Nephrol Dial Transplant (2000) 15: 1729-1731
© 2000 European Renal Association-European Dialysis and Transplant Association
Editorial Comments
Transdifferentiation comes of age
Department of Nephrology and Rheumatology, Georg-August-University Medical Center, Robert-Koch-Str. 40, D-37075 Göttingen, Germany
Introduction
Five years ago we wrote an editorial in this journal about the possible transdifferentiation of tubular epithelial cells to fibroblasts [1]. We had just cloned a fibroblast specific protein (FSP)-1 in murine kidney fibroblasts and had described that de novo expression of this protein could be detected in selected tubular epithelial cells during late stages of renal fibrogenesis, indicating possible epithelial–mesenchymal transformation (EMT) or transdifferentiation [2]. After this passage of time, we would like to readdress the issue of transdifferentiation and the scientific evidence collected in the meantime supporting its existence.
As was pointed out in that first editorial, transdifferentiation is defined as the loss of one phenotype and the acquisition of a new one. Transdifferentiation is a physiological process during development but has also been described in a number of adult organs including the liver, the thyroid and the mammary glands (reviewed in [3]). The kidney seems particularly well suited for changes in phenotype since, with the exception of collecting duct cells (which are derived from the ureteric epithelium), all kidney cells are derivatives of the metanephric mesenchyme. Recently, Ng and colleagues [4] were able to demonstrate that glomerular epithelial cells may undergo transdifferentiation to myofibroblasts. Regarding renal fibroblasts, transdifferentiation occurs regularly although it is usually not so termed. During fibrogenesis, resident interstitial fibroblasts undergo a change of phenotype expressing new molecules such as 
-smooth-muscle actin whose expression is normally confined to vascular smooth-muscle cells. Hence the new name ‘myofibroblasts’ for these cells. The number of interstitial -smooth-muscle actin-positive cells correlated with the degree of fibrosis and renal function in animal studies [5] and in human disease such as IgA nephropathy [6]. It is widely assumed that these activated ‘myofibroblasts’ proliferate and synthesize the extracellular matrix substances responsible for the widening of the tubulointerstitial space and the subsequent loss of renal function. However, these cells remain incompletely defined. Unlike in rodents, interstitial cells constitutively expressing -smooth-muscle actin can be detected even under physiological conditions in humans, as was pointed out by Alpers and colleagues [7]. In addition, whereas in dermal myofibroblasts, desmin expression has been described in some of these cells and has been used to classify them accordingly [8], no such expression has been noted in kidney fibroblasts. Still, it seems clear that not all interstitial fibroblasts convert into myofibroblasts and it is highly likely that at least some fibroblasts are not derived from resident interstitial fibroblasts, but also from tubular epithelial cells.
In vivo evidence of tubular epithelial–mesenchymal transdifferentiation
As stated above, the concept of renal transdifferentiation of tubular epithelial cells resulted from the de novo expression of an otherwise fibroblast-specific protein in tubular epithelial cells during fibrogenesis in a mouse model of anti-tubular basement membrane disease. Similar observations were made in a model of anti-glomerular basement membrane disease and in a model of toxic nephropathy (unpublished observations). Nadasdy et al. had already described mesenchymally appearing cells within the interstitium of human kidneys with marked fibrosis. These cells, though of fibroblast appearance and localization, still expressed several epithelial marker proteins, indicating that they may represent transitional cells between epithelial and mesenchymal state [9]. These observations were corroborated in a well-performed study by Ng and co-workers [10] in the rat model of 5/6 nephrectomy. Using immunohistochemistry and in-situ hybridizations, the group noted de novo expression of the myofibroblast marker -smooth-muscle actin in some tubular epithelial cells [10]. By electron microscopy, these cells lost apical–basal polarity and tight junctions and acquired a fusiform cell shape characteristic of fibroblasts. Moreover, these cells separated from neighbouring cells, lost contact with the tubular basement membrane and seemed to migrate to the interstitium. Interestingly, the process of transdifferentiation was restricted to areas where the tubular basement membrane was disrupted, underlining the importance of an intact anchoring of tubular epithelial cells. The number of -smooth-muscle actin-positive tubular epithelial cells correlated with the number of cells expressing the activation marker within the interstitium and also with the degree of interstitial fibrosis.
