PAI-1 promotes the accumulation of exudate macrophages and worsens pulmonary fibrosis following type II alveolar epithelial cell injury

Fibrotic disorders of the lung are associated with perturbations in the plasminogen activation system. Specifically, plasminogen activator inhibitor-1 (PAI-1) expression is increased relative to the plasminogen activators. A direct role for this imbalance in modulating the severity of lung scarring following injury has been substantiated in the bleomycin model of pulmonary fibrosis. However, it remains unclear whether derangements in the plasminogen activation system contribute more generally to the pathogenesis of lung fibrosis beyond bleomycin injury. To answer this question, we employed an alternative model of lung scarring, in which type II alveolar epithelial cells (AECs) are specifically injured by administering diphtheria toxin (DT) to mice genetically engineered to express the human DT receptor (DTR) off the surfactant protein C promoter. This targeted AEC injury results in the diffuse accumulation of interstitial collagen. In the present study, we found that this targeted type II cell insult also increases PAI-1 expression in the alveolar compartment. We identified AECs and lung macrophages to be sources of PAI-1 production. To determine whether this elevated PAI-1 concentration was directly related to the severity of fibrosis, DTR(+) mice were crossed into a PAI-1-deficient background (DTR(+) : PAI-1(-/-) ). DT administration to DTR(+) : PAI-1(-/-) animals caused significantly less fibrosis than was measured in DTR(+) mice with intact PAI-1 production. PAI-1 deficiency also abrogated the accumulation of CD11b(+) exudate macrophages that were found to express PAI-1 and type-1 collagen. These observations substantiate the critical function of PAI-1 in pulmonary fibrosis pathogenesis and provide new insight into a potential mechanism by which this pro-fibrotic molecule influences collagen accumulation. Copyright © 2012 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Increased survivin expression contributes to apoptosis-resistance in IPF fibroblasts

Fibroblasts perform critical functions during the normal host response to tissue injury, but the inappropriate accumulation and persistent activation of these cells results in the development of tissue fibrosis. The mechanisms accounting for the aberrant accumulation of fibroblasts during fibrotic repair are poorly understood, although evidence supports a role for fibroblast resistance to apoptosis as a contributing factor. We have shown that TGF-β1 and endothelin-1 (ET-1), soluble mediators implicated in fibrogenesis, promote fibroblast resistance to apoptosis. Moreover, we recently found that ET-1 induced apoptosis resistance in normal lung fibroblasts through the upregulation of survivin, a member of the Inhibitor of Apoptosis (IAP) protein family. In the current study, we sought to determine the role of survivin in the apoptosis resistance of primary fibroblasts isolated from the lungs of patients with Idiopathic Pulmonary Fibrosis (IPF), a fibrotic lung disease of unclear etiology for which there is no definitive therapy. First, we examined survivin expression in lung tissue from patients with IPF and found that there is robust expression in the fibroblasts residing within fibroblastic foci (the "active" lesions in IPF which correlate with mortality). Next, we show that survivin expression is increased in fibroblasts isolated from IPF lung tissue compared to cells from normal lung tissue. Consistent with a role in fibrogenesis, we demonstrate that TGF-β1 increases survivin expression in normal lung fibroblasts. Finally, we show that inhibition of survivin enhances susceptibility of a subset of IPF fibroblasts to apoptosis. Collectively, these findings suggest that increased survivin expression represents one mechanism contributing an apoptosis-resistant phenotype in IPF fibroblasts.

Survivin expression induced by endothelin-1 promotes myofibroblast resistance to apoptosis

