Fatty Acids Attenuate Insulin Regulation of 5′-AMP–Activated Protein Kinase and Insulin Cardioprotection After Ischemia

Protein lysine acetylation does not contribute to the high rates of fatty acid oxidation seen in the post-ischemic heart

High rates of cardiac fatty acid oxidation during reperfusion of ischemic hearts contribute to contractile dysfunction. This study aimed to investigate whether lysine acetylation affects fatty acid oxidation rates and recovery in post-ischemic hearts. Isolated working hearts from Sprague Dawley rats were perfused with 1.2 mM palmitate and 5 mM glucose and subjected to 30 min of ischemia and 40 min of reperfusion. Cardiac function, fatty acid oxidation, glucose oxidation, and glycolysis rates were compared between pre- and post-ischemic hearts. The acetylation status of enzymes involved in cardiac energy metabolism was assessed in both groups. Reperfusion after ischemia resulted in only a 41% recovery of cardiac work. Fatty acid oxidation and glycolysis rates increased while glucose oxidation rates decreased. The contribution of fatty acid oxidation to ATP production and TCA cycle activity increased from 90 to 93% and from 94.9 to 98.3%, respectively, in post-ischemic hearts. However, the overall acetylation status and acetylation levels of metabolic enzymes did not change in response to ischemia and reperfusion. These findings suggest that acetylation may not contribute to the high rates of fatty acid oxidation and reduced glucose oxidation observed in post-ischemic hearts perfused with high levels of palmitate substrate.

Insulin signaling in the heart

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

The insulin receptor family and protein kinase B (Akt) are activated in the heart by alkaline pH and α1-adrenergic receptors

Insulin and insulin-like growth factor stimulate protein synthesis and cardioprotection in the heart, acting through their receptors (INSRs, IGF1Rs) and signalling via protein kinase B (PKB, also known as Akt). Protein synthesis is increased in hearts perfused at alkaline pH_o to the same extent as with insulin. Moreover, α₁-adrenergic receptor (α₁-AR) agonists (e.g. phenylephrine) increase protein synthesis in cardiomyocytes, activating PKB/Akt. In both cases, the mechanisms are not understood. Our aim was to determine if insulin receptor-related receptors (INSRRs, activated in kidney by alkaline pH) may account for the effects of alkaline pH_o on cardiac protein synthesis, and establish if α₁-ARs signal through the insulin receptor family. Alkaline pH_o activated PKB/Akt signalling to the same degree as insulin in perfused adult male rat hearts. INSRRs were expressed in rat hearts and, by immunoblotting for phosphorylation (activation) of INSRRs/INSRs/IGF1Rs, we established that INSRRs, together with INSRs/IGF1Rs, are activated by alkaline pH_o. The INSRR/INSR/IGF1R kinase inhibitor, linsitinib, prevented PKB/Akt activation by alkaline pH_o, indicating that INSRRs/INSRs/IGF1Rs are required. Activation of PKB/Akt in cardiomyocytes by α₁-AR agonists was also inhibited by linsitinib. Furthermore, linsitinib inhibited cardiomyocyte hypertrophy induced by α₁-ARs in cultured cells, reduced the initial cardiac adaptation (24 h) to phenylephrine in vivo (assessed by echocardiography) and increased cardiac fibrosis over 4 days. We conclude that INSRRs are expressed in the heart and, together with INSRs/IGF1Rs, the insulin receptor family provide a potent system for promoting protein synthesis and cardioprotection. Moreover, this system is required for adaptive hypertrophy induced by α₁-ARs.

Targeting Adrenergic Receptors in Metabolic Therapies for Heart Failure

The heart has a reduced capacity to generate sufficient energy when failing, resulting in an energy-starved condition with diminished functions. Studies have identified numerous changes in metabolic pathways in the failing heart that result in reduced oxidation of both glucose and fatty acid substrates, defects in mitochondrial functions and oxidative phosphorylation, and inefficient substrate utilization for the ATP that is produced. Recent early-phase clinical studies indicate that inhibitors of fatty acid oxidation and antioxidants that target the mitochondria may improve heart function during failure by increasing compensatory glucose oxidation. Adrenergic receptors (α1 and β) are a key sympathetic nervous system regulator that controls cardiac function. β-AR blockers are an established treatment for heart failure and α1A-AR agonists have potential therapeutic benefit. Besides regulating inotropy and chronotropy, α1- and β-adrenergic receptors also regulate metabolic functions in the heart that underlie many cardiac benefits. This review will highlight recent studies that describe how adrenergic receptor-mediated metabolic pathways may be able to restore cardiac energetics to non-failing levels that may offer promising therapeutic strategies.

Energy substrate metabolism and mitochondrial oxidative stress in cardiac ischemia/reperfusion injury

The heart is the most metabolically flexible organ with respect to the use of substrates available in different states of energy metabolism. Cardiac mitochondria sense substrate availability and ensure the efficiency of oxidative phosphorylation and heart function. Mitochondria also play a critical role in cardiac ischemia/reperfusion injury, during which they are directly involved in ROS-producing pathophysiological mechanisms. This review explores the mechanisms of ROS production within the energy metabolism pathways and focuses on the impact of different substrates. We describe the main metabolites accumulating during ischemia in the glucose, fatty acid, and Krebs cycle pathways. Hyperglycemia, often present in the acute stress condition of ischemia/reperfusion, increases cytosolic ROS concentrations through the activation of NADPH oxidase 2 and increases mitochondrial ROS through the metabolic overloading and decreased binding of hexokinase II to mitochondria. Fatty acid-linked ROS production is related to the increased fatty acid flux and corresponding accumulation of long-chain acylcarnitines. Succinate that accumulates during anoxia/ischemia is suggested to be the main source of ROS, and the role of itaconate as an inhibitor of succinate dehydrogenase is emerging. We discuss the strategies to modulate and counteract the accumulation of substrates that yield ROS and the therapeutic implications of this concept.

How Hypertension Affects Heart Metabolism

Hypertension is one of the most frequently observed cardiovascular diseases, which precedes heart failure in 75% of its cases. It is well-established that hypertensive patients have whole body metabolic complications such as hyperlipidemia, hyperglycemia, decreased insulin sensitivity or diabetes mellitus. Since myocardial metabolism is strictly dependent on hormonal status as well as substrate milieu, the above mentioned disturbances may affect energy generation status in the heart. Interestingly, it was found that hypertension induces a shift in substrate preference toward increased glucose utilization in cardiac muscle, prior to structural changes development. The present work reports advances in the aspect of heart metabolism under high blood pressure conditions, including human and the most common animal models of hypertension.

Uncoupling of glycolysis from glucose oxidation accompanies the development of heart failure with preserved ejection fraction

BACKGROUND

Alterations in cardiac energy metabolism contribute to the development and severity of heart failure (HF). In severe HF, overall mitochondrial oxidative metabolism is significantly decreased resulting in a reduced energy reserve. However, despite the high prevalence of HF with preserved ejection fraction (HFpEF) in our society, it is not clear what changes in cardiac energy metabolism occur in HFpEF, and whether alterations in energy metabolism contribute to the development of contractile dysfunction.

METHODS

We directly assessed overall energy metabolism during the development of HFpEF in Dahl salt-sensitive rats fed a high salt diet (HSD) for 3, 6 and 9 weeks.

