Plasminogen mediates the pathological effects of urokinase-type plasminogen activator overexpression

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

Simple Summary

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

Abstract

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

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

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

Novel LRAP-binding partner revealing the plasminogen activation system as a regulator of cementoblast differentiation and mineral nodule formation in vitro

Amelogenin isoforms, including full-length amelogenin (AMEL) and leucine-rich amelogenin peptide (LRAP), are major components of the enamel matrix, and are considered as signaling molecules in epithelial-mesenchymal interactions regulating tooth development and periodontal regeneration. Nevertheless, the molecular mechanisms involved are still poorly understood. The aim of the present study was to identify novel binding partners for amelogenin isoforms in the cementoblast (OCCM-30), using an affinity purification assay (GST pull-down) followed by mass spectrometry and immunoblotting. Protein-protein interaction analysis for AMEL and LRAP evidenced the plasminogen activation system (PAS) as a potential player regulating OCCM-30 response to amelogenin isoforms. For functional assays, PAS was either activated (plasmin) or inhibited (ε-aminocaproic acid [aminocaproic]) in OCCM-30 cells and the cell morphology, mineral nodule formation, and gene expression were assessed. PAS inhibition (EACA 100 mM) dramatically decreased mineral nodule formation and expression of OCCM-30 differentiation markers, including osteocalcin (Bglap), bone sialoprotein (Ibsp), osteopontin (Spp1), tissue-nonspecific alkaline phosphatase (Alpl) and collagen type I (Col1a1), and had no effect on runt-related transcription factor 2 (Runx2) and Osterix (Osx) mRNA levels. PAS activation (plasmin 5 µg/µl) significantly increased Col1a1 and decreased Bglap mRNA levels (p < .05). Together, our findings shed new light on the potential role of plasminogen signaling pathway in the control of the amelogenin isoform-mediated response in cementoblasts and provide new insights into the development of targeted therapies.

The overexpression of uPA promotes the proliferation and fibrinolytic activity of human umbilical vein endothelial cells

The purpose of this article is to study whether the overexpression of urokinase-type plasminogen activator (uPA) can promote the proliferation and fibrinolytic activity of human umbilical vein endothelial cells (HUVECs). The recombinant adenovirus vectors containing the human uPA gene were constructed and transfected into HUVECs. In this study, the mRNA of uPA was detected by qPCR, and the uPA protein was measured by Western blot. The cell proliferation was measured using MTT. The fibrinolytic activity of uPA was quantified using a colorimetric assay. We also measured MMP2 (metalloproteinase-2), MMP9 (metalloproteinase-9), and VEGF (vascular endothelial growth factor) proteins using ELISA. The results showed that the levels of the uPA mRNA and the protein in the overexpression group were significantly higher compared to the other groups, (P < 0.05). The cell proliferation and uPA activity were increased significantly in the overexpression group, compared to the other groups, (P < 0.05). The secretions of MMP2, MMP9, and VEGF in the overexpression group were significantly higher than they were in the other two groups (P < 0.05). In conclusion, we successfully transfected a recombined adenovirus vector carrying uPA into a HUVEC. The exogenous uPA gene could transcribe and secrete the uPA protein in the HUVECs. The overexpression of uPA can increase cell proliferation and uPA activity. It can improve the invasion and angiogenesis ability in HUVECs by promoting their secretions of MMP2, MMP9, and VEGF.

Plasmin in urine from patients with type 2 diabetes and treatment-resistant hypertension activates ENaC in vitro

BACKGROUND

Aberrant filtration of plasminogen from plasma and subsequent activation to plasmin in the urinary space may activate proteolytically the epithelial sodium channel, ENaC. In conditions with chronic albuminuria, this may cause hypertension. It was hypothesized that patients with type 2 diabetes mellitus (T2DM) and treatment-resistant hypertension excrete plasmin(ogen) in urine in proportion to albumin and that plasmin confers to urine the ability to activate ENaC.

