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American Journal of Respiratory Cell and Molecular Biology. Vol. 37, pp. 1-2, 2007
© 2007 American Thoracic Society
DOI: 10.1165/rcmb.2007-0001ED

Editorial

Endothelin-1 as Initiator of Epithelial–Mesenchymal Transition

Potential New Role for Endothelin-1 during Pulmonary Fibrosis

Kevin K. Kim and Harold A. Chapman

Department of Medicine and The Cardiovascular Research Institute, University of California, San Francisco, California

Despite years of research into the pathogenesis of pulmonary fibrosis and several clinical trials, Idiopathic Pulmonary Fibrosis (IPF) continues to be a devastating disease without good therapy (1). This is in part due to an incomplete understanding of the mechanisms involved during lung fibrogenesis. Recent attention has focused on factors leading to fibroblast proliferation, activation, and transdifferentiation into myofibroblasts (2). The epithelium is also a likely contributor to this process. Signals from the epithelium are vital for mesenchymal expansion during lung development. Under repair conditions, lung epithelial cells are sites of expression and activation of several pro-fibrotic factors, including TGF-beta1, connective tissue growth factor (CTGF), and platelet-derived growth factor (PDGF) (3). Epithelial cells are also known to either undergo apoptosis or proliferate during fibrogenesis. The consensus of current thinking is that loss of epithelial cell integrity, whether functionally or by apoptosis, and recovery of lining layer integrity by epithelial proliferation and repopulation of alveolar surfaces, are key determinants of the degree of injury and efficiency of repair, respectively (2). Recently, there have been hints of additional mechanisms of epithelial involvement during lung repair.

Some epithelial cells can exert their own plasticity during tissue repair, taking on fibroblast-like features, a process termed epithelial–mesenchymal transition (EMT). EMT involves epithelial cell deaggregation, detachment from the basement membrane, cytoskeletal rearrangement, and commonly invasion of provisional matrices at sites of injury (4, 5). These phenotypic changes require alterations in gene expression, with loss of epithelial-specific proteins and up-regulation of mesenchymal markers. It is important to note that EMT is a process and not a single event, meaning multiple steps are involved, and that the process is commonly reversible. Amelioration of the driving signals for EMT may lead to apoptosis of the altered cells or their reversion to an epithelial phenotype (6). EMT has been well established as an important pathway during embryonic development and carcinogenesis (4, 5). A role for EMT in tissue fibrosis has only recently been described and suggests that in response to injury epithelial cells themselves acquire the ability to migrate and deposit extracellular matrix proteins characteristic of pathologic tissue fibrosis (7). In organs with a predominance of epithelial cells, such as the kidney and lung, EMT could ultimately contribute a significant proportion of the fibrogenic cells, though this hypothesis is far from settled.

Accumulating evidence supports EMT as occurring during pulmonary fibrosis. There is considerable overlap between signaling pathways thought to be important to EMT and to IPF. A number of factors, such as TGF-beta1, TNF-{alpha}, and PDGF, are activated in IPF (2, 3) and have been shown to promote EMT in some systems (8). As discussed by Jain and colleagues (9) in this issue of the AJRCMB (pp. 38–47), endothelin-1 (ET-1) can be added to this list. Thus, the microenvironment of the IPF lung seems conducive for EMT. The direct lines of evidence supporting EMT during pulmonary fibrosis include: IPF lung biopsies demonstrating epithelial cells with some mesenchymal features (10, 11), the ability of primary alveolar epithelial cells to undergo EMT in vitro (10, 11), and demonstration of EMT in vivo in an animal model of fibrosis using mice with fate-mapped alveolar epithelial cells (10). While the evidence that EMT occurs during experimental pulmonary fibrogenesis is robust, important questions remain: Does EMT occur during the pathogenesis of IPF or other fibrotic lung diseases? To what extent does EMT contribute to fibrosis? How is EMT during fibrogenesis regulated?

Jain and colleagues add new information to this last question. ET-1 is known to be up-regulated in fibrotic lung diseases and in animal models of fibrosis (12–14). Evidence that ET-1 can induce EMT has previously been reported in an ovarian cancer cell line (15). Jain and coworkers show in an alveolar epithelial cell line (RLE-6TN) that ET1–1, acting through its G protein–coupled receptor, activates TGF-beta1 signaling and affects loss of an epithelial marker (pro–surfactant protein-B) and gain of one mesenchymal marker ({alpha}-smooth muscle actin). TGF-beta1 is a well-described inducer of EMT and many pathways can lead to TGF-beta1 activation (16). Inhibition of ET-1 did not block EMT of RLE-6TN cells by exogenously added TGF-beta1; thus ET-1 may not be required for EMT if other pathways can activate TGF-beta1 signaling. Demonstration of ET-1–induced EMT in an alveolar epithelial cell line (RLE-6TN) offers some support that ET-1–mediated EMT may be a relevant pathway during pulmonary fibrogenesis.

