Glycosylation profile of a recombinant urokinase-type plasminogen activator receptor expressed in Chinese hamster ovary cells

suPAR in cardiovascular disease

Soluble urokinase plasminogen activator receptor (suPAR), the soluble counterpart of urokinase plasminogen activator receptor, is found in the circulation at various levels. suPAR and its parent molecule, cell surface uPAR, exhibit similar structure and extracellular functional roles facilitating fibrinolysis, cellular adhesion, and migration. Studies have assessed the correlation between suPAR in cardiovascular disease (CVD). It is postulated that suPAR may serve as an indicator of inflammatory activation and burden during CVD progression. Increased suPAR independently predicts poorer outcomes in acute coronary syndromes, in heart failure, as well as in coronary artery disease and atherosclerosis. To guide translation into clinical utization, suPAR has been assessed in numerous CVD settings for improved risk discrimination independently or in association with established traditional risk factors. Whilst the involvement of suPAR has been explored in other diseases such as kidney diseases and cancer, there is only emerging evidence of suPAR's mechanistic involvement in cardiovascular disease. In this review, we provide a background into suPAR and its potential role as a biomarker in CVD.

Identification, characterization, and engineering of glycosylation in thrombolyticsa

Cardiovascular diseases, such as myocardial infarction, ischemic stroke, and pulmonary embolism, are the most common causes of disability and death worldwide. Blood clot hydrolysis by thrombolytic enzymes and thrombectomy are key clinical interventions. The most widely used thrombolytic enzyme is alteplase, which has been used in clinical practice since 1986. Another clinically used thrombolytic protein is tenecteplase, which has modified epitopes and engineered glycosylation sites, suggesting that carbohydrate modification in thrombolytic enzymes is a viable strategy for their improvement. This comprehensive review summarizes current knowledge on computational and experimental identification of glycosylation sites and glycan identity, together with methods used for their reengineering. Practical examples from previous studies focus on modification of glycosylations in thrombolytics, e.g., alteplase, tenecteplase, reteplase, urokinase, saruplase, and desmoteplase. Collected clinical data on these glycoproteins demonstrate the great potential of this engineering strategy. Outstanding combinatorics originating from multiple glycosylation sites and the vast variety of covalently attached glycan species can be addressed by directed evolution or rational design. Directed evolution pipelines would benefit from more efficient cell-free expression and high-throughput screening assays, while rational design must employ structure prediction by machine learning and in silico characterization by supercomputing. Perspectives on challenges and opportunities for improvement of thrombolytic enzymes by engineering and evolution of protein glycosylation are provided.

Crystal structure and cellular functions of uPAR dimer

Receptor dimerization of urokinase-type plasminogen activator receptor (uPAR) was previously identified at protein level and on cell surface. Recently, a dimeric form of mouse uPAR isoform 2 was proposed to induce kidney disease. Here, we report the crystal structure of human uPAR dimer at 2.96 Å. The structure reveals enormous conformational changes of the dimer compared to the monomeric structure: D1 of uPAR opens up into a large expanded ring that captures a β-hairpin loop of a neighboring uPAR to form an expanded β-sheet, leading to an elongated, highly intertwined dimeric uPAR. Based on the structure, we identify E49P as a mutation promoting dimer formation. The mutation increases receptor binding to the amino terminal fragment of its primary ligand uPA, induces the receptor to distribute to the basal membrane, promotes cell proliferation, and alters cell morphology via β1 integrin signaling. These results reveal the structural basis for uPAR dimerization, its effect on cellular functions, and provide a basis to further study this multifunctional receptor.

The Urokinase Receptor (uPAR) as a "Trojan Horse" in Targeted Cancer Therapy: Challenges and Opportunities

Simple Summary

Discovered more than three decades ago, the urokinase-type plasminogen activator receptor (uPAR) has now firmly established itself as a versatile molecular target holding promise for the treatment of aggressive malignancies. The copious abundance of uPAR in virtually all human cancerous tissues versus their healthy counterparts has fostered a gradual shift in the therapeutic landscape targeting this receptor from function inhibition to cytotoxic approaches to selectively eradicate the uPAR-expressing cells by delivering a targeted cytotoxic insult. Multiple avenues are being explored in a preclinical setting, including the more innovative immune- or stroma targeting therapies. This review discusses the current state of these strategies, their potentialities, and challenges, along with future directions in the field of uPAR targeting.

Abstract

One of the largest challenges to the implementation of precision oncology is identifying and validating selective tumor-driving targets to enhance the therapeutic efficacy while limiting off-target toxicity. In this context, the urokinase-type plasminogen activator receptor (uPAR) has progressively emerged as a promising therapeutic target in the management of aggressive malignancies. By focalizing the plasminogen activation cascade and subsequent extracellular proteolysis on the cell surface of migrating cells, uPAR endows malignant cells with a high proteolytic and migratory potential to dissolve the restraining extracellular matrix (ECM) barriers and metastasize to distant sites. uPAR is also assumed to choreograph multiple other neoplastic stages via a complex molecular interplay with distinct cancer-associated signaling pathways. Accordingly, high uPAR expression is observed in virtually all human cancers and is frequently associated with poor patient prognosis and survival. The promising therapeutic potential unveiled by the pleiotropic nature of this receptor has prompted the development of distinct targeted intervention strategies. The present review will focus on recently emerged cytotoxic approaches emphasizing the novel technologies and related limits hindering their application in the clinical setting. Finally, future research directions and emerging opportunities in the field of uPAR targeting are also discussed.

suPAR, a Circulating Kidney Disease Factor

Urokinase plasminogen activator receptor (uPAR) is a multifaceted, GPI-anchored three-domain protein. Release of the receptor results in variable levels of soluble uPAR (suPAR) in the blood circulation. suPAR levels have been linked to many disease states. In this mini-review, we discuss suPAR as a key circulating molecule mediating kidney disease with a particular focus on differently spliced isoforms.

Targeting the Urokinase-Type Plasminogen Activator Receptor (uPAR) in Human Diseases With a View to Non-invasive Imaging and Therapeutic Intervention

The interaction between the serine protease urokinase-type plasminogen activator (uPA) and its glycolipid-anchored receptor (uPAR) focalizes plasminogen activation to cell surfaces, thereby regulating extravascular fibrinolysis, cell adhesion, and migration. uPAR belongs to the Ly6/uPAR (LU) gene superfamily and the high-affinity binding site for uPA is assembled by a dynamic association of its three consecutive LU domains. In most human solid cancers, uPAR is expressed at the invasive areas of the tumor-stromal microenvironment. High levels of uPAR in resected tumors or shed to the plasma of cancer patients are robustly associated with poor prognosis and increased risk of relapse and metastasis. Over the years, a plethora of different strategies to inhibit uPA and uPAR function have been designed and investigated in vitro and in vivo in mouse models, but so far none have been implemented in the clinics. In recent years, uPAR-targeting with the intent of cytotoxic eradication of uPAR-expressing cells have nonetheless gained increasing momentum. Another avenue that is currently being explored is non-invasive imaging with specific uPAR-targeted reporter-molecules containing positron emitting radionuclides or near-infrared (NIR) florescence probes with the overarching aim of being able to: (i) localize disease dissemination using positron emission tomography (PET) and (ii) assist fluorescence guided surgery using optical imaging. In this review, we will discuss these advancements with special emphasis on applications using a small 9-mer peptide antagonist that targets uPAR with high affinity.

