Mechanism of osteoclast stimulatory transmembrane protein promoting silicosis fibrosis by inducing ferroptosis

Background Osteoclast stimulatory transmembrane protein (OC-STAMP) is involved in silicosis fibrosis induced by silicon oxide (SiO₂) exposure. Its role in silicosis fibrosis by inducing ferroptosis of alveolar type II epithelial cells and its related mechanism remain unclear.

Objective To explore the effect and possible mechanism of OC-STAMP on ferroptosis of alveolar type II epithelial cells and silicosis fibrosis in rats under SiO₂ exposure.

Methods Twenty male Wistar rats of SPF grade were randomly divided into two groups: control (Sham) group and SiO₂ group, 15 rats in each group. Rats in the SiO₂ group were given 1 mL of 50 mg·L⁻¹ SiO₂ suspension at one time through the non-exposed intratracheal instillation method to establish an animal model of silicosis, and rats in the Sham group were give 1 mL of 0.9% sodium chloride solution in the same way. Rats were sacrificed after 8 weeks. Samples of lung tissue were fixed in glutaraldehyde or paraformaldehyde for observing ultrastructure of mitochondria by transmission electron microscopy; HE, Masson, VG, and Prussian blue were used to observe changes in lung tissue structure and iron deposition. The expression level of OC-STAMP and the degree of lung fibrosis were evaluated by immunohistochemistry and immunofluorescence. The expression level of OC-STAMP in rat lung tissue was detected and the transfection effect of OC-STAMP was verified by real-time fluorescence quantitative polymerase chain reaction (RT-PCR). Overexpression (OCS group) and inhibition expression (SI-OC group) models were constructed by OC-STAMP plasmid and OC-STAMP small interfering RNA (siRNA) transfection to cultured MLE-12 cells, respectively. The relative expression levels of glutathione peroxidase 4 (GPX4), solute carrier family 7 member 11 (SLC7A11), and other proteins in lung tissue and MLE-12 were detected by Western blotting.

Results The results of HE, Masson, and VG staining showed that the silicosis modeling was successful after 8 weeks of SiO₂ exposure. The immunofluorescence results showed that OC-STAMP and ATP binding cassette subfamily A member 3 (ABCA3) co-localized in alveolar type II epithelium. The immunohistochemical results showed that the levels of OC-STAMP and collagen I in the SiO₂ group were significantly higher than those in the Sham group (P<0.01). The RT-PCR results showed that the OC-STAMP mRNA in the lung tissue of the SiO₂ group was significantly higher than that of the Sham group (P<0.01). The Prussian blue staining in the lung tissue of the SiO₂ group showed positive brownish-yellow particles. Compared with the Sham group which showed normal mitochondrial structure, the mitochondrial structure was generally swollen and the mitochondrial cristae dissolved and disappeared in the SiO₂ group by transmission electron microscope observation. The Western blotting results showed that the expression levels of SLC7A11 and GPX4 both decreased in the lung tissue of the SiO₂ group (P<0.05, P<0.01), and the expression level of Vimentin increased (P<0.01). In the transfected MLE-12 cells, compared with the Sham group, the expression levels of SLC7A11 and GPX4 in the OCS group were significantly reduced (P<0.05, P<0.01).

Conclusion OC-STAMP may affect the expression of proteins related to ferroptosis, and promote lung fibrosis induced by SiO₂ exposure.