help button home button
AJRCMB
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
This Article
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Schissel, S. L.
Right arrow Articles by Layne, M. D.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Schissel, S. L.
Right arrow Articles by Layne, M. D.
American Journal of Respiratory Cell and Molecular Biology. Vol. 34, pp. 520-522, 2006
© 2006 American Thoracic Society
DOI: 10.1165/rcmb.F312

Editorials

Telomerase, Myofibroblasts, and Pulmonary Fibrosis

Scott L. Schissel and Matthew D. Layne

Brigham and Women's Hospital, Boston, Massachusetts

The pathologic hallmark of idiopathic pulmonary fibrosis (IPF) and other fibrotic pulmonary diseases are fibroblastic foci, areas rich in mesenchymal cells and extracellular matrix. Although mesenchymal cell phenotypes in fibrotic lungs range from proliferating fibroblasts to fully differentiated smooth muscle cells, the predominant cell type is the myofibroblast, a fibroblast-like cell that expresses {alpha}-smooth muscle actin ({alpha}-SMA) (1, 2). Through expression of procollagens and pro-fibrotic cytokines, including transforming growth factor-beta1 (TGF-beta1), myofibroblasts increase extracellular matrix deposition and remodeling. Surrounded by an altered extracellular matrix, the myofibroblasts may account for the hypercontractile properties and low compliance of fibrotic lung (1–6). Furthermore, in contrast to their abrupt disappearance after normal wound healing (7), myofibroblasts persist through all stages of pulmonary fibrosis. While myofibroblasts are central to fibrogenesis, significant uncertainty remains regarding their cellular origin(s) and the molecular mechanisms regulating their differentiation, proliferation, and death.

Over the last several decades, various experimental approaches have addressed whether the myofibroblast arises from differentiation of fibroblasts, de-differentiation of smooth muscle cells, or from circulating, bone marrow–derived fibroblast precursor cells called fibrocytes. Immunohistochemical studies of human IPF and bleomycin-treated rodent lung tissue have revealed that myofibroblasts express {alpha}-SMA, vimentin, and desmin; they do not typically express smooth muscle myosin (1, 2, 6). Moreover, the collagen expression profile and ultrastructural features of myofibroblasts are more similar to those of fibroblasts than smooth muscle cells (8). Finally, tissue-derived fibroblasts in vitro can express {alpha}-SMA and acquire features of myofibroblasts after treatment with TGF-beta1 or interleukin-4 (IL-4) (9, 10). Together, these data support the hypothesis that myofibroblasts are derived from differentiated fibroblasts rather than de-differentiated smooth muscle cells.

The assumption has been that fibroblasts and myofibroblasts in injured and fibrotic lung arise from "resident" fibroblasts present in the adventitia of perivascular and peribronchial tissue. Recent experiments in mice, however, raise the possibility that fibroblasts recruited to the lung after bleomycin injury are derived from fibrocytes (11). In these experiments, irradiated mice received bone marrow cells from green fluorescent protein (GFP)-expressing transgenic mice; after marrow engraftment, the mice were treated with bleomycin. Interestingly, while nearly 30% of collagen-producing cells in the lung were GFP+, very few {alpha}-SMA–expressing cells were GFP+, indicating that fibroblasts, but not myofibroblasts, were bone marrow derived. One conclusion from these data is that myofibroblasts do not arise from fibroblasts, at least in this experimental model; alternatively, it is possible that bone marrow–derived fibroblasts have distinct functions from resident fibroblasts, with only the latter differentiating into myofibroblasts.

