TIE2 RECEPTOR LIGANDS, ANGIOPOIETIN-1 AND ANGIOPOIETIN-2, MODULATE VEGF-INDUCED POSTNATAL NEOVASCULARIZATION

Angiopoietin-1 (Ang1) has been recently identified as the major physio logical ligand for the tyrosine kinase receptor Tie2 and assigned resp onsibility for recruiting and sustaining periendothelial support cells . Angiopoietin-2 (Ang2) was found to disrupt blood vessel formation in the developing embryo by antagonizing the effects of Ang1 and Tie2 an d was thus considered to represent a natural Ang1/Tie2 inhibitor. In v ivo effects of either angiopoietin on postnatal neovascularization, ho wever, have not been previously described. Accordingly, we used the co rnea micropocket assay of neovascularization to investigate the impact of angiopoietins on neovascularization in vivo. Neither Ang1 nor Ang2 alone promoted neovascularization. Pellets containing vascular endoth elial growth factor (VEGF) alone induced corneal neovascularity extend ing from the limbus across the cornea. Addition of Ang1 to VEGF (Ang1VEGF) produced an increase in macroscopically evident perfusion of the corneal neovasculature without affecting macroscopic measurements of length (0.58+/-0.03 mm) or circumferential neovascularity (136+/-10 de grees). In contrast, pellets containing Ang2+VEGF promoted significant ly longer (0.67+/-0.05 mm) and more circumferential (160+/-15 degrees) neovascularity than VEGF alone or Ang1+VEGF (P<0.05). Excess soluble Tie2 receptor (sTie2-Fc) precluded modulation of VEGF-induced neovascu larization by both Ang2 and Ang1. Fluorescent microscopic findings dem onstrated enhanced capillary density (fluorescence intensity, 2.55+/-0 .23 e(+9) versus 1.23+/-0.17 e+9, P<0.01) and increased luminal diamet er of the basal limbus artery (39.0+/-2.8 versus 27.9+/-1.3 mu m, P<0. 01) for Ang1+VEGF compared with VEGF alone. In contrast to Ang1+VEGF, Ang2+VEGF produced longer vessels and, at the tip of the developing ca pillaries, frequent isolated sprouting cells. In the case of Ang2+VEGF , however, luminal diameter of the basal limbus artery was not increas ed (26.7+/-1.9 versus 27.9+/-1.3, P=NS). These findings constitute wha t is to our knowledge the first direct demonstration of postnatal bioa ctivity associated with either angiopoietin. In particular, these resu lts indicate that angiopoietins may potentiate the effects of other an giogenic cytokines. Moreover, these findings provide in vivo evidence that Ang1 promotes vascular network maturation, whereas Ang2 works to initiate neovascularization.