AMP-activated protein kinase control of energy metabolism in the ischemic heart

Alleviation effects of dexmedetomidine on myocardial ischemia/reperfusion injury through fatty acid metabolism pathway via Elovl6

Dexmedetomidine (Dex) is widely used in the sedation in intensive care units and as an anesthetic adjunct. Considering the anti-inflammatory and antioxidant properties of Dex, we applied in vivo rat model as well as in vitro cardiomyocyte models (embryonic rat cardiomyocytes H9c2 cells and neonatal rat cardiomyocytes, NRCMs) to evaluate the effects of Dex against myocardial ischemia reperfusion (I/R) injury. Transcriptomic sequencing for gene expression in heart tissues from control rats and Dex-treated rats identified that genes related to fatty acid metabolism were significantly regulated by Dex. Among these genes, the elongation of long-chain fatty acids (ELOVL) family member 6 (Elovl6) was most increased upon Dex-treatment. By comparing the effects of Dex on both wild type and Elovl6-knockdown H9c2 cells and NRCMs under oxygen-glucose deprivation/reoxygenation (OGD/R) challenge, we found that Elovl6 knockdown attenuated the protection efficiency of Dex, which was supported by the cytotoxicity endpoints (cell viability and lactate dehydrogenase release) and apoptosis as well as key gene expressions. These results indicate that Dex exhibited the protective function against myocardial I/R injury via fatty acid metabolism pathways and Elovl6 plays a key role in the process, which was further confirmed using palmitate exposure in both cells, as well as in an in vivo rat model. Overall, this study systematically evaluates the protective effects of Dex on the myocardial I/R injury and provides better understanding on the fatty acid metabolism underlying the beneficial effects of Dex.

Dexmedetomidine as a myocardial protector in pediatric heart surgery using cardiopulmonary bypass: a systematic review

Background

In recent years, dexmedetomidine has been studied as a cardioprotective agent. However, studies on its application in pediatric heart surgery using cardiopulmonary bypass (CPB) remain limited. This systematic review aimed to provide information on the cardioprotective effect of dexmedetomidine in children undergoing heart surgery using CPB.

Methods

The authors searched several databases (MEDLINE, Embase, Cochrane Library, etc.) to identify all trials comparing the levels of myocardial injury via biomarkers, including pediatric patients undergoing heart surgery using CPB who received dexmedetomidine versus placebo or other anesthetic agents. Literatures from non-primary studies were excluded. Two reviewers independently screened studies for eligibility and extracted data. The Cochrane Risk-of-Bias tool was implemented to evaluate any potential biases. Information from eligible studies was summarized and correspondingly reviewed based on any quantitative outcomes.

Results

We identified six trials composed of 419 participants, three of which (n=241) showed significantly reduced interleukin-6 (IL-6) levels in the dexmedetomidine group, while one study (n=40) showed no IL-6 difference between groups. Cardiac troponin I (cTnI) and creatinine kinase-myocardial band (CK-MB), as myocardial injury biomarkers, were found to be lower in two trials (n=180). Despite several limitations hindering this review from pooling the data objectively, the majority of published studies indicated that dexmedetomidine is a seemingly efficacious agent protecting against cardiac injury during bypass.

Conclusions

These studies suggest that dexmedetomidine has cardioprotective effects through the lowering of cardiac injury biomarkers while improving its clinical outcomes after heart surgery using bypass.

Abnormalities of glucose and lipid metabolism in myocardial ischemia-reperfusion injury

Myocardial ischemia-reperfusion injury is a common condition in cardiovascular diseases, and the mechanism of its occurrence involves multiple complex metabolic pathways and signaling pathways. Among these pathways, glucose metabolism and lipid metabolism play important roles in regulating myocardial energy metabolism. Therefore, this article focuses on the roles of glucose metabolism and lipid metabolism in myocardial ischemia-reperfusion injury, including glycolysis, glucose uptake and transport, glycogen metabolism and the pentose phosphate pathway; and triglyceride metabolism, fatty acid uptake and transport, phospholipid metabolism, lipoprotein metabolism, and cholesterol metabolism. Finally, due to the different alterations and development of glucose metabolism and lipid metabolism in myocardial ischemia-reperfusion, there are also complex interregulatory relationships between them. In the future, modulating the equilibrium between glucose metabolism and lipid metabolism in cardiomyocytes and ameliorating aberrations in myocardial energy metabolism represent highly promising novel strategies for addressing myocardial ischemia-reperfusion injury. Therefore, a comprehensive exploration of glycolipid metabolism can offer novel theoretical and clinical insights into the prevention and treatment of myocardial ischemia-reperfusion injury.

Emerging Roles of SIRT3 in Cardiac Metabolism

The heart is a highly metabolically active organ that predominantly utilizes fatty acids as an energy substrate. The heart also derives some part of its energy by oxidation of other substrates, including glucose, lactose, amino acids and ketones. The critical feature of cardiac pathology is metabolic remodeling and loss of metabolic flexibility. Sirtuin 3 (SIRT3) is one of the seven mammalian sirtuins (SIRT1 to SIRT7), with NAD⁺ dependent deacetylase activity. SIRT3 is expressed in high levels in healthy hearts but downregulated in the aged or diseased hearts. Experimental evidence shows that increasing SIRT3 levels or activity can ameliorate several cardiac pathologies. The primary deacetylation targets of SIRT3 are mitochondrial proteins, most of which are involved in energy metabolism. Thus, SIRT3 improves cardiac health by modulating cardiac energetics. In this review, we discuss the essential role of SIRT3 in regulating cardiac metabolism in the context of physiology and pathology. Specifically, we summarize the recent advancements that emphasize the critical role of SIRT3 as a master regulator of cardiac metabolism. We also present a comprehensive view of all known activators of SIRT3, and elaborate on their therapeutic potential to ameliorate energetic abnormalities in various cardiac pathologies.

Dynamic Regulation of Cysteine Oxidation and Phosphorylation in Myocardial Ischemia-Reperfusion Injury

Myocardial ischemia-reperfusion (I/R) injury significantly alters heart function following infarct and increases the risk of heart failure. Many studies have sought to preserve irreplaceable myocardium, termed cardioprotection, but few, if any, treatments have yielded a substantial reduction in clinical I/R injury. More research is needed to fully understand the molecular pathways that govern cardioprotection. Redox mechanisms, specifically cysteine oxidations, are acute and key regulators of molecular signaling cascades mediated by kinases. Here, we review the role of reactive oxygen species in modifying cysteine residues and how these modifications affect kinase function to impact cardioprotection. This exciting area of research may provide novel insight into mechanisms and likely lead to new treatments for I/R injury.

Adiponectin Synthesis, Secretion and Extravasation from Circulation to Interstitial Space

Adiponectin, an adipokine that circulates as multiple multimeric complexes at high levels in serum, has antidiabetic, anti-inflammatory, antiatherogenic, and cardioprotective properties. Understanding the mechanisms regulating adiponectin's physiological effects is likely to provide critical insight into the development of adiponectin-based therapeutics to treat various metabolic-related diseases. In this review, we summarize our current understanding on adiponectin action in its various target tissues and in cellular models. We also focus on recent advances in two particular regulatory aspects; namely, the regulation of adiponectin gene expression, multimerization, and secretion, as well as extravasation of circulating adiponectin to the interstitial space and its degradation. Finally, we discuss some potential therapeutic approaches using adiponectin as a target and the current challenges facing adiponectin-based therapeutic interventions.

