Cell-induced potentiation of the plasminogen activation system is abolished by a monoclonal antibody that recognizes the NH2-terminal domain of the urokinase receptor

Targeting uPAR with an antibody-drug conjugate suppresses tumor growth and reshapes the immune landscape in pancreatic cancer models

Antibody-drug conjugates (ADCs) hold promise to advance targeted therapy of pancreatic ductal adenocarcinoma (PDAC), where the desmoplastic tumor stroma challenges effective treatment. Here, we explored the urokinase plasminogen activator receptor (uPAR) as a candidate ADC target in PDAC, harnessing its massive tumoral and stromal expression in this stroma-dense tumor. We generated a site-specific ADC offering high-affinity, cross-species reactivity, and efficient internalization of the anti-uPAR monoclonal antibody, FL1, carrying a potent anthracycline derivative (PNU-158692). In vitro, FL1-PNU exhibited potent and specific cytotoxicity against uPAR-expressing PDAC cell lines, stromal and immune cells, and bystander killing of uPAR-negative cells. In vivo, the ADC induced remission or sustained tumor regression and extended survival in xenograft models. In syngeneic orthotopic models, the antitumor effect promoted immunomodulation by enhancing infiltrating immune effectors and decreasing immunosuppressive cells. This study lays grounds for further exploring FL1-PNU as a putative clinical ADC candidate, potentially providing a promising therapeutic avenue for PDAC as a monotherapy or in combinatorial regimens.

The endocytic receptor uPARAP is a regulator of extracellular thrombospondin-1

Thrombospondin-1 (TSP-1) is a matricellular protein with a multitude of functions in the pericellular and extracellular environment. We report a novel pathway for the regulation of extracellular TSP-1, governed by the endocytic collagen receptor, uPARAP (urokinase plasminogen activator receptor-associated protein; MRC2 gene product, also designated Endo180, CD280). First, using a novel proteomic approach for unbiased identification of ligands for endocytosis, we identify TSP-1 as a candidate ligand for specific uptake by uPARAP. We then show that uPARAP can efficiently internalize TSP-1 for lysosomal degradation, that this capability is not shared by other, closely related endocytic receptors and that uPARAP serves to regulate the extracellular levels of TSP-1 in vitro. Using wild type and uPARAP null mice, we also demonstrate uPARAP-mediated endocytosis of TSP-1 in dermal fibroblasts in vivo. Unlike other uPARAP ligands, the interaction with TSP-1 is sensitive to heparin and the responsible molecular motifs in uPARAP are overlapping, but not identical with those governing the interaction with collagens. Finally, we show that uPARAP can also mediate the endocytosis of TSP-2, a thrombospondin closely related to TSP-1, but not the more distantly related members of the same protein family, TSP-3, -4 and -5. These findings indicate that the role of uPARAP in ECM remodeling is not limited to the uptake of collagen for degradation but also includes an orchestrator function in the regulation of thrombospondins with numerous downstream effects. This is likely to be an important factor in the physiological and pathological roles of uPARAP in bone biology, fibrosis and cancer. The proteomic data has been deposited to the ProteomeXchange Consortium via the PRIDE partner repository with the data set identifier PXD031272.

Expression and one-step purification of active lipoprotein lipase contemplated by biophysical considerations

Lipoprotein lipase (LPL) is essential for intravascular lipid metabolism and is of high medical relevance. Since LPL is notoriously unstable, there is an unmet need for a robust expression system producing high quantities of active and pure recombinant human LPL. We showed previously that bovine LPL purified from milk is unstable at body temperature (T_m is 34.8 °C), but in the presence of the endothelial transporter glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1) LPL is stabile (T_m increases to 57.6 °C). Building on this information, we now designed an expression system for human LPL using Drosophila S2 cells grown in suspension at high cell density and at an advantageous temperature of 25 °C. We co-transfected S2 cells with human LPL, LMF1 and soluble GPIHBP1 to provide an efficient chaperoning and stabilization of LPL in all compartments during synthesis and after secretion into the conditioned medium. For LPL purification, we used heparin-Sepharose affinity chromatography, which disrupted LPL-GPIHBP1 complexes causing GPIHBP1 to elute with the flow-through of the conditioned media. This one-step purification procedure yielded high quantities of pure and active LPL (4‒28 mg/L). Purification of several human LPL variants (furin-cleavage resistant mutant R297A, active-site mutant S132A, and lipid-binding-deficient mutant W390A-W393A-W394A) as well as murine LPL underscores the versatility and robustness of this protocol. Notably, we were able to produce and purify LPL containing the cognate furin-cleavage site. This method provides an efficient and cost-effective approach to produce large quantities of LPL for biophysical and large-scale drug discovery studies.

D2A sequence of the urokinase receptor induces cell growth through αvβ3 integrin and EGFR

The urokinase receptor (uPAR) stimulates cell proliferation by forming a macromolecular complex with αvβ3 integrin and the epidermal growth factor receptor (EGFR, ErbB1 or HER1) that we name the uPAR proliferasome. uPAR transactivates EGFR, which in turn mediates uPAR-initiated mitogenic signal to the cell. EGFR activation and EGFR-dependent cell growth are blocked in the absence of uPAR expression or when uPAR activity is inhibited by antibodies against either uPAR or EGFR. The mitogenic sequence of uPAR corresponds to the D2A motif present in domain 2. NMR analysis revealed that D2A synthetic peptide has a particular three-dimensional structure, which is atypical for short peptides. D2A peptide is as effective as EGF in promoting EGFR phosphorylation and cell proliferation that were inhibited by AG1478, a specific inhibitor of the tyrosine kinase activity of EGFR. Both D2A and EGF failed to induce proliferation of NR6-EGFR-K721A cells expressing a kinase-defective mutant of EGFR. Moreover, D2A peptide and EGF phosphorylate ERK demonstrating the involvement of the MAP kinase signalling pathway. Altogether, this study reveals the importance of sequence D2A of uPAR, and the interdependence of uPAR and EGFR.

N-Formyl Peptide Receptors Induce Radical Oxygen Production in Fibroblasts Derived From Systemic Sclerosis by Interacting With a Cleaved Form of Urokinase Receptor

Systemic sclerosis (SSc) is a chronic autoimmune disease characterized by fibrosis, alteration in the microvasculature and immunologic abnormalities. It has been hypothesized that an abnormal redox state could regulate the persistent fibrotic phenotype in SSc patients. N-Formyl peptide receptors (FPRs) are chemotactic receptors overexpressed in fibroblasts derived from SSc patients. In this study, we demonstrated that stimulation of FPRs promotes the generation of reactive oxygen species (ROS) in skin fibroblasts. In fibroblast cells, ROS production was due to FPRs interaction with the urokinase receptor (uPAR) and to β₁ integrin engagement. FPRs cross-talk with uPAR and integrins led to Rac1 and ERKs activation. FPRs stimulation increased gp91phox and p67phox expression as well as the direct interaction between GTP-Rac1 and p67phox, thus promoting assembly and activation of the NADPH oxidase complex. FPRs functions occur through interaction with a specific domain of uPAR (residues ₈₈SRSRY₉₂) that can be exposed on the cell membrane by protease-mediated receptor cleavage. Immunohistochemistry analysis with a specific anti-SRSRY antibody showed increased expression of uPAR in a cleaved form, which exposes the SRSRY sequence at its N-terminus (DIIDIII-uPAR88–92) in skin biopsies from SSc patients. As expected by the increased expression of both FPRs and DII-DIII-uPAR_88-92, fibroblasts derived from SSc patients showed a significantly increase in ROS generation both at a basal level than after FPRs stimulation, as compared to fibroblasts from normal subjects. C37, a small molecule blocking the interaction between FPRs and uPAR, and selumetinib, a clinically approved MAPKK/ERK inhibitor, significantly inhibited FPRs-mediated ROS production in fibroblasts derived from SSc patients. Thus, FPRs, through the interaction with the uPA/uPAR system, can induce ROS generation in fibroblasts by activating the NADPH oxidase, playing a role in the alteration of the redox state observed in SSc.

Is the Urokinase-type Plasminogen Activator System a Reliable Prognostic Factor in Gastric Cancer?

