PURPOSE: Endostatin, a peptide derived from proteolysis of collagen XVIII, is an 
endogenous inhibitor of angiogenesis and tumor growth. We have synthesized five 
peptide fragments designed to cover the whole length of the endostatin molecule 
(containing 40-50 amino acids each) with the aim of exploring the possibility 
that a specific sequence within the molecule might be responsible for its 
antiangiogenic effects.
EXPERIMENTAL DESIGN: The five peptide sequences, termed fragments I, II, III, 
IV, and IVox, the latter bearing the original disulfide bond Cys(135)-Cys(165), 
were investigated for their effects on cultured endothelial cells, on enzyme 
activities related to angiogenesis, on tube formation in three-dimensional gel 
matrices, in vivo in the avascular rabbit cornea assay, and in an experimental 
tumor burden paradigm.
RESULTS: Both the fragment covering the COOH-terminal endostatin region, 
fragment IV, and particularly fragment IVox, retained the angioinhibitory 
effects of endostatin. Fragment IVox strongly inhibited endothelial cell 
migration and proliferation, in vitro tube formation, and in vivo angiogenesis, 
displaying a potency comparable with that of endostatin. When tested in vivo on 
tumor growth, fragment IVox demonstrated to be more effective than full-length 
endostatin. Unexpectedly, fragment III exhibited proangiogenic activity, 
increasing endothelial cell migration, producing neovascularization to an extent 
similar to that of vascular endothelial growth factor, and enhancing the 
angiogenic response to vascular endothelial growth factor in the cornea assay. 
Peptides encompassing the NH(2)-terminal region of endostatin (fragments I and 
II) had negligible effects on angiogenesis.
CONCLUSIONS: In view of these results, which show strikingly distinct profiles 
of endostatin fragments, we propose that the amino acid sequence of endostatin 
contains both angiosuppressive and angiostimulatory domains.