Pulmonary Pharmacology & Therapeutics (1998) 11, 141–146Article No. pu980128

PULMONARYPHARMACOLOGY& THERAPEUTICS

Review

Endothelin-1: An Interesting Peptide or an Important Mediatorin Pulmonary Diseases?

D. W. P. Hay

Department of Pulmonary Pharmacology, SmithKline Beecham Pharmaceuticals, 709 Swedeland Road,King of Prussia, Pennsylvania 19406, USA

Distribution, synthesis, release and metabolism ofINTRODUCTIONET-1

Since its discovery by Yanagisawa et al in 1988,1

ET-1 is abundant in mammalian lung, including thatendothelin-1 (ET-1) has been comprehensively re-

of humans. The cellular localization of ET-1 includessearched. The extent of this investigation is highlighted the endothelium, epithelium, submucosal glands,by the more than 8000 publications on ET-1, of which neuroendocrine cells and some inflammatory cells,several hundred relate to research in the pulmonary such as macrophages.2–5 In support of these ob-system. Thus, and as outlined in previous chapters, servations in vitro and in vivo studies indicate thatthere is an increasing amount of information to im- the enzyme responsible for the formation of ET-plicate ET-1 as a potentially significant mediator in 1, endothelin-converting enzyme (ECE), is found inpulmonary diseases, in particular asthma and pul- mammalian lung.6,7

monary hypertension. A brief review will be given of ET is released from cultured airway epithelial cells;the evidence in support and at odds with ET-1 playing release is increased by endotoxin, thrombin and vari-a key role in the pathophysiology of lung diseases; ous cytokines.8–10 The metabolism of ET-1 in the lungparticular attention will be focused on the results is predominantly via epithelium-derived, phos-obtained using human cells and tissues. In addition, phoramidon-sensitive, neutral endopeptidase.11,12 Thesome key issues and questions related to our current lung is a major site for the clearance of ET-1,13,14

understanding of the effects of ET-1 in the lung will probably by a mechanism involving the ETB receptorbe discussed. subtype.14

ET receptors in the lungCRITERIA FOR A PATHOPHYSIOLOGICALLYRELEVANT MEDIATOR The diverse biological effects of ET-1 are mediated via

ET receptors, which are members of the superfamilyAs with any pathophysiologically relevant mediator, of G-protein-coupled, seven transmembrane-spanningseveral standard criteria must be fulfilled before receptors. Two subtypes of ET receptor, ETA and ETB,ET-1 can be considered unequivocally to contribute have been cloned, sequenced and characterized: ETA

to the pathogenesis of lung disorders: (1) pathways has higher affinity for ET-1 and ET-2 than ET-3,for the synthesis, release and metabolism of the whereas the ETB receptor has equivalent affinity forET-1 exist in the lung; (2) ET receptors are located the three ET ligands.15 ETA and ETB receptors arein relevant cells in the pulmonary system; (3) ET-1 found in mammalian airways.16,17 Binding and auto-mimics several of the features of the particular disease; radiographic studies in human lung indicate the pres-(4) ET-1 levels are elevated in patients with lung ence of binding sites for radiolabeled ET-1, primarilydisorders, with a correlation between levels and disease to airway and vascular smooth muscle, with someseverity; (5) drugs which inhibit the synthesis and/or associated with nerves; information about binding toantagonize the biological effects of the ET-1 have the epithelium is equivocal.16–19 In non-asthmatic lungtherapeutic benefit. A summary of the data related to the percentages of receptor subtypes are 88% ETB andthe fulfillment of these individual criteria is presented 12% ETA in bronchial smooth muscle,17 70% ETB and

30% ETA in peripheral lung,20 and 7% ETB and 93%below.

