HSP105 recruits protein phosphatase 2A To dephosphorylate ß-catenin

The Wnt/ß-catenin pathway causes accumulation of ß-catenin in the cytoplasm and its subsequent translocation into the nucleus to initiate the transcription of the target genes. Without Wnt stimulation, ß-catenin forms a complex with axin (axis inhibitor), adenomatous polyposis coli (APC), casein kinase 1α (CK1α), and glycogen synthase kinase 3ß (GSK3ß) and undergoes phosphorylation-dependent ubiquitination. Phosphatases, such as protein phosphatase 2A (PP2A), interestingly, also are components of this degradation complex; therefore, a balance must be reached between phosphorylation and dephosphorylation. How this balance is regulated is largely unknown. Here we show that a heat shock protein, HSP105, is a previously unidentified component of the ß-catenin degradation complex. HSP105 is required for Wnt signaling, since depletion of HSP105 compromises ß-catenin accumulation and target gene transcription upon Wnt stimulation. Mechanistically, HSP105 depletion disrupts the integration of PP2A into the ß-catenin degradation complex, favoring the hyperphosphorylation and degradation of ß-catenin. HSP105 is overexpressed in many types of tumors, correlating with increased nuclear ß-catenin protein levels and Wnt target gene upregulation. Furthermore, overexpression of HSP105 is a prognostic biomarker that correlates with poor overall survival in breast cancer patients as well as melanoma patients participating in the BRIM2 clinical study.