Direct cell adhesion to the angiopoietins mediated by integrins

Genetic ablation of angiopoietin-1 (Ang-l) or of its cognate receptor, Tie2 , disrupts angiogenesis in mouse embryos. The endothelial cells in growing blood vessels of Ang-l knockout mice have a rounded appearance and are poor ly associated with one another and their underlying basement membranes (Dum ont, D. J., Gradwohl, G., Fong, G. H., purl, M. C., Gertsenstein, M., Auerb ach, A, and Breitman, M. L. (1994) Genes Dev. 8, 1897-1909; Sate, T. N., To zawa, Y., Deutsch, U., Wolburg-Buchholz, K., Fujiwara, Y., Gendron-Maguire, M., Gridley, T., Wolburg, H., Risau, W., and Qin, Y. (1995) Nature 376, 70 -74; Suri, C., Jones, P. F., Patan, S., Bartunkova, S., Maisonpierre, P. C. , Davis, S., Sate, T. N., and Yancopoulos, G. D. (1996) Cell 87, 1171-1180) . It is therefore possible that Ang-l regulates endothelial cell adhesion. In this study we asked whether Ang-l might act as a direct substrate for ce ll adhesion. Human umbilical vein endothelial cells (HUVECs) plated for a b rief period on different substrates were found to adhere and spread well on Ang-l. Similar results were seen on angiopoietin-2 (Ang-2)-coated surfaces , although cells did not spread well on Ang-2. Ang-l, but not Ang-2, suppor ted HUVEC migration, and this was independent of growth factor activity. Wh en the same experiments were done with fibroblasts that either lacked, or s tably expressed, Tie2, results similar to those with HUVECs were seen, sugg esting that adhesion to the angiopoietins was independent of Tie2 and not l imited to endothelial cells. Interestingly, when integrin-blocking agents w ere included in these assays, adhesion to either angiopoietin was significa ntly reduced. Moreover, Chinese hamster ovary-B2 cells lacking the a, integ rin subunit did not adhere to Ang-l, but they did adhere to Ang-8. Stable e xpression of the human a, integrin subunit in these cells rescued adhesion to Ang-l and promoted an increase in adhesion to Ang-a, We also found that Ang-l and Ang-8 bind rather selectively to vitronectin. These results sugge st that, beyond their role in modulating Tie2 signaling, Ang-l and Ang-2 ca n directly support cell adhesion mediated by integrins.