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(Circulation Research. 2003;93:896.)
© 2003 American Heart Association, Inc.

Reviews

What Is the Role of ß-Adrenergic Signaling in Heart Failure?

Martin J. Lohse, Stefan Engelhardt, Thomas Eschenhagen

From the Institute of Pharmacology (M.J.L., S.E.), Wuerzburg, Germany; Institute of Experimental and Clinical Pharmacology (T.E.), Universitaetsklinikum Hamburg-Eppendorf, Germany.

Correspondence to Martin Lohse, Institute of Pharmacology, Versbacher Straße 9, 97078 Wuerzburg, Germany. E-mail lohse{at}toxi.uni-wuerzburg.de

Steven Houser Guest Editor

This Review is part of a thematic series on Unanswered Questions in Heart Failure, which includes the following articles:


   Is Depressed Myocyte Contractility Centrally Involved in Heart Failure?
   What Is the Role of ß-Adrenergic Signaling in Heart Failure?
   What Causes Sudden Death in Heart Failure?
   Is Abnormal Cell Growth and Hypertrophy the Cause of Heart Failure?
   Does Energy Starvation Cause Heart Failure?
   What Mechanisms Underlie Diastolic Dysfunction in Heart Failure?

	Abstract

Top Abstract Introduction ß-Adrenergic Receptor... Alterations of the ß... Role of GRKs in... Cardiac ß-Adrenergic... Which Mechanisms Mediate... Impact on Clinical Medicine ß-Blockade in Heart... Conclusions References

 
This review addresses open questions about the role of ß-adrenergic receptors in cardiac function and failure. Cardiomyocytes express all three ß-adrenergic receptor subtypes—ß₁, ß₂, and, at least in some species, ß₃. The ß₁ subtype is the most prominent one and is mainly responsible for positive chronotropic and inotropic effects of catecholamines. The ß₂ subtype also increases cardiac function, but its ability to activate nonclassical signaling pathways suggests a function distinct from the ß₁ subtype. In heart failure, the sympathetic system is activated, cardiac ß-receptor number and function are decreased, and downstream mechanisms are altered. However, in spite of a wealth of data, we still do not know whether and to what extent these alterations are adaptive/protective or detrimental, or both. Clinically, ß-adrenergic antagonists represent the most important advance in heart failure therapy, but it is still debated whether they act by blocking or by resensitizing the ß-adrenergic receptor system. Newer experimental therapeutic strategies aim at the receptor desensitization machinery and at downstream signaling steps.

Key Words: ß-adrenergic receptors • G proteins • transgenic mice • cardiac hypertrophy • apoptosis

	Introduction

Top Abstract Introduction ß-Adrenergic Receptor... Alterations of the ß... Role of GRKs in... Cardiac ß-Adrenergic... Which Mechanisms Mediate... Impact on Clinical Medicine ß-Blockade in Heart... Conclusions References

 
The human heart expresses ß₁- and ß₂-adrenergic receptors at a ratio of about 70:30; both subtypes increase cardiac frequency and contractility.^1,2 In addition, ß₃-receptors have been described to mediate negative inotropic effects,³ but their role remains uncertain.⁴ All three subtypes appear to occur in cardiomyocytes, but they seem to possess distinct intracellular signaling and functional properties.^5,6

Two well-established lines of evidence suggest that the ß-adrenergic receptor system plays a major role in heart failure. First, there is a pronounced activation of the sympathetic system in patients with heart failure that is inversely correlated with survival.⁷ Second, cardiac ß-receptors, in particular the ß₁ subtype, are downregulated in heart failure,^1,8 and the remaining receptors are uncoupled from G_s, presumably via increased activity of the receptor kinases GRK2 and/or GRK5.^9–11 Furthermore, an increase in G_i subunits antagonizes ß-adrenergic signaling.^12–14 Clinically, the use of ß-receptor antagonists in heart failure, pioneered in the 1970s,¹⁵ is now standard treatment.^16–19

In spite of these major advances, many fundamental questions have remained unanswered. Why does the heart express three different ß-receptors and what are the differences between the subtypes? Are the alterations of the ß-receptors in heart failure detrimental or beneficial? Does the ß-receptor system contribute to the pathogenesis of heart failure? And why are ß-blockers effective in the treatment of heart failure? This review will attempt to elucidate open questions in understanding myocardial ß-adrenergic signaling with respect to its role in cardiac hypertrophy and failure.

