Reduced heart size and increased myocardial fuel substrate oxidation in ACC2 mutant mice

SGLT2 inhibitors: role in protective reprogramming of cardiac nutrient transport and metabolism

Sodium-glucose cotransporter 2 (SGLT2) inhibitors reduce heart failure events by direct action on the failing heart that is independent of changes in renal tubular function. In the failing heart, nutrient transport into cardiomyocytes is increased, but nutrient utilization is impaired, leading to deficient ATP production and the cytosolic accumulation of deleterious glucose and lipid by-products. These by-products trigger downregulation of cytoprotective nutrient-deprivation pathways, thereby promoting cellular stress and undermining cellular survival. SGLT2 inhibitors restore cellular homeostasis through three complementary mechanisms: they might bind directly to nutrient-deprivation and nutrient-surplus sensors to promote their cytoprotective actions; they can increase the synthesis of ATP by promoting mitochondrial health (mediated by increasing autophagic flux) and potentially by alleviating the cytosolic deficiency in ferrous iron; and they might directly inhibit glucose transporter type 1, thereby diminishing the cytosolic accumulation of toxic metabolic by-products and promoting the oxidation of long-chain fatty acids. The increase in autophagic flux mediated by SGLT2 inhibitors also promotes the clearance of harmful glucose and lipid by-products and the disposal of dysfunctional mitochondria, allowing for mitochondrial renewal through mitochondrial biogenesis. This Review describes the orchestrated interplay between nutrient transport and metabolism and nutrient-deprivation and nutrient-surplus signalling, to explain how SGLT2 inhibitors reverse the profound nutrient, metabolic and cellular abnormalities observed in heart failure, thereby restoring the myocardium to a healthy molecular and cellular phenotype.

Integrated Control of Fatty Acid Metabolism in Heart Failure

Disrupted fatty acid metabolism is one of the most important metabolic features in heart failure. The heart obtains energy from fatty acids via oxidation. However, heart failure results in markedly decreased fatty acid oxidation and is accompanied by the accumulation of excess lipid moieties that lead to cardiac lipotoxicity. Herein, we summarized and discussed the current understanding of the integrated regulation of fatty acid metabolism (including fatty acid uptake, lipogenesis, lipolysis, and fatty acid oxidation) in the pathogenesis of heart failure. The functions of many enzymes and regulatory factors in fatty acid homeostasis were characterized. We reviewed their contributions to the development of heart failure and highlighted potential targets that may serve as promising new therapeutic strategies.

Acetyl-CoA Carboxylases and Diseases

Acetyl-CoA carboxylases (ACCs) are enzymes that catalyze the carboxylation of acetyl-CoA to produce malonyl-CoA. In mammals, ACC1 and ACC2 are two members of ACCs. ACC1 localizes in the cytosol and acts as the first and rate-limiting enzyme in the de novo fatty acid synthesis pathway. ACC2 localizes on the outer membrane of mitochondria and produces malonyl-CoA to regulate the activity of carnitine palmitoyltransferase 1 (CPT1) that involves in the β-oxidation of fatty acid. Fatty acid synthesis is central in a myriad of physiological and pathological conditions. ACC1 is the major member of ACCs in mammalian, mountains of documents record the roles of ACC1 in various diseases, such as cancer, diabetes, obesity. Besides, acetyl-CoA and malonyl-CoA are cofactors in protein acetylation and malonylation, respectively, so that the manipulation of acetyl-CoA and malonyl-CoA by ACC1 can also markedly influence the profile of protein post-translational modifications, resulting in alternated biological processes in mammalian cells. In the review, we summarize our understandings of ACCs, including their structural features, regulatory mechanisms, and roles in diseases. ACC1 has emerged as a promising target for diseases treatment, so that the specific inhibitors of ACC1 for diseases treatment are also discussed.

