ABBS 2008,40(07): Wnt signaling: the good and the bad

Review

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Acta Biochim Biophys

Sin 2008, 40: 577-594

doi:10.1111/j.1745-7270.2008.00440.x

Wnt signaling: the good and the bad

Xi Chen, Jun Yang, Paul M Evans, and Chunming Liu*

Sealy Center for Cancer Cell Biology,

Department of Biochemistry and Molecular Biology, University of Texas Medical

Branch, 301 University Blvd, Galveston, Texas 77555-1048, USA

Received: April 24,

2008       

Accepted: May 16,

2008

This work was

supported by the grants from the Sealy Center for Cancer Cell Biology and the

National Institutes of Health (T32CA117834)

*Corresponding

author: Tel, 409-747-1909; E-mail, [email protected]

Since the first Wnt gene was identified in

1982, the functions and mechanisms of Wnt signaling have been extensively

studied. Wnt signaling is conserved from invertebrates to vertebrates and

regulates early embryonic development as well as the homeostasis of adult

tissues. In addition, both embryonic stem cells and adult stem cells are

regulated by Wnt signaling. Deregulation of Wnt signaling is associated with

many human diseases, particularly cancers. In this review, we will discuss in

detail the functions of many components involved in the Wnt signal transduction

pathway. Then, we will explore what is known about the role of Wnt signaling in

stem cells and cancers.

Keywords        Wnt signaling; b-catenin;

cancer; stem cell

The orchestration of proliferation and differentiation of each cell

in a specific spatial and temporal manner is critical in the development of any

multicellular organism. To this effect, multiple signaling pathways have

evolved to help coordinate these events and facilitate between cells. The Wnt

signaling pathway is exploited in a wide variety of contexts to achieve these

aims. Wnt signaling controls many events in early development including axis

determination, patterning of organs, and cell fate. In the adult organism, Wnt

signaling is critically involved in the homeostasis of many tissues, including

the intestine, skin, bone and hematopoietic system [1,2]. In addition, recent

research suggests that Wnt signaling is also essential in stem cell

self-renewal [1,2]. Moreover, the Wnt pathway as a whole and the majority of

its components are preserved in many organisms used in biological research, including

Drosophila melanogaster, Caenorhabditis elegans, Xenopus

laevis, and Mus musculus.The Wnt-1 gene was first identified as a preferential insertion site

for the murine mammary tumor virus, resulting in overexpression of the Wnt-1

ligand and the formation of mammary tumors [3]. Wnt-1 was originally called

int-1. Its Drosophila homolog, Wingless (Wg), controls segment polarity

in early development [4], whereas injection of Wnt-1 mRNA into ventral side of

a Xenopus embryo induces body axis duplication [5]. Wnt proteins

activate three different downstream pathways: the canonical pathway, the planar

cell polarity (PCP) pathway and the Wnt/Ca2+

pathway. This review will focus on the canonical pathway, which regulates

cellular responses through b-catenin. Readers interested in the PCP pathway and Ca2+ pathway are directed to several excellent reviews already written

on the subject [6–9]. Deregulated Wnt signaling has been implicated in many hereditary

diseases and cancers. Constitutive activation of Wnt signaling is the

initiating event in both colorectal cancer and hepatocellular carcinoma,

whereas mutations that result in decreased or absent Wnt signaling have been

found in several disorders as well. For example, osteoporosis-pseudoglioma syndrome,

which causes defects in bone density, and Familial Exudative Vitreoretinopathy,

which results in defective vasculogenesis of the retina [1,2]. A comprehensive

resource of information about Wnt signaling can be found on the web at http://www.stanford.edu/~rnusse/Wntwindow.html.  

Major Components of the Wnt Pathway

Wnt genes have been identified in many organisms, including insects,

nematodes, Cnidaria and vertebrates [10]. In both human and mouse

genomes, 19 Wnt genes have been found. Each individual Wnt protein can have

drastically different effects on the target. Some activate the canonical

pathway, whereas others activate the PCP pathway, and/or the Ca2+ pathway. The latter two pathways are collectively referred to as

the “non-canonical” pathways. Thus, Wnt species are generally classified

according to which particular pathways they activate [1,2].  In the canonical Wnt pathway, a large number of components work

together to transduce an external signal into changes in gene expression within

the target cell (Figs. 1 and 2). Wnt is a secreted ligand that

binds to its receptor at the cell membrane. The major effect of Wnt binding its

ligand is the stabilization of cytoplasmic b-catenin through inhibition

of the ?-catenin degradation complex. b-catenin is then free to

enter the nucleus and activate Wnt-regulated genes through its interaction with

TCF (T-cell factor) family transcription factors and concomitant recruitment of

co-activators such as p300/CBP, Pygopus, and BCL9/Legless. This section will

explore the many different components of the Wnt pathway in more detail.

Wnt is a secreted ligandDespite the differences individual Wnts can have on target cells,

all Wnt proteins are similar in that they have an N-terminal signal peptide,

one or more N-linked glycosylation sites, and 23 conserved cysteine residues

[2]. In addition, most Wnt proteins are lipid-modified and are thus hydrophobic

in nature, explaining the difficulty many have had in their purification [11].

The acyl-transferase protein Porcupine regulates lipid modification of Wnt

proteins in the endoplasmic reticulum and is critical in the transport and

secretion of Wnt (Fig. 1) [11,12]. However, the secretion of Drosophila

WntD does not require lipid modification [13].Wntless (Wls, also known as Evenness Interrupted, EVI, or Sprinter,

SRT) is a multi-trans-membrane protein localized in the Golgi apparatus and in

the cell membrane that also regulates the secretion of Wnt [14–16]. Moreover,

Wntless genes have been found in both worms and mammals, suggesting that it is

a conserved member of the Wnt pathway. Wntless directly interacts with Wnt and

may act as a receptor for transporting Wnt from the trans-Golgi network to

endosomes. In both the Golgi and endosomes, Wntless co-localizes with retromer,

a highly conserved multi-protein complex involved in cell sorting. Furthermore,

in C. elegans, RNA interference (RNAi)-mediated knockdown of a key

component of the retromer complex, Vps35, hampers the formation of Wnt

gradients [17]. Additional studies in Drosophila and C. elegans

suggest that retromer regulates the retrieval of Wntless (mig14 in C.

elegans). For example, when Vps35 is inhibited, Wntless is targeted to the

lysosome for degradation, compromising the secretion of Wnt [18,19].

The Wnt receptors––Frizzled, and

others

After Wnt is secreted from a cell, it diffuses to nearby cells and

binds to its receptor, Frizzled (Fz) (Fig. 2). Fz was originally

identified as the receptor in Drosophila through biochemical and genetic

means [20]. Many vertebrate homologs of Fz were identified soon after [21]. Fz

receptors are seven trans-membrane repeat proteins that belong to a family of

G-protein coupled receptors. Interestingly, this family also includes

Smoothened, a key component in Hedgehog signaling [22]. The extra-cellular

N-terminus of Fz contains a cysteine-rich domain (CRD) that directly interacts

with Wnt. In vitro, each Wnt can bind to many different Fz receptors

[23,24]. In vivo, specificity may be achieved by additional

factors or by the restricted temporal expression of both Wnt and Fz. The

cytoplasmic tail of Fz has a conserved KTxxxW motif that interacts with a

downstream mediator of Wnt signaling, Dishevelled (Dvl), through its PDZ domain

[25,26]. In addition, some Fz receptors have an S/TxV motif that may also bind

to the PDZ domain of Dvl [27]. In addition to signaling through Dvl, it has

also been suggested that Fz may activate signaling through heterotrimeric G

protein [28,29]. Low-density-lipoprotein receptor-related proteins 5 and 6 (Lrp5/6) are co-receptors of Fz, whereas

arrow is the Drosophila homolog [30–32]. Genetic deletion of Lrp5/6 in mice results in a phenotype

that resembles a Wnt null mutation, suggesting that Lrp5/6 is a critical component of the Wnt pathway [30,32,33].

Lrp5/6 is a single trans-membrane

protein. The extracellular domain contains a YWTD b-propeller and an EGF-like

domain, and is required for binding Wnt as well as signal transduction [31,34].

On the cytoplasmic side, the C-terminus of Lrp5/6

contains five conserved PPP(S/T)P motifs which are equally indispensable for

the transduction of Wnt signaling (see below). The proper expression and

localization of Lrp5/6 is itself

subject to regulation. Mesd, an ER chaperone protein, is involved in the

maturation of the Lrp5/6 receptor

and its transport to the cell surface. Boca is its Drosophila homolog

[35,36].Additional receptors for Wnt have also been reported. For example, the

atypical receptor tyrosine kinase Ryk binds Wnt and regulates neurite

outgrowth. Derail, its Drosophila homolog, similarly mediates axon

guidance [37,38]. Wnt5a can also signal through Ror2, a receptor tyrosine

kinase, to regulate convergent extension through the activation of PI3K and

Cdc42 [39–41].In addition to Wnt, several other ligands can bind Fz receptors and

activate the canonical pathway. For example, Norrin binds Fz4 and this

interaction appears to be required for vascular development in eye and ear

[42]. In addition, mutations in either Norrin or Fz4 result in the retinal

vascular defects found in both Norrie disease and Familial Exudative

Vitreoretinopathy (FEVR) [42].Similarly, R-spondin family members bind Fz8 and Lrp6 and activate expression of Wnt

target genes [43–45]. In vertebrates, the R-spondin family contains four members.

R-Spondin2 activates Wnt signaling in Xenopus embryo [46], whereas the

intestinal epithelium of mice overexpressing R-spondin1 has hyperplasia and

elevated b-catenin levels [46]. Additional roles of R-spondin family members

have been reported in limb and lung development as well as sex determination

[47–49].Similarly, R-spondin family members bind Fz8 and Lrp6 and activate expression of Wnt

target genes [43–45]. In vertebrates, the R-spondin family contains four members.

R-Spondin2 activates Wnt signaling in Xenopus embryo [46], whereas the

intestinal epithelium of mice overexpressing R-spondin1 has hyperplasia and

elevated b-catenin levels [46]. Additional roles of R-spondin family members

have been reported in limb and lung development as well as sex determination

[47–49].

Extracellular Wnt antagonistsMany extracellular inhibitors of Wnt signaling have been reported.

For example, both secreted Frizzled-related protein (sFRP) and Wnt-inhibitory

factor (WIF) antagonize Wnt signaling by sequestering the Wnt protein in the

extracellular matrix. sFRP binds Wnt through its CRD domain, whereas [50,51]

WIF protein binds Wnt through its WIF domain [52].  Using an entirely different mechanism Wise [53], SOST [54,55], and

Dickkopf (Dkk) [56] antagonize Wnt through interactions with LRP. In addition,

Dkk1 bridges LRP6 and another transmembrane protein, Kremen, inducing

endocytosis of Lrp6. Without Lrp6 available at the cell surface, Wnt

signaling is effectively inhibited [57].

Crossing the membraneThe extracellular binding of Wnt to Fz and Lrp5/6 modulates intracellular components of the Wnt pathway

through at least two mechanisms (Fig. 2).  First, the binding of Wnt induces structural changes in the receptor

that result in recruitment of Dvl to the cytoplasmic tail of Fz through its

intracellular KTxxxW motif [25,26,58,59]. Dvl is an important cytoplasmic

component of the Wnt pathway that is conserved in both flies and vertebrates

[60–62],

and genetic epistasis studies place Dishevelled downstream of Fz, but upstream

of GSK3 (glycogen synthase kinase 3) and b-catenin [63]. Wnt also

induces phosphorylation of Dvl [64], although the role of the phosphorylation

of Dvl is still unclear. However, several kinases phosphorylate Dvl, including

casein kinase I (CKI), CKII, and Par-1 [65–67]. Second, the C-terminus of LRP5/6 interacts with Axin, which is an

inhibitory downstream component of the Wnt pathway. Recruitment of Axin to the

cell membrane inhibits its function, resulting in the stabilization of b-catenin [34].

The binding of Wnt also induces phosphorylation of the co-receptor Lrp5/6 at its PPPSP motif, creating a

docking site for Axin [68]. Phosphorylation of the PPPSP motif is mediated by

membrane-bound GSK3 and CKI [69]. In response to Wnt stimulation, LRP5/6 is

phosphorylated at an additional site, N-terminal to PPPSP motif by the

membrane-bound protein casein kinase Ig (CKIg). Moreover, phosphorylation at this site is

indispensable for Wnt signaling [70]. Although it is thought that the physical proximity of Fz and LRP5/6

affected by Wnt binding is crucial in activating downstream components,

surprisingly, Arrow mutant flies were only inefficiently rescued by a

Frizzled-arrow fusion protein. To explain this, a two-step signaling mechanism

was recently proposed. The initiation step requires both Fz and arrow, whereas

the amplification step depends only on arrow [71]. In support of this, both Fz

and Disheveled are required for phosphorylation of LRP6 [72–74]. Thus, the

PPPSP motif of LRP5/6 may function as an amplifier of Wnt signaling. Using live

imaging of vertebrate cells, one study found that Wnt induces the organization

of phosphorylated LRP6 into aggregates known as signalosomes, in a

Dvl-dependent manner [74]. These findings suggest that Fz, LRP5/6, Dvl and Axin

must organize into macromolecular complex in order to efficiently mediate Wnt

signaling.

