Myocardial fatty acid utilization as a determinant of cardiac efficiency and function

Cardiac Metabolism and MiRNA Interference

The aberrant increase in cardio-metabolic diseases over the past couple of decades has drawn researchers' attention to explore and unveil the novel mechanisms implicated in cardiometabolic diseases. Recent evidence disclosed that the derangement of cardiac energy substrate metabolism plays a predominant role in the development and progression of chronic cardiometabolic diseases. Hence, in-depth comprehension of the novel molecular mechanisms behind impaired cardiac metabolism-mediated diseases is crucial to expand treatment strategies. The complex and dynamic pathways of cardiac metabolism are systematically controlled by the novel executor, microRNAs (miRNAs). miRNAs regulate target gene expression by either mRNA degradation or translational repression through base pairing between miRNA and the target transcript, precisely at the 3' seed sequence and conserved heptametrical sequence in the 5' end, respectively. Multiple miRNAs are involved throughout every cardiac energy substrate metabolism and play a differential role based on the variety of target transcripts. Novel theoretical strategies have even entered the clinical phase for treating cardiometabolic diseases, but experimental evidence remains inadequate. In this review, we identify the potent miRNAs, their direct target transcripts, and discuss the remodeling of cardiac metabolism to cast light on further clinical studies and further the expansion of novel therapeutic strategies. This review is categorized into four sections which encompass (i) a review of the fundamental mechanism of cardiac metabolism, (ii) a divulgence of the regulatory role of specific miRNAs on cardiac metabolic pathways, (iii) an understanding of the association between miRNA and impaired cardiac metabolism, and (iv) summary of available miRNA targeting therapeutic approaches.

Lipotoxicity in a Vicious Cycle of Pancreatic Beta Cell Exhaustion

Hyperlipidemia is a common metabolic disorder in modern society and may precede hyperglycemia and diabetes by several years. Exactly how disorders of lipid and glucose metabolism are related is still a mystery in many respects. We analyze the effects of hyperlipidemia, particularly free fatty acids, on pancreatic beta cells and insulin secretion. We have developed a computational model to quantitatively estimate the effects of specific metabolic pathways on insulin secretion and to assess the effects of short- and long-term exposure of beta cells to elevated concentrations of free fatty acids. We show that the major trigger for insulin secretion is the anaplerotic pathway via the phosphoenolpyruvate cycle, which is affected by free fatty acids via uncoupling protein 2 and proton leak and is particularly destructive in long-term chronic exposure to free fatty acids, leading to increased insulin secretion at low blood glucose and inadequate insulin secretion at high blood glucose. This results in beta cells remaining highly active in the “resting” state at low glucose and being unable to respond to anaplerotic signals at high pyruvate levels, as is the case with high blood glucose. The observed fatty-acid-induced disruption of anaplerotic pathways makes sense in the context of the physiological role of insulin as one of the major anabolic hormones.

The Hormetic Effect of Metformin: "Less Is More"?

Metformin (MTF) is the first-line therapy for type 2 diabetes (T2DM). The euglycemic effect of MTF is due to the inhibition of hepatic glucose production. Literature reports that the principal molecular mechanism of MTF is the activation of 5′-AMP-activated protein kinase (AMPK) due to the decrement of ATP intracellular content consequent to the inhibition of Complex I, although this effect is obtained only at millimolar concentrations. Conversely, micromolar MTF seems to activate the mitochondrial electron transport chain, increasing ATP production and limiting oxidative stress. This evidence sustains the idea that MTF exerts a hormetic effect based on its concentration in the target tissue. Therefore, in this review we describe the effects of MTF on T2DM on the principal target organs, such as liver, gut, adipose tissue, endothelium, heart, and skeletal muscle. In particular, data indicate that all organs, except the gut, accumulate MTF in the micromolar range when administered in therapeutic doses, unmasking molecular mechanisms that do not depend on Complex I inhibition.

