Urokinase-type plasminogen activator is induced in migrating capillary endothelial cells

Cellular migration is an essential component of invasive biological processes, many of which have been correlated with an increase in plasminogen activator production. Endothelial cell migration occurs in vivo during repair of vascular lesions and angiogenesis, and can be induced in vitro by wounding a confluent monolayer of cells. By combining the wounded monolayer model with a substrate overlay technique, we show that cells migrating from the edges of an experimental wound display an increase in urokinase-type plasminogen activator (uPA) activity, and that this activity reverts to background levels upon cessation of movement, when the wound has closed. Our results demonstrate a direct temporal relationship between endothelial cell migration and uPA activity, and suggest that induction of uPA activity is a component of the migratory process.     

Plasminogen activator inhibitor-1 regulates tumor growth and angiogenesis.

Elevated expression of plasminogen activator inhibitor-1 (PAI-1) in tumors is associated with a poor prognosis in many cancers. Reduced tumor growth and angiogenesis have also been reported in mice deficient in PAI-1. These results suggest that PAI-1 may be required for efficient angiogenesis and tumor growth. In the present study, we demonstrate that PAI-1 can both enhance and inhibit the growth of M21 human melanoma tumors in nude mice and that this appears to be due to PAI-1 regulation of angiogenesis. Quantitative analysis of angiogenesis in a Matrigel implant assay indicated that in PAI-1 null mice angiogenesis was reduced approximately 60% compared with wild-type mice, while in mice overexpressing PAI-1, angiogenesis was increased nearly 3-fold. Furthermore, addition of PAI-1 to implants in wild-type mice enhanced angiogenesis up to 3-fold at low concentrations but inhibited angiogenesis nearly completely at high concentrations. Together, these data demonstrate that PAI-1 is a potent regulator of angiogenesis and hence of tumor growth and suggest that understanding the mechanism of this activity may lead to the development of important new therapeutic agents for controlling pathologic angiogenesis.     

Plasminogen activator inhibitor-1 in the pathogenesis of asthma

Plasminogen activator inhibitor (PAI)-1 is the main inhibitor of the fibrinolytic system and is known to play an essential role in tissue remodeling. Recent evidence indicates that chronic asthma may lead to tissue remodeling such as subepithelial fibrosis and extracellular matrix (ECM) deposition in the airways. However, the role of PAI-1 in asthma is unknown. Recently the mast cell (MC), which plays a major role in asthma, was found as a novel source of PAI-1, and a large number of MCs expressing PAI-1 are infiltrated in the airways of patients with severe asthma. Furthermore, PAI-1-deficient mice show reduced ECM deposition in the airways of a murine model of chronic asthma by inhibiting MMP-9 activity and fibrinolysis. In a human study, the 4G allele frequency was significantly higher in the asthmatic patients than in the control group. In view of the findings that the 4G allele is associated with elevated plasma PAI-1 level, elevated PAI-1 level in the lung may contribute to the development of asthma. In summary, PAI-1 may play an important role in the pathogenesis of asthma and further studies evaluating the mechanisms of PAI-1 action may lead to the development of a novel therapeutic target for the treatment and prevention of asthma.     

Plasminogen activator inhibitor-1 polymorphism in women with pre-eclampsia

We determined whether or not genetic variability in the promoter region of the gene encoding plasminogen activator inhibitor-1 (PAI1) contributes to individual differences in susceptibility to the development of preeclampsia. The study involved 133 preeclamptic and 115 healthy control pregnant women who were genotyped for a single-nucleotide insertion/deletion polymorphism (4G/5G) at position -675 in the PAI1 gene promoter. Furthermore, the frequencies of the alleles in the general middle-aged population are presented for comparison. Chi-square analysis was used to assess genotype and allele frequency differences between preeclamptic women and controls. A similar allelic distribution of PAI1 4G/5G polymorphism was observed in the two groups, with the frequency of the variant 4G allele being 50.4% in the preeclampsia group and 54.3% in the control group (p = 0.377; OR = 0.85, 95% CI = 0.60-1.22). Accordingly, the genotype distribution of the PAI1 4G/5G polymorphism in the preeclamptic and control groups was found to be similar (p = 0.68). Overall, this genotype data on fertile women is almost identical to that in the general middle-aged Finnish population. The 4G/5G polymorphism of the PAI1 gene promoter is unlikely to be a major genetic predisposing factor as regards preeclampsia in subjects from eastern Finland. These results are not suggestive of an important contribution of the PAI1 genotype on preeclampsia across populations.     

