Inhibition of the interaction of urokinase-type plasminogen activator (uPA) with its receptor (uPAR) by synthetic peptides

Piptide Chemotypes for Perturbation of the Interaction of Urokinase with Its Receptor

Only a few small molecules that disrupt the uPA and uPA receptor (uPAR) interaction have been discovered despite decades of research in the area, and none have been approved in clinical trials. Research reported here features two new ways of considering the problem of discovering small molecules to disrupt uPA•uPAR, specifically in terms of chemotype design and method of evaluation. Chemotypes used in this work are piptides (Arancillo . Angew. Chem., Int. Ed., 2021, 60, 6653-6659) with side chains corresponding to the uPA loop that binds uPAR. Further, hybrids of 1 and another uPAR ligand developed in these labs (2), i.e., 3 and 4, were also designed and tested. All the piptide chemotypes bound uPAR at concentrations of 50 μM or less. Members of this series had K_i values <3 μM and showed favorable responses in cellular assays; these data are comparable with the best small molecule uPA•uPAR disruptors in the literature (from conventional screening).

Urokinase-type plasminogen activator receptor (uPAR) as a therapeutic target in cancer

Urokinase-type plasminogen activator receptor (uPAR) is an attractive target for the treatment of cancer, because it is expressed at low levels in healthy tissues but at high levels in malignant tumours. uPAR is closely related to the invasion and metastasis of malignant tumours, plays important roles in the degradation of extracellular matrix (ECM), tumour angiogenesis, cell proliferation and apoptosis, and is associated with the multidrug resistance (MDR) of tumour cells, which has important guiding significance for the judgement of tumor malignancy and prognosis. Several uPAR-targeted antitumour therapeutic agents have been developed to suppress tumour growth, metastatic processes and drug resistance. Here, we review the recent advances in the development of uPAR-targeted antitumor therapeutic strategies, including nanoplatforms carrying therapeutic agents, photodynamic therapy (PDT)/photothermal therapy (PTT) platforms, oncolytic virotherapy, gene therapy technologies, monoclonal antibody therapy and tumour immunotherapy, to promote the translation of these therapeutic agents to clinical applications.

A versatile insertion point on albumin to accommodate peptides and maintain their activities

Genetic fusion of human serum albumin to peptides is an important strategy to enhance the plasma half-life of the peptide. An inherent challenge of such method is the reduction of specific activity of the cargo peptides upon connecting at N- or C-termini of albumin. Here, we report a finding that residue 363-364 of albumin can be inserted with a peptide while maintaining the peptide activities. We insert a peptide inhibitor into this site, and at the N-terminus of albumin, for comparison. The chimeric protein displays potent inhibition (IC₅₀ value of 30 nM) to its target (uPAR), but not the N-terminally fused construct. We also study the chimera of HSA with a cyclic peptide inhibitor of murine urokinase-type plasminogen activator grafted at either the internal site or the N-terminus. The internally peptide-grafted protein possesses a much more potent inhibition compared to the N-terminally located fusion (IC₅₀ value of 32 nM vs 19 μM). We further demonstrate that such internal fusion does not affect albumin expression, secondary structure, and inherent drug binding activity. Thus, this work identifies a versatile insertion point inside albumin for maintaining fusion peptide activity, and opens a new avenue to expand the applications of albumin fusion technology.

uPAR antibody (huATN-658) and Zometa reduce breast cancer growth and skeletal lesions

Urokinase plasminogen activator receptor (uPAR) is implicated in tumor growth and metastasis due to its ability to activate latent growth factors, proteases, and different oncogenic signaling pathways upon binding to different ligands. Elevated uPAR expression is correlated with the increased aggressiveness of cancer cells, which led to its credentialing as an attractive diagnostic and therapeutic target in advanced solid cancer. Here, we examine the antitumor effects of a humanized anti-uPAR antibody (huATN-658) alone and in combination with the approved bisphosphonate Zometa (Zoledronic acid) on skeletal lesion through a series of studies in vitro and in vivo. Treatment with huATN-658 or Zometa alone significantly decreased human MDA-MB-231 cell proliferation and invasion in vitro, effects which were more pronounced when huATN-658 was combined with Zometa. In vivo studies demonstrated that huATN-658 treatment significantly reduced MDA-MB-231 primary tumor growth compared with controls. In a model of breast tumor-induced bone disease, huATN-658 and Zometa were equally effective in reducing skeletal lesions. The skeletal lesions were significantly reduced in animals receiving the combination of huATN-658 + Zometa compared with monotherapy treatment. These effects were due to a significant decrease in osteoclastic activity and tumor cell proliferation in the combination treatment group. Transcriptome analysis revealed that combination treatment significantly changes the expression of genes from signaling pathways implicated in tumor progression and bone remodeling. Results from these studies provide a rationale for the continued development of huATN-658 as a monotherapy and in combination with currently approved agents such as Zometa in patients with metastatic breast cancer.

The urokinase plasminogen activator binding to its receptor: a quantum biochemistry description within an in/homogeneous dielectric function framework with application to uPA–uPAR peptide inhibitors

DFT calculations using the MFCC fragment-based model considering a spatial-dependent dielectric function based on the Poisson–Boltzmann approximation were performed to describe the uPA–uPAR interactions.

