The role of urokinase and urokinase inhibitor in tumour cell metastasis

Abstract

In this paper we summarize experiments in which murine melanoma B16-F1 cells of low metastatic potential were transfected with the human gene for prep rourokinase. B16-F1 cells selected for their secretion of high amounts of the human urokinase gene product (3- to 4-fold higher plasminogen activator activity than control cells) formed between 4- and 16-fold greater numbers of lung tumours in C57BL mice after tail vein injection. In correlative results, highly malignant B16-F10 melanoma cells transfected with a urokinase antisense sequence showed a significant inhibition of endogenous urokinase expression and a corresponding significant decrease in the number of tumours formed after tail vein injection. The human urokinase gene product expressed by the transfected cells does not bind to murine cell surface urokinase receptors, and the results may provide direct evidence for a role of secreted (non-surface bound) urokinase in the extravasation steps of tumour cell metastasis. These human urokinase secreting cells also showed a greater ability to spontaneously metastasize from a primary footpad tumour to the mouse lung. Extending the study of urokinase to the role of its natural inhibitor, plasminogen activator inhibitor 2 (PAI-2), we determined the constitutive and phorbol ester inducibility of the mRNA for PAI-2 and urokinase in human A375 melanoma cell lines and human prostate PC3 carcinoma cell lines of different metastatic potentials. Analysis of the changes in PAI-2 and urokinase mRNA levels indicate that variation in the levels of PAI-2 may be more important than urokinase in explaining the different metastatic abilities of A375 and PC3 sublines. Urokinase activity may promote metastasis by its initiation of a specific proteolysis pathway leading to the activation of metalloproteinases. We have previously shown that the metalloproteinase inhibitor, recombinant tissue inhibitor of metalloproteinases (rTIMP), inhibits B16 melanoma cell lung colonization.