Cardiovascular Research Centre, Mazankowski Alberta Heart Institute, University of Alberta, Edmonton, AB, Canada
Address correspondence to: Gary Lopaschuk, 423 Heritage Medical Research Center, University of Alberta, Edmonton, AB, Canada T6G 2S2. Tel: +780-492-2170. Fax: +780-492-9753. E-mail: firstname.lastname@example.org
Heart disease is the leading cause of death worldwide . This is evident despite the many recent advances made in the prevention, diagnosis, and treatment of heart diseases. As a result, new cardioprotective approaches are needed to treat heart disease. Metabolic modulation is one such promising strategy for the treatment of heart disease . This is because alterations in energy metabolism are involved in the progression of many types of heart disease . Inadequate oxygen and energy supply to the heart is a major cause of cell death in ischemic heart disease . Impaired cardiac energetics also contributes to the severity of heart failure [3-6]. In addition, switches in energy substrate utilization by the heart can also contribute to the severity of heart disease . For instance, excessive use of fatty acids as an energy source can contribute to the development of cardiomyopathies in diabetes and obesity [2, 7, 8]. Importantly, this is likely not confounded by changes in body weight since, for example, PPARα overexpressing transgenic mice have impaired cardiac function but do not have a significantly different body weight than their wild-type (WT) counterparts . It is important to mention, however, that in heart failure, fatty acids are not necessarily detrimental as there are a number of studies that have reported that a high-fat/low-carbohydrate diet when compared with a low-fat/high-carbohydrate diet can prevent and treat heart failure in weight neutral circumstances . The reason for this is not well understood but it is a fatty acid-type specific effect and the unsaturated fatty acids, a major component of many of these high-fat diets (HFDs), partially exert their beneficial effects by decreasing inflammation . A switch from mitochondrial oxidative metabolism to glycolysis in ischemic heart disease and heart failure also has the potential to contribute to contractile failure and cell death [2, 10, 11]. In general, a decrease in cardiac efficiency caused by a rise in cardiac fatty acid oxidation and/or an increase in the uncoupling of glycolysis from glucose oxidation can impair cardiac function and is a common contributing factor to the severity of cardiac disease.
One potential therapeutic strategy that has potential in the treatment of heart disease is inhibition of fatty acid oxidation. One approach being investigated that holds promise is inhibition of a key protein involved in the regulation of malonyl CoA, malonyl CoA decarboxylase (MCD). Preclinical studies have shown that MCD inhibition can be beneficial in the setting of ischemia, heart failure, and insulin resistance [8, 12-16]. In this review, we focus on the importance of malonyl CoA in the regulation of cardiac energy metabolism in the normal and diseased heart and will discuss the evidence that modulating cardiac malonyl CoA levels is a promising strategy to treat cardiac disease.
Overview of Cardiac Fatty Acid Oxidation
The heart derives most of its energy from the metabolism of fatty acids and carbohydrates, primarily through mitochondrial oxidative metabolism. Fatty acids are taken up by the cell via fatty acid transports, such as CD36 [17-20]. The fatty acid is then esterified to CoA to form long-chain acyl CoA, prior to mitochondrial uptake of the fatty acids. A key enzyme involved in mitochondrial uptake of fatty acids is carnitine palmitoyl transferase 1 (CPT1), which transfers the fatty acids from long-chain acyl CoA to carnitine to form long-chain acylcarnitine (Fig. 1). This long-chain acylcarnitine is then transferred into the mitochondrial matrix by a carnitine translocase, where it is subsequently converted back to long-chain acyl CoA by CPT2 . Mitochondrial long-chain acyl CoA then enters the fatty acid β-oxidation pathway, which is coupled to the tricarboxylic acid (TCA) cycle and the electron transport chain for eventual production of ATP.