Aim

The aim of this prospective study was to evaluate the clinical and prognostic impact of immunohisto-chemically assessed uPA and PAI-1 in patients with gastric cancer.

Methods

This prospective study analyzed specimens obtained from 105 gastric cancer patients who underwent gastrectomy with extended lymphadenectomy. The immunohistochemical expression of uPA and PAI-1 was studied semiquantitatively in the tumor epithelium and was correlated with the clinicopathological features of each patient.

Results

Univariate analysis revealed no statistically significant association of uPA levels with pT and pN category (p=0.655 and 0.053, respectively), grading (p=0.374), depth of tumor invasion (p=0.665), UICC classification (p=0.21) and the Laurén classification (p=0.578). PAI-1 expression showed no statistically significant correlation with pT, pN and M category (p=0.589, 0.414, and 0.167, respectively), grading (p=0.273), and the Laurén classification (p=0.368). Only the UICC classification was significantly correlated with PAI-1 (p=0.016). Kaplan-Meier analysis revealed no significant association of uPA and PAI-1 with overall survival (p=0.0929 and 0.0870, respectively).

Conclusions

Our results could not verify any prognostic value of uPA and PAI-1 levels in patients with gastric carcinoma. Therefore, the uPA-system as a biologically defined prognostic marker to identify high-risk gastric cancers should be applied with caution. However, considering the number of patients involved and the borderline level of significance observed in this study, a larger number of events may have resulted in significant differences.

Urokinase-type plasminogen activator receptor (uPAR), tissue factor (TF) and epidermal growth factor receptor (EGFR): tumor expression patterns and prognostic value in oral cancer

Background

Tumor-specific biomarkers are a prerequisite for the development of targeted imaging and therapy in oral squamous cell carcinoma (OSCC). urokinase-type Plasminogen Activator Receptor (uPAR), Tissue Factor (TF) and Epidermal Growth Factor Receptor (EGFR) are three biomarkers that exhibit enhanced expression in many types of cancers, and have been investigated as potential biomarkers for targeted strategies and prognostication. The aim of the study was to investigate the expression patterns of uPAR, TF and EGFR and their potential prognostic value in OSCC.

Methods

Immunohistochemical expression of uPAR, TF and EGFR in tumor resection specimens from 191 patients with primary OSCC was analyzed. Overall (OS) and disease-free survival (DFS) was calculated. Associations between biomarker expression, clinicopathological factors and patient survival was analyzed using the Cox proportional hazards model for univariate and multivariate analysis, log rank and Kaplan-Meier statistics.

Results

uPAR and TF exhibited a highly tumor-specific expression pattern while EGFR also showed expression in normal tissues outside the tumor compartment. The overall positive expression rate of uPAR, TF and EGFR was 95%, 58% and 98%, respectively. High uPAR expression across the entire cohort was negatively associated with OS (p = 0.031, HR = 1.595 (95%CI 1.044–2.439)) in univariate analysis. The 5-year OS for high and low uPAR expression was 39% and 56%, respectively. The expression of TF and EGFR was not associated with survival outcome.

Conclusions

This study may suggest that uPAR and TF could potentially be attractive targets for molecular imaging and therapy in OSCC due to high positive expression rates and tumor-specific expression patterns. High uPAR expression was significantly associated with a reduced survival. uPAR seems to be a prognostic biomarker in oral cancer.

Urokinase-type plasminogen activator modulates mammalian circadian clock phase regulation in tissue-type plasminogen activator knockout mice

Glutamate phase shifts the circadian clock in the mammalian suprachiasmatic nucleus (SCN) by activating NMDA receptors. Tissue-type plasminogen activator (tPA) gates phase shifts by activating plasmin to generate m(ature) BDNF, which binds TrkB receptors allowing clock phase shifts. Here, we investigate phase shifting in tPA knockout (tPA^-/- ; B6.129S2-Plat^tm1Mlg /J) mice, and identify urokinase-type plasminogen activator (uPA) as an additional circadian clock regulator. Behavioral activity rhythms in tPA^-/- mice entrain to a light-dark (LD) cycle and phase shift in response to nocturnal light pulses with no apparent loss in sensitivity. When the LD cycle is inverted, tPA^-/- mice take significantly longer to entrain than C57BL/6J wild-type (WT) mice. SCN brain slices from tPA^-/- mice exhibit entrained neuronal activity rhythms and phase shift in response to nocturnal glutamate with no change in dose-dependency. Pre-treating slices with the tPA/uPA inhibitor, plasminogen activator inhibitor-1 (PAI-1), inhibits glutamate-induced phase delays in tPA^-/- slices. Selective inhibition of uPA with UK122 prevents glutamate-induced phase resetting in tPA^-/- but not WT SCN slices. tPA expression is higher at night than the day in WT SCN, while uPA expression remains constant in WT and tPA^-/- slices. Casein-plasminogen zymography reveals that neither tPA nor uPA total proteolytic activity is under circadian control in WT or tPA^-/- SCN. Finally, tPA^-/- SCN tissue has lower mBDNF levels than WT tissue, while UK122 does not affect mBDNF levels in either strain. Together, these results suggest that either tPA or uPA can support photic/glutamatergic phase shifts of the SCN circadian clock, possibly acting through distinct mechanisms.

Urokinase links plasminogen activation and cell adhesion by cleavage of the RGD motif in vitronectin

Components of the plasminogen activation system including urokinase (uPA), its inhibitor (PAI-1) and its cell surface receptor (uPAR) have been implicated in a wide variety of biological processes related to tissue homoeostasis. Firstly, the binding of uPA to uPAR favours extracellular proteolysis by enhancing cell surface plasminogen activation. Secondly, it promotes cell adhesion and signalling through binding of the provisional matrix protein vitronectin. We now report that uPA and plasmin induces a potent negative feedback on cell adhesion through specific cleavage of the RGD motif in vitronectin. Cleavage of vitronectin by uPA displays a remarkable receptor dependence and requires concomitant binding of both uPA and vitronectin to uPAR Moreover, we show that PAI-1 counteracts the negative feedback and behaves as a proteolysis-triggered stabilizer of uPAR-mediated cell adhesion to vitronectin. These findings identify a novel and highly specific function for the plasminogen activation system in the regulation of cell adhesion to vitronectin. The cleavage of vitronectin by uPA and plasmin results in the release of N-terminal vitronectin fragments that can be detected in vivo, underscoring the potential physiological relevance of the process.

A CCR2 macrophage endocytic pathway mediates extravascular fibrin clearance in vivo

Extravascular fibrin deposition accompanies many human diseases and causes chronic inflammation and organ damage, unless removed in a timely manner. Here, we used intravital microscopy to investigate how fibrin is removed from extravascular space. Fibrin placed into the dermis of mice underwent cellular endocytosis and lysosomal targeting, revealing a novel intracellular pathway for extravascular fibrin degradation. A C-C chemokine receptor type 2 (CCR2)-positive macrophage subpopulation constituted the majority of fibrin-uptaking cells. Consequently, cellular fibrin uptake was diminished by elimination of CCR2-expressing cells. The CCR2-positive macrophage subtype was different from collagen-internalizing M2-like macrophages. Cellular fibrin uptake was strictly dependent on plasminogen and plasminogen activator. Surprisingly, however, fibrin endocytosis was unimpeded by the absence of the fibrin(ogen) receptors, αMβ2 and ICAM-1, the myeloid cell integrin-binding site on fibrin or the endocytic collagen receptor, the mannose receptor. The study identifies a novel fibrin endocytic pathway engaged in extravascular fibrin clearance and shows that interstitial fibrin and collagen are cleared by different subsets of macrophages employing distinct molecular pathways.

