**Dietary Compound Isoliquiritigenin, an Antioxidant from Licorice, Suppresses Triple-Negative Breast Tumor Growth via Apoptotic Death Program Activation in Cell and Xenograft Animal Models**

**Po-Han Lin 1, Yi-Fen Chiang 1,**†**, Tzong-Ming Shieh 2,3,**†**, Hsin-Yuan Chen 1, Chun-Kuang Shih 1, Tong-Hong Wang 4,5, Kai-Lee Wang 6, Tsui-Chin Huang 7, Yong-Han Hong 8, Sing-Chung Li 1 and Shih-Min Hsia 1,9,10,11,\***


Received: 22 January 2020; Accepted: 7 March 2020; Published: 10 March 2020

**Abstract:** Patients with triple-negative breast cancer have few therapeutic strategy options. In this study, we investigated the effect of isoliquiritigenin (ISL) on the proliferation of triple-negative breast cancer cells. We found that treatment with ISL inhibited triple-negative breast cancer cell line (MDA-MB-231) cell growth and increased cytotoxicity. ISL reduced cell cycle progression through the reduction of cyclin D1 protein expression and increased the sub-G1 phase population. The ISL-induced apoptotic cell population was observed by flow cytometry analysis. The expression of Bcl-2 protein was reduced by ISL treatment, whereas the Bax protein level increased; subsequently, the downstream signaling molecules caspase-3 and poly ADP-ribose polymerase (PARP) were activated. Moreover, ISL reduced the expression of total and phosphorylated mammalian target of rapamycin (mTOR), ULK1, and cathepsin B, whereas the expression of autophagic-associated proteins p62, Beclin1, and LC3 was increased. The decreased cathepsin B cause the p62 accumulation to induce caspase-8 mediated apoptosis. In vivo studies further showed that preventive treatment with ISL could inhibit breast cancer growth and induce apoptotic and autophagic-mediated apoptosis cell death. Taken together, ISL exerts an effect on the inhibition of triple-negative MDA-MB-231 breast cancer cell growth through autophagy-mediated apoptosis. Therefore, future studies of ISL as a supplement or alternative therapeutic agen<sup>t</sup> for clinical trials against breast cancer are warranted.

**Keywords:** isoliquiritigenin (ISL); triple-negative breast cancer; apoptosis; autophagy
