Clinical significance of epithelial-mesenchymal transition
Steinestel et al. Clinical and Translational Medicine
0 Department of Urology, Ulm University Medical Center , Prittwitzstrasse 43, Ulm 89075 , Germany
1 Institute of Pathology and Molecular Pathology, Bundeswehrkrankenhaus Ulm , Oberer Eselsberg 40, Ulm 89081 , Germany
2 Bundeswehr Institute of Radiobiology , Neuherbergstrasse 11, Munich 80937 , Germany
Clinical significance of epithelial-mesenchymal
Clinical significance of epithelial-mesenchymal
Konrad Steinestel1,2*, Stefan Eder1, Andres Jan Schrader3 and Julie Steinestel3
The concept of epithelial-mesenchymal transition (EMT), a process where cells change their epithelial towards a
mesenchymal phenotype, has gained overwhelming attention especially in the cancer research community.
Thousands of scientific reports investigated changes in gene, mRNA and protein expression compatible with EMT
and their possible correlation with tumor invasion, metastatic spread or patient prognosis; however, up to now, a
proof of clinical significance of the concept is still missing. This review, with a main focus on the role of EMT in
tumors, will summarize the basic molecular events underlying EMT including the signaling pathways capable of its
induction as well as changes in EMT-associated protein expression and will very briefly touch the role of microRNAs
in EMT. We then outline protein markers that are used most frequently for the assessment of EMT in research and
diagnostic evaluation of tumor specimens and depict the link between EMT, a cancer stem cell (CSC) phenotype
and resistance to conventional antineoplastic therapies. Furthermore, we evaluate a possible correlation between
EMT marker expression and patient prognosis as well as current therapeutic concepts targeting the EMT process
to slow down or prevent metastatic spread of malignant tumors.
Epithelial-mesenchymal transition (EMT) is a central
element of embryonic development, wound healing and
tumor cell migration, and has thus obtained much
attention by the research community since Greenburg and
Hay firstly described a mesenchymal-like transformation
of epithelial cells when suspended in collagen gels .
Basically, the term describes a process in which cells lose
epithelial and gain mesenchymal characteristics; this is
accompanied by a loss of cell-cell cohesiveness, leading
to enhanced migratory capacity . Multiple genes as
well as proteins that seem to play a central role in EMT
have so far been identified and are either up- or
downregulated during the process, thus serving as possible
markers in the assessment of EMT. Since it seems to be
a key element in wound healing and tumor cell migration,
there is also great interest in EMT as a pharmaceutical
target; recent publications even proposed vaccination against
1Bundeswehr Institute of Radiobiology, Neuherbergstrasse 11, Munich 80937,
2Institute of Pathology and Molecular Pathology, Bundeswehrkrankenhaus
Ulm, Oberer Eselsberg 40, Ulm 89081, Germany
Full list of author information is available at the end of the article
drivers of EMT as an immunotherapeutic approach
against tumor progression .
However, since many studies on EMT are based on
in vitro results and not all findings could be confirmed
in vivo, the clinical significance of the concept remains
unclear . This review lays the main focus on EMT in
tumor cells and aims at recapitulating what is known
about the molecular basis of EMT. Furthermore, we will
summarize current markers of EMT that are in clinical
and/or diagnostic use and, finally, evaluate EMT from a
translational point of view and in the context of clinical
The molecular basis of EMT
Basically, EMT stands for a loss of epithelial and a gain
of mesenchymal cellular characteristics that enhance
migration and invasion by the cell . This process
includes loss of cell cohesiveness as well as fundamental
reorganization of the cytoskeleton inducing a switch
from apical-basal to front-rear polarity, and may
furthermore be associated with the acquisition of invasive
properties through the secretion of lytic proteases as well as
resistance to senescence and apoptosis . EMT is
under tight control of multiple regulatory pathways; first
and foremost, transforming growth factor (TGF-)
signaling activity is enhanced in many physiological and
pathological conditions in which EMT is observed, such
as organogenesis, inflammation and tumor invasion [7,8].
