Double-edged effects and mechanisms of Zn2+ microenvironments on osteogenic activity of BMSCs: osteogenic differentiation or apoptosis

Zinc-incorporated biomaterials show promoting effects on osteogenesis; however, excessive zinc ions lead to cytotoxic reactions and also have other adverse effects. Therefore, the double-edged effects of Zn2+ microenvironments on osteogenesis may become critical issues for new material development. This study systematically investigated the bidirectional influences of diverse Zn2+ microenvironments on the cell adhesion, proliferation, osteogenic differentiation and apoptosis of rBMSCs. Furthermore, the mechanisms of zinc-induced osteogenic differentiation of rBMSCs and of cell apoptosis induced by high concentration of Zn2+ were both discussed in detail. The results indicated that the Zn2+ microenvironments of 2 μg mL−1 and 5 μg mL−1 effectively improved the initial adhesion and proliferation of rBMSCs, while that of 15 μg mL−1 had exactly the opposite effect. More importantly, the suitable Zn2+ microenvironments (2 μg mL−1 and 5 μg mL−1) moderately increased the intracellular Zn2+ concentration by regulating zinc transportation, and then activated the MAPK/ERK signaling pathway to induce the osteogenic differentiation of rBMSCs. In contrast, the high Zn2+ concentration (15 μg mL−1) not only inhibited the osteogenic differentiation of rBMSCs by damaging intracellular zinc homeostasis, but also induced rBMSC apoptosis by enhancing intracellular ROS generation. The current study clarified the double-edged effects of Zn2+ microenvironments on the osteogenic properties of rBMSCs and the related mechanisms, and may provide valuable guidance for optimizing the design of zinc-doped biomaterials and zinc-based alloys.