Abstract P3-11-01: Increasing DAXX Expression in ER+ Breast Cancer Cells to Overcome Endocrine Therapy Resistance

Abstract Background: New treatment paradigms are needed to overcome resistance to endocrine therapy (ET; tamoxifen or aromatase inhibitors, AI) in ER+ breast cancer (BC). ET resistance is due to survival of breast cancer stem cells (BCSCs) that contribute to relapse of ER+ BC. Notch signaling drives BCSCs. In order to identify Notch specific biomarkers for the purpose of patient selection for anti-Notch therapy, we conducted a pre-surgical biomarker window study combining ET plus MK-0752, a -secretase inhibitor (GSI). Death Associated Protein 6 (DAXX) was discovered to be a novel Notch1 target gene and necessary for GSI-mediated inhibition of BCSCs. Subsequently, we found that DAXX alone was sufficient to inhibit BCSCs. In this current study, we investigated the mechanism by which high DAXX expression inhibited growth of ET resistant ER+ BC cells in vitro and in vivo. Methods: Isogenic ER+ BC cell lines (parental MCF-7, ET resistant MCF-7/5C, parental T47D, and ET resistant T47D-ED) were used. Cells were cultured in estrogen-deprived medium for more than 1 year to mimic AI use. DAXX was depleted using siRNA or overexpressed using a pCMV-expression vector. Bulk cell proliferation was analyzed in response to estrogen depletion or increasing concentrations of 17-estradiol. BCSC survival was measured using the mammosphere-forming assay. Tumor onset and burden were measured by injecting DAXX-expressing or depleted mammospheres into mammary fat pads of female, athymic nude mice. Recurrence of an ER+ PDX tumor (BCM 5097) was measured after withdrawal of estrogen. RNA sequencing identified enriched genes and pathways that required DAXX. Based on these results, cell death was assessed using Annexin V/7-AAD flow cytometry, PARP-1 and Caspase 8 cleavage, phosphorylation of JNK, and expression of apoptotic protein regulators, BIM, BAX, Bcl-2, and Bcl-xL. JNK signaling was inhibited using SB600125. Results: Estradiol stimulated proliferation of parental, ER+ MCF-7 and T47D cells. In contrast, estradiol inhibited proliferation of isogenic ET resistant MCF-7/5C and T47D-ED cells in a concentration and ER-dependent manner. Estradiol induced the DAXX protein in both ET sensitive and resistant cells. DAXX was required for BCSC survival in ET sensitive cells. However, once ER+ cells acquired resistance to ET, DAXX was necessary and sufficient to inhibit both bulk cell proliferation and BCSC survival, suggesting that increasing DAXX might be a novel approach to overcome ET resistance. In mice, high DAXX expression significantly inhibited tumor onset and burden of ET resistant tumors compared to DAXX-depleted tumors. Low DAXX expression was significantly associated with recurrence of an ER+ PDX tumor (BCM 5097) after withdrawal of estrogen. RNA sequencing revealed that DAXX activated an anti-neoplastic gene signature, including transcription factors that regulate cell death genes including the Bcl2-family. DAXX was required for high BIM expression and low levels of Bcl-xL. DAXX was necessary and sufficient to induce apoptosis, PARP-1 cleavage, and phosphorylation of JNK in ET resistant cells. A selective JNK inhibitor, SB600125 rescued DAXX-mediated inhibition of ET resistant bulk cell proliferation and BCSC survival, suggesting that high DAXX expression activates JNK signaling to regulate apoptotic proteins to induce cell death of BCSCs-derived from ET resistant BC. Conclusions: Expressing high DAXX levels is a potent method to inhibit ET-resistant BC cell proliferation and BCSC survival. The mechanism by which DAXX inhibits ET-resistant BC is through activation of JNK signaling, regulation of pro-apoptotic genes, and induction of apoptosis. The translational impact of this research is to identify novel agents that can increase DAXX expression and test them pre-clinically and in clinical trials for patients with ET-resistant breast cancer. Citation Format: Clodia Osipo, Kathy S. Albain, Daniel Peiffer, Debra Wyatt. Increasing DAXX Expression in ER+ Breast Cancer Cells to Overcome Endocrine Therapy Resistance [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P3-11-01.