Effects of two Gbetagamma-binding proteins--N-terminally truncated phosducin and beta-adrenergic receptor kinase C terminus (betaARKct)--in heart failure

Medium-chain fatty acids modulate myocardial function via a cardiac odorant receptor

Several studies have demonstrated the expression of odorant receptors (OR) in various human tissues and their involvement in different physiological and pathophysiological processes. However, the functional role of ORs in the human heart is still unclear. Here, we firstly report the functional characterization of an OR in the human heart. Initial next-generation sequencing analysis revealed the OR expression pattern in the adult and fetal human heart and identified the fatty acid-sensing OR51E1 as the most highly expressed OR in both cardiac development stages. An extensive characterization of the OR51E1 ligand profile by luciferase reporter gene activation assay identified 2-ethylhexanoic acid as a receptor antagonist and various structurally related fatty acids as novel OR51E1 ligands, some of which were detected at receptor-activating concentrations in plasma and epicardial adipose tissue. Functional investigation of the endogenous receptor was carried out by Ca²⁺ imaging of human stem cell-derived cardiomyocytes. Application of OR51E1 ligands induced negative chronotropic effects that depended on activation of the OR. OR51E1 activation also provoked a negative inotropic action in cardiac trabeculae and slice preparations of human explanted ventricles. These findings indicate that OR51E1 may play a role as metabolic regulator of cardiac function.

Chapter Three - Ubiquitination and Protein Turnover of G-Protein-Coupled Receptor Kinases in GPCR Signaling and Cellular Regulation

G-protein-coupled receptors (GPCRs) are responsible for regulating a wide variety of physiological processes, and distinct mechanisms for GPCR inactivation exist to guarantee correct receptor functionality. One of the widely used mechanisms is receptor phosphorylation by specific G-protein-coupled receptor kinases (GRKs), leading to uncoupling from G proteins (desensitization) and receptor internalization. GRKs and β-arrestins also participate in the assembly of receptor-associated multimolecular complexes, thus initiating alternative G-protein-independent signaling events. In addition, the abundant GRK2 kinase has diverse "effector" functions in cellular migration, proliferation, and metabolism homeostasis by means of the phosphorylation or interaction with non-GPCR partners. Altered expression of GRKs (particularly of GRK2 and GRK5) occurs during pathological conditions characterized by impaired GPCR signaling including inflammatory syndromes, cardiovascular disease, and tumor contexts. It is increasingly appreciated that different pathways governing GRK protein stability play a role in the modulation of kinase levels in normal and pathological conditions. Thus, enhanced GRK2 degradation by the proteasome pathway occurs upon GPCR stimulation, what allows cellular adaptation to chronic stimulation in a physiological setting. β-arrestins participate in this process by facilitating GRK2 phosphorylation by different kinases and by recruiting diverse E3 ubiquitin ligase to the receptor complex. Different proteolytic systems (ubiquitin-proteasome, calpains), chaperone activities and signaling pathways influence the stability of GRKs in different ways, thus endowing specificity to GPCR regulation as protein turnover of GRKs can be differentially affected. Therefore, modulation of protein stability of GRKs emerges as a versatile mechanism for feedback regulation of GPCR signaling and basic cellular processes.

Differential effects of genetically-encoded Gβγ scavengers on receptor-activated and basal Kir3.1/Kir3.4 channel current in rat atrial myocytes

