Characterization of rat liver malonyl-CoA decarboxylase and the study of its role in regulating fatty acid metabolism

In the liver, malonyl-CoA is central to many cellular processes, including both fatty acid biosynthesis and oxidation. Malonyl-CoA decarboxylase (MCD) is involved in the control of cellular malonyl-CoA levels, and functions to decarboxylate malonyl-CoA to acetyl-CoA. MCD may play an essential role in regulating energy utilization in the liver by regulating malonyl-CoA levels in response to various nutritional or pathological states. The purpose of the present study was to investigate the role of liver MCD in the regulation of fatty acid oxidation in situations where lipid metabolism is altered. A single MCD enzyme of molecular mass 50.7 kDa was purified from rat liver using a sequential column chromatography procedure and the cDNA was subsequently cloned and sequenced. The liver MCD cDNA was identical to rat pancreatic beta-cell MCD cDNA, and contained two potential translational start sites, producing proteins of 50.7 kDa and 54.7 kDa. Western blot analysis using polyclonal antibodies generated against rat liver MCD showed that the 50.7 kDa isoform of MCD is most abundant in heart and liver, and of relatively low abundance in skeletal muscle (despite elevated MCD transcript levels in skeletal muscle). Tissue distribution experiments demonstrated that the pancreas is the only rat tissue so far identified that contains both the 50.7 kDa and 54. 7 kDa isoforms of MCD. In addition, transfection of the full-length rat liver MCD cDNA into COS cells produced two isoforms of MCD. This indicated either that both initiating methionines are functionally active, generating two proteins, or that the 54.7 kDa isoform is the only MCD protein translated and removal of the putative mitochondrial targeting pre-sequence generates a protein of approx. 50.7 kDa in size. To address this, we transiently transfected a mutated MCD expression plasmid (second ATG to GCG) into COS-7 cells and performed Western blot analysis using our anti-MCD antibody. Western blot analysis revealed that two isoforms of MCD were still present, demonstrating that the second ATG may not be responsible for translation of the 50.7 kDa isoform of MCD. These data also suggest that the smaller isoform of MCD may originate from intracellular processing. To ascertain the functional role of the 50. 7 kDa isoform of rat liver MCD, we measured liver MCD activity and expression in rats subjected to conditions which are known to alter fatty acid metabolism. The activity of MCD was significantly elevated under conditions in which hepatic fatty acid oxidation is known to increase, such as streptozotocin-induced diabetes or following a 48 h fast. A 2-fold increase in expression was observed in the streptozotocin-diabetic rats compared with control rats. In addition, MCD activity was shown to be enhanced by alkaline phosphatase treatment, suggesting phosphorylation-related control of the enzyme. Taken together, our data demonstrate that rat liver expresses a 50.7 kDa form of MCD which does not originate from the second methionine of the cDNA sequence. This MCD is regulated by at least two mechanisms (only one of which is phosphorylation), and its activity and expression are increased under conditions where fatty acid oxidation increases.

The antianginal drug trimetazidine shifts cardiac energy metabolism from fatty acid oxidation to glucose oxidation by inhibiting mitochondrial long-chain 3-ketoacyl coenzyme A thiolase

