AMPK and metabolic adaptation by the heart to pressure overload

Increased acylcarnitines in infant heart failure indicate fatty acid oxidation inhibition: towards therapeutic options?

AIMS

Heart failure in adults is characterized by reduction of long-chain fatty acid oxidation in favour of carbohydrate metabolism. This adaptive phenomenon becomes maladaptive because energy conversion decreases and lipid toxic derivatives known to impair cardiac function are accumulating. No data are available concerning metabolic modification in heart failure in children.

METHODS AND RESULTS

In order to evaluate the fatty acid oxidation in children suffering from heart failure, acylcarnitine profiles on dried blood spots were obtained from children under 16 years old with dilated cardiomyopathy and clinical heart failure (DCM-HF) and control children. Nine children were included in the DCM-HF group and eight in the control group. Acylcarnitine profiles revealed a significant 3.1-fold increase of total acylcarnitines (sum of C3 to C18 acylcarnitine species) in DCM-HF children compared with controls. This result persisted considering the sum of long-chain acylcarnitines (sum of C14 to C18 species), medium-chain acylcarnitines (sum of C8 to C12 species), and short-chain acylcarnitines (sum of C3 to C6 species), respectively, 2.0-, 2.6-, and 1.9-fold increase compared with the control group. A significant linear correlation was found between left ventricular dilatation or ejection fraction and acylcarnitines accumulation. Finally, acylcarnitine ratio C16OH/C16 and C18OH/C18 enhanced in the DCM-HF group, suggesting a diminution of the long-chain hydroxyl acyl-CoA dehydrogenase activity.

CONCLUSIONS

Our results suggest down-regulation of fatty acid oxidation in children with heart failure. Such lipidomic alteration could worsen heart function and may suggest considering a metabolic treatment of heart failure in children.

Integrated Control of Fatty Acid Metabolism in Heart Failure

Disrupted fatty acid metabolism is one of the most important metabolic features in heart failure. The heart obtains energy from fatty acids via oxidation. However, heart failure results in markedly decreased fatty acid oxidation and is accompanied by the accumulation of excess lipid moieties that lead to cardiac lipotoxicity. Herein, we summarized and discussed the current understanding of the integrated regulation of fatty acid metabolism (including fatty acid uptake, lipogenesis, lipolysis, and fatty acid oxidation) in the pathogenesis of heart failure. The functions of many enzymes and regulatory factors in fatty acid homeostasis were characterized. We reviewed their contributions to the development of heart failure and highlighted potential targets that may serve as promising new therapeutic strategies.

Glycometabolism reprogramming: Implications for cardiovascular diseases

Glycometabolism is well known for its roles as the main source of energy, which mainly includes three metabolic pathways: oxidative phosphorylation, glycolysis and pentose phosphate pathway. The orderly progress of glycometabolism is the basis for the maintenance of cardiovascular function. However, upon exposure to harmful stimuli, the intracellular glycometabolism changes or tends to shift toward another glycometabolism pathway more suitable for its own development and adaptation. This shift away from the normal glycometabolism is also known as glycometabolism reprogramming, which is commonly related to the occurrence and aggravation of cardiovascular diseases. In this review, we elucidate the physiological role of glycometabolism in the cardiovascular system and summarize the mechanisms by which glycometabolism drives cardiovascular diseases, including diabetes, cardiac hypertrophy, heart failure, atherosclerosis, and pulmonary hypertension. Collectively, directing GMR back to normal glycometabolism might provide a therapeutic strategy for the prevention and treatment of related cardiovascular diseases.

Effects of intestinal bacteria on cardiovascular disease

In the long process of human evolution, the Intestinal Bacteria has become intimately related to human health, producing many metabolites in the intestines that can affect cardiovascular disease. Today, the incidence of cardiovascular disease is rising, its treatment is becoming increasingly important, and new therapeutic targets are needed. Here we describe the effects of trimethylamine oxide (TMAO), lipid metabolism, phenolic compounds, indole sulfate (IS), oleuropein (OL), and hydroxytyrosol (HT) on atherosclerosis, heart failure, hypertension, and other cardiovascular diseases, as well as their mechanism of action. This study provides new ideas, new methods, and new directions for the treatment of cardiovascular disease.

Metformin Protects Against Sunitinib-induced Cardiotoxicity: Investigating the Role of AMPK

ABSTRACT

Sunitinib is associated with cardiotoxicity through inhibition of AMP-protein kinase (AMPK) signaling. By contrast, the common antidiabetic agent metformin has demonstrated cardioprotection through indirect AMPK activation. In this study, we investigate the effects of metformin during sunitinib-induced cytotoxicity. Left ventricular developed pressure, coronary flow, heart rate, and infarct size were measured in Langendorff-perfused rat hearts treated with 1 µM sunitinib ±50 µM metformin ±1 µM human equilibrative nucleoside transporter inhibitor S-(4-Nitrobenzyl)-6-thioinosine (NBTI). Western blot analysis was performed for p-AMPKα levels. Primary isolated cardiac myocytes from the left ventricular tissue were used to measure live cell population levels. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay was used to assess adjunctive treatment of and metformin in human hepatoma G2 and promyelocytic leukemia (HL-60) cells treated with 0.1-100 µM sunitinib ±50 µM metformin. In the perfused hearts, coadministration of metformin attenuated the sunitinib-induced changes to left ventricular developed pressure, infarct size, and cardiac myocyte population. Western blot analysis revealed a significant decrease in p-AMPKα during sunitinib treatment, which was attenuated after coadministration with metformin. All metformin-induced effects were attenuated, and NBTI was coadministered. The MTT assay demonstrated an increase in the EC50 value during coadministration of metformin with sunitinib compared with sunitinib monotherapy in hepatoma G2 and HL-60 cell lines, demonstrating the impact and complexity of metformin coadministration and the possible role of AMPK signaling. This study highlights the novel cardioprotective properties of metformin and AMPK activation during sunitinib-induced cardiotoxicity when administered together in the Langendorff heart model.

Testosterone activates glucose metabolism through AMPK and androgen signaling in cardiomyocyte hypertrophy

Background

Testosterone regulates nutrient and energy balance to maintain protein synthesis and metabolism in cardiomyocytes, but supraphysiological concentrations induce cardiac hypertrophy. Previously, we determined that testosterone increased glucose uptake—via AMP-activated protein kinase (AMPK)—after acute treatment in cardiomyocytes. However, whether elevated glucose uptake is involved in long-term changes of glucose metabolism or is required during cardiomyocyte growth remained unknown. In this study, we hypothesized that glucose uptake and glycolysis increase in testosterone-treated cardiomyocytes through AMPK and androgen receptor (AR).

Methods

Cultured cardiomyocytes were stimulated with 100 nM testosterone for 24 h, and hypertrophy was verified by increased cell size and mRNA levels of β-myosin heavy chain (β-mhc). Glucose uptake was assessed by 2-NBDG. Glycolysis and glycolytic capacity were determined by measuring extracellular acidification rate (ECAR).