Epithelial and stromal cell urokinase plasminogen activator receptor expression differentially correlates with survival in rectal cancer stages B and C patients

Urokinase plasminogen activator receptor (uPAR) has been proposed as a potential prognostic factor for colorectal cancer (CRC) patient survival. However, CRC uPAR expression remains controversial, especially regarding cell types where uPAR is overexpressed (e.g., epithelium (uPAR^E) or stroma-associated cells (uPAR^S)) and associated prognostic relevance. In this study, two epitope-specific anti-uPAR monoclonal antibodies (MAbs) could discriminate expression of uPAR^E from uPAR^S and were used to examine this association with survival of stages B and C rectal cancer (RC) patients. Using immunohistochemistry, MAbs #3937 and R4 were used to discriminate uPAR^E from uPAR^S respectively in the central and invasive frontal regions of 170 stage B and 179 stage C RC specimens. Kaplan-Meier and Cox regression analyses were used to determine association with survival. uPAR expression occurred in both epithelial and stromal compartments with differential expression observed in many cases, indicating uPAR^E and uPAR^S have different cellular roles. In the central and invasive frontal regions, uPAR^E was adversely associated with overall stage B survival (HR = 1.9; p = 0.014 and HR = 1.5; p = 0.031, respectively) reproducing results from previous studies. uPAR^S at the invasive front was associated with longer stage C survival (HR = 0.6; p = 0.007), reflecting studies demonstrating that macrophage peritumoural accumulation is associated with longer survival. This study demonstrates that different uPAR epitopes should be considered as being expressed on different cell types during tumour progression and at different stages in RC. Understanding how uPAR^E and uPAR^S expression affects survival is anticipated to be a useful clinical prognostic marker of stages B and C RC.

Urokinase-type plasminogen activator receptor (uPAR) on tumor-associated macrophages is a marker of poor prognosis in colorectal cancer

Patients were identified from a population-based prospective study of 4990 individuals with symptoms associated with colorectal cancer (CRC). A total of 244 CRC tissue samples were available for immunohistochemical staining of uPAR, semiquantitatively scored at the invasive front, and in the tumor core on cancer cells, macrophages, and myofibroblasts. In addition, the levels of the intact and cleaved uPAR-forms in blood from the same patients are evaluated in this study. In a univariate analysis, the number of uPAR-positive versus uPAR-negative macrophages (HR = 2.26, [95% CI: 1.39-3.66, P = 0.0009]) and cancer cells (HR=1.49, [95% CI: 1.01-2.20, P = 0.047]) located in the tumor core were significantly associated to overall survival. In a multivariate analysis, uPAR-positive versus uPAR-negative macrophages located in the tumor core showed the best separation of patients with positive score associated to poor prognosis (HR = 1.84 [95% CI: 1.12-3.04, P = 0.017]). In a multivariate analysis including clinical covariates and soluble uPAR(I), the latter was significantly associated to overall survival (HR = 2.68 [95% CI: 1.90-3.79, P < 0.0001]) and uPAR-positive macrophages in the tumor core remained significantly associated to overall survival (HR = 1.81 [95% CI: 1.08-3.01, P = 0.023]). Membrane-bound uPAR showed additive effects with the circulating uPAR(I) and stage, giving a hazard ratio of 12 between low and high scores. Thus, combining stage, uPAR(I) in blood and uPAR on macrophages in the tumor core increase the prognostic precision more than tenfold, as compared to stage alone.

uPAR regulates pericellular proteolysis through a mechanism involving integrins and fMLF-receptors

The expression of the urokinase-type plasminogen activator (uPA) and its receptor (uPAR) can be regulated by several hormones, cytokines, and tumour promoters. uPAR is a glycosyl-phosphatidyl inositol (GPI)-linked cell-surface protein; however, it is capable to transduce signals inside the cell by interacting with other cell-surface proteins, such as integrins and G-protein coupled (GPC) receptors. We previously reported that uPAR cell-surface expression can be positively regulated by its ligand, uPA, independently of its proteolytic activity. We now demonstrate that uPAR overexpression induces or increases uPA secretion both in uPAR-negative and in uPAR-expressing cells. Accordingly, uPAR depletion impairs uPA expression in cells which constitutively express both uPA and its receptor. uPAR exerts its regulatory effect through the activation of the ERK mitogen-activated protein kinases (MAPKs), whereas the p-38 MAPK is not involved. Overexpression of truncated forms of uPAR, lacking the N-terminal domain (DI) and not able to interact with membrane co-receptors, failed to increase uPA expression. Inhibition of uPAR-integrin interaction by the specific P-25 peptide, as well as Gi-protein inhibition by cholera pertussin toxin or depletion of the GPC receptors for fMLF (fMLF-Rs) also impaired uPAR capability to regulate uPA expression. These findings demonstrate that uPAR, whose expression is regulated by uPA, can, in turn, regulate uPA expression through a mechanism involving its functional interaction with integrins and fMLF-Rs.

Targeting tumor cell invasion and dissemination in vivo by an aptamer that inhibits urokinase-type plasminogen activator through a novel multifunctional mechanism

Data accumulated over the latest two decades have established that the serine protease urokinase-type plasminogen activator (uPA) is a potential therapeutic target in cancer. When designing inhibitors of the proteolytic activity of serine proteases, obtaining sufficient specificity is problematic, because the topology of the proteases' active sites are highly similar. In an effort to generate highly specific uPA inhibitors with new inhibitory modalities, we isolated uPA-binding RNA aptamers by screening a library of 35 nucleotides long 2'-fluoro-pyrimidine RNA molecules using a version of human pro-uPA lacking the epidermal growth factor-like and kringle domains as bait. One pro-uPA-binding aptamer sequence, referred to as upanap-126, proved to be highly specific for human uPA. Upanap-126 delayed the proteolytic conversion of human pro-uPA to active uPA, but did not inhibit plasminogen activation catalyzed by two-chain uPA. The aptamer also inhibited the binding of pro-uPA to uPAR and the binding of vitronectin to the preformed pro-uPA/uPAR complex, both in cell-free systems and on cell surfaces. Furthermore, upanap-126 inhibited human tumor cell invasion in vitro in the Matrigel assay and in vivo in the chick embryo assay of cell escape from microtumors. Finally, upanap-126 significantly reduced the levels of tumor cell intravasation and dissemination in the chick embryo model of spontaneous metastasis. Together, our findings show that usage of upanap-126 represents a novel multifunctional mechanistic modality for inhibition of uPA-dependent processes involved in tumor cell spread.

Soluble urokinase receptor and focal segmental glomerulosclerosis

PURPOSE OF REVIEW

We discuss recent work on the pathogenetic mechanism of focal segmental glomerulosclerosis (FSGS) and its relationship to soluble urokinase receptor (suPAR).

RECENT FINDINGS

Primary FSGS can affect both children and adults. The disease can recur after kidney transplantation suggesting the involvement of a causative circulating factor. Recent work suggests that approximately two-thirds of FSGS patients have elevated levels of serum suPAR, which confers risk to both native and posttransplant FSGS.

SUMMARY

Future studies that are geared to understand the clinical implications of high suPAR levels in native FSGS as well as in the pretransplant and posttransplant setting will allow for better patient risk stratification and more targeted treatment options.

Prognosis in adenocarcinomas of lower oesophagus, gastro-oesophageal junction and cardia evaluated by uPAR-immunohistochemistry

Adenocarcinomas of lower oesophagus, gastro-oesophageal junction and cardia in humans are highly invasive tumours with poor prognosis. The localisation of urokinase-type plasminogen activator receptor (uPAR) was determined in 66 patients; 60 with adenocarcinomas and six cases with Barrett's oesophagus. uPAR was expressed in nearly all cases of invasive adenocarcinomas by populations of cancer cells, macrophages and myofibroblasts at both the invasion front and the tumour core. In areas with high-grade dysplasia or with Barrett's metaplasia adjacent to the tumour tissue, no uPAR-immunoreactivity was found. High local expression of uPAR, therefore, appears to be a characteristic marker for invasive behaviour in this tumour, suggesting that uPAR's contribution to matrix degradation during invasive growth is a late event in carcinogenesis. Using a scoring system for semiquantitative estimation of uPAR-positivity on immmunohistochemically stained specimens, a significant association was found between poor overall survival and high uPAR-score for cancer cells in the tumour core and for macrophages peripherally at the tumour invasion zone. In multivariate analysis, these two uPAR-scores were confirmed as highly significant prognostic parameters independent of Tumour, Node, Metastasis (TNM)-stage and World Health Organization (WHO) classification. The proteolytic action of these malignant and nonmalignant accessory cells thus seemed to follow two main patterns: one dominated by uPAR positive cancer cells and one by uPAR-positive macrophages. Scoring of uPAR-positivity might be a useful parameter for onset of invasion and prognosis in these adenocarcinomas.

