The urokinase receptor

Soluble Urokinase Plasminogen Activator Receptor (suPAR) in Autoimmune Rheumatic and Non Rheumatic Diseases

The soluble urokinase plasminogen activator receptor (suPAR) is the bioactive form of uPAR, a membrane-bound glycoprotein, and it is primarily expressed on the surface of immunologically active cells. Mirroring local inflammation and immune activation, suPAR has gained interest as a potential prognostic biomarker in several inflammatory diseases. Indeed, in many diseases, including cancer, diabetes, cardiovascular diseases, kidney diseases, and inflammatory disorders, higher suPAR concentrations have been associated with disease severity, disease relapse, and mortality. Our review describes and discusses the supporting literature concerning the promising role of suPAR as a biomarker in different autoimmune rheumatic and non-rheumatic diseases.

Circulating Soluble Urokinase Plasminogen Activator Receptor as a Predictive Indicator for COVID-19-Associated Acute Kidney Injury and Mortality: Clinical and Bioinformatics Analysis

Urokinase receptors regulate the interplay between inflammation, immunity, and blood clotting. The soluble urokinase plasminogen activator system is an immunologic regulator affecting endothelial function and its related receptor; the soluble urokinase plasminogen activator receptor (suPAR) has been reported to impact kidney injury. This work aims to measure serum levels of suPAR in COVID-19 patients and correlate the measurements with variable clinicolaboratory parameters and patient outcomes. In this prospective cohort study, 150 COVID-19 patients and 50 controls were included. The circulating suPAR levels were quantified by Enzyme-linked immunosorbent assay (ELISA). Routine COVID-19 laboratory assessments, including CBC, CRP, LDH, serum creatinine, and estimated glomerular filtration rates, were performed. The need for oxygen therapy, CO-RAD score, and survival rates was assessed. Bioinformatic analysis and molecular docking were run to explore the urokinase receptor structure/function and to characterize molecules as potential anti-suPAR therapeutic targets, respectively. We found higher circulating suPAR levels in COVID-19 patients vs. controls (p < 0.001). Circulating suPAR levels positively correlated with COVID-19 severity, the need for O₂ therapy, the total leukocytes count, and the neutrophils to lymphocyte ratio, while they were negatively correlated with the O₂ saturation level, albumin, blood calcium, lymphocytic count, and GFR. In addition, the suPAR levels were associated with poor prognostic outcomes such as a high incidence of acute kidney injury (AKI) and mortality rate. Kaplan-Meier curves showed a lower survival rate with higher suPAR levels. The logistic regression analysis confirmed the significant association of suPAR levels with the occurrence of AKI related to COVID-19 and with increased mortality probability within three months of COVID-19 follow-up. Some compounds that can act similarly to uPAR were discovered and tested by molecular docking to identify the possible ligand-protein interactions. In conclusion, higher circulating suPAR levels were associated with COVID-19 severity and could be considered a putative predictor of AKI development and mortality.

The Association of uPA, uPAR, and suPAR System with Inflammation and Joint Damage in Rheumatoid Arthritis: suPAR as a Biomarker in the Light of a Personalized Medicine Perspective

BACKGROUND

In recent years, the involvement of the soluble urokinase Plasminogen Activator Receptor (suPAR) in the pathophysiological modulation of Rheumatoid Arthritis (RA) has been documented, resulting in the activation of several intracellular inflammatory pathways.

METHODS

We investigated the correlation of urokinase Plasminogen Activator (uPA)/urokinase Plasminogen Activator Receptor (uPAR) expression and suPAR with inflammation and joint damage in RA, evaluating their potential role in a precision medicine context.

RESULTS

Currently, suPAR has been shown to be a potential biomarker for the monitoring of Systemic Chronic Inflammation (SCI) and COVID-19. However, the effects due to suPAR interaction in immune cells are also involved in both RA onset and progression. To date, the literature data on suPAR in RA endorse its potential application as a biomarker of inflammation and subsequent joint damage.

CONCLUSION

Available evidence about suPAR utility in the RA field is promising, and future research should further investigate its use in clinical practice, resulting in a big step forward for precision medicine. As it is elevated in different types of inflammation, suPAR could potentially work as an adjunctive tool for the screening of RA patients. In addition, a suPAR system has been shown to be involved in RA pathogenesis, so new data about the therapeutic response to Jak inhibitors can represent a possible way to develop further studies.

Fibrinolytic Serine Proteases, Therapeutic Serpins and Inflammation: Fire Dancers and Firestorms

The making and breaking of clots orchestrated by the thrombotic and thrombolytic serine protease cascades are critical determinants of morbidity and mortality during infection and with vascular or tissue injury. Both the clot forming (thrombotic) and the clot dissolving (thrombolytic or fibrinolytic) cascades are composed of a highly sensitive and complex relationship of sequentially activated serine proteases and their regulatory inhibitors in the circulating blood. The proteases and inhibitors interact continuously throughout all branches of the cardiovascular system in the human body, representing one of the most abundant groups of proteins in the blood. There is an intricate interaction of the coagulation cascades with endothelial cell surface receptors lining the vascular tree, circulating immune cells, platelets and connective tissue encasing the arterial layers. Beyond their role in control of bleeding and clotting, the thrombotic and thrombolytic cascades initiate immune cell responses, representing a front line, "off-the-shelf" system for inducing inflammatory responses. These hemostatic pathways are one of the first response systems after injury with the fibrinolytic cascade being one of the earliest to evolve in primordial immune responses. An equally important contributor and parallel ancient component of these thrombotic and thrombolytic serine protease cascades are the serine protease inhibitors, termed serpins. Serpins are metastable suicide inhibitors with ubiquitous roles in coagulation and fibrinolysis as well as multiple central regulatory pathways throughout the body. Serpins are now known to also modulate the immune response, either via control of thrombotic and thrombolytic cascades or via direct effects on cellular phenotypes, among many other functions. Here we review the co-evolution of the thrombolytic cascade and the immune response in disease and in treatment. We will focus on the relevance of these recent advances in the context of the ongoing COVID-19 pandemic. SARS-CoV-2 is a "respiratory" coronavirus that causes extensive cardiovascular pathogenesis, with microthrombi throughout the vascular tree, resulting in severe and potentially fatal coagulopathies.

Increased Urokinase-Type Plasminogen Activator Receptor Expression on Circulating Monocytes Is Correlated with Clinical Instability and Long-Term Adverse Cardiac Events in Patients with Coronary Artery Disease

OBJECTIVES

This study sought to investigate the clinical correlates and prognostic roles of urokinase-type plasminogen activator receptor (uPAR) on circulating monocytes in patients with coronary artery disease (CAD).

