Published online before print April 22, 2004, doi: 10.1161/01.RES.0000129255.19569.8f
© 2004 American Heart Association, Inc.
UltraRapid Communication |
Malonyl Coenzyme A Decarboxylase Inhibition Protects the Ischemic Heart by Inhibiting Fatty Acid Oxidation and Stimulating Glucose Oxidation
From the Cardiovascular Research Group (J.R.B.D., G.D.L.), Departments of Pediatrics and Pharmacology, Faculty of Medicine, University of Alberta, Edmonton, Alberta, Canada; Chugai Pharma USA LLC (J.-F.C., S.B., D.W., T.A., C.H., G.Y., A.M.N.), San Diego, Calif; Department of Physiology and Biophysics (W.C.S., M.P.C.), Case Western Reserve University, Cleveland, Ohio; Metabolic Modulators Research Ltd. (J.R.B.D., R.B., G.D.L.) 2020 Research Transition Facility, University of Alberta, Edmonton, Alberta, Canada.
Correspondence to Dr Jason R.B. Dyck, 474 Heritage Medical Research Centre, The University of Alberta, Edmonton, Alberta, T6G 2S2 Canada. E-mail Jason.Dyck{at}UAlberta.ca; or Dr Gary D. Lopaschuk, 423 Heritage Medical Research Centre, The University of Alberta, Edmonton, Alberta, T6G 2S2 Canada. E-mail Gary.Lopaschuk@UAlberta.ca
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Abstract |
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Abnormally high rates of fatty acid oxidation and low rates of glucose oxidation are important contributors to the severity of ischemic heart disease. Malonyl coenzyme A (CoA) regulates fatty acid oxidation by inhibiting mitochondrial uptake of fatty acids. Malonyl CoA decarboxylase (MCD) is involved in the decarboxylation of malonyl CoA to acetyl CoA. Therefore, inhibition of MCD may decrease fatty acid oxidation and protect the ischemic heart, secondary to increasing malonyl CoA levels. Ex vivo working rat hearts aerobically perfused in the presence of newly developed MCD inhibitors showed an increase in malonyl CoA levels, which was accompanied by both a significant decrease in fatty acid oxidation rates and an increase in glucose oxidation rates compared with controls. Using a model of demand-induced ischemia in pigs, MCD inhibition significantly increased glucose oxidation rates and reduced lactate production compared with vehicle-treated hearts, which was accompanied by a significant increase in cardiac work compared with controls. In a more severe rat heart global ischemia/reperfusion model, glucose oxidation was significantly increased and cardiac function was significantly improved during reperfusion in hearts treated with the MCD inhibitor compared with controls. Together, our data show that MCD inhibitors, which increase myocardial malonyl CoA levels, decrease fatty acid oxidation and accelerate glucose oxidation in both ex vivo rat hearts and in vivo pig hearts. This switch in energy substrate preference improves cardiac function during and after ischemia, suggesting that pharmacological inhibition of MCD may be a novel approach to treating ischemic heart disease.
Key Words: metabolism • fatty acids • glucose • ischemia • malonyl CoA decarboxylase
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Introduction |
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The heart has a high-energy demand that is primarily met by the oxidation of fatty acids in the mitochondria. The oxidation of fatty acids is a highly regulated process ensuring that energy supply matches energy demand. Carnitine palmitoyltransferase 1 (CPT-1) is the key enzyme involved in the mitochondrial uptake of fatty acids1,2 and is inhibited by endogenous malonyl coenzyme A (CoA).2–5 Therefore, malonyl CoA acts as physiologic regulator of fatty acid oxidation in the heart through inhibition of CPT-1, which decreases fatty acid transport into the mitochondria and its subsequent oxidation. Malonyl CoA has a rapid turnover in the heart6 and is produced primarily by the carboxylation of acetyl CoA by acetyl CoA carboxylase (ACC). Studies from our laboratory7 and others8–12 have implicated malonyl CoA decarboxylase (MCD) as being responsible for malonyl CoA degradation back to acetyl CoA. Although it has been suggested that MCD modulates malonyl CoA12 and subsequently regulates fatty acid oxidation in the heart,7,13 the evidence supporting this has only been correlative.
