Polyol pathway and modulation of ischemia-reperfusion injury in Type 2 diabetic BBZ rat hearts

The role of glycolytic metabolic pathways in cardiovascular disease and potential therapeutic approaches

Cardiovascular disease (CVD) is a major threat to human health, accounting for 46% of non-communicable disease deaths. Glycolysis is a conserved and rigorous biological process that breaks down glucose into pyruvate, and its primary function is to provide the body with the energy and intermediate products needed for life activities. The non-glycolytic actions of enzymes associated with the glycolytic pathway have long been found to be associated with the development of CVD, typically exemplified by metabolic remodeling in heart failure, which is a condition in which the heart exhibits a rapid adaptive response to hypoxic and hypoxic conditions, occurring early in the course of heart failure. It is mainly characterized by a decrease in oxidative phosphorylation and a rise in the glycolytic pathway, and the rise in glycolysis is considered a hallmark of metabolic remodeling. In addition to this, the glycolytic metabolic pathway is the main source of energy for cardiomyocytes during ischemia-reperfusion. Not only that, the auxiliary pathways of glycolysis, such as the polyol pathway, hexosamine pathway, and pentose phosphate pathway, are also closely related to CVD. Therefore, targeting glycolysis is very attractive for therapeutic intervention in CVD. However, the relationship between glycolytic pathway and CVD is very complex, and some preclinical studies have confirmed that targeting glycolysis does have a certain degree of efficacy, but its specific role in the development of CVD has yet to be explored. This article aims to summarize the current knowledge regarding the glycolytic pathway and its key enzymes (including hexokinase (HK), phosphoglucose isomerase (PGI), phosphofructokinase-1 (PFK1), aldolase (Aldolase), phosphoglycerate metatase (PGAM), enolase (ENO) pyruvate kinase (PKM) lactate dehydrogenase (LDH)) for their role in cardiovascular diseases (e.g., heart failure, myocardial infarction, atherosclerosis) and possible emerging therapeutic targets.

Synthesis and characterization of novel acyl hydrazones derived from vanillin as potential aldose reductase inhibitors

In the polyol pathway, aldose reductase (AR) catalyzes the formation of sorbitol from glucose. In order to detoxify some dangerous aldehydes, AR is essential. However, due to the effects of the active polyol pathway, AR overexpression in the hyperglycemic state leads to microvascular and macrovascular diabetic problems. As a result, AR inhibition has been recognized as a potential treatment for issues linked to diabetes and has been studied by numerous researchers worldwide. In the present study, a series of acyl hydrazones were obtained from the reaction of vanillin derivatized with acyl groups and phenolic Mannich bases with hydrazides containing pharmacological groups such as morpholine, piperazine, and tetrahydroisoquinoline. The resulting 21 novel acyl hydrazone compounds were investigated as an inhibitor of the AR enzyme. All the novel acyl hydrazones derived from vanillin demonstrated activity in nanomolar levels as AR inhibitors with IC50 and KI values in the range of 94.21 ± 2.33 to 430.00 ± 2.33 nM and 49.22 ± 3.64 to 897.20 ± 43.63 nM, respectively. Compounds 11c and 10b against AR enzyme activity were identified as highly potent inhibitors and showed 17.38 and 10.78-fold more effectiveness than standard drug epalrestat. The synthesized molecules' absorption, distribution, metabolism, and excretion (ADME) effects were also assessed. The probable-binding mechanisms of these inhibitors against AR were investigated using molecular-docking simulations.

Aldose reductase inhibition alleviates diabetic cardiomyopathy and is associated with a decrease in myocardial fatty acid oxidation

BACKGROUND

Cardiovascular diseases, including diabetic cardiomyopathy, are major causes of death in people with type 2 diabetes. Aldose reductase activity is enhanced in hyperglycemic conditions, leading to altered cardiac energy metabolism and deterioration of cardiac function with adverse remodeling. Because disturbances in cardiac energy metabolism can promote cardiac inefficiency, we hypothesized that aldose reductase inhibition may mitigate diabetic cardiomyopathy via normalization of cardiac energy metabolism.

METHODS

Male C57BL/6J mice (8-week-old) were subjected to experimental type 2 diabetes/diabetic cardiomyopathy (high-fat diet [60% kcal from lard] for 10 weeks with a single intraperitoneal injection of streptozotocin (75 mg/kg) at 4 weeks), following which animals were randomized to treatment with either vehicle or AT-001, a next-generation aldose reductase inhibitor (40 mg/kg/day) for 3 weeks. At study completion, hearts were perfused in the isolated working mode to assess energy metabolism.

RESULTS

Aldose reductase inhibition by AT-001 treatment improved diastolic function and cardiac efficiency in mice subjected to experimental type 2 diabetes. This attenuation of diabetic cardiomyopathy was associated with decreased myocardial fatty acid oxidation rates (1.15 ± 0.19 vs 0.5 ± 0.1 µmol min-1 g dry wt-1 in the presence of insulin) but no change in glucose oxidation rates compared to the control group. In addition, cardiac fibrosis and hypertrophy were also mitigated via AT-001 treatment in mice with diabetic cardiomyopathy.

CONCLUSIONS

Inhibiting aldose reductase activity ameliorates diastolic dysfunction in mice with experimental type 2 diabetes, which may be due to the decline in myocardial fatty acid oxidation, indicating that treatment with AT-001 may be a novel approach to alleviate diabetic cardiomyopathy in patients with diabetes.

Ameliorative effects of chickpea flavonoids on redox imbalance and mitochondrial complex I dysfunction in type 2 diabetic rats

Chickpeas are an important source of flavonoids in the human diet, and researchers have demonstrated that flavonoids have antidiabetic compositions in chickpeas. Because the NAD⁺/NADH redox balance is heavily perturbed in diabetes and complex I is the only site for NADH oxidation and NAD⁺ regeneration, in the present study, mitochondrial complex I was used as a target for anti-diabetes. The objective of this study was to investigate the effects of a crude chickpea flavonoid extract (CCFE) on NAD⁺/NADH redox imbalance and mitochondrial complex I dysfunction in the pancreas as well as oxidative stress in type 2 diabetes mellitus (T2DM) rats. Our results demonstrated that the degree of NAD⁺/NADH redox imbalance in the pancreas of T2DM rats was alleviated by CCFE, which is likely attributed to the inhibition of the polyol pathway and the decrease in poly ADP ribose polymerase (PARP) and sirtuin 3 (Sirt3) activities. Moreover, mitochondrial complex I dysfunction in the pancreas of T2DM rats was ameliorated by CCFE through the suppression of the activity of complex I. Furthermore, CCFE treatment could attenuate oxidative stress in T2DM rats, which was proven by the reduction in hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) as well as the upregulation of glutathione peroxidase (GSH-Px) and superoxide dismutase (SOD) in serum. CCFE treatment significantly improved dyslipidemia in T2DM rats.