Opening of G-protein-activated inward-rectifying K(+) (GIRK, Kir3) channels is regulated by interaction with βγ-subunits of Pertussis-toxin-sensitive G proteins upon activation of appropriate GPCRs. In atrial and neuronal cells agonist-independent activity (I(basal)) contributes to the background K(+) conductance, important for stabilizing resting potential. Data obtained from the Kir3 signaling pathway reconstituted in Xenopus oocytes suggest that I(basal) requires free G(βγ). In cells with intrinsic expression of Kir3 channels this issue has been scarcely addressed experimentally. Two G(βγ)-binding proteins (myristoylated phosducin - mPhos - and G(αi1)) were expressed in atrial myocytes using adenoviral gene transfer, to interrupt G(βγ)-signaling. Agonist-induced and basal currents were recorded using whole cell voltage-clamp. Expression of mPhos and G(αi1) reduced activation of Kir3 current via muscarinic M(2) receptors (IK(ACh)). Inhibition of IK(ACh) by mPhos consisted of an irreversible component and an agonist-dependent reversible component. Reduction in density of IK(ACh) by overexpressed Gαi1, in contrast to mPhos, was paralleled by substantial slowing of activation, suggesting a reduction in density of functional M2 receptors, rather than G(βγ)-scavenging as underlying mechanism. In line with this notion, current density and activation kinetics were rescued by fusing the αi1-subunit to an Adenosine A(1) receptor. Neither mPhos nor G(αi1) had a significant effect on I(basal), defined by the inhibitory peptide tertiapin-Q. These data demonstrate that basal Kir3 current in a native environment is unrelated to G-protein signaling or agonist-independent free G(βγ). Moreover, our results illustrate the importance of physiological expression levels of the signaling components in shaping key parameters of the response to an agonist.

Cardioprotective effects of lixisenatide in rat myocardial ischemia-reperfusion injury studies

Background

Lixisenatide is a glucagon-like peptide-1 analog which stimulates insulin secretion and inhibits glucagon secretion and gastric emptying. We investigated cardioprotective effects of lixisenatide in rodent models reflecting the clinical situation.

Methods

The acute cardiac effects of lixisenatide were investigated in isolated rat hearts subjected to brief ischemia and reperfusion. Effects of chronic treatment with lixisenatide on cardiac function were assessed in a modified rat heart failure model after only transient coronary occlusion followed by long-term reperfusion. Freshly isolated cardiomyocytes were used to investigate cell-type specific mechanisms of lixisenatide action.

Results

In the acute setting of ischemia-reperfusion, lixisenatide reduced the infarct-size/area at risk by 36% ratio without changes on coronary flow, left-ventricular pressure and heart rate. Treatment with lixisenatide for 10 weeks, starting after cardiac ischemia and reperfusion, improved left ventricular end-diastolic pressure and relaxation time and prevented lung congestion in comparison to placebo. No anti-fibrotic effect was observed. Gene expression analysis revealed a change in remodeling genes comparable to the ACE inhibitor ramipril. In isolated cardiomyocytes lixisenatide reduced apoptosis and increased fractional shortening. Glucagon-like peptide-1 receptor (GLP1R) mRNA expression could not be detected in rat heart samples or isolated cardiomyocytes. Surprisingly, cardiomyocytes isolated from GLP-1 receptor knockout mice still responded to lixisenatide.

Conclusions

In rodent models, lixisenatide reduced in an acute setting infarct-size and improved cardiac function when administered long-term after ischemia-reperfusion injury. GLP-1 receptor independent mechanisms contribute to the described cardioprotective effect of lixisenatide. Based in part on these preclinical findings patients with cardiac dysfunction are currently being recruited for a randomized, double-blind, placebo-controlled, multicenter study with lixisenatide.

Trial registration

(ELIXA, ClinicalTrials.gov Identifier: NCT01147250)

Cardiomyocyte-restricted inhibition of G protein-coupled receptor kinase-3 attenuates cardiac dysfunction after chronic pressure overload

