Genetic and phenotypic targeting of β-adrenergic signaling in heart failure

Heart failure is a leading cause of hospitalization worldwide. No major significant improvements in prognosis have been achieved for heart failure over the last several decades despite advances in disease management. Heart failure itself represents a final common endpoint for several disease entities, including hypertension and coronary artery disease. On a molecular level, certain biochemical features remain common to failing myocardium. Among these are alterations in the -adrenergic receptor (-AR) signaling cascade. Recent advances in transgenic and gene therapy techniques have presented novel therapeutic strategies for management of heart failure via genetic manipulation of -AR signaling including the targeted inhibition of the -AR kinase (ARK1 or GRK2). In this review, we will discuss the -AR signaling changes that accompany heart failure as well as corresponding therapeutic strategies. We will then review the evidence from transgenic mouse work supporting the use of -AR manipulation in the failing heart and more recent in vivo applications of gene therapy directed at reversing or preventing heart failure. (Mol Cell Biochem 263: 5–9, 2004)     

Modification of beta-adrenoceptor signal transduction pathway by genetic manipulation and heart failure

The beta-adrenoceptor (beta-AR) mediated signal transduction pathway in cardiomyocytes is known to involve beta1- and beta2-ARs, stimulatory (Gs) and inhibitory (Gi) guanine nucleotide binding proteins, adenylyl cyclase (AC) and cAMP-dependent protein kinase (PKA). The activation of beta1- and beta2-ARs has been shown to increase heart function by increasing Ca2+ -movements across the sarcolemmal membrane and sarcoplasmic reticulum through the stimulation of Gs-proteins, activation of AC and PKA enzymes and phosphorylation of the target sites. The activation of PKA has also been reported to increase phosphorylation of some myofibrillar proteins (for promoting cardiac relaxation) and nuclear proteins (for cardiac hypertrophy). The activation of beta2-AR has also been shown to affect Gi-proteins, stimulate mitogen activated protein kinase and increase protein synthesis by enhancing gene expression. Beta1- and beta2-ARs as well as AC are considered to be regulated by PKA- and protein kinase C (PKC)-mediated phosphorylations directly; both PKA and PKC also regulate beta-AR indirectly through the involvement of beta-AR kinase (betaARK), beta-arrestins and Gbeta gamma-protein subunits. Genetic manipulation of different components and regulators of beta-AR signal transduction pathway by employing transgenic and knockout mouse models has provided insight into their functional and regulatory characteristics in cardiomyocytes. The genetic studies have also helped in understanding the pathophysiological role of PARK in heart dysfunction and therapeutic role of betaARK inhibitors in the treatment of heart failure. Varying degrees of defects in the beta-AR signal transduction system have been identified in different types of heart failure to explain the attenuated response of the failing heart to sympathetic stimulation or catecholamine infusion. A decrease in beta1-AR density, an increase in the level of G1-proteins and overexpression of betaARK are usually associated with heart failure; however, these attenuations have been shown to be dependent upon the type and stage of heart failure as well as region of the heart. Both local and circulating renin-angiotensin systems, sympathetic nervous system and endothelial cell function appears to regulate the status of beta-AR signal transduction pathway in the failing heart. Thus different components and regulators of the beta-AR signal transduction pathway appears to represent important targets for the development of therapeutic interventions for the treatment of heart failure.     

Cardiac hypertrophy and altered beta-adrenergic signaling in transgenic mice that express the amino terminus of beta-ARK1

The G protein-coupled receptor (GPCR) kinase beta-adrenergic receptor (beta-AR) kinase-1 (beta-ARK1) is elevated during heart failure; however, its role is not fully understood. Beta-ARK1 contains several domains that are capable of protein-protein interactions that may play critical roles in the regulation of GPCR signaling. In this study, we developed a novel line of transgenic mice that express an amino-terminal peptide of beta-ARK1 that is comprised of amino acid residues 50-145 (beta-ARKnt) in the heart to determine whether this domain has any functional significance in vivo. Surprisingly, the beta-ARKnt transgenic mice presented with cardiac hypertrophy. Our data suggest that the phenotype was driven via an enhanced beta-AR system, as beta-ARKnt mice had elevated cardiac beta-AR density. Moreover, administration of a beta-AR antagonist reversed hypertrophy in these mice. Interestingly, signaling through the beta-AR in response to agonist stimulation was not enhanced in these mice. Thus the amino terminus of beta-ARK1 appears to be critical for normal beta-AR regulation in vivo, which further supports the hypothesis that beta-ARK1 plays a key role in normal and compromised cardiac GPCR signaling.     