Designing tracers for PET imaging of the urokinase-type plasminogen activator receptor from a cyclic uPA-derived peptide: first in vitro evaluations

The treatment of cancer remains a major challenge, especially after tumour cell dissemination and metastases formation. Expression of the urokinase-type plasminogen activation system including urokinase (uPA) and its receptor (uPAR) has been associated with the complex process of cell migration, a tumour's invasive potential as well as a reduced overall and disease-free survival of patients with solid cancers and haematological disorders. A cyclic peptide cyclo[21,29][d-Cys²¹ ,Cys²⁹ ]-uPA_21-30 was designed from the growth factor-like domain (GFD) of urokinase whose binding to uPAR was found to inhibit tumour growth and spread of human ovarian cancer cells in mice. With the aim of visualising uPAR expression using PET imaging to attempt an estimate on the tumour's aggressiveness, the cyclic peptide was modified with an either C- or N-terminally attached variable spacer and chelator. The free ligands were evaluated for their binding affinities to the isolated human uPAR and labelled with ⁶⁸ Ga and ¹⁷⁷ Lu to assess their lipophilicities and stabilities in human serum. Although retaining the full binding potential displayed by cyclo[21,29][d-Cys²¹ ,Cys²⁹ ]-uPA_21-30 to its target was found to be a challenging task upon both C- and N-terminal modification, chelator-bearing ligands were identified that can serve as promising starting points in the development of uPAR-addressing PET tracers.

Multifaceted Role of the Urokinase-Type Plasminogen Activator (uPA) and Its Receptor (uPAR): Diagnostic, Prognostic, and Therapeutic Applications

The plasminogen activator (PA) system is an extracellular proteolytic enzyme system associated with various physiological and pathophysiological processes. A large body of evidence support that among the various components of the PA system, urokinase-type plasminogen activator (uPA), its receptor (uPAR), and plasminogen activator inhibitor-1 and -2 (PAI-1 and PAI-2) play a major role in tumor progression and metastasis. The binding of uPA with uPAR is instrumental for the activation of plasminogen to plasmin, which in turn initiates a series of proteolytic cascade to degrade the components of the extracellular matrix, and thereby, cause tumor cell migration from the primary site of origin to a distant secondary organ. The components of the PA system show altered expression patterns in several common malignancies, which have identified them as ideal diagnostic, prognostic, and therapeutic targets to reduce cancer-associated morbidity and mortality. This review summarizes the various components of the PA system and focuses on the role of uPA-uPAR in different biological processes especially in the context of malignancy. We also discuss the current state of knowledge of uPA-uPAR-targeted diagnostic and therapeutic strategies for various malignancies.

A novel peptide blocking cancer cell invasion by structure-based drug design

The receptor for the urokinase-type plasminogen activator (uPA), uPAR, facilitates tumor cell invasion and metastasis by focusing on several ligands, including uPA, integrins and vitronectin. With computational prediction algorithms and structure-based drug design, we identified peptides containing the Gly-Lys-Gly-Glu-Gly-Glu-Gly-Lys-Gly sequence (peptide H1), which strongly interacts with uPAR. The aim of the present study was to investigate the effect of allosteric inhibition at the uPAR interface using a novel synthetic peptide and its function on ovarian cancer cell invasion. The molecular and functional mechanisms of H1 were determined by complementary biochemical and biological methods in the promyeloid U937 cell line as well as ovarian cancer cell lines, including serous carcinoma SKOV3 and clear cell carcinoma TOV21G. The effects of H1 treatment on cancer cell invasion were evaluated in vitro. H1 inhibited cancer cell invasion, without affecting cell viability, accompanied by the suppression of extracellular signal-regulated kinase (ERK)-1 phosphorylation and then matrix metalloproteinase (MMP)-9 expression. H1 failed to block the interaction of uPA-uPAR protein-protein interaction in cells, but antagonized the uPA function. H1 failed to disrupt the uPA-uPAR complex, but abolished the invasion of ovarian cancer cells at least through suppression of the ERK-MMP-9 signaling pathway. Further studies are needed to confirm our observations and to describe the underlying molecular mechanism.

Urokinase plasminogen activator receptor (uPAR) targeted nuclear imaging and radionuclide therapy

Urokinase-type plasminogen activator receptor (uPAR) is a glycosylphosphatidylinositol (GPI)-anchored protein. Besides regulating proteolysis, uPAR could also activate many intracellular signaling pathways that promote cell motility, invasion, proliferation, and survival through cooperating with transmembrane receptors. uPAR is overexpressed across a variety of tumors and is associated with cancer invasion and metastasis. In order to meet the demand for a rapid development and potential clinical application of anti-cancer therapy based on uPA/uPAR system, it is desirable to develop non-invasive imaging methods to visualize and quantify uPAR expression in vivo. In this review, we will discuss recent advances in the development of uPAR-targeted nuclear imaging and radionuclide therapy agents. The successful development of molecular imaging probes to visualize uPAR expression in vivo would not only assist preclinical researches on uPAR function, but also eventually impact patient management.

Role of urokinase receptor in tumor progression and development

Elevated level of urokinase receptor (uPAR) is detected in various aggressive cancer types and is closely associated with poor prognosis of cancers. Binding of uPA to uPAR triggers the conversion of plasminogen to plasmin and the subsequent activation of metalloproteinases. These events confer tumor cells with the capability to degrade the components of the surrounding extracellular matrix, thus contributing to tumor cell invasion and metastasis. uPA-uPAR interaction also elicits signals that stimulate cell proliferation/survival and the expression of tumor-promoting genes, thus assisting tumor development. In addition to its interaction with uPA, uPAR also interacts with vitronectin and this interaction promotes cancer metastasis by activating Rac and stimulating cell migration. Although underlying mechanisms are yet to be fully elucidated, uPAR has been shown to facilitate epithelial-mesenchymal transition (EMT) and induce cancer stem cell-like properties in breast cancer cells. The fact that uPAR lacks intracellular domain suggests that its signaling must be mediated through its co-receptors. Indeed, uPAR interacts with diverse transmembrane proteins including integrins, ENDO180, G protein-coupled receptors and growth factor receptors in cancer cells and these interactions are proven to be critical for the role of uPAR in tumorigenesis. Inhibitory peptide that prevents uPA-uPAR interaction has shown the promise to prolong patients' survival in the early stage of clinical trial. The importance of uPAR's co-receptor in uPAR's tumor-promoting effects implicate that anti-cancer therapeutic agents may also be developed by disrupting the interactions between uPAR and its functional partners.

