The Epithelial-Mesenchymal Transition (EMT) is a critical biological process that plays a role in embryogenesis and wound healing. However, it is also a key process involved in cancer progression and metastasis. The EMT is a key transition from a primary tumour to an invasive metastatic tumour. Hence, the role of the EMT in cancer biology has been a growing area of interest as a novel therapeutic avenue for cancer treatments targeting metastasis. 

Metastasis: When cancer spreads 

Metastasis is the process where cancer spreads from a primary tumour to other sites in the body to form secondary tumours.¹ Invasion and Metastasis are hallmarks of cancer, constituting the primary cause of death for >90% of patients with cancer.² Cancer can spread to many parts of the body, however, different types of cancer are more likely to spread to certain areas than others.¹ The most common targets of metastatic migration are the bones, liver, and lungs.¹ Metastasis is a multistep process involving several key steps, with the EMT being a crucial factor facilitating the spread of tumour cells. 

What is the EMT? 

The transformation of epithelial cells into mesenchymal cells, known as epithelial-mesenchymal transition, is critical in facilitating the increase in migration and invasion to promote the metastatic ability of tumour cells.⁵ The EMT involves the transformation of a polarised, stationary epithelial cell to undergo biochemical changes to transform into a mesenchymal cell phenotype.⁴ EMT represents a continuum of molecular changes involving the transformation of a polarised, stationary epithelial cell to undergo biochemical changes to transform into an invasive mesenchymal cell phenotype.⁴

Characteristics of Epithelial cells vs. Mesenchymal cells

Morphology 

Epithelial cells are cuboidal with a flat or columnar shape, arranged into tightly packed layers. In contrast, Mesenchymal cells have an elongated morphology found in an irregular arrangement.⁴ 

Cell polarity 

Epithelial phenotypes exhibit an Apical-Basal Polarity, meaning a distinct apical surface faces the extracellular space and a basal surface is attached to the basement membrane. As a result of this apical-basal polarity, the epithelial cells have polarised functions.⁴ In contrast, mesenchymal phenotypes lack apical-basal polarity, rather exhibiting front-back polarity, which supports directional movement and promotes invasion and migration. 

Cell-Cell adhesion 

Mesenchymal cells exhibit reduced cell-cell junctions compared to endothelial phenotypes due to the downregulation of E-Cadherin, leading to the loss of tight cell-cell junctions.⁴ In addition, mesenchymal cells form focal adhesions with the ECM mediated by integrins to facilitate migration and invasion into surrounding tissues. 

Cytoskeleton reorganisation 

Epithelial cells exhibit a highly organised actin cytoskeleton mediated by keratins, while mesenchymal cells reorganise their cytoskeleton to form stress fibres. This switch is known as the intermediate filament switch and involves the switch from keratin to vimentin.⁹ This enables the mesenchymal phenotype to adopt migratory properties to promote tumour invasiveness. 

Interaction with the extracellular matrix (ECM)

Epithelial cells are firmly attached to a basement membrane, a specialised ECM separating the epithelial cells from underlying tissues.¹¹ In contrast, mesenchymal cells are embedded within and actively remodel the components of the ECM, including collagen, fibronectin, and proteoglycans.¹⁰ This remodelling is enabled by mesenchymal cell secretion of Matrix Metalloproteinases (MMPs), which degrade the ECM, facilitating invasion into surrounding tissues.¹⁰

EMT in the metastatic cascade

There are five key stages of metastasis. EMT is integral for enabling the progression of these various stages, culminating in the development of a secondary tumour.

Local invasion 

Local invasion is the consequence of epithelial cells acquiring mesenchymal traits that enable the ability to infiltrate local adjacent tissues by degrading the basement membrane.⁵ This phenotypic progression towards increased invasiveness depends on the activation of EMT.⁵ 

Intravasation

Intravasation into the blood and lymphatic vessels is mediated by EMT, causing the acquired migratory capacity of mesenchymal cells. Additionally, EMT facilitates vascular permeability, further promoting cancer cells' intravasation into blood and lymphatic vessels. This occurs via multiple mechanisms, including the disruption of cell-cell junctions, cytoskeletal remodelling, ECM degradation by secretion of MMPs, and the production of pro-permeability factors such as vascular endothelial growth factor (VEGF), which promotes vascular permeability by affecting endothelial cell junctions.⁶

Circulation and survival in the vasculature

Within the circulation, EMT provides cancer cells with survival advantages, such as resistance to anoikis (detachment-induced cell death) and promoting the evasion of immune surveillance. Acceleration of Cancer metastasis has been associated with immunosuppression during Snail-induced EMT of cancer cells⁷. This causes the downregulation of surface antigens and the production of immunosuppressive factors, enabling enhanced circulation survival. 

Extravasation 

Extravasation involves the exit of cancer cells from circulation into secondary tissues, a process mediated by the expression of cell surface molecules like integrins and selectins, which interact with endothelial cell receptors. EMT-induced cancer cells secrete factors that disrupt endothelial junctions, making it easier to penetrate the endothelial layer. Furthermore, EMT induces various mechanisms of survival to establish a metastatic niche. 

Colonisation and metastatic outgrowth 

The Mesenchymal-to-Epithelial Transition (MET) at secondary sites enables the establishment of secondary tumours. The switching between epithelial and mesenchymal states is known as plasticity and is a hallmark of metastatic cancer cells. 

