Plasmin cleaves the juxtamembrane domain and releases truncated species of the urokinase receptor (CD87) from human bronchial epithelial cells

Plasmin(ogen) serves as a favorable biomarker for prediction of survival in advanced high-grade serous ovarian cancer

In serous ovarian cancer, the clinical relevance of tumor cell-expressed plasmin(ogen) (PLG) has not yet been evaluated. Due to its proteolytic activity, plasmin supports tumorigenesis, however, angiostatin(-like) fragments, derived from PLG, can also function as potent anti-tumorigenic factors. In the present study, we assessed PLG protein expression in 103 cases of advanced high-grade serous ovarian cancer (FIGO III/IV) by immunohistochemistry (IHC). In 70/103 cases, positive staining of tumor cells was observed. In univariate Cox regression analysis, PLG staining was positively associated with prolonged overall survival (OS) [hazard ratio (HR)=0.59, p=0.026] of the patients. In multivariable analysis, PLG, together with residual tumor mass, remained a statistically significant independent prognostic marker (HR=0.49, p=0.009). In another small patient cohort (n=29), we assessed mRNA expression levels of PLG by quantitative PCR. Here, elevated PLG mRNA levels were also significantly associated with prolonged OS of patients (Kaplan-Meier analysis; p=0.001). This finding was validated by in silico analysis of a microarray data set (n=398) from The Cancer Genome Atlas (Kaplan-Meier analysis; p=0.031). In summary, these data indicate that elevated PLG expression represents a favorable prognostic biomarker in advanced (FIGO III/IV) high-grade serous ovarian cancer.

Soluble urokinase plasminogen activator receptor--a valuable biomarker in systemic lupus erythematosus?

Systemic lupus erythematosus (SLE) is a potentially severe autoimmune condition with an unpredictable disease course, often with fluctuations in disease activity over time. Long term inflammation and drug-related side-effects may subsequently lead to permanent organ damage, a consequence which is intimately connected to decreased quality of life and mortality. New lupus biomarkers that convey information regarding inflammation and/or organ damage are thus warranted. Today, there is no clinical biomarker that indicates the risk of damage accrual. Herein we highlight the urokinase plasminogen activator receptor (uPAR) and especially its soluble form (suPAR) that besides having biological functions in e.g. proteolysis, cell migration and tissue homeostasis, recently has emerged as a promising biomarker of inflammation and prognosis of several disorders. A strong association between suPAR and organ damage in SLE was recently demonstrated, and preliminary data (presented in this review) suggests the possibility of a predictive value of suPAR blood levels. The involvement of suPAR in the pathogenesis of SLE remains obscure, but its effects in leukocyte recruitment, phagocytic uptake of dying cells (efferocytosis) and complement regulation suggests that the central parts of the SLE pathogenesis could be regulated by suPAR, and vice versa.

Involvement of urokinase-type plasminogen activator system in cancer: an overview

Currently, there are several studies supporting the role of urokinase-type plasminogen activator (uPA) system in cancer. The association of uPA to its receptor triggers the conversion of plasminogen into plasmin. This process is regulated by the uPA inhibitors (PAI-1 and PAI-2). Plasmin promotes degradation of basement membrane and extracellular matrix (ECM) components as well as activation of ECM latent matrix metalloproteases. Degradation and remodeling of the surrounding tissues is crucial in the early steps of tumor progression by facilitating expansion of the tumor mass, release of tumor growth factors, activation of cytokines as well as induction of tumor cell proliferation, migration, and invasion. Hence, many tumors showed a correlation between uPA system component levels and tumor aggressiveness and survival. Therefore, this review summarizes the structure of the uPA system, its contribution to cancer progression, and the clinical relevance of uPA family members in cancer diagnosis. In addition, the review evaluates the significance of uPA system in the development of cancer-targeted therapies.

