Background
Epithelial-mesenchymal transition (EMT) is a biological process in which non-motile, polarized epithelial cells undergo a series of biochemical alterations, becoming motile non-polarized mesenchymal cells with invasive capacity, resistance to apoptosis and altered biosynthesis of extracellular matrix (ECM) components. Epithelial and mesenchymal cells differ in their morphology and tissue organization. In a typical epithelium, cells are organized either as a single layer or in multi-layered sheets. In the latter case, structure is maintained through cell-cell interactions including tight junctions, gap junctions, cadherin based adherent junctions, desmosomes and ECM interactions [
1,
2]. These junctions and interactions impede the movement of individual cells within the epithelial monolayer [
3,
4]. Mesenchymal cells rarely establish tight junctions with surrounding cells and are embedded inside the extracellular matrix [
5].
Cytoskeletal changes and cell signaling pathways are altered as a cell undergoes EMT. Processes known to contribute to EMT include the activation of transcription factors (TFs) such as SNAIL, SLUG and TWIST, altered expression of specific cell-surface proteins, reorganization and expression of cytoskeletal proteins, production of ECM-degrading enzymes, and changes in the expression of specific microRNAs [
6,
7]. EMT is initiated following the dissolution of tight junctions resulting in the loss of apical-basal cell polarity [
8,
9]. Other types of cell junctions are disassembled as well, such as gap and adherent junctions, leading to the loss of basement membrane integrity. The cytoskeleton also undergoes characteristic reorganization such as increased allocation of actin into stress fiber formation and the replacement of cytokeratin intermediate filaments by vimentin. These alterations enable the transition into a spindle-shaped cell morphology from a cuboidal/columnar precursor, correspond with an increased ability to invade surround tissue [
10‐
12]. A cell is considered to have undergone EMT following the loss of epithelial marker expression in tandem with the development of mesenchymal marker expression. Key epithelial markers lost include E-cadherin (
CHD1), Mucin-1, Cytokeratins (such as CK19, CK18, CK8), Occludin and Desmoplakin. Oppositely, markers gained during the process include N-cadherin, Vimentin, Smooth Muscle Actin (αSMA), Fibronectin, and Vitronectin, which together comprise the key mesenchymal markers [
6,
13‐
16]. In addition numerous proteins not located on the cell surface also undergo key changes in localization. β-catenin, a component of the cadherin complex is one such example. During EMT, β-catenin dissociates from the cadherin complex and is translocated into the nucleus where it behaves as a transcription factor, regulating the expression of several genes in key pathways such as Wnt signaling. Importantly, the changes observed in cells to revert back to a epithelial-like phenotype upon arrival at a suitable location to colonize, a process prudently entitled mesenchymal to epithelial transition (MET) [
5].
EMT has been classified into three categories: type I, type II and type III [
17,
18]. Type I occurs during embryogenesis where cells need to migrate to adjacent tissues in order to form new organs and tissues [
5]. Type II is associated with the wound healing, whereby fibroblasts repair or rebuild tissues [
6]. Unlike types I and II which perform necessary physiologic functions, type III is a pathophysiologic adaptation of the process, and is closely associated with progression of neoplasia occurring in cells containing certain epigenetic and genetic changes [
4,
19]. It is currently theorized that exploitation of the normal EMT signaling pathways provides the molecular genetic basis for how neoplastic (but differentiated) cells can shed their epithelial characteristics and acquire migratory properties. Having undergone such a change, the cells are subsequently able to invade tissues surrounding the primary tumor, extravasate into lymphatics or blood vessels, travel to distant sites through the circulation, and ultimately colonize a metastatic niche [
18,
20]. It is important to highlight that oncogenic EMT is a transient process that may function in a paracrine fashion and is followed by MET once the tumor cells reach the metastatic site [
21].
