Urokinase (urinary-type plasminogen activator or uPA (International Union of Biochemistry classification number: EC3.4.21.31)) is a proteolytic enzyme which is highly specific for a single peptide bond in plasminogen. Plasminogen activation (cleavage of this bond by the urokinase enzyme) results in formation of plasmin, a potent general protease.
Many cell types use urokinase as a key initiator of plasmin-mediated proteolytic degradation or modification of extracellular support structures such as extracellular matrix (ECM) and basement membrane (BM). Cells exist, move and interact with each other in tissues and organs within the physical framework provided by ECM and BM. Movement of cells within ECM or across BM requires local proteolytic degradation or modification of the structures and allows cells to invade adjacent areas previously unavailable prior to the degradation or modification.
Cellular invasiveness initiated by urokinase is central to a variety of normal and disease-state physiological processes (Blasi, F., Vassalli, J. D., and Dano, K. J. Cell Biol. 104:801-804, (1987); Dano, K., Anderson, P. A., Grondahl-Hansen, J., Kristensen, P., Nielsen, L. S., and Skriver, L. Adv. Cancer Res. 44:139-266, (1985); Littlefield, B. A. Ann. N. Y. Acad. Sci. 622:167-175, (1991); Saksela, O., Biochim. Biophys. Acta 823:35-65, (1985); Testa, J. E. and Quigley, J. P. Cancer Metast. Rev. 9:353-367, (1990)). Such processes include, but are not limited to, angiogenesis (neovascularization), bone restructuring, embryo implantation in the uterus, infiltration of immune cells into inflammatory sites, ovulation, spermatogenesis, tissue remodelling during wound repair and organ differentiation, fibrosis, tumor invasion, metastatic spread of tumor cells from primary to secondary sites and tissue destruction in arthritis. Amiloride, for example, a known urokinase inhibitor of only moderate potency, has been reported to inhibit tumor metastasis in vivo (Kellen, J. A., Mirakian, A. Kolin, A. Anticancer Res. 8:1373-1376, (1988)) and angiogenesis/capillary network formation in vitro (Alliegro, M. C. and Glaser, B. M. J. Cell Biol. 115[3 Pt 2]: 402a, (1991)).
Inhibitors of urokinase, therefore, have mechanism-based anti-angiogenic, anti-arthritic, anti-inflammatory, anti-retinopathic (for angiogenesis-dependent retinopathies), contraceptive and tumoristatic uses.