Background Pulmonary fibrosis currently lacks screening and diagnostic methods in the early stages and effective treatments in the later stages, so there is an urgent need to explore the mechanisms and develop targeted treatments.

Objective To screen the expression of differentially expressed circular RNA (circRNA) hsa_circUCK2 under pathological conditions and to explore its effect on pulmonary fibrosis.

Methods In the cell-based experiments, hsa_circUCK2 was knocked down in HPF-a cells using small interfering RNA (siRNA), and HPF-a cells were stimulated by TGF-β1. Four groups were set up: si-NC group, si-circUCK2 group, si-NC+TGF-β1-treated group, and si-circUCK2+TGF-β1-treated group. Western blot assay was used to detect the expression of fibronectin (FN1) in HPF-a cells of the four groups, scratch assay was used to detect the migration ability of HPF-a cells, and CCK-8 assay was used to detect the proliferation ability of HPF-a cells in the two groups with TGF-β1 stimulation, the si-NC+TGF-β1-treated group and the si-circUCK2+TGF-β1-treated group. In the animal experiments, forty-eighty healthy SPF-grade male C57BL/6 mice were randomly divided into four groups: saline+si-con group, saline+si-circ_0000115 group, SiO₂+si-con group, and SiO₂+si-circ_0000115 group. Mouse lung circRNA mmu_circ_0000115 (mouse homolog of hsa_circUCK2) was knocked down by tracheal drip injection of siRNA, and a mouse lung fibrosis model was constructed by tracheal drip injection of SiO₂ suspension (0.2 g·kg⁻¹, 50 mg·mL⁻¹) after 48 h. Real-time fluorescence quantitative PCR was used to detect the knockout efficiency in each organ of the mouse, Western blot was applied to detect the expression of type I collagen α2 (COL1A2) in the lung tissues, and Sirius red was used to detect collagen synthesis in the lung tissues.

Results In the cell-based experiments, after the knockdown of hsa_circUCK2, the Western blot results showed that the expression level of the FN1 protein in TGF-β1-stimulated HPF-a cells was significantly down-regulated (P <0.05); the CCK-8 assay and cell scratch assay showed that the down-regulation of hsa_circUCK2 gene significantly inhibited the proliferation and migration of HPF-a cells (P<0.01). In the animal experiments, the real-time fluorescence quantitative PCR results showed that among the detected organs, mmu_circ_0000115 was significantly knocked down only in the lung tissues (P<0.0001); the Western blot results showed that knocking down mmu_circ_0000115 significantly reduced the COL1A2 protein expression level when compared with the SiO₂+si-con group (P<0.0001); the Sirius red results showed that knocking down mmu_circ_0000115 significantly reduced collagen production and deposition in lung tissues of mice in the model group.

Conclusion Knockdown of hsa_circUCK2 inhibits fibroblast activation and reduces collagen deposition in lung fibrosis model mice. It is suggested that the hsa_circUCK2 is involved in the process of pulmonary fibrosis and may be a potential therapeutic target for pulmonary fibrosis.