Apoptosis Resistance in Fibroblasts Precedes Progressive Scarring in Pulmonary Fibrosis and Is Partially Mediated by Toll-Like Receptor 4 Activation

Our plain-English summary of Apoptosis Resistance in Fibroblasts Precedes Progressive Scarring in Pulmonary Fibrosis and Is Partially Mediated by Toll-Like Receptor 4 Activation, by K.M. Hanson, E.B. Hernady, C.K. Reed, et al (Toxicological Sciences, 2019).

The study

• Authors: K.M. Hanson, E.B. Hernady, C.K. Reed, C.J. Johnston, A.M. Groves, J.N. Finkelstein
• Journal: Toxicological Sciences
• Published: 2019;170(2):489-498
• Cited by: 23 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Inhalation of environmental toxicants such as cigarette smoke, metal or wood dust, silica, or asbestos is associated with increased risk for idiopathic pulmonary fibrosis (IPF). IPF involves progressive scarring of lung tissue, which interferes with normal respiration and is ultimately fatal; however, the complex cellular mechanisms of IPF pathogenesis remain unclear. Fibroblast apoptosis is essential in normal wound healing but is dysregulated in IPF. Recent studies suggest that Toll-like receptor 4 (TLR4) is key in the onset of IPF. Here, radiation-induced PF was used as a model for IPF because it very closely mimics the progressive and intractable nature of IPF. Female C57BL/6J (C57) and C57BL/6J TLR4-/- mice were exposed to a single dose of 13 Gy whole-thorax ionizing radiation. Although both strains showed similar levels of immediate radiation-induced damage, C57 mice exhibited more extensive fibrosis at 22-week postirradiation (PI) than TLR4-/- mice. Isolated C57 primary 1° MLFs showed decreased apoptosis susceptibility as early as 8-week postirradiation, a phenotype that persisted for the remainder of the radiation response. TLR4-/- 1° mouse lung fibroblasts did not exhibit significant apoptosis resistance at any point. Systemic release of high mobility group box 1, a TLR4 agonist, during the pneumonitis phase of the radiation response may act through TLR4 to contribute to fibroblast apoptosis resistance and thus interfere with wound resolution. These findings demonstrate that apoptosis resistance occurs earlier in pulmonary fibrosis pathogenesis than previously assumed, and that TLR4 signaling is a key mediator in this process.

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In plain English

This was a laboratory experiment in mice. Asbestos was not studied; it appears only as background, as one of several inhaled agents linked to idiopathic pulmonary fibrosis. The authors used radiation-induced lung scarring as a model. Female C57BL/6J mice, and mice of the same strain lacking Toll-like receptor 4 (TLR4, an immune receptor), received a single 13 Gy dose of radiation to the chest. Sham-irradiated mice served as controls. Both strains showed similar early radiation injury. At 22 weeks, the TLR4-lacking mice had significantly less lung scarring than the normal mice (p less than 0.01), although they did develop some fibrosis. Lung fibroblasts (scar-forming cells) from irradiated normal mice resisted programmed cell death from 8 weeks onward, though the difference was not statistically significant at 16 weeks. Fibroblasts from TLR4-lacking mice showed no significant resistance at any time. Blood levels of HMGB1, a protein that can activate TLR4, rose in both strains at 24 hours and 8 weeks. The authors concluded that this resistance occurs earlier than currently believed and that TLR4 is a key mediator. They noted small sample sizes and that a causal link involving HMGB1 remains to be established.

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: animal experiment using a mouse model of radiation-induced pulmonary fibrosis
• Who was studied: Female C57BL/6J mice and C57BL/6J-background TLR4-deficient (TLR4-/-) mice, 6 weeks old, obtained from Jackson Laboratory and studied at the University of Rochester Medical Center. Groups were small (for example, 4-5 mice per group for the fibrosis measurements), sampled from 24 hours to 22 weeks after irradiation. Study dates were not given.
• Exposure: A single 13 Gy dose of thorax-only gamma radiation from a cesium-137 source, compared with sham-irradiated (0 Gy) controls. No asbestos or other fiber exposure was studied.
• Main results:
  • Both strains showed similar levels of acute radiation injury, measured by DNA damage staining and loss of airway SCGB1A1-positive cells.
  • Macrophage density in the lungs rose significantly in both irradiated strains at 8 and 12 weeks after irradiation. Irradiated TLR4-/- mice generally showed more macrophage density than irradiated C57 mice.
  • At 22 weeks after irradiation, irradiated TLR4-/- lungs had significantly less fibrosis than irradiated C57 lungs, by both percent fibrotic area and modified Ashcroft score (p < 0.01 for both; 4-5 mice per group).
  • The TLR4-/- mice still developed some fibrosis, though less than the C57 mice.
  • At 24 hours after irradiation, fibroblast apoptosis susceptibility in both strains did not differ from controls.
  • Lung fibroblasts from irradiated C57 mice showed apoptosis resistance (significantly lower apoptosis susceptibility than controls) from 8 weeks after irradiation onward. The difference did not reach statistical significance at 16 weeks.
  • Fibroblasts from irradiated TLR4-/- mice did not show a significant decrease in apoptosis susceptibility at any time point.
  • Plasma HMGB1 was elevated in both strains at 24 hours and 8 weeks after irradiation, and slightly but not significantly at 12 weeks. There were no statistically significant differences between strains.
• The authors' conclusion: The authors concluded that fibroblast apoptosis resistance occurs much earlier in pulmonary fibrosis development than currently believed, and that TLR4 is a key mediator of this aspect of the disease.
• Limitations the authors state:
  • Several findings were based on limited sample sizes; the authors said larger samples and greater statistical power would strengthen the conclusions.
  • Limited sample size prevented study of fibroblast characteristics other than apoptosis susceptibility.
  • A causal link between HMGB1 and TLR4-mediated fibroblast apoptosis resistance was not established; this would require studies that inhibit HMGB1.
  • The molecular signaling pathway linking TLR4 activation to apoptosis resistance in lung fibroblasts was not clarified.

Topics

Medical terms in this study, explained in our Lexicon: Apoptosis, Fibroblast, Pulmonary Fibrosis, Idiopathic, Thorax, Fibrosis, High Mobility Group Box 1 (HMGB1), Pneumonitis.

Related studies

• NOX4 regulates macrophage apoptosis resistance to induce fibrotic progression — J.L. Larson-Casey, L. Gu, et al. (2021)
• Mineral fibers induce apoptosis in Syrian hamster embryo fibroblasts — E. Dopp, B. Nebe, et al. (1995)
• The Its Not JUST Idiopathic pulmonary fibrosis Study (INJUSTIS): description of the protocol for a multicentre prospective observational cohort study identifying biomarkers of progressive fibrotic lung disease — F. Khan, I. Stewart, et al. (2019)
• Alveolar macrophage stimulation of lung fibroblast growth in asbestos-induced pulmonary fibrosis — I. Lemaire, H. Beaudoin, et al. (1986)
• A Single Aspiration of Rod-like Carbon Nanotubes Induces Asbestos-like Pulmonary Inflammation Mediated in Part by the IL-1 Receptor — E.M. Rydman, M. Ilves, et al. (2015)
• 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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