Mechanism research of baicalein in improving alveolar epithelial cell senescence via downregulating TGF-β1/Smad3 signal

This study aims to investigate the effect and mechanism of baicalein in improving alveolar epithelial cell senescence. A cellular model of alveolar epithelial cell senescence induced by transforming growth factor-β1(TGF-β1) was established. Cells were randomly divided into control group, model group, baicalein group, small interfering RNA(siRNA) targeting Smad family member 2(Smad2) group, siRNA targeting Smad3 group, and baicalein + Smad3 plasmid group. Western blot was employed to detect the protein expression level of plasminogen activator inhibitor-1(PAI-1), a marker protein of alveolar epithelial cell senescence. Senescence-associated β-galactosidase(SA-β-gal) staining was used to measure the percentage of SA-β-gal-positive cells. The results show that baicalein can significantly inhibit the protein expression of PAI-1 in TGF-β1-induced alveolar epithelial cells and markedly reduce the percentage of SA-β-gal-positive cells in TGF-β1-induced alveolar epithelial cells. Baicalein can also significantly decrease the phosphorylation levels of Smad2 protein and Smad3 protein in TGF-β1-induced alveolar epithelial cells. Knockdown of Smad2 does not affect the protein expression level of PAI-1 in TGF-β1-induced alveolar epithelial cells, nor does it alter the percentage of SA-β-gal-positive cells in TGF-β1-induced alveolar epithelial cells. In contrast, knockdown of Smad3 can significantly downregulate the protein expression level of PAI-1 and reduce the percentage of SA-β-gal-positive cells in TGF-β1-induced alveolar epithelial cells. Overexpression of Smad3 can notably reverse the inhibitory effects of baicalein on PAI-1 protein expression and the percentage of SA-β-gal-positive cells in TGF-β1-induced alveolar epithelial cells. In conclusion, baicalein improves alveolar epithelial cell senescence by downregulating the TGF-β1/Smad3 signaling, but this effect might be independent of Smad2 signaling.