RUNX3 plays an important role in mediating the BMP9-induced osteogenic differentiation of mesenchymal stem cells.

Although bone morphogenetic protein 9 (BMP9) is highly capable of promoting the osteogenic differentiation of mesenchymal stem cells (MSCs) both in vitro and in vivo, the molecular mechanisms involved remain to be fully elucidated. Runt-related transcription factor (RUNX)3 is an essential regulator of osteoblast/chondrocyte maturation. However, the exact role of RUNX3 in BMP9 osteoinductive activity is unknown. In this study, we sought to investigate the functional role of RUNX3 in the BMP9-induced osteogenic differentiation of MSCs. We found that BMP9 upregulated the endogenous expression of RUNX3 in MSCs. The overexpression or/and knockdown of RUNX3 both increased the levels of alkaline phosphatase (ALP) a marker of BMP9-induced early osteogenic differentiation. Nevertheless, matrix mineralization, a marker of BMP9-induced late osteogenic differentiation was enhanced by the overexpression of RUNX3, whereas it was inhibited by the knockdown of RUNX3. The BMP9-induced expression of osteogenic pivotal transcription factors [inhibitor of differentiation (Id)3, distal-less homeobox 5 (DLX5) and RUNX2)] was further increased by the overexpression of RUNX3; however, it was reduced by the knockdown of RUNX3. However, the expression levels of Id1 and Id2 were both enhanced by the overexpression or/and knockdown of RUNX3. The BMP9-induced phosphorylation of Smad1/5/8 was increased with the overexpression of RUNX3, and yet was decreased with the knockdown of RUNX3. Collectively, our findings suggest that RUNX3 is an essential modulator of the BMP9-induced osteoblast lineage differentiation of MSCs.