Asbestos induces apoptosis in human alveolar macrophages

Our plain-English summary of Asbestos induces apoptosis in human alveolar macrophages, by R.F. Hamilton, L.L. Iyer, A. Holian (The American Journal of Physiology, 1996).

The study

• Authors: R.F. Hamilton, L.L. Iyer, A. Holian
• Journal: The American Journal of Physiology
• Published: 1996;271(5 Pt 1):L813-9
• Cited by: 63 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Asbestos refers to a group of fibrous minerals implicated in the development of several lung diseases, including fibrosis (asbestosis), cancer, and malignant mesothelioma. Although major health risks exist in occupationally exposed individuals, low-level exposures of asbestos may still contribute to health problems. The mechanism by which asbestos causes lung disease is not clearly understood but has been proposed to involve the alveolar macrophage (AM). We propose that asbestos induces apoptosis of AM, resulting in the development of an inflammatory state. In this study, we examined two forms of asbestos, chrysotile (CHR) and crocidolite (CRO), along with a control fiber, wollastonite (WOL), to characterize their relative cytotoxicity and ability to stimulate apoptosis in vitro. AM were cultured for 24 h with these particulates and examined for cell viability (trypan blue exclusion) and apoptosis (morphology, levels of cytosolic oligonucleosomal DNA fragments, and DNA ladder). In the absence of a decrease in cell viability, both CHR and CRO produced changes in cell morphology consistent with apoptosis. In addition, levels of cytoplasmic oligonucleosomal DNA (Cell Death Detection enzyme-linked immunosorbent assay) were significantly enhanced for CHR (3-25 micrograms/ml) and CRO (25-75 micrograms/ml) in a dose-dependent manner (a process that was inhibitable by 10 microM Z-Val-Ala-Asp fluoromethyl ketone, an interleukin-converting enzyme inhibitor). In contrast, WOL (up to 400 micrograms/ml) produced no significant DNA fragmentation in a 24-h culture. Neither CHR nor CRO caused DNA ladder formation in 24-h cell cultures. However, in 48-h cell cultures, both CHR- and CRO-exposed cells, but not WOL, resulted in the formation of DNA ladders characteristic of apoptosis. In summary, these results suggest that, unlike nonfibrogenic particulates, low doses of asbestos fibers cause apoptosis in cultured human AM that may be an early step in the development of lung fibrosis.

In plain English

This laboratory study used lung immune cells called alveolar macrophages, collected by lung washing from healthy nonsmoking volunteers. The cells were grown for 24 or 48 hours with chrysotile asbestos, crocidolite asbestos, or wollastonite, a comparison fiber the authors called nonfibrogenic. The cells were then checked for apoptosis, a form of programmed cell death. Only chrysotile at 25 µg/ml significantly lowered cell survival, though 90% of cells stayed alive. By 24 hours, chrysotile and crocidolite raised the share of apoptotic-looking cells by 10% and 9%, rising to 21% and 19% by 48 hours. Both asbestos types increased DNA fragments linked to apoptosis: chrysotile at concentrations down to 3 µg/ml, crocidolite only at higher concentrations. Wollastonite, at concentrations up to 400 µg/ml, produced no significant DNA fragmentation in 24-hour cultures. An enzyme inhibitor significantly reduced the asbestos-related increase. DNA 'ladders' typical of apoptosis appeared with both asbestos types at 48 hours but not at 24 hours. The authors concluded that both asbestos forms caused apoptosis in these cells in the lab and proposed a possible direct link to lung fibrosis. They noted that their results did not account for how fast the fibers clear from the lung.

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) experiment on human cells grown in culture
• Who was studied: Alveolar macrophages obtained by bronchoalveolar lavage (lung washing) from normal nonsmoking volunteers. The number of individuals per experiment is given as n in the figures (for example, n = 9). No total number of volunteers is stated.
• Exposure: Chrysotile asbestos (25 µg/ml), crocidolite asbestos (75 µg/ml) and wollastonite (200 µg/ml), with an unexposed control. The concentrations were chosen to give roughly equal fiber surface area. Separate dose-response tests used varying concentrations, with wollastonite tested up to 400 µg/ml. Cultures ran for 24 and 48 hours.
• Main results:
  • Cell viability at 24 hours: only chrysotile at 25 µg/ml produced a statistically significant decrease compared with control, but 90% of the cells still remained viable.
  • Cell appearance (morphology): by 24 hours, chrysotile and crocidolite increased the percentage of apoptotic cells by 10% and 9% respectively. By 48 hours this had risen to 21% and 19%.
  • At 24 hours, cells treated with chrysotile and crocidolite showed a significant increase in apoptosis-related DNA fragments compared with control cells.
  • This increase was significantly inhibited by the ICE inhibitor (Z-Val-Ala-Asp-fluoromethyl ketone). The scanned text appears to give the size of the reduction as about 40% for each asbestos type.
  • DNA fragment values in wollastonite-treated cultures did not differ significantly from control.
  • Dose response in 24-hour cultures (average of two experiments): chrysotile produced DNA fragments at concentrations down to 3 µg/ml. Crocidolite required higher concentrations (around 25 µg/ml). Wollastonite produced no significant DNA fragmentation at any concentration tested, up to a maximum of 400 µg/ml.
  • None of the fibers produced a DNA ladder at 24 hours.
  • At 48 hours, both chrysotile and crocidolite produced DNA ladders, while wollastonite-exposed cells did not differ from control cells.
• The authors' conclusion: The authors concluded that chrysotile and crocidolite caused apoptosis in alveolar macrophages in vitro and that wollastonite did not. They proposed that there may be a direct cause-and-effect relationship between apoptosis and fibrosis, and that particle-induced apoptosis should be considered when evaluating the relative risk of respirable particles.
• Limitations the authors state:
  • The in vitro results do not take into account the relative clearance rates of the two asbestos fibers from the lung.
  • The exact relationship between apoptosis and fibrosis is not certain.
  • The inhibitors are not completely specific, so the role of individual ICE-family enzymes could not be worked out.
  • The dose-response data were averages of only two observations, with no standard error available.

Topics

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

Medical terms in this study, explained in our Lexicon: Apoptosis, Alveolar Macrophage, Fibrosis, Malignant Mesothelioma, Chrysotile, Crocidolite, Wollastonite.

Related studies

• Long-lasting production of TGF-beta1 by alveolar macrophages exposed to low doses of asbestos without apoptosis — Y. Nishimura, T. Nishiike-Wada, et al. (2007)
• Involvement of the ICE family of proteases in silica-induced apoptosis in human alveolar macrophages — R. Iyer, A. Holian (1997)
• Cytokine and free radical responses of alveolar macrophages in vitro to asbestos fibres — L.C. Mongan, T. Jones, et al. (2000)
• Asbestos fibers and interferon-gamma up-regulate nitric oxide production in rat alveolar macrophages — G. Thomas, T. Ando, et al. (1994)
• Asbestos and silica-induced changes in human alveolar macrophage phenotype — A. Holian, M.O. Uthman, et al. (1997)
• Magnetometric evaluation for the effect of chrysotile on alveolar macrophages — T. Keira, M. Okada, et al. (1998)

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