Modulators of the urokinase-type plasminogen activation system for cancer

IMPORTANCE OF THE FIELD

The serine protease urokinase-type plasminogen activator (uPA) and its receptor uPAR as well as two specific inhibitors, the plasminogen activator inhibitor type-1 (PAI-1) and type-2 (PAI-2), are involved in the control of extracellular matrix turnover and tumor growth. Data accumulating over the past 20 years have made increasingly clear that the uPA system has a multifunctional role in neoplastic evolution, affecting cancer cell proliferation, tumor angiogenesis, adhesion and migration.

AREAS COVERED IN THIS REVIEW

Several therapeutic strategies inhibiting the uPA system have been or are currently being developed for suppression of tumor growth. This review examines the role of the uPA system in tumor progression and assesses the various therapeutic strategies developed to selectively exploit this system.

WHAT WILL THE READER GAIN

We focus on the therapeutic developments of the last 15 years. In addition to antibodies and recombinant uPA- or uPAR-derived proteins, various antagonistic peptides as well as small molecules have been designed and synthesized that inhibit the uPA system, leading to reduced tumor progression.

TAKE HOME MESSAGE

The multifunctional potential of the uPA system in cancer has rendered this system an attractive novel target for anticancer therapy. A few novel tumor biology-based therapeutic strategies reported here, opening new ways for patient-optimized and individualized cancer therapy. It may be the right time to evaluate the hypothesis that the uPA system plays a pivotal role in cancer progression and that targeting this system will lead to clinical benefit in cancer patients.

Urokinase plasminogen activator system as a potential target for cancer therapy

Proteolysis of extracellular matrix (ECM) and basement membrane is an essential mechanism used by cancer cells for their invasion and metastasis. The ECM proteinases are divided into three groups: metalloproteinases, cysteine proteinases and serine proteinases. The urokinase plasminogen activator (uPA) system is one of the serine proteinase systems involved in ECM degradation. Members of this system, including uPA and its receptor (uPAR), are overexpressed in several malignant tumors. This system plays a major role in adhesion, migration, invasion and metastasis of cancer cells, thus making it an important target for anticancer drug therapy. Several strategies, including the use of antisense oligodeoxynucleotides, ribozymes, DNAzyme, RNAi, uPA inhibitors, soluble uPAR, catalytically inactive uPA fragments, synthetic peptides and synthetic hybrids are under study, as they interfere with the expression and/or activity of uPA or uPAR in tumor cells. Herein, we discuss the various pharmaceutical strategies under investigation to combat the uPA activity in cancer.

Development and evaluation of peptidic ligands targeting tumour-associated urokinase plasminogen activator receptor (uPAR) for use in alpha-emitter therapy for disseminated ovarian cancer

PURPOSE

Among gynecologic malignancies, ovarian cancer has the highest mortality due to rapid peritoneal dissemination. Treatment failure particularly arises from failure to eliminate disseminated cells. Our aim was to develop peptidic radioligands targeting tumour cell-associated urokinase receptor (uPAR, CD87) for alpha-emitter therapy for advanced ovarian cancer.

METHODS

DOTA-conjugated, uPAR-directed ligands were synthesised on solid-phase. Binding of peptides to human cells expressing uPAR was assayed by flow cytofluorometry or, in case of (213)Bi-labelled peptides, by measuring cell-bound radioactivity. Bio-distribution of the (213)Bi-labelled peptide P-P4D was analysed in nude mice 28 days after intraperitoneal inoculation of OV-MZ-6 ovarian cancer cells in the absence or presence of the plasma expander gelofusine.

RESULTS

uPAR-selective ligands were developed based on published high-affinity uPAR-binding peptides. For preparation of N-terminally cross-linked divalent ligands, a novel solid-phase procedure was developed. Specific binding of (213)Bi-labelled peptides to monocytoid U937 and OV-MZ-6 cells was demonstrated using the natural ligand of uPAR, pro-uPA, or a soluble form of uPAR, suPAR, as competitors. The pseudo-symmetrical covalent dimer (213)Bi-P-P4D displayed superior binding to OV-MZ-6 cells in vitro. Accumulation of (213)Bi-P-P4D in tumour tissue was demonstrated by bio-distribution analysis in nude mice bearing intraperitoneal OV-MZ-6-derived tumours. Gelofusine reduced kidney uptake of (213)Bi-P-P4D by half.

CONCLUSION

Ovarian cancer cells overexpressing uPAR were specifically targeted in vitro and in vivo by (213)Bi-P-P4D. Kidney uptake of (213)Bi-P-P4D was distinctly reduced using gelofusine. Thus, this radiopeptide may represent a promising option for therapy for disseminated ovarian cancer.

The crystal structures of 3-TAPAP in complexes with the urokinase-type plasminogen activator and picrate

The urokinase-type plasminogen activator (uPA) is a protein involved in tissue remodeling and other biological processes. The inhibitors of uPA have been shown to prevent the spread of metastasis and tumor growth, and accordingly this enzyme is widely accepted as a promising anticancer target. In this work, we have investigated the conformation of the uPA inhibitor 3-TAPAP in two different crystalline environments of a picrate and a uPA complex. These structures were compared to the known structure of the 3-TAPAP in the complex with trypsin. In the complexes with the proteins, trypsin, and uPA, the binding mode of 3-TAPAP is similar. A larger difference in the conformation, in the comparison to these structures, has been observed by us in the 3-TAPAP picrate crystal. This observation contradicts the hypothesis that 3-TAPAP derivatives inhibit serine proteinases in preformed stable conformations.

