MET Increases the Sensitivity of Gefitinib-Resistant Cells to SN-38, an Active Metabolite of Irinotecan, by Up-Regulating the Topoisomerase I Activity

Most non–small-cell lung cancer tumors with epidermal growth factor receptor mutations are responsive to EGFR tyrosine kinase inhibitors, such as gefitinib and erlotinib, but almost all such tumors ultimately acquire resistance. We previously found that a gefitinib-resistant cell line, PC-9/Met in which MET (MNNG-HOS transforming gene) is amplified, was more sensitive than its parent cell line (PC-9) to 7-ethyl-10-hydroxy-camptothecin (SN-38), an active metabolite of irinotecan. The purpose of this study was to investigate the mechanisms responsible for the increased sensitivity of the gefitinib-resistant cell line to SN-38. The sensitivity of PC-9 and PC-9/Met to SN-38 was assessed by performing water soluble tetrazolium salt (WST-1) assays. Topoisomerase I (topo I) activities were determined for the cell lines cultured in the presence of hepatocyte growth factor and for those of which MET expression was knocked down by introducing a MET-specific small interfering RNA. PC-9/Met exhibited higher topo I activities, and higher topo I gene and protein expression levels than PC-9 did. Suppression of MET expression by a MET-specific small interfering RNA led to a decrease in the topo I protein expression in the PC-9/Met cells. The stimulation of PC-9 with hepatocyte growth factor caused an increase in the topo I protein level via the activation of MET. The increased sensitivity of PC-9/Met cells to SN-38 compared with that of PC-9 cells was partially because of topo I activities resulting from increased topo I mRNA and protein expression caused by MET signaling.