Endothelin-1 induces bronchial myofibroblast differentiation

Endothelin-1 mediates Aspergillus fumigatus-induced airway inflammation and remodelling

BACKGROUND

Asthma is a chronic inflammatory condition of the airways and patients sensitized to airborne fungi such as Aspergillus fumigatus have more severe asthma. Thickening of the bronchial subepithelial layer is a contributing factor to asthma severity for which no current treatment exists. Airway epithelium acts as an initial defence barrier to inhaled spores, orchestrating an inflammatory response and contributing to subepithelial fibrosis.

OBJECTIVE

We aimed to analyse the production of pro-fibrogenic factors by airway epithelium in response to A fumigatus, in order to propose novel anti-fibrotic strategies for fungal-induced asthma.

METHODS

We assessed the induction of key pro-fibrogenic factors, TGF-β1, TGF-β2, periostin and endothelin-1, by human airway epithelial cells and in mice exposed to A fumigatus spores or secreted fungal factors.

RESULTS

Aspergillus fumigatus specifically caused production of endothelin-1 by epithelial cells in vitro but not any of the other pro-fibrogenic factors assessed. A fumigatus also induced endothelin-1 in murine lungs, associated with extensive inflammation and airway remodelling. Using a selective endothelin-1 receptor antagonist, we demonstrated for the first time that endothelin-1 drives many features of airway remodelling and inflammation elicited by A fumigatus.

CONCLUSION

Our findings are consistent with the hypothesis that elevated endothelin-1 levels contribute to subepithelial thickening and highlight this factor as a possible therapeutic target for difficult-to-treat fungal-induced asthma.

The myofibroblast, a key cell in normal and pathological tissue repair

Myofibroblasts are characterized by their expression of α-smooth muscle actin, their enhanced contractility when compared to normal fibroblasts and their increased synthetic activity of extracellular matrix proteins. Myofibroblasts play an important role in normal tissue repair processes, particularly in the skin where they were first described. During normal tissue repair, they appear transiently and are then lost via apoptosis. However, the chronic presence and continued activity of myofibroblasts characterize many fibrotic pathologies, in the skin and internal organs including the liver, kidney and lung. More recently, it has become clear that myofibroblasts also play a role in many types of cancer as stromal or cancer-associated myofibroblast. The fact that myofibroblasts are now known to be key players in many pathologies makes understanding their functions, origin and the regulation of their differentiation important to enable them to be regulated in normal physiology and targeted in fibrosis, scarring and cancer.

The profibrotic role of endothelin-1: is the door still open for the treatment of fibrotic diseases?

The endothelin (ET) system consists of two G-protein-coupled receptors (ETA and ETB), three peptide ligands (ET-1, ET-2 and ET-3), and two activating peptidases (endothelin-converting enzyme-, ECE-1 and ECE-2). While initially described as a vasoregulatory factor, shown to influence several cardiovascular diseases, from hypertension to heart failure, ET-1, the predominant form in most cells and tissues, has expanded its pathophysiological relevance by recent evidences implicating this factor in the regulation of fibrosis. In this article, we review the current knowledge of the role of ET-1 in the development of fibrosis, with particular focus on the regulation of its biosynthesis and the molecular mechanisms involved in its profibrotic actions. We summarize also the contribution of ET-1 to fibrotic disorders in several organs and tissues. The development and availability of specific ET receptor antagonists have greatly stimulated a number of clinical trials in these pathologies that unfortunately have so far given negative or inconclusive results. This review finally discusses the circumstances underlying these disappointing results, as well as provides basic and clinical researchers with arguments to keep exploring the complex physiology of ET-1 and its therapeutic potential in the process of fibrosis.

Extracellular matrix remodelling properties of human fibrocytes

The fibrocytes are thought to serve as a source of newly deposited collagens I and III during reparative processes and in certain fibrotic disorders, but their matrix remodelling properties are incompletely understood. We evaluated their ability to produce several extracellular matrix (ECM) components, in comparison with fibroblasts, and to participate in collagen turnover. The collagen gene expression profile of fibrocytes differed from that of fibroblasts because fibrocytes constitutively expressed relatively high levels of the mRNA encoding collagen VI and significantly lower levels of the mRNA encoding collagens I, III and V. The proteoglycan (PG) gene expression profile was also different in fibrocytes and fibroblasts because fibrocytes constitutively expressed the mRNA encoding perlecan and versican at relatively high levels and the mRNA encoding biglycan and decorin at low and very low levels, respectively. Moreover, fibrocytes expressed the mRNA for hyaluronan synthase 2 at higher level than fibroblasts. Significant differences between the two cell populations were also demonstrated by metabolic labelling and analysis of the secreted collagenous proteins, PGs and hyaluronan. Fibrocytes constitutively expressed the scavenger receptors CD163 and CD204 as well as the mannose receptors CD206 and Endo180, and internalized and degraded collagen fragments through an Endo180-mediated mechanism. The results of this study demonstrate that human fibrocytes exhibit ECM remodelling properties previously unexplored, including the ability to participate in collagen turnover. The observed differences in collagen and PG expression profile between fibrocytes and fibroblasts suggest that fibrocytes may predominantly have a matrix-stabilizing function.

