A growth factor for cardiac myocytes is produced by cardiac nonmyocytes

FNDC5/irisin mitigates the cardiotoxic impacts of cancer chemotherapeutics by modulating ROS-dependent and -independent mechanisms

Cardiotoxicity remains a major limiting factor in the clinical implementation of anthracycline chemotherapy. Though the etiology of doxorubicin-dependent heart damage has yet to be fully elucidated, the ability of doxorubicin to damage DNA and trigger oxidative stress have been heavily implicated in the pathogenesis of chemotherapy-associated cardiomyopathy. Here, we demonstrate that fibronectin type III domain-containing protein 5 (FNDC5), the precursor protein for myokine irisin, is depleted in the hearts of human cancer patients or mice exposed to chemotherapeutics. In cardiomyocytes, restoration of FNDC5 expression was sufficient to mitigate reactive oxygen species (ROS) accumulation and apoptosis following doxorubicin exposure, effects dependent on the irisin encoding domain of FNDC5 as well as signaling via the putative irisin integrin receptor. Intriguingly, we identified two parallel signaling cascades impacted by FNDC5 in cardiomyocytes: the ROS-driven intrinsic mitochondrial apoptosis pathway and the ROS-independent Ataxia Telangiectasia and Rad3-Related Protein (ATR)/Checkpoint Kinase 1 (Chk1) pathway. In fact, FNDC5 forms a co-precipitable complex with Chk1 alluding to possible intracellular actions for this canonically membrane-associated protein. Whereas FNDC5 overexpression in murine heart was cardioprotective, introduction of FNDC5-targeted shRNA into the myocardium was sufficient to trigger Bax up-regulation, ATR/Chk1 activation, oxidative stress, cardiac fibrosis, loss of ventricular function, and compromised animal survival. The detrimental impact of FNDC5 depletion on heart function could be mitigated via treatment with a Chk1 inhibitor identifying Chk1 hyperactivity as a causative factor in cardiac disease. Though our data point to the potential clinical utility of FNDC5/irisin-targeted agents in the treatment of chemotherapy-induced cardiotoxicity, we also found significant down regulation in FNDC5 expression in the hearts of aged mice that attenuated the cardioprotective impacts of FNDC5 overexpression following doxorubicin exposure. Together our data underscore the importance of FNDC5/irisin in maintenance of cardiac health over the lifespan.

Cardiomyocyte crosstalk with endothelium modulates cardiac structure, function, and ischemia-reperfusion injury susceptibility through erythropoietin

Erythropoietin (EPO) exerts non-canonical roles beyond erythropoiesis that are developmentally, structurally, and physiologically relevant for the heart as a paracrine factor. The role for paracrine EPO signalling and cellular crosstalk in the adult is uncertain. Here, we provided novel evidence showing cardiomyocyte restricted loss of function in Epo in adult mice induced hyper-compensatory increases in Epo expression by adjacent cardiac endothelial cells via HIF-2α independent mechanisms. These hearts showed concentric cellular hypertrophy, elevated contractility and relaxation, and greater resistance to ischemia-reperfusion injury. Voluntary exercise capacity compared to control hearts was improved independent of any changes to whole-body metabolism or blood O2 content or delivery (i.e., hematocrit). Our findings suggest cardiac EPO had a localized effect within the normoxic heart, which was regulated by cell-specific EPO-reciprocity between cardiomyocytes and endothelium. Within the heart, hyper-compensated endothelial Epo expression was accompanied by elevated Vegfr1 and Vegfb RNA, that upon pharmacological pan-inhibition of VEGF-VEGFR signaling, resulted in a paradoxical upregulation in whole-heart Epo. Thus, we provide the first evidence that a novel EPO-EPOR/VEGF-VEGFR axis exists to carefully mediate cardiac homeostasis via cardiomyocyte-endothelial EPO crosstalk.

RGS6 drives cardiomyocyte death following nucleolar stress by suppressing Nucleolin/miRNA-21

BACKGROUND

Prior evidence demonstrated that Regulator of G protein Signaling 6 (RGS6) translocates to the nucleolus in response to cytotoxic stress though the functional significance of this phenomenon remains unknown.

METHODS

Utilizing in vivo gene manipulations in mice, primary murine cardiac cells, human cell lines and human patient samples we dissect the participation of a RGS6-nucleolin complex in chemotherapy-dependent cardiotoxicity.

RESULTS

Here we demonstrate that RGS6 binds to a key nucleolar protein, Nucleolin, and controls its expression and activity in cardiomyocytes. In the human myocyte AC-16 cell line, induced pluripotent stem cell derived cardiomyocytes, primary murine cardiomyocytes, and the intact murine myocardium tuning RGS6 levels via overexpression or knockdown resulted in diametrically opposed impacts on Nucleolin mRNA, protein, and phosphorylation.RGS6 depletion provided marked protection against nucleolar stress-mediated cell death in vitro, and, conversely, RGS6 overexpression suppressed ribosomal RNA production, a key output of the nucleolus, and triggered death of myocytes. Importantly, overexpression of either Nucleolin or Nucleolin effector miRNA-21 counteracted the pro-apoptotic effects of RGS6. In both human and murine heart tissue, exposure to the genotoxic stressor doxorubicin was associated with an increase in the ratio of RGS6/Nucleolin. Preventing RGS6 induction via introduction of RGS6-directed shRNA via intracardiac injection proved cardioprotective in mice and was accompanied by restored Nucleolin/miRNA-21 expression, decreased nucleolar stress, and decreased expression of pro-apoptotic, hypertrophy, and oxidative stress markers in heart.

CONCLUSION

Together, these data implicate RGS6 as a driver of nucleolar stress-dependent cell death in cardiomyocytes via its ability to modulate Nucleolin. This work represents the first demonstration of a functional role for an RGS protein in the nucleolus and identifies the RGS6/Nucleolin interaction as a possible new therapeutic target in the prevention of cardiotoxicity.

RGS7 balances acetylation/de-acetylation of p65 to control chemotherapy-dependent cardiac inflammation

Cardiotoxicity remains a major limitation in the clinical utility of anthracycline chemotherapeutics. Regulator of G-protein Signaling 7 (RGS7) and inflammatory markers are up-regulated in the hearts of patients with a history of chemotherapy particularly those with reduced left-ventricular function. RGS7 knockdown in either the murine myocardium or isolated murine ventricular cardiac myocytes (VCM) or cultured human VCM provided marked protection against doxorubicin-dependent oxidative stress, NF-κB activation, inflammatory cytokine production, and cell death. In exploring possible mechanisms causally linking RGS7 to pro-inflammatory signaling cascades, we found that RGS7 forms a complex with acetylase Tip60 and deacetylase sirtuin 1 (SIRT1) and controls the acetylation status of the p65 subunit of NF-κB. In VCM, the detrimental impact of RGS7 could be mitigated by inhibiting Tip60 or activating SIRT1, indicating that the ability of RGS7 to modulate cellular acetylation capacity is critical for its pro-inflammatory actions. Further, RGS7-driven, Tip60/SIRT1-dependent cytokines released from ventricular cardiac myocytes and transplanted onto cardiac fibroblasts increased oxidative stress, markers of transdifferentiation, and activity of extracellular matrix remodelers emphasizing the importance of the RGS7-Tip60-SIRT1 complex in paracrine signaling in the myocardium. Importantly, while RGS7 overexpression in heart resulted in sterile inflammation, fibrotic remodeling, and compromised left-ventricular function, activation of SIRT1 counteracted the detrimental impact of RGS7 in heart confirming that RGS7 increases acetylation of SIRT1 substrates and thereby drives cardiac dysfunction. Together, our data identify RGS7 as an amplifier of inflammatory signaling in heart and possible therapeutic target in chemotherapeutic drug-induced cardiotoxicity.

A RGS7-CaMKII complex drives myocyte-intrinsic and myocyte-extrinsic mechanisms of chemotherapy-induced cardiotoxicity

Dose-limiting cardiotoxicity remains a major limitation in the clinical use of cancer chemotherapeutics. Here, we describe a role for Regulator of G protein Signaling 7 (RGS7) in chemotherapy-dependent heart damage, the demonstration for a functional role of RGS7 outside of the nervous system and retina. Though expressed at low levels basally, we observed robust up-regulation of RGS7 in the human and murine myocardium following chemotherapy exposure. In ventricular cardiomyocytes (VCM), RGS7 forms a complex with Ca²⁺/calmodulin-dependent protein kinase (CaMKII) supported by key residues (K412 and P391) in the RGS domain of RGS7. In VCM treated with chemotherapeutic drugs, RGS7 facilitates CaMKII oxidation and phosphorylation and CaMKII-dependent oxidative stress, mitochondrial dysfunction, and apoptosis. Cardiac-specific RGS7 knockdown protected the heart against chemotherapy-dependent oxidative stress, fibrosis, and myocyte loss and improved left ventricular function in mice treated with doxorubicin. Conversely, RGS7 overexpression induced fibrosis, reactive oxygen species generation, and cell death in the murine myocardium that were mitigated following CaMKII inhibition. RGS7 also drives production and release of the cardiokine neuregulin-1, which facilitates paracrine communication between VCM and neighboring vascular endothelial cells (EC), a maladaptive mechanism contributing to VCM dysfunction in the failing heart. Importantly, while RGS7 was both necessary and sufficient to facilitate chemotherapy-dependent cytotoxicity in VCM, RGS7 is dispensable for the cancer-killing actions of these same drugs. These selective myocyte-intrinsic and myocyte-extrinsic actions of RGS7 in heart identify RGS7 as an attractive therapeutic target in the mitigation of chemotherapy-driven cardiotoxicity.