Maspin regulates hypoxia-mediated stimulation of uPA/uPAR complex in invasive breast cancer cells

Maspin, a unique serine proteinase inhibitor (serpin), plays a key role in mammary gland development and is silenced during breast cancer progression. Maspin has been shown to inhibit tumor cell motility and invasion in cell culture, as well as growth and metastasis in animal models. In this study, we investigated the effect of maspin on the regulation of hypoxia-induced expression of urokinase-type plasminogen activator (uPA) and its receptor (uPAR), with respect to invasive potential in metastatic breast cells MDA-MB-231. We hypothesized that maspin can neutralize or mitigate hypoxia-induced expression of uPA/uPAR in metastatic breast cancer cells, resulting in suppression of their invasive potential. To test our hypothesis, we employed the highly invasive MDA-MB-231 breast cancer cells that are devoid of maspin, and transfected them with the maspin gene, and then determined the effect of hypoxia on uPA/uPAR expression. Normal mammary epithelial cells 1436N1 were used as a control. Our findings demonstrate that maspin downregulated the basal and hypoxia-induced uPA/uPAR expression and reduced the stimulatory effect of hypoxia on the in vitro invasive ability of MDA-MB-231-cells. In addition, maspin also inhibited the enzymatic activity of secreted and cell associated uPA in MDA-MB-231 cells. These results indicate that maspin inhibits hypoxia-induced invasion of metastatic breast cancer cells by blocking the uPA system, thus illuminating an important molecular pathway for therapeutic consideration.

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.

Inhibition of the invasion capacity of carcinoma cells by WX-UK1, a novel synthetic inhibitor of the urokinase-type plasminogen activator system

The overall survival rate of patients suffering from carcinomas has remained poor and nearly unchanged over the last decades. This is mainly due to the so-called minimal residual disease, i.e., remaining tumor cells that overcome surgery and/or radiotherapy and are the cause of locoregional and distant metastases. To metastasize, tumor cells take advantage of proteases to invade and remodel surrounding tissues. Here, we analyzed the efficiency of WX-UK1, a novel 3-amidinophenylalanine-based inhibitor of the uPA system, at inhibiting the invasive capacity of carcinoma cells. First, uPAR expression was characterized in different carcinoma cell lines, including SCCHN, breast and cervical carcinoma. Thereafter, the invasive potential of these cell lines was determined using Matrigel invasion chambers and a spheroid cocultivation model with human fibroblasts. uPAR expression levels correlated positively with invasion capacity, which could be significantly inhibited by WX-UK1. A decrease of tumor cell invasion by up to 50% was achieved in both models with the SCCHN line FaDu and the cervical carcinoma line HeLa after treatment with WX-UK1. Thus, our results demonstrate the potential of WX-UK1 in vitro as a promising adjuvant antimetastatic therapy of carcinomas.

Urokinase plasminogen activator system: a multifunctional role in tumor progression and metastasis

The urokinase plasminogen activator (uPA) system is central to a spectrum of biologic processes including fibrinoloysis, inflammation, atherosclerotic plaque formation, matrix remodeling during wound healing, tumor invasion, angiogenesis, and metastasis. Binding of uPA with its receptor (uPAR) initiates a proteolytic cascade that results in the conversion of plasminogen to plasmin. Plasmin through its own proteolytic function degrades a range of extracellular basement membrane components and activates others such as the metalloproteinases. Independent of catalytic activity, uPAR also is involved in cell signaling, interactions with integrins, cell motility, adhesion and invasion, and angiogenesis. Over expression of uPA or uPAR is a feature of malignancy and is correlated with tumor progression and metastasis. In contrast, inhibition of expression of these components leads to a reduction in the invasive and metastatic capacity of many tumors. Strategies that target uPA or its receptor with the aim of disrupting the interaction between the two or the ligand independent actions of uPAR include antisense technology, monoclonal antibodies, cytotoxic antibiotics, and synthetic inhibitors of uPA. Targeted therapy is a goal of future cancer treatment and the uPA system is a likely candidate for manipulation.

The kringle stabilizes urokinase binding to the urokinase receptor

The structural basis of the interaction between single-chain urokinase-type plasminogen activator (scuPA) and its receptor (uPAR) is incompletely defined. Several observations indicated the kringle facilitates the binding of uPA to uPAR. A scuPA variant lacking the kringle (Delta K-scuPA) bound to soluble uPAR (suPAR) with the similar "on-rate" but with a faster "off-rate" than wild-type (WT)-scuPA. Binding of Delta K-scuPA, but not WT-scuPA, to suPAR was comparably inhibited by its growth factor domain (GFD) and amino-terminal fragment (ATF). ATF and WT-scuPA, but not GFD, scuPA lacking the GFD (Delta GFD-scuPA), or Delta K-scuPA reconstituted the isolated domains of uPAR. ATF completely inhibited the enzymatic activity of WT-scuPA-suPAR unlike comparable concentrations of GFD. Variants containing mutations that alter the charge, length, or flexibility of linker sequence (residues 43-49) between the GFD and the kringle displayed a lower affinity for uPAR, were unable to reconstitute uPAR domains, and their binding to uPAR was inhibited by GFD in the same manner as Delta K-scuPA. A scuPA variant in which the charged amino acids in the heparin binding site (HBS) in the kringle domain were mutated to alanines behaved like Delta K-scuPA, indicating that that the structure of the kringle as well as its interaction with the GFD govern receptor binding. These data demonstrate an important role for the kringle in stabilizing the binding of scuPA to uPAR.

