Redistribution of pulmonary EC-SOD after exposure to asbestos

Our plain-English summary of Redistribution of pulmonary EC-SOD after exposure to asbestos, by R.J. Tan, C.L. Fattman, S.C. Watkins, et al (Journal of Applied Physiology, 2004).

The study

• Authors: R.J. Tan, C.L. Fattman, S.C. Watkins, T.D. Oury
• Journal: Journal of Applied Physiology
• Published: 2004;97(5):2006-13
• Cited by: 60 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Inhalation of asbestos fibers leads to interstitial lung disease (asbestosis) characterized by inflammation and fibrosis. The pathogenesis of asbestosis is not fully understood, but reactive oxygen species are thought to play a central role. Extracellular superoxide dismutase (EC-SOD) is an antioxidant enzyme that protects the lung in a bleomycin-induced pulmonary fibrosis model, but its role has not been studied in asbestos-mediated disease. EC-SOD is found in high levels in the extracellular matrix of lung alveoli because of its positively charged heparin-binding domain. Proteolytic removal of this domain results in clearance of EC-SOD from the matrix of tissues. We treated wild-type C57BL/6 mice with 0.1 mg of crocidolite asbestos by intratracheal instillation and euthanized them 24 h later. Compared with saline- or titanium dioxide-treated control mice, bronchoalveolar lavage fluid (BALF) from asbestos-treated mice contained significantly higher total protein levels and increased numbers of inflammatory cells, predominantly neutrophils, indicating acute lung injury in response to asbestos. Decreased EC-SOD protein and activity were found in the lungs of asbestos-treated mice, whereas more EC-SOD was found in the BALF of these mice. The EC-SOD in the BALF was predominantly in the proteolyzed form, which lacks the heparin-binding domain. This redistribution of EC-SOD correlated with development of fibrosis 14 days after asbestos exposure. These data suggest that asbestos injury leads to enhanced proteolysis and clearance of EC-SOD from lung parenchyma into the air spaces. The depletion of EC-SOD from the extracellular matrix may increase susceptibility of the lung to oxidative stress during asbestos-mediated lung injury.

In plain English

This was a laboratory experiment in mice, not a study of people. The authors placed 0.1 mg of crocidolite asbestos into the windpipes of male C57BL/6 mice. Control mice received saline or 0.1 mg of titanium dioxide, which the authors used as an inert particle control. The mice were examined 24 hours or 14 days later. The study focused on EC-SOD, an antioxidant enzyme normally held in lung tissue. At 24 hours, fluid washed from the lungs of asbestos-treated mice had significantly more protein and more inflammatory cells, mostly neutrophils, than fluid from control mice. In lung tissue, EC-SOD activity and the amount of EC-SOD were significantly lower after asbestos. The washed fluid contained more EC-SOD, with a higher share of it in a cut form. At 14 days, EC-SOD was still raised in the fluid, and asbestos-treated mice had more lung fibrosis (scarring) than titanium dioxide controls. The authors concluded that EC-SOD appears to be cut and removed from lung tissue after asbestos. They suggested this may leave the lung more vulnerable to oxidative stress and may be an early step toward asbestosis.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Animal experiment (mice) comparing asbestos-treated mice with saline- and titanium dioxide-treated control mice
• Who was studied: Male C57BL/6 mice, 8 to 10 weeks old, at the University of Pittsburgh. Group sizes in Table 1: saline at 24 h, 22 mice; titanium dioxide at 24 h, 4; asbestos at 24 h, 23; titanium dioxide at 14 days, 8; asbestos at 14 days, 8. Some individual tests used smaller groups.
• Exposure: A single intratracheal instillation (fluid placed into the windpipe) of 0.1 mg crocidolite asbestos. Controls received 0.1 mg titanium dioxide, used as an inert particulate control, or 0.9% saline. Mice were examined 24 hours or 14 days after treatment.
• Main results:
  • At 24 hours, lung wash fluid (BALF) from asbestos-treated mice had significantly greater total protein than saline-treated controls. It also held more cells, most of them neutrophils.
  • Neutrophil counts in titanium dioxide-treated mice were higher than in saline-treated mice, but this increase was not statistically significant. Asbestos-treated mice had significantly more neutrophils than titanium dioxide-treated mice.
  • At 24 hours, EC-SOD activity in lung tissue was significantly lower in asbestos-treated mice than in controls (9 mice per group).
  • On Western blots, lung tissue from asbestos-treated mice (5 mice) had significantly less EC-SOD protein than lung tissue from saline-treated (4) and titanium dioxide-treated (4) controls.
  • Lung wash fluid from asbestos-treated mice (7 mice) had significantly more EC-SOD than fluid from saline-treated controls (5 mice). It also had a higher ratio of cut to uncut EC-SOD, which the authors took to indicate increased cutting (proteolysis) of the enzyme.
  • Tissue staining showed significantly less EC-SOD in the alveolar septa (the walls of the air sacs) of asbestos-treated mice (5) than of saline-treated mice (6).
  • At 14 days, lung wash protein and white blood cell counts in asbestos-treated mice were slightly lower than at 24 hours, but still significantly higher than in titanium dioxide controls. EC-SOD in the lung wash fluid remained higher in asbestos-treated mice (4 mice per group).
  • At 14 days, asbestos-treated mice had more lung fibrosis on microscope scoring than titanium dioxide controls. They also had significantly greater collagen deposition measured by hydroxyproline (4 mice per group).
• The authors' conclusion: The authors concluded that EC-SOD is redistributed 24 hours after asbestos is instilled: it appears to be cut and removed from lung tissue and then builds up in the air spaces. They suggested that this loss may make lung tissue more vulnerable to oxidative stress. They also suggested it may be a key early event in the progression to asbestosis and other asbestos-related diseases.
• Limitations the authors state:
  • The authors stated that they do not know which protease cuts the heparin-binding domain of EC-SOD in their model.
  • The authors stated that it remains to be determined whether loss of EC-SOD from the lung contributes to inflammation or is a by-product of it.

Topics

Medical terms in this study, explained in our Lexicon: Interstitial Lung Disease, Inflammation, Fibrosis, Reactive Oxygen Species (ROS), Superoxide, Pulmonary Fibrosis, Alveoli, Crocidolite.

Related studies

• Leukocyte-derived extracellular superoxide dismutase does not contribute to airspace EC-SOD after interstitial pulmonary injury — M.L. Manni, M.W. Epperly, et al. (2012)
• Inflammatory cells as a source of airspace extracellular superoxide dismutase after pulmonary injury — R.J. Tan, J.S. Lee, et al. (2006)
• Extracellular superoxide dismutase inhibits inflammation by preventing oxidative fragmentation of hyaluronan — F. Gao, J.R. Koenitzer, et al. (2008)
• Extracellular superoxide dismutase protects against matrix degradation of heparan sulfate in the lung — C.R. Kliment, J.M. Tobolewski, et al. (2008)
• Comparative proteomics and pulmonary toxicity of instilled single-walled carbon nanotubes, crocidolite asbestos, and ultrafine carbon black in mice — J.G. Teeguarden, B.J. Webb-Robertson, et al. (2011)
• Increased sensitivity to asbestos-induced lung injury in mice lacking extracellular superoxide dismutase — C.L. Fattman, R.J. Tan, et al. (2006)

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