Epithelial mesenchymal transition from a natural gestational orchestration to a bizarre cancer disturbance

Abstract

The epithelial to mesenchymal transition (EMT), a pathologic phenomenon in cancer, has a twin in the embryonic period of life. In the first one, its promotion will cause metastasis to become a life‐threatening stage of cancer, while in the second it will lead to organogenesis, which is necessary for all living creatures. There is one more from this phenomenon, which occurs during the wound healing process and if dys‐regulated can lead to fibrosis. In both there are stimulants in common and one that are different. Stages start from cell–cell junction dissociation followed by morphological changes and behavioral and essence alterations. To control the EMT as a bizarre disturbance in cancer and metastasis, initially it is better to understand the wonder of natural gestational orchestration in early life. In this review, first the structure of the two heads of the spectrum is described followed by the cellular and micro‐environmental alterations during this phenomenon. Understanding cellular behavior in this process and what makes them invasive resistant stemness cells will be of great importance in highlighting roads to cancer treatment. (Cancer Sci 2013; 104: 28–35)

The epithelial‐mesenchymal transition (EMT) is a phenomenon with a chain of events that starts from alteration in the cell–cell junction and cytoskeleton. It can lead to cell–ECM changes and release of epithelial cells from the surrounding tissue.1 A physiological mechanism that is present during the developmental stage of gestation is called gastrula, which includes mesoderm and neural tube configuration. It can have numerous pathological conditions, i.e. renal interstitial fibrosis, endometrial adhesion, and cancer metastasis.2

This was first defined as an aspect of embryogenesis, which is vital for morphogenesis during embryonic development. Then it was found during the translation of early stage tumor into invasive malignancies.3 Increasing evidence suggests that tumor progression is critically involved with the acquisition of an EMT phenotype, which allows tumor cells to acquire the capacity to infiltrate surrounding tissues, and thus license these cells to metastasize in distant sites.4 There are excellent detailed reviews about this phenomena of signaling with clinical points of view.5, 6 Yet its importance as a critical target for cancer management makes it a necessary subject of investigation. In this review the phenomenon is discussed in general along with its physiological aspects during cell–cell and cell–ECM interaction with an emphasis on the importance of targeting of EMT.

Structure of epithelial layer

The epithelium was most likely the earliest tissue type to have appeared in the evolution of eukaryotic living things, and is the initial differentiated structure to emerge during embryogenesis. The characteristics depend on three features: intercellular adhesion; mitotic spindle orientation, which leads to contact inhibition; and apical/basal polarity.7 Therefore, the integrity of their structural design is critical. The majority of cancers arise from epithelial tissues, and display loss of cell polarity and often, as a consequence, tissue disorganization.8 Cell junctions provide interaction between adjacent cells or cells and the ECM. Generally, there are three major types of junction: the adherens junction, tight junction and gap junction.9 The molecular structure in the cell junction is illustrated in Figure 1.

Figure 1.

Construction of Epithelial cells. Cell to cell adherence junctions are based on E‐cadherin adhesion, which is anchored by α‐catenin followed by β‐catenin to actin filaments. Such joints make a strong attachment structure between two epithelial cells with contact inhibitory characteristics. Tight Junction and Gap Junctions are also important structures illustrated in this figure.

The adherens junction has a mechanical interaction role between cells, and catenins and cadherins have great importance in EMT. “Catenins and cadherins” are important molecules in cell–cell adhesion, which has an important role in the EMT phenomenon of epithelial structures. The name “catenin” was derived from the Latin word “catena” meaning “chain” because of its role in the correlation with cadherins to the cytoskeleton junction.10 Catenins belong to protein families that come as complexes with the cadherin cell junction molecules of eukaryotic cells. The first two catenins that were identified were named α and β‐catenin.11 α‐catenin binds to the β‐catenin and also actin filaments. β‐catenin connects the cytoplasmic domain of cadherins. There are other molecules from the catenin family such as γ and δ‐catenin. Adherence junctions create and maintain epithelial layers and help regulate cell growth. They typically include cadherin, β‐catenin, and α‐catenin.12