Inhibition of the tumor-associated urokinase-type plasminogen activation system: effects of high-level synthesis of soluble urokinase receptor in ovarian and breast cancer cells in vitro and in vivo

Tumor cell invasion and metastasis depend on the coordinated and temporal expression of proteolytic enzymes to degrade the surrounding extracellular matrix and of adhesion molecules to remodel cell-cell and/or cell-matrix attachments. The tumor cell-associated urokinase-type plasminogen activator system, consisting of the serine protease uPA, its substrate plasminogen, its membrane-bound receptor uPAR, as well as its inhibitors PAI-1 and PAI-2, plays an important role in these pericellular processes. Especially, association of the proteolytic activity of uPA with the cell surface via interaction with uPAR significantly increases the invasive capacity of tumor cells. Consequently, various approaches have been pursued to interfere with the expression or activity of uPA and/or uPAR, including antisense strategies and the development of active-site inhibitors of uPA or inhibitors of uPA/uPAR interaction. In this review, we focus on the results obtained in vitro and in vivo with tumor cells producing high levels of a recombinant soluble form of uPAR, which efficiently inhibits uPA binding to cell surface-associated uPAR and, by this, acts as a scavenger for uPA.

The urokinase receptor (uPAR, CD87) as a target for tumor therapy: uPA-silica particles (SP-uPA) as a new tool for assessing synthetic peptides to interfere with uPA/uPA-receptor interaction

Many different processes in the physiology and pathophysiology of human beings are regulated protein/protein interactions such as receptor/ligand interactions. A more detailed knowledge of the nature of receptor/ligand binding sites and mechanisms of interaction is necessary as well in order to understand the process of cancer spread and metastasis. For instance, the cell surface receptor uPAR (CD87) and its ligand, the serine protease urokinase-type plasminogen activator (uPA), facilitate tumor invasion and metastasis in solid malignant tumors. Besides its proteolytic function in activating the zymogen plasminogen into the serine protease plasmin, binding of uPA to tumor cell-associated uPAR initiates various cell responses such as tumor cell migration, adhesion, proliferation, and differentiation. Hence, the tumor-associated uPA/uPAR system is considered a potential target for cancer therapy. Here we briefly describe a new technology using micro-silica particles coated with uPA (yields SP-uPA) and reaction of SP-uPA with recombinant soluble uPAR (suPAR) to test the competitive antagonistic potential of synthetic uPA peptides by flow cytofluorometry (FACS). We discuss the data obtained with the SP-uPA system from two different points of view: (1) The enhanced potential of improved uPA-derived synthetic peptides compared to previously described peptides, and (2) comparison of the new technique to other test systems currently used to identify uPA/uPAR or other protein/protein interactions.

Peptides with anticancer use or potential

This review is an attempt to illustrate the diversity of peptides reported for a potential or an established use in cancer therapy. With 612 references, this work aims at covering the patents and publications up to year 2000 with many inroads in years 2001-2002. The peptides are classed according to four categories of effective (or plausible) biological mechanisms of action: receptor-interacting compounds; inhibitors of protein-protein interaction; enzymes inhibitors; nucleic acid-interacting compounds. The fifth group is made of the peptides for which no mechanism of action has been found yet. Incidentally this work provides an overview of many of the modern targets of anticancer research.

Suppression of invasion and peritoneal carcinomatosis of ovarian cancer cell line by overexpression of bikunin

Bikunin (bik), a Kunitz-type protease inhibitor, also known as urinary trypsin inhibitor, is proposed as a main participant in the inhibition of tumor cell invasion and metastasis, possibly through the direct inhibition of cell-associated plasmin activity and suppression of urokinase-type plasminogen activator (uPA) mRNA expression. In the present study, we transfected the human ovarian carcinoma cell line HRA, highly invasive cells, with an expression vector harboring a cDNA encoding for human bik. Our study was designed to investigate the effect of bik overexpression and changes in tumor cell phenotype and invasiveness in the stably transfected clones. Bik gene transfection of HRA gave the following results: 1) transfection of HRA with the bik cDNA resulted in 5 variants stably expressing functional bik; 2) bik(+) clones exhibited a significantly reduced uPA mRNA expression as compared to the parental cells; 3) bikunin negatively regulates the ERK1/2 activity; 4) secretion-blocking treatments of bik(+) clones abrogated bik-mediated suppression of ERK1/2 activation and uPA expression; 5) the regulation of invasion seen in the HRA cells is mainly mediated by the uPA-plasmin-MMP-2 system; 6) transfection of HRA with the bik gene significantly reduced invasion, but not proliferation, adhesion, or migration relative to the parental cells; and 7) animals with bik(+) clones induced reduced peritoneal dissemination and long term survival. We conclude that transfection of HRA cells with the bik cDNA constitutively suppresses ERK1/2 activation, which results in inhibition of uPA expression and subsequently reduces dissemination of bik(+) clones.

Modulation of invasive properties of human glioblastoma cells stably expressing amino-terminal fragment of urokinase-type plasminogen activator

The binding of urokinase-type plasminogen activator (uPA) to its receptor (uPAR) on the surface of tumor cells is involved in the activation of proteolytic cascades responsible for the invasiveness of those cells. The diffuse, extensive infiltration of glioblastomas into the surrounding normal brain tissue is believed to rely on modifications of the proteolysis of extracellular matrix components; blocking the interaction between uPA and uPAR might be a suitable approach for inhibiting glioma tumorigenesis. We assessed how expression of an amino-terminal fragment (ATF) of uPA that contains binding site to uPAR affects the invasiveness of SNB19 human glioblastoma cells. SNB19 cells were transfected with an expression plasmid (pcDNA3-ATF) containing a cDNA sequence of ATF-uPA. The resulting ATF-uPA-expressing clones showed markedly less cell adhesion, spreading, and clonogenicity than did control cells. Endogenous ATF expression also significantly decreased the invasive capacity of transfected glioblastoma cells in Matrigel and spheroid-rat brain cell aggregate models. ATF-uPA transfectants were also markedly less invasive than parental SNB19 cells after injection into the brains of nude mice, suggesting that competitive inhibition of the uPA-uPAR interaction on SNB19 cells by means of transfection with ATF cDNA could be a useful therapeutic strategy for inhibiting tumor progression.

