Urokinase

A rapidly expanding body of evidence supports the concept that abnormalities of pathways of fibrin turnover and extravascular fibrin deposition contribute to the pathogenesis of acute lung injury (1). Concurrently increased intraalveolar coagulation and depressed fibrinolysis in the acutely injured lung favors formation of a transitional fibrin neomatrix, which undergoes organization and eventual fibrotic repair. This progression recapitulates that associated with wound healing (2).

Morphologic observations suggest that remodeling of transitional fibrin contributes to alveolar fibrosis in chronic inflammatory lung diseases as well as in acute lung injury. In accelerated pulmonary fibrosis after acute lung injury and in the active phase of alveolitis associated with interstitial lung diseases, alveolar fibrin deposition is characteristic as it is in the early stages of acute respiratory distress syndrome (1). Similarly, in bleomycin-induced lung injury, early fibrin deposition occurs during the initial phase of exudative alveolitis and persists as alveolar and interstitial fibrosis ensues (3). More recent studies of transgenic mice overexpressing or deficient in components of the fibrinolytic pathways further support a linkage between aberrant pulmonary fibrin turnover and subsequent fibrotic repair of the injured lung. Intraalveolar fibrin was blocked after hyperoxic challenge in mice deficient in plasminogen activator inhibitor-1 (4), suggesting that blockade of locally expressed plasminogen activators by this inhibitor plays a crucial role in alveolar fibrin clearance. Similarly, pulmonary fibrosis was likewise increased in bleomycin-treated mice overexpressing PAI-1 and fibrosis was decreased in PAI-1 deficient mice (5). These effects on histologic and biochemical indices of fibrosis were presumably attributable to modulation of the fibrinolytic activity and extent of transitional fibrin in the acutely injured lung.

Gunther and colleagues extend this work and used a novel aerosolization strategy to prevent pulmonary fibrosis in bleomycin-challenged rabbits (6). The authors chose to administer an anticoagulant, heparin, or a fibrinolysin, urokinase plasminogen activator, and show that both interventions protect the lungs against the development of pulmonary fibrosis. The key findings were that "early" therapy with aerosolized heparin, within days 2-12 days of bleomycin challenge, or "late" therapy with urokinase, days 14-24 post-bleomycin, most effectively attenuated the fibrotic response. A strength of this study is that the assessments of fibrosis are comprehensive and clinically germane, including normalization of lung compliance as well as consistent improvements in biochemical, radiographic and histologic analyses.

The new wrinkle of this provocative study is that aerosolized delivery of either heparin or urokinase can attenuate the fibrotic response, suggesting that reversal of the increased procoagulant and depressed fibrinolytic responses in the alveolar compartment is responsible for the salutary effect. Gunther and colleagues now show that either agent, when aerosolized, remains active in lavage fluids from the injured lungs, in support of this concept. The findings build upon and validate previously published work that shows that intratracheal administration or upregulation of urokinase in the lungs attenuates the fibrotic response in bleomycin-induced pulmonary fibrosis (7, 8). Parenteral heparin has also been shown to be effective in this model (9). The ability of heparin to best attenuate the fibrotic response through earlier delivery suggests that beneficial effects were achieved through prevention of extravascular fibrin deposition. Conversely, the efficacy of late delivery of urokinase suggests the likelihood that accelerated fibrin clearance may have been more important after florid extravascular fibrin deposition is established.

This study also raises the intriguing possibility that alternative anticoagulant or fibrinolytic agents could likewise be used to effectively attenuate the fibrotic response. Inhibition of thrombin or administration of activated protein C are likewise capable of inhibiting bleomycin-induced pulmonary fibrosis (10, 11). In a related vein, alternative anticoagulants have recently been shown to protect against the development of sepsis-associated pulmonary injury (12). The efficacy of blockade of tissue factor by site inactivated factor VII or tissue factor pathway inhibitor in this context raises the possibility that fibrotic sequellae of acute lung injury could also be reversed by their administration. Ongoing preclinical trials in primates are now in progress to address this possibility. Recently, single chain urokinase, which is less susceptible to inhibition by plasminogen activator inhibitor-1, was shown to provide virtually complete protection against the development of fibrin-rich collagenous adhesions between the visceral and parietal pleural surfaces in tetracycline-induced pleural fibrosis (13). Could this agent likewise more effectively block accelerated parenchymal lung fibrosis than two-chain form of urokinase used by Gunther and colleagues? Extension of this work may help us to identify the most effective interventional agents and the best routes for their administration.