Although many mechanisms contribute to ischemic injury,14 there is clear evidence that contractile dysfunction during and after myocardial ischemia is mediated, at least in part, by the type of energy substrate metabolized by the heart. For instance, an excessive use of fatty acids by the heart during and after ischemia contributes to contractile dysfunction and ischemic injury.15 These detrimental effects of high fatty acid oxidation appear to occur secondary to an inhibition of glucose oxidation in the heart.15 During ischemia, glycolysis is stimulated, and as a result, low rates of glucose oxidation increase the production of protons and lactate. When this occurs, there is a decrease in cardiac efficiency as energy produced by the heart, in the form of ATP, is redirected away from contractile function and toward re-establishment of H+, Na+, and Ca2+ ionic homeostasis.16–18 After ischemia, fatty acid oxidation also dominates as a source of energy by the heart.15,19,20 This results in a continued low rate of glucose oxidation and a continued decrease in cardiac efficiency during the critical period of reperfusion.
The primary reasons why fatty acid oxidation rates are high during and after ischemia are attributable to the fact that circulating plasma levels of fatty acids are dramatically elevated during and after ischemia21 and that there are direct alterations in the subcellular control of fatty acid oxidation in the heart. One of these changes in fatty acid oxidation control is a dramatic decrease in malonyl CoA levels during and after ischemia.3,7 This is because of an activation of AMP-activated protein kinase, which phosphorylates and inhibits ACC activity and subsequent malonyl CoA production.5 The combination of high-circulating fatty acid levels and a decrease in malonyl CoA control of mitochondrial fatty acid uptake results in the preferential use of fatty acids as an oxidative substrate over glucose during and after ischemia.15,17,22,23 This phenomenon is based on the Randle cycle, in which fatty acid-derived acetyl CoA can decrease the production of glucose-derived acetyl CoA via inhibition of the pyruvate dehydrogenase complex. Because the regulation of malonyl CoA levels is central to the control of fatty acid oxidation rates in the heart, MCD may play a major role in controlling the extent of ischemic injury by promoting glucose oxidation.
The purpose of this study was to directly determine the importance of MCD in controlling malonyl CoA and fatty acid oxidation in the heart and to determine whether inhibition of MCD could increase malonyl CoA levels, decrease fatty acid oxidation, increase glucose oxidation, and protect the ischemic heart. This was achieved by using novel MCD inhibitors (MCDis) in both the in vivo and ex vivo heart.
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Materials and Methods |
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Animal Care
The University of Alberta adheres to the principles for biomedical research involving rats developed by the Council for International Organizations of Medical Sciences and complied with the Canadian Council on Animal Care guidelines. Pig studies were conducted in accordance with the Guide for the Care and Use of Laboratory Animals (National Institutes of Health publication No. 85-23, revised 1996) and the Institutional Animal Care and Use Committee at Case Western Reserve University.
In Vitro MCD Inhibitory Activity Assay
The decarboxylase activity of recombinant human MCD (hMCD) was measured spectrophotometrically by monitoring the acetyl CoA formation using the malate dehydrogenase/citrate synthase-coupling system.24 The conversion of acetyl CoA from malonyl CoA was assayed using a modified protocol as described previously.7 Assays were initiated by the addition of malonyl CoA, and the rates were corrected for the background rate determined in the absence of hMCD. Kinetic data analysis was performed as described (see online data supplement available at http://circres.ahajournals.org).
MCD Inhibitors
MCDis were synthesized at Chugai Pharma USA, LLC. The detailed account of discovery and development of small molecule MCDis will be published in separate articles.
