Wendy Keung

Inhibition of de novo ceramide synthesis reverses diet-induced insulin resistance and enhances whole body oxygen consumption

OBJECTIVE It has been proposed that skeletal muscle insulin resistance arises from the accumulation of intramyocellular lipid metabolites that impede insulin signaling, including diacylglycerol and ceramide. We determined the role of de novo ceramide synthesis in mediating muscle insulin resistance. RESEARCH DESIGN AND METHODS Mice were subjected to 12 weeks of diet-induced obesity (DIO), and then treated for 4 weeks with myriocin, an inhibitor of serine palmitoyl transferase-1 (SPT1), the rate-limiting enzyme of de novo ceramide synthesis. RESULTS After 12 weeks of DIO, C57BL/6 mice demonstrated a doubling in gastrocnemius ceramide content, which was completely reversed (141.5 ± 15.8 vs. 94.6 ± 10.2 nmol/g dry wt) via treatment with myriocin, whereas hepatic ceramide content was unaffected by DIO. Interestingly, myriocin treatment did not alter the DIO-associated increase in gastrocnemius diacyglycerol content, and the only correlation observed between lipid metabolite accumulation and glucose intolerance occurred with ceramide (R = 0.61). DIO mice treated with myriocin showed a complete reversal of glucose intolerance and insulin resistance which was associated with enhanced insulin-stimulated Akt and glycogen synthase kinase 3β phosphorylation. Furthermore, myriocin treatment also decreased intramyocellular ceramide content and prevented insulin resistance development in db/db mice. Finally, myriocin-treated DIO mice displayed enhanced oxygen consumption rates (3,041 ± 124 vs. 2,407 ± 124 ml/kg/h) versus their control counterparts. CONCLUSIONS Our results demonstrate that the intramyocellular accumulation of ceramide correlates strongly with the development of insulin resistance, and suggests that inhibition of SPT1 is a potentially promising target for the treatment of insulin resistance.

Targeting fatty acid and carbohydrate oxidation — A novel therapeutic intervention in the ischemic and failing heart

Cardiac ischemia and its consequences including heart failure, which itself has emerged as the leading cause of morbidity and mortality in developed countries are accompanied by complex alterations in myocardial energy substrate metabolism. In contrast to the normal heart, where fatty acid and glucose metabolism are tightly regulated, the dynamic relationship between fatty acid β-oxidation and glucose oxidation is perturbed in ischemic and ischemic-reperfused hearts, as well as in the failing heart. These metabolic alterations negatively impact both cardiac efficiency and function. Specifically there is an increased reliance on glycolysis during ischemia and fatty acid β-oxidation during reperfusion following ischemia as sources of adenosine triphosphate (ATP) production. Depending on the severity of heart failure, the contribution of overall myocardial oxidative metabolism (fatty acid β-oxidation and glucose oxidation) to adenosine triphosphate production can be depressed, while that of glycolysis can be increased. Nonetheless, the balance between fatty acid β-oxidation and glucose oxidation is amenable to pharmacological intervention at multiple levels of each metabolic pathway. This review will focus on the pathways of cardiac fatty acid and glucose metabolism, and the metabolic phenotypes of ischemic and ischemic/reperfused hearts, as well as the metabolic phenotype of the failing heart. Furthermore, as energy substrate metabolism has emerged as a novel therapeutic intervention in these cardiac pathologies, this review will describe the mechanistic bases and rationale for the use of pharmacological agents that modify energy substrate metabolism to improve cardiac function in the ischemic and failing heart. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.

