Exploring the effects of mitochondrial estrogen receptors on mitochondrial priming and endocrine therapy response in breast cancer cells

Official URL: http://risc01.sabanciuniv.edu/record=b1649228 (Table of Contents)

Breast cancer is the most common malignancy in women, only in Europe with an incidence of 450,332 cases in 2008. Estrogen receptors (ERs) and estrogen signaling regulate key events in breast cancer development and progression. Estrogen receptor status is an important prognostic factor and antiestrogen (endocrine) therapy is the choice of first-line treatment in ER-positive breast cancer cases. However, most tumors develop resistance to endocrine therapies during the course of treatment. Thus, a better understanding of estrogen receptor signaling, including non-genomic, non-transcriptional components, is essential for developing effective treatments and predicting endocrine therapy response. Programmed cell death/apoptosis is essential for development and homeostasis of tissues and the mitochondrial apoptotic pathway is regulated by members of the BCL-2 protein family. Impaired cell death signaling promotes cancer cell survival in response to a myriad of cellular damage or stress, including chemotherapy, hypoxia and oncogenic stress. BH3-only BCL-2 proteins integrate cellular damage into mitochondrial apoptotic pathway by selectively interacting with antiapoptotic BCL-2 proteins. This selective binding pattern is translated into a functional mitochondrial cell death priming assay, designated as BH3 profiling, which allows us to assess the apoptotic blocks employed by cancer cells. The goal of our study was to determine how mitochondrial estrogen receptors affect mitochondrial cell death priming and endocrine therapy response in breast cancer cells by using innovative techniques such as confocal immunofluorescence microscopy, coimmunoprecipitation, 3D spheroid cell culture and BH3 profiling assay. Our results demonstrate the differential expression of estrogen receptor isoforms at both RNA and protein levels. ER-α expression is selectively but ER-β is ubiquitously expressed in breast cancer cells and in normal cells. Endocrine therapy agents have similar EC50 values regardless of ER-α and ER-β expression statuses pointing out that these agents exert their anti-cancer actions by growth arrest, consequently resulting in cell death. Breast cancer cells have different mitochondrial priming status. According to the non-parametric Spearman correlation test, there is a correlation between BH3 profiles and endocrine therapy response as following: Sensitivity to Tamoxifen correlates with Bmf and Noxa levels in all breast cancer cell lines (n=40) (p < 0.05). Sensitivity to Anastrozole correlates with Hrk levels in all breast cancer cell lines (n=40) (p < 0.05). Sensitivity to Tamoxifen correlates with Hrk and Bmf levels in normal breast epithelial cell lines (n=4) (p < 0.05). Immunofluorescence analysis revealed mitochondrial localization of both receptors in addition to nucleus and cytoplasm. These studies provide new information on mitochondrial estrogen signaling, which help in the design of applicable tests to predict endocrine therapy resistance and facilitate tailored pharmacological interventions targeting estrogen receptors in breast cancer.