Conformational regulation of urokinase receptor function: impact of receptor occupancy and epitope-mapped monoclonal antibodies on lamellipodia induction

The urokinase-type plasminogen activator receptor (uPAR) is a glycolipid-anchored membrane protein with an established role in focalizing uPA-mediated plasminogen activation on cell surfaces. Distinct from this function, uPAR also modulates cell adhesion and migration on vitronectin-rich matrices. Although uPA and vitronectin engage structurally distinct binding sites on uPAR, they nonetheless cooperate functionally, as uPA binding potentiates uPAR-dependent induction of lamellipodia on vitronectin matrices. We now present data advancing the possibility that it is the burial of the β-hairpin in uPA per se into the hydrophobic ligand binding cavity of uPAR that modulates the function of this receptor. Based on these data, we now propose a model in which the inherent interdomain mobility in uPAR plays a major role in modulating its function. Particularly one uPAR conformation, which is stabilized by engagement of the β-hairpin in uPA, favors the proper assembly of an active, compact receptor structure that stimulates lamellipodia induction on vitronectin. This molecular model has wide implications for drug development targeting uPAR function.

Density enhanced phosphatase-1 down-regulates urokinase receptor surface expression in confluent endothelial cells

VEGF(165), the major angiogenic growth factor, is known to activate various steps in proangiogenic endothelial cell behavior, such as endothelial cell migration and invasion, or endothelial cell survival. Thereby, the urokinase-type plasminogen activator (uPA) system has been shown to play an essential role not only by its proteolytic capacities, but also by induction of intracellular signal transduction. Therefore, expression of its cell surface receptor uPAR is thought to be an essential regulatory mechanism in angiogenesis. We found that uPAR expression on the surface of confluent endothelial cells was down-regulated compared with subconfluent proliferating endothelial cells. Regulation of uPAR expression was most probably affected by extracellular signal-regulated kinase 1/2 (ERK1/2) activation, a downstream signaling event of the VEGF/VEGF-receptor system. Consistently, the receptor-like protein tyrosine phosphatase DEP-1 (density enhanced phosphatase-1/CD148), which is abundantly expressed in confluent endothelial cells, inhibited the VEGF-dependent activation of ERK1/2, leading to down-regulation of uPAR expression. Overexpression of active ERK1 rescued the DEP-1 effect on uPAR. That DEP-1 plays a biologic role in angiogenic endothelial cell behavior was demonstrated in endothelial cell migration, proliferation, and capillary-like tube formation assays in vitro.

Prognostic relevance of tumour cell-associated uPAR expression in invasive ductal breast carcinoma

AIMS

The urokinase-type plasminogen activator receptor (uPAR) is a key molecule for pericellular proteolysis in tumour cell invasion and metastasis. The aim was to evaluate the prognostic impact of uPAR in invasive breast cancer dependent on which cell types within the tumour express uPAR.

METHODS AND RESULTS

uPAR expression was analysed by immunohistochemistry in 270 tumour tissue specimens of invasive ductal breast carcinomas using tissue microarrays. For evaluation of uPAR immunoexpression we used the epitope-mapped, uPAR domain II-specific monoclonal antibody IID7. High uPAR score values in both tumour cells (uPAR-Tc) and stromal cells were significantly related to high tumour grade (G3), and inversely correlated with oestrogen receptor status. On multivariate analysis, high uPAR-Tc values contributed independent prognostic information for disease-free survival (hazard ratio 1.93, P = 0.007) when adjusted for prognostically relevant clinicopathological parameters, whereas uPAR expression in stromal cells was not related to prognosis. In addition, elevated uPAR-Tc values were found to be prognostic indicators in clinically relevant subgroups of patients with invasive breast cancer.

CONCLUSIONS

In invasive breast cancer uPAR expression in invasive carcinoma cells, but not in stromal cells, has a significant impact on patients' prognosis, and contributes to a more aggressive tumour phenotype.

Urokinase plasminogen activator receptor is expressed in invasive cells in gastric carcinomas from high- and low-risk countries

Gastric cancer is the second cancer causing death worldwide. Both incidence and mortality rates vary according to geographical regions. The receptor for urokinase plasminogen activator (uPAR) is involved in extracellular matrix degradation by mediating cell surface associated plasminogen activation, and its presence on gastric cancer cells is linked to micro-metastasis and poor prognosis. Immunohistochemical analyses of a set of 44 gastric cancer lesions from Costa Rica showed expression of uPAR in cancer cells in both intestinal subtype (14 of 27) and diffuse subtype (10 of 17). We compared the expression pattern of uPAR in gastric cancers from a high-risk country (Costa Rica) with a low-risk country (Norway). We found uPAR on gastric cancer cells in 24 of 44 cases (54%) from Costa Rica and in 13 of 23 cases (56%) from Norway. uPAR was seen in macrophages and neutrophils in all cases. We also examined the nonneoplastic mucosa and found that uPAR was more frequently seen in epithelial cells located at the luminal edge of the crypts in cases with Helicobacter pylori infection than in similar epithelial cells in noninfected mucosa (p = 0.033; chi(2) = 4.54). In conclusion, the expression of uPAR in cancer cells in more than half of the gastric cancer cases suggests that their uPAR-positivity do not contribute to explain the different mortality rates between the 2 countries, however, the actual prevalence of uPAR-positive cancer cells in the gastric cancers may still provide prognostic information.

Two distinct expression patterns of urokinase, urokinase receptor and plasminogen activator inhibitor-1 in colon cancer liver metastases

Metastatic growth and invasion by colon cancer cells in the liver requires the ability of the cancer cells to interact with the new tissue environment. Plasmin(ogen) is activated on cell surfaces by urokinase-type PA (uPA), and is regulated by uPAR and plasminogen activator inhibitor-1 (PAI-1). To compare the expression patterns of uPA, uPAR and PAI-1 in colon cancer with that in their liver metastases, we analysed matched samples from 14 patients. In all 14 primary colon cancers, we found upregulation of uPAR, uPA mRNA and PAI-1 in primarily stromal cells at the invasive front. In 5 of the 14 liver metastases, we found intense expression of uPAR, uPA-mRNA and PAI-1 in primarily stromal cells at the metastases periphery, and in an expression pattern similar to that found in the primary tumours. In the remaining 9 liver metastases, uPAR and uPA-mRNA were only seen associated with the presence of necrosis within the liver metastases. In addition, PAI-1-immunoreactivity was in all liver metastases seen in hepatocytes at the metastases periphery. Interestingly, the former 5 liver metastases positive for uPAR, uPA mRNA and PAI-1 at the metastasis periphery all had a predominantly desmoplastic reaction, whereas 8 of the remaining 9 showed direct contact between the cancer cells and the liver parenchyma. We conclude that there are 2 distinct patterns of expression of uPAR, uPA and PAI-1 in colon cancer liver metastases and that these correlate closely with 2 morphological growth patterns. These findings may have implication for the treatment of patients with metastatic disease.

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.

Prognostic value analysis of urokinase-type plasminogen activator receptor in oral squamous cell carcinoma: an immunohistochemical study

Background

Oral squamous cell carcinoma (OSCC) represents the most common oral malignancy. Despite recent advances in therapy, up to 50% of the cases have relapse and/or metastasis. There is therefore a strong need for the identification of new biological markers able to predict the clinical behaviour of these lesions in order to improve quality of life and overall survival. Among tumour progression biomarkers, already known for their involvement in other neoplasia, a crucial role is ascribed to the urokinase-type plasminogen activator receptor (uPAR), which plays a multiple role in extracellular proteolysis, cell migration and tissue remodelling not only as a receptor for the zymogen pro-uPA but also as a component for cell adhesion and as a chemoattractant. The purpose of this study was to gain information on the expression of uPAR in OSCC and to verify whether this molecule can have a role as a prognostic/predictive marker for this neoplasia.

