Reduction in hexokinase II levels results in decreased cardiac function and altered remodeling after ischemia/reperfusion injury

GLUT and HK: Two primary and essential key players in tumor glycolysis

Cancer cells reprogram their metabolism to become "glycolysis-dominant," which enables them to meet their energy and macromolecule needs and enhancing their rate of survival. This glycolytic-dominancy is known as the "Warburg effect", a significant factor in the growth and invasion of malignant tumors. Many studies confirmed that members of the GLUT family, specifically HK-II from the HK family play a pivotal role in the Warburg effect, and are closely associated with glucose transportation followed by glucose metabolism in cancer cells. Overexpression of GLUTs and HK-II correlates with aggressive tumor behaviour and tumor microenvironment making them attractive therapeutic targets. Several studies have proven that the regulation of GLUTs and HK-II expression improves the treatment outcome for various tumors. Therefore, small molecule inhibitors targeting GLUT and HK-II show promise in sensitizing cancer cells to treatment, either alone or in combination with existing therapies including chemotherapy, radiotherapy, immunotherapy, and photodynamic therapy. Despite existing therapies, viable methods to target the glycolysis of cancer cells are currently lacking to increase the effectiveness of cancer treatment. This review explores the current understanding of GLUT and HK-II in cancer metabolism, recent inhibitor developments, and strategies for future drug development, offering insights into improving cancer treatment efficacy.

Network-based identification of diagnosis-specific trans-omic biomarkers via integration of multiple omics data

The integration of multiple omics data promises to reveal new insights into the pathogenic mechanisms of complex human diseases, with the potential to identify avenues for the development of targeted therapies for disease subtypes. However, the extraction of diagnostic/disease-specific biomarkers from multiple omics data with biological pathway knowledge is a challenging issue in precision medicine. In this paper, we present a novel computational method to identify diagnosis-specific trans-omic biomarkers from multiple omics data. In the algorithm, we integrated multi-class sparse canonical correlation analysis (MSCCA) and molecular pathway analysis in order to derive discriminative molecular features that are correlated across different omics layers. We applied our proposed method to analyzing proteome and metabolome data of heart failure (HF), and extracted trans-omic biomarkers for HF subtypes; specifically, ischemic cardiomyopathy (ICM) and dilated cardiomyopathy (DCM). We were able to detect not only individual proteins that were previously reported from single-omics studies but also correlated protein-metabolite pairs characteristic of HF disease subtypes. For example, we identified hexokinase1(HK1)-d-fructose-6-phosphate as a paired trans-omic biomarker for DCM, which could significantly perturb amino-sugar metabolism. Our proposed method is expected to be useful for various applications in precision medicine.

The role of glycolytic metabolic pathways in cardiovascular disease and potential therapeutic approaches

Cardiovascular disease (CVD) is a major threat to human health, accounting for 46% of non-communicable disease deaths. Glycolysis is a conserved and rigorous biological process that breaks down glucose into pyruvate, and its primary function is to provide the body with the energy and intermediate products needed for life activities. The non-glycolytic actions of enzymes associated with the glycolytic pathway have long been found to be associated with the development of CVD, typically exemplified by metabolic remodeling in heart failure, which is a condition in which the heart exhibits a rapid adaptive response to hypoxic and hypoxic conditions, occurring early in the course of heart failure. It is mainly characterized by a decrease in oxidative phosphorylation and a rise in the glycolytic pathway, and the rise in glycolysis is considered a hallmark of metabolic remodeling. In addition to this, the glycolytic metabolic pathway is the main source of energy for cardiomyocytes during ischemia-reperfusion. Not only that, the auxiliary pathways of glycolysis, such as the polyol pathway, hexosamine pathway, and pentose phosphate pathway, are also closely related to CVD. Therefore, targeting glycolysis is very attractive for therapeutic intervention in CVD. However, the relationship between glycolytic pathway and CVD is very complex, and some preclinical studies have confirmed that targeting glycolysis does have a certain degree of efficacy, but its specific role in the development of CVD has yet to be explored. This article aims to summarize the current knowledge regarding the glycolytic pathway and its key enzymes (including hexokinase (HK), phosphoglucose isomerase (PGI), phosphofructokinase-1 (PFK1), aldolase (Aldolase), phosphoglycerate metatase (PGAM), enolase (ENO) pyruvate kinase (PKM) lactate dehydrogenase (LDH)) for their role in cardiovascular diseases (e.g., heart failure, myocardial infarction, atherosclerosis) and possible emerging therapeutic targets.

Transcriptomic profiling of mare endometrium at different stages of endometrosis

In the current study, transcriptome profiles of mare endometrium, classified into categories I, IIA, and IIB according to Kenney and Doig, were compared using RNA sequencing, analyzed, and functionally annotated using in silico analysis. In the mild stage (IIA) of endometrosis compared to category I endometrium, differentially expressed genes (DEGs) were annotated to inflammation, abnormal metabolism, wound healing, and quantity of connective tissue. In the moderate stage (IIB) of endometrosis compared to category I endometrium, DEGs were annotated to inflammation, fibrosis, cellular homeostasis, mitochondrial dysfunction, and pregnancy disorders. Ingenuity pathway analysis (IPA) identified cytokines such as transforming growth factor (TGF)-β1, interleukin (IL)-4, IL-13, and IL-17 as upstream regulators of DEGs associated with cellular homeostasis, metabolism, and fibrosis signaling pathways. In vitro studies showed the effect of these cytokines on DEGs such as ADAMTS1, -4, -5, -9, and HK2 in endometrial fibroblasts at different stages of endometrosis. The effect of cytokines on ADAMTS members' gene transcription in fibroblasts differs according to the severity of endometrosis. The identified transcriptomic changes associated with endometrosis suggest that inflammation and metabolic changes are features of mild and moderate stages of endometrosis. The changes of ADAMTS-1, -4, -5, -9, in fibrotic endometrium as well as in endometrial fibroblast in response to TGF-β1, IL-4, IL-13, and IL-17 suggest the important role of these factors in the development of endometrosis.

SIRT2 inhibition protects against cardiac hypertrophy and ischemic injury

Sirtuins (SIRT) exhibit deacetylation or ADP-ribosyltransferase activity and regulate a wide range of cellular processes in the nucleus, mitochondria, and cytoplasm. The role of the only sirtuin that resides in the cytoplasm, SIRT2, in the development of ischemic injury and cardiac hypertrophy is not known. In this paper, we show that the hearts of mice with deletion of Sirt2 (Sirt2-/-) display improved cardiac function after ischemia-reperfusion (I/R) and pressure overload (PO), suggesting that SIRT2 exerts maladaptive effects in the heart in response to stress. Similar results were obtained in mice with cardiomyocyte-specific Sirt2 deletion. Mechanistic studies suggest that SIRT2 modulates cellular levels and activity of nuclear factor (erythroid-derived 2)-like 2 (NRF2), which results in reduced expression of antioxidant proteins. Deletion of Nrf2 in the hearts of Sirt2-/- mice reversed protection after PO. Finally, treatment of mouse hearts with a specific SIRT2 inhibitor reduced cardiac size and attenuates cardiac hypertrophy in response to PO. These data indicate that SIRT2 has detrimental effects in the heart and plays a role in cardiac response to injury and the progression of cardiac hypertrophy, which makes this protein a unique member of the SIRT family. Additionally, our studies provide a novel approach for treatment of cardiac hypertrophy and injury by targeting SIRT2 pharmacologically, providing a novel avenue for the treatment of these disorders.

Upregulation of glycolytic enzyme PFKFB3 by deubiquitinase OTUD4 promotes cardiac fibrosis post myocardial infarction

Metabolic dysregulations have emerged as a major mediator of cardiovascular disorders and fibrotic diseases. Metabolic reprogramming contributes a lot to cardiac fibroblast activation and cardiac fibrosis post-myocardial infarction (MI), yet the mechanism remains incompletely understood. Our work aimed to determine whether or not glycolytic reprogramming, regulated by phosphofructokinase-2/fructose-2,6-bisphosphatase 3 (PFKFB3), is a therapeutic target for alleviating post-MI cardiac fibrosis. Here, we showed that cardiac fibroblasts displayed cell energy phenotype toward augmented glycolysis in response to transforming growth factor-beta 1 (TGF-β1), evidenced by significant extracellular acidification rate (ECAR) increase and lactate accumulation. The expression of glycolytic enzyme PFKFB3, a master activator of glycolysis, was up-regulated in TGF-β1-treated cardiac fibroblasts and in cardiac fibroblasts of post-MI mice. Pharmacological inhibition of PFKFB3 by 3PO diminished TGF-β1-mediated profibrotic phenotypes, attenuated cardiac fibrosis, and preserved cardiac functions in post-MI mice. Meanwhile, the genetic inhibition of PFKFB3 decreased the cardiac fibroblast activation and reversed the differentiated phenotypes in vitro and in vivo. Mechanistically, we identified deubiquitinase OTUD4 as a new binding protein of PFKFB3, and their interaction blocked PFKFB3 degradation via OTUD4-mediated deubiquitylation. Taken together, this work characterized a key role for PFKFB3 in cardiac fibroblast activation and suggested that inhibiting PFKFB3-involved glycolysis is an alternative way to alleviate post-MI cardiac fibrosis. KEY MESSAGES: PFKFB3, a master activator of glycolysis, was highly expressed in ischemic cardiac fibroblasts to enhance cardiac fibrosis The deubiquitinase OTUD4 was identified as a new binding protein of PFKFB3 TGF-β1 blunted the ubiquitination-mediated degradation of PFKFB3 via OTUD4-mediated deubiquitylation Blockade of PFKFB3 contributed to ameliorating ischemia-induced cardiac fibrosis.

