ß 2014. Published by The Company of Biologists Ltd | Journal of Cell Science (2014) 127, 709–717 doi:10.1242/jcs.140103
COMMENTARY
The Hippo-YAP signaling pathway and contact inhibition of growth
ABSTRACT
The Hippo-YAP pathway mediates the control of cell proliferation by
contact inhibition as well as other attributes of the physical state of
cells in tissues. Several mechanisms sense the spatial and physical
organization of cells, and function through distinct upstream modules
to stimulate Hippo-YAP signaling: adherens junction or cadherin–
catenin complexes, epithelial polarity and tight junction complexes,
the FAT-Dachsous morphogen pathway, as well as cell shape,
actomyosin or mechanotransduction. Soluble extracellular factors
also regulate Hippo pathway signaling, often inhibiting its activity.
Indeed, the Hippo pathway mediates a reciprocal relationship
between contact inhibition and mitogenic signaling. As a result,
cells at the edges of a colony, a wound in a tissue or a tumor are more
sensitive to ambient levels of growth factors and more likely to
proliferate, migrate or differentiate through a YAP and/or TAZdependent process. Thus, the Hippo-YAP pathway senses and
responds to the physical organization of cells in tissues and
coordinates these physical cues with classic growth-factormediated signaling pathways. This Commentary is focused on the
biological significance of Hippo-YAP signaling and how upstream
regulatory modules of the pathway interact to produce biological
outcomes.
KEY WORDS: Hippo, YAP, Cadherin, Mechanotransduction,
Mitogenesis, Polarity
Introduction
Contact inhibition of cell proliferation has been recognized for a
long time to be an important phenomenon controlling tissue
growth (Fagotto and Gumbiner, 1996; Perrais et al., 2007;
McClatchey and Yap, 2012). Contact inhibition is thought to
occur as cells in a tissue or in monolayer culture reach a high
density. Other phenomena have also been invoked to explain
density-dependent growth, including accessibility to growth
factors and changes in cell shape and/or tension (Dupont et al.,
2011; Halder et al., 2012; Aragona et al., 2013), but there is also
direct evidence for a role of physical contact (Perrais et al., 2007;
Nishioka et al., 2009; Kim et al., 2011; McClatchey and Yap,
2012). A number of signaling pathways have been implicated in
contact inhibition, but the underlying mechanisms were still
unclear. The Hippo pathway was found to mediate contact
inhibition of growth (Zhao et al., 2007), and recent findings on
Hippo-YAP signaling have provided mechanistic insights into
contact inhibition as well as a way to understand how various
aspects of the physical state and/or organization of cells in tissues
are integrated to regulate tissue growth.
Department of Cell Biology, University of Virginia School of Medicine,
Charlottesville, VA 22908, USA.
*Author for correspondence (gumbiner@virginia.edu)
Contact inhibition is most frequently invoked in the context of
cancer. Cancer cells are often thought to have lost contact
inhibition of growth, and they proliferate despite the normal
confines of the tissue structure they reside in. Contact inhibition
has been considered an important regulator of tissue growth, but
much less frequently in the context of development. Indeed, cells
in rapidly growing tissues in embryos do not appear to be strongly
contact inhibited, as there can be a very rapid proliferation of
cells that are completely surrounded by neighbors. This indicates
that contact inhibition must be a highly regulated process rather
than a constitutive property of cells; often turned on in fully
developed and differentiated tissues, and switched off or
attenuated in normally growing cells during development, in
tissues with high turnover or in regeneration. Therefore, we can
expect to see considerable variations in the degree of contact
inhibition in various tissues or developmental stages. In cancer
cells, contact inhibition may be misregulated, causing differentiated
cells to revert to a more embryonic and/or developmental state, just
as they acquire other properties of embryonic cells such as
enhanced cell migration.
Many different signaling pathways have been implicated in the
contact inhibition of growth, and a number of pathways seem to
be altered as a function of cell density (Polyak et al., 1994;
Wieser et al., 1999; Heit et al., 2001; Faust et al., 2005; Li et al.,
2012; McClatchey and Yap, 2012). However, the Hippo pathway
has been found to have an important role in contact inhibition and
growth regulation through physical properties of cells. Indeed, it
may be the main locus where various pathways that sense cell
contact, cell shape, cell density and tissue organization are
integrated to control cell growth. The Hippo pathway was first
discovered as a result of genetic studies of tissue growth in
Drosophila, and this system is still a main engine of discovery for
this pathway (Halder and Johnson, 2011; Tumaneng et al., 2012;
Bossuyt et al., 2013). In Drosophila, the Hippo pathway is
thought to mediate organ size control, because loss of Hippo
pathway activity leads to enlarged but fairly normally organized
tissues, such as imaginal disks. Many Hippo pathway components
and genes are also expressed in mammalian cells and tissues, and
play a similar role in tissue growth and organ size, as well
as tumor growth. The core components of the Hippo pathway
are shared between Drosophila and mammals, but there are
some differences in the upstream regulators. As we will see
below, these differences reveal interesting variations on growth
regulation in different tissues.
Hippo pathway basics
The core of the Hippo pathway consists of a serine kinase cascade
with associated regulatory and/or scaffolding proteins that act on
a transcriptional complex to regulate the expression of genes that
control growth (Halder and Johnson, 2011; Halder et al., 2012;
Tumaneng et al., 2012; Bossuyt et al., 2013; Harvey et al., 2013;
Yu and Guan, 2013) (see Fig. 1). The Ser/Thr kinase Hippo in
Drosophila (Mst 1 and Mst2 in mammals; officially known as
709
Journal of Cell Science
Barry M. Gumbiner* and Nam-Gyun Kim
COMMENTARY
Journal of Cell Science (2014) 127, 709–717 doi:10.1242/jcs.140103
A
iv
RTK
GPCRs
v
i
TJ/
AP
Mst1/2
Sav1
ii
AJ
Lats1/2
Mob
iii
Mechanotransduction
YAP
YAP
Fat
Dachsous
Hippo signaling
kinase cascade
P
CTGF
AREG
YAP
TEAD
B i) Polarity and tight junction proteins
ii) AJ–cadherin-catenin complex
Apical
Crb
E-cad
PatJ
Pals1
AMOT
Ex
β-cat
Mer
NHERF
Kibra
TJ
NF2
Kibra
Par3
aPKC
Par6
Mst1/2
Mob
Basolateral
α-cat
Lats1/2
Echinoid
Echinoid
Lats1/2
Lgl
Scrib
Dlg
iii) Mechanotransduction
Sav1
AJ
Sav1
Sav
iv) Secreted mitogens
Mob
Warts
Hippo
YAP
Mst1/2
Mats
v) FAT/morphogen gradient
Growth factors
RTK
Stiff ECM
(stretched)
GPCRs
MAPK
`
Dachsons
Dachs
ex
Jub/WTIP
Mst1/2
YAP
Mob
Sav1
Lats1/2
Fat
Dco
PKA
PDK
Actomyosin
tension
Soft ECM
(dense)
PCP
signaling
Fj
D
Hippo
V
Sav
Mats
Warts
Drosophila
wing imaginal disc
Fig. 1. Control of the core Hippo signaling pathway through interacting upstream modules. (A) Overview of the interactions of various modules with
the core pathway. The Hippo pathway consists of a core kinase cascade in which the transcriptional co-activators YAP/TAZ are phosphorylated and inactivated
by either their exclusion from the nucleus or their enhanced degradation. The nuclear activity of YAP/TAZ promotes cell growth. (B) Upstream modules. (Panels i, ii)
Two upstream cell surface regulators, epithelial polarity or tight junction (TJ) complexes (i) and adherens junction (AJ) or cadherin–catenin complexes may function
together to sense the integrity of the epithelial layer. (Panel iii) Cell shape and mechanotransduction can regulate the activity of YAP/TAZ independently of Lats kinase, but
Lats-dependent regulation of YAP/TAZ through the actin cytoskeleton has also been observed. (Panel iv) Extracellular soluble growth factors act reciprocally – with contact
inhibition – through the Hippo pathway to integrate mitogenesis with growth inhibitory mechanisms. (Panel v) The atypical cadherins FAT and Dachsous set up a
morphogen gradient to control the spatial patterning of both cell proliferation (through Hippo pathway signaling) and PCP. b-cat, b-catenin; a-cat, a-catenin, AP, apical
polarity complexes; Dco, Discs overgrown; E-cad, E-cadherin; ECM, extracellular matrix; ex, Expanded; GPCRs, G-protein-coupled receptors; RTK, receptor tyrosine
kinase; PCP, planar cell polarity.
