Asbestos-induced alveolar epithelial cell apoptosis. The role of endoplasmic reticulum stress response

Our plain-English summary of Asbestos-induced alveolar epithelial cell apoptosis. The role of endoplasmic reticulum stress response, by D.W. Kamp, G. Liu, P. Cheresh, et al (American Journal of Respiratory Cell and Molecular Biology, 2013).

The study

• Authors: D.W. Kamp, G. Liu, P. Cheresh, S.J. Kim, A. Mueller, A.P. Lam, H. Trejo, D. Williams, S. Tulasiram, M. Baker, K. Ridge, N.S. Chandel, R. Beri
• Journal: American Journal of Respiratory Cell and Molecular Biology
• Published: 2013;49(6):892-901
• Cited by: 53 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Asbestos exposure results in pulmonary fibrosis (asbestosis) and malignancies (bronchogenic lung cancer and mesothelioma) by mechanisms that are not fully understood. Alveolar epithelial cell (AEC) apoptosis is important in the development of pulmonary fibrosis after exposure to an array of toxins, including asbestos. An endoplasmic reticulum (ER) stress response and mitochondria-regulated (intrinsic) apoptosis occur in AECs of patients with idiopathic pulmonary fibrosis, a disease with similarities to asbestosis. Asbestos induces AEC intrinsic apoptosis, but the role of the ER is unclear. The objective of this study was to determine whether asbestos causes an AEC ER stress response that promotes apoptosis. Using human A549 and rat primary isolated alveolar type II cells, amosite asbestos fibers increased AEC mRNA and protein expression of ER stress proteins involved in the unfolded protein response, such as inositol-requiring kinase (IRE) 1 and X-box-binding protein-1, as well as ER Ca²(2+) release ,as assessed by a FURA-2 assay. Eukarion-134, a superoxide dismutase/catalase mimetic, as well as overexpression of Bcl-XL in A549 cells each attenuate asbestos-induced AEC ER stress (IRE-1 and X-box-binding protein-1 protein expression; ER Ca²(2+) release) and apoptosis. Thapsigargin, a known ER stress inducer, augments AEC apoptosis, and eukarion-134 or Bcl-XL overexpression are protective. Finally, 4-phenylbutyric acid, a chemical chaperone that attenuates ER stress, blocks asbestos- and thapsigargin-induced AEC IRE-1 protein expression, but does not reduce ER Ca²(2+) release or apoptosis. These results show that asbestos triggers an AEC ER stress response and subsequent intrinsic apoptosis that is mediated in part by ER Ca²(2+) release.

In plain English

In this laboratory study, the authors exposed cultured human A549 cells (a lung adenocarcinoma cell line with some features of alveolar type II cells) and alveolar type II cells isolated from rats to amosite asbestos. They compared it with hydrogen peroxide and with thapsigargin, a known trigger of endoplasmic reticulum (ER) stress. Amosite increased ER stress markers. Messenger RNA for IRE-1, XBP-1, CHOP and GRP78/BiP rose as early as 30 minutes, and IRE-1 and spliced XBP-1 protein rose 1.5- to 2-fold from 1 hour through 24 hours. Asbestos triggered calcium release within 5 to 10 minutes, including in calcium-free media, and about doubled cell death (apoptosis) in A549 cells. An antioxidant compound, Euk-134, and extra Bcl-XL protein each reduced these stress signals and apoptosis. A calcium-binding agent, BAPTA, modestly reduced apoptosis, a difference that was not statistically significant. The chemical 4-PBA lowered IRE-1 but did not reduce calcium release or apoptosis. The authors concluded that ER calcium release has an important role in promoting this cell death. They said in vivo studies are needed.

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 culture experiments
• Who was studied: Human A549 cells (a lung adenocarcinoma cell line with some features of alveolar epithelial type II cells, including A549 cells engineered to overexpress Bcl-XL) and primary alveolar type II cells isolated from the lungs of Sprague-Dawley rats; no human subjects
• Exposure: Amosite asbestos (UICC reference standard samples), mostly at 5 to 25 µg/cm2 and in some apoptosis experiments 25 to 50 µg/cm2, compared with hydrogen peroxide (H2O2) and thapsigargin, a known ER stress inducer
• Main results:
  • In A549 cells, amosite asbestos at 5 µg/cm2 significantly increased mRNA of IRE-1, XBP-1, CHOP and GRP78/BiP compared with control as early as 30 minutes; for XBP-1 and CHOP the increases persisted over 24 hours
  • Asbestos increased IRE-1 and spliced XBP-1 protein by 1.5- to 2-fold, seen from 1 hour and persisting over 24 hours
  • Amosite asbestos (25 µg/cm2) caused calcium release in A549 and rat AT2 cells within 5 to 10 minutes. In A549 cells it also occurred in calcium-free media, which the authors took to implicate the ER as the source
  • In A549 cells, asbestos and H2O2 increased apoptosis by 2-fold and 2.5-fold respectively. The calcium chelator BAPTA modestly reduced this, but the differences were not statistically significant
  • In A549 cells, Euk-134 (an SOD/catalase mimetic) prevented asbestos- and thapsigargin-induced IRE-1 protein expression at 4 hours and apoptosis at 24 hours
  • Compared with empty-vector control A549 cells, A549 cells overexpressing Bcl-XL had significantly reduced apoptosis induced by amosite (25 to 50 µg/cm2) and thapsigargin
  • 4-PBA blocked asbestos- and thapsigargin-induced IRE-1 protein expression but did not reduce ER calcium release or apoptosis
  • In A549 cells, nearly 90% of control cells showed co-localization of mitochondria and ER. Amosite (5 to 25 µg/cm2) caused negligible change in co-localization (about 38% low-level and about 45% high-level, of more than 200 cells per condition), though it caused mitochondrial dysfunction
• The authors' conclusion: The authors concluded that asbestos and H2O2 activate an ER stress response in alveolar epithelial cells and that oxidant-induced ER calcium release has an important role in promoting apoptosis of these cells
• Limitations the authors state:
  • The findings are from cells in culture; the authors state that animal (in vivo) studies are needed to assess whether ER stress causes asbestosis
  • Sources of reactive oxygen species other than mitochondria may contribute to the protective effect of Euk-134
  • The study focused on the IRE-1 arm and calcium release; other ER stress pathways (PERK and ATF6) were not examined
  • The detailed molecular mechanism linking ER and mitochondria in asbestos-exposed cells remains unclear

Topics

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

Medical terms in this study, explained in our Lexicon: Apoptosis, Asbestos Exposure, Pulmonary Fibrosis, Bronchogenic Carcinoma, Lung Cancer, Mesothelioma, Idiopathic, Amosite.

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• Repression of GADD153/CHOP by NF-kappaB: a possible cellular defense against endoplasmic reticulum stress-induced cell death — S. Nozaki, G.W. Sledge, et al. (2001)
• Increased sensitivity to apoptosis upon endoplasmic reticulum stress-induced activation of the unfolded protein response in chemotherapy-resistant malignant pleural mesothelioma — D. Xu, S.Q. Liang, et al. (2018)
• The c-Jun N-terminal kinase signaling pathway mediates chrysotile asbestos-induced alveolar epithelial cell apoptosis — P. Li, T. Liu, et al. (2015)
• SIRT3 deficiency promotes lung fibrosis by augmenting alveolar epithelial cell mitochondrial DNA damage and apoptosis — R.P. Jablonski, S.J. Kim, et al. (2017)
• The Role of Mitochondrial DNA in Mediating Alveolar Epithelial Cell Apoptosis and Pulmonary Fibrosis — S.J. Kim, P. Cheresh, et al. (2015)

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