Extracellular Engagement of α6 Integrin Inhibited uPA 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 extra capsular 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 (uPA) 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 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 SCID mouse model of extravasation and bone metastasis produced detectable, progressive osteolytic lesions within three weeks of intracardiac injections. Injection of tumor cells, pre-treated 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 6 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.