Plasminogen activator inhibitor-1 detaches cells from extracellular matrices by inactivating integrins

The binding of urokinase plaminogen activator (uPA) to its cell surface receptor (uPAR; CD87) promotes cell adhesion by increasing the affinity of the receptor for both vitronectin (VN) and integrins. We provide evidence that plasminogen activator inhibitor (PAI)-1 can detach cells by disrupting uPAR-VN and integrin-VN interactions and that it does so by binding to the uPA present in uPA-uPAR-integrin complexes on the cell surface. The detached cells cannot reattach to VN unless their surface integrins are first activated by treatment with MnCl2. Immunoprecipitation and subcellular fractionation experiments reveal that PAI-1 treatment triggers deactivation and disengagement of uPA-uPAR-integrin complexes and their endocytic clearance by the low density lipoprotein receptor-related protein. Transfection experiments demonstrate that efficient cell detachment by PAI-1 requires an excess of matrix-engaged uPA-uPAR-integrin complexes over free engaged integrins and that changes in this ratio alter the efficacy of PAI-1. Together, these results suggest a VN-independent, uPA-uPAR-dependent mechanism by which PAI-1 induces cell detachment. This pathway may represent a general mechanism, since PAI-1 also can detach cells from fibronectin and type-1 collagen. This novel "deadhesive" activity of PAI-1 toward a variety of cells growing on different extracellular matrices may begin to explain why high PAI-1 levels often are associated with a poor prognosis in human metastatic disease.     

Junctional communication is induced in migrating capillary endothelial cells

Using an in vitro model in which a confluent monolayer of capillary endothelial cells is mechanically wounded, gap junction-mediated intercellular communication has been studied by loading the cells with the fluorescent dye, Lucifer Yellow. Approximately 40-50% of the cells in a nonwounded confluent monolayer were coupled in groups of four to five cells (basal level). Basal levels of communication were also observed in sparse and preconfluent cultures, but were reduced in postconfluent monolayers. 30 min after wounding, coupling was markedly reduced between cells lining the wound. Communication at the wound was partially reestablished by 2 h, exceeded basal levels after 6 h and reached a maximum after 24 h, at which stage approximately 90% of the cells were coupled in groups of six to seven cells. When the wound had closed (after 8 d), the increase in communication was no longer observed. Induction of wound-associated communication was unaffected by exposure of the cells to the DNA synthesis inhibitor mitomycin C, but was prevented by the protein synthesis inhibitor, cycloheximide. The induction of wound-associated communication was also inhibited when migration was prevented by placing the cells immediately after wounding at 22 degrees C or after exposure to cytochalasin D, suggesting that the increase in communication is dependent on cells migrating into the wound area. In contrast, migration was not prevented when coupling was blocked by exposure of the cells to retinoic acid, although this agent did disrupt the characteristic sheet-like pattern of migration typically seen during endothelial repair. These results suggest that junctional communication may play an important role in wound repair, possibly by coordinating capillary endothelial cell migration.     

Plasminogen activator inhibitor-1 deficiency suppresses osteoblastic differentiation of mesenchymal stem cells in mice