RGS11-CaMKII complex mediated redox control attenuates chemotherapy-induced cardiac fibrosis

Dose limiting cardiotoxicity remains a major limiting factor in the clinical use of several cancer chemotherapeutics including anthracyclines and the antimetabolite 5-fluorouracil (5-FU). Prior work has demonstrated that chemotherapeutics increase expression of R7 family regulator of G protein signaling (RGS) protein-binding partner Gβ5, which drives myocyte cytotoxicity. However, though several R7 family members are expressed in heart, the exact role of each protein in chemotherapy driven heart damage remains unclear. Here, we demonstrate that RGS11, downregulated in the human heart following chemotherapy exposure, possesses potent anti-apoptotic actions, in direct opposition to the actions of fellow R7 family member RGS6. RGS11 forms a direct complex with the apoptotic kinase CaMKII and stress responsive transcription factor ATF3 and acts to counterbalance the ability of CaMKII and ATF3 to trigger oxidative stress, mitochondrial dysfunction, cell death, and release of the cardiokine neuregulin-1 (NRG1), which mediates pathological intercommunication between myocytes and endothelial cells. Doxorubicin triggers RGS11 depletion in the murine myocardium, and cardiac-specific OE of RGS11 decreases doxorubicin-induced fibrosis, myocyte hypertrophy, apoptosis, oxidative stress, and cell loss and aids in the maintenance of left ventricular function. Conversely, RGS11 knockdown in heart promotes cardiac fibrosis associated with CaMKII activation and ATF3/NRG1 induction. Indeed, inhibition of CaMKII largely prevents the fibrotic remodeling resulting from cardiac RGS11 depletion underscoring the functional importance of the RGS11-CaMKII interaction in the pathogenesis of cardiac fibrosis. These data describe an entirely new role for RGS11 in heart and identify RGS11 as a potential new target for amelioration of chemotherapy-induced cardiotoxicity.

Exosomal lncRNA AK139128 Derived from Hypoxic Cardiomyocytes Promotes Apoptosis and Inhibits Cell Proliferation in Cardiac Fibroblasts

Introduction

Myocardial infarction (MI) is the leading cause of congestive heart failure and mortality. Hypoxia is an important trigger in the cardiac remodeling of the myocardium in the development and progression of cardiac diseases.

Objective

Thus, we aimed to investigate the effect of hypoxia-induced exosomes on cardiac fibroblasts (CFs) and its related mechanisms.

Materials and Methods

In this study, we successfully isolated and identified the exosomes from hypoxic cardiomyocytes (CMs). Exosomes derived from hypoxic CMs promoted apoptosis and inhibited proliferation, migration, and invasion in CFs. RNA-Seq assay suggested that long noncoding RNA AK139128 (lncRNA AK139128) was found to overexpress in both hypoxic CMs and CMs-secreting exosomes. After coculturing with CFs, hypoxic exosomes increased the expression of AK139128 in recipient CFs. Moreover, exosomal AK139128 derived from hypoxic CMs stimulated CFs apoptosis and inhibited proliferation, migration, and invasion. Furthermore, the effect of exosomal AK139128 derived from hypoxic CMs could also exacerbate MI in the rat model.

Conclusion

Taken together, hypoxia upregulated the level of AK139128 in CMs and exosomes and exosomal AK139128 derived from hypoxic CMs modulated cellular activities of CFs in vitro and in vivo. This study provides a new understanding of the mechanism underlying hypoxia-related cardiac diseases and insight into developing new therapeutic strategies.

Physiologic, Pathologic, and Therapeutic Paracrine Modulation of Cardiac Excitation-Contraction Coupling

Cardiac excitation-contraction coupling (ECC) is the orchestrated process of initial myocyte electrical excitation, which leads to calcium entry, intracellular trafficking, and subsequent sarcomere shortening and myofibrillar contraction. Neurohumoral β-adrenergic signaling is a well-established mediator of ECC; other signaling mechanisms, such as paracrine signaling, have also demonstrated significant impact on ECC but are less well understood. For example, resident heart endothelial cells are well-known physiological paracrine modulators of cardiac myocyte ECC mainly via NO and endothelin-1. Moreover, recent studies have demonstrated other resident noncardiomyocyte heart cells (eg, physiological fibroblasts and pathological myofibroblasts), and even experimental cardiotherapeutic cells (eg, mesenchymal stem cells) are also capable of altering cardiomyocyte ECC through paracrine mechanisms. In this review, we first focus on the paracrine-mediated effects of resident and therapeutic noncardiomyocytes on cardiomyocyte hypertrophy, electrophysiology, and calcium handling, each of which can modulate ECC, and then discuss the current knowledge about key paracrine factors and their underlying mechanisms of action. Next, we provide a case example demonstrating the promise of tissue-engineering approaches to study paracrine effects on tissue-level contractility. More specifically, we present new functional and molecular data on the effects of human adult cardiac fibroblast conditioned media on human engineered cardiac tissue contractility and ion channel gene expression that generally agrees with previous murine studies but also suggests possible species-specific differences. By contrast, paracrine secretions by human dermal fibroblasts had no discernible effect on human engineered cardiac tissue contractile function and gene expression. Finally, we discuss systems biology approaches to help identify key stem cell paracrine mediators of ECC and their associated mechanistic pathways. Such integration of tissue-engineering and systems biology methods shows promise to reveal novel insights into paracrine mediators of ECC and their underlying mechanisms of action, ultimately leading to improved cell-based therapies for patients with heart disease.

Fish, the better model in human heart research? Zebrafish Heart aggregates as a 3D spontaneously cardiomyogenic in vitro model system

The zebrafish (ZF) has become an essential model for biomedical, pharmacological and eco-toxicological heart research. Despite the anatomical differences between fish and human hearts, similarities in cellular structure and conservation of genes as well as pathways across vertebrates have led to an increase in the popularity of ZF as a model for human cardiac research. ZF research benefits from an entirely sequenced genome, which allows us to establish and study cardiovascular mutants to better understand cardiovascular diseases. In this review, we will discuss the importance of in vitro model systems for cardiac research and summarise results of in vitro 3D heart-like cell aggregates, consisting of myocardial tissue formed spontaneously from enzymatically digested whole embryonic ZF larvae (Zebrafish Heart Aggregate - ZFHA). We will give an overview of the similarities and differences of ZF versus human hearts and highlight why ZF complement established mammalian models (i.e. murine and large animal models) for cardiac research. At this stage, the ZFHA model system is being refined into a high-throughput (more ZFHA generated than larvae prepared) and stable in vitro test system to accomplish the same longevity of previously successful salmonid models. ZFHA have potential for the use of high-throughput-screenings of different factors like small molecules, nucleic acids, proteins and lipids which is difficult to achieve in the zebrafish in vivo screening models with lethal mutations as well as to explore ion channel disorders and to find appropriate drugs for safety screening.

Gq-activated fibroblasts induce cardiomyocyte action potential prolongation and automaticity in a three-dimensional microtissue environment

Cardiac fibroblasts (CFs) are known to regulate cardiomyocyte (CM) function in vivo and in two-dimensional in vitro cultures. This study examined the effect of CF activation on the regulation of CM electrical activity in a three-dimensional (3-D) microtissue environment. Using a scaffold-free 3-D platform with interspersed neonatal rat ventricular CMs and CFs, G_q-mediated signaling was selectively enhanced in CFs by Gα_q adenoviral infection before coseeding with CMs in nonadhesive hydrogels. After 3 days, the microtissues were analyzed by signaling assay, histological staining, quantitative PCR, Western blots, optical mapping with voltage- or Ca²⁺-sensitive dyes, and microelectrode recordings of CF resting membrane potential (RMP_CF). Enhanced G_q signaling in CFs increased microtissue size and profibrotic and prohypertrophic markers. Expression of constitutively active Gα_q in CFs prolonged CM action potential duration (by 33%) and rise time (by 31%), prolonged Ca²⁺ transient duration (by 98%) and rise time (by 65%), and caused abnormal electrical activity based on depolarization-induced automaticity. Constitutive G_q activation in CFs also depolarized RMP_CF from -33 to -20 mV and increased connexin 43 and connexin 45 expression. Computational modeling confers that elevated RMP_CF and increased cell-cell coupling between CMs and CFs in a 3-D environment could lead to automaticity. In conclusion, our data demonstrate that CF activation alone is capable of altering action potential and Ca²⁺ transient characteristics of CMs, leading to proarrhythmic electrical activity. Our results also emphasize the importance of a 3-D environment where cell-cell interactions are prevalent, underscoring that CF activation in 3-D tissue plays a significant role in modulating CM electrophysiology and arrhythmias.NEW & NOTEWORTHY In a three-dimensional microtissue model, which lowers baseline activation of cardiac fibroblasts but enables cell-cell, paracrine, and cell-extracellular matrix interactions, we demonstrate that selective cardiac fibroblast activation by enhanced G_q signaling, a pathophysiological trigger in the diseased heart, modulates cardiomyocyte electrical activity, leading to proarrhythmogenic automaticity.