SIRT2 inhibition protects against cardiac hypertrophy and heart failure

Sirtuins (SIRT) exhibit deacetylation or ADP-ribosyltransferase activity and regulate a wide range of cellular processes in the nucleus, mitochondria and cytoplasm. The role of the only sirtuin that resides in the cytoplasm, SIRT2, in the development of heart failure (HF) and cardiac hypertrophy is not known. In this paper, we show that the hearts of mice with deletion of Sirt2 ( Sirt2 -/- ) display improved cardiac function after ischemia-reperfusion (I/R) and pressure overload (PO), suggesting that SIRT2 exerts maladaptive effects in the heart in response to stress. Similar results were obtained in mice with cardiomyocyte-specific Sirt2 deletion. Mechanistic studies suggest that SIRT2 modulates cellular levels and activity of nuclear factor (erythroid-derived 2)-like 2 (NRF2), which results in reduced expression of antioxidant proteins. Deletion of Nrf2 in the hearts of Sirt2 -/- mice reversed protection after PO. Finally, treatment of mouse hearts with a specific SIRT2 inhibitors reduces cardiac size and attenuates cardiac hypertrophy in response to PO. These data indicate that SIRT2 has detrimental effects in the heart and plays a role in the progression of HF and cardiac hypertrophy, which makes this protein a unique member of the SIRT family. Additionally, our studies provide a novel approach for treatment of cardiac hypertrophy by targeting SIRT2 pharmacologically, providing a novel avenue for the treatment of this disorder.

Oncometabolism: A Paradigm for the Metabolic Remodeling of the Failing Heart

Heart failure is associated with profound alterations in cardiac intermediary metabolism. One of the prevailing hypotheses is that metabolic remodeling leads to a mismatch between cardiac energy (ATP) production and demand, thereby impairing cardiac function. However, even after decades of research, the relevance of metabolic remodeling in the pathogenesis of heart failure has remained elusive. Here we propose that cardiac metabolic remodeling should be looked upon from more perspectives than the mere production of ATP needed for cardiac contraction and relaxation. Recently, advances in cancer research have revealed that the metabolic rewiring of cancer cells, often coined as oncometabolism, directly impacts cellular phenotype and function. Accordingly, it is well feasible that the rewiring of cardiac cellular metabolism during the development of heart failure serves similar functions. In this review, we reflect on the influence of principal metabolic pathways on cellular phenotype as originally described in cancer cells and discuss their potential relevance for cardiac pathogenesis. We discuss current knowledge of metabolism-driven phenotypical alterations in the different cell types of the heart and evaluate their impact on cardiac pathogenesis and therapy.

Molecular Signaling to Preserve Mitochondrial Integrity against Ischemic Stress in the Heart: Rescue or Remove Mitochondria in Danger

Cardiovascular diseases are one of the leading causes of death and global health problems worldwide, and ischemic heart disease is the most common cause of heart failure (HF). The heart is a high-energy demanding organ, and myocardial energy reserves are limited. Mitochondria are the powerhouses of the cell, but under stress conditions, they become damaged, release necrotic and apoptotic factors, and contribute to cell death. Loss of cardiomyocytes plays a significant role in ischemic heart disease. In response to stress, protective signaling pathways are activated to limit mitochondrial deterioration and protect the heart. To prevent mitochondrial death pathways, damaged mitochondria are removed by mitochondrial autophagy (mitophagy). Mitochondrial quality control mediated by mitophagy is functionally linked to mitochondrial dynamics. This review provides a current understanding of the signaling mechanisms by which the integrity of mitochondria is preserved in the heart against ischemic stress.

Lactate and Myocadiac Energy Metabolism

The myocardium is capable of utilizing different energy substrates, which is referred to as “metabolic flexibility.” This process assures ATP production from fatty acids, glucose, lactate, amino acids, and ketones, in the face of varying metabolic contexts. In the normal physiological state, the oxidation of fatty acids contributes to approximately 60% of energy required, and the oxidation of other substrates provides the rest. The accumulation of lactate in ischemic and hypoxic tissues has traditionally be considered as a by-product, and of little utility. However, recent evidence suggests that lactate may represent an important fuel for the myocardium during exercise or myocadiac stress. This new paradigm drives increasing interest in understanding its role in cardiac metabolism under both physiological and pathological conditions. In recent years, blood lactate has been regarded as a signal of stress in cardiac disease, linking to prognosis in patients with myocardial ischemia or heart failure. In this review, we discuss the importance of lactate as an energy source and its relevance to the progression and management of heart diseases.

The Role of Metabolism in Heart Failure and Regeneration

Heart failure is the leading cause of death worldwide. The inability of the adult mammalian heart to regenerate following injury results in the development of systolic heart failure. Thus, identifying novel approaches toward regenerating the adult heart has enormous therapeutic potential for adult heart failure. Mitochondrial metabolism is an essential homeostatic process for maintaining growth and survival. The emerging role of mitochondrial metabolism in controlling cell fate and function is beginning to be appreciated. Recent evidence suggests that metabolism controls biological processes including cell proliferation and differentiation, which has profound implications during development and regeneration. The regenerative potential of the mammalian heart is lost by the first week of postnatal development when cardiomyocytes exit the cell cycle and become terminally differentiated. This inability to regenerate following injury is correlated with the metabolic shift from glycolysis to fatty acid oxidation that occurs during heart maturation in the postnatal heart. Thus, understanding the mechanisms that regulate cardiac metabolism is key to unlocking metabolic interventions during development, disease, and regeneration. In this review, we will focus on the emerging role of metabolism in cardiac development and regeneration and discuss the potential of targeting metabolism for treatment of heart failure.

Neutralization of hexokinase 2-targeting miRNA attenuates the oxidative stress-induced cardiomyocyte apoptosis

Hexokinase 2 (HK2) is a metabolic sensor that couples glycolysis and oxidative phosphorylation of mitochondria by binding to the outer mitochondrial membrane (OMM), and it also has been implicated in induction of apoptotic process by regulating the integrity of OMM. When HK2 detaches from the mitochondria, it triggers permeability increase of the OMM and subsequently facilitates the cytosolic release of cytochrome c, a major apoptosis-inducing factor. According to previous studies, a harsh microenvironment created by ischemic heart disease such as low tissue oxygen and nutrients, and increased reactive oxygen species (ROS) can cause cardiomyocyte apoptosis. Under these conditions, the expression of HK2 in heart significantly decrease and such down-regulation of HK2 was correlated to the increased apoptosis of cardiomyocytes. Therefore, prevention of HK2 down-regulation may salvage cardiomyocytes from apoptosis. MicroRNAs are short, non-coding RNAs that either inhibit transcription of target mRNAs or degrade the targeted mRNAs via complementary binding to the 3'UTR (untranslated region) of the targeted mRNAs. Since miRNAs are known to be involved in virtually every biological processes, it is reasonable to assume that the expression of HK2 is also regulated by miRNAs. Currently, to my best knowledge, there is no previous study examined the miRNA-mediated regulation of HK2 in cardiomyocytes. Thus, in the present study, miRNA-mediated modulation of HK2 during ROS (H2O2)-induced cardiomyocyte apoptosis was investigated. First, the expression of HK2 in cardiomyocytes exposed to H2O2 was evaluated. H2O2 (500 μM) induced cardiomyocyte apoptosis and it also decreased the mitochondrial expression of HK2. Based on miRNA-target prediction databases and empirical data, miR-181a was identified as a HK2-targeting miRNA. To further examine the effect of negative regulation of the selected HK2-targeting miRNA on cardiomyocyte apoptosis, anti-miR-181a, which neutralizes endogenous miR-181a, was utilized. Delivery of anti-miR-181a significantly abrogated the H2O2-induced suppression of HK2 expression and subsequent disruption of mitochondrial membrane potential, improving the survival of cardiomyocytes exposed to H2O2. These findings suggest that miR-181a-mediated down-regulation of HK2 contributes to the apoptosis of cardiomyocytes exposed to ROS. Neutralizing miR-181a can be a viable and effective means to prevent cardiomyocyte from apoptosis in ischemic heart disease.

Hexokinase 2 in Cancer: A Prima Donna Playing Multiple Characters

Hexokinases are a family of ubiquitous exose-phosphorylating enzymes that prime glucose for intracellular utilization. Hexokinase 2 (HK2) is the most active isozyme of the family, mainly expressed in insulin-sensitive tissues. HK2 induction in most neoplastic cells contributes to their metabolic rewiring towards aerobic glycolysis, and its genetic ablation inhibits malignant growth in mouse models. HK2 can dock to mitochondria, where it performs additional functions in autophagy regulation and cell death inhibition that are independent of its enzymatic activity. The recent definition of HK2 localization to contact points between mitochondria and endoplasmic reticulum called Mitochondria Associated Membranes (MAMs) has unveiled a novel HK2 role in regulating intracellular Ca²⁺ fluxes. Here, we propose that HK2 localization in MAMs of tumor cells is key in sustaining neoplastic progression, as it acts as an intersection node between metabolic and survival pathways. Disrupting these functions by targeting HK2 subcellular localization can constitute a promising anti-tumor strategy.

