Heart and Metabolism

Potential of gene therapy for the treatment of metabolic abnormalities in the heart*

Jason R.B. Dyck1,2, Martinus T. Spoor1,2,3 1Cardiovascular Research Group, Departments of 2Pediatrics and 3Cardiac Surgery, Faculty of Medicine, University of Alberta, Edmonton, Alberta, Canada

Correspondence: Dr Jason R.B. Dyck, 474 Heritage Medical Research Centre,
The University of Alberta, Edmonton, Alberta, Canada, T6G 2S2.
Tel: +1 780 4920314, fax: +1 780 4929753, e-mail: jason.dyck@ualberta.ca

Abstract

The heart is able to use glucose, fatty acids, or lactate as energy sources but meets its heavy metabolic demands via the production of ATP derived principally from fatty acid metabolism.
Abnormalities in heart metabolism have been described in both the newborn and adult populations as a result of inborn metabolic abnormalities or acquired heart disease. Inherited fatty acid oxidation and/or mitochondrial defects have clear associations with newborn heart pathology. In addition, cardiac hypertrophy and failure have well-documented abnormalities in heart metabolism, which may contribute to progression of the disease. While significant progress has been made in the understanding of the pathophysiology of heart failure, the main treatment modalities remain limited to medical and/or surgical therapy. Recent advances in the pharmacological manipulation of heart metabolism and promising clinical results from these treatments may produce more treatment options for heart failure patients. In addition to pharmacological approaches, there is the potential for gene therapy for the treatment of metabolic abnormalities in the heart. Gene therapy of the heart is already well studied at the bench and is being introduced at the bedside. The potential of metabolic manipulation of the heart using gene therapy has not been well appreciated but may produce novel approaches to the treatment of a variety of heart disorders. Heart Metab. 2002;18:4–9.

Keywords: Heart, metabolism, gene therapy

Introduction
The heart is capable of utilizing a variety of substrates in order to meet its extremely high energy demand. Under normal circumstances, the main fuels involved in maintaining cardiac function are glucose, lactate, and fatty acids. These substrates are used to produce ATP, which provides the heart with the energy needed to perform contractile work. Many of the enzymes that regulate the pathways involved in controlling substrate utilization are essential for proper energy metabolism.
However, disturbances or deficiencies in any of these enzymes have the potential to lead to metabolic cardiomyopathies that can severely impair the heart’s ability to perform properly [1]. The most severe impairments would presumably result in early embryonic death while the less severe defects would cause metabolic cardiomyopathies, which would present during the newborn period. Many of the identified metabolic cardiomyopathies originate from inherited metabolic defects, although specific defects can develop later in life. In particular, some of the more common metabolic alterations that occur later in life are associated with diabetes mellitus, ischemic heart disease, hypertrophy, and/or heart failure.
This review will discuss some of the metabolic defects identified in newborn patients and the consequences of metabolic abnormalities in the failing adult heart. The potential of gene therapy for the treatment of metabolic abnormalities in both the newborn and the adult heart will also be discussed.

Energy metabolism in the healthy heart
Cardiac function is dependent on the production of intracellular ATP derived from carbohydrate and fatty acid metabolism. The initiation of carbohydrate metabolism begins with the breakdown of glucose, which consists of both glycolysis and glucose oxidation.
Glycolysis is the initial sequence of reactions involved in the breakdown of glucose to
pyruvate and can occur without the presence of oxygen. The pyruvate generated from
glycolysis (or from oxidation of lactate) can be metabolized within the mitochondria by the pyruvate dehydrogenase complex to generate acetyl-CoA. Acetyl-CoA can then enter the Krebs’ cycle and undergo a series of oxidative degradations that produce substrates for the electron transport chain. This eventually generates the majority of carbohydrate-derived ATP (Figure 1A).