Transgenic mice with cardiac-specific expression of a peptide inhibitor of G protein-coupled receptor kinase (GRK)3 [transgenic COOH-terminal GRK3 (GRK3ct) mice] display myocardial hypercontractility without hypertrophy and enhanced α(1)-adrenergic receptor signaling. A role for GRK3 in the pathogenesis of heart failure (HF) has not been investigated, but inhibition of its isozyme, GRK2, has been beneficial in several HF models. Here, we tested whether inhibition of GRK3 modulated evolving cardiac hypertrophy and dysfunction after pressure overload. Weight-matched male GRK3ct transgenic and nontransgenic littermate control (NLC) mice subjected to chronic pressure overload by abdominal aortic banding (AB) were compared with sham-operated (SH) mice. At 6 wk after AB, a significant increase of cardiac mass consistent with induction of hypertrophy was found, but no differences between GRK3ct-AB and NLC-AB mice were discerned. Simultaneous left ventricular (LV) pressure-volume analysis of electrically paced, ex vivo perfused working hearts revealed substantially reduced systolic and diastolic function in NLC-AB mice (n = 7), which was completely preserved in GRK3ct-AB mice (n = 7). An additional cohort was subjected to in vivo cardiac catheterization and LV pressure-volume analysis at 12 wk after AB. NLC-AB mice (n = 11) displayed elevated end-diastolic pressure (8.5 ± 3.1 vs. 2.9 ± 1.2 mmHg, P < 0.05), reduced cardiac output (3,448 ± 323 vs. 4,488 ± 342 μl/min, P < 0.05), and reduced dP/dt(max) and dP/dt(min) (both P < 0.05) compared with GRK3ct-AB mice (n = 16), corroborating the preserved cardiac structure and function observed in GRK3ct-AB hearts assessed ex vivo. Increased cardiac mass and myocardial mRNA expression of β-myosin heavy chain confirmed the similar induction of cardiac hypertrophy in both AB groups, but only NLC-AB hearts displayed significantly elevated mRNA levels of brain natriuretic peptide and myocardial collagen contents as well as reduced β(1)-adrenergic receptor responsiveness to isoproterenol, indicating increased LV wall stress and the transition to HF. Inhibition of cardiac GRK3 in mice does not alter the hypertrophic response but attenuates cardiac dysfunction and HF after chronic pressure overload.

Targeting GRK2 by gene therapy for heart failure: benefits above β-blockade

Heart failure (HF) is a common pathological end point for several cardiac diseases. Despite reasonable achievements in pharmacological, electrophysiological and surgical treatments, prognosis for chronic HF remains poor. Modern therapies are generally symptom oriented and do not currently address specific intracellular molecular signaling abnormalities. Therefore, new and innovative therapeutic approaches are warranted and, ideally, these could at least complement established therapeutic options if not replace them. Gene therapy has potential to serve in this regard in HF as vectors can be directed toward diseased myocytes and directly target intracellular signaling abnormalities. Within this review, we will dissect the adrenergic system contributing to HF development and progression with special emphasis on G-protein-coupled receptor kinase 2 (GRK2). The levels and activity of GRK2 are increased in HF and we and others have demonstrated that this kinase is a major molecular culprit in HF. We will cover the evidence supporting gene therapy directed against myocardial as well as adrenal GRK2 to improve the function and structure of the failing heart and how these strategies may offer complementary and synergistic effects with the existing HF mainstay therapy of β-adrenergic receptor antagonism.

G protein coupled receptor kinases as therapeutic targets in cardiovascular disease

G protein-coupled receptors (GPCRs) represent the largest family of membrane receptors and are responsible for regulating a wide variety of physiological processes. This is accomplished via ligand binding to GPCRs, activating associated heterotrimeric G proteins and intracellular signaling pathways. G protein-coupled receptor kinases (GRKs), in concert with β-arrestins, classically desensitize receptor signal transduction, thus preventing hyperactivation of GPCR second-messenger cascades. As changes in GRK expression have featured prominently in many cardiovascular pathologies, including heart failure, myocardial infarction, hypertension, and cardiac hypertrophy, GRKs have been intensively studied as potential diagnostic or therapeutic targets. Herein, we review our evolving understanding of the role of GRKs in cardiovascular pathophysiology.

The physiological roles of phosducin: from retinal function to stress-dependent hypertension

In the time since its discovery, phosducin's functions have been intensively studied both in vivo and in vitro. Phosducin's most important biochemical feature in in vitro studies is its binding to heterotrimeric G protein βγ-subunits. Data on phosducin's in vivo relevance, however, have only recently been published but expand the range of biological actions, as shown both in animal models as well as in human studies. This review gives an overview of different aspects of phosducin biology ranging from structure, phylogeny of phosducin family members, posttranscriptional modification, biochemical features, localization and levels of expression to its physiological functions. Special emphasis will be placed on phosducin's function in the regulation of blood pressure. In the second part of this article, findings concerning cardiovascular regulation and their clinical relevance will be discussed on the basis of recently published data from gene-targeted mouse models and human genetic studies.