Eleven genomic loci affect plasma levels of chronic inflammation marker soluble urokinase-type plasminogen activator receptor

Soluble urokinase-type plasminogen activator receptor (suPAR) is a chronic inflammation marker associated with the development of a range of diseases, including cancer and cardiovascular disease. The genetics of suPAR remain unexplored but may shed light on the biology of the marker and its connection to outcomes. We report a heritability estimate of 60% for the variation in suPAR and performed a genome-wide association meta-analysis on suPAR levels measured in Iceland (N = 35,559) and in Denmark (N = 12,177). We identified 13 independently genome-wide significant sequence variants associated with suPAR across 11 distinct loci. Associated variants were found in and around genes encoding uPAR (PLAUR), its ligand uPA (PLAU), the kidney-disease-associated gene PLA2R1 as well as genes with relations to glycosylation, glycoprotein biosynthesis, and the immune response. These findings provide new insight into the causes of variation in suPAR plasma levels, which may clarify suPAR's potential role in associated diseases, as well as the underlying mechanisms that give suPAR its prognostic value as a unique marker of chronic inflammation.

Relating GPI-Anchored Ly6 Proteins uPAR and CD59 to Viral Infection

The Ly6 (lymphocyte antigen-6)/uPAR (urokinase-type plasminogen activator receptor) superfamily protein is a group of molecules that share limited sequence homology but conserved three-fingered structures. Despite diverse cellular functions, such as in regulating host immunity, cell adhesion, and migration, the physiological roles of these factors in vivo remain poorly characterized. Notably, increasing research has focused on the interplays between Ly6/uPAR proteins and viral pathogens, the results of which have provided new insight into viral entry and virus-host interactions. While LY6E (lymphocyte antigen 6 family member E), one key member of the Ly6E/uPAR-family proteins, has been extensively studied, other members have not been well characterized. Here, we summarize current knowledge of Ly6/uPAR proteins related to viral infection, with a focus on uPAR and CD59. Our goal is to provide an up-to-date view of the Ly6/uPAR-family proteins and associated virus-host interaction and viral pathogenesis.

Crystal structure of the unoccupied murine urokinase-type plasminogen activator receptor (uPAR) reveals a tightly packed DII-DIII unit

The urokinase-type plasminogen activator receptor (uPAR) is a cell surface receptor that is capable of binding to a range of extracellular proteins and triggering a series of proteolytic and signaling events. Previous structural studies of uPAR with its ligands uPA and vitronectin revealed that its three domains (DI, DII, and DIII) form a large hydrophobic cavity to accommodate uPA. In the present study, the structure of unoccupied murine uPAR (muPAR) is determined. The structure of DII and DIII of muPAR is well defined and forms a compact globular unit, while DI could not be traced. Molecular dynamic simulations further confirm the rigid binding interface between DII and DIII. This study shows overall structural flexibility of uPAR in the absence of uPA.

Did evolution create a flexible ligand-binding cavity in the urokinase receptor through deletion of a plesiotypic disulfide bond?

The urokinase receptor (uPAR) is a founding member of a small protein family with multiple Ly6/uPAR (LU) domains. The motif defining these LU domains contains five plesiotypic disulfide bonds stabilizing its prototypical three-fingered fold having three protruding loops. Notwithstanding the detailed knowledge on structure-function relationships in uPAR, one puzzling enigma remains unexplored. Why does the first LU domain in uPAR (DI) lack one of its consensus disulfide bonds, when the absence of this particular disulfide bond impairs the correct folding of other single LU domain-containing proteins? Here, using a variety of contemporary biophysical methods, we found that reintroducing the two missing half-cystines in uPAR DI caused the spontaneous formation of the corresponding consensus 7-8 LU domain disulfide bond. Importantly, constraints due to this cross-link impaired (i) the binding of uPAR to its primary ligand urokinase and (ii) the flexible interdomain assembly of the three LU domains in uPAR. We conclude that the evolutionary deletion of this particular disulfide bond in uPAR DI may have enabled the assembly of a high-affinity urokinase-binding cavity involving all three LU domains in uPAR. Of note, an analogous neofunctionalization occurred in snake venom α-neurotoxins upon loss of another pair of the plesiotypic LU domain half-cystines. In summary, elimination of the 7-8 consensus disulfide bond in the first LU domain of uPAR did have significant functional and structural consequences.

Characterization of Site-Specific N-Glycosylation

Even if a consensus sequence has been identified for a posttranslational modification, the presence of such a sequence motif only indicates the possibility, not the certainty that the modification actually occurs. Proteins can be glycosylated on certain amino acid side chains, and these modifications are designated as C-, N-, and O-glycosylation. C-mannosylation occurs on Trp residues within a relatively loosely defined consensus motif. N-glycosylated species are modified at Asn residues of Asn-Xxx-Ser/Thr/Cys sequons (where Xxx can be any amino acid except proline). N-linked oligosaccharides share a common core structure of GlcNAc₂Man₃. In addition, an enzyme, peptide N-glycosidase F (PNGase F), removes most of the common N-linked carbohydrates unaltered from proteins while hydrolyzing the originally glycosylated Asn residue to Asp. O-glycosylation occurs at Ser, Thr, and Tyr residues, usually in sequence stretches rich in hydroxy-amino acids. O-glycosylation lacks a common core structure. Mammalian proteins have been reported bearing O-linked N-acetylgalactosamine, fucose, glucose, xylose, mannose, and corresponding elongated structures, as well as N-acetylglucosamine. Chemical methods are used to liberate these oligosaccharides because no enzyme would remove all the different O-linked carbohydrates. Characterization of both N- and O-glycosylation is complicated by the fact that the same positions within a population of protein molecules may feature an array of different carbohydrate structures, or remain unmodified. This site-specific heterogeneity may vary by species and tissue, and may also be affected by physiological changes. For addressing site-specific carbohydrate heterogeneity mass spectrometry has become the method of choice. Reversed-phase HPLC directly coupled with electrospray ionization mass spectrometry (LC/ESI-MS/MS) offers the best solution. Using a mass spectrometer as online detector not only assures the analysis of every component eluting (mass mapping), but also at the same time diagnostic carbohydrate ions can be generated by collisional activation that permits the selective and specific detection of glycopeptides. In addition, ESI-compatible alternative MS/MS techniques, electron-capture and electron-transfer dissociation, aid glycopeptide identification as well as modification site assignments.