In canonical TGF- signaling, binding of TGF- to its cell
surface receptors (type I-III) activates complex formation
of Smad family transcription factors, which translocate to
the nucleus and cooperate with transcription factors from
the Snail and Twist family, so-called EMT master
genes [9,10]. Non-Smad signaling molecules
downstream TFG- and supportive of EMT include activated
Rho-like GTPases, Phosphatidylinositol-3-kinase (PI3K)
and mitogen-associated protein kinase (MAPK; the
various signaling pathways mediating TGF- signaling in
EMT are excellently reviewed in ). Taken together,
these effectors mediate transcriptional repression of
genes that are involved in cell polarity and cell-cell
adhesion, such as RhoA and E-cadherin (Figure 1A) [11,12].
The latter is mediated by the recruitment of histone
deacetylases (HDACs) and other repressors to E-box
elements in the E-cadherin promoter, leading to chromatin
condensation and transcriptional repression . At the
same time, the expression of N-cadherin, another member
of the cadherin family that allows for enhanced adhesion
between mesenchymal cells, is upregulated; this balanced
change in cadherin expression has thus been designated
cadherin switch and is regarded a hallmark of EMT .
Not only the expression, but also specific membraneous
targeting of E-cadherin is repressed in EMT via loss of the
epithelial-specific intermediate filament keratin; therefore,
loss of keratin immunostaining is widely regarded as a
marker for ongoing EMT [15,16]. Further mechanisms
that lead to degradation of cell-cell junctions include a
repression of claudin and occludin expression, while
zonula occludens 1 (ZO-1) is subsequently lost in a
posttranscriptional manner [17-19]. This repression is
maintained throughout further progression of EMT . Since
protein complexes (such as partitioning defective PAR)
that define the apical compartment of the cell are
normally associated with intercellular junctions, degradation
Figure 1 Basic molecular changes underlying EMT. A, Signaling along canonical TGF- pathway activates EMT-promoting transcription factor
(such as Twist, ZEB or Snail) to repress transcription of E-cadherin that initially forms the adherens junction (AJ) complex together with -catenin.
Extinction of E-cadherin from the AJ complex as well as concomitant phosphorylation via activated growth factor receptors lead to cytoplasmic
accumulation and nuclear translocation of -catenin, where it acts as a transcription factor for migration-associated genes. B, Enhanced expression
of Vimentin in migrating tumor cells protects phosphorylated MAPK from cytoplasmic phosphatases, thus ensuring signaling activity along the
EGFR/MAPK axis. This supports pro-migratory effects on the cytoskeleton (such as Rac-mediated actin polymerization) and secretion of lytic matrix
metalloproteinases that cleave the surrounding extracellular matrix to allow for cell migration.
of the junctions also weakens the apical-basal polarity
cellular phenotype . Moreover, the TGF--facilitated
signaling along the MAPK axis exerts pro-proliferative
and anti-apoptotic effects on the cell, while Ras/MAPK
activity alone without TGF- induction - has also been
linked to enhanced EMT [21-23]. After losing
cohesiveness due to degradation of cell-cell junction complexes,
mesenchymal-like tumor cells are able to invade through
the basement membrane into underlying tissue by the
secretion of lytic enzymes such as matrix metalloproteinases
(MMPs) and MAPK-mediated reorganization of the actin
cytoskeleton which is enhanced by the expression of
Vimentin (Figure 1B) . In detail, migration and
invasion of moving cells is facilitated by specialized cellular
protrusions, such as filopodia, lamellipodia and
invadopodia. While filopodia, consisting of actin filaments arranged
in a parallel fashion, seem to sense changes to the cellular
microenvironment and act as a guide through the
surrounding matrix, lamellipodia are built upon a branched
actin network and allow for actin-myosin interactions as a
prerequisite for cellular movement [25,26]. Both filopodia
and lamellipodia have been linked to an EMT-like
phenotype in migrating tumor cells [27,28]. Invadopodia are
closely related to lamellipodia in a sense that they also
consist of a branched network of actin filaments, but have
the ability to degrade the extracellular matrix (ECM)
through the secretion of lytic proteases, such as MMP-1,
MMP-7 and MMP-9 (Figure 1B) . Invadopodia
formation has been linked to activity of the EMT transcription
factor Twist1 cancer, and own results showed high
expression of invadopodia-associated proteins, such as Cortactin
and Abelson interactor 1 (Abi1), in a colorectal carcinoma
cell line with an EMT-like phenotype shown by loss of
E-cadherin [29,30]. Accordingly, TGF- signaling activates
small GTPases that enhance local reorganization of the
actin cytoskeleton as a prerequisite for lamellipodia and
filopodia formation, such as Rho, Rac and Cdc42 .
