Abstract 3246: APC loss prevents doxorubicin-induced cell death by effluxing drug and increasing a chemoresistant cell population

Abstract Chemoresistance is a leading cause of breast cancer related deaths. Therefore, understanding the molecular basis for chemoresistance is essential for novel therapeutic advancement and improving patient outcome. The Adenomatous Polyposis Coli (APC) tumor suppressor is lost 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 made the novel observation that APC loss decreased doxorubicin (DOX) induced apoptosis. Therefore, we examined the mechanisms contributing to DOX resistance with APC loss to identify combination therapy options. We previously showed 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). ATP-binding cassette (ABC) transporters, such as MDR1, are well established to contribute to drug resistance through exporting drugs from the cell. Furthermore, tumor initiating cells (TICs) have increased expression of ABC transporters and are known to be chemoresistant. We showed that MMTV-PyMT;ApcMin/+ cells have an increased TIC population. In addition, decreased intracellular DOX accumulation was observed in APC-deficient cells suggesting enhanced drug efflux. To investigate this decreased intracellular DOX accumulation, we measured the expression and activity of the drug export pumps, MDR1 and multidrug resistance protein 1 (MRP1). Using a small molecule inhibitor (Valspodar), we found that MDR1 inhibition restored the DOX-induced cleaved caspase 3 expression, demonstrating that MDR1 contributes to DOX resistance. MDR1 inhibition also increased the intracellular DOX accumulation in the APC-deficient cells and reduced the TICs. Genetic manipulation to silence MRP1 also increased DOX-induced apoptosis, increased DOX accumulation, and decreased the TICs in MMTV-PyMT;ApcMin/+ cells. To target both MDR1 and MRP1, a small molecule inhibitor (Reversan) was used to see if affecting both would offer more benefit. We again found that Reversan restored DOX sensitivity in MMTV-PyMT;ApcMin/+ cells through increasing DOX accumulation and decreasing TICs. Future studies include using a panel of human breast cancer cell lines to determine correlation of APC expression and DOX sensitivity. In addition, we will also evaluate patient tumor protein lysates for APC, MDR1, and MRP1 expression, followed by correlation studies to determine whether expression of APC inversely correlates with MDR1 or MRP1. Taken together, APC loss mediates DOX resistance via increasing DOX export and the TIC population demonstrating the potential use of combination therapy to overcome chemoresistance. Citation Format: Casey D. Stefanski, Janna Kotsen, Amy Bernard, Jenifer Prosperi. APC loss prevents doxorubicin-induced cell death by effluxing drug and increasing a chemoresistant cell population [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3246.