beta-Hydroxybutyrate inhibits myocardial fatty acid oxidation in vivo independent of changes in malonyl-CoA content

Alterations of fatty acid β-oxidation capability in the liver of ketotic cows

Dairy cows are highly susceptible to ketosis after parturition. In the present study, we evaluated the expression of fatty acid β-oxidation-related enzymes in the liver of ketotic (n=6) and nonketotic (n=6) cows. Serum levels of nonesterified fatty acids (NEFA), β-hydroxybutyrate (BHBA), and glucose were determined by using standard biochemical techniques. The mRNA abundance and protein content of acyl-CoA synthetase long-chain (ACSL), carnitine palmitoyltransferase I (CPT I), carnitine palmitoyltransferase II (CPT II), acyl-CoA dehydrogenase long chain (ACADL), 3-hydroxy-3-methylglutaryl-CoA synthase (HMGCS), and acetyl-CoA carboxylase (ACC) were evaluated by real-time PCR and ELISA. We found that serum glucose levels were lower in ketotic cows than in nonketotic cows, but serum BHBA and NEFA concentrations were higher. Messenger RNA and protein levels of ACSL were significantly higher in livers of ketotic cows than those in nonketotic cows. In contrast, mRNA levels of CPT I and mRNA and protein levels of CPT II, ACADL, HMGCS, and ACC were decreased in the liver of ketotic cows. Serum NEFA concentration positively correlated with ACSL protein levels and negatively correlated with protein levels of CPT II, HMGCS, ACADL, and ACC. In addition, serum BHBA concentration negatively correlated with protein levels of CPT II, HMGCS, and ACADL. Overall, fatty acid β-oxidation capability was altered in the liver of ketotic compared with nonketotic cows. Furthermore, high serum NEFA and BHBA concentrations play key roles in affecting pathways of fatty acid metabolism in the liver.

Metabolism, hypoxia and the diabetic heart

The diabetic heart becomes metabolically remodelled as a consequence of exposure to abnormal circulating substrates and hormones. Fatty acid uptake and metabolism are increased in the type 2 diabetic heart, resulting in accumulation of intracellular lipid intermediates and an increased contribution of fatty acids towards energy generation. Cardiac glucose uptake and oxidation are decreased, predominantly due to increased fatty acid metabolism, which suppresses glucose utilisation via the Randle cycle. These metabolic changes decrease cardiac efficiency and energetics in both humans and animal models of diabetes. Diabetic hearts have decreased recovery following ischemia, indicating a reduced tolerance to oxygen-limited conditions. There is evidence that diabetic hearts have a compromised hypoxia signalling pathway, as hypoxia-inducible factor (HIF) and downstream signalling from HIF are reduced following ischemia. Failure to activate HIF under oxygen-limited conditions results in less angiogenesis, and an inability to upregulate glycolytic ATP generation. Given that glycolysis is already suppressed in the diabetic heart under normoxic conditions, the inability to upregulate glycolysis in response to hypoxia may have deleterious effects on ATP production. Thus, impaired HIF signalling may contribute to metabolic and energetic abnormalities, and impaired collateral vessel development following myocardial infarction in the type 2 diabetic heart.

Adaptation of myocardial substrate metabolism to a ketogenic nutrient environment

Heart muscle is metabolically versatile, converting energy stored in fatty acids, glucose, lactate, amino acids, and ketone bodies. Here, we use mouse models in ketotic nutritional states (24 h of fasting and a very low carbohydrate ketogenic diet) to demonstrate that heart muscle engages a metabolic response that limits ketone body utilization. Pathway reconstruction from microarray data sets, gene expression analysis, protein immunoblotting, and immunohistochemical analysis of myocardial tissue from nutritionally modified mouse models reveal that ketotic states promote transcriptional suppression of the key ketolytic enzyme, succinyl-CoA:3-oxoacid CoA transferase (SCOT; encoded by Oxct1), as well as peroxisome proliferator-activated receptor alpha-dependent induction of the key ketogenic enzyme HMGCS2. Consistent with reduction of SCOT, NMR profiling demonstrates that maintenance on a ketogenic diet causes a 25% reduction of myocardial (13)C enrichment of glutamate when (13)C-labeled ketone bodies are delivered in vivo or ex vivo, indicating reduced procession of ketones through oxidative metabolism. Accordingly, unmetabolized substrate concentrations are higher within the hearts of ketogenic diet-fed mice challenged with ketones compared with those of chow-fed controls. Furthermore, reduced ketone body oxidation correlates with failure of ketone bodies to inhibit fatty acid oxidation. These results indicate that ketotic nutrient environments engage mechanisms that curtail ketolytic capacity, controlling the utilization of ketone bodies in ketotic states.

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.

