on the sample median. Outcome variables were mortality, major adverse cardiovascular events (MACE), and diabetic kidney disease (DKD). Logistic regression adjusting for covariates including glycemic control and comorbidities were performed.Results: In the adjusted model, insulin use was significantly associated with increased risk of mortality (OR: 2.39, 95% CI: 1.136-5.010) having a MACE (OR: 2.45, 95% CI: 1.137-4.550), and developing DKD (OR: 1.89, 95% CI: 1.119-3.198) in the high HOMA-IR group. The association between insulin use and the outcome variables was not statistically significant in patients within the low HOMA-IR group.Conclusions: Insulin use was associated with increased risk of mortality, MACE, and DKD in patients within the high IR group, but the association was not significant within the low IR group. Our findings indicate that insulin therapy could be less beneficial in patients with high IR. Prospective studies are needed to identify subsets of individuals with type 2 diabetes who would benefit the most from insulin therapy, and for which patients, insulin should be avoided.

Remodeling of substrate consumption in the murine sTAC model of heart failure

BACKGROUND

Energy metabolism and substrate selection are key aspects of correct myocardial mechanical function. Myocardial preference for oxidizable substrates changes in both hypertrophy and in overt failure. Previous work has shown that glucose oxidation is upregulated in overpressure hypertrophy, but its fate in overt failure is less clear. Anaplerotic flux of pyruvate into the tricarboxylic acid cycle (TCA) has been posited as a secondary fate of glycolysis, aside from pyruvate oxidation or lactate production.

METHODS AND RESULTS

A model of heart failure that emulates both valvular and hypertensive heart disease, the severe transaortic constriction (sTAC) mouse, was assayed for changes in substrate preference using metabolomic and carbon-13 flux measurements. Quantitative measures of O₂ consumption in the Langendorff perfused mouse heart were paired with ¹³C isotopomer analysis to assess TCA cycle turnover. Since the heart accommodates oxidation of all physiological energy sources, the utilization of carbohydrates, fatty acids, and ketones were measured simultaneously using a triple-tracer NMR method. The fractional contribution of glucose to acetyl-CoA production was upregulated in heart failure, while other sources were not significantly different. A model that includes both pyruvate carboxylation and anaplerosis through succinyl-CoA produced superior fits to the data compared to a model using only pyruvate carboxylation. In the sTAC heart, anaplerosis through succinyl-CoA is elevated, while pyruvate carboxylation was not. Metabolomic data showed depleted TCA cycle intermediate pool sizes versus the control, in agreement with previous results.

CONCLUSION

In the sTAC heart failure model, the glucose contribution to acetyl-CoA production was significantly higher, with compensatory changes in fatty acid and ketone oxidation not reaching a significant level. Anaplerosis through succinyl-CoA is also upregulated, and is likely used to preserve TCA cycle intermediate pool sizes. The triple tracer method used here is new, and can be used to assess sources of acetyl-CoA production in any oxidative tissue.

Immunometabolic cross-talk in the inflamed heart

Inflammatory processes underlie many diseases associated with injury of the heart muscle, including conditions without an obvious inflammatory pathogenic component such as hypertensive and diabetic cardiomyopathy. Persistence of cardiac inflammation can cause irreversible structural and functional deficits. Some are induced by direct damage of the heart muscle by cellular and soluble mediators but also by metabolic adaptations sustained by the inflammatory microenvironment. It is well established that both cardiomyocytes and immune cells undergo metabolic reprogramming in the site of inflammation, which allow them to deal with decreased availability of nutrients and oxygen. However, like in cancer, competition for nutrients and increased production of signalling metabolites such as lactate initiate a metabolic cross-talk between immune cells and cardiomyocytes which, we propose, might tip the balance between resolution of the inflammation versus adverse cardiac remodeling. Here we review our current understanding of the metabolic reprogramming of both heart tissue and immune cells during inflammation, and we discuss potential key mechanisms by which these metabolic responses intersect and influence each other and ultimately define the prognosis of the inflammatory process in the heart.

