Department of Toxicology, College of Pharmacy, Chungnam National University, Daejeon, South Korea.
Toxicol Appl Pharmacol. 2013 Sep 1;271(2):229-38. doi: 10.1016/j.taap.2013.05.010. Epub 2013 May 21.
Resistance to therapy is the major obstacle to more effective cancer treatment. Heme oxygenase-1 (HO-1) is often highly up-regulated in tumor tissues, and its expression is further increased in response to therapies. It has been suggested that inhibition of HO-1 expression is a potential therapeutic approach to sensitize tumors to chemotherapy and radiotherapy. In this study, we tested the hypothesis that the anti-tumor effects of metformin are mediated by suppression of HO-1 expression in cancer cells. Our results indicate that metformin strongly suppresses HO-1 mRNA and protein expression in human hepatic carcinoma HepG2, cervical cancer HeLa, and non-small-cell lung cancer A549 cells. Metformin also markedly reduced Nrf2 mRNA and protein levels in whole cell lysates and suppressed tert-butylhydroquinone (tBHQ)-induced Nrf2 protein stability and antioxidant response element (ARE)-luciferase activity in HepG2 cells. We also found that metformin regulation of Nrf2 expression is mediated by a Keap1-independent mechanism and that metformin significantly attenuated Raf-ERK signaling to suppress Nrf2 expression in cancer cells. Inhibition of Raf-ERK signaling by PD98059 decreased Nrf2 mRNA expression in HepG2 cells, confirming that the inhibition of Nrf2 expression is mediated by an attenuation of Raf-ERK signaling in cancer cells. The inactivation of AMPK by siRNA, DN-AMPK or the pharmacological AMPK inhibitor compound C, revealed that metformin reduced HO-1 expression in an AMPK-independent manner. These results highlight the Raf-ERK-Nrf2 axis as a new molecular target in anticancer therapy in response to metformin treatment.
耐药性是提高癌症治疗效果的主要障碍。血红素加氧酶-1（HO-1）在肿瘤组织中常高度上调，且其表达在应对治疗时进一步增加。有人认为抑制 HO-1 的表达是使肿瘤对化疗和放疗敏感的一种潜在治疗方法。在这项研究中，我们测试了二甲双胍的抗肿瘤作用是通过抑制癌细胞中 HO-1 表达来介导的假设。我们的结果表明，二甲双胍强烈抑制人肝癌 HepG2、宫颈癌 HeLa 和非小细胞肺癌 A549 细胞中的 HO-1 mRNA 和蛋白表达。二甲双胍还显著降低了整个细胞裂解物中的 Nrf2 mRNA 和蛋白水平，并抑制了tert-butylhydroquinone（tBHQ）诱导的 HepG2 细胞中 Nrf2 蛋白稳定性和抗氧化反应元件（ARE）-荧光素酶活性。我们还发现，二甲双胍对 Nrf2 表达的调节是通过 Keap1 非依赖性机制介导的，并且二甲双胍显著减弱了 Raf-ERK 信号以抑制癌细胞中的 Nrf2 表达。PD98059 抑制 Raf-ERK 信号可降低 HepG2 细胞中的 Nrf2 mRNA 表达，证实抑制 Nrf2 表达是通过减弱癌细胞中的 Raf-ERK 信号来介导的。siRNA、DN-AMPK 或药理学 AMPK 抑制剂化合物 C 对 AMPK 的失活表明，二甲双胍以 AMPK 非依赖性方式降低 HO-1 表达。这些结果强调了 Raf-ERK-Nrf2 轴作为二甲双胍治疗反应中抗肿瘤治疗的新分子靶标。
