Vascular endothelin-B receptor system in vivo plays a favorable inhibitory role in vascular remodeling after injury revealed by endothelin-B receptor-knockout mice

Ambient pressure upregulates nitric oxide synthase in a phosphorylated-extracellular regulated kinase– and protein kinase C–dependent manner

PURPOSE

Using endothelial cell/smooth muscle cell (SMC) cocultures, we have demonstrated that pressurized endothelial cell coculture inhibits SMC proliferation and promotes apoptosis, and that this effect is transferable through pressurized endothelial medium. We now hypothesized that endothelial nitric oxide synthase (eNOS) plays a significant role in mediating these pressure-induced effects.

METHODS

Conditioned media from endothelial cells and SMCs exposed to ambient and increased pressure were transferred to recipient SMCs. We counted cells after 5 days of incubation with these media and evaluated eNOS and inducible NOS (iNOS) levels by Western blot.

RESULTS

Conditioned media from pressurized endothelial cells significantly decreased recipient SMC counts. This effect was sustained when N-nitro-L-arginine-methyl ester (L-NAME) was added to recipient cells but abolished when L-NAME was added to donor cells. SMCs were then exposed to control and pressurized conditions in monoculture or in coculture with endothelial cells. Pressure and coculture caused similar increase in iNOS levels but had no additive effect in combination. Finally, endothelial cells were exposed to control and pressurized environments. Pressure caused a 24% +/- 1.6% increase in eNOS protein (P = .04, n = 12). This effect was sustained when cells were treated with L-NAME (32% +/- 1.6% increase, P = .02) but abolished when endothelial cells were treated with calphostin C or PD98059 to block protein kinase C (PKC) or extracellular regulated kinase (ERK). Pressure also increased endothelial phosphorylated ERK (p-ERK) by 1.8-fold to 2.6-fold compared with control conditions after exposure of 2, 4, and 6 hours (P = .02, n = 4). This increase was sustained after pretreatment with calphostin C.

CONCLUSION

Pressure modulates endothelial cell effects on SMC growth by increasing eNOS in an ERK-dependent and PKC-dependent manner.

CLINICAL RELEVANCE

Intimal hyperplasia is the main cause for restenosis that complicates 10% to 30% of all such vascular procedures and 30% to 40% of endovascular procedures. This article provides some novel information about smooth muscle cell/endothelial cell interaction, one of the main regulators of vascular remodeling and intimal hyperplasia. The role of endothelial cell/smooth muscle cell interaction cannot be studied well in vivo because these interactions cannot be distinguished from other factors that coexist in vivo, such as flow dynamics, matrix proteins, inflammatory factors, and interactions with other cells in the vascular wall and in the bloodstream. In this work, we use pressure as a triggering stimulus to alter in vitro endothelial behavior and identify important changes in endothelial regulation of smooth muscle cell biology. The pathways involved in this process and discussed in this article could ultimately be used to manipulate endothelial cell/smooth muscle cell interaction in clinical disease.

Endothelin receptor antagonists

Endothelin receptor antagonists (ERAs) have been developed to block the effects of endothelin-1 (ET-1) in a variety of cardiovascular conditions. ET-1 is a powerful vasoconstrictor with mitogenic or co-mitogenic properties, which acts through the stimulation of 2 subtypes of receptors [endothelin receptor subtype A (ETA) and endothelin receptor subtype B (ETB) receptors]. Endogenous ET-1 is involved in a variety of conditions including systemic and pulmonary hypertension (PH), congestive heart failure (CHF), vascular remodeling (restenosis, atherosclerosis), renal failure, cancer, and cerebrovascular disease. The first dual ETA/ETB receptor blocker, bosentan, has already been approved by the Food and Drug Administration for the treatment of pulmonary arterial hypertension (PAH). Trials of endothelin receptor antagonists in heart failure have been completed with mixed results so far. Studies are ongoing on the effects of selective ETA antagonists or dual ETA/ETB antagonists in lung fibrosis, cancer, and subarachnoid hemorrhage. While non-peptidic ET-1 receptor antagonists suitable for oral intake with excellent bioavailability have become available, proven efficacy is limited to pulmonary hypertension, but it is possible that these agents might find a place in the treatment of several cardiovascular and non-cardiovascular diseases in the coming future.