Gene and cell therapy for heart disease

Heart disease is the most common cause of morbidity and mortality in Western society and the incidence is projected to increase significantly over the next few decades as our population ages. Heart failure occurs when the heart is unable to pump blood at a rate to commensurate with tissue metabolic requirements and represents the end stage of a variety of pathological conditions. Causes of heart failure include ischemia, hypertension, coronary artery disease, and idiopathic dilated cardiomyopathy. Hypertension and ischemia both cause infarction with loss of function and a consequent contractile deficit that promotes ventricular remodeling. Remodeling results in dramatic alterations in the size, shape, and composition of the walls and chambers of the heart and can have both positive and negative effects on function. In 30-40% of patients with heart failure, left ventricular systolic function is relatively unaffected while diastolic dysfunction predominates. Recent progress in our understanding of the molecular and cellular bases of heart disease has provided new therapeutic targets and led to novel approaches including the delivery of proteins, genes, and cells to replace defective or deficient components and restore function to the diseased heart. This review focuses on three such strategies that are currently under development: (a) gene transfer to modulate contractility, (b) therapeutic angiogenesis for the treatment of ischemia, and (c) embryonic and adult stem cell transfer to replace damaged myocardium.     

Hope for a broken heart?

Heart failure affects 23 million people worldwide and results from cardiac dysfunction characterized by decreased responsiveness to beta-adrenergic stimulation. A recent publication by W.J. Koch and colleagues highlights evidence for targeted beta-adrenergic receptor kinase (betaARK1) inhibition by gene transfer to improve contractile function and beta-adrenergic responsiveness in failing human myocardium. This proof-of-concept study has great importance for future heart failure therapy because it provides evidence for the therapeutic effectiveness of betaARK1 inhibition in failing human myocardium.     

Cardiac histone acetylation--therapeutic opportunities abound

Diverse etiologic factors trigger a cardiac remodeling process in which the heart becomes abnormally enlarged with a consequent decline in cardiac function and eventual heart failure. Heart failure is traditionally treated with drugs that antagonize early signaling events at or near the cell membrane. Although such approaches have short-term efficacy, the five-year mortality rate for patients with late-stage heart failure continues to exceed 50%. Because of the redundant nature of the signaling networks that drive cardiac pathogenesis, targeting the common downstream elements of the cascades would be a more effective therapeutic strategy. Recent studies point to the importance of enzymes that control histone acetylation as stress-responsive regulators of gene expression in the heart. Given their role as nuclear integrators that couple divergent upstream signals to the gene program for cardiac remodeling, we propose that these chromatin-modifying factors represent auspicious targets for the pharmacological manipulation of cardiac disease.     

Alterations of adenylyl cyclase and G proteins in aortocaval shunt-induced heart failure

Unlike most other experimental models of congestive heart failure, the volume overload model induced by aortocaval shunt (AVS) in rats was found to exhibit enhanced beta-adrenoceptor (beta-AR) signaling. To study whether the adenylyl cyclase (AC)-G protein system is involved in such a change, we examined cardiac AC activity and protein content as well as G(s)alpha and G(i)alpha activities, protein contents, and mRNA levels in both left (LV) and right (RV) ventricles at the failing stage (16 wk after surgery). Basal and forskolin-stimulated AC activities were significantly increased in both LV and RV from the failing hearts; this change was associated with an upregulation of type V/VI AC protein. In contrast to 5'-guanylyl imidodiphosphate and NaF, the stimulatory effect of isoproterenol on AC was increased in the failing heart. Although G(s)alpha and G(i)alpha protein contents in the failing hearts were not altered, the mRNA level for G(s)alpha was decreased by 20% and that for G(i)alpha was increased by 20%. In addition, the activity of G(s)alpha, but not G(i)alpha, as assessed by toxin-catalyzed ADP ribosylation, was significantly decreased in the failing heart. Losartan and imidapril treatments improved cardiac function and attenuated alterations in mRNA levels for G(s)alpha and G(i)alpha proteins, as well as G(s)alpha activity, without affecting changes in AC protein content or activities in heart failure due to volume overload. These data suggest that increased AC activity may contribute to the enhanced beta-AR signaling in the AVS model of heart failure, whereas alterations in gene expression for G proteins may be of an adaptive nature at this stage of heart failure.     