RESULTS

Over the course of 9 weeks, the HSD caused a progressive decrease in diastolic function (assessed by echocardiography assessment of E'/A'). This was accompanied by a progressive increase in cardiac glycolysis rates (assessed in isolated working hearts obtained at 3, 6, and 9 weeks of HSD). In contrast, the subsequent oxidation of pyruvate from glycolysis (glucose oxidation) was not altered, resulting in an uncoupling of glucose metabolism and a significant increase in proton production. Increased glucose transporter (GLUT)1 expression accompanied this elevation in glycolysis. Decreases in cardiac fatty acid oxidation and overall adenosine triphosphate (ATP) production rates were not observed in early HF, but both significantly decreased as HF progressed to HF with reduced EF (i.e. 9 weeks of HSD).

CONCLUSIONS

Overall, we show that increased glycolysis is the earliest energy metabolic change that occurs during HFpEF development. The resultant increased proton production from uncoupling of glycolysis and glucose oxidation may contribute to the development of HFpEF.

Glucagon-like peptide-1 receptor mediated control of cardiac energy metabolism

Glucagon-like peptide-1 receptor (GLP-1R) agonists are frequently used to improve glycemia in patients with type 2 diabetes (T2D). Recent data from cardiovascular outcomes trials for the GLP-1R agonists, liraglutide and semaglutide, have also demonstrated significant reductions in death rates from cardiovascular causes in patients with T2D. As cardiovascular death is the number one cause of death in patients with T2D, understanding the mechanisms by which GLP-1R agonists such as liraglutide and semaglutide improve cardiac function is essential. Yet despite strong evidence from preclinical and clinical studies supporting the cardioprotective actions of GLP-1R agonists, the precise mechanism(s) by which this drug-class for T2D may produce these beneficial actions remains enigmatic. Negligible GLP-1R expression in ventricular cardiac myocytes suggests that GLP-1R agonist-induced cardioprotection is likely partially attributed to indirect actions on peripheral tissues other than the heart. Because insulin increases glucose oxidation, whereas glucagon increases fatty acid oxidation in the heart, GLP-1R agonist-induced increases and decreases in insulin and glucagon secretion, respectively, may modify cardiac energy metabolism in T2D patients. This may represent a potential mechanism for GLP-1R agonist-induced cardioprotection in T2D, as increases in fatty acid oxidation and decreases in glucose oxidation are frequently observed in the hearts of animals and human subjects with T2D.

Metabolic and Biochemical Stressors in Diabetic Cardiomyopathy

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

Hyperglycemia Abrogates Ischemic Postconditioning Cardioprotection by Impairing AdipoR1/Caveolin-3/STAT3 Signaling in Diabetic Rats

Signal transducer and activator of transcription 3 (STAT3) activation is key for ischemic postconditioning (IPo) to attenuate myocardial ischemia-reperfusion injury (MIRI), but IPo loses cardioprotection in diabetes in which cardiac STAT3 activation is impaired and adiponectin (APN) reduced. We found that IPo increased postischemic cardiomyocyte-derived APN, activated mitochondrial STAT3 (mitoSTAT3), improved mitochondrial function, and attenuated MIRI in wild-type but not in APN knockout (Adipo(-/-)) mice subjected to 30 min coronary occlusion, followed by 2 or 24 h of reperfusion. Hypoxic postconditioning-induced protection against hypoxia/reoxygenation injury was lost in Adipo(-/-) cardiomyocytes but restored by recombinant APN, but this APN beneficial effect was abolished by specific STAT3 or APN receptor 1 (AdipoR1) gene knockdown, or caveolin-3 (Cav3) disruption. APN activated cardiac STAT3 and restored IPo cardioprotection in 4-week diabetic rats where AdipoR1 and Cav3 were functionally interactive but not in 8-week diabetic rats whose cardiac Cav3 was severely reduced and AdipoR1/Cav3 signaling impaired. We concluded that IPo activates mitoSTAT3 through APN/AdipoR1/Cav3 pathway to confer cardioprotection, whereas in diabetes, IPo loses cardioprotection due to impaired APN/AdipoR1/Cav3 signaling. Therefore, effective means that may concomitantly activate APN and repair APN signaling (i.e., AdipoR1/Cav3) in diabetes may represent promising avenues in the treatment of MIRI in diabetes.

Clinical diabetic cardiomyopathy: a two-faced disease with restrictive and dilated phenotypes

Diabetes mellitus-related cardiomyopathy (DMCMP) was originally described as a dilated phenotype with eccentric left ventricular (LV) remodelling and systolic LV dysfunction. Recently however, clinical studies on DMCMP mainly describe a restrictive phenotype with concentric LV remodelling and diastolic LV dysfunction. Both phenotypes are not successive stages of DMCMP but evolve independently to respectively heart failure with preserved left ventricular ejection fraction (HFPEF) or reduced left ventricular ejection fraction (HFREF). Phenotype-specific pathophysiological mechanisms were recently proposed for LV remodelling and dysfunction in HFPEF and HFREF consisting of coronary microvascular endothelial dysfunction in HFPEF and cardiomyocyte cell death in HFREF. A similar preferential involvement of endothelial or cardiomyocyte cell compartments explains DMCMP development into distinct restrictive/HFPEF or dilated/HFREF phenotypes. Diabetes mellitus (DM)-related metabolic derangements such as hyperglycaemia, lipotoxicity, and hyperinsulinaemia favour development of DMCMP with restrictive/HFPEF phenotype, which is more prevalent in obese type 2 DM patients. In contrast, autoimmunity predisposes to a dilated/HFREF phenotype, which manifests itself more in autoimmune-prone type 1 DM patients. Finally, coronary microvascular rarefaction and advanced glycation end-products deposition are relevant to both phenotypes. Diagnosis of DMCMP requires impaired glucose metabolism and exclusion of coronary, valvular, hypertensive, or congenital heart disease and of viral, toxic, familial, or infiltrative cardiomyopathy. In addition, diagnosis of DMCMP with restrictive/HFPEF phenotype requires normal systolic LV function and diastolic LV dysfunction, whereas diagnosis of DMCMP with dilated/HFREF phenotype requires systolic LV dysfunction. Treatment of DMCMP with restrictive/HFPEF phenotype is limited to diuretics and lifestyle modification, whereas DMCMP with dilated/HFREF phenotype is treated in accordance to HF guidelines.

Propofol (Diprivan®) and Intralipid® exacerbate insulin resistance in type-2 diabetic hearts by impairing GLUT4 trafficking

BACKGROUND

The IV anesthetic, propofol, when administered as fat emulsion-based formulation (Diprivan) promotes insulin resistance, but the direct effects of propofol and its solvent, Intralipid, on cardiac insulin resistance are unknown.

METHODS

Hearts of healthy and type-2 diabetic rats (generated by fructose feeding) were aerobically perfused for 60 minutes with 10 μM propofol in the formulation of Diprivan or an equivalent concentration of its solvent Intralipid (25 μM) ± insulin (100 mU•L). Glucose uptake, glycolysis, and glycogen metabolism were measured using [H]glucose. Activation of Akt, GSK3β, AMPK, ERK1/2, p38MAPK, S6K1, JNK, protein kinase Cθ (PKCθ), and protein kinase CCβII (PKCβII) was determined using immunoblotting. GLUT4 trafficking and phosphorylations of insulin receptor substrate-1 (IRS-1) at Ser307(h312), Ser1100(h1101), and Tyr608(hTyr612) were measured. Mass spectrometry was used to determine acylcarnitines, phospholipids, and sphingolipids.