Fibrosis of the lungs and other organs is characterized by the accumulation of myofibroblasts, effectors of wound-repair that are responsible for the deposition and organization of new extracellular matrix (ECM) in response to tissue injury. During the resolution phase of normal wound repair, myofibroblast apoptosis limits the continued deposition of ECM. Mounting evidence suggests that myofibroblasts from fibrotic wounds acquire resistance to apoptosis, but the mechanisms regulating this resistance have not been fully elucidated. Endothelin-1 (ET-1), a soluble peptide strongly associated with fibrogenesis, decreases myofibroblast susceptibility to apoptosis through activation of phosphatidylinositol 3'-OH kinase (PI3K)/AKT. Focal adhesion kinase (FAK) also promotes myofibroblast resistance to apoptosis through PI3K/AKT-dependent and -independent mechanisms, although the role of FAK in ET-1 mediated resistance to apoptosis has not been explored. The goal of this study was to investigate whether FAK contributes to ET-1 mediated myofibroblast resistance to apoptosis and to examine potential mechanisms downstream of FAK and PI3K/AKT by which ET-1 regulates myofibroblast survival. Here, we show that ET-1 regulates myofibroblast survival by Rho/ROCK-dependent activation of FAK. The anti-apoptotic actions of FAK are, in turn, dependent on activation of PI3K/AKT and the subsequent increased expression of Survivin, a member of the inhibitor of apoptosis protein (IAP) family. Collectively, these studies define a novel mechanism by which ET-1 promotes myofibroblast resistance to apoptosis through upregulation of Survivin.

Plasmin overcomes resistance to prostaglandin E2 in fibrotic lung fibroblasts by reorganizing protein kinase A signaling

Collagen deposition by fibroblasts contributes to scarring in fibrotic diseases. Activation of protein kinase A (PKA) by cAMP represents a pivotal brake on fibroblast activation, and the lipid mediator prostaglandin E(2) (PGE(2)) exerts its well known anti-fibrotic actions through cAMP signaling. However, fibrotic fibroblasts from the lungs of patients with idiopathic pulmonary fibrosis, or of mice with bleomycin-induced fibrosis, are resistant to the normal collagen-inhibiting action of PGE(2). In this study, we demonstrate that plasminogen activation to plasmin restores PGE(2) sensitivity in fibrotic lung fibroblasts from human and mouse. This involves amplified PKA signaling resulting from the promotion of new interactions between AKAP9 and PKA regulatory subunit II in the perinuclear region as well as from the inhibition of protein phosphatase 2A. This is the first report to show that an extracellular mediator can dramatically reorganize and amplify the intracellular PKA-A-kinase anchoring protein signaling network and suggests a new strategy to control collagen deposition by fibrotic fibroblasts.

Macrophage-specific expression of urokinase-type plasminogen activator promotes skeletal muscle regeneration

Macrophages (Mp) and the plasminogen system play important roles in tissue repair following injury. We hypothesized that Mp-specific expression of urokinase-type plasminogen activator (uPA) is sufficient for Mp to migrate into damaged muscle and for efficient muscle regeneration. We generated transgenic mice expressing uPA only in Mp, and we assessed the ability of these mice to repair muscle injury. Mp-only uPA expression was sufficient to induce wild-type levels of Mp accumulation, angiogenesis, and new muscle fiber formation. In mice with wild-type uPA expression, Mp-specific overexpression further increased Mp accumulation and enhanced muscle fiber regeneration. Furthermore, Mp expression of uPA regulated the level of active hepatocyte growth factor, which is required for muscle fiber regeneration, in damaged muscle. In vitro studies demonstrated that uPA promotes Mp migration through proteolytic and nonproteolytic mechanisms, including proteolytic activation of hepatocyte growth factor. In summary, Mp-derived uPA promotes muscle regeneration by inducing Mp migration, angiogenesis, and myogenesis.

Latent infection by γherpesvirus stimulates profibrotic mediator release from multiple cell types

Although γherpesvirus infections are associated with enhanced lung fibrosis in both clinical and animal studies, there is limited understanding about fibrotic effects of γherpesviruses on cell types present in the lung, particularly during latent infection. Wild-type mice were intranasally infected with a murine γherpesvirus (γHV-68) or mock-infected with saline. Twenty-eight days postinfection (dpi), ∼14 days following clearance of the lytic infection, alveolar macrophages (AMs), mesenchymal cells, and CD19-enriched cell populations from the lung and spleen express M(3) and/or glycoprotein B (gB) viral mRNA and harbor viral genome. AMs from infected mice express more transforming growth factor (TGF)-β(1), CCL2, CCL12, TNF-α, and IFN-γ than AMs from mock-infected mice. Mesenchymal cells express more total TGF-β(1), CCL12, and TNF-α than mesenchymal cells from mock-infected mice. Lung and spleen CD19-enriched cells express more total TGF-β(1) 28 dpi compared with controls. The CD19-negative fraction of the spleen overexpresses TGF-β(1) and harbors viral genome, but this likely represents infection of monocytes. Purified T cells from the lung harbor almost no viral genome. Purified T cells overexpress IL-10 but not TGF-β(1). Intracellular cytokine staining demonstrated that lung T cells at 28 dpi produce IFN-γ but not IL-4. Thus infection with a murine γherpesvirus is sufficient to upregulate profibrotic and proinflammatory factors in a variety of lung resident and circulating cell types 28 dpi. Our results provide new information about possible contributions of these cells to fibrogenesis in the lungs of individuals harboring a γherpesvirus infection and may help explain why γHV-68 infection can augment or exacerbate fibrotic responses in mice.