Was the attenuation of fibrosis in this study strictly attributable to anticoagulant or fibrinolytic activity or to alternative properties of the interventional agents? The observation that fibrinogen is not required for the development of bleomycin-induced fibrosis strongly suggests that alternative mechanisms are likely operative (14). While it appears that the responses in this study were attributable at least in part to effects on altered pathways of fibrin turnover, analysis of the broad scope of possible antiinflammatory effects of these agents was reasonably beyond the scope of this study. Heparin, like other selective anticoagulants such as activated protein C, site-inactivated factor VII or tissue factor pathway inhibitor, is capable of exerting a range of such effects. Interestingly, urokinase was recently shown to be able to induce expression of selected cytokines from neutrophils in response to lipopolysaccharide in the acutely injured lung (15). Whether urokinase somehow favorably altered the temporal expression of fibrogenic cytokines or other mediators in this model remains to be determined.

While there is still much more to learn about the underlying mechanisms, this study confirms the feasibility of using aerosolized anticoagulants or fibrinolysins to attenuate pulmonary fibrosis. The observations are both promising and practical, raising the possibility that a similar approach could provide clinical benefits. Endorsing the message of another recent editorial (16), this is an appropriate time to contemplate clinical trials of anticoagulant or fibrinolytic interventions for patients with accelerated or chronic pulmonary fibrosis, particularly given increasing concerns about the efficacy of conventional immunotherapy. Based on the findings of Gunther and colleagues, aerosolized delivery of these agents should be considered in the design of these trials.

Conflict of Interest Statement: S.I. has no declared conflict of interest.

References

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2. Dvorak HF. Tumors: wounds that do not heal. Similarities between tumor stroma generation and wound healing. N Engl J Meet 1986;315:1650-1659.

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8. Hart DA, Whidden P, Green F, Henkin J, Woods DE. Partial reversal of established bleomycin-induced pulmonary librosis by rh-urokinase in a rat model. Clin Invest Med 1994;17:69-76.

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10. Howell DC, Goldsack NR, Marshall RP, McAnulty RJ, Starke R, Purdy G, Laurent GJ, Chambers RC. Direct thrombin inhibition reduces lung collagen, accumulation, and connective tissue growth factor mRNA levels in bleomycin-induced pulmonary fibrosis. Am J Pathol 2001;159:1383-1395.

11. Yasui H, Gabazza EC, Tamaki S, Kobayashi T, Hataji O, Yuda H, Shimizu S, Suzuki K, Adachi Y, Taguchi O. Intratracheal administration of activated protein C inhibits bleomycin-induced lung fibrosis in the mouse. Am J Respir Crit Care Med 2001;163:1660-1668.

12. Carraway MS, Welly-Wolf KE, Ghio AJ, Miller DM, Carter JG, Idell S, Ortel TL, Piantadosi CA. Blockade of tissue factor attenuates lung inflammation in baboons with established sepsis. Am J Respir Crit Care Med 2002;165:A384.

13. Idell S, Mazar A, Cines D, Kuo A, Parry G, Gawlak S, Juarez J, Koenig K, Azghani A, Hadden W, et al. Single-chain urokinase alone or complexed to its receptor in tetracycline-induced pleuritis in rabbits. Am J Respir Crit Care Med 2002;166:920-926.

14. Hattori N, Degen JL, Sisson TH, Liu H, Moore BB, Pandrangi RG, Simon RH, Drew AF. Bleomycin-induced pulmonary fibrosis in fibrinogen-null mice. J Clin invest 2000;106:1341-1350.

15. Abraham E, Gyetko M, Kuhn K, Arcoli J, Strassheim D, Park JS, Shetty S, Idell S. Urokinase-type plasminogen activator potentiates LPS-induced neutrophil activation. J Immunol 2003;170:5644-5651.

16. Strieter RM. To clot or not to clot, that is the question in pulmonary fibrosis. Am J Respir Crit Care Med 2003;167:1589-1590.

DOI: 10.1164/rccm.2309012

STEVEN IDELL, M.D., PH.D.

The University of Texas Health Center at Tyler

Tyler, Texas

Copyright American Thoracic Society Dec 1, 2003
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