Endothelin in pulmonary fibrosis

The endothelin receptor-ligand system includes a family of polypeptides and G-protein-coupled receptors, which, in addition to their classic activity in the regulation of vascular tone (both directly and through the control of nitric oxide), were implicated in a wide variety of other key biological processes. In this regard, the endothelins are potent mitogens and motogens for mesenchymal cells, and can induce cell differentiation, increasing both the synthesis and deposition of extracellular matrix components and contractile ability. The endothelins are produced as inactive pre-pro-polypeptides, with gene transcription (as well as the proteolytic processing to mature active forms) under the influence of many factors, including cytokines, hypoxia, biomechanical and shear stress, pathogen products, and many growth factors. These complex regulatory events underlie the association and potential role of endothelins in a number of human diseases affecting many different target organs, including the vasculature (atherosclerosis and hypertension), kidney (renal crisis and chronic kidney disease), heart (coronary heart disease), and lungs (pulmonary fibrosis and pulmonary hypertension). This review focuses on the biochemistry of endothelin and the pathobiology of endothelin in lung fibrosis, with particular emphasis on idiopathic pulmonary fibrosis, and examines the antifibrotic potential of endothelin receptor antagonism.

IPF: new insight on pathogenesis and treatment

Recent years have seen a robust influx of exciting new observations regarding the mechanisms that regulate the initiation and progression of pulmonary fibrosis but the pathogenesis remains poorly understood. The search for an alternative hypothesis to unremitting, chronic inflammation as the primary explanation for the pathophysiology of idiopathic pulmonary fibrosis (IPF) derives, in part, from the lack of therapeutic efficacy of high-dose immunosuppressive therapy in patients with IPF. The inflammatory hypothesis of IPF has since been challenged by the epithelial injury hypothesis, in which fibrosis is believed to result from epithelial injury, activation, and/or apoptosis with abnormal wound healing. This hypothesis suggests that recurrent unknown injury to distal pulmonary parenchyma causes repeated epithelial injury and apoptosis. The resultant loss of alveolar epithelium exposes the underlying basement membrane to oxidative damage and degradation. Emerging concepts suggest that IPF is the result of epithelial-mesenchymal interaction. The initiation of this fibrotic response may depend upon genetic factors and environmental triggers; the role of Th1 or Th2 cell-derived cytokines may also be important. This process appears to be unique to usual interstitial pneumonia/IPF. It is clear that IPF is a heterogeneous disease with variations in pathology, high-resolution computed tomography findings, and patterns of progression. Idiopathic pulmonary fibrosis is a complex disorder, and no unifying hypothesis has been identified at present that explains all the abnormalities.

Gastrin-releasing peptide, immune responses, and lung disease

Gastrin-releasing peptide (GRP) is produced by pulmonary neuroendocrine cells (PNECs), with highest numbers of GRP-positive cells present in fetal lung. Normally GRP-positive PNECs are relatively infrequent after birth, but PNEC hyperplasia is frequently associated with chronic lung diseases. To address the hypothesis that GRP mediates chronic lung injury, we present the cumulative evidence implicating GRP in bronchopulmonary dysplasia (BPD), the chronic lung disease of premature infants who survive acute respiratory distress syndrome. The availability of well-characterized animal models of BPD was a critical tool for demonstrating that GRP plays a direct role in the early pathogenesis of this disease. Potential mechanisms by which GRP contributes to injury are analyzed, with the main focus on innate immunity. Autoreactive T cells may contribute to lung injury late in the course of disease. A working model is proposed with GRP triggering multiple cell types in both the innate and adaptive immune systems, promoting cascades culminating in chronic lung disease. These observations represent a paradigm shift in the understanding of the early pathogenesis of BPD, and suggest that GRP blockade could be a novel treatment to prevent this lung disease in premature infants.

Augmented epithelial endothelin-1 expression in refractory asthma

BACKGROUND

Airway remodeling in patients with severe steroid-refractory asthma might result from a reduced ability of steroid therapy to limit the transcription of remodeling factors by the bronchial epithelium.

