Protease nexin-1-urokinase complexes are internalized and degraded through a mechanism that requires both urokinase receptor and alpha 2-macroglobulin receptor

Novel insights into the multifaceted and tissue-specific roles of the endocytic receptor LRP1

Receptor-mediated endocytosis provides a mechanism for the selective uptake of specific molecules thereby controlling the composition of the extracellular environment and biological processes. The low-density lipoprotein receptor-related protein 1 (LRP1) is a widely expressed endocytic receptor that regulates cellular events by modulating the levels of numerous extracellular molecules via rapid endocytic removal. LRP1 also participates in signalling pathways through this modulation as well as in the interaction with membrane receptors and cytoplasmic adaptor proteins. LRP1 SNPs are associated with several diseases and conditions such as migraines, aortic aneurysms, cardiopulmonary dysfunction, corneal clouding, and bone dysmorphology and mineral density. Studies using Lrp1 KO mice revealed a critical, nonredundant and tissue-specific role of LRP1 in regulating various physiological events. However, exactly how LRP1 functions to regulate so many distinct and specific processes is still not fully clear. Our recent proteomics studies have identified more than 300 secreted proteins that either directly interact with LRP1 or are modulated by LRP1 in various tissues. This review will highlight the remarkable ability of this receptor to regulate secreted molecules in a tissue-specific manner and discuss potential mechanisms underpinning such specificity. Uncovering the depth of these "hidden" specific interactions modulated by LRP1 will provide novel insights into a dynamic and complex extracellular environment that is involved in diverse biological and pathological processes.

Recent Advances in Nanomaterials for Diagnosis, Treatments, and Neurorestoration in Ischemic Stroke

Stroke is a major public health issue, corresponding to the second cause of mortality and the first cause of severe disability. Ischemic stroke is the most common type of stroke, accounting for 87% of all strokes, where early detection and clinical intervention are well known to decrease its morbidity and mortality. However, the diagnosis of ischemic stroke has been limited to the late stages, and its therapeutic window is too narrow to provide rational and effective treatment. In addition, clinical thrombolytics suffer from a short half-life, inactivation, allergic reactions, and non-specific tissue targeting. Another problem is the limited ability of current neuroprotective agents to promote recovery of the ischemic brain tissue after stroke, which contributes to the progressive and irreversible nature of ischemic stroke and also the severity of the outcome. Fortunately, because of biomaterials’ inherent biochemical and biophysical properties, including biocompatibility, biodegradability, renewability, nontoxicity, long blood circulation time, and targeting ability. Utilization of them has been pursued as an innovative and promising strategy to tackle these challenges. In this review, special emphasis will be placed on the recent advances in the study of nanomaterials for the diagnosis and therapy of ischemic stroke. Meanwhile, nanomaterials provide much promise for neural tissue salvage and regeneration in brain ischemia, which is also highlighted.

The Dual Role of Low-Density Lipoprotein Receptor-Related Protein 1 in Atherosclerosis

Low-density lipoprotein receptor–related protein-1 (LRP1) is a large endocytic and signaling receptor belonging to the LDL receptor (LDLR) gene family and that is widely expressed in several tissues. LRP1 comprises a large extracellular domain (ECD; 515 kDa, α chain) and a small intracellular domain (ICD; 85 kDa, β chain). The deletion of LRP1 leads to embryonic lethality in mice, revealing a crucial but yet undefined role in embryogenesis and development. LRP1 has been postulated to participate in numerous diverse physiological and pathological processes ranging from plasma lipoprotein homeostasis, atherosclerosis, tumor evolution, and fibrinolysis to neuronal regeneration and survival. Many studies using cultured cells and in vivo animal models have revealed the important roles of LRP1 in vascular remodeling, foam cell biology, inflammation and atherosclerosis. However, its role in atherosclerosis remains controversial. LRP1 not only participates in the removal of atherogenic lipoproteins and proatherogenic ligands in the liver but also mediates the uptake of aggregated LDL to promote the formation of macrophage- and vascular smooth muscle cell (VSMC)-derived foam cells, which causes a prothrombotic transformation of the vascular wall. The dual and opposing roles of LRP1 may also represent an interesting target for atherosclerosis therapeutics. This review highlights the influence of LRP1 during atherosclerosis development, focusing on its dual role in vascular cells and immune cells.

Low Density Receptor-Related Protein 1 Interactions With the Extracellular Matrix: More Than Meets the Eye

The extracellular matrix (ECM) is a biological substrate composed of collagens, proteoglycans and glycoproteins that ensures proper cell migration and adhesion and keeps the cell architecture intact. The regulation of the ECM composition is a vital process strictly controlled by, among others, proteases, growth factors and adhesion receptors. As it appears, ECM remodeling is also essential for proper neuronal and glial development and the establishment of adequate synaptic signaling. Hence, disturbances in ECM functioning are often present in neurodegenerative diseases like Alzheimer’s disease. Moreover, mutations in ECM molecules are found in some forms of epilepsy and malfunctioning of ECM-related genes and pathways can be seen in, for example, cancer or ischemic injury. Low density lipoprotein receptor-related protein 1 (Lrp1) is a member of the low density lipoprotein receptor family. Lrp1 is involved not only in ligand uptake, receptor mediated endocytosis and lipoprotein transport—functions shared by low density lipoprotein receptor family members—but also regulates cell surface protease activity, controls cellular entry and binding of toxins and viruses, protects against atherosclerosis and acts on many cell signaling pathways. Given the plethora of functions, it is not surprising that Lrp1 also impacts the ECM and is involved in its remodeling. This review focuses on the role of Lrp1 and some of its major ligands on ECM function. Specifically, interactions with two Lrp1 ligands, integrins and tissue plasminogen activator are described in more detail.

Overexpression of SerpinE2/protease nexin-1 Contribute to Pathological Cardiac Fibrosis via increasing Collagen Deposition

Although increases in cardiovascular load (pressure overload) are known to elicit ventricular remodeling including cardiomyocyte hypertrophy and interstitial fibrosis, the molecular mechanisms of pressure overload or AngII -induced cardiac interstitial fibrosis remain elusive. In this study, serpinE2/protease nexin-1 was over-expressed in a cardiac fibrosis model induced by pressure-overloaded via transverse aortic constriction (TAC) in mouse. Knockdown of serpinE2 attenuates cardiac fibrosis in a mouse model of TAC. At meantime, the results showed that serpinE2 significantly were increased with collagen accumulations induced by AngII or TGF-β stimulation in vitro. Intriguingly, extracellular collagen in myocardial fibroblast was reduced by knockdown of serpinE2 compared with the control in vitro. In stark contrast, the addition of exogenous PN-1 up-regulated the content of collagen in myocardial fibroblast. The MEK1/2- ERK1/2 signaling probably promoted the expression of serpinE2 via transcription factors Elk1 in myocardial fibroblast. In conclusion, stress-induced the ERK1/2 signaling pathway activation up-regulated serpinE2 expression, consequently led accumulation of collagen protein, and contributed to cardiac fibrosis.

PEDF and its roles in physiological and pathological conditions: implication in diabetic and hypoxia-induced angiogenic diseases

Pigment epithelium-derived factor (PEDF) is a broadly expressed multifunctional member of the serine proteinase inhibitor (serpin) family. This widely studied protein plays critical roles in many physiological and pathophysiological processes, including neuroprotection, angiogenesis, fibrogenesis and inflammation. The present review summarizes the temporal and spatial distribution patterns of PEDF in a variety of developing and adult organs, and discusses its functions in maintaining physiological homoeostasis. The major focus of the present review is to discuss the implication of PEDF in diabetic and hypoxia-induced angiogenesis, and the pathways mediating PEDF's effects under these conditions. Furthermore, the regulatory mechanisms of PEDF expression, function and degradation are also reviewed. Finally, the therapeutic potential of PEDF as an anti-angiogenic drug is briefly summarized.

