Identification of the rat Heymann nephritis autoantigen (GP330) as a receptor site for plasminogen

Previous results have demonstrated the binding of a 76- and 80-kDa serum protein to the Heymann nephritis autoantigen, gp330. This 76-kDa serum protein was purified by column chromatography and preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A rabbit polyclonal antibody for the serum protein was produced and used to screen a rat liver cDNA expression library. Sequence analysis of an isolated clone identified the serum protein as plasminogen. Plasminogen was isolated from rat serum by standard techniques, and the binding of plasminogen to gp330 was confirmed by Western analysis. Enzyme-linked immunosorbent assay results demonstrated a time-dependent, saturable, and specifically inhibitable binding of plasminogen to gp330. There was no significant difference in the binding of the two carbohydrate forms of plasminogen to gp330. Plasminogen binding to gp330 could be completely inhibited by the addition of exogenous gp330. This binding could also be partially inhibited by benzamidine but only slightly by the lysine analogue, epsilon-aminocaproic acid. However, even a combination of these two inhibitors could not completely block the binding of plasminogen to gp330 indicating that gp330 may be binding to plasminogen through some other unknown interactions. These results demonstrate that gp330 is a receptor site for plasminogen.

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.

Growth factor-like effect of urokinase type plasminogen activator in human renal cells

Human renal glomerular epithelial cells possess membrane urokinase receptors. Addition of purified active urokinase to these cells in serum free minimum medium induced a dose-dependent increase in 3H-thymidine incorporation and a doubling of cell number after 48 hours of incubation. Both receptor occupancy and enzymatic activity of u-PA were required to stimulate cell proliferation. This effect was inhibited by down regulation of protein kinase C (PKC) or by H7, an inhibitor of PKC. It involved a pertussis toxin-sensitive pathway. This effect of urokinase was additive with EGF but not with thrombin growth factor activity and was not inhibited by aprotinin, an inhibitor of plasmin.

A soluble, ligand binding mutant of the human urokinase plasminogen activator receptor

A truncated version of the human urokinase plasminogen activator receptor has been obtained by in vitro mutagenesis by insertion of a premature nonsense codon in the urokinase plasminogen activator receptor cDNA. This results in a protein truncated immediately upstream of the region which appears to be required for membrane attachment of the receptor via a glycolipid anchor. The modified receptor cDNA inserted into an expression vector has been transfected into mouse LB6 cells. Transfectants produce a urokinase plasminogen activator (u-PA)-binding protein that is secreted into the medium. It can be cross-linked to iodinated ATF (amino-terminal fragment of u-PA) and can also inhibit binding of iodinated ATF to mouse LB6 cells that express the wild type human receptor. The soluble u-PA receptor will be used in a variety of experiments aimed at identifying the role and mechanism of u-PA in physiological and pathological invasive processes, as well as in therapeutical attempts to block or decrease cancer cell invasion and in general u-PA-mediated tissue destruction.

Plasmin binding to the plasminogen receptor enhances catalytic efficiency and activates the receptor for subsequent ligand binding

Specific cell surface receptors for plasminogen (Pg) are expressed by a wide variety of cell types. The colocalization of receptors for Pg and its activators restricts plasmin (Pm) activity to specific sites and serves to promote fibrinolysis and local Pg activation. These studies show that both Pg and Pm bind to cellular receptors on monocytoid U937 cells. Limited Pm pretreatment of the cells enhances total Pg binding and alters the kinetics of Pm binding. Furthermore, surface-bound Pg is converted to Pm in the absence of exogenous activators. Cell-bound Pm exhibits a 12-fold increase in catalytic efficiency (kcat/Km) relative to Pm free in solution. These studies demonstrate that Pg/Pm receptor occupancy can be regulated by Pm in the microenvironment and may play a significant regulatory role in fibrinolysis and extravascular proteolysis.

Urokinase-type plasminogen activator is expressed in stromal cells and its receptor in cancer cells at invasive foci in human colon adenocarcinomas

In this study in situ hybridization methods were used to examine biopsy samples from 13 adenocarcinomas of the colon for the presence of mRNA for the urokinase-type plasminogen activator (u-PA) and its specific cell-surface receptor (u-PAR). In all cases, u-PA mRNA was present in fibroblastlike cells in the stroma adjacent to the invasive tumor nodules. Urokinase-type plasminogen activator mRNA was not detected in the malignant cells. All specimens also contained u-PAR mRNA in cells located at the tumoral-stromal interface of invasive foci, but in contrast at least some of these cells were in all but one case identified as being of malignant origin. Stromal cells, probably tumor-infiltrating macrophages and neutrophils, also were positive in these areas. These results support the view that components of the plasminogen activation system may act to influence proteolytic events occurring at the interface between stroma and malignant cells in adenocarcinomas of the colon in humans.

The ligand-binding domain of the cell surface receptor for urokinase-type plasminogen activator

The purified urokinase plasminogen activator receptor (u-PAR) was cleaved into two fragments by mild chymotrypsin treatment. The smaller fragment (apparent Mr 16,000) possessed the ligand-binding capability, as shown by chemical cross-linking analysis. This fragment constituted the NH2-terminal part of the intact receptor, probably including the whole sequence 1-87, and contained N-linked carbohydrate. After detergent phase separation in the Triton X-114 system, the fragment was present in the water phase where its binding activity could be demonstrated in the absence of the rest of the protein. An analysis of internal homology in the amino acid sequence of u-PAR revealed the presence of three repeats of approximately 90 residues each. The ligand-binding fragment corresponds to the first repeat, supporting that this unit is a structurally autonomous domain. Domains homologous with the internal repeats of u-PAR constitute the extracellular part of Ly-6 antigens and of the squid glycoprotein Sgp-2. Like u-PAR, these proteins are attached to the membrane by a glycosyl-phosphatidylinositol anchor. The hydrophilic, ligand-binding u-PAR domain identified in the present study has potential applications in interfering with cell-surface plasmin-mediated proteolysis.

Modulation of surface-associated urokinase: binding, interiorization, delivery to lysosomes, and degradation in human keratinocytes

Receptor-mediated endocytosis of urokinase-type plasminogen activator (u-PA) was characterized with the human keratinocyte cell line NCTC, by both biochemical and ultrastructural methods. Binding to specific cell surface receptors at low temperature occurs with both catalytically active and inhibited u-PA. At 37 degrees C a single cohort of bound u-PA molecules is rapidly reduced at the surface level by both membrane dissociation and intracellular accumulation of the ligand, with no difference between active and inhibited u-PA. After a short lag period, both intact u-PA and u-PA degradation products are released into the culture medium. In the continued presence of native and inhibited u-PA at 37 degrees C the cumulative ligand uptake largely exceeds the total cellular capacity of binding sites measured at low temperature, consistent with receptor recycling. Catalytically inhibited u-PA shows a reduced interiorization rate, consistent with a requirement of an intact catalytic site which becomes evident in the presence of multiple cycles of endo-exocytosis. In the presence of a molar excess of anti-plasminogen activator inhibitor-type 1 (PAI-1) antibodies the interiorization rate is similar to that observed with catalytically inhibited u-PA, suggesting that PAI-1 molecules can modulate the intracellular accumulation of u-PA in this cell line. Parallel electron microscopy studies of a u-PA-colloidal gold complex have shown that membrane-associated u-PA molecules are concentrated in clusters before invagination of the underlying membrane to form endosomes which then fuse with lysosomes, where at least a part of u-PA degradation is likely to occur. Also, ultrastructural studies have confirmed the decrease in intracellular u-PA accumulation after inhibition of u-PA catalytic site. We conclude that cell surface-associated u-PA modulation in human keratinocytes involves ligand binding, uptake, and degradation, mediated by the classic receptor system for u-PA A chain, which can be modulated by membrane-associated PAI-1 molecules.

