The Hippo signaling pathway plays an important role in both tissue regeneration and cancer formation. In regeneration, transient activation of YAP and TAZ promotes proliferation of stem and progenitor cells to replace damaged tissue. However, in cancer YAP and TAZ are often deregulated and constantly active, driving cell proliferation and tumor growth. While suppressing YAP/TAZ could be a cancer therapy, activating them may facilitate tissue regeneration; however more research is still needed to understand how to precisely control their activity for these opposing outcomes. The dual roles of the Hippo pathway highlight the complex links between signaling pathways involved in normal development and disease.

1. Surveying double-edge effects of Hippo pathway in
regeneration medicine and tumor formation
By: Simindokht Afra
Spring 1398
1

2. The mammalian Hippo
pathway
• The Salvador/Warts/Hippo pathway
• The protein kinase Hippo (Hpo)
• Restraining cell proliferation and promoting apoptosis
• Discovered initially by Drosophila mosaic genetic screens
• Highly conservation from Drosophila to mammals
Drosophila
melanogaster
Human ortholog(s)
Merlin (Mer)
NF2 (Neurofibromin 2)
Hippo (Hpo) MST1/2 (protein kinase)
Salvador (Sav) SAV1
Warts (Wts) LATS1/2(large tumor suppressor)
Mob as tumor
suppressor (Mats)
MOBKL1A, MOBKL1B
Yorkie (Yki)
YAP, TAZ (oncogenes)
(Zhao, Li et al. 2010)(Juan and Hong 2016)
How an organ
knows to stop
growing after
reaching a particular
size????
2

3. RASSF, a
subgroup of Ras
effector proteins,
may also activate
Mst1/2
MST1/2+ Sav1
stimulates LATS
kinase.
phosphorylating
LATS1/2 and
MOB1
Phosphorylation
YAP(S127) and
TAZ (S89),
inhibiting
translocation
Cytoplasmic
retention
Phosphorylation
on YAP S381 by
casein kinase 1
Recruitment
of β-TRCP E3
ubiquitin
ligase
(Zhao, Li et al. 2010)(Lei, Zhang et al. 2008)
YAP poly
ubiquitination
Hippo pathway is on
3

4. Hippo pathway is off
 Cis-regulatory elements
 Interacting TEAD
 YAP/TAZ/TEAD control the expression
of their targets by binding to distant
enhancers
 AREG is induced by YAP through an
unidentified transcription factor
 Interaction of WW domains of YAP
with Smad1 in the BMP signaling
pathway
 TAZ is required to maintain self-
renewal markers and loss of TAZ leads
to inhibition of TGFβ signaling and
induce differentiation into endoderm
lineage.
YAP mediate BMP target
gene expression in mouse
embryonic stem (ES) cells,
which relies on BMP
signal for pluripotency
maintenance
TAZ in maintaining
Smad2/3 nuclear
localization and target
gene expression in
response to TGF-β
signaling
TAAZ YAP
(Zhang, Ji et al. 2009)(Alarcón, Zaromytidou et al. 2009)(Varelas, Sakuma et al. 2008)(Mo, Park et al. 2014)
4

5. Two important co-activators (YAP/TAZ) in cancer
(Zanconato, Cordenonsi et al. 2016)
 YAP/TAZ promotes cell proliferation
 By activation factors involved in
 DNA synthesis
 S-phase entry
 Completion of mitosis
 Activation c-Myc
 upregulate Bcl2 proteins and
decrease the expression of Bcl-2
associated X protein (Bax)
 Overcome the alternative apoptosis cascade
initiated by tumor necrosis factor alpha (TNF-α)
and FAS ligands
 YAP/TAZ are active in the tumor's
cancer stem cell (CSC) fraction, and
functionally instrumental and
required for CSC
YAP/T
AZ
AREG CTGF Angiogenesis
Chemoattractant of T
cell suppressing
Immune
tolerance
5

