Tie2 belongs to the receptor tyrosine kinase family and functions as a receptor for Angiopoietin-1 (Ang1). Gene-targeting analyses of either Ang1 or Tie2 in mice reveal a critical role of Ang1-Tie2 signalling in developmental vascular formation. It remains elusive how the Tie2 signalling pathway plays distinct roles in both vascular quiescence and angiogenesis. We demonstrate here that Ang1 bridges Tie2 at cell-cell contacts, resulting in trans-association of Tie2 in the presence of cell-cell contacts. In clear contrast, in isolated cells, extracellular matrix-bound Ang1 locates Tie2 at cell-substratum contacts. Furthermore, Tie2 activated at cell-cell or cell-substratum contacts leads to preferential activation of Akt and Erk, respectively. Microarray analyses and real-time PCR validation clearly show the differential gene expression profile in vascular endothelial cells upon Ang1 stimulation in the presence or absence of cell-cell contacts, implying downstream signalling is dependent upon the spatial localization of Tie2.

Vascular endothelial (VE)-cadherin is a cell-cell adhesion molecule involved in endothelial barrier function. Here, we show that initial circumferential actin bundling induced by cyclic AMP-Epac-Rap1 signal and its linkage to VE-cadherin through α- and β-catenins lead to the stabilization of VE-cadherin at cell-cell contacts.

Angiopoietin-1 (Ang1) binds to and activates Tie2 receptor tyrosine kinase. Ang1-Tie2 signal has been proposed to exhibit two opposite roles in the controlling blood vessels. One is vascular stabilization and the other is vascular angiogenesis. There has been no answer to the question as to how Tie2 induces two opposite responses to the same ligand. Our group and Dr. Alitalo's group have demonstrated that trans-associated Tie2 at cell-cell contacts and extracellular matrix (ECM)-anchored Tie2 play distinct roles in the endothelial cells. The complex formation depends on the presence or absence of cell-cell adhesion. Here, we review how Ang1-Tie2 signal regulates vascular maintenance and angiogenesis. We further point to the unanswered questions that must be clarified to extend our knowledge of vascular biology and to progress basic knowledge to the treatment of the diseases in which Ang1-Tie2-mediated signal is central.