A cleaved form of the receptor for urokinase-type plasminogen activator in invasive transplanted human and murine tumors

Seroprevalence of Helicobacter pylori/CagA Antibodies in Guatemalan Gastric Cancer Patients: Association of Seropositivity with Increased Plasma Levels of Pepsinogens but not Soluble Urokinase Plasminogen Activator Receptor

Infection by Helicobacter pylori is a major risk factor for gastric cancer (GC), the second leading cause of cancer-related death worldwide. Although biomarkers such as pepsinogens (PGs) and soluble urokinase plasminogen activator receptor (suPAR) may have diagnostic and/or prognostic value in patients with GC, their levels may be affected by H. pylori infection. The aim of this study was to investigate the association of the presence of antibodies to H. pylori and cytotoxin-associated gene A (CagA) with plasma levels of PGs and suPAR in a cohort of Guatemalan GC patients and controls. To this end, levels of suPAR, Pepsinogens I and II (PGI and PGII), and antibodies to H. pylori and CagA toxin were determined by ELISA in plasma samples from 67 GC patients and 136 matched healthy controls. Seropositivity for CagA was significantly higher in patients with GC than in controls. Pepsinogens II and suPAR levels were higher and PGI/PGII ratios were lower in GC patients than in controls. There was a significant association of H. pylori seropositivity status with increased levels of PGII and lower PGI/PGII ratios, particularly in the control (non-GC) population. The levels of suPAR were not significantly affected by H. pylori or CagA seropositivity status. These results suggest that the seropositivity status for H. pylori and CagA need to be taken into account during the GC diagnostic process.

Soluble Urokinase Plasminogen Activator Receptor Measurements: Influence of Sample Handling

Aim

The influence of sample handling on soluble urokinase plasminogen activator receptor (suPAR) concentrations in serum and EDTA plasma was studied in 16 healthy premenopausal women.

Method

Blood was collected in dry tubes and tubes containing EDTA and kept at 4°C or 20°C for 1, 3, 8, 24 or 72 hours before processing into serum or EDTA plasma. In addition, serum and EDTA plasma were frozen and thawed 1–8 times. All suPAR measurements were performed by ELISA.

Results

No significant differences were found between serum or EDTA plasma suPAR concentrations when whole blood samples were kept for 1, 3, 8 or 24 hours. Significantly higher suPAR levels were found in samples kept for 72 hours at 20°C compared to samples processed into serum or EDTA plasma after short-term storage for no more than 24 hours after collection. No significant differences were observed when whole blood was kept at 4°C for up to 72 hours. Repeated freezing and thawing had no significant effect on the serum and EDTA plasma suPAR levels.

Conclusion

suPAR values in blood samples are dependent on the handling procedures of the samples. All samples of whole blood must be processed into EDTA plasma or serum within 24 hours if kept at 20°C and within 72 hours if kept at 4°C. However, repeated freezing/thawing cycles had no influence on suPAR values in the samples.

Diversity and functional evolution of the plasminogen activator system

The urokinase plasminogen activator system is a family of serine proteases which consists of uPA (urokinase plasminogen activator), uPAR (urokinase type plasminogen activator receptor) and PAI-1 (plasminogen activator inhibitor 1). In addition to their significant roles in activation, these proteases act as key regulators of the tumor microenvironment and are involved in the metastatic process in many cancers. High levels of uPA system proteases in many human cancer predicts poor patient prognosis and strongly indicated a key role of uPA system in cancer metastasis. Individual components of uPA system are found to be differentially expressed in cancer cells compared to normal cells and therefore are potential therapeutic targets. In this review, we present the molecular and cellular mechanisms underlying the role of uPA system in cancer progression. Epithelial to mesenchymal transitions (EMT) is the main cause of the cancer cell metastasis. We have also attempted to relate the role of uPA signaling in EMT of cancer cells.

Copenhagen uPAR prostate cancer (CuPCa) database: protocol and early results

AIM

Urokinase plasminogen activator receptor (uPAR) plays a central role during cancer invasion by facilitating pericellular proteolysis. We initiated the prospective 'Copenhagen uPAR Prostate Cancer' study to investigate the significance of uPAR levels in prostate cancer (PCa) patients.

METHODS

Plasma samples and clinical data from patients with newly diagnosed PCa have been collected prospectively. The uPAR forms have been measured in plasma using time-resolved fluorescence immunoassays.

RESULTS

The level of intact uPAR(I-III) did not differ. Plasma uPAR(I-III) + uPAR(II-III) levels and uPAR(I) levels were significantly higher in hormone-naive and castrate-resistant patients compared with patients with localized disease (both: p < 0.0001).

CONCLUSION

Our results show that cleaved uPAR forms are significantly increased in patients with advanced PCa.

Tumour microenvironments induce expression of urokinase plasminogen activator receptor (uPAR) and concomitant activation of gelatinolytic enzymes

Background

The urokinase plasminogen activator receptor (uPAR) is associated with poor prognosis in oral squamous cell carcinoma (OSCC), and increased expression of uPAR is often found at the invasive tumour front. The aim of the current study was to elucidate the role of uPAR in invasion and metastasis of OSCC, and the effects of various tumour microenvironments in these processes. Furthermore, we wanted to study whether the cells’ expression level of uPAR affected the activity of gelatinolytic enzymes.

Methods

The Plaur gene was both overexpressed and knocked-down in the murine OSCC cell line AT84. Tongue and skin tumours were established in syngeneic mice, and cells were also studied in an ex vivo leiomyoma invasion model. Soluble factors derived from leiomyoma tissue, as well as purified extracellular matrix (ECM) proteins, were assessed for their ability to affect uPAR expression, glycosylation and cleavage. Activity of gelatinolytic enzymes in the tissues were assessed by in situ zymography.

Results

We found that increased levels of uPAR did not induce tumour invasion or metastasis. However, cells expressing low endogenous levels of uPAR in vitro up-regulated uPAR expression both in tongue, skin and leiomyoma tissue. Various ECM proteins had no effect on uPAR expression, while soluble factors originating from the leiomyoma tissue increased both the expression and glycosylation of uPAR, and possibly also affected the proteolytic processing of uPAR. Tumours with high levels of uPAR, as well as cells invading leiomyoma tissue with up-regulated uPAR expression, all displayed enhanced activity of gelatinolytic enzymes.

Conclusions

Although high levels of uPAR are not sufficient to induce invasion and metastasis, the activity of gelatinolytic enzymes was increased. Furthermore, several tumour microenvironments have the capacity to induce up-regulation of uPAR expression, and soluble factors in the tumour microenvironment may have an important role in the regulation of posttranslational modification of uPAR.

Discrimination of different forms of the murine urokinase plasminogen activator receptor on the cell surface using monoclonal antibodies

The urokinase plasminogen activator receptor (uPAR) is a versatile three-domain GPI-anchored protein, which binds urokinase plasminogen activator (uPA) and thereby focalises plasminogen activation on the cell surface. Generation of a proteolytic potential is essential in both normal physiological and pathological extracellular tissue remodelling processes. uPA can also cleave uPAR, resulting in liberation of the amino-terminal domain I, which encompasses binding sites for both uPA and the adhesion molecule, vitronectin. In order to localise the different uPAR forms on the plasma membrane of murine monocyte macrophage-like P388D.1 cells, we have now generated and characterised two high-affinity murine mAbs, mR3 and mR4, raised against murine uPAR. mR3 was found to recognise an epitope located in domain I of uPAR. Surface plasmon resonance analyses and cell binding studies revealed that this mAb was able to bind preformed complexes of murine pro-uPA and murine uPAR. In contrast, mR4 recognises domains II-III in uPAR and does not bind preformed pro-uPA-uPAR complexes in similar analyses. Immunofluorescence microscopy of P388D.1 cells revealed that mR3 stained the cells equally well in the presence or absence of saturation with the amino-terminal fragment of uPA, ATF. However, the signal intensity obtained using another uPAR domain I specific mAb, mR1, was significantly reduced upon ATF saturation. Furthermore, when adding ATF, mR4 selectively stained the cleaved receptor. Applying these newly generated mAbs, we additionally demonstrated that cleaved and intact uPAR was evenly distributed on the surface of these cells.

