Hogarth L A, Redfern C P F, Teodoridis J M, Hall A G, Anderson H, Case M C, Coulthard S A
Leukaemia Research Group, Northern Institute for Cancer Research, Newcastle University, Newcastle upon Tyne NE2 4HH, UK.
Biochem Pharmacol. 2008 Oct 15;76(8):1024-35. doi: 10.1016/j.bcp.2008.07.026. Epub 2008 Jul 30.
The thiopurine drugs 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG) are well-established agents for the treatment of leukaemia but their main modes of action are controversial. Thiopurine methyltransferase (TPMT) metabolises thiopurine drugs and influences their cytotoxic activity. TPMT, like DNA methyltransferases (DNMTs), transfers methyl groups from S-adenosylmethionine (SAM) and generates S-adenosylhomocysteine (SAH). Since SAM levels are dependent on de novo purine synthesis (DNPS) and the metabolic products of 6-TG and 6-MP differ in their ability to inhibit DNPS, we postulated that 6-TG compared to 6-MP would have differential effects on changes in SAM and SAH levels and global DNA methylation, depending on TPMT status. To test this hypothesis, we used a human embryonic kidney cell line with inducible TPMT. Although changes in SAM and SAH levels occurred with each drug, decrease in global DNA methylation more closely reflected a decrease in DNMT activity. Inhibition was influenced by TPMT for 6-TG, but not 6-MP. The decrease in global methylation and DNMT activity with 6-MP, or with 6-TG when TPMT expression was low, were comparable to 5-aza-2'-deoxycytidine. However, this was not reflected in changes in methylation at the level of an individual marker gene (MAGE1A). The results suggest that a non-TPMT metabolised metabolite of 6-MP and 6-TG and the TPMT-metabolised 6-MP metabolite 6-methylthioguanosine 5'-monophosphate, contribute to a decrease in DNMT levels and global DNA methylation. As demethylating agents have shown promise in leukaemia treatment, inhibition of DNA methylation by the thiopurine drugs may contribute to their cytotoxic affects.
硫嘌呤类药物6-巯基嘌呤（6-MP）和6-硫鸟嘌呤（6-TG）是治疗白血病的常用药物，但其主要作用方式仍存在争议。硫嘌呤甲基转移酶（TPMT）可代谢硫嘌呤类药物并影响其细胞毒性活性。TPMT与DNA甲基转移酶（DNMTs）一样，从S-腺苷甲硫氨酸（SAM）转移甲基基团并生成S-腺苷同型半胱氨酸（SAH）。由于SAM水平依赖于从头嘌呤合成（DNPS），且6-TG和6-MP的代谢产物抑制DNPS的能力不同，我们推测，与6-MP相比，6-TG对SAM和SAH水平变化以及整体DNA甲基化的影响会因TPMT状态而异。为验证这一假设，我们使用了可诱导TPMT的人胚肾细胞系。尽管每种药物都会引起SAM和SAH水平的变化，但整体DNA甲基化的降低更能反映DNMT活性的降低。6-TG的抑制作用受TPMT影响，而6-MP则不受影响。6-MP或TPMT表达较低时6-TG导致的整体甲基化和DNMT活性降低与5-氮杂-2'-脱氧胞苷相当。然而，在单个标记基因（MAGE1A）水平的甲基化变化中并未体现这一点。结果表明，6-MP和6-TG的非TPMT代谢产物以及TPMT代谢的6-MP代谢产物6-甲基硫鸟苷5'-单磷酸，会导致DNMT水平和整体DNA甲基化降低。由于去甲基化剂在白血病治疗中已显示出前景，硫嘌呤类药物对DNA甲基化的抑制作用可能有助于其细胞毒性作用。