Plasminogen activator inhibitor-1 (PAI-1) is known as an inhibitor of fibrinolytic system. Previous studies suggest that PAI-1 is involved in the pathogenesis of osteoporosis induced by ovariectomy, diabetes, and glucocorticoid excess in mice. However, the roles of PAI-1 in early-stage osteogenic differentiation have remained unknown. In the current study, we investigated the roles of PAI-1 in osteoblastic differentiation of mesenchymal stem cells (MSCs) using wild-type (WT) and PAI-1-deficient (PAI-1 KO) mice. PAI-1 mRNA levels were increased with time during osteoblastic differentiation of MSCs or mesenchymal ST-2 cells. However, the increased PAI-1 levels declined at the mineralization phase in the experiment using MC3T3-E1 cells. PAI-1 deficiency significantly blunted the expression of osteogenic gene, such as osterix and alkaline phosphatase enhanced by bone morphogenetic protein (BMP)-2 in bone marrow-derived MSCs (BM-MSCs), adipose-tissue-derived MSCs (AD-MSCs), and bone marrow stromal cells of mice. Moreover, a reduction in endogenous PAI-1 levels by small interfering RNA significantly suppressed the expression of osteogenic gene in ST-2 cells. Plasmin did not affect osteoblastic differentiation of AD-MSCs induced by BMP-2 with or without PAI-1 deficiency. PAI-1 deficiency and a reduction in endogenous PAI-1 levels did not affect the phosphorylations of receptor-specific Smads by BMP-2 and transforming growth factor-β in AD-MSCs and ST-2 cells, respectively. In conclusion, we first showed that PAI-1 is crucial for the differentiation of MSCs into osteoblasts in mice.     

Plasminogen activator inhibitor-1, adipose tissue and insulin resistance

PURPOSE OF REVIEW: Plasminogen activator inhibitor (PAI)-1 is a physiological inhibitor of plasminogen activators (urokinase and tissue types) and vitronectin. It is synthesized by adipose tissue, and its levels in plasma are increased in obesity and reduced with weight loss. Circulating PAI-1 level predicts development of type 2 diabetes, suggesting that it may be causally related to development of obesity. A role for PAI-1 in development of obesity has only partially been established, however. This review summarizes current knowledge, gives context to developments thus far and discusses controversies. RECENT FINDINGS: In addition to its role in atherothrombosis, PAI-1 might be involved in adipose tissue development. PAI-1 is produced by ectopic fat depots under the influence of inducers. Among the most recently described inducers are inflammation, oxidative stress and circadian clock protein. PAI-1 may play several roles in contributing to obesity: through indirect effects on insulin signalling, by influencing adipocyte differentiation and by regulating recruitment of inflammatory cells within adipose tissue. SUMMARY: These recent findings emphasize the involvement of PAI-1 in controlling the biology of adipose tissue; PAI-1 is an attractive new therapeutic target to retard the metabolic complications that accompany obesity.     

Natriuretic peptides reduce plasminogen activator inhibitor-1 expression in human endothelial cells

Plasma concentrations of natriuretic peptides increase in some pathological conditions, but very little is known about the effect of these vasodilator peptides on the regulation of the blood coagulation system. The fundamental role in the regulation of fibrinolysis is played by plasminogen activator inhibitor type 1 (PAI-1). Recent studies demonstrate that natriuretic peptides can modulate PAI-1 expression in bovine aortic smooth muscle cells and rat aortic endothelial cells. In this report, we tested the effect of natriuretic peptides on PAI-1 expression in the human endothelial cell line (EA.hy 926). For this purpose, we treated the cell cultures with ANP, BNP and CNP, and modulation of PAI-1 synthesis was evaluated. We compared the effect of natriuretic peptides on synthesis and release of PAI-1 in unstimulated cells, and after activation with tumour necrosis factor alpha (TNFalpha). Natriuretic peptides abolished TNFalpha - induced upregulation of PAI-1 expression at both the PAI-1 mRNA and the antigen levels. The inhibitory efficiency was higher in the case of CNP when compared to that produced by ANP and BNP, particularly when TNFalpha-stimulated cells were used. We observed an inhibition of stimulatory effect of TNFalpha on PAI-1 expression also at the level of the PAI-1 promoter in cells transfected with a PAI-1 promoter fragment (+71 to -800). The PAI-1 promoter activity was markedly inhibited by C-type natriuretic peptide, already at a very low (0.001 micro M) concentration of the peptide.     

