Mechanisms responsible for enhanced fatty acid utilization by perfused hearts from type 2 diabetic db/db mice

CD36 as a gatekeeper of myocardial lipid metabolism and therapeutic target for metabolic disease

The multifunctional membrane glycoprotein CD36 is expressed in different types of cells and plays a key regulatory role in cellular lipid metabolism, especially in cardiac muscle. CD36 facilitates the cellular uptake of long-chain fatty acids, mediates lipid signaling, and regulates storage and oxidation of lipids in various tissues with active lipid metabolism. CD36 deficiency leads to marked impairments in peripheral lipid metabolism, which consequently impact on the cellular utilization of multiple different fuels because of the integrated nature of metabolism. The functional presence of CD36 at the plasma membrane is regulated by its reversible subcellular recycling from and to endosomes and is under the control of mechanical, hormonal, and nutritional factors. Aberrations in this dynamic role of CD36 are causally associated with various metabolic diseases, in particular insulin resistance, diabetic cardiomyopathy, and cardiac hypertrophy. Recent research in cardiac muscle has disclosed the endosomal proton pump vacuolar-type H+-ATPase (v-ATPase) as a key enzyme regulating subcellular CD36 recycling and being the site of interaction between various substrates to determine cellular substrate preference. In addition, evidence is accumulating that interventions targeting CD36 directly or modulating its subcellular recycling are effective for the treatment of metabolic diseases. In conclusion, subcellular CD36 localization is the major adaptive regulator of cellular uptake and metabolism of long-chain fatty acids and appears a suitable target for metabolic modulation therapy to mend failing hearts.

Fasting Substrate Concentrations Predict Cardiovascular Outcomes in the CANagliflozin cardioVascular Assessment Study (CANVAS)

OBJECTIVE

To examine whether the circulating substrate mix may be related to the incidence of heart failure (HF) and cardiovascular (CV) mortality and how it is altered by canagliflozin treatment.

RESEARCH DESIGN AND METHODS

We measured fasting glucose, free fatty acids (FFA), glycerol, β-hydroxybutyrate, acetoacetate, lactate, and pyruvate concentrations in 3,581 samples from the CANagliflozin cardioVascular Assessment Study (CANVAS) trial at baseline and at 1 and 2 years after randomization. Results were analyzed by univariate and multivariate Cox proportional hazards models.

RESULTS

Patients in the lowest baseline FFA tertile were more often men with a longer duration of type 2 diabetes (T2D), higher urinary albumin excretion, lower HDL-cholesterol levels, higher history of CV disease (CVD), and higher use of statins and insulin. When all seven metabolites were used as predictors, FFA were inversely associated with incident hospitalized HF (hazard ratio [HR] 0.33 [95% CI 0.21-0.55]), while glycerol was a positive predictor (2.21 [1.45-3.35]). In a model further adjusted for 16 potential confounders, including prior HF and CVD and pharmacologic therapies, FFA remained a significant negative predictor. FFA and glycerol also predicted CV mortality (HR 0.53 [95% CI 0.35-0.81] and 1.81 [1.26-2.58], respectively) and all-cause death (0.50 [0.36-0.70] and 1.64 [1.22-2.18]). When added to these models, background insulin therapy was an independent positive predictor of risk of death. Canagliflozin treatment significantly increased plasma FFA and β-hydroxybutyrate regardless of background antihyperglycemic therapy.

CONCLUSIONS

A constitutive metabolic setup consisting of higher lipolysis may be beneficial in delaying or preventing hospitalized HF; a further stimulation of lipolysis by canagliflozin may reinforce this influence.

The Contribution of Cardiac Fatty Acid Oxidation to Diabetic Cardiomyopathy Severity

Diabetes is a major risk factor for the development of cardiovascular disease via contributing and/or triggering significant cellular signaling and metabolic and structural alterations at the level of the heart and the whole body. The main cause of mortality and morbidity in diabetic patients is cardiovascular disease including diabetic cardiomyopathy. Therefore, understanding how diabetes increases the incidence of diabetic cardiomyopathy and how it mediates the major perturbations in cell signaling and energy metabolism should help in the development of therapeutics to prevent these perturbations. One of the significant metabolic alterations in diabetes is a marked increase in cardiac fatty acid oxidation rates and the domination of fatty acids as the major energy source in the heart. This increased reliance of the heart on fatty acids in the diabetic has a negative impact on cardiac function and structure through a number of mechanisms. It also has a detrimental effect on cardiac efficiency and worsens the energy status in diabetes, mainly through inhibiting cardiac glucose oxidation. Furthermore, accelerated cardiac fatty acid oxidation rates in diabetes also make the heart more vulnerable to ischemic injury. In this review, we discuss how cardiac energy metabolism is altered in diabetic cardiomyopathy and the impact of cardiac insulin resistance on the contribution of glucose and fatty acid to overall cardiac ATP production and cardiac efficiency. Furthermore, how diabetes influences the susceptibility of the myocardium to ischemia/reperfusion injury and the role of the changes in glucose and fatty acid oxidation in mediating these effects are also discussed.

Concurrent diabetes and heart failure: interplay and novel therapeutic approaches

Diabetes mellitus increases the risk of developing heart failure, and the co-existence of both diseases worsens cardiovascular outcomes, hospitalization and the progression of heart failure. Despite current advancements on therapeutic strategies to manage hyperglycemia, the likelihood of developing diabetes-induced heart failure is still significant, especially with the accelerating global prevalence of diabetes and an ageing population. This raises the likelihood of other contributing mechanisms beyond hyperglycemia in predisposing diabetic patients to cardiovascular disease risk. There has been considerable interest in understanding the alterations in cardiac structure and function in the diabetic patients, collectively termed as "diabetic cardiomyopathy". However, the factors that contribute to the development of diabetic cardiomyopathies is not fully understood. This review summarizes the main characteristics of diabetic cardiomyopathies, and the basic mechanisms that contribute to its occurrence. This includes perturbations in insulin resistance, fuel preference, reactive oxygen species generation, inflammation, cell death pathways, neurohormonal mechanisms, advanced glycated end-products accumulation, lipotoxicity, glucotoxicity, and posttranslational modifications in the heart of the diabetic. This review also discusses the impact of antihyperglycemic therapies on the development of heart failure, as well as how current heart failure therapies influence glycemic control in diabetic patients. We also highlight the current knowledge gaps in understanding how diabetes induces heart failure.

Particulate matter inhalation and the exacerbation of cardiopulmonary toxicity due to metabolic disease

Particulate matter is a significant public health issue in the United States and globally. Inhalation of particulate matter is associated with a number of systemic and organ-specific adverse health outcomes, with the pulmonary and cardiovascular systems being particularly vulnerable. Certain subpopulations are well-recognized as being more susceptible to inhalation exposures, such as the elderly and those with pre-existing respiratory disease. Metabolic syndrome is becoming increasingly prevalent in our society and has known adverse effects on the heart, lungs, and vascular systems. The limited evaluations of individuals with metabolic syndromehave demonstrated that theymay compose a sensitive subpopulation to particulate exposures. However, the toxicological mechanisms responsible for this increased vulnerability are not fully understood. This review evaluates the currently available literature regarding how the response of an individual's pulmonary and cardiovascular systems is influenced by metabolic syndrome and metabolic syndrome-associated conditions such as hypertension, dyslipidemia, and diabetes. Further, we will discuss potential therapeutic agents and targets for the alleviation and treatment of particulate-matter induced metabolic illness. The information reviewed here may contribute to the understanding of metabolic illness as a risk factor for particulate matter exposure and further the development of therapeutic approaches to treat vulnerable subpopulations, such as those with metabolic diseases.

Impact of Obesity-Related Inflammation on Cardiac Metabolism and Function

This review focuses on the role of adipose tissue in obese individuals in the development of metabolic diseases, and their consequences for metabolic and functional derangements in the heart. The general idea is that the expansion of adipocytes during the development of obesity gives rise to unhealthy adipose tissue, characterized by low-grade inflammation and the release of proinflammatory adipokines and fatty acids (FAs). This condition, in turn, causes systemic inflammation and elevated FA concentrations in the circulation, which links obesity to several pathologies, including impaired insulin signaling in cardiac muscle and a subsequent shift in myocardial substrate oxidation in favor of FAs and reduced cardiac efficiency. This review also argues that efforts to prevent obesity-related cardiometabolic disease should focus on anti-obesogenic strategies to restore normal adipose tissue metabolism.

High fat diet altered cardiac metabolic gene profile in Psammomys obesus gerbils

Background

In metabolic disorders, myocardial fatty infiltration is critically associated with lipotoxic cardiomyopathy.

Methods

Twenty Psammomys obesus gerbils were randomly assigned to normal plant or high fat diet. Sixteen weeks later, myocardium was sampled for pathobiological evaluation.