Cardiometabolism as an Interlocking Puzzle between the Healthy and Diseased Heart: New Frontiers in Therapeutic Applications

Cardiac metabolism represents a crucial and essential connecting bridge between the healthy and diseased heart. The cardiac muscle, which may be considered an omnivore organ with regard to the energy substrate utilization, under physiological conditions mainly draws energy by fatty acids oxidation. Within cardiomyocytes and their mitochondria, through well-concerted enzymatic reactions, substrates converge on the production of ATP, the basic chemical energy that cardiac muscle converts into mechanical energy, i.e., contraction. When a perturbation of homeostasis occurs, such as an ischemic event, the heart is forced to switch its fatty acid-based metabolism to the carbohydrate utilization as a protective mechanism that allows the maintenance of its key role within the whole organism. Consequently, the flexibility of the cardiac metabolic networks deeply influences the ability of the heart to respond, by adapting to pathophysiological changes. The aim of the present review is to summarize the main metabolic changes detectable in the heart under acute and chronic cardiac pathologies, analyzing possible therapeutic targets to be used. On this basis, cardiometabolism can be described as a crucial mechanism in keeping the physiological structure and function of the heart; furthermore, it can be considered a promising goal for future pharmacological agents able to appropriately modulate the rate-limiting steps of heart metabolic pathways.

A systematic review of post-translational modifications in the mitochondrial permeability transition pore complex associated with cardiac diseases

The mitochondrial permeability transition pore (mPTP) opening is involved in the pathophysiology of multiple cardiac diseases, such as ischemia/reperfusion injury and heart failure. A growing number of evidence provided by proteomic screening techniques has demonstrated the role of post-translational modifications (PTMs) in several key components of the pore in response to changes in the extra/intracellular environment and bioenergetic demand. This could lead to a fine, complex regulatory mechanism that, under pathological conditions, can shift the state of mitochondrial functions and, thus, the cell's fate. Understanding the complex relationship between these PTMs is still under investigation and can provide new, promising therapeutic targets and treatment approaches. This review, using a systematic review of the literature, presents the current knowledge on PTMs of the mPTP and their role in health and cardiac disease.

Forced expression of miR-143 and -145 in cardiomyocytes induces cardiomyopathy with a reductive redox shift

BACKGROUND

Animal model studies show that reductive stress is involved in cardiomyopathy and myopathy, but the exact physiological relevance remains unknown. In addition, the microRNAs miR-143 and miR-145 have been shown to be upregulated in cardiac diseases, but the underlying mechanisms associated with these regulators have yet to be explored.

METHODS

We developed transgenic mouse lines expressing exogenous miR-143 and miR-145 under the control of the alpha-myosin heavy chain (αMHC) promoter/enhancer.

RESULTS

The two transgenic lines showed dilated cardiomyopathy-like characteristics and early lethality with markedly increased expression of miR-143. The expression of hexokinase 2 (HK2), a cardioprotective gene that is a target of miR-143, was strongly suppressed in the transgenic hearts, but the in vitro HK activity and adenosine triphosphate (ATP) content were comparable to those observed in wild-type mice. In addition, transgenic complementation of HK2 expression did not reduce mortality rates. Although HK2 is crucial for the pentose phosphate pathway (PPP) and glycolysis, the ratio of reduced glutathione (GSH) to oxidized glutathione (GSSG) was unexpectedly higher in the hearts of transgenic mice. The expression of gamma-glutamylcysteine synthetase heavy subunit (γ-GCSc) and the in vitro activity of glutathione reductase (GR) were also higher, suggesting that the recycling of GSH and its de novo biosynthesis were augmented in transgenic hearts. Furthermore, the expression levels of glucose-6-phosphate dehydrogenase (G6PD, a rate-limiting enzyme for the PPP) and p62/SQSTM1 (a potent inducer of glycolysis and glutathione production) were elevated, while p62/SQSTM1 was upregulated at the mRNA level rather than as a result of autophagy inhibition. Consistent with this observation, nuclear factor erythroid-2 related factor 2 (Nrf2), Jun N-terminal kinase (JNK) and inositol-requiring enzyme 1 alpha (IRE1α) were activated, all of which are known to induce p62/SQSTM1 expression.

CONCLUSIONS

Overexpression of miR-143 and miR-145 leads to a unique dilated cardiomyopathy phenotype with a reductive redox shift despite marked downregulation of HK2 expression. Reductive stress may be involved in a wider range of cardiomyopathies than previously thought.

Delayed ischaemic contracture onset by empagliflozin associates with NHE1 inhibition and is dependent on insulin in isolated mouse hearts

AIMS

Sodium glucose cotransporter 2 (SGLT2) inhibitors have sodium-hydrogen exchanger (NHE) inhibition properties in isolated cardiomyocytes, but it is unknown whether these properties extend to the intact heart during ischaemia-reperfusion (IR) conditions. NHE inhibitors as Cariporide delay time to onset of contracture (TOC) during ischaemia and reduce IR injury. We hypothesized that, in the ex vivo heart, Empagliflozin (Empa) mimics Cariporide during IR by delaying TOC and reducing IR injury. To facilitate translation to in vivo conditions with insulin present, effects were examined in the absence and presence of insulin.

METHODS AND RESULTS

Isolated C57Bl/6NCrl mouse hearts were subjected to 25 min I and 120 min R without and with 50 mU/L insulin. Without insulin, Empa and Cari delayed TOC by 100 and 129 s, respectively, yet only Cariporide reduced IR injury [infarct size (mean ± SEM in %) from 51 ± 6 to 34 ± 5]. Empa did not delay TOC in the presence of the NHE1 inhibitor Eniporide. Insulin perfusion increased tissue glycogen content at baseline (from 2 ± 2 µmol to 42 ± 1 µmol glycosyl units/g heart dry weight), amplified G6P and lactate accumulation at end-ischaemia, thereby decreased mtHKII and exacerbated IR injury. Under these conditions, Empa (1 µM) and Cariporide (10 µM) were without effect on TOC and IR injury. Empa and Cariporide both inhibited NHE activity, in isolated cardiomyocytes, independent of insulin.

CONCLUSIONS

In the absence of insulin, Empa and Cariporide strongly delayed the time to onset of contracture during ischaemia. In the presence of insulin, both Empa and Cari were without effect on IR, possibly because of severe ischaemic acidification. Insulin exacerbates IR injury through increased glycogen depletion during ischaemia and consequently mtHKII dissociation. The data suggest that also in the ex vivo intact heart Empa exerts direct cardiac effects by inhibiting NHE during ischaemia, but not during reperfusion.

Cardiac metabolism as a driver and therapeutic target of myocardial infarction

Reducing infarct size during a cardiac ischaemic‐reperfusion episode is still of paramount importance, because the extension of myocardial necrosis is an important risk factor for developing heart failure. Cardiac ischaemia‐reperfusion injury (IRI) is in principle a metabolic pathology as it is caused by abruptly halted metabolism during the ischaemic episode and exacerbated by sudden restart of specific metabolic pathways at reperfusion. It should therefore not come as a surprise that therapy directed at metabolic pathways can modulate IRI. Here, we summarize the current knowledge of important metabolic pathways as therapeutic targets to combat cardiac IRI. Activating metabolic pathways such as glycolysis (eg AMPK activators), glucose oxidation (activating pyruvate dehydrogenase complex), ketone oxidation (increasing ketone plasma levels), hexosamine biosynthesis pathway (O‐GlcNAcylation; administration of glucosamine/glutamine) and deacetylation (activating sirtuins 1 or 3; administration of NAD⁺‐boosting compounds) all seem to hold promise to reduce acute IRI. In contrast, some metabolic pathways may offer protection through diminished activity. These pathways comprise the malate‐aspartate shuttle (in need of novel specific reversible inhibitors), mitochondrial oxygen consumption, fatty acid oxidation (CD36 inhibitors, malonyl‐CoA decarboxylase inhibitors) and mitochondrial succinate metabolism (malonate). Additionally, protecting the cristae structure of the mitochondria during IR, by maintaining the association of hexokinase II or creatine kinase with mitochondria, or inhibiting destabilization of F_OF₁‐ATPase dimers, prevents mitochondrial damage and thereby reduces cardiac IRI. Currently, the most promising and druggable metabolic therapy against cardiac IRI seems to be the singular or combined targeting of glycolysis, O‐GlcNAcylation and metabolism of ketones, fatty acids and succinate.

