PREVENTION OF METASTASIS BY INHIBITION OF THE UROKINASE RECEPTOR

The plasminogen activator urokinase (u-PA) mediates proteolysis by a v ariety of human tumor cells. Competitive displacement of u-PA from cel lular binding sites results in decreased proteolysis in vitro, suggest ing that the cell surface is the preferred site for u-PA-mediated prot ein degradation. We studied the effect of u-PA receptor blockade on th e metastatic capacity of human PC3 prostate carcinoma cells, using tra nsfectants which expressed chloramphenicol acetyl-transferase (CAT). E ight weeks after subcutaneous inoculation of these cells into nude mic e, CAT activity was detected in regional lymph nodes, femurs, lungs, a nd brain, thereby mimicking the organ tropism observed for naturally o ccurring metastases of prostate cancer. In a second transfection, CAT- expressing PC3 cells received cDNA encoding a mutant u-PA (Ser356 --> Ala) which lacks enzymatic activity but which retains full receptor bi nding affinity. Three mutant u-PA expressors, each with <5% of wild-ty pe cell-associated u-PA activity, were compared in vivo with independe ntly derived controls. Primary tumor growth was similar in each group of animals and all tumors expressed comparable CAT activity. In contra st, metastasis (as assessed by CAT activity) was markedly inhibited wh en cell surface u-PA activity was blocked. Levels of CAT activity were reduced by a factor of >300 in regional lymph nodes, 40-100 in brain tissue, and 10-20 in lung tissue. Metastatic capacity was inhibited si milarly when animals were given intermittent intraperitoneal injection s of a u-PA/IgG fusion protein capable of displacing u-PA activity fro m the tumor cell surface. Our results indicate that cell surface u-PA activity is essential to the metastatic process. In addition, the assa y system employed in these experiments may be generally useful in test ing other therapeutic modalities to limit the spread of primary tumors .