A Synopsis of Research in Cardiac Apoptosis and Its Application to Congestive Heart Failure

Abstract

Cardiac apoptosis diminishes the contractile mass, which leads to heart failure. Apoptosis of cardiac non-myocytes also contributes to maladaptive remodeling and the transition to decompensated congestive heart failure. New antiapoptotic interventions and medications will be available within the next decade. The aim of this study is to provide a critical synopsis of research projects on cardiocyte apoptosis that have implications for current and future practice and to identify methods to prevent or attenuate apoptosis in patients who have poor ventricular function.

A retrospective literature review reveals a great many important publications. However, very few investigators discuss the clinical ramifications of cardiocyte apoptosis, nor do they address the clinician who sees poor ventricular contractility daily.

Most studies are still investigational and involve antiapoptotic agents such as broad-spectrum caspase inhibitors, antioxidants, calcium channel blockers, insulin-like growth-factor 1, and poly(adenosine diphosphate ribose) synthetase inhibitors.

Some options have already been incorporated into the clinical practices of cardiologists and cardiac surgeons: repairing or replacing diseased or damaged valves before ventricular function deteriorates; reducing afterload with medication or intra-aortic balloon pulsation in patients who display acute increases in afterload; decreasing catecholamine-induced cardiotoxicity in hemodynamically compromised patients, by using β-blockers and phosphodiesterase inhibitors; and inserting intra-aortic balloon pumps or ventricular assist devices early in cases of failing myocardium. Coronary revascularization early in myocardial infarction is effective antiapoptotic therapy. Other therapeutic targets are cardiopulmonary bypass and aortic cross-clamping, both of which require reductions in associated myocardial apoptosis.

Cardiac failure is the end stage of all heart disease and is a major cause of morbidity and death. National hospital discharge surveys indicate that approximately 4.8 million Americans have heart failure.¹ Despite significant advances in the medical and surgical treatment of heart failure, this important challenge remains: during the last 2 decades, congestive heart failure has become an increasingly frequent reason for hospital admission. Clearly, it is a major health problem.

Within the past decade, there has been increasing evidence that apoptosis contributes substantially to the pathogenesis of heart failure. Cardiocyte apoptosis, a morphologically different mode of cell death from necrosis, is an important component of the remodeling process and of the transition from an adaptive myocardial condition to end-stage cardiac failure (Figs. 1 and 2). Cardiac apoptosis is a genetically programmed and energy-requiring process that is executed by a family of ubiquitously expressed cysteine proteases that are termed caspases. Caspases are present in the cell as inactive pro-caspases, which are activated in response to apoptotic stimuli (Fig. 2). An understanding of the physiology of apoptosis and its clinical implications is important for the physician, because the therapeutic options may improve the outcome of patients who have heart failure. In fact, the attenuation and prevention of apoptotic pathways are new modes of therapy for congestive heart failure that will be applied in clinical practice within the next decade.

Fig. 1 The most relevant myocardial conditions and agents that induce cardiomyocyte apoptosis are listed (left). These can lead to other clinical entities (right).

ANF = antinuclear factor; TNF-α = tumor necrosis factor-α; p-53 = protein 53

Fig. 2 Early stages of apoptosis are characterized by cell shrinkage and aggregation of chromosomal DNA into small masses and preparation for exocytosis. These apoptotic bodies are membrane bound and are subsequently phagocytosed by macro-phages and neutrophils. Necrosis, on the other hand, is asso-ciated with loss of transmembrane ion gradient and with membrane disruption secondary to depletion of intracellular adenosine tri-phosphate, which causes an inflammatory reaction in surrounding tissue. Specific DNA fragmentation is a hallmark of apoptosis, and it is used as a detection method in “DNA-laddering.”

Cardiocyte apoptosis in heart failure has been the topic of research in many recent studies. However, very few of these articles are of clinical use to the physician who treats patients with congestive heart failure. The aims of this paper are to present a critical review of the studies on antiapoptotic therapy for conditions associated with congestive heart failure, to provide ramifications for physicians who treat patients who have congestive heart failure, and to translate the current knowledge into modern clinical practice, both in surgery and in medicine.

Methods

A literature review that used the Ovid search engine to find “heart failure and apoptosis” and its subheadings yielded 62,008 publications. In order to keep our study concise, we subsequently limited the search to articles in English and focused on studies that involved human beings. The articles were evaluated for their validity, their importance, and their applicability to clinical practice. The detection methods were critically reviewed for methodological accuracy, because detection methods with poor sensitivity can lead to skewed results and incorrect conclusions. The articles were reviewed, and clinically important data were collected and incorporated.