N-Terminal Proteolysis of the Endothelin B Receptor Abolishes Its Ability to Induce EGF Receptor Transactivation and Contractile Protein Expression in Vascular Smooth Muscle Cells

OBJECTIVE

The extracellular N terminus of the endothelin B (ETB) receptor is cleaved by a metalloprotease in an agonist-dependent manner, but the physiological role of this N-terminal proteolysis is not known. In this study, we aimed to determine the functional role of the ETB receptor and of its N-terminal cleavage in vascular smooth muscle cells (VSMCs).

METHODS AND RESULTS

VSMCs expressing either the full-length ETB receptor or an N-terminally truncated ETB receptor (corresponding to the N-terminally cleaved receptor) were analyzed for ligand-induced mitogen-activated protein kinase activation and expression of contractile proteins. In VSMCs expressing the full-length ETB receptor, IRL1620 (an ETB-selective agonist) induced a biphasic extracellular signal-regulated kinase 1/2 (ERK1/2) activation and increased expression of contractile proteins (smooth muscle myosin-1 [SM-1]/SM-2, SM22alpha, and alpha-actin). Interestingly, the second phase of ERK1/2 activation required metalloprotease activity, epidermal growth factor (EGF) receptor transactivation, and predominantly activation of Gi proteins. In contrast, in VSMCs expressing N-terminally truncated ETB receptors, IRL1620 did not elicit EGF transactivation and failed to increase contractile protein expression.

CONCLUSIONS

This study is the first to show that stimulation of full-length ETB receptors promotes expression of contractile proteins and may thus participate in the differentiation of VSMCs.

Increased expression of cyclooxygenase-2 mediates enhanced contraction to endothelin ETA receptor stimulation in endothelial nitric oxide synthase knockout mice

The aim of this study was to determine whether prolonged loss of NO activity, in endothelial NO synthase knockout (eNOS(-/-)) mice, influences endothelin (ET) ETA receptor-mediated smooth muscle contraction and, if so, to define the underlying mechanism(s). In isolated endothelium-denuded abdominal aortas, contractions to the selective ETA receptor agonist ET-1(1-31) were significantly increased in aortas from eNOS(-/-) compared with wild-type (WT) mice. In contrast, contractions to the alpha1-adrenergic agonist phenylephrine or the thromboxane (TX) A2 analog U-46619 were similar between eNOS(-/-) and WT mice. Immunofluorescent and Western blot analysis demonstrated that the aortic expression of ETA receptors was decreased in eNOS(-/-) compared with WT mice. Contractions evoked by ET-1(1-31), but not phenylephrine, were reduced by inhibition of cyclooxygenase-2 (COX-2) (indomethacin or celecoxib) or of TXA2/prostaglandin H2 receptors (SQ-29548). After COX inhibition, contractions to ET-1(1-31) were no longer increased and were actually decreased in eNOS(-/-) compared with WT aortas. Western blot analysis revealed that endothelium-denuded abdominal aortas express COX-2, but not COX-1, and that expression of COX-2 was significantly increased in eNOS(-/-) compared with WT mice. Contractions to the COX substrate arachidonic acid were also increased in eNOS(-/-) aortas. Furthermore, ET-1(1-31) but not phenylephrine stimulated production of the TXA2 metabolite TXB2, which was increased in eNOS(-/-) compared with WT aortas. Therefore, COX-2 plays a crucial and selective role in ETA-mediated smooth muscle contraction. Furthermore, COX-2 expression is increased in eNOS(-/-) mice, which overcomes a reduced expression of ETA receptors and enables a selective increase in contraction to ETA receptor stimulation.

Treatment of pulmonary arterial hypertension with bosentan: from pathophysiology to clinical evidence

In addition to its potent vasoconstricting effect, endothelin (ET)-1 induces proliferation of pulmonary vascular cells and appears to play a pathogenic role in the development of pulmonary arterial hypertension (PAH). Blockade of the ET receptors has been proposed for the treatment of this condition. Bosentan (Tracleer, Actelion Pharmaceuticals), an oral ETA/ETB receptor antagonist, has been shown to improve exercise capacity, quality of life, haemodynamics and time to clinical worsening of patients with PAH in short-term placebo-controlled trials. These improvements were sustained, and a long-term observational study on idiopathic PAH patients suggested a favourable effect on survival in this subset. In the present report, the pharmacology, clinical efficacy and safety profile of bosentan are summarised. The place of bosentan among the current therapies available for the treatment of PAH is also discussed.