Pum2 mediates Sirt1 mRNA decay and exacerbates hypoxia/reoxygenation-induced cardiomyocyte apoptosis

Pum2 is a ribonucleic acid binding protein that controls target mRNA turnover. It has been reported to be potentially associated with cardiac fibrosis. However, little is known about the role of Pum2 in cardiac disease. In this study, we found that Pum2 was upregulated in the rat heart tissue subjected to ischemia/reperfusion procedure and cultured neonatal rat ventricular cardiomyocytes (NRVMs) with hypoxia/reoxygenation (H/R) treatment. Further, knockdown of Pum2 showed a beneficial effect on H/R treated NRVMs through decreasing caspase 3-associated apoptosis, whereas overexpression of Pum2 increased H/R-induced NRVMs apoptosis. Moreover, our results demonstrated that Sirt1 was identified as the target of Pum2-mediated mRNA decay in cardiomyocytes, and two Pum2 binding elements were found in the 3'-untranslated region of Sirt1 mRNA. Additionally, overexpression of Pum2 prompted the acetylation of LKB1 by decreasing Sirt1's mRNA level, which in turn repressed the activity of AMPK pathway in both normoxic and H/R-treated NRVMs. Finally, our data indicated that the pro-apoptotic effect of Pum2 was dependent on Sirt1 and AMPK. Collectively, our results provide the evidence that Pum2-mediated Sirt1 mRNA decay plays a detrimental role in H/R-induced cardiomyocytes injury.

Mechanical Postconditioning Promotes Glucose Metabolism and AMPK Activity in Parallel with Improved Post-Ischemic Recovery in an Isolated Rat Heart Model of Donation after Circulatory Death

Donation after circulatory death (DCD) could improve donor heart availability; however, warm ischemia-reperfusion injury raises concerns about graft quality. Mechanical postconditioning (MPC) may limit injury, but mechanisms remain incompletely characterized. Therefore, we investigated the roles of glucose metabolism and key signaling molecules in MPC using an isolated rat heart model of DCD. Hearts underwent 20 min perfusion, 30 min global ischemia, and 60 minu reperfusion with or without MPC (two cycles: 30 s reperfusion—30 s ischemia). Despite identical perfusion conditions, MPC either significantly decreased (low recovery = LoR; 32 ± 5%; p < 0.05), or increased (high recovery = HiR; 59 ± 7%; p < 0.05) the recovery of left ventricular work compared with no MPC (47 ± 9%). Glucose uptake and glycolysis were increased in HiR vs. LoR hearts (p < 0.05), but glucose oxidation was unchanged. Furthermore, in HiR vs. LoR hearts, phosphorylation of raptor, a downstream target of AMPK, increased (p < 0.05), cytochrome c release (p < 0.05) decreased, and TNFα content tended to decrease. Increased glucose uptake and glycolysis, lower mitochondrial damage, and a trend towards decreased pro-inflammatory cytokines occurred specifically in HiR vs. LoR MPC hearts, which may result from greater AMPK activation. Thus, we identify endogenous cellular mechanisms that occur specifically with cardioprotective MPC, which could be elicited in the development of effective reperfusion strategies for DCD cardiac grafts.

RNA sequencing revealing the role of AMP-activated protein kinase signaling in mice myocardial ischemia reperfusion injury

Long non-coding RNAs (lncRNA) and circular RNAs (circRNA) that sponge miRNAs could indirectly regulate gene expression, contributing to certain biological processes. This study aimed to investigate the role of non-coding RNAs in the pathogenesis of myocardial ischemia reperfusion-injury (MIRI). MIRI in male C57B/6J mice was induced by left anterior descending coronary artery ligation occlusion for 30 min, and 4 h of reperfusion. RNA sequencing was performed to obtain the mRNA and non-coding RNA expression profiles of the MIRI and sham groups. Bioinformatic methods were used to analyze the co-expression RNAs, miRNA binding sites and competitive endogenous RNA (ceRNA) pairs. Differentially expressed RNAs were identified with a cutoff fold change > 2 and p < 0.05. A total of 64 mRNAs were upregulated and 98 mRNAs were downregulated, and 10 lncRNAs were upregulated and 10 lncRNAs were downregulated. All altered (p < 0.05) mRNAs were selected for gene ontology and pathway analysis. The AMP-activated protein kinase (AMPK) signaling pathway was enriched in the downregulated genes, and the activation of AMPK was confirmed by western blotting. The lncRNA co-expression network and ceRNA network base on genes in AMPK signaling pathway were then constructed, revealing that ENSMUST00000147762.7 and TUCP_000184 might be key regulators in MIRI induced AMPK activation. The expression levels of AMPK signaling-related RNAs and those involved in the ceRNA network were validated using qRT-PCR. Overall, this study identified potential new targets on AMPK signaling in MIRI.

Glucose Lowering Strategies for Cardiac Benefits: Pathophysiological Mechanisms

Recent trials in Type 2 diabetes (T2D) have shown cardiovascular benefits with specific GLP-1 receptor agonists and SGLT2 inhibitors. We discuss the landscape of outcome trials in T2D from a pathophysiology viewpoint, review current knowledge gaps in underlying mechanisms, propose a caloric fuel routing hypothesis, and highlight areas of future research.

Targeting the glucagon receptor improves cardiac function and enhances insulin sensitivity following a myocardial infarction

Background

In heart failure the myocardium becomes insulin resistant which negatively influences cardiac energy metabolism and function, while increasing cardiac insulin signalling improves cardiac function and prevents adverse remodelling in the failing heart. Glucagon’s action on cardiac glucose and lipid homeostasis counteract that of insulin’s action. We hypothesised that pharmacological antagonism of myocardial glucagon action, using a human monoclonal antibody (mAb A) against glucagon receptor (GCGR), a G-protein coupled receptor, will enhance insulin sensitivity and improve cardiac energy metabolism and function post myocardial infarction (MI).

Methods

Male C57BL/6 mice were subjected to a permanent left anterior descending coronary artery ligation to induce MI, following which they received either saline or mAb A (4 mg kg⁻¹ week⁻¹ starting at 1 week post-MI) for 3 weeks.

Results

Echocardiographic assessment at 4 weeks post-MI showed that mAb A treatment improved % ejection fraction (40.0 ± 2.3% vs 30.7 ± 1.7% in vehicle-treated MI heart, p < 0.05) and limited adverse remodelling (LV mass: 129 ± 7 vs 176 ± 14 mg in vehicle-treated MI hearts, p < 0.05) post MI. In isolated working hearts an increase in insulin-stimulated glucose oxidation was evident in the mAb A-treated MI hearts (1661 ± 192 vs 924 ± 165 nmol g dry wt⁻¹ min⁻¹ in vehicle-treated MI hearts, p < 0.05), concomitant with a decrease in ketone oxidation and fatty acid oxidation rates. The increase in insulin stimulated glucose oxidation was accompanied by activation of the IRS-1/Akt/AS160/GSK-3β pathway, an increase in GLUT4 expression and a reduction in pyruvate dehydrogenase phosphorylation. This enhancement in insulin sensitivity occurred in parallel with a reduction in cardiac branched chain amino acids content (374 ± 27 vs 183 ± 41 µmol g protein⁻¹ in vehicle-treated MI hearts, p < 0.05) and inhibition of the mTOR/P70S6K hypertrophic signalling pathway. The MI-induced increase in the phosphorylation of transforming growth factor β-activated kinase 1 (p-TAK1) and p38 MAPK was also reduced by mAb A treatment.

Conclusions

mAb A-mediated cardioprotection post-myocardial infarction is associated with improved insulin sensitivity and a selective enhancement of glucose oxidation via, at least in part, enhancing branched chain amino acids catabolism. Antagonizing glucagon action represents a novel and effective pharmacological intervention to alleviate cardiac dysfunction and adverse remodelling post-myocardial infarction.