Results

Cardiocyte apoptosis, a part of the phenotype of the failing heart, has been detected in studies of both animals and human beings.^2–5 Compensatory mechanisms that are closely associated with failing myocardium are left ventricular (LV) hypertrophy and dilation, enhanced and sustained activity of the renin-angiotensin-aldoster-one system, chronically increased intracellular calcium, and increased sympathetic response. These same mechanisms—considered to be potent inducers of apoptosis—are responsible for cardiac remodeling and for the progression of heart failure.^6–8 It is well known, of course, that heart failure can result from the acute loss of cardiocytes in an infarcted area; but not infrequently, heart failure is precipitated by delayed remodeling of the myocardium.

Valvular regurgitation is the leading cause of LV dilatation. The volume overload associated with aortic and mitral regurgitation increases LV end-diastolic pressure and isometric stretching of the myocardium. In 1 prominent study that used a rodent model, the papillary muscle was subjected to 50 mN/mm isometric tension.⁹ The authors found a 21-fold higher apoptotic index than was associated with baseline tension (7–8 mN/mm).⁹ This evidence supports and explains the occurrence of cardio-cyte apoptosis in volume-overloaded ventricles. Abbate and coworkers¹⁰ studied the role of cardiocyte apoptosis in the genesis of LV dilatation and thinning of non-infarcted myocardium after large transmural myocardial infarction. In this postmortem analysis, the apoptotic index in remote non-infarcted regions correlated strongly with dilatation and thinning of the myocardium.¹⁰

Ventricular hypertrophy is another compensatory mechanism associated with heart failure; it can be the result of increased afterload, long-lasting hypertension, and aortic or pulmonic stenosis. Cardiocyte apoptosis has been detected in hypertrophied hearts regardless of the cause of the hypertrophy.^11,12 In a rodent model of LV hypertrophy, banded rats convincingly showed a substantially higher apoptotic index than did rats in a control group.¹² Using isolated adult human cardiocytes, Kajstura and colleagues⁷ showed that the apoptotic index rose in cardiocytes treated with angiotensin II (AT-II). In the same study, apoptotic cell death was prevented with angiotensin-converting enzyme (ACE) inhibitors. Angiotensin II activates proapoptotic p-53 transcription,¹³ induces apoptogenic genes (such as cMyc, c-Fos, and c-Jun), and up-regulates genes that encode for proapoptotic atrial natriuretic factor.^14,15 Both ACE inhibitors and AT-II blockers can inhibit this effect and reduce cardiocyte death due to apoptosis. Both agents counteract the effects of AT-II, including increase of afterload, ventricular remodeling, and cardiocytic and endothelial necrosis and apoptosis; they also counteract diastolic heart failure due to fibroblast proliferation and collagen deposition in the LV wall.^7,16 There is level-1 evidence supporting the use of ACE inhibitors and AT-II blockers,^17,18 especially after surgery on patients who have poorly functioning or dilated ventricles.

Congestive heart failure patients have increased baseline intracellular calcium due to high levels of AT-II and catecholamines.^7,14,19 An increase in cellular calcium ions is a proapoptotic stimulus.⁶ Because calcium channel blockers and sodium–hydrogen exchanger inhibitors reduce cellular calcium (the latter by decreasing cellular sodium), they have the potential to decrease the apoptotic index in cardiocytes at risk. Oral administration of sodium–hydrogen exchanger inhibitors has reduced death, arrhythmias, infarct size, apoptotic index, and Bcl-2/Bax ratio in an ischemic rodent model.²⁰ Although this experiment in rodents is promising, the literature still lacks a convincing study in a human model of heart failure. Calcium channel blockers have reduced the direct toxic effects of norepinephrine by decreasing intracellular calcium,¹⁹ but no studies that might show decreased apoptosis in heart failure models have been undertaken. In randomized controlled studies of human heart failure, there is no level-1 evidence that calcium channel blockers are of benefit.²¹ Because calcium channel blockers do not influence the structural cardiac remodeling process, they fail to improve outcomes of patients with congestive heart failure.²²

The level of catecholamine activity, which is elevated in failing myocardium, is associated with direct toxicity and with apoptosis of cardiocytes.^8,19,23 In 2 independent studies,^5,8 the direct toxic and apoptogenic effects of norepinephrine were abolished with β-adrenergic receptor antagonists, but not with an α-receptor antagonist. The antiapoptotic effects of β-adrenergic receptor antagonists on norepinephrine-induced apoptosis have been studied and have proved true for atenolol and car-vedilol.^24,25

