Abstract
The epidermal growth factor receptor (EGFR) tyrosine‐kinase inhibitor gefitinib (Iressa®, ZD1839) has shown promising activity preclinically and clinically. Because comparative investigations of drug‐resistant sublines with their parental cells are useful approaches to identifying the mechanism of gefitinib resistance and select factors that determine sensitivity to gefitinib, we established a human non‐small cell lung carcinoma subline (PC‐9/ZD) that is resistant to gefitinib. PC‐9/ZD cells are ∼180‐fold more resistant to gefitinib than their parental PC‐9 cells and PC‐9/ZD cells do not exhibit cross‐resistance to conventional anticancer agents or other tyrosine kinase inhibitors, except AG‐1478, a specific inhibitor of EGFR. PC‐9/ZD cells also display significant resistance to gefitinib in a tumor‐bearing animal model. To elucidate the mechanism of resistance, we characterized PC‐9/ZD cells. The basal level of EGFR in PC‐9 and PC‐9/ZD cells was comparable. A deletion mutation was identified within the kinase domain of EGFR in both PC‐9 and PC‐9/ZD, but no difference in the sequence of EGFR cDNA was detected in either cell line. Increased EGFR/HER2 (and EGFR/HER3) heterodimer formations were demonstrated in PC‐9/ZD cells by chemical cross‐linking and immunoprecipitation analysis in cells unexposed to gefitinib. Exposure to gefitinib increased heterodimer formation in PC‐9 cells, but not in PC‐9/ZD cells. Gefitinib inhibits EGFR autophosphorylation in a dose‐dependent manner in PC‐9 cells but not in PC‐9/ZD cells. A marked difference in inhibition of site‐specific phosphorylation of EGFR was observed at Tyr1068 compared to other tyrosine residues (Tyr845, 992 and 1045). To elucidate the downstream signaling in the PC9/ZD cellular machinery, complex formation between EGFR and its adaptor proteins GRB2, SOS, and Shc was examined. A marked reduction in the GRB2‐EGFR complex and absence of SOS‐EGFR were observed in PC‐9/ZD cells, even though the protein levels of GRB2 and SOS in PC‐9 and PC‐9/ZD cells were comparable. Expression of phosphorylated AKT was increased in PC‐9 cells and inhibited by 0.02 μM gefitinib. But the inhibition was not significant in PC‐9/ZD cells. These results suggest that alterations of adaptor‐protein‐mediated signal transduction from EGFR to AKT is a possible mechanism of the resistance to gefitinib in PC‐9/ZD cells. These phenotypes including EGFR–SOS complex and heterodimer formation of HER family members are potential biomarkers for predicting resistance to gefitinib. © 2005 Wiley‐Liss, Inc.