Synthesis, solution structure, and biological evaluation of urokinase type plasminogen activator (uPA)-derived receptor binding domain mimetics

Tumor cell migration and metastasis in cancer are facilitated by interaction of the serine protease urokinase type plasminogen activator (uPA) with its receptor uPAR (CD 87). Overexpression of uPA and uPAR in cancer tissues is associated with a high incidence of disease recurrence and early death. In agreement with these findings, disruption of the protein-protein interaction between uPAR present on tumor cells and its ligand uPA evolved as an attractive intervention strategy to impair tumor growth and metastasis. For this, the uPAR antagonist cyclo[19,31][D-Cys(19)]-uPA(19)(-)(31) was optimized to efficiently interrupt binding of uPA to cellular uPAR. First, the disulfide bridge of this lead compound was shifted and then the modified peptide was shortened from the amino and carboxy terminus to generate cyclo[21,29][Cys(21,29)]-uPA(21)(-)(30). Next, cyclo[21,29][D-Cys(21)Cys(29)]-uPA(21)(-)(30) was yielded by changing the chirality of Cys(21) to D-Cys(21). For analysis of uPAR binding activity, we employed competitive flow cytofluorometric receptor binding assays, using FITC-uPA as the ligand and U937 promyeloid leukemia cells as the cellular source of uPAR. As demonstrated for cyclo[21,29][D-Cys(21)Cys(29)]-uPA(21)(-)(30), the achieved peptide modifications maintained receptor binding activity (IC(50) = 0.04 microM), which is close in order to that of the parent protein ligand, uPA (IC(50) = 0.01 microM). A detailed NMR analysis with restrained and free molecular dynamics calculations in explicit H(2)O exhibits a well-defined structure with characteristic features such as an omega-loop with two betaI-turns about Lys(3), Tyr(4), Ser(6), and Asn(7). Hydrophobic clustering of the side chains of Tyr(4), Phe(5), Ile(8), and Trp(10) is observed. Side chain mobility is analyzed with time-dependent distance restraints. The NMR structure of cyclo[21,29][D-Cys(21)Cys(29)]-uPA(21)(-)(30) is very similar to the previously reported structure of the amino terminal fragment of uPA. Systematic point mutations led to cyclo[21,29][D-Cys(21)Nle(23)Cys(29)]-uPA(21)(-)(30), which still binds to uPAR but is resistant to proteolytic cleavage, e.g., by the tumor-associated serine proteases uPA and plasmin, and is stable in blood serum or plasma. In conclusion, small cyclic peptides were created, which mimic the structure and activity of the binding epitope of uPA to uPAR and which may serve as novel therapeutic agents in cancer metastasis.

Flow cytometric analysis of ligand-receptor interactions and molecular assemblies

Flow cytometers make homogeneous real-time measurements of ligand-receptor interactions and, simultaneously, the physiological responses of cells. Their multiparameter capabilities are also useful in resolving multicomponent assemblies or in developing multiplexed assays. Recent advances suggest that these approaches can be extended in several important ways. Sample delivery in the millisecond time domain is applicable to the analysis of complex binding kinetics and reaction mechanisms. The homogeneous discrimination of free components and particle-based assemblies can be extended into the micromolar concentration range. Measurements can be made of molecular assemblies among proteins, DNA, RNA, lipids, and carbohydrates on beads. The topography and assembly of components within cells can be evaluated with resonance energy transfer. Temperature dependence can be evaluated with Peltier temperature control. Many assembly endpoints can be assessed through new tools for high-throughput flow cytometry using plate-based assay formats and small volume samples.

Angiotensin II inhibits human trophoblast invasion through AT1 receptor activation

Trophoblast implantation depends, in part, on the controlled production of plasmin from plasminogen, a process regulated by plasminogen activators and plasminogen activator inhibitors. We have determined that angiotensin II (Ang II) stimulates plasminogen activator inhibitor-1 (PAI-1) synthesis and secretion in human trophoblasts in a time- and concentration-dependent manner. Our results indicate that Ang II activates PAI-1 gene expression through the AT1 receptor and involves the calcium-dependent activation of calcineurin and the nuclear translocation of NFAT. Increased PAI-1 synthesis and secretion is associated with reduced trophoblast invasion as judged by an in vitro invasion assay. These studies are the first to link the renin-angiotensin system with the fibrinolytic system to regulate trophoblast invasion.

uPA-silica-Particles (SP-uPA): a novel analytical system to investigate uPA-uPAR interaction and to test synthetic uPAR antagonists as potential cancer therapeutics

The urokinase-type plasminogen activation system, including the serine protease uPA (urokinase-type plasminogen activator) and its cell surface receptor (uPAR, CD87), are important key molecules in tumor invasion and metastasis. Besides its proteolytic function, binding of uPA to uPAR on tumor cells exerts various cell responses such as migration, adhesion, proliferation, and differentiation. Hence, the uPA/uPAR system is a potential target for tumor therapy. We have designed a new generation of uPA-derived synthetic cyclic peptides suited to interfere with the binding of uPA to uPAR and present a new technology involving micro silica particles coated with uPA (SP-uPA) and reacting with recombinant soluble uPAR (suPAR), to rapidly assess the antagonistic potential of uPA-peptides by flow cytofluorometry (FACS). For this, we used silica particles of 10 microm in diameter to which HMW-uPA is coupled using the EDC/NHS method. Soluble, recombinant suPAR was added and the interaction of SP-uPA with suPAR verified by reaction with monoclonal antibody HD13.1 directed to uPAR, followed by a cyan dye (cy5)-labeled antibody directed against mouse IgG. Thereby it was possible to test naturally occurring ligands of uPAR (HMW-uPA, ATF) as well as highly effective, synthetic cyclic uPA-derived peptides (cyclo21,29[D-Cys21Cys29]-UPA21-30, cyclo21,29[D-Cys21Nle28Cys29]-uPA21-30, cyclo21,29[D-Cys(21)2-Nal24Cys29]-uPA21-30, and cyclo21,29[D-Cys21Orn23Thi24Thi25Cys29]-uPA21-30. The results obtained with the noncellular SP-uPA/uPAR system are highly comparable to those obtained with a cellular system involving FITC-uPA and the promyeloid cell line U937 as the source of uPAR.

