Enhanced expression of nicotinamide nucleotide transhydrogenase (NNT) and its role in a human T cell line continuously exposed to asbestos

Our plain-English summary of Enhanced expression of nicotinamide nucleotide transhydrogenase (NNT) and its role in a human T cell line continuously exposed to asbestos, by S. Yamamoto, S. Lee, H. Matsuzaki, et al (Environment International, 2020).

The study

• Authors: S. Yamamoto, S. Lee, H. Matsuzaki, N. Kumagai-Takei, K. Yoshitome, N. Sada, Y. Shimizu, T. Ito, Y. Nishimura, T. Otsuki
• Journal: Environment International
• Published: 2020;138:105654
• Cited by: 8 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The effects of asbestos fibers on human immune cells have not been well documented. We have developed a continuously exposed cell line model using the human T-lymphotropic virus 1 (HTLV-1)-immortalized human T cell line MT-2. Sublines continuously exposed to chrysotile (CH) or crocidolite (CR) showed acquired resistance to asbestos-induced apoptosis following transient and high-dose re-exposure with fibers. These sublines in addition to other immune cells such as natural killer cells or cytotoxic T lymphocytes exposed to asbestos showed a reduction in anti-tumor immunity. In this study, the expression of genes and molecules related to antioxidative stress was examined. Furthermore, complexes related to oxidative phosphorylation were investigated since the production of reactive oxygen species (ROS) is important when considering the effects of asbestos in carcinogenesis and the mechanisms involved in resistance to asbestos-induced apoptosis. In sublines continuously exposed to CH or CR, the expression of thioredoxin decreased. Interestingly, nicotinamide nucleotide transhydrogenase (NNT) expression was markedly enhanced. Thus, knockdown of NNT was then performed. Although the knockdown clones did not show any changes in proliferation or occurrence of apoptosis, these clones showed recovery of ROS production with returning NADPH/NADP+ ratio that increased with decreased production of ROS in continuously exposed sublines. These results indicated that NNT is a key factor in preventing ROS-induced cytotoxicity in T cells continuously exposed to asbestos. Considering that these sublines showed a reduction in anti-tumor immunity, modification of NNT may contribute to recovery of the anti-tumor effects in asbestos-exposed T cells.

Copyright © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.

In plain English

This was a laboratory study of cells, not people. The authors used a human T cell line, MT-2, and two sublines earlier grown under continuous low-dose exposure to chrysotile (CB1) or crocidolite (CR1). They compared antioxidant molecules, the energy-producing complexes in mitochondria, and an enzyme called nicotinamide nucleotide transhydrogenase (NNT). Antioxidant proteins and genes did not differ statistically between the cells. Thioredoxin gene expression seemed lower in the sublines, but this was not statistically significant. NNT was markedly higher in both sublines than in the original cells. The authors then reduced NNT in CB1 cells. This did not change the cells' growth or their resistance to cell death caused by chrysotile. However, production of reactive oxygen species (ROS, damaging forms of oxygen) tended to rise with chrysotile dose, as in the original cells. A chemical NNT blocker also increased ROS in both sublines. The authors concluded that NNT is a key factor in preventing ROS-induced cell damage in T cells continuously exposed to asbestos. They added that NNT is not directly related to acquiring resistance to asbestos-induced apoptosis (cell death).

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 (in vitro) cell line experiment, including gene knockdown of NNT
• Who was studied: The human T-lymphotropic virus 1-immortalized human T cell line MT-2 (MT-2Org), its sublines CB1 and CR1, NNT knockdown clones KD4 and KD5 made from CB1, and a scrambled-sequence control clone (Scr). No human participants.
• Exposure: Continuous relatively low-dose exposure to chrysotile (CH-B, from Canada) for subline CB1 and to crocidolite for subline CR1. Cells were later re-exposed for short periods to 50, 100 or 200 μg/ml chrysotile, or 100 μg/ml crocidolite, depending on the experiment.
• Main results:
  • Antioxidant-related proteins (catalase, SOD2, PRDX3, thioredoxin 2, GLRX2) showed no statistically significant differences between MT-2Org and the sublines, with or without high-dose fiber exposure.
  • Thioredoxin gene expression seemed reduced in CB1 and CR1 compared with MT-2Org, but no antioxidant-related gene differed statistically between the cells.
  • NNT expression was markedly higher in CB1 and CR1 than in MT-2Org. For NNT mRNA, comparisons between groups gave: MT-2Org vs CB1 p = 0.007, MT-2Org vs CR1 p = 0.019. For NNT protein by Western blot: MT-2Org vs CB1 p = 0.001, MT-2Org vs CR1 p = 0.002.
  • The amount of mitochondria did not differ significantly among MT-2Org, CB1 and CR1 (p = 0.438).
  • Reducing NNT in CB1 cells did not change cell growth after exposure to 200 μg/ml chrysotile. Growth did not differ significantly among CB1, the knockdown clone KD5 and the control clone Scr.
  • Early apoptosis (cell death) after 200 μg/ml chrysotile was unchanged among CB1, KD5 and Scr. All three showed lower apoptosis than MT-2Org.
  • ROS production tended to increase with chrysotile dose (0 to 200 μg/ml) in MT-2Org and the NNT knockdown clone, but not in CB1 or Scr.
  • The NNT inhibitor palmitoyl CoA-K increased ROS production in CB1 and CR1 cells exposed to 100 μg/ml chrysotile or crocidolite, respectively.
  • The NADPH to NADP+ ratio tended to be higher in CB1 than in MT-2Org, but this was not statistically significant. The ratio in the knockdown clone returned to the MT-2Org level. The GSSG to GSH ratio did not change significantly.
• The authors' conclusion: The authors concluded that NNT is a key factor in preventing ROS-induced cytotoxicity in T cells continuously exposed to asbestos. They stated that NNT is not directly related to the acquisition of resistance to asbestos-induced apoptosis. They suggested that modifying NNT may contribute to recovery of anti-tumor effects in asbestos-exposed T cells.
• Limitations the authors state:
  • The doses used in the cell experiments were not directly compared with human exposure doses, including doses reaching immune cells in the lung and pleural regions.
  • NNT activity was not measured in cells treated with the NNT inhibitor, and the authors did not confirm that the inhibitor enters the cells.
  • Other mitochondrial enzymes linked to NADP (IDH2 and malic enzyme) were not measured in the NNT knockdown cells.

Topics

Medical terms in this study, explained in our Lexicon: Chrysotile, Crocidolite, Apoptosis, Reactive Oxygen Species (ROS), Carcinogenesis.

Related studies

• Aberrant expression of FoxP3 in a human T cell line possessing regulatory T cell‑like function and exposed continuously to asbestos fibers — M. Maeda, H. Matsuzaki, et al. (2018)
• Inflammatory Alteration of Human T Cells Exposed Continuously to Asbestos — N. Kumagai-Takei, S. Yamamoto, et al. (2018)
• FoxO1 regulates apoptosis induced by asbestos in the MT-2 human T-cell line — H. Matsuzaki, S. Lee, et al. (2016)
• Decrease in Intracellular Perforin Levels and IFN-γ Production in Human CD8+ T Cell Line following Long-Term Exposure to Asbestos Fibers — N. Kumagai-Takei, Y. Nishimura, et al. (2018)
• Accelerated cell cycle progression of human regulatory T cell-like cell line caused by continuous exposure to asbestos fibers — S. Lee, H. Matsuzaki, et al. (2017)
• Chronic exposure to asbestos enhances TGF-β1 production in the human adult T cell leukemia virus-immortalized T cell line MT-2 — M. Maeda, Y. Chen, et al. (2014)

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