Angiopoietins in angiogenesis and beyond

The angiopoietin (Ang) family of growth factors includes four members, all of which bind to the endothelial receptor tyrosine kinase Tie2. Two of the Angs, Ang-1 and Ang-4, activate the Tie2 receptor, whereas Ang-2 and Ang-3 inhibit Ang-1-induced Tie2 phosphorylation. While genetic models have underscored the importance of Angs in the developing cardiovascular system, other studies have demonstrated that Ang-1 promotes endothelial cell survival, sprouting and tube formation. More recently, a new aspect of the biology of this class of growth factors has emerged, namely the ability of Ang-1 to reduce inflammation. This review presents an outline of Angs and their receptors, examining their structure, expression, signalling, regulation and biological significance and comments on the role and potential usefulness of Angs in medicine.