OBJECTIVE

We sought to compare the levels of transcripts encoding remodeling factors in bronchial epithelium of healthy volunteers and of asthmatic patients with either steroid-sensitive or steroid-refractory disease and to correlate these levels with hallmarks of airway remodeling.

METHODS

By means of real-time quantitative PCR, we assessed the levels of 14 transcripts encoding remodeling factors, matrix metalloproteinases, and extracellular matrix proteins in laser-capture microdissected bronchial epithelium of healthy volunteers, patients with mild steroid-untreated asthma, and patients with steroid-sensitive and steroid-refractory asthma (n = 8-10 in each group). Histologic features of airway remodeling and endothelin-1 (EDN1) immunolocalization were determined by using frozen specimens.

RESULTS

Patients with steroid-refractory asthma had greater levels of EDN1 transcripts (4.1-fold increase, P = .026) and protein (P = .0009) in their bronchial epithelium compared with patients with steroid-sensitive asthma. EDN1 mRNA levels and protein expression in asthmatic patients were negatively correlated with prebronchodilator and postbronchodilator FEV(1) value (r(2) >or= 0.193, P <or= .03), and they were positively related to airway smooth muscle areas (r(2) = 0.253, P = .01 and r(2) = 0.281, P = .005 for EDN1 mRNA and protein expression, respectively).

CONCLUSION

Increased EDN1 synthesis by the bronchial epithelium characterizes severe refractory asthma and correlates with airway remodeling and airflow obstruction.

CLINICAL IMPLICATIONS

Targeting EDN1 might represent a novel therapeutic strategy for severe steroid-refractory asthma.

Leukotriene D4 enhances the function of endothelin-1-primed fibroblasts

Airway inflammation is accompanied by structural changes, termed remodeling, that lead to lung dysfunction over the long term. Although both endothelin-1 (ET-1) and cysteinyl leukotrienes (CysLTs) appear to be involved in airway remodeling in several lung diseases, how these molecules interact remains largely unknown. In this study, we examined the effects of leukotriene (LT) D(4) on the function of ET-1-primed fibroblasts. ET-1 at 10(-7) M up-regulated the expression of the CysLT receptors at both the mRNA and protein levels in human lung fibroblasts. LTD(4) enhanced matrix metalloproteinase-2 and pro-collagen production, and alpha-smooth muscle actin expression of ET-1-primed fibroblasts, but had little or no effect on unprimed fibroblasts. The CysLT1 receptor antagonist montelukast completely abrogated the effects of LTD(4). Our data suggested that LTD(4) may act as a precipitating factor during ET-1-mediated airway remodeling and that CysLT1 receptor antagonists may have a role in preventing aberrant extracellular matrix degradation.

In vitro wounding: effects of hypoxia and transforming growth factor β1 on proliferation, migration and myofibroblastic differentiation in an endothelial cell-fibroblast co-culture model

The adequate reconstitution of human soft tissue wounds requires the coordinated interaction of endothelial cells and fibroblasts during the proliferation phase of healing. Endothelial cells assure neoangiogenesis, fibroblasts fill the defect and provide extracellular matrix proteins, and myofibroblasts are believed to support the reconstitution of microvessels. In the present study, we combined in vitro-wound size measurement and multicolour immunocytochemical staining of co-cultured human dermal microvascular endothelial cells and normal human dermal fibroblasts, recently introduced as co-culture scratch-wound migration assay. Applying antibodies for alpha-smooth-muscle actin, von Willebrand factor, extra domain A fibronectin and endothelin-1, we were able to monitor proliferation, migration and the differentiation process from fibroblasts to myofibroblasts as a response to hypoxia. Furthermore, we verified, whether transforming growth factor beta1 (TGFbeta1) and endothelin-1 are able to mediate this response. We show, that proliferation and migration of endothelial cells and fibroblasts decreased under hypoxia. The additional administration of TGFbeta1 did not significantly attenuate this decrease. Solely the myofibroblast population in co-culture adapted well to hypoxia, when cultures were supplemented with TGFbeta1. Considerating the data concerning TGFbeta1 and endothelin-1, we propose a model explaining the cellular interaction during early and late proliferation phase of human wound healing.

