Activation of thrombin-inactivated single-chain urokinase-type plasminogen activator by dipeptidyl peptidase I (cathepsin C)

Cathepsin C inhibitors as anti-inflammatory drug discovery: Challenges and opportunities

Cathepsin C, an important lysosomal cysteine protease, mediates the maturation process of neutrophil serine proteases, and participates in the inflammation and immune regulation process associated with polymorphonuclear neutrophils. Therefore, cathepsin C is considered to be an attractive target for treating inflammatory diseases. With INS1007 (trade name: brensocatib) being granted a breakthrough drug designation by FDA for the treatment of Adult Non-cystic Fibrosis Bronchiectasis and Coronavirus Disease 2019, the development of cathepsin C inhibitor will attract attentions from medicinal chemists in the future soon. Here, we summarized the research results of cathepsin C as a therapeutic target, focusing on the development of cathepsin C inhibitor, and provided guidance and reference opinions for the upcoming development boom of cathepsin C inhibitor.

The NADPH Oxidase and Microbial Killing by Neutrophils, With a Particular Emphasis on the Proposed Antimicrobial Role of Myeloperoxidase within the Phagocytic Vacuole

This review is devoted to a consideration of the way in which the NADPH oxidase of neutrophils, NOX2, functions to enable the efficient killing of bacteria and fungi. It includes a critical examination of the current dogma that its primary purpose is the generation of hydrogen peroxide as substrate for myeloperoxidase-catalyzed generation of hypochlorite. Instead, it is demonstrated that NADPH oxidase functions to optimize the ionic and pH conditions within the vacuole for the solubilization and optimal activity of the proteins released into this compartment from the cytoplasmic granules, which kill and digest the microbes. The general role of other NOX systems as electrochemical generators to alter the pH and ionic composition in compartments on either side of a membrane in plants and animals will also be examined.

Leukocyte cathepsin C deficiency attenuates atherosclerotic lesion progression by selective tuning of innate and adaptive immune responses

OBJECTIVE

The protein degrading activity of cathepsin C (CatC), combined with its role in leukocyte granule activation, suggests a contribution of this cystein protease in atherosclerosis. However, no experimental data are available to validate this concept.

APPROACH AND RESULTS

CatC gene and protein expression were increased in ruptured versus advanced stable human carotid artery lesions. To assess causal involvement of CatC in plaque progression and stability, we generated LDLr(-/-)//CatC(-/-) chimeras by bone marrow transplantation. CatC(-/-) chimeras presented attenuated plaque burden in carotids, descending aorta, aortic arch and root, at both the early and advanced plaque stage. CatC was abundantly expressed by plaque macrophages and foam cells. CatC expression and activity were dramatically downregulated in plaques of CatC(-/-) chimeras, supporting a hematopoietic origin of plaque CatC. Our studies unveiled an unexpected feedback of CatC deficiency on macrophage activation programs and T helper cell differentiation in as much as that CatC expression was upregulated in M1 macrophages, whereas its deficiency led to combined M2 (in vitro) and Th2 polarization (in vivo).

CONCLUSIONS

Our data implicate CatC has a role in the selective tuning of innate and adaptive immune responses, relevant to a chronic immune disease, such as atherosclerosis.

Activation of single-chain urokinase-type plasminogen activator by platelet-associated plasminogen: a mechanism for stimulation of fibrinolysis by platelets

BACKGROUND AND OBJECTIVE

Platelets are essential for hemostasis, and they cause resistance to fibrinolysis by tissue-type plasminogen activator. In contrast, platelets enhance fibrinolysis mediated by single-chain urokinase-type plasminogen activator (scu-PA). This study investigated the mechanism behind this profibrinolytic role of platelets.