Methods

In a retrospective study, a cohort of 189 OSCC patients was investigated for uPAR expression and its cellular localization by immunohistochemistry. As standard controls, 8 normal oral mucosal tissues free of malignancy, obtained from patients with no evidence or history of oral cavity tumours, were similarly investigated. After grouping for uPAR expression, OSCCs were statistically analyzed for the variables age, gender, histological grading (G), tumour size, recurrence, TNM staging and overall survival rate.

Results

In our immunohistochemical study, 74 cases (39.1%) of OSCC showed a mostly cytoplasmic positivity for uPAR, whereas 115 were negative. uPAR expression correlated with tumour differentiation grade and prognosis: percentage of positive cases was the greatest in G3 (70.4%) and patients positives for uPAR expression had an expectation of life lower than those for uPAR negatives.

Conclusion

The results obtained in this study suggest a role of uPAR as a potential biomarker useful to identify higher risk subgroups of OSCC patients.

Mannose 6-phosphate receptor targeting and its applications in human diseases

The cation-independent mannose 6-phosphate receptor is a multifunctional protein which binds at the cell surface to two distinct classes of ligands, the mannose 6-phosphate (M6P) bearing proteins and IGF-II. Its major function is to bind and transport M6P-enzymes to lysosomes, but it can also modulate the activity of a variety of extracellular M6P-glycoproteins (i.e., latent TGFbeta precursor, urokinase-type plasminogen activator receptor, Granzyme B, growth factors, Herpes virus). The purpose of this review is to highlight the synthesis and potential use of high affinity M6P analogues able to target this receptor. Several M6P analogues with phosphonate, carboxylate or malonate groups display a higher affinity and a stronger stability in human serum than M6P itself. These derivatives could be used to favour the delivery of specific therapeutic compounds to lysosomes, notably in enzyme replacement therapies of lysosomal diseases or in neoplastic drug targeting. In addition, their potential applications in preventing clinical disorders, which are associated with the activities of other M6P-proteins involved in wound healing, cell growth or viral infection, will be discussed.

Plasticity in urokinase-type plasminogen activator receptor (uPAR) display in colon cancer yields metastable subpopulations oscillating in cell surface uPAR density--implications in tumor progression

It is becoming increasingly clear that tumor growth and progression is not entirely due to genetic aberrations but also reflective of tumor cell plasticity. It follows therefore that proteins contributing to tumor progression oscillate in their expression a contention yet to be shown. Because the urokinase-type plasminogen activator receptor (uPAR) promotes tumor growth and invasion, we determined whether its expression is itself plastic. In fluorescence-activated cell sorting (FACS), three independent colon cancer clonal populations revealed the expected Gaussian distribution for cell surface uPAR display. However, subcloning of cells collected from the trailing edge of the FACS yielded subpopulations, displaying low cell surface uPAR number. Importantly, these subclones spontaneously reverted to cells enriched in uPAR display, indicating a metastable phenotype. uPAR display plasticity was associated with divergent in vivo behavior with weak tumor growth and progression segregating with receptor deficiency. Mechanistically, reduced uPAR display reflected not repressed gene expression but a switch in uPAR protein trafficking from membrane insertion to shedding. To our knowledge, this is the first demonstration that uPAR cell surface density is oscillatory and we propose that such an event might well contribute to tumor progression.

Systemic administration of anti-urokinase plasminogen activator receptor monoclonal antibodies induces hepatic fibrin deposition in tissue-type plasminogen activator deficient mice

BACKGROUND

Degradation of extracellular matrix proteins, such as fibrin, is pivotal to tumor invasion. Inhibition of the interaction between urokinase plasminogen activator (u-PA) and its receptor (u-PAR), and hence pro-u-PA activation, is an attractive approach to anti-invasive cancer therapy. A number of inhibitors exist for the human system, but because of species specificity none of these are efficient in mice. We have recently generated an inhibitory monoclonal antibody (mAb) against mouse u-PAR (mR1) by immunization of u-PAR-deficient mice.

OBJECTIVES

To evaluate the effect of mR1 in vivo in a physiological setting sensitive to deregulated fibrinolysis, we have administered mR1 systemically and quantitated the effect on liver fibrin accumulation.

METHODS

Wild-type and tissue-type plasminogen activator (t-PA) deficient mice were administered with mR1, or control antibody, during 6 weeks. Thereafter, the livers were retrieved and the amount of liver fibrin measured by unbiased morphometrical analysis of immunofluorescence signal.

RESULTS

Systemic administration of mR1 caused significantly increased fibrin signal in anti-u-PAR treated t-PA-deficient mice compared to mock-treated, which mimics the phenotype of u-PAR;t-PA double-deficient mice. Fibrin and fibronectin accumulated within the sinusoidal space and was infiltrated by inflammatory cells. Analysis of small and rare hepatic fibrin plaques observed in t-PA-deficient mice showed infiltrating macrophages that, contrary to surrounding Kuppfer cells, expressed u-PAR.

CONCLUSION

We show that u-PAR-expressing macrophages are involved in cell-mediated fibrinolysis of liver fibrin deposits, and that the antimouse-u-PAR mAb is effective in vivo and thus suited for studies of the effect of targeting the u-PA/u-PAR interaction in mouse cancer models.

Mapping of the Vitronectin-binding Site on the Urokinase Receptor

The urokinase-type plasminogen activator receptor (uPAR) has been implicated as a modulator of several biochemical processes that are active during tumor invasion and metastasis, e.g. extracellular proteolysis, cell adhesion, and cell motility. The structural basis for the high affinity interaction between the urokinase-type plasminogen activator (uPA) and uPAR, which focuses cell surface-associated plasminogen activation in vivo, is now thoroughly characterized by site-directed mutagenesis studies and x-ray crystallography. In contrast, the structural basis for the interaction between uPAR and the extracellular matrix protein vitronectin, which is involved in the regulation of cell adhesion and motility, remains to be clarified. In this study, we have identified the functional epitope on uPAR that is responsible for its interaction with the full-length, extended form of vitronectin by using a comprehensive alanine-scanning library of purified single-site uPAR mutants (244 positions tested). Interestingly, the five residues identified as "hot spots" for vitronectin binding form a contiguous epitope consisting of two exposed loops connecting the central fourstranded beta-sheet in uPAR domain I (Trp(32), Arg(58), and Ile(63)) as well as a proximal region of the flexible linker peptide connecting uPAR domains I and II (Arg(91) and Tyr(92)). This binding topology provides the molecular basis for the observation that uPAR can form a ternary complex with uPA and vitronectin. Furthermore, it raises the intriguing possibility that the canonical receptor and inhibitor for uPA (uPAR and PAI-1) may have reached a convergent solution for binding to the somatomedin B domain of vitronectin.

One-step affinity purification of recombinant urokinase-type plasminogen activator receptor using a synthetic peptide developed by combinatorial chemistry

Several lines of evidence have pointed to a role of urokinase-type plasminogen activator receptor (uPAR) as a modulator of certain biochemical processes that are active during tumor invasion and metastasis. Consequently, the structure and function of this receptor have been studied extensively, using recombinantly produced uPAR that has been purified by either affinity chromatography using its cognate ligand, the urokinase-type plasminogen activator (uPA), or a monoclonal anti-uPAR antibody (R2), or by hydroxyapatite. Here, we present a new method for the efficient one-step affinity purification of recombinant uPAR exploiting a high-affinity synthetic peptide antagonist (AE152). The corresponding parent peptide was originally identified in a random phage-display library and subsequently subjected to affinity maturation by combinatorial chemistry. This study compares the affinity purification of a soluble, recombinant uPAR using the monoclonal antibody R2 or the peptide AE152 immobilized on Sepharose. The two affinity ligands perform equally well in purifying uPAR from Drosophila melanogaster Schneider 2 cell culture medium and yield products of comparable purity, activity, and stability as judged by SDS-PAGE, size exclusion chromatography and surface plasmon resonance analysis. The general availability of peptide synthesis renders the present AE152-based affinity purification of uPAR more accessible than the traditional protein-based affinity purification strategies. In this way, large amounts of recombinant uPAR can conveniently be purified for further structural and functional studies.

