Hypoxia inhibits cardiomyocyte proliferation in fetal rat hearts via upregulating TIMP-4

Cardiac programming in the placentally restricted sheep fetus in early gestation

Fetal growth restriction (FGR) occurs in 8% of human pregnancies, and the growth restricted newborn is at a greater risk of developing heart disease in later adult life. In sheep, experimental restriction of placental growth (PR) from conception results in FGR, a decrease in cardiomyocyte endowment and an upregulation of pathological hypertrophic signalling in the fetal heart in late gestation. However, there is no change in the expression of markers of cellular proliferation nor in the level of cardiomyocyte apoptosis in the heart of the PR fetus in late gestation. This suggests that FGR arises early in gestation and programs a decrease in cardiomyocyte endowment in early, rather than late, gestation. Here, control and PR fetal sheep were humanely killed at 55 days' gestation (term, 150 days). Fetal body and heart weight were lower in PR compared with control fetuses and there was evidence of sparing of fetal brain growth. While there was no change in the proportion of cardiomyocytes that were proliferating in the early gestation PR heart, there was an increase in measures of apoptosis, and markers of autophagy and pathological hypertrophy in the PR fetal heart. These changes in early gestation highlight that FGR is associated with evidence of early cell death and compensatory hypertrophic responses of cardiomyocytes in the fetal heart. The data suggest that early placental restriction results in a decrease in the pool of proliferative cardiomyocytes in early gestation, which would limit cardiomyocyte endowment in the heart of the PR fetus in late gestation. KEY POINTS: Placental restriction leading to fetal growth restriction (FGR) and chronic fetal hypoxaemia in sheep results in a decrease in cardiomyocyte endowment in late gestation. FGR did not change cardiomyocyte proliferation during early gestation but did result in increased apoptosis and markers of autophagy in the fetal heart, which may result in the decreased endowment of cardiomyocytes observed in late gestation. FGR in early gestation also results in increased hypoxia inducible factor signalling in the fetal heart, which in turn may result in the altered expression of epigenetic regulators, increased expression of insulin-like growth factor 2 and cardiomyocyte hypertrophy during late gestation and after birth.

Targeting cardiomyocyte cell cycle regulation in heart failure

Heart failure continues to be a significant global health concern, causing substantial morbidity and mortality. The limited ability of the adult heart to regenerate has posed challenges in finding effective treatments for cardiac pathologies. While various medications and surgical interventions have been used to improve cardiac function, they are not able to address the extensive loss of functioning cardiomyocytes that occurs during cardiac injury. As a result, there is growing interest in understanding how the cell cycle is regulated and exploring the potential for stimulating cardiomyocyte proliferation as a means of promoting heart regeneration. This review aims to provide an overview of current knowledge on cell cycle regulation and mechanisms underlying cardiomyocyte proliferation in cases of heart failure, while also highlighting established and novel therapeutic strategies targeting this area for treatment purposes.

Cardioprotective Effects of Dexmedetomidine in an Oxidative-Stress In Vitro Model of Neonatal Rat Cardiomyocytes

Preterm birth is a risk factor for cardiometabolic disease. The preterm heart before terminal differentiation is in a phase that is crucial for the number and structure of cardiomyocytes in further development, with adverse effects of hypoxic and hyperoxic events. Pharmacological intervention could attenuate the negative effects of oxygen. Dexmedetomidine (DEX) is an α2-adrenoceptor agonist and has been mentioned in connection with cardio-protective benefits. In this study, H9c2 myocytes and primary fetal rat cardiomyocytes (NRCM) were cultured for 24 h under hypoxic condition (5% O₂), corresponding to fetal physioxia (pO₂ 32-45 mmHg), ambient oxygen (21% O₂, pO₂ ~150 mmHg), or hyperoxic conditions (80% O₂, pO₂ ~300 mmHg). Subsequently, the effects of DEX preconditioning (0.1 µM, 1 µM, 10 µM) were analyzed. Modulated oxygen tension reduced both proliferating cardiomyocytes and transcripts (CycD2). High-oxygen tension induced hypertrophy in H9c2 cells. Cell-death-associated transcripts for caspase-dependent apoptosis (Casp3/8) increased, whereas caspase-independent transcripts (AIF) increased in H9c2 cells and decreased in NRCMs. Autophagy-related mediators (Atg5/12) were induced in H9c2 under both oxygen conditions, whereas they were downregulated in NRCMs. DEX preconditioning protected H9c2 and NRCMs from oxidative stress through inhibition of transcription of the oxidative stress marker GCLC, and inhibited the transcription of both the redox-sensitive transcription factors Nrf2 under hyperoxia and Hif1α under hypoxia. In addition, DEX normalized the gene expression of Hippo-pathway mediators (YAP1, Tead1, Lats2, Cul7) that exhibited abnormalities due to differential oxygen tensions compared with normoxia, suggesting that DEX modulates the activation of the Hippo pathway. This, in the context of the protective impact of redox-sensitive factors, may provide a possible rationale for the cardio-protective effects of DEX in oxygen-modulated requirements on survival-promoting transcripts of immortalized and fetal cardiomyocytes.

