Beta-adrenergic receptor desensitization in cardiac hypertrophy and heart failure

Functional Characterization of the Obesity-Linked Variant of the β3-Adrenergic Receptor

Adrenergic receptor β₃ (ADRβ₃) is a member of the rhodopsin-like G protein-coupled receptor family. The binding of the ligand to ADRβ₃ activates adenylate cyclase and increases cAMP in the cells. ADRβ₃ is highly expressed in white and brown adipocytes and controls key regulatory pathways of lipid metabolism. Trp64Arg (W64R) polymorphism in the ADRβ₃ is associated with the early development of type 2 diabetes mellitus, lower resting metabolic rate, abdominal obesity, and insulin resistance. It is unclear how the substitution of W64R affects the functioning of ADRβ₃. This study was initiated to functionally characterize this obesity-linked variant of ADRβ₃. We evaluated in detail the expression, subcellular distribution, and post-activation behavior of the WT and W64R ADRβ₃ using single cell quantitative fluorescence microscopy. When expressed in HEK 293 cells, ADRβ₃ shows a typical distribution displayed by other GPCRs with a predominant localization at the cell surface. Unlike adrenergic receptor β₂ (ADRβ₂), agonist-induced desensitization of ADRβ₃ does not involve loss of cell surface expression. WT and W64R variant of ADRβ₃ displayed comparable biochemical properties, and there was no significant impact of the substitution of tryptophan with arginine on the expression, cellular distribution, signaling, and post-activation behavior of ADRβ₃. The obesity-linked W64R variant of ADRβ₃ is indistinguishable from the WT ADRβ₃ in terms of expression, cellular distribution, signaling, and post-activation behavior.

Endothelial Dysfunction in Chronic Heart Failure: Assessment, Findings, Significance, and Potential Therapeutic Targets

Chronic heart failure (CHF) is a complex syndrome that results from structural and functional disturbances that affect the ability of the heart to supply oxygen to tissues. It largely affects and reduces the patient's quality of life, socio-economic status, and imposes great costs on health care systems worldwide. Endothelial dysfunction (ED) is a newly discovered phenomenon that contributes greatly to the pathophysiology of numerous cardiovascular conditions and commonly co-exists with chronic heart failure. However, the literature lacks clarity as to which heart failure patients might be affected, its significance in CHF patients, and its reversibility with pharmacological and non-pharmacological means. This review will emphasize all these points and summarize them for future researchers interested in vascular pathophysiology in this particular patient population. It will help to direct future studies for better characterization of these two phenomena for the potential discovery of therapeutic targets that might reduce future morbidity and mortality in this "at risk" population.

Investigating β-adrenergic-induced cardiac hypertrophy through computational approach: classical and non-classical pathways

The chronic stimulation of β-adrenergic receptors plays a crucial role in cardiac hypertrophy and its progression to heart failure. In β-adrenergic signaling, in addition to the well-established classical pathway, Gs/AC/cAMP/PKA, activation of non-classical pathways such as Gi/PI3K/Akt/GSK3β and Gi/Ras/Raf/MEK/ERK contribute in cardiac hypertrophy. The signaling network of β-adrenergic-induced hypertrophy is very complex and not fully understood. So, we use a computational approach to investigate the dynamic response and contribution of β-adrenergic mediators in cardiac hypertrophy. The proposed computational model provides insights into the effects of β-adrenergic classical and non-classical pathways on the activity of hypertrophic transcription factors CREB and GATA4. The results illustrate that the model captures the dynamics of the main signaling mediators and reproduces the experimental observations well. The results also show that despite the low portion of β2 receptors out of total cardiac β-adrenergic receptors, their contribution in the activation of hypertrophic mediators and regulation of β-adrenergic-induced hypertrophy is noticeable and variations in β1/β2 receptors ratio greatly affect the ISO-induced hypertrophic response. The model results illustrate that GSK3β deactivation after β-adrenergic receptor stimulation has a major influence on CREB and GATA4 activation and consequent cardiac hypertrophy. Also, it is found through sensitivity analysis that PKB (Akt) activation has both pro-hypertrophic and anti-hypertrophic effects in β-adrenergic signaling.

Cardiac myocyte-secreted cAMP exerts paracrine action via adenosine receptor activation

Acute stimulation of cardiac β-adrenoceptors is crucial to increasing cardiac function under stress; however, sustained β-adrenergic stimulation has been implicated in pathological myocardial remodeling and heart failure. Here, we have demonstrated that export of cAMP from cardiac myocytes is an intrinsic cardioprotective mechanism in response to cardiac stress. We report that infusion of cAMP into mice averted myocardial hypertrophy and fibrosis in a disease model of cardiac pressure overload. The protective effect of exogenous cAMP required adenosine receptor signaling. This observation led to the identification of a potent paracrine mechanism that is dependent on secreted cAMP. Specifically, FRET-based imaging of cAMP formation in primary cells and in myocardial tissue from murine hearts revealed that cardiomyocytes depend on the transporter ABCC4 to export cAMP as an extracellular signal. Extracellular cAMP, through its metabolite adenosine, reduced cardiomyocyte cAMP formation and hypertrophy by activating A1 adenosine receptors while delivering an antifibrotic signal to cardiac fibroblasts by A2 adenosine receptor activation. Together, our data reveal a paracrine role for secreted cAMP in intercellular signaling in the myocardium, and we postulate that secreted cAMP may also constitute an important signal in other tissues.