Polyol pathway and redox balance in diabetes

Diabetes is a major public health disease that is globally approaching epidemic proportions. One of the major causes of type 2 diabetes is either a defect in insulin secretion or insulin action which is usually caused by a combination of genetic and environmental factors. Not only these factors but others such as deregulation of various pathways, and oxidative stress are also known to trigger the redox imbalance in diabetics. Increasing evidences suggest that there are tight interactions between the development of diabetes and redox imbalance. An alternate pathway of glucose metabolism, the polyol pathway, becomes active in patients with diabetes that disturbs the balance between NADH and NAD+ . The occurrence of such redox imbalance supports other pathways that lead to oxidative damage to DNA, lipids, and proteins and consequently to oxidative stress which further ascend diabetes and its complications. However, the precise mechanism through which oxidative stress regulates diabetes progression remains to be elucidated. The understanding of how antioxidants and oxidants are controlled and impact the generation of oxidative stress and progression of diabetes is essential. The main focus of this review is to provide an overview of redox imbalance caused by oxidative stress through the polyol pathway. Understanding the pathological role of oxidative stress in diabetes will help to design potential therapeutic strategies against diabetes.

Pathophysiological Mechanisms That Alter the Autonomic Brain-Liver Communication in Metabolic Diseases

The brain influences liver metabolism through many neuroendocrine and autonomic mechanisms that have evolved to protect the organism against starvation and hypoglycemia. Unfortunately, this effective way of preventing death has become dysregulated in modern obesogenic environments, although the pathophysiological mechanisms behind metabolic dyshomeostasis are still unclear. In this Mini-Review, we provide our thoughts regarding obesity and type 2 diabetes as diseases of the autonomic nervous system. We discuss the pathophysiological mechanisms that alter the autonomic brain-liver communication in these diseases, and how they could represent important targets to prevent or treat metabolic dysfunctions. We discuss how sympathetic hyperactivity to the liver may represent an early event in the progression of metabolic diseases and could progressively lead to hepatic neuropathy. We hope that this discussion will inspire and help to frame a model based on better understanding of the chronology of autonomic dysfunctions in the liver, enabling the application of the right strategy at the right time.

Physiological and Pathological Roles of Aldose Reductase

Aldose reductase (AR) is an aldo-keto reductase that catalyzes the first step in the polyol pathway which converts glucose to sorbitol. Under normal glucose homeostasis the pathway represents a minor route of glucose metabolism that operates in parallel with glycolysis. However, during hyperglycemia the flux of glucose via the polyol pathway increases significantly, leading to excessive formation of sorbitol. The polyol pathway-driven accumulation of osmotically active sorbitol has been implicated in the development of secondary diabetic complications such as retinopathy, nephropathy, and neuropathy. Based on the notion that inhibition of AR could prevent these complications a range of AR inhibitors have been developed and tested; however, their clinical efficacy has been found to be marginal at best. Moreover, recent work has shown that AR participates in the detoxification of aldehydes that are derived from lipid peroxidation and their glutathione conjugates. Although in some contexts this antioxidant function of AR helps protect against tissue injury and dysfunction, the metabolic transformation of the glutathione conjugates of lipid peroxidation-derived aldehydes could also lead to the generation of reactive metabolites that can stimulate mitogenic or inflammatory signaling events. Thus, inhibition of AR could have both salutary and injurious outcomes. Nevertheless, accumulating evidence suggests that inhibition of AR could modify the effects of cardiovascular disease, asthma, neuropathy, sepsis, and cancer; therefore, additional work is required to selectively target AR inhibitors to specific disease states. Despite past challenges, we opine that a more gainful consideration of therapeutic modulation of AR activity awaits clearer identification of the specific role(s) of the AR enzyme in health and disease.

Fructose Metabolism and Cardiac Metabolic Stress

Cardiovascular disease is one of the leading causes of mortality in diabetes. High fructose consumption has been linked with the development of diabetes and cardiovascular disease. Serum and cardiac tissue fructose levels are elevated in diabetic patients, and cardiac production of fructose via the intracellular polyol pathway is upregulated. The question of whether direct myocardial fructose exposure and upregulated fructose metabolism have potential to induce cardiac fructose toxicity in metabolic stress settings arises. Unlike tightly-regulated glucose metabolism, fructose bypasses the rate-limiting glycolytic enzyme, phosphofructokinase, and proceeds through glycolysis in an unregulated manner. In vivo rodent studies have shown that high dietary fructose induces cardiac metabolic stress and functional disturbance. In vitro, studies have demonstrated that cardiomyocytes cultured in high fructose exhibit lipid accumulation, inflammation, hypertrophy and low viability. Intracellular fructose mediates post-translational modification of proteins, and this activity provides an important mechanistic pathway for fructose-related cardiomyocyte signaling and functional effect. Additionally, fructose has been shown to provide a fuel source for the stressed myocardium. Elucidating the mechanisms of fructose toxicity in the heart may have important implications for understanding cardiac pathology in metabolic stress settings.

Therapeutic potential of targeting oxidative stress in diabetic cardiomyopathy

Even in the absence of coronary artery disease and hypertension, diabetes mellitus (DM) may increase the risk for heart failure development. This risk evolves from functional and structural alterations induced by diabetes in the heart, a cardiac entity termed diabetic cardiomyopathy (DbCM). Oxidative stress, defined as the imbalance of reactive oxygen species (ROS) has been increasingly proposed to contribute to the development of DbCM. There are several sources of ROS production including the mitochondria, NAD(P)H oxidase, xanthine oxidase, and uncoupled nitric oxide synthase. Overproduction of ROS in DbCM is thought to be counterbalanced by elevated antioxidant defense enzymes such as catalase and superoxide dismutase. Excess ROS in the cardiomyocyte results in further ROS production, mitochondrial DNA damage, lipid peroxidation, post-translational modifications of proteins and ultimately cell death and cardiac dysfunction. Furthermore, ROS modulates transcription factors responsible for expression of antioxidant enzymes. Lastly, evidence exists that several pharmacological agents may convey cardiovascular benefit by antioxidant mechanisms. As such, increasing our understanding of the pathways that lead to increased ROS production and impaired antioxidant defense may enable the development of therapeutic strategies against the progression of DbCM. Herein, we review the current knowledge about causes and consequences of ROS in DbCM, as well as the therapeutic potential and strategies of targeting oxidative stress in the diabetic heart.