RESULTS

Diprivan and Intralipid reduced insulin-induced glucose uptake and redirected glucose to glycogen stores in diabetic hearts. Reduced glucose uptake was accompanied by lower GLUT4 trafficking to the sarcolemma. Diprivan and Intralipid inactivated GSK3β but activated AMPK and ERK1/2 in diabetic hearts. Only Diprivan increased phosphorylation of Akt(Ser473/Thr308) and translocated PKCθ and PKCβII to the sarcolemma in healthy hearts, whereas it activated S6K1 and p38MAPK and translocated PKCβII in diabetic hearts. Furthermore, only Diprivan phosphorylated IRS-1 at Ser1100(h1101) in healthy and diabetic hearts. JNK expression, phosphorylation of Ser307(h312) of IRS-1, and PKCθ expression and translocation were increased, whereas GLUT4 expression was reduced in insulin-treated diabetic hearts. Phosphatidylglycerol, phosphatidylethanolamine, and C18-sphingolipids accumulated in Diprivan-perfused and Intralipid-perfused diabetic hearts.

CONCLUSIONS

Propofol and Intralipid promote insulin resistance predominantly in type-2 diabetic hearts.

High-fat feeding-induced hyperinsulinemia increases cardiac glucose uptake and mitochondrial function despite peripheral insulin resistance

In obesity, reduced cardiac glucose uptake and mitochondrial abnormalities are putative causes of cardiac dysfunction. However, high-fat diet (HFD) does not consistently induce cardiac insulin resistance and mitochondrial damage, and recent studies suggest HFD may be cardioprotective. To determine cardiac responses to HFD, we investigated cardiac function, glucose uptake, and mitochondrial respiration in young (3-month-old) and middle-aged (MA) (12-month-old) male Ldlr(-/-) mice fed chow or 3 months HFD to induce obesity, systemic insulin resistance, and hyperinsulinemia. In MA Ldlr(-/-) mice, HFD induced accelerated atherosclerosis and nonalcoholic steatohepatitis, common complications of human obesity. Surprisingly, HFD-fed mice demonstrated increased cardiac glucose uptake, which was most prominent in MA mice, in the absence of cardiac contractile dysfunction or hypertrophy. Moreover, hearts of HFD-fed mice had enhanced mitochondrial oxidation of palmitoyl carnitine, glutamate, and succinate and greater basal insulin signaling compared with those of chow-fed mice, suggesting cardiac insulin sensitivity was maintained, despite systemic insulin resistance. Streptozotocin-induced ablation of insulin production markedly reduced cardiac glucose uptake and mitochondrial dysfunction in HFD-fed, but not in chow-fed, mice. Insulin injection reversed these effects, suggesting that insulin may protect cardiac mitochondria during HFD. These results have implications for cardiac metabolism and preservation of mitochondrial function in obesity.

Acute regulation of 5'-AMP-activated protein kinase by long-chain fatty acid, glucose and insulin in rat primary adipocytes

Palmitate increased AMPK (5'-AMP-activated protein kinase) activity, glucose utilization and 2-DOG (2-deoxyglucose) transport in rat adipocytes. All three effects were blocked by the AMPK inhibitor Compound C, leading to the conclusion that in response to an increase in long-chain NEFA (non-esterified fatty acid) concentration AMPK mediated an enhancement of adipocyte glucose transport, thereby providing increased glycerol 3-phosphate for FA (fatty acid) esterification to TAG (triacylglycerol). Activation of AMPK in response to palmitate was not due to an increase in the adipocyte AMP:ATP ratio. Glucose decreased AMPK activity and effects of palmitate and glucose on AMPK activity were antagonistic. While insulin had no effect on basal AMPK activity insulin did decrease AMPK activity in the presence of palmitate and also decreased the percentage effectiveness of palmitate to increase the transport of 2-DOG. It is suggested that activation of adipocyte AMPK by NEFA, as well as decreasing the activity of hormone-sensitive lipase, could modulate adipose tissue dynamics by increasing FA esterification and, under certain circumstances, FA synthesis.

Rethinking Heart Failure

An increasing body of clinical observations and experimental evidence suggests that cardiac dysfunction results from autonomic dysregulation of the contractile output of the heart. Excessive activation of the sympathetic nervous system and a decrease in parasympathetic tone are associated with increased mortality. Elevated levels of circulating catecholamines closely correlate with the severity and poor prognosis in heart failure. Sympathetic over-stimulation causes increased levels of catecholamines, which induce excessive aerobic metabolism leading to excessive cardiac oxygen consumption. Resulting impaired mitochondrial function causes acidosis, which results in reduction in blood flow by impairment of contractility. To the extent that the excessive aerobic metabolism resulting from adrenergic stimulation comes to a halt the energy deficit has to be compensated for by anaerobic metabolism. Glucose and glycogen become the essential nutrients. Beta-adrenergic blockade is used successfully to decrease hyperadrenergic drive. Neurohumoral antagonists block adrenergic over-stimulation but do not provide the heart with fuel for compensatory anaerobic metabolism. The endogenous hormone ouabain reduces catecholamine levels in healthy volunteers, promotes the secretion of insulin, induces release of acetylcholine from synaptosomes and potentiates the stimulation of glucose metabolism by insulin and acetylcholine. Ouabain stimulates glycogen synthesis and increases lactate utilisation by the myocardium. Decades of clinical experience with ouabain confirm the cardioprotective effects of this endogenous hormone. The so far neglected sympatholytic and vagotonic effects of ouabain on myocardial metabolism clearly make a clinical re-evaluation of this endogenous hormone necessary. Clinical studies with ouabain that correspond to current standards are warranted.

AMPK in myocardial infarction and diabetes: the yin/yang effect

There has been considerable progress in our understanding of cardiac cell metabolism in health and disease, yet important gaps remain in basic knowledge and its translation to clinical care. AMP-activated protein kinase (AMPK) functions either to conserve ATP or to promote alternative methods of ATP generation. Since the discovery of AMPK more than three decades ago and demonstration of its expression in the heart, interest has grown exponentially in this major fuel gauge as a modulator of the cellular response to ischemia. Such pathway may potentially explain the strong association between metabolic syndrome and ischemic heart disease. Still missing from our most recent cardiology textbooks, this article aims to summarize our understanding so far of the role of AMPK in coordinating the cellular response to ischemic stress and reperfusion injury in the heart. We aim to provide a focused update on the pharmacological agents activating AMPK for treatment of diabetes that show potential cardioprotective effects. Our hope is to stimulate future researchers to the potential benefits of harnessing the AMPK signaling pathway, or better one of its novel downstream targets for the treatment of myocardial ischemia.

Inhibition of serine palmitoyl transferase I reduces cardiac ceramide levels and increases glycolysis rates following diet-induced insulin resistance

OBJECTIVE

Diet-induced obesity (DIO) leads to an accumulation of intra-myocardial lipid metabolites implicated in causing cardiac insulin resistance and contractile dysfunction. One such metabolite is ceramide, and our aim was to determine the effects of inhibiting de novo ceramide synthesis on cardiac function and insulin stimulated glucose utilization in mice subjected to DIO.

MATERIALS AND METHODS

C57BL/6 mice were fed a low fat diet or subjected to DIO for 12 weeks, and then treated for 4 weeks with either vehicle control or the serine palmitoyl transferase I (SPT I) inhibitor, myriocin. In vivo cardiac function was assessed via ultrasound echocardiography, while glucose metabolism was assessed in isolated working hearts.