Discovery of novel small molecule inhibitors of cardiac hypertrophy using high throughput, high content imaging

Chronic cardiac hypertrophy is maladaptive and contributes to the pathogenesis of heart failure. The objective of this study was to identify small molecule inhibitors of pathological cardiomyocyte hypertrophy. High content screening was performed with primary neonatal rat ventricular myocytes (NRVMs) cultured on 96-well plates and treated with a library of 3241 distinct small molecules. Non-toxic hit compounds that blocked hypertrophy in response to phenylephrine (PE) and phorbol myristate acetate (PMA) were identified based on their ability to reduce cell size and inhibit expression of atrial natriuretic factor (ANF), which is a biomarker of pathological cardiac hypertrophy. Many of the hit compounds are existing drugs that have not previously been evaluated for benefit in the setting of cardiovascular disease. One such compound, the anti-malarial drug artesunate, blocked left ventricular hypertrophy (LVH) and improved cardiac function in adult mice subjected to transverse aortic constriction (TAC). These findings demonstrate that phenotypic screening with primary cardiomyocytes can be used to discover anti-hypertrophic lead compounds for heart failure drug discovery. Using annotated libraries of compounds with known selectivity profiles, this screening methodology also facilitates chemical biological dissection of signaling networks that control pathological growth of the heart.

Novel therapeutic strategies targeting fibroblasts and fibrosis in heart disease

Our understanding of the functions of cardiac fibroblasts has moved beyond their roles in heart structure and extracellular matrix generation and now includes their contributions to paracrine, mechanical and electrical signalling during ontogenesis and normal cardiac activity. Fibroblasts also have central roles in pathogenic remodelling during myocardial ischaemia, hypertension and heart failure. As key contributors to scar formation, they are crucial for tissue repair after interventions including surgery and ablation. Novel experimental approaches targeting cardiac fibroblasts are promising potential therapies for heart disease. Indeed, several existing drugs act, at least partially, through effects on cardiac connective tissue. This Review outlines the origins and roles of fibroblasts in cardiac development, homeostasis and disease; illustrates the involvement of fibroblasts in current and emerging clinical interventions; and identifies future targets for research and development.

Oxygen cycling to improve survival of stem cells for myocardial repair: A review

Heart disease represents the leading cause of death among Americans. There is currently no clinical treatment to regenerate viable myocardium following myocardial infarction, and patients may suffer progressive deterioration and decreased myocardial function from the effects of remodeling of the necrotic myocardium. New therapeutic strategies hold promise for patients who suffer from ischemic heart disease by directly addressing the restoration of functional myocardium following death of cardiomyocytes. Therapeutic stem cell transplantation has shown modest benefit in clinical human trials with decreased fibrosis and increased functional myocardium. Moreover, autologous transplantation holds the potential to implement these therapies while avoiding the immunomodulation concerns of heart transplantation. Despite these benefits, stem cell therapy has been characterized by poor survival and low engraftment of injected stem cells. The hypoxic tissue environment of the ischemic/infracting myocardium impedes stem cell survival and engraftment in myocardial tissue. Hypoxic preconditioning has been suggested as a viable strategy to increase hypoxic tolerance of stem cells. A number of in vivo and in vitro studies have demonstrated improved stem cell viability by altering stem cell secretion of protein signals and up-regulation of numerous paracrine signaling pathways that affect inflammatory, survival, and angiogenic signaling pathways. This review will discuss both the mechanisms of hypoxic preconditioning as well as the effects of hypoxic preconditioning in different cell and animal models, examining the pitfalls in current research and the next steps into potentially implementing this methodology in clinical research trials.

Cardiac fibroblasts form and function

The formation and structure of the extracellular matrix (ECM) that makes up the cardiac interstitum is well known yet the underlying mechanisms that regulate the interstitum are poorly known. This review focuses on the role of the cardiac fibroblast in the formation and regulation of the ECM components during cardiac development and in response to physiological and pathological stimulation. The role of ECM receptors (integrins), cellular phenotype, and chemical and mechanical signaling by cardiac fibroblasts are discussed.

Alpha-1-adrenergic receptors in heart failure: the adaptive arm of the cardiac response to chronic catecholamine stimulation

Alpha-1-adrenergic receptors (ARs) are G protein-coupled receptors activated by catecholamines. The alpha-1A and alpha-1B subtypes are expressed in mouse and human myocardium, whereas the alpha-1D protein is found only in coronary arteries. There are far fewer alpha-1-ARs than beta-ARs in the nonfailing heart, but their abundance is maintained or increased in the setting of heart failure, which is characterized by pronounced chronic elevation of catecholamines and beta-AR dysfunction. Decades of evidence from gain and loss-of-function studies in isolated cardiac myocytes and numerous animal models demonstrate important adaptive functions for cardiac alpha-1-ARs to include physiological hypertrophy, positive inotropy, ischemic preconditioning, and protection from cell death. Clinical trial data indicate that blocking alpha-1-ARs is associated with incident heart failure in patients with hypertension. Collectively, these findings suggest that alpha-1-AR activation might mitigate the well-recognized toxic effects of beta-ARs in the hyperadrenergic setting of chronic heart failure. Thus, exogenous cardioselective activation of alpha-1-ARs might represent a novel and viable approach to the treatment of heart failure.

Effects of mechanical stimulation induced by compression and medium perfusion on cardiac tissue engineering

Cardiac tissue engineering presents a challenge due to the complexity of the muscle tissue and the need for multiple signals to induce tissue regeneration in vitro. We investigated the effects of compression (1 Hz, 15% strain) combined with fluid shear stress (10(-2) -10(-1) dynes/cm(2) ) provided by medium perfusion on the outcome of cardiac tissue engineering. Neonatal rat cardiac cells were seeded in Arginine-Glycine-Aspartate (RGD)-attached alginate scaffolds, and the constructs were cultivated in a compression bioreactor. A daily, short-term (30 min) compression (i.e., "intermittent compression") for 4 days induced the formation of cardiac tissue with typical striation, while in the continuously compressed constructs (i.e., "continuous compression"), the cells remained spherical. By Western blot, on day 4 the expression of the gap junction protein connexin 43 was significantly greater in the "intermittent compression" constructs and the cardiomyocyte markers (α-actinin and N-cadherin) showed a trend of better preservation compared to the noncompressed constructs. This regime of compression had no effect on the proliferation of nonmyocyte cells, which maintained low expression level of proliferating cell nuclear antigen. Elevated secretion levels of basic fibroblast growth factor and transforming growth factor-β in the daily, intermittently compressed constructs likely attributed to tissue formation. Our study thus establishes the formation of an improved cardiac tissue in vitro, when induced by combined mechanical signals of compression and fluid shear stress provided by perfusion.

Automated image analysis identifies signaling pathways regulating distinct signatures of cardiac myocyte hypertrophy

Cardiac hypertrophy is controlled by a complex signal transduction and gene regulatory network, containing multiple layers of crosstalk and feedback. While numerous individual components of this network have been identified, understanding how these elements are coordinated to regulate heart growth remains a challenge. Past approaches to measure cardiac myocyte hypertrophy have been manual and often qualitative, hindering the ability to systematically characterize the network's higher-order control structure and identify therapeutic targets. Here, we develop and validate an automated image analysis approach for objectively quantifying multiple hypertrophic phenotypes from immunofluorescence images. This approach incorporates cardiac myocyte-specific optimizations and provides quantitative measures of myocyte size, elongation, circularity, sarcomeric organization, and cell-cell contact. As a proof-of-concept, we examined the hypertrophic response to α-adrenergic, β-adrenergic, tumor necrosis factor (TNFα), insulin-like growth factor-1 (IGF-1), and fetal bovine serum pathways. While all five hypertrophic pathways increased myocyte size, other hypertrophic metrics were differentially regulated, forming a distinct phenotype signature for each pathway. Sarcomeric organization was uniquely enhanced by α-adrenergic signaling. TNFα and α-adrenergic pathways markedly decreased cell circularity due to increased myocyte protrusion. Surprisingly, adrenergic and IGF-1 pathways differentially regulated myocyte-myocyte contact, potentially forming a feed-forward loop that regulates hypertrophy. Automated image analysis unlocks a range of new quantitative phenotypic data, aiding dissection of the complex hypertrophic signaling network and enabling myocyte-based high-content drug screening.

Biomaterial strategies for alleviation of myocardial infarction

World Health Organization estimated that heart failure initiated by coronary artery disease and myocardial infarction (MI) leads to 29 per cent of deaths worldwide. Heart failure is one of the leading causes of death in industrialized countries and is expected to become a global epidemic within the twenty-first century. MI, the main cause of heart failure, leads to a loss of cardiac tissue impairment of left ventricular function. The damaged left ventricle undergoes progressive ‘remodelling’ and chamber dilation, with myocyte slippage and fibroblast proliferation. Repair of diseased myocardium with in vitro-engineered cardiac muscle patch/injectable biopolymers with cells may become a viable option for heart failure patients. These events reflect an apparent lack of effective intrinsic mechanism for myocardial repair and regeneration. Motivated by the desire to develop minimally invasive procedures, the last 10 years observed growing efforts to develop injectable biomaterials with and without cells to treat cardiac failure. Biomaterials evaluated include alginate, fibrin, collagen, chitosan, self-assembling peptides, biopolymers and a range of synthetic hydrogels. The ultimate goal in therapeutic cardiac tissue engineering is to generate biocompatible, non-immunogenic heart muscle with morphological and functional properties similar to natural myocardium to repair MI. This review summarizes the properties of biomaterial substrates having sufficient mechanical stability, which stimulates the native collagen fibril structure for differentiating pluripotent stem cells and mesenchymal stem cells into cardiomyocytes for cardiac tissue engineering.