The antifibrotic effects of plasminogen activation occur via prostaglandin E2 synthesis in humans and mice

Plasminogen activation to plasmin protects from lung fibrosis, but the mechanism underlying this antifibrotic effect remains unclear. We found that mice lacking plasminogen activation inhibitor-1 (PAI-1), which are protected from bleomycin-induced pulmonary fibrosis, exhibit lung overproduction of the antifibrotic lipid mediator prostaglandin E2 (PGE2). Plasminogen activation upregulated PGE2 synthesis in alveolar epithelial cells, lung fibroblasts, and lung fibrocytes from saline- and bleomycin-treated mice, as well as in normal fetal and adult primary human lung fibroblasts. This response was exaggerated in cells from Pai1-/- mice. Although enhanced PGE2 formation required the generation of plasmin, it was independent of proteinase-activated receptor 1 (PAR-1) and instead reflected proteolytic activation and release of HGF with subsequent induction of COX-2. That the HGF/COX-2/PGE2 axis mediates in vivo protection from fibrosis in Pai1-/- mice was demonstrated by experiments showing that a selective inhibitor of the HGF receptor c-Met increased lung collagen to WT levels while reducing COX-2 protein and PGE2 levels. Of clinical interest, fibroblasts from patients with idiopathic pulmonary fibrosis were found to be defective in their ability to induce COX-2 and, therefore, unable to upregulate PGE2 synthesis in response to plasmin or HGF. These studies demonstrate crosstalk between plasminogen activation and PGE2 generation in the lung and provide a mechanism for the well-known antifibrotic actions of the fibrinolytic pathway.

Targeted injury of type II alveolar epithelial cells induces pulmonary fibrosis

RATIONALE

Ineffective repair of a damaged alveolar epithelium has been postulated to cause pulmonary fibrosis. In support of this theory, epithelial cell abnormalities, including hyperplasia, apoptosis, and persistent denudation of the alveolar basement membrane, are found in the lungs of humans with idiopathic pulmonary fibrosis and in animal models of fibrotic lung disease. Furthermore, mutations in genes that affect regenerative capacity or that cause injury/apoptosis of type II alveolar epithelial cells have been identified in familial forms of pulmonary fibrosis. Although these findings are compelling, there are no studies that demonstrate a direct role for the alveolar epithelium or, more specifically, type II cells in the scarring process.

OBJECTIVES

To determine if a targeted injury to type II cells would result in pulmonary fibrosis.

METHODS

A transgenic mouse was generated to express the human diphtheria toxin receptor on type II alveolar epithelial cells. Diphtheria toxin was administered to these animals to specifically target the type II epithelium for injury. Lung fibrosis was assessed by histology and hydroxyproline measurement.

MEASUREMENTS AND MAIN RESULTS

Transgenic mice treated with diphtheria toxin developed an approximately twofold increase in their lung hydroxyproline content on Days 21 and 28 after diphtheria toxin treatment. The fibrosis developed in conjunction with type II cell injury. Histological evaluation revealed diffuse collagen deposition with patchy areas of more confluent scarring and associated alveolar contraction.

CONCLUSIONS

The development of lung fibrosis in the setting of type II cell injury in our model provides evidence for a causal link between the epithelial defects seen in idiopathic pulmonary fibrosis and the corresponding areas of scarring.