In addition to providing insight into the fibroblast origins of myofibroblasts, recent animal studies have led to a better understanding of the enzyme telomerase in proliferating lung fibroblasts after lung injury. Telomerase, a specialized ribonucleoprotein-containing enzyme, synthesizes telomere DNA to prevent degeneration of chromosomal ends in actively dividing cells. After bleomycin-induced lung injury in rats, telomerase activity in lung fibroblasts markedly increases both in vivo and in vitro compared with saline-treated controls; telomerase activity peaks 14 d after bleomycin treatment and returns to baseline levels by 28 d (12). While telomerase activity is clearly required for cancer cell propagation, the role of telomerase induction in the injured lung is unknown. Telomerase activity, however, is induced in other organs after injury, including the liver after surgical resection and in chronic hepatitis (13). Moreover, telomerase activity increases in proliferating pulmonary vascular smooth muscle cells after a hypoxic insult (14). Interestingly, bleomycin-induced telomerase activity in lung tissue appears restricted to proliferating fibroblasts and is not seen in {alpha}-SMA–positive cells (12). Furthermore, bleomycin-induced telomerase activity in lung fibroblasts increases after treatment in vitro with basic fibroblast growth factor (FGF2), a cytokine that induces fibroblast proliferation, and decreases after treatment with IL-4, a cytokine that promotes expression of {alpha}-SMA (15). Finally, a return of telomerase activity back to baseline levels after lung injury is associated with an increase in myofibroblasts. A direct causal relationship between telomerase activity and fibroblast to myofibroblast differentiation has not been shown.

In this issue of the American Journal of Respiratory Cell and Molecular Biology (pp. 625–633), Liu and colleagues provide evidence that telomerase inhibition induces fibroblast to myofibroblast differentiation (16). The authors again demonstrate telomerase induction in lung fibroblasts derived from rats treated with bleomycin compared with saline-treated controls. Interestingly, inhibition of induced-telomerase activity in lung fibroblasts, by 3'-azido-2', 3'-dideoxythymidine (AZT) or antisense oligonucleotides to telomerase RNA, increases fibroblast {alpha}-SMA expression; this effect occurs at doses of telomerase inhibitors that do not affect fibroblast proliferation. In a complementary experiment, telomerase inhibition blocks FGF2-mediated suppression of {alpha}-SMA expression in lung fibroblasts. Thus, direct telomerase inhibition increases lung fibroblast {alpha}-SMA expression in vitro. Figure 1 summarizes these findings in a model depicting the potential role of telomerase in fibroblast to myofibroblast differentiation.


Figure 1
View larger version (12K):
[in this window]
[in a new window]
  		Figure 1. Telomerase regulation of fibroblast to myofibroblast differentiation. The uninjured lung contains quiescent fibroblasts with little telomerase activity. After lung injury, fibroblasts become activated, proliferate, and have increased telomerase activity; high telomerase activity inhibits their expression of {alpha}-SMA. As telomerase activity decreases, activated lung fibroblasts express more {alpha}-SMA, a feature consistent with myofibroblast differentiation. While myofibroblasts are central to the pathogenesis of pulmonary fibrosis, the direct effects of telomerase activity on pulmonary fibrosis remain unknown.

 
These data raise several interesting questions at the molecular, cellular, and disease levels. At the molecular level, what are the mechanisms by which telomerase inhibition increases {alpha}-SMA transcription? While specific activation of serum response factor (SRF) binding to the {alpha}-SMA promoter may occur, telomerase inhibition likely has broad effects on gene expression. For example, telomerase inhibition in human glioma cells induces expression of glial fibrillary acidic protein and cyclin-dependent kinase inhibitors, both markers of glial cell differentiation (17). Thus, cell differentiation induced by telomerase inhibition is not fibroblast specific. Furthermore, increased telomerase activity and {alpha}-SMA expression are not mutually exclusive, as immortalized cells can express {alpha}-SMA in vitro (18).

At the cellular level, what effects does telomerase inhibition have on lung myofibroblast function? For example, is telomerase inhibition sufficient to increase myofibroblast contractility, procollagen and TGF-beta1 expression, or to induce ultrastructural changes characteristic of myofibroblasts? It will be interesting to determine whether the telomerase-expressing fibroblasts used in the current study are a special subset of resident lung fibroblasts able to differentiate into myofibroblasts, since the authors reported in a prior study that after bleomycin injury most telomerase-positive lung fibroblasts, but not myofibroblasts, are bone marrow derived (11).