A novel function of plasminogen activator inhibitor-1 in modulation of the AKT pathway in wild-type and plasminogen activator inhibitor-1-deficient endothelial cells

Cell proliferation, an event associated with angiogenesis, involves coordinated activities of a number of proteins. The role of plasminogen activator inhibitor-1 (PAI-1) in angiogenesis remains controversial. Utilizing proliferating PAI-1-/- endothelial cells (EC), the impact of a host PAI-1 deficiency on Akt activation was evaluated. Hyperactivation of Akt(Ser(P)473) was observed in PAI-1-/- EC, and this was probably due to enhanced inactivation of tumor suppressor PTEN, thus rendering the cells resistant to apoptotic signals. Higher levels of inactivated caspase-9 in PAI-1-/- EC led to lower levels of procaspase-3 and cleaved caspase-3, thereby promoting survival. These effects were reversed when recombinant PAI-1 was added to PAI-1-/- EC. Additional studies demonstrated that regulation of proliferation is dependent on its interaction with low density lipoprotein receptor-related protein. Thus, PAI-1 is a negative regulator of cell growth, exerting its effect on the phosphatidylinositol 3-kinase/Akt pathway and allowing controlled cell proliferation.     

Plasminogen activator activity by cultured bovine aortic endothelial cells

This communication reports the observations that bovine aortic endothelial cells (EC) in culture during their log phase of growth secrete plasminogen activator. Hydrocortisone, dibutyryl cAMP, theophylline, colchicine and cycloheximide, dependent upon concentration, inhibit plasminogen activator activity. Several substances associated with inflammation and thrombosis, such as thrombin, serotonin,catecholomines, histamine, vasopressin, endotoxin, and indomethacin, at the concentrations tested, did not significantly alter plasminogen activator activity when compared with controls.     

Plasminogen activator inhibitor-1 deficient mice are protected from angiotensin II-induced fibrosis

Fungal methionine synthase, Met6p, transfers a methyl group from 5-methyl-tetrahydrofolate to homocysteine to generate methionine. The enzyme is essential to fungal growth and is a potential anti-fungal drug design target. We have characterized the enzyme from the pathogen Candida albicans but were unable to crystallize it in native form. We converted Lys103, Lys104, and Glu107 all to Tyr (Met6pY), Thr (Met6pT) and Ala (Met6pA). All variants showed wild-type kinetic activity and formed useful crystals, each with unique crystal packing. In each case the mutated residues participated in beneficial crystal contacts. We have solved the three structures at 2.0–2.8 Å resolution and analyzed crystal packing, active-site residues, and similarity to other known methionine synthase structures. C. albicans Met6p has a two domain structure with each of the domains having a (βα)₈-barrel fold. The barrels are arranged face-to-face and the active site is located in a cleft between the two domains. Met6p utilizes a zinc ion for catalysis that is bound in the C-terminal domain and ligated by four conserved residues: His657, Cys659, Glu679 and Cys739.     

Plasminogen activator inhibitor-1 deficient mice are protected from angiotensin II-induced fibrosis

PAI-1 has been shown to be both profibrotic and antifibrotic in animal models of hepatic fibrosis. Although these models have similarities to human fibrotic liver disease, no rodent model completely recapitulates the clinical situation; indeed, transaminase values in most models of hepatic fibrosis are much higher than in chronic liver diseases in humans. Here, wild-type and PAI-1^−/− mice were administered AngII (500 ng/kg/min) for 4 weeks. ECM accumulation was evaluated by Sirius red staining, hydroxyproline content, and fibrin and collagen immunostaining. Induction of pro-fibrotic genes was assessed by real-time RT-PCR. Despite the absence of any significant liver damage, AngII infusion increased the deposition of hepatic collagen and fibrin ECM, with a perisinusoidal pattern. PAI-1^−/− mice were protected from these ECM changes, indicating a causal role of PAI-1 in this fibrosis model. Protection in the knockout strain correlated with a blunted increase in αSMA, and elevated activities of matrix metalloproteinases (MMP2, MMP9). These data suggest that PAI-1 plays a critical role in mediating fibrosis caused by AngII and lends weight-of-evidence to a pro-fibrotic role of this protein in liver. Furthermore, the current study proposes a new model of ‘pure’ hepatic fibrosis in mice with little inflammation or hepatocyte death.     

