The GLP-1 Receptor Agonist Liraglutide Increases Myocardial Glucose Oxidation Rates via Indirect Mechanisms and Mitigates Experimental Diabetic Cardiomyopathy.

BACKGROUND: Type 2 diabetes (T2D) increases risk for cardiovascular disease. Of interest, liraglutide, a therapy for T2D that activates the glucagon-like peptide-1 receptor to augment insulin secretion, reduces cardiovascular-related death in people with T2D, though it remains unknown how liraglutide produces these actions. Notably, the glucagon-like peptide-1 receptor is not expressed in ventricular cardiac myocytes, making it likely that ventricular myocardium-independent actions are involved. We hypothesized that augmented insulin secretion may explain how liraglutide indirectly mediates cardioprotection, which thereby increases myocardial glucose oxidation. METHODS: C57BL/6J male mice were fed either a low-fat diet (lean) or were subjected to experimental T2D and treated with either saline or liraglutide 3× over a 24-hour period. Mice were subsequently euthanized and had their hearts perfused in the working mode to assess energy metabolism. A separate cohort of mice with T2D were treated with either vehicle control or liraglutide for 2 weeks for the assessment of cardiac function via ultrasound echocardiography. RESULTS: Treatment of lean mice with liraglutide increased myocardial glucose oxidation without affecting glycolysis. Conversely, direct treatment of the isolated working heart with liraglutide had no effect on glucose oxidation. These findings were recapitulated in mice with T2D and associated with increased circulating insulin levels. Furthermore, liraglutide treatment alleviated diastolic dysfunction in mice with T2D, which was associated with enhanced pyruvate dehydrogenase activity, the rate-limiting enzyme of glucose oxidation. CONCLUSIONS: Our data demonstrate that liraglutide augments myocardial glucose oxidation via indirect mechanisms, which may contribute to how liraglutide improves cardiovascular outcomes in people with T2D.

l-Citrulline supplementation improves glucose and exercise tolerance in obese male mice.

NEW FINDINGS: What is the central question of the study? Does the action of l-citrulline, which has been shown to augment performance in animals and athletes, possibly via increasing mitochondrial function, translate to obese animals, and does this improve glycaemia? What is the main finding and its importance? Chronic supplementation with l-citrulline improves not only exercise capacity, but also glycaemia in obese mice, which would be beneficial as obese individuals are at increased risk for type 2 diabetes. However, l-citrulline supplementation also caused a mild impairment in insulin signalling and insulin tolerance in obese mice. ABSTRACT: l-Citrulline is an organic α-amino acid that has been shown to have a number of salutary actions on whole-body physiology, including reducing muscle wasting and augmenting exercise and muscle performance. The latter has been suggested to arise from elevations in mitochondrial function. Because enhancing mitochondrial function has been proposed as a novel strategy to mitigate insulin resistance, our goal was to determine whether supplementation with l-citrulline could also improve glycaemia in an experimental mouse model of obesity. We hypothesized that l-citrulline treatment would improve glycaemia in obese mice, and this would be associated with elevations in skeletal muscle mitochondrial function. Ten-week-old C57BL/6J mice were fed either a low-fat (10% kcal from lard) or a high-fat (60% kcal from lard) diet, while receiving drinking water supplemented with either vehicle or l-citrulline (0.6 g l-1 ) for 15 weeks. Glucose homeostasis was assessed via glucose/insulin tolerance testing, while in vivo metabolism was assessed via indirect calorimetry, and forced exercise treadmill testing was utilized to assess endurance. As expected, obese mice supplemented with l-citrulline exhibited an increase in exercise capacity, which was associated with an improvement in glucose tolerance. Consistent with augmented mitochondrial function, we observed an increase in whole body oxygen consumption rates in obese mice supplemented with l-citrulline. Surprisingly, l-citrulline supplementation worsened insulin tolerance and reduced insulin signalling in obese mice. Taken together, although l-citrulline supplementation improves both glucose tolerance and exercise capacity in obese mice, caution must be applied with its broad use as a nutraceutical due to a potential deterioration of insulin sensitivity.

