Analyzing the genes and peptide growth factors expressed in lung cells in vivo consequent to asbestos exposure and in vitro

Our plain-English summary of Analyzing the genes and peptide growth factors expressed in lung cells in vivo consequent to asbestos exposure and in vitro, by A.R. Brody, J.Y. Liu, D. Brass, et al (Environmental Health Perspectives, 1997).

The study

• Authors: A.R. Brody, J.Y. Liu, D. Brass, M. Corti
• Journal: Environmental Health Perspectives
• Published: 1997;105 Suppl 5(Suppl 5):1165-71
• Type of article: Review
• Cited by: 30 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Inhalation of fibrogenic particles causes injury to the bronchiolar-alveolar epithelium. Consequently, there is a rapid proliferative response as the epithelium recovers and interstitial mesenchymal cells divide and produce connective tissue. In our model of brief (5-hr) exposure to chrysotile asbestos (approximately 1000 fibers/cc) in rats and mice, these events result in focal scarring at the bronchiolar-alveolar duct junctions in a histopathologic pattern identical to that seen in asbestos-exposed individuals. After 3 consecutive days of exposure, these lesions persist for at least 6 months postexposure. We postulate that cell proliferation and production of extracellular matrix is mediated in large part by three peptide growth factors, transforming growth factors alpha and beta (TGF-alpha and -beta), and platelet-derived growth factor (PDGF) A- and B-chains. To test this hypothesis in part, we have asked whether the genes that code for these growth factor proteins are activated at sites of asbestos-induced lung injury. If these genes were not activated, it would be reasonable to suspect that other potent growth factors and cytokines released during lung injury could be the primary mediators of fibroproliferative lung disease. In the studies reported here, we show, by in situ hybridization (ISH) and immunohistochemistry, that the four genes and their concomitant proteins are expressed within 24 hr in the bronchiolar-alveolar epithelium and underlying mesenchymal cells. RNase protection assay and ISH showed that the PDGF gene was upregulated during the first 5 hr of exposure and all the gene products remained above control levels for at least 2 weeks postexposure. TGF-alpha is a potent mitogen for epithelial cells, whereas the PDGF isoforms are potent growth factors for mesenchymal cells. TGF-beta retards fibroblast growth but stimulates extracellular matrix synthesis. Further studies using gene knockouts, appropriate antibodies, or antisense technology will be necessary to prove whether any of the growth factors are playing a significant role in fibrogenic lung disease. In addition, we have carried out a series of studies using type II alveolar epithelial cells purified from adult mouse lungs and maintained for up to 8 weeks in serum-free culture. These cells exhibit high transepithelial resistance values and they release TGF-beta 1 and -beta 2. This cell type also has been cultured from TGF-alpha knockout mice, resulting in monolayers with increased transepithelial resistance. This combination of studies in vivo and in vitro will allow us to pursue the mechanisms through which growth factors mediate lung fibrosis.

In plain English

This article summarized the authors' ongoing laboratory work on how chrysotile asbestos causes lung scarring (fibrosis) in rats and mice. Animals breathed high concentrations of chrysotile (1000-5000 fibers/cc) for brief periods of 1 to 5 hours. In asbestos-exposed rats, genes for the A- and B-chains of a growth factor called PDGF gave a strong signal right after a 5-hour exposure. The PDGF message stayed elevated for several weeks, but only at sites of injury. No signal was seen in unexposed rats or in rats exposed to iron spheres. Genes for two other growth factors, TGF-alpha and TGF-beta1, were also expressed after exposure. Among mouse strains given the same dose, C57 mice had the most severe lesions, while 129 mice were essentially protected. In a single experiment, a blinded pathologist could not tell asbestos-exposed mice lacking TNF-alpha receptors from unexposed controls. In cultured mouse lung cells, only the highest chrysotile concentration significantly reduced electrical resistance across the cell layer. The authors cautioned that descriptive findings like theirs are far from proof that a growth factor actually drives the disease.

