Biochemical and pharmacological profile of a potent and selective endothelin B-receptor antagonist, BQ-788

The role of endothelin receptor antagonists in kidney disease

Kidney diseases are among the most prevalent conditions worldwide, impacting over 850 million individuals. They are categorized into acute kidney injury and chronic kidney disease. Current preclinical and clinical trials have demonstrated that endothelin (ET) is linked to the onset and progression of kidney disease. In kidney diseases, pathological conditions such as hyperglycemia, acidosis, insulin resistance, and elevated angiotensin II levels lead to an increase in ET. This elevation activates endothelin receptor type A, resulting in harmful effects like proteinuria and a reduced glomerular filtration rate (GFR). Therefore, to slow the progression of kidney disease, endothelin receptor antagonists (ERAs) have been proposed as promising new therapies. Numerous studies have demonstrated the efficacy of ERAs in significantly reducing proteinuria and improving GFR, thereby slowing the progression of kidney diseases. This review discusses the mechanisms of action of ERAs in treating kidney disease, their efficacy and safety in preclinical and clinical studies, and explores future prospects for ERAs.

The impact of manganese on vascular endothelium

Manganese (Mn) is an essential trace element involved in various physiological processes, but excessive exposure may lead to toxicity. The vascular endothelium, a monolayer of endothelial cells within blood vessels, is a primary target of Mn toxicity. This review provides a comprehensive overview of the impact of Mn on vascular endothelium, focusing on both peripheral and brain endothelial cells. In vitro studies have demonstrated that high concentrations of Mn can induce endothelial cell cytotoxicity, increase permeability, and disrupt cell-cell junctions through mechanisms involving oxidative stress, mitochondrial damage, and activation of signaling pathways, such as Smad2/3-Snail. Conversely, low concentrations of Mn may protect endothelial cells from the deleterious effects of high glucose and advanced glycation end-products. In the central nervous system, Mn can cross the blood-brain barrier (BBB) and accumulate in the brain parenchyma, leading to neurotoxicity. Several transport mechanisms, including ZIP8, ZIP14, and SPCA1, have been identified for Mn uptake by brain endothelial cells. Mn exposure can impair BBB integrity by disrupting tight junctions and increasing permeability. In vivo studies have corroborated these findings, highlighting the importance of endothelial barriers in mediating Mn toxicity in the brain and kidneys. Maintaining optimal Mn homeostasis is crucial for preserving endothelial function, and further research is needed to develop targeted therapeutic strategies to prevent or mitigate the adverse effects of Mn overexposure.

Graphical Abstract

Antecubital venous endothelial ETB receptor protein expression is preserved with aging in men

Changes in endothelial function precede the development of cardiovascular disease (CVD). We have previously shown that age-related declines in endothelial function in women are due in part to a reduction in endothelial cell endothelin-B receptor (ETBR) protein expression. However, it is not known if ETBR protein expression changes with aging in men. The purpose of this study was to test the hypothesis that ETBR protein expression is attenuated in older men (OM) compared with younger men (YM). Primary endothelial cells were harvested from the antecubital vein of 14 OM (60 ± 6 yr; 26 ± 3 kg/m2) and 17 YM (24 ± 5 yr; 24 ± 2 kg/m2). Cells were stained with 4',6-diamidino-2-phenylindole, vascular endothelial cadherin, and ETBR. Images were quantified using immunocytochemistry. Endothelial function was assessed using brachial artery flow-mediated dilation (FMD). Systolic BP was similar (OM, 123 ± 11 vs. YM, 122 ± 10 mmHg) whereas diastolic BP was higher in OM (OM, 77 ± 7 vs. YM, 70 ± 6 mmHg; P < 0.01). Total testosterone was lower in OM (OM, 6.28 ± 4.21 vs. YM, 9.10 ± 2.68 ng/mL; P = 0.03). As expected, FMD was lower in OM (OM, 3.85 ± 1.51 vs. YM, 6.40 ± 2.68%; P < 0.01). However, ETBR protein expression was similar between OM and YM (OM, 0.39 ± 0.17 vs. YM, 0.42 ± 0.17 AU; P = 0.66). These data suggest that ETBR protein expression is not altered with age in men. These findings contrast with our previous data in women and further support sex differences in the endothelin system.NEW & NOTEWORTHY Our laboratory has previously shown that age-related declines in endothelial function are associated with a reduction in endothelial cell ETBR protein expression in women. However, it is unclear if endothelial cell ETBR protein expression is reduced with aging in men. This study demonstrates that endothelial cell ETBR protein expression is preserved with aging in men, and provides additional evidence for sex differences in the endothelin system.

Aerobic exercise training reduces ET-1-mediated vasoconstriction and improves endothelium-dependent vasodilation in postmenopausal women

Endothelin-1 (ET-1) contributes to vascular dysfunction in postmenopausal women (PMW). Although aerobic exercise is beneficial in reducing ET-1-mediated vasoconstrictor tone in men, it is unknown whether this favorable vascular effect occurs in women. We tested the hypothesis that aerobic exercise training reduces ET-1-mediated vasoconstriction in PMW. We further hypothesized that reductions in ET-1 vasoconstrictor tone underly exercise-induced improvements in endothelium-dependent vasodilatation in PMW. Forearm blood flow (FBF) responses to intra-arterial infusion of selective ETA receptor blockade (BQ-123, 100 nmol/min for 60 min) and acetylcholine (4.0, 8.0, and 16.0 μg/100 mL tissue/min) in the absence and presence of ETA receptor blockade were determined before and after a 12-wk aerobic exercise training intervention in 18 healthy, sedentary PMW (58 ± 4 yr). Women exercised an average of 4.9 ± 0.7 day/wk for 51 ± 7 min/day at 71 ± 3% of maximal heart rate. Before exercise, BQ-123 significantly increased FBF (∼25%) in sedentary PMW; however, this effect was abolished following the exercise intervention. FBF responses to acetylcholine were also significantly higher after exercise training (from 4.2 ± 1.2 to 14.0 ± 3.8 mL/100 mL tissue/min) versus before (from 4.1 ± 1.0 to 11.4 ± 3.3 mL/100 mL tissue/min; ∼25% increase; P < 0.05). Before exercise training, coinfusion of BQ-123 with acetylcholine enhanced (∼25%; P < 0.05) the vasodilator response (from 4.4 ± 1.1 to 13.9 ± 4.2 mL/100 mL tissue/min) compared with acetylcholine alone; after exercise training, the presence of BQ-123 did not significantly affect the vasodilator response to acetylcholine. Aerobic exercise training reduces ET-1-mediated vasoconstriction in PMW. Furthermore, decreased ET-1-mediated vasoconstriction is an important mechanism underlying aerobic exercise-induced improvement in endothelium-dependent vasodilation in PMW.NEW & NOTEWORTHY Endothelin-1 (ET-1) contributes to declines in endothelial function in postmenopausal women. To our knowledge, we show for the first time that aerobic exercise reduces ET-1-mediated vasoconstriction in previously sedentary postmenopausal women. Moreover, aerobic exercise improved endothelial-dependent dilation due in part to the reductions in ET-1-mediated vasoconstriction.

Endothelin and the Cardiovascular System: The Long Journey and Where We Are Going

Simple Summary

In this review, we describe the basic functions of endothelin and related molecules, including their receptors and enzymes. Furthermore, we discuss the important role of endothelin in several cardiovascular diseases, the relevant clinical evidence for targeting the endothelin pathway, and the scope of endothelin-targeting treatments in the future. We highlight the present uses of endothelin receptor antagonists and the advancements in the development of future treatment options, thereby providing an overview of endothelin research over the years and its future scope.

Abstract

Endothelin was first discovered more than 30 years ago as a potent vasoconstrictor. In subsequent years, three isoforms, two canonical receptors, and two converting enzymes were identified, and their basic functions were elucidated by numerous preclinical and clinical studies. Over the years, the endothelin system has been found to be critical in the pathogenesis of several cardiovascular diseases, including hypertension, pulmonary arterial hypertension, heart failure, and coronary artery disease. In this review, we summarize the current knowledge on endothelin and its role in cardiovascular diseases. Furthermore, we discuss how endothelin-targeting therapies, such as endothelin receptor antagonists, have been employed to treat cardiovascular diseases with varying degrees of success. Lastly, we provide a glimpse of what could be in store for endothelin-targeting treatment options for cardiovascular diseases in the future.

