CONVERGENCE OF GLUCOSE- AND FATTY ACID-INDUCED ABNORMAL MYOCARDIAL EXCITATION-CONTRACTION COUPLING AND INSULIN SIGNALLING

Uridine and its role in metabolic diseases, tumors, and neurodegenerative diseases

Uridine is a pyrimidine nucleoside found in plasma and cerebrospinal fluid with a concentration higher than the other nucleosides. As a simple metabolite, uridine plays a pivotal role in various biological processes. In addition to nucleic acid synthesis, uridine is critical to glycogen synthesis through the formation of uridine diphosphate glucose in which promotes the production of UDP-GlcNAc in the hexosamine biosynthetic pathway and supplies UDP-GlcNAc for O-GlcNAcylation. This process can regulate protein modification and affect its function. Moreover, Uridine has an effect on body temperature and circadian rhythms, which can regulate the metabolic rate and the expression of metabolic genes. Abnormal levels of blood uridine have been found in people with diabetes and obesity, suggesting a link of uridine dysregulation and metabolic disorders. At present, the role of uridine in glucose metabolism and lipid metabolism is controversial, and the mechanism is not clear, but it shows the trend of long-term damage and short-term benefit. Therefore, maintaining uridine homeostasis is essential for maintaining basic functions and normal metabolism. This article summarizes the latest findings about the metabolic effects of uridine and the potential of uridine metabolism as therapeutic target in treatment of metabolic disorders.

Metabolic and Biochemical Stressors in Diabetic Cardiomyopathy

Diabetic cardiomyopathy (DCM) or diabetes-induced cardiac dysfunction is a direct consequence of uncontrolled metabolic syndrome and is widespread in US population and worldwide. Despite of the heterogeneous and distinct features of DCM, the clinical relevance of DCM is now becoming established. DCM progresses to pathological cardiac remodeling with the higher risk of heart attack and subsequent heart failure in diabetic patients. In this review, we emphasize lipid substrate quality and the phenotypic, metabolic, and biochemical stressors of DCM in the rodent and human pathophysiology. We discuss lipoxygenase signaling in the inflammatory pathway with multiple contributing and confounding factors leading to DCM. Additionally, emerging biochemical pathways are emphasized to make progress toward therapeutic advancement to treat DCM.

Insights into the role of maladaptive hexosamine biosynthesis and O-GlcNAcylation in development of diabetic cardiac complications

Diabetes mellitus significantly increases the risk of heart failure, independent of coronary artery disease. The mechanisms implicated in the development of diabetic heart disease, commonly termed diabetic cardiomyopathy, are complex, but much of the impact of diabetes on the heart can be attributed to impaired glucose handling. It has been shown that the maladaptive nutrient-sensing hexosamine biosynthesis pathway (HBP) contributes to diabetic complications in many non-cardiac tissues. Glucose metabolism by the HBP leads to enzymatically-regulated, O-linked attachment of a sugar moiety molecule, β-N-acetylglucosamine (O-GlcNAc), to proteins, affecting their biological activity (similar to phosphorylation). In normal physiology, transient activation of HBP/O-GlcNAc mechanisms is an adaptive, protective means to enhance cell survival; interventions that acutely suppress this pathway decrease tolerance to stress. Conversely, chronic dysregulation of HBP/O-GlcNAc mechanisms has been shown to be detrimental in certain pathological settings, including diabetes and cancer. Most of our understanding of the impact of sustained maladaptive HBP and O-GlcNAc protein modifications has been derived from adipose tissue, skeletal muscle and other non-cardiac tissues, as a contributing mechanism to insulin resistance and progression of diabetic complications. However, the long-term consequences of persistent activation of cardiac HBP and O-GlcNAc are not well-understood; therefore, the goal of this timely review is to highlight current understanding of the role of the HBP pathway in development of diabetic cardiomyopathy.