The vascular endothelin system in hypertension--recent patents and discoveries

The discovery of endothelin two decades ago has now evolved into an intricate vascular endothelin (ET) system. Several ET isoforms, receptors, signaling pathways, agonists, antagonists, and clinical applications have been identified and documented in first-rate patents. The role of ET as one of the most potent endothelium-derived vasoconstricting factors is now complemented by a newly discovered role in vascular relaxation. ET synthesis is initiated by the transcription of ET genes in endothelial cells and the generation of the gene products preproET and big ET, which are further cleaved by specific ET converting enzymes into ET-1, -2, -3 and -4 isoforms. ET isoforms bind with different affinities to ET(A) and ET(B2) receptors in vascular smooth muscle, and stimulate [Ca(2+)](i), protein kinase C, mitogen-activated protein kinase and other signaling mechanisms of smooth muscle contraction, growth and proliferation. ET also binds to endothelial ET(B1) receptors, which mediate the release of vasodilator substances such as nitric oxide, prostacyclin and endothelium-derived hyperpolarizing factor. Endothelial ET(B1) receptors may also function in ET re-uptake and clearance. Although the effects of ET on vascular function and growth are well-recognized, the role of ET and its receptors in the regulation of blood pressure and in the pathogenesis of hypertension is not clearly established. Salt-dependent hypertension in experimental animals and some forms of moderate to severe hypertension in human may show elevated levels of plasma or vascular ET; however, other forms of hypertension show normal ET levels. The currently available ET receptor antagonists reduce blood pressure in some forms of experimental hypertension. Careful examination of recent patents may identify more effective and specific modulators of the vascular ET system for clinical use in human hypertension.

Development of occlusive neointimal lesions in distal pulmonary arteries of endothelin B receptor-deficient rats: a new model of severe pulmonary arterial hypertension

BACKGROUND

Human pulmonary arterial hypertension (PAH) is characterized by proliferation of vascular smooth muscle and, in its more severe form, by the development of occlusive neointimal lesions. However, few animal models of pulmonary neointimal proliferation exist, thereby limiting a complete understanding of the pathobiology of PAH. Recent studies of the endothelin (ET) system demonstrate that deficiency of the ET(B) receptor predisposes adult rats to acute and chronic hypoxic PAH, yet these animals fail to develop neointimal lesions. Herein, we determined and thereafter showed that exposure of ET(B) receptor-deficient rats to the endothelial toxin monocrotaline (MCT) leads to the development of neointimal lesions that share hallmarks of human PAH.

METHODS AND RESULTS

The pulmonary hemodynamic and morphometric effects of 60 mg/kg MCT in control (MCT(+/+)) and ET(B) receptor-deficient (MCT(sl/sl)) rats at 6 weeks of age were assessed. MCT(sl/sl) rats developed more severe PAH, characterized by elevated pulmonary artery pressure, diminished cardiac output, and right ventricular hypertrophy. In MCT(sl/sl) rats, morphometric evaluation revealed the presence of neointimal lesions within small distal pulmonary arteries, increased medial wall thickness, and decreased arterial-to-alveolar ratio. In keeping with this, barium angiography revealed diminished distal pulmonary vasculature of MCT(sl/sl) rat lungs. Cells within neointimal lesions expressed smooth muscle and endothelial cell markers. Moreover, cells within neointimal lesions exhibited increased levels of proliferation and were located in a tissue microenvironment enriched with vascular endothelial growth factor, tenascin-C, and activated matrix metalloproteinase-9, factors already implicated in human PAH. Finally, assessment of steady state mRNA showed that whereas expression of ET(B) receptors was decreased in MCT(sl/sl) rat lungs, ET(A) receptor expression increased.

CONCLUSIONS

Deficiency of the ET(B) receptor markedly accelerates the progression of PAH in rats treated with MCT and enhances the appearance of cellular and molecular markers associated with the pathobiology of PAH. Collectively, these results suggest an overall antiproliferative effect of the ET(B) receptor in pulmonary vascular homeostasis.

Bosentan therapy for pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is characterized by the progressive increase in pulmonary vascular resistance potentially leading to right heart failure and death. Since endothelins may play a pathogenic role in the development of the disease, endothelin receptor antagonists have been proposed for the treatment of this condition. Bosentan (Tracleer®), an oral nonselective ETA/ETB endothelin receptor antagonist, has been shown to improve exercise capacity, quality of life and hemodynamics of patients with PAH in short-term trials. These improvements were sustained and a long-term observational study on idiopathic PAH patients suggested a favorable effect on survival in this subset. The present report summarizes the pharmacology, clinical efficacy and safety profile of bosentan with an overview of the current therapies available for the treatment of PAH.

