SAT-316 Androgen receptor antagonism inhibits mitochondrial oxidative phosphorylation and promotes palmitate lipotoxicity in ER-mutant breast cancer

Abstract Disclosure: D. Sessions: None. N. Spoelstra: None. M. Yu: None. J. Richer: None. Introduction: Breast cancer (BC) is the highest incidence cancer in women, and two-thirds of primary tumors are positive for estrogen receptor (ER). Aromatase inhibitors (AI) block synthesis of estrogens from androgens, but inadvertently increase circulating androgen levels. AI-resistant metastases often occur and ∼40% of tumors demonstrate activating mutations in the ER ligand binding domain, the most common being Y537S and D538G. Androgen receptor (AR) levels increase with long term estrogen deprivation (LTED) and anchorage independent survival. Although the role of AR in ER+ BC is controversial, our studies and others show that AR is elevated in BC resistant to tamoxifen, fulvestrant and AI therapy. However, the mechanisms by which AR supports resistant metastatic ER-mutant BC remain elusive. Recent work identified mitochondrial oxidative phosphorylation (OXPHOS) and fatty acid oxidation (FAO) as critical to persistent anti-estrogen-resistant BC. Interestingly, the common metastatic sites of liver and lung have high interstitial concentrations of palmitate (PA), a long-chain saturated fatty acid (FA) that supports metastases. PA is a substrate for FAO but can cause toxicity through endoplasmic reticulum stress (ERS), ROS, and apoptosis. Hypothesis: We postulate that AR supports AI-resistant BC by promoting OXPHOS and FAO and protecting against palmitate lipotoxicity at metastatic sites.Methods: To model the hormonal milieu of ER-mutant BC in women on AI therapy, we performed prolonged (>3 months) culture of CRISPR knock-in ER Y537S or D538G MCF7, T47D, and patient-derived UCD4 (D538G) cells in media with phenol red free charcoal stripped FBS and 1 nM dihydrotestosterone (DHT). We used the AR antagonist enzalutamide to interrogate effects of AR inhibition (ARi) on cell viability, FAO by Seahorse assay, apoptosis, ERS, and ROS in the context of PA exposure. Results: ARi decreased ER-mutant BC proliferation and decreased viability of PA-treated cells by 1.8-2.75 fold compared to vehicle (p<0.05, One-way ANOVA + Sidak’s test). ARi promoted PA-induced PARP cleavage and apoptosis in an ERS-independent manner that is not rescued by antioxidants. ARi decreased OXPHOS capacity by 50-70% and FAO specifically by 50-75% (p<0.001, One-way ANOVA + Sidak’s test). The effects of ARi on FAO do not appear to be due to AR regulation of the FAO rate-limiting enzyme CPT1A or FA transporter CD36. Conclusions: We conclude that activated AR promotes growth, OXPHOS, and resistance to palmitate lipotoxicity in ER-mutant BC. Since OXPHOS and FAO are critical to persistence under LTED (mimicking AI therapy), ARi may be an attractive means of anti-mitochondrial therapy to reduce mortality due to AI-resistant metastatic BC. Presentation: Saturday, July 12, 2025