Parallel induction of fibrinolysis and receptors for plasminogen and urokinase by interferon gamma on U937 cells

Plasminogen activator production in a rat model of Pneumocystis carinii pneumonia

Several studies have indicated that the serine protease urokinase-plasminogen-activator (uPA) is an important factor in host defense against pulmonary pathogens. To gain a better insight into the role of uPA in Pneumocystis carinii (P. carinii) pneumonia (PCP), we evaluated PA production in alveolar macrophages (AMs) obtained from rats with steroid-induced PCP. Treatment with cortisone acetate favored PCP in 91% of rats. In the bronchoalveolar lavage (BAL) samples of immunosuppressed rats both with and without PCP, we observed a decrease in uPA activity as well as a decrease in cell number. Urokinase-PA production by AMs was reduced in rats treated with cortisone alone. However, an increase in cell-associated uPA was observed in rats with PCP. This increase appears to be produced in response to P carinii infection. In fact, when AMs obtained from untreated healthy or immunosuppressed uninfected rats were challenged with P carinii, a significant increase in PA activity in cell lysates was observed, though a lower response was obtained in cortisone-treated animals. Our results suggest that healthy AMs respond to the presence of P carinii with an increase in uPA production and that this response in immunodepressed rat-AMs is partially impaired.

Regulation of plasminogen binding to neutrophils

Plasminogen plays an integral role in the inflammatory response, and this participation is likely to depend on its interaction with cell surfaces. It has previously been reported that isolation of human neutrophils from blood leads to a spontaneous increase in their plasminogen-binding capacity, and the basis for this up-regulation has been explored as a model for mechanisms for modulation of plasminogen receptor expression. Freshly isolated human peripheral blood neutrophils exhibited relatively low plasminogen binding, but when cultured for 20 hours, they increased this capacity dramatically, up to 50-fold. This increase was abolished by soybean trypsin inhibitor and was susceptible to carboxypeptidase B treatment, implicating proteolysis and exposure of carboxy-terminal lysines in the enhanced interaction. In support of this hypothesis, treatment of neutrophils with elastase, cathepsin G, or plasmin increased their plasminogen binding, and specific inhibitors of elastase and cathepsin G suppressed the up-regulation that occurred during neutrophil culture. When neutrophils were stimulated with phorbol ester, their plasminogen binding increased rapidly, but this increase was insensitive to the protease inhibitors. These results indicate that plasminogen binding to neutrophils can be up-regulated by 2 distinct pathways. A major pathway with the propensity to markedly up-regulate plasminogen binding depends upon the proteolytic remodeling of the cell surface. In response to thioglycollate, neutrophils recruited into the peritoneum of mice were shown to bind more plasminogen than those in peripheral blood, suggesting that modulation of plasminogen binding by these or other pathways may also occur in vivo.

Gamma interferon administration to patients with atopic dermatitis inhibits fibrinolysis and elevates C1 inhibitor

Recombinant human gamma interferon was used to treat 10 atopic dermatitis patients. Recombinant gamma interferon was administered weekly for three consecutive days at 50 microg/M2 SQ for four weeks. All patients' dermatitis improved with recombinant gamma interferon therapy and plasma tumor necrosis factor-alpha levels rose with treatment. Recombinant gamma interferon treatment positively correlated with reduced total plasma fibrinolysis as measured by the fibrin lysis plate, plasmin-alpha2antiplasmin complexes, and tissue type plasminogen activator levels. Accordingly, plasminogen activator inhibitor levels increased. Treatment also was associated with a transient increase in thrombin-antithrombin III complexes. Recombinant gamma interferon resulted in a significant increase in C1 inhibitor antigen but not activity. Plasma prekallikrein, high molecular weight kininogen, and factor XII levels were not decreased. However, 5 of the 10 atopic dermatitis patients before therapy had circulating cleaved plasma high molecular weight kininogen detected on immunoblot, indicating prior kallikrein formation. The cleaved, circulating plasma high molecular weight kininogen disappeared in four out of the five original patients who were reexamined at one year after treatment. These combined data indicated that recombinant gamma interferon treatment reduced total plasma fibrinolysis. In untreated atopic dermatitis, circulating cleaved high molecular weight kininogen also may be a presenting manifestation.