Monocyte urokinase-type plasminogen activator up-regulation reduces thrombus size in a model of venous thrombosis

Background

Our previous studies showed that the direct injection of an adenovirus construct expressing urokinase-type plasminogen activator (uPA) into experimental venous thrombi significantly reduces thrombus weight. The systemic use of adenovirus vectors is limited by inherent hepatic tropism and inflammatory response. As macrophages are recruited into venous thrombi, it is reasonable to speculate that these cells could be used to target the adenovirus uPA (ad-uPA) gene construct to the thrombus. The aims of this study were to determine whether macrophages transduced with ad-uPA have increased fibrinolytic activity and whether systemic injection of transduced cells could be used to target uPA expression to the thrombus and reduce its size.

Methods

The effect of up-regulating uPA was examined in an immortalized macrophage cell line (MM6) and macrophages differentiated from human blood monocyte-derived macrophages (HBMMs). Cells were infected with ad-uPA or blank control virus (ad-blank). Fibrinolytic mediator expression, cell viability, and cytokine expression were measured by activity assays and enzyme-linked immunosorbent assays. Monocyte migration was measured using a modified Boyden chamber assay. A model of venous thrombosis was developed and characterized in mice with severe combined immunodeficiency (SCID). This model was used to study whether systemically administered macrophages over-expressing uPA reduced thrombus size. Uptake of HBMMs into the thrombus induced in these mice was confirmed by a combination of PKH2-labeled cell tracking and colocalization with human leukocyte antigen (HLA) by immunohistology.

Results

Compared with ad-blank, treated HBMMs transduction with ad-uPA increased uPA production by >1000-fold (P = .003), uPA activity by 150-fold (P = .0001), and soluble uPA receptor (uPAR) by almost twofold (P = .043). Expression of plasminogen activator inhibitor (PAI-1) and PAI-2 was decreased by about twofold (P = .011) and threefold (P = .005), respectively. Up-regulation of uPA had no effect on cell viability or inflammatory cytokine production compared with ad-blank or untreated cells. Ad-uPA transduction increased the migration rate of HBMMs (about 20%, P = .03) and MM6 cells (>twofold, P = .005) compared with ad-blank treated controls. Human macrophage recruitment into the mouse thrombus was confirmed by the colocalization of HLA with the PKH2-marked cells. Systemic injection of uPA-up-regulated HBMMs reduced thrombus weight by approximately 20% compared with ad-blank (P = .038) or sham-treated controls (P = .0028).

Conclusion

Transduction of HBBM with ad-uPA increases their fibrinolytic activity. Systemic administration of uPA up-regulated HBBMs reduced thrombus size in an experimental model of venous thrombosis. Alternative methods of delivering fibrinolytic agents are worth exploring.

The use of thrombolysis in the treatment of acute iliofemoral deep vein thrombosis is not suitable for all patients. Our previous studies have shown that direct injection of an adenovirus construct expressing urokinase plasminogen activator (uPA) into experimental venous thrombi significantly reduced thrombus weight. The systemic use of adenovirus vectors is, however, limited by both their inherent hepatic tropism, which precludes targeted delivery to disease sites, and by the associated host inflammatory response. As macrophages are recruited into venous thrombi, these cells could be used to target uPA gene constructs to the thrombus after systemic administration.

Inhalation of urokinase-type plasminogen activator reduces airway remodeling in a murine asthma model

The airway remodeling that occurs in asthma is characterized by an excess of extracellular matrix deposition in the submucosa, hyperplasia/hypertrophy of smooth muscle, goblet cell metaplasia, and accumulation of fibroblasts/myofibroblasts. The urokinase-type plasminogen activator (uPA)/plasmin system participates in pericellular proteolysis and is capable of directly degrading matrix components, activating latent proteinases, and activating growth factors. In a mouse ovalbumin (OVA) asthma model, we increased plasminogen activator activity in the lung by administering exogenous uPA or by using mice genetically deficient in the uPA inhibitor plasminogen activator inhibitor-1 (PAI-1) to assess the role of this system in asthma pathogenesis. After intraperitoneal OVA sensitization, mice inhaled OVA plus uPA (500 IU/mouse) or saline by ultrasonic nebulization for 3 wk. When studied 24 h after the final exposure, the groups with upregulated plasmin activity had significantly reduced subepithelial fibrosis within the airway walls and had decreased airway hyperresponsiveness (AHR) to methacholine. Morphometric analysis showed that subepithelial wall thickening of the bronchi (subepithelial area ratio) was also reduced, as were collagen and alpha-smooth muscle actin. Upregulation of plasmin activity also increased the level of hepatocyte growth factor activity in bronchoalveolar lavage fluid, whereas the release of transforming growth factor-beta was decreased. The administration of uPA 1 wk after the last OVA inhalation also significantly reduced lung hydroxyproline content and AHR. These results show that enhancing uPA/plasmin activity lessens the airway remodeling in a murine asthma model.

