Receptor-independent role of urokinase-type plasminogen activator in pericellular plasmin and matrix metalloproteinase proteolysis during vascular wound healing in mice

Plasmin activity promotes amyloid deposition in a transgenic model of human transthyretin amyloidosis

Cardiac ATTR amyloidosis, a serious but much under-diagnosed form of cardiomyopathy, is caused by deposition of amyloid fibrils derived from the plasma protein transthyretin (TTR), but its pathogenesis is poorly understood and informative in vivo models have proved elusive. Here we report the generation of a mouse model of cardiac ATTR amyloidosis with transgenic expression of human TTR^S52P. The model is characterised by substantial ATTR amyloid deposits in the heart and tongue. The amyloid fibrils contain both full-length human TTR protomers and the residue 49-127 cleavage fragment which are present in ATTR amyloidosis patients. Urokinase-type plasminogen activator (uPA) and plasmin are abundant within the cardiac and lingual amyloid deposits, which contain marked serine protease activity; knockout of α₂-antiplasmin, the physiological inhibitor of plasmin, enhances amyloid formation. Together, these findings indicate that cardiac ATTR amyloid deposition involves local uPA-mediated generation of plasmin and cleavage of TTR, consistent with the previously described mechano-enzymatic hypothesis for cardiac ATTR amyloid formation. This experimental model of ATTR cardiomyopathy has potential to allow further investigations of the factors that influence human ATTR amyloid deposition and the development of new treatments.

ATTR amyloidosis causes heart failure through the accumulation of misfolded transthyretin in cardiac muscle. Here the authors report a mouse model of ATTR amyloidosis and demonstrate the involvement of protease activity in ATTR amyloid deposition.

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.

Urokinase-type plasminogen activator-mediated crosstalk between N-cadherin and β-catenin promotes wound healing

Urokinase-type plasminogen activator (uPA; encoded by Plau) is a serine proteinase that, in the central nervous system, induces astrocytic activation. β-Catenin is a protein that links the cytoplasmic tail of cadherins to the actin cytoskeleton, thus securing the formation of cadherin-mediated cell adhesion complexes. Disruption of cell-cell contacts leads to the detachment of β-catenin from cadherins, and β-catenin is then degraded by the proteasome following its phosphorylation by GSK3β. Here, we show that astrocytes release uPA following a scratch injury, and that this uPA promotes wound healing via a plasminogen-independent mechanism. We found that uPA induces the detachment of β-catenin from the cytoplasmic tail of N-cadherin (NCAD; also known as CDH2) by triggering its phosphorylation at Tyr654. Surprisingly, this is not followed by degradation of β-catenin because uPA also induces the phosphorylation of the low density lipoprotein receptor-related protein 6 (LRP6) at Ser1490, which then blocks the kinase activity of GSK3β. Our work indicates that the ensuing cytoplasmic accumulation of β-catenin is followed by its nuclear translocation and β-catenin-triggered transcription of the receptor for uPA (Plaur), which in turn is required for uPA to induce astrocytic wound healing.

Therapeutics targeting the fibrinolytic system

The function of the fibrinolytic system was first identified to dissolve fibrin to maintain vascular patency. Connections between the fibrinolytic system and many other physiological and pathological processes have been well established. Dysregulation of the fibrinolytic system is closely associated with multiple pathological conditions, including thrombosis, inflammation, cancer progression, and neuropathies. Thus, molecules in the fibrinolytic system are potent therapeutic and diagnostic targets. This review summarizes the currently used agents targeting this system and the development of novel therapeutic strategies in experimental studies. Future directions for the development of modulators of the fibrinolytic system are also discussed.

The fibrinolytic system was originally identified to dissolve blood clots, and is shown to have important roles in other pathological processes, including cancer progression, inflammation, and thrombosis. Molecules or therapeutics targeting fibrinolytic system have been successfully used in the clinical treatments of cancer and thrombotic diseases. The clinical studies and experimental models targeting fibrinolytic system are reviewed by Haili Lin at Sanming First Hosipital, Mingdong Huang at Fuzhou University in China, and Peng Xu at A*STAR in Singapore to demonstrate fibrinolytic system as novel therapeutic targets. As an example, the inhibition of fibrinolytic system protein can be used to suppress cancer prolifieration and metastasis. This review also discusses the potential therapeutic effects of inhibitiors of fibrinolytic system on inflammatory disorders.

Contribution of platelets, the coagulation and fibrinolytic systems to cutaneous wound healing

Wound healing is a complex process that consists of multiple phases, each of which are indispensable for adequate repair. Timely initiation and resolution of each of these phases namely, hemostasis, inflammation, proliferation and tissue remodeling, is critical for promoting healing and avoiding excess scar formation. While platelets have long been known to influence the healing process, other components of blood particularly coagulation factors and the fibrinolytic system also contribute to efficient wound repair. This review aims to summarize our current understanding of the role of platelets, the coagulation and fibrinolytic systems in cutaneous wound healing, with a focus on how these components communicate with immune and non-immune cells in the wound microenvironment. We also outline current and potential therapeutic strategies to improve the management of chronic, non-healing wounds.

TEM8 functions as a receptor for uPA and mediates uPA-stimulated EGFR phosphorylation

Background

TEM8 is a cell membrane protein predominantly expressed in tumor endothelium, which serves as a receptor for the protective antigen (PA) of anthrax toxin. However, the physiological ligands for TEM8 remain unknown.

Results

Here we identified uPA as an interacting partner of TEM8. Binding of uPA stimulated the phosphorylation of TEM8 and augmented phosphorylation of EGFR and ERK1/2. Finally, TEM8-Fc, a recombinant fusion protein comprising the extracellular domain of human TEM8 linked to the Fc portion of human IgG1, efficiently abrogated the interaction between uPA and TEM8, blocked uPA-induced migration of HepG2 cells in vitro and inhibited the growth and metastasis of human MCF-7 xenografts in vivo. uPA, TEM8 and EGFR overexpression and ERK1/2 phosphorylation were found co-located on frozen cancer tissue sections.

Conclusions

Taken together, our data provide evidence that TEM8 is a novel receptor for uPA, which may play a significant role in the regulation of tumor growth and metastasis.

Electronic supplementary material

The online version of this article (10.1186/s12964-018-0272-8) contains supplementary material, which is available to authorized users.

D2A sequence of the urokinase receptor induces cell growth through αvβ3 integrin and EGFR

The urokinase receptor (uPAR) stimulates cell proliferation by forming a macromolecular complex with αvβ3 integrin and the epidermal growth factor receptor (EGFR, ErbB1 or HER1) that we name the uPAR proliferasome. uPAR transactivates EGFR, which in turn mediates uPAR-initiated mitogenic signal to the cell. EGFR activation and EGFR-dependent cell growth are blocked in the absence of uPAR expression or when uPAR activity is inhibited by antibodies against either uPAR or EGFR. The mitogenic sequence of uPAR corresponds to the D2A motif present in domain 2. NMR analysis revealed that D2A synthetic peptide has a particular three-dimensional structure, which is atypical for short peptides. D2A peptide is as effective as EGF in promoting EGFR phosphorylation and cell proliferation that were inhibited by AG1478, a specific inhibitor of the tyrosine kinase activity of EGFR. Both D2A and EGF failed to induce proliferation of NR6-EGFR-K721A cells expressing a kinase-defective mutant of EGFR. Moreover, D2A peptide and EGF phosphorylate ERK demonstrating the involvement of the MAP kinase signalling pathway. Altogether, this study reveals the importance of sequence D2A of uPAR, and the interdependence of uPAR and EGFR.

