Analyzing the Impact of EGFR-induced c-Met Phosphorylation in Non-Small Cell Lung Cancer

BackgroundLung cancer is currently the second most prevalent form of cancer in the United States and is the leading cause of cancer-related deaths. Currently, there are no effective therapies for those diagnosed in the later stages of lung cancer. The c-Met receptor is a potential therapeutic target for NSCLC along with its ligand, hepatocyte growth factor (HGF). Signaling interactions between c-Met and the mutant Epidermal Growth Factor Receptor (EGFR) have been studied extensively, but the biological importance of lateral signaling to c-Met in EGFR wild-type tumors is minimally understood. Principal FindingsOur observations indicate that wild-type EGFR, the receptor most often found in NSCLC tumors, can initiate delayed c-Met activation in NSCLC cell lines. EGFR ligands induce accumulation of activated c-Met which begins at 8 h and continues for 48 h. This effect is accompanied by phosphorylation of critical c-Met tyrosine residues. Gene transcription is required for delayed c-Met activation; however, phosphorylation of c-Met by EGFR occurs without production of HGF or secretion of other factors, supporting an internal mechanism that is independent of c-Met ligand. Lateral signaling is blocked by two selective c-Met tyrosine kinase inhibitors (TKIs), PF2341066 and SU11274, or with gefitinib, an EGFR TKI, suggesting kinase activities of both receptors are required for this effect. The c-Src pathway is essential for EGFR to c-Met communication. This appears to be up- and downstream of delayed c-Met activation. Pre-treatment with pan-SFK inhibitors, PP2 and dasatinib, abolishes delayed c-Met phosphorylation. A c-Src dominant-negative construct reduces EGF-induced c-Met phosphorylation compared to control, further confirming a c-Src requirement. Additionally, delayed c-Src association with c-Met and prolonged c-Src activation are observed following EGF addition. Inhibition of c-Met with PF2341066 and siRNA decreases the EGF-induced phenotypes of invasion by ~86% and motility by ~81%, suggesting that delayed c-Met activation is utilized by EGFR to potentiate its full biological effects possibly through STAT3. Combined targeting of c-Met and EGFR pathways lead to increased NSCLC xenograft anti-tumor activity.Conclusions and SignificanceCollectively, these data provide an alternative working model of prolonged EGFR signaling, whereby c-Met activation in NSCLC cell lines initiated by wild-type, non-amplified EGFR with wild-type, non-amplified c-Met maximizes EGFR-induced cell motility and invasion. With the identification of this novel pathway, the studies presented here demonstrate that inhibition of both EGFR downstream signaling and EGFR lateral signaling through the EGFR-c-Src-c-Met axis might be effective in treatment of NSCLC. Taken together, these findings will aid in the future development of combination EGFR and c-Met TKI treatments in clinical trials for NSCLC tumors that are wild-type for both EGFR and c-Met