The EMT program is activated by multiple signaling pathways as well as several epigenetic and post-translational modifications such as methylation, acetylation, phosphorylation, glycosylation, hydroxylation and SUMOylation. Epigenetic modifications including modification of histone protein tails, and DNA promoter regions, play a key role in regulating gene expression by defining whether chromatin at a given genomic locus will be transcriptionally active or inactive [
22]. Post translational modifications are covalent modifications that occur after transcript has been translated into protein [
23]. Improving our understanding of how these modifications function in the regulation of EMT is of crucial importance, and likely instance where novel therapeutics might be developed to better treat diseases such as cancer [
24]. Since the EMT program is regulated dually by epigenetic and post translational modifications, we will focus closely on these two mechanisms as they pertain to EMT in this review. In addition, we will provide a current overview of the various therapeutic approaches currently being investigated to undermine EMT.
E-cadherin as a key epithelial marker
The
CDH1 gene is located on chromosome 16q22.1 and codes for the subtype of cadherin protein expressed by epithelial cells (E-cadherin). Functionally, E-cadherin behaves as a tumor suppressor gene and plays diverse roles in regulating cell polarity, differentiation, migration and stem cell-like properties. In the context of cell polarity, E-cadherin binds to adjacent cells creating an intercellular complex that forms epithelial barriers. The extracellular portion of E-cadherin binds to cadherins on an adjacent cell creating a bridge between the cytoskeletons of contiguous cells. The intracellular domain of E-cadherin interacts with β-catenin, which itself is linked actin filaments within the cells via a linker protein called α-catenin [
25‐
27].
Down-regulation or inactivation of
CDH1 has been frequently observed during tumor cell progression, and several mechanisms have been proposed [
28]. These include germline mutations [
29,
30], promoter hypermethylation [
31,
32] and upregulation of E-cadherin transcriptional repressors [
10], alternatively known as EMT transcription factors (EMT-TFs). Transcription factors such as SNAIL, SLUG, ZEB1, and ZEB2/SIP1 are considered direct repressors of E-cadherin as they bind to E-boxes present on the
CDH1 promoter [
10,
33,
34]. Indirect repressors include bHLH proteins (TWIST1 and TWIST2), homeobox proteins (GSC and SIX1), the bHLH factor E2.2 and the forkhead-box protein FOXC2 [
2,
10]. Additionally, while the TWIST proteins are commonly recognized as an indirect repressors of
CDH1, they can also bind directly on E-boxes 2 and 3 present on the
CDH1 promoter to repress its expression [
35].
Conclusion
Approximately 90 % of cancer mortalities occur in patients with tumors derived from epithelial tissues, and the primary cause of death in such cases results from dissemination of tumor cells to distant organs [
139]. As such, understanding the cellular mechanisms contributing to metastasis is paramount in the effort to improve outcomes. EMT is a process in which tumor cells within the primary tumor lose their cell junctions and their epithelial morphology changes to fibroblastoid morphology. These changes allow the cells to invade the surrounding tissue of the primary tumor, intravasate into the bloodstream and lymphatic vessels as circulating tumor cells (CTC), and extravasate to distant sites where they may colonize distant organs as epithelial metastasis. Although EMT is a process that occurs under normal conditions such as wound healing and embryogenesis, the misappropriation of these pathways during tumor progression is an unpredictable and disastrous event with the simultaneous activation of different molecular cascades.
Many pharmacological approaches, including chemical inhibitors and monoclonal antibodies that target several proteins that regulate cancer progression have been devised and show promising results for the treatment of a variety of cancers. However, very little research has been done to target post translational modifications of proteins in cancers, and thus we believe that identifying inhibitors for post-translational modifications represents an underexplored area which may hold significant potential, and thus should be a high priority in the development of future cancer treatments. Furthermore, the identification of these post-transcriptional and post-translational modifications is important given that these changes could be identified in the primary tumor before metastasis occurs. Such knowledge would allow clinicians to better predict which patients have genotypes more likely to follow an aggressive clinical course prone to development of metastases. These patients could then be treated with different approaches from the onset of disease to reduce the risk of metastasis, and allow for better prognoses and ultimately, enhance survival.
Competing interestss
The authors declare that they have no competing interests.
Authors’ contributions
SA conceptualized, SG wrote, and SA, MM and SG finalized the manuscript together. All authors read and approved the final manuscript.