Mechanisms of silica-induced IL-8 release from A549 cells: initial kinase-activation does not require EGFR activation or particle uptake

Our plain-English summary of Mechanisms of silica-induced IL-8 release from A549 cells: initial kinase-activation does not require EGFR activation or particle uptake, by J. Øvrevik, M. Refsnes, E. Namork, et al (Toxicology, 2006).

The study

• Authors: J. Øvrevik, M. Refsnes, E. Namork, R. Becher, D. Sandnes, P.E. Schwarze, M. Låg
• Journal: Toxicology
• Published: 2006;227(1-2):105-16
• Cited by: 35 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Understanding how mineral particles trigger cellular responses is crucial in order to elucidate what characteristics determine their harmful effects. It is not clear whether cellular effects are triggered through the cell membrane or require particle uptake. However, studies with asbestos suggest that activation of the epidermal growth factor receptor (EGFR) may be important. We have previously reported that crystalline silica-induced interleukin (IL)-8 release from human lung epithelial cells (A549) was regulated through Src family kinases (SFKs) and the mitogen-activated protein kinases (MAPKs) p38 and extracellular signal-regulated kinase (ERK)-1 and -2. The present study shows that SFK and p38 phosphorylation increased almost immediately upon crystalline silica exposure, whereas ERK1/2 phosphorylation increased after 10 min of exposure. The p38 inhibitor SB202190 increased the silica-induced ERK1/2 phosphorylation suggesting that p38 activity may attenuate activation of ERK1/2. Scanning electron microscopy showed that some silica particles were phagocytosed between 1 and 4h of exposure, but that the majority remained bound by microvilli on the cell surface. The EGFR inhibitor AG1478 attenuated both silica-induced IL-8 release and phosphorylation of SFKs and ERK1/2. However, AG1478 also inhibited the respective background levels, and the EGFR was not phosphorylated at the onset of silica exposure. The results suggest that crystalline silica triggers p38 and SFK-ERK1/2 signaling through interactions with membrane components as both pathways were rapidly activated prior to particle internalization. However, the silica-induced up-regulation of IL-8 release through the SFK-ERK1/2 pathway does not appear to be initiated through activation of the EGFR, although basal EGFR activity may affect the magnitude of the responses.

In plain English

This laboratory study looked at how crystalline silica (quartz) particles trigger chemical signals in lung cells. It did not test asbestos. The article mentions asbestos only as background from other research. The authors exposed a human lung cell line (A549) to a commercial ground silica, MIN-U-SIL 5. They tracked signaling enzymes and the release of IL-8, a signal that drives inflammation. The enzymes p38 and Src family kinases were activated within minutes, with p38 appearing to peak at 10 minutes. ERK1/2 rose later, from 10 to 30 minutes. Electron microscopy showed no particle uptake at 30 minutes and negligible uptake at 1 hour. By 4 hours many particles had been engulfed, but most stayed on the cell surface. A drug that blocks the EGF receptor cut silica-induced IL-8 release by 41%, but it also cut IL-8 in unexposed cells by 38%. Silica did not visibly activate the receptor at 15 minutes. The authors concluded that the signaling was most likely triggered through cell-membrane components other than the EGF receptor, before particles entered the cells. They noted that their microscopy method was not quantitative.

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 experiment
• Who was studied: The human lung epithelial cell line A549. Some experiments also used primary type-II lung cells taken from six male Wistar Kyoto rats aged 6 to 8 weeks.
• Exposure: Crystalline silica (MIN-U-SIL 5 ground silica, at least 98–99% SiO2). Doses were printed as 60 'g/cm2' for kinase experiments and 40 (A549) or 20 (rat cells) 'g/cm2' for chemokine experiments. The micro sign appears to have been lost from the text. No asbestos was tested.
• Main results:
  • In silica-exposed A549 cells, p38 phosphorylation rose rapidly, appeared to peak after 10 minutes, then gradually decreased.
  • In silica-exposed A549 cells, ERK1/2 phosphorylation increased later, from 10 to 30 minutes.
  • In silica-exposed A549 cells, phosphorylated c-Src (a Src family kinase) increased rapidly during the first 10 minutes.
  • In A549 cells, the p38 inhibitor SB202190 enhanced silica-induced ERK1/2 phosphorylation.
  • In A549 cells, the EGFR inhibitor AG1478 inhibited silica-induced IL-8 release by 41%.
  • AG1478 also reduced IL-8 in unexposed control A549 cells, by 38%.
  • In A549 cells, AG1478 reduced silica-induced phosphorylation of ERK1/2 and Src family kinases. It also appeared to reduce background levels in unexposed cells to a certain extent.
  • In primary rat type-II cells, AG1478 did not reduce silica-induced MIP-2 release. The release was rather enhanced.
  • After 15 minutes, EGF (the positive control) clearly phosphorylated the EGFR in A549 cells, while silica had no observable effect.
  • Scanning electron microscopy of A549 cells found no particle uptake at 30 minutes. At 1 hour, the number of internalized particles was negligible compared with those on the cell surface.
  • After 4 hours, a considerable amount of particles had been engulfed by A549 cells, but most remained on the cell surface.
  • Particle settling in culture medium, measured as loss of light absorbance, was 72% at 1 hour and 85% at 2 hours.
• The authors' conclusion: The authors concluded that silica-induced signaling started within minutes, before particle uptake. They concluded it was most likely triggered through cell-membrane components other than the EGFR, although basal EGFR activity may affect the size of the IL-8 response.
• Limitations the authors state:
  • Scanning electron microscopy is non-quantitative, so partial effects of the kinase inhibitors on particle uptake might have been overlooked.
  • Scanning electron microscopy may be less sensitive for detecting particle uptake than other methods.
  • Because particles settled slowly, early responses may reflect rising particle concentration as well as time.
  • Finding no EGFR phosphorylation at 15 minutes does not rule out activation at a later time.
  • The combined time-course figure merged data from different experiments, so relative levels should be interpreted with caution.

Topics

Medical terms in this study, explained in our Lexicon: A549 Cells, EGFR, Crystalline, Scanning Electron Microscopy (SEM).

Related studies

• Indications for distinct pathogenic mechanisms of asbestos and silica through gene expression profiling of the response of lung epithelial cells — T.N. Perkins, P.M. Peeters, et al. (2015)
• Role of extracellular signal-regulated protein kinases in apoptosis by asbestos and H2O2 — L.A. Jiménez, C. Zanella, et al. (1997)
• Mineral particles of varying composition induce differential chemokine release from epithelial lung cells: importance of physico-chemical characteristics — J. Ovrevik, T. Myran, et al. (2005)
• Src-dependent ERK5 and Src/EGFR-dependent ERK1/2 activation is required for cell proliferation by asbestos — L. Scapoli, M.E. Ramos-Nino, et al. (2004)
• IL-8 response of cyclically stretching alveolar epithelial cells exposed to non-fibrous particles — S.M. Mijailovich, K. Hamada, et al. (2007)
• Combined inhibition of MET and EGFR suppresses proliferation of malignant mesothelioma cells — K. Kawaguchi, H. Murakami, et al. (2009)

Browse more research in Toxicology, or return to medical research.