Induction of tube formation by angiopoietin-1 in endothelial cell/fibroblast co-culture is dependent on endogenous VEGF

The angiopoietin-1 (Ang1)/Tie2 receptor system is known to be important for angiogenesis and vascular remodeling. However, its contribution to the survival and morphogenesis of endothelial cells is still not well elucidated. In this study, we analyzed the role of the Ang1 /Tie2 pathway in cell survival and tube formation using a human umbilical vein endothelial (HUVE) cell and fibroblast co-culture system. In this system, which mimics angiogenesis in vivo, fibroblasts secrete a basal level of vascular endothelial growth factor (VEGF), and Ang1 stimulated tube formation. However, anti-VEGF or anti-VEGF receptor-2 neutralizing antibody blocked the Ang1-induced tube formation. Furthermore, other angiogenic factors such as hepatic growth factor (HGF) and basic fibroblast growth factor (bFGF) showed the same phenotype as Ang1, i.e., a stimulatory effect only in the presence of endogenous VEGF. The Ang1-promoted tube formation was mainly due to suppression of HUVE cell apoptosis in a PI3-kinase-dependent manner. These findings suggest that Ang1 stimulates tube formation in vivo via the PI3-kinase/Akt pathway, but this effect takes place only in a VEGF-dependent manner.