Urokinase-type plasminogen activator (uPA) and its receptor (uPAR): development of antagonists of uPA/uPAR interaction and their effects in vitro and in vivo

In cancer, increased levels of the tumor-associated serine protease uPA (urokinase-type plasminogen activator) and its receptor uPAR (CD87) are linked to tumor progression, metastasis, and shortened survival in patients afflicted with this disease. Strong clinical and experimental evidence has accumulated that the cell surface interaction of uPA with uPAR facilitates extravasation and intravasation of cancer cells by regulating local proteolysis and attachment of the cells to components of the extracellular matrix. Moreover, the uPA/uPAR system is also implicated in proliferation of some tumor cells and migration of tumor and endothelial cells. Thus, metastasis formation is facilitated via tumor cell spread through the blood circulation system and neovascularization at the metastatic site. This multifunctional potential has rendered the uPA/uPAR system an attractive novel target for anti-metastatic therapy. Consequently, inhibitors of the uPA/uPAR interaction have been and are currently developed for suppression of tumor growth and angiogenesis. In addition to antibodies and recombinant uPA- or uPAR-derived proteins, various linear and cyclic peptides as well as small molecules have been designed and synthesized which potently interfere with the uPA/uPAR interaction, leading to reduced tumor progression in experimental animals. Such compounds affecting the uPA/uPAR system represent novel tumor biology-based therapeutic agents, thereby opening new ways for patient optimized and individualized cancer therapy.     

The urokinase-type plasminogen activator (uPA) system as a biomarker and therapeutic target in human malignancies

Introduction: The urokinase plasminogen activator (uPA) system, comprising the serine protease uPA, its cognate receptor, uPAR, and two endogenous inhibitors, plasminogen activator inhibitor-1 (PAI-1) and plasminogen activator inhibitor-2 (PAI-2), is a key player in the break-down of extracellular matrix (ECM) and basement membrane. Elevated expression of uPA and uPAR is observed in numerous cancer types and associated with poor prognosis.

Areas covered: In addition to the aberrant expression during tumor development, the components of uPA system are functionally involved in various processes that are prerequisite for cancer progression. These processes include, but not limited to, ECM degradation, angiogenesis, cell proliferation, adhesion, migration and epithelial–mesenchymal transition. All of these findings implicate uPA system as a target for cancer treatment. Thus, therapeutic agents and approaches to targeting the constituents of uPA system, mainly at their expression level and biological activities, have been extensively used in antineoplastic investigations.

Expert opinion: Because of promising results obtained from previous preclinical studies, several clinical trials aimed at inhibiting the expression or function of uPA/uPAR have been completed or are ongoing. In these trials, favorable outcomes in reducing metastatic spread and extending the lifespan of cancer patients have been reported, and no severe adverse events were observed.     

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.     

uPAR as anti-cancer target: evaluation of biomarker potential, histological localization, and antibody-based therapy

Degradation of proteins in the extracellular matrix is crucial for the multistep process of cancer invasion and metastasis. Compelling evidence has demonstrated the urokinase receptor (uPAR) and its cognate ligand, the urokinase plasminogen activator (uPA), to play critical roles in the concerted action of several proteolytic systems in generation of a high proteolytic potential required for tissue remodeling processes. uPAR is additionally cleaved by uPA on the cell surface, liberating domain I, resulting in abrogated pericellular proteolysis. The expression of both uPAR and uPA is significantly up-regulated during cancer progression and is primarily confined to the tumor-associated stromal compartment. Furthermore, both uPAR and uPA have proven to be prognostic markers in several types of cancer; high levels indicating poor survival. The cleaved forms of uPAR are also prognostic markers, and a potential diagnostic and predictive impact of the different uPAR forms has been reported. Hence, pericellular proteolysis seems to be a suitable target for anti-cancer therapy and numerous approaches have been pursued. Targeting of this process may be achieved by preventing the binding of uPA to uPAR on the cell surface and/or by direct inhibition of the catalytic activity of uPA. Both strategies have been pursued and inhibition of these functions has shown effect in xenogenic cancer models. Pericellular proteolysis has also been inhibited in vivo in mouse models of wound healing and hepatic fibrinolysis using mouse monoclonal antibodies (mAbs) against mouse uPA or uPAR. These reagents will target uPA and uPAR in both stromal cells and cancer cells, and their therapeutic potential can now be assessed in syngenic mouse cancer models.     