	ß-Adrenergic Receptor Subtypes and Their Signaling

Top Abstract Introduction ß-Adrenergic Receptor... Alterations of the ß... Role of GRKs in... Cardiac ß-Adrenergic... Which Mechanisms Mediate... Impact on Clinical Medicine ß-Blockade in Heart... Conclusions References

 
ß-Adrenergic Receptors in the Heart
The classical subdivision of ß-receptors defines the ß₁ subtype as the one that stimulates cardiac muscle, and the ß₂ subtype that relaxes smooth muscle.²⁰ Both receptors couple to G_s and thereby elevate cAMP, but distinct downstream signaling decreases contractility in smooth muscle cells and increases it in cardiomyocytes. However, the ß₁-receptor contributes to relaxation of blood vessels²¹ and the ß₂-receptor to cardiac contractility.² Even though the predominant cardiac ß₁ subtype (70% to 80% depending on species) is the strongest stimulus for cardiac function, the expression levels are quite small: no more than 50 to 70 fmol/mg membrane protein in most species. Therefore, there is little receptor reserve.

Expression of the ß₃ subtype is essentially limited to adipose tissue,²² but several groups have reported ß₃-receptor effects and mRNA in human, guinea pig, and canine heart and cardiomyocytes.^3,23,24 However, in contrast to its G_s-mediated signaling in adipose tissue, these reports suggest coupling to a nonclassical G_i/nitric oxide pathway mediating negative inotropic effects. In mice, cardiac-specific overexpression of ß₃-receptors enhanced cardiac contractility,²⁵ and experiments with ß₁/ß₂ knockout mice provided no or very modest ß₃-receptor effects.^4,26,27 Thus, the importance of this subtype remains to be defined.

A fourth receptor subtype, ß₄, had been postulated to mediate cardiostimulatory effects of the antagonist CGP12177. However, studies with ß₁- and ß₂-receptor knockout mice have led to the conclusion that the ß₄ effects are mediated by the ß₁-receptor.^28,29

ß-Receptors occur also on nonmyocyte cells in the heart, and these cells can—eg, by paracrine effects—affect cardiomyocyte function and fate. The communication between these cells will constitute a major research topic.

ß-Adrenergic Receptor Signaling
Why should the heart express several different subtypes of ß-adrenergic receptors? Evidence has been accumulating that subtype differences are important for cardiac function and failure.

First, the three receptor subtypes have different affinities for their ligands (Figure 1). Second, there is increasing evidence for specific subcellular localizations and distinct signaling pathways. The basic hypothesis is that spatial segregation of receptors allows their association with other—equally segregated—proteins to form "signalosomes" that mediate subtype-specific responses.

View larger version (128K): [in this window] [in a new window]   Figure 1. Agonist activation and coupling/signaling properties of ß-adrenergic receptor subtypes. GRK indicates G protein–coupled receptor kinase; ßArr, ß-arrestin; PDE, phosphodiesterase; PI3K, phosphatidylinositol 3-kinase; and AC, adenylyl cyclase. Data from Hoffmann et al.184

Signaling by cardiac ß-receptors has been studied in great detail (Figures 1 and 2). The classical common pathway is activation of adenylyl cyclases via G_s, resulting in increased cAMP levels. The primary target for cAMP is protein kinase A (PKA). PKA phosphorylates several proteins that are essential for cardiac function: L-type calcium channels,^30,31 phospholamban,³² troponin I,³³ ryanodine receptors,³⁴ myosin binding protein-C (MyBP-C),³⁵ and protein phosphatase inhibitor-1.³⁶ This affects cardiomyocyte contractile behavior by increasing Ca²⁺ influx (L-type channel), increasing Ca²⁺ reuptake into the sarcoplasmic reticulum (phospholamban/SERCA), and modulating myofilament Ca²⁺ sensitivity (troponin I, MyBP-C). Another target of cAMP are cAMP-gated HCN pacemaker channels.³⁷

View larger version (98K): [in this window] [in a new window]   Figure 2. Calcium cycling in cardiac myocytes and regulation by PKA. AC indicates adenylyl cyclase; RyR, ryanodine receptor; PLB, phospholamban; SERCA, sarcoplasmic reticulum calcium ATPase; CaM, calmodulin; CaMK, calmodulin-dependent kinase; CaN, calcineurin; GRK, G protein–coupled receptor kinase; NCX, sodium-calcium exchanger; NHE, sodium-proton exchanger; and PP, protein phosphatase.

