Abstract 5428: Sigma-2 receptor-induced cell death: a novel approach to triple-negative breast cancer treatment

Triple negative breast cancers (TNBC) comprise 10-20% of all breast cancers. The 5-year survival rate of women with metastatic TNBC is less than 30%, and the mortality rate is nearly 100%, despite adjuvant chemotherapy (Lehmann, B.D. et al., 2011). TNBC lacks estrogen receptors (ER), progesterone receptors (PR), and human epidermal growth factor (HER2) amplification, and is particularly aggressive and drug-resistant. There is a lack of targeted therapies available for TNBC, as current endocrine and drug therapies that target ER, PR, or HER2 are ineffective (Foulkes, W.D. et al., 2010). Sigma-2 receptors are highly upregulated in all tumor cell lines studied thus far, and sigma-2 agonists induce cell death in these cancer cells (Crawford and Bowen, 2002). We previously reported that CM572, an irreversible sigma-2 partial agonist, had cytotoxic effects in MCF-7 breast cancer cells (Nicholson et al., Proc. Amer. Assoc. Cancer Res. 74: #3237, 2014). Here, we present initial findings that validate the sigma-2 receptor as a potential chemotherapeutic target for the treatment of TNBC. In the MDA-MB-231 TNBC cell line, the estimated B max value for sigma-2 receptors (65.7 ± 17.0 fmol/mg protein) is approximately 20-fold higher than the B max for sigma-1 receptors (3.4 ± 2.4 fmol/mg protein). The sigma-2 selective ligands CM572, SV119, and siramesine (sigma-2 K i values of 14.7 nM, 5.2 nM, and 0.12 nM, respectively) caused comparable cell death in a dose-dependent manner in TNBC cells and the much less aggressive MCF-7 cells (which have ER and PR) after 48 hour treatment. CM572 had an EC 50 of 1.93 μM in MDA-MB-231 cells (95.0 ± 2.52% maximal cytotoxicity) and 1.04 μM in MCF-7 cells (89.1 ± 7.59% maximal cytotoxicity). SV119 had an EC 50 of 23.3 μM in MDA-MB-231 cells (100 ± 0.49% maximal cytotoxicity) and 17.6 μM in MCF-7 cells (100 ± 1.11% maximal cytotoxicity). Siramesine had an EC 50 of 5.14 μM in MDA-MB-231 cells (99.7 ± 0.47% maximal cytotoxicity) and 2.81 μM in MCF-7 cells (99.8 ± 0.26% maximal cytotoxicity). Therefore, sigma-2 ligands have similar potencies in TNBC and non-triple negative breast cancer cells, indicating that these drugs have cytotoxic effects that are independent of triple negative receptor status. Treatment with 30 μM of SV119 for 24-48 hours yielded decreased protein levels of full-length Bid, indicating activation of this pro-apoptotic member of the Bcl-2 family. Bid cleavage was previously found in sigma-2 ligand-mediated apoptosis in SK-N-SH neuroblastoma cells (Nicholson et al., Proc. Amer. Assoc. Cancer Res. 74: #3237, 2014), and it may be caused by the involvement of caspases or other proteases. In summary, the data show that the sigma-2 receptor is able to induce apoptotic cell death in TNBC cells. Sigma-2 selective ligands CM572, SV119, and siramesine produced significant cytotoxic effects on TNBC cells, suggesting that sigma-2 receptors are an exciting novel therapeutic target for TNBC. Citation Format: Zongyi Liu, Hilary E. Nicholson, Wayne D. Bowen. Sigma-2 receptor-induced cell death: a novel approach to triple-negative breast cancer treatment. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5428. doi:10.1158/1538-7445.AM2015-5428