Abstract
Bone morphogenetic proteins (BMPs) induce not only bone formation in vivo but also osteoblast differentiation of mesenchymal cells in vitro. Tumor necrosis factor alpha (TNFalpha) inhibits both osteoblast differentiation and bone formation induced by BMPs. However, the molecular mechanisms of these inhibitions remain unknown. In this study, we found that TNFalpha inhibited the alkaline phosphatase activity and markedly reduced BMP2- and Smad-induced reporter activity in MC3T3-E1 cells. TNFalpha had no effect on the phosphorylation of Smad1, Smad5, and Smad8 or on the nuclear translocation of the Smad1-Smad4 complex. In p65-deficient mouse embryonic fibroblasts, overexpression of p65, a subunit of NF-kappaB, inhibited BMP2- and Smad-induced reporter activity in a dose-dependent manner. Furthermore, this p65-mediated inhibition of BMP2- and Smad-responsive promoter activity was restored after inhibition of NF-kappaB by the overexpression of the dominant negative IkappaBalpha. Although TNFalpha failed to affect receptor-dependent formation of the Smad1-Smad4 complex, p65 associated with the complex. Chromatin immunoprecipitation and electrophoresis mobility shift assays revealed that TNFalpha suppressed the DNA binding of Smad proteins to the target gene. Importantly, the specific NF-kappaB inhibitor, BAY11-7082, abolished these phenomena. These results suggest that TNFalpha inhibits BMP signaling by interfering with the DNA binding of Smads through the activation of NF-kappaB.