BackgroundDelayed fracture healing causes substantial disability and usually requires additional surgical treatments. Pharmacologic management to improve fracture repair would substantially improve patient outcome. The signaling pathways regulating bone healing are beginning to be unraveled, and they provide clues into pharmacologic management. The β-catenin signaling pathway, which activates T cell factor (TCF)-dependent transcription, has emerged as a key regulator in embryonic skeletogenesis, positively regulating osteoblasts. However, its role in bone repair is unknown. The goal of this study was to explore the role of β-catenin signaling in bone repair.Methods and FindingsWestern blot analysis showed significant up-regulation of β-catenin during the bone healing process. Using a β-Gal activity assay to observe activation during healing of tibia fractures in a transgenic mouse model expressing a TCF reporter, we found that β-catenin-mediated, TCF-dependent transcription was activated in both bone and cartilage formation during fracture repair. Using reverse transcription-PCR, we observed that several WNT ligands were expressed during fracture repair. Treatment with DKK1 (an antagonist of WNT/β-catenin pathway) inhibited β-catenin signaling and the healing process, suggesting that WNT ligands regulate β-catenin. Healing was significantly repressed in mice conditionally expressing either null or stabilized β-catenin alleles induced by an adenovirus expressing Cre recombinase. Fracture repair was also inhibited in mice expressing osteoblast-specific β-catenin null alleles. In stark contrast, there was dramatically enhanced bone healing in mice expressing an activated form of β-catenin, whose expression was restricted to osteoblasts. Treating mice with lithium activated β-catenin in the healing fracture, but healing was enhanced only when treatment was started subsequent to the fracture.ConclusionsThese results demonstrate that β-catenin functions differently at different stages of fracture repair. In early stages, precise regulation of β-catenin is required for pluripotent mesenchymal cells to differentiate to either osteoblasts or chondrocytes. Once these undifferentiated cells have become committed to the osteoblast lineage, β-catenin positively regulates osteoblasts. This is a different function for β-catenin than has previously been reported during development. Activation of β-catenin by lithium treatment has potential to improve fracture healing, but only when utilized in later phases of repair, after mesenchymal cells have become committed to the osteoblast lineage.

Fibroproliferative processes are a group of disorders in which there is excessive proliferation of spindle (mesenchymal fibroblast-like) cells. They range from hypertrophic scars to neoplasms such as aggressive fibromatosis. Cells from these disorders share cytologic similarity with fibroblasts present during the proliferative phase of wound healing, suggesting that they represent a prolonged wounding response. A critical role for ␤-catenin in mesenchymal cells in fibroproliferative processes is suggested by its high rate of somatic mutation in aggressive fibromatosis. Using a Tcf-reporter mouse we found that ␤-catenin protein level and Tcf-transcriptional activity are elevated in fibroblasts during the proliferative phase of healing. We generated a transgenic mouse in which stabilized ␤-catenin is expressed in mesenchymal cells under control of a tetracycline-regulated promoter. Fibroblasts from the transgenic mice exhibited increased proliferation, motility, and invasiveness when expressing stabilized ␤-catenin and induced tumors after induction of the transgene when grafted into nude mice. Mice developed aggressive fibromatoses and hyperplastic gastrointestinal polyps after 3 months of transgene induction and healed with hyperplastic cutaneous wounds compared with control mice, which demonstrates an important function for ␤-catenin in mesenchymal cells and shows a central role for ␤-catenin in wound healing and fibroproliferative disorders.