Human endothelial cells inhibit BMSC differentiation into mature osteoblasts in vitro by interfering with osterix expression

It is well accepted, that there is communication between osteoblasts and endothelial cells. However, the influence of endothelial cells on the differentiation of bone precursors into mature osteoblasts is not yet well understood. We therefore studied the effect of human umbilical vein endothelial cells (HUVEC) on human bone marrow stromal cell (BMSC) differentiation towards an osteoblastic phenotype by culturing them in two different types of HUVEC-BMSC cultures (indirect contact, HUVEC-conditioned medium). Typical bone markers (Ca-45 incorporation, ALP activity, and gene expression of collagen 1 (COLI), osteonectin (ON), matrix metalloproteinase 13 (MMP-13), bone morphogenetic protein 2 (BMP-2)) and transcription factors (Runx2, osterix (OSX)) were evaluated at different time points. The effect of stimulating HUVEC with vascular endothelial growth factor (VEGF) before co-cultures with BMSC was also evaluated. As expected, BMSC in osteogenic medium (OM) (with dexamethasone) differentiated towards the osteoblastic phenotype, as measured by increased matrix mineralization, high ALP activity, and elevated expression of specific osteoblastic marker genes, when compared to BMSC in non-OM. HUVEC reversibly inhibited osteoblastic differentiation of BMSC in OM, independent of the co-culture type. When HUVEC were removed from the cultures, BMSC differentiated into osteoblasts, albeit at a faster rate than BMSC that were never co-cultured with HUVEC. Stimulation of HUVEC with VEGF before co-culture enhanced the inhibitory effect of HUVEC on BMSC differentiation. This inhibitory effect was connected to a reversible suppression of OSX gene expression and was specific to endothelial cells. We conclude that HUVEC can inhibit dexamethasone-induced BMSC differentiation into osteoblasts in vitro, by interfering with OSX expression, thereby arresting BMSC differentiation at a preosteoblastic stage. J. Cell. Biochem. 98: 992-1006, 2006. (c) 2006 Wiley-Liss, Inc.