Aldose Reductase: An Emerging Target for Development of Interventions for Diabetic Cardiovascular Complications

Diabetes is a leading cause of cardiovascular morbidity and mortality. Despite numerous treatments for cardiovascular disease (CVD), for patients with diabetes, these therapies provide less benefit for protection from CVD. These considerations spur the concept that diabetes-specific, disease-modifying therapies are essential to identify especially as the diabetes epidemic continues to expand. In this context, high levels of blood glucose stimulate the flux via aldose reductase (AR) pathway leading to metabolic and signaling changes in cells of the cardiovascular system. In animal models flux via AR in hearts is increased by diabetes and ischemia and its inhibition protects diabetic and non-diabetic hearts from ischemia-reperfusion injury. In mouse models of diabetic atherosclerosis, human AR expression accelerates progression and impairs regression of atherosclerotic plaques. Genetic studies have revealed that single nucleotide polymorphisms (SNPs) of the ALD2 (human AR gene) is associated with diabetic complications, including cardiorenal complications. This Review presents current knowledge regarding the roles for AR in the causes and consequences of diabetic cardiovascular disease and the status of AR inhibitors in clinical trials. Studies from both human subjects and animal models are presented to highlight the breadth of evidence linking AR to the cardiovascular consequences of diabetes.

Elevated myocardial fructose and sorbitol levels are associated with diastolic dysfunction in diabetic patients, and cardiomyocyte lipid inclusions in vitro

Diabetes is associated with cardiac metabolic disturbances and increased heart failure risk. Plasma fructose levels are elevated in diabetic patients. A direct role for fructose involvement in diabetic heart pathology has not been investigated. The goals of this study were to clinically evaluate links between myocardial fructose and sorbitol (a polyol pathway fructose precursor) levels with evidence of cardiac dysfunction, and to experimentally assess the cardiomyocyte mechanisms involved in mediating the metabolic effects of elevated fructose. Fructose and sorbitol levels were increased in right atrial appendage tissues of type 2 diabetic patients (2.8- and 1.5-fold increase respectively). Elevated cardiac fructose levels were confirmed in type 2 diabetic rats. Diastolic dysfunction (increased E/e’, echocardiography) was significantly correlated with cardiac sorbitol levels. Elevated myocardial mRNA expression of the fructose-specific transporter, Glut5 (43% increase), and the key fructose-metabolizing enzyme, Fructokinase-A (50% increase) was observed in type 2 diabetic rats (Zucker diabetic fatty rat). In neonatal rat ventricular myocytes, fructose increased glycolytic capacity and cytosolic lipid inclusions (28% increase in lipid droplets/cell). This study provides the first evidence that elevated myocardial fructose and sorbitol are associated with diastolic dysfunction in diabetic patients. Experimental evidence suggests that fructose promotes the formation of cardiomyocyte cytosolic lipid inclusions, and may contribute to lipotoxicity in the diabetic heart.

Diabetic Retinopathy: The Role of Mitochondria in the Neural Retina and Microvascular Disease

Diabetic retinopathy (DR), a common chronic complication of diabetes mellitus and the leading cause of vision loss in the working-age population, is clinically defined as a microvascular disease that involves damage of the retinal capillaries with secondary visual impairment. While its clinical diagnosis is based on vascular pathology, DR is associated with early abnormalities in the electroretinogram, indicating alterations of the neural retina and impaired visual signaling. The pathogenesis of DR is complex and likely involves the simultaneous dysregulation of multiple metabolic and signaling pathways through the retinal neurovascular unit. There is evidence that microvascular disease in DR is caused in part by altered energetic metabolism in the neural retina and specifically from signals originating in the photoreceptors. In this review, we discuss the main pathogenic mechanisms that link alterations in neural retina bioenergetics with vascular regression in DR. We focus specifically on the recent developments related to alterations in mitochondrial metabolism including energetic substrate selection, mitochondrial function, oxidation-reduction (redox) imbalance, and oxidative stress, and critically discuss the mechanisms of these changes and their consequences on retinal function. We also acknowledge implications for emerging therapeutic approaches and future research directions to find novel mitochondria-targeted therapeutic strategies to correct bioenergetics in diabetes. We conclude that retinal bioenergetics is affected in the early stages of diabetes with consequences beyond changes in ATP content, and that maintaining mitochondrial integrity may alleviate retinal disease.

Role of pseudohypoxia in the pathogenesis of type 2 diabetes

Type 2 diabetes is caused by persistent high blood glucose, which is known as diabetic hyperglycemia. This hyperglycemic situation, when not controlled, can overproduce NADH and lower nicotinamide adenine dinucleotide (NAD), thereby creating NADH/NAD redox imbalance and leading to cellular pseudohypoxia. In this review, we discussed two major enzymatic systems that are activated by diabetic hyperglycemia and are involved in creation of this pseudohypoxic condition. One system is aldose reductase in the polyol pathway, and the other is poly (ADP ribose) polymerase. While aldose reductase drives overproduction of NADH, PARP could in contrast deplete NAD. Therefore, activation of the two pathways underlies the major mechanisms of NADH/NAD redox imbalance and diabetic pseudohypoxia. Consequently, reductive stress occurs, followed by oxidative stress and eventual cell death and tissue dysfunction. Additionally, fructose formed in the polyol pathway can also cause metabolic syndrome such as hypertension and nonalcoholic fatty liver disease. Moreover, pseudohypoxia can also lower sirtuin protein contents and induce protein acetylation which can impair protein function. Finally, we discussed the possibility of using nicotinamide riboside, an NAD precursor, as a promising therapeutic agent for restoring NADH/NAD redox balance and for preventing the occurrence of diabetic pseudohypoxia.

