2. Terminology
• Wnt signaling pathway- is a group of signal transduction pathway
which is made of proteins that pass signals into a cell through cell
surface receptors.
• There are three Wnt signaling pathways: canonical (beta-catenin
dependent pathway); non-canonical planar cell polarity pathway
(beta-catenin independent) and non-canonical Wnt/calcium pathway.
• The Wnt ligand is a secreted lipid-modified glycoprotein that binds to
its cell surface receptor ligand Frizzled (Fz).
• There are 19 Wnt genes and 12 Fz receptors in vertebrates.
• The Fz receptor is a seven- transmembrane (7TM) spanning protein.
According to the International Union of Pharmacology and other
published reports, Fz receptors are G protein-coupled receptors
(Nichols et al., 2013).
• Beta-catenin-is key mediator of Wnt signaling and is a dual function
protein which regulates the coordination of cell to cell adhesion and
gene transcription (Heuberger and Birchmeier, 2009; De, 2011).
3. At a glance
The large number of Wnt ligands in
vertebrates along with the many branches of
signalling that are triggered by Wnt ligands
have led to the question of whether a
specific Wnt ligand may selectively activate
one particular branch of signalling. However,
the answer to this question remains
unresolved (Komiya and Kabas, 2008).
4. .
1) Cell fate determination
2) Cell migration
3) Cell polarity
4) Neural patterning
5) Organogenesis during embryonic development. Most
recently, this pathway has been implicated in stem cell
renewal
Wnt functions
History of Wnt
The name “Wnt” is derived from the gene “wingless” (Wg) found in
Drosophila and the gene INT, which functions homologously as the
Wg gene due to its evolutionary similarity in amino acid sequence.
The two proteins resulting from these genes came to be part of the
large Wnt family of proteins. The Wnt1 gene, originally named Int-1,
was identified in 1982 as a gene activated by integration of mouse
mammary tumor virus proviral DNA in virally induced breast tumors.
5. If something goes wrong in Wnt
signaling…
Perturbation of the levels of WNT ligands, or
altered activities of the proteins that are
necessary for WNT signal transduction, can
result in defects in embryonic development;
additionally, abnormal WNT signaling in
adults may contribute to disease etiology
(Anastas and Moon, 2013).
6. Wnt proteins are secreted glycoproteins that bind to the N-terminal
extra-cellular cysteine-rich domain of the Frizzled. The Wnt/Frizzled
signaling pathway controls numerous cellular processes such as
proliferation, differentiation, cell-fate decisions, migration and plays a
crucial role during embryonic development (Dijksterhuis et al., 2014).
(Nichols et al., 2013)
Wnt/Fz signaling
7. Absence of Wnt
β-
catenin
Dishevelled
APC
A
x
i
n
CK1γ GS
K3
TCF
Target
genes
In the absence of WNT, β-catenin
is targeted to a multimeric protein
complex called destruction
complex for its phosphorylation.
This is achieved by CK1-
mediated phosphorylation at
Ser45, followed by Ser33, Ser37,
and Thr41 phosphorylation by
glycogen synthase kinase
(GSK)3β.This phosphorylation
targets β-catenin for β-TRCo-
mediated ubiquitination and its
subsequent degradation by the
proteasome. (Rao and Kuhl,
2010).
8. Presence of Wnt
Activation of the Wnt pathway occurs upon binding of a soluble Wnt
protein to a membrane-associated receptor Fz and leads to the
disruption and inhibition of a protein complex responsible for
the phosphorylation and breakdown of β-catenin. Studies have
uncovered that phosphoprotein Dishevelled can indirectly interact
with FZ. Axin is a negative regulator of Wnt pathway. Hallmark >
canonical Wnt pathway is accumulation and translocation of the
adherens junction associated protein beta-catenin into the nucleus.
In the presence of signalling b-catenin displaces Groucho from TCF
(Heuberger and Birchmeier, 2009; Luis et al., 2011).
9. WNTs and their downstream effectors regulate various
processes that are important for cancer progression,
including:
Misregulation of these processes can lead to tumor development via excess cell
proliferation. Specifically, the gene expression signature of the WNT/β-catenin
pathway as well as aberrant β-catenin localization are implicated in conferring a
poorer prognosis in patients (Anastas and Moon, 2012; Pai et al., 2016).
10. Targeted therapies: Exploiting APC function as
a novel cancer therapy
In the majority of
colorectal cancers,
the β-catenin
destruction
complex is not
properly formed
due to truncation
of APC.