Urokinase receptor mRNA level and gene transcription are strongly and rapidly increased by phorbol myristate acetate in human monocyte-like U937 cells

We have studied the effect of the tumor promotor phorbol myristate acetate (PMA) on the level of mRNA for the receptor for urokinase-type plasminogen activator (u-PAR) in the human monocyte-like cell line U937. PMA causes an early increase in the u-PAR mRNA level which reaches a maximal 50-fold enhancement after 24 h of treatment. Half-maximal stimulation occurs at approximately 5 nM PMA. The effect is observed only with phorbol esters that also act as tumor promotors. The protein synthesis inhibitor cycloheximide (10 micrograms/ml) also increases the level of u-PAR mRNA. Nuclear run-on experiments show a time-dependent increase in the u-PAR gene transcription rate after exposure of the cells to PMA. The PMA-induced increase in u-PAR mRNA is paralleled by a time-dependent increase in u-PAR protein as detected by cross-linking studies with radiolabeled ligand. We conclude that PMA stimulates transcription of the u-PAR gene in U937 cells, and this is responsible at least in part for the accumulation of the u-PAR mRNA and for the subsequent increase in urokinase-binding capacity.

An autocrine role for urokinase in phorbol ester-mediated differentiation of myeloid cell lines

The human myeloid cell line HL60 secretes urokinase-type plasminogen activator (uPA) and expresses its receptor. When stimulated with phorbol myristate acetate (PMA), both secretion of uPA and the expression of its receptor are up-regulated, and these cells differentiate to an adherent phenotype. This adhesive response is markedly reduced in the presence of uPA antibodies. The PMA response is restored by the addition of native uPA, an amino-terminal fragment of uPA (residues 1-143) devoid of proteolytic activity, or a synthetic peptide (residues 12-32) from the uPA growth factor domain known to mediate receptor binding. In contrast, the addition of catalytically active low molecular weight uPA, which is missing the growth factor domain, or a peptide from the catalytic domain (residues 247-266) is ineffective. The influence of uPA antibodies on a second marker of macrophage differentiation, cysteine proteinase activity, was also examined. Cysteine proteinase activity of HL60 cells is increased in PMA-treated cells after 24 h but it fails to increase in the presence of anti-uPA. This increase in cathepsin B-like activity is also restored by exogenous uPA. These experiments indicate that an autocrine interaction of the growth factor domain of uPA with its receptor mediates an essential step in PMA-mediated myeloid cell differentiation.

Malignant transformation of human fibroblasts correlates with increased activity of receptor-bound plasminogen activator

To determine whether a relationship exists among urokinase plasminogen activator (u-PA) activity, tissue plasminogen activator (t-PA) activity, and the malignant transformation of human fibroblasts, we measured receptor-bound and secreted u-PAs and t-PA activity in fibroblast cell strains of a unique cell lineage and compared the results with the values obtained in human fibrosarcoma-derived cell lines and control cell lines. The lineage consists of four nonmalignant, infinite life span cell strains, clonally derived from a finite life span, neonatal foreskin-derived cell line or one of its derivatives and 10 malignant cell strains clonally derived from that same derivative. Seven of the latter were malignantly transformed by K-, H-, or N-ras oncogene transfection, two were obtained following carcinogen treatment, and one arose spontaneously. All 10 malignant strains in this lineage exhibited significantly higher levels of activity of receptor-bound u-PA than was found in the cell strain from which they arose or the nonmalignant cell strains derived from it. The ras oncogene-transformed malignant strains also exhibited significantly higher levels of activity of receptor-bound t-PA than their cell strain of origin. The other three malignant strains showed undetectable levels, consistent with their attaining the malignant state by an alternate process. The five fully malignant fibrosarcoma-derived cell lines tested also showed high levels of receptor-bound u-PA and t-PA. The majority (greater than or equal to 80%) of the nonmalignant control cell lines did not do so. The 10 malignant cell strains in the lineage also exhibited higher levels of activity of secreted high molecular weight u-PA or t-PA than did their cell strain of origin and the nonmalignant cell strains derived from it, as did the malignant fibrosarcoma-derived cell lines. The data suggest that the malignant state of human fibroblasts is always associated with high levels of activity of receptor-bound u-PA, and in addition cells transformed to the malignant state are very likely to exhibit high levels of receptor-bound t-PA and secreted forms of plasminogen activators.

Role of cell-surface lysines in plasminogen binding to cells: identification of alpha-enolase as a candidate plasminogen receptor

Plasminogen binding to cell surfaces results in enhanced plasminogen activation, localization of the proteolytic activity of plasmin on cell surfaces, and protection of plasmin from alpha 2-antiplasmin. We sought to characterize candidate plasminogen binding sites on nucleated cells, using the U937 monocytoid cell as a model, specifically focusing on the role of cell-surface proteins with appropriately placed lysine residues as candidate plasminogen receptors. Lysine derivatives with free alpha-carboxyl groups and peptides with carboxy-terminal lysyl residues were effective inhibitors of plasminogen binding to the cells. One of the peptides, representing the carboxy-terminal 19 amino acids of alpha 2-antiplasmin, was approximately 5-fold more effective than others with carboxy-terminal lysines. Thus, in addition to a carboxy-terminal lysyl residue, other structural features of the cell-surface proteins may influence their affinity for plasminogen. Affinity chromatography has been used to isolate candidate plasminogen receptors from U937 cells. A major protein of Mr 54,000 was recovered and identified as alpha-enolase by immunochemical and functional criteria. alpha-Enolase was present on the cell surface and was capable of binding plasminogen in ligand blotting analyses. Plasminogen binding activity of a molecular weight similar to alpha-enolase also was present in a variety of other cell types. Carboxypeptidase B treatment of alpha-enolase abolished its ability to bind plasminogen, consistent with the presence of a C-terminal lysyl residue. Thus, cell-surface proteins with carboxy-terminal lysyl residues appear to function as plasminogen binding sites, and alpha-enolase has been identified as a prominent representative of this class of receptors.