Cardiometabolism as an Interlocking Puzzle between the Healthy and Diseased Heart: New Frontiers in Therapeutic Applications

Cardiac metabolism represents a crucial and essential connecting bridge between the healthy and diseased heart. The cardiac muscle, which may be considered an omnivore organ with regard to the energy substrate utilization, under physiological conditions mainly draws energy by fatty acids oxidation. Within cardiomyocytes and their mitochondria, through well-concerted enzymatic reactions, substrates converge on the production of ATP, the basic chemical energy that cardiac muscle converts into mechanical energy, i.e., contraction. When a perturbation of homeostasis occurs, such as an ischemic event, the heart is forced to switch its fatty acid-based metabolism to the carbohydrate utilization as a protective mechanism that allows the maintenance of its key role within the whole organism. Consequently, the flexibility of the cardiac metabolic networks deeply influences the ability of the heart to respond, by adapting to pathophysiological changes. The aim of the present review is to summarize the main metabolic changes detectable in the heart under acute and chronic cardiac pathologies, analyzing possible therapeutic targets to be used. On this basis, cardiometabolism can be described as a crucial mechanism in keeping the physiological structure and function of the heart; furthermore, it can be considered a promising goal for future pharmacological agents able to appropriately modulate the rate-limiting steps of heart metabolic pathways.

Enhancing fatty acid oxidation negatively regulates PPARs signaling in the heart

High fatty acid oxidation (FAO) is associated with lipotoxicity, but whether it causes lipotoxic cardiomyopathy remains controversial. Molecular mechanisms that may be responsible for FAO-induced lipotoxic cardiomyopathy are also elusive. In this study, increasing FAO by genetic deletion of acetyl-CoA carboxylase 2 (ACC2) did not induce cardiac dysfunction after 16 weeks of high fat diet (HFD) feeding. This suggests that increasing FAO, per se, does not cause metabolic cardiomyopathy in obese mice. We compared transcriptomes of control and ACC2 deficient mouse hearts under chow- or HFD-fed conditions. ACC2 deletion had a significant impact on the global transcriptome including downregulation of the peroxisome proliferator-activated receptors (PPARs) signaling and fatty acid degradation pathways. Increasing fatty acids by HFD feeding normalized expression of fatty acid degradation genes in ACC2 deficient mouse hearts to the same level as the control mice. In contrast, cardiac transcriptome analysis of the lipotoxic mouse model (db/db) showed an upregulation of PPARs signaling and fatty acid degradation pathways. Our results suggest that enhancing FAO by genetic deletion of ACC2 negatively regulates PPARs signaling through depleting endogenous PPAR ligands, which can serve as a negative feedback mechanism to prevent excess activation of PPAR signaling under non-obese condition. In obesity, excessive lipid availability negates the feedback mechanism resulting in over activation of PPAR cascade, thus contributes to the development of cardiac lipotoxicity.

Increased cardiac fatty acid oxidation in a mouse model with decreased malonyl-CoA sensitivity of CPT1B

Aims

Mitochondrial fatty acid oxidation (FAO) is an important energy provider for cardiac work and changes in cardiac substrate preference are associated with different heart diseases. Carnitine palmitoyltransferase 1B (CPT1B) is thought to perform the rate limiting enzyme step in FAO and is inhibited by malonyl-CoA. The role of CPT1B in cardiac metabolism has been addressed by inhibiting or decreasing CPT1B protein or after modulation of tissue malonyl-CoA metabolism. We assessed the role of CPT1B malonyl-CoA sensitivity in cardiac metabolism.

Methods and results

We generated and characterized a knock in mouse model expressing the CPT1BE3A mutant enzyme, which has reduced sensitivity to malonyl-CoA. In isolated perfused hearts, FAO was 1.9-fold higher in Cpt1bE3A/E3A hearts compared with Cpt1bWT/WT hearts. Metabolomic, proteomic and transcriptomic analysis showed increased levels of malonylcarnitine, decreased concentration of CPT1B protein and a small but coordinated downregulation of the mRNA expression of genes involved in FAO in Cpt1bE3A/E3A hearts, all of which aim to limit FAO. In vivo assessment of cardiac function revealed only minor changes, cardiac hypertrophy was absent and histological analysis did not reveal fibrosis.