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: Summary of the authors' ongoing laboratory work: animal inhalation experiments in rats and mice plus cell-culture (in vitro) studies; some findings previously published or submitted
• Who was studied: Rats; four mouse strains (C57, 129, SJL, and C57 x SJL hybrids); TNF-alpha receptor knockout mice and comparison groups (four groups, four animals per group at each sacrifice time); cultured type II alveolar epithelial cells from mice, including TGF-alpha knockout mice. Work done at Tulane University Medical Center, New Orleans; dates of the experiments not given
• Exposure: Brief inhalation (1 to 5 hours) of chrysotile asbestos at 1000-5000 fibers/cc; nonfibrogenic iron spheres used as a control exposure; in culture, chrysotile, latex and iron particles added to cell layers
• Main results:
  • In asbestos-exposed rats, the genes for both the PDGF A-chain and B-chain proteins were expressed, mainly in epithelial cells and macrophages
  • In asbestos-exposed rats, PDGF gene expression gave a strong signal immediately after a 5-hour chrysotile exposure
  • In asbestos-exposed rats, the PDGF message stayed elevated for several weeks after exposure, but only in the bronchiolar-alveolar duct regions
  • No PDGF message was detected at any time in unexposed rats or in rats exposed to iron spheres
  • In exposed rats, TGF-alpha message and protein could not be measured until 24 hours after the 5-hour asbestos exposure (work previously published by the authors)
  • mRNA for TGF-beta1 was expressed in rats exposed to asbestos for 5 hours
  • No TGF-beta signal was detected in sham-exposed or iron-exposed rats
  • Among four mouse strains given the same chrysotile dose, C57 mice had the most severe lesions and 129 mice were essentially protected
  • Exposed C57 mice had about 10-fold increases in dividing (BrdU-positive) cells; 129 mice had significantly fewer dividing cells through 2 weeks after exposure
  • A blinded histopathologist could not distinguish the four asbestos-exposed TNF-alpha receptor knockout mice from unexposed controls
  • Cell division was about 10-fold over normal in exposed C57 and hybrid mice; unexposed and knockout mice showed normal low levels
  • In the authors' laboratory (Riva et al.), cultured mouse type II cells produced TGF-beta1 and TGF-beta2 on their own
  • Type II cells from TGF-alpha knockout mice had electrical resistances averaging 40% higher than controls
  • In cultured mouse alveolar epithelial cell layers, only the highest chrysotile concentration used significantly reduced electrical resistance across the layer
• The authors' conclusion: The authors concluded that TNF-alpha appears to play a key role in the early fibroproliferative (scarring) responses to inhaled asbestos
• Limitations the authors state:
  • Descriptive findings on timing and location of gene expression are far from proof that PDGF is an essential actor in the disease
  • The growth factors studied may be side effects rather than the real mediators of the disease
  • The TNF-alpha receptor knockout findings came from a single inhalation experiment
  • Stronger TGF-beta staining in C57 mice may simply reflect more cells being present rather than increased production

Topics

Medical terms in this study, explained in our Lexicon: Asbestos Exposure, Fibrogenic, Epithelium, Interstitial, Chrysotile, Platelet-Derived Growth Factor (PDGF), Cytokine, Immunohistochemistry.

Related studies

• Growth factor responses and protooncogene expression of murine mesothelial cell lines derived from asbestos-induced mesotheliomas — L.A. Goodglick, C.A. Vaslet, et al. (1997)
• 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)
• Inhalation of Asbestos Fibers and Consequent Expression of Peptide Growth Factors — A.R. Brody (2000)
• Primary lung fibroblasts from the 129 mouse strain exhibit reduced growth factor responsiveness in vitro — D.M. Brass, S.Y. Tsai, 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)
• Susceptibility to asbestos-induced and transforming growth factor-beta1-induced fibroproliferative lung disease in two strains of mice — G.S. Warshamana, D.A. Pociask, et al. (2002)

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