Background
The mechanisms of silicon dioxide (SiO2)-induced inflammation and cell injury in pulmonary macrophages are not fully characterized.
Objective
To investigate the potential roles of inhibition of toll-like receptor 4 (TLR4)/nuclear factor-kappa B (NF-κB) signaling in inflammation and macrophage polarization in mouse Raw264.7 cells in response to SiO2 stimulation.
Methods
Sixteen 6- to 8-week-old C57BL/6 mice, half male and half female, were intratracheally instilled with 50 µL of SiO2 (50 mg·mL−1 in saline) or normal saline via oropharyngeal route, and the lungs of mice were harvested at 14 d and 28 d post the first challenge of SiO2. HE staining of mouse lung was used for histopathological analysis. The expressions of TLR4 signaling-related proteins were detected by Western blotting (WB) and immunofluorescent (IF) assay, including TLR4, myeloid differentiation factor 88 (Myd88), and TNF receptor associated factor 6 (TRAF6). Raw264.7 cells were stimulated with SiO2 (100 μg·cm2) for 12 h in absence or presence of TLR4 inhibitor M62812 for 13 h before the culture supernatants and cell lysates were harvested for analysis. The expressions of key components of TLR4 signaling cascade including TLR4, Myd88, and phosphorylated nuclear factor-kappa B P65 (P-NF-κB P65), P-1NF-kappa-B inhibitor α (P-1κbα), tumor necrosis factor-α (TNF-α), and interleukin 6 (IL-6), M1 phenotype markers inducible nitric oxide synthase (iNOS) and cluster of differentiation 86 (CD86), as well as M2 phenotype arginase-1 (Arg-1) were accessed by WB and IF. The expressions of inflammation factors IL-6 and TNF-α in supernatants were determined by enzyme-linked immunosorbent assay (ELISA).
Results
After SiO2 intratracheal instillation for 14 d, the HE staining results showed obvious fibrotic nodules in the lung tissues of mice. The results of WB analysis revealed more abundant TLR4, Myd88, and TRAF6 in the silicosis mouse lung samples than in the controls. The results of IF assay showed an increased abundance of TLR4 and Myd88 proteins in the lung samples of silicosis mice at 14 d post the silica challenge, compared to the controls, indicating TLR4 signaling activation. As seen in the in vitro experiment, significant upregulations after the exposure to 100 μg·cm2 SiO2 were observed in TLR4 and P-1κbα at 6, 12, and 24 h (P<0.05); Myd88 at 12 and 24 h (P <0.05); and P-NF-κB P65 at 12 h ( P<0.05). The inhibitor significantly suppressed the expressions of TLR4, Myd88, TRAF6, P-NF-κB P65, TNF-α, and IL-6 in Raw264.7 cells. In addition, the SiO2-induced M1 phenotype marker iNOS was significantly suppressed, but the M2 phenotype marker Arg-1 was increased in the Raw264.7 cells.
Conclusion
The inhibition of TLR4/NF-κB signaling could result in a reduction of the inflammation response and the transition of M1 toward M2 phenotypes of macrophages in response to SiO2 challenge.
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