METHOD

Patients (n = 113) with T2DM and resistant hypertension, defined as systolic blood pressure (SBP) more than 130 mmHg and/or diastolic blood pressure (DBP) more than 80 mmHg despite use of at least three drugs with one diuretic and one renin-angiotensin system inhibitor, were included. Urine was analyzed for albumin, creatinine, plasmin(ogen), protease activity, and ability to activate inward current in single collecting duct cells.

RESULTS

Mean ambulatory SBP/DBP was 143 ± 1/77 ± 0.7 mmHg; HbA1c 7.35%; and eGFR 81.0 ml/min per 1.73 m (geometric means). Patients with microalbuminuria (39%) and macroalbuminuria (13%) displayed significantly elevated levels of urinary plasmin(ogen) normalized to urine creatinine compared with patients with normal excretion of albumin (48%). Urinary plasminogen correlated significantly to urine albumin. Western immunoblotting and gelatine zymography confirmed active plasmin in urine samples from patients with microalbuminuria and macroalbuminuria. Single collecting duct cells displayed significantly increased, amiloride-sensitive, inward current when superfused with urine from albuminuric patients compared with patients with normal albumin excretion. Urinary plasminogen/creatinine ratio correlated significantly with 24-h ambulatory blood pressure.

CONCLUSION

Aberrant presence of plasmin in preurine may inappropriately activate ENaC in patients with type 2 diabetes and microalbuminuria. This may contribute to treatment-resistant hypertension.

Genome resequencing and bioinformatic analysis of SNP containing candidate genes in the autoimmune vitiligo Smyth line chicken model

BACKGROUND

The Smyth line (SL) chicken is the only animal model for autoimmune vitiligo that spontaneously displays all clinical and biological manifestations of the human disorder. To understand the genetic components underlying the susceptibility to develop SL vitiligo (SLV), whole genome resequencing analysis was performed in SLV chickens compared with non-vitiliginous parental Brown line (BL) chickens, which maintain a very low incidence rate of vitiligo.

RESULTS

Illumina sequencing technology and reference based assembly on Red Jungle Fowl genome sequences were used. Results of genome resequencing of pooled DNA of each 10 BL and SL chickens reached 5.1x and 7.0x coverage, respectively. The total number of SNPs was 4.8 and 5.5 million in BL and SL genome, respectively. Through a series of filtering processes, a total of ~1 million unique SNPs were found in the SL alone. Eventually of the 156 reliable marker SNPs, which can induce non-synonymous-, frameshift-, nonsense-, and no-start mutations in amino acid sequences in proteins, 139 genes were chosen for further analysis. Of these, 14 randomly chosen SNPs were examined for SNP verification by PCR and Sanger sequencing to detect SNP positions in 20 BL and 70 SL chickens. The results of the analysis of the 14 SNPs clearly showed differential frequencies of nucleotide bases in the SNP positions between BL and SL chickens. Bioinformatic analysis showed that the 156 most reliable marker SNPs included genes involved in dermatological diseases/conditions such as ADAMTS13, ASPM, ATP6V0A2, BRCA2, COL12A1, GRM5, LRP2, OBSCN, PLAU, RNF168, STAB2, and XIRP1. Intermolecular gene network analysis revealed that candidate genes identified in SLV play a role in networks centered on protein kinases (MAPK, ERK1/2, PKC, PRKDC), phosphatase (PPP1CA), ubiquitinylation (UBC) and amyloid production (APP).

CONCLUSIONS

Various potential genetic markers showing amino acid changes and potential roles in vitiligo development were identified in the SLV chicken through genome resequencing. The genetic markers and bioinformatic interpretations of amino acid mutations found in SLV chickens may provide insight into the genetic component responsible for the onset and the progression of autoimmune vitiligo and serve as valuable markers to develop diagnostic tools to detect vitiligo susceptibility.