Vimentin, which is frequently upregulated in cells with an
EMT-like phenotype, is then required for the further
maturation of invadopodia . Besides clearing the way
for migrating tumor cells, MMPs that are released during
tumor cell invasion are themselves further fueling the
EMT process; the same effect is achieved via liberated
TGF- from the ECM [33-35]. In a mouse model of
gastric cancer, it could be shown that EMT cooperates with
MMP activity to gain access to lymph vessels and to
spread distant metastases . Accordingly, blood and
lymph vessel infiltration by triple-negative breast cancer
cells is associated with the expression of EMT
transcription factor Zeb1 in surrounding stroma . Alterations
in MMP expression are linked to changes in the integrin
repertoire with downregulation of some (epithelial) and
upregulation of other integrins that facilitate interaction
with extracellular matrix components such as collagen .
Targeting transmembrane proteins - like E-cadherin - or
increasing the levels of intracellular reactive oxygen
species via enhanced activation of Rac1b are further
mechanisms of MMP-induced EMT [38,39].
Upon arrival at the site of metastasis, it seems a
prerequisite for metastatic colonization that tumor cells
undergo a partial reversal of the EMT, the so-called
mesenchymal-epithelial transition (MET) [40,41].
During that process, tumor cells regain the expression of
epithelial markers, such as E-cadherin, while the expression
of EMT-associated transcription factors, such as Twist1, is
repressed . Thus, EMT can be seen as a reversible and
transient process that enables epithelial tumor cells to gain
access to the vasculature, allowing for the formation of
Besides TGF-, other signaling pathways have also
been implied in the activation of EMT; for example,
hypoxia-inducible factor (HIF) contributes to EMT in
tissue fibrosis and cancer cell invasion by modulating
the activity of pro-EMT transcription factors Notch and
-catenin [42-44]. HIF1 induces Twist and Snail
expression in endothelial as well as ovarian carcinoma cells
[45-47]. Additionally, activation of several receptor
tyrosine kinases (RTKs) may result in induction of EMT; in
these scenarios, growth factor binding to RTKs as well
as activating mutations in oncogenes downstream of the
receptors leads to enhanced signaling along the Ras/
MAPK or Akt/mTOR axis, resulting in upregulation of
Snail expression . Finally, it has been shown that
enhanced wnt signaling activity as well as an upregulation
of chemokine receptors (such as CXCR-1) also support
the process of EMT [48,49]. Here, wnt signaling leads to
an inhibition of glycogen synthase kinase 3
(GSK3)mediated phosphorylation of -catenin; the resulting
decrease in proteosomal degradation and cytoplasmic
accumulation of -catenin supports its translocation to
the nucleus, where it acts as a transcriptional co-activator
of EMT-associated gene expression .