The plasminogen activation system in lung disease

The importance of the plasminogen activator (PA) system in multiple pulmonary disorders has become increasingly apparent as methods to analyze its components have improved. Early investigations discovered that the pulmonary alveolar space is normally a pro-fibrinolytic environment that is diminished in a variety of lung diseases. Interest in these observations was greatly increased when animal experiments revealed that manipulations of the PA system significantly modulated the tissue fibrosis that follows many types of lung injury. In particular, enhancement of PA activity was found to consistently decrease the extent of scarring induced by lung damage. Based upon these early observations, it was hypothesized that fibrin was necessary for the pathogenesis of lung fibrosis, and that an increase in PA activity would reduce collagen accumulation by accelerating the clearance of fibrin from the provisional matrix. However, as is often the case with simple hypotheses, subsequent studies revealed that the actual role of the PA system in pulmonary disease is much more complex. Possible mechanisms beyond fibrinolysis include degradation of other matrix proteins, activation of protease cascades including those involving matrix metalloproteinases, activation and release of growth factors from sites of production and sequestration, and modulation of cell adhesion and motility. In each of these processes, the serpin plasminogen activator inhibitor-1 (PAI-1) plays a central role. For these reasons, it has become apparent that PAI-1 presents an attractive target to influence multiple disease processes within the lung, particularly those that lead to lung fibrosis.

Plasminogen activation induced pericellular fibronectin proteolysis promotes fibroblast apoptosis

Apoptosis of fibroblasts/myofibroblasts is a critical event in the resolution of tissue repair responses; however, mechanisms for the regulation of (myo)fibroblast apoptosis/survival remain unclear. In this study, we demonstrate counter-regulatory interactions between the plasminogen activation system and transforming growth factor-beta1 (TGF-beta1) in the control of fibroblast apoptosis. Plasmin treatment induced fibroblast apoptosis in a time- and dose-dependent manner in association with proteolytic degradation of extracellular matrix proteins, as detected by the release of soluble fibronectin peptides. Plasminogen, which was activated to plasmin by fibroblasts, also induced fibronectin proteolysis and fibroblast apoptosis, both of which were blocked by alpha2-antiplasmin but not by inhibition of matrix metalloproteinase activity. TGF-beta1 protected fibroblasts from apoptosis induced by plasminogen but not from apoptosis induced by exogenous plasmin. The protection from plasminogen-induced apoptosis conferred by TGF-beta1 is associated with the up-regulation of plasminogen activator-1 (PAI-1) expression and inhibition of plasminogen activation. Moreover, lung fibroblasts from mice genetically deficient in PAI-1 lose the protective effect of TGF-beta1 against plasminogen-induced apoptosis. These findings support a novel role for the plasminogen activation system in the regulation of fibroblast apoptosis and a potential role of TGF-beta1/PAI-1 in promoting (myo)fibroblast survival in chronic fibrotic disorders.