Chronic Inhibition of Mitochondrial Dihydrolipoamide Dehydrogenase (DLDH) as an Approach to Managing Diabetic Oxidative Stress

Mitochondrial dihydrolipoamide dehydrogenase (DLDH) is a redox enzyme involved in decarboxylation of pyruvate to form acetyl-CoA during the cascade of glucose metabolism and mitochondrial adenine triphosphate (ATP) production. Depending on physiological or pathophysiological conditions, DLDH can either enhance or attenuate the production of reactive oxygen species (ROS) and reactive nitrogen species. Recent research in our laboratory has demonstrated that inhibition of DLDH induced antioxidative responses and could serve as a protective approach against oxidative stress in stroke injury. In this perspective article, we postulated that chronic inhibition of DLDH could also attenuate oxidative stress in type 2 diabetes. We discussed DLDH-involving mitochondrial metabolic pathways and metabolic intermediates that could accumulate upon DLDH inhibition and their corresponding roles in abrogating oxidative stress in diabetes. We also discussed a couple of DLDH inhibitors that could be tested in animal models of type 2 diabetes. It is our belief that DLDH inhibition could be a novel approach to fighting type 2 diabetes.

KRT1 gene silencing ameliorates myocardial ischemia-reperfusion injury via the activation of the Notch signaling pathway in mouse models

Myocardial ischemia and reperfusion injury (MIRI) includes major drawbacks, such as excessive formation of free radicals and also overload of calcium, which lead to cell death, tissue scarring, and remodeling. The current study aims to explore whether KRT1 silencing may ameliorate MIRI via the Notch signaling pathway in mouse models. Myocardial tissues were used for the determination of the positive rate of KRT1 protein expression, apoptosis of myocardial cells, creatine kinase (CK) and lactate dehydrogenase (LDH) expression, expression of related biomarkers as well as myocardial infarction area. The transfected myocardial cells were treated with KRT1-siRNA, Jagged1, and DAPT (inhibitor of Notch-1 signaling pathway). The expression of KRT1, NICD, Hes1, Bcl-2, and Bax protein was detected. The MTT assay was applied for cell proliferation and flow cytometry was used for cell apoptosis. Mice with MIRI had a higher positive rate of KRT1 protein expression, apoptosis of myocardial cells, CK and LDH expression, myocardial infarction area, increased expression of MDA, NO, SDH, IL-1, IL-6, TNF-α, CRP, KRT1, Bax protein, CK, and LDH, and decreased expression of SOD, NICD, Hes1, and Bcl-2. The downregulation of KRT1 led to decreased expression of KRT1 and Bax protein, increased expression of NICD, Hes1, and Bcl-2, decreased cell apoptosis, and improved cell proliferation. The inhibition of the Notch signaling pathway leads to reduced expression of Bax, increased expression of NICD, Hes1, and Bcl 2, and also decreased cell apoptosis and increased cell proliferation. Our data conclude that KRT1 silencing is able to make MIRI better by activating the Notch signaling pathway in mice.

Redox imbalance stress in diabetes mellitus: Role of the polyol pathway

In diabetes mellitus, the polyol pathway is highly active and consumes approximately 30% glucose in the body. This pathway contains 2 reactions catalyzed by aldose reductase (AR) and sorbitol dehydrogenase, respectively. AR reduces glucose to sorbitol at the expense of NADPH, while sorbitol dehydrogenase converts sorbitol to fructose at the expense of NAD⁺, leading to NADH production. Consumption of NADPH, accumulation of sorbitol, and generation of fructose and NADH have all been implicated in the pathogenesis of diabetes and its complications. In this review, the roles of this pathway in NADH/NAD⁺ redox imbalance stress and oxidative stress in diabetes are highlighted. A potential intervention using nicotinamide riboside to restore redox balance as an approach to fighting diabetes is also discussed.

Reductive Stress in Inflammation-Associated Diseases and the Pro-Oxidant Effect of Antioxidant Agents

Abstract: Reductive stress (RS) is the counterpart oxidative stress (OS), and can occur in response to conditions that shift the redox balance of important biological redox couples, such as the NAD⁺/NADH, NADP⁺/NADPH, and GSH/GSSG, to a more reducing state. Overexpression of antioxidant enzymatic systems leads to excess reducing equivalents that can deplete reactive oxidative species, driving the cells to RS. A feedback regulation is established in which chronic RS induces OS, which in turn, stimulates again RS. Excess reducing equivalents may regulate cellular signaling pathways, modify transcriptional activity, induce alterations in the formation of disulfide bonds in proteins, reduce mitochondrial function, decrease cellular metabolism, and thus, contribute to the development of some diseases in which NF-κB, a redox-sensitive transcription factor, participates. Here, we described the diseases in which an inflammatory condition is associated to RS, and where delayed folding, disordered transport, failed oxidation, and aggregation are found. Some of these diseases are aggregation protein cardiomyopathy, hypertrophic cardiomyopathy, muscular dystrophy, pulmonary hypertension, rheumatoid arthritis, Alzheimer's disease, and metabolic syndrome, among others. Moreover, chronic consumption of antioxidant supplements, such as vitamins and/or flavonoids, may have pro-oxidant effects that may alter the redox cellular equilibrium and contribute to RS, even diminishing life expectancy.

Effects of the New Aldose Reductase Inhibitor Benzofuroxane Derivative BF-5m on High Glucose Induced Prolongation of Cardiac QT Interval and Increase of Coronary Perfusion Pressure

This study investigated the effects of the new aldose reductase inhibitor benzofuroxane derivative 5(6)-(benzo[d]thiazol-2-ylmethoxy)benzofuroxane (BF-5m) on the prolongation of cardiac QT interval and increase of coronary perfusion pressure (CPP) in isolated, high glucose (33.3 mM D-glucose) perfused rat hearts. BF-5m was dissolved in the Krebs solution at a final concentration of 0.01 μM, 0.05 μM, and 0.1 μM. 33.3 mM D-glucose caused a prolongation of the QT interval and increase of CPP up to values of 190 ± 12 ms and 110 ± 8 mmHg with respect to the values of hearts perfused with standard Krebs solution (11.1 mM D-glucose). The QT prolongation was reduced by 10%, 32%, and 41%, respectively, for the concentration of BF-5m 0.01 μM, 0.05 μM, and 0.1 μM. Similarly, the CPP was reduced by 20% for BF-5m 0.05 μM and by 32% for BF-5m 0.1 μM. BF-5m also increased the expression levels of sirtuin 1, MnSOD, eNOS, and FOXO-1, into the heart. The beneficial actions of BF-5m were partly abolished by the pretreatment of the rats with the inhibitor of the sirtuin 1 activity EX527 (10 mg/kg/day/7 days i.p.) prior to perfusion of the hearts with high glucose + BF-5m (0.1 μM). Therefore, BF-5m supplies cardioprotection from the high glucose induced QT prolongation and increase of CPP.