Rat Heart Perfusions
Sprague-Dawley rats (300 to 350 g) were anesthetized with sodium pentobarbital (60 mg·kg–1). Hearts were then excised and placed in ice-cold Krebs-Henseleit bicarbonate solution and perfused as described (see online data supplement) with either [5-3H/U-14C]-labeled glucose or [9,10-3H]-labeled palmitate for glycolysis, glucose oxidation, and palmitate oxidation measurements, respectively. Spontaneously beating hearts were perfused for: (1) a 60-minute aerobic period with the vehicle or 20 µmol/L CBM-300864 (MCDi) present for the entire 60 minutes; (2) a 30-minute aerobic period followed by 30 minutes of mild ischemia (35% reduction in coronary flow) with the vehicle or 20 µmol/L CBM-300864 (MCDi) present for the entire 60 minutes; or (3) 30 minutes of aerobic perfusion followed by 30 minutes of global no-flow ischemia and 60 minutes of aerobic reperfusion with the vehicle or 1 µmol/L CBM-301940 added, either at the onset of the perfusion (preischemic) or at the onset of aerobic reperfusion after ischemia (postischemic). For the global ischemia/reperfusion protocols, coronary flow was reduced to zero for a 30-minute period, after which hearts were aerobically reperfused as described previously.7 For the mild ischemia model, the perfusion apparatus contained a 1-way valve in the aortic outflow line that does not impede systolic ejection but restricts diastolic perfusion of the coronary arteries. Coronary flow, measured continuously from the difference in flow between cardiac output (left atrial inflow) and aortic flow, was controlled by a back-flow controller that permitted ejected perfusate to bypass the 1-way valve and return to enter the coronary circulation. At the end of the aerobic period (60 minutes), mild ischemic periods, or reperfusion periods (60 minutes), the ventricle from the heart was frozen immediately in liquid nitrogen (N2) and the mass recorded.
Measurement of Fatty Acid Oxidation, Glucose Oxidation, and Glycolysis
Rates of glucose oxidation, glycolysis, and palmitate oxidation were measured in 2 separate series of heart perfusions as described (see online data supplement).
Calculated Proton Production
Proton production from glucose metabolism is calculated as 2x(rate of glycolysis–rate of glucose oxidation), which takes into consideration the net production of 2 protons per molecule of glucose that passes through glycolysis that is not subsequently oxidized. An increase in proton production indicates increased uncoupling of glycolysis from glucose oxidation.
Determination of CoA Esters
Detection and quantification of CoA esters were performed by extracting CoA esters from powdered tissue into 6% perchloric acid and measuring with a modified high-performance liquid chromatography (HPLC) as described.7 (see online data supplement).
Enzymatic Assays
Either the mitochondrial or cytosolic fractions of rat heart tissue were assayed, as described for MCD12, ACC,7 CPT-1,25 and long-chain 3-ketoacyl CoA thiolase activities.26
Model of Demand-Induced Ischemia in Pigs
Experiments were performed on 16 domestic pigs (vehicle n=8, mean weight 35.9±1.8 kg; CBM-300864 n=8, mean weight 37.3±1.0 kg). Pigs were fasted overnight, sedated, and the left anterior descending coronary artery (LAD) was perfused via a roller pump with blood supplied from the femoral artery as described in detail previously.27 This preparation allowed the LAD perfusion bed to be subjected to demand-induced ischemia by decreasing LAD flow by 20% and infusing dobutamine to increase heart rate and cardiac contractility. Animals were heparinized and given an intravenous infusion of 20% triglyceride emulsion to increase fatty acid concentration to levels observed in angina patients (
0.8 mmol/L).27 After completion of the instrumentation, a continuous infusion of [U-14C] glucose (0.2 µCi/min) was introduced into the proximal end of the coronary perfusion line. After 30 minutes of tracer infusion, an infusion of either vehicle or CBM-300864 was initiated directly into the coronary perfusion circuit at a rate set to result in a step increase of 100 µmol/L CBM-300864 in the coronary arterial blood. Arterial and interventricular venous samples were drawn at 20 and 27 minutes after treatment. Thirty minutes after starting treatment, demand-induced ischemia was initiated with an infusion of dobutamine (15 µg · kg–1 · min–1) to increase myocardial oxygen demand and by reducing LAD blood flow by 20% for a period of 15 minutes. Arterial and anterior interventricular venous blood samples were then taken at 3, 6, 10, and 15 minutes after demand-induced ischemia. Blood samples were analyzed for the concentrations of oxygen, lactate, and glucose in blood and plasma-free fatty acids. In addition, samples were analyzed for14C-glucose and14CO2 calculation of the rates of glucose oxidation, as described previously.27 Heart rate, left ventricular pressure (LVP), peak positive and negative rates of developed pressure, and segment length were continuously recorded using a commercial online data acquisition system. After 18 minutes of ischemia, myocardial biopsies from the LAD perfusion bed (2 g) were freeze clamped (3 to 5 seconds) on aluminum blocks precooled in N2 and stored at –80°C for subsequent analysis of tissue malonyl CoA content. The LVP-segment-length loop area was used as an index of external wall work of the anterior-free wall as described previously.27 Vehicle and treatment groups were compared using an unpaired t test with significance set at P<0.05.
