Actin polymerization plays a significant role in asbestos-induced inflammasome activation in mesothelial cells in vitro

Our plain-English summary of Actin polymerization plays a significant role in asbestos-induced inflammasome activation in mesothelial cells in vitro, by M. MacPherson, C. Westbom, H. Kogan, et al (Histochemistry and Cell Biology, 2017).

The study

• Authors: M. MacPherson, C. Westbom, H. Kogan, A. Shukla
• Journal: Histochemistry and Cell Biology
• Published: 2017;147(5):595-604
• Cited by: 10 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Asbestos exposure leads to malignant mesothelioma (MM), a deadly neoplasm of mesothelial cells of various locations. Although there is no doubt about the role of asbestos in MM tumorigenesis, mechanisms are still not well explored. Recently, our group demonstrated that asbestos causes inflammasome priming and activation in mesothelial cells, which in part is dependent on oxidative stress. Our current study sheds light on yet another mechanism of inflammasome activation by asbestos. Here we show the role of actin polymerization in asbestos-induced activation of the nod-like receptor pyrin domain-containing protein 3 (NLRP3) inflammasome. Using human mesothelial cells, we first demonstrate that asbestos and carbon nanotubes induced caspase-1 activation and high-mobility group box 1, interleukin 1 beta and interleukin 18 secretion was blocked by Cytochalasin D (Cyto D) an actin polymerization inhibitor. Next, to understand the mechanism, we assessed whether phagocytosis of fibers by mesothelial cells is affected by actin polymerization inhibition. Transmission electron microscopy showed the inhibition of fiber uptake by mesothelial cells in the presence of Cyto D. Furthermore, localization of components of the inflammasome, apoptotic speck-like protein containing a CARD domain (ASC) and NLRP3, to the perinuclear space in mitochondria or endoplasmic reticulum in response to fiber exposure was also interrupted in the presence of Cyto D. Taken together, our studies suggest that actin polymerization plays important roles in inflammasome activation by fibers via regulation of phagocytosis and/or spatial localization of inflammasome components.

In plain English

This was a laboratory study of human mesothelial cells (the LP9 cell line) grown in dishes. The authors exposed the cells to crocidolite asbestos or multiwalled carbon nanotubes, with or without first adding cytochalasin D, which blocks actin polymerization, a process cells use to take in particles. Asbestos and carbon nanotubes caused significant cell death. They also activated the NLRP3 inflammasome, a cell structure that triggers inflammation, measured by release of caspase-1, HMGB1, IL-1 beta and IL-18. Cytochalasin D prevented the cell death and significantly reduced these signals. Under the electron microscope, asbestos fibers filled an average of 1.0% of cell area without cytochalasin D and 0.2% with it, an 80% decrease. Cytochalasin D also reduced asbestos-induced movement of the inflammasome parts ASC and NLRP3 to mitochondria and the endoplasmic reticulum. The authors reported no effect of a cathepsin B blocker on asbestos-induced inflammasome activation, as measured by NLRP3 protein levels and caspase-1p20 release, but did not show these data. The authors concluded this activation is in part dependent on actin polymerization, and said the exact pathway needs further study.

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) experiment on cultured human cells
• Who was studied: Human mesothelial cells (the LP9 cell line) grown in culture, obtained from Brigham and Women's Hospital, Boston
• Exposure: A National Institute of Environmental Health Sciences reference sample of crocidolite asbestos; multiwalled carbon nanotubes (Mitsui-7) at two doses; carbon black and glass beads as negative controls. All particles were given at equal particle surface area concentrations. Exposures lasted 24 hours, or 4 hours for electron microscopy. Some cells were first treated with the actin polymerization inhibitor cytochalasin D (mainly 0.1 μg/mL).
• Main results:
  • Asbestos or carbon nanotube exposure for 24 hours caused significant death of the mesothelial cells. Pre-treatment with cytochalasin D (0.1 μg/mL) prevented this death.
  • Asbestos or carbon nanotubes activated the inflammasome, measured by release of caspase-1p20, HMGB1, IL-1 beta and IL-18. Cytochalasin D pre-treatment significantly reduced these levels and therefore NLRP3 inflammasome activation.
  • In electron microscope images of asbestos-exposed cells without cytochalasin D, an average of 1.0% of the cell area contained asbestos.
  • With cytochalasin D pre-treatment, asbestos took up only 0.2% of cell area, which the authors described as an 80% decrease in fiber uptake.
  • The authors reported observing no effect of a cathepsin B inhibitor (Ca-074Me) on asbestos-induced inflammasome priming or activation, as measured by NLRP3 protein levels or caspase-1p20 release. The data were not shown. The authors took this to suggest no significant role for lysosome disruption.
  • Asbestos caused strong localization of ASC to mitochondria. Cytochalasin D pre-treatment before fiber exposure reduced this.
  • Asbestos increased localization of ASC and NLRP3 to the endoplasmic reticulum, and cytochalasin D inhibited this. Carbon nanotubes did not move ASC to the endoplasmic reticulum at any dose.
• The authors' conclusion: The authors concluded that asbestos-induced inflammasome activation in mesothelial cells is in part dependent on actin polymerization.
• Limitations the authors state:
  • The authors stated it remains to be investigated whether actin polymerization acts through phagocytosis and reactive oxygen species generation, or through direct recruitment of inflammasome components.

Topics

Medical terms in this study, explained in our Lexicon: Mesothelial Cells, Asbestos Exposure, Malignant Mesothelioma, Neoplasm, Tumorigenesis, Oxidative Stress, Phagocytosis, Transmission Electron Microscopy (TEM).

Related studies

• Differential activation of the inflammasome in THP-1 cells exposed to chrysotile asbestos and Libby "six-mix" amphiboles and subsequent activation of BEAS-2B cells — M. Li, M.E. Gunter, et al. (2012)
• The Synthetic Lignan Secoisolariciresinol Diglucoside Prevents Asbestos-Induced NLRP3 Inflammasome Activation in Murine Macrophages — R.A. Pietrofesa, P. Woodruff, et al. (2017)
• Double-walled carbon nanotubes trigger IL-1β release in human monocytes through Nlrp3 inflammasome activation — E. Meunier, A. Coste, et al. (2012)
• Orthopedic wear debris mediated inflammatory osteolysis is mediated in part by NALP3 inflammasome activation — L. Burton, D. Paget, et al. (2013)
• Distribution of transforming growth factor-beta 1, fibronectin, and smooth muscle actin in asbestos-induced pulmonary fibrosis in rats — T.D. Perdue, A.R. Brody (1994)
• Vimentin regulates activation of the NLRP3 inflammasome — G. dos Santos, M.R. Rogel, et al. (2015)

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