Direct and indirect effects of transforming growth factor-beta on osteoclast-mediated bone remodeling using a new in vitro bone matrix model.

Transforming growth factor-beta (TGF-β), a cytokine embedded in the bone matrix, is released during bone resorption, influencing osteoclast differentiation and coupling factor production, which affect osteoblasts and osteocytes. This study investigates the role of TGF-β in bone remodeling using an in vitro model with calcium phosphate-coated plates covalently bonded to latent TGF-β (LTGF-β(+)-CaP plates). This model replicates the natural release of TGF-β and its effects on RAW264 macrophage-like cells, which differentiate into osteoclasts upon stimulation of RANKL. Cells cultured on LTGF-β(+)-CaP plates formed resorption pits and released TGF-β, upregulating osteoclast differentiation- and resorption-related genes during early differentiation. During the resorption phase, TGF-β-enhanced osteoblast activation and coupling factor expression supporting bone formation in surrounding cells. In osteocytes, it differentially regulated gene expression by upregulating osteoprotegerin and downregulating sclerostin, suggesting a dual role in remodeling. Our findings demonstrate that TGF-β plays a critical role in bone homeostasis by directly promoting osteoclast differentiation and resorption while indirectly facilitating osteoblast differentiation through coupling factors. These results provide insights into the dynamic interactions between osteoclasts, osteoblasts, and osteocytes, emphasizing TGF-β's role in linking bone resorption and formation. This study establishes a novel in vitro platform to examine TGF-β-mediated bone remodeling and its underlying molecular mechanisms. Furthermore, our model can be used to explore how TGF-β signaling affects cellular communication in the bone and may contribute to identifying new therapeutic targets for osteoporosis and other bone-resorptive disorders.