In clinical application, β-adrenergic receptor antag-o-nists reduce overall death and hospitalization in congestive heart failure patients.^26,27 In patients with low-output syndrome after open-heart surgery, acute myocardial infarction, or other conditions that are associated with increased endogenous or exogenous catecholamine, ino-tropic support of the heart can be accomplished with alternative medications, such as milrinone. Physicians with experience in the insertion of intra-aortic balloon pumps or ventricular assist devices might want to use them in patients who display clinically low output, as a bridge to recovery or as destination therapy. The ReMATCH trial²⁸ has provided us with level-1 evidence for the survival benefits associated with ventricular assist devices. Moreover, the literature²⁹ reveals decreased apoptotic DNA laddering and apoptotic DNA fragmentation after the insertion of a ventricular assist device. de Jonge and associates³⁰ have shown a decreased apoptotic index after unloading the heart with a LV assist device, while Rivello and colleagues³¹ have documented (by nuclear staining) decreased cardiomyocyte nuclear size and ploidy status. Therefore, timely placement of a ventricular assist device is likely to improve the prospect of myocardial recovery by reducing the apoptotic index.

Myocardial infarction is a prominent inducer of cardiocyte apoptosis and is the leading cause of heart failure. Myocytic loss due to apoptotic cell death during the acute stage of myocardial infarction has been well established in both animal and human studies. In rat hearts, an increase in the apoptotic index was present as early as 3 hours and as late as 1 month after coronary occlusion.³² Apoptotic human cardiocytes have been found chiefly in the hypoperfused border zone between the central infarct area and non-ischemic cardiac tissue.³³

Broad-spectrum caspase inhibitors can control the signal pathway of cardiocyte apoptosis. Such antiapoptotic agents have been used successfully in a rodent model of acute myocardial infarction.^34,35 Armstrong and co-workers³⁵ found that IDN-6734 reduces the size of myocardial infarction in rats if the poly-caspase inhibitor is given before reperfusion of the left anterior descending artery (47% reduction) or 1 hour after reperfusion (45% reduction). In another rodent study, zVAD.fmk reduced infarction size, improved hemodynamics, and attenuated apoptosis, in comparison with a control group.³⁴

Discussion

Cardiocyte apoptosis is a precisely orchestrated process that is hard-wired into all metazoan cells. Apoptosis contributes to the development of congestive heart failure in at least 2 distinct ways. First, it reduces the number of contractile cardiocytes through programmed cell death. This low, but abnormal, rate of cardiocyte apoptosis amounts to a considerable loss for adult human myocardium, because adult human cardiocytes have lost the replicative potential of neonatal cardiocytes. Second, apoptosis of cardiac non-myocytes can contribute substantially to the progressive nature of failing myocardium: Fujiwara and coworkers^36,37 have supported the hypothesis that apoptosis of granulation cells is related to cardiac remodeling after myocardial infarction.

For the reasons mentioned above, the apoptotic sig-naling pathway in cardiocytes and non-cardiocytes is an important factor in the transition from compensated to decompensated heart failure. Antiapoptotic therapeutic interventions offer an appealing platform for devising ways in which to retard the maladaptative growth associated with congestive heart failure.

Numerous myocardial conditions and agents have been identified as inducers of cardiocyte apoptosis (Fig. 1). Table I presents a summary of the most prominent antiapoptotic therapies, which are primarily investigational and without direct application to clinical practice. The use of β-adrenergic receptor antagonists, AT-II blockers, and ACE inhibitors has been associated with decreased apoptotic activity. These medications have proved in randomized controlled studies to be of substantial clinical benefit to heart failure patients. What remains unclear is the specific survival benefit that might be attached to the antiapoptotic characteristics of these medications.

TABLE I. Investigational Options for Treatment of the Failing Myocardium

Most antiapoptotic treatment of heart failure is pharmaceutical in nature. The investigations have been led mostly by our nonsurgical colleagues and, therefore, are rarely applicable to the practice of surgeons who treat the heart-failure population. Myocardial infarction, volume and pressure overload, and clinical conditions associated with sustained or acute increases in catecholamine levels are all stimuli of apoptosis that are clinically relevant to the surgeon. The therapeutic ramifications of these events have, in some cases, already been incorporated into the clinical practice of modern heart failure medicine and surgery (Table II). The pertinent preventive measures on the surgical side include reducing myocardial ischemia time with prompt coronary artery revascularization, early valvular repair or replacement (before deterioration of ventricular function), and afterload reduction with intra-aortic balloon pulsation when cardiac function has been compromised after myocardial infarction or ischemic mitral regurgitation.