METHODS

263 angina patients were included in this study. The percentage of uPAR expressing monocytes (PUEM) and the mean fluorescence intensity (MFI) index of uPAR were measured using flow cytometry. Patient follow-up was on average 604 days. Major adverse cardiac events (MACE) were defined as a composite of cardiac death, reinfarction, acute heart failure and hospitalization for revascularization.

RESULTS

The PUEM and MFI index levels were significantly more elevated in acute coronary syndrome patients than in stable ones. uPAR expressions on circulating monocytes at admission were correlated to inflammatory biomarkers and myocardial necrosis. Logistic regression analysis revealed that PUEM ≥15% (OR 21.96, 95% CI 7.31-65.98, p < 0.001) and uPAR MFI index ≥3.00 (OR 3.54, 95% CI 1.18-10.59, p = 0.024) were independent determinants of clinical instability in patients with CAD. When followed up, a high PUEM level at admission was an independent prognostic parameter for long-term MACE (HR 3.99, 95% CI 1.31-12.11, p = 0.015).

CONCLUSIONS

uPAR expression on circulating monocytes is associated with clinical instability and myocardial necrosis and independently predicts the risk of MACE in patients with CAD.

Proteases as prognostic markers in human and canine cancers

The extracellular matrix (ECM) is composed of several types of proteins, which interact and form dynamic networks. These components can modulate cell behaviour and actively influence the growth and differentiation of tissues. ECM is also important in several pathological processes, such as cancer invasion and metastasis, by creating favourable microenvironments. Proteolysis in neoplastic tissues is mediated by proteinases, whose regulation involves complex interactions between neoplastic cells and non-neoplastic stromal cells. In this review, we discuss aspects of proteinase expression and tumor behaviour in humans and dogs. Different classes of proteases are summarized, with special emphasis being placed on molecules that have been shown to correlate with prognosis, reinforcing the need for a better understanding of the regulation of this microenvironment and its influences in tumor progression and metastasis, which should significantly aid the development of improved prognosis and treatment.

Soluble urokinase plasminogen activator receptor (suPAR) in the assessment of inflammatory activity of rheumatoid arthritis patients in remission

BACKGROUND

Soluble urokinase plasminogen activator receptor (suPAR) is a biomarker increasingly used for the assessment of systemic inflammation. We aimed to evaluate suPAR for the assessment of inflammatory activity in rheumatoid arthritis (RA) patients in remission.

METHODS

In our cross-sectional study we measured plasma suPAR and C-reactive protein (CRP) levels as well as erythrocyte sedimentation rate (ESR) in 120 RA patients at various stages of disease activity and 29 healthy age-matched controls.

RESULTS

suPAR, CRP and ESR values were higher in RA patients compared to healthy individuals. When suPAR levels were analyzed according to DAS28 scores of RA patients, suPAR level in the subgroup with DAS28≤2.6 was lower than in the subgroup with DAS28>2.6, but still higher than in controls [4.45 (3.33-5.56) ng/mL vs. 3.66 (3.10-4.67) ng/mL vs. 2.80 (2.06-3.42) ng/mL, p<0.0001, median (interquartile range)]. In contrast, CRP and ESR values were comparable in the subgroup with DAS28≤2.6 and in healthy individuals. We further analyzed the correlation between the number of tender and/or swollen joints and suPAR levels in RA patients in remission. suPAR values were significantly higher in patients with four tender and/or swollen joints than in patients with 2-3 or 0-1 tender and/or swollen joints.

CONCLUSIONS

While CRP and ESR values indicate remission of the chronic inflammatory process in RA, suPAR values are still elevated compared to healthy individuals. suPAR might be particularly valuable in the recognition of inflammatory activity in patients who are in remission according to DAS28 scores but have symptoms of tender and/or swollen joints.

Relevance of novel inflammatory markers in stroke-induced immunosuppression

BACKGROUND

Acute ischemic stroke (AIS) has a biphasic effect on the peripheral immune system. The initial inflammatory response is followed by systemic immunosuppression, referred to as stroke-induced immunosuppression (SIIS), leading to severe complications in stroke patients. We aimed to identify an inflammatory marker that best represents this biphasic immunological response after AIS.

METHODS

We investigated the alteration of CRP, WBC, neutrophil count, suPAR levels, CD4+ CD25high Tregs, CD64+ and CD177+ neutrophils and monocytes in 12 acute ischemic stroke patients free of infection within 6 hours and one week after the insult. As controls, 14 age-matched healthy individuals were included.

RESULTS

CRP, WBC and neutrophil count values were comparable in stroke patients within 6 hours and controls, however, they were elevated in stroke one week after the insult. suPAR levels were higher in both stroke groups compared to controls. The prevalence of CD64+ neutrophils was higher in stroke patients within 6 hours than in controls and it decreased in stroke one week after the insult below the level in controls (5.95 [5.41-8.75] % vs. 32.38 [9.21-43.93] % vs. 4.06 [1.73-6.77] %, p < 0.05).

CONCLUSIONS

Our pilot study identified that the prevalence of CD64+ neutrophils may reflect a biphasic alteration of the immune response following AIS. Since its level decreases below baseline after one week of the CNS insult in stroke patients without infection, it might serve as a reliable candidate to identify the developing inflammatory response due to infection after stroke in the future.

Relationship of circulating soluble urokinase plasminogen activator receptor (suPAR) levels to disease control in asthma and asthmatic pregnancy

Asthma has a high burden of morbidity if not controlled and may frequently complicate pregnancy, posing a risk for pregnancy outcomes. Elevated plasma level of the inflammatory biomarker soluble urokinase plasminogen activator receptor (suPAR) is related to a worse prognosis in many conditions such as infectious, autoimmune, or pregnancy-related diseases; however the value of suPAR in asthma and asthmatic pregnancy is unknown. The present study aimed to investigate the suPAR, CRP and IL-6 levels in asthma (asthmatic non-pregnant, ANP; N = 38; female N = 27) and asthmatic pregnancy (AP; N = 15), compared to healthy non-pregnant controls (HNP; N = 29; female N = 19) and to healthy pregnant women (HP; N = 58). The relationship between suPAR levels and asthma control was also evaluated. The diagnostic efficacy of suPAR in asthma control was analyzed using ROC analysis. IL-6 and CRP levels were comparable in all study groups. Circulating suPAR levels were lower in HP and AP than in HNP and ANP subjects, respectively (2.01 [1.81–2.38] and 2.39 [2.07–2.69] vs. 2.60 [1.82–3.49] and 2.84 [2.33–3.72] ng/mL, respectively, p = 0.0001). suPAR and airway resistance correlated in ANP (r = 0.47, p = 0.004). ROC analysis of suPAR values in ANP patients with PEF above and below 80% yielded an AUC of 0.75 (95% CI: 0.57–0.92, p = 0.023) and with ACT total score above and below 20 an AUC of 0.80 (95% CI: 0.64–0.95, p = 0.006). The cut-off value of suPAR to discriminate between controlled and not controlled AP and ANP was 4.04 ng/mL. In conclusion, suPAR may help the objective assessment of asthma control, since it correlates with airway resistance and has good sensitivity in the detection of impaired asthma control. Decrease in circulating suPAR levels detected both in healthy and asthmatic pregnant women presumably represents pregnancy induced immune tolerance.