In the recent years, the role of small, non-coding
RNAs in the EMT process has also been further
elucidated. Methylation-depedent expression changes in
levels of miR-200c and miR-141, for example, regulate
invasion and metastasis in colorectal cancer via altered
miR-200c target gene expression; miR-375 is
downregulated in tamoxifen-resistant breast cancer cells with
EMT-like properties, and its reexpression partly reverses
EMT [51,52]. Other miRNAs that have been discussed
to play a central role in EMT are, among others, miR-1,
9, 24, 29b, 30a, 31, 124, 155, 192/215 and 661 (reviewed
in ). Their mechanisms of action include
posttranscriptional regulation of EMT master genes or of
genes defining the epithelial or mesenchymal phenotype
of the cell (such as E-/N-cadherin or vimentin).
However, a thorough review of the role of miRNAs in EMT
and their clinical significance would lie beyond the
scope of this text, where we would like focus on the role
of well-characterized proteins in EMT.
Tissue markers of EMT
Unlike the various mechanisms that are known to initiate
or repress EMT, the observed hallmarks of established or
ongoing EMT are quite consistent. As previously
mentioned, loss or degradation of proteins associated with
epithelial homeostasis, cell polarity and cell adhesion, such as
E-cadherin, RhoA or Plakophilin 2 is frequently observed
in EMT (Figure 1A); some proteins that play key roles in
cell-cell adhesion when attached to the membrane, such
as -catenin, are redistributed to the cytoplasm [11,12,17].
Moreover, cells undergoing EMT show decreased
expression of epithelial cytokeratin filaments, such as keratins 8
and 18 . On the other hand, the intermediate filament
protein Vimentin is frequently overexpressed and
contributes to cell migration as well as invasion-associated gene
expression by stabilizing the phosphorylated state of
MAPK and is thus regarded as a stable marker of EMT;
moreover, its presence is a prerequisite for the maturation
of invadopodia which are indispensable for cell invasion
[32,54,55]. Dysregulated expression of transcription
factors, such as Notch1, Slug, Snail, Twist or Zeb1 has been
described in invasive tumors displaying EMT; these
markers are therefore designated as EMT master genes.
Table 1 provides an overview over selected dysregulated
protein markers that have been and still are frequently
used in the assessment of EMT.
EMT, tumor invasion and metastasis
The highest clinical significance of the EMT process is
linked to its role in tumor cell invasion and metastasis.
In a transgenic mouse model of pancreatic beta-cell
carcinogenesis, the switch from noninvasive adenoma to
invasive carcinoma is associated with a loss of E-cadherin
expression ; moreover, it has been shown that loss of
membraneous -catenin is associated with tumor cell
budding, a morphologic hallmark of invasive tumor
phenotype and tumor aggressivity in colorectal cancer
tissue specimens [83-85]. In samples from 49 breast
cancer patients, the single-cell infiltration pattern that is
observed in some lobular carcinomas has been linked to
protein truncation mutations in the CDH1 gene
encoding for E-cadherin , and hypoxia-induced
upregulation of Slug and Snail is associated with increased breast
cancer cell migration and invasion in vitro .
Accordingly, expression of Vimentin can be found in many
aggressive breast cancer cell lines . As mentioned above,
to allow for tumor cell invasion into the vasculature as a
prerequisite for metastatic seeding, EMT cooperates with
Table 1 Frequently used protein markers for epithelial-mesenchymal transition (EMT)
Downregulated in EMT
Cell adhesion molecule
Cell adhesion molecule
Cell adhesion molecule
Cell adhesion molecule
Cell adhesion molecule
Tyrosine kinase receptor
Cell adhesion molecule
Colon, Breast, Lung, Ovary, Esophagus, Prostate, Cervix
Pancreas, Breast, Lung
Breast, Esophagus, Cervix
1Membraneous depletion, but cytoplasmic accumulation/nuclear translocation.
NET, neuroendocrine tumor; EGFR, epidermal growth factor receptor; TTF-1, thyroid transcription factor-1; ZEB1, Zinc finger E-box-binding homeobox 1.
invadopodia formation and MMP activity [36,37];
circulating tumor cells (CTCs) obtained from peripheral blood of
breast cancer patients frequently show an EMT-like
phenotype [88,89]. In human and murine malignant
melanoma cells, metastatic dissemination is enhanced and
accelerated via Snail-induced EMT , and bone
metastases of human prostate carcinomas show significant
overexpression of Notch-1 compared to the primary
tumors . In lung carcinoma surgical specimens, tumor
dedifferentiation as well as lymphogenous metastasis are
also associated with reduced E-cadherin expression .