Concomitant lack of MMP9 and uPA disturbs physiological tissue remodeling

Urokinase-type plasminogen activator (uPA) and matrix metalloproteinase-9 (MMP9, gelatinase B) have separately been recognized to play important roles in various tissue remodeling processes. In this study, we demonstrate that deficiency for MMP9 in combination with ablation of either uPA- or tissue-type plasminogen activator (tPA)-catalyzed plasminogen activation is critical to accomplish normal gestation in mice. Gestation was also affected by simultaneous lack of MMP9 and the uPA receptor (uPAR). Interestingly, uPA-deficiency additionally exacerbated the effect of MMP9-deficiency on bone growth and an additive effect caused by combined lack in MMP9 and uPA was observed during healing of cutaneous wounds. By comparison, MMP9-deficiency combined with absence of either tPA or uPAR resulted in no significant effect on wound healing, indicating that the role of uPA during wound healing is independent of uPAR, when MMP9 is absent. Notably, compensatory upregulation of uPA activity was seen in wounds from MMP9-deficient mice. Taken together, these studies reveal essential functional dependency between MMP9 and uPA during gestation and tissue repair.

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

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

Cleaved forms of the urokinase plasminogen activator receptor in plasma have diagnostic potential and predict postoperative survival in patients with ovarian cancer

PURPOSE

To evaluate the plasma level of different forms of soluble urokinase plasminogen activator receptor (suPAR) as discriminators between malignant, borderline, and benign ovarian tumors and as prognostic markers in patients with ovarian cancer.

EXPERIMENTAL DESIGN

The different suPAR forms were measured in preoperative plasma samples obtained from 335 patients with adnexal lesions using three different time-resolved fluoresence assays (TR-FIA): TR-FIA 1 measuring intact suPAR, suPAR(I-III), TR-FIA 2 measuring the total amount of suPAR(I-III) and the cleaved form, suPAR(II-III), and TR-FIA 3 measuring the liberated uPAR(I). Tumors were classified as benign (n = 211), borderline (possibly malignant; n = 30), and well (n = 19), moderately (n = 15), and poorly (n = 60) differentiated malignant.

RESULTS

All uPAR forms as well as CA125 were statistically significant in univariate analysis discriminating between benign, borderline, and invasive tumors. Restricting the analysis of invasive tumors to early stage (I and II) showed similar results. A combination of CA125 and suPAR(I-III) + suPAR(II-III) discriminated between malignant (all stages) and benign tumors [AUC, 0.94; 95% confidence interval (95% CI), 0.90-0.98] as well as borderline and benign tumors (AUC, 0.78; 95% CI, 0.67-0.89). All suPAR forms were markers for poor prognosis in univariate analyses, and high preoperative plasma level of uPAR(I) is an independent predictor of poor prognosis (hazard ratio, 1.84; 95% CI, 1.15-2.95; P = 0.011) in multivariate analyses including age and CA125.

CONCLUSIONS

High concentration of plasma uPAR(I) is an independent preoperative marker of poor prognosis in patients with ovarian cancer. The combination of plasma suPAR(I-III) + suPAR(II-III) and CA125 discriminates between malignant and benign tumors with an AUC of 0.94.

Mapping of the Vitronectin-binding Site on the Urokinase Receptor

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

Extracellular proteolysis in transgenic mouse models of breast cancer

Growth and invasion of breast cancer require extracellular proteolysis in order to physically restructure the tissue microenvironment of the mammary gland. This pathological tissue remodeling process depends on a collaboration of epithelial and stromal cells. In fact, the majority of extracellular proteases are provided by stromal cells rather than cancer cells. This distinct expression pattern is seen in human breast cancers and also in transgenic mouse models of breast cancer. The similar expression patterns suggest that transgenic mouse models are ideally suited to study the role of extracellular proteases in cancer progression. Here we give a status report on protease intervention studies in transgenic models. These studies demonstrate that proteases are involved in all stages of breast cancer progression from carcinogenesis to metastasis. Transgenic models are now beginning to provide vital mechanistic insight that will allow us to combat breast cancer invasion and metastasis with new protease-targeted drugs.