METHODS AND RESULTS

Platelets enhanced scu-PA activity, but not urokinase-type plasminogen activator (u-PA) activity, in plasma clot lysis and chromogenic assays. We established, using the non-cleavable scu-PA mutant (Lys158-->Glu) and protease inhibitors, that platelets increased activation to u-PA by a serine protease. Activation of scu-PA was platelet-dependent, even in plasma. It occurred in platelet-rich but not in platelet-poor plasma, as assessed by sodium dodecylsulfate polyacrylamide gel electrophoresis and zymography after addition of plasminogen activator inhibitor-1. Candidate proteases that are known to activate scu-PA and are present in platelet preparations were investigated. Factor VII activating protease was detected in platelet preparations by western blotting, but its inhibition by antibodies did not inhibit activation of scu-PA by platelets. Plasmin and plasma kallikrein both mimicked the platelet effect, but were distinguished by their responses to a range of inhibitors. Analysis of platelet-associated protease activity and the time course of scu-PA activation pointed towards plasminogen, and the data were consistent with a mechanism of reciprocal activation. The essential role of plasminogen was revealed using platelets from plasminogen-deficient mice, which could not activate scu-PA. Local plasminogen on platelet membranes was markedly more effective than solution-phase plasminogen in activation of scu-PA.

CONCLUSIONS

Platelets enhance fibrinolysis by scu-PA through reciprocal activation of scu-PA and platelet-associated plasminogen, a system that is potentially important in the lysis of platelet-rich thrombi.

Dual modulation of prothrombin activation by the cyclopentapeptide plactin

Plactin, a family of cyclopentapeptides, enhances fibrinolytic activity by elevating the activity of cellular urokinase-type plasminogen activator (u-PA), a protease involved in a variety of extracellular proteolytic events. Factor(s) in the blood plasma is an absolute requirement for this plactin activity. In this study, we found that plactin promoted plasma cofactor-dependent conversion of inactive single-chain u-PA to active two-chain u-PA on U937 cells. Using plactin-affinity chromatography, we identified prothrombin as one of the plasma cofactors. In incubations of U937 cells with prothrombin and Xa, plactin increased the formation of thrombin, which cleaved single-chain u-PA to afford the inactive two-chain form. Thrombin-cleaved two-chain u-PA was alternatively activated by cellular cystatin-sensitive peptidase activity, yielding fully active two-chain u-PA. In a purified system, plactin bound to prothrombin, altered its conformation and dually modulated factor Xa-mediated proteolytic activation of prothrombin to alpha-thrombin. Plactin inhibited the activation catalyzed by Xa in complex with Va, Ca(2+) and phospholipids (prothrombinase), whereas the activations catalyzed by nonmembrane-associated Xa were enhanced markedly by plactin. Plactin inhibited in vitro plasma coagulation, which involved prothrombinase formation. Plactin did not cause prothrombin activation or thrombosis in normal mice at doses that produced a protective effect in a thrombin-induced pulmonary embolism mouse model. Therefore, the dual modulation of prothrombin activation by plactin may be interpreted as leading to anticoagulation under physiological coagulating conditions.

Molecular cloning and mRNA expression of cathepsin C gene in black tiger shrimp (Penaeus monodon)

Cathepsin C (dipeptidyl-peptidase I, DPPI) is a lysosomal cysteine proteinase belonging to the papain superfamily, which is capable of removing dipeptides sequentially from the amino terminus of peptide and protein substrates. In the present study, the cDNA of a cathepsin C was cloned from black tiger shrimp Penaeus monodon (designated PmcathepsinC) by homology cloning and rapid amplification of cDNA ends (RACE) approaches. The full-length cDNA of PmcathepsinC consisted of 2051 nucleotides with a canonical polyadenylation signal sequence AATAAA and a poly(A) tail, and an open reading frame (ORF) of 1350 bp encoding a polypeptide of 449 amino acid residues with a predicted molecular weight of 50.0 kDa and theoretical isoelectric point of 5.65. The high identity of PmcathepsinC with Cathepsin C in other organisms indicated that PmcathepsinC should be a new member of the Cathepsin C family. By fluorescent quantitative real-time PCR, mRNA transcript of PmcathepsinC was detectable in all the examined tissues with higher level in ovary and heart. The temporal expression of PmcathepsinC mRNA in the hepatopancreas was up-regulated by lipopolysaccharide (LPS) stimulation and reached the maximum level at 4 h post-stimulation, and then dropped back to the original level gradually. These results indicated that PmcathepsinC was a constitutive and inducible acute-phase protein that perhaps involved in the immune defense of P. monodon.

