Axin prevents Wnt-3a-induced accumulation of beta-catenin. (1/400)

When Axin, a negative regulator of the Wnt signaling pathway, was expressed in COS cells, it coeluted with glycogen synthase kinase-3beta (GSK-3beta), beta-catenin, and adenomatous polyposis coli protein (APC) in a high molecular weight fraction on gel filtration column chromatography. In this fraction, GSK-3beta, beta-catenin, and APC were co-precipitated with Axin. Although beta-catenin was detected in the high molecular weight fraction in L cells on gel filtration column chromatography, addition of conditioned medium expressing Wnt-3a to the cells increased beta-catenin in the low molecular weight fraction. However, Wnt-3a-dependent accumulation of beta-catenin was greatly inhibited in L cells stably expressing Axin. Axin also suppressed Wnt-3a-dependent activation of Tcf-4 which binds to beta-catenin and acts as a transcription factor. These results suggest that Axin forms a complex with GSK-3beta, beta-catenin, and APC, resulting in the stimulation of the degradation of beta-catenin and that Wnt-3a induces the dissociation of beta-catenin from the Axin complex and accumulates beta-catenin.  (+info)

Xenopus axin interacts with glycogen synthase kinase-3 beta and is expressed in the anterior midbrain. (2/400)

Axin is encoded by the fused locus in mice and is required for normal vertebrate axis formation. It has recently been shown that axin associates with APC, beta-catenin and glycogen synthase kinase-3 (GSK-3) in a complex that appears to regulate the level of cytoplasmic beta-catenin. We have identified the Xenopus homologue of axin through its interaction with GSK-3b. Xenopus axin (Xaxin) is expressed maternally and throughout early development with a low level of ubiquitous expression. Xaxin also shows remarkably high expression in the anterior mesencephalon adjacent to the forebrain-midbrain boundary.  (+info)

Negative regulation of Wingless signaling by D-axin, a Drosophila homolog of axin. (3/400)

Wnt/Wingless directs many cell fates during development. Wnt/Wingless signaling increases the amount of beta-catenin/Armadillo, which in turn activates gene transcription. Here the Drosophila protein D-Axin was shown to interact with Armadillo and D-APC. Mutation of d-axin resulted in the accumulation of cytoplasmic Armadillo and one of the Wingless target gene products, Distal-less. Ectopic expression of d-axin inhibited Wingless signaling. Hence, D-Axin negatively regulates Wingless signaling by down-regulating the level of Armadillo. These results establish the importance of the Axin family of proteins in Wnt/Wingless signaling in Drosophila.  (+info)

Phosphorylation of axin, a Wnt signal negative regulator, by glycogen synthase kinase-3beta regulates its stability. (4/400)

Axin forms a complex with glycogen synthase kinase-3beta (GSK-3beta) and beta-catenin and promotes GSK-3beta-dependent phosphorylation of beta-catenin, thereby stimulating the degradation of beta-catenin. Because GSK-3beta also phosphorylates Axin in the complex, the physiological significance of the phosphorylation of Axin was examined. Treatment of COS cells with LiCl, a GSK-3beta inhibitor, and okadaic acid, a protein phosphatase inhibitor, decreased and increased, respectively, the cellular protein level of Axin. Pulse-chase analyses showed that the phosphorylated form of Axin was more stable than the unphosphorylated form and that an Axin mutant, in which the possible phosphorylation sites for GSK-3beta were mutated, exhibited a shorter half-life than wild type Axin. Dvl-1, which was genetically shown to function upstream of GSK-3beta, inhibited the phosphorylation of Axin by GSK-3beta in vitro. Furthermore, Wnt-3a-containing conditioned medium down-regulated Axin and accumulated beta-catenin in L cells and expression of Dvl-1(DeltaPDZ), in which the PDZ domain was deleted, suppressed this action of Wnt-3a. These results suggest that the phosphorylation of Axin is important for the regulation of its stability and that Wnt down-regulates Axin through Dvl.  (+info)

An F-box protein, FWD1, mediates ubiquitin-dependent proteolysis of beta-catenin. (5/400)