Plasma soluble urokinase plasminogen activator receptor (suPAR) levels in systemic lupus erythematosus

OBJECTIVE

Soluble urokinase plasminogen activator receptor (suPAR) is a biomarker of systemic inflammation. We aimed to characterize plasma suPAR levels in SLE patients.

METHODS

We measured plasma suPAR, C reactive protein (CRP) and erythrocyte sedimentation rate (ESR) in 89 SLE patients and 29 healthy controls.

RESULTS

suPAR and ESR values were higher in SLE than in controls, while CRP levels were comparable. ROC analysis of suPAR levels indicated a cut-off value of 5.70 ng/mL to distinguish patients with high disease activity (SLEDAI >8).

CONCLUSION

suPAR might be an objective marker for identifying SLE patients with active disease.

Role of hepatocyte growth factor/c-Met signaling in regulating urokinase plasminogen activator on invasiveness in human hepatocellular carcinoma: a potential therapeutic target

Hepatocyte growth factor (HGF), its transmembrane tyrosine kinase receptor (c-Met), and urokinase type plasminogen activator (uPA) is a key protein in the plasminogen activation system, which plays a proteolytically important role in the invasion and metastasis of various types of cancers. However, the mechanisms by which HGF/c-Met signaling mediates cancer progression and metastasis are unclear. This study was designed to investigate the roles of HGF/c-Met in tumor progression and metastasis in HepG2 and Hep3B hepatoma cell lines. Treatment with HGF increased c-Met phosphorylation in a dose-dependent manner. Activity of c-Met phosphorylation peaked 1-3 min after HGF treatment and then declined. HGF enhanced the protein level and the activity of uPA in HepG2 and Hep3B cells, and the uPAR protein level also increased in a HGF dose-dependent manner. HGF increased cell invasion through the Matrigel. A monoclonal antibody against human uPA receptor, mAb 3936, inhibited HGF-mediated tumor cell invasion in a dose-dependent manner. Down-regulation of uPA using uPA-shRNA induced a decrease in in vitro cell invasion. These results suggest that hepatoma cells express functional c-Met, which may provide a target for a therapeutic basis to interfere with metastases of cancer cells by inhibiting uPA system-mediated proteolysis.

Plasma Levels of Intact and Cleaved Urokinase Receptor Decrease in HIV-1-Infected Patients Initiating Highly Active Antiretroviral Therapy

Elevated blood levels of soluble urokinase receptor (suPAR) measured by ELISA decrease in human immunodeficiency virus-1 (HIV-1)-infected patients initiating highly active antiretroviral therapy (HAART). As the suPAR ELISA measures both three- and two-domain suPAR [suPAR(I-III), suPAR(II-III)] and suPAR(I-III)-ligand complexes, the amount by which the individual suPAR forms (suPAR(I-III), suPAR(II-III) and one-domain suPAR [suPAR(I)]) decrease in plasma in HIV-1-infected patients initiating HAART is unknown. Consequently, the objective of this study was to investigate HAART-induced changes in the individual plasma suPAR forms in HIV-1-infected patients. Plasma suPAR was measured by three time-resolved fluorescence immunoassays detecting suPAR(I-III), suPAR(I-III) + suPAR(II-III) and suPAR(I) in 29 treatment-naïve HIV-1-infected patients followed annually for 5 years after initiation of HAART and in 20 age- and gender-matched healthy individuals. In addition, plasma levels of the following inflammatory markers were also investigated: soluble tumour necrosis factor receptor (sTNFr)-II, TNF-alpha, interleukins (IL)-10, IL-6, IL-4, IL-2 and interferon (IFN)-gamma. In HIV-1-infected patients, plasma suPAR(I-III), suPAR(II-III) and suPAR(I) decreased within the first treatment year (all P < 0.05) and suPAR(I-III) and suPAR(II-III) remained above normal throughout follow-up (both P < 0.05). Plasma sTNFrII, IL-6, IFN-gamma and IL-10 also decreased during HAART (all P < 0.05). In HIV-1-infected patients, sTNFrII correlated with all suPAR forms before (all P < 0.01) and after 5 years HAART (all P < 0.001), whereas sTNFrII and suPAR did not correlate in healthy individuals. Intact and cleaved plasma suPAR decreased in HIV-1-infected patients initiating HAART but remained above normal. The positive correlation with sTNFrII suggests that the individual plasma suPAR forms are linked to immune activation in HIV-1 infection.

The complex between urokinase (uPA) and its type-1 inhibitor (PAI-1) in pulmonary adenocarcinoma: Relation to prognosis

In a lung cancer population comprising tumor tissue from 99 pulmonary adenocarcinoma patients, the relationship between tumor tissue level of the complex formed of urokinase (uPA) and its type-1 inhibitor (PAI-1) and survival was studied. The study included patient material previously investigated for the prognostic impact of PAI-1 on survival. Standard clinical parameters were available and the patients had a median survival time of 25 months. An ELISA established to measure preformed uPA-PAI-1 complexes was applied to the tumor extracts and previously measured data on uPA and PAI-1 levels were available. The amounts of uPA-PAI-1 complex measured in pulmonary adenocarcinoma tissue were within the same range as previously reported in breast cancer tissue (0.11-5.74 ng/mg protein). uPA and PAI-1 levels were weakly correlated to the uPA-PAI-1 complex, r = 0.52 and r = 0.47, respectively, and no relation was found between uPA-PAI-1 complex and any of the clinical parameters. However, a significant prognostic impact of PAI-1 on prognosis was demonstrated (HR = 1.62, p = 0.04). Patients with high PAI-1 and low uPA-PAI-1 complex were found to have a significantly poorer survival than patients with low PAI-1 and high uPA-PAI-1 complex (HR = 3.06, p = 0.01). This is the first investigation of the prognostic impact of uPA-PAI-1 complex in a tumor type other than breast cancer, showing low levels of uPA-PAI-1 complex in combination with high levels of PAI-1 to be associated with poor prognosis. To understand these interactions and the clinical importance of the tissue levels of uPA, PAI-1 and uPA-PAI-1 complex, the results suggest further exploratory studies of the components in pulmonary adenocarcinomas and other cancers.

