Epidermal growth factor (EGFR) activating mutations occur in 10% to 40% of non-small cell lung cancer (NSCLC), and can be successfully targeted with EGFR tyrosine kinase inhibitors (EGFR-TKIs), achieving up to 80% response rate in patient populations selected for presence of activating mutations. However, almost all patients develop resistance to treatment within a year, and in at least 30% of cases the cause of resistance is unknown. To identify novel mechanisms of resistance we executed a genome-wide siRNA screen in PC9 cells, an EGFR-TKI sensitive NSCLC cell line. We completed the screen in untreated and drug treated conditions, and selected hits as those genes that had a significantly different effect on survival in drug treated conditions compared to untreated controls. RNAi-mediated knockdown of DEPTOR, a negative regulator of mTOR, increased resistance of PC9 cells to erlotinib (an EGFR-TKI). Cells depleted of DEPTOR have higher levels of mTOR signaling as seen by greater phosphorylation of AKT, and SGK1 substrate NDRG1. DEPTOR loss leads to enhanced resistance to erlotinib treatment by increasing abundance of EGFR protein, thereby augmenting MAPK signaling in basal and erlotinib treated conditions. This upregulation of EGFR appears dependent on mTOR activation, as mTOR kinase inhibitors can resensitize DEPTOR deficient cells to erlotinib treatment. Also, combination treatment of erlotinib with a MEK inhibitor restores sensitivity to erlotinib of cells in which DEPTOR expression has been reduced, indicating increased MAPK signaling contributes to resistance in these cells. mTOR activation is known to cause DEPTOR degradation, therefore our results may have implications for a wide subset of NSCLC where mTOR signaling is upregulated.

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©2012 American Association for Cancer Research.
2012