Regulation of the uPAR/uPA system expressed on monocytes by the deactivating cytokines, IL-4, IL-10 and IL-13: consequences on cell adhesion to vitronectin and fibrinogen

Urokinase (uPA) and its receptor (uPAR) have been proposed to be involved in monocyte migration by inducing degradation of matrix proteins. In addition, uPAR is also implicated in cell adhesion to the vascular wall. The adhesive function of uPAR depends on a direct interaction with vitronectin which is increased by uPA and by modification of cell surface integrin (such as CD11b-CD18) when associated to uPAR. In this study we analysed the role of three deactivating cytokines, IL-4, IL-10 and IL-13, on the surface expression of uPA, uPAR and CD11b by monocytes and their consequences on monocyte adhesion to immobilized fibrinogen and vitronectin. IL-10 induced a decrease in uPA and CD11b after 18 h incubation and a delayed decrease in uPAR which was only significant after 48 h incubation. These results may explain the decrease in monocyte adhesion, which was observed after an 18 h incubation with IL-10, on immobilized vitronectin and fibrinogen. In contrast, IL-4 and IL-13 induced a decrease in uPAR after 18 h and a significant increase in uPA both in the cell lysates and at the cell surface, as well as an increase in cell surface associated CD11b. These cytokines did not modify cell adhesiveness to vitronectin or fibrinogen despite the increase in CD11b-CD18. This could be due to the decrease in uPAR because CD11b-CD18/uPAR forms a cell adhesion complex. In addition, the increase in uPA induced by IL-4 could counterbalance the direct interaction of uPAR with vitronectin. The increase in uPA suggests that IL-4 and IL-13 could induce plaque fissuring by monocytes, whereas IL-10 may induce protection against matrix protein degradation by decreasing uPA.

Tumor cell-conditioned medium stimulates expression of the urokinase receptor in vascular endothelial cells

We have previously reported that culture medium conditioned by human SK-Hep1 hepatoma cells or mouse S180 sarcoma cells induces in vitro angiogenesis and stimulates production of urokinase plasminogen activator (uPA) in vascular endothelial cells. These activities are mediated by a 3.5-10 kDa, heparin-binding peptide that upregulates endothelial cell expression of basic fibroblast growth factor (bFGF; Peverali et al., 1994, J. Cell. Physiol. 161:1-14.) We now report that SK-Hep 1 or S180 cell-conditioned medium rapidly induces a 4- to 5-fold increase in cell-bound uPA activity and in the high-affinity binding of 125I-prouPA to vascular endothelial cells. Ligand blotting and purification experiments show an equivalent increase in the synthesis of a cell surface protein corresponding to the endothelial cell uPA receptor (uPAR) on the basis of M, (45-50 kDa) and sensitivity to phosphatidylinositol-specific phospholipase C (PI-PLC). The tumor cell-conditioned media also upregulate uPAR mRNA levels in endothelial cells. Thus, the increase in uPA binding capacity of endothelial cells is mediated by an increased expression of uPAR. The uPAR-inducing activity of SK-Hep 1 or S180 cell-conditioned medium is not neutralized by antibodies to bFGF, and is associated with a peptide that has a M, higher than 10 kDa and no affinity for heparin. Therefore, it appears to be distinct from the bFGF/uPA-inducing factor secreted by the same cells, and from other heparin-binding cytokines that upregulate uPAR expression in endothelial cells.