Ablation of MEK Kinase 1 Suppresses Intimal Hyperplasia by Impairing Smooth Muscle Cell Migration and Urokinase Plasminogen Activator Expression in a Mouse Blood-Flow Cessation Model

Background— Migration, proliferation, and matrix-degrading protease expression of smooth muscle cells (SMCs) are major features of intimal hyperplasia after vascular injury. Although MEK kinase 1 (MEKK1) has been shown to regulate cell migration and urokinase plasminogen activator (uPA) expression, the precise role of MEKK1 in this process remains unknown.

Methods and Results— We triggered a vascular remodeling model by complete ligation of the right common carotid artery in wild-type (WT) and MEKK1-null (MEKK1 −/− ) mice. The intimal areas 28 days after ligation were significantly decreased in the ligated MEKK1 −/− arteries compared with WT arteries (28±8 versus 65±17 μm 2 , P <0.05). There were no differences in the ratios of proliferating cell nuclear antigen (PCNA)–positive cells to total cells within the arterial wall between WT and MEKK1 −/− arteries. Proliferation capacity also did not differ between WT and MEKK1 −/− cultured aortic smooth muscle cells (AoSMCs). In contrast, the number of intimal PCNA-positive cells 7 days after ligation was significantly smaller in MEKK1 −/− arteries. Three different migration assays revealed that migration and invasion of MEKK1 −/− AoSMCs were markedly impaired. Addition of full-length MEKK1 restored the migration capacity of MEKK1 −/− AoSMCs. The number of MEKK1 −/− AoSMCs showing lamellipodia formation by epithelial growth factor was significantly smaller compared with those of WT SMCs. Furthermore, uPA expression after ligation was markedly decreased in MEKK1 −/− arteries.

Conclusions— MEKK1 is implicated in vascular remodeling after blood-flow cessation by regulating the migration and uPA expression of SMCs. MEKK1 is a potential target for drug development to prevent vascular remodeling.

Sitaxsentan, a selective endothelin-A receptor antagonist for the treatment of pulmonary arterial hypertension

Sitaxsentan, a highly selective endothelin-A (ET(A)) receptor antagonist (6500-fold more selective for ET(A) receptors than endothelin-B (ET(B)) receptors), may benefit patients with pulmonary artery hypertension (PAH) by blocking the vasoconstrictor effects of ET(A) receptors while maintaining the vasodilator/clearance functions of ET(B) receptors. In its first randomised, placebo-controlled study, Sitaxsentan to Relieve Impaired Exercise-1 (STRIDE-1), sitaxsentan improved exercise capacity assessed by 6 min walk, New York Heart Association functional class, cardiac index and pulmonary vascular resistance in New York Heart Association class II, III and IV patients with idiopathic PAH, PAH related to connective tissue disease or PAH related to congenital heart disease. In STRIDE-1, doses of 100 and 300 mg/day p.o. were evaluated. Although both doses showed equivalent efficacy, the lower dose had a more tolerable safety profile. Additional studies are ongoing to assess the relative safety and efficacy of 50 and 100 mg/day doses, both in de novo patients and in patients previously treated with the ET(A)/ET(B) receptor antagonist bosentan. Long-term comparative studies are necessary to determine whether there is a clinically meaningful difference between selective ET(A) receptor antagonism and ET(A)/ET(B) receptor antagonism.

Sustained symptomatic, functional, and hemodynamic benefit with the selective endothelin-A receptor antagonist, sitaxsentan, in patients with pulmonary arterial hypertension: a 1-year follow-up study

STUDY OBJECTIVES

To examine the long-term efficacy and safety of the selective endothelin-A receptor (ET-A) antagonist, sitaxsentan sodium, after 1 year of therapy in patients with pulmonary arterial hypertension (PAH).

DESIGN

The study was a Canadian, open-label extension of at least 1-year total of active therapy (sitaxsentan, 100 mg/d), following a preceding, blinded, 12-week placebo controlled trial of sitaxsentan (placebo, or sitaxsentan, 100 mg/d or 300 mg/d), which had then been followed by a blinded active-therapy continuation study (sitaxsentan, 100 mg/d or 300 mg/d).