Most studies agree that in cardiomyocytes the cAMP pathway is stimulated through ß₁- as well as ß₂-receptors. However, even though the ß₂ subtype causes greater adenylyl cyclase stimulation than the ß₁ subtype in transfected cells^38,39 and in cardiomyocytes,^40,41 the ß₁ subtype confers greater functional effects in cardiomyocytes.² One explanation for this difference is that cAMP generated by ß₂-receptor stimulation is not equivalent to cAMP-generated via ß₁-receptors. Another explanation is the existence of additional signaling pathways that modify the G_s/adenylyl cyclase/PKA pathway.

Such additional nonclassical signaling is particularly important for the ß₂ (and perhaps the ß₃) subtype, but less for the ß₁-receptor. Many coupling proteins have been identified for the three ß-receptor subtypes,⁴² but only a few of them have been demonstrated in the heart (Figure 1). Nonclassical signaling for the ß₁-adrenergic receptor—studied less—may include a calcium signal not inhibitable by inactive cAMP analogues.⁴³ It will be a major task to elucidate the physiological role(s) of these nonclassical signaling pathways.

Compartmentation of ß-Adrenergic Signaling
Receptor-generated signals are usually measured as global changes in second messenger concentrations, which are implicitly assumed to change in a uniform manner. This simplistic view would mean that intracellular signaling uses neither spatial nor temporal information. However, evidence is accumulating that this is not the case, and that compartmentation of intracellular signaling may be more than an excuse for results that are difficult to interpret.

Studies on intact hearts and isolated cardiomyocytes helped to establish this concept.^44,45 Work in the 1970s suggested that PKA activation by prostaglandin E and isoprenaline exerted differential effects on glycogen phosphorylase phosphorylation in rat heart.⁴⁶ Similarly, cAMP accumulation via the glucagon-like peptide receptor was recently found to be completely uncoupled from inotropic effects in cardiomyocytes.⁴⁷ Different "efficacies" of cAMP generated via ß-receptor activation or via direct adenylyl cyclase stimulation with forskolin further suggested spatial compartmentation of cAMP in cardiomyocytes.⁴⁸ Recent electrophysiological studies showed that localized stimulation of ß-receptors on frog cardiomyocytes activated L-type Ca²⁺ channels in the vicinity of the receptors, whereas forskolin activated also distant Ca²⁺ channels.⁴⁹ Inhibition of isoprenaline effects by acetylcholine⁵⁰ and NO were also locally restricted.⁵¹ Imaging of cAMP in neonatal cardiomyocytes showed that the noradrenaline-induced cAMP signal diffused only 1 µm, but phosphodiesterase inhibition led to generalized cAMP elevation.⁵² Thus, cAMP degradation by phosphodiesterases may spatially limit cAMP signals.

Several pieces of evidence point toward differences in compartmentation between ß₁- and ß₂-receptors.^5,6 First, ß₂-receptors can couple, in addition to G_s, also to G_i, whereas ß₁-receptors couple only to G_s.⁵³ G_i coupling is enhanced by PKA-mediated ß₂-receptor phosphorylation. Second, the PKA phosphorylation pattern induced by ß₁- and ß₂-receptor stimulation seems different, at least in some species. Two groups found only ß₁-induced troponin I phosphorylation in rats,^54,55 whereas others described phosphorylation via both subtypes in human heart.⁵⁶ Phospholamban phosphorylation has been reported after ß₁- but not ß₂-receptor activation in canine and human cardiomyocytes,^54,57 while in other studies, particularly in human heart, both subtypes caused phosphorylation.^55,56 ß₁-Receptor stimulation increased PKA activity in the particulate fraction of cardiomyocytes, whereas after ß₂-receptor stimulation this increase was limited to the soluble fraction.⁵⁵ In adult rat cardiomyocytes, the complete absence of a cAMP signal after ß₂-receptor stimulation has been reported.⁵⁸

Which mechanisms permit spatial compartmentation of cardiomyocyte ß-adrenergic signaling? First, receptors might have different cell surface localizations. In one study, the ß₂ subtype was copurified with cardiomyocyte caveolae, while the ß₁ subtype was more evenly distributed.⁵⁹ In another study, the ß₁ subtype was preferentially localized in caveolae on rat neonatal cardiomyocytes, and this was taken as a reason for efficient adenylyl cyclase coupling.⁴⁰ Second, the receptors are probably embedded into large signalosomes, which might differ between the ß₁ and the ß₂ subtype. ß₂-Receptor signalosomes containing an entire signaling chain have recently been demonstrated in neurons.⁶⁰ And third, postreceptor signaling may also be spatially organized.⁶¹ cAMP signals might be spatially regulated via their site of generation (receptor localization) and via localized destruction (phosphodiesterases). Several experiments show loss of spatial localization after phosphodiesterase inhibition.^48,52,62 ß₂ Receptors can actively recruit phosphodiesterase 4 to the plasma membrane.⁶³ Downstream signaling proteins also have specific localizations. In particular, PKA is spatially localized via binding to A-kinase anchoring proteins⁶⁴ and the same applies for protein phosphatases⁶⁵ and possibly their inhibitors.

