Changes in beta-adrenoceptors and G-proteins during the transition from cardiac hypertrophy to heart failure

The Arg16Gly-β(2)-adrenoceptor single nucleotide polymorphism: exercise capacity and survival in patients with end-stage heart failure

Heart failure (HF) is characterized by impaired myocardial β-adrenergic signal transduction. Single nucleotide polymorphisms (SNPs) within the β(1)- (Ser49Gly, Arg389Gly) and β(2)-adrenoceptor (Arg16Gly, Gln27Glu, Thr164Ile) have been associated with alterations in adrenoceptor (AR) function sensitivity in vitro and in vivo and possibly contribute to HF progression. The present study evaluated the relation of those SNPs to morbidity and mortality in patients with end-stage HF. A total of 226 patients with end-stage HF (ejection fraction ≤35%) were genotyped for the two β(1)AR SNPs and the three β(2)AR SNPs. Outcome (death, heart transplantation (HTX)) was determined from May 2003 to June 2004. Heart rate, systolic and diastolic blood pressure, and peak oxygen uptake were measured during graded treadmill exercise. Left ventricular end-diastolic and end-systolic diameters, ejection fraction, and fractional shortening at rest were measured using two-dimensional echocardiography. Minor allele frequencies were 0.12 for Gly49 and 0.27 for Gly389 (β(1)AR) and 0.37 for Arg16, 0.43 for Glu27 and 0.01 for Ile164 (β(2)AR). During follow-up, 45 patients died (20%), and 27 patients underwent HTX (12%). No significant differences in the incidence or in the time-to-endpoint of death and HTX between genotypes of the different SNPs within the β(1)- and β(2)AR were detected. However, patients carrying the Arg16-β(2)AR tended to have lower exercise capacity and a higher probability for death/HTX within 45 months (survival proportion 46%) than patients carrying the Gly16Gly-β(2)AR (survival proportion 64%). In conclusion, the Arg16Gly-β(2)AR might impact on exercise capacity and outcome in end-stage heart failure.

Regression of copper-deficient heart hypertrophy: reduction in the size of hypertrophic cardiomyocytes

Dietary copper (Cu) deficiency causes cardiac hypertrophy and its transition to heart failure in a mouse model. Cu repletion results in rapid regression of cardiac hypertrophy and prevention of heart failure. The present study was undertaken to understand dynamic changes of cardiomyocytes in the hypertrophic heart during the regression. Dams of FVB mice were fed a Cu-deficient (CuD) diet (0.3 mg Cu/kg) starting on Day 3 post-delivery, and weanling pups were fed the same diet until Cu repletion (6.0 mg Cu/kg) in the diet at 31 days of age. Heart samples were obtained at the end of CuD feeding or at 3, 7, 14 or 28 days after Cu repletion. Cu deficiency resulted in increases in the size and reduction in the number of cardiomyocytes in the heart. Cu repletion led to regression in the size of hypertrophic cardiomyocytes and normalization of the total number of cardiomyocytes. Although a direct reduction in the cell size would be significantly responsible for the regression of heart hypertrophy, some hypertrophic cardiomyocytes upon Cu repletion reentered the cell cycle as determined by Ki-67 staining in the cardiomyocyte-specific alpha-sarcomeric actin-stained cells and underwent division as determined by a mitosis-specific marker, phospho-histone 3. Quantitative analysis indicated that the replication of hypertrophic cardiomyocytes made a contribution of about one-third to the total mitosis of the regenerated myocardium. This study suggests that a direct reduction in the size of some hypertrophic cardiomyocytes and a replication of other hypertrophic cardiomyocytes with reduced size make a significant contribution to the regression of CuD heart hypertrophy, leading to normalization of the size and the number of cardiomyocytes in the heart.

