Opposite effects of bone morphogenetic protein-2 and transforming growth factor-beta1 on osteoblast differentiation.

Several members of the transforming growth factor-beta (TGF-beta) superfamily have been demonstrated to play regulatory roles in osteoblast differentiation and maturation, but the mechanisms by which they act on different cells at different developmental stages remain largely unknown. We studied the effects of TGF-beta1 and bone morphogenetic protein-2 (BMP-2) on the differentiation/maturation of osteoblasts using the murine cell lines MC3T3-E1 and C3H10T1/2. BMP-2 induced or enhanced the expression of the osteoblast differentiation markers alkaline phosphatase (ALP) and osteocalcin (OC) in both cells. In contrast, TGF-beta1 was not only unable to induce these markers, but it dramatically inhibited BMP-2-mediated OC gene expression and ALP activity. In addition, TGF-beta1 inhibited the ability of BMP-2 to induce MC3T3-E1 mineralization. TGF-beta1 did not sensibly modify the increase of Osf2/Cbfa1 gene expression mediated by BMP-2, thus demonstrating that the inhibitory effect of TGF-beta1 on osteoblast differentiation/maturation mediated by BMP-2 was independent of Osf2/Cbfa1 gene expression. Finally, it is shown that TGF-beta1 does not affect BMP-2-induced Smad1 transcriptional activity in the mesenchymal pluripotent cells studied herein. Our data indicate that in vitro BMP-2 and TGF-beta1 exert opposite effects on osteoblast differentiation and maturation.