In addition to spatial compartmentation, ß-receptor coupling is also temporally regulated. First, ß-receptors desensitize. This is most prominent for the ß₂ subtype and small for the ß₃ subtype.^66,67 Desensitization occurs via receptor phosphorylation either by PKA or by G protein–coupled receptor kinases, GRKs,^68,69 plus ß-arrestins.⁷⁰ Both mechanisms uncouple receptors from G proteins.⁷¹ In addition, the receptor/ß-arrestin complex recruits several proteins that initiate nonclassical signaling pathways. Since the GRK/ß-arrestin mechanism is most prominent for the ß₂ subtype, this explains why nonclassical signaling pathways are most prominent for this subtype. Furthermore, PKA-induced phosphorylation of ß₂-receptors promotes switching from G_s to G_i.^27,72 And finally, prolonged stimulation results in receptor downregulation, ie, a reduction in receptor number.⁷³ In summary, ß-receptor signaling is a multifaceted process, and our current understanding seems still incomplete.

	Alterations of the ß-Adrenergic System in Heart Failure

Top Abstract Introduction ß-Adrenergic Receptor... Alterations of the ß... Role of GRKs in... Cardiac ß-Adrenergic... Which Mechanisms Mediate... Impact on Clinical Medicine ß-Blockade in Heart... Conclusions References

 
Numerous studies show alterations of the cardiac ß-receptor system in failing hearts.^1,2,74–76 They include a reduction of the ß₁ subtype and its mRNA by up to 50%, correlated to disease severity,^75,76 while the ß₂-receptor levels remained unchanged in most studies. It is still not clear why downregulation in heart failure is specific for the ß₁ subtype. The remaining ß-receptors are desensitized, presumably mostly via GRKs (see below). In addition, up to 2-fold increases of G_i—particularly G_i2—and its mRNA occur early in heart failure^12–14,77 and may cause reduced responsiveness of many G_s-coupled receptor systems.^78–81 In addition, canine heart failure models show downregulation of G_s⁸² and of adenylyl cyclases V and VI,⁸³ which are rate-limiting in the ß-receptor system.⁸⁴ Heart failure–induced elevated catecholamine levels most likely cause all these alterations that functionally limit the contractile reserve.

Even though these changes have been confirmed repeatedly, their interpretation is uncertain. They can be interpreted either as a beneficial mechanism that protects the heart from the detrimental effects of chronic ß-receptor stimulation, including arrhythmias, energy dysbalance, hypertrophy, and apoptosis—even though they deprive the heart from the benefits of short-term ß-adrenergic responsiveness. Alternatively, they may lead to further deterioration of heart failure, since they disable the heart to meet its demands. Depending on these interpretations, therapeutic strategies might attempt either to inhibit the ß-receptor system even further or to restore its sensitivity. Both apparently contrasting strategies are currently being pursued.

	Role of GRKs in Heart Failure

Top Abstract Introduction ß-Adrenergic Receptor... Alterations of the ß... Role of GRKs in... Cardiac ß-Adrenergic... Which Mechanisms Mediate... Impact on Clinical Medicine ß-Blockade in Heart... Conclusions References

 
Increased GRK activity appears to be a major factor contributing to ß-receptor desensitization in failing hearts.^85,86 Numerous studies have demonstrated upregulation of GRK activity and GRK2 mRNA in patients and animal models of heart failure and hypertrophy.^11,87,88 This has led to the hypothesis that cardiac function might be restored by inhibiting GRKs.⁸⁶ No GRK inhibitors have so far been described that would allow a testing of this hypothesis, but several studies with the C-terminus of GRK2 ("ßARKct") appear to support it. This 184 amino acid C-terminus inhibits GRK-mediated receptor phosphorylation,⁸⁹ and its overexpression has led to reduction of heart failure in several heart failure models.^86,90,91

However, again the alternative hypothesis is that increased GRK activity is part of a protective adaptation. This hypothesis would be compatible with the long-term damage caused by chronic stimulation or transgenic ß₁-receptor overexpression,^92,93 and with the beneficial effects of ß-blockers in heart failure patients.¹⁶ In this case, the beneficial effects of ßARKct might be mediated by mechanisms distinct from GRK2 inhibition.