Enhanced Discrimination of Benign from Malignant Prostatic Disease by Selective Measurements of Cleaved Forms of Urokinase Receptor in Serum

Background: Early detection of prostate cancer (PCa) centers on measurements of prostate-specific antigen (PSA), but current testing practices suffer from lack of specificity and generate many unnecessary prostate biopsies. Soluble urokinase plasminogen activator receptor (uPAR) is present in blood in both intact and cleaved forms. Increased uPAR in blood is correlated with poor prognosis in various cancers, but uPAR has not been shown to be useful in PCa diagnostics. We assessed the ability of immunoassays for specific uPAR forms to discriminate PCa from benign conditions.

Methods: We measured total PSA (tPSA), free PSA (fPSA), intact uPAR [uPAR(I-III)], intact uPAR + cleaved uPAR domains II+III [uPAR(I-III) + uPAR(II-III)], and cleaved uPAR domain I [uPAR(I)] in sera from 224 men with and 166 men without PCa. We assessed differences in serum concentrations between the PCa and noncancer groups within the entire cohort and in men with tPSA concentrations of 2–10 μg/L. The diagnostic accuracy of individual analytes and analyte combinations was explored by logistic regression and ROC analyses and evaluations of sensitivity and specificity pairs.

Results: Serum uPAR(I) and uPAR(II-III) were higher in PCa than in benign disease. In men with tPSA between 2 and 10 μg/L, the combination of %fPSA with the ratio uPAR(I)/uPAR(I-III) had a greater area under the ROC curve (0.73) than did %fPSA (0.68).

Conclusions: Specific measurements of different uPAR forms in serum improve the specificity of PCa detection. The uPAR forms may therefore be complementary to PSA for PCa detection, most importantly in men with moderately increased PSA.

Requirement of the enzymatic and signaling activities of plasmin for phorbol-ester-induced scattering of colon cancer cells

Colon cancer progression is associated with the activation of protein kinase C (PKC), the downregulation of functional E-cadherin and an increased expression of the serine protease urokinase (u-PA) and its receptor (u-PAR). HT29-M6 intestinal epithelial cells represent an in vitro model to study colon cancer progression. These cells are induced to scatter and to invade by phorbol esters. Using proteolytic and cell signaling inhibitors, we show that HT29-M6 cells require plasminogen for the acquisition of the scattering response to PMA. Our results indicate that, prior to inducing a state of competency for plasminogen-dependent scattering, PMA triggers an ordered succession of events where upregulation of the activity of u-PA precedes proteolysis of u-PAR and active degradation of the extracellular matrix (ECM). These events poise HT29-M6 cells to a scatter-competent state that allows the subsequent localized proteolytic activation of plasminogen to plasmin, required for the execution of scattering. Finally, we show that, in addition to its enzymatic activity directed at the degradation of ECM, plasmin generates an intracellular signal resulting in the phosphorylation of ERK 1/2. For a full motogenic activity, plasmin requires this signal since the use of a MEK inhibitor (PD98059) specifically blocks the plasmin-dependent phase of cell scattering. Our observations suggest that plasmin exerts a dual role in PMA-induced scattering of HT29-M6 cells, one directed extracellularly to promote proteolysis of the ECM and one directed to generate intracellular signaling.

Gelatinase-mediated migration and invasion of cancer cells

The matrix metalloproteinases(MMP)-2 and -9, also known as the gelatinases have been long recognized as major contributors to the proteolytic degradation of extracellular matrix during tumor invasion. In the recent years, a plethora of non-matrix proteins have also been identified as gelatinase substrates thus significantly broadening our understanding of these enzymes as proteolytic executors and regulators in various physiological and pathological states including embryonic growth and development, angiogenesis and tumor progression, inflammation, infective diseases, degenerative diseases of the brain and vascular diseases. Although the effect of broad-spectrum inhibitors of MMPs in the treatment of cancer has been disappointing in clinical trials, novel mechanisms of gelatinase inhibition have been now identified. Inhibition of the association of the gelatinases with cell-surface integrins appears to offer highly specific means to target these enzymes without inhibiting their catalytic activity in multiple cell types including endothelial cells, tumor cells and leukocytes. Here, we review the multiple functions of the gelatinases in cancer, and especially their role in the tumor cell migration and invasion.

Plasmin cleaves the juxtamembrane domain and releases truncated species of the urokinase receptor (CD87) from human bronchial epithelial cells

The three-domain (D1D2D3) urokinase receptor (CD87) is highly susceptible to cleavage within the D1-D2 linker sequence, but also within the juxtamembrane region by yet poorly characterized proteinases, allowing the release of D1 and D2D3 species in various (patho)physiological body fluids. Using immunoblot analysis and ELISA applied to a recombinant soluble CD87 and to CD87-expressing epithelial cells, we establish that exogenous or in situ generated plasmin proteolyzes CD87 in the D1-D2 linker and D3 carboxyterminal sequences, producing a major soluble D2D3 species. Mass spectrometry analysis of the fragmentation of CD87-related synthetic peptides, and aminoterminal sequencing of D2D3 reveal Arg83, Arg89, and Arg281 as residues targeted by plasmin within human CD87.

Specific Immunoassays for Detection of Intact and Cleaved Forms of the Urokinase Receptor

Background: The cell surface receptor (uPAR) for urokinase plasminogen activator (uPA) is a strong prognostic marker in several types of cancer. uPA cleaves the three-domain protein uPAR(I-III) into two fragments: uPAR(I), which contains domain I; and uPAR(II-III), which contains domains II and III. Established immunoassays measure a combination of uPAR forms. Our aim was to design immunoassays for specific quantification of the individual forms of uPAR.

Methods: Using appropriate combinations of epitope-mapped monoclonal antibodies (Mabs) for capture and europium-labeled detection Mabs, we designed two-site sandwich time-resolved fluorescence immunoassays (TR-FIAs): TR-FIA 1 to measure uPAR(I-III) alone; TR-FIA 2 to measure both uPAR(I-III) and uPAR(II-III); and TR-FIA 3 to measure uPAR(I). To avoid detection of uPAR(I-III) in TR-FIA 3, we used a combination of the peptide uPAR antagonist AE120 and a domain I antibody, R3. AE120 blocks the binding of R3 to uPAR(I-III). In contrast, AE120 does not interact with liberated domain I and therefore does not interfere with the binding of R3 to uPAR(I).

Results: The limits of quantification (CV &lt;20%) determined by adding the proteins to uPAR-depleted plasma were &lt;3 pmol/L in all three assays. The interassay CVs in plasma with added analytes were &lt;11%, and recoveries were between 93% and 105%. Cross-reactivities of purified proteins in the three TR-FIAs were no more than 4%. Studies on chymotrypsin cleavage of uPAR and size-exclusion chromatography of plasma with and without added protein further supported the specificity of the assays.

