Vascular basement membrane is an important regulator of angiogenesis and undergoes many alterations during angiogenesis and these changes are speculated to influence neovascularization. Recently, fragments of collagen molecules have been identified to possess antiangiogenic activity. Tumstatin (␣3(IV)NC1 domain) is one such novel molecule with distinct anti-tumor properties and possesses an N-terminal (amino acids 54 -132) anti-angiogenic and a C-terminal (amino acids 185-203) anti-tumor cell

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... tivity (Maeshima, Y., et al. 2000) J. Biol. Chem. 275, 21340 -21348). Previous studies have identified the 185-203 amino acid sequence as a ligand for ␣ v ␤ 3 integrin (Shahan, T. A., et al. (1999) Cancer Res. 59, 4584 -4590). In the present study, we found distinct additional RGD-independent ␣ v ␤ 3 integrin binding site within 54 -132 amino acids of tumstatin. This site is not essential for inhibition of tumor cell proliferation but necessary for the anti-angiogenic activity. A fragment of tumstatin containing 54 -132 amino acid (tum-2) binds both endothelial cells and melanoma cells but only inhibited proliferation of endothelial cells, with no effect on tumor cell proliferation. A similar experiment with fragment of tumstatin containing the 185-203 amino acid (tum-4) demonstrates that it binds both endothelial cells and melanoma cells but only inhibits the proliferation of melanoma cells. The presence of cyclic RGD peptides did not affect the ␣ v ␤ 3 integrin-mediated activity of tumstatin, although significant inhibition of endothelial cell binding to vitronectin was observed. The two distinct RGD-independent binding sites on tumstatin suggest unique ␣ v ␤ 3 integrin-mediated mechanisms governing the two distinct anti-tumor properties of tumstatin.