Background Quartz dust cannot be degraded in the lungs, and inhalation of a large amount of quartz dust in the occupational production process will lead to the occurrence of pulmonary fibrosis, and then develop into silicosis. In recent years, studies have found that exosomes may be involved in the pathogenesis of fibrotic diseases by carrying microribonucleic acid (miRNA), but the mechanism of their actions in silicosis still needs to be studied.

Objective To investigate the role of exosome-derived miRNA-21-5p/mothers against decapentaplegic homolog 7 (Smad7) in quartz dust-induced pulmonary fibrosis in rats.

Methods Twenty-four healthy male SD rats were randomly divided into four groups (six rats in each group): control 4-week group, control 16-week group, quartz 4-week group, and quartz 16-week group. At the beginning of the experiment, 1 mL of quartz suspension (50 mg·mL⁻¹) and 1 mL of normal saline were injected into the trachea of rats in the quartz group and the control group, respectively, by means of one-time non-exposure intratracheal dust staining. Alveolar lavage was performed at the 4th and 16th weeks after dust staining, the exosomes in lavage solution were extracted by polyethylene glycol (PEG) precipitation, morphological identification was conducted by transmission electron microscopy (TEM), particle size of exosomes was detected by nano-tracking analysis (NTA), and the marker proteins CD9 and CD63 of exosomes were detected by Western blotting (WB). The expression of miRNA-21-5p in exosomes was determined by reverse transcription polymerase chain reaction (RT-PCR). The degree of lung tissue injury and fibrosis was observed by hematoxylin-eosin staining (HE) and Masson staining. The collagen content of lung tissue was detected by hydroxyproline (HYP) method. The expression of Smad7 protein in lung tissue was detected by WB.

Results The results of pathological staining showed that compared with the control group, lung inflammatory cell infiltration, alveolar wall thickening, and collagen increase were observed after 4 weeks of dusting, and collagen deposition and silicon nodules appeared after 16 weeks of dusting. Compared with the control group, the expression level of HYP in the lung tissue of the quartz group was increased after 4 weeks and 16 weeks of dust staining (P<0.05). Transmission electron microscopy showed that exosomes were saucer-shaped, and the average particle size of exosomes was 95.8 nm by NTA. Positive expression of exosome marker proteins CD9 and CD81 was found by WB. Compared with the control group, the expression of exosome-derived miRNA-21-5p in alveolar lavage fluid in the quartz group increased in the 4th week and the 16th week (P<0.05), and the expression of Smad7 protein in lung tissue decreased (P<0.05).

Conclusion Exosome-derived miRNA-21-5p and Smad7 may be involved in the mechanism of quartz dust-induced pulmonary fibrosis in rats.