Soluble urokinase receptor levels in plasma during 5 years of highly active antiretroviral therapy in HIV-1-infected patients

High blood levels of the soluble urokinase receptor (suPAR) strongly predict increased mortality in human immunodeficiency virus-1 (HIV-1)-infected patients. This study investigated the plasma concentration of suPAR in 29 treatment-naive HIV-1-infected patients during 5 years treatment with highly active antiretroviral therapy (HAART). Plasma suPAR decreased after introducing HAART, most pronounced during the first treatment year. The change in plasma suPAR was independent of changes in viral replication and CD4+ cells but it was strongly correlated with plasma levels of the soluble TNF receptor II. Compared with healthy individuals, plasma suPAR and sTN-FrII was increased in untreated patients. After initiating HAART, plasma sTNFrII remained increased whereas plasma suPAR decreased to a level comparable with healthy individuals. The present data indicate that the circulating suPAR level is linked to inflammation in untreated as well as HAART-treated HIV-1-infected patients.

High plasma levels of intact and cleaved soluble urokinase receptor reflect immune activation and are independent predictors of mortality in HIV-1-infected patients

BACKGROUND

High blood levels of soluble urokinase receptor (suPAR) measured by enzyme-linked immunoassay (ELISA) (bulk measurement of 3-domain and 2-domain suPAR [suPAR(I-III), suPAR(II-III)], and suPAR(I-III) ligand complexes) strongly predict mortality in HIV-1-infected patients. This study investigated plasma levels of suPAR(I-III), suPAR(II-III), and 1-domain suPAR [suPAR(I)] and their predictive value for survival in HIV patients.

METHODS

Plasma suPAR was measured by ELISA and 3 different time-resolved fluorescence immunoassays detecting suPAR(I-III), suPAR(I-III) plus suPAR(II-III), and suPAR(I) in 99 HIV patients and 59 healthy individuals.

RESULTS

Plasma suPAR(I-III), suPAR(II-III), and suPAR(I) were increased in HIV patients and increased with HIV disease progression (P < 0.001 for all). In multivariate linear regression analysis, soluble immune activation markers and hemoglobin were independent predictors of plasma suPAR in HIV patients, whereas the neutrophil concentration was the only independent predictor of plasma suPAR in controls. In univariate Cox analysis, higher levels of suPAR(I-III), suPAR(II-III), and suPAR(I) predicted increased mortality risk (P < 0.001 for all). In multivariate Cox analysis adjusting for CD4+ count, HIV RNA, beta2-microglobulin, hemoglobin and clinical stage, higher levels of suPAR(I-III) and suPAR(II-III) were independent predictors of increased mortality risk (P < 0.05 for both), whereas suPAR(I) was not.

CONCLUSIONS

Plasma levels of different suPAR forms are increased and associated with immune activation in HIV patients, and suPAR(I-III) and suPAR(II-III) are independent predictors of mortality in these patients.

Quantification of tissue inhibitor of metalloproteinases 2 in plasma from healthy donors and cancer patients

Tissue inhibitor of metalloproteinases (TIMP)-2 is a highly conserved molecule, which binds both active and latent matrix metalloproteinase (MMP)-2. TIMP-2 is also involved in the activation of MMP-2 on the cell surface. A quantitative enzyme-linked immunosorbent assay (ELISA) was established and optimized for measurement of TIMP-2 in plasma. The capturing antibody in the ELISA was a monoclonal, while the detecting antibody was a chicken polyclonal antibody recognizing the native form of human TIMP-2. The levels of TIMP-2 were measured in ethylenediaminetetraacetic acid (EDTA) and citrate plasma from healthy donors. The median values were determined as 163 ng/ml (n = 186) with a range of 109-253 ng/ml for EDTA plasma and 139 ng/ml (n = 77) with a range of 95-223 ng/ml for citrate plasma. The TIMP-2 concentration in citrate plasma from 15 patients with advanced, stage IV breast cancer had a median value of 160 ng/ml, only slightly higher but statistically distinguishable from the level found in citrate plasma from the healthy donors. In addition, the TIMP-2 concentration in EDTA plasma from colorectal cancer patients revealed a significantly higher level in plasma from patients with Dukes stage A (P = 0.01) compared with patients with more advanced Dukes stages.

Reduced Release of Intact and Cleaved Urokinase Receptor in Stimulated Whole-Blood Cultures from Human Immunodeficiency Virus-1-Infected Patients

The blood levels of the soluble forms of the urokinase receptor (suPAR) are increased in human immunodeficiency virus (HIV)-1-infected patients. This study investigated whether the release of urokinase-type plasminogen activator receptor (uPAR) in whole-blood cultures was affected by HIV infection. The release of different uPAR forms in whole-blood cultures incubated 24 h with or without phytohemagglutinin and lipopolysaccharide was analysed in 47 HIV patients and 19 controls. suPAR was measured by enzyme-linked immunosorbent assay (ELISA) (bulk-suPAR) and three different time-resolved fluorescence immunoassays measuring three-domain suPAR [suPAR(I-III)], three- and two-domain suPAR [suPAR(I-III) + suPAR(II-III)] and one-domain suPAR [suPAR(I)]. The uPAR release was correlated to leucocyte subpopulations and plasma levels of suPAR. The stimulated net whole-blood culture release of bulk-uPAR, uPAR(I-III), uPAR(II-III) and uPAR(I) was reduced in HIV patients (all P < 0.01), whereas the spontaneous bulk-uPAR and uPAR(I-III) release was increased in HIV patients (both P < 0.05). The stimulated uPAR release in whole-blood cultures correlated well to leucocytes and circulating suPAR levels in controls, whereas the correlation was weaker to leucocytes and nonexisting to circulating suPAR levels in HIV patients. These findings demonstrate that HIV infection affects stimulated and spontaneous uPAR release in whole-blood cultures. Given that high blood levels of suPAR in HIV patients are linked to immune activation, the perturbations in uPAR release in whole-blood cultures from HIV patients may also reflect immune activation.