Conclusions: The three novel TR-FIAs accurately quantify uPAR(I-III) alone, uPAR(I-III) together with uPAR(II-III), and uPAR(I), respectively, in biological samples, including plasma, and thus are well suited for studies of the diagnostic and prognostic value of individual uPAR forms in cancer patients.

The urokinase receptor (uPAR) and the uPAR-associated protein (uPARAP/Endo180): membrane proteins engaged in matrix turnover during tissue remodeling

The breakdown of the barriers formed by extracellular matrix proteins is a pre-requisite for all processes of tissue remodeling. Matrix degradation reactions take part in specific physiological events in the healthy organism but also represent a crucial step in cancer invasion. These degradation processes involve a highly organized interplay between proteases and their cellular binding sites as well as specific substrates and internalization receptors. This review article is focused on two components, the urokinase plasminogen activator receptor (uPAR) and the uPAR-associated protein (uPARAP, also designated Endo180), that are considered crucially engaged in matrix degradation. uPAR and uPARAP have highly diverse functions, but on certain cell types they interact with each other in a process that is still incompletely understood. uPAR is a glycosyl-phosphatidylinositol-anchored glycoprotein on the surface of various cell types that serves to bind the urokinase plasminogen activator and localize the activation reactions in the proteolytic cascade system of plasminogen activation. uPARAP is an integral membrane protein with a pronounced role in the internalization of collagen for intracellular degradation. Both receptors have additional functions that are currently being unraveled. The present discussion of uPAR and uPARAP is centered on their protein structure and molecular and cellular function.

Proteolytic Regulation of the Urokinase Receptor/CD87 on Monocytic Cells by Neutrophil Elastase and Cathepsin G

The urokinase receptor (CD87) participates to the pericellular proteolytic potential of migrating cells and to the recruitment of leukocytes during inflammation. It consists of three structurally homologous domains, with the C-terminal domain D3 attached to cell membranes through a GPI anchor. CD87 is susceptible to an endoproteolytic processing removing the N-terminal domain D1 and generating truncated D2D3 membrane species, thus modulating CD87-associated functions. Full-length or truncated CD87 can be also released from cells via juxtamembrane cleavage by phospholipases and/or by yet unidentified proteinases. Using a recombinant CD87 and the CD87-positive monocytic U937 cell line and isolated blood monocytes, we show by protein immunoblotting and flow immunocytometry that the human neutrophil serine-proteinases elastase and cathepsin G cleave CD87 within the D1-D2 linker sequence, while in addition cathepsin G is highly efficient in cleaving the C terminus of D3. The combination of cathepsin G and elastase provided by degranulated neutrophils results in enzymatic cooperation leading to the release from monocytic cells of a truncated D2D3 species resembling that previously described in pathological body fluids. Using mass spectrometry analysis, the proteolytic fragmentation of synthetic peptides mapping the D1-D2 linker and D3 C-terminal domains identifies potential cleavage sites for each enzyme and suggests the existence of a mechanism regulating the CD87(D1-D2)-associated chemotactic activity. Finally, isolated or combined elastase and cathepsin G drastically reduce the capacity of cells to bind urokinase. Secretable leukocyte serine-proteinases are thus endowed with high potential for the regulation of CD87 expression and function on inflammatory cells.

The urokinase receptor can be induced by Borrelia burgdorferi through receptors of the innate immune system

Monocytic cells exposed to Borrelia burgdorferi, through unknown receptors, overexpress the urokinase receptor (uPAR), a key mediator of the plasminogen activation system. We show that combined blockade of CD14 and TLR2 causes a significant inhibition of B. burgdorferi-induced uPAR in Mono Mac 6 (MM6) cells. Other pattern recognition receptors tested (CD11b/CD18, the mannose receptor, and the N-formyl-methionyl-leucyl-phenylalanine receptor) did not have demonstrated roles in B. burgdorferi-mediated uPAR induction. We dissected the result for CD14 andTLR2 by investigating the singular contributions of each. Independent functional blockade of CD14 or TLR2 failed to inhibit B. burgdorferi-mediated uPAR induction. 1,25-Dihydroxyvitamin D(3) differentiation of MM6 cells increased CD14 expression 12-fold but did not augment B. burgdorferi-mediated uPAR expression. Peritoneal exudate macrophages (PEM) from CD14- or TLR2-deficient mice were not defective in B. burgdorferi-mediated synthesis of uPAR mRNA and protein. Increased uPAR mRNA or protein or both were apparent in PEM from transgenic and control mice, even at a ratio of one Borrelia spirochete per cell. We conclude that signaling for the uPAR response, as mediated by B. burgdorferi, proceeds with CD14 and TLR2 as partial contributors. That part under control of CD14 and TLR2 represents a new link between the host plasminogen activation and innate immunity systems.

VEGF-induced paracellular permeability in cultured endothelial cells involves urokinase and its receptor

Vascular endothelial growth factor/vascular permeability factor (VEGF) has been implicated in blood/tissue barrier dysfunctions associated with pathological angiogenesis, but the mechanisms of VEGF-induced permeability increase are poorly understood. Here, the role of VEGF-induced extracellular proteolytic activities on the endothelial cell permeability increase is evaluated. Confluent monolayers of bovine retinal microvascular endothelial (BRE) cells grown on porous membrane were treated with VEGF or urokinase plasminogen activator (uPA), and permeability changes were analyzed. uPA-induced permeability was rapid and sustained, but VEGF-induced permeability showed a biphasic pattern: a rapid and transient phase (1-2 h) followed by delayed and sustained phase (6-24 h). The delayed, but not the early phase of VEGF-induced permeability, was blocked by anti-uPA or anti-uPAR (uPA receptor) antibodies and was accompanied by reduced transendothelial electrical resistance, indicating the paracellular route of permeability. Confocal microscopy and Western blotting showed that VEGF treatment increased free cytosolic beta-catenin, which was followed by beta-catenin nuclear translocation, upregulation of uPAR, and downregulation of occludin. Membrane-bound occludin was released immediately after uPA treatment, but with a long delay after VEGF treatment, suggesting a requirement for uPAR gene expression. In conclusion, VEGF induces a sustained paracellular permeability in capillary endothelial cells that is mediated by activation of the uPA/uPAR system.

The Cleavage of the Urokinase Receptor Regulates Its Multiple Functions

The urokinase-type plasminogen activator (uPA) is able to cleave its cell surface receptor (uPAR) anchored to the cell membrane through a glycophosphatidylinositol tail. The cleavage leads to the formation of cell surface truncated forms, devoid of the N-terminal domain 1 (D1) and unmasks or disrupts, depending on the cleavage site, a sequence in the D1-D2 linker region (residues 88-92), which in the soluble form is a potent chemoattractant for monocyte-like cells. To investigate the possible role(s) of the cleaved forms of cell surface glycophosphatidylinositol-anchored uPAR, uPAR-negative human embrional kidney 293 cells were transfected with the cDNA of intact uPAR (uPAR-293) or with cDNAs corresponding to the truncated forms of uPAR exposing (D2D3-293) or lacking (D2D3wc-293) the peptide 88-92 (P88-92). Cell adhesion assays and co-immunoprecipitation experiments indicated that the removal of D1, independently of the presence of P88-92, abolished the lateral interaction of uPAR with integrins and its capability to regulate integrin adhesive functions. The expression of intact uPAR induced also a moderate increase in 293 cell proliferation, which was accompanied by the activation of ERK. Also this effect was abolished by D1 removal, independently of the presence of P88-92. The expression of intact and truncated uPARs regulated cell directional migration toward uPA, the specific uPAR ligand, and toward fMLP, a bacterial chemotactic peptide. In fact, the uPA-dependent cell migration required the expression of intact uPAR, including D1, whereas the fMLP-dependent cell migration required the expression of a P88-92 containing uPAR and was independent of the presence of D1. Together these observations indicate that uPA-mediated uPAR cleavage and D1 removal, occurring on the cell surface of several cell types, can play a fundamental role in the regulation of multiple uPAR functions.