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MCD Inhibitors
To understand the role of MCD in the regulation of fatty acid oxidation in the heart and to evaluate the potential therapeutic value of MCD inhibition, an MCDi discovery program was initiated using human recombinant MBP fusion MCD protein. A high throughput screening for MCD inhibitory activity was performed on an in-house small molecule compound library at Chugai, which led to the discovery of several weak inhibitors. Structural optimization of 1 of these series led to the development of 2 potent inhibitors: CBM-300864 (Ki=98 nM) and CBM-301940 (Ki=28 nM) (Table 1). In addition, CBM-301940 and CBM-300864 significantly inhibited both rat and pig heart MCD in an in vitro assay to the same extent with CBM-300864, inhibiting rat heart MCD from 8472±833 to 416±23 nmol · g dry wt–1 · min–1 and pig heart MCD from 4167±278 to 250±23 nmol · g dry wt–1 · min–1. Neither of these compounds inhibited other key enzymes that control malonyl CoA or fatty acid oxidation in the heart, including ACC, CPT-1, or long-chain 3-ketoacyl CoA thiolase activities (data not shown).
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MCD Inhibition Increases Glucose Oxidation in Isolated Working Rat Hearts
With potent MCDis in hand, it became possible to directly examine the importance of MCD in regulating fatty acid oxidation. Ex vivo working rat hearts were aerobically perfused for 60 minutes with Krebs-Henseleit solution containing either 5 mmol/L [U-14C]glucose and 1.2 mmol/L [9,10-3H]palmitate or 5 mmol/L [5-3H]glucose and 1.2 mmol/L [1-14C]palmitate.17 The concentration of fatty acid used is equivalent to plasma concentrations that have been observed clinically in patients experiencing myocardial ischemia.21,28 Rates of glycolysis, glucose oxidation, and fatty acid oxidation were measured directly from the production of3H2O and14CO2 as described.18 In all instances, either 20 µmol/L CBM-300864 dissolved in 0.1% dimethyl sulfoxide (DMSO; drug treatment group) or 0.1% DMSO alone (control group) was added at the beginning of the perfusion. The mechanical function of the MCDi-treated hearts was not significantly different from control hearts (data not shown), demonstrating that MCD inhibition has no adverse effects on cardiac function or coronary flow. At the end of the perfusion, hearts were frozen and malonyl CoA levels were determined by HPLC analysis.29 Treatment with the MCDi CBM-300864 effectively blocked malonyl CoA degradation, as demonstrated by a 7-fold increase in myocardial malonyl CoA content (Figure 1A). In addition, inhibition of MCD significantly suppressed fatty acid oxidation (Figure 1B) and increased glucose oxidation compared with control hearts (Figure 1C) without altering the rate of glycolysis (Figure 1D). MCD inhibition resulted in a dramatic switch in energy substrate preference such that glucose metabolism became the predominant source of energy in the heart (Figure 1E). Therefore, our data show that MCD controls intracellular malonyl CoA levels and is a major regulator of cardiac fatty acid oxidation rates. These data also demonstrate that MCD inhibition can significantly increase glucose oxidation secondary to decreasing palmitate oxidation while having no effect on glycolytic rates.
