Relationship of fiber surface iron and active oxygen species to expression of procollagen, PDGF-A, and TGF-beta(1) in tracheal explants exposed to amosite asbestos

Our plain-English summary of Relationship of fiber surface iron and active oxygen species to expression of procollagen, PDGF-A, and TGF-beta(1) in tracheal explants exposed to amosite asbestos, by J. Dai, A. Churg (American Journal of Respiratory Cell and Molecular Biology, 2001).

The study

• Authors: J. Dai, A. Churg
• Journal: American Journal of Respiratory Cell and Molecular Biology
• Published: 2001;24(4):427-35
• Cited by: 29 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

To investigate the role of iron and active oxygen species (AOS) in asbestos-induced fibrosis, we loaded increasing amounts of Fe(II)/Fe(III) onto the surface of amosite asbestos fibers and then applied the fibers to rat tracheal explants. Explants were harvested after 7 d in air organ culture. Asbestos by itself doubled procollagen gene expression, and a further increase was seen with increasing iron loading; actual collagen content measured as hydroxyproline was increased in a similar pattern. Iron loading also increased gene expression of platelet-derived growth factor (PDGF)-A and transforming growth factor (TGF)-beta(1). Neither asbestos alone nor iron-loaded asbestos affected gene expression of PDGF-B, tumor necrosis factor-alpha, or TGF-alpha. The AOS scavenger tetramethylthiourea or treatment of fibers with the iron chelator deferoxamine prevented asbestos-induced increases in procollagen, PDGF-A, and TGF-beta gene expression, whereas glutathione had no effect. The proteasome inhibitor MG-132 abolished asbestos-induced increases in procollagen gene expression but did not affect increases in PDGF-A or TGF-beta(1) expression, whereas the extracellular signal-regulated protein kinase (ERK) inhibitor PD98059 had exactly the opposite effect. We conclude that surface iron as well as the iron-catalyzed generation of AOS play a role in asbestos-induced matrix (procollagen) production and that this process is driven in part through oxidant-induced nuclear factor kappa B activation. Surface iron and AOS also play a role in PDGF-A and TGF-beta gene expression, but through an ERK-dependent mechanism.

In plain English

This laboratory experiment asked whether iron on amosite asbestos fiber surfaces, and the reactive oxygen molecules it helps form, drive changes linked to scarring. The authors loaded increasing amounts of iron onto amosite fibers, applied them to rat windpipe tissue, and cultured it for 7 days. Asbestos alone roughly doubled gene activity for procollagen, a building block of collagen. More surface iron raised it further. Tissue collagen (hydroxyproline) rose similarly; iron added without fibers did not raise it. Asbestos also increased gene activity for the growth factors PDGF-A and TGF-beta1, with further rises from added iron. Neither plain nor iron-loaded asbestos increased gene activity for TNF-alpha, PDGF-B or TGF-alpha. Treating the fibers with the iron-binding drug deferoxamine, or adding the scavenger tetramethylthiourea, blocked the rises in procollagen, PDGF-A and TGF-beta gene activity; glutathione did not. Blocking the NF-kappa B pathway stopped the procollagen rise, and blocking the ERK pathway stopped the growth-factor rises. The authors concluded that surface iron and iron-generated oxygen species play a role in procollagen production, partly through NF-kappa B, and in growth-factor gene activity through a separate ERK-dependent pathway.

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: Laboratory experiment on animal tissue explants (pieces of windpipe kept in organ culture)
• Who was studied: Tracheal (windpipe) explants from male Sprague-Dawley animals, described as rats in the abstract. Explants from several animals were mixed for each data point, and each test group had three data points.
• Exposure: Amosite asbestos (UICC standard reference sample), either alone or loaded with increasing amounts of an iron(II)/iron(III) mixture, applied for 1 hour. Tissue was then cultured for 7 days, with or without added inhibitors or scavengers, or with fibers pretreated with the chelator deferoxamine.
• Main results:
  • In explants exposed to amosite alone, procollagen gene expression roughly doubled. It rose further as surface iron increased.
  • Tissue hydroxyproline (collagen content) was significantly higher after asbestos than in controls, and higher again with 1 mM iron loading.
  • Iron added to control explants without dust did not raise hydroxyproline.
  • Deferoxamine (used to treat the fibers before use) and the scavenger tetramethylthiourea each completely abolished the asbestos-induced increase in procollagen gene expression; glutathione did not.
  • For TGF-beta1 and PDGF-A gene expression, deferoxamine and tetramethylthiourea again abolished the asbestos effect; glutathione did not.
  • The proteasome inhibitor MG-132 (used to block NF-kappa B activation) prevented the procollagen increase. The ERK inhibitor PD98059 did not.
  • Asbestos increased NF-kappa B activity in the explants. Adding MG-132 prevented this.
  • Asbestos alone increased TGF-beta1 and PDGF-A gene expression, and surface iron increased it further.
  • For PDGF-A and TGF-beta, the pattern was the reverse of procollagen: the ERK inhibitor PD98059 prevented the increases and MG-132 did not.
  • Neither asbestos alone nor iron-loaded asbestos increased TNF-alpha gene expression. The same was seen for PDGF-B and TGF-alpha.
  • TGF-beta protein measured by ELISA was at the bottom of the test's range and showed no differences among groups.
• The authors' conclusion: The authors concluded that surface iron and iron-catalyzed active oxygen species play a role in asbestos-induced procollagen production, driven in part through oxidant-induced NF-kappa B activation. Surface iron and active oxygen species also play a role in PDGF-A and TGF-beta gene expression, but through a separate ERK-dependent mechanism. In this model, one pathway directly affects procollagen production and the other affects expression of fibrogenic mediators.
• Limitations the authors state:
  • The explant system needs relatively high dust loadings to produce effects.
  • The authors could not tell whether the increased TGF-beta gene expression was translated into protein, or whether protein levels were simply too low to detect.
  • The authors said the peculiarities of the explant system, including slow dust uptake that requires 7-day cultures, should be kept in mind when interpreting the results.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Amosite.

Medical terms in this study, explained in our Lexicon: Platelet-Derived Growth Factor (PDGF), Amosite, Fibrosis, Collagen, Tumor Necrosis Factor-alpha (TNF-α), Glutathione (GSH), Nuclear Factor kappa B (NF-κB).

Related studies

• Scavengers of active oxygen species prevent cigarette smoke-induced asbestos fiber penetration in rat tracheal explants — A. Churg, J. Hobson, et al. (1989)
• Active oxygen species mediate asbestos fiber uptake by tracheal epithelial cells — J. Hobson, J.L. Wright, et al. (1990)
• Cigarette smoke increases amosite asbestos fiber binding to the surface of tracheal epithelial cells — A. Churg, J. Sun, et al. (1998)
• Iron enhances uptake of mineral particles and increases lipid peroxidation in tracheal epithelial cells — B. Keeling, K.Y. Li, et al. (1994)
• Airway wall remodeling induced by occupational mineral dusts and air pollutant particles — A. Churg, J.L. Wright (2002)
• Asbestos-derived reactive oxygen species activate TGF-beta1 — D.A. Pociask, P.J. Sime, et al. (2004)

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