S100B-RAGE dependent VEGF secretion by cardiac myocytes induces myofibroblast proliferation

Post-infarct remodeling is associated with the upregulation of the receptor for advanced glycation end products (RAGE), the induction of its ligand the calcium binding protein S100B and the release of the potent endothelial-cell specific mitogen vascular endothelial growth factor (VEGF). To determine a possible functional interaction between S100B, RAGE and VEGF we stimulated rat neonatal cardiac myocyte cultures transfected with either RAGE or a dominant-negative cytoplasmic deletion mutant of RAGE with S100B for 48 h. Under baseline conditions, cardiac myocytes express low levels of RAGE and VEGF and secrete VEGF in the medium as measured by ELISA. In RAGE overexpressing myocytes, S100B (100 nM) resulted in increases in VEGF mRNA, VEGF protein, VEGF secretion, and activation of the transcription factor NF-κB. Pre-treatment of RAGE overexpressing myocytes with the NF-κB inhibitor caffeic acid phenethyl ester inhibited increases in VEGF mRNA, VEGF protein and VEGF in the medium by S100B. In myocytes expressing dominant-negative RAGE, S100B did not induce VEGF mRNA, VEGF protein, VEGF secretion or NF-κB activation. In culture, rat neonatal and adult cardiac fibroblasts undergo phenotypic transition to myofibroblasts. Treatment of neonatal and adult myofibroblasts with VEGF (10 ng/mL) induces VEGFR-2 (flk-1/KDR) tyrosine kinase phosphorylation, ERK1/2 phosphorylation and myofibroblast proliferation. Together these data demonstrate that secreted VEGF by cardiac myocytes in response to S100B via RAGE ligation induces myofibroblast proliferation potentially contributing to scar formation observed in infarcted myocardium. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

Cyclophilin A promotes cardiac hypertrophy in apolipoprotein E-deficient mice

OBJECTIVE

Cyclophilin A (CyPA, encoded by Ppia) is a proinflammatory protein secreted in response to oxidative stress in mice and humans. We recently demonstrated that CyPA increased angiotensin II (Ang II)-induced reactive oxygen species (ROS) production in the aortas of apolipoprotein E (Apoe)-/- mice. In this study, we sought to evaluate the role of CyPA in Ang II-induced cardiac hypertrophy.

METHODS AND RESULTS

Cardiac hypertrophy was not significantly different between Ppia+/+ and Ppia-/- mice infused with Ang II (1000 ng/min per kg for 4 weeks). Therefore, we investigated the effect of CyPA under conditions of high ROS and inflammation using the Apoe-/- mice. In contrast to Apoe-/- mice, Apoe-/-Ppia-/- mice exhibited significantly less Ang II-induced cardiac hypertrophy. Bone marrow cell transplantation showed that CyPA in cells intrinsic to the heart plays an important role in the cardiac hypertrophic response. Ang II-induced ROS production, cardiac fibroblast proliferation, and cardiac fibroblast migration were markedly decreased in Apoe-/-Ppia-/- cardiac fibroblasts. Furthermore, CyPA directly induced the hypertrophy of cultured neonatal cardiac myocytes.

CONCLUSIONS

CyPA is required for Ang II-mediated cardiac hypertrophy by directly potentiating ROS production, stimulating the proliferation and migration of cardiac fibroblasts, and promoting cardiac myocyte hypertrophy.

Adventitial fibroblasts in vascular structure and function: the role of oxidative stress and beyondThis review is one of a selection of papers published in a Special Issue on Oxidative Stress in Health and Disease

The vascular adventitia, defined as the area between the external elastic lamina and the outermost edge of the blood vessel, is composed primarily of fibroblasts and for years was thought to be merely a passive structural support for the blood vessel. Consequently, studies pertaining to the role of the adventitia in regulating vascular function have been far outnumbered by those regarding the vascular endothelium. However, recent work has begun to reveal the dynamic properties of the adventitia. It was therefore the aim of this review to provide an overview of the existing knowledge demonstrating the role of the adventitia in regulating vessel structure and function. The main topics covered in this review include the cellular composition of the adventitia and the role of the adventitia in vascular oxidative stress, vasomotor responses, extracellular matrix protein expression, growth factor expression, and endothelin-1 (ET-1) expression. Recent evidence suggests that the adventitia is a major producer of vascular reactive oxygen species. It displays a distinct response to injury, hypoxia, and pulmonary hypertension, mediating vascular remodelling, repair, and extracellular matrix deposition. It may also play a role in regulating vascular tone. More recently, it has been reported that adventitial fibroblasts can produce ET-1 after Ang II treatment. Additionally, emerging evidence suggests that the adventitia may be a potent source of vasoactive hormones such as growth factors and ET-1, which may regulate vascular structure and function via autocrine or paracrine signalling mechanisms. Despite these findings, many important questions regarding the role of the vascular adventitia remain unanswered, suggesting the need for further research to determine its exact function in health and disease.

Cardiac cell-specific apoptotic and cytokine responses to reovirus infection: determinants of myocarditic phenotype

BACKGROUND

The pathophysiologic mechanisms underlying viral myocarditis are not well defined. As a result, effective treatments do not exist and viral myocarditis remains a potentially lethal infection of the heart.

METHODS AND RESULTS

We used cultured rat cardiac myocytes and fibroblasts to investigate apoptosis and cytokine production in response to infection by myocarditic vs. non-myocarditic strains of reovirus. Myocarditic reovirus strain 8B and non-myocarditic strain DB188 replicate comparably in each cardiac cell type. However, strain 8B and related myocarditic reoviruses preferentially increase apoptosis of myocytes relative to fibroblasts, whereas DB188 and nonmyocarditic strains preferentially increase fibroblast apoptosis. Infection of cardiac fibroblasts with the nonmyocarditic strain DB188 elicits substantial increases in a panel of cytokines compared to fibroblasts infected with strain 8B or mock-infected controls. Analysis of culture supernatants using cytometric bead arrays revealed that DB188 enhanced release of interleukin (IL)-1beta, IL-4, IL-6, IL-10, IL-12(p70), GRO-KC, tumor necrosis factor-alpha, and MCP-1 relative to 8B or mock-infected controls (all P < .05).

CONCLUSION

We hypothesize that differential cytokine production and cell-specific apoptosis are important determinants of myocarditic potential of reoviral strains. Therapies that target the beneficial effects of cytokines in limiting cytopathic damage may offer an effective and novel treatment approach to viral myocarditis.

Iron-loaded cardiac myocytes stimulate cardiac myofibroblast DNA synthesis

Cardiac fibrosis in iron overload disorders may arise from activation of the interstitial fibroblast. However, the cardiac myocyte, and not the fibroblast, is the main target for iron deposition. We hypothesized that fibroblasts respond to the presence of iron-loaded myocytes with increased proliferative capacity. Cardiac fibroblasts were either co-cultured with myocytes on porous filters or treated with medium conditioned by growth of myocyte cultures. In both circumstances myocytes suppressed [(3)H]thymidine incorporation by fibroblasts over 24 h, compared to stimulation of quiescent fibroblasts with fresh, unconditioned medium. However, when the myocytes were preloaded with iron, the suppressive effect was lost and DNA synthesis was restored to levels seen in unconditioned medium. This effect was not due to early events in cell cycle entry; activation of Erk at 15 min and expression of c-fos mRNA at 30 min were similar in media from control and iron-loaded myocytes. Early markers of progression of G1, namely cyclin D and phosphoretinoblastoma protein, were not significantly different in fibroblasts treated with either conditioned medium. However, cyclin E expression, a marker of the G1/S transition, was significantly increased by conditioned medium from the iron-loaded cells, compared to control-conditioned medium. We conclude that myocytes can suppress proliferation of fibroblasts by cumulative effects on late G1 events leading to DNA synthesis, and these effects are diminished with myocyte iron accumulation.

Expression of S100A6 in cardiac myocytes limits apoptosis induced by tumor necrosis factor-alpha

S100A6 is induced in myocardium post-infarction in vivo and in response to growth factors and inflammatory cytokines in vitro. Forced expression of S100A6 in cardiomyocytes inhibits regulation of cardiac specific gene expression in response to trophic stimulation. To define regulation and function of S100A6, we characterized the human S100A6 promoter and mapped upstream regulatory elements in rat neonatal cardiac myocytes, fibroblasts, and vascular smooth muscle cells and defined a functional role for S100A6 in tumor necrosis factor-alpha-induced myocyte apoptosis. The functional S100A6 promoter was localized to region -167/+134 containing 167 upstream base pairs. The S100A6 promoter is regulated by positive (-361/-167 and -588/-361) and negative (-1371/-1194) elements. Tumor necrosis factor-alpha induced the maximal S100A6 promoter and transcription factor NF-kappaB (p65 subunit). Electrophoretic mobility shift showed that tumor necrosis factor-alpha induced p65 binding to a potential NF-kappaB-binding site at -460/-451. Chromatin immunoprecipitation analysis revealed p65 is recruited to the S100A6 promoter upon tumor necrosis factor-alpha stimulation. The NF-kappaB inhibitor caffeic acid phenethyl ester and mutation of the NF-kappaB-binding site inhibited S100A6 promoter activation by tumor necrosis factor-alpha. Tumor necrosis factor-alpha induced cardiac myocyte apoptosis. Specific inhibition of S100A6 using a small interfering RNA directed against S100A6 potentiated tumor necrosis factor-alpha-induced myocyte apoptosis, whereas overexpression of S100A6 by gene transfer prevented tumor necrosis factor-alpha-induced myocyte apoptosis by interfering with p53 phosphorylation. These results demonstrate that S100A6 is induced by tumor necrosis factor-alpha via an NF-kappaB-dependent mechanism, serving a role in homeostasis to limit tumor necrosis factor-alpha-induced apoptosis by regulating p53 phosphorylation.