尿激酶型纤溶酶原激活剂及其受体在胰腺癌中表达的意义

目的探讨尿激酶型纤溶酶原激活剂(urokinaseplasminogenactivator,uPA)、尿激酶型纤溶酶原激活剂受体(urokinaseplasminogenactivatorreceptor,uPAR)在胰腺癌中表达的意义。方法用反转录聚合酶链反应(RT鄄PCR)和免疫组织化学方法分别检测uPA、uPAR在85例胰腺癌和10例胰腺炎组织中mRNA和蛋白抗原的表达。HPIAS图像分析各组标本相对密度,t检验各组相对密度差异,字2检验uPA、uPAR与胰腺癌肿瘤临床病理之间的相关性。结果uPA、uPARmRNA和蛋白抗原在胰腺炎组织中低表达,胰腺癌组织中uPA、uPARmRNA和蛋白抗原的高表达,胰腺癌中伴有转移者uPA、uPAR的mRNA和蛋白抗原的表达明显高于无转移者。uPA、uPAR的表达与胰腺癌的转移程度高度相关。结论uPA、uPAR是反映胰腺肿瘤进展和生物学特性的指标,是临床上判断预后的良好指标之一     

Inhibition of protein synthesis by chromium(VI) differentially affects expression of urokinase and its receptor in human type II pneumocytes.

Exposure to chromium(VI) increases the incidence of cancer, respiratory distress, and pulmonary fibrosis. The latter is a pathological disorder characterized by decreased urokinase-type plasminogen activator (uPA) activity and fibrinolysis. In this study, treatment of alveolar type II cells (A549) with 1 to 5 microM chromium(VI) for 4 and 12 h decreased both the specific activity and the amount of uPA protein. Chromium reduced uPA protein levels by inhibiting protein synthesis and had no effect on uPA mRNA levels or the rate of uPA protein degradation. In contrast, both mRNA and protein levels for the uPA receptor (uPAR) were increased by treatment with concentrations of chromium(VI) that did not completely inhibit protein synthesis. The chromium-induced increase in uPAR resulted from increased message stability. These data indicate that chromium has differential effects on expression of the proteins in the pulmonary fibrinolytic cascade. The net loss of uPA activity may promote fibrosis following inhalation of chromium(VI).     

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.     

Structural basis for therapeutic intervention of uPA/uPAR system

Urokinase-type plasminogen activator (uPA) is one of the two physiological serine proteases responsible for the activation of plasminongen to plasmin. uPA activity is regulated by its inhibitors (PAI-1 and PAI-2) and its receptor (uPAR), and an expanding list of their interacting proteins. In addition to plasminogen activation, this system also plays important roles in the regulation of many cellular processes including cell proliferation, adhesion and migration. It is beyond reasonable doubt that this enzyme system plays a central role in tumor biology and represents a high potential target for therapeutic intervention of tumor growth and metastasis. During the past fifteen years, crystal structures of uPA and its inhibitors have facilitated the development of uPA inhibitors. Many crystal structures of proteins in the uPA/uPAR system have also been reported recently, especially a series of structures of uPAR and its complexes with vitronectin and uPA, facilitating the development and evaluation of uPAR inhibitors. Recent progress on uPA inhibitors will be summarized in this article. The unique structural features and the druggable potentials of these new structures will also be discussed.     

Clinical applications of the urokinase receptor (uPAR) for cancer patients

Since decades the urokinase plasminogen activator (uPA) system has been associated with the invasion of malignant cells. The receptor of urokinase (uPAR) is one of the key players in this proteolytic cascade, because it focuses uPA's proteolytic activity to the cell surface and in addition functions as a signaling receptor. uPAR is highly expressed in virtually all human cancers, suggesting possible clinical applications as diagnostic marker, predictive tool of survival or clinical response, and as a target for therapy and imaging. This review summarizes the possibilities of uPAR in clinical applications for cancer patients.     

