Background/Aims: Osteogenic differentiation of mesenchymal stem cells (MSCs) plays a crucial role in bone regeneration and bone reparation. This complex process is regulated precisely and firmly by specific factors. Recent studies have demonstrated that miR-125b regulates osteogenic differentiation, but little is known about the molecular mechanisms of this regulation. Furthermore, how miR-125b regulates the osteogenic differentiation of MSCs still needs elucidation. Methods: In the present study, human bone marrow-derived mesenchymal stem cells (hBMSCs) were isolated and induced to osteoblasts with miR-125b inhibition or overexpression. qRT-PCR and western blot analysis were used to detect the expression of osteogenic marker genes and proteins. Alkaline phosphatase (ALP) and Alizarin Red (ARS) staining were performed to evaluate the osteoblast phenotype. TargetScan, PicTar and miRanda database were used to predict the target gene of miR-125b. Dual luciferase reporter assay and RNA interference were performed to verify the target gene. Micro-CT imaging and histochemical staining were used to investigate the bone defect repair capacity of miR-125b in vivo. Results: We observed that miR-125b was expressed at a low level during the osteogenic differentiation of hBMSCs. Then, we found that osteogenic marker genes were negatively regulated by miR-125b during the course of osteogenic differentiation, suggesting that miR-125b down regulation plays an important role in the process of osteogenic differentiation. Bioinformatics approaches using miRNA target prediction algorithms indicated that the bone morphogenetic protein type Ib receptor (BMPR1b) is a potential target of miR-125b. The results of the dual luciferase reporter assay indicated that miR-125b binds to the 3’-UTR of the BMPR1b gene. We observed that knockdown of BMPR1b by siRNA inhibited the osteogenic differentiation of hBMSCs. Furthermore, by co-transfecting cells with an miR-125b inhibitor and si-BMPR1b, we found that the osteogenic capacity of the cells transfected with miR-125b inhibitor was blocked upon knockdown of BMPR1b. In vivo, demineralized bone matrix (DBM) was composited with hBMSCs as a scaffold to repair segmental femoral defects. By inhibiting the expression of miR-125b, hBMSCs showed a better capacity to repair bone defects. Conclusions: Taken together, our study demonstrated that miR-125b regulated the osteogenic differentiation of hBMSCs by targeting BMPR1b and that inhibiting miR-125b expression could enhance the capacity of bone defect repair in vivo.

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