Paracrine effects of hypoxic fibroblast-derived factors on the MPT-ROS threshold and viability of adult rat cardiac myocytes

Cardiac fibroblasts contribute to multiple aspects of myocardial function and pathophysiology. The pathogenetic relevance of cytokine production by these cells under hypoxia, however, remains unexplored. With the use of an in vitro cell culture model, this study evaluated cytokine production by hypoxic cardiac fibroblasts and examined two distinct effects of hypoxic fibroblast-conditioned medium (HFCM) on cardiac myocytes and fibroblasts. Hypoxia caused a marked increase in the production of tumor necrosis factor (TNF)-alpha by cardiac fibroblasts. HFCM significantly enhanced the susceptibility of cardiac myocytes to reactive oxygen species (ROS)-induced mitochondrial permeability transition (MPT), determined by high-precision confocal line-scan imaging following controlled, photoexcitation-induced ROS production within individual mitochondria. Furthermore, exposure of cardiac myocytes to HFCM for 5 h led to loss of viability, as evidenced by change in morphology and annexin staining. HFCM also decreased DNA synthesis in cardiac fibroblasts. Normoxic fibroblast-conditioned medium spiked with TNF-alpha at 200 pg/ml, a concentration comparable to that in HFCM, promoted loss of myocyte viability and decreased DNA synthesis in cardiac fibroblasts. These effects of HFCM are similar to the reported effects of hypoxia per se on these cell types, showing that hypoxic fibroblast-derived factors may amplify the distinct effects of hypoxia on cardiac cells. Importantly, because both hypoxia and oxidant stress prevail in a setting of ischemia and reperfusion, the effects of soluble factors from hypoxic fibroblasts on the MPT-ROS threshold and viability of myocytes may represent a novel paracrine mechanism that could exacerbate ischemia-reperfusion injury to cardiomyocytes.

Inhibition of PPAR-α activity in mice with cardiac-restricted expression of tumor necrosis factor: potential role of TGF-β/Smad3

A shift in energy substrate utilization from fatty acids to glucose has been reported in failing hearts, resulting in improved oxygen efficiency yet perhaps also contributing to a state of energy deficiency. Peroxisome proliferator-activated receptor (PPAR)-α, the principal transcriptional regulator of cardiac fatty acid β-oxidation (FAO) genes, is downregulated in heart failure, and this may contribute to reduced fatty acid utilization. Cardiomyopathic states are also accompanied by elevated levels of circulating cytokines, such as tumor necrosis factor (TNF), as well as increased local production of cytokines and profibrotic factors, such as transforming growth factor (TGF)-β. However, whether these molecular pathways directly modulate cardiac energy metabolism and PPAR-α activity is not known. Therefore, FAO capacity and FAO gene expression were determined in mice with cardiac-restricted overexpression of TNF (MHCsTNF3). MHCsTNF3 hearts had significantly lower FAO capacity and decreased expression of PPAR-α and FAO target genes compared with control hearts. Surprisingly, TNF had little effect on PPAR-α activity and FAO rates in cultured ventricular myocytes, suggesting that TNF acts indirectly on myocyte FAO in vivo. We found that TGF-β expression was upregulated in MHCsTNF3 hearts and that treatment of cultured myocytes with TGF-β significantly suppressed FAO rates and directly impaired PPAR-α activity, a result reproduced by Smad3 overexpression. This work demonstrates that TGF-β signaling pathways directly suppress PPAR-α activity and reduce FAO in cardiac myocytes, perhaps in response to locally elevated TNF. Although speculative, TGF-β-driven repair mechanisms may also include the additional benefit of limiting FAO in injured myocardium.

IL-1β stimulates rat cardiac fibroblast migration via MAP kinase pathways

The pro-inflammatory cytokines interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) are elevated following acute myocardial infarction (MI) and have been implicated in the pathophysiology of cardiac disease progression. The cardiac fibroblast represents an important effector cell target for cytokine actions. In particular, cytokine-directed cardiac fibroblast migration is likely to impact both myocardial repair following acute MI and pathological myocardial remodeling in the progression to heart failure. In the present study, we examined the migratory response of neonatal rat cardiac fibroblasts to pro-inflammatory cytokines using modified Boyden chamber assays. On the basis of the knowledge of migration in other cell types, we hypothesized that members of the mitogen-activated protein kinase (MAPK) family may regulate this process. This possibility was addressed with the use of immunoblot detection of active phosphorylated MAPK species and pharmacological inhibitors for individual members of the MAPK cascades. IL-1β stimulated robust and concentration-dependent increases in migration (maximum, 20-fold over control cells). TNF-α had lesser effect (fourfold increase over control). IL-6 did not induce migration. Activation of all three MAPK subfamilies (extracellular signal-regulated kinases, c-Jun NH2-terminal kinases, and p38) was shown to occur in response to cytokine stimulation. Fibroblast migration was attenuated by pharmacological inhibition of each MAPK subfamily. Understanding the regulation of cardiac fibroblast migration may provide insights in the search for therapies aimed at enhancing the functional nature of the remodeling process.

Cardiac fibroblasts influence cardiomyocyte phenotype in vitro

Cardiac fibroblasts impact myocardial development and remodeling through intercellular contact with cardiomyocytes, but less is known about noncontact, profibrotic signals whereby fibroblasts alter cardiomyocyte behavior. Fibroblasts and cardiomyocytes were harvested from newborn rat ventricles and separated by serial digestion and gradient centrifugation. Cardiomyocytes were cultured in 1) standard medium, 2) standard medium diluted 1:1 with PBS, or 3) standard medium diluted 1:1 with medium conditioned > or =72 h by cardiac fibroblasts. Serum concentrations were held constant under all media conditions, and complete medium exchanges were performed daily. Cardiomyocytes began contracting within 24 h at clonal or mass densities with <5% of cells expressing vimentin. Immunocytochemical analysis revealed progressive expression of alpha-smooth muscle actin in cardiomyocytes after 24 h in all conditions. Only cardiomyocytes in fibroblast-conditioned medium stopped contracting by 72 h. There was a significant, sustained increase in vimentin expression specific to these cultures (means +/- SD: conditioned 46.3 +/- 6.0 vs. control 5.3 +/- 2.9%, P < 0.00025) typically with cardiac myosin heavy chain coexpression. Proteomics assays revealed 10 cytokines (VEGF, GRO/KC, monocyte chemoattractant protein-1, leptin, macrophage inflammatory protein-1alpha, IL-6, IL-10, IL-12p70, IL-17, and tumor necrosis factor-alpha) at or below detection levels in unconditioned medium that were significantly elevated in fibroblast-conditioned medium. Latent transforming growth factor-beta and RANTES were present in unconditioned medium but rose to higher levels in conditioned medium. Only granulocyte-macrophage colony-stimulating factor was present above threshold levels in standard medium but decreased with fibroblast conditioning. These data indicated that under the influence of fibroblast-conditioned medium, cardiomyocytes exhibited marked hypertrophy, diminished contractile capacity, and phenotype plasticity distinct from the dedifferentiation program present under standard culture conditions.

Role of endocytic inhibitory drugs on internalization of amyloidogenic light chains by cardiac fibroblasts

Amyloidosis is a disease of protein misfolding that ultimately impairs organ function. Previously, we demonstrated that amyloidogenic light chains (kappa1, lambda6, and lambda3 subtypes), internalized by cardiac fibroblasts, enhanced sulfation of secreted glycosaminoglycans. In this study, we investigated the internalization and cellular trafficking of urinary immunoglobulin light chains into cardiac fibroblasts. We demonstrate that these light chains have the ability to form annular rings in solution. Internalization was assessed by incubating cells in the presence of light chain conjugated to Oregon Green 488 followed by monitoring with live cell confocal imaging. The rate of light chain internalization was reduced by treatment with methyl-beta-cyclodextrin but not filipin. Amyloid light chain did co-localize with dextran-Texas Red. Once internalized, the light chains were detected in lysosomes and then secreted into the extracellular medium. The light chain detected in the cell lysate and medium possessed a lower hydrophobic species. Nocodazole, a microtubule inhibitor, did not disperse aggregates. In addition, internalization and retention of the light chain proteins was altered in the presence of the proteasomal inhibitor MG132. These results indicate that the cell internalizes light chain by a fluid phase endocytosis, which is then modified and ultimately compromises the cell.