Intracranial therapy of glioblastoma with the fusion protein DTAT in immunodeficient mice

A gene splicing technique was used to create a hybrid fusion protein DTAT encoding the 390 amino acid portion of diphtheria toxin (DT(390)), a linker, and the downstream 135-amino terminal fragment portion of human urokinase plasminogen activator. DTAT was assembled to target human glioblastoma cell lines in a murine intracranial model. Previously published in vitro studies demonstrated that DTAT was highly selective and toxic to human glioblastoma cell lines in a flank tumor model. The purpose of this study was to determine the toxicity, specificity and possible therapeutic efficacy of DTAT in an intracranial model. Convection enhanced delivery of DTAT resulted in about a 16-fold increase in maximum tolerated dose. Intracranial administration of DTAT on an every-other-day basis in nude mice with established U87 MG brain tumors resulted in significant reductions in tumor volume and significantly prolonged survival (p < 0.0001). Magnetic resonance imaging proved to be a powerful tool in mice and rats for demonstrating tumor growth in a xenograft intracranial model, assessing the efficacy of DTAT in tumor volume reduction and detecting DTAT-associated intracranial toxicity and vascular damage. These results suggest that the DTAT recombinant fusion protein is highly effective in an intracranial model and DTAT might be an effective treatment for glioblastoma.

Stromal cells associated with early invasive foci in human mammary ductal carcinoma in situ coexpress urokinase and urokinase receptor

The transition from ductal carcinoma in situ (DCIS) of the breast to invasive ductal carcinoma is facilitated by proteolytic degradation of basement membrane. The transition can be identified as microinvasive foci in a small proportion of DCIS lesions. We have previously found that MMP-13 is frequently expressed in such foci. To establish whether plasmin-directed proteolysis is likely to be involved in early invasion, we have here studied the expression of urokinase plasminogen activator (uPA) and its receptor (uPAR) in human DCIS lesions with and without microinvasion. uPA mRNA was detected in periductal stromal cells in all of 9 DCIS lesions with microinvasion and in 2 of 9 DCIS lesions without microinvasion by in situ hybridization. The uPA mRNA signal was seen in numerous stromal cells in microinvasive areas together with MMP-13 mRNA expressing cells. Double immunofluorescence analyses, using emission fingerprinting, showed that the uPA expressing stromal cells included both myofibroblasts and macrophages. The early invasive carcinoma cells were negative for uPA. uPAR immunoreactivity was focally upregulated in periductal stromal cells in all of the 9 DCIS lesions with microinvasion and in only 2 of the 9 DCIS lesions without microinvasion. uPAR was seen in both macrophages and myofibroblasts in microinvasive areas, and it was evident that uPA and uPAR colocalized in both fibroblast-like cells and macrophage-like cells. We conclude that periductal macrophages and myofibroblasts are strongly involved in the initial steps of breast cancer invasion by focally upregulating the expression of the plasminogen activation system and MMP-13.

Increased expression of the collagen internalization receptor uPARAP/Endo180 in the stroma of head and neck cancer

Local growth, invasion, and metastasis of malignancies of the head and neck involve extensive degradation and remodeling of the underlying, collagen-rich connective tissue. Urokinase plasminogen activator receptor-associated protein (uPARAP)/Endo180 is an endocytic receptor recently shown to play a critical role in the uptake and intracellular degradation of collagen by mesenchymal cells. As a step toward determining the putative function of uPARAP/Endo180 in head and neck cancer progression, we used immunohistochemistry to determine the expression of this collagen internalization receptor in 112 human squamous cell carcinomas and 19 normal or tumor-adjacent head and neck tissue samples from the tongue, gingiva, cheek, tonsils, palate, floor of mouth, larynx, maxillary sinus, upper jaw, nasopharynx/nasal cavity, and lymph nodes. Specificity of detection was verified by staining of serial sections with two different monoclonal antibodies against two non-overlapping epitopes on uPARAP/Endo180 and by the use of isotype-matched non-immune antibodies. uPARAP/Endo180 expression was observed in stromal fibroblast-like, vimentin-positive cells. Furthermore, expression of the collagen internalization receptor was increased in tumor stroma compared with tumor-adjacent connective tissue or normal submucosal connective tissue and was most prominent in poorly differentiated tumors. These data suggest that uPARAP/Endo180 participates in the connective tissue destruction during head and neck squamous cell carcinoma progression by mediating cellular uptake and lysosomal degradation of collagen.

EGF-stimulated migration in ovarian cancer cells is associated with decreased internalization, increased surface expression, and increased shedding of the urokinase plasminogen activator receptor

OBJECTIVES

The EGFR is expressed in malignant ovarian tumor tissue, and tissue content of EGFR has been directly associated with poor prognosis in patients with ovarian cancer. The uPA system plays a role in pericellular proteolysis, cell migration, invasion, and is over-expressed in ovarian cancer. This study explored the effects of EGF on uPAR expression in the ovarian cancer cell line OVCAR-3.

METHODS

We used OVCAR-3 cells and the following methods: cell migration assay, time-lapse video microscopy, real-time PCR, assays for cellular binding of 125I-uPA and cellular degradation of 125I-uPA:PAI-1 complex, biosynthetic labeling using 35S-methionin, Western blot, Northern blot, and ELISAs for uPA, PAI-1, and uPAR.

RESULTS

EGF up-regulates both protein and mRNA not only for uPAR, but also for the ligand uPA and its inhibitor PAI-1. Cell surface uPAR, in control as well as EGF-stimulated cells, is present only in the intact, not the cleaved, form. Ligand binding experiments showed an increase of endogenously occupied uPAR, whereas non-occupied receptor sites were not increased. In addition, EGF treatment resulted in decreased degradation of radiolabeled uPA:PAI-1 complex. This suggests decreased internalization of uPAR, since the complex is internalized together with uPAR. Like EGF, colchicine, which inhibits endocytosis, increased cell surface expression of uPAR. In addition, we found an immediate increase of uPAR after exposing the cells to EGF and this was accompanied by a transient increase of cell migration. The increase of cell surface uPAR in response to EGF is accompanied by increased release of the soluble form of uPAR (suPAR) to the medium as well as by increased cell migration. Both uPAR and suPAR increased in cells treated with the endocytosis inhibitor colchicine even though cell migration was inhibited, suggesting that the mechanism of uPAR shedding is not related to cell migration.

CONCLUSION

Increased cell surface uPAR in response to EGF stimulation results from mobilization of uPAR from detergent-resistant domains, increased expression of uPAR mRNA, and decreased internalization and degradation of uPAR. Both the anti-uPAR antibody R3, which inhibits binding of uPA, and the EGFR phosphorylation inhibitor Iressa inhibited cell migration in response to uPA as well as to EGF, suggesting that EGFR and uPAR are engaged in the same multiprotein assembly on the cell surface.

A region in urokinase plasminogen receptor domain III controlling a functional association with alpha5beta1 integrin and tumor growth

Highly expressed urokinase plasminogen activator receptor (uPAR) can interact with alpha5beta1 integrin leading to persistent ERK activation and tumorigenicity. Disrupting this interaction reduces ERK activity, forcing cancer cells into dormancy. We identified a site in uPAR domain III that is indispensable for these effects. A 9-mer peptide derived from a sequence in domain III (residues 240-248) binds purified alpha5beta1 integrin. Substituting a single amino acid (S245A) in this peptide, or in full-length soluble uPAR, impairs binding of the purified integrin. In the recently solved crystal structure of uPAR the Ser-245 is confined to the large external surface of the receptor, a location that is well separated from the central urokinase plasminogen binding cavity. The impact of this site on alpha5beta1 integrin-dependent cell functions was examined by comparing cells induced to express uPAR(wt) or the uPAR(S245A) mutant. Transfecting uPAR(wt) into cells with low endogenous levels of uPAR, inactive integrin, low ERK activity, and a dormant phenotype in vivo restores these functions and reinstates growth in vivo. In contrast, transfection of the same cells with uPAR(S245A) elicits only very small changes. Incubation of highly malignant cells with the wild-type, but not the S245A mutant peptide, disrupts the uPAR integrin interaction leading to down-regulation of ERK activity. The relevance of this binding site, and of the lateral uPAR-alpha5beta1 integrin interaction, to ERK pathway activation and tumor growth implicates it as a possible specific target for cancer therapy.