Plasminogen-mediated activation and release of hepatocyte growth factor from extracellular matrix

Interventions that enhance plasminogen activation within the lung consistently limit the fibrosis that follows alveolar injury. However, this protective effect cannot be attributed solely to accelerated clearance of fibrin that forms as a provisional matrix after lung injury. To explore other mechanisms, we considered interactions between the plasminogen activation system and hepatocyte growth factor (HGF). HGF is known to have antifibrotic activity, but to do so, it must be both released from its sites of sequestration within extracellular matrix (ECM) and activated by proteolytic cleavage. A recent study using bleomycin-exposed mice showed that manipulations of the plasminogen activation system influenced the amount of free HGF within bronchoalveolar lavage fluid without affecting total lung HGF mRNA or protein. To elucidate the mechanisms, we studied the role of plasminogen activation in fibroblast-mediated HGF release and activation. We found that NIH3T3 and mouse lung fibroblasts release ECM-bound HGF in a plasminogen-dependent fashion. The plasminogen effect was lost when lung fibroblasts from urokinase-type plasminogen activator (uPA)-deficient mice were used, and was increased by fibroblasts from plasminogen activator inhibitor (PAI)-1-deficient mice. Plasminogen addition to NIH3T3 or mouse lung fibroblasts increased conversion of pro-HGF to its active form. The plasminogen effect on activation was lost when uPA-deficient fibroblasts were used and accentuated by PAI-1-deficient fibroblasts. In conjunction with the previous in vivo study, these results suggest that plasminogen activation can protect the lung against fibrosis by increasing the availability of active HGF.

Expression of the reverse tetracycline-transactivator gene causes emphysema-like changes in mice

The doxycycline-inducible, gene regulatory system allows tight control of transgene expression for the study of organ development and disease pathogenesis. Multiple recent reports have employed this model to investigate various lung diseases including emphysema. For our study, we used this transgenic system to test whether prolonged, lung-specific, overexpression of the serine protease urokinase plasminogen activator (uPA) would result in alveolar wall destruction. Double transgenic mice were generated that possessed: (1) the rat Clara cell secretory protein promoter controlling the reverse tetracycline transactivator gene (CCSP:rtTA) and (2) the tetracycline operator controlling the murine uPA cDNA (tet[O]:muPA). Mice were treated with doxycycline beginning at age 6 wk to initiate uPA overexpression. Single transgenic and wild-type animals served as controls. A second group of double transgenic and control animals were maintained off of doxycycline. At ages 10, 18, and 30 wk, the mice underwent measurements of alveolar size, lung compliance, and total lung capacity. We found that, although the uPA overexpressing mice demonstrated an emphysema phenotype, similar abnormalities occurred in the CCSP-rtTA control animals. These CCSP-rtTA-related alterations occurred even without doxycycline exposure. Evaluation of a second transgenic line possessing the human surfactant protein C promoter controlling rtTA expression also exhibited lung abnormalities consistent with emphysema. These findings indicate that pulmonary epithelial expression of rtTA alone causes an emphysema phenotype in mice. Therefore, when using this system to study emphysema pathogenesis, the inclusion of proper controls is essential for accurate data interpretation.

Mice deficient in plasminogen activator inhibitor-1 have improved skeletal muscle regeneration

Skeletal muscle possesses a remarkable capacity for regeneration. Although the regulation of this process at the molecular level remains largely undefined, the plasminogen system appears to play a critical role. Specifically, mice deficient in either urokinase-type plasminogen activator (uPA−/−mice) or plasminogen demonstrate markedly impaired muscle regeneration after injury. In the present study, we tested the hypothesis that loss of the primary inhibitor of uPA, plasminogen activator inhibitor-1 (PAI-1), would improve muscle regeneration. Repair of the extensor digitorum longus muscle was assessed after cardiotoxin injury in wild-type, uPA−/−, and PAI-1-deficient (PAI-1−/−) mice. As expected, there was no uPA activity in the injured muscles of uPA−/−mice, and muscles from these transgenic animals demonstrated impaired regeneration. On the other hand, uPA activity was increased in injured muscle from PAI-1−/−mice to a greater extent than in wild-type controls. Furthermore, PAI-1−/−mice demonstrated increased expression of MyoD and developmental myosin after injury as well as accelerated recovery of muscle morphology, protein levels, and muscle force compared with wild-type animals. The injured muscles of PAI-1-null mice also demonstrated increased macrophage accumulation, contrasting with impaired macrophage accumulation in uPA-deficient mice. The extent of macrophage accumulation correlated with both the clearance of protein after injury and the efficiency of regeneration. Taken together, these results indicate that PAI-1 deficiency promotes muscle regeneration, and this protease inhibitor represents a therapeutic target for enhancing muscle regeneration.