Results

Testosterone induced cardiomyocyte hypertrophy that was accompanied by increased glucose uptake, glycolysis enhancement and upregulated mRNA expression of hexokinase 2. In addition, testosterone increased AMPK phosphorylation (Thr172), while inhibition of both AMPK and AR blocked glycolysis and cardiomyocyte hypertrophy induced by testosterone. Moreover, testosterone supplementation in adult male rats by 5 weeks induced cardiac hypertrophy and upregulated β-mhc, Hk2 and Pfk2 mRNA levels.

Conclusion

These results indicate that testosterone stimulates glucose metabolism by activation of AMPK and AR signaling which are critical to induce cardiomyocyte hypertrophy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40659-021-00328-4.

Reactivation of fatty acid oxidation by medium chain fatty acid prevents myocyte hypertrophy in H9c2 cell line

Metabolic shift is an important contributory factor for progression of hypertension-induced left ventricular hypertrophy into cardiac failure. Under hypertrophic conditions, heart switches its substrate preference from fatty acid to glucose. Prolonged dependence on glucose for energy production has adverse cardiovascular consequences. It was reported earlier that reactivation of fatty acid metabolism with medium chain triglycerides ameliorated cardiac hypertrophy, oxidative stress and energy level in spontaneously hypertensive rat. However, the molecular mechanism mediating the beneficial effect of medium chain triglycerides remained elusive. It was hypothesized that reduction of cardiomyocyte hypertrophy by medium chain fatty acid (MCFA) is mediated by modulation of signaling pathways over expressed in cardiac hypertrophy. The protective effect of medium chain fatty acid (MCFA) was evaluated in cellular model of myocyte hypertrophy. H9c2 cells were stimulated with Arginine vasopressin (AVP) for the induction of hypertrophy. Cell volume and secretion of brain natriuretic peptide (BNP) were used for assessment of cardiomyocyte hypertrophy. Cells were pretreated with MCFA (Caprylic acid) and metabolic modulation was assessed from the expression of medium-chain acyl-CoA dehydrogenase (MCAD), cluster of differentiation-36 (CD36) and peroxisome proliferator-activated receptor (PPAR)-α mRNA. The signaling molecules modified by MCFA was evaluated from protein expression of mitogen activated protein kinases (MAPK: ERK1/2, p38 and JNK) and Calcineurin A. Pretreatment with MCFA stimulated fatty acid metabolism in hypertrophic H9c2, with concomitant reduction of cell volume and BNP secretion. MCFA reduced activated ERK1/2, JNK and calicineurin A expression mediated by AVP. In conclusion, the beneficial effect of MCFA is possibly mediated by stimulation of fatty acid metabolism and modulation of MAPK and Calcineurin A.

Beneficial Effects of Baduanjin Exercise on Left Ventricular Remodelling in Patients after Acute Myocardial Infarction: an Exploratory Clinical Trial and Proteomic Analysis

BACKGROUND

The beneficial effects of physical exercise on cardiac remodelling improvement after myocardial infarction have already been suggested. However, the results of previous clinical trials have not been consistent. Moreover, the putative molecular mechanisms leading to the clinically observed effects of physical exercise still remain elusive.

AIM

We aimed to evaluate whether the well-defined and strictly controlled traditional Chinese Qigong Baduanjin exercise (BE) would attenuate the adverse left ventricular (LV) remodelling in patients with ST-elevation myocardial infarction (STEMI).

METHODS

A total of 110 clinically stable STEMI patients, following successful revascularization of their infarcted coronary arteries, were randomized and enrolled in two groups: 56 were subjected to a 12-week BE-based cardiac rehabilitation programme (BE group), and the remaining 54 were exposed to the usual physical exercise (control group) for the same time period. The primary outcome was the change from baseline to 6 months in the echocardiographic LV end-diastolic volume index (ΔLVEDVi). Proteomic analysis was also performed to uncover associated mechanisms.

RESULTS

Compared with the control group, the BE group showed significantly lower ΔLVEDVi (-5.1 ± 1.1 vs. 0.3 ± 1.2 mL/m², P < 0.01). Proteomic analysis revealed BE-induced variations in the expression of 80 proteins linked to regulation the of metabolic process, immune process, and extracellular matrix reorganization. Furthermore, correlation analyses between the validated serum proteomes and primary endpoint demonstrated a positive association between ΔLVEDVi and MMP-9 expression, but a negative correlation between ΔLVEDVi and CXCL1 expression.

CONCLUSION

This is the first study indicating that BE in STEMI patients can alleviate adverse LV remodelling associated with beneficial energy metabolism adaptation, inflammation curbing, and extracellular matrix organization adjustment.

Cardiac ketone body metabolism

The ketone bodies, d-β-hydroxybutyrate and acetoacetate, are soluble 4-carbon compounds derived principally from fatty acids, that can be metabolised by many oxidative tissues, including heart, in carbohydrate-depleted conditions as glucose-sparing energy substrates. They also have important signalling functions, acting through G-protein coupled receptors and histone deacetylases to regulate metabolism and gene expression including that associated with anti-oxidant activity. Their concentration, and hence availability, increases in diabetes mellitus and heart failure. Whilst known to be substrates for ATP production, especially in starvation, their role(s) in the heart, and in heart disease, is uncertain. Recent evidence, reviewed here, indicates that increased ketone body metabolism is a feature of heart failure, and is accompanied by other changes in substrate selection. Whether the change in myocardial ketone body metabolism is adaptive or maladaptive is unknown, but it offers the possibility of using exogenous ketones to treat the failing heart.

Androgen-Regulated Cardiac Metabolism in Aging Men

The prevalence of cardiovascular mortality is higher in men than in age-matched premenopausal women. Gender differences are linked to circulating sex-related steroid hormone levels and their cardio-specific actions, which are critical factors involved in the prevalence and features of age-associated cardiovascular disease. In women, estrogens have been described as cardioprotective agents, while in men, testosterone is the main sex steroid hormone. The effects of testosterone as a metabolic regulator and cardioprotective agent in aging men are poorly understood. With advancing age, testosterone levels gradually decrease in men, an effect associated with increasing fat mass, decrease in lean body mass, dyslipidemia, insulin resistance and adjustment in energy substrate metabolism. Aging is associated with a decline in metabolism, characterized by modifications in cardiac function, excitation-contraction coupling, and lower efficacy to generate energy. Testosterone deficiency -as found in elderly men- rapidly becomes an epidemic condition, associated with prominent cardiometabolic disorders. Therefore, it is highly probable that senior men showing low testosterone levels will display symptoms of androgen deficiency, presenting an unfavorable metabolic profile and increased cardiovascular risk. Moreover, recent reports establish that testosterone replacement improves cardiomyocyte bioenergetics, increases glucose metabolism and reduces insulin resistance in elderly men. Thus, testosterone-related metabolic signaling and gene expression may constitute relevant therapeutic target for preventing, or treating, age- and gender-related cardiometabolic diseases in men. Here, we will discuss the impact of current evidence showing how cardiac metabolism is regulated by androgen levels in aging men.

