Abstract 1026: Combating resistance to EGFR inhibitors: eIF4B as a novel target

Triple negative breast cancer (TNBC) patients suffer from a highly malignant and aggressive cancer that currently has no efficacious therapy. These patients have a high rate of relapse and often develop resistance to chemotherapy. Many TNBCs, both in vitro and in vivo, have elevated levels of epidermal growth factor receptor (EGFR) but are resistant to EGFR inhibitors as a monotherapy. The TNBC cell line, BT20, has increased levels of EGFR and is resistant to EGFR inhibitors. To identify the signaling pathways that remain phosphorylated after treatment with gefitinib, an EGFR tyrosine kinase inhibitor (TKI), we used mass spectrometry based phospho-proteomics. Through these assays we identified many components of the mTOR pathway phosphorylated in the presence of gefitinib and further sought to explore this pathway as a mechanism of resistance to EGFR inhibitors in TNBC. mTOR inhibitors have been investigated in the clinic due to their ability to inhibit the PI3Kinase/Akt pathway thus decreasing cell survival and proliferation. Inhibiting mTOR also importantly inhibits translation. Despite activation of these pathways our TNBC cell lines, BT20, MDA-MB-123, and MDA-MB-468 are resistant to temsirolimus, an mTOR inhibitor. Our studies have found that dual treatment with an mTOR inhibitor and an EGFR TKI has a synergistic effect on decreasing TNBC cell viability, but the mechanism of this synergy is not understood. We have found that the abrogation of both EGFR and mTOR signaling does not alter the phosphorylation status of key signaling pathways including MAPK and Akt. Instead, our preliminary data have identified the translational control protein eIF4B as potentially key fragile point in EGFR and mTOR inhibitor synergy. Therefore, in this study we hypothesized that mTOR inhibition will sensitize TNBC cells to EGFR TKIs through the inhibition of eIF4B phosphorylation. Small molecules have yet to be identified to abrogate the function of eIF4B, which when phosphorylated enhances the helicase activity of eIF4A critical to translation. Therefore, we knockdown eIF4B expression and found a decrease in cell survival comparable to the decrease observed with gefitinib and temsirolimus treatment. In addition, we have identified p70S6K and p90RSK as kinases directly responsible for eIF4B phosphorylation, such that both molecules need to be inactivated in order for eIF4B phosphorylation to be inhibited. This inactivation correlates with a loss of cell growth and viabilit