Therapeuti
cs for
various
molecular
componen
ts of the
Wnt
signaling
pathway.
Great majority of colorectal
cancers (>80%) have mutations
in the adenomatous polyposis
coli (APC) tumor suppressor
gene. APC mutations usually
result in a truncated form of the
protein lacking the carboxy-
terminal region resulting in loss
of function (Masuda et al., 2015).
11. Whether aberrant β-catenin expression
is driven by CTNNB1 (β-catenin
encoding gene) activating mutations?
Alternatively, β-catenin nuclear expression may stem from
mutations affecting other exons of the CTNNB1 gene or
other genes in the Wnt pathway (eg, the APC gene). It has
been demonstrated that in colorectal cancers, copy number
losses of APC gene may cooperate with inactivating APC
mutations for complete APC protein loss of function and
promote nuclear β-catenin translocation in tumour cells
(Geyer, et al., 2011).
12. Future research
• Oncogenes-do epigenetic changes occur in WNT
signalling?
• How has our knowledge of TSGs (e.g. APC) led to the
development of novel therapies?
• Cell cycle control-targeted therapies including
angiogenesis and apoptosis.
• Possible protein-protein interactions. E.g., Once inside
the nucleus beta-catenin can bind with other proteins
including Groucho, SMADs and CBP to up-regulate
proteins for cell division, ultimately leading to tumour
formation.
13.
14. Bibliography
• Anastas, J. and Moon, R. (2012). WNT signalling pathways as therapeutic targets in cancer. Nature Reviews
Cancer, 13(1), pp.11-26.
• Clevers, H. and Nusse, R. (2012). Wnt/β-Catenin Signaling and Disease. Cell, 149(6), pp.1192-1205.
• De, A. (2011). Wnt/Ca2+ signaling pathway: a brief overview. Acta Biochimica et Biophysica Sinica, 43(10),
pp.745-756.
• Dijksterhuis, J., Petersen, J. and Schulte, G. (2014). WNT/Frizzled signalling: receptor-ligand selectivity with focus
on FZD-G protein signalling and its physiological relevance: IUPHAR Review 3. British Journal of Pharmacology,
171(5), pp.1195-1209.
• Geyer, F., Lacroix-Triki, M., Savage, K., Arnedos, M., Lambros, M., MacKay, A., Natrajan, R. and Reis-Filho, J.
(2010). β-Catenin pathway activation in breast cancer is associated with triple-negative phenotype but not with
CTNNB1 mutation. Mod Pathol, 24(2), pp.209-231.
• Heuberger, J. and Birchmeier, W. (2009). Interplay of Cadherin-Mediated Cell Adhesion and Canonical Wnt
Signaling. Cold Spring Harbor Perspectives in Biology, 2(2), pp.a002915-a002915.
• Komiya, Y., Habas, R. (2008) Wnt signal transduction pathways. Organogenesis. 4(2): 68–75.
• Luis, T., Naber, B., Roozen, P., Brugman, M., de Haas, E., Ghazvini, M., Fibbe, W., van Dongen, J., Fodde, R. and
Staal, F. (2011). Canonical Wnt Signaling Regulates Hematopoiesis in a Dosage-Dependent Fashion. Cell Stem
Cell, 9(4), pp.345-356.
• MacDonald, B., Tamai, K. and He, X. (2009). Wnt/β-Catenin Signaling: Components, Mechanisms, and Diseases.
Developmental Cell, 17(1), pp.9-26.
• Masuda, M., Sawa, M. and Yamada, T. (2015). Therapeutic targets in the Wnt signaling pathway: Feasibility of
targeting TNIK in colorectal cancer. Pharmacology & Therapeutics, 156, pp.1-9.
• Nichols, A., Floyd, D., Bruinsma, S., Narzinski, K. and Baranski, T. (2013). Frizzled receptors signal through G
proteins. Cellular Signalling, 25(6), pp.1468-1475.
• Pai, P., Rachagani, S., Lakshmanan, I., Macha, M., Sheinin, Y., Smith, L., Ponnusamy, M. and Batra, S. (2016).
The canonical Wnt pathway regulates the metastasis-promoting mucin MUC4 in pancreatic ductal
adenocarcinoma. Molecular Oncology, 10(2), pp.224-239.
• Rao, T. and Kuhl, M. (2010). An Updated Overview on Wnt Signaling Pathways: A Prelude for More. Circulation
Research, 106(12), pp.1798-1806.
• Seth, C. and Ruiz i Altaba, A. (2016). Metastases and Colon Cancer Tumor Growth Display Divergent Responses
to Modulation of Canonical WNT Signaling. PLOS ONE, 11(3), p.e0150697.