Loss of GCN5L1 in cardiac cells limits mitochondrial respiratory capacity under hyperglycemic conditions

The mitochondrial acetyltransferase-related protein GCN5L1 controls the activity of fuel substrate metabolism enzymes in several tissues. While previous studies have demonstrated that GCN5L1 regulates fatty acid oxidation in the prediabetic heart, our understanding of its role in overt diabetes is not fully developed. In this study, we examined how hyperglycemic conditions regulate GCN5L1 expression in cardiac tissues, and modeled the subsequent effect in cardiac cells in vitro. We show that GCN5L1 abundance is significantly reduced under diabetic conditions in vivo, which correlated with reduced acetylation of known GCN5L1 fuel metabolism substrate enzymes. Treatment of cardiac cells with high glucose reduced Gcn5l1 expression in vitro, while expression of the counteracting deacetylase enzyme, Sirt3, was unchanged. Finally, we show that genetic depletion of GCN5L1 in H9c2 cells leads to reduced mitochondrial oxidative capacity under high glucose conditions. These data suggest that GCN5L1 expression is highly responsive to changes in cellular glucose levels, and that loss of GCN5L1 activity under hyperglycemic conditions impairs cardiac energy metabolism.

Insulin resistance and insulin hypersecretion in the metabolic syndrome and type 2 diabetes: Time for a conceptual framework shift

While few dispute the existence of the metabolic syndrome as a clustering of factors indicative of poor metabolic health, its utility above that of its individual components in the clinical care of individual patients is questioned. This is likely a consequence of the failure of clinicians and scientists to agree on a unifying mechanism to explain the metabolic syndrome. Insulin resistance has most commonly been proposed for this role and is generally considered to be a root causative factor for not only metabolic syndrome but also for its associated conditions of non-alcoholic fatty liver disease (NAFLD), polycystic ovary syndrome (PCOS), obesity-related type 2 diabetes (T2D) and atherosclerotic cardiovascular disease (ASCVD). An alternative view, for which evidence is mounting, is that hyper-responsiveness of islet β-cells to a hostile environment, such as westernised lifestyle, is primary and that the resulting hyperinsulinaemia drives the other components of the metabolic syndrome. Importantly, within this new conceptual framework, insulin resistance, while always a biomarker and state of poor metabolic health, is not considered to be harmful, but a protective adaptive response of critical tissues including the myocardium against insulin-induced metabolic stress. This major shift in how metabolic syndrome can be considered puts insulin hypersecretion into position as the unifying mechanism. If shown to be correct, this new conceptual framework has major implications for the future prevention and management of the metabolic syndrome, including its associated conditions of NAFLD, PCOS, obesity-related T2D and ASCVD.

Adropin treatment restores cardiac glucose oxidation in pre-diabetic obese mice

Exposure to a high fat (HF) diet promotes increased fatty acid uptake, fatty acid oxidation and lipid accumulation in the heart. These maladaptive changes impact cellular energy metabolism and may promote the development of cardiac dysfunction. Attempts to increase cardiac glucose utilization have been proposed as a way to reverse cardiomyopathy in obese and diabetic individuals. Adropin is a nutrient-regulated metabolic hormone shown to promote glucose oxidation over fatty acid oxidation in skeletal muscle homogenates in vitro. The focus of the current study was to investigate whether adropin can regulate substrate metabolism in the heart following prolonged exposure to a HF diet in vivo. Mice on a long-term HF diet received serial intraperitoneal injections of vehicle or adropin over three days. Cardiac glucose oxidation was significantly reduced in HF animals, which was rescued by acute adropin treatment. Significant decreases in cardiac pyruvate dehydrogenase activity were observed in HF animals, which were also reversed by adropin treatment. In contrast to previous studies, this change was unrelated to Pdk4 expression, which remained elevated in both vehicle- and adropin-treated HF mice. Instead, we show that adropin modulated the expression of the mitochondrial acetyltransferase enzyme GCN5L1, which altered the acetylation status and activity of fuel metabolism enzymes to favor glucose utilization. Our findings indicate that adropin exposure leads to increased cardiac glucose oxidation under HF conditions, and may provide a future therapeutic avenue in the treatment of diabetic cardiomyopathy.