Cyclo19,31[D-Cys19]-uPA19-31 is a potent competitive antagonist of the interaction of urokinase-type plasminogen activator with its receptor (CD87)

Urokinase-type plasminogen activator (uPA) represents a central molecule in pericellular proteolysis and is implicated in a variety of physiological and pathophysiological processes such as tissue remodelling, wound healing, tumor invasion, and metastasis. uPA binds with high affinity to a specific cell surface receptor, uPAR (CD87), via a well defined sequence within the N-terminal region of uPA (uPA19-31). This interaction directs the proteolytic activity of uPA to the cell surface which represents an important step in tumor cell proliferation, invasion, and metastasis. Due to its fundamental role in these processes, the uPA/uPAR-system has emerged as a novel target for tumor therapy. Previously, we have identified a synthetic, cyclic, uPA-derived peptide, cyclo19,31uPA19-31, as a lead structure for the development of low molecular weight uPA-analogues, capable of blocking uPA/uPAR-interaction [Burgle et al., Biol. Chem. 378 (1997), 231-237]. We now searched for peptide variants of cyclo19,31uPA19-31 with elevated affinities for uPAR binding. Among other tasks, we performed a systematic D-amino acid scan of uPA19-31, in which each of the 13 L-amino acids was individually substituted by the corresponding D-amino acid. This led to the identification of cyclo19,31[D-Cys19]-uPA19-31 as a potent inhibitor of uPA/uPAR-interaction, displaying only a 20 to 40-fold lower binding capacity as compared to the naturally occurring uPAR-ligands uPA and its amino-terminal fragment. Cyclo19,31[D-Cys19]-uPA19-31 not only blocks binding of uPA to uPAR but is also capable of efficiently displacing uPAR-bound uPA from the cell surface and to inhibit uPA-mediated, tumor cell-associated plasminogen activation and fibrin degradation. Thus, cyclo19,31[D-Cys19]-uPA19-31 represents a promising therapeutic agent to significantly affect the tumor-associated uPA/uPAR-system.

Interference with the urokinase plasminogen activator system: a promising therapy concept for solid tumours

There is abundant evidence that the plasminogen activator (PA) system with its key components uPA (urokinase-type plasminogen activator), its cell surface receptor uPA-R (CD87) and its inhibitor PAI-1 plays a key role in tumour invasion and metastasis. Elevated levels of these factors in tumour tissue are associated with tumour aggressiveness and poor patient outcome. Animal models suggest that the PA system is not essential for fertility or survival under physiological conditions. Thus, it seems well suited as a therapeutic target for patients with solid malignant tumours. Novel therapy concepts targeting the uPA system are currently being explored. A variety of different synthetic uPA inhibitor classes have been developed over the last decades. First generation inhibitors displayed a low uPA inhibitory potency combined with broad specificity. More recently, structure based design, x-ray crystallographic screening or NMR based screening have revealed a large number of new, potent and selective uPA-inhibitors. A few modern compounds have shown promising results in preclinical testing and are now ready for Phase I clinical studies. Other therapeutic strategies such as antagonists of uPA/uPA-R interaction or gene therapeutic approaches to suppress the uPA-system are still being evaluated in in vitro and in vivo models. For clinical application, a combination therapy targeting more than one of the interacting proteolytic pathways may be required for effective antiproteolytic therapy. In addition, antiproteolytic agents may provide additive or synergistic treatment benefits if used in combination together with conventional therapeutics, in particular in those solid tumours for which potent conventional regimens already exist.

The urokinase plasminogen activator receptor (uPAR) as a target for the diagnosis and therapy of cancer

The identification and characterization of validated molecular targets for cancer drug and diagnostic development is rapidly changing the way that promising new anti-cancer compounds are developed and evaluated. A significant body of in vitro and in vivo data has established the urokinase plasminogen activator (uPA) system as a promising target for cancer drug development. The uPA system has been demonstrated to have pleiotropic activities in the development of tumors, and in tumor progression and angiogenesis. There are multiple ways to target this system, the most straightforward being the development of small molecule active site inhibitors of the serine protease, uPA. However, compounds of this type have not entered into clinical trials, and issues related to selectivity and specificity of this class of inhibitors have yet to be satisfactorily resolved. Recent evidence suggests that in addition to uPA, its specific cell surface receptor (uPAR) may also be a suitable target for the design and development of cancer therapeutic and diagnostic agents. uPAR is central to several pathways implicated in tumor progression and angiogenesis. The binding of the uPA zymogen (scuPA) to uPAR appears to be a pre-requisite for efficient cell-surface activation of scuPA to the active two-chain form (tcuPA) by plasmin, and simple ligand occupancy of uPAR by scuPA initiates various signaling pathways leading to alterations in cell motility and adhesion. One therapeutic rationale that is currently being investigated is the simple displacement of scuPA or tcuPA from suPAR, which may effectively inhibit both the proteolytic and signal-transducing cascades. In addition, other approaches to the modulation of the activity of this system that may also be useful include blocking the interaction of uPAR with integrins and extracellular matrix proteins as well as strategies to down-regulate the expression of uPA and uPAR in target cells. This review will summarize these approaches, and also describe the targeting of uPAR for diagnosis and imaging.