Translational research on u-PAR

The urokinase receptor (u-PAR) is one of the most critical molecules in migration, invasion, intravasation, and metastasis and is also a key regulator between tumour cell proliferation and dormancy. It is overexpressed in most human solid cancer types, which has led to increasing translational and clinical research on this molecule. The current review discusses in particular the in vivo, translational, and putative clinical relevance of u-PAR in the context of this latest development. It outlines how u-PAR is already being used and might increasingly be applied as a diagnostic tool, for example, in distinguishing benign from malignant neoplasms, as a molecular marker for predicting clinical response to chemotherapy or novel targeted therapy, and finally as a promising tool for the development of novel cancer therapeutics.

The urokinase receptor: focused cell surface proteolysis, cell adhesion and signaling

Plasma membrane urokinase-type plasminogen activator (uPA)-receptor (uPAR) is a GPI-anchored protein that binds with high-affinity and activates the serine protease uPA, thus regulating proteolytic activity at the cell surface. In addition, uPAR is a signaling receptor that often does not require its protease ligand or its proteolytic function. uPAR is highly expressed during tissue reorganization, inflammation, and in virtually all human cancers. Since its discovery, in vitro and in vivo models, as well as retrospective clinical studies have shown that over-expression of components of the uPA/uPAR-system correlates with increased proliferation, migration, and invasion affecting the malignant phenotype of cancer. uPAR regulates the cells-extracellular matrix interactions promoting its degradation and turnover through the plasminogen activation cascade.

Regulation of arterial remodeling and angiogenesis by urokinase-type plasminogen activatorThis article is one of a selection of papers from the NATO Advanced Research Workshop on Translational Knowledge for Heart Health (published in part 2 of a 2-part Special Issue)

A wide variety of disorders are associated with an imbalance in the plasminogen activator system, including inflammatory diseases, atherosclerosis, intimal hyperplasia, the response mechanism to vascular injury, and restenosis. Urokinase-type plasminogen activator (uPA) is a multifunctional protein that in addition to its fibrinolytic and matrix degradation capabilities also affects growth factor bioavailability, cytokine modulation, receptor shedding, cell migration and proliferation, phenotypic modulation, protein expression, and cascade activation of proteases, inhibitors, receptors, and modulators. uPA is the crucial protein for neointimal growth and vascular remodeling. Moreover, it was recently shown to be implicated in the stimulation of angiogenesis, which makes it a promising multipurpose therapeutic target. This review is focused on the mechanisms by which uPA can regulate arterial remodeling, angiogenesis, and cell migration and proliferation after arterial injury and the means by which it modulates gene expression in vascular cells. The role of domain specificity of urokinase in these processes is also discussed.

The urokinase receptor (u-PAR)--a link between tumor cell dormancy and minimal residual disease in bone marrow?

Minimal residual disease (MRD) is hypothesized to be the major cause of tumor recurrence and metastasis even years and decades after primary cancer diagnosis and curative solid tumor resection. In these patients disseminated tumor cells reflecting MRD can be detected in the bone marrow years after treatment. It is to be assumed that genetic determinants and a complex interplay between the disseminated tumor cells and their microenvironment in the bone marrow are responsible for tumor cell dormancy and the final reactivation towards metastasis. The urokinase receptor (u-PAR), a critical regulator of invasion, intravasation, and metastasis, is found to be a key player in regulating the shift between single cell tumor dormancy and proliferation. This has mainly been attributed to a regulation by u-PAR of integrins, and the ability of the latter to propagate signals from fibronectin through the EGF-receptor, ERK, and p38 signaling. Interestingly, u-PAR is found in disseminated tumor cells in the bone marrow of solid cancer patients, and is associated with the expansion of these cells and clinical prognosis. Here we summarize and discuss findings on disseminated tumor cells in the bone marrow, MRD and the role of u-PAR in tumor biology, especially focusing on its specific role in providing a switch between tumor cell proliferation and dormancy. Finally, we discuss the hypothesis that u-PAR might be an essential molecule in bone marrow disseminated tumor cells for long-term survival during dormancy, and/or reactivation of their proliferation years after primary treatment.

The inhibitory effect of HKa in endothelial cell tube formation is mediated by disrupting the uPA-uPAR complex and inhibiting its signaling and internalization

In two-dimensional (2-D) culture systems, we have previously shown that cleaved two-chain high-molecular-weight kininogen (HKa) or its domain 5 induced apoptosis by disrupting urokinase plasminogen activator (uPA) receptor (uPAR)-integrin signal complex formation. In the present study, we used a three-dimensional (3-D) collagen-fibrinogen culture system to monitor the effects of HKa on tube formation. In a 3-D system, HKa significantly inhibited tube and vacuole formation as low as 10 nM, which represents 1.5% of the physiological concentration of high-molecular-weigh kininogen (660 nM), without apparent apoptosis. However, HKa (300 nM) completely inhibited tube formation and increased apoptotic cells about 2-fold by 20-24 h of incubation. uPA-dependent ERK activation and uPAR internalization regulate cell survival and migration. In a 2-D system, we found that exogenous uPA-induced ERK phosphorylation and uPAR internalization were blocked by HKa. In a 3-D system, we found that not only uPA-uPAR association but also the activation of ERK were inhibited by HKa. HKa disrupts the uPA-uPAR complex, inhibiting the signaling pathways, and also inhibits uPAR internalization and regeneration to the cell surface, thereby interfering with uPAR-mediated cell migration, proliferation, and survival. Thus, our data suggest that the suppression of ERK activation and uPAR internalization by HKa contributes to the inhibition of tube formation. We conclude that in this 3-D collagen-fibrinogen gel, HKa modulates the multiple functions of uPAR in endothelial cell tube formation, a process that is closely related to in vivo angiogenesis.

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

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

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

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

Molecular regulation of an invasion-related molecule – options for tumour staging and clinical strategies

This review provides a summary of the European Association for Cancer Research Award Lecture, presented at the ECCO13 meeting in Paris in November 2005. It is a brief overview on the biological and clinical relevance of the urokinase receptor (u-PAR), an essential molecule to promote invasive and metastatic tumour phenotype and shown to be associated with early relapse and poor prognosis in many different types of cancers. The review summarizes the most important transcriptional mechanisms regulating u-PAR gene, and will focus on the differential binding of transcription factors to u-PAR promoter elements from studies in resected tumour and normal tissues of colorectal and gastric cancer patients. These studies conducted by our group may help to understand transcriptional mechanisms, which are employed to promote invasion and metastasis, in subpopulations of cancer patients. Such studies could lead to a more target-oriented patient selection and therapy against transcriptional and oncogeneic regulators in cancer.

Urokinase system expression in gastric carcinoma: prognostic impact in an independent patient series and first evidence of predictive value in preoperative biopsy and intestinal metaplasia specimens

BACKGROUND

The prognostic relevance of urokinase-type plasminogen activator (u-PA), u-PA receptor (u-PAR), and plasminogen activator inhibitor 1 (PAI-1) in gastric carcinoma was demonstrated in an independent patient series. To the authors' knowledge,the roles of these activators as predictors of aggressive phenotypes in preoperative biopsies, Helicobacter pylori infection, and intestinal metaplasia have to date not been investigated simultaneously in resected tumors. The objectives of the current study were 1) to demonstrate the prognostic relevance of u-PA, u-PAR, and PAI-1 in an independent series; 2) to evaluate u-PA system expression in preoperative biopsy specimens compared with resected tumors; and 3) to evaluate u-PA system expression in intestinal metaplasias and samples with H. pylori infection.