Heparan sulfate proteoglycans mediate factor XIIa binding to the cell surface

Hageman factor (FXIIa) initiates the intrinsic coagulation pathway and triggers the kallikrein-kinin and the complement systems. In addition, it functions as a growth factor by expressing promitogenic activities toward several cell types. FXIIa binds to the cell surface via a number of structurally unrelated surface receptors; however, the underlying mechanisms are not yet fully understood. Here, we demonstrate that FXIIa utilizes cell membrane-bound glycosaminoglycans to interact with the cell surface of human lung fibroblasts (HLF). The combination of enzymatic, inhibitory, and overexpression approaches identified a heparan sulfate (HS) component of proteoglycans as an important determinant of the FXIIa binding capacity of HLF. Moreover, cell-free assays and competition experiments revealed preferential binding of FXIIa to HS and heparin over dextran sulfate, dermatan sulfate, and chondroitin sulfate A and C. Finally, we demonstrate that fibroblasts isolated from the lungs of the patients suffering from idiopathic pulmonary fibrosis (IPF) exhibit enhanced FXIIa binding capacity. Increased sulfation of HS resulting from elevated HS 6-O-sulfotransferase-1 expression in IPF HLF accounted, in part, for this phenomenon. Application of RNA interference technology and inhibitors of intracellular sulfation revealed the cooperative action of cell surface-associated HS and urokinase-type plasminogen activator receptor in the accumulation of FXIIa on the cell surface of IPF HLF. Moreover, FXIIa stimulated IPF HLF migration, which was abrogated by pretreatment of cells with heparinase I. Collectively, our study uncovers a novel role of HS-type glycosaminoglycans in a local accumulation of FXIIa on the cell membrane. The enhanced association of FXIIa with IPF HLF suggests its contribution to fibrogenesis.

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

Background

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

Methods

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

Results

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

Conclusions

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

Structure-driven design of radionuclide tracers for non-invasive imaging of uPAR and targeted radiotherapy. The tale of a synthetic peptide antagonist

Research performed during the last two decades has provided a wealth of information to highlight the role of the urokinase-type plasminogen activator receptor (uPAR) in the progression and dissemination of invasive and metastatic cancer. In parallel, our perception of the structure-function relationships in uPAR has been refined to such a level that a rational design of uPAR function as well as compounds specifically targeting defined functions of uPAR are now realistic options. This knowledge opens new avenues for developing therapeutic intervention regimens targeting uPAR as well as for monitoring the effects of such treatments by non-invasive imaging using e.g. positron emission tomography. This mini-review will focus on recent advancements in translational research devoted to non-invasive targeting of uPAR, with a view to molecular imaging of its expression in live individuals as well as specific eradication of these cells by targeted radiotherapy.

Urokinase receptor orchestrates the plasminogen system in airway epithelial cell function

PURPOSE

The plasminogen system plays many roles in normal epithelial cell function, and components are elevated in diseases, such as cancer and asthma. The relative contribution of each component to epithelial function is unclear. We characterized normal cell function in airway epithelial cells with increased expression of selected pathway components.

METHODS

BEAS-2B R1 bronchial epithelial cells stably overexpressing membrane urokinase plasminogen activator receptor (muPAR), soluble spliced uPAR (ssuPAR), the ligand (uPA) or inhibitors (PAI1 or PAI2), were characterized for pathway expression. Cell function was examined using proliferation, apoptosis, and scratch wound assays. A549 alveolar epithelial cells overexpressing muPAR were similarly characterized and downstream plasmin activity, MMP-1, and MMP-9 measured.

RESULTS

Elevated expression of individual components led to changes in the plasminogen system expression profile, indicating coordinated regulation of the pathway. Increased muPAR expression augmented wound healing rate in BEAS-2B R1 and attenuated repair in A549 cells. Elevated expression of other system components had no effect on cell function in BEAS-2B R1 cells. This is the first study to investigate activity of the splice variant ssuPAR, with results suggesting that this variant plays a limited role in epithelial cell function in this model.

CONCLUSIONS

Our data highlight muPAR as the critical molecule orchestrating effects of the plasminogen system on airway epithelial cell function. These data have implications for diseases, such as cancer and asthma, and suggest uPAR as the key therapeutic target for the pathway in approaches to alter epithelial cell function.

A flexible multidomain structure drives the function of the urokinase-type plasminogen activator receptor (uPAR)

The urokinase-type plasminogen activator receptor (uPAR) provides a rendezvous between proteolytic degradation of the extracellular matrix and integrin-mediated adhesion to vitronectin. These processes are, however, tightly linked because the high affinity binding of urokinase regulates the binding of uPAR to matrix-embedded vitronectin. Although crystal structures exist to define the corresponding static bi- and trimolecular receptor complexes, it is evident that the dynamic property of uPAR plays a decisive role in its function. In the present study, we combine small angle x-ray scattering, hydrogen-deuterium exchange, and surface plasmon resonance to develop a structural model describing the allosteric regulation of uPAR. We show that the flexibility of its N-terminal domain provides the key for understanding this allosteric mechanism. Importantly, our model has direct implications for understanding uPAR-assisted cell adhesion and migration as well as for translational research, including targeted intervention therapy and non-invasive tumor imaging in vivo.

Lectin chromatography/mass spectrometry discovery workflow identifies putative biomarkers of aggressive breast cancers

We used a lectin chromatography/MS-based approach to screen conditioned medium from a panel of luminal (less aggressive) and triple negative (more aggressive) breast cancer cell lines (n=5/subtype). The samples were fractionated using the lectins Aleuria aurantia (AAL) and Sambucus nigra agglutinin (SNA), which recognize fucose and sialic acid, respectively. The bound fractions were enzymatically N-deglycosylated and analyzed by LC-MS/MS. In total, we identified 533 glycoproteins, ∼90% of which were components of the cell surface or extracellular matrix. We observed 1011 glycosites, 100 of which were solely detected in ≥3 triple negative lines. Statistical analyses suggested that a number of these glycosites were triple negative-specific and thus potential biomarkers for this tumor subtype. An analysis of RNaseq data revealed that approximately half of the mRNAs encoding the protein scaffolds that carried potential biomarker glycosites were up-regulated in triple negative vs luminal cell lines, and that a number of genes encoding fucosyl- or sialyltransferases were differentially expressed between the two subtypes, suggesting that alterations in glycosylation may also drive candidate identification. Notably, the glycoproteins from which these putative biomarker candidates were derived are involved in cancer-related processes. Thus, they may represent novel therapeutic targets for this aggressive tumor subtype.