Role of Endothelin-1 Receptors in Limiting Leg Blood Flow and Glucose Uptake During Hyperinsulinemia in Type 2 Diabetes

Skeletal muscle insulin resistance is a hallmark of individuals with type 2 diabetes mellitus (T2D). In healthy individuals insulin stimulates vasodilation, which is markedly blunted in T2D; however, the mechanism(s) remain incompletely understood. Investigations in rodents indicate augmented endothelin-1 (ET-1) action as a major contributor. Human studies have been limited to young obese participants and focused exclusively on the ET-1 A (ETA) receptor. Herein, we have hypothesized that ETA receptor antagonism would improve insulin-stimulated vasodilation and glucose uptake in T2D, with further improvements observed during concurrent ETA + ET-1 B (ETB) antagonism. Arterial pressure (arterial line), leg blood flow (LBF; Doppler), and leg glucose uptake (LGU) were measured at rest, during hyperinsulinemia alone, and hyperinsulinemia with (1) femoral artery infusion of BQ-123, the selective ETA receptor antagonist (n = 10 control, n = 9 T2D) and then (2) addition of BQ-788 (selective ETB antagonist) for blockade of ETA and ETB receptors (n = 7 each). The LBF responses to hyperinsulinemia alone tended to be lower in T2D (controls: ∆161 ± 160 mL/minute; T2D: ∆58 ± 43 mL/minute, P = .08). BQ-123 during hyperinsulinemia augmented LBF to a greater extent in T2D (% change: controls: 14 ± 23%; T2D: 38 ± 21%, P = .029). LGU following BQ-123 increased similarly between groups (P = .85). Concurrent ETA + ETB antagonism did not further increase LBF or LGU in either group. Collectively, these findings suggest that during hyperinsulinemia ETA receptor activation restrains vasodilation more in T2D than controls while limiting glucose uptake similarly in both groups, with no further effect of ETB receptors (NCT04907838).

Endothelin receptor type A is involved in the development of oxaliplatin-induced mechanical allodynia and cold allodynia acting through spinal and peripheral mechanisms in rats

Oxaliplatin, a platinum-based chemotherapeutic agent, frequently causes severe neuropathic pain typically encompassing cold allodynia and long-lasting mechanical allodynia. Endothelin has been shown to modulate nociceptive transmission in a variety of pain disorders. However, the action of endothelin varies greatly depending on many variables, including pain causes, receptor types (endothelin type A (ET_A) and B (ET_B) receptors) and organs (periphery and spinal cord). Therefore, in this study, we investigated the role of endothelin in a Sprague–Dawley rat model of oxaliplatin-induced neuropathic pain. Intraperitoneal administration of bosentan, a dual ET_A/ET_B receptor antagonist, effectively blocked the development or prevented the onset of both cold allodynia and mechanical allodynia. The preventive effects were exclusively mediated by ET_A receptor antagonism. Intrathecal administration of an ET_A receptor antagonist prevented development of long-lasting mechanical allodynia but not cold allodynia. In marked contrast, an intraplantar ET_A receptor antagonist had a suppressive effect on cold allodynia but only had a partial and transient effect on mechanical allodynia. In conclusion, ET_A receptor antagonism effectively prevented long-lasting mechanical allodynia through spinal and peripheral actions, while cold allodynia was prevented through peripheral actions.

ETB receptor-mediated vasodilation is regulated by estradiol in young women

The endothelin-B (ET_B) receptor is a key regulator of vascular endothelial function in women. We have previously shown that the ET_B receptor mediates vasodilation in young women, an effect that is lost after menopause. However, the direct impact of changes in estradiol (E₂) on ET_B receptor function in women remains unclear. Therefore, the purpose of this study was to test the hypothesis that E₂ exposure modulates ET_B receptor-mediated dilation in young women. Fifteen young women (24 ± 4 yr, 24 ± 3 kg/m²) completed the study. Endogenous sex hormone production was suppressed with daily administration of a gonadotropin-releasing hormone antagonist (GnRHant; Ganirelix) for 10 days; E₂ (0.1 mg/day, Vivelle-Dot patch) was added back on days 4-10. We measured vasodilation in the cutaneous microcirculation (microvascular endothelial function) via local heating (42°C) on day 4 (GnRHant) and day 10 (GnRHant + E₂) using laser Doppler flowmetry coupled with intradermal microdialysis during perfusions of lactated Ringer's (control) and ET_B receptor antagonist (BQ-788, 300 nM). During GnRHant, vasodilatory responses to local heating were enhanced with ET_B receptor blockade (control: 83 ± 9 vs. BQ-788: 90 ± 5%CVC_max, P = 0.004). E₂ administration improved vasodilation in the control site (GnRHant: 83 ± 9 vs. GnRHant + E₂: 89 ± 8%CVC_max, P = 0.036). Furthermore, cutaneous vasodilatory responses during ET_B receptor blockade were blunted after E₂ administration (control: 89 ± 8 vs. BQ-788: 84 ± 8%CVC_max, P = 0.047). These data demonstrate that ovarian hormones, specifically E₂, modulate ET_B receptor function and contribute to the regulation of microvascular endothelial function in young women.NEW & NOTEWORTHY The endothelin-B (ETB) receptor mediates vasodilation in young women, an effect lost following menopause. It is unclear whether these alterations are due to aging or changes in estradiol (E2). During endogenous hormone suppression (GnRH antagonist), blockade of ETB receptors enhanced cutaneous microvascular vasodilation. However, during E2 administration, blockade of ETB receptors attenuated vasodilation, indicating that the ETB receptor mediates dilation in the presence of E2. In young women, ETB receptors mediate vasodilation in the presence of E2, an effect that is lost when E2 is suppressed.

Endothelin ETB Receptor-Mediated Astrocytic Activation: Pathological Roles in Brain Disorders

In brain disorders, reactive astrocytes, which are characterized by hypertrophy of the cell body and proliferative properties, are commonly observed. As reactive astrocytes are involved in the pathogenesis of several brain disorders, the control of astrocytic function has been proposed as a therapeutic strategy, and target molecules to effectively control astrocytic functions have been investigated. The production of brain endothelin-1 (ET-1), which increases in brain disorders, is involved in the pathophysiological response of the nervous system. Endothelin B (ET_B) receptors are highly expressed in reactive astrocytes and are upregulated by brain injury. Activation of astrocyte ET_B receptors promotes the induction of reactive astrocytes. In addition, the production of various astrocyte-derived factors, including neurotrophic factors and vascular permeability regulators, is regulated by ET_B receptors. In animal models of Alzheimer’s disease, brain ischemia, neuropathic pain, and traumatic brain injury, ET_B-receptor-mediated regulation of astrocytic activation has been reported to improve brain disorders. Therefore, the astrocytic ET_B receptor is expected to be a promising drug target to improve several brain disorders. This article reviews the roles of ET_B receptors in astrocytic activation and discusses its possible applications in the treatment of brain disorders.

Sex Differences in Cardiovascular Aging and Heart Failure

PURPOSE OF THE REVIEW

This review summarizes sex-related changes in the heart and vasculature that occur with aging, both in the presence and absence of cardiovascular disease (CVD).

RECENT FINDINGS

In the presence of CVD risk factors and/or overt CVD, sex-specific changes in the number of cardiomyocytes, extent of the myocardial extracellular matrix, and myocellular hypertrophy promote unique patterns of LV remodeling in men and women. In addition, age- and sex-specific vascular stiffening is also well established, driven by changes in endothelial dysfunction, elastin-collagen content, microvascular dysfunction, and neurohormonal signaling. Together, these changes in LV chamber geometry and morphology, coupled with heightened vascular stiffness, appear to drive both age-related increases in systolic function and declines in diastolic function, particularly in postmenopausal women. Accordingly, estrogen has been implicated as a key mediator, given its direct vasodilating properties, association with nitric oxide excretion, and involvement in myocellular Ca2+ handling, mitochondrial energy production, and oxidative stress. The culmination of the abovementioned sex-specific cardiac and vascular changes across the lifespan provides important insight into heart failure development, particularly of the preserved ejection fraction variety, while offering promise for future preventive strategies and therapeutic approaches.