Treatment with a fixed dose combination antiretroviral therapy drug containing tenofovir, emtricitabine and efavirenz is associated with cardioprotection in high calorie diet-induced obese rats

HIV-infection, certain antiretroviral drug classes, especially protease inhibitors (PI), and obesity are associated with increased ischaemic heart disease (IHD) risk. However, the effect of PI-free fixed dose combination (FDC) antiretroviral therapy (ART) on hearts exposed to ischaemia-reperfusion injury (I/R) is unknown, particularly in obesity. This is becoming relevant as World Health Organisation guidelines recommend a FDC ART containing (non-) nucleoside reverse transcriptase inhibitors (tenofovir (TDF), emtricitabine (FTC) and efavirenz (EFV)) as first-line HIV treatment. Additionally, obesity rates are rising in HIV-infected populations, not only in ART-experienced individuals, but also at the time of ART initiation, which may further increase the risk of IHD. Therefore, we investigated the effects of PI-free FDC ART in myocardial I/R-exposed hearts from obese rats. Obesity was induced in male wistar rats via a 16-week high calorie diet. At week 10, treatment with a FDC ART drug containing TDF/FTC/EFV was initiated. Biometric and metabolic parameters, as well as myocardial functional recovery and infract size (IS), and myocardial signalling proteins following I/R were assessed after 16 weeks. Obese rats presented with increased body and intraperitoneal fat mass, elevated triglyceride and TBARS levels, whilst the hearts responded to I/R with impaired functional performance and increased IS. The FDC ART treatment did not alter biometric and metabolic parameters in obese rats. In a novel finding, ART protected obese hearts against I/R as shown by improved functional performance and smaller IS vs. untreated obese hearts. Cardioprotection was underscored by increased myocardial phosphorylated endothelial nitric oxide synthase (eNOS) and reduced AMP-kinase levels. In conclusion, these results demonstrate for the first time, that 6-weeks treatment of obese rats with a FDC ART drug specifically containing TDF/FTC/EFV conferred cardioprotection against I/R. The FDC ART-induced cardioprotection was seemingly unrelated to metabolic changes, but rather due to direct cardiac mechanisms including the up-regulation of myocardial eNOS.

Differential effects of AMP-activated protein kinase in isolated rat atria subjected to simulated ischemia-reperfusion depending on the energetic substrates available

AMP-activated protein kinase (AMPK) is a serine-threonine kinase that functions primarily as a metabolic sensor to coordinate anabolic and catabolic processes in the cell, via phosphorylation of multiple proteins involved in metabolic pathways, aimed to re-establish energy homeostasis at a cell-autonomous level. Myocardial ischemia and reperfusion represents a metabolic stress situation for myocytes. Whether AMPK plays a critical role in the metabolic and functional responses involved in these conditions remains uncertain. In this study, in order to gain a deeper insight into the role of endogenous AMPK activation during myocardial ischemia and reperfusion, we explored the effects of the pharmacological inhibition of AMPK on contractile function rat, contractile reserve, tissue lactate production, tissue ATP content, and cellular viability. For this aim, isolated atria subjected to simulated 75 min ischemia-75 min reperfusion (Is-Rs) in the presence or absence of the pharmacological inhibitor of AMPK (compound C) were used. Since in most clinical situations of ischemia-reperfusion the heart is exposed to high levels of fatty acids, the influence of palmitate present in the incubation medium was also investigated. The present results suggest that AMPK activity significantly increases during Is, remaining activated during Rs. The results support that intrinsic activation of AMPK has functional protective effects in the reperfused atria when glucose is the only available energetic substrate whereas it is deleterious when palmitate is also available. Cellular viability was not affected by either of these conditions.

Activation of AMPK Attenuated Cardiac Fibrosis by Inhibiting CDK2 via p21/p27 and miR-29 Family Pathways in Rats

Cardiac fibrosis is pathological damage associated with nearly all forms of heart disease. AMP-activated protein kinase (AMPK) is an evolutionary conserved energy-sensing enzyme. Emerging evidences indicate that AMPK plays an important role in cardiac fibrosis and cell proliferation. However, less is known about the detailed mechanism of AMPK activation on cardiac fibrosis. In this study, we found the AMPK activation improved the impaired cardiac function of cardiac fibrosis rats and decreased interstitial fibrosis. Further results indicated AMPK activation promoted p21 and p27 and inhibited CDK2 and cyclin E protein expressions both in vivo and in vitro. Moreover, AMPK activation repressed downstream transcription factor hepatocyte nuclear factor 4 alpha (HNF-4α) expression and decreased the binding of HNF-4α to TGF-β₁ promoters, which eventually resulted in TGF-β₁ downregulation and miR-29 family upregulation. Furthermore, miR-29, in turn, inhibited the progression of cardiac fibrosis through suppressing its target CDK2. Taken together, activation of AMPK, on the one hand, upregulated p21 and p27 expression, further inhibited CDK2 and cyclin E complex, and finally suppressed the progression of cardiac fibrosis, and, on the other hand, repressed HNF-4α expression, further downregulated the activity of TGF-β₁ promoter, promoted miR-29 expression, and finally prevented the development of cardiac fibrosis.

Signal transduction analysis of the NLRP3-inflammasome pathway after cellular damage and its paracrine regulation

Activation of the NLRP3-inflammasome pathway and production of the inflammatory cytokine IL-1B after cellular damage caused by infarct or infection is a key process in several diseases such as acute myocardial infarction and inflammatory bowel disease. However, while the molecular triggers of the NLRP3-pathway after cellular damage are well known, the mechanisms that sustain or confine its activity are currently under investigation. We present here an Ordinary Differential Equation-based model that investigates the mechanisms of inflammasome activation and regulation in monocytes to predict IL-1β activation kinetics upon a two-step activation by Damage-Associate-Molecular-Particles (DAMP) and extracellular ATP. Assuming both activation signals to be concomitantly present or present with a delay of 12h, the model predicted a transient IL-1β activation at different concentration levels dependent on signal synchronisation. Introducing a positive feedback loop mediated by active IL-1β resulted in a sustained IL-1β activation, hence arguing for a paracrine signalling between inflammatory cells to guarantee a temporally stable inflammatory response. We then investigate mechanisms that control termination of inflammation using two recently identified molecular intervention points in the inflammasome pathway. We found that a more upstream regulation, by attenuating production of the IL-1β-proform, was more potent in attenuating active IL-1β production than direct inhibition of the NLRP3-inflammasome. Interestingly, ablating this upstream negative feedback led to a high variability of IL-1β production in monocytes from different subjects, consistent with a recent pre-clinical study. We finally discuss the relevance and implications of our findings in disease models of acute myocardial infarction and spontaneous colitis.

The role of AMPK in cardiomyocyte health and survival

Cellular energy homeostasis is a fundamental process that governs the overall health of the cell and is paramount to cell survival. Central to this is the control of ATP generation and utilization, which is regulated by a complex myriad of enzymatic reactions controlling cellular metabolism. In the cardiomyocyte, ATP generated from substrate catabolism is used for numerous cellular processes including maintaining ionic homeostasis, cell repair, protein synthesis and turnover, organelle turnover, and contractile function. In many instances, cardiovascular disease is associated with impaired cardiac energetics and thus the signalling that regulates pathways involved in cardiomyocyte metabolism may be potential targets for pharmacotherapy designed to help treat cardiovascular disease. An important regulator of cardiomyocyte energy homeostasis is adenosine monophosphate-activated protein kinase (AMPK). AMPK is a serine-threonine kinase that functions primarily as a metabolic sensor to coordinate anabolic and catabolic activities in the cell via the phosphorylation of multiple proteins involved in metabolic pathways. In addition to the direct role that AMPK plays in the regulation of cardiomyocyte metabolism, AMPK can also either directly or indirectly influence other cellular processes such as regulating mitochondrial function, post-translation acetylation, autophagy, mitophagy, endoplasmic reticulum stress, and apoptosis. Thus, AMPK is implicated in the control of a wide variety of cellular processes that can influence cardiomyocyte health and survival. In this review, we will discuss the important role that AMPK plays in regulating cardiac metabolism, as well as the additional cellular processes that may contribute to cardiomyocyte function and survival in the healthy and the diseased heart. This article is part of a Special Issue entitled: The role of post-translational protein modifications on heart and vascular metabolism edited by Jason R.B. Dyck & Jan. F.C. Glatz.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Berberine Pretreatment Confers Cardioprotection Against Ischemia-Reperfusion Injury in a Rat Model of Type 2 Diabetes