In the cytoplasm

b-catenin was originally

identified as E-cadherin binding partner and important in cell-cell adhesion,

prior to the discovery of its involvement in the Wnt pathway [75]. However, it

was found that mutations of the Drosophila homolog of ?-catenin,

Armadillo (Arm) [76,77], gave a similar phenotype as the Wg mutant [4],

suggesting that Arm might be part of the Wg signaling pathway. Further studies

found that injection of b-catenin mRNA into the ventral side of Xenopus embryos

induced a secondary axis, a hallmark of Wnt signaling [78]. Thus, it became

clear that b-catenin/Arm functions downstream of Wnt/Wg [79], in addition to its

previously characterized role in cell-cell adhesion. Many of the other components of the Wnt pathway were soon found by a

variety of means. In mice, Axin is encoded by the fused locus, and Axis

duplication was observed in fused homozygous mutant embryo [80]. Similar

results were found in Xenopus embryos, implicating Axin as a negative

regulator of Wnt signaling [80]. The serine/threonine kinase GSK-3b was initially

found as a key regulator of glycogen metabolism, as it can phosphorylate and

inactivate glycogen-synthase. However, it was later found to be essential in

several signaling pathways as well [81]. Zeste-White 3, the Drosophila

homologue of GSK-3b, was found to be a negative regulator of segment polarity

downstream of Wg [82]. In the Xenopus embryo, GSK-3b suppresses axis

formation induced by Wnt [83]. Adenomatous polyposis coli (APC) is a tumor suppressor protein

frequently mutated in colorectal cancer [84,85]. In early attempts to identify

the function of APC, it was found that APC could directly interact with b-catenin and

decrease levels of cytoplasmic b-catenin [86–88], whereas mutations of APC found in human colorectal cancer lead

to accumulation of b-catenin [89,90]. In addition, APC also binds and is phosphorylated

by GSK-3b [89]. Thus, it is now clear that the central task of canonical Wnt

signaling is to regulate b-catenin stability. The level of cytoplasmic b-catenin is

tightly controlled by the cytoplasmic degradation complex (Fig. 2),

which contains the scaffold protein Axin, as well as b-catenin, CKI, GSK3 and APC

[91–95].

In unstimulated cells, this complex mediates the degradation of cytoplasmic b-catenin through

a multi-step process. First, b-catenin is phosphorylated at the N-terminus by CKIa [95,96] and

GSK-3b [97]. Phosphorylated b-catenin is then ubiquitinated by b-Trcp, a

component of an E3 ubiquitin ligase complex [98–102]. Ubiquitinated b-catenin is then

rapidly degraded by proteasome. The structure of part of this destruction complex has been solved

[103–112]. The structures of central armadillo repeats as well as the

full-length b-catenin have also been solved [109,113]. These studies suggest that

b-catenin

degradation is regulated by a dynamic protein complex. APC may regulate the

assembly of Axin complex. When b-catenin is phosphorylated by CKI and GSK-3b within this complex, APC

is also phosphorylated. Phosphorylated APC binds b-catenin with a significantly

higher affinity, displacing b-catenin from the Axin complex [108,114,115]. Axin is the least

abundant protein among the destruction complex proteins and appears to be the

rate-limiting factor [116]. b-catenin can be phosphorylated in colon cancer cell line SW480,

which contains truncated APC. However, b-catenin ubiquitination

cannot be detected in SW480 cells, suggesting that separate domains of APC are

required for phosphorylation and ubiquitination. In addition, overexpression of

a functional APC fragment can restore b-catenin ubiquitination and degradation,

further suggesting that APC regulates b-catenin phosphorylation and degradation by

distinct domains and steps.   As mentioned earlier, Wnt proteins bind Fz and Lrp5/6, resulting in phosphorylation of

cytoplasmic tail of Lrp5/6 and

recruitment of Dvl [73,74]. Additionally, phosphorylated Lrp5/6 relocates Axin to the cell

membrane, inhibiting the cytoplasmic degradation complex through a mechanism

that is not completely understood. b-catenin is then free to accumulate and

translocate into the nucleus. Moreover, Axin degradation upon Wnt stimulation

provides another way to stabilize b-catenin [34,117–119].Since phosphorylation plays important roles in Wnt signaling, the

many associated kinases and phosphatases have been extensively studied. For

example, Axin binds the catalytic domain of protein phosphatase 2A (PP2A)

[120]. However, both positive and negative roles for PP2A in Wnt signaling have

been reported [121–128]. The exact role of PP2A in Wnt signaling may depend on the

composition of different PP2A regulatory subunits and needs further

examination. Protein phosphatase 1 (PPI) has a clear positive role in Wnt

signaling [129]. PP1 binds and de-phosphorylates Axin, decreasing its affinity

for GSK-3b, therefore leading to stabilization of b-catenin [129]. By tandem-affinity purification WTX (Wilms tumor suppressor X

chromosome), was found to interact with b-catenin, Axin, APC and b-Trcp.  Moreover, WTX promotes b-catenin

degradation and ubiquitination in mammalian cells, as well as zebrafish and Xenopus

[130]. As WTX is inactivated in one third of Wilms tumors [131], it will be

interesting to investigate its role in other types of tumors.

Into the nucleus

b-catenin does not contain a nuclear localization

sequence. It has been suggested that b-catenin

can directly interact with nuclear pore components, bypassing the

importin/karyopherin proteins, in order to enter the nucleus [132,133].

Recently, it was discovered that JNK2 phosphorylates b-catenin at Ser191 and Ser605 in response to Rac1

activation, and that phosphorylation at these two serine controls nuclear

translocation of b-catenin [134].

b-catenin contains a nuclear export

sequence, which is consistent with its ability to shuttle in and out of the

nucleus in response to changes in Wnt signaling; however, how other factors

regulate its export is still a contentious issue. One model suggests that Axin

[135] or APC [136–138] actively export b-catenin from the nucleus, in addition to

their more fully characterized role in b-catenin degradation. An alternate model suggests that

these factors do not actively participate in shuttling, but rather as an

“anchor” to retain b-catenin within their

respective compartments [139]. In this model TCF-4, Pygopus, and BCL9 function

as nuclear “anchors” [140], and Axin functions as cytoplasmic “anchor” [141].

In the nucleus b-catenin interacts with the TCF family of

transcription factors [142,143]. The TCF family includes TCF-1, LEF-1 (lymphoid

enhancer factor-1), TCF-3, and TCF-4. Among them, TCF-4 is the primary member

of the TCF family that is regulated by b-catenin in response to Wnt signaling in the

intestine. In the unbound state, TCF/LEF family members actively recruit

co-repressors such as CtBP [144], HDAC1 [145,146], and Groucho/TLE [147–149] to inhibit transcription. Groucho/TLE,

in turn, interacts with hypo-acetylated histone H3, presumably to help maintain

a repressive chromatin environment [150]. However, once b-catenin enters the nucleus, it binds TCF-4

through its central armadillo repeats, displaces Groucho/TLE-1 from TCF/LEF

[151] and recruits co-activators through its N- and C-terminal transactivation

domains (Fig. 2).

The N-terminal transactivation domain of b-catenin, extends from the region just

C-terminal to the regulatory region involved in its stability, to the first

four Armadillo repeats [152]. This transactivation domain directly associates

with BCL9/Legless, which in turn recruits the transcriptional co-activator

Pygopus [153–156]. Pygopus contains a plant homeodomain

(PHD). PHD domain can interact with tri-methylated histone H3, and is thought

to regulate epigenetic modifications on target genes [157]. In addition,

Pygopus can dimerize through this domain in vitro [110].

The C-terminus of b-catenin contains a strong transactivation domain

[142,158,159]. This transactivation domain recruits p300/CBP, which is required

for Wnt signaling [158,160]. p300 and CBP are paralogous transcriptional

co-activators; they acetylate nearby histones, loosening chromatin in order to

facilitate binding of other transcription factors [161,162]. In addition, the

C-terminal transactivation domain associates with Parafibromin, a component of

PAF1 complex, and is recruited after Pygopus. PAF1 is important for the

initiation and elongation steps of transcription through its interaction with

RNA polymerase II. The association of b-catenin

with the PAF1 complex is required for transactivation. Overexpression of

Parafibromin compensated for loss of Legless in vivo [163].

Other data suggest that b-catenin can interact with the co-activator

FHL2 [164], the basal transcription factor TBP [165], the ATP-dependent

chromatin remodeling factors Brg-1/Brahma [166 ] and the ATP-dependent helicase

TIP49a/Pontin52 [167,168]. However, these interactions have not been fully

characterized.

Wnt Signaling in Stem Cells

The cells of mammalian organisms are highly dynamic. Every day, millions

of cells are replaced due to physical, chemical and immunologic injuries. Stem

cells are required to maintain the architecture and function of organisms.

These cells reside in a special micro-environment called a niche and they

maintain the proliferative potential of tissues throughout the life of an

organism. Key features of stem cells are self-renewal and their ability to give

rise to different cell lineages. Wnt signaling is critical in the self-renewal

of stem cells in many different tissues, including the skin, intestine, brain

and blood. This section will further explore the role of Wnt signaling in this

context.

Intestinal stem cells

The gut is a tube-like organ that originates from all three germ

layers: the endoderm, mesoderm and ectoderm. The luminal surface of the gut is

covered by a continuous sheet of epithelial cells derived from endoderm. In the

epithelium of the small of intestine, this sheet folds into finger-like

protrusions that extend into the lumen, called villi. In between each villus,

the epithelial sheet additionally invaginates inward to form the crypts of

Lieberk?hn [169] (Fig. 3). Notably, no villi are present in the colon

and instead the colonic epithelium consists entirely of crypts. Stem cells that

replenish the intestinal epithelium are located at the bottom of crypts. Crypt

stem cells produce transit-amplifying cells that ultimately differentiate into

enterocytes, goblet cells, and enteroendocrine cells (Fig. 3). In the

small intestine, transit-amplifying cells additionally differentiate into

Paneth cells [169,170]. Enterocytes are the most abundant cell type of the

intestine, and carry out its primary absorptive function. Goblet cells secrete

mucin that protects the luminal surface. Enteroendocrine cells are located throughout

the crypt-villus axis and secrete intestinal hormones. Paneth cells are found

at the bottom of crypts and release lysozyme as well as other anti-microbial

molecules. With the exception of Paneth cells, terminally differentiated cells

migrate along the crypt-villi axis and are shed into lumen after 5–7 days. In each

crypt, stem cells have to generate around 300 cells per day in order to

replenish those lost [170].Wnt signaling is critical in the regulation of intestinal

homeostasis. TCF-4, encoded by Tcf7l2 gene, is a downstream target of

Wnt signaling and is highly expressed in the intestinal epithelium. TCF-4

knockout mice lack crypts, suggesting that TCF-4 is essential for maintenance

of epithelial stem cell compartment [171]. Overexpression of Dkk1, a Wnt

inhibitor, in intestine causes loss of crypt and secretory cell lineages [172].

These data suggest that Wnt is essential for the homeostasis of intestine

epithelium. A similar phenotype was observed when Dkk1 was overexpressed using

adenoviruses [173]. As mentioned earlier, APC is a negative regulator of b-catenin and a

tumor suppressor in colorectal cancer. Targeted deletion of APC in the mouse

intestine activates Wnt signaling and results in expansion of the crypts [174].

In addition, Goblet cells are lost, and Paneth cells are mis-positioned

throughout the crypts-villus axis [174], resembling loss of the cell-sorting

receptor EphB3 [175]. Expression of the Wnt agonist R-spondin1 in mice induces

crypt cell proliferation [46]. Crypt epithelial cells consistently produce

Wnt3, Wnt6, and Wnt9b [176], suggesting that Wnt might function in a paracrine

or autocrine manner. MYC is a well-established Wnt target gene [177].

Interestingly, deletion of MYC in APC–/– intestine rescues the defects in proliferation and migration found

in APC–/– mice [178].

