Up-regulated expression of transforming growth factor-alpha in the bronchiolar-alveolar duct regions of asbestos-exposed rats

Our plain-English summary of Up-regulated expression of transforming growth factor-alpha in the bronchiolar-alveolar duct regions of asbestos-exposed rats, by J.Y. Liu, G.F. Morris, W.H. Lei, et al (The American Journal of Pathology, 1996).

The study

• Authors: J.Y. Liu, G.F. Morris, W.H. Lei, M. Corti, A.R. Brody
• Journal: The American Journal of Pathology
• Published: 1996;149(1):205-17
• Cited by: 56 articles, as counted by NIH iCite
• Read it: PubMed · Free full text

Abstract

It has become apparent that the numerous growth factors and cytokines are produced during the development of fibroproliferative lung disease. Investigators must sort out which combinations of these factors are playing mechanistic roles in the disease process. Here we demonstrate that transforming growth factor (TGF)-alpha, a potent epithelial and mesenchymal cell mitogen, is upregulated specifically at the sites of asbestos fiber deposition in the lungs of rats exposed for 5 hours. Unexposed animals and those exposed to high concentrations of iron spheres exhibited no increase in TGF-alpha expression at any time during the experiment. Inhaled asbestos fibers deposit initially at the bronchiolar-alveolar duct regions and alveolar macrophages accumulate at these sites within hours. Non-isotopic in situ hybridization and immunohistochemistry were used to show that the mRNA that codes for TGF-alpha along with the peptide were clearly up-regulated at the bronchiolar-alveolar duct regions by 24 hours after the single asbestos exposure. The numbers of labeled cells demonstrated that expression of the mRNA and protein remained significantly above background for at least 2 weeks after exposure along with increased cell proliferation assessed by staining for proliferating cell nuclear antigen. This, to our knowledge, is the first demonstration of TGF-alpha expression at sites of lung injury in developing fibroproliferative disease. This finding supports the hypothesis that the growth factor is involved in the dramatic epithelial and mesenchymal proliferation we documented previously, although additional experiments will be essential to establish the precise role of TGF-alpha.

In plain English

This was an animal experiment. Male Sprague-Dawley rats, 35 per group, breathed chrysotile asbestos (10 mg/m3 respirable mass), carbonyl iron spheres (50 mg/m3) or room air once, for 5 hours. The experiment was carried out twice, several months apart. Lungs were examined from immediately after exposure to 4 weeks later. The researchers looked for the growth factor TGF-alpha and for PCNA, a marker of cell division that, the authors noted, also functions in DNA repair. In asbestos-exposed rats, TGF-alpha messenger RNA appeared strongly in the lining cells of the small airways and alveolar ducts within 48 hours, and TGF-alpha protein was first increased by 24 hours. Both TGF-alpha and PCNA stayed significantly raised in these lining cells for 2 weeks, and both returned to normal by 1 month. PCNA labeling rose about 20-fold. Unexposed and iron-exposed rats had about 1% PCNA-positive cells and no increase in TGF-alpha messenger RNA. The authors concluded that the findings support the idea that TGF-alpha contributes to the early cell-division response at asbestos deposition sites, but said further studies are needed to prove this.

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 (inhalation) experiment: a single 5-hour exposure, with groups of rats sacrificed at set times from 0 hours to 4 weeks afterward. The whole experiment was run twice. Lungs were studied by in situ hybridization (for TGF-alpha messenger RNA) and by immunohistochemistry (for TGF-alpha protein, PCNA and macrophages).
• Who was studied: Pathogen-free male Sprague-Dawley rats, 8 to 10 weeks old, weighing 200 to 250 g. There were 35 rats per exposure group, with three groups (asbestos, iron, room air). Place and dates of the experiment are not stated beyond the authors' affiliation (Tulane University School of Medicine, New Orleans).
• Exposure: Chrysotile asbestos aerosol at 10 mg/m3 respirable mass for 5 hours. Comparison groups breathed carbonyl iron spheres at 50 mg/m3 or room air (sham) for the same 5 hours.
• Main results:
  • In asbestos-exposed rats, numerous macrophages (ED1-positive cells) gathered on the alveolar duct bifurcation surfaces during the first 24 hours after exposure.
  • In iron-exposed rats, few macrophages accumulated, and only for a brief period.
  • Unexposed and iron-exposed rats had low numbers of PCNA-positive cells, about 1%.
  • In asbestos-exposed rats, PCNA labeling increased roughly 20-fold. The authors described the increase as highly significant (P less than 0.001); the scanned text prints the figure as '-20-fold'.
  • In asbestos-exposed rats, PCNA labeling returned to normal low levels by 1 month after exposure.
  • Unexposed and iron-exposed rats showed no increase in TGF-alpha messenger RNA at any time point.
  • In asbestos-exposed rats, within 48 hours there was strong TGF-alpha messenger RNA signal in bronchiolar Clara cells and in the epithelial cells lining the alveolar duct walls and bifurcations.
  • In asbestos-exposed rats, the TGF-alpha messenger RNA signal was reduced by 2 weeks but was still obvious in every animal.
  • Counts of stained cells showed the first increase in TGF-alpha protein in asbestos-exposed rats by 24 hours after exposure.
  • In the epithelium (lining cells) of asbestos-exposed rats, both TGF-alpha and PCNA remained significantly elevated for 2 weeks. TGF-alpha returned to normal by 1 month.
  • In the lung interstitial cells of asbestos-exposed rats, PCNA but not TGF-alpha was significantly increased for 2 weeks after exposure.
• The authors' conclusion: The authors concluded that their findings support the idea that TGF-alpha contributes to the rapid cell-division response at sites of asbestos deposition, where fibrogenic lesions develop in the lungs of exposed rats.
• Limitations the authors state:
  • The authors stated that the observations support a role for TGF-alpha but that additional studies are needed to prove it.
  • The authors noted that PCNA, used as the marker of cell proliferation, also functions as a DNA repair protein.

Topics

Medical terms in this study, explained in our Lexicon: Cytokine, Deposition, Alveolar Macrophage, Immunohistochemistry, Asbestos Exposure.

Related studies

• Reduced tumor necrosis factor-alpha and transforming growth factor-beta1 expression in the lungs of inbred mice that fail to develop fibroproliferative lesions consequent to asbestos exposure — D.M. Brass, G.W. Hoyle, et al. (1999)
• Incorporation of tritiated thymidine by epithelial and interstitial cells in bronchiolar-alveolar regions of asbestos-exposed rats — A.R. Brody, L.H. Overby (1989)
• Upregulation of the PDGF-alpha receptor precedes asbestos-induced lung fibrosis in rats — J.A. Lasky, B. Tonthat, et al. (1998)
• Distribution of transforming growth factor-beta 1, fibronectin, and smooth muscle actin in asbestos-induced pulmonary fibrosis in rats — T.D. Perdue, A.R. Brody (1994)
• Transforming growth factor-beta(1) overexpression in tumor necrosis factor-alpha receptor knockout mice induces fibroproliferative lung disease — J.Y. Liu, P.J. Sime, et al. (2001)
• Chrysotile asbestos upregulates gene expression and production of alpha-receptors for platelet-derived growth factor (PDGF-AA) on rat lung fibroblasts — J.C. Bonner, A.L. Goodell, et al. (1993)

Browse more research in The American Journal of Pathology, or return to medical research.