RESUMO
BACKGROUND: Silicosis features foci of inflammation where macrophages and lymphocytes precede and accompany fibroblast proliferation, alveolar epithelial hyperplasia, and increased deposition of connective tissue matrix material. In the mouse following silica inhalation there is recruitment of natural killer-, B-, and CD4+ and CD8+ lymphocytes to the alveolar spaces, enlargement of bronchial-associated lymphoid tissues (BALT), and aggregation of lymphocytes surrounding small airways and blood vessels. A substantial fraction of the recruited lung lymphocytes produce interferon-gamma (IFN-gamma), and IFN-gamma gene-deleted mice develop less silicosis than wild-type mice. Interleukin-12 (IL-12) is an important pathway for driving the adaptive immune response towards a TH1-like phenotype. We hypothesized that IL-12 might stimulate lymphocyte activation and the up-regulation of IFN-gamma, and consequently be an essential mediator for silicosis. RESULTS: C57Bl/6 wild-type (WT) and IL-12 deficient (IL-12 KO) mice were exposed to sham-air or crystobalite silica (61 mg/m3) by inhalation for 5 hours/day for 12 days and then studied from 1 to 112 days after exposure. Mice exposed to sham-air had normal lung histology at all time points. WT mice exposed to titanium dioxide (72 mg/m3) showed pulmonary macrophage recruitment but no increase in lung collagen. Both WT and IL-12 KO mice exposed to silica showed similar progressive lung pathology, increased wet lung weight and increased total lung collagen (hydroxyproline). IL-12 p35 mRNA was not increased in either strain after silica exposure; IL-12 p40 mRNA was up-regulated after silica in WT mice and constitutively absent in the IL-12 KO mice. IL-18 mRNA was not increased after silica exposure. The expression of IL-15 (an important driver for innate immunity, Natural Killer cell activation, and IFN-gamma production) was abundant in air-exposed mice and was increased slightly in the lungs of mice with silicosis. CONCLUSION: The axis of IL-12 driving IFN-gamma production is not essential for the full manifestations of silicosis in mice exposed to a crystobalite silica aerosol.
RESUMO
Nitration of proteins by peroxynitrite (ONOO-) has been shown to critically alter protein function in vitro. We have shown previously that asbestos inhalation induced nitrotyrosine formation, a marker of ONOO- production, in the rat lung. To determine whether asbestos-induced protein nitration may affect mitogen-activated protein kinase (MAPK) signaling pathways, lung lysates from crocidolite and chrysotile asbestos-exposed rats and from sham-exposed rats were immunoprecipitated with anti-nitrotyrosine antibody, and captured proteins were subjected to Western blotting with anti-phospho-extracellular signal-regulated kinase (ERK)1/2 antibodies. Both types of asbestos inhalation induced significantly greater phosphorylation of ERK1/2 in rat lung lysates than was noted after sham exposure. Phosphorylated ERK proteins co-immunoprecipitated with nitrotyrosine. Moreover, in MAPK functional assays using Elk-1 substrate, immunoprecipitated phospho-ERK1/2 in lung lysates from both crocidolite-exposed and chrysotile-exposed rats demonstrated significantly greater phosphorylation of Elk-1 than was noted after sham exposure. Asbestos inhalation also induced ERK phosphorylation in bronchoalveolar lavage cells. Lung sections from rats exposed to crocidolite or chrysotile (but not from sham-exposed rats nor from rats exposed to "inert" carbonyl iron particles) demonstrated strong immunoreactivity for nitrotyrosine and phospho-ERK1/2 in alveolar macrophages and bronchiolar epithelium. These findings suggest that asbestos fibers may activate the ERK signaling pathway by generating ONOO- or other nitrating species that induce tyrosine nitration and phosphorylation of critical signaling molecules.