Interleukin-6 infusion during human endotoxaemia inhibits in vitro release of the urokinase receptor from peripheral blood mononuclear cells

Leucocyte expression of the urokinase receptor [urokinase-type plasminogen activator receptor (uPAR)] is regulated by inflammatory mediators. This study investigated the in vivo effect of endotoxin, interleukin (IL)-6 and tumour necrosis factor (TNF)-alpha on uPAR-release in vivo and in vitro in humans. Healthy subjects received intravenous endotoxin injection [high-dose, 2 ng/kg (n=8) and low-dose, 0.06 ng/kg (n=7)], coadministration of 0.06 ng/kg endotoxin and 3 h recombinant human (rh)IL-6 infusion (n=7) or 3 h infusion of rhIL-6 (n=6), rhTNF-alpha (n=6) or NaCl (n=5). Soluble uPAR (suPAR) was measured by enzyme-linked immunosorbent assay in plasma and supernatants from unstimulated and phytohaemagglutinin and lipopolysaccharide-stimulated peripheral blood mononuclear cell (PBMC) cultures incubated for 24 h. The spontaneous and stimulated uPAR-release from PBMC cultures was enhanced 5 h after low-dose endotoxin (both P <0.05), but coadministration of rhIL-6 during low-dose endotoxaemia abolished this enhanced uPAR release. High-dose endotoxin increased plasma suPAR levels (P <0.001) whereas low-dose endotoxin, rhIL-6 or TNF-alpha did not influence uPAR release in vivo to such degree that a systemic effect on the plasma suPAR level was detectable. Even subclinical doses of endotoxin in vivo enhance the capacity of PBMC to release uPAR after incubation in vitro. The inhibitory effect of IL-6 on endotoxin-mediated uPAR-release in vitro suggests that IL-6 has anti-inflammatory effects on endotoxin-mediated inflammation.

Detection of urokinase plasminogen activator receptor and c-erbB-2 in sera of patients with breast and ovarian carcinoma

The role of urokinase plasminogen activator receptor (uPAR) and c-erbB-2 in breast and ovarian cancer was investigated. Eighty patients of breast and ovarian cancer and benign lesions, as well as twenty normal controls were evaluated for the expression of c-erbB-2 by Western blotting and uPAR levels by ELISA. The c-erbB-2 and uPAR showed a significant increase in both types of cancer investigated compared to normal control and benign lesions. The frequency of c-erbB-2 was significantly higher in breast cancer lesions (p < 0.01). Levels of CA15.3 in breast cancer and CA125 in ovarian cancer were significantly higher in cases expressing c-erbB-2 (p < 0.01) than in negative c-erbB-2 cases. The uPAR showed a significant positive correlation with advanced stages of breast cancer (r = 0.7971) and ovarian cancer (r = 0.83662), while significant correlations were found for CA15.3 in breast cancer (r = 0.64967) and CA125 in ovarian cancer (r = 0.83996). Taken together, our data suggest that the c-erbB-2 and uPAR in the sera of ovarian and breast cancer act as valuable markers for the evaluation of the patients preoperatively.

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.

The myofibroblast is the predominant plasminogen activator inhibitor-1-expressing cell type in human breast carcinomas

The tumor level of plasminogen activator inhibitor-1 (PAI-1) is an informative biochemical marker of a poor prognosis in several cancer types. However, the tumor biological functions of PAI-1 and the identity of PAI-1-expressing cells are controversial. With the aim of immunohistochemically localizing PAI-1 in formalin-fixed, paraffin-embedded invasive ductal breast carcinoma samples, we raised new polyclonal antibodies against PAI-1 from different expression systems. The antibodies were affinity purified by absorption on immobilized preparations of PAI-1 different from those used for immunization. The specificity of the antibodies was ensured by immunoblotting analysis. In immunohistochemistry, the staining pattern obtained with the antibodies showed a good correlation with the PAI-1 mRNA expression pattern. In all 25 cases analyzed, PAI-1 immunoreactivity was predominantly localized in fibroblast-like cells. Double-immunofluorescence analyses showed co-expression of PAI-1 and alpha-smooth muscle actin in these cells, suggesting that they are myofibroblasts. PAI-1 was also seen in some myoepithelial cells surrounding occasional foci of ductal carcinoma in situ (9 of 25), some endothelial cells (8 of 25), some cancer cells (3 of 25), and some mast cells (6 of 25). In conclusion, we have provided a robust immunohistochemical procedure for detection of PAI-1 and shown that the majority of the PAI-1-expressing cells in invasive ductal breast carcinomas are myofibroblasts.

Generation of antibodies to the urokinase receptor (uPAR) by DNA immunization of uPAR knockout mice: membrane-bound uPAR is not required for an antibody response

The urokinase receptor (uPAR) is a glycolipid-anchored cell surface glycoprotein that plays a central role in extracellular proteolysis during tissue remodeling processes including cancer invasion. Furthermore, uPAR is found on the surface of both dendritic cells (DCs) and T cells, and has been proposed to play a role in DC-induced T-cell activation and, therefore, in the induction of an immune response. In order to investigate the possibility of using DNA immunization for the generation of poly- and monoclonal antibodies to uPAR, we injected wild-type mice and mice deficient in uPAR (uPAR knockouts) intramuscularly with plasmid DNA encoding a carboxy-terminal truncated soluble form of the human uPAR. Multiple injections of 100 micro g of DNA resulted in a strong and specific antibody response in all mice irrespective of genotype. Antisera with a maximum titre of 32,000 were obtained, comparable with that obtained after immunization with recombinant uPAR. The subclass distribution of uPAR-specific antibodies in the sera demonstrated the induction of a mixed TH1/TH2 response, irrespective of the genotype of the mice. Our results demonstrate the possibility of generating high titre antibodies to uPAR by DNA immunization of wild-type as well as uPAR knockout mice, and that cell surface uPAR is not indispensable for the generation of a humoral immune response.

Urokinase receptor is up-regulated in endothelial cells and macrophages associated with fibrinoid deposits in the human placenta

Clearance of fibrin deposits within the human placenta is an ongoing process during normal placental development. Plasminogen is a circulating fibrinolytic protease zymogen activated in situ by plasminogen activators. We have previously reported that the receptor for urokinase plasminogen activator (uPAR) is expressed by cells either covering or enmeshed within the perivillous fibrinoid deposits. Whereas these cells seemed likely to be trophoblasts, a definitive identification was lacking, and this question is central to the understanding of the cellular mechanisms directing fibrinolysis in the placenta. In this study we have performed immunohistochemical co-localization studies and found that the uPAR-positive cells covering fibrinoid deposits are immunoreactive for CD31 and vWF, indicating that they are actually endothelial cells. In addition, we found that perivillous fibrinoid deposits not covered with uPAR-positive endothelial cells were covered with platelets identified by integrin alpha(IIb)beta(3)-immunoreactivity. Also surprisingly, the uPAR-positive cells enmeshed within fibrinoid deposits express a cell specific marker indicating that they are macrophages. Both uPAR-positive cell populations also express uPA immunoreactivity. Taken together, the data suggest that both fibrinoid-covering endothelial cells and fibrinoid-enmeshed macrophages can participate in the clearance process of perivillous fibrinoid deposits formed in the human placenta.

