Free radical activity of synthetic vitreous fibers: iron chelation inhibits hydroxyl radical generation by refractory ceramic fiber

Our plain-English summary of Free radical activity of synthetic vitreous fibers: iron chelation inhibits hydroxyl radical generation by refractory ceramic fiber, by D.M. Brown, C. Fisher, K. Donaldson (Journal of Toxicology and Environmental Health. Part a, 1998).

The study

• Authors: D.M. Brown, C. Fisher, K. Donaldson
• Journal: Journal of Toxicology and Environmental Health. Part a
• Published: 1998;53(7):545-61
• Cited by: 17 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Synthetic vitreous fibers are in widespread use but the parameters that dictate their carcinogenicity are still a matter of scientific debate. The free radical activities of a panel comprising an asbestos sample and five different respirable synthetic vitreous fiber samples were determined, to address the hypothesis that carcinogenic fibers have greater free radical activity than noncarcinogenic fibers. On the basis of recent inhalation studies, the six samples were divided into three carcinogenic fibers-amphibole asbestos, silicon carbide, and refractory ceramic fiber 1 (designated with the abbreviation RCF 1)-and three noncarcinogenic fibers--man-made vitreous fiber 10 (a glass fiber sample designated with the abbreviation MMVF 10), Code 100/475 glass fiber, and RCF4. All experiments were carried out with equal fiber numbers. Of the two assays of free radical activity used, the plasmid assay of DNA scission showed only amosite asbestos to have free radical activity, while the salicylate assay of hydroxyl activity showed that both amosite asbestos and RCF1 release hydroxyl radicals; silicon carbide fibers had no free radical activity in either of the assays. None of the noncarcinogenic fibers demonstrated free radical activity in either of the assays. The differences in the two assays in demonstrating free radical activity with RCF1 may be due to increased release of Fe from RCF1 under the more acid conditions of the salicylate assay, which was confirmed by the fact that soluble iron caused hydroxylation of salicylate. Presence of an iron chelator inhibited the ability of the RCF1 fibers to cause hydroxylation of salicylate, demonstrating that RCF1 generates hydroxyl radical by Fenton chemical reaction in the same way as amphibole asbestos.

In plain English

This was a test-tube study of fibers, not of people or disease. The authors tested six fiber samples for free radical activity at equal fiber numbers. Earlier animal inhalation studies had classed long-fiber amosite asbestos, silicon carbide and refractory ceramic fiber RCF1 as carcinogenic. RCF4, Code 100/475 glass fiber and MMVF10 had been classed as noncarcinogenic. The authors cautioned that this did not show RCF4's composition was noncarcinogenic, as RCF4 was shorter than RCF1. In a DNA-damage test, only amosite had a significant effect: 55% depletion of supercoiled plasmid DNA. The other fibers ranged from 5 to 20%, not significantly different from control. In a hydroxyl radical test, a product was detected only with amosite and RCF1. Coating these fibers with rat lung lining fluid, or adding mannitol or the iron-binding chelator desferal, significantly reduced it. RCF1 released more iron under that test's more acidic conditions. The test was positive for two of three carcinogenic fibers and none of the noncarcinogenic ones. The authors concluded that a negative result cannot show a fiber is noncarcinogenic, because silicon carbide was negative.