Metabolic alterations induce oxidative stress in diabetic and failing hearts: different pathways, same outcome

SIGNIFICANCE

Several authors have proposed a link between altered cardiac energy substrate metabolism and reactive oxygen species (ROS) generation. A cogent evidence of this association has been found in diabetic cardiomyopathy (dCM); however, experimental findings in animal models of heart failure (HF) and in human myocardium also seem to support the coexistence of the two alterations in HF.

CRITICAL ISSUES

Two important questions remain open: whether pathological changes in metabolism play an important role in enhancing oxidative stress and whether there is a common pathway linking altered substrate utilization and activation of ROS-generating enzymes, independently of the underlying cardiac pathology. In this regard, the comparison between dCM and HF is intriguing, in that these pathological conditions display very different cardiac metabolic phenotypes.

RECENT ADVANCES

Our literature review on this topic indicates that a vast body of knowledge is now available documenting the relationship between the metabolism of energy substrates and ROS generation in dCM. In some cases, biochemical mechanisms have been identified. On the other hand, only a few and relatively recent studies have explored this phenomenon in HF and their conclusions are not consistent.

FUTURE DIRECTIONS

Better methods of investigation, especially in vivo, will be necessary to test whether the metabolic fate of certain substrates is causally linked to ROS production. If successful, these studies will place a new emphasis on the potential clinical relevance of metabolic modulators, which might indirectly mitigate cardiac oxidative stress in dCM, HF, and, possibly, in other pathological conditions.

Sorbitol dehydrogenase inhibitor protects the liver from ischemia/reperfusion-induced injury via elevated glycolytic flux and enhanced sirtuin 1 activity

Sorbitol dehydrogenase (SDH), a key enzyme of the polyol pathway, has recently been demonstrated to have an important role in mediating tissue ischemia/reperfusion (I/R) injury. The present study investigated how this enzyme may affect the ischemic liver and the mechanism underlying its effect. Firstly, C57BL/6 mice were subjected to oral administration of CP-470,711 (5 mg/kg body weight/day for five days) and 70% hepatic I/R. Next the present study further investigated the changes in liver function, histology, inflammation, apoptosis and necrosis; the cytosolic adenosine triphosphate (ATP) and nictotinamide adenine dinucleotide [NAD(H)] contents and the protein level of caspase 3 and sirtuin 1 (SIRT1). The data demonstrated that sorbitol dehydrogenase inhibitor (SDI)-administration significantly alleviated I/R-induced liver injury, palliated histological changes and lowered the level of hepatocyte apoptosis and necrosis. In addition, SDI-pretreatment in ischemic liver markedly maintained the cytosolic ATP and NAD(H) proportion, enhanced SIRT1 and suppressed the activation of caspase 3 at the protein level. The findings in the present study revealed that the flux through SDH may render the liver more vulnerable to I/R-induced injury and interventions targeting this enzyme may provide a novel adjunctive approach to protect from severe tissue injury following liver ischemia.

Biology of diabetic neuropathy

More than half of all patients with diabetes develop neuropathic disorders affecting the distal sensory and/or motor nerves, or autonomic or cranial nerve functions. Glycemic control can decrease the incidence of neuropathy but is not adequate alone to prevent or treat the disease. This chapter introduces diabetic neuropathy with a morphological description of the disease then describes our current understanding of metabolic and molecular mechanisms that contribute to neurovascular dysfunctions. Key mechanisms include glucose and lipid imbalances and insulin resistance that are interconnected via oxidative stress, inflammation, and altered gene expression. These complex interactions should be considered for the development of new treatment strategies against the onset or progression of neuropathy. Advances in understanding the combined metabolic stressors and the novel study of epigenetics suggest new therapeutic targets to combat this morbid and intractable disease affecting millions of patients with type 1 or type 2 diabetes.

Pathogenesis of chronic hyperglycemia: from reductive stress to oxidative stress

Chronic overnutrition creates chronic hyperglycemia that can gradually induce insulin resistance and insulin secretion impairment. These disorders, if not intervened, will eventually be followed by appearance of frank diabetes. The mechanisms of this chronic pathogenic process are complex but have been suggested to involve production of reactive oxygen species (ROS) and oxidative stress. In this review, I highlight evidence that reductive stress imposed by overflux of NADH through the mitochondrial electron transport chain is the source of oxidative stress, which is based on establishments that more NADH recycling by mitochondrial complex I leads to more electron leakage and thus more ROS production. The elevated levels of both NADH and ROS can inhibit and inactivate glyceraldehyde 3-phosphate dehydrogenase (GAPDH), respectively, resulting in blockage of the glycolytic pathway and accumulation of glycerol 3-phospate and its prior metabolites along the pathway. This accumulation then initiates all those alternative glucose metabolic pathways such as the polyol pathway and the advanced glycation pathways that otherwise are minor and insignificant under euglycemic conditions. Importantly, all these alternative pathways lead to ROS production, thus aggravating cellular oxidative stress. Therefore, reductive stress followed by oxidative stress comprises a major mechanism of hyperglycemia-induced metabolic syndrome.

Role of hyperglycemia-mediated erythrocyte redox state alteration in the development of diabetic retinopathy

PURPOSE

To evaluate erythrocyte redox state and its surrogates in patients with different stages of diabetic retinopathy and their association with cellular metabolic derangement developed in retinal microvascular cells.

METHODS

Sixty type 2 diabetic patients with nonproliferative diabetic retinopathy (NPDR), 85 patients with proliferative diabetic retinopathy (PDR), and 70 patients with diabetes but without retinopathy were considered as diabetic control (DC) for the study. In addition, 65 normal individuals without diabetes were enrolled as healthy control in this study. Erythrocyte oxidized nicotinamide adenine dinucleotide phosphate / reduced nicotinamide adenine dinucleotide phosphate (NADP / NADPH), oxidized nicotinamide adenine dinucleotide / reduced nicotinamide adenine dinucleotide (NAD / NADH) glutathione, plasma and vitreous lactate, and pyruvate levels were determined by enzymatic reaction-based spectrophotometric assay for the patients and individuals.

