eIF4E Expression Predicts Apoptosis in Response to Epidermal Growth Factor Receptor Inhibition and Mammalian Target of Rapamycin Inhibition in Triple Negative Breast Cancers.

Background: Triple negative (TN) breast cancers account for 15% of all breast cancers, are two-fold higher in African American women, have high histological grade, a poor prognosis and until recently no viable therapeutic targets. Epidermal growth factor receptor (EGFR) is over-expressed in more than 50% of triple negative breast cancers, but the use of agents blocking EGFR has produced disappointing results in metastatic breast cancer. Inhibitors of Mammalian Target of Rapamycin (mTOR) have demonstrated moderate activity as single agents in metastatic breast cancer. mTOR inhibitors have been demonstrated to activate the Akt pathway by a possible feedback mechanism, which could potentially sensitize TN breast cancer cells to upstream inhibitors. We have previously demonstrated synergistic antitumor effects with combined mTOR and EGFR inhibition in TN breast cancers. Eukaryotic translation initiation factor (eIF4E) is a protein downstream of mTOR and a target of PI3K/Akt and Erk signaling, and is important in mRNA translation, cell proliferation and apoptotic resistance. We evaluated the effects of EGFR and mTOR inhibition on apoptotic markers, and correlated these effects with eIF4E protein expression.Methods: Apoptotic assay and colony formation analysis were performed following mTOR inhibition with rapamycin and EGFR inhibition with lapatinib or erlotinib in TN breast cancer cells (MDA-MB-231, MDA-MB-468, HCC1806). Effects of EGFR and mTOR inhibition on downstream proteins in TN breast cancer in vitro and in vivo were examined through western blotting analysis with p-EGFR, pAkt, p-Erk, p-S6, and p-eIF4E.Results: We observed differential apoptotic effects of EGFR and mTOR inhibition in TN breast cancers cells. The combination resulted in a significant increase in apoptosis in MDA-MB-468 cells in vitro, but no increase in apoptosis was seen in MDA-MB-231 or HCC1806 cells. As expected, correlating with the apoptotic effects, expression of p-eIF4E was decreased in MDA-MB-468 cells treated with the combination of mTOR and EGFR inhibition in vitro. In contrast, both MDA-MB-231 and HCC1806 cells had high baseline levels of eIF4E, which increased in response to treatment with EGFR and mTOR inhibitors. Using a colony forming assay, we demonstrated that the combination of EGFR and mTOR inhibition resulted in cytostatic effects in HCC1806 cells. The combination of EGFR and mTOR inhibition did not result in apoptosis in MDA-MB-231 cancers in vivo, and these xenograft tumors had high level of eIF4E at baseline.We are currently evaluating the effects of lapatinib and rapamycin on apoptotic pathways in MDA-MB-468 cancers in vivo.Conclusion: Based on our previous findings, we are developing a clinical trial in which patients with metastatic TN breast cancers are treated with lapatinib and everolimus. These results suggest that this combination may be more effective in TN breast cancers with low levels of eIF4E, and this will be explored using metastatic research biopsies.

Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 5078.