The effect of iron binding on the ability of crocidolite asbestos to catalyze DNA single-strand breaks

Our plain-English summary of The effect of iron binding on the ability of crocidolite asbestos to catalyze DNA single-strand breaks, by J.A. Hardy, A.E. Aust (Carcinogenesis, 1995).

The study

• Authors: J.A. Hardy, A.E. Aust
• Journal: Carcinogenesis
• Published: 1995;16(2):319-25
• Cited by: 68 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Crocidolite or crocidolite pretreated with desferrioxamine-B (DF crocidolite) was exposed to ferrous chloride solutions to determine whether iron could be bound from solution. Native crocidolite was capable of binding up to 57 nmol Fe2+/mg fiber in 60 min, while the DF crocidolite was capable of binding only 5.5 nmol Fe2+/mg fiber. The rate of iron binding for the first 5 min of exposure was independent of the concentration of iron in the solution, suggesting that there was a group of rapidly saturable sites, approximately 1.5 x 10(18) binding sites/m2 crocidolite surface, which were responsible for the immediate binding. This process was followed by a slower binding phase, likely occurring at other sites. Crocidolite and DF crocidolite, with various amounts of iron bound, were assayed for their abilities to catalyze the formation of DNA single-strand breaks (SSBs) in phi X174 RFI DNA. Native crocidolite with additional iron bound did not significantly change in its ability to cause DNA SSBs in 15 or 30 min incubations, even though more iron could be mobilized from the iron-treated crocidolite at 4 or 24 h. DF crocidolite, after the addition of iron, had a significantly increased ability to form DNA SSBs. DF crocidolite with 0, 3.0 or 5.5 mmol Fe2+/mg catalyzed the formation of DNA SSBs in 21, 42 or 51% of the DNA respectively in the presence of EDTA and ascorbate. Fibers were also incubated in tissue culture medium with or without iron salts. The fibers incubated in the iron-containing medium had an increased ability to form DNA SSBs. These results suggest that fibers such as crocidolite may be capable of binding iron from intracellular sources. This additional iron may be as reactive as the intrinsic iron and may increase the reactive lifetime of the fiber.

In plain English

In this laboratory study, the authors tested whether crocidolite asbestos fibers could take up extra iron, and whether that changed their ability to break DNA strands in a test tube. No people or animals were studied. They used untreated (native) crocidolite, crocidolite soaked 90 days in sodium bicarbonate solution, and crocidolite partly stripped of iron by 90 days in desferrioxamine-B (DF crocidolite). Native crocidolite bound up to 57 nmol of ferrous iron per mg in 60 minutes; DF crocidolite, at most 5.5 nmol per mg. Extra iron did not significantly change DNA breakage by native crocidolite in 15 or 30 minute tests. However, iron-treated native fibers placed in the chelators citrate or EDTA released significantly more iron over 4 or 24 hours than untreated fibers; without a chelator, neither released iron. In DF crocidolite, added iron significantly increased DNA breakage; 5.5 nmol per mg more than doubled it. Fibers kept in iron-containing cell culture medium also caused more DNA breaks. The authors concluded that fibers such as crocidolite may bind iron from sources inside cells, possibly increasing the fiber's reactive lifetime.

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 asbestos fibers and purified DNA; no human or animal subjects
• Who was studied: Samples of crocidolite asbestos supplied by NIEHS/NTP, containing 26% iron by weight, tested with phiX174 RFI DNA at Utah State University, Logan, Utah
• Exposure: Three forms of crocidolite were tested: native crocidolite; crocidolite soaked 90 days in sodium bicarbonate solution (36 mM); and DF crocidolite, treated 90 days with desferrioxamine-B (1 mM) to remove iron. Each was exposed to ferrous chloride solutions (25, 50 and 100 micromolar) or to Ham's F-12 tissue culture medium with or without iron.
• Main results:
  • Native crocidolite bound up to 57 nmol of ferrous iron per mg of fiber in 60 minutes.
  • DF crocidolite placed in 25, 50 or 100 micromolar ferrous chloride for 1 hour bound only 3.0 plus or minus 0.6, 4.4 plus or minus 0.9 or 5.5 plus or minus 0.8 nmol ferrous iron per mg, respectively.
  • Based on iron bound in the first 5 minutes, the authors calculated about 1.49 x 10^18 rapid-binding sites per square meter of native crocidolite surface.
  • Native and soaked crocidolite with 0, 25 or 57 nmol per mg of added iron caused the same amounts of DNA single-strand breaks.
  • For DF crocidolite, adding 5.5 nmol ferrous iron per mg more than doubled the DNA single-strand breaks compared with no added iron.
  • With EDTA and ascorbate present, DF crocidolite with 0, 3.0 or 5.5 nmol per mg of added iron broke 21, 42 or 51% of the DNA, according to the article's abstract. Table I gives 58 plus or minus 1% for the 5.5 nmol level.
  • Native crocidolite loaded with 57 nmol iron per mg released more iron to citrate or EDTA over 4 or 24 hours than untreated native fibers, and these increases were statistically significant. For example, with EDTA at 24 hours Table II gives 85 versus 57 nmol per mg.
  • Without a chelator, neither native nor iron-treated crocidolite released iron into solution over the same periods.
  • After incubation in iron-containing tissue culture medium, compared with iron-free medium, native crocidolite showed increases in DNA single-strand break activity of 5, 3 or 7%. These figures are for assays with ascorbate alone, ascorbate plus citrate, and ascorbate plus EDTA, respectively.
  • Under the same three assay conditions, DF crocidolite incubated in iron-containing medium showed increases of 6, 13 or 24%.
• The authors' conclusion: The authors concluded that fibers such as crocidolite may be capable of binding iron from sources inside cells. They suggested this added iron may be as reactive as the fiber's own iron and may increase the reactive lifetime of the fiber.
• Limitations the authors state:
  • The authors suggested that the lack of an effect of added iron on native crocidolite may reflect the large amount of iron already on the fibers combined with the sensitivity of the DNA break assay.
  • The authors noted that desferrioxamine-B may have bound to the fibers during the pretreatment.
  • The authors noted that iron in tissue culture medium can bind to fibers and may affect the results of experiments performed in culture medium.

Topics

Medical terms in this study, explained in our Lexicon: Crocidolite.

Related studies

• Iron mobilization from crocidolite asbestos greatly enhances crocidolite-dependent formation of DNA single-strand breaks in phi X174 RFI DNA — L.G. Lund, A.E. Aust (1992)
• Iron associated with asbestos bodies is responsible for the formation of single strand breaks in phi X174 RFI DNA — L.G. Lund, M.G. Williams, et al. (1994)
• Modulation of DNA single-strand breaks by intracellular glutathione in human lung cells exposed to asbestos fibers — A. Puhakka, T. Ollikainen, et al. (2002)
• Asbestos causes DNA strand breaks in cultured pulmonary epithelial cells: role of iron-catalyzed free radicals — D.W. Kamp, V.A. Israbian, et al. (1995)
• The role of catalytic iron in asbestos induced lipid peroxidation and DNA-strand breakage in C3H10T1/2 cells — C.J. Turver, R.C. Brown (1987)
• Iron-related toxicity of single-walled carbon nanotubes and crocidolite fibres in human mesothelial cells investigated by Synchrotron XRF microscopy — F. Cammisuli, S. Giordani, et al. (2018)

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