Abstract 2110: Molecular characterization of everolimus-resistant cell lines established from estrogen depletion-resistant MCF-7

Abstract

Abstract mTOR inhibitor everolimus has been considered as a promising agent for breast cancer treatment especially among estrogen receptor (ER) positive postmenopausal advanced breast cancer. As expected, combination therapy of everolimus and steroidal aromatase inhibitor (AI) showed remarkable clinical efficacy for the first line nonsteroidal AI treatment failure patients in BOLERO-2 trial. However, the subsequent treatment after refractory to that therapy remains a great matter. We already reported several types of long term estrogen-depletion resistant (EDR) cell lines from MCF-7-E10, which has been constitutively ERE-GFP-transfected to monitor ER activity in living cells (Fujiki N et al., J Steroid Biochem Mol Biol. 2013 Oct 13;139C:61-72). They mimic AI resistance and these EDR cells have been characterized as the followings. 1) Type 1 as ER positive (ERE-GFP positive), constitutive ER overexpression without estrogen, and PI3K/Akt/mTOR pathway upregulated. 2) Type 2 as ER negative conversion (ERE-GFP negative) and IGF-1R/JNK signaling pathway upregulated. Then with those two types of sub-cell lines, we generated everolimus resistant cells after long term exposure to everolimus in estrogen depleted medium. Those everolimus resistant (EVR) cells are equivalent to resistance to the combination therapy of AI and everolimus as the second line endocrine treatment after the first line AI treatment failure. Though everolimus extremely inhibited cell proliferation of two EDR cell lines, everolimus resistant cells named as Type 1-EVR, Type 2-EVR no longer responded to everolimus. ER expression and estrogen sensitivity of two types of EVR cells were similar to those of the parental cells, respectively. Pure anti-estrogen fulvestrant inhibited cell proliferation of Type1-EVR cells. On the other hand, Type2-EVR did not respond to fulvestrant but JNK inhibitor inhibited the cell growth. Phosphorylation status of p70S6K and 4E-BP1 were different among individual parental EDR cells and EVR cells. Type1-EVR showed much higher expression of phosphorylation of MAP-kinase than the parental cells. In contrast, Type2-EVR expressed phosphorylation of MAP-kinase less than the parental cells. As above, the mechanism of acquired resistance to everolimus was likely to be different between those two sub-cell lines. This finding implies that different mechanisms of AI resistance follow different types of everolimus resistance. Citation Format: Mariko Kimura, Toru Hanamura, Toshifumi Niwa, Yuri Yamaguchi, Itaru Endo, Shin-ichi Hayashi. Molecular characterization of everolimus-resistant cell lines established from estrogen depletion-resistant MCF-7. [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 2110. doi:10.1158/1538-7445.AM2014-2110