Mobilization of iron from urban particulates leads to generation of reactive oxygen species in vitro and induction of ferritin synthesis in human lung epithelial cells

Our plain-English summary of Mobilization of iron from urban particulates leads to generation of reactive oxygen species in vitro and induction of ferritin synthesis in human lung epithelial cells, by K.R. Smith, A.E. Aust (Chemical Research in Toxicology, 1997).

The study

• Authors: K.R. Smith, A.E. Aust
• Journal: Chemical Research in Toxicology
• Published: 1997;10(7):828-34
• Cited by: 81 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Many of the biochemical effects of asbestos in cultured cells have been shown to be due to iron, which can be as high as 27% by weight. Urban air particulates also contain iron, and some of the pathological effects after inhalation may be due to reactive oxygen species produced by iron-catalyzed reactions. Two standard reference material (SRM) urban air particulate samples were used for the studies described here. SRM 1648 (3.9% iron by weight) was collected in the St. Louis, MO, area, and SRM 1649 (3% iron by weight) was collected in the Washington, DC, area. To determine if iron associated with urban particulates could be mobilized, as it is from asbestos, SRMs 1648 and 1649 were incubated with 1 mM citrate or EDTA, in the presence or absence of ascorbate. Iron was mobilized from both particulates by either chelator, especially in the presence of ascorbate. Citrate, in the presence of ascorbate, mobilized 30.9 nmol of Fe/mg of SRM 1648 and 65.1 nmol of Fe/mg of SRM 1649 in 24 h. EDTA, in the presence of ascorbate, mobilized 53.8 nmol of Fe/mg of SRM 1648 and 98.8 nmol of Fe/mg of SRM 1649 in 24 h. To determine whether reactive oxygen species were being produced by the particulate iron, each particulate was incubated with phi X174 RFI DNA in the presence or absence of ascorbate. Single-strand breaks (SSBs) were produced by either particulate, but only in the presence of ascorbate. Incubation of SRM 1648 or 1649 (0.5 mg/mL) with DNA in the presence of ascorbate and citrate resulted in 20% or 34% DNA with SSBs, respectively. Incubation of SRM 1648 or 1649 (0.1 mg/mL) with DNA in the presence of ascorbate and EDTA resulted in 26% or 45% DNA with SSBs, respectively. To determine if iron associated with urban particulates could be mobilized by human lung epithelial cells (A549), cells were treated with particulates and the amount of the iron storage protein ferritin was determined at the end of treatment. The 6.4- or 8.4-fold increase in ferritin observed in cells treated with SRM 1648 or 1649, respectively, over that of control (untreated) cells strongly suggested that iron was mobilized in the cultured cells. If similar mobilization and reactivity of the iron occurs in the lung, this may explain some of the pathological effects of urban particulates.

In plain English

This laboratory study asked whether iron in city air dust behaves like iron in crocidolite asbestos. The authors used two standard urban dust samples, one from St. Louis (3.9% iron by weight) and one from Washington, DC (3% iron). The iron-binding chemicals citrate and EDTA released iron from both dusts, especially when ascorbate (vitamin C) was added. Citrate plus ascorbate released 30.9 nmol of iron per mg of the St. Louis dust and 65.1 nmol per mg of the Washington dust in 24 hours. Both dusts broke strands of test-tube DNA, but only when ascorbate was present. The released iron alone, with the dust removed, caused as much DNA damage as the dust itself. An iron-blocking chemical stopped the breaks completely. Human lung cells (the A549 line) treated with the dusts had 6.4 times (St. Louis) and 8.4 times (Washington) more of the iron-storage protein ferritin than untreated cells. The authors concluded that iron was responsible for the DNA breaks. They suggested that if the same happens in the lung, it may explain some harmful effects of urban dust. No people were studied.

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) study: cell-free chemical and DNA experiments, plus experiments on a cultured human lung cell line
• Who was studied: Two standard reference urban air particulate samples (SRM 1648, collected in St. Louis, MO, 1974 to 1976; SRM 1649, collected in Washington, DC, 1975 to 1976), and the A549 human lung epithelial cell line
• Exposure: Urban air particulates containing 3.9% (SRM 1648) and 3.0% (SRM 1649) iron by weight, incubated with 1 mM citrate or EDTA, with or without 1 mM ascorbate. Cells were treated with 100 µg/cm2 of particulates for 24 h. Crocidolite asbestos (27% iron by weight) was used as a comparison, partly using data from earlier studies.
• Main results:
  • With ascorbate, over 24 h, citrate released 30.9 nmol of iron per mg of SRM 1648 and 65.1 nmol per mg of SRM 1649.
  • With ascorbate, over 24 h, EDTA released 53.8 nmol of iron per mg of SRM 1648 and 98.8 nmol per mg of SRM 1649.
  • Both particulates caused single-strand breaks in DNA, but only when ascorbate was present.
  • At 0.5 mg/mL with ascorbate and citrate, 20% (SRM 1648) and 34% (SRM 1649) of the DNA had single-strand breaks.
  • At 0.1 mg/mL with ascorbate and EDTA, 26% (SRM 1648) and 45% (SRM 1649) of the DNA had single-strand breaks.
  • Liquid with the particulates removed, containing only the released iron, caused the same amount of DNA breaks as samples with particulates. The authors interpreted this as the breaks being due to released iron.
  • Pre-treatment with the iron-binding chemical desferrioxamine B completely blocked the DNA breaks.
  • DNA breaks with either particulate were considerably greater than those previously reported for crocidolite in an earlier study.
  • The percentage of iron in each particulate did not appear to be directly related to the amount of DNA breaks it caused.
  • Ferritin in A549 lung cells rose 6.4-fold after treatment with SRM 1648 and 8.4-fold after SRM 1649, compared with untreated cells (1.15 and 1.51 versus 0.18 ng ferritin per µg total protein).
• The authors' conclusion: The authors concluded that iron was responsible for the particulate-dependent DNA breaks. They suggested that if similar iron release and reactivity occur in the lung, this may explain some of the harmful effects of urban particulates.
• Limitations the authors state:
  • Iron release inside cells could not be measured directly; ferritin levels were used as an indirect indicator.
  • The authors could not explain an initial dip in one iron-release curve (ascorbate with SRM 1649).

Topics

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

Medical terms in this study, explained in our Lexicon: Reactive Oxygen Species (ROS), Ferritin.

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