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Axin prevents Wnt-3a-induced accumulation of beta-catenin
M Kishida1, S Koyama, S Kishida
1Department of Biochemistry, Hiroshima University School of Medicine, Japan.
Abstract:
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.
Insights
Axin, a Wnt signaling regulator, forms a complex with GSK-3beta, beta-catenin, and APC, promoting beta-catenin degradation. Wnt-3a signaling disrupts this complex, leading to beta-catenin accumulation.
Area of Science:
- Molecular Biology
- Cell Signaling
- Biochemistry
Background:
- The Wnt signaling pathway is crucial for cellular processes.
- Axin acts as a negative regulator within this pathway.
- Understanding Axin's interactions is key to deciphering Wnt signaling.
Purpose of the Study:
- To investigate the molecular interactions of Axin in the Wnt signaling pathway.
- To elucidate the mechanism by which Axin regulates beta-catenin.
- To determine the effect of Wnt-3a on the Axin-beta-catenin complex.
Main Methods:
- Gel filtration column chromatography to analyze protein complex formation.
- Co-precipitation assays to confirm protein interactions.
- Stable cell line expression to study functional inhibition.
Main Results:
- Axin coelutes and co-precipitates with GSK-3beta, beta-catenin, and APC.
- Axin forms a high molecular weight complex with these proteins.
- Wnt-3a treatment leads to beta-catenin dissociation from the Axin complex and accumulation.
- Axin expression inhibits Wnt-3a-induced beta-catenin accumulation and Tcf-4 activation.
Conclusions:
- Axin forms a multi-protein complex that stimulates beta-catenin degradation.
- Wnt-3a signaling induces dissociation of beta-catenin from the Axin complex.
- This dissociation results in the accumulation of beta-catenin, a key event in Wnt pathway activation.