Results

A sixteen-week high fat diet resulted in myocardial structure disorganization, with collagen deposits, lipid accumulation, cardiomyocyte apoptosis and inflammatory cell infiltration. Myocardial expressions of glucose transporter GLUT1 and pyruvate dehydrogenase (PDH) inhibitor, PDH kinase (PDK)4 increased, while insulin-regulated GLUT4 expression remained unchanged. Myocardial expressions of molecules regulating fatty acid transport, CD36 and fatty acid binding protein (FABP)3, were increased, while expression of rate-controlling fatty acid β-oxidation, carnitine palmitoyl transferase (CPT)1B decreased. Myocardial expression of AMP-activated protein kinase (AMPK), decreased, while expression of peroxisome proliferator activated receptors (PPAR)-α and -γ did not change.

Conclusion

In high fat diet fed Psammomys obesus, an original experimental model of nutritionally induced metabolic syndrome mixing genetic predisposition and environment interactions, a short period of high fat feeding was sufficient to induce myocardial structural alterations, associated with altered myocardial metabolic gene expression in favor of lipid accumulation.

Understanding the distinct subcellular trafficking of CD36 and GLUT4 during the development of myocardial insulin resistance

CD36 and GLUT4 are the main cardiac trans-sarcolemmal transporters for long-chain fatty acids and glucose, respectively. Together they secure the majority of cardiac energy demands. Moreover, these transporters each represent key governing kinetic steps in cardiac fatty acid and glucose fluxes, thereby offering major sites of regulation. The underlying mechanism of this regulation involves a perpetual vesicle-mediated trafficking (recycling) of both transporters between intracellular stores (endosomes) and the cell surface. In the healthy heart, CD36 and GLUT4 translocation to the cell surface is under short-term control of the same physiological stimuli, most notably increased contraction and insulin secretion. However, under chronic lipid overload, a condition that accompanies a Western lifestyle, CD36 and GLUT4 recycling are affected distinctly, with CD36 being expelled to the sarcolemma while GLUT4 is imprisoned within the endosomes. Moreover, the increased CD36 translocation towards the cell surface is a key early step, setting the heart on a route towards insulin resistance and subsequent contractile dysfunction. Therefore, the proteins making up the trafficking machinery of CD36 need to be identified with special focus to the differences with the protein composition of the GLUT4 trafficking machinery. These proteins that are uniquely dedicated to either CD36 or GLUT4 traffic may offer targets to rectify aberrant substrate uptake seen in the lipid-overloaded heart. Specifically, CD36-dedicated trafficking regulators should be inhibited, whereas such GLUT4-dedicated proteins would need to be activated. Recent advances in the identification of CD36-dedicated trafficking proteins have disclosed the involvement of vacuolar-type H⁺-ATPase and of specific vesicle-associated membrane proteins (VAMPs). In this review, we summarize these recent findings and sketch a roadmap of CD36 and GLUT4 trafficking compatible with experimental findings.

Metabolic Coordination of Physiological and Pathological Cardiac Remodeling

Metabolic pathways integrate to support tissue homeostasis and to prompt changes in cell phenotype. In particular, the heart consumes relatively large amounts of substrate not only to regenerate ATP for contraction but also to sustain biosynthetic reactions for replacement of cellular building blocks. Metabolic pathways also control intracellular redox state, and metabolic intermediates and end products provide signals that prompt changes in enzymatic activity and gene expression. Mounting evidence suggests that the changes in cardiac metabolism that occur during development, exercise, and pregnancy as well as with pathological stress (eg, myocardial infarction, pressure overload) are causative in cardiac remodeling. Metabolism-mediated changes in gene expression, metabolite signaling, and the channeling of glucose-derived carbon toward anabolic pathways seem critical for physiological growth of the heart, and metabolic inefficiency and loss of coordinated anabolic activity are emerging as proximal causes of pathological remodeling. This review integrates knowledge of different forms of cardiac remodeling to develop general models of how relationships between catabolic and anabolic glucose metabolism may fortify cardiac health or promote (mal)adaptive myocardial remodeling. Adoption of conceptual frameworks based in relational biology may enable further understanding of how metabolism regulates cardiac structure and function.

Mitochondrial NAD+/NADH Redox State and Diabetic Cardiomyopathy

Significance: Diabetic cardiomyopathy (DCM) is a frequent complication occurring even in well-controlled asymptomatic diabetic patients, and it may advance to heart failure (HF). Recent Advances: The diabetic heart is characterized by a state of "metabolic rigidity" involving enhanced rates of fatty acid uptake and mitochondrial oxidation as the predominant energy source, and it exhibits mitochondrial electron transport chain defects. These alterations promote redox state changes evidenced by a decreased NAD⁺/NADH ratio associated with an increase in acetyl-CoA/CoA ratio. NAD⁺ is a co-substrate for deacetylases, sirtuins, and a critical molecule in metabolism and redox signaling; whereas acetyl-CoA promotes protein lysine acetylation, affecting mitochondrial integrity and causing epigenetic changes. Critical Issues: DCM lacks specific therapies with treatment only in later disease stages using standard, palliative HF interventions. Traditional therapy targeting neurohormonal signaling and hemodynamics failed to improve mortality rates. Though mitochondrial redox state changes occur in the heart with obesity and diabetes, how the mitochondrial NAD⁺/NADH redox couple connects the remodeled energy metabolism with mitochondrial and cytosolic antioxidant defense and nuclear epigenetic changes remains to be determined. Mitochondrial therapies targeting the mitochondrial NAD⁺/NADH redox ratio may alleviate cardiac dysfunction. Future Directions: Specific therapies must be supported by an optimal understanding of changes in mitochondrial redox state and how it influences other cellular compartments; this field has begun to surface as a therapeutic target for the diabetic heart. We propose an approach based on an alternate mitochondrial electron transport that normalizes the mitochondrial redox state and improves cardiac function in diabetes.

Inhibition of sarcolemmal FAT/CD36 by sulfo-N-succinimidyl oleate rapidly corrects metabolism and restores function in the diabetic heart following hypoxia/reoxygenation

Aims

The type 2 diabetic heart oxidizes more fat and less glucose, which can impair metabolic flexibility and function. Increased sarcolemmal fatty acid translocase (FAT/CD36) imports more fatty acid into the diabetic myocardium, feeding increased fatty acid oxidation and elevated lipid deposition. Unlike other metabolic modulators that target mitochondrial fatty acid oxidation, we proposed that pharmacologically inhibiting fatty acid uptake, as the primary step in the pathway, would provide an alternative mechanism to rebalance metabolism and prevent lipid accumulation following hypoxic stress.

Methods and results

Hearts from type 2 diabetic and control male Wistar rats were perfused in normoxia, hypoxia and reoxygenation, with the FAT/CD36 inhibitor sulfo-N-succinimidyl oleate (SSO) infused 4 min before hypoxia. SSO infusion into diabetic hearts decreased the fatty acid oxidation rate by 29% and myocardial triglyceride concentration by 48% compared with untreated diabetic hearts, restoring fatty acid metabolism to control levels following hypoxia-reoxygenation. SSO infusion increased the glycolytic rate by 46% in diabetic hearts during hypoxia, increased pyruvate dehydrogenase activity by 53% and decreased lactate efflux rate by 56% compared with untreated diabetic hearts during reoxygenation. In addition, SSO treatment of diabetic hearts increased intermediates within the second span of the Krebs cycle, namely fumarate, oxaloacetate, and the FAD total pool. The cardiac dysfunction in diabetic hearts following decreased oxygen availability was prevented by SSO-infusion prior to the hypoxic stress. Infusing SSO into diabetic hearts increased rate pressure product by 60% during hypoxia and by 32% following reoxygenation, restoring function to control levels.

Conclusions

Diabetic hearts have limited metabolic flexibility and cardiac dysfunction when stressed, which can be rapidly rectified by reducing fatty acid uptake with the FAT/CD36 inhibitor, SSO. This novel therapeutic approach not only reduces fat oxidation but also lipotoxicity, by targeting the primary step in the fatty acid metabolism pathway.

MG53 and disordered metabolism in striated muscle

MG53 is a member of tripartite motif family (TRIM) that expressed most abundantly in striated muscle. Using rodent models, many studies have demonstrated the MG53 not only facilitates membrane repair after ischemia reperfusion injury, but also contributes to the protective effects of both pre- and post-conditioning. Recently, however, it has been shown that MG53 participates in the regulation of many metabolic processes, especially insulin signaling pathway. Thus, sustained overexpression of MG53 may contribute to the development of various metabolic disorders in striated muscle. In this review, using cardiac muscle as an example, we will discuss muscle metabolic disturbances associated with diabetes and the current understanding of the underlying molecular mechanisms; in particular, the pathogenesis of diabetic cardiomyopathy. We will focus on the pathways that MG53 regulates and how the dysregulation of MG53 leads to metabolic disorders, thereby establishing a causal relationship between sustained upregulation of MG53 and the development of muscle insulin resistance and metabolic disorders. This article is part of a Special issue entitled Cardiac adaptations to obesity, diabetes and insulin resistance, edited by Professors Jan F.C. Glatz, Jason R.B. Dyck and Christine Des Rosiers.

