Page 121 - Read Online

Ponnusamy et al. Cancer Drug Resist 2019;2:297-312 I http://dx.doi.org/10.20517/cdr.2018.11 Page 305

be activated by concurrent pathways that are associated with EMT-CSCs, hence the co-existence is often
observed in resistant population [Figure 2] [20,53] .

This section of the review further describes the reported epigenetic alterations on target genes, transcription
factors, and signaling pathways that mediate the EMT-CSC pathways and associated acquired resistance
in breast cancer. Studies involving genome-wide mapping and global profiling postulate that EMT is
characterized by reprogramming of selective chromatin domains across the genome [53,62,63] . To begin with,
loss of E-Cadherin (CDH1) expression, the initial step in EMT process, shown to occur due to promoter
methylation, chromatin modification and associated transcriptional repression . For example, loss of
[8]
E-cadherin and ESR1 due to promoter hypermethylation have been reported in primary breast carcinoma
cells and doxorubicin-resistant breast cancer cells and demethylation restored the expression levels of
[64]
[53]
these genes . Genome-wide methylation in doxorubicin resistant breast cancer cells found EMT pathway
[64]
as a critical process for acquisition of resistance . Likewise, Chekhun et al. revealed that pronounced
[53]
[12]
global epigenetic changes such as loss of global DNA methylation, elevated phosporylation of H3S10, loss
of H4K20me3, reduced Suv4-20h2 activity in chemoresistant breast cancer MCF-7 . In the same study,
[53]
hypomethylation of Activin A, a negative regulator for E-Cadherin and FOXK1, a positive regulator for
Snail-associated EMT were also observed. Correspondingly, the HAT p300 in co-ordination with C-terminal
binding protein (CtBP) initiate EMT through the acetylation of SMADs in the TGF-β signaling pathway.
Elevated HAT p300 increased vimentin expression and associated with migration, tumor recurrence, and
chemoresistance . This notion was further supported through depletion p300 that induced epithelial
[65]
phenotype characterized by decreased vimentin and elevated E-cadherin .
[66]
Aberrations in EMT-TFs and associated epigenetic changes have been evident in breast cancer acquired
resistance. Overexpression of Snail 1, ZEB1, and Twist linked to poor treatment response and acquired
resistance in breast cancer through the EMT pathway [67,68] . Epigenetic modulations in support of these EMT-
TFs indicate that Twist, Snail, Slug, and ZEBs coordinate to recruit chromatin modifying complexes after
binding with E-cadherin (CDH1) promoter [8,68] . For example, Snail 1 interacts with DNMTs, G9a, Suv39H1,
EZH2, and LSD1 to silence the E-cadherin promoter during activation of EMT signaling. Specifically,
Snail 1 recruit the repressor complex SIN3/ HDAC1/2 to CDH1 promoter, and coordinates with LSD1 to
demethylate the CDH1 followed by interaction with G9a/ Suv39H1 to induce deacetylation of histone H3/
H4 and trimethylation of H3K9 to repress E-cadherin expression Similarly, ZEBs interact with CtBP
[43]
to form ZEB1/p300/PCAF complex that activate ataxia-telangiectasia mutated (ATM) kinase pathway .
[68]
Activated ATM signaling facilitate efficient DNA repair and associated chemoresistance . Notably, Twist
[68]
interact with nucleosome remodeling and deacetylase complex containing Mi2/NuRD-MTA2, RbAp46 and
HDAC2 that result in H3K9 deacetylation and H3K9 methylation to mediate promoter-associated silencing
of E-cadherin and ER-α . These findings further support that epigenetic alterations take part in EMT
[69]
induction to facilitate resistance development.

Elevated SIRT1 deacetylase modulate the nuclear localization of FOXO1, an apoptosis associated EMT-TF
and deacetylate the acetylated DNMT1 thus inhibition of SIRT1 enhance the DNMT1-mediated silencing of
ER-α and CDH1 reflecting mesenchymal acquisition in MDA-MB-231 breast cancer cells . Overexpression
[70]
of epigenetic repressor EZH2 involve in EMT, self-renewal potential of CSCs, aggressiveness, poor survival,
tumor recurrence and resistance in breast cancer . Basal-like breast cancer cells that express mesenchymal
[71]
characteristics overexpress EZH2, co-ordinates with SUZ12 to form PRC2 complex and recruited to CDH1
promoter to represses E-cadherin . ZEB1/2-mediated repression of E-cadherin associated with DNA
[8]
methylation-regulated silencing of polycomb protein Mel-18 to induce EMT in breast cancer cells.
[72]
An EMT-associated HMT, G9a, increase de novo methylation of CDH1 promoter and loss of its expression .
[43]
The SET8 localized to CDH promoter and methylate H4K20 that causes loss of E-Cadherin and gain of