Plasminogen activator inhibitor-1 impairs alveolar epithelial repair by binding to vitronectin

The pathogenesis of pulmonary fibrosis is thought to involve alveolar epithelial injury that, when successfully repaired, can limit subsequent scarring. The plasminogen system participates in this process with the balance between urokinase-type plasminogen activator (uPA) and plasminogen activator inhibitor-1 (PAI-1) being a critical determinant of the extent of collagen accumulation that follows lung injury. Because the plasminogen system is known to influence the rate of migration of epithelial cells, including keratinocytes and bronchial epithelial cells, we hypothesized that the balance of uPA and PAI-1 would affect the efficiency of alveolar epithelial cell (AEC) wound repair. Using an in vitro model of AEC wounding, we show that the efficiency of repair is adversely affected by a deficiency in uPA or by the exogenous administration of PAI-1. By using PAI-1 variants and AEC from mice transgenically deficient in vitronectin (Vn), we demonstrate that the PAI-1 effect requires its Vn-binding activity. Furthermore, we have found that cell motility is enhanced by the availability of Vn in the matrix and that the AEC-Vn interaction is mediated, in part, by the alpha(v)beta(1) integrin. The significant effect of uPA and PAI-1 on epithelial repair suggests a mechanism by which the plasminogen system may modulate pulmonary fibrosis.

Localization of plasminogen activator activity within normal and injured lungs by in situ zymography

During inflammatory lung injury, the fibrinolytic activity that is normally present within bronchoalveolar lavage (BAL) fluid (BALF) is often suppressed due to increased levels of inhibitors, including plasminogen activator inhibitor (PAI)-1. Despite this suppression, BALF frequently contains fibrin degradation products, indicating persistence of fibrinolytic activity within the lung. To address this discrepancy and determine the sites where plasminogen activation is occurring, we developed an in situ zymographic technique for frozen sections of lung tissue that localizes plasminogen activator activity at the cellular level. After validating the method using enzyme inhibitors and mice with genetic manipulations of their plasminogen system genes, we applied the technique to lungs of normal and bleomycin-exposed mice. In normal mice, plasminogen activator activity was localized to bronchial epithelial cells, cells of the alveolar walls, and alveolar macrophages. After bleomycin exposure, in situ zymography showed that, despite loss of fibrinolytic activity within BALF, abundant enzymatic activity was associated with aggregates of inflammatory cells. PAI-1-deficient mice that are protected from bleomycin-induced fibrosis had preserved plasminogen activator activity in BALF and increased tissue activity, as determined by in situ zymography. We conclude that analysis of BALF does not adequately reflect the fibrinolytic activity that persists within microenvironments of the lung during inflammation.

The plasminogen activation system reduces fibrosis in the lung by a hepatocyte growth factor-dependent mechanism

Mice deficient in the plasminogen activator inhibitor-1 gene (PAI-1-/- mice) are relatively protected from developing pulmonary fibrosis from bleomycin administration. We hypothesized that one of the protective mechanisms may be the ability of the plasminogen system to enhance hepatocyte growth factor (HGF) effects, which have been reported to be anti-fibrotic in the lung. HGF is known to be sequestered in tissues by binding to extracellular matrix components. Following bleomycin administration, we found that HGF protein levels were higher in bronchoalveolar lavage fluid from PAI-1-/- mice compared to wild-type (PAI-1+/+) mice. This increase could be suppressed by administering tranexamic acid, which inhibits plasmin activity. Conversely, intratracheal instillation of urokinase into bleomycin-injured PAI-1+/+ mice to activate plasminogen caused a significant increase in HGF within bronchoalveolar lavage and caused less collagen accumulation in the lungs. Administration of an anti-HGF neutralizing antibody markedly increased collagen accumulation in the lungs of bleomycin-injured PAI-1-/- mice. These results support the hypothesis that increasing the availability of HGF, possibly by enhancing its release from extracellular matrix by a plasmin-dependent mechanism, is an important means by which activation of the plasminogen system can limit pulmonary fibrosis.