One important limitation of all of the work to date with ET-1 is that the experiments use epithelial cell lines that often have significantly altered biology compared with that of their primary epithelial cell counterparts due to transformation and serial in vitro passage. The extensive studies of EMT in cancer research indicate that epithelial cell transformation itself is likely to favor EMT and may bypass necessary steps that would be required for EMT in a nontransformed epithelial cell (17). Many epithelial cell lines express fairly high levels of common mesenchymal markers, such as {alpha}-smooth muscle actin and vimentin, compared with primary epithelial cells. Jain and colleagues demonstrate that primary rat alveolar epithelial cells express both ET-1 and the ET-1 receptors, the minimum requirements for autocrine signaling, suggesting that this pathway could occur in these primary cells and perhaps in vivo. The limitations of in vitro cell culture experiments in reflecting in vivo biology may be especially true in studies of EMT because specific cell culture conditions can greatly affect the EMT response. For example, the extracellular matrix on which the cells are attached dictates the EMT response (10, 18). This may be due to multiple factors including integrin signaling, cytoskeletal signaling through small G proteins, and signaling through growth factors bound to the matrix (4, 19). This input into the EMT response in vitro likely reflects an important in vivo point of regulation. Thus, it remains unknown if ET-1 stimulation is sufficient to induce EMT in a primary epithelial cell and if ET-1 can induce EMT in vivo during fibrogenesis. While studying EMT using primary cells and in vivo models is technically challenging, it may be necessary in defining important pathways in EMT.

There is already interest in ET-1 signaling during pulmonary fibrosis. Transgenic mice that overexpress ET-1 spontaneously develop kidney and lung fibrosis (20). Treatment with bosentan, an endothelin antagonist, partially protects rats from bleomycin-induced pulmonary fibrosis (13), and there is currently a phase III clinical trial underway investigating the effectiveness of bosentan in patients with IPF. Previously, the profibrotic role of ET-1 has been attributed to inducing fibroblast proliferation/activation, regulating vascular tone/remodeling, and regulating inflammation (13). The findings presented in the article by Jain and coworkers suggest that the profibrotic effects of ET-1 may include induction of EMT. If these findings prove true in vivo, then inhibition of ET-1 could potentially block the accumulation of activated fibroblasts derived from EMT and from resident fibroblasts. Effective treatment of fibrotic lung diseases may require blockade of activated fibroblasts from multiple sources (resident fibroblasts, EMT, and bone marrow–derived cells).

The real excitement for EMT during pulmonary fibrosis is the possibility of discovering previously unrecognized pathways that may be important during pulmonary fibrogenesis. Studies into molecules and signaling pathways that are epithelial cell–specific or prominent in epithelial cells may reveal new insights into the pathogenesis of pulmonary fibrosis and may potentially offer new targets for therapy. Several important signaling pathways have been identified from studies of EMT during development and tumor progression that may also be important in EMT during fibrosis. Apart from TGF-beta1, signaling through the Wnt/catenin, integrin/ILK, and Notch pathways have emerged as important for EMT (16). Interestingly, all of these pathways cross-talk with the TGF-beta1 signaling pathway (16). In addition, a number of different transcription factors, including Slug/Snail, Twist, beta-catenin, and FTS-1, have been shown in some systems to regulate transcriptional reprogramming during EMT (4, 5, 7). Several factors, most notably BMP-7, have been shown to inhibit EMT and fibrosis in animal models (21). The relevance of these pathways to alveolar epithelial cell EMT is still unclear, but the pathways are promising avenues of future investigation. Evidence that EMT pathways are activated in animals models of pulmonary fibrosis and in samples from patients with fibrotic lung diseases would support a functionally important role for EMT in lung fibrosis.

In summary, Jain and colleagues have defined the potential for ET-1 as an inducer of alveolar epithelial cell EMT. Their findings compel us to reevaluate the function of ET-1 as a profibrotic mediator and may affect interpretation of results from ongoing clinical trials in IPF. Further studies into the regulation of alveolar epithelial cell EMT may lead to a greater understanding into the pathogenesis of fibrotic lung diseases.

Footnotes

Conflict of Interest Statement: H.A.C. received $8,000 over the last 3 years from Biogen Idec for serving on their fibrosis advisory board. K.K.K. has no financial relationship with a commercial entity that has an interest in the subject of this manuscript.

