Abstract 4995: Glucose-regulated protein 78 regulates crosstalk between apoptosis and autophagy to determine antiestrogen responsiveness

Abstract

Abstract While over 70% of breast cancers express estrogen receptor α (ER+), drugs targeting these receptors such as Tamoxifen (TAM) or Faslodex (fulvestrant, ICI) often fail to cure these patients. Many ER+ patients fail to respond (de novo resistance) or lose drug responsiveness (acquired resistance), making endocrine resistance a major clinically relevant problem. The molecular mechanisms of endocrine therapy resistance still remain unclear. We show that antiestrogen resistant cells with an acquired resistant phenotype overexpress glucose-regulated protein 78 (GRP78), which is not observed in de novo resistance. Moreover, we show increased expression of GRP78 in MCF7/LCC9 (estrogen independent, ICI resistant, TAM cross-resistant) when compared to the MCF7/LCC1 (estrogen independent, TAM and ICI sensitive) breast cancer cells and tumors. These data reflect the response of aromatase inhibitor resistant breast cancers to second line antiestrogen therapy, and further support the hypothesis that antiestrogen resistance and estrogen independence are separate phenotypes. Targeting GRP78 with RNAi restored antiestrogen sensitivity in endocrine therapy resistant cells (MCF7-RR and MCF7/LCC9), whereas expressing GRP78 cDNA conferred resistance on antiestrogen sensitive breast cancer cells (MCF7 and MCF7/LCC1). We show that GRP78 integrates cellular signaling to inhibit apoptosis (as determined by cleaved caspase-7, cleaved PARP, and annexin V staining) and stimulate prosurvival autophagy (as determined by LC3-II, p62, and GFP-LC3 punctate). Moreover, GRP78-stimulated autophagy is dependent on TSC2/AMPK-mediated mTOR inhibition. Endocrine resistance induced by GRP78 overexpression is prevented through an inhibition of autophagy by ATG5 knockdown, while caspase inhibition prevented GRP78 RNAi-mediated therapy resensitization. Furthermore, knockdown of GRP78 and beclin 1 (BECN1) synergistically restores antiestrogen sensitivity in resistant cells. The use of GRP78 to integrate the cellular functions of apoptosis and autophagy to control cell fate raises the provocative question that this signaling may be widely applicable and represent a major stress response that explains how many different cell types could respond to stressors and acquire resistance to therapeutic interventions. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4995. doi:1538-7445.AM2012-4995