Nearly 40 000 women die annually from breast cancer in the United States. Clinically available targeted breast cancer therapy is largely ineffective in triple negative breast cancer (TNBC), characterized by tumors that lack expression of the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (Her2). TNBC is associated with a poor prognosis. Previous reports show that aryl hydrocarbon receptor (AhR) partial agonist 2‐(4‐amino‐3‐methylphenyl)‐5‐fluorobenzothiazole (5F 203) selectively inhibits the growth of breast cancer cells, including those of the TNBC subtype. We previously demonstrated that 5F 203 induced the expression of putative tumor suppressor gene cytoglobin (CYGB) in breast cancer cells. In the current study, we determined that 5F 203 induces apoptosis and caspase‐3 activation in MDA‐MB‐468 TNBC cells and in T47D ER+ PR + Her2 − breast cancer cells. We also show that caspases and CYGB promote 5F 203–mediated apoptosis in MDA‐MB‐468 cells. 5F 203 induced lysosomal membrane permeabilization (LMP) and cathepsin B release in MDA‐MB‐468 and T47D cells. In addition, silencing CYGB attenuated the ability of 5F 203 to induce caspase‐3/‐7 activation, proapoptotic gene expression, LMP, and cathepsin B release in MDA‐MB‐468 cells. Moreover, 5F 203 induced CYGB protein expression, proapoptotic protein expression, and caspase‐3 cleavage in MDA‐MB‐468 cells and in MDA‐MB‐468 xenograft tumors grown orthotopically in athymic mice. These data provide a basis for the development of AhR ligands with the potential to restore CYGB expression as a novel strategy to treat TNBC.

Despite advances in targeted therapeutic agents for women with breast cancer that either express estrogen receptor (ER) alpha or over-expresses human epidermal growth factor receptor 2 (Her2), nearly 40,000 women die each year due to this malignancy. An investigational anticancer agent 2-(4-amino-3-methylphenyl)-5-benzothiazole (5F 203) potently inhibits the growth of breast cancer cells irrespective of either ER or Her2 status. In our current studies we used the Annexin V/PI assay to determine that 5F 203 induced a dose-dependent increase in the percentage of ER/Her2-positive T47D cells and ER/Her2-negative MDA-MB-468 breast cancer cells in early apoptosis. Cell death was not entirely caspase-dependent as pretreatment with pan-caspase inhibitor z-VAD-fmk only partially suppressed apoptosis. A pathway specific PCR-array revealed the ability for 5F 203 to increase the mRNA expression of pro-apoptotic genes bcl-2-antagonist/killer 1 (BAK1) and lymphotoxin alpha (LTA). Using the Acridine Orange assay, we found that 5F 203-mediated cell death was associated with lysosomal membrane permeabilization in both T47D and MDA-MB-468 cells. Additionally, we determined using the Cathepsin B assay that 5F 203 promoted the release of cathepsin B from the lysosomes in both breast cancer cell types. These data indicate 5F 203 upregulates BAK1 and LTA expression and promotes lysosomal cell death triggering the release of cathepsin B in breast cancer cells to confer its anticancer action. Citation Format: Eileen J. Brantley, Gabriell Thorne, Devin Daly, Dain Zylstra, Lancelot McLean. Investigational agent 5F 203 modulates apoptotic regulatory gene expression and induces lysosomal membrane permeabilization in breast cancer cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2102. doi:10.1158/1538-7445.AM2013-2102