Acetylation of mitochondrial proteins by GCN5L1 promotes enhanced fatty acid oxidation in the heart

Lysine acetylation is a reversible posttranslational modification and is particularly important in the regulation of mitochondrial metabolic enzymes. Acetylation uses acetyl-CoA derived from fuel metabolism as a cofactor, thereby linking nutrition to metabolic activity. In the present study, we investigated how mitochondrial acetylation status in the heart is controlled by food intake and how these changes affect mitochondrial metabolism. We found that there was a significant increase in cardiac mitochondrial protein acetylation in mice fed a long-term high-fat diet and that this change correlated with an increase in the abundance of the mitochondrial acetyltransferase-related protein GCN5L1. We showed that the acetylation status of several mitochondrial fatty acid oxidation enzymes (long-chain acyl-CoA dehydrogenase, short-chain acyl-CoA dehydrogenase, and hydroxyacyl-CoA dehydrogenase) and a pyruvate oxidation enzyme (pyruvate dehydrogenase) was significantly upregulated in high-fat diet-fed mice and that the increase in long-chain and short-chain acyl-CoA dehydrogenase acetylation correlated with increased enzymatic activity. Finally, we demonstrated that the acetylation of mitochondrial fatty acid oxidation proteins was decreased after GCN5L1 knockdown and that the reduced acetylation led to diminished fatty acid oxidation in cultured H9C2 cells. These data indicate that lysine acetylation promotes fatty acid oxidation in the heart and that this modification is regulated in part by the activity of GCN5L1.NEW & NOTEWORTHY Recent research has shown that acetylation of mitochondrial fatty acid oxidation enzymes has greatly contrasting effects on their activity in different tissues. Here, we provide new evidence that acetylation of cardiac mitochondrial fatty acid oxidation enzymes by GCN5L1 significantly upregulates their activity in diet-induced obese mice.

Cardiovascular consequences of metabolic syndrome

The metabolic syndrome (MetS) is defined as the concurrence of obesity-associated cardiovascular risk factors including abdominal obesity, impaired glucose tolerance, hypertriglyceridemia, decreased HDL cholesterol, and/or hypertension. Earlier conceptualizations of the MetS focused on insulin resistance as a core feature, and it is clearly coincident with the above list of features. Each component of the MetS is an independent risk factor for cardiovascular disease and the combination of these risk factors elevates rates and severity of cardiovascular disease, related to a spectrum of cardiovascular conditions including microvascular dysfunction, coronary atherosclerosis and calcification, cardiac dysfunction, myocardial infarction, and heart failure. While advances in understanding the etiology and consequences of this complex disorder have been made, the underlying pathophysiological mechanisms remain incompletely understood, and it is unclear how these concurrent risk factors conspire to produce the variety of obesity-associated adverse cardiovascular diseases. In this review, we highlight current knowledge regarding the pathophysiological consequences of obesity and the MetS on cardiovascular function and disease, including considerations of potential physiological and molecular mechanisms that may contribute to these adverse outcomes.

MicroRNAs 33, 122, and 208: a potential novel targets in the treatment of obesity, diabetes, and heart-related diseases

Despite decades of research, obesity and diabetes remain major health problems in the USA and worldwide. Among the many complications associated with diabetes is an increased risk of cardiovascular diseases, including myocardial infarction and heart failure. Recently, microRNAs have emerged as important players in heart disease and energy regulation. However, little work has investigated the role of microRNAs in cardiac energy regulation. Both human and animal studies have reported a significant increase in circulating free fatty acids and triacylglycerol, increased cardiac reliance on fatty acid oxidation, and subsequent decrease in glucose oxidation which all contributes to insulin resistance and lipotoxicity seen in obesity and diabetes. Importantly, MED13 was initially identified as a negative regulator of lipid accumulation in Drosophilia. Various metabolic genes were downregulated in MED13 transgenic heart, including sterol regulatory element-binding protein. Moreover, miR-33 and miR-122 have recently revealed as key regulators of lipid metabolism. In this review, we will focus on the role of microRNAs in regulation of cardiac and total body energy metabolism. We will also discuss the pharmacological and non-pharmacological interventions that target microRNAs for the treatment of obesity and diabetes.

Cardiac fatty acid oxidation in heart failure associated with obesity and diabetes

Obesity and diabetes are major public health problems, and are linked to the development of heart failure. Emerging data highlight the importance of alterations in cardiac energy metabolism as a major contributor to cardiac dysfunction related to obesity and diabetes. Increased rates of fatty acid oxidation and decreased rates of glucose utilization are two prominent changes in cardiac energy metabolism that occur in obesity and diabetes. This metabolic profile is probably both a cause and consequence of a prominent cardiac insulin resistance, which is accompanied by a decrease in both cardiac function and efficiency, and by the accumulation of potentially toxic lipid metabolites in the heart that can further exaggerate insulin resistance and cardiac dysfunction. The high cardiac fatty acid oxidation rates seen in obesity and diabetes are attributable to several factors, including: 1) increased fatty acid supply and uptake into the cardiomyocyte, 2) increased transcription of fatty acid metabolic enzymes, 3) decreased allosteric control of mitochondrial fatty acid uptake and fatty acid oxidation, and 4) increased post-translational acetylation control of various fatty acid oxidative enzymes. Emerging evidence suggests that therapeutic approaches aimed at switching the balance of cardiac energy substrate preference from fatty acid oxidation to glucose use can prevent cardiac dysfunction associated with obesity and diabetes. Modulating acetylation control of fatty acid oxidative enzymes is also a potentially attractive strategy, although presently this is limited to precursors of nicotinamide adenine or nonspecific activators of deacetylation such as resveratrol. This review will focus on the metabolic alterations in the heart that occur in obesity and diabetes, as well as on the molecular mechanisms controlling these metabolic changes. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.

Assessment of myocardial metabolic flexibility and work efficiency in human type 2 diabetes using 16-[18F]fluoro-4-thiapalmitate, a novel PET fatty acid tracer

Altered myocardial fuel selection likely underlies cardiac disease risk in diabetes, affecting oxygen demand and myocardial metabolic flexibility. We investigated myocardial fuel selection and metabolic flexibility in human type 2 diabetes mellitus (T2DM), using positron emission tomography to measure rates of myocardial fatty acid oxidation {16-[(18)F]fluoro-4-thia-palmitate (FTP)} and myocardial perfusion and total oxidation ([(11)C]acetate). Participants underwent paired studies under fasting conditions, comparing 3-h insulin + glucose euglycemic clamp conditions (120 mU·m(-2)·min(-1)) to 3-h saline infusion. Lean controls (n = 10) were compared with glycemically controlled volunteers with T2DM (n = 8). Insulin augmented heart rate, blood pressure, and stroke index in both groups (all P < 0.01) and significantly increased myocardial oxygen consumption (P = 0.04) and perfusion (P = 0.01) in both groups. Insulin suppressed available nonesterified fatty acids (P < 0.0001), but fatty acid concentrations were higher in T2DM under both conditions (P < 0.001). Insulin-induced suppression of fatty acid oxidation was seen in both groups (P < 0.0001). However, fatty acid oxidation rates were higher under both conditions in T2DM (P = 0.003). Myocardial work efficiency was lower in T2DM (P = 0.006) and decreased in both groups with the insulin-induced increase in work and shift in fuel utilization (P = 0.01). Augmented fatty acid oxidation is present under baseline and insulin-treated conditions in T2DM, with impaired insulin-induced shifts away from fatty acid oxidation. This is accompanied by reduced work efficiency, possibly due to greater oxygen consumption with fatty acid metabolism. These observations suggest that improved fatty acid suppression, or reductions in myocardial fatty acid uptake and retention, could be therapeutic targets to improve myocardial ischemia tolerance in T2DM.

Myocardial metabolism in diabetic cardiomyopathy: potential therapeutic targets

SIGNIFICANCE

Cardiovascular complications in diabetes are particularly serious and represent the primary cause of morbidity and mortality in diabetic patients. Despite early observations of cardiac dysfunction in diabetic humans, cardiomyopathy unique to diabetes has only recently been recognized.

RECENT ADVANCES

Research has focused on understanding the pathogenic mechanisms underlying the initiation and development of diabetic cardiomyopathy. Emerging data highlight the importance of altered mitochondrial function as a major contributor to cardiac dysfunction in diabetes. Mitochondrial dysfunction occurs by several mechanisms involving altered cardiac substrate metabolism, lipotoxicity, impaired cardiac insulin and glucose homeostasis, impaired cellular and mitochondrial calcium handling, oxidative stress, and mitochondrial uncoupling.