Type 2 diabetes, mitochondrial biology and the heart

Diabetes is recognized as an independent risk factor for cardiovascular morbidity and mortality. This is due, in large part, to premature atherosclerosis, enhanced thrombogenicity and activation of systemic inflammatory programs with resultant vascular dysfunction. More enigmatic mechanisms underpinning diabetes-associated cardiac pathophysiology include the direct metabolic consequences of this disease on the myocardium. Nevertheless, a role for diabetes-associated disruption in cardiac contractile mechanics and in increasing cardiomyocyte susceptibility to ischemic-stress has been implicated independent of vascular pathology. This review will focus broadly on the direct effects of diabetes on the cardiac myocardium with more specific reference to the role of the modulation of cardiomyocyte mitochondrial function in these disease processes. This focus in part, stems from the growing recognition that in some instances mitochondrial dysfunction is central to the development of insulin resistance and diabetes, and in others, diabetes associated disruption in mitochondrial function exacerbates and accentuates the pathophysiology of diabetes.

Oxidation of Intracellular and Extracellular Fatty Acids in Skeletal Muscle: Application of kinetic modeling, stable isotopes and liquid chromatography/electrospray ionization ion-trap tandem mass spectrometry technology

Fatty acids are a major fuel for many tissues and abnormal utilization is implicated in diseases. However, tissue fatty acid oxidation has not been determined reliably in vivo. Furthermore, fatty acid oxidation has not been partitioned into intracellular and extracellular components. In this report, a one-pool model is described that enables direct quantitation of fluxes of intracellular and plasma fatty acids to mitochondria in skeletal muscle using dual stable isotopes and liquid chromatography/electrospray ionization ion-trap tandem mass spectrometry (LC/ESI-itMS²) technology. It is validated by the determination of palmitate oxidation by skeletal muscle in lean and obese rats and the regulation by insulin. Resting postabsorptive intramyocellular and plasma palmitate oxidation by gastrocnemius muscle was determined to be 3.47±0.8 and 2.06±0.5 nmol/g min in lean and 6.96±1.8 and 1.34±0.2 nmol/g min in obese rats, respectively. In obese rats, hyperinsulinemia (1 nmol/l) suppressed intramyocellular (by 59±5% to 2.88±0.3 nmol/g min P<0.05) but not plasma (1.41±0.14 nmol/g min, P>0.05) palmitate oxidation. The fractional turnover rate of palmitoylcarnitine (0.34±0.1/min vs. 0.83±0.2/min, P<0.05) was also suppressed by insulin. In obese and lean rats, there are 83% and 51%, respectively (P=0.08), of plasma fatty acids traverse triglyceride pool before being oxidized. The results demonstrated that the methodology is feasible and sensitive to metabolic alterations and thus can be used to study fatty acid utilization at tissue level in a compartmentalized manner for the firs time.

Free fatty acids normalize a rosiglitazone-induced visfatin release

The detrimental effect of elevated free fatty acids (FFAs) on insulin sensitivity can be improved by thiazolidinediones (TZDs) in patients with type 2 diabetes mellitus. It is unknown whether this salutary action of TZD is associated with altered release of the insulin-mimetic adipocytokine visfatin. In this study, we investigated whether visfatin concentrations are altered by FFA and TZD treatment. In a randomized, double-blind, placebo-controlled, parallel-group study 16 healthy volunteers received an infusion of triglycerides/heparin to increase plasma FFA after 3 wk of treatment with rosiglitazone (8 mg/day, n = 8) or placebo (n = 8), and circulating plasma visfatin was measured. As a corollary, human adipocytes were incubated with synthetic fatty acids and rosiglitazone to assess visfatin release in vitro. The results were that rosiglitazone treatment increased systemic plasma visfatin concentrations from 0.6 +/- 0.1 to 1.7 +/- 0.2 ng/ml (P < 0.01). Lipid infusion caused a marked elevation of plasma FFA but had no effect on circulating visfatin in controls. In contrast, elevated visfatin concentrations in subjects receiving rosiglitazone were normalized by lipid infusion. In isolated adipocytes, visfatin was released into supernatant medium by acute addition and long-term treatment of rosiglitazone. This secretion was blocked by synthetic fatty acids and by inhibition of phosphatidylinositol 3-kinase or Akt. In conclusion, release of the insulin-mimetic visfatin may represent a major mechanism of metabolic TZD action. The presence of FFA antagonizes this action, which may have implications for visfatin bioactivity.

