Aspect ratio has no effect on genotoxicity of multi-wall carbon nanotubes

Our plain-English summary of Aspect ratio has no effect on genotoxicity of multi-wall carbon nanotubes, by J.S. Kim, K. Lee, Y.H. Lee, et al (Archives of Toxicology, 2011).

The study

• Authors: J.S. Kim, K. Lee, Y.H. Lee, H.S. Cho, K.H. Kim, K.H. Choi, S.H. Lee, K.S. Song, C.S. Kang, I.J. Yu
• Journal: Archives of Toxicology
• Published: 2011;85(7):775-86
• Cited by: 45 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Carbon nanotubes (CNTs) have specific physico-chemical and electrical properties that are useful for telecommunications, medicine, materials, manufacturing processes and the environmental and energy sectors. Yet, despite their many advantages, it is also important to determine whether CNTs may represent a hazard to the environment and human health. Like asbestos, the aspect ratio (length:diameter) and metal components of CNTs are known to have an effect on the toxicity of carbon nanotubes. Thus, to evaluate the toxic potential of CNTs in relation to their aspect ratio and metal contamination, in vivo and in vitro genotoxicity tests were conducted using high-aspect-ratio (diameter: 10-15 nm, length: ~10 μm) and low-aspect-ratio multi-wall carbon nanotubes (MWCNTs, diameter: 10-15 nm, length: ~150 nm) according to OECD test guidelines 471 (bacterial reverse mutation test), 473 (in vitro chromosome aberration test), and 474 (in vivo micronuclei test) with a good laboratory practice system. To determine the treatment concentration for all the tests, a solubility and dispersive test was performed, and a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) solution found to be more suitable than distilled water. Neither the high- nor the low-aspect-ratio MWCNTs induced any genotoxicity in a bacterial reverse mutation test (~1,000 μg/plate), in vitro chromosome aberration test (without S9: ~6.25 μg/ml, with S9: ~50 μg/ml), or in vivo micronuclei test (~50 mg/kg). However, the high-aspect-ratio MWCNTs were found to be more toxic than the low-aspect-ratio MWCNTs. Thus, while high-aspect-ratio MWCNTs do not induce direct genotoxicity or metabolic activation-mediated genotoxicity, genotoxicity could still be induced indirectly through oxidative stress or inflammation.

In plain English

In this Korean study, the authors tested whether multi-wall carbon nanotubes (MWCNTs), which they liken to asbestos, damage genetic material and whether length matters. They compared commercial MWCNTs about 10 micrometers long with a shortened version about 150 nanometers long. They used bacterial mutation tests, chromosome tests in Chinese hamster ovary cells, and a bone marrow micronucleus test in male mice injected abdominally with 12.5, 25 or 50 mg/kg. Neither type caused a statistically significant increase in mutations, chromosome aberrations or micronuclei compared with controls. Long nanotubes were more toxic to cells: the dose halving cell growth was 12.94 to 41.90 micrograms per ml, versus 40.48 to 93.19 for short ones. In mice, nanotubes stayed in the abdominal cavity. The authors concluded that these three tests found no genotoxicity, though long nanotubes might still cause it indirectly through oxidative stress or inflammation, and that the nanotubes harmed cell growth and viability. They noted the long nanotubes clumped at high doses and attributed the negative mouse result to nanotubes not reaching bone marrow. Mesothelioma and asbestos were not tested.

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: a battery of genetic-toxicity tests following OECD test guidelines 471 (bacterial reverse mutation), 473 (in vitro chromosome aberration) and 474 (in vivo mouse micronucleus), under good laboratory practice
• Who was studied: Four Salmonella typhimurium strains and one Escherichia coli strain; Chinese hamster ovary cells (CHO-k1); seven-week-old male ICR mice from OrientBio, Korea, with 6 animals per dose group (Table 5)
• Exposure: Commercial multi-wall carbon nanotubes (Hanwha Nanotech, CM-95; 95% carbon, about 5% iron). These were the high-aspect-ratio nanotubes, about 10 micrometers long and 10 to 15 nm in diameter. A low-aspect-ratio version, about 150 nm long, was made by strong-acid oxidation. Doses were up to 1,000 micrograms per plate in the bacterial test and 3.125 to 200 micrograms per ml in the cell cytotoxicity test. Mice received 12.5, 25 or 50 mg/kg by intraperitoneal injection.
• Main results:
  • Bacterial reverse mutation test: neither nanotube type significantly increased revertant colonies compared with the negative control, in any strain, with or without metabolic activation.
  • Chromosome aberration test in CHO-k1 cells: neither the high- nor the low-aspect-ratio nanotubes produced a statistically significant increase in cells with chromosome aberrations compared with the negative control, at any dose tested.
  • Cytotoxicity in CHO-k1 cells: the dose causing 50% growth inhibition (GI50) was lower, meaning the material was more toxic, for the high-aspect-ratio nanotubes. Values were 12.94 vs 60.20 micrograms per ml (24 h without S9), 12.94 vs 40.48 micrograms per ml (6 h without S9), and 41.90 vs 93.19 micrograms per ml (6 h with S9), high vs low aspect ratio.
  • Iron oxide, tested up to 12 micrograms per ml as a check on the iron impurity, did not reduce CHO-k1 cell growth.
  • Mouse micronucleus test: micronucleated cells per 2,000 polychromatic erythrocytes at 12.5, 25 and 50 mg/kg were 2.6, 3.0 and 2.2 (high-aspect-ratio; vehicle control 1.6) and 1.0, 2.6 and 4.0 (low-aspect-ratio; vehicle control 2.4). There was no significant dose-related increase.
  • In the mice, both nanotube types were found in the abdominal cavity but had not entered the organs or bloodstream.
• The authors' conclusion: The authors concluded that no genotoxicity was exhibited by the commercial MWCNTs in the bacterial reverse mutation, in vitro chromosome aberration and in vivo micronucleus tests, but that the nanotubes had an adverse effect on mammalian cell proliferation and viability. They left open that genotoxicity from high-aspect-ratio MWCNTs could still be induced indirectly, through oxidative stress or inflammation.
• Limitations the authors state:
  • The high-aspect-ratio nanotubes aggregated severely at 100 micrograms per ml or above in the culture medium and showed low or no cytotoxicity at those high doses.
  • The authors attributed the absence of effects in mouse bone marrow to the nanotubes not translocating there.
  • Only less than 0.5% MWCNTs could be wetted in the dispersion agent, which limited the treatment concentrations.

Topics

Medical terms in this study, explained in our Lexicon: Aspect Ratio (Length-to-diameter ratio), Genotoxicity, Micronuclei, Oxidative Stress, Inflammation.

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