Abstract 2908: Characteristics of triple negative breast cancer that result in sensitivity to englerin A
Authors: Shaffer Corena V. , Risinger April L. , Beutler John A. , Mooberry Susan L. 
Published: August 2018

Abstract
New approaches are needed to improve long term survival for patients with metastatic triple negative breast cancers (TNBCs). Patients with metastatic TNBC have a poorer 3-year survival compared to patients with metastatic HER2+ or ER/PR+ disease. Identifying new therapeutic targets of TNBC subgroups could provide opportunities for more effective treatments. We conduct a screening program to identify compounds with selective activity against a panel of TNBC cell lines, reasoning that mechanism of action studies of selective cytotoxins could identify new therapeutic targets. Englerin A was identified as a compound with highly selective actions against two TNBC cell lines: BT-549 (&gt;1800-fold selectively) and Hs578T (&gt;600-fold selectivity) as compared to other TNBC cell lines. Previous studies showed that englerin A is an agonist of TRPC1/4/5 non-specific cation channels and PKCθ in renal cancer cells.1–4 RT-PCR analyses showed that BT-549 and Hs578T cells express more TRPC4 than either the insensitive MDA-MB-231 TNBC cells or the A498 renal cancer cells. If BT-549 and Hs578T cells are sensitive to englerin A due to high expression of TRPC4 channels, then treatment with TRPC4 channel inhibitors should decrease sensitivity to englerin A. These studies show that the TRPC1/4/5 channel inhibitor Pico1455 caused a rightward shift in the concentration response curve of englerin A in both BT-549 and Hs578T cells. These data are consistent with TRPC1/4/5 channel activation contributing to englerin A-induced cell death of BT-549 and Hs578T cells. TRPC channels are non-specific cation channels, therefore their activation is expected to cause an influx of cations including calcium and sodium. Live cell calcium imaging showed the influx of calcium into BT-549 cells following englerin A treatment, further supporting the hypothesis that englerin A-induced cell death in these cell lines is the result of TRPC channel activation. TRPC channel activation initiates disruption of intracellular cation homeostasis, therefore we tested ouabain, a Na+/K+ATPase inhibitor, to determine if BT-549 and Hs578T cells were also sensitive to TRPC-independent methods of intracellular cation homeostasis disruption. BT-549 and Hs578T cells were 12- and 2-fold more sensitive to ouabain as compared to other TNBC cell lines, suggesting that some TNBC cells lines are particularly sensitive to disruption of intracellular cation homeostasis. Ongoing work is aimed at identifying additional factors in englerin A's mechanism of action in BT-549 and Hs578T cells. Our studies suggest that effective treatments for this subgroup of TNBC may include agents which disrupt cation homeostasis.
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Citation Format: Corena V. Shaffer, April L. Risinger, John A. Beutler, Susan L. Mooberry. Characteristics of triple negative breast cancer that result in sensitivity to englerin A [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 2908.