Cell associated urokinase activity and colonic epithelial cells in health and disease

It is not known if urokinase-type plasminogen activator (uPA) is associated with normal colonic epithelial cells. The aims of this study were to determine if normal colonic epithelial cells have uPA activity and whether this is concentrated at the cell membrane. In addition, the contribution of colonic epithelial cell associated uPA activity to disease related pertubations of mucosal uPA activity were examined. A highly enriched population of colonic epithelial cells was isolated from resected colon or biopsy specimens by an enzymatic technique. uPA activity was measured in cell homogenates by a specific and sensitive colorimetric method and expressed relative to cellular DNA. In two experiments subcellular fractionation of colonic epithelial cells was performed by nitrogen cavitation followed by ultracentrifugation over a linear sucrose gradient. The fractions collected were analysed for uPA and organelle-specific enzyme activities. Normal colonic epithelial cells have cell associated uPA activity (mean (SEM) 5.6 (1.1) IU/mg, n = 18). This colocalised with fractions enriched for leucine-beta-naphthylamidase and 5'-nucleotidase, markers of plasma membrane. uPA activities in epithelial cells from cancerous colons (9.8 (3.1) n = 7) or from mucosa affected by inflammatory bowel disease (3.8 (0.7) n = 15) were not significantly different from normal (paired t test), while that in epithelial cells from greatly inflamed mucosa was similar to that from autologous normal or mildly inflamed areas (4.4 (1.2) v 5.9 (3.6), n = 9). Thus normal colonic epithelial cells have cell associated uPA activity which is concentrated on the plasma membranes, suggesting the presence of uPA receptors. Increased mucosal levels of uPA previously reported in patients with inflammatory bowel disease are not due to increased colonic epithelial cell associated uPA.

Two-color cytofluorometry and cellular properties of the urokinase receptor associated with a human metastatic carcinomatous cell line

Purified human urokinase was labeled with either fluorescein isothiocyanate or iodine-125 and used as a probe for binding to the human metastatic carcinomatous cell line, Detroit 562. Cytofluorometry showed that the ligand bound preferentially to cells that had been exposed to acidic pH. The binding was competitive and decreased after mild tryptic digestion. The bound ligand could be removed by restoration of the cells to a low pH. Therefore, the cells had specific binding sites. The bound urokinase was involved in the breakdown of fibrin. Two-color cytofluorometric maps were constructed by counterstaining with propidium iodide. Results suggested that there were different cell populations that had different numbers of receptors and amounts of DNA. We cloned cells and found that single clones had homogeneous levels of receptors with different dissociation constants (from 10(-13) to 10(-11) mol/mg protein) for different clones. Cells of one clone, C5, which had high levels of receptor production, moved characteristically on a glass substratum coated with gold particles and reacted with wheat germ agglutinin, but not with concanavalin A. The receptors were found together with adhesion proteins at the sites where the cells adhered to the substrate. These results and the data obtained by zymography of the cellular proteins suggested that the urokinase-type plasminogen activators were bound to the receptors. The membrane-associated activator may stimulate local proteolysis, facilitating the migration of the tumor cell across the substrate.

Cellular receptor for urokinase plasminogen activator. Carboxyl-terminal processing and membrane anchoring by glycosyl-phosphatidylinositol

The cellular receptor for human urokinase-type plasminogen activator (u-PAR) is shown by several independent criteria to be a true member of a family of integral membrane proteins, anchored to the plasma membrane exclusively by a COOH-terminal glycosyl-phosphatidylinositol moiety. 1) Amino acid analysis of u-PAR after micropurification by affinity chromatography and N-[2-hydroxy-1,1-bis(hydroxymethyl)-ethyl]glycine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed the presence of 2-3 mol of ethanolamine/mol protein. 2) Membrane-bound u-PAR is efficiently released from the surface of human U937 cells by trace amounts of purified bacterial phosphatidylinositol-specific phospholipase C. This soluble form of u-PAR retains the binding specificity toward both u-PA and its amino-terminal fragment holding the receptor-binding domain. 3) Treatment of purified u-PAR with phosphatidylinositol-specific phospholipase C or mild alkali completely alters the hydrophobic properties of the receptor as judged by temperature-induced detergent-phase separation and charge-shift electrophoresis. 4) Biosynthetic labeling of u-PAR was obtained with [3H]ethanolamine and myo-[3H]inositol. 5) Finally, comparison of amino acid compositions derived from cDNA sequence and amino acid analysis shows that a polypeptide of medium hydrophobicity is excised from the COOH terminus of the nascent u-PAR. A similar proteolytic processing has been reported for other proteins that are linked to the plasma membrane by a glycosyl-phosphatidylinositol membrane anchor.

Plasminogen receptors in the mediation of pericellular proteolysis

A wide variety of cells bind plasminogen with very high capacity, with similar affinity and recognize the same structural features within the plasminogen molecule. As a consequence of binding to cell surfaces, plasminogen is more readily activated to plasmin. Plasmin remains cell-bound where it can degrade matrix constituents and is protected from inactivation by alpha 2-antiplasmin. Thus, the functional consequence of plasminogen binding to cells is pericellular proteolysis, permitting cell migration. Both proteins and nonprotein cell-surface constituents function as plasminogen binding sites. Gangliosides exhibit the appropriate properties of the non-protein plasminogen receptors.

Growth factor control of extracellular proteolysis

The involvement of proteases and growth factors in angiogenesis is complex. The angiogenic factor basic fibroblast growth factor (bFGF) induces increased synthesis of both plasminogen activator and collagenase in endothelial cells. In addition, bFGF increases the number of plasminogen activator receptors on the cell surface. Increased production of plasmin may be responsible for the release of soluble complexes of heparan sulfate-bFGF which may be the active form of bFGF. The activity of a negative regulator of angiogenesis, transforming growth factor beta (TGF-beta), is also regulated by proteases since the released latent form of TGF-beta is activated by a surface proteolytic assembly plasminogen activator and plasmin. Since TGF-beta induces an inhibitor of plasminogen activator, the activation reaction is self-regulatory.

Alveolar macrophage urokinase receptors localize enzyme activity to the cell surface

Human alveolar macrophages are known to synthesize urokinase (uPA) and a specific plasminogen activator inhibitor, PAI-2. In this study we have identified a uPA receptor expressed by these cells and defined the influence of PAI-2 on the interaction of uPA with its receptor. Alveolar macrophages from four normal volunteers were incubated with 55 kDa 125I-labeled uPA (0.24-8 nM) in the presence or absence of excess unlabeled uPA. Specific and saturable binding was demonstrable in all cases. Scatchard plots were linear; regression analysis revealed a mean Kd of 5.25 nM (range 3.2-6.7) and mean Bmax of 30.7 femtomoles/10(5) cells (range 21.5-34.5). The structure of the uPA receptor was defined by electroblotting membrane fractions of macrophages and sequentially exposing filters to uPA and uPA antibodies. Membranes from macrophages demonstrated binding of either uPA or a 15-kDa amino-terminal fragment of uPA to a 55- to 60-kDa glycosylated membrane protein. Binding of uPA to filters was blocked by a synthetic oligopeptide containing the known receptor binding region of native uPA. Preincubation of 125I-uPA with PAI-2 dramatically reduced the rate of association of uPA with macrophage uPA receptor. Conversely, receptor-bound uPA activity was less susceptible to inhibition by PAI-2 than soluble uPA activity. These data indicate that normal alveolar macrophages express uPA receptors. The receptor preferentially binds and protects free uPA over complexed enzyme, indicating that one function of the receptor is to allow the cells to express active uPA in an inhibitor-rich environment.

