Alpha1-adrenergic receptors prevent a maladaptive cardiac response to pressure overload

Knockout of beta(1)- and beta(2)-adrenoceptors attenuates pressure overload-induced cardiac hypertrophy and fibrosis

BACKGROUND AND PURPOSE

The role of beta-adrenoceptors in heart disease remains controversial. Although beta-blockers ameliorate the progression of heart disease, the mechanism remains undefined. We investigated the effect of beta-adrenoceptors on cardiac hypertrophic growth using beta(1)- and beta(2)-adrenoreceptor knockout and wild-type (WT) mice.

EXPERIMENTAL APPROACH

Mice were subjected to aortic banding or sham surgery, and their cardiac function was determined by echocardiography and micromanometry.

KEY RESULTS

At 4 and 12 weeks after aortic banding, the left ventricle:body mass ratio was increased by 80-87% in wild-type mice, but only by 15% in knockouts, relative to sham-operated groups. Despite the blunted hypertrophic growth, ventricular function in knockouts was maintained. WT mice responded to pressure overload with up-regulation of gene expression of inflammatory cytokines and fibrogenic growth factors, and with severe cardiac fibrosis. All these effects were absent in the knockout animals.

CONCLUSION AND IMPLICATIONS

Our findings of a markedly attenuated cardiac hypertrophy and fibrosis following pressure overload in this knockout model emphasize that beta-adrenoceptor signalling plays a central role in cardiac hypertrophy and maladaptation following pressure overload.

Testosterone protects rat hearts against ischaemic insults by enhancing the effects of alpha(1)-adrenoceptor stimulation

BACKGROUND AND PURPOSE

Testosterone alleviates symptoms in patients with ischaemic heart disease. Androgen receptors are present in the heart, and testosterone upregulates gene expression of cardiac beta(1)-adrenoceptors. We hypothesize that testosterone may confer cardioprotection by interacting with adrenoceptors.

EXPERIMENTAL APPROACH

In isolated perfused hearts and ventricular myocytes from orchidectomized rats without or with testosterone (200 microg/100 g) replacement, we first determined the effect of ischaemia/reperfusion in the presence of noradrenaline (10(-7) M). Then we determined the contribution of interactions between testosterone and alpha(1)- or beta(1)-adrenoceptors in cardiac injury/protection (infarct size, release of lactate dehydrogenase, viability of myocytes, recovery of contractile function and incidence of arrhythmias) upon ischaemia/reperfusion by pharmacological manipulation using selective adrenoceptor agonists (alpha(1)-adrenoceptor agonist: phenylephrine 10(-6) M; non-selective beta-adrenoceptor agonist: isoprenaline 10(-7) M) and antagonists (alpha(1): prazosin or benoxathian 10(-6) M; beta(1): CGP 20712A 5 x 10(-7) M). We also determined the expression of alpha(1) and beta(1)-adrenoceptor in the hearts from rats with and without testosterone.

KEY RESULTS

Testosterone reduced injury induced by ischaemia/reperfusion and noradrenaline. This was achieved by enhancing the beneficial effect of alpha(1)-adrenoceptor stimulation, which was greater than the deleterious effect of beta(1)-adrenoceptor stimulation (also enhanced by testosterone). The effects of testosterone were abolished or attenuated by blockade of androgen receptors. Testosterone also enhanced the expression of alpha(1A) and beta(1)-adrenoceptor.

CONCLUSIONS AND IMPLICATIONS

Testosterone conferred cardioprotection by upregulating the cardiac alpha(1)-adrenoceptor and enhancing the effects of stimulation of this adrenoceptor. The effect of testosterone was at least partly mediated by androgen receptors.

Reversal of cardiac remodeling by modulation of adrenergic receptors: a new frontier in heart failure

PURPOSE OF REVIEW

Heart failure is a common clinical syndrome, and despite intensive medical therapy it remains a leading cause of global morbidity and mortality. Pathological stimuli promote a general remodeling process in the heart.

RECENT FINDINGS

Recent animal studies have highlighted very promising novel therapeutic possibilities, based on the regulation of adrenergic receptor function, and novel signaling pathways are being discovered that could be relevant for future molecular approaches.

SUMMARY

This review highlights some of the novel approaches to reverse pathological remodeling and improve cardiac dysfunction, placing emphasis on strategies targeting the adrenergic receptors.