Taking the heart failure battle inside the cell: small molecule targeting of Gβγ subunits

Heart failure (HF) is devastating disease with poor prognosis. Elevated sympathetic nervous system activity and outflow, leading to pathologic attenuation and desensitization of β-adrenergic receptors (β-ARs) signaling and responsiveness, are salient characteristic of HF progression. These pathologic effects on β-AR signaling and HF progression occur in part due to Gβγ-mediated signaling, including recruitment of receptor desensitizing kinases such as G-protein coupled receptor (GPCR) kinase 2 (GRK2) and phosphoinositide 3-kinase (PI3K), which subsequently phosphorylate agonist occupied GPCRs. Additionally, chronic GPCR signaling signals chronically dissociated Gβγ subunits to interact with multiple effector molecules that activate various signaling cascades involved in HF pathophysiology. Importantly, targeting Gβγ signaling with large peptide inhibitors has proven a promising therapeutic paradigm in the treatment of HF. We recently described an approach to identify small molecule Gβγ inhibitors that selectively block particular Gβγ functions by specifically targeting a Gβγ protein-protein interaction "hot spot." Here we describe their effects on Gβγ downstream signaling pathways, including their role in HF pathophysiology. We suggest a promising therapeutic role for small molecule inhibition of pathologic Gβγ signaling in the treatment of HF. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

The inotropic peptide βARKct improves βAR responsiveness in normal and failing cardiomyocytes through G(βγ)-mediated L-type calcium current disinhibition

RATIONALE

The G(βγ)-sequestering peptide β-adrenergic receptor kinase (βARK)ct derived from the G-protein-coupled receptor kinase (GRK)2 carboxyl terminus has emerged as a promising target for gene-based heart failure therapy. Enhanced downstream cAMP signaling has been proposed as the underlying mechanism for increased β-adrenergic receptor (βAR) responsiveness. However, molecular targets mediating improved cardiac contractile performance by βARKct and its impact on G(βγ)-mediated signaling have yet to be fully elucidated.

OBJECTIVE

We sought to identify G(βγ)-regulated targets and signaling mechanisms conveying βARKct-mediated enhanced βAR responsiveness in normal (NC) and failing (FC) adult rat ventricular cardiomyocytes.

METHODS AND RESULTS

Assessing viral-based βARKct gene delivery with electrophysiological techniques, analysis of contractile performance, subcellular Ca²(+) handling, and site-specific protein phosphorylation, we demonstrate that βARKct enhances the cardiac L-type Ca²(+) channel (LCC) current (I(Ca)) both in NCs and FCs on βAR stimulation. Mechanistically, βARKct augments I(Ca) by preventing enhanced inhibitory interaction between the α1-LCC subunit (Cav1.2α) and liberated G(βγ) subunits downstream of activated βARs. Despite improved βAR contractile responsiveness, βARKct neither increased nor restored cAMP-dependent protein kinase (PKA) and calmodulin-dependent kinase II signaling including unchanged protein kinase (PK)Cε, extracellular signal-regulated kinase (ERK)1/2, Akt, ERK5, and p38 activation both in NCs and FCs. Accordingly, although βARKct significantly increases I(Ca) and Ca²(+) transients, being susceptible to suppression by recombinant G(βγ) protein and use-dependent LCC blocker, βARKct-expressing cardiomyocytes exhibit equal basal and βAR-stimulated sarcoplasmic reticulum Ca²(+) load, spontaneous diastolic Ca²(+) leakage, and survival rates and were less susceptible to field-stimulated Ca²(+) waves compared with controls.

CONCLUSION

Our study identifies a G(βγ)-dependent signaling pathway attenuating cardiomyocyte I(Ca) on βAR as molecular target for the G(βγ)-sequestering peptide βARKct. Targeted interruption of this inhibitory signaling pathway by βARKct confers improved βAR contractile responsiveness through increased I(Ca) without enhancing regular or restoring abnormal cAMP-signaling. βARKct-mediated improvement of I(Ca) rendered cardiomyocytes neither susceptible to βAR-induced damage nor arrhythmogenic sarcoplasmic reticulum Ca²(+) leakage.

