|Year : 2012 | Volume
| Issue : 2 | Page : 82-86
Epithelial-mesenchymal transition: Understanding the basic concept
Suresh Babu Ghanta, Neha Nayan, N Govind Raj Kumar, Swetha Pasupuleti
Department of Oral Pathology, Vishnu Dental College, Bhimavaram, Andhra Pradesh, India
|Date of Web Publication||17-Jan-2013|
Department of Oral Pathology, Vishnu Dental College, Bhimavaram - 534 202, Andhra Pradesh
Source of Support: None, Conflict of Interest: None
The epithelial-mesenchymal transition (EMT) is described as a rapid and reversible process of change of cell phenotype seen during embryonic development, organ fibrosis, and tumor progression. EMT was first described by Gary Greenberg and Elizabeth Hay in 1982. During EMT the epithelial cells alter their cell polarity, reorganize their cytoskeleton thus become isolated and motile. Depending upon the biological context in which they occur, EMT is divided into three types namely EMT type I, II, III. The article describes the process of EMT implicated in the oral cavity as in palate and root development (type I EMT), gingival fibromatosis and oral sub-mucous fibrosis (type II EMT), and oral squamous cell carcinoma (type III EMT). The reverse process of EMT is called as mesenchymal-epithelial transition seen in association with kidney formation.
Keywords: Epithelial-mesenchymal transition, E-Cadherin, mesenchymal-epithelial transition, snail gene
|How to cite this article:|
Ghanta SB, Nayan N, Raj Kumar N G, Pasupuleti S. Epithelial-mesenchymal transition: Understanding the basic concept. J Orofac Sci 2012;4:82-6
|How to cite this URL:|
Ghanta SB, Nayan N, Raj Kumar N G, Pasupuleti S. Epithelial-mesenchymal transition: Understanding the basic concept. J Orofac Sci [serial online] 2012 [cited 2019 Sep 16];4:82-6. Available from: http://www.jofs.in/text.asp?2012/4/2/82/106190
| Introduction|| |
"One plus one is one" is the dogma in developmental biology of multicellular organisms as in metazoan where fusion of female oocyte with the male sperm results in a single cell called zygote. Further proliferation of zygote form a solid mass of 16 cells called morula. The fascinating events that follow represent "many manifestations of one." In the embryonic development the process known as gastrulation, establishes three germ layers: Ectoderm, mesoderm, and endoderm. The mesoderm and endoderm contribute to the formation of many mesenchymal tissues and the ectoderm give rise to the epithelial structures. Epithelial and mesenchymal structures show distinct morphological and functional differences. Epithelial cells are closely packed with minimal inter-cellular substance and adhere to one another by intercellular adhesion molecules. The basal cells adhere to the basement membrane that separates them from the adjacent mesenchymal tissue. In addition, they exhibit apico-basal polarity. On the contrary, mesenchymal cells move throughout the extracellular matrix, non-polarized and lack intercellular adhesions or junctions.
| Epithelial-Mesenchymal Transition/Transformation|| |
Embryonic development is characterized by conversion of cells from epithelium to mesenchyme and vice versa. They are known as "epithelial-mesenchymal transition" (EMT) and "mesenchymal-epithelial transition" (MET), respectively.  It was first identified by Gary Greenburg and Elizabeth Hay about 40 years ago who defined these concepts first under the term "epithelial-mesenchymal transformation (EMT)." 
The plasticity of cellular event was described with many terms as "epithelial-mesenchymal transformation, interactions, or transition" and "epithelial mesenchymal trans-differentiation." The term "transformation" describes a permanent alteration of the cell. Epithelial-mesenchymal interaction refers proximate cell signaling between epithelium to adjacent mesenchyme as in ameloblast and dental papilla mesenchymal cell changing into odontoblasts. The term "trans-differentiation" refers to differentiated cells changing into other differentiated cell. Therefore, the term "EMT" is considered as an appropriate term to indicate this dynamic event. 