A cleavage-resistant urokinase plasminogen activator receptor exhibits dysregulated cell-surface clearance

Urokinase plasminogen activator receptor (u-PAR) binds urokinase plasminogen activator (u-PA) and participates in plasminogen activation in addition to modulating several cellular processes such as adhesion, proliferation, and migration. u-PAR is susceptible to proteolysis by its cognate ligand and several other proteases. To elucidate the biological significance of receptor cleavage by u-PA, we engineered and expressed a two-chain urokinase plasminogen activator (tcu-PA) cleavage-resistant u-PAR (cr-u-PAR). This mutated receptor was similar to wild-type u-PAR in binding u-PA and initiating plasminogen activation. However, cr-u-PAR exhibited accelerated internalization and resurfacing due to direct association with the endocytic receptor alpha(2)-macroglobulin receptor/low density lipoprotein receptor-related protein in the absence of the enzyme x inhibitor complex of tcu-PA and plasminogen activator inhibitor-1 (tcu-PA.PAI-1). cr-u-PAR-expressing cells had enhanced migration compared with wild-type u-PAR-expressing cells, and cr-u-PAR was less sensitive to chymotrypsin cleavage as compared with wt u-PAR. Our studies suggest that these mutations in the linker region result in a rearrangement within the cr-u-PAR structure that makes it resemble its ligand-bound form. This constitutively active variant may mimic highly glycosylated cleavage-resistant u-PAR expressed in certain highly malignant cancer-cells.

Structure-based engineering of species selectivity in the interaction between urokinase and its receptor: implication for preclinical cancer therapy

The high affinity interaction between the urokinase-type plasminogen activator (uPA) and its glycolipid-anchored receptor (uPAR) is decisive for cell surface-associated plasminogen activation. Because plasmin activity controls fibrinolysis in a variety of pathological conditions, including cancer and wound healing, several intervention studies have focused on targeting the uPA.uPAR interaction in vivo. Evaluations of such studies in xenotransplanted tumor models are, however, complicated by the pronounced species selectivity in this interaction. We now report the molecular basis underlying this difference by solving the crystal structure for the murine uPA.uPAR complex and demonstrate by extensive surface plasmon resonance studies that the kinetic rate constants for this interaction can be swapped completely between these orthologs by exchanging only two residues. This study not only discloses the structural basis required for a successful rational design of the species selectivity in the uPA.uPAR interaction, which is highly relevant for functional studies in mouse models, but it also suggests the possible development of general inhibitors that will target the uPA.uPAR interaction across species barriers.

Regulation of cell signalling by uPAR

Urokinase-type plasminogen activator receptor (uPAR) expression is elevated during inflammation and tissue remodelling and in many human cancers, in which it frequently indicates poor prognosis. uPAR regulates proteolysis by binding the extracellular protease urokinase-type plasminogen activator (uPA; also known as urokinase) and also activates many intracellular signalling pathways. Coordination of extracellular matrix (ECM) proteolysis and cell signalling by uPAR underlies its important function in cell migration, proliferation and survival and makes it an attractive therapeutic target in cancer and inflammatory diseases. uPAR lacks transmembrane and intracellular domains and so requires transmembrane co-receptors for signalling. Integrins are essential uPAR signalling co-receptors and a second uPAR ligand, the ECM protein vitronectin, is also crucial for this process.

Characterisation of urokinase plasminogen activator receptor variants in human airway and peripheral cells

Background

Expression of the urokinase plasminogen activator receptor (UPAR) has been shown to have clinical relevance in various cancers. We have recently identified UPAR as an asthma susceptibility gene and there is evidence to suggest that uPAR may be upregulated in lung diseases such as COPD and asthma. uPAR is a key receptor involved in the formation of the serine protease plasmin by interacting with uPA and has been implicated in many physiological processes including proliferation and migration. The current aim was to determine key regulatory regions and splice variants of UPAR and quantify its expression in primary human tissues and cells (including lung, bronchial epithelium (HBEC), airway smooth muscle (HASM) and peripheral cells).

Results

Using Rapid Amplification of cDNA Ends (RACE) a conserved transcription start site (-42 to -77 relative to ATG) was identified and multiple transcription factor binding sites predicted. Seven major splice variants were identified (>5% total expression), including multiple exon deletions and an alternative exon 7b (encoding a truncated, soluble, 229aa protein). Variants were differentially expressed, with a high proportion of E7b usage in lung tissue and structural cells (55–87% of transcripts), whereas classical exon 7 (encoding the GPI-linked protein) was preferentially expressed in peripheral cells (~80% of transcripts), often with exon 6 or 5+6 deletions. Real-time PCR confirmed expression of uPAR mRNA in lung, as well as airway and peripheral cell types with ~50–100 fold greater expression in peripheral cells versus airway cells and confirmed RACE data. Protein analysis confirmed expression of multiple different forms of uPAR in the same cells as well as expression of soluble uPAR in cell supernatants. The pattern of expression did not directly reflect that seen at the mRNA level, indicating that post-translational mechanisms of regulation may also play an important role.

Conclusion

We have identified multiple uPAR isoforms in the lung and immune cells and shown that expression is cell specific. These data provide a novel mechanism for uPAR regulation, as different exon splicing may determine uPAR function e.g. alternative E7b results in a soluble isoform due to the loss of the GPI anchor and exon deletions may affect uPA (ligand) and/or integrin binding and therefore influence downstream pathways. Expression of different isoforms within the lung should be taken into consideration in studies of uPAR in respiratory disease.

