Trichothecin Induces Cell Death in NF-κB Constitutively Activated Human Cancer Cells via Inhibition of IKKβ Phosphorylation

Constitutive activation of the transcription factor nuclear factor-κB (NF-κB) is involved in tumorigenesis and chemo-resistance. As the key regulator of NF-κB, IKKβ is a major therapeutic target for various cancers. Trichothecin (TCN) is a metabolite isolated from an endophytic fungus of the herbal plant Maytenus hookeri Loes. In this study, we evaluated the anti-tumor activity of TCN and found that TCN markedly inhibits the growth of cancer cells with constitutively activated NF-κB. TCN induces G0/G1 cell cycle arrest and apoptosis in cancer cells, activating pro-apoptotic proteins, including caspase-3, -8 and PARP-1, and decreasing the expression of anti-apoptotic proteins Bcl-2, Bcl-xL, and survivin. Reporter activity assay and target genes expression analysis illustrated that TCN works as a potent inhibitor of the NF-κB signaling pathway. TCN inhibits the phosphorylation and degradation of IκBα and blocks the nuclear translocation of p65, and thus inhibits the expression of NF-κB target genes XIAP, cyclin D1, and Bcl-xL. Though TCN does not directly interfere with IKKβ kinase, it suppresses the phosphorylation of IKKβ. Overexpression of constitutively activated IKKβ aborted TCN induced cancer cell apoptosis, whereas knockdown of endogenous IKKβ with siRNA sensitized cancer cells toward apoptosis induced by TCN. Moreover, TCN showed a markedly weaker effect on normal cells. These findings suggest that TCN may be a potential therapeutic candidate for cancer treatment, targeting NF-κB signaling.

转录因子核因子κB（nuclear factor-κB，NF-κB）的组成型激活参与肿瘤发生与化疗耐药过程。作为NF-κB的关键调控因子，IκB激酶β（IKKβ）是多种癌症的重要治疗靶点。单端孢霉烯（Trichothecin，TCN）是从药用植物美登木（Maytenus hookeri Loes.）的内生真菌中分离得到的代谢产物。本研究评估了TCN的抗肿瘤活性，发现其可显著抑制组成型激活NF-κB的癌细胞增殖。TCN可诱导癌细胞发生G0/G1期细胞周期阻滞与凋亡，激活半胱天冬氨酸蛋白酶（caspase）-3、-8及多聚ADP核糖聚合酶-1（PARP-1）等促凋亡蛋白，并下调抗凋亡蛋白Bcl-2、Bcl-xL与生存素（survivin）的表达。报告基因活性检测与靶基因表达分析结果显示，TCN是NF-κB信号通路的强效抑制剂。TCN可抑制IκBα的磷酸化与降解，阻断p65的核转位，进而抑制NF-κB靶基因XIAP、细胞周期蛋白D1（cyclin D1）与Bcl-xL的表达。尽管TCN不直接作用于IKKβ激酶，但可抑制其磷酸化。组成型激活型IKKβ的过表达可抵消TCN诱导的癌细胞凋亡，而通过小干扰RNA（siRNA）敲低内源性IKKβ则可增强癌细胞对TCN诱导凋亡的敏感性。此外，TCN对正常细胞的抑制作用显著较弱。上述研究结果表明，TCN或可成为靶向NF-κB信号通路的潜在抗肿瘤治疗候选药物。