Conclusions

Malonyl-CoA-dependent inhibition of CPT1B plays a crucial role in regulating FAO rate in the heart. Chronic elevation of FAO has a relatively subtle impact on cardiac function at least under baseline conditions.

Pressure Overload Impairs Cardiac Function in Long-Chain Fatty Acid Transporter CD36-Knockout Mice

CD36 is one of the important transporters of long-chain fatty acids (LCFAs) in the myocardium. We previously reported that CD36-deficient patients demonstrate a marked reduction of myocardial uptake of LCFA, while myocardial glucose uptake shows a compensatory increase, and are often accompanied by cardiomyopathy. However, the molecular mechanisms and functional role of CD36 in the myocardium remain unknown.The current study aimed to explore the pathophysiological role of CD36 in the heart. Methods: Using wild type (WT) and knockout (KO) mice, we generated pressure overload by transverse aortic constriction (TAC) and analyzed cardiac functions by echocardiography. To assess cardiac hypertrophy and fibrosis, histological and molecular analyses and measurement of ATP concentration in mouse hearts were performed.By applying TAC, the survival rate was significantly lower in KO than that in WT mice. After TAC, KO mice showed significantly higher heart weight-to-tibial length ratio and larger cross-sectional area of cardiomyocytes than those of WT. Although left ventricular (LV) wall thickness in the KO mice was similar to that in the WT mice, the KO mice showed a significant enlargement of LV cavity and reduced LV fractional shortening compared to the WT mice with TAC. A tendency for decreased myocardial ATP concentration was observed in the KO mice compared to the WT mice after TAC operation.These data suggest that the LCFA transporter CD36 is required for the maintenance of energy provision, systolic function, and myocardial structure.

Metabolism in cardiomyopathy: every substrate matters

Cardiac metabolism is highly adaptive to changes in fuel availability and the energy demand of the heart. This metabolic flexibility is key for the heart to maintain its output during the development and in response to stress. Alterations in substrate preference have been observed in multiple disease states; a clear understanding of their impact on cardiac function in the long term is critical for the development of metabolic therapies. In addition, the contribution of cellular metabolism to growth, survival, and other signalling pathways through the generation of metabolic intermediates has been increasingly noted, adding another layer of complexity to the impact of metabolism on cardiac function. In a quest to understand the complexity of the cardiac metabolic network, genetic tools have been engaged to manipulate cardiac metabolism in a variety of mouse models. The ability to engineer cardiac metabolism in vivo has provided tremendous insights and brought about conceptual innovations. In this review, we will provide an overview of the cardiac metabolic network and highlight alterations observed during cardiac development and pathological hypertrophy. We will focus on consequences of altered substrate preference on cardiac response to chronic stresses through energy providing and non-energy providing pathways.

Lipid Use and Misuse by the Heart

The heart utilizes large amounts of fatty acids as energy providing substrates. The physiological balance of lipid uptake and oxidation prevents accumulation of excess lipids. Several processes that affect cardiac function, including ischemia, obesity, diabetes mellitus, sepsis, and most forms of heart failure lead to altered fatty acid oxidation and often also to the accumulation of lipids. There is now mounting evidence associating certain species of these lipids with cardiac lipotoxicity and subsequent myocardial dysfunction. Experimental and clinical data are discussed and paths to reduction of toxic lipids as a means to improve cardiac function are suggested.

Cardiac Metabolism in Perspective

The heart is a biological pump that converts chemical to mechanical energy. This process of energy conversion is highly regulated to the extent that energy substrate metabolism matches energy use for contraction on a beat-to-beat basis. The biochemistry of cardiac metabolism includes the biochemistry of energy transfer, metabolic regulation, and transcriptional, translational as well as posttranslational control of enzymatic activities. Pathways of energy substrate metabolism in the heart are complex and dynamic, but all of them conform to the First Law of Thermodynamics. The perspectives expand on the overall idea that cardiac metabolism is inextricably linked to both physiology and molecular biology of the heart. The article ends with an outlook on emerging concepts of cardiac metabolism based on new molecular models and new analytical tools. © 2016 American Physiological Society. Compr Physiol 6:1675-1699, 2016.

