SMAD6 contributes to patient survival in non-small cell lung cancer and its knockdown reestablishes TGF-β homeostasis in lung cancer cells

The malignant transformation in several types of cancer, including lung cancer, results in a loss of growth inhibition by transforming growth factor-β (TGF-β). Here, we show that SMAD6 expression is associated with a reduced survival in lung cancer patients. Short hairpin RNA (shRNA)-mediated knockdown of SMAD6 in lung cancer cell lines resulted in reduced cell viability and increased apoptosis as well as inhibition of cell cycle progression. However, these results were not seen in Beas2B, a normal bronchial epithelial cell line. To better understand the mechanism underlying the association of SMAD6 with poor patient survival, we used a lentivirus construct carrying shRNA for SMAD6 to knock down expression of the targeted gene. Through gene expression analysis, we observed that knockdown of SMAD6 led to the activation of TGF-β signaling through up-regulation of plasminogen activator inhibitor-1 and phosphorylation of SMAD2/3. Furthermore, SMAD6 knockdown activated the c-Jun NH₂-terminal kinase pathway and reduced phosphorylation of Rb-1, resulting in increased G₀-G₁ cell arrest and apoptosis in the lung cancer cell line H1299. These results jointly suggest that SMAD6 plays a critical role in supporting lung cancer cell growth and survival. Targeted inactivation of SMAD6 may provide a novel therapeutic strategy for lung cancers expressing this gene.