Urokinase activates the Jak/Stat signal transduction pathway in human vascular endothelial cells

3'-Sialyllactose protects against LPS-induced endothelial dysfunction by inhibiting superoxide-mediated ERK1/2/STAT1 activation and HMGB1/RAGE axis

AIM

Endothelial hyperpermeability is an early stage of endothelial dysfunction associated with the progression and development of atherosclerosis. 3'-Sialyllactose (3'-SL) is the most abundant compound in human milk oligosaccharides, and it has the potential to regulate endothelial dysfunction. This study investigated the beneficial effects of 3'-SL on lipopolysaccharide (LPS)-induced endothelial dysfunction in vitro and in vivo.

MAIN METHODS

We established LPS-induced endothelial dysfunction models in both cultured bovine aortic endothelial cells (BAECs) and mouse models to determine the effects of 3'-SL. Western blotting, qRT-PCR analysis, immunofluorescence staining, and en face staining were employed to clarify underlying mechanisms. Superoxide production was measured by 2',7'-dichlorofluorescin diacetate, and dihydroethidium staining.

KEY FINDINGS

LPS significantly decreased cell viability, whereas 3'-SL treatment mitigated these effects via inhibiting ERK1/2 activation. Mechanistically, 3'-SL ameliorated LPS-induced ROS accumulation leading to ERK1/2 activation-mediated STAT1 phosphorylation and subsequent inhibition of downstream transcriptional target genes, including VCAM-1, TNF-α, IL-1β, and MCP-1. Interestingly, LPS-induced ERK1/2/STAT1 activation leads to the HMGB1 release from the nucleus into the extracellular space, where it binds to RAGE, while 3'-SL suppressed EC hyperpermeability by suppressing the HMGB1/RAGE axis. This interaction also led to VE-cadherin endothelial junction disassembly and endothelial cell monolayer disruption through ERK1/2/STAT1 modulation. In mouse endothelium, en face staining revealed that 3'-SL abolished LPS-stimulated ROS production and VCAM-1 overexpression.

SIGNIFICANCE

Our findings suggest that 3'-SL inhibits LPS-induced endothelial hyperpermeability by suppressing superoxide-mediated ERK1/2/STAT1 activation and HMGB1/RAGE axis. Therefore, 3'-SL may be a potential therapeutic agent for preventing the progression of atherosclerosis.

JAK-STAT inhibition reduces endothelial prothrombotic activation and leukocyte-endothelial proadhesive interactions

BACKGROUND

Vascular activation is characterized by increased proinflammatory, pro thrombotic, and proadhesive signaling. Several chronic and acute conditions, including Bcr-abl-negative myeloproliferative neoplasms (MPNs), graft-vs-host disease, and COVID-19 have been noted to have increased activation of the janus kinase (JAK)-signal transducer and downstream activator of transcription (STAT) pathways. Two notable inhibitors of the JAK-STAT pathway are ruxolitinib (JAK1/2 inhibitor) and fedratinib (JAK2 inhibitor), which are currently used to treat MPN patients. However, in some conditions, it has been noted that JAK inhibitors can increase the risk of thromboembolic complications.

OBJECTIVES

We sought to define the anti-inflammatory and antithrombotic effects of JAK-STAT inhibitors in vascular endothelial cells.

METHODS

We assessed endothelial activation in the presence or absence of ruxolitinib or fedratinib by using immunoblots, immunofluorescence, qRT-PCR, and function coagulation assays. Finally, we used endothelialized microfluidics perfused with blood from normal and JAK2V617F+ individuals to evaluate whether ruxolitinib and fedratinib changed cell adhesion.

RESULTS

We found that both ruxolitinib and fedratinib reduced endothelial cell phospho-STAT1 and STAT3 signaling and attenuated nuclear phospho-NK-κB and phospho-c-Jun localization. JAK-STAT inhibition also limited secretion of proadhesive and procoagulant P-selectin and von Willebrand factor and proinflammatory IL-6. Likewise, we found that JAK-STAT inhibition reduced endothelial tissue factor and urokinase plasminogen activator expression and activity.

CONCLUSIONS

By using endothelialized microfluidics perfused with whole blood samples, we demonstrated that endothelial treatment with JAK-STAT inhibitors prevented rolling of both healthy control and JAK2V617F MPN leukocytes. Together, these findings demonstrate that JAK-STAT inhibitors reduce the upregulation of critical prothrombotic pathways and prevent increased leukocyte-endothelial adhesion.

Tyrosine kinase 2 - Surveillant of tumours and bona fide oncogene

Tyrosine kinase 2 (TYK2) is a member of the Janus kinase (JAK) family, which transduces cytokine and growth factor signalling. Analysis of TYK2 loss-of-function revealed its important role in immunity to infection, (auto-) immunity and (auto-) inflammation. TYK2-deficient patients unravelled high similarity between mice and men with respect to cellular signalling functions and basic immunology. Genome-wide association studies link TYK2 to several autoimmune and inflammatory diseases as well as carcinogenesis. Due to its cytokine signalling functions TYK2 was found to be essential in tumour surveillance. Lately TYK2 activating mutants and fusion proteins were detected in patients diagnosed with leukaemic diseases suggesting that TYK2 is a potent oncogene. Here we review the cell intrinsic and extrinsic functions of TYK2 in the characteristics preventing and enabling carcinogenesis. In addition we describe an unexpected function of kinase-inactive TYK2 in tumour rejection.

