Repression of Runx2 function by TGF-β through recruitment of class II histone deacetylases by Smad3

Transforming growth factor-β (TGF-β) inhibits osteoblast differentiation through inhibition of the function of Runx2 (Cbfa1) by Smad3. The mechanism through which TGF-β/Smad3 inhibits Runx2 function has not been characterized. We show that TGF-β induces histone deacetylation, primarily of histone H4, at the osteocalcin promoter, which is repressed by TGF-β, and that histone deacetylation is required for repression of Runx2 by TGF-β. This repression occurs through the action of the class IIa histone deacetylases (HDAC)4 and 5, which are recruited through interaction with Smad3 to the Smad3/Runx2 complex at the Runx2-binding DNA sequence. Accordingly, HDAC4 or 5 is required for efficient TGF-β-mediated inhibition of Runx2 function and is involved in osteoblast differentiation. Our results indicate that class IIa HDACs act as corepressors for TGF-β/Smad3-mediated transcriptional repression of Runx2 function in differentiating osteoblasts and are cell-intrinsic regulators of osteoblast differentiation.