The most important role of catenins is linking cadherins to actin filaments.13 β‐catenin has a double role in the cell. Initially, it binds to the intracellular cytoplasmic domains of the cadherin molecule, acting as an essential constituent of adherence junctions that helps cells maintain epithelial layers and has an important role in contact inhibition of the epithelial layer.14 Its next role is participating in the Wnt signaling pathway as a downstream target.15 Generally, in the absence of Wnt, GSK‐3β can phosphorylate β‐catenin, which forms into a complex that includes β‐catenin, AXIN1, AXIN2, APC, CSNK1A1, and GSK‐3β. Following phosphorylation of the N‐terminal Ser and Thr residues of β‐catenin, ΒTRC promotes its ubiquitination, which causes it to be degraded by the TrCP/SKP complex. On the other hand, when Wnt is present, GSK‐3β is displaced from the previously mentioned complex by inactivation (in this case phosphorylation). This means that β‐catenin will not be phosphorylated, followed by ubiquitination. Consequently, its level inside the cell stabilizes and eventually, accumulated β‐catenin will transpose into the nucleus with Rac1corporation. At that moment, β‐catenin becomes a co‐activator for TCF (T Cell Factor) and LEF (lymphoid enhancer factor) and turns on Wnt genes by displacing Groucho and HDAC transcription repressors.16

It is expected that α‐catenin acts in association with vinculin in order to bind to actin and help to stabilize the junctions.14 Lack of α‐catenin can actually stimulate aberrant transcription, which can cause cancer.17 In contrast to this finding that increasing β‐catenin levels may be associated with carci‐nogenesis through Wnt/beta‐catenin pathway,18 the level of α‐catenin in cancers are almost downregulated.19

Other members of the catenin family are δ and γ‐catenin. δ‐catenin has been concerned as a regulator of the nuclear factor (NF)‐κB transcription factor.20 The other name for γ‐catenin is junction plakoglobin or JUP. Mutation of the gene encoding plakoglobin has been associated to be one of the causes of the cardiomyopathy, which is recognized as arrhythmogenic right ventricular dysplasia (ARVD).21

Cadherins, the name of which is derived from “calcium‐dependent adhesion”, belong to type‐1 transmembrane proteins.22 The cadherin superfamily includes cadherins, protocadherins, desmogleins, and desmocollins, etc.23 It has been detected that cells with specific cadherin classes tend to cluster.24 Cadherins are categorized into four groups: classical, desmosomal, protocadherins, and unconventional.25 The classical type of cadherins has an important role in EMT and epithelial structure and will be discussed in this review.

There are various members of the classical cadherin family, which are present in different places. Cadherin‐1 which is also identified as CAM 120/80, epithelial cadherin (E‐cadherin) or uvomorulin is a protein that is encoded by the CDH1 gene in humans.26 It is well‐known as a tumor suppressor gene.27 E‐cadherin downexpression has been associated with cancer progression and metastasis through decreasing cellular adhesion, which leads to increasing in cellular motility. Then cancer cells will acquire the capability to cross the basement membrane and initiate the invasion.28 In immunohistochemistry analysis, expression of E‐cadherin is distinctly reduced in the great majority of invasive lobular carcinomas.29

Cadherin‐2, which is also identified as neural cadherin (NCAD) is encoded by the CDH2 gene.30 It is frequently detectable in cancer cells and associates in mechanisms for trans‐endothelial migration through upregulating the sac kinase pathway when cancer cells adhere to the endothelial cells of a blood vessel. The intercellular junction between the two adjacent endothelial cells will be loose and permits the cancer cell to get through.31 The other two cadherins, i.e. cadherin‐12 encoded by the CDH12 gene32 and cadherin‐3 also identified as P‐cadherin and encoded by the CDH3 gene,33 will not be discussed in this review.

Tight junctions are the strictly intercellular junctions in which membranes intersect together to make a virtually impermeable barrier to fluid. Their location on the intracellular side of the plasma membrane and anchoring with actin shows that tight junction associates cytoskeletons of adjacent cells. They tighten cells together and help to preserve the polarity of cells by avoiding the lateral diffusion of integral membrane proteins and prevent the flow of molecules and ions through the space between cells.9

Claudins, derived from Latin “claudere” meaning “to close”, are the most important constituents of the tight junctions that control the flow of molecules in the intercellular space.34, 35 Occludin, which is encoded by the OCLN gene is a 65‐kDa integral plasma‐membrane protein located at the tight junctions.36, 37 Studies have shown that during EMT, the expression of both molecules are downregulated followed by overexpression of Snail transcription factor.38 It can explain the loss of cell polarity during EMT phenomenon.