These data suggest that Wnt is an essential mitogen in the crypt. Because no specific marker for intestinal stem cells has been found,

the exact position of these cells within the crypts is still unclear. However,

experiments following the retention of labeled DNA suggest that the stem cell

might be found at the +4 position, just above the crypt base columnar, and

Paneth cells at the base of the crypt [170]. In addition crypt base columnar

cells have been suggested to have stem cell activity [179]. LGR5 is an orphan

G-protein coupled receptor first identified as Wnt target gene in micro-array

studies [180]. In situ hybridization and reporter knock-in studies

suggest that LGR5 is an intestinal stem cell marker and that the crypt columnar

base cells are the stem cells of the intestine [181]. However, the relationship

between +4 cells and the crypt columnar base cells remain to be determined. Recent studies suggest that a small subset of cells in tumors have

stem cell-like characteristics. Two groups independently reported the

identification of a colorectal cancer stem cell based on the surface marker

CD133 [182,183]. Since various Wnt signaling pathway components, such as APC,

Axin or b-catenin, are mutated in more than 90% of colorectal cancer

patients, it is of great interest to know the role of Wnt signaling in

colorectal cancer stem cells.

Hematopoietic stem cells

Hematopoietic stem cells (HSCs) are multi-potent cells that are able

to give rise to all blood cell lineages. The fates of HSCs progeny are

determined in a stepwise, hierarchical fashion. HSCs give rise to common

myeloid progenitor (CMP) and common lymphoid progenitor (CLP) cells. Red blood

cells, macrophages, granulocytes and platelets derive from CMP cells, whereas T

cells, B cells, dendritic cells and natural killer cells derive from the CLP

cells. In the early embryo, HSCs derive from the mesoderm. Hematopoiesis begins

in the yolk sac but then quickly shifts to the liver. Later, fetal HSCs home to

the bone marrow, where they reside throughout the life of the adult [184].  HSCs are the best characterized type of stem cell because of the

ability to purify them to homogeneity based on cell surface markers. For example,

mouse HSCs express cell surface proteins C-kit and Sca-1 but are negative for

lineage markers (Lin-Sca-1+c-Kit+, or LSK

cells) [185]. Recent studies have begun to uncover the role of Wnt in

hematopoiesis. TCF-1 and LEF-1, transcription factors targeted by the Wnt

signaling pathway, and are expressed in a specific pattern, suggests Wnt

signaling might be important in hematopoiesis, both in self-renewal and

differentiation [186,187]. For example, knockout TCF-1 or LEF-1 in HSCs blocks

T-cell differentiation [188], whereas in vitro purified Wnt3a stimulates

self-renewal of HSCs [11]. Overexpression of activated b-catenin promotes the

growth of HSCs and maintains an immature phenotype in long term culture. In

addition, these expanded HSCs reconstitute the blood system more efficiently in

lethally irradiated mice [189]. Inhibition of Wnt signaling by overexpressing

Axin leads to slow growth of HSCs and a reduced percentage of reconstituted

stem cells [189]. However, other reports have found that knocking down expression of b-catenin had no

effect on the ability of HSC to reconstitute hematopoiesis in an irradiated

host mouse [190]. Another study found that simultaneous knockout b-catenin and g-catenin did not

impair reconstitution [191,192]. Using a synthetic reporter, however, another

report found that canonical Wnt signaling was still active in these double

knockout cells [192]. Enhancing Wnt signaling by treating lethally irradiated

mice with a GSK3 inhibitor increases the likelihood of hematopoietic repopulation

[193]. On the other hand, overexpression of b-catenin in transgenic mice

blocks lineage differentiation and results in an inability to repopulate

irradiated hosts [194,195]. Although complete resolution of these discrepancies

will require additional carefully designed experiments, it is likely that

supra-physiologic levels of b-catenin enforced cell cycling of HSCs and exhausted the long-term

stem cell pool [195]. Thus, it appears that maintaining a critical level of b-catenin might

be important for the normal function of HSCs. 

Additional data suggest that non-canonical Wnt signaling also has

role in hematopoiesis. For example, treating HSC cells with Wnt5a enhances

their ability to reconstitute hematopoiesis [196]. In addition, Wnt5a antagonizes

canonical Wnt signaling, keeping HSCs in the quiescent, G0 state, and increases the ability of HSCs to repopulate the

irradiated hosts [197].In addition, the bone marrow itself provides cues for HSCs in deciding

between self-renewal and differentiation. For example, both osteoblasts and

vascular cells have been implicated in maintaining the stem cell niche.

Osteoblasts may function to retain HSCs in the bone marrow and regulate their

stemness [184]. For example, using a transgene driven by a collagen 1a

promoter, targeted overexpression of Dkk1 in osteoblasts results in HSCs that

are unable to reconstitute bone marrow in lethally irradiated mice. Since Dkk1

is a Wnt antagonist, this implies that Wnt signaling is required for

self-renewal, although interestingly, the transgenic donor mice themselves have

a relatively normal hematopoietic cell population. However, analysis of these

HSCs by flow cytometry suggests they are not quiescent, suggesting that Wnt signals

from neighboring osteoblast cells may keep HSCs in the quiescent phase and able

to maintain their stemness [198].

Skin stem cells

Skin is the largest organ of the human body. It separates an

organism from the outside world, is the first barrier to fight against

microbes, and protects the body from chemical and physical injury. To protect

itself from permanent injury, the epithelium of the skin rapidly turns over,

replacing the entire barrier every 4 weeks.Similar to the intestinal epithelium, the skin relies on stem cells

to replenish lost cells. Some evidence suggests that the epidermal stem cells

are found within the basal cell compartment. Stem cell progeny migrate upward

towards the surface as they deposit cytokeratins. Terminally differentiated

keratinocytes become enucleated and contain cross-linked cytokeratins.

Eventually, these cells are sloughed off at the surface [199].  In addition to the replenishing keratinocytes, the skin must also

repopulate the cells that constitute a hair follicle. Hair follicle stem cells

reside in the bulge region, located in the middle of hair follicles. Bulge

cells are quiescent and retain BrdU labeling after administration. Activated

bulge stem cells move out of this niche and proliferate to supply the hair regeneration

at the beginning of a new hair cycle [199]. The importance of Wnt signaling in the skin homeostasis has long

been observed and several Wnt genes are expressed in skin. In addition, LEF-1

deficient mice lacked body hair and whiskers [200]. Conditional ablation of

?-catenin in the skin blocks placode formation and hair follicle growth

[201].  Blocking Wnt signaling by

expressing Dkk1 in the skin results in a similar phenotype [202]. Transient or

continuous overexpression of a stabilized b-catenin mutant in skin

causes excess follicle formation [203–206]. Wnt10a and Wnt10b are specifically expressed in placode [207]. Using

a galactosidase reporter in transgenic mice, Wnt signaling appears to be active

in the cortical cells of the hair shaft, whereas the bulge region is largely

inactive [208]. LEF-1 may mediate Wnt activity in cortex by the fact the

co-staining of nuclear b-catenin and galactosidase reporter in the precotex [209,210]. TCF-3

is expressed in the bulge region and keeps the follicle stem cell in a

quiescent state by inhibiting Wnt signaling [209,210]. In studying hair

follicle regeneration after wounding the skin, it was found that the

regeneration process was enhanced by overexpression of Wnt7a, whereas it was

blocked by express Dkk1 in skin [211]. These data highlight the critical role

of Wnt in hair follicle formation.

Neural stem cells

Adult neural stem cells are present in the subventricular zone of

the lateral ventricles and in the subgranular zone of the hippocampus. Neurons

from the subventricular zone migrate towards the olfactory bulb, while neurons

from the subgranular zone integrate into the existing circuitry [212].In situ hybridization studies suggest

that Wnt3 is expressed close to the subgranular zone [213]. Staining patterns

in a transgenic mouse with Wnt signaling reporter demonstrate that Wnt

signaling is active in the subgranular zone and dentate the granule cell layer

[213]. Overexpressing Wnt3 in purified hippocampal stem cells increases

neuronal production [213], whereas blocking Wnt signaling with the dominant

negative Wnt1 blocks neurogenesis at the subgranular zone [213]. In addition,

expressing Wnt3 in the subgranular zone region by injecting lentiviruses

enhances neurogenesis in the hippocampus [213]. Details of the role of Wnt

signaling in the adult neural stem cell will be uncovered by further analysis,

using tissue-specific knockout and transgenic animals.

Embryonic stem cells

Embryonic stem (ES) cells derive from the inner cell mass (ICM) of

mouse blastocysts. In contrast to tissue stem cells, embryonic stem cells are

pluripotent. When injected into an embryo, an ES cell is able to give rise to

all cell lineages in the adult. Pluripotency of the ES cell is controlled by an

intricate signaling network [214].Individual APC mutations have marked differences in their ability to

regulate the level of b-catenin in ES cells. In addition, these ES cells show different

levels of Wnt signaling, as demonstrated by activity of the TOPFlash reporter.

Differentiation patterns of teratomas generated by the APC1638T/1638T ES cell are indistinguishable from wild type teratomas, whereas APC1638N/1638N ES cells, which have low levels of APC expression, have

differentiation defects in the neuroectodermal, dorsal mesodermal and endodermal

lineages. ES cells from APCMin/Min mice on the other hand

could not form teratomas at all. Interestingly, ES cells with a stabilizing

mutation in b-catenin are able to form teratomas with only limited

differentiation capabilities [215]. Activation of Wnt signaling by a GSK3 inhibitor maintains the

undifferentiated state of ES cells [216]. Small molecule IQ-1 targets PR72/130

subunit of PP2A. It increases b-catenin/CBP mediated expression at the expense of b-catenin/p300

mediated expression. Through this mechanism it helps maintain the pluripotency

of embryonic stem cell [217]. Oct4 and Nanog are two important transcriptional

factors that control ES cell pluripotency. In ES cells, TCF-3, Oct4, and Nanog

co-occupy the promoters of many genes throughout the genome [218]. Knockdown

TCF-3 expression or treatment with Wnt-conditioned medium stimulates Oct4 and

Nanog expression and facilitates the maintenance of pluripotency [218]. Thus,

the function of TCF-3 may be to balance self-renewal and differentiation in ES

cells. These data suggest that the Wnt signaling levels are critical in

regulation of ES cell differentiation and self-renewal.

Wnt Signaling in Cancers

As a central pathway in both development and

homeostasis, the Wnt pathway regulates cell growth, survival and movement, and

uncontrolled activation of this pathway can result in neoplasia and cancer.

Mutations of b-catenin, APC, and Axin have been found in

many cancers, including colon, liver, ovary, brain, prostate, uterus, and skin

cancer [219].

Colorectal cancer (CRC)

Aberrant Wnt signaling was first linked to

cancer by the observation that familial adenomatous polyposis (FAP) patients

had a mutation in the APC gene [220–222].

In addition, aberrations in Wnt signaling have been identified in 90% of

sporadic CRCs [223]. The absence of functional APC protein results in chronic

activation of Wnt signaling, resulting in the formation of adenomas that

ultimately progress to adeno-carcinomas. Genetic studies using the APCmin/+ mouse model clearly demonstrate the role

of mutant APC in initiating the formation of tumors in the intestine [224,225].

These mice are heterozygous for a C-terminal truncated form of the APC gene.

Loss-of-heterozygosity of the wild-type allele leaves only mutant APC, which is

deficient to participate in the cytoplasmic degradation complex. This allows b-catenin to accumulate and results in

constitutively active Wnt signaling [90,226]. Moreover, conditional deletion of

the APC gene in the mouse adult intestine results in a “crypt progenitor-like”

phenotype with altered patterns of proliferation and differentiation [174,227],

and eventually leads to the formation of tumors [228].

In sporadic colorectal tumors that retain

wild-type APC, mutations are frequently found in the b-catenin gene (CTNNB1) [226,229] or Axin2 [230].

Moreover, targeted deletion of the N-terminus of b-catenin in the intestinal epithelium of mice produces

thousands of adenomatous polyps within weeks [231]. Finally, in a mouse model

of colitis-associated colorectal carcinoma, using 1,2-dimethylhydrazine and

dextran sulfate sodium, mice develop dysplastic lesions and invasive colorectal

cancer that strongly stains for b-catenin

in the nuclei [232].

Although APC and b-catenin mutations are the initiating step of colonic

tumorigenesis [85], downregulation of other tumor suppressor genes may also

contribute to the development of colon cancer. For example, Kr?ppel like factor

4 (KLF4), interacts with b-catenin,

repressing Wnt signaling and inhibiting tumor growth [233]. KLF+/–/APCMin/+ mice developed, on average, 59% more intestinal

adenomas than APCMin/+ mice [234]. It is

important to further analyze the cross-talk between Wnt signaling and other

signaling pathways, such as PTEN/Akt, Notch, bone morphogenetic protein (BMP) and Hedgehog in the tumorigenesis of

colon cancer as well as other cancers.

