Upregulating Noxa by ER Stress, Celastrol Exerts Synergistic Anti-Cancer Activity in Combination with ABT-737 in Human Hepatocellular Carcinoma Cells

The human hepatocellular carcinoma (HCC) represents biologically aggressive and chemo-resistant cancers. Owing to the low affinity with the apoptotic factor Mcl-1, the BH3 mimetic drug ABT-737 failed to exert potent cancer-killing activities in variety of cancer models including HCC. The current study demonstrated that combining ABT-737 and Celastrol synergistically suppressed HCC cell proliferation, and induced apoptosis which was accompanied with the activation of caspase cascade and release of cytochrome c from mitochondria. Further study revealed that the enhanced Noxa caused by Celastrol was the key factor for the synergy, since small interfering RNA-mediated knockdown of Noxa expression in HCC cells resulted in decreased apoptosis and attenuated anti-proliferative effects of the combination. In addition, our study unraveled that, upon Celastrol exposure, the activation of endoplasmic reticulum (ER) stress, specifically, the eIF2α-ATF4 pathway played indispensable roles in the activation of Noxa, which was validated by the observation that depletion of ATF4 significantly abrogated the Noxa elevation by Celastrol. Our findings highlight a novel signaling pathway through which Celastrol increase Noxa expression, and suggest the potential use of ATF4-mediated regulation of Noxa as a promising strategy to improve the anti-cancer activities of ABT-737.

人类肝细胞癌（human hepatocellular carcinoma, HCC）是一类兼具生物学侵袭性与化疗耐药性的恶性肿瘤。由于其与凋亡因子Mcl-1（apoptotic factor Mcl-1）的亲和力较低，BH3模拟物ABT-737无法在包括HCC在内的多种癌症模型中展现出强效的抗肿瘤活性。本研究证实，联合应用ABT-737与雷公藤红素（Celastrol）可协同抑制HCC细胞增殖，并诱导细胞凋亡；该凋亡过程伴随半胱天冬酶级联反应（caspase cascade）的激活，以及线粒体（mitochondria）中细胞色素c（cytochrome c）的释放。进一步研究表明，雷公藤红素诱导的Noxa表达上调是产生协同效应的核心机制：在HCC细胞中通过小干扰RNA（small interfering RNA）敲低Noxa的表达，可显著削弱细胞凋亡水平，并减弱联合用药的抗增殖作用。此外，本研究还揭示，经雷公藤红素处理后，内质网应激（endoplasmic reticulum stress, ER）通路的激活——尤其是eIF2α-ATF4信号通路——在Noxa的激活过程中发挥了不可或缺的作用；这一结论得到了实验验证：敲除ATF4可显著抵消雷公藤红素对Noxa的上调作用。本研究发现了一条雷公藤红素上调Noxa表达的全新信号通路，并提示通过ATF4介导的Noxa调控策略，有望增强ABT-737的抗癌活性。