Abstract 1068: Inhibition of the phosphorylation of eIF4 molecules sensitizes EGFR expressing breast cancers to EGFR inhibitors
Authors: Madden Julie , Mueller Kelly , Bollig-Fischer Aliccia , Stemmer Paul , Boerner Julie 
Published: August 2012

Abstract
Triple-negative breast cancers have a poor prognosis and lack an effective targeted therapeutic. Even though ∼50% of triple-negative breast cancers express epidermal growth factor receptor (EGFR), they are resistant to EGFR tyrosine kinase inhibitors (TKI). Although the activation of several tyrosine kinases including Met, c-Src, and IGF-IR have been found to correlate with EGFR TKI resistance, the mechanism of this de novo resistance is still being elucidated. The BT20 breast cancer cell line has amplified EGFR expression yet is resistant to EGFR TKIs. Phospho-proteomic analysis by mass spectrometry was used to identify proteins that remain phosphorylated after gefitinib treatment. These proteins included Raptor, PDK1, and GSK3, implicating the Akt and mTOR pathways in EGFR TKI resistance. Despite activation of these pathways, BT20 cells were resistance to the mTOR antagonist temsirolimus. However, temsirolimus was able to sensitize BT20 cells to gefitinib which led to a synergistic decrease in cell survival. Similar results have been observed by others using different inhibitors; however, the mechanism of this synergistic decrease in cell survival has not been identified. These data suggest that EGFR and mTOR activation regulate cell survival through parallel signaling pathways and that both pathways need to be inhibited to abrogate growth and survival. This is supported by the observation that the PI3Kinase/Akt signaling pathway was unaffected by gefitinib or temsirolimus treatment alone. In addition, the combination of EGFR and mTOR inhibitors had no effect on PI3Kinase/Akt or Ras/MAPK signaling, despite the synergistic decrease in cell survival. However, further immunoblotting showed a pronounced inhibition in activation of the eIF4 family in the mTOR translational control pathway with dual treatment of gefitinib and temsirolimus, an abrogation not significantly observed with either treatment alone. These results suggest that inhibiting mTOR signaling alone is insufficient to modulate the translational control pathway in BT20 cells and that EGFR signals independent of mTOR to activate this pathway. These data demonstrate that phosphorylation and subsequent regulation of these translational control molecules may be critical to the survival of BT20 cells. Indeed, the combination of gefitinib and temsirolimus stimulated apoptosis in BT20 cells, supporting the idea that inhibiting the mTOR pathway will sensitize breast cancers expressing EGFR to gefitinib inhibition. In summary, these data provide strong evidence for the presence of a de novo EGFR TKI resistance pathway in breast cancers that occurs through the activation of the mTOR pathway and therapy for EGFR positive breast cancers could be improved with dual treatment of EGFR TKI and an mTOR antagonist.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1068. doi:1538-7445.AM2012-1068