In fact, ßARKct impairs many Gß pathways, and several lines of evidence suggest that Gß inhibition ("scavenging") is important for its effects. First, the detrimental ß-receptor–mediated effects appear to be due more to the cAMP than to nonclassical signaling since (1) transgenic overexpression of the ß₁ subtype is more detrimental than that of the ß₂ subtype^93,94 even though the ß₁ subtype activates essentially only the cAMP pathway and does not internalize well,^6,95 and (2) the detrimental effects of ß₁-receptor overexpression are very similar to those of overexpression of G_s or PKA.^96–98 Second, GRK2 transgenic mice show no overt cardiac pathology,⁹⁹ arguing against a detrimental role of GRK activity per se. Third, in a transgenic mouse model of heart failure, ß-blockers conferred a benefit in addition to ßARKct, suggesting an unrelated mechanism of action for the two principles.⁹¹ And fourth, the protective effects of ßARKct on heart failure progression have been reproduced with N-terminally truncated phosducin,¹⁰⁰ a supposedly "pure" Gß-binding protein¹⁰¹ that did not restore the cAMP signal.

Thus, ß-receptor blockade and ßARKct might be regarded as independent and complementary therapeutic principles in heart failure. The usefulness and the mechanisms of ßARKct and of "pure" GRK inhibitors will be questions for future investigations.

	Cardiac ß-Adrenergic Receptor Transgenic Mice

Top Abstract Introduction ß-Adrenergic Receptor... Alterations of the ß... Role of GRKs in... Cardiac ß-Adrenergic... Which Mechanisms Mediate... Impact on Clinical Medicine ß-Blockade in Heart... Conclusions References

 
Transgenic cardiac overexpression of ß₂-receptors in mice at 20 to 30 pmol/mg protein led to marked enhancement of basal contractility⁹⁴ but caused no overt cardiac pathology.¹⁰² Adenovirus-mediated ß₂-receptor overexpression enhanced myocardial contractility in a rabbit heart failure model.¹⁰³ However, later studies showed that 50-fold overexpression of ß₂-receptors was well tolerated, whereas 350-fold overexpression induced cardiac pathology.¹⁰⁴ High-density ß₂-receptor overexpression rescued left ventricular contractility after myocardial infarction¹⁰⁵ but worsened cardiac function after aortic banding¹⁰⁶ and negatively affected several genetic heart failure models,⁸⁶ including G_q overexpression.¹⁰⁷ Lower levels (30-fold overexpression) had beneficial effects in the same model,¹⁰⁷ suggesting an expression optimum for enhancing cardiac function via ß₂-receptors.

Cardiac overexpression of ß₁-receptors in transgenic mice caused cardiomyocyte hypertrophy, followed by fibrosis and heart failure.^93,108 Calcium transients were prolonged, and expression of the sarcoplasmic reticulum (SR) protein junction was reduced.¹⁰⁹ Interestingly, ß₁-receptor transgenic mice develop marked cardiomyocyte hypertrophy without a major increase in heart weight,⁹³ indicating a dramatic loss of ventricular cardiomyocytes, perhaps via apoptosis.¹⁰⁸ ß₁ as well as ₁ agonists have long been known to cause hypertrophy.¹¹⁰ These data indicate that the ß₁-receptor system is ideally suited for short-term increases in cardiac function but causes marked damage after prolonged activation.

The differences reported between ß₁-versus ß₂-receptor overexpression are remarkable. Since both subtypes activate cAMP signaling, they must be due to nonclassical, receptor-specific pathways such as ß₂-receptor coupling to G_i and mitogen-activated protein (MAP) kinases.^72,111,112 In addition, compartmentation of cAMP signals might cause differences between ß₁ and ß₂-receptor–generated cAMP.⁹⁸ The potential clinical implications of these differences are obvious. For example, ß₁-selective antagonists might be superior to nonselective blockers in heart failure, because they would leave the ß₂-receptor operational. And increasing ß₂-receptors to an optimum level might also be beneficial.¹⁰³

	Which Mechanisms Mediate Detrimental ß1-Receptor Effects?

