Abstract P1-08-22: The role of DAXX as a critical growth regulator of triple negative breast cancer
Authors: Osipo Clodia , Wyatt Debra , Fernandez Michelle , Peiffer Daniel S , Albain Kathy S 
Published: March 2022

Abstract
Background: Triple negative breast cancer (TNBC) is a heterogeneous disease that frequently presents at a metastatic stage or upon initial early stage diagnosis, and may recur quickly. Treatment options are limited outside of cytotoxic chemotherapy in the adjuvant or metastatic setting and there is an immediate need to identify and validate novel biomarkers that predict sensitivity to chemotherapy. Previously, it was shown that the pro-apoptotic protein DAXX is required for paclitaxel-mediated breast cancer cell death. Based on these data and our recent discoveries that DAXX is a potent inhibitor of breast cancer stem cells in ER+ breast cancer (Peiffer et al., Cancer Res. 2019; Peiffer et al., NPJ Breast Cancer. 2020), we next explored the role of DAXX in TNBC. We found that high DAXX expression in TNBC cells regulates cell proliferation and activities of PARP-1 and JNK; two pathways required for cell death. The goals of the current work were to investigate the mechanism by which DAXX regulates proliferation at the level of PARP-1 and/or JNK and test if DAXX expression predicts sensitivity to carboplatin, paclitaxel, doxorubicin, the PARP-1 inhibitor olaparib, or the JNK inhibitor SP600125 in TNBC cells. Methods: Proliferation and cell cycle analysis were performed in three TNBC cell lines (MDA-MB-231, BT549, and MDA-MB-468) under DAXX expression and depletion conditions using an siRNA approach. Sensitivity to carboplatin, paclitaxel, and doxorubicin was determined by measuring proliferation in a dose-dependent manner. PARP-1 activity was assessed by detecting global protein PARylation levels using Western blotting. JNK activation was analyzed by measuring phosphorylated JNK levels. A JNK inhibitor (SP600125) was used to determine if JNK signaling was responsible for restricting cell proliferation and if DAXX was required for the elevated JNK activity. Olaparib was used to assess the role of PARP-1 activation in DAXX-depleted cells. Results: The TNBC cell lines expressed high levels of DAXX. Depletion of DAXX increased cell proliferation by promoting cell cycle progression through the S-phase. The consequence of this increased cell proliferation was decreased sensitivity to carboplatin and paclitaxel, but not to doxorubicin. Mechanistically, at least one chemotherapeutic agent, carboplatin induced JNK activation and DAXX was required for the increased JNK activity. The JNK inhibitor (SP600125) partially reversed resistance to carboplatin in DAXX-depleted cells. Additionally, genetic knockdown of DAXX resulted in elevated PARP-1 activity as measured by global PARylation of proteins. PARP-1 blockade using olaparib also partially reversed resistance to carboplatin in DAXX-depleted cells. Conclusions: These results suggest that DAXX is a critical growth regulator and predictor of response to carboplatin and paclitaxel in triple negative breast cancer cells. DAXX is necessary to activate the JNK pathway in response to carboplatin and it is this carboplatin-mediated JNK signaling that inhibits cell proliferation. Furthermore, DAXX limits PARP-1 activity and is necessary for increased sensitivity to carboplatin or paclitaxel. Data herein support assessing DAXX expression in triple negative tumors to determine if high DAXX expression predicts increased sensitivity to carboplatin, paclitaxel, and if DAXX-low expressing TNBC cells require addition of a JNK or PARP-1 inhibitor.
Citation Format: Clodia Osipo, Debra Wyatt, Michelle Fernandez, Daniel S Peiffer, Kathy S Albain. The role of DAXX as a critical growth regulator of triple negative 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 P1-08-22.