Pulmonary arteriole gene expression signature in idiopathic pulmonary fibrosis

A third of patients with idiopathic pulmonary fibrosis (IPF) develop pulmonary hypertension (PH-IPF), which is associated with increased mortality. Whether an altered gene expression profile in the pulmonary vasculature precedes the clinical onset of PH-IPF is unknown. We compared gene expression in the pulmonary vasculature of IPF patients with and without PH with controls. Pulmonary arterioles were isolated using laser capture microdissection from 16 IPF patients: eight with PH (PH-IPF) and eight with no PH (NPH-IPF), and seven controls. Probe was prepared from extracted RNA, and hybridised to Affymetrix Hu133 2.0 Plus genechips. Biometric Research Branch array tools and Ingenuity Pathway Analysis software were used for analysis of the microarray data. Univariate analysis revealed 255 genes that distinguished IPF arterioles from controls (p<0.001). Mediators of vascular smooth muscle and endothelial cell proliferation, Wnt signalling and apoptosis were differentially expressed in IPF arterioles. Unsupervised and supervised clustering analyses revealed similar gene expression in PH-IPF and NPH-IPF arterioles. The pulmonary arteriolar gene expression profile is similar in IPF patients with and without coexistent PH. Pathways involved in vascular proliferation and aberrant apoptosis, which may contribute to pulmonary vascular remodelling, are activated in IPF patients.

uPA and uPAR shRNA inhibit angiogenesis via enhanced secretion of SVEGFR1 independent of GM-CSF but dependent on TIMP-1 in endothelial and glioblastoma cells

The uPA/uPAR system is known to play a critical role in angiogenesis of glioblastoma. Previously, we have shown that shRNA against uPA and uPAR attenuates angiogenesis by blocking nuclear translocation of angiogenin, inhibition of angiopoietin/Tie2 signaling, and regulating several other pro-angiogenic, angiostatic and anti-angiogenic molecules. Further analysis revealed that GM-CSF, a pleiotropic cytokine, was significantly inhibited in U87MG and 4910 co-cultures with endothelial cells transfected with shRNA against uPA and uPAR. The role of the uPA/uPAR system in this process is not completely understood. Analysis of tumor conditioned medium of U87MG, 4910 and HMECs transfected with shRNA against uPA or uPAR alone or in combination (pU2) revealed inhibition of GM-CSF-enhanced secretion of SVEGFR1 as shown by Western blotting and ELISA. Moreover, phosphorylation of JAK2 and STAT5, the downstream effectors of GM-CSF signaling, was also inhibited in all three cell lines. Phosphorylation at Tyr 166 position of the GM-CSFRβ subunit, the signal activating subunit of the GM-CSF receptor, was inhibited in HMEC, U87MG and 4910 cells. Further analysis revealed that shRNA against uPA and/or uPAR increased secretion of TIMP-1, which is known to enhance SVEGFR1 secretion in endothelial cells. Moreover, addition of purified uPA (with and without GM-CSF) activated JAK2/STAT5 signaling in HMEC. Exogenous addition of SVEGFR1 to pU2 tumor conditioned medium enhanced inhibition of VEGF-induced endothelial capillary tube formation as assessed by an in vitro angiogenesis assay. To determine the significance of these events in vivo, nude mice with pre-established tumors treated with shRNA against uPA and/or uPAR showed decreased levels of GM-CSF and increased levels of SVEGFR1 and TIMP-1 when compared with controls. Enhanced secretion of SVEGFR1 by puPA, puPAR and pU2 in endothelial and GBM cells was mediated indirectly by MMP-7 and augmented by ectodomain shedding of VEGFr1 by tyrosine phosphorylation at the 1213 position. Taken together, these results suggest that the uPA/uPAR system could prove beneficial as an indirect target for inhibition of angiogenesis in glioblastoma.

Members of the microRNA-17-92 cluster exhibit a cell-intrinsic antiangiogenic function in endothelial cells

MicroRNAs are endogenously expressed small noncoding RNAs that regulate gene expression on the posttranscriptional level. The miR-17-92 cluster (encoding miR-17, -18a, -19a/b, -20a, and miR-92a) is highly expressed in tumor cells and is up-regulated by ischemia. Whereas miR-92a was recently identified as negative regulator of angiogenesis, the specific functions of the other members of the cluster are less clear. Here we demonstrate that overexpression of miR-17, -18a, -19a, and -20a significantly inhibited 3-dimensional spheroid sprouting in vitro, whereas inhibition of miR-17, -18a, and -20a augmented endothelial cell sprout formation. Inhibition of miR-17 and miR-20a in vivo using antagomirs significantly increased the number of perfused vessels in Matrigel plugs, whereas antagomirs that specifically target miR-18a and miR-19a were less effective. However, systemic inhibition of miR-17/20 did not affect tumor angiogenesis. Further mechanistic studies showed that miR-17/20 targets several proangiogenic genes. Specifically, Janus kinase 1 was shown to be a direct target of miR-17. In summary, we show that miR-17/20 exhibit a cell-intrinsic antiangiogenic activity in endothelial cells. Inhibition of miR-17/20 specifically augmented neovascularization of Matrigel plugs but did not affect tumor angiogenesis indicating a context-dependent regulation of angiogenesis by miR-17/20 in vivo.

Urokinase receptor expression involves tyrosine phosphorylation of phosphoglycerate kinase

The interaction of urokinase-type plasminogen activator (uPA) with its receptor, uPAR, plays a central role in several pathophysiological processes, including cancer. uPA induces its own cell surface receptor expression through stabilization of uPAR mRNA. The mechanism involves binding of a 51 nt uPAR mRNA coding sequence with phosphoglycerate kinase (PGK) to down regulate cell surface uPAR expression. Tyrosine phosphorylation of PGK mediated by uPA treatment enhances uPAR mRNA stabilization. In contrast, inhibition of tyrosine phosphorylation augments PGK binding to uPAR mRNA and attenuates uPA-induced uPAR expression. Mapping the specific peptide region of PGK indicated that its first quarter (amino acids 1-100) interacts with uPAR mRNA. To determine if uPAR expression by uPA is regulated through activation of tyrosine residues of PGK, we mutated the specific tyrosine residue and tested mutant PGK for its ability to interfere with uPAR expression. Inhibition of tyrosine phosphorylation by mutating Y76 residue abolished uPAR expression induced by uPA treatment. These findings collectively demonstrate that Y76 residue present in the first quarter of the PGK molecule is involved in lung epithelial cell surface uPAR expression. This region can effectively mimic the function of a whole PGK molecule in inhibiting tumor cell growth.

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.