CRITICAL ISSUES

Currently, treatment is not specifically tailored for diabetic patients with cardiac dysfunction. Given the multifactorial development and progression of diabetic cardiomyopathy, traditional treatments such as anti-diabetic agents, as well as cellular and mitochondrial fatty acid uptake inhibitors aimed at shifting the balance of cardiac metabolism from utilizing fat to glucose may not adequately target all aspects of this condition. Thus, an alternative treatment such as resveratrol, which targets multiple facets of diabetes, may represent a safe and promising supplement to currently recommended clinical therapy and lifestyle changes.

FUTURE DIRECTIONS

Elucidation of the mechanisms underlying the initiation and progression of diabetic cardiomyopathy is essential for development of effective and targeted treatment strategies. Of particular interest is the investigation of alternative therapies such as resveratrol, which can function as both preventative and mitigating agents in the management of diabetic cardiomyopathy.

Mitochondrial fatty acid oxidation alterations in heart failure, ischaemic heart disease and diabetic cardiomyopathy

Heart disease is a leading cause of death worldwide. In many forms of heart disease, including heart failure, ischaemic heart disease and diabetic cardiomyopathies, changes in cardiac mitochondrial energy metabolism contribute to contractile dysfunction and to a decrease in cardiac efficiency. Specific metabolic changes include a relative increase in cardiac fatty acid oxidation rates and an uncoupling of glycolysis from glucose oxidation. In heart failure, overall mitochondrial oxidative metabolism can be impaired while, in ischaemic heart disease, energy production is impaired due to a limitation of oxygen supply. In both of these conditions, residual mitochondrial fatty acid oxidation dominates over mitochondrial glucose oxidation. In diabetes, the ratio of cardiac fatty acid oxidation to glucose oxidation also increases, although primarily due to an increase in fatty acid oxidation and an inhibition of glucose oxidation. Recent evidence suggests that therapeutically regulating cardiac energy metabolism by reducing fatty acid oxidation and/or increasing glucose oxidation can improve cardiac function of the ischaemic heart, the failing heart and in diabetic cardiomyopathies. In this article, we review the cardiac mitochondrial energy metabolic changes that occur in these forms of heart disease, what role alterations in mitochondrial fatty acid oxidation have in contributing to cardiac dysfunction and the potential for targeting fatty acid oxidation to treat these forms of heart disease.

Ischemic conditioning: the challenge of protecting the diabetic heart

The successful clinical translation of novel therapeutic strategies to attenuate lethal myocardial ischemia-reperfusion injury and limit infarct size has been identified as a major unmet need, and is of particular importance in patients with type-2 diabetes. There is a wealth of preclinical evidence that ischemic conditioning (encompassing the three paradigms of preconditioning, postconditioning and remote conditioning) is profoundly cardioprotective and, via up-regulation of endogenous signaling cascades, renders the heart resistant to infarction. However, current phase II trials aimed at exploiting ischemic conditioning for the clinical treatment of myocardial ischemia-reperfusion injury have yielded mixed results, possibly reflecting the emerging concern that the efficacy of conditioning-induced cardioprotection may be compromised in the diabetic heart. Our goal in this review is to provide a summary of our present understanding of the effect of type-2 diabetes on the infarct-sparing effect of ischemic conditioning, and the challenges of limiting ischemia-reperfusion injury in the diabetic heart.

Obesity-induced lysine acetylation increases cardiac fatty acid oxidation and impairs insulin signalling

AIMS

Lysine acetylation is a novel post-translational pathway that regulates the activities of enzymes involved in both fatty acid and glucose metabolism. We examined whether lysine acetylation controls heart glucose and fatty acid oxidation in high-fat diet (HFD) obese and SIRT3 knockout (KO) mice.

METHODS AND RESULTS

C57BL/6 mice were placed on either a HFD (60% fat) or a low-fat diet (LFD; 4% fat) for 16 or 18 weeks. Cardiac fatty acid oxidation rates were significantly increased in HFD vs. LFD mice (845 ± 76 vs. 551 ± 87 nmol/g dry wt min, P < 0.05). Activities of the fatty acid oxidation enzymes, long-chain acyl-CoA dehydrogenase (LCAD), and β-hydroxyacyl-CoA dehydrogenase (β-HAD) were increased in hearts from HFD vs. LFD mice, and were associated with LCAD and β-HAD hyperacetylation. Cardiac protein hyperacetylation in HFD-fed mice was associated with a decrease in SIRT3 expression, while expression of the mitochondrial acetylase, general control of amino acid synthesis 5 (GCN5)-like 1 (GCN5L1), did not change. Interestingly, SIRT3 deletion in mice also led to an increase in cardiac fatty acid oxidation compared with wild-type (WT) mice (422 ± 29 vs. 291 ± 17 nmol/g dry wt min, P < 0.05). Cardiac lysine acetylation was increased in SIRT3 KO mice compared with WT mice, including increased acetylation and activity of LCAD and β-HAD. Although the HFD and SIRT3 deletion decreased glucose oxidation, pyruvate dehydrogenase acetylation was unaltered. However, the HFD did increase Akt acetylation, while decreasing its phosphorylation and activity.

CONCLUSION

We conclude that increased cardiac fatty acid oxidation in response to high-fat feeding is controlled, in part, via the down-regulation of SIRT3 and concomitant increased acetylation of mitochondrial β-oxidation enzymes.

Matrix revisited: mechanisms linking energy substrate metabolism to the function of the heart

Metabolic signaling mechanisms are increasingly recognized to mediate the cellular response to alterations in workload demand, as a consequence of physiological and pathophysiological challenges. Thus, an understanding of the metabolic mechanisms coordinating activity in the cytosol with the energy-providing pathways in the mitochondrial matrix becomes critical for deepening our insights into the pathogenic changes that occur in the stressed cardiomyocyte. Processes that exchange both metabolic intermediates and cations between the cytosol and mitochondria enable transduction of dynamic changes in contractile state to the mitochondrial compartment of the cell. Disruption of such metabolic transduction pathways has severe consequences for the energetic support of contractile function in the heart and is implicated in the pathogenesis of heart failure. Deficiencies in metabolic reserve and impaired metabolic transduction in the cardiomyocyte can result from inherent deficiencies in metabolic phenotype or maladaptive changes in metabolic enzyme expression and regulation in the response to pathogenic stress. This review examines both current and emerging concepts of the functional linkage between the cytosol and the mitochondrial matrix with a specific focus on metabolic reserve and energetic efficiency. These principles of exchange and transport mechanisms across the mitochondrial membrane are reviewed for the failing heart from the perspectives of chronic pressure overload and diabetes mellitus.

Angiotensin 1-7 ameliorates diabetic cardiomyopathy and diastolic dysfunction in db/db mice by reducing lipotoxicity and inflammation

BACKGROUND

The angiotensin-converting enzyme 2 and angiotensin-(1-7) (Ang 1-7)/MasR (Mas receptor) axis are emerging as a key pathway that can modulate the development of diabetic cardiomyopathy. We studied the effects of Ang 1-7 on diabetic cardiomyopathy in db/db diabetic mice to elucidate the therapeutic effects and mechanism of action.

METHODS AND RESULTS

Ang 1-7 was administered to 5-month-old male db/db mice for 28 days via implanted micro-osmotic pumps. Ang 1-7 treatment ameliorated myocardial hypertrophy and fibrosis with normalization of diastolic dysfunction assessed by pressure-volume loop analysis and echocardiography. The functional improvement by Ang 1-7 was accompanied by a reduction in myocardial lipid accumulation and systemic fat mass and inflammation and increased insulin-stimulated myocardial glucose oxidation. Increased myocardial protein kinase C levels and loss of phosphorylation of extracellular signal-regulated kinase 1/2 were prevented by Ang 1-7. Furthermore, Ang 1-7 treatment decreased cardiac triacylglycerol and ceramide levels in db/db mice, concomitantly with an increase in myocardial adipose triglyceride lipase expression. Changes in adipose triglyceride lipase expression correlated with increased SIRT1 (silent mating type information regulation 2 homolog 1) levels and deacetylation of FOXO1 (forkhead box O1).

CONCLUSIONS

We identified a novel beneficial effect of Ang 1-7 on diabetic cardiomyopathy that involved a reduction in cardiac hypertrophy and lipotoxicity, adipose inflammation, and an upregulation of adipose triglyceride lipase. Ang 1-7 completely rescued the diastolic dysfunction in the db/db model. Ang 1-7 represents a promising therapy for diabetic cardiomyopathy associated with type 2 diabetes mellitus.

