Endothelin-1 as a protective factor against beta-adrenergic agonist-induced apoptosis in cardiac myocytes

The role of endothelin B receptor in bone modelling during orthodontic tooth movement: a study on ETB knockout rats

The endothelin system has an important role in bone modelling during orthodontic tooth movement (OTM); however, little is known about the involvement of endothelin B receptors (ET_B) in this process. The aim of this study was to evaluate the role of ET_B in bone modelling during OTM using ET_B knockout rats (ET_B-KO). Thirty-two male rats were divided into 4 groups (n = 8 per group): the ET_B-KO appliance group, ET_B-KO control group, wild type (ET_B-WT) appliance group, and ET_B-WT control group. The appliance consisted of a super-elastic closed-coil spring placed between the first and second left maxillary molar and the incisors. Tooth movement was measured on days 0 and 35, and maxillary alveolar bone volume, osteoblast, and osteoclast volume were determined histomorphometrically on day 35 of OTM. Next, we determined the serum endothelin 1 (ET-1) level and gene expression levels of the osteoclast activity marker cathepsin K and osteoblast activity markers osteocalcin and dentin matrix acidic phosphoprotein 1 (DMP1) on day 35. The ET_B-KO appliance group showed significantly lower osteoblast activity, diminished alveolar bone volume and less OTM than the ET_B-WT appliance group. Our results showed that ET_B is involved in bone modelling in the late stage of OTM.

Signaling Pathways in Cardiac Myocyte Apoptosis

Cardiovascular diseases, the number 1 cause of death worldwide, are frequently associated with apoptotic death of cardiac myocytes. Since cardiomyocyte apoptosis is a highly regulated process, pharmacological intervention of apoptosis pathways may represent a promising therapeutic strategy for a number of cardiovascular diseases and disorders including myocardial infarction, ischemia/reperfusion injury, chemotherapy cardiotoxicity, and end-stage heart failure. Despite rapid growth of our knowledge in apoptosis signaling pathways, a clinically applicable treatment targeting this cellular process is currently unavailable. To help identify potential innovative directions for future research, it is necessary to have a full understanding of the apoptotic pathways currently known to be functional in cardiac myocytes. Here, we summarize recent progress in the regulation of cardiomyocyte apoptosis by multiple signaling molecules and pathways, with a focus on the involvement of these pathways in the pathogenesis of heart disease. In addition, we provide an update regarding bench to bedside translation of this knowledge and discuss unanswered questions that need further investigation.

Endothelin axis is upregulated in human and rat right ventricular hypertrophy

RATIONALE

Right ventricular (RV) function is the most important determinant of morbidity and mortality in pulmonary arterial hypertension (PAH). Endothelin (ET)-1 receptor antagonists (ERAs) are approved therapies for PAH. It is not known whether ERAs have effects on the RV, in addition to their vasodilating/antiproliferative effects in pulmonary arteries.

OBJECTIVE

We hypothesized that the ET axis is upregulated in RV hypertrophy (RVH) and that ERAs have direct effects on the RV myocardium.

METHODS AND RESULTS

RV myocardial samples from 34 patients with RVH were compared with 16 nonhypertrophied RV samples, and from rats with normal RV versus RVH attributable to PAH. Confocal immunohistochemistry showed that RVH myocardial ET type A (but not type B) receptor and ET-1 protein levels were increased compared with the nonhypertrophied RVs and positively correlated with the degree of RVH (RV thickness/body surface area; r(2)=0.838 and r(2)=0.818, respectively; P<0.01). These results were recapitulated in the rat model. In modified Langendorff perfusions, ERAs (BQ-123 and bosentan 10(-7,-6,-5) mol/L) decreased contractility in the hypertrophied, but not normal RV, in a dose-dependent manner (P<0.01).

CONCLUSIONS

Patients and rats with PAH have an upregulation of the myocardial ET axis in RVH. This might be a compensatory mechanism to preserve RV contractility, as the afterload increases. ERAs use might potentially worsen RV function, and this could explain some of the peripheral edema noted clinically with these agents. Further studies are required to evaluate the effects of ERAs on the RV in patients with RVH and PAH.

β1-Integrin is up-regulated via Rac1-dependent reactive oxygen species as part of the hypertrophic cardiomyocyte response

β(1)-Integrin mediates cardiomyocyte growth and survival and its proper regulation is essential for the structural and functional integrity of the heart. β(1)-Integrin expression is enhanced in hypertrophy, but the mechanism and significance of its up-regulation are unknown. Because reactive oxygen species (ROS) are important mediators of myocardial remodeling we examined their role in regulated β(1)-integrin expression. Hypertrophy was induced in neonatal cardiomyocytes by endothelin-1 (ET-1), which activated the regulatory NADPH oxidase subunit Rac1, evoked ROS, and enhanced fetal gene expression and cardiomyocyte size. ET-1 also enhanced cell adhesion and FAK phosphorylation and inhibited oxidative stress-induced cardiomyocyte apoptosis. Further, ET-1 increased β(1)-integrin mRNA and protein expression via Rac1-ROS-dependent MEK/ERK and EGF receptor-PI3K/Akt activation as shown by adenoviral dominant-negative Rac1 or overexpression of copper/zinc-superoxide dismutase. The relevance of regulated β(1)-integrin expression was examined in cardiomyocytes, in which targeting siRNA impeded the ET-1-induced β(1)-integrin up-regulation. In these cells, ET-1-induced cell adhesion, FAK phosphorylation, and hypertrophic response were significantly blunted, whereas its antiapoptotic effect was predominantly unchanged, suggesting at least partial dissociation of prohypertrophic and prosurvival signaling elicited by ET-1. In conclusion, β(1)-integrin up-regulation in response to ET-1 is mediated via Rac1-ROS-dependent activation of prohypertrophic pathways and is mandatory for ET-1-induced FAK activation, cell adhesion, and hypertrophic response.

Prolonged overcirculation-induced pulmonary arterial hypertension as a cause of right ventricular failure

AIMS

Three-month chronic systemic-to-pulmonary shunting in growing piglets has been reported as an early pulmonary arterial hypertension (PAH) model with preserved right ventricular (RV) function. We sought to determine whether prolonged shunting might be associated with more severe PAH and RV failure.