1094–5539/98/020141+06 $30.00/0 1998 Academic Press141

142 D. W. P. Hay

ETA in human pulmonary artery.21 The number and reason for this descrepancy between in vitro and inrelative proportions of ETA and ETB receptors in vivo studies is not known but may reflect regionalairway smooth muscle and peripheral lung from differences in the release and subsequent sensitivitypatients with asthma are not significantly different to ET-1.from control tissues.17,20 The effects of ETA receptoractivation include airway smooth muscle and pul-monary vascular smooth muscle contraction, airway Airways remodeling. Airway smooth muscle hyper-and vascular smooth muscle proliferation, mediator trophy and hyperplasia occurs in chronic asthma.release, inflammatory cell recruitment and po- Although in one study ET-1 produced a small (abouttentiation of nerve-induced responses, whereas for two-fold), albeit potent, stimulation of proliferationETB receptor stimulation they include airway smooth in human cultured bronchial smooth muscle cells,29

muscle and pulmonary smooth muscle contraction another report demonstrated no direct stimulation ofand potentiation of nerve-induced responses.16,22,23 The DNA synthesis in human tracheal smooth muscle cellsreceptor(s) mediating some of the effects of ET-1 in by ET-1.30 However, in the latter study ET-1 potentlythe lung has still to be elucidated. Although evidence potentiated the effects of the mitogen, epidermalfrom pharmacological studies has suggested the pres- growth factor.30 Both the direct and potentiating in-ence of novel ET receptor subtypes, including in fluences of ET-1 were due to stimulation of ETAhuman lung,24 caution should be exercised before receptors.29,30 Other effects of ET which support aproposing extension of the current ET receptor clas- potential impact on lung remodeling include its abilitysification, without information from structural, op- to stimulate proliferation of rat pulmonary arteryerational and molecular biological experiments. fibroblasts,31 and porcine tracheal epithelial cells,32

and to increase expression and release of fibronectinin human bronchial epithelial cells.33Mimicry of pulmonary diseases by ET-1

AsthmaRecruitment and activation of inflammatory cells. Thereis growing evidence that ET-1 activates inflammatoryAsthma is a common, chronic, inflammatory diseasecells and also elicits their recruitment into the lung. Forcharacterized by airways obstruction, airways in-example, ET-1 potently stimulated human monocytesflammation, airways hyperresponsiveness and lungand monocyte-derived macrophages to release TNF-remodeling. Parameters which contribute to these

features of asthma include: airway smooth muscle a, IL-1b and IL-6,34 and activated mast cells in thecontraction, mucus hypersecretion, epithelial cell dam- presence of IL-4.35 There are various reports of ET-1age, microvascular leakage and edema formation, increasing the release of inflammatory cell pro-airway smooth muscle hyperplasia and hypertrophy, ducts,36,37 although another study found no effect oncollagen deposition and thickening of the extracellular human peripheral blood monocyte activation, chemo-matrix including epithelial basement membrane, in- taxis or adhesion.38 In a mouse model of lung in-flammatory cell recruitment and activation, and dys- flammation antigen-induced influx of eosinophils andfunction in neuronal inputs. neutrophils in BAL was inhibited by the mixed ETA/

ETB receptor antagonist, SB 209670, and the ETA

receptor antagonist, BQ-123, suggesting an influenceAirway smooth muscle contraction. ET-1 is amongstof ET-1 on the recruitment of these cells via ETAthe most potent contractile agonists in human isolatedreceptor stimulation.39 In guinea-pigs sensitized toairway smooth muscle.16–18,22–26 The contraction is me-Ascaris suum, antigen challenge induced an increasediated predominantly by stimulation of ETB receptors,in immunoreactive ET (ir-ET) in bronchoalveolar17 although there is evidence for a contribution fromlavage fluid (BAL) which correlated, to some extent,non-ETB receptors, including ETA receptors.17,24,26 Thewith airway influx of eosinophils.40 In a rat model ofsensitivity of isolated bronchial smooth muscle fromairway inflammation, induced by exposure to Se-asthmatics to ET-1, and to the ETB receptor-selectivephadex beads, there was a marked increase in ir-ETagonist, sarafotoxin S6c,27 was less than that of tissuesin BAL, but not plasma, which correlated with levelsfrom non-asthmatics. This provides evidence that theof eosinophils and neutrophils; there was also anlarge airways are hyporesponsive to ETB receptorincrease in ET-like staining in airway epithelium.41stimulation, perhaps as a result of chronic exposureThe mixed ETA/ETB receptor antagonist, bosentan,to increased ET-1 release.17 In contrast, aerosol ad-inhibited the increase in eosinophils in this model.41ministration of ET-1 produced bronchoconstriction inHowever, in guinea pigs several studies have reportedasthmatic individuals, with little effect on pulmonarythat iv or aerosol administration of ET-1 did notfunction in non-asthmatic subjects, suggesting that

