Abstract P6-04-26: PI3K inhibitors restored the sensitivity to anti-estrogen drugs in endocrine therapy resistant cell lines

Background The Combination of endocrine therapy with molecular targeted drugs has become one of the standard treatment option for ER positive advanced or metastatic breast cancer. The therapeutic effects of PI3K-mTOR inhibitors, which inhibit intracellular phosphorylation of signaling pathways, have been clarified in clinical trials. However, the effects of these drugs on ER signal pathways in endocrine-resistant breast cancer have not been fully elucidated. We investigated the changes in the expression and functions of ER when endocrine-resistant cell lines, established in our laboratory, were treated with PI3K-mTOR inhibitors Methods Long-term estrogen deprivation-resistant (EDR) cell lines and fulvestrant-resistant cell lines (MFR) were established from MCF-7 cells in our previous studies. These cell lines exhibited different ER expression levels, including high (EDR-1), low (EDR-2), or no expression (MFR). All these cell lines had the same PIK3CA mutations as the parental cell lines. We used buparlisib (BUP, pan-class1 PI3K inhibitor) and alpelisib (ALP, α-specific PI3K inhibitor) as PI3K inhibitors and everolimus (EVE) as an mTOR inhibitor. Tamoxifen and fulvestrant were used in anti-estrogen therapy. In this study, the procedure to examine histone modifications was CHIP-qtPCR analysis. Results EDR-1, -2 and MFR were treated with the PI3K-TOR inhibitors for 24 h. There were no changes in ER expression and ER activity on EDR-1 and MFR before and after the addition of PI3K-mTOR inhibitors. Interestingly, ER expression and ER activity in EDR-2 increased after the addition of PI3K-mTOR inhibitors. In particular, a high re-expression of ER was observed when the PI3K inhibitors were added. Next, in order to evaluate the effects of endocrine therapy on these cell lines Using BUP, ALP and EVE, each drug resistant cell lines were established from EDR-2. When the cell proliferation assay was performed in the established resistant cell line, EDR-2 which showed re-expression of ER on adding the PI3K inhibitors, showed cell growth-inhibitory effects with the addition of anti-estrogen agents. Previously, we reported that epigenetic regulation contributes to the reduction of ER expression in EDR-2. (Tsuboi et al, J Steroid Biochem Mol Biol 2017). Therefore, we investigated the possibility that epigenetic regulation could contribute to the mechanism of ER re-expression when PI3K inhibitors were used in EDR-2. It was revealed that the change of H3K27me3, one of the histonemodifications, correlated with the expression of ER. Conclusion PI3K inhibitor could restore the expression of ER and sensitivity to anti-estrogen drugs in endocrine therapy resistant cell line in which ER expression was lost during acquired resistant period. This restoration might be due to a histone modification (H3K27me3). Citation Format: Emi Tokuda, Shunta Sasaki, Kouki Tsuboi, Masafumi Iida, Toshifumi Niwa, Shigehira Saji, Shin-Ichi Hayashi. PI3K inhibitors restored the sensitivity to anti-estrogen drugs in endocrine therapy resistant cell lines [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-04-26.