Nox2-containing NADPH oxidase and Akt activation play a key role in angiotensin II-induced cardiomyocyte hypertrophy

Angiotensin II (ANG II) has profound effects on the development and progression of pathological cardiac hypertrophy; however, the intracellular signaling mechanisms are not fully understood. In this study, we used genetic tools to test the hypothesis that increased formation of superoxide (O2−·) radicals from a Rac1-regulated Nox2-containing NADPH oxidase is a key upstream mediator of ANG II-induced activation of serine-threonine kinase Akt, and that this signaling cascade plays a crucial role in ANG II-dependent cardiomyocyte hypertrophy. ANG II caused a significant time-dependent increase in Rac1 activation and O2−· production in primary neonatal rat cardiomyocytes, and these responses were abolished by adenoviral (Ad)-mediated expression of a dominant-negative Rac1 (AdN17Rac1) or cytoplasmic Cu/ZnSOD (AdCu/ZnSOD). Moreover, both AdN17Rac1 and AdCu/ZnSOD significantly attenuated ANG II-stimulated increases in cardiomyocyte size. Quantitative real-time PCR analysis demonstrated that Nox2 is the homolog expressed at highest levels in primary neonatal cardiomyocytes, and small interference RNA (siRNA) directed against it selectively decreased Nox2 expression by >95% and abolished both ANG II-induced O2−· generation and cardiomyocyte hypertrophy. Finally, ANG II caused a time-dependent increase in Akt activity via activation of AT1 receptors, and this response was abolished by Ad-mediated expression of cytosolic human O2−· dismutase (AdCu/ZnSOD). Furthermore, pretreatment of cardiomyocytes with dominant-negative Akt (AdDNAkt) abolished ANG II-induced cellular hypertrophy. These findings suggest that O2−· generated by a Nox2-containing NADPH oxidase is a central mediator of ANG II-induced Akt activation and cardiomyocyte hypertrophy, and that dysregulation of this signaling cascade may play an important role in cardiac hypertrophy.

A beta1-adrenergic receptor CaM kinase II-dependent pathway mediates cardiac myocyte fetal gene induction

Beta-adrenergic signaling plays an important role in the natural history of dilated cardiomyopathies. Chronic activation of beta-adrenergic receptors (beta1-AR and beta2-AR) during periods of cardiac stress ultimately harms the failing heart by mechanisms that include alterations in gene expression. Here, we show that stimulation of beta-ARs with isoproterenol in neonate rat ventricular myocytes causes a "fetal" response in the relative activities of the human cardiac fetal and/or adult gene promoters that includes repression of the human and rat alpha-myosin heavy chain (alpha-MyHC) promoters with simultaneous activation of the human atrial natriuretic peptide (ANP) and rat beta-MyHC promoters. We also show that the promoter changes correlate with changes in endogenous gene expression as measured by mRNA expression. Furthermore, we show that these changes are specifically mediated by the beta1-AR, but not the beta2-AR, and are independent of alpha1-AR stimulation. We also demonstrate that the fetal gene response is independent of cAMP and protein kinase A, whereas inhibition of Ca2+/calmodulin-dependent protein kinase (CaMK) pathway blocks isoproterenol-mediated fetal gene program induction. Finally, we show that induction of the fetal program is dependent on activation of the L-type Ca2+ channel. We conclude that in neonatal rat cardiac myocytes, agonist-occupied beta1-AR mobilizes Ca2+ stores to activate fetal gene induction through cAMP independent pathways that involve CaMK.

Differential cardiotoxic/cardioprotective effects of beta-adrenergic receptor subtypes in myocytes and fibroblasts in doxorubicin cardiomyopathy

beta-Adrenoceptor (beta-AR) subtypes act through different signaling pathways to regulate cardiac function and remodeling. Previous in vivo data show a markedly enhanced cardiotoxic response to doxorubicin in beta2-/- mice, which is rescued by the additional deletion of the beta1-AR. We determined whether this differential response was myocyte specific by examining the effects of doxorubicin in myocytes and fibroblasts from WT and beta1, beta2 and beta1/beta2-/- mice. Cells were exposed to doxorubicin at 1-50 microM and viability and apoptosis assessed at 6, 24 and 48 h. WT myocytes showed a time and dose-dependent decrease in viability (42% decrease at 1 microM after 24 h). beta2-/- Myocytes showed a greater decrease in viability vs. WT (20.8% less at 6 h; 14% less at 24 h, P<0.05); beta1-/- and beta1/beta2-/- myocytes showed enhanced survival (beta1-/- 11%; beta1/beta2-/- 18% greater than WT, P<0.05). TUNEL staining demonstrated a similar differential susceptibility (WT 26% apoptotic nuclei, beta2-/- 45.9%, beta1/beta2-/- 16.8%, P<0.05). beta2-/- Fibroblasts also showed enhanced toxicity. Pertussis toxin pretreatment of WT cells decreased survival similar to the beta2-/-, suggesting a role for Gi signaling. JNK was differentially activated in beta2-/- myocytes after doxorubicin and its inhibition increased cardiotoxicity. In conclusion, the differential cardioprotective/cardiotoxic effects mediated by beta1 vs. beta2-AR subtypes in knockout mice are recapitulated in myocytes isolated from these mice. beta2-ARs appear to play a cardioprotective role, whereas beta1-ARs a cardiotoxic role.

T-type Ca2+ channels are involved in high glucose-induced rat neonatal cardiomyocyte proliferation

Infants develop hypertrophic cardiomyopathy in approximately 30% of diabetic pregnancies. We have characterized the effects of glucose on voltage-gated T-type Ca2+ channels and intracellular free calcium concentration, [Ca2+]i in neonatal rat cardiomyocytes. We found that T-type Ca2+ channel current density increased significantly in primary culture neonatal cardiac myocytes that were treated with 25 mM glucose for 48 h when compared with those that were treated with 5 mM glucose. High-glucose treatment also caused a higher Ca2+ influx elicited by 50 mM KCl in the myocytes. KCl-induced Ca2+ influx was attenuated when nickel was present. Real-time PCR studies demonstrated that mRNA levels of both alpha1G (Ca(v)3.1) and alpha1H (Ca(v)3.2) T-type Ca2+ channels were elevated after high-glucose treatment. High-glucose also significantly increased ventricular cell proliferation as well as the proportion of cells in the S-phase of the cell cycle; both effects were reversed by nickel or mibefradil. These results indicate that high glucose causes a rise in [Ca2+]i in neonatal cardiac myocytes by a mechanism that is associated with the regulation of the T-type Ca2+ channel activity.

Pharmacological properties of angiotensin II receptors in cultured rabbit gingival fibroblasts

We demonstrated that angiotensin II (Ang II, 10-1000 nM) induced proliferation of cultured rabbit gingival fibroblasts in a concentration-dependent manner. The Ang II-induced proliferation was inhibited by CV-11974 (AT1 antagonist; 1 microM) and saralasin (AT1/AT2 antagonist; 1 microM), but not by PD123,319 (AT2 antagonist; 1 microM), suggesting that Ang II-induced proliferation was mediated via AT1 receptors present in and/or on gingival fibroblasts. The results of Western blot analysis indicated the presence of AT1 and AT2 receptors in/on the fibroblasts. In a subsequent radioligand binding assay, the binding of [3H]Ang II to the fibroblasts was specific and saturable with both high- and low-affinity sites. Competition binding experiments indicated that Ang II completely displaced [3H]Ang II binding, and CV-11974 and PD123,319 maximally displaced up to approximately 63% and 37% of the total binding, respectively. Ang II and CV-11974 completely displaced the [3H]DuP753 binding but PD123,319 did not, indicating a single population of binding site. These findings demonstrate that gingival fibroblasts contain both AT1 and AT2 receptor subtypes for Ang II, and support that Ang II stimulation of AT1 receptors results in proliferation of the fibroblasts.

Cardiac tissue engineering for replacement therapy

Cell therapy is a new concept to repair diseased organs. For patients with myocardial infarction, heart failure, and congenital heart diseases cell based therapies might represent a potential cure. The field can be subdivided into two principally different approaches: (1) Implantation of isolated cells and (2) implantation of in vitro engineered tissue constructs. This review will focus on the latter approach. Cardiac tissue engineering comprises the fields of material sciences and cell biology. In general, scaffold materials such as gelatin, collagen, alginate, or synthetic polymers and cardiac cells are utilized to reconstitute tissue-like constructs in vitro. Ideally, these constructs display properties of native myocardium such as coherent contractions, low diastolic tension, and syncytial propagation of action potentials. To be applicable for surgical repair of diseased myocardium engineered tissue constructs should have the propensity to integrate and remain contractile in vivo. Size and mechanical properties of engineered constructs are critical for surgical repair of large tissue defects. Successful application of tissue engineering in men will depend on the utilization of an autologous or non-immunogeneic cell source and scaffold material to avoid life long immunosuppression. This review will give an overview of recent approaches in cardiac tissue engineering and its first applications in vivo. We will discuss materials and cell sources for cardiac tissue engineering. Further, principle obstacles will be addressed. Cardiac tissue engineering for replacement therapy has an intriguing perspective, but is in its early days. Its true value remains to be thoroughly evaluated.

