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Abstract
Vascular endothelial growth factor (VEGF), which acts via members of a family of endothelial-specific
receptor tyrosine kinases, is the only factor that has been shown definitively to
play a role in the formation of the embryonic vasculature. Only one other family of
receptor tyrosine kinases, comprising TIE1 and TIE2, is largely endothelial cell specific.
We have recently cloned a ligand for TIE2, termed Angiopoietin-1. Here we show that
mice engineered to lack Angiopoietin-1 display angiogenic deficits reminiscent of
those previously seen in mice lacking TIE2, demonstrating that Angiopoietin-1 is a
primary physiologic ligand for TIE2 and that it has critical in vivo angiogenic actions
that are distinct from VEGF and that are not reflected in the classic in vitro assays
used to characterize VEGF. Angiopoietin-1 seems to play a crucial role in mediating
reciprocal interactions between the endothelium and surrounding matrix and mesenchyme.