Rational design of a selective covalent modifier of G protein βγ subunits

G protein-coupled receptors transduce signals through heterotrimeric G protein Gα and Gβγ subunits, both of which interact with downstream effectors to regulate cell function. Gβγ signaling has been implicated in the pathophysiology of several diseases, suggesting that Gβγ could be an important pharmaceutical target. Previously, we used a combination of virtual and manual screening to find small molecules that bind to a protein-protein interaction "hot spot" on Gβγ and block regulation of physiological effectors. One of the most potent and effective compounds from this screen was selenocystamine. In this study, we investigated the mechanism of action of selenocystamine and found that selenocysteamine forms a covalent complex with Gβγ by a reversible redox mechanism. Mass spectrometry and site-directed mutagenesis suggest that selenocysteamine preferentially modifies GβCys204, but also a second undefined site. The high potency of selenocystamine in Gβγ inhibition seems to arise from both high reactivity of the diselenide group and binding to a specific site on Gβ. Using structural information about the "hot spot," we developed a strategy to selectively target redox reversible compounds to a specific site on Gβγ using peptide carriers such as SIGCAFKILGY(-cysteamine) [SIGC(-cysteamine)]. Mass spectrometry and site-directed mutagenesis indicate that SIGC(-cysteamine) specifically and efficiently leads to cysteamine (half-cystamine) modification of a single site on Gβ, likely GβCys204, and inhibits Gβγ more than a hundred times more potently than cystamine. These data support the concept that covalent modifiers can be specifically targeted to the Gβγ "hot spot" through rational incorporation into molecules that noncovalently bind to Gβγ.

Small molecule disruption of G beta gamma signaling inhibits the progression of heart failure

RATIONALE

Excess signaling through cardiac Gbetagamma subunits is an important component of heart failure (HF) pathophysiology. They recruit elevated levels of cytosolic G protein-coupled receptor kinase (GRK)2 to agonist-stimulated beta-adrenergic receptors (beta-ARs) in HF, leading to chronic beta-AR desensitization and downregulation; these events are all hallmarks of HF. Previous data suggested that inhibiting Gbetagamma signaling and its interaction with GRK2 could be of therapeutic value in HF.

OBJECTIVE

We sought to investigate small molecule Gbetagamma inhibition in HF.

METHODS AND RESULTS

We recently described novel small molecule Gbetagamma inhibitors that selectively block Gbetagamma-binding interactions, including M119 and its highly related analog, gallein. These compounds blocked interaction of Gbetagamma and GRK2 in vitro and in HL60 cells. Here, we show they reduced beta-AR-mediated membrane recruitment of GRK2 in isolated adult mouse cardiomyocytes. Furthermore, M119 enhanced both adenylyl cyclase activity and cardiomyocyte contractility in response to beta-AR agonist. To evaluate their cardiac-specific effects in vivo, we initially used an acute pharmacological HF model (30 mg/kg per day isoproterenol, 7 days). Concurrent daily injections prevented HF and partially normalized cardiac morphology and GRK2 expression in this acute HF model. To investigate possible efficacy in halting progression of preexisting HF, calsequestrin cardiac transgenic mice (CSQ) with extant HF received daily injections for 28 days. The compound alone halted HF progression and partially normalized heart size, morphology, and cardiac expression of HF marker genes (GRK2, atrial natriuretic factor, and beta-myosin heavy chain).

CONCLUSIONS

These data suggest a promising therapeutic role for small molecule inhibition of pathological Gbetagamma signaling in the treatment of HF.