Influence of sex on heightened vasoconstrictor mechanisms in the non-Hispanic black population

Cardiovascular disease (CVD) affects individuals of all races and ethnicities; however, its prevalence is highest in non-Hispanic black individuals (BL) relative to other populations. While previous research has provided valuable insight into elevated CVD risk in the BL population, this work has been almost exclusively conducted in men. This is alarming given that BL women suffer from CVD at an equivalent rate to BL men and each has a greater prevalence when compared to all other ethnicities, regardless of sex. The importance of investigating sex differences in mechanisms of cardiovascular function is highlighted by the National Institute of Health requiring sex to be considered as a biological variable in research studies to better our "understanding of key sex influences on health processes and outcomes." The mechanism(s) responsible for the elevated CVD risk in BL women remains unclear and is likely multifactorial. Limited studies in BL women suggest that, while impaired vasodilator capacity is involved, heightened vasoconstrictor tone and/or responsiveness may also contribute. Within this mini-review, we will discuss potential mechanisms of elevated rates of hypertension and other CVDs in BL individuals with a particular focus on young, otherwise healthy, college-aged women. To stimulate academic thought and future research, we will also discuss potential mechanisms for impaired vascular function in BL women, as well as possible divergent mechanisms between BL men and women based on either preliminary data or plausible speculation extending from findings in the existing literature. Last, we will conclude with potential future research directions aimed at better understanding the elevated risk for hypertension and CVD in BL women.

Hyperglycemia Augments Endothelin-1-Induced Constriction of Human Retinal Venules

Purpose

Endothelin-1 (ET-1) is a potent vasoactive factor implicated in development of diabetic retinopathy, which is commonly associated with retinal edema and hyperglycemia. Although the vasomotor activity of venules contributes to the regulation of tissue fluid homeostasis, responses of human retinal venules to ET-1 under euglycemia and hyperglycemia remain unknown and the ET-1 receptor subtype corresponding to vasomotor function has not been determined. Herein, we addressed these issues by examining the reactivity of isolated human retinal venules to ET-1, and results from porcine retinal venules were compared.

Methods

Retinal tissues were obtained from patients undergoing enucleation. Human and porcine retinal venules were isolated and pressurized to assess diameter changes in response to ET-1 after exposure to 5 mM control glucose or 25 mM high glucose for 2 hours.

Results

Both human and porcine retinal venules exposed to control glucose developed similar basal tone and constricted comparably to ET-1 in a concentration-dependent manner. ET-1–induced constrictions of human and porcine retinal venules were abolished by ET_A receptor antagonist BQ123. During high glucose exposure, basal tone of human and porcine retinal venules was unaltered but ET-1–induced vasoconstrictions were enhanced.

Conclusions

ET-1 elicits comparable constriction of human and porcine retinal venules by activation of ET_A receptors. In vitro hyperglycemia augments human and porcine retinal venular responses to ET-1.

Translational Relevance

Similarities in vasoconstriction to ET-1 between human and porcine retinal venules support the latter as an effective model of the human retinal microcirculation to help identify vascular targets for the treatment of retinal complications in patients with diabetes.

Phenotypic assessment and ligand screening of ETA/ETB receptors with label-free dynamic mass redistribution assay

Endothelin receptors, consisting of two subtypes, ETA and ETB, are expressed in various tissues and widely regulate cardiovascular systems. The two receptors show distinct biological characteristics and are involved in different downstream pathways. Hence, to evaluate the ETA and ETB receptors on the same platform is helpful to display their pharmacological features. In this study, we developed a label-free dynamic mass redistribution (DMR) assay to investigate the phenotypic features of the ETA and ETB receptors in native cell lines. Meanwhile, specific agonists and antagonists were investigated for their pharmacological parameters. Results indicated that the DMR response of endothelin 1 (ET-1, an endogenous ETA/ETB agonist) was cell line dependent on ETA receptors and this ligand generated a biphasic dose-response curve in SH-SY5Y as well as PC3 cell lines. ET-1 and IRL 1620 (an ETB agonist) showed different DMR responses in U251 cells. IC₅₀ values of antagonists were consistent with the K_i values previously reported. Furthermore, a list of compounds was screened on the ETA and ETB receptor models established by the high-throughput DMR assays. This study demonstrated that the DMR assay had great potential in the phenotypic-based investigation and ligand screening of GPCRs.

Crystal structure of human endothelin ETB receptor in complex with peptide inverse agonist IRL2500

Endothelin receptors (ETA and ETB) are G-protein-coupled receptors activated by endothelin-1 and are involved in blood pressure regulation. IRL2500 is a peptide-mimetic of the C-terminal tripeptide of endothelin-1, and has been characterized as a potent ETB-selective antagonist, which has preventive effects against brain edema. Here, we report the crystal structure of the human ETB receptor in complex with IRL2500 at 2.7 Å-resolution. The structure revealed the different binding modes between IRL2500 and endothelin-1, and provides structural insights into its ETB-selectivity. Notably, the biphenyl group of IRL2500 penetrates into the transmembrane core proximal to D2.50, thus stabilizing the inactive conformation. Using the newly-established constitutively active mutant, we clearly demonstrate that IRL2500 functions as an inverse agonist for the ETB receptor. The current findings will expand the chemical space of ETR antagonists and facilitate the design of inverse agonists for other class A GPCRs.

Androgens drive microvascular endothelial dysfunction in women with polycystic ovary syndrome: role of the endothelin B receptor

KEY POINTS

Polycystic ovary syndrome (PCOS) is a complex syndrome with cardiovascular risk factors, including obesity and insulin resistance. PCOS is also associated with high androgens, increases the risk of cardiovascular dysfunction in women. Due to the complexity of PCOS, had it has been challenging to isolate specific causes of the cardiovascular dysfunction. Our measure of cardiovascular dysfunction (endothelial dysfunction) was most profound in lean women with PCOS. The endothelin-1-induced vasodilation in these PCOS subject, was dependent on the ET_B R but was not NO-dependent. We also demonstrated oestrogen administration improved endothelial function in lean and obese women with PCOS likely because oestrogen increased NO availability. Our studies indicate a primary role for androgens in cardiovascular dysfunction in PCOS.

ABSTRACT

Endothelin-1 (ET-1) is an indicator of endothelial injury and dysfunction and is elevated in women with androgen excess polycystic ovary syndrome (AE-PCOS). The endothelin B receptor (ET_B R) subtype mediates vasodilatation, but is blunted in women with PCOS. We hypothesized that androgen drives endothelial dysfunction in AE-PCOS women and oestradiol (EE) administration reverses these effects. We assessed microvascular endothelial function in women with (7 lean and 7 obese) and without AE-PCOS (controls, 6 lean, 7 obese). Only obese AE-PCOS women were insulin resistant (IR). We evaluated cutaneous vascular conductance (%CVC_max ) with laser Doppler flowmetry during low dose intradermal microdialysis ET-1 perfusions (1, 3, 4, 5 and 7 pmol) with either lactated Ringer solution alone, or with ET_B R (BQ-788), or nitric oxide (NO) inhibition (l-NAME). Log[ET-1]-%maxCVC dose-response curves demonstrated reduced vasodilatory responses to ET-1 in lean AE-PCOS (logED₅₀ , 0.59 ± 0.08) versus lean controls (logED₅₀ , 0.49 ± 0.09, P < 0.05), but not compared to obese AE-PCOS (logED₅₀ , 0.65 ± 0.09). ET_B R inhibition decreased ET-1-induced vasodilatation in AE-PCOS women (logED₅₀ , 0.64 ± 0. 22, P < 0.05). This was mechanistically observed at the cellular level, with ET-1-induced, DAF-FM-measurable endothelial cell NO production, which was abrogated by dihydrotestosterone in an androgen receptor-dependent manner. EE augmented the cutaneous vasodilating response to ET-1(logED₅₀ 0.29 ± 0.21, 0.47 ± 0.09, P < 0.05 for lean and obese, respectively). Androgens drive endothelial dysfunction in lean and obese AE-PCOS. We propose that the attenuated ET-1-induced vasodilatation in AE-PCOS is a consequence of androgen receptor-mediated, suppressed ET_B R-stimulated NO production, and is reversed with EE.

