ANGIOPOIETIN-1 INDUCES SPROUTING ANGIOGENESIS IN-VITRO

Sprouting of new capillaries from pre-existing blood vessels is a hall mark of angiogenesis during embryonic development and solid tumor grow th [1]. In addition to the vascular endothelial growth factor (VEGF) a nd its receptors, the Tie receptors and their newly identified ligands , the angiopoietins, have been implicated in the control of blood vess el formation [2 3], Although 'knockouts' of the gene encoding the Tie2 receptor, or its activating ligand angiopoietin-1 (Ang1), result in e mbryonic lethality in mice due to ail absence of remodeling and sprout ing of blood vessels [4,5], biological activity in vitro has not yet b een described for this receptor-ligand system. In an assay in which a monolayer of endothelial cells were cultured on microcarrier beads and embedded in three-dimensional fibrin gels, recombinant Ang1 (0.5-10 n M) induced the formation of capillary sprouts in a dose-dependent mann er that was completely inhibited by soluble Tie2 receptor extracellula r domains. In contrast with VEGF, which also induced sprouting of capi llaries, Ang1 was only very weakly mitogenic for endothelial cells. Su boptimal concentrations of VEGF and Ang1 acted synergistically to indu ce sprout formation. Thus, the biological activity of Ang1 in vitro is consistent with the specific phenotype of mice deficient in Tie2 or A ng1. The data suggest that, like in other developmental systems, blood vessel formation requires a hierarchy of master-control genes in whic h VEGF and angiopoietins, along with their receptors, are amongst the most important regulators. (C) Current Biology Ltd ISSN 0960-9822.