Regulation of plasminogen receptor expression on monocytoid cells by beta1-integrin-dependent cellular adherence to extracellular matrix proteins

Plasminogen binding sites function to arm cell surfaces with the proteolytic activity of plasmin, critical for degradation of extracellular matrices. We have assessed the effects of adhesion of the representative monocytoid cell lines, THP-1 and U937, to purified extracellular matrix proteins on their expression of plasminogen receptors. After adhesion to immobilized fibronectin, adherent and nonadherent subpopulations of cells were separated. Plasminogen binding to the nonadherent population of cells increased 3-fold, whereas binding to the adherent population decreased by 60%. These changes were due to differences in the plasminogen binding capacities of the cells, while the affinities of the cells for plasminogen were unchanged. The up-regulation of receptor expression in the nonadherent cell population was: 1) induced rapidly and reversibly, 2) independent of new protein synthesis, 3) required an interaction between adherent and nonadherent cell populations, and 4) associated with an enhanced ability of the cells to promote plasminogen activation and to degrade fibronectin. Other immobilized adhesive proteins, laminin and vitronectin, also supported up-regulation of plasminogen receptors in the nonadherent cells. Carboxypeptidase B treatment eliminated the increment in the plasminogen binding capacity of the nonadherent cells, suggesting that the increase in binding was due to exposure of new carboxyl-terminal lysyl residues on the cell surfaces. Furthermore, both the adherence of the cells and up-regulation of plasminogen binding sites was abolished by beta1-integrin monoclonal antibodies. These results suggest that proteins found in extracellular matrices have the capacity to modulate the expression of plasminogen binding sites, thus regulating local proteolysis and cell migration.

Prostaglandin E2 regulates production of plasminogen activator isoenzymes, urokinase receptor, and plasminogen activator inhibitor-1 in primary cultures of rat calvarial osteoblasts

The bone resorbing agent, prostaglandin E2 (PGE2), was found to alter several components of the plasminogen activator (PA)/plasmin pathway in primary cultures of rat neonatal osteoblast-like cells. The mRNA and activities of both urokinase-type PA (uPA) and tissue-type PA (tPA) were enhanced by PGE2 treatment. The presence of mRNA for the uPA receptor (uPAR) has been demonstrated in these cells and steady-state levels shown to be greatly enhanced, the response being rapid and sustained for at least 24 hours. mRNA for plasminogen activator inhibitor 1 (PAI-1) was modulated in a biphasic manner, with inhibition of the constitutive level apparent at 4 hours of treatment and stimulation apparent at 12 hours and longer, while PAI-1 protein, measured by an ELISA assay for rat PAI-1, was diminished over this period. Neither PAI-2 mRNA nor mRNA for the broad spectrum protease inhibitor, protease nexin-1 (PN-1), was found to be modulated by PGE2. Therefore, PGE2 is likely to stimulate cell surface proteolytic activity, since uPA mRNA and cell-associated activity were elevated, as was mRNA for the cellular receptor for uPA. Although it was not possible to measure uPAR number and affinity it seems likely that elevated uPAR mRNA would translate into increased uPARs which would localize the increased uPA activity to the pericellular region. tPA mRNA and activity were also increased transiently with the activity inhibited with prolonged incubations, apparently by PAI-1. Elevation of tPA mRNA and activity may result in elevated activity within the extracellular matrix as tPA has been reported to associate with several matrix proteins. Thus the early effect of PGE2 would be to promote proteolysis, both pericellularly and in the extracellular matrix. The inhibition of PAI-1 mRNA and protein, which would contribute to the elevation of activity, is due to PGE2, but the later stimulatory effect on PAI-1 mRNA may be due to feedback regulation by transforming growth factor beta (TGF beta), secreted by osteoblasts and activated by elevated levels of PA.

