Troglitazone Attenuates TGF-b1-Induced EMT in Alveolar Epithelial Cells via a PPARc-Independent Mechanism

Peroxisome proliferator activated receptor c (PPARc) agonists are effective antifibrotic agents in a number of tissues. Effects of these agents on epithelial-mesenchymal transition (EMT) of primary alveolar epithelial cells (AEC) and potential mechanisms underlying effects on EMT have not been well delineated. We examined effects of troglitazone, a synthetic PPARc agonist, on transforming growth factor (TGF)-b1-induced EMT in primary rat AEC and an alveolar epithelial type II (AT2) cell line (RLE-6TN). TGF-b1 (2.5 ng/mL) induced EMT in both cell types, as evidenced by acquisition of spindle-like morphology, increased expression of the mesenchymal marker a-smooth muscle actin (a-SMA) and downregulation of the tight junctional protein zonula occludens-1 (ZO-1). Concurrent treatment with troglitazone (or rosiglitazone), ameliorated effects of TGF-b1. Furthermore, following stimulation with TGF-b1 for 6 days, troglitazone reversed EMT-related morphological changes and restored both epithelial and mesenchymal markers to control levels. Treatment with GW9662 (an irreversible PPARc antagonist), or overexpression of a PPARc dominant negative construct, failed to inhibit these effects of troglitazone in AEC. Troglitazone not only attenuated TGF-b1-induced phosphorylation of Akt and glycogen synthase kinase (GSK)-3b, but also inhibited nuclear translocation of b-catenin, phosphorylation of Smad2 and Smad3 and upregulation of the EMT-associated transcription factor SNAI1. These results demonstrate inhibitory actions of troglitazone on TGF-b1-induced EMT in AEC via a PPARc-independent mechanism likely through inhibition of b-catenin-dependen