RESULTS

DIO was not associated with an accumulation of intra-myocardial ceramide, but rather, an accumulation of intra-myocardial DAG (2.63±0.41 vs. 4.80±0.97 nmol/g dry weight). Nonetheless, treatment of DIO mice with myriocin decreased intra-myocardial ceramide levels (50.3±7.7 vs. 26.9±2.7 nmol/g dry weight) and prevented the DIO-associated increase in intra-myocardial DAG levels. Interestingly, although DIO impaired myocardial glycolysis rates (7789±1267 vs. 2671±326 nmol/min/g dry weight), hearts from myriocin treated DIO mice exhibited an increase in glycolysis rates.

CONCLUSIONS

Our data reveal that although intra-myocardial ceramide does not accumulate following DIO, inhibition of de novo ceramide synthesis nonetheless reduces intra-myocardial ceramide levels and prevents the accumulation of intra-myocardial DAG. These effects improved the DIO-associated impairment of cardiac glycolysis rates, suggesting that SPT I inhibition increases cardiac glucose utilization.

Effect of diabetes on alteration of metabolism in cardiac myocytes: therapeutic implications

Diabetic cardiomyopathy is a distinct entity in humans. It leads to ventricular dysfunction independent of and additive to coronary artery disease and hypertension. Clinical and experimental studies have pointed to the role of metabolic derangements in the development of diabetic cardiomyopathy. Altered insulin signaling in diabetes leads to decreased myocyte glucose uptake and utilization, associated with an increased concentration of free fatty acids. This results in decreased glucose oxidation and increased fatty acid oxidation. Fatty acids increase mitochondrial oxygen consumption for ATP production and stimulate the uncoupling proteins in mitochondria. These proteins decrease the mitochondrial protein gradient, leading to fall in ATP production. The resultant defect in myocardial energy production impairs myocyte contraction and diastolic function. This is the hallmark of diabetic cardiomyopathy at earlier stages. In later stages diabetes impairs the myocyte ischemic defense mechanism, leading to increased cardiovascular morbidity and mortality. Other factors contributing toward causation of diabetic cardiomyopathy are collagen accumulation leading to reduced myocardial compliance, accumulation of advanced glycation end product-modified extracellular matrix proteins with subsequent inelasticity of vessel walls and myocytes, abnormal myocardial calcium handling leading to altered mechanics, endothelial dysfunction, cardiac autonomic neuropathy, and impairment of ischemic preconditioning. Trimetazidine acts a metabolic switch, favoring glucose over free fatty acids as the substrate for metabolism in cardiac myocytes.

Targeting fatty acid and carbohydrate oxidation--a novel therapeutic intervention in the ischemic and failing heart

Cardiac ischemia and its consequences including heart failure, which itself has emerged as the leading cause of morbidity and mortality in developed countries are accompanied by complex alterations in myocardial energy substrate metabolism. In contrast to the normal heart, where fatty acid and glucose metabolism are tightly regulated, the dynamic relationship between fatty acid β-oxidation and glucose oxidation is perturbed in ischemic and ischemic-reperfused hearts, as well as in the failing heart. These metabolic alterations negatively impact both cardiac efficiency and function. Specifically there is an increased reliance on glycolysis during ischemia and fatty acid β-oxidation during reperfusion following ischemia as sources of adenosine triphosphate (ATP) production. Depending on the severity of heart failure, the contribution of overall myocardial oxidative metabolism (fatty acid β-oxidation and glucose oxidation) to adenosine triphosphate production can be depressed, while that of glycolysis can be increased. Nonetheless, the balance between fatty acid β-oxidation and glucose oxidation is amenable to pharmacological intervention at multiple levels of each metabolic pathway. This review will focus on the pathways of cardiac fatty acid and glucose metabolism, and the metabolic phenotypes of ischemic and ischemic/reperfused hearts, as well as the metabolic phenotype of the failing heart. Furthermore, as energy substrate metabolism has emerged as a novel therapeutic intervention in these cardiac pathologies, this review will describe the mechanistic bases and rationale for the use of pharmacological agents that modify energy substrate metabolism to improve cardiac function in the ischemic and failing heart. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.

Glucose-insulin-potassium reduces the incidence of low cardiac output episodes after aortic valve replacement for aortic stenosis in patients with left ventricular hypertrophy: results from the Hypertrophy, Insulin, Glucose, and Electrolytes (HINGE) trial

BACKGROUND

Patients undergoing aortic valve replacement for critical aortic stenosis often have significant left ventricular hypertrophy. Left ventricular hypertrophy has been identified as an independent predictor of poor outcome after aortic valve replacement as a result of a combination of maladaptive myocardial changes and inadequate myocardial protection at the time of surgery. Glucose-insulin-potassium (GIK) is a potentially useful adjunct to myocardial protection. This study was designed to evaluate the effects of GIK infusion in patients undergoing aortic valve replacement surgery.

METHODS AND RESULTS

Patients undergoing aortic valve replacement for aortic stenosis with evidence of left ventricular hypertrophy were randomly assigned to GIK or placebo. The trial was double-blind and conducted at a single center. The primary outcome was the incidence of low cardiac output syndrome. Left ventricular biopsies were analyzed to assess changes in 5' adenosine monophosphate-activated protein kinase (AMPK), Akt phosphorylation, and protein O-linked β-N-acetylglucosamination (O-GlcNAcylation). Over a 4-year period, 217 patients were randomized (107 control, 110 GIK). GIK treatment was associated with a significant reduction in the incidence of low cardiac output state (odds ratio, 0.22; 95% confidence interval, 0.10 to 0.47; P=0.0001) and a significant reduction in inotrope use 6 to 12 hours postoperatively (odds ratio, 0.30; 95% confidence interval, 0.15 to 0.60; P=0.0007). These changes were associated with a substantial increase in AMPK and Akt phosphorylation and a significant increase in the O-GlcNAcylation of selected protein bands.

CONCLUSIONS

Perioperative treatment with GIK was associated with a significant reduction in the incidence of low cardiac output state and the need for inotropic support. This benefit was associated with increased signaling protein phosphorylation and O-GlcNAcylation. Multicenter studies and late follow-up will determine whether routine use of GIK improves patient prognosis.

Elevated levels of activated NHE1 protect the myocardium and improve metabolism following ischemia/reperfusion injury

In the myocardium, the Na(+)/H(+) exchanger isoform 1 (NHE1) is a plasma membrane protein that regulates intracellular pH. Inhibition of NHE1 activity has been shown to be beneficial in cardiovascular disease. However, recent reports have suggested that elevation of NHE1 levels has beneficial effects in hearts subjected to ischemia/reperfusion. We determined if activated and non-activated NHE1 proteins have varying cardioprotective and metabolic effects with ischemia/reperfusion in the isolated perfused working mouse heart. We used transgenic mice hearts that specifically expressed wild type NHE1 (N-line) or activated NHE1 protein (K-line). Intact hearts 10-12 weeks of age were perfused under working conditions, with fatty acids and glucose present as substrates. Hearts were subjected to 30 min of aerobic perfusion, followed by 20 min of global no-flow ischemia and 40 min of aerobic reperfusion. We examined changes in contractility and substrate use and ATP levels. K-line hearts expressing activated NHE1, recovered to a much greater extent than N-line and control hearts recovering almost 75% of their preischemic function. In addition, K-line hearts had elevated fatty acid oxidation, increased glycolysis rates and elevated ATP levels relative to N-line mice or controls. An examination of kinase activation showed that there were no differences between controls and transgenics in ERK, p38, p90(rsk) or pGSK3β levels. The results demonstrate that elevated levels of NHE1 induce cardioprotection and alter cardiac metabolism. However, in the working heart model, with glucose and fatty acid as substrates, this required an activated NHE1 protein.

