Combined Treatment with Erlotinib and a Transforming Growth Factor-β Type I Receptor Inhibitor Effectively Suppresses the Enhanced Motility of Erlotinib-Resistant Non–Small-Cell Lung Cancer Cells

IntroductionDespite an initial dramatic response to the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) gefitinib and erlotinib, the majority of non–small cell lung cancer (NSCLC) patients with EGFR-activating mutations develop acquired resistance. Therefore, there is an urgent need to elucidate the unknown mechanisms and biological behaviors of EGFR TKI–resistant lung tumors. We investigated the motility of EGFR TKI–resistant cells, as these characteristics are relevant to cancer metastasis.MethodsErlotinib-resistant PC-9ER cells were generated from PC-9 NSCLC cells, which harbor an EGFR-activating mutation, and used in this study. We investigated the involvement of the transforming growth factor beta (TGF-β) pathway in cell motility, and tested the effects of erlotinib and TGF-β type I receptor (RI) inhibition on cell motility.ResultsPC-9ER cells displayed enhanced motility resulting from autocrine activation of the TGF-β pathway. Increased TGF-β2 secretion resulting from TGF-β2 up-regulation at the transcriptional level was suggested to be responsible for the phosphorylation of Smad2 and the subsequently elevated transcriptional regulatory activity in PC-9ER cells. The motility of PC-9ER cells was suppressed by treatment with either the TGF-βRI inhibitor LY364947 or erlotinib, and greater suppression was observed when used in combination. LY364947 or erlotinib exerted no growth-inhibitory effects, suggesting that motility and growth are driven by different signaling pathways in PC-9ER cells.ConclusionsOur results imply that blockade of the TGF-β signaling pathway combined with continuous EGFR TKI treatment will be beneficial in preventing metastasis in patients with EGFR TKI–resistant NSCLC without the EGFR T790M resistance mutation.