Prostate cancer

Prostate cancer is the most commonly diagnosed malignancy in

American males. The prostate gland is an organ dependent on androgen. Androgen,

via the androgen receptor (AR), controls the initial growth of prostatic tumor.

Androgen ablation therapy causes tumor regression in the early stages of

prostate cancer [235,236], clearly highlighting the dependence of tumor growth

on androgens. In prostate cells, the binding of androgen hormones to AR allows

AR to interact with b-catenin and stimulate AR-mediated transcriptional activity

[237–244]. In prostate cancer, ?-catenin similarly binds AR and activates

AR target gene expression [245]. On the other hand, AR can promote

?-catenin nuclear translocation in prostate cells [238]. Mutations in

components of the Wnt pathway have also been found in prostate cancer. In stark

contrast to colorectal cancer, mutations in APC are rarely detected [246], and

instead N-terminal stabilizing mutations of b-catenin are much more

frequent [245,247]. Mutant b-catenin induces hyperplasia, squamous cell trans-differentiation and

prostate intraepithelial neoplasia (PIN) in mice, suggesting that b-catenin can

induce neoplastic transformation in the prostate [248,249]. b-catenin

activity is also regulated by other molecules in prostate cancer. For example,

growth factors such as IGF and HGF activate b-catenin [250,251]. The

tumor suppressor PTEN, which is frequently mutated in prostate cancer, inhibits

b-catenin

signaling [250]. This suggests some degree of cross-talk between the Wnt and

PI3K pathways in the context of prostate cancer.

Liver cancer

Wnt signaling also plays a central role in

regulating liver cell proliferation during development [252–254] and in

governing essential functions of the adult liver

[255–257]. Moreover, aberrant reactivation of Wnt signaling due to accumulation of b-catenin is evident in many different tumors

of the liver [258]. Mutations in the b-catenin and Axin genes that lead to constitutive activation of b-catenin have been found in

hepatocellular carcinoma (HCC) and hepatoblastoma. In addition, frequent

overexpression of the Wnt receptor Fz7 is a common

early event in hepatocarcinogenesis [259,260]. Genotype-phenotype correlation

analysis in hepatocellular adenoma showed that mutation of b-catenin occurs

in only 12% of adenomas but in 46% of these adenomas progressed to HCC [261],

suggesting a role for b-catenin in the progression of pre-cancerous lesions to HCC.

Furthermore, simultaneous mutation of b-catenin and H-ras leads to 100% incidence of

HCC in mice [262]. These findings suggest that the aberrant Wnt signaling is

important in the progression of HCC.

Skin cancer

Wnt signaling regulates hair morphogenesis. Mice expressing a

truncated form of b-catenin have abnormal hair follicle morphogenesis [203].

Pilomatricoma is a common benign skin adnexal tumor showing differentiation

towards the matrix cells of the hair follicle. About 75% of pilomatricomas have

an activating mutation in b-catenin at the N-terminal phosphorylation site, which results in

cytoplasmic accumulation and nuclear translocation of b-catenin, resulting in

transcriptional activation of many target genes, such as c-Myc, cyclin D1 [263]. Isolation of CD34+/K14+ cells

from early mouse epidermal tumors results in a population of cells that are

more than 100-fold more potent in initiating secondary tumors than the original

heterogeneous mixture of cells isolated from the tumor. These cells express

many markers of bulge skin stem cells, suggesting that the CD34+/K14+ cells might be a type of cancer stem cell.

Nuclear b-catenin and high expression level of Axin2, both hallmarks of Wnt

signaling, are also evident in these skin tumors. Moreover, their

tumor-initiating ability depends on b-catenin signaling as loss of b-catenin results

in tumor regression [264]. Research from the last two decades

strongly emphasizes the importance of Wnt/b-catenin signaling in stem cell and cancers.

Whether Wnt signaling has a general role in cancer stem cells is not yet known.

However, clearly a deeper understanding of the molecular mechanisms of Wnt

signaling in human cancers will lead to translational research regarding novel

methods in cancer diagnosis and treatment.

References

  1   Clevers H. Wnt/b-catenin signaling

in development and disease. Cell 2006, 127: 469–480

  2   Logan CY, Nusse

R. The Wnt signaling pathway in development and disease. Annu Rev Cell Dev Biol

2004, 20: 781–810

  3   Nusse R,

Varmus HE. Many tumors induced by the mouse mammary tumor virus contain a

provirus integrated in the same region of the host genome. Cell 1982, 31: 99–109

  4   Nusslein-Volhard

C, Wieschaus E. Mutations affecting segment number and polarity in Drosophila.

Nature 1980, 287: 795–801

  5   McMahon AP,

Moon RT. Ectopic expression of the proto-oncogene int-1 in Xenopus

embryos leads to duplication of the embryonic axis. Cell 1989, 58: 1075–1084

  7   Veeman MT,

Axelrod JD, Moon RT. A second canon. Functions and mechanisms of b-catenin-independent

Wnt signaling. Dev Cell 2003, 5: 367–377

  8   Wallingford

JB, Fraser SE, Harland RM. Convergent extension: the molecular control of

polarized cell movement during embryonic development. Dev Cell 2002, 2: 695–706

  9   Kohn AD,

Moon RT. Wnt and calcium signaling: b-catenin-independent pathways.

Cell Calcium 2005, 38: 439–446

 10 Guder C, Philipp I, Lengfeld T,

Watanabe H, Hobmayer B, Holstein TW. The Wnt code: cnidarians signal the way.

Oncogene 2006, 25: 7450–7460

 11 Willert K, Brown JD, Danenberg

E, Duncan AW, Weissman IL, Reya T, Yates JR 3rd et al. Wnt proteins are

lipid-modified and can act as stem cell growth factors. Nature 2003, 423: 448–452

 12 Takada R, Satomi Y, Kurata T, Ueno

N, Norioka S, Kondoh H, Takao T et al. Monounsaturated fatty acid

modification of Wnt protein: its role in Wnt secretion. Dev Cell 2006, 11: 791–801

 13 Ching W, Hang HC, Nusse R.

Lipid-independent secretion of a Drosophila Wnt protein. J Biol Chem 2008,

Apr 22. [Epub ahead of print]

 14 Banziger C, Soldini D, Schutt

C, Zipperlen P, Hausmann G, Basler K. Wntless, a conserved membrane protein

dedicated to the secretion of Wnt proteins from signaling cells. Cell 2006,

125: 509–522

 15 Bartscherer K, Pelte N,

Ingelfinger D, Boutros M. Secretion of Wnt ligands requires Evi, a conserved

transmembrane protein. Cell 2006, 125: 523–533

 16 Goodman RM, Thombre S, Firtina

Z, Gray D, Betts D, Roebuck J, Spana EP et al. Sprinter: a novel

transmembrane protein required for Wg secretion and signaling. Development

2006, 133: 4901–4911

 17 Coudreuse DY, Roel G, Betist

MC, Destree O, Korswagen HC. Wnt gradient formation requires retromer function

in Wnt-producing cells. Science 2006, 312: 921–924

 18 Port F, Kuster M, Herr P,

Furger E, Banziger C, Hausmann G, Basler K. Wingless secretion promotes and

requires retromer-dependent cycling of Wntless. Nat Cell Biol 2008, 10: 178–185

 19 Yang PT, Lorenowicz MJ,

Silhankova M, Coudreuse DY, Betist MC, Korswagen HC. Wnt signaling requires

retromer-dependent recycling of MIG-14/Wntless in Wnt-producing cells. Dev Cell

2008, 14: 140–147

 20 Bhanot P, Brink M, Samos CH,

Hsieh JC, Wang Y, Macke JP, Andrew D et al. A new member of the Frizzled

family from Drosophila functions as a Wingless receptor. Nature 1996,

382: 225–230

 21 Yang-Snyder J, Miller JR, Brown

JD, Lai CJ, Moon RT. A Frizzled homolog functions in a vertebrate Wnt signaling

pathway. Curr Biol 1996, 6: 1302–1306

 22 Foord SM, Bonner TI, Neubig RR,

Rosser EM, Pin JP, Davenport AP, Spedding M et al. International Union

of Pharmacology. XLVI. G protein-coupled receptor list. Pharmacol Rev 2005, 57:

279–288

 23 Dann CE, Hsieh JC, Rattner A,

Sharma D, Nathans J, Leahy DJ. Insights into Wnt binding and signaling from the

structures of two Frizzled cysteine-rich domains. Nature 2001, 412: 86–90

 24 Hsieh JC, Rattner A, Smallwood

PM, Nathans J. Biochemical characterization of Wnt-Frizzled interactions using

a soluble, biologically active vertebrate Wnt protein. Proc Natl Acad Sci USA

1999, 96: 3546–3551

 25 Umbhauer M, Djiane A, Goisset

C, Penzo-Mendez A, Riou JF, Boucaut JC, Shi DL. The C-terminal cytoplasmic

Lys-Thr-X-X-X-Trp motif in Frizzled receptors mediates Wnt/b-catenin signaling.

EMBO J 2000, 19: 4944–4954

 26 Wong HC, Bourdelas A, Krauss A,

Lee HJ, Shao Y, Wu D, Mlodzik M et al. Direct binding of the PDZ domain

of Dishevelled to a conserved internal sequence in the C-terminal region of

Frizzled. Mol Cell 2003, 12: 1251–1260

 27 Schulte G, Bryja V. The

Frizzled family of unconventional G-protein-coupled receptors. Trends Pharmacol

Sci 2007, 28: 518–525

 28 Katanaev VL, Ponzielli R,

Semeriva M, Tomlinson A. Trimeric G protein-dependent Frizzled signaling in Drosophila.

Cell 2005, 120: 111–122

 29 Liu X, Rubin JS, Kimmel AR.

Rapid, Wnt-induced changes in GSK-3b associations that regulate b-catenin

stabilization are mediated by Ga proteins. Curr Biol 2005, 15:

1989–1997

 30 Pinson KI, Brennan J, Monkley

S, Avery BJ, Skarnes WC. An LDL-receptor-related protein mediates Wnt signaling

in mice. Nature 2000, 407: 535–538

 31 Tamai K, Semenov M, Kato Y,

Spokony R, Liu C, Katsuyama Y, Hess F et al. LDL-receptor-related

proteins in Wnt signal transduction. Nature 2000, 407: 530–535

 32 Wehrli M, Dougan ST, Caldwell

K, O’Keefe L, Schwartz S, Vaizel-Ohayon D, Schejter E et al. Arrow

encodes an LDL-receptor-related protein essential for Wingless signaling.

Nature 2000, 407: 527–530

 33 Kelly OG, Pinson KI, Skarnes

WC. The Wnt co-receptors LRP5 and LRP6 are essential for gastrulation in mice.

Development 2004, 131: 2803–2815

 34 Mao J, Wang J, Liu B, Pan W,

Farr GH 3rd, Flynn C, Yuan H et al. Low-density lipoprotein

receptor-related protein-5 binds to Axin and regulates the canonical Wnt

signaling pathway. Mol Cell 2001, 7: 801–809

 35 Culi J, Mann RS. Boca, an

endoplasmic reticulum protein required for wingless signaling and trafficking

of LDL receptor family members in Drosophila. Cell 2003, 112: 343–354

 36 Hsieh JC, Lee L, Zhang L, Wefer

S, Brown K, DeRossi C, Wines ME et al. Mesd encodes an LRP5/6 chaperone

essential for specification of mouse embryonic polarity. Cell 2003, 112: 355–367

 37 Lu W, Yamamoto V, Ortega B,

Baltimore D. Mammalian Ryk is a Wnt coreceptor required for stimulation of

neurite outgrowth. Cell 2004, 119: 97–108

 38 Yoshikawa S, McKinnon RD, Kokel

M, Thomas JB. Wnt-mediated axon guidance via the Drosophila Derailed

receptor. Nature 2003, 422: 583–588

 39 Hikasa H, Shibata M, Hiratani

I, Taira M. The Xenopus receptor tyrosine kinase Xror2 modulates

morphogenetic movements of the axial mesoderm and neuroectoderm via Wnt

signaling. Development 2002, 129: 5227–5239

 40 Mikels AJ, Nusse R. Wnts as

ligands: processing, secretion and reception. Oncogene 2006, 25: 7461–7468

 41 Schambony A, Wedlich D.

Wnt-5A/Ror2 regulate expression of XPAPC through an alternative noncanonical

signaling pathway. Dev Cell 2007, 12: 779–792

 42 Xu Q, Wang Y, Dabdoub A,

Smallwood PM, Williams J, Woods C, Kelley MW et al. Vascular development

in the retina and inner ear: control by Norrin and Frizzled-4, a high-affinity

ligand-receptor pair. Cell 2004, 116: 883–895

 43 Nam JS, Turcotte TJ, Smith PF,

Choi S, Yoon JK. Mouse cristin/R-spondin family proteins are novel ligands for

the Frizzled 8 and LRP6 receptors and activate b-catenin-dependent

gene expression. J Biol Chem 2006, 281: 13247–13257

 44 Kazanskaya O, Glinka A, del

Barco Barrantes I, Stannek P, Niehrs C, Wu W. R-Spondin2 is a secreted

activator of Wnt/b-catenin signaling and is required for Xenopus

myogenesis. Dev Cell 2004, 7: 525–534

 45 Wei Q, Yokota C, Semenov MV,

Doble B, Woodgett J, He X. R-spondin1 is a high affinity ligand for LRP6 and

induces LRP6 phosphorylation and b-catenin signaling. J Biol

Chem 2007, 282: 15903–15911

 46 Kim KA, Kakitani M, Zhao J,

Oshima T, Tang T, Binnerts M, Liu Y et al. Mitogenic influence of human

R-spondin1 on the intestinal epithelium. Science 2005, 309: 1256–1259

 47 Bell SM, Schreiner CM, Wert SE,

Mucenski ML, Scott WJ, Whitsett JA. R-spondin 2 is required for normal

laryngeal-tracheal, lung and limb morphogenesis. Development 2008, 135: 1049–1058

 48 Tomaselli S, Megiorni F, De

Bernardo C, Felici A, Marrocco G, Maggiulli G, Grammatico B et al.

