ABSTRACT
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.
Subject(s)
HSP110 Heat-Shock Proteins/metabolism , Protein Phosphatase 2/metabolism , beta Catenin/metabolism , Adenomatous Polyposis Coli/genetics , Breast/metabolism , Breast/pathology , Breast Neoplasms/diagnosis , Breast Neoplasms/genetics , Breast Neoplasms/metabolism , Female , Gene Expression Regulation, Neoplastic , HEK293 Cells , HSP110 Heat-Shock Proteins/analysis , HSP110 Heat-Shock Proteins/genetics , Humans , Mutation , Phosphorylation , Prognosis , Protein Interaction Maps , Proteolysis , RNA Interference , RNA, Small Interfering/genetics , Wnt Signaling PathwayABSTRACT
Receptor-interacting protein kinase 4 (RIPK4) is required for epidermal differentiation and is mutated in Bartsocas-Papas syndrome. RIPK4 binds to protein kinase C, but its signaling mechanisms are largely unknown. Ectopic RIPK4, but not catalytically inactive or Bartsocas-Papas RIPK4 mutants, induced accumulation of cytosolic ß-catenin and a transcriptional program similar to that caused by Wnt3a. In Xenopus embryos, Ripk4 synergized with coexpressed Xwnt8, whereas Ripk4 morpholinos or catalytic inactive Ripk4 antagonized Wnt signaling. RIPK4 interacted constitutively with the adaptor protein DVL2 and, after Wnt3a stimulation, with the co-receptor LRP6. Phosphorylation of DVL2 by RIPK4 favored canonical Wnt signaling. Wnt-dependent growth of xenografted human tumor cells was suppressed by RIPK4 knockdown, suggesting that RIPK4 overexpression may contribute to the growth of certain tumor types.