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 (cell-free, in vitro) experiment. Free radical activity of fibers was measured with a plasmid DNA scission assay and with a salicylate hydroxylation assay read by HPLC.
• Who was studied: Six fiber samples. Three were classed as carcinogenic from earlier animal studies: long-fiber amosite, silicon carbide and RCF1. Three were classed as noncarcinogenic: RCF4, Code 100/475 glass fiber and MMVF10. Lung lining fluid from female Wistar rats was used to coat fibers in some tests. The work was done at Napier University, Edinburgh.
• Exposure: Fibers were tested at equal fiber numbers: 8.24 × 10^7 fibers per ml, and 924,900 fibers per 20 µl in the plasmid assay. RCF4 was a heated or after-service sample of RCF1.
• Main results:
  • In the plasmid DNA assay, only long-fiber amosite had a significant effect: 55% depletion of supercoiled DNA.
  • The other five fibers showed 5 to 20% activity in the plasmid assay. These values were not significantly different from control.
  • With amosite in the plasmid assay, adding mannitol brought DNA depletion to a level not significantly different from control. Coating the fibers with rat surfactant did the same.
  • In the salicylate assay, the hydroxyl radical product (2,3-DHBA) was detected only with long-fiber amosite and RCF1.
  • Mean 2,3-DHBA production was 0.45 (SEM 0.05) pmol per 100 µl for long-fiber amosite and 1.0 (SEM 0.05) for RCF1, from 3 separate experiments.
  • For both amosite and RCF1, mannitol, surfactant coating and the iron chelator desferal each significantly reduced 2,3-DHBA production.
  • At 20 hours, RCF1 released 76.84 nmol of iron (Fe3+) under salicylate assay conditions and 52.52 nmol under plasmid assay conditions. Both figures are per 8.24 × 10^7 fibers. The authors called this an approximate 50% increase.
  • Added soluble iron (Fe2+ and Fe3+) hydroxylated salicylate. Fe2+ produced almost twice as much product.
  • Silicon carbide showed no free radical activity in either assay.
• The authors' conclusion: The authors concluded that, for this small panel, hydroxyl radical generation was not a 100% reliable way to separate carcinogenic from noncarcinogenic fibers. The salicylate assay was positive for amosite and RCF1, and in both cases iron was involved. They stated that detecting such activity supports a fiber being carcinogenic. They also stated that failing to detect it cannot be taken as evidence of noncarcinogenicity, because silicon carbide was negative.
• Limitations the authors state:
  • The noncarcinogenic label for RCF4 came from inhalation studies. The authors stated that those results do not show the RCF4 composition is noncarcinogenic.
  • RCF4 was shorter and thicker than RCF1. The authors said this could explain its lack of carcinogenicity in the inhalation studies, apart from its composition.
  • RCF4, although classed as noncarcinogenic, caused fibrosis in the highest exposure group.
  • Fibers were matched by fiber number, not surface area. The data needed to equalize by surface area were not available.
  • The negative silicon carbide result may mean the assays were not sensitive enough to detect its free radical generation.
  • Only a small panel of six fibers was tested. The authors said a greater range of fibers should be tested.
  • Producing 2,3-DHBA from soluble iron needed iron concentrations about 100-fold higher than those found after incubating the fibers.

Topics

Medical terms in this study, explained in our Lexicon: Vitreous, Hydroxyl Radical, Carcinogenic, Amphibole Asbestos, Amosite.

Related studies

• Free radical activity of industrial fibers: role of iron in oxidative stress and activation of transcription factors — P.S. Gilmour, D.M. Brown, et al. (1997)
• Supercoiled plasmid DNA as a model target for assessing the generation of free radicals at the surface of fibres — K. Donaldson, P.S. Gilmour, et al. (1995)
• Detection of surface free radical activity of respirable industrial fibres using supercoiled phi X174 RF1 plasmid DNA — P.S. Gilmour, P.H. Beswick, et al. (1995)
• Perspectives on refractory ceramic fiber (RCF) carcinogenicity: comparisons with other fibers — H. Greim, M.J. Utell, et al. (2014)
• Free radical generation in the toxicity of inhaled mineral particles: the role of iron speciation at the surface of asbestos and silica — I. Fenoglio, L. Prandi, et al. (2001)
• A study of the synergistic interaction of asbestos fibers with cigarette tar extracts for the generation of hydroxyl radicals in aqueous buffer solution — A. Valavanidis, H. Balomenou, et al. (1996)

Browse more research in Journal of Toxicology and Environmental Health. Part a, or return to medical research.