Figure 1. The regulation of fatty acid and glucose oxidation in the heart: a simplified schematic diagram showing the effects of elevated fatty acids on fatty acid and glucose oxidation in the heart. Fatty acids are converted to fatty acyl-CoA by fatty acyl-CoA synthetase (FACS). These fatty acyl-CoA esters are then converted to fatty acyl carnitine and shuttled into the mitochondria via carnitine translocase (CT). Once inside the mitochondria, fatty acyl carnitines are converted back into fatty acyl-CoA esters and enter into the ‚-oxidation spiral to produce acetyl-CoA. In addition to fatty acid metabolism, glucose can also be used to produce acetyl-CoA, beginning with the conversion of glucose to pyruvate (termed glycolysis). Pyruvate enters into the mitochondria via the pyruvate carrier (PC) and then is converted to acetyl-CoA by the pyruvate dehydrogenase complex (PDH). Fatty acid-derived or glucose-derived acetyl-CoA enters into the Krebs’ cycle to produce reducing equivalents that are then used to produce ATP in the electron transport chain. With normal levels of plasma fatty acids (A) the oxidation of both fatty acids and glucose contribute to ATP production. However, when fatty acid levels are elevated (B, page 7), (fatty acid-derived acetyl-CoA is elevated and is able to stimulate pyruvate dehydrogenase kinase (PDHK), which phosphorylates and inhibits PDH. This dramatically reduces the amount of pyruvate being converted into acetyl-CoA and inhibits glucose oxidation. This pyruvate can then be converted to lactate and H+, resulting in a decrease in intracellular pH. CPT, Carnitine palmitoyl transferase.

However, under normal physiological conditions the adult heart obtains approximately 60% to 80% of the myocardial acetyl-CoA-derived ATP from fatty acids [2]. Fatty acids are taken up into the mitochondria by the carnitine shuttle (carnitine palmitoyl transferase-1 and -2), where they undergo b-oxidation. This oxidative process produces acetyl-CoA that, upon entering into the Krebs’ cycle, undergoes further metabolism to eventually produce ATP (Figure 1A). One important link between fatty acid metabolism and glucose metabolism is that elevated levels of fatty acids can result in inhibition of the pyruvate dehydrogenase complex and thus glucose-derived acetyl-CoA (termed the Randle glucose-fatty acid cycle; Figure 1B). This inhibition of glucose oxidation occurs in a variety of pathological conditions, such as diabetes and ischemic heart disease. The switch away from glucose and towards fatty acids contributes to fatty acid-induced ischemic damage and can be detrimental to the function of the myocardium even in the absence of ischemia.

Figure 1B. When fatty acid levels are elevated, (fatty acid-derived acetyl-CoA is elevated and is able to stimulate pyruvate dehydrogenase kinase (PDHK), which phosphorylates and inhibits PDH. This dramatically reduces the amount of pyruvate being converted into acetyl-CoA and inhibits glucose oxidation. This pyruvate can then be converted to lactate and H+, resulting in a decrease in intracellular pH. CPT, Carnitine palmitoyl transferase.

Metabolic abnormalities in the newborn heart
In utero, the primary source of energy for the heart is derived from carbohydrates via lactate [3]. Within days of birth, the heart undergoes a rapid transformation and begins to use fatty acids as the main energy source. In the newborn, a wide variety of cardiac metabolic abnormalities stem from disturbances in the ability of the heart to efficiently utilize fuel substrates. Of these, there appear to be a disproportionate number of defects associated with lipid metabolism and/or mitochondrial dysfunction [1]. This may be a consequence of inherited fatty acid oxidation or mitochondrial disorders resulting in more obvious
clinical problems [4] and therefore may be more readily diagnosed.
Defects in the enzymes involved in mitochondrial b-oxidation impair the heart’s ability to oxidize fatty acids, its major fuel source, resulting in impaired performance. In addition, a direct link between defects in fatty acid oxidation pathways and impaired cardiac function has been demonstrated. Studies involving pediatric patients have shown that enzymatic defects involved in the fatty acid oxidation pathways are associated with cardiac dysfunction and arrhythmias [4]. These data provide evidence of the importance of fatty acid oxidation in the development and maintenance of a healthy heart. Unfortunately, mitochondrial and other fatty acid oxidation disorders are often incurable using conventional therapies. Therefore, patients with these disorders would potentially benefit
the most from some form of gene replacement therapy.