TABLE II. Antiapoptotic Strategies in Treatment of the Failing Myocardium

The use of cardiopulmonary bypass has been associated with increased apoptotic indices. Blood contact with foreign surfaces raises the serum level of Fas and Fasligand in human beings who undergo cardiopulmonary bypass,⁵⁶ which in turn increases apoptotic cell death in the myocardium and other organ tissues through the cellular death receptor pathway. Some of the features of post-bypass syndrome, including cerebral dysfunction and renal insufficiency, may be secondary to apoptotic injury to other organs.^57,58 Although the apoptosis associated with coronary artery bypass grafting can be avoided by performing the operation without cardio-pulmonary bypass, off-pump processes are not feasible in all currently performed coronary operations or in any other open-heart operation involving valve repair or replacement. However, an important therapeutic goal can still be attained if the pro-inflammatory and proapoptotic state of cardiopulmonary bypass can be ameliorated. Potential therapeutic measures are the reduction of foreign surfaces, the shortening of bypass time, and the addition of antiapoptotic medication to the priming solution in the cardiopulmonary bypass circuit. Myocardial protection itself can be altered to reduce apoptosis. A myocardial protection plan that depends heavily upon hypothermia will result in longer bypass times and increased apoptotic indices. Alternatively, surgeons who use normothermic techniques will achieve faster restoration of cardiac performance, decreased reperfusion time, and less use of catecholamines. Such procedures are beneficial in reducing the apoptotic rate in the failing myocardium.

Reperfusion injury after cardioplegia, another apop-totic state that confronts cardiac surgeons on a daily basis, is another treatment target. Antioxidants, poly(adenosine diphosphate ribose) synthetase inhibitors, and calcium channel blockers may be useful in decreasing reactive oxygen radicals, reducing neutro-phil-mediated inflammatory cascade, and attenuating cellular calcium overload.39,40,49–51 However, all of these agents are investigational for clinical purposes. In order to receive FDA approval for clinical practice, they must be studied and endorsed by a heart failure specialist as a sole treatment, or by a heart failure surgeon as an adjunct to cardioplegic solution or to the cardiopulmonary bypass circuit.

In order to present a critical overview of the current studies on apoptosis in failing myocardium, it is important to understand that our current knowledge about cardiocyte apoptosis has substantial limitations. It is unknown, for example, whether apoptosis is the primary or the secondary event in pathogenic cardiac conditions. Also, further investigation is needed to determine whether inhibition of apoptosis can delay disease progression in animal models and in subsequent clinical applications in human beings. Antiapoptotic pharmaceutical agents have the highest potential to become clinically important as a therapeutic option. These medications could be administered preoperatively as an adjunct to the priming solution of the cardiopulmonary circuit or intraoperatively as an adjunct to the cardioplegic solution. However, the safety and long-term consequences of these therapies have not been adequately investigated. Such therapies could possibly lead to alternative modes of cellular death, such as necrosis, or could increase autoimmune and lymphoproliferative disorders. Also of concern is the imprecision of the techniques used to detect cardiocyte apoptosis. The positive predictive value of terminal deoxynucleotidyl-transferase-mediated dUTP nick end-labeling (TUNEL), which is the method most commonly used to detect apoptosis, is less than satisfactory. The heterogeneity of examined tissue samples can reduce the accuracy of DNA-laddering, because it cannot specify the cell type that is undergoing apoptosis in a tissue sample comprising multiple types of cells. The brevity of apoptosis is another reason for the low sensitivity rates of current diagnostic methods. Indeed, these factors could explain inconsistencies in the reported frequency of human cardiocyte apoptosis (Table III). More accurate detection tools are now under development.

TABLE III. Published Series on the Frequency of Human Cardiomyocyte Apoptosis and on the Method of Its Detection

The prevention or attenuation of cardiocytic apoptosis is a very appealing therapeutic goal in the treatment of congestive heart failure, and as we accumulate more data, the spectrum of therapeutic methods will widen. It is vital for physicians to understand the treatment options and to incorporate them to the extent possible into the practice of modern heart failure medicine and surgery.

Acknowledgment

The authors wish to acknowledge Rita Laire for her help in preparing the manuscript.