Synergistic effect of strontium and silicon in strontium-substituted sub-micron bioactive glass for enhanced osteogenesis.

Strontium-substituted sub-micron bioactive glasses (Sr-SBG) have been reported to have enhanced osteogenic differentiation capacity compared to sub-micron bioactive glasses (SBG) in our previous study. However, the underlying molecular mechanisms of such beneficial effect of Sr-SBG are still not fully understood. In this study, we synthesized Sr-SBG, studied the effects of Sr-SBG on proliferation and osteogenic differentiation of mouse mesenchymal stem cells (mMSCs), and identified the molecular mechanisms of the enhancement effect of Sr-SBG on mMSCs. The results demonstrated that Sr-SBG had more profound promotion effect on proliferation and osteogenic differentiation of mMSCs than SBG and SrCl2 group which containing identical Sr concentration with Sr-SBG group. RT-qPCR and western blot analysis showed that the mRNA expressions and protein expressions involved in NFATc and Wnt/β-catenin signaling pathways were all upregulated mediated by Sr-SBG, while only Wnt/β-catenin signaling pathway related genes upregulated in SBG group and only NFATc signaling pathway activated in SrCl2 group, suggesting that NFATc and Wnt/β-catenin signaling pathways played important roles in osteogenesis enhancement induced by Sr-SBG. To conform the above conclusion, cyclosporin A (CSA) was applied to inhibit NFATc signaling pathway. It was found that the enhanced osteogenic differentiation of mMSCs induced by Sr-SBG was partially abrogated and the activated Wnt/β-catenin signaling pathway was also inhibited in part. However, the effects of SBG on proliferation and osteogenesis of mMSCs were unimpaired, yet the effects of SrCl2 were greatly suppressed. Taken together, these results indicated that strontium activated NFATc signaling pathway and silicate activated Wnt/β-catenin signaling pathway might synergistically mediated the enhanced osteogenesis induced by Sr-SBG.