RESULT

Erythrocyte NADP+ to NADPH ratio to NADPH ratio was found to be significantly higher among NPDR and PDR patients compared with DC subjects (P < 0.0001). Erythrocyte-reduced glutathione was significantly decreased in patients of NPDR (P = 0.0004) and patients of PDR (P = 0.0157) compared to DC. Erythrocyte NAD to NADH ratio was also significantly decreased in patients of NPDR (P < 0.0001) and PDR (P < 0.0001) compared to DC subjects. Lactate to pyruvate ratio of plasma was elevated significantly in patients with NPDR compared with DC (P < 0.0001) and those having PDR (P = 0.0046). In the vitreous fluid, the lactate to pyruvate ratios were found to be significantly lower in normal individuals without diabetes compared with patients having PDR (P < 0.0001).

CONCLUSION

Hyperglycemia-mediated erythrocyte redox state alterations might be a potential risk factor for the development of NPDR in poorly controlled diabetic subjects.

Cardiomyocyte aldose reductase causes heart failure and impairs recovery from ischemia

Aldose reductase (AR), an enzyme mediating the first step in the polyol pathway of glucose metabolism, is associated with complications of diabetes mellitus and increased cardiac ischemic injury. We investigated whether deleterious effects of AR are due to its actions specifically in cardiomyocytes. We created mice with cardiac specific expression of human AR (hAR) using the α–myosin heavy chain (MHC) promoter and studied these animals during aging and with reduced fatty acid (FA) oxidation. hAR transgenic expression did not alter cardiac function or glucose and FA oxidation gene expression in young mice. However, cardiac overexpression of hAR caused cardiac dysfunction in older mice. We then assessed whether hAR altered heart function during ischemia reperfusion. hAR transgenic mice had greater infarct area and reduced functional recovery than non-transgenic littermates. When the hAR transgene was crossed onto the PPAR alpha knockout background, another example of greater heart glucose oxidation, hAR expressing mice had increased heart fructose content, cardiac fibrosis, ROS, and apoptosis. In conclusion, overexpression of hAR in cardiomyocytes leads to cardiac dysfunction with aging and in the setting of reduced FA and increased glucose metabolism. These results suggest that pharmacological inhibition of AR will be beneficial during ischemia and in some forms of heart failure.

Aldose reductase, oxidative stress and diabetic cardiovascular complications

Cardiovascular disease represents the major cause of morbidity and mortality in patients with diabetes mellitus. Studies by us and others have implicated increased flux via aldose reductase (AR) as a key player in mediating diabetic complications, including cardiovascular complications. Data suggest that increased flux via AR in diabetics perpetuates increased injury after myocardial infarction, accelerates atherosclerotic lesion formation, and promotes restenosis via multiple mechanisms. Most importantly, studies have shown that increased generation of reactive oxygen species due to flux via AR has been a common feature in animal models of diabetic cardiovascular disease. Taken together, these considerations place AR in the center of biochemical and molecular stresses that characterize the cardiovascular complications of diabetes. Stopping AR-dependent signaling may hold the key to interrupting cycles of cellular perturbation and tissue damage in diabetic cardiovascular complications.

Human aldose reductase expression accelerates atherosclerosis in diabetic apolipoprotein E-/- mice

OBJECTIVE

There are several pathways that mediate the aberrant metabolism of glucose and that might induce greater vascular damage in the setting of diabetes. The polyol pathway mediated by aldose reductase (AR) has been postulated to be one such pathway. However, it has been reported that AR reduces toxic lipid aldehydes and, under some circumstances, might be antiatherogenic.

METHODS AND RESULTS

Atherosclerosis development was quantified in 2 lines of transgenic mice expressing human AR (hAR) crossed on the apolipoprotein E knockout background. The transgenes were used to increase the normally low levels of this enzyme in wild-type mice. Both generalized hAR overexpression and hAR expression via the Tie 2 promoter increased lesion size in streptozotocin diabetic mice. In addition, pharmacological inhibition of AR reduced lesion size.

CONCLUSIONS

Although in some settings AR expression might reduce levels of toxic aldehydes, transgenic expression of this enzyme within the artery wall leads to greater atherosclerosis.

Aldose reductase pathway contributes to vulnerability of aging myocardium to ischemic injury

Aging men and women display both increased incidence of cardiovascular disease and complications of myocardial infarction and heart failure. We hypothesized that altered glucose metabolism, in particular, flux of glucose via the polyol pathway (PP) may be responsible, in part, for the enhanced vulnerability of aging myocardium to ischemic injury, even in the absence of superimposed disease processes linked to PP flux, such as diabetes. To test our hypothesis, we determined the expression and products of PP enzymes aldose reductase (AR) and sorbitol dehydrogenase (SDH) in hearts from Fischer 344 aged (26 months) and young (4 months) rats subjected to global ischemia followed by reperfusion in the presence or absence of blockers of PP and the measures of ischemic injury and functional recovery were determined. Expression and activities of AR and SDH were significantly higher in aged vs. young hearts, and induction of ischemia further increased AR and SDH activity in the aged hearts. Myocardial ischemic injury was significantly greater in aged vs. young hearts, and blockade of AR reduced ischemic injury and improved cardiac functional recovery on reperfusion in aged hearts. These data indicate that innate increases in activity of the PP enzymes augment myocardial vulnerability to I/R injury in aging, and that blockers of PP protect the vulnerable aging hearts.

X-ray crystal structure and small-angle X-ray scattering of sheep liver sorbitol dehydrogenase

The X-ray crystal structure of sheep liver sorbitol dehydrogenase (slSDH) has been determined using the crystal structure of human sorbitol dehydrogenase (hSDH) as a molecular-replacement model. slSDH crystallized in space group I222 with one monomer in the asymmetric unit. A conserved tetramer that superposes well with that seen in hSDH (despite belonging to a different space group) and obeying the 222 crystal symmetry is seen in slSDH. An acetate molecule is bound in the active site, coordinating to the active-site zinc through a water molecule. Glycerol, a substrate of slSDH, also occupies the substrate-binding pocket together with the acetate designed by nature to fit large polyol substrates. The substrate-binding pocket is seen to be in close proximity to the tetramer interface, which explains the need for the structural integrity of the tetramer for enzyme activity. Small-angle X-ray scattering was also used to identify the quaternary structure of the tetramer of slSDH in solution.