The antianginal ranolazine mitigates obesity-induced nonalcoholic fatty liver disease and increases hepatic pyruvate dehydrogenase activity.

Obese individuals are often at risk for nonalcoholic fatty liver disease (NAFLD), insulin resistance, type 2 diabetes (T2D), and cardiovascular diseases such as angina, thereby requiring combination therapies for their comorbidities. Ranolazine is a second-line antianginal agent that also improves glycemia, and our aim was to determine whether ranolazine modifies the progression of obesity-induced NAFLD. Twelve-week-old C57BL/6J male mice were fed a low-fat or high-fat diet for 10 weeks and then treated for 30 days with either vehicle control or ranolazine (50 mg/kg via daily s.c. injection). Glycemia was monitored via glucose/pyruvate/insulin tolerance testing, whereas in vivo metabolism was assessed via indirect calorimetry. Hepatic triacylglycerol content was quantified via the Bligh and Dyer method. Consistent with previous reports, ranolazine treatment reversed obesity-induced glucose intolerance, which was associated with reduced body weight and hepatic steatosis, as well as increased hepatic pyruvate dehydrogenase (PDH) activity. Ranolazine's actions on hepatic PDH activity may be directly mediated, as ranolazine treatment reduced PDH phosphorylation (indicative of increased PDH activity) in HepG2 cells. Therefore, in addition to mitigating angina, ranolazine also reverses NAFLD, which may contribute to its documented glucose-lowering actions, situating ranolazine as an ideal antianginal therapy for obese patients comorbid for NAFLD and T2D.

Skeletal muscle-specific Cre recombinase expression, controlled by the human α-skeletal actin promoter, improves glucose tolerance in mice fed a high-fat diet.

AIMS/HYPOTHESIS: Cre-loxP systems are frequently used in mouse genetics as research tools for studying tissue-specific functions of numerous genes/proteins. However, the expression of Cre recombinase in a tissue-specific manner often produces undesirable changes in mouse biology that can confound data interpretation when using these tools to generate tissue-specific gene knockout mice. Our objective was to characterise the actions of Cre recombinase in skeletal muscle, and we anticipated that skeletal muscle-specific Cre recombinase expression driven by the human α-skeletal actin (HSA) promoter would influence glucose homeostasis. METHODS: Eight-week-old HSA-Cre expressing mice and their wild-type littermates were fed a low- or high-fat diet for 12 weeks. Glucose homeostasis (glucose/insulin tolerance testing) and whole-body energy metabolism (indirect calorimetry) were assessed. We also measured circulating insulin levels and the muscle expression of key regulators of energy metabolism. RESULTS: Whereas tamoxifen-treated HSA-Cre mice fed a low-fat diet exhibited no alterations in glucose homeostasis, we observed marked improvements in glucose tolerance in tamoxifen-treated, but not corn-oil-treated, HSA-Cre mice fed a high-fat diet vs their wild-type littermates. Moreover, Cre dissociation from heat shock protein 90 and translocation to the nucleus was only seen following tamoxifen treatment. These improvements in glucose tolerance were not due to improvements in insulin sensitivity/signalling or enhanced energy metabolism, but appeared to stem from increases in circulating insulin. CONCLUSIONS/INTERPRETATION: The intrinsic glycaemia phenotype in the HSA-Cre mouse necessitates the use of HSA-Cre controls, treated with tamoxifen, when using Cre-loxP models to investigate skeletal muscle-specific gene/protein function and glucose homeostasis.

Female offspring born to obese and insulin-resistant dams are not at increased risk for obesity and metabolic dysfunction during early development.

