Chemical and molecular regulation of enzymes that detoxify carcinogens.

Inductions of detoxication (phase 2) enzymes, such as glutathione transferases and NAD(P)H:(quinone-acceptor) oxidoreductase, are a major mechanism for protecting animals and their cells against the toxic and neoplastic effects of carcinogens. These inductions result from enhanced transcription, and they are evoked by diverse chemical agents: oxidizable diphenols and phenylenediamines; Michael reaction acceptors; organic isothiocyanates; other electrophiles--e.g., alkyl and aryl halides; metal ions--e.g., HgCl2 and CdCl2; trivalent arsenic derivatives; vicinal dimercaptans; organic hydroperoxides and hydrogen peroxide; and 1,2-dithiole-3-thiones. The molecular mechanisms of these inductions were analyzed with the help of a construct containing a 41-bp enhancer element derived from the 5' upstream region of the mouse liver glutathione transferase Ya subunit gene ligated to the 5' end of the isolated promoter region of this gene, and inserted into a plasmid containing a human growth hormone reporter gene. When this construct was transfected into Hep G2 human hepatoma cells, the concentrations of 28 compounds (from the above classes) required to double growth hormone production, and the concentrations required to double quinone reductase specific activities in Hepa 1c1c7 cells, spanned a range of four orders of magnitude but were closely linearly correlated. Six compounds tested were inactive in both systems. A 26-bp subregion of the above enhancer oligonucleotide (containing the two tandem "AP-1-like" sites but lacking the preceding ETS protein binding sequence) was considerably less responsive to the same inducers. We conclude that the 41-bp enhancer element mediates most, if not all, of the phase 2 enzyme inducer activity of all of these widely different classes of compounds. IMAGES:

解毒（第二相，phase 2）酶的诱导表达——例如谷胱甘肽转移酶（glutathione transferases）与NAD(P)H：（醌受体）氧化还原酶[NAD(P)H:(quinone-acceptor) oxidoreductase]——是保护动物及其细胞免受致癌物毒性与致瘤效应的核心机制之一。这类诱导表达源于转录增强，且可被多种化学物质诱发：可氧化的二酚与苯二胺、迈克尔反应受体、有机异硫氰酸盐、其他亲电试剂（如烷基与芳基卤化物）、金属离子（如氯化汞HgCl₂与氯化镉CdCl₂）、三价砷衍生物、邻二巯基化合物、有机氢过氧化物与过氧化氢，以及1,2-二硫杂环戊烯-3-硫酮（1,2-dithiole-3-thiones）。本研究通过构建体分析了此类诱导的分子机制：将源自小鼠肝脏谷胱甘肽转移酶Ya亚基基因5'上游区域的41bp增强子元件，连接至该基因分离得到的启动子区域的5'端，并插入携带人生长激素报告基因的质粒中。将该构建体转染至Hep G2人肝癌细胞后，检测到28种来自上述类别的化合物使生长激素产量翻倍所需的浓度，以及使Hepa 1c1c7细胞中醌还原酶比活性翻倍所需的浓度，跨度达四个数量级，但二者呈显著线性相关。另有6种受试化合物在两个系统中均无活性。上述增强子寡核苷酸的26bp亚区域（包含两个串联的类AP-1（AP-1-like）位点，但缺失上游的ETS蛋白结合序列）对同一诱导剂的响应性显著降低。综上，41bp增强子元件介导了上述各类迥异化合物对第二相酶的绝大多数（即便非全部）诱导活性。图像：