METHODS

In 104 patients with gastric carcinoma (median follow-up, 68 mos), u-PA, u-PAR, and PAI-1 in tumors and metaplasias were evaluated immunohistochemically. Preoperative biopsies were evaluated in a subset of patients. Patients were screened for H. pylori (urease) and tumor cells in bone marrow (u-PAR/CK18).

RESULTS

u-PA and PAI-1 were confirmed as independent prognostic parameters, and u-PAR was associated with a trend toward a poor prognosis. u-PA system tumor expression was found to be correlated significantly with u-PAR in disseminated tumor cells and H. pylori-infected tumors, implicating a role of H. pylori in protease induction. There was a significant correlation noted between u-PA system staining between preoperative biopsies and the results in resected tumors. The expression of u-PAR and PAI-1 in intestinal metaplasias was found to be associated significantly with advanced tumor stage (depth of invasion; pathologic tumor status) and lymph node involvement (pathologic lymph node status) and was correlated significantly with u-PA system expression in tumors.

CONCLUSIONS

To the author's know the current study is the first to date to demonstrate that u-PA system expression may serve as a predictor of risk in intestinal metaplasias and preoperative biopsies, implicating consequences for neoadjuvant therapy. The independent impact on recurrence and survival and a correlation with u-PAR-expression of minimal residual disease were identified in this independent series.

Combination of an SRC kinase inhibitor with a novel pharmacological antagonist of the urokinase receptor diminishes in vitro colon cancer invasiveness

PURPOSE

The urokinase-type plasminogen activator receptor (u-PAR) contributes to colon cancer invasion and metastases. We have shown previously that u-PAR expression in colon cancer is driven by the Src tyrosine kinase. In the current study, we determined the ability of PP2 (4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine), a Src kinase inhibitor, to reduce u-PAR expression and colon cancer invasion.

EXPERIMENTAL DESIGN

Western blotting, Northern blotting, and u-PAR promoter-reporter assays were performed to determine whether PP2 represses u-PAR expression. In vitro invasion assays were used to determine whether this kinase inhibitor, with or without a novel u-PAR antagonist, diminished cultured colon cancer invasiveness.

RESULTS

A constitutively active c-Src increased in vitro invasiveness of SW480 cells, whereas HT-29 cells expressing antisense c-Src showed diminished invasiveness, validating c-Src as a target for low molecular weight compound(s). The Src inhibitor PP2 reduced u-PAR transcription in HT-29 cells over the concentration range that blocked Src kinase activity. PP2 also reduced u-PAR protein amounts in three other colon cancer cell lines with modest to high constitutive Src activity. Treatment of HT-29 cells and 2C8 cells (a SW480 clone expressing a constitutively active Src) with PP2 diminished their in vitro invasiveness. Furthermore, combination of the Src inhibitor with a novel u-PAR peptide antagonist (NI-5.12) proved superior to the individual agents in suppressing invasiveness.

CONCLUSIONS

A c-Src kinase inhibitor represses u-PAR expression and, alone or in combination with a u-PAR antagonist, diminishes colon cancer invasiveness. Thus, concurrent targeting of c-Src expression and pharmacological blockade of the u-PAR may represent a novel means of controlling colon cancer spread.

Kinetic analysis of plasminogen activator inhibitor type-2: urokinase complex formation and subsequent internalisation by carcinoma cell lines

The overexpression of urokinase (uPA), which plays a key role in tumour invasion and metastasis, is an established prognostic marker and potential therapeutic target. Plasminogen activator inhibitor type 2 (PAI-2), an efficient and specific inhibitor of uPA, has been shown to selectively deliver potent cytotoxins to tumour cells. However, a direct quantitative analysis of both the inhibition kinetics and subsequent fate of PAI-2 upon interaction with cell-surface uPA has not been previously undertaken. In this study, we analysed specific PAI-2 binding to receptor-bound uPA on human breast and prostate cancer cell lines to directly measure inhibition kinetics. Cell-surface uPA:PAI-2 complex formation, which is reflective of complete uPA inhibition, was found to be very efficient (inactivation constant [K(I)] = 60-80 pM, depending on cell line used) and rapid (inactivation rate constant [k(inact)] = 0.32-0.47 min(-1) at 37 degrees C, depending on cell line used). To directly quantify and visualise cellular internalisation and localisation, we developed a novel assay based on the use of PAI-2 labelled with Alexa(488) fluorochrome and a polyclonal antibody to quench Alexa(488) fluorescence. The efficient and rapid formation of uPA:PAI-2 complexes was thus shown to be associated with specific and rapid internalisation of PAI-2, which could be localised within endosomes and lysosomes. PAI-2 was subsequently degraded, presumably within lysosomes. This study is the first to provide definitive evidence for uPA/uPAR-mediated PAI-2 endocytosis.

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.

Vascular endothelial growth factor receptor-2-induced initial endothelial cell migration depends on the presence of the urokinase receptor

The angiogenic response of endothelial cells initiated by different growth factors is accompanied by assembly of cell surface-bound proteolytic machinery as a prerequisite for focal invasion. We have shown previously how the vascular endothelial growth factor (VEGF) initiates proteolysis by activation of pro-urokinase (pro-PA) via the VEGF receptor-2 (VEGFR-2). We now show that the cell surface receptor of the uPA-system, the urokinase receptor (uPAR), is redistributed to focal adhesions at the leading edge of endothelial cells in response to VEGF. VEGF165 and VEGF-E, both interacting with VEGFR-2, but not PlGF exclusively stimulating VEGFR-1, induce within minutes internalization of uPAR via an LDL receptor-like molecule, dependent on generation of active uPA and the presence of plasminogen activator inhibitor-1 (PAI-1). uPAR seems to play a pivotal role in VEGFR-2-induced endothelial cell migration because cleavage of surface uPAR impaired the migratory response of endothelial cells toward VEGF-E, but not toward PlGF.

Reciprocal regulation of urokinase receptor (CD87)-mediated cell adhesion by plasminogen activator inhibitor-1 and protease nexin-1

Protease nexin-1 (PN-1) and plasminogen activator inhibitor-1 (PAI-1) are serine protease inhibitors that bind to the extracellular matrix protein vitronectin (VN) with high affinity. PAI-1 is known to inhibit cell adhesion and migration by binding to VN and inhibiting the interaction with integrins or the urokinase receptor (uPAR). Unexpectedly, PN-1 was found to increase the association between VN and uPAR in the presence of enzymatically active uPA. Through this mechanism PN-1 also stimulated uPAR-dependent cell adhesion to immobilized VN. In contrast to PAI-1, PN-1 did not influence VN binding to integrins or integrin-mediated cell adhesion. Upon adhesion of monocytes to VN there was an accumulation of uPAR and PN-1 at the interface between the cell and the matrix, whereas on fibronectin (FN) both components were distributed evenly over the whole cell as visualized by confocal microscopy. Immunohistochemistry of atherosclerotic vessels indicated that PN-1 was found associated with smooth muscle cells, macrophages and platelets. In some regions of the diseased vessels PN-1 was in close proximity to VN and uPAR, but no PN-1 was present in normal vessels. These results indicate a novel function of PN-1 linked to complex formation with uPA that leads to the regulation of VN-dependent adhesion of leukocytes.