Thrombin-thrombomodulin inhibits prourokinase-mediated pleural mesothelial cell-dependent fibrinolysis

Fibrin deposition is a hallmark of pleural inflammation and loculation but understanding of mechanisms by which mesothelial cells regulate intrapleural fibrinolysins remains incomplete. We speculated that pleural mesothelial cells regulate local fibrinolytic capacity via processing of single chain urokinase type plasminogen activator (scuPA). Pretreatment of human pleural mesothelial (MeT-5A) cells with TGF-beta or thrombin, either alone or in combination, inhibited urokinase (uPA)-mediated fibrinolysis by MeT-5A. Thrombin, unlike TGF-beta, inhibited fibrinolysis without induction of PAI-1, suggesting that thrombin-mediated cleavage of scuPA inhibits the fibrinolytic capacity of MeT-5A cells. Thrombin cleaves both purified scuPA as well as that secreted by MeT-5A cells and cell surface thrombomodulin accelerates thrombin-mediated cleavage of scuPA to inhibit cellular fibrinolytic activity. Molecular dynamics analyses demonstrated that thrombin-cleaved scuPA (uPAt) do not acquire a catalytically active conformation and that secondary plasminogen binding sites of uPA implicated in plasminogen activation are distorted in uPAt, explaining, at least in part, why uPAt is a poor enzyme. uPAt was detectable in transudative and exudative pleural effusions from patients. Intrapleural administration of scuPA generated increased levels of uPAt in PF of rabbits with pleural injury and loculation induced by tetracycline in vivo. This pathway is operative in diverse forms of pleural injury, restricts the urokinase-dependent fibrinolytic capacity of pleural mesothelial cells and contributes to local control of fibrinolytic activity via processing of endogenous or exogenous scuPA within the pleural compartment.

Pro-urokinase-type plasminogen activator is a substrate for hepsin

Hepsin, a type II transmembrane serine protease, is strongly up-regulated in prostate cancer. Hepsin overexpression in a mouse prostate cancer model resulted in tumor progression and metastasis, associated with basement membrane disorganization. We investigated whether hepsin enzymatic activity was linked to the basement membrane defects by examining its ability to initiate the plasminogen/plasmin proteolytic pathway. Because plasminogen is not processed by hepsin, we investigated the upstream activators, urokinase-type plasminogen activator (uPA) and tissue-type plasminogen activator. Enzymatic assays with a recombinant soluble form of hepsin demonstrated that hepsin did not cleave pro-tissue-type plasminogen activator but efficiently converted pro-uPA into high molecular weight uPA by cleavage at the Lys158-Ile159 (P1-P1') peptide bond. uPA generated by hepsin displayed enzymatic activity toward small synthetic and macromolecular substrates indistinguishable from uPA produced by plasmin. The catalytic efficiency of pro-uPA activation by hepsin (kcat/Km 4.8 x 10(5) m(-1) s(-1)) was similar to that of plasmin, which is considered the most potent pro-uPA activator and was about 6-fold higher than that of matriptase. Conversion of pro-uPA was also demonstrated with cell surface-expressed full-length hepsin. A stable hepsinoverexpressing LnCaP cell line converted pro-uPA into high molecular weight uPA at a rate of 6.6 +/- 1.9 nm uPA h(-1), which was about 3-fold higher than LnCaP cells expressing lower hepsin levels on their surface. In conclusion, the ability of hepsin to efficiently activate pro-uPA suggests that it may initiate plasmin-mediated proteolytic pathways at the tumor/stroma interface that lead to basement membrane disruption and tumor progression.