Diabetes-related cardiomyopathy: The sweet story of glucose overload from epidemiology to cellular pathways

Type 2 diabetes (T2D) is a major risk factor for heart failure (HF). Although the number of cases of myocardial infarction in the T2D population has been reduced by 25% over the last 10 years, the incidence of HF is continuously increasing, making it the most worrying diabetes complication. This strongly reinforces the urgent need for innovative therapeutic interventions to prevent cardiac dysfunction in T2D patients. To this end, epidemiological, imaging and animal studies have aimed to highlight the mechanisms involved in the development of diabetic cardiomyopathy. Epidemiological observations clearly show that hyperglycaemia correlates with severity of cardiac dysfunction and mortality in T2D patients. Both animal and cellular studies have demonstrated that, in the context of diabetes, the heart loses its ability to utilize glucose, therefore leading to glucose overload in cardiomyocytes that, in turn, promotes oxidative stress, accumulation of advanced glycation end-products (AGEs) and chronic activation of the hexosamine pathway. These have all been found to activate apoptosis and to alter heart contractility, calcium signalling and mitochondrial function. Although, in the past, tight glycaemic control has failed to improve cardiac function in T2D patients, recent clinical trials have reported cardiovascular benefit with hypoglycaemic antidiabetic drugs of the SGLT2-inhibitor family. This review, based on clinical evidence from mechanistic studies as well as several large clinical trials, covers 15 years of research, and strongly supports the idea that hyperglycaemia and glucose overload play a central role in the pathophysiology of diabetic cardiomyopathy.

A novel inhibitor of pyruvate dehydrogenase kinase stimulates myocardial carbohydrate oxidation in diet-induced obesity

The pyruvate dehydrogenase complex (PDC) is a key control point of energy metabolism and is subject to regulation by multiple mechanisms, including posttranslational phosphorylation by pyruvate dehydrogenase kinase (PDK). Pharmacological modulation of PDC activity could provide a new treatment for diabetic cardiomyopathy, as dysregulated substrate selection is concomitant with decreased heart function. Dichloroacetate (DCA), a classic PDK inhibitor, has been used to treat diabetic cardiomyopathy, but the lack of specificity and side effects of DCA indicate a more specific inhibitor of PDK is needed. This study was designed to determine the effects of a novel and highly selective PDK inhibitor, 2((2,4-dihydroxyphenyl)sulfonyl) isoindoline-4,6-diol (designated PS10), on pyruvate oxidation in diet-induced obese (DIO) mouse hearts compared with DCA-treated hearts. Four groups of mice were studied: lean control, DIO, DIO + DCA, and DIO + PS10. Both DCA and PS10 improved glucose tolerance in the intact animal. Pyruvate metabolism was studied in perfused hearts supplied with physiological mixtures of long chain fatty acids, lactate, and pyruvate. Analysis was performed using conventional ¹H and ¹³C isotopomer methods in combination with hyperpolarized [1-¹³C]pyruvate in the same hearts. PS10 and DCA both stimulated flux through PDC as measured by the appearance of hyperpolarized [¹³C]bicarbonate. DCA but not PS10 increased hyperpolarized [1-¹³C]lactate production. Total carbohydrate oxidation was reduced in DIO mouse hearts but increased by DCA and PS10, the latter doing so without increasing lactate production. The present results suggest that PS10 is a more suitable PDK inhibitor for treatment of diabetic cardiomyopathy.