Molecular regulation of urokinase-receptor gene expression as one potential concept for molecular staging and therapy

The urokinase-receptor (u-PAR) is a central molecule of invasion and metastasis promoting plasminogen-dependent extracellular matrix degradation in diverse carcinoma types such as gastric or colon cancer. Overexpression of u-PAR has been reported to occur mainly at the transcriptional level in malignant cells, and has been shown to indicate a poor clinical prognosis of cancer patients. This review will give an overview on experimental findings on u-PAR and its function, molecular mechanisms of its regulation, and its impact for future clinical decision planning and potential therapeutic concepts.

Transgenic mice demonstrate novel promoter regions for tissue-specific expression of the urokinase receptor gene

The urokinase-type plasminogen activator receptor (u-PAR) contributes to cell migration and proteolysis in normal and cancerous tissues. Currently, there are no reports on the regulatory regions directing tissue-specific expression. Consequently, we undertook a study to identify novel promoter regions required for expression of this gene in transgenic mice bearing a LacZ reporter regulated by varying amounts (0.4, 1.5, and 8.5 kb) of upstream sequence. The 0.4-kb u-PAR upstream sequence directed weak and strong LacZ expression in the placenta and epididymis, respectively, both of which are tissues that express endogenous u-PAR. Conversely, transgene expression in the apical cells of the colon positive for endogenous u-PAR protein required 1.5 kb of upstream sequence for optimal expression. Furthermore, chromatin accessibility assays coupled with real-time polymerase chain reaction suggested a putative regulatory region spanning -1295/-1192 driving u-PAR expression in colonic cells. Interestingly, placental transgene expression was augmented with the 8.5-kb upstream fragment compared with the shorter 1.5-kb fragment indicating contributing element(s) between -1.5 and -8.5 kb. Thus, while 0.4 kb of upstream sequence directs u-PAR expression in the epididymis, sequences located between -0.4 and -1.5 kb and between -1.5 and -8.5 kb are required for optimal tissue-specific expression in the colon and the placenta, respectively.

Induction of plasminogen activator inhibitor-1 by urokinase in lung epithelial cells

The plasminogen/plasmin system, urokinase-type plasminogen activator (uPA), its receptor (uPAR), and its inhibitor (PAI-1), influence extracellular proteolysis and cell migration in lung injury or neoplasia. In this study, we sought to determine whether tcuPA (two chain uPA) alters expression of its major inhibitor PAI-1 in lung epithelial cells. The expression of PAI-1 was evaluated at the protein and mRNA level by Western blot, immunoprecipitation, and Northern blot analyses. We found that tcuPA treatment enhanced PAI-1 protein and mRNA expression in Beas2B lung epithelial cells in a time- and concentration-dependent manner. The tcuPA-mediated induction of PAI-1 involves post-transcriptional control involving stabilization of PAI-1 mRNA. Inactivation of the catalytic activity of tcuPA had little effect on PAI-1 induction and the activity of the isolated amino-terminal fragment was comparable with full-length single- or two-chain uPA. In contrast, deletion of either the uPA receptor binding growth factor domain or kringle domain (kringle) from full-length single chain uPA markedly attenuated the induction of PAI-1. Induction of PAI-1 by exposure of lung epithelial cells to uPA is a newly recognized pathway by which PAI-1 could regulate local fibrinolysis and urokinase-dependent cellular responses in the setting of lung inflammation or neoplasia.

Transcriptional regulation of the urokinase receptor (u-PAR)--a central molecule of invasion and metastasis

The phenomenon of tumor-associated proteolysis has been acknowledged as a decisive step in the progression of cancer. This short review focuses on the urokinase receptor (u-PAR), a central molecule involved in tumor-associated invasion and metastasis, and summarizes the transcriptional regulation of u-PAR. The urokinase receptor (u-PAR) is a heavily glycosylated cell surface protein and binds the serine protease urokinase specifically and with high affinity. It consists of three similar cysteine-rich repeats and is anchored to the cell membrane via a GPI-anchor. The u-PAR gene comprises 7 exons and is located on chromosome 19q13. Transcriptional activation of the u-PAR promoter region can be induced by binding of transcription factors (Sp1, AP-1, AP-2, NF-kappa B). One current study gives an example for transcriptional downregulation of u-PAR through a PEA3/ets transcriptional silencing element. Knowledge of the molecular regulation of this molecule in tumor cells could be very important for diagnosis and therapy in the near future.

Low density lipoprotein (LDL) receptor-related protein 1B impairs urokinase receptor regeneration on the cell surface and inhibits cell migration

The low density lipoprotein (LDL) receptor-related protein 1B (LRP1B) is a newly identified member of the LDL receptor family and is closely related to LRP. It was discovered as a putative tumor suppressor and is frequently inactivated in lung cancer cells. In the present study, we used an LRP1B minireceptor (mLRP1B4), which mimics the function and trafficking of LRP1B, to explore the roles of LRP1B on the plasminogen activation system. We found that mLRP1B4 and urokinase plasminogen activator receptor (uPAR) form immunoprecipitable complexes on the cell surface in the presence of complexes of uPA and its inhibitor, plasminogen activator inhibitor type-1 (PAI-1). However, compared with cells expressing the analogous LRP minireceptor (mLRP4), cells expressing mLRP1B4 display a substantially slower rate of uPA.PAI-1 complex internalization. Expression of mLRP1B4, or an mLRP4 mutant deficient in endocytosis, leads to an accumulation of uPAR at the cell surface and increased cell-associated uPA and PAI-1 when compared with cells expressing mLRP4. In addition, we found that expression of mLRP1B or the mLRP4 endocytosis mutant impairs the regeneration of unoccupied uPAR on the cell surface and that this correlates with a diminished rate of cell migration. Taken together, these results demonstrate that LRP1B can function as a negative regulator of uPAR regeneration and cell migration.

SERPIN regulation of factor XIa. The novel observation that protease nexin 1 in the presence of heparin is a more potent inhibitor of factor XIa than C1 inhibitor

In the present studies we have made the novel observation that protease nexin 1 (PN1), a member of the serine protease inhibitor (SERPIN) superfamily, is a potent inhibitor of the blood coagulation Factor XIa (FXIa). The inhibitory complexes formed between PN1 and FXIa are stable when subjected to reducing agents, SDS, and boiling, a characteristic of the acyl linkage formed between SERPINs and their cognate proteases. Using a sensitive fluorescence-quenched peptide substrate, the K(assoc) of PN1 for FXIa was determined to be 7.9 x 10(4) m(-)(1) s(-)(1) in the absence of heparin. In the presence of heparin, this rate was accelerated to 1.7 x 10(6), M(-)(1) s(-)(1), making PN1 a far better inhibitor of FXIa than C1 inhibitor, which is the only other SERPIN known to significantly inhibit FXIa. FXIa-PN1 complexes are shown to be internalized and degraded by human fibroblasts, most likely via the low density lipoprotein receptor-related protein (LRP), since degradation was strongly inhibited by the LRP agonist, receptor-associated protein. Since FXIa proteolytically modifies the amyloid precursor protein, this observation may suggest an accessory role for PN1 in the pathobiogenesis of Alzheimer's disease.

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.

