Inui H, Watatani M, Yasutomi M, Nishisho I
First Department of Surgery, Kinki University Medical School, Osaka, Japan.
Carcinogenesis. 1994 Mar;15(3):459-63. doi: 10.1093/carcin/15.3.459.
Activation of the Ha-ras oncogene in N-methyl-N-nitrosourea (MNU)-induced rat mammary tumors has been well documented. Such Ha-ras activation is thought to be brought about by direct action of carcinogens resulting in a G-->A transition at the second nucleotide of codon 12. However, a DNA repair enzyme, O6-methylguanine-DNA methyltransferase (MGMT), can specifically remove methyl groups from O6-methylguanine, which is a major mutagenic and carcinogenic DNA lesion leading to the G-->A transition. In this study, we compared the amount of MGMT mRNA in MNU-induced rat mammary tumors with and without such Ha-ras activation. A single injection of MNU into 82 female Sprague-Dawley rats induced 80 mammary carcinomas. RNase protection analysis and subsequent sequencing revealed that 42 of 65 randomly selected tumors contained Ha-ras oncogenes activated by the G-->A transition. The amount of MGMT mRNA was then measured by means of reverse transcriptase-mediated polymerase chain reaction (RT-PCR) amplification and Southern hybridization. No obvious difference in the level of MGMT mRNA was detected between the two tumor groups. In addition, in the course of our experiment, five of 42 tumors classified as containing activated Ha-ras oncogenes proved to contain low percentages of tumor cells with the Ha-ras activation. These results suggest that Ha-ras activation in MNU-induced rat mammary tumors may not necessarily be influenced by differences in MGMT activity. They also raise the possibility that activation of other oncogenes and/or inactivation of unidentified tumor suppressor gene(s) may be involved in development of a certain proportion of tumors with activated Ha-ras oncogenes, as is suspected in the case of tumors without Ha-ras activation.
N-甲基-N-亚硝基脲（MNU）诱导的大鼠乳腺肿瘤中Ha-ras癌基因的激活已有充分的文献记载。这种Ha-ras激活被认为是致癌物直接作用导致密码子12第二个核苷酸处发生G→A转换的结果。然而，一种DNA修复酶，O6-甲基鸟嘌呤-DNA甲基转移酶（MGMT），可以特异性地从O6-甲基鸟嘌呤上去除甲基，O6-甲基鸟嘌呤是导致G→A转换的主要诱变和致癌DNA损伤。在本研究中，我们比较了有和没有这种Ha-ras激活的MNU诱导的大鼠乳腺肿瘤中MGMT mRNA的量。向82只雌性Sprague-Dawley大鼠单次注射MNU诱导出80个乳腺癌。核糖核酸酶保护分析及随后的测序显示，在随机选择的65个肿瘤中，有42个含有通过G→A转换激活的Ha-ras癌基因。然后通过逆转录酶介导的聚合酶链反应（RT-PCR）扩增和Southern杂交来测量MGMT mRNA的量。在两个肿瘤组之间未检测到MGMT mRNA水平有明显差异。此外，在我们的实验过程中，被归类为含有激活的Ha-ras癌基因的42个肿瘤中有5个被证明含有低比例的具有Ha-ras激活的肿瘤细胞。这些结果表明，MNU诱导的大鼠乳腺肿瘤中Ha-ras激活不一定受MGMT活性差异的影响。它们还提出了一种可能性，即其他癌基因的激活和/或未鉴定的肿瘤抑制基因的失活可能参与了一定比例具有激活的Ha-ras癌基因的肿瘤的发生，就像在没有Ha-ras激活的肿瘤中所怀疑的那样。
