Runx2 Is a Common Target of Transforming Growth Factor β1 and Bone Morphogenetic Protein 2, and Cooperation between Runx2 and Smad5 Induces Osteoblast-Specific Gene Expression in the Pluripotent Mesenchymal Precursor Cell Line C2C12

TLDR

TGF‑β1 blocks myogenic differentiation of C2C12 cells, while BMP‑2 both blocks myogenesis and induces osteoblast differentiation, but the underlying mechanisms remain unclear. We found that Runx2/PEBP2αA/Cbfa1, a key osteogenic regulator, is strongly induced by both TGF‑β1 and BMP‑2 in C2C12 cells. Runx2 mediates common effects of TGF‑β1 and BMP‑2—promoting matrix gene expression, suppressing MyoD, and inhibiting myotube formation—but osteoblast‑specific gene expression requires additional BMP‑activated Smad signaling, and the combined action of Runx2 and Smads drives osteoblast differentiation.

Abstract

When C2C12 pluripotent mesenchymal precursor cells are treated with transforming growth factor β1 (TGF-β1), 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-β1 and BMP-2 to both induce ligand-specific responses and inhibit myogenic differentiation are not known. We identified Runx2/PEBP2αA/Cbfa1, a global regulator of osteogenesis, as a major TGF-β1-responsive element binding protein induced by TGF-β1 and BMP-2 in C2C12 cells. Consistent with the observation that Runx2 can be induced by either TGF-β1 or BMP-2, the exogenous expression of Runx2 mediated some of the effects of TGF-β1 and BMP-2 but not osteoblast-specific gene expression. Runx2 mimicked common effects of TGF-β1 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-β 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.