Uncoupling protein 3 deficiency impairs myocardial fatty acid oxidation and contractile recovery following ischemia/reperfusion

Patients with insulin resistance and type 2 diabetes have poor cardiac outcomes following myocardial infarction (MI). The mitochondrial uncoupling protein 3 (UCP3) is down-regulated in the heart with insulin resistance. We hypothesized that decreased UCP3 levels contribute to poor cardiac recovery following ischemia/reperfusion (I/R). After confirming that myocardial UCP3 levels were systematically decreased by 20-49% in animal models of insulin resistance and type 2 diabetes, we genetically engineered Sprague-Dawley rats with partial loss of UCP3 (ucp3^+/-). Wild-type littermates (ucp3^+/+) were used as controls. Isolated working hearts from ucp3^+/- rats were characterized by impaired recovery of cardiac power and decreased long-chain fatty acid (LCFA) oxidation following I/R. Mitochondria isolated from ucp3^+/- hearts subjected to I/R in vivo displayed increased reactive oxygen species (ROS) generation and decreased respiratory complex I activity. Supplying ucp3^+/- cardiac mitochondria with the medium-chain fatty acid (MCFA) octanoate slowed electron transport through the respiratory chain and reduced ROS generation. This was accompanied by improvement of cardiac LCFA oxidation and recovery of contractile function post ischemia. In conclusion, we demonstrated that normal cardiac UCP3 levels are essential to recovery of LCFA oxidation, mitochondrial respiratory capacity, and contractile function following I/R. These results reveal a potential mechanism for the poor prognosis of type 2 diabetic patients following MI and expose MCFA supplementation as a feasible metabolic intervention to improve recovery of these patients at reperfusion.

Medium-chain fatty acids modulate myocardial function via a cardiac odorant receptor

Several studies have demonstrated the expression of odorant receptors (OR) in various human tissues and their involvement in different physiological and pathophysiological processes. However, the functional role of ORs in the human heart is still unclear. Here, we firstly report the functional characterization of an OR in the human heart. Initial next-generation sequencing analysis revealed the OR expression pattern in the adult and fetal human heart and identified the fatty acid-sensing OR51E1 as the most highly expressed OR in both cardiac development stages. An extensive characterization of the OR51E1 ligand profile by luciferase reporter gene activation assay identified 2-ethylhexanoic acid as a receptor antagonist and various structurally related fatty acids as novel OR51E1 ligands, some of which were detected at receptor-activating concentrations in plasma and epicardial adipose tissue. Functional investigation of the endogenous receptor was carried out by Ca²⁺ imaging of human stem cell-derived cardiomyocytes. Application of OR51E1 ligands induced negative chronotropic effects that depended on activation of the OR. OR51E1 activation also provoked a negative inotropic action in cardiac trabeculae and slice preparations of human explanted ventricles. These findings indicate that OR51E1 may play a role as metabolic regulator of cardiac function.

Cardiac Ryanodine Receptor (Ryr2)-mediated Calcium Signals Specifically Promote Glucose Oxidation via Pyruvate Dehydrogenase

Cardiac ryanodine receptor (Ryr2) Ca²⁺ release channels and cellular metabolism are both disrupted in heart disease. Recently, we demonstrated that total loss of Ryr2 leads to cardiomyocyte contractile dysfunction, arrhythmia, and reduced heart rate. Acute total Ryr2 ablation also impaired metabolism, but it was not clear whether this was a cause or consequence of heart failure. Previous in vitro studies revealed that Ca²⁺ flux into the mitochondria helps pace oxidative metabolism, but there is limited in vivo evidence supporting this concept. Here, we studied heart-specific, inducible Ryr2 haploinsufficient (cRyr2Δ50) mice with a stable 50% reduction in Ryr2 protein. This manipulation decreased the amplitude and frequency of cytosolic and mitochondrial Ca²⁺ signals in isolated cardiomyocytes, without changes in cardiomyocyte contraction. Remarkably, in the context of well preserved contractile function in perfused hearts, we observed decreased glucose oxidation, but not fat oxidation, with increased glycolysis. cRyr2Δ50 hearts exhibited hyperphosphorylation and inhibition of pyruvate dehydrogenase, the key Ca²⁺-sensitive gatekeeper to glucose oxidation. Metabolomic, proteomic, and transcriptomic analyses revealed additional functional networks associated with altered metabolism in this model. These results demonstrate that Ryr2 controls mitochondrial Ca²⁺ dynamics and plays a specific, critical role in promoting glucose oxidation in cardiomyocytes. Our findings indicate that partial RYR2 loss is sufficient to cause metabolic abnormalities seen in heart disease.

Metabolic efficiency promotes protection from pressure overload in hearts expressing slow skeletal troponin I

BACKGROUND

The failing heart displays increased glycolytic flux that is not matched by a commensurate increase in glucose oxidation. This mismatch induces increased anaplerotic flux and inefficient glucose metabolism. We previously found adult transgenic mouse hearts expressing the fetal troponin I isoform, (ssTnI) to be protected from ischemia by increased glycolysis. In this study, we investigated the metabolic response of adult mouse hearts expressing ssTnI to chronic pressure overload.

METHODS AND RESULTS

At 2 to 3 months of age, ssTnI mice or their nontransgenic littermates underwent aortic constriction (TAC). TAC induced a 25% increase in nontransgenic heart size but only a 7% increase in ssTnI hearts (P<0.05). Nontransgenic TAC developed diastolic dysfunction (65% increase in E/A ratio), whereas the E/A ratio actually decreased in ssTnI TAC. Isolated perfused hearts from nontransgenic TAC mice showed reduced cardiac function and reduced creatine phosphate:ATP (16% reduction), but ssTnI TAC hearts maintained cardiac function and energy charge. Contrasting nontransgenic TAC, ssTnI TAC significantly increased glucose oxidation at the expense of palmitate oxidation, preventing the increase in anaplerosis observed in nontransgenic TAC hearts. Elevated glucose oxidation was mediated by a reduction in pyruvate dehydrogenase kinase 4 expression, enabling pyruvate dehydrogenase to compete against anaplerotic enzymes for pyruvate carboxylation.

CONCLUSIONS

Expression of a single fetal myofilament protein into adulthood in the ssTnI-transgenic mouse heart induced downregulation of the gene expression response for pyruvate dehydrogenase kinase to pressure overload. The consequence of elevated pyruvate oxidation in ssTnI during TAC reduced anaplerotic flux, ameliorating inefficiencies in glucose oxidation, with energetic and functional protection against cardiac decompensation.

Advances in the research of AMPK and its subunit genes

AMP-activated kinase (AMPK) is a heterotrimeric complex composed of three subunits and is the core energy sensor of the cell. The AMPK activity is important for survival during periods of stress and starvation and also has implications in type II diabetes, obesity, metabolic syndrome, longevity and cancer, etc. The activation of AMPK is triggered through binding of Adenosine Monophosphate Activated Proteins (AMP) to the Bateman domains of the gamma subunit, leading to increased phosphorylation of the threonine 172 on the alpha subunit by inducing allosteric activation and inhibiting dephosphorylation. AMPK and its subunits have been the focuses of many researchers dealing with genetic and metabolic issues. The study makes a comprehensive review on the structure, function, distribution, enzyme activity, the genetic mutation and other aspects of AMPK and its subunit genes, with the aim to outline main aspects of present researches on AMPK and its subunits in animal genetics.