METHODS AND RESULTS

Fourteen growing piglets were randomized to a sham operation or the anastomosis of the left innominate artery to the pulmonary arterial trunk. Six months later, the shunt was closed and the animals underwent haemodynamic evaluation followed by tissue sampling for pathobiological assessment. Prolonged shunting had resulted in increased mean pulmonary artery pressure (22 ± 2 versus 17 ± 1 mmHg) and pulmonary arteriolar medial thickness, while cardiac output was decreased. However, RV-arterial coupling was markedly deteriorated, with a ~50% decrease in the ratio of end-systolic to pulmonary arterial elastances (Ees/Ea). Lung tissue expressions of endothelin-1, angiopoietin-1, and bone morphogenetic protein receptor-2 were similarly altered compared with previously observed after 3-month shunting. At the RV tissue level, pro-apoptotic ratio of Bax-to-Bcl-2 expressions and caspase-3 activation were increased, along with an increase in cardiomyocyte size, while expressions in voltage-gated potassium channels (Kv1.5 and Kv2.1) and angiogenic factors (angiopoietin-2 and vascular endothelial growth factor) were decreased. Right ventricular expressions of pro-inflammatory cytokines [interleukin (IL)-1α, IL-1β, tumour necrosis factor-α (TNF-α)] and natriuretic peptide precursors (NPPA and NPPB) were increased. There was an inverse correlation between RV Ees/Ea and pro-apoptotic Bax/Bcl-2 ratios.

CONCLUSIONS

Prolonged left-to-right shunting in piglets does not further aggravate pulmonary vasculopathy, but is a cause of RV failure, which appears related to an activation of apoptosis and inflammation.

Endothelin signalling regulates volume-sensitive Cl- current via NADPH oxidase and mitochondrial reactive oxygen species

AIMS

We assessed regulation of volume-sensitive Cl(-) current (I(Cl,swell)) by endothelin-1 (ET-1) and characterized the signalling pathway responsible for its activation in rabbit atrial and ventricular myocytes.

METHODS AND RESULTS

ET-1 elicited I(Cl,swell) under isosmotic conditions. Outwardly rectified Cl(-) current was blocked by the I(Cl,swell)-selective inhibitor DCPIB or osmotic shrinkage and involved ET(A) but not ET(B) receptors. ET-1-induced current was abolished by inhibiting epidermal growth factor receptor (EGFR) kinase or phosphoinositide-3-kinase (PI-3K), indicating that these kinases were downstream. Regarding upstream events, activation of I(Cl,swell) by osmotic swelling or angiotensin II (AngII) was suppressed by ET(A) blockade, whereas AngII AT(1) receptor blockade failed to alter ET-1-induced current. Reactive oxygen species (ROS) produced by NADPH oxidase (NOX) stimulate I(Cl,swell). As expected, blockade of NOX suppressed ET-1-induced I(Cl,swell), but blockade of mitochondrial ROS production with rotenone also suppressed I(Cl,swell). I(Cl,swell) was activated by augmenting complex III ROS production with antimycin A or diazoxide; in this case, I(Cl,swell) was insensitive to NOX inhibitors, indicating that mitochondria were downstream from NOX. ROS generation in HL-1 cardiomyocytes measured by flow cytometry confirmed the electrophysiological findings. ET-1-induced ROS production was inhibited by blocking either NOX or mitochondrial complex I, whereas complex III-induced ROS production was insensitive to NOX blockade.

CONCLUSION

ET-1-ET(A) signalling activated I(Cl,swell) via EGFR kinase, PI-3K, and NOX ROS production, which triggered mitochondrial ROS production. ET(A) receptors were downstream effectors when I(Cl,swell) was elicited by osmotic swelling or AngII. These data suggest that ET-1-induced ROS-dependent I(Cl,swell) is likely to participate in multiple physiological and pathophysiological processes.

Cardioprotective signaling by endothelin

The endothelin axis promotes vasoconstriction, suggesting that antagonists of endothelin signaling might be useful in treatment of heart failure. However, promising results from animal trials have not been recapitulated in heart failure patients. Here we review the role of major signaling pathways in the heart that are involved in cell survival initiated by ET-1. These pathways include mitogen-activated protein kinase, phosphatidyl inositol-1,4,5-triphosphate kinase (PI3K-AKT), nuclear factor-kappaB (NF-kappaB), and calcineurin signaling. A better understanding of endothelin-mediated signaling in cardiac cell survival may allow a reevaluation of endothelin receptor antagonists (ETRAs) in the treatment of heart failure.

Antagonism of endothelin-1 inhibits hypoxia-induced apoptosis in cardiomyocytes

Apoptosis is well documented to be a common feature of many pathological processes of the heart. Exogenous endothelin-1 (ET-1) has been shown to be proapoptotic or antiapoptotic, depending on ET-1 concentration, cell type, and the ratio of ETA/ETB receptor subtypes. The role of endogenous ET-1 in cardiomyocyte apoptosis, however, is not clarified. This study observed the effects of the ETA-receptor antagonists BQ610 and BQ123 and the ETB-receptor antagonist BQ788 on hypoxia-induced apoptosis in primary cultured neonatal rat cardiomyocytes. Hypoxic apoptosis was induced by incubating cardiomyocytes in serum-free medium under 3% O2 and 5% CO2 for 24 h and evaluated by TUNEL analysis and flow cytometry. TUNEL analysis showed that the apoptotic cardiomyocytes constituted 24.2% +/- 2.2% of the total cells under hypoxic conditions. Treatment with BQ610 (5 micromol/L) significantly reduced the apoptosis rate to 13.2% +/- 3.7% (data from 4 independent experiments, p < 0.01 vs. hypoxia). Flow cytometry showed that the percentage of apoptotic cells positively stained with annexin V and propidium iodide was 42.76% +/- 4.44% (n = 12) in cultures subjected to hypoxia. BQ123 at 0.04, 0.2, and 1.0 micromol/L dose-dependently reduced the apoptosis rate to 34.00% +/- 10.35% (n = 6, p < 0.05), 30.38% +/- 8.28% (n = 6, p < 0.01), and 22.89% +/- 4.19% (n = 6, p < 0.01), respectively. In contrast, BQ788 did not affect hypoxic apoptosis. These findings suggested that endogenous ET-1 contributed to hypoxia-induced apoptosis in cultured cardiomyocytes, which was mediated by ETA receptors, but not by ETB receptors.