Leptin activates hypothalamic acetyl-CoA carboxylase to inhibit food intake

Hypothalamic fatty acid metabolism has recently been implicated in the controls of food intake and energy homeostasis. We report that intracerebroventricular (ICV) injection of leptin, concomitant with inhibiting AMP-activated kinase (AMPK), activates acetyl-CoA carboxylase (ACC), the key regulatory enzyme in fatty acid biosynthesis, in the arcuate nucleus (Arc) and paraventricular nucleus (PVN) in the hypothalamus. Arc overexpression of constitutively active AMPK prevents the Arc ACC activation in response to ICV leptin, supporting the hypothesis that AMPK lies upstream of ACC in leptins Arc intracellular signaling pathway. Inhibiting hypothalamic ACC with 5-tetradecyloxy-2-furoic acid, a specific ACC inhibitor, blocks leptin-mediated decreases in food intake, body weight, and mRNA level of the orexigenic neuropeptide NPY. These results show that hypothalamic ACC activation makes an important contribution to leptins anorectic effects. Furthermore, we find that ICV leptin up-regulates the level of malonyl-CoA (the intermediate of fatty acid biosynthesis) specifically in the Arc and increases the level of palmitoyl-CoA (a major product of fatty acid biosynthesis) specifically in the PVN. The rises of both levels are blocked by 5-tetradecyloxy-2-furoic acid along with the blockade of leptin-mediated hypophagia. These data suggest malonyl-CoA as a downstream mediator of ACC in leptins signaling pathway in the Arc and imply that palmitoyl-CoA, instead of malonyl-CoA, could be an effector in relaying ACC signaling in the PVN. Together, these findings highlight site-specific impacts of hypothalamic ACC activation in leptins anorectic signaling cascade.

Role of fatty acid uptake and fatty acid β-oxidation in mediating insulin resistance in heart and skeletal muscle

Fatty acids are a major fuel source used to sustain contractile function in heart and oxidative skeletal muscle. To meet the energy demands of these muscles, the uptake and beta-oxidation of fatty acids must be coordinately regulated in order to ensure an adequate, but not excessive, supply for mitochondrial beta-oxidation. However, imbalance between fatty acid uptake and beta-oxidation has the potential to contribute to muscle insulin resistance. The action of insulin is initiated by binding to its receptor and activation of the intrinsic protein tyrosine kinase activity of the receptor, resulting in the initiation of an intracellular signaling cascade that eventually leads to insulin-mediated alterations in a number of cellular processes, including an increase in glucose transport. Accumulation of fatty acids and lipid metabolites (such as long chain acyl CoA, diacylglycerol, triacylglycerol, and/or ceramide) can lead to alterations in this insulin signaling pathway. An imbalance between fatty acid uptake and oxidation is believed to be responsible for this lipid accumulation, and is thought to be a major cause of insulin resistance in obesity and diabetes, due to lipid accumulation and inhibition of one or more steps in the insulin-signaling cascade. As a result, decreasing muscle fatty acid uptake can improve insulin sensitivity. However, the potential role of increasing fatty acid beta-oxidation in the heart or skeletal muscle in order to prevent cytoplasmic lipid accumulation and decrease insulin resistance is controversial. While increased fatty acid beta-oxidation may lower cytoplasmic lipid accumulation, increasing fatty acid beta-oxidation can decrease muscle glucose metabolism, and incomplete fatty acid oxidation has the potential to also contribute to insulin resistance. In this review, we discuss the proposed mechanisms by which alterations in fatty acid uptake and oxidation contribute to insulin resistance, and how targeting fatty acid uptake and oxidation is a potential therapeutic approach to treat insulin resistance.

Stimulation of glucose oxidation protects against acute myocardial infarction and reperfusion injury

AIMS During reperfusion of the ischaemic myocardium, fatty acid oxidation rates quickly recover, while glucose oxidation rates remain depressed. Direct stimulation of glucose oxidation via activation of pyruvate dehydrogenase (PDH), or secondary to an inhibition of malonyl CoA decarboxylase (MCD), improves cardiac functional recovery during reperfusion following ischaemia. However, the effects of such interventions on the evolution of myocardial infarction are unknown. The purpose of this study was to determine whether infarct size is decreased in response to increased glucose oxidation. METHODS AND RESULTS In vivo, direct stimulation of PDH in mice with the PDH kinase (PDHK) inhibitor, dichloroacetate, significantly decreased infarct size following temporary ligation of the left anterior descending coronary artery. These results were recapitulated in PDHK 4-deficient (PDHK4-/-) mice, which have enhanced myocardial PDH activity. These interventions also protected against ischaemia/reperfusion injury in the working heart, and dichloroacetate failed to protect in PDHK4-/- mice. In addition, there was a dramatic reduction in the infarct size in malonyl CoA decarboxylase-deficient (MCD-/-) mice, in which glucose oxidation rates are enhanced (secondary to an inhibition of fatty acid oxidation) relative to their wild-type littermates (10.8 ± 3.8 vs. 39.5 ± 4.7%). This cardioprotective effect in MCD-/- mice was associated with increased PDH activity in the ischaemic area at risk (1.89 ± 0.18 vs. 1.52 ± 0.05 μmol/g wet weight/min). CONCLUSION These findings demonstrate that stimulating glucose oxidation via targeting either PDH or MCD decreases the infarct size, validating the concept that optimizing myocardial metabolism is a novel therapy for ischaemic heart disease.