Syndromic true hermaphroditism due to an R-spondin1 (RSPO1) homozygous

mutation. Hum Mutat 2008, 29: 220–226

 49 Blaydon DC, Ishii Y, O’Toole

EA, Unsworth HC, Teh MT, Ruschendorf F, Sinclair C et al. The gene

encoding R-spondin 4 (RSPO4), a secreted protein implicated in Wnt signaling,

is mutated in inherited anonychia. Nat Genet 2006, 38: 1245–1247

 50 Hoang B, Moos M Jr, Vukicevic

S, Luyten FP. Primary structure and tissue distribution of FRZB, a novel

protein related to Drosophila Frizzled, suggest a role in skeletal

morphogenesis. J Biol Chem 1996, 271: 26131–26137

 51 Rattner A, Hsieh JC, Smallwood

PM, Gilbert DJ, Copeland NG, Jenkins NA, Nathans J. A family of secreted

proteins contains homology to the cysteine-rich ligand-binding domain of

frizzled receptors. Proc Natl Acad Sci USA 1997, 94: 2859–2863

 52 Hsieh JC, Kodjabachian L,

Rebbert ML, Rattner A, Smallwood PM, Samos CH, Nusse R et al. A new

secreted protein that binds to Wnt proteins and inhibits their activities.

Nature 1999, 398: 431–436

 53 Itasaki N, Jones CM, Mercurio S,

Rowe A, Domingos PM, Smith JC, Krumlauf R. Wise, a context-dependent activator

and inhibitor of Wnt signaling. Development 2003, 130: 4295–4305

 54 Li X, Zhang Y, Kang H, Liu W,

Liu P, Zhang J, Harris SE et al. Sclerostin binds to LRP5/6 and

antagonizes canonical Wnt signaling. J Biol Chem 2005, 280: 19883–19887

 55 Semenov M, Tamai K, He X. SOST

is a ligand for LRP5/LRP6 and a Wnt signaling inhibitor. J Biol Chem 2005, 280:

26770–26775

 56 Glinka A, Wu W, Delius H,

Monaghan AP, Blumenstock C, Niehrs C. Dickkopf-1 is a member of a new family of

secreted proteins and functions in head induction. Nature 1998, 391: 357–362

 57 Mao B, Niehrs C. Kremen2

modulates Dickkopf2 activity during Wnt/LRP6 signaling. Gene 2003, 302: 179–183

 58 Rothbacher U, Laurent MN,

Deardorff MA, Klein PS, Cho KW, Fraser SE. Dishevelled phosphorylation,

subcellular localization and multimerization regulate its role in early

embryogenesis. EMBO J 2000, 19: 1010–1022

 59 Axelrod JD, Miller JR, Shulman

JM, Moon RT, Perrimon N. Differential recruitment of Dishevelled provides

signaling specificity in the planar cell polarity and Wingless signaling

pathways. Genes Dev 1998, 12: 2610–2622

 60 Sokol SY, Klingensmith J,

Perrimon N, Itoh K. Dorsalizing and neuralizing properties of Xdsh, a

maternally expressed Xenopus homolog of Dishevelled. Development 1995,

121: 1637–1647

 61 Klingensmith J, Nusse R,

Perrimon N. The Drosophila segment polarity gene dishevelled encodes a

novel protein required for response to the Wingless signal. Genes Dev 1994, 8:

118–130

 62 Sussman DJ, Klingensmith J,

Salinas P, Adams PS, Nusse R, Perrimon N. Isolation and characterization of a

mouse homolog of the Drosophila segment polarity gene Dishevelled. Dev

Biol 1994, 166: 73–86

 63 Noordermeer J, Klingensmith J,

Perrimon N, Nusse R. Dishevelled and armadillo act in the wingless signaling

pathway in Drosophila. Nature 1994, 367: 80–83

 64 Yanagawa S, van Leeuwen F,

Wodarz A, Klingensmith J, Nusse R. The Dishevelled protein is modified by

Wingless signaling in Drosophila. Genes Dev 1995, 9: 1087–1097

 65 Peters JM, McKay RM, McKay JP,

Graff JM. Casein kinase I transduces Wnt signals. Nature 1999, 401: 345–350

 66 Willert K, Brink M, Wodarz A,

Varmus H, Nusse R. Casein kinase 2 associates with and phosphorylates

dishevelled. EMBO J 1997, 16: 3089–3096

 67 Sun TQ, Lu B, Feng JJ, Reinhard

C, Jan YN, Fantl WJ, Williams LT. PAR-1 is a Dishevelled-associated kinase and

a positive regulator of Wnt signaling. Nat Cell Biol 2001, 3: 628–636

 68 Tamai K, Zeng X, Liu C, Zhang

X, Harada Y, Chang Z, He X. A mechanism for Wnt coreceptor activation. Mol Cell

2004, 13: 149–156

 69 Zeng X, Tamai K, Doble B, Li S,

Huang H, Habas R, Okamura H et al. A dual-kinase mechanism for Wnt

co-receptor phosphorylation and activation. Nature 2005, 438: 873–877

 70 Davidson G, Wu W, Shen J, Bilic

J, Fenger U, Stannek P, Glinka A et al. Casein kinase 1g

couples

Wnt receptor activation to cytoplasmic signal transduction. Nature 2005, 438:

867–872

 71 Baig-Lewis S, Peterson-Nedry W,

Wehrli M. Wingless/Wnt signal transduction requires distinct initiation and

amplification steps that both depend on Arrow/LRP. Dev Biol 2007, 306: 94–111

 72 Macdonald BT, Yokota C, Tamai

K, Zeng X, He X. Wnt signal amplification: activity, cooperativity and

regulation of multiple intracellular PPPSP motifs in the Wnt coreceptor LRP6. J

Biol Chem 2008, 283: 16115–16123

 73 Zeng X, Huang H, Tamai K, Zhang

X, Harada Y, Yokota C, Almeida K et al. Initiation of Wnt signaling:

control of Wnt coreceptor Lrp6 phosphorylation/activation via frizzled,

dishevelled and axin functions. Development 2008, 135: 367–375

 74 Bilic J, Huang YL, Davidson G,

Zimmermann T, Cruciat CM, Bienz M, Niehrs C. Wnt induces LRP6 signalosomes and

promotes dishevelled-dependent LRP6 phosphorylation. Science 2007, 316: 1619–1622

 75 Ozawa M, Baribault H, Kemler R.

The cytoplasmic domain of the cell adhesion molecule uvomorulin associates with

three independent proteins structurally related in different species. EMBO J

1989, 8: 1711–1717

 76 McCrea PD, Turck CW, Gumbiner

B. A homolog of the armadillo protein in Drosophila (plakoglobin)

associated with E-cadherin. Science 1991, 254: 1359–1361

 77 Peifer M, Wieschaus E. The

segment polarity gene armadillo encodes a functionally modular protein that is

the Drosophila homolog of human plakoglobin. Cell 1990, 63: 1167–1176

 78 Funayama N, Fagotto F, McCrea

P, Gumbiner BM. Embryonic axis induction by the armadillo repeat domain of b-catenin: evidence

for intracellular signaling. J Cell Biol 1995, 128: 959–968

 79 Siegfried E, Wilder EL,

Perrimon N. Components of Wingless signaling in Drosophila. Nature 1994,

367: 76–80

 80 Zeng L, Fagotto F, Zhang T, Hsu

W, Vasicek TJ, Perry WL 3rd, Lee JJ et al. The mouse Fused locus encodes

Axin, an inhibitor of the Wnt signaling pathway that regulates embryonic axis

formation. Cell 1997, 90: 181–192

 81 Forde JE, Dale TC. Glycogen

synthase kinase 3: a key regulator of cellular fate. Cell Mol Life Sci 2007,

64: 1930–1944

 82 Siegfried E, Chou TB, Perrimon N.

Wingless signaling acts through zeste-white 3, the Drosophila homolog of

glycogen synthase kinase-3, to regulate engrailed and establish cell fate. Cell

1992, 71: 1167–1179

 83 He X, Saint-Jeannet JP,

Woodgett JR, Varmus HE, Dawid IB. Glycogen synthase kinase-3 and dorsoventral

patterning in Xenopus embryos. Nature 1995, 374: 617–622

 84 Polakis P. The adenomatous

polyposis coli (APC) tumor suppressor. Biochim Biophys Acta 1997, 1332: F127–F147

 85 Kinzler KW, Vogelstein B.

Lessons from hereditary colorectal cancer. Cell 1996, 87: 159–170

 86 Su LK, Vogelstein B, Kinzler

KW. Association of the APC tumor suppressor protein with catenins. Science

1993, 262: 1734–1737

 87 Rubinfeld B, Souza B, Albert I,

Muller O, Chamberlain SH, Masiarz FR, Munemitsu S et al. Association of

the APC gene product with b-catenin. Science 1993, 262:

1731–1734

 88 Munemitsu S, Albert I, Souza B,

Rubinfeld B, Polakis P. Regulation of intracellular b-catenin levels by the

adenomatous polyposis coli (APC) tumor-suppressor protein. Proc Natl Acad Sci

USA 1995, 92: 3046–3050

 89 Rubinfeld B, Albert I, Porfiri

E, Fiol C, Munemitsu S, Polakis P. Binding of GSK3b to the APC-b-catenin complex

and regulation of complex assembly. Science 1996, 272: 1023–1026

 90 Korinek V, Barker N, Morin PJ,

van Wichen D, de Weger R, Kinzler KW, Vogelstein B et al. Constitutive

transcriptional activation by a b-catenin-TCF complex in APC–/–

colon carcinoma. Science 1997, 275: 1784–1787

 91 Hart MJ, de los Santos R,

Albert IN, Rubinfeld B, Polakis P. Downregulation of b-catenin by human

Axin and its association with the APC tumor suppressor, b-catenin and GSK3b. Curr Biol 1998,

8: 573–581

 92 Ikeda S, Kishida S, Yamamoto H,

Murai H, Koyama S, Kikuchi A. Axin, a negative regulator of the Wnt signaling

pathway, forms a complex with GSK-3b and b-catenin and

promotes GSK-3b-dependent phosphorylation of b-catenin. EMBO J

1998, 17: 1371–1384

 93 Behrens J, Jerchow BA, Wurtele

M, Grimm J, Asbrand C, Wirtz R, Kuhl M et al. Functional interaction of

an axin homolog, conductin, with b-catenin, APC, and GSK3b. Science 1998,

280: 596–599

 94 Kishida S, Yamamoto H, Ikeda S,

Kishida M, Sakamoto I, Koyama S, Kikuchi A. Axin, a negative regulator of the

Wnt signaling pathway, directly interacts with adenomatous polyposis coli and

regulates the stabilization of b-catenin. J Biol Chem 1998,

273: 10823–10826

 95 Liu C, Li Y, Semenov M, Han C,

Baeg GH, Tan Y, Zhang Z et al. Control of b-catenin phosphorylation/degradation

by a dual-kinase mechanism. Cell 2002, 108: 837–847

 96 Amit S, Hatzubai A, Birman Y,

Andersen JS, Ben-Shushan E, Mann M, Ben-Neriah Y et al. Axin-mediated

CKI phosphorylation of b-catenin at Ser45: a molecular

switch for the Wnt pathway. Genes Dev 2002, 16: 1066–1076

 97 Yost C, Torres M, Miller JR,

Huang E, Kimelman D, Moon RT. The axis-inducing activity, stability, and

subcellular distribution of b-catenin is regulated in Xenopus

embryos by glycogen synthase kinase 3. Genes Dev 1996, 10: 1443–1454

 98 Hart M, Concordet JP, Lassot I,

Albert I, del los Santos R, Durand H, Perret C et al. The F-box protein b-TrCP associates

with phosphorylated b-catenin and regulates its activity in the cell. Curr

Biol 1999, 9: 207–210

 99 Jiang J, Struhl G. Regulation

of the Hedgehog and Wingless signaling pathways by the F-box/WD40-repeat

protein Slimb. Nature 1998, 391: 493–496

100 Liu C, Kato Y, Zhang Z, Do VM,

Yankner BA, He X. b-Trcp couples b-catenin phosphorylation-degradation

and regulates Xenopus axis formation. Proc Natl Acad Sci USA 1999, 96:

6273–6278

101 Spencer E, Jiang J, Chen ZJ.

Signal-induced ubiquitination of IkBa by the F-box

protein Slimb/b-TrCP. Genes Dev 1999, 13: 284–294

102 Winston JT, Strack P,

Beer-Romero P, Chu CY, Elledge SJ, Harper JW. The SCFb-TRCP-ubiquitin

ligase complex associates specifically with phosphorylated destruction motifs

in IkBa and b-catenin and stimulates IkBa ubiquitination in

vitro. Genes Dev 1999, 13: 270–283

103 Eklof Spink K, Fridman SG,

Weis WI. Molecular mechanisms of b-catenin recognition by

adenomatous polyposis coli revealed by the structure of an APC-b-catenin complex.

