Mini ReviewWnt/β-catenin signaling
Introduction
The Wnt signaling transduction pathway plays an important role in a number of developmental processes, including body axis formation, development of the central nervous system, axial specification in limb development and mouse mammary gland development [1], [2]. The Wingless signaling transduction pathway is the Drosophila homolog of the Wnt signaling pathway and is important for correct cellular patterning within the embryo and imaginal discs [3], [4]. Furthermore, the Wnt signaling transduction pathway is also known to be involved in tumorigenesis [1], [5], [6]. The best studied Wnt gene, Wnt-1, was first identified as a proto-oncogene present adjacent to the integration site of mouse mammary tumor virus [5]. Mis-expression of Wnt-1 promotes mammary tumorigenesis [7], [8] or neural tube hyperplasia [9], [10]. More recently, the colorectal tumor suppressor adenomatous polyposis coli (APC) has been found to function as a negative regulator of the Wnt signaling pathway [6], [11], [12]. APC induces the degradation of β-catenin, an essential player in the Wnt signaling pathway, and the mutant APCs identified in colon cancers are defective in this activity. Thus, in colon cancer cells, β-catenin levels are elevated and Wnt signaling is constitutively activated. In this review, the focus is on recent works concerning β-catenin, APC, Axin and TCF/LEF.
Section snippets
Outline of the Wnt/Wingless signaling pathway
A series of genetic, cell biology and molecular biology studies have defined the nature and relative order of components within the Wnt/Wingless signaling pathway [1], [2] (Fig. 1). The Wnt family of proteins consists of more than 15 closely related secreted glycoproteins. Receptors for the Wnt proteins are members of the frizzled family of transmembrane proteins, and the Wnt signal is transduced to a cytoplasmic protein, Dishevelled (Dvl). Upon activation by the Wnt signa, Dvl then inhibits
β-Catenin
β-Catenin was originally identified by its association with the cytoplasmic domain of cadherins and was found to play an important role in Ca2+-dependent cell adhesion [18], [19], [20]. In addition to Wnt signaling and cell adhesion, β-catenin plays various roles by interacting with a number of proteins including the actin-binding protein Facin and Presenilins [21], [22], [23].
Homozygous inactivation of β-catenin results in embryonic lethality at day 6.5–7 post coitum [24]. β-Catenin−/− mouse
Structure of β-catenin
β-Catenin consists of an N-terminal region of approximately 130 amino acids, a central region of 550 amino acids, and a C-terminal region of 100 amino acids [1], [2] (Fig. 2). The N-terminal region contains consensus phosphorylation sites for GSK-β, while the C-terminal region possesses the transactivator function required for activation of target genes. The central region contains 12 imperfect sequence repeats of 42 amino acids known as armadillo repeats, which are required for the interaction
The colorectal tumor suppressor APC induces the degradation of β-catenin
The intracellular amount of β-catenin is negatively regulated by the tumor suppressor gene APC. APC was identified as a gene responsible for the onset of familial adenomatous polyposis (FAP), an autosomal dominantly inherited disease that predisposes patients to multiple colorectal polyps and cancers [6], [11], [12]. APC is also somatically mutated in the majority of sporadic colorectal tumors. Consistent with its role as a tumor suppressor, overexpression of APC blocks cell cycle transition
Mutations in β-catenin
The amino-terminal domain of β-catenin contains four consensus motifs for phosphorylation by GSK-3β, and these sites have been found to be mutated in some colorectal tumors with normal APC [42], [43]. Furthermore, β-catenin has also been found to be mutated in melanoma, prostate cancer, hepatocellular carcinoma, hepatoblastomas, endometrial carcinomas, ovarian cancer, medalloblastomas and pilomatricomas [44], [45]. β-Catenins mutated at the consensus sites for phosphorylation by GSK-3β are
The APC family of proteins
In human, a second APC, APCL/APC2, which is specifically expressed in the brain, has been identified [51], [52] (Fig. 3). APCL/APC2 also interacts with β-catenin and is able to induce the degradation of β-catenin when overexpressed in SW480 cells.
The APC gene is highly conserved among species and is found in Xenopus [53], Drosophila [54], and C. elegans [55], in addition to mammals (Fig. 3). A Drosophila homolog of APC, D-APC, also interacts with a Drosophila homolog of β-catenin, Armadillo
Axin negatively regulates β-catenin stability
It has recently been shown that Axin is involved in the degradation of β-catenin. Axin was identified as a product of the fused locus [60], [61], [62]. The most remarkable abnormality of embryos homozygous for fused is the formation of axial duplications. In addition, injection of Axin mRNA into Xenopus embryos inhibits dorsal axis formation, while injection of mutant Axin mRNA induces an ectopic axis, apparently through a dominant negative mechanism [62].