The plasminogen system in regulating stem cell mobilization

The treatment of patients with hematopoietic progenitor and stem cells (HPSCs) to reconstitute hematopoiesis after myeloablative therapy or to repair ischemia after myocardial infarction has significantly improved clinical outcomes. Successful blood or bone marrow transplants require a sufficient number of HPSCs capable of homing to the injured site to regenerate tissue. Granulocyte-colony stimulating factor (G-CSF) is widely used clinically for stem cell mobilization. However, in some patients the response is poor, thus a better understanding of the mechanisms underlying G-CSF-regulated stem cell mobilization is needed. The pasminogen (Plg) system is the primary fibrinolytic pathway responsible for clot dissolution after thrombosis. Recent evidence suggests that Plg plays a pivotal role in stem cell mobilization from the bone marrow to the peripheral circulation, particularly in HPSC mobilization in response to G-CSF. This paper will discuss the potential mechanisms by which the Plg system regulates stem cell mobilization, focusing on stepwise proteolysis and signal transduction during HPSC egress from their bone marrow niche. Clear elucidation of the underlying mechanisms may lead to the development of new Plg-based therapeutic strategies to improve stem cell mobilization in treating hematological and cardiovascular diseases.

Engineering kunitz domain 1 (KD1) of human tissue factor pathway inhibitor-2 to selectively inhibit fibrinolysis: properties of KD1-L17R variant

Tissue factor pathway inhibitor-2 (TFPI-2) inhibits factor XIa, plasma kallikrein, and factor VIIa/tissue factor; accordingly, it has been proposed for use as an anticoagulant. Full-length TFPI-2 or its isolated first Kunitz domain (KD1) also inhibits plasmin; therefore, it has been proposed for use as an antifibrinolytic agent. However, the anticoagulant properties of TFPI-2 or KD1 would diminish its antifibrinolytic function. In this study, structure-based investigations and analysis of the serine protease profiles revealed that coagulation enzymes prefer a hydrophobic residue at the P2' position in their substrates/inhibitors, whereas plasmin prefers a positively charged arginine residue at the corresponding position in its substrates/inhibitors. Based upon this observation, we changed the P2' residue Leu-17 in KD1 to Arg (KD1-L17R) and compared its inhibitory properties with wild-type KD1 (KD1-WT). Both WT and KD1-L17R were expressed in Escherichia coli, folded, and purified to homogeneity. N-terminal sequences and mass spectra confirmed proper expression of KD1-WT and KD1-L17R. Compared with KD1-WT, the KD1-L17R did not inhibit factor XIa, plasma kallikrein, or factor VIIa/tissue factor. Furthermore, KD1-L17R inhibited plasmin with ∼6-fold increased affinity and effectively prevented plasma clot fibrinolysis induced by tissue plasminogen activator. Similarly, in a mouse liver laceration bleeding model, KD1-L17R was ∼8-fold more effective than KD1-WT in preventing blood loss. Importantly, in this bleeding model, KD1-L17R was equally or more effective than aprotinin or tranexamic acid, which have been used as antifibrinolytic agents to prevent blood loss during major surgery/trauma. Furthermore, as compared with aprotinin, renal toxicity was not observed with KD1-L17R.

Release of the soluble urokinase-type plasminogen activator receptor (suPAR) by activated neutrophils in rheumatoid arthritis

Soluble form of the urokinase-type plasminogen activator receptor (suPAR) is markedly increased in biological fluids during different inflammatory conditions. It has previously been observed that the highest suPAR concentrations in inflammatory exudates tend to be associated with the presence of high number of neutrophils. Guided by this observation and our recent finding that activated neutrophils release suPAR we investigated whether neutrophils can be a source of suPAR during the inflammatory response in vivo. To address this question we conducted the comparative analysis of neutrophils isolated from the paired samples of synovial fluid (SF) and peripheral blood (PB) of rheumatoid arthritis patients. Freshly isolated SF neutrophils released significantly (p < 0.01) higher amounts of suPAR compared with PB neutrophils. We demonstrated that neutrophils from both sources release predominantly the truncated D2D3 form of suPAR. Migration of formyl peptide receptor-like 1 (FPRL1)-transfected human embryonic kidney (HEK) 293 cells toward the supernatants harvested from in vitro cultured SF neutrophils was significantly diminished when D2D3 form of suPAR was immunodepleted from the supernatants. Taken together, these data demonstrate that neutrophils, first, contribute to or are responsible for the generation of the increased suPAR levels during the inflammatory response and, second, release the chemotactically active form of suPAR that might be involved in the recruitment of formyl peptide receptors-expressing leukocytes into the inflamed tissues.

