Incorporation of tritiated thymidine by epithelial and interstitial cells in bronchiolar-alveolar regions of asbestos-exposed rats

Our plain-English summary of Incorporation of tritiated thymidine by epithelial and interstitial cells in bronchiolar-alveolar regions of asbestos-exposed rats, by A.R. Brody, L.H. Overby (The American Journal of Pathology, 1989).

The study

• Authors: A.R. Brody, L.H. Overby
• Journal: The American Journal of Pathology
• Published: 1989;134(1):133-40
• Cited by: 97 articles, as counted by NIH iCite
• Read it: PubMed · Free full text

Abstract

Inhaled asbestos causes progressive interstitial lung disease. The authors have performed a series of studies to elucidate early pathogenetic events at sites of fiber deposition in asbestos-exposed rats. This study reports that a single 5-hour exposure to chrysotile asbestos induces significant increases in incorporation of tritiated thymidine (3HTdR) into nuclei of epithelial and interstitial cells of bronchiolar-alveolar regions. No cell populations in air-exposed or carbonyl iron-exposed control animals exhibited more than 1% labeling at any point in time. Immediately after the 5-hour asbestos exposure, incorporation was normal. By 19 hours after exposure there was a significant increase in incorporation of 3HTdR, particularly by Type II epithelial cells of the first alveolar duct bifurcations. The greatest increase in degree of incorporation (up to 18-fold) was observed 24 hours after exposure, and increased percentages of 3HTdR-labeled cells were maintained through the 48 hours postexposure period. Normal labeling was present by 8 days after exposure, and this level remained through the 1-month period studied. This apparent mitogenic response correlates with increased numbers of bronchiolar-alveolar epithelial and interstitial cells demonstrated by ultrastructural morphometry in correlative studies. The authors speculate that the incorporation of 3HTdR could be induced by the direct effects of inhaled fibers or by mitogenic factors released from macrophages attracted to the inhaled asbestos.

In plain English

This was an animal experiment in rats. Groups of five young male rats breathed room air, carbonyl iron spheres, or chrysotile asbestos (printed as 10 mg/sq m respirable mass) for a single 5-hour period. At set times from 0 hours to 4 weeks afterwards, the rats were injected with tritiated thymidine, a labeled substance that cells take up when they copy their DNA. Lung tissue around the smallest airways was then examined. In air-exposed and iron-exposed rats, no more than 1% of cells were labeled at any time. In asbestos-exposed rats, labeling was normal immediately after exposure and rose by 19 hours. At 1 day, epithelial cells of the first alveolar duct bifurcations showed an 18-fold increase over control levels. Labeled cells included Type II alveolar cells, Clara-like airway cells and interstitial cells. Labeling was returning to normal by 2 days. None of 1500 macrophages gathered in air spaces and airways was labeled. The authors concluded that a single brief chrysotile exposure significantly increased thymidine uptake in these rats. They noted they had no direct evidence that the cells actually divided.

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 (inhalation study in rats with air-exposed and iron-exposed control groups, using tritiated thymidine autoradiography)
• Who was studied: Male CD (SD)BR rats (Charles River), 8 weeks old, 5 rats per group. Two terminal bronchiole-alveolar duct regions were examined per animal (10 anatomic units per group). Rats were studied at recovery times of 0, 19, 24, 33 and 48 hours and 1, 2 and 4 weeks after exposure.
• Exposure: A single 5-hour inhalation exposure to chrysotile asbestos aerosol (10 mg/sq m respirable mass, as printed). Controls breathed room air (sham) or carbonyl iron spheres (30 mg/sq m, as printed).
• Main results:
  • Air-exposed (sham) control rats never showed more than 1% labeled cells in any anatomic compartment at any time.
  • In iron-exposed rats, over 2000 epithelial cells and almost 1000 interstitial cells were counted between 18 hours and 2 weeks after exposure. In no animal did more than 1% of cells have labeled nuclei.
  • In asbestos-exposed rats, there was no increase in labeling in any cell type immediately after the 5-hour exposure.
  • In asbestos-exposed rats, increased labeling was first seen 19 hours after exposure and continued through the 2-day period.
  • In asbestos-exposed rats, epithelial cells of the first alveolar duct bifurcations showed an 18-fold increase over control levels 1 day after exposure.
  • In asbestos-exposed rats at 24, 33 and 48 hours, 100% of terminal bronchioles and 75-80% of duct bifurcations had at least one labeled epithelial or interstitial cell (data not shown in the article).
  • At the alveolar level, the labeled epithelial cells were Type II cells.
  • In the terminal bronchioles of asbestos-exposed rats, nonciliated Clara-like cells appeared to be the only epithelial cell type labeled.
  • Twenty-two cells identified by light microscopy as labeled interstitial cells were examined by electron microscopy. All proved to be interstitial but apparently undifferentiated. Two of these appeared monocyte- or macrophage-like.
  • In asbestos-exposed rats, macrophages accumulated in alveolar spaces and airways at 19, 24, 33 and 48 hours. No labeling was detected in 1500 of these macrophages observed in 20 animals.
  • Alveolar ducts beyond the first bifurcation showed no significant increase in labeling in any cell type.
  • By 2 days after exposure, labeling in epithelial and interstitial compartments was returning to normal. Normal levels were maintained for the rest of the 30-day period.
• The authors' conclusion: The authors concluded that a single brief exposure to chrysotile asbestos caused a significant increase in thymidine incorporation by epithelial and interstitial cells of bronchiolar-alveolar regions in rats. This increase was not seen with air or iron spheres. They suggested that direct fiber-induced cell injury and/or mitogenic factors secreted by lung macrophages could be responsible, while stating that the mechanisms are not clear.
• Limitations the authors state:
  • The authors stated they had no direct evidence that the epithelial cells actually divided rather than only incorporating thymidine into DNA.
  • The labeled interstitial cells appeared undifferentiated, so the precise identity of each cell type could not be determined.
  • The precise nature of the labeled fibroblast-like cells of the airway submucosa was not determined.

Topics

Medical terms in this study, explained in our Lexicon: Interstitial, Interstitial Lung Disease, Deposition, Chrysotile, Asbestos Exposure, Macrophage.

Related studies

• Chrysotile asbestos inhalation induces tritiated thymidine incorporation by epithelial cells of distal bronchioles — P.D. McGavran, A.R. Brody (1989)
• Up-regulated expression of transforming growth factor-alpha in the bronchiolar-alveolar duct regions of asbestos-exposed rats — J.Y. Liu, G.F. Morris, et al. (1996)
• Asbestos and benzo[a]pyrene act synergistically to induce squamous metaplasia and incorporation of [3H]thymidine in hamster tracheal epithelium — B.T. Mossman, A. Eastman, et al. (1984)
• Bronchiolar and alveolar lesions in the pathogenesis of crocidolite-induced pulmonary fibrosis in mice — D.H. Bowden, I.Y. Adamson (1985)
• Asbestos causes apoptosis in alveolar epithelial cells: role of iron-induced free radicals — A. Aljandali, H. Pollack, et al. (2001)
• Thymidine incorporation by lung fibroblasts as a sensitive assay for biological activity of asbestos — I. Lemaire, D. Gingras, et al. (1982)

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