STK4 and STK3, respectively), activates another kinase, Warts
(Lats 1 and Lats2 in mammals), that phosphorylates the
transcriptional activator Yorkie (YAP and TAZ in mammals),
causing it to be excluded from the nucleus and retained in the
cytoplasm; indeed, the localization and phosphorylation of Yorkie/
YAP are often taken as a measure of the activity of the Hippo
pathway. The scaffolding proteins that are required for Hippo
710
pathway activity include Salvador/Sav1 and Mats/Mob1. Some
membrane-associated proteins, including the Merlin/NF2 tumor
suppressor and Kibra, also interact with the core kinase components
and act as common upstream activators of the pathway. The Hippo
signaling pathway has also been reported to lead to the degradation
of TAZ or YAP under some circumstances (Liu et al., 2010; Zhao
et al., 2010). In the nucleus, YAP interacts with the DNA-binding
Journal of Cell Science
PI3K
Ras
TEAD transcription factors (Scalloped in Drosophila) to turn on the
expression of growth-promoting and apoptosis-inhibiting genes.
Thus, the growth inhibitory activity of the Hippo pathway is to
prevent the nuclear accumulation of YAP and activation of growthpromoting genes. Nuclear YAP and Yorkie have also been found to
interact with other transcription factors, including Smads, PEBP2,
tumor protein p73, ErbB4, and Traffic Jam/MAF to regulate various
aspects of growth and differentiation (Yagi et al., 1999; Basu et al.,
2003; Komuro et al., 2003; Varelas et al., 2010a; Jukam et al.,
2013).
In recent years there was a plethora of discoveries of molecules
and pathways that act upstream of the core Hippo pathway, to
either activate or inhibit YAP/TAZ, or the transcriptional activity
of Yorkie. These include a variety of kinases (Harvey et al., 2013;
Yu and Guan, 2013), other signaling pathways, such as the Wnt
(Varelas et al., 2010b) and RTK pathways (Straßburger et al.,
2012; Fan et al., 2013; Reddy and Irvine, 2013), cell adhesion and
cell junction proteins, cell polarity proteins, and the state of the
actin cytoskeleton (Nishioka et al., 2009; Kim et al., 2011; Zhao
et al., 2011; Boggiano and Fehon, 2012; Halder et al., 2012;
Aragona et al., 2013; Hirate et al., 2013). The reader is referred to
the numerous recent review articles cited above that describe
these mechanisms in detail; these descriptions will not be
repeated here, except when used to illustrate certain points. A
couple of general important issues should be noted, however.
First, it has been claimed that, in some cases, the nuclear
localization of YAP can be regulated by upstream components
independent of the core Hippo cascade or YAP phosphorylation
by Lats (Dupont et al., 2011; Silvis et al., 2011; Aragona et al.,
2013). Ideally, these examples ought not be referred to as the
Hippo pathway; however, it is common for YAP regulation to be
generally discussed as being related to the Hippo pathway. A
more minor exception is the finding that, in some cell types,
regulation of the core Hippo complex and YAP phosphorylation
by Lats can be controlled by kinases other than the Hippo
homologues Mst1 and Mst2 (Zhou et al., 2009). Despite this
variation, the core mechanism is typically still considered as
the ‘Hippo pathway’. The second important issue is where in
the cell this kinase cascade occurs. Originally, it was thought
that it functioned in the cytosol, but several studies have discovered
that many pathway components, including the downstream
components Lats and YAP, interact with membrane-associated
proteins, including Merlin/NF2 and proteins found in tight
junctions, adherens junctions and apical polarity complexes
(Silvis et al., 2011; Zhao et al., 2011; Boggiano and Fehon,
2012; Bossuyt et al., 2013; Hirate et al., 2013; Yin et al., 2013).
Also, the scaffold protein Salvador interacts with both the
membrane protein Echinoid and the Hippo kinase in Drosophila
(Yin et al., 2013). One frequent interpretation of these findings in
the literature is that YAP is simply sequestered out of the nucleus
because it directly binds to these membrane-associated proteins
(Schlegelmilch et al., 2011; Harvey et al., 2013). However, this
explanation is likely to be incorrect because the vast majority of
extranuclear YAP protein is present in the cytosol, with little if any
accumulating at the membrane (Kim et al., 2011; Fan et al., 2013).
A rather better explanation is that the membrane association of the
Hippo pathway components is transient and catalytic, leading to
phosphorylation or other modifications of YAP. How this occurs
and how it is regulated by various cell junction and membrane
proteins is presently not very well understood but will be key to
furthering our understanding of the mechanisms that underly
contact inhibition.
Journal of Cell Science (2014) 127, 709–717 doi:10.1242/jcs.140103
Interacting upstream modules regulate Hippo-YAP signaling
Several inter-related mechanisms signal through the Hippo
pathway and sense the integrity and organization of cells in
tissues; together they control contact and density-dependent
regulation of growth. This can be thought of as several distinct
upstream modules that control Hippo-YAP signaling (Fig. 1), but
they interact in various ways. For the purpose of the present
discussion, these can be divided in five categories: (1) adherens
junction or cadherin–catenin complexes as adhesive elements, (2)
epithelial polarity and tight junction proteins, (3) the FATDachsous planar cell polarity (PCP) pathway, (4) cell shape or
the actin cytoskeletal mechanotransduction pathway and (5)
regulation by soluble extracellular growth factors.
Adherens junctions and the cadherin-catenin complex have
been found to activate the Hippo signaling pathway and inhibit
cell growth (Nishioka et al., 2009; Kim et al., 2011; Hirate et al.,
2013). Cadherin-mediated stimulation of the Hippo signaling
pathway requires cadherin ligation and the formation of a
homophilic bond – consistent with a role in cell-cell contact –
and works owing to phosphorylation of YAP by Lats and nuclear
exclusion of YAP. a- and b-catenin are required for cadherinmediated stimulation of the Hippo signaling pathway, although a
cadherin-independent role of YAP regulation by a-catenin has
been reported as well (Schlegelmilch et al., 2011; Silvis et al.,
2011). Merlin and Kibra appear to link cadherin ligation to the
Hippo cascade, although Merlin itself might be dispensable in
some cell types because cadherins can stimulate the pathway in
the Merlin-deficient MDA-MB-231 cell line (Kim et al., 2011). In
Drosophila, the adherens junction protein Echinoid activates
Hippo signaling through its direct interaction with Salvador (Yue
et al., 2012). There is no obvious homologous adherens junction
protein in mammals, but it is important to note that Echinoid also
interacts with E-cadherin to control the formation of adherens
junctions in flies (Wei et al., 2005).
