Constitutive β-catenin activation in osteoblasts impairs terminal osteoblast differentiation and bone quality

Accumulating evidence suggests that Wnt/beta-catenin signaling plays a central role in controlling bone mass. We previously reported that constitutive activation of eta-catenin (CA-beta-catenin) in osteoblasts potentially has side effects on the bone growth and bone remodeling process, although it could increase bone mass. The present study aimed to observe the effects of osteoblastic CA-beta-catenin on bone quality and to investigate possible mechanisms of these effects. It was found that CA-beta-catenin mice exhibited lower mineralization levels and disorganized collagen in long bones as confirmed by von Kossa staining and sirius red staining, respectively. Also, bone strength decreased significantly in CA-beta-catenin mice. Then the effect of CA-beta-catenin on biological functions of osteoblasts were investigated and it was found that the expression levels of osteocalcin, a marker for the late differentiation of osteoblasts, decreased in CA-beta-catenin mice, while the expression levels of osterix and alkaline phosphatase, two markers for the early differentiation of osteoblasts, increased in CA-beta-catenin mice. Furthermore, higher proliferation rate were revealed in osteoblasts that were isolated from CA-beta-catenin mice. The Real-time PCR and western blot examination found that the expression level of c-myc and cyclin Dl, two G1 progression-related molecules, increased in osteoblasts that were isolated from the CA-beta-catenin mice, and the expression levels of CDK14 and cyclin Y, two mitotic-related molecules that can accelerate cells entering into S and G2/M phases, increased in osteoblasts that were isolated from the CA-beta-catenin mice. In summary, osteoblastic CA-beta-catenin kept osteoblasts in high proliferative state and impaired the terminal osteoblast differentiation, and this led to changed bone structure and decreased bone strength.