Activation of AMP-activated protein kinase signaling pathway by adiponectin and insulin in mouse adipocytes: requirement of acyl-CoA synthetases FATP1 and Acsl1 and association with an elevation in AMP/ATP ratio

Adiponectin activates AMP-activated protein kinase (AMPK) in adipocytes, but the underlying mechanism remains unclear. Here we tested the hypothesis that AMP, generated in activating fatty acids to their CoA derivatives, catalyzed by acyl-CoA synthetases, is involved in AMPK activation by adiponectin. Moreover, in adipocytes, insulin affects the subcellular localization of acyl-CoA synthetase FATP1. Thus, we also tested whether insulin activates AMPK in these cells and, if so, whether it activates through a similar mechanism. We examined these hypotheses by measuring the AMP/ATP ratio and AMPK activation on adiponectin and insulin stimulation and after knocking down acyl-CoA synthetases in adipocytes. We show that adiponectin activation of AMPK is accompanied by an ∼2-fold increase in the cellular AMP/ATP ratio. Moreover, FATP1 and Acsl1, the 2 major acyl-CoA synthetase isoforms in adipocytes, are essential for AMPK activation by adiponectin. We also show that after 40 min. insulin activated AMPK in adipocytes, which was coupled with a 5-fold increase in the cellular AMP/ATP ratio. Knockdown studies show that FATP1 and Acsl1 are required for these processes, as well as for stimulation of long-chain fatty acid uptake by adiponection and insulin. These studies demonstrate that a change in cellular energy state is associated with AMPK activation by both adiponectin and insulin, which requires the activity of FATP1 and Acsl1.

Novel O-palmitolylated beta-E1 subunit of pyruvate dehydrogenase is phosphorylated during ischemia/reperfusion injury

BACKGROUND

During and following myocardial ischemia, glucose oxidation rates are low and fatty acids dominate as a source of oxidative metabolism. This metabolic phenotype is associated with contractile dysfunction during reperfusion. To determine the mechanism of this reliance on fatty acid oxidation as a source of ATP generation, a functional proteomics approach was utilized.

RESULTS

2-D gel electrophoresis of mitochondria from working rat hearts subjected to 25 minutes of global no flow ischemia followed by 40 minutes of aerobic reperfusion identified 32 changes in protein abundance compared to aerobic controls. Of the five proteins with the greatest change in abundance, two were increased (long chain acyl-coenzyme A dehydrogenase (48 +/- 1 versus 39 +/- 3 arbitrary units, n = 3, P < 0.05) and alpha subunit of ATP synthase (189 +/- 15 versus 113 +/- 23 arbitrary units, n = 3, P < 0.05)), while two were decreased (24 kDa subunit of NADH-ubiquinone oxidoreductase (94 +/- 7 versus 127 +/- 9 arbitrary units, n = 3, P < 0.05) and D subunit of ATP synthase (230 +/- 11 versus 368 +/- 47 arbitrary units, n = 3, P < 05)). Two forms of pyruvate dehydrogenase betaE1 subunit, the rate-limiting enzyme for glucose oxidation, were also identified. The protein level of the more acidic form of pyruvate dehydrogenase was reduced during reperfusion (37 +/- 4 versus 56 +/- 7 arbitrary units, n = 3, P < 05), while the more basic form remained unchanged. The more acidic isoform was found to be O-palmitoylated, while both isoforms exhibited ischemia/reperfusion-induced phosphorylation. In silico analysis identified the putative kinases as the insulin receptor kinase for the more basic form and protein kinase Czeta or protein kinase A for the more acidic form. These modifications of pyruvate dehydrogenase are associated with a 35% decrease in glucose oxidation during reperfusion.

CONCLUSIONS

Cardiac ischemia/reperfusion induces significant changes to a number of metabolic proteins of the mitochondrial proteome. In particular, ischemia/reperfusion induced the post-translational modification of pyruvate dehydrogenase, the rate-limiting step of glucose oxidation, which is associated with a 35% decrease in glucose oxidation during reperfusion. Therefore these post-translational modifications may have important implications in the regulation of myocardial energy metabolism.

Diabetic cardiomyopathy: signaling defects and therapeutic approaches

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

Phospholipase C as a potential target for cardioprotection during oxidative stressThis review is one of a selection of papers published in a Special Issue on Oxidative Stress in Health and Disease

Cardiac dysfunction due to ischemia–reperfusion (I/R) is associated with marked changes in membrane function and subsequent Ca2+-handling abnormalities in cardiomyocytes. The membrane abnormalities in hearts subjected to I/R arise primarily from oxidative stress as a consequence of increased formation of reactive oxygen species and other oxidants, as well as reduced antioxidant defenses. Little is known, however, about the nature and mechanisms of the sarcolemmal membrane changes with respect to phospholipase C (PLC)-related signaling events. In addition, the mechanisms involved in protection of the postischemic myocardium and in ischemic preconditioning with respect to PLC function need to be established. Accordingly, this article reviews the historical and current information on PLC-mediated signal transduction mechanisms in I/R, as well as outlining future directions that should be addressed. Such information will extend our knowledge of ischemic heart disease and help improve its therapy.

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.

Effects of hormone-sensitive lipase disruption on cardiac energy metabolism in response to fasting and refeeding

Increased fatty acid (FA) flux and intracellular lipid accumulation (steatosis) give rise to cardiac lipotoxicity in both pathological and physiological conditions. Since hormone-sensitive lipase (HSL) contributes to intracellular lipolysis in adipose tissue and heart, we investigated the impact of HSL disruption on cardiac energy metabolism in response to fasting and refeeding. HSL-knockout (KO) mice and wild-type (WT) littermates were fasted for 24 h, followed by ∼6 h of refeeding. Plasma FA concentration in WT mice was elevated twofold with fasting, whereas KO mice lacked this elevation, resulting in twofold lower cardiac FA uptake compared with WT mice. Echocardiography showed that fractional shortening was 15% decreased during fasting in WT mice and was associated with steatosis, whereas both of these changes were absent in KO mice. Compared with Langendorff-perfused hearts isolated from fasted WT mice, the isolated KO hearts also displayed higher contractile function and a blunted response to FA. Although cardiac glucose uptake in KO mice was comparable with WT mice under all conditions tested, cardiac VLDL uptake and lipoprotein lipase (LPL) activity were twofold higher in KO mice during fasting. The KO hearts showed undetectable activity of neutral cholesteryl esterase and 40% lower non-LPL triglyceride lipase activity compared with WT hearts in refed conditions accompanied by overt steatosis, normal cardiac function, and increased mRNA expression of adipose differentiation-related protein. Thus, the dissociation between cardiac steatosis and functional sequelae observed in HSL-KO mice suggests that excess FA influx, rather than steatosis per se, appears to play an important role in the pathogenesis of cardiac lipotoxicity.