Acute endurance exercise increases plasma membrane fatty acid transport proteins in rat and human skeletal muscle

Fatty acid transport proteins are present on the plasma membrane and are involved in the uptake of long-chain fatty acids into skeletal muscle. The present study determined whether acute endurance exercise increased the plasma membrane content of fatty acid transport proteins in rat and human skeletal muscle and whether the increase was accompanied by an increase in long-chain fatty acid transport in rat skeletal muscle. Sixteen subjects cycled for 120 min at ∼60 ± 2% Vo(2) peak. Two skeletal muscle biopsies were taken at rest and again following cycling. In a parallel study, eight Sprague-Dawley rats ran for 120 min at 20 m/min, whereas eight rats acted as nonrunning controls. Giant sarcolemmal vesicles were prepared, and protein content of FAT/CD36 and FABPpm was measured in human and rat vesicles and whole muscle homogenate. Palmitate uptake was measured in the rat vesicles. In human muscle, plasma membrane FAT/CD36 and FABPpm protein contents increased 75 and 20%, respectively, following 120 min of exercise. In rat muscle, plasma membrane FAT/CD36 and FABPpm increased 20 and 30%, respectively, and correlated with a 30% increase in palmitate transport following 120 min of running. These data suggest that the translocation of FAT/CD36 and FABPpm to the plasma membrane in rat skeletal muscle is related to the increase in fatty acid transport and oxidation that occurs with endurance running. This study is also the first to demonstrate that endurance cycling induces an increase in plasma membrane FAT/CD36 and FABPpm content in human skeletal muscle, which is predicted to increase fatty acid transport.

Inactivation of the AMP-activated protein kinase by glucose in cardiac myocytes: a role for the pentose phosphate pathway

Incubation of adult rat cardiac myocytes with increasing glucose concentrations decreased phosphorylation (αThr172) and activity of AMPK (AMP-activated protein kinase). The effect could be demonstrated without measurable changes in adenine nucleotide contents. The glucose effect was additive to the decrease in AMPK activity caused by insulin, was attenuated by adrenaline, was not mimicked by glucose analogues, lactate or pyruvate and was not due to changes in myocyte glycogen content. AMPK activity was decreased by xylitol and PMS (phenazine methosulfate) and was increased by the glucose-6-phosphate dehydrogenase inhibitor DHEA (dehydroepiandrosterone) and by thiamine. PMS and DHEA respectively, increased and decreased CO2 formation by the PPP (pentose phosphate pathway). AMPK activity was inversely related to the myocyte content of Xu5P (xylulose 5-phosphate), an intermediate of the non-oxidative arm of the PPP. Endothall, an inhibitor of PP2A (protein phosphatase 2A), abolished the glucose effect on AMPK activity. Further studies are needed to define the ‘active component’ that mediates the glucose effect and whether its site of action is PP2A.

High fat diet induced diabetic cardiomyopathy

In response to a chronic high plasma concentration of long-chain fatty acids (FAs), the heart is forced to increase the uptake of FA at the cost of glucose. This switch in metabolic substrate uptake is accompanied by an increased presence of the FA transporter CD36 at the cardiac plasma membrane and over time results in the development of cardiac insulin resistance and ultimately diabetic cardiomyopathy. FA can interact with peroxisome proliferator-activated receptors (PPARs), which induce upregulation of the expression of enzymes necessary for their disposal through mitochondrial β-oxidation, but also stimulate FA uptake. This then leads to a further increase in FA concentration in the cytoplasm of cardiomyocytes. These metabolic changes are supposed to play an important role in the development of cardiomyopathy. Although the onset of this pathology is an increased FA utilization by the heart, the subsequent lipid overload results in an increased production of reactive oxygen species (ROS) and accumulation of lipid intermediates such as diacylglycerols (DAG) and ceramide. These compounds have a profound impact on signaling pathways, in particular insulin signaling. Over time the metabolic changes will introduce structural changes that affect cardiac contractile characteristics. The present mini-review will focus on the lipid-induced changes that link metabolic perturbation, characteristic for type 2 diabetes, with cardiac remodeling and dysfunction.

Protein-mediated Fatty Acid Uptake in the Heart

Long chain fatty acids (LCFAs) provide 70-80% of the energy for cardiac contractile activity. LCFAs are also essential for many other cellular functions, such as transcriptional regulation of proteins involved in lipid metabolism, modulation of intracellular signalling pathways, and as substrates for membrane constituents. When LCFA uptake exceeds the capacity for their cardiac utilization, the intracellular lipids accumulate and are thought to contribute to contractile dysfunction, arrhythmias, cardiac myocyte apoptosis and congestive heart failure. Moreover, increased cardiac myocyte triacylglycerol, diacylglycerol and ceramide depots are cardinal features associated with obesity and type 2 diabetes. In recent years considerable evidence has accumulated to suggest that, the rate of entry of long chain fatty acids (LCFAs) into the cardiac myocyte is a key factor contributing to a) regulating cardiac LCFA metabolism and b) lipotoxicity in the obese and diabetic heart. In the present review we i) examine the evidence indicating that LCFA transport into the heart involves a protein-mediated mechanism, ii) discuss the proteins involved in this process, including FAT/CD36, FABPpm and FATP1, iii) discuss the mechanisms involved in regulating LCFA transport by some of these proteins (including signaling pathways), as well as iv) the possible interactions of these proteins in regulating LCFA transport into the heart. In addition, v) we discuss how LCFA transport and transporters are altered in the obese/diabetic heart.

Subcellular trafficking of the substrate transporters GLUT4 and CD36 in cardiomyocytes

Cardiomyocytes use glucose as well as fatty acids for ATP production. These substrates are transported into the cell by glucose transporter 4 (GLUT4) and the fatty acid transporter CD36. Besides being located at the sarcolemma, GLUT4 and CD36 are stored in intracellular compartments. Raised plasma insulin concentrations and increased cardiac work will stimulate GLUT4 as well as CD36 to translocate to the sarcolemma. As so far studied, signaling pathways that regulate GLUT4 translocation similarly affect CD36 translocation. During the development of insulin resistance and type 2 diabetes, CD36 becomes permanently localized at the sarcolemma, whereas GLUT4 internalizes. This juxtaposed positioning of GLUT4 and CD36 is important for aberrant substrate uptake in the diabetic heart: chronically increased fatty acid uptake at the expense of glucose. To explain the differences in subcellular localization of GLUT4 and CD36 in type 2 diabetes, recent research has focused on the role of proteins involved in trafficking of cargo between subcellular compartments. Several of these proteins appear to be similarly involved in both GLUT4 and CD36 translocation. Others, however, have different roles in either GLUT4 or CD36 translocation. These trafficking components, which are differently involved in GLUT4 or CD36 translocation, may be considered novel targets for the development of therapies to restore the imbalanced substrate utilization that occurs in obesity, insulin resistance and diabetic cardiomyopathy.

Chronic and acute exposure of mouse hearts to fatty acids increases oxygen cost of excitation-contraction coupling

The aim of the present study was to evaluate the underlying processes involved in the oxygen wasting induced by inotropic drugs and acute and chronic elevation of fatty acid (FA) supply, using unloaded perfused mouse hearts from normal and type 2 diabetic ( db/db) mice. We found that an acute elevation of the FA supply in normal hearts, as well as a chronic (in vivo) exposure to elevated FA as in db/db hearts, increased myocardial oxygen consumption (MV̇o2unloaded) due to increased oxygen cost for basal metabolism and for excitation-contraction (EC) coupling. Isoproterenol stimulation, on top of a high FA supply, led to an additive increase in MV̇o2unloaded, because of a further increase in oxygen cost for EC coupling. In db/db hearts, the acute elevation of FA did not further increase MV̇o2. Since the elevation in the FA supply is accompanied by increased rates of myocardial FA oxidation, the present study compared MV̇o2 following increased FA load versus FA oxidation rate by exposing normal hearts to normal and high FA concentration (NF and HF, respectively) and to compounds that either stimulate (GW-610742) or inhibit [dichloroacetate (DCA)] FA oxidation. While HF and NF + GW-610742 increased FA oxidation to the same extent, only HF increased MV̇o2unloaded. Although DCA counteracted the HF-induced increase in FA oxidation, DCA did not reduce MV̇o2unloaded. Thus, in normal hearts, acute FA-induced oxygen waste is 1) due to an increase in the oxygen cost for both basal metabolism and EC coupling and 2) not dependent on the myocardial FA oxidation rate per se, but on processes initiated by the presence of FAs. In diabetic hearts, chronic exposure to elevated circulating FAs leads to adaptations that afford protection against the detrimental effect of an acute FA load, suggesting different underlying mechanisms behind the increased MV̇o2 following acute and chronic FA load.

