Peroxisome Proliferator-Activated Receptor-γ Activation Inhibits Tumor Metastasis by Antagonizing Smad3-Mediated Epithelial-Mesenchymal Transition

Epithelial-mesenchymal transition (EMT) was shown to confer tumor cells with abilities essential for metastasis, including migratory phenotype, invasiveness, resistance to apoptosis, evading immune surveillance, and tumor stem cell traits. Therefore, inhibition of EMT can be an important therapeutic strategy to inhibit tumor metastasis. Here, we show that activation of peroxisome proliferator-activated receptor gamma (PPAR-gamma) inhibits transforming growth factor beta (TGF-beta)-induced EMT in lung cancer cells and prevents metastasis by antagonizing Smad3 function. Activation of PPAR-gamma by synthetic ligands (troglitazone and rosiglitazone) or by a constitutively active form of PPAR-gamma prevents TGF-beta-induced loss of E-cadherin expression and inhibits the induction of mesenchymal markers (vimentin, N-cadherin, fibronectin) and matrix metalloproteases. Consistently, activation of PPAR-gamma also inhibited EMT-induced migration and invasion of lung cancer cells. Furthermore, effects of PPAR-gamma ligands were attenuated by siRNA-mediated knockdown of PPAR-gamma, indicating that the ligand-induced responses are PPAR-gamma dependent. Selective knockdown of Smad2 and Smad3 by siRNA showed that TGF-beta-induced EMT is Smad3 dependent in lung cancer cells. Activation of PPAR-gamma inhibits TGF-beta-induced Smad transcriptional activity but had no effect on the phosphorylation or nuclear translocation of Smads. Consistently, PPAR-gamma activation prevented TGF-beta-induced transcriptional repression of E-cadherin promoter and inhibited transcriptional activation of Ncadherin promoter. Finally, treatment of mice with troglitazone or knockdown of Smad3 in tumor cells significantly inhibited TGF-beta-induced experimental metastasis in SCID-Beige mice. Together, with the low toxicity profile of PPAR-gamma ligands, our data show that these ligands may serve as potential therapeutic agents to inhibit metastasis. Mol Cancer Ther; 9(12); 3221-32. (C) 2010 AACR.