Silencing RNA for MMPs May Be Utilized for Cardioprotection

Ischemia/reperfusion (I/R) injury is accompanied by an increase of matrix metalloproteinase 2 (MMP-2) activity, which degrades heart contractile proteins. The aim of the study was to investigate the effect of MMP-2 small interfering RNA (MMP-2 siRNA) administration on I/R heart. Isolated rat hearts perfused by the Langendorff method were subjected to I/R in the presence or absence of MMP-2 siRNA. The hemodynamic parameters of heart function were monitored. Lactate dehydrogenase (LDH) activity was measured in coronary effluents. Activity and concentration of MMPs in the hearts were measured. Concentration of troponin I (TnI) in coronary effluents was examined as a target for MMP-2 degradation. Recovery of heart mechanical function was reduced after I/R; however, administration of MMP-2 siRNA resulted in restoration of proper mechanical function (p < 0.001). LDH activity was decreased after the use of MMP-2 siRNA (p = 0.02), providing evidence for reduced cardiac damage. Both MMP-2 and MMP-9 syntheses as well as their activity were inhibited in the I/R hearts after siRNA administration (p < 0.05). MMP-2 siRNA administration inhibited TnI release into the coronary effluents (p < 0.001). The use of MMP-2 siRNA contributed to the improvement of heart mechanical function and reduction of contractile proteins degradation during I/R; therefore, MMP-2 siRNA may be considered a cardioprotective agent.

Cardiomyocyte Proliferation from Fetal- to Adult- and from Normal- to Hypertrophy and Failing Hearts

Simple Summary

Death from injury to the heart from a variety of causes remains a major cause of mortality worldwide. The cardiomyocyte, the major contracting cell of the heart, is responsible for pumping blood to the rest of the body. During fetal development, these immature cardiomyocytes are small and rapidly divide to complete development of the heart by birth when they develop structural and functional characteristics of mature cells which prevent further division. All further growth of the heart after birth is due to an increase in the size of cardiomyocytes, hypertrophy. Following the loss of functional cardiomyocytes due to coronary artery occlusion or other causes, the heart is unable to replace the lost cells. One of the significant research goals has been to induce adult cardiomyocytes to reactivate the cell cycle and repair cardiac injury. This review explores the developmental, structural, and functional changes of the growing cardiomyocyte, and particularly the sarcomere, responsible for force generation, from the early fetal period of reproductive cell growth through the neonatal period and on to adulthood, as well as during pathological response to different forms of myocardial diseases or injury. Multiple issues relative to cardiomyocyte cell-cycle regulation in normal or diseased conditions are discussed.

Abstract

The cardiomyocyte undergoes dramatic changes in structure, metabolism, and function from the early fetal stage of hyperplastic cell growth, through birth and the conversion to hypertrophic cell growth, continuing to the adult stage and responding to various forms of stress on the myocardium, often leading to myocardial failure. The fetal cell with incompletely formed sarcomeres and other cellular and extracellular components is actively undergoing mitosis, organelle dispersion, and formation of daughter cells. In the first few days of neonatal life, the heart is able to repair fully from injury, but not after conversion to hypertrophic growth. Structural and metabolic changes occur following conversion to hypertrophic growth which forms a barrier to further cardiomyocyte division, though interstitial components continue dividing to keep pace with cardiac growth. Both intra- and extracellular structural changes occur in the stressed myocardium which together with hemodynamic alterations lead to metabolic and functional alterations of myocardial failure. This review probes some of the questions regarding conditions that regulate normal and pathologic growth of the heart.

Developmental programming of cardiovascular function: a translational perspective

The developmental origins of health and disease (DOHaD) is a concept linking pre- and early postnatal exposures to environmental influences with long-term health outcomes and susceptibility to disease. It has provided a new perspective on the etiology and evolution of chronic disease risk, and as such is a classic example of a paradigm shift. What first emerged as the 'fetal origins of disease', the evolution of the DOHaD conceptual framework is a storied one in which preclinical studies played an important role. With its potential clinical applications of DOHaD, there is increasing desire to leverage this growing body of preclinical work to improve health outcomes in populations all over the world. In this review, we provide a perspective on the values and limitations of preclinical research, and the challenges that impede its translation. The review focuses largely on the developmental programming of cardiovascular function and begins with a brief discussion on the emergence of the 'Barker hypothesis', and its subsequent evolution into the more-encompassing DOHaD framework. We then discuss some fundamental pathophysiological processes by which developmental programming may occur, and attempt to define these as 'instigator' and 'effector' mechanisms, according to their role in early adversity. We conclude with a brief discussion of some notable challenges that hinder the translation of this preclinical work.

Changes in Cardiomyocyte Cell Cycle and Hypertrophic Growth During Fetal to Adult in Mammals

The failure of adult cardiomyocytes to reproduce themselves to repair an injury results in the development of severe cardiac disability leading to death in many cases. The quest for an understanding of the inability of cardiac myocytes to repair an injury has been ongoing for decades with the identification of various factors which have a temporary effect on cell‐cycle activity. Fetal cardiac myocytes are continuously replicating until the time that the developing fetus reaches a stage of maturity sufficient for postnatal life around the time of birth. Recent reports of the ability for early neonatal mice and pigs to completely repair after the severe injury has stimulated further study of the regulators of the cardiomyocyte cell cycle to promote replication for the remuscularization of injured heart. In all mammals just before or after birth, single‐nucleated hyperplastically growing cardiomyocytes, 1X2N, undergo ≥1 additional DNA replications not followed by cytokinesis, resulting in cells with ≥2 nuclei or as in primates, multiple DNA replications (polyploidy) of 1 nucleus, 2X2(+)N or 1X4(+)N. All further growth of the heart is attributable to hypertrophy of cardiomyocytes. Animal studies ranging from zebrafish with 100% 1X2N cells in the adult to some strains of mice with up to 98% 2X2N cells in the adult and other species with variable ratios of 1X2N and 2X2N cells are reviewed relative to the time of conversion. Various structural, physiologic, metabolic, genetic, hormonal, oxygenation, and other factors that play a key role in the inability of post‐neonatal and adult myocytes to undergo additional cytokinesis are also reviewed.

