Chemical carcinogen-mediated decreases in DNA 5-methylcytosine content of BALB/3T3 cells.
Chemical carcinogen-mediated decreases in DNA 5-methylcytosine content of BALB/3T3 cells.
复制标题
化学致癌剂介导的 BALB/3T3 细胞 DNA 5-甲基胞嘧啶含量降低。
作者：
V. L. Wilson;Peter A. Jones
在几种芳香烃致癌物处理的BALB/3T3 A31 CL1-13细胞中，DNA胞嘧啶甲基化的基因组水平显著降低。在16 h处理结束时测定，苯并[a]芘诱导的DNA 5-甲基胞嘧啶水平下降在0.1至1.0微克/毫升范围内呈浓度依赖性。DNA胞嘧啶残基的酶甲基化在处理后48小时对抑制最敏感，因为低至0.033微克/毫升的苯并[a]芘浓度会导致5-甲基胞嘧啶水平显著降低。这种DNA胞嘧啶甲基化的抑制可能是由DNA的烷基化介导的。用(+/-)-r-7，t-8-二羟基-t-9,10-环氧-7,8,9,10-四氢苯并[a]芘（抗bpde）、(+/-)-r，7-t-8-二羟基-c-9,10-环氧-7,8,9,10-四氢苯并[a]芘（syn-BPDE）和(+/-)-苯并[a]芘-4,5-环氧处理半甲基化dna，在小鼠脾脏甲基转移酶活性存在的情况下，抑制了[3H] s -腺苷蛋氨酸甲基部分向未甲基化链中胞嘧啶残基的转移。BPDE的syn异构体在这一作用中是最有效的，而母体化合物苯并[a]芘并没有显著降低处理过的dna接受甲基的能力。所有经过测试的化学致癌物，在处理后48小时内，已知会转化BALB/3T3细胞，导致5-甲基胞嘧啶形成显著减少。然而，一些不能转化这些细胞的碳氢化合物的浓度比转化致癌物的有效浓度高5到50倍，也显著降低了DNA胞嘧啶甲基化。因此，抑制DNA甲基化可能是启动BALB/3T3细胞致癌转化的一个重要步骤，但DNA 5-甲基胞嘧啶水平的降低并不能单独解释这一多步骤过程的开始。
The genomic level of DNA cytosine methylation was significantly diminished in dividing BALB/3T3 A31 CL1-13 cells treated with several aromatic hydrocarbon carcinogens. Benzo[a]pyrene-induced decreases in DNA 5-methylcytosine levels were concentration dependent over the range of 0.1 to 1.0 microgram/ml when determined at the end of the 16 h treatment period. The enzymatic methylation of DNA cytosine residues was the most sensitive to inhibition 48 h after treatment as concentrations of benzo[a]pyrene as low as 0.033 micrograms/ml initiated significant reductions in 5-methylcytosine levels. This inhibition of DNA cytosine methylation may be mediated by alkylation of the DNA. Treatment of hemimethylated DNAs with (+/-)-r-7,t-8-dihydroxy-t-9,10-epoxy-7,8,9,10-tetrahydro benzo[a]pyrene (anti-BPDE), (+/-)-r,7-t-8-dihydroxy-c-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (syn-BPDE), and (+/-)-benzo[a]pyrene-4,5-epoxide inhibited the transfer of methyl moieties from [3H]S-adenosylmethionine to cytosine residues in the undermethylated strand in the presence of mouse spleen methyltransferase activity. The syn isomer of BPDE was the most potent in this action while the parent compound, benzo[a]pyrene, did not significantly decrease the methyl accepting abilities of treated DNAs. All chemical carcinogens that were tested and are known to transform BALB/3T3 cells initiated significant reductions in 5-methylcytosine formation by 48 h post-treatment. However, concentrations of some hydrocarbons which do not transform these cells 5- to 50-fold above effective concentrations of the transforming carcinogens also provided significant reductions in DNA cytosine methylation. Thus the inhibition of DNA methylation may be an important step in the initiation of oncogenic transformation of BALB/3T3 cells, but decreases in DNA 5-methylcytosine levels alone cannot account for the onset of this multi-step process.