Role of cAMP in Double Switch of Glucagon Secretion

Glucose metabolism plays a crucial role in modulating glucagon secretion in pancreatic alpha cells. However, the downstream effects of glucose metabolism and the activated signaling pathways influencing glucagon granule exocytosis are still obscure. We developed a computational alpha cell model, implementing metabolic pathways of glucose and free fatty acids (FFA) catabolism and an intrinsically activated cAMP signaling pathway. According to the model predictions, increased catabolic activity is able to suppress the cAMP signaling pathway, reducing exocytosis in a Ca²⁺-dependent and Ca²⁺ independent manner. The effect is synergistic to the pathway involving ATP-dependent closure of K_ATP channels and consequent reduction of Ca²⁺. We analyze the contribution of each pathway to glucagon secretion and show that both play decisive roles, providing a kind of "secure double switch". The cAMP-driven signaling switch plays a dominant role, while the ATP-driven metabolic switch is less favored. The ratio is approximately 60:40, according to the most recent experimental evidence.

Modelling of energy-driven switch for glucagon and insulin secretion

We present a mathematical model of the energy-driven metabolic switch for glucagon and insulin secretion from pancreatic alpha and beta cells, respectively. The energy status related to hormone secretion is studied for various glucose concentrations. Additionally, the physiological response is studied with regards to the presence of other metabolites, particularly the free-fatty acids. At low glucose, the ATP production in alpha cells is high due to free-fatty acids oxidation in mitochondria, which enables glucagon secretion. When the glucose concentration is elevated above the threshold value, the glucagon secretion is switched off due to the contribution of glycolytic ATP production, representing an "anaerobic switch". On the other hand, during hypoglycemia, the ATP production in beta cells is low, reflecting a "waiting state" for glucose as the main metabolite. When glucose is elevated above the threshold value, the oxidative fate of glucose in mitochondria is the main source of energy required for effective insulin secretion, i.e. the "aerobic switch". Our results show the importance of well-regulated and fine-tuned energetic processes in pancreatic alpha and beta cells required for efficient hormone secretion and hence effective blood glucose regulation. These energetic processes have to be appropriately switched on and off based on the sensing of different metabolites by alpha and beta cells. Our computational results indicate that disturbances in cell energetics (e.g. mitochondrial dysfunction), and dysfunctional metabolite sensing and distribution throughout the cell might be related to pathologies such as metabolic syndrome and diabetes.

Grape skin fermentation by Lactobacillus fermentum CQPC04 has anti-oxidative effects on human embryonic kidney cells and apoptosis-promoting effects on human hepatoma cells

Studies on the antioxidant effects of grapes have attracted increasing interest. We used Lactobacillus fermentum CQPC04 to ferment grape skins. Components of the fermentation solution were separated and identified via high-performance liquid chromatography, and polyphenol compounds, including resveratrol and epicatechin, were isolated and identified from the fermentation solution. The major fermentation production components were assessed for their antioxidative abilities when administered under H2O2-induced oxidative damage in cell culture models. The fermentation solution significantly reduced oxidative damage, increased the expressions of the superoxide dismutase (SOD), catalase (CAT), glutathione (GSH), and GSH-peroxidase (GSH-Px) antioxidant genes and proteins in human embryonic kidney (293T) cells, stimulated the indices of total antioxidant capacity (T-AOC), SOD, CAT, GSH, and GSH-Px, and inhibited the indices of lactate dehydrogenase (LDH), malondialdehyde (MDA), and nitric oxide (NO), and the fermentation solution alleviated the increase in glutathione oxidized (GSSG) caused by oxidative damage, and the ratio of GSH/GSSG was up-regulated compared to the damage group. The fermentation solution also accelerated Human hepatoma (HepG2) cell death. Applying the fermentation solution to HepG2 cells significantly altered the cell morphology. HepG2 cell apoptosis and cell cycles were detected via flow cytometry. The fermentation solution promoted the apoptotic rate, and more cells were retained in the G2 phase, which prevented cells from further dividing. In the fermented group, the mRNA expression levels of Bcl-2, cox-2, PCNA, CD1, C-myc, CDK4, NF-κB and pRb1 were significantly decreased, and the expression levels of Caspase-3, Caspase-7, Caspase-8, Caspase-9, p53, TGF-β, and p21 were higher than those in the normal group. Phospho-NF-κB (p65), Bax and Caspase-8 protein expression increased, and NF-κB (p65) protein expression decreased. Protein expression levels also promoted apoptosis. Fermented grape skin solution is bioavailable in vitro and may help prevent oxidation and cancer cell proliferation.