Silibinin protects against isoproterenol-induced rat cardiac myocyte injury through mitochondrial pathway after up-regulation of SIRT1

Terminally differentiated adult injured cardiac myocytes have been used for various animal models of heart failure. It has recently been shown that isoproterenol induces injury in rat neonatal cardiac myocytes via a beta-adrenergic pathway, suggesting that it might be one of the factors involved in myocardial cell injury in heart failure in vivo. In the study, silibinin, a plant flavanoid from milk thistle was first evaluated for its protective effect against beta-adrenergic agonist isoproterenol-induced injury in cultured rat neonatal cardiac myocytes. The viability, activation of lactate dehydrogenase (LDH), and content of maleic dialdehyde (MDA) were chosen for measuring the degree of cardiac myocytes injury. As a result, silibinin protected isoproterenol-treated rat cardiac myocytes from death and significantly decreased LDH release and MDA production. Silibinin increased superoxide dismutase activity, decreased [Ca(2+)](i), and increased mitochondrial membrane potential (DeltaPsi). Furthermore, the release of pro-apoptotic cytochrome c from mitochondria was reduced by silibinin. Silibinin increased the expression of anti-apoptotic Bcl-2 family protein Bcl-2, and up-regulation of SIRT1 inhibited the translocation of Bax from cytoplasm to mitochondria, which caused mitochondrial dysfunction and cell injury. These results demonstrate that silibinin protects against isoproterenol-induced cardiac myocytes injury through resuming mitochondrial function and regulating the expression of SIRT1 and Bcl-2 family members.

Endogenous endothelin-1 is required for cardiomyocyte survival in vivo

BACKGROUND

Endothelin-1 (ET-1) has potent vasoconstrictor and hypertrophic actions. Pharmacological antagonists of endothelin receptors attenuate cardiac hypertrophy, have been approved for treatment of pulmonary hypertension, and are under investigation for treatment of heart failure. To investigate the role of ET-1 in the heart, we created mice with cardiomyocyte deletion of ET-1.

METHODS AND RESULTS

Mice with cardiomyocyte-specific deletion of ET-1 are phenotypically normal when young. Remarkably, as the mice age or when young animals are subjected to aortic banding, they develop an unexpected phenotype of progressive systolic dysfunction and cardiac dilation. Echocardiography, necropsy, histology, and molecular phenotype confirm a dilated cardiomyopathy. Terminal deoxynucleotidyl transferase-mediated dUTP nick-end-labeling analysis reveals greater abundance of apoptotic nuclei in the ET-1-deficient hearts. Transcriptional and Western analyses suggest enhanced tumor necrosis factor (TNF)-mediated apoptosis with increases in caspase-8 activity. These ET-1-deficient hearts also have diminished nuclear factor (NF)-kappaB activity, resulting in diminution of downstream inhibitors of TNF signaling.

CONCLUSIONS

Local ET-1 gene expression is necessary to maintain normal cardiac function and cardiomyocyte survival in mice with both age and hemodynamic stress. This cardiac-protective effect is mediated by paracrine ET-1 modulation of TNF-related apoptosis, in part through upregulation of NF-kappaB signaling.

FHL2/SLIM3 decreases cardiomyocyte survival by inhibitory interaction with sphingosine kinase-1

Sphingosine kinase-1 (SK1) is a key enzyme catalyzing the phosphorylation of sphingosine to sphingosine-1-phosphate (S1P). Recent studies suggest that SK1, and its product S1P, regulate diverse biological functions, including cell growth, differentiation, proliferation, and apoptosis. S1P may also play an important role in cardiac development and ischemic preconditioning, but the mechanism underlying these effects is not known. Using a yeast 2-hybrid screen with SK1 as bait and a cardiac cDNA library to identify novel proteins involved in regulating SK1 activity in cardiomyocytes, we identified the LIM-only factor FHL2 (SLIM3) as a SK1-interacting protein in both yeast and mammalian cells. FHL2, but not FHL1 or FHL3, interacted with SK1, and FHL2 colocalized with SK1 in the cytoplasm. The interaction sites with SK1 consisted of at least 4 LIM domains in FHL2, whereas the C-terminal portion of SK1 mediates the binding of FHL2 in SK1. Overexpression of FHL2 attenuated the activity and antiapoptotic effects of SK1. Indeed, endothelin-1, which is a potent survival factor in cardiomyocytes, inhibited FHL2-SK1 association and increased SK1 activity. These findings indicate that FHL2 is a novel inhibitor of SK1 activity in cardiomyocytes and suggest that targeting FHL2 for inhibition may prevent myocardial apoptosis through activation of SK1.

Cardiovascular endothelins: essential regulators of cardiovascular homeostasis

The endothelin (ET) system consists of 3 ET isopeptides, several isoforms of activating peptidases, and 2 G-protein-coupled receptors, ETA and ETB, that are linked to multiple signaling pathways. In the cardiovascular system, the components of the ET family are expressed in several tissues, notably the vascular endothelium, smooth muscle cells, and cardiomyocytes. There is general agreement that ETs play important physiological roles in the regulation of normal cardiovascular function, and excessive generation of ET isopeptides has been linked to major cardiovascular pathologies, including hypertension and heart failure. However, several recent clinical trials with ET receptor antagonists were disappointing. In the present review, the authors take the stance that ETs are mainly and foremost essential regulators of cardiovascular function, hence that antagonizing normal ET actions, even in patients, will potentially do more harm than good. To support this notion, we describe the predominant roles of ETs in blood vessels, which are (indirect) vasodilatation and ET clearance from plasma and interstitial spaces, against the background of the subcellular mechanisms mediating these effects. Furthermore, important roles of ETs in regulating and adapting heart functions to different needs are addressed, including recent progress in understanding the effects of ETs on diastolic function, adaptations to changes in preload, and the interactions between endocardial-derived ET-1 and myocardial pump function. Finally, the potential dangers (and gains) resulting from the suppression of excessive generation or activity of ETs occurring in some cardiovascular pathological states, such as hypertension, myocardial ischemia, and heart failure, are discussed.

