Mini Review
Wnt/β-catenin signaling

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Abstract

The Wnt/Wingless signaling transduction pathway plays an important role in both embryonic development and tumorigenesis. β-Catenin, a key component of the Wnt signaling pathway, interacts with the TCF/LEF family of transcription factors and activates transcription of Wnt target genes. Recent studies have revealed that a number of proteins such as, the tumor suppressor APC and Axin are involved in the regulation of the Wnt signaling pathway. Furthermore, mutations in APC or β-catenin have been found to be responsible for the genesis of human cancers.

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

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