Revisiting the biological roles of PAI2 (SERPINB2) in cancer

Tumour expression of the urokinase plasminogen activator correlates with invasive capacity. Consequently, inhibition of this serine protease by physiological inhibitors should decrease invasion and metastasis. However, of the two main urokinase inhibitors, high tumour levels of the type 1 inhibitor actually promote tumour progression, whereas high levels of the type 2 inhibitor decrease tumour growth and metastasis. We propose that the basis of this apparently paradoxical action of two similar serine protease inhibitors lies in key structural differences controlling interactions with components of the extracellular matrix and endocytosis-signalling co-receptors.

Interleukin-21 regulates expression of key Epstein-Barr virus oncoproteins, EBNA2 and LMP1, in infected human B cells

Epstein-Barr virus (EBV) persists for the life of the host by accessing the long-lived memory B cell pool. It has been proposed that EBV uses different combinations of viral proteins, known as latency types, to drive infected B cells to make the transition from resting B cells to memory cells. This process is normally antigen-driven. A major unresolved question is what factors coordinate expression of EBV latency proteins. We have recently described novel type III latency EBV+ B cell lines (OCI-BCLs) that were induced to differentiate into late plasmablasts/early plasma cells in culture with interleukin-21 (IL-21), mimicking normal B cell development. The objective of this study was to determine whether IL-21-mediated signals also regulate the expression of key EBV latent proteins during this window of development. Here we show that IL-21-reduced gene and protein expression of growth-transforming EBV nuclear antigen 2 (EBNA2) in OCI-BCLs. By contrast, the expression of CD40-like, latent membrane protein 1 (LMP1) strongly increased in these cells suggesting an EBNA2-independent mode of regulation. Same results were also observed in Burkitt's lymphoma line Jijoye and B95-8 transformed lymphoblastoid cell lines. The effect of IL-21 on EBNA2 and LMP1 expression was attenuated by a pharmacological JAK inhibitor indicating involvement of JAK/STAT signalling in this process. Our study also shows that IL-21 induced transcription of ebna1 from the viral Q promoter (Qp).

JAK kinases promote invasiveness in VHL-mediated renal cell carcinoma by a suppressor of cytokine signaling-regulated, HIF-independent mechanism

von Hippel-Lindau (VHL) disease is a cancer syndrome, which includes renal cell carcinoma (RCC), and is caused by VHL mutations. Most, but not all VHL phenotypes are due to failure of mutant VHL to regulate constitutive proteolysis of hypoxia-inducible factors (HIFs). Janus kinases (JAK1, 2, 3, and TYK2) promote cell survival and proliferation, processes tightly controlled by SOCS proteins, which have sequence and structural homology to VHL. We hypothesized that in VHL disease, RCC pathogenesis results from enhanced SOCS1 degradation, leading to upregulated JAK activity. We find that baseline JAK2, JAK3, and TYK2 activities are increased in RCC cell lines, even after serum deprivation or coincubation with cytokine inhibitors. Furthermore, JAK activity is sustained in RCC stably expressing HIF2α shRNA. Invasion through Matrigel and migration in wound-healing assays, in vitro correlates of metastasis, are significantly greater in VHL mutant RCC compared with wild-type cells, and blocked by dominant-negative JAK expression or JAK inhibitors. Finally, we observe enhanced SOCS2/SOCS1 coprecipitation and reduced SOCS1 expression due to proteasomal degradation in VHL-null RCC compared with wild-type cells. The data support a new HIF-independent mechanism of RCC metastasis, whereby SOCS2 recruits SOCS1 for ubiquitination and proteasome degradation, which lead to unrestricted JAK-dependent RCC invasion. In addition to commonly proposed RCC treatment strategies that target HIFs, our data suggest that JAK inhibition represents an alternative therapeutic approach.

Expression of the GPI-anchored receptor Prv-1 enhances thrombopoietin and IL-3-induced proliferation in hematopoietic cell lines

Prv-1 is a hematopoietic cell surface receptor that has been shown to be overexpressed in patients diagnosed with polycythemia vera (PV) and essential thrombocythemia (ET), yet its cellular function remains unclear. In this study, we assessed the role of Prv-1 in thrombopoietin (Tpo)/Mpl signaling with the goal of identifying molecular mechanisms which augment Tpo-induced proliferation. By engineering the cytokine-dependent hematopoietic cell line BaF3 to express both Prv-1 and wild-type or mutant forms of Mpl, we were able to follow the time course of Tpo-dependent proliferation. We report that the overexpression of Prv-1 increased Tpo as well as IL-3-induced proliferation of BaF3/Mpl and BaF3 cells. Cells co-expressing Prv-1 and an Mpl receptor containing a Box 1 motif mutation, which fails to activate Jak2, was completely deficient in Tpo-dependent proliferation. In addition, BaF3 and BaF3/Prv-1 cells stimulated with IL-3 in the presence of the Jak2 inhibitor, AG490, abrogated the proliferative response, indicating that Prv-1 requires a functional Jak2 for its signaling activities. Western blot analysis showed an increase in Tpo and IL-3-induced Stat3 and Stat5 tyrosine phosphorylation in BaF3/Mpl and BaF3 cells expressing Prv-1. These results indicate a novel function for Prv-1 as a signaling molecule in cytokine signaling cascades and may lead to a greater understanding of the mechanism of overexpression of Prv-1 in myeloproliferative disorders.

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.

Urokinase receptor cleavage: a crucial step in fibroblast-to-myofibroblast differentiation

Fibroblasts migrate into and repopulate connective tissue wounds. At the wound edge, fibroblasts differentiate into myofibroblasts, and they promote wound closure. Regulated fibroblast-to-myofibroblast differentiation is critical for regenerative healing. Previous studies have focused on the role in fibroblasts of urokinase plasmingen activator/urokinase plasmingen activator receptor (uPA/uPAR), an extracellular protease system that promotes matrix remodeling, growth factor activation, and cell migration. Whereas fibroblasts have substantial uPA activity and uPAR expression, we discovered that cultured myofibroblasts eventually lost cell surface uPA/uPAR. This led us to investigate the relevance of uPA/uPAR activity to myofibroblast differentiation. We found that fibroblasts expressed increased amounts of full-length cell surface uPAR (D1D2D3) compared with myofibroblasts, which had reduced expression of D1D2D3 but increased expression of the truncated form of uPAR (D2D3) on their cell surface. Retaining full-length uPAR was found to be essential for regulating myofibroblast differentiation, because 1) protease inhibitors that prevented uPAR cleavage also prevented myofibroblast differentiation, and 2) overexpression of cDNA for a noncleavable form of uPAR inhibited myofibroblast differentiation. These data support a novel hypothesis that maintaining full-length uPAR on the cell surface regulates the fibroblast to myofibroblast transition and that down-regulation of uPAR is necessary for myofibroblast differentiation.

