Abstract • 187

Integrins are transmembrane proteins responsible for cell adhesion to extracellular matrices and the induction of intracellular signals important for the immune response, angiogenesis and tumor invasion. Tumor expression of integrins αvβ3 and αvβ5, and their extracellular matrix ligand, vitronectin, has been shown in anaplastic gliomas and medulloblastomas, the two most commonly invasive pediatric brain tumors. The resistance of these tumors to conventional therapies prompted us to explore the importance of the αv integrins for tumor invasion, with the aim of identifying an exploitable novel therapeutic target. We investigated the effect of monoclonal antibodies (mAbs) to human αv integrins on the adhesion, migration and survival of human U87MG malignant glioma and Daoy medulloblastoma cells plated on vitronectin. FACS analysis confirmed high expression of αv integrin on the surface of both cell lines. Treatment of the cells with anti-αv mAbs for 15 minutes resulted in the appearance of rounded and loosely attached apoptotic cells. Complete detachment of the cells from vitronectin was observed following 1-hour incubation. By comparison, an isotype matched control antibody had no effect on cell adhesion. In addition, anti-αvβ3 but not anti-αvβ5 mAb inhibited the migration of the cells on vitronectin-coated Transwell Boyden chambers, suggesting that an alternative mechanism to adhesion had been specifically disrupted by the anti-αvβ3 mAb. We have previously shown that ligation of the urokinase receptor (uPAR) by the amino-terminus of urokinase is critical for the migration of these cells. Treatment with the anti-αvβ3 mAb, but not the αvβ5 mAb, completely blocked uPAR-mediated cell migration, presumably by receptor cross talk. Finally, similar results were obtained when the pentapeptide cyclo-(Arg-Gly-Asp-D-Phe-Val), which specifically binds αvβ3/αvβ5 integrins, was used instead of αv mAbs. The pentapeptide induced apoptosis in vitro and inhibited the growth and invasion of these cell lines xenografted intracranially in athymic mice. These results indicate that αv integrin blockade with this peptide, in addition to its anti-angiogenic action, may exert a direct anti-tumor effect and could thus represent a novel therapeutic agent for brain tumors expressing αv integrins.