MECHANISMS IN ENDOCRINOLOGY: Diabetic cardiomyopathy: pathophysiology and potential metabolic interventions state of the art review

Heart failure is a major cause of morbidity and mortality in type 2 diabetes. Type 2 diabetes contributes to the development of heart failure through a variety of mechanisms, including disease-specific myocardial structural, functional and metabolic changes. This review will focus on the contemporary contributions of state of the art non-invasive technologies to our understanding of diabetic cardiomyopathy, including data on cardiac disease phenotype, cardiac energy metabolism and energetic deficiency, ectopic and visceral adiposity, diabetic liver disease, metabolic modulation strategies and cardiovascular outcomes with new classes of glucose-lowering therapies.

Chronic O-GlcNAcylation and Diabetic Cardiomyopathy: The Bitterness of Glucose

Type 2 diabetes (T2D) is a major risk factor for heart failure. Diabetic cardiomyopathy (DC) is characterized by diastolic dysfunction and left ventricular hypertrophy. Epidemiological data suggest that hyperglycaemia contributes to the development of DC. Several cellular pathways have been implicated in the deleterious effects of high glucose concentrations in the heart: oxidative stress, accumulation of advanced glycation end products (AGE), and chronic hexosamine biosynthetic pathway (HBP) activation. In the present review, we focus on the effect of chronic activation of the HBP on diabetic heart function. The HBP supplies N-acetylglucosamine moiety (O-GlcNAc) that is O-linked by O-GlcNAc transferase (OGT) to proteins on serine or threonine residues. This post-translational protein modification modulates the activity of the targeted proteins. In the heart, acute activation of the HBP in response to ischaemia-reperfusion injury appears to be protective. Conversely, chronic activation of the HBP in the diabetic heart affects Ca²⁺ handling, contractile properties, and mitochondrial function and promotes stress signaling, such as left ventricular hypertrophy and endoplasmic reticulum stress. Many studies have shown that O-GlcNAc impairs the function of key protein targets involved in these pathways, such as phospholamban, calmodulin kinase II, troponin I, and FOXO1. The data show that excessive O-GlcNAcylation is a major trigger of the glucotoxic events that affect heart function under chronic hyperglycaemia. Supporting this finding, pharmacological or genetic inhibition of the HBP in the diabetic heart improves heart function. In addition, the SGLT2 inhibitor dapagliflozin, a glucose lowering agent, has recently been shown to lower cardiac HBP in a lipodystophic T2D mice model and to concomitantly improve the diastolic dysfunction of these mice. Therefore, targeting cardiac-excessive O-GlcNAcylation or specific target proteins represents a potential therapeutic option to treat glucotoxicity in the diabetic heart.

Probing the cardiac malate-aspartate shuttle non-invasively using hyperpolarized [1,2-13 C2 ]pyruvate

Previous studies have demonstrated that using hyperpolarized [2-¹³ C]pyruvate as a contrast agent can reveal ¹³ C signals from metabolites associated with the tricarboxylic acid (TCA) cycle. However, the metabolites detectable from TCA cycle-mediated oxidation of [2-¹³ C]pyruvate are the result of several metabolic steps. In the instance of the [5-¹³ C]glutamate signal, the amplitude can be modulated by changes to the rates of pyruvate dehydrogenase (PDH) flux, TCA cycle flux and metabolite pool size. Also key is the malate-aspartate shuttle, which facilitates the transport of cytosolic reducing equivalents into the mitochondria for oxidation via the malate-α-ketoglutarate transporter, a process coupled to the exchange of cytosolic malate for mitochondrial α-ketoglutarate. In this study, we investigated the mechanism driving the observed changes to hyperpolarized [2-¹³ C]pyruvate metabolism. Using hyperpolarized [1,2-¹³ C]pyruvate with magnetic resonance spectroscopy (MRS) in the porcine heart with different workloads, it was possible to probe ¹³ C-glutamate labeling relative to rates of cytosolic metabolism, PDH flux and TCA cycle turnover in a single experiment non-invasively. Via the [1-¹³ C]pyruvate label, we observed more than a five-fold increase in the cytosolic conversion of pyruvate to [1-¹³ C]lactate and [1-¹³ C]alanine with higher workload. ¹³ C-Bicarbonate production by PDH was increased by a factor of 2.2. Cardiac cine imaging measured a two-fold increase in cardiac output, which is known to couple to TCA cycle turnover. Via the [2-¹³ C]pyruvate label, we observed that ¹³ C-acetylcarnitine production increased 2.5-fold in proportion to the ¹³ C-bicarbonate signal, whereas the ¹³ C-glutamate metabolic flux remained constant on adrenergic activation. Thus, the ¹³ C-glutamate signal relative to the amount of ¹³ C-labeled acetyl-coenzyme A (acetyl-CoA) entering the TCA cycle was decreased by 40%. The data strongly suggest that NADH (reduced form of nicotinamide adenine dinucleotide) shuttling from the cytosol to the mitochondria via the malate-aspartate shuttle is limited on adrenergic activation. Changes in [5-¹³ C]glutamate production from [2-¹³ C]pyruvate may play an important future role in non-invasive myocardial assessment in patients with cardiovascular diseases, but careful interpretation of the results is required.

