Extracellular engagement of α₆ integrin inhibited urokinase-type plasminogen activator-mediated cleavage and delayed human prostate bone metastasis

Expression of α₆ 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 α₆ integrin, called α₆p, generated by urokinase-type plasminogen activator-dependent cleavage of the lamininbinding 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 α₆ integrin was present on prostate carcinoma escaping the gland via nerves. Both endogenous and inducible levels of α₆p were inhibited by engaging the extracellular domain of α₆ 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 α₆ cleavage effect on extravasation was confirmed through a genetic approach using tumor cells transfected with uncleavable α₆ integrin. Uncleavable α₆ integrin significantly delayed the onset and progression of osseous metastases out to six weeks post-injection. The results suggest that α₆ integrin cleavage permits extravasation of human prostate cancer cells from circulation to bone and can be manipulated to prevent metastasis.