Fibrose pulmonar idiopática: uma década de progressos

Embora diagnósticos de fibrose pulmonar idiopática continuem sendo devastadores, avanços recentes têm melhorado nossa compreensão a respeito de muitas das facetas desta doença. Estas descobertas, juntamente com o aumento da disponibilidade geral de ensaios terapêuticos, encerram a promessa de um futuro mais promissor para pacientes com fibrose pulmonar idiopática. Por exemplo, nós temos agora uma compreensão mais abrangente a respeito dos critérios diagnósticos e da história natural da doença. Vários estudos têm mostrado que a mensuração simples da fisiologia pulmonar ou troca gasosa pode ser usada para prever a sobrevida do paciente. Através da identificação de várias vias moleculares que têm papéis importantes na patogênese da fibrose pulmonar idiopática, os pesquisadores têm produzido uma lista crescente de possíveis novos alvos terapêuticos para a doença. Vários ensaios terapêuticos prospectivos e controlados têm sido realizados. Outros estão em andamento ou ainda estão em fase de planejamento. Estes esforços têm avançado nosso conhecimento atual sobre fibrose pulmonar idiopática e levantado novas questões importantes, assim como têm gerado o interesse e o impulso necessários para avançar terreno na luta contra esta doença desafiadora. Este artigo oferece ao leitor um panorama dos avanços recentes nas pesquisas sobre fibrose pulmonar idiopática, tendo como foco a história natural, patogênese e tratamento.

Therapeutic Management of Idiopathic Pulmonary Fibrosis: An Evidence-Based Approach

The treatment of idiopathic pulmonary fibrosis (IPF) remains controversial. The benefits of conventional treatment with corticosteroids plus either azathioprine or cyclophosphamide have not been established in randomized, controlled trials. Other treatment strategies have been suggested as understanding of the pathogenesis of IPF has increased. Recent clinical and animal studies suggest that modulating the effects of profibrotic growth factors and cytokines holds significant promise. As additional well controlled prospective studies are completed, these trials should provide the clinical evidence necessary for identifying optimal treatment strategies for future patients who have IPF.

Idiopathic Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive, and usually fatal pulmonary disease for which there are no proven drug therapies. Anti-inflammatory and immunosuppressive agents have been largely ineffective. The precise relationship of IPF to other idiopathic interstitial pneumonias (IIPs) is not known, despite the observation that different histopathologic patterns of IIP may coexist in the same patient. We propose that these different histopathologic 'reaction' patterns may be determined by complex interactions between host and environmental factors that alter the local alveolar milieu. Recent paradigms in IPF pathogenesis have focused on dysregulated epithelial-mesenchymal interactions, an imbalance in T(H)1/T(H)2 cytokine profile and potential roles for aberrant angiogenesis. In this review, we discuss these evolving concepts in disease pathogenesis and emerging therapies designed to target pro-fibrogenic pathways in IPF.

Medical treatment for pulmonary fibrosis: current trends, concepts, and prospects

A diagnosis of idiopathic pulmonary fibrosis (IPF) carries a poor prognosis, with our currently available therapies offering little clinical benefit. Unfortunately, recent major advances in our understanding of the clinical and biologic features of this disease have not been matched by similar advances in treatment. This is likely because of the complex cascade of biologic and pathobiologic events that occurs in IPF. The necessary, and desperately needed, next generation of therapies, focused on specific molecular targets thought to play pivotal roles in the development and progression of fibrosis, are under active investigation.

Leukotriene D4 stimulates collagen production from myofibroblasts transformed by TGF-beta

BACKGROUND

Airway remodeling has an important role in the pathogenesis of bronchial asthma. Many mediators that influence the pathophysiology of bronchial asthma, especially cysteinyl leukotrienes (CysLTs) and TGF-beta1, are involved in airway remodeling.

OBJECTIVE

To know whether TGF-beta1 alters fibroblast responsiveness to CysLTs, we examined the effects of leukotriene (LT) D4 on collagen production from fibroblasts and from myofibroblasts transformed by TGF-beta1. We also examined whether TGF-beta1 upregulates CysLT1 receptor (CysLT1R) expression in fibroblasts.

METHODS

Concentrations of procollagen in the human fetal lung fibroblast (HFL) 1 cell supernatant were measured by using an enzyme immunoassay kit in the presence or absence of various concentrations of LTD4, TGF-beta1, CysLT1R antagonist, or some combination of these. The mRNA expression of CysLT1R and alpha-smooth muscle actin as a marker of myofibroblasts was measured by means of real-time PCR. Furthermore, protein expression of CysLT1R on fibroblasts was measured by means of flow cytometric analysis.

RESULTS

TGF-beta1 stimulated collagen production from HFL-1 cells, but LTD4 alone did not. LTD4 in combination with TGF-beta1 increased collagen production compared with TGF-beta1 alone. Real-time PCR showed that stimulation with TGF-beta1 significantly upregulated CysLT1R and alpha-smooth muscle actin mRNA expression in HFL-1 cells.

