Abstract B07: Estrogen-related receptor alpha regulates S6K1 expression and tamoxifen sensitivity in breast cancer

Abstract

Abstract Estrogen related receptor alpha (ERRα) is an orphan nuclear receptor that plays important role in the metabolic activities and proliferation of cancer cells. We investigated the role of ERRα in regulating the expression of S6K1 in breast cancer cells. First, using RT-qPCR and immunoblotting we found that ERRα overexpression in estrogen receptor (ER) positive MCF7 cells results in decreased S6K1 expression and activity. Conversely, downregulation of ERRα expression resulted in increased S6K1 expression and activity in ER-negative MDA-MB-231 cells. Second, overexpression of ERRα in MCF7 cells also resulted in decreased transcriptional activity of the RPS6KB1 gene promoter as revealed by luciferase reporter assay. On the other hand, ERRα knockdown in MDA-MB-231 cells resulted in increased promoter activity. Third, because the proximal 1 kilobase (Kb) promoter region of S6K1 contains several putative estrogen receptor DNA binding elements (EREs), we investigated whether ERRα modulates S6K1 expression by direct promoter binding using chromatin immunoprecipitation (ChIP) technique. We detected two binding peaks approximately 300 and 600 kb away from the transcription start site (TSS). In addition, we showed that downregulation of ERRα caused increased cell proliferation and sensitivity to the mTOR inhibitor rapamycin. Suprisingly, tamoxifen treatment increased the proliferative capacity of these cells. Interestingly, cytoplasmic and nuclear staining of ERRα in tumor specimens derived from patients in the Stockholm breast cancer trial of adjuvant tamoxifen revealed that (1) in triple-negative tumors, the low-expressing cytoplasmic ERRα group had a worse prognosis, whereas a clear improvement in recurrence-free survival was found in the group with high ERRα-expressing tumors; and (2) in the ERα-negative subgroup, high ERRα rendered a clear tamoxifen benefit, which is very interesting, as this group is not receiving tamoxifen today and may constitute a subgroup which could gain from tamoxifen. Overall, our data indicate that ERRα regulates S6K1 expression and affects the sensitivity to rapamycin tamoxifen. Our long-term goal is to establish ERRα as a marker for endocrine and rapamycin therapy response, as well as a novel target in combination therapy approaches. Citation Format: Subrata Manna, Josefine Bostner, Anya Alayev, Olle Stal, Marina K. Holz. Estrogen-related receptor alpha regulates S6K1 expression and tamoxifen sensitivity in breast cancer. [abstract]. In: Proceedings of the AACR Special Conference: Targeting the PI3K-mTOR Network in Cancer; Sep 14-17, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(7 Suppl):Abstract nr B07.