Comparison of the dual receptor endothelin antagonist enrasentan with enalapril in asymptomatic left ventricular systolic dysfunction: a cardiovascular magnetic resonance study

OBJECTIVE

To compare the effect of the dual endothelin A/B receptor antagonist enrasentan with enalapril on left ventricular (LV) remodelling.

METHODS

Multicentre, randomised, double blind, parallel group study of 72 asymptomatic patients with LV dysfunction. Patients received enrasentan (60-90 mg/day) or enalapril (10-20 mg/day). The primary end point was the change in LV end diastolic volume index (EDVI) after six months' treatment.

RESULTS

LV EDVI increased with enrasentan but decreased with enalapril (3.9 (1.8) v -3.4 (1.4) ml/m2, p = 0.001). Enrasentan increased resting cardiac index compared with enalapril (0.11 (0.07) v -0.10 (0.07) l/m2, p = 0.04), as well as LV mass index (0.67 (1.6) v -3.6 (1.6) g/m2, p = 0.04). Other variables were comparable between groups. Enalapril lowered brain natriuretic peptide more than enrasentan (-19.3 (9.4) v -5.8 (6.9) pg/ml, p = 0.005). Noradrenaline (norepinephrine) (p = 0.02) increased more with enrasentan than with enalapril. Enrasentan was associated with more serious adverse events compared with enalapril (six (16.7%) patients v one (2.8%), p = 0.02); the rate of progression of heart failure did not differ.

CONCLUSION

In asymptomatic patients with LV dysfunction, LV EDVI increased over six months with enrasentan compared with enalapril treatment, with adverse neurohormonal effects. This suggests that enrasentan at a dose of 60-90 mg/day over six months causes adverse ventricular remodelling despite an increase in the resting cardiac index.

TGF-beta-dependent pathogenesis of mitral valve prolapse in a mouse model of Marfan syndrome

Mitral valve prolapse (MVP) is a common human phenotype, yet little is known about the pathogenesis of this condition. MVP can occur in the context of genetic syndromes, including Marfan syndrome (MFS), an autosomal-dominant connective tissue disorder caused by mutations in fibrillin-1. Fibrillin-1 contributes to the regulated activation of the cytokine TGF-beta, and enhanced signaling is a consequence of fibrillin-1 deficiency. We thus hypothesized that increased TGF-beta signaling may contribute to the multisystem pathogenesis of MFS, including the development of myxomatous changes of the atrioventricular valves. Mitral valves from fibrillin-1-deficient mice exhibited postnatally acquired alterations in architecture that correlated both temporally and spatially with increased cell proliferation, decreased apoptosis, and excess TGF-beta activation and signaling. In addition, TGF-beta antagonism in vivo rescued the valve phenotype, suggesting a cause and effect relationship. Expression analyses identified increased expression of numerous TGF-beta-related genes that regulate cell proliferation and survival and plausibly contribute to myxomatous valve disease. These studies validate a novel, genetically engineered murine model of myxomatous changes of the mitral valve and provide critical insight into the pathogenetic mechanism of such changes in MFS and perhaps more common nonsyndromic variants of mitral valve disease.

Intracellular signaling pathways for norepinephrine- and endothelin-1-mediated regulation of myocardial cell apoptosis

Accumulating data support the idea that apoptosis in cardiac myocytes, in part, contributes to the development of heart failure. Since a number of neurohormonal factors are activated in this state, these factors may be involved in the positive and negative regulation of apoptosis in cardiac myocytes. Norepinephrine is one such factor and induces apoptosis in cardiac myocytes via a beta-adrenergic receptor pathway. beta-adrenergic agonist-induced apoptosis in cardiac myocytes is dependent on the activation of the cAMP/protein kinase A pathway. Interestingly, the activation of this pathway protects PC12 cells from apoptosis, suggesting that cAMP/protein kinase A regulates apoptosis in a cell type-specific manner. Another neurohormonal factor activated in heart failure is endothelin-1, which acts as a potent survival factor against myocardial cell apoptosis. Intracellular signaling pathways for endothelin-1-mediated protection include activation of MEK-1 /ERK1/2 and PI3 kinase. In addition to these protective pathways common among cell types, endothelin- activates the calcium-activated phosphatase calcineurin, which is necessary for the nuclear import of NFAT transcription factors. These factors interact with the cardiac-restricted zinc finger protein GATA-4 and induce transcription and expression of anti-apoptotic molecule bcl-2. Thus, myocardial cell apoptosis is regulated by pathways unique to cardiac myocytes as well as by those common among cell types. It should be further determined whether agents that specifically block myocardial cell apoptosis will attenuate the progression of heart failure.

Activators of PPARgamma antagonize protection of cardiac myocytes by endothelin-1

Endothelin-1 (ET-1) is a potent survival factor against myocardial cell apoptosis. This anti-apoptotic effect of ET-1 is mediated in part through calcineurin/NFATc-dependent induction of bcl-2 expression. Since it has been reported that peroxisome proliferator-activated receptor-gamma (PPARgamma) interacts with NFATc, we investigated the effects of PPARgamma ligands on anti-apoptotic effects of ET-1 in cardiac myocytes. In primary cardiac myocytes from neonatal rats, administration of PPARgamma activators (15-deoxy-delta12,14-prostaglandin J2 and troglitazone) attenuated the anti-apoptotic effects of ET-1. These activators abolished the ET-1-stimulated increase in bcl-2 expression and in binding of cardiac NFATc to the bcl-2 NFAT site. These findings demonstrate that activators of PPARgamma perturb the anti-apoptotic effects of ET-1 in cardiac myocytes and that this perturbation is, in part, based on functional transcriptional cross-talk between NFATc and PPARgamma.