Preclinical and clinical studies have demonstrated that berberine (BBR) improves diabetic complications and reduces mortality of patients with congestive heart failure. The therapeutic effects of BBR have been reported to be mediated by its regulation of adenosine monophosphate (AMP)-activated protein kinase (AMPK). We previously reported that BBR protects against ischemia-reperfusion injury via regulating AMPK activity in both ischemic and nonischemic areas of the rat heart. Since diabetic hearts are more sensitive to ischemia-reperfusion injury, we examined whether BBR treatment exhibited cardioprotective effects in the diabetic heart. Type 2 diabetic rats were pretreated plus or minus BBR for 7 days and subjected to 30-minute ischemia followed by 120-minute reperfusion. Pretreatment of type 2 diabetic rats with BBR reduced ischemia-reperfusion injury infarct size and attenuated arrhythmia compared to untreated diabetic controls. Subsequent to ischemia-reperfusion, serum triglyceride, total cholesterol, and malondialdehyde levels were reduced by pretreatment of type 2 diabetic rats with BBR compared to untreated diabetic controls. In contrast, serum glucose and superoxide dismutase levels were unaltered. The mechanism for the BBR-mediated cardioprotective effect was examined. Pretreatment with BBR did not alter AMPK activity in ischemic areas at risk but increased AMPK activity in nonischemic areas compared to untreated diabetic controls. The increased AMPK activity in nonischemic areas was due an elevated ratio of AMP to adenosine triphosphate (ATP) and adenosine diphosphate to ATP. In addition, pretreatment with BBR increased protein kinase B (AKT) phosphorylation and reduced glycogen synthase kinase 3β (GSK3β) activity in nonischemic areas compared to untreated diabetic controls. These findings indicate that BBR protects the diabetic heart from ischemia-reperfusion injury. In addition, BBR may mediate this cardioprotective effect through AMPK activation, AKT phosphorylation, and GSK3β inhibition in the nonischemic areas of the diabetic heart.

GSK-3β controls autophagy by modulating LKB1-AMPK pathway in prostate cancer cells

BACKGROUND

Glycogen synthase kinase 3β (GSK3B, GSK-3β) is a multi-functional protein kinase involved in various cellular processes and its activity elevates after serum deprivation. We have shown that inhibition of GSK-3β activity triggered a profound autophagic response and subsequent necrotic cell death after serum deprivation in prostate cancer cells. In this study, we dissected the mechanisms involved in GSK-3β inhibition-triggered autophagy.

METHODS

Prostate cancer PC-3 and DU145 cells were used in the study. Multiple GSK-3β specific inhibitors were used including small chemicals TDZD8, Tideglusib, TWS119, and peptide L803-mts. Western blot assay coupled with phospho-specific antibodies were used in detecting signal pathway activation. ATP levels were assessed with ATPLite kit and HPLC methods. Autophagy response was determined by evaluating Microtubule-associated proteins 1A/1B light chain 3B (LC3B) processing and p62 protein stability in Western blot assays. Immunofluorescent microscopy was used to detect LKB1 translocation.

RESULTS

Inhibition of GSK-3β activity resulted in a significant decline of cellular ATP production, leading to a significant increase of AMP/ATP ratio, a strong trigger of AMP-activated protein kinase (AMPK) activation in prostate cancer PC-3 cells. In parallel with increased LC-3B biosynthesis and p62 protein reduction, the classical sign of autophagy induction, AMPK was activated after inhibition of GSK-3β activity. Further analysis revealed that Liver kinase B1 (LKB1) but not Calcium/calmodulin-dependent protein kinase kinase β (CaMKKβ) is involved in AMPK activation and autophagy induction triggered by GSK-3β inhibition. Meanwhile, GSK-3β inhibition promoted LKB1 translocation from nuclear to cytoplasmic compartment and enhanced LKB1 interaction with its regulatory partners Mouse protein-25 (MO25) and STE20-related adaptor (STRAD).

CONCLUSIONS

In conclusion, our data suggest that GSK-3β plays an important role in controlling autophagy induction by modulating the activation of LKB1-AMPK pathway after serum deprivation.

AMPK in Cardiovascular Diseases

This chapter summarizes the implication of AMP-activated protein kinase (AMPK) in the regulation of various physiological and pathological cellular events of great importance for the maintenance of cardiac function. These include the control of both metabolic and non-metabolic elements targeting the different cellular components of the cardiac tissue, i.e., cardiomyocytes, fibroblasts, and vascular cells. The description of the multifaceted action of the two AMPK catalytic isoforms, α1 and α2, emphasizes the general protective action of this protein kinase against the development of critical pathologies like myocardial ischemia, cardiac hypertrophy, diabetic cardiomyopathy, and heart failure.

AMPK is critical for mitochondrial function during reperfusion after myocardial ischemia

AMP-activated kinase (AMPK) is a stress responsive kinase that regulates cellular metabolism and protects against cardiomyocyte injury during ischemia-reperfusion (IR). Mitochondria play an important role in cell survival, but the specific actions of activated AMPK in maintaining mitochondrial integrity and function during reperfusion are unknown. Thus, we assessed the consequences of AMPK inactivation on heart mitochondrial function during reperfusion. Mouse hearts expressing wild type (WT) or kinase-dead (KD) AMPK were studied. Mitochondria isolated from KD hearts during reperfusion had intact membrane integrity, but demonstrated reduced oxidative capacity, increased hydrogen peroxide production and decreased resistance to mitochondrial permeability transition pore opening compared to WT. KD hearts showed increased activation of the mitogen activated protein kinase kinase 4 (MKK4) and downstream c-Jun terminal kinase (JNK) and greater necrosis during reperfusion after coronary occlusion. Transgenic expression of mitochondrial catalase (MCAT) prevented the excessive cardiac JNK activation and attenuated the increased myocardial necrosis observed during reperfusion in KD mice. Inhibition of JNK increased the resistance of KD hearts to mPTP opening, contractile dysfunction and necrosis during IR. Thus, intrinsic activation of AMPK is critical to prevent excess mitochondrial reactive oxygen production and consequent JNK signaling during reperfusion, thereby protecting against mPTP opening, irreversible mitochondrial damage and myocardial injury.

Total saponins from Aralia taibaiensis protect against myocardial ischemia/reperfusion injury through AMPK pathway

It was previously shown that total saponins extracted from Aralia taibaiensis (sAT) have potent antioxidant activities for treating diabetes mellitus and attenuate D-galactose-induced aging. Since diabetes mellitus and aging are closely associated with cardiac dysfunction, particularly ischemic heart disease, sAT may have potential protective activity against myocardial ischemia/reperfusion injury (MI/RI). However, the anti-MI/RI effects of sAT have yet to be examined, and the possible molecular mechanisms remain to be determined. The present study was undertaken to investigate the anti-MI/RI activities of sAT and to elucidate the mechanisms underlying these effects in rats using TUNEL and Hoechst 33258 staining. The results confirmed the cardioprotective effects in vivo and elucidated the potential molecular mechanisms of sAT in vitro. Pretreatment with sAT significantly reduced infarct size, decreased the levels of lactate dehydrogenase and creatine kinase in the serum and blocked apoptosis. In addition, sAT inhibited A/R-induced apoptosis by decreasing DNA strand breaks, caspase-3 activity and cytochrome c release in H9c2 cells. Furthermore, sAT markedly increased the phosphorylation of AMP-activated protein kinase (AMPK) and acetyl CoA carboxylase and elevated the Bcl2/Bcl-2-associated X protein ratio. These effects were blocked by compound C. The results suggested that sAT pretreatment exerts protective effects on myocardial cells in vitro and in vivo against MI/RI-induced apoptosis by activating AMPK pathway.