However, as mentioned above, some authors also
reported reexpression of epithelial markers, such as
E-cadherin, along with loss of EMT-associated
transcription factors in established metastases . This
apparent reversal of EMT, often referred to as
mesenchymalepithelial transition (MET), has been described for
metastases of colorectal carcinoma, non-small cell lung
cancer and transitional cell carcinoma [92-94]. There is
an ongoing debate regarding the extent to which these
findings reflect a basic mechanism in the establishment
of metastases or if they are restricted to certain tumor
entities or reflect distinct circumjacent conditions [4,41].
There are also critical voices that doubt the role of EMT
in invasion at all, since in most histopathologic
specimens, many tumors invade and metastasize by cohesive
and multicellular rather than single-cell migration, and
histopathologists rarely see abundant mesenchymal-like
tumor cells in routine surgical specimens [4,95,96]. This
apparent contradiction might in part be explained by
regarding EMT as a transient state of a small proportion
of migrating tumor cells, with only single tumor cells
or small clusters of cells obtaining the ideal dynamic
configuration for different stages of invasion and
metastasis; this reasonable compromise has been referred
to as spatial and temporal heterogeneity of EMT by
Voulgari et al. (Figure 1) [97,98].
Notably, there is another controversy regarding the
point whether the EMT program is associated with
enhanced or attenuated proliferative activity of the cell.
While under normal circumstances TGF- signaling
exerts an anti-proliferative and pro-apoptotic effect, there
is experimental evidence that tumor cells having
undergone EMT do in fact show enhanced proliferation and
resistance to apoptosis [99,100]. This apparent
contradiction might also be explained by a possible heterogeneity in
the course and the extent of EMT, with specialized cell
populations exerting different roles during invasion and
metastasis; this is in line with findings that highly
metastatic breast cancer cells in fact show strong activity of the
TGF- signaling pathway . It has also been proposed
that the two oppositional endpoints of TGF- signaling
might be distinguished by loss of Smad4 in tumor tissue,
which promotes TGF--mediated tumorigenesis, while in
parallel abolishing its tumor-suppressive functions .
Additionally, as described above, signaling along various
non-TGF--dependent pathways might be capable of
overcoming the original anti-tumorigenic effect of TGF-
in the course of an unfriendly takeover of central TGF-
signaling nodes and target genes; concurrent PI3K/AKT
signaling, for example, thwarts the pro-apoptotic effect of
TGF-, thus selectively allowing for the pro-metastatic
effects of the pathway to occur [13,49,50,77].
EMT, cancer stem cells and therapy resistance
Concerning the role of EMT in antitumoral therapies, it
has been shown that an EMT-like cellular phenotype in
both surgical specimens and cell lines is associated with
increased resistance to most conventional approaches,
such as chemotherapy [103-105], radiotherapy  or
hormone withdrawal [107,108]. The observed changes in
gene expression during EMT show striking similarity to
a rather dedifferentiated state of the cell; in
immortalized mammary epithelial cells, induction of EMT not
only leads to the gain of a mesenchymal phenotype, but
also induces the expression of certain stem cell markers
(CD44+/CD24) . This generation of breast cancer
cells with both cancer stem cell and mesenchymal-like
characteristics has again been shown to be dependent on
an activation Ras/MAPK signaling, and the link between
EMT and cancer cell stemness is supported by the fact
that genes associated with angiogenesis, invasion and
metastasis are overexpressed in stem cell- like CD44+/CD24
breast cancer cells; notably, after chemotherapy for breast
cancer, residual tumor cells frequently display a stem
celllike phenotype and increased mammosphere formation
efficiency [101,110,111]. The sensitivity of non-small cell
lung cancer cells to EGFR kinase inhibition depends on
their respective EMT phenotype, with mesenchymal-like
cells (that express Vimentin or Fibronectin) being less
sensitive to EGFR inhibition . In the NSCLC model, it
has also been shown that this resistance might be
mediated via EGFR-independent MEK-Erk pathway activation
and PDGFR, FGFR and TGF- receptor acquisition in
mesenchymal-like tumor cells . An EMT-like gene
expression profile in lung cancer cell lines is in fact
associated with increased resistance to both EGFR and PI3K/
Akt pathway inhibitors, a finding that could even be
confirmed in a small patient cohort . Thus, the
mechanisms of resistance to antineoplastic therapies might be
due to stem-cell like properties of tumor cells that have
undergone EMT, allowing for self-renewing of a
proportion of cells within the tumor based on the activation of
central signaling pathways that are common to both
processes, such as TGF-, wnt, Notch and Hedgehog .