EMBO J 2001, 20: 6203–6212

104 Dajani R, Fraser E, Roe SM,

Yeo M, Good VM, Thompson V, Dale TC et al. Structural basis for

recruitment of glycogen synthase kinase 3b to the axin-APC

scaffold complex. EMBO J 2003, 22: 494–501

105 Ha NC, Tonozuka T, Stamos JL,

Choi HJ, Weis WI. Mechanism of phosphorylation-dependent binding of APC to b-catenin and its

role in b-catenin degradation. Mol Cell 2004, 15: 511–521

106 Spink KE, Polakis P, Weis WI.

Structural basis of the Axin-adenomatous polyposis coli interaction. EMBO J

2000, 19: 2270–2279

107 Xing Y, Clements WK, Kimelman

D, Xu W. Crystal structure of a b-catenin/axin complex suggests

a mechanism for the b-catenin destruction complex. Genes Dev 2003, 17:

2753–2764

108 Xing Y, Clements WK, Le Trong

I, Hinds TR, Stenkamp R, Kimelman D, Xu W. Crystal structure of a b-catenin/APC

complex reveals a critical role for APC phosphorylation in APC function. Mol

Cell 2004, 15: 523–533

109 Xing Y, Takemaru K, Liu J,

Berndt JD, Zheng JJ, Moon RT, Xu W. Crystal structure of a full-length b-catenin. Structure

2008, 16: 478–487

110  Nakamura Y, Umehara T, Hamana

H, Hayashizaki Y, Inoue M, Kigawa T, Shirouzu M et al. Crystal structure

analysis of the PHD domain of the transcription co-activator Pygopus. J Mol

Biol 2007, 370: 80–92

111  Sampietro J, Dahlberg CL, Cho US,

Hinds TR, Kimelman D, Xu W. Crystal structure of a b-catenin/BCL9/TCF4

complex. Mol Cell 2006, 24: 293–300

112 Wu G, Xu G, Schulman BA,

Jeffrey PD, Harper JW, Pavletich NP. Structure of a b-TrCP1-Skp1-b-catenin complex:

destruction motif binding and lysine specificity of the SCF (b-TrCP1) ubiquitin

ligase. Mol Cell 2003, 11: 1445–1456

113 Huber AH, Nelson WJ, Weis WI.

Three-dimensional structure of the armadillo repeat region of b-catenin. Cell

1997, 90: 871–882

114 Liu J, Xing Y, Hinds TR, Zheng

J, Xu W. The third 20 amino acid repeat is the tightest binding site of APC for

b-catenin. J Mol Biol 2006, 360: 133–144

115 Yang J, Zhang W, Evans PM,

Chen X, He X, Liu C. Adenomatous polyposis coli (APC) differentially regulates b-catenin

phosphorylation and ubiquitination in colon cancer cells. J Biol Chem 2006,

281: 17751–17757

116 Lee E, Salic A, Kruger R,

Heinrich R, Kirschner MW. The roles of APC and Axin derived from experimental

and theoretical analysis of the Wnt pathway. PLoS Biol 2003, 1: e10

117  Willert K, Shibamoto S, Nusse

R. Wnt-induced dephosphorylation of axin releases b-catenin from the

axin complex. Genes Dev 1999, 13: 1768–1773

118 Yamamoto H, Kishida S, Kishida

M, Ikeda S, Takada S, Kikuchi A. Phosphorylation of axin, a Wnt signal negative

regulator, by glycogen synthase kinase-3b regulates its

stability. J Biol Chem 1999, 274: 10681–10684

119 Hocevar BA, Mou F, Rennolds

JL, Morris SM, Cooper JA, Howe PH. Regulation of the Wnt signaling pathway by

disabled-2 (Dab2). EMBO J 2003, 22: 3084–3094

120 Hsu W, Zeng L, Costantini F.

Identification of a domain of Axin that binds to the serine/threonine protein

phosphatase 2A and a self-binding domain. J Biol Chem 1999, 274: 3439–3445

121 Yang J, Wu J, Tan C, Klein PS.

PP2A:B56epsilon is required for Wnt/b-catenin signaling during

embryonic development. Development 2003, 130: 5569–5578

122 Seeling JM, Miller JR, Gil R,

Moon RT, White R, Virshup DM. Regulation of b-catenin signaling

by the B56 subunit of protein phosphatase 2A. Science 1999, 283: 2089–2091

123 Gao ZH, Seeling JM, Hill V,

Yochum A, Virshup DM. Casein kinase I phosphorylates and destabilizes the b-catenin

degradation complex. Proc Natl Acad Sci USA 2002, 99: 1182–1187

124 Bos CL, Diks SH, Hardwick JC, Walburg

KV, Peppelenbosch MP, Richel DJ. Protein phosphatase 2A is required for

mesalazine-dependent inhibition of Wnt/b-catenin pathway

activity. Carcinogenesis 2006, 27: 2371–2382

125 Ratcliffe MJ, Itoh K, Sokol

SY. A positive role for the PP2A catalytic subunit in Wnt signal transduction.

J Biol Chem 2000, 275: 35680–35683

126 Bos CL, Kodach LL, van den

Brink GR, Diks SH, van Santen MM, Richel DJ, Peppelenbosch MP et al.

Effect of aspirin on the Wnt/b-catenin pathway is mediated

via protein phosphatase 2A. Oncogene 2006, 25: 6447–6456

127 Li X, Yost HJ, Virshup DM,

Seeling JM. Protein phosphatase 2A and its B56 regulatory subunit inhibit Wnt

signaling in Xenopus. EMBO J 2001, 20: 4122–4131

128 Bajpai R, Makhijani K, Rao PR,

Shashidhara LS. Drosophila Twins regulates Armadillo levels in response

to Wg/Wnt signal. Development 2004, 131: 1007–1016

129 Luo W, Peterson A, Garcia BA,

Coombs G, Kofahl B, Heinrich R, Shabanowitz J et al. Protein phosphatase

1 regulates assembly and function of the b-catenin degradation

complex. EMBO J 2007, 26: 1511–1521

130 Major MB, Camp ND, Berndt JD,

Yi X, Goldenberg SJ, Hubbert C, Biechele TL et al. Wilms tumor

suppressor WTX negatively regulates WNT/b-catenin signaling.

Science 2007, 316: 1043–1046

131 Rivera MN, Kim WJ, Wells J,

Driscoll DR, Brannigan BW, Han M, Kim JC et al. An X chromosome gene, WTX,

is commonly inactivated in Wilms tumor. Science 2007, 315: 642–645

132 Fagotto F, Gluck U, Gumbiner

BM. Nuclear localization signal-independent and importin/karyopherin-independent

nuclear import of b-catenin. Curr Biol 1998, 8: 181–190

133 Yokoya F, Imamoto N, Tachibana

T, Yoneda Y. b-catenin can be transported into the nucleus in a

Ran-unassisted manner. Mol Biol Cell 1999, 10: 1119–1131

134 Wu X, Tu X, Joeng KS, Hilton

MJ, Williams DA, Long X. Rac1 activation controls nuclear localization of

?-catenin during canonical Wnt signaling. Cell 2008, 133: 340–353

135 Cong F, Varmus H.

Nuclear-cytoplasmic shuttling of Axin regulates subcellular localization of b-catenin. Proc Natl

Acad Sci USA 2004, 101: 2882–2887

136 Rosin-Arbesfeld R, Townsley F,

Bienz M. The APC tumor suppressor has a nuclear export function. Nature 2000,

406: 1009–1012

137 Henderson BR.

Nuclear-cytoplasmic shuttling of APC regulates b-catenin

subcellular localization and turnover. Nat Cell Biol 2000, 2: 653–660

138 Neufeld KL, Zhang F, Cullen

BR, White RL. APC-mediated downregulation of b-catenin activity

involves nuclear sequestration and nuclear export. EMBO Rep 2000, 1: 519–523

139 Krieghoff E, Behrens J, Mayr

B. Nucleo-cytoplasmic distribution of b-catenin is regulated by

retention. J Cell Sci 2006, 119: 1453–1463

140 Townsley FM, Cliffe A, Bienz

M. Pygopus and Legless target Armadillo/b-catenin to the

nucleus to enable its transcriptional co-activator function. Nat Cell Biol

2004, 6: 626–633

141 Tolwinski NS, Wieschaus E.

Armadillo nuclear import is regulated by cytoplasmic anchor Axin and nuclear

anchor dTCF/Pan. Development 2001, 128: 2107–2117

142 van de Wetering M, Cavallo R,

Dooijes D, van Beest M, van Es J, Loureiro J, Ypma A et al. Armadillo

coactivates transcription driven by the product of the Drosophila

segment polarity gene dTCF. Cell 1997, 88: 789–799

143 Brunner E, Peter O, Schweizer

L, Basler K. Pangolin encodes a LEF-1 homologue that acts downstream of

Armadillo to transduce the Wingless signal in Drosophila. Nature 1997,

385: 829–833

144 Brannon M, Brown JD, Bates R,

Kimelman D, Moon RT. XCtBP is a XTCF-3 co-repressor with roles throughout Xenopus

development. Development 1999, 126: 3159–3170

145 Billin AN, Thirlwell H, Ayer

DE. b-catenin-histone deacetylase interactions regulate the transition of

LEF1 from a transcriptional repressor to an activator. Mol Cell Biol 2000, 20:

6882–6890

146 Kioussi C, Briata P, Baek SH,

Rose DW, Hamblet NS, Herman T, Ohgi KA et al. Identification of a

Wnt/Dvl/b-catenin®Pitx2 pathway mediating cell-type-specific

proliferation during development. Cell 2002, 111: 673–685

147 Cavallo RA, Cox RT, Moline MM,

Roose J, Polevoy GA, Clevers H, Peifer M et al. Drosophila TCF

and Groucho interact to repress Wingless signaling activity. Nature 1998, 395:

604–608

148 Roose J, Molenaar M, Peterson

J, Hurenkamp J, Brantjes H, Moerer P, van de Wetering M et al. The Xenopus

Wnt effector XTCF-3 interacts with Groucho-related transcriptional repressors.

Nature 1998, 395: 608–612

149 Levanon D, Goldstein RE,

Bernstein Y, Tang H, Goldenberg D, Stifani S, Paroush Z et al.

Transcriptional repression by AML1 and LEF-1 is mediated by the TLE/Groucho

corepressors. Proc Natl Acad Sci USA 1998, 95: 11590–11595

150 Palaparti A, Baratz A, Stifani

S. The Groucho/transducin-like enhancer of split transcriptional repressors

interact with the genetically defined amino-terminal silencing domain of

histone H3. J Biol Chem 1997, 272: 26604–26610

151 Daniels DL, Weis WI. b-catenin directly

displaces Groucho/TLE repressors from Tcf/Lef in Wnt-mediated transcription

activation. Nat Struct Mol Biol 2005, 12: 364–371

152 Hsu SC, Galceran J, Grosschedl

R. Modulation of transcriptional regulation by LEF-1 in response to Wnt-1

signaling and association with b-catenin. Mol Cell Biol 1998,

18: 4807–4818

153 Belenkaya TY, Han C, Standley

HJ, Lin X, Houston DW, Heasman J, Lin X. Pygopus encodes a nuclear protein

essential for Wingless/Wnt signaling. Development 2002, 129: 4089–4101

154 Kramps T, Peter O, Brunner E,

Nellen D, Froesch B, Chatterjee S, Murone M et al. Wnt/Wingless

signaling requires BCL9/Legless-mediated recruitment of pygopus to the nuclear b-catenin-TCF

complex. Cell 2002, 109: 47–60

155 Parker DS, Jemison J, Cadigan

KM. Pygopus, a nuclear PHD-finger protein required for Wingless signaling in Drosophila.