Targeted intracellular accumulation of macrophage migration inhibitory factor in the reperfused heart mediates cardioprotection

S-nitrosation of macrophage migration inhibitory factor (MIF) has been shown to be cytoprotective in myocardial ischaemia/reperfusion (I/R) injury. Since the exact mechanism of action is unknown, we here characterise the cardioprotective effects of targeted intracellular accumulation of MIF in myocardial I/R injury. We used different in vivo, ex vivo and in vitro models of myocardial I/R and hypoxia/reoxygenation (H/R) injury to determine MIF levels by immunoblots and ELISA in different phases of reperfusion and reoxygenation, respectively. We discovered a rapid decrease of cardiac MIF that was specific to the early phase of reperfusion. Posttranslational modification of MIF via S-nitrosation--proofed by a modified version of the Biotin Switch Assay--prevented this rapid decrease, leading to a targeted intracellular accumulation of MIF in the early phase of reperfusion. Intracellular MIF accumulation preserved the intracellular ability of MIF to reduce oxidative stress as shown by hydrogen peroxide and aconitase activity measurements. Infarct size measurements by TTC staining showed an overall enhanced cardioprotective effect of this protein by reduction of reperfusion injury. In summary, we have unravelled a novel mechanism of MIF-mediated cardioprotection. Targeted intracellular accumulation of MIF by S-nitrosation may offer a novel therapeutic approach in the treatment of myocardial I/R-injury.

Tolerance to ischaemic injury in remodelled mouse hearts: less ischaemic glycogenolysis and preserved metabolic efficiency

AIMS

Post-infarction remodelled failing hearts have reduced metabolic efficiency. Paradoxically, they have increased tolerance to further ischaemic injury. This study was designed to investigate the metabolic mechanisms that may contribute to this phenomenon and to examine the relationship between ischaemic tolerance and metabolic efficiency during post-ischaemic reperfusion.

METHODS AND RESULTS

Male C57BL/6 mice were subjected to coronary artery ligation (CAL) or SHAM surgery. After 4 weeks, in vivo mechanical function was assessed by echocardiography, and then isolated working hearts were perfused in this sequence: 45 min aerobic, 15 min global no-flow ischaemia, and 30 min aerobic reperfusion. Left ventricular (LV) function, metabolic rates, and metabolic efficiency were measured. Relative to SHAM, both in vivo and in vitro CAL hearts had depressed cardiac function under aerobic conditions (45 and 36%, respectively), but they had a greater recovery of LV function during post-ischaemic reperfusion (67 vs. 49%, P < 0.05). While metabolic efficiency (LV work per ATP produced) was 50% lower during reperfusion of SHAM hearts, metabolic efficiency in CAL hearts did not decrease. During ischaemia, glycogenolysis was 28% lower in CAL hearts, indicative of lower ischaemic proton production. There were no differences in mitochondrial abundance, calcium handling proteins, or key metabolic enzymes.

CONCLUSION

Compared with SHAM, remodelled CAL hearts are more tolerant to ischaemic injury and undergo no further deterioration of metabolic efficiency during reperfusion. Less glycogen utilization in CAL hearts during ischaemia may contribute to increased ischaemic tolerance by limiting ischaemic proton production that may improve ion homeostasis during early reperfusion.

Pyrrolidinyl caffeamide against ischemia/reperfusion injury in cardiomyocytes through AMPK/AKT pathways

Background

Coronary heart disease is a leading cause of death in the world and therapy to reduce injury is still needed. The uncoupling of glycolysis and glucose oxidation induces lactate accumulation during myocardial ischemia/reperfusion (I/R) injury. Cell death occurs and finally leads to myocardial infarction. Caffeic acid, one of the major phenolic constituents in nature, acts as an antioxidant. Pyrrolidinyl caffeamide (PLCA), a new derivative of caffeic acid, was synthesized by our team. We aimed to investigate the effect of PLCA on hypoxia/reoxygenation (H/R) in neonatal rat ventricular myocytes (NRVM) and on myocardial I/R in rats.

Results

Cardiomyocytes were isolated and subjected to 6 h hypoxia followed by 18 h reperfusion. PLCA (0.1 to 3 μM) and metformin (30 μM) were added before hypoxia was initiated. PLCA at 1 μM and metformin at 30 μM exerted similar effects on the improvement of cell viability and the alleviation of cell apoptosis in NRVM after H/R. PLCA promoted p-AMPK, p-AKT, and GLUT4 upregulation to induce a cardioprotective effect in both cell and animal model. The accumulation of cardiac lactate was attenuated by PLCA during myocardial I/R, and infarct size was smaller in rats treated with PLCA (1 mg/kg) than in those treated with caffeic acid (1 mg/kg).

Conclusions

AMPK and AKT are synergistically activated by PLCA, which lead facilities glucose utilization, thereby attenuating lactate accumulation and cell death. The cardioprotective dose of PLCA was lower than those of metformin and caffeic acid. We provide a new insight into this potential drug for the treatment of myocardial I/R injury.

Direct effects of adipokines on the heart: focus on adiponectin

Cardiovascular disease, including heart failure, is a principal cause of death in individuals with obesity and diabetes. However, the mechanisms of obesity- and diabetes-induced heart disease are multifaceted and remain to be clearly defined. Of relevance to this review, there is currently great research and clinical interest in the endocrine effects of adipokines on the myocardium and their role in heart failure. We will discuss the potential significance of adipokines in the pathogenesis of heart failure via their ability to regulate remodeling events including metabolism, hypertrophy, fibrosis, and cell death. As an excellent example, we will first focus on adiponectin which is best known to confer numerous cardioprotective effects. However, we comprehensively discuss the existing literature that highlights it would be naive to assume that this was always the case. We also focus on lipocalin-2 which mediates pro-inflammatory and pro-apoptotic effects. It is important when studying actions of adipokines to integrate cellular and mechanistic analyses and translate these to physiologically relevant in vivo models and clinical studies. However, assimilating studies on numerous cardiac remodeling events which ultimately dictate cardiac dysfunction into a unifying conclusion is challenging. Nevertheless, there is undoubted potential for the use of adipokines as robust biomarkers and appropriate therapeutic targets in heart failure.

AMPK-regulated and Akt-dependent enhancement of glucose uptake is essential in ischemic preconditioning-alleviated reperfusion injury

AIMS

Ischemic preconditioning (IPC) is a potent form of endogenous protection. However, IPC-induced cardioprotective effect is significantly blunted in insulin resistance-related diseases and the underlying mechanism is unclear. This study aimed to determine the role of glucose metabolism in IPC-reduced reperfusion injury.

METHODS

Normal or streptozotocin (STZ)-treated diabetic rats subjected to 2 cycles of 5 min ischemia/5 min reperfusion prior to myocardial ischemia (30 min)/reperfusion (3 h). Myocardial glucose uptake was determined by (18)F-fluorodeoxyglucose-positron emission tomography (PET) scan and gamma-counter biodistribution assay.

RESULTS

IPC exerted significant cardioprotection and markedly improved myocardial glucose uptake 1 h after reperfusion (P<0.01) as evidenced by PET images and gamma-counter biodistribution assay in ischemia/reperfused rats. Meanwhile, myocardial translocation of glucose transporter 4 (GLUT4) to plasma membrane together with myocardial Akt and AMPK phosphorylation were significantly enhanced in preconditioned hearts. Intramyocardial injection of GLUT4 siRNA markedly decreased GLUT4 expression and blocked the cardioprotection of IPC as evidence by increased myocardial infarct size. Moreover, the PI3K inhibitor wortmannin significantly inhibited activation of Akt and AMPK, reduced GLUT4 translocation, glucose uptake and ultimately, depressed IPC-induced cardioprotection. Furthermore, IPC-afforded antiapoptotic effect was markedly blunted in STZ-treated diabetic rats. Exogenous insulin supplementation significantly improved glucose uptake via co-activation of myocardial AMPK and Akt and alleviated ischemia/reperfusion injury as evidenced by reduced myocardial apoptosis and infarction size in STZ-treated rats (P<0.05).

CONCLUSIONS

The present study firstly examined the role of myocardial glucose metabolism during reperfusion in IPC using direct genetic modulation in vivo. Augmented glucose uptake via co-activation of myocardial AMPK and Akt in reperfused myocardium is essential to IPC-alleviated reperfusion injury. This intrinsic metabolic modulation and cardioprotective capacity are present in STZ-treated hearts and can be triggered by insulin.

Berberine attenuates ischemia-reperfusion injury via regulation of adenosine-5'-monophosphate kinase activity in both non-ischemic and ischemic areas of the rat heart

BACKGROUND

Berberine exhibits numerous pharmacological effects, but the mechanism for its protective effects against ischemia-reperfusion cardiac injury is unknown.