Cardiac lipotoxicity: molecular pathways and therapeutic implications

Diabetes and obesity are both associated with lipotoxic cardiomyopathy exclusive of coronary artery disease and hypertension. Lipotoxicities have become a public health concern and are responsible for a significant portion of clinical cardiac disease. These abnormalities may be the result of a toxic metabolic shift to more fatty acid and less glucose oxidation with concomitant accumulation of toxic lipids. Lipids can directly alter cellular structures and activate downstream pathways leading to toxicity. Recent data have implicated fatty acids and fatty acyl coenzyme A, diacylglycerol, and ceramide in cellular lipotoxicity, which may be caused by apoptosis, defective insulin signaling, endoplasmic reticulum stress, activation of protein kinase C, MAPK activation, or modulation of PPARs.

Structure and function of biotin-dependent carboxylases

Biotin-dependent carboxylases include acetyl-CoA carboxylase (ACC), propionyl-CoA carboxylase (PCC), 3-methylcrotonyl-CoA carboxylase (MCC), geranyl-CoA carboxylase, pyruvate carboxylase (PC), and urea carboxylase (UC). They contain biotin carboxylase (BC), carboxyltransferase (CT), and biotin-carboxyl carrier protein components. These enzymes are widely distributed in nature and have important functions in fatty acid metabolism, amino acid metabolism, carbohydrate metabolism, polyketide biosynthesis, urea utilization, and other cellular processes. ACCs are also attractive targets for drug discovery against type 2 diabetes, obesity, cancer, microbial infections, and other diseases, and the plastid ACC of grasses is the target of action of three classes of commercial herbicides. Deficiencies in the activities of PCC, MCC, or PC are linked to serious diseases in humans. Our understanding of these enzymes has been greatly enhanced over the past few years by the crystal structures of the holoenzymes of PCC, MCC, PC, and UC. The structures reveal unanticipated features in the architectures of the holoenzymes, including the presence of previously unrecognized domains, and provide a molecular basis for understanding their catalytic mechanism as well as the large collection of disease-causing mutations in PCC, MCC, and PC. This review will summarize the recent advances in our knowledge on the structure and function of these important metabolic enzymes.

Cardiac-specific deletion of acetyl CoA carboxylase 2 prevents metabolic remodeling during pressure-overload hypertrophy

RATIONALE

Decreased fatty acid oxidation (FAO) with increased reliance on glucose are hallmarks of metabolic remodeling that occurs in pathological cardiac hypertrophy and is associated with decreased myocardial energetics and impaired cardiac function. To date, it has not been tested whether prevention of the metabolic switch that occurs during the development of cardiac hypertrophy has unequivocal benefits on cardiac function and energetics.

OBJECTIVE

Because malonyl CoA production via acetyl CoA carboxylase 2 (ACC2) inhibits the entry of long chain fatty acids into the mitochondria, we hypothesized that mice with a cardiac-specific deletion of ACC2 (ACC2H-/-) would maintain cardiac FAO and improve function and energetics during the development of pressure-overload hypertrophy.

METHODS AND RESULTS

ACC2 deletion led to a significant reduction in cardiac malonyl CoA levels. In isolated perfused heart experiments, left ventricular function and oxygen consumption were similar in ACC2H-/- mice despite an ≈60% increase in FAO compared with controls (CON). After 8 weeks of pressure overload via transverse aortic constriction (TAC), ACC2H-/- mice exhibited a substrate utilization profile similar to sham animals, whereas CON-TAC hearts had decreased FAO with increased glycolysis and anaplerosis. Myocardial energetics, assessed by 31P nuclear magnetic resonance spectroscopy, and cardiac function were maintained in ACC2H-/- after 8 weeks of TAC. Furthermore, ACC2H-/--TAC demonstrated an attenuation of cardiac hypertrophy with a significant reduction in fibrosis relative to CON-TAC.

CONCLUSIONS

These data suggest that reversion to the fetal metabolic profile in chronic pathological hypertrophy is associated with impaired myocardial function and energetics and maintenance of the inherent cardiac metabolic profile and mitochondrial oxidative capacity is a viable therapeutic strategy.