Elevated urokinase-specific surface receptor expression is maintained through its interaction with urokinase plasminogen activator

Urokinase plasminogen activator (uPA) and its receptor (uPAR) are overexpressed in various neoplasms, and play a key role in tumor progression and metastasis. In this study, we examined uPA and uPAR expression in a variety of human breast cancer cell lines and found that lines with elevated uPA expression also exhibited high uPAR expression, suggesting the possibility that uPA and uPAR are regulated in concert. To test this possibility, we introduced antisense uPA RNA and antisense uPAR RNA in MDA-MB-231 and BT-549 lines that express high levels of uPA and uPAR. Antisense uPA RNA not only downregulated uPA expression, but also greatly reduced uPAR expression in both lines. However, antisense uPAR RNA-reduced uPAR expression with no apparent inhibitory effect on the levels of uPA. These results indicate that expression of uPAR requires uPA but not vice versa. With a panel of uPA and uPAR monoclonal antibodies (mAbs), we observed that the mAbs disrupting uPA and uPAR interaction, rather than mAb inhibiting uPA protease activity, reduced uPAR expression. Moreover, adding soluble single chain uPA (scuPA) to MDA-MB-231 or BT-549 cells expressing antisense uPA mRNA-restored uPAR expression. These findings suggest that uPA dictates uPAR expression and that uPA binding to uPAR transmits signals for uPAR expression. Finally, we provided evidence that Fyn, a Src family kinase, is involved in uPA-induced uPAR expression.

Regulation of urokinase receptor expression by protein tyrosine phosphatases

Urokinase-type plasminogen activator (uPA) and its receptor (uPAR) play a major role in several physiological processes such as cell migration, proliferation, morphogenesis, and regulation of gene expression. Many of the biological activities of uPA depend on its association with uPAR. uPAR expression and its induction by uPA are regulated at the posttranscriptional level. Inhibition of protein tyrosine phosphatase-mediated dephosphorylation by sodium orthovanadate induces uPAR expression and, with uPA, additively induces cell surface uPAR expression. Sodium orthovanadate induces uPAR by increasing uPAR mRNA in a time- and concentration-dependent manner. Both sodium orthovanadate and uPA induce uPAR mRNA stability, indicating that dephosphorylation could contribute to uPA-induced posttranscriptional regulation of uPAR expression. Induction of the tyrosine phosphatase SHP2 in Beas2B and H157 cells inhibits basal cell surface uPAR expression and uPA-induced uPAR expression. Sodium orthovanadate also increases uPAR expression by decreasing the interaction of a uPAR mRNA coding region sequence with phosphoglycerate kinase (PGK) as well as by enhancing the interaction between a uPAR mRNA 3' untranslated sequence with heterogeneous nuclear ribonucleoprotein C (hnRNPC). On the contrary, overexpression of SHP2 in Beas2B cells increased interaction of PGK with the uPAR mRNA coding region and inhibited hnRNPC binding to the 3' untranslated sequence. These findings confirm a novel mechanism by which uPAR expression of lung airway epithelial cells is regulated at the level of mRNA stability by inhibition of protein tyrosine phosphatase-mediated dephosphorylation of uPAR mRNA binding proteins and demonstrate that the process involves SHP2.

Expression of Chicken Ovalbumin Upstream Promoter-Transcription Factor II Enhances Invasiveness of Human Lung Carcinoma Cells

Chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII) plays an essential role in angiogenesis and development. It is differentially expressed in tumor cell lines, but its role in carcinogenesis is largely unknown. We demonstrate here that noninvasive human lung cancer cells become invasive when COUP-TFII was expressed. The expression of extracellular matrix degrading proteinases, such as matrix metalloproteinase 2 and urokinase-type plasminogen activator, was up-regulated in these cells. This finding was confirmed by transduction of different human lung cancer cell lines with COUP-TFII protein and also by using antisense expression. We observed disorganization of actin filaments and focal adhesion kinase phosphorylation in COUP-TFII-transfected human lung cancer cells in addition to the increase in extracellular metalloproteinase activity. These results suggest that COUP-TFII may be considered as a new target for anticancer therapies.

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

Increased expression of urokinase-type plasminogen activator (uPA) and its receptor (uPAR) is associated with different pathological conditions. Both uPAR-mediated signaling and plasmin-catalyzed extracellular proteolysis may contribute to pathogenesis. To evaluate the involvement of plasminogen in such circumstances, we have taken advantage of transgenic mouse models in which overexpression of uPA and/or uPAR in enamel epithelium, basal epidermis, and hair follicles leads to a pathological phenotype; uPA transgenic mice have chalky-white incisors and, when uPAR is co-expressed, develop extensive alopecia, epidermal thickening, and subepidermal blisters. We report here that when these transgenic mice were backcrossed into a plasminogen-deficient (Plg-/-) background, the dental and skin phenotypes appeared completely normal. Heterozygous Plg+/- transgenic mice exhibited a haplo-insufficiency, with an intermediate or normal phenotype. These results do not argue in favor of a role for uPAR-mediated signaling in our experimental model; rather, they demonstrate an essential, dose-dependent, requirement for plasminogen in uPA-mediated tissue alterations. They also support the hypothesis that plasminogen could play a part in certain skin diseases.