Top Abstract Introduction ß-Adrenergic Receptor... Alterations of the ß... Role of GRKs in... Cardiac ß-Adrenergic... Which Mechanisms Mediate... Impact on Clinical Medicine ß-Blockade in Heart... Conclusions References

 
Candidate Downstream Targets
The potentially ß₁-selective damage must be mediated via ß₁-initiated signals. Since the major ß₁ signal in cardiomyocytes is the cAMP/PKA pathway, the prime suspects are the targets of PKA-mediated phosphorylation. Identifying the relevant targets is difficult since our knowledge is mostly derived from acute ß-receptor stimulation, while in heart failure ß-adrenergic activation lasts for years. Adaptive changes and transcriptional/translational alterations may dominate long-term responses. A striking example of opposite effects of short-versus long-term ß-adrenergic stimulation is the sodium proton exchanger NHE1.

Phospholamban
The most prominent cardiac target of PKA is phospholamban (PLB), a 52 amino acid phosphoprotein that controls SR Ca²⁺ uptake by inhibiting SERCA.³² PKA-mediated phosphorylation of PLB relieves this inhibition. Experiments with PLB knockout mice indicate that PLB mediates the positive lusitropic and part of the positive inotropic ß-receptor effects.^113,114 Even though phospholamban is a major PKA target in cardiomyocytes, it is probably not responsible for the detrimental ß₁-receptor effects. First, PLB knockout rescued several (but not all^115,116) heart failure models including ß₁-receptor transgenic mice,¹¹⁷ suggesting that phospholamban inhibition by PKA is not detrimental. Second, gene therapy approaches inhibiting phospholamban^118,119 or augmenting SERCA^120,121 ameliorated several heart failure models. However, there may be important differences between mice and humans, since two inactivating mutations of phospholamban have recently been reported to cause heart failure in patients.^122,123 At present, it is unclear how these findings can be integrated.

Other Calcium Regulatory Proteins
Ryanodine receptor (RyR) hyperphosphorylation by PKA leading to increased open probability has been implicated in the pathogenesis of heart failure.³⁴ Others have disputed this observation¹²⁴ or attributed the PKA-induced increase in SR Ca²⁺ release to an indirect mechanism involving phospholamban.¹²⁵ Thus, the role of RyR phosphorylation in heart failure awaits clarification.¹²⁶

PKA-mediated phosphorylation opens L-type Ca²⁺ channels triggering SR Ca²⁺ release through the RyR.¹¹⁴ Basal L-type currents are maintained¹²⁷ in heart failure, but single-channel recordings show an increased open probability.¹²⁸ Further PKA targets are MyBP-C and troponin I, which may disinhibit myosin actin interaction³⁵ and reduce Ca²⁺ sensitivity of myofilaments,¹²⁹ respectively. It is not clear whether these effects contribute to heart failure.

A functional imbalance between pathways in or decreasing diastolic Ca²⁺ might lead to elevated cytosolic Ca²⁺ levels as a common final pathway in failing cardiomyocytes.¹³⁰ The downstream mechanisms exerting possible detrimental Ca²⁺ effects and the roles of Ca²⁺-dependent proteins such as calcineurin and CaM kinase remain to be elucidated.¹³¹

Protein Phosphatases and Protein Phosphatase Inhibitor-1
Recent evidence indicates a regulatory role for phosphatases in cardiomyocytes.¹³² Phosphatase 2A occurs in ß₂-receptor signalosomes in neurons⁶⁰ and colocalizes with the RyR.³⁴ Heart failure is accompanied by increased global protein phosphatase (PP) activity.¹³² The protein phosphatase inhibitor-1, PPI-1, inhibits PP1 only in its PKA-phosphorylated form and seems to amplify ß-adrenergic signaling in cardiomyocytes.^133,134 PPI-1 mRNA, protein, and phosphorylation are reduced by 2- to 5-fold in failing human hearts,^134,135 leading to reduced PP1 inhibition.¹³² Expression of constitutively active PPI-1 rescued the function of failing cardiomyocytes.¹³³ In order to delineate a role of PPI-1 in heart failure, the proteins regulated by PPI-1–sensitive dephosphorylation need to be identified.

