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THE ROLE OF EMT IN TUMOUR IMMUNE EVASION AND DRUG RESISTANCE
EMT is a crucially important causative factor in tumour immune evasion and drug resistance. EMT has
been shown to suppress cytotoxic T-lymphocyte mediated immunity in MCF7 cells by activating autophagy
[30]
instead of apoptosis . EMT-activated MCF7 cells express high levels of programmed death-ligand 1 (PD-L1)
which contributes to suppression of tumour immunity. A ZEB1 transcription factor is important in this
[31]
process and miR200 family members negatively regulate it . EMT-related gene expression contributes to
immunotherapy resistance against programmed cell death protein 1 inhibitors in urothelial cancers, non-
small cell lung cancers (NSCLC) and triple negative breast cancers (TNBC) [32,33] . EMT contributes in the
development ofchemoresistance in breast, pancreatic and bladder cancers [14,29,34-36] . Both adriamycin- and
vinblastine-resistant MCF-7 breast cancer cells lose epithelial markers like keratin-19 and uvomorulin
expression, and overexpress mesenchymal markers such as vimentin. They also show reduced desmosome
formation and tight junctions [37,38] . This resistance is achieved by a variety of signalling events that
contribute to EMT. Gottesman et al. reviewed the role of ATP-binding cassette (ABC) transporters
[39]
in the multi-drug resistance of cancers. These are overexpressed in many cancers and the inhibition of
them contributes to the reversal of resistance. MRP1 (ABCC1) is a multidrug ABC transporter that gives
resistance to lung, breast, and prostate cancers, and neuroblastoma, and is expressed by the transcription
[40]
factor N-myc proto-oncogene (MYCN) . Breast cancer resistance protein (BRCP/ABCG2) is one more
example of an ABC transporter whose normal function removing toxins and xenobiotics in healthy
[41]
tissues are repurposed in cancers towards multidrug resistance . Most of these ABC transporters
are overexpressed by transcription factors such as Twist, Snail, and FOXC2, which contribute to the
progression of EMT . Thus, targeting EMT via these transcription factors is a viable strategy to overcome
[42]
[43]
drug resistance. Witta et al. proved that the reversal of EMT by overexpressing E-cadherin successfully
sensitized NSCLC towards epidermal growth factor receptor (EGFR) inhibitors. They did this by pre-
treating resistant cells with MS-275, a histone deacetylase (HDAC) inhibitor which activated E-cadherin,
followed by treatmentwith gefitinib and erlotinib, which are EGFR inhibitors . Recent clinical work also
[43]
suggests that EMT contributes to drug resistance by increasing cancer stem cell (CSC) markers. Treatment
of a patient with metastatic prostate cancer with the PARP inhibitor talazoparib increased NANOG, CD133,
[44]
CD44v6, and ALDH1, which are CSC markers . TGF-β1-induced EMT increases ALDH expression and
leads to the generation of CSCs. This contributes to decreased drug sensitivity, increased proliferation,
[45]
invasion and metastasis, and poor prognosis . Owing to the fact that EMT is often regulated by many
signalling pathways, it is nearly impossible to target it with single-agent therapies . Optimal combinatorial
[46]
approaches are needed for specific types of cancer after understanding the molecular drivers of EMT.

EXISTING SCREENING TECHNOLOGIES FOR TARGETING EMT AND THEIR LIMITATIONS
Many drugs were tested and their variable efficiency against EMT has been demonstrated. These drugs
encompass all the pathways which contributeto EMT development . The scope of this review is the
[47]
methodologies for screening drugs against EMT. They can be broadly divided into two categories:
2-dimensional (2D) models, and 3-dimensional (3D) models. Alongwith increased dimensionality, the
complexity of 3D models increases compared to 2D models, and there is decreased ease of handling.

2D models
As EMT is a 3-dimensional phenomenon, it is not possible to model it in 2-dimensions. Thus, these
2D models utilize only certain EMT features for designing the assays. These are easy to handle and well
amenable to high throughput screening (HTS) systems. Most of these assays are based on reporter constructs
[48]
which can measure the activation of CDH1 promoter (for E-cadherin protein) elements. Chua et al.
developed a high throughput assay based on EMT spot migration, where they seeded epithelial cells as a
spot and induced it with EMT inducers. The resulting mesenchymal cells will be loosely arranged and the
area of the spot increases. They screened several drugs on inhibition of this spot area and found that ALK5,