Repair gene O6-methylguanine-DNA methyltransferase is controlled by SP1 and up-regulated by glucocorticoids, but not by temozolomide and radiation
Repair gene O6-methylguanine-DNA methyltransferase is controlled by SP1 and up-regulated by glucocorticoids, but not by temozolomide and radiation
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DOI：
10.1111/jnc.14262
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发表时间：
2018-01-01
影响因子：
4.7
通讯作者：
Christmann, Markus
中科院分区：
文献类型：
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作者：
Aasland, Dorthe;Reich, Thomas R.;Christmann, Markus
恶性胶质瘤的治疗依赖于O-6甲基化药物替莫唑胺(TMZ)联合电离辐射治疗，然后辅以TMZ。对于复发的治疗，DNA氯乙基化药物也被使用。这些药物的主要致死损伤是O-6-烷基鸟嘌呤。由于这种损伤是由O-6-甲基鸟嘌呤-DNA甲基转移酶(MGMT)修复的，修复酶是耐药的最重要因素，限制了恶性高级别胶质瘤的治疗。虽然已经证明，在基因毒性应激后，啮齿动物的MGMT在转录水平上上调，但目前仍不清楚人类MGMT是否受到上调的影响。在这里，我们使用启动子分析，解决了烷化药物或电离辐射后胶质瘤细胞中的MGMT是否增强的问题。我们还检查了胶质瘤细胞系对地塞米松的反应。在一系列实验中，我们没有发现证据表明，在TMZ、氯乙基化试剂尼莫司汀或辐射处理后，人MGMT启动子显著上调。然而，它是被地塞米松激活的。利用缺失结构，我们进一步证明了MGMT的基础水平主要由转录因子SP1决定。MGMT启动子中大量的SP1位点可能通过中和诱导信号来阻止遗传毒性应激后转录上调。迪格尔基因敲除实验表明，miRNAs对MGMT的调控作用很小。由于大剂量地塞米松联合替莫唑胺常用于治疗胶质母细胞瘤，在MGMT启动子未甲基化的情况下，通过糖皮质激素诱导MGMT基因可能会进一步提高耐药性，而放射和烷化药物似乎不会在转录水平上诱导MGMT。
Therapy of malignant glioma relies on treatment with the O-6-methylating agent temozolomide (TMZ) concomitant with ionizing radiation followed by adjuvant TMZ. For the treatment of recurrences, DNA chloroethylating drugs are also used. The main killing lesion induced by these drugs is O-6-alkylguanine. Since this damage is repaired by O-6-methylguanine-DNA methyltransferase (MGMT), the repair enzyme represents a most important factor of drug resistance, limiting the therapy of malignant high-grade gliomas. Although MGMT has been shown to be transcriptionally up-regulated in rodents following genotoxic stress, it is still unclear whether human MGMT is subject to up-regulation. Here, we addressed the question whether MGMT in glioma cells is enhanced following alkylating drugs or ionizing radiation, using promoter assays. We also checked the response of glioma cell lines to dexamethasone. In a series of experiments, we found no evidence that the human MGMT promoter is significantly up-regulated following treatment with TMZ, the chloroethylating agent nimustine or radiation. It was activated, however, by dexamethasone. Using deletion constructs, we further show that the basal level of MGMT is mainly determined by the transcription factor SP1. The high amount of SP1 sites in the MGMT promoter likely prevents transcriptional up-regulation following genotoxic stress by neutralizing inducible signals. The regulation of MGMT by miRNAs plays only a minor role, as shown by DICER knockdown experiments. Since high dose dexamethasone concomitant with temozolomide is frequently used in glioblastoma therapy, induction of the MGMT gene through glucocorticoids in MGMT promoter unmethylated cases might cause further elevation of drug resistance, while radiation and alkylating drugs seem not to induce MGMT at transcriptional level.