Cyclic tensile strain promotes the osteogenic differentiation of a bone marrow stromal cell and vascular endothelial cell co-culture system

• Optimization of BMSC/VEC co-culture ratios and cyclic tensile loading conditions for best osteogenic effect.
• Reveal the synergic effect of cyclic tensile strain and co-cultured VECs on BMSC osteogenic differentiation.
• The combined effect of tensile strain and VECs on BMSC osteogenesis relies on VEGF paracrine signaling pathway.

Mechanical stimuli and neovascularization are closely coupled to osteogenic differentiation and new bone formation. The purpose of present study was to detect the effect of cyclic mechanical strain on a co-culture system of bone marrow stromal cells (BMSCs) and vascular endothelial cells (VECs) and to clarify the related mechanisms. Primary BMSCs and VECs were isolated from Sprague-Dawley rats and co-cultured at various ratios (1:0, 1:2, 1:4, 4:1, 2:1, 1:1, and 0:1). To determine optimized loading conditions, the cells were then subjected to various cyclic tensile strains (0%, 3%, 6% and 9%) using a Flexcell 5000 mechanical loading system. A protocol of 6% strain on the co-cultured cells at a 1:1 ratio was selected as the optimized culture conditions based on the best osteogenic effects, which included increased ALP activity, matrix mineralization and the expressions of VEGF, Runx-2 and Col-1. The VEGF-R inhibitor tivozanib was used to analyze the paracrine role of VEGF, and the osteogenesis-promoting effects of 6% tensile strain were abrogated in the co-cultured cells treated with tivozanib. These results demonstrate that cyclic tensile strain promotes osteogenic differentiation in BMSC/VEC co-culture systems, possibly via a VEC-mediated paracrine effect of VEGF on BMSCs.