Urokinase-type Plasminogen Activator mRNA is Expressed in Normal Developing Teeth and Leads to Abnormal Incisor Enamel in αMUPA Transgenic Mice

The urokinase-type plasminogen activator (uPA) is a secreted, inducible serine protease implicated in extracellular proteolysis and tissue remodeling. Here we detected uPA mRNA through in situ hybridization in developing molar and incisor teeth of normal mice at multiple sites of the cap and bell developmental stages. The mRNA was confined to epithelial cells, however, was undetectable in ameloblasts or their progenitor preameloblasts and the inner enamel epithelium. Furthermore, mice of five lines of previously described alphaMUPA transgenic mice, carrying a transgene consisting of the uPA cDNA linked downstream from the alphaA-crystallin promoter, overexpressed uPA mRNA in the same epithelial sites. In addition, alphaMUPA mice showed remarkably high levels of uPA mRNA in ameloblasts, however, exclusively in two specific sites late in incisor development. First, at the late secretory stage, but only on sides of the ameloblast layer. Second, in a limited zone of ameloblasts near the incisal end, coinciding with a striking morphological change of the ameloblast layer and the enamel matrix. In adult alphaMUPA mice, the incisor teeth displayed discoloration and tip fragility, and reduction of the outer enamel as determined by scanning electron microscopy. These results suggest that balanced uPA activity could play a role in normal tooth development. The alphaMUPA tooth phenotype demonstrates a remarkable sensitivity to excessive extracellular proteolysis at the incisor maturation stage of amelogenesis.

Deregulated matriptase causes ras-independent multistage carcinogenesis and promotes ras-mediated malignant transformation

Overexpression of the type II transmembrane serine protease matriptase is a highly consistent feature of human epithelial tumors. Here we show that matriptase possesses a strong oncogenic potential when unopposed by its endogenous inhibitor, HAI-1. Modest orthotopic overexpression of matriptase in the skin of transgenic mice caused spontaneous squamous cell carcinoma and dramatically potentiated carcinogen-induced tumor formation. Matriptase-induced malignant conversion was preceded by progressive interfollicular hyperplasia, dysplasia, follicular transdifferentiation, fibrosis, and dermal inflammation. Furthermore, matriptase induced activation of the pro-tumorigenic PI3K-Akt signaling pathway. This activation was frequently accompanied by H-ras or K-ras mutations in carcinogen-induced tumors, whereas matriptase-induced spontaneous carcinoma formation occurred independently of ras activation. Increasing epidermal HAI-1 expression completely negated the oncogenic effects of matriptase. The data implicate dysregulated matriptase expression in malignant epithelial transformation.

Crystal structure of the human urokinase plasminogen activator receptor bound to an antagonist peptide

We report the crystal structure of a soluble form of human urokinase-type plasminogen activator receptor (uPAR/CD87), which is expressed at the invasive areas of the tumor-stromal microenvironment in many human cancers. The structure was solved at 2.7 A in association with a competitive peptide inhibitor of the urokinase-type plasminogen activator (uPA)-uPAR interaction. uPAR is composed of three consecutive three-finger domains organized in an almost circular manner, which generates both a deep internal cavity where the peptide binds in a helical conformation, and a large external surface. This knowledge combined with the discovery of a convergent binding motif shared by the antagonist peptide and uPA allowed us to build a model of the human uPA-uPAR complex. This model reveals that the receptor-binding module of uPA engages the uPAR central cavity, thus leaving the external receptor surface accessible for other protein interactions (vitronectin and integrins). By this unique structural assembly, uPAR can orchestrate the fine interplay with the partners that are required to guide uPA-focalized proteolysis on the cell surface and control cell adhesion and migration.