Promotion of lung adenocarcinoma following inhalation exposure to multi-walled carbon nanotubes

Our plain-English summary of Promotion of lung adenocarcinoma following inhalation exposure to multi-walled carbon nanotubes, by L.M. Sargent, D.W. Porter, L.M. Staska, et al (Particle and Fibre Toxicology, 2014).

The study

• Authors: L.M. Sargent, D.W. Porter, L.M. Staska, A.F. Hubbs, D.T. Lowry, L. Battelli, K.J. Siegrist, M.L. Kashon, R.R. Mercer, A.K. Bauer, B.T. Chen, J.L. Salisbury, D. Frazer, W. McKinney, M. Andrew, S. Tsuruoka, M. Endo, K.L. Fluharty, V. Castranova, S.H. Reynolds
• Journal: Particle and Fibre Toxicology
• Published: 2014;11:3
• Cited by: 185 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: Engineered carbon nanotubes are currently used in many consumer and industrial products such as paints, sunscreens, cosmetics, toiletries, electronic processes and industrial lubricants. Carbon nanotubes are among the more widely used nanoparticles and come in two major commercial forms, single-walled carbon nanotubes (SWCNT) and the more rigid, multi-walled carbon nanotubes (MWCNT). The low density and small size of these particles makes respiratory exposures likely. Many of the potential health hazards have not been investigated, including their potential for carcinogenicity. We, therefore, utilized a two stage initiation/promotion protocol to determine whether inhaled MWCNT act as a complete carcinogen and/or promote the growth of cells with existing DNA damage. Six week old, male, B6C3F1 mice received a single intraperitoneal (ip) injection of either the initiator methylcholanthrene(MCA, 10 μg/g BW, i.p.), or vehicle (corn oil). One week after i.p. injections, mice were exposed by inhalation to MWCNT (5 mg/m³, 5 hours/day, 5 days/week) or filtered air (controls) for a total of 15 days. At 17 months post-exposure, mice were euthanized and examined for lung tumor formation.

Results: Twenty-three percent of the filtered air controls, 26.5% of the MWCNT-exposed, and 51.9% of the MCA-exposed mice, had lung bronchiolo-alveolar adenomas and lung adenocarcinomas. The average number of tumors per mouse was 0.25, 0.81 and 0.38 respectively. By contrast, 90.5% of the mice which received MCA followed by MWCNT had bronchiolo-alveolar adenomas and adenocarcinomas with an average of 2.9 tumors per mouse 17 months after exposure. Indeed, 62% of the mice exposed to MCA followed by MWCNT had bronchiolo-alveolar adenocarcinomas compared to 13% of the mice that received filtered air, 22% of the MCA-exposed, or 14% of the MWCNT-exposed. Mice with early morbidity resulting in euthanasia had the highest rate of metastatic disease. Three mice exposed to both MCA and MWCNT that were euthanized early had lung adenocarcinoma with evidence of metastasis (5.5%). Five mice (9%) exposed to MCA and MWCNT and 1 (1.6%) exposed to MCA developed serosal tumors morphologically consistent with sarcomatous mesotheliomas, whereas mice administered MWCNT or air alone did not develop similar neoplasms.

Conclusions: These data demonstrate that some MWCNT exposures promote the growth and neoplastic progression of initiated lung cells in B6C3F1 mice. In this study, the mouse MWCNT lung burden of 31.2 μg/mouse approximates feasible human occupational exposures. Therefore, the results of this study indicate that caution should be used to limit human exposures to MWCNT.