Diabetic cardiomyopathy: mechanisms and new treatment strategies targeting antioxidant signaling pathways

Cardiovascular disease is the primary cause of morbidity and mortality among the diabetic population. Both experimental and clinical evidence suggest that diabetic subjects are predisposed to a distinct cardiomyopathy, independent of concomitant macro- and microvascular disorders. 'Diabetic cardiomyopathy' is characterized by early impairments in diastolic function, accompanied by the development of cardiomyocyte hypertrophy, myocardial fibrosis and cardiomyocyte apoptosis. The pathophysiology underlying diabetes-induced cardiac damage is complex and multifactorial, with elevated oxidative stress as a key contributor. We now review the current evidence of molecular disturbances present in the diabetic heart, and their role in the development of diabetes-induced impairments in myocardial function and structure. Our focus incorporates both the contribution of increased reactive oxygen species production and reduced antioxidant defenses to diabetic cardiomyopathy, together with modulation of protein signaling pathways and the emerging role of protein O-GlcNAcylation and miRNA dysregulation in the progression of diabetic heart disease. Lastly, we discuss both conventional and novel therapeutic approaches for the treatment of left ventricular dysfunction in diabetic patients, from inhibition of the renin-angiotensin-aldosterone-system, through recent evidence favoring supplementation of endogenous antioxidants for the treatment of diabetic cardiomyopathy. Novel therapeutic strategies, such as gene therapy targeting the phosphoinositide 3-kinase PI3K(p110α) signaling pathway, and miRNA dysregulation, are also reviewed. Targeting redox stress and protective protein signaling pathways may represent a future strategy for combating the ever-increasing incidence of heart failure in the diabetic population.

Fructose modulates cardiomyocyte excitation-contraction coupling and Ca²⁺ handling in vitro

BACKGROUND

High dietary fructose has structural and metabolic cardiac impact, but the potential for fructose to exert direct myocardial action is uncertain. Cardiomyocyte functional responsiveness to fructose, and capacity to transport fructose has not been previously demonstrated.

OBJECTIVE

The aim of the present study was to seek evidence of fructose-induced modulation of cardiomyocyte excitation-contraction coupling in an acute, in vitro setting.

METHODS AND RESULTS

The functional effects of fructose on isolated adult rat cardiomyocyte contractility and Ca²⁺ handling were evaluated under physiological conditions (37°C, 2 mM Ca²⁺, HEPES buffer, 4 Hz stimulation) using video edge detection and microfluorimetry (Fura2) methods. Compared with control glucose (11 mM) superfusate, 2-deoxyglucose (2 DG, 11 mM) substitution prolonged both the contraction and relaxation phases of the twitch (by 16 and 36% respectively, p<0.05) and this effect was completely abrogated with fructose supplementation (11 mM). Similarly, fructose prevented the Ca²⁺ transient delay induced by exposure to 2 DG (time to peak Ca²⁺ transient: 2 DG: 29.0±2.1 ms vs. glucose: 23.6±1.1 ms vs. fructose +2 DG: 23.7±1.0 ms; p<0.05). The presence of the fructose transporter, GLUT5 (Slc2a5) was demonstrated in ventricular cardiomyocytes using real time RT-PCR and this was confirmed by conventional RT-PCR.

CONCLUSION

This is the first demonstration of an acute influence of fructose on cardiomyocyte excitation-contraction coupling. The findings indicate cardiomyocyte capacity to transport and functionally utilize exogenously supplied fructose. This study provides the impetus for future research directed towards characterizing myocardial fructose metabolism and understanding how long term high fructose intake may contribute to modulating cardiac function.

Elevated dietary sugar and the heart: experimental models and myocardial remodeling

A dramatic rise in the prevalence of insulin resistance has been paralleled by increasing dietary consumption of sugar. The use of added sweeteners containing fructose (sucrose and high-fructose corn syrup) has increased by 25% over the past 3 decades. High fructose intake has the potential to adversely influence systemic and cellular metabolism via insulin resistance and glycolytic dysregulation. As a tissue that is both insulin sensitive and glycolysis dependent, the heart may be especially vulnerable to fructose over-consumption. In this review, experimental studies of elevated dietary sugar intake are evaluated, including sucrose and fructose dietary manipulation models. The possible role of the GLUT5 transporter as a mediator of cardiomyocyte fructose uptake is considered. The impact of dietary sucrose and fructose on cardiac insulin-dependent signaling in the context of perturbed systemic metabolic response is detailed. Myocardial dysfunction, modified growth, and oxidative stress responses associated with high dietary sugar intake are discussed. Finally, the involvement of the renin-angiotensin system in mediating fructose cardiopathology is considered. This review highlights the importance of obtaining new mechanistic data that can contribute to a more developed understanding of how high sugar intake directly contributes to structural and functional cardiomyopathy.