Characterization of site-specific N-glycosylation

Even if a consensus sequence has been identified for a post- translational modification, the presence of such a sequence motif only indicates the possibility, not the certainty that the modification actually occurs. Proteins can be glycosylated on certain amino acid side-chains, and these modifications are designated as N- and O-glycosylation. N-glycosylated species are modified at Asn residues. There is a consensus sequence for N-glycosylation: AsnXxxSer/Thr/Cys, where Xxx can be any amino acid except proline. N-linked oligosaccharides share a common core structure of GlcNAc2Man3. In addition, an enzyme, peptide N-glycosidase F (PNGase F), removes unaltered most of the common N-linked carbohydrates from proteins while hydrolyzing the originally glycosylated Asn residue to Asp. O- glycosylation occurs at Ser or Thr-residues, usually in sequence-stretches rich in hydroxy amino acids, but there has been no consensus sequence determined for this modification. In addition, O-glycosylation lacks a common core structure: mammalian proteins have been reported bearing O-linked N-acetylgalactosamine, fucose, glucose, and corresponding elongated structures, as well as N-acetylglucosamine. Chemical methods are used to liberate these oligosaccharides because no enzyme has been discovered that would cleave all the different O-linked carbohydrates. Characterization of both types of glycosylation is complicated by the fact that the same amino acids within a population of protein molecules may be derivatized with an array of different carbohydrate structures, or remain unmodified. This site-specific heterogeneity may vary by species, tissue, and may be affected by physiological changes, and so on. For addressing site-specific carbohydrate heterogeneity mass spectrometry has become the method of choice. Although matrix-assisted laser desorption ionization mass spectrometry of collected HPLC-fractions has been used successfully for this purpose, reversed phase HPLC directly coupled with electrospray ionization mass spectrometry (LC/ESIMS) offers better resolution. Using a mass spectrometer as on-line detector not only assures the analysis of every component eluting (mass mapping), but at the same time diagnostic carbohydrate ions can be generated by collisional activation in the ion-source that permit the selective detection of glycopeptides.

Glycosylation of Asn-76 in mouse GPIHBP1 is critical for its appearance on the cell surface and the binding of chylomicrons and lipoprotein lipase

GPIHBP1 is a glycosylphosphatidylinositol-anchored protein in the lymphocyte antigen 6 (Ly-6) family that recently was identified as a platform for the lipolytic processing of triglyceride-rich lipoproteins. GPIHBP1 binds both LPL and chylomicrons and is expressed on the luminal face of microvascular endothelial cells. Here, we show that mouse GPIHBP1 is N-glycosylated at Asn-76 within the Ly-6 domain. Human GPIHBP1 is also glycosylated. The N-linked glycan could be released from mouse GPIHBP1 with N-glycosidase F, endoglycosidase H, or endoglycosidase F1. The glycan was marginally sensitive to endoglycosidase F2 digestion but resistant to endoglycosidase F3 digestion, suggesting that the glycan on GPIHBP1 is of the oligomannose type. Mutating the N-glycosylation site in mouse GPIHBP1 results in an accumulation of GPIHBP1 in the endoplasmic reticulum and a markedly reduced amount of the protein on the cell surface. Consistent with this finding, cells expressing a nonglycosylated GPIHBP1 lack the ability to bind LPL or chylomicrons. Eliminating the N-glycosylation site in a truncated soluble version of GPIHBP1 causes a modest reduction in the secretion of the protein. These studies demonstrate that N-glycosylation of GPIHBP1 is important for the trafficking of GPIHBP1 to the cell surface.

Tumour cell expression of C4.4A, a structural homologue of the urokinase receptor, correlates with poor prognosis in non-small cell lung cancer

PURPOSE

C4.4A expression has been implicated in human cancer progression. This protein is a structural homologue of the urokinase receptor, uPAR, which constitutes a well-established prognostic marker in various human cancers. Nonetheless, little is known about the prognostic significance of C4.4A expression. In the present study, we therefore explored the possible association between C4.4A expression and prognosis in patients with non-small cell lung cancer (NSCLC).

EXPERIMENTAL DESIGN

Tissue sections from 108 NSCLC patients were subjected to immunohistochemical staining using a polyclonal antibody that specifically recognises human C4.4A. Staining frequency and intensity was scored semiquantitatively and grouped into cancers with high and low expression of C4.4A. Kaplan-Meier survival curves were generated to evaluate the significance of C4.4A expression in prognosis of NSCLC patients.

RESULTS

High C4.4A expression was observed in 42% of the NSCLC specimens analysed, and this correlates with overall survival (p = 0.012). A remarkably strong correlation was noted between high expression of C4.4A in pulmonary adenocarcinoma and survival (p < 0.0001). Multivariate Cox regression analysis shows that high C4.4A expression is an independent predictor of poor disease outcome in NSCLC (risk ratio, 1.42; 95% confidence interval, 1.09-1.86; p = 0.009). Although histological type is not a predictor of outcome in NSCLC, high C4.4A expression in adenocarcinoma is nevertheless a very strong predictor of poor disease outcome (risk ratio, 1.62; 95% confidence interval, 1.24-2.09; p = 0.001).

CONCLUSIONS

High tumour cell C4.4A expression is associated with shorter survival for NSCLC patients. Patients with pulmonary adenocarcinoma have a particularly poor prognosis if this histological type is combined with high tumour cell C4.4A expression.

A new tagging system for production of recombinant proteins in Drosophila S2 cells using the third domain of the urokinase receptor

The use of protein fusion tag technology greatly facilitates detection, expression and purification of recombinant proteins, and the demands for new and more effective systems are therefore expanding. We have used a soluble truncated form of the third domain of the urokinase receptor as a convenient C-terminal fusion partner for various recombinant extracellular human proteins used in basic cancer research. The stability of this cystein-rich domain, which structure adopts a three-finger fold, provides an important asset for its applicability as a fusion tag for expression of recombinant proteins. Up to 20mg of intact fusion protein were expressed by stably transfected Drosophila S2 cells per liter of culture using this strategy. Purification of these secreted fusion proteins from the conditioned serum free medium of S2 cells was accompanied by an efficient one-step immunoaffinity chromatography procedure using the immobilized anti-uPAR monoclonal antibody R2. An optional enterokinase cleavage site is included between the various recombinant proteins and the linker region of the tag, which enables generation of highly pure preparations of tag-free recombinant proteins. Using this system we successfully produced soluble and intact recombinant forms of extracellular proteins such as CD59, C4.4A and vitronectin, as well as a number of truncated domain constructs of these proteins. In conclusion, the present tagging system offers a convenient general method for the robust expression and efficient purification of a variety of recombinant proteins.

Structural basis of interaction between urokinase-type plasminogen activator and its receptor

Recent studies indicate that binding of the urokinase-type plasminogen activator (uPA) to its high-affinity receptor (uPAR) orchestrates uPAR interactions with other cellular components that play a pivotal role in diverse (patho-)physiological processes, including wound healing, angiogenesis, inflammation, and cancer metastasis. However, notwithstanding the wealth of biochemical data available describing the activities of uPAR, little is known about the exact mode of uPAR/uPA interactions or the presumed conformational changes that accompany uPA/uPAR engagement. Here, we report the crystal structure of soluble urokinase plasminogen activator receptor (suPAR), which contains the three domains of the wild-type receptor but lacks the cell-surface anchoring sequence, in complex with the amino-terminal fragment of urokinase-type plasminogen activator (ATF), at the resolution of 2.8 A. We report the 1.9 A crystal structure of free ATF. Our results provide a structural basis, represented by conformational changes induced in uPAR, for several published biochemical observations describing the nature of uPAR/uPA interactions and provide insight into mechanisms that may be responsible for the cellular responses induced by uPA binding.