The fibrinolytic receptor for urokinase activates the G protein-coupled chemotactic receptor FPRL1/LXA4R

The function of urokinase and its receptor is essential for cell migration in pathological conditions, as shown by the analysis of knockout mice phenotypes. How a protease of a fibrinolytic pathway can induce migration is not understood and no link between this protease and migration-promoting G protein-coupled receptors has been described. We now show that FPRL1/LXA4R, a G protein-coupled receptor for a number of polypeptides and for the endogenous lipoxin A4 (LXA4), is the link between urokinase-type plasminogen activator (uPA) and migration as it directly interacts with an activated, soluble, cleaved form of uPA receptor (uPAR) (D2D3(88-274)) to induce chemotaxis. In this article we show that (i) both uPAR and FPRL1/LXA4R are necessary for the chemotactic activity of uPA whereas FPRL1/LXA4R is sufficient to mediate D2D3(88-274)-induced cell migration. (ii) Inhibition or desensitization of FPRL1/LXA4R by antibodies or specific ligands specifically prevents chemotaxis induced by D2D3(88-274) in THP-1 cells and human peripheral blood monocytes. (iii) Desensitization of FPRL1/LXA4R prevents the activation of tyrosine kinase Hck induced by D2D3(88-274). (iv) D2D3(88-274) directly binds to FPRL1/LXA4R and is competed by two specific FPRL1/LXA4R agonists, the synthetic MMK-1 peptide and a stable analog of LXA4. Thus, a naturally produced cleaved form of uPAR is a unique endogenous chemotactic agonist for FPRL1/LXA4R receptor and its activity can be antagonized by specific ligands. These results provide the first direct link, to our knowledge, between the fibrinolytic machinery and the inflammatory response, demonstrating that uPA-derived peptide fragments can activate a specific chemotactic receptor.

Urokinase receptors promote beta1 integrin function through interactions with integrin alpha3beta1

The urokinase receptor (uPAR) is linked to cellular migration through its capacity to promote pericellular proteolysis, regulate integrin function, and mediate cell signaling in response to urokinase (uPA) binding. The mechanisms for these activities remain incompletely defined, although uPAR was recently identified as a cis-acting ligand for the beta2 integrin CD11b/CD18 (Mac-1). Here we show that a major beta1 integrin partner for uPAR/uPA signaling is alpha3. In uPAR-transfected 293 cells uPAR complexed (>90%) with alpha3beta1 and antibodies to alpha3 blocked uPAR-dependent vitronectin (Vn) adhesion. Soluble uPAR bound to recombinant alpha3beta1 in a uPA-dependent manner (K(d) < 20 nM) and binding was blocked by a 17-mer alpha3beta1 integrin peptide (alpha325) homologous to the CD11b uPAR-binding site. uPAR colocalized with alpha3beta1 in MDA-MB-231 cells and uPA (1 nM) enhanced spreading and focal adhesion kinase phosphorylation on fibronectin (Fn) or collagen type I (Col) in a pertussis toxin- and alpha325-sensitive manner. A critical role of alpha3beta1 in uPA signaling was verified by studies of epithelial cells from alpha3-deficient mice. Thus, uPAR preferentially complexes with alpha3beta1, promoting direct (Vn) and indirect (Fn, Col) pathways of cell adhesion, the latter a heterotrimeric G protein-dependent mechanism of signaling between alpha3beta1 and other beta1 integrins.

Urokinase-catalysed cleavage of the urokinase receptor requires an intact glycolipid anchor

Urokinase (uPA) has the striking ability to cleave its receptor, uPAR, thereby inactivating the binding potential of this molecule. Here we demonstrate that the glycosylphosphatidylinositol (GPI) anchor of uPAR, which is attached to the third domain, is an important determinant in governing this reaction, even though the actual cleavage occurs between the first and second domains. Purified full-length GPI-anchored uPAR (GPI-uPAR) proved much more susceptible to uPA-mediated cleavage than recombinant truncated soluble uPAR (suPAR), which lacks the glycolipid anchor. This was not a general difference in proteolytic susceptibility since GPI-uPAR and suPAR were cleaved with equal efficiency by plasmin. Since the amino acid sequences of GPI-uPAR and suPAR are identical except for the C-terminal truncation, the different cleavage patterns suggest that the two uPAR variants differ in the conformation or the flexibility of the linker region between domains 1 and 2. This was supported by the fact that an antibody to the peptide AVTYSRSRYLE, amino acids 84-94 in the linker region, recognizes GPI-uPAR but not suPAR. This difference in the linker region is thus caused by a difference in a remote hydrophobic region. In accordance with this model, when the hydrophobic lipid moiety was removed from the glycolipid anchor by phospholipase C, low concentrations of uPA could no longer cleave the modified GPI-uPAR and the reactivity to the peptide antibody was greatly decreased. Naturally occurring suPAR, purified from plasma, was found to have a similar resistance to uPA cleavage as phospholipase C-treated GPI-uPAR and recombinant suPAR.

Urokinase-receptor/integrin complexes are functionally involved in adhesion and progression of human breast cancer in vivo

Interactions between specific cell-surface molecules, which include the urokinase receptor (uPAR) and integrins, are crucial to processes of tumor invasion and metastasis. Here we demonstrate that uPAR and beta1-integrins may cluster at distinct sites at the cell surface of metastatic MDA-MB-231 breast cancer cells and form functional complexes. Attachment assays performed in the presence of a synthetic peptide (p25), which interferes with the formation of uPAR-integrin complexes, reveal that uPAR is able to regulate the adhesive function of integrins in breast cancer cells. On dissociation of the uPAR-integrin complexes by p25, tumor cell attachment to the extracellular matrix was either decreased (vitronectin) or increased (fibronectin). Moreover, the tumor cells display remarkable morphological changes when cultured on fibronectin in the continuous presence of p25, leading to increased cell spreading and attachment. In marked contrast to control conditions, increased cellular adhesion to fibronectin after p25 treatment was entirely beta1-integrin-mediated. The role of uPAR-integrin complexes in tumor progression was studied in an in vivo bone xenograft model. Stably transfected MDA-MB-231 cells that overexpress p25 showed a significant reduction in tumor progression in bone (P < or = 0.0001 versus mock-control). In line with these observations, continuous administration of p25 (25 microg/mouse/day, osmotic minipumps) for 28 days resulted in significantly reduced tumor progression of MDA-MB-231 cells in bone (P < or = 0.005) when compared to scrambled control peptide. In conclusion, our data demonstrate that uPAR can act as an adhesion receptor in breast cancer and is capable of regulating integrin function. Our findings strongly suggest that adhesive and proteolytic events are tightly associated in metastatic breast cancer cells and that functional integrin-uPAR complexes are involved in tumor progression in vivo.