A functional activating protein 1 (AP-1) site regulates matrix metalloproteinase 2 (MMP-2) transcription by cardiac cells through interactions with JunB-Fra1 and JunB-FosB heterodimers

Enhanced synthesis of a specific matrix metalloproteinase, MMP-2, has been demonstrated in experimental models of ventricular failure and in cardiac extracts from patients with ischaemic cardiomyopathy. Cultured neonatal rat cardiac fibroblasts and myocytes were used to analyse the determinants of MMP-2 synthesis, including the effects of hypoxia. Culture of rat cardiac fibroblasts for 24 h in 1% oxygen enhanced MMP-2 synthesis by more than 5-fold and augmented the MMP-2 synthetic responses of these cells to endothelin-1, angiotensin II and interleukin 1beta. A series of MMP-2 promoter-luciferase constructs were used to map the specific enhancer element(s) that drive MMP-2 transcription in cardiac cells. Deletion studies mapped a region of potent transactivating function within the 91 bp region from -1433 to -1342 bp, the activity of which was increased by hypoxia. Oligonucleotides from this region were cloned in front of a heterologous simian-virus-40 (SV40) promoter and mapped the enhancer activity to a region between -1410 and -1362 bp that included a potential activating protein 1 (AP-1)-binding sequence, C(-1394)CTGACCTCC. Site-specific mutagenesis of the core TGAC sequence (indicated in bold) eliminated the transactivating activity within the -1410 to -1362 bp sequence. Electrophoretic mobility shift assays (EMSAs) using the -1410 to -1362 bp oligonucleotide and rat cardiac fibroblast nuclear extracts demonstrated specific nuclear-protein binding that was eliminated by cold competitor oligonucleotide, but not by the AP-1-mutated oligonucleotide. Antibody-supershift EMSAs of nuclear extracts from normoxic rat cardiac fibroblasts demonstrated Fra1 and JunB binding to the -1410 to -1362 bp oligonucleotide. Nuclear extracts isolated from hypoxic rat cardiac fibroblasts contained Fra1, JunB and also included FosB. Co-transfection of cardiac fibroblasts with Fra1-JunB and FosB-JunB expression plasmids led to significant increases in transcriptional activity. These studies demonstrate that a functional AP-1 site mediates MMP-2 transcription in cardiac cells through the binding of distinctive Fra1-JunB and FosB-JunB heterodimers. The synthesis of MMP-2 is widely considered, in contrast with many members of the MMP gene family, to be independent of the AP-1 transcriptional complex. This report is the first demonstration that defined members of the Fos and Jun transcription-factor families specifically regulate this gene under conditions relevant to critical pathophysiological processes.

Regulation of the S100B gene by alpha 1-adrenergic stimulation in cardiac myocytes

We previously reported that S100B, a 20-kDa Ca(2+)-binding homodimer, inhibited the postinfarct myocardial hypertrophic response mediated by alpha(1)-adrenergic stimulation through the protein kinase C (PKC) signaling pathway. In the present study, we examined whether the same pathway induced the S100B gene, supporting the hypothesis that S100B is a feedback negative regulator of this pathway. We transfected cultured neonatal rat cardiac myocytes with a luciferase reporter gene driven by the maximal human S100B promoter and progressively shorter segments of this promoter sequentially deleted from the 5' end. We identified a basic promoter essential for transcription spanning 162 bp upstream of the transcription initiation site and positive (at -782/-162 and -6,689/-4,463) and negative (at -4,463/-782) myocyte-selective regulatory elements. We showed that the basic and maximal S100B promoters were activated specifically by alpha(1)-adrenergic agonists through the alpha(1A)-adrenergic receptor, but not by any other trophic hormonal stimuli. The activation of the S100B promoter was mediated through the PKC signaling pathway. Transcription enhancer factor-1 (TEF-1) and related to TEF-1 (RTEF-1) influenced transcription from the maximal, but not the basic, promoter implicating active MCAT elements upstream from the basic promoter. Acting in opposing fashions, TEF-1 transrepressed the S100B promoter and RTEF-1 transactivated the promoter. Our results suggest that alpha(1)-adrenergic stimulation induces the S100B gene after myocardial infarction through the PKC signaling pathway and that this induction is modulated by TEF-1 and RTEF-1.

Demonstration of direct effects of growth hormone on neonatal cardiomyocytes

The cellular and molecular basis of growth hormone (GH) actions on the heart remain poorly defined, and it is unclear whether GH effects on the myocardium are direct or mediated at least in part via insulin-like growth factor (IGF-1). Here, we demonstrate that the cultured neonatal cardiomyocyte is not an appropriate model to study the effects of GH because of artifactual loss of GH receptors (GHRs). To circumvent this problem, rat neonatal cardiomyocytes were infected with a recombinant adenovirus expressing the murine GHR. Functional integrity of GHR was suggested by GH-induced activation of the cognate JAK2/STAT5, MAPK, and Akt intracellular pathways in the cells expressing GHR. Although exposure to GH resulted in a significant increase in the size of the cardiomyocyte and increased expression of c-fos, myosin light chain 2, and skeletal alpha-actin mRNAs, there were no significant changes in IGF-1 or atrial natriuretic factor mRNA levels in response to GH stimulation. In this model, GH increased incorporation of leucine, uptake of palmitic acid, and abundance of fatty acid transport protein mRNA. In contrast, GH decreased uptake of 2-deoxy-d-glucose and levels of Glut1 protein. Thus, in isolated rat neonatal cardiomyocytes expressing GHR, GH induces hypertrophy and causes alterations in cellular metabolic profile in the absence of demonstrable changes in IGF-1 mRNA, suggesting that these effects may be independent of IGF-1.

A cardiac myocyte vascular endothelial growth factor paracrine pathway is required to maintain cardiac function

The role of the cardiac myocyte as a mediator of paracrine signaling in the heart has remained unclear. To address this issue, we generated mice with cardiac myocyte-specific deletion of the vascular endothelial growth factor gene, thereby producing a cardiomyocyte-specific knockout of a secreted factor. The hearts of these mice had fewer coronary microvessels, thinned ventricular walls, depressed basal contractile function, induction of hypoxia-responsive genes involved in energy metabolism, and an abnormal response to beta-adrenergic stimulation. These findings establish the critical importance of cardiac myocyte-derived vascular endothelial growth factor in cardiac morphogenesis and determination of heart function. Further, they establish an adult murine model of hypovascular nonnecrotic cardiac contractile dysfunction.

Cloning and characterization of the mouse alpha1C/A-adrenergic receptor gene and analysis of an alpha1C promoter in cardiac myocytes: role of an MCAT element that binds transcriptional enhancer factor-1 (TEF-1)

alpha1-Adrenergic receptor (AR) subtypes in the heart are expressed by myocytes but not by fibroblasts, a feature that distinguishes alpha1-ARs from beta-ARs. Here we studied myocyte-specific expression of alpha1-ARs, focusing on the subtype alpha1C (also called alpha1A), a subtype implicated in cardiac hypertrophic signaling in rat models. We first cloned the mouse alpha1C-AR gene, which consisted of two exons with an 18 kb intron, similar to the alpha1B-AR gene. The receptor coding sequence was >90% homologous to that of rat and human. alpha1C-AR transcription in mouse heart was initiated from a single Inr consensus sequence at -588 from the ATG; this and a putative polyadenylation sequence 8.5 kb 3' could account for the predominant 11 kb alpha1C mRNA in mouse heart. A 5'-nontranscribed fragment of 4.4 kb was active as a promoter in cardiac myocytes but not in fibroblasts. Promoter activity in myocytes required a single muscle CAT (MCAT) element, and this MCAT bound in vitro to recombinant and endogenous transcriptional enhancer factor-1. Thus, alpha1C-AR transcription in cardiac myocytes shares MCAT dependence with other cardiac-specific genes, including the alpha- and beta-myosin heavy chains, skeletal alpha-actin, and brain natriuretic peptide. However, the mouse alpha1C gene was not transcribed in the neonatal heart and was not activated by alpha1-AR and other hypertrophic agonists in rat myocytes, and thus differed from other MCAT-dependent genes and the rat alpha1C gene.

Release of pro- and anti-angiogenic factors by human cardiac fibroblasts: effects on DNA synthesis and protection under hypoxia in human endothelial cells

Myocardium consists of diverse cell types suggesting a role for cell-cell interaction in maintaining the structural and functional integrity of the heart. Cardiac fibroblasts are the source of extracellular matrix, growth factors and cytokines in the heart and their interactions with cardiac myocytes are recognized. Their effects on biological responses of endothelial cells, however, are vastly unexplored. Proliferation of endothelial cells is an essential stage of angiogenesis and contributes to development of coronary collaterals. This study was designed to evaluate the effect of soluble factors produced by cardiac fibroblasts on endothelial cell proliferation. Human cardiac fibroblast-conditioned medium (CF-CM) caused a significant increase (47%, P < 0.0001) in DNA synthesis in human umbilical vein endothelial cells (HUVEC), as determined by [(3)H]thymidine incorporation. This effect was dependent on de novo protein synthesis and activation of MAP kinases. Consistently, CF-CM induced the expression and activation of ERK2 in HUVEC. The CF-CM from which heparin-binding proteins were removed, had a significantly enhanced stimulatory effect on DNA synthesis in HUVEC compared to that of 'whole CF-CM'. Western analysis showed the presence of VEGF, bFGF, PDGF, TGF-beta(1), fibronectin and thrombospondin-1 in whole CF-CM. The individual immunodepletion of each factor from whole CF-CM showed that all were necessary for full activity of CF-CM. CF-CM caused a significant reversal of hypoxia-induced inhibition of DNA synthesis and enhanced expression of survival-associated protein, Bcl(2), in HUVEC. Together, these data show that cardiac fibroblasts release inhibitory and stimulatory factors, the net effect of which is an enhancement of DNA synthesis in endothelial cells. These results point to the role that cardiac fibroblasts may play in angiogenesis in the heart.