Collagenase-3 binds to a specific receptor and requires the low density lipoprotein receptor-related protein for internalization

We have previously identified a specific receptor for collagenase-3 that mediates the binding, internalization, and degradation of this ligand in UMR 106-01 rat osteoblastic osteosarcoma cells. In the present study, we show that collagenase-3 binding is calcium-dependent and occurs in a variety of cell types, including osteoblastic and fibroblastic cells. We also present evidence supporting a two-step mechanism of collagenase-3 binding and internalization involving both a specific collagenase-3 receptor and the low density lipoprotein receptor-related protein. Ligand blot analysis shows that (125)I-collagenase-3 binds specifically to two proteins ( approximately 170 kDa and approximately 600 kDa) present in UMR 106-01 cells. Western blotting identified the 600-kDa protein as the low density lipoprotein receptor-related protein. Our data suggest that the 170-kDa protein is a specific collagenase-3 receptor. Low density lipoprotein receptor-related protein-null mouse embryo fibroblasts bind but fail to internalize collagenase-3, whereas UMR 106-01 and wild-type mouse embryo fibroblasts bind and internalize collagenase-3. Internalization, but not binding, is inhibited by the 39-kDa receptor-associated protein. We conclude that the internalization of collagenase-3 requires the participation of the low density lipoprotein receptor-related protein and propose a model in which the cell surface interaction of this ligand requires a sequential contribution from two receptors, with the collagenase-3 receptor acting as a high affinity primary binding site and the low density lipoprotein receptor-related protein mediating internalization.

The light chain of factor VIII comprises a binding site for low density lipoprotein receptor-related protein

In the present study, the interaction between the endocytic receptor low density lipoprotein receptor-related protein (LRP) and coagulation factor VIII (FVIII) was investigated. Using purified components, FVIII was found to bind to LRP in a reversible and dose-dependent manner (K(d) approximately 60 nM). The interaction appeared to be specific because the LRP antagonist receptor-associated protein readily inhibited binding of FVIII to LRP (IC(50) approximately 1 nM). In addition, a 12-fold molar excess of the physiological carrier of FVIII, i.e. von Willebrand factor (vWF), reduced the binding of FVIII to LRP by over 90%. Cellular degradation of (125)I-labeled FVIII by LRP-expressing cells ( approximately 8 fmol/10(5) cells after a 4.5-h incubation) was reduced by approximately 70% in the presence of receptor-associated protein. LRP-directed antibodies inhibited degradation to a similar extent, indicating that LRP indeed contributes to binding and transport of FVIII to the intracellular degradation pathway. Degradation of FVIII was completely inhibited by vWF. Because vWF binding by FVIII involves its light chain, LRP binding to this subunit was studied. In ligand blotting experiments, binding of FVIII light chain to LRP could be visualized. More detailed analysis revealed that FVIII light chain interacts with LRP with moderate affinity (k(on) approximately 5 x 10(4) M(-1) s(-1); k(off) approximately 2.5 x 10(-3) s(-1); K(d) approximately 50 nM). Furthermore, experiments using recombinant FVIII C2 domain showed that this domain contributes to the interaction with LRP. In contrast, no association of FVIII heavy chain to LRP could be detected under the same experimental conditions. Collectively, our data demonstrate that in vitro LRP is able to bind FVIII at the cell surface and to mediate its transport to the intracellular degradation pathway. FVIII-LRP interaction involves the FVIII light chain, and FVIII-vWF complex formation plays a regulatory role in LRP binding. Our findings may explain the beneficial effect of vWF on the in vivo survival of FVIII.

The mammalian low-density lipoprotein receptor family

The low-density lipoprotein (LDL) receptor (LDL-R) family consists of cell-surface receptors that recognize extracellular ligands and internalize them for degradation by lysosomes. The LDL-R is the prototype of this family, which also contains very-low-density lipoprotein receptors (VLDL-R), apolipoprotein E receptor 2, LRP, and megalin. The family members contain four major structural modules: the cysteine-rich complement-type repeats, epidermal growth factor precursor-like repeats, a transmembrane domain, and a cytoplasmic domain. Each structural module serves distinct and important functions. These receptors bind several structurally dissimilar ligands. It is proposed that instead of a primary sequence, positive electrostatic potential in different ligands constitutes a receptor binding domain. This family of receptors plays crucial roles in various physiologic functions. LDL-R plays an important role in cholesterol homeostasis. Mutations cause familial hypercholesterolemia and premature coronary artery disease. LDL-R-related protein plays an important role in the clearance of plasma-activated alpha 2-macroglobulin and apolipoprotein E-enriched lipoproteins. It is essential for fetal development and has been associated with Alzheimer's disease. Megalin is the major receptor in absorptive epithelial cells of the proximal tubules and an antigenic determinant for Heymann nephritis in rats. Mutations in a chicken homolog of VLDL-R cause female sterility and premature atherosclerosis. This receptor is not expressed in liver tissue; however, transgenic expression of VLDL-R in liver corrects hypercholesterolemia in experiment animals, which suggests that it can be a candidate for gene therapy for various hyperlipidemias. The functional importance of individual receptors may lie in their differential tissue expression. The regulation of expression of these receptors occurs at the transcriptional level. Expression of the LDL-R is regulated by intracellular sterol levels involving novel membrane-bound transcription factors. Other members of the family are not regulated by sterols. All the members are, however, regulated by hormones and growth factors, but the mechanisms of regulation by hormones have not been elucidated. Studies of these receptors have provided important insights into receptor structure-function and mechanisms of ligand removal and catabolism. It is anticipated that increased knowledge about the LDL-R family members will open new avenues for the treatment of many disorders.

Targeted disruption of the K-ras oncogene in an invasive colon cancer cell line down-regulates urokinase receptor expression and plasminogen-dependent proteolysis

The urokinase receptor, overexpressed in invasive colon cancer, promotes tumour cell invasion. Since K-Ras is activated in many colon cancers, we determined if urokinase receptor overexpression is a consequence of this activated oncogene. Accordingly, urokinase receptor expression was compared in HCT 116 colon cancer cells containing either a mutation-activated K-Ras or disrupted for this oncogene (by homologous recombination). HCT 116 cells containing the disrupted K-Ras oncogene expressed between 50 and 85% less urokinase receptor protein compared with the parental HCT 116 cells. Reduced urokinase receptor expression in cells containing the disrupted mutated K-Ras was not due to a physical impairment of the urokinase receptor gene since phorbol ester treatment was inductive for its expression. Constitutive urokinase receptor expression in HCT 116 cells required an intact AP-1 motif in the promoter (at -184) and electrophoretic mobility shifting assays indicated less c-Jun, JunD, c-Fos and Fra-1 bound to this motif in the K-Ras-disrupted cells. Since the urokinase receptor accelerates proteolysis, laminin degradation was compared in cells containing the mutation-activated and disrupted K-Ras oncogene. The latter cells displaying fewer urokinase receptors, degraded 80% less laminin. This is the first study to demonstrate a role for K-Ras as a regulator of the constitutive expression of the urokinase receptor.

Transcriptional induction of the urokinase receptor gene by a constitutively active Src. Requirement of an upstream motif (-152/-135) bound with Sp1

Since c-src overexpression increases colonic cell invasiveness and because both Src activity and urokinase receptor protein are elevated in invasive colon cancers, the present study was undertaken: 1) to determine if a constitutively active Src regulates urokinase receptor expression and 2) to identify required cis-elements and trans-acting factors. SW480 colon cancer cells transfected with an expression plasmid (c-srcY527F) encoding a constitutively active Src protein manifested increased urokinase receptor gene expression and Src activity. Treatment of the src transfectants with a Src-inhibitor (PD173955) reduced urokinase receptor protein levels and laminin degradation. Inasmuch as we recently implicated an upstream region of the urokinase receptor promoter (-152/-135) in constitutive urokinase receptor expression, we determined its role for the induction by src. Whereas the activity of a CAT reporter driven by this region was stimulated by c-srcY527F, the u-PAR promoter mutated at the Sp1-binding motif in the -152/-135 region was not. Nuclear extracts from the src transfectants demonstrated increased Sp1 binding to region -152/-135 compared with those from SW480 cells. Finally, endogenous urokinase receptor protein amounts in 10 colon cancers and corresponding normal colon correlated with Src specific activity. These data suggest that urokinase receptor gene expression is regulated by Src partly via increased Sp1 binding.