Plasminogen activator inhibitor-1 is a critical downstream target of p53 in the induction of replicative senescence

p53 limits the proliferation of primary diploid fibroblasts by inducing a state of growth arrest named replicative senescence - a process which protects against oncogenic transformation and requires integrity of the p53 tumour suppressor pathway. However, little is known about the downstream target genes of p53 in this growth-limiting response. Here, we report that suppression of the p53 target gene encoding plasminogen activator inhibitor-1 (PAI-1) by RNA interference (RNAi) leads to escape from replicative senescence both in primary mouse embryo fibroblasts and primary human BJ fibroblasts. PAI-1 knockdown results in sustained activation of the PI(3)K-PKB-GSK3beta pathway and nuclear retention of cyclin D1, consistent with a role for PAI-1 in regulating growth factor signalling. In agreement with this, we find that the PI(3)K-PKB-GSK3beta-cyclin D1 pathway is also causally involved in cellular senescence. Conversely, ectopic expression of PAI-1 in proliferating p53-deficient murine or human fibroblasts induces a phenotype displaying all the hallmarks of replicative senescence. Our data indicate that PAI-1 is not merely a marker of senescence, but is both necessary and sufficient for the induction of replicative senescence downstream of p53.     

Plasminogen activator inhibitor-1 binds to fibrin and inhibits tissue-type plasminogen activator-mediated fibrin dissolution.

Plasminogen activator inhibitor-1 (PAI-1) accumulates within thrombi and forming whole blood clots. To explore this phenomenon at the molecular level, PAI-1 binding to fibrin was examined. The experiments were performed by adding 125I-PAI-1, which retains its complete tissue-type plasminogen (t-PA) inhibitory activity, to fibrin matrices formed in 2-cm2 tissue culture wells. Guanidine HCl-activated PAI-1 binding was reversible and was inhibited in the presence of excess, unlabeled PAI-1. Activated 125I-PAI-1 recognized 2 sites on fibrin: a very small number of high affinity sites (Kd less than 1 nM) and principally a large number of low affinity sites with an approximate Kd of 3.8 microM. Latent PAI-1 bound to fibrin at a site indistinguishable from the lower affinity site recognized by activated PAI-1. Fibrin, pretreated with activated PAI-1, was protected from t-PA-mediated plasmin degradation in a PAI-1 dose-responsive manner (IC50 = 12.3 nM). Clot protection correlated with partial occupancy of the low affinity PAI-1 binding site on fibrin and was due to the formation of sodium dodecyl sulfate-stable, PAI-1.t-PA complexes. Latent PAI-1 (27 nM) did not protect the fibrin from dissolution. The localization of PAI-1 to a thrombus by virtue of its fibrin binding potential could result in significant protection of the thrombus from the degradative effects of the fibrinolytic system.     

Plasminogen activator inhibitor-1 is an inhibitor of factor VII-activating protease in patients with acute respiratory distress syndrome

Factor VII-activating protease (FSAP) is a novel plasma-derived serine protease structurally homologous to tissue-type and urokinase-type plasminogen activators. We demonstrate that plasminogen activator inhibitor-1 (PAI-1), the predominant inhibitor of tissue-type and urokinase-type plasminogen activators in plasma and tissues, is an inhibitor of FSAP as well. We detected PAI-1.FSAP complexes in addition to high levels of extracellular RNA, an important FSAP cofactor, in bronchoalveolar lavage fluids from patients with acute respiratory distress syndrome. Hydrolytic activity of FSAP was inhibited by PAI-1 with a second-order inhibition rate constant (K(a)) of 3.38 +/- 1.12 x 10(5) m(-1).s(-1). Residue Arg(346) was a critical recognition element on PAI-1 for interaction with FSAP. RNA, but not DNA, fragments (>400 nucleotides in length) dramatically enhanced the reactivity of PAI-1 with FSAP, and 4 microg.ml(-1) RNA increased the K(a) to 1.61 +/- 0.94 x 10(6) m(-1).s(-1). RNA also stabilized the active conformation of PAI-1, increasing the half-life for spontaneous conversion of active to latent PAI-1 from 48.4 +/- 8 min to 114.6 +/- 5 min. In contrast, little effect of DNA on PAI-1 stability was apparent. Residues Arg(76) and Lys(80) in PAI-1 were key elements mediating binding of nucleic acids to PAI-1. FSAP-driven inhibition of vascular smooth muscle cell proliferation was antagonized by PAI-1, suggesting functional consequences for the FSAP-PAI-1 interaction. These data indicate that extracellular RNA and PAI-1 can regulate FSAP activity, thereby playing a potentially important role in hemostasis and cell functions under various pathophysiological conditions, such as acute respiratory distress syndrome.