PATIENTS

Eleven patients with PAH were enrolled. The condition of one patient worsened at 7 months of therapy, and the patient transferred to epoprostenol therapy. The remaining 10 patients (idiopathic [n = 3], connective tissue disease [n = 3], congenital heart disease [n = 4]) completed the evaluation after 1 year of active therapy.

INTERVENTIONS

The end points of the study included the 6-min walk test, World Health Organization (WHO) functional class, and cardiopulmonary hemodynamic parameters.

RESULTS

After 1 year of sitaxsentan therapy, there were significant improvements in 6-min walk distance (50-m treatment effect), WHO functional class, and hemodynamics, as compared to baseline. There were no serious adverse events, and no instances of hepatotoxicity or bleeding.

CONCLUSION

Long-term selective ET-A blockade with sitaxsentan sodium is safe and may improve exercise capacity, functional class, and hemodynamics in patients with PAH.

Influence of growth factors on the proliferation of vascular smooth muscle cells isolated from subtotally nephrectomized rats after endothelin or angiotensin II antagonism

BACKGROUND

Cardiovascular disease is the most important cause of death in patients with end-stage renal disease. In uraemia, the renin-angiotensin-aldosterone and endothelin (ET) systems are activated. It is not known whether inhibition of these systems attenuates the proliferation of isolated smooth muscle cells of uraemic rats.

METHODS

Subtotally nephrectomized (SNX) rats were treated with an ET(A) receptor antagonist, an ET(AB) receptor antagonist, the angiotensin type 1 (AT1) receptor antagonist losartan (all 10 mg/kg body weight/day) or the angiotensin-converting enzyme (ACE) inhibitor trandolapril (0.1 mg/kg body weight/day) or received no medication (SNX) for 12 weeks. Then, aortal smooth muscle cells (SMCs) were isolated and cultivated. After incubation of SMCs with different growth factors (5-7 days), proliferation was measured using a bromodeoxyuridine enzyme-linked immunosorbent assay (BrdU ELISA).

RESULTS

Higher maximum levels of proliferation were found in SMCs from untreated SNX rats than in SMCs from control animals [platelet-derived growth factor-BB (PDGF-BB) 486.60+/-8.27 vs 346.74+/-4.60%, basic fibroblast growth factor (bFGF) 176.68+/-6.50 vs 123.71+/-1.49%, tumour necrosis factor-alpha (TNF-alpha) 153.38+/-10.16 vs 122.27+/-1.41%]. Treatment with ET receptor antagonists or losartan attenuated growth factor-stimulated proliferation (PDGF-BB: ET(A) receptor antagonist, 135.71+/-1.08%; ET(AB) receptor antagonist, 122.72+/-0.58%; losartan: 103.69+/-1.83%, n = 8). SMCs from trandolapril-treated rats showed an increased response (PDGF-BB 663.48+/-7.00%, n = 8).

CONCLUSIONS

Treatment of SNX rats with ET receptor antagonists or losartan reduced growth factor-induced SMC proliferation in vitro. However, further investigations with uraemic patients have to clarify whether angiotensin or ET receptor antagonists inhibit the development of atherosclerosis.

Function and survival of dendritic cells depend on endothelin-1 and endothelin receptor autocrine loops

The biologic effects of endothelin-1 (ET-1) are not limited to its potent vasoconstricting activity. The endothelin receptors, ETA and ETB, have differential tissue and functional distributions. Here we showed that dendritic cells (DCs), the major antigen-presenting cells in the adaptive limb of the immune system, produce large amounts of ET-1 and significantly increase the expression of endothelin receptors upon maturation. Selective blockade of the ETA receptor significantly reduced expression of the mature DC marker CD83, decreased the production of the immunostimulatory cytokine interleukin-12, down-regulated DC ability to stimulate T cells, and promoted DC apoptosis. Selective ETB receptor blockade, on the other hand, resulted in increased expression of CD83 and improved DC survival. Therefore, ET-1/ETA/ETB autocrine/paracrine loops on DCs appear to be essential for the normal maturation and function of human DCs, presenting a unique target for immunomodulatory therapies.