Foetal hypoxia impacts methylome and transcriptome in developmental programming of heart disease

AIMS

Antenatal hypoxia negatively impacts foetal heart development, and increases the risk of heart disease later in life. The molecular mechanisms remain largely elusive. Here, we conducted a genome-wide analysis to study the impact of antenatal hypoxia on DNA methylome and transcriptome profiling in foetal and adult offspring hearts.

METHODS AND RESULTS

Pregnant rats were treated with normoxia or hypoxia (10.5% O2) from Day 15 to Day 21 of gestation. Hearts were isolated from near-term foetuses and 5-month-old male and female offsprings, and DNA methylome and RNA-seq were performed. Methylome data shows a sharp dip in CpG methylation centred at the transcription start site (TSS). CpG islands (CGIs) and CpG island shores (CGSs) within 10 kb upstream of the TSS are hypomethylated, compared with CGIs and CGSs within gene bodies. Combining transcriptome, data indicate an inverse relation between gene expression and CpG methylation around the TSS. Of interest, antenatal hypoxia induces opposite changes in methylation patterns in foetal and adult hearts, with hypermethylation in the foetus and hypomethylation in the adult. Also, there is significant sex dimorphism of changes in gene expression patterns in the adult offspring heart. Notably, pathway analysis indicates that enrichment of inflammation-related pathways are significantly greater in the adult male heart than those in the female heart.

CONCLUSION

Our study provides an initial framework and new insights into foetal hypoxia-mediated epigenetic programming of pro-inflammatory phenotype in the heart development, linking antenatal stress, and developmental programming of heart vulnerability to disease later in life.

How to Stimulate Myocardial Regeneration in Adult Mammalian Heart: Existing Views and New Approaches

Stem cell-based therapy has been considered as a promising option in the treatment of ischemic heart disease. Although stem cell administration resulted in the temporary improvement of myocardial contractility in the majority of studies, the formation of new cardiomyocytes within the injured myocardium has not been conclusively demonstrated. Consequently, the focus of research in the field has since shifted to stem cell-derived paracrine factors, including cytokines, growth factors, mRNA, and miRNA. Notably, both mRNA and miRNA can enter into the extracellular space either in soluble form or packed into membrane vesicles. Stem cell-derived paracrine factors have been shown to suppress inflammation and apoptosis, stimulate angiogenesis, and amplify the proliferation and differentiation of resident cardiac stem cells (CSCs). Such features have led to exosomes being considered as potential drug candidates affording myocardial regeneration. The search for chemical signals capable of stimulating cardiomyogenesis is ongoing despite continuous debates regarding the ability of mature cardiomyocytes to divide or dedifferentiate, transdifferentiation of other cells into cardiomyocytes, and the ability of CSCs to differentiate into cardiomyocytes. Future research is aimed at identifying novel cell candidates capable of differentiating into cardiomyocytes. The observation that CSCs can undergo intracellular development with the formation of “cell-in-cell structure” and subsequent release of transitory amplifying cells with the capacity to differentiate into cardiomyocytes may provide clues for stimulating regenerative cardiomyogenesis.

Multi-Omics Integration Reveals Short and Long-Term Effects of Gestational Hypoxia on the Heart Development

Antenatal hypoxia caused epigenetic reprogramming of methylome and transcriptome in the developing heart and increased the risk of heart disease later in life. Herein, we investigated the impact of gestational hypoxia in proteome and metabolome in the hearts of fetus and adult offspring. Pregnant rats were treated with normoxia or hypoxia (10.5% O₂) from day 15 to 21 of gestation. Hearts were isolated from near-term fetuses and 5 month-old offspring, and proteomics and metabolomics profiling was determined. The data demonstrated that antenatal hypoxia altered proteomics and metabolomics profiling in the heart, impacting energy metabolism, lipid metabolism, oxidative stress, and inflammation-related pathways in a developmental and sex dependent manner. Of importance, integrating multi-omics data of transcriptomics, proteomics, and metabolomics profiling revealed reprogramming of the mitochondrion, especially in two clusters: (a) the cluster associated with "mitochondrial translation"/"aminoacyl t-RNA biosynthesis"/"one-carbon pool of folate"/"DNA methylation"; and (b) the cluster with "mitochondrion"/"TCA cycle and respiratory electron transfer"/"acyl-CoA dehydrogenase"/"oxidative phosphorylation"/"complex I"/"troponin myosin cardiac complex". Our study provides a powerful means of multi-omics data integration and reveals new insights into phenotypic reprogramming of the mitochondrion in the developing heart by fetal hypoxia, contributing to an increase in the heart vulnerability to disease later in life.

Effects of hypoxia on cardiomyocyte proliferation and association with stage of development

BACKGROUND

Hypoxia has been suggested to be both beneficial and harmful to the proliferation of cardiomyocytes. This controversy remains unresolved, and the underlying mechanism by which hypoxia exerts its effects remains unclear. We here hypothesize that cardiomyocyte developmental stage may play a role.

METHODS AND RESULTS

The embryonic ventricular myocyte cell line H9C2, primary isolated fetal cardiomyocytes, and neonatal cardiomyocytes were cultured with normal O₂ (21% O₂) or under hypoxic conditions (10% O₂) for 7 days, and then harvested for Western blotting, qRT-PCR, and immunostaining. When cultured under hypoxic conditions, proliferating marker-Ki67, mRNA level, and the percentage of Ki67-positive cardiomyocytes were significantly lower in H9C2 and fetal cardiomyocytes but higher in neonatal cardiomyocytes. Consistently, the mRNA and protein levels and induced nuclear localization of yes associated protein 1(YAP1), one of the most important regulators of cardiomyocyte proliferation, were significantly lower in H9C2 and fetal cardiomyocytes but up-regulated in neonatal cardiomyocytes when treated with hypoxia. Compared to neonatal cardiomyocytes, there was a lower level of troponin T mRNA and protein expression in H9C2 and fetal cardiomyocytes. When H9C2 or fetal cardiomyocytes overexpressing troponin T in were cultured under hypoxic condition, their ability to proliferate increased.

