Abstract 3821: Taxane resistance in breast cancer controlled by the APC tumor suppressor
Authors: Astarita Emily , Dieringer Megan , Prosperi Jenifer R. 
Published: August 2019

Abstract
Adenomatous Polyposis Coli (APC) is a multi-domain tumor suppressor protein that binds to proteins including β-catenin, axin, and microtubules (MTs). APC is lost in many epithelial cancers and up to 70% of sporadic breast cancers, with a tendency towards triple negative breast cancers (TNBCs). In a mouse breast cancer model of APC loss, MMTV-PyMT;ApcMin/+, our laboratory previously demonstrated that APC loss resulted in metaplastic-like tumors, a subtype of TNBCs that often develops resistance to chemotherapy. Using the human breast cancer cell line, MDA-MB-157, we created APC knockdown cells (APCKD) using lentiviral mediated shRNA knockdown of APC, which demonstrated resistance to the Taxane family chemotherapeutic agent, Paclitaxel (PTX). Given that PTX and APC both alter MT dynamics, we sought to understand the molecular mechanisms of APC-mediated resistance. Based on the mechanism of action of PTX, we hypothesized that genes involved in the G2/M transition would be responsible for PTX resistance. We have also performed an unbiased analysis of transcriptomic changes downstream of APC loss to identify potential therapeutic targets to overcome PTX resistance. Cell cycle analysis demonstrated that the cell cycle profile of APCKD cells was not drastically changed after PTX treatment. However, PTX induction of p21 specifically in the APCKD cells suggests that APCKD cells may become senescent to avoid apoptosis. We next analyzed the expression of the G2/M checkpoint proteins, CDK1 and cyclin B1, by western blot. CDK1 expression was significantly increased in APCKD cells compared to control with no changes in CDK1 phosphorylation (Thr14, Thr161, Tyr15). Interestingly, there was no effect of PTX treatment in either cell line. Based on these findings, we performed combination studies using a CDK1 inhibitor (RO-3306) with PTX treatment. PARP (total and cleaved) was used to measure apoptosis, and showed enhanced cleavage in the APCKD cells specifically after combination treatment. In addition to the cell cycle analysis, we performed RNA-sequencing on parent and APCKD cells to understand the transcriptomic changes downstream of APC loss. Upon validation of results by qRT-PCR and western blot, we identified that APCKD cells had altered expression of ZNF366, ADAMTSL1, SELENBP1, and CYB5R2. These 4 genes are known to be involved in the development of breast cancer and response to chemotherapy. Ongoing studies in the laboratory will investigate the effect of manipulating expression of these genes. Overall, our studies strive to understand the molecular nature of PTX resistance in APCKD cells, and to identify a therapeutic target to work as a targeted treatment for some TNBC types.
Citation Format: Emily Astarita, Megan Dieringer, Jenifer R. Prosperi. Taxane resistance in breast cancer controlled by the APC tumor suppressor [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 3821.