Endothelin-1-induced responses in isolated mouse vessels: the expression and function of receptor types

Mice have been increasingly used as models for investigating cardiovascular diseases. However, the responsiveness of mouse vasculature to endothelin (ET)-1 has not been clearly established. The goal of this study was to determine the role of ET receptors (ET(A) and ET(B)) in mouse vessels using isometric force measurements. Results showed that in the abdominal aorta ET-1 induced a concentration-dependent contraction (EC(50): 1.4 nM) with maximum reaching 89.5 +/- 4.9% (10 nM) of that induced by 60 mM K(+) [with nitric oxide synthase (NOS) inhibitor N(omega)-nitro-l-arginine methyl ester (l-NAME)]. However, in the thoracic aorta or the carotid artery, ET-1 was poorly effective. RT-PCR revealed that in the endothelium-denuded abdominal aorta, the PCR product for ET(B) receptors was very low compared with ET(A). Similarly in tissues treated with l-NAME, the ET(B) receptor-specific agonist sarafotoxin 6c (S6c; 100 nM) induced only a minimal contraction (<5%). Meanwhile, the ET(A) antagonist BQ-123 (1 microM) completely inhibited the maximum ET-1 (10 nM) contractile response. Furthermore, we found that in the abdominal aorta that had not been treated with l-NAME, ET-1-induced contraction significantly decreased. However, in such specimens, S6c was unable to induce any relaxation on phenylephrine-induced contraction. These results indicate that the role of ET receptors differs considerably among mouse vessels. In the abdominal aorta, ET(A) receptor mediates a potent vasoconstrictor response, whereas ET(B) has, if any, only a minimal functional presence. Also, our data suggest that ET-1 might involve a NOS-dependent vasodilation in the abdominal aorta, which remains to be further defined.

Differential vasoconstrictions induced by angiotensin II: role of AT1 and AT2 receptors in isolated C57BL/6J mouse blood vessels

Genetically altered mice are increasingly used as experimental models. However, ANG II responses in mouse blood vessels have not been well defined. Therefore, the aim of this study was to determine the role of ANG II in regulating major blood vessels in C57/BL6J mice with isometric force measurements. Our results showed that in mouse abdominal aorta ANG II induced a concentration-dependent contraction (EC50 4.6 nM) with a maximum contraction of 75.1 +/- 4.9% at 100 nM compared with that of 60 mM K+. Similarly, femoral artery also exhibited a contractile response of 76.0 +/- 3.4% to the maximum concentration of ANG II (100 nM). In contrast, ANG II (100 nM)-induced contraction was significantly less in carotid artery (24.5 +/- 6.6%) and only minimal (3.5 +/- 0.31%) in thoracic aorta. The nitric oxide synthase inhibitor N omega-nitro-L-arginine methyl ester and the AT2 antagonist PD-123319 failed to enhance ANG II-induced contractions. However, an AT1 antagonist, losartan (10 microM), completely inhibited ANG II (100 nM) response in abdominal aorta and carotid artery. An AT1 agonist, [Sar1]-ANG II (100 nM), behaved similarly to ANG II (100 nM) in abdominal aorta and carotid artery. RT-PCR analyses showed that mouse thoracic aorta has a significantly lower AT1 mRNA level than abdominal aorta. These results demonstrate that major mouse vessels exhibit differential contractions to ANG II, possibly because of varied AT1 receptor levels.

Endothelin-1 activates endothelial cell nitric-oxide synthase via heterotrimeric G-protein betagamma subunit signaling to protein jinase B/Akt

Endothelin-1 has dual vasoactive effects, mediating vasoconstriction via ETA receptor activation of vascular smooth muscle cells and vasorelaxation via ETB receptor activation of endothelial cells. Although it is commonly accepted that endothelin-1 binding to endothelial cell ETB receptors stimulates nitric oxide (NO) synthesis and subsequent smooth muscle relaxation, the signaling pathways downstream of ETB receptor activation are unknown. Here, using a model in which we have utilized isolated primary endothelial cells, we demonstrate that ET-1 binding to sinusoidal endothelial cell ETB receptors led to increased protein kinase B/Akt phosphorylation, endothelial cell nitric-oxide synthase (eNOS) phosphorylation, and NO synthesis. Furthermore, eNOS activation was not dependent on tyrosine phosphorylation, and pretreatment of endothelial cells with pertussis toxin as well as overexpression of a dominant negative G-protein-coupled receptor kinase construct that sequesters betagamma subunits inhibited Akt phosphorylation and NO synthesis. Taken together, the data elucidate a G-protein-coupled receptor signaling pathway for ETB receptor-mediated NO production and call attention to the absolute requirement for heterotrimeric G-protein betagamma subunits in this cascade.

