INTRODUCTION: Estrogen receptor-positive (ER+) breast cancer accounts for nearly 70% of all cases. Common targeted anti-estrogen therapies such tamoxifen, fulvestrant and aromatase inhibitors have shown success in the clinic, but unfortunately, often lead to resistance. The fatty acid synthase enzyme (FASN) is responsible for endogenously synthesizing long-chain fatty acids, such as palmitate, which can contribute to protein modification, phospholipid biosynthesis for membranes, and lipid raft signaling that favors tumorigenesis. The depletion of intracellular palmitate leads to an alteration of lipid composition within lipid rafts of the plasma membrane as well as lipids within the endoplasmic reticulum membrane. Endoplasmic reticulum stress elicits an inhibition of protein translation through the phosphorylation of eukaryotic initiating factor 2-α (p-eIF2α) and is activated by various stimuli including altered phospholipid composition within the membrane. Our preliminary findings illustrated a degradation of the ERα upon treatment with the FASN inhibitor, TVB-3166, in tamoxifen-resistant breast cancer both in vivo and in vitro. Moreover, previous studies have illustrated FASN inhibition to induce endoplasmic reticulum stress concomitant with a loss of the androgen receptor (AR) in castration-resistant prostate cancer. Additionally, palmitate treatment rescued AR expression that was accompanied by an attenuation in endoplasmic reticulum stress. HYPOTHESIS: We hypothesize FASN inhibition leads to a degradation of ERα in tamoxifen-resistant breast cancer through the induction of endoplasmic reticulum stress. METHODS: Patient tumor explants were incubated for 72h on gelatin sponges in culture medium in the absence or presence of 200nM TVB-3166. Tissue were fixed in 10% formalin and processed into paraffin blocks and stained for ERα and Ki67. To investigate TVB induced endoplasmic reticulum stress, tamoxifen-Resistant (TamR) MCF-7 and MCF-7 cells were treated with TVB-3166 or vehicle control followed by treatment with either palmitate or ER stress inhibitor, 1,2-Bis(2-Aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA). Expression of ERα, p-eIF2α, eIF2α were measured by western blot. RESULTS: TVB-3166 treatment of primary tumor explants decreased their proliferation (Ki67) compared to untreated controls (14% vs 36%, p<0.01). Both IHC and Western blotting demonstrated a reduction in ERα upon treatment with TVB-3166. In addition to the decreased expression of ERα, there was an increase in the phosphorylation of eukaryotic initiation factor 2α (p-eIF2α). The expression of ERα was rescued upon palmitate treatment while resulting in decreased p-eIF2α. Inhibition of endoplasmic stress using BAPTA also rescued ERα expression after TVB-3166 treatment. CONCLUSION: FASN is a potentially viable target in tamoxifen-resistant breast cancer.

Citation Format: Bryan Mcclellan, Aleksandra Gruslova, Christopher Jolly, Linda deGraffenried, Andrew Brenner. Fatty acid synthase inhibition targets ERα in tamoxifen-resistant breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS17-26.