Proteins that control apical-basolateral polarity in epithelia
have also been strongly implicated in Hippo-YAP signaling
(Grusche et al., 2010; Boggiano and Fehon, 2012; Tepass, 2012).
The apical polarity regulators, including the crumbs complex and
the aPKC–Par6–Par3 complex, interact with components of the
Hippo pathway and stimulate signaling in both Drosophila and
mammalian cells (Chen et al., 2010; Grzeschik et al., 2010;
Robinson et al., 2010; Varelas et al., 2010a; Hirate et al., 2013).
Mutations in the basolateral polarity genes scribble, Lgl and Dlg
cause substantial tissue overgrowth in Drosophila, which is due at
least in part to loss of Hippo pathway signaling (Grzeschik et al.,
2010; Chen et al., 2012). In mammals, tight junctions are thought
to be analogous to the apical determining polarity complex in
Drosophila (Tepass, 2012) and, indeed, tight-junction-associated
proteins, especially angiomotin, activate Hippo pathway signaling
(Zhao et al., 2011; Hirate et al., 2013); similarly, Scribble has been
found to antagonize YAP transcriptional activity (Skouloudaki
et al., 2009).
Regulation of Hippo pathway activity by adherens junctions or
cadherins and polarity proteins could operate through distinct
molecular mechanisms, and many of the findings published so far
are consistent with this notion. However, it is important to
remember that tight junctions or associated polarity complexes and
adherens junction or cadherin complexes are functionally highly
interdependent. On one hand, cadherin-mediated adhesion is a
basic early cell-cell interaction that helps to promote the formation
of tight junctions and the development of polarized membrane
domains (Gumbiner et al., 1988; Nejsum and Nelson, 2009). On
711
Journal of Cell Science
COMMENTARY
COMMENTARY
Journal of Cell Science (2014) 127, 709–717 doi:10.1242/jcs.140103
the other hand, polarity proteins facilitate the formation and near
apical positioning of the adherens junctions (Harris and Peifer,
2004; Tepass, 2012). Moreover, cadherin–catenin or adherens
junction proteins have been found to interact with some tight
junction and polarity proteins and, therefore, it is possible that they
regulate the Hippo pathway through some common mechanisms.
Stimulation of Hippo pathway signaling, both by adherens
junctions or cadherins and by polarity proteins, together suggests
that they function to sense the integrity of the epithelium.
Disruptions or discontinuities in the epithelium, reflected by the
state of adhesion and polarity complexes, should lead to nuclear
activity of YAP and/or TAZ, and activation of growth-promoting
genes. The importance of this idea for contact inhibition is clear;
cells at the edge of growing or damaged regions of an epithelium or
at the edge of an epithelial tumor would be allowed to grow faster
than cells well inside the epithelium.
Regulation of Hippo-YAP signaling by cell contact or cell
density also appears to occur in non-epithelial cells (Zhao et al.,
2007; Zhao et al., 2010; Zhao et al., 2011; Zhao et al., 2012), in
which epithelial polarity and epithelial integrity are not thought to
be especially important factors for growth control. In these cases,
the cadherin–catenin-mediated mechanism and/or the cell shape
or mechanotransduction mechanism (see below) may account
for Hippo pathway activity. Moreover, several tight-junctionassociated and polarity proteins are still expressed in nonepithelial cells, and they might have a role in Hippo-YAP
signaling independent of their function in establishing apical and
basolateral membrane domains.
Indeed, changes in the functional interactions between the
adherens junction or cadherin adhesion system and the epithelial
polarity system provide important regulatory mechanisms and a
way to diversify the roles of Hippo pathway signaling in response
to tissue organization. The formation of the first polarized
epithelium during compaction of the early mouse embryo, the
trophectoderm, was found to switch off Hippo pathway activity
and to lead to YAP nuclear accumulation and YAP- and TEADmediated gene expression (Nishioka et al., 2009; Hirate et al.,
2013) (Fig. 2). In this case, the pathway regulates one of the first
and most crucial cell fate decisions in mammalian embryos rather
than cell proliferation. It determines the distinction between the
inner cell mass, which leads to the formation of the embryo proper
(the source of embryonic stem cells) and the trophectoderm – the
outer epithelial cell layer that gives rise to extra-embryonic tissues
and the placenta. Although the inner cells do not exhibit epithelial
polarity, they do have cadherin-mediated junctions; and recruitment
of the (normally) tight-junction-associated protein angiomotin to
the cadherin-mediated junctions leads to activation of the Hippo
signaling pathway and nuclear exclusion of YAP. When outer cells
undergo epithelialization during compaction, polarity proteins act
to segregate angiomotin into the apical domain, which leads to a
loss of adherens-junction–angiomotin and/or cadherin–angiomotin
stimulation of the Hippo pathway. Thus, establishment of cell
Polarized
outer cell
(TE differentiation)
AMOT
TJ
β-cat
AJ
Nanog
Lats1/2
Merlin/
NF2
α-cat
TE
YAP
Cdx2
TEAD
ICM
P
β-cat
E-cad
YAP
Cdx2
AMOT
Lats1/2
Merlin/
NF2
YAP P
α-cat
Early blastocyst
Non-polarized
inner cell
(pluripotent ESCs)
Cdx2
Fig. 2. Hippo pathway-mediated regulation of cell lineage and pluripotency by adhesion and epithelial polarization in the early mouse embryo.
Compaction of the mouse embryo at the pre-implantation stage leads to the development of two different cell lineages, trophectoderm (TE) and inner cell mass
(ICM) (shown on the left). The TE is a polarized epithelium with E-cadherin (E-cad)-mediated adherens junctions (AJs), tight junctions (TJs) and apical–
basolateral polarity (illustrated on the right); it differentiates into an extra-embryonic tissue, the placenta. The ICM contains adhesive cells that are nonpolarized and form pluripotent embryonic stem cells (ESCs), which give rise to the embryo proper. Recruitment of angiomotin (AMOT) to the E-cad–catenin
complexes of inner cells stimulates Hippo pathway signaling and restricts the transcriptional activity of YAP through its exclusion from the nucleus, thereby
allowing expression of the pluripotency gene Nanog. AMOT becomes restricted to the apical domain of the outer cells by the action of polarity protein complexes
during epithelialization, which leads to a loss of Hippo pathway signaling, nuclear accumulation of YAP and transcriptional activation of TE-specific transcription
factor Cdx2. b-cat, b-catenin; a-cat, a-catenin.
712
Journal of Cell Science
TEAD
polarity during compaction turns off Hippo pathway signaling,
resulting in activation of YAP and TEAD target genes.
Hippo-YAP signaling during mouse embryo compaction is
regulated in a way that is completely opposite to what would be
expected from previous studies, in several important ways. First,
although both inner cell mass and trophectoderm express Ecadherin, activation of strong E-cadherin-mediated adhesion and
formation of a complete zonular adherens type adherens junction
occurs during compaction to make the trophectoderm (Vestweber
et al., 1987; Fleming and Johnson, 1988). Thus, a loss of Hippo
pathway signaling in the trophectoderm occurs despite a greater
extent of cadherin adhesion and formation of adherens junctions.
