Characteristics and modifying factors of asbestos-induced oxidative DNA damage

Our plain-English summary of Characteristics and modifying factors of asbestos-induced oxidative DNA damage, by L. Jiang, H. Nagai, H. Ohara, et al (Cancer Science, 2008).

The study

• Authors: L. Jiang, H. Nagai, H. Ohara, S. Hara, M. Tachibana, S. Hirano, Y. Shinohara, N. Kohyama, S. Akatsuka, S. Toyokuni
• Journal: Cancer Science
• Published: 2008;99(11):2142-51
• Cited by: 61 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Respiratory exposure to asbestos has been linked with mesothelioma in humans. However, its carcinogenic mechanism is still unclear. Here we studied the ability of chrysotile, crocidolite and amosite fibers to induce oxidative DNA damage and the modifying factors using four distinct approaches. Electron spin resonance analyses revealed that crocidolite and amosite containing high amounts of iron, but not chrysotile, catalyzed hydroxyl radical generation in the presence of H(2)O(2), which was enhanced by an iron chelator, nitrilotriacetic acid, and suppressed by desferal. Natural iron chelators, such as citrate, adenosine 5'-triphosphate and guanosine 5'-triphosphate, did not inhibit this reaction. Second, we used time-lapse video microscopy to evaluate how cells cope with asbestos fibers. RAW264.7 cells, MeT-5 A and HeLa cells engulfed asbestos fibers, which reached not only cytoplasm but also the nucleus. Third, we utilized supercoiled plasmid DNA to evaluate the ability of each asbestos to induce DNA double strand breaks (DSB). Crocidolite and amosite, but not chrysotile, induced DNA DSB in the presence of iron chelators. We cloned the fragments to identify break sites. DSB occurred preferentially within repeat sequences and between two G:C sequences. Finally, i.p. administration of each asbestos to rats induced not only formation of nuclear 8-hydroxy-2'-deoxyguanosine in the mesothelia, spleen, liver and kidney but also significant iron deposits in the spleen. Together with the established carcinogenicity of i.p. chrysotile, our data suggest that asbestos-associated catalytic iron, whether constitutional or induced by other mechanisms, plays an important role in asbestos-induced carcinogenesis and that chemoprevention may be possible through targeting the catalytic iron.

In plain English

This laboratory and animal study from Japan examined how three standard asbestos types (chrysotile, crocidolite and amosite) might cause oxidative DNA damage, and how iron-binding chemicals ('iron chelators') change that damage. In test-tube experiments with hydrogen peroxide, amosite was the strongest generator of hydroxyl radicals (a reactive form of oxygen) and crocidolite was also strong. Chrysotile was quite weak. The chelator NTA increased radical generation, while DTPA and desferal suppressed it. Several cell types, including mesothelial cells, took up crocidolite fibers, and some fibers reached the cell nucleus. With certain chelators present, crocidolite and amosite broke both DNA strands in a test plasmid, but chrysotile did not. The breaks fell mostly within repeat sequences (74.3% and 78.1% of breaks, against 47.1% of plasmid sites). In rats given a single abdominal injection, all three fiber types were followed by iron deposits in the spleen and liver. They were also followed by higher levels of a DNA-damage marker (8-OHdG) in the kidney, spleen, liver and mesothelium. The authors concluded that iron may be one of the major candidate mechanisms for asbestos-caused cancer. They noted that abdominal injection is a different situation from human exposure.

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 experiments (chemical tests, cultured cell lines, plasmid DNA) plus an animal experiment in rats
• Who was studied: 21 eight-week-old male Wistar rats in seven groups of three, each given a single intraperitoneal (abdominal) injection of 10 mg asbestos except untreated controls, and killed 1 or 4 weeks later. The study also used the cell lines RAW264.7 (macrophage), MeT-5A (non-transformed mesothelial), FRCC562 (rat kidney cancer) and HeLa.
• Exposure: UICC standard asbestos samples (chrysotile A, crocidolite and amosite), with and without hydrogen peroxide and various iron chelators. Silica was used for comparison in some tests.
• Main results:
  • In test-tube measurements with hydrogen peroxide, amosite was the strongest catalyst of hydroxyl radical generation, crocidolite was also strong, and chrysotile was quite weak.
  • The iron chelator NTA increased radical generation for all three asbestos types. DTPA and desferal suppressed it for all three.
  • Asbestos was toxic to all four cell lines in a dose-dependent way, and amosite was the most cytotoxic.
  • In RAW264.7 cells, chrysotile did not catalyze hydroxyl radical generation. Signals with crocidolite or amosite increased with dose.
  • Macrophage, mesothelial, FRCC562 and HeLa cells took up crocidolite fibers, which reached the cytoplasm and eventually the nuclei.
  • With certain iron chelators present, crocidolite and amosite produced double-strand breaks (linear form III DNA) in plasmid DNA. Chrysotile did not produce significant form III.
  • Double-strand breaks fell within repeat sequences in 74.3% of cases for crocidolite (P less than 0.005) and 78.1% for amosite (P less than 0.001). Repeat sequences make up 47.1% of sites in the plasmid.
  • Breaks between two G:C base pairs made up 34.3% of crocidolite breaks and 43.8% of amosite breaks.
  • The predicted share of G:C-to-G:C sites in the plasmid was 28.4%. The comparison was reported as P less than 0.05.
  • In rats, all asbestos types caused iron deposition in the spleen and liver. Deposition was especially high in the spleen 4 weeks after chrysotile.
  • Four weeks after a single injection of any of the three fiber types, rat kidney tubules showed higher levels of the DNA-damage marker 8-OHdG, most intensely with chrysotile. This was judged by immunostaining; no numbers were given.
• The authors' conclusion: The authors concluded that iron may be one of the major candidate mechanisms for asbestos-caused cancer, whether it is part of the fiber or results from asbestos-induced changes in the body. They suggested that some iron chelators might work for prevention.
• Limitations the authors state:
  • Injecting asbestos into the abdomen of rats differs from human exposure.
  • The study does not explain why mesothelial cells are the primary targets of asbestos-induced cancer.

Topics

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

Medical terms in this study, explained in our Lexicon: Mesothelioma, Carcinogenic, Chrysotile, Crocidolite, Amosite, Hydroxyl Radical, Carcinogenesis.

Related studies

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• Asbestos surface provides a niche for oxidative modification — H. Nagai, T. Ishihara, et al. (2011)
• Forsterite exposure causes less oxidative DNA damage and lung injury than chrysotile exposure in rats — A. Takata, H. Yamauchi, et al. (2009)
• Detection of 8-oxo-2'-deoxyguanosine, a marker of oxidative DNA damage, in culture medium from human mesothelial cells exposed to crocidolite asbestos — Q. Chen, J. Marsh, et al. (1996)
• Tobacco smoke: involvement of reactive oxygen species and stable free radicals in mechanisms of oxidative damage, carcinogenesis and synergistic effects with other respirable particles — A. Valavanidis, T. Vlachogianni, et al. (2009)
• Iron overload signature in chrysotile-induced malignant mesothelioma — L. Jiang, S. Akatsuka, et al. (2012)

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