The molecular basis of myocardial hypertrophy and heart failure

Heart failure (HF) is the inability of the heart to cope with the metabolic demands of the periphery. It is the common end-stage of many frequent cardiac diseases and is characterized by relentless progression. Mechanisms of progression include renal sodium and water retention, neurohumoral activation and alterations of the protein composition (gene programme) of the heart itself. In this review, we explain the often confusing terminology in the subject, briefly touch upon the peripheral mechanisms of HF, and then focus on the changes in the gene programme of the failing heart and the molecular mechanisms leading to them. Understanding the basic processes underlying HF will help uninitiated readers to gain insight into recent novel approaches to its treatment.     

Changes in beta-adrenoceptors in heart failure due to myocardial infarction are attenuated by blockade of renin-angiotensin system

Earlier studies have revealed an improvement of cardiac function in animals with congestive heart failure (CHF) due to myocardial infarction (MI) by treatment with angiotensin converting enzyme (ACE) inhibitors. Since heart failure is also associated with attenuated responses to catecholamines, we examined the effects of imidapril, an ACE inhibitor, on the beta-adrenoceptor (beta-AR) signal transduction in the failing heart. Heart failure in rats was induced by occluding the coronary artery, and 3 weeks later the animals were treated with g/(kg x day) (orally) imidapril for 4 weeks. The animals were assessed for their left ventricular function and inotropic responses to isoproterenol. Cardiomyocytes and crude membranes were isolated from the non-ischemic viable left ventricle and examined for the intracellular concentration of Ca2+ [Ca2+]i and beta-ARs as well as adenylyl cyclase (AC) activity, respectively. Animals with heart failure exhibited depressions in ventricular function and positive inotropic response to isoproterenol as well as isoproterenol-induced increase in [Ca2+]i in cardiomyocytes; these changes were attenuated by imidapril treatment. Both beta1-AR receptor density and isoproterenol-stimulated AC activity were decreased in the failing heart and these alterations were prevented by imidapril treatment. Alterations in cardiac function, positive inotropic effect of isoproterenol, beta1-AR density and isoproterenol-stimulated AC activity in the failing heart were also attenuated by treatment with another ACE inhibitor, enalapril and an angiotensin II receptor antagonist, losartan. The results indicate that imidapril not only attenuates cardiac dysfunction but also prevents changes in beta-AR signal transduction in CHF due to MI. These beneficial effects are similar to those of enalapril or losartan and thus appear to be due to blockade of the renin-angiotensin system.     

Sarcoplasmic reticulum Ca-ATPase-phospholamban interactions and dilated cardiomyopathy

Dilated cardiomyopathy is a disease of the heart muscle resulting from a diverse array of conditions that damages the heart and impairs myocardial function. Heart failure occurs when the heart is unable to pump blood at a rate which can accommodate the heart muscle's metabolic requirements. Several signaling pathways have been shown to be involved in the induction of cardiac disease and heart failure. Many of these pathways are linked to cardiac sarcoplasmic reticulum (SR) Ca cycling directly or indirectly. A large body of evidence points to the central role of abnormal Ca handling by SR proteins, Ca-ATPase pump (SERCA2a) and phospholamban (PLN), in pathophysiological heart conditions, compromising the contractile state of the cardiomyocytes. This review summarizes studies which highlight the key role of these two SR proteins in the regulation of cardiac function, the significance of SERCA2a-PLN interactions using transgenic approaches, and the recent discoveries of human PLN mutations leading to disease states. Finally, we will discuss extrapolation of experimental paradigms generated in animal models to the human condition.     

Model-specific selection of molecular targets for heart failure gene therapy

Heart failure (HF) is a complex multifaceted problem of abnormal ventricular function and structure. In recent years, new information has been accumulated allowing for a more detailed understanding of the cellular and molecular alterations that are the underpinnings of diverse causes of HF, including myocardial ischemia, pressure-overload, volume-overload or intrinsic cardiomyopathy. Modern pharmacological approaches to treat HF have had a significant impact on the course of the disease, although they do not reverse the underlying pathological state of the heart. Therefore gene-based therapy holds a great potential as a targeted treatment for cardiovascular diseases. Here, we survey the relative therapeutic efficacy of genetic modulation of β-adrenergic receptor signaling, Ca(2+) handling proteins and angiogenesis in the most common extrinsic models of HF.     