Molecular mechanisms underlying EMT

The molecular mechanisms underlying EMT are complex and involve multiple signalling pathways and transcription factors. 

Activation of EMT transcription factors

Key transcription factors include Snail, Slug, Twist, and ZEB1/2, which are involved in the repression of epithelial markers such as E-cadherin and cytokeratins while promoting the expression of mesenchymal markers such as N-cadherin, vimentin, fibronectin, and MMPs.⁴

Activation of Signalling pathways 

TGF-β pathway

TGF-β induced EMT by activating SMAD-dependent and SMAD-independent pathways, leading to the activation of EMT-TFs involved in the repression of epithelial characteristics in exchange for mesenchymal characteristics to promote invasiveness.⁴

Wnt/β-catenin pathway

Wnt signalling stabilises β-catenin, which moves to the nucleus and activates EMT-related genes.¹³

Notch pathway

Notch signalling promotes EMT by upregulating Snail and other EMT-TFs.¹³

Hedgehog pathway 

The Hedgehog pathway induces EMT by regulating the expression of Transcription factors such as Gli and Snail.¹³

Receptor Tyrosine Kinase (RTK) pathways

Growth factors, such as epithelial growth factors, activate RTKs to trigger downstream signalling cascades. These signalling cascades include the PI3K/AKT pathway and RAS/RAF/MEK/ERK, which both have downstream effects that promote EMT.¹³

The tumour microenvironment (TME) and EMT

The Tumour Microenvironment has been recently uncovered as an active promoter of cancer progression and metastasis rather than a silent bystander as previously assumed.³ A tumour is not purely a group of cancer cells, but rather a heterogeneous collection of infiltrating and resident host cells, secreted factors and extracellular matrix constituting the TME. There is a dynamic relationship between the TME and cancer cells, with the TME promoting cancer cell survival, local invasion and metastatic dissemination.¹⁴ This support is observed through interactions between cancer cells and the stroma, including cancer-associated fibroblasts (CAFs) which secrete MMPs to remodel the extracellular matrix to facilitate tumour invasiveness.¹⁴ Additionally, the TME secretes inflammatory cytokines and growth factors such as IL-6, TNF-α, and TGF-β.¹⁴ These factors further support the induction of EMT and aid in sustaining the mesenchymal phenotype to promote further invasiveness and metastasis. 

Therapeutic implications of EMT

Phylogenetic evidence of the evolution of metastasis provides evidence that metastases are seeded at an early stage before the disease is manifested clinically.⁸ This highlights the promising strategy of targeting EMT to inhibit cancer metastasis and improve patient outcomes.

Targeting EMT pathways 

Inhibiting key signalling pathways that regulate EMT, such as TGF-β, Wnt/β-catenin, and Notch pathways, are promising therapeutic strategies.¹⁵ When these pathways are dysregulated, the EMT process can be disrupted, preventing cancer cells from acquiring the traits necessary for metastasis.

Small molecule inhibitors and monoclonal antibodies are alternative strategies for dysregulating EMT pathways. Small molecule inhibitors block the activity of enzymes and kinases involved in EMT signalling, while monoclonal antibodies can target growth factors and their receptors, preventing the initiation of EMT.¹⁵

Mesenchymal-epithelial transition (MET) inducers have been developed to reverse EMT to restore epithelial characteristics, reducing their migratory and invasive capabilities, which worsens patient prognosis.¹⁵

Combination therapies 

Combination therapies offer improved clinical outcomes for cancer patients by targeting multiple aspects of tumour progression. Therefore, combining EMT inhibitors with chemotherapy enhances the overall efficacy of cancer treatment by targeting rapidly dividing cells and preventing the cancer cells from becoming invasive and metastatic. 

EMT inhibitors have been shown to promote immunotherapeutic strategies targeting cancer cells.¹⁶ Through the reversal of EMT, cancer cells become more easily detectable by the immune system compared to their usual immune evasiveness. Therefore, this synergistic interaction between EMT inhibitors and immunotherapeutic approaches is a promising avenue for cancer therapeutics.¹⁶

Challenges 

Drug resistance and EMT plasticity pose a challenge to cancer therapeutic development.¹⁷ The dynamic nature of EMT enables cancer cells to gain resistance to therapies which are only able to target one state. Therapies targeting EMT and MET states are currently being developed.¹⁷

Future directions 

Personalised medicine may hold the answer for the future of EMT-targeting therapies. This allows for more effective targeting of individual signalling pathways. However, before this is a reality, advancements in diagnostic tools to assess EMT status are required. 

Summary 

• EMT plays a crucial role in cancer progression and the key stages of the metastatic cascade
• EMT drives the transformation of epithelial cancer cells into invasive mesenchymal phenotypes with enhanced migratory properties and increased invasiveness
• Molecular mechanisms, including the activation of EMT transcription factors, downstream signalling cascades and interactions with the tumour microenvironment, facilitate the metastatic cascade and contribute to the formation of secondary tumours
• Understanding the interaction between the EMT and metastasis is a promising strategy for developing novel therapeutic strategies to improve cancer patient outcomes
• Through a combination of MET inducers, signalling pathway inhibitors, and synergistic treatment strategies, the challenges posed by EMT can be addressed, paving the way for single therapies targeting multiple EMT states and personalised cancer therapies