Roles of alpha1A- and alpha1B-adrenoceptors in heart: insights from studies of genetically modified mice

1. Several mouse strains have been prepared in which different subtypes of the alpha1-adrenoceptor (AR) are overexpressed or deleted. The phenotypes of the animals generated vary depending on whether the receptors are expressed specifically in heart or generally throughout the animal, but some overall conclusions can be drawn. 2. Heightened activity of alpha1B-AR by overexpressing the receptors leads to depressed contractile responses to beta-AR activation, which may be related to activation of the inhibitory G-protein Gi. In contrast, alpha1A-AR cause substantially heightened contractility when overexpressed in heart. 3. Overexpressed alpha1B-AR predispose hearts to hypertrophy and worsen heart failure caused by pressure overload, whereas increased alpha1A-AR expression does not influence hypertrophic responses and, furthermore, improves outcomes after pressure overload or myocardial infarction. 4. Alpha1A-adrenoceptors mediate a preconditioning action to improve functional recovery after acute ischaemic insult, whereas alpha1B-AR are ineffective. Both subtypes appear to protect from inositol 1,4,5-trisphosphate generation and arrhythmogenesis in early postischaemic reperfusion. 5. Although some of the protective effects of heightened alpha1A-AR drive may be related to the enhanced contractility, it is also possible that alpha1A-AR protect from cardiomyocyte apoptotic responses.

G-protein coupled receptor signaling in myocardium: not for the faint of heart

Catecholamines, endothelin-1 and angiotensin II are among a diverse group of diffusible extracellular signals that regulate pump function of the heart by binding to G-protein coupled receptors (GPCR). When the body demands a temporary boost of power output or if temporary budgeting of resources is required, these signals can adjust heart rate and contractile strength to maintain continuous perfusion of all vascular beds with nutrient- and oxygen-rich blood. Given adequate time in the face of prolonged challenges, activation of GPCRs can also promote "remodeling of the heart" by increasing cell size, organ size, and chamber dimensions, or by varying tissue composition and altering the expression of protein isoforms controlling excitability and contractility. A common feature of heart disease is the state of chronic activation of GPCR signaling systems. Paradoxically, whereas acute activation is beneficial, chronic activation often contributes to further deterioration of cardiac performance. A better understanding of how chronic GPCR activation contributes to the development of heart disease is needed so that it can be translated into better prevention and therapeutic strategies in the clinic.

Alpha1-adrenoceptors are required for normal male sexual function

BACKGROUND AND PURPOSE

Alpha(1)-adrenoceptor antagonists are extensively used in the treatment of hypertension and lower urinary tract symptoms associated with benign prostatic hyperplasia. Among the side effects, ejaculatory dysfunction occurs more frequently with drugs that are relatively selective for alpha(1A)-adrenoceptors compared with other drugs of this class. This suggests that alpha(1A)-adrenoceptors may contribute to ejaculation. However, this has not been studied at the molecular level.

EXPERIMENTAL APPROACH

The physiological contribution of each alpha(1)-adrenoceptor subtype was characterized using alpha(1)-adrenoceptor subtype-selective knockout (KO) mice (alpha(1A)-, alpha(1B)- and alpha(1D)-AR KO mice) since the subtype-specific drugs available are only moderately selective. We analysed the role of alpha(1)-adrenoceptors in the blood pressure and vascular response as well as ejaculation by determining these variables in alpha(1)-adrenoceptor subtype-selective KO mice and in mice with all their alpha(1)-adrenoceptor subtypes deleted (alpha(1)-AR triple-KO mice).

KEY RESULTS

The pregnancy rate was reduced by 50% in alpha(1A)-adrenoceptor KO mice, and this reduction was dramatically enhanced in alpha(1)-adrenoceptor triple-KO mice. Contractile tension of the vas deferens in response to noradrenaline was markedly decreased in alpha(1A)-adrenoceptor KO mice, and this contraction was completely abolished in alpha(1)-adrenoceptor triple-KO mice. This attenuation of contractility was also observed in the electrically stimulated vas deferens.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that alpha(1)-adrenoceptors, particularly alpha(1A)-adrenoceptors, are required for normal contractility of the vas deferens and consequent sperm ejaculation as well as having a function in fertility.

Divergence of hypertrophic growth and fetal gene profile: the influence of beta-blockers

While the expression patterns of cardiac hypertrophy-related genes have been well documented and widely used as markers for hypertrophy, recent research has revealed uncoupling of hypertrophy-related gene profiles and hypertrophic growth. The role of beta-adrenergic signalling in the development of hypertrophy is incompletely understood. The finding of an upregulated expression of hypertrophy-related genes but a suppressed hypertrophy following beta-blockade reveals previously unrecognized sympatho-adrenergic mechanisms of hypertrophic growth.