Urokinase receptor antagonists: novel agents for the treatment of cancer

The interaction between the urokinase receptor (uPAR) and its ligand urokinase (uPA) mediates phenomena such as tissue remodelling, chemotaxis, tumour invasion, dissemination, proliferation, and angiogenesis. The broad-spectrum of biological processes that the uPA/uPAR interaction plays a role in has led researchers to speculate that this interaction may be a useful molecular target for therapeutic intervention in several pathological conditions, particularly in the prevention and inhibition of the dissemination of cancer cells. In syngeneic and xenograft murine tumour models, in which metastasis is driven by the uPA/uPAR interaction, inhibition of primary tumour growth, metastasis and angiogenesis has been shown with several proteins acting as uPAR antagonists. Immunohistochemistry, in conjunction with prognostic studies, has implicated the uPA/uPAR interaction in the dissemination of tumours, such as malignant melanoma, colon cancer, non-small cell lung cancer (NSCLC) and stomach cancer, as well as breast and ovarian carcinomas. A potential inhibitor of the uPA/uPAR interaction should result in a significant increase in the disease-free interval and survival time following resection of the primary tumour in a clinical Minimal Residual Disease (MRD) setting. Low molecular weight uPAR antagonists should be orally active, and have few side-effects, excellent bioavailability, favourable pharmacokinetic properties and a long half-life. Furthermore, these compounds should be able to inhibit the dissemination of cancer cells without the need for targeted drug and vector delivery.     

Urokinase receptor antagonists: novel agents for the treatment of cancer

The interaction between the urokinase receptor (uPAR) and its ligand urokinase (uPA) mediates phenomena such as tissue remodelling, chemotaxis, tumour invasion, dissemination, proliferation, and angiogenesis. The broad-spectrum of biological processes that the uPA/uPAR interaction plays a role in has led researchers to speculate that this interaction may be a useful molecular target for therapeutic intervention in several pathological conditions, particularly in the prevention and inhibition of the dissemination of cancer cells. In syngeneic and xenograft murine tumour models, in which metastasis is driven by the uPA/uPAR interaction, inhibition of primary tumour growth, metastasis and angiogenesis has been shown with several proteins acting as uPAR antagonists. Immunohistochemistry, in conjunction with prognostic studies, has implicated the uPA/uPAR interaction in the dissemination of tumours, such as malignant melanoma, colon cancer, non-small cell lung cancer (NSCLC) and stomach cancer, as well as breast and ovarian carcinomas. A potential inhibitor of the uPA/uPAR interaction should result in a significant increase in the disease-free interval and survival time following resection of the primary tumour in a clinical Minimal Residual Disease (MRD) setting. Low molecular weight uPAR antagonists should be orally active, and have few side-effects, excellent bioavailability, favourable pharmacokinetic properties and a long half-life. Furthermore, these compounds should be able to inhibit the dissemination of cancer cells without the need for targeted drug and vector delivery.     

