Tie2 is a tyrosine kinase receptor on endothelial cells that regulates the formation and maintenance of new blood vessels. Its ligands are the angiopoietins Ang1 and Ang2. Ang1 stimulates Tie2. Ang2 is more controversial: sometimes it appears to activate the receptor; sometimes it inhibits it. On p. 3551, Daniel Dumont and co-workers reveal that this could be because of what happens after ligand binding. They find that both angiopoietins activate Tie2 but Ang2 is less effective, and only Ang1 induces internalization of the receptor by endothelial cells. Surprisingly, the authors observe that, after binding to Tie2, Ang1 and Ang2 are released back into the medium before the receptor is internalized (most tyrosine kinase receptors are instead internalized with their ligands). They also observe that Ang2 is released more quickly than Ang1. Since Ang1/Ang2 can re-bind to fresh cells once they have been released, the authors propose that they may be recycled and/or reused by endothelial cells. Given the different rates of Ang1 and Ang2 release, this could account for some of the differences in their effects.
- © The Company of Biologists Limited 2006