Angiostatin, the N-terminal four kringles (K1-4) of plasminogen, blocks tumor-mediated angiogenesis and has great therapeutic potential. However, angiostatin's mechanism of anti-angiogenic action is unclear. We found that bovine arterial endothelial (BAE) cells adhere to angiostatin in an integrin-dependent manner and that integrins αvβ 3, α9β1, and to a lesser extent α4β1, specifically bind to angiostatin. αvβ3 is a predominant receptor for angiostatin on BAE cells, since a function-blocking antibody to αvβ 3 effectively blocks adhesion of BAE cells to angiostatin, but an antibody to α9β1 does not. ε-Aminocaproic acid, a Lys analogue, effectively blocks angiostatin binding to BAE cells, indicating that an unoccupied Lys-binding site of the kringles may be required for integrin binding. It is known that other plasminogen fragments containing three or five kringles (K1-3 or K1-5) have an anti-angiogenic effect, but plasminogen itself does not. We found that K1-3 and K1-5 bind to α vβ3, but plasminogen does not. These results suggest that the anti-angiogenic action of angiostatin may be mediated via interaction with αvβ3. Angiostatin binding to α vβ3 does not strongly induce stress-fiber formation, suggesting that angiostatin may prevent angiogenesis by perturbing the αvβ3-mediated signal transduction that may be necessary for angiogenesis.
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