Targeting the MEK1 cascade in lung epithelium inhibits proliferation and fibrogenesis by asbestos

Our plain-English summary of Targeting the MEK1 cascade in lung epithelium inhibits proliferation and fibrogenesis by asbestos, by C.B. Manning, T. Sabo-Attwood, R.F. Robledo, et al (American Journal of Respiratory Cell and Molecular Biology, 2008).

The study

• Authors: C.B. Manning, T. Sabo-Attwood, R.F. Robledo, M.B. Macpherson, M. Rincón, P. Vacek, D. Hemenway, D.J. Taatjes, P.J. Lee, B.T. Mossman
• Journal: American Journal of Respiratory Cell and Molecular Biology
• Published: 2008;38(5):618-26
• Cited by: 13 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

The extracellular signal-regulated kinases 1 and 2 (ERK1/2) are phosphorylated after inhalation of asbestos. The effect of blocking this signaling pathway in lung epithelium is unclear. Asbestos-exposed transgenic mice expressing a dominant-negative mitogen-activated protein kinase kinase-1 (dnMEK1) (i.e., the upstream kinase necessary for phosphorylation of ERK1/2) targeted to lung epithelium exhibited morphologic and molecular changes in lung. Transgene-positive (Tg+) (i.e., dnMEK1) and transgene-negative (Tg-) littermates were exposed to crocidolite asbestos for 2, 4, 9, and 32 days or maintained in clean air (sham controls). Distal bronchiolar epithelium was isolated using laser capture microdissection and mRNA analyzed for molecular markers of proliferation and Clara cell secretory protein (CCSP). Lungs and bronchoalveolar lavage fluids were analyzed for inflammatory and proliferative changes and molecular markers of fibrogenesis. Distal bronchiolar epithelium of asbestos-exposed wild-type mice showed increased expression of c-fos at 2 days. Elevated mRNA levels of histone H3 and numbers of Ki-67-labeled proliferating bronchiolar epithelial cells were decreased at 4 days in asbestos-exposed Tg+ mice. At 32 days, distal bronchioles normally composed of Clara cells in asbestos-exposed Tg+ mouse lungs exhibited nonreplicating ciliated and mucin-secreting cells as well as decreased mRNA levels of CCSP. Gene expression (procollagen 3-a-1, procollagen 1-a-1, and IL-6) linked to fibrogenesis was also increased in lung homogenates of asbestos-exposed Tg- mice, but reduced in asbestos-exposed Tg+ mice. These results suggest a critical role of MEK1 signaling in epithelial cell proliferation and lung remodeling after toxic injury.

In plain English

In this animal experiment, mice inhaled crocidolite asbestos (about 7 mg per cubic meter) for 2, 4, 9 or 32 days, or clean air. Some mice carried a gene blocking a signaling protein, MEK1, in cells lining the small airways; their littermates did not. In ordinary mice, asbestos raised c-fos gene activity in these lining cells at 2 days. At 4 days, asbestos increased the number of dividing lining cells, but significantly fewer divided in MEK1-blocked mice. Inflammatory cells in lung washings rose similarly in both groups, and lung tissue looked similar under the microscope. At 32 days, about 25% of small-airway lining cells in exposed MEK1-blocked mice were ciliated, more than in other groups; these mice also had more mucus-producing cells. Four of 5 of these mice had mucus-producing cells, versus 1 of 6 exposed littermates. Lung markers linked to fibrosis (IL-6 and two procollagen genes) rose in exposed littermates but not in MEK1-blocked mice. The authors concluded that their results suggest a critical role for MEK1 signaling in airway cell growth and lung remodeling after toxic injury.

