Asbestos-induced disruption of calcium homeostasis induces endoplasmic reticulum stress in macrophages

Our plain-English summary of Asbestos-induced disruption of calcium homeostasis induces endoplasmic reticulum stress in macrophages, by A.J. Ryan, J.L. Larson-Casey, C. He, et al (The Journal of Biological Chemistry, 2014).

The study

• Authors: A.J. Ryan, J.L. Larson-Casey, C. He, S. Murthy, A.B. Carter
• Journal: The Journal of Biological Chemistry
• Published: 2014;289(48):33391-403
• Cited by: 40 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Although the mechanisms for fibrosis development remain largely unknown, recent evidence indicates that endoplasmic reticulum (ER) stress and activation of the unfolded protein response (UPR) may act as an important fibrotic stimulus in diseased lungs. ER stress is observed in lungs of patients with idiopathic pulmonary fibrosis. In this study we evaluated if ER stress and the UPR was present in macrophages exposed to chrysotile asbestos and if ER stress in macrophages was associated with asbestos-induced pulmonary fibrosis. Macrophages exposed to chrysotile had elevated transcript levels of several ER stress genes. Macrophages loaded with the Ca(2+)-sensitive dye Fura2-AM showed that cytosolic Ca(2+) increased significantly within minutes after chrysotile exposure and remained elevated for a prolonged time. Chrysotile-induced increases in cytosolic Ca(2+) were partially inhibited by either anisomycin, an inhibitor of passive Ca(2+) leak from the ER, or 1,2-bis(2-aminophenoxyl)ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM), an intracellular Ca(2+) chelator known to deplete ER Ca(2+) stores. Anisomycin inhibited X-box-binding protein 1 (XBP1) mRNA splicing and reduced immunoglobulin-binding protein (BiP) levels, whereas BAPTA-AM increased XBP1 splicing and BiP expression, suggesting that ER calcium depletion may be one factor contributing to ER stress in cells exposed to chrysotile. To evaluate ER stress in vivo, asbestos-exposed mice showed fibrosis development, and alveolar macrophages from fibrotic mice showed increased expression of BiP. Bronchoalveolar macrophages from asbestosis patients showed increased expression of several ER stress genes compared with normal subjects. These findings suggest that alveolar macrophages undergo ER stress, which is associated with fibrosis development.

© 2014 by The American Society for Biochemistry and Molecular Biology, Inc.

In plain English

This laboratory study asked whether chrysotile asbestos induces endoplasmic reticulum (ER) stress in macrophages. Human THP-1 cells and mouse bone marrow-derived macrophages were exposed to chrysotile, with thapsigargin and tunicamycin used as comparison ER stressors. Chrysotile-exposed macrophages showed XBP1 mRNA splicing at 24 hours but not earlier, and raised BiP and ATF6 protein at 24 and 48 hours. Cytosolic calcium rose within minutes of chrysotile exposure and stayed elevated, reaching 5 to 6 times control values, and rose also in calcium-free medium. Anisomycin, an inhibitor of ER calcium leak, partly blocked the rise and reduced ER stress markers, while the chelator BAPTA-AM lowered cytosolic calcium but itself triggered XBP1 splicing and BiP. Knockdown of IP3R-1 did not alter the calcium response. Mice given 125 micrograms chrysotile intratracheally showed collagen deposition and raised lung hydroxyproline at 21 days versus titanium dioxide. Their alveolar macrophages had elevated BiP protein and BiP mRNA, BiP mRNA was not raised in whole lung tissue, and caspase-3 did not differ. Alveolar macrophages from asbestosis patients had higher BiP, ATF6, ERO1-alpha and CHOP than normal subjects.

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: Experimental laboratory study combining in vitro macrophage cell experiments, an in vivo mouse asbestos exposure experiment, and comparison of bronchoalveolar lavage macrophages from asbestosis patients and normal volunteers
• Who was studied: Human THP-1 monocytic cell line and bone marrow-derived macrophages from C57BL/6 mice; C57BL/6 mice exposed intratracheally; alveolar macrophages obtained by bronchoalveolar lavage from normal volunteers and patients with asbestosis at the University of Iowa
• Exposure: Chrysotile asbestos, 10 micrograms per square centimetre in cell culture and 125 micrograms intratracheally in mice (titanium dioxide as the comparison dust); thapsigargin, tunicamycin, anisomycin, BAPTA-AM and ionomycin used as pharmacological agents
• Main results:
  • In macrophages, chrysotile caused XBP1 mRNA splicing only at 24 hours, while thapsigargin did so within 3-6 hours
  • Chrysotile-exposed macrophages had increased BiP and ATF6 protein at 24 and 48 hours
  • Cytosolic calcium in chrysotile-exposed macrophages rose and stayed raised, reaching 5-6 times control values
  • Cytosolic calcium also rose when chrysotile was given in calcium-free medium, which the authors read as implicating the ER as a calcium source
  • The chelator BAPTA-AM alone induced ER stress markers in macrophages
  • Anisomycin reduced ER stress markers whereas BAPTA increased them in chrysotile-exposed macrophages
  • IP3R-1 knockdown lowered IP3R-1 mRNA but did not change the cytosolic calcium response to chrysotile
  • Alveolar macrophages from chrysotile-exposed fibrotic mice had elevated BiP protein compared with TiO2-exposed mice
  • In chrysotile-exposed mice, BiP mRNA was elevated in bronchoalveolar lavage macrophages but not in whole lung tissue
  • BiP was raised in alveolar macrophages of chrysotile-exposed fibrotic mice and of asbestosis patients
  • Alveolar macrophages from asbestosis patients had higher BiP, ERO1-alpha and CHOP mRNA than normal subjects (n = 3 per group)
  • Active TGF-beta1 in bronchoalveolar lavage fluid was higher in the chrysotile-exposed fibrotic mice
  • No significant apoptosis difference (caspase-3) was seen in alveolar macrophages of chrysotile-exposed mice
• The authors' conclusion: The authors concluded that ER stress in alveolar macrophages is linked to asbestos-induced pulmonary fibrosis and suggested that relieving ER stress might be a therapeutic target
• Limitations the authors state:
  • The authors state their calcium measurements came from whole cell populations and may lack the resolution to detect changes in single cells or mitochondria
  • The authors state that ER calcium leak may not be the only contributing factor and other mechanisms, including IP3R-1, are not excluded

Topics

The National Library of Medicine indexes this study under subjects our site covers: Asbestosis, Chrysotile.

Medical terms in this study, explained in our Lexicon: Macrophage, Fibrosis, Idiopathic, Pulmonary Fibrosis, Chrysotile, Alveolar Macrophage.

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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)
• Rapid screening of possible cytotoxic effects of particulate air pollutants by measurement of changes in cytoplasmic free calcium, cytosolic pH, and plasma membrane potential in alveolar macrophages by flow cytometry — A. Tárnok, M. Dörger, et al. (2001)
• Mineral fiber-induced leukocyte activation: the role of intra- and extracellular calcium — M. Ruotsalainen, J. Naarala, et al. (1995)
• Alternative activation of macrophages and pulmonary fibrosis are modulated by scavenger receptor, macrophage receptor with collagenous structure — S. Murthy, J.L. Larson-Casey, et al. (2015)

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