Lysosomal degradation of receptor-bound urokinase-type plasminogen activator is enhanced by its inhibitors in human trophoblastic choriocarcinoma cells

We have studied the effect of plasminogen activator inhibitors PAI-1 and PAI-2 on the binding of urokinase-type plasminogen activator (u-PA) to its receptor in the human choriocarcinoma cell line JAR. With 125I-labeled ligands in whole-cell binding assays, both uncomplexed u-PA and u-PA-inhibitor complexes bound to the receptor with a Kd of approximately 100 pM at 4 degrees C. Transferring the cells to 37 degrees C led to degradation to amino acids of up to 50% of the cell-bound u-PA-inhibitor complexes, whereas the degradation of uncomplexed u-PA was 15%; the remaining ligand was recovered in an apparently intact form in the medium or was still cell associated. The degradation could be inhibited by inhibitors of vesicle transport and lysosomal hydrolases. By electron microscopic autoradiography, both 125I-u-PA and 125I-u-PA-inhibitor complexes were located over the cell membrane at 4 degrees C, with the highest density of grains over the membrane at cell-cell interphases, but, after incubation at 37 degrees C, 17 and 27% of the grains for u-PA and u-PA-PAI-1 complexes, respectively, appeared over lysosomal-like bodies. These findings suggest that the u-PA receptor possesses a clearance function for the removal of u-PA after its complex formation with a specific inhibitor. The data suggest a novel mechanism by which receptor-mediated endocytosis is initiated by the binding of a secondary ligand.

Receptors for human plasminogen on the biological response modifier OK-432

The biological response modifier OK-432, constituting cell wall fragments from a group A Streptococcus strain and used in anticancer therapy trials, was tested for its ability to interact with different plasma proteins. The uptake of 125I-labelled protein was measured using a panel of six different plasma proteins all known to react with receptors on a majority of streptococcal strains. Of the proteins tested, plasminogen demonstrated the most substantial uptake, with uptake values ranging from 70 to 79%. A slight interaction with fibrinogen was also detected whereas no significant interaction was found with either human immunoglobulin (Ig)A, IgG, serum albumin, or mouse albumin. The results with plasminogen suggest the possibility of a new explanation of the antitumor activity described for OK-432.

Influence of urokinase on cell proliferation and invasion

The urokinase-dependent plasminogen activating system is regulated not only by zymogen to enzyme conversion of pro-urokinase and inhibition of the active enzyme by plasminogen activator inhibitors, but also by regulated expression of urokinase receptors on the cell surface. Receptor-bound pro-urokinase in turn becomes activated and is capable of activating plasminogen probably bound site by site to urokinase to a cell surface receptor. Plasmin by itself or via activation of pro-collagenase to collagenase is capable of degrading the extracellular matrix, in turn mediating processes like invasion, metastasis and tumour growth. In addition, in some cell lines the urokinase-dependent system mediated via receptor-bound active urokinase is also capable of eliciting a mitogenic response of the cells. Therefore, the urokinase-dependent plasminogen activating system might not only be responsible for mediating extravascular proteolysis but might also be an autocrine mitogen for some cell lines.

Characterization of human endothelial cell urokinase-type plasminogen activator receptor protein and messenger RNA

Human umbilical vein endothelial cells in culture (HUVEC) express receptors for urokinase-type plasminogen activators (u-PA). The immunochemical nature of this receptor and its relationship to u-PA receptors expressed by other cell types is unknown. Cross-linking active site-blocked u-PA to HUVEC lead to an increase in its apparent molecular mass by approximately 40 Kd. The predominant u-PA binding protein isolated from whole cell detergent extracts migrated with a molecular mass of approximately 36 Kd using affinity chromatography. In contrast, when only cell surface proteins were radiolabeled before extraction, the predominant labeled u-PA binding protein isolated migrated with a molecular mass of approximately 46 Kd. Several pieces of evidence suggested that the difference in molecular mass between these two u-PA binding proteins resulted from glycosylation of a single receptor protein. First, a polyclonal antibody against u-PA receptor isolated from phorbol myristate acetate (PMA) stimulated U-937 cells reacted with both the 36- and 46-Kd proteins on Western blotting. Second, the size of the unmodified receptor was estimated by amplifying a full-length cDNA for u-PA receptor from an endothelial cell cDNA library using the polymerase chain reaction (PCR) and oligonucleotide primers corresponding to the DNA sequence of the receptor cloned from transformed human fibroblasts (Roldan et al, EMBO J 9:467, 1990). The size of the cDNA (approximately 1,054 base pairs, bp) and the presence of a single 1.4-kilobase (Kb) mRNA transcript on Northern blot analysis predict an unglycosylated receptor protein of approximately 35 Kd. Third, synthesis of 35S-labeled 46-Kd cell surface receptor protein was inhibited when the cells were grown in the presence of tunicamycin, while the synthesis of the 36-Kd species was unaffected. Moreover, the apparent molecular mass of purified surface-labeled receptor (approximately 46 Kd) was reduced by N-glycanase. These studies suggest that the u-PA receptor on the surface of HUVEC is a glycoprotein derived from a protein of approximately 35 Kd which is similar immunologically to u-PA receptors on other cell types.

A high-affinity receptor for urokinase plasminogen activator on human keratinocytes: characterization and potential modulation during migration

Low passage cultures of normal human keratinocytes produce several components of the plasminogen activator/plasmin proteolytic cascade, including urokinase plasminogen activator (uPA), tissue plasminogen activator (tPA), and two specific inhibitors. Studies here presented demonstrate that these cells also contain a high-affinity (Kd = 3 x 10(-10) M) plasma membrane-binding site for uPA. High molecular weight uPA, either as the single-chain precursor or two-chain activated form, bound to the receptor; however, low molecular weight (33 kD) uPA, tPA, or epidermal growth factor did not compete for binding, demonstrating specificity. Acid treatment, which removed endogenous uPA from the receptor, was required to detect maximal binding (45,000 sites per cell). To investigate the possibility that the uPA receptor on keratinocytes may be involved in epithelial migration during wound repair, cultures were wounded and allowed to migrate into the wounded site. Binding sites for uPA were localized by autoradiographic analysis of 125I-uPA binding as well as by immunocytochemical studies using anti-uPA IgG. With both techniques uPA binding sites were detected selectively on the plasma membrane of cells at the leading edge of the migrating epithelial sheet. This localization pattern suggests that uPA receptor expression on keratinocytes may be coupled to cell migration during cutaneous wounding.

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.