Metabolic abnormalities in the failing heart
During cardiac hypertrophy and heart failure, fatty acid oxidation is reduced [5, 6] and overall oxidative metabolism is depressed. Preceding hypertrophy-induced heart failure, the hypertrophic heart demonstrates accelerated glycolytic rates with no concomitant increase in glucose oxidation [5]. In the failing heart, the perturbation of glucose oxidation is not well understood although the failing heart does downregulate metabolic genes that control fatty acid oxidation and upregulate genes that control glycolysis [6]. In the hypertrophic heart, and possibly the failing heart, the acceleration of glycolysis with no parallel increase in glucose oxidation leads to the excess production of glycolytic byproducts including lactate and protons (Figure 1B). The ATP used to clear these by-products of glycolysis and to maintain ion homeostasis decreases the available ATP needed for contractile activity. This ultimately leads to a decrease in cardiac efficiency [7] and may contribute to the inevitable progression of heart failure.
Numerous pharmacological approaches designed to stimulate cardiac glucose oxidation have been proposed and tested under various conditions [8]. Although the pharmacological agents designed to stimulate myocardial glucose oxidation have proven to be extremely efficacious in reducing ischemic injury [9–14], less information is available as to the effectiveness of these compounds in the failing heart. However, modifying the genes responsible for controlling glucose oxidation may prove to be a novel approach to treating hypertrophy-induced heart failure in the coming years.

Gene therapy for modifying cardiac energy metabolism in the newborn
The genetic understanding of heart pathology has been rapidly increasing. For instance, specific gene defects have been identified for hypertrophic and dilated cardiomyopathies, mitochondrial cardiomyopathies, Marfan syndrome, Williams syndrome, familial supravalvar aortic stenosis, CATCH 22 syndrome, and atrioventricular canal [15]. While there is considerable interest in the development of gene therapy approaches to the treatment of cardiovascular diseases (see article by Baker, page 28–35, this issue), little attention has been paid to the correction of cardiac metabolic abnormalities using these approaches. Gene therapy techniques designed to correct metabolic abnormalities in the newborn have great potential but await the development of efficient, safe, and well-tested gene delivery methods. Great strides have been made in the development of effective gene delivery procedures [16–23] and it may only be a matter of time before gene therapy is routinely used to treat or correct metabolic abnormalities in the heart. A significant advancement in the use of gene therapy would be to develop the ability to detect and correct metabolic abnormalities in utero. Research is currently ongoing in this area [24–28].

Gene therapy for modifying cardiac energy metabolism in the adult
In the hypertrophic or failing adult heart the approach of making the heart more efficient by delivering genes that accelerate glucose oxidation is a relatively new concept for the field of gene therapy. However, care must be taken in choosing the appropriate gene(s) for delivery. For example, simply promoting glucose uptake via delivery of glucose transporter 4 to the heart may only prove to accelerate glycolysis and not glucose oxidation. This would promote further uncoupling of glycolysis from glucose oxidation and exacerbate the problem. An attractive gene to consider delivering may be a dominant-negative form of pyruvate dehydrogenase kinase (PDHK). This would inhibit PDHK activity, increase pyruvate dehydrogenase complex activity, and promote glucose oxidation. Alternatively, one could deliver a gene that would decrease fatty acid oxidation, thereby accelerating glucose oxidation via the Randle glucose-fatty acid cycle [29].
Irrespective of the gene(s) that may be delivered to optimize glucose oxidation, this type of metabolic gene delivery approach could be applied to patients with cardiac hypertrophy and lessen the probability of, or even prevent, the progression to heart failure. In addition, metabolic gene therapy could be given to patients with early signs of heart failure (independent of the initiating event) and help reduce the rate at which the disease progresses.

Conclusion
Gene therapy for the treatment of metabolic abnormalities of the heart represents an exciting treatment modality that potentially could be administered to a variety of heart patients from the fetus and newborn to the adult. Together with increased understanding of the metabolic and cellular abnormalities associated with both congenital and acquired heart disease, gene therapy could potentially be applied in a variety of clinical settings from the operating room to the clinic. Future gene delivery strategies targeting metabolic pathways will have exciting implications in the treatment of heart disease.

REFERENCES

1: Int J Exp Pathol. 2000 Dec;81(6):349-72.