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 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.

Characterization of Protein N‐Glycosylation

Although mass spectrometry (MS)-based protein identification is a straightforward task, the characterization of most posttranslational modifications still represents a challenge. N-glycosylation with its well known consensus sequence, common core structure, and "universally" active endoglycosidase seems to belong to the easier category. In this chapter, MS methods for the analysis of N-glycosylated proteins are reviewed. In particular, LC-MS analysis of glycoprotein digests is discussed in detail. The examples included in this chapter illustrate the improved detection sensitivities achieved during the last decade. The characterization of site heterogeneity and of site occupancy is addressed. Low-energy collision-induced dissociation (CID) fragmentation of N-linked glycopeptides and their sodium-adducts is also described.

Dynamics of Urokinase Receptor Interaction with Peptide Antagonists Studied by Amide Hydrogen Exchange and Mass Spectrometry

Using amide hydrogen exchange combined with electrospray ionization mass spectrometry, we have in this study determined the number of amide hydrogens on several peptides that become solvent-inaccessible as a result of their high-affinity interaction with the urokinase-type plasminogen activator receptor (uPAR). These experiments reveal that at least six out of eight amide hydrogens in a synthetic nine-mer peptide antagonist (AE105) become sequestered upon engagement in uPAR binding. Various uPAR mutants with decreased affinity for this peptide antagonist gave similar results, thereby indicating that deletion of the favorable interactions involving the side chains of these residues in uPAR does not affect the number of hydrogen bonds established by the main chain of the peptide ligand. The isolated growth factor-like domain (GFD) of the cognate serine protease ligand for uPAR showed 11 protected amide hydrogens in the receptor complex. Interestingly, a naturally occurring O-linked fucose on Thr(18) confers protection of two additional amide hydrogens in GFD when it forms a complex with uPAR. Dissociation of the uPAR-peptide complexes is accompanied by a correlated exchange of nearly all amide hydrogens on the peptide ligand. This yields bimodal isotope patterns from which dissociation rate constants can be determined. In addition, the distinct bimodal isotope distributions also allow investigation of the exchange kinetics of receptor-bound peptides providing information about the local structural motions at the interface. These exchange experiments therefore provide both structural and kinetic information on the interaction between uPAR and these small peptide antagonists, which in model systems show promise as inhibitors of intravasation of human cancer cells.

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.

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.

Characterization of low-glycosylated forms of soluble human urokinase receptor expressed in Drosophila Schneider 2 cells after deletion of glycosylation-sites

The urokinase-type plasminogen activator receptor (uPAR) is a glycolipid-anchored membrane protein that is thought to play an active role during cancer cell invasion and metastasis. We have expressed a truncated soluble form of human uPAR using its native signal peptide in stably transfected Drosophila Schneider 2 (S2) cells. This recombinant product, denoted suPAR (residues 1-283), is secreted in high quantities in serum-free medium and can be isolated in very high purity. Characterization by SDS-PAGE and mass spectrometry reveals that suPAR produced in this system carries a uniform glycosylation composed of biantennary carbohydrates. In contrast, suPAR produced in stably transfected Chinese hamster ovary (CHO) cells carries predominantly complex-type glycosylation and exhibits in addition a site-specific microheterogeneity of the individual N-linked carbohydrates. Measurement of binding kinetics for the interaction with uPA by surface plasmon resonance reveals that S2-produced suPAR exhibits binding properties similar to those of suPAR produced by CHO cells. By site-directed mutagenesis we have furthermore removed the five potential N-linked glycosylation-sites either individually or in various combinations and studied the effect thereof on secretion and ligand-binding. Only suPAR completely deprived of N-linked glycosylation exhibits an impaired level of secretion. All the other mutants showed comparable secretion levels and retained the ligand-binding properties of suPAR-wt. In conclusion, stable expression of suPAR in Drosophila S2 cells offers a convenient and attractive method for the large scale production of homogeneous preparations of several uPAR mutants, which may be required for future attempts to solve the three-dimensional structure of uPAR by X-ray crystallography.

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

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

Using optimized collision energies and high resolution, high accuracy fragment ion selection to improve glycopeptide detection by precursor ion scanning

Glycosylation is the most widespread protein modification and is known to modulate signal transduction and several biologically important interactions. In order to understand and evaluate the biological role of glycosylation it is important to identify the glycosylated protein and localize the site glycosylation under particular biological conditions. To identify glycosylated peptides from simple mixtures, i.e., in-gel digests from single SDS PAGE bands we performed high resolution, high accuracy precursor ion scanning using a quadrupole TOF instrument equipped with the Q(2) pulsing function. The high resolving power of the quadrupole TOF instrument results in the selective detection of glycan specific fragment ions minimizing the interference of peptide derived fragment ions with the same nominal mass. Precursor ion scanning has been previously described for these glycan derived ions. However the use of this method has been limited by the low specificity of the method. The analysis using precursor ion scanning can be applied to any peptide mixture from a protein digest without having previous knowledge of the glycosylation of the protein. In addition to the low femtomole (nanomolar) detection limits, this method has the advantage that no prior derivatization or enzymatic treatment of the peptide mixtures is required.

Binding of urokinase-type plasminogen activator receptor (uPAR) to the mannose 6-phosphate/insulin-like growth factor II receptor: contrasting interactions of full-length and soluble forms of uPAR

Urokinase-type plasminogen activator receptor (uPAR) binding by the mannose 6-phosphate/insulin-like growth factor II receptor (Man-6-P/IGF2R) is considered important to Man-6-P/IGF2R tumor suppressor function via regulation of cell surface proteolytic activity. Our goal was to map the uPAR binding site of the Man-6-P/IGF2R by analyzing the uPAR binding characteristics of a panel of minireceptors containing different regions of the Man-6-P/IGF2R extracytoplasmic domain. Coimmunoprecipitation assays revealed that soluble recombinant uPAR (suPAR) bound the Man-6-P/IGF2R at two distinct sites, one localized to the amino-terminal end of the Man-6-P/IGF2R extracytoplasmic domain (repeats 1-3) and the other to the more carboxyl-terminal end (repeats 7-9). These sites correspond with the positions of the two Man-6-P binding domains of Man-6-P/IGF2R. Indeed, the suPAR-Man-6-P/IGF2R interaction was inhibited by Man-6-P, and binding-competent su-PAR species represented a minor percentage (8-30%) of the suPAR present. In contrast, Man-6-P/IGF2R binding of endogenous, full-length uPAR solubilized from plasma membranes of the prostate cancer cell line, PC-3, was not inhibited by Man-6-P. Further studies showed that very little (<5%) endogenous uPAR was Man-6-P/IGF2R binding-competent. We conclude that, contrary to previous reports, the interaction between uPAR and Man-6-P/IGF2R is a low percentage binding event and that suPAR and full-length uPAR bind the Man-6-P/IGF2R by different mechanisms.

