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MoLEtuLAR BIOLOGY INTELLIGENCE UNIT
Rise and Fall of Epithelial Phenotype: Concepts of Epithelial-Mesenchymal Transition
Pierre Savagner, Ph.D., D.V.M. Genotypes et Phenotypes Tumoraux
INSERM Batiment de Recherche en Cancerologie
CRLC Val d'Aurelle-Paul Lamarque Montpellier Cedex, France
L A N D E S B I O S C I E N C E / EUREKAH.COM K L U W E R ACADEMIC / PLENUM PUBUSHERS
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RISE AND FALL OF EPITHELIAL PHENOTYPE: CONCEPTS OF EPITHELIAL-MESENCHYMAL TRANSITION
Molecular Biology Intelligence Unit
Eurekah.com / Landes Bioscience Kluwer Academic / Plenum Publishers
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ISBN: 0-306-48239-8
Rise and Fall of Epithelial Phenotype: Concepts of Epithelial-Mesenchymal Transition, edited by Pierre Savagner, Landes / Kluwer dual imprint / Landes series: Molecular Biology Intelligence Unit
While the authors, editors and publisher believe that drug selection and dosage and the specifications and usage of equipment and devices, as set forth in this book, are in accord with current recommendations and practice at the time of publication, they make no warranty, expressed or implied, with respect to material described in this book. In view of the ongoing research, equipment development, changes in governmental regulations and the rapid accumulation of information relating to the biomedical sciences, the reader is urged to carefully review and evaluate the information provided herein.
Library o f Congress Cataloging- in-Publ icat ion Data
Rise and fall of epithelial phenotype : concepts of epithelial-mesenchymal transition / [edited by] Pierre Savagner.
p. ; cm. — (Molecular biology intelligence unit) Includes index. ISBN 0-306-48239-8 1. Vertebrates—Embryology. 2. Epithelium. 3. Mesenchyme. I. Savagner, Pierre. II. Series:
Molecular biology intelligence unit (Unnumbered) [DNLM: 1. Mesoderm—physiology. 2. Cell Differentiation-physiology. 3. Epithelial Cells-physiology. 4. Phenotype. W Q 2 0 5 R5945 2005] QL959.R57 2005 571.8'636-dc22
2005005382
To Mary, who holds my hand
CONTENTS
Preface xvii
1. EMT Concept and Examples from the Vertebrate Embryo 1 Elizabeth D. Hay
Formation of the Mesenchymal Cell in the Embryo 2 Definition of the Mesenchymal Cell 4 Examples of EMT in Higher Vertebrate Embryos 4
2. Epithelium-Mesenchyme Transitions Are Crucial Morphogenetic Events Occurring during Early Development 12 Olivier G. Morali, Pierre Savagner and Lionel Larue
Supracellular Architecture: The Dichotomy between Epithelium and Mesenchyme 14
Embryonic Morphogenesis: A Harmonious Series of Transitions from One Cellular Architectural Type to Another 15
3. The Neural Crest: A Model Developmental EMT 29 Donald F. Newgreen and Sonja J. McKeown
The N C Is a Valuable Model for EMT 29 Defining the N C 29 The EMT of die N C Is Stereotyped 29 The Induction, EMT and Migration of N C Are Linked 32 The N C Is Induced by Cell Interactions. 32 Genes and Molecules in the N C during Induction,
EMT and Migration 33 Specific Transcription Factors Are Expressed by Nascent N C Cells 33 Motor Molecules and Genes in the N C that Contribute to EMT 34
4. Epithelial-Mesenchymal Transformation in the Embryonic Heart 40 Raymond B. Runyan, Ronald L. Heimarky ToddD. Camenisch
and Scott E. Klewer Description of EMT in Vivo 41 Development of Collagen Gel Culture 43 Identification of Components of the EMT Process 43 TGFP as a Mediator of EMT 45 TGFP Receptors ^G TGFp Activation AG Extracellular Matrix in EMT \1 Other Growth Factors in EMT 48 Signal Transduction during EMT 48 Cell-Cell Regulation of EMT 49 Transcriptional Regulation of EMT 49 Clinical Significance of EMT in the Heart 50 Questions and Future Directions 50
