Apigenin is a bioactive flavone in several herbs including parsley, thyme, and peppermint. Apigenin possesses anti-cancer and anti-inflammatory properties; however, whether apigenin enhances TRAIL-mediated apoptosis in cancer cells is unknown. In the current study, we found that apigenin enhanced TRAIL-induced apoptosis by promoting caspase activation and death receptor 5 (DR5) expression and a chimeric antibody against DR5 completely blocked the apoptosis. Apigenin also upregulated reactive oxygen species (ROS) generation; however, intriguingly, ROS inhibitors, glutathione (GSH) or N-acetyl-l-cysteine (NAC), moderately increased apigenin/TRAIL-induced apoptosis. Additional results showed that an autophagy inducer, rapamycin, enhanced apigenin/TRAIL-mediated apoptosis by a slight increase of ROS generation. Accordingly, NAC and GSH rather decreased apigenin-induced autophagy formation, suggesting that apigenin-induced ROS generation increased autophagy formation. However, autophagy inhibitors, bafilomycin (BAF) and 3-methyladenine (3-MA), showed different result in apigenin/TRAIL-mediated apoptosis without ROS generation. 3-MA upregulated the apoptosis but remained ROS levels; however, no changes on apoptosis and ROS generation were observed by BAF treatment. Taken together, these findings reveal that apigenin enhances TRAIL-induced apoptosis by activating apoptotic caspases by upregulating DR5 expression regardless of ROS generation, which may be a promising strategy for an adjuvant of TRAIL.

芹菜素是包括欧芹，百里香和薄荷在内的多种草药中的一种生物活性黄酮。芹菜素具有抗癌和抗炎作用；然而，芹菜素是否能增强TRAIL介导的癌细胞凋亡。在当前的研究中，我们发现芹菜素通过促进caspase激活和死亡受体5（DR5）表达来增强TRAIL诱导的细胞凋亡，而针对DR5的嵌合抗体则完全阻断了细胞凋亡。芹菜素还上调了活性氧（ROS）的产生。然而，有趣的是，ROS抑制剂，谷胱甘肽（GSH）或N-乙酰基-升-半胱氨酸（NAC），适度增加芹菜素/ TRAIL诱导的细胞凋亡。其他结果表明，自噬诱导剂雷帕霉素通过略微增加ROS的产生来增强芹菜素/ TRAIL介导的细胞凋亡。因此，NAC和GSH宁可减少芹菜素诱导的自噬形成，这表明芹菜素诱导的ROS生成增加了自噬形成。但是，自噬抑制剂巴氟霉素（BAF）和3-甲基腺嘌呤（3-MA）在芹菜素/ TRAIL介导的细胞凋亡中显示出不同的结果，而没有ROS的产生。3-MA上调细胞凋亡，但仍保持ROS水平。然而，BAF处理未观察到细胞凋亡和ROS产生的变化。综上所述，这些发现表明芹菜素可通过上调DR5表达来激活凋亡的胱天蛋白酶，从而增强TRAIL诱导的细胞凋亡，而与ROS的产生无关，