Proteases as prognostic markers in human and canine cancers

The extracellular matrix (ECM) is composed of several types of proteins, which interact and form dynamic networks. These components can modulate cell behaviour and actively influence the growth and differentiation of tissues. ECM is also important in several pathological processes, such as cancer invasion and metastasis, by creating favourable microenvironments. Proteolysis in neoplastic tissues is mediated by proteinases, whose regulation involves complex interactions between neoplastic cells and non-neoplastic stromal cells. In this review, we discuss aspects of proteinase expression and tumor behaviour in humans and dogs. Different classes of proteases are summarized, with special emphasis being placed on molecules that have been shown to correlate with prognosis, reinforcing the need for a better understanding of the regulation of this microenvironment and its influences in tumor progression and metastasis, which should significantly aid the development of improved prognosis and treatment.

A CCR2 macrophage endocytic pathway mediates extravascular fibrin clearance in vivo

Extravascular fibrin deposition accompanies many human diseases and causes chronic inflammation and organ damage, unless removed in a timely manner. Here, we used intravital microscopy to investigate how fibrin is removed from extravascular space. Fibrin placed into the dermis of mice underwent cellular endocytosis and lysosomal targeting, revealing a novel intracellular pathway for extravascular fibrin degradation. A C-C chemokine receptor type 2 (CCR2)-positive macrophage subpopulation constituted the majority of fibrin-uptaking cells. Consequently, cellular fibrin uptake was diminished by elimination of CCR2-expressing cells. The CCR2-positive macrophage subtype was different from collagen-internalizing M2-like macrophages. Cellular fibrin uptake was strictly dependent on plasminogen and plasminogen activator. Surprisingly, however, fibrin endocytosis was unimpeded by the absence of the fibrin(ogen) receptors, αMβ2 and ICAM-1, the myeloid cell integrin-binding site on fibrin or the endocytic collagen receptor, the mannose receptor. The study identifies a novel fibrin endocytic pathway engaged in extravascular fibrin clearance and shows that interstitial fibrin and collagen are cleared by different subsets of macrophages employing distinct molecular pathways.

MHJ_0461 is a multifunctional leucine aminopeptidase on the surface of Mycoplasma hyopneumoniae

Aminopeptidases are part of the arsenal of virulence factors produced by bacterial pathogens that inactivate host immune peptides. Mycoplasma hyopneumoniae is a genome-reduced pathogen of swine that lacks the genetic repertoire to synthesize amino acids and relies on the host for availability of amino acids for growth. M. hyopneumoniae recruits plasmin(ogen) onto its cell surface via the P97 and P102 adhesins and the glutamyl aminopeptidase MHJ_0125. Plasmin plays an important role in regulating the inflammatory response in the lungs of pigs infected with M. hyopneumoniae. We show that recombinant MHJ_0461 (rMHJ_0461) functions as a leucine aminopeptidase (LAP) with broad substrate specificity for leucine, alanine, phenylalanine, methionine and arginine and that MHJ_0461 resides on the surface of M. hyopneumoniae. rMHJ_0461 also binds heparin, plasminogen and foreign DNA. Plasminogen bound to rMHJ_0461 was readily converted to plasmin in the presence of tPA. Computational modelling identified putative DNA and heparin-binding motifs on solvent-exposed sites around a large pore on the LAP hexamer. We conclude that MHJ_0461 is a LAP that moonlights as a multifunctional adhesin on the cell surface of M. hyopneumoniae.

Molecular functions of thyroid hormones and their clinical significance in liver-related diseases

Thyroid hormones (THs) are potent mediators of several physiological processes, including embryonic development, cellular differentiation, metabolism, and cell growth. Triiodothyronine (T3) is the most biologically active TH form. Thyroid hormone receptors (TRs) belong to the nuclear receptor superfamily and mediate the biological functions of T3 via transcriptional regulation. TRs generally form heterodimers with the retinoid X receptor (RXR) and regulate target genes upon T3 stimulation. Research over the past few decades has revealed that disruption of cellular TH signaling triggers chronic liver diseases, including alcoholic or nonalcoholic fatty liver disease and hepatocellular carcinoma (HCC). Animal model experiments and epidemiologic studies to date imply close associations between high TH levels and prevention of liver disease. Moreover, several investigations spanning four decades have reported the therapeutic potential of T3 analogs in lowering lipids, preventing chronic liver disease, and as anticancer agents. Thus, elucidating downstream genes/signaling pathways and molecular mechanisms of TH actions is critical for the treatment of significant public health issues. Here, we have reviewed recent studies focusing on the roles of THs and TRs in several disorders, in particular, liver diseases. We also discuss the potential therapeutic applications of THs and underlying molecular mechanisms.

A novel autologous cell-based therapy to promote diabetic wound healing

OBJECTIVES

We have previously shown that stromal cell-derived factor-1α (SDF-1α) is downregulated within diabetic cutaneous wounds, and that direct application of recombinant SDF-1α increases wound closure rates, neovascularization, and endothelial progenitor cell (EPC) recruitment. However, increased wound levels of exogenous SDF-1α results in elevated systemic levels of this proangiogenic chemokine that raises concerns for tumorigenesis and inflammation. We now seek to test the efficacy of a novel, safer cell-based therapy (CBT) employing ex vivo primed bone marrow-derived stem cells (BMDSC) with SDF-1α. We also elucidate the mechanism of action of this new approach for accelerating diabetic wound healing.

METHODS

Unfractionated BMDSC from diabetic Lepr mice were incubated for 20 hours with SDF-1α (100 ng/mL) or bovine serum albumin (control). Pretreated BMDSC (1 × 10) were injected subcutaneously into full-thickness skin wounds in Lepr mice (n = 8 per group). Wound closure rates, capillary density, and the recruitment of EPC were assessed with serial photography, DiI perfusion, confocal microscopy, and immunohistochemistry. The expression of molecular targets, which may mediate prohealing/proangiogenic effects of SDF-1α-primed BMDSC was evaluated by polymerase chain reaction array and immunoblotting assay. The biological function of a potential mediator was tested in a mouse wound-healing model. Serum SDF-1α levels were measured with enzyme-linked immunosorbent assay (ELISA).

RESULTS

SDF-1α-primed BMDSC significantly promote wound healing (P < 0.0001), neovascularization (P = 0.0028), and EPC recruitment (P = 0.0059). Gene/protein expression studies demonstrate upregulation of Ephrin Receptor B4 and plasminogen as downstream targets potentially mediating the prohealing and proangiogenic responses. Ex vivo BMDSC activation and the subsequent inoculation of cells into wounds does not increase systemic SDF-1α levels.

CONCLUSIONS

We report a novel CBT that is highly effective in promoting healing and neovascularization in a murine model of type 2 diabetes. Furthermore, we identify new molecular targets that may be important for advancing the field of wound healing.

