Preuss I, Thust R, Kaina B
Division of Applied Toxicology, Institute of Toxicology, University of Mainz, Germany.
Int J Cancer. 1996 Feb 8;65(4):506-12. doi: 10.1002/(SICI)1097-0215(19960208)65:4<506::AID-IJC19>3.0.CO;2-7.
The DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) removes alkyl groups from the O6 position of guanine in DNA and thus may protect cells against genotoxic effects of agents inducing this lesion. To analyze quantitatively the level of protection mediated by MGMT against antineoplastic drugs, we determined the cytotoxic and recombinogenic (sister-chromatid exchange inducing) effects of various chemotherapeutic agents in a pair of isogenic Chinese hamster cell lines deficient and proficient for MGMT, generated upon transfection with human MGMT cDNA. Furthermore, we compared the responses of the human cell lines HeLa MR (MGMT deficient) and HeLa S3 (MGMT proficient) to the various agents. It is shown that: (1) MGMT proficient cells are resistant in cell killing to the methylating drug streptozotocin and all the chloroethylating nitrosoureas tested. There was a marked agent specificity in protection. The level of resistance provoked by MGMT increased in the order BCNU < CCNU < ACNU < HeCNU < streptozotocin. (2) MGMT did not protect cells against killing induced by chlorambucil, cisplatin, melphalan, activated cyclophosphamide (malosfamide) and activated ifosfamide (4-hydroperoxy-ifosfamide). (3) MGMT caused protection against the recombinogenic effect of all nitrosoureas tested. The lowest level of protection was again observed for BCNU, followed by CCNU, ACNU < HeCNU < streptozotocin. (4) MGMT proficient cells did not exhibit resistance in SCE induction towards cyclophosphamide (activated by microsomes), 4-hydroperoxy-ifosfamide, mafosfamide, chlorambucil and melphalan. Some protection was afforded, however, against cisplatin (and transplatin). This effect was abolished by pretreatment of cells with O6-benzylguanine, which depletes MGMT, indicating that some lesion(s) induced by cisplatin giving rise to SCEs can be repaired by MGMT. Taken together, these results indicate that streptozotocin, HeCNU and ACNU are more selective than CCNU and BCNU in killing MGMT deficient cells, and that in the cases of cyclophosphamide, ifosfamide, chlorambucil, cisplatin and melphalan MGMT is not involved in mediating cytotoxic drug resistance.
DNA修复蛋白O6-甲基鸟嘌呤-DNA甲基转移酶（MGMT）可去除DNA中鸟嘌呤O6位上的烷基，因此可能保护细胞免受诱导该损伤的药物的基因毒性作用。为了定量分析MGMT介导的对抗肿瘤药物的保护水平，我们测定了多种化疗药物在一对经人MGMT cDNA转染后产生的同基因中国仓鼠细胞系中的细胞毒性和致重组作用（诱导姐妹染色单体交换），这对细胞系中一个缺乏MGMT，另一个富含MGMT。此外，我们比较了人细胞系HeLa MR（MGMT缺陷型）和HeLa S3（MGMT富集型）对各种药物的反应。结果表明：（1）富含MGMT的细胞在细胞杀伤方面对甲基化药物链脲佐菌素和所有测试的氯乙基亚硝脲具有抗性。保护作用具有明显的药物特异性。MGMT引发的抗性水平按以下顺序增加：卡莫司汀＜洛莫司汀＜醋硝香豆素＜司莫司汀＜链脲佐菌素。（2）MGMT不能保护细胞免受苯丁酸氮芥、顺铂、美法仑、活化环磷酰胺（马法兰磷酰胺）和活化异环磷酰胺（4-氢过氧异环磷酰胺）诱导的杀伤。（3）MGMT对所有测试的亚硝脲的致重组作用具有保护作用。卡莫司汀的保护水平再次最低，其次是洛莫司汀、醋硝香豆素＜司莫司汀＜链脲佐菌素。（4）富含MGMT的细胞在SCE诱导方面对环磷酰胺（经微粒体活化）、4-氢过氧异环磷酰胺、马法兰磷酰胺、苯丁酸氮芥和美法仑不表现出抗性。然而，对顺铂（和反铂）有一定的保护作用。用O6-苄基鸟嘌呤预处理细胞可消除这种作用，O6-苄基鸟嘌呤会耗尽MGMT，这表明顺铂诱导产生SCE的某些损伤可被MGMT修复。综上所述，这些结果表明链脲佐菌素、司莫司汀和醋硝香豆素在杀死MGMT缺陷细胞方面比洛莫司汀和卡莫司汀更具选择性，并且在环磷酰胺、异环磷酰胺、苯丁酸氮芥、顺铂和美法仑的情况下，MGMT不参与介导细胞毒性药物抗性。