Monocyte-expressed urokinase inhibits vascular smooth muscle cell growth by activating Stat1

After vascular injury, a remodeling process occurs that features leukocyte migration and infiltration. Loss of endothelial integrity allows the leukocytes to interact with vascular smooth muscle cells (VSMCs) and to elicit "marching orders"; however, the signaling processes are poorly understood. We found that human monocytes inhibit VSMC proliferation and induce a migratory potential. The monocytes signal the VSMCs through the urokinase-type plasminogen activator (uPA). The VSMC uPA receptor (uPAR) receives the signal and activates the transcription factor Stat1 that, in turn, mediates the antiproliferative effects. These results provide the first evidence that monocytes signal VSMCs by mechanisms involving the fibrinolytic system, and they imply an important link between the uPA/uPAR-related signaling machinery and human vascular disease.

The HIF family member EPAS1/HIF-2alpha is required for normal hematopoiesis in mice

Hypoxic stress plays a role in pathophysiologic states such as myocardial infarction and cerebral vascular events as well as in normal physiologic conditions including development and hematopoiesis. Members of the hypoxia inducible factor (HIF) family function as transcriptional regulators of genes involved in the hypoxic response. After generating adult mice that globally lack endothelial PAS domain protein 1 (EPAS1, also known as HIF-2alpha/HRF/HLF/MOP3), the second member of the HIF family, characterization of the hematopoietic cell population indicated that the loss of EPAS1/HIF-2alpha resulted in pancytopenia. Using bone marrow reconstitution experiments of lethally irradiated hosts, we have defined the extent and site of hematopoietic impairment in the EPAS1/HIF-2alpha null mice. These data suggest a critical role for EPAS1/HIF-2alpha in maintaining a functional microenvironment in the bone marrow for effective hematopoiesis.

The role of the plasminogen activation cascade in glioma cell invasion: a review

Tumour cell invasion is a dynamic process that depends on a co-ordinated series of biochemical events. This review discusses the role of the proteolytic enzyme system, the plasminogen activation cascade, in glioma cell invasion.

Binding of urokinase plasminogen activator to gp130 via a putative urokinase-binding consensus sequence

Urokinase-type plasminogen activator (uPA) and its receptor (uPAR) are instrumental in cellular activities during inflammation, angiogenesis and tumor metastasis. Recent studies suggest that uPA might exert its function on cell proliferation and migration in a uPAR-independent manner or through an adaptor to the uPA-uPAR system. By applying phage display technology, we have identified a putative uPA-binding consensus sequence BXXSSXXB (where B represents a basic amino acid and X represents any amino acid), which has no apparent sequence correlation to uPAR. This uPA-binding motif apparently recognizes the kringle domain of the protease and has an agonistic effect on uPA binding to immobilized uPAR, thereby possibly serving as part of an adaptor component for uPAR signaling. As a result of protein database searches, this motif was found in the extracellular domain of several cell surface proteins, some of which were proposed to be associated with the uPA-uPAR system. Among these, gp130, a common signal transducer for cytokines, was identified as a uPA-binding protein. The specificity of this interaction was demonstrated by inhibition of uPA binding to immobilized gp130 with soluble gp130. Furthermore, the binding could be partially inhibited by a uPA-binding consensus sequence-containing fusion protein in a dose-dependent manner, with an IC50 of approximately 1 microM, indicating that the uPA-binding motif is apparently involved in the uPA-gp130 interaction. The association of gp130 with uPA may link the uPA-uPAR system to various signal transduction pathways.

Increased expression of the urokinase plasminogen-activator receptor in amyloid beta peptide-treated human brain microglia and in AD brains

The urokinase plasminogen-activator receptor (uPAR) is involved in many processes in inflammation including the migration of inflammatory-associated cells to sites of tissue damage. This receptor, also designated as CD87, is induced in response to a range of stimuli and is a marker of macrophage activation. Its role in inflammatory responses of microglia in Alzheimer's disease (AD) has not been previously investigated. In this study we demonstrate that uPAR mRNA and protein expression is induced following incubation of human post-mortem brain-derived microglia with fibrillar amyloid beta (Abeta) peptide. This response was stronger with Abeta peptide than with other tested pro-inflammatory agents. Induction of uPAR surface expression by microglia was inhibited by the antioxidant N-acetyl-cysteine, indicating that this gene may be induced as a result of oxidative stress-related mechanisms. The significance of these findings to AD was investigated. UPAR protein levels were significantly increased in human brain tissues from the hippocampus, superior frontal gyrus and inferior temporal gyrus of AD cases compared with similar tissues from non-demented cases. Increased uPAR expression was not demonstrated in AD cerebellum. Finally, increased uPAR immunoreactivity was demonstrated in activated microglia in AD brain samples using two different antibodies to uPAR. These results provide a connection between the induction of oxidative stress in AD and microglial activation, and establish a possible involvement of uPAR in AD pathogenesis.

The urokinase plasminogen activator receptor in the regulation of the actin cytoskeleton and cell motility

Cell migration is a complex process requiring tight control of several mechanisms including dynamic reorganization of the actin cytoskeleton and adhesion to the extracellular matrix. The GPI-anchored urokinase plasminogen activator receptor (uPAR) has an important role in the regulation of cell motility in many cell types. This is partly due to the localization of proteolytic activity on the cell surface by binding of the serine protease uPA. Results accumulated over the last decade suggest that uPAR is also involved in motility control through other mechanisms. These include induction of signal transduction events after ligation with uPA, binding to the extracellular matrix molecule vitronectin (VN), and association with integrins and other transmembrane partners. In this review these mechanisms will be discussed with a special emphasis on how the GPI-linked receptor transmits signals to the intracellular milieu and how uPAR participates in the regulation of actin cytoskeleton reorganization and cell adhesion during cell migration.

