Figure 8. Schematic illustration showing novel evidence for inhibitory effect of pirfenidone on MUC1-CT bioactivation in IPF.

TGF-β1 binds to TbRI/II, leading to the recruitment and phosphorylation of SMAD3. Phosphorylated SMAD3 promotes the phosphorylation of MUC1-CT at Thr⁴¹ and Tyr⁴⁶, thus increasing the active form of β-catenin. MUC1-CT forms protein complexes with phospho (p)-SMAD3 and active (act)-β-catenin in response to TGF-β1; this stimulus promotes nuclear translocation of the phospho-SMAD3/MUC1-CT/act-β-catenin complex, which is required to activate the SMAD-binding element (SBE) DNA sequence and in turn promote pro-fibrotic gene expression, proliferation or cell senescence. Pirfenidone inhibits the TGF-β1-induced Thr⁴¹ (1224) and Tyr⁴⁶ (1229) MUC1-CT phosphorylations, thus reducing the amount of act-β-catenin. Therefore, the nuclear translocation of the p-SMAD3/MUC1-CT/act-β-catenin complex and induction of SBE is also inhibited by pirfenidone. Additionally, we suggest that pirfenidone also inhibits the MUC1-CT phosphorylation induced by galectin-3 and growth factors such as epidermal growth factor (EGF), fibroblast growth factor (FGF) and platelet derived growth factor (PDGF).