CONCLUSIONS

The effect of hypoxia on the proliferation of cardiomyocyte is associated with their developmental stage. YAP1 expression is positively correlated with the change in cardiomyocyte proliferation in response to hypoxia. Developmental stage- specific sarcomere component troponin T may partly account for the underlying mechanism.

Cardiomyocyte nuclearity and ploidy: when is double trouble?

Considerable effort has gone into investigating mechanisms that underlie the developmental transition in which mammalian cardiomyocytes (CMs) switch from being able to proliferate during development, to essentially having lost that ability at maturity. This problem is interesting not only for scientific curiosity, but also for its clinical relevance because controlling the ability of mature CMs to replicate would provide a much-needed approach for restoring cardiac function in damaged hearts. In this review, we focus on the propensity of mature mammalian CMs to be multinucleated and polyploid, and the extent to which this may be necessary for normal physiology yet possibly disadvantageous in some circumstances. In this context, we explore whether the concept of the myonuclear domain (MND) in multinucleated skeletal muscle fibers might apply to cardiomyocytes, and whether cardio-MND size might be related to the transition of CMs to become multinuclear. Nuclei in CMs are almost certainly integrators of not only biochemical, but also-because of their central location within the myofibrils-mechanical information, and this multimodal, integrative function in adult CMs-involving molecules that have been extensively studied along with newly identified possibilities-could influence both gene expression as well as replication of the genome and the nuclei themselves.

Morphological features of atrial myocardium embryonic development and its changes caused by hypoxia effect

Mortality and morbidity during the prenatal period of development remain a real problem at the present time. The Scientific Committee EURO-PERISTAT has revealed that mortality of fetuses associated with congenital abnormalities is on average 15–20% across Europe. Hypoxia is one of the top causes of death of fetuses. Since the heart begins to function before birth, influence of teratogenic factors leads to formation of anomalies of its development. Congenital heart defects are the most common of these and occur with a frequency of 24%. Abnormalities associated with the atrium occur with frequency of 6.4 per 10,000 cases. Investigation of structural changes of the atrial myocardium is a key for understanding of pathogenic mechanisms of cardiovascular diseases that are caused by influence of hypoxia. Nowadays, a great deal of research is being dedicated to normal cardiogenesis and much less work is focused on abnormal heart development. There are numerous teratogenic factors such as alcohol, retinoic acid, hyperthermia, hypoxia that are most common causes of heart diseases. The attention of researchers has been predominantly focused on study of changes of the ventricular myocardium under the effect of hypoxia. It is known that the atrium is different from the ventricles by derivation, development and structure. Therefore, the effects of pathological factors on the atrial myocardium will be different as complared to their effect on the ventricles. Also, almost all research has focused on study of consequences of hypoxia at the late stages of cardiogenesis. However, the greatest number of abnormalities is associated with the early embryonic period, as structures that continue development are more sensitive to the effects of harmful factors. Thus, comparative analysis of scientific research devoted to morphological study of atrial myocardium transformations on the cellular and ultrastructural levels under the influence of hypoxia during the stages of cardiogenesis is an important task.

Oxygen as a key regulator of cardiomyocyte proliferation: New results about cell culture conditions!

The goal of the new therapeutically strategies aimed to treat cardiovascular diseases (CVDs) is to enhance the natural ability of the heart to regenerate. This represents a great challenge for the coming years as all the mechanisms underlying the replacement of dying cells by functional cells of the same type are not completely elucidated. Among these, stimulating cardiomyocyte proliferation seems to be crucial for the restoration of normal cardiac function after CVDs. In this review, we summarized the recent advances about the modulation of cardiomyocyte proliferation in physiological (during ageing) and pathological conditions. We highlighted the role of oxygen and we presented new results demonstrating that performing neonatal cardiomyocyte cell cultures in "normoxic" oxygen conditions (i.e. 3% oxygen) increases their proliferation rate, when compared to "hyperoxic" conventional conditions (i.e. 20% oxygen). Thus, oxygen concentration seems to be a key factor in the control of cardiomyocyte proliferation.

Combined use of external therapeutic ultrasound and tirofiban has synergistic therapeutic effects on no-reflow after myocardial reperfusion

OBJECTIVE

This study aimed to evaluate the effects of the combined use of external therapeutic ultrasound (ETUS) and the specific glycoprotein IIb/IIIa inhibitor tirofiban on myocardial no-reflow in a canine model of acute myocardial infarction after reperfusion.

METHODS

The canine myocardial no-reflow model was established by a 3-hour occlusion of the left anterior desecending coronary artery followed by a 2-hour reperfusion. Twenty-four canines were divided into four groups (6/group): (1) control, (2) tirofiban alone, (3) ETUS combined with tirofiban (ETUS + tirofiban), and (4) ETUS alone.

RESULTS

The area of no-reflow in each of the three treatment groups was significantly decreased, compared with the control group, with the ETUS + tirofiban group having the smallest area. Also, the ETUS + tirofiban group had the highest recanalized rate of microvessels in the no-reflow area and fewer impaired cellular organelles. The recovery rates of the endocardial and middle circumferential strain as well as longitudinal strain in the ETUS + tirofiban group were significantly greater than those of the tirofiban group. Moreover, the expression of hypoxia-inducible factor-1α (HIF-1α) was significantly increased in the ETUS + tirofiban group, compared with the other groups.

CONCLUSIONS

The combined use of ETUS and tirofiban offers synergistic benefits for the treatment of myocardial no-reflow.