Gap junction is a specialized intercellular construction, which directly connects the cytoplasm of two cells and permits various molecules and ions to flow freely among cells. One gap junction channel is composed of two connexons (or hemichannels) which connect across the intercellular space.39 In general, gap junctions consist of primarily homo‐ or hetero‐hexamers of connexin proteins. To date, 20 isoforms of connexins are known40 and they are classified into different groups according to sequence similarity (such as A, B, C like GJA1 or GJC1). Alternative names based on molecular weight (like Connexin 43) are also present.39 Studies have shown that during the EMT phenomenon, gap junction will be transformed just like other cell–cell structures. For example, it has been demonstrated that cadherins facilitate assembly of connexin 43 in rat liver epithelial cells in an opposite manner.41

Structure of mesenchymal cells

The term mesenchyme is an embryonic name for cells that form mesoderm and are multipotent stromal cells with the capability of developing into a variety of cell types in connective tissue, such as osteoblasts, chondrocytes, adipocytes, the lymphatic system, the circulatory system, and have a huge capability for self‐renewal while preserving multipotency.42 Such cells neither have restricted structure nor tight intracellular junctions. They have irregular shape, uneven in arrangement or density. They have loose adhesion, which allows them to migrate. They also have a more prolonged shape. These cells move individually (Fig. 2).4

Figure 2.

Structure of mesenchymal cells. There is no trace of E‐cadherin junction in these cells. They are free to move and express vimentin filaments inside with integrin in the surface. Expression and secretion of matrix metalloproteinases (MMP) starts the extracellular matrix (ECM) remodeling, which is caused by collagen degradation, loosening the ECM and preparing the microenvironment as an area for easy movement of such cells.

The development of various cell types from mesenchymal cells is influenced by the expression of several transcription factors. For example, Smad family members are important for chondrogenic differentiation, and Forkhead box (Fox) proteins promote osteogenesis.43 In addition, transcription factors, such as SOX2, are important regulators of mesenchymal stem cell self‐renewal capacity and multipotency.44

What is going on during EMT?

For the transition to mesenchymal cells, there should be an orchestration of processes from the cells inside into the ECM.

Intracellular alterations will be morphology to cell structure, adhesion capability, acquiring new (mesenchymal‐like) molecular characteristics and abilities such as migration. In this case, EMT markers such as N‐cadherin and vimentin upregulate. As a result of β‐catenin stabilization, induction of the transcription factors such as Snail1 (Snail), Snail2 (Slug), Twist, EF1/ZEB1, SIP1/ZEB2, and/or E47 will also upregulate, which will repress E‐cadherin expression. This is followed by losing cell–cell adhesion through E‐cadherin junction. Cells will dissociate and morphologically change to more prolonged forms compared with the conservative shape of epithelium with actin remodeling. Cells will acquire a motility capability. Many studies have demonstrated that in addition to Wnt signaling, other pathways also can induce these phenomena. EGF signaling is one of the EMT inducers that can lead to E‐cadherin downregulation.45 Transforming growth factor‐β (TGF‐β) is another major Smad3 dependent inducer of EMT.46 These and other factors are demonstrated in Figure 3.

Figure 3.

Major signaling pathways involved in epithelial to mesenchymal transition (EMT) phenomenon. Wnt (Wingless) through Frizzled family member followed by GSK3β (Glycogen Synthase Kinase 3 β) phosphorylation (inactivation) which releases β‐catenin from the complex. It translocates into nucleus and binds to Lef1/Tcf (Lymphoid enhancer factor1/T cell factor1) transcription factor and upregulation of target genes followed by epithelial structural protein meaning E‐cadherin suppression. There are also signaling pathways other than Wnt; for example TGF‐β (transforming growth factor β) through TGFβR (receptor) and Smad stimulation, EGF (epidermal growth factor) through EGFR, Ras and MAPK, IGF (insulin‐like growth factor) through IGF1R, PI3K (phosphoinositide 3‐kinase) and Ras same as FGF (fibroblast growth factor) and PDGF (platelet derived growth factor) through Akt (Serine/threonine kinase) and mTOR (mammalian target of rapamycin) inducing EMT phenotype.

In EMT, the signaling pathways are mediated by micro‐environmental stimuli, initiating and controlling EMT processes followed by micro‐environmental alteration in ECM due to enzymatic activity of mesenchymal cells. Among numerous signaling pathways, Wnt, TGF‐β, Hedgehog, Notch, and NF‐κB signaling pathways are found to be important for EMT induction. These signaling pathways arrange a combination and elaborate gene program and protein network necessary for the formation of mesenchymal phenotypes after dissociation of the basic foundations in epithelial architecture, like cell–cell junctions and cell polarity.47 Then other processes are initiated by releasing matrix remodeling enzymes called the MMP family. Although these proteins can induce EMT, especially during wound healing when they release from macrophages, secretion of these enzymes from transformed cells will facilitate cell migration.