The percentage of women who are obese at the time of conception or during pregnancy is increasing, with animal and human studies demonstrating that offspring born to obese dams or mothers are at increased risk for obesity and the metabolic syndrome. Our goal was to confirm in an experimental model of metabolic syndrome in the dam, whether the offspring would be at increased risk of obesity. Conversely, we observed that male offspring born to dams with metabolic syndrome had no alterations in their body mass profiles, whereas female offspring born to dams with metabolic syndrome were heavier at weaning, but exhibited no perturbations in energy metabolism. Moreover, they gained weight at a reduced rate versus female offspring born to healthy dams, and thus weighed less at study completion. Hence, our findings suggest that factors other than increased adiposity and insulin resistance during pregnancy are responsible for the increased risk of obesity in children born to obese mothers.

FoxO1 regulates myocardial glucose oxidation rates via transcriptional control of pyruvate dehydrogenase kinase 4 expression.

Pyruvate dehydrogenase (PDH) is the rate-limiting enzyme for glucose oxidation and a critical regulator of metabolic flexibility during the fasting to feeding transition. PDH is regulated via both PDH kinases (PDHK) and PDH phosphatases, which phosphorylate/inactivate and dephosphorylate/activate PDH, respectively. Our goal was to determine whether the transcription factor forkhead box O1 (FoxO1) regulates PDH activity and glucose oxidation in the heart via increasing the expression of Pdk4, the gene encoding PDHK4. To address this question, we differentiated H9c2 myoblasts into cardiac myocytes and modulated FoxO1 activity, after which Pdk4/PDHK4 expression and PDH phosphorylation/activity were assessed. We assessed binding of FoxO1 to the Pdk4 promoter in cardiac myocytes in conjunction with measuring the role of FoxO1 on glucose oxidation in the isolated working heart. Both pharmacological (1 µM AS1842856) and genetic (siRNA mediated) inhibition of FoxO1 decreased Pdk4/PDHK4 expression and subsequent PDH phosphorylation in H9c2 cardiac myocytes, whereas 10 µM dexamethasone-induced Pdk4/PDHK4 expression was abolished via pretreatment with 1 µM AS1842856. Furthermore, transfection of H9c2 cardiac myocytes with a vector expressing FoxO1 increased luciferase activity driven by a Pdk4 promoter construct containing the FoxO1 DNA-binding element region, but not in a Pdk4 promoter construct lacking this region. Finally, AS1842856 treatment in fasted mice enhanced glucose oxidation rates during aerobic isolated working heart perfusions. Taken together, FoxO1 directly regulates Pdk4 transcription in the heart, thereby controlling PDH activity and subsequent glucose oxidation rates.NEW & NOTEWORTHY Although studies have shown an association between FoxO1 activity and pyruvate dehydrogenase kinase 4 expression, our study demonstrated that pyruvate dehydrogenase kinase 4 is a direct transcriptional target of FoxO1 (but not FoxO3/FoxO4) in the heart. Furthermore, we report here, for the first time, that FoxO1 inhibition increases glucose oxidation in the isolated working mouse heart.

Targeting ceramide metabolism in obesity.

Obesity is a major health concern that increases the risk for insulin resistance, type 2 diabetes (T2D), and cardiovascular disease. Thus, an enormous research effort has been invested into understanding how obesity-associated dyslipidemia and obesity-induced alterations in lipid metabolism increase the risk for these diseases. Accordingly, it has been proposed that the accumulation of lipid metabolites in organs such as the liver, skeletal muscle, and heart is critical to these obesity-induced pathologies. Ceramide is one such lipid metabolite that accumulates in tissues in response to obesity, and both pharmacological and genetic strategies that reduce tissue ceramide levels yield salutary actions on overall metabolic health. We will review herein why ceramide accumulates in tissues during obesity and how an increase in intracellular ceramide impacts cellular signaling and function as well as potential mechanisms by which reducing intracellular ceramide levels improves insulin resistance, T2D, atherosclerosis, and heart failure. Because a reduction in skeletal muscle ceramide levels is frequently associated with improvements in insulin sensitivity in humans, the beneficial findings reported for reducing ceramides in preclinical studies may have clinical application in humans. Therefore, modulating ceramide metabolism may be a novel, exciting target for preventing and/or treating obesity-related diseases.