BMP2 and VEGF Promote Angiogenesis but Retard Terminal Differentiation of Osteoblasts in Bone Regeneration by Up-regulating Id1

Inadequate vascularization limits the repair of bone defects. In order to improve angiogenesis and accelerate osteogenesis, the synergism of co-cultured cells with genetic modification in bone regeneration was investigated in this study. Endothelial progenitor cells (EPCs) and bone marrow stem cells (BMSCs) were transfected with the genes of vascular endothelial growth factor (VEGF) and bone morphogenetic protein 2 (BMP2) by adenovirus, respectively. The co-cultured cells, designated as four groups including BMSC + EPC, Ad-BMP2-BMSC + EPC, BMSC + Ad-VEGF-EPC, and Ad-BMP2-BMSC + Ad-VEGF-EPC groups, were seeded on an alginate gel and then implanted into rat intramuscularly to evaluate the effects on angiogenesis and osteogenesis. Both VEGF and BMP2 could induce the overexpression of inhibitor of DNA-binding 1(Id1) gene which significantly promoted tube formation in vitro and increase the amount of blood vessels in the Ad-BMP2-BMSC + Ad-VEGF-EPC group after implantation. Nevertheless, overexpression of Id1 retarded the terminal differentiation of osteoblasts and the bone formation. Later, osteogenic gene expression at transcriptional level, calcium nodules, and alkaline phosphatase (ALP) activity showed a gradual decrease and the amount of newly formed osteogenesis area exhibited a small increase in the Ad-BMP2-BMSC + Ad-VEGF-EPC group. This finding suggests that a balanced regulation of Id1 expression in VEGF-EPCs and BMP2-BMSCs may be critical to cell-based and gene-based approaches for bone regeneration.