Suppression of 5'-AMP-activated protein kinase activity does not impair recovery of contractile function during reperfusion of ischemic hearts

Activation of 5'-AMP-activated protein kinase (AMPK) may benefit the heart during ischemia-reperfusion by increasing energy production. While AMPK stimulates glycolysis, mitochondrial oxidative metabolism is the major source of ATP production during reperfusion of ischemic hearts. Stimulating AMPK increases mitochondrial fatty acid oxidation, but this is usually accompanied by a decrease in glucose oxidation, which can impair the functional recovery of ischemic hearts. To examine the relationship between AMPK and cardiac energy substrate metabolism, we subjected isolated working mouse hearts expressing a dominant negative (DN) alpha(2)-subunit of AMPK (AMPK-alpha(2) DN) to 20 min of global no-flow ischemia and 40 min of reperfusion with Krebs-Henseleit solution containing 5 mM [U-(14)C]glucose, 0.4 mM [9, 10-(3)H]palmitate, and 100 microU/ml insulin. AMPK-alpha(2) DN hearts had reduced AMPK activity at the end of reperfusion (82 +/- 9 vs. 141 +/- 7 pmol.mg(-1).min(-1)) with no changes in high-energy phosphates. Despite this, AMPK-alpha(2) DN hearts had improved recovery of function during reperfusion (14.9 +/- 0.8 vs. 9.4 +/- 1.4 beats.min(-1).mmHg.10(-3)). During reperfusion, fatty acid oxidation provided 44.0 +/- 2.8% of total acetyl-CoA in AMPK-alpha(2) DN hearts compared with 55.0 +/- 3.2% in control hearts. Since insulin can inhibit both AMPK activation and fatty acid oxidation, we also examined functional recovery in the absence of insulin. Functional recovery was similar in both groups despite a decrease in AMPK activity and a decreased reliance on fatty acid oxidation during reperfusion (66.4 +/- 9.4% vs. 85.3 +/- 4.3%). These data demonstrate that the suppression of cardiac AMPK activity does not produce an energetically compromised phenotype and does not impair, but may in fact improve, the recovery of function after ischemia.

Adenylate kinase and AMP signaling networks: metabolic monitoring, signal communication and body energy sensing

Adenylate kinase and downstream AMP signaling is an integrated metabolic monitoring system which reads the cellular energy state in order to tune and report signals to metabolic sensors. A network of adenylate kinase isoforms (AK1-AK7) are distributed throughout intracellular compartments, interstitial space and body fluids to regulate energetic and metabolic signaling circuits, securing efficient cell energy economy, signal communication and stress response. The dynamics of adenylate kinase-catalyzed phosphotransfer regulates multiple intracellular and extracellular energy-dependent and nucleotide signaling processes, including excitation-contraction coupling, hormone secretion, cell and ciliary motility, nuclear transport, energetics of cell cycle, DNA synthesis and repair, and developmental programming. Metabolomic analyses indicate that cellular, interstitial and blood AMP levels are potential metabolic signals associated with vital functions including body energy sensing, sleep, hibernation and food intake. Either low or excess AMP signaling has been linked to human disease such as diabetes, obesity and hypertrophic cardiomyopathy. Recent studies indicate that derangements in adenylate kinase-mediated energetic signaling due to mutations in AK1, AK2 or AK7 isoforms are associated with hemolytic anemia, reticular dysgenesis and ciliary dyskinesia. Moreover, hormonal, food and antidiabetic drug actions are frequently coupled to alterations of cellular AMP levels and associated signaling. Thus, by monitoring energy state and generating and distributing AMP metabolic signals adenylate kinase represents a unique hub within the cellular homeostatic network.

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.

Malonyl-CoA, a key signaling molecule in mammalian cells

Malonyl-CoA can be formed within the mitochondria, peroxisomes, and cytosol of mammalian cells. Besides being an intermediate in the pathways of de novo fatty acid biosynthesis and fatty acid elongation, malonyl-CoA has an important signaling function through its allosteric inhibition of carnitine palmitoyltransferase 1, the enzyme that normally exerts flux control over mitochondrial beta-oxidation. Malonyl-CoA is rapidly turned over in mammalian cells, and the activities of acetyl-CoA carboxylase and malonyl-CoA decarboxylase are important determinants of its cytosolic concentration. It is now recognized that malonyl-CoA participates in a diverse range of physiological or pathological responses and systems. These include the ketogenic response of the liver to fasting and diabetes, carbohydrate versus fat fuel selection in muscle tissues, metabolic changes in muscle during contracture, alterations in fatty acid metabolism during cardiac ischemia and postischemic reperfusion, stimulation of B cell insulin secretion by glucose, and the hypothalamic control of appetite.

Maternal overnutrition suppresses the phosphorylation of 5'-AMP-activated protein kinase in liver, but not skeletal muscle, in the fetal and neonatal sheep

Epidemiological studies have shown that infants exposed to an increased supply of nutrients before birth are at increased risk of type 2 diabetes in later life. We have investigated the hypothesis that fetal overnutrition results in reduced expression and phosphorylation of the cellular fuel sensor, AMP-activated kinase (AMPK) in liver and skeletal muscle before and after birth. From 115 days gestation, ewes were fed either at or approximately 55% above maintenance energy requirements. Postmortem was performed on lamb fetuses at 139-141 days gestation (n = 14) and lambs at 30 days of postnatal age (n = 21), and liver and quadriceps muscle were collected at each time point. The expression of AMPKalpha1 and AMPKalpha2 mRNA was determined by quantitative RT-PCR (qRT-PCR). The abundance of AMPKalpha and phospho-AMPKalpha (P-AMPKalpha) was determined by Western blot analysis, and the proportion of the total AMPKalpha pool that was phosphorylated in each sample (%P-AMPKalpha) was determined. The ratio of AMPKalpha2 to AMPKalpha1 mRNA expression was lower in fetuses compared with lambs in both liver and muscle, independent of maternal nutrition. Hepatic %P-AMPKalpha was lower in both fetuses and lambs in the Overfed group and %P-AMPKalpha in the lamb liver was inversely related to plasma glucose concentrations in the first 24 h after birth (r = 0.73, P < 0.025). There was no effect of maternal overnutrition on total AMPKalpha or P-AMPKalpha abundance in liver or skeletal muscle. We have, therefore, demonstrated that AMPKalpha responds to signals of increased nutrient availability in the fetal liver. Suppression of hepatic AMPK phosphorylation may contribute to increased glucose production, and basal hyperglycemia, present in lambs of overfed ewes in early postnatal life.

H(2)O(2)-induced left ventricular dysfunction in isolated working rat hearts is independent of calcium accumulation

Reactive oxygen species (ROS) and intracellular Ca(2+) overload play key roles in myocardial ischemia-reperfusion (IR) injury but the relationships among ROS, Ca(2+) overload and LV mechanical dysfunction remain unclear. We tested the hypothesis that H(2)O(2) impairs LV function by causing Ca(2+) overload by increasing late sodium current (I(Na)), similar to Sea Anemone Toxin II (ATX-II). Diastolic and systolic Ca(2+) concentrations (d[Ca(2+)](i) and s[Ca(2+)](i)) were measured by indo-1 fluorescence simultaneously with LV work in isolated working rat hearts. H(2)O(2) (100 microM, 30 min) increased d[Ca(2+)](i) and s[Ca(2+)](i). LV work increased transiently then declined to 32% of baseline before recovering to 70%. ATX-II (12 nM, 30 min) caused greater increases in d[Ca(2+)](i) and s[Ca(2+)](i). LV work increased transiently before declining gradually to 17%. Ouabain (80 microM) exerted similar effects to ATX-II. Late I(Na) inhibitors, lidocaine (10 microM) or R56865 (2 microM), reduced effects of ATX-II on [Ca(2+)](i) and LV function, but did not alter effects of H(2)O(2). The antioxidant, N-(2-mercaptopropionyl)glycine (MPG, 1 mM) prevented H(2)O(2)-induced LV dysfunction, but did not alter [Ca(2+)](i). Paradoxically, further increases in [Ca(2+)](i) by ATX-II or ouabain, given 10 min after H(2)O(2), improved function. The failure of late I(Na) inhibitors to prevent H(2)O(2)-induced LV dysfunction, and the ability of MPG to prevent H(2)O(2)-induced LV dysfunction independent of changes in [Ca(2+)](i) indicate that impaired contractility is not due to Ca(2+) overload. The ability of further increases in [Ca(2+)](i) to reverse H(2)O(2)-induced LV dysfunction suggests that Ca(2+) desensitization is the predominant mechanism of ROS-induced contractile dysfunction.