Endothelin receptor antagonists in heart failure: current status and future directions

Experimental evidence suggests that endothelin substantially contributes to left ventricular remodelling and progression of heart failure. Plasma endothelin (ET)-1 levels are increased in patients with heart failure, independent of the aetiology, and correlate with the severity of the disease. Furthermore, tissue endothelin levels and endothelin receptors are upregulated in myocardium from animals and humans with heart failure. In several experimental models of left ventricular remodelling and/or heart failure, treatment with nonselective ET-A and -B as well as selective ET-A antagonists exerted beneficial cardiovascular effects. In patients with heart failure, short-term studies of treatment with endothelin antagonists demonstrated an improvement of haemodynamic parameters; however, long-term treatment with these drugs did not significantly improve combined morbidity/mortality endpoints. Furthermore, in the recently completed Endothelin-A Receptor Antagonist Trial in Heart Failure (EARTH) trial in patients with chronic heart failure, the selective ET-A receptor antagonist darusentan did not significantly affect left ventricular remodelling as assessed by cardiac magnetic resonance imaging. Potential reasons for the lack of beneficial effects of long-term treatment with ET antagonists in patients with heart failure include the following. Firstly, adverse effects on left ventricular healing have been observed when endothelin antagonist therapy was introduced early after myocardial infarction in rats. Secondly, the role of the ET-B receptor in the pathophysiology of heart failure and remodelling processes has not been clearly defined. Finally, for the detection of improvement in left ventricular remodelling, a study needs to be conducted in patients with recent myocardial infarction and signs of heart failure.

Endothelin-1-dependent nuclear factor of activated T lymphocyte signaling associates with transcriptional coactivator p300 in the activation of the B cell leukemia-2 promoter in cardiac myocytes

Endothelin-1 (ET-1) is a potent survival factor that protects cardiac myocytes from apoptosis. ET-1 induces cardiac gene transcription and protein expression of antiapoptotic B cell leukemia-2 (bcl-2) in a calcineurin-dependent manner. A cellular target of adenovirus early region 1A (E1A) oncoprotein, p300 also activates bcl-2 transcription in cardiac myocytes and is required for their survival. p300 acts as a calcineurin-regulated nuclear factors of activated T lymphocytes (NFATc), downstream targets of calcineurin. In addition, the bcl-2 promoter contains multiple NFAT consensus sequences. These findings prompted us to investigate the role of NFATc in ET-1-dependent and p300-dependent bcl-2 transcription in cardiac myocytes. In primary cardiac myocytes prepared from neonatal rats, mutation of 2 NFAT sites within the bcl-2 promoter completely abolished the ET-1- and p300-induced increases in the activity of this promoter. We show here that p300 markedly potentiates the binding of NFATc1 to the bcl-2 NFAT element by interacting with NFATc1 in an E1A-dependent manner. On the other hand, stimulation of cardiac myocytes with ET-1 causes nuclear translocation of NFATc1, which interacts with p300 and increases DNA binding. Expression of E1A did not change the cardiac nuclear localization of NFATc1 but blocked its interaction with p300, DNA binding, and bcl-2 promoter activation. These findings suggest that ET-1-dependent NFATc signaling associates with p300 in the transactivation of bcl-2 gene in cardiac myocytes.

Regulation of cardiac myocyte apoptosis by the GATA-4 transcription factor

Apoptosis of cardiac muscle cells plays important roles in the development of various heart diseases including myocardial infarction and anthracycline-induced cardiomyopathy. Understanding the regulatory mechanisms of cardiac myocyte apoptosis and survival is important for establishing therapeutic strategies against heart disease. Our recent experiments demonstrate that the GATA-4 transcription factor not only mediates cardiac hypertrophy, but also regulates apoptosis and survival of adult cardiac muscle cells. Apoptosis induced by anthracyclines is associated with decreased expression of GATA-4, while the restoration of GATA-4 levels via ectopic expression attenuated the apoptosis. Survival factors of cardiac myocytes such as hepatocyte growth factor and endothelin-1 activate GATA-4, and this signal transduction mechanism at least in part serves to protect the heart against oxidative stress.

Growth factor signaling for cardioprotection against oxidative stress-induced apoptosis

The heart is subjected to oxidative stress during various clinical situations, such as ischemia-reperfusion injury and anthracycline chemotherapy. The loss of cardiac myocytes is the major problem in heart failure; thus, it is important to protect cardiac myocytes against cell death. Various growth factors, including insulin like growth factor, hepatocyte growth factor, endothelin-1, fibroblast growth factor, and transforming growth factor, have been shown to protect the heart against oxidative stress. The mechanism of growth factor-mediated cardioprotection may involve the attenuation of cardiac myocyte apoptosis. The present article summarizes the current knowledge on the molecular mechanisms of growth factor-mediated antiapoptotic signaling in cardiac myocytes. Insulin-like growth factor-1 activates phosphatidylinositol 3' -kinase and extracellular signal-regulated kinase pathways. Recent data showed that GATA-4 might be an important mediator of cardiac myocyte survival by endothelin-1 and hepatocyte growth factor. These growth factors, as well as mediators of growth factor-signaling, may be useful in therapeutic strategies against oxidative stress-induced cardiac injury.

The opposing effects of endothelin-1 and C-type natriuretic peptide on apoptosis of neonatal rat cardiac myocytes

C-type natriuretic peptide (CNP) and endothelin-1 are paracrine peptides with opposing effects on cardiac myocyte contraction and intracellular cGMP production. Elevated levels of both endothelin-1 and CNP are found in patients with congestive heart failure. These factors may be related to positive and negative regulation of cell apoptosis in the failing heart. To evaluate the effect of CNP and endothelin-1 on apoptosis of cardiac myocytes and the possible mechanisms involved, primary cardiac myocytes were prepared from neonatal Sabra rats. Cardiomyocyte apoptosis was evaluated by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) and Annexin V in situ staining. The TUNEL method was used to measure the apoptotic index. CNP and the cGMP derivative, 8-br-cGMP, induced apoptosis of cardiac myocytes. CNP-induced apoptosis could be blocked by HS 142-1 (a mixture of 20-30 kinds of linear beta-1, 6-glucan esterified by capronic acid, an antagonist of type A and B natriuretic peptide receptors), and KT 5823 (C29H25N3O5), the inhibitor of cGMP-dependent protein kinase). Alpha-difluoromethylornithine (DFMO), the irreversible inhibitor of ornithine decarboxylase, also induced apoptosis to a similar extent. CNP and 8-br-cGMP caused a marked reduction of intracellular ornithine decarboxylase expression, as determined by Western blot analysis and immunohistochemical assay. Preincubation with endothelin-1 attenuated CNP- and 8-br-cGMP-induced cardiomyocyte apoptosis. Endothelin-1 also antagonized the CNP- and 8-br-cGMP-induced reduction of intracellular ornithine decarboxylase expression. These results suggest that CNP has a proapoptotic effect on neonatal rat cardiac myocytes. The effect is mediated via natriuretic peptide receptors and is due to an elevation of intracellular cGMP, which reduces the expression of intracellular ornithine decarboxylase and probably the production of polyamines. Endothelin-1 protects cardiac myocytes against CNP-induced apoptosis by influencing the cGMP-dependent pathway, and this effect is probably mediated through both a reduction of cGMP and antagonism of the CNP-induced reduction of intracellular ornithine decarboxylase expression.