Trimetazidine is a clinically effective antianginal agent that has no negative inotropic or vasodilator properties. Although it is thought to have direct cytoprotective actions on the myocardium, the mechanism(s) by which this occurs is as yet undefined. In this study, we determined what effects trimetazidine has on both fatty acid and glucose metabolism in isolated working rat hearts and on the activities of various enzymes involved in fatty acid oxidation. Hearts were perfused with Krebs-Henseleit solution containing 100 microU/mL insulin, 3% albumin, 5 mmol/L glucose, and fatty acids of different chain lengths. Both glucose and fatty acids were appropriately radiolabeled with either (3)H or (14)C for measurement of glycolysis, glucose oxidation, and fatty acid oxidation. Trimetazidine had no effect on myocardial oxygen consumption or cardiac work under any aerobic perfusion condition used. In hearts perfused with 5 mmol/L glucose and 0.4 mmol/L palmitate, trimetazidine decreased the rate of palmitate oxidation from 488+/-24 to 408+/-15 nmol x g dry weight(-1) x minute(-1) (P<0.05), whereas it increased rates of glucose oxidation from 1889+/-119 to 2378+/-166 nmol x g dry weight(-1) x minute(-1) (P<0.05). In hearts subjected to low-flow ischemia, trimetazidine resulted in a 210% increase in glucose oxidation rates. In both aerobic and ischemic hearts, glycolytic rates were unaltered by trimetazidine. The effects of trimetazidine on glucose oxidation were accompanied by a 37% increase in the active form of pyruvate dehydrogenase, the rate-limiting enzyme for glucose oxidation. No effect of trimetazidine was observed on glycolysis, glucose oxidation, fatty acid oxidation, or active pyruvate dehydrogenase when palmitate was substituted with 0.8 mmol/L octanoate or 1.6 mmol/L butyrate, suggesting that trimetazidine directly inhibits long-chain fatty acid oxidation. This reduction in fatty acid oxidation was accompanied by a significant decrease in the activity of the long-chain isoform of the last enzyme involved in fatty acid beta-oxidation, 3-ketoacyl coenzyme A (CoA) thiolase activity (IC(50) of 75 nmol/L). In contrast, concentrations of trimetazidine in excess of 10 and 100 micromol/L were needed to inhibit the medium- and short-chain forms of 3-ketoacyl CoA thiolase, respectively. Previous studies have shown that inhibition of fatty acid oxidation and stimulation of glucose oxidation can protect the ischemic heart. Therefore, our data suggest that the antianginal effects of trimetazidine may occur because of an inhibition of long-chain 3-ketoacyl CoA thiolase activity, which results in a reduction in fatty acid oxidation and a stimulation of glucose oxidation.

Fatty acid oxidation in the reperfused ischemic heart

Myocardial ATP production is dependent chiefly on the oxidative decarboxylation of glucose and fatty acids. The co-utilization of these and other substrates is determined by both the amount of any given substrate supplied to the heart as well as by complex intracellular regulatory mechanisms. This regulated balance is altered during and after ischemia. During aerobic reperfusion of ischemic myocardium, a rapid recovery of energy production is desirable for the complete recovery of muscle contractile function. It is now clear that the type of energy substrate used by the heart during reperfusion will directly influence this contractile recovery. By increasing the relative proportion of glucose oxidized to that of fatty acids, the mechanical function of the reperfused heart can be improved. However, fatty acid oxidation recovers quickly during reperfusion and dominates as a source of oxygen consumption. These high rates of fatty acid oxidation occur at the expense of glucose oxidation, resulting in a decreased recovery of both cardiac function and efficiency during reperfusion. One contributory factor to these high rates of fatty acid oxidation is a decrease in myocardial malonyl-coenzyme A (CoA) levels. Malonyl-CoA, which is synthesized by acetyl-CoA carboxylase, is an essential metabolic intermediary in the regulation of fatty acid oxidation. A decrease in malonyl-CoA level results in an increase of carnitine palmitoyl transferase-1 mediated fatty acid uptake into the mitochondria. This mechanism seems important in the regulation of fatty acid oxidation in the postischemic heart and is discussed in detail in this review, with reference to specific clinical scenarios of ischemia and reperfusion and options for modulating cardiac energy metabolism.

Volume overload hypertrophy of the newborn heart slows the maturation of enzymes involved in the regulation of fatty acid metabolism

OBJECTIVES

The purpose of this study was to determine the effect of volume overload hypertrophy in the newborn heart on the cardiac enzymes controlling fatty acid metabolism.