Acetyl-CoA carboxylase 2-/- mutant mice are protected against fatty liver under high-fat, high-carbohydrate dietary and de novo lipogenic conditions

Hepatic fat accumulation resulting from increased de novo fatty acid synthesis leads to hepatic steatosis and hepatic insulin resistance. We have shown previously that acetyl-CoA carboxylase 2 (Acc2(-/-)) mutant mice, when fed a high-fat (HF) or high-fat, high-carbohydrate (HFHC) diet, are protected against diet-induced obesity and maintained whole body and hepatic insulin sensitivity. To determine the effect of an ACC2 deletion on hepatic fat metabolism, we studied the regulation of the enzymes involved in the lipogenic pathway under Western HFHC dietary and de novo lipogenic conditions. After completing the HFHC regimen, Acc2(-/-) mutant mice were found to have lower body weight, smaller epididymal fat pads, lower blood levels of nonesterified fatty acids and triglycerides, and higher hepatic cholesterol than wild-type mice. Significant up-regulation of lipogenic enzymes and an elevation in hepatic peroxisome proliferator-activated receptor-γ (PPAR-γ) protein were found in Acc2(-/-) mutant mice under de novo lipogenic conditions. The increase in lipogenic enzyme levels was accompanied by up-regulation of the transcription factors, sterol regulatory element-binding proteins 1 and 2, and carbohydrate response element-binding protein. In contrast, hepatic levels of the PPAR-γ and PPAR-α proteins were significantly lower in the Acc2(-/-) mutant mice fed an HFHC diet. When compared with wild-type mice fed the same diet, Acc2(-/-) mutant mice exhibited a similar level of AKT but with a significant increase in pAKT. Hence, deleting ACC2 ameliorates the metabolic syndrome and protects against fatty liver despite increased de novo lipogenesis and dietary conditions known to induce obesity and diabetes.

Mouse cardiac acyl coenzyme a synthetase 1 deficiency impairs Fatty Acid oxidation and induces cardiac hypertrophy

Long-chain acyl coenzyme A (acyl-CoA) synthetase isoform 1 (ACSL1) catalyzes the synthesis of acyl-CoA from long-chain fatty acids and contributes the majority of cardiac long-chain acyl-CoA synthetase activity. To understand its functional role in the heart, we studied mice lacking ACSL1 globally (Acsl1(T-/-)) and mice lacking ACSL1 in heart ventricles (Acsl1(H-/-)) at different times. Compared to littermate controls, heart ventricular ACSL activity in Acsl1(T-/-) mice was reduced more than 90%, acyl-CoA content was 65% lower, and long-chain acyl-carnitine content was 80 to 90% lower. The rate of [(14)C]palmitate oxidation in both heart homogenate and mitochondria was 90% lower than in the controls, and the maximal rates of [(14)C]pyruvate and [(14)C]glucose oxidation were each 20% higher. The mitochondrial area was 54% greater than in the controls with twice as much mitochondrial DNA, and the mRNA abundance of Pgc1α and Errα increased by 100% and 41%, respectively. Compared to the controls, Acsl1(T-/-) and Acsl1(H-/-) hearts were hypertrophied, and the phosphorylation of S6 kinase, a target of mammalian target of rapamycin (mTOR) kinase, increased 5-fold. Our data suggest that ACSL1 is required to synthesize the acyl-CoAs that are oxidized by the heart, and that without ACSL1, diminished fatty acid (FA) oxidation and compensatory catabolism of glucose and amino acids lead to mTOR activation and cardiac hypertrophy without lipid accumulation or immediate cardiac dysfunction.