CONCLUSIONS

LTD4 increased collagen production by upregulating CysLT1R induced by TGF-beta1. In the TGF-beta-rich milieu, activated myofibroblasts expressing CysLT1R can respond to CysLTs and produce large amounts of extracellular matrix, thereby contributing to airway remodeling. These data suggest that treatment with leukotriene receptor antagonists might prevent airway remodeling in patients with asthma.

The remodelled tracheal basement membrane zone of infant rhesus monkeys after 6 months of recovery

BACKGROUND

In previous studies, we showed that repeated exposure to (1) house dust mite allergen (HDMA) (Dermatophagoides farinae) caused thickening of the basement membrane zone (BMZ) and (2) HDMA+ozone (O3) caused depletion of BMZ perlecan and atypical development of BMZ collagen (irregular thin areas<2.0 microm in width).

OBJECTIVE

The purpose of this study was to determine if these remodelling changes were reversible after 6 months of recovery.

METHODS

Rhesus monkeys were exposed to a regimen of HDMA and or O3 or filtered air (FA) for 6 months. After the exposure protocol was completed FA and O3 groups were allowed to recover in FA for 6 months. The HDMA and HDMA+O3 exposure groups recovered in a modified environment. They were re-exposed to HDMA aerosol for 2 h at monthly intervals during recovery in order to maintain sensitization for pulmonary function testing. To detect structural changes in the BMZ, collagen I and perlecan immunoreactivity were measured and compared to data from the previous papers.

RESULTS

The remodelled HDMA group had a significantly thicker BMZ and after 6 months of recovery the width had not regressed. In the remodelled BMZ of the HDMA+O3 group, perlecan had returned to the BMZ after 6 months of the recovery protocol, and the thin, irregular, collagen BMZ had been resolved.

CONCLUSION

In summary, this study has shown that: (1) The width of the remodelled HDMA BMZ did not regress during a recovery protocol that included a sensitizing dose of HDMA. (2) The atypical collagen BMZ in the HDMA+O3 BMZ was resolved in the absence of O3. (3) Depletion of perlecan from the BMZ by O3 was reversed by recovery in the absence of O3.

Dog peritoneal and pleural cavities as bioreactors to grow autologous vascular grafts

OBJECTIVE

The purpose of this study was to grow "artificial blood vessels" for autologous transplantation as arterial interposition grafts in a large animal model (dog).

METHOD AND RESULTS

Tubing up to 250 mm long, either bare or wrapped in biodegradable polyglycolic acid (Dexon) or nonbiodegradable polypropylene (Prolene) mesh, was inserted in the peritoneal or pleural cavity of dogs, using minimally invasive techniques, and tethered at one end to the wall with a loose suture. After 3 weeks the tubes and their tissue capsules were harvested, and the inert tubing was discarded. The wall of living tissue was uniformly 1-1.5 mm thick throughout its length, and consisted of multiple layers of myofibroblasts and matrix overlaid with a single layer of mesothelium. The myofibroblasts stained for alpha-smooth muscle actin, vimentin, and desmin. The bursting strength of tissue tubes with no biodegradable mesh scaffolds was in excess of 2500 mm Hg, and the suture holding strength was 11.5 N, both similar to that in dog carotid and femoral arteries. Eleven tissue tubes were transplanted as interposition grafts into the femoral artery of the same dog in which they were grown, and were harvested after 3 to 6.5 months. Eight remained patent during this time. At harvest, their lumens were lined with endothelium-like cells, and wall cells stained for alpha-actin, smooth muscle myosin, desmin and smoothelin; there was also a thick "adventitia" containing vasa vasorum.

CONCLUSION

Peritoneal and pleural cavities of large animals can function as bioreactors to grow myofibroblast tubes for use as autologous vascular grafts.

Endothelin-1 promotes osteoprogenitor proliferation and differentiation in fetal rat calvarial cell cultures

Endothelin-1 (ET-1), a peptide produced by vascular endothelial cells, has been suggested to be one of the signaling factors between vascular and osteoblastic cells during bone growth and remodeling. The osteoinductive effects of ET-1 were tested on fetal rat calvaria which have the ability to form bone nodules in culture. ET-1 (10(-10) to 10(-6) M) dose-dependently increased cell proliferation. The effect of ET-1 (10(-8) M) on proliferation was greater than that of dexamethasone (Dex; 10(-8) M). ET-1 also increased the number of bone nodules by 146% over untreated cells, which coincided with a 3.1-fold increase in alkaline phosphatase activity. Limiting dilution assays showed that ET-1 treatment increased the number of osteoprogenitors (CFU-AP and CFU-OB) beyond what would be expected by a proliferative effect alone, indicating that ET-1 also stimulated osteoblast differentiation. Osteocalcin mRNA expression was upregulated as shown by Northern blot analysis. Using cDNA microarray analysis, ET-1 treatment resulted in an expression profile that included an upregulation of 163 genes and expressed sequence tags. Simultaneous addition of ET-1 and Dex to the medium further increased the number of bone nodules and alkaline phosphatase activity over either treatment alone. Our results show that ET-1 promotes both osteoblastic proliferation and differentiation and that the effects of ET-1 and Dex on differentiation are cooperative.

