TiO₂ Nanotubes Enhance Vascularization and Osteogenic Differentiation Through Stimulating Interactions Between Bone Marrow Stromal Cells and Endothelial Cells.

The effects of titanium dioxide nanotubes (TNs) on bone repair through affecting the behaviors of mesenchymal stem cells and endothelial cells have been extensively investigated. However, their effects on cell-cell interactions have rarely been reported. Herein, the structural effects (especially the nanotube diameter) of TNs on the cellular behaviors and cell-cell interaction of human bone marrow stromal cells (HBMSCs) and human umbilical vein endothelial cells (HUVECs) were investigated. Results demonstrated that, as compared to pure Ti foil, the TNs substrates could significantly improve cell proliferation of HBMSCs and HUVECs. In addition, co-cultured HBMSCs and HUVECs showed better adhesion and spreading than mono-cultured cells when they were cultured on the same substrates. Further investigation on co-cultures suggested that the structural cues of TNs could effectively enhance the cell-cell interactions of co-cultured HBMSCs-HVUECs by stimulating cell-cell communications through cell junctions and paracrine effects, which subsequently upregulated the expression of typical osteogenic differentiation genes and vascularization genes and accelerated the vascularization of co-cultured HUVECs and osteogenic differentiation of co-cultured HBMSCs. Besides, TNs with diameter of 100 nm showed preferable stimulatory effects on cell proliferation and differentiation than TNs with diameter of 30 nm. Taken together, structural cues of TNs can significantly affect cell-cell interactions and it can be anticipated that vascularized bone tissues can be constructed by regulating the cell-cell interactions of osteogenic cells and endothelial cells with optimized TNs structural cues, which could eventually result in the enhancement of bone healing in vivo.