Inhibition of Carnitine Palmitoyltransferase-1 Activity Alleviates Insulin Resistance in Diet-Induced Obese Mice

Impaired skeletal muscle fatty acid oxidation has been suggested to contribute to insulin resistance and glucose intolerance. However, increasing muscle fatty acid oxidation may cause a reciprocal decrease in glucose oxidation, which might impair insulin sensitivity and glucose tolerance. We therefore investigated what effect inhibition of mitochondrial fatty acid uptake has on whole-body glucose tolerance and insulin sensitivity in obese insulin-resistant mice. C57BL/6 mice were fed a high-fat diet (60% calories from fat) for 12 weeks to develop insulin resistance. Subsequent treatment of mice for 4 weeks with the carnitine palmitoyltransferase-1 inhibitor, oxfenicine (150 mg/kg i.p. daily), resulted in improved whole-body glucose tolerance and insulin sensitivity. Exercise capacity was increased in oxfenicine-treated mice, which was accompanied by an increased respiratory exchange ratio. In the gastrocnemius muscle, oxfenicine increased pyruvate dehydrogenase activity, membrane GLUT4 content, and insulin-stimulated Akt phosphorylation. Intramyocellular levels of lipid intermediates, including ceramide, long-chain acyl CoA, and diacylglycerol, were also decreased. Our results demonstrate that inhibition of mitochondrial fatty acid uptake improves insulin sensitivity in diet-induced obese mice. This is associated with increased carbohydrate utilization and improved insulin signaling in the skeletal muscle, suggestive of an operating Randle Cycle in muscle.

Inhibition of Serine Palmitoyl Transferase I Reduces Cardiac Ceramide Levels and Increases Glycolysis Rates following Diet-Induced Insulin Resistance

Objective Diet-induced obesity (DIO) leads to an accumulation of intra-myocardial lipid metabolites implicated in causing cardiac insulin resistance and contractile dysfunction. One such metabolite is ceramide, and our aim was to determine the effects of inhibiting de novo ceramide synthesis on cardiac function and insulin stimulated glucose utilization in mice subjected to DIO. Materials and Methods C57BL/6 mice were fed a low fat diet or subjected to DIO for 12 weeks, and then treated for 4 weeks with either vehicle control or the serine palmitoyl transferase I (SPT I) inhibitor, myriocin. In vivo cardiac function was assessed via ultrasound echocardiography, while glucose metabolism was assessed in isolated working hearts. Results DIO was not associated with an accumulation of intra-myocardial ceramide, but rather, an accumulation of intra-myocardial DAG (2.63±0.41 vs. 4.80±0.97 nmol/g dry weight). Nonetheless, treatment of DIO mice with myriocin decreased intra-myocardial ceramide levels (50.3±7.7 vs. 26.9±2.7 nmol/g dry weight) and prevented the DIO-associated increase in intra-myocardial DAG levels. Interestingly, although DIO impaired myocardial glycolysis rates (7789±1267 vs. 2671±326 nmol/min/g dry weight), hearts from myriocin treated DIO mice exhibited an increase in glycolysis rates. Conclusions Our data reveal that although intra-myocardial ceramide does not accumulate following DIO, inhibition of de novo ceramide synthesis nonetheless reduces intra-myocardial ceramide levels and prevents the accumulation of intra-myocardial DAG. These effects improved the DIO-associated impairment of cardiac glycolysis rates, suggesting that SPT I inhibition increases cardiac glucose utilization.

