Oleanolic acid exerts an osteoprotective effect in ovariectomy-induced osteoporotic rats and stimulates the osteoblastic differentiation of bone mesenchymal stem cells in vitro

Objective: Oleanolic acid (OA) and its glycosides have been reported to prevent bone loss by inhibiting the formation of osteoclasts. However, because bone formation and resorption are balanced processes in bone metabolism, no studies have described the effect of OA on osteogenesis. The aim of the present study was to evaluate the osteoprotective effect of OA in rats with ovariectomy (OVX)-induced osteoporosis and to search for the molecular targets of OA in bone mesenchymal stem cells (bMSCs). Methods: Two-month-old female mice that underwent OVX were treated with OA (20 mg/kg a day). After 2 weeks and after 3 months, bone mass was evaluated by micro-CT, morphometry, and immunohistochemical detection. In addition, the expression of 256 genes was measured via microarray and confirmed by real-time reverse transcription- polymerase chain reaction. The effects of OA on the activities of bMSCs were also observed in vitro using alkaline phosphatase and cell proliferation assays. Results: Micro-CT displayed only a tendency for bone loss at 2 weeks but a decrease in bone mass at 3 months after OVX. OA treatment increased osteoblast number, increasing osteocalcin and runt-related protein 2 protein levels in vivo and facilitating the osteoblastic differentiation of bMSCs in vitro at doses of 10 ^-6 and 10 ^-5 M. Gene expression profile analysis revealed that OVX caused a marked dysregulation of gene expression, especially at 2 weeks, some of which was rescued by OA. Few of these genes overlapped, but their functions were involved in the Notch signaling pathway between two phases of the osteoporotic process. Conclusions: OA exerts an osteoprotective effect in OVX-induced osteoporotic rats and stimulates the osteoblastic differentiation of bMSCs in vitro. The molecular mechanism of this effect might be related to the Notch signaling pathway and requires further investigation.