Regenerating cardiac cells: insights from the bench and the clinic

A major challenge in cardiovascular regenerative medicine is the development of novel therapeutic strategies to restore the function of cardiac muscle in the failing heart. The heart has historically been regarded as a terminally differentiated organ that does not have the potential to regenerate. This concept has been updated by the discovery of cardiac stem and progenitor cells that reside in the adult mammalian heart. Whereas diverse types of adult cardiac stem or progenitor cells have been described, we still do not know whether these cells share a common origin. A better understanding of the physiology of cardiac stem and progenitor cells should advance the successful use of regenerative medicine as a viable therapy for heart disease. In this review, we summarize current knowledge of the various adult cardiac stem and progenitor cell types that have been discovered. We also review clinical trials presently being undertaken with adult stem cells to repair the injured myocardium in patients with coronary artery disease.     

Differential changes in beta-adrenoceptor signal transduction in left and right ventricles of infarcted rats

Although different experimental and clinical studies have revealed varying degrees of defects in beta-adrenoceptors (beta-ARs) during the development of heart failure, the mechanisms for differences in beta-AR signal transduction between the left (LV) and right ventricle (RV) are not understood. Because biochemical alterations in the myocardium depend on the stage of heart disease, this study was undertaken to assess the status of beta-ARs in the LV and RV at different stages of heart failure. Myocardial infarction was induced in rats by occluding the left coronary artery for 8 and 24 weeks. The beta-AR signal transduction was monitored by measuring beta1-AR density, the isoproterenol-induced positive inotropic effect, the increase in [Ca2+]i in cardiomyocytes, and the activation of adenylyl cyclase. The beta-AR signal transduction parameters in the 8- and 24-week failing LV were depressed, whereas the RV showed upregulation at 8 weeks and downregulation at 24 weeks of these mechanisms. These results suggest that beta-AR-mediated signal transduction in the LV and RV are differentially regulated and are dependent upon the stage of development of congestive heart failure due to myocardial infarction.     

Genetics of heart failure

Heart failure (HF) occurs when the cardiac output, no longer compensated by endogenous mechanisms, fails to meet the metabolic demands of the body. In most populations, the prevalence of heart failure continues to rise, constituting a major public health burden, especially in developed countries. There is some evidence that the risk of HF in the general population depends on genetic predisposition, necessarily characterised by a very complex architecture. In a small, but probably underestimated proportion, HF is caused by Mendelian inherited forms of myocardial disease. The genetic background of these genetic conditions is a matter of intensive research that is already shedding light onto the genetics of common sporadic forms of HF. In this review, we briefly review the insights provided by candidate gene and genome-wide association approaches in common HF and then describe the main genetic causes of inherited heart muscle disease. Finally we present the current challenges and future research needs for both forms of HF. This article is part of a Special Issue entitled: Heart failure pathogenesis and emerging diagnostic and therapeutic interventions.     

心力衰竭中线粒体的生物合成及治疗策略

心力衰竭是目前全球共同面对的公共卫生问题之一,是各种心血管疾病发展的最终阶段。传统的强心、利尿、扩张外周血管等治疗措施仅能缓解心力衰竭的症状,但无法逆转在心肌细胞中发生的分子变化。心力衰竭发生、发展的病理生理机制是复杂的、多方面的,包括神经-体液的调节、炎症反应、细胞的肥大及凋亡等机制,其中线粒体的功能障碍是心力衰竭进展中的关键因素之一。心力衰竭中心肌细胞虽然发生代谢障碍,但仍然保持活性,且存在逆转的可能性。因此,在心力衰竭的治疗上不能局限在缓解症状,而应针对心力衰竭中潜在的分子机制,逆转损伤的心肌。研究线粒体在衰竭心肌中发生的病理生理变化,对于逆转心肌的收缩功能具有重要意义。本文就线粒体的生物起源及其针对其起源在心力衰竭中的治疗措施作一综述。     

Heart failure and the brain: new perspectives

Despite recent therapeutic advances, the prognosis for patients with heart failure remains dismal. Unchecked neurohumoral excitation is a critical element in the progressive clinical deterioration associated with the heart failure syndrome, and its peripheral manifestations have become the principal targets for intervention. The link between peripheral systems activated in heart failure and the central nervous system as a source of neurohumoral drive has therefore come under close scrutiny. In this context, the forebrain and particularly the paraventricular nucleus of the hypothalamus have emerged as sites that sense humoral signals generated peripherally in response to the stresses of heart failure and contribute to the altered volume regulation and augmented sympathetic drive that characterize the heart failure syndrome. This brief review summarizes recent studies from our laboratory supporting the concept that the forebrain plays a critical role in the pathogenesis of ischemia-induced heart failure and suggesting that the forebrain contribution must be considered in designing therapeutic strategies. Forebrain signaling by neuroactive products of the renin-angiotensin system and the immune system are emphasized.     