Second, epithelial polarization as mediated by polarity protein
complexes leads to loss of Hippo pathway activity instead of
stimulating it as has been observed for Drosophila and many
studies of epithelial cells in culture. Nonetheless, the Hippo
pathway still reads an aspect of tissue structure – inside versus
outside cell layers, and still utilizes the same set of epithelial
adhesion and polarity proteins, albeit in a different way. It is also
worth noting that, in the early mouse embryo, YAP is excluded
from the nucleus in the pluripotent cells of the inner cell mass,
while its nuclear activity has been found to be associated with
stem cell properties in somatic stem cells (Tremblay and
Camargo, 2012). Therefore, there is a diversity of molecular
regulatory mechanisms to control Hippo-YAP signaling activity as
well as diverse outcomes, which we will have to elucidate to truly
understand the role of Hippo-YAP signaling in tissue organizationand contact- regulation of growth and differentiation.
The Fat-Dachsous signaling pathway is a cell-contactdependent mechanism that regulates the Hippo pathway in a
very different context (Irvine, 2012). In Drosophila, it is a very
strong regulator of the Hippo pathway; Fat acts upstream of
Warts and Yorkie, and loss of Fat function results in very strong
Hippo pathway phenotypes. Dachsous and Fat are both very large
cell surface proteins containing a number of cadherin repeat
domains, although they do not seem to mediate traditional
physical cell adhesion. Neither do they appear to have a role in
the local integrity of cells in the epithelial sheet, in contrast to Ecadherin and polarity proteins. Instead, they mediate a cellcontact-dependent morphogen gradient that patterns the overall
tissue; in this case the whole imaginal disk, the primordium that
gives rise to adult tissues. In fact the Fat-Dachsous system is not
specific to Hippo signaling, because it is also a main mediator of
planar PCP signaling, again modulating PCP across the whole
imaginal disk rather than locally the Frizzled-mediated regulation
of the PCP pathway. However, the PCP and Hippo signaling
activities of Fat are separable and mechanistically distinct (Irvine,
2012; Bossuyt et al., 2013). Nonetheless, both pathways can
detect a morphogen gradient set up by gradients of Dachsous and
the Golgi kinase Four-jointed (Fj) that regulates Fat activity.
Thus, Fat regulates cell proliferation across the imaginal disk as a
way to pattern growth over a distance and to help shape the
growth of the disk.
Homologues of Fat and other components of the Fat-Hippo
pathway are expressed in mammals; of those, Fat4 is the one most
similar to Drosophila Fat. It was found to have a role in PCP
signaling in mammals (Saburi et al., 2012), but so far there has
been minimal evidence that it functions in Hippo-YAP signaling
in mammals or other vertebrates. One possibility is that none of
the previous studies have looked in the right places or contexts for
Fat-Hippo pathway signaling, e.g. over small regions of tissue
where patterning by a local morphogen gradient might be
Journal of Cell Science (2014) 127, 709–717 doi:10.1242/jcs.140103
important. Nonetheless, a recent study argued that Fat regulation
of the Hippo pathway is specific to the arthropod lineage (Bossuyt
et al., 2013). A molecular dissection of the Fat cytoplasmic tail
reveals regions that function differentially in Hippo pathway
signaling and PCP signaling; whereas the PCP signaling portions
are conserved across a wide range of species, the Hippo pathway
signaling-specific regions are not. Other molecules and molecular
interactions also appear to diverge between Drosophila and
vertebrates (Bossuyt et al., 2013). Notably, the vertebrate junctionassociated adaptor protein angiomotin (Amot), which was found to
have key roles in mammalian Hippo-YAP signaling (Zhao et al.,
2011; Adler et al., 2013; Hirate et al., 2013), has no homologue in
Drosophila. Nonetheless, most of the downstream core components
of the Hippo-YAP signaling pathway, as well as some upstream
regulators such as Merlin/NF2, function similarly in Drosophila and
vertebrates.
Cell shape and mechanotransduction also have been found to
regulate the Hippo-YAP signaling (Dupont et al., 2011; Wada
et al., 2011; Halder et al., 2012) and might be involved in contact
inhibition and/or cell-density-dependent growth (Aragona et al.,
2013). As cells grow into dense colonies, they change shape and
mechanical properties together with changes in cell-cell
adhesions and junctions, and it can be difficult to separate out
these different factors. Nonetheless, cell shape or tension has
been found to regulate the nuclear accumulation of YAP and TAZ
independently of the formation of cell-cell contacts (Dupont
et al., 2011; Aragona et al., 2013). Highly spread individual cells
that are grown on a hard substrate are under greater tension,
contain numerous stress fibers and focal adhesions, and exhibit
strong nuclear accumulation of YAP/TAZ. By contrast, rounded
individual cells grown on soft substrates strongly exclude YAP/
TAZ from the nucleus. There is evidence that this mechanical or
shape regulation of the nuclear localization of YAP occurs
independently of the Hippo pathway, i.e. that it does not
involve the Hippo or Lats kinases, or the phosphorylation of
YAP/TAZ on Lats target sites (Dupont et al., 2011; Aragona
et al., 2013). This is consistent with the idea that the mechano- or
Cell contact
Cadherin,
TJ, other
Hippo pathway
Growth factor
Receptor
(RTK, GPCR)
Mitogenic
signaling
Nuclear YAP
Proliferation
Fig. 3. The Hippo pathway mediates the reciprocal regulation of cell
proliferation by contact inhibition and mitogenic signaling. Cell-cell
contact inhibits cell proliferation through stimulation of the Hippo pathway.
Mitogenic growth factors stimulate proliferation by well-known signaling
mechanisms, but also counteract the growth inhibitory effects of the Hippo
signaling pathway. Thus, the Hippo pathway can coordinate classic
mitogenic signaling with the physical state of the cells in a tissue. TJ, tight
junction; RTK, receptor tyrosine kinase; GPCR, G-protein-coupled receptor.
713
Journal of Cell Science
COMMENTARY
COMMENTARY
Journal of Cell Science (2014) 127, 709–717 doi:10.1242/jcs.140103
shape-regulation of YAP/TAZ is independent of cell-cell contact
per se, which does work through the Hippo pathway. Of course,
cell-cell contact can also regulate cell shape and tension. The
molecular mechanisms by which shape and tension regulate YAP/
TAZ is unclear, but they appear to require an intact Rho-Rock
pathway, as expected for a mechanical component.
The state of the actin cytoskeleton is very important in
mediating the effects of shape and tension of nuclear YAP/TAZ.
Perturbations of the actin cytoskeleton have strong effects on
YAP localization (Wada et al., 2011; Zhao et al., 2012; Kim
et al., 2013). Remarkably, genetic deficiencies in actin-capping
proteins both in Drosophila and mammalian cells strongly
activate nuclear Yorkie and YAP activity, respectively
(Sansores-Garcia et al., 2011; Aragona et al., 2013). These
findings are consistent with the idea that actomyosin-dependent
tension regulates YAP/TAZ signaling, i.e. that such mutations
reflect the cell shape or the mechanotransduction-mediated
pathway described above. However, numerous studies have
found that actin perturbations do, indeed, act through the Hippo
pathway or, at least through YAP phosphorylation by Lats
(Sansores-Garcia et al., 2011; Wada et al., 2011; Zhao et al.,
2012; Kim et al., 2013) – in contrast to the studies on the role of
the cell shape (Dupont et al., 2011; Aragona et al., 2013). To
explain this apparent discrepancy, it has been proposed that
mechanical cues can initiate both Lats-dependent and Latsindependent regulation of YAP/TAZ (Halder et al., 2012). These
studies have not yet discerned which part of the actin
cytoskeleton is involved in the regulation of Hippo-YAP
signaling; for example, whether actin associated with focaladhesion-associated stress fibers, cell junctions, other parts of the
cell cortex, or the global state of actin effect cell-contactdependent Hippo-YAP signaling.
