Emerging evidence suggests that the transforming growth factor (TGF)-β1-induced epithelial-mesenchymal transition (EMT) of alveolar epithelial cells (AEC) may contribute to airway remodeling in severe asthma and fibrotic lung diseases. Studies have shown that extracts from propolis protect chemical-induced cardiac and liver fibrosis in animals. This study assesses the inhibitory effect of propolis on TGF-β1-induced EMT in serum-deprived A549 cells (human AECs). Experimental results show progressive cell morphological changes, decreased E-cadherin, increased N-cadherin production, intracellular F-actin rearrangement, increased reactive oxygen species (ROS) production, and increased cell motility with increasing TGF-β1 concentration. A549 cells pretreated with propolis and then treated with TGF-β1 for 24 h regained epithelial cell morphology, decreased the production of N-cadherin and ROS, and had reduced motility. Propolis prevents the effects of TGF-β1-induced Smad2 and AKT activation pathways and Snail expression. Moreover, propolis pretreatment may prevent the TGF-β1-induced down-regulation of nuclear hormone receptors and peroxisome proliferator-activated receptor gamma (PPARγ) protein in A549 cells, whose effect was blocked by adding PPARγ antagonist, GW9662. Two active components of propolis, caffeic acid phenethyl ester (CAPE) and pinocembrin (PIN), only had partial effects on TGF-β1-induced EMT in A549 cells. The results of this study suggest that natural propolis extracts may prevent TGF-β1-induced EMT in immortalized type II AECs via multiple inhibitory pathways, which may be clinically applied in the prevention and/or treatment of EMT-related fibrotic diseases as well as airway remodeling in chronic asthma.
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