An important intracellular regulator of cardiac fatty acid oxidation is malonyl CoA. Malonyl CoA is a potent inhibitor of CPT1, and increases in cellular malonyl CoA levels result in a decreased uptake of long-chain fatty acids into mitochondria, and therefore a decrease in fatty acid oxidation rates [21-23]. Malonyl CoA levels in the heart are regulated by two proteins, acetyl CoA carboxylase (ACC) and MCD. ACC synthesizes malonyl CoA from acetyl CoA, whereas MCD degrades malonyl CoA back to acetyl CoA. Therefore, there are two main ways to increase malonyl CoA levels in the heart, increase ACC activity or decrease MCD activity [13, 15, 24]. ACC exists as two isoforms in the heart, ACC1 and ACC2, both of which are under a high degree of phosphorylation control. For instance, AMP-activated protein kinase (AMPK) phosphorylation of ACC inhibits enzyme activity, thereby decreasing cardiac malonyl CoA levels [25, 26]. AMPK phosphorylation of ACC is a major regulator of ACC activity, and increases in AMPK activity can decrease cardiac malonyl CoA levels and increase fatty acid oxidation in many forms of cardiac disease, as well as during cardiac development [27, 28]. In contrast to ACC, MCD is not highly regulated by phosphorylation, but rather by transcriptional control [29-31]. Increased expression and activity of MCD occurs under numerous conditions associated with increased cardiac fatty acid oxidation rates, which includes diabetes, obesity, and during postnatal development [27, 31, 32]. Increased cardiac MCD activity is associated with a decrease in malonyl CoA levels and a subsequent increase in mitochondrial fatty acid uptake and oxidation [27, 30, 32].
Fatty Acid Oxidation Regulation of Cardiac Function
The source of fuel for energy production in the heart can have important implications on cardiac function. For instance, elevated fatty acid oxidation rates can contribute to a decrease in cardiac function in a number of ways, including via an inhibition of glucose oxidation and a decrease in cardiac efficiency . Fatty acid oxidation and glucose metabolism are highly integrated in the heart, and the Randle Cycle shows that fatty acids can inhibit the metabolism both at the level of glycolysis and glucose oxidation . Fatty acid oxidation inhibits a number of key enzymes involved in glucose oxidation and glycolysis. The acetyl CoA and NADH produced from fatty acid oxidation inhibits pyruvate dehydrogenase (PDH) activity [35, 36]. In addition, the glycolytic enzyme phosphofructokinase 1 is inhibited by citrate produced by fatty acid oxidation [2, 37]. Therefore, increased rates of fatty acid oxidation result in decreased rates of glycolysis and glucose oxidation. However, in the heart, fatty acids inhibit glucose oxidation to a much greater extent than glycolysis, which can lead to an uncoupling of glycolysis from glucose oxidation . The consequence of this is decreased cardiac efficiency. Indeed, increasing fatty acid oxidation rates at the expense of glucose metabolism can dramatically increase the oxygen cost of contractility .
Decreased cardiac efficiency in hearts oxidizing high amounts of fatty acids occurs due to a number of reasons. This includes an increase in futile cycling of fatty acids through triacylglycerol and a dissipation of the mitochondrial proton gradient . In addition, fatty acids require more oxygen to produce the same amount of ATP. This is because while the fatty acid palmitate provides 105 ATP, it requires 23 O2, but the full oxidation of a single glucose molecule is more efficient providing 31 ATP while using 6 O2. Another reason for the decreased cardiac efficiency seen in hearts oxidizing high amounts of fatty acids is related to the increased uncoupling of glycolysis from glucose oxidation because of the dramatic inhibition of glucose oxidation when fatty acid oxidation increases [10, 33]. Uncoupling of glycolysis from glucose oxidation results in the production of H+s, which when cleared from the heart often involves an ATP-consuming process, thereby resulting in a reduction in cardiac efficiency. For instance, if the H+ is transported out of the cell via the Na+/H+ exchanger, the excess intracellular Na+ is then transported out of the cell by the Na+/Ca2+ exchanger or the Na+/K+ ATPase . The intracellular Ca2+ levels originating from the Na+/Ca2+ exchanger must then be transported out of the cell or into intracellular vesicles by Ca2+ ATPases . As a result, re-establishment of intracellular ionic homeostasis due to elevated H+ production increases ATP consumption for noncontractile processes, thereby decreasing cardiac efficiency .