Urokinase-type plasminogen activator-like proteases in teleosts lack genuine receptor-binding epidermal growth factor-like domains

Plasminogen activation catalyzed by urokinase-type plasminogen activator (uPA) plays an important role in normal and pathological tissue remodeling processes. Since its discovery in the mid-1980s, the cell membrane-anchored urokinase-type plasminogen activator receptor (uPAR) has been believed to be central to the functions of uPA, as uPA-catalyzed plasminogen activation activity appeared to be confined to cell surfaces through the binding of uPA to uPAR. However, a functional uPAR has so far only been identified in mammals. We have now cloned, recombinantly produced, and characterized two zebrafish proteases, zfuPA-a and zfuPA-b, which by several criteria are the fish orthologs of mammalian uPA. Thus, both proteases catalyze the activation of fish plasminogen efficiently and both proteases are inhibited rapidly by plasminogen activator inhibitor-1 (PAI-1). But zfuPA-a differs from mammalian uPA by lacking the exon encoding the uPAR-binding epidermal growth factor-like domain; zfuPA-b differs from mammalian uPA by lacking two cysteines of the epidermal growth factor-like domain and a uPAR-binding sequence comparable with that found in mammalian uPA. Accordingly, no zfuPA-b binding activity could be found in fish white blood cells or fish cell lines. We therefore propose that the current consensus of uPA-catalyzed plasminogen activation taking place on cell surfaces, derived from observations with mammals, is too narrow. Fish uPAs appear incapable of receptor binding in the manner known from mammals and uPA-catalyzed plasminogen activation in fish may occur mainly in solution. Studies with nonmammalian vertebrate species are needed to obtain a comprehensive understanding of the mechanism of plasminogen activation.

Urokinase-type plasminogen activator deficiency in bone marrow-derived cells augments rupture of angiotensin II-induced abdominal aortic aneurysms

OBJECTIVE

Abdominal aortic aneurysms (AAAs) are associated with fragmentation of extracellular matrix during development of aortic dilation and rupture. Therefore, it is important to identify specific protease systems involved in extracellular matrix degradation during AAA formation. The present study determined the contribution of the urokinase system to AAA formation and rupture.

METHODS AND RESULTS

Angiotensin II (Ang II)-induced AAAs were associated with increased aortic abundance of both urokinase-type plasminogen activator receptor (uPAR) and urokinase-type plasminogen activator (uPA) proteins. However, this increased presence was unrelated to AAA formation because deficiencies of either uPAR or uPA had no effect on either the incidence or size of Ang II-induced AAAs in both normolipidemic mice and low-density lipoprotein receptor-/- mice fed a saturated fat-enriched diet. Although uPA deficiency did not affect development of AAAs, there was an effect of increasing mortality rate from AAA rupture in hypercholesterolemic mice. Bone marrow transplantation demonstrated that enhanced aneurysmal rupture was attributable to deficiency of uPA in leukocytes. uPA deficiency led to an increased propensity for impaired resolution of the thrombotic material within the aneurysmal tissue. Neither uPAR nor uPA deficiency had any effect on Ang II-induced atherosclerosis in low-density lipoprotein receptor-/- mice.

CONCLUSIONS

The uPA-uPAR axis has no effect on the formation of Ang II-induced AAAs, but uPA deficiency promotes aneurysmal rupture.

Selective abrogation of the uPA-uPAR interaction in vivo reveals a novel role in suppression of fibrin-associated inflammation

The urokinase plasminogen activator receptor (uPAR) has emerged as a potential regulator of cell adhesion, cell migration, proliferation, differentiation, and cell survival in multiple physiologic and pathologic contexts. The urokinase plasminogen activator (uPA) was the first identified ligand for uPAR, but elucidation of the specific functions of the uPA-uPAR interaction in vivo has been difficult because uPA has important physiologic functions that are independent of binding to uPAR and because uPAR engages multiple ligands. Here, we developed a new mouse strain (Plau(GFDhu/GFDhu)) in which the interaction between endogenous uPA and uPAR is selectively abrogated, whereas other functions of both the protease and its receptor are retained. Specifically, we introduced 4 amino acid substitutions into the growth factor domain (GFD) of uPA that abrogate uPAR binding while preserving the overall structure of the domain. Analysis of Plau(GFDhu/GFDhu) mice revealed an unanticipated role of the uPA-uPAR interaction in suppressing inflammation secondary to fibrin deposition. In contrast, leukocyte recruitment and tissue regeneration were unaffected by the loss of uPA binding to uPAR. This study identifies a principal in vivo role of the uPA-uPAR interaction in cell-associated fibrinolysis critical for suppression of fibrin accumulation and fibrin-associated inflammation and provides a valuable model for further exploration of this multifunctional receptor.

Down-regulation of uPA and uPAR by 3,3'-diindolylmethane contributes to the inhibition of cell growth and migration of breast cancer cells

3,3'-Diindolylmethane (DIM) is a known anti-tumor agent against breast and other cancers; however, its exact mechanism of action remains unclear. The urokinase plasminogen activator (uPA) and its receptor (uPAR) system are involved in the degradation of basement membrane and extracellular matrix, leading to tumor cell invasion and metastasis. Since uPA-uPAR system is highly activated in aggressive breast cancer, we hypothesized that the biological activity of B-DIM could be mediated via inactivation of uPA-uPAR system. We found that B-DIM treatment as well as silencing of uPA-uPAR led to the inhibition of cell growth and motility of MDA-MB-231 cells, which was in part due to inhibition of VEGF and MMP-9. Moreover, silencing of uPA-uPAR led to decreased sensitivity of these cells to B-DIM indicating an important role of uPA-uPAR in B-DIM-mediated inhibition of cell growth and migration. We also found similar effects of B-DIM on MCF-7, cells expressing low levels of uPA-uPAR, which was due to direct down-regulation of MMP-9 and VEGF, independent of uPA-uPAR system. Interestingly, over-expression of uPA-uPAR in MCF-7 cells attenuated the inhibitory effects of B-DIM. Our results, therefore, suggest that B-DIM down-regulates uPA-uPAR in aggressive breast cancers but in the absence of uPA-uPAR, B-DIM can directly inhibit VEGF and MMP-9 leading to the inhibition of cell growth and migration of breast cancer cells.

Inactivation of uPA and its receptor uPAR by 3,3'-diindolylmethane (DIM) leads to the inhibition of prostate cancer cell growth and migration

3,3'-Diindolylmethane (DIM) has been studied for its putative anti-cancer properties, especially against prostate cancer; however, its exact mechanism of action remains unclear. We recently provided preliminary data suggesting down-regulation of uPA during B-DIM (a clinically active DIM)-induced inhibition of invasion and angiogenesis in prostate cancer cells. Since the expression and activation of uPA plays important role in tumorigenicity, and high endogenous levels of uPA and uPAR are found in advanced metastatic cancers, we investigated their role in B-DIM-mediated inhibition of prostate cancer cell growth and motility. Using PC3 cells, we found that B-DIM treatment as well as the silencing of uPA and uPAR by siRNAs led to the inhibition of cell growth and motility. Conversely, over-expression of uPA/uPAR in LNCaP and C4-2B cells resulted in increased cell growth and motility, which was effectively inhibited by B-DIM. Moreover, we found that uPA as well as uPAR induced the production of VEGF and MMP-9, and that the down-regulation of uPA/uPAR by siRNAs or B-DIM treatment resulted in the inhibition of VEGF and MMP-9 secretion which could be responsible for the observed inhibition of cell migration. Interestingly, silencing of uPA/uPAR led to decreased sensitivity to B-DIM indicating important role of uPA/uPAR in B-DIM-mediated regulation of prostate cancer cell growth and migration. Our data suggest that chemopreventive and/or therapeutic activity of B-DIM is in part due to down-regulation of uPA-uPAR leading to reduced production of VEGF/MMP-9 which ultimately leads to the inhibition of cell growth and migration of aggressive prostate cancer cells.