Receptors for plasminogen and t-PA: an update

Over the past decade, the existence of cell-surface receptors for components of the plasminogen system, t-PA, u-PA, plasminogen and plasmin, has been demonstrated. Plasminogen receptors have been detected on virtually all cell types tested, and occupancy has also been demonstrated in biological settings. Characteristic features of plasminogen receptors include their relatively low affinity and their extraordinarily high density on many cells. These receptors recognize the lysine binding sites associated with the kringles of plasminogen. Plasminogen receptors include proteins with carboxyl-terminal lysine residues (enolase and annexin II are representatives) and nonproteins, such as gangliosides. Plasminogen binding to cells enhances plasmin activity by augmenting plasminogen activation, increasing the enzymatic activity of plasmin, and protecting plasmin for inactivation by inhibitors. t-PA receptors serve two major functions, clearance and cell-surface localization. The liver is the main organ for t-PA clearance; parenchymal, endothelial and Kupffer cells are all capable of t-PA uptake. Clearance receptors on these cells are heterogeneous and include ones which recognize the carbohydrate side chains of t-PA and ones which take up t-PA: PAI-1 complexes. Receptors which recognize free t-PA also mediate liver clearance, and alpha 2-MR/LRP is a representative of this latter category. Receptors that localize t-PA on cell surfaces serve a profibrinolytic function. Vascular endothelial cells are rich in such receptors, and annexin II is a representative of these t-PA binding sites. Circulating blood cells also bind t-PA, and some of the sites on these cells are shared with plasminogen. Cells of neuronal origin are capable of binding t-PA with high affinity; and amphoterin, a protein involved in neurite outgrowth, may be a neuronal t-PA receptor. Overall, the plasminogen system is one of the most widely distributed and versatile of the cell surface-proteinase systems. By activating bound plasminogen by cell-bound plasminogen activators, the cell harnesses the broad proteolytic activity of plasmin. Cells can then utilize this activity to perform functions such as assisting in cell migration.

Blockage of the urokinase receptor on the cell surface: construction and characterization of a hybrid protein consisting of the N-terminal fragment of human urokinase and human albumin

Receptor-bound urokinase is likely to be a crucial determinant in both tumor invasion and angiogenesis. We report here that a yeast-derived genetic conjugate between human serum albumin and the 1-135 N-terminal residues of urokinase (u-PA) competitively inhibits the binding of exogenous and endogenous u-PA to its cell-anchored receptor (u-PAR). This hybrid molecule (ATF-HSA) also inhibits in vitro pro-urokinase-dependent plasminogen activation in the presence of u-PAR bearing cells. These effects are probably responsible for the observed in vitro inhibition of tumor cell invasion in a reconstituted basement membrane extract (Matrigel).

Regulation of macrophage alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein by lipopolysaccharide and interferon-gamma

alpha 2-Macroglobulin receptor/low density lipoprotein receptor-related protein (alpha 2M-R/LRP) is a broad specificity receptor that may function in lipoprotein metabolism, proteinase regulation, and growth factor regulation. In this study, we demonstrated that alpha 2M-R/LRP expression in macrophages can be markedly decreased by LPS and by IFN-gamma. Regulation of alpha 2M-R/LRP in RAW 264.7 cells was demonstrated at the mRNA, antigen, and receptor-function levels. In receptor-function studies, the decrease in alpha 2M-R/LRP expression was detected as a 90% decrease in the Bmax or maximum receptor binding capacity for activated alpha 2M after treatment with LPS or IFN-gamma. Western blot analysis of whole cell lysates demonstrated significant loss of alpha 2M-R/LRP heavy-chain. Northern blot analysis of poly(A)+ RNA revealed a marked decrease in alpha 2M-R/LRP mRNA after treatment with LPS (79% decrease) or IFN-gamma (70% decrease). Other cytokines, including tumor necrosis factor-alpha, transforming growth factor-beta-1, and interleukin-6 did not regulate alpha 2M-R/LRP. The ability of LPS and IFN-gamma to regulate alpha 2M-R/LRP was confirmed in experiments with primary cultures of murine bone marrow macrophages. These studies demonstrate that macrophage alpha 2M-R/LRP is subject to significant downregulation by physiologically significant cytokines and signaling macromolecules.

Comparative study of fibrinolytic activity on 937 line after stimulation by interferon gamma, 1,25 dihydroxyvitamin D3 and their combination

