GSK-3 Phosphorylates δ-Catenin and Negatively Regulates Its Stability via Ubiquitination/Proteosome-mediated Proteolysis

δ-Catenin was first identified because of its interaction with presenilin-1, and its aberrant expression has been reported in various human tumors and in patients with Cri-du-Chat syndrome, a form of mental retardation. However, the mechanism whereby δ-catenin is regulated in cells has not been fully elucidated. We investigated the possibility that glycogen-synthase kinase-3 (GSK-3) phosphorylates δ-catenin and thus affects its stability. Initially, we found that the level of δ-catenin was greater and the half-life of δ-catenin was longer in GSK-3β−/− fibroblasts than those in GSK-3β+/+ fibroblasts. Furthermore, four different approaches designed to specifically inhibit GSK-3 activity, i.e. GSK-3-specific chemical inhibitors, Wnt-3a conditioned media, small interfering RNAs, and GSK-3α and -3β kinase dead constructs, consistently showed that the levels of endogenous δ-catenin in CWR22Rv-1 prostate carcinoma cells and primary cortical neurons were increased by inhibiting GSK-3 activity. In addition, it was found that both GSK-3α and -3β interact with and phosphorylate δ-catenin. The phosphorylation of ΔC207-δ-catenin (lacking 207 C-terminal residues) and T1078A δ-catenin by GSK-3 was noticeably reduced compared with that of wild type δ-catenin, and the data from liquid chromatography-tandem mass spectrometry analyses suggest that the Thr1078 residue of δ-catenin is one of the GSK-3 phosphorylation sites. Treatment with MG132 or ALLN, specific inhibitors of proteosome-dependent proteolysis, increased δ-catenin levels and caused an accumulation of ubiquitinated δ-catenin. It was also found that GSK-3 triggers the ubiquitination of δ-catenin. These results suggest that GSK-3 interacts with and phosphorylates δ-catenin and thereby negatively affects its stability by enabling its ubiquitination/proteosome-mediated proteolysis.