Soluble growth factors
Several recent studies have discovered that soluble hormones or
growth factor can regulate Hippo-YAP in addition to its
regulation by cell contact, polarity, and mechanical properties
that are important for tissue organization. In most cases, these
soluble growth factors inhibit Hippo-YAP signaling and
counteract the effects of cell contact (Straßburger et al., 2012;
Yu et al., 2012; Fan et al., 2013; Reddy and Irvine, 2013),
suggesting that the Hippo pathway mediates a reciprocal
relationship between contact inhibition and mitogenic signaling
(Fig. 3). Several growth factors, including EGF, IGF, serum and
LPA, inhibit Hippo-pathway signaling and lead to nuclear
accumulation of Yorkie/YAP, which then activates growthrelated genes. Significantly, Yorkie/YAP is found to be
required for growth-factor-induced proliferation, suggesting that
inhibition of Hippo pathway signaling is a crucial component of
mitogenic signaling. Using genetics studies, this was shown to be
true in vivo in Drosophila, but how important it is for mitogenic
growth factor signaling in vivo in mammalian tissues or cancer
remains to be determined.
A few different mechanisms have been found to mediate the
regulation of Hippo-YAP signaling by soluble factors. In
mammalian cells, EGF, serum and LPA were found to rapidly
stimulate the nuclear accumulation of YAP by a novel
downstream effect of PI 3-kinase (PI3K) and PDPK1 (PDK1)
on the Hippo pathway, largely independent of Akt signaling.
PDK1 was found to interact with the core Hippo kinase complex,
and stimulation of PI3K by EGF causes the complex to dissociate,
which presumably impairs the ability of Lats to phosphorylate
YAP (Fan et al., 2013) (Fig. 4). The PI3K-PDK pathway was
similarly found to mediate the inhibition of the Hippo pathway
through IGF signaling in Drosophila (Straßburger et al., 2012).
Hippo ON
(No growth factors)
EGF, IGF receptor
(RTK)
Hippo OFF
(Growth factor action)
LPA receptor
(GPCR)
EGF, IGF receptor
(RTK)
LPA receptor
(GPCR)
Ligand
Ligand
PIP2 PIP3 PDK1 PIP3 PIP2
PI3k
PDK1
Mst1/2
Ras
PI3K
Sav1
Ajuba
MAPK
Lats
YAP
Mst1/2
Lats
YAP
P
P
Sav1
Ajuba
YAP
Sav
Wts
Yorkie
No proliferation
Yorkie
YAP
Nucleus
Proliferation
Fig. 4. Two mechanisms for growth-factor-mediated regulation of the Hippo pathway: activation of Ras-MAPK signaling and activation of PI3K-PDK1
signaling. (Left panel, Hippo pathway on) In confluent cells in the absence of growth factors, PDK1 forms a complex with Hippo pathway components
(Lats, Mst and Sav1) and the Hippo pathway is active. Mst phosphorylates Mob and Lats, which then phosphorylate YAP. YAP is then excluded from the nucleus
and cell growth is arrested. (Right panel, Hippo pathway off) Activation of the Ras-MAPK pathway by EGF signaling phosphorylates Ajuba, which binds to and
inhibits the activity of the Sav–Wts complex, leading to dephosphorylation of Yorkie, its accumulation in the nucleus and increased cell proliferation. Growth
factors (EGF, LPA or serum) can also activate PI3K and recruit PDK1 to the membrane, resulting in the dissociation of the PDK1–Hippo complex. As a
result, the regulation of Lats by Mst is prevented, which eventually leads to nuclear accumulation of YAP and cell proliferation. Figure was modified with
permission (Fan et al., 2013). PIP2, PtdIns(4,5)P2; PIP3, PtdIns(3,4,5)P3.
714
Journal of Cell Science
Nucleus
Journal of Cell Science (2014) 127, 709–717 doi:10.1242/jcs.140103
respond to a steady level of a growth factor depending on its
location relative to other cells in the culture or – presumably – in
a tissue. Although there are many potential mechanisms to
explain this phenomenon, the Hippo pathway is ideally suited to
mediate this effect, with the nuclear localization of YAP more
likely to occur in cells at the edge of the cluster of cells (Fig. 5).
In this way, cells at the edge of a colony or a wound in a tissue, or
at the edge of a tumor would be more sensitive to ambient levels
of growth factors and more likely to proliferate, migrate or
differentiate in a YAP/TAZ-dependent process.
The Hippo pathway also plays a role in responding to
boundaries between populations of cells that have different
growth potential within the same tissue, even when they have
continuous contact. Cell competition is a homeostatic mechanism
that can eliminate abnormal or cancerous cells from a tissue
during development (Johnston, 2009). Moreover, a recent study
found that regulation of Hippo pathway signaling by normal cells
prevents the overgrowth of tumor cells in Drosophila imaginal
disks (Chen et al., 2012). Null-mutations in the scribble gene
(scrib–) cause neoplastic tumor-like growths when all cells in the
tissue harbor the mutation (Chen et al., 2012). However, in
mosaic tissues, the growth of scrib– clones is completely
suppressed by neighboring clones of wild-type cells. Wild-type
clones prevent the overproliferation of scrib– clones by
suppressing Yorkie/YAP activity in the mutant clones (Chen
et al., 2012). Thus, cell competition through a regulation of Hippo
pathway signaling acts as a tumor suppressor mechanism. It will
Broader implications – what is contact inhibition for?
Fig. 5. Role of contact inhibition and Hippo-YAP signaling in the
spatial control of mitogenic signaling by growth factors. (Top) The
graph shows the dose-response curves for growth factor-stimulated
proliferation. A high degree of cell-cell contact shifts the curve on the x-axis,
such that a higher concentration of growth factors is required to elicit the
same response. This graph is an interpretation of the findings of Kim et al.,
2009. (Bottom) Illustration how cell-cell contact regulates transcription
factor activity through the Hippo pathway. At high cell density in the middle
of the cell monolayer, the Hippo pathway is active, leading to nuclear
exclusion (NE) of YAP and/or TEAD (TEAD is labeled by
immunofluorescence staining in this example). At the edge of the culture
where cells have lost contact inhibition, YAP and/or TEAD (as shown here)
accumulate in the nuclei (N) and stimulate proliferation. Thus, proliferation is
spatially regulated despite uniform levels of growth factors acting on these
cells.