Identification of determinants involved in binding of tissue-type plasminogen activator-plasminogen activator inhibitor type 1 complexes to HepG2 cells

Complexes between tissue-type plasminogen activator (t-PA) and its rapidly acting inhibitor plasminogen activator inhibitor type 1 (PAI-1) are bound, internalized, and degraded by HepG2 cells. The mechanism involves endocytosis mediated by a specific high-affinity receptor. However, the particular domains of the complex that are recognized by the receptor have not been elucidated. To identify the determinants involved in ligand binding to the receptor, several variants of t-PA were assessed for their ability to form complexes with PAI-1 and thereby to inhibit specific cellular binding of complexes between structurally unmodified 125I-t-PA and PAI-1. Catalytically active variants lacking selected structural domains form complexes with PAI-1 and inhibit 125I-t-PA.PAI-1 binding to HepG2 cells. In addition, several forms of the plasminogen activator urokinase (u-PA), which shares partial structural homology with t-PA, were evaluated as competitors of cellular binding. The catalytically active two-chain forms of u-PA, but not the inactive proenzyme single-chain form, complex with PAI-1 and inhibit specific binding of 125I-t-PA.PAI-1, suggesting that the serine protease domain, rather than other domains, may confer the determinants required for cellular binding. However, a mutant t-PA with markedly reduced catalytic activity, resulting from replacement of the active site serine with threonine, not only forms complexes with PAI-1 but also inhibits specific cellular binding of unmodified 125I-t-PA.PAI-1. These data indicate that specific binding of t-PA.PAI-1 to HepG2 cells does not require a serine-containing catalytic site in the protease domain. To determine whether binding of the complex is mediated through other components of t-PA or through structural elements of PAI-1, both t-PA and PAI-1 were examined separately for capacity to bind directly to HepG2 cells. To exclude potential interactions with components of the extracellular matrix which contains binding sites for PAI-1, ligand binding to HepG2 cells in suspension was assessed. Although neither t-PA nor PAI-1 alone binds specifically to HepG2 cells, the preformed t-PA.PAI-1 complexes do. These findings suggest that specific binding of t-PA.PAI-1 requires elements of the PAI-1 moiety and/or parts of the protease domain of t-PA.

Absence of specific cell-surface binding of tissue plasminogen activator in uterine cells

Tissue plasminogen activator (tPA), an arginine-specific serine protease, is an oestrogen-regulated protein in uterine and breast cancer tissue. It contains a domain which shares homology with epidermal growth factor (EGF). The aim of the present study was to determine whether specific tPA receptors or EGF receptors mediate the binding of tPA to cells and whether tPA possesses intrinsic mitogenic activity. The binding of 125I-labelled tPA to rat uterine and liver membranes was shown to be non-specific and could not be displaced by unlabelled tPA or EGF. Furthermore, acid washing of cell membranes did not unmask specific tPA-binding sites. In contrast, 125I-labelled EGF binding to both rat uterine and liver membranes was displaced in a dose-dependent manner by unlabelled EGF, and Scatchard analysis of the binding data revealed dissociation constant (Kd) values of 2.4 and 0.71 nM respectively. Unlabelled tPA (up to 20,000-fold excess) did not displace 125I-labelled EGF binding to these membranes. A study of the binding of 125I-labelled tPA and 125I-labelled EGF to endometrial carcinoma cells (Ishikawa), cervical carcinoma cells (HOG-1) and vulval carcinoma cells (A431) showed that up to a 100-fold excess of EGF or a 1000-fold excess of tPA did not displace 125I-labelled tPA binding to these cells. In contrast, 125I-labelled EGF binding was displaced by unlabelled EGF (Kd values for Ishikawa and HOG-1 cells were 2.72 and 1.92 nM respectively) but not by unlabelled tPA (1000-fold excess).(ABSTRACT TRUNCATED AT 250 WORDS)

The receptor for urokinase type plasminogen activator polarizes expression of the protease to the leading edge of migrating monocytes and promotes degradation of enzyme inhibitor complexes

Receptor-bound urokinase-type plasminogen activator (uPA) remains associated to the surface of human monocytes for many hours. Monocytes induced to migrate in a chemotactic gradient of f-Met-Leu-Phe rapidly polarize their uPA receptors to the leading front of the cells. Receptor-bound enzyme can be inhibited by plasminogen activator inhibitor 2 (PAI-2), with a kinetics comparable to that determined for the free enzyme, and uPA/PAI-2 complexes can bind to the uPA receptor. In contrast to the active enzyme, the uPA/PAI-2 complex is rapidly cleared from the monocyte cell surface; this involves an initial cleavage of the complex at the cell surface, followed by endocytosis and degradation. These results indicate that the uPA receptor can function both to focus plasmin-mediated extracellular matrix degradation in front of migrating cells, and to target uPA/PAI-2 enzyme/inhibitor complexes for degradation; they suggest that this receptor is a key determinant in the control of uPA-catalyzed extracellular proteolysis.

Plasminogen carbohydrate side chains in receptor binding and enzyme activation: a study of C6 glioma cells and primary cultures of rat hepatocytes

The human [Glu1]-plasminogen carbohydrate isozymes, plasminogen type I (Pg 1) and plasminogen type II (Pg 2), were separated by chromatography and studied in cell binding experiments at 4 degrees C with primary cultures of rat hepatocytes and rat C6 glioma cells. In both cell systems, Pg 1 and Pg 2 bound to an equivalent number of receptors, apparently representing the same population of surface molecules. The affinity for Pg 2 was slightly higher. With hepatocytes, the KD for Pg 1 was 3.2 +/- 0.2 microM, and the KD for Pg 2 was 1.9 +/- 0.1 microM, as determined from Scatchard transformations of the binding isotherms. The Bmax was approximately the same for both isozymes. With C6 cells, the KD for Pg 1 was 2.2 +/- 0.1 microM vs. 1.5 +/- 0.2 microM for Pg 2. Again, the Bmax was similar with both isozymes. 125I-Pg 1 and 125I-Pg 2 were displaced from specific binding sites by either nonradiolabeled isozyme. The KI for Pg 2 was slightly lower than the KI for Pg 1 with hepatocytes (0.9 vs. 1.3 microM) and with C6 cells (0.6 vs. 1.1 microM). No displacement was detected with miniplasminogen at concentrations up to 5.0 microM. Activation of Pg 1 and Pg 2 by recombinant two-chain tissue-plasminogen activator (rt-PA) was enhanced by hepatocyte cultures. The enhancing effect was greater with Pg 2. Hepatocyte cultures did not affect the activation of miniplasminogen by rt-PA or the activation of plasminogen by streptokinase. Unlike the hepatocytes, C6 cells did not enhance the activation of plasminogen by rt-PA or streptokinase; however, plasmin generated in the presence of C6 cells reacted less readily with alpha 2-antiplasmin.