17 Beta-estradiol regulates the expression of endothelin receptor type B in the heart

(1) Little is known about the interaction of 17beta-estradiol (E2) and the vasoactive endothelin system in the heart. Endothelin signaling is activated in a failing heart and may contribute to myocardial dysfunction and remodeling. Therefore, we investigated the regulation of proteins of the endothelin system (ppET-1, ECE and ETA-R and ETB-R) in the hearts of female spontaneously hypertensive rats (SHR) with respect to E2. (2) Relative expression levels of the respective cardiac mRNA obtained from sham-operated, ovariectomized and ovariectomized E2-substituted SHR were quantified by real-time PCR. Ovariectomy led to a significant upregulation of the ETB-R mRNA (2.6+/-0.8-fold) in the left ventricular myocardium, which was not attendant with an alteration of ETA-R, ECE and ppET-1 mRNA expression. (3) An upregulation of the relative expression level of ETB-R protein due to ovariectomy was also demonstrated by radioligand binding assay. (4) Upregulation of both ETB-R mRNA and ETB-R protein expression was completely inhibited by E2 replacement. (5) To confirm these results in in vitro experiments, we quantified the mRNA of ET-R subtypes from isolated cardiomyocytes in the presence and absence of E2 (10-8 m, 24 h). Our data showed a markedly downregulated level of ETB-R mRNA in cardiomyocytes stimulated with E2. ETB-R downregulation was not attendant with the alteration of ETA-R, ECE and ppET-1 mRNA expression. (6) Taken together, these data demonstrate that estrogen regulates the expression of ETB-R in rat ventricular myocardium in vivo and in vitro. These observations may help to understand gender-based differences found in cardiovascular disease.

Glucose-dependent insulinotropic peptide stimulates thymidine incorporation in endothelial cells: role of endothelin-1

We have previously characterized the receptor for glucose-dependent insulinotropic polypeptide (GIPR) in vascular endothelial cells (EC). Different EC types were found to contain distinct GIPR splice variants. To determine whether activation of the GIPR splice variants resulted in different cellular responses, we examined GIP effects on human umbilical vein endothelial cells (HUVEC), which contain two GIPR splice variants, and compared them with a spontaneously transformed human umbilical vein EC line, ECV 304, which contains four GIPR splice variants. GIP dose-dependently stimulated HUVEC and ECV 304 proliferation as measured by [3H]thymidine incorporation. GIP increased endothelin-1 (ET-1) secretion from HUVEC but not from ECV 304. Use of the endothelin B receptor blocker BQ-788 resulted in an inhibition of [3H]thymidine incorporation in HUVEC but not in ECV 304. These findings suggest that, although GIP increases [3H]thymidine incorporation in both HUVEC and ECV 304, this proliferative response is mediated by ET-1 only in HUVEC. These differences in cellular response to GIP may be related to differences in activation of GIPR splice variants.

[New expansion of endothelin research: perspectives for clinical application of endothelin-receptor antagonists]

Three isopeptides of endothelin (ET-1, -2, and -3) exert various actions through stimulation of two sub-types of receptor (ETA and ETB). Vascular endothelial cells produce only ET-1. In addition to its powerful vasoconstrictor action, ET-1 has direct mitogenic actions on cardiovascular tissues, as well as comitogennic actions with a wide variety of growth factors and vasoactive substances. ET-1 also promotes the synthesis and secretion of growth factors and various substances, including extracellular constituents. These effects of endogenous ET-1 would naturally be thought to be concerned with the development and/or aggravation of chronic cardiovascular diseases; e.g., hypertension, pulmonary hypertension, vascular remodeling (stenosis, atherosclerosis), renal failure, and heart failure. A large number of peptide and orally active non-peptide endothelin receptor antagonists have been developed, and utilized to analyze physiological and pathophysiological roles of endogenous ET-1. These antagonists have been shown to exert excellent therapeutic effects in animal models of various kinds of diseases by either acute or chronic treatment. Therapeutic treatment of patients suffering from the above-mentioned cardiovascular diseases with ET-receptor antagonists have also been taking place, and bosentan (ETA/ETB antagonist) was recently approved by the FDA as a formal therapeutic drug for pulmonary hypertension. In this review, perspectives for therapeutic applicability of ET-receptor antagonists will be explored.