5. Epithelial-Extracellular Matrix (Cell-ECM) Interactions in Hydra 56 Michael P. SarraSyJr.
General Introduction to Hydra Structure and Tissue Dynamics 56 Composition and Supramolecular Organization
of Hydra Extracellular Matrix 57 Biogenesis of Hydra ECM during Regeneration, Epithelial Repair
along the Longitudinal Axis, and during Normal Cell Turnover in the Adult Polyp 62
Epithelial Morphogenesis in Hydra Is Dependent on ECM Biogenesis as Monitored during Head Regeneration and Epithelial Repair Following Surgical Incision of the Bilayer GG
Role of Metalloproteinases in the Regulation of Cell-ECM Interactions in Hydra 68
6. Regulation of the Epithelial-to-Mesenchymal Transition in Sea Urchin Embryos 77 Gary M. Wessel and Hideki Katow
Sea Urchin Development: Summary 77 The Extracellular Matrix Involved in the EMT 80 Morphogenesis of the EMT 83 Molecular Mechanisms Involved in EMT 90 Signal Transduction in PMC Cell Surface Modifications 93 Gene Regulation in Cells that Undergo EMT 94 Future Implications 96
7. Change of Epithelial Fate: Lessons from Gastrulation in Drosophila 101 Atish Ganguly and Y. Tony Ip
Gastrulation in the Fly Embryo 101 Cellular Changes in the Presumptive Mesoderm 103 Dorsal-Ventral Patterning of the Blastoderm 103 Multifaceted Control of Gastrulation by Snail 104
8. Cutaneous Wound Reepithelialization: A Partial and Reversible EMT I l l Valine Amot4Xy Christophe Come, Donna F. Kusewitt,
Laurie G. Hudson and Pierre Savagner Cutaneous Wound Healing, a Multistep Process 112 Keratinocyte Activation, a Partial EMT 113 Migration of a Semi-Cohesive Sheet of Cells 116 Contribution of the Inflammatory Response to Reepithelialization ... 120 Transcriptional Control of Keratinocyte Migration 123
9. Epithelial-Mesenchymal Transitions in Human Cancer 135 Veerle L. Van March and Marc E. Bracke
The Molecular Basis of EMT 136 Features of EMT in Pathology 137 Morphological Markers of Cell Differentiation 140 Examples of EMT 142 Epithelium-Stroma Interactions 148
10. Structural and Functional Regulation of Desmosomes 160 Spiro GetsioSy LisaM. Godsel and Kathleen J. Green
Molecular Components of the Desmosome 160 Functional Regulation of Desmosome Assembly 168
11. Epithelial Cell Plasticity by Dynamic Transcriptional Regulation ofE-Cadherin 178 Geert Berx and Frans Van Roy
E-Cadherin Transcriptional Regulation 179 Transcriptional Up-Regulation of E-Cadherin 180 Transcriptional Down-Regulation of E-Cadherin 181 E-Cadherin Transrepression in Human Tumors 185 E-Cadherin Gene Silencing by CpG Hypermethylation 185
12. The Regulation of Catenins in Cancer 191 Maralice Conacci-SorrellandAvri Ben-Ze^ev
Gene Deletion or Inactivation of Catenins and E-Cadherin in Cancer Cells 191
The Wnt Signaling Pathway 193 Transcriptional Targets of P-Catenin in Cancer 194 Antiproliferative Responses to Oncogenic P-Catenin
that Involve p53 and PML 194 Signaling by Plakoglobin 196 The Reciprocal Relationship between Cadherin-Mediated
Adhesion and P-Catenin Signaling 197 Regulation of Cell-Cell Adhesion by Growth Factor Receptor
Driven Phosphorylation 197
13. Hepatocyte Growth Factor Regulates Transitions between Epithelial and Mesenchymal Cellular Phenotypes during Normal Development and in Disease 203 Regina M. Day, Angelina Felici and Donald P. Bottaro
Developmental Transitions between Epithelial and Mesenchymal Phenotypes 204
The Role of HGF Signaling in Tumor-Associated EMT 206 Occurrence of EMT in Tissue Fibrosis and Its Inhibition by HGF ... 209