METHODS

Male Wistar rats were treated with berberine (100 mg/Kg/day, ig) for 14 days and controls treated with water. Hearts were isolated in vitro and perfused in the Langendorff mode and subjected to 30 min of global ischemia followed by 30 min of reperfusion and hemodynamic data examined. In a separate set of experiments, hearts were subjected in vivo to left anterior descending coronary artery ligation for 30 min followed by 120 min reperfusion and hemodynamic data, type and duration of arrhythmias, and myocardial infarct size determined. AMP-activated protein kinase (AMPK) level, ADP/ATP and AMP/ATP ratios were examined in non-ischemic areas and risk areas of the heart.

RESULTS

Subsequent to ischemia-reperfusion injury, left ventricular developed pressure, left ventricular end diastolic pressure and maximum rate of intraventricular pressure contractility and relaxation were significantly improved in the berberine treatment groups compared to controls. Berberine treatment decreased infarct size and diminished the duration and incidence of arrhythmias compared to controls. Berberine treatment significantly decreased AMPK protein concentration, and the ratio of ADP/ATP and AMP/ATP in the myocardial risk areas. In contrast, berberine treatment significantly increased AMPK protein concentration, and the ratio of ADP/ATP and AMP/ATP in the non-ischemia areas compared to controls.

CONCLUSION

These findings suggest that berberine may exert its cardioprotective effect on ischemia-reperfusion injury via regulation of AMPK activity in both non-ischemic areas and risk areas of the heart.

Polysaccharide from Fuzi Likely Protects Against Starvation-Induced Cytotoxicity in H9c2 Cells by Increasing Autophagy Through Activation of the AMPK/mTOR Pathway

There is increasing evidence that starvation induces autophagy, which may be protective during starvation, in an AMPK-dependent manner. Polysaccharides from Fuzi (FPS) reportedly have protective effects on nutrition-limited livers. The present study was designed to determine whether FPS protected H9c2 cells against starvation-induced cytotoxicity using an AMPK/mTOR-dependent mechanism. H9c2 cells were incubated in serum and glucose starvation media for 12 hours to establish a cell injury model. 3-Methyladenine (3MA, an autophagy inhibitor) was used to identify the exact role of autophagy in starvation. Cells were incubated with different FPS concentrations, and the cell injury levels, autophagy activity and AMPK/mTOR phosphorylation were measured. Adenine 9-β-D-arabinofuranoside (Ara-A, an AMPK inhibitor) and 5-amino-4-imidazole-carboxamide riboside (AICAR, an AMPK activator) were used to identify whether the AMPK/mTOR pathway was involved in FPS-mediated cardioprotection. We demonstrated that starvation decreased cell viability in a time-dependent manner, and 3MA-induced autophagy inhibition aggravated the reduced cell viability. FPS treatment attenuated the cell viability decrement and the starvation-induced decline in the mitochondrial membrane potential (MMP), and autophagy; also, the AMPK/mTOR pathways were activated during treatment. Ara-A treatment abolished the protective effect of FPS, while AICAR treatment had a similar effect to FPS. We conclude that autophagy attenuates starvation-induced cardiomyocyte death, and FPS increases autophagy activity to protect against starvation-induced cytotoxicity in H9c2 cells, likely through AMPK/mTOR pathway activation.

AMPK signalling and the control of substrate use in the heart

All mammalian cells rely on adenosine triphosphate (ATP) to maintain function and for survival. The heart has the highest basal ATP demand of any organ due to the necessity for continuous contraction. As such, the ability of the cardiomyocyte to monitor cellular energy status and adapt the supply of substrates to match the energy demand is crucial. One important serine/threonine protein kinase that monitors cellular energy status in the heart is adenosine monophosphate activated protein kinase (AMPK). AMPK is also a key enzyme that controls multiple catabolic and anabolic biochemical pathways in the heart and indirectly plays a crucial role in regulating cardiac function in both physiological and pathophysiological conditions. Herein, we review the involvement of AMPK in myocardial fatty acid and glucose transport and utilization, as it relates to basal cardiac function. We also assess the literature amassed on cardiac AMPK and discuss the controversies surrounding the role of AMPK in physiological and pathophysiological processes in the heart. The work reviewed herein also emphasizes areas that require further investigation for the purpose of eventually translating this information into improved patient care.

Regulating cardiac energy metabolism and bioenergetics by targeting the DNA damage repair protein BRCA1

OBJECTIVE

Alterations in cardiac energy and substrate metabolism play a critical role in the development and clinical course of heart failure. We hypothesized that the cardioprotective role of the breast cancer 1, early onset (BRCA1) gene might be mediated in part by alterations in cardiac bioenergetics.

METHODS

We generated cardiomyocyte-specific BRCA1 homozygous and heterozygous knockout mice using the Cre-loxP technology and evaluated the key molecules and pathways involved in glucose metabolism, fatty acid metabolism, and mitochondrial bioenergetics.

RESULTS

Cardiomyocyte-specific BRCA1-deficient mice showed reduced cardiac expression of glucose and fatty acid transporters, reduced acetyl-coenzyme A carboxylase 2 and malonyl-coenzyme A decarboxylase (key enzymes that control malonyl coenzyme A, which in turn controls fatty acid oxidation), and reduced carnitine palmitoyltransferase I, a rate-limiting enzyme for mitochondrial fatty acid uptake. Peroxisome proliferator-activated receptor α and γ and carnitine palmitoyltransferase I levels were also downregulated in these hearts. Rates of glucose and fatty acid oxidation were reduced in the hearts of heterozygous cardiomyocyte-restricted BRCA1-deficient mice, resulting in a decrease in the rate of adenosine triphosphate production. This decrease in metabolism and adenosine triphosphate production occurred despite an increase in 5'-adenosine monophosphate-activated protein kinase and AKT activation in the heart.

CONCLUSIONS

Cardiomyocyte-specific loss of BRCA1 alters critical pathways of fatty acid and glucose metabolism, leading to an energy starved heart. BRCA1-based cell or gene therapy might serve as a novel target to improve cardiac bioenergetics in patients with heart failure.

Antihyperglycaemic activity of 2,4:3,5-dibenzylidene-D-xylose-diethyl dithioacetal in diabetic mice

We have recently generated lipophilic D-xylose derivatives that increase the rate of glucose uptake in cultured skeletal muscle cells in an AMP-activated protein kinase (AMPK)-dependent manner. The derivative 2,4:3,5-dibenzylidene-D-xylose-diethyl dithioacetal (EH-36) stimulated the rate of glucose transport by increasing the abundance of glucose transporter-4 in the plasma membrane of cultured myotubes. The present study aimed at investigating potential antihyperglycaemic effects of EH-36 in animal models of diabetes. Two animal models were treated subcutaneously with EH-36: streptozotocin-induced diabetes in C57BL/6 mice (a model of insulin-deficient type 1 diabetes), and spontaneously diabetic KKAy mice (Kuo Kondo rats carrying the A(y) yellow obese gene; insulin-resistant type 2 diabetes). The in vivo biodistribution of glucose in control and treated mice was followed with the glucose analogue 2-deoxy-2-[(18) F]-D-glucose; the rate of glucose uptake in excised soleus muscles was measured with [(3) H]-2-deoxy-D-glucose. Pharmacokinetic parameters were determined by non-compartmental analysis of the in vivo data. The effective blood EH-36 concentration in treated animals was 2 μM. It reduced significantly the blood glucose levels in both types of diabetic mice and also corrected the typical compensatory hyperinsulinaemia of KKAy mice. EH-36 markedly increased glucose transport in vivo into skeletal muscle and heart, but not to adipose tissue. This stimulatory effect was mediated by Thr(172) -phosphorylation in AMPK. Biochemical tests in treated animals and acute toxicological examinations showed that EH-36 was well tolerated and not toxic to the mice. These findings indicate that EH-36 is a promising prototype molecule for the development of novel antidiabetic drugs.

