Chrysotile asbestos stimulates platelet-derived growth factor-AA production by rat lung fibroblasts in vitro: evidence for an autocrine loop

Our plain-English summary of Chrysotile asbestos stimulates platelet-derived growth factor-AA production by rat lung fibroblasts in vitro: evidence for an autocrine loop, by J.A. Lasky, P.G. Coin, P.M. Lindroos, et al (American Journal of Respiratory Cell and Molecular Biology, 1995).

The study

• Authors: J.A. Lasky, P.G. Coin, P.M. Lindroos, L.E. Ostrowski, A.R. Brody, J.C. Bonner
• Journal: American Journal of Respiratory Cell and Molecular Biology
• Published: 1995;12(2):162-70
• Cited by: 43 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

We have investigated the mitogenic and chemotactic role of platelet-derived growth factor (PDGF) in pulmonary fibrogenesis induced by chrysotile asbestos. Since fibroblasts phagocytize asbestos in the lung interstitium, we have sought to learn whether the fibers alter the production of PDGF-like molecules by rat lung fibroblasts or induce mitogenesis of these fibroblasts in vitro. Conditioned medium as well as cell lysates from fibroblasts exposed to asbestos contained approximately 4-fold more PDGF than unexposed cells as detected by Western blot. Two distinct molecular weight forms of PDGF (36 and 18 kD) were detected by Western blotting. We postulate that these PDGF-like molecules are homologues of human PDGF-AA since we could not detect any PDGF in a sensitive enzyme immunoassay that recognized only PDGF-BB and PDGF-AB. Furthermore, PDGF-A chain mRNA was readily detected by Northern analysis, whereas PDGF-B chain mRNA was not detected by conventional Northern analysis. However, message amplification using a reverse transcriptase polymerase chain reaction allowed detection of the B-chain message. A significant dose-dependent mitogenic effect of asbestos was found by using both a cell proliferation assay and nuclear labeling with bromodeoxyuridine when fibroblasts were exposed under serum-free conditions. This mitogenesis induced directly by asbestos was blocked almost entirely with an anti-PDGF antibody that neutralized all three PDGF isoforms. Thus, these data support our hypothesis that an autocrine loop for PDGF-AA is operative in vitro following exposure to asbestos in lung fibroblasts, and we suggest that this signaling pathway could be significant in the pathogenesis of pulmonary fibrosis.

In plain English

In this laboratory study, the authors exposed cultured rat lung fibroblasts (connective-tissue cells) to chrysotile asbestos. They measured platelet-derived growth factor (PDGF), which makes cells divide. The cells took up asbestos fibers. Culture fluid from asbestos-treated cells held about 4-fold more PDGF than fluid from untreated cells, roughly 4 versus 1 ng/ml. Several tests pointed to the PDGF-AA form. A test detecting only the AB and BB forms found no PDGF. The cells readily showed A-chain messenger RNA; B-chain messages needed a more sensitive method. Asbestos increased cell numbers only in resting (quiescent) cells, peaking at 0.5 to 1 micrograms per square centimeter; at 5 micrograms they did not rise, although that dose produced the most PDGF. In actively growing cells, doses above 0.5 micrograms per square centimeter were toxic. An antibody that blocks PDGF prevented most, though not all, of this growth. The authors concluded the results support a self-stimulating (autocrine) loop, which they suggested could matter in pulmonary fibrosis. They noted it is unknown whether this occurs in living animals, and that the dose was large compared with lung burdens after inhalation.

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 (in vitro) cell-culture experiment
• Who was studied: Fibroblasts isolated from adult rat lungs, used at the third passage. Staining suggested a myofibroblast type. The Western blots were representative of four separate experiments.
• Exposure: Chrysotile asbestos supplied by the National Institute of Environmental Health Sciences was added to the cultures at doses including 1 and 5 micrograms per square centimeter. Cells were kept in serum-free medium.
• Main results:
  • In culture fluid, asbestos-treated fibroblasts showed about a 4-fold increase in PDGF (36 and 18 kD bands combined) compared with unexposed fibroblasts.
  • The approximate PDGF concentrations were 4 ng/ml in culture fluid from asbestos-treated cells and 1 ng/ml from unexposed cells.
  • No PDGF was detected in fibroblast culture fluid by an immunoassay that detects PDGF-BB and PDGF-AB but not PDGF-AA.
  • Blocking the antibody with PDGF-AA reduced the fibroblast PDGF bands, indicating that at least half of the PDGF was A-chain.
  • Asbestos-exposed and unexposed fibroblasts expressed about the same level of PDGF-A gene messages. PDGF-B chain messages were detected only by reverse transcriptase PCR, not by conventional Northern analysis.
  • In quiescent fibroblasts, the authors reported a significant dose-dependent rise in cell number. The proliferative response was maximal at 0.5 to 1 micrograms per square centimeter of asbestos. The peak at 1 microgram per square centimeter was about 40% of the maximal response to recombinant PDGF-BB.
  • In quiescent fibroblasts, cell numbers did not increase at 5 micrograms per square centimeter, even though that dose induced the highest PDGF secretion after 48 h. The authors attributed this probably to the proliferation assay being measured after 3 days.
  • In fibroblasts that were actively growing (log phase), asbestos was cytotoxic rather than growth-promoting at concentrations above 0.5 micrograms per square centimeter. This graph was based on two experiments.
  • DNA-synthesis (BrdU) labeling from 24 to 48 h in asbestos-exposed cells was twice that of control cells.
  • An anti-PDGF antibody blocked the asbestos-induced fibroblast proliferation. Control IgG had a small but significant effect in the BrdU assay.
• The authors' conclusion: The authors concluded that the data support an autocrine loop for PDGF-AA in lung fibroblasts after asbestos exposure in vitro. They suggested this signaling pathway could be significant in the development of pulmonary fibrosis.
• Limitations the authors state:
  • The authors do not know whether the effect seen in culture occurs in living animals.
  • The asbestos concentration used in culture was large compared with the amount retrieved from rat lungs after inhalation exposure.
  • The anti-PDGF antibody did not completely block the growth effect. The authors said other growth factors could be involved.

Topics

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

Medical terms in this study, explained in our Lexicon: Chrysotile, Platelet-Derived Growth Factor (PDGF), Fibroblast, Pulmonary Fibrosis.

Related studies

• 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)
• Differential proliferation of rat lung fibroblasts induced by the platelet-derived growth factor-AA, -AB, and -BB isoforms secreted by rat alveolar macrophages — J.C. Bonner, A.R. Osornio-Vargas, et al. (1991)
• Rat alveolar macrophage-derived platelet-derived growth factor is chemotactic for rat lung fibroblasts — A.R. Osornio-Vargas, J.C. Bonner, et al. (1990)
• Inorganic particles induce secretion of a macrophage homologue of platelet-derived growth factor in a density-and time-dependent manner in vitro — R.M. Schapira, A.R. Osornio-Vargas, et al. (1991)
• Chrysotile asbestos induces PDGF-A chain-dependent proliferation in human and rat lung fibroblasts in vitro — J.A. Lasky, J.C. Bonner, et al. (1996)
• Primary lung fibroblasts from the 129 mouse strain exhibit reduced growth factor responsiveness in vitro — D.M. Brass, S.Y. Tsai, et al. (2001)

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