Competition between acetate and oleate for the formation of malonyl-CoA and mitochondrial acetyl-CoA in the perfused rat heart

We previously showed that, in the perfused rat heart, the capacity of n-fatty acids to generate mitochondrial acetyl-CoA decreases as their chain length increases. In the present study, we investigated whether the oxidation of a long-chain fatty acid, oleate, is inhibited by short-chain fatty acids, acetate or propionate (which do and do not generate mitochondrial acetyl-CoA, respectively). We perfused rat hearts with buffer containing 4 mM glucose, 0.2 mM pyruvate, 1 mM lactate, and various concentrations of either (i) [U-(13)C]acetate, (ii) [U-(13)C]acetate plus [1-(13)C]oleate, or (iii) unlabeled propionate plus [1-(13)C]oleate. Using mass isotopomer analysis, we determined the contributions of the labeled substrates to the acetyl moiety of citrate (a probe of mitochondrial acetyl-CoA) and to malonyl-CoA. We found that acetate, even at low concentration, markedly inhibits the oxidation of [1-(13)C]oleate in the heart, without change in malonyl-CoA concentration. We also found that propionate, at a concentration higher than 1 mM, decreases (i) the contribution of [1-(13)C]oleate to mitochondrial acetyl-CoA and (ii) malonyl-CoA concentration. The inhibition by acetate or propionate of acetyl-CoA production from oleate probably results from a competition for mitochondrial CoA between the CoA-utilizing enzymes.

Cardiac metabolism in mice: tracer method developments and in vivo application revealing profound metabolic inflexibility in diabetes

Studies of cardiac fuel metabolism in mice have been almost exclusively conducted ex vivo. The major aim of this study was to assess in vivo plasma FFA and glucose utilization by the hearts of healthy control (db/+) and diabetic (db/db) mice, based on cardiac uptake of (R)-2-[9,10-(3)H]bromopalmitate ([3H]R-BrP) and 2-deoxy-D-[U-14C]glucose tracers. To obtain quantitative information about the evaluation of cardiac FFA utilization with [3H]R-BrP, simultaneous comparisons of [3H]R-BrP and [14C]palmitate ([14C]P) uptake were first made in isolated perfused working hearts from db/+ mice. It was found that [3H]R-BrP uptake was closely correlated with [14C]P oxidation (r2 = 0.94, P < 0.001). Then, methods for in vivo application of [3H]R-BrP and [14C]2-DG previously developed for application in the rat were specially adapted for use in the mouse. The method yields indexes of cardiac FFA utilization (R(f)*) and clearance (K(f)*), as well as glucose utilization (R(g)'). Finally, in the main part of the study, the ability of the heart to switch between FFA and glucose fuels (metabolic flexibility) was investigated by studying anesthetized, 8-h-fasted control and db/db mice in either the basal state or during glucose infusion. In control mice, glucose infusion raised plasma levels of glucose and insulin, raised R(g)' (+58%), and lowered plasma FFA level (-48%), K(f)* (-45%), and R(f)* (-70%). This apparent reciprocal regulation of glucose and FFA utilization by control hearts illustrates metabolic flexibility for substrate use. By contrast, in the db/db mice, glucose infusion raised glucose levels with no apparent influence on cardiac FFA or glucose utilization. In conclusion, tracer methodology for assessing in vivo tissue-specific plasma FFA and glucose utilization has been adapted for use in mice and reveals a profound loss of metabolic flexibility in the diabetic db/db heart, suggesting a fixed level of FFA oxidation in fasted and glucose-infused states.

Regulation of myocardial substrate metabolism during increased energy expenditure: insights from computational studies

In response to exercise, the heart increases its metabolic rate severalfold while maintaining energy species (e.g., ATP, ADP, and Pi) concentrations constant; however, the mechanisms that regulate this response are unclear. Limited experimental studies show that the classic regulatory species NADH and NAD+ are also maintained nearly constant with increased cardiac power generation, but current measurements lump the cytosol and mitochondria and do not provide dynamic information during the early phase of the transition from low to high work states. In the present study, we modified our previously published computational model of cardiac metabolism by incorporating parallel activation of ATP hydrolysis, glycolysis, mitochondrial dehydrogenases, the electron transport chain, and oxidative phosphorylation, and simulated the metabolic responses of the heart to an abrupt increase in energy expenditure. Model simulations showed that myocardial oxygen consumption, pyruvate oxidation, fatty acids oxidation, and ATP generation were all increased with increased energy expenditure, whereas ATP and ADP remained constant. Both cytosolic and mitochondrial NADH/NAD+ increased during the first minutes (by 40% and 20%, respectively) and returned to the resting values by 10-15 min. Furthermore, model simulations showed that an altered substrate selection, induced by either elevated arterial lactate or diabetic conditions, affected cytosolic NADH/NAD+ but had minimal effects on the mitochondrial NADH/NAD+, myocardial oxygen consumption, or ATP production. In conclusion, these results support the concept of parallel activation of metabolic processes generating reducing equivalents during an abrupt increase in cardiac energy expenditure and suggest there is a transient increase in the mitochondrial NADH/NAD+ ratio that is independent of substrate supply.