Direct, autocrine and paracrine effects of cyclic stretch on growth of myocytes and fibroblasts isolated from neonatal rat ventricles

Several studies have demonstrated that static stretch of cardiomyocytes induces cardiomyocyte hypertrophy. We investigated the effects of cyclic stretch, a more physiological stimulus, on protein synthesis and DNA synthesis of rat ventricular cardiomyocytes and cardiofibroblasts. Further-more, we investigated whether these effects are caused by autocrine mechanisms. In addition, we studied the paracrine influences of stretched cardiofibroblasts on cardiomyocyte growth. Short-term cyclic stretch (0-24 h) of cardiomyocytes induced a growth response indicative of cardiomyocyte hypertrophy, given the fact that increased rates of protein synthesis and DNA synthesis were accompanied by an elevated release of atrial natriuretic peptide into the culture medium. In cardiofibroblasts, short-term cyclic stretch also induced a growth response as indicated by an increased rate of protein synthesis and DNA synthesis. Furthermore, incubation of stationary cardiofibroblasts with conditioned medium derived from stretched cardiofibroblasts revealed an autocrine effect of stretch as illustrated by an increased rate of protein synthesis in stationary cardiofibroblasts. In analogy, there was an autocrine effect of stretch on stationary cardiomyocytes incubated with conditioned medium derived from stretched cardiomyocytes. Moreover, we observed a paracrine effect of the conditioned medium derived from stretched cardiofibroblasts on stationary cardiomyocytes. Thus, short-term cyclic stretch of cardiomyocytes and cardiofibroblasts induces growth responses that are the result of direct, autocrine, and paracrine effects. These autocrine/paracrine effects of stretch are most probably due to release of factors from stretched cells.

Autonomous and growth factor-induced hypertrophy in cultured neonatal mouse cardiac myocytes. Comparison with rat

Cultured neonatal rat cardiac myocytes have been used extensively to study cellular and molecular mechanisms of cardiac hypertrophy. However, there are only a few studies in cultured mouse myocytes despite the increasing use of genetically engineered mouse models of cardiac hypertrophy. Therefore, we characterized hypertrophic responses in low-density, serum-free cultures of neonatal mouse cardiac myocytes and compared them with rat myocytes. In mouse myocyte cultures, triiodothyronine (T3), norepinephrine (NE) through a beta-adrenergic receptor, and leukemia inhibitory factor induced hypertrophy by a 20% to 30% increase in [(3)H]phenylalanine-labeled protein content. T3 and NE also increased alpha-myosin heavy chain (MyHC) mRNA and reduced beta-MyHC. In contrast, hypertrophic stimuli in rat myocytes, including alpha(1)-adrenergic agonists, endothelin-1, prostaglandin F(2alpha), interleukin 1beta, and phorbol 12-myristate 13-acetate (PMA), had no effect on mouse myocyte protein content. In further contrast with the rat, none of these agents increased atrial natriuretic factor or beta-MyHC mRNAs. Acute PMA signaling was intact by extracellular signal-regulated kinase (ERK1/2) and immediate-early gene (fos/jun) activation. Remarkably, mouse but not rat myocytes had hypertrophy in the absence of added growth factors, with increases in cell area, protein content, and the mRNAs for atrial natriuretic factor and beta-MyHC. We conclude that mouse myocytes have a unique autonomous hypertrophy. On this background, T3, NE, and leukemia inhibitory factor activate hypertrophy with different mRNA phenotypes, but certain Gq- and protein kinase C-coupled agonists do not.

Interleukin-6 family of cytokines mediate angiotensin II-induced cardiac hypertrophy in rodent cardiomyocytes

This study was designed to investigate whether angiotensin II induces the interleukin (IL)-6 family of cytokines in cardiac fibroblasts and, if so, whether these cytokines can augment cardiac hypertrophy. Angiotensin II increased IL-6, leukemia inhibitory factor (LIF) and cardiotrophin-1 mRNA by 6.5-, 10.2-, and 2.0-fold, respectively, but did not affect IL-11, ciliary neurotrophic factor, or oncostatin M in cardiac fibroblasts. Enzyme-linked immunosorbent assay revealed that angiotensin II-stimulated conditioned medium from cardiac fibroblasts contained 9.3 ng/ml IL-6 at 24 h, which was 24-fold higher than the control. It phosphorylated gp130 and STAT3 in cardiomyocytes, which was reduced with RX435 (anti-gp130 blocking antibody). It increased [(3)H]phenylalanine uptake and cell area by 44% and 86% in cardiomyocytes compared with mock medium. RX435 suppressed these increases by 26% and 38%, while TAK044 (endothelin-A/B-R blocker) suppressed them by 52% and 52%, respectively. Antisense oligonucleotides against LIF and cardiotrophin-1 blocked their up-regulation, and attenuated the conditioned medium-induced increase in [(3)H]phenylalanine uptake by 21% and 13%, respectively. The combination of antisense oligonucleotides to LIF and cardiotrophin-1 decreased their uptake by 33%. These results indicated that angiotensin II induced IL-6, LIF, and cardiotrophin-1 in cardiac fibroblasts, and that these cytokines, particularly LIF and cardiotrophin-1, activated gp130-linked signaling and contributed to angiotensin II-induced cardiomyocyte hypertrophy.

A novel angiotensin-converting enzyme-related carboxypeptidase (ACE2) converts angiotensin I to angiotensin 1-9

ACE2, the first known human homologue of angiotensin-converting enzyme (ACE), was identified from 5' sequencing of a human heart failure ventricle cDNA library. ACE2 has an apparent signal peptide, a single metalloprotease active site, and a transmembrane domain. The metalloprotease catalytic domains of ACE2 and ACE are 42% identical, and comparison of the genomic structures indicates that the two genes arose through duplication. In contrast to the more ubiquitous ACE, ACE2 transcripts are found only in heart, kidney, and testis of 23 human tissues examined. Immunohistochemistry shows ACE2 protein predominantly in the endothelium of coronary and intrarenal vessels and in renal tubular epithelium. Active ACE2 enzyme is secreted from transfected cells by cleavage N-terminal to the transmembrane domain. Recombinant ACE2 hydrolyzes the carboxy terminal leucine from angiotensin I to generate angiotensin 1-9, which is converted to smaller angiotensin peptides by ACE in vitro and by cardiomyocytes in culture. ACE2 can also cleave des-Arg bradykinin and neurotensin but not bradykinin or 15 other vasoactive and hormonal peptides tested. ACE2 is not inhibited by lisinopril or captopril. The organ- and cell-specific expression of ACE2 and its unique cleavage of key vasoactive peptides suggest an essential role for ACE2 in the local renin-angiotensin system of the heart and kidney. The full text of this article is available at http://www. circresaha.org.

Signaling properties and functions of two distinct cardiomyocyte protease-activated receptors

Previous studies have established that cardiomyocytes express protease-activated receptor (PAR)-1, a high-affinity receptor for thrombin, which is also activated by the tethered-ligand domain sequence (SFLLRN) and which promotes inositol trisphosphate accumulation, stimulates extracellular signal-regulated protein kinase, and modulates contractile function. A single previous report identified PAR-1 as a hypertrophic stimulus, but there have been no subsequent investigations of the mechanism. This study reveals the coexpression of PAR-1 and PAR-2 (a second PAR, which is activated by trypsin/tryptase but not thrombin) by Northern blot analysis and compares their signaling properties in neonatal rat ventricular cardiomyocytes. SFLLRN and SLIGRL (an agonist peptide for PAR-2) promote inositol trisphosphate accumulation, stimulate mitogen-activated protein kinases (extracellular signal-regulated protein kinase and p38-mitogen-activated protein kinase), elevate calcium concentration, and increase spontaneous automaticity. SFLLRN (but not SLIGRL) also activates c-Jun NH(2)-terminal kinase and AKT. In keeping with their linkage to pathways that have been associated with growth and/or survival, SFLLRN and SLIGRL both induce hypertrophy. However, PAR agonists promote cell elongation, a morphology that is distinct from the uniform increase in cell dimension induced by alpha(1)-adrenergic receptor activation. These studies provide novel evidence that cardiomyocytes coexpress 2 functional PARs, which link to a common set of signals that culminate in changes in contractile function and hypertrophic growth. PAR actions may assume clinical importance in the border zone surrounding an infarction, where local proteolysis of PARs by serine proteases generated during inflammatory or thrombogenic pathways would elevate calcium concentration (setting the stage for arrhythmias), promote hypertrophic growth, and/or influence cardiomyocyte survival.

Insulin-like growth factor I as a cardiac hormone: physiological and pathophysiological implications in heart disease

Accumulating evidence has indicated that insulin-like growth factor-1 (IGF-1) plays a specific role in the intricate cascade of events of cardiovascular function, in addition to its well established growth-promoting and metabolic effects. IGF-1 is believed to mediate many effects of growth hormone (GH), IGF-1 promotes cardiac growth, improves cardiac contractility, cardiac output, stroke volume, and ejection fraction. In humans, IGF-1 improves cardiac function after myocardial infarction by stimulating contractility and promoting tissue remodeling. Furthermore, IGF-1 facilitates glucose metabolism, lowers insulin levels, increases insulin sensitivity, and improves the lipid profile. These data suggest an attractive therapeutic potential of IGF-1. Both clinically observed and experimentally induced impairments of cardiac function are also found to be associated with abnormal IGF-1 levels. IGF-1 and its binding proteins have been considered as markers for the presence of certain cardiac abnormalities, indicating that IGF-1 may be a risk factor for certain cardiac disorders. The present review will emphasize the role of IGF-1 in the regulation of cardiac growth and function, and the potential pathophysiological role of IGF-1 in cardiac function.