A new type of ERK1/2 autophosphorylation causes cardiac hypertrophy

The extracellular-regulated kinases ERK1 and ERK2 (commonly referred to as ERK1/2) have a crucial role in cardiac hypertrophy. ERK1/2 is activated by mitogen-activated protein kinase kinase-1 (MEK1) and MEK2 (commonly referred to as MEK1/2)-dependent phosphorylation in the TEY motif of the activation loop, but how ERK1/2 is targeted toward specific substrates is not well understood. Here we show that autophosphorylation of ERK1/2 on Thr188 directs ERK1/2 to phosphorylate nuclear targets known to cause cardiac hypertrophy. Thr188 autophosphorylation requires the activation and assembly of the entire Raf-MEK-ERK kinase cascade, phosphorylation of the TEY motif, dimerization of ERK1/2 and binding to G protein betagamma subunits released from activated G(q). Thr188 phosphorylation of ERK1/2 was observed in isolated cardiomyocytes induced to undergo hypertrophic growth, in mice upon stimulation of G(q)-coupled receptors or after aortic banding and in failing human hearts. Experiments using transgenic mouse models carrying mutations at the Thr188 phosphorylation site of ERK2 suggested a causal relationship to cardiac hypertrophy. We propose that specific phosphorylation events on ERK1/2 integrate differing upstream signals (Raf1-MEK1/2 or G protein-coupled receptor-G(q)) to induce cardiac hypertrophy.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

G protein-coupled receptor kinase 2 ablation in cardiac myocytes before or after myocardial infarction prevents heart failure

Myocardial G protein-coupled receptor kinase (GRK)2 is a critical regulator of cardiac beta-adrenergic receptor (betaAR) signaling and cardiac function. Its upregulation in heart failure may further depress cardiac function and contribute to mortality in this syndrome. Preventing GRK2 translocation to activated betaAR with a GRK2-derived peptide that binds G(beta)gamma (betaARKct) has benefited some models of heart failure, but the precise mechanism is uncertain, because GRK2 is still present and betaARKct has other potential effects. We generated mice in which cardiac myocyte GRK2 expression was normal during embryonic development but was ablated after birth (alphaMHC-Cre x GRK2 fl/fl) or only after administration of tamoxifen (alphaMHC-MerCreMer x GRK2 fl/fl) and examined the consequences of GRK2 ablation before and after surgical coronary artery ligation on cardiac adaptation after myocardial infarction. Absence of GRK2 before coronary artery ligation prevented maladaptive postinfarction remodeling and preserved betaAR responsiveness. Strikingly, GRK2 ablation initiated 10 days after infarction increased survival, enhanced cardiac contractile performance, and halted ventricular remodeling. These results demonstrate a specific causal role for GRK2 in postinfarction cardiac remodeling and heart failure and support therapeutic approaches of targeting GRK2 or restoring betaAR signaling by other means to improve outcomes in heart failure.

G protein betagamma subunits as targets for small molecule therapeutic development

G proteins mediate the action of G protein coupled receptors (GPCRs), a major target of current pharmaceuticals and a major target of interest in future drug development. Most pharmaceutical interest has been in the development of selective GPCR agonists and antagonists that activate or inhibit specific GPCRs. Some recent thinking has focused on the idea that some pathologies are the result of the actions of an array of GPCRs suggesting that targeting single receptors may have limited efficacy. Thus, targeting pathways common to multiple GPCRs that control critical pathways involved in disease has potential therapeutic relevance. G protein betagamma subunits released from some GPCRs upon receptor activation regulate a variety of downstream pathways to control various aspects of mammalian physiology. There is evidence from cell- based and animal models that excess Gbetagamma signaling can be detrimental and blocking Gbetagamma signaling has salutary effects in a number of pathological models. Gbetagamma regulates downstream pathways through modulation of enzymes that produce cellular second messengers or through regulation of ion channels by direct protein-protein interactions. Thus, blocking Gbetagamma functions requires development of small molecule agents that disrupt Gbetagamma protein interactions with downstream partners. Here we discuss evidence that small molecule targeting Gbetagamma could be of therapeutic value. The concept of disruption of protein-protein interactions by targeting a "hot spot" on Gbetagamma is delineated and the biochemical and virtual screening strategies for identification of small molecules that selectively target Gbetagamma functions are outlined. Evaluation of the effectiveness of virtual screening indicates that computational screening enhanced identification of true Gbetagamma binding molecules. However, further refinement of the approach could significantly improve the yield of Gbetagamma binding molecules from this screen that could result in multiple candidate leads for future drug development.