Dissecting the urokinase activation pathway using urokinase-activated anthrax toxin

Anthrax toxin is a three-part toxin secreted by Bacillus anthracis, consisting of protective antigen (PrAg), edema factor (EF), and lethal factor (LF). To intoxicate host mammalian cells, PrAg, the cell-binding moiety of the toxin, binds to cells and is then proteolytically activated by furin on the cell surface, resulting in the active heptameric form of PrAg. This heptamer serves as a protein-conducting channel that translocates EF and LF, the two enzymatic moieties of the toxin, into the cytosol of the cells where they exert cytotoxic effects. The anthrax toxin delivery system has been well characterized. The amino-terminal PrAg-binding domain of LF (residues 1-254, LFn) is sufficient to allow translocation of fused "passenger" polypeptides, such as the ADP-ribosylation domain of Pseudomonas exotoxin A, to the cytosol of the cells in a PrAg-dependent process. The protease specificity of the anthrax toxin delivery system can also be reengineered by replacing the furin cleavage target sequence of PrAg with other protease substrate sequences. PrAg-U2 is such a PrAg variant, one that is selectively activated by urokinase plasminogen activator (uPA). The uPA-dependent proteolytic activation of PrAg-U2 on the cell surface is readily detected by western blotting analysis of cell lysates in vitro, or cell or animal death in vivo. Here, we describe the use of PrAg-U2 as a molecular reporter tool to test the controversial question of what components are required for uPAR-mediated cell surface pro-uPA activation. The results demonstrate that both uPAR and plasminogen play critical roles in pro-uPA activation both in vitro and in vivo.

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.

Hic-5, an adaptor protein expressed in vascular smooth muscle cells, modulates the arterial response to injury in vivo

Focal adhesion components are targets for biochemical and mechanical stimuli that evoke crucial injury. Hic-5 (hydrogen peroxide-inducible clone 5) is a multidomain adaptor protein which is implicated in the regulation of integrin signaling in focal adhesion. The aim of this research was to test the hypothesis that Hic-5, a focal adhesion LIM protein expressed in smooth muscle cells, is involved in dynamic processes by pathological stimuli in the vessel wall. Here, we describe the analysis of the function of Hic-5 using a mouse model of vascular injury that may mimic balloon angioplasty. At 4 days after vascular injury, marked down-regulation of the Hic-5 expression was observed in the smooth muscle layer, and local delivery of the Hic-5 using adenovirus vectors repressed injury-induced neointimal expansion. In addition, Hic-5 reduced cells migration into three-dimensional collagen gels, and the forced expression of Hic-5 in cells embedded in the collagen gel matrix repressed the expression of uPA that participates in smooth muscle cell migration. These results suggest that Hic-5 modulates cellular responses to pathological stimuli in the vessel wall.

Immunomodulatory effects of plasminogen activators on hepatic fibrogenesis

Tissue-type plasminogen activators (tPA) and urokinase-type plasminogen activators (uPA) are involved in liver repair. We examined the potential immunomodulatory actions of uPA, tPA and uPA-receptor (uPAR) in carbon-tetrachloride-induced hepatic fibrosis in wild-type (WT), tPA-/-, uPA-/- and uPAR-/- mice. Carbon-tetrachloride treatment increased fibrosis in four groups but significantly less in three knock-out models. Serum cytokines and intrahepatic T cells elevated significantly following fibrosis process in WT animals but not in the knock-out groups. In culture, uPA increased lymphocyte proliferation significantly in WT and uPA-/- but not uPAR-/- animals. Following uPA exposure in vivo, there was CD8 predominance. To isolate uPA's effect on lymphocytes, WT mice were irradiated sublethally and then reconstituted with WT or uPA-/- lymphocytes. In these animals fibrosis was decreased and T cells were reduced in the uPA-/- recipients. Based on these data we postulate that plasminogen activators affect fibrosis in part by liver-specific activation of CD8 subsets that govern the fibrogenic activity of hepatic stellate cells.

Urinary-type plasminogen activator (uPA) and its receptor (uPAR) in squamous cell carcinoma of the oral cavity

OSCC (oral squamous cell carcinoma) is the most common oral malignancy and is estimated to affect approx. 350000 new patients worldwide this year. OSCC is characterized by a high degree of morbidity and mortality, as most patients exhibit local, regional and distant metastasis at the time of diagnosis. Recent genome-wide screening efforts have identified the serine proteinase uPA (urinary-type plasminogen activator, also known as urokinase) as a strong biomarker for prediction of poor disease outcome and a key candidate for molecular classification of oral neoplasms using a 'gene signature' approach. The proteinase uPA binds a surface-anchored receptor designated uPAR (uPA receptor), focalizing proteolytic activity to the pericellular milieu. Furthermore, uPA-uPAR can interact with transmembrane proteins to modify multiple signal transduction pathways and influence a wide variety of cellular behaviours. Correlative clinical data show elevated uPA-uPAR in oral tumour tissues, with tumours exhibiting high levels of both uPA and uPAR as the most invasive. Combined in vitro, pre-clinical and clinical data support the need for further analysis of uPA-uPAR as a prognostic indicator as well as a potential therapeutic target in OSCC.

Evolving role of uPA/uPAR system in human cancers

Recent advancements in cancer research have led to some major breakthroughs; however, the impact on overall cancer-related death rate remains unacceptable, suggesting that further insight into tumor markers and development of targeted therapies is urgently needed. The urokinase plasminogen activator (uPA) system represents a family of serine proteases that are involved in the degradation of basement membrane and the extracellular matrix, leading to tumor cell invasion and metastasis. In this review, we have provided an overview of emerging data, from basic research as well as clinical studies, highlighting the evolving role of uPA/uPAR system in tumor progression. It is currently believed that the expression and activation of uPA plays an important role in tumorigenicity, and high endogenous levels of uPA and uPAR are associated with advanced metastatic cancers. The endogenous inhibitors of this system, PAI-1 and PAI-2, regulate uPA-uPAR activity by either direct inhibition or affecting cell surface expression and internalization. PAI-1's role in cancers is rather unusual; on one hand, it inhibits uPA-uPAR leading to inhibition of invasion and metastasis and on the other it has been reported to facilitate tumor growth and angiogenesis. Individual components of uPA/uPAR system are reported to be differentially expressed in cancer tissues compared to normal tissues and, thus, have the potential to be developed as prognostic and/or therapeutic targets. Therefore, this system represents a highly attractive target that warrants further in-depth studies. Such studies are likely to contribute towards the development of molecularly-driven targeted therapies in the near future.

uPA deficiency exacerbates muscular dystrophy in MDX mice

Duchenne muscular dystrophy (DMD) is a fatal and incurable muscle degenerative disorder. We identify a function of the protease urokinase plasminogen activator (uPA) in mdx mice, a mouse model of DMD. The expression of uPA is induced in mdx dystrophic muscle, and the genetic loss of uPA in mdx mice exacerbated muscle dystrophy and reduced muscular function. Bone marrow (BM) transplantation experiments revealed a critical function for BM-derived uPA in mdx muscle repair via three mechanisms: (1) by promoting the infiltration of BM-derived inflammatory cells; (2) by preventing the excessive deposition of fibrin; and (3) by promoting myoblast migration. Interestingly, genetic loss of the uPA receptor in mdx mice did not exacerbate muscular dystrophy in mdx mice, suggesting that uPA exerts its effects independently of its receptor. These findings underscore the importance of uPA in muscular dystrophy.