Alterations in fatty acid oxidation occur in many forms of heart disease. In heart failure, there is a consensus that both cardiac ATP and phosphocreatine levels are decreased [2, 4, 5, 39-41]. This is likely due to the fact that the energy metabolism of the heart shifts back toward a fetal metabolism with reduced oxidative metabolism and increased rates of glycolysis [2, 4, 5, 11, 33, 39, 40, 42, 43]. In pressure- or volume-overload hypertrophied hearts, fatty acid oxidation rates are either normal or reduced, whereas glycolysis is elevated, which is accompanied by alterations in the expression and activity of glycolytic and oxidative enzymes [44-50]. Heart failure induced by coronary artery ligation is not associated with altered fatty acid oxidation, although these hearts do have increased uncoupling of glycolysis from glucose oxidation and reduced cardiac efficiency . During reperfusion of ischemic hearts, overall fatty acid oxidation rates are high due to increased circulating fatty acids and to a decrease in malonyl CoA control of fatty acid oxidation . In addition, cardiac fatty acid oxidation rates are elevated and glucose oxidation rates are reduced in diabetes and obesity [51-56]. Interestingly, these changes in cardiac energy metabolism occur before the development of cardiac insulin and glucose intolerance or cardiac hypertrophy . A decrease in glucose oxidation and insulin stimulation of glucose oxidation is also observed within 3 weeks of initiating a HFD in mice .
Role of Malonyl CoA in the Regulation of Cardiac Function and Energy Metabolism
Malonyl CoA is an important regulator of cardiac energy metabolism under both physiological and pathophysiological conditions. Cardiac malonyl CoA levels, which are reduced in the ischemic and diabetic heart [59, 60], may be responsible for increased fatty acid oxidation rates in the reperfused ischemic heart and the diabetic heart [10, 51-56]. A progressive decrease in malonyl CoA levels also occurs in the postnatal heart, which is accompanied by a dramatic rise in cardiac fatty acid oxidation rates . The decreases in malonyl CoA levels in the ischemic heart and the developing heart are due to an increase in AMPK activity and a decrease in ACC activity [27, 32, 59, 61], whereas an increase in the expression and activity of MCD contributes to the decrease in malonyl CoA levels in diabetic hearts and in developing hearts . Unlike diabetes, cardiac malonyl CoA levels are not altered in the hearts of obese mice fed a HFD [58, 62]. In the setting of heart failure, the literature is inconclusive as to whether cardiac malonyl CoA levels are altered or unchanged [63-65].
One key factor that may contribute to changes in malonyl CoA levels in the heart is fatty acids. Circulating fatty acids are elevated in diabetes, ischemia/reperfusion, and heart failure . Through their actions as PPAR agonists, fatty acids may regulate the expression of genes involved in regulating malonyl CoA levels. PPARα transcriptionally regulates MCD gene expression [30, 66]. This decrease in MCD expression is probably responsible for the elevated cardiac malonyl CoA levels accompanied by reduced cardiac fatty acid oxidation rates and, as expected, increased glucose oxidation rates in PPARα knockout (KO) mice . Furthermore, WY14643, a specific PPARα agonist, increases MCD mRNA expression in the heart . Cardiac MCD mRNA levels are elevated under conditions in which circulating fatty acids are elevated, including HFD, fasting, and streptozotocin-induced diabetes [31, 67]. However, this does not necessarily translate into changes in MCD activity or malonyl CoA levels as MCD activity is actually reduced in response to a HFD and in the Otusuka Long-Evans Tokushima Fatty Diabetic rat [31, 67].
Overall, these studies indicate that malonyl CoA levels are modulated under physiological and pathophysiological conditions as a mechanism to control fatty acid oxidation rates. As elevating fatty acid oxidation has been shown to decrease cardiac efficiency and cardiac function, malonyl CoA levels also modulate cardiac function. Therefore, elevating malonyl CoA levels could be an effective strategy for the treatment of heart disease.