Effects of endothelin-1 on mitochondrial function during the protection against myocardial cell apoptosis

Endothelin-1 is a potent survival factor against myocardial cell apoptosis. While apoptotic stimuli often perturb mitochondrial function by decreasing the membrane potential as well as oxygen consumption, it is unknown whether ET-1 can rescue such perturbation by apoptotic stimuli. Administration of endothelin-1 inhibited the H(2)O(2)-induced release of cytochrome c from mitochondria to the cytosol in cardiac myocytes, indicating the involvement of the mitochondria-dependent pathway in the anti-apoptotic effect of endothelin-1. We showed here by cytofluorimetric analysis that endothelin-1 prevented the H(2)O(2)-induced decrease of membrane potential. However, endothelin-1 was unable to reverse the H(2)O(2)-mediated decrease in oxygen consumption and electron transport in the mitochondria of cardiac myocytes. Endothelin-1 was unable to rescue cardiac myocytes from apoptosis when administered after the decrease in mitochondrial membrane potential. These data suggest that endothelin-1 does not target the mitochondrial respiratory chain, but rather stabilizes the mitochondrial membrane during the protection against apoptosis.

Role of endothelins in congestive heart failure

Despite major advances in conventional medical therapy, patients with heart failure continue to experience significant morbidity and mortality. Endothelin-1 (ET-1) is a potent vasocontrictor and mitogenic peptide that is activated in heart failure. There is increasing experimental and clinical evidence in support of an important role of ET-1 in the pathophysiology of heart failure. Manipulation of the activity of ET-1, especially using endothelin receptor blockers, has allowed for the further elucidation of the role of this neurohormonal system and development of novel therapeutic strategies in heart failure. Published clinical studies of these agents to date have involved relatively small numbers of patients with severe heart failure, followed for a relatively short period of time, and have mainly examined surrogate endpoints. Large-scale trials that address to hard clinical outcomes are ongoing and their results forthcoming. A key question that remains concerns whether selective ETA or dual ETA-ETB receptor blockade will be more effective.

beta1 integrins expression in adult rat ventricular myocytes and its role in the regulation of beta-adrenergic receptor-stimulated apoptosis

We have shown that the stimulation of beta-adrenergic receptors (beta-AR) increases apoptosis in adult rat ventricular myocytes (ARVMs). Integrins, a family of alphabeta-heterodimeric cell surface receptors, are postulated to play a role in ventricular remodeling. Here, we show that norepinephrine (NE) increases beta1 integrins expression in ARVMs via the stimulation of alpha1-AR, not beta-AR. Inhibition of ERK1/2 using PD 98059, an inhibitor of ERK1/2 pathway, inhibited alpha1-AR-stimulated increases in beta1 integrins expression. Activation of beta1 integrins signaling pathway using laminin (LN) inhibited beta-AR-stimulated apoptosis as measured by terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL)-staining and flow cytometry. Likewise, ligation of beta1 integrins with anti-beta1 integrin antibodies prevented beta-AR-stimulated apoptosis. Treatment of cells using LN or anti-beta1 integrin antibodies activated ERK1/2 pathway. PD 98059 inhibited activation of ERK1/2 by LN, and prevented the anti-apoptotic effects of LN. Thus (1) stimulation of alpha1-AR regulates beta1 integrins expression via the activation of ERK1/2, (2) beta1 integrins signaling protects ARVMs from beta-AR-stimulated apoptosis, (3) activation of ERK1/2 plays a critical role in the anti-apoptotic effects of beta1-integrin signaling. These data suggest that beta1 integrin signaling protects ARVMs against beta-AR-stimulated apoptosis possibly via the involvement of ERK1/2.

Antiapoptotic effect of endothelin-1 in rat cardiomyocytes in vitro

Apoptosis of cardiac myocytes is thought to be a feature of many pathological disorders, including congestive heart failure (CHF) and ischemic heart disease (IHD). Because recent investigations indicate that endothelin-1 (ET-1) plays an important role in CHF and IHD, we investigated the effect of ET-1 on cardiomyocyte apoptosis. The presence of apoptosis in rat cardiomyocytes (H9c2 and neonatal) was evaluated by morphological criteria, electrophoresis of DNA fragments, 4',6'-diamidine-2'-phenylindole staining, and TUNEL analysis. ET-1, but not angiotensin II, prevented apoptosis induced by serum deprivation via ETA receptors in a dose-dependent manner (1 to 100 nmol/L). ET-1 also prevented cytochrome c release from mitochondria to the cytosol. The use of specific pharmacological inhibitors demonstrated that the antiapoptotic effect of ET-1 was mediated through a tyrosine kinase pathway (genistein and AG490) but not through protein kinase C (PKC; calphostin C), mitogen-activated protein kinases (PD98059 and SB203580), or PKA (KT5270) pathways. Adenovirus-mediated gene transfer of kinase-inactive (KI) c-Src reversed the antiapoptotic effect of ET-1. We further investigated whether Bcl-xL, an antiapoptotic molecule, would be upregulated by using a luciferase-based reporter system. ET-1 upregulated Bcl-xL, and this upregulation was inhibited by genistein or AG490 but not by calphostin C. The experiments with KI mutants for various tyrosine kinases revealed that c-Src and Pyk2 (but not JAK1, Jak2, Syk, and Tec) are involved in ET-1-induced upregulation of Bcl-xL expression. These findings suggest that ET-1 prevents apoptosis in cardiac myocytes through the ETA receptor and the subsequent c-Src/Bcl-xL-dependent pathway.