14. The Role of Insulin-Like Growth Factors in the Epithelial to Mesenchymal Transition 215 Sylvia Julien-Grille, Robert Moore, Laurence Denaty Olivier G. Morali,
Vironique Delmas, Alfonso Bellacosa and Lionel Larue IGFs and Regulation of Cell-Cell Adhesion:
Cellular and Molecular Aspects 218 Signaling Pathways Activated by IGF-IR 220 Pathways Activated by IGF and Implicated in the Induction
o f theEMT 226
15. TGFP-Dependent EpitheHal-Mesenchymal Transition 236 Marie-Luce Vignais and Patrick Fafet
TGFP Signaling 237 Role of TGFp in Tumorigenesis and EMT 238 Role of the Three TGFp Isoforms and of Their Receptors
in the EMT Process 239 Mechanisms and Signaling Pathways Involved
in TGFp-Dependent EMTs 240 RoieoftheSmads 240 Role of Smad-Independent Signaling Pathways 240 Cross-Talk between Signaling Pathways 241
16. The Ras and Src Signaling Cascades Involved in Epithelial Cell Scattering 245 Brigitte Boyer
The Signaling Pathways Leading to EMT 245 Src Involvement in EMT 247
17. Regulation of E-Cadherin-Mediated Cell-Cell Adhesion by Rho Family GTPases 255 Masato Nakagawa, Nanae Izumi andKozo Kaibuchi
Cadherin-Mediated Cell-Cell Adhesions 255 Rho GTPases Regulate Cadherin-Mediated Cell-Cell Adhesion 259 Cadherin-Mediated Cell-Cell Adhesion
Affects Rho GTPase Activity 261
18. Wnt Signaling Networks and Embryonic Patterning 267 Michael W. Klymkowsky
Wnts and Their Receptors 268 Downstream of Frizzled, a Networking Nightmare 269 Analyzing Interactions 270 The Canonical Pathway 270 Dishevelled 273 Catenin Degradation 273 P-Catenin and Transcription Factor Interactions 274 Catenins and LEF/TCFs 274 Xenopus P-Catenin and LEF/TCFs 276 SOXs as Modulators of the Catenin Wnt Pathway 278 SOX-Catenin Interactions 279 SOX Modulation of Catenin-Signaling 279
19. Cadherin-Mediated Cell-Cell Adhesion and the Microtubule Network 288 Cdcile Gauthier-Rouvihey Marie Causerety Franck Comunale
and Sophie Charrasse Delivery of Cadherin and Catenin to the Cell Periphery
through Microtubules 291 Cadherin-Dependent Cell-Cell Contact
Regulates Microtubules Stability 292 Catenin-Dependent Regulation of Spindle Localization 292
20. Matrix Metalloproteases and Epithelial-to-Mesenchymal Transition: Implications for Carcinoma Metastasis 297 Christine Gilles, Donald F. Newgreen, Hiroshi Sato
and Erik W. Thompson General Considerations of the EMT 297 Matrix Metalloproteases 298 MMPs in Carcinoma Model Systems for the EMT 300 MMPs in Developmental EMT Systems 305 Regulation of MMPs by EMT-Associated Transcription Factors 309
Index 317
EDITOR
Pierre Savagner Genotypes et Ph^notypes Tumoraux
INSERM Batiment de Recherche en Cancerologie
CRLC Val d'Aurelle-Paul Lamarque Montpellier Cedex, France
Email: [email protected] Chapters 2y 8
CONTRIBUTORS Valerie Arnoux EMI 0229 INSERM: Genotypes
et Phenotypes Tumoraux CRLC Val d'Aurelle-Paul Lamarque Mompellier, France Chapter 8
Alfonso Bellacosa Developmental Genetics of Melanocytes UMR 146 CNRS-Institut Curie Orsay Cedex, France Chapter 14
Avri Ben-Ze'ev Department of Molecular Cell Biology The Weizmann Institute of Science Rehovot, Israel Email: [email protected] Chapter 12
Geert Berx Unit of Molecular and Cellular
Oncology VIB-Ghent University Ghent, Belgium Email: [email protected] Chapter 11
Donald P. Bottaro Urologic Oncology Branch Center for Cancer Research, NIH Bethesda, Maryland, U.S.A. Email: [email protected] Chapter 13
Brigitte Boyer Laboratoire Oncogenese et Regulations
Cellulaires UMRCNRS146 Institut Curie Section de Recherche
Batiment Orsay, France Email: [email protected] Chapter 16