Hexokinase II Upregulation Contributes to Asiaticoside-Induced Protection of H9c2 Cardioblasts During Oxygen-Glucose Deprivation/Reoxygenation

Asiaticoside (AS), one of the main functional components of Centella asiatica, has been reported to protect neurons from ischemia-hypoxia-induced injury. However, the role of AS in myocardial oxygen-glucose deprivation/reoxygenation (OGD/R) injury has not been investigated. The aim of this study was to investigate the role of AS in OGD/R-treated H9c2 cardiomyocytes and the underlying mechanism involved. Cell viability was detected using MTT assay. Cell apoptosis was measured using flow cytometry. The oxidative stress was assessed by detecting the malonaldehyde (MDA) content and activities of superoxide dismutase, glutathione peroxidase, and catalase (CAT). The glucose consumption and lactate production were determined to reflect glycolysis rate. The expression levels of hexokinase II (HK2) were detected using reverse transcription quantitative polymerase chain reaction and Western blot. H9c2 cells were transfected with small interfering RNA targeting HK2 (si-HK2) to knockdown HK2. Results showed that AS improved cell viability and inhibited apoptosis in OGD/R-injured H9c2 cells. AS pretreatment prevented OGD/R-induced oxidative stress, as evidenced by the decreased MDA content, and increased activities of superoxide dismutase, glutathione peroxidase, and CAT. The decreased glucose consumption and lactate production in OGD/R-injured H9c2 cells were reversed after AS treatment. Mechanically, AS induced the expression of HK2 in OGD/R-injured H9c2 cells. Knockdown of HK2 abolished the protective effects of AS on OGD/R-injured H9c2 cells. In conclusion, the protective effects of AS on cardiomyocytes from OGD/R-induced injury were mediated at least partly by upregulating HK2.

Insulin resistance and diabetes in hyperthyroidism: a possible role for oxygen and nitrogen reactive species

In addition to insulin, glycemic control involves thyroid hormones. However, an excess of thyroid hormone can disturb the blood glucose equilibrium, leading to alterations of carbohydrate metabolism and, eventually, diabetes. Indeed, experimental and clinical hyperthyroidism is often accompanied by abnormal glucose tolerance. A common characteristic of hyperthyroidism and type 2 diabetes is the altered mitochondrial efficiency caused by the enhanced production of reactive oxygen and nitrogen species. It is known that an excess of thyroid hormone leads to increased oxidant production and mitochondrial oxidative damage. It can be hypothesised that these species represent the link between hyperthyroidism and development of insulin resistance and diabetes, even though direct evidence of this relationship is lacking. In this review, we examine the literature concerning the effects of insulin and thyroid hormones on glucose metabolism and discuss alterations of glucose metabolism in hyperthyroid conditions and the cellular and molecular mechanisms that may underline them.

Intracellular Energy-Transfer Networks and High-Resolution Respirometry: A Convenient Approach for Studying Their Function

Compartmentalization of high-energy phosphate carriers between intracellular micro-compartments is a phenomenon that ensures efficient energy use. To connect these sites, creatine kinase (CK) and adenylate kinase (AK) energy-transfer networks, which are functionally coupled to oxidative phosphorylation (OXPHOS), could serve as important regulators of cellular energy fluxes. Here, we introduce how selective permeabilization of cellular outer membrane and high-resolution respirometry can be used to study functional coupling between CK or AK pathways and OXPHOS in different cells and tissues. Using the protocols presented here the ability of creatine or adenosine monophosphate to stimulate OXPHOS through CK and AK reactions, respectively, is easily observable and quantifiable. Additionally, functional coupling between hexokinase and mitochondria can be investigated by monitoring the effect of glucose on respiration. Taken together, high-resolution respirometry in combination with permeabilization is a convenient approach for investigating energy-transfer networks in small quantities of cells and tissues in health and in pathology.

Doxorubicin triggers bioenergetic failure and p53 activation in mouse stem cell-derived cardiomyocytes

Doxorubicin (DOX) is a widely used anticancer drug that could be even more effective if its clinical dosage was not limited because of delayed cardiotoxicity. Beating stem cell-derived cardiomyocytes are a preferred in vitro model to further uncover the mechanisms of DOX-induced cardiotoxicity. Our objective was to use cultured induced-pluripotent stem cell(iPSC)-derived mouse cardiomyocytes (Cor.At) to investigate the effects of DOX on cell and mitochondrial metabolism, as well as on stress responses. Non-proliferating and beating Cor.At cells were treated with 0.5 or 1 μM DOX for 24 h, and morphological, functional and biochemical changes associated with mitochondrial bioenergetics, DNA-damage response and apoptosis were measured. Both DOX concentrations decreased ATP levels and SOD2 protein levels and induced p53-dependent caspase activation. However, differential effects were observed for the two DOX concentrations. The highest concentration induced a high degree of apoptosis, with increased nuclear apoptotic morphology, PARP-1 cleavage and decrease of some OXPHOS protein subunits. At the lowest concentration, DOX increased the expression of p53 target transcripts associated with mitochondria-dependent apoptosis and decreased transcripts related with DNA-damage response and glycolysis. Interestingly, cells treated with 0.5 μM DOX presented an increase in PDK4 transcript levels, accompanied by an increase in phospho-PDH and decreased PDH activity. This was accompanied by an apparent decrease in basal and maximal oxygen consumption rates (OCR) and in basal extracellular acidification rate (ECAR). Cells pre-treated with the PDK inhibitor dichloroacetate (DCA), with the aim of restoring PDH activity, partially recovered OCR and ECAR. The results suggest that the higher DOX concentration mainly induces p53-dependent apoptosis, whereas for the lower DOX concentration the cardiotoxic effects involve bioenergetic failure, unveiling PDH as a possible therapeutic target to decrease DOX cardiotoxicity.

Effects of dietary energy sources on early postmortem muscle metabolism of finishing pigs

OBJECTIVE

This study investigated the effects of different dietary energy sources on early postmortem muscle metabolism of finishing pigs.

METHODS

Seventy-two barrow (Duroc×Landrace×Yorkshire, DLY) pigs (65.0±2.0 kg) were allotted to three iso-energetic and iso-nitrogenous diets: A (44.1% starch, 5.9% crude fat, and 12.6% neutral detergent fibre [NDF]), B (37.6% starch, 9.5% crude fat, and 15.4% NDF) or C (30.9% starch, 14.3% crude fat, and 17.8% NDF). After the duration of 28-day feeding experiment, 24 pigs (eight per treatment) were slaughtered and the M. longissimus lumborum (LL) samples at 45 min postmortem were collected.

RESULTS

Compared with diet A, diet C resulted in greater adenosine triphosphate and decreased phosphocreatine (PCr) concentrations, greater activity of creatine kinase and reduced percentage bound activities of hexokinase (HK), and pyruvate kinase (PK) in LL muscles (p<0.05). Moreover, diet C decreased the phosphor-AKT level and increased the hydroxy-hypoxia-inducible factor-1α (HIF-1α) level, as well as decreased the bound protein expressions of HK II, PKM2, and lactate dehydrogenase A (p<0.05).

CONCLUSION

Diet C with the lowest level of starch and the highest levels of fat and NDF could enhance the PCr utilization and attenuate glycolysis early postmortem in LL muscle of finishing pigs.

Effects of dietary starch types on early postmortem muscle energy metabolism in finishing pigs

This study aimed to investigate the effects of different dietary starch types on early postmortem muscle energy metabolism in finishing pigs. Ninety barrows (68.0±2.0kg) were randomly allotted to three experimental diets with five replicates of six pigs, containing pure waxy maize starch (WMS), nonwaxy maize starch (NMS), and pea starch (PS) (amylose/amylopectin were 0.07, 0.19 and 0.28 respectively). Compared with the WMS diet, pigs fed the PS diet exhibited greater creatine kinase activity, higher adenosine triphosphate and adenosine diphosphate contents, lower phosphocreatine (PCr), adenosine monophosphate and glycogen contents, and lower glycolytic potential (P<0.05). Moreover, the PS diet led to reduced percentage of bound hexokinase activity, decreased level of phosphorylated AKT (P<0.05) and increased level of hypoxia-inducible factor-1α (P<0.05). In conclusion, diet with high amylose content might promote PCr degradation and inhibit the rate of glycolysis, followed by attenuation of early postmortem glycolysis in finishing pigs.

Cyclophilin D ablation is associated with increased end-ischemic mitochondrial hexokinase activity

Both the absence of cyclophilin D (CypD) and the presence of mitochondrial bound hexokinase II (mtHKII) protect the heart against ischemia/reperfusion (I/R) injury. It is unknown whether CypD determines the amount of mtHKII in the heart. We examined whether CypD affects mtHK in normoxic, ischemic and preconditioned isolated mouse hearts. Wild type (WT) and CypD-/- mouse hearts were perfused with glucose only and subjected to 25 min ischemia and reperfusion. At baseline, cytosolic and mtHK was similar between hearts. CypD ablation protected against I/R injury and increased ischemic preconditioning (IPC) effects, without affecting end-ischemic mtHK. When hearts were perfused with glucose, glutamine, pyruvate and lactate, the preparation was more stable and CypD ablation-resulted in more protection that was associated with increased mtHK activity, leaving little room for additional protection by IPC. In conclusion, in glucose only-perfused hearts, deletion of CypD is not associated with end-ischemic mitochondrial-HK binding. In contrast, in the physiologically more relevant multiple-substrate perfusion model, deletion of CypD is associated with an increased mtHK activity, possibly explaining the increased protection against I/R injury.

Improvement of obesity-linked skeletal muscle insulin resistance by strength and endurance training

Obesity-linked insulin resistance is mainly due to fatty acid overload in non-adipose tissues, particularly skeletal muscle and liver, where it results in high production of reactive oxygen species and mitochondrial dysfunction. Accumulating evidence indicates that resistance and endurance training alone and in combination can counteract the harmful effects of obesity increasing insulin sensitivity, thus preventing diabetes. This review focuses the mechanisms underlying the exercise role in opposing skeletal muscle insulin resistance-linked metabolic dysfunction. It is apparent that exercise acts through two mechanisms: (1) it stimulates glucose transport by activating an insulin-independent pathway and (2) it protects against mitochondrial dysfunction-induced insulin resistance by increasing muscle antioxidant defenses and mitochondrial biogenesis. However, antioxidant supplementation combined with endurance training increases glucose transport in insulin-resistant skeletal muscle in an additive fashion only when antioxidants that are able to increase the expression of antioxidant enzymes and/or the activity of components of the insulin signaling pathway are used.