Matrix metalloproteinases, which were first described in vertebrates in 1962,48 are zinc‐dependent endopeptidases, belonging to the metzincin family of enzymes that are known to exploit a zinc ion in their active sites. They are degrading different substrates of the ECM, which finally lead to tissue remodeling. In normal circumstances, they are regulated by tissue inhibitors of MMP (TIMP). Disproportion between the levels of MMP and TIMP plays an important role in pathologic status.49 They can degrade various types of ECM proteins, develop some bioactive molecules, capable of cleavage of cell surface receptors, releasing apoptotic ligands (like FAS ligand), and chemokine/cytokine activity.50

The MMP family includes a “pre” domain and, a “pro” domain, which maintains the MMP in an inactive form and a “catalytic” domain. Most MMPs also contain a c‐terminal hemopexin‐like domain, which mediates interactions with substrates and or in substrate binding. Therefore, cell adhesion molecules (i.e. E‐cadherin, CD44, αv integrin)51 or growth factor receptors (i.e. FGF receptor 1, members of the EGF receptor family HER2 and HER4, c‐met)52 can be developed by the MMP‐dependent process through proteolysis. Their activity is under the control of specific tissue inhibitors of MMPs (TIMP‐1 to ‐4).53 Some drugs like All Trans Retinoic Acid (ATRA) can change the TIMP/MMP balance.54

Gelatinases MMP‐2 and MMP‐9 (gelatinase A and B, respectively), can specifically degrade the type IV collagen, which forms a major component of the basement membranes. Loss of the basement membrane is one of the most reliable signs of poor prognosis in most carcinomas.55 Activation of MMPs take place on the cell surface by a membrane‐associated subclass of MMPs called the membrane‐type MMPs (MT‐MMP).56 There are six identified MT‐MMPs.56 They can also involve cell migration and invasion processes by degrading specific substrates. They play a key role in the peri‐cellular proteolysis associated with cell migration and invasion.57 Generally, MMPs have been concerned with different stages of malignancy, such as primary growth of tumor, angiogenesis, invasion and metastasis.52, 53

Types of EMT

There are three different types of EMT, which are illustrated in Figure 4.

Figure 4.

Different types of epithelial to mesenchymal transition (EMT). (a) Gastrulation in early stages of life, which is occurring followed by formation of primitive streak (invagination in the middle shown by arrows). (b) During inflammation and wound healing, which rise at the edges of the wound and is shown with arrows, misregulation in such circumstances will lead to tissue fibrosis. (c) Epithelial to mesenchymal transition in metastasis, which leads to cell migration and microvasculature invasion.

Type 1 in embryogenesis (gastrulation)

During the initiation of embryogenesis, the embryo implantation and the placenta formation are both associated with an EMT.58 The trophoectoderm cells, undergo an EMT for endometrial invasion.59 In gastrulation, three germ layers are created. This initiates with the generation of primitive streak (invagination in the midline of the epiblast) in the epiblast layer60 and will be associated with programmed alterations of the epiblast, which leads to cell migration and differentiation.61 The primitive streak, creates the mesendoderm, followed by formation of the mesoderm and the endoderm via an EMT.62 The EMT associated with gastrulation is a natural gestational orchestration that is dependent on Wnt signaling.63The major effectors on type 1 EMT have been mentioned in part in Table 1.64, 65, 66, 67, 68, 69, 70

Table 1.

Major factors involved in epithelial to mesenchymal transition (EMT)