Marc E. Bracke Department of Experimental
Cancerology University Hospital Ghent Ghent, Belgium Email: [email protected] Chapter 9
Todd D. Camenisch Departments of Pharmacology
and Toxicology University of Arizona Tucson, Arizona, U.S.A. Chapter 4
Marie Causeret Centre de Recherches de Biochimie
Macromoldculaire Centre de la Recherche Scientifique Montpellier Cedex, France Chapter 19
Sophie Charrasse Centre de Recherches de Biochimie
Macromol^culaire Centre de la Recherche Scientifique Montpellier Cedex, France Chapter 19
Christophe Come EMI 0229 INSERM: Genotypes
et Ph^notypes Tumoraux CRLC Val d'Aurelle-Paul Lamarque Montpellier, France Chapter 8
Franck Comunale Centre de Recherches de Biochimie
Macromol^culaire Centre de la Recherche Scientifique Montpellier Cedex, France Chapter 19
Maralice Conacci-Sorrell Department of Molecular Cell Biology The Weizmann Institute of Science Rehovot, Israel Chapter 12
Regina M. Day Pulmonary and Critical Care Division Tupper Research Institute Tufts University/New England Medical
Center Boston, Massachusetts, U.S.A. Chapter 13
V^ronique Delmas Developmental Genetics of Melanocytes UMR 146 CNRS-Institut Curie Orsay Cedex, France Chapter 14
Laurence Denat Developmental Genetics of Melanocytes UMR 1 AG CNRS-Institut Curie Orsay Cedex, France Chapter 14
Patrick Fafet Institut de Genetique Moleculaire
de Montpellier CNRS Montpellier Cedex, France Chapter 15
Angelina Felici Laboratory of Cell Regulation
and Carcinogenesis National Cancer Institute National Institutes of Health Bethesda, Maryland, U.S.A. Chapter 13
Atish Ganguly Department of Molecular Genetics
and Microbiology University of Massachusetts Medical
School Worcester, Massachusetts, U.S.A. Chapter 7
C^cile Gauthier-Rouvi^re Centre de Recherches de Biochimie
Macromol^culaire Centre de la Recherche Scientifique Montpellier Cedex, France Email: [email protected] Chapter 19
Spiro Getsios Departments of Pathology
and Dermatology and the Robert H. Lurie Cancer Center
Northwestern University Feinberg School of Medicine
Chicago, Illinois, U.S.A. Chapter 10
Christine Gilles Laboratory of Tumor
and Developmental Biology University of Liege CHU-Sart-Tilman Liege, Belgium Chapter 20
Lisa M. Godsel Departments of Pathology
and Dermatology and the Robert H. Lurie Cancer Center
Northwestern University Feinberg School of Medicine
Chicago, Illinois, U.S.A. Chapter 10
Kathleen J. Green Department of Pathology Northwestern University Medical School Chicago, Illinois, U.S.A. Email: [email protected] Chapter 10
Elizabeth D. Hay Harvard Medical School Department of Cell Biology Boston, Massachusetts, U.S.A. Chapter 1
Ronald L. Heimark Department of Surgery University of Arizona Tucson, Arizona, U.S.A. Chapter 4
Laurie G. Hudson UNM Health Sciences Center Albuquerque, New Mexico, U.S.A. Chapter 8
Y. Tony Ip Program in Molecidar Medicine University of Massachusetts School
of Medicine Worcester, Massachusetts, U.S.A. Chapter 7
Nanae Izumi Department of Cell Pharmacology Nagoya University, Graduate School
of Medicine Showa, Nagoya, Japan Chapter 17
Sylvia Julien-Grille Developmental Genetics of Melanocytes UMR 146 CNRS-Institut Curie Orsay Cedex, France Chapter 14
Kozo Kaibuchi Department of Cell Pharmacology Nagoya University Graduate School of Medicine Showa, Nagoya, Japan Email: [email protected] Chapter 17
Hideki Katow Marine Biological Station Graduate School of Science University of Tohoku Asamushi, Aomori, Japan Email: [email protected] Chapter 6