Previous study showed that the secretion of urokinase (UK) by monoblastic cell line U 937 and the number of binding sites for urokinase and for plasminogen (Plg) on the cell surfaces were augmented by interferon gamma (INF tau). This induction led to an increase in fibrinolytic activity on cell surfaces. A similar increase was also observed when treating the U 937 cells with 1,25-dihydroxyvitamin D3 (1,25 (OH)2D3. Here we report that the combination of these two agents induced a 2.7 fold increase in the plasminogen activator activity on U 937 cell surfaces in comparison with 1 fold increase induced by INF tau and 1.3 fold increase by 1,25(OH)2D3. As evaluated by a flow cytometer, the increased fibrinolytic activity induced by the combination of INF tau and 1,25(OH)2D3 could be attributed to the increase of the number of binding sites both for UK (3.7 x 10(4) vs 1.2 x 10(4) per cell) and for Plg (16.2 x 10(4) vs 3.6 x 10(4) per cell), accompanied by an increased expression of CD 14, which is an antigen of differentiation on cell surfaces. These results suggest that the expression of urokinase receptors and plasminogen receptors may be coupled together by unknown intracellular mechanisms during cell differentiation, and support the idea that the concomitant regulation of these two receptors for UK and Plg is an important aspect in cell associated-fibrinolytic activity.

Regulation of the endothelial cell urokinase-type plasminogen activator receptor. Evidence for cyclic AMP-dependent and protein kinase C-dependent pathways

Binding of urokinase-type plasminogen activator (u-PA) to specific receptors (u-PAR) on the surface of endothelial cells contributes to the regulation of plasmin-dependent processes such as fibrinolysis and angiogenesis. We studied the effect of raising intracellular levels of cyclic AMP (cAMP) and/or activating protein kinase C on the expression of u-PAR in cultured human umbilical vein endothelial cells (HUVEC). Incubation of HUVEC with forskolin stimulated a time- and concentration-dependent increase in the expression of u-PAR, measured both by an increase in the specific binding of radiolabeled single-chain u-PA (scu-PA) and by increased binding of anti-u-PAR antibodies. Maximal increase in u-PAR expression (81 +/- 11% above control, n = 11) was not associated with a change in receptor affinity for scu-PA when HUVEC were incubated for 20 hours at 37 degrees C with 50 microM forskolin. Receptor induction by forskolin was inhibited when HUVEC were preincubated with deoxyadenosine monophosphate (DAM), an inhibitor of adenylyl cyclase. A similar increase in receptor expression (128 +/- 27% above control, n = 3) was induced by the cAMP analogue 8-bromoadenosine 3':5'-cyclic monophosphate (50 mM). Forskolin induced an approximately twofold increase in the expression of a single approximately 1.4-kb u-PAR messenger RNA (mRNA) transcript within 2 hours. Phorbol myristate acetate (PMA) also stimulated a time- and concentration-dependent increase in specific scu-PA binding. The maximal increase in u-PAR expression (254 +/- 27% above control, n = 11) was observed when HUVEC were preincubated with 10 nM PMA for 20 hours. Induction of u-PAR by PMA was inhibited when HUVEC were preincubated with either cycloheximide or H7 but was unaffected by DAM. u-PAR induced by PMA showed a reduced affinity for scu-PA (Kd, 14 +/- 2 nM versus 3.6 +/- 0.6 nM, p < 0.001; n = 8). PMA stimulation for 20 hours resulted in a sixfold increase in a single approximately 1.4-kb u-PAR mRNA transcript, with increased levels detectable within 30 minutes. Coincubation of HUVEC with optimal concentrations of forskolin and PMA for 20 hours produced a fully additive increase in u-PAR expression at both the mRNA and protein levels. These data suggest that both cAMP-dependent and protein kinase C-dependent protein kinase pathways may independently regulate u-PAR expression in human endothelial cells.

Modulation of receptor bound urokinase-type plasminogen activator on human monocytes by non-steroidal antiinflammatory drugs

It has been shown previously that cell surface bound fibrinolytic activity on human monocytes is significantly increased in patients with rheumatoid arthritis (RA) as compared to monocytes from healthy volunteers. Studies on the modulation of receptor-bound urokinase type plasminogen activator on peripheral blood monocytes of patients with RA showed that tenoxicam, a non-steroidal antiinflammatory drug with long half life and good tissue permeability, is able to downregulate the total number of urokinase receptors. Furthermore the degree of endogenous occupation of the urokinase receptor was significantly decreased in post-treatment monocytes. These data provide evidence that the non-steroidal antiinflammatory drug tenoxicam is able to downregulate cell bound fibrinolytic activity, known to contribute to pronounced degradation of cartilage and connective tissue in patients with RA.