Urokinase gene transfer augments angiogenesis in ischemic skeletal and myocardial muscle

Urokinase plasminogen activator (uPA) is required for both endogenous and vascular endothelial growth factor (VEGF)-augmented angiogenesis in normal tissues, leading us to hypothesize that uPA augmentation by gene transfer might promote angiogenesis in ischemic tissues. Overexpression of uPA was studied in rat myocardial infarction (MI) and mouse hind limb ischemia models and compared with VEGF overexpression effects. Animals were divided into control and three experimental groups (n = 6), receiving intramuscular injections of plasmids as follows: (i) control (empty vector or expressing beta-galactosidase); (ii) uPA; (iii) VEGF(165); (iv) a 1:1 mixture of uPA and VEGF(165). The capillary densities in both ischemic models were greater (P < 0.05) in tissues treated with uPA, VEGF, or a combination of both than in controls. Infarct size was reduced in hearts from uPA and VEGF experimental groups compared with controls (P < 0.05). Local overexpression of uPA induced a marked increase in the number of macrophages and myofibroblasts present within infarcts. Hind limb blood flow was greater in all experimental groups by day 10 (P < 0.05). Overall, the effects of uPA and VEGF were uniformly comparable. Additional analysis revealed association of local edema with VEGF but not with uPA treatment. This study established that uPA gene therapy effectively induces functionally significant angiogenesis in models of acute MI and hind limb ischemia.

The urokinase-type plasminogen activator receptor is not required for skeletal muscle inflammation or regeneration

The hypothesis of this study was the urokinase-type plasminogen activator receptor (uPAR) is required for accumulation of inflammatory cells in injured skeletal muscle and for efficient muscle regeneration. Expression of uPAR was elevated at 1 and 3 days after cardiotoxin-induced muscle injury in wild-type mice before returning to baseline levels. Neutrophil accumulation peaked 1 day postinjury in muscle from both wild-type (WT) and uPAR null mice, while macrophage accumulation peaked between 3 and 5 days postinjury, with no differences between strains. Histological analyses confirmed efficient muscle regeneration in both wild-type and uPAR null mice, with no difference between strains in the formation or growth of regenerating fibers, or recovery of normal morphology. Furthermore, in vitro experiments demonstrated that chemotaxis is not different between WT and uPAR null macrophages. Finally, fusion of cultured satellite cells into multinucleated myotubes was not different between cells isolated from WT and uPAR null mice. These results demonstrate that uPAR is not required for the accumulation of inflammatory cells or the regeneration of skeletal muscle following injury, suggesting uPA can act independently of uPAR to regulate events critical for muscle regeneration.

A guide to murine fibrinolytic factor structure, function, assays, and genetic alterations

The components and functions of the murine fibrinolytic system are quite similar to those of humans. Because of these similarities and the adaptability of mice to genetic manipulation, murine fibrinolysis has been studied extensively. These studies have yielded important information regarding the function of the several components of fibrinolysis. This review presents information on the structure, function and assay of mouse fibrinolytic parameters and it discusses the results of the extensive studies of genetically modified mice. It is intended to be a convenient reference resource for investigators of fibrinolysis.

Haemostasis

When the continuity of the vascular endothelium is disrupted, platelets and fibrin seal off the defect. Haemostatic processes are classified as primary (mainly involving platelets) and secondary (mainly related to fibrin formation or blood coagulation). When the blood clot is no longer required for haemostasis, the fibrinolytic system will dissolve it. The pivotal ligand for initial platelet recruitment to injured vessel wall components is von Willebrand factor (vWF), a multimeric protein present in the subendothelium and in plasma, where it is conformationally activated by shear forces. Adhering activated platelets recruit additional platelets, which are in turn activated and form a platelet aggregate. Coagulation is initiated by a reaction, activating factors IX and X. Once critical amounts of factor Xa are generated, thrombin generation is initiated and soluble fibrinogen is converted into insoluble fibrin. Excessive thrombin generation is prevented via inhibition by antithrombin and also via downregulation of its further generation by activation of the protein C pathway. Activation of the fibrinolytic system results from conversion of the proenzyme plasminogen into the active serine proteinase plasmin by tissue-type or urokinase-type plasminogen activators. Plasmin digests the fibrin component of a blood clot. Inhibition of the fibrinolytic system occurs at the level of the plasminogen activator (by plasminogen activator inhibitors) or at the level of plasmin (by alpha2-antiplasmin). Together, these physiological processes act to maintain normal functioning blood vessels and a non-thrombotic state.

Modulation of Smooth Muscle Cell Migration by Members of the Low-Density Lipoprotein Receptor Family

Low-density lipoprotein receptor family members (LRs) play a key role in the catabolism of many membrane-associated proteins, such as complexes between proteinases and their receptors, in addition to being involved in lipoprotein metabolism as suspected by the hitherto well-established functions of low-density lipoprotein receptor, in a variety of tissues. Recent studies using receptor-deficient or -overexpressing animals and cells have suggested that certain LRs are important regulators of the migration (and proliferation) of vascular smooth muscle cells (SMCs). LR expression is markedly induced in intimal or medial SMCs during the formation of atherosclerotic lesions. Because LRs can modulate the activity of the urokinase-type plasminogen activator (uPA) receptor and possibly of the platelet-derived growth factor (PDGF) receptor, LRs may influence the migration of SMCs through functional modulation of these membrane receptors. Therefore, SMC migration may be regulated by time-restricted expression of LRs. In agreement with the concept of functional interaction between LRs and membrane signaling receptors, a negative regulator of uPA receptor protein catabolism, LR11, has been identified. Statins modulate the PDGF-induced migration of intimal SMCs via the LR11/uPA receptor cascade. Selective modification of the LRs/uPA receptor/PDGF receptor systems in SMCs may be important for suppression of atherosclerotic plaque formation as well as for preventing intimal thickening after angioplasty.