Soluble urokinase plasminogen activator receptor in preoperatively obtained plasma from patients with gynecological cancer or benign gynecological diseases

OBJECTIVE

The present study was planned to measure preoperative levels of soluble urokinase plasminogen activator receptor (suPAR) in plasma from patients with gynecological diseases, and to test for a relationship to clinical and biochemical patient characteristics.

METHODS

Using a specific and sensitive kinetic ELISA, suPAR levels were determined in preoperative citrate plasma samples from 53 ovarian, 34 endometrial, and 30 cervical cancer patients, 17 patients with benign ovarian tumors, and 28 patients with benign endometrial diseases. In addition, suPAR was measured in citrate samples from 31 female blood donors.

RESULTS

suPAR was measurable in all samples. No significant difference was found between plasma suPAR in the blood donors and the patients with benign diseases (P = 0.58). The groups of cancer patients had suPAR levels that were significantly higher than those found in the blood donors (P < 0.0001, P < 0.0001, and P = 0.001 for patients with ovarian, endometrial, and cervical cancer, respectively). In all groups of cancer patients a trend toward increasing suPAR levels with increasing FIGO stage was noted (P = 0.0003, P = 0.02, and P = 0.01 for patients with ovarian, endometrial, and cervical cancer, respectively). Using the median suPAR level to dichotomize the ovarian cancer patients, FIGO stages I-III, a significantly increased risk of progression/relapse was found for patients with high suPAR levels (Hazard ratio (HR) = 3.1, 95% CI: 1.1-8.8, P = 0.03). A multivariate analysis was performed, including suPAR, FIGO stage, and CA-125. Only FIGO stage III compared with FIGO stage I was significant (HR = 15, 95% CI: 1.8-129, P = 0.01). Survival analyses were not performed in the endometrial or cervical cancer patients due to few progressions/relapses during the follow-up period.

CONCLUSION

This study concludes that patients with gynecological cancers have elevated plasma suPAR levels as compared with healthy female blood donors and patients with benign gynecological diseases. In addition, high preoperative plasma levels of suPAR are significantly associated with poor outcome of ovarian cancer patients. However, additional studies are needed to further validate the clinical usefulness of plasma suPAR measurements in the management of ovarian cancer patients.

Increased stromal expression of murine urokinase plasminogen activator in a human breast cancer xenograft model following treatment with the matrix metalloprotease inhibitor, batimastat

The matrix metalloprotease (MMP) family of enzymes and the urokinase plasminogen activator (uPA) pathway have both been implicated in tumor invasion and metastasis and in poor prognosis of cancer. We have previously shown that treatment with batimastat, a synthetic MMP inhibitor, leads to significant retardation but not regression of tumor growth in a human breast cancer xenograft model. In addition, batimastat treatment did not inhibit local tumor invasion, nor did it encourage stromal encapsulation of the tumor, suggesting the additional involvement of non-MMP proteolytic mechanisms. To investigate the presence of an alternative extracellular matrix protease whose activity is known to be important in breast cancer, but which is not inhibited by batimastat, expression of murine and human uPA were examined by in situ hybridization and ELISA. No differences were observed between untreated and batimastat-treated tumors regarding human uPA mRNA and protein. In contrast, murine uPA mRNA expression was increased at the tumor-stromal junction in batimastat-treated tumors in comparison with the control tumors. In agreement with these results, batimastat treatment was shown to significantly induce murine uPA protein content in the tumors. Inoculating MDA435/LCC-6 cells into immunodeficient, uPA-deficient mice resulted in tumor growth retardation as compared to tumor growth in littermate wild-type controls, while addition of batimastat treatment to uPA-/- mice did not result in further growth inhibition. The increased expression of stromal uPA may represent a cellular response to MMP inhibition and may demonstrate a new level of plasticity in the malignant progression of the disease. These results may have important implications for the clinical applications of MMP inhibitors, as well as for development of other anti-invasion drugs.

Enhanced angiogenic capacity and urokinase-type plasminogen activator expression by endothelial cells isolated from human endometrium

The endometrium is a tissue unique for its cyclic destruction and rapid regeneration of blood vessels. Angiogenesis, indispensable for the regeneration process, provides a richly vascularized, receptive endometrium fundamental for implantation, placentation, and embryogenesis. Human endometrial microvascular endothelial cells (hEMVEC) were isolated to better understand the properties and angiogenic behavior of these cells. Unlike human foreskin microvascular endothelial cells (hFMVEC), which proliferated better upon stimulation by basic fibroblast growth factor, hEMVEC were much more sensitive to vascular endothelial growth factor A (VEGF-A) stimulation, probably due to enhanced VEGF receptor 2 expression. In addition, hEMVEC displayed an enhanced expression of the urokinase-type plasminogen activator (u-PA) compared with hFMVEC. No differences were found in tissue-type PA, PA inhibitor-1, and u-PA receptor expression. The high expression of u-PA by hEMVEC was also found in tissue sections. hEMVEC formed capillary-like structures when cultured in 20% human serum on top of three-dimensional fibrin matrices, and VEGF-A or basic fibroblast growth factor increased this tube formation. This is in contrast with hFMVEC, which formed tubes only after simultaneous stimulation by a growth factor and tumor necrosis factor-alpha. The high basal level of u-PA contributes to and may explain the higher angiogenic properties of hEMVEC (in vitro).

Proteolysis of the urokinase-type plasminogen activator receptor by metalloproteinase-12: implication for angiogenesis in fibrin matrices

Pericellular proteolysis plays an important role in cell migration and the formation of new capillary structures. The plasminogen activator/plasmin and matrix degrading metalloproteinase (MMP) cascades act together in the remodeling of matrix and cell-matrix contacts. Previously we have shown that the formation of capillary structures by human foreskin microvascular endothelial cells (hMVECs) in a 3-dimensional fibrin matrix requires a functional urokinase-type plasminogen activator receptor (u-PAR). Here we report on the unexpected finding that inhibition of hMVEC-derived MMP activity by BB94 (batimastat) increased the outgrowth of capillary structures in a fibrin matrix. BB94 prevented the release of the u-PA binding domain D1 of u-PAR and thereby increased the number of functional u-PARs on hMVECs without affecting the u-PAR messenger RNA levels. Comparison of various types of protease inhibitors pointed to the prime involvement of MMP activity. Using recombinant MMPs it was shown that MMP-12 activity was able to release the D1 domain of cellularly expressed u-PAR. In addition, the expression of MMP-12 in control and basic fibroblast growth factor/tumor necrosis factor-alpha-stimulated hMVECs was shown by reverse transcriptase-polymerase chain reaction, suggesting that endothelial cell-derived MMP-12 may be involved in angiogenesis-related u-PAR shedding. This new mechanism of u-PAR cleavage provides new insights into the mutual interactions between the MMP and u-PA/plasmin systems. Moreover, it may be helpful in the interpretation of recent data on the use of specific MMP inhibitors in the treatment of several types of cancer.