Expression of plasminogen activator inhibitor-1, urokinase receptor and laminin γ-2 chain is an early coordinated event in incipient oral squamous cell carcinoma

Cancer cell invasion is facilitated by extracellular matrix degrading proteases such as plasmin. We have studied the expression of plasminogen activator inhibitor-1 (PAI-1) and urokinase receptor (uPAR) together with the gamma2-chain of laminin-5 (lam-gamma2) by immunohistochemistry in 20 cases with incipient oral squamous cell carcinoma (SCC). PAI-1-positive neoplastic cells located at the tip of the putative invasive front of grade 1 (incipient) carcinoma were seen in 16 of the 20 cases (75%), whereas adjacent normal and dysplastic epithelium was PAI-1-negative. Clusters of putative invasive neoplastic cells located in the lamina propria were PAI-1-positive in areas with grade 2 incipient carcinoma as were invasive cancer cells in areas of grade 3-4 invasive carcinoma. uPAR immunoreactivity was strongly expressed in numerous stromal cells in the carcinoma area in all 20 lesions, while a few uPAR-positive stromal cells were found in areas with normal and dysplastic epithelium. uPAR-positive neoplastic cell islands located at the front of the lesions were seen in 15 of the 20 cases. The expression pattern of lam-gamma2 was very similar to that of PAI-1; however, lam-gamma2-positive neoplastic cells were only detected in 11 of the 20 cases (55%) in areas of grade 1 incipient carcinoma. Direct comparison of the 3 components revealed colocalization in neoplastic cell islands in both incipient and invasive SCC. Our results suggest that PAI-1 is a novel potential marker of initial invasion in oral SCC, and that the coordinated expression of PAI-1 with uPAR and lam-gamma2 sustain the features of the early invasive cancer cells.

An uncleavable uPAR mutant allows dissection of signaling pathways in uPA-dependent cell migration

Urokinase-type plasminogen activator (uPA) binding to uPAR induces migration, adhesion, and proliferation through multiple interactions with G proteins-coupled receptor FPRL1, integrins, or the epidermal growth factor (EGF) receptor (EGFR). At least two forms of uPAR are present on the cell surface: full-length and cleaved uPAR, each specifically interacting with one or more transmembrane proteins. The connection between these interactions and the effects on the signaling pathways activation is not clear. We have exploited an uPAR mutant (hcr, human cleavage resistant) to dissect the pathways involved in uPA-induced cell migration. This mutant is not cleaved by proteases, is glycosylphosphatidylinositol anchored, and binds uPA with a normal K(d). Both wild-type (wt) and hcr-uPAR are able to mediate uPA-induced migration, are constitutively associated with the EGFR, and associate with alpha3beta1 integrin upon uPA binding. However, they engage different pathways in response to uPA. wt-uPAR requires both integrins and FPRL1 to mediate uPA-induced migration, and association of wt-uPAR to alpha3beta1 results in uPAR cleavage and extracellular signal-regulated kinase (ERK) activation. On the contrary, hcr-uPAR does not activate ERK and does not engage FPRL1 or any other G protein-coupled receptor, but it activates an alternative pathway initiated by the formation of a triple complex (uPAR-alpha3beta1-EGFR) and resulting in the autotyrosine phosphorylation of EGFR.

Plasminogen activator inhibitor-1 deficiency retards diabetic nephropathy

BACKGROUND

Plasminogen activator inhibitor-1 (PAI-1) is increased in kidneys of humans and animals with diabetic nephropathy and is associated with extracellular matrix (ECM) accumulation. PAI-1 may promote ECM buildup by preventing plasmin and matrix metalloproteinase (MMP) activation. However, the importance and mechanism of PAI-1 action in the pathogenesis of diabetic nephropathy is unknown.

METHODS

We investigated the effect of streptozotocin (STZ)-induced diabetes in wild-type (PAI-1(+/+)) mice and mice null for PAI-1 (PAI-1(-/-)). After 1 month of diabetes, animals were placed in metabolic cages for 24-hour urine collection. Total RNA was isolated from kidney cortex for reverse transcription-polymerase chain reaction (RT-PCR) and Northern blot analysis, and Western blots were quantitated from cortical protein. Primary mesangial cells were grown from Sprague-Dawley rats and used in signal transduction studies.

RESULTS

Urinary albumin excretion (UAE) in diabetic PAI-1(+/+) mice increased >threefold, but remained unchanged in PAI-1(-/-) mice. Transforming growth factor-beta (TGF-beta) and fibronectin message and protein levels were lower in diabetic PAI-1(-/-) vs. PAI-1(+/+) mice, suggesting that PAI-1 deficiency impaired TGF-beta expression despite diabetes. Indeed, recombinant PAI-1 directly stimulated TGF-beta message and protein via mitogen-activated protein kinase (MAPK) signal transduction in cultured mesangial cells. Urokinase plasminogen activator (uPA) inhibited this PAI-1 action in a dose-dependent manner. The inhibitory effect of antibody to uPA receptor (uPAR) on PAI-1-induced TGF-beta function suggested that uPAR mediated the cellular effect of PAI-1.

CONCLUSION

PAI-1 can regulate TGF-beta expression by binding to uPAR and activating the extracellular-regulated signal kinase (ERK)/MAPK pathway. Therefore, PAI-1 contributes to diabetic nephropathy by regulating TGF-beta and renal ECM production and may be a therapeutic target in diabetic nephropathy.

Urokinase-induced signaling in human vascular smooth muscle cells is mediated by PDGFR-beta

Urokinase (uPA)-induced signaling in human vascular smooth muscle cells (VSMC) elicits important cellular functional responses, such as cell migration and proliferation. However, how intracellular signaling is linked to glycolipid-anchored uPA receptor (uPAR) is unknown. We provide evidence that uPAR activation by uPA induces its association with platelet-derived growth factor receptor (PDGFR)-beta. The interaction results in PDGF-independent PDGFR-beta activation by phosphorylation of cytoplasmic tyrosine kinase domains and receptor dimerization. Association of the receptors as well as the tyrosine kinase activity of PDGFR-beta are decisive in mediating uPA-induced downstream signaling that regulates VSMC migration and proliferation. These findings provide a molecular basis for mechanisms VSMC use to induce uPAR- and PDGFR-directed signaling. The processes may be relevant to VSMC function and vascular remodeling.

Soluble urokinase plasminogen activator receptor in blood transfusion components

Post-transfusion infectious complications associated with allogeneic blood components may depend on storage time and may be related to extracellular accumulation of bioactive substances during storage. The glycoprotein, soluble urokinase plasminogen activator receptor (suPAR), which is located in specific granules of neutrophils, plays a role in inflammation and remodelling of the extracellular matrix. Using enzyme-linked immunosorbent assay, suPAR was determined in serum, plasma and blood cell lysates. In addition, suPAR was measured in whole blood (WB), buffy-coat-depleted saline-adenine-glucose-mannitol (SAGM) blood, platelet-rich plasma (PRP) and buffy-coat-derived platelet (BCP) pools with and without pre-storage leucofiltration, and in non-filtered WB, SAGM blood and platelet concentrates prepared using apheresis (APC) at different time points during storage. Mean suPAR concentration was significantly higher in cell lysates, compared to that in a corresponding serum (P = 0.007) and in plasma samples (P = 0.004). Mean suPAR levels in WB, BCP and SAGM were significantly reduced using leucofiltration (WB: 3.4 versus 2.0 ng mL(-1); BCP: 1.6 versus 1.1 ng mL(-1); SAGM: 2.8 versus 0.19 ng mL(-1)), whereas no difference was observed in PRP. In non-filtered WB, SAGM and APC, extracellular suPAR accumulated significantly in a storage-time-dependent manner (WB: P < 0.01; SAGM: P < 0.001; APC: P < 0.001). The present study demonstrates that cell lysates contain significantly more suPAR, compared to both serum and plasma. This can be explained by the release of suPAR from intracellular granules during cell lysis. The amount of suPAR is significantly increased during storage of blood transfusion components, an accumulation that is reduced using pre-storage leucofiltration.