asthmatic airways are hyperresponsive to ET-1.28 The increase inflammatory cell influx into the lung.42–44

Mediator Role of ET-1 in Pulmonary Diseases 143

Neuromodulation. Dysfunction in neuronal inputs may ET-1 levels in lung disordersbe involved in the pathophysiology of lung disorders,including asthma and chronic obstructive pulmonary Asthmadisease (COPD). Binding and localization studies in-dicate that ET receptors are associated with nerves, Many studies have demonstrated increased expressionincluding ganglia, in mammalian lung.16,19,20,25 ET lig- and/or release of ET-1, measured in BAL, bronchialands potentiated cholinergic nerve-induced responses biopsies, peripheral blood and alveolar macro-in human bronchus, by a prejunctional rather than a phages,16,22,23,55–57 in patients with asthma comparedpostjunctional mechanism, involving stimulation of with asymptomatic asthmatic or non-asthmatic in-ETA and ETB receptors;45 similar effects have been dividuals. Immunohistochemical experiments suggestdemonstrated in other species.16 that the major cellular source of ET-1 is the airway

epithelium.55 However, other studies did not detect anincrease in ET-1 in induced sputum samples or inOther effects. Some studies have shown that ET-1peripheral blood of asthmatics vs. control in-mimics other pertinent features of asthma includingdividuals.58,59 An acute elevation in ir-ET in BAL wasoedema/microvascular permeability, mucus hyper-not observed after segmental bronchial challenge withsecretion and airway hyperresponsiveness. However,allergen.60 A reduction in BAL ir-ET levels was dem-other reports indicate no effect of ET-1, or even anonstrated in patients with nocturnal asthma.59

inhibition, in the case of mucus secretion.16,22,23,46

There is conflicting information about a correlationOverall, ET-1 mimics several of the features ofbetween ET levels and expression and disease severity,asthma, in particular bronchoconstriction, cell pro-assessed by measuring parameters such as bronchialliferation and modulation of nerve inputs. However,hyperresponsiveness, and percentage predicted FEV1.some of the evidence, especially with respect to otherCurrent therapies for asthma, such as b-adrenoceptorparameters, is equivocal.agonists and aerosol corticosteroids, decreased airwayET levels, concomitant with improvements in lungPulmonary hypertension and chronic obstructivefunction.56,61,62

pulmonary disease

Pulmonary vascular disease, including pulmonaryPulmonary hypertension and COPDhypertension, is a common, potentially serious com-

plication of COPD. Alveolar hypoxia, which produces An increase in plasma ET levels in patients, boththe two major features of pulmonary hypertension, adults and children, with pulmonary hypertension haspulmonary vasoconstriction and vascular remodeling, been demonstrated in several studies, with a cor-is a key contributing factor to the development and relation between levels and disease severity apparentmaintenance of this condition. ET-1 mimics these in some cases.16,23,63,64 There have been few studiestwo aspects of pulmonary hypertension in human investigating ET levels and expression in patients withpulmonary vessels in vitro: potent constriction via ETA COPD. In three reports there was no increase in BALand ETB receptors,25,47,48 and mitogenesis of human or in resected lung tissue from smokers with airflowpulmonary artery smooth muscle cells by an ETA obstruction,61,65,66 whereas in another study an eleva-receptor-mediated mechanism.49 The recruitment and tion in plasma ET levels was demonstrated in mildactivation of neutrophils and alveolar macrophages, COPD patients.67

and stimulation of mucus secretion,5,16,23,50–52 are othereffects of ET-1 which may be relevant to COPD.