AMPK activation represses the human gene promoter of the cardiac isoform of acetyl-CoA carboxylase: Role of nuclear respiratory factor-1

The cardiac-enriched isoform of acetyl-CoA carboxylase (ACCbeta) produces malonyl-CoA, a potent inhibitor of carnitine palmitoyltransferase-1. AMPK inhibits ACCbeta activity, lowering malonyl-CoA levels and promoting mitochondrial fatty acid beta-oxidation. Previously, AMPK increased promoter binding of nuclear respiratory factor-1 (NRF-1), a pivotal transcriptional modulator controlling gene expression of mitochondrial proteins. We therefore hypothesized that NRF-1 inhibits myocardial ACCbeta promoter activity via AMPK activation. A human ACCbeta promoter-luciferase construct was transiently transfected into neonatal cardiomyocytes+/-a NRF-1 expression construct. NRF-1 overexpression decreased ACCbeta gene promoter activity by 71+/-4.6% (p<0.001 vs. control). Transfections with 5'-end serial promoter deletions revealed that NRF-1-mediated repression of ACCbeta was abolished with a pPIIbeta-18/+65-Luc deletion construct. AMPK activation dose-dependently reduced ACCbeta promoter activity, while NRF-1 addition did not further decrease it. We also investigated NRF-1 inhibition in the presence of upstream stimulatory factor 1 (USF1), a known transactivator of the human ACCbeta gene promoter. Here NRF-1 blunted USF1-dependent induction of ACCbeta promoter activity by 58+/-7.5% (p<0.001 vs. control), reversed with a dominant negative NRF-1 construct. NRF-1 also suppressed endogenous USF1 transcriptional activity by 55+/-6.2% (p<0.001 vs. control). This study demonstrates that NRF-1 is a novel transcriptional inhibitor of the human ACCbeta gene promoter in the mammalian heart. Our data extends AMPK regulation of ACCbeta to the transcriptional level.

Liquid-liquid extraction coupled with LC/MS/MS for monitoring of malonyl-CoA in rat brain tissue

The formation of malonyl-CoA is catalyzed by acetyl-CoA carboxylase (ACC), the rate-limiting enzyme of de novo fatty acid synthesis. Monitoring the changes of malonyl-CoA concentration in the brain in response to treatments such as pharmaceutical intervention (via ACC inhibitors) or different dietary conditions (such as varied feeding regimes) is of great interest and could help increase the understanding of how this molecule contributes to feeding behavior and overall energy balance. We have developed a sensitive analytical method for the determination of malonyl-CoA levels in rat brain tissue. The assay involved removal of tissue lipids by liquid-liquid extraction followed by LC/MS/MS analysis of the aqueous layer for malonyl-CoA. The method was sensitive enough (limit of quantitation = 50 ng/mL, or approximately 0.018 nmol/g brain tissue) to determine malonyl-CoA in individual rat brain preparations. The assay performance was sufficiently rugged to support drug discovery screening efforts and provided an additional analytical tool for monitoring brain malonyl-CoA levels.

Infusion of a lipid emulsion modulates AMPK and related proteins in rat liver, muscle, and adipose tissues

The primary objective of this study was to investigate the impact of lipid oversupply on the AMPK pathway in skeletal muscle, liver, and adipose tissue. Male Wistar rats were infused with lipid emulsion (LE) or phosphate-buffered saline for 5 h/day for 6 days. Muscles exposed to LE for 6 days exhibited increased AMPK and acetyl-CoA carboxylase (ACC) phosphorylation, along with a greater association between AMPK and Ca(2+)/calmodulin-dependent protein kinase kinase (CaMKK). No differences in muscle protein phosphatase 2C (PP2C) activity, LKB1 phosphorylation or AMPK and LKB1 association were observed. Muscle ACCbeta, and adiponectin receptor 1 (AdipoR1) mRNA levels and PPARgamma-co-activator 1alpha (PGC1alpha) protein levels were also increased in LE-treated rats. In contrast, AMPK and ACC phosphorylation decreased and PP2C activity increased in rat livers exposed to LE. Hepatic mRNA levels of ACCalpha, PPARalpha, AdipoR1, AdipoR2, and sterol regulatory element-binding protein-1c (SREBP1c) were also reduced after LE infusion. In adipose tissue, there was no significant alteration in AMPK or ACC phosphorylation. These results demonstrate that following lipid oversupply the AMPK pathway was enhanced in rat skeletal muscle while diminished in the liver and was unchanged in adipose tissue. CaMKK in skeletal muscle and PP2C in the liver, at least in part, appear to mediate these alterations. Alterations in AMPK pathway in the liver induced metabolic defects associated with lipid oversupply.