AMP-activated protein kinase regulation and biological actions in the heart

AMP-activated protein kinase (AMPK) is a stress-activated kinase that functions as a cellular fuel gauge and master metabolic regulator. Recent investigation has elucidated novel molecular mechanisms of AMPK regulation and important biological actions of the AMPK pathway that are highly relevant to cardiovascular disease. Activation of the intrinsic AMPK pathway plays an important role in the myocardial response to ischemia, pressure overload, and heart failure. Pharmacological activation of AMPK shows promise as a therapeutic strategy in the treatment of heart disease. The purpose of this review is to assess how recent discoveries have extended and in some cases challenged existing paradigms, providing new insights into the regulation of AMPK, its diverse biological actions, and therapeutic potential in the heart.

Evidence that AMP-activated protein kinase can negatively modulate ornithine decarboxylase activity in cardiac myoblasts

The responses of AMP-activated protein kinase (AMPK) and Ornithine decarboxylase (ODC) to isoproterenol have been examined in H9c2 cardiomyoblasts, AMPK represents the link between cell growth and energy availability whereas ODC, the key enzyme in polyamine biosynthesis, is essential for all growth processes and it is thought to have a role in the development of cardiac hypertrophy. Isoproterenol rapidly induced ODC activity in H9c2 cardiomyoblasts by promoting the synthesis of the enzyme protein and this effect was counteracted by inhibitors of the PI3K/Akt pathway. The increase in enzyme activity became significant between 15 and 30min after the treatment. At the same time, isoproterenol stimulated the phosphorylation of AMPKα catalytic subunits (Thr172), that was associated to an increase in acetyl coenzyme A carboxylase (Ser72) phosphorylation. Downregulation of both α1 and α2 isoforms of the AMPK catalytic subunit by siRNA to knockdown AMPK enzymatic activity, led to superinduction of ODC in isoproterenol-treated cardiomyoblasts. Downregulation of AMPKα increased ODC activity even in cells treated with other adrenergic agonists and in control cells. Analogue results were obtained in SH-SY5Y neuroblastoma cells transfected with a shRNA construct against AMPKα. In conclusion, isoproterenol quickly activates in H9c2 cardiomyoblasts two events that seem to contrast one another. The first one, an increase in ODC activity, is linked to cell growth, whereas the second, AMPK activation, is a homeostatic mechanism that negatively modulates the first. The modulation of ODC activity by AMPK represents a mechanism that may contribute to control cell growth processes.

Cardiac lipoprotein lipase activity in the hypertrophied heart may be regulated by fatty acid flux

Cardiac hypertrophy is characterised by an imbalance between lipid uptake and fatty acid β-oxidation leading to an accumulation of lipids, particularly triacylglycerol (TAG). It is unclear whether uptake mechanisms such as lipoprotein lipase (LPL) can be attenuated to diminish this uptake. Rats were cold acclimated to induce cardiac hypertrophy and increase cardiac LPL. Lipid uptake and metabolism were altered by feeding a ‘Western-style’ high fat diet (WSD) or feeding oxfenicine (2 g/L) in the drinking water. Diastolic stiffness (increased volume change/unit pressure change) was induced in hypertrophied hearts for rats fed WSD (P < 0.05) or WSD + oxfenicine (P < 0.01), although absolute performance of cardiac muscle, estimated from stress–strain calculations was unchanged. Cold acclimation increased cardiac endothelial LPL (P < 0.05) but this was diminished following oxfenicine. Following WSD LPL was further decreased below WSD-fed control hearts (P < 0.05) with no further decrease by oxfenicine supplementation. A negative correlation was noted between plasma TAG and endothelial LPL (correlation coefficient = − 0.654; P < 0.001) but not cardiac TAG concentration. Transcript levels of angiopoietin-like protein-4 (ANGPTL4) were increased 6-fold by WSD (P < 0.05) and increased 15-fold following WSD + oxfenicine (P < 0.001). For CA-hearts fed WSD or WSD + oxfenicine ANGPTL4 mRNA levels were preserved at chow-fed levels. VLDLR protein levels were increased 10-fold (P < 0.01) by CA. ANGPTL4 protein levels were increased 2-fold (P < 0.05) by WSD, but restored following oxfenicine. For CA-hearts WSD increased ANGPTL4 protein levels 3-fold (P < 0.01) with WSD + oxfenicine increasing ANGPTL4 protein 4-fold (P < 0.01). These data suggest that endothelial LPL levels in the heart are altered to maintain FA flux and may exploit ANGPTL4.

Glucose metabolism and cardiac hypertrophy

The most notable change in the metabolic profile of hypertrophied hearts is an increased reliance on glucose with an overall reduced oxidative metabolism, i.e. a reappearance of the foetal metabolic pattern. In animal models, this change is attributed to the down-regulation of the transcriptional cascades promoting gene expression for fatty acid oxidation and mitochondrial oxidative phosphorylation in adult hearts. Impaired myocardial energetics in cardiac hypertrophy also triggers AMP-activated protein kinase (AMPK), leading to increased glucose uptake and glycolysis. Aside from increased reliance on glucose as an energy source, changes in other glucose metabolism pathways, e.g. the pentose phosphate pathway, the glucosamine biosynthesis pathway, and anaplerosis, are also noted in the hypertrophied hearts. Studies using transgenic mouse models and pharmacological compounds to mimic or counter the switch of substrate preference in cardiac hypertrophy have demonstrated that increased glucose metabolism in adult heart is not harmful and can be beneficial when it provides sufficient fuel for oxidative metabolism. However, improvement in the oxidative capacity and efficiency rather than the selection of the substrate is likely the ultimate goal for metabolic therapies.

Ivabradine reduces heart rate while preserving metabolic fluxes and energy status of healthy normoxic working hearts

Heart rate reduction (HRR) is an important target in the management of patients with chronic stable angina. Most available drugs for HRR, such as β-blockers, have adverse effects, including on cardiac energy substrate metabolism, a well-recognized determinant of cardiac homeostasis. This study aimed at 1) testing whether HRR by ivabradine (IVA) alters substrate metabolism in the healthy normoxic working heart and 2) comparing the effect of IVA with that of the β-blocker metoprolol (METO). This was assessed using our well-established model of ex vivo mouse heart perfusion in the working mode, which enables concomitant evaluation of myocardial contractility and metabolic fluxes using (13)C-labeled substrates. Hearts were perfused in the absence (controls; n = 10) or presence of IVA (n = 10, 3 μM) with or without atrial pacing to abolish HRR in the IVA group. IVA significantly reduced HR (35 ± 5%) and increased stroke volume (39 ± 9%) while maintaining similar cardiac output, contractility, power, and efficiency. Effects of IVA on HR and stroke volume were reversed by atrial pacing. At the metabolic level, IVA did not impact on substrate selection to citrate formation, rates of glycolysis, or tissue levels of high-energy phosphates. In contrast, METO, at concentrations up to 40 μM, decreased markedly cardiac function (flow: 25 ± 6%; stroke volume: 30 ± 10%; contractility: 31 ± 9%) as well as glycolysis (2.9-fold) but marginally affected HR. Collectively, these results demonstrate that IVA selectively reduces HR while preserving energy substrate metabolism of normoxic healthy working mouse hearts perfused ex vivo, a model that mimics to some extent the denervated transplanted heart. Our results provide the impetus for testing selective HRR by IVA on cardiac substrate metabolism in pathological models.