Role of tissue stroma in cancer cell invasion

Maintenance of epithelial tissues needs the stroma. When the epithelium changes, the stroma inevitably follows. In cancer, changes in the stroma drive invasion and metastasis, the hallmarks of malignancy. Stromal changes at the invasion front include the appearance of myofibroblasts, cells sharing characteristics with fibroblasts and smooth muscle cells. The main precursors of myofibroblasts are fibroblasts. The transdifferentiation of fibroblasts into myofibroblasts is modulated by cancer cell-derived cytokines, such as transforming growth factor-beta (TGF-beta). TGF-beta causes cancer progression through paracrine and autocrine effects. Paracrine effects of TGF-beta implicate stimulation of angiogenesis, escape from immunosurveillance and recruitment of myofibroblasts. Autocrine effects of TGF-beta in cancer cells with a functional TGF-beta receptor complex may be caused by a convergence between TGF-beta signalling and beta-catenin or activating Ras mutations. Experimental and clinical observations indicate that myofibroblasts produce pro-invasive signals. Such signals may also be implicated in cancer pain. N-Cadherin and its soluble form act as invasion-promoters. N-Cadherin is expressed in invasive cancer cells and in host cells such as myofibroblasts, neurons, smooth muscle cells, and endothelial cells. N-Cadherin-dependent heterotypic contacts may promote matrix invasion, perineural invasion, muscular invasion, and transendothelial migration; the extracellular, the juxtamembrane and the beta-catenin binding domain of N-cadherin are implicated in positive invasion signalling pathways. A better understanding of stromal contributions to cancer progression will likely increase our awareness of the importance of the combinatorial signals that support and promote growth, dedifferentiation, invasion, and ectopic survival and eventually result in the identification of new therapeutics targeting the stroma.

Identification of circulating fibrocytes as precursors of bronchial myofibroblasts in asthma

The mechanisms contributing to airway wall remodeling in asthma are under investigation to identify appropriate therapeutic targets. Bronchial myofibroblasts would represent an important target because they play a crucial role in the genesis of subepithelial fibrosis, a characteristic feature of the remodeling process, but their origin is poorly understood. We hypothesized that they originate from fibrocytes, circulating cells with the unique characteristic of expressing the hemopoietic stem cell Ag CD34 and collagen I. In this study we show that allergen exposure induces the accumulation of fibrocyte-like cells in the bronchial mucosa of patients with allergic asthma. These cells are CD34-positive; express collagen I and alpha-smooth muscle actin, a marker of myofibroblasts; and localize to areas of collagen deposition below the epithelium. By tracking labeled circulating fibrocytes in a mouse model of allergic asthma, we provide evidence that fibrocytes are indeed recruited into the bronchial tissue following allergen exposure and differentiate into myofibroblasts. We also show that human circulating fibrocytes acquire the myofibroblast phenotype under in vitro stimulation with fibrogenic cytokines that are produced in exaggerated quantities in asthmatic airways. These results indicate that circulating fibrocytes may function as myofibroblast precursors and may contribute to the genesis of subepithelial fibrosis in asthma.

Role of endothelin-1 in lung disease

Endothelin-1 (ET-1) is a 21 amino acid peptide with diverse biological activity that has been implicated in numerous diseases. ET-1 is a potent mitogen regulator of smooth muscle tone, and inflammatory mediator that may play a key role in diseases of the airways, pulmonary circulation, and inflammatory lung diseases, both acute and chronic. This review will focus on the biology of ET-1 and its role in lung disease.