Mechanism of thrombin clearance by human astrocytoma cells

Astroglial cells secrete a variety of factors that contribute to the regulation of neurite initiation and continued outgrowth, among them proteases and protease inhibitors. An alteration in the balance between these proteins has been implicated in Alzheimer's disease, resulting in an accumulation of thrombin:protease nexin 1 (PN1) complexes in the brains of these patients. This report aims at providing a biochemical explanation for this phenomenon. We show that human astrocytoma cells bind and internalize thrombin and thrombin:PN1 complexes efficiently by a PN1-dependent mechanism. Binding was potently inhibited by soluble heparin and did not occur with the mutant PN1 (K7E) deficient in heparin binding. Receptor-associated protein, an antagonist of the low-density lipoprotein receptor-related protein (LRP), inhibited internalization of thrombin by the astrocytoma cells, but did not affect cell-surface binding. The results are consistent with a mechanism by which astrocytoma cells clear thrombin in a sequential manner: thrombin is first complexed with PN1, then bound to cell-surface heparins, and finally internalized by LRP. This mechanism provides a link between the neuronal growth regulators thrombin and PN1 and proteins genetically associated with Alzheimer's disease, such as LRP.

Transactivation of the urokinase-type plasminogen activator receptor gene through a novel promoter motif bound with an activator protein-2alpha-related factor

The urokinase receptor overexpressed in invasive cancers promotes laminin degradation. The current study was undertaken to identify cis elements and trans-acting factors activating urokinase receptor expression through a footprinted (-148/-124) region of the promoter containing putative activator protein-2- and Sp1-binding motifs. Mobility shifting experiments using nuclear extract from a high urokinase receptor-expressing cell line (RKO) indicated that Sp1, Sp3, and a factor similar to, but distinct from, activator protein-2alpha bound to this region. Mutations preventing the binding of the activator protein 2alpha-related factor diminished urokinase receptor promoter activity. In RKO cells, the expression of a negative regulator of activator protein-2 function diminished urokinase receptor promoter activity, protein, and laminin degradation. Conversely, urokinase receptor promoter activity in low urokinase receptor-expressing GEO cells was increased by activator protein-2alphaA expression. Although using GEO nuclear extract, little activator protein-2alpha-related factor bound to the footprinted region, phorbol 12-myristate 13-acetate treatment, which induces urokinase receptor expression, increased complex formation. Mutations preventing the activator protein-2alpha-related factor and Sp1/Sp3 binding reduced urokinase receptor promoter stimulation by this agent. Thus, the constitutive and phorbol 12-myristate 13-acetate-inducible expression of the urokinase receptor is mediated partly through trans-activation of the promoter via a sequence (-152/-135) bound with an activator protein-2alpha-related factor.

Analysis of a structural determinant in thrombin-protease nexin 1 complexes that mediates clearance by the low density lipoprotein receptor-related protein

We recently identified a synthetic peptide, Pro47-Ile58, derived from the mature protease nexin 1 (PN1) sequence, that inhibited the low density lipoprotein receptor-related protein (LRP)-mediated internalization of thrombin-PN1 (Th-PN1) complexes. Presently, we have analyzed this sequence in Th-PN1 complex catabolism using two independent approaches: 1) An antibody was generated against Pro47-Ile58, which inhibited complex degradation by 70% but had no effect on the binding of the complexes to cell surface heparins. This places the structural determinant in PN1 mediating complex internalization by the LRP outside of the heparin-binding site. 2) Site-directed genetic variants of PN1 with a single Ala substitution at His48, or two Ala substitutions, one at His48 and another at Asp49, were expressed in Sf9 insect cells. The catabolic rate of complexes formed between Th and the singly substituted and doubly substituted variants was lowered to 50 and 15%, respectively, when compared with the catabolic rate of native Th-PN1 complexes. This is the first analysis of a structural determinant in a serine protease inhibitor (SERPIN) required for LRP-mediated internalization and in part may explain the cryptic nature of this site in the unreacted serine protease inhibitor.

Urokinase-type plasminogen activator receptor is internalized by different mechanisms in polarized and nonpolarized Madin-Darby canine kidney epithelial cells

Accumulated data indicate that endocytosis of the glycosylphosphatidyl-inositol-anchored protein urokinase plasminogen activator receptor (uPAR) depends on binding of the ligand uPA:plasminogen activator inhibitor-1 (PAI-1) and subsequent interaction with internalization receptors of the low-density lipoprotein receptor family, which are internalized through clathrin-coated pits. This interaction is inhibited by receptor-associated protein (RAP). We show that uPAR with bound uPA:PAI-1 is capable of entering cells in a clathrin-independent process. First, HeLaK44A cells expressing mutant dynamin efficiently internalized uPA:PAI-1 under conditions in which transferrin endocytosis was blocked. Second, in polarized Madin-Darby canine kidney (MDCK) cells, which expressed human uPAR apically, the low basal rate of uPAR ligand endocytosis, which could not be inhibited by RAP, was increased by forskolin or phorbol ester (phorbol 12-myristate 13-acetate), which selectively up-regulate clathrin-independent endocytosis from the apical domain of epithelial cells. Third, in subconfluent nonpolarized MDCK cells, endocytosis of uPA:PAI-1 was only decreased marginally by RAP. At the ultrastructural level uPAR was largely excluded from clathrin-coated pits in these cells and localized in invaginated caveolae only in the presence of cross-linking antibodies. Interestingly, a larger fraction of uPAR in nonpolarized relative to polarized MDCK cells was insoluble in Triton X-100 at 0 degreesC, and by surface labeling with biotin we also show that internalized uPAR was mainly detergent insoluble, suggesting a correlation between association with detergent-resistant membrane microdomains and higher degree of clathrin-independent endocytosis. Furthermore, by cryoimmunogold labeling we show that 5-10% of internalized uPAR in nonpolarized, but not polarized, MDCK cells is targeted to lysosomes by a mechanism that is regulated by ligand occupancy.