Aldose reductase inhibition suppresses oxidative stress-induced inflammatory disorders

Oxidative stress-induced inflammation is a major contributor to several disease conditions including sepsis, carcinogenesis and metastasis, diabetic complications, allergic asthma, uveitis and after cataract surgery posterior capsular opacification. Since reactive oxygen species (ROS)-mediated activation of redox-sensitive transcription factors and subsequent expression of inflammatory cytokines, chemokines and growth factors are characteristics of inflammatory disorders, we envisioned that by blocking the molecular signals of ROS that activate redox-sensitive transcription factors, various inflammatory diseases could be ameliorated. We have indeed demonstrated that ROS-induced lipid peroxidation-derived lipid aldehydes such as 4-hydroxy-trans-2-nonenal (HNE) and their glutathione-conjugates (e.g. GS-HNE) are efficiently reduced by aldose reductase to corresponding alcohols which mediate the inflammatory signals. Our results showed that inhibition of aldose reductase (AKR1B1) significantly prevented the inflammatory signals induced by cytokines, growth factors, endotoxins, high glucose, allergens and auto-immune reactions in cellular as well as animal models. We have demonstrated that AKR1B1 inhibitor, fidarestat, significantly prevents tumor necrosis factor-alpha (TNF-α)-, growth factors-, lipopolysachharide (LPS)-, and environmental allergens-induced inflammatory signals that cause various inflammatory diseases. In animal models of inflammatory diseases such as diabetes, cardiovascular, uveitis, asthma, and cancer (colon, breast, prostate and lung) and metastasis, inhibition of AKR1B1 significantly ameliorated the disease. Our results from various cellular and animal models representing a number of inflammatory conditions suggest that ROS-induced inflammatory response could be reduced by inhibition of AKR1B1, thereby decreasing the progression of the disease and if the therapy is initiated early, the disease could be eliminated. Since fidarestat has already undergone phase III clinical trial for diabetic neuropathy and found to be safe, though clinically not very effective, our results indicate that it can be developed for the therapy of a number of inflammation-related diseases. Our results thus offer a novel therapeutic approach to treat a wide array of inflammatory diseases.

Evaluation of the aldose reductase inhibitor fidarestat on ischemia-reperfusion injury in rat retina

This study evaluated the effects of retinal ischemia-reperfusion (IR) injury and pre-treatment with the potent and specific aldose reductase inhibitor fidarestat on apoptosis, aldose reductase and sorbitol dehydrogenase expression, sorbitol pathway intermediate concentrations, and oxidative-nitrosative stress. Female Wistar rats were pre-treated with either vehicle (N-methyl-D-glucamine) or fidarestat, 32 mg kg(-1) d(-1) for both, in the right jugular vein, for 3 consecutive days. A group of vehicle- and fidarestat-treated rats were subjected to 45-min retinal ischemia followed by 24-h reperfusion. Ischemia was induced 30 min after the last vehicle or fidarestat administration. Retinal IR resulted in a remarkable increase in retinal cell death. The number of TUNEL-positive nuclei increased 48-fold in the IR group compared with non-ischemic controls (p<0.01), and this increase was partially prevented by fidarestat. AR expression (Western blot analysis) increased by 19% in the IR group (p<0.05), and this increase was prevented by fidarestat. Sorbitol dehydrogenase and nitrated protein expressions were similar among all experimental groups. Retinal sorbitol concentrations tended to increase in the IR group but the difference with non-ischemic controls did not achieve statistical significance (p=0.08). Retinal fructose concentrations were 2.2-fold greater in the IR group than in the non-ischemic controls (p<0.05). Fidarestat pre-treatment of rats subjected to IR reduced retinal sorbitol concentration to the levels in non-ischemic controls. Retinal fructose concentrations were reduced by 41% in fidarestat-pre-treated IR group vs. untreated ischemic controls (p=0.0517), but remained 30% higher than in the non-ischemic control group. In conclusion, IR injury to rat retina is associated with a dramatic increase in cell death, elevated AR expression and sorbitol pathway intermediate accumulation. These changes were prevented or alleviated by the AR inhibitor fidarestat. The results identify AR as an important therapeutic target for diseases involving IR injury, and provide the rationale for development of fidarestat and other AR inhibitors.

Reactive oxygen species originating from mitochondria regulate the cardiac sodium channel

RATIONALE

Pyridine nucleotides regulate the cardiac Na(+) current (I(Na)) through generation of reactive oxygen species (ROS).

OBJECTIVE

We investigated the source of ROS induced by elevated NADH.

METHODS AND RESULTS

In human embryonic kidney (HEK) cells stably expressing the cardiac Na(+) channel, the decrease of I(Na) (52±9%; P<0.01) induced by cytosolic NADH application (100 μmol/L) was reversed by mitoTEMPO, rotenone, malonate, DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid), PK11195, and 4'-chlorodiazepam, a specific scavenger of mitochondrial superoxide and inhibitors of the mitochondrial complex I, complex II, voltage-dependent anion channels, and benzodiazepine receptor, respectively. Anti-mycin A (20 μmol/L), a complex III inhibitor known to generate ROS, decreased I(Na) (51±4%, P<0.01). This effect was blocked by NAD(+), forskolin, or rotenone. Inhibitors of complex IV, nitric oxide synthase, the NAD(P)H oxidases, xanthine oxidases, the mitochondrial permeability transition pore, and the mitochondrial ATP-sensitive K(+) channel did not change the NADH effect on I(Na). Analogous results were observed in cardiomyocytes. Rotenone, mitoTEMPO, and 4'-chlorodiazepam also blocked the mutant A280V GPD1-L (glycerol-3-phosphate dehydrogenase 1-like) effect on reducing I(Na), indicating a role for mitochondria in the Brugada syndrome caused by this mutation. Fluorescent microscopy confirmed mitochondrial ROS generation with elevated NADH and ROS inhibition by NAD(+).

CONCLUSIONS

Altering the oxidized to reduced NAD(H) balance can activate mitochondrial ROS production, leading to reduced I(Na). This signaling cascade may help explain the link between altered metabolism, conduction block, and arrhythmic risk.