References

  1. Gross TJ, Hunninghake GW. Idiopathic pulmonary fibrosis. N Engl J Med 2001;345:517–525.[Free Full Text]
  2. Thannickal VJ, Toews GB, White ES, Lynch JP III, Martinez FJ. Mechanisms of pulmonary fibrosis. Annu Rev Med 2004;55:395–417.[CrossRef][Medline]
  3. Selman M, King TE, Pardo A. Idiopathic pulmonary fibrosis: prevailing and evolving hypotheses about its pathogenesis and implications for therapy. Ann Intern Med 2001;134:136–151.[Abstract/Free Full Text]
  4. Savagner P. Leaving the neighborhood: molecular mechanisms involved during epithelial-mesenchymal transition. Bioessays 2001;23:912–923.[CrossRef][Medline]
  5. Thiery JP. Epithelial-mesenchymal transitions in development and pathologies. Curr Opin Cell Biol 2003;15:740–746.[CrossRef][Medline]
  6. Liu Y. Epithelial to mesenchymal transition in renal fibrogenesis: pathologic significance, molecular mechanism, and therapeutic intervention. J Am Soc Nephrol 2004;15:1–12.[Abstract/Free Full Text]
  7. Kalluri R, Neilson EG. Epithelial-mesenchymal transition and its implications for fibrosis. J Clin Invest 2003;112:1776–1784.[CrossRef][Medline]
  8. Radisky DC. Epithelial-mesenchymal transition. J Cell Sci 2005;118:4325–4326.[Free Full Text]
  9. Jain R, Shaul PW, Borok Z, Willis BC. Endothelin-1 induces alveolar epithelial–mesenchymal transition through endothelin type A receptor–mediated production of TGF-beta1. Am J Respir Cell Mol Biol 2007;37:38–47.
  10. Kim KK, Kugler MC, Wolters PJ, Robillard L, Galvez MG, Brumwell AN, Sheppard D, Chapman HA. Alveolar epithelial cell mesenchymal transition develops in vivo during pulmonary fibrosis and is regulated by the extracellular matrix. Proc Natl Acad Sci USA 2006;103:13180–13185.[Abstract/Free Full Text]
  11. Willis BC, Liebler JM, Luby-Phelps K, Nicholson AG, Crandall ED, du Bois RM, Borok Z. Induction of epithelial-mesenchymal transition in alveolar epithelial cells by transforming growth factor-beta1: potential role in idiopathic pulmonary fibrosis. Am J Pathol 2005;166:1321–1332.[Abstract/Free Full Text]
  12. Giaid A, Michel RP, Stewart DJ, Sheppard M, Corrin B, Hamid Q. Expression of endothelin-1 in lungs of patients with cryptogenic fibrosing alveolitis. Lancet 1993;341:1550–1554.[CrossRef][Medline]
  13. Park SH, Saleh D, Giaid A, Michel RP. Increased endothelin-1 in bleomycin-induced pulmonary fibrosis and the effect of an endothelin receptor antagonist. Am J Respir Crit Care Med 1997;156:600–608.[Abstract/Free Full Text]
  14. Uguccioni M, Pulsatelli L, Grigolo B, Facchini A, Fasano L, Cinti C, Fabbri M, Gasbarrini G, Meliconi R. Endothelin-1 in idiopathic pulmonary fibrosis. J Clin Pathol 1995;48:330–334.[Abstract/Free Full Text]
  15. Rosano L, Spinella F, Di Castro V, Decandia S, Nicotra MR, Natali PG, Bagnato A. Endothelin-1 is required during epithelial to mesenchymal transition in ovarian cancer progression. Exp Biol Med (Maywood) 2006;231:1128–1131.[Abstract/Free Full Text]
  16. Zavadil J, Bottinger EP. TGF-beta and epithelial-to-mesenchymal transitions. Oncogene 2005;24:5764–5774.[CrossRef][Medline]
  17. Thiery JP. Epithelial-mesenchymal transitions in tumour progression. Nat Rev Cancer 2002;2:442–454.[CrossRef][Medline]
  18. Zeisberg M, Bonner G, Maeshima Y, Colorado P, Muller GA, Strutz F, Kalluri R. Renal fibrosis: collagen composition and assembly regulates epithelial-mesenchymal transdifferentiation. Am J Pathol 2001;159:1313–1321.[Abstract/Free Full Text]
  19. Werb Z. ECM and cell surface proteolysis: regulating cellular ecology. Cell 1997;91:439–442.[CrossRef][Medline]
  20. Hocher B, Schwarz A, Fagan KA, Thone-Reineke C, El-Hag K, Kusserow H, Elitok S, Bauer C, Neumayer HH, Rodman DM, et al. Pulmonary fibrosis and chronic lung inflammation in ET-1 transgenic mice. Am J Respir Cell Mol Biol 2000;23:19–26.[Abstract/Free Full Text]
  21. Zeisberg M, Hanai J, Sugimoto H, Mammoto T, Charytan D, Strutz F, Kalluri R. BMP-7 counteracts TGF-beta1-induced epithelial-to-mesenchymal transition and reverses chronic renal injury. Nat Med 2003;9:964–968.[CrossRef][Medline]



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