The biological process of EMT allows a polarized epithelial cell to undergo multiple biochemical changes by interacting with basement membrane via its basal surface. This result in a phenotypic change and cell assume a mesenchymal cell both morphologically and functionally. The change imparts enhanced migratory capacity, invasiveness, elevated resistance to apoptosis, and greatly increased production of extracellular matrix components. The EMT is completed by the degradation of underlying basement membrane and the formation of a mesenchymal cell that can migrate away from the epithelial layer in which it originated. EMT is critical for appropriate embryonic development, and this process is re-engaged in adults during wound healing, tissue regeneration, organ fibrosis, and cancer progression. The phenotypic plasticity afforded by an EMT is revealed by the occurrence of the reverse process - A MET, which involves the conversion of mesenchymal cells to epithelial derivatives. Relatively little is known about this process; the best-studied example is the MET-associated kidney formation. 
Classification of epithelial-mesenchymal transition
EMTs are encountered in three distinct biological settings that carry very different functional consequences. A proposal to classify EMTs into three different biological subtypes based on the biological context in which they occur was discussed at 2007 meeting on EMT in Poland and a subsequent meeting in March 2008 at Cold Spring Harbor Laboratories.
EMTs are classified as:
EMT type I: Epithelial-mesenchymal transition during implantation, embryogenesis, and organ development
- EMT type I: EMT during implantation, embryogenesis, and organ development.
- EMT type II: EMT associated with tissue regeneration and organ fibrosis.
- EMT type III: EMT associated with malignancies.
Primary developmental EMTs are one of the morphogenic mechanisms driving germ layer reorganization of the initial primary embryonic epithelium. This occurs during gastrulation, neurulation, and neural crest formation. The biological steps in primary EMT is summarized below.
Patterning and mapping out the area of EMT starts from a restricted zone by the arrest of cell division and cytoskeletal modification. The process progresses gradually so that both physiological and morphological continuity of the remaining epithelium are maintained by proper gene expression and protein synthesis to change cell shape and allow cell motility. The free or forced movement of the cells to the site of EMT is decided by the epithelial morphogenesis. Then there are alterations or disruption of basal lamina by the ingressing cells by the activity of matrix metalloproteinases (MMPs). The two common mechanisms involved in the EMT process are ingression and de-epithelialization.
Term ingression implies withdrawal of the moving cell's apex from the superficial epithelial layer into the deeper layers whereas de-epithelialization is the loss of epithelial phenotype by loss of the coherent contact between the epithelial cells and apical contact. Ingression follows de-epithelialization and adoption of basal mesenchymal characteristics like active motility and strong traction on deep tissues to pull the cell out of epithelium.
After the alteration of basal lamina by the ingressing cells there will be change in cell shape. These ingressing cells then constrict apically and assume a bottle shape by three ways i.e., by an apical actimyosin contractile mechanism and changes in adhesion or reduction of apical membrane by endocytosis. This allows the displacement of their intracellular contents basally and initiates movement out of the epithelium. Finally, the basal lamina is broken by the ingressing cells. The differentiated cell's behavior and organization characterizes mesenchymal phenotype. The process of ingression leaves a hole or wound in the epithelium which is closed by two ways. The first method is by wound healing in which the hole or the small opening is quickly sealed by zipping up of adjacent loose ingressed cells. The second method is the formation of extensions by the adjacent cells that arch over the top of the ingressing cell forming additional bridging junctions above these, leading to the closure of the opening. Finally the process of remodeling occurs which involves complete removal of cell adhesion molecules by endocytosis and replacement by mesenchymal type adhesion molecules and matrix receptors. The cytoskeleton is remodeled with vimentin, actin polymerization, microtubule formation, and myosin function for protrusive activity. 
