Circular RNA circSmad4 controls pulmonary fibrosis

Pulmonary fibrosis is a progressive and irreversible lung disease characterized by excessive fibroblast activation and extracellular matrix (ECM) deposition, leading to respiratory failure. Despite recent advances in understanding its molecular mechanisms, effective therapies remain limited. Circular RNAs have emerged as key regulators of gene expression, yet their role in pulmonary fibrosis is poorly understood. Here, we investigated circSmad4 and its therapeutic potential. Our results demonstrate that circSmad4 expression was markedly upregulated in bleomycin-induced pulmonary fibrosis, suggesting a role in fibrotic progression. Silencing circSmad4 by siRNA significantly alleviated lung fibrosis, reducing lung weight, collagen deposition, and inflammatory cytokine expression. Mechanistically, we identified that circSmad4 exerted its pro-fibrotic effects through the miR-671-5p/Fgfr2 axis, suppressing miR-671-5p and increasing Fgfr2 expression, thereby enhancing fibroblast activation. Additionally, si-circSmad4 treatment also downregulated pro-inflammatory cytokines (IL-6, TNF-α, and TGF-β1) and inhibited ECM protein expression. Furthermore, in vitro experiments using TGF-β1-induced fibroblast activation models showed that circSmad4 knockdown mitigated fibroblast activation by lowering the expression of fibrosis-related genes (Acta2, Col1a1, Col3a1, Ctgf) and collagen secretion. Consistently, pharmacological inhibition of FGFR2 with FGFR2-IN-1 also suppressed the pro-fibrotic effects of TGF-β1, mimicking si-circSmad4. These findings suggest that circSmad4 functions as a central regulator of pulmonary fibrosis by modulating fibroblast activation, ECM deposition, and inflammation. In conclusion, circSmad4 represents a novel driver of pulmonary fibrosis, and targeting circSmad4 may offer a promising therapeutic strategy.