Sodium Proton Exchanger NHE1
A remarkable example of how fundamentally short- and long-term effects can differ is the involvement of the cardiac sodium proton exchanger NHE1 in the detrimental effects of chronic ß-receptor stimulation. Acute ß-receptor stimulation may inhibit NHE1.¹³⁶ However, in ß₁-receptor transgenic mice NHE1 is upregulated, and pharmacological NHE1 inhibition prevented the development of hypertrophy, fibrosis, and heart failure.¹³⁷ The mechanisms of the protective effects of NHE1 inhibition, the roles of intracellular sodium and calcium, and NHE1 regulation remain to be investigated.^137,138

Apoptosis
ß-Receptor stimulation causes apoptosis of isolated rat cardiomyocytes. Different approaches demonstrate proapoptotic effects of ß₁ and antiapoptotic effects of ß₂-receptors.^139–141 These in vitro studies are paralleled by findings in transgenic mice where ß₁-receptor and G_s overexpression markedly increased cardiomyocyte apoptosis.^108,142 However, the studies differ as to the responsible intracellular signaling pathways. ß₂-Receptor–mediated stimulation of p38 MAP kinase¹¹¹ and activation of Akt kinase via G_i¹⁴¹ have been proposed as antiapoptotic mediators. The proapoptotic effect of ß₁-receptors has been found to be dependent on reactive oxygen species,¹⁴³ while others imply PKA-independent activation of CaMK.⁴³ Thus, the mechanistic links for the opposing effects of ß₁-versus ß₂-receptor stimulation on cardiomyocyte apoptosis remain uncertain. Nor is it clear how much ß₁-receptor–mediated apoptosis contributes to heart failure.¹⁴⁴

	Impact on Clinical Medicine

Top Abstract Introduction ß-Adrenergic Receptor... Alterations of the ß... Role of GRKs in... Cardiac ß-Adrenergic... Which Mechanisms Mediate... Impact on Clinical Medicine ß-Blockade in Heart... Conclusions References

 
ß-Adrenergic Receptor Polymorphisms
Genes for all three ß-receptor subtypes contain single nucleotide polymorphisms. Eighteen ß₁-receptor variants¹⁴⁵ and 13 ß₂-receptor variants have been described,¹⁴⁶ but only two ß₁ and three ß₂ variant genes are common and have been extensively studied with respect to cardiovascular function (Table). Two attractive hypotheses arise from these studies.

View this table: [in this window] [in a new window]   Table 1. Cardiovascular Role of ß1-Receptor Arg389Gly and ß2-Receptor Thr164Ile Polymorphisms

First, the ß₁-receptor Gly³⁸⁹ variant (allele frequency 25% in whites, 42% in blacks) shows reduced cAMP signals¹⁴⁷ and may represent an impaired variant of the potentially harmful ß₁-receptor. Clinical studies have given mixed results (Table), suggesting that the more active Arg³⁸⁹ variant may be detrimental in some contexts. For example, the risk for heart failure in blacks was increased for the Arg³⁸⁹ variant only when combined with the _2C-receptor deletion polymorphism (allele frequency 4% in whites, 38% in blacks),¹⁴⁸ which is a risk factor by itself.¹⁴⁹ Large cohort studies investigating more complete sets of such polymorphisms including complete haplotypes will be needed for an answer.

Second, the ß₂-receptor Ile¹⁶⁴ variant is a rare variant of the potentially protective ß₂-receptor displaying reduced agonist-binding and activity when expressed in fibroblasts³⁸ or transgenic mice.¹⁵⁰ Healthy volunteers or patients heterozygous for the Ile¹⁶⁴ variant exhibited lower chronotropic and inotropic response to the ß₂ agonist terbutaline, reduced exercise capacity, and decreased survival in heart failure (Table). These findings support the hypothesis that an impaired ß₂-receptor represents a risk factor for cardiovascular disease and particularly heart failure.

The Gly¹⁶ and the Gln²⁷ variants of the ß₂-receptor may exhibit altered agonist-induced downregulation and have been associated with decreased exercise capacity. These data are inconsistent and have been discussed elsewhere.¹⁵¹

	ß-Blockade in Heart Failure

Top Abstract Introduction ß-Adrenergic Receptor... Alterations of the ß... Role of GRKs in... Cardiac ß-Adrenergic... Which Mechanisms Mediate... Impact on Clinical Medicine ß-Blockade in Heart... Conclusions References

 
Since the pioneering work in the 1970s,¹⁵² it took two decades until ß-blockers turned from contraindication to standard treatment in heart failure.¹⁶ Blocking ß-receptors when cardiac function depends on sympathoadrenergic drive long appeared counterintuitive. Today, three large studies with bisoprolol, metoprolol, and carvedilol show a similar reduction in the risk of death by a third or more, a benefit greater than of any other drug used in heart failure.¹⁶

Why Do ß-Blockers Work in Heart Failure?
How can long-term application of negative inotropic compounds increase cardiac index, exercise capacity, and survival? Two basic mechanisms might explain this paradox: a block of the detrimental consequences of sustained ß₁-receptor stimulation or resensitization of the cardiac ß-receptor system. We are not aware of studies that would permit a definitive decision, but several arguments support the role of blocking detrimental ß₁ effects.