Axin interacts with β-catenin, GSK-3β
Regulation of the function of Axin
Axin itself is also phosphorylated by GSK-3β in vitro. Wnt signaling induces dephosphorylation of Axin [74], [75]. The dephosphorylated Axin binds β-catenin less efficiently than the phosphorylated form, and is more unstable than the phosphorylated form. Thus, Wnt-induced dephosphorylation may be important to prevent the phosphorylation of β-catenin by GSK-3β so that β-catenin can accumulate to high levels and activate transcription in concert with TCF/LEF.
Dvl-1 may be involved in Wnt-mediated
APC requires axin to downregulate β-catenin
In Axin and conductin/Axil, the sites responsible for binding to APC are the G-protein signaling (RGS) domains (Fig. 4). The complementary sites in APC responsible for binding to Axin and conductin/Axil reside between the 20 amino acid repeat numbers 3 and 4, 4 and 5, and downstream of repeat 7 [69], [79] (Fig. 3). The region of APC containing these sites is located just downstream of the mutation cluster region, suggesting that the interaction of APC with Axin and conductin/Axil is important
APC and phosphatase
The B56 subunit of protein phosphatase 2 (PP2A) interacts with APC [80]. Expression of B56 reduces the amount of β-catenin and inhibits transcription of β-catenin target genes in mammalian cells and Xenopus embryo explants. The B56-dependent decrease in β-catenin is not observed in the colon cancer cell lines that contain mutant β-catenin or APC. Thus, B56 may direct PP2A to dephosphorylate specific components of the APC signaling complex. Interestingly, it has also been shown that the PP2A C
β-TrCP targets the degradation of phosphorylated β-catenin
β-Catenin is turned over by the ubiquitin-dependent proteolysis system [81]. Mutations in the GSK-3β phosphorylation consensus motif of β-catenin inhibit its ubiquitination and results in its stabilization [82], [83], [84], [85]. Thus, β-catenin phosphorylated by GSK-3β in the Axin complex is thought to be subjected to the ubiquitin-dependent degradation. The Drosophila gene slimb, loss of whose function results in an accumulation of high levels of Armadillo and Cubitus, encodes a conserved
Interaction of β-catenin with TCF/LEF
β-catenin stabilized by Wnt signaling associates with the TCF/LEF family of transcription factors and activates Wnt target genes. T-cell factor (TCF) and lymphoid enhancer factor (LEF-1) were originally identified as factors that bind to the enhancers of T cell-specific genes [13], [14], [15], [16], [17]. To date, four members of this family have been identified in mammals; LEF1, TCF1, TCF3 and TCF4 [41], [91], [92], [93]. These proteins contain the high mobility group (HMG) domain, and
Significance of TCF/LEF in development
TCF1 is expressed in T lymphocytes, and TCF1−/− knockout mice are impaired in the generation of T cells [95]. However, TCF1−/− mice are fully immunocompetent having functional peripheral T cells and live for over a year [96]. More recently, it was found that TCF1−/− mice develop adenomas in gut and mammary glands [97]. LEF1 is expressed in pre-B and T lymphocytes of adult mice and in the neural crest, mesencephalon, tooth germs, whisker follicles and other tissues during embryogenesis. LEF1−/−
Negative regulation of TCF/LEF activity
It has recently been reported that in the absence of β-catenin, TCF is associated with members of the Groucho family of proteins and acts as a transcriptional repressor of Wnt/Wingless target genes [100], [101]. This finding explains the previous data obtained in Drosophila, Xenopus and Caenorhabditis elegans showing that dTCF/Pan can function as either an activator or a repressor of Wingless-responsive genes depending on the state of the Wingless signaling pathway and possibly on the amount of
Target genes for the β-catenin–TCF complex
The best studied target genes of the β-catenin–TCF/LEF complex are Xenopus Siamois and Twin, dorsal-specific genes required for the activation of the Spemann organizer and the subsequent development of the embryonic axes [105], [106]. Other candidates for target genes include Xenopus nodal-related 3 (Xnr-3) in Xenopus, and Engrailed and Ultrabithorax (Ubx) in Drosophila [14], [107]. In human, cyclin D1 has been shown to be a target gene of the β-catenin–TCF complex [108]. The cyclin D1 promoter
References (109)
- et al.
Signaling by wingless in Drosophila
Dev. Biol.
(1994) Signal transduction of beta-catenin
Curr. Opin. Cell Biol.
(1995)- et al.
Wnt genes
Cell
(1992) The adenomatous polyposis coli (APC) tumor suppressor
Biochim. Biophys. Acta.
(1997)- et al.
Expression of the int-1 gene in transgenic mice is associated with mammary gland hyperplasia and adenocarcinomas in male and female mice
Cell
(1988) - et al.
