Abstract 3478: APC loss mediates doxorubicin response in breast cancer via changes in intracellular drug concentration and DNA repair pathways
Authors: Stefanski Casey , Keffler Kaitlyn , McClintock Stephanie , Milac Lauren , Prosperi Jenifer 
Published: August 2019

Abstract
Chemoresistance is one of the leading causes of breast cancer related deaths. Understanding the molecular basis for chemoresistance is essential for novel therapeutic advancement to improve patient outcome. The Adenomatous Polyposis Coli (APC) tumor suppressor is either mutated or hypermethylated in up to 70% of sporadic breast cancer; however, little is known about how APC loss contributes to chemoresistance. Using mammary tumor cells isolated from the ApcMin/+ mouse crossed to the Polyoma middle T antigen (PyMT) transgenic model, we demonstrated that APC loss decreased doxorubicin (DOX) induced apoptosis. DOX, a commonly used chemotherapeutic in breast cancer, inhibits topoisomerase IIa, resulting in double stranded DNA breaks, resulting in cell cycle arrest to allow repair or apoptosis. We made the novel observation that APC loss in MMTV-PyMT;ApcMin/+ cells activated signal transducer and activator of transcription 3 (STAT3) thereby increasing the expression of the drug efflux pump, multidrug resistance protein 1 (MDR1). Therefore, we hypothesized that APC loss prevents doxorubicin-mediated cell death through: 1) reduced intracellular DOX and 2) alterations in DNA damage and repair. To investigate the intracellular accumulation of DOX, we first used calcein incorporation assay and demonstrated that APC loss increased MDR1 activity, which was restored by an MDR1 inhibitor. In addition, MDR1 inhibition sensitized the MMTV-PyMT;ApcMin/+ cells to DOX-mediated apoptosis. To investigate the effect of APC loss on DNA damage repair pathways, we initially monitored damage recognition pathways after 24-hour DOX treatment. The MMTV-PyMT;ApcMin/+ cells exhibited decreased γH2AX and ataxia-telangiectasia mutated (ATM) phosphorylation following DOX treatment compared to controls, suggesting decreased DNA damage either through decreased intracellular DOX or enhanced DNA repair. Decreased phosphorylation of Chk1 and Chk2 was also observed in DOX-treated MMTV-PyMT;ApcMin/+ cells. Preliminary data to investigate if this decreased DNA damage was due to increased DNA damage repair, via monitoring γH2AX expression throughout treatment and recovery from DOX, suggest enhanced DNA repair in MMTV-PyMT;ApcMin/+ cells. Using the ATM inhibitor, we observed an increase in DOX sensitivity in MMTV-PyMT;ApcMin/+ cells. Alternatively, ATR inhibition did not affect DOX-mediated apoptosis. We will further investigate the efficiency of repair pathways through reporter plasmids and radiation induced damage to separate the drug transport alterations. Finally, we will test whether combination therapy of doxorubicin with an MDR1 inhibitor will reduce tumor burden in vivo. Taken together, APC loss mediates DOX resistance via reducing intracellular DOX and increasing DNA damage repair demonstrating the potential use of combination therapy to overcome chemoresistance.
Citation Format: Casey Stefanski, Kaitlyn Keffler, Stephanie McClintock, Lauren Milac, Jenifer Prosperi. APC loss mediates doxorubicin response in breast cancer via changes in intracellular drug concentration and DNA repair pathways [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3478.