Abstract
A high-throughput small-molecule screen was conducted to identify inhibitors of epithelial-mesenchymal transition (EMT) that could be used as tool compounds to test the importance of EMT signaling in vivo during fibrogenesis. Transforming growth factor (TGF)-β1-induced fibronectin expression and E-cadherin repression in A549 cells were used as 48-hour endpoints in a cell-based imaging screen. Compounds that directly blocked Smad2/3 phosphorylation were excluded. From 2,100 bioactive compounds, methacycline was identified as an inhibitor of A549 EMT with the half maximal inhibitory concentration (IC50) of roughly 5 μM. In vitro, methacycline inhibited TGF-β1-induced α-smooth muscle actin, Snail1, and collagen I of primary alveolar epithelial cells . Methacycline inhibited TGF-β1-induced non-Smad pathways, including c-Jun N-terminal kinase, p38, and Akt activation, but not Smad or β-catenin transcriptional activity. Methacycline had no effect on baseline c-Jun N-terminal kinase, p38, or Akt activities or lung fibroblast responses to TGF-β1. In vivo, 100 mg/kg intraperitoneal methacycline delivered daily beginning 10 days after intratracheal bleomycin improved survival at Day 17 (P < 0.01). Bleomycin-induced canonical EMT markers, Snail1, Twist1, collagen I, as well as fibronectin protein and mRNA, were attenuated by methacycline (Day 17). Methacycline did not attenuate inflammatory cell accumulation or alter TGF-β1-responsive genes in alveolar macrophages. These studies identify a novel inhibitor of EMT as a potent suppressor of fibrogenesis, further supporting the concept that EMT signaling is important to lung fibrosis. The findings also provide support for testing the impact of methacycline or doxycycline, an active analog, on progression of human pulmonary fibrosis.