Mitochondrial adaptations to physiological vs. pathological cardiac hypertrophy

Cardiac hypertrophy is a stereotypic response of the heart to increased workload. The nature of the workload increase may vary depending on the stimulus (repetitive, chronic, pressure, or volume overload). If the heart fully adapts to the new loading condition, the hypertrophic response is considered physiological. If the hypertrophic response is associated with the ultimate development of contractile dysfunction and heart failure, the response is considered pathological. Although divergent signalling mechanisms may lead to these distinct patterns of hypertrophy, there is some overlap. Given the close relationship between workload and energy demand, any form of cardiac hypertrophy will impact the energy generation by mitochondria, which are the key organelles for cellular ATP production. Significant changes in the expression of nuclear and mitochondrially encoded transcripts that impact mitochondrial function as well as altered mitochondrial proteome composition and mitochondrial energetics have been described in various forms of cardiac hypertrophy. Here, we review mitochondrial alterations in pathological and physiological hypertrophy. We suggest that mitochondrial adaptations to pathological and physiological hypertrophy are distinct, and we shall review potential mechanisms that might account for these differences.

Does long-term metformin treatment increase cardiac lipoprotein lipase?

Acute activation of adenosine monophosphate-activated protein kinase (AMPK) or jumps in cardiac work increased cardiac endothelial lipoprotein lipase (LPL), yet it is unclear whether chronic AMPK activation maintains this elevated LPL. To activate AMPK chronically, metformin at low (300 mg/kg/d) and high dose (600 mg/kg/d) was administered in drinking water for 14 days. Control, metformin-treated, and 5-amino-imidazole-4-carboxamide riboside (AICAR)-treated (0.5 mmol/L) ex vivo hearts were perfused to investigate uptake of triacylglycerol and cardiac LPL activity. For perfused rat hearts, increased uptake of labeled Intralipid and β-oxidation of Intralipid-fatty acid were noted for both AICAR (P < .05) and high-dose metformin (P < .01). Intralipid incorporation into tissue lipids was decreased by AICAR (P < .05) and increased after high-dose metformin (P < .05), the increase manifest as enhanced triacylglycerol deposition (P < .05). Low-dose metformin did not alter lipid uptake or tissue deposition. Both high-dose metformin and AICAR decreased cardiac acetyl-coenzyme A carboxylase activity (P < .01). Heparin-releasable LPL was increased after treatment with AICAR (P < .05) and high-dose metformin (P < .01). Low-dose metformin did not alter cardiac LPL. High-dose metformin doubled immunoreactive AMPK and phospho-AMPK protein (P < .001) and increased phosphorylation of p38-mitogen-activated protein kinase (P < .05). After heparin pretreatment, the rate of recruitment of LPL to the cardiac endothelium was increased by AICAR (P < .05) but not by high-dose metformin. These data suggest that AMPK activation increased cardiac endothelial LPL, yet acute and chronic activation of AMPK may yield increased LPL through differing mechanisms.

AMPK - Activated Protein Kinase and its Role in Energy Metabolism of the Heart

Adenosine monophosphate - activated kinase (AMPK) plays a key role in the coordination of the heart's anabolic and catabolic pathways. It induces a cellular cascade at the center of maintaining energy homeostasis in the cardiomyocytes.. The activated AMPK is a heterotrimeric protein, separated into a catalytic α - subunit (63kDa), a regulating β - subunit (38kDa) and a γ - subunit (38kDa), which is allosterically adjusted by adenosine triphosphate (ATP) and adenosine monophosphate (AMP). The actual binding of AMP to the γ - subunit is the step which activates AMPK. AMPK serves also as a protein kinase in several metabolic pathways of the heart, including cellular energy sensoring or cardiovascular protection. The AMPK cascade represents a sensitive system, activated by cellular stresses that deplete ATP and acts as an indicator of intracellular ATP/AMP. In the context of cellular stressors (i.e. hypoxia, pressure overload, hypertrophy or ATP deficiency) the increasing levels of AMP promote allosteric activation and phosphorylation of AMPK. As the concentration of AMP begins to increase, ATP competitively inhibits further phosphorylation of AMPK. The increase of AMP may also be induced either from an iatrogenic emboli, percutaneous coronary intervention, or from atherosclerotic plaque rupture leading to an ischemia in the microcirculation. To modulate energy metabolism by phosphorylation and dephosphorylation is vital in terms of ATP usage, maintaining transmembrane transporters and preserving membrane potential. In this article, we review AMPK and its role as an important regulatory enzyme during periods of myocardial stress, regulating energy metabolism, protein synthesis and cardiovascular protection.

A high-fructose diet worsens eccentric left ventricular hypertrophy in experimental volume overload

The development of left ventricular (LV) hypertrophy (LVH) can be affected by diet manipulation. Concentric LVH resulting from pressure overload can be worsened by feeding rats with a high-fructose diet. Eccentric LVH is a different type of hypertrophy and is associated with volume overload (VO) diseases. The impact of an abnormal diet on the development of eccentric LVH and on ventricular function in chronic VO is unknown. This study therefore examined the effects of a fructose-rich diet on LV eccentric hypertrophy, ventricular function, and myocardial metabolic enzymes in rats with chronic VO caused by severe aortic valve regurgitation (AR). Wistar rats were divided in four groups: sham-operated on control diet (SC; n = 13) or fructose-rich diet (SF; n = 13) and severe aortic regurgitation fed with the same diets [aortic regurgitation on control diet (ARC), n = 16, and aortic regurgitation on fructose-rich diet (ARF), n = 13]. Fructose-rich diet was started 1 wk before surgery, and the animals were euthanized 9 wk later. SF and ARF had high circulating triglycerides. ARC and ARF developed significant LV eccentric hypertrophy after 8 wk as expected. However, ARF developed more LVH than ARC. LV ejection fraction was slightly lower in the ARF compared with ARC. The increased LVH and decreased ejection fraction could not be explained by differences in hemodynamic load. SF, ARC, and ARF had lower phosphorylation levels of the AMP kinase compared with SC. A fructose-rich diet worsened LV eccentric hypertrophy and decreased LV function in a model of chronic VO caused by AR in rats. Normal animals fed the same diet did not develop these abnormalities. Hypertriglyceridemia may play a central role in this phenomenon as well as AMP kinase activity.