Serum tetranectin is an independent prognostic marker in colorectal cancer and weakly correlated with plasma suPAR, plasma PAI-1 and serum CEA

Soluble tetranectin (TN) was measured preoperatively in serum from 567 patients with primary colorectal cancer and levels were tested for association with prognosis. The prognostic significance of TN was also compared to that of plasminogen-activator inhibitor-1 (PAI-1), urokinase plasminogen activator (uPAR) and carcinoembryonic antigen (CEA). Significantly shorter survival was found for patients with TN levels below a cut-off point of 7.5 mg/l compared to patients with levels above, as illustrated by Kaplan-Meier curves. By Cox analyses, log TN, log soluble uPAR as well as log CEA were found to have an independent prognostic value for survival (log TN: HR = 0.47, 95% CI: 0.29-0.76); log soluble uPAR: HR = 1.65, 95% CI: 1.18-2.31; log CEA: HR = 1.I1, 95% CI: 1.03-1.20). Based on the multivariate model, a patient with a combination of low levels of TN and PAI-1 and elevated levels of soluble uPAR and CEA had a 2.43 increased risk as compared to a patient with median levels of these biochemical markers. Significant correlations were found with Dukes' stages for all the biochemical markers and between the respective biochemical markers. The findings confirm that TN is a strong prognostic factor in patients with colorectal cancer. TN may be valuable as a prognostic variable in future studies evaluating new treatment strategies for colorectal cancer.

Differential binding properties of human pregnancy zone protein- and alpha2-macroglobulin-proteinase complexes to low-density lipoprotein receptor-related protein

Human pregnancy zone protein (PZP) is a major pregnancy-associated plasma protein strongly related to alpha2-macroglobulin (alpha2-M). Both alpha-macroglobulins (alpha-Ms) covalently bind proteinases, which is accompanied by the exposure of carboxy terminal receptor recognition domains important for the rapid clearance from the circulation and tissues. It is accepted that the molecule responsible for the clearance of alpha2-M- and PZP-proteinase complexes is the low-density lipoprotein receptor-related protein (LRP). Although both alpha-M-proteinase complexes bind to the same receptor, differences in the binding properties have been reported. In addition, although it is known that the binding of alpha2-M-proteinase complexes to LRP can be blocked by Ni2+, the effect on PZP-proteinase has never been examined. In order to investigate differences in the binding properties of both alpha-Ms to the receptor, we purified LRP from human placenta by affinity chromatography and then analyzed the specificity and affinity of binding of alpha2-M- and PZP-proteinase complexes to the receptor by enzyme immunoassay. Our results clearly established that although both alpha-M-proteinase complexes specifically bind to LRP, PZP-chymotrypsin complexes bind to the receptor with lesser apparent affinity (Kd approximately equal 320 nM) than alpha2-M-chymotrypsin complexes (Kd approximately equal 40 nM). We also demonstrated that Ni2+ blocks the binding of alpha2-M-chymotrypsin complexes, but not PZP-chymotrypsin complexes, to LRP. These data suggest that the binding to LRP involves conformational differences between both alpha-Ms in a region immediately upstream of the carboxy terminal receptor recognition domain. The possibility that PZP-proteinase complexes interact with other receptors not available to alpha2-M-proteinase complexes could be considered.

The urokinase plasminogen activator receptor in the regulation of the actin cytoskeleton and cell motility

Cell migration is a complex process requiring tight control of several mechanisms including dynamic reorganization of the actin cytoskeleton and adhesion to the extracellular matrix. The GPI-anchored urokinase plasminogen activator receptor (uPAR) has an important role in the regulation of cell motility in many cell types. This is partly due to the localization of proteolytic activity on the cell surface by binding of the serine protease uPA. Results accumulated over the last decade suggest that uPAR is also involved in motility control through other mechanisms. These include induction of signal transduction events after ligation with uPA, binding to the extracellular matrix molecule vitronectin (VN), and association with integrins and other transmembrane partners. In this review these mechanisms will be discussed with a special emphasis on how the GPI-linked receptor transmits signals to the intracellular milieu and how uPAR participates in the regulation of actin cytoskeleton reorganization and cell adhesion during cell migration.

The molecular basis for anti-proteolytic and non-proteolytic functions of plasminogen activator inhibitor type-1: roles of the reactive centre loop, the shutter region, the flexible joint region and the small serpin fragment

The serine proteinase inhibitor plasminogen activator inhibitor type-1 (PAI-1) is the primary physiological inhibitor of the tissue-type and the urokinase-type plasminogen activator (tPA and uPA, respectively) and as such an important regulator of proteolytic events taking place in the circulation and in the extracellular matrix. Moreover, a few non-proteolytic functions have been ascribed to PAI-1, mediated by its interaction with vitronectin or the interaction between the uPA-PAI-1 complex bound to the uPA receptor and members of the low density lipoprotein receptor family. PAI-1 belongs to the serpin family, characterised by an unusual conformational flexibility, which governs its molecular interactions. In this review we describe the anti-proteolytic and non-proteolytic functions of PAI-1 from both a biological and a biochemical point of view. We will relate the various biological roles of PAI-1 to its biochemistry in general and to the different conformations of PAI-1 in particular. We put emphasis on the intramolecular rearrangements of PAI-1 that are required for its antiproteolytic as well as its non-proteolytic functions.

Ligand binding regions in the receptor for urokinase-type plasminogen activator

The interaction between urokinase plasminogen activator (uPA) and its cellular receptor (uPAR) is a key event in cell surface-associated plasminogen activation, relevant for cell migration and invasion. In order to define receptor recognition sites for uPA, we have expressed uPAR fragments as fusion products with the minor coat protein on the surface of M13 bacteriophages. Sequence analysis of cDNA fragments encoding uPA-binding peptides indicated the existence of a composite uPA-binding structure including all three uPAR domains. This finding was confirmed by experiments using an overlapping 15-mer peptide array covering the entire uPAR molecule. Four regions within the uPAR sequence were found to directly bind to uPA: two distinct regions containing amino acids 13--20 and amino acids 74--84 of the uPAR domain I, and regions in the putative loop 3 of the domains II and III. All the uPA-binding fragments from the three domains were shown to have an agonistic effect on uPA binding to immobilized uPAR. Furthermore, uPAR-(154--176) increased uPAR-transfected BAF3-cell adhesion on vitronectin in the presence of uPA, whereas uPAR-(247--276) stimulated the cell adhesion both in the absence or presence of uPA. The latter fragment was also able to augment the binding of vitronectin to uPAR in a purified system, thereby mimicking the effect of uPA on this interaction. These results indicate that uPA binding can take place to particular part(s) on several uPAR molecules and that direct uPAR-uPAR contacts may contribute to receptor activation and ligand binding.

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.

