Advanced Bone Formation in Mice with a Dominant-negative Mutation in the Thyroid Hormone Receptor β Gene due to Activation of Wnt/β-Catenin Protein Signaling

Abstract Thyroid hormone (T3) acts in chondrocytes and bone-forming osteoblasts to control bone development and maintenance, but the signaling pathways mediating these effects are poorly understood. ThrbPV/PV mice have a severely impaired pituitary-thyroid axis and elevated thyroid hormone levels due to a dominant-negative mutant T3 receptor (TRβPV) that cannot bind T3 and interferes with the actions of wild-type TR. ThrbPV/PV mice have accelerated skeletal development due to unknown mechanisms. We performed microarray studies in primary osteoblasts from wild-type mice and ThrbPV/PV mice. Activation of the canonical Wnt signaling in ThrbPV/PV mice was confirmed by in situ hybridization analysis of Wnt target gene expression in bone during postnatal growth. By contrast, T3 treatment inhibited Wnt signaling in osteoblastic cells, suggesting that T3 inhibits the Wnt pathway by facilitating proteasomal degradation of β-catenin and preventing its accumulation in the nucleus. Activation of the Wnt pathway in ThrbPV/PV mice, however, results from a gain of function for TRβPV that stabilizes β-catenin despite the presence of increased thyroid hormone levels. These studies demonstrate novel interactions between T3 and Wnt signaling pathways in the regulation of skeletal development and bone formation.