Physiological role of endothelin-1 in flow-mediated vasodilatation in humans and impact of cardiovascular risk factors

OBJECTIVES

The current study addressed the hypothesis that the local decrease in endothelin-1 (ET-1) bioavailability during sustained flow increases contributes to endothelium-dependent, flow-mediated dilatation (FMD) of conduit arteries and is altered in presence of cardiovascular risk factors.

METHODS AND RESULTS

In nine young healthy individuals, the decrease in local ET-1 plasma levels and radial artery FMD in response to hand skin heating (from 34 to 44 °C) was not affected by endothelin type A (ETA) receptor blockade, achieved using the brachial infusion of BQ-123 (100 nmol/min per l of forearm), as compared with physiological saline (0.9% NaCl) infusion. In contrast, endothelin type B (ETB) receptor blockade with BQ-788 (10 nmol/min per l) suppressed the decrease in plasma ET-1 during heating and reduced FMD, without altering nitric oxide release. The coinfusion of BQ-123 did not affect the inhibitory effect of ETB receptor blockade on the decrease in ET-1 plasma levels during heating but prevented the reduction in FMD. Basal radial artery parameters, systemic hemodynamics, and endothelium-independent dilatation to glyceryl trinitrate were not modified by ETA and/or ETB blockade. In a general population of 40 participants without treatment or major cardiovascular diseases, including the nine healthy individuals, the reduction in endothelin-1 level during heating was correlated with FMD (r = -0.55, P < 0.001) and decreased with increased age (r = 0.49, P = 0.001), mean arterial blood pressure (r = 0.48, P = 0.002), and total cholesterol level (r = 0.37, P = 0.024).

CONCLUSION

The uptake of endothelin-1 by ETB receptors contributes to conduit artery FMD, preventing its vasoconstrictor action mediated by ETA receptors. The alteration of this mechanism by cardiovascular risk factors may contribute to endothelial dysfunction.

Na+ /K+ -ATPase coupled to endothelin receptor type B stimulates peripheral nerve regeneration via lactate signalling

We have recently demonstrated that endothelin (ET) is functionally coupled to Na_x , a Na⁺ concentration-sensitive Na⁺ channel for lactate release via ET receptor type B (ET_B R) and is involved in peripheral nerve regeneration in a sciatic nerve transection-regeneration mouse model. Na_x is known to interact directly with Na⁺ /K⁺ -ATPase, leading to lactate production in the brain. To investigate the role of Na⁺ /K⁺ -ATPase in peripheral nerve regeneration, in this study, we applied ouabain, a Na⁺ /K⁺ -ATPase inhibitor, to the cut site for 4 weeks with an osmotic pump. While functional recovery and nerve reinnervation to the toe started at 5 weeks after axotomy and were completed by 7 weeks, ouabain delayed them by 2 weeks. The delay by ouabain was improved by lactate, and its effect was blocked by α-cyano-4-hydroxy-cinnamic acid (CIN), a broad monocarboxylate transporter (MCT) inhibitor. In primary cultures of dorsal root ganglia, neurite outgrowth of neurons and lactate release into the culture medium was inhibited by ouabain. Conversely, lactate enhanced the neurite outgrowth, which was blocked by CIN, but not by AR-C155858, a MCT1/2-selective inhibitor. ET-1 and ET-3 increased neurite outgrowth of neurons, which was attenuated by an ET_B R antagonist, ouabain and 2 protein kinase C inhibitors. Taken together with the finding that ET_B R was expressed in Schwann cells, these results demonstrate that ET enhanced neurite outgrowth of neurons mediated by Na⁺ /K⁺ -ATPase via ET_B R in Schwann cells. This study suggests that Na⁺ /K⁺ -ATPase coupled to the ET-ET_B R system plays a critical role in peripheral nerve regeneration via lactate signalling.

The effects of endothelin-1 on satellite glial cells in peripheral ganglia

Endothelins (ET) are a family of highly active neuropeptides with manifold influences via ET receptors (ETR) in both the peripheral and central nervous systems. We have shown previously that satellite glial cells (SGCs) in mouse trigeminal ganglia (TG) are extremely sensitive to ET-1 in evoking [Ca²⁺]_in increase, apparently via ET_BR activation, but there is no functional information on ETR in SGCs of other peripheral ganglia. Here we tested the effects of ET-1 on SGCs in nodose ganglia (NG), which is sensory, and superior cervical ganglia (Sup-CG), which is part of the sympathetic nervous system, and further investigated the influence of ET-1 on SGCs in TG. Using calcium imaging we found that SGCs in intact, freshly isolated NG and Sup-CG are highly sensitive to ET-1, with threshold concentration at 0.1nM. Our results showed that [Ca²⁺]_in elevation in response to ET-1 was partially due to Ca²⁺ influx from the extracellular space and partially to Ca²⁺ release from intracellular stores. Using receptor selective ETR agonists and antagonists, we found that the responses were mediated by mixed ET_AR/ET_BR in SGCs of NG and predominantly by ET_BR in SGCs of Sup-CG. By employing intracellular dye injection we examined coupling among SGCs around different neurons in the presence of 5nM ET-1 and observed coupling inhibition in all the three ganglion types. In summary, our work showed that SGCs in mouse sensory and sympathetic ganglia are highly sensitive to ET-1 and that this peptide markedly reduces SGCs coupling. We conclude that ET-1, which may participate in neuron-glia communications, has similar functions in wide range of peripheral ganglia.

ETB receptor contribution to vascular dysfunction in postmenopausal women

Endothelin-1 (ET-1) contributes to age-related endothelial dysfunction in men via the ET_A receptor. However, there are sex differences in the ET-1 system, and ET_B receptors are modulated by sex hormones. The purpose of this study was to test the hypothesis that ET_B receptors contribute to impaired vasodilatory function in postmenopausal women (PMW). We measured flow-mediated dilation (FMD) using ultrasound, and cutaneous nitric oxide-mediated vasodilation during local heating (42°C) via laser Doppler flowmetry in 18 young women (YW; 22 ± 1 yr) and 16 PMW (56 ± 1 yr). Cutaneous microdialysis perfusions of lactated Ringer (control), an ET_B receptor antagonist (BQ-788, 300 nM), and an ET_A receptor antagonist (BQ-123, 500 nM), were done through separate fibers, followed by perfusions of sodium nitroprusside (28 mM) and local heating to 43°C (max). Cutaneous vascular conductance (CVC) was calculated as cutaneous blood flow/mean arterial pressure and expressed as a percent of maximal dilation. FMD (YW: 7.5 ± 0.5 vs. PMW: 5.6 ± 0.6%) and cutaneous vasodilation (YW: 93 ± 2 vs. PMW: 83 ± 4%CVC_max) were lower in PMW (both P < 0.05). Blockade of ET_B receptors decreased cutaneous vasodilation in YW (87 ± 2%CVC_max; P < 0.05 vs. control) but increased vasodilation in PMW (93 ± 1%CVC_max; P < 0.05 vs. control). ET_A receptor blockade had minimal effect in YW (92 ± 1%CVC_max) but increased cutaneous vasodilation in PMW (91 ± 2%CVC_max; P < 0.05 vs. control). In conclusion, ET_B receptors mediate vasodilation in YW, but this effect is lost after menopause. Impaired vasodilatory function in PMW is due in part to a loss of ET_B-mediated dilation.

