Mechanisms of osteopontin and CD44 as metastatic principles in prostate cancer cells

Background: The expression level of osteopontin correlates with the metastatic potential of several tumors. Osteopontin is a well-characterized ligand for the alpha v beta 3 integrin. The present study was undertaken to elucidate the possible role of osteopontin/alpha v beta 3 signaling in prostate cancer cell migration. Results: We generated stable prostate cancer cell (PC3) lines that over-express osteopontin (PC3/OPN), mutant OPN in the integrin binding-site (PC3/RGD Delta RGA), and null for OPN (PC3/ SiRNA). The following observations were made in PC3/ OPN cells as compared with PC3 cells: 1) an increase in multinucleated giant cells and RANKL expression; 2) an increase in CD44 surface expression, interaction of CD44/MMP-9 on the cell surface, MMP-9 activity in the conditioned medium, and cell migration; 3) western blot analysis of concentrated conditioned medium exhibited equal levels of MMP-9 protein in all PC3 cells. However, zymography analysis demonstrated that the levels of MMP-9 activity in the conditioned media reflect the CD44 surface expression pattern of the PC3 cell lines; 4) although MMP- 9 and MMP- 2 are secreted by PC3 cells, only the secretion of MMP- 9 is regulated by OPN expression. A strong down regulation of the above- mentioned processes was observed in PC3/OPN (RGA) and PC3/SiRNA cells. PC3/OPN cells treated with bisphosphonate (BP) reproduce the down-regulation observed in PC3/OPN (RGA) and PC3/ SiRNA cells. Conclusion: Rho signaling plays a crucial role in CD44 surface expression. BPs inhibits the mevalonate pathway, which in turn, prevents the prenylation of a number of small GTPases. Attenuation of Rho GTPase activation by BPs may have contributed to the down regulation of cell surface CD44/MMP-9 interaction, MMP-9 activation/secretion, and cell migration. Taken together, these observations suggest that CD44 surface expression is an important event in the activation of MMP- 9 and migration of prostate cancer cells. The various steps involved in the above mentioned signaling pathway and/or the molecules regulating the activation of MMP-9 are potential therapeutic target.