Abstract P3-05-04: Estrogen receptor α-regulation of nucleocytoplasmic transport pathways as modulators of breast cancer therapy effectiveness

Estrogen Receptor α (ERα), a member of the large superfamily of nuclear receptors, exerts profound effects on gene expression, cellular response programs, and phenotypic properties of estrogen target cells. Because of these broad and important actions, ERα is considered the single most important predictor of breast cancer prognosis and is the target of endocrine therapies. The importance of kinases in cancer biology is well known, as increased kinase activity through phosphorylation, mutations or increased expression is often observed in clinical samples and is associated with a poorer prognosis. It is believed that, in therapy-resistant breast cancers, control of cellular physiology switches from ERα nuclear-initiated pathways to extranuclear-activated protein kinase pathways, which enable these cells to adopt a more aggressive phenotype. However, the mechanisms underlying the interplay between ERα and protein kinase pathways in cancer, and the processes by which ERα influences these pathways are poorly understood. Our aim in this study was to elucidate how ERα modulated extranuclear- initiated kinase signaling through alteration of the subcellular localization of ERK5. We have previously shown that upon estradiol treatment ERα elicits nuclear localization of ERK5 and Cofilin in ERα-positive breast cancer cells. This event diminishes ERK5 and Cofilin localization to regions of high actin remodeling in the cytoplasm, thereby possibly accounting for the reduced invasiveness and metastatic potential that is characteristic of many ERα-positive vs. ERα-negative breast cancer cells (PMID: 24505128). In our studies involving an endocrine-resistance cell model, we have found that ERK5 and Cofilin become localized to the cytoplasm as resistance progresses. Using several publicly available tumor gene expression databases, we have identified a signature of genes regulating nucleocytoplasmic transport that are differentially upregulated in more aggressive tumors. Additionally, using RPPA data from TCGA, we find that high expression of the signature genes correlates with higher phosphorylation of key signaling molecules like TAZ, indicative of their mislocalization in invasive breast carcinomas. Low expression of signature genes would successfully predict those patients that would respond to endocrine therapy with Luminal B type tumors that are generally more aggressive and harder to manage clinically. We verified our findings in cell line models by modulating levels and activities of these proteins by overexpression, knock-down and use of small molecule inhibitors. These findings provide insight into how ERα regulates extranuclear initiated kinase signaling by modulating nuclear transport of key kinases in breast cancer. They also suggest that therapeutic targeting of nucleocytoplasmic transport machinery in breast cancer might decrease aggressiveness of breast cancers and increase the efficiency of endocrine therapies by sequestering factors in their proper subcellular localizations where they would contribute to effective anti-estrogenic actions of endocrine targeting agents. Citation Format: Madak-Erdogan Z, Chen-Zhao Y, Wrobel K. Estrogen receptor α-regulation of nucleocytoplasmic transport pathways as modulators of breast cancer therapy effectiveness. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P3-05-04.