Runx2 is a common target of transforming growth factor beta 1 and bone morphogenetic protein 2, and cooperation between Runx2 and Smad5 induces osteoblast-specific gene expression in the pluripotent mesenchymal precursor cell line C2C12

When C2C12 pluripotent mesenchymal precursor cells are treated with transfo rming growth factor beta2 (TGF-beta1), terminal differentiation into myotub es is blocked. Treatment with bone morphogenetic protein 2 (BMP-2) not only blocks myogenic differentiation of C2C12 cells but also induces osteoblast differentiation. The molecular mechanisms governing the ability of TGF-bet a1 and BMP-2 to both induce ligand-specific responses and inhibit myogenic differentiation are not known. We identified Runx2/PEBP2 alphaA/Cbfa1, a gl obal regulator of osteogenesis, as a major TGF-beta1-responsive element bin ding protein induced by TGF-beta1 and BMP-2 in C2C12 cells. Consistent with the observation that Runx2 can be induced by either TGF-beta1 or BMP-2, th e exogenous expression of Runx2 mediated some of the effects of TGF-beta1 a nd BMP-2 but not osteoblast-specific gene expression, Runx2 mimicked common effects of TGF-beta1 and BMP-2 by inducing expression of matrix gene produ cts (for example, collagen and fibronectin), suppressing MyoD expression, a nd inhibiting myotube formation of C2C12 cells. For osteoblast differentiat ion, an additional effector, BMP-specific Smad protein, was required. Our r esults indicate that Runx2 is a major target gene shared by TGF-beta and BM P signaling pathways and that the coordinated action of Runx2 and BMP-activ ated Smads leads to the induction of osteoblast-specific gene expression in C2C12 cells.