Compartmentalization of beta-adrenergic signals in cardiomyocytes

Activation of adrenergic receptors (AR) represents the primary mechanism to increase cardiac performance under stress. Activated βAR couple to Gs protein, leading to adenylyl cyclase-dependent increases in secondary-messenger cyclic adenosine monophosphate (cAMP) to activate protein kinase A. The increased protein kinase A activities promote phosphorylation of diversified substrates, ranging from the receptor and its associated partners to proteins involved in increases in contractility and heart rate. Recent progress with live-cell imaging has drastically advanced our understanding of the βAR-induced cAMP and protein kinase A activities that are precisely regulated in a spatiotemporal fashion in highly differentiated myocytes. Several features stand out: membrane location of βAR and its associated complexes dictates the cellular compartmentalization of signaling; βAR agonist dose-dependent equilibrium between cAMP production and cAMP degradation shapes persistent increases in cAMP signals for sustained cardiac contraction response; and arrestin acts as an agonist dose-dependent master switch to promote cAMP diffusion and propagation into intracellular compartments by sequestrating phosphodiesterase isoforms associated with the βAR signaling cascades. These features and the underlying molecular mechanisms of dynamic regulation of βAR complexes with adenylyl cyclase and phosphodiesterase enzymes and the implication in heart failure are discussed.

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

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

Inhibition of receptor-localized PI3K preserves cardiac beta-adrenergic receptor function and ameliorates pressure overload heart failure

beta-Adrenergic receptor (betaAR) downregulation and desensitization are hallmarks of the failing heart. However, whether abnormalities in betaAR function are mechanistically linked to the cause of heart failure is not known. We hypothesized that downregulation of cardiac betaARs can be prevented through inhibition of PI3K activity within the receptor complex, because PI3K is necessary for betaAR internalization. Here we show that in genetically modified mice, disrupting the recruitment of PI3K to agonist-activated betaARs in vivo prevents receptor downregulation in response to chronic catecholamine administration and ameliorates the development of heart failure with pressure overload. Disruption of PI3K/betaAR colocalization is required to preserve betaAR signaling, since deletion of a single PI3K isoform (PI3Kgamma knockout) is insufficient to prevent the recruitment of other PI3K isoforms and subsequent betaAR downregulation with catecholamine stress. These data demonstrate a specific role for receptor-localized PI3K in the regulation of betaAR turnover and show that abnormalities in betaAR function are associated with the development of heart failure. Thus, a strategy that blocks the membrane translocation of PI3K and leads to the inhibition of betaAR-localized PI3K activity represents a novel therapeutic approach to restore normal betaAR signaling and preserve cardiac function in the pressure overloaded failing heart.

Exercise restores beta-adrenergic vasorelaxation in aged rat carotid arteries

Aging is associated with alterations in beta-adrenergic receptor (beta-AR) signaling and reduction in cardiovascular responses to beta-AR stimulation. Because exercise can attenuate age-related impairment in myocardial beta-AR signaling and function, we tested whether training could also exert favorable effects on vascular beta-AR responses. We evaluated common carotid artery responsiveness in isolated vessel ring preparations from 8 aged male Wistar-Kyoto (WKY) rats trained for 6 wk in a 5 days/wk swimming protocol, 10 untrained age-matched rats, and 10 young WKY rats. Vessels were preconstricted with phenylephrine (10-6 M), and vasodilation was assessed in response to the beta-AR agonist isoproterenol (10-10-3 x 10-8 M), the alpha2-AR agonist UK-14304 (10-9-10-6 M), the muscarinic receptor agonist ACh (10-9-10-6 M), and nitroprusside (10-8-10-5 M). beta-AR density and cytoplasmic beta-AR kinase (beta-ARK) activity were tested on pooled carotid arteries. beta-ARK expression was assessed in two endothelial cell lines from bovine aorta and aorta isolated from a 12-wk WKY rat. beta-AR, alpha2-AR, and muscarinic responses, but not that to nitroprusside, were depressed in untrained aged vs. young animals. Exercise training restored beta-AR and muscarinic responses but did not affect vasodilation induced by UK-14304 and nitroprusside. Aged carotid arteries showed reduced beta-AR number and increased beta-ARK activity. Training counterbalanced these phenomena and restored beta-AR density and beta-ARK activity to levels observed in young rat carotids. Our data indicate that age impairs beta-AR vasorelaxation in rat carotid arteries through beta-AR downregulation and desensitization. Exercise restores this response and reverts age-related modification in beta-ARs and beta-ARK. Our data support an important role for beta-ARK in vascular beta-AR vasorelaxation.