Endothelial overexpression of endothelin-1 modulates aortic, carotid, iliac and renal arterial responses in obese mice

Endothelin-1 (ET-1) is essential for mammalian development and life, but it has also been implicated in increased cardiovascular risk under pathophysiological conditions. The aim of this study was to determine the impact of endothelial overexpression of the prepro-endothelin-1 gene on endothelium-dependent and endothelium-independent responses in the conduit and renal arteries of lean and obese mice. Obesity was induced by high-fat-diet (HFD) consumption in mice with Tie-1 promoter-driven, endothelium-specific overexpression of the prepro-endothelin-1 gene (TET^het) and in wild-type (WT) littermates on a C57BL/6N background. Isometric tension was measured in rings (with endothelium) of the aorta (A), carotid (CA) and iliac (IA) arteries as well as the main (MRA) and segmental renal (SRA) arteries; all experiments were conducted in the absence or presence of L-NAME and/or the COX inhibitor meclofenamate. The release of prostacyclin and thromboxane A2 was measured by ELISA. In the MRA, TET^het per se increased contractions to endothelin-1, but the response was decreased in SRA in response to serotonin; there were also improved relaxations to acetylcholine but not insulin in the SRA in the presence of L-NAME. HFD per se augmented the contractions to endothelin-1 (MRA) and to the thromboxane prostanoid (TP) receptor agonist U46619 (CA, MRA) as well as facilitated relaxations to isoproterenol (A). The combination of HFD and TET^het overexpression increased the contractions of MRA and SRA to vasoconstrictors but not in the presence of meclofenamate; this combination also augmented further relaxations to isoproterenol in the A. Contractions to endothelin-1 in the IA were prevented by endothelin-A receptor antagonist BQ-123 but only attenuated in obese mice by BQ-788. The COX-1 inhibitor FR122047 abolished the contractions of CA to acetylcholine. The release of prostacyclin during the latter condition was augmented in samples from obese TET^het mice and abolished by FR122047. These findings suggest that endothelial TET^het overexpression in lean animals has minimal effects on vascular responsiveness. However, if comorbid with obesity, endothelin-1-modulated, prostanoid-mediated renal arterial dysfunction becomes apparent.

Endothelin B Receptors on Primary Chicken Müller Cells and the Human MIO-M1 Müller Cell Line Activate ERK Signaling via Transactivation of Epidermal Growth Factor Receptors

Injury to the eye or retina triggers Müller cells, the major glia cell of the retina, to dedifferentiate and proliferate. In some species they attain retinal progenitor properties and have the capacity to generate new neurons. The epidermal growth factor receptor (EGFR) system and extracellular signal-regulated kinase (ERK) signaling are key regulators of these processes in Müller cells. The extracellular signals that modulate and control these processes are not fully understood. In this work we studied whether endothelin receptor signaling can activate EGFR and ERK signaling in Müller cells. Endothelin expression is robustly upregulated at retinal injury and endothelin receptors have been shown to transactivate EGFRs in other cell types. We analyzed the endothelin signaling system in chicken retina and cultured primary chicken Müller cells as well as the human Müller cell line MIO-M1. The Müller cells were stimulated with receptor agonists and treated with specific blockers to key enzymes in the signaling pathway or with siRNAs. We focused on endothelin receptor mediated transactivation of EGFRs by using western blot analysis, quantitative reverse transcriptase PCR and immunocytochemistry. The results showed that chicken Müller cells and the human Müller cell line MIO-M1 express endothelin receptor B. Stimulation by the endothelin receptor B agonist IRL1620 triggered phosphorylation of ERK1/2 and autophosphorylation of (Y1173) EGFR. The effects could be blocked by Src-kinase inhibitors (PP1, PP2), EGFR-inhibitor (AG1478), EGFR-siRNA and by inhibitors to extracellular matrix metalloproteinases (GM6001), consistent with a Src-kinase mediated endothelin receptor response that engage ligand-dependent and ligand-independent EGFR activation. Our data suggest a mechanism for how injury-induced endothelins, produced in the retina, may modulate the Müller cell responses by Src-mediated transactivation of EGFRs. The data give support to a view in which endothelins among several other functions, serve as an injury-signal that regulate the gliotic response of Müller cells.

Endothelin-1 (ET-1) induces resistance to bortezomib in human multiple myeloma cells via a pathway involving the ETB receptor and upregulation of proteasomal activity

PURPOSE

Bortezomib (BTZ) is used for the treatment of multiple myeloma (MM). However, a significant proportion of patients may be refractory to the drug. This study aimed to investigate whether the endothelin (ET-1) axis may act as an escape mechanism to treatment with bortezomib in MM cells.

METHODS

NCI-H929 and RPMI-8226 (human MM cell lines) were cultured with or without ET-1, BTZ, and inhibitors of the endothelin receptors. ET-1 levels were determined by ELISA, while the protein levels of its receptors and of the PI3K and MAPK pathways' components by western blot. Effects of ET-1 on cell proliferation were studied by MTT and on the ubiquitin proteasome pathway by assessing the chymotryptic activity of the 20S proteasome in cell lysates.

RESULTS

Endothelin receptors A and B (ETAR and ETBR, respectively) were found to be expressed in both cell lines, with the RPMI-8226 cells that are considered resistant to BTZ, expressing higher levels of ETBR and in addition secreting ET-1. Treatment of the NCI-H929 cells with ET-1 increased proliferation, while co-incubation of these cells with ET-1 and BTZ decreased BTZ efficacy with concomitant upregulation of 20S proteasomal activity. Si-RNA silencing or chemical blockade of ETBR abrogated the protective effects of ET-1. Finally, data suggest that the predominant signaling pathway involved in ET-1/ETBR-induced BTZ resistance in MM cells may be the MAPK pathway.

CONCLUSION

Our data suggest a possible role of the ET-1/ETBR axis in regulating the sensitivity of MM cells to BTZ. Thus, combining bortezomib with strategies to target the ET-1 axis could prove to be a novel promising therapeutic approach in MM.

Plasma levels of cell adhesion molecules during hyperinsulinemia and modulation of vasoactive mediators

Endothelial expression of cell adhesion molecules (CAMs) plays an important role in atherosclerosis. Atherosclerosis is increased in hyperinsulinemic states, but whether insulin per se is proatherogenic remains unclear. To investigate the effects of hyperinsulinemia on CAM expression, plasma levels of ICAM-1, VCAM-1 and E-selectin were measured before and after forearm infusion of insulin in healthy subjects. Insulin administration for 2 h resulted in signifi-cant hyperinsulinemia, whereas no significant change was observed in soluble CAMs (all p > 0.05). Because insulin stimulates endothelial release of both endothelin-1 (ET-1) and nitric oxide (NO), which may modulate the expression of CAMs, we also investigated the response of CAMs to ET-1 receptor blockade, alone and in combination with NO synthesis inhibition. ET-1 receptor blockade during hyperinsulinemia resulted in a vasodilator response, but did not affect soluble CAMs (all p > 0.05). Superimposition of NO inhibition by l-NMMA reversed the vasodilator effect of ET-1 blockade, without affecting soluble CAMs (all p > 0.05). In conclusion, acute hyperinsulinemia, alone or during ET-1 and NO pathway blockade, does not affect soluble CAMs. These results do not support a direct effect of insulin on endothelial cells to affect leukocyte adhesiveness to the vascular wall.

Endothelin-Receptor Antagonists beyond Pulmonary Arterial Hypertension: Cancer and Fibrosis

The endothelin axis and in particular the two endothelin receptors, ETA and ETB, are targets for therapeutic intervention in human diseases. Endothelin-receptor antagonists are in clinical use to treat pulmonary arterial hypertension and have been under clinical investigation for the treatment of several other diseases, such as systemic hypertension, cancer, vasospasm, and fibrogenic diseases. In this Perspective, we review the molecules that have been evaluated in human clinical trials for the treatment of pulmonary arterial hypertension, as well as other cardiovascular diseases, cancer, and fibrosis. We will also discuss the therapeutic consequences of receptor selectivity with regard to ETA-selective, ETB-selective, or dual ETA/ETB antagonists. We will also consider which chemical characteristics are relevant to clinical use and the properties of molecules necessary for efficacy in treating diseases against which known molecules displayed suboptimal efficacy.