Metabolic reserve of the heart: the forgotten link between contraction and coronary flow

Myocardial energy substrate metabolism entails a complex system of enzyme catalyzed reactions, in which the heart efficiently converts chemical to mechanical energy. The system is highly regulated and responsive to changes in workload as well as in substrate and hormone supply to the heart. Akin to the terms "contractile reserve" and "coronary flow reserve" we propose the term "metabolic reserve" to reflect the heart's capacity to respond to increases in workload. The heart's metabolic response to inotropic stimulation involves the ability to increase oxidative metabolism over a wide range, by activating the oxidation of glycogen and carbohydrate substrates. Here we review the known biochemical mechanisms responsible for those changes. Specifically, we explore the notion that disturbances in the metabolic reserve result in contractile dysfunction of the stressed heart.

Transient activation of p38 MAP kinase and up-regulation of Pim-1 kinase in cardiac hypertrophy despite no activation of AMPK

AMP-activated protein kinase (AMPK), is an important regulator of cardiac metabolism, but its role is not clearly understood in pressure overload induced hypertrophy. In addition, the relationship between AMPK and other important protein kinases such as p38 MAP kinase, Akt and Pim-1 is unclear. Thus we studied the time course of AMPK activity and phosphorylation of Thr-172 of its alpha-subunit during the development of cardiac hypertrophy. In parallel, we examined the expression and activation of key kinases known to be involved in cardiac hypertrophy that could interact with AMPK (i.e. p38 MAP kinase, Akt and Pim-1). Male C57BL/6J mice underwent sham or transverse aortic constriction (TAC) surgery and the hearts were harvested 2, 4, 6 and 8 weeks later. Despite significant left ventricular (LV) hypertrophy, LV dilation and impaired LV contractile function at all time points in TAC compared to sham mice, the activity and phosphorylation of AMPK were similar to sham. In contrast, p38 and Pim-1 protein expression was transiently increased in TAC mice at 2 and 4 weeks and at 2, 4 and 6 weeks, respectively. In addition, p38 activation by phosphorylation was also transiently increased at 2 to 6 weeks. There were no differences between sham and TAC mice in p38, Akt or Pim-1 at 8 weeks. In conclusion, TAC resulted in a transient up-regulation in the expression of p38 and Pim-1 despite no activation of AMPK or Akt.

Role of glucose metabolism in the recovery of postischemic LV mechanical function: effects of insulin and other metabolic modulators

The role of proton (H+) production from glucose metabolism in the recovery of myocardial function during postischemic reperfusion and its alteration by insulin and other metabolic modulators were examined. Rat hearts were perfused in vitro with Krebs-Henseleit solution containing palmitate (1.2 mmol/l) and glucose (11 mmol/l) under nonischemic conditions or during reperfusion following no-flow ischemia. Perfusate contained normal insulin (n-Ins, 50 mU/l), zero insulin (0-Ins), or supplemental insulin (s-Ins, 1,000 mU/l) or other metabolic modulators [dichloroacetate (DCA) at 3 mmol/l, oxfenicine at 1 mmol/l, and N6-cyclohexyladenosine (CHA) at 0.5 μmol/l]. Relative to n-Ins, 0-Ins depressed rates of glycolysis and glucose oxidation in nonischemic hearts and impaired recovery of postischemic function. Relative to n-Ins, s-Ins did not affect aerobic glucose metabolism and did not improve recovery when present during reperfusion. When present during ischemia and reperfusion, s-Ins impaired recovery. Combinations of metabolic modulators with s-Ins stimulated glucose oxidation ∼2.5-fold in nonischemic hearts and reduced H+ production. DCA and CHA, in combination with s-Ins, improved recovery of function, but addition of oxfenicine to this combination provided no further benefit. Although DCA and CHA were each partially protective in hearts perfused with n-Ins, optimal protection was achieved with DCA + CHA; recovery of function was inversely proportional to H+ production during reperfusion. Although supplemental insulin is not beneficial, elimination of H+ production from glucose metabolism by simultaneous inhibition of glycolysis and stimulation of glucose oxidation optimizes recovery of postischemic mechanical function.

The malonyl CoA axis as a potential target for treating ischaemic heart disease

Cardiovascular disease is the leading cause of death and disability for people living in western societies, with ischaemic heart disease accounting for the majority of this health burden. The primary treatment for ischaemic heart disease consists of either improving blood and oxygen supply to the heart or reducing the heart's oxygen demand. Unfortunately, despite recent advances with these approaches, ischaemic heart disease still remains a major health problem. Therefore, the development of new treatment strategies is still required. One exciting new approach is to optimize cardiac energy metabolism, particularly by decreasing the use of fatty acids as a fuel and by increasing the use of glucose as a fuel. This approach is beneficial in the setting of ischaemic heart disease, as it allows the heart to produce energy more efficiently and it reduces the degree of acidosis associated with ischaemia/reperfusion. Malonyl CoA is a potent endogenous inhibitor of cardiac fatty acid oxidation, secondary to inhibiting carnitine palmitoyl transferase-I, the rate-limiting enzyme in the mitochondrial uptake of fatty acids. Malonyl CoA is synthesized in the heart by acetyl CoA carboxylase, which in turn is phosphorylated and inhibited by 5'AMP-activated protein kinase. The degradation of myocardial malonyl CoA occurs via malonyl CoA decarboxylase (MCD). Previous studies have shown that inhibiting MCD will significantly increase cardiac malonyl CoA levels. This is associated with an increase in glucose oxidation, a decrease in acidosis, and an improvement in cardiac function and efficiency during and following ischaemia. Hence, the malonyl CoA axis represents an exciting new target for the treatment of ischaemic heart disease.

Cardiac phosphatase-deficient 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase increases glycolysis, hypertrophy, and myocyte resistance to hypoxia

During ischemia and heart failure, there is an increase in cardiac glycolysis. To understand if this is beneficial or detrimental to the heart, we chronically elevated glycolysis by cardiac-specific overexpression of phosphatase-deficient 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2) in transgenic mice. PFK-2 controls the level of fructose-2,6-bisphosphate (Fru-2,6-P2), an important regulator of phosphofructokinase and glycolysis. Transgenic mice had over a threefold elevation in levels of Fru-2,6-P2. Cardiac metabolites upstream of phosphofructokinase were significantly reduced, as would be expected by the activation of phosphofructokinase. In perfused hearts, the transgene caused a significant increase in glycolysis that was less sensitive to inhibition by palmitate. Conversely, oxidation of palmitate was reduced by close to 50%. The elevation in glycolysis made isolated cardiomyocytes highly resistant to contractile inhibition by hypoxia, but in vivo the transgene had no effect on ischemia-reperfusion injury. Transgenic hearts exhibited pathology: the heart weight-to-body weight ratio was increased 17%, cardiomyocyte length was greater, and cardiac fibrosis was increased. However, the transgene did not change insulin sensitivity. These results show that the elevation in glycolysis provides acute benefits against hypoxia, but the chronic increase in glycolysis or reduction in fatty acid oxidation interferes with normal cardiac metabolism, which may be detrimental to the heart.

