Abstract. Triple-negative breast cancer (TNBC) is an aggressive heterogeneous cancer subgroup with a higher rate of distant recurrence and a poorer prognosis compared to other subgroups. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is an attractive molecule that induces cell death in various tumor cells without causing cytotoxicity to normal cells; however, primary or acquired resistance to TRAIL often limits its efficacy in cancer patients. To develop combination therapies to improve TRAIL efficacy and/or to overcome the resistant mechanism, we screened 138 medicinal plant extracts against TRAIL-sensitive and -insensitive TNBC cell lines, MDA-MB-231 and MDA-MB-468. Among them, 5 plant extracts, Uvaria dac, Artemisia vulgaris, Cortia depressa, Dichasia bengalensis and Cinnamomum obtusifolium did not cause apparent cytotoxicity (<20%) as a single regimen, but showed significant synergistic effects in combination with TRAIL against both cell lines. Moreover, Uvaria dac, Artemisia vulgaris and Cinnamomum obtusifolium were found to suppress the phosphorylation of p65 that is involved in TRAIL-resistant mechanisms. These observations suggest that the identified plant extracts in combination with TRAIL could lead to potential therapeutic benefits for cancer patients in the clinical setting. IntroductionBreast cancer is one of the most frequently diagnosed types of cancer and is also the leading cause of cancer-related mortality among women, representing 23% of total cancer cases and 14% of cancer-related mortality (1). Breast cancer is classified into different categories according to the expression of three receptors, estrogen receptor (ER), progesterone receptor (PR), and HER2/Neu (2). A triple-negative breast cancer (TNBC), which is ER-negative, PR-negative, and HER2-negative, is one of the most aggressive forms that accounts for 15-25% of all breast cancer cases and is associated with a poor prognosis and unresponsiveness to the usual endocrine therapies (3-5).The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a member of the TNF superfamily that can initiate apoptosis via the activation of death receptor 4 (DR4) and death receptor 5 (DR5). As TRAIL can selectively induce apoptosis in cancer cells, including breast cancer cells, without causing toxicity to normal cells (6-8), it could be a safe medication for cancer patients; however, primary or acquired resistance to TRAIL is often observed which may limit its efficacy in cancer patients (6). Therefore, increasing TRAIL efficacy by combining with natural medicines or other chemotherapeutic agents is an important strategy in the treatment of breast cancer in the clinical setting. We previously identified that vanillin, an active constituent of vanilla, enhances TRAIL-induced apoptosis in cancer cells via suppression of NF-κB (9). This finding prompted us to search for other natural products that can sensitize cancer cells to TRAIL or that can overcome the TRAIL-resistant mechanism.In the present study, we screened 138 medicin...

Materials and Methods Drugs and reagentsCinobufagin and cinobufotalin were purchased from Wako Pure