Membrane-anchored uPAR regulates the proliferation, marrow pool size, engraftment, and mobilization of mouse hematopoietic stem/progenitor cells

The mechanisms of BM hematopoietic stem/progenitor cell (HSPC) adhesion, engraftment, and mobilization remain incompletely identified. Here, using WT and transgenic mice, we have shown that membrane-anchored plasminogen activator, urokinase receptor (MuPAR) marks a subset of HSPCs and promotes the preservation of the size of this pool of cells in the BM. Loss or inhibition of MuPAR increased HSPC proliferation and impaired their homing, engraftment, and adhesion to the BM microenvironment. During mobilization, MuPAR was inactivated by plasmin via proteolytic cleavage. Cell-autonomous loss of the gene encoding MuPAR also impaired long-term engraftment and multilineage repopulation in primary and secondary recipient mice. These findings identify MuPAR and plasmin as regulators of the proliferation, marrow pool size, homing, engraftment, and mobilization of HSPCs and possibly also of HSCs.

Mannose 6-phosphate/insulin-like growth factor 2 receptor limits cell invasion by controlling alphaVbeta3 integrin expression and proteolytic processing of urokinase-type plasminogen activator receptor

The multifunctional mannose 6-phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R) is considered a tumor suppressor. We report here that RNA interference with M6P/IGF2R expression in urokinase-type plasminogen activator (uPA)/urokinase-type plasminogen activator receptor (uPAR) expressing human cancer and endothelial cells resulted in increased pericellular plasminogen activation, cell adhesion, and higher invasive potential through matrigel. M6P/IGF2R silencing led also to the cell surface accumulation of urokinase and plasminogen and enhanced expression of alphaV integrins. Genetic rescue experiments and inhibitor studies revealed that the enhanced plasminogen activation was due to a direct effect of M6P/IGF2R on uPAR, whereas increased cell adhesion to vitronectin was dependent on alphaV integrin expression and not uPAR. Increased cell invasion of M6P/IGF2R knockdown cells was rescued by cosilencing both uPAR and alphaV integrin. Furthermore, we found that M6P/IGF2R expression accelerates the cleavage of uPAR. M6P/IGF2R silencing resulted in an increased ratio of full-length uPAR to the truncated D2D3 fragment, incapable of binding most uPAR ligands. We conclude that M6P/IGF2R controls cell invasion by regulating alphaV integrin expression and by accelerating uPAR cleavage, leading to the loss of the urokinase/vitronectin/integrin-binding site on uPAR.

Activated human neutrophils rapidly release the chemotactically active D2D3 form of the urokinase-type plasminogen activator receptor (uPAR/CD87)

