Bone marrow stromal cells and endothelial cells exhibit an instructional reciprocal relationship

Objectives: The aim of these studies was to determine if endothelial cells (ECs) and bone marrow stromal cells (BMSCs) modulate the growth and differentiation of each other . Microvascular ECs have been shown to be an integral component of the bone microenvironment, and it has previously been shown that osteogenic cells can produce angiogenic factors. Therefore, BMSCs could potentially secrete angiogenic factors that accelerate the vasculature of developing bone tissue, thereby enhancing growth of bone tissue. Methods: ECs were cultured in the presence of BMSC conditioned media (CM), and EC survival and differentiation (sprout formation) quantified. ELISAs were used to quantify levels of Vascular Endothelial Growth Factor (VEGF) in BMSC CM, and experiments utilizing VEGF blocking antibodies were performed to confirm its role in EC survival and differentiation. RNA was isolated from ECs and analyzed by RT-PCR to determine expression of bone morphogenetic proteins (BMPs). Co-culture studies were performed to evaluate the effects of ECs on BMSC differentiation. Results: BMSCs secreted 2.65 ng VEGF/million cells/day and the addition of BMSC CM to endothelial cell cultures significantly enhanced endothelial cell survival and sprouting in culture. Addition of VEGF blocking Abs completely eliminated the BMSC CM effects on ECs. In RNA samples from ECs, BMP-2 mRNA, but not BMP-4 and BMP-7, was detected. In addition, alkaline phosphatase activity was higher in co-cultures of ECs and BMSCs than in cultures of BMSCs alone. Conclusions: The results suggest that BMSCs secrete sufficient VEGF to enhance the growth and differentiation of ECs. Conversely, we found that ECs express BMP-2 and when co-cultured with BMSCs, enhance differentiation of BMSCs. These studies indicate that ECs and BMSCs can augment the growth of one another, an interaction critical to enhancing bone formation, particularly in tissue engineering applications. DK was supported by a grant from the NIDCR (F30-DE05747).