Site-specific characterization of the N-linked oligosaccharides of a murine immunoglobulin M by high-performance liquid chromatography/electrospray mass spectrometry

Immunoglobulin M is an especially important product of the immune system because it plays a critical role in early protection against infections. In this report, the glycosylation pattern of the protective murine monoclonal IgM 12A1 to Cryptococcus neoformans polysaccharide was analyzed by high-performance liquid chromatography coupled with electrospray ionization mass spectrometry. Peptide mapping studies covering 88% of the deduced amino acid sequence indicated that of the six potential N-glycosylation sites in this antibody only five were utilized, as the tryptic peptide derived from monoclonal IgM 12A1 containing Asn-260 was recovered without carbohydrates. The oligosaccharide side chains of monoclonal IgM 12A1 were characterized at each of the N-glycosylation sites. Asn-166 possessed 20 monosialylated and nonsialylated, and fucosylated and nonfucosylated complex- and hybrid-type oligosaccharides and one high-mannose-type oligosaccharide. Thirteen oligosaccharides were attached to the site at Asn-401, including six complex-type, four hybrid-type, and three high-mannose-type oligosaccharides. Twelve hybrid-type oligosaccharides were attached to Asn-378, three of which had terminal sialic acids. Eleven hybrid-type oligosaccharides were attached to Asn-331, seven of which had terminal sialic acids. Only two high-mannose type oligosaccharides were attached to Asn-363. These results indicated great complexity in the structure and composition of oligosaccharides attached to individual IgM glycosylation sites.

Urokinase regulates vitronectin binding by controlling urokinase receptor oligomerization

Adhesion of monocytes to the extracellular matrix is mediated by a direct high affinity interaction between cell-surface urokinase-type plasminogen activator (uPA) receptor (uPAR) and the extracellular matrix protein vitronectin. We demonstrate a tight connection between uPA-regulated uPAR oligomerization and high affinity binding to immobilized vitronectin. We find that binding of soluble uPAR (suPAR) to immobilized vitronectin is strictly ligand-dependent with a linear relationship between the observed binding and the concentration of ligand added. Nevertheless, a comparison of experimentally obtained binding curves to those generated using a simple equilibrium model suggests that the high affinity vitronectin-binding pro-uPA.suPAR complex contains two molecules of suPAR. In co-immunoprecipitation experiments, using different epitope-tagged suPAR molecules, suPAR/suPAR co-immunoprecipitation displayed a similar uPA dose dependence as that observed for vitronectin binding, demonstrating that the high affinity vitronectin-binding complex indeed contains oligomeric suPAR. Structurally, the kringle domain of uPA was found to be critical for the formation of the vitronectin-binding competent complex because the amino-terminal fragment, but not the growth factor-like domain, behaved as a full-length uPA. Our data represent the first demonstration of functional, ligand-induced uPAR oligomerization having extensive implications for glycosylphosphatidylinositol-anchored receptors in general, and for the biology of the uPA/uPAR system in particular.

Peptide-derived antagonists of the urokinase receptor. affinity maturation by combinatorial chemistry, identification of functional epitopes, and inhibitory effect on cancer cell intravasation

The high-affinity interaction between urokinase-type plasminogen activator (uPA) and its glycolipid-anchored receptor (uPAR) plays an important role in pericellular plasminogen activation. Since proteolytic degradation of the extracellular matrix has an established role in tumor invasion and metastasis, the uPA-uPAR interaction represents a potential target for therapeutic intervention. By affinity maturation using combinatorial chemistry we have now developed and characterized a 9-mer, linear peptide antagonist of the uPA-uPAR interaction demonstrating specific, high-affinity binding to human uPAR (K(d) approximately 0.4 nM). Studies by surface plasmon resonance reveal that the off-rate for this receptor-peptide complex is comparable to that measured for the natural protein ligand, uPA. The functional epitope on human uPAR for this antagonist has been delineated by site-directed mutagenesis, and its assignment to loop 3 of uPAR domain III (Met(246), His(249), His(251), and Phe(256)) corroborates data previously obtained by photoaffinity labeling and provides a molecular explanation for the extreme selectivity observed for the antagonist toward human compared to mouse, monkey, and hamster uPAR. When human HEp-3 cancer cells were inoculated in the presence of this peptide antagonist, a specific inhibition of cancer cell intravasation was observed in a chicken chorioallantoic membrane assay. These data imply that design of small organic molecules mimicking the binding determinants of this 9-mer peptide antagonist may have a potential application in combination therapy for certain types of cancer.

beta(3)A-integrin downregulates the urokinase-type plasminogen activator receptor (u-PAR) through a PEA3/ets transcriptional silencing element in the u-PAR promoter

Migration of cells requires interactions with the extracellular matrix mediated, in part, by integrins, proteases, and their receptors. Previous studies have shown that beta(3)-integrin interacts with the urokinase-type plasminogen activator receptor (u-PAR) at the cell surface. Since integrins mediate signaling into the cell, the current study was undertaken to determine if in addition beta(3)-integrin regulates u-PAR expression. Overexpression of beta(3)-integrin in CHO cells, which are avid expressers of the receptor, downregulated u-PAR protein and mRNA expression. The u-PAR promoter (-1,469 bp) that is normally constitutively active in CHO cells was downregulated by induced beta(3)-integrin expression. A region between -398 and -197 bp of the u-PAR promoter was critical for beta(3)-integrin-induced downregulation of u-PAR promoter activity. Deletion of the PEA3/ets motif at -248 bp substantially impaired the ability of beta(3)-integrin to downregulate the u-PAR promoter, suggesting that the PEA3/ets site acts as a silencing element. An expression vector encoding the transcription factor PEA3 caused inhibition of the wild-type but not the PEA3/ets-deleted u-PAR promoter. The PEA3/ets site bound nuclear factors from CHO cells specifically, but binding was enhanced when beta(3)-integrin was overexpressed. A PEA3 antibody inhibited DNA-protein complex formation, indicating the presence of PEA3. Downregulation of the u-PAR promoter was achieved by the beta(3)A-integrin isoform but not by other beta(3)-integrin isoforms and required the cytoplasmic membrane NITY(759) motif. Moreover, overexpression of the short but not the long isoform of the beta(3)-integrin adapter protein beta(3)-endonexin blocked u-PAR promoter activity through the PEA3/ets binding site. Thus, besides the physical interaction of beta(3)-integrin and u-PAR at the cell surface, beta(3) signaling is implicated in the regulation of u-PAR gene transcription, suggesting a mutual regulation of adhesion and proteolysis receptors.