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 (inhalation study in transgenic mice with MEK1 blocked in airway lining cells, their non-transgenic littermates, and wild-type mice)
• Who was studied: Transgenic mice carrying a dominant-negative MEK1 gene in bronchiolar lining cells and their transgene-negative littermates, 3 to 6 per group per time point, in experiments run in duplicate. Wild-type C57BL/6 mice (5 to 6 per group) were used for the initial gene-expression profiling. Location and dates of the experiments are not stated; the authors are at the University of Vermont and Yale University.
• Exposure: Inhaled crocidolite asbestos (NIEHS reference standard) at about 7 mg per cubic meter of air for 2, 4, 9 or 32 days; comparison mice breathed clean air (sham controls).
• Main results:
  • In wild-type mice, asbestos increased c-fos mRNA (but not c-jun) in distal bronchiolar lining cells at 2 days, but not at other time points (P less than 0.05).
  • At 4 days, asbestos increased the number of Ki-67-positive (dividing) bronchiolar lining cells compared with sham controls. This increase was significantly smaller in transgene-positive (MEK1-blocked) mice than in transgene-negative littermates (P less than 0.05 for both comparisons).
  • Dividing cells beneath the airway lining (peribronchiolar cells) increased after asbestos but were comparable in transgene-positive and transgene-negative mice.
  • Inflammatory cells in bronchoalveolar lavage fluid at 9 days increased with asbestos in both groups; inflammatory profiles were not altered in transgene-positive mice.
  • Lung histopathology after asbestos was similar in transgene-positive mice and transgene-negative littermates.
  • At 32 days, ciliated cells made up a larger proportion of the epithelial cells in distal bronchioles of asbestos-exposed transgene-positive mice, about 25% (P less than 0.05). This was not seen in asbestos-exposed transgene-negative mice. Ciliated cells were counted per total number of bronchiolar epithelial cells.
  • At 32 days, mucin-positive cells were found in distal bronchioles of 4 of 5 asbestos-exposed transgene-positive mice (13 of 32 bronchioles). In asbestos-exposed transgene-negative mice the figure was 1 of 6 mice (2 of 18 bronchioles). No sham mice had them. The difference was significant (P less than 0.05) for the transgene-positive group compared with the other groups.
  • At 32 days, mRNA for Clara cell secretory protein (CCSP) was significantly reduced in bronchioles of asbestos-exposed transgene-positive mice (P less than 0.05).
  • At 32 days, IL-6 mRNA in whole lung increased in asbestos-exposed transgene-negative mice but not in transgene-positive mice.
  • Procollagen 1-a-1 and procollagen 3-a-1 mRNA increased in asbestos-exposed transgene-negative mice compared with sham transgene-negative mice. They were not elevated in asbestos-exposed transgene-positive mice.
  • TNF-alpha mRNA showed trends in both asbestos-exposed groups but was not significantly increased.
• The authors' conclusion: The authors concluded that their results suggest a critical role of MEK1 signaling in epithelial cell proliferation and lung remodeling after toxic injury.
• Limitations the authors state:
  • The focal fibrosis seen in these mice at about 30 days was modest and could not be measured with biochemical hydroxyproline assays, so the authors relied on gene-expression markers.
  • Because too few transgene-positive mice were available, sham transgene-negative mice were used as the comparison group for the whole-lung gene-expression measurements.
  • Blocking MEK1 did not completely stop asbestos-induced cell division, which the authors suggest means other pathways are also involved.
  • The authors note that long-term MEK1 inhibition in the airway lining might itself change how the mice respond to inhaled asbestos.

Topics

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

Medical terms in this study, explained in our Lexicon: Epithelium, Crocidolite, Bronchoalveolar Lavage (BAL), Ki-67.

Related studies

• The duration of nuclear extracellular signal-regulated kinase 1 and 2 signaling during cell cycle reentry distinguishes proliferation from apoptosis in response to asbestos — Z. Yuan, D.J. Taatjes, et al. (2004)
• Asbestos-induced activation of cell signaling pathways in human bronchial epithelial cells — X. Wang, J.M. Samet, et al. (2006)
• Persistent localization of activated extracellular signal-regulated kinases (ERK1/2) is epithelial cell-specific in an inhalation model of asbestosis — A.B. Cummins, C. Palmer, et al. (2003)
• Gene expression profiles reveal increased mClca3 (Gob5) expression and mucin production in a murine model of asbestos-induced fibrogenesis — T. Sabo-Attwood, M. Ramos-Nino, et al. (2005)
• Long Noncoding RNA FENDRR Inhibits Lung Fibroblast Proliferation via a Reduction of β-Catenin — L.K. Senavirathna, Y. Liang, et al. (2021)
• Imatinib mesylate inhibits fibrogenesis in asbestos-induced interstitial pneumonia — K. Vuorinen, F. Gao, et al. (2007)

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