AMPK in cardiovascular health and disease

AbstractAdenosine Monophosphate-activated Protein Kinase (AMPK), a serine/threonine kinase and a member of the Snf1/AMPK protein kinase family, consists of three protein subunits that together make a functional enzyme. AMPK, which is expressed in a number of tissues, including the liver, brain, and skeletal muscle, is allosterically activated by a rise in the AMP: ATP ratio (ie in a low ATP or energy depleted state). The net effect of AMPK activation is to halt energy consuming (anabolic) pathways but to promote energy conserving (catabolic) cellular pathways. AMPK has therefore often been dubbed the "metabolic master switch". AMPK also plays a critical physiological role in the cardiovascular system. Increasing evidence suggest that AMPK might also function as a sensor by responding to oxidative stress. Mostly importantly, AMPK modulates endogenous antioxidant gene expression and/or suppress the production of oxidants. AMPK promotes cardiovascular homeostasis by ensuring an optimum redox balance on the heart and vascular tissues. Dysfunctional AMPK is thought to underlie several cardiovascular pathologies. Here we review this kinase from its structure and discovery to current knowledge of its adaptive and maladaptive role in the cardiovascular system.

A distinct AMP-activated protein kinase phosphorylation site characterizes cardiac hypertrophy induced by L-thyroxine and angiotensin II

1. The purpose of the present study was to evaluate differences in the AMP-activated protein kinase (AMPK) phosphorylation sites in cardiac hypertrophy induced by L-thyroxine and angiotensin (Ang) II. 2. Cardiac hypertrophy was induced in wild-type and AMPKalpha2-knockout mice by treatment with 1 mg/kg, i.p., thyroxine or 1.44 mg/kg per day AngII for 14 days. The phenotype of the hypertrophy was evaluated using echocardiographic measurements and histological analyses. The phosphorylation of AMPK at alpha-Ser(485/491) and alpha-Thr(172) was determined by western blot analysis. 3. In wild-type mice, the phosphorylation of AMPKalpha-Ser(485/491) was significantly elevated in the AngII-treated group, but not in the thyroxine-treated group, compared with the vehicle control group. In contrast, the phosphorylation of AMPKalpha-Thr(172) was significantly increased by thyroxine, but not AngII, treatment compared with the vehicle control group. Furthermore, knockout of the AMPKalpha2 subunit abolished phosphorylation at the alpha-Ser(485/491) site and significantly suppressed phosphorylation at the alpha-Thr(172) site, resulting in alleviation of thyroxine- but not AngII-induced hypertrophy. 4. In conclusion, L-thyroxine and AngII induce the phosphorylation of distinct sites of AMPK in cardiac hypertrophy. Phosphorylation of AMPK alpha-Thr(172) may contribute to thyroxine-induced cardiac hypertrophy.

Differential AMPK phosphorylation sites associated with phenylephrine vs. antihypertrophic effects of adenosine agonists in neonatal rat ventricular myocytes

Stimulation of cardiac AMP-activated protein kinase (AMPK) has been demonstrated in both prohypertrophic and antihypertrophic settings, although the reasons for such discrepant results are not well understood. We determined how AMPK is regulated in response to phenylephrine-induced cardiomyocyte hypertrophy and assessed whether AMPK activity may be a factor underlying the antihypertrophic effect of adenosine receptor agonists. The role of AMPK in hypertrophic responses was determined by assessing the effect of the AMPK activator 5-aminoimidazole-4-carboxyamide ribonucleoside on three hypertrophic indexes, including protein synthesis, cell surface area, and fetal gene expression. The changes in phosphorylation of the catalytic alpha-subunit of AMPK at two different sites, Thr(172) and Ser(485/491), in response to phenylephrine and adenosine receptor agonists were also examined. 5-Aminoimidazole-4-carboxyamide ribonucleoside completely abolished phenylephrine-induced increases in protein synthesis, cell surface area, and fetal gene expression. AMPK phosphorylation time course studies revealed that phenylephrine induced a time-dependent activation at site Ser(485/491), in contrast to adenosine receptor agonists, which demonstrated rapid AMPK phosphorylation at Thr(172). Furthermore, the phosphorylation at Ser(485/491) by phenylephrine was not affected by the addition of adenosine receptor agonists, although, conversely, phosphorylation of AMPK at Thr(172) by adenosine receptor agonists was abrogated by the addition of phenylephrine. We propose from these results that cardiomyocyte hypertrophic and antihypertrophic responses, at least with respect to inhibition of phenylephrine-induced hypertrophy by adenosine receptor agonists, are mediated by multisite AMPK regulation. The latter are reflected by increased phosphorylation at Ser(485/491) and at Thr(172), associated with prohypertrophic and antihypertrophic responses, respectively.

Stimulation of cardiac fatty acid oxidation by leptin is mediated by a nitric oxide-p38 MAPK-dependent mechanism

Leptin has previously been shown to stimulate fatty acid oxidation independent of AMP-activated protein kinase (AMPK). Nitric oxide and p38 mitogen activated protein kinase (MAPK) are known effectors of leptin signaling. The aim of the present study was to determine whether nitric oxide and p38 MAPK mediate the stimulation of leptin by MAPK. Hearts from male Sprague-Dawley rats were mounted on the isolated perfused working heart in the presence or absence of leptin (1.9 nM), N-Nitro-L-Arginine Methyl Ester (L-NAME) (3 microM), the specific p38 MAPK inhibitor 4-[4-(4-Fluorophenyl)-5-(4-pyridinyl)-1H-imidazol-2-yl] phenol (SB202190, 2 microM) and the specific STAT-3 inhibitor (E)-2-Cyano-3-(3,4-dihydrophenyl)-N-(phenylmethyl)-2-propenamide (AG490, 5 microM) for the measurement of substrate metabolism and function. AMPK and carnitine palimitoyltransferase-1 activity, nitrate/nitrite levels, STAT-3 phosphorylation and p38 MAPK phosphorylation were measured. To assess mitochondrial function, hearts were perfused with or without leptin prior to the isolation of mitochondria. Leptin stimulated fatty acid oxidation and decreased cardiac function, associated with the activation of STAT-3 and p38 MAPK and an increase in tissue nitrate/nitrite levels; the effect on function was ameliorated and the effect on fatty acid oxidation was prevented by L-NAME, B202190 and AG490. L-NAME lowered tissue nitrate/nitrite levels, and prevented the phosphorylation of p38, whereas SB202190 had no effect on tissue nitrate/nitrite levels. AG490 also lowered tissue nitrate/nitrite levels. Leptin had no effect on fatty acid-dependent mitochondrial respiration or uncoupling activity, but, surprisingly, stimulated pyruvate-dependent mitochondrial respiration. These data indicate that leptin stimulates fatty acid oxidation by a STAT-3-nitric oxide-p38 MAPK-dependent mechanism. The target of the pathway is upstream of the mitochondria.

