Borate-Ion-Stimulated Macrophages Promote Osteogenic Differentiation of Mesenchymal Stem Cells

Immune cells, such as macrophages, stimulated by several types of inorganic ions released from bioactive glasses secrete cytokines that promote and inhibit bone formation. In this study, the effects of borate-ion-stimulated mouse macrophages (RAW264) on the osteogenic differentiation of mouse bone marrow-derived mesenchymal stem cells (KUSA-A1) are investigated. KUSA-A1 is cultured with a borate-ion-containing medium and RAW264-conditioned medium, which contained the secretome released from boron-stimulated RAW264, and its osteogenic differentiation is evaluated. Results indicated that the secretome of RAW264 stimulated by more than 3 mg L^-1 borate ions promoted the expression level of Gla-osteocalcin in cells, and that of RAW264 stimulated by more than 10 mg L^-1 borate ions promoted collagen production. Borate ions inhibited KUSA-A1 calcification, whereas the RAW264-conditioned medium promoted it. On the contrary, borate ions promoted gene expression levels in RAW264 cells, including anti-inflammatory cytokine-related and bone morphogenetic protein-2-related genes. Therefore, the immune response of RAW264 stimulated by borate ions can promote the osteogenic differentiation and mineralization of KUSA-A1, indicating that the ability to elute borate ions will be useful in the design of glass materials for bone regeneration.