Gestational Hypoxia and Developmental Plasticity

Hypoxia is one of the most common and severe challenges to the maintenance of homeostasis. Oxygen sensing is a property of all tissues, and the response to hypoxia is multidimensional involving complicated intracellular networks concerned with the transduction of hypoxia-induced responses. Of all the stresses to which the fetus and newborn infant are subjected, perhaps the most important and clinically relevant is that of hypoxia. Hypoxia during gestation impacts both the mother and fetal development through interactions with an individual's genetic traits acquired over multiple generations by natural selection and changes in gene expression patterns by altering the epigenetic code. Changes in the epigenome determine "genomic plasticity," i.e., the ability of genes to be differentially expressed according to environmental cues. The genomic plasticity defined by epigenomic mechanisms including DNA methylation, histone modifications, and noncoding RNAs during development is the mechanistic substrate for phenotypic programming that determines physiological response and risk for healthy or deleterious outcomes. This review explores the impact of gestational hypoxia on maternal health and fetal development, and epigenetic mechanisms of developmental plasticity with emphasis on the uteroplacental circulation, heart development, cerebral circulation, pulmonary development, and the hypothalamic-pituitary-adrenal axis and adipose tissue. The complex molecular and epigenetic interactions that may impact an individual's physiology and developmental programming of health and disease later in life are discussed.

MicroRNA-210 suppresses glucocorticoid receptor expression in response to hypoxia in fetal rat cardiomyocytes

Hypoxia is a common intrauterine stressor, often resulting in intrauterine growth restriction and increased risk for cardiovascular disease later in life. The aim of this work was to test the hypothesis that microRNA-210 (miR-210) mediates the detrimental suppression of glucocorticoid receptor (GR) in response to hypoxia in fetal rat cardiomyocytes. Cardiomyocytes isolated from gestational day 21 Sprague Dawley fetal rats showed increased miR-210 levels and reduced GR abundance after exposure to ex vivo hypoxia (1% O2). In regard to mechanisms, the different contributions of hypoxia response elements (HREs) motifs in the regulation of miR-210 promoter activity and the miR-210-mediated repression of GR expression were determined in rat embryonic heart-derived myogenic cell line H9c2. Moreover, using a cell culture-based model of hypoxia-reoxygenation injury, we assessed the cytotoxic effects of GR suppression under hypoxic conditions. The results showed that hypoxia induced HIF-1α-dependent miR-210 production, as well as miR-210-mediated GR suppression, in cardiomyocytes. Furthermore, inhibition or knockdown of GR exacerbated cell death in response to hypoxia-reoxygenation injury. Altogether, the present study demonstrates that the HIF-1α-dependent miR-210-mediated suppression of GR in fetal rat cardiomyocytes increases cell death in response to hypoxia, providing novel evidence for a possible mechanistic link between fetal hypoxia and programming of ischemic-sensitive phenotype in the developing heart.

Irisin Ameliorates Hypoxia/Reoxygenation-Induced Injury through Modulation of Histone Deacetylase 4

Irisin is a recently identified myokine which brings increases in energy expenditure and contributes to the beneficial effects of exercise through the browning of white adipose tissues. However, its effects in the heart remains unknown. This study sought to determine the effects of irisin on hypoxia/reoxygenation injury and its relationship with HDAC4. Wild type and stable HDAC4-overexpression cells were generated from H9c2 cardiomyoblasts. HDAC4 overexpression cells and wild type H9c2 cells were exposed to 24 hours of hypoxia followed by one hour of reoxygenation in vitro in the presence or absence of irisin (5 ng/ml). Cell cytotoxicity, apoptosis, mitochondrial respiration, and mitochondrial permeability transition pore (mPTP) were determined. Western blotting was employed to determine active-caspase 3, annexin V, and HDAC4 expression. As compared to wild type H9c2 group, HDAC4 overexpression remarkably led to a great increase in cell death as evident by the increased lactate dehydrogenase (LDH) leakage, ratio of caspase-3-positive cells as well as the upregulated levels of active-caspase 3 and annexin V shown by western blot analysis. In addition, HDAC4 overexpression also induced much severe mitochondrial dysfunction, as indicated by apoptotic mitochondria and increased mPTP. However, irisin treatment significantly attenuated all of these effects. Though irisin treatment did not influence the expression of HDAC4 at the transcriptional level, western blot analysis showed that HDAC4 protein levels decreased in a time-dependent way after administration of irisin, which is associated with the degradation of HDAC4 mediated by small ubiquitin-like modification (SUMO). Our results are the first to demonstrate that the protective effects of irisin in cardiomyoblasts exposed to hypoxia/reoxygenation might be associated with HDAC4 degradation.

Proteomic Analysis of Endothelin-1 Targets in the Regulation of Cardiomyocyte Proliferation

Hypoxia is a fetal stressor that leads to the production of endothelin-1 (ET-1). Previous work has shown that ET-1 treatment leads to the premature terminal differentiation of fetal cardiomyocytes. However, the precise mechanism is unknown. We tested the hypothesis that the fetal cardiomyocyte proteome will be greatly altered due to ET-1-treatment, which reveals a potential molecular mechanism of ET-1-induced terminal differentiation. Over a thousand proteins were detected in the fetal cardiomyocytes and among them 75 proteins were significantly altered due to ET-1 treatment. Using IPA pathway analysis, the merged network depicted several key proteins that appeared to be involved in regulating proliferation, including: EED, UBC, ERK1/2, MAPK, Akt, and EGFR. EED protein, which is associated with regulating proliferation via epigenetic mechanisms, is of particular interest. Herein we propose a model of the molecular mechanism by which ET-1 induced cardiomyocyte terminal differentiation occurs.