In vivo clearance of ternary complexes of vitronectin-thrombin-antithrombin is mediated by hepatic heparan sulfate proteoglycans

Thrombin is inhibited by its cognate plasma inhibitor antithrombin, through the formation of covalent thrombin-antithrombin (TAT) complexes that are found as ternary complexes with vitronectin (VN-TAT). To determine whether the metabolism of VN-TAT ternary complexes is different from that previously reported for binary TAT complexes, plasma clearance studies were done in rabbits using human VN-TAT. 125I-VN-TAT was shown to be cleared rapidly from the circulation (t1/2alpha = 3.8 min) in a biphasic manner mainly by the liver. 125I-TAT had a similar initial clearance (t1/2alpha = 5.3 min) but had a significantly faster beta-phase clearance (t1/2beta = 42.8 min versus 85.4 min for VN-TAT; p = 0.005). Protamine sulfate and heparin abolished the rapid initial alpha-phase of 125I-VN-TAT clearance and reduced its liver-specific association and in vivo degradation. Heparin also reduced the alpha-phase clearance of 125I-TAT and was associated with the appearance of high molecular weight complexes, suggesting enhanced complex formation between VN and TAT. 125I-VN-TAT binding to HepG2 cells was reduced by competition with VN-TAT or heparin but to a much lesser extent in the presence of TAT. The binding of VN-TAT to HepG2 cells was not inhibited by competition with the low density lipoprotein receptor-related protein ligand, methylamine-alpha2-macroglobulin. 125I-VN-TAT binding was also inhibited by treating HepG2 cells with heparinase or by growing the cells in the presence of beta-D-xyloside. Finally, both heparin and chloroquine, but not methylamine-alpha2-macroglobulin, reduced the internalization and degradation of VN-TAT by HepG2 cells. Taken together, these data indicate the importance of VN in TAT metabolism and demonstrate that VN-TAT binds to liver-associated heparan sulfate proteoglycans, which mediate its internalization and subsequent intracellular degradation.

Plasminogen activator inhibitor-1 contains a cryptic high affinity binding site for the low density lipoprotein receptor-related protein

Much of the controversy surrounding the binding of plasminogen activator inhibitor-1 (PAI-1) to the low density lipoprotein receptor-related protein (LRP) may be due to the labile structure of PAI-1 and the distinct conformations that it can adopt. To examine this possibility and to test the hypothesis that PAI-1 contains a specific high affinity binding site for LRP, a sensitive and quantitative assay for PAI-1 binding to LRP was developed. This assay utilizes a unique PAI-1 mutant that was constructed with a hexapeptide tag at the NH2 terminus, which is recognized by the protein kinase, heart muscle kinase and can be specifically labeled with 32P. Our results show that only 32P-PAI-1 in complex with a proteinase binds LRP with high affinity and is efficiently endocytosed by cells, indicating that a high affinity site for LRP is generated on PAI-1 only when in complex with a proteinase. In addition, PAI-1 in complex with different proteinases is shown to cross-compete for LRP binding, demonstrating that the binding site is independent of the proteinase and therefore must reside on the PAI-1 portion of the complex. Finally, mutagenesis of PAI-1 results in loss of LRP binding, confirming that the high affinity binding site is located on PAI-1 and suggesting that the LRP binding site lays within a region of PAI-1 previously shown to contain the heparin binding domain.

Prognostic factors in gastric cancer

BACKGROUND

Despite gastric cancer being common, its prognosis has not been improved significantly in recent years. Now, greater insight has been gained into the biological properties of tumour cells, how they become malignant and what mechanisms they may use to invade and metastasize. This involves tumour-associated protease systems, loss or mutation of adhesion molecules and changes in genetics. The view of gastric cancer is changing: it is not only a solid tumour but also exhibits a minimal residual disease component even in the early stages of disease. Such biological tumour characteristics may provide new prognostic factors and also potential new therapeutic options.

METHODS

This is an update of prognostic factors in gastric cancer, emphasizing new biological features, some of which have been investigated by this group over the past few years. Current results are discussed in the light of 212 references obtained from the Medline database from 1979 to 1997.

RESULTS

There is high probability that some of the factors reviewed, such as c-erbB-2, individual course and phenotyping of disseminated tumour cells will become significant new prognostic variables. This is true also, to a lesser extent, of cathepsin D, matrix metalloproteinase 2 combined with activators or tissue inhibitor of metalloproteinases 2, CD44, E-cadherin, p53 and cripto. Plasminogen activator inhibitor 1 (PAI-1), a member of the urokinase-type plasminogen activator (uPA) system, can already be defined as an established new prognostic factor in gastric cancer.

CONCLUSION

PAI-1 should be considered prognostically in addition to established tumour classifications. Moreover, the uPA system is a target for future therapeutic concepts. Further analysis of factors describing tumour biology should lead to new, functionally orientated, tumour classifications in gastric cancer.

The efficient catabolism of thrombin-protease nexin 1 complexes is a synergistic mechanism that requires both the LDL receptor-related protein and cell surface heparins

Protease nexin 1 (PN1) is a serine protease inhibitor (SERPIN) that acts as a suicide substrate for thrombin (Th) and urokinase-type plasminogen activator (uPA). PN1 forms 1:1 stoichiometric complexes with these proteases, which are then rapidly bound, internalized, and degraded. The low density lipoprotein receptor-related protein (LRP) is the receptor responsible for the internalization of protease-PN1 complexes. However, we found that the LRP is not significantly involved in the initial cell surface binding of thrombin-PN1, leading us to investigate what cellular component was responsible for this initial interaction. Since Th-PN1 complexes retain a high-affinity for heparin after complex formation, unlike several of the other SERPINs, we tested the possibility that cell surface heparins were involved in initial complex binding. Soluble heparin was found to be a potent inhibitor of the binding of Th-PN1 to the cell surface and greatly facilitated the dissociation of Th-PN1 complexes pre-bound in the absence of soluble heparin. To ascertain the role of cell surface heparins, further studies were done using complexes of thrombin and PN1(K7E), a variant of PN1 in which the heparin binding site was rendered non-functional. When added at equal initial concentrations of complexes, Th-PN1(K7E) was catabolized 5- to 10-fold less efficiently than Th-PN1, a direct result of the greatly diminished initial binding of the Th-PN1(K7E) complexes. These data demonstrate the sizable contribution of cell surface heparins to Thrombin-PN1 complex binding and support a model in which these heparins act to concentrate the complexes at the cell surface facilitating their subsequent LRP-dependent endocytosis.

Cytokeratin 18 is expressed on the hepatocyte plasma membrane surface and interacts with thrombin-antithrombin complexes

During experiments to identify putative hepatic receptors for thrombin-antithrombin (TAT) complexes, a 45-kDa protein was identified by ligand blotting. Following gel purification, amino acid sequencing revealed the 45-kDa TAT-binding polypeptide to be cytokeratin 18 (CK18). The presence of CK18 on the surface of intact rat hepatoma cells was demonstrated by binding of 125I-anti-CK18 antibodies. Anti-CK18 antibodies reduced the binding and internalization of 125I-TAT by rat hepatoma cells. Immunocytochemical analysis, to determine the location of CK18 in vivo, revealed a periportal gradient of CK18 staining; with hepatocytes around the portal triads demonstrating striking pericellular staining. In addition, anti-CK18 IgG associated with perfused livers to a significantly greater extent than preimmune IgG. Taken together, these data provide evidence that CK18 is found on the extracellular surface of hepatocytes and could play a role in TAT removal. Finally, these data, in conjunction with recent reports of CK8 (Hembrough, T. A., Li, L., and Gonias, S. L. (1996) J. Biol. Chem. 271, 25684-25691) and CK1 cell membrane surface expression (Schmaier, A. H. (1997) Thromb. Hemostasis 78, 101-107), indicate a novel role for these proteins as putative cellular receptors or cofactors to cellular receptors.