Upregulation of C4.4A expression during progression of melanoma

We have previously described the human homolog of a rat metastasis-associated molecule, hC4.4 A, with a weak homology to the urokinase-type plasminogen activator receptor. By the restricted expression in nontransformed tissues as opposed to expression in roughly 50% of a variety of carcinoma lines of different origins, a possible correlation between hC4.4 A and tumor progression emerged. This was explored in more detail in melanoma by quantitative polymerase chain reaction and in situ hybridization. As shown before, normal human skin weakly expresses hC4.4 A. Melanocytes and nevi are negative, but up to 60% of primary malignant melanoma and 100% of lymph node and skin metastases of melanoma are hC4.4 A positive. Signal intensity in both polymerase chain reaction and in situ hybridization varied considerably between individual samples, which is indicative for regulated expression of hC4.4 A. To test the hypothesis, melanoma lines were incubated with human serum. Whereas expression of hC4.4 was not influenced by heat-inactivated human serum, all melanoma lines responded to noninactivated human serum with upregulation of hC4.4 A expression. Regulated expression with highest level expression on metastases is a feature that hC4.4 A shares with the urokinase-type plasminogen activator receptor. This feature points towards functional activity of hC4.4 A in tumor progression.

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

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

Cloning of the human homologue of the metastasis-associated rat C4.4A

We have previously described a rat metastasis-associated molecule, C4.4A, which has some common features with the uPAR. Because of its restricted expression in non-transformed tissues a search for the human homologue became of interest. Human C4.4A was cloned from a placental cDNA library. As in the rat, the human uPAR and the human C4.4A genes appear to belong to the same family. Both genes are located on chromosome 19q13.1-q13.2 and both molecules have a glycolipid anchor site and are composed of three extracellular domains. Only domains one and two of the human C4.4A and the uPAR protein show a significant degree of identity. Expression of the human C4.4A was observed by RT-PCR and Northern blotting in placental tissue, skin, esophagus and peripheral blood leukocytes, but not in brain, lung, liver, kidney, stomach, colon and lymphoid organs. Yet, tumors derived from the latter tissues frequently contained C4.4A mRNA. As demonstrated for malignant melanoma, C4.4A mRNA expression correlated with tumor progression. While nevi were negative and only a minority of primary malignant melanoma expressed C4.4A, all metastases were C4.4A-positive. Taking into account the high degree of homology between rat and human C4.4A, the conformity of the expression profiles and the association of rat C4.4A with tumor progression, human C4.4A might well become a prognostic marker and possibly a target of therapy.

A novel type of bifunctional inhibitor directed against proteolytic activity and receptor/ligand interaction. Cystatin with a urokinase receptor binding site

Cancer invasion and metastasis is a process requiring a coordinated series of (anti-)adhesive, migratory, and pericellular proteolytic events involving various proteases such as urokinase-type plasminogen activator (uPA)/plasmin, cathepsins B and L, and matrix metalloproteases. Novel types of double-headed inhibitors directed to different tumor-associated proteolytic systems were generated by substitution of a loop in chicken cystatin, which is nonessential for cysteine protease inhibition, with uPA-derived peptides covering the human uPA receptor binding sequence uPA-(19-31). The inhibition constants of these hybrids toward cysteine proteases are similar to those of wild-type cystatin (K(i), papain (pm), 1.9-2.4; K(i), cathepsin B (nm), 1.0-1.7; K(i), cathepsin L (pm), 0.12-0.61). FACS analyses revealed that the hybrids compete for binding of uPA to the cell surface-associated uPA receptor (uPAR) expressed on human U937 cells. The simultaneous interaction of the hybrid molecules with papain and uPAR was analyzed by surface plasmon resonance. The measured K(D) value of a papain-bound cystatin variant harboring the uPAR binding sequence of uPA (chCys-uPA-(19-31)) and soluble uPAR was 17 nm (K(D) value for uPA/uPAR interaction, 5 nm). These results indicate that cystatins with a uPAR binding site are efficient inhibitors of cysteine proteases and uPA/uPAR interaction at the same time. Therefore, these compact and small bifunctional inhibitors may represent promising agents for the therapy of solid tumors.

Crystals of the urokinase type plasminogen activator variant beta(c)-uPAin complex with small molecule inhibitors open the way towards structure-based drug design

Urokinase is a serine protease involved in cancer growth and metastasis. Here we present the first urokinase crystal structure in complex with reversible inhibitors at 2.1 and 2.6 A resolution. These inhibitor complex structures have been obtained from crystals of engineered urokinase type plasminogen activator designed to obtain a crystal form open for inhibitor soaking. The mutant C122S loses its flexible A-chain upon activation cleavage and crystallizes in the presence of benzamidine, which was later displaced by the desired inhibitor. This new soakable crystal form turned out to be of great value in the process of structure-based drug design. The evaluated binding mode of amiloride, and UKI-1D revealed a new subsite of the primary specificity pocket of urokinase that will be employed in the future ligand optimisation process.

Urokinase induces proliferation of human ovarian cancer cells: characterization of structural elements required for growth factor function

Ovarian cancer metastasis is associated with an increase in the urokinase-type plasminogen activator (uPA) and its receptor uPAR. We present evidence that binding of uPA to uPAR provokes a mitogenic response in the human ovarian cancer cell line OV-MZ-6 in which endogenous uPA production had been significantly reduced by stable uPA 'antisense' transfection. High molecular weight (HMW) uPA, independent of its enzymatic activity, produced an up to 95% increase in cell number concomitant with 2-fold elevated [3H]thymidine incorporation as did the catalytically inactive but uPAR binding amino-terminal fragment of uPA, ATF. uPA-induced cell proliferation was significantly decreased by blocking uPA/uPAR interaction by the monoclonal antibody IIIF10 and by soluble uPAR. The efficiency of the uPAR binding synthetic peptide cyclo19,31 uPA19-31 to enhance OV-MZ-6 cell growth proved this molecular domain to be the minimal structural determinant for uPA mitogenic activity. Dependence of uPA-provoked cell proliferation on uPAR was further demonstrated in Raji cells which do not express uPAR and were thus not induced by uPA. However, upon transfection with full-length uPAR, Raji cells acquired a significant growth response to HMW uPA and ATF.