Allergic rhinitis

Allergic rhinitis In one study an elevation in expression of ir-ET,and ir-ECE-1, was detected in patients with chronicir-ET is present in human nasal mucosa, including inrhinitis.68 Note, the expression levels of ECE in mostthe vascular endothelium and the serous cells in thelung diseases have not been reported, although such ansubmucosal glands.5 ET-1 increased secretions fromincrease was demonstrated in patients with idiopathicserous and mucous cells,5 and prostanoid release frompulmonary fibrosis.69

human cultured nasal mucosa.53 Intranasal ad-ministration of ET-1 increased nasal secretion weights,lysozyme secretion and symptoms of rhinorrhea, itch Other lung diseasesand sneezing in allergic and non-allergic volunteers.54

Increased amounts of ETs have been detected inThe effects of ET-1 on these parameters was moreseveral other lung diseases including: lung tumors,pronounced in allergic subjects, suggesting that al-emphysema, acute respiratory failure, idiopathic pul-lergic inflammation increases the sensitivity of the

upper airways to ET-1. monary fibrosis, interstitial lung disease, cryptogenic

144 D. W. P. Hay

fibrosing alveolitis, cystic fibrosis, acute respiratory or can simply be added to the list of substances withdistress syndrome, sarcoidosis, scleroderma- several effects in the pulmonary system.associated fibrotic lung disease.16,23 There remain key issues and questions regarding

A key question related to all of the studies dem- the role of ET-1 in the lung. For example, what is theonstrating an elevation in ET levels in pulmonary desired therapeutic profile with regard to the receptordiseases is whether these findings represent a ‘cause subtype selectivity (e.g. ETA receptor-selective, ETB

or effect’ phenomenon. This issue can only be sat- receptor-selective or a mixed ETA/ETB receptor an-isfactorily addressed by exploring the clinical effects tagonist?) for an optimally, clinically effective an-of drugs modifying the ET pathway (receptor ant- tagonist in pulmonary disorders, and will this profileagonists or ECE inhibitors). differ depending upon the specific disorder? In relation

to this, does ET-1 have some beneficial as well asdeleterious effects in the lung? It is noteworthy that

Therapeutic benefit in lung diseases of drugs which knockout mice deficient in ET-1 (heterozygous) hadblock the synthesis or effects of ET airway hyperreactivity to aerosol-delivered metha-

choline suggesting that a reduction in ET-1 levelsUnfortunately, there is no clinical information on theplays a role in this phenomenon; there was no changeeffects of ET receptor antagonists or ECE inhibitorsin the sensitivity to antigen.74 It will be of interest toin pulmonary disease. However, many potent andconduct similar studies with ET receptor knockoutselective members of the former class of compoundsanimals. In addition, the ETB receptor has been pro-have been characterized and the hope is that some ofposed to mediated ET-1-induced vasodilation,75 andthese will soon be evaluated clinically in pulmonaryto act as the clearance receptor in the lung,14 suggestingdiseases, including asthma, pulmonary hypertensiona potential beneficial influence of this ET receptorand COPD. There is a considerable amount of in-subtype. Are there novel ET receptors in the lung andformation indicating the effectiveness of ET receptorwhat are the effects of their activation? Furthermore,antagonists in animal models of lung diseases, inclarification is needed regarding the ET receptor sub-particular, asthma,39–41 and pulmonary hyper-type mediating some of the effects of ET-1 in the lung.tension.70–73 In some of the latter studies, several com-

pounds have been shown not only to prevent, butalso to reverse hypoxia-induced functional (pul-

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