Effects of matrix components and cytokines on transdifferentiation
A number of studies have analysed the possible effects of cytokines and matrix components on EMT. Healy and colleagues [11] incubated human proximal tubular epithelial cells with supernatant from activated peripheral blood mononuclear cells and observed changes in phenotype to a more fibroblast-like morphology. Moreover, this treatment resulted in a significant decrease in transepithelial resistance and in expression of the epithelial junctional proteins E-cadherin and occludin. Recently we have shown that culture of tubular epithelial cells on collagen type I gels or disruption of the tubular basement membrane by soluble NC1 domains promoted tubular epithelial transdifferentiation, whereas culture on type IV collagen stabilized the epithelial phenotype [12]. Furthermore, we were able to demonstrate that the process of EMT could be induced by cytokines, the combination of basic fibroblast growth factor (FGF-2) and transforming growth factor (TGF)-ß1 being the most effective (unpublished observations). This combination resulted in reduced expressions of cytokeratin and E-cadherin, whereas FSP-1 and -smooth muscle actin were upregulated by up to 6.8-fold.
Fan and co-workers [13] analysed the effects of TGF-ß1 on a rat tubular epithelial cell line and found a dose-dependent increase in the percentage of cells expressing -smooth-muscle actin, whereas expression of the epithelial cell adhesion molecule E-cadherin was completely lost after culture in 50 ng/ml TGF-ß1 for 6 days [13]. In addition, TGF-ß1 in combination with EGF was found to be a potent inducer of transdifferentiation in a study by Okada et al. [14]. These studies demonstrate clearly that the differentiation state of the tubular epithelial cell is not stable but depends on the surrounding extracellular matrix and on cytokines secreted by adjacent cells.
Tubular epithelial transdifferentiation and proliferation
In the study by Healy et al. [11] tubular epithelial transdifferentiation was accompanied by sustained activation of the p38 and p42/44 mitogen-activated protein (MAP) kinases which are typically associated with a proliferative state. MAP kinase activation is associated with cellular differentiation in a number of cell systems. Schramek et al. [15] could demonstrate that stable expression of the MAP kinase/extracellular signal-related kinase (ERK) induced epithelial transdifferentiation to fibroblast-like cells in Madin–Darby canine kidney-C7 cells. Moreover, the addition of the potent tubular mitogens EGF or FGF-2 made the conversion from epithelium to a fibroblast-like cell more complete in the studies by Okada et al. [14] and our own investigations. Thus, there is some evidence that EMT may be linked to tubular epithelial proliferation, at least in some cases. But does proliferation occur in renal fibrogenesis? Many researchers have dismissed the idea of tubular cell proliferation in progressive renal disease. However, as was already demonstrated by Nadasdy and colleagues [9], high proliferative activity within tubules and interstitium could be demonstrated even in end-stage kidneys. These findings were corroborated by a recent study from our group [16] where robust tubular staining for the Ki 67 antigen was noted even in severely fibrosed kidneys. Nevertheless, further studies are necessary to define precisely the relationship between transdifferentiation and proliferation.
In summary, as indicated by the title of this editorial, transdifferentiation has come of age. There is not much question that tubular epithelial cells may transdifferentiate into mesenchymal cells. However, the question remains as to whether these cells are capable of synthesizing interstitial extracellular matrix proteins. Our own in vitro studies determined that tubular epithelial cells did synthesize fibronectin and collagen type I after stimulation with EGF, FGF-2, or TGF-ß1, however, only TGF-ß1 resulted also in an increase in secretion of the two matrix proteins. In vivo data regarding synthesis of extracellular matrix proteins are still scarce. Dautheville and co-workers [17] used transgenic mice for pro-2-collagen (I) promoter linked to a lacZ reporter gene construct to specifically localize collagen synthesizing cells. Using the unilateral ureteral obstruction model, the group was able to find two distinct cell populations synthesizing the 2 chain of type I collagen: interstitial, fibroblast like cells and tubular epithelial cells.