Recombinant yeast screen for new inhibitors of human acetyl-CoA carboxylase 2 identifies potential drugs to treat obesity

Acetyl-CoA carboxylase (ACC) is a key enzyme of fatty acid metabolism with multiple isozymes often expressed in different eukaryotic cellular compartments. ACC-made malonyl-CoA serves as a precursor for fatty acids; it also regulates fatty acid oxidation and feeding behavior in animals. ACC provides an important target for new drugs to treat human diseases. We have developed an inexpensive nonradioactive high-throughput screening system to identify new ACC inhibitors. The screen uses yeast gene-replacement strains depending for growth on cloned human ACC1 and ACC2. In "proof of concept" experiments, growth of such strains was inhibited by compounds known to target human ACCs. The screen is sensitive and robust. Medium-size chemical libraries yielded new specific inhibitors of human ACC2. The target of the best of these inhibitors was confirmed with in vitro enzymatic assays. This compound is a new drug chemotype inhibiting human ACC2 with 2.8 muM IC(50) and having no effect on human ACC1 at 100 muM.

Gene knockout of Acc2 has little effect on body weight, fat mass, or food intake

Deletion of acetyl CoA carboxylase-2 (Acc2) reportedly causes leanness in the setting of hyperphagia. To determine the cellular basis for these effects, we generated a mouse model in which Acc2 can be selectively deleted by the action of Cre recombinase. Deletion of Acc2 from skeletal muscle, the predominant site of Acc2 expression, had no effect on body weight, food intake, or body composition. When Acc2 was inactivated in the germline, Acc2 knockout (Acc2KO) mice displayed no differences in body weight, food intake, body composition, or glucose homeostasis as compared to controls on chow or high fat diet. Total malonyl CoA content and fatty acid oxidation rates in skeletal muscle of Acc2KO mice were unchanged, suggesting metabolic compensation in response to the loss of Acc2. The limited impact of Acc2 deletion on energy balance raises the possibility that selective pharmacological inhibition of Acc2 for the treatment of obesity may be ineffective.

Mitochondrial dysfunction in diabetes: from molecular mechanisms to functional significance and therapeutic opportunities

Given their essential function in aerobic metabolism, mitochondria are intuitively of interest in regard to the pathophysiology of diabetes. Qualitative, quantitative, and functional perturbations in mitochondria have been identified and affect the cause and complications of diabetes. Moreover, as a consequence of fuel oxidation, mitochondria generate considerable reactive oxygen species (ROS). Evidence is accumulating that these radicals per se are important in the pathophysiology of diabetes and its complications. In this review, we first present basic concepts underlying mitochondrial physiology. We then address mitochondrial function and ROS as related to diabetes. We consider different forms of diabetes and address both insulin secretion and insulin sensitivity. We also address the role of mitochondrial uncoupling and coenzyme Q. Finally, we address the potential for targeting mitochondria in the therapy of diabetes.

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.

The Randle cycle revisited: a new head for an old hat

In 1963, Lancet published a paper by Randle et al. that proposed a "glucose-fatty acid cycle" to describe fuel flux between and fuel selection by tissues. The original biochemical mechanism explained the inhibition of glucose oxidation by fatty acids. Since then, the principle has been confirmed by many investigators. At the same time, many new mechanisms controlling the utilization of glucose and fatty acids have been discovered. Here, we review the known short- and long-term mechanisms involved in the control of glucose and fatty acid utilization at the cytoplasmic and mitochondrial level in mammalian muscle and liver under normal and pathophysiological conditions. They include allosteric control, reversible phosphorylation, and the expression of key enzymes. However, the complexity is formidable. We suggest that not all chapters of the Randle cycle have been written.