Targeting of urokinase plasminogen activator receptor in human pancreatic carcinoma cells inhibits c-Met- and insulin-like growth factor-I receptor-mediated migration and invasion and orthotopic tumor growth in mice

Pancreatic carcinomas express high levels of urokinase-type plasminogen activator (uPA) and its receptor (uPAR), both of which mediate cell migration and invasion. We investigated the hypotheses that (a) insulin-like growth factor-I (IGF-I)- and hepatocyte growth factor (HGF)-mediated migration and invasion of human pancreatic carcinoma cells require uPA and uPAR function and (b) inhibition of uPAR inhibits tumor growth, retroperitoneal invasion, and hepatic metastasis of human pancreatic carcinomas in mice. Using transwell assays, we investigated the effect of IGF-I and HGF on L3.6pl migration and invasion. We measured the induction of uPA and uPAR following treatment of cells with IGF-I and HGF using immunoprecipitation and Western blot analysis. The importance of uPA and uPAR on L3.6pl cell migration and invasion was studied by inhibiting their activities with amiloride and antibodies before cytokine treatment. In an orthotopic mouse model of human pancreatic carcinoma, we evaluated the effect of anti-uPAR monoclonal antibodies with and without gemcitabine on primary tumor growth, retroperitoneal invasion, and hepatic metastasis. IGF-I and HGF mediated cell migration and invasion in L3.6pl cells. In addition, IGF-I and HGF induced uPA and uPAR expression in L3.6pl cells. In vitro, blockade of uPA and uPAR activity inhibited IGF-I- and HGF-mediated cell migration and invasion. Treatment of mice with anti-uPAR monoclonal antibody significantly decreased pancreatic tumor growth and hepatic metastasis and completely inhibited retroperitoneal invasion. Our study shows the importance of the uPA/uPAR system in pancreatic carcinoma cell migration and invasion. These findings suggest that uPAR is a potential target for therapy in patients with pancreatic cancer.

Fibrin-induced skin fibrosis in mice deficient in tissue plasminogen activator

The deposition of fibrin is an integral part of the tissue repair process, but its persistence is also associated with a number of fibrotic conditions. This study addressed the hypothesis that reduced fibrinolysis and fibrin persistence are associated with an enhanced accumulation of collagen and the development of skin fibrosis. Decreased fibrinolysis was confirmed in fibrin gel cultures that contained human dermal fibroblasts plus the specific plasmin inhibitor alpha(2)-antiplasmin or dermal fibroblasts isolated from plasminogen activator (PA)-deficient mice. Collagen accumulation was significantly increased in the presence of inhibitor and in tPA-deficient, but not uPA-deficient, fibroblasts compared with controls. These findings were also confirmed using a skin fibrosis model in which multiple injections of fibrin were given subcutaneously to PA-deficient mice. Injection sites from tPA-deficient mice displayed significantly increased collagen levels compared with uPA-deficient mice and wild-type controls. Up-regulation of fibroblast procollagen gene expression and reduced activation of pro-MMP-1 appeared to mediate the increase in collagen by human dermal fibroblasts in the presence of alpha2-antiplasmin. These findings suggest that persistent fibrin is associated with enhanced collagen accumulation that may result in the development of fibrotic skin disorders in which reduced fibrinolysis is a feature.

Arterial Wall Response to ex vivo Exposure to Oscillatory Shear Stress

BACKGROUND

The aim of this study was to analyze the arterial wall response to plaque-prone hemodynamic environments, known to occur mainly in areas of arterial trees such as bifurcations and branching points. In these areas, the vasculature is exposed to cyclically reversing flow that induces an endothelial dysfunction predisposing thus arteries to local development of atherosclerotic plaques.

METHODS

We used an ex vivo perfusion system that allows culturing arterial segments under different hemodynamic conditions. Porcine carotid arteries were exposed for 3 days to unidirectional high and low shear stress (6 +/- 3 and 0.3 +/- 0.1 dyn/cm(2)) as well as to oscillatory shear stress (0.3 +/- 3 dyn/cm(2)). This latter condition mimics the hemodynamics present at plaque-prone areas. At the end of the perfusion, the influence of different flow patterns on arterial metabolism was assessed in terms of matrix turnover as well as of smooth muscle cell function, differentiation and migration.

RESULTS

Our results show that after 3 days of perfusion none of the applied conditions influence smooth muscle cell phenotype retaining their full contraction capacity. However, an increase in the expression level of matrix metalloproteinase-2 and -9, as well as a decrease in plasminogen activator inhibitor-1 expression were observed in arteries exposed to oscillatory shear stress when compared to arteries exposed to unidirectional shear stress.

CONCLUSION

These observations suggest that plaque-prone hemodynamic environment triggers a vascular wall remodelling process and promotes changes in arterial wall metabolism, with important implication in atherogenesis.

Normal cutaneous wound healing: clinical correlation with cellular and molecular events

BACKGROUND

Cutaneous wound healing is a normal physiologic function, observed and described for centuries by those afflicted with wounds and by those caring for them. Recently, tremendous progress has been made in discovering the cellular and molecular mechanisms responsible for wound healing. Counseling patients appropriately and planning future therapeutic interventions in delayed or abnormal wound healing may be improved by a thorough understanding of the relationship between clinical, cellular, and subcellular events occurring during the normal healing process.

MATERIALS AND METHODS

A review of the wound healing literature from the past several decades, with a focus on the past 5 to 10 years in particular, along with illustrative case examples from our clinical practice over the past decade.

RESULTS

Traditional clinical stages of wounding healing are still relevant, but more overlap between stages is likely a more accurate depiction of events. The role of cells such as platelets, macrophages, leukocytes, fibroblasts, endothelial cells, and keratinocytes is much better known, particularly during the inflammatory and proliferation stages of healing. Molecules such as interferon, integrins, proteoglycans and glycosaminoglycans, matrix metalloproteinases, and other regulatory cytokines play a critical role in the regulation of healing mechanisms.

CONCLUSION

Cutaneous wound healing in normal hosts follows an orderly clinical process. The scientific underpinnings for healing are better understood than ever, although much remains to be discovered. Eventually, such improved understanding of cellular and subcellular physiology may lead to new or better forms of therapy for patients with acute, chronic, and surgical skin wounds.

Normal cutaneous wound healing: clinical correlation with cellular and molecular events

BACKGROUND

Cutaneous wound healing is a normal physiologic function, observed and described for centuries by those afflicted with wounds and by those caring for them. Recently, tremendous progress has been made in discovering the cellular and molecular mechanisms responsible for wound healing. Counseling patients appropriately and planning future therapeutic interventions in delayed or abnormal wound healing may be improved by a thorough understanding of the relationship between clinical, cellular, and subcellular events occurring during the normal healing process.

MATERIALS AND METHODS

A review of the wound healing literature from the past several decades, with a focus on the past 5 to 10 years in particular, along with illustrative case examples from our clinical practice over the past decade.

RESULTS

Traditional clinical stages of wounding healing are still relevant, but more overlap between stages is likely a more accurate depiction of events. The role of cells such as platelets, macrophages, leukocytes, fibroblasts, endothelial cells, and keratinocytes is much better known, particularly during the inflammatory and proliferation stages of healing. Molecules such as interferon, integrins, proteoglycans and glycosaminoglycans, matrix metalloproteinases, and other regulatory cytokines play a critical role in the regulation of healing mechanisms.

CONCLUSION

Cutaneous wound healing in normal hosts follows an orderly clinical process. The scientific underpinnings for healing are better understood than ever, although much remains to be discovered. Eventually, such improved understanding of cellular and subcellular physiology may lead to new or better forms of therapy for patients with acute, chronic, and surgical skin wounds.