Insulin decreases atherosclerosis by inducing endothelin receptor B expression

Endothelial cell (EC) insulin resistance and dysfunction, caused by diabetes, accelerates atherosclerosis. It is unknown whether specifically enhancing EC-targeted insulin action can decrease atherosclerosis in diabetes. Accordingly, overexpressing insulin receptor substrate-1 (IRS1) in the endothelia of Apoe^-/- mice (Irs1/Apoe^-/-) increased insulin signaling and function in the aorta. Atherosclerosis was significantly reduced in Irs1/ApoE^-/- mice on diet-induced hyperinsulinemia and hyperglycemia. The mechanism of insulin's enhanced antiatherogenic actions in EC was related to remarkable induction of NO action, which increases endothelin receptor B (EDNRB) expression and intracellular [Ca²⁺]. Using the mice with knockin mutation of eNOS, which had Ser1176 mutated to alanine (AKI), deleting the only known mechanism for insulin to activate eNOS/NO pathway, we observed that IRS1 overexpression in the endothelia of Aki/ApoE^-/- mice significantly decreased atherosclerosis. Interestingly, endothelial EDNRB expression was selectively reduced in intima of arteries from diabetic patients and rodents. However, endothelial EDNRB expression was upregulated by insulin via P13K/Akt pathway. Finally EDNRB deletion in EC of Ldlr^-/- and Irs1/Ldlr^-/- mice decreased NO production and accelerated atherosclerosis, compared with Ldlr^-/- mice. Accelerated atherosclerosis in diabetes may be reduced by improving insulin signaling selectively via IRS1/Akt in the EC by inducing EDNRB expression and NO production.

Endothelin

The endothelins comprise three structurally similar 21-amino acid peptides. Endothelin-1 and -2 activate two G-protein coupled receptors, ETA and ETB, with equal affinity, whereas endothelin-3 has a lower affinity for the ETA subtype. Genes encoding the peptides are present only among vertebrates. The ligand-receptor signaling pathway is a vertebrate innovation and may reflect the evolution of endothelin-1 as the most potent vasoconstrictor in the human cardiovascular system with remarkably long lasting action. Highly selective peptide ETA and ETB antagonists and ETB agonists together with radiolabeled analogs have accurately delineated endothelin pharmacology in humans and animal models, although surprisingly no ETA agonist has been discovered. ET antagonists (bosentan, ambrisentan) have revolutionized the treatment of pulmonary arterial hypertension, with the next generation of antagonists exhibiting improved efficacy (macitentan). Clinical trials continue to explore new applications, particularly in renal failure and for reducing proteinuria in diabetic nephropathy. Translational studies suggest a potential benefit of ETB agonists in chemotherapy and neuroprotection. However, demonstrating clinical efficacy of combined inhibitors of the endothelin converting enzyme and neutral endopeptidase has proved elusive. Over 28 genetic modifications have been made to the ET system in mice through global or cell-specific knockouts, knock ins, or alterations in gene expression of endothelin ligands or their target receptors. These studies have identified key roles for the endothelin isoforms and new therapeutic targets in development, fluid-electrolyte homeostasis, and cardiovascular and neuronal function. For the future, novel pharmacological strategies are emerging via small molecule epigenetic modulators, biologicals such as ETB monoclonal antibodies and the potential of signaling pathway biased agonists and antagonists.

Sulfenic Acid Modification of Endothelin B Receptor is Responsible for the Benefit of a Nonsteroidal Mineralocorticoid Receptor Antagonist in Renal Ischemia

AKI is associated with high mortality rates and the development of CKD. Ischemia/reperfusion (IR) is an important cause of AKI. Unfortunately, there is no available pharmacologic approach to prevent or limit renal IR injury in common clinical practice. Renal IR is characterized by diminished nitric oxide bioavailability and reduced renal blood flow; however, the mechanisms leading to these alterations are poorly understood. In a rat model of renal IR, we investigated whether the administration of the novel nonsteroidal mineralocorticoid receptor (MR) antagonist BR-4628 can prevent or treat the renal dysfunction and tubular injury induced by IR. Renal injury induced by ischemia was associated with increased oxidant damage, which led to a cysteine sulfenic acid modification in endothelin B receptor and consequently decreased endothelial nitric oxide synthase activation. These modifications were efficiently prevented by nonsteroidal MR antagonism. Furthermore, we demonstrated that the protective effect of BR-4628 against IR was lost when a selective endothelin B receptor antagonist was coadministered. These data describe a new mechanism for reduced endothelial nitric oxide synthase activation during renal IR that can be blocked by MR antagonism with BR-4628.

Endothelin receptors and their antagonists

All three members of the endothelin (ET) family of peptides, ET-1, ET-2, and ET-3, are expressed in the human kidney, with ET-1 being the predominant isoform. ET-1 and ET-2 bind to two G-protein-coupled receptors, ETA and ETB, whereas at physiological concentrations ET-3 has little affinity for the ET(A) receptor. The human kidney is unusual among the peripheral organs in expressing a high density of ET(B). The renal vascular endothelium only expresses the ET(B) subtype and ET-1 acts in an autocrine or paracrine manner to release vasodilators. Endothelial ETB in kidney, as well as liver and lungs, also has a critical role in scavenging ET-1 from the plasma. The third major function is ET-1 activation of ET(B) in in the nephron to reduce salt and water re-absorption. In contrast, ET(A) predominate on smooth muscle, causing vasoconstriction and mediating many of the pathophysiological actions of ET-1. The role of the two receptors has been delineated using highly selective ET(A) (BQ123, TAK-044) and ET(B) (BQ788) peptide antagonists. Nonpeptide antagonists, bosentan, macitentan, and ambrisentan, that are either mixed ET(A)/ET(B) antagonists or display ET(A) selectivity, have been approved for clinical use but to date are limited to pulmonary hypertension. Ambrisentan is in clinical trials in patients with type 2 diabetic nephropathy. This review summarizes ET-receptor antagonism in the human kidney, and considers the relative merits of selective versus nonselective antagonism in renal disease.

Endothelin@25 - new agonists, antagonists, inhibitors and emerging research frontiers: IUPHAR Review 12

Since the discovery of endothelin (ET)-1 in 1988, the main components of the signalling pathway have become established, comprising three structurally similar endogenous 21-amino acid peptides, ET-1, ET-2 and ET-3, that activate two GPCRs, ETA and ETB . Our aim in this review is to highlight the recent progress in ET research. The ET-like domain peptide, corresponding to prepro-ET-193-166 , has been proposed to be co-synthesized and released with ET-1, to modulate the actions of the peptide. ET-1 remains the most potent vasoconstrictor in the human cardiovascular system with a particularly long-lasting action. To date, the major therapeutic strategy to block the unwanted actions of ET in disease, principally in pulmonary arterial hypertension, has been to use antagonists that are selective for the ETA receptor (ambrisentan) or that block both receptor subtypes (bosentan). Macitentan represents the next generation of antagonists, being more potent than bosentan, with longer receptor occupancy and it is converted to an active metabolite; properties contributing to greater pharmacodynamic and pharmacokinetic efficacy. A second strategy is now being more widely tested in clinical trials and uses combined inhibitors of ET-converting enzyme and neutral endopeptidase such as SLV306 (daglutril). A third strategy based on activating the ETB receptor, has led to the renaissance of the modified peptide agonist IRL1620 as a clinical candidate in delivering anti-tumour drugs and as a pharmacological tool to investigate experimental pathophysiological conditions. Finally, we discuss biased signalling, epigenetic regulation and targeting with monoclonal antibodies as prospective new areas for ET research.