EMT type I in oral tissues: Palate and root development
The epithelium covering the tip of the opposing palatal shelves adheres, intercalates, and thins into a single layer midline epithelial seam (MES) during fusion of the palate. Disintegration of this MES results in the confluence of palatal mesenchyme. EMT is one of the proposed mechanisms that regulate the medial edge epithelial (MEE) cell fate. In-vitro culture study of palatal fusion in which MEE cells were labeled with carboxy dichloro flouorescin diacetate succimidyl ester showed the cells move actively and confocal live recording showed that a portion of MEE cells survive and undergo EMT. Transmission electron micrographic study reveals that during palatal fusion, the epithelial cells extend filopodia and pseudopodia through breaks in the surrounding basement membrane thus exhibiting the characteristic features of EMT. 
The Hertwig's epithelial root sheath (HERS) cells help in the synthesis of enamel proteins initially followed by morphologic and phenotypic changes along with the expression of transcripts for several cementum-associated proteins, support the view that HERS cells undergo EMT to become functional cementoblasts. Transcription factor, Lymphoid enhancer-binding factor 1, a regulator of E-Cadherin, was expressed just at later stages of HERS cultures. 
EMT type II: Epithelial-mesenchymal transition associated with tissue regeneration and organ fibrosis
Accumulation of fibroblasts, excess collagen, and other matrix components at sites of chronic inflammation lead to scar formation and progressive tissue injury. These fibroblasts derive not only from the bone marrow but also from an EMT of cells at injury sites. EMT is likely to be involved in the progressive fibrotic diseases of the heart lung liver and kidney. Fibro-specific protein 1 (FSP1) positive cells appear during kidney fibrosis and in Immunoglobulin A nephropathy and increased expression of FSP1 is correlated with prognosis and extent of fibrosis. The ablation of FSP1 cells attenuates fibrosis and collagen deposition, indicating a causal role for these cells in fibrotic disease. 
EMT type II in oral tissues: Gingival fibromatosis and oral submucous fibrosis
The histopathology of gingival overgrowth is characterized by extension of epithelial cells as thin elongated rete ridges deep into the area of fibrosis. The observation of increased levels of connective tissue growth factor found in various conditions like renal proximal tubular epithelial cells and alveolar epithelial cells in vitro in fibrosis of lung in which markers of EMT were also identified. This suggests the possibility of the EMT playing a contributory role in gingival fibromatosis. 
EMT can be inhibited by bone morphogenic protein 7 and hepatocyte growth factor.  It is considered that if EMT contributes significantly to gingival fibrosis, this new understanding could provide major new therapeutic modalities in treatment protocol. 
Oral submucous fibrosis
The observation of many cytoskeleton proteins (CK 18 decreased and vimentin increased), nucleus proteins (Smad-2/3 and nuclear factor kappa-light-chain-enhancer of activated B cells [NF-kβ] increased), cytokines like transforming growth factor β (TGF-β), fibroblast growth factor, tumor necrosis factor α, interleukin 1, platelet-derived growth factor, endothelin-1, and interstitial growth factor are increased. ECM molecules like MMP2 and MMP 9 are elevated and signaling pathways involved in EMT had been expressed in oral submucous fibrosis (OSMF) and in in-vitro experiments. The betal-arecanut quid-induced tissue injury release reactive oxygen species which could mediate TGF-β1 induced EMT. As the elements associated with EMT are also observed in OSMF, it is considered that EMT plays a role in the fibrosis of OSMF. Further studies are required to explain the other histo-pathological features seen in OSMF. 
EMT type III: Epithelial-mesenchymal transition associated with malignancies
Studies of epithelial malignancies have consistently demonstrated that loss of cell adhesion and acquisition of mesenchymal features, the process of EMT precedes their invasion and progression. The contribution of Spanish scientists to the existence of EMT in tumors goes back almost more than 100 years, when Cajal referred to some "pear-like cells, not attached to each other" in his description of human breast carcinomas. 