Dexamethasone Induces Cardiomyocyte Terminal Differentiation via Epigenetic Repression of Cyclin D2 Gene

Dexamethasone treatment of newborn rats inhibited cardiomyocyte proliferation and stimulated premature terminal differentiation of cardiomyocytes in the developing heart. Yet mechanisms remain undetermined. The present study tested the hypothesis that the direct effect of glucocorticoid receptor-mediated epigenetic repression of cyclin D2 gene in the cardiomyocyte plays a key role in the dexamethasone-mediated effects in the developing heart. Cardiomyocytes were isolated from 2-day-old rats. Cells were stained with a cardiomyocyte marker α-actinin and a proliferation marker Ki67. Cyclin D2 expression was evaluated by Western blot and quantitative real-time polymerase chain reaction. Promoter methylation of CcnD2 was determined by methylated DNA immunoprecipitation (MeDIP). Overexpression of Cyclin D2 was conducted by transfection of FlexiCcnD2 (+CcnD2) construct. Treatment of cardiomyocytes isolated from newborn rats with dexamethasone for 48 hours significantly inhibited cardiomyocyte proliferation with increased binucleation and decreased cyclin D2 protein abundance. These effects were blocked with Ru486 (mifepristone). In addition, the dexamethasone treatment significantly increased cyclin D2 gene promoter methylation in newborn rat cardiomyocytes. 5-Aza-2'-deoxycytidine inhibited dexamethasone-mediated promoter methylation, recovered dexamethasone-induced cyclin D2 gene repression, and blocked the dexamethasone-elicited effects on cardiomyocyte proliferation and binucleation. In addition, the overexpression of cyclin D2 restored the dexamethasone-mediated inhibition of proliferation and increase in binucleation in newborn rat cardiomyocytes. The results demonstrate that dexamethasone acting on glucocorticoid receptors has a direct effect and inhibits proliferation and stimulates premature terminal differentiation of cardiomyocytes in the developing heart via epigenetic repression of cyclin D2 gene.

Hypoxia induces autophagy in cardiomyocytes via a hypoxia-inducible factor 1-dependent mechanism

Hypoxia frequently accompanies such vascular disorders as atherosclerosis, thrombosis and ischemia/reperfusion injury. Myocardial ischemia/reperfusion, in particular, is a major contributor to cardiomyocyte impairment. Autophagy is a dynamic, self-catabolic process that has been implicated in a wide range of physiological processes and the pathogenesis of diverse diseases. The aim of the present study was to investigate the promotion of autophagy by hypoxia in a rat H9c2 heart cell line and determine the regulatory role of hypoxia-inducible factor 1 (HIF-1) in the hypoxia-induced autophagy in H9c2 cells, using quantitative green fluorescent protein-microtubule-associated protein 1 light chain 3 analysis and electron microscopy of autophagic vesicles. In addition, western blot and quantitative polymerase chain reaction analysis of autophagy-associated markers was conducted. In addition, the role of HIF-1-mediated autophagy in the hypoxia-induced impairment of H9c2 cells was examined, as a measure of cellular viability, using an MTT assay. The results demonstrated that autophagy was induced in H9c2 cells under hypoxia, and the autophagy induction triggered by hypoxia could be enhanced by HIF-1α overexpression and inhibited by HIF-1α knockdown. Furthermore, the HIF-1-mediated autophagy ameliorated the reduction in the H9c2 cell viability induced by hypoxia. These findings provide a novel insight into the hypoxic-ischemic injury to cardiomyocytes and give evidence for the occurrence of HIF-1-mediated autophagy in myocardial ischemia.

Tissue Inhibitor of Metalloproteinase-4 Triggers Apoptosis in Cervical Cancer Cells

Tissue inhibitor of metalloproteinase-4 (TIMP-4) is a member of extracellular matrix (ECM) metalloproteinases inhibitors that has pleiotropic functions. However, TIMP-4 roles in carcinogenesis are not well understood. Cell viability and flow cytometer assays were employed to evaluate cell death differences between H-Vector and H-TIMP-4 cell lines. Immunobloting and semi-quantitative RT-PCR were used to evaluate the expression of apoptosis regulators. We showed that TIMP-4 has apoptosis-sensitizing effects towards several death stimuli. Consistent with these findings, regulators of apoptosis from Inhibitors of Apoptosis Proteins (IAP), FLICE-like inhibitor proteins (FLIP) and Bcl-2 family members were modulated by TIMP-4. In addition, TIMP-4 knockdown resulted in cell survival increase after serum deprivation, as assessed by clonogenic cell analyses. This report shows that TIMP-4 regulates carcinogenesis through apoptosis activation in cervical cancer cells. Understanding TIMP-4 effects in tumorigenesis may provide clues for future therapies.

HDAC4 degradation mediates HDAC inhibition-induced protective effects against hypoxia/reoxygenation injury

Histone deacetylases (HDACs) play a crucial role in the regulation of gene expression through remodeling of chromatin structures. However, the molecular mechanisms involved in this event remain unknown. In this study, we sought to examine whether HDAC inhibition-mediated protective effects involved HDAC4 sumoylation, degradation, and the proteasome pathway. Isolated neonatal mouse ventricular myocytes (NMVM) and H9c2 cardiomyoblasts were subjected to 48 h of hypoxia (H) (1% O2 ) and 2 h of reoxygenation (R). Treatment of cardiomyocytes with trichostatin A (TSA) attenuated H/R-elicited injury, as indicated by a reduction of lactate dehydrogenase (LDH) leakage, an increase in cell viability, and decrease in apoptotic positive cardiomyocytes. MG132, a potent proteasome pathway inhibitor, abrogated TSA-induced protective effects, which was associated with the accumulation of ubiquitinated HDAC4. NMVM transduced with adenoviral HDAC4 led to an exaggeration of H/R-induced injury. TSA treatment resulted in a decrease in HDAC4 in cardiomyocytes infected with adenoviral HDAC4, and HDAC4-induced injury was attenuated by TSA. HDAC inhibition resulted in a significant reduction in reactive oxygen species (ROS) in cardiomyoblasts exposed to H/R, which was attenuated by blockade of the proteasome pathway. Cardiomyoblasts carrying wild type and sumoylation mutation (K559R) were established to examine effects of HDAC4 sumoylation and ubiquitination on H/R injury. Disruption of HDAC4 sumoylation brought about HDAC4 accumulation and impairment of HDAC4 ubiquitination in association with enhanced susceptibility of cardiomyoblasts to H/R. Taken together, these results demonstrated that HDAC inhibition stimulates proteasome dependent degradation of HDAC4, which is associated with HDAC4 sumoylation to induce these protective effects.

