Mitochondria-derived reactive intermediate species mediate asbestos-induced genotoxicity and oxidative stress-responsive signaling pathways

Our plain-English summary of Mitochondria-derived reactive intermediate species mediate asbestos-induced genotoxicity and oxidative stress-responsive signaling pathways, by S.X. Huang, M.A. Partridge, S.A. Ghandhi, et al (Environmental Health Perspectives, 2012).

The study

• Authors: S.X. Huang, M.A. Partridge, S.A. Ghandhi, M.M. Davidson, S.A. Amundson, T.K. Hei
• Journal: Environmental Health Perspectives
• Published: 2012;120(6):840-7
• Cited by: 27 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: The incidence of asbestos-induced human cancers is increasing worldwide, and considerable evidence suggests that reactive oxygen species (ROS) are important mediators of these diseases. Our previous studies suggested that mitochondria might be involved in the initiation of oxidative stress in asbestos-exposed mammalian cells.

Objective: We investigated whether mitochondria are a potential cytoplasmic target of asbestos using a mitochondrial DNA-depleted (ρ(0)) human small airway epithelial (SAE) cell model: ρ(0) SAE cells lack the capacity to produce mitochondrial ROS.

Methods: We examined nuclear DNA damage, micronuclei (MN), intracellular ROS production, and the expression of inflammation-related nuclear genes in both parental and ρ(0) SAE cells in response to asbestos treatment.

Results: Asbestos induced a dose-dependent increase in nuclear DNA oxidative damage and MN in SAE cells. Furthermore, there was a significant increase in intracellular oxidant production and activation of genes involved in nuclear factor κB and proinflammatory signaling pathways in SAE cells. In contrast, the effects of asbestos were minimal in ρ(0) SAE cells.

Conclusions: Mitochondria are a major cytoplasmic target of asbestos. Asbestos may initiate mitochondria-associated ROS, which mediate asbestos-induced nuclear mutagenic events and inflammatory signaling pathways in exposed cells. These data provide new insights into the molecular mechanisms of asbestos-induced genotoxicity.

In plain English

In this laboratory cell-culture study, human telomerase-immortalized small airway epithelial (SAE) cells and mitochondrial DNA-depleted (ρ0) SAE cells, generated by two months of ethidium bromide treatment, were exposed to UICC chrysotile or crocidolite at 0.5, 1, 2 and/or 4 µg/cm2 for 12, 24 or 48 hr. Baseline 8-OHdG fluorescence was higher in untreated ρ0 cells (20.96 ± 7.72) than in untreated parental cells (7.45 ± 2.05), but asbestos-induced 8-OHdG was much lower in ρ0 cells (p < 0.01). In parental cells the background-subtracted share of micronucleus-positive binucleated cells (background 4.9 ± 1.14%) rose with chrysotile from 6.7 ± 1.9% at 0.5 µg/cm2 to 17.6 ± 4.3% at 4 µg/cm2, and with crocidolite from 1.7 ± 0.5% to 5 ± 1.2%. In parental cells 0.5 µg/cm2 chrysotile for 24 hr tripled intracellular oxidant (CM-DCF) fluorescence, which DMSO prevented. Of 178 immune- and inflammation-related genes, 26 at 24 hr and 32 at 48 hr were altered in parental cells, linked by pathway analysis to NF-κB and AP-1 signaling, with little effect in ρ0 cells. The authors concluded mitochondria are a major cytoplasmic target of asbestos.

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: in vitro laboratory experiment comparing parental and mitochondrial DNA-depleted human small airway epithelial cells exposed to asbestos
• Who was studied: Cultured human telomerase reverse transcriptase-immortalized small airway epithelial (SAE) cells and a mitochondrial DNA-depleted (ρ0) SAE cell line derived from them by ethidium bromide treatment; laboratory study, Columbia University, published 2012
• Exposure: UICC standard reference chrysotile and crocidolite asbestos at 0.5, 1, 2 and/or 4 µg/cm2 for 12, 24 or 48 hr; hydrogen peroxide as positive control and DMSO as radical scavenger
• Main results:
  • Untreated ρ0 SAE cells had a higher baseline 8-OHdG fluorescence level (20.96 ± 7.72) than untreated parental SAE cells (7.45 ± 2.05), but asbestos-induced 8-OHdG was much lower in ρ0 cells than in parental cells (p < 0.01)
  • 8-OHdG was measured as mean fluorescence intensity per cell from about 200 cells per sample after 48 hr of treatment
  • In parental SAE cells, the frequency of micronucleus-positive cells rose dose-dependently with chrysotile from 6.7 ± 1.9% at 0.5 µg/cm2 to 17.6 ± 4.3% at 4 µg/cm2 (r = 0.89), and with crocidolite from 1.7 ± 0.5% to 5 ± 1.2% (r = 0.9)
  • The plotted micronucleus yields were background-subtracted percentages of micronucleus-positive binucleated cells
  • ρ0 SAE cells had higher background micronuclei (9.1 ± 2.34%) than parental SAE cells (4.9 ± 1.14%)
  • In ρ0 SAE cells, chrysotile produced a slight dose-dependent increase in micronuclei (1.2 ± 0.4% to 7.6 ± 0.2%) and crocidolite produced none at any dose
  • ρ0 SAE cells were significantly less sensitive than parental cells to asbestos-induced micronucleus formation at every dose, and chrysotile was more potent than crocidolite in both cell lines
  • In parental SAE cells treated with 0.5 µg/cm2 chrysotile for 24 hr, CM-DCF fluorescence tripled over background (p < 0.05) and DMSO pre- and co-treatment prevented this
  • In ρ0 SAE cells the baseline oxidant level was higher but asbestos induced little or no increase in intracellular oxidants
  • The CM-DCF experiment used 0.5 µg/cm2 chrysotile for 24 hr in parental and ρ0 SAE cells
  • 178 immune- and inflammation-related genes were assayed in total
  • In parental SAE cells, 26 genes at 24 hr and 32 genes at 48 hr were significantly altered, mostly up-regulated
  • In ρ0 SAE cells, six genes were up-regulated at 48 hr with fold changes all below 2 except MC2R (5.66) and CCR4 (3.24)
  • Pathway analysis of the 32 asbestos-responsive genes in parental SAE cells linked them to NF-κB and AP-1 activation and downstream proinflammatory signaling
• The authors' conclusion: The authors concluded that mitochondria are a major cytoplasmic target of asbestos and that mitochondria-associated oxidants mediate asbestos-associated nuclear damage and initiate redox-responsive signaling cascades in target cells
• Limitations the authors state:
  • The authors state the extended ethidium bromide treatment needed to make the ρ0 cells is a potential drawback of the model, and that micronuclei may have been increased by that treatment
  • The authors note chrysotile was more DNA damaging than crocidolite in their system and suggest the fibers' properties may have been modified during processing or vary between batches
  • The authors state the source of the raised background oxidants in ρ0 cells is unidentified and remains to be investigated

Topics

Medical terms in this study, explained in our Lexicon: Genotoxicity, Oxidative Stress, Reactive Oxygen Species (ROS), Micronuclei, Inflammation.

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