Abstract 830: Transcriptional modulation of estrogen-induced apoptosis through activation of c-Fos/c-Jun in long-term estrogen deprived breast cancer cells. (2013)

Abstract Recent clinical trials have demonstrated that estrogen (E2) alone reduces breast cancer incidence in postmenopausal women and has therapeutic effects on aromatase inhibitor resistant patients which both are related with the effect of E2-induced apoptosis. We have shown that E2 induces apoptosis in long-term E2 deprived breast cancer cells (MCF-7:5C) through stress responses, but the molecular mechanism underlying E2-induced stress remains to be elucidated. Here, we report that E2 activated the sensors of unfolded protein response (UPR) inositol-requiring protein 1 alpha (IRE1α) and PRK-like endoplasmic reticulum kinase (PERK) within 24 hours. Knockdown of PERK and IRE-1α through small interferon RNAs (siRNA) partially prevented E2-induced apoptosis, which suggested that endoplasmic reticulum stress was involved in the E2-induced apoptosis. Further examination showed E2 activated both classical estrogen responsive element (ERE) pathway and nonclassical activating protein-1 (AP-1) pathway in MCF-7:5C cells. Classical ERE regulated genes were not directly involved in E2-induced apoptosis. However, the transcription factor c-Fos acted as a critical trigger involved in stress responses induced by E2 in MCF-7:5C cells. E2 immediately elevated c-Fos expression in MCF-7:5C cells but not in wild-type breast cancer cells and 4-hydroxytamoxifen blocked this stimulation which suggested that E2 activated c-Fos through estrogen receptor (ER). E2 increased phosphorylation of c-Jun after 24 hours treatment but did not significantly enhance the abundance of c-Jun as c-Fos. c-Fos protein forms stable heterodimers with c-Jun which preferentially bind to phorbol 12-O-tetradecanoate-13-acetate (TPA)-responsive element (TRE). Interestingly, low doses of TPA which activated TRE activity could induce apoptosis and activated apoptosis-related genes similar to E2 in MCF-7:5C cells. Knockdown of c-Fos and c-Jun with specific siRNAs blocked E2-induced apoptosis, which confirmed the activation of AP-1 played a critical role in the process of apoptosis induced by E2. Although c-Fos closely associates with c-Jun to form a stable heterodimer, they had differential functions in modulation of UPR and other nuclear transcription factors. Knockdown of c-Fos but not c-Jun could inhibit the activation of UPR by E2 in MCF-7:5C cells. Furthermore, downregulation of c-Fos abolished the expression of multiple nuclear transcription factors such as SRC-3, NF-κB, and p300 etc in MCF-7:5C cells. Overall, these data illustrate that c-Jun might provide a dimerizing site for c-Fos, whereas c-Fos was identified as a pivotal regulatory target of E2 in MCF-7:5C cells to trigger the apoptotic cascades. This study provides an important rationale for further exploration of E2-induced apoptosis in endocrine resistant breast cancer to improve clinical benefit. Citation Format: Ping Fan, Fadeke Agboke, Obi L. Griffith, Russell E. McDaniel, Xiaojun Zou, Karen Creswell, Joe W. Gray, Virgil Craig Jordan. Transcriptional modulation of estrogen-induced apoptosis through activation of c-Fos/c-Jun in long-term estrogen deprived breast cancer cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 830. doi:10.1158/1538-7445.AM2013-830