Ischemia/reperfusion-induced myosin light chain 1 phosphorylation increases its degradation by matrix metalloproteinase 2

Degradation of myosin light chain 1 (MLC1) by matrix metalloproteinase 2 (MMP-2) during myocardial ischemia/reperfusion (I/R) has been demonstrated. However, the exact mechanisms controlling this process remain unknown. I/R increases the phosphorylation of MLC1, but the consequences of this modification are not known. We hypothesized that phosphorylation of MLC1 plays an important role in its degradation by MMP-2. To examine this, isolated perfused rat hearts were subjected to 20 min global ischemia followed by 30 min of aerobic reperfusion. I/R increased phosphorylation of MLC1 (as measured by mass spectrometry). When hearts were subjected to I/R in the presence of ML-7 (a myosin light-chain kinase inhibitor) or doxycycline (an MMP inhibitor), improved recovery of contractile function was observed compared to aerobic controls, and MLC1 was protected from degradation. Enzyme kinetic studies revealed an increased affinity of MMP-2 for the phosphorylated form of MLC1 compared to non-phosphorylated MLC1. We conclude that MLC1 phosphorylation is an important mechanism controlling the intracellular action of MMP-2 and promoting degradation of MLC1. These results further support previous findings implicating post-translational modifications of contractile proteins as a key factor in the pathology of cardiac dysfunction during and following ischemia.

Targeting myocardial substrate metabolism in heart failure: potential for new therapies

The incidence and prevalence of heart failure have increased significantly over the past few decades. Available data suggest that patients with heart failure independent of the aetiology have viable but dysfunctional myocardium that is potentially salvageable. Although a great deal of research effort has focused on characterizing the molecular basis of heart failure, cardiac metabolism in this disorder remains an understudied discipline. It is known that many aspects of cardiomyocyte energetics are altered in heart failure. These include a shift from fatty acid to glucose as a preferred substrate and a decline in the levels of ATP. Despite these demonstrated changes, there are currently no approved drugs that target metabolic enzymes or proteins in heart failure. This is partly due to our limited knowledge of the mechanisms and pathways that regulate cardiac metabolism. Better characterization of these pathways may potentially lead to new therapies for heart failure. Targeting myocardial energetics in the viable and potentially salvageable tissue may be particularly effective in the treatment of heart failure. Here, we will review metabolic changes that occur in fatty acid and glucose metabolism and AMP-activated kinase in heart failure. We propose that cardiac energetics should be considered as a potential target for therapy in heart failure and more research should be done in this area.

5'-AMP-activated protein kinase is inactivated by adrenergic signalling in adult cardiac myocytes

In adult rat cardiac myocytes adrenaline decreased AMPK (AMP-activated protein kinase) activity with a half-time of approximately 4 min, decreased phosphorylation of AMPK (α-Thr172) and decreased phosphorylation of ACC (acetyl-CoA carboxylase). Inactivation of AMPK by adrenaline was through both α1- and β-ARs (adrenergic receptors), but did not involve cAMP or calcium signalling, was not blocked by the PKC (protein kinase C) inhibitor BIM I (bisindoylmaleimide I), by the ERK (extracellular-signal-regulated kinase) cascade inhibitor U0126 or by PTX (pertussis toxin). Adrenaline caused no measurable change in LKB1 activity. Adrenaline decreased AMPK activity through a process that was distinct from AMPK inactivation in response to insulin or PMA. Neither adrenaline nor PMA altered the myocyte AMP:ATP ratio although the adrenaline effect was attenuated by oligomycin and by AICAR (5-amino-4-imidazolecarboxamide-1-β-D-ribofuranoside), agents that mimic 'metabolic stress'. Inactivation of AMPK by adrenaline was abolished by 1 μM okadaic acid suggesting that activation of PP2A (phosphoprotein phosphatase 2A) might mediate the adrenaline effect. However, no change in PP2A activity was detected in myocyte extracts. Adrenaline increased phosphorylation of the AMPK β-subunit in vitro but there was no detectable change in vivo in phosphorylation of previously identified AMPK sites (β-Ser24, β-Ser108 or β-Ser182) suggesting that another site(s) is targeted.

Myocardial ischemia reperfusion injury: from basic science to clinical bedside

Myocardial ischemia reperfusion injury contributes to adverse cardiovascular outcomes after myocardial ischemia, cardiac surgery or circulatory arrest. Primarily, no blood flow to the heart causes an imbalance between oxygen demand and supply, named ischemia (from the Greek isch, restriction; and haema, blood), resulting in damage or dysfunction of the cardiac tissue. Instinctively, early and fast restoration of blood flow has been established to be the treatment of choice to prevent further tissue injury. Indeed, the use of thrombolytic therapy or primary percutaneous coronary intervention is the most effective strategy for reducing the size of a myocardial infarct and improving the clinical outcome. Unfortunately, restoring blood flow to the ischemic myocardium, named reperfusion, can also induce injury. This phenomenon was therefore termed myocardial ischemia reperfusion injury. Subsequent studies in animal models of acute myocardial infarction suggest that myocardial ischemia reperfusion injury accounts for up to 50% of the final size of a myocardial infarct. Consequently, many researchers aim to understand the underlying molecular mechanism of myocardial ischemia reperfusion injury to find therapeutic strategies ultimately reducing the final infarct size. Despite the identification of numerous therapeutic strategies at the bench, many of them are just in the process of being translated to bedside. The current review discusses the most striking basic science findings made during the past decades that are currently under clinical evaluation, with the ultimate goal to treat patients who are suffering from myocardial ischemia reperfusion-associated tissue injury.

Inactivation of the AMP-activated protein kinase by glucose in cardiac myocytes: a role for the pentose phosphate pathway

Incubation of adult rat cardiac myocytes with increasing glucose concentrations decreased phosphorylation (αThr172) and activity of AMPK (AMP-activated protein kinase). The effect could be demonstrated without measurable changes in adenine nucleotide contents. The glucose effect was additive to the decrease in AMPK activity caused by insulin, was attenuated by adrenaline, was not mimicked by glucose analogues, lactate or pyruvate and was not due to changes in myocyte glycogen content. AMPK activity was decreased by xylitol and PMS (phenazine methosulfate) and was increased by the glucose-6-phosphate dehydrogenase inhibitor DHEA (dehydroepiandrosterone) and by thiamine. PMS and DHEA respectively, increased and decreased CO2 formation by the PPP (pentose phosphate pathway). AMPK activity was inversely related to the myocyte content of Xu5P (xylulose 5-phosphate), an intermediate of the non-oxidative arm of the PPP. Endothall, an inhibitor of PP2A (protein phosphatase 2A), abolished the glucose effect on AMPK activity. Further studies are needed to define the ‘active component’ that mediates the glucose effect and whether its site of action is PP2A.

Pharmacological AMP-kinase activators have compartment-specific effects on cell physiology

5'-AMP-activated kinase (AMPK) regulates numerous biological events and is an essential target for the treatment of type 2 diabetes. The objectives of the present study were first to determine the compartment-specific effects of three established AMPK activators on Thr172 phosphorylation of the α-subunit, an indicator of AMPK activation. Second, we examined how cytoplasmic and nuclear processes are modulated by pharmacological AMPK activators. Specifically, the impact of phenformin, resveratrol, and 5-aminoimidazole-4-carboxamide riboside (AICAR) on Thr172 phosphorylation in the cytoplasm and nucleus was quantified by different methods. To analyze how these activators change cell physiology, we measured the inactivation of acetyl-CoA-carboxylase 1, a predominantly cytoplasmic enzyme that is crucial for lipid metabolism. As a criterion for activities associated with the nucleus, de novo RNA synthesis in nucleoli was quantified. Our studies demonstrate that pharmacological activators of AMPK can alter the balance between nuclear and cytoplasmic AMPK pools. Thus, phenformin and resveratrol caused a strong activation of AMPK in the cytoplasm, whereas the effect was less pronounced in nuclei. By contrast, AICAR elicited a comparable rise in Thr172 phosphorylation in both compartments. Notably, these activators differed drastically in their effects on physiological processes that are located in distinct subcellular compartments. All compounds led to a substantial inactivation of acetyl-CoA-carboxylase 1 in the cytoplasm, with only minor changes to the nuclear enzyme. In the nucleolus, transcription was strongly inhibited by resveratrol, while a moderate inhibition was observed with phenformin and AICAR. Taken together, the compartment-specific phosphorylation of AMPK and downstream events are determined by the activator.