L-type Ca2+ channel density and regulation are altered in failing human ventricular myocytes and recover after support with mechanical assist devices

Ca2+ influx through the L-type calcium channel (LTCC) induces Ca2+ release from the sarcoplasmic reticulum (SR) and maintains SR Ca2+ loading. Alterations in LTCC properties, their contribution to the blunted adrenergic responsiveness in failing hearts and their recovery after support with LV assist devices (LVAD) were studied. L-type Ca2+ current (I(Ca,L)) was measured under basal conditions and in the presence of isoproterenol (ISO), dibutyryl-cAMP (db-cAMP), Bay K 8644 (BayK), Okadaic acid (OA, a phosphatase inhibitor), and phosphatase 2A (PP2A) in nonfailing (NF), failing (F), and LVAD-supported human left ventricular myocytes (HVMs). Basal I(Ca,L) density was not different in the 3 groups but I(Ca,L) was activated at more negative voltages in F- and LVAD- versus NF-HVMs (V(0.5): -7.18+/-1.4 and -7.0+/-0.9 versus 0.46+/-1.1 mV). Both ISO and db-cAMP increased I(Ca,L) in NF- and LVAD- significantly more than in F-HVMs (NF >LVAD> F: ISO: 90+/-15% versus 77+/-19% versus 24+/-12%; db-cAMP: 235%>172%>90%). ISO caused a significant leftward shift of the I(Ca,L) activation curve in NF- and LVAD- but not in F-HVMs. After ISO and db-cAMP, the I(Ca,L) activation was not significantly different between groups. BayK also increased I(Ca,L) more in NF- (81+/-30%) and LVAD- (70+/-15%) than in F- (51+/-8%) HVMs. OA increased I(Ca, L) by 85.6% in NF-HVMs but had no effect in F-HVMs, while PP2A decreased I(Ca, L) in F-HVMs by 35% but had no effect in NF-HVMs. These results suggest that the density of LTCC is reduced in F-HVMs but basal I(Ca,L) density is maintained by increasing in LTCC phosphorylation.

Cellular and molecular aspects of myocardial dysfunction

Disruption of any one of a large number of balanced systems that maintain cardiomyocyte structure and function can cause myocardial dysfunction. Such disruption can occur either in response to acute stresses such as cardiac surgery with cardiopulmonary bypass and cross-clamping of the aorta or because of more chronic stresses resulting from factors such as genetic abnormalities, infection, or chronic ischemia. Several currently available therapies such as beta-adrenergic receptor agonists and antagonists, phosphodiesterase inhibitors, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, and other agents affect cardiomyocytes in ways that are more far reaching than initially appreciated when these agents were first introduced into clinical practice. As our knowledge and understanding of myocardial dysfunction increases, particularly in the neonatal and pediatric patient, we will be able to further target interventions to highly specific perturbations of cellular function and individual genetic variability.

Modification of the beta 2-adrenergic receptor to engineer a receptor-effector complex for gene therapy

Depressed G-protein-coupled receptor (GPCR) signaling has been implicated as a component of the pathophysiology of a number of complex diseases including heart failure and asthma, and augmentation or restoration of signaling by various means has been shown to improve organ function. Because some properties of native GPCRs are disadvantageous for ectopic therapeutic expression, we utilized the beta(2)-adrenergic receptor (beta(2)AR) as a scaffold to construct a highly modified therapeutic receptor-effector complex (TREC) suitable for gene therapy. Altogether, 19 modifications were made to the receptor. The ligand-binding site was re-engineered in TM-3 so that a beta-hydroxylmethyl side chain acts as a proton donor for the binding of a novel ligand. In addition, sites critical for agonist-promoted down-regulation in the amino terminus and for phosphorylation by GPCR kinases, and protein kinases A and C, in the third intracellular loop and the carboxyl terminus of the receptor were altered. These modifications of the receptor resulted in depressed agonist-stimulated adenylyl cyclase activity (26.8 +/- 2.1 versus 41.4 +/- 8 pmol/min/mg for wild-type beta(2)AR). This was fully restored by fusing the carboxyl terminus of the modified receptor to G alpha(s) (43.3 +/- 2.7 pmol/min/mg). The fully modified fused receptor was not activated by beta-agonists but rather by a nonbiogenic amine agonist that itself failed to activate the wild-type beta(2)AR. This two-way selectivity thus provides targeted activation based on physiologic status. Furthermore, the TREC did not display tachyphylaxis to prolonged agonist exposure (desensitization was 1 +/- 5% versus 55 +/- 4% for wild-type beta(2)AR). Thus, despite extensive alterations in regions of conformational lability, the beta(2)AR can be tailored to have optimal signaling characteristics for gene therapy. As a general paradigm, TRECs for enhancement of other G-protein signaling appear to be feasible for modification of other pathologic states.

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

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