The recent observations that regulation of Hippo-YAP signaling
is intimately coordinated with signaling through traditional
growth factor signaling pathways have additional implications
for control of tissue growth; in particular, for activities at the edge
of tissues or boundaries between tissue domains. How this might
work was nicely demonstrated in a study describing the
relationship between contact inhibition and mitogenic signaling
(Kim et al., 2009). Contact inhibition is not an absolute switch
that shuts off cell proliferation. Rather, increasing cell contact
appears to shift the dose-dependence of cell proliferation in
response to a mitogenic growth factor such as EGF to a higher
threshold response (Fig. 5). As a result, a cell can differentially
Proliferation
EGF signaling was also found to inhibit Hippo pathway signaling
in Drosophila, but in this case it was found to be mediated by
MAP-kinase and the regulation of Warts by Ajuba (Reddy and
Irvine, 2013). In some cases, hormones can stimulate, rather than
inhibit, the Hippo pathway; for instance through G-proteincoupled receptor-mediated stimulation of adenyl-cyclase–PKA
signaling to stimulate Lats activity (Yu et al., 2012; Yu et al.,
2013). It will be interesting to see how widespread the regulation
of the Hippo pathway by soluble factors is.
Soluble growth-factor-mediated regulation of the Hippo
pathway may explain some of the cell nonautonomous features
of Hippo-YAP signaling. Two of the well-established target
genes for YAP and TEAD are connective tissue growth factor
(CTGF) and amphiregulin (AREG), which can mediate effects of
Hippo-YAP signaling on neighboring cells (Zhao et al., 2008;
Zhang et al., 2009). Increased production of amphiregulin occurs
at low-cell density due to loss of contact inhibition, decreased
Hippo pathway signaling and increased concentration of nuclear
YAP, which allows cells to proliferate rapidly (Zhang et al.,
2009). Because amphiregulin is an EGF receptor (EGFR) ligand
it can, in turn, inhibit Hippo pathway signaling and increase the
nuclear accumulation of YAP (Fan et al., 2013), creating a
positive autocrine or paracrine feedback loop to stimulate and
spread the transcriptional activity of YAP. Several other
examples of cell nonautonomy in Hippo pathway signaling
have been observed, including Src kinase activation of Yorkie in
neighboring cells (Enomoto and Igaki, 2013) and the effects of
cell-cell competition on Hippo pathway signaling (Chen et al.,
2012) (see below). In these cases the mediators that act between
cells are not yet known, but soluble growth factors are likely to be
candidates.
The findings that inhibition of Hippo pathway signaling can
be a component of mitogenic signaling has important general
implications and raises questions with regard to many different
biological processes. How common is it for regulation of HippoYAP signaling to be coordinated with mitogenic signaling
pathways during embryonic and/or tissue development? Is this
coordination important for cancer growth? For example, mutational
activation of the PI3K pathway, due to constitutively active
mutations in PI3K or inactivation of PTEN, or activation of the
EGFR pathway are known to drive the growth of many tumors
(Engelman et al., 2006). Stimulation of PI3K signaling, including
the presence of mutations that render PI3K constitutively active,
cause nuclear accumulation of YAP (Fan et al., 2013), which
suggests that YAP regulation by PI3K signaling is an important
general mechanism in cancer. Understanding the role of HippoYAP signaling inhibition in mitogenic signaling in vivo is likely to
be important for future research.
Low contact
High contact
Growth factor or mitogen concentration
N
NE
715
Journal of Cell Science
COMMENTARY
be interesting to see whether a similar mechanism suppresses
tumor growth in mammals.
Hippo-YAP signaling might be particularly important in sensing
other three-dimensional features of tissues. This is beautifully
illustrated by the process of mouse embryo compaction described
above, in which cells in the outside layer of a tissue differentiate
along a completely different path than the inner cells. In this case,
the difference in Hippo-YAP signaling between the two layers is
brought about by the acquisition of cell polarity and by switching
off of the stimulation of the pathway that is dependent on cadherinangiomotin coupling. Another potential way in which the position
of outer versus inner cells of a tissue might be sensed is through
the mechanotransduction or tension-mediated regulation of YAP,
because outer cells may be more spread and, thus, might
experience a higher surface tension.
Concluding remarks
One remarkable and particularly interesting aspect of the HippoYAP signaling pathway is that it senses and responds in so many
ways to the physical organization of cells in tissues; through cellcell adhesion, cell junctions and other contact-dependent signals,
cell polarity mechanisms, cell shape, tension and, finally, actin
organization. Thus, Hippo signaling provides an important
explanation for many of the observed phenomena by which
physical cues regulate tissue growth and differentiation. Future
discoveries in this fast-moving field are likely to reveal even
greater insights into the physical mechanisms that control cell
growth and differentiation, as well as their coordination with
classic growth-factor-mediated pathways.
Acknowledgements
We thank our colleagues Jing Yu and Xiaowei Lu for many valuable discussions
about the ideas presented in this Commentary, and for reading and making
comments on the manuscript.
Competing interests
The authors declare no conflict of interest.
Funding
This work was supported by a National Institutes of Health grant to B.M.G.
Deposited in PMC for release after 12 months.
References
Adler, J. J., Johnson, D. E., Heller, B. L., Bringman, L. R., Ranahan, W. P.,
Conwell, M. D., Sun, Y., Hudmon, A. and Wells, C. D. (2013). Serum
deprivation inhibits the transcriptional co-activator YAP and cell growth via
phosphorylation of the 130-kDa isoform of Angiomotin by the LATS1/2 protein
kinases. Proc. Natl. Acad. Sci. USA 110, 17368-17373.
Aragona, M., Panciera, T., Manfrin, A., Giulitti, S., Michielin, F., Elvassore, N.,
Dupont, S. and Piccolo, S. (2013). A mechanical checkpoint controls
multicellular growth through YAP/TAZ regulation by actin-processing factors.
Cell 154, 1047-1059.
Basu, S., Totty, N. F., Irwin, M. S., Sudol, M. and Downward, J. (2003). Akt
phosphorylates the Yes-associated protein, YAP, to induce interaction with
14-3-3 and attenuation of p73-mediated apoptosis. Mol. Cell 11, 11-23.
Boggiano, J. C. and Fehon, R. G. (2012). Growth control by committee:
intercellular junctions, cell polarity, and the cytoskeleton regulate Hippo
signaling. Dev. Cell 22, 695-702.
Bossuyt, W., Chen, C. L., Chen, Q., Sudol, M., McNeill, H., Pan, D., Kopp, A.
and Halder, G. (2013). An evolutionary shift in the regulation of the Hippo
pathway between mice and flies. Oncogene.
Chen, C. L., Gajewski, K. M., Hamaratoglu, F., Bossuyt, W., Sansores-Garcia,
L., Tao, C. and Halder, G. (2010). The apical-basal cell polarity determinant
Crumbs regulates Hippo signaling in Drosophila. Proc. Natl. Acad. Sci. USA
107, 15810-15815.
Chen, C. L., Schroeder, M. C., Kango-Singh, M., Tao, C. and Halder, G. (2012).
Tumor suppression by cell competition through regulation of the Hippo pathway.
Proc. Natl. Acad. Sci. USA 109, 484-489.
Dupont, S., Morsut, L., Aragona, M., Enzo, E., Giulitti, S., Cordenonsi, M.,
Zanconato, F., Le Digabel, J., Forcato, M., Bicciato, S. et al. (2011). Role of
YAP/TAZ in mechanotransduction. Nature 474, 179-183.
716
Journal of Cell Science (2014) 127, 709–717 doi:10.1242/jcs.140103
Engelman, J. A., Luo, J. and Cantley, L. C. (2006). The evolution of
phosphatidylinositol 3-kinases as regulators of growth and metabolism. Nat.
Rev. Genet. 7, 606-619.