Dipeptidyl peptidase I: importance of progranzyme activation sequences, other dipeptide sequences, and the N-terminal amino group of synthetic substrates for enzyme activity

The broadly reactive cysteine protease dipeptidyl peptidase I (DPPI, cathepsin C) is thought to activate all progranzymes (zymogens of lymphocyte serine proteases) to form mature granzymes. We synthesized dipeptide 7-amino-4-methylcoumarin (AMC) substrates containing progranzyme activation sequences and showed that they were efficiently hydrolyzed by DPPI. However, DPPI will not hydrolyze Ile-Ile-AMC, the N-terminal dipeptide sequence found in mature granzymes. Introduction of the nonphysiological homophenylalanine (Hph) residue at P1 resulted in the best substrate Ala-Hph-AMC for DPPI (k(cat)/K(m)=9,000,000M(-1)s(-1)). The charged N-terminal amino group of the substrate was essential and replacement of the NH(2) group with OH or NH(CH(3)) in Gly-Phe-AMC reduced the k(cat)/K(m) value by two to three orders of magnitude. A hydrazide azaglycine analog, NH(2)NHCO-Phe-AMC, was not hydrolyzed at pH 5.5, but underwent slow hydrolysis at lower pHs where the amino group is partially protonated. DPPI also failed to hydrolyze NH(2)COCH(2)-Phe-AMC, where the NH(2) group is unprotonated. The results reported in this paper should be useful in the design of better DPPI inhibitors to block granzyme maturation and granzyme-dependent apoptosis.

Identification of an alternative splicing variant of cathepsin C/dipeptidyl-peptidase I

Cathepsin C/dipeptidyl-peptidase I is a papain-like lysosomal cysteine proteinase implicated in the processing of various proenzymes to their active forms. In this study, we identified an alternative splicing variant of cathepsin C in both human and mouse species for the first time. The variant messenger RNA (mRNA) encodes 137 amino acids corresponding to the first and second exons, followed by additional 31 amino acids. The two newly recognized exons are located in the former intron 2. The variant mRNA is distributed ubiquitously, but predominantly in kidney, placenta, and lymph nodes. Furthermore, both interleukin 4 (IL-4) and IL-13, but not a range of cytokines induce expression of the variant in bronchial epithelial cells. These results indicate that the variant may play a role in regulating the biological activities of cathepsin C, involved in the pathogenesis of bronchial asthma.

Structure of human dipeptidyl peptidase I (cathepsin C): exclusion domain added to an endopeptidase framework creates the machine for activation of granular serine proteases

Dipeptidyl peptidase I (DPPI) or cathepsin C is the physiological activator of groups of serine proteases from immune and inflammatory cells vital for defense of an organism. The structure presented shows how an additional domain transforms the framework of a papain-like endopeptidase into a robust oligomeric protease-processing enzyme. The tetrahedral arrangement of the active sites exposed to solvent allows approach of proteins in their native state; the massive body of the exclusion domain fastened within the tetrahedral framework excludes approach of a polypeptide chain apart from its termini; and the carboxylic group of Asp1 positions the N-terminal amino group of the substrate. Based on a structural comparison and interactions within the active site cleft, it is suggested that the exclusion domain originates from a metallo-protease inhibitor. The location of missense mutations, characterized in people suffering from Haim-Munk and Papillon-Lefevre syndromes, suggests how they disrupt the fold and function of the enzyme.

Identification of the epidermal growth factor-like domains of thrombomodulin essential for the acceleration of thrombin-mediated inactivation of single-chain urokinase-type plasminogen activator

Single-chain urokinase-type plasminogen activator (scu-PA) can be cleaved by thrombin into a virtually inactive form called thrombin-cleaved two-chain urokinase-type plasminogen activator (tcu-PA/T), a process accelerated by thrombomodulin, which contains six epidermal growth factor (EGF)-like domains. In this study, we identified the EGF-like domains of thrombomodulin required for the acceleration of the inactivation of scu-PA by thrombin using various forms of thrombomodulin (TM). scu-PA was treated with thrombin in the absence and presence of full-length rabbit TM (containing EGF1-6), recombinant TM comprising all of the extracellular domains including EGF1-6 (TMLEO) and recombinant TM comprising EGF4-6 plus the interconnecting region between EGF3 and EGF4 (TMEi4-6), and the tcu-PA/T generated was quantitated in each case. Rabbit TM accelerated the inactivation of scu-PA approximately 35-fold, while both recombinant forms accelerated it only threefold due to the absence of a critical chondroitin sulfate moiety. Subsequently, TME5-6 was prepared by cyanogen bromide digestion of TMEi4-6. TME5-6 bound to thrombin but did not accelerate the activation of protein C. In contrast, the inactivation of scu-PA by thrombin was accelerated to the same extent as that induced by TMLEO and TMEi4-6. This study demonstrates that, in addition to the chondroitin sulfate moiety, only EGF-like domains 5 and 6 are essential for the acceleration of the inactivation of scu-PA by thrombin. This differs from the domains that are critical for activation of protein C (EGF-like domains i4-6) and thrombin activatable fibrinolysis inhibitor (EGF-like domains 3-6).