Cardiac endothelial-myocardial signaling: its role in cardiac growth, contractile performance, and rhythmicity

Experimental work during the past 15 years has demonstrated that endothelial cells in the heart play an obligatory role in regulating and maintaining cardiac function, in particular, at the endocardium and in the myocardial capillaries where endothelial cells directly interact with adjacent cardiomyocytes. The emerging field of targeted gene manipulation has led to the contention that cardiac endothelial-cardiomyocytal interaction is a prerequisite for normal cardiac development and growth. Some of the molecular mechanisms and cellular signals governing this interaction, such as neuregulin, vascular endothelial growth factor, and angiopoietin, continue to maintain phenotype and survival of cardiomyocytes in the adult heart. Cardiac endothelial cells, like vascular endothelial cells, also express and release a variety of auto- and paracrine agents, such as nitric oxide, endothelin, prostaglandin I(2), and angiotensin II, which directly influence cardiac metabolism, growth, contractile performance, and rhythmicity of the adult heart. The synthesis, secretion, and, most importantly, the activities of these endothelium-derived substances in the heart are closely linked, interrelated, and interactive. It may therefore be simplistic to try and define their properties independently from one another. Moreover, in relation specifically to the endocardial endothelium, an active transendothelial physicochemical gradient for various ions, or blood-heart barrier, has been demonstrated. Linkage of this blood-heart barrier to the various other endothelium-mediated signaling pathways or to the putative vascular endothelium-derived hyperpolarizing factors remains to be determined. At the early stages of cardiac failure, all major cardiovascular risk factors may cause cardiac endothelial activation as an adaptive response often followed by cardiac endothelial dysfunction. Because of the interdependency of all endothelial signaling pathways, activation or disturbance of any will necessarily affect the others leading to a disturbance of their normal balance, leading to further progression of cardiac failure.

Glucagon-like peptide-2 receptor activation engages bad and glycogen synthase kinase-3 in a protein kinase A-dependent manner and prevents apoptosis following inhibition of phosphatidylinositol 3-kinase

Activation of glucagon-like peptide-2 receptor (GLP-2R) signaling promotes expansion of the mucosal epithelium indirectly via activation of growth and anti-apoptotic pathways; however, the cellular mechanisms coupling direct GLP-2R activation to cell survival remain poorly understood. We now demonstrate that GLP-2, in a cycloheximide-insensitive manner, enhanced survival in baby hamster kidney cells stably transfected with the rat GLP-2R; reduced mitochondrial cytochrome c efflux; and attenuated the caspase-dependent cleavage of Akt, poly(ADP-ribose) polymerase, and beta-catenin following inhibition of phosphatidylinositol 3-kinase (PI3K) by LY294002. The prosurvival effects of GLP-2 on LY294002-induced cell death were independent of Akt, p90(Rsk), or p70 S6 kinase activation; were mimicked by forskolin; and were abrogated by inhibition of protein kinase A (PKA) activity. GLP-2 inhibited activation of glycogen synthase kinase-3 (GSK-3) through phosphorylation at Ser(21) in GSK-3alpha and at Ser(9) in GSK-3beta in a PI3K-independent, PKA-dependent manner. GLP-2 reduced LY294002-induced mitochondrial association of endogenous Bad and Bax and stimulated phosphorylation of a transfected Bad fusion protein at Ser(155) in a PI3K-independent, but H89-sensitive manner, a modification known to suppress Bad pro-apoptotic activity. These results suggest that GLP-2R signaling enhances cell survival independently of PI3K/Akt by inhibiting the activity of a subset of pro-apoptotic downstream targets of Akt in a PKA-dependent manner.

Modulation of the beta-adrenergic receptor system of vascular smooth muscle cells in vitro and in vivo by chronically elevated endothelin-1 levels

Endothelin-1 (ET-1) levels are chronically elevated in several cardiovascular diseases and correlate with an increased mortality. However, in contrast to acute biological activities such as vasoconstriction, little is known about long-term effects of ET-1. In this study we determined the effects of ET-1 on the beta(2)-adrenergic receptor (AR) system. Incubation of smooth muscle cells with ET-1 for 72 hr led to increased beta(2)AR density as determined by radioligand binding. Experiments with inhibitors of protein and RNA synthesis as well as RT-PCR revealed that beta(2)AR upregulation required de novo synthesis. In addition, protein kinase C but neither NO nor prostaglandin metabolism were involved in this effect. The enhanced expression of beta(2)AR was associated with an increased expression of its stimulatory G-protein and the receptor's ability to stimulate adenylyl cyclase. To study chronic effects of ET-1 in vivo, rats were infused with ET-1 for 3 weeks. Similarly as in cultured cells, prolonged ET-1 exposure led to increased betaAR expression in vivo. As a consequence, beta(2)AR-induced vasodilatation was increased in aortic rings from ET-1-treated animals. Our results therefore suggest that chronically elevated ET-1 levels in vitro and in vivo induce counterregulatory mechanisms by increasing betaARs that attenuate the vasoconstrictive effects of ET-1.

Endothelin-1 stimulates cardiomyocyte injury during mitochondrial dysfunction in culture

To understand the pathophysiological role of endothelin-1 in the failing heart, we constructed a cellular mitochondrial impairment model and demonstrated the effect of endothelin-1. Primary cultured cardiomyocytes from neonatal rats were pretreated with rotenone, a mitochondrial complex I inhibitor, and the cytotoxic effect of endothelin-1 on the cardiomyocytes was demonstrated. Rotenone gradually decreased the pH of the culture medium with incubation time and caused slight cell injury. Endothelin-1 markedly enhanced the effect of rotenone that decreased the pH of the medium and enhanced cellular injury. The enhancement of the decrease in pH and cell injury induced by endothelin-1 was counteracted by the endothelin ET(A) receptor antagonist BQ123 or by maintaining the pH of the medium by the addition of 50 mM HEPES. Endothelin-1 markedly increased the uptake of 2-deoxyglucose and lactic acid production when the cardiomyocytes were pretreated with rotenone. These findings suggest that the stimulation of glucose uptake and anaerobic glycolysis followed by the increase in lactic acid accumulation in cardiomyocytes under the condition of mitochondrial impairment may be involved, at least in part, in the cellular injury by endothelin-1. Moreover, these findings suggest the possibility that the effect of endothelin-1 on myocardium is reversed by the condition of the mitochondria, and endogenous endothelin-1 may deteriorate cardiac failure with mitochondrial dysfunction. This may contribute to clarify the beneficial effect of endothelin receptor blockade in improving heart failures.