Finally, at the disease level, what are the net effects of telomerase activity versus inhibition on the development and progression of pulmonary fibrosis? While loss of telomerase activity in select populations of fibroblasts may prove important for myofibroblast differentiation, myofibroblast persistence in IPF lesions suggests a need for telomerase activity to maintain a "supply" of activated fibroblasts. Although the effects of either increasing or decreasing telomerase activity on pulmonary fibrosis are difficult to predict, one human disease may provide insight. Dyskeratosis congenita is a rare disorder caused by loss of human telomerase activity; the X-linked form of the disease is due to mutations in dyskerin (DSK1), a protein necessary for processing the RNA component of telomerase, and the autosomal dominant form is caused by mutations in the reverse transcriptase domain of telomerase. While the disease is classically characterized by skin hyperpigmentation, oral leukoplakia, nail dystrophy, and often fatal aplastic anemia, ~ 20% of patients develop pulmonary fibrosis with clinical, radiographic, and pathologic features resembling IPF (19, 20). Although the pulmonary fibrosis may be secondary to other organ dysfunction, fibrosis can occur independently of overt bone marrow failure. Thus, the phenotype in dyskeratosis congenita supports a causal relationship between low telomerase activity and pulmonary fibrosis. Additional studies potentially using telomerase null mice may clarify the role of telomerase in IPF.

In conclusion, fibroblast telomerase activity increases after lung injury and the subsequent decrease in telomerase activity is likely important for fibroblast to myofibroblast differentiation. Future studies will help further define the role of telomerase in myofibroblast biology and may provide a novel target for the treatment of pulmonary fibrosis.

Footnotes

Conflict of Interest Statement: Neither author has a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