Sex Differences in Cardiac Mitochondria in the New Zealand Obese Mouse

Background: Obesity is a risk factor for diseases including type 2 diabetes mellitus (T2DM) and cardiovascular disorders. Diabetes itself contributes to cardiac damage. Thus, studying cardiovascular events and establishing therapeutic intervention in the period of type T2DM onset and manifestation are of highest importance. Mitochondrial dysfunction is one of the pathophysiological mechanisms leading to impaired cardiac function. Methods: An adequate animal model for studying pathophysiology of T2DM is the New Zealand Obese (NZO) mouse. These mice were maintained on a high-fat diet (HFD) without carbohydrates for 13 weeks followed by 4 week HFD with carbohydrates. NZO mice developed severe obesity and only male mice developed manifest T2DM. We determined cardiac phenotypes and mitochondrial function as well as cardiomyocyte signaling in this model. Results: The development of an obese phenotype and T2DM in male mice was accompanied by an impaired systolic function as judged by echocardiography and MyH6/7 expression. Moreover, the mitochondrial function only in male NZO hearts was significantly reduced and ERK1/2 and AMPK protein levels were altered. Conclusions: This is the first report demonstrating that the cardiac phenotype in male diabetic NZO mice is associated with impaired cardiac energy function and signaling events.

Catabolic and anabolic faces of insulin resistance and their disorders: a new insight into circadian control of metabolic disorders leading to diabetes

Maintenance of glucose homeostasis during circadian behavioral cycles is critical. The processes controlling the switch between predominant lipolysis/fatty oxidation during fasting and predominant lipid storage/glucose oxidation following feeding are determined principally by insulin. Chronic elevated threshold of insulin resistance (IR) is a key pathological feature of obesity, Type 2 diabetes, sepsis and cancer cachexia; however, a temporal reduced threshold of IR is widely met in fasting/hibernation, pregnancy, antibacterial immunity, exercise and stress. Paradoxically, some of these cases are associated with catabolic metabolism, whereas others are related to anabolic pathways. This article considers the possible causes of circadian disorders in glucose and lipid metabolism that act as a driving force for obesity-promoted development of Type 2 diabetes. This is intended to provide improved insight into the pathogenesis of chronic circadian disorders that increase the risk of diabetes, and consider new targets for its metabolic and drug correction.

Insulin resistance (IR) is a common adaptive mechanism, acting under opposite anabolic and catabolic conditions. However, chronic IR is a key pathological feature of obesity, Type 2 diabetes and cancer cachexia, whereas a temporal IR is widely seen in fasting, pregnancy, exercise and stress. Therefore, it is important to understand when this transient IR-mechanism shifts to chronic IR-associated diseases. What factors result in the switch between the anabolic and catabolic conditions and what defect(s) in this switch is associated with chronic IR induction? The present opinion article aimed to address these questions to the metabolic changes typical for circadian regulation in lean, obese and diabetic patients.