Norepinephrine-induced aortic hyperplasia and extracellular matrix deposition are endothelin-dependent

BACKGROUND

Sympathetic hyperactivity is observed in several disease states and may contribute to cardiovascular hypertrophic remodeling. Endothelin has been suggested to be a mediator of hypertrophy.

OBJECTIVE

To examine the involvement of endothelin in maintaining the growth response induced by exogenous norepinephrine.

DESIGN AND METHODS

Rats were treated with norepinephrine (2.5 microg/Kg per min subcutaneously) for 2 and 4 weeks, alone or in association with the selective endothelin-A (ETA) receptor antagonist, darusentan (LU135252, 30 mg/Kg per day orally) for weeks 3 and 4.

RESULTS

Increases in medial cell number and accumulation of collagen and elastin characterized norepinephrine-induced aortic remodeling. These effects occurred without marked changes of mean arterial pressure, but may be related to enhanced pressure variability in addition to direct effects of norepinephrine. Inhibition of ETA receptors by darusentan reversed aortic alterations produced by infusion of norepinephrine. Evaluation of medial apoptosis did not reveal any significant change in any group at 4 weeks.

CONCLUSIONS

Antagonism of ETA receptors effectively and rapidly reversed norepinephrine-induced aortic structural and compositional changes, suggesting a central role of endothelin in mediating this response. Thus, ETA receptor antagonists may help to regress large artery remodeling in conditions of increased circulating catecholamine concentrations.

Calcineurin pathway is required for endothelin-1-mediated protection against oxidant stress-induced apoptosis in cardiac myocytes

Endothelin-1 (ET-1) acts not only as a growth-promoting peptide but also as a potent survival factor against myocardial cell apoptosis. However, the signaling pathways leading to myocardial cell protection by ET-1 are poorly understood. Using a culture system of primary cardiac myocytes derived from neonatal rats, we show in the present study that ET-1 almost completely blocked the hydrogen peroxide-induced increase in the percentage of TdT-mediated dUTP-biotin nick-end labeling-positive myocytes. Apoptosis inhibition by ET-1 was confirmed by cytofluorometric analysis as well as by examination of the ladder formation, morphological features, and caspase-3 cleavage. We have found that ET-1 converts the nuclear factor of activated T lymphocytes (NFATc) in cardiac myocytes into high-mobility forms and translocates cytoplasmic NFATc to the nuclei. In addition, ET-1 stimulates the interaction between NFATc and the cardiac-restricted zinc-finger protein GATA4 in these cells. The immunosuppressants cyclosporin A and FK506, which antagonize calcineurin, negated the inhibitory effect of ET-1 on apoptosis. Calcineurin activation de novo was sufficient to inhibit hydrogen peroxide-induced apoptosis. ET-1 induced the expression of an antiapoptotic protein bcl-2 in cardiac myocytes in a cyclosporin A-dependent manner, but it did not alter the expression of bax. Cyclosporin A also attenuated the ET-1-stimulated transcription of the bcl-2 gene in these cells. These findings demonstrate that the calcineurin pathway is required for the inhibitory effect of ET-1 on oxidant stress-induced apoptosis in cardiac myocytes.

Molecular mechanisms of apoptosis in the cardiac myocyte

Cardiac myocytes can undergo programmed cell death in response to a variety of insults and apoptotic elimination of myocytes from the adult myocardium can lead directly to cardiomyopathy and death. Although it remains to be shown that therapy specifically targeting apoptosis will improve the prognosis of ischemic heart disease or heart failure, a number of studies in the past year have shed light on potential ways to intervene in the process. Progress in the past year includes a better understanding of the importance of mitochondria-initiated events in cardiac myocyte apoptosis, of factors inducing apoptosis during hypoxia, and of the dual pro-apoptotic and anti-apoptotic effects of hypertrophic stimuli such as beta-adrenoceptor agonists, nitric oxide and calcineurin. Further evidence supports the pathophysiologic relevance of apoptosis in human heart disease. The tracking of cytoprotective and apoptotic signal transduction pathways has revealed important new insights into the roles of the mitogen-activated protein (MAP) kinases p38, extracellular signal regulated kinase (ERK) and c-Jun N-terminus kinase (JNK) in cardiac cell fate.

Pericardial fluid from patients with ischemic heart disease induces myocardial cell apoptotis via an oxidant stress-sensitive p38 mitogen-activated protein kinase pathway

Factors produced by the heart are accumulated at high concentrations in pericardial fluid. We recently reported that pericardial fluid from patients with ischemic heart disease induces apoptosis in an F2 cell line. To characterize factors in pericardial fluid from patients with ischemic heart disease, we investigated signaling pathways by which this pericardial fluid induces apoptosis in cardiac myocytes. Pericardial fluid from patients with ischemic heart disease markedly increased the percentage of TUNEL-positive myocytes compared with fetal bovine serum. Apoptosis was also confirmed by ladder formation and morphologic features. Apoptosis mediated by this pericardial fluid occurs as readily in cardiac myocytes prepared from neonatal mice nullizygous for p53 as in wild-type littermates. This indicates that p53 is not required for this process. We have found that pericardial fluid from ischemic heart disease elicits a robust increase in phosphorylation of p38 mitogen-activated protein kinase. Specific inhibition of the p38 mitogen-activated protein kinase pathway with SB 203580 almost completely blocked apoptosis mediated by pericardial fluid from ischemic heart disease. Activation of p38 mitogen-activated protein kinase is caused by cellular stress, including oxidants. We have also found that anti-oxidant catalase inhibited pericardial fluid-induced activation of p38 mitogen-activated protein kinase and apoptosis. These findings demonstrate that myocardial cell apoptosis induced by pericardial fluid from patients with ischemic heart disease is mediated by an oxidant stress-sensitive p38 mitogen-activated protein kinase pathway. A possible application of SB 203580 to preserve cardiac function in patients with ischemic heart disease should be discussed.