The murine receptor for urokinase-type plasminogen activator is primarily expressed in tissues actively undergoing remodeling

uPAR is a cellular receptor for urokinase plasminogen activator, an enzyme involved in extracellular matrix degradation during processes involving tissue remodeling. We have expressed a recombinant soluble form of murine uPAR and raised rabbit polyclonal antibodies to study the expression of uPAR by immunohistochemistry. The immunohistochemical localization of uPAR was determined in normal mouse organs and in tumors formed by the highly metastatic Lewis lung carcinoma. uPAR immunoreactivity was found in the lungs, kidneys, and spleen, and in endothelial cells in the uterus, urinary bladder, thymus, heart, liver, and testis. No uPAR immunoreactivity was detected in muscle. In general, strong uPAR immunoreactivity was observed in organs undergoing extensive tissue remodeling, as exemplified by trophoblast cells in placenta, and in migrating, but not resting, keratinocytes at the edge of incisional wounds. Staining was not detected in any tissue sections derived from uPAR-deficient mice, thus confirming the specificity of the immunohistochemical staining of uPAR in normal mouse tissues. In Lewis lung carcinoma, uPAR immunoreactivity was found in the tumor cells of the primary tumor and in lung metastases. (J Histochem Cytochem 49:237-246, 2001)

Borrelia burgdorferi and other bacterial products induce expression and release of the urokinase receptor (CD87)

The urokinase-type plasminogen activator receptor (uPAR, CD87) is a highly glycosylated 55- to 60-kDa protein anchored to the cell membrane through a glycosylphosphatidylinositol moiety that promotes the acquisition of plasmin on the surface of cells and subsequent cell movement and migration by binding urokinase-type plasminogen activator. uPAR also occurs in a soluble form in body fluids and tumor extracts, and both membrane and soluble uPAR are overexpressed in patients with tumors. uPAR may be a factor in inflammatory disorders as well. We investigated whether Borrelia burgdorferi could stimulate up-regulation of cell membrane uPAR in vitro. B. burgdorferi, purified native outer surface protein A, and a synthetic outer surface protein A hexalipopeptide stimulated human monocytes to up-regulate membrane uPAR as measured by immunofluorescence/FACS and Western blot. The presence of soluble uPAR in culture supernatants, measured by Ag capture ELISA, was also observed. LPS from Salmonella typhimurium and lipotechoic acid from Streptococcus pyogenes also induced the up-regulation of both membrane and soluble uPAR protein by monocytes. Up-regulation of uPAR was induced by conditioned medium from B. burgdorferi/monocyte cocultures. The up-regulation of uPAR by B. burgdorferi was concomitant with an increase in uPAR mRNA, indicating that synthesis was de novo. The expression and release of uPAR in response to B. burgdorferi and other bacterial components suggests a role in the pathogenesis of Lyme disease as well as in other bacterial infections.

Urokinase-type plasminogen activator and its receptor synergize to promote pathogenic proteolysis

Urokinase-type plasminogen activator (uPA) is a potent catalyst of extracellular proteolysis, which also binds to a high-affinity plasma membrane receptor (uPAR). Binding of uPA may influence pericellular proteolysis and/or activate intracellular signal transduction. Transgenic mice overexpressing either uPA or uPAR in basal epidermis and hair follicles had no detectable cutaneous alterations. In contrast, bi-transgenic mice overexpressing both uPA and uPAR, obtained by crossing the two transgenic lines, developed extensive alopecia induced by involution of hair follicles, epidermal thickening and sub-epidermal blisters. The phenotype was due to uPA catalytic activity since combined overexpression of uPAR and uPAR-binding but catalytically inactive uPA in the same tissue was not detrimental in another bi-transgenic line. It was accompanied by increased plasmin-generating capacity, up-regulation and activation of matrix metalloproteinases type-2 and -9, and cleavage of uPAR. Thus, combined overexpression of uPA and uPAR acts in synergy to promote pathogenic extracellular proteolysis.

Shedding and cleavage of the urokinase receptor (uPAR): identification and characterisation of uPAR fragments in vitro and in vivo

Applying a novel, highly specific and sensitive immunoabsorption/Western blotting technique we have identified in vitro in conditioned cell culture medium and in vivo in human urine different soluble forms of the urokinase-type plasminogen activator receptor (uPAR/CD87). These include the uPAR fragment D2D3 and the never before identified domain 1 (D1) fragment. These forms correspond to fragments previously characterised as biologically active as inducers of chemotaxis and cell adhesion. We find that stimulation of U937 cells is associated with increased uPAR expression, cleavage of surface uPAR, and release of soluble fragments to the culture medium suggesting that monocytes are a source of the circulating and urinary soluble uPAR fragments found in vivo. Our study demonstrates that potentially biologically active uPAR fragments are produced in the human body, indicating a possible function in the regulation of not only proteolysis but also signal transduction related processes.

Domain 1 of the urokinase receptor (uPAR) is required for uPAR-mediated cell binding to vitronectin

In the present paper we have analyzed uPAR-mediated cellular binding to vitronectin using the murine erythroid progenitor cell line 32D. We show that expression of uPAR in 32D cells promotes cellular binding to vitronectin, but fails to support cell spreading. The strength of binding is correlated to the expression level of uPAR and is strongly stimulated by the presence of uPAR ligands. Using a truncated variant of uPAR lacking domain 1 and by antibody inhibition experiments, we demonstrate that domain 1 plays a crucial role in uPAR-mediated cellular binding. The failure of the mutant uPAR to promote cellular binding is paralleled by a strong reduction in the affinity for vitronectin in vitro.

Generation of high-affinity rabbit polyclonal antibodies to the murine urokinase receptor using DNA immunization

The urokinase receptor (uPAR) is a glycolipid anchored cell surface glycoprotein that plays a central role in extracellular proteolysis during tissue remodeling processes and cancer invasion. By intramuscular (i.m.) injection of rabbits with plasmid DNA coding for a carboxy-terminally truncated secreted form of the murine uPAR (muPAR), specific anti-sera with a titer of 64,000, as measured by ELISA, have been obtained. Rabbits received a total of 10 monthly injections of 1 mg DNA in phosphate-buffered saline. The antibody titer peaked between the 5th and 7th injection and slowly declined after the 8th injection. After the final immunization the immune response persisted for at least 6 months without further injections. The antibodies generated by DNA immunization were useful for immunohistochemistry and immunoblotting, recognizing the antigen both in its native and in its reduced and alkylated form. Using the antibodies in immunoblotting muPAR was identified in lysates of peritoneal macrophages, spleen and lung tissue. Both the intact and cleaved form of muPAR were identified in lysates of a murine monocyte cell line P388D.1. No cross-reaction with human uPAR was observed. In immunohistochemical analysis of normal mouse lung tissue uPAR immunoreactivity was located in the alveoli and pulmonary vessels, whereas the bronchial epithelium was negative. These results demonstrate that DNA immunization of rabbits using i.m. injection is a very effective and easy method to raise polyclonal antibodies which can be used for characterization and localization of muPAR in mouse tissue.

Cleavage of urokinase receptor regulates its interaction with integrins in thyroid cells

The urokinase-type plasminogen activator uPA-R can regulate integrin functions by associating with several types of beta-subunit. We have recently shown that normal thyroid TAD-2 cells express both a native and a cleaved form of uPA-R which lacks the binding domain for uPA. We found this cleaved form to be present in reduced amounts in papillary and follicular thyroid carcinoma cells and completely absent in cells derived from an anaplastic thyroid carcinoma (ARO). We now report that in normal thyroid cells the intact form of uPA-R strongly associates with beta-1 integrins, whereas its cleaved form does not. uPA-R expressed by ARO cells shows a stronger resistance to the cleavage mediated by uPA, plasmin and chymotrypsin than does uPA-R expressed by normal thyroid cells. This resistance to cleavage correlates with the higher level of glycosylation of uPA-R of ARO cells as compared to that of cleavable uPA-R of normal thyroid cells. These results suggest that uPA-R cleavage, which occurs in several cell types, represents a mechanism regulating the interactions of uPA-R with integrins and, possibly, the subsequent integrin-mediated cell adhesion. Moreover we hypothesize that glycosylation regulates uPA-R cleavage and, indirectly, its interaction with integrins.