Transcriptional activation of the urokinase receptor gene by endothelin-1

Using an immortalized human glomerular epithelial cell line (E71 A1), we studied the effect of endothelin-1 (ET-1), a potent vasoconstrictor peptide, on the synthesis of urokinase type plasminogen activator (u-PA) and its receptor (u-PAR). The results show that ET-1 had no effect on u-PA synthesis but induced an increase in u-PAR number (2.8 +/- 0.6 x 10(4) vs 1.2 +/- 0.5 x 10(4) sites per cell, p less than 0.001) without change in receptor affinity (280 +/- 80 pM vs 250 +/- 50 pM, NS), maximal effect being observed at 10(-7) M. Time course shows that a plateau was reached after a 24 hour incubation. ET-1 induced-increase in binding capacity was abolished by cycloheximide. ET-1 also induced an increase in u-PAR mRNA level, which was completely blocked by alpha-amanitin (5 micrograms/ml). Cycloheximide (1 microgram/ml) alone induced an increase in u-PAR mRNA level and this effect was enhanced when cycloheximide was combined with ET-1. Our data show that ET-1 can induce an increase in membrane expression of u-PAR through activation of the transcription of the u-PAR gene and de novo protein synthesis.

D-dimer and CA 125 levels in patients with ovarian cancer during antineoplastic therapy. Prognostic significance for the success of anti-cancer treatment

In patients with ovarian cancer before they receive chemotherapy, the level of fibrin degradation products (D-dimer), is correlated with the tumor load. In this study, the evolution of D-dimer was compared in patients receiving antineoplastic therapy with the evolution of the disease. The patients could be classified into three groups. In Group 1 (nine patients), both plasma CA 125 (a tumor-associated antigen) and D-dimer remained elevated; the prognosis was always poor. In Group 2 (eight patients), CA 125 and D-dimer decreased simultaneously, complete remission was observed in two patients, and significant residual tumor was observed in the others. In Group 3 (nine patients), despite an important decrease in CA 125, D-dimer remained elevated during therapy. In this group, complete remission was observed in six patients, and three others showed a large decrease in their tumor load. The combination of a decrease in CA 125 levels with a continuous enhanced level of D-dimer during chemotherapy identified a subgroup of patients with a favorable prognosis.

Expression of the urokinase receptor in vascular endothelial cells is stimulated by basic fibroblast growth factor

Basic fibroblast growth factor, a potent angiogenesis inducer, stimulates urokinase (uPA) production by vascular endothelial cells. In both basic fibroblast growth factor-stimulated and -nonstimulated bovine capillary endothelial and human umbilical vein endothelial cells single-chain uPA binding is mediated by a membrane protein with a Mr of 42,000. Exposure of bovine capillary or endothelial human umbilical vein endothelial cells to pmolar concentrations of basic fibroblast growth factor results in a dose-dependent, protein synthesis-dependent increase in the number of membrane receptors for uPA (19,500-187,000) and in a parallel decrease in their affinity (KD = 0.144-0.790 nM). With both cells, single-chain uPA binding is competed by synthetic peptides whose sequence corresponds to the receptor-binding sequence in the NH2-terminal domain of uPA. Exposure of bovine capillary endothelial cells to transforming growth factor beta 1, which inhibits uPA production and upregulates type 1 plasminogen activator inhibitor, the major endothelial cell plasminogen activator inhibitor, has no effect on uPA receptor levels. These results show that basic fibroblast growth factor, besides stimulating uPA production by vascular endothelial cells, also increases the production of receptors, which modulates their capacity to focalize this enzyme on the cell surface. This effect may be important in the degradative processes that occur during angiogenesis.