The inactivation of single-chain urokinase-type plasminogen activator by thrombin on cultured human endothelial cells

Single-chain urokinase-type plasminogen activator (scu-PA) is cleaved by thrombin, resulting in an inactive molecule called thrombin-cleaved two-chain urokinase-type plasminogen activator (tcu-PA/T). There is no knowledge about cell-mediated inactivation of scu-PA. We have studied whether scu-PA bound to cultured human umbilical vein endothelial cells (HUVEC) could be inactivated by thrombin. High molecular weight scu-PA was bound to HUVEC and incubated with increasing amounts of thrombin for 30 min at 37 degrees C. Cell-bound urokinase-type plasminogen activator (u-PA) was released and levels of scu-PA, tcu-PA/T and active two-chain u-PA were measured using sensitive bioimmunoassays. Cell-bound scu-PA was efficiently inactivated by thrombin. Fifty percent inactivation of scu-PA occurred at about 0.2 nM thrombin. In the presence of monoclonal anti-urokinase receptor IgG, at least 50% of the binding of scu-PA to HUVEC was inhibited. The relative amount of tcu-PA/T that was generated by thrombin was not affected by the monoclonal antibody. These results indicated that scu-PA bound to HUVEC via the urokinase receptor can be inactivated by thrombin. The efficient inactivation of cell-bound scu-PA suggests that a cofactor for thrombin may be involved, like thrombomodulin or glycosaminoglycans. It is concluded that scu-PA bound to the urokinase receptor on a cell surface can be inactivated by thrombin, which may have profound effects on u-PA-mediated local fibrinolysis and extracellular proteolysis during processes in which thrombin is also involved.

Arginine-based structures are specific inhibitors of cathepsin C. Application of peptide combinatorial libraries

Novel synthetic peptide inhibitors of lysosomal cysteine proteinase cathepsin C have been designed through the use of soluble peptide combinatorial libraries. The uncovered structural inhibitory module consists of the N-terminal cluster of L-arginine residues. Its modification with D-amino acids or arginine derivatives did not increase the inhibition strength. Inhibitory potency of oligoarginines improves with the elongation of peptide chain reaching a maximum for octa-L-arginine. The oligoarginines specifically interact with the cathepsin C active site as shown by competitive-type inhibition kinetics (Ki approximately 10-5 M) and intrinsic fluorescence measurements. The inhibitory interaction of oligoarginines is established through the specific spatial contact of a net of guanidino groups in the arginine side-chains, as indicated by comparison with inhibitory action of low molecular mass guanidine derivatives (Ki approximately 10-3 M). Nonarginine polyionic compounds cannot mimic the inhibitory effect of oligoarginines. The arginine-based peptide inhibitors were selective towards cathepsin C among other cysteine proteinases tested.

Plasminogen-independent initiation of the pro-urokinase activation cascade in vivo. Activation of pro-urokinase by glandular kallikrein (mGK-6) in plasminogen-deficient mice

