Plasminogen activators (PAs) are serine proteases which convert the abundant extracellular zymogen, plasminogen, into plasmin, an active protease which can promote degradation of all components of the extracellular matrix. (Dano et al. Adv. Cancer Res. 44: 139-266, 1985).
Two different types of PAs have been recognised in mammalian tissues:
(1) Tissue-type Plasminogen Activator (t-PA). t-PA is a serine protease with a molecular weight of about 70,000, composed of one polypeptide chain containing 527 amino acids. Upon limited digestion with plasmin the molecule is converted to a two-chain activator linked by one disulphide bond. This occurs by cleavage of the Arg 275 - Ile 276 peptide bond yielding a heavy chain (M.sub.r 38,000) derived from the N-terminal part of the molecule and a light chain (M.sub.r 32,000) comprising the COOH-terminal region. The catalytic site located in the light chain of t-PA is composed of His 322, Asp 371 and Ser 478. t-PA specifically catalyses the hydrolysis of an Arg 560 - Val 561 bond in plasminogen. Fibrin has been found to strongly stimulate plasminogen activation by t-PA. PA0 (ii) Urokinase-type Plasminogen Activator (U-PA). u-PA has an M.sub.r of 50,000 and occurs in a one-polypeptide and a two polypeptide chain form. The one chain form is an inactive proenzyme, while the two-chain form is the active enzyme. u-PA has a substantial plasminogen activator activity in the absence of fibrin and is not stimulated by its presence. t-PA's high affinity for fibrin suggests that it is mostly associated with a fibrinolytic function while u-PA is associated with extracellular proteolytic events such as tissue remodelling and destruction (i.e. organ involution, inflammatory reactions and particularly in the invasive growth and metastatic spread of malignant tumours). PA0 1) Endothelial cell type inhibitor, PAI-1. PA0 2) Placental type PA-inhibitor, PAI-2. PA0 3) Urinary type PA-inhibitor, PAI-3. PA0 4) Protease Nexin I, PNI.
Experimental use of t-PA and single chain u-PA as thrombolytic agents in man has been promising. However, it has become apparent that PAs may have a less pronounced fibrin specificity in man than was anticipated from several animal models, suggesting a need for further improvement either of the agents or of their administrative schemes in clinical thrombolytic therapy. One possibility is the use of specific fast-acting protein inhibitors of PAs to modulate the systemic fibrinolytic effects of PAs.
Recent evidence suggests that urokinase-mediated plasminogen activation may also play a role in the invasive behaviour of malignant cells. With few exceptions malignant cells release PAs in abnormally high amounts. Ossowski and Reich (Cell 35: 611-619, 1983) reported that anti-urokinase antibodies inhibited the metastasis of human epidermoid carcinoma cells seeded onto chick embryo chorioallantoic membranes. Bergman et al (Proc. Natl. Acad. Sci. 83: 996-1000, 1986) have shown that protease nexin I, a fibroblast-secreted inhibitor of urokinase and plasmin, effectively inhibits the cell mediated degradation of extracellular matrix (ECM) by human fibrosarcoma (HT1080) cells. Finally, Sullivan and Quigley (Cell 45: 905-915, 1986) have demonstrated that a monoclonal antibody to PA inhibits the degradation of ECM by Rous sarcoma virus-transformed chick fibroblasts. It follows from these observations and those of others [e.g. Mignatti et al., Cell 47: 487 (1986); Ossowski, Cell 52: 321 (1988); Reich et al, Cancer Res. 48: 3307 (1988)] that specific protease inhibitors of urokinase may play a critical role in altering the levels of active tumour cell PA in tumour tissue and therefore influence tumour growth and invasion in vivo.
There are other indications that a specific inhibitor of urokinase-type plasminogen activator has a role in modern medicine. PAs are involved in a range of inflammatory conditions such as arthritis. Plasmin can degrade cartilage [Lack, CH & Rogers, HJ (1958) Nature 182: 948] and low levels of fibrinolytic activity due to plasmin have been detected histochemically in synovial membranes. The PA/plasmin system has been detected in rheumatoid cell cultures [Werb, Z et al. (1977) New Engl J Med 296, 1017] and elevated levels of uPA have been noted in rheumatoid synovial fluid [Mochan, E. & Uhl, J. (1984) J. Rheumatol 11, 123]. Hence, the use of a specific inhibitor of uPA in arthritis could reverse the tissue destruction associated with this disease.
Other conditions where the application of a specific PA inhibitor may be of use include diseases or conditions such as osteoarthritis, multiple sclerosis, colitis ulcerosa, SLE-like disease, psoriasis, pemphigus, corneal ulcer, gastroduodenal ulcer, purpura, periodontitis, haemorrhage and muscular dystrophy. Finally, a PA inhibitor could have a significant role in skin wound healing and tissue repair especially since two trypsin inhibitors have been shown to enhance formation of connective tissue with increased tensile strength of the wound tissue [Kwaan, HC and Astrup, T (1969) Exp. Molec. Path. 11, 82] and keratinocytes are known to produce both uPA and tPA [Grondahl-Hansen, J et al. (1988) J. Invest Dermatol].
PA inhibitors, members of the serpin gene family (Sprengers and Kluft, Blood 69: 381-387, 1987), have been classified into four immunologically different groups:
PAI-2 (M.sub.r about 46,000) has been purified from placental tissue, monocytes and the human monocytic cell line U937. The PAI-2 inhibitors from these different sources are immunologically related and recent cDNA sequence analyses of PAI-2 derived from human placenta and the human U937 cell line confirmed they are identical, although two forms of the molecule exist differing in only 3 single amino acid residues. Both cDNA forms have been isolated from U937 cells. (Schleuning et al. Mol. Cell. Biol. 7: 4564-4567, 1987; Antalis et al. Proc. Natl. Acad. Sci. 85: 985-989, 1988). PAI-2 reacts with both u-PA and t-PA (better with two chain t-PA than with single chain t-PA) to form SDS stable complexes. PAI-2 does not bind to fibrin or to fibrin-bound t-PA.
As is the case with most potent biologically active proteins, PAI-2 is produced in very small amounts in vivo and as such is difficult to purify and characterise by conventional biochemical approaches. The recent expression of PAI-2 in bacterial cells (Antalis et al. Proc. Natl. Acad. Sci. 85: 985-989, 1988; Bunn et al, Abstracts of the Second International Workshop on the Molec. and Cell. Biol. of Plasminogen Activation, Brookhaven National Lab., April 1989), now allows the production of quantities of purified PAI-2 needed to evaluate its biological efficacy in the various potential clinical applications described above.