Cardiac contractile dysfunction during acute hyperglycemia due to impairment of SERCA by polyol pathway-mediated oxidative stress

Hyperglycemia is an indication of poor outcome for heart attack patients, even for nondiabetic patients with stress-induced hyperglycemia. Previous studies showed that inhibition of aldose reductase, the first and rate-limiting enzyme of the polyol pathway, attenuated contractile dysfunction in diabetic animals, but the mechanism is unclear. We therefore wanted to find out whether the polyol pathway also contributes to acute hyperglycemia-induced cardiac contractile dysfunction, and determine the mechanism involved. Rat hearts were isolated and retrogradely perfused with Krebs buffer containing either normal or high concentrations of glucose for 2 h. Short exposure to high-glucose medium led to contractile dysfunction as indicated by decreased -dP/dt(max), as well as elevation in left ventricular end-diastolic pressure. Cardiomyocytes incubated in high-glucose medium showed abnormal Ca2+ signaling, most likely because of decreased activity of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) inactivated by oxidative stress. Inhibition of aldose reductase or sorbitol dehydrogenase, the second enzyme in the polyol pathway, ameliorated contractile dysfunction, attenuated oxidative stress, and normalized Ca2+ signaling and SERCA activity caused by high glucose, indicating that the polyol pathway is the major contributor to acute hyperglycemia-induced oxidative stress leading to the inactivation of SERCA and contractile dysfunction.

Aldose reductase and cardiovascular diseases, creating human-like diabetic complications in an experimental model

Hyperglycemia and reduced insulin actions affect many biological processes. One theory is that aberrant metabolism of glucose via several pathways including the polyol pathway causes cellular toxicity. Aldose reductase (AR) is a multifunctional enzyme that reduces aldehydes. Under diabetic conditions AR converts glucose into sorbitol, which is then converted to fructose. This article reviews the biology and pathobiology of AR actions. AR expression varies considerably among species. In humans and rats, the higher level of AR expression is associated with toxicity. Flux via AR is increased by ischemia and its inhibition during ischemia reperfusion reduces injury. However, similar pharmacological effects are not observed in mice unless they express a human AR transgene. This is because mice have much lower levels of AR expression, probably insufficient to generate toxic byproducts. Human AR expression in LDL receptor knockout mice exacerbates vascular disease, but only under diabetic conditions. In contrast, a recent report suggests that genetic ablation of AR increased atherosclerosis and increased hydroxynonenal in arteries. It was hypothesized that AR knockout prevented reduction of toxic aldehydes. Like many in vivo effects found in genetically manipulated animals, interpretation requires the reproduction of human-like physiology. For AR, this will require tissue specific expression of AR in sites and at levels that approximate those in humans.

Polyol pathway impairs the function of SERCA and RyR in ischemic-reperfused rat hearts by increasing oxidative modifications of these proteins

A number of studies have shown that the polyol pathway, consisting of aldose reductase (AR) and sorbitol dehydrogenase (SDH), contributes to ischemia-reperfusion (I/R)-induced myocardial infarction due to depletion of ATP. In this report we show that the polyol pathway in I/R heart also contributes to the impairment of sacro/endoplasmic reticulum Ca(2+)-ATPase (SERCA) and ryanodine receptor (RyR), two key players in Ca(2+) signaling that regulate cardiac contraction. Rat hearts were isolated and retrogradely perfused with either Krebs' buffer containing 1 microM AR inhibitor, zopolrestat, or 200 nM SDH inhibitor, CP-170,711, and challenged by 30 min of regional ischemia and 45 min of reperfusion. We found that post-ischemic contractile function of the isolated perfused hearts was improved by pharmacological inhibition of the polyol pathway. I/R-induced contractile dysfunction is most likely due to impairment in Ca(2+) signaling and the activities of SERCA and RyR. All these abnormalities were significantly ameliorated by treatment with ARI or SDI. We showed that the polyol pathway activities increase the level of peroxynitrite, which enhances the tyrosine nitration of SERCA and irreversibly modifies it to form SERCAC674-SO(3)H. This leads to reduced level of S-glutathiolated SERCA, contributing to its inactivation. The polyol pathway activities also deplete the level of GSH, leading to decreased active RyR, the S-glutathiolated RyR. Thus, in I/R heart, inhibition of polyol pathway improved the function of SERCA and RyR by protecting them from irreversible oxidation.

Cardiac Na+ current regulation by pyridine nucleotides

RATIONALE

Mutations in glycerol-3-phosphate dehydrogenase 1-like (GPD1-L) protein reduce cardiac Na+ current (I(Na)) and cause Brugada Syndrome (BrS). GPD1-L has >80% amino acid homology with glycerol-3-phosphate dehydrogenase, which is involved in NAD-dependent energy metabolism.

OBJECTIVE

Therefore, we tested whether NAD(H) could regulate human cardiac sodium channels (Na(v)1.5).

METHODS AND RESULTS

HEK293 cells stably expressing Na(v)1.5 and rat neonatal cardiomyocytes were used. The influence of NADH/NAD+ on arrhythmic risk was evaluated in wild-type or SCN5A(+/-) mouse heart. A280V GPD1-L caused a 2.48+/-0.17-fold increase in intracellular NADH level (P<0.001). NADH application or cotransfection with A280V GPD1-L resulted in decreased I(Na) (0.48+/-0.09 or 0.19+/-0.04 of control group, respectively; P<0.01), which was reversed by NAD+, chelerythrine, or superoxide dismutase. NAD+ antagonism of the Na+ channel downregulation by A280V GPD1-L or NADH was prevented by a protein kinase (PK)A inhibitor, PKAI(6-22). The effects of NADH and NAD+ were mimicked by a phorbol ester and forskolin, respectively. Increasing intracellular NADH was associated with an increased risk of ventricular tachycardia in wild-type mouse hearts. Extracellular application of NAD+ to SCN5A(+/-) mouse hearts ameliorated the risk of ventricular tachycardia.

CONCLUSIONS

Our results show that Na(v)1.5 is regulated by pyridine nucleotides, suggesting a link between metabolism and I(Na). This effect required protein kinase C activation and was mediated by oxidative stress. NAD+ could prevent this effect by activating PKA. Mutations of GPD1-L may downregulate Na(v)1.5 by altering the oxidized to reduced NAD(H) balance.