Development 2002, 129: 2565–2576

156 Thompson B, Townsley F,

Rosin-Arbesfeld R, Musisi H, Bienz M. A new nuclear component of the Wnt

signaling pathway. Nat Cell Biol 2002, 4: 367–373

157 Soliman MA, Riabowol K. After

a decade of study-ING, a PHD for a versatile family of proteins. Trends Biochem

Sci 2007, 32: 509–519

158 Hecht A, Vleminckx K, Stemmler

MP, van Roy F, Kemler R. The p300/CBP acetyltransferases function as

transcriptional coactivators of b-catenin in vertebrates. EMBO

J 2000, 19: 1839–1850

159 Cox RT, Pai LM, Kirkpatrick C,

Stein J, Peifer M. Roles of the C terminus of Armadillo in Wingless signaling

in Drosophila. Genetics 1999, 153: 319–332

160 Takemaru KI, Moon RT. The

transcriptional coactivator CBP interacts with b-catenin to

activate gene expression. J Cell Biol 2000, 149: 249–254

161 Ogryzko VV, Schiltz RL, Russanova

V, Howard BH, Nakatani Y. The transcriptional coactivators p300 and CBP are

histone acetyltransferases. Cell 1996, 87: 953–959

162 Goldman PS, Tran VK, Goodman

RH. The multifunctional role of the co-activator CBP in transcriptional

regulation. Recent Prog Horm Res 1997, 52: 103–119

163 Mosimann C, Hausmann G, Basler

K. Parafibromin/Hyrax activates Wnt/Wg target gene transcription by direct

association with b-catenin/Armadillo. Cell 2006, 125: 327–341

164 Wei Y, Renard CA, Labalette C,

Wu Y, Levy L, Neuveut C, Prieur X et al. Identification of the LIM

protein FHL2 as a coactivator of b-catenin. J Biol Chem 2003,

278: 5188–5194

165 Hecht A, Litterst CM, Huber O,

Kemler R. Functional characterization of multiple transactivating elements in b-catenin, some of

which interact with the TATA-binding protein in vitro. J Biol Chem 1999,

274: 18017–18025

166 Barker N, Hurlstone A, Musisi

H, Miles A, Bienz M, Clevers H. The chromatin remodelling factor Brg-1

interacts with b-catenin to promote target gene activation. EMBO J

2001, 20: 4935–4943

167 Bauer A, Huber O, Kemler R.

Pontin52, an interaction partner of b-catenin, binds to the TATA

box binding protein. Proc Natl Acad Sci USA 1998, 95: 14787–14792

168 Bauer A, Chauvet S, Huber O,

Usseglio F, Rothbacher U, Aragnol D, Kemler R et al. Pontin52 and

reptin52 function as antagonistic regulators of b-catenin signaling

activity. EMBO J 2000, 19: 6121–6130

169 Crosnier C, Stamataki D, Lewis

J. Organizing cell renewal in the intestine: stem cells, signals and

combinatorial control. Nat Rev Genet 2006, 7: 349–359

170 Marshman E, Booth C, Potten

CS. The intestinal epithelial stem cell. Bioessays 2002, 24: 91–98

171 Korinek V, Barker N, Moerer P,

van Donselaar E, Huls G, Peters PJ, Clevers H. Depletion of epithelial

stem-cell compartments in the small intestine of mice lacking TCF-4. Nat Genet

1998, 19: 379–383

172 Pinto D, Gregorieff A, Begthel

H, Clevers H. Canonical Wnt signals are essential for homeostasis of the

intestinal epithelium. Genes Dev 2003, 17: 1709–1713

173 Kuhnert F, Davis CR, Wang HT,

Chu P, Lee M, Yuan J, Nusse R et al. Essential requirement for Wnt

signaling in proliferation of adult small intestine and colon revealed by

adenoviral expression of Dickkopf-1. Proc Natl Acad Sci USA 2004, 101: 266–271

174 Sansom OJ, Reed KR, Hayes AJ,

Ireland H, Brinkmann H, Newton IP, Batlle E et al. Loss of APC in

vivo immediately perturbs Wnt signaling, differentiation, and migration.

Genes Dev 2004, 18: 1385–1390

175 Batlle E, Henderson JT,

Beghtel H, van den Born MM, Sancho E, Huls G, Meeldijk J et al. b-catenin and TCF

mediate cell positioning in the intestinal epithelium by controlling the

expression of EphB/ephrinB. Cell 2002, 111: 251–263

176 Gregorieff A, Pinto D, Begthel

H, Destree O, Kielman M, Clevers H. Expression pattern of Wnt signaling

components in the adult intestine. Gastroenterology 2005, 129: 626–638

177 He TC, Sparks AB, Rago C,

Hermeking H, Zawel L, da Costa LT, Morin PJ et al. Identification of

c-Myc as a target of the APC pathway. Science 1998, 281: 1509–1512

178 Sansom OJ, Meniel VS, Muncan

V, Phesse TJ, Wilkins JA, Reed KR, Vass JK et al. Myc deletion rescues

APC deficiency in the small intestine. Nature 2007, 446: 676–679

179 Bjerknes M, Cheng H. The

stem-cell zone of the small intestinal epithelium. III. Evidence from columnar,

enteroendocrine, and mucous cells in the adult mouse. Am J Anat 1981, 160: 77–91

180  van de Wetering M, Sancho E,

Verweij C, de Lau W, Oving I, Hurlstone A, van der Horn K et al. The b-catenin/TCF-4

complex imposes a crypt progenitor phenotype on colorectal cancer cells. Cell

2002, 111: 241–250

181 Barker N, van Es JH, Kuipers

J, Kujala P, van den Born M, Cozijnsen M, Haegebarth A et al.

Identification of stem cells in small intestine and colon by marker gene Lgr5.

Nature 2007, 449: 1003–1007

182 O’Brien CA, Pollett A,

Gallinger S, Dick JE. A human colon cancer cell capable of initiating tumor

growth in immunodeficient mice. Nature 2007, 445: 106–110

183 Ricci-Vitiani L, Lombardi DG,

Pilozzi E, Biffoni M, Todaro M, Peschle C, De Maria R. Identification and

expansion of human colon-cancer-initiating cells. Nature 2007, 445: 111–115

184 Orkin SH, Zon LI.

Hematopoiesis: an evolving paradigm for stem cell biology. Cell 2008, 132: 631–644

185 Kondo M, Wagers AJ, Manz MG,

Prohaska SS, Scherer DC, Beilhack GF, Shizuru JA et al. Biology of

hematopoietic stem cells and progenitors: implications for clinical

application. Annu Rev Immunol 2003, 21: 759–806

186 van de Wetering M, Oosterwegel

M, Dooijes D, Clevers H. Identification and cloning of TCF-1, a T

lymphocyte-specific transcription factor containing a sequence-specific HMG

box. EMBO J 1991, 10: 123–132

187 Travis A, Amsterdam A,

Belanger C, Grosschedl R. LEF-1, a gene encoding a lymphoid-specific

protein with an HMG domain, regulates T-cell receptor alpha enhancer function.

Genes Dev 1991, 5: 880–894

188 Okamura RM, Sigvardsson M,

Galceran J, Verbeek S, Clevers H, Grosschedl R. Redundant regulation of T cell

differentiation and TCRa gene expression by the

transcription factors LEF-1 and TCF-1. Immunity 1998, 8: 11–20

189 Reya T, Duncan AW, Ailles L,

Domen J, Scherer DC, Willert K, Hintz L et al. A role for Wnt signaling

in self-renewal of haematopoietic stem cells. Nature 2003, 423: 409–414

190 Cobas M, Wilson A, Ernst B,

Mancini SJ, MacDonald HR, Kemler R, Radtke F. b-catenin is

dispensable for hematopoiesis and lymphopoiesis. J Exp Med 2004, 199: 221–229

191 Koch U, Wilson A, Cobas M,

Kemler R, Macdonald HR, Radtke F. Simultaneous loss of b– and g-catenin does not

perturb hematopoiesis or lymphopoiesis. Blood 2008, 111: 160–164

192 Jeannet G, Scheller M,

Scarpellino L, Duboux S, Gardiol N, Back J, Kuttler F et al. Long-term,

multilineage hematopoiesis occurs in the combined absence of b-catenin and g-catenin. Blood

2008, 111: 142–149

193 Trowbridge JJ, Xenocostas A,

Moon RT, Bhatia M. Glycogen synthase kinase-3 is an in vivo regulator of

hematopoietic stem cell repopulation. Nat Med 2006, 12: 89–98

194 Kirstetter P, Anderson K,

Porse BT, Jacobsen SE, Nerlov C. Activation of the canonical Wnt pathway leads

to loss of hematopoietic stem cell repopulation and multilineage

differentiation block. Nat Immunol 2006, 7: 1048–1056

195 Scheller M, Huelsken J, Rosenbauer

F, Taketo MM, Birchmeier W, Tenen DG, Leutz A. Hematopoietic stem cell and

multilineage defects generated by constitutive b-catenin

activation. Nat Immunol 2006, 7: 1037–1047

196 Murdoch B, Chadwick K, Martin

M, Shojaei F, Shah KV, Gallacher L, Moon RT et al. Wnt-5a augments

repopulating capacity and primitive hematopoietic development of human blood

stem cells in vivo. Proc Natl Acad Sci USA 2003, 100: 3422–3427

197 Nemeth MJ, Topol L, Anderson

SM, Yang Y, Bodine DM. Wnt5a inhibits canonical Wnt signaling in hematopoietic

stem cells and enhances repopulation. Proc Natl Acad Sci USA 2007, 104: 15436–15441

198 Fleming HE, Janzen V, Lo Celso

C, Guo J, Leahy KM, Kronenberg HM, Scadden DT. Wnt signaling in the niche

enforces hematopoietic stem cell quiescence and is necessary to preserve

self-renewal in vivo. Cell Stem Cell 2008, 2: 274–283

199 Fuchs E. Scratching the

surface of skin development. Nature 2007, 445: 834–842

200 van Genderen C, Okamura RM,

Farinas I, Quo RG, Parslow TG, Bruhn L, Grosschedl R. Development of several

organs that require inductive epithelial-mesenchymal interactions is impaired

in LEF-1-deficient mice. Genes Dev 1994, 8: 2691–2703

201 Huelsken J, Vogel R, Erdmann

B, Cotsarelis G, Birchmeier W. b-catenin controls hair follicle

morphogenesis and stem cell differentiation in the skin. Cell 2001, 105: 533–545

202 Andl T, Reddy ST, Gaddapara T,

Millar SE. WNT signals are required for the initiation of hair follicle

development. Dev Cell 2002, 2: 643–653

203 Gat U, DasGupta R, Degenstein

L, Fuchs E. De novo hair follicle morphogenesis and hair tumors in mice

expressing a truncated b-catenin in skin. Cell 1998,

95: 605–614

204  Lo Celso C, Prowse DM, Watt

FM. Transient activation of b-catenin signaling in adult

mouse epidermis is sufficient to induce new hair follicles but continuous

activation is required to maintain hair follicle tumors. Development 2004, 131:

1787–1799

205 Lowry WE, Blanpain C, Nowak

JA, Guasch G, Lewis L, Fuchs E. Defining the impact of b-catenin/TCF transactivation

on epithelial stem cells. Genes Dev 2005, 19: 1596–1611

206  van Mater D, Kolligs FT,

Dlugosz AA, Fearon ER. Transient activation of b-catenin signaling

in cutaneous keratinocytes is sufficient to trigger the active growth phase of

the hair cycle in mice. Genes Dev 2003, 17: 1219–1224

207 Reddy S, Andl T, Bagasra A, Lu

MM, Epstein DJ, Morrisey EE, Millar SE. Characterization of Wnt gene expression

in developing and postnatal hair follicles and identification of Wnt5a as a

target of Sonic hedgehog in hair follicle morphogenesis. Mech Dev 2001, 107: 69–82

208 DasGupta R, Fuchs E. Multiple

roles for activated LEF/TCF transcription complexes during hair follicle

development and differentiation. Development 1999, 126: 4557–4568

209 Merrill BJ, Gat U, DasGupta R,

Fuchs E. TCF3 and LEF1 regulate lineage differentiation of multipotent stem

cells in skin. Genes Dev 2001, 15: 1688–1705

210 Nguyen H, Rendl M, Fuchs E.

Tcf3 governs stem cell features and represses cell fate determination in skin.

