A Protein Interaction between b-Catenin and Dnmt1 Regulates Wnt Signaling and DNA Methylation in Colorectal Cancer Cells

Aberrant activation of the Wnt signaling pathway is an important step in the initiation and progression of tumor development in diverse cancers. The central effector of canonical Wnt signaling, b-catenin (CTNNB1), is a multifunctional protein, and has been extensively studied with respect to its roles in cell–cell adhesion and in regulation of Wnt-driven transcription. Here, a novel mass spectrometry–based proteomics technique in colorectal cancer cells expressing stabilized b-catenin, was used to identify a protein–protein interaction between b-catenin and DNA methyltransferase I (Dnmt1) protein, the primary regulator of DNA methylation patterns in mammalian cells. Dnmt1 and b-catenin strongly colocalized in the nuclei of colorectal cancer cells, and the interaction is mediated by the central domain of the Dnmt1 protein. Dnmt1 protein abundance is dependent upon the levels of b-catenin, and is increased in cells expressing stabilized mutant b-catenin. Conversely, the Dnmt1 regulates the levels of nuclear b-catenin and b-catenin/TCF–driven transcription. In addition, lysinespecific demethylase 1 (LSD1/KDM1A), a regulator of DNMT1 stability, was identified as a component of the Dnmt1–b-catenin protein complex and perturbation of the Dnmt1–b-catenin interaction altered DNA methylation. In summary, a functional protein–protein interaction was identified between two critically important oncoproteins, in turn revealing a link between Wnt signaling and downstream nuclear functions mediated by Dnmt1. Implications: Two critical oncoproteins, Dnmt1 and b-catenin, mutually regulate one each other's levels and activities in colorectal cancer cells. Mol Cancer Res; 13(6); 969–81. 2015 AACR.