Associations between EMT-like properties and a stem-cell
like cellular phenotype have not only been described in
carcinoma of the breast and in NSCLC, but also in
urinary bladder, head and neck, pancreas, and colorectal
carcinoma; here, increased resistance to anti-epithelial
growth factor receptor (EGFR)-directed therapy is also
associated with an EMT-like phenotype of the tumor
EMT and patient prognosis
Since the metastatic spread of malignant tumors accounts
for the majority of cancer-specific deaths [116-118],
possible correlations between EMT markers and patient
prognosis have been intensely studied in multiple tumor
entities. However, there is still controversy regarding the
impact of the EMT concept on the actual situation in
human malignancies . Therefore, much effort has been
put into linking the expression of EMT markers to data
on patient survival. In colon cancer, the upregulation of
genes involved in EMT/matrix remodeling defines a
molecularly distinct subtype with very unfavorable prognosis;
downregulation of E-cadherin in patient samples, on the
other hand, seems to be associated with high TNM stages
and distant metastasis [120,121]. Accordingly, basal-like,
triple-negative breast cancers that show upregulation of
Vimentin have a poor prognosis [54,87]. In a
metaanalysis of 1107 breast cancer samples, Tobin et al.
showed reduced recurrence free survival in tumors
displaying increased gene expression of EMT markers
SNAI2, TWIST1 and VIM, and decreased levels of
CDH1 (encoding for E-cadherin) . In contrast, only
recently Lee et al. were unable to confirm an impact of
the tissue expression of EMT markers on disease-free
survival or overall survival in breast cancer patients
. In prostate cancer, expression levels of EMT
markers Twist and Vimentin - as assessed by
immunohistochemistry in radical prostatectomy specimens - are
independent predictors for biochemical recurrence as
defined by a resurgence in serum prostate-specific
antigen (PSA) levels following surgery . Additionally,
loss of membraneous E-cadherin staining seems to be
associated with increased Gleason score, advanced
clinical stage, and poor prognosis in prostate cancer .
In tissue samples from 354 primary tumors and 30
metastases of endometrial carcinomas, Tanaka et al. reported
that EMT status (E-cadherin-negative/ Snail-positive
immunostaining) correlated with histological type, FIGO
stage, myometrial invasion and positive peritoneal
cytology while it was inversely associated with both
progression-free survival (HR = 0.443) and overall
survival (HR = 0.366) .