Regulation of plasminogen receptor expression on human monocytes and monocytoid cell lines

The capacity of human monocytoid cell lines and peripheral blood monocytes to modulate their expression of plasminogen receptors has been assessed. After PMA stimulation, THP-1 or U937 monocytoid cells were separated into adherent and nonadherent populations. Plasminogen bound to adherent cells with similar capacity and affinity as to nonstimulated cells. In contrast, the nonadherent cells bound plasminogen with 5-17-fold higher capacity (without a change in affinity). This increase was selective as urokinase bound with similar affinity and capacity to the adherent and nonadherent populations. Upregulation of plasminogen receptors on the nonadherent monocytoid cells was rapid, detectable within 30 min, and reversible, adhesion of the nonadherent cells resulted in a sixfold decrease in plasminogen binding within 90 min. The increase in plasminogen binding to the nonadherent cells was associated with a marked increase in their capacity to generate plasmin activity from cell-bound plasminogen. PMA stimulation of human peripheral blood monocytes increased their expression of plasminogen receptors by two- to fourfold. This increase was observed in both adherent and nonadherent monocytes. Freshly isolated monocytes maximally bound 5.0 x 10(5) plasminogen molecules per cell, whereas monocytes cultured for 18 h or more maximally bound 1.7 x 10(7) molecules per cell, a 30-fold difference in receptor number. These results indicate that both monocytes and monocytoid cell lines can rapidly and markedly regulate their expression of plasminogen binding sites. As enhanced plasminogen binding is correlated with an increased capacity to generate plasmin, an enzyme with broad substrate recognition, modulation of plasminogen receptors may have profound functional consequences.

Functional inhibition of endogenously produced urokinase decreases cell proliferation in a human melanoma cell line

Binding of urokinase-type plasminogen activator (u-PA) to its receptor has been shown not only to focus proteolytic activity to the cell surface but also to exert a mitogenic effect on the human epidermal tumor cell line CCL 20.2. This report shows that u-PA is an autocrine mitogen in the human melanoma cell line GUBSB and that inhibition of receptor-bound u-PA by specific anti-u-PA antibodies causes a significant suppression of cell proliferation in this system. The GUBSB cell line secretes 70-80% of the u-PA in its active form and expresses high-affinity u-PA receptors with a Kd of 5.2 x 10(-10) M and 2.8 x 10(4) binding sites per cell. Approximately 70% of the u-PA receptors on these cells are occupied by endogenously secreted u-PA. Addition of the monoclonal anti-u-PA antibody MPW5UK (10 nM), directed against the active site of u-PA, twice daily to the cell cultures resulted in a significant decrease of [3H]thymidine incorporation by the tumor cells, whereas a 10 times higher concentration of the monoclonal antibody MPW4UK, which does not inhibit plasminogen activator activity of u-PA, was necessary to achieve the same effect. In addition, diisopropyl fluorophosphate-inactivated u-PA, in a concentration 50-fold higher than the concentration necessary to saturate the u-PA receptor (250 pM), decreased [3H]thymidine incorporation similarly to the specific antibody, proving that active u-PA is required for the mitogenic effect. Inhibition of endogenous u-PA production by cycloheximide reduced [3H]thymidine incorporation significantly; after addition of exogenous u-PA, [3H]thymidine incorporation increased again in the cycloheximide-treated cells. Therefore, inhibition of receptor-bound u-PA might represent a tool not only to inactivate cell-bound proteolytic activity, necessary for invasion, but also to exert a specific antiproliferative effect on certain tumor cells.

A potential basis for the thrombotic risks associated with lipoprotein(a)

Lipoprotein(a) (Lp(a)) has been strongly linked with atherosclerosis and is an independent risk factor for myocardial infarction. Distinguishing Lp(a) from other low-density lipoprotein particles is its content of a unique apoprotein, apo(a). The recently described sequence of apo(a) indicates a remarkable homology with plasminogen, the zymogen of the primary thrombolytic enzyme, plasmin. Lp(a) may contain 37 or more disulphide-looped kringle structures, which are 75-85% identical to the fourth kringle of plasminogen. Plasminogen receptors are widely distributed on blood cells and are present at extremely high density on endothelial cells. These receptors promote thrombolysis by accelerating plasminogen activation and protecting plasmin from inhibition. If, by molecular mimicry, Lp(a) competes with plasminogen for receptors, then thrombolysis would be inhibited and thrombosis promoted. Here we provide support for such a mechanism being responsible for the thrombotic risks associated with elevated Lp(a) by demonstrating that Lp(a) inhibits plasminogen binding to cells.