Graphical abstract: Early circadian IR disorders caused by overweight and obesity are associated with increased risk for diabetes via formation of a vicious cycle between lipid anabolic and catabolic programs thus distorting insulin and lipid levels in day/night period.

Diabetes-Related Cardiac Dysfunction

The proposal that diabetes plays a role in the development of heart failure is supported by the increased risk associated with this disease, even after correcting for all other known risk factors. However, the precise mechanisms contributing to the condition referred to as diabetic cardiomyopathy have remained elusive, as does defining the disease itself. Decades of study have defined numerous potential factors that each contribute to disease susceptibility, progression, and severity. Many recent detailed reviews have been published on mechanisms involving insulin resistance, dysregulation of microRNAs, and increased reactive oxygen species, as well as causes including both modifiable and non-modifiable risk factors. As such, the focus of the current review is to highlight aspects of each of these topics and to provide specific examples of recent advances in each area.

Effects of exenatide on cardiac function, perfusion, and energetics in type 2 diabetic patients with cardiomyopathy: a randomized controlled trial against insulin glargine

BACKGROUND

Multiple bloodglucose-lowering agents have been linked to cardiovascular events. Preliminary studies showed improvement in left ventricular (LV) function during glucagon-like peptide-1 receptor agonist administration. Underlying mechanisms, however, are unclear. The purpose of this study was to investigate myocardial perfusion and oxidative metabolism in type 2 diabetic (T2DM) patients with LV systolic dysfunction as compared to healthy controls. Furthermore, effects of 26-weeks of exenatide versus insulin glargine administration on cardiac function, perfusion and oxidative metabolism in T2DM patients with LV dysfunction were explored.

METHODS AND RESULTS

Twenty-six T2DM patients with LV systolic dysfunction (cardiac magnetic resonance (CMR) derived LV ejection fraction (LVEF) of 47 ± 13%) and 10 controls (LVEF of 59 ± 4%, P < 0.01 as compared to patients) were analyzed. Both myocardial perfusion during adenosine-induced hyperemia (P < 0.01), and coronary flow reserve (P < 0.01), measured by [¹⁵O]H₂O positron emission tomography (PET), were impaired in T2DM patients as compared to healthy controls. Myocardial oxygen consumption and myocardial efficiency, measured using [¹¹C]acetate PET and CMR derived stroke volume, were not different between the groups. Eleven patients in the exenatide group and 12 patients in the insulin glargine group completed the trial. Systemic metabolic control was improved after both treatments, although, no changes in cardiac function, perfusion and metabolism were seen after exenatide or insulin glargine.

CONCLUSIONS

T2DM patients with LV systolic dysfunction did not have altered myocardial efficiency as compared to healthy controls. Exenatide or insulin glargine had no effects on cardiac function, perfusion or oxidative metabolism. Trial registration NCT00766857.

Exercise Intolerance In Heart Failure With Preserved Ejection Fraction

More than 50% of Americans with heart failure have preserved ejection fraction (HFpEF). Exercise intolerance is a hallmark of HFpEF, but the pathophysiology is not well understood. Diverse etiologies and incomplete mechanistic understanding have resulted in ineffective management strategies to improve the outcomes of HFpEF. Traditional therapies that have been beneficial in the treatment of heart failure with reduced ejection fraction (HFrEF), neurohormonal blockade in particular, have not been effective in treating HFpEF. In this review, we address underlying mechanisms of HFpEF and present the rationale supporting exercise as a component of comprehensive management.

