Davidson N E, Egner P A, Kensler T W
Oncology Center, Johns Hopkins Medical Institutions, Baltimore, Maryland 21205.
Cancer Res. 1990 Apr 15;50(8):2251-5.
The substituted 1,2-dithiole-3-thione oltipraz [5-(2-pyrazinyl)-4-methyl-1,2-dithiole-3-thione] protects against the acute and chronic toxicities of many xenobiotics, including aflatoxin B1, in rodents. These protective effects are mediated, in part, through elevation of glutathione S-transferase (GST) activities. Because studies by Coles et al. [Carcinogenesis (Lond.), 6: 693-697, 1985] suggested that the detoxication of aflatoxin through conjugation with glutathione is principally catalyzed by GST homodimer YaYa, we have investigated the regulation of the gene coding for the Ya subunit in the liver of F344 rats following dietary administration of oltipraz. Overall GST activity, as measured by conjugation with 1,2-dichloro-4-nitrobenzene or 1-chloro-2,4-dinitrobenzene, as well as the levels of GST Ya protein, was elevated 1.5-fold by 24 h and maximally (2.7- to 3.5-fold) and persistently after 5 days on a purified diet supplemented with 0.075% oltipraz. Steady state mRNA levels for GST subunit Ya, as quantified by slot blot analysis using rat liver GST complementary DNA clone pGTB38, were also elevated by 24 h, with a maximal elevation of 3-fold observed at 3 days. However, mRNA levels decreased thereafter, despite continued feeding of oltipraz. Northern blot analyses demonstrated that oltipraz did not alter the size of GST mRNA. Transcriptional activity of the GST Ya gene, as determined by nuclear run-off analysis, was increased 2-fold after 24-h feeding of oltipraz, was maximally induced 2.4-fold at 3 days, and returned to near control levels at 7 days, despite sustained feeding of oltipraz. Modulation of GST activity by oltipraz was not accompanied by changes in the methylation pattern at internal sites of the GST Ya gene. These results show that the initial induction of hepatic GST activity during oltipraz exposure correlates with changes in steady state levels of GST mRNA and rates of GST gene transcription; however, the continued elevation of GST enzymatic activities and GST Ya protein levels in the face of declining GST Ya mRNA levels and transcription rates suggests that additional mechanisms may be involved in regulating GST Ya expression by oltipraz.
取代的1,2 - 二硫杂环戊烯 - 3 - 硫酮奥替普拉（5 - (2 - 吡嗪基)-4 - 甲基 - 1,2 - 二硫杂环戊烯 - 3 - 硫酮）可保护啮齿动物免受包括黄曲霉毒素B1在内的多种异生物素的急性和慢性毒性影响。这些保护作用部分是通过提高谷胱甘肽S - 转移酶（GST）的活性来介导的。由于科尔斯等人[《癌变（伦敦）》，6: 693 - 697，1985]的研究表明，黄曲霉毒素与谷胱甘肽结合的解毒作用主要由GST同型二聚体YaYa催化，我们研究了在给F344大鼠喂食奥替普拉后，肝脏中编码Ya亚基的基因的调控情况。通过与1,2 - 二氯 - 4 - 硝基苯或1 - 氯 - 2,4 - 二硝基苯结合来测定的总体GST活性，以及GST Ya蛋白的水平，在补充了0.075%奥替普拉的纯化饮食喂养24小时后升高了1.5倍，在5天后达到最大值（2.7至3.5倍）并持续保持。通过使用大鼠肝脏GST互补DNA克隆pGTB38进行斑点印迹分析定量的GST亚基Ya的稳态mRNA水平，在24小时时也升高了，在3天时观察到最大升高3倍。然而，尽管继续喂食奥替普拉，此后mRNA水平下降。Northern印迹分析表明，奥替普拉不会改变GST mRNA的大小。通过核转录分析确定的GST Ya基因的转录活性，在喂食奥替普拉24小时后增加了2倍，在3天时最大诱导2.4倍，尽管持续喂食奥替普拉，但在7天时恢复到接近对照水平。奥替普拉对GST活性的调节并未伴随着GST Ya基因内部位点甲基化模式的改变。这些结果表明，在奥替普拉暴露期间肝脏GST活性的初始诱导与GST mRNA的稳态水平和GST基因转录速率的变化相关；然而，面对GST Ya mRNA水平和转录速率下降，GST酶活性和GST Ya蛋白水平持续升高，这表明可能有其他机制参与奥替普拉对GST Ya表达的调节。