Regression of dietary copper restriction-induced cardiomyopathy by copper repletion in mice

Dietary copper deficiency (CuD)(3) leads to cardiac hypertrophy in various animal models. We showed recently that heart failure develops after hypertrophy in FVB mice fed a CuD diet. The present study was undertaken to determine whether CuD-induced cardiac failure is reversible upon copper repletion (CuR). Dams of FVB mice were fed a CuD diet (0.3 mg/kg) starting from d 3 postdelivery; the weanling pups were fed the same diet until CuR with 6.0 mg/kg Cu in the diet at 4 or 5 wk of age. CuR at 4 wk of age prevented the body weight loss; at 5 wk of age, it resulted in the regaining of the lost weight caused by CuD. A significant regression of CuD-induced cardiac hypertrophy was observed in the CuR mice. Histopathological examination revealed that CuR eliminated CuD-caused lipid deposition in the myocardium, and electron microscopy demonstrated that CuD-induced ultrastructural changes such as mitochondrial swelling and organelle structural disarray were all reversed in the CuR mice. Hemodynamic analysis showed that the CuD-depressed systolic and diastolic parameters such as the maximal rate of left ventricular pressure rise (+dP/dt) and decline (-dP/dt), and the contraction and relaxation times were completely recovered in the CuR mice. Furthermore, the CuD-blunted myocardial responses to the beta-adrenergic agonist, isoproterenol, were also restored in the CuR mice. This study thus demonstrates for the first time that CuR results in the regression of heart failure induced by CuD as demonstrated by the reversal of depressed cardiac hemodynamic and contractile function and the restored responsiveness to beta-adrenergic stimulation.

Cardiomegaly induced by pressure overload in newborn rats is accompanied by altered expression of the long isoform of G(s)alpha protein and deranged signaling of adenylyl cyclase

G proteins-coupled signaling pathways appear to play a role in the development of cardiac hypertrophy and its progression to heart failure. The present study aimed to investigate trimeric G proteins and adenylyl cyclase signaling in immature as well as in adult rat myocardium during this process caused by pressure overload. Pressure overload was induced in newborn (2-day-old) rats by abdominal aortic banding and myocardial preparations from left ventricular myocardium of immature (10-day-old) and adult (90-day-old) animals were analyzed for the relative content of different G protein subunits and adenylyl cyclase (AC) by immunoblotting with specific antibodies. A functional status of the AC signaling system was also evaluated. Normal maturation of rat heart was accompanied by increased expression of AC type V/VI and VII and of the long isoform (G(s)alphaL) of G(s)alpha protein. In parallel, the amounts of myocardial G(i)alpha/G(o)alpha proteins tended to decrease, and G(q)alpha/G(11)alpha and Gbeta did not change. Interestingly, whereas fluoride-stimulated AC activity increased in the course of maturation, activity of AC measured under other experimental conditions (stimulation by Mn2+, forskolin or isoproterenol) was lower in adult than in young rat myocardium. Pressure overload did not influence distribution of G proteins in immature myocardium, but considerably decreased the content of G(s)alphaL and increased G(o)alpha proteins in hearts of 90-day-old rats. These hearts exhibited worsened functional reserve as compared to age-matched controls and activity of AC was also markedly lower. A considerable reduction in Mn(2+)-stimulated AC activity together with similar decrease in AC activity determined under other stimulation conditions suggests that it is a function of AC catalytic subunit that is primarily impaired in this model of pressure overload.

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.

Changes of beta-adrenergic signaling in compensated human cardiac hypertrophy depend on the underlying disease

In human heart failure, desensitization of the beta-adrenergic signal transduction has been reported to be one of the main pathophysiological alterations. However, data on the beta-adrenergic system in human compensated cardiac hypertrophy are very limited. Therefore, we studied the myocardial beta-adrenergic signaling in patients suffering from hypertrophic obstructive cardiomyopathy (HOCM, n = 9) or from aortic valve stenosis (AoSt, n = 8). beta-Adrenoceptor density determined by [(125)I]iodocyanopindolol binding was reduced in HOCM and AoSt compared with nonhypertrophied, nonfailing myocardium (NF) of seven organ donors. In HOCM the protein expression of stimulatory G protein alpha-subunit (G(s)alpha) measured by immunoblotting was unchanged, whereas the inhibitory G protein alpha-subunit (Galpha(i-2)) was increased. In contrast, in AoSt, Galpha(i-2) protein was unchanged, but G(s)alpha protein was increased. Adenylyl cyclase stimulation by isoproterenol was reduced in HOCM but not in AoSt. Plasma catecholamine levels were normal in all patients. In conclusion, both forms of hypertrophy are associated with beta-adrenoceptor downregulation but with different changes at the G protein level that occur before symptomatic heart failure due to progressive dilatation of the left ventricle develops and are not due to elevated plasma catecholamine levels.