Modulation of Malonyl CoA Levels in Heart Disease
Studies have indicated that malonyl CoA modulation could be an effective treatment for heart disease. Specifically, studies that have directly modulated the expression of a key protein that regulates malonyl CoA levels, MCD, provide some of the key evidence for the concept that malonyl CoA is an important regulator of cardiac energy metabolism and could therefore be an effective therapeutic target. For instance, administration of MCD inhibitors to pigs reduces cardiac fatty acid oxidation and increases glucose oxidation . This is accompanied by a decrease in lactate production in ischemic hearts . Furthermore, in the in vitro rat heart MCD inhibition improves the recovery of cardiac function following ischemia [12, 13]. As expected, during and following ischemia, MCD inhibition also reduced cardiac fatty acid oxidation and lactate production and increases glucose oxidation when compared with the control group [12, 13]. MCD inhibitors are also effective in treating inflammation. They reverse LPS induction of neonatal rat cardiomyocyte inflammation and increased fatty acid oxidation .
The results from studies using MCD KO mice also suggest that increasing malonyl CoA levels is beneficial in the setting of heart disease. As expected, cardiac malonyl CoA levels are elevated in MCD KO mice [8, 16]. Under normal conditions, the cardiac function of these mice is not different from WT mice [8, 16]. Despite this, there is no significant difference in fatty acid oxidation, glucose oxidation, or glycolysis rates between healthy MCD KO and WT hearts under normal aerobic conditions . The absence of reduced rates of palmitate oxidation or increased glucose oxidation may be due to changes in MCD KO metabolic protein expression. The changes that have been reported include an increase in CD36, UCP3, CPT1, and PDK4 protein expression in MCD KO hearts . Interestingly, even in the presence of insulin and/or HFD, cardiac fatty acid oxidation rates are not significantly different between WT and MCD KO mice . However, beneficial effects of MCD deletion can be seen under conditions that normally elevate fatty acid oxidation in the heart. For example, in WT obese mice fed a HFD, there is a marked decrease in insulin-stimulated glucose oxidation in the heart . In MCD KO mice, this HFD-induced insulin resistance is decreased, and glucose oxidation rates are significantly increased [8, 58]. In addition, although there is no difference in palmitate oxidation and glycolysis rates in reperfused MCD KO and WT hearts, MCD KO hearts do have higher glucose oxidation rates . When compared with WT mice, MCD KO mice subjected to a permanent coronary artery ligation that results in chronic heart failure have an improved cardiac function and an increased cardiac efficiency . In the settings of a HFD, ischemia/reperfusion, or heart failure, the absence of MCD protein does not impair cardiac function and, in the setting of heart failure and acute myocardial infarction, MCD deletion significantly improves cardiac function [8, 14-16]. Further, benefits have also been observed in MCD KO mice subjected to an acute myocardial infarction or to a HFD. When subjected to reversible coronary artery ligation and reperfusion, MCD KO mouse hearts show a lower infarct size and lower reperfusion-induced H+ production when compared with WT mice . MCD KO mice subjected to a HFD are also protected against glucose intolerance and insulin resistance [8, 58]. Some changes in protein expression may explain this increase in insulin sensitivity. The activity of PDH, the rate-limiting enzyme of glucose oxidation, is higher in MCD KO when compared with WT hearts of mice subjected to a HFD . In addition, cardiac pAkt and pGSK3β are higher in MCD KO mice when compared with WT mice on a HFD, an observation consistent with the improved insulin signaling seen in these hearts . Based on these studies, increasing malonyl CoA levels appear to be beneficial in the setting of heart disease.