BACKGROUND

Shortly after birth, a rise in 5'-adenosine monophosphate-activated protein kinase (AMPK) activity results in the phosphorylation and inhibition of acetyl coenzyme A (CoA) carboxylase (ACC), and a decline in myocardial malonyl CoA levels with increased fatty acid oxidation rates. Whether the early onset of hypertrophy in the newborn heart alters this maturational increase in fatty acid oxidation is unknown.

METHODS

Newborn piglets underwent endovascular stenting of the ductus arteriosus on day 1 of life with a 4.5-mm diameter stent, resulting in a left to right shunt, and left ventricular (LV) volume loading. Left ventricular and right ventricular samples from fetal, newborn, three-week control and three-week stented animals were compared.

RESULTS

Stenting resulted in echocardiographic evidence of volume overload and myocardial hypertrophy. In control animals, left ventricular ACC activity declined from 274 +/- 30 pmol/mg/min on day 1 to 115 +/- 12 after three weeks (p < 0.05), but did not display this maturation drop in hypertrophied hearts, remaining elevated (270 +/- 50 pmol/mg/min, p < 0.05). At three weeks, malonyl CoA levels remained 2.8-fold higher in hypertrophied hearts than in control hearts. In control hearts, LV AMPK activity increased 178% between day 1 and three weeks, whereas in hypertrophied hearts AMPK activity at three weeks was only 71% of control values, due to a significant decrease in expression of the catalytic subunit of AMPK.

CONCLUSIONS

Early onset LV volume overload with hypertrophy results in a delay in the normal maturation of fatty acid oxidation in the newborn heart.

Maturation of fatty acid and carbohydrate metabolism in the newborn heart

During fetal life, myocardial ATP is derived predominantly from glycolysis and lactate oxidation. Following birth, a rapid maturational increase in fatty acid oxidation occurs along with a decline in glycolytic and lactate oxidative rates, thus changing the major source of myocardial ATP production. This shift in energy substrate preference occurs in response to changes in the circulating substrate content of newborn plasma with the onset of suckling, and is also due to alterations in circulating levels of hormones, such as insulin and glucagon. Important changes in subcellular regulatory mechanisms of both fatty acid and carbohydrate metabolism in the heart also characterize this response. This review deals with recent advances in the understanding of these subcellular mechanisms which regulate this important shift in myocardial energy metabolism, with particular emphasis on the molecular events occurring in the heart during the transition from fetal to newborn life.

Reduction of ischemic K+ loss and arrhythmias in rat hearts. Effect of glibenclamide, a sulfonylurea

Glibenclamide, one of the antidiabetic sulfonylureas, is known to block ATP-dependent K+ channels. We used this drug to determine to what extent K+ loss from acutely ischemic myocardium is mediated via these channels. We also investigated whether glibenclamide would influence ischemic arrhythmias. Isolated rat hearts rendered globally ischemic showed no correlation between early lactate and K+ efflux rates. Cumulative K+ loss during 11 minutes of global ischemia (0.5 ml min-1 g-1) was reduced, from 3.2 +/- 0.3 to 2.5 +/- 0.1 mueq/g (p less than 0.025) by 1 microM glibenclamide and from 3.3 +/- 0.2 to 1.9 +/- 0.2 mueq/g (p less than 0.005) by 10 microM glibenclamide, while lactate efflux was unaltered by the drug. Glibenclamide also exhibited potent antifibrillatory activity, abolishing irreversible ventricular fibrillation during regional ischemia (0/6 vs. 5/6 controls; p less than 0.02) and during global ischemia (0/7 vs. 9/9 controls; p less than 0.01). Heart rate, coronary flow rate, peak systolic pressure, and myocardial oxygen consumption were unaltered by the drug (1 microM). Similarly, glibenclamide (1 microM) did not alter myocardial ATP, phosphocreatine or lactate content, or glucose utilization. Ventricular fibrillation threshold during normoxia was also unaltered by glibenclamide (1 microM). We conclude that K+ loss during acute myocardial ischemia is mediated partly by ATP-dependent K+ channels, and not by a tightly coupled co-efflux with anionic lactate.