Adaptive regulation of endothelin receptor type-A and type-B in vascular smooth muscle cells during pregnancy in rats

Normal pregnancy is associated with systemic vasodilation and decreased vascular contraction, partly due to increased release of endothelium-derived vasodilator substances. Endothelin-1 (ET-1) is an endothelium-derived vasoconstrictor acting via endothelin receptor type A (ETA R) and possibly type B (ETB R) in vascular smooth muscle cells (VSMCs), with additional vasodilator effects via endothelial ETB R. However, the role of ET-1 receptor subtypes in the regulation of vascular function during pregnancy is unclear. We investigated whether the decreased vascular contraction during pregnancy reflects changes in the expression/activity of ETAR and ETBR. Contraction was measured in single aortic VSMCs isolated from virgin, mid-pregnant (mid-Preg, day 12), and late-Preg (day 19) Sprague-Dawley rats, and the mRNA expression, protein amount, tissue and cellular distribution of ETAR and ETBR were examined using RT-PCR, Western blots, immunohistochemistry, and immunofluorescence. Phenylephrine (Phe, 10(-5)  M), KCl (51 mM), and ET-1 (10(-6)  M) caused VSMC contraction that was in late-Preg < mid-Preg and virgin rats. In VSMCs treated with ETB R antagonist BQ788, ET-1 caused significant contraction that was still in late-Preg < mid-Preg and virgin rats. In VSMCs treated with the ETAR antagonist BQ123, ET-1 caused a small contraction; and the ETBR agonists IRL-1620 and sarafotoxin 6c (S6c) caused similar contraction that was in late-Preg < mid-Preg and virgin rats. RT-PCR revealed similar ETAR, but greater ETBR mRNA expression in pregnant versus virgin rats. Western blots revealed similar ETAR, and greater protein amount of ETBR in endothelium-intact vessels, but reduced ETBR in endothelium-denuded vessels of pregnant versus virgin rats. Immunohistochemistry revealed prominent ETBR staining in the intima, but reduced ETAR and ETBR in the aortic media of pregnant rats. Immunofluorescence signal for ETAR and ETBR was less in VSMCs of pregnant versus virgin rats. The pregnancy-associated decrease in ETAR- and ETBR-mediated VSMC contraction appears to involve downregulation of ETAR and ETBR expression/activity in VSM, and may play a role in the adaptive vasodilation during pregnancy.

Androgens influence microvascular dilation in PCOS through ET-A and ET-B receptors

Hyperandrogenism and vascular dysfunction often coexist in women with polycystic ovary syndrome (PCOS). We hypothesized that testosterone compromises cutaneous microvascular dilation in women with PCOS via the endothelin-1 ET-B subtype receptor. To control and isolate testosterone's effects on microvascular dilation, we administered a gonadotropin-releasing hormone antagonist (GnRHant) for 11 days in obese, otherwise healthy women [controls, 22.0 (4) yr, 36.0 (3.2) kg/m(2)] or women with PCOS [23 (4) yr, 35.4 (1.3) kg/m(2)], adding testosterone (T; 2.5 mg/day) on days 8-11. Using laser Doppler flowmetry and cutaneous microdialysis, we measured changes in skin microcirculatory responsiveness (ΔCVC) to local heating while perfusing ET-A (BQ-123) and ET-B (BQ-788) receptor antagonists under three experimental conditions: baseline (BL; prehormone intervention), GnRHant (day 4 of administration), and T administration. At BL, ET-A receptor inhibition enhanced heat-induced vasodilation in both groups [ΔCVC control 2.03 (0.65), PCOS 2.10 (0.25), AU/mmHg, P < 0.05]; ET-B receptor inhibition reduced vasodilation in controls only [ΔCVC 0.98 (0.39), 1.41 (0.45) AU/mmHg for controls, PCOS] compared with saline [ΔCVC controls 1.27 (0.48), PCOS 1.31 (0.13) AU/mmHg]. GnRHant enhanced vasodilation in PCOS [saline ΔCVC 1.69 (0.23) AU/mmHg vs. BL, P < 0.05] and abolished the ET-A effect in both groups, a response reasserted with T in controls. ET-B receptor inhibition reduced heat-induced vasodilation in both groups during GnRHant and T [ΔCVC, controls: 0.95 (0.21) vs. 0.51 (13); PCOS: 1.27 (0.23) vs. 0.84 (0.27); for GnRHant vs. T, P < 0.05]. These data demonstrate that androgen suppression improves microvascular dilation in PCOS via ET-A and ET-B receptors.

Pro-fibrotic processes in human lung fibroblasts are driven by an autocrine/paracrine endothelinergic system

BACKGROUND AND PURPOSE

Since endothelin (ET) may act as pro-fibrotic mediator, expression and release of ET isoforms, their receptors and potential pro-fibrotic ET effects were studied in human lung fibroblasts.

EXPERIMENTAL APPROACH

MRC-5 and primary human lung fibroblasts (phLFb) were cultured. Expression of prepro-ET isoforms was determined by qPCR and release of ET-1 by elisa. ET receptor function was analysed by real-time measurement of dynamic mass redistribution (DMR). Incorporation of [(3) H]-thymidine was determined as measure of proliferation and that of [(3) H]-proline for collagen synthesis. Phospho-p42/44 MAP kinase was determined by Western blot.

KEY RESULTS

ET-1 is the predominant ET in human lung fibroblasts (hLF), and TGF-β caused a further, selective and sustained up-regulation of ET-1 resulting in increased extracellular ET-1 accumulation. hLFb express mRNA encoding ET-A and ET-B receptors. Expression of both receptors was confirmed at protein level. ET-1 induced marked DMR signals, an effect that involved ET-A and ET-B receptors. Stimulatory effects of ET-1 on hLFb proliferation and collagen synthesis were mediated exclusively via ET-A receptors. ET-1, again via ET-A receptors, induced rapid activation of ERK MAPK, shown to be a crucial cellular signal in ET-1-induced collagen synthesis. ET-1-induced activation of ERK and collagen synthesis was, in contrast to corresponding effect of a muscarinic agonist, largely insensitive to pertussis toxin.

CONCLUSIONS AND IMPLICATIONS

hLFb are endowed with all elements necessary to build a functional autocrine/paracrine endothelinergic system, which appears to drive pro-fibrotic airway and lung remodelling processes, effects for which only ET-A, but not ET-B receptors appear to be of significance.

Identification of endothelin 2 as an inflammatory factor that promotes central nervous system remyelination

The development of new regenerative therapies for multiple sclerosis is hindered by the lack of potential targets for enhancing remyelination. The study of naturally regenerative processes such as the innate immune response represents a powerful approach for target discovery to solve this problem. By 'mining' these processes using transcriptional profiling we can identify candidate factors that can then be tested individually in clinically-relevant models of demyelination and remyelination. Here, therefore, we have examined a previously described in vivo model of the innate immune response in which zymosan-induced macrophage activation in the retina promotes myelin sheath formation by oligodendrocytes generated from transplanted precursor cells. While this model is not itself clinically relevant, it does provide a logical starting point for this study as factors that promote myelination must be present. Microarray analysis of zymosan-treated retinae identified several cytokines (CXCL13, endothelin 2, CCL20 and CXCL2) to be significantly upregulated. When tested in a cerebellar slice culture model, CXCL13 and endothelin 2 promoted myelination and endothelin 2 also promoted remyelination. In studies to identify the receptor responsible for this regenerative effect of endothelin 2, analysis of both remyelination following experimental demyelination and of different stages of multiple sclerosis lesions in human post-mortem tissue revealed high levels of endothelin receptor type B in oligodendrocyte lineage cells. Confirming a role for this receptor in remyelination, small molecule agonists and antagonists of endothelin receptor type B administered in slice cultures promoted and inhibited remyelination, respectively. Antagonists of endothelin receptor type B also inhibited remyelination of experimentally-generated demyelination in vivo. Our work therefore identifies endothelin 2 and the endothelin receptor type B as a regenerative pathway and suggests that endothelin receptor type B agonists represent a promising therapeutic approach to promote myelin regeneration.