Inhibition of receptor-bound urokinase by plasminogen-activator inhibitors

Urokinase-type plasminogen activator (uPA) binds to a specific receptor on various cell types, the bound molecule retaining its enzymatic activity against plasminogen. We have now investigated whether receptor-bound uPA also retains the ability to react with and be inhibited by plasminogen activator inhibitors (PAI-1 and PAI-2). uPA bound to its receptor on human U937 monocyte-like cells was inhibited by PAI-1 (in its active form in the presence of vitronectin fragments) with an association rate constant of 4.5 x 10(6) M-1 s-1, which was 40% lower than that obtained for uPA in solution (7.9 x 10(6) M-1 s-1). The inhibition of uPA by PAI-2 was decreased to a similar extent by receptor binding, falling from 5.3 x 10(5) to 3.3 x 10(5) M-1 s-1. Stimulation of U937 cells with phorbol 12-myristate 13-acetate was accompanied by a further reduction in receptor-bound uPA inhibition by PAI-1 and PAI-2 to 1.7 x 10(6) and 1.1 x 10(5) M-1 s-1, respectively. These constants although lower than those for uPA in solution still represent rather rapid inhibition of the enzyme, and demonstrate that uPA bound to its specific cellular receptor remains available for efficient inhibition by PAI's, which may therefore play a major role in controlling cell-surface plasminogen activation and extracellular proteolytic activity.

Inhibition of urokinase activity and prevention of urokinase receptor binding by monoclonal antibodies

Two murine monoclonal antibodies produced against human urokinase-type plasminogen activator were characterized with respect to their antigen-binding specificity and their effects on urokinase activity and urokinase receptor binding. One of the antibodies binds to the protease domain of urokinase (Kass = 2.1 X 10(7) M-1). Antibody binding inhibits catalysis of plasminogen activation. It does not, however, affect amidolytic activity of urokinase towards the chromogenic substrate D-Val-Leu-Arg-p-nitroanilide. The antibody thus appears to interfere with plasminogen binding without directly affecting catalytically active amino acid residues of the enzyme. The other antibody binds to the aminoterminal fragment of urokinase (Kass = 1.0 X 10(7) M-1) and prevents binding of the enzyme to high affinity receptors on human granulocytes. Binding of this antibody neither influences plasminogen activation nor the amidolytic activity of urokinase. Both antibodies are potentially useful for the further analysis and manipulation of urokinase function.

Urokinase receptors in human monocytes

Receptors for the 54 kDa plasminogen activator urokinase were characterized in freshly isolated and 5-14 day cultured human monocytes. The half saturation constant was about 55 pM in freshly isolated monocytes at 4 degrees C and 140 pM at 37 degrees C. Diisopropylfluorophosphate-inactivated urokinase was bound with the same affinity as catalytically active urokinase. Binding per cell of 2-5 pM urokinase increased progressively during cell culture with a concomitant decrease in the apparent affinity. By 14 days, binding had increased 5-7-fold and the half-saturation constant had increased to 500 pM at 4 degrees C, indicating a large increase in the binding capacity. Affinity cross-linking of labelled urokinase to receptors showed a 110 kDa complex in both freshly isolated and cultured monocytes. When cells with labelled urokinase (prebound at 4 degrees C) were incubated at 37 degrees C, about 80% of the urokinase dissociated as the intact molecule, whereas about 20% was degraded to iodide and iodotyrosine. Electron microscopic autoradiography of cultured monocytes incubated at 4 degrees C showed a marked heterogeneity between cells with regard to bound urokinase. Autoradiographic grains were mainly seen over the plasma membrane in areas rich in microvilli and invaginations. Transfer of the cells to 37 degrees C caused no major alteration in the distribution of grains. Thus, freshly prepared monocytes have urokinase receptors (approx. 55 kDa) of high affinity. Development to macrophage-like cells in culture causes a decrease in affinity and a large increase in capacity. The receptors are confined mainly to certain areas of the plasma membrane. Internalization and degradation of the ligand occurs only to a minor extent.

Different receptors mediate the hepatic catabolism of tissue-type plasminogen activator and urokinase

Tissue-type plasminogen activator (t-PA) and urokinase (u-PA) are proteins with partial structural similarity and which are of importance in the therapy of thrombotic diseases. Both are known to be cleared from the circulation in vivo by uptake in the liver. The present study investigated whether the hepatic catabolism of u-PA and t-PA is mediated by a common receptor system. Four experimental protocols of increasing complexity were used: hepatocyte plasma membranes, isolated primary hepatocytes, liver perfusion and whole animals. For t-PA, a specific high-affinity binding site to hepatocytes and plasma membranes could be defined with a mean Kd of 4 +/- 3 nM, whereas the Kd for u-PA was less than 300 nM. Binding of t-PA could not be competed for by u-PA, and vice versa. Furthermore, clearance of t-PA in isolated perfused rat livers and in rabbits in vivo was 3-fold higher than that of u-PA, and a 50-100-fold molar excess of u-PA failed to inhibit clearance of t-PA in either system, and vice versa. Taken together, the results imply that hepatic elimination of t-PA and u-PA is mediated by distinct receptor systems of differing affinity.

The human receptor for urokinase plasminogen activator. NH2-terminal amino acid sequence and glycosylation variants

The receptor for human urokinase-type plasminogen activator (u-PA) was purified from phorbol 12-myristate 13-acetate-stimulated U937 cells by temperature-induced phase separation of detergent extracts, followed by affinity chromatography with immobilized diisopropyl fluorophosphate-treated u-PA. The purified protein shows a single 55-60 kDa band after sodium dodecyl sulfate-polyacrylamide gel electrophoresis and silver staining. It is a heavily glycosylated protein, the deglycosylated polypeptide chain comprising only 35 kDa. The glycosylated protein contains N-acetyl-D-glucosamine and sialic acid, but no N-acetyl-D-galactosamine. Glycosylation is responsible for substantial heterogeneity in the receptor on phorbol ester-stimulated U937 cells, and also for molecular weight variations among various cell lines. The amino acid composition and the NH2-terminal amino acid sequence are reported. The protein has a high content of cysteine residues. The NH2-terminal sequence is not closely related to any known sequence. The identification of the purified and sequenced protein with the human u-PA receptor is based on the following findings: 1) the ability of the purified protein to bind u-PA and its amino-terminal fragment; 2) the identical electrophoretic mobilities observed for cross-linked conjugates, formed between either the purified protein or the u-PA receptor on intact U937 cells and the above ligands; 3) the identity of the apparent molecular weight of the purified protein to that predicted for the u-PA receptor in the same cross-linking studies; 4) the identical extent of glycosylation of the purified protein and of the u-PA receptor in crude membrane fractions, as detected after cross-linking; 5) the ability of antibodies raised against the purified protein to inhibit cellular binding of the amino-terminal fragment of u-PA.