Malonyl-CoA mediates leptin hypothalamic control of feeding independent of inhibition of CPT-1a

Hypothalamic fatty acid metabolism is involved in central nervous system controls of feeding and energy balance. Malonyl-CoA, an intermediate of fatty acid biosynthesis, is emerging as a significant player in these processes. Notably, hypothalamic malonyl-CoA has been implicated in leptins feeding effect. Leptin treatment increases malonyl-CoA level in the hypothalamic arcuate nucleus (Arc), and this increase is required for leptin-induced decrease in food intake. However, the intracellular downstream mediators of malonyl-CoAs feeding effect have not been identified. A primary biochemical action of malonyl-CoA is the inhibition of the acyltransferase activity of carnitine palmitoyltransferase-1 (CPT-1). In the hypothalamus, the predominant isoform of CPT-1 that possesses the acyltransferase activity is CPT-1 liver type (CPT-1a). To address the role of CPT-1a in malonyl-CoAs anorectic action, we used a recombinant adenovirus expressing a mutant CPT-1a that is insensitive to malonyl-CoA inhibition. We show that Arc overexpression of the mutant CPT-1a blocked the malonyl-CoA-mediated inhibition of CPT-1 activity. However, the overexpression of this mutant did not affect the anorectic actions of leptin or central cerulenin for which an increase in Arc malonyl-CoA level is also required. Thus, CPT-1a does not appear to be involved in the malonyl-CoAs anorectic actions induced by leptin. Furthermore, long-chain fatty acyl-CoAs, substrates of CPT-1a, dissociate from malonyl-CoAs actions in the Arc under different feeding states. Together, our results suggest that Arc intracellular mechanisms of malonyl-CoAs anorectic actions induced by leptin are independent of CPT-1a. The data suggest that target(s), rather than CPT-1a, mediates malonyl-CoA action on feeding.

Trimetazidine Therapy Prevents Obesity-Induced Cardiomyopathy in Mice

Obesity is a significant risk factor for the development of cardiovascular disease. Inhibiting fatty acid oxidation has emerged as a novel approach for the treatment of ischemic heart disease. Our aim was to determine whether pharmacologic inhibition of 3-ketoacyl-coenzyme A thiolase (3-KAT), which catalyzes the final step of fatty acid oxidation, could improve obesity-induced cardiomyopathy. A 3-week treatment with the 3-KAT inhibitor trimetazidine prevented obesity-induced reduction in both systolic and diastolic function. Therefore, targeting cardiac fatty acid oxidation may be a novel therapeutic approach to alleviate the growing burden of obesity-related cardiomyopathy.

Differential effects of central ghrelin on fatty acid metabolism in hypothalamic ventral medial and arcuate nuclei

Fatty acid metabolism is an important pathway involved in the hypothalamus-mediated control of food intake. Previous studies using whole hypothalamic tissue lysates have shown that fatty acid metabolism plays a key role in ghrelins effect on feeding. Here, we report site-specific effects of central ghrelin on fatty acid metabolism in two critical hypothalamic nuclei, the ventral medial nucleus (VMN) and the arcuate nucleus (Arc). Intracerebroventricular administration of ghrelin to rats activates AMP-activated protein kinase in both the VMN and the Arc, while ghrelin treatment has a site-specific effect on fatty acid metabolic pathways in these two nuclei. In the VMN, central ghrelin increases the phosphorylation level of ACC, indicating the decrease in activity, and decreases the level of malonyl-CoA (the product of ACC). Malonyl-CoA is an inhibitor of carnitine palmitoyltransferase-1 (CPT-1) that is a key enzyme in mitochondrial fatty acid oxidation. Consistent with this action of malonyl-CoA on CPT-1, central ghrelin treatment increases the activity of CPT-1 in the VMN. In contrast, in the Arc, neither malonyl-CoA level nor CPT-1 activity is affected following central ghrelin. Taken together, our data suggest ghrelin exerts differential effects on fatty acid metabolic pathways in the VMN and the Arc.