Interaction of mite allergens Der p3 and Der p9 with protease-activated receptor-2 expressed by lung epithelial cells

The respiratory epithelium represents the first barrier encountered by airborne Ags. Two major dust mite Ags, Der p3 and Der p9, are serine proteases that may activate lung epithelial cells by interaction with the protease-activated receptor 2 (PAR-2). In this study both Der p3 and Der p9 cleaved the peptide corresponding to the N terminus of PAR-2 at the activation site. Both Ags sequentially stimulated phosphoinositide hydrolysis, transient cytosolic Ca(2+) mobilization, and release of GM-CSF and eotaxin in human pulmonary epithelial cells. These responses were similar to those observed with trypsin and a specific PAR-2 agonist and were related to the serine protease activity of Der p3 and Der p9. Cell exposure to the Ags resulted in a refractory period, indicating that a PAR had been cleaved. Partial desensitization to Der p3 and Der p9 by the PAR-2 agonist suggested that PAR-2 was one target of the Ags. However, PAR-2 was not the only target, because the PAR-2 agonist caused less desensitization to Der p3 and Der p9 than did trypsin. A phospholipase C inhibitor prevented the cytokine-releasing effect of the PAR-2 agonist and abolished or reduced (>70%) the cytokine-releasing effects of Der p3 and Der p9. Our results suggest that Der p 3 and Der p9 may induce a nonallergic inflammatory response in the airways through the release of proinflammatory cytokines from the bronchial epithelium and that this effect is at least in part mediated by PAR-2.

Airway remodeling in the pathogenesis of asthma

Asthma is characterized by a chronic inflammatory process of the airways followed by healing, the end result of which is an altered structure referred to as airway remodeling. Although the mechanisms responsible for such structural alterations appear to be heterogeneous, it is likely that abnormal airway cell dedifferentiation, migration, and redifferentiation, together with changes in connective tissue deposition, contribute to the altered restitution of airway structure and function. This altered restitution is often seen as fibrosis and increased smooth muscle, mucus gland mass, and vessel area. As a consequence of these structural changes, the airway wall in asthma is usually characterized by increased thickness and markedly and permanently reduced airway caliber. These features may result in increased airflow resistance, particularly when there is bronchial contraction and bronchial hyperresponsiveness. The effect on airflow is compounded by increased mucus secretion and inflammatory exudate, which not only block the airway passages but also cause increased surface tension favoring airway closure.

Triggering the induction of myofibroblast and fibrogenesis by airway epithelial shedding

Myofibroblasts have been thought to participate in subepithelial fibrosis in asthma, but the mechanism of myofibroblast induction has not been fully understood. In this study we investigated injury-related myofibroblast induction in a coculture system of guinea-pig epithelial cells and fibroblasts cocultured in a human amnion chamber. After pseudostratified epithelial cells were mechanically scraped, migrated flat epithelial cells differentiated into cuboidal appearances on Day 4 and then returned to their original shapes on Day 8. During the course of the epithelial redifferentiation, it was found by Northern blot analysis, immunohistochemistry for alpha-smooth muscle actin, and electron microscopic observation that the myofibroblasts were transiently induced on Day 4. The myofibroblast induction was inhibited by the blocking of transforming growth factor (TGF)-beta1 and thrombospondin (TSP)-1, indicating that the activation of TGF-beta1 by TSP-1 would induce myofibroblasts. This finding was also supported by a transient upregulation of TSP immunoreactivity and TSP-1 messenger RNA (mRNA) in fibroblasts. Interestingly, epithelial injury reduced TGF-beta1 immunoreactivity in the amnion membrane but did not affect TGF-beta1 mRNA in epithelial cells and fibroblasts, indicating that TGF-beta1 supplied from the extracellular matrix can participate in myofibroblast induction. Concurrently with myofibroblast induction, procollagen type I and III mRNAs were upregulated in fibroblasts, and obvious collagen deposition was observed ultrastructurally around the myofibroblasts compared with the fibroblasts. These results indicate that induced myofibroblasts can be functionally more active in producing collagen than are resting fibroblasts. The present study suggests that epithelial injury stimulates TGF-beta1 release from the extracellular matrix and its activation via TSP-1 production, causing collagen synthesis through myofibroblast induction.

Tissue remodeling as a feature of persistent asthma

A chronic inflammatory process is almost invariably associated with tissue damage and healing. Healing results in repair and replacement of dead or damaged cells by viable cells. Repair usually involves 2 distinct processes: regeneration, which is the replacement of injured tissue by parenchymal cells of the same type, and replacement by connective tissue and its eventual maturation into scar tissue. In many instances both processes contribute to the healing response. Chronic inflammatory disease can therefore lead to a wide variety of consequences, from complete or partial restitution of organ structure and function to fibrosis. Asthma is characterized by a chronic inflammatory process of the airways. The ensuing healing process results in structural alterations referred to as a remodeling of the airways. The mechanisms underlying these structural alterations are still largely unknown. They are likely to be heterogeneous, leading-through the highly dynamic process of cell de-differentiation, migration, differentiation, and maturation-to changes in connective tissue deposition and to the altered restitution of airways structure, resulting in mucus gland hyperplasia, neovascularization, fibrosis, and an increase in smooth muscle mass.