直肠癌组织uPA系统和VEGF表达与浸润转移的关系

目的研究尿激酶型纤溶酶原激活系统uPA,uPAR,PAI-1蛋白及血管内皮生长因子（VEGF）在直肠癌组织中的表达及其与直肠癌癌生物学行为的关系。方法2005年1月至2006年1月,采用免疫组化S-P法检测67例直肠癌及癌旁组织中uPA,uPAR,PAI-1及VEGF蛋白表达情况,同时对临床病理学资料进行回顾性分析。结果癌和癌周比较uPA,uPAR,PAI-1及VEGF蛋白阳性表达率明显升高（P〈0.01）,阳性产物主要分布在癌细胞胞质。侵及浆膜组阳性率明显高于未及浆膜组（P=0.000,0.014,0.016,0.000）。uPA,uPAR蛋白在淋巴结转移组中的阳性率明显高于无淋巴结转移组（P=0.002,0.008）。PAI-1蛋白在低分化组中的阳性率明显低于中分化和高分化组（P〈0.05）。uPA,uPAR及VEGF蛋白阳性率在直肠癌组织不同分化程度间差异无显著性。在直肠癌组织中uPA,uPAR蛋白的表达呈显著正相关（r=0.653,P〈0.001）。在uPA,uPAR蛋白同时阳性的41例病例中,有28例发生淋巴结转移,与阴性组（4/15）相比,转移发生率明显增高（P〈0.01）。直肠癌组织中uPA蛋白表达与VEGF表达显著正相关（r=0.300,P〈0.05）。PAI-1蛋白表达与VEGF表达具有极显著正相关性（r=0.413,P〈0.01）。应用COX回归模型进行多变量分析并采用向前逐步回归法,仅浸润深度、uPA表达和病理类型对淋巴结转移有影响。结论uPA,uPAR,PAI-1及VEGF蛋白在直肠癌组织中表达增强,uPA,uPAR与直肠癌浸润转移密切相关。     

The urokinase plasminogen activator system: role in malignancy

The urokinase plasminogen activator (uPA) system consists of the serine protease uPA, its glycolipid-anchored receptor, uPAR and its 2 serpin inhibitors, plasminogen activator inhibitor-1 (PAI-1) and plasminogen activator inhibitor-2 (PAI-2). Recent findings suggest that the uPA system is causally involved at multiple steps in cancer progression. In particular, uPA has been implicated in remodelling of the extracellular matrix, enhancing both cell proliferation and migration and modulating cell adhesion. Consistent with its role in cancer progression, multiple groups have shown that high levels of uPA in primary breast cancers are independently associated with adverse outcome. Paradoxically, high levels of PAI-1 also correlate with poor prognosis in patients with breast cancer. The prognostic value of uPA/PAI-1 in axillary node-negative breast cancer patients was recently validated using both a prospective randomised trial and a pooled analysis, i.e., in 2 different Level 1 Evidence studies. Assay of uPA and PAI-1 may thus help identify low risk node-negative patients for whom adjuvant chemotherapy is unnecessary. Finally, preclinical studies show that either inhibition of uPA catalytic activity or prevention of uPA binding to its receptor reduces tumor growth, angiogenesis and metastasis.     

Development of novel therapeutics targeting the urokinase plasminogen activator receptor (uPAR) and their translation toward the clinic

The urokinase plasminogen activator receptor (uPAR) mediates cell motility and tissue remodeling. Although uPAR may be expressed transiently in many tissues during development and wound healing, its constitutive expression appears to be associated with several pathological conditions, including cancer. uPAR expression has been demonstrated in most solid tumors and several hematologic malignancies including multiple myeloma and acute leukemias.Unlike many tumor antigens, uPAR is present not only in tumor cells but also in a number of tumor-associated cells including angiogenic endothelial cells and macrophages. The expression of uPAR has been shown to be fairly high in tumor compared to normal, quiescent tissues, which has led to uPAR being proposed as a therapeutic target, as well as a targeting agent, for the treatment of cancer. The majority of therapeutic approaches that have been investigated to date have focused on inhibiting the urokinase plasminogen activator (uPA)-uPAR interaction but these have not led to the development of a viable uPAR targeted clinical candidate. Genetic knockdown approaches e.g. siRNA, shRNA focused on decreasing uPAR expression have demonstrated robust antitumor activity in pre-clinical studies but have been hampered by the obstacles of stability and drug delivery that have limited the field of RNA nucleic acid based therapeutics. More recently, novel approaches that target interactions of uPAR that are downstream of uPA binding e.g. with integrins or that exploit observations describing the biology of uPAR such as mediating uPA internalization and signaling have generated novel uPAR targeted candidates that are now advancing towards clinic evaluation. This review will discuss some of the pitfalls that have delayed progress on uPAR-targeted interventions and will summarize recent progress in the development of uPAR-targeted therapeutics.     