Activation of the hexosamine biosynthesis pathway and protein O-GlcNAcylation modulate hypertrophic and cell signaling pathways in cardiomyocytes from diabetic mice

Patients with diabetes have a much greater risk of developing heart failure than non-diabetic patients, particularly in response to an additional hemodynamic stress such as hypertension or infarction. Previous studies have shown that increased glucose metabolism via the hexosamine biosynthesis pathway (HBP) and associated increase in O-linked-β-N-acetylglucosamine (O-GlcNAc) levels on proteins contributed to the adverse effects of diabetes on the heart. Therefore, in this study we tested the hypothesis that diabetes leads to impaired cardiomyocyte hypertrophic and cell signaling pathways due to increased HBP flux and O-GlcNAc modification on proteins. Cardiomyocytes isolated from type 2 diabetic db/db mice and non-diabetic controls were treated with 1 μM ANG angiotensin II (ANG) and 10 μM phenylephrine (PE) for 24 h. Activation of hypertrophic and cell signaling pathways was determined by assessing protein expression levels of atrial natriuretic peptide (ANP), α-sarcomeric actin, p53, Bax and Bcl-2 and phosphorylation of p38, ERK and Akt. ANG II and PE significantly increased levels of ANP and α-actin and phosphorylation of p38 and ERK in the non-diabetic but not in the diabetic group; phosphorylation of Akt was unchanged irrespective of group or treatment. Constitutive Bcl-2 levels were lower in diabetic hearts, while there was no difference in p53 and Bax. Activation of the HBP and increased protein O-GlcNAcylation in non-diabetic cardiomyocytes exhibited a significantly decreased hypertrophic signaling response to ANG or PE compared to control cells. Inhibition of the HBP partially restored the hypertrophic signaling response of diabetic cardiomyocytes. These results suggest that activation of the HBP and protein O-GlcNAcylation modulates hypertrophic and cell signaling pathways in type 2 diabetes.

Diabetic cardiomyopathy: signaling defects and therapeutic approaches

Diabetes mellitus is the world's fastest growing disease with high morbidity and mortality rates, predominantly as a result of heart failure. A significant number of diabetic patients exhibit diabetic cardiomyopathy; that is, left ventricular dysfunction independent of coronary artery disease or hypertension. The pathogenesis of diabetic cardiomyopathy is complex, and is characterized by dysregulated lipid metabolism, insulin resistance, mitochondrial dysfunction and disturbances in adipokine secretion and signaling. These abnormalities lead to impaired calcium homeostasis, ultimately resulting in lusitropic and inotropic defects. This article discusses the impact of these hallmark factors in diabetic cardiomyopathy, and concludes with a survey of available and emerging therapeutic modalities.

Protein O-GlcNAcylation: a new signaling paradigm for the cardiovascular system

The posttranslational modification of serine and threonine residues of nuclear and cytoplasmic proteins by the O-linked attachment of the monosaccharide beta-N-acetylglucosamine (O-GlcNAc) is a highly dynamic and ubiquitous protein modification. Protein O-GlcNAcylation is rapidly emerging as a key regulator of critical biological processes including nuclear transport, translation and transcription, signal transduction, cytoskeletal reorganization, proteasomal degradation, and apoptosis. Increased levels of O-GlcNAc have been implicated as a pathogenic contributor to glucose toxicity and insulin resistance, which are both major hallmarks of diabetes mellitus and diabetes-related cardiovascular complications. Conversely, there is a growing body of data demonstrating that the acute activation of O-GlcNAc levels is an endogenous stress response designed to enhance cell survival. Reports on the effect of altered O-GlcNAc levels on the heart and cardiovascular system have been growing rapidly over the past few years and have implicated a role for O-GlcNAc in contributing to the adverse effects of diabetes on cardiovascular function as well as mediating the response to ischemic injury. Here, we summarize our present understanding of protein O-GlcNAcylation and its effect on the regulation of cardiovascular function. We examine the pathways regulating protein O-GlcNAcylation and discuss, in more detail, our understanding of the role of O-GlcNAc in both mediating the adverse effects of diabetes as well as its role in mediating cellular protective mechanisms in the cardiovascular system. In addition, we also explore the parallels between O-GlcNAc signaling and redox signaling, as an alternative paradigm for understanding the role of O-GlcNAcylation in regulating cell function.