Chronic activation of PPARα is detrimental to cardiac recovery after ischemia

High fatty acid oxidation (FAO) rates contribute to ischemia-reperfusion injury of the myocardium. Because peroxisome proliferator-activated receptor (PPAR)α regulates transcription of several FAO enzymes in the heart, we examined the response of mice with cardiac-restricted overexpression of PPARα (MHC-PPARα) or whole body PPARα deletion including the heart (PPARα−/−) to myocardial ischemia-reperfusion injury. Isolated working hearts from MHC-PPARα and nontransgenic (NTG) littermates were subjected to no-flow global ischemia followed by reperfusion. MHC-PPARα hearts had significantly higher FAO rates during aerobic and postischemic reperfusion (aerobic 1,479 ± 171 vs. 699 ± 117, reperfusion 1,062 ± 214 vs. 601 ± 70 nmol·g dry wt−1·min−1; P < 0.05) and significantly lower glucose oxidation rates compared with NTG hearts (aerobic 225 ± 36 vs. 1,563 ± 165, reperfusion 402 ± 54 vs. 1,758 ± 165 nmol·g dry wt−1·min−1; P < 0.05). In hearts from PPARα−/−mice, FAO was significantly lower during aerobic and reperfusion (aerobic 235 ± 36 vs. 442 ± 75, reperfusion 205 ± 25 vs. 346 ± 38 nmol·g dry wt−1·min−1; P < 0.05) whereas glucose oxidation was significantly higher compared with wild-type (WT) hearts (aerobic 2,491 ± 631 vs. 901 ± 119, reperfusion 2,690 ± 562 vs. 1,315 ± 172 nmol·g dry wt−1·min−1; P < 0.05). Increased FAO rates in MHC-PPARα hearts were associated with a markedly lower recovery of cardiac power (45 ± 9% vs. 71 ± 6% of preischemic levels in NTG hearts; P < 0.05). In contrast, the percent recovery of cardiac power of PPARα−/−hearts was not significantly different from that of WT hearts (80 ± 8% vs. 75 ± 9%). This study demonstrates that chronic activation of PPARα is detrimental to the cardiac recovery during reperfusion after ischemia.

Myocardial substrate metabolism in the normal and failing heart

The alterations in myocardial energy substrate metabolism that occur in heart failure, and the causes and consequences of these abnormalities, are poorly understood. There is evidence to suggest that impaired substrate metabolism contributes to contractile dysfunction and to the progressive left ventricular remodeling that are characteristic of the heart failure state. The general concept that has recently emerged is that myocardial substrate selection is relatively normal during the early stages of heart failure; however, in the advanced stages there is a downregulation in fatty acid oxidation, increased glycolysis and glucose oxidation, reduced respiratory chain activity, and an impaired reserve for mitochondrial oxidative flux. This review discusses 1) the metabolic changes that occur in chronic heart failure, with emphasis on the mechanisms that regulate the changes in the expression of metabolic genes and the function of metabolic pathways; 2) the consequences of these metabolic changes on cardiac function; 3) the role of changes in myocardial substrate metabolism on ventricular remodeling and disease progression; and 4) the therapeutic potential of acute and long-term manipulation of cardiac substrate metabolism in heart failure.

Regulation of cardiac malonyl-CoA content and fatty acid oxidation during increased cardiac power

Myocardial fatty acid oxidation is regulated by carnitine palmitoyltransferase I (CPT I), which is inhibited by malonyl-CoA. Increased cardiac power causes a fall in malonyl-CoA content and accelerated fatty acid oxidation; however, the mechanism for the decrease in malonyl-CoA is unclear. Malonyl-CoA is formed by acetyl-CoA carboxylase (ACC) and degraded by malonyl-CoA decarboxylase (MCD); thus a fall in malonyl-CoA could be due to activation of MCD, inhibition of ACC, or both. This study assessed the effects of increased cardiac power on malonyl-CoA content and ACC and MCD activities. Anesthetized pigs were studied under control conditions and during increased cardiac power in response to dobutamine infusion and aortic constriction alone, under hyperglycemic conditions, or with the CPT I inhibitor oxfenicine. An increase in cardiac power was accompanied by increased myocardial O(2) consumption, decreased malonyl-CoA concentration, and increased fatty acid oxidation. There were no differences among groups in activity of ACC or AMP-activated protein kinase (AMPK), which physiologically inhibits ACC. There also were no differences in V(max) or K(m) of MCD. Previous studies have demonstrated that AMPK can be inhibited by protein kinase B (PKB); however, PKB was activated by dobutamine and the elevated insulin that accompanied hyperglycemia, but there was no effect on AMPK activity. In conclusion, the fall in malonyl-CoA and increase in fatty acid oxidation that occur with increased cardiac work were not due to inhibition of ACC or activation of MCD, suggesting alternative regulatory mechanisms for the work-induced decrease in malonyl-CoA concentration.