The urokinase-type plasminogen activator receptor (uPAR/CD87) exists both in cell-bound and soluble forms. Neutrophils contain extensive intracellular pools of uPAR that are translocated to the plasma membrane upon activation. In the present study, we investigated the ability of human neutrophils to shed uPAR from cell surface following activation and addressed the possible involvement of the released receptor in the inflammatory response. We first observed that the spontaneous release of suPAR by resting neutrophils was strongly and rapidly (within minutes) enhanced by calcium ionophore ionomycin and to a lesser extent when cells were primed with TNF-alpha and then stimulated with fMLP or IL-8. We demonstrated that suPAR is produced by resting and activated neutrophils predominantly as a truncated form devoid of N-terminal D1 domain (D2D3 form) that lacks GPI anchor. Migration of formyl peptide receptor-like 1 (FPRL1)-transfected human embryonic kidney (HEK) 293 cells toward the supernatants harvested from activated neutrophils was significantly diminished when D2D3 form of suPAR was immunodepleted from the supernatants. We conclude that activated neutrophils release the chemotactically active D2D3 form of suPAR that acts as a ligand of FPRL1. Interestingly, we present evidence that GPI-specific phospholipase D (GPI-PLD) that has previously been shown to shed uPAR in cancer cells is not involved in suPAR release from human neutrophils. We suggest that production of the chemotactically active D2D3 form of suPAR by activated human neutrophils in vivo could contribute to the recruitment of monocytes and other formyl peptide receptors-expressing cells to the sites of acute inflammation where neutrophil accumulation and activation occur.

Pharmacoproteomics of a metalloproteinase hydroxamate inhibitor in breast cancer cells: dynamics of membrane type 1 matrix metalloproteinase-mediated membrane protein shedding

Broad-spectrum matrix metalloproteinase (MMP) inhibitors (MMPI) were unsuccessful in cancer clinical trials, partly due to side effects resulting from limited knowledge of the full repertoire of MMP substrates, termed the substrate degradome, and hence the in vivo functions of MMPs. To gain further insight into the degradome of MMP-14 (membrane type 1 MMP) an MMPI, prinomastat (drug code AG3340), was used to reduce proteolytic processing and ectodomain shedding in human MDA-MB-231 breast cancer cells transfected with MMP-14. We report a quantitative proteomic evaluation of the targets and effects of the inhibitor in this cell-based system. Proteins in cell-conditioned medium (the secretome) and membrane fractions with levels that were modulated by the MMPI were identified by isotope-coded affinity tag (ICAT) labeling and tandem mass spectrometry. Comparisons of the expression of MMP-14 with that of a vector control resulted in increased MMP-14/vector ICAT ratios for many proteins in conditioned medium, indicating MMP-14-mediated ectodomain shedding. Following MMPI treatment, the MMPI/vehicle ICAT ratio was reversed, suggesting that MMP-14-mediated shedding of these proteins was blocked by the inhibitor. The reduction in shedding or the release of substrates from pericellular sites in the presence of the MMPI was frequently accompanied by the accumulation of the protein in the plasma membrane, as indicated by high MMPI/vehicle ICAT ratios. Considered together, this is a strong predictor of biologically relevant substrates cleaved in the cellular context that led to the identification of many undescribed MMP-14 substrates, 20 of which we validated biochemically, including DJ-1, galectin-1, Hsp90alpha, pentraxin 3, progranulin, Cyr61, peptidyl-prolyl cis-trans isomerase A, and dickkopf-1. Other proteins with altered levels, such as Kunitz-type protease inhibitor 1 and beta-2-microglobulin, were not substrates in biochemical assays, suggesting an indirect affect of the MMPI, which might be important in drug development as biomarkers or, in preclinical phases, to predict systemic drug actions and adverse side effects. Hence, this approach describes the dynamic pattern of cell membrane ectodomain shedding and its perturbation upon metalloproteinase drug treatment.

The Pseudomonas aeruginosa LasB metalloproteinase regulates the human urokinase-type plasminogen activator receptor through domain-specific endoproteolysis