Proteins involved in type 1 EMT
Wnt8c	Formation of the primitive streak
TGF‐β superfamily (Nodal, Vg1)	Mediating of Wnt function
FGF	Regulation of EMT associate with gastrulation through Wnt
Eomes, and Mesps	Transcription factors in type 1 EMT orchestration
Snail	Represses E‐cadherin and induces EMT through cell adhesion (occludins and claudins) and polarity genes (Dlg, Crb3)
Sox, Snail, SlugandFoxD3	Migratory neural crest cells, which will differentiate to melanocytes
Wnts, FGFs, BMPs, c‐MybandMsx‐1	Migratory property of neural crest cells
Major fibrosis EMT markers
Monocyte chemoattractant Protein 1(MCP1)	Recruits monocytes, memory T cells, and dendritic cells to sites of tissue injury, infection, and inflammation
Fibroblast‐specific protein 1 (FSP1)	A fibroblast marker in different organs undergoing tissue remodeling
Discoidin domain receptor Tyrosine kinase 2(DDR2)	Has a key role in the communication of cells with their microenvironment.
Vimentin, Desmin, α‐SMA	Mesenchymal cell cytoskeletal protein
Major elements in cancer induced EMT signaling
HGF, EGF, PDGF, TGF‐β	Growth factors
Snail, Slug, zinc finger E‐box binding homeobox 1 (ZEB1), Twist, Goosecoid, and FOXC2	Transcription factors
ERK, MAPK, PI3K, Akt, Smads, RhoB, β‐catenin, lymphoid enhancer binding factor (LEF), Ras, and c‐Fos	Signal‐transducing proteins
β4 integrins, α5β1 integrin, and αVβ6 integrin	Surface proteins

BMP, bone morphogenetic protein; EGF, epidermal growth factor; ERK, extracellular signal‐regulated kinase; FGF, fibroblast growth factor; HGF, hepatocyte growth factor; MAPK, mitogen‐activated protein kinase; PDGF, platelet derived growth factor; SMA, smooth muscle actin; TGF, transforming growth factor.

Type 2 in inflammation and wound healing (fibrosis)

Type 2 of EMT, known as wound healing or fibrosis, which occurs in a number of epithelial tissues, is mediated by inflammatory cells and fibroblasts that secret different inflammatory components that interact with ECM, including collagens, laminins, elastin, and tenacins. Such EMTs are identified in association with fibrosis occurring in the kidney, liver, lung, and intestine.71 This type of EMT is associated with inflammation and generation of various classes of inflammatory molecules in adjacent activated fibroblasts (myofibroblasts).These molecules further cause epithelial layers dissociation by basement membrane degradation. The major markers in fibrosis have come in Table 1.72, 73

Endothelial cells in the microvasculature can also go under the process of mesenchymal transition during fibrosis, known as EndMT,71 which is similar to the normal process occurring during development.74 During embryogenesis, it occurs throughout the formation of the endocardial cushion and the heart valves, while in pathologic conditions, such as post‐ischemic injury cardiac fibrosis it involves both the endocardium and the microvascular endothelium by the emergence of newly formed fibroblasts.74

Transforming growth factor‐β is one of the well‐known inducers of EMT and progression of organ fibrosis that has been demonstrated in studies using BMP‐7, an antagonist of TGF‐β signaling, in kidney, liver, billiary tract, lung, and intestinal fibrosis of animal subjects.73 BMP‐7 functions as an endogenous inhibitor of TGF‐β‐induced EMT.73

Type 3 in cancer (metastasis)

Unlimited epithelial cell growth and angiogenesis are characteristics in initiation and early growth of primary epithelial tumors.75 Cells will acquire invasiveness in advanced stages, which in the initial step is marked by invasion through the basement membrane that leads eventually to metastatic dissemination.

It has been demonstrated that carcinoma cells can acquire mesenchymal phenotype and expression of mesenchymal markers such as α‐smooth muscle actin (SMA), FSP1, vimentin, and desmin.76 These cells are typically found in the invasive stage of tumors followed by invasion and metastasis, which is initiated by the invasion through basement membrane, incursion to the microvasculature, transporting via circulation, extravasation, seeding as micrometastases, and ultimately colonization.77 It is suggested that the genetic and epigenetic misregulation during tumor formation especially make them responsive for EMT‐inducing heterotypic signals originating in the tumor‐associated stroma. They may also take part in senescence prevention, which is induced by oncogenes, by the means of facilitating the following aggressive spreading of the cells.78 The major elements in signaling have been cited in Table 1.79, 80

Transforming growth factor‐β is an important suppressor for epithelial cell proliferation in early stages of cancer, while it has been shown that in latent stages it is also capable of positive regulation of tumor progression and metastasis.81 The relationship between E‐cadherin downregulation by cancer cells and transition through mesenchymal cells has been established by different studies.82 There is contrary relationship between levels of E‐cadherin and patient survival.83 A study has shown that downregulation of actin binding protein gelsolin, which is known as cytoskeletal tumor suppressor,84 can induce E‐cadherin downregulation and EMT through Wnt signaling pathway.85 Cytoplasmic sequestration of β‐catenin is important for the preservation of epithelial features in cancer cells. As β‐catenin transports to the nucleus, cells acquire the mesenchymal phenotype.86

