Role of Urokinase in the Fibrogenic Response of the Lung to Mineral Particles

Abstract

The lung plasminogen activator (PA) response was examined in four different models of particle-induced pulmonary lesions in NMRI mice (single intratracheal administration, 0.75 to 5 mg/mouse). Sequential changes in cellular (total and differential counts) and biochemical markers of alveolitis (lactate dehydrogenase [LDH], total proteins) were monitored in bronchoalveolar fluid (BALF) and the fibrotic lung response was assessed histologically. An intense but spontaneously resolving alveolitis was produced by manganese dioxide (MnO2) and a fibrosing alveolitis was elicited by crystalline silica (DQ12). Minimal and noninflammatory responses were obtained after instillation of titanium dioxide (TiO2) and tungsten carbide (WC), respectively. The comparison between the resolving and the fibrosing alveolitis model was especially taken into consideration in an attempt to identify fibrinolytic changes associated with the development of fibrosis. At the alveolitis stage, similarly increased BALF PA activities were measured in both the resolving and the fibrosing alveolitis models whereas only slight and no PA modifications were noted after administration of TiO2 and WC, respectively. Persistently (up to 120 d) increased BALF PA activity was selectively associated with the progression to fibrosis (DQ12), suggesting that PA is involved in the fibrotic process. ELISA measurements demonstrated that the changes in BALF PA activity were exclusively related to changes in urokinase (uPA), not tissue-type PA. A rapid and persisting (up to Day 30) upregulation of cell-associated PA activity occurred after DQ12, MnO2, and TiO2 treatment only. Cellular PA activity was however significantly higher in fibrogenic inflammatory cells recovered from DQ12 than from MnO2-treated mice suggesting that the intensity of cellular PA upregulation may represent an early indicator of the progression to fibrosis. The implication of urokinase in the pathogenesis of silica-induced fibrosis was demonstrated by the use of a uPA knockout mice. The acceleration of the fibrotic process in uPA-deficient compared with the wild type animals demonstrated the contribution of uPA to limit the fibrotic process.

Expression of plasminogen activator inhibitors type-1 and type-2 in the mouse lung after administration of crystalline silica

“…We previously demonstrated a marked upregulation of lung urokinase during the course of both alveolitis and fibrosis induced by silica particles in the mouse [7]. In the present study we describe a concomitant increase in both BAL fluid PAI activity and PAI-1 protein in the same model.…”

“…In an attempt to clarify the involvement of both PAIs in interstitial lung disease, we characterized the time course of lung PAI-1 and PAI-2 changes accompanying the lung reaction induced by crystalline silica in a murine model. Silica-induced lung injury is an established murine model of pulmonary fibrosis [7,33]. A single dose of the highly toxic DQ12 silica [7,34,35] induced a severe and persistent alveolitis characterized by significant increases in lactate dehydrogenase (LDH) and total protein content in BAL fluid.…”

“…Silica-induced lung injury is an established murine model of pulmonary fibrosis [7,33]. A single dose of the highly toxic DQ12 silica [7,34,35] induced a severe and persistent alveolitis characterized by significant increases in lactate dehydrogenase (LDH) and total protein content in BAL fluid. Concomitantly, a rapid and significant recruitment of monocytes/macrophages and neutrophils was also noted in response to silica (table 2).…”

“…Inappropriate control of uPA activity either by altered expression of the protein or altered expression of its inhibitors (PAIs) is, therefore, of potential relevance in interstitial lung diseases. We have previously demonstrated an upregulation of uPA in the lung compartment in response to toxic particulate materials [7].…”

“…The trachea was exposed using a sterile technique and 5 mg DQ12 suspended in 100 µl sterile saline solution was slowly instilled into the trachea lumen. This dose of silica was demonstrated in preliminary experiments to cause a fibrotic response within 1 month [7]. Controls were treated with an equivalent volume of saline.…”