TNFR1/phox interaction and TNFR1 mitochondrial translocation Thwart silica-induced pulmonary fibrosis.

Macrophages play a fundamental role in innate immunity and the pathogenesis of silicosis. Phagocytosis of silica particles is associated with the generation of reactive oxygen species (ROS), secretion of cytokines, such as TNF, and cell death that contribute to silica-induced lung disease. In macrophages, ROS production is executed primarily by activation of the NADPH oxidase (Phox) and by generation of mitochondrial ROS (mtROS); however, the relative contribution is unclear, and the effects on macrophage function and fate are unknown. In this study, we used primary human and mouse macrophages (C57BL/6, BALB/c, and p47(phox-/-)) and macrophage cell lines (RAW 264.7 and IC21) to investigate the contribution of Phox and mtROS to silica-induced lung injury. We demonstrate that reduced p47(phox) expression in IC21 macrophages is linked to enhanced mtROS generation, cardiolipin oxidation, and accumulation of cardiolipin hydrolysis products, culminating in cell death. mtROS production is also observed in p47(phox-/-) macrophages, and p47(phox-/-) mice exhibit increased inflammation and fibrosis in the lung following silica exposure. Silica induces interaction between TNFR1 and Phox in RAW 264.7 macrophages. Moreover, TNFR1 expression in mitochondria decreased mtROS production and increased RAW 264.7 macrophage survival to silica. These results identify TNFR1/Phox interaction as a key event in the pathogenesis of silicosis that prevents mtROS formation and reduces macrophage apoptosis.

Secreted Phosphoprotein 1 and Sex-Specific Differences in Silica-Induced Pulmonary Fibrosis in Mice.

BACKGROUND: Fibrotic lung diseases occur predominantly in males, and reports describe better survival in affected females. Male mice are more sensitive to silica-induced lung fibrosis than silica-treated female mice. Secreted phosphoprotein 1 (SPP1, also known as osteopontin) increases in pulmonary fibrosis, and Spp1 transcription may be regulated by estrogen or estrogen receptor-related receptors. OBJECTIVE: We determined whether differences in silica-induced SPP1 levels contribute to sex differences in lung fibrosis. METHODS: Male and female mice were treated with 0.2 g/kg intratracheal silica, and lung injury was assessed 1, 3, or 14 days post-exposure. Gene-targeted (Spp1-/-) mice, control Spp1+/+ (C57BL/6J) mice, ovariectomized (OVX) female mice, and estrogen-treated male mice were treated with silica, and lung injury was assessed. RESULTS: Silica-induced SPP1 in lung tissue, bronchoalveolar lavage, and serum increased more in male than in female mice. Following silica treatment, bronchoalveolar lavage cell infiltrates decreased in female Spp1-/- mice compared with female Spp1+/+ mice, and lung hydroxyproline decreased in male Spp1-/- mice compared with male Spp1+/+ mice. OVX female mice had increased lung SPP1 expression in response to silica compared with silica-treated sham female mice. Silica-induced lung collagen and hydroxyproline (markers of fibrosis), and SPP1 levels decreased in estrogen-treated males compared with untreated males. CONCLUSION: These findings suggest that sex-specific differences in SPP1 levels contribute to the differential sensitivity of male and female mice to the development of silica-induced fibrosis. CITATION: Latoche JD, Ufelle AC, Fazzi F, Ganguly K, Leikauf GD, Fattman CL. 2016. Secreted phosphoprotein 1 and sex-specific differences in silica-induced pulmonary fibrosis in mice. Environ Health Perspect 124:1199-1207; http://dx.doi.org/10.1289/ehp.1510335.