Endothelial Cell Modulation of Bone Marrow Stromal Cell Osteogenic Potential

Objectives: Microvascular endothelial cells (ECs) have been shown to be an integral component of the bone microenvironment, comprising blood vessels which deliver oxygen and nutrients to developing bone tissue. ECs themselves may also directly regulate the growth and differentiation of osteo-progenitor cells through production of osteoinductive factors. We have previously shown that bone marrow stromal cells (BMSCs) can enhance endothelial cell survival and differentiation. The aim of these studies was to determine if ECs reciprocally modulate the osteogenic differentiation of BMSCs. Methods: BMSCs were cultured in the presence of EC conditioned medium (CM), in medium shared with ECs, and in co-culture with endothelial cells to evaluate the effects of ECs on BMSC differentiation. Osteogenic differentiation was quantified through alkaline phosphatase activity. RNA and protein were isolated from ECs and analyzed to determine production of bone morphogenetic proteins (BMPs). Results: In co-cultures of BMSCs and ECs, BMSCs exhibited as much as a two-fold increase in osteogenic differentiation compared to cultures of BMSCs alone (control). Osteogenic differentiation was also enhanced as a function of increased EC number. CM from ECs did not affect BMSC differentiation, although BMSCs cultured in the presence of shared medium with ECs showed increased osteogenic differentiation compared to controls. In RNA samples from ECs, BMP-2 mRNA, but neither BMP-4 nor BMP-7, was detected. Western blotting also confirmed the production of BMP-2 by ECs. Conclusions: The results indicate that ECs enhance the osteogenic activity of BMSCs when in direct contact and through the production of a soluble factor. The observed BMP-2 production by ECs could play a key role in this interaction. These studies demonstrate that ECs can directly modulate the differentiation of BMSCs, a finding with significant implications for bone regeneration and tissue engineering. DK was supported by a grant from the NIDCR (F30-DE05747).