Kemper B
Department of Molecular and Integrative Physiology, College of Medicine at Urbana-Champaign, University of Illinois 61801, USA.
Prog Nucleic Acid Res Mol Biol. 1998;61:23-64.
The ability of phenobarbital to induce levels of drug metabolism in mammals has been known for over 40 years. However, the molecular mechanisms underlying increased expression of the genes of the key enzyme in drug metabolism, cytochrome P450, have not been elucidated, primarily because in vitro model systems in which the induction could be studied were not available. Transfected primary cultured hepatocytes, transfection of liver in situ, and transgenic mice now provide suitable models for phenobarbital induction. In this review, progress toward understanding the mechanism of phenobarbital induction of gene expression is discussed with an emphasis on the mammalian genes, CYP2B1, CYP2B2, and Cyp2b10, which are most highly inducible by phenobarbital. Barbiturate induction of P450s in Bacillus megaterium, which is the system best understood, and its relevance to mammalian mechanisms of induction are also discussed. In B. megaterium, the binding of a repressor to several motifs is reversed by direct effects of barbiturates and by induction of positively acting factors. One of the repressor binding sites, the barbie box, is present in many mammalian phenobarbital-inducible genes, including the promimal promoter regions of CYP2B1, CYP2B2, and Cyp2B10. In the mammalian P450 genes, evidence has been proposed for phenobarbital-regulated elements both in the proximal promoter region and in a distal enhancer region. The role of the proximal region is controversial. A positively acting element that overlaps the barbie box sequence and a negative element have been proposed to mediate induction of CYP2B1/2, based primarily on protein binding and cell-free transcription assays. In contrast, other investigators have not found differences in phenobarbital-dependent protein binding in the proximal promoter region nor mediation of phenobarbital induction by this region. A distal gene fragment, at about -2000 kb in CYP2B1, CYP2B2, and Cyp2b10, has been shown to be a phenobarbital-responsive enhancer independent of proximal promoter elements. This fragment contains several binding sites for proteins and several functional elements, including an NF-1 site, and, therefore, has been designated as a phenobarbital-responsive unit. Possible models are presented in which phenobarbital treatment induces altered chromatin structure, which allows the binding of positively acting factors, or activates factors already bound, to the distal enhancer and the proximal promoter.
苯巴比妥诱导哺乳动物体内药物代谢水平的能力已为人所知达40多年。然而，药物代谢关键酶细胞色素P450基因表达增加的分子机制尚未阐明，主要是因为缺乏可用于研究这种诱导作用的体外模型系统。转染的原代培养肝细胞、原位肝脏转染以及转基因小鼠现在为苯巴比妥诱导提供了合适的模型。在这篇综述中，我们讨论了在理解苯巴比妥诱导基因表达机制方面取得的进展，重点是哺乳动物基因CYP2B1、CYP2B2和Cyp2b10，它们对苯巴比妥的诱导最为敏感。还讨论了巨大芽孢杆菌中巴比妥酸盐对P450s的诱导作用，这是目前理解得最清楚的系统，以及它与哺乳动物诱导机制的相关性。在巨大芽孢杆菌中，巴比妥酸盐的直接作用以及正性作用因子的诱导可逆转阻遏物与几个基序的结合。阻遏物结合位点之一，巴比妥框，存在于许多哺乳动物苯巴比妥诱导型基因中，包括CYP2B1、CYP2B2和Cyp2B10的近端启动子区域。在哺乳动物P450基因中，已经提出在近端启动子区域和远端增强子区域都存在苯巴比妥调节元件。近端区域的作用存在争议。主要基于蛋白质结合和无细胞转录分析，有人提出一个与巴比妥框序列重叠的正性作用元件和一个负性元件介导CYP2B1/2的诱导。相比之下，其他研究人员在近端启动子区域未发现苯巴比妥依赖性蛋白质结合的差异，也未发现该区域介导苯巴比妥诱导作用。在CYP2B1、CYP2B2和Cyp2b10中，大约位于 -2000 kb处的一个远端基因片段已被证明是一个独立于近端启动子元件的苯巴比妥反应性增强子。该片段包含几个蛋白质结合位点和几个功能元件，包括一个NF-1位点，因此被指定为苯巴比妥反应单元。文中提出了一些可能的模型，其中苯巴比妥处理可诱导染色质结构改变，从而使正性作用因子能够结合到远端增强子和近端启动子上，或者激活已经结合的因子。