Trimetazidine protects cardiomyocytes against hypoxia-induced injury through ameliorates calcium homeostasis

Intracellular calcium (Ca(2+)i) overload induced by chronic hypoxia alters Ca(2+)i homeostasis, which plays an important role on mediating myocardial injury. We tested the hypothesis that treatment with trimetazidine (TMZ) would improve Ca(2+)i handling in hypoxic myocardial injury. Cardiomyocytes isolated from neonatal Sprague-Dawley rats were exposed to chronic hypoxia (1% O2, 5% CO2, 37 °C). Intracellular calcium concentration ([Ca(2+)]i) was measured with Fura-2/AM. Perfusion of cardiomyocytes with a high concentration of caffeine (10 mM) was carried out to verify the function of the cardiac Na(+)/Ca(2+) exchanger (NCX) and the activity of sarco(endo)-plasmic reticulum Ca(2+)-ATPase (SERCA2a). For TMZ-treated cardiomyocytes exposured in hypoxia, we observed a decrease in mRNA expression of proapoptotic Bax, caspase-3 activation and enhanced expression of anti-apoptotic Bcl-2. The cardiomyocyte hypertrophy were also alleviated in hypoxic cardiomyocyte treated with TMZ. Moreover, we found that TMZ treatment cardiomyocytes enhanced "metabolic shift" from lipid oxidation to glucose oxidation. Compared with hypoxic cardiomyocyte, the diastolic [Ca(2+)]i was decreased, the amplitude of Ca(2+)i oscillations and sarcoplasmic reticulum Ca(2+) load were recovered, the activities of ryanodine receptor 2 (RyR2), NCX and SERCA2a were increased in cardiomyocytes treated with TMZ. TMZ attenuated abnormal changes of RyR2 and SERCA2a genes in hypoxic cardiomyocytes. In addition, cholinergic signaling are involved in hypoxic stress and the cardioprotective effects of TMZ. These results suggest that TMZ ameliorates Ca(2+)i homeostasis through switch of lipid to glucose metabolism, thereby producing the cardioprotective effect and reduction in hypoxic cardiomyocytes damage.

Newborn hypoxia/anoxia inhibits cardiomyocyte proliferation and decreases cardiomyocyte endowment in the developing heart: role of endothelin-1

In the developing heart, cardiomyocytes undergo terminal differentiation during a critical window around birth. Hypoxia is a major stress to preterm infants, yet its effect on the development and maturation of the heart remains unknown. We tested the hypothesis in a rat model that newborn anoxia accelerates cardiomyocyte terminal differentiation and results in reduced cardiomyocyte endowment in the developing heart via an endothelin-1-dependent mechanism. Newborn rats were exposed to anoxia twice daily from postnatal day 1 to 3, and hearts were isolated and studied at postnatal day 4 (P4), 7 (P7), and 14 (P14). Anoxia significantly increased HIF-1α protein expression and pre-proET-1 mRNA abundance in P4 neonatal hearts. Cardiomyocyte proliferation was significantly decreased by anoxia in P4 and P7, resulting in a significant reduction of cardiomyocyte number per heart weight in the P14 neonates. Furthermore, the expression of cyclin D2 was significantly decreased due to anoxia, while p27 expression was increased. Anoxia has no significant effect on cardiomyocyte binucleation or myocyte size. Consistently, prenatal hypoxia significantly decreased cardiomyocyte proliferation but had no effect on binucleation in the fetal heart. Newborn administration of PD156707, an ET_A-receptor antagonist, significantly increased cardiomyocyte proliferation at P4 and cell size at P7, resulting in an increase in the heart to body weight ratio in P7 neonates. In addition, PD156707 abrogated the anoxia-mediated effects. The results suggest that hypoxia and anoxia via activation of endothelin-1 at the critical window of heart development inhibits cardiomyocyte proliferation and decreases myocyte endowment in the developing heart, which may negatively impact cardiac function later in life.

Overexpression of CyclinA2 ameliorates hypoxia-impaired proliferation of cardiomyocytes