Although studies involving MCD deletion are consistent with an increase in malonyl CoA and a decrease in fatty acid oxidation being beneficial in heart disease, other studies have proposed the opposite, that is, decreasing malonyl CoA and increasing fatty acid oxidation is beneficial in cardiac hypertrophy. This includes a recent study in which ACC2 in the heart is deleted, resulting in a decrease in malonyl CoA levels and an increase in fatty acid oxidation [24, 69]. Under normal conditions, cardiac function on ACC2 KO mice was not significantly different, and fatty acid oxidation rates were elevated when compared with WT mice [24, 69]. Interestingly, cardiac glucose oxidation rates were also elevated in the ACC2 KO mouse . In addition, left ventricular mass was lower in whole-body ACC2 KO mice . Overall, glucose uptake was increased in these hearts as well . In addition, PPARα, MCD, PDK4, and CPT1 protein expression are reduced in these hearts . Interestingly, if ACC2 KO mice were subjected to a pressure-overload cardiac hypertrophy by transverse aortic constriction (TAC), they showed less contractile dysfunction and improved energetics when compared with WT mice subjected to TAC . Although these results would suggest that lowering malonyl CoA may actually be desirable (as opposed to increasing it), a number of interesting observations about this study should be noted: 1) TAC-induced cardiac hypertrophy was also lower in ACC2 KO when compared with WT mice, 2) malonyl CoA levels did not actually decrease in hearts of ACC2 KO mice subjected to TAC, and 3) although TAC produced a marked increase in glycolysis and lactate production in WT hearts (consistent with an uncoupling of glycolysis from glucose oxidation), this was not observed in hearts of ACC2 KO mice subjected to TAC . As a result, although the beneficial effect on cardiac function following both MCD deletion and ACC2 deletion may seem at first paradoxical, it is not, because malonyl CoA levels are not reduced in ACCKO hearts and both scenarios result in improved coupling of glycolysis to glucose oxidation. We speculate that in the case of MCD deletion, this increased coupling is due to a switch from fatty acid oxidation to glucose oxidation, whereas in the case of ACC2 deletion, an increase in overall mitochondrial oxidative metabolism results in a decreased need for a compensatory increase in glycolysis. Therefore, elevating malonyl CoA levels still remain a promising strategy to improve cardiac energy metabolism and cardiac function.
Malonyl CoA Regulation: A Promising Therapy for Cardiac Disease
As discussed, there is an abundance of preclinical evidence that raising malonyl CoA levels and inhibiting fatty acid oxidation may be a beneficial therapy for heart disease (Fig. 2; see refs.  and  for reviews). In support of this, a number of clinical studies have shown that inhibiting fatty acid oxidation not only can be used to treat ischemic heart disease  but also heart failure [72-75]. Approaches used clinically to inhibit fatty acid oxidation include pharmacological inhibition of CPT 1 with perhexeline or etomoxir or direct inhibition of fatty acid β-oxidation with trimetazidine [71-75]. We propose that therapeutic strategies aimed at increasing cardiac malonyl CoA levels should decrease fatty acid oxidation, increase the coupling of glycolysis to glucose oxidation leading to improved cardiac efficiency, and improve cardiac function. MCD inhibitors are a promising class of drugs for the treatment of heart disease that increase malonyl CoA levels. There is evidence that MCD inhibitors improve cardiac efficiency by decreasing fatty acid oxidation and increasing the coupling of glycolysis and glucose oxidation resulting in improved heart function in healthy and diseased hearts [12, 13, 76, 77]. As mentioned earlier, malonyl CoA levels can be altered in heart disease. However, this is not always the case, such as in heart failure [63-65]. Despite this, MCD KO mice when compared with WT mice subjected to heart failure do show improved cardiac efficiency and function . This suggests that these drugs can be used to restore cardiac function in a broad range of cardiac diseases. It is also interesting to note that some of the other drugs that increase cardiac efficiency and function also increase cardiac malonyl CoA levels. For example, dichloroacetate, a drug that increases PDH activity, increases cardiac glucose oxidation, decreases fatty acid oxidation, and increases malonyl CoA levels . Future studies will need to be done to determine whether MCD inhibitors are effective in the long-term treatment of many diseases of the heart, including heart failure and diabetic cardiomyopathy.
Impaired cardiac efficiency is an important contributor to the severity of heart disease. Cardiac efficiency and cardiac function can be improved by inhibiting fatty acid oxidation. This can be achieved by increasing cardiac malonyl CoA levels in the heart, a key endogenous inhibitor of cardiac fatty acid oxidation. Overall, increasing malonyl CoA levels may be a promising strategy to treat many types of cardiac disease, including ischemia/reperfusion, heart failure, and insulin resistance. MCD inhibition is one approach to increasing cardiac malonyl CoA levels that can lead to improvement in cardiac function by decreasing fatty acid oxidation and increasing cardiac efficiency. Further work still needs to be done to verify that modulating the level of malonyl CoA, with drugs such as MCD inhibitors, is an effective strategy to treat heart disease in the clinical setting.
This study was supported by a grant from the Canadian Institutes of Health Research. G.D. Lopaschuk is an Alberta Heritage Foundation for Medical Research Scientist. N. Fillmore holds an Alberta Innovates Health Solutions Studentship.