Endothelin-1 is a transcriptional target of p53 in epidermal keratinocytes and regulates ultraviolet-induced melanocyte homeostasis

Keratinocytes contribute to melanocyte activity by influencing their microenvironment, in part, through secretion of paracrine factors. Here, we discovered that p53 directly regulates Edn1 expression in epidermal keratinocytes and controls UV-induced melanocyte homeostasis. Selective ablation of endothelin-1 (EDN1) in murine epidermis (EDN1(ep-/-) ) does not alter melanocyte homeostasis in newborn skin but decreases dermal melanocytes in adult skin. Results showed that keratinocytic EDN1 in a non-cell autonomous manner controls melanocyte proliferation, migration, DNA damage, and apoptosis after ultraviolet B (UVB) irradiation. Expression of other keratinocyte-derived paracrine factors did not compensate for the loss of EDN1. Topical treatment with EDN1 receptor (EDNRB) antagonist BQ788 abrogated UV-induced melanocyte activation and recapitulated the phenotype seen in EDN1(ep-/-) mice. Altogether, the present studies establish an essential role of EDN1 in epidermal keratinocytes to mediate UV-induced melanocyte homeostasis in vivo.

Agonist-dependent modulation of arterial endothelinA receptor function

BACKGROUND AND PURPOSE Endothelin-1 (ET-1) causes long-lasting vasoconstrictions. These can be prevented by ET(A) receptor antagonists but are only poorly reversed by these drugs. We tested the hypothesis that endothelin ET(A) receptors are susceptible to allosteric modulation by endogenous agonists and exogenous ligands. EXPERIMENTAL APPROACH Rat isolated mesenteric resistance arteries were pretreated with capsaicin and studied in wire myographs, in the presence of L-NAME and indomethacin to concentrate on arterial smooth muscle responses. KEY RESULTS Endothelins caused contractions with equal maximum but differing potency (ET-1 = ET-2 > ET-3). ET-1(1-15) neither mimicked nor antagonized these effects in the absence and presence of ET(16-21). 4(Ala) ET-1 (ET(B) agonist) and BQ788 (ET(B) antagonist) were without effects. BQ123 (peptide ET(A) antagonist) reduced the sensitivity and relaxed the contractile responses to endothelins. Both effects depended on the agonist (pK(B): ET-3 = ET-1 > ET-2; % relaxation: ET-3 = ET-2 > ET-1). Also, with PD156707 (non-peptide ET(A) antagonist) agonist-dependence and a discrepancy between preventive and inhibitory effects were observed. The latter was even more marked with bulky analogues of BQ123 and PD156707. CONCLUSIONS AND IMPLICATIONS These findings indicate allosteric modulation of arterial smooth muscle ET(A) receptor function by endogenous agonists and by exogenous endothelin receptor antagonists. This may have consequences for the diagnosis and pharmacotherapy of diseases involving endothelins.

Endothelin-1 stimulates human trophoblast cell migration through Cdc42 activation

PURPOSE

This study investigated the role and mechanism of Cdc42 in Endothelin-1 (ET-1)-induced trophoblast cell migration.

METHODS

We examined ET-1-mediated stimulation of trophoblast migration with HTR-8/SVneo cells. Cdc42 activation was measured after ET-1 treatment of HTR-8/SVneo cells. To determine the ET receptor subtype involved in ET-1-mediated Cdc42 activation, experiments were performed in the presence of ET(A) and ET(B) receptor antagonists. Finally, using siRNA we knocked down the expression of Cdc42 to examine the involvement of Cdc42 in the regulation of ET-1-stimulated trophoblast cell migration.

RESULTS

ET-1 was shown to have a dose-dependent effect on trophoblast migration. At low concentrations of ET-1 (0.1 nmol/L) ET-1 had a stimulatory effect on cell migration. ET-1 (10 nmol/L) increased HTR-8/svneo cell migration index by 2.5 fold. ET-1 (10 nmol/L) elevated protein level and activity of Cdc42. ET-1 induced activation of Cdc42 GTPase was mediated by both ET(A) and ET(B). ET-1-induced cell migration was shown to be inhibited by Cdc42 siRNA.The inhibition was not mitigated by the addition of ET-1, suggesting that Cdc42 plays an important role in trophoblast migration and is obligatory for ET-1 action.

CONCLUSIONS

ET-1 stimulates EVT migration through Cdc42 activation.

The role of endothelin and endothelin antagonists in traumatic brain injury: a review of the literature

OBJECTIVES

To date, there is increasing evidence for the role of endothelins in the pathophysiological development of cerebral vasospasms associated with a variety of neurological diseases, e.g., stroke and subarachnoid hemorrhage. In contrast, only little is known regarding the role of endothelins in impaired cerebral hemodynamics after traumatic brain injury. Therapeutic work in blocking the endothelin system has led to the discovery of a number of antagonists potentially useful in restoring cerebral blood flow after traumatic brain injury, potentially reducing the detrimental effects of secondary brain injury. Therefore, the present work provides an overview of background topics such as structures and biosynthesis of endothelins, different types as well as potential mechanisms and sites of action. In addition, the role of age for the effects of endothelins on cerebral hemodynamics after traumatic brain injury is discussed.

RESULTS

Description of data supporting the role of the endothelins play in a host of neurological deficits.

CONCLUSIONS

Endothelin antagonists may be effective as novel treatments for various neuropathologies.

Endothelin and endothelin receptors in the renal and cardiovascular systems

Endothelin-1 (ET-1) is a multifunctional hormone which regulates the physiology of the cardiovascular and renal systems. ET-1 modulates cardiac contractility, systemic and renal vascular resistance, salt and water renal reabsorption, and glomerular function. ET-1 is responsible for a variety of cellular events: contraction, proliferation, apoptosis, etc. These effects take place after the activation of the two endothelin receptors ET(A) and ET(B), which are present - among others - on cardiomyocytes, fibroblasts, smooth muscle and endothelial cells, glomerular and tubular cells of the kidney. The complex and numerous intracellular pathways, which can be contradictory in term of functional response depending on the receptor type, cell type and physiological situation, are described in this review. Many diseases share an enhanced ET-1 expression as part of the pathophysiology. However, the use of endothelin blockers is currently restricted to pulmonary arterial hypertension, and more recently to digital ulcer. The complexity of the endothelin system does not facilitate the translation of the molecular knowledge to clinical applications. Endothelin antagonists can prevent disease development but secondary undesirable effects limit their usage. Nevertheless, the increasing understanding of the effects of ET-1 on the cardiac and renal physiology maintains the endothelin system as a promising therapeutic target.

Kinase-dependent activation of voltage-gated Ca2+ channels by ET-1 in pulmonary arterial myocytes during chronic hypoxia

Exposure to chronic hypoxia (CH) causes pulmonary hypertension. The vasoconstrictor endothelin-1 (ET-1) is thought to play a role in the development of hypoxic pulmonary hypertension. In pulmonary arterial smooth muscle cells (PASMCs) from chronically hypoxic rats, ET-1 signaling is altered, with the ET-1-induced change in intracellular calcium concentration (Δ[Ca(2+)](i)) occurring through activation of voltage-dependent Ca(2+) channels (VDCC) even though ET-1-induced depolarization via inhibition of K(+) channels is lost. The mechanism underlying this response is unclear. We hypothesized that activation of VDCCs by ET-1 following CH might be mediated by protein kinase C (PKC) and/or Rho kinase, both of which have been shown to phosphorylate and activate VDCCs. To test this hypothesis, we examined the effects of PKC and Rho kinase inhibitors on the ET-1-induced Δ[Ca(2+)](i) in PASMCs from rats exposed to CH (10% O(2), 3 wk) using the Ca(2+)-sensitive dye fura 2-AM and fluorescent microscopy techniques. We found that staurosporine and GF109203X, inhibitors of PKC, and Y-27632 and HA 1077, Rho kinase inhibitors, reduced the ET-1-induced Δ[Ca(2+)](i) by >70%. Inhibition of tyrosine kinases (TKs) with genistein or tyrphostin A23, or combined inhibition of PKC, TKs, and Rho kinase, reduced the Δ[Ca(2+)](i) to a similar extent as inhibition of either PKC or Rho kinase alone. The ability of PKC or Rho kinase to activate VDCCs in our cells was verified using phorbol 12-myristate 13-acetate and GTP-γ-S. These results suggest that following CH, the ET-1-induced Δ[Ca(2+)](i) in PASMCs occurs via Ca(2+) influx through VDCCs mediated primarily by PKC, TKs, and Rho kinase.