Urokinase-type plasminogen activator receptor (CD87) is a ligand for integrins and mediates cell-cell interaction

Binding of urokinase-type plasminogen activator (uPA) to its receptor (uPAR/CD87) regulates cellular adhesion, migration, and tumor cell invasion. However, it is unclear how glycosyl phosphatidylinositol-anchored uPAR, which lacks a transmembrane structure, mediates signal transduction. It has been proposed that uPAR forms cis-interactions with integrins as an associated protein and thereby transduces proliferative or migratory signals to cells upon binding of uPA. We provide evidence that soluble uPAR (suPAR) specifically binds to integrins alpha4beta1, alpha6beta1, alpha9beta1, and alphavbeta3 on Chinese hamster ovary cells in a cation-dependent manner. Anti-integrin and anti-uPAR antibodies effectively block binding of suPAR to these integrins. Binding of suPAR to alpha4beta1 and alphavbeta3 is blocked by known soluble ligands and by the integrin mutations that inhibit ligand binding. These results suggest that uPAR is an integrin ligand rather than, or in addition to, an integrin-associated protein. In addition, we demonstrate that glycosyl phosphatidylinositol-anchored uPAR on the cell surface specifically binds to integrins on the apposing cells, suggesting that uPAR-integrin interaction may mediate cell-cell interaction (trans-interaction). These previously unrecognized uPAR-integrin interactions may allow uPAR to transduce signals through the engaged integrin without a hypothetical transmembrane adapter and may provide a potential therapeutic target for control of inflammation and cancer.

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

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

Urokinase receptor and integrin partnership: coordination of signaling for cell adhesion, migration and growth

Urokinase-type plasminogen activator receptor (uPAR) binds the urokinase-type plasminogen activator (uPA) and facilitates a proteolytic cascade focused at the cell surface. More recently, uPAR was recognized as a multifunctional protein that, through its interactions with integrins, initiates signaling events that alter cell adhesion, migration and proliferation. Results obtained recently have led to new insights into the structural aspects of uPAR interaction with integrins, provided a more detailed description of the signaling pathway they induce, and determined that uPAR signaling plays a role in cell migration and tumorigenicity.

Roles of the heparin and low density lipid receptor-related protein-binding sites of protease nexin 1 (PN1) in urokinase-PN1 complex catabolism. The PN1 heparin-binding site mediates complex retention and degradation but not cell surface binding or internalization

We have previously described thrombin (Th)-protease nexin 1 (PN1) inhibitory complex binding to cell surface heparins and subsequent low density lipid receptor-related protein (LRP)-mediated internalization. Our present studies examine the catabolism of urinary plasminogen activator (uPA)-PN1 inhibitory complexes, which, unlike Th.PN1 complexes, bind almost exclusively through the uPA receptor. In addition, the binding site in PN1 required for the LRP-mediated internalization of Th.PN1 complexes is not required for the LRP-mediated internalization of uPA.PN1 complexes. Thus, the protease moiety of the complex partially determines the mechanistic route of entry. Because cell surface heparins are only minimally involved in the binding and internalization of uPA.PN1 complexes, we then predicted that complexes between uPA and the heparin binding-deficient PN1 variant, PN1(K7E), should be catabolized at the same rate as complexes formed with native PN1. Surprisingly, the uPA.PN1(K7E) complexes were degraded at only a fraction of the rate of native complexes. Internalization studies revealed that both uPA. PN1(K7E) and native uPA.PN1 complexes were initially internalized at the same rate, but uPA.PN1(K7E) complexes were rapidly retro-endocytosed in an intact form. By examining the pH dependence of complex binding in the range of 4.0-7.0, it was determined that the uPA.PN1 inhibitory complexes must specifically bind to endosomal heparins at pH 5.5 to be retained and sorted to lysosomes. These studies are the first to document a role for heparins in the catabolism of SERPIN-protease complexes at a point further in the pathway than cell surface binding, and this role may extend to other heparin-binding LRP-internalized ligands.

Plasminogen-independent initiation of the pro-urokinase activation cascade in vivo. Activation of pro-urokinase by glandular kallikrein (mGK-6) in plasminogen-deficient mice

The plasminogen activation (PA) system is involved in the degradation of fibrin and various extracellular matrix proteins, taking part in a number of physiological and pathological tissue remodeling processes including cancer invasion. This system is organized as a classical proteolytic cascade, and as for other cascade systems, understanding the physiological initiation mechanism is of central importance. The attempts to identify initiation routes for activation of the proform of the key enzyme urokinase-type plasminogen activator (pro-uPA) in vivo have been hampered by the strong activator potency of the plasmin, that is generated during the progress of the cascade. Using gene-targeted mice deficient in plasminogen (Plg -/- mice) [Bugge, T. H., Flick, M. J., Daugherty, C. C., and Degen, J. L. (1995) Genes Dev. 9, 794-807], we have now demonstrated and identified a component capable of initiating the cascade by activating pro-uPA. The urine from Plg -/- mice contained active two-chain uPA as well as a proteinase capable of activating exogenously added pro-uPA. The active component was purified and identified by mass spectrometry-based peptide mapping as mouse glandular kallikrein mGK-6 (true tissue kallikrein). The pro-uPA converting activity of the mGK-6 enzyme, as well as its ability to cleave a synthetic substrate for glandular kallikrein, was inhibited by the serine proteinase inhibitor leupeptin but not by other serine proteinase inhibitors such as aprotinin, antithrombin III, or alpha(1)-antitrypsin. We suggest that mouse glandular kallikrein mGK-6 is an activator of pro-uPA in the mouse urinary tract in vivo. Since this kallikrein is expressed in a number of tissues and also occurs in plasma, it can also be considered a candidate for a physiological pro-uPA activator in other locations.

MALDI-TOF mass spectrometry in protein chemistry

Mass spectrometry has in the last decade been accepted as a key analytical technique in protein chemistry. It is now the preferred technique for identification of proteins separated by one- or two-dimensional polyacrylamide gel electrophoresis, i.e. in proteome analysis. It is the dominating technique for determination of posttranslational modifications in proteins. The two ionization techniques presently widely used in protein studies are matrix-assisted laser desorption/ionization (MALDI) in combination with time-of-flight (TOF) mass analyzers and electrospray ionization (ESI) in combination with a variety of mass analyzers. In this chapter the principles and performance of MALDI-TOF mass spectrometry will be described as well as the application of this technique to a variety of applications.