Role of nitric oxide in regulating aldose reductase activation in the ischemic heart

Aldose reductase (AR) catalyzes the reduction of several aldehydes ranging from lipid peroxidation products to glucose. The activity of AR is increased in the ischemic heart due to oxidation of its cysteine residues, but the underlying mechanisms remain unclear. To examine signaling mechanisms regulating AR activation, we studied the role of nitric oxide (NO). Treatment with the NO synthase (NOS) inhibitor, N-nitro-l-arginine methyl ester prevented ischemia-induced AR activation and myocardial sorbitol accumulation in rat hearts subjected to global ischemia ex vivo or coronary ligation in situ, whereas inhibition of inducible NOS and neuronal NOS had no effect. Activation of AR in the ischemic heart was abolished by pretreatment with peroxynitrite scavengers hesperetin or 5, 10, 15, 20-tetrakis-[4-sulfonatophenyl]-porphyrinato-iron [III]. Site-directed mutagenesis and electrospray ionization mass spectrometry analyses showed that Cys-298 of AR was readily oxidized to sulfenic acid by peroxynitrite. Treatment with bradykinin and insulin led to a phosphatidylinositol 3-kinase (PI3K)-dependent increase in the phosphorylation of endothelial NOS at Ser-1177 and, even in the absence of ischemia, was sufficient in activating AR. Activation of AR by bradykinin and insulin was reversed upon reduction with dithiothreitol or by inhibiting NOS or PI3K. Treatment with AR inhibitors sorbinil or tolrestat reduced post-ischemic recovery in the rat hearts subjected to global ischemia and increased the infarct size when given before ischemia or upon reperfusion. These results suggest that AR is a cardioprotective protein and that its activation in the ischemic heart is due to peroxynitrite-mediated oxidation of Cys-298 to sulfenic acid via the PI3K/Akt/endothelial NOS pathway.

Myocardial susceptibility to ischemic-reperfusion injury in a prediabetic model of dietary-induced obesity

We assessed the myocardial susceptibility to ischemic-reperfusion injury in obese rat hearts in the absence and the presence of predicted circulating concentrations of insulin and fatty acids. Feeding rats a high-calorie diet resulted in increases in body weight, visceral fat content, cardiac hypertrophy, plasma insulin, nonesterified free fatty acid, and triglyceride concentrations. In the absence of both insulin and fatty acids in the coronary perfusate, the hearts of obese rats developed an increased infarct size (41.9 +/- 1.9% for obese vs. 22.9 +/- 2.3% for control, P < 0.05) and a reduced percent recovery of aortic output (4.2 +/- 4.2% for obese vs. 27.7 +/- 3.4% for controls, P < 0.05) after coronary artery occlusion and reperfusion. In the presence of insulin in the coronary perfusate, a cardioprotective effect was noted in both groups, an action that was greater in hearts from obese compared with control rats and which abolished the obesity-induced changes in infarct size (13.8 +/- 1.2% for controls vs. 21.0 +/- 1.6% for obese), and percent recovery of aortic output (60.2 +/- 4.7% for controls vs. 45.7 +/- 9.4% for obese). Fatty acids (0.7 mM, control; and 1.5 mM, obese) added to the coronary perfusate with in vivo concentrations of insulin dramatically increased infarct size (48.2 +/- 3.1% for obese, and 37.5 +/- 2.7% for control; P < 0.05 vs. without fatty acids) and decreased percent aortic output recovery (control, 10.4 +/- 5.2%, and obese 7.8 +/- 3.5%; P < 0.05 vs. without fatty acids) in both groups to similar values. In conclusion, in obesity, the impact of an increased susceptibility of the myocardium to ischemic-reperfusion injury on myocardial injury is likely to be overshadowed by the comparatively greater roles played by predicted increases in circulating insulin and fatty acids found in vivo. These data support the notion that adiposity per se is unlikely to be a valuable predictor of outcomes in ischemic-reperfusion injury.

Metabolic response to an acute jump in cardiac workload: effects on malonyl-CoA, mechanical efficiency, and fatty acid oxidation

Inhibition of myocardial fatty acid oxidation can improve left ventricular (LV) mechanical efficiency by increasing LV power for a given rate of myocardial energy expenditure. This phenomenon has not been assessed at high workloads in nonischemic myocardium; therefore, we subjected in vivo pig hearts to a high workload for 5 min and assessed whether blocking mitochondrial fatty acid oxidation with the carnitine palmitoyltransferase-I inhibitor oxfenicine would improve LV mechanical efficiency. In addition, the cardiac content of malonyl-CoA (an endogenous inhibitor of carnitine palmitoyltransferase-I) and activity of acetyl-CoA carboxylase (which synthesizes malonyl-CoA) were assessed. Increased workload was induced by aortic constriction and dobutamine infusion, and LV efficiency was calculated from the LV pressure-volume loop and LV energy expenditure. In untreated pigs, the increase in LV power resulted in a 2.5-fold increase in fatty acid oxidation and cardiac malonyl-CoA content but did not affect the activation state of acetyl-CoA carboxylase. The activation state of the acetyl-CoA carboxylase inhibitory kinase AMP-activated protein kinase decreased by 40% with increased cardiac workload. Pretreatment with oxfenicine inhibited fatty acid oxidation by 75% and had no effect on cardiac energy expenditure but significantly increased LV power and LV efficiency (37 +/- 5% vs. 26 +/- 5%, P < 0.05) at high workload. In conclusion, 1) myocardial fatty acid oxidation increases with a short-term increase in cardiac workload, despite an increase in malonyl-CoA concentration, and 2) inhibition of fatty acid oxidation improves LV mechanical efficiency by increasing LV power without affecting cardiac energy expenditure.

Increased hepatic CD36 expression contributes to dyslipidemia associated with diet-induced obesity

OBJECTIVE

The etiology of type 2 diabetes often involves diet-induced obesity (DIO), which is associated with elevated plasma fatty acids and lipoprotein associated triglycerides. Since aberrant hepatic fatty acid uptake may contribute to this, we investigated whether increased expression of a fatty acid transport protein (CD36) in the liver during DIO contributes to the dyslipidemia that precedes development of type 2 diabetes.

RESEARCH DESIGN AND METHODS

We determined the effect DIO has on hepatic CD36 protein expression and the functional consequence of this in terms of hepatic triglyceride storage and secretion. In addition, in vivo adenoviral gene delivery of CD36 to the livers of lean mice was performed to determine if increased hepatic CD36 protein was sufficient to alter hepatic fatty acid uptake and triglyceride storage and secretion.

RESULTS

During DIO, CD36 protein levels in the liver are significantly elevated, and these elevated levels correlate with increased hepatic triglyceride storage and secretion. These alterations in liver lipid storage and secretion were also observed upon forced expression of hepatic CD36 in the absence of DIO and were accompanied with a marked rise in hepatic fatty acid uptake in vivo, demonstrating that increased CD36 expression is sufficient to recapitulate the aberrant liver lipid handling observed in DIO.

CONCLUSIONS

Increased expression of hepatic CD36 protein in response to DIO is sufficient to exacerbate hepatic triglyceride storage and secretion. As these CD36-mediated effects contribute to the dyslipidemia that often precedes the development of type 2 diabetes, increased hepatic CD36 expression likely plays a causative role in the pathogenesis of type 2 diabetes.