Department of Marine Life Sciences, Jeju National University, Jeju 63243, Republic of Korea.
Department of Bioprocess Technology, Faculty of Technology, Rajarata University of Sri Lanka, Mihintale 50300, Sri Lanka.
Food Chem Toxicol. 2019 May;127:53-60. doi: 10.1016/j.fct.2019.03.001. Epub 2019 Mar 6.
Camptothecin (CPT), a quinoline alkaloid isolated from Camptotheca acuminate, targets topoisomerase I, which is continuously expressed in cancer cells. However, the molecular mechanisms responsible for CPT-induced telomerase inhibition remain unclear. Unexpectedly, we found that CPT upregulates hTERT expression and concomitantly increases telomerase activity. However, transfection of hTERT-targeting siRNA had no effect on CPT-induced G/M phase arrest, suggesting that CPT-induced telomerase activation was not related to G/M phase arrest. CPT simultaneously increased Nrf2 expression and the level of intracellular reactive oxygen species (ROS), whereas pretreatment with the antioxidants N-acetyl-cysteine (NAC) or glutathione (GSH) strongly attenuated ROS production, which was accompanied by hTERT downregulation. Additionally, transient Nrf2 knockdown enhanced CPT-induced ROS production and hTERT promoter activity. CPT also upregulated hTERT expression and telomerase activity by inducing c-Myc and Sp1 expression and activity. Moreover, c-Myc stimulated ROS production in response to CPT, leading to Sp1 activation, which promoted hTERT expression and telomerase activity. CPT treatment enhanced the phosphorylation of PI3K and Akt, which led to hTERT phosphorylation into the nucleus. These findings demonstrate that CPT positively regulates telomerase activity by upregulating hTERT expression and phosphorylation via the c-Myc/ROS/Sp1 and PI3K/Akt axis.
喜树碱（CPT）是一种从喜树中分离出来的喹啉生物碱，靶向拓扑异构酶 I，该酶在癌细胞中持续表达。然而，CPT 诱导端粒酶抑制的分子机制尚不清楚。出乎意料的是，我们发现 CPT 上调 hTERT 表达并同时增加端粒酶活性。然而，hTERT 靶向 siRNA 的转染对 CPT 诱导的 G/M 期阻滞没有影响，表明 CPT 诱导的端粒酶激活与 G/M 期阻滞无关。CPT 同时增加 Nrf2 表达和细胞内活性氧（ROS）水平，而抗氧化剂 N-乙酰半胱氨酸（NAC）或谷胱甘肽（GSH）预处理强烈减弱 ROS 产生，同时 hTERT 下调。此外，瞬时 Nrf2 敲低增强了 CPT 诱导的 ROS 产生和 hTERT 启动子活性。CPT 还通过诱导 c-Myc 和 Sp1 表达和活性来上调 hTERT 表达和端粒酶活性。此外，c-Myc 响应 CPT 刺激 ROS 产生，导致 Sp1 激活，从而促进 hTERT 表达和端粒酶活性。CPT 处理增强了 PI3K 和 Akt 的磷酸化，导致 hTERT 磷酸化入核。这些发现表明，CPT 通过 c-Myc/ROS/Sp1 和 PI3K/Akt 轴正向调节端粒酶活性，上调 hTERT 表达和磷酸化。