References

  1. Kuhn C, McDonald JA. The roles of the myofibroblast in idiopathic pulmonary fibrosis: ultrastructural and immunohistochemical features of sites of active extracellular matrix synthesis. Am J Pathol 1991;138:1257–1265.[Abstract]
  2. Zhang K, Rekhter MD, Gordon D, Phan SH. Myofibroblasts and their role in lung collagen gene expression during pulmonary fibrosis: a combined immunohistochemical and in situ hybridization study. Am J Pathol 1994;145:114–125.[Abstract]
  3. Zhang K, Flanders KC, Phan SH. Cellular localization of transforming growth factor-beta expression in bleomycin-induced pulmonary fibrosis. Am J Pathol 1995;147:352–361.[Abstract]
  4. Adler KB, Low RB, Leslie KO, Mitchell J, Evans JN. Contractile cells in normal and fibrotic lung. Lab Invest 1989;60:473–485.[Medline]
  5. Evans JN, Kelley J, Low RB, Adler KB. Increased contractility of isolated lung parenchyma in an animal model of pulmonary fibrosis induced by bleomycin. Am Rev Respir Dis 1982;125:89–94.[Medline]
  6. Mitchell J, Woodcock-Mitchell J, Reynolds S, Low R, Leslie K, Adler K, Gabbiani G, Skalli O. Alpha-smooth muscle actin in parenchymal cells of bleomycin-injured rat lung. Lab Invest 1989;60:643–650.[Medline]
  7. Darby I, Skalli O, Gabbiani G. Alpha-smooth muscle actin is transiently expressed by myofibroblasts during experimental wound healing. Lab Invest 1990;63:21–29.[Medline]
  8. Oda D, Gown AM, Vande Berg JS, Stern R. The fibroblast-like nature of myofibroblasts. Exp Mol Pathol 1988;49:316–329.[CrossRef][Medline]
  9. Desmouliere A, Geinoz A, Gabbiani F, Gabbiani G. Transforming growth factor-beta 1 induces alpha-smooth muscle actin expression in granulation tissue myofibroblasts and in quiescent and growing cultured fibroblasts. J Cell Biol 1993;122:103–111.[Abstract/Free Full Text]
  10. Mattey DL, Dawes PT, Nixon NB, Slater H. Transforming growth factor beta1 and interleukin 4 induced {alpha} smooth muscle actin expression and myofibroblast-like differentiation in human synovial fibroblasts in vitro: modulation by basic fibroblast growth factor. Ann Rheum Dis 1997;56:426–431.[Abstract/Free Full Text]
  11. Hashimoto N, Jin H, Liu T, Chensue SW, Phan SH. Bone marrow-derived progenitor cells in pulmonary fibrosis. J Clin Invest 2004;113:243–252.[CrossRef][Medline]
  12. Nozaki Y, Liu T, Hatano K, Gharaee-Kermani M, Phan SH. Induction of telomerase activity in fibroblasts from bleomycin-injured lungs. Am J Respir Cell Mol Biol 2000;23:460–465.[Abstract/Free Full Text]
  13. Tahara H, Nakanishi T, Kitamoto M, Nakashio R, Shay JW, Tahara E, Kajiyama G, Ide T. Telomerase activity in human liver tissues: comparison between chronic liver disease and hepatocellular carcinomas. Cancer Res 1995;55:2734–2736.[Abstract/Free Full Text]
  14. Minamino T, Mitsialis SA, Kourembanas S. Hypoxia extends the life span of vascular smooth muscle cells through telomerase activation. Mol Cell Biol 2001;21:3336–3342.[Abstract/Free Full Text]
  15. Liu T, Nozaki Y, Phan SH. Regulation of telomerase activity in rat lung fibroblasts. Am J Respir Cell Mol Biol 2002;26:534–540.[Abstract/Free Full Text]
  16. Liu T, Hu B, Chung MJ, Ullenbruch M, Jin H, Phan SH. Telomerase regulation of myofibroblast differentiation. Am J Respir Cell Mol Biol 2006;34:625–633.[Abstract/Free Full Text]
  17. Kondo S, Tanaka Y, Kondo Y, Hitomi M, Barnett GH, Ishizaka Y, Liu J, Haqqi T, Nishiyama A, Villeponteau B, et al. Antisense telomerase treatment: induction of two distinct pathways, apoptosis and differentiation. FASEB J 1998;12:801–811.[Abstract/Free Full Text]
  18. Bonin LR, Madden K, Shera K, Ihle J, Matthews C, Aziz S, Perez-Reyes N, McDougall JK, Conroy SC. Generation and characterization of human smooth muscle cell lines derived from atherosclerotic plaque. Arterioscler Thromb Vasc Biol 1999;19:575–587.[Abstract/Free Full Text]
  19. Armanios M, Chen JL, Chang YP, Brodsky RA, Hawkins A, Griffin CA, Eshleman JR, Cohen AR, Chakravarti A, Hamosh A, et al. Haploinsufficiency of telomerase reverse transcriptase leads to anticipation in autosomal dominant dyskeratosis congenita. Proc Natl Acad Sci USA 2005;102:15960–15964.[Abstract/Free Full Text]
  20. Utz JP, Ryu JH, Myers JL, Michels VV. Usual interstitial pneumonia complicating dyskeratosis congenita. Mayo Clin Proc 2005;80:817–821.[Medline]



This article has been cited by other articles:


Home page
 Am. J. Physiol. Lung Cell. Mol. Physiol.Home page
J. E. Milam, V. G. Keshamouni, S. H. Phan, B. Hu, S. R. Gangireddy, C. M. Hogaboam, T. J. Standiford, V. J. Thannickal, and R. C. Reddy
PPAR-{gamma} agonists inhibit profibrotic phenotypes in human lung fibroblasts and bleomycin-induced pulmonary fibrosis
Am J Physiol Lung Cell Mol Physiol, May 1, 2008; 294(5): L891 - L901.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Respir. Cell Mol. Bio.Home page
J. S. Hagood and M. A. Olman
Muscle Fatigue: MK2 Signaling and Myofibroblast Differentiation
Am. J. Respir. Cell Mol. Biol., November 1, 2007; 37(5): 503 - 506.
[Full Text] [PDF]

This Article
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Schissel, S. L.
Right arrow Articles by Layne, M. D.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Schissel, S. L.
Right arrow Articles by Layne, M. D.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Proc. Am. Thorac. Soc. Am. J. Respir. Crit. Care Med.
Copyright © 2006 American Thoracic Society.