Editor's Notes
First point-The Wnt1 proto-oncogene encodes a secreted, cysteine-rich protein. The fly Wingless (wg) gene, which controls segment polarity during larval development was later shown to be a homolog of Wnt1. By 1994, epistasis experiments examining the relationships among segment polarity mutations delineated the core of this developmental signal transduction cascade in Drosophila (e.g., porcupine, dishevelled, armadillo (b-catenin), and zeste-white 3/GSK3 gene.
Third point-The lipid modifications involve covalent attachment of palmitic acid on the first cysteine residue and palmitoleic acid on the highly conserved serine residue. The palmitoylation of Wnt is required for its binding to cognate receptor Fz, initiating a signal transduction pathway; glycosylation is required for its secretion
4th point-Both receptor and ligand are highly conserved during evolution and cross reactivity between them is very common. WNTs are a family of 19 secreted glycoproteins that have crucial roles in the regulation of diverse processes.
6th point β-Catenin forms a direct link between cadherins and α-catenin, whereas α-catenin also talks with the actin cytoskeleton. This link is essential for strong cadherin-mediated cell adhesion (Heuberger and Birchmeier, 2009)
Below, we discuss these, taking the reader from Wnt secretion through Wnt reception and signal transduction to the nuclear response of the recipient cell.
A role for WNTs in cancer was first described three decades ago in mouse models of mammary cancer and in human and mouse colon cancer. Aberrant overexpression of WNT1 induced by a proviral insertion at the Wnt1 (also known as int1) locus induces spontaneous mammary hyperplasia and tumours in mice2,3, and Wnt1 transgenic mice similarly develop mammary tumours, suggesting a causative role for WNT1 in mammary tumorigenesis.
Deregulated Wnt signaling has catastrophic consequences for the developing embryo and it is now well appreciated that defective Wnt signaling is a causative factor for a number of pleiotropic human pathologies.Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously. An example is phenylketonuria, a human disease that affects multiple systems but is caused by one gene defect. Most notably, these pathologies include cancers of
the breast, colon and skin, skeletal defects and human birth defect
disorders including the most common human neural tube closure
birth; spina bifida.4
Potential mechanisms of direct Frizzled-G protein interactions
A) Frizzled may act as a classic guanine nucleotide exchange factor (GEF) to catalyze the exchange of GDP for GTP on the Gαs subunit, resulting in the release of GTP-bound Gα and Gβγ. B) Frizzled may act like a guanine dissociation inhibitor (GDI) by sequestering Gα in the GDP-bound form and releasing Gβγ without catalyzing guanine nucleotide exchange. If Frizzled acts as a GEF, then a reduction in the level of the corresponding G protein would be predicted to enhance the Frizzled loss-of-function phenotype. In contrast, if Frizzled acts as a GDI, a reduction in G protein level would be predicted to suppress the Frizzled phenotype.
The protein kinases phosphorylate beta-catenin targeting it for ubiquitination and degradation in the proteosome meaning there is no beta-catenin free to move into the nucleus. In the absence of beta-catenin transcriptional co-repressor bind to TCF transcription factors and this prevents the expression of certain genes (Rao and Kuhl, 2010).
WNT signaling is critical for human colon cancers, which is perhaps best exemplified by two key findings 1)the vast majority of colon cancers harbor mutations in the tumour suppressor APC, a key WNT-TCF pathway antagonist 2) restoration of Apc function in apc compromised mice results in the loss of intestinal tumours (Seth and Ruiz i Altaba, 2016). It is known that Wnt/β-catenin signals supports cardiomyogenesis, whereas inhibition promotes adipogenesis This could explain the reduced production of cardiomyocytes and the increased production of adipocytes, which is observed in ARVC. These data nicely show how a switch from cadherin-mediated cell-adhesion to modulated Wnt signaling might result in the complex properties of a human disease (Rao and Kuhl, 2010) >>>Adhesion and Wnt/β-Catenin Signaling in Cardiomyopathy
Abbreviations: APC, adenomatous polyposis coli; β-Cat, β-catanin; CBP, cAMP response element binding protein (CREB)-binding protein; CK, casein kinase; Dvl, disheveled; GSK3β, glycogen synthase kinase 3β; LRP, low-density lipoprotein receptor-related protein; TCF4, T-cell factor-4, TNIK, TRAF2 and NCK-interacting protein kinase.
Epigenetic-that is not changing the sequence but it is involved in dna methylation or histone modifications