cDNA-RDA of genes expressed in fetal and adult lungs identifies factors important in development and function

The identification of genetic factors important in lung development and function will help in understanding the underlying molecular mechanisms of respiratory disease. Representational difference analysis of cDNA (cDNA-RDA) is a PCR-based subtractive enrichment procedure for the isolation of differentially expressed genes. We performed cDNA-RDA and isolated genes expressed more abundantly in fetal and adult lungs. Fifty-four clones potentially representing genes with higher transcript levels in the fetal lung were sequenced. Sequence similarity searches indicated that these clones included 12 known genes, a discoidin-like domain-containing gene, six expressed sequence tags (ESTs), and one novel sequence. Fifty-six clones potentially representing genes expressed more abundantly in the adult lung were also cloned and sequenced. Of these, 16 known human genes were represented along with two sequences significantly similar to known mouse genes and two novel sequences. Several of these known genes are implicated in stress response and lung protection. Thus cDNA-RDA was successfully used to isolate known and novel differentially expressed genes, which putatively play an important role in human lung development.

The N-cadherin/catenin complex in colon fibroblasts and myofibroblasts

Fibroblasts and myofibroblasts were isolated respectively from normal colon mucosa and from colon cancers. Immunostaining with an antibody against alpha-smooth muscle actin (alpha-SMA) of the tissues of origin and of early passage cultures showed equal proportions of alpha-SMA positive myofibroblasts in vivo as in vitro. Immunocytochemistry, immunoprecipitation of metabolically labelled cells followed by Western blotting and RT-PCR of RNA isolates demonstrated the presence of a N-cadherin/catenin complex in both fibroblasts and myofibroblasts. This complex was found preferentially at the cell cell boundaries. Immunocytochemistry and, to a lesser extent, co-immunoprecipitation indicated partial colocalisation of catenins and alpha-SMA. Transforming growth factor beta1 (TGF-beta1) greatly enhanced the expression of alpha-SMA, but left the N-cadherin/catenin complex unaltered. We speculate that the N-cadherin/catenin complex may have different functions in myofibroblasts than in fibroblasts because of its interaction with alpha-SMA.

Endothelins and asthma

In the decade since endothelin-1 (ET-1) and related endogenous peptides were first identified as vascular endothelium-derived spasmogens, with potential pathophysiological roles in vascular diseases, there has been a significant accumulation of evidence pointing to mediator roles in obstructive respiratory diseases such as asthma. Critical pieces of evidence for this concept include the fact that ET-1 is an extremely potent spasmogen in human and animal airway smooth muscle and that it is synthesised in and released from the bronchial epithelium. Importantly, symptomatic asthma involves a marked enhancement of these processes, whereas asthmatics treated with anti-inflammatory glucocorticoids exhibit reductions in these previously elevated indices. Despite this profile, a causal link between ET-1 and asthma has not been definitively established. This review attempts to bring together some of the evidence suggesting the potential mediator roles for ET-1 in this disease.

Expression of sarcomeric proteins and assembly of myofibrils in the putative myofibroblast cell line BHK-21/C13

The expression and organization patterns of several myofibrillar proteins were analysed in the putative myofibroblast cell line BHK-21/C13. Although this cell line originates from renal tissue, the majority of the cells express titin. In these cells, titin is, under standard culture conditions, detected in myofibril-like structures (MLSs), where it alternates with non-muscle myosin (NMM). Expression of sarcomeric myosin heavy chain (sMyHC) is observed in a small minority of cells, while other sarcomeric proteins, such as nebulin, myosin binding protein C (MyBP-C), myomesin and M-protein are not expressed at all. By changing the culture conditions in a way equal to conditions that induce differentiation of skeletal muscle cells, a process reminiscent of sarcomerogenesis in vitro is induced. Within one day after the switch to a low-nutrition medium, myofibrillar proteins can be detected in a subset of cells, and after two to five days, all myofibrillar proteins examined are organized in typical sarcomeric patterns. Frequently, cross-striations are visible with phase contrast optics. Transfection of these cells with truncated myomesin fragments showed that a specific part of the myomesin molecule, known to contain a titin-binding site, binds to MLSs, whereas other parts do not. These results demonstrate that this cell line could serve as a powerful model to study the assembly of myofibrils. At the same time, its transfectability offers an invaluable tool for in vivo studies concerning binding properties of sarcomeric proteins.