Taken together, numerous studies in a variety of tumor
entities show statistical correlations between patient
prognosis and alterations of various markers compatible
with EMT. However, it may be difficult to yield reliable
prognostic information for an individual patient from
the expression pattern of EMT markers in surgical
specimens; this is in part due to high variability of
marker expression patterns in different tumor areas in a
heterogeneous sample [127,128]. Moreover, artificial
induction of EMT in vitro (under certain cell culture
conditions) as well as in vivo (in surgical specimens
subjected to ischemia) has been shown [129,130]. Another key
problem is the lack of a standardized diagnostic definition
of which gene or which extent of expression changes is
sufficient to determine EMT; in many reports, expression
changes of one or two genes are already referred to as
EMT or partial EMT, thus impairing the comparability
of studies [4,131]. Furthermore, as has already been
discussed above, it is still unclear whether the gene expression
changes observed in EMT reflect passenger mutations
caused by genetic instability during tumor dedifferentiation
rather than a real mesenchymal transdifferentiation state of
the cell . From this point of view, the expression of
EMT markers simply represents a more primitive
differentiation state of the cancer cell that is associated with
oncogenic activation of a variety of signaling molecules .
EMT as a potential target for antineoplastic therapies
Since the population of stem cell-like tumor cells will
always bear considerable resistance to conventional
therapies and since the hallmarks of EMT have been identified
in a significant proportion of these cells as described
above, efforts have been made to develop antineoplastic
therapies that directly target EMT. The aim of most
therapeutic approaches is to block or slow down invasion and
metastasis in tumors or, in benign conditions associated
with EMT, impede fibrotic organ remodeling [96,133].
Table 2 shows some current therapeutic approaches that
are aiming at EMT, most of them targeting kinase
signaling pathways upstream EMT master gene expression. In
mouse hepatocytes that have undergone EMT, it has been
shown that inhibition of STAT3 signaling, for example,
reduces EMT-like changes; in renal tubular epithelium, ALK
receptor activation via recombinant BMP-7 acts
antagonistically to TGF- and leads to reexpression of E-cadherin
[134-136]. Inhibition of kinase signaling downstream
FGFR3, ILK, Ras/MAPK or PI3K/AKT downregulates
tumor formation, EMT master gene expression and
invasive potential in colorectal, lung and pancreatic carcinoma
cells in vitro, while in some models, re-expression of
E-cadherin could be shown upon treatment with kinase
inhibitors [137-140]. In a mouse model of
hepatocellular carcinoma, transformation with kinase-inactivated
integrin-linked kinase (KI-ILK) partially restored the
sensitivity to anti-EGFR treatment . Accordingly,
our own group showed that Ras-driven EMT is
attenuated via Sorafenib-mediated inhibition of Urokinase
plasminogen activator (uPA) expression in RT112
urothelial carcinoma cells .
Table 2 Therapeutic approaches targeting EMT in benign and malignant processes
Urothelial carcinoma Urokinase
in situ (UCIS) plasminogen activator
STAT3, Signal transducer and activator of transcription 3; TGF-, transforming growth factor ; ALK3, activin-like kinase 3; BMP-7, bone morphogenetic protein 7, FGFR4,
fibroblast growth factor receptor 4; Src, sarcoma kinase; MEK, mitogen-associated protein kinase kinase; ERK, extracellular signal-related kinase; siRNA, small interfering
RNA; ILK, integrin-linked kinase; AKT, protein kinase B; HAT, histone acetyltransferase; HDAC, histone deacetylase; EGFR, epidermal growth factor receptor; RTK, receptor
tyrosine kinase; NSCLC, non-small cell lung cancer; LYN, Lck/Yes-related novel protein tyrosine kinase; MMP, matrix metalloproteinase; uPA, urokinase plasminogen
activator; ZEB1, Zinc finger E-box-binding homeobox 1; PI3K, phosphatidylinositol-3-kinase.
Tumor formation ,
Twist1 , ZEB1 ,
MMP-3, MMP-9 ,
Inhibition of STAT3
Antagonistic ALK receptor
Reduction of Src and
Reduction of Akt signaling
Sensitivity to anti-EGFR therapy 
HDAC expression, possibly via
inhibition of Ras/Raf/MAPK and
T-cell mediated cytotoxicity
Inhibition of Axl phosphorylation
Inhibition of LYN kinase activity
Changes in histone acetylation
and transcriptional repression
of EMT-related genes
Growth of mesenchymal
NSCLC xenograft tumors
Inhibition of Ras/MAPK signaling
Gli1, Ptch (Hedgehog
Inhibition of Hedgehog signaling Snail ,
siRNA Knockdown Inhibition of MAPK and
PI3K/AKT kinase signaling
From the knowledge of the diverse kinase-dependent
signaling pathways that are activated during EMT, it is not
surprising that the application of multi-kinase inhibitors
such as Sorafenib is capable of reversing the process to a
certain extent. Up to now, concepts that are directly
targeting EMT master genes or their effectors are rare.