First, ß-blockers showed benefits in all patient subgroups, including those with high and low heart rate, blood pressure, and ejection fraction.¹⁸ Thus, the benefit is not restricted to patients with high heart rate. Second, ß-blockers exert antiarrhythmic effects, which may explain why they caused larger reductions in sudden deaths than in total mortality. Antiarrhythmic effects are particularly important since altered calcium handling makes failing hearts very susceptible to arrhythmias. Third, ß-blockers might prevent the hypertrophic, proapoptotic, and pronecrotic effects of cardiomyocyte ß₁-receptor stimulation. Fourth, ß-blockers might improve the energy balance in failing hearts, which show energy starvation¹⁵³ and high-energy phosphate depletion,¹⁵⁴ since they reduce heart rate and improve diastolic filling and blood flow. ß-Blockers apparently induce a partial switch from fatty acid to glucose metabolism by inhibiting carnitine palmitoyl transferase.⁷ And finally, ß-blockers reverse failure-specific alterations in cardiac gene expression, which may be involved in progression of the disease.^155–158

The resensitization hypothesis is supported by the fact ß-blockers upregulate ß-receptor levels and normalize elevated GRK and G_i levels.^155–159 While these receptors are partially available even in the continued presence of ß-blockers,¹⁶⁰ it is doubtful whether their increase is functionally relevant.^161,162 However, resensitization of downstream elements does result in enhanced responses to phosphodiesterase inhibitors.¹⁶¹

Differences Between ß-Blockers
Bisoprolol, carvedilol, and metoprolol have been shown to be beneficial in heart failure, and others may follow. Metoprolol and bisoprolol are ß₁-selective and have modest inverse activity (ie, they decrease the spontaneous activity of the receptor in the absence of agonist),¹⁶³ whereas carvedilol is nonselective, shows no inverse activity,¹⁶³ dissociates slowly from the receptor,¹⁶⁴ and is a radical scavenger and ₁-receptor antagonist. All three compounds led to similar clinical results, but carvedilol was superior to metoprolol in the recent COMET trial.¹⁶⁵ Methodological criticism aside, this trial does not answer the questions of possibly protective ß₂-receptors and whether the additional properties of carvedilol are important. A role for intrinsic activity is supported by the observations that the strong partial agonist xamoterol¹⁶³ was detrimental in heart failure,¹⁹ and the weak partial agonist bucindolol¹⁶⁶ was ineffective in the BEST trial.¹⁶⁷ The beneficial ß-blockers are neutral (carvedilol) or inverse agonistic (bisoprolol, metoprolol).¹⁶³ However, because of the low constitutive activity of the ß₁-receptor, it is uncertain how important inverse agonism is. Taken together, the question is still open which pharmacological properties of ß-blockers make them effective in heart failure.

	Conclusions

Top Abstract Introduction ß-Adrenergic Receptor... Alterations of the ß... Role of GRKs in... Cardiac ß-Adrenergic... Which Mechanisms Mediate... Impact on Clinical Medicine ß-Blockade in Heart... Conclusions References

 
Overwhelming evidence supports a major role for the ß-adrenergic system in heart failure. While this system is ideally suited for short-term increases in cardiac performance, its long-term activation is apparently detrimental. These damaging effects appear to be mainly due to stimulation of the ß₁ subtype, but the responsible signaling pathways need to be identified. Relevant beneficial effects of the ß₂ subtype remain to be confirmed, again together with the elucidation of the responsible—presumably nonclassical—signaling pathways. And the role of the ß₃-receptor in the heart is still unclear.

The many changes in the ß-adrenergic system in heart failure are most likely a protective adaptation. ß-Blockers presumably act by (further) inhibiting the detrimental effects of ß₁-receptor stimulation, but perhaps also by resensitizing downstream signaling elements. Future questions include the role of resensitization, the essential properties of clinically effective ß-blockers, and the importance of the many downstream signaling steps in finding new strategies for the treatment of heart failure.

	Footnotes

 
Original received May 21, 2003; revision received October 3, 2003; accepted October 8, 2003.

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Top Abstract Introduction ß-Adrenergic Receptor... Alterations of the ß... Role of GRKs in... Cardiac ß-Adrenergic... Which Mechanisms Mediate... Impact on Clinical Medicine ß-Blockade in Heart... Conclusions References

 

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