Abstract

This study investigated the mechanism of TGF-β1-induced Nox4 expression in pulmonary fibrosis (PF) and the anti-fibrotic effects of Tanshinone IIA (Tan-IIA). In a bleomycin-induced pulmonary fibrosis mouse model and in TGF-β1-stimulated fibroblasts, Tan-IIA attenuated fibrosis, oxidative stress, and fibroblast activation. Pharmacological inhibition revealed that the JNK/c-Jun and Smad3 pathways cooperatively mediate TGF-β1-induced expression of Nox4 and fibrotic markers (Collagen I/III, α-SMA). Tan-IIA exerted these effects by dually inhibiting the JNK/c-Jun and Smad2/3 pathways, reducing their phosphorylation and nuclear signaling, which consequently suppressed Nox4 transcription and protein expression. The combination of Tan-IIA with JNK or Smad3 inhibitors synergistically enhanced these effects. We identified a tandem c-Jun/Smad binding element in the Nox4 promoter that is critical for TGF-β1 response. Reporter assays and CUT&RUN experiments confirmed that TGF-β1-induced transcriptional activation depends on an intact c-Jun/Smad binding element and recruitment of c-Jun and Smad2/3. Moreover, Tan-IIA inhibited the enrichment of c-Jun and Smad2/3 at the Nox4 promoter. Collectively, our findings demonstrate that a c-Jun/Smad element integrates profibrotic JNK and Smad signaling to drive Nox4 expression. Tan-IIA presents a novel therapeutic strategy for fibrosis by simultaneously targeting these two key pathways, thereby mitigating Nox4-dependent oxidative stress and fibroblast activation.