Noncoding microRNAs, such as microRNA 200 (miR200) and miR205 (suggested recently as a possible biomarker in TNBC (Triple Negative Breast Cancer)87 (inhibiting the repressors of E‐cadherin expression, ZEB1 and ZEB2), help in maintaining the epithelial cell phenotype.88 Loss of miR200 in breast carcinoma correlates with vimentin upregulation and E‐cadherin downregulation.88 On the other hand, actingmiR21 is upregulated in many cancers and facilitates TGF‐β‐induced EMT.89

MET versus EMT

The EMT transformation followed by acquisition of epithelial cells in the distant metastasis colonies is called MET (mesenchymal epithelial transition) during the course of secondary tumor formation (Fig. 5).90 It is more likely that microenvironment alteration is responsible for such reversed phenomenon possibly through the absence of the heterotypic signals they experienced in the primary tumor that were responsible for inducing the EMT in the first place.77 It is suggested that both phenomena are important for distant metastasis followed by metastatic cell colonization.

Figure 5.

Epithelial to mesenchymal transition (EMT) versus mesenchymal epithelial transition (MET). During metastasis of tumor to the distant organ, cells invade through EMT phenomena which is facilitated by releasing MMPs and collagen degradation (dashed lines) move into the microvasculature and after seeding into the distant target change into epithelial phenotype (which is called MET) probably by microenvironment element alterations.

Brabletz et al. have shown that in metastases derived from tumors (with nuclear localization of β‐catenin and downregulation of E‐cadherin), β‐catenin was transported to the cytoplasm and E‐cadherin was upregulated, which was the hallmark of MET.91 Also, variants of the metastatic T24/TSU‐Pr1 bladder carcinoma line have been reported by Christine Chaffer (Bernard O'Brien, Institute of Microsurgery, Melbourne, Australia) with selection for enhanced metastatic capability and have more epithelial markers (i.e. E‐cadherin and keratins) than their less metastatic complements, but with some mesenchymal markers (e.g. vimentin and MMPs). The ability of both epithelial and mesenchymal phenotypes was referred to by Savagner as a “metastable phenotype”. The ability of metastasis in colorectal cancer cells undergoing EMT is followed by the expression of stem cell markers and it is suggested that acquisition of such characteristics could happen in various organs. Such alterations may explain the troubles of EMT phenotype monitoring in cancer development by means of transient characteristics for mesenchymal phenotype during later phases of tumorigenesis.4

Cancer stem cell and EMT

As it was mentioned before, the EMT phenomenon was first found in the embryonic stage of life (type 1) among the embryonic stem cells. In recent reports, it has been demonstrated that the emergence of CSCs occurs in part as a result of EMT.92 On the other hand the individual overexpression of essential regulators of the embryonic EMT may be sufficient to efficiently drive the ontogeny of the breast cancer stem cell molecular signature independently of EMT phenotype.93 Understanding the direct relation of the phenomena with cancer stem cell has a critical importance in targeting the therapy of cancers especially in case of drug resistance.92

Future and overview

Recently, EMT has become the headline of medical scientific investigation stories. While there are many clarified mechanisms in EMT and their role in the initial steps of metastasis, still many questions are unanswered. What are the first key factors in the initiation of EMT? Are they in ECM or are they in the cancer cells themselves? Of course, both could be possible as it has been mentioned in previously.

The explanation in this review shows a kinetic and step by step phenomenon, which can elongate from hours to weeks in in vitro experiments. However, it will be difficult for practical and clinical detection because of its plasticity nature. Therefore it is wise to implement more in vivo studies to elucidate the dark side of this enigma and finding more detailed molecules in different signaling pathways during EMT and interactions between various factors such as cadherin and integrin, Snail and β‐catenin, TGF‐β platelet derived growth factor (PDGF). With more answers to the un‐responded aspects of these bizarre disturbances, controlling the EMT phenomenon in clinical and practical challenges in cancer will be more clarified and facilitated.

Disclosure statement

This study was supported by Cancer Research Center of Cancer Institute of Iran (Tehran University of Medical Sciences). The author has no conflict of interest.

(Cancer Sci, doi: 10.1111/cas.12074, 2012)