β-adrenoceptor blocker treatment and the cardiac β-adrenoceptor-G-protein(s)-adenylyl cyclase system in chronic heart failure

Recent studies showed that chronic beta-adrenoceptor (AR) blocker treatment exerts beneficial effects in patients with chronic heart failure (CHF). In CHF, sympathetic drive to the heart is increased, and this causes pathological changes in cardiac beta-AR-G-protein(s)-adenylyl cyclase system: Cardiac beta-1 AR are decreased, and amount and activity of cardiac G(i)-protein and G-protein-coupled receptor kinase (GRK) are increased resulting in diminished cardiac beta-AR functional responsiveness. One possible mechanism of beneficial effects of beta-AR blockers could be that they prevent adverse effects of increased sympathetic activity and up-regulate cardiac (and vascular) beta-AR density, and by this, enhance beta-AR-mediated effects. Another possibility could be that chronic beta-AR blocker treatment normalizes activity of G(i)-protein and may thereby restore beta-AR functional responsiveness. Moreover, failing human heart exhibits an inverse force-frequency relationship. beta-AR blockers reduce heart rate; this may, therefore, improve force of contraction. One of the strongest stimuli to activate GRK is increased sympathetic activity (as in CHF) via beta-AR stimulation. beta-AR blockers, by blocking beta-AR, can prevent GRK activation and/or can reduce the (previously enhanced) GRK activity, and this might-at least partly-contribute to beneficial effects of beta-AR blockers in CHF treatment. Finally, the "loss-of-function" Arg389Gly beta-1 AR polymorphism seems to determine heart rate and blood pressure responses to beta-1 AR blocker administration: Arg389Arg beta-1 AR subjects exhibit stronger effects than subjects with one or two Gly389 alleles. Thus, it might be predicted that patients homozygous Arg389 beta-1 AR should be good responders, whereas patients homozygous Gly389 beta-1 AR polymorphism should be poor or non-responders.

Cardiac adrenoceptors: physiological and pathophysiological relevance

At present, nine adrenoceptor (AR) subtypes have been identified: alpha(1A)-, alpha(1B)-, alpha(1D)-, alpha(2A)-, alpha(2B)-, alpha(2C)-, beta(1)-, beta(2)-, and beta(3)AR. In the human heart, beta(1)- and beta(2)AR are the most powerful physiologic mechanism to acutely increase cardiac performance. Changes in betaAR play an important role in chronic heart failure (CHF). Thus, due to increased sympathetic activity in CHF, betaAR are chronically (over)stimulated, and that results in beta(1)AR desensitization and alterations of down-stream mechanisms. However, several questions remain open: What is the role of beta(2)AR in CHF? What is the role of increases in cardiac G(i)-protein in CHF? Do increases in G-protein-coupled receptor kinase (GRK)s play a role in CHF? Does betaAR-blocker treatment cause its beneficial effects in CHF, at least partly, by reducing GRK-activity? In this review these aspects of cardiac AR pharmacology in CHF are discussed. In addition, new insights into the functional importance of beta(1)- and beta(2)AR gene polymorphisms are discussed. At present it seems that for cardiovascular diseases, betaAR polymorphisms do not play a role as disease-causing genes; however, they might be risk factors, might modify disease, and/or might influence progression of disease. Furthermore, betaAR polymorphisms might influence drug responses. Thus, evidence has accumulated that a beta(1)AR polymorphism (the Arg389Gly beta(1)AR) may affect the response to betaAR-blocker treatment.

Gene therapy targeted at calcium handling as an approach to the treatment of heart failure