In plain English

This was an animal inhalation experiment using a two-stage initiation-promotion protocol. Six-week-old male B6C3F1 mice (60 per group) received a single intraperitoneal injection of methylcholanthrene (MCA, 10 micrograms per gram body weight) or corn oil vehicle. One week later they were exposed by whole-body inhalation to Mitsui-7 multi-walled carbon nanotubes (MWCNT) at 5 mg/m3, 5 hours per day for 15 days, or to filtered air. The initial lung burden was 31.2 plus or minus 0.9 micrograms MWCNT per lung. Mice were examined 17 months after exposure; those euthanized earlier because of illness were analyzed separately. At terminal sacrifice, combined bronchiolo-alveolar adenoma and adenocarcinoma incidence was 23.2% (air), 51.9% (MCA), 26.5% (MWCNT) and 90.5% (MCA plus MWCNT), with means of 0.25, 0.81, 0.38 and 2.9 tumors per mouse. Adenocarcinoma incidences were 13%, 22%, 14% and 62%. Malignant serosal tumors morphologically consistent with sarcomatous mesothelioma occurred in five mice (9%) given MCA plus MWCNT and one (2%) given MCA. The authors concluded that inhaled MWCNT are strong promoters of pulmonary adenomas and adenocarcinomas in these mice.

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: animal experiment: two-stage initiation-promotion inhalation carcinogenicity study in mice
• Who was studied: Six-week-old male B6C3F1 mice, 60 per group, randomly assigned to four treatment groups. They were examined 17 months after exposure (terminal group sizes 56 air, 54 MCA, 49 MWCNT, 42 MCA plus MWCNT). Mice euthanized early for morbidity (4 air, 6 MCA, 6 MWCNT, 13 MCA plus MWCNT) were analyzed separately.
• Exposure: Whole-body inhalation of Mitsui-7 multi-walled carbon nanotubes at 5 mg/m3, 5 hours/day for 15 days, with or without prior intraperitoneal methylcholanthrene (10 micrograms per gram body weight). The initial lung burden was 31.2 plus or minus 0.9 micrograms per lung.
• Main results:
  • The initial MWCNT lung burden in MWCNT-exposed mice was 31.2 plus or minus 0.9 micrograms per lung
  • Percent of animals with lung tumors and tumors per mouse at terminal sacrifice: air 23.2% and 0.25; MCA 51.9% and 0.81; MWCNT 26.5% and 0.38; MCA plus MWCNT 90.5% and 2.9
  • Bronchiolo-alveolar adenocarcinoma incidence at terminal sacrifice: 62% in MCA plus MWCNT, 22% in MCA, 13% in air, 14% in MWCNT
  • Mice euthanized early because of morbidity were analyzed separately from those euthanized at 17 months
  • MWCNT alone produced hyperplasia and a moderate increase in adenomas that was not statistically significant, and no increase in adenocarcinomas
  • Focal adenomatous alveolar hyperplasia, macrophage infiltration and foreign material counts by group (air, MCA, MWCNT, MCA plus MWCNT)
  • Malignant serosal tumors morphologically consistent with sarcomatous mesothelioma were found in 5 mice (9%) given MCA plus MWCNT and 1 mouse (2%) given MCA. These tumors stained positive for podoplanin, and their cytokeratin staining was negative to equivocal.
  • Three mice (5.5%) exposed to MCA and MWCNT that were euthanized early had tumors with local invasion of lung tissue
  • Lung volume occupied by tumor was greatest in the MCA plus MWCNT group
  • No significant difference in survival between treatment groups (log-rank p = 0.1609)
• The authors' conclusion: The authors concluded that inhaled MWCNT acted as strong promoters of lung adenomas and adenocarcinomas in initiated B6C3F1 mice. They also concluded that the mouse lung burdens approximate feasible human occupational exposures, and that caution should be taken to limit human inhalation exposure.
• Limitations the authors state:
  • No suitable non-carcinogenic particle control exists for this model
  • The serosal tumors could not be definitively diagnosed as mesothelioma
  • The authors state extrapolation to human health is premature
  • Negative to equivocal cytokeratin staining must be interpreted cautiously because mouse mesothelioma marker techniques are less advanced than human ones
  • The role of nanotube dimensions and physical properties in carcinogenicity was not resolved and needs further study

Topics

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

Medical terms in this study, explained in our Lexicon: Lung Adenocarcinoma, Adenocarcinoma, Metastasis, Mesothelioma, Neoplasm, Occupational Exposure.

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