On the mechanism of the electrophysiological changes and membrane lesions induced by asbestos fiber exposure in Xenopus laevis oocytes

Our plain-English summary of On the mechanism of the electrophysiological changes and membrane lesions induced by asbestos fiber exposure in Xenopus laevis oocytes, by A. Bernareggi, G. Conte, A. Constanti, et al (Scientific Reports, 2019).

The study

• Authors: A. Bernareggi, G. Conte, A. Constanti, V. Borelli, F. Vita, G. Zabucchi
• Journal: Scientific Reports
• Published: 2019;9(1):2014
• Cited by: 8 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

The so-called amphibole asbestos fibers are enriched with mineral iron ions, able to stimulate ROS production. We recently reported that crocidolite asbestos was able to interact with the cell membranes of Xenopus laevis oocytes, to alter their electrical membrane properties. Here, we found that applied iron ions (Fe3+) or H2O2 (for ROS generation) mimicked these effects, suggesting that at least one effect of iron-containing asbestos fiber exposure was mediated by ROS production. Furthermore, combined Fe3+ and H2O2 acted synergistically, producing a membrane effect stronger than that induced by these factors alone. Similar to crocidolite, these changes peaked within 30 minutes of incubation and vanished almost completely after 120 min. However, in the presence of cytochalasin D, which inhibits membrane actin repair mechanisms, crocidolite or applied Fe3+/H2O2 invariably produced oocyte cell death. While the electrophysiological modifications induced by crocidolite suggested a modification of an intrinsic chloride ion channel, the morphological appearance of the treated oocytes also indicated the formation of membrane "pores"; the effects of asbestos exposure may therefore consist of multiple (not necessarily exclusive) underlying mechanisms. In conclusion, using Xenopus oocytes allowed us for the first time, to focus on a specific membrane effect of crocidolite asbestos exposure, which deserves to be tested also on human lung cell lines. Much available evidence suggests that asbestos fibers damage cells through the production of ROS. Our present data confirm that crocidolite fibers can indeed trigger ROS-mediated damaging effects in the oocyte cell membrane, provided iron ions and H2O2 are available for ROS production.

In plain English

This laboratory study used egg cells (oocytes) from African clawed frogs (Xenopus laevis) to examine how crocidolite asbestos alters cell membranes. It did not study people. The authors exposed oocytes to standard crocidolite (15 micrograms per millilitre) and compared its effects with those of added hydrogen peroxide and iron. About 63% of crocidolite-treated cells responded with lower resting membrane potential and membrane resistance. The changes were consistent in the first 30 minutes and tended to reverse after more than 120 minutes, although the resting potential stayed slightly depolarized. In samples of five oocytes over 30 minutes, crocidolite increased the hydrogen peroxide released into the medium (0.43 versus 0.17 nanomoles without crocidolite). Catalase, an enzyme that breaks down hydrogen peroxide, counteracted the crocidolite effect. Hydrogen peroxide and ferric iron each mimicked crocidolite, and together their effect exceeded the sum of each alone. With the cells' actin repair system blocked by cytochalasin D, crocidolite produced pore-like membrane lesions, and co-treatment beyond 120 minutes killed 100% of cells. The authors concluded that crocidolite can trigger damage driven by reactive oxygen species, which deserves testing in human lung cell lines.

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 experiment on isolated frog egg cells (oocytes), using electrical recordings, chemical assays and scanning electron microscopy
• Who was studied: Stage VI oocytes taken from the ovaries of adult female Xenopus laevis frogs, studied at the University of Trieste, Italy. The number of frogs is not stated, and group sizes ranged from 3 to 41 oocytes per condition.
• Exposure: UICC standard South African crocidolite at 15 micrograms per millilitre. Comparison treatments were ferric iron (FeCl3, 400 micromolar), ferrous iron (FeSO4, 1 millimolar), hydrogen peroxide (50 micromolar to 1 millimolar), catalase (250 U/ml) and cytochalasin D (5 micromolar).
• Main results:
  • About 63% of crocidolite-exposed oocytes responded, with lowered resting membrane potential and membrane resistance.
  • The crocidolite effect was consistent during the first 30 minutes and tended to reverse after more than 120 minutes. The one exception was the resting potential, which remained slightly depolarized.
  • Each sample of five oocytes in 1 ml was measured over 30 minutes. Hydrogen peroxide released into the medium was 0.17 (SD 0.11) nanomoles for untreated samples and 0.43 (SD 0.14) nanomoles for crocidolite-exposed samples (n = 7 each). Total production was 3.17 versus 2.91 nanomoles.
  • Adding catalase to crocidolite-treated cells made the resting potential more negative: -26.2 mV with crocidolite alone versus -40.71 mV with crocidolite plus catalase.
  • Added hydrogen peroxide (1 mM) mimicked the crocidolite effect, and catalase abolished this effect.
  • Ferrous iron (Fe2+) up to 1 mM did not change membrane properties. Ferric iron (Fe3+, 400 micromolar) produced crocidolite-like changes that fully recovered after 120 minutes.
  • Combined ferric iron and hydrogen peroxide produced an effect greater than the sum of each alone.
  • With cytochalasin D present, crocidolite produced many pore-like membrane lesions with a mean diameter of 5.0 (SD 1.4). No asbestos fibers were seen associated with the lesions.
  • Oocytes treated with crocidolite plus cytochalasin D died as follows: 32% with short co-treatment, 20% when cytochalasin D was removed after short co-treatment, and 100% when co-treatment lasted more than 120 minutes.
  • Ferric iron plus hydrogen peroxide with cytochalasin D killed about 70% of cells after 2 hours at 100 micromolar hydrogen peroxide, and 25% at 50 micromolar.
  • Manganese, which blocks a calcium-activated chloride channel, reduced crocidolite-induced currents in 4 of 5 treated cells.
• The authors' conclusion: The authors concluded that crocidolite fibers can trigger membrane damage mediated by reactive oxygen species while iron and hydrogen peroxide are available, that the cytoskeleton can repair this damage in this model, and that the effect deserves testing in human lung cell lines.
• Limitations the authors state:
  • The authors state that it is unclear how these frog oocyte changes relate to asbestos toxicity in mammalian cells.
  • The authors could not exclude that ion channels other than the calcium-activated chloride channel were involved.
  • The authors could not exclude that the cytochalasin D effect also reflected more fibers entering through the pores.
  • The authors state that whether the same reaction damages genetic material or starts cancerous change was not studied.

Topics

Medical terms in this study, explained in our Lexicon: Amphibole Asbestos, Crocidolite, Asbestos Exposure.

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