Chronic congestive heart failure primarily of ischemic origin remains a leading cause of morbidity and mortality in the United States and other leading countries. The current main stream of therapy is, however, palliative and uses a complex regimen of drugs, the actions of which are not understood completely. On the other hand, unfavorable remodeling after cardiac injuries of multiple causes has been thought to lead to cardiac contractile dysfunction in heart failure, and a body of scientific evidence points to a central role of intrinsic defects in intracellular calcium handling in cardiomyocytes that arise from the distorted functions of several key regulatory molecules on plasma membrane or sarcoplasmic reticulum (SR), a muscle-specific intracellular membrane complex that stores calcium at high concentration. Accordingly, the initial appetite to use gene transfer strategies to modulate calcium regulatory proteins was to validate molecular targets for the development of new pharmaceuticals; however, remarkable therapeutic efficacies found in an initial series of studies using various heart failure animal models immediately promoted us to seek ways to directly apply gene transfer to cure clinical heart failure. The first part of this article reviews our up-to-date knowledge of various functional components to regulate calcium handling in cardiomyocytes, including beta-adrenergic receptor, L-type calcium channel, ryanodine receptor (RyR) and its associated proteins, sarco-endoplasmic reticulum calcium ATPase (SERCA), and phospholamban (PLN), and their abnormalities in failing hearts. A series of new somatic gene transfer attempts targeting calcium handling in cardiomyocytes are discussed thereafter.

Alterations in cAMP-mediated signaling and their role in the pathophysiology of dilated cardiomyopathy

Dilated cardiomyopathy is a disease characterized by enlargement of the chambers of the heart and a decrease in contractility of the heart muscle. The process involves several alterations in proteins involved in cyclic adenosine monophosphate (cAMP) generation that result in a decrease in intracellular cAMP content per unit of adrenergic stimulation in cardiac myocytes. A fundamental question is whether these changes constitute a pathologic mechanism that contributes to chamber enlargement and hypocontractility or a compensatory adaptation that protects the heart from the adverse effects of increased catecholamine stimulation. Clinical studies in humans suggest that the latter effect may be more important. Studies in animal models, however, make the picture more complex: changes in cAMP-mediated signaling can have different effects depending on the specific protein whose expression or function is altered and the setting in which the alteration occurs. It may be that dilated cardiomyopathy represents a collection of different diseases in which alterations in cAMP-mediated signaling have different roles in the pathophysiology of the disease, and, furthermore, that changes in the phosphorylation of individual substrates of cAMP-dependent protein kinase may be either beneficial or harmful. Identifying differences among patients with dilated cardiomyopathy with respect to the role of altered cAMP-mediated signaling in their pathology, and identifying the "good" and "bad" substrates of cAMP-dependent protein kinase, are important areas for further research.

Models of Dilated Cardiomyopathy in Small Animals and Novel Positive Inotropic Therapies

Several randomized clinical trials of vesnarinone and milrinone in patients with heart failure left disappointing results in the 1990s. Thereafter, use of positive inotropic agents has been avoided. Exceptions are the use of digitalis glycosides to treat mild-moderate heart failure and the intravenous administration of catecholamines and phosphodiesterase inhibitors in patients with acute and/or refractory heart failure. It is not, however, exactly known whether chronic enhancement of cardiac contractility indeed has harmful effects, besides increased risk of arrhythmia and mortality. We investigated the potential chronic benefit of positive inotropic modification to treat progressive cardiomyopathy and associated heart failure using a genetic complementation strategy of muscle lim-protein and phospholamban (PLN) double mutagenesis in the mouse and found clear evidence of positive effects. Subsequent somatic modification of PLN function via gene transfer with recombinant adeno-associated virus vectors in small animal models of dilated cardiomyopathy further supported the chronic benefit of enhanced cardiac function achieved in an beta-adrenergic stimulus-independent manner. This study examines current small animal models of dilated cardiomyopathy and recent multiple attempts to use these models as novel gene-based inotropic therapies.

What is the role of beta-adrenergic signaling in heart failure?

This review addresses open questions about the role of beta-adrenergic receptors in cardiac function and failure. Cardiomyocytes express all three beta-adrenergic receptor subtypes-beta1, beta2, and, at least in some species, beta3. The beta1 subtype is the most prominent one and is mainly responsible for positive chronotropic and inotropic effects of catecholamines. The beta2 subtype also increases cardiac function, but its ability to activate nonclassical signaling pathways suggests a function distinct from the beta1 subtype. In heart failure, the sympathetic system is activated, cardiac beta-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, beta-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 beta-adrenergic receptor system. Newer experimental therapeutic strategies aim at the receptor desensitization machinery and at downstream signaling steps.