Localization of urokinase-type plasminogen activator receptor on U937 cells: phorbol ester PMA induces heterogeneity

The binding of human urokinase-type plasminogen activator (u-PA) to the surface of the human monocytic cell line U937 was studied by immunological detection of bound u-PA or binding of biotinylated diisopropyl fluorophosphate-inactivated human u-PA visualized by light or electron microscopy. Untreated U937 cells showed a characteristic binding pattern, with the majority of the u-PA bound to the microvillar-containing protruding pole of the cells. After treatment with the phorbol ester PMA, the resulting adherent cell population was very heterogeneous with respect to both cellular morphology and u-PA binding. The bound u-PA was distributed on both the dorsal and the substrate side of the cells, and the patches of bound u-PA could not be correlated to any typical membrane conformations or cell-cell or cell-substratum contacts. When a monoclonal antibody directed against the amino-terminal fragment (ATF) of u-PA was used, the results were identical regardless of whether intact u-PA or ATF was used for binding to the cells. In contrast, when a monoclonal antibody recognizing the non-receptor-binding protease domain of u-PA was used, bound ATF showed no staining, while bound intact u-PA was stained as efficiently as above. The alteration of u-PA receptor distribution following treatment with PMA could be related to the changes in glycosylation and ligand affinity of the purified u-PA receptor previously described following PMA treatment of U937 cells.

Receptor-mediated internalization and degradation of urokinase is caused by its specific inhibitor PAI-1

The receptor for urokinase plasminogen activator (uPA) has been previously shown not to internalize its ligand, but rather to focalize its activity at the cell surface, allowing a regulated cell surface plasmin dependent proteolysis. The receptor in fact binds the proenzyme pro-uPA and allows its very efficient conversion to the active two chains form. Receptor bound active uPA can also interact with its specific type 1 inhibiror (PAI-1) which is therefore able to inhibit the cell surface plasmin formation. In this paper we show that the uPA-PAI-1 complex bound to the uPA receptor is internalized and degraded. U937 cells were incubated at 4 degrees C with labeled uPA-PAI-1 (and other ligands), the temperature then raised to 37 degrees C and the fate of the ligand followed for 3 h thereafter. The uPA-PAI-1 complex was internalized into the cells (i.e. could not be dissociated by acid treatment) and thereafter degraded (i.e. appeared in the supernatant in a non TCA-precipitable form). Other ligands (free uPA, ATF and DFP-treated uPA) were not internalized nor degraded. The degradation of the uPA-PAI-1 complex is preceded by internalization and is inhibited by chloroquine, an inhibitor of lysosomal protein degradation. These data suggest the existence of a cellular cycle of uPA. After synthesis pro-uPA is secreted, bound to the receptor and activated to two chain uPA. On the surface, uPA can activate surface bound plasminogen to produce surface bound plasmin. In the presence of PAI-1 uPA activity is inhibited and plasmin production interrupted, while the uPA-PAI-1 complex is internalized and degraded.

Role of specific membrane receptors in urokinase-dependent migration of human keratinocytes

On the basis of both 125I-labeled plasminogen activator binding analysis and transmission electron microscopy studies of the interaction of a plasminogen activator/gold complex with cell membranes, we have found that human keratinocytes have specific receptors for human urokinase-type plasminogen activator distributed on the cell surface as singlets, or as small or large clusters. The use in binding experiments of the purified A chain of urokinase-plasminogen activator and of anti-A chain monoclonal antibodies has indicated that cell receptors are specific for a sequence present on the A chain, as previously reported for other cells. The interaction of both the native molecule and the purified A chain with such receptors stimulates mobilization of keratinocytes in an in vitro cell model system (Boyden chamber), when present in the lower compartment of the migration apparatus in nanomolar concentrations. Preincubation of chemoattractants with a monoclonal antibody which prevents receptor/ligand interaction also prevents plasminogen activator-induced cell migration. These data suggest that, under the conditions used in this in vitro model system, the plasminogen activator-dependent mobilization of keratinocytes depends on the interaction of the ligand with free receptors on the cell surface, and is independent of plasmin generation.

Urokinase receptors in lung cancer and normal lung

Cell membranes from ten non-small cell lung cancers and four specimens of adjacent lung tissue were assessed for the presence of urokinase type plasminogen activator (uPA) receptors. Displacement binding studies using 125I labelled urokinase showed specific binding on lung cancer and lung membrane preparations. Scatchard analysis showed that the dissociation constant of high affinity sites on tumour membranes was 2.9 x 10(-11) M/1 and on lung membranes was 2 x 10(-9) M/1. The concentration of high affinity binding sites on tumour membrane was 54 fmol/mg of membrane protein and on normal lung membrane was 170 fmol/mg protein. Two-point binding assays showed specific binding of urokinase on five of eight tumour membranes and one of three normal lung membranes. There was no correlation between the amount of urokinase bound and tumour subtype or extent of disease. Because of interactions between uPA and epidermal growth factor receptors (EGFr) in cell culture and because lung cancers express increased EGFr we studied the association of uPA receptors and EGFr. Seven tumours expressed EGFr at 6.8-67.6 fmol/mg of protein of EGFr and four normal lung membranes had EGFr at 5.2-15.6 fmol/mg protein EGFr. There was no correlation between uPA receptors and EGFr in this series. We conclude that non-small cell lung cancers carry receptors for urokinase and this provides a novel mechanism for control of local proteolysis.

Interaction of urokinase A chain with the receptor of human keratinocytes stimulates release of urokinase-like plasminogen activator

On the basis of a fibrinolytic assay with 125I-fibrin, zymography, and immunoprobing with anti-human urokinase antibody, we have observed that the in vitro established NCTC human keratinocyte cell line releases into the culture medium a 54,000-Da plasminogen activator which is indistinguishable from human urokinase. Only the early release following the washing of keratinocyte monolayers is accounted for by secretion of preformed enzyme, while late secretory events require the de novo synthesis of urokinase. The released enzyme can interact by autocriny with its own receptor present on keratinocytes. The addition to the keratinocyte culture medium of the urokinase A chain can stimulate a concentration-dependent urokinase oversecretion, which is not paralleled by oversecretion of plasminogen activator inhibitor-1. Since stimulation of urokinase production can be obtained by an A chain concentration (5 ng/ml) which was previously shown to be efficient in inducing keratinocyte mobilization in an in vitro migration model system, we hypothesize that this mechanism may be important in vivo during the process of wound repair.

Interaction of single-chain urokinase-type plasminogen activator with human endothelial cells

The interaction of urokinase-type plasminogen activators with receptors on the surface of endothelial cells may play an important role in the regulation of fibrinolysis and cell migration. Therefore, we investigated whether human umbilical vein endothelial cells (HUVEC) express receptors for single-chain urokinase (scu-PA) on the cell surface and examined the effect of such binding on plasminogen activator activity. Binding of 125I-labeled scu-PA to HUVEC, performed at 4 degrees C, was saturable, reversible, and specific (k+1 4 +/- 1 X 10(6) min-1 M-1, k-1 6.2 +/- 1.4 X 10(-3) min-1, Kd 2.8 +/- 0.1 nM; Bmax 2.2 +/- 0.1 X 10(5) sites/cell; mean +/- S.E.). Binding of radiolabeled scu-PA was inhibited by both natural and recombinant wild-type scu-PA, high molecular weight two-chain u-PA (tcu-PA), catalytic site-inactivated tcu-PA, an amino-terminal fragment of u-PA (amino acids 1-143), and a smaller peptide (amino acids 4-42) corresponding primarily to the epidermal growth factor-like domain. Binding was not inhibited by low molecular weight urokinase or by a recombinant scu-PA missing amino acids 9-45. Cell-bound scu-PA migrated at its native molecular mass on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In the presence of plasminogen, scu-PA bound to endothelial cells generated greater plasmin activity than did scu-PA in the absence of cells. In contrast, when tcu-PA was added directly to HUVEC, sodium dodecyl sulfate-stable complexes formed with cell or matrix-associated plasminogen activator inhibitors with a loss of plasminogen activator activity. These studies suggest that endothelial cells in culture express high affinity binding sites for the epidermal growth factor domain of scu-PA. Interaction of scu-PA with these receptors may permit plasminogen activator activity to be expressed at discrete sites on the endothelial cell membrane.