Modulation of myocardial metabolism: an emerging therapeutic principle

PURPOSE OF REVIEW

We focus on the molecular and cellular basis of the improvement in myocardial energetics, which might represent an attractive therapeutic option in some forms of acute and chronic heart disease.

RECENT FINDINGS

Myocardial dysfunction, whether related to left ventricular hypertrophy, heart failure or myocardial ischaemia, is frequently associated with impairment of myocardial energy balance. It is now apparent that this energetic impairment plays a pivotal role, not only in the evolution and outcomes of these disorders but also frequently in their pathogenesis. Despite the fact that energetic impairment may arise for many complex reasons, and the difficulty both in assessing the impairment in vivo and in determining its precise mechanism(s), a number of drugs have become available for treatment of ischaemia and heart failure, as well as potentially for limitation of pathological left ventricular hypertrophy, which act primarily by altering myocardial metabolism so as to improve energetic status. Recent studies with perhexiline and trimetazidine, agents which induce a 'metabolic shift' from long-chain fatty acid to glucose utilization, have demonstrated the utility of this therapeutic principle.

SUMMARY

There is ongoing need for more complete mechanistic understanding of the 'metabolic agents', as well as for the large-scale clinical trials of their impact on health outcomes.

Chronic inhibition of pyruvate dehydrogenase in heart triggers an adaptive metabolic response

Diabetic cardiac dysfunction is associated with decreased rates of myocardial glucose oxidation (GO) and increased fatty acid oxidation (FAO), a fuel shift that has been shown to sensitize the heart to ischemic insult and ventricular dysfunction. We sought to evaluate the metabolic and functional consequences of chronic suppression of GO in heart as modeled by transgenic mice with cardiac-specific overexpression of pyruvate dehydrogenase kinase 4 (myosin heavy chain (MHC)-PDK4 mice), an inhibitor of pyruvate dehydrogenase. Hearts of MHC-PDK4 mice were shown to exhibit an insulin-resistant substrate utilization profile, characterized by low GO rates and high FAO flux. Surprisingly, MHC-PDK4 mice were not sensitized to cardiac ischemia-reperfusion injury despite a fuel utilization pattern that phenocopied the diabetic heart. In addition, MHC-PDK4 mice were protected against high fat diet-induced myocyte lipid accumulation, likely related to increased capacity for FAO. The high rates of mitochondrial FAO in the MHC-PDK4 heart were related to heightened activity of the AMP-activated protein kinase, reduced levels of malonyl-CoA, and increased capacity for mitochondrial uncoupled respiration. The expression of the known AMP-activated protein kinase target, peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α), a master regulator of mitochondrial function and biogenesis, was also activated in the MHC-PDK4 heart. These results demonstrate that chronic activation of PDK4 triggers transcriptional and post-transcriptional mechanisms that re-program the heart for chronic high rates of FAO without the expected deleterious functional or metabolic consequences.

Regulation of AMPK by the ubiquitin proteasome system

The 5'-AMP-activated protein kinase (AMPK) functions as a metabolic fuel gauge that is activated in response to environmental stressors to restore cellular energy balance. In the heart, AMPK coordinates the activation of glucose and fatty acid metabolic pathways to ensure increased production of myocardial ATP when required, such as during cardiac ischemia/reperfusion and hypertrophy, causing an increase in AMPK activity that can be viewed as both protective and maladaptive. While we understand the basic regulation of AMPK activity by kinases, recent studies have introduced the concept that AMPK is regulated by other post-translational modifications, specifically ubiquitination. These studies reported that the ubiquitin ligase cell death-inducing DFFA-like effector a ubiquitinates the β subunit of AMPK to regulate its steady-state protein levels. Other investigators found that AMPK regulatory components, including the AMPK α subunit and AMPK kinases NUAK1 and MARK4, can be ubiquitinated with atypical ubiquitin chains. The USP9X-deubiquitinating enzyme was identified to remove ubiquitination from both NUAK1 and MARK4. Lastly, AMPK activation increases the expression of the ubiquitin ligases MAFBx/Atrogin-1 and MuRF1. These ubiquitin ligases regulate key cardiac transcription factors to control cardiomyocyte mass and remodeling, thus suggesting another mechanism by which AMPK may function in the heart. The relevance of AMPK ubiquitination in cardiac disease has yet to be tested directly, but it likely represents an important mechanism that occurs in common cardiac diseases that may be targeted for therapy.

Long-term effects of intrauterine growth restriction on cardiac metabolism and susceptibility to ischaemia/reperfusion

AIMS

Adult offspring who are born intrauterine growth restricted (IUGR) are at risk of developing cardiovascular diseases during adulthood. Additionally, several cardiac diseases are associated with changes in myocardial energy metabolism. However, the potential long-term effects of being born IUGR on cardiac energetics are unknown. The aim of this study was to assess the long-term effect of IUGR on cardiac performance and energy metabolism under aerobic conditions and after ischaemia/reperfusion (IR) injury.

METHODS AND RESULTS

To induce IUGR, pregnant Sprague-Dawley rats were randomly assigned to hypoxic (11.5% O(2)) or control (21% O(2)) environments from day 15 to 21 of pregnancy. Cardiac susceptibility to IR was evaluated in male and female offspring at 4 (young-adult) or 12 (ageing) months of age using isolated working hearts. Cardiac production of energy was evaluated using radiolabelled substrates. Both male and female IUGR offspring exhibited an increased susceptibility to IR injury compared with controls (P< 0.05) as well as an increased post-ischaemic production of protons (P< 0.001) secondary to a mismatch between myocardial glycolysis and glucose oxidation rates. Moreover, offspring born IUGR exhibited an increased myocardial production of acetyl-CoA during reperfusion. The mismatch between energy production and cardiac performance indicates that in IUGR offspring, cardiac efficiency during reperfusion was decreased relative to controls.

CONCLUSION

Our results suggest that hypoxia-induced IUGR has long-term effects on cardiac susceptibility to IR injury that are independent of sex and age. Moreover, we identified a mismatch in glucose metabolism, leading to proton accumulation in the post-ischaemic myocardium of offspring born IUGR as a potential mechanism involved.

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.

Transitory activation of AMPK at reperfusion protects the ischaemic-reperfused rat myocardium against infarction

PURPOSE

AMPK plays a crucial role in the regulation of the energy metabolism of the heart. During ischaemia, AMPK activation is a known adaptative prosurvival mechanism that helps to maintain the energy levels of the myocardium. However, it still remains unclear if activation of AMPK during reperfusion is beneficial for the heart. Two known AMPK activators (metformin and AICAR) were used to verify the hypothesis that a transitory activation of AMPK at reperfusion may exert cardioprotection, as reflected in a reduction in myocardial infarct size.

METHODS

Perfused rat hearts were subjected to 35 min ischaemia and 120 min reperfusion. Metformin (50 microM) or AICAR (0.5 mM) were added for 15 min at the onset of reperfusion alone or with Compound C (CC, 10 microM), an AMPK inhibitor. Infarct size and alpha-AMPK phosphorylation were measured.

RESULTS

Metformin significantly reduced infarct size from 47.8 +/- 1.7% in control to 31.4 +/- 2.9%, an effect abolished by CC when the drugs were given concomitantly. Similarly, AICAR also induced a significant reduction in infarct size to 32.3 +/- 4.8%, an effect also abrogated by CC. However, metformin's protection was not abolished if CC was administered later in reperfusion. In addition, alpha-AMPK phosphorylation was significantly increased in the metformin treated group during the initial 30 min of reperfusion.

CONCLUSIONS

Our data demonstrated that, in our ex vivo model of myocardial ischaemia-reperfusion injury, AMPK activation in early reperfusion is associated with a reduction in infarct size.