Calcium sparks

The calcium ion (Ca(2+)) is the simplest and most versatile intracellular messenger known. The discovery of Ca(2+) sparks and a related family of elementary Ca(2+) signaling events has revealed fundamental principles of the Ca(2+) signaling system. A newly appreciated "digital" subsystem consisting of brief, high Ca(2+) concentration over short distances (nanometers to microns) comingles with an "analog" global Ca(2+) signaling subsystem. Over the past 15 years, much has been learned about the theoretical and practical aspects of spark formation and detection. The quest for the spark mechanisms [the activation, coordination, and termination of Ca(2+) release units (CRUs)] has met unexpected challenges, however, and raised vexing questions about CRU operation in situ. Ample evidence shows that Ca(2+) sparks catalyze many high-threshold Ca(2+) processes involved in cardiac and skeletal muscle excitation-contraction coupling, vascular tone regulation, membrane excitability, and neuronal secretion. Investigation of Ca(2+) sparks in diseases has also begun to provide novel insights into hypertension, cardiac arrhythmias, heart failure, and muscular dystrophy. An emerging view is that spatially and temporally patterned activation of the digital subsystem confers on intracellular Ca(2+) signaling an exquisite architecture in space, time, and intensity, which underpins signaling efficiency, stability, specificity, and diversity. These recent advances in "sparkology" thus promise to unify the simplicity and complexity of Ca(2+) signaling in biology.

Interplay between impaired calcium regulation and insulin signaling abnormalities in diabetic cardiomyopathy

According to the International Diabetes Federation the number of people between the ages of 20 and 79 years diagnosed with diabetes mellitus is projected to reach 380 million worldwide by 2025. Cardiovascular disease, including heart failure, is the major cause of death in patients with diabetes. A contributing factor to heart failure in such patients is the development of diabetic cardiomyopathy--a clinical myocardial condition distinguished by ventricular dysfunction that can present independently of other risk factors such as hypertension or coronary artery disease. This disorder has been associated with both type 1 and type 2 diabetes, and is characterized by early-onset diastolic dysfunction and late-onset systolic dysfunction. The development of diabetic cardiomyopathy and the cellular and molecular perturbations associated with the pathology are complex and multifactorial. Hallmark mechanisms include abnormalities in regulation of calcium homeostasis, and associated abnormal ventricular excitation-contraction coupling, metabolic disturbances, and alterations in insulin signaling. An emerging concept is that disruptions in calcium homeostasis might be linked to diminished insulin responsiveness. An understanding of the cellular effect of these abnormalities on cardiomyocytes should be useful in predicting the maladaptive cardiac structural and functional consequences of diabetes.

Aging leads to increased levels of protein O-linked N-acetylglucosamine in heart, aorta, brain and skeletal muscle in Brown-Norway rats

Changes in the levels of O-linked N-acetyl-glucosamine (O-GlcNAc) on nucleocytoplasmic protein have been associated with a number of age-related diseases such as Alzheimer's and diabetes; however, there is relatively little information regarding the impact of age on tissue O-GlcNAc levels. Therefore, the goal of this study was to determine whether senescence was associated with alterations in O-GlcNAc in heart, aorta, brain and skeletal muscle and if so whether there were also changes in the expression of enzymes critical in regulating O-GlcNAc levels, namely, O-GlcNAc transferase (OGT), O-GlcNAcase and glutamine:fructose-6-phosphate amidotransferase (GFAT). Tissues were harvested from 5- and 24-month old Brown-Norway rats; UDP-GlcNAc, a precursor of O-GlcNAc was assessed by HPLC, O-GlcNAc and OGT levels were assessed by immunoblot analysis and GFAT1/2, OGT, O-GlcNAcase mRNA levels were determined by RT-PCR. In the 24-month old animals serum insulin and triglyceride levels were significantly increased compared to the 5-month old group; however, glucose levels were unchanged. Protein O-GlcNAc levels were significantly increased with age (30-107%) in all tissues examined; however, paradoxically the expression of OGT, which catalyzes O-GlcNAc formation, was decreased by approximately 30% in the heart, aorta and brain. In the heart increased O-GlcNAc was associated with increased UDP-GlcNAc levels and elevated GFAT mRNA while in other tissues we found no difference in UDP-GlcNAc or GFAT mRNA levels. These results demonstrate that senescence is associated with increased O-GlcNAc levels in multiple tissues and support the notion that dysregulation of pathways leading to O-GlcNAc formation may play an important role in the development of age-related diseases.

