Signal transduction in myocardial ischaemia and reperfusion

β-Adrenergic Receptor Signaling and Heart Failure: From Bench to Bedside

Despite improvements in management and therapeutic approach in the last decades, heart failure is still associated with high mortality rates. The sustained enhancement in the sympathetic nervous system tone, observed in patients with heart failure, causes alteration in β-adrenergic receptor signaling and function. This latter phenomenon is the result of several heart failure-related molecular abnormalities involving adrenergic receptors, G-protein-coupled receptor kinases, and β-arrestins. This article summarizes novel encouraging preclinical strategies to reactivate β-adrenergic receptor signaling in heart failure, including pharmacologic and gene therapy approaches, and attempts to translate acquired notions into the clinical setting.

α1-adrenoceptor activation induces phosphorylation of β2-adrenoceptors in human prostate tissue

OBJECTIVE

• To test whether β1-adrenoceptor activation leads to phosphorylation of the β2-adrenoceptor in human prostate tissue.

PATIENTS AND METHODS

• Prostate tissue from patients undergoing radical prostatectomy was stimulated in vitro with the α1-adrenergic agonist phenylephrine (10 µM). • α2-adrenoceptor phosphorylation at serines 345/346 was studied using Western blot analysis with a phospho-specific antibody. • The role of second messenger kinases was assessed by studying the effects of the protein kinase C (PKC) inhibitor Ro 31-8425 and the protein kinase A (PKA) inhibitor H89 on phenylephrine-induced phosphorylation. • The expression of G protein-coupled receptor kinases (GRKs) 2/3 was analysed using quantitative reverse-transcriptase-polymerase chain reaction (RT-PCR), Western blot analysis and immunohistochemistry.

RESULTS

• Stimulation of prostate tissue with phenylephrine resulted in phosphorylation of the β2-adrenoceptor (5, 10 and 20 min after stimulation). • This α1-adrenoceptor-induced phosphorylation of β2-adrenoceptors was resistant to inhibition of PKC and PKA. • Changes in phosphorylation levels were not attributable to changes in receptor levels, as these remained constant during stimulation. • RT-PCR and Western blot analysis showed expression of GRK2/3 in human prostate tissues. • Immunohistochemical staining showed that GRK2/3 expression in human prostate tissue is located to stromal and smooth muscle cells.

CONCLUSIONS

• Activation of α1-adrenoceptors causes phosphorylation of β2-adrenoceptors in the human prostate. This may enhance α1-adrenergic contraction and is possibly mediated by GRK2, which is expressed in prostate smooth muscle. • Mutual regulation between different adrenergic receptors might be involved in the therapeutic effects of α1-blockers in patients with benign prostate hyperplasia.

Role of specific protein kinase C isoforms in modulation of β1- and β2-adrenergic receptors

The function of beta-adrenergic receptor (betaAR) is modulated by the activity status of alpha1-adrenergic receptors (alpha1ARs) via molecular crosstalk, and this becomes evident when measuring cardiac contractile responses to adrenergic stimulation. The molecular mechanism underlying this crosstalk is unknown. We have previously demonstrated that overexpression of alpha1B-adrenergic receptor (alpha1BAR) in transgenic mice leads to a marked desensitization of betaAR-mediated adenylyl cyclase stimulation which is correlated with increased levels of activated protein kinase C (PKC) beta, delta and [J. Mol. Cell. Cardiol. 30 (1998) 1827]. Therefore, we wished to determine which PKC isoforms play a role in heterologous betaAR desensitization and also which isoforms of the betaAR were the molecular target(s) for PKC. In experiments using constitutively activated PKC expression constructs transfected into HEK 293 cells also expressing the beta2AR, constitutively active (CA)-PKC overexpression was first confirmed by immunoblots using specific anti-PKC antibodies. We then demonstrated that the different PKC subtypes lead to a decreased maximal cAMP accumulation following isoproterenol stimulation with a rank order of PKCalpha > or = PKCzeta>PKC>PKCbetaII. However, a much more dramatic desensitization of adenylyl cyclase stimulation was observed in cells co-transfected with different PKC isoforms and beta1AR. Further, the modulation of beta1AR by PKC isoforms had a different rank order than for the beta2AR: PKCbetaII>PKCalpha>PKC>PKCzeta. PKC-mediated desensitization was reduced by mutating consensus cAMP-dependent protein kinase (PKA)/PKC sites in the third intracellular loop and/or the carboxy-terminal tail of either receptor. Our results demonstrate therefore that the beta1AR is the most likely molecular target for PKC-mediated heterologous desensitization in the mammalian heart and that modulation of adrenergic receptor activity in any given cell type will depend on the complement of PKC isoforms present.

Cardiac-directed overexpression of wild-type alpha1B-adrenergic receptor induces dilated cardiomyopathy

Using transgenesis as a paradigm, we show here that alpha1-adrenergic receptors (alpha1AR) play an important role in cardiac homeostasis. Cardiomyocyte-specific overexpression of the alpha(1B)AR subtype resulted in the development of dilated cardiomyopathy and death at ~9 mo of age with typical signs of heart failure. Histological analyses showed the enlargement of all four cardiac chambers and cardiomyocyte disarray in the failing hearts. Transgenic animals showed increased left ventricular areas, as assessed by echocardiography. In addition, a progressive decrease in left ventricular systolic function was revealed. The abundance and activity of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2) were reduced, and the ratio of phospholamban to SERCA2 was increased. alpha-Myosin heavy chain (MHC) mRNA was less abundant in older transgenic ventricles, whereas beta-MHC was induced in the failing hearts. Titin mRNA abundance was decreased at 9 mo, whereas atrial natriuretic factor mRNA was elevated at all times. This model mimics structural and functional features of idiopathic dilated cardiomyopathy. The results of this study suggest that chronic alpha1AR activity is deleterious for cardiac function.

Overexpression of alpha1B-adrenergic receptor induces left ventricular dysfunction in the absence of hypertrophy

The stimulation of cardiac alpha1-adrenergic receptors (AR) modulates the heart's inotropic response and plays a role in the induction of cardiomyocyte hypertrophy. We have analyzed transgenic mouse lines overexpressing a wild-type alpha1B-AR specifically in the heart. Basal level systolic and diastolic left ventricular (LV) contractile function was depressed both in the anesthetized closed-chest mouse and the perfused working-heart preparation. Intrinsic LV function was further characterized under controlled preload and afterload conditions using the perfusion model. Contractile parameters were restored by chronic treatment with the alpha-AR antagonist prazosin. In ventricular function curves, the load-dependent force increases (length-tension effects) remained intact, although the transgenic curve was shifted to lower levels. The basal level contractile deficits were paralleled by a decrease in calcium transients in isolated LV cardiomyocytes. LV function comparable to controls was restored by isoproterenol stimulation. The physiological changes occurred in the absence of cardiomyocyte hypertrophy. This transgenic model will be useful for studying the potential role of alpha1-AR in cardiac contractility and hypertrophy.