Epidermal growth factor and pro-inflammatory cytokines regulate the expression of components of plasminogen activation system in U373-MG astrocytoma cells

Cytokines and growth factors that influence both secretion of the extracellular matrix (ECM) proteins and migration of the cells decide about the final outcome of tissue remodelling. We have examined expression of the components of the plasminogen activation system in human astrocytoma U373-MG cells and found that interleukin 1beta (IL-1beta), tumour necrosis factor alpha TNF-alpha), interferon gamma (INF-gamma) and epidermal growth factor (EGF) specifically regulate the expression of tissue-type plasminogen activator (t-PA), urokinase-type plasminogen activator (u-PA), plasminogen activator inhibitor type 1 (PAI-1) and protease nexin-1 (PN-1). We conclude that EGF and IFN-gamma are new important regulators of the plasminogen activation system in astrocytoma cells and, therefore, may influence turnover of extracellular matrix and migration of cells within the brain.

PTP-epsilon, a tyrosine phosphatase expressed in endothelium, negatively regulates endothelial cell proliferation

The vascular endothelium is a dynamic interface between the blood vessel and circulating factors and, as such, plays a critical role in vascular events like inflammation, angiogenesis, and hemostasis. Whereas specific protein tyrosine kinases have been identified in these processes, less is known about their protein tyrosine phosphatase (PTP) counterparts. We utilized a RT-PCR/differential hybridization assay to identify PTP-epsilon as a highly abundant endothelial cell PTP. PTP-epsilon mRNA expression is growth factor responsive, suggesting a role for this enzyme in endothelial cell proliferation. Overexpression of PTP-epsilon decreases proliferation by 60% in human umbilical vein endothelial cells (HUVEC) but not in smooth muscle cells or fibroblasts. In contrast, overexpression of PTP-epsilon (D284A), a catalytically inactive mutant, has no significant effect on HUVEC proliferation. These data provide the first functional characterization of PTP-epsilon in endothelial cells and identify a novel pathway that negatively regulates endothelial cell growth. Such a pathway may have important implications in vascular development and angiogenesis.

Plasmin-dependent elimination of the growth-factor-like domain in urokinase causes its rapid cellular uptake and degradation

Urokinase-type plasminogen activator (uPA) and its receptor (uPAR) act in concert to mediate pericellular proteolysis and to stimulate intracellular signalling responsible for cell migration and proliferation. uPA is composed of three domains, a proteolytic domain (PD), a kringle domain (KD) and a growth-factor-like domain (GFD), the last of which mediates the interaction with uPAR. We demonstrate that uPA, associated with the surface of U937 cells, undergoes plasmin-mediated cleavage of the Lys(46)-Ser(47) bond with elimination of the GFD. Using recombinant forms of uPA, we show that a uPA variant lacking the GFD (r-uPADeltaGFD) and unable to associate with uPAR is rapidly cleared from the cell surface. Binding and internalization of r-uPADeltaGFD are markedly decreased in the presence of 39 kDa receptor-associated protein (RAP), the antagonist of several endocytic receptors of the low-density lipoprotein receptor family, suggesting that this protein clearance pathway is used for r-uPADeltaGFD. In contrast with rapidly internalized r-uPADeltaGFD, the intact recombinant single-chain urokinase with wild-type structure (r-uPAwt) bound to uPAR is retained on the cell surface. Soluble uPAR protects uPA from cleavage by plasmin that results in the elimination of GFD, suggesting that uPAR might protect cell-bound urokinase from plasmin-mediated cleavage between the GFD and KD and subsequent degradation.

Urokinase stimulates human vascular smooth muscle cell migration via a phosphatidylinositol 3-kinase-Tyk2 interaction

Janus kinases Jak1 and Tyk2 play an important role in urokinase-type plasminogen activator (uPA)-dependent signaling. We have recently demonstrated that both kinases are associated with the uPA receptor (uPAR) and mediate uPA-induced activation of signal transducers and activators of transcription (Stat1, Stat2, and Stat4) in human vascular smooth muscle cells (VSMC). Janus kinases are not only required for Stat activation but may also interfere with other intracellular signaling pathways. Here we report that in VSMC, Tyk2 interacts with a downstream signaling cascade involving phosphatidylinositol 3-kinase (PI3-K). We demonstrate that uPA induces PI3-K activation, which is abolished in VSMC expressing the dominant negative form of Tyk2. The regulatory subunit p85 of PI3-K co-immunoprecipitates with Tyk2 but not with Jak1, Jak2, or Jak3, and uPA stimulation increases the PI3-K activity in Tyk2 immunoprecipitates. Tyk2 directly binds to either of the two Src homology 2(SH2)p85 domains in a uPA-dependent fashion. We provide evidence that the Tyk2-mediated PI3-K activation in response to uPA is required for VSMC migration. Thus, two unrelated structurally distinct specific inhibitors of PI3-K, wortmannin and LY294002, prevent VSMC migration induced by uPA. No migratory effect of uPA was observed in VSMC expressing the dominant negative form of Tyk2. Our results underscore the versatile function of Tyk2 in uPA-related intracellular signaling and indicate that PI3-K plays a selective role in the regulation of VSMC migration.

Vascular endothelial growth factor activates STAT proteins in aortic endothelial cells

Vascular endothelial growth factor (VEGF) intracellular signaling in endothelial cells is initiated by the activation of distinct tyrosine kinase receptors, VEGFR1 (Flt-1) and VEGFR2 (Flk-1/KDR). Because the tyrosine kinase-dependent transcription factors known as STAT (signal transducers and activators of transcription) proteins are important modulators of cell growth responses induced by other growth factor receptors, we have determined the effects VEGF of on STAT activation in BAEC (bovine aortic endothelial cells). Here, we show that VEGF induces tyrosine phosphorylation and nuclear translocation of STAT1 and STAT6. VEGF also stimulates STAT3 tyrosine phosphorylation, but nuclear translocation does not occur. We found that placenta growth factor, which selectively activates VEGFR1, has no effect on the STATs. However, upon VEGF stimulation, STAT1 associates with the VEGFR2 in a tyrosine kinase-dependent manner, indicating that VEGF-induced STAT1 activation is mediated primarily by VEGFR2. Thus, our study shows for the first time that VEGF activates the STAT pathway through VEGFR2. Because the growth-promoting activity of VEGF depends upon VEGFR2 activation, these findings suggest a role for the STATs in the regulation of gene expression associated with the angiogenic effects of VEGF.