6. The role of Hippo pathway in various types of cancers
Lung Cancer
I. lung adenocarcinomas (LAC)
II. lung squamous cell carcinomas (LSCC)
III. large-cell lung carcinomas
all generally defined as non-small-cell lung cancer
(NSCLC)
 Elevated expression/nuclear localization of YAP or
TAZ correlates with malignant features
 YAP/TAZ activation might not be sufficient to promote
NSCLC formation: expression of an active form of YAP
in the mouse airway epithelium only causes
hyperplasia of the tissue but is insufficient to trigger
tumor formation
Breast Cancer
 In datasets of BC patients, elevated expression of gene
signatures for YAP/TAZ activity correlate with high
histological grade, enrichment of stem cell signatures,
metastasis.
I. Site of origin (ductal or lobular),
II. Ability to grow in the lumen of the gland (carcinoma in
situ), or in the stroma surrounding the gland (invasive
carcinoma)
Invasive ductal carcinomas, the most common forms of BC,
I. Classified into hormone (estrogen and/or progesterone)-
receptor-positive BC (often referred as luminal BC), HER2-
positive BC
II. BCs that are negative for all three receptors (triple-
negative BC [TNBC])
(Noguchi, Saito et al. 2014)(Cheng, Zhang et al. 2016) 6

7. The role of Hippo pathway in various types of cancers
Colorectal Cancer
 Colorectal carcinoma (CRC) derives from benign
tumors (polyps and adenomas) that arise from the
mucosal epithelium of the lower tract of the
digestive system (colon and rectum)
 Over activation of the Wnt signaling pathway
 High levels of expression of a gene signature for
YAP activity have been found to be prognostic for
bad outcome in four datasets of CRC patients and
correlated with cetuximab resistance
 YAP and/or TAZ knockdown in human CRC cell
lines suppresses their growth and ability to trigger
tumor formation after injection in mice
Liver Cancer
(Azzolin, Zanconato et al. 2012)(Lee, Lee et al. 2015)
I. Hepatocellular carcinoma (HCC), arising from
hepatocytes;
II. Cholangiocarcinomas (CC) that derive from the bile
ducts;
III. Hepatoblastoma (HB), a pediatric tumor formed by
immature liver cells
 High levels of YAP protein expression have also been
detected in a majority of human CC and HB samples
 YAP or TAZ knockdown strongly reduces subcutaneous
tumor growth of human and mouse HCC cell lines
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8. The role of Hippo pathway in various types of cancers
Pancreatic Cancer
I. pancreatic adenocarcinoma (PDAC)
II. pancreatic intraepithelial neoplasia (PanIN)
I. Similarly, YAP/TAZ expression and nuclear
localization can be detected in most PanINs
and PDACs arising in mouse models of
pancreatic cancer, and in tumors derived
from human PDAC cell lines growing as
xenografts
Gliomas
I. low-grade gliomas
II. high-grade gliomas: are very aggressive tumors that
are classified as glioblastoma multiform (GBM)
I. TAZ levels are detected in the majority of GBM, and
high TAZ mRNA levels are associated with reduced
survival
II. High levels of YAP expression are found in subsets of
gliomas of all grades, and correlated with shorter
survival of glioma patients
(Tian, Li et al. 2015)(Orr, Bai et al. 2011)(Zhang, Li et al. 2013)
I. Esophageal squamous cell carcinomas
(ESCC)
II. Esophageal adeno-carcinomas (EAC)
Esophageal cancer I. Elevated YAP nuclear staining is a predictor of
worse prognosis in ESCC and is associated with
therapy resistance in EAC
II. YAP overexpression confers CSC properties
III. Resistance to chemotherapy
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9. YAP levels are
upregulated in
about 80% of OS
compared with
normal bone tissue
(Chan, Wang et al. 2014)(Zhang, Li et al. 2013)(Rao, Salloum et al. 2016)
Elevated nuclear staining of YAP
or TAZ correlates with tumor
recurrence, resistance to
radiotherapy, and poor outcome
GNAQ and GNA11
mutations in uveal
melanoma
Elevated YAP nuclear staining
in melanoma induced by
overexpression of oncogenic
GNAQ
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10.  Regenerative medicine refers to medical approaches which promote functional regeneration
of damaged tissues or organs, such as stimulation of intrinsic regenerative/repair mechanisms
by molecular therapy, or transplantation of tissues or stem/progenitor cells cultured in
laboratories
The role of Hippo pathway in regenerative medicine
I. the tissues surrounding the damaged sites need to induce cell proliferation and differentiation