The uPA/uPA receptor system as a target for tumor therapy

Invasiveness of a variety of tumors depends on the regulated expression of proteolytic enzymes that degrade the surrounding extracellular matrix and dissociate cell-cell and/or cell-matrix attachments. The tumor cell surface-associated urokinase-type plasminogen activator (uPA) system plays an especially important role in tumor cell invasion and metastasis. It consists of the serine protease uPA, its membrane-bound receptor (uPAR, CD87) and one of the natural inhibitors PAI-1 or PAI-2. There are strong indications based on animal experiments that interference with this system by inhibiting the enzymatic activity of uPA and/or antagonizing its binding to the receptor is of therapeutic relevance. With the recent solution of various X-ray structures of uPA/inhibitor complexes, structural information is available for optimizing existing lead compounds in their affinity and selectivity for uPA. Furthermore, peptide compounds capable of mimicking the structural epitope of uPA responsible for binding to the receptor efficiently antagonize this recognition process. Thus, both approaches prove to be well suited for the design of highly promising drugs in human medicine.     

Modulators of the urokinase-type plasminogen activation system for cancer

The urokinase-type plasminogen activator (uPA) is a serine protease which acts as an initiator of extracellular matrix degradation and cell invasion. Urokinase acts with its cell surface receptor, uPAR, to promote cell adhesion, migration and invasion in co-operation with a number of other key molecular players, including plasminogen activator inhibitor-1 and integrins. Work in pharmaceutical and biotechnology companies has focused on the development of inhibitors of the urokinase enzyme activity and agents which prevent binding of uPA to uPAR for treatment of cancer progression. The challenge with the former is to generate orally-bioavailable small molecule inhibitors of sufficient selectivity with respect to other members of the trypsin family of serine proteases, especially tissue-type plasminogen activator and Factor Xa. A total of 14 patents have been published in this area from nine different companies over the last three years. With respect to inhibitors of the uPA:uPAR interaction the challenges have been to identify small molecules with significant potency and appropriate physicochemical properties. Most of the activity in this area has been with peptidic inhibitors. Six different companies have disclosed a total of 13 patents in this area. There are no announced clinical candidates in this field to date, although given the number of companies involved, this is likely to change in the near future.     

Molecular interactions involving urokinase plasminogen activator (uPA), its receptor (uPAR) and its inhibitor,plasminogen activator inhibitor-1 (PAI-1), as new targets for tumour therapy

In the promotion of cancer progression, a classical role had previously been ascribed to the plasminogen activation system on the basis of urokinase plasminogen activator (uPA) proteolytic activity and plasminogen activation triggering a focalised pericellular activation cascade involving matrix metalloproteinases (MMPs). As a result, many pharmaceutical companies have undertaken the development of synthetic uPA inhibitors. However, during the last few years, data have accumulated that uPA, as well as urokinase-type plasminogen activator receptor (uPAR) and plasminogen activator inhibitor-1 (PAI-1), are likely to play an essential role in tumour progression through non-proteolysis-related activities. Such activities endow them with new and likely key functions in tumour progression-associated events, such as cellular adhesion, migration, invasion and angiogenesis. Since these activities essentially depend upon protein-protein interactions, they represent new therapeutic targets.     

Rational targeting of the urokinase receptor (uPAR): development of antagonists and non-invasive imaging probes

In the last two decades, the urokinase-type plasminogen activator receptor (uPAR) has been implicated in a number of human pathologies such as cancer, bacterial infections, and paroxysmal nocturnal hemoglobinuria. The primary function of this glycolipid-anchored receptor is to focalize uPA-mediated plasminogen activation at the cell surface, which is accomplished by its high-affinity interaction with the growth factor-like domain of uPA. Detailed insights into the molecular basis underlying the interactions between uPAR and its two bona fide ligands, uPA and vitronectin, have been obtained recently by X-ray crystallography and surface plasmon resonance studies. Importantly, these structural studies also define possible druggable target sites in uPAR for small molecules and provide guidelines for the development of reporter groups applicable for non-invasive molecular imaging of uPAR expression in vivo by positron emission tomography. In this review, we will discuss recent advances in our perception of the structure-function relationships of uPAR ligation and how these may assist translational research in preclinical intervention studies of uPAR function.