Hypoxia is a primary mediator for cell survival, and has been reported to inhibit cardiomyocyte proliferation in fetal and neonatal hearts. CyclinA2 is a key regulator of cell proliferation. Whether CyclinA2 affects cardiomyocyte proliferation in hypoxic conditions remains unexamined. This study was designed to investigate the roles of CyclinA2 expression on hypoxia-impaired cardiomyocyte proliferation. Cardiomyocytes were isolated from neonatal rats and randomly separated into six groups: Control, hypoxia, enhanced green fluorescent protein (EGFP)-Adv, EGFP-Ccna2, EGFP-Adv + hypoxia and EGFP-Ccna2 + hypoxia. The cells in the control group were cultured in a general cell incubator; the cells in the hypoxia group were placed in a hypoxic chamber for 12 h; the cells in the EGFP-Adv and EGFP-Ccna2 groups were separately transfected with EGFP-adenovirus capsids or EGFP-adenovirus capsids with CyclinA2 cDNA for 18 h, and then placed in a general incubator for an additional 12 h; the cells in the EGFP-Adv + hypoxia and EGFP-Ccna2 + hypoxia groups were separately transfected with EGFP-adenovirus capsids or EGFP-adenovirus capsids with CyclinA2 cDNA for 18 h, and then placed in a hypoxia chamber for an additional 12 h. CyclinA2 expression was measured using immunochemical staining and western blot analysis, and cardiomyocyte proliferation was measured using the cell counting kit 8. GFP fluorescence indicated a high transfection efficiency (>80%), and immunochemical staining showed that CyclinA2 was mainly distributed in the nucleus. CyclinA2 expression was downregulated following exposure to hypoxia for 12 h. Cardiomyocyte proliferation was also significantly decreased following exposure to hypoxia for 12 h. However, compared with the EGFP-Adv group, CyclinA2 expression and cardiomyocyte proliferation was markedly increased in the EGFP-Ccna2 group. Furthermore, compared with the EGFP-Adv + hypoxia group, CyclinA2 expression and cell proliferation were markedly increased in the EGFP-Ccna2 + hypoxia group. These findings indicate that CyclinA2 upregulation improves cardiomyocyte proliferation in hypoxic conditions.

Myocardial recovery from ischemia-reperfusion is compromised in the absence of tissue inhibitor of metalloproteinase 4

BACKGROUND

Myocardial reperfusion after ischemia (I/R), although an effective approach in rescuing the ischemic myocardium, can itself trigger several adverse effects including aberrant remodeling of the myocardium and its extracellular matrix. Tissue inhibitor of metalloproteinases (TIMPs) protect the extracellular matrix against excess degradation by matrix metalloproteinases (MMPs). TIMP4 levels are reduced in myocardial infarction; however, its causal role in progression of post-I/R injury has not been explored.

METHODS AND RESULTS

In vivo I/R (20-minute ischemia, 1-week reperfusion) resulted in more severe systolic and diastolic dysfunction in TIMP4(-/-) mice with enhanced inflammation, oxidative stress (1 day post-I/R), hypertrophy, and interstitial fibrosis (1 week). After an initial increase in TIMP4 (1 day post-I/R), TIMP4 mRNA and protein decreased in the ischemic myocardium from wild-type mice by 1 week post-I/R and in tissue samples from patients with myocardial infarction, which correlated with enhanced activity of membrane-bound MMP, membrane-type 1 MMP. By 4 weeks post-I/R, wild-type mice showed no cardiac dysfunction, elevated TIMP4 levels (to baseline), and normalized membrane-type 1 MMP activity. TIMP4-deficient mice, however, showed exacerbated diastolic dysfunction, sustained elevation of membrane-type 1 MMP activity, and worsened myocardial hypertrophy and fibrosis. Ex vivo I/R (20- or 30-minute ischemia, 45-minute reperfusion) resulted in comparable cardiac dysfunction in wild-type and TIMP4(-/-) mice.

CONCLUSIONS

TIMP4 is essential for recovery from myocardial I/R in vivo, primarily because of its membrane-type 1 MMP inhibitory function. TIMP4 deficiency does not increase susceptibility to ex vivo I/R injury. Replenishment of myocardial TIMP4 could serve as an effective therapy in post-I/R recovery for patients with reduced TIMP4.

Endothelin-1 promotes cardiomyocyte terminal differentiation in the developing heart via heightened DNA methylation

AIMS

Hypoxia is a major stress on fetal development and leads to induction of endothelin-1 (ET-1) expression. We tested the hypothesis that ET-1 stimulates the terminal differentiation of cardiomyocytes from mononucleate to binucleate in the developing heart.

METHODS AND RESULTS

Hypoxia (10.5% O2) treatment of pregnant rats from day 15 to day 21 resulted in a significant increase in prepro-ET-1 mRNA expression in fetal hearts. ET-1 ex vivo treatment of fetal rat cardiomyocytes increased percent binucleate cells and decreased Ki-67 expression, a marker for proliferation, under both control and hypoxic conditions. Hypoxia alone decreased Ki-67 expression and in conjunction with ET-1 treatment decreased cardiomyocyte size. PD145065, a non-selective ET-receptor antagonist, blocked the changes in binucleation and proliferation caused by ET-1. DNA methylation in fetal cardiomyocytes was significantly increased with ET-1 treatment, which was blocked by 5-aza-2'-deoxycytidine, a DNA methylation inhibitor. In addition, 5-aza-2'-deoxycytidine treatment abrogated the increase in binucleation and decrease in proliferation induced by ET-1.

CONCLUSIONS

Hypoxic stress and synthesis of ET-1 increases DNA methylation and promotes terminal differentiation of cardiomyocytes in the developing heart. This premature exit of the cell cycle may lead to a reduced cardiomyocyte endowment in the heart and have a negative impact on cardiac function.

Binucleation of cardiomyocytes: the transition from a proliferative to a terminally differentiated state

Cardiomyocytes possess a unique ability to transition from mononucleate to the mature binucleate phenotype in late fetal development and around birth. Mononucleate cells are proliferative, whereas binucleate cells exit the cell cycle and no longer proliferate. This crucial period of terminal differentiation dictates cardiomyocyte endowment for life. Adverse early life events can influence development of the heart, affecting cardiomyocyte number and contributing to heart disease late in life. Although much is still unknown about the mechanisms underlying the binucleation process, many studies are focused on molecules involved in cell cycle regulation and cytokinesis as well as epigenetic modifications that can occur during this transition. Better understanding of these mechanisms could provide a basis for recovering the proliferative capacity of cardiomyocytes.