Cardiac abnormalities in patients with nonalcoholic fatty liver disease : Insights from auxiliary examinations

Nonalcoholic fatty liver disease (NAFLD) is one of the most common forms of chronic liver disease in developed countries and is associated with type 2 diabetes mellitus, obesity, hypertension, dyslipidemia, and metabolic syndrome. It is defined as steatosis in over 5% of hepatocytes. The disease spectrum of NAFLD ranges from simple fatty liver to nonalcoholic steatohepatitis, liver fibrosis, even hepatic cirrhosis. The disease affects various extra-hepatic systems such as the cardiovascular system and urinary system. Heart-related disease is identified as the leading cause of mortality in NAFLD patients rather than liver-related disease. In this review, we summarize the cardiac abnormalities (structural, functional, arrhythmic cardiac complications etc.) seen in NAFLD patients with the assistance of auxiliary examinations, such as electrocardiography, echocardiography, computed tomography, magnetic resonance imaging etc. In addition, the epidemiology of NAFLD and how NAFLD affects the myocardium are also discussed.

Adropin regulates cardiac energy metabolism and improves cardiac function and efficiency

BACKGROUND

Impaired cardiac insulin signalling and high cardiac fatty acid oxidation rates are characteristics of conditions of insulin resistance and diabetic cardiomyopathies. The potential role of liver-derived peptides such as adropin in mediating these changes in cardiac energy metabolism is unclear, despite the fact that in skeletal muscle adropin can preferentially promote glucose metabolism and improve insulin sensitivity.

OBJECTIVES

To determine the influence of adropin on cardiac energy metabolism, insulin signalling and cardiac efficiency.

METHODS

C57Bl/6 mice were injected with either vehicle or a secretable form of adropin (450 nmol/kg, i.p.) three times over a 24-h period. The mice were fasted to accentuate the differences between animals in adropin plasma levels before their hearts were isolated and perfused using a working heart system. In addition, direct addition of adropin to the perfusate of ex vivo hearts isolated from non-fasting mice was utilized to investigate the acute effects of the peptide on heart metabolism and ex vivo function.

RESULTS

In contrast to the observed fasting-induced predominance of fatty acid oxidation as a source of ATP production in control hearts, insulin inhibition of fatty acid oxidation was preserved by adropin treatment. Adropin-treated mouse hearts also showed a higher cardiac work, which was accompanied by improved cardiac efficiency and enhanced insulin signalling compared to control hearts. Interestingly, acute adropin administration to isolated working hearts also resulted in an inhibition of fatty acid oxidation, accompanied by a robust stimulation of glucose oxidation compared to vehicle-treated hearts. Adropin also increased activation of downstream cardiac insulin signalling. Moreover, both in vivo and ex vivo treatment protocols induced a reduction in the inhibitory phosphorylation of pyruvate dehydrogenase (PDH), the major enzyme of glucose oxidation, and the protein levels of the responsible kinase PDH kinase 4 and the insulin-signalling inhibitory phosphorylation of JNK (p-T¹⁸³/Y¹⁸⁵) and IRS-1 (p-S³⁰⁷), suggesting acute receptor- and/or post-translational modification-mediated mechanisms.

CONCLUSIONS

These results demonstrate that adropin has important effects on energy metabolism in the heart and may be a putative candidate for the treatment of cardiac disease associated with impaired insulin sensitivity.

Cardiac Resynchronisation Therapy and Cellular Bioenergetics: Effects Beyond Chamber Mechanics

Cardiac resynchronisation therapy is a cornerstone in the treatment of advanced dyssynchronous heart failure. However, despite its widespread clinical application, precise mechanisms through which it exerts its beneficial effects remain elusive. Several studies have pointed to a metabolic component suggesting that, both in concert with alterations in chamber mechanics and independently of them, resynchronisation reverses detrimental changes to cellular metabolism, increasing energy efficiency and metabolic reserve. These actions could partially account for the existence of responders that improve functionally but not echocardiographically. This article will attempt to summarise key components of cardiomyocyte metabolism in health and heart failure, with a focus on the dyssynchronous variant. Both chamber mechanics-related and -unrelated pathways of resynchronisation effects on bioenergetics – stemming from the ultramicroscopic level – and a possible common underlying mechanism relating mechanosensing to metabolism through the cytoskeleton will be presented. Improved insights regarding the cellular and molecular effects of resynchronisation on bioenergetics will promote our understanding of non-response, optimal device programming and lead to better patient care.