Scott E. Klewer Department of Pediatrics University of Arizona Tucson, Arizona, U.S.A. Chapter 4
Michael W. Klymkowsky Molecular, Cellular and Developmental
Biology University of Colorado, Boulder Boulder, Colorado, U.S.A. Email: [email protected] Chapter 18
Donna F. Kusewitt Department of Veterinary Biosciences The Ohio State University Columbus, Ohio, U.S.A. Chapter 8
Lionel Larue Developmental Genetics of Melanoqrtes UMR 146 CNRS-Institut Curie Orsay Cedex, France Chapters 2, 14
Sonja J. McKeown Murdoch Children*s Research Institute Department of Paediatrics Royal Children's Hospital Parkville, Victoria, Australia Chapter 3
Robert Moore Developmental Genetics of Melanocytes UMR 146 CNRS-Institut Curie Orsay Cedex, France Chapter 14
Olivier G. Morali Developmental Genetics of Melanocytes UMR 146 CNRS-Institut Curie Orsay Cedex, France Chapters 2, 14
Masato Nakagawa Department of Cell Pharmacology Nagoya University, Graduate School
of Medicine Showa, Nagoya, Japan Chapter 17
Donald F. Newgreen Embryology Laboratory Murdoch Children's Research Institute Royal Children's Hospital Melbourne, Australia Chapters 3y 20
Raymond B. Runyan University of Arizona Departments of Cell Biology
and Anatomy Tucson, Arizona, U.S.A. Email: [email protected] Chapter 4
Michael P. Sarras, Jr. Department of Anatomy
and Cell Biology University of Kansas Medical School Kansas City, Kansas, U.S.A. Email: [email protected] and Department of Cell Biology
and Anatomy Chicago Medical School North Chicago, Illinois, U.S.A. Email: [email protected] Chapter 5
Hiroshi Sato Department of Molecular Virology
and Oncology Cancer Research Institute Kanazawa University Kanazawa, Japan Chapter 20
Erik W. Thompson University of Melbourne Department of Surgery Melbourne, Australia Email: [email protected] Chapter 20
Veerle L. Van Marck Department of Experimental
Cancerology University Hospital Ghent Ghent, Belgium Email: [email protected] Chapter 9
Frans Van Roy Unit of Molecular Cell Biology Department for Molecular Biomedical
Research VIB-Ghent, Belgium Chapter 11
Marie-Luce Vignais Institut de Genetique Moleculaire
de Montpellier CNRS Montpellier Cedex, France Email: [email protected] Chapter 15
Gary M. Wessel Department of Molecular
and Cellular Biology Department of Biochemistry Brown University Providence, Rhode Island, U.S.A. Email: [email protected] Chapter 6
PREFACE
Cell phenotype is a comprehensive term describing the general appearance and behavior of the cell. It reflects a dynamic stage of differentiation, proliferation, or apoptosis. It also reflects cell group organization, cell motility and cell interaction status with the local cellular and extracellular environment. The two "classic" cell phenotypes, epithelial and mesenchymal, are distinguished by a number of characteristics. Essentially, epithelial phenotype is defined at the level of a group of cells. Epithelial cells form cohesive groups usually ordered along a bidimensional layer and organized in mono- or pluristratified structures. Epithelial phenotype can be modulated under a variety of physiological and pathological conditions. The rapid and sometimes reversible conversion to a mesenchymal-like phenotype is called epithelial-mesenchymal transition (EMT). This term does not imply that the epithelial cell fully transdifferentiates into a fibroblast, a tissue-specific and differentiated cell type. Rather, it means the cell adopts a fibroblast-like phenotype by going through an activation process presenting common features in a wide range of apparently unrelated situations.
