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Extracellular Engagement of ₆ Integrin Inhibited Urokinase-Type Plasminogen Activator–Mediated Cleavage and Delayed Human Prostate Bone Metastasis

¹ Cancer Biology Interdisciplinary Graduate Program and Departments of ² Pathology, ³ Radiology, and ⁴ Cell Biology and Anatomy, University of Arizona, The Arizona Cancer Center, Tucson, Arizona

Requests for reprints: Anne E. Cress, 1515 North Campbell Avenue, Tucson, AZ 85724-5044. Phone: 520-626-7553; Fax: 520-626-4979; E-mail: acress{at}azcc.arizona.edu.

Key Words: integrin • bone • metastasis • cleavage • uPA

Expression of ₆ integrin, a laminin receptor, on tumor cell surfaces is associated with reduced patient survival and increased metastasis in a variety of tumors. In prostate cancer, tumor extracapsular escape occurs in part via laminin-coated nerves and vascular dissemination, resulting in clinically significant bone metastases. We previously identified a novel form of ₆ integrin, called ₆p, generated by urokinase-type plasminogen activator-dependent cleavage of the laminin-binding domain from the tumor cell surface. Cleavage increased laminin-dependent migration. Currently, we used the known conformation sensitivity of integrin function to determine if engagement of the extracellular domain inhibited integrin cleavage and the extravasation step of metastasis. We show that ₆ integrin was present on prostate carcinoma escaping the gland via nerves. Both endogenous and inducible levels of ₆p were inhibited by engaging the extracellular domain of ₆ with monoclonal antibody J8H. J8H inhibited tumor cell invasion through Matrigel. A severe combined immunodeficient mouse model of extravasation and bone metastasis produced detectable, progressive osteolytic lesions within 3 weeks of intracardiac injections. Injection of tumor cells, pretreated with J8H, delayed the appearance of metastases. Validation of the ₆ cleavage effect on extravasation was confirmed through a genetic approach using tumor cells transfected with uncleavable ₆ integrin. Uncleavable ₆ integrin significantly delayed the onset and progression of osseous metastases out to six weeks post-injection. The results suggest that ₆ integrin cleavage permits extravasation of human prostate cancer cells from circulation to bone and can be manipulated to prevent metastasis. [Cancer Res 2009;69(12):5007–14]