ABSTRACT
Wnt signaling, named after the secreted proteins that bind to cell surface receptors to activate the pathway, plays critical roles both in embryonic development and the maintenance of homeostasis in many adult tissues. Two particularly important cellular programs orchestrated by Wnt signaling are proliferation and stem cell self-renewal. Constitutive activation of the Wnt pathway resulting from mutation or improper modulation of pathway components contributes to cancer development in various tissues. Colon cancers frequently bear inactivating mutations of the adenomatous polyposis coli (APC) gene, whose product is an important component of the destruction complex that regulates ß-catenin levels. Stabilization and nuclear localization of ß-catenin result in the expression of a panel of Wnt target genes. We previously showed that Mule/Huwe1/Arf-BP1 (Mule) controls murine intestinal stem and progenitor cell proliferation by modulating the Wnt pathway via c-Myc. Here we extend our investigation of Mule's influence on oncogenesis by showing that Mule interacts directly with ß-catenin and targets it for degradation under conditions of hyperactive Wnt signaling. Our findings suggest that Mule uses various mechanisms to fine-tune the Wnt pathway and provides multiple safeguards against tumorigenesis.
Subject(s)
Tumor Suppressor Proteins/physiology , Ubiquitin-Protein Ligases/physiology , Wnt Signaling Pathway , beta Catenin/antagonists & inhibitors , Adenomatous Polyposis Coli Protein/deficiency , Animals , Axin Protein/biosynthesis , Axin Protein/genetics , Cell Nucleus/metabolism , Cell Nucleus/ultrastructure , Colonic Neoplasms/metabolism , Cyclin D1/biosynthesis , Cyclin D1/genetics , Down-Regulation , Genes, APC , Genes, Tumor Suppressor , HEK293 Cells , Humans , Mice , Mice, Knockout , Neoplasm Proteins/physiology , Organoids/metabolism , Organoids/ultrastructure , Protein Binding , Protein Processing, Post-Translational , Proteolysis , RNA Interference , RNA, Small Interfering/genetics , Recombinant Proteins/metabolism , Tumor Suppressor Proteins/antagonists & inhibitors , Tumor Suppressor Proteins/deficiency , Tumor Suppressor Proteins/genetics , Ubiquitin-Protein Ligases/antagonists & inhibitors , Ubiquitin-Protein Ligases/deficiency , Ubiquitin-Protein Ligases/genetics , UbiquitinationABSTRACT
Enchondromas are benign cartilage tumors and precursors to malignant chondrosarcomas. Somatic mutations in the isocitrate dehydrogenase genes (IDH1 and IDH2) are present in the majority of these tumor types. How these mutations cause enchondromas is unclear. Here, we identified the spectrum of IDH mutations in human enchondromas and chondrosarcomas and studied their effects in mice. A broad range of mutations was identified, including the previously unreported IDH1-R132Q mutation. These mutations harbored enzymatic activity to catalyze α-ketoglutarate to d-2-hydroxyglutarate (d-2HG). Mice expressing Idh1-R132Q in one allele in cells expressing type 2 collagen showed a disordered growth plate, with persistence of type X-expressing chondrocytes. Chondrocyte cell cultures from these animals or controls showed that there was an increase in proliferation and expression of genes characteristic of hypertrophic chondrocytes with expression of Idh1-R132Q or 2HG treatment. Col2a1-Cre;Idh1-R132Q mutant knock-in mice (mutant allele expressed in chondrocytes) did not survive after the neonatal stage. Col2a1-Cre/ERT2;Idh1-R132 mutant conditional knock-in mice, in which Cre was induced by tamoxifen after weaning, developed multiple enchondroma-like lesions. Taken together, these data show that mutant IDH or d-2HG causes persistence of chondrocytes, giving rise to rests of growth-plate cells that persist in the bone as enchondromas.