Insulin-stimulated cardiac glucose oxidation is increased in high-fat diet-induced obese mice lacking malonyl CoA decarboxylase

OBJECTIVE

Whereas an impaired ability to oxidize fatty acids is thought to contribute to intracellular lipid accumulation, insulin resistance, and cardiac dysfunction, high rates of fatty acid oxidation could also impair glucose metabolism and function. We therefore determined the effects of diet-induced obesity (DIO) in wild-type (WT) mice and mice deficient for malonyl CoA decarboxylase (MCD(-/-); an enzyme promoting mitochondrial fatty acid oxidation) on insulin-sensitive cardiac glucose oxidation.

RESEARCH DESIGN AND METHODS

WT and MCD(-/-) mice were fed a low- or high-fat diet for 12 weeks, and intramyocardial lipid metabolite accumulation was assessed. A parallel feeding study was performed to assess myocardial function and energy metabolism (nanomoles per gram of dry weight per minute) in isolated working hearts (+/- insulin).

RESULTS

DIO markedly reduced insulin-stimulated glucose oxidation compared with low fat-fed WT mice (167 +/- 31 vs. 734 +/- 125; P < 0.05). MCD(-/-) mice subjected to DIO displayed a more robust insulin-stimulated glucose oxidation (554 +/- 82 vs. 167 +/- 31; P < 0.05) and less incomplete fatty acid oxidation, evidenced by a decrease in long-chain acylcarnitines compared with WT counterparts. MCD(-/-) mice had long-chain acyl CoAs similar to those of WT mice subjected to DIO but had increased triacylglycerol levels (10.92 +/- 3.72 vs. 3.29 +/- 0.62 mumol/g wet wt; P < 0.05).

CONCLUSIONS

DIO does not impair cardiac fatty acid oxidation or function, and there exists disassociation between myocardial lipid accumulation and insulin sensitivity. Our results suggest that MCD deficiency is not detrimental to the heart in obesity.

Targeting malonyl CoA inhibition of mitochondrial fatty acid uptake as an approach to treat cardiac ischemia/reperfusion

Cardiovascular disease is the major cause of death and disability in the world, with ischemic heart disease accounting for the vast majority of this health problem. Current treatments for ischemic heart disease are primarily aimed at either increasing blood and oxygen supply to the heart or decreasing the heart's oxygen demand. A novel treatment strategy involves increasing the efficiency of oxygen use by the heart. During and following ischemia, the heart can become inefficient in using oxygen, due in part to an excessive use of fatty acids as a source of fuel. One potential strategy to increase cardiac efficiency is to inhibit this use of fatty acid oxidation as a fuel source, while stimulating the use of glucose oxidation as a fuel source, which allows the heart to produce energy more efficiently and reduces the acidosis associated with ischemia/reperfusion, both of which are beneficial to the heart. Malonyl CoA is a potent endogenous inhibitor of cardiac fatty acid oxidation, secondary to inhibition of carnitine palmitoyl transferase-I, the gatekeeper of mitochondrial fatty acid uptake. Malonyl CoA is synthesized in the heart by acetyl CoA carboxylase and degraded by malonyl CoA decarboxylase (MCD). Strategies aimed at increasing cardiac malonyl CoA levels, such as via inhibition of MCD, are associated with a decrease in fatty acid oxidation rates, and a parallel increase in glucose oxidation rates. This is associated with a decrease in acidosis and an improvement in cardiac function and efficiency during and following ischemia. Therefore, targeting malonyl CoA is a novel exciting approach for the treatment of cardiac ischemia/reperfusion.

Fatty acid metabolism: target for metabolic syndrome

Fatty acids are a major energy source and important constituents of membrane lipids, and they serve as cellular signaling molecules that play an important role in the etiology of the metabolic syndrome. Acetyl-CoA carboxylases 1 and 2 (ACC1 and ACC2) catalyze the synthesis of malonyl-CoA, the substrate for fatty acid synthesis and the regulator of fatty acid oxidation. They are highly regulated and play important roles in the energy metabolism of fatty acids in animals, including humans. They are presently considered as an attractive target to regulate the human diseases of obesity, diabetes, cancer, and cardiovascular complications. In this review we discuss the role of fatty acid metabolism and its key players, ACC1 and ACC2, in animal evolution and physiology, as related to health and disease.