Reduction in fibrotic tissue formation in mice genetically deficient in plasminogen activator inhibitor-1

Mice with homozygous deletion of the plasminogen activator inhibitor-1 gene (PAI-1(-/-)) are relatively protected from bleomycin-induced pulmonary fibrosis. At least part of the protective effect appears to occur during the latter stages of the pathological process when fibrotic tissue is being deposited. To investigate the effect of PAI-1 deficiency on fibrosis, we studied the accumulation of fibrotic tissue within subcutaneously implanted polyvinyl alcohol sponges. Similar to the effect of PAI-1 deficiency on bleomycin-induced pulmonary fibrosis, the accumulation of fibrotic tissue within implanted sponges occurred more slowly in PAI-1(-/-) compared to wild-type mice. Another striking difference observed in the PAI-1(-/-) mice was the rapid removal of a fibrin-rich matrix that formed within the sponges by 1 day after implantation in both wild-type and PAI-1(-/-) mice. The pattern of connective tissue invasion also differed: cells in wild-type mice infiltrated as individually penetrating cells whereas in PAI-1(-/-) mice they did so as a well-demarcated advancing front. Providing an alternative provisional matrix by impregnating sponges with a low concentration of collagen before implantation corrected the changes induced by PAI-1 deficiency. In conclusion, PAI-1 deficiency appears to affect fibrotic tissue formation in part by altering the provisional matrix that forms soon after tissue injury.

A plasminogen activator inhibitor-1 promoter polymorphism and idiopathic interstitial pneumonia

The normal fibrinolytic activity within the alveolar space is suppressed in fibrotic lung diseases in part because of increased levels of plasminogen activator inhibitor-1 (PAI-1). Studies with animals have shown that inhibition of the plasminogen system by PAI-1 increases the generation of pulmonary fibrosis. To determine if a similar relationship occurs in human fibrotic lung diseases, we took advantage of a polymorphism (4G/5G) that occurs in the promoter region of the human PAI-1 gene and influences the expression of PAI-1. We hypothesized that the 4G/4G genotype, because of its association with higher levels of PAI-1, would occur in patients with idiopathic interstitial pneumonia more frequently than in a control population. PAI-1 promoter genotype was determined in 88 well-characterized patients with idiopathic interstitial pneumonia consisting of 62 patients with usual interstitial pneumonia and 26 with nonspecific interstitial pneumonia. DNA was extracted from paraffin-embedded biopsy tissue and the genotype identified by polymerase chain reaction and restriction endonuclease digestion. We found that the distribution of PAI-1 genotypes in the idiopathic interstitial pneumonia population was similar to that of a large control population. However, subgroup analysis showed that patients with nonspecific interstitial pneumonia were more likely than the control population to have the promoter genotype (4G/4G) that is associated with higher levels of PAI-1. A similar pattern in PAI-1 polymorphism was not seen in the usual interstitial pneumonia subgroup. The results of this study support the conclusion that PAl-1 expression influences the development of nonspecific interstitial pneumonia in a similar manner to what occurs in animal models of pulmonary fibrosis. Patients with usual interstitial pneumonia did not show the same relationship with PAl-1 genotype.

Inducible lung-specific urokinase expression reduces fibrosis and mortality after lung injury in mice

Plasminogen activator inhibitor-1 (PAI-1)-deficient transgenic mice have improved survival and less fibrosis after intratracheal bleomycin instillation. We hypothesize that PAI-1 deficiency limits scarring through unopposed plasminogen activation. If this is indeed true, then we would expect increased urokinase-type plasminogen activator (uPA) expression to result in a similar reduction in scarring and improvement in mortality. To test our hypothesis, using the tetracycline gene regulatory system, we have generated a transgenic mouse model with the features of inducible, lung-specific uPA production. After doxycycline administration, these transgenic animals expressed increased levels of uPA in their bronchoalveolar lavage (BAL) fluid that accelerated intrapulmonary fibrin clearance. Importantly, this increased plasminogen activator production led to a reduction in both lung collagen accumulation and mortality after bleomycin-induced injury. These results suggest that PAI-1 deficiency does protect against the effects of bleomycin-induced lung injury through unopposed plasmin generation. By allowing the manipulation of plasminogen activation at different phases of the fibrotic process, this model will serve as a powerful tool in further investigations into the pathogenesis of pulmonary fibrosis.