Abstract: BACKGROUND: Bone marrow mesenchymal stem cells play a pivotal role in tissue engineering and bone regeneration. However, promoting the osteogenic differentiation of bone marrow mesenchymal stem cells poses a significant challenge.  
OBJECTIVE: To examine the influence of Fe3O4@ZIF-8 nanoparticles on the osteogenic differentiation of bone marrow mesenchymal stem cells under magnetic stimulation.
METHODS: Zeolite imidazolate skeleton (ZIF-8) was synthesized by hydrothermal method, and magnetic Fe3O4@ZIF-8 nanoparticles were synthesized by one-pot method (2.5, 5, 10, and 20 µg Fe3O4 were added to the preparation materials, respectively). The Fe3O4@ZIF-8 nanoparticles were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and vibration sample magnetometer detection, and suitable materials were selected for subsequent experiments. Bone marrow mesenchymal stem cells of 4-week-old SD rats were extracted and co-cultured with Fe3O4@ZIF-8 nanoparticle solution with different mass concentrations (25, 50, 75, 100, and 125 μg/mL), respectively. Cell proliferation was detected by CCK-8 assay, and the optimal material solution mass concentration was selected. After the mass concentration of the material solution was screened, magnetic stimulation was applied (magnetic field intensity was 0, 50, 100, and 150 MT, respectively). Cell proliferation was detected by CCK-8 assay, and the best magnetic field intensity and Fe3O4@ZIF-8 nanoparticles were selected for the experiment of induced differentiation of bone marrow mesenchymal stem cells. SD rat bone marrow mesenchymal stem cells were co-cultured with ZIF-8, Fe3O4@ZIF-8, and Fe3O4@ZIF-8 (magnetic field intervention) nanoparticle solution, respectively. The single cultured cells were used as blank controls. Lipid induction was followed by oil red O staining. After osteogenesis induction, alkaline phosphatase, alizarin red staining and Runx2 protein concentration were detected. 
RESULTS AND CONCLUSION: (1) Under scanning electron microscopy, Fe3O4@ZIF-8 nanoparticles showed a dodecahedral structure. With the increase of Fe3O4 content in the material, the particle size of the nanoparticles increased. Fe3O4@ZIF-8 nanoparticles (5 and 10 µg Fe3O4 was added to the material preparation) with a particle size of about 250 nm (stable functional and biosafety of nanoparticles at this particle size) were selected. (2) The results of CCK-8 assay showed that 50 μg/mL Fe3O4@ZIF-8 nanoparticles (with 10 µg Fe3O4 added to the preparation of the material) could significantly promote the proliferation of bone marrow mesenchymal stem cells under a 100 MT magnetic field. The nanoparticles under this condition were selected for the osteogenic induction differentiation experiment of bone marrow mesenchymal stem cells. (3) After osteogenic induction, the alkaline phosphatase activity, extracellular matrix mineralization degree, and Runx2 protein mass concentration of bone marrow mesenchymal stem cells in Fe3O4@ZIF-8 (magnetic field intervention) group were higher than those in other three groups (P < 0.05). After adipogenic induction, the lipid droplet formation of bone marrow mesenchymal stem cells in Fe3O4@ZIF-8 (magnetic field intervention) group was lower than that in the other three groups (P < 0.05). (4) The results show that Fe3O4@ZIF-8 nanoparticles can promote osteogenic differentiation of bone marrow mesenchymal stem cells under specific magnetic field conditions.

Key words: Fe3O4@ZIF-8, magnetic nanoparticle, magnetic stimulation, bone marrow mesenchymal stem cell, osteogenic differentiation