Cardiac GPCRs: GPCR signaling in healthy and failing hearts

G protein-coupled receptors (GPCRs) are widely implicated in human heart disease, making them an important target for cardiac drug therapy. The most commonly studied and clinically targeted cardiac GPCRs include the adrenergic, angiotensin, endothelin, and adenosine receptors. Treatment options focusing on the complex and integrated signaling pathways of these GPCRs are critical for the understanding and amelioration of heart disease. The focus of this review is to highlight the most commonly studied and clinically targeted cardiac GPCRs, placing emphasis on their common signaling components implicated in cardiac disease.

An α1A-Adrenergic–Extracellular Signal-Regulated Kinase Survival Signaling Pathway in Cardiac Myocytes

Background— In α1-AR knockout (α1ABKO) mice that lacked cardiac myocyte α1-adrenergic receptor (α1-AR) binding, aortic constriction induced apoptosis, dilated cardiomyopathy, and death. However, it was unclear whether these effects were attributable to a lack of cardiac myocyte α1-ARs and whether the α1A, α1B, or both subtypes mediated protection. Therefore, we investigated α1A and α1B subtype–specific survival signaling in cultured cardiac myocytes to test for a direct protective effect of α1-ARs in cardiac myocytes.

Methods and Results— We cultured α1ABKO myocytes and reconstituted α1-AR signaling with adenoviruses expressing α1-GFP fusion proteins. Myocyte death was induced by norepinephrine, doxorubicin, or H 2 O 2 and was measured by annexin V/propidium iodide staining. In α1ABKO myocytes, all 3 stimuli significantly increased apoptosis and necrosis. Reconstitution of the α1A subtype, but not the α1B, rescued α1ABKO myocytes from cell death induced by each stimulus. To address the mechanism, we examined α1-AR activation of extracellular signal-regulated kinase (ERK). In α1ABKO hearts, aortic constriction failed to activate ERK, and in α1ABKO myocytes, expression of a constitutively active MEK1 rescued α1ABKO myocytes from norepinephrine-induced death. In addition, only the α1A-AR activated ERK in α1ABKO myocytes, and expression of a dominant-negative MEK1 completely blocked α1A survival signaling in α1ABKO myocytes.

Conclusions— Our results demonstrate a direct protective effect of the α1A subtype in cardiac myocytes and define an α1A-ERK signaling pathway that is required for myocyte survival. Absence of the α1A-ERK pathway can explain the failure to activate ERK after aortic constriction in α1ABKO mice and can contribute to the development of apoptosis, dilated cardiomyopathy, and death.

Crosstalk between signaling pathways of adrenoreceptors and signal transducers and activators of transcription 3 (STAT3) in heart

Recently, there have been important advancements in our understanding of the signaling mechanisms of adrenoreceptors (AR) and signal transducers and activators of transcription 3 (STAT3). While their crucial roles in the pathological processes of the heart are well established, accumulating evidence suggests there is a complex pattern of crosstalk between these 2 signaling pathways. Moreover, the potential for crosstalk occurs at multiple levels in each signaling cascade and involves receptor transactivation, G proteins, small GTPases, cyclic adenosine 3',5'-monophosphate/protein kinase A, protein kinase C, scaffold/adaptor proteins, protein tyrosine kinases, and mitogen-activated protein kinases. In addition, post-translational modification (eg acetylation) of STAT3 may provide a link between STAT3 and AR signaling. In particular, crosstalk between these 2 systems in the heart would appear to be dependent upon the species/tissue studied, developmental stage, and eliciting stimulus. This at least partly accounts for the epigenetic effects on biological function that is mediated by the 2 signaling pathways. Elucidation of these mechanisms will provide new targets in the development of novel clinical strategies for heart disorders.

Cardiac 7-transmembrane-spanning domain receptor portfolios: diversify, diversify, diversify

Enhanced signaling in myocytes by the G protein Gq has been implicated in cardiac hypertrophy and the transition to heart failure. alpha1-Adrenergic receptors (alpha1-ARs) are members of the 7-transmembrane-spanning domain (7-TM) receptor family and signal via interaction with Gq in the heart. The specific effects of a loss of alpha1-AR signaling in the heart are explored by O'Connell et al. in this issue of the JCI (see the related article beginning on page 1005). Paradoxically, gene ablation of the alpha 1A and alpha 1B subtypes in mice results in a maladaptive form of reactive cardiac hypertrophy from pressure overload, with a predisposition to heart failure. Thus signaling to the alpha1-AR (compared with signaling from other receptors such as angiotensin receptors, which also couple to Gq) appears to be specifically required for a normal hypertrophic response. This represents another example of how receptors that share common G proteins have diversified, developing unique signaling programs. These findings may have particular clinical relevance because of the widespread use of alpha1-AR antagonists in the treatment of hypertension and symptomatic prostate enlargement.