As was pointed out 5 years ago, further studies are still necessary in order to better define the role of epithelial–mesenchymal transformation in renal fibrogenesis. However, unlike 5 years ago, there is now ample evidence from different groups of researchers around the globe that the process exists and that it can be induced by a variety of factors. Figure 1
summarizes our current knowledge of tubular epithelial transdifferentiation via a transitional cell type to a myofibroblast.
|
Acknowledgments
This work is dedicated to our collaborators around the world. It was supported in part by a grant from the Deutsche Forschungsgemeinschaft (DFG) to F. Strutz (DFG Str 388/3-1). The authors wish to acknowledge the excellent technical assistance of M. Dietrich and A. Renziehausen in the original work.
Notes
Correspondence and offprint requests to: Frank Strutz MD, Department of Nephrology and Rheumatology, Georg-August-University Medical Center, Robert-Koch-Str. 40, D-37075 Göttingen, Germany. 
References
-
Strutz F. The fibroblast — a (trans-)differentiated cell? Nephrol Dial Transplant1995; 10: 1504–1506
[Free Full Text] -
Strutz F, Okada H, Lo CW et al. Identification and characterization of fibroblast-specific protein 1 (FSP1). J Cell Biol1995; 130: 393–405
[Abstract/Free Full Text] - Hay ED, Zuk A. Transformations between epithelium and mesenchyme: normal, pathological, and experimentally induced. Am J Kidney Dis1995; 26: 678–690[Web of Science][Medline]
-
Ng YY, Fan JM, Mu W et al. Glomerular epithelial-myofibroblast transdifferentiation in the evolution of glomerular crescent formation. Nephrol Dial Transplant1999; 14: 2860–2872
[Abstract/Free Full Text] -
Muchaneta-Kubara EC, el Nahas AM. Myofibroblast phenotypes expression in experimental renal scarring. Nephrol Dial Transplant1997; 12: 904–915
[Abstract/Free Full Text] - Hewitson TD, Becker GJ. Interstitial myofibroblasts in IgA glomerulonephritis. Am J Nephrol1995; 15: 111–1117[Web of Science][Medline]
- Alpers CE, Hudkins KL, Floege J, Johnson RJ. Human renal cortical interstitial cells with some features of smooth muscle cells participate in tubulointerstitial and crescentic glomerular injury. J Am Soc Nephrol1994; 5: 201–209[Abstract]
- Gabbiani G. The biology of the myofibroblast. Kidney Int1992; 41: 530–532[Web of Science][Medline]
- Nadasdy T, Laszik Z, Blick KE, Johnson DL, Silva FG. Tubular atrophy in the end-stage kidney: a lectin and immunohistochemical study. Hum Pathol1994; 25: 22–28[Web of Science][Medline]
- Ng YY, Huang TP, Yang WC et al. Tubular epithelial–myofibroblast transdifferentiation in progressive tubulointerstitial fibrosis in 5/6 nephrectomized rats. Kidney Int1998; 54: 864–876[Web of Science][Medline]
- Healy E, Leonard M, Madrigal-Estebas L, O'Farrelly C, Watson AJ, Ryan MP. Factors produced by activated leukocytes alter renal epithelial cell differentiation. Kidney Int1999; 56: 1266–1269[Web of Science][Medline]
-
Zeisberg M, Strutz F, Müller GA, Kalluri R. Epithelial-mesenchymal transdifferentiation (EMT) in renal fibrogenesis: disruption of type IV collagen with soluble NC1 domains induces EMT. Kidney Blood Pres Res1999; 22: 255–256 (Abstract)
- Fan JM, Ng YY, Hill PA et al. Transforming growth factor-beta regulates tubular epithelial–myofibroblast transdifferentiation in vitro. Kidney Int1999; 56: 1455–1467[Web of Science][Medline]
-
Okada H, Danoff TM, Kalluri R, Neilson EG. Early role of Fsp1 in epithelial–mesenchymal transformation. Am J Physiol1997; 273: F563–574
[Abstract/Free Full Text] -
Schramek H, Feifel E, Healy E, Pollack V. Constitutively active mutant of the mitogen-activated protein kinase kinase MEK1 induces epithelial dedifferentiation and growth inhibition in Madin–Darby canine kidney-C7 cells. J Biol Chem1997; 272: 11426–11433
