Asbestos-Induced Lung Inflammation and Epithelial Cell Proliferation Are Altered in Myeloperoxidase-Null Mice

Abstract

Asbestos fibers are carcinogens causing oxidative stress and inflammation, but the sources and ramifications of oxidant production by asbestos are poorly understood. Here, we show that inhaled chrysotile asbestos fibers cause increased myeloperoxidase activity in bronchoalveolar lavage fluids (BALF) and myeloperoxidase immunoreactivity in epithelial cells lining distal bronchioles and alveolar ducts, sites of initial lung deposition of asbestos fibers. In comparison with sham mice, asbestos-exposed myeloperoxidase-null (MPOÀ/À) and normal (MPO+/+) mice exhibited comparable increases in polymorphonuclear leukocytes, predominately neutrophils, in BALF after 9 days of asbestos inhalation. Differential cell counts on BALF revealed decreased proportions of macrophages and increased lymphocytes in all mice exposed to asbestos, but numbers were decreased overall in asbestosexposed myeloperoxidase-null versus normal mice. Asbestosassociated lung inflammation in myeloperoxidase-null mice was reduced (P V 0.05) in comparison with normal asbestosexposed mice at 9 days. Decreased lung inflammation in asbestos-exposed myeloperoxidase-null mice at 9 days was accompanied by increases (P V 0.05) in Ki-67-and cyclin D1-positive immunoreactive cells, markers of cell cycle reentry, in the distal bronchiolar epithelium. Asbestos-induced epithelial cell proliferation in myeloperoxidase-null mice at 30 days was comparable to that found at 9 days. In contrast, inflammation and epithelial cell proliferation in asbestos-exposed normal mice increased over time. These results support the hypothesis that myeloperoxidase status modulates early asbestos-induced oxidative stress, epithelial cell proliferation, and inflammation.

Airway Epithelial NF-κB Activation Modulates Asbestos-Induced Inflammation and Mucin Production In Vivo

“…We have shown that other survival pathways such as extracellular signal-regulated kinases (ERK1/2 and ERK5) are important mediators of early epithelial cell proliferation by asbestos (27) and it is likely that cross-talk between the ERK/ AP-1 and NF-B signaling pathways occur in our model. Our recent studies also reveal increased expression of cyclin D1 in the distal bronchiolar epithelium of mice inhaling chrysotile asbestos at 9 days (17). Cyclin D1 and its partner, cyclin-dependent kinase 4, are critical in promoting the G 1 -S phase progression via phosphorylation of the retinoblastoma protein, and expression of cyclin D1 is regulated by NF-B in a myoblast model of proliferation and growth (28).…”

“…These time points were selected based on previous work showing that they represented peak times of epithelial cell proliferation, inflammation, and fibrogenesis, respectively, in lungs of chrysotile asbestos-exposed C57BL/6 mice (4,5,17). The chemical and physical characteristics of National Institute of Environmental Health Sciences chrysotile asbestos have been previously described (18), and fibers were endotoxin free as determined by the Limulus amebocyte lysate gel clot assay (Endosage; Charles River Laboratories).…”

“…Ki-67 is a marker of cycling cells undergoing proliferation (17,20). Lung sections were deparaffinized in xylene, rehydrated through graded ethanols and equilibrated in PBS (17).…”

“…Lung sections were deparaffinized in xylene, rehydrated through graded ethanols and equilibrated in PBS (17). Ag retrieval was then performed using a 1/10 dilution in PBS of 10ϫ Dako Target Retrieval Solution (DakoCytomation) in a 95°C water bath for 40 min followed by 20 min cooling to room temperature.…”

“…5B). Because the development of fibrosis was minimal in chrysotile asbestos-exposed mice as has been previously reported (4,5,17) and unlikely to reveal transgene differences, we screened lung homogenates from 40-day mice using a custom-made RPA template for genes linked to fibrogenesis (Fig. 6A).…”