Interesting new approaches include the previously mentioned
vaccination against Brachyury-positive tumor cells and the
transcriptional repression of EMT master gene expression
by the anti-diabetic drug Metformin (Table 2) [3,72,143].
Resveratol, a dietary polyphenol, downregulated
expression of EMT master genes Zeb1, Snail and Slug and
impaired CSC self-renewal capacity, tumor growth and
invasion in a mouse model of pancreatic ductal
adenocarcinoma . However, despite the abundance of
literature on effectors of EMT, there is a lack of studies that
show a solid effect of a specific compound in an in vivo
system additionally to cell culture data, and to our best
knowledge, a study that rescued the EMT phenotype after
application of a certain compound - for example by
overexpressing an EMT-inducing transcription factor - has
so far not been conducted. Therefore, most of the data
on drugs targeting EMT has to be regarded as
preliminary, and further research is needed to identify valuable
pharmacologic targets during the induction or
progression of the EMT process.
Taken together, the concept of EMT is a valuable model
for the morphologic and molecular changes observed in
tumor cell invasion as well as tissue fibrosis. However, it
is still unclear whether or to which extent cells in fact
do undergo a complete conversion of cell type or show
only transient changes in cellular morphology and
protein expression patterns that are supportive of a
migratory phenotype. Despite the controversies dealing with
the definition and extent of EMT, the association
between an EMT-like cellular phenotype - as shown by
changes in marker protein expression - and tumor
aggressivity has been well-proven in a variety of
malignancies. In recent years, first promising results have been
reported concerning a possible use of the EMT process
as a pharmacological target, especially with multi-kinase
inhibitors such as Sorafenib. However, since most of
these results are actually derived from in vitro data and
definite proof of druggable EMT in vivo is still missing,
the clinical utility of these approaches remains to be
elucidated in future studies.
AJ: Adherens junction; CXCR-1: CXC motif chemokine receptor
1/Interleukin8-receptor alpha; ECM: Extracellular matrix; EMT: Epithelial-mesenchymal
transition; FGFR: Fibroblast growth factor receptor; FIGO: Fdration
internationale de Gyncologie et dObsttrique; GSK3: Glycogen synthase
kinase 3; HDAC: Histone deacetylase; HIF: Hypoxia-inducible factor;
MAPK: Mitogen-associated protein kinase; MEK: Mitogen-associated protein
kinase kinase; MET: Mesenchymal-epithelial transition; MMP: Matrix
metalloproteinase; NSCLC: Non-small cell lung cancer; PAR: Partitioning
defective; PDGFR: Platelet-derived growth factor receptor;
PI3K: Phosphatidylinositol-3-kinase; RTK: Receptor tyrosine kinase;
TGF-: Transforming growth factor ; TNM: Tumor/Nodes/Metastasis
(clinical classification system for tumor spread); uPA: Urokinase plasminogen
activator; ZO-1: Zonula occludens 1.
Prof AJ Schrader receives compensation as a consultant for Bayer Healthcare
AG, which manufactures Sorafenib (Nexavar) for clinical application.
All authors participated in the design of this review. KS, SE and JS reviewed
literature on the molecular basis of EMT, on EMT markers and on EMT in
tumor invasion and metastasis; KS and AJS reviewed literature on the
association between EMT and cancer stem cells, therapy resistance and
patient prognosis as well as on EMT as a pharmaceutical target. All authors
read and approved the final manuscript.
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