The plasminogen activation (PA) system is involved in the degradation of fibrin and various extracellular matrix proteins, taking part in a number of physiological and pathological tissue remodeling processes including cancer invasion. This system is organized as a classical proteolytic cascade, and as for other cascade systems, understanding the physiological initiation mechanism is of central importance. The attempts to identify initiation routes for activation of the proform of the key enzyme urokinase-type plasminogen activator (pro-uPA) in vivo have been hampered by the strong activator potency of the plasmin, that is generated during the progress of the cascade. Using gene-targeted mice deficient in plasminogen (Plg -/- mice) [Bugge, T. H., Flick, M. J., Daugherty, C. C., and Degen, J. L. (1995) Genes Dev. 9, 794-807], we have now demonstrated and identified a component capable of initiating the cascade by activating pro-uPA. The urine from Plg -/- mice contained active two-chain uPA as well as a proteinase capable of activating exogenously added pro-uPA. The active component was purified and identified by mass spectrometry-based peptide mapping as mouse glandular kallikrein mGK-6 (true tissue kallikrein). The pro-uPA converting activity of the mGK-6 enzyme, as well as its ability to cleave a synthetic substrate for glandular kallikrein, was inhibited by the serine proteinase inhibitor leupeptin but not by other serine proteinase inhibitors such as aprotinin, antithrombin III, or alpha(1)-antitrypsin. We suggest that mouse glandular kallikrein mGK-6 is an activator of pro-uPA in the mouse urinary tract in vivo. Since this kallikrein is expressed in a number of tissues and also occurs in plasma, it can also be considered a candidate for a physiological pro-uPA activator in other locations.

Inactivation of single-chain urokinase-type plasminogen activator by thrombin in human subjects

Thrombin cleaves single-chain urokinase-type plasminogen activator (scu-PA) into a virtually inactive two-chain form (tcu-PA/T), a process that may protect a blood clot from early fibrinolysis. It is not known under what circumstances tcu-PA/T can be generated in vivo. We have studied the occurrence of tcu-PA/T in human subjects with a varying degree of hypercoagulability. tcu-PA/T was assessed in the plasma of patients with disseminated intravascular coagulation (DIC), endotoxin-treated volunteers, patients with unstable angina pectoris, and patients selected for hip replacement. Relationships between tcu-PA/T and several markers reflecting thrombin generation were examined. tcu-PA/T was observed only in the plasma of patients with DIC and was associated with all thrombin markers and with scu-PA and urokinase antigen. Prothrombin fragment 1 + 2 and urokinase antigen were independent predictors of tcu-PA/T. The fact that tcu-PA/T could not be detected in the other three groups was explained by a lower extent of thrombin generation, a greater inhibition of thrombin by antithrombin, or less available urokinase antigen in these groups. The contribution of scu-PA to total urokinase antigen was decreased in the patients with DIC because of inactivation by thrombin, which may be an additional explanation for the inadequate fibrinolysis observed in these patients. These findings show that scu-PA can be inactivated in the circulation under severe pathophysiologic circumstances and that the process of inactivation depends not only on the generation of thrombin but also on the control of thrombin activity by its inhibitor antithrombin.

Thrombin Stimulation of Platelets Induces Plasminogen Activation Mediated by Endogenous Urokinase-Type Plasminogen Activator

Gene knockout mice studies indicate that urokinase-type plasminogen activator (u-PA) is importantly involved in fibrinolysis, but its physiologic mechanism of action remains poorly understood. We postulated that platelets may be involved in this mechanism, as they carry a novel receptor for u-PA and a portion of the single-chain u-PA (scu-PA) intrinsic to blood is tightly associated with platelets. Therefore, plasminogen activation by platelet-associated u-PA was studied. When washed platelets were incubated with plasminogen, no plasmin was generated as detected by plasmin synthetic substrate (S2403) hydrolysis; however, after the addition of thrombin, but not other agonists, platelet-dependent plasminogen activation occurred. Plasminogen activation was surface-related, being inhibited by blocking platelet fibrinogen receptors or by preventing plasminogen binding to the thrombin-activated platelet surface. U-PA was identified as the only plasminogen activator responsible and enrichment of platelets with exogenous scu-PA significantly augmented plasminogen activation. These findings appeared paradoxical because thrombin inactivates scu-PA. Indeed, zymograms showed inactivation of scu-PA during the first hour of incubation with even the lowest dose of thrombin used (1 u/mL). However, this was followed by a thrombin dose-dependent (1 to 10 u/mL) partial return of u-PA activity. Reactivation of u-PA was not due to the direct action of thrombin, but required platelets and was found to be related to a platelet lysosomal thiol protease, consistent with cathepsin C. In conclusion, a new pathway of plasminogen activation by platelet-associated endogenous or exogenous scu-PA was demonstrated, which is specifically triggered by thrombin activation of platelets. These findings may help explain u-PA–mediated physiological fibrinolysis and have implications for therapeutic thrombolysis with scu-PA.