Alterations of anchoring intercellular junctions are considered as the hallmark of cancer invasion and progression. E-Cadherin is a 120 kDal, calcium binding trans-membrane glycoprotein encoded by the CDH1 gene located in chromosome16q21. The extracellular domain interacts with adjacent cell receptors, whereas the intracellular domain binds to catenines which in turn interacts with the actin cytoskeleton to form tight cell junctions. During tumor progression, E-Cadherin can be functionally inactivated or silenced by different mechanisms like post-translational modification, somatic mutation, and/or transcriptional repression leading to loss or repression of E-Cadherin activity. Many studies have shown that E-Cadherin plays a transcriptional and regulatory role in invasion, metastasis, and poor prognosis for several epithelial malignancies. EMT can be induced by a variety of cellular growth factors and signaling pathways. Src, a protein tyrosine kinase, is a common target of different growth factor activation in EMT. Src has shown to down regulate both focal adhesions resulting in reduced cell contact and enhanced cell migration. In addition Src-mediated phosphorylation induces endocytosis of E-Cadherin, affects cell junction assembly, and cell motility. Recently, macrophage-derived TGF-β was identified by Immunohistochemistry in patients with non-small cell lung cancer. Tumor-associated macrophage cluster with intra-epithelial fibroblatoid cells suggest the contribution in tumor-EMT and tumor progression. 
EMT type III: Epithelial-mesenchymal transition in head and neck and oral squamous cell carcinoma
Many studies have shown that E-Cadherin plays a transcriptional and regulatory role in invasion, metastasis, and poor prognosis for several epithelial malignancies. Chemokines, are related to heparin-binding proteins that attract different types of blood leukocytes to sites of infection and inflammation. They act on leukocytes through selective membrane bound G-Protein-coupled receptors and are produced locally in the tissues. The stromal-derived factor-1 (SDF-1) also referred to as the Chemokine(C-X-C motif) ligand 12(CXCL12) / C-X-C chemokine receptor type 4 (CXCR4) system are among the chemokines that has been shown to be involved in lymph node and distant metastasis of several types of cancer. Their role in human oral squamous cell carcinoma (OSCC) on regional lymph node metastasis and not to distant metastasis was reported. Culture study with OSCC showed that they lose their epithelial morphology due to SDF-1 which was reversible with the withdrawal of SDF-1. It appears that for the intravasation into lymphatics, OSCC use SDF-1/CXCR4 for the alteration of their shape. When the cancer cells complete the intravasation into lymphatics, oral SCC cells use the same system for the establishment of lymph node metastasis. 
Signaling pathways in epithelial-mesenchymal transition
Turning an epithelial cell into a mesenchymal cell requires alterations in morphology, cellular architecture, adhesions, and migration capacity. Commonly used molecular markers of EMT could be grouped under the following ways:
Decrease in the amount of few proteins (E-Cadherin, Desmoplakin, Cytokeratin, and Occludin). Increased activity of selected proteins (K, GSK-3b, and Rho). Accumulation of proteins within the nucleus (β-Catenin, Smad 2/3, NF-kβ, Snail1, Snail2, and Twist).  Snail genes play important role both in development and pathology as in fibrosis and cancer. Function of Snail genes and their properties in relation to development and cancer is represented as acquisition of migratory properties, decreased proliferative activity, and resistance to cell death. Snail genes act primarily as survival factors and inducers of cell movement, rather than as inducers of EMT or cell fate. 
- Abundance of proteins, decrease in the amount of proteins, increased activity of selected proteins or accumulation of proteins within the nucleus.
- Abundance of proteins (N-Cadherin, Vimentin, Fibronectin, Snai1 (Snail), Snail2 (Slug), Twist, MMP-2, MMP-3, MMP-9, and Integrin).
| Conclusion|| |
EMT is a dynamic patho-physiologic process and it depends upon the interaction of co-ordinated molecular signaling pathways. It plays a key role during embryonic development allowing cell migration necessary for tissue and organ development. Postnatally, it explains the tissue changes as seen in wound healing and fibrosis and explains the mechanism involved in cancer invasion and progression. Understanding of molecular mechanisms involved in EMT may open up new areas in cancer research.
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