Abstract: Objective To study the role of miR-30a in bone morphogenetic protein 9 (BMP9) induced osteogenic differentiation of mesenchymal stem cell line C2C12. Methods C2C12 cells were treated with BMP9 conditioned medium to induce osteogenic differentiation. Real-time PCR was used to detect the expression of miR-30 family members in BMP9-induced osteogenic differentiation of C2C12 cells. Effects of miR-30a on BMP9-induced osteogenic differentiation of C2C12 cells were detected by alkaline phosphatase(ALP)，osteocalcin(OCN) and calcium deposition. MTT and cell cycle analysis were used to reveale cell proliferation. Results The expression of only one miR-30 family member, miR-30a, first decreased and then increased during BMP9-induced osteogenic differentiation of C2C12 cells.Over-expression of miR-30a ，which targets on Runx2，led to expression of an early osteogenic marker and a reduction in ALP expression. In addition, we observed decreases in the expression of later osteogenic markers Osteocalcin，as well as calcium deposition. Conclusions BMP9-induced osteogenic differentiation is partially inhibited by miR-30a in mesenchymal stem cell line C2C12.

Key words: miR-30a , Bone morphogenetic protein 9, C2C12, osteogenic differentiation