Enomoto, M. and Igaki, T. (2013). Src controls tumorigenesis via JNK-dependent
regulation of the Hippo pathway in Drosophila. EMBO Rep. 14, 65-72.
Fagotto, F. and Gumbiner, B. M. (1996). Cell contact-dependent signaling. Dev.
Biol. 180, 445-454.
Fan, R., Kim, N. G. and Gumbiner, B. M. (2013). Regulation of Hippo pathway by
mitogenic growth factors via phosphoinositide 3-kinase and phosphoinositidedependent kinase-1. Proc. Natl. Acad. Sci. USA 110, 2569-2574.
Faust, D., Dolado, I., Cuadrado, A., Oesch, F., Weiss, C., Nebreda, A. R. and
Dietrich, C. (2005). p38alpha MAPK is required for contact inhibition. Oncogene
24, 7941-7945.
Fleming, T. P. and Johnson, M. H. (1988). From egg to epithelium. Annu. Rev.
Cell Biol. 4, 459-485.
Grusche, F. A., Richardson, H. E. and Harvey, K. F. (2010). Upstream regulation
of the hippo size control pathway. Curr. Biol. 20, R574-R582.
Grzeschik, N. A., Parsons, L. M., Allott, M. L., Harvey, K. F. and Richardson,
H. E. (2010). Lgl, aPKC, and Crumbs regulate the Salvador/Warts/Hippo
pathway through two distinct mechanisms. Curr. Biol. 20, 573-581.
Gumbiner, B., Stevenson, B. and Grimaldi, A. (1988). The role of the cell
adhesion molecule uvomorulin in the formation and maintenance of the
epithelial junctional complex. J. Cell Biol. 107, 1575-1587.
Halder, G. and Johnson, R. L. (2011). Hippo signaling: growth control and
beyond. Development 138, 9-22.
Halder, G., Dupont, S. and Piccolo, S. (2012). Transduction of mechanical and
cytoskeletal cues by YAP and TAZ. Nat. Rev. Mol. Cell Biol. 13, 591-600.
Harris, T. J. and Peifer, M. (2004). Adherens junction-dependent and
-independent steps in the establishment of epithelial cell polarity in
Drosophila. J. Cell Biol. 167, 135-147.
Harvey, K. F., Zhang, X. and Thomas, D. M. (2013). The Hippo pathway and
human cancer. Nat. Rev. Cancer 13, 246-257.
Heit, I., Wieser, R. J., Herget, T., Faust, D., Borchert-Stuhlträger, M., Oesch, F.
and Dietrich, C. (2001). Involvement of protein kinase Cdelta in contactdependent inhibition of growth in human and murine fibroblasts. Oncogene 20,
5143-5154.
Hirate, Y., Hirahara, S., Inoue, K., Suzuki, A., Alarcon, V. B., Akimoto, K., Hirai,
T., Hara, T., Adachi, M., Chida, K. et al. (2013). Polarity-dependent distribution
of angiomotin localizes Hippo signaling in preimplantation embryos. Curr. Biol.
23, 1181-1194.
Irvine, K. D. (2012). Integration of intercellular signaling through the Hippo
pathway. Semin. Cell Dev. Biol. 23, 812-817.
Johnston, L. A. (2009). Competitive interactions between cells: death, growth,
and geography. Science 324, 1679-1682.
Jukam, D., Xie, B., Rister, J., Terrell, D., Charlton-Perkins, M., Pistillo, D.,
Gebelein, B., Desplan, C. and Cook, T. (2013). Opposite feedbacks in the
Hippo pathway for growth control and neural fate. Science 342, 1238016.
Kim, J. H., Kushiro, K., Graham, N. A. and Asthagiri, A. R. (2009). Tunable
interplay between epidermal growth factor and cell-cell contact governs the spatial
dynamics of epithelial growth. Proc. Natl. Acad. Sci. USA 106, 11149-11153.
Kim, N. G., Koh, E., Chen, X. and Gumbiner, B. M. (2011). E-cadherin mediates
contact inhibition of proliferation through Hippo signaling-pathway components.
Proc. Natl. Acad. Sci. USA 108, 11930-11935.
Kim, M., Kim, M., Lee, S., Kuninaka, S., Saya, H., Lee, H., Lee, S. and Lim,
D. S. (2013). cAMP/PKA signalling reinforces the LATS-YAP pathway to fully
suppress YAP in response to actin cytoskeletal changes. EMBO J. 32, 15431555.
Komuro, A., Nagai, M., Navin, N. E. and Sudol, M. (2003). WW domaincontaining protein YAP associates with ErbB-4 and acts as a co-transcriptional
activator for the carboxyl-terminal fragment of ErbB-4 that translocates to the
nucleus. J. Biol. Chem. 278, 33334-33341.
Li, W., Cooper, J., Karajannis, M. A. and Giancotti, F. G. (2012). Merlin: a
tumour suppressor with functions at the cell cortex and in the nucleus. EMBO
Rep. 13, 204-215.
Liu, C. Y., Zha, Z. Y., Zhou, X., Zhang, H., Huang, W., Zhao, D., Li, T., Chan,
S. W., Lim, C. J., Hong, W. et al. (2010). The hippo tumor pathway promotes
TAZ degradation by phosphorylating a phosphodegron and recruiting the
SCFbeta-TrCP E3 ligase. J. Biol. Chem. 285, 37159-37169.
McClatchey, A. I. and Yap, A. S. (2012). Contact inhibition (of proliferation) redux.
Curr. Opin. Cell Biol. 24, 685-694.
Nejsum, L. N. and Nelson, W. J. (2009). Epithelial cell surface polarity: the early
steps. Front. Biosci. (Landmark Ed.) 14, 1088-1098.
Nishioka, N., Inoue, K., Adachi, K., Kiyonari, H., Ota, M., Ralston, A., Yabuta,
N., Hirahara, S., Stephenson, R. O., Ogonuki, N. et al. (2009). The Hippo
signaling pathway components Lats and Yap pattern Tead4 activity to
distinguish mouse trophectoderm from inner cell mass. Dev. Cell 16, 398-410.
Perrais, M., Chen, X., Perez-Moreno, M. and Gumbiner, B. M. (2007). Ecadherin homophilic ligation inhibits cell growth and epidermal growth factor
receptor signaling independently of other cell interactions. Mol. Biol. Cell 18,
2013-2025.
Polyak, K., Kato, J. Y., Solomon, M. J., Sherr, C. J., Massague, J., Roberts,
J. M. and Koff, A. (1994). p27Kip1, a cyclin-Cdk inhibitor, links transforming
growth factor-beta and contact inhibition to cell cycle arrest. Genes Dev. 8, 9-22.
Reddy, B. V. and Irvine, K. D. (2013). Regulation of Hippo signaling by EGFRMAPK signaling through Ajuba family proteins. Dev. Cell 24, 459-471.
Journal of Cell Science
COMMENTARY
Robinson, B. S., Huang, J., Hong, Y. and Moberg, K. H. (2010). Crumbs
regulates Salvador/Warts/Hippo signaling in Drosophila via the FERM-domain
protein Expanded. Curr. Biol. 20, 582-590.
Saburi, S., Hester, I., Goodrich, L. and McNeill, H. (2012). Functional
interactions between Fat family cadherins in tissue morphogenesis and planar
polarity. Development 139, 1806-1820.
Sansores-Garcia, L., Bossuyt, W., Wada, K., Yonemura, S., Tao, C., Sasaki, H.
and Halder, G. (2011). Modulating F-actin organization induces organ growth by
affecting the Hippo pathway. EMBO J. 30, 2325-2335.