The role of succinate and ROS in reperfusion injury - A critical appraisal

We critically assess the proposal that succinate-fuelled reverse electron flow (REF) drives mitochondrial matrix superoxide production from Complex I early in reperfusion, thus acting as a key mediator of ischemia/reperfusion (IR) injury. Real-time surface fluorescence measurements of NAD(P)H and flavoprotein redox state suggest that conditions are unfavourable for REF during early reperfusion. Furthermore, rapid loss of succinate accumulated during ischemia can be explained by its efflux rather than oxidation. Moreover, succinate accumulation during ischemia is not attenuated by ischemic preconditioning (IP) despite powerful cardioprotection. In addition, measurement of intracellular reactive oxygen species (ROS) during reperfusion using surface fluorescence and mitochondrial aconitase activity detected major increases in ROS only after mitochondrial permeability transition pore (mPTP) opening was first detected. We conclude that mPTP opening is probably triggered initially by factors other than ROS, including increased mitochondrial [Ca²⁺]. However, IP only attenuates [Ca²⁺] increases later in reperfusion, again after initial mPTP opening, implying that IP regulates mPTP opening through additional mechanisms. One such is mitochondria-bound hexokinase 2 (HK2) which dissociates from mitochondria during ischemia in control hearts but not those subject to IP. Indeed, there is a strong correlation between the extent of HK2 loss from mitochondria during ischemia and infarct size on subsequent reperfusion. Mechanisms linking HK2 dissociation to mPTP sensitisation remain to be fully established but several related processes have been implicated including VDAC1 oligomerisation, the stability of contact sites between the inner and outer membranes, cristae morphology, Bcl-2 family members and mitochondrial fission proteins such as Drp1.

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Mitochondrial ROS production during early reperfusion (RPF) occurs after mPTP opening.

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Conditions do not favour ROS production by reverse electron flow during early RPF.

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Ischemic preconditioning (IP) does not attenuate succinate accumulation in ischemia.

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IP reduction of ROS and Ca²⁺ during RPF is secondary to attenuation of mPTP opening.

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IP attenuates mPTP opening by mechanisms independent of ROS and Ca²⁺ such as HK2.

Acute detachment of hexokinase II from mitochondria modestly increases oxygen consumption of the intact mouse heart

OBJECTIVE

Cardiac hexokinase II (HKII) can translocate between cytosol and mitochondria and change its cellular expression with pathologies such as ischemia-reperfusion, diabetes and heart failure. The cardiac metabolic consequences of these changes are unknown. Here we measured energy substrate utilization in cytosol and mitochondria using stabile isotopes and oxygen consumption of the intact perfused heart for 1) an acute decrease in mitochondrial HKII (mtHKII), and 2) a chronic decrease in total cellular HKII.

METHODS/RESULTS

We first examined effects of 200nM TAT (Trans-Activator of Transcription)-HKII peptide treatment, which was previously shown to acutely decrease mtHKII by ~30%. In Langendorff-perfused hearts TAT-HKII resulted in a modest, but significant, increased oxygen consumption, while cardiac performance was unchanged. At the metabolic level, there was a nonsignificant (p=0.076) ~40% decrease in glucose contribution to pyruvate and lactate formation through glycolysis and to mitochondrial citrate synthase flux (6.6±1.1 vs. 11.2±2.2%), and an 35% increase in tissue pyruvate (27±2 vs. 20±2pmol/mg; p=0.033). Secondly, we compared WT and HKII^+/- hearts (50% chronic decrease in total HKII). RNA sequencing revealed no differential gene expression between WT and HKII^+/- hearts indicating an absence of metabolic reprogramming at the transcriptional level. Langendorff-perfused hearts showed no significant differences in glycolysis (0.34±0.03μmol/min), glucose contribution to citrate synthase flux (35±2.3%), palmitate contribution to citrate synthase flux (20±1.1%), oxygen consumption or mechanical performance between WT and HKII^+/- hearts.

CONCLUSIONS

These results indicate that acute albeit not chronic changes in mitochondrial HKII modestly affect cardiac oxygen consumption and energy substrate metabolism.

Intrinsic remote conditioning of the myocardium as a comprehensive cardiac response to ischemia and reperfusion

We have previously shown that distal anterior wall ischemia/reperfusion induces gene expression changes in the proximal anterior myocardial area, involving genes responsible for cardiac remodeling. Here we investigated the molecular signals of the ischemia non-affected remote lateral and posterior regions and present gene expression profiles of the entire left ventricle by using our novel and straightforward method of 2D and 3D image reconstruction. Five or 24h after repetitive 10min ischemia/reperfusion without subsequent infarction, pig hearts were explanted and myocardial samples from 52 equally distributed locations of the left ventricle were collected. Expressional changes of seven genes of interest (HIF-1α; caspase-3, transcription factor GATA4; myocyte enhancer factor 2C /MEF2c/; hexokinase 2 /HK2/; clusterin /CLU/ and excision repair cross-complementation group 4 /ERCC4/) were measured by qPCR. 2D and 3D gene expression maps were constructed by projecting the fold changes on the NOGA anatomical mapping coordinates. Caspase-3, GATA4, HK2, CLU, and ERCC4 were up-regulated region-specifically in the ischemic zone at 5 h post ischemia/reperfusion injury. Overexpression of GATA4, clusterin and ERCC4 persisted after 24 h. HK2 showed strong up-regulation in the ischemic zone and down-regulation in remote areas at 5 h, and was severely reduced in all heart regions at 24 h. These results indicate a quick onset of regulation of apoptosis-related genes, which is partially reversed in the late phase of ischemia/reperfusion cardioprotection, and highlight variations between ischemic and unaffected myocardium over time. The NOGA 2D and 3D construction system is an attractive method to visualize expressional variations in the myocardium.

(-)-Epigallocatechin-3-gallate attenuates myocardial injury induced by ischemia/reperfusion in diabetic rats and in H9c2 cells under hyperglycemic conditions

(−)-Epigallocatechin gallate (EGCG) exerts multiple beneficial effects on cardiovascular performance. In this study, we aimed to examine the effects of EGCG on diabetic cardiomyopathy during myocardial ischemia/reperfusion (I/R) injury. EGCG (100 mg/kg/day) was administered at week 6 for 2 weeks to diabetic rats following the induction of type 1 diabetes by streptozotocin (STZ). At the end of week 8, the animals were subjected to myocardial I/R injury. The EGCG-elicited structural and functional effects were analyzed. Additionally, EGCG (20 μM) was administered for 24 h to cultured cardiac H9c2 cells under hyperglycemic conditions (30 mM glucose) prior to hypoxia/reoxygenation (H/R) challenge, and its effects on oxidative stress were compared to H9c2 cells transfecteed with silent information regulator 1 (SIRT1) small interfering RNA (siRNA). In rats with STZ-induced diabetes, EGCG treatment ameliorated post-ischemic cardiac dysfunction, decreased the myocardial infarct size, apoptosis and cardiac fibrosis, and reduced the elevated lactate dehydrogenase (LDH) and malonaldehyde (MDA) levels, and attenuated oxidative stress. Furthermore, EGCG significantly reduced H/R injury in cardiac H9c2 cells exposed to high glucose as evidenced by reduced apoptotic cell death and oxidative stress. The protein expression levels of SIRT1 and manganese superoxide dismutase (MnSOD) were reduced in the diabetic rats and the H9c2 cells under hyperglycemic conditions, compared with the control rats following I/R injury and H9c2 cells under normal glucose conditions. EGCG pre-treatment significantly upregulated the levels of htese proteins in vitro and in vivo. However, treatment with EX527 and SIRT1 siRNA blocked the EGCG-mediated cardioprotective effects. Taken together, our data indicate that SIRT1 plays a critical role in the EGCG-mediated amelioration of I/R injury in diabetic rats, which suggests that EGCG may be a promising dietary supplement for the prevention of diabetic cardiomyopathy.

FGF-dependent metabolic control of vascular development

Blood and lymphatic vasculatures are intimately involved in tissue oxygenation and fluid homeostasis maintenance. Assembly of these vascular networks involves sprouting, migration and proliferation of endothelial cells. Recent studies have suggested that changes in cellular metabolism are important to these processes. Although much is known about vascular endothelial growth factor (VEGF)-dependent regulation of vascular development and metabolism, little is understood about the role of fibroblast growth factors (FGFs) in this context. Here we identify FGF receptor (FGFR) signalling as a critical regulator of vascular development. This is achieved by FGF-dependent control of c-MYC (MYC) expression that, in turn, regulates expression of the glycolytic enzyme hexokinase 2 (HK2). A decrease in HK2 levels in the absence of FGF signalling inputs results in decreased glycolysis, leading to impaired endothelial cell proliferation and migration. Pan-endothelial- and lymphatic-specific Hk2 knockouts phenocopy blood and/or lymphatic vascular defects seen in Fgfr1/Fgfr3 double mutant mice, while HK2 overexpression partly rescues the defects caused by suppression of FGF signalling. Thus, FGF-dependent regulation of endothelial glycolysis is a pivotal process in developmental and adult vascular growth and development.

