TTC3 contributes to TGF-β1-induced epithelial-mesenchymal transition and myofibroblast differentiation, potentially through SMURF2 ubiquitylation and degradation

Transforming Growth Factor-β (TGF-β) acts as a key cytokine in epithelial-mesenchymal transition (EMT) and myofibroblast differentiation, which are important for normal tissue repair and fibrotic diseases. Ubiquitylation and proteasomal degradation of TGF-β signaling proteins acts as a regulatory mechanism for the precise control of TGF-β signaling. SMAD-specific ubiquitin E3 ligase (SMAD ubiquitination regulatory factor 2, SMURF2) controls TGF-β signaling proteins including the TGF-β Receptor (TGFR) and SMAD2/3. Here, we report that tetratricopeptide repeat domain 3 (TTC3), a ubiquitin E3 ligase, positively regulates TGF-β₁-induced EMT and myofibroblast differentiation, through inducing ubiquitylation and proteasomal degradation of SMURF2. In human bronchial epithelial cells (BEAS-2B) and normal human lung fibroblasts, TTC3 knockdown suppressed TGF-β₁-induced EMT and myofibroblast differentiation, respectively. Similarly, when TTC3 expression was suppressed, the TGF-β₁-stimulated elevation of p-SMAD2, SMAD2, p-SMAD3, and SMAD3 were inhibited. In contrast, overexpression of TTC3 caused both EMT and myofibroblast differentiation in the absence of TGF-β₁ treatment. TGF-β₁ reduced SMURF2 levels and TTC3 overexpression led to a further decrease in SMURF2 levels, while TTC3 knockdown inhibited TGF-β₁-induced SMURF2 reduction. In cell and in vitro ubiquitylation assays demonstrated TTC3-mediated SMURF2 ubiquitylation, and coimmunoprecipitation assays established the binding between SMURF2 and TTC3. TGF-β₁-induced TTC3 expression was inhibited by the knockdown of SMAD2 and SMAD3. Finally, Ttc3 mRNA levels were significantly increased and Smurf2 protein levels were significantly decreased in the lungs of mice treated with bleomycin as compared with the lungs of control mice. Collectively, these data suggest that TTC3 may contribute to TGF-β₁-induced EMT and myofibroblast differentiation, potentially through SMURF2 ubiquitylation/proteasomal degradation and subsequent inhibition of SMURF2-mediated suppression of SMAD2 and SMAD3, which in turn induces TTC3 expression.