Diabetes provides an unfavorable environment for muscle mass and function after muscle injury in mice

It is of common knowledge that diabetes decreases skeletal muscle contractility and induces atrophy. However, how hyperglycemia and insulin deficiency modify muscle mass and neuromuscular recovery after muscle injury is not well known. We have analyzed two models of diabetes: streptozotocin (STZ)-treated Swiss mice and Akita mice that spontaneously develop diabetes. A fast muscle, the tibialis anterior, was injured following injection of a myotoxic agent (cardiotoxin). Neuromuscular function was evaluated by examining in situ isometric contractile properties of regenerating muscles in response to nerve stimulation 14, 28 and 56 days after myotoxic injury. We found that STZ-induced diabetes reduces muscle weight and absolute maximal tetanic force in both regenerating and uninjured muscles (p = 0.0001). Moreover, it increases specific maximal tetanic force and tetanic fusion in regenerating and uninjured muscles (p = 0.04). In the Akita mice, diabetes decreases muscle weight and absolute maximal tetanic force, and increases tetanic fusion in both regenerating and uninjured muscles (p < or = 0.003). Interestingly, STZ-induced diabetes exerts more marked effects than diabetes of genetic origin, in particular on muscle weight. This reduction in muscle mass was not due to an increased expression of the atrogenes MuRF1 and atrogin-1 during STZ-induced diabetes. The present study in mice demonstrates that both models of diabetes impair regenerating muscles as well as uninjured muscles. Regenerating fast muscles are weaker, lighter and slower in diabetic compared with nondiabetic mice.

Cardiovascular actions of insulin

Insulin has important vascular actions to stimulate production of nitric oxide from endothelium. This leads to capillary recruitment, vasodilation, increased blood flow, and subsequent augmentation of glucose disposal in classical insulin target tissues (e.g., skeletal muscle). Phosphatidylinositol 3-kinase-dependent insulin-signaling pathways regulating endothelial production of nitric oxide share striking parallels with metabolic insulin-signaling pathways. Distinct MAPK-dependent insulin-signaling pathways (largely unrelated to metabolic actions of insulin) regulate secretion of the vasoconstrictor endothelin-1 from endothelium. These and other cardiovascular actions of insulin contribute to coupling metabolic and hemodynamic homeostasis under healthy conditions. Cardiovascular diseases are the leading cause of morbidity and mortality in insulin-resistant individuals. Insulin resistance is typically defined as decreased sensitivity and/or responsiveness to metabolic actions of insulin. This cardinal feature of diabetes, obesity, and dyslipidemia is also a prominent component of hypertension, coronary heart disease, and atherosclerosis that are all characterized by endothelial dysfunction. Conversely, endothelial dysfunction is often present in metabolic diseases. Insulin resistance is characterized by pathway-specific impairment in phosphatidylinositol 3-kinase-dependent signaling that in vascular endothelium contributes to a reciprocal relationship between insulin resistance and endothelial dysfunction. The clinical relevance of this coupling is highlighted by the findings that specific therapeutic interventions targeting insulin resistance often also ameliorate endothelial dysfunction (and vice versa). In this review, we discuss molecular mechanisms underlying cardiovascular actions of insulin, the reciprocal relationships between insulin resistance and endothelial dysfunction, and implications for developing beneficial therapeutic strategies that simultaneously target metabolic and cardiovascular diseases.

Impairment of cardiac insulin signaling and myocardial contractile performance in high-cholesterol/fructose-fed rats

Although insulin resistance is recognized as a potent and prevalent risk factor for coronary heart disease, less is known as to whether insulin resistance causes an altered cardiac phenotype independent of coronary atherosclerosis. In this study, we investigated the relationship between insulin resistance and cardiac contractile dysfunctions by generating a new insulin resistance animal model with rats on high cholesterol-fructose diet. Male Sprague-Dawley rats were given high cholesterol-fructose (HCF) diet for 15 wk; the rats developed insulin resistance syndrome characterized by elevated blood pressure, hyperlipidemia, hyperinsulinemia, impaired glucose tolerance, and insulin resistance. The results show that HCF induced insulin resistance not only in metabolic-response tissues (i.e., liver and muscle) but also in the heart as well. Insulin-stimulated cardiac glucose uptake was significantly reduced after 15 wk of HCF feeding, and cardiac insulin resistance was associated with blunted Akt-mediated insulin signaling along with glucose transporter GLUT4 translocation. Basal fatty acid transporter FATP1 levels were increased in HCF rat hearts. The cardiac performance of the HCF rats exhibited a marked reduction in cardiac output, ejection fraction, stroke volume, and end-diastolic volume. It also showed decreases in left ventricular end-systolic elasticity, whereas the effective arterial elasticity was increased. In addition, the relaxation time constant of left ventricular pressure was prolonged in the HCF group. Overall, these results indicate that insulin resistance reduction of cardiac glucose uptake is associated with defects in insulin signaling. The cardiac metabolic alterations that impair contractile functions may lead to the development of cardiomyopathy.