Abstract 5488: Everolimus inhibits the proliferation of aromatase inhibitor-resistant breast cancer cells

Abstract

Abstract Aromatase inhibitors (AIs) are currently the standard treatment for estrogen receptor positive breast cancer in postmenopausal women. Unfortunately, the majority of patients treated with AIs eventually develop resistance to these drugs. The mechanism by which AI resistance occurs is still not known, however, there is strong evidence that upregulation of the PI3K/Akt/mTOR pathway might play an important role. As such, mTOR inhibition is now under study in clinical trials as a breast cancer treatment. The newest mTOR inhibitor, Everolimus (Afinitor®), has been FDA approved for the treatment of metastatic breast cancer. We have previously reported the development of two AI-resistant breast cancer cell lines (MCF-7:5C and MCF-7:2A) which were clonally derived from parental MCF-7 cells following long term estrogen deprivation. Both MCF-7:5C and MCF-7:2A cells have been shown to overexpress members of the PI3K/Akt/mTOR pathway. In this study, we examined the therapeutic benefits of everolimus in AI-resistant MCF-7:5C and MCF-7:2A cells compared to endocrine sensitive MCF-7 breast cancer cells and normal breast epithelium MCF10A cells. Results from cell viability assay showed that everolimus markedly inhibited the proliferation of endocrine sensitive MCF-7 cells and AI-resistant MCF-7:5C and MCF-7:2A cells in a time-dependent and concentration-dependent manner with IC50s of 30 nM, 40 nM and 10 nM, respectively. Notably, the MCF-7:2A cells were the most sensitive to everolimus which caused a 70% reduction in proliferation at 24 hours whereas in MCF-7 and MCF-7:5C cells only 30% inhibition was observed at the same time point. The inhibitory effect of everolimus was due primarily to G1 arrest and was associated with significant downregulation of phospho-mTOR, phospho-p70S6K, phospho-Akt, GSK3β, cyclin D1 and p21. Colony formation assay in soft agar indicated that AI-resistant MCF-7:5C and MCF-7:2A cells formed significantly more colonies than MCF-7 cells and that treatment with everolimus notably reduced the size and number of colonies for all three cell lines. Finally, we found that exposure of AI-resistant MCF-7:5C and MCF-7:2A cells to a single dose of gamma-irradiation (6 Gy) significantly enhanced the anti-proliferative effect of everolimus on these cells. In summary, these studies showed that everolimus significantly inhibited the proliferation and tumorigenicity of AI-resistant breast cancer cells and that everolimus might be an effective treatment option for patients with AI-resistant breast cancer. Citation Format: Asona Lui, Joan Lewis-Wambi. Everolimus inhibits the proliferation of aromatase inhibitor-resistant breast cancer cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5488. doi:10.1158/1538-7445.AM2014-5488