Soluble urokinase receptor released from human carcinoma cells: a plasma parameter for xenograft tumour studies

The urokinase plasminogen activator receptor (uPAR) plays a critical role in urokinase-mediated plasminogen activation and thereby in the process leading to invasion and metastasis. Soluble urokinase receptor (suPAR) is released from tumours, and in cancer patients the blood level of soluble receptor is increased. Using an enzyme-linked, immunosorbent assay (ELISA)-specific for the human urokinase receptor, release of soluble receptor was measured in cultures of human breast carcinoma cells, in tumour extracts and in plasma from mice with xenografted human tumours. Soluble human urokinase receptor (shuPAR) was released into culture supernatant during the growth of the human breast cancer cell line MDA-MB-231 BAG, and the level of shuPAR in conditioned medium determined by ELISA was a linear function of both viable cell number and time of incubation. Western blotting showed that the form of shuPAR measured by ELISA in conditioned medium consisted virtually exclusively of the three-domain full-length protein, while uPAR in cell lysates consisted of full-length uPAR as well as the domains (2+3) cleavage product. shuPAR was also released into the plasma of nude mice during growth of MDA-MB-231 BAG, MDA-MB-435 BAG and HCT 116 cells as subcutaneously xenografted tumours. Western blotting demonstrated that the shuPAR released from the xenografted human tumours into plasma consisted of the three-domain full-length protein, despite the finding of some cleaved uPAR in detergent extracts of tumour tissue. The levels of shuPAR determined by ELISA in the plasma of host mice during the growth of xenografted cell lines were highly correlated with tumour volume.

The urokinase plasminogen activator system as a target for prognostic studies in breast cancer

The identification of patients at high risk of relapse is currently one of the most important issues in breast cancer research. However, the selection of high-risk patients continues to be difficult due to the unpredictable course of this disease. Axillary lymph node status is currently recognized as the best clinical discriminant between good and poor prognosis, yet almost 30% of node-negative patients and 65% of node-positive patients will experience a relapse. Additional prognostic markers are therefore urgently needed. Since metastatic disease is the main cause of cancer patient morbidity and mortality, the measurement of molecules functionally involved in the regulation of tumor invasion and metastasis is attractive as a means to predict prognosis. Cancer invasion is a complex process in which degradation of the extracellular matrix plays a crucial role. This degradation is accomplished by the concerted action of several proteolytic enzyme systems, including generation of plasmin by the urokinase pathway of plasminogen activation, matrix metalloproteases, and other extracellular proteases. Increased expression and secretion of urokinase plasminogen activator (uPA) strongly correlates with the malignant phenotype of many types of cells, and the central role of uPA in tumor invasion is now well established. This review will focus on the prognostic impact of components of the urokinase plasminogen activation system in breast cancer with emphasize on methodological issues.

The urokinase plasminogen activator receptor in blood from healthy individuals and patients with cancer

The cell surface plasminogen activation system functions in promoting tumor dissemination, and is facilitated by a glycolipid anchored three domain receptor for urokinase. This receptor can also be found in a soluble form (suPAR) in extracts of tumors, as well as in plasma from both healthy individuals and cancer patients. The suPAR in plasma consists of the intact three domain protein, but neither the precise mechanism of its release from cell surfaces, nor its biological function are understood. Increased levels of plasma suPAR have been found in patients with cancers of the lung, breast, ovary, and colon, and recent data now indicates that the level of the molecule is related to patient prognosis.

Urokinase-type plasminogen activator receptor is internalized by different mechanisms in polarized and nonpolarized Madin-Darby canine kidney epithelial cells

Accumulated data indicate that endocytosis of the glycosylphosphatidyl-inositol-anchored protein urokinase plasminogen activator receptor (uPAR) depends on binding of the ligand uPA:plasminogen activator inhibitor-1 (PAI-1) and subsequent interaction with internalization receptors of the low-density lipoprotein receptor family, which are internalized through clathrin-coated pits. This interaction is inhibited by receptor-associated protein (RAP). We show that uPAR with bound uPA:PAI-1 is capable of entering cells in a clathrin-independent process. First, HeLaK44A cells expressing mutant dynamin efficiently internalized uPA:PAI-1 under conditions in which transferrin endocytosis was blocked. Second, in polarized Madin-Darby canine kidney (MDCK) cells, which expressed human uPAR apically, the low basal rate of uPAR ligand endocytosis, which could not be inhibited by RAP, was increased by forskolin or phorbol ester (phorbol 12-myristate 13-acetate), which selectively up-regulate clathrin-independent endocytosis from the apical domain of epithelial cells. Third, in subconfluent nonpolarized MDCK cells, endocytosis of uPA:PAI-1 was only decreased marginally by RAP. At the ultrastructural level uPAR was largely excluded from clathrin-coated pits in these cells and localized in invaginated caveolae only in the presence of cross-linking antibodies. Interestingly, a larger fraction of uPAR in nonpolarized relative to polarized MDCK cells was insoluble in Triton X-100 at 0 degreesC, and by surface labeling with biotin we also show that internalized uPAR was mainly detergent insoluble, suggesting a correlation between association with detergent-resistant membrane microdomains and higher degree of clathrin-independent endocytosis. Furthermore, by cryoimmunogold labeling we show that 5-10% of internalized uPAR in nonpolarized, but not polarized, MDCK cells is targeted to lysosomes by a mechanism that is regulated by ligand occupancy.

Immunohistochemical identification of the receptor for urokinase plasminogen activator associated with fibrin deposition in normal and ectopic human placenta

The receptor for urokinase plasminogen activator (uPAR) is a key molecule in cell surface-directed plasminogen activation. uPAR binds urokinase plasminogen activator (uPA) and thereby focuses plasminogen activation on the cell surface. Plasmin dissolves fibrin deposits and facilitates cell migration during tissue repair processes by degrading the extracellular matrix. During human implantation and placental development, plasmin is considered important for both trophoblast migration/invasion and for fibrin surveillance. This study examined the expression of uPAR in normal and ectopic human placentae by immunohistochemistry. In first and third trimester normal placentae as well as in tubal ectopic placental tissues, a high uPAR expression was seen in the trophoblast associated with deposits of fibrin-type fibrinoid. Extravillous trophoblast of the basal plate, of the cell islands, and of the cell columns was also positive for uPAR in the first trimester whereas at term the expression of the protein was decreased. Moreover, uPAR immunostaining was observed in decidual cells throughout normal gestation and in endometrial tissues of patients with ectopic pregnancies. These findings suggest that uPAR participates in placental development and in trophoblast invasion particularly in the first trimester of pregnancy and that uPAR is involved in repair mechanisms of the trophoblast and fibrin surveillance.

Mannose 6-phosphate/insulin-like growth factor-II receptor targets the urokinase receptor to lysosomes via a novel binding interaction

The urokinase-type plasminogen activator receptor (uPAR) plays an important role on the cell surface in mediating extracellular degradative processes and formation of active TGF-beta, and in nonproteolytic events such as cell adhesion, migration, and transmembrane signaling. We have searched for mechanisms that determine the cellular location of uPAR and may participate in its disposal. When using purified receptor preparations, we find that uPAR binds to the cation-independent, mannose 6-phosphate/insulin-like growth factor-II (IGF-II) receptor (CIMPR) with an affinity in the low micromolar range, but not to the 46-kD, cation-dependent, mannose 6-phosphate receptor (CDMPR). The binding is not perturbed by uPA and appears to involve domains DII + DIII of the uPAR protein moiety, but not the glycosylphosphatidylinositol anchor. The binding occurs at site(s) on the CIMPR different from those engaged in binding of mannose 6-phosphate epitopes or IGF-II. To evaluate the significance of the binding, immunofluorescence and immunoelectron microscopy studies were performed in transfected cells, and the results show that wild-type CIMPR, but not CIMPR lacking an intact sorting signal, modulates the subcellular distribution of uPAR and is capable of directing it to lysosomes. We conclude that a site within CIMPR, distinct from its previously known ligand binding sites, binds uPAR and modulates its subcellular distribution.