Metabolism, hypoxia and the diabetic heart

The diabetic heart becomes metabolically remodelled as a consequence of exposure to abnormal circulating substrates and hormones. Fatty acid uptake and metabolism are increased in the type 2 diabetic heart, resulting in accumulation of intracellular lipid intermediates and an increased contribution of fatty acids towards energy generation. Cardiac glucose uptake and oxidation are decreased, predominantly due to increased fatty acid metabolism, which suppresses glucose utilisation via the Randle cycle. These metabolic changes decrease cardiac efficiency and energetics in both humans and animal models of diabetes. Diabetic hearts have decreased recovery following ischemia, indicating a reduced tolerance to oxygen-limited conditions. There is evidence that diabetic hearts have a compromised hypoxia signalling pathway, as hypoxia-inducible factor (HIF) and downstream signalling from HIF are reduced following ischemia. Failure to activate HIF under oxygen-limited conditions results in less angiogenesis, and an inability to upregulate glycolytic ATP generation. Given that glycolysis is already suppressed in the diabetic heart under normoxic conditions, the inability to upregulate glycolysis in response to hypoxia may have deleterious effects on ATP production. Thus, impaired HIF signalling may contribute to metabolic and energetic abnormalities, and impaired collateral vessel development following myocardial infarction in the type 2 diabetic heart.

Berberine attenuates cardiac dysfunction in hyperglycemic and hypercholesterolemic rats

The positive effects of berberine (30 mg/kg/day, i.g. for 6 weeks) on cardiac dysfunction were evaluated in the rat model of hyperglycemia and hypercholesterolemia. Hyperglycemia and hypercholesterolemia were induced by feeding high-sucrose/fat diet (HSFD) consisting of 20% sucrose, 10% lard, 2.5% cholesterol, 1% bile salt for 12 weeks and streptozotocin (30 mg/kg, i.p.). The plasma sugar, total cholesterol, and triglyceride levels were significantly increased (422, 194 and 82%, respectively) in the HSFD/streptozotocin-treated rats, when compared with control animals receiving normal diet and vehicle. Berberine treatment reduced the plasma sugar and lipid levels by 24-69% in the rat model of hyperglycemia and hypercholesterolemia. Cardiac functions signed as values of cardiac output, left ventricular systolic pressure, the maximum rate of myocardial contraction (+dp/dtmax), left ventricular end diastolic pressure and the maximum rate of myocardial diastole (-dp/dtmax) were injured by 16-55% in the hyperglycemic/hypercholesterolemic rats. Berberine increased cardiac output, left ventricular systolic pressure and +dp/dtmax by 64, 16 and 79%, but decreased left ventricular end diastolic pressure and -dp/dtmax by 121 and 61% in the rats receiving HSFD/streptozotocin, respectively, when compared with the drug-untreated rats of hyperglycemia and hypercholesterolemia. Berberine caused significant increase in cardiac fatty acid transport protein-1 (159%), fatty acid transport proteins (56%), fatty acid beta-oxidase (52%), as well as glucose transporter-4 and peroxisome proliferator-activated receptor-γ (PPARγ), but decrease in PPARα mRNA and protein expression in hyperglycemic/hypercholesterolemic rats. These results indicated that berberine exerted protective effects on cardiac dysfunction induced by hyperglycemia/hypercholesterolemia through alleviating cardiac lipid accumulation and promoting glucose transport.

A high fat diet increases mitochondrial fatty acid oxidation and uncoupling to decrease efficiency in rat heart

Elevated levels of cardiac mitochondrial uncoupling protein 3 (UCP3) and decreased cardiac efficiency (hydraulic power/oxygen consumption) with abnormal cardiac function occur in obese, diabetic mice. To determine whether cardiac mitochondrial uncoupling occurs in non-genetic obesity, we fed rats a high fat diet (55% kcal from fat) or standard laboratory chow (7% kcal from fat) for 3 weeks, after which we measured cardiac function in vivo using cine MRI, efficiency in isolated working hearts and respiration rates and ADP/O ratios in isolated interfibrillar mitochondria; also, measured were medium chain acyl-CoA dehydrogenase (MCAD) and citrate synthase activities plus uncoupling protein 3 (UCP3), mitochondrial thioesterase 1 (MTE-1), adenine nucleotide translocase (ANT) and ATP synthase protein levels. We found that in vivo cardiac function was the same for all rats, yet oxygen consumption was 19% higher in high fat-fed rat hearts, therefore, efficiency was 21% lower than in controls. We found that mitochondrial fatty acid oxidation rates were 25% higher, and MCAD activity was 23% higher, in hearts from rats fed the high fat diet when compared with controls. Mitochondria from high fat-fed rat hearts had lower ADP/O ratios than controls, indicating increased respiratory uncoupling, which was ameliorated by GDP, a UCP3 inhibitor. Mitochondrial UCP3 and MTE-1 levels were both increased by 20% in high fat-fed rat hearts when compared with controls, with no significant change in ATP synthase or ANT levels, or citrate synthase activity. We conclude that increased cardiac oxygen utilisation, and thereby decreased cardiac efficiency, occurs in non-genetic obesity, which is associated with increased mitochondrial uncoupling due to elevated UCP3 and MTE-1 levels.

Myocardial fatty acid metabolism in health and disease

There is a constant high demand for energy to sustain the continuous contractile activity of the heart, which is met primarily by the beta-oxidation of long-chain fatty acids. The control of fatty acid beta-oxidation is complex and is aimed at ensuring that the supply and oxidation of the fatty acids is sufficient to meet the energy demands of the heart. The metabolism of fatty acids via beta-oxidation is not regulated in isolation; rather, it occurs in response to alterations in contractile work, the presence of competing substrates (i.e., glucose, lactate, ketones, amino acids), changes in hormonal milieu, and limitations in oxygen supply. Alterations in fatty acid metabolism can contribute to cardiac pathology. For instance, the excessive uptake and beta-oxidation of fatty acids in obesity and diabetes can compromise cardiac function. Furthermore, alterations in fatty acid beta-oxidation both during and after ischemia and in the failing heart can also contribute to cardiac pathology. This paper reviews the regulation of myocardial fatty acid beta-oxidation and how alterations in fatty acid beta-oxidation can contribute to heart disease. The implications of inhibiting fatty acid beta-oxidation as a potential novel therapeutic approach for the treatment of various forms of heart disease are also discussed.

Membrane Fatty Acid Transporters as Regulators of Lipid Metabolism: Implications for Metabolic Disease

Long-chain fatty acids and lipids serve a wide variety of functions in mammalian homeostasis, particularly in the formation and dynamic properties of biological membranes and as fuels for energy production in tissues such as heart and skeletal muscle. On the other hand, long-chain fatty acid metabolites may exert toxic effects on cellular functions and cause cell injury. Therefore, fatty acid uptake into the cell and intracellular handling need to be carefully controlled. In the last few years, our knowledge of the regulation of cellular fatty acid uptake has dramatically increased. Notably, fatty acid uptake was found to occur by a mechanism that resembles that of cellular glucose uptake. Thus, following an acute stimulus, particularly insulin or muscle contraction, specific fatty acid transporters translocate from intracellular stores to the plasma membrane to facilitate fatty acid uptake, just as these same stimuli recruit glucose transporters to increase glucose uptake. This regulatory mechanism is important to clear lipids from the circulation postprandially and to rapidly facilitate substrate provision when the metabolic demands of heart and muscle are increased by contractile activity. Studies in both humans and animal models have implicated fatty acid transporters in the pathogenesis of diseases such as the progression of obesity to insulin resistance and type 2 diabetes. As a result, membrane fatty acid transporters are now being regarded as a promising therapeutic target to redirect lipid fluxes in the body in an organ-specific fashion.

Cardiac and skeletal muscle fatty acid transport and transporters and triacylglycerol and fatty acid oxidation in lean and Zucker diabetic fatty rats

We examined fatty acid transporters, transport, and metabolism in hearts and red and white muscles of lean and insulin-resistant ( week 6) and type 2 diabetic ( week 24) Zucker diabetic fatty (ZDF) rats. Cardiac fatty acid transport was similar in lean and ZDF hearts at week 6 but was reduced at week 24 (−40%) in lean but not ZDF hearts. Red muscle of ZDF rats exhibited an early susceptibility to upregulation (+66%) of fatty acid transport at week 6 that was increased by 50% in lean and ZDF rats at week 24 but remained 44% greater in red muscle of ZDF rats. In white muscle, no differences were observed in fatty acid transport between groups or from week 6 to week 24. In all tissues (heart and red and white muscle), FAT/CD36 protein and plasmalemmal content paralleled the changes in fatty acid transport. Triacylglycerol content in red and white muscles, but not heart, in lean and ZDF rats correlated with fatty acid transport ( r = 0.91) and sarcolemmal FAT/CD36 ( r = 0.98). Red and white muscle fatty acid oxidation by isolated mitochondria was not impaired in ZDF rats but was reduced by 18–24% in red muscle of lean rats at week 24. Thus, in red, but not white, muscle of insulin-resistant and type 2 diabetic animals, a marked upregulation in fatty acid transport and intramuscular triacylglycerol was associated with increased levels of FAT/CD36 expression and plasmalemmal content. In heart, greater rates of fatty acid transport and FAT/CD36 in ZDF rats ( week 24) were attributable to the inhibition of age-related reductions in these parameters. However, intramuscular triacylglycerol did not accumulate in hearts of ZDF rats. Thus insulin resistance and type 2 diabetes are accompanied by tissue-specific differences in FAT/CD36 and fatty acid transport and metabolism. Upregulation of fatty acid transport increased red muscle, but not cardiac, triacylglycerol accumulation. White muscle lipid metabolism dysregulation was not observed.