Glucagon-like peptide-1 receptor mediated control of cardiac energy metabolism

Glucagon-like peptide-1 receptor (GLP-1R) agonists are frequently used to improve glycemia in patients with type 2 diabetes (T2D). Recent data from cardiovascular outcomes trials for the GLP-1R agonists, liraglutide and semaglutide, have also demonstrated significant reductions in death rates from cardiovascular causes in patients with T2D. As cardiovascular death is the number one cause of death in patients with T2D, understanding the mechanisms by which GLP-1R agonists such as liraglutide and semaglutide improve cardiac function is essential. Yet despite strong evidence from preclinical and clinical studies supporting the cardioprotective actions of GLP-1R agonists, the precise mechanism(s) by which this drug-class for T2D may produce these beneficial actions remains enigmatic. Negligible GLP-1R expression in ventricular cardiac myocytes suggests that GLP-1R agonist-induced cardioprotection is likely partially attributed to indirect actions on peripheral tissues other than the heart. Because insulin increases glucose oxidation, whereas glucagon increases fatty acid oxidation in the heart, GLP-1R agonist-induced increases and decreases in insulin and glucagon secretion, respectively, may modify cardiac energy metabolism in T2D patients. This may represent a potential mechanism for GLP-1R agonist-induced cardioprotection in T2D, as increases in fatty acid oxidation and decreases in glucose oxidation are frequently observed in the hearts of animals and human subjects with T2D.

The role of cardiac lipotoxicity in the pathogenesis of diabetic cardiomyopathy

Cardiomyopathy, the presence of cardiac dysfunction independent of ischemic heart disease and/or hypertension, is becoming a more prominent condition in our diabetic patient population. Unfortunately, we do not yet understand the mechanism(s) responsible for causing diabetic cardiomyopathy. With the recent explosion in the obesity and Type 2 diabetes epidemic, our understanding of dyslipidemia and the adverse effects of lipid surplus on cellular and organ function has grown considerably. Numerous studies now illustrate that excess lipid accumulation may exert direct toxic effects on cellular function, a term coined 'lipotoxicity'. As obesity and Type 2 diabetes are significant risk factors for cardiovascular disease, cardiac lipotoxicity may represent a significant component mediating the diabetic cardiomyopathy phenotype. Therefore, a more complete understanding of how cardiac lipotoxicity is regulated and how different lipid metabolites cause cellular dysfunction may lead to the discovery of novel targets to treat cardiomyopathy in our diabetic patient population.

Gut microbiota metabolism of L-carnitine and cardiovascular risk

In recent years, a number of studies have alluded to the importance of the intestinal microflora in controlling whole-body metabolic homeostasis and organ physiology. In particular, it has been suggested that the hepatic production of trimethylamine-N-oxide (TMAO) from gut microbiota-derived trimethylamine (TMA) may enhance cardiovascular risk via promoting atherosclerotic lesion development. The source of TMA production via the gut microbiota appears to originate from 2 principle sources, either phosphatidylcholine/choline and/or L-carnitine. Therefore, it has been postulated that consumption of these dietary sources, which are often found in large quantities in red meats, may be critical factors promoting cardiovascular risk. In contrast, a number of studies demonstrate beneficial properties for l-carnitine consumption against metabolic diseases including skeletal muscle insulin resistance and ischemic heart disease. Furthermore, fish are a significant source of TMAO, but dietary fish consumption and fish oil supplementation may exhibit positive effects on cardiovascular health. In this mini-review we will discuss the discrepancies regarding L-carnitine supplementation and its possible negative effects on cardiovascular risk through potential increases in TMAO production, as well as its positive effects on metabolic health via increasing glucose metabolism in the muscle and heart.