Loss or inhibition of uPA or MMP-9 attenuates LV remodeling and dysfunction after acute pressure overload in mice

Left ventricular (LV) hypertrophy is a natural response of the heart to increased pressure loading, but accompanying fibrosis and dilatation may result in irreversible life-threatening heart failure. Matrix metalloproteinases (MMPs) have been invoked in various cardiac diseases, however, direct genetic evidence for a role of the plasminogen activator (PA) and MMP systems in pressure overload-induced LV hypertrophy and in heart failure is lacking. Therefore, the consequences of transverse aortic banding (TAB) were analyzed in mice lacking tissue-type PA (t-PA(-/-)), urokinase-type PA (u-PA(-/-)), or gelatinase-B (MMP-9(-/-)), and in wild-type (WT) mice after adenoviral gene transfer of the PA-inhibitor PAI-1 or the MMP-inhibitor TIMP-1. TAB elevated LV pressure comparably in all genotypes. In WT and t-PA(-/-) mice, cardiomyocyte hypertrophy was associated with myocardial fibrosis, LV dilatation and dysfunction, and pump failure after 7 weeks. In contrast, in u-PA(-/-) mice or in WT mice after PAI-1- and TIMP-1-gene transfer, cardiomyocyte hypertrophy was moderate and only minimally associated with cardiac fibrosis and LV dilatation, resulting in better preservation of pump function. Deficiency of MMP-9 had an intermediate effect. These findings suggest that the use of u-PA- or MMP-inhibitors might preserve cardiac pump function in LV pressure overloading.

Overexpression of urokinase receptor and cell surface urokinase-type plasminogen activator in the human vessel wall with different types of atherosclerotic lesions

Urokinase-type plasminogen activator (UPA) has been implicated in a broad spectrum of pathogenic processes involved in the formation and disruption of atherosclerotic lesions. Up to now, there is no consensus on the contribution of membrane-bound UPA and its receptor CD87 (UPAR) to the development of atherosclerosis. In this study, we determined comparatively the levels of UPAR and UPAR-bound UPA in segments of human coronary and aortic vessels with different degrees of atherosclerotic lesions (macroscopically normal areas, early atherosclerotic lesions, fibrous and calcified plaques). The UPAR content increased progressively with the severity of atherosclerosis. In aortic segments, in which intima and media layers were analyzed separately, the content of UPAR in the intima significantly exceeded the levels measured in the media. Using a detergent-phase separation method with a Triton X-114-containing buffer, we could demonstrate that the levels of membrane (glycosylphosphatidylinositol)-anchored UPAR were significantly higher in the intima of early atherosclerotic lesions as well as in the cap areas of fibrous plaques compared with macroscopically normal areas. However, only 20-25% of the intimal and 30-50% of the medial glycosylphosphatidylinositol-UPAR was occupied by UPA as determined on a molar basis. These data confirm that the overexpression of UPAR in advanced atherosclerotic lesions contributes to lesion development. Whether UPAR's excess over cell surface UPA provides an additional role for this receptor in atherogenesis besides UPA-mediated proteolysis remains to be elucidated.

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.

Clinical significance of urokinase-type plasminogen activator receptor (uPAR) expression in cancer

The involvement of the urokinase-type plasminogen activator (uPA) system in particular has been extensively studied in the pathogenesis of cancer. The molecular role of the uPA receptor (uPAR) is well characterized with its participation in cell migration and extracellular matrix (ECM) degradation. Over-expression of uPAR in cancer has been demonstrated in many studies and is considered an attractive target for anticancer agents. We and others have down-regulated uPAR expression in an attempt to inhibit cancer metastasis based on its molecular role. Uniquely, uPAR which is a glycosyl phosphatidylinositol anchored protein is not only bound to the cell surface but also has a soluble form, suPAR. There is now accumulated clinical and experimental evidence supporting the significant role of uPAR and its soluble counterpart in a number of solid cancers. The expression of uPAR can be associated with tumor cells or stromal cells or both. Differences observed in the expression of uPAR using immunohistochemistry (IHC) are likely explained by the use of different antibodies and techniques rather than true cellular differences and are reviewed here. This review summarizes the clinical relevance of uPAR and its soluble form in the prognosis and diagnosis of different cancers.

Integrin αMβ2 Orchestrates and Accelerates Plasminogen Activation and Fibrinolysis by Neutrophils

Plasmin, the pivotal thrombolytic enzyme, is generated on the surface of many cell types, where urokinase receptor (uPAR)-bound urokinase (uPA) activates cell-bound plasminogen (Plg). It has been reported that neutrophils mediate endogenous thrombolysis involving a uPA-dependent mechanism, and we previously demonstrated that both uPAR and integrin alpha(M)beta(2) recognize uPA to control cell migration and adhesion. In the present study, we report that the alpha(M)beta(2) regulates neutrophil-dependent fibrinolysis. Phorbol 12-myristate 13-acetate (PMA)-stimulated but not resting neutrophils dissolved fibrin clots, and this activity was not only uPA- and Plg-dependent but also alpha(M)beta(2)-dependent. Purified alpha(M)beta(2) directly bound uPA (K(d) = 40 nm) and Plg (K(d) = 1 microm) in a dose-dependent and saturable manner. In Plg activation assays, addition of purified alpha(M)beta(2), but not a control protein, to a single chain uPA (sc-uPA)/Plg mixture, decreased the K(m) from 2 to 0.1 microm, thereby augmenting the overall reaction efficiency by 50-fold. The binding of sc-uPA to alpha(M)beta(2) was critical for the alpha(M)beta(2)-mediated enhancement of plasmin (Plm) generation, because this effect was lost when WT-sc-uPA was replaced with a kringle-less mutant (DeltaK-sc-uPA), which does not bind to alpha(M)beta(2). Plm inactivation by alpha(2)-antiplasmin was significantly delayed when Plm was preincubated with purified, soluble alpha(M)beta(2). When Plg was added to PMA-stimulated neutrophils, both uPA and Plg were co-immunoprecipitated with alpha(M)beta(2.) Thus, assembly of Plg and uPA on integrin alpha(M)beta(2) regulates Plm activity and, thereby, plays a crucial role in neutrophil-mediated thrombolysis.

Role of matrix metalloproteinases in vascular remodeling

Vascular remodeling, defined as lasting structural changes in the vessel wall in response to hemodynamic stimuli, plays a role in many (patho)physiological processes requiring cell migration and degradation of extracellular matrix(ECM). Matrix metalloproteinase(MMP) system can degrade most ECM components. Several lines of evidence support a role for MMP system components in the development and progression of atherosclerosis and restenosis after angioplasty. This review article focuses on the role of MMPs in vascular remodeling relevant to atherosclerosis and restenosis after angioplasty.