Metabolic Remodeling in Diabetic Cardiomyopathy

Diabetes is a risk factor for heart failure and cardiovascular mortality with specific changes to myocardial metabolism, energetics, structure, and function. The gradual impairment of insulin production and signalling in diabetes is associated with elevated plasma fatty acids and increased myocardial free fatty acid uptake and activation of the transcription factor PPARα. The increased free fatty acid uptake results in accumulation of toxic metabolites, such as ceramide and diacylglycerol, activation of protein kinase C, and elevation of uncoupling protein-3. Insulin signalling and glucose uptake/oxidation become further impaired, and mitochondrial function and ATP production become compromised. Increased oxidative stress also impairs mitochondrial function and disrupts metabolic pathways. The diabetic heart relies on free fatty acids (FFA) as the major substrate for oxidative phosphorylation and is unable to increase glucose oxidation during ischaemia or hypoxia, thereby increasing myocardial injury, especially in ageing female diabetic animals. Pharmacological activation of PPARγ in adipose tissue may lower plasma FFA and improve recovery from myocardial ischaemic injury in diabetes. Not only is the diabetic heart energetically-impaired, it also has early diastolic dysfunction and concentric remodelling. The contractile function of the diabetic myocardium negatively correlates with epicardial adipose tissue, which secretes proinflammatory cytokines, resulting in interstitial fibrosis. Novel pharmacological strategies targeting oxidative stress seem promising in preventing progression of diabetic cardiomyopathy, although clinical evidence is lacking. Metabolic agents that lower plasma FFA or glucose, including PPARγ agonism and SGLT2 inhibition, may therefore be promising options.

My Sweetheart Is Broken: Role of Glucose in Diabetic Cardiomyopathy

Despite overall reductions in heart disease prevalence, the risk of developing heart failure has remained 2-fold greater among people with diabetes. Growing evidence has supported that fluctuations in glucose level and uptake contribute to cardiovascular disease (CVD) by modifying proteins, DNA, and gene expression. In the case of glucose, clinical studies have shown that increased dietary sugars for healthy individuals or poor glycemic control in diabetic patients further increased CVD risk. Furthermore, even after decades of maintaining tight glycemic control, susceptibility to disease progression can persist following a period of poor glycemic control through a process termed "glycemic memory." In response to chronically elevated glucose levels, a number of studies have identified molecular targets of the glucose-mediated protein posttranslational modification by the addition of an O-linked N-acetylglucosamine to impair contractility, calcium sensitivity, and mitochondrial protein function. Additionally, elevated glucose contributes to dysfunction in coupling glycolysis to glucose oxidation, pentose phosphate pathway, and polyol pathway. Therefore, in the "sweetened" environment associated with hyperglycemia, there are a number of pathways contributing to increased susceptibly to "breaking" the heart of diabetics. In this review we will discuss the unique contribution of glucose to heart disease and recent advances in defining mechanisms of action.

Sodium-myoinositol cotransporter-1, SMIT1, mediates the production of reactive oxygen species induced by hyperglycemia in the heart

Hyperglycemia (HG) stimulates the production of reactive oxygen species in the heart through activation of NADPH oxidase 2 (NOX2). This production is independent of glucose metabolism but requires sodium/glucose cotransporters (SGLT). Seven SGLT isoforms (SGLT1 to 6 and sodium-myoinositol cotransporter-1, SMIT1) are known, although their expression and function in the heart remain elusive. We investigated these 7 isoforms and found that only SGLT1 and SMIT1 were expressed in mouse, rat and human hearts. In cardiomyocytes, galactose (transported through SGLT1) did not activate NOX2. Accordingly, SGLT1 deficiency did not prevent HG-induced NOX2 activation, ruling it out in the cellular response to HG. In contrast, myo-inositol (transported through SMIT1) reproduced the toxic effects of HG. SMIT1 overexpression exacerbated glucotoxicity and sensitized cardiomyocytes to HG, whereas its deletion prevented HG-induced NOX2 activation. In conclusion, our results show that heart SMIT1 senses HG and triggers NOX2 activation. This could participate in the redox signaling in hyperglycemic heart and contribute to the pathophysiology of diabetic cardiomyopathy.