Thr160 of Axin1 is critical for the formation and function of the β-catenin destruction complex

Upon binding of a Wnt ligand to the frizzled (FZD)-low density lipoprotein receptor related protein 5/6 (LRP5/6) receptor complex, the β-catenin destruction complex, composed of Axin1, adenomatous polyposis coli (APC), glycogen synthase kinase 3 (GSK3) and casein kinase 1 (CK1), is immediately inactivated, which causes β-catenin stabilization. However, the molecular mechanism of signal transduction from the receptor complex to the β-catenin destruction complex is controversial. Here we show that Wnt3a treatment promotes the dissociation of the Axin1-APC complex in glioblastoma cells cultured in serum-free medium. Experiments with the GSK3 inhibitor BIO suggest that Axin1-APC dissociation was controlled by phosphorylation. Introduction of a phosphomimetic mutation into Thr160 of Axin1, located in the APC-binding region RGS, abrogated the interaction of Axin1 with APC. Consistent with these observations, the Axin1 phosphomimetic mutant lost the ability to reduce β-catenin stability and to repress β-catenin/TCF-dependent transcription. Taken together, our results suggest a novel mechanism of Wnt signaling through the dissociation of the β-catenin destruction complex by Axin1 Thr160 modification.