Cell 2006, 127: 171–183

211 Ito M, Yang Z, Andl T, Cui C,

Kim N, Millar SE, Cotsarelis G. Wnt-dependent de novo hair follicle

regeneration in adult mouse skin after wounding. Nature 2007, 447: 316–320

212 Zhao C, Deng W, Gage FH.

Mechanisms and functional implications of adult neurogenesis. Cell 2008, 132:

645–660

213 Lie DC, Colamarino SA, Song

HJ, Desire L, Mira H, Consiglio A, Lein ES et al. Wnt signaling

regulates adult hippocampal neurogenesis. Nature 2005, 437: 1370–1375

214 Niwa H. How is pluripotency

determined and maintained? Development 2007, 134: 635–646

215 Kielman MF, Rindapaa M, Gaspar

C, van Poppel N, Breukel C, van Leeuwen S, Taketo MM et al. APC

modulates embryonic stem-cell differentiation by controlling the dosage of b-catenin signaling.

Nat Genet 2002, 32: 594–605

216 Sato N, Meijer L, Skaltsounis

L, Greengard P, Brivanlou AH. Maintenance of pluripotency in human and mouse

embryonic stem cells through activation of Wnt signaling by a pharmacological

GSK-3-specific inhibitor. Nat Med 2004, 10: 55–63

217  Miyabayashi T, Teo JL,

Yamamoto M, McMillan M, Nguyen C, Kahn M. Wnt/b-catenin/CBP

signaling maintains long-term murine embryonic stem cell pluripotency. Proc

Natl Acad Sci USA 2007, 104: 5668–5673

218 Cole MF, Johnstone SE, Newman

JJ, Kagey MH, Young RA. Tcf3 is an integral component of the core regulatory

circuitry of embryonic stem cells. Genes Dev 2008, 22: 746–755

219 Polakis P. Wnt signaling and

cancer. Genes Dev 2000, 14: 1837–1851

220 Groden J, Thliveris A,

Samowitz W, Carlson M, Gelbert L, Albertsen H, Joslyn G et al.

Identification and characterization of the familial adenomatous polyposis coli

gene. Cell 1991, 66: 589–600

221 Kinzler KW, Nilbert MC, Su LK,

Vogelstein B, Bryan TM, Levy DB, Smith KJ et al. Identification of FAP

locus genes from chromosome 5q21. Science 1991, 253: 661–665

222 Nishisho I, Nakamura Y,

Miyoshi Y, Miki Y, Ando H, Horii A, Koyama K et al. Mutations of

chromosome 5q21 genes in FAP and colorectal cancer patients. Science 1991, 253:

665–669

223 Powell SM, Zilz N,

Beazer-Barclay Y, Bryan TM, Hamilton SR, Thibodeau SN, Vogelstein B et al.

APC mutations occur early during colorectal tumorigenesis. Nature 1992, 359:

235–237

224 Moser AR, Pitot HC, Dove WF. A

dominant mutation that predisposes to multiple intestinal neoplasia in the

mouse. Science 1990, 247: 322–324

225 Su LK, Kinzler KW, Vogelstein

B, Preisinger AC, Moser AR, Luongo C, Gould KA et al. Multiple

intestinal neoplasia caused by a mutation in the murine homolog of the APC

gene. Science 1992, 256: 668–670

226 Morin PJ, Sparks AB, Korinek

V, Barker N, Clevers H, Vogelstein B, Kinzler KW. Activation of b-catenin-TCF

signaling in colon cancer by mutations in b-catenin or APC.

Science 1997, 275: 1787–1790

227 Andreu P, Colnot S, Godard C, Gad

S, Chafey P, Niwa-Kawakita M, Laurent-Puig P et al. Crypt-restricted

proliferation and commitment to the Paneth cell lineage following APC loss in

the mouse intestine. Development 2005, 132: 1443–1451

228 Shibata H, Toyama K, Shioya H,

Ito M, Hirota M, Hasegawa S, Matsumoto H et al. Rapid colorectal adenoma

formation initiated by conditional targeting of the APC gene. Science 1997,

278: 120–123

229 Munemitsu S, Albert I,

Rubinfeld B, Polakis P. Deletion of an amino-terminal sequence b-catenin in vivo

and promotes hyperphosporylation of the adenomatous polyposis coli tumor

suppressor protein. Mol Cell Biol 1996, 16: 4088–4094

230 Liu W, Dong X, Mai M, Seelan

RS, Taniguchi K, Krishnadath KK, Halling KC et al. Mutations in Axin2

cause colorectal cancer with defective mismatch repair by activating b-catenin/TCF

signaling. Nat Genet 2000, 26: 146–147

231 Harada N, Tamai Y, Ishikawa T,

Sauer B, Takaku K, Oshima M, Taketo MM. Intestinal polyposis in mice with a

dominant stable mutation of the b-catenin gene. EMBO J 1999,

18: 5931–5942

232 Wang JG, Wang DF, Lv BJ, Si

JM. A novel mouse model for colitis-associated colon carcinogenesis induced by

1,2-dimethylhydrazine and dextran sulfate sodium. World J Gastroenterol 2004,

10: 2958–2962

233 Zhang W, Chen X, Kato Y, Evans

PM, Yuan S, Yang J, Rychahou PG et al. Novel cross talk of Kruppel-like

factor 4 and b-catenin regulates normal intestinal homeostasis and

tumor repression. Mol Cell Biol 2006, 26: 2055–2064

234  Ghaleb AM, McConnell BB,

Nandan MO, Katz JP, Kaestner KH, Yang VW. Haploinsufficiency of Kruppel-like

factor 4 promotes adenomatous polyposis coli dependent intestinal

tumorigenesis. Cancer Res 2007, 67: 7147–7154

235  Isaacs W, De Marzo A, Nelson

WG. Focus on prostate cancer. Cancer Cell 2002, 2: 113–116

236  Feldman BJ, Feldman D. The

development of androgen-independent prostate cancer. Nature Rev 2001, 1: 34–45

237  Yang F, Li X, Sharma M, Sasaki

CY, Longo DL, Lim B, Sun Z. Linking b-catenin to androgen-signaling

pathway. J Biol Chem 2002, 277: 11336–11344

238 Mulholland DJ, Cheng H, Reid K,

Rennie PS, Nelson CC. The androgen receptor can promote b-catenin nuclear

translocation independently of adenomatous polyposis coli. J Biol Chem 2002,

277: 17933–17943

239  Chesire DR, Ewing CM, Gage WR,

Isaacs WB. In vitro evidence for complex modes of nuclear b-catenin signaling

during prostate growth and tumorigenesis. Oncogene 2002, 21: 2679–2694

240 Sharma M, Chuang WW, Sun Z.

Phosphatidylinositol 3-kinase/Akt stimulates androgen pathway through GSK3b inhibition and

nuclear b-catenin accumulation. J Biol Chem 2002, 277: 30935–30941

241 Mulholland DJ, Read JT, Rennie

PS, Cox ME, Nelson CC. Functional localization and competition between the

androgen receptor and T-cell factor for nuclear b-catenin: a mean

for inhibition of the TCF signaling axis. Oncogene 2003, 22: 5602–5613

242 Song LN, Herrell R, Byers S,

Shah S, Wilson EM, Gelmann EP. b-catenin binds to the

activation function 2 region of the androgen receptor and modulates the effects

of the N-terminal domain and TIF2 on ligand-dependent transcription. Mol Cell

Biol 2003, 23: 1674–1687

243 Masiello D, Chen SY, Xu Y,

Verhoeven MC, Choi E, Hollenberg AN, Balk SP. Recruitment of b-catenin by

wild-type or mutant androgen receptors correlates with ligand-stimulated growth

of prostate cancer cells. Mol Endocrinol 2004, 18: 2388–2401

244 Verras M, Brown J, Li X, Nusse

R, Sun Z. Wnt3a growth factor induces androgen receptor-mediated transcription

and enhances cell growth in human prostate cancer cells. Cancer Res 2004, 64:

8860–8866

245  Gerstein AV, Almeida TA, Zhao

G, Chess E, Shih IeM, Buhler K, Pienta K et al. APC/CTNNB1 (b-catenin) pathway

alterations in human prostate cancers. Genes Chromosomes Cancer 2002, 34: 9–16

246  Watanabe M, Kakiuchi H, Kato

H, Shiraishi T, Yatani R, Sugimura T, Nagao M. APC gene mutations in human

prostate cancer.  J Clin Oncol 1996, 26:

77–81

247  Voeller HJ, Truica CI, Gelmann

EP. b-catenin mutations in human prostate cancer. Cancer Res 1998, 58: 2520–2523

248  Gounari F, Signoretti S,

Bronson R, Klein L, Sellers WR, Kum J, Siermann A et al. Stabilization

of b-catenin induces lesions reminiscent of prostatic intraepithelial

neoplasia, but terminal squamous transdifferentiation of other secretory

epithelia. Oncogene 2002, 21: 4099–4107

249  Bierie B, Nozawa M, Renou JP,

Shillingford JM, Morgan F, Oka T, Taketo MM et al. Activation of b-catenin in

prostate epithelium induces hyperplasias and squamous transdifferentiation.

Oncogene 2003, 22: 3875–3887

250 Persad S, Troussard AA, McPhee

TR, Mulholland DJ, Dedhar S. Tumor suppressor PTEN inhibits nuclear

accumulation of b-catenin and T cell/lymphoid enhancer factor

1-mediated transcriptional activation. J Cell Biol 2001, 153: 1161–1174

251 Verras M, Sun Z. b-catenin is

involved in insulin-like growth factor 1-mediated transactivation of the

androgen receptor. Mol Endocrinol 2005, 19: 391–398

252 Suksaweang S, Lin CM, Jiang

TX, Hughes MW, Widelitz RB, Chuong CM. Morphogenesis of chicken liver: identification

of localized growth zones and the role of b-catenin/Wnt in

size regulation. Dev Biol 2004, 266: 109–122

253  Monga SP, Monga HK, Tan X,

Mule K, Pediaditakis P, Michalopoulos GK. b-catenin antisense

studies in embryonic liver cultures: role in proliferation, apoptosis, and

lineage specification. Gastroenterology 2003, 124: 202–216

254 Micsenyi A, Tan X, Sneddon T,

Luo JH, Michalopoulos GK, Monga SP. b-catenin is temporally

regulated during normal liver development. Gastroenterology 2004, 126: 1134–1146

255 Cadoret A, Ovejero C, Terris

B, Souil E, Levy L, Lamers WH, Kitajewski J et al. New targets of b-catenin signaling

in the liver are involved in the glutamine metabolism. Oncogene 2002, 21: 8293–8301

256 Sekine S, Lan BY, Bedolli M,

Feng S, Hebrok M. Liver-specific loss of b-catenin blocks

glutamine synthesis pathway activity and cytochrome p450 expression in mice.

Hepatology 2006, 43: 817–825

257 Benhamouche S, Decaens T,

Godard C, Chambrey R, Rickman DS, Moinard C, Vasseur-Cognet M et al. APC

tumor suppressor gene is the “zonation-keeper” of mouse liver. Dev Cell 2006,

10: 759–770

258 Colnot S, Decaens T,

Niwa-Kawakita M, Godard C, Hamard G, Kahn A, Giovannini M et al.

Liver-targeted disruption of APC in mice activates b-catenin signaling

and leads to hepatocellular carcinomas. Proc Natl Acad Sci USA 2004, 101: 17216–17221

259 Buendia MA. Genetics of

hepatocellular carcinoma. Semin Cancer Biol 2000, 10: 185–200

260 Thorgeirsson SS, Grisham JW.

Molecular pathogenesis of human hepatocellular carcinoma. Nat Genet 2002, 31:

339–346

261 Zucman-Rossi J, Jeannot E,

Nhieu JT, Scoazec JY, Guettier C, Rebouissou S, Bacq Y et al.

Genotype-phenotype correlation in hepatocellular adenoma: new classification

and relationship with HCC. Hepatology 2006, 43: 515–524

262 Harada N, Oshima H, Katoh M,

Tamai Y, Oshima M, Taketo MM. Hepatocarcinogenesis in mice with b-catenin and Ha-ras

gene mutations. Cancer Res 2004, 64: 48–54

263 Chan E, Gat U, McNiff JM,

Fuchs E. A common human skin tumor is caused by activating mutations in b-catenin. Nature

Genet 1999, 21: 410–413

264 Malanchi I, Peinado H, Kassen

D, Hussenet T, Metzger D, Chambon P, Huber M et al. Cutaneous cancer

stem cell maintenance is dependent on b-catenin signaling. Nature

2008, 452: 650–653