Abstract P4-10-01: Estrogen induced apoptosis can be mimicked by targeting unfolded protein response
Authors: Sengupta S , Sevigny C , Clarke R 
Published: March 2017

Abstract
Estrogen signaling is considered to promote growth in estrogen receptor positive (ER+) breast cancers and its actions can be blocked by antagonists like tamoxifen or by inhibiting the synthesis of estrogen by aromatase inhibitors. Both of these classes of drug are used to treat ER+ breast cancers in the clinic. Paradoxically, before the discovery of tamoxifen and aromatase inhibitors high dose estrogen (HDE) was the choice of endocrine therapy to treat post-menopausal breast cancers. Recent clinical trials have observed 30% clinical benefit rate with high as well as low doses of estrogen-therapy in aromatase-inhibitor resistant breast cancers. Despite its clinical success, the precise underlying mechanism of estrogen-therapy by which it triggers the tumor regression remains unknown. Studies in the laboratory have indicated that unfolded protein response (UPR) and apoptotic pathways may play important role in estrogen-induced apoptosis. Using MCF7:5C cells, which can proliferate independent of estrogen and are hyper-sensitive to estrogen as evident by induction of apoptosis, we demonstrate that increased global protein translational load as the trigger for estrogen-induced apoptosis. This subsequently leads to endoplasmic reticulum (EnR) stress and activates the protein kinase-RNA-like endoplasmic reticulum kinase (PERK) pathway of UPR. Our results also suggest that sustained phosphorylation of eukaryotic initiation factor 2-alpha (eIF2-α), a downstream target of PERK activation, may be crucial in estrogen-induced apoptosis. Phosphorylation of eIF2-α attenuated global translation but preferentially allowed high expression of transcription factors, including, activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP). ATF4 and CHOP are known to activate apoptosis. Notably, we were able to recapitulate this phenotype by pharmacologically inhibiting the regulatory subunits of the protein phosphatase 1, GADD34 (growth arrest and DNA damage inducible protein) and CReP (constitutive repressor of eIF2α phosphorylation), that are responsible for de-phosphorylation of eIF2-α. This was evident in MCF7:5C cells as well as another estrogen-independent breast cancer cell line LCC9 that is resistant to both tamoxifen and fulvestrant but does not undergo estrogen mediated apoptosis. We further observed that the combination of 4-hydroxy-tamoxifen (4OHT) and pharmacological inhibitors of GADD34 and CReP potentiated its apoptotic action in both LCC9 and MCF7:5C cells. These results not only enhance our understanding of the apoptotic mechanism of estrogen but also provides crucial evidence that estrogen-induced apoptosis can be mimicked by manipulating the unfolded protein response even in breast cancer cells that are not susceptible to estrogen mediated apoptosis.
Citation Format: Sengupta S, Sevigny C, Clarke R. Estrogen induced apoptosis can be mimicked by targeting unfolded protein response [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P4-10-01.