The urokinase/urokinase-receptor system and cancer invasion

u-PA binds with high affinity to its specific GPI-anchored receptor on the cell surface. The binding has at least two important consequences: (1) it enhances the rate of plasminogen activation on the cell surface; and (2) it focuses the u-PA proteolytic activity at the leading front of migrating cells. Several recent findings suggest that surface-bound u-PA is essential for the invasive ability of tumour cells, even if a picture is emerging indicating a concerted action with other proteases, like collagenases and cathepsin B (Kobayashi et al, 1992; Ossowski, 1992; Schmitt et al, 1992; (Danø et al, 1994). In some tumours, e.g. colon, breast and skin cancer, in situ hybridization studies have given an insight into the u-PA/u-PAR tumour biology showing a complex interplay between stromal and cancer cells Danø et al, 1994). u-PA, u-PAR, and PAI-1 tumour content are now well established prognostic factor in breast cancer. This body of knowledge could be used for theurapeutic purposes. For example, a large study with 671 patients has allowed the identification of node-negative patients which, according to their u-PA levels, would need adjuvant therapy (Foekens et al, 1992). Many other tumours, especially colorectal cancer, expect a direct clinical evaluation of u-PA, u-PAR and serpins as prognostic factors.

Plasminogen activators and plasminogen activator inhibitors: biochemical aspects

Although this chapter does not represent a historical review, it will be clear how the biochemistry of t-PA, u-PA, PAI-1 and PAI-2 has evolved and where we stand in 1994. While the functional activities of the proteins were recognized at least three to four decades ago, highly purified preparations became available around 1980. In the mid-eighties the cDNAs of the proteins were cloned, representing a major breakthrough in the biochemistry of the four proteins. Amino acid sequences were derived from the nucleotide sequences, homologies with other proteins were recognized and larger amounts of (recombinant) proteins became available for research. In addition, mutant proteins were prepared by recombinant DNA technology, enabling investigation of structure-function relationships. This report is mainly based on the latter studies. Detailed information about three-dimensional structures of the proteins and the mode of interaction with other macromolecules is still lacking. To obtain this information will be the goal for biochemists in the coming years.

Plasminogen activation by human keratinocytes: molecular pathways and cell-biological consequences

Keratinocytes are the major cellular constituent of stratified epithelia. Defects in these epithelia are re-epithelialized by keratinocytes migrating from the edge of the defect into the wound. The cells form a monolayer with subsequent differentiation into a multilayered epithelium. It is thought that plasminogen activation by migrating keratinocytes is an important event during re-epithelialization. In the present report we summarize the studies on plasminogen activation by human keratinocytes in vitro and in vivo. Under the aspect of pericellular proteolysis the discussion is focused on the molecular mechanisms of plasminogen activation at the keratinocyte surface and on the cell-biological consequences of pericellular plasmin formation. We describe a cell surface-associated pathway of plasminogen activation which crucially depends on cell surface receptors for (pro)-uPA and plasmin(ogen). uPA bound to its receptor converts cell-bound plasminogen into the active protease plasmin. Compared to plasminogen activation in solution, activation at the keratinocyte cell surface is accelerated by a factor of approx. 7-10, and the plasmin generated and bound at the cell surface is protected against its specific inhibitor alpha 2-antiplasmin. Plasmin thus provided in the pericellular space leads to detachment of cultured keratinocytes from the growth substratum. Plasmin interferes with the adhesion of keratinocytes to fibrin, but not with the adhesion to collagen type I. By demonstrating that keratinocytes of the epithelial outgrowth in healing skin wounds express uPA and the uPA-R and that plasmin(ogen) is colocalized with uPA and/or uPA-R, indirect evidence is provided that this pathway may be operative in vivo. In view of previous findings that plasminogen activation is also observed under certain pathologic conditions in the epidermis, we conclude that plasminogen activation by keratinocytes is rather related to tissue damage and subsequent repair mechanisms than to a specific pathologic situation.