The original concept of EMT arose from in vivo studies characterizing developmental stages involving dramatic phenotype remodeling. The original definition of EMT was later extended, based on in vitro studies of epithelial cells that became individualized motile cells, as part of an activation process stimulated by growth factors or extracellular matrix components. Early examples of in vitro EMT include lens epithelial cell transformation described by Prof E. Hay (see Chapter 1), FGF-treated NBT-II carcinoma cells,^ and HGF-treated MDCK cells.^'^ Many more models have been described since. More recently, the concept of EMT has been extended to describe "dedifferentiation' occurring during pathological events such as chronic fibrosis pathologies affecting the kidney and other organs. It may be more appropriate to use the term epithelial-mesenchymal transformation to describe such pathological situations involving a dysregulation of cell phenotype. These processes probably reflect hyperactivation of signaling pathways, for example marked activation of TGpp-stimulated pathways in the case of kidney fibrosis.^ EMT-related transformation taking place during chronic fibrosis will be reviewed in the next edition of this book.
It is attractive to hypothesize that EMT takes place during carcinoma progression, as EMT in vitro typically generates motile and invasive cells that are apparently well suited for cancer progression. Nevertheless, the occurrence of EMT during cancer progression remains controversial (see Chapter 9, Van Marck et al). It has been suggested that EMT might take place during basement membrane invasion and intravasation stages by carcinoma cells.^^ Beyond the clinically rare example of carcinosarcoma in which there is good evidence for EMT, observations in clinical carcinoma samples do
not provide clear indications of EMT, but rather emphasize the complexity of tumor structure and organization in vivo. Carcinoma cells typically express marked phenotypic heterogeneity related to tumor type and stage. Generally, tissue architecture and cell organization are significantly disrupted, in association with a significant remodeling of cytoskeleton, cell-cell and cell-matrix adhesion structures in tumor cells. Some tumors, such as those of invasive lobular carcinoma of the breast, no longer express cell-cell adhesion structures, a clear EMT-like transformation. However, there is no evidence these tumors are more aggressive than invasive ductal carcinoma, the dominant breast carcinoma type, which still expresses E-cadherin at the protein level.
In fact, it is possible that only limited numbers of tumor cells undergo EMT-like events to generate individualized cells responsible for invasion and metastasis. However, this has been difficult to demonstrate. More typically, tumor cells show partial downregulation of cell-cell adhesion structures while migrating as sheets, cords, tubules or isolated cells. Maintenance of partial but decreased cohesiveness is also found in motile cell populations during physiological events such as wound healing (see Chapter 8) or branching and tubulogenesis during organogenesis. Such a mechanism could be involved in the "mass invasion" process seen in solid tumors. It appears that maintenance of some level of cell-cell adhesion in invasive tumors can actually serve as a strategy for tumor progression and metastasis. ̂ '̂ °
In this book, the concept of EMT is first described by the pioneer in the field. Prof E. Hay, in Chapter 1. EMT concept rised from a combination of in vitro and in vivo observations. Accordingly, Part I reviews early morphogenetic events involving EMT in various animal models. The first developmental stage to involve EMT is gastrulation, when the mesoderm first emerges. This very intricate process is reviewed in three different chapters, covering mouse (Morali et al. Chapter 2), Drosophila (Ganguly et al. Chapter 7) and sea urchin (Wessel, Chapter 6). The second classic and well documented EMT example occurs during emergence and migration of neural crest cells from the neural tube. Neural crest cells individualize and emigrate from a cohesive epithelial sheet, the neuroepithelium, to undergo wide-ranging migrations before undergoing further differentiation into a variety of cell types (Newgreen, Chapter 3). A third classic example of EMT is provided by heart morphogenesis in vertebrate embryos. The multistep differentiation of the mitral and tricuspid valves and the interventricular septum includes an EMT step that has been extensively studied in chick models (Runyan et al. Chapter 4). In these three examples of EMT during development, extracellular matrix components are crucial in providing an appropriate substrate to differentiating cells.
