Andrographolide attenuates epithelial‐mesenchymal transition induced by TGF‐β1 in alveolar epithelial cells

Andrographolide (Andro), a component from Chinese medicinal herb Andrographis paniculata, could alleviate pulmonary fibrosis in rodents. Yet, whether and how Andro mitigates epithelial‐mesenchymal transition (EMT) induced by TGF‐β1 remain unknown. This study aimed to explore the effect of Andro on TGF‐β1‐induced EMT in human alveolar epithelial cells (AECs) and the mechanisms involved. We illustrated that Andro inhibited TGF‐β1‐induced EMT and EMT‐related transcription factors in alveolar epithelial A549 cells. Andro also reduced TGF‐β1‐induced cell migration and synthesis of pro‐fibrotic factors (ie CCN‐2, TGF‐β1), matrix metalloproteinases (ie MMP‐2, MMP‐9) and extracellular matrix (ECM) components (ie collagen 1), implying the inhibiting effect of Andro on TGF‐β1‐induced EMT‐like cell behaviours. Mechanistically, Andro treatment not only repressed TGF‐β1‐induced Smad2/3 phosphorylation and Smad4 nuclear translocation, but also suppressed TGF‐β1‐induced Erk1/2 phosphorylation and nuclear translocation in A549 cells. And treatment with ALK5 inhibitor (SB431542) or Erk1/2 inhibitors (SCH772984 and PD98059) remarkably reduced EMT evoked by TGF‐β1. In addition, Andro also reduced TGF‐β1‐induced intracellular ROS generation and NOX4 expression, and elevated antioxidant superoxide dismutase 2 (SOD2) expression, demonstrating the inhibiting effect of Andro on TGF‐β1‐induced oxidative stress, which is closely linked to EMT. Furthermore, Andro remarkably attenuated TGF‐β1‐induced down‐regulation of sirtuin1 (Sirt1) and forkhead box O3 (FOXO3), implying that Andro protects AECs from EMT partially by activating Sirt1/FOXO3‐mediated anti‐oxidative stress pathway. In conclusion, Andro represses TGF‐β1‐induced EMT in AECs by suppressing Smad2/3 and Erk1/2 signalling pathways and is also closely linked to the activation of sirt1/FOXO3‐mediated anti‐oxidative stress pathway.