Abstract: BACKGROUND: Icariin is the main active ingredient of epimedium, a commonly used Chinese herbal medicine for the treatment of osteoporosis, but its molecular mechanism needs to be studied in depth.
OBJECTIVE: To explore the effect and mechanism of icariin on osteoporosis in mice.
METHODS: The osteoporosis models of C57BL/6 mice were constructed by subcutaneous injection of dexamethasone for 5 weeks. Totally 20 successfully modeled mice were randomly divided into a model group and a treatment group (n=10). An additional 10 non-modeled C57BL/6 mice were used as a control group. The treatment group was given a suspension of icariin and saline by intragastric administration; the model group and the control group were given the same amount of saline, once a day. The state of the mice was monitored every day during the administration, and the body weight was measured. Two months after the treatment, Micro-CT machine was used to scan the proximal tibia metaphysis to analyze the microstructure of the bone tissue in mice, and to study the therapeutic effect of icariin on osteoporosis in mice. Simultaneously, bone marrow mesenchymal stem cells were isolated and cultured in each group. The expression levels of alkaline phosphatase, osteogenic specific transcription factor, osteocalcin, transforming growth factor β, and RUNX family transcription factor 2 were detected by flow cytometry. The expression levels of osteopontin, bone sialoprotein, and peroxisome proliferator activated receptor γ mRNA were determined using qRT-PCR. Alizarin red staining and oil red O staining were applied to detect osteogenic and adipogenic differentiation.  
RESULTS AND CONCLUSION: (1) Compared with the control group, the bone wet weight, bone volume fraction, trabecular thickness, and trabecular number significantly decreased (P < 0.01), trabecular separation became larger (P < 0.001), and bone cortical thickness became thinner in the model group (P < 0.001). Compared with the model group, above indicators of mice in the treatment group were significantly improved (P < 0.05). (2) Compared with the model group, the expression of osteogenic related genes or proteins was significantly increased (P < 0.05), and the expression level of lipogenic related gene peroxisome proliferator activated receptor gamma mRNA was significantly decreased in the treatment group. (3) Compared with the model group, the absorbance value of alizarin red staining was significantly increased in mouse bone marrow mesenchymal stem cells of the treatment group (P < 0.05), while the absorbance value of oil red O staining was significantly reduced (P < 0.05). (4) The results suggest that icariin may relieve osteoporosis by promoting osteogenic differentiation of mouse bone marrow mesenchymal stem cells, inhibiting their adipogenic differentiation, improving bone marrow microenvironment, and finally promoting trabecular bone formation.

Key words: icariin, osteoporosis, trabecular bone, bone marrow mesenchymal stem cells, osteogenic differentiation