br tected by western blotting analysis
tected by western blotting analysis.
Results: Caudatin enhanced TRAIL-induced apoptosis in human breast cancer cells. This sensitization was
achieved by upregulating death receptor 5 (DR5). Knockdown of DR5 abolished the enhancing eﬀect of caudatin
on TRAIL responses. The caudatin-induced upregulation of DR5 was accompanied by increased expression of
CHOP and phosphorylation of p38 MAPK and JNK. CHOP knockdown blocked caudatin-upregulated DR5 ex-
pression. Moreover, cotreatment of breast cancer U46619 with p38 MAPK and JNK inhibitors significantly coun-
teracted the caudatin-induced expression of DR5.
Conclusion: Our results showed that caudatin sensitized breast cancer cells to TRAIL-induced apoptosis through
activation of CHOP, p38 MAPK and JNK-mediated upregulation of DR5 expression. The combination of TRAIL
and caudatin may be a promising therapeutic approach for the treatment of breast cancer.
Breast cancer is now the most common cancer in women in both developed and less-developed regions. Breast cancer is also the leading cause of cancer death in women in China, with nearly 268.6 thousand new cases having been diagnosed in 2015 (Chen et al., 2016b). Al-though good progress has been achieved and many promising cancer drugs have been widely used in the past years, significant side eﬀects are induced by continuous cancer treatment (López-Tarruella et al., 2017; Trail et al., 2018). Therefore, the development of potential therapeutic agents that induce cancer-selective apoptosis is essential for breast cancer treatment.
Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), a
member of the TNF cytokine superfamily, has attracted extensive at-tention because of its capacity to selectively induce apoptosis in cancer cells by binding to TRAIL-R1 (DR4) or TRAIL-R2 (DR5) (Lemke et al., 2014; Tse et al., 2014). Upon TRAIL binding, extracellular DR4 and DR5 trigger the formation of the death-inducing signaling complex (DISC), characterized by recruitment of the adaptor protein FAS-asso-ciated death domain protein (FADD), which binds to the intracellular death domain of the receptors and the initiator caspase-8. Conse-quently, downstream eﬀector caspases are activated and then digest cellular proteins to cause apoptotic cell death (Haimovici et al., 2017; Thorburn A, 2007).
However, certain types of cancer cells have demonstrated TRAIL resistance, caused by overexpression of anti-apoptotic genes, defects in
Abbreviations: CHOP, C/EBP homologous protein; DR4, death receptor 4; DR5, death receptor 5; ER stress, endoplasmic reticulum stress; ERK, extracellular signal-regulated kinase; JNK, c-Jun N-terminal kinase; MAPKs, mitogen-activated protein kinases; TRAIL, tumor necrosis factor-related apoptosis-inducing ligand
E-mail address: [email protected] (F.-Z. Wang). 1 These authors contributed equally.
TRAIL signaling components, or decreased expression or mutation of DR4 or DR5 (Zhu et al., 2016; Twomey et al., 2015); these resistant cells present new challenges for long-term cancer treatment. Interestingly, there is growing interest in combining chemotherapeutic drugs with TRAIL to overcome this resistance, and many studies have shown that cancer cells with TRAIL resistance can be sensitized by bioactive compounds from natural products, including auriculasin (Cho et al., 2019), curcumin (Park et al., 2013) and tanshinone IIA (Lin et al., 2015).
As an excellent source for the development of modern drugs used in the treatment of cancer, natural products play critical roles in modern drug development and cancer chemoprevention due to their excellent pharmacological activities and low toxicity (Kim et al., 2017; Yuan et al., 2017). Caudatin is a species of C-21 steroidal glycosides isolated from the roots of Cynanchum auriculatum that has been shown to induce cell cycle arrest and caspase-dependent apoptosis in HepG2 cells (Fei et al., 2012a). Caudatin targets the TNFAIP1/NF-κB signaling pathway to suppress tumor progression in human uterine cancer, con-firming its anti-tumor activity (Tan et al., 2016). Our previous study also showed that caudatin could inhibit human lung cancer cell growth and angiogenesis by modulating the GSK3β/β-catenin pathway (Fei et al., 2012b).
In this study, we tested whether caudatin could potentiate TRAIL-induced apoptosis in both the human estrogen receptor-positive MCF-7 and human triple-negative MDA-MB-231 breast carcinoma cell lines. Our results showed that caudatin could increase the rate of TRAIL-as-sociated cell death by upregulating DR5 expression in a CHOP-depen-dent manner. Moreover, inhibition of p38 MAPK and JNK blocked caudatin-induced DR5 expression. Our findings lay the groundwork for the planning of future studies on the anticancer activity of caudatin.