Abstract 554: APC loss alters DNA damage repair in breast cancer cells

Abstract

Abstract Breast cancer is the second leading cause of cancer deaths in U.S. women. Many of these deaths have been attributed to the development of chemotherapeutic resistance. Thus, understanding the process of chemotherapeutic resistance will help develop new approaches to combat breast cancer and ultimately cause a decrease in breast cancer related deaths. The Adenomatous Polyposis Coli (APC) tumor suppressor is silenced or mutated in up to 70% of sporadic breast cancer; however, the possible effects of APC loss on chemoresistance has not been elucidated. Using the ApcMin/+ mouse crossed to the Polyoma middle T antigen (PyMT) transgenic model, we previously showed that APC loss decreased cisplatin and doxorubicin-induced apoptosis. We also have shown that APC loss increased mRNA expression of the multidrug resistance protein 1 (MDR1), which was augmented by doxorubicin treatment. Treatment with doxorubicin and A69, a STAT3 inhibitor, normalized MDR1 gene expression in MMTV-PyMT;ApcMin/+ cells. Here, we hypothesize that loss of APC causes an increase in doxorubicin efflux and a decrease in DNA damage. First, we show that APC loss increased MDR1 activity as measured by calcein flux. To determine whether MDR1 inhibition would restore doxorubicin induced DNA damage, a combination treatment of doxorubicin and the MDR1 inhibitor, Valspodar, was able to restore doxorubicin sensitivity in MMTV-PyMT;ApcMin/+ cells compared to the control MMTV-PyMT;Apc+/+ cells. In addition, we have shown a decrease of γH2AX, a marker of damaged DNA, in MMTV-PyMT;ApcMin/+ cells treated with doxorubicin suggesting that the MMTV-PyMT;ApcMin/+ cells either have less damage due to increased drug efflux via MDR1 or due to increased DNA damage repair. Due to APC binding to Topoisomerase IIa, we also examined the degree of Topoisomerase IIa inhibition (via doxorubicin and etoposide) in MMTV-PyMT;ApcMin/+ compared to MMTV-PyMT;Apc+/+ cells. Topoisomerase IIa activity will also be measured to determine the effect of APC loss on Topoisomerase IIa. To test the hypothesis that loss of APC affected DNA damage repair, the expression of phosphorylated H2AX was measured in MMTV-PyMT;ApcMin/+ and MMTV-PyMT;Apc+/+ cells treated with the following: doxorubicin, etoposide, Valspodar, or a combination treatment. The longevity of doxorubicin-induced double stranded breaks was observed. Future studies will measure DNA damage repair pathway efficiency via reporter plasmids. We also will measure downstream signaling of yH2AX. Overall with an understanding of the role of APC in DNA damage repair, the use of a combination therapy could be advantageous to overcome chemotherapeutic resistance. Citation Format: Stephanie M. McClintock Batista. APC loss alters DNA damage repair 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 554.