Sarcolemmal fatty acid uptake vs. mitochondrial beta-oxidation as target to regress cardiac insulin resistance

Cardiomyopathy and heart failure are frequent comorbid conditions in type-2 diabetic patients. However, it has become increasingly evident that insulin resistance, type-2 diabetes, and cardiomyopathy are not independent variables, and are linked through changes in metabolism. Specifically, elevated intracellular levels of long-chain fatty acid (LCFA) metabolites are a central feature in the development of cardiac insulin resistance, and their prolonged accumulation is an important cause of heart failure. In the insulin-resistant heart, the abundance of the LCFA transporters CD36 and FABPpm at the sarcolemma of cardiac myocytes appears to be markedly increased. Because circulating LCFA levels are increased in insulin resistance, the cardiac LCFA metabolizing machinery is confronted with drastic increases in substrate supply. Indeed, LCFA esterification into triacylglycerol and other lipid intermediates is increased, as is beta-oxidation and reactive oxygen species production. Therapeutic strategies to normalize the cardiac LCFA flux would be most successful when the target is the rate-limiting step in cardiac LCFA utilization. Carnitine palmitoyltransferase (CPT)-I has long been considered to be this rate-limiting site and, accordingly, pharmacological inhibition of CPT-I, or beta-oxidation enzymes, has been proposed as an insulin-resistance-antagonizing strategy. However, recent evidence indicates that, instead, sarcolemmal LCFA transport mediated by CD36 in concert with FABPpm provides a major site of flux control. In this review, it is proposed that a pharmacologically imposed net internalization of CD36 and FABPpm is the preferable strategy to limit LCFA entry and accumulation of LCFA metabolites, to regress cardiac insulin resistance and, eventually, prevent diabetic heart failure.

Increased O2 cost of basal metabolism and excitation-contraction coupling in hearts from type 2 diabetic mice

We have reported previously that hearts from type 2 diabetic ( db/ db) mice show decreased cardiac efficiency due to increased work-independent myocardial O2 consumption (unloaded MV̇o2), indicating higher O2 use for nonmechanical processes such as basal metabolism (MV̇o2BM) and excitation-contraction coupling (MV̇o2ECC). Although alterations in cardiac metabolism and/or Ca2+ handling may contribute to increased energy expenditure in diabetic hearts, direct measurements of the O2 cost for these individual processes have not been determined. In this study, we 1) validate a procedure for measuring unloaded MV̇o2 directly (MV̇o2unloaded) and for determining MV̇o2BM and MV̇o2ECC separately in isolated perfused mouse hearts and 2) determine O2 cost for these processes in hearts from db/ db mice. Unloaded MV̇o2, extrapolated from the relationship between cardiac work (measured as pressure-volume area, PVA) and MV̇o2, was found to correspond with MV̇o2 measured directly in unloaded retrograde perfused hearts (MV̇o2unloaded). MV̇o2 in K+-arrested hearts was defined as MV̇o2BM; the difference between MV̇o2unloaded and MV̇o2BM represented MV̇o2ECC. This procedure was validated by demonstrating that elevations in perfusate fatty acid (FA) and/or Ca2+ concentrations resulted in changes in either MV̇o2BM and/or MV̇o2ECC. The higher MV̇o2unloaded in db/ db mice was due to both a higher MV̇o2BM and MV̇o2ECC. Elevation of glucose and insulin decreased FA oxidation and reduced both MV̇o2unloaded and MV̇o2BM. In conclusion, this study provides direct evidence that MV̇o2BM and MV̇o2ECC are elevated in diabetes and that acute metabolic interventions can have a therapeutic benefit in diabetic hearts due to a MV̇o2-lowering effect.

Etomoxir-induced partial carnitine palmitoyltransferase-I (CPT-I) inhibition in vivo does not alter cardiac long-chain fatty acid uptake and oxidation rates

Although CPT-I (carnitine palmitoyltransferase-I) is generally regarded to present a major rate-controlling site in mitochondrial β-oxidation, it is incompletely understood whether CPT-I is rate-limiting in the overall LCFA (long-chain fatty acid) flux in the heart. Another important site of regulation of the LCFA flux in the heart is trans-sarcolemmal LCFA transport facilitated by CD36 and FABPpm (plasma membrane fatty acid-binding protein). Therefore, we explored to what extent a chronic pharmacological blockade of the LCFA flux at the level of mitochondrial entry of LCFA-CoA would affect sarcolemmal LCFA uptake. Rats were injected daily with saline or etomoxir, a specific CPT-I inhibitor, for 8 days at 20 mg/kg of body mass. Etomoxir-treated rats displayed a 44% reduced cardiac CPT-I activity. Sarcolemmal contents of CD36 and FABPpm, as well as the LCFA transport capacity, were not altered in the hearts of etomoxir-treated versus control rats. Furthermore, rates of LCFA uptake and oxidation, and glucose uptake by cardiac myocytes from etomoxir-treated rats were not different from control rats, neither under basal nor under acutely induced maximal metabolic demands. Finally, hearts from etomoxir-treated rats did not display triacylglycerol accumulation. Therefore CPT-I appears not to present a major rate-controlling site in total cardiac LCFA flux. It is likely that sarcolemmal LCFA entry rather than mitochondrial LCFA-CoA entry is a promising target for normalizing LCFA flux in cardiac metabolic diseases.

Partial A1 adenosine receptor agonist regulates cardiac substrate utilization in insulin-resistant rats in vivo

Reducing the availability and uptake of fatty acids is a plausible pharmaceutical target to ameliorate glucose intolerance and insulin resistance. CVT-3619 [2-{6-[((1R,2R)-2-hydroxycyclopentyl) amino]purin-9-yl(4S,5S,2R,3R)-5-[(2-fluorophenylthio)methyl]oxolane-3,4-diol] is a partial A(1) adenosine receptor agonist with antilipolytic properties. Aims of the present study were to examine the acute effects of CVT-3619 on whole-body and cardiac glucose and fatty acid kinetics in vivo in normal and diet-induced insulin-resistant rats. Male Sprague-Dawley rats were fed either a chow (CH) or high-fat (HF) diet for 4 weeks. Catheters were then chronically implanted in the carotid artery and jugular vein for sampling and infusions, respectively. After 5 days of recovery, fasted animals (10 h) received either saline or CVT-3619 (0.4 mg/kg bolus + 1 mg/kg/h). Indices of glucose and fatty acid utilization were obtained by the administration of 2-deoxy[(14)C]glucose and [9,10-(3)H]-(R)-2-bromopalmitate. HF feeding resulted in elevated, fasting insulin and free fatty acid (FFA) levels compared with CH. CVT-3619 caused a 64 and 86% reduction of FFA and insulin in HF (p < 0.05) but less (N.S.) in CH diet-fed animals. In HF diet-fed rats, CVT-3619 increased whole-body glucose clearance with no change in fatty acid kinetics. Likewise, analysis of cardiac tissue metabolism showed that CVT-3619 caused an increased glucose but not fatty acid clearance in HF-fed animals. Results show that the acute administration of CVT-3619 lowers circulating fatty acid levels, leading to improved whole-body and cardiac glucose clearance in a model of diet-induced insulin resistance. As such, CVT-3619 may be a treatment option for the restoration of substrate balance in the insulin-resistant heart.

Metabolic implications of reduced heart-type fatty acid binding protein in insulin resistant cardiac muscle

Insulin resistance is characterized by elevated rates of cardiac fatty acid utilization resulting in reduced efficiency and cardiomyopathy. One potential therapeutic approach is to limit the uptake and oxidation of fatty acids. The aims of this study were to determine whether a quantitative reduction in heart-type fatty acid binding protein (FABP3) normalizes cardiac substrate utilization without altering cardiac function. Transgenic (FABP3(+/-)) and wild-type (WT) littermates were studied following low fat (LF) or high fat (HF) diets, with HF resulting in obese, insulin-resistant mice. Cardiovascular function (systolic blood pressure, % fractional shortening) and heart dimension were measured at weaning and every month afterward for 3 mo. During this period cardiovascular function was the same independent of genotype and diet. Catheters were surgically implanted in the carotid artery and jugular vein for sampling and infusions in mice at 4 mo of age. Following 5 d recovery, mice underwent either a saline infusion or a hyperinsulinemic-euglycemic clamp (4 mU kg(-1) min(-1)). Indices of long chain fatty acid and glucose utilization (R(f), R(g); mumol g wet weight(-1) min(-1)) were obtained using 2-deoxy[(3)H]glucose and [(125)I]-15-rho-iodophenyl)-3-R,S-methylpentadecanoic acid. FABP3(+/-) had enhanced cardiac R(g) compared with WT during saline infusion in both LF and HF. FABP3(+/-) abrogated the HF-induced decrement in insulin-stimulated cardiac R(g). On a HF diet, FABP(+/-) but not WT had an increased reliance on fatty acids (R(f)) during insulin stimulation. In conclusion, cardiac insulin resistance and glucose uptake is largely corrected by a reduction in FABP3 in vivo without contemporaneous deleterious effects on cardiac function.