Skeletal muscle insulin resistance: role of mitochondria and other ROS sources

At present, obesity is one of the most important public health problems in the world because it causes several diseases and reduces life expectancy. Although it is well known that insulin resistance plays a pivotal role in the development of type 2 diabetes mellitus (the more frequent disease in obese people) the link between obesity and insulin resistance is yet a matter of debate. One of the most deleterious effects of obesity is the deposition of lipids in non-adipose tissues when the capacity of adipose tissue is overwhelmed. During the last decade, reduced mitochondrial function has been considered as an important contributor to 'toxic' lipid metabolite accumulation and consequent insulin resistance. More recent reports suggest that mitochondrial dysfunction is not an early event in the development of insulin resistance, but rather a complication of the hyperlipidemia-induced reactive oxygen species (ROS) production in skeletal muscle, which might promote mitochondrial alterations, lipid accumulation and inhibition of insulin action. Here, we review the literature dealing with the mitochondria-centered mechanisms proposed to explain the onset of obesity-linked IR in skeletal muscle. We conclude that the different pathways leading to insulin resistance may act synergistically because ROS production by mitochondria and other sources can result in mitochondrial dysfunction, which in turn can further increase ROS production leading to the establishment of a harmful positive feedback loop.

Reducing mitochondrial bound hexokinase II mediates transition from non-injurious into injurious ischemia/reperfusion of the intact heart

Ischemia/reperfusion (I/R) of the heart becomes injurious when duration of the ischemic insult exceeds a certain threshold (approximately ≥20 min). Mitochondrial bound hexokinase II (mtHKII) protects against I/R injury, with the amount of mtHKII correlating with injury. Here, we examine whether mtHKII can induce the transition from non-injurious to injurious I/R, by detaching HKII from mitochondria during a non-injurious I/R interval. Additionally, we examine possible underlying mechanisms (increased reactive oxygen species (ROS), increased oxygen consumption (MVO₂) and decreased cardiac energetics) associated with this transition. Langendorff perfused rat hearts were treated for 20 min with saline, TAT-only or 200 nM TAT-HKII, a peptide that translocates HKII from mitochondria. Then, hearts were exposed to non-injurious 15-min ischemia, followed by 30-min reperfusion. I/R injury was determined by necrosis (LDH release) and cardiac mechanical recovery. ROS were measured by DHE fluorescence. Changes in cardiac respiratory activity (cardiac MVO₂ and efficiency and mitochondrial oxygen tension (mitoPO₂) using protoporphyrin IX) and cardiac energetics (ATP, PCr, ∆G_ATP) were determined following peptide treatment. When exposed to 15-min ischemia, control hearts had no necrosis and 85% recovery of function. Conversely, TAT-HKII treatment resulted in significant LDH release and reduced cardiac recovery (25%), indicating injurious I/R. This was associated with increased ROS during ischemia and reperfusion. TAT-HKII treatment reduced MVO₂ and improved energetics (increased PCr) before ischemia, without affecting MVO₂/RPP ratio or mitoPO₂. In conclusion, a reduction in mtHKII turns non-injurious I/R into injurious I/R. Loss of mtHKII was associated with increased ROS during ischemia and reperfusion, but not with increased MVO₂ or decreased cardiac energetics before damage occurs.

Ginsenoside Rg5 increases cardiomyocyte resistance to ischemic injury through regulation of mitochondrial hexokinase-II and dynamin-related protein 1

Hexokinase-II (HK-II) and dynamin-related protein 1 (Drp1) regulate mitochondrial function differently. This study was designed to investigate the cardioprotective effect of ginsenoside Rg5 (Rg5) with emphasis on the regulation of mitochondrial HK-II and Drp1. Saturated acid palmitate (PA) stimulation increased lactate accumulation and induced cellular acidification by impairing the activity of pyruvate dehydrogenase (PDH) in cardiomyocytes, leading to HK-II dissociation from mitochondria. Rg5 improved PDH activity and prevented cellular acidification by combating fatty-acid oxidation, contributing to protecting mitochondrial HK-II. HK-II binding to mitochondria prevented mitochondrial Drp1 recruitment, whereas Drp1 activation decreased the content of mitochondrial HK-II, demonstrating the reciprocal control for binding to mitochondria. Rg5 promoted Akt translocation to mitochondria and increased HK-II binding to mitochondria while coordinately suppressing Drp1 recruitment and mitochondrial fission. Akt inhibitor triciribine or knockdown of Akt with small interfering RNA diminished the effects of Rg5, indicating that Rg5 inhibited Drp1 activation and promoted HK-II mitochondrial binding through Akt activation. Rg5 prevented the opening of mitochondrial permeability transition pore and increased ATP production, resultantly increasing cardiomyocyte resistance to hypoxia/reoxygenation injury. Meanwhile, Rg5 prevented cell apoptosis with increased HK-II binding and reduced Drp1 recruitment to mitochondria in isoproterenol-induced ischemic heart of mice. Taken together, these findings not only established a previously unrecognized role of ginsenosides in cardioprotection but also suggest that mitochondrial HK-II binding and Drp1 recruitment could be targeted therapeutically to prevent ischemic injury in the heart.

Snf1-related kinase improves cardiac mitochondrial efficiency and decreases mitochondrial uncoupling

Ischaemic heart disease limits oxygen and metabolic substrate availability to the heart, resulting in tissue death. Here, we demonstrate that the AMP-activated protein kinase (AMPK)-related protein Snf1-related kinase (SNRK) decreases cardiac metabolic substrate usage and mitochondrial uncoupling, and protects against ischaemia/reperfusion. Hearts from transgenic mice overexpressing SNRK have decreased glucose and palmitate metabolism and oxygen consumption, but maintained power and function. They also exhibit decreased uncoupling protein 3 (UCP3) and mitochondrial uncoupling. Conversely, Snrk knockout mouse hearts have increased glucose and palmitate oxidation and UCP3. SNRK knockdown in cardiac cells decreases mitochondrial efficiency, which is abolished with UCP3 knockdown. We show that Tribbles homologue 3 (Trib3) binds to SNRK, and downregulates UCP3 through PPARα. Finally, SNRK is increased in cardiomyopathy patients, and SNRK reduces infarct size after ischaemia/reperfusion. SNRK also decreases cardiac cell death in a UCP3-dependent manner. Our results suggest that SNRK improves cardiac mitochondrial efficiency and ischaemic protection.

The Snf1-related kinase (SNRK) is widely expressed and yet its function is poorly understood. Here the authors show that SNRK regulates mitochondrial coupling via the Trib3-PPARα-UCP3 pathway and that cardiac overexpression of SNRK decreases metabolic substrate usage and oxygen consumption but maintains cardiac function and energy in mice.

Reduction in mitochondrial iron alleviates cardiac damage during injury

Excess cellular iron increases reactive oxygen species (ROS) production and causes cellular damage. Mitochondria are the major site of iron metabolism and ROS production; however, few studies have investigated the role of mitochondrial iron in the development of cardiac disorders, such as ischemic heart disease or cardiomyopathy (CM). We observe increased mitochondrial iron in mice after ischemia/reperfusion (I/R) and in human hearts with ischemic CM, and hypothesize that decreasing mitochondrial iron protects against I/R damage and the development of CM. Reducing mitochondrial iron genetically through cardiac‐specific overexpression of a mitochondrial iron export protein or pharmacologically using a mitochondria‐permeable iron chelator protects mice against I/R injury. Furthermore, decreasing mitochondrial iron protects the murine hearts in a model of spontaneous CM with mitochondrial iron accumulation. Reduced mitochondrial ROS that is independent of alterations in the electron transport chain's ROS producing capacity contributes to the protective effects. Overall, our findings suggest that mitochondrial iron contributes to cardiac ischemic damage, and may be a novel therapeutic target against ischemic heart disease.

Cardiac Metabolism in Perspective

The heart is a biological pump that converts chemical to mechanical energy. This process of energy conversion is highly regulated to the extent that energy substrate metabolism matches energy use for contraction on a beat-to-beat basis. The biochemistry of cardiac metabolism includes the biochemistry of energy transfer, metabolic regulation, and transcriptional, translational as well as posttranslational control of enzymatic activities. Pathways of energy substrate metabolism in the heart are complex and dynamic, but all of them conform to the First Law of Thermodynamics. The perspectives expand on the overall idea that cardiac metabolism is inextricably linked to both physiology and molecular biology of the heart. The article ends with an outlook on emerging concepts of cardiac metabolism based on new molecular models and new analytical tools. © 2016 American Physiological Society. Compr Physiol 6:1675-1699, 2016.