Neointima formation and thrombosis after vascular injury in transgenic mice overexpressing plasminogen activator inhibitor-1 (PAI-1)

The controversial role of plasminogen activator inhibitor-1 (PAI-1) in neointima formation and restenosis was studied with the use of a vascular injury model in transgenic mice overexpressing murine PAI-1 (PAI-1 Tg) and in wild-type (WT) controls. Despite the high circulating PAI-1 levels in the PAI-1 Tg mice (52 +/- 9.8 ng mL-1 vs. 0.76 +/- 0.17 ng mL-1 in WT mice), no significant fibrin deposition was observed in non-injured femoral arteries of 8- to 12-week-old mice. Two weeks after severe electric injury, extensive and comparable fibrin deposition was observed in both genotypes, despite a significantly reduced in situ fibrinolytic activity in arterial sections of the PAI-1 Tg mice. The neointimal and medial areas were similar in WT and PAI-1 Tg mice, resulting in comparable intima/media ratios (e.g. 0.94 +/- 0.25 and 1.04 +/- 0.17 at the center of the injury). Nuclear cell counts in cross-sectional areas of the neointima of the injured region were also comparable in arteries from WT and PAI-1 Tg mice (224 +/- 63, 233 +/- 20), and the distribution pattern of alpha-actin-positive smooth muscle cells was similar. These findings indicate that in a vascular injury model that induces extensive and persistent fibrin deposition in femoral arteries of mice, overexpression of PAI-1 does not affect neointima formation.

Expression of antisense uPAR and antisense uPA from a bicistronic adenoviral construct inhibits glioma cell invasion, tumor growth, and angiogenesis

Urokinase-type plasminogen activator (uPA) and its receptor (uPAR) play an important role in the invasiveness of gliomas and other infiltrative tumors. In glioma cell lines and tumors, high grade correlates with increased expression of uPAR and uPA. We report here the downregulation of uPAR and uPA by delivery of antisense sequences of uPAR and uPA in a single adenoviral vector, Ad-uPAR-uPA (Ad, adenovirus). The bicistronic construct (Ad-uPAR-uPA) infected glioblastoma cell line had significantly reduced levels of uPAR, uPA enzymatic activity and immunoreactivity for these proteins when compared to controls. The Ad-uPAR-uPA infected cells showed a markedly lower level of invasion in the Matrigel invasion assays, and their spheroids failed to invade the fetal rat brain aggregates in the coculture system. Intracranial injection of SNB19 cells with the Ad-uPAR-uPA antisense bicistronic construct showed inhibited invasiveness and tumorigenicity. Subcutaneous injections of bicistronic antisense constructs into established tumors (U87 MG) caused regression of those tumors. Our results support the therapeutic potential of targeting the individual components of the uPAR-uPA system by using a single adenovirus construct for the treatment of glioma and other invasive cancers.

Anthrax toxin: structures, functions and tumour targeting

Anthrax toxin, the major virulence factor of Bacillus anthracis, consists of three polypeptides: protective antigen (PrAg), lethal factor (LF) and oedema factor (EF). To intoxicate mammalian cells, PrAg binds to its cellular receptors and is subsequently activated via proteolysis, yielding a carboxyl-terminal fragment which coordinately assembles to form heptamers that bind and translocate LF and EF into the cytosol to exert their cytotoxic effects. Substantial progress has been made in recent years towards the characterisation of the structure and function of anthrax toxin, and this has greatly facilitated rational drug design of antianthrax agents. There is also emerging evidence that toxins can be manipulated for cancer therapy. LF can efficiently inactivate several mitogen-activated protein kinase kinases (MAPKKs) via cleavage of their amino-terminal sequences. Consequently, antitumour effects of wild type lethal toxin were observed after treatment of mitogen-activated protein kinase (MAPK)-dependent tumours such as human melanomas. Modification of the toxin's proteolytic activation site limits its cytotoxicity to certain cell types and creates a versatile method of treatment. One approach that has successfully achieved specific tumour targeting is the alteration of the furin cleavage of PrAg so that it is not activated by furin, but, alternatively, by proteases that are highly expressed by tumour tissues, including matrix metalloproteases and urokinase.

Receptor-independent role of the urokinase-type plasminogen activator during arteriogenesis

To define the role of the plasminogen activators (PAs) urokinase PA (uPA) and tissue PA (tPA) as well as the uPA receptor (uPAR) in arteriogenesis, we investigated their impact in a rabbit and mouse model of adaptive collateral artery growth. Collateral artery growth was induced by occlusion of the femoral artery in rabbit and wild-type (WT) mice and in mice with targeted inactivation of uPA (uPA-/-), tPA (tPA-/-), or uPAR (uPAR-/-). Northern blot results revealed a significant up-regulation of uPA but not uPAR or tPA in the early phase of arteriogenesis in rabbit and WT mice. This up-regulation on RNA level was followed by an increased protein level and enzymatic activity. Impaired perfusion recovery upon femoral artery ligation was observed by laser Doppler analysis in vivo in uPA-deficient mice but not in uPAR or tPA deficiency compared with WT mice. Immunohistochemical studies revealed an association of leukocyte infiltration with arteriogenesis in WT mice that was strongly reduced in uPA-/- but not in uPAR- or tPA-deficient mice. We conclude that arteriogenesis is promoted by an uPA-mediated infiltration of leukocytes that is not dependent on uPAR.

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.

Urokinase receptor modulates cellular and angiogenic responses in obstructive nephropathy

Interstitial cells have been implicated in the pathogenesis of renal fibrosis. Given that the urokinase receptor (uPAR) is known to play a role in cell adhesion, migration, and angiogenesis, the present study was designed to evaluate the role of uPAR in the regulation of the phenotypic composition of interstitial cells (macrophages, myofibroblasts, capillaries) in response to chronic renal injury. Groups of uPAR wild-type (+/+) and knockout (-/-) mice were investigated between 3 and 14 d after unilateral ureteral obstruction (UUO) or sham surgery (n = 8 mice per group). The density of F4/80+ interstitial macrophages (Mphi) was significantly lower in the -/- mice (3.3 +/- 0.4 versus 6.9 +/- 1.7% area at day 3 UUO; 10.8 +/- 1.6 versus 15.7 +/- 1.0% at day 14 UUO; -/- versus +/+). In contrast, in the -/- mice there were significantly more alpha smooth muscle actin (alphaSMA)-positive cells (12.9 +/- 3.2 versus 7.8 +/- 1.5% area at day 3 UUO; 21.0 +/- 4.7 versus 9.7 +/- 1.9% at day 14 UUO) and CD34-positive endothelial cells (8.4 +/- 1.9 versus 4.0 +/- 1.1% area at day 14 UUO). These differences were associated with significantly more interstitial fibrosis in the -/- mice based on Sirius red staining (4.6 +/- 0.9 versus 2.3 +/- 0.9% area at 14 d UUO). Absence of the uPAR scavenger receptor was associated with significantly greater accumulation of plasminogen activator inhibitor-1 protein (PAI-1) (20.5 +/- 3.5 versus 9.1 +/- 2.9% area, day 14 UUO) and vitronectin protein (2.4 +/- 1.1 versus 0.9 +/- 0.4% area, day 14 UUO). By immunostaining alphaSMA+ cells, CD34+ cells, vitronectin and PAI-1 co-localized to the same tubulointerstitial area. The number of apoptotic cells increased in response to UUO but was significantly higher in the -/- mice (2.0 +/- 0.2 versus 1.2 +/- 0.2 per 100 tubulointerstitial cells, day 14 UUO) while the number of proliferating cells was significantly lower in the uPAR-/- mice. These data suggest that uPAR deficiency suppresses renal Mphi recruitment, but the absence of this scavenger receptor actually accentuates the fibrogenic response, likely due in part to the delayed clearance of angiogenic/profibrotic molecules such as PAI-1 and decreased receptor-associated uPA activity.