Identification and characterization of human endothelial cell membrane binding sites for tissue plasminogen activator and urokinase

Cultured human endothelial cells synthesize and secrete two types of plasminogen activator, tissue plasminogen activator (t-PA) and urokinase (u-PA). Previous work from this laboratory (Hajjar, K.A., Hamel, N. M., Harpel, P. C., and Nachman, R. L. (1987) J. Clin. Invest. 80, 1712-1719) has demonstrated dose-dependent, saturable, and high affinity binding of t-PA to two sites associated with cultural endothelial cell monolayers. We now report that an isolated plasma membrane-enriched endothelial cell fraction specifically binds 125I-t-PA at a single saturable site (Kd 9.1 nM; Bmax 3.1 pmol/mg membrane protein). Ligand blotting experiments demonstrated that both single and double-chain t-PA specifically bound to a Mr 40,000 membrane protein present in detergent extracts of isolated membranes, while high molecular weight, low molecular weight, and single-chain u-PA associated with a Mr 48,000 protein. Both binding interactions were reversible and cell-specific and were inhibitable by pretreatment of intact cells with nanomolar concentrations of trypsin. The relevant binding proteins were not found in subendothelial cell matrix, failed to react with antibodies to plasminogen activator inhibitor type 1 and interacted with their respective ligands in an active site-independent manner. The isolated t-PA binding site was resistant to reduction and preserved the capacity for plasmin generation. In contrast, the isolated u-PA binding protein was sensitive to reduction, and did not maintain the catalytic activity of the ligand on the blot. The results suggest that in addition to sharing a matrix-associated binding site (plasminogen activator inhibitor type 1), both t-PA and u-PA have unique membrane binding sites which may regulate their function. The results also provide further support for the hypothesis that plasminogen and t-PA can assemble on the endothelial cell surface in a manner which enhances cell surface generation of plasmin.

Cloning and expression of the receptor for human urokinase plasminogen activator, a central molecule in cell surface, plasmin dependent proteolysis

The surface receptor for urokinase plasminogen activator (uPAR) has been recognized in recent years as a key molecule in regulating plasminogen mediated extracellular proteolysis. Surface plasminogen activation controls the connections between cells, basement membrane and extracellular matrix, and therefore the capacity of cells to migrate and invade neighboring tissues. We have isolated a 1.4 kb cDNA clone coding for the entire human uPAR. An oligonucleotide synthesized on the basis of the N-terminal sequence of the purified protein was used to screen a cDNA library made from SV40 transformed human fibroblasts [Okayama and Berg (1983) Mol. Cell Biol., 3, 280-289]. The cDNA encodes a protein of 313 amino acids, preceded by a 21 residue signal peptide. A hydrophobicity plot suggests the presence of a membrane spanning domain close to the C-terminus. The cDNA hybridizes to a 1.4 kb mRNA from human cells, a size very close to that of the cloned cDNA. Expression of the uPAR cDNA in mouse cells confirms that the clone is complete and expresses a functional uPA binding protein, located on the cell surface and with properties similar to the human uPAR. Caseinolytic plaque assay, immunofluorescence analysis, direct binding studies and cross-linking experiments show that the transfected mouse LB6 cells specifically bind human uPA, which in turn activates plasminogen. The Mr of the mature human receptor expressed in mouse cells is approximately 55,000, in accordance with the naturally occurring, highly glycosylated human uPAR. The Mr calculated on the basis of the cDNA sequence, approximately 35,000, agrees well with that of the deglycosylated receptor.

Receptors for human plasminogen on gram-negative bacteria

A total of 188 strains representing 11 species of gram-negative bacteria were examined for the ability to interact with human plasminogen. Highly purified human plasminogen was labeled with 125I, and its uptake by different bacterial strains was measured. All 14 strains of Haemophilus influenzae and all 13 strains of Branhamella catarrhalis tested were positive with respect to plasminogen uptake. Also, eight species belonging to the family Enterobacteriaceae were tested, and of those, Proteus mirabilis demonstrated the most substantial uptake, with 28 of 39 strains taking up more than 10% of the plasminogen. Ten strains of Pseudomonas aeruginosa were also tested, of which seven showed uptake values higher than 10%. With H. influenzae and B. catarrhalis strains, Scatchard analysis indicated a two-phase receptor interaction, one more-avid receptor with a Kd of 6 to 8 nM and 2,000 to 2,500 sites per bacterium and a second receptor with a Kd of 50 to 80 nM and 9,000 sites per bacterium. With Pseudomonas aeruginosa strains, a single receptor interaction was detected with a Kd of 60 nM and the number of sites was estimated as 8,000 per bacterium. Scatchard analysis with strains of P. mirabilis indicated binding of a less-specific nature. However, plasminogen uptake by this species could be reduced by 50% by the addition of 2 mM unlabeled plasminogen. This estimate of Kd, as well as uptake studies with plasminogen fragments, suggests different properties of this receptor. With all receptor types, the addition of plasmin-aprotinin complex inhibited plasminogen uptake, which demonstrates that both forms of the molecule react with the same receptors. Plasminogen uptake could be eliminated by the addition of lysine or epsilon-aminocaproic acid, which suggests that the lysine-binding sites of the plasminogen molecule are involved in the receptor-ligand interaction.

Local function of urokinase receptor at the adhesion contact sites of a metastatic tumor cell

The local activity of urokinase and its receptor associated with a cloned cell (C5) obtained from the cloning of Detroit 562 was investigated. The cellular binding sites, similar structure to adhesion plaques, were visualized by fluorescein labeled urokinase and the number was determined to be 300 per cell. The binding sites for radioiodinated urokinase were found to be 30 thousand per cell. Thus, about as many as 100 receptor molecules per site was estimated to be associated with the cellular membrane domains. Immunofluorescence studies demonstrated that the receptors were colocalized with a set of adhesion and cytoskeletal proteins such as vinculin, alpha-actinin and actin; localizing at the adhesion sites. These proteins soluble in 9 M urea were able to be reconstituted by dialyzing out the urea against low ionic buffer solution. It was demonstrated that vinculin and actin were co-associated. Since cell bound urokinase revealed fibrinolytic activity, it was suggested that the focal adhesions of the migrating cells would facilitate proteolytic action when cells move across the matrix architectures.