An impaired metabolism of nucleotides underpins a novel mechanism of cardiac remodeling leading to Huntington's disease related cardiomyopathy

Huntington's disease (HD) is mainly thought of as a neurological disease, but multiple epidemiological studies have demonstrated a number of cardiovascular events leading to heart failure in HD patients. Our recent studies showed an increased risk of heart contractile dysfunction and dilated cardiomyopathy in HD pre-clinical models. This could potentially involve metabolic remodeling, that is a typical feature of the failing heart, with reduced activities of high energy phosphate generating pathways. In this study, we sought to identify metabolic abnormalities leading to HD-related cardiomyopathy in pre-clinical and clinical settings. We found that HD mouse models developed a profound deterioration in cardiac energy equilibrium, despite AMP-activated protein kinase hyperphosphorylation. This was accompanied by a reduced glucose usage and a significant deregulation of genes involved in de novo purine biosynthesis, in conversion of adenine nucleotides, and in adenosine metabolism. Consequently, we observed increased levels of nucleotide catabolites such as inosine, hypoxanthine, xanthine and uric acid, in murine and human HD serum. These effects may be caused locally by mutant HTT, via gain or loss of function effects, or distally by a lack of trophic signals from central nerve stimulation. Either may lead to energy equilibrium imbalances in cardiac cells, with activation of nucleotide catabolism plus an inhibition of re-synthesis. Our study suggests that future therapies should target cardiac mitochondrial dysfunction to ameliorate energetic dysfunction. Importantly, we describe the first set of biomarkers related to heart and skeletal muscle dysfunction in both pre-clinical and clinical HD settings.

Cardiomyocyte-Specific Ablation of Med1 Subunit of the Mediator Complex Causes Lethal Dilated Cardiomyopathy in Mice

Mediator, an evolutionarily conserved multi-protein complex consisting of about 30 subunits, is a key component of the polymerase II mediated gene transcription. Germline deletion of the Mediator subunit 1 (Med1) of the Mediator in mice results in mid-gestational embryonic lethality with developmental impairment of multiple organs including heart. Here we show that cardiomyocyte-specific deletion of Med1 in mice (csMed1-/-) during late gestational and early postnatal development by intercrossing Med1fl/fl mice to α-MyHC-Cre transgenic mice results in lethality within 10 days after weaning due to dilated cardiomyopathy-related ventricular dilation and heart failure. The csMed1-/- mouse heart manifests mitochondrial damage, increased apoptosis and interstitial fibrosis. Global gene expression analysis revealed that loss of Med1 in heart down-regulates more than 200 genes including Acadm, Cacna1s, Atp2a2, Ryr2, Pde1c, Pln, PGC1α, and PGC1β that are critical for calcium signaling, cardiac muscle contraction, arrhythmogenic right ventricular cardiomyopathy, dilated cardiomyopathy and peroxisome proliferator-activated receptor regulated energy metabolism. Many genes essential for oxidative phosphorylation and proper mitochondrial function such as genes coding for the succinate dehydrogenase subunits of the mitochondrial complex II are also down-regulated in csMed1-/- heart contributing to myocardial injury. Data also showed up-regulation of about 180 genes including Tgfb2, Ace, Atf3, Ctgf, Angpt14, Col9a2, Wisp2, Nppa, Nppb, and Actn1 that are linked to cardiac muscle contraction, cardiac hypertrophy, cardiac fibrosis and myocardial injury. Furthermore, we demonstrate that cardiac specific deletion of Med1 in adult mice using tamoxifen-inducible Cre approach (TmcsMed1-/-), results in rapid development of cardiomyopathy and death within 4 weeks. We found that the key findings of the csMed1-/- studies described above are highly reproducible in TmcsMed1-/- mouse heart. Collectively, these observations suggest that Med1 plays a critical role in the maintenance of heart function impacting on multiple metabolic, compensatory and reparative pathways with a likely therapeutic potential in the management of heart failure.

Microarray and Co-expression Network Analysis of Genes Associated with Acute Doxorubicin Cardiomyopathy in Mice

Clinical use of doxorubicin (DOX) in cancer therapy is limited by its dose-dependent cardiotoxicity. But molecular mechanisms underlying this phenomenon have not been well defined. This study was to investigate the effect of DOX on the changes of global genomics in hearts. Acute cardiotoxicity was induced by giving C57BL/6J mice a single intraperitoneal injection of DOX (15 mg/kg). Cardiac function and apoptosis were monitored using echocardiography and TUNEL assay at days 1, 3 and 5. Myocardial glucose and ATP levels were measured. Microarray assays were used to screen gene expression profiles in the hearts at day 5, and the results were confirmed with qPCR analysis. DOX administration caused decreased cardiac function, increased cardiomyocyte apoptosis and decreased glucose and ATP levels. Microarrays showed 747 up-regulated genes and 438 down-regulated genes involved in seven main functional categories. Among them, metabolic pathway was the most affected by DOX. Several key genes, including 2,3-bisphosphoglycerate mutase (Bpgm), hexokinase 2, pyruvate dehydrogenase kinase, isoenzyme 4 and fructose-2,6-bisphosphate 2-phosphatase, are closely related to glucose metabolism. Gene co-expression networks suggested the core role of Bpgm in DOX cardiomyopathy. These results obtained in mice were further confirmed in cultured cardiomyocytes. In conclusion, genes involved in glucose metabolism, especially Bpgm, may play a central role in the pathogenesis of DOX-induced cardiotoxicity.

Mitochondrial autophagy in cardiomyopathy

Cardiac mitochondria produce vast amounts of ATP through oxidative phosphorylation to maintain contractile function. They are also the primary source of reactive oxygen species, which contribute to mitochondrial dysfunction, cardiomyocyte death, and heart failure. To protect against mitochondrial damage, cardiomyocytes develop well-coordinated quality control mechanisms that maintain the overall mitochondrial health through mitochondrial biogenesis, mitochondrial dynamics, and mitochondrial autophagy (mitophagy). Mitophagy removes dysfunctional mitochondria in the heart not only under normal physiological conditions, but also in response to pathological stresses. Accumulating evidence suggests that mitophagy dysregulation can induce cardiomyocyte death and cardiomyopathy. In this review, we discuss what is currently known about mitophagic mechanisms, regulatory pathways, and function in the heart.

Nutrient-sensing mTORC1: Integration of metabolic and autophagic signals

The ability of adult cardiomyocytes to regenerate is limited, and irreversible loss by cell death plays a crucial role in heart diseases. Autophagy is an evolutionarily conserved cellular catabolic process through which long-lived proteins and damaged organelles are targeted for lysosomal degradation. Autophagy is important in cardiac homeostasis and can serve as a protective mechanism by providing an energy source, especially in the face of sustained starvation. Cellular metabolism is closely associated with cell survival, and recent evidence suggests that metabolic and autophagic signaling pathways exhibit a high degree of crosstalk and are functionally interdependent. In this review, we discuss recent progress in our understanding of regulation of autophagy and its crosstalk with metabolic signaling, with a focus on the nutrient-sensing mTOR complex 1 (mTORC1) pathway.

Mg(++) requirement for MtHK binding, and Mg(++) stabilization of mitochondrial membranes via activation of MtHK & MtCK and promotion of mitochondrial permeability transition pore closure: A hypothesis on mechanisms underlying Mg(++)'s antioxidant and cytoprotective effects

Evidence points to magnesium's antioxidant, anti-necrotic, and anti-apoptotic effects in cardio- and neuroprotection. With magnesium being involved in over 300 biochemical reactions, the mechanisms underlying its cytoprotective and antioxidant effects have remained elusive. The profound anti-apoptotic, anabolic, and antioxidant effects of mitochondrion bound hexokinase (MtHk), and the anti-apoptotic, anti-necrotic, and antioxidant functions of mitochondrial creatine kinase (MtCK) have been established over the past few decades. As powerful regulators of the mitochondrial permeability transition pore (PTP), MtHK and MtCK promote anti-apoptosis and anti-necrosis by stabilizing mitochondrial outer and inner membranes. In this article, it is proposed that magnesium is essentially and directly involved in mitochondrial membrane stabilization via (i) Mg(++) ion requirement for the binding of mitochondrial hexokinase (ii) Mg(++)'s allosteric activation of mitochondrial bound hexokinase, and stimulation of mitochondrial bound creatine kinase activities, and (iii) Mg(++) inhibition of PTP opening by Ca(++) ions. These effects of Mg(++) ions are indirectly supplanted by the stimulatory effect of magnesium on the Akt kinase survival pathway. The "Magnesium/Calcium Yin Yang Hypothesis" proposes here that because of the antagonistic effects of Ca(++) and Mg(++) ions in the presence of high Ca(++) ion concentration at MtHK, MtCK, and PTP, magnesium supplementation may provide cytoprotective effects in the treatment of some degenerative diseases and cytopathies with high intracellular [Ca(++)]/ [Mg(++)] ratio at these sites, whether of genetic, developmental, drug induced, ischemic, immune based, toxic, or infectious etiology.

Suppression of miR-181a attenuates H2O2-induced death of mesenchymal stem cells by maintaining hexokinase II expression

BACKGROUND

Low survival rate of transplanted cells compromises the efficacy of cell therapy. Hexokinase II (HKII) is known to have anti-apoptotic activity through its interaction with mitochondria. The objective was to identify miRNAs targeting HKII and investigate whether miRNA-mediated modulation of HKII could improve the survival of mesenchymal stem cells (MSCs) exposed to H2O2. The expression of HKII in MSCs exposed to H2O2 was evaluated, and HKII-targeting miRNA was screened based on miRNA-target prediction databases. The effect of H2O2 on the expression of the selected HKII-targeting miRNA was examined and the effect of modulation of the selected HKII-targeting miRNA using anti-miRNA on H2O2-induced apoptosis of MSC was evaluated.