Cancer cell expression of urokinase-type plasminogen activator receptor mRNA in squamous cell carcinomas of the skin

In this study we have used in situ hybridization with radiolabeled antisense RNA probes to examine the expression of mRNA for urokinase-type plasminogen activator and its receptor in histologic samples of squamous cell (n = 7) and basal cell (n = 7) carcinomas of the skin. Messenger RNA for both urokinase-type plasminogen activator and its receptor were expressed in all of the squamous cell carcinomas, but could not be detected in the basal cell carcinomas. In all of the seven squamous cell carcinomas a signal for urokinase-type plasminogen activator receptor mRNA was detected focally in well-differentiated cancer cells surrounding keratinized pearls, and in four specimens urokinase-type plasminogen activator receptor mRNA was in addition expressed by cancer cells at the edge of invasively growing strands of tumor. Urokinase-type plasminogen activator mRNA expression was found in virtually all the cancer cells of the squamous cell carcinomas, and importantly we found, by hybridizations for urokinase-type plasminogen activator and its receptor mRNA on adjacent sections of squamous cell carcinomas, that it was exactly the invading cancer cells that simultaneously expressed both these components required for plasmin-mediated proteolysis at the cell surface. We have previously shown that both urokinase-type plasminogen activator and its receptor mRNA are expressed by the leading-edge keratinocytes in regenerating epidermis during mouse skin wound healing, and that wound healing is impaired in mice made deficient in plasminogen by targeted gene disruption. We propose that there are similarities between the mechanisms of generation and regulation of extracellular proteolysis during skin re-epithelialization and squamous cell carcinoma invasion. The ability of the squamous carcinoma cells to mimic the "invasive" phenotype of re-epithelializing keratinocytes may be one of the factors that make squamous cell carcinomas more aggressive tumors than basal cell carcinomas.

Differential Binding of Urokinase and Peptide Antagonists to the Urokinase Receptor: Evidence from Characterization of the Receptor in Four Primate Species

The urokinase plasminogen activator receptor (uPAR) is a membrane protein active in localizing the plasminogen activation cascade system on the cell surface. The resulting pericellular proteolytic activity is responsible for degradation reactions in the extracellular matrix that are needed for the invasion of cancer cells, thus making uPAR a potential target for anti-invasive therapy based on binding antagonists. A remarkable property of the uPA-uPAR system is a pronounced species specificity in ligand recognition. We have now cloned and studied uPAR from four primate species and show that even though these sequences contain very few substitutions relative to the human uPAR, the receptor protein products differ markedly in terms of ligand selectivity. Thus, a well described competitive peptide antagonist directed against the human uPAR reacts with only one of the monkey receptors (chimpanzee uPAR), in spite of the fact that uPAR from all of the four species cross-reacts with human uPA. Notably, uPAR from African green monkey, which is completely devoid of reactivity with the peptide, contains only three substitutions relative to chimpanzee uPAR in the molecular regions critical for binding. These findings aid the elucidation of the structure/function relationship of uPAR and, unexpectedly, identify a structural distinction governing the binding of uPA and a very similar peptide antagonist.

Enhanced release of soluble urokinase receptor by endothelial cells in contact with peripheral blood cells

The urokinase receptor (uPAR) on the cell surface plays an important role in extracellular proteolysis, cell migration and adhesion. Soluble uPAR (suPAR) has been recently discovered in plasma, but its origin is unclear. Our results now demonstrate that both unstimulated blood mononuclear and endothelial cells can release suPAR and that the release is enhanced when either mononuclear cells or thrombocytes are cultured together with endothelial cells. Co-culture without cell-cell contacts fails to enhance suPAR release. We also found suPAR fragments, known to be potent inducers of chemotaxis, in co-culture growth medium samples. Taken together, our results suggest that normal plasma suPAR can be produced by endothelial and mononuclear cells and that suPAR release in cell-cell contacts may have a regulatory role in cell adhesion.

Urokinase receptor: a molecular organizer in cellular communication

In a variety of cell types, the glycolipid-anchored urokinase receptor (uPAR) is colocalized pericellularly with components of the plasminogen activation system and endocytosis receptors. uPAR is also coexpressed with caveolin and members of the integrin adhesion receptor superfamily. The formation of functional units with these various proteins allows the uPAR to mediate the focused proteolysis required for cell migration and invasion and to contribute both directly and indirectly to cell adhesive processes in a non-proteolytic fashion. This dual activity, together with the initiation of signal transduction pathways by uPAR, is believed to influence cellular behaviour in angiogenesis, inflammation, wound repair and tumor progression/metastasis and open up the way for uPAR-based therapeutic approaches.

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

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

High-molecular-weight serum protein complexes differentially promote cell migration and the focal adhesion localization of the urokinase receptor in human glioma cells

The distribution of the urokinase-type plasminogen activator receptor (uPAR) on human glioma cells was examined as a function of culture conditions, using immunofluorescence and immunophotoelectron microscopy. Both uPAR colocalization with focal adhesion proteins and glioma cell motility were maximal in medium containing whole serum or a serum fraction retained by a 500,000 mol wt cutoff centrifugal concentration filter. High motility also took place in medium containing a serum fraction passed by the 500,000 cutoff filter but retained by a 100,000 cutoff filter and in minimal medium containing added vitronectin; however, under these conditions only a small percentage of the otherwise abundant focal adhesions contained colocalized uPAR. Glioma cells in minimal medium with added laminin migrated with a highly elongated morphology but without either classical focal adhesions or well-defined uPAR labeling. In contrast, glioma cells in minimal medium with no additions did not migrate, nor did they adhere well or display defined labeling patterns for focal adhesion proteins or uPAR. The results indicate that high-molecular-weight serum protein complexes promote both uPAR-focal adhesion colocalization and cell migration in glioma cells. However, conditions can be selected in which migration takes place with minimal uPAR-focal adhesion localization, as well as in the absence of apparent focal adhesions.

Recombinant toxins that bind to the urokinase receptor are cytotoxic without requiring binding to the alpha(2)-macroglobulin receptor

The alpha(2-)macroglobulin receptor (alpha(2)MR) has been reported to mediate the internalization of the urokinase plasminogen activator receptor (uPAR) via ligand binding to both receptors. To target malignant uPAR-expressing cells and to determine whether uPAR can internalize without ligand binding to alpha(2)MR, we engineered two recombinant toxins, ATF-PE38 and ATF-PE38KDEL. Each consists of the amino-terminal fragment (ATF) of human urokinase and a truncated form of Pseudomonas exotoxin (PE) devoid of domain Ia, which binds alpha(2)MR. ATF-PE38 and ATF-PE38KDEL were cytotoxic toward malignant uPAR-bearing cells, with IC(50) values as low as 0.02 ng/ml (0.3 pM). Cytotoxicity could be blocked using either recombinant urokinase or free ATF, indicating that the cytotoxicity of the recombinant toxins was specific. Radiolabeled ATF-PE38 had high affinity for uPAR (K(d) = 0.4-8 nM) on a variety of different malignant cell types and internalized at a rate similar to that of ATF. The cytotoxicity was not diminished by receptor-associated protein, which binds and shields the alpha(2)MR from other proteins, or by incubation with phorbol myristate acetate, which is known to decrease the number of alpha(2)MRs in U937 cells or by antibodies to alpha(2)MR. Therefore, these recombinant toxins appear to internalize via uPAR without association with the alpha(2)MR.