Pseudomonas aeruginosa is an opportunistic pathogen in human lungs, where its secretable LasB metalloproteinase can be a virulence factor. The urokinase-type plasminogen activator receptor (uPAR) participates in pericellular proteolysis and the adherence/migration of epithelial cells and leukocytes recruited during infection and shows functional regulation by various proteinases via limited endoproteolysis occurring within its three domains (D1 to D3). We thus examined the proteolytic activity of LasB on uPAR by using recombinant uPAR as well as uPAR-expressing, human monocytic, and bronchial epithelial cell lines. Protein immunoblotting and flow immunocytometry using a panel of domain-specific anti-uPAR antibodies showed that LasB is able to cleave uPAR both within the sequence linking D1 to D2 and at the carboxy terminus of D3. Comparison of LasB-producing and LasB-deficient bacterial strains indicated that LasB is entirely responsible for the uPAR cleavage ability of P. aeruginosa. Based on amino-terminal protein microsequencing and mass spectrometry analysis of the cleavage of peptides mimicking the uPAR sequences targeted by LasB, cleavage sites were determined to be Ala(84)-Val(85) and Thr(86)-Tyr(87) (D1-D2) and Gln(279)-Tyr(280) (D3). Such a dual cleavage of uPAR led to the removal of amino-terminal D1, the generation of a truncated D2D3 species, and the shedding of D2D3 from cells. This proteolytic processing of uPAR was found to (i) drastically reduce the capacity of cells to bind urokinase and (ii) abrogate the interaction between uPAR and the matrix adhesive protein vitronectin. The LasB proteinase is thus endowed with a high potential for the alteration of uPAR expression and functioning on inflammatory cells during infections by P. aeruginosa.

The human airway trypsin-like protease modulates the urokinase receptor (uPAR, CD87) structure and functions

The human airway trypsin-like protease (HAT) is a respiratory epithelium-associated, type II transmembrane serine protease, which is also detected as an extracellular enzyme in lung fluids during airway inflammatory disorders. We have evaluated its capacity to affect the urokinase-type plasminogen activator receptor (uPAR), a membrane glycolipid-anchored, three-domain (D1D2D3) glycoprotein that plays a crucial role in innate immunity and inflammation by supporting cell migration and matrix degradation, with structure and biological properties that can be regulated via limited endoproteolysis. With the use of immunoblotting, flow immunocytometry, and ELISA analyses applied to a recombinant uPAR protein and to uPAR-expressing monocytic and human bronchial epithelial cells, it was shown that exposure of uPAR to soluble HAT in the range of 10-500 nM resulted in the proteolytic processing of the full-length (D1D2D3) into the truncated (D2D3) species, with cleavage occurring in the D1 to D2 linker sequence after arginine residues at position 83 and 89. Using immunohistochemistry, we found that both HAT and uPAR were expressed in the human bronchial epithelium. Moreover, transient cotransfection in epithelial cells showed that membrane coexpression of the two partners produced a constitutive and extensive shedding of the D1 domain, occurring for membrane-associated HAT concentrations in the nanomolar range. Because the truncated receptor was found to be unable to bind two of the major uPAR ligands, the adhesive matrix protein vitronectin and the serine protease urokinase, it thus appears that proteolytic regulation of uPAR by HAT is likely to modulate cell adherence and motility, as well as tissue remodeling during the inflammatory response in the airways.

Interplay of human tissue kallikrein 4 (hK4) with the plasminogen activation system: hK4 regulates the structure and functions of the urokinase-type plasminogen activator receptor (uPAR)

The plasminogen activation system is involved in cancer progression and metastasis. Among other proteolytic factors, it includes the serine protease urokinase-type plasminogen activator (uPA) and its three-domain (D1D2D3) receptor uPAR (CD87), which focuses plasminogen activation to the cell surface. The function of uPAR is regulated in part through shedding of domain D1 by proteases, e.g., uPA itself or plasmin. Human tissue kallikrein 4 (hK4), which is highly expressed in prostate and ovarian tumor tissue, was previously shown to cleave and activate the pro-enzyme forms of prostate-specific antigen (PSA, tissue kallikrein hK3) and uPA. Here we demonstrate that uPAR is also a target for hK4, being cleaved in the D1-D2 linker sequence and, to a lesser extent, in its D3 juxtamembrane domain. hK4 may thus modulate the tumor-associated uPA/uPAR-system activity by either activating the pro-enzyme form of uPA or cleaving the cell surface-associated uPA receptor.