Abstract 4463: Loss of APC mediates doxorubicin resistance in breast cancer cells

Abstract

Abstract Resistance to chemotherapy is one of the leading causes of breast cancer related deaths. Understanding the molecular basis for chemoresistance is essential for the advancement of novel therapeutic approaches to ultimately improve patient outcomes. 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 loss of APC contributes to chemoresistance. Using the ApcMin/+ mouse crossed to the Polyoma middle T antigen (PyMT) transgenic model, we demonstrated that APC loss decreased cisplatin and doxorubicin-induced apoptosis. We previously showed that loss of APC in MMTV-PyMT;ApcMin/+ cells induced STAT3-activation resulting in an enhanced tumor initiating cell (TIC) population and increased multidrug resistance protein 1 (MDR1) expression. Therefore, we hypothesized that APC loss increased MDR1 activity and decreased doxorubicin-mediated DNA damage, ultimately preventing cell death. Using calcein incorporation, we first demonstrated that APC loss increased MDR1 activity which was normalized to WT control levels by the MDR1 inhibitor, Valspodar. We next showed that MDR1 inhibition in MMTV-PyMT;ApcMin/+ cells could restore doxorubicin-mediated apoptosis to that of MMTV-PyMT;Apc+/+ treated cells. Due to STAT3 inducing MDR1 activity and TIC population, MDR1 inhibition in MMTV-PyMT;ApcMin/+ cells was also shown to reduce the TIC population. In addition, we are investigating the effect of APC loss on DNA damage repair pathways. Preliminary studies have shown decreased γH2AX in MMTV-PyMT;ApcMin/+ cells treated with doxorubicin, suggesting a decrease in DNA damage. Downstream signaling of γH2AX, measured by phosphorylation of ATM, also demonstrated decreased DNA damage with ongoing studies measuring phosphorylation of ATR. Due to APC binding to topoisomerase IIα (topo IIα), a target of doxorubicin, the activity of topo IIα was measured to address whether this decreased DNA damage was attributed to impaired topo IIα activity or decreased intracellular doxorubicin due to increased MDR1 efflux. Future studies will investigate whether MDR1 inhibition in combination with doxorubicin will reduce tumor burden in vivo. Taken together, APC loss mediates doxorubicin resistance via enhanced MDR1 activity and altered DNA damage repair pathways demonstrating the potential use of combination therapy to overcome resistance to chemotherapy. Citation Format: Casey D. Stefanski, Kaitlyn Keffler, Stephanie McClintock, Jenifer R. Prosperi. Loss of APC mediates doxorubicin resistance in breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4463.