Biphasic effects of transforming growth factor β on bone morphogenetic protein-induced osteoblast differentiation

Bone morphogenetic proteins (BMPs) exert an important role in skeletal development, adult bone homeostasis, and fracture healing and have demonstrated clinical utility for bone regeneration. However, BMPs fall short as regenerative agents because high doses need to be used to obtain therapeutic effects. Determining the molecular mechanisms controlling BMP-induced bone formation may lead to the development of more effective BMP-based therapies. To identify kinases mediating BMP-induced osteoblast differentiation, we performed an siRNA screen to find kinases modulating BMP-6-induced alkaline phosphatase (ALP) activity. Surprisingly, although transforming growth factor β (TGF-β) generally is considered to antagonize BMP-induced osteoblast differentiation, C2C12  cells transfected with siRNAs targeting TGF-β receptors displayed reduced BMP-6-induced ALP activity. Furthermore, pharmacologic inhibitors blocking the TGF-β type I receptor impaired BMP-induced ALP activity in KS483 and C2C12 cells and mineralization of KS483 cells. Consistently, costimulation with BMPs and TGF-β further increased expression of osteoblast-specific genes, ALP activity, and mineralization of KS483 cells and primary mesenchymal stem cells compared with BMPs alone. The stimulatory and inhibitory effects of TGF-β were found to depend on timing and duration of the costimulation. TGF-β inhibited BMP-induced activation of a BMP-Smad-dependent luciferase reporter, suggesting that the stimulatory effect of TGF-β is not due to increased BMP-Smad activity. TGF-β also inhibited the BMP-induced expression of the BMP antagonist noggin and prolonged BMP activity. In conclusion, TGF-β, besides acting as an inhibitor, also can, by dampening the noggin-mediated negative-feedback loop, enhance BMP-induced osteoblast differentiation, which might be beneficial in fracture healing.