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MCD Inhibition Decreases Proton Production During Mild Ischemia in Rat Hearts
Because it is well established that there is a benefit to partially suppressing myocardial fatty acid oxidation and increasing glucose oxidation during myocardial ischemia, we evaluated the effects of CBM-300864 using a model of mild global ischemia. Ex vivo working rat hearts were subjected to a mild global ischemia by reducing coronary flow by 35% for a 30-minute period, which reduced cardiac function to 40% to 50% of normal aerobic values (Table 2). Despite the high levels of palmitate in the perfusate, MCD inhibition significantly reduced fatty acid oxidation in the ischemic period (484±55 versus 328±50 nmol · g dry wt–1 · min–1, control and MCDi, respectively) and accelerated glucose oxidation during both the aerobic and the ischemic periods (Figure 1F). Because accelerated glucose oxidation rates improve the coupling of glucose oxidation to glycolysis, the drug-treated group demonstrated a significant reduction in calculated proton production from anaerobic glucose metabolism compared with control hearts during the ischemic period (Figure 1G).
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MCD Inhibition Improves Cardiac Function in Demand-Induced Ischemia in Pigs
Increasing glucose oxidation without affecting glycolytic rate reduces myocardial lactate production during ischemia. A reduction in lactate production may be the mechanism by which partial inhibitors of myocardial fatty acid oxidation improve symptoms during demand-induced ischemia in patients with chronic stable angina.27,30 Therefore, we tested the effects of MCD inhibition in a swine model of demand-induced ischemia and measured rates of glucose oxidation, lactate production, and cardiac work. This model produces ischemia as a result of flow restriction and dobutamine stimulation of heart rate and contractility, with no change in myocardial oxygen consumption.27,31 Treatment with CBM-300864 (100 µmol/L in arterial blood) during the normal flow period had no effect on heart rate, peak systolic LVP, peak positive or negative LVdP/dt, or LV external work (as measured from the segment length-LVP loop area). Pig hearts treated with the MCDi CBM-300864 showed a significant increase in malonyl CoA levels during the demand-induced ischemic period compared with vehicle-treated hearts (Figure 2A). During this period, the percentage of glucose taken up by the myocardium that underwent immediate oxidation to CO2 was doubled in the MCDi-treated animals, whereas lactate production was significantly reduced (Figure 2B and 2C). Accompanying this decrease in lactate production was a significant increase in external work in the ischemic region of the left ventricle (Figure 2D), demonstrating that MCD inhibition improves cardiac function during demand-induced ischemia.
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MCD Inhibition Improves Cardiac Function During Reperfusion of Severely Ischemic Rat Hearts
To determine whether MCD inhibition can improve cardiac function during reperfusion of severely ischemic hearts, the effects of MCD inhibition on functional recovery of reperfused ischemic hearts were determined. To lessen the likelihood that our MCDi was inhibiting another enzyme, a more potent novel MCDi (CBM-301940) was identified and used in ex vivo aerobically perfused rat hearts. At 1 µmol/L, CBM-301940 was equally efficacious in stimulating glucose oxidation as CBM-300864 (Figure 3B, aerobic period) but was significantly more potent. The MCDi CBM-301940 (1 µmol/L) was examined in ex vivo working rat hearts aerobically perfused for 30 minutes, followed by 30 minutes of global no-flow ischemia and 60 minutes of aerobic reperfusion, as described previously.18 Hearts were treated with the MCDi either before ischemia or at the onset of reperfusion to determine whether MCD inhibition was protective during or after ischemia. Hearts treated with the MCDi before ischemia had no significant alterations in mechanical function. However, cardiac function on aerobic reperfusion after ischemia was significantly improved in hearts treated with CBM-301940 when added either before or after ischemia compared with control hearts (Table 3, Figure 3A and 3C). In addition, MCD inhibition significantly improved rate pressure product, cardiac output, aortic flow, coronary flow, and cardiac power on aerobic reperfusion of ischemic hearts (Table 3). Moreover, MCD inhibition stimulated glucose oxidation rates both before and after ischemia compared with vehicle-treated hearts (Figure 3B and 3D). Similar effects of CBM-301940 on recovery of cardiac function (Table 3, Figure 3C) and glucose oxidation (Figure 3D) were seen when the drug was added in reperfusion, suggesting that the primary beneficial effect was occurring during reperfusion. These data show that the alteration in cardiac substrate preferences produced by MCD inhibition leads to improved cardiac function during reperfusion of severely ischemic hearts.