The intact urokinase receptor is required for efficient vitronectin binding: receptor cleavage prevents ligand interaction

The urokinase receptor (uPAR) is a receptor for both urokinase plasminogen activator (uPA) and the adhesion protein vitronectin. There are two forms of cell surface-bound uPAR; intact uPAR and a cleaved form, uPAR(2+3), which is formed by uPA-catalyzed cleavage of uPAR. In ligand-blotting experiments we found that vitronectin binds uPAR but not uPAR(2+3). In real-time biomolecular interaction analysis using recombinant, soluble uPAR (suPAR) both plasma and multimeric forms of vitronectin bound to intact, antibody-immobilized suPAR. Monoclonal antibodies against domain 1 of uPAR blocked suPAR binding to vitronectin and vitronectin did not interact with suPAR(2+3). Both suPAR(2+3) and the isolated domain 1 failed to compete with the intact suPAR in binding to vitronectin. We therefore conclude that the intact receptor is required for efficient vitronectin binding.

A urokinase-sensitive region of the human urokinase receptor is responsible for its chemotactic activity

The role of urokinase-type plasminogen activator (uPA) and its receptor (uPAR/CD87) in cell migration and invasion is well substantiated. Recently, uPA has been shown to be essential in cell migration, since uPA-/- mice are greatly impaired in inflammatory cell recruitment. We have shown previously that the uPA-induced chemotaxis requires interaction with and modification of uPAR/CD87, which is the true chemoattracting molecule acting through an unidentified cell surface component which mediates this cell surface chemokine activity. By expressing and testing several uPAR/CD87 variants, we have located and functionally characterized a potent uPAR/CD87 epitope that mimics the effects of the uPA-uPAR interaction. The chemotactic activity lies in the region linking domains 1 and 2, the only protease-sensitive region of uPAR/CD87, efficiently cleaved by uPA at physiological concentrations. Synthetic peptides carrying this epitope promote chemotaxis and activate p56/p59(hck) tyrosine kinase. Both chemotaxis and kinase activation are pertussis toxin sensitive, involving a Gi/o protein in the pathway.

ELISA determination of soluble urokinase receptor in blood from healthy donors and cancer patients

Measurement of urokinase receptor (uPAR) in tumor extracts has prognostic value, but assay of the soluble uPAR (suPAR) in peripheral blood may offer wider applications in cancer patient management. A tumor extract uPAR ELISA was modified to eliminate nonspecific plasma protein interference, enabling specific detection of suPAR in plasma and sera with &gt;90% recovery of added calibrator. suPAR concentrations in citrate plasma correlated with sera in 93 healthy blood donors (r = 0.84, P &lt;0.0001), with a median value for both of 1.2 μg/L. The plasma median for 19 advanced breast cancer patients was 2.9 μg/L suPAR, and a similar increase was found for 10 advanced colon cancer patients, consistent with release of suPAR from tumors into blood. Repetitive monitoring of suPAR in cancer patients’ blood may have value in assessment of prognosis and tumor recurrence.

ELISA for complexes between urokinase-type plasminogen activator and its receptor in lung cancer tissue extracts

A sandwich-type ELISA has been developed for the assessment of complexes between urokinase-type plasminogen activator (uPA) and its receptor (uPAR) in extracts of squamous cell lung carcinomas. The assay is based on a combination of rabbit polyclonal anti-uPA antibodies and a biotinylated mouse anti-uPAR monoclonal antibody (MAb). The detection limit of the assay is approximately 0.5 fmol/ml. A linear dose-response is obtained with up to 40 fmol/ml of uPA:uPAR complexes, while uPA and uPAR separately do not cause any response in the ELISA. A buffer which has been used previously for optimal extraction of uPAR yields the highest amounts of uPA:uPAR complexes. Absorption of tumor extracts with anti-uPA or anti-uPAR MAbs results in a complete disappearance of the ELISA signal, demonstrating the specificity of the ELISA. The recovery of chemically cross-linked uPA:uPAR complexes added to tumor extracts varies between 80% and 105%. The intra- and inter-assay variation coefficients are 5.3% and 9.8%, respectively. Furthermore, a peptide antagonist for uPAR was employed to evaluate de novo uPA:uPAR complex formation during tumor tissue extraction and the immunoassay procedure. Our results strongly indicate that de novo complex formation is a major factor to consider and that complexes analyzed in the presence of this antagonist represent original uPA:uPAR complexes present prior to tumor tissue processing. The present ELISA appears suitable for studying the potential prognostic impact of uPA:uPAR complexes in lung tumor tissue as well as other types of cancer.

The urokinase-type plasminogen activator system in cancer metastasis: a review

The urokinase-type plasminogen activator (u-PA) system consists of the serine proteinases plasmin and u-PA; the serpin inhibitors alpha2-anti-plasmin, PAI-1 and PAI-2; and the u-PA receptor (u-PAR). Two lines of evidence have strongly suggested an important and apparently causal role for the u-PA system in cancer metastasis: results from experimental model systems with animal tumor metastasis and the finding that high levels of u-PA, PAI-1 and u-PAR in many tumor types predict poor patient prognosis. We discuss here recent observations related to the molecular and cellular mechanisms underlying this role of the u-PA system. Many findings suggest that the system does not support tumor metastasis by the unrestricted enzyme activity of u-PA and plasmin. Rather, pericellular molecular and functional interactions between u-PA, u-PAR, PAI-1, extracellular matrix proteins, integrins, endocytosis receptors and growth factors appear to allow temporal and spatial re-organizations of the system during cell migration and a selective degradation of extracellular matrix proteins during invasion. Differential expression of components of the system by cancer and non-cancer cells, regulated by paracrine mechanisms, appear to determine the involvement of the system in cancer cell-directed tissue remodeling. A detailed knowledge of these processes is necessary for utilization of the therapeutic potential of interfering with the action of the system in cancers.

Elevated plasma levels of urokinase plasminogen activator receptor in non-small cell lung cancer patients

The urokinase plasminogen activator (uPA) is involved in extracellular matrix degradation during cancer invasion. Binding of uPA to a specific cell surface receptor (uPAR) is a key step in this process. We have previously reported that high levels of uPAR in squamous cell lung cancer tissue extracts are associated with poor prognosis (Pedersen et al., Cancer Res 1994, 54, 4671-4675). Recently we found that uPAR is present in blood plasma from healthy donors as determined by enzyme-linked immunosorbent assay (ELISA) and chemical cross-linking. We now report that uPAR in plasma from 17 patients with non-small cell lung cancer (NSCLC) was significantly higher than in 30 healthy controls (P = 0.0004), while no significant increase was found in plasma from 14 patients with small cell lung cancer (SCLC). The increased levels of uPAR in the plasma from NSCLC patients is likely to be due to release of uPAR from the tumour tissue, and may, therefore, be related to prognosis.