Expression of urokinase plasminogen activator, its receptor and type-1 inhibitor in malignant and benign prostate tissue

The plasminogen activation (PA) cascade participates in degradation of extracellular matrix during cancer invasion. We have studied the expression of urokinase-type plasminogen activator (uPA) mRNA, uPA receptor (uPAR) mRNA and immunoreactivity, and type-1 plasminogen activator inhibitor (PAI-1) mRNA and immunoreactivity in 16 prostate adenocarcinomas and 9 benign prostate hyperplasias. uPA mRNA and uPAR mRNA expression were found in 9 and 8 of the adenocarcinomas, respectively, and in 7 and 6 of the benign hyperplasias, respectively. In both malignant and benign lesions, expression of these 2 mRNAs was predominantly seen in cells identified as macrophages, which in most of the carcinomas (approximately 90%) were located in the interstitial tissue between the tumor cell islands, while in most of the benign hyperplasias they were located in the lumen of the glands and were in only a few cases (approximately 30%) found in the interstitial tissue. uPAR immunoreactivity correlated with the mRNA expression and was, in addition, found in neutrophils. PAI-1 mRNA was detected in 13 of the 16 carcinomas and in 8 of the 9 benign hyperplasias, located in scattered fibroblast-like cells in both groups, in some vascular structures and in a few macrophages located in the interstitial tissue of both malignant and benign lesions. A similar expression pattern was found for PAI-1 immunoreactivity. In 8 of the 16 carcinomas, all 3 components were present, and in several areas colocalization was observed in stromal cells in close proximity to cancer cell islands. No immunoreactivity and/or mRNA expression of uPA, uPAR or PAI-1 was observed in cancer cells or in other epithelial cells in any of the cases.

Regulation of urokinase receptor proteolytic function by the tetraspanin CD82

The high affinity interaction between the urokinase-type plasminogen activator (uPA) and its glycolipid-anchored cellular receptor (uPAR) promotes plasminogen activation and the efficient generation of pericellular proteolytic activity. We demonstrate here that expression of the tetraspanin CD82/KAI1 (a tumor metastasis suppressor) leads to a profound effect on uPAR function. Pericellular plasminogen activation was reduced by approximately 50-fold in the presence of CD82, although levels of components of the plasminogen activation system were unchanged. uPAR was present on the cell surface and molecularly intact, but radioligand binding analysis with uPA and anti-uPAR antibodies revealed that it was in a previously undetected cryptic form unable to bind uPA. This was not due to direct interactions between uPAR and CD82, as they neither co-localized on the cell surface nor could be co-immunoprecipitated. However, expression of CD82 led to a redistribution of uPAR to focal adhesions, where it was shown by double immunofluorescence labeling to co-localize with the integrin alpha(5)beta(1), which was also redistributed in the presence of CD82. Co-immunoprecipitation experiments showed that, in the presence of CD82, uPAR preferentially formed stable associations with alpha(5)beta(1), but not with a variety of other integrins, including alpha(3)beta(1). These data suggest that CD82 inhibits the proteolytic function of uPAR indirectly, directing uPAR and alpha(5)beta(1) to focal adhesions and promoting their association with a resultant loss of uPA binding. This represents a novel mechanism whereby tetraspanins, integrins, and uPAR, systems involved in cell adhesion and migration, cooperate to regulate pericellular proteolytic activity and may suggest a mechanism for the tumor-suppressive effects of CD82/KAI1.

Structural analysis and tissue localization of human C4.4A: a protein homologue of the urokinase receptor

C4.4A, a structural homologue of the urokinase-type plasminogen activator receptor (uPAR), was originally identified as a metastasis-associated membrane protein, but little is known about its structural and functional properties. Therefore, we expressed, purified and characterized a soluble truncated form of human C4.4A, and used this protein to produce specific polyclonal anti-C4.4A antibodies. By immunohistochemistry we observed a pronounced surface staining for C4.4A in suprabasal keratinocytes of chronic human wounds and found C4.4A expression markedly upregulated in migrating keratinocytes during re-epithelisation of incisional skin wounds. Phorbol-ester-induced hyperplasia of mouse skin is also accompanied by a significant induction of C4.4A expression in the multilayered, suprabasal keratinocytes. C4.4A contains two Ly-6 (leucocyte antigen 6)/uPAR/alpha-neurotoxin modules. Our recombinant human C4.4A is extensively modified by post-translational glycosylation, which include 5-6 N-linked carbohydrates primarily located in or close to its second Ly-6/uPAR/alpha-neurotoxin module and approximately 15 O-linked carbohydrates clustered in a Ser/Thr/Pro-rich region at the C-terminus. A highly protease-sensitive region (Tyr200-Arg204) is located between these two clusters of N- and O-linked carbohydrates. The natural, glycolipid-anchored C4.4A from amnion membranes of human term placenta exhibits similar properties. Using recombinant, soluble C4.4A or MCF 7 cells, which express significant amounts of GPI-anchored C4.4A, we find no evidence for an interaction between C4.4A and uPA, a property suggested previously for rat C4.4A. Collectively these data indicate that C4.4A, although being a structural homologue of uPAR, is unlikely to have a functional overlap with uPAR.

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.

Involvement of the urokinase-type plasminogen activator receptor in hematopoietic stem cell mobilization

We investigated the involvement of the urokinase-type plasminogen-activator receptor (uPAR) in granulocyte-colony-stimulating factor (G-CSF)-induced mobilization of CD34+ hematopoietic stem cells (HSCs) from 16 healthy donors. Analysis of peripheral blood mononuclear cells (PBMNCs) showed an increased uPAR expression after G-CSF treatment in CD33+ myeloid and CD14+ monocytic cells, whereas mobilized CD34+ HSCs remained uPAR negative. G-CSF treatment also induced an increase in serum levels of soluble uPAR (suPAR). Cleaved forms of suPAR (c-suPAR) were released in vitro by PBMNCs and were also detected in the serum of G-CSF-treated donors. c-suPAR was able to chemoattract CD34+ KG1 leukemia cells and CD34+ HSCs, as documented by their in vitro migratory response to a chemotactic suPAR-derived peptide (uPAR84-95). uPAR84-95 induced CD34+ KG1 and CD34+ HSC migration by activating the high-affinity fMet-Leu-Phe (fMLP) receptor (FPR). In addition, uPAR84-95 inhibited CD34+ KG1 and CD34+ HSC in vitro migration toward the stromal-derived factor 1 (SDF1), thus suggesting the heterologous desensitization of its receptor, CXCR4. Finally, uPAR84-95 treatment significantly increased the output of clonogenic progenitors from long-term cultures of CD34+ HSCs. Our findings demonstrate that G-CSF-induced upregulation of uPAR on circulating CD33+ and CD14+ cells is associated with increased uPAR shedding, which leads to the appearance of serum c-suPAR. c-suPAR could contribute to the mobilization of HSCs by promoting their FPR-mediated migration and by inducing CXCR4 desensitization.

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.

Comparative evaluation of urokinase-type plasminogen activator receptor expression in primary breast carcinomas and on metastatic tumor cells

The urokinase-type plasminogen activator receptor (uPAR, CD87) plays a central role in the plasminogen activation cascade, which participates in extracellular matrix degradation, cell migration and invasion. Here we performed a comprehensive immmunohistochemical evaluation of uPAR expression in primary tumor cells, tumor-surrounding fibroblasts, lymph node metastases and micrometastatic cells in bone marrow of patients with breast carcinomas at the time of primary diagnosis. Variable degrees of uPAR staining of tumor cells were observed in 84 of 93 (90%) carcinomas, whereas intratumoral fibroblasts were uPAR-positive in 70 (75%) carcinomas. The fraction of uPAR-positive primary tumor cells but not fibroblasts was positively correlated with the presence of tumor cells in bone marrow (p = 0.037), whereas no correlation with lymph node metastasis was found. Immunophenotyping of bone marrow and lymph node specimens revealed expression of uPAR on metastatic tumor cells in 10 of 13 and 22 of 23 cases, respectively. Direct comparison to the autologous primary tumor cells showed different uPAR staining scores in most patients with evidence for both up- and downregulation of uPAR on metastatic cells. Our results indicate that uPAR plays an active role in breast cancer metastasis and may therefore be a promising target for new biologic therapies.