What are the biochemical mechanisms responsible for enhanced fatty acid utilization by perfused hearts from type 2 diabetic db/db mice?

INTRODUCTION

It is generally accepted that diabetic hearts have an altered metabolic phenotype, with enhanced fatty acid (FA) utilization. The over-utilization of FA by diabetic hearts can have deleterious functional consequences, contributing to a distinct diabetic cardiomyopathy. The objective of this review will be to examine which biochemical mechanisms are responsible for enhanced FA utilization by diabetic hearts.

METHODOLOGY AND RESULTS

Studies were performed with db/db mice, a monogenic model of type 2 diabetes with extreme obesity and hyperglycemia. Perfused db/db hearts exhibit enhanced FA oxidation and esterification. Hypothesis 1: Cardiac FA uptake is enhanced in db/db hearts. The plasma membrane content of two FA transporters, fatty acid translocase/CD36 (FAT/CD36) and plasma membrane fatty acid binding protein (FABPpm), was increased in db/db hearts, consistent with hypothesis 1. Hypothesis 2: Cardiac FA oxidation is enhanced in db/db hearts due to mitochondrial alterations. However, the activity of carnitine palmitoyl transferase-1 (CPT-1) and sensitivity to inhibition by malonyl CoA was unchanged in mitochondria from db/db hearts. Furthermore, total malonyl CoA content was increased, not decreased as predicted for elevated FA oxidation. Finally, the content of uncoupling protein-3 was unchanged in db/db heart mitochondria.

CONCLUSION

Increased plasma membrane content of FA transporters (FAT/CD36 and FABPpm) will increase FA uptake into db/db cardiomyocytes and thus increase FA utilization. On the other hand, mitochondrial mechanisms do not contribute to elevated rates of FA oxidation in db/db hearts.

Regulation of sarcolemmal glucose and fatty acid transporters in cardiac disease

Circulating long-chain fatty acids (LCFA) and glucose are the main sources for energy production in the heart. In the healthy heart the ratio of glucose and LCFA oxidation is sensitively balanced and chronic alterations in this substrate mix are closely associated with cardiac dysfunction. While it has been accepted for several years that cardiac glucose uptake is mediated by facilitated transport, i.e. by means of the glucose transport proteins GLUT1 and GLUT4, only in the last few years it has become clear that proteins with high-affinity binding sites to LCFA, referred to as LCFA transporters, are responsible for bulk LCFA uptake. Similar to the GLUTs, the LCFA transporters CD36 and FABP(pm) can be recruited from an intracellular storage compartment to the sarcolemma to increase the rate of substrate uptake. Permanent relocation of LCFA transporters, mainly CD36, from intracellular stores to the sarcolemma is accompanied by accumulation of lipids and lipid metabolites in the heart. As a consequence, insulin signalling and glucose utilization are impaired, leading to decreased contractile activity of the heart. These observations underline the particular role and interplay of substrate carriers for glucose and LCFA in modulating cardiac metabolism, and the development of heart failure. The signalling and trafficking pathways and subcellular machinery regulating translocation of glucose and LCFA transporters are beginning to be unravelled. More knowledge on substrate transporter recycling, especially the similarities and differences between glucose and LCFA transporters, is expected to enable novel therapies aimed at changing the subcellular distribution of glucose and LCFA transporters, thereby manipulating the substrate preference of the diseased heart to help restore cardiac function.

Evidence for concerted action of FAT/CD36 and FABPpm to increase fatty acid transport across the plasma membrane

There is substantial molecular, biochemical and physiologic evidence that long-chain fatty acid transport involves a protein-mediated process. A number of fatty acid transport proteins have been identified, and for unknown reasons, some of these are coexpressed in the same tissues. In muscle and heart FAT/CD36 and FABPpm appear to be key transporters. Both proteins are regulated acutely (within minutes) and chronically (hours to days) by selected physiologic stimuli (insulin, AMP kinase activation). Acute regulation involves the translocation of FAT/CD36 by insulin, muscle contraction and AMP kinase activation, while FABPpm is induced to translocate by muscle contraction and AMP kinase activation, but not by insulin. Protein expression ofFAT/CD36 and FABPpm is regulated by prolonged AMP kinase activation (heart) or increased muscle contraction. Prolonged insulin exposure increases the expression of FAT/CD36 but not FABPpm. Trafficking of fatty acid transporters between an intracellular compartment(s) and the plasma membrane is altered in insulin-resistant skeletal muscle, as some FAT/CD36 is permanently relocated to plasma membrane, thereby contributing to insulin resistance due to the increased influx of fatty acids into muscle cells. Studies in FAT/CD36 null mice have revealed that this transporter is key to regulating the increase in the rate of fatty acid metabolism in heart and skeletal muscle. It appears based on a number of experiments that FAT/CD36 and FABPpm may collaborate to increase the rates of fatty acid transport, as these proteins co-immunoprecipitate.

Rosiglitazone treatment improves cardiac efficiency in hearts from diabetic mice

Isolated perfused hearts from type 2 diabetic (db/db) mice show impaired ventricular function, as well as altered cardiac metabolism. Assessment of the relationship between myocardial oxygen consumption (MVO(2)) and ventricular pressure-volume area (PVA) has also demonstrated reduced cardiac efficiency in db/db hearts. We hypothesized that lowering the plasma fatty acid supply and subsequent normalization of altered cardiac metabolism by chronic treatment with a peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist will improve cardiac efficiency in db/db hearts. Rosiglitazone (23 mg/kg body weight/day) was administered as a food admixture to db/db mice for five weeks. Ventricular function and PVA were assessed using a miniaturized (1.4 Fr) pressure-volume catheter; MVO(2) was measured using a fibre-optic oxygen sensor. Chronic rosiglitazone treatment of db/db mice normalized plasma glucose and lipid concentrations, restored rates of cardiac glucose and fatty acid oxidation, and improved cardiac efficiency. The improved cardiac efficiency was due to a significant decrease in unloaded MVO(2), while contractile efficiency was unchanged. Rosiglitazone treatment also improved functional recovery after low-flow ischemia. In conclusion, the present study demonstrates that in vivo PPARgamma-treatment restores cardiac efficiency and improves ventricular function in perfused hearts from type 2 diabetic mice.

Fenofibrate modulates cardiac and hepatic metabolism and increases ischemic tolerance in diet-induced obese mice

Peroxisome proliferator-activated receptors (PPARs) play an important role in the transcriptional regulation of lipid utilization and storage in several organs, including liver and heart. Our working hypothesis is that treatment of obesity/hyperlipedemia with the PPARalpha ligand fenofibrate leads to drainage of plasma lipids by the liver, resulting in reduced myocardial lipid supply, reduced myocardial fatty acid oxidation and improved myocardial tolerance to ischemic stress. Thus, we investigated changes in substrate utilization in heart and liver, as well as post-ischemic functional recovery in hearts from diet-induced obese (DIO) mice following long-term (11-12 weeks) treatment with fenofibrate. The present study shows that DIO mice express increased plasma lipids and glucose, as well as increased myocardial fatty acid oxidation and a concomitant decrease in glucose oxidation. The lipid-lowering effect of fenofibrate was associated with increased hepatic mitochondrial and peroxisomal fatty acid oxidation, as indicated by a more than 30% increase in hepatic palmiotyl-CoA oxidation and more than a 10-fold increase in acyl-CoA oxidase (ACO) activity. In line with an adaptation to the reduced myocardial lipid supply, isolated hearts from fenofibrate-treated DIO mice showed increased glucose oxidation and decreased fatty acid oxidation, as well as reduced ACO activity. Fenofibrate treatment also prevented the diet-induced decrease in cardiac function and improved post-ischemic functional recovery. We also found that, while fenofibrate treatment markedly increased the expression of PPARalpha target genes in the liver, there were no such changes in the heart. These data demonstrate that fenofibrate results in a direct activation of PPARalpha in the liver with increased hepatic drainage of plasma lipids, while the cardiac effect of the compound most likely is secondary to its lipid-lowering effect.