II. the vascular, nervous, and immune systems as well as the extracellular matrix (ECM) need to
be restored to maintain functionality of the new tissue
origin
of
these
“new”
cells
in
regeneration
1) Proliferation of terminally differentiated cells
2) Dedifferentiation of mature cells
3) Expansion and differentiation of resident progenitor cells
4) Influx of stem cells from other tissues
(Wang, Yu et al. 2017)(Brockes and Kumar 2002)
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11. The role of Hippo pathway in regenerative medicine
The function of the Hippo pathway has been
well studied in both embryonic stem cells (ESCs)
and induced pluripotent stem cells (iPSCs)
YAP is highly expressed in self-renewing ESCs
but is inactivated during differentiation
Overexpression of Yap inhibits ESC
differentiation and maintains stem-like
properties and self-renewal even under
differentiation conditions, while Yap/Taz
knockdown is sufficient to result in the loss of
the ES cells phenotype
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12. (Wang, Liu et al. 2018)
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13. The role of Hippo pathway in mammalian tissue regeneration
Intestine  Rapid turnover(less than a week)
 Intestinal stem cells (ISCs)
(leucine-rich repeat containing
GPCR5 (Lgr5+)
 niche factors provided by
surrounding cells
 Paneth cells
 Myofibroblasts
Tissue
injuries
in
mice
Dextran
sodium
sulfate
(DSS)
Gamma
radiation
 YAP protein levels are
dramatically induced following
DSS treatment,
 Distributed in both cytoplasm
and nuclei
 YAP is also activated following
gamma irradiation, and YAP
shows predominant nuclear
localization
(Li and Clevers 2010)
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14. The role of Hippo pathway in mammalian tissue regeneration
Liver
In response to liver injury, mature hepatocytes proliferate to compensate for cell loss.
YAP upregulates TGF-β signaling to trigger
proliferation of biliary epithelial cells (BEC)
In adult liver, YAP is mainly localized to the bile
ductal epithelium
Upon liver injury and inflammation, YAP is
transiently activated, which promotes
proliferation
On the other hand, YAP activity is decreased
during hepatocyte differentiation and mature
hepatocyte have low YAP expression and
nuclear accumulation
(Lee, Park et al. 2016)(Zhou, Conrad et al. 2009)
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15. The role of Hippo pathway in mammalian tissue regeneration
Skin
The epidermis is continuously renewed to
maintain skin homeostasis.
 Epidermal tissue self-renewal and wound
healing are mainly dependent on epidermal
stem cells
Epidermis-specific deletion of Yap at early
embryonic stage causes lethality, the skin of
these mice is thinner.
Mice carrying a constitutively- active form
of YAP have thicker epidermis
Deletion of Yap leads to a reduction of cell
growth of keratinocyte, and delay in wound
healing
(Goodell, Nguyen et al. 2015)(Schlegelmilch, Mohseni et al. 2011)
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16. The role of Hippo pathway in mammalian tissue regeneration
Heart growth I. Fetal heart growth is mainly due to the proliferation of cardiomyocytes
II. Soon after birth, cardiomyocytes stop proliferating, and heart size is principally
controlled by the size of cardiomyocytes
 Cardiomyocyte loss is a major pathogenic
mechanism leading to heart failure
 Non-regenerative organ
Deletion
of
Yap
 Increased myocardial fibrosis
 Cardiomyocyte apoptosis
 Decreased cardiomyocyte proliferation
 Reduction myocardia injury
 Promotion cardiac function
 Stimulates IGF-1 and Akt signaling to
reduce cardiomyocyte apoptosis
Activation
Yap
(Porrello and Olson 2014)(Lin, Guo et al. 2016)(Xin, Kim et al. 2011)(Wang, Liu et al. 2018) 16

17. Concluding the two face of Hippo path way
YAP and TAZ regulate the balance between stem, progenitor and differentiated cells
 Deregulated YAP and TAZ activity and long term activation of them promotes
multiple cancer cells
Targeting the Hippo pathway as an anticancer therapeutic strategy would aim to
suppress YAP and TAZ activity, while targeting the Hippo pathway to facilitate
regeneration and reprogramming of adult cells and tissues would aim at elevating
YAP and TAZ activity
but
 many important questions await answers
(Zhao, Tumaneng et al. 2011)(Johnson and Halder 2014)

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Thank you for your attention
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