Abstract 4281: LT-IIc, a bacterial type II heat-labile enterotoxin, induces specific lethality in triple negative breast cancer cells by modulaton of autophagy and induction of apoptosis and necroptosis

Despite the recent advances in breast cancer treatment, triple negative breast cancer (TNBC) remains a serious health problem with limited treatment options. Poor prognosis is due to the development of chemoresistance. To discover novel therapeutic approaches to treat TNBC, we screened cholera toxin (CT) and the members of the bacterial type II heat-labile enterotoxin family (LT-IIa, LT-IIb, and LT-IIc) for their capacity to induce cell death in TNBC cells. LT-IIc, but not the other toxins, significantly reduced viability of the TNBC cell lines BT549 and MDA-MB-231 (IC50 = 82.32 nM). LT-IIc had no significant cytotoxic effects on MCF10A (IC50 = 2600 nM), a non-tumorigenic breast epithelial cell line, and minimal effects on MCF7 and T47D, ER+ breast cancer cells, or SKBR-3, HER2+ cells. LT-IIc stimulated autophagy through inhibition of the mTOR pathway, while simultaneously inhibiting autophagic progression, as observed by the accumulation of LC3B-II and p62 proteins. Morphologically, LT-IIc induced accumulation of enlarged LAMP2+ autolysosomes selectively in TNBC cells. Bafilomycin A1, an inhibitor of autophagic flux, blocked the formation and/or retention of these autolysosomes. The increase in caspase 3, 7 activity and annexin V staining indicated that LT-IIc induced an apoptotic response. Co-treatment with necrostatin-1, however, demonstrated that the lethal response to LT-IIc is elicited, at least in part, by concomitant induction of necroptosis. Collectively, these experiments demonstrate that LT-IIc acts bifunctionally, inducing autophagy while simultaneously blocking autolysosomal progression in TNBC cells, resulting in specific cytotoxicity in this breast cancer subtype. Citation Format: Patricia A. Masso-Welch, Sofia Girald Berlingeri, Natalie D. King-Lyons, Lorrie Mandell, John C. Hu, Christopher Greene, Matthew Federowicz, Peter Cao, Yasser Heakal. LT-IIc, a bacterial type II heat-labile enterotoxin, induces specific lethality in triple negative breast cancer cells by modulaton of autophagy and induction of apoptosis and necroptosis [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 4281.