Hedgehog Signalling in EMT - Therapeutic Strategies in Cancer Biology and Pathology - page 33

Hh signalling is known to interact with the Wnt pathway and so expected to
modulate EMT ( Figure 4.1 ). Wnt5A occurs as two isoforms with alternative exons
1A and 1B. Smad binding elements and bHLH binding sties occur in the promoter
A close 5′ to exon 1A, whilst NF-κB and two Smad binding sites occur within the
promoter B region, 5′-adjacent to exon I B. In the absence of Gli binding site, it
seems likely that the TGF-β can directly upregulate the expression of Wnt5a pos-
sibly through the agency of Smad and indirectly by the Smad/NF-κB route (Katoh
and Katoh, 2008, 2009). TGF-β is capable of upregulating SHh in NSCLC cells
accompanied by the acquisition of EMT features. Inhibition of Hh signalling leads to
reduction in the expression of ZEB-1 and fibronectin and upregulation of E-cadherin
indicating the reversal of EMT. Inhibition of SHh also reduced EMT inducing effects
of TGF-β (Maitah et al., 2011). Besides TGF-β, Hh can also function in conjunction
with EGF, FGF, HGF, together with the transcription factors Snail, Slug, Twist and
ZEB and often implicated in this way in many facets of cell proliferation, embryonic
development and pattern formation and possibly also in stem cell pluripotency and
perpetuation (Sherbet, 2011a).
Besides Hh and TGF-β, Wnt can interact with Notch signalling in differenti-
ating systems as well as in the activation of EMT. Indeed, Notch, Wnt and TGF-β
and other members of the TGF-β family are linked together and possibly also func-
tionally by the Notch ligand Delta-like 1 and lef and Smad components of signal-
ling downstream (Bordonaro et al., 2011). There is a strong possibility that activation
of Notch signalling might downregulate E-cadherin expression. Overexpression
of Notch-1 activates EMT results in the expression of ZEB-1 which suppresses
E-cadherin and other markers such as CD44 and EPCAM, the epithelial cell adhe-
sion molecule which is overexpressed in many tumours (see also Figure 4.3 ). The
expression of a number of miRNAs is also altered; the expression of miRNA-21
is upregulated, and that of miRNA-200b, miRNA-200c, let-7a, let-7b and let-7c is
downregulated. Forced re-expression of miRNA-200b decreased the expression of
ZEB-1 and vimentin, and increased expression of E-cadherin (Bao et al., 2011a). The
intervention of miRNA in Notch-mediated induction of EMT is supported by other
studies. Brabletz et al. (2011) showed that downregulation of miRNA-200 leads to
enhanced expression of ZEB-1, a suppressor of E-cadherin and activator of EMT, and
Jagged 1, which is a downstream effector of Notch. A general reading of these data
would be that Notch suppressed the expression of E-cadherin and activated EMT.
On the other hand, it has been argued that enhanced E-cadherin inhibits the Notch
pathway and conversely that Notch activity is enhanced in E-cadherin deficient cells
(Ferreira et al., 2012). Inhibition of Notch as indicated by using DAPT (( N -[ N -(3,5-
difluorophenacetyl)-l-alanyl]- S -phenylglycine t -butyl ester) also suppressed growth
of E-cadherin deficient cells. Since DAPT functions independently of SHh, the pos-
sibility of influences of SHh can be ruled out. However, the status of E-cadherin
expression upon drug treatment is uncertain, but this is somewhat important. For it is
possible that Notch indeed could be inhibiting E-cadherin expression. A further pos-
sibility is that in certain differentiation systems the canonical Wnt/β-catenin signal-
ling enters into a reciprocal interaction and regulation with Notch (Li et al., 2012a).