AMP-activated protein kinase influences metabolic remodeling in H9c2 cells hypertrophied by arginine vasopressin

Substrate use switches from fatty acids toward glucose in pressure overload-induced cardiac hypertrophy with an acceleration of glycolysis being characteristic. The activation of AMP-activated protein kinase (AMPK) observed in hypertrophied hearts provides one potential mechanism for the acceleration of glycolysis. Here, we directly tested the hypothesis that AMPK causes the acceleration of glycolysis in hypertrophied heart muscle cells. The H9c2 cell line, derived from the embryonic rat heart, was treated with arginine vasopressin (AVP; 1 μM) to induce a cellular model of hypertrophy. Rates of glycolysis and oxidation of glucose and palmitate were measured in nonhypertrophied and hypertrophied H9c2 cells, and the effects of inhibition of AMPK were determined. AMPK activity was inhibited by 6-[4-(2-piperidin-1- yl-ethoxy)-phenyl]-3-pyridin-4-yl-pyrrazolo-[1,5-a]pyrimidine (compound C) or by adenovirus-mediated transfer of dominant negative AMPK. Compared with nonhypertrophied cells, glycolysis was accelerated and palmitate oxidation was reduced with no significant alteration in glucose oxidation in hypertrophied cells, a metabolic profile similar to that of intact hypertrophied hearts. Inhibition of AMPK resulted in the partial reduction of glycolysis in AVP-treated hypertrophied H9c2 cells. Acute exposure of H9c2 cells to AVP also activated AMPK and accelerated glycolysis. These elevated rates of glycolysis were not altered by AMPK inhibition but were blocked by agents that interfere with Ca2+ signaling, including extracellular EGTA, dantrolene, and 2-aminoethoxydiphenyl borate. We conclude that the acceleration of glycolysis in AVP-treated hypertrophied heart muscle cells is partially dependent on AMPK, whereas the acute glycolytic effects of AVP are AMPK independent and at least partially Ca2+ dependent.

AMP-activated protein kinase: a core signalling pathway in the heart

Over the past decade, AMP-activated protein kinase (AMPK) has emerged as an important intracellular signalling pathway in the heart. Activated AMPK stimulates the production of ATP by regulating key steps in both glucose and fatty acid metabolism. It has an inhibitory effect on cardiac protein synthesis. AMPK also interacts with additional intracellular signalling pathways in a coordinated network that modulates essential cellular processes in the heart. Evidence is accumulating that AMPK may protect the heart from ischaemic injury and limit the development of cardiac myocyte hypertrophy to various stimuli. Heart AMPK is activated by hormones, cytokines and oral hypoglycaemic drugs that are used in the treatment of type 2 diabetes. The tumour suppressor LKB1 is the major regulator of AMPK activity, but additional upstream kinases and protein phosphatases also contribute. Mutations in the regulatory gamma2 subunit of AMPK lead to an inherited syndrome of hypertrophic cardiomyopathy and ventricular pre-excitation, which appears to be due to intracellular glycogen accumulation. Future research promises to elucidate the molecular mechanisms responsible for AMPK activation, novel downstream AMPK targets, and the therapeutic potential of targeting AMPK for the prevention and treatment of myocardial ischaemia or cardiac hypertrophy.

Resveratrol prevents the prohypertrophic effects of oxidative stress on LKB1

BACKGROUND

Master regulators of protein synthesis such as mammalian target of rapamycin (mTOR) and p70S6 kinase contribute to left ventricular hypertrophy. These prohypertrophic pathways are modulated by a number of kinase cascades, including the hierarchical LKB1/AMP-activated protein kinase (AMPK) energy-sensing pathway. Because oxidative stress inhibits the LKB1/AMPK signaling axis to promote abnormal cell growth in cancer cells, we investigated whether oxidative stress associated with hypertension also results in the inhibition of this kinase circuit to contribute to left ventricular hypertrophy.

METHODS AND RESULTS

In the spontaneously hypertensive rat, a well-established genetic model of hypertension and subsequent cardiac hypertrophy, the development of left ventricular hypertrophy is associated with an increase in the electrophilic lipid peroxidation byproduct 4-hydroxy-2-nonenal (HNE). Using isolated cardiomyocytes, we show that elevated levels of HNE result in the formation of HNE-LKB1 adducts that inhibit LKB1 and subsequent AMPK activity. Consistent with inhibition of the LKB1/AMPK signaling pathway, the mTOR/p70S6 kinase system is activated, which is permissive for cardiac myocyte cell growth. Treatment of cardiomyocytes with resveratrol prevents HNE modification of the LKB1/AMPK signaling axis and blunts the prohypertrophic p70S6 kinase response. Furthermore, administration of resveratrol to spontaneously hypertensive rats results in increased AMPK phosphorylation and activity and reduced left ventricular hypertrophy.

CONCLUSIONS

Our data identify a molecular mechanism in the cardiomyocyte involving the oxidative stress-derived lipid peroxidation byproduct HNE and the LKB1/AMPK signaling pathway that contributes to the development of left ventricular hypertrophy. We also suggest that resveratrol may be a potential therapy for patients at risk for developing pathological cardiac hypertrophy by preventing this prohypertrophic process.

Energy metabolism in heart failure and remodelling

Myocytes of the failing heart undergo impressive metabolic remodelling. The time line for changes in the pathways for ATP synthesis in compensated hypertrophy is: flux through the creatine kinase (CK) reaction falls as both creatine concentration ([Cr]) and CK activity fall; increases in [ADP] and [AMP] lead to increases in glucose uptake and utilization; fatty acid oxidation either remains the same or decreases. In uncompensated hypertrophy and in other forms of heart failure, CK flux and fatty acid oxidation are both lower; any increases in glucose uptake and utilization are not sufficient to compensate for overall decreases in the capacity for ATP supply and [ATP] falls. Metabolic remodelling is under transcriptional and post-transcriptional control. The lower metabolic reserve of the failing heart contributes to impaired contractile reserve.

Cold acclimation induces physiological cardiac hypertrophy and increases assimilation of triacylglycerol metabolism through lipoprotein lipase

The contribution of triacylglycerol to energy provision in the hypertrophied heart, mediated through lipoprotein lipase (LPL) is largely unknown and the contribution of very-low-density lipoprotein (VLDL) receptor to control of LPL presentation at the endothelium is unclear. For isolated perfused rat hearts, cold acclimation (CA) induced volume-overload hypertrophy, with decreased developed pressure (P < 0.01), increased end-diastolic volume of the left ventricle (P < 0.001) and a loss of contractile reserve in response to dobutamine challenge (P < 0.01). Oleate utilisation by perfused hearts was unchanged by CA, however uptake of intralipid emulsion increased 3-fold (P < 0.01). CA increased the proportion of lipid deposited in tissue lipids from 10% in euthermic controls to 40% (P < 0.01) although the overall contribution of individual lipid classes was unaffected. Cold acclimation significantly increased heparin-releasable LPL (P < 0.05) and tissue residual LPL (P < 0.01). Western blot analysis indicated preserved expression of proteins coding for SERCA2, muscle-CPT1 and VLDL-receptor following CA, while AMPKα2 and phospho-AMPKα2 were unaffected. These observations indicate that for physiological hypertrophy AMPK phosphorylation does not mediate the enhanced translocation of LPL to cardiac endothelium.