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Discussion |
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High fatty acid oxidation rates inhibit glucose oxidation, which can decrease cardiac efficiency and contractile function during and after ischemia. Although fatty acid oxidation does not increase during ischemia, it does predominate as the major residual source of oxidative metabolism. During ischemia, mitochondrial oxidative metabolism decreases in proportion to the decrease in oxygen consumption.32 This includes both a decrease in fatty acid oxidation and a decrease in glucose oxidation.31,33,34 However, the residual fatty acid oxidation predominates over glucose oxidation as the main source of residual oxidative metabolism (which is attributable, in part, to the high levels of fatty acids to which the ischemic myocardium is exposed). As originally shown by Neely et al,33 in isolated working rat hearts perfused with 1.2 mmol/L palmitate, although oxidative rates decrease during ischemia, oxidation of palmitate accounts for the largest proportion of oxygen consumed during moderate ischemia (70%) and severe ischemia (90%). This finding has been confirmed in the in vivo pig heart.33,34 This predominance of fatty acid oxidation to residual oxidative metabolism, at the expense of glucose oxidation, occurs during a period when glycolysis has increased to compensate for the decrease in ATP supply. As a result, an uncoupling of glycolysis from glucose oxidation occurs, resulting in lactate production and acidosis (Figure 1G). For this reason, we believe that inhibition of "residual" fatty acid oxidation during ischemia will increase glucose oxidation, improve the coupling of glycolysis to glucose oxidation, lessen lactate and proton production, and improve cardiac efficiency. Therefore, a therapeutic goal in the treatment of ischemia would be to inhibit fatty acid oxidation, thereby stimulating glucose oxidation.
In this report, we provide evidence that MCD is a major regulator of cardiac fatty acid oxidation, secondary to modifying intracellular malonyl CoA levels. Rat hearts perfused in the presence of the MCDi (CBM-300864) showed a significant increase in intracellular malonyl CoA levels and a corresponding decrease in palmitate oxidation rates. This was accompanied by large increases in glucose oxidation rates. Because palmitate provides 8-acetyl CoA molecules to the Krebs cycle compared with 2-acetyl CoA molecules from glucose, a decrease in the oxidation of 1 palmitate molecule will result in an increase in the oxidation of 4 glucose molecules. As a result, inhibition of fatty acid oxidation by MCD inhibition resulted in dramatic increases in glucose oxidation (Figures 1, 2, and 3
). Even in the presence of high levels of fatty acids, in which glucose oxidation provided only 13% of total ATP production under control conditions (Figure 1E), the presence of an MCDi increased the contribution of glucose oxidation to ATP production by 3.8-fold. This resulted in glucose oxidation becoming a major source of energy (49% of total ATP). These data suggest the importance of MCD in energy substrate selection in the heart.
