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 transforming growth factor beta 1 (TGF- beta 1), terminal differentiation into myotubes 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- beta 1 and BMP-2 to both induce ligand-specific responses and inhibit myogenic differentiation are not known. We identified Runx2/PEBP2 alpha A/Cbfa1, a global regulator of osteogenesis, as a major TGF- beta 1-responsive element binding protein induced by TGF- beta 1 and BMP-2 in C2C12 cells. Consistent with the observation that Runx2 can be induced by either TGF- beta 1 or BMP-2, the exogenous expression of Runx2 mediated some of the effects of TGF- beta 1 and BMP-2 but not osteoblast-specific gene expression. Runx2 mimicked common effects of TGF- beta 1 and BMP-2 by inducing expression of matrix gene products (for example, collagen and fibronectin), suppressing MyoD expression, and inhibiting myotube formation of C2C12 cells. For osteoblast differentiation, an additional effector, BMP-specific Smad protein, was required. Our results indicate that Runx2 is a major target gene shared by TGF- beta and BMP signaling pathways and that the coordinated action of Runx2 and BMP-activated Smads leads to the induction of osteoblast-specific gene expression in C2C12 cells.