Characterization of cancer cells that have lost their oncogene addiction

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Epidermal growth factor receptor (EGFR) is frequently mutated and/or over-expressed in non-small cell lung cancer (NSCLC). These cancer cells may depend on aberrant EGFR signaling to proliferate and survive; i.e. they are addicted to the EGFR oncogene. Mutations in EGFR have been discovered which either render the cells sensitive or resistant to inhibitors of the kinase. It has also been shown that somatic deletions in the kinase domain of EGFR are associated with increased EGFR gene copy number. Development of resistance to small molecule kinase inhibitors for the treatment of cancer is a major concern. The most common mechanism of resistance known to date appears to be structural alterations in the EGFR kinase domain, leading to the inability of the inhibitor to bind in the active site. For instance, a secondary mutation in EGFR (T790M) accounts for almost half the cases of acquired clinical resistance to Iressa (gefitinib). Another possible mechanism is the ability of a cancer cell that is dependent on one kinase to activate alternative signaling pathways for growth when that kinase is inhibited. The NSCLC cell line HCC827, which over-expresses and contains an activating deletion mutation in EGFR (del E746-A750), is dependent on EGFR signaling for proliferation and survival and is highly sensitive to EGFR inhibitors, notably gefitinib and erlotinib. Here we show data comparing the parental HCC827 line to an established HCC827 gefitinib-resistant line (HCC827R), which was once highly sensitive and is now resistant to both gefitinib and erlotinib. Based on our sequencing data, the HCC827R cells do not appear to have become resistant to EGFR inhibitors through secondary mutations of the kinase. It is apparent by Western analysis that the HCC827R cells have down regulated EGFR expression, and gefitinib is no longer able to effectively inhibit the phosphorylation of ERK, a predominant downstream signaling protein. These resistant cells do not show cross-resistance to other anti-cancer cytotoxic agents. The Src inhibitor dasatinib is highly potent in both parent and the resistant lines, and the ability of dasatinib to inhibit ERK phosphorylation is similar in both lines. The elucidation of the mechanisms of losing EGFR oncogene addiction in HCC827R cells could help circumvent the problem and initiate the development of possible solutions for acquired resistance, a highly relevant issue in the clinic.