Metabolic actions of metformin in the heart can occur by AMPK-independent mechanisms

The metabolic actions of the antidiabetic agent metformin reportedly occur via the activation of the AMP-activated protein kinase (AMPK) in the heart and other tissues in the presence or absence of changes in cellular energy status. In this study, we tested the hypothesis that metformin has AMPK-independent effects on metabolism in heart muscle. Fatty acid oxidation and glucose utilization (glycolysis and glucose uptake) were measured in isolated working hearts from halothane-anesthetized male Sprague-Dawley rats and in cultured heart-derived H9c2 cells in the absence or in the presence of metformin (2 mM). Fatty acid oxidation and glucose utilization were significantly altered by metformin in hearts and H9c2 cells. AMPK activity was not measurably altered by metformin in either model system, and no impairment of energetic state was observed in the intact hearts. Furthermore, the inhibition of AMPK by 6-[4-(2-piperidin-1-yl-ethoxy)-phenyl]-3-pyridin-4-yl-pyyrazolo[1,5-a] pyrimidine (Compound C), a well-recognized pharmacological inhibitor of AMPK, or the overexpression of a dominant-negative form of AMPK failed to prevent the metabolic actions of metformin in H9c2 cells. The exposure of H9c2 cells to inhibitors of p38 mitogen-activated protein kinase (p38 MAPK) or protein kinase C (PKC) partially or completely abrogated metformin-induced alterations in metabolism in these cells, respectively. Thus the metabolic actions of metformin in the heart muscle can occur independent of changes in AMPK activity and may be mediated by p38 MAPK- and PKC-dependent mechanisms.

Inhibitory crosstalk between ERK and AMPK in the growth and proliferation of cardiac fibroblasts

Extracellular signal-regulated kinase (ERK) is one of the key protein kinases that regulate the growth and proliferation in cardiac fibroblasts (CFs). As an energy sensor of cellular metabolism, AMP-activated protein kinase (AMPK) is found recently to be involved in myocardial remodeling. In this study, we investigated the crosstalk between ERK and AMPK in the growth and proliferation of CFs. In neonatal rat cardiac fibroblasts (NRCFs), we found that serum significantly inhibited basal AMPK phosphorylation between 10min and 24h and also partially inhibited AMPK phosphorylation by AMPK activator, 5-aminoimidazole-4-carboxamide-ribonucleoside (AICAR). Furthermore, ERK inhibitor could greatly reverse the inhibition of AMPK by serum. Conversely, activation of AMPK by AICAR also showed a significant inhibition of basal and serum-induced ERK phosphorylation but it showed a delayed and steadfast inhibition which appeared after 60min and lasted until 12h. Moreover, inhibition of ERK could repress the activation of p70S6K, an important kinase in cardiac proliferation, and AICAR could also inhibit p70S6K phosphorylation. In addition, under both serum and serum-free medium, AICAR significantly inhibited the DNA synthesis and cell numbers, and reduced cells at S phase. In conclusion, AMPK activation with AICAR inhibited growth and proliferation in cardiac fibroblasts, which involved inhibitory interactions between ERK and AMPK. This is the first report that AMPK could be a target of ERK in growth factors-induced proliferation, which may give a new mechanism that growth factors utilize in their promotion of proliferation in cardiac fibroblasts.

Metoprolol improves cardiac function and modulates cardiac metabolism in the streptozotocin-diabetic rat

The effects of diabetes on heart function may be initiated or compounded by the exaggerated reliance of the diabetic heart on fatty acids and ketones as metabolic fuels. beta-Blocking agents such as metoprolol have been proposed to inhibit fatty acid oxidation. We hypothesized that metoprolol would improve cardiac function by inhibiting fatty acid oxidation and promoting a compensatory increase in glucose utilization. We measured ex vivo cardiac function and substrate utilization after chronic metoprolol treatment and acute metoprolol perfusion. Chronic metoprolol treatment attenuated the development of cardiac dysfunction in streptozotocin (STZ)-diabetic rats. After chronic treatment with metoprolol, palmitate oxidation was increased in control hearts but decreased in diabetic hearts without affecting myocardial energetics. Acute treatment with metoprolol during heart perfusions led to reduced rates of palmitate oxidation, stimulation of glucose oxidation, and increased tissue ATP levels. Metoprolol lowered malonyl-CoA levels in control hearts only, but no changes in acetyl-CoA carboxylase phosphorylation or AMP-activated protein kinase activity were observed. Both acute metoprolol perfusion and chronic in vivo metoprolol treatment led to decreased maximum activity and decreased sensitivity of carnitine palmitoyltransferase I to malonyl-CoA. Metoprolol also increased sarco(endo)plasmic reticulum Ca(2+)-ATPase expression and prevented the reexpression of atrial natriuretic peptide in diabetic hearts. These data demonstrate that metoprolol ameliorates diabetic cardiomyopathy and inhibits fatty acid oxidation in streptozotocin-induced diabetes. Since malonyl-CoA levels are not increased, the reduction in total carnitine palmitoyltransferase I activity is the most likely factor to explain the decrease in fatty acid oxidation. The metabolism changes occur in parallel with changes in gene expression.

Cardiac expression of adiponectin and its receptors in streptozotocin-induced diabetic rats

Adiponectin can improve both glucose metabolism and insulin resistance via the adenosine monophosphate-activated protein kinase (AMPK) signaling pathway. Activated AMPK phosphorylates a variety of intracellular proteins, including acetyl coenzyme A carboxylase (ACC) that is involved in fatty acid oxidation. Adenosine monophosphate-activated protein kinase increases glucose transport by stimulating the translocation of glucose transporter 4 (GLUT4) to the sarcolemma in the heart. Adiponectin exerts its effect through adiponectin receptors, which are predominantly expressed in the liver and skeletal muscle. It is unknown whether the cardiac expression of adiponectin and its receptors is changed in diabetic rats. In the present study, we investigated the protein expression of adiponectin and its receptors in streptozotocin (STZ)-induced diabetic rat hearts. We also explored whether the levels of AMPK, ACC, and GLUT4 will be altered with the changed adiponectin and its receptors in STZ diabetic rat hearts. Plasma and cardiac adiponectin levels were measured by radioimmunoassay. Plasma and cardiac interleukin 6 and plasma tumor necrosis factor alpha (TNF-alpha) were assayed by enzyme-linked immunosorbent assay. Cardiac adiponectin receptors, AMPK-alpha, ACC, GLUT4, and TNF-alpha were analyzed by Western blot in control and STZ diabetic rats. The plasma adiponectin level was decreased, but the cardiac protein expression of adiponectin receptor 1 was increased in diabetic rats. There was no difference in the cardiac adiponectin level and the cardiac adiponectin receptor 2 protein expression between control and diabetic rats. The phosphorylation of AMPK-alpha and protein expression of GLUT4 were decreased, but the phosphorylation of ACC was unchanged in diabetic rat hearts. Plasma and cardiac levels of interleukin 6 and TNF-alpha were increased in diabetic rats. In conclusion, STZ-induced diabetes up-regulates adiponectin receptors in the heart. Despite an increase in cardiac adiponectin receptor 1 expression, there is an increased cardiac inflammatory response and a decreased GLUT4 protein expression associated with a reduction in circulating adiponectin.