During and after ischemia, rates of glycolysis are high and glucose oxidation rates are low.15,19,20 This uncoupling of glucose oxidation from glycolysis increases the production of protons and lactate and decreases cardiac efficiency.17 Using a model of mild global ischemia, we show that MCD inhibition can decrease fatty acid oxidation and increase glucose oxidation during ischemia and improve the coupling of glucose oxidation to glycolysis, thus reducing proton production during the ischemic period. Furthermore, using a demand-induced ischemia model in pigs, we show that MCD inhibition increases malonyl CoA levels, increases glucose oxidation, reduces lactate production, and increases external work in the ischemic region of the left ventricle. Together, these data demonstrate that MCD inhibition improves cardiac function during mild ischemia.
We also show that MCD inhibition improves cardiac function during reperfusion of severely ischemic hearts. Hearts treated with MCDis before ischemia had significantly improved cardiac function compared with control hearts, which were accompanied by an increase in glucose oxidation during reperfusion. Although the energetics during ischemia are extremely important, it has been shown that after ischemia, fatty acid oxidation also dominates as a source of energy by the heart.4,5,15,19,20 Although we did not measure fatty acid oxidation rates during reperfusion in this study, it has already been shown that during reperfusion of ischemic hearts, there is a close relationship between fatty acid oxidation and glucose oxidation, with high rates of fatty acid oxidation resulting in stoichiometric decreases in glucose oxidation.15 This results in a continued decrease in cardiac efficiency during the critical period of reperfusion. We have observed previously that stimulation of glucose oxidation during reperfusion can improve function recovery,4,15–18 an effect we also observed with MCD inhibition. For this reason, MCDis also appear to be beneficial when added after ischemia and before reperfusion. These data suggest that the alteration in cardiac substrate preferences produced by MCD inhibition leads to improved cardiac function during reperfusion of severely ischemic hearts.
In this study, we show that MCD inhibition increases malonyl CoA levels and switches energy substrate preference from fatty acid oxidation to glucose oxidation. We propose that this increase in glucose oxidation is responsible for the beneficial effect of malonyl CoA on contractile function and cardiac efficiency. However, it cannot be ruled out that malonyl CoA may be acting by altering other mechanisms, including calcium handling, pH, altering redox potential or ATP levels. To our knowledge, there are no reports of malonyl CoA directly impacting either calcium or pH regulatory enzymes in the heart. However, stimulation of glucose oxidation in the heart will decrease proton production and acidosis in the heart.17 These beneficial effects on pH have the potential to decrease calcium accumulation, secondary to a decrease in Na+/H+ exchange and Na+/Ca2+ exchange activity. It is also possible that increasing malonyl CoA alters redox state or ATP levels. However, alterations in these parameters would be expected to occur because of an increase in energy supply, as opposed to a switch in energy substrate preference. Alternatively, redox state and ATP could be increased but secondary to an increase in cardiac function (which was observed in this study). As a result, further studies are needed to examine the relationship between alterations in malonyl CoA levels and calcium handling, pH, redox state, and ATP levels.
In summary, this report describes the effects of novel MCDis on energy metabolism in the heart and provides evidence that MCD is a major regulator of cardiac fatty acid oxidation, secondary to modifying intracellular malonyl CoA levels. Our data also show that MCD inhibition accelerates glucose oxidation in both ex vivo and in vivo hearts, and that MCD inhibition improves both functional recovery during reperfusion of an ischemic rat heart and cardiac work during demand-induced ischemia. Although MCDis require further evaluation, our data suggest that pharmacological inhibition of MCD may be a viable approach to the treatment of clinical pathologies associated with myocardial ischemia.
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Acknowledgments |
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J.R.B.D. is a scholar of the Alberta Heritage Foundation for Medical Research and a Canadian Institutes of Health Research New Investigator. G.D.L. is a medical scientist of the Alberta Heritage Foundation for Medical Research.
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Footnotes |
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A portion of this work was conducted in collaboration with Chugai Pharma USA, LLC of San Diego, California, the company that developed, synthesized, and owns the MCDis described in this publication. In addition, Metabolic Modulators Research Ltd, a University of Alberta spin-off company, was also involved in the testing of these new inhibitors. The authors and their respective affiliations have been disclosed in the author list.
Original received January 22, 2004; revision accepted April 13, 2004.
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References |
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