Troglitazone Attenuates TGF-β1-Induced EMT in Alveolar Epithelial Cells via a PPARγ-Independent Mechanism

Peroxisome proliferator activated receptor γ (PPARγ) 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 PPARγ agonist, on transforming growth factor (TGF)-β1-induced EMT in primary rat AEC and an alveolar epithelial type II (AT2) cell line (RLE-6TN). TGF-β1 (2.5 ng/mL) induced EMT in both cell types, as evidenced by acquisition of spindle-like morphology, increased expression of the mesenchymal marker α-smooth muscle actin (α-SMA) and downregulation of the tight junctional protein zonula occludens-1 (ZO-1). Concurrent treatment with troglitazone (or rosiglitazone), ameliorated effects of TGF-β1. Furthermore, following stimulation with TGF-β1 for 6 days, troglitazone reversed EMT-related morphological changes and restored both epithelial and mesenchymal markers to control levels. Treatment with GW9662 (an irreversible PPARγ antagonist), or overexpression of a PPARγ dominant negative construct, failed to inhibit these effects of troglitazone in AEC. Troglitazone not only attenuated TGF-β1-induced phosphorylation of Akt and glycogen synthase kinase (GSK)-3β, but also inhibited nuclear translocation of β-catenin, phosphorylation of Smad2 and Smad3 and upregulation of the EMT-associated transcription factor SNAI1. These results demonstrate inhibitory actions of troglitazone on TGF-β1-induced EMT in AEC via a PPARγ-independent mechanism likely through inhibition of β-catenin-dependent signaling downstream of TGF-β1, supporting a role for interactions between TGF-β and Wnt/β-catenin signaling pathways in EMT.