beta-catenin plays an essential role in the Wingless/Wnt signaling cascade and is a component of the cadherin cell adhesion complex. Deregulation of beta-catenin accumulation as a result of mutations in adenomatous polyposis coli (APC) tumor suppressor protein is believed to initiate colorectal neoplasia. beta-catenin levels are regulated by the ubiquitin-dependent proteolysis system and beta-catenin ubiquitination is preceded by phosphorylation of its N-terminal region by the glycogen synthase kinase-3beta (GSK-3beta)/Axin kinase complex. Here we show that FWD1 (the mouse homologue of Slimb/betaTrCP), an F-box/WD40-repeat protein, specifically formed a multi-molecular complex with beta-catenin, Axin, GSK-3beta and APC. Mutations at the signal-induced phosphorylation site of beta-catenin inhibited its association with FWD1. FWD1 facilitated ubiquitination and promoted degradation of beta-catenin, resulting in reduced cytoplasmic beta-catenin levels. In contrast, a dominant-negative mutant form of FWD1 inhibited the ubiquitination process and stabilized beta-catenin. These results suggest that the Skp1/Cullin/F-box protein FWD1 (SCFFWD1)-ubiquitin ligase complex is involved in beta-catenin ubiquitination and that FWD1 serves as an intracellular receptor for phosphorylated beta-catenin. FWD1 also links the phosphorylation machinery to the ubiquitin-proteasome pathway to ensure prompt and efficient proteolysis of beta-catenin in response to external signals. SCFFWD1 may be critical for tumor development and suppression through regulation of beta-catenin protein stability.  (+info)

Functional domains of axin. Importance of the C terminus as an oligomerization domain. (6/400)

To understand the mechanism of how Axin acts as an inhibitory molecule in the Wnt pathway, we generated a series of mutated forms of Axin. From the binding experiments, we defined the domains of Axin that bind glycogen synthase kinase-3beta (GSK-3beta) and beta-catenin. We also examined the ability of each Axin mutant to inhibit lymphoid enhancer factor-1 (Lef-1) reporter activity in a cell line expressing high levels of beta-catenin. Axin mutants that did not bind GSK-3beta or beta-catenin were ineffective in suppressing Lef-1 reporter activity. Binding GSK-3beta and beta-catenin was not sufficient for this inhibitory effect of Axin. Axin mutants with C-terminal truncations lacked the ability to inhibit Lef-1 reporter activity, even though they bound GSK-3beta and beta-catenin. The C-terminal region was required for binding to Axin itself. Substitution of the C-terminal region with an unrelated dimerizing molecule, the retinoid X receptor restored its inhibitory effect on Lef-1-dependent transcription. The oligomerization of Axin through its C terminus is important for its function in regulation of beta-catenin-mediated response.  (+info)

The cyclin D1 gene is a target of the beta-catenin/LEF-1 pathway. (7/400)

beta-Catenin plays a dual role in the cell: one in linking the cytoplasmic side of cadherin-mediated cell-cell contacts to the actin cytoskeleton and an additional role in signaling that involves transactivation in complex with transcription factors of the lymphoid enhancing factor (LEF-1) family. Elevated beta-catenin levels in colorectal cancer caused by mutations in beta-catenin or by the adenomatous polyposis coli molecule, which regulates beta-catenin degradation, result in the binding of beta-catenin to LEF-1 and increased transcriptional activation of mostly unknown target genes. Here, we show that the cyclin D1 gene is a direct target for transactivation by the beta-catenin/LEF-1 pathway through a LEF-1 binding site in the cyclin D1 promoter. Inhibitors of beta-catenin activation, wild-type adenomatous polyposis coli, axin, and the cytoplasmic tail of cadherin suppressed cyclin D1 promoter activity in colon cancer cells. Cyclin D1 protein levels were induced by beta-catenin overexpression and reduced in cells overexpressing the cadherin cytoplasmic domain. Increased beta-catenin levels may thus promote neoplastic conversion by triggering cyclin D1 gene expression and, consequently, uncontrolled progression into the cell cycle.  (+info)

Interaction of axin and Dvl-2 proteins regulates Dvl-2-stimulated TCF-dependent transcription. (8/400)

Axin promotes the phosphorylation of beta-catenin by GSK-3beta, leading to beta-catenin degradation. Wnt signals interfere with beta-catenin turnover, resulting in enhanced transcription of target genes through the increased formation of beta-catenin complexes containing TCF transcription factors. Little is known about how GSK-3beta-mediated beta-catenin turnover is regulated in response to Wnt signals. We have explored the relationship between Axin and Dvl-2, a member of the Dishevelled family of proteins that function upstream of GSK-3beta. Expression of Dvl-2 activated TCF-dependent transcription. This was blocked by co-expression of GSK-3beta or Axin. Expression of a 59 amino acid GSK-3beta-binding region from Axin strongly activated transcription in the absence of an upstream signal. Introduction of a point mutation into full-length Axin that prevented GSK-3beta binding also generated a transcriptional activator. When co-expressed, Axin and Dvl-2 co-localized within expressing cells. When Dvl-2 localization was altered using a C-terminal CAAX motif, Axin was also redistributed, suggesting a close association between the two proteins, a conclusion supported by co-immunoprecipitation data. Deletion analysis suggested that Dvl-association determinants within Axin were contained between residues 603 and 810. The association of Axin with Dvl-2 may be important in the transmission of Wnt signals from Dvl-2 to GSK-3beta.  (+info)

• 1
• 2
• 3
• 4
• 5
• 6
• 7
• 8
• 9
• 10