Influence of cardiac function and failure on sleep-disordered breathing: evidence for a causative role.

Heart failure is an increasingly common public health problem that is strongly linked to both central and obstructive sleep apnea, collectively referred to as sleep-disordered breathing. Much attention has been given to the deleterious effects of sleep-disordered breathing on the failing heart and potential mechanisms by which treatment of sleep-disordered breathing may result in improved cardiac performance and long-term outcomes. However, there is compelling evidence that cardiac dysfunction may contribute to sleep-disordered breathing. Although there is recognized overlap between pathophysiological mechanisms in central sleep apnea and obstructive sleep apnea, data supporting the role of cardiac function are certain forms of central sleep apnea are well established, whereas investigation into the relationship with obstructive sleep apnea is less mature but continues to evolve. This review will examine experimental and observational data that explore possible pathophysiological mechanisms and potential targets for therapy in heart failure and sleep-disordered breathing.     

beta-adrenergic mechanisms in cardiac diseases: a perspective

Several lines of evidence show that neurohumoral systems, especially those involving catecholamines, play a crucial role in cardiac diseases. Changes in the beta-adrenergic receptor (beta-AR) system such as receptor down-regulation, uncoupling from G-proteins, receptor internalization and receptor degradation may account for some of the abnormalities of contractile function in this disease. Increases in the level of inhibitory G-protein subunits also appears to be involved in attenuating the beta-AR signal. Finally beta-AR signalling is strongly regulated by members of the G-protein-coupled receptor kinase family (GRKs), the best known of which is beta-adrenergic receptor kinase 1 (beta-ARK1). beta-ARK1 mRNA, protein level and enzymatic activity is increased in heart disease, further contributing to an attenuation in beta-AR signalling. The combination of these negative alterations are presumably related to the contractile dysfunction seen in human heart disease. The combination of biochemical, physiological and molecular biological studies bearing on the normal function and regulation of these various molecules should provide strategies for elucidating the pharmacological basis of the regulation of myocardial contractility in the normal and failing heart.     

Exercise training and beta-blocker treatment ameliorate age-dependent impairment of beta-adrenergic receptor signaling and enhance cardiac responsiveness to adrenergic stimulation

Cardiac beta-adrenergic receptor (beta-AR) signaling and left ventricular (LV) responses to beta-AR stimulation are impaired with aging. It is shown that exercise and beta-AR blockade have a favorable effect on cardiac and vascular beta-AR signaling in several cardiovascular diseases. In the present study, we examined the effects of these two different strategies on beta-AR dysregulation and LV inotropic reserve in the aging heart. Forty male Wistar-Kyoto aged rats were randomized to sedentary, exercise (12 wk treadmill training), metoprolol (250 mg.kg(-1).day(-1) for 4 wk), and exercise plus metoprolol treatment protocols. Ten male Wistar-Kyoto sedentary young rats were also used as a control group. Old trained, old metoprolol-treated, and old trained plus metoprolol-treated rats showed significantly improved LV maximal and minimal first derivative of the pressure rise responses to beta-AR stimulation (isoproterenol) compared with old untrained animals. We found a significant reduction in cardiac sarcolemmal membrane beta-AR density and adenylyl cyclase activity in old untrained animals compared with young controls. Exercise training and metoprolol, alone or combined, restored cardiac beta-AR density and G-protein-dependent adenylyl cyclase activation in old rats. Although cardiac membrane G-protein-receptor kinase 2 levels were not upregulated in untrained old compared with young control rats, both exercise and metoprolol treatment resulted in a dramatic reduction of G-protein-receptor kinase 2 protein levels, which is a further indication of beta-AR signaling amelioration in the aged heart induced by these treatment modalities. In conclusion, we demonstrate for the first time that exercise and beta-AR blockade can similarly ameliorate beta-AR signaling in the aged heart, leading to improved beta-AR responsiveness and corresponding LV inotropic reserve.