EGFR inhibition evokes innate drug resistance in lung cancer cells by preventing Akt activity and thus inactivating Ets-1 function

Abstract

Nonsmall cell lung cancer (NSCLC) is the leading cause of cancer death worldwide. About 14% of NSCLCs harbor mutations in epidermal growth factor receptor (EGFR). Despite remarkable progress in treatment with tyrosine kinase inhibitors (TKIs), only 5% of patients achieve tumor reduction >90%. The limited primary responses are attributed partly to drug resistance inherent in the tumor cells before therapy begins. Recent reports showed that activation of receptor tyrosine kinases (RTKs) is an important determinant of this innate drug resistance. In contrast, we demonstrate that EGFR inhibition promotes innate drug resistance despite blockade of RTK activity in NSCLC cells. EGFR TKIs decrease both the mitogen-activated protein kinase (MAPK) and Akt protein kinase pathways for a short time, after which the Ras/MAPK pathway becomes reactivated. Akt inhibition selectively blocks the transcriptional activation of Ets-1, which inhibits its target gene, dual specificity phosphatase 6 (DUSP6), a negative regulator specific for ERK1/2. As a result, ERK1/2 is activated. Furthermore, elevated c-Src stimulates Ras GTP-loading and activates Raf and MEK kinases. These observations suggest that not only ERK1/2 but also Akt activity is essential to maintain Ets-1 in an active state. Therefore, despite high levels of ERK1/2, Ets-1 target genes including DUSP6 and cyclins D1, D3, and E2 remain suppressed by Akt inhibition. Reduction of DUSP6 in combination with elevated c-Src renews activation of the Ras/MAPK pathway, which enhances cell survival by accelerating Bim protein turnover. Thus, EGFR TKIs evoke innate drug resistance by preventing Akt activity and inactivating Ets-1 function in NSCLC cells.nonsmall cell lung cancer | EGFR inhibition | tyrosine kinase inhibitors | ERK1/2 paradoxical activation | innate drug resistance

Drug Resistance to EGFR Inhibitors in Lung Cancer

“…As mentioned above, we recently studied the mechanism by which a small subset of cells remains viable after EGFR inhibition, despite cell death in the vast majority [96-98, 106, 107]. Our study demonstrated that EGFR inhibition in lung cancer cells generates a drug-tolerant subpopulation by blocking AKT activity and thus inactivating Ets-1 function (Figure 1).…”

“…The origin of resistant cells remains to be elucidated, but they must arise from surviving populations [96-98]. These cells lie temporarily dormant or quiescent as a means of circumventing the effects of the given therapy, but eventually regain proliferative capacity [99].…”

“…In lung cancers, drug-tolerant persisters emerge during treatment as a subset of the cells within primary tumors [96, 102]. Although initially quiescent, the persister cells can soon begin to propagate in the presence of EGFR TKIs, becoming drug-tolerant expanded persisters.…”

Resistance to EGFR-targeted therapy by Ets-1 inactivation

“…3 For example, we found that EGFR TKIinduced activation of Ras accompanied phosphorylation of ERK1/2, but not Akt. Therefore, we question the role of Ras as the immediate upstream regulator of the phosphoinositide 3-kinase (PI3K)/Akt pathway and suggest an important role for RTKs in Ras-mediated activation of this pathway.…”

“…3 Therefore, to reduce the probability of emergent resistance to EGFR TKIs in NSCLCs, combined TKI and MEK inhibitor treatment should be considered. A recent report from other laboratory has also proposed this novel combined therapy, which is thought to be effective not only in innate resistance, but also in acquired resistance with T790M second-site EGFR mutation.…”