Loss of ELISA specificity due to biotinylation of monoclonal antibodies

A significant degree of nonspecificity was found in ELISA determinations of soluble urokinase receptor (suPAR) in human blood plasma when biotinylated monoclonal antibodies (Mabs) were used for the detection layer. Surface plasmon resonance studies using both nonbiotinylated and biotinylated antibodies demonstrated that biotinylation reduced specific binding of the antibodies to their target antigen, suPAR. Furthermore, biotinylation produced a new interaction with unknown human plasma protein(s), unrelated to suPAR. Nonspecific interaction with plasma protein(s) was also observed after biotinylation of a Mab having no specific target antigen in human plasma and, in both cases, the level of nonspecific interaction was directly related to the degree of antibody biotinylation. These results reinforce earlier observations that biotinylation of antibodies can reduce the affinity of antibodies, but also indicate that, in addition, biotinylation can reduce the specificity of immunoassays for plasma proteins.

Expression and cellular localization of the urokinase-type plasminogen activator and its receptor in human hepatocellular carcinoma

The urokinase-type plasminogen activator (uPA) and its receptor (uPAR) play an important role in tumour invasion. Previous studies have shown by RT-PCR that uPA and uPAR mRNAs are expressed in human hepatocellular carcinoma (HCC). Here, in situ hybridization, immunohistochemistry, and double immunofluorescence were used to identify the cells expressing uPA and uPAR in 26 HCCs. The results indicate that uPA and uPAR were expressed in every case, almost exclusively in stromal cells, mostly myofibroblasts and macrophages, except for rare tumoural hepatocytes expressing cytokeratin 7. These results show the important role of stromal cells of HCC in the pericellular proteolysis which facilitates cancer cell invasion.

A urokinase receptor-associated protein with specific collagen binding properties

The plasminogen activation cascade system, directed by urokinase and the urokinase receptor, plays a key role in extracellular proteolysis during tissue remodeling. To identify molecular interaction partners of these trigger proteins on the cell, we combined covalent protein cross-linking with mass spectrometry based methods for peptide mapping and primary structure analysis of electrophoretically isolated protein conjugates. A specific tri-molecular complex was observed upon addition of pro-urokinase to human U937 cells. This complex included the urokinase receptor, pro-urokinase, and an unknown, high molecular weight urokinase receptor-associated protein. The tryptic peptide mixture derived from a cross-linked complex of pro-urokinase and the latter protein was analyzed by nanoelectrospray tandem mass spectrometric sequencing. This analysis identified the novel protein as the human homologue of a murine membrane-bound lectin with hitherto unknown function. The human cDNA was cloned and sequenced. The protein, designated uPARAP, is a member of the macrophage mannose receptor protein family and contains a putative collagen-binding (fibronectin type II) domain in addition to 8 C-type carbohydrate recognition domains. It proved capable of binding strongly to a single type of collagen, collagen V. This collagen binding reaction at the exact site of plasminogen activation on the cell may lead to adhesive functions as well as a contribution to cellular degradation of collagen matrices.

Determination of the Complex between Urokinase and Its Type-1 Inhibitor in Plasma from Healthy Donors and Breast Cancer Patients

Background: The complex between urokinase (uPA) and its type-1 inhibitor (PAI-1) is formed exclusively from the active forms of these components; thus, the complex concentration in a biological sample may reflect the ongoing degree of plasminogen activation. Our aim was to establish an ELISA for specific quantification of the uPA:PAI-1 complex in plasma of healthy donors and breast cancer patients.

Methods: A kinetic sandwich format immunoassay was developed, validated, and applied to plasma from 19 advanced-stage breast cancer patients, 39 age-matched healthy women, and 31 men.

Results: The assay detection limit was &lt;2 ng/L, and the detection of complex in plasma was validated using immunoabsorption, competition, and recovery tests. Eighteen cancer patients had a measurable complex concentration (median, 68 ng/L; range, &lt;16 to 8700 ng/L), whereas for healthy females and males the median signal values were below the detection limit (median, &lt;16 ng/L; range, &lt;16 to 200 ng/L; P &lt;0.0001). For patient plasma, a comparison with total uPA and PAI-1 showed that the complex represented a variable, minor fraction of the uPA and PAI-1 concentrations of each sample.

Conclusion: The reported ELISA enables detection of the uPA:PAI-1 complex in blood and, therefore, the evaluation of the complex as a prognostic marker in cancer.

Different mechanisms are involved in the antibody mediated inhibition of ligand binding to the urokinase receptor: a study based on biosensor technology

Certain monoclonal antibodies are capable of inhibiting the biological binding reactions of their target proteins. At the molecular level, this type of effect may be brought about by completely different mechanisms, such as competition for common binding determinants, steric hindrance or interference with conformational properties of the receptor critical for ligand binding. This distinction is central when employing the antibodies as tools in the elucidation of the structure-function relationship of the protein in question. We have studied the effect of monoclonal antibodies against the urokinase plasminogen activator receptor (uPAR), a protein located on the surface of various types of malignant and normal cells which is involved in the direction of proteolytic degradation reactions in the extracellular matrix. We show that surface plasmon resonance/biomolecular interaction analysis (BIA) can be employed as a highly useful tool to characterize the inhibitory mechanism of specific antagonist antibodies. Two inhibitory antibodies against uPAR, mAb R3 and mAb R5, were shown to exhibit competitive and non-competitive inhibition, respectively, of ligand binding to the receptor. The former antibody efficiently blocked the receptor against subsequent ligand binding but was unable to promote the dissociation of a preformed receptor-ligand complex. The latter antibody was capable of binding the preformed complex, forming a transient trimolecular assembly, and promoting the dissociation of the uPA/uPAR complex. The continuous recording of binding and dissociation, obtained in BIA, is central in characterizing these phenomena. The identification of a non-competitive inhibitory mechanism against this receptor reveals the presence of a determinant which influences the binding properties of a remote site in the molecular structure and which could be an important target for a putative synthetic antagonist.