The impact of current and novel anti-diabetic therapies on cardiovascular risk

Type 2 diabetes mellitus (T2DM) has become an overwhelming health condition that is no longer just a threat to developed nations, but to undeveloped nations as well. Current therapies for T2DM are relatively effective in controlling hyperglycemia; examples include metformin, thiazolidinediones, sulfonylurea derivatives, α-glucosidase inhibitors, glucagon-like peptide-1 receptor agonists and dipeptidyl peptidase-4 inhibitors. Despite their efficacy in controlling hyperglycemia, due to recent findings of increased cardiovascular risk following treatment with either rosiglitazone or intensive glucose lowering, new guidelines from the US FDA recommend that new therapies for diabetes not only improve glycemia, but exert no adverse cardiovascular effects. Based on cardiovascular risk profiles, metformin appears to be the superior anti-diabetic therapy, although studies in humans with glucagon-like peptide-1 receptor agonists are encouraging. As patients with T2DM also often have cardiovascular disease, the increased rigor in drug development should ultimately reduce the health burden of both of these conditions.

Pyridine nucleotide regulation of cardiac intermediary metabolism

The pyridine nucleotides NAD(+) and NADP(+) play a pivotal role in regulating intermediary metabolism in the heart. The intracellular NAD(+)/NADH ratio controls flux through various dehydrogenase enzymes involved in both anaerobic and aerobic metabolism and also regulates posttranslational protein modification. The intracellular NADP(+)/NADPH ratio controls flux through the pentose phosphate pathway (PPP) and the polyol pathway, while also regulating ion channel function and oxidative stress. Not only does the NAD(+)/NADH ratio regulate the rates of ATP production, it can also modify energy substrate preference. For instance, in many forms of heart disease a greater contribution from fatty acids for oxidative energy metabolism increases fatty acid β-oxidation-derived NADH, which can activate pyruvate dehydrogenase (PDH) kinase isoforms that inhibit PDH and subsequent glucose oxidation. As such, novel therapies that overcome fatty acid β-oxidation-induced inhibition of PDH improve cardiac efficiency and subsequent function during ischemia/reperfusion and in heart failure. Furthermore, recent studies have implicated a pivotal role for increased PPP-derived NADPH in mediating oxidative stress observed in heart failure. In this article, we review the multiple actions of NAD(+)/NADH and NADP(+)/NADPH in regulating intermediary metabolism in the heart. A better understanding of the roles of NAD(+)/NADH and NADP(+)/NADPH in cellular physiology and pathology could potentially be used to exploit pyridine nucleotide modification in the treatment of a number of different forms of heart disease.

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.

Targeting intermediary metabolism in the hypothalamus as a mechanism to regulate appetite

The central nervous system mediates energy balance (energy intake and energy expenditure) in the body; the hypothalamus has a key role in this process. Recent evidence has demonstrated an important role for hypothalamic malonyl CoA in mediating energy balance. Malonyl CoA is generated by the carboxylation of acetyl CoA by acetyl CoA carboxylase and is then either incorporated into long-chain fatty acids by fatty acid synthase, or converted back to acetyl-CoA by malonyl CoA decarboxylase. Increased hypothalamic malonyl CoA is an indicator of energy surplus, resulting in a decrease in food intake and an increase in energy expenditure. In contrast, a decrease in hypothalamic malonyl CoA signals an energy deficit, resulting in an increased appetite and a decrease in body energy expenditure. A number of hormonal and neural orexigenic and anorexigenic signaling pathways have now been shown to be associated with changes in malonyl CoA levels in the arcuate nucleus (ARC) of the hypothalamus. Despite compelling evidence that malonyl CoA is an important mediator in the hypothalamic ARC control of food intake and regulation of energy balance, the mechanism(s) by which this occurs has not been established. Malonyl CoA inhibits carnitine palmitoyltransferase-1 (CPT-1), and it has been proposed that the substrate of CPT-1, long-chain acyl CoA(s), may act as a mediator(s) of appetite and energy balance. However, recent evidence has challenged the role of long-chain acyl CoA(s) in this process, as well as the involvement of CPT-1 in hypothalamic malonyl CoA signaling. A better understanding of how malonyl CoA regulates energy balance should provide novel approaches to targeting intermediary metabolism in the hypothalamus as a mechanism to control appetite and body weight. Here, we review the data supporting an important role for malonyl CoA in mediating hypothalamic control of energy balance, and recent evidence suggesting that targeting malonyl CoA synthesis or degradation may be a novel approach to favorably modify appetite and weight gain.