The role of Wnt genes in vertebrate development
Curr. Opin. Genet. Dev.
(1992) - et al.
Lessons from hereditary colorectal cancer
Cell
(1996) APC: the plot thickens
Curr. Opin. Genet. Dev.
(1999)- et al.
XTcf-3 transcription factor mediates beta-catenin-induced axis formation in Xenopus embryos
Cell
(1996) - et al.
LEF-1, a nuclear factor coordinating signaling inputs from wingless and decapentaplegic
Cell
(1997)
Armadillo coactivates transcription driven by the product of the Drosophila segment polarity gene dTCF
Cell
Single amino acid substitutions in proteins of the armadillo gene family abolish their binding to alpha-catenin
J. Biol. Chem.
Cadherins in the developing central nervous system: an adhesive code for segmental and functional subdivisions
Dev. Biol.
Three-dimensional structure of the armadillo repeat region of beta-catenin
Cell
The APC protein and E-cadherin form similar but independent complexes with alpha-catenin, beta-catenin, and plakoglobin
J. Biol. Chem.
The tumor suppressor protein APC colocalizes with beta-catenin in the colon epithelial cells
Biochem. Biophys. Res. Commun.
The oncogenic activation of beta-catenin
Curr. Opin. Genet. Dev.
De Novo hair follicle morphogenesis and hair tumors in mice expressing a truncated beta-catenin in skin
Cell
Wnt signaling and an APC-related gene specify endoderm in early C. elegans embryos
Cell
Regulation of armadillo by a Drosophila APC inhibits neuronal apoptosis during retinal development
Cell
The mouse Fused locus encodes Axin, an inhibitor of the Wnt signaling pathway that regulates embryonic axis formation
Cell
Downregulation of beta-catenin by human Axin and its association with the APC tumor suppressor, beta-catenin and GSK3 beta
Curr. Biol.
Axis determination in Xenopus involves biochemical interactions of axin, glycogen synthase kinase 3 and beta-catenin
Curr. Biol.
Identification of a domain of Axin that binds to the serine/threonine protein phosphatase 2A and a self-binding domain
J. Biol. Chem.
GBP, an inhibitor of GSK-3, is implicated in Xenopus development and oncogenesis
Cell
Direct and long-range action of a wingless morphogen gradient
Cell
A novel human WD protein, h-beta TrCp, that interacts with HIV-1 Vpu connects CD4 to the ER degradation pathway through an F-box motif
Mol. Cell
The HMG domain of lymphoid enhancer factor 1 bends DNA and facilitates assembly of functional nucleoprotein structures
Cell
Wnt signaling: a common theme in animal development
Genes Dev.
Signal transduction through beta-catenin and specification of cell fate during embryogenesis
Genes Dev.
Deficiency of p53 accelerates mammary tumorigenesis in Wnt-1 transgenic mice and promotes chromosomal instability
Genes Dev.
Evidence for a mitogenic effect of Wnt-1 in the developing mammalian central nervous system
Development
Functional interaction of beta-catenin with the transcription factor LEF-1
Nature
Pangolin encodes a Lef-1 homologue that acts downstream of Armadillo to transduce the Wingless signal in Drosophila
Nature
Cadherin cell adhesion receptors as a morphogenetic regulator
Science
Beta-catenin associates with the actin-bundling protein fascin in a noncadherin complex
J. Cell Biol.
Presenilin mutations associated with Alzheimer disease cause defective intracellular trafficking of beta-catenin, a component of the presenilin protein complex
Nat. Med.
Presenilins interact with armadillo proteins including neural-specific plakophilin-related protein and beta-catenin
J. Neurochem.
Lack of beta-catenin affects mouse development at gastrulation
Development
Requirement for beta-catenin in anterior–posterior axis formation in mice
J. Cell Biol.
Association of the APC gene product with beta-catenin
Science
An in vivo structure-function study of armadillo, the beta-catenin homologue, reveals both separate and overlapping regions of the protein required for cell adhesion and for wingless signaling
J. Cell Biol.
Drosophila alpha-catenin and E-cadherin bind to distinct regions of Drosophila Armadillo
J. Biol. Chem.
The tumour suppressor gene product APC blocks cell cycle progression from G0/G1 to S phase
Embo. J.
Apoptosis and APC in colorectal tumorigenesis
Proc. Natl. Acad. Sci. USA
The APC gene product in normal and tumor cells
Proc. Natl. Acad. Sci. USA
Subcellular localization of the APC protein: immunoelectron microscopic study of the association of the APC protein with catenin
Oncogene
The adenomatous polyposis coli tumor suppressor protein localizes to plasma membrane sites involved in active cell migration
J. Cell Biol.
Association of the APC tumor suppressor protein with catenins
Science
APC binds to the novel protein EB1
Cancer Res.
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