Abstract P4-02-12: Oxidative phosphorylation and NRF2 activation mediate resistance to estrogen deprivation in ER+ breast cancer
Authors: Tau Steven , Miller Todd W. 
Published: March 2022

Abstract
Despite the success of adjuvant endocrine therapy in the treatment of ER+ breast cancer, recurrences occur in ~1/3 of patients with most becoming metastatic and ultimately fatal. Recurrence can be traced to drug-tolerant persister cancer cells (DTPs) that can survive years of endocrine therapy. Targeting DTPs is an attractive therapeutic approach to prevent cancer recurrence, but it has been hampered by a lack of understanding of DTP biology. We observed that subpopulations of ER+ breast cancer cells persist when challenged with estrogen deprivation and exhibit slower cell cycling, which is reversed when estrogen is restored. A CRISPR/Cas9 knock-out screen in MCF-7 DTPs identified genes that modulate fitness and survival during estrogen deprivation. Pathways analysis of the genes most essential for estrogen-independent growth revealed that mitochondrial function and oxidative phosphorylation (OXPHOS) was a top enriched pathway. Validation experiments revealed that knock-out of Mitochondrial Complex I genes conferred sensitivity to estrogen deprivation. We measured the extent of metabolic reprogramming in ER+ breast cancer cells induced by estrogen deprivation. A global proteomics study revealed widespread changes in the abundance of proteins involved in metabolism; specifically, estrogen deprivation decreased levels of glycolytic proteins while maintaining high levels of Complex I and mitoribosome proteins. Following 3 weeks of estrogen deprivation, restoration of estrogen remediated metabolic reprogramming from estrogen deprivation with higher levels of glycolysis proteins and lower mitochondrial Complex I and mitoribosome proteins. MitoTracker dye staining for mitochondria revealed increased mitochondrial content in estrogen-deprived cells, reversed by estrogen restoration in both MCF-7 and T47D DTPs. Measurement of oxygen consumption and extracellular acidification rate by Seahorse assay confirmed a shift towards greater ATP productivity by mitochondria in the setting of estrogen deprivation. The therapeutic potential of targeting mitochondrial function in DTPs was tested with the Complex I inhibitor IACS-010759. Greater drug sensitivity occurred with longer periods of estrogen deprivation in DTPs, with increased cell death at 14 and 28 days of estrogen deprivation compared to cells without previous hormone deprivation. We postulated that increased mitochondrial oxidative phosphorylation subsequently increases reactive oxygen species and oxidative stress. Indeed, DTPs exhibited an anti-oxidant stress response driven by the transcription factor nuclear factor erythroid 2-related factor 2 (NRF2), a signature exhibited by ER+ tumors undergoing neoadjuvant endocrine therapy. Knock-down of NRF2 decreased survival in estrogen-deprived DTPs but not in estrogen-replete cells. The survival of NRF2-deficient DTPs was rescued by the antioxidant N-acetyl cysteine (NAC), affirming the role of an anti-oxidative stress response in preserving DTP survival. Decreased DTP survival was concomitant with an increase in ROS. These findings establish the scope of metabolic reprogramming in DTPs and offer Mitochondrial Complex I and NRF2 inhibition as novel therapeutic strategies for eradicating DTPs in ER+ breast cancer.
Citation Format: Steven Tau, Todd W. Miller. Oxidative phosphorylation and NRF2 activation mediate resistance to estrogen deprivation in ER+ breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-02-12.