Schlegelmilch, K., Mohseni, M., Kirak, O., Pruszak, J., Rodriguez, J. R., Zhou,
D., Kreger, B. T., Vasioukhin, V., Avruch, J., Brummelkamp, T. R. et al.
(2011). Yap1 acts downstream of a-catenin to control epidermal proliferation.
Cell 144, 782-795.
Silvis, M. R., Kreger, B. T., Lien, W. H., Klezovitch, O., Rudakova, G. M.,
Camargo, F. D., Lantz, D. M., Seykora, J. T. and Vasioukhin, V. (2011). acatenin is a tumor suppressor that controls cell accumulation by regulating the
localization and activity of the transcriptional coactivator Yap1. Sci. Signal. 4, ra33.
Skouloudaki, K., Puetz, M., Simons, M., Courbard, J. R., Boehlke, C.,
Hartleben, B., Engel, C., Moeller, M. J., Englert, C., Bollig, F. et al. (2009).
Scribble participates in Hippo signaling and is required for normal zebrafish
pronephros development. Proc. Natl. Acad. Sci. USA 106, 8579-8584.
Straßburger, K., Tiebe, M., Pinna, F., Breuhahn, K. and Teleman, A. A. (2012).
Insulin/IGF signaling drives cell proliferation in part via Yorkie/YAP. Dev. Biol.
367, 187-196.
Tepass, U. (2012). The apical polarity protein network in Drosophila epithelial
cells: regulation of polarity, junctions, morphogenesis, cell growth, and survival.
Annu. Rev. Cell Dev. Biol. 28, 655-685.
Tremblay, A. M. and Camargo, F. D. (2012). Hippo signaling in mammalian stem
cells. Semin. Cell Dev. Biol. 23, 818-826.
Tumaneng, K., Russell, R. C. and Guan, K. L. (2012). Organ size control by
Hippo and TOR pathways. Curr. Biol. 22, R368-R379.
Varelas, X., Samavarchi-Tehrani, P., Narimatsu, M., Weiss, A., Cockburn, K.,
Larsen, B. G., Rossant, J. and Wrana, J. L. (2010a). The Crumbs complex
couples cell density sensing to Hippo-dependent control of the TGF-b-SMAD
pathway. Dev. Cell 19, 831-844.
Varelas, X., Miller, B. W., Sopko, R., Song, S., Gregorieff, A., Fellouse, F. A.,
Sakuma, R., Pawson, T., Hunziker, W., McNeill, H. et al. (2010b). The Hippo
pathway regulates Wnt/beta-catenin signaling. Dev. Cell 18, 579-591.
Vestweber, D., Gossler, A., Boller, K. and Kemler, R. (1987). Expression and
distribution of cell adhesion molecule uvomorulin in mouse preimplantation
embryos. Dev. Biol. 124, 451-456.
Wada, K., Itoga, K., Okano, T., Yonemura, S. and Sasaki, H. (2011). Hippo pathway
regulation by cell morphology and stress fibers. Development 138, 3907-3914.
Wei, S. Y., Escudero, L. M., Yu, F., Chang, L. H., Chen, L. Y., Ho, Y. H., Lin,
C. M., Chou, C. S., Chia, W., Modolell, J. et al. (2005). Echinoid is a
Journal of Cell Science (2014) 127, 709–717 doi:10.1242/jcs.140103
component of adherens junctions that cooperates with DE-Cadherin to mediate
cell adhesion. Dev. Cell 8, 493-504.
Wieser, R. J., Faust, D., Dietrich, C. and Oesch, F. (1999). p16INK4 mediates
contact-inhibition of growth. Oncogene 18, 277-281.
Yagi, R., Chen, L. F., Shigesada, K., Murakami, Y. and Ito, Y. (1999). A WW
domain-containing yes-associated protein (YAP) is a novel transcriptional coactivator. EMBO J. 18, 2551-2562.
Yin, F., Yu, J., Zheng, Y., Chen, Q., Zhang, N. and Pan, D. (2013). Spatial
organization of Hippo signaling at the plasma membrane mediated by the tumor
suppressor Merlin/NF2. Cell 154, 1342-1355.
Yu, F. X. and Guan, K. L. (2013). The Hippo pathway: regulators and regulations.
Genes Dev. 27, 355-371.
Yu, F. X., Zhao, B., Panupinthu, N., Jewell, J. L., Lian, I., Wang, L. H., Zhao, J.,
Yuan, H., Tumaneng, K., Li, H. et al. (2012). Regulation of the Hippo-YAP
pathway by G-protein-coupled receptor signaling. Cell 150, 780-791.
Yu, F. X., Zhang, Y., Park, H. W., Jewell, J. L., Chen, Q., Deng, Y., Pan, D., Taylor, S.
S., Lai, Z. C. and Guan, K. L. (2013). Protein kinase A activates the Hippo pathway
to modulate cell proliferation and differentiation. Genes Dev. 27, 1223-1232.
Yue, T., Tian, A. and Jiang, J. (2012). The cell adhesion molecule echinoid
functions as a tumor suppressor and upstream regulator of the Hippo signaling
pathway. Dev. Cell 22, 255-267.
Zhang, J., Ji, J. Y., Yu, M., Overholtzer, M., Smolen, G. A., Wang, R., Brugge,
J. S., Dyson, N. J. and Haber, D. A. (2009). YAP-dependent induction
of amphiregulin identifies a non-cell-autonomous component of the Hippo
pathway. Nat. Cell Biol. 11, 1444-1450.
Zhao, B., Wei, X., Li, W., Udan, R. S., Yang, Q., Kim, J., Xie, J., Ikenoue, T., Yu,
J., Li, L. et al. (2007). Inactivation of YAP oncoprotein by the Hippo pathway is
involved in cell contact inhibition and tissue growth control. Genes Dev. 21,
2747-2761.
Zhao, B., Ye, X., Yu, J., Li, L., Li, W., Li, S., Yu, J., Lin, J. D., Wang, C. Y.,
Chinnaiyan, A. M. et al. (2008). TEAD mediates YAP-dependent gene
induction and growth control. Genes Dev. 22, 1962-1971.
Zhao, B., Li, L., Tumaneng, K., Wang, C. Y. and Guan, K. L. (2010). A
coordinated phosphorylation by Lats and CK1 regulates YAP stability through
SCF(beta-TRCP). Genes Dev. 24, 72-85.
Zhao, B., Li, L., Lu, Q., Wang, L. H., Liu, C. Y., Lei, Q. and Guan, K. L. (2011).
Angiomotin is a novel Hippo pathway component that inhibits YAP oncoprotein.
Genes Dev. 25, 51-63.
Zhao, B., Li, L., Wang, L., Wang, C. Y., Yu, J. and Guan, K. L. (2012). Cell
detachment activates the Hippo pathway via cytoskeleton reorganization to
induce anoikis. Genes Dev. 26, 54-68.
Zhou, D., Conrad, C., Xia, F., Park, J. S., Payer, B., Yin, Y., Lauwers, G. Y.,
Thasler, W., Lee, J. T., Avruch, J. et al. (2009). Mst1 and Mst2 maintain
hepatocyte quiescence and suppress hepatocellular carcinoma development
through inactivation of the Yap1 oncogene. Cancer Cell 16, 425-438.
Journal of Cell Science
COMMENTARY
717