RESULTS

H2O2 (600 μM) induced cell death of MSCs and decreased mitochondrial HKII expression. We have identified miR-181a as a HKII-targeting miRNA and H2O2 increased the expression of miR-181a in MSCs. Delivery of anti-miR-181a, which neutralizes endogenous miR-181a, significantly attenuated H2O2-induced decrease of HKII expression and disruption of mitochondrial membrane potential, improving the survival of MSCs exposed to H2O2.

CONCLUSIONS

These findings suggest that H2O2-induced up-regulation of miR-181a contributes to the cell death of MSCs by down-regulating HKII. Neutralizing miR-181a can be an effective way to prime MSCs for transplantation into ischemic tissues.

Assessing the early changes of cerebral glucose metabolism via dynamic (18)FDG-PET/CT during cardiac arrest

To study the changes of cerebral glucose metabolism (CGM) during the phase of return of spontaneous circulation (ROSC) after cardiac arrest (CA), we used 18-fluorodeoxyglucose-positron emission tomography/computed tomography ((18)FDG-PET/CT) to measure the CGM changes in six beagle canine models. After the baseline (18)FDG-PET/CT was recorded, ventricular fibrillation (VF) was induced for 6 min, followed by close-chest cardiopulmonary resuscitation (CPR) in conjunction with intravenous (IV) administration of epinephrine and external defibrillator shocks until ROSC was achieved, within 30 min. The (18)FDG was recorded prior to intravenous administration at 0 h (baseline), and at 4, 24, and 48 h after CA with ROSC. We evaluated the expression of two key control factors in canine CGM, hexokinase I (HXK I) and HXK II, by immunohistochemistry at the four above mentioned time points. Electrically induced VF of 6 min duration was successfully induced in the dogs. Resuscitation was then performed to maintain blood pressure stability. Serial (18)FDG-PET/CT scans found that the CGM decreased at 4 h after ROSC and remained lower than the baseline even at 48 h. The expression of HXK I and II levels were consistent with the changes in CGM. These data from our present work showed that (18)FDG-PET/CT imaging can be used to detect decreased CGM during CA and was consistent with the results of CMRgl. Furthermore, there were also concomitant changes in the expression of HXK I and HXK II. The decrease in CGM may be an early sign of hyperacute global cerebral ischemia.

Targeting hexokinase II to mitochondria to modulate energy metabolism and reduce ischaemia-reperfusion injury in heart

Mitochondrially bound hexokinase II (mtHKII) has long been known to confer cancer cells with their resilience against cell death. More recently, mtHKII has emerged as a powerful protector against cardiac cell death. mtHKII protects against ischaemia-reperfusion (IR) injury in skeletal muscle and heart, attenuates cardiac hypertrophy and remodelling, and is one of the major end-effectors through which ischaemic preconditioning protects against myocardial IR injury. Mechanisms of mtHKII cardioprotection against reperfusion injury entail the maintenance of regulated outer mitochondrial membrane (OMM) permeability during ischaemia and reperfusion resulting in stabilization of mitochondrial membrane potential, the prevention of OMM breakage and cytochrome C release, and reduced reactive oxygen species production. Increasing mtHK may also have important metabolic consequences, such as improvement of glucose-induced insulin release, prevention of acidosis through enhanced coupling of glycolysis and glucose oxidation, and inhibition of fatty acid oxidation. Deficiencies in expression and distorted cellular signalling of HKII may contribute to the altered sensitivity of diabetes to cardiac ischaemic diseases. The interaction of HKII with the mitochondrion constitutes a powerful endogenous molecular mechanism to protect against cell death in almost all cell types examined (neurons, tumours, kidney, lung, skeletal muscle, heart). The challenge now is to harness mtHKII in the treatment of infarction, stroke, elective surgery and transplantation. Remote ischaemic preconditioning, metformin administration and miR-155/miR-144 manipulations are potential means of doing just that.

Targeting hexokinase II to mitochondria to modulate energy metabolism and reduce ischaemia-reperfusion injury in heart

Mitochondrially bound hexokinase II (mtHKII) has long been known to confer cancer cells with their resilience against cell death. More recently, mtHKII has emerged as a powerful protector against cardiac cell death. mtHKII protects against ischaemia-reperfusion (IR) injury in skeletal muscle and heart, attenuates cardiac hypertrophy and remodelling, and is one of the major end-effectors through which ischaemic preconditioning protects against myocardial IR injury. Mechanisms of mtHKII cardioprotection against reperfusion injury entail the maintenance of regulated outer mitochondrial membrane (OMM) permeability during ischaemia and reperfusion resulting in stabilization of mitochondrial membrane potential, the prevention of OMM breakage and cytochrome C release, and reduced reactive oxygen species production. Increasing mtHK may also have important metabolic consequences, such as improvement of glucose-induced insulin release, prevention of acidosis through enhanced coupling of glycolysis and glucose oxidation, and inhibition of fatty acid oxidation. Deficiencies in expression and distorted cellular signalling of HKII may contribute to the altered sensitivity of diabetes to cardiac ischaemic diseases. The interaction of HKII with the mitochondrion constitutes a powerful endogenous molecular mechanism to protect against cell death in almost all cell types examined (neurons, tumours, kidney, lung, skeletal muscle, heart). The challenge now is to harness mtHKII in the treatment of infarction, stroke, elective surgery and transplantation. Remote ischaemic preconditioning, metformin administration and miR-155/miR-144 manipulations are potential means of doing just that.

The role of hexokinase in cardioprotection - mechanism and potential for translation

Mitochondrial permeability transition pore (mPTP) opening plays a critical role in cardiac reperfusion injury and its prevention is cardioprotective. Tumour cell mitochondria usually have high levels of hexokinase isoform 2 (HK2) bound to their outer mitochondrial membranes (OMM) and HK2 binding to heart mitochondria has also been implicated in resistance to reperfusion injury. HK2 dissociates from heart mitochondria during ischaemia, and the extent of this correlates with the infarct size on reperfusion. Here we review the mechanisms and regulations of HK2 binding to mitochondria and how this inhibits mPTP opening and consequent reperfusion injury. Major determinants of HK2 dissociation are the elevated glucose‐6‐phosphate concentrations and decreased pH in ischaemia. These are modulated by the myriad of signalling pathways implicated in preconditioning protocols as a result of a decrease in pre‐ischaemic glycogen content. Loss of mitochondrial HK2 during ischaemia is associated with permeabilization of the OMM to cytochrome c, which leads to greater reactive oxygen species production and mPTP opening during reperfusion. Potential interactions between HK2 and OMM proteins associated with mitochondrial fission (e.g. Drp1) and apoptosis (B‐cell lymphoma 2 family members) in these processes are examined. Also considered is the role of HK2 binding in stabilizing contact sites between the OMM and the inner membrane. Breakage of these during ischaemia is proposed to facilitate cytochrome c loss during ischaemia while increasing mPTP opening and compromising cellular bioenergetics during reperfusion. We end by highlighting the many unanswered questions and discussing the potential of modulating mitochondrial HK2 binding as a pharmacological target.

Linked Articles

This article is part of a themed section on Conditioning the Heart – Pathways to Translation. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue‐8

Cardiac-specific ablation of ARNT leads to lipotoxicity and cardiomyopathy

Patients with type 2 diabetes often present with cardiovascular complications; however, it is not clear how diabetes promotes cardiac dysfunction. In murine models, deletion of the gene encoding aryl hydrocarbon nuclear translocator (ARNT, also known as HIF1β) in the liver or pancreas leads to a diabetic phenotype; however, the role of ARNT in cardiac metabolism is unknown. Here, we determined that cardiac-specific deletion of Arnt in adult mice results in rapid development of cardiomyopathy (CM) that is characterized by accumulation of lipid droplets. Compared with hearts from ARNT-expressing mice, ex vivo analysis of ARNT-deficient hearts revealed a 2-fold increase in fatty acid (FA) oxidation as well as a substantial increase in the expression of PPARα and its target genes. Furthermore, deletion of both Arnt and Ppara preserved cardiac function, improved survival, and completely reversed the FA accumulation phenotype, indicating that PPARα mediates the detrimental effects of Arnt deletion in the heart. Finally, we determined that ARNT directly regulates Ppara expression by binding to its promoter and forming a complex with HIF2α. Together, these findings suggest that ARNT is a critical regulator of myocardial FA metabolism and that its deletion leads to CM and an increase in triglyceride accumulation through PPARα.

Hexokinase II integrates energy metabolism and cellular protection: Akting on mitochondria and TORCing to autophagy

Accumulating evidence reveals that metabolic and cell survival pathways are closely related, sharing common signaling molecules. Hexokinase catalyzes the phosphorylation of glucose, the rate-limiting first step of glycolysis. Hexokinase II (HK-II) is a predominant isoform in insulin-sensitive tissues such as heart, skeletal muscle, and adipose tissues. It is also upregulated in many types of tumors associated with enhanced aerobic glycolysis in tumor cells, the Warburg effect. In addition to the fundamental role in glycolysis, HK-II is increasingly recognized as a component of a survival signaling nexus. This review summarizes recent advances in understanding the protective role of HK-II, controlling cellular growth, preventing mitochondrial death pathway and enhancing autophagy, with a particular focus on the interaction between HK-II and Akt/mTOR pathway to integrate metabolic status with the control of cell survival.