Abstract 1257: In vivo inhibition of GRP78 potentiates antiestrogen therapy effectiveness in ER+ breast tumors

Abstract

Abstract Over 232,000 new cases of invasive breast cancer are diagnosed annually within the United States; 70% express the estrogen receptor-α (ER) and may be treated with therapies targeting ER, such as tamoxifen (TAM). While initially effective, over 50% of patients treated with these therapies will develop or not respond, thereby making understanding the molecular mechanism of antiestrogen resistance critical component of breast cancer biology. We have previously determined that the unfolded protein response protein chaperone, glucose regulated protein 78 (GRP78), mediates antiestrogen resistance in ER+ breast cancer cells. Mice bearing LCC1 (TAM sensitive) or LCC9 (TAM resistant) xenografts were treated with 30μM 1x weekly IP injection of human GRP78-targeting morpholino and/or a diet containing 400 ppm tamoxifen citrate for 4-6 weeks. Tumor area was measured using calipers and tumor wet weight measured at sacrifice. Combination of GRP78 knockdown and tamoxifen treatment significantly reduced LCC9 tumor area and tumor wet weight, suggesting inhibition of GRP78 restores antiestrogen sensitivity in vivo. TAM and GRP78 morpholino+TAM treatments both reduced LCC1 tumor growth, however the combination of GRP78 inhibition and TAM significantly reduced tumor area and tumor wet weight when compared to TAM treatment alone. These data suggest that inhibiting GRP78 in combination with antiestrogens is more effective for the treatment of breast cancer than single agent endocrine therapy. Metabolic analysis of LCC1 and LCC9 breast cancer cells indicate that inhibition of GRP78 through RNAi results in an increase in the intracellular concentrations of the n-3 and n-6 essential polyunsaturated fats (PUFA) linoleic and linolenic acid. We further show that the accumulation of essential fatty acids is due to an inhibition of mitochondrial fatty acid transport resulting from a reduction in SREBP1/SREBP2 transcription and CPT1A protein expression. The reduced mitochondrial fatty acid transport is also associated with an inhibition of fatty acid oxidation, suggesting a novel role of GRP78 mediating cellular metabolism. We validated the effect of the GRP78-regulated metabolites by treating mice bearing LCC9 xenografts with TAM, 0.25 mg/day linoleic acid by oral gavage, or linoleic acid+TAM for 6 weeks. LCC9 tumors treated with linoleic acid+TAM demonstrated reduced tumor area and tumor weight when compared with TAM or linoleic acid treatment alone. These data suggest that the in vivo effect of GRP78 inhibition is mediated, in part, by increased linoleic acid concentrations. Moreover, the novel role of GRP78 controlling fatty acid oxidation may represent a new homeostatic ability of GRP78 to modify fatty acid metabolism and demonstrates the importance of targeting GRP78 in breast cancer treatment. Citation Format: Katherine L. Cook, Robert Clarke. In vivo inhibition of GRP78 potentiates antiestrogen therapy effectiveness in ER+ breast tumors. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1257. doi:10.1158/1538-7445.AM2015-1257