Urokinase receptor expression on human microvascular endothelial cells is increased by hypoxia: implications for capillary-like tube formation in a fibrin matrix

Hypoxia stimulates angiogenesis, the formation of new blood vessels. This study evaluates the direct effect of hypoxia (1% oxygen) on the angiogenic response of human microvascular endothelial cells (hMVECs) seeded on top of a 3-dimensional fibrin matrix. hMVECs stimulated with fibroblast growth factor–2 (FGF-2) or vascular endothelial growth factor (VEGF) together with tumor necrosis factor–α (TNF-α) formed 2- to 3-fold more tubular structures under hypoxic conditions than in normoxic (20% oxygen) conditions. In both conditions the in-growth of capillary-like tubular structures into fibrin required cell-bound urokinase-type plasminogen activator (uPA) and plasmin activities. The hypoxia-induced increase in tube formation was accompanied by a decrease in uPA accumulation in the conditioned medium. This decrease in uPA level was completely abolished by uPA receptor-blocking antibodies. During hypoxic culturing uPA receptor activity and messenger RNA (mRNA) were indeed increased. This increase and, as a consequence, an increase in plasmin formation contribute to the hypoxia-induced stimulation of tube formation. A possible contribution of VEGF-A to the increased formation under hypoxic conditions is unlikely because there was no increased VEGF-A expression detected under hypoxic conditions, and the hypoxia-induced tube formation by FGF-2 and TNF-α was not inhibited by soluble VEGFR-1 (sVEGFR-1), or by antibodies blocking VEGFR-2. Furthermore, although the αv-integrin subunit was enhanced by hypoxia, blocking antibodies against αvβ3- and αvβ5-integrins had no effect on hypoxia-induced tube formation. Hypoxia increases uPA association and the angiogenic response of human endothelial cells in a fibrin matrix; the increase in the uPA receptor is an important determinant in this process.

Urokinase receptor expression on human microvascular endothelial cells is increased by hypoxia: implications for capillary-like tube formation in a fibrin matrix

Hypoxia stimulates angiogenesis, the formation of new blood vessels. This study evaluates the direct effect of hypoxia (1% oxygen) on the angiogenic response of human microvascular endothelial cells (hMVECs) seeded on top of a 3-dimensional fibrin matrix. hMVECs stimulated with fibroblast growth factor–2 (FGF-2) or vascular endothelial growth factor (VEGF) together with tumor necrosis factor–α (TNF-α) formed 2- to 3-fold more tubular structures under hypoxic conditions than in normoxic (20% oxygen) conditions. In both conditions the in-growth of capillary-like tubular structures into fibrin required cell-bound urokinase-type plasminogen activator (uPA) and plasmin activities. The hypoxia-induced increase in tube formation was accompanied by a decrease in uPA accumulation in the conditioned medium. This decrease in uPA level was completely abolished by uPA receptor-blocking antibodies. During hypoxic culturing uPA receptor activity and messenger RNA (mRNA) were indeed increased. This increase and, as a consequence, an increase in plasmin formation contribute to the hypoxia-induced stimulation of tube formation. A possible contribution of VEGF-A to the increased formation under hypoxic conditions is unlikely because there was no increased VEGF-A expression detected under hypoxic conditions, and the hypoxia-induced tube formation by FGF-2 and TNF-α was not inhibited by soluble VEGFR-1 (sVEGFR-1), or by antibodies blocking VEGFR-2. Furthermore, although the αv-integrin subunit was enhanced by hypoxia, blocking antibodies against αvβ3- and αvβ5-integrins had no effect on hypoxia-induced tube formation. Hypoxia increases uPA association and the angiogenic response of human endothelial cells in a fibrin matrix; the increase in the uPA receptor is an important determinant in this process.

Extracellular signal-regulated kinase functions in the urokinase receptor-dependent pathway by which neutralization of low density lipoprotein receptor-related protein promotes fibrosarcoma cell migration and matrigel invasion

The low density lipoprotein receptor-related protein (LRP) has been reported to regulate cellular migration. In this study, an antisense RNA expression strategy was used to reduce LRP to undetectable levels in HT 1080 fibrosarcoma cells. The LRP-deficient cells demonstrated increased levels of cell-surface uPAR, higher levels of uPA in conditioned medium, increased migration on vitronectin-coated surfaces, and increased invasion of Matrigel. LRP-deficient cells also demonstrated increased levels of phosphorylated extracellular signal-regulated kinase (ERK) in the absence of exogenous stimulants. Antibodies which block binding of endogenously produced uPA to uPAR reduced ERK phosphorylation and migration of LRP-deficient cells to the levels observed with control cells. Inhibitors of ERK activation, including PD098059 and dominant-negative MEK1, also decreased the migration of LRP-deficient but not control cells. By contrast, constitutively active MEK1 stimulated the migration of control but not LRP-deficient cells. Although Matrigel invasion by LRP-deficient cells was inhibited by the proteinase inhibitor, aprotinin, PD098059 in combination with aprotinin was necessary for an optimal effect. Expression of the VLDL receptor in LRP-deficient cells reversed the changes in cellular migration and invasion. These studies demonstrate that binding of endogenously produced uPA to uPAR may serve as a major determinant of basal levels of activated ERK and, by this mechanism, regulate cellular migration and invasion. By regulating the uPA/uPAR system, LRP may also regulate ERK activation, cellular migration, and invasion.

Urokinase-induced mitogenesis is mediated by casein kinase 2 and nucleolin

BACKGROUND

Urokinase (uPA) and the urokinase receptor (uPAR) form a multifunctional system capable of concurrently regulating pericellular proteolysis, cell-surface adhesion, and mitogenesis. The role of uPA and uPAR in directed proteolysis is well established and its function in cellular adhesiveness has recently been clarified by numerous studies. The molecular mechanisms underlying the mitogenic effects of uPA and uPAR are still unclear, however.