The role of extracellular matrix in cell plasticity is also described during epithelial regeneration in hydra, a diblastic animal devoid of mesenchyme (Chapter 5, Sarras). It may be surprising to find a chapter on epithelial regeneration in diblastic organisms in this book, but the whole process
shows interesting similarities to developmental EMT and to cutaneous wound healing in vertebrates, another process involving EMT-like stages. Reepithelialization is initiated by a cell activation stage, including partial and transient cell-cell dissociation and cell migration (see Chapter 8, Arnoux et al). It is striking that this physiological process involves molecular pathways that are known for their involvement in other instances of EMT.
Characterizing the pathways driving EMT is key to understanding the whole process and is therefore the focus of Part II. Because of the diversity of physiological and pathological situations in which EMT occurs, it is still problematic to define "EMT-specific" genes or pathways. Initiation of EMT is typically linked to downregulation of cell-cell adhesion structures, particularly desmosomes and adherens junctions. Regulation of desmosomes is reviewed in Chapter 10 (Getsios et al). Desmosomes are sturdy cell-cell adhesion structures that show unexpected plasticity. One of the best known cell-cell adhesion molecules is E-cadherin, a critical component of the adherens junction. Its role in maintaining cell-cell cohesiveness has been demonstrated both in vitro and in vivo. Its integrity depends on associated proteins, the catenins, that link E-cadherin to the cytoskeleton. Regulation of the main molecular components of adherens junctions, E-cadherin and catenins, is reviewed in Chapters 11 (G. Berx et al), and 12 (M. Conacci-Sorrell et al). Destabilization of cell-cell adhesion structures is a general feature of EMT and commonly involves downregulation of these components at protein or RNA level. Induction of EMT typically involves growth factors, including members of HGF (Chapter 13, R.M. Day et al), FGF, IGF (Chapter 14, S. Julien-GriUe et al) andTGF|3 (Chapter 15, M.L. Vignais et al) families. Mesenchymal cells are often responsible for secreting these factors. The specificity of the induction signal depends on the ability of the target cell to recognize these growth factors through specific receptors, usually tyrosine-kinase receptors. The growth factor signal typically activates ras and sarc pathways, as described in Chapter 16 (B. Boyer). In response, the cytoskeleton undergoes dramatic remodeling mediated by the Rho family members and associated proteins (Chapter 17, M. Nakagawa). Expression of cytokeratin intermediate filaments is altered at the transcriptional level, and the actin cytoskeleton undergoes drastic reorganization, reflecting the induction of motility. Accordingly, the microtubule network is also modulated, as reviewed in Chapter 19 (C. Gauthier-Riviere). A distinct EMT pathway described both in vitro and in vivo implicates a large family of extracellular activators, known as Wnt, as signals for EMT. Their complex biological functions, mediated by the Frizzled family of receptors, include developmental EMT, reviewed in Chapter 18 by M. Klymkowsky. Activation of matrix metalloproteases is usually associated with EMT. Matrix metalloprotease substrates include extracellular matrix molecules as well as membrane proteins involved in cell-cell adhesion, as reviewed in Chapter 20 by C. Gilles et al. Eventually, induction of EMT is controlled at the transcriptional level and the EMT response, in most cases, depends on members
of the snail family of transcription factors, described in several chapters. However, none of the pathways described above is functionally specific for EMT, since these pathways can also be involved in unrelated functions such as cell proliferation or apoptosis, in other circumstances. The specificity of these pathways for EMT induction is apparently provided by the cellular environment of the target cells and the timing of the signaling.
In conclusion, this book attempts, for the first time, to bring together multiple perspectives on EMT. It is now very clear that the concept of EMT links processes involving common molecular pathways and can bring valuable conceptual tools to the study of dynamic processes ranging from early embryo development to wound healing and carcinoma progression. The field is expanding very rapidly with the institution of an international conference in 2003 and the recent establishment of TEMTIA (The EMT International Association: ht tp: / /www.magicdatabases.com/TEMTIA/ temtia.html). Some newer aspects of EMT, including molecular pathways and potential inducers have been omitted in this book and will be included in an updated version. I am personally very thankful to the numerous authors who contributed to this multidisciplinary effort.
Acknowledgments I thank D.F. Kusewitt and M. Savagner for helpfiil suggestions. Finan
cial support is provided by the Fondation de France and Ligue Nationale contre le Cancer.
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