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independent TGF-β signaling in breast cancer stem cell induction is not yet clearly understood. Moreover,
elicitation of TGF-β modulates Wnt and NF-κB signaling to support vimentin-dependent mesenchymal
state, enrich CSCs, and maintain stemness as well as tolerance [14,27] . TGF-β mediated EMT-CSC-based
resistance has been evident from various studies. For example, doxorubicin resistant MCF-7 breast cancer
cells acquired signature mesenchymal and invasive phenotype. This aggressive phenotype accompanied with
Snail 1-mediated loss of E-cadherin and ESR, estrogen independency, tolerance to TNF-mediated apoptosis
and concurrent alteration in TGF-β and NF-κB signaling [20,31] .

Wnt-β-Catenin signaling
Among all the EMT-CSC-shared signaling pathways, Wnt/β-catenin signaling is crucial for EMT and
proliferation and self-renewal of normal as well as cancer stem cells [32,33] . Induction of Wnt signaling
modulates β-catenin stability and its nuclear translocation, to recruit histone modifying co-activators
including HAT CBP/p300 and BRG1 to activate downstream transcription of various targets including
Snail, Twist, Slug, ZEB1, vimentin, fibronectin, and MMPs to promote EMT and associated migration .
[34]
Wnt signaling not only induce EMT-TFs such as Slug and Twist but also shown to be highly activated in
mammary stem cells contributing to their increased potential for self-renewal [32,35] . Through transcriptional
regulation of MDR1 gene, Wnt/B-catenin signaling supports emergence of chemoresistance with acquisition
of EMT phenotype . Wnt antagonist secreted frizzled-related protein 1 (SFRP1) known to sensitize breast
[33]
CSCs to doxorubicin/cisplatin-induced apoptosis . Besides, depletion of Wnt signaling abrogated stem cell
[36]
sub-population with CD44-high/ CD24-low, and ALDH1 markers and decreased metastatic potential in vivo .
[32]
Chemotherapy resistant and non-responsive breast cancer cells found to have elevated anti-apoptotic protein
Survivin which is also a Wnt-β catenin target and has been linked to transient acquisition of EMT and
stemness .
[37]
Hedgehog signaling
Hedgehog (Hh) signaling have been shown to be de-regulated in various solid tumors and associated
with EMT and stemness . Activated Hh signaling increases Snail expression to repress E-Cadherin and
[14]
to promote EMT and stemness which may eventually contribute to chemoresistance phenotype [38,39] . Hh
signaling pathway also known to interact with Wnt-β-catenin signaling to promote carcinogenesis and
cancer aggressiveness. This interaction of signaling can modulate both EMT and CSC pathways through
elevating key TFs (such as Snail, Slug, ZEB1, ZEB 2, TWIST2 and FOXC2) and stem cell markers (such as
BMI1, CD44 and CD133) . Enrichment of aberrant Hh signaling mechanisms in aggressive, metastatic and
[38]
chemoresistant breast cancers types lead to low levels of E-cadherin and increased expression of FOXC2,
SIP1, Snail, and Twist, vimentin, fibronectin and N-cadherin [14,24,38] . Modulation of these proteins sensitize
resistant cancer cells to chemotherapy which further establish the role of Hh signaling in chemoresistance.

Notch signaling
Notch signaling influences cell proliferation, differentiation and dictates cell fate and apoptosis. Notch
signaling is regulated through Notch receptors (1-4) and ligands such as Delta-like ligand (DDL 1/3/4) and
Jagged 1/2 . Notch signaling co-operates with transcription factors of EMT (Snail and Slug) and stemness
[40]
(SOX2, Nanog and OCT4) and facilitate acquisition of both EMT and stemness [14,40] . Notch, coupled with
TGF-β, induces Slug expression to mediate EMT . Notch signaling also regulates different target genes
[40]
related to CSCs and mediate chemoresistance [41,42] . For example, IL-6, a Notch target gene supports self-
renewal potential of CSCs and Notch-mediated activation of PKB protect cells from apoptosis to induce
resistance . Deactivation of Notch signaling kill progenitor cells that are similar to CSCs which further
[41]
supports the role of Notch in stemness and associated chemoresistance. MDR1 is highly expressed in CSCs
while Notch signal coupled with NF-κB and associated PI3K/Akt activation regulate MRP2 transporter that
favors maintenance of stemness .
[20]