Rhotekin contributes to osteoblast differentiation and bone homeostasis by interacting with Chibby 1

Rhotekin, an effector protein for Rho proteins, is implicated in tumorigenesis, yet its function in other biological processes remains largely unexplored. In the current study, we investigate the function of Rhotekin in osteoblast differentiation and bone homeostasis. Our data show that Rhotekin is expressed in bone and adipose tissue, with its levels increasing as mesenchymal progenitor cells differentiate toward both osteoblasts and adipocytes. When Rhotekin is overexpressed in progenitor cells, it facilitates osteogenic differentiation while inhibiting adipogenic differentiation, whereas knockdown of Rhotekin leads to opposite effects. Moreover, in vivo silencing of Rhotekin in bone marrow results in an increase in adipocytes and a decrease in osteoblasts, ultimately leading to a reduction in cancellous bone mass in mice. Further mechanistic investigations unveil that Rhotekin interacts with Chibby 1 (CBY1), an antagonistic regulator of β-catenin, and competes with β-catenin for binding to CBY1, thereby activating the canonical Wnt/β-catenin pathway. CBY1 is found to suppress osteoblast differentiation, an effect that is counteracted by overexpression of Rhotekin. Moreover, Rhotekin activates the focal adhesion kinase (FAK)/AKT signaling. Collectively, this study provides compelling evidence that Rhotekin interacts with CBY1, relieves its antagonistic effect on β-catenin, and thereby activates canonical Wnt/β-catenin signaling. Together with the concomitant activation of the FAK/AKT pathway, these events contribute to osteoblast differentiation and maintenance of bone homeostasis. Rhotekin may represent a promising therapeutic target for metabolic bone disorders such as osteoporosis.