Involvement of let-7c in alveolar macrophage M2 polarization in coal mine dust-induced epithelial-mesenchymal transition in rat lung tissue

Background Coal workers' pneumoconiosis (CWP) is a serious occupational lung disease and one of the prescript occupational diseases in China. Epithelial-mesenchymal transition (EMT) is involved in the diffuse fibrosis of lung tissue of pneumoconiosis patients, and its mechanism may be related to the polarization of macrophages regulated by let-7c.

Objective To investigate the effect of let-7c on the regulation of macrophage polarization in EMT in rats induced by coal dust exposure with different content of free SiO₂.

Methods SD rats were randomly divided into a control group, a 5% SiO₂ group, a 30% SiO₂ group, and a 99.9% SiO₂ group, with 16 rats in each group. The rats in each group were tracheally titrated with 100 μL of 20 mg·mL⁻¹ suspension (5% SiO₂, 30% SiO₂, and 99.9% SiO₂) or normal saline, respectively. Alveolar lavage fluid was collected at the ends of the 1st month and the 3rd month. The relative expression levels of M1 or M2 markers, CD86 or CD206, in alveolar macrophages (AMs) were detected by immunofluorescence. The inflammation and fibrosis of lung tissue were observed by hematoxylin-eosin (HE) staining and Masson staining. The expression levels of transforming growth factor-β1 (TGF-β1), E-cadherin, and vimentin were detected by Western blotting. The expression levels of let-7c and c/EBP-δ genes were detected by real-time fluorescence quantitative PCR.

Results The HE and Masson staining results showed that compared with the control group, the degree of pulmonary fibrosis in the 5% SiO₂ group, the 30% SiO₂ group, and the 99.9% SiO₂ group gradually increased with the increase of dust exposure time. Compared with the control group, the expressions of CD86 and CD206 in the 5% SiO₂ group, the 30% SiO₂ group, and the 99.9% SiO₂ group gradually increased at the end of the 1st month (F=330.904, 146.801, P<0.05), and the expression of CD86 in each group decreased gradually at the end of the 3rd month (F=331.781, P<0.05), but the expression of CD206 increased (F=1164.190, P<0.05). At the end of the 1st month, the expressions of TGF-β1 (F=8.847, P<0.05) and vimentin (F=13.275, P<0.05) gradually increased, and the expression of E-cadherin (F=6.253, P<0.05) gradually reduced in the 5% SiO₂ group, the 30% SiO₂ group, and the 99.9% SiO₂ group. At the end of the 3rd month, the expressions of TGF-β1 (F=16.833, P<0.05) and vimentin (F=55.021, P<0.05) increased, and the expression of E-cadherin (F=12.790, P<0.05) gradually decreased in all groups. The PCR results showed that compared with the control group, the expression of let-7c mRNA in the 5% SiO₂ group, the 30% SiO₂ group, and the 99.9% SiO₂ group increased at the ends of the 1st month and the 3rd month (F=11.251, 28.136, P<0.05). The expression of c/EBP-δ mRNA decreased in all groups at the ends of the 1st month and the 3rd month (F=49.204, 177.090, P<0.05).

Conclusion In response to dust stimulation, let-7c promotes EMT by modulating macrophage polarization, which is involved in the formation of pulmonary fibrosis and thus influences the progression of CWP .