RESULTS

We identified mechanisms that might participate in uPA-related mitogenesis in human vascular smooth muscle cells and demonstrated that uPA induces activation of a unique signaling complex. This complex contains uPAR and two additional proteins, nucleolin and casein kinase 2, which are implicated in cell proliferation. Both proteins were isolated by affinity chromatography on uPA-conjugated cyanogen-bromide-activated Sepharose 4B and were identified using nano-electrospray mass spectrometry and immunoblotting. We used laser scanning and immunoelectron microscopy studies to further demonstrate that nucleolin and casein kinase 2 are located on the cell surface where they colocalize with the uPAR. Moreover, the proteins were co-internalized into the cell as an entire complex. Immunoprecipitation experiments in combination with an in vitro kinase assay demonstrated a specific association of uPAR with nucleolin and casein kinase 2 and revealed a uPA-induced activation of casein kinase 2, which presumably led to phosphorylation of nucleolin. Blockade of nucleolin and casein kinase 2 with specific modulators led to the inhibition of uPA-induced cell proliferation.

CONCLUSIONS

We conclude that in human vascular smooth muscle cells, uPA induces the formation and activation of a newly identified signaling complex comprising uPAR, nucleolin, and casein kinase 2, that is responsible for the uPA-related mitogenic response. The complex is not a unique feature of vascular smooth muscle cells, as it was also found in other uPAR-expressing cell types.

TAC-101 (4-[3,5-bis(trimethylsilyl)benzamido]benzoic acid) inhibits spontaneous mediastinal lymph node metastasis produced by orthotopic implantation of Lewis lung carcinoma

The anti-tumor and anti-metastatic effects of 4-[3,5-bis(trimethylsilyl)benzamido]benzoic acid (TAC-101) were investigated using our established lung cancer model. Orthotopic implantation of Lewis lung carcinoma (LLC) cells into the lung parenchyma produced a solitary tumor nodule in the lung followed by mediastinal lymph node metastasis. Daily oral administration of TAC-101 at doses ranging from 4 to 16 mg/kg resulted in a significant inhibition of lymphatic metastasis (inhibition rate=57 to 76%), while only the dose of 16 mg/kg significantly inhibited tumor growth at the implanted sites (inhibition rate=46%). Combined treatment with cis-diamminedichloroplatinum (CDDP) and TAC-101 (8 mg/kg, p.o., daily) enhanced the anti-tumor effect of CDDP (7 mg/kg, i.v., bolus) against both the growth of implanted tumor and lymphatic metastasis. In addition, this combined treatment significantly prolonged the survival time of LLC tumor-bearing mice as compared to treatment with each agent alone. The anti-activating protein-1 (AP-1) activity of TAC-101 caused inhibition of LLC cell invasion through the repression of expression of urokinase-type plasminogen activator and its receptor. The anti-invasive activity of TAC-101 may be involved in its in vivo anti-metastatic activity. These findings suggest that TAC-101 is a novel anti-cancer agent that may improve the therapeutic modalities for lung cancer patients with metastatic disease.

Urokinase activates calcium-dependent potassium channels in U937 cells via calcium release from intracellular stores

The urokinase receptor (uPAR) is highly expressed in the human promyelocytic cell line U937 and contributes to transmembrane signalling. However, the signalling mechanisms are poorly understood. We used the patch-clamp technique to demonstrate that urokinase (uPA) binds to uPAR and thereby stimulates Ca(2+)-activated K+ channels in U937 cells. uPA transiently increased K+ currents within 30 s. The K+ currents were pertussis toxin-sensitive and were also observed in Ca(2+)-free solution. However, when cells were dialysed with EGTA, uPA did not affect K+ currents. The intracellular Ca2+ response to uPA was independent of extracellular Ca2+, was pertussis toxin-sensitive, and was blocked by both thapsigargin and the phospholipase C inhibitor U-73122. The uPA-induced increase in intracellular Ca2+ was independent of uPA proteolytic activity. Furthermore, uPA initiated a rapid formation of inositol 1,4, 5-trisphosphate [Ins(1,4,5)P3]. The amino-terminal uPA fragment and uPA inactivated with diisopropyl fluorophosphate or with inhibitory antibody, elicited the same Ca2+ signal. On the other hand, Ca2+ signalling required the intact uPAR because the effects were abrogated by PtdIns-specific phospholipase C, which removes the uPAR from the cell surface. The prevention of glycosyl phosphatidylinositol moiety synthesis and interference with uPAR anchoring to the cell surface using mannosamine also abolished Ca2+ signals. Taken together, our findings indicate that uPA binds to uPAR and stimulates the production of Ins(1,4,5)P3 via a G-protein- and phospholipase C-dependent mechanism. Ins(1,4,5)P3 in turn liberates Ca2+ from intracellular stores, which leads to the stimulation of Ca(2+)-activated K+ channels.

Urokinase induces activation and formation of Stat4 and Stat1-Stat2 complexes in human vascular smooth muscle cells

Urokinase-type plasminogen activator (uPA) and its specific receptor (uPAR) act in concert to stimulate cytoplasmic signaling machinery and transcription factors responsible for cell migration and proliferation. Recently we demonstrated that uPA activates the Janus kinase/signal transducers and activators of transcription (Stat1) signaling in human vascular smooth muscle and endothelial cells. However, the important question whether other transcription factors of the Stat family, in addition to Stat1, are involved in the uPAR-related signaling has not been addressed. In this study, we demonstrate that Stat4 and Stat2, but not Stat3, Stat5, or Stat6, are rapidly activated in response to uPA. We demonstrate further that Stat4 and Stat2 rapidly and transiently translocate to the cell nucleus where they bind specifically to the regulatory DNA elements. Analysis of Stat complexes formed in response to uPA revealed a Stat2-Stat1 heterodimer, which lacks p48, a DNA-binding protein known to combine with Stat1-Stat2. This new uPA-induced Stat2-Stat1 heterodimer binds to GAS (the interferon-gamma activation site) distinct from the interferon-stimulated response element to which the p48 protein containing complexes generally bind. We conclude that uPA activates a specific and unusual subset of latent cytoplasmic transcription factors in human vascular smooth muscle cells that suggests a critical role of uPA in these cells.