Specificity of serine proteinase/serpin complex binding to very-low-density lipoprotein receptor and alpha2-macroglobulin receptor/low-density-lipoprotein-receptor-related protein

Very-low-density lipoprotein receptor (VLDLR) and alpha2-macroglobulin receptor/low-density-lipoprotein-receptor-related protein (alpha2MR/LRP) are multifunctional endocytosis receptors of the low-density lipoprotein receptor family. Both have been shown to mediate endocytosis and degradation of complex between plasminogen activators and type-1 plasminogen-activator inhibitor (PAI-1) by cultured cells. We have now studied the specificity of binding and endocytosis by VLDLR and alpha2MR/LRP among a variety of serine proteinase/serpin complexes, including various combinations of the serine proteinases urokinase-type and tissue-type plasminogen activators, plasmin, thrombin, human leukocyte elastase, cathepsin G, and plasma kallikrein with the serpins PAI-1, horse leukocyte elastase inhibitor, protein C inhibitor, C1-inhibitor, alpha2-antiplasmin, alpha1-proteinase inhibitor, alpha1-antichymotrypsin, protease nexin-1, heparin cofactor II, and antithrombin III. Binding was estimated with radiolabelled ligands in ligand blotting analysis and microtiter well assays. Endocytosis was estimated by measuring receptor-associated protein (RAP)-sensitive degradation of radiolabelled complexes by Chinese hamster ovary cells transfected with VLDLR cDNA and by COS-1 cells, which have a high endogenous expression of alpha2MR/LRP. We found that the receptors bind with high affinity to some, but not all, combinations of plasminogen activators and thrombin with PAI-1, protease nexin-1, protein C inhibitor, and antithrombin III, while complexes of many serine proteinases with their primary inhibitor, i.e. plasmin/alpha2-antiplasmin complex, do not bind, or bind with a very low affinity. Both the serine proteinase and the serpin moieties contribute to the binding specificity. The binding specificities of VLDLR and alpha2MR/LRP are overlapping, but not identical. The results suggest that VLDLR and alpha2MR/LRP have different biological functions by having different binding specificities as well as by being expressed by different cell types.

The urokinase-type plasminogen activator system in cancer metastasis: a review

The urokinase-type plasminogen activator (u-PA) system consists of the serine proteinases plasmin and u-PA; the serpin inhibitors alpha2-anti-plasmin, PAI-1 and PAI-2; and the u-PA receptor (u-PAR). Two lines of evidence have strongly suggested an important and apparently causal role for the u-PA system in cancer metastasis: results from experimental model systems with animal tumor metastasis and the finding that high levels of u-PA, PAI-1 and u-PAR in many tumor types predict poor patient prognosis. We discuss here recent observations related to the molecular and cellular mechanisms underlying this role of the u-PA system. Many findings suggest that the system does not support tumor metastasis by the unrestricted enzyme activity of u-PA and plasmin. Rather, pericellular molecular and functional interactions between u-PA, u-PAR, PAI-1, extracellular matrix proteins, integrins, endocytosis receptors and growth factors appear to allow temporal and spatial re-organizations of the system during cell migration and a selective degradation of extracellular matrix proteins during invasion. Differential expression of components of the system by cancer and non-cancer cells, regulated by paracrine mechanisms, appear to determine the involvement of the system in cancer cell-directed tissue remodeling. A detailed knowledge of these processes is necessary for utilization of the therapeutic potential of interfering with the action of the system in cancers.

Identification of a binding site in protease nexin I (PN1) required for the receptor mediated internalization of PN1-thrombin complexes

An overlapping synthetic peptide library was constructed representing most of the mature protease nexin I (PN1) sequence from the amino terminus to the reactive center. This library, along with peptides from the heparin binding domain and from the region carboxyl-terminal to the P1 residue of the cleavage site, was screened for the inhibition of 125I-thrombin (Th)-PN1 complex binding and degradation. A peptide corresponding to residues Pro47-Ile58 in the PN1 sequence was identified as a potent inhibitor of 125I-Th-PN1 complex degradation, although it did not affect binding significantly. Pro47-Ile58 was shown to competitively inhibit the low density lipoprotein receptor-related protein (LRP)/alpha2-macroglobulin receptor-mediated endocytosis of 125I-Th-PN1 complexes in mouse embryo fibroblasts. Pro47-Ile58 is an apparent transition sequence in PN1, separating sheet-6B and helix-B. The sequence of Pro47-Ile58, PHDNIVISPHGI, suggests that it forms a loop structure defined by the seven underlined amino acids bordered by proline residues at each end. These studies are the first to identify a putative binding site in a serine protease inhibitor that is required for LRP-mediated internalization.

Phosphorylation of human pro-urokinase on Ser138/303 impairs its receptor-dependent ability to promote myelomonocytic adherence and motility

Serine phosphorylation of human pro-urokinase (pro-uPA) by A431 human carcinoma cells results in a catalytically active molecule with reduced sensitivity to plasminogen activator inhibitor type 1. We mapped the phosphorylated seryl residues by analyzing the in vivo phosphorylation state of engineered pro-uPA variants carrying a COOH-terminal poly-histidine tag. Stably transfected A431 cells do not incorporate radioactive phosphate into tagged pro-uPA in which the serines 138 and 303 have been replaced with glutamic residues, although endogenous nontagged pro-uPA is 32P-labeled on A and B chains. Moreover, the catalytic-independent ability of the mono- and di-substituted "phosphorylation-like" variants to bind to the GPI-anchored urokinase receptor (uPAR) and promote adherence of differentiating U937, HL-60, and THP-1 myelomonocytic cells was examined. We found that glutamic residues as well as the naturally occurring phosphoserines at positions 138 and 303 abolish proadhesive ability, although they do not interfere with receptor binding. In addition, pro-uPA carrying Glu138/303 lacks the capability to induce a chemotactic response of THP-1 cells. The exclusive presence of Glu138 reduces pro-uPA proadhesive and chemotactic ability by 70-80%, indicating that a phosphoserine residue at the same position plays a major inhibitory role of myeloid cell response to pro-urokinase. The di-substitution does not affect pro-uPA ability to interact with vitronectin or to enhance binding of urea-denatured vitronectin to uPAR. However, unlike wild-type tagged pro-uPA, the di-substituted variant does not induce receptor polarization in pre-adherent U937 cells. Taken together, the data support the possibility that pro-uPA phosphorylation on Ser138/303 can modulate uPAR transducing ability.

Molecular and functional characterization of the urokinase receptor on human mast cells

The urokinase receptor system is involved in several biological processes including extracellular proteolysis, cell invasion, and chemotaxis. Mast cells are multifunctional perivascular cells that play an important role in the regulation of microenvironmental events. We report that primary human mast cells and the human mast cell line HMC-1 express the receptor for urokinase. As assessed by Northern blotting and reverse transcription polymerase chain reaction technique, purified human lung mast cells and HMC-1 cells expressed urokinase receptor mRNA in a constitutive manner. Using a toluidine blue/immunofluorescence double staining technique and monoclonal antibodies, surface expression of urokinase receptor was demonstrable in lung, skin, uterus, heart, and tonsil mast cells, whereas the low density lipoprotein receptor-related protein was not detectable. Binding of monoclonal antibody VIM5 (recognizing the urokinase binding domain of urokinase receptor) to HMC-1 could be blocked by high molecular weight but not low molecular weight urokinase. Binding analyses performed with 123I-urokinase revealed expression of 271,000 +/- 55,000 high affinity urokinase binding sites per HMC-1 cell, with a calculated dissociation constant of 1. 29 +/- 0.3 nM. Purified urokinase induced dose-dependent migration of primary mast cells and HMC-1 in a chemotaxis assay without inducing release of histamine. The mast cell agonist stem cell factor also induced migration of HMC-1 and caused up-regulation of expression of urokinase receptor mRNA. Together, our data show that human mast cells express functional receptors for urokinase. Expression of urokinase receptors on mast cells may have implications for mast cell-dependent microvascular processes associated with fibrinolysis, migration, or local tissue repair.