[Abstract/Free Full Text] - Strutz F, Zeisberg M, Hemmerlein B et al. Basic fibroblast growth factor (FGF-2) expression is increased in human renal fibrogenesis and may mediate autocrine fibroblast proliferation. Kidney Int2000; 57: 1521–1538[Web of Science][Medline]
- Dautheville S, Fischer E, Mougenot B, Delauche M, Ronco P, Rossert J. Identification of type I collagen-producing cells during interstitial fibrosis induces by unilateral ureteral obstruction (UUO). J Am Soc Nephrol1998; 9: 514A (Abstract)
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  P. L. Dudas, R. L. Argentieri, and F. X. Farrell BMP-7 fails to attenuate TGF-{beta}1-induced epithelial-to-mesenchymal transition in human proximal tubule epithelial cells Nephrol. Dial. Transplant., May 1, 2009; 24(5): 1406 - 1416. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. Zeng, Y. Yao, M. Han, X. Zhao, X.-C. Liu, J. Wei, Y. Luo, J. Zhang, J. Zhou, S. Wang, et al. Biliverdin Reductase Mediates Hypoxia-Induced EMT via PI3-Kinase and Akt J. Am. Soc. Nephrol., February 1, 2008; 19(2): 380 - 387. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. El Chaar, J. Chen, S. V. Seshan, S. Jha, I. Richardson, S. R. Ledbetter, E. D. Vaughan Jr, D. P. Poppas, and D. Felsen Effect of combination therapy with enalapril and the TGF-beta antagonist 1D11 in unilateral ureteral obstruction Am J Physiol Renal Physiol, April 1, 2007; 292(4): F1291 - F1301. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
T. McMorrow, M. M. Gaffney, C. Slattery, E. Campbell, and M. P. Ryan Cyclosporine A induced epithelial-mesenchymal transition in human renal proximal tubular epithelial cells Nephrol. Dial. Transplant., October 1, 2005; 20(10): 2215 - 2225. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. Slattery, E. Campbell, T. McMorrow, and M. P. Ryan Cyclosporine A-Induced Renal Fibrosis: A Role for Epithelial-Mesenchymal Transition Am. J. Pathol., August 1, 2005; 167(2): 395 - 407. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 D. M. Sadlier, S. B. Connolly, N. E. Kieran, S. Roxburgh, D. P. Brazil, L. Kairaitis, Y. Wang, D. C. H. Harris, P. Doran, and H. R. Brady Sequential Extracellular Matrix-focused and Baited-global Cluster Analysis of Serial Transcriptomic Profiles Identifies Candidate Modulators of Renal Tubulointerstitial Fibrosis in Murine Adriamycin-induced Nephropathy J. Biol. Chem., July 9, 2004; 279(28): 29670 - 29680. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Y. Liu Epithelial to Mesenchymal Transition in Renal Fibrogenesis: Pathologic Significance, Molecular Mechanism, and Therapeutic Intervention J. Am. Soc. Nephrol., January 1, 2004; 15(1): 1 - 12. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
R. Poulsom, M. R. Alison, T. Cook, R. Jeffery, E. Ryan, S. J. Forbes, T. Hunt, S. Wyles, and N. A. Wright Bone Marrow Stem Cells Contribute to Healing of the Kidney J. Am. Soc. Nephrol., June 1, 2003; 14(90001): S48 - 54. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Zeisberg, Y. Maeshima, B. Mosterman, and R. Kalluri Renal Fibrosis : Extracellular Matrix Microenvironment Regulates Migratory Behavior of Activated Tubular Epithelial Cells Am. J. Pathol., June 1, 2002; 160(6): 2001 - 2008. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Zeisberg, G. Bonner, Y. Maeshima, P. Colorado, G. A. Muller, F. Strutz, and R. Kalluri Renal Fibrosis : Collagen Composition and Assembly Regulates Epithelial-Mesenchymal Transdifferentiation Am. J. Pathol., October 1, 2001; 159(4): 1313 - 1321. [Abstract] [Full Text]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||