Tzameli I, Pissios P, Schuetz E G, Moore D D
Department of Molecular Biology, Baylor College of Medicine, Houston, Texas 77030, USA.
Mol Cell Biol. 2000 May;20(9):2951-8. doi: 10.1128/MCB.20.9.2951-2958.2000.
A wide range of xenobiotic compounds are metabolized by cytochrome P450 (CYP) enzymes, and the genes that encode these enzymes are often induced in the presence of such compounds. Here, we show that the nuclear receptor CAR can recognize response elements present in the promoters of xenobiotic-responsive CYP genes, as well as other novel sites. CAR has previously been shown to be an apparently constitutive transactivator, and this constitutive activity is inhibited by androstanes acting as inverse agonists. As expected, the ability of CAR to transactivate the CYP promoter elements is blocked by the inhibitory inverse agonists. However, CAR transactivation is increased in the presence of 1,4-bis[2-(3, 5-dichloropyridyloxy)]benzene (TCPOBOP), the most potent known member of the phenobarbital-like class of CYP-inducing agents. Three independent lines of evidence demonstrate that TCPOBOP is an agonist ligand for CAR. The first is that TCPOBOP acts in a dose-dependent manner as a direct agonist to compete with the inhibitory effect of the inverse agonists. The second is that TCPOBOP acts directly to stimulate coactivator interaction with the CAR ligand binding domain, both in vitro and in vivo. The third is that mutations designed to block ligand binding block not only the inhibitory effect of the androstanes but also the stimulatory effect of TCPOBOP. Importantly, these mutations do not block the apparently constitutive transactivation by CAR, suggesting that this activity is truly ligand independent. Both its ability to target CYP genes and its activation by TCPOBOP demonstrate that CAR is a novel xenobiotic receptor that may contribute to the metabolic response to such compounds.
多种外源性化合物可被细胞色素P450（CYP）酶代谢，编码这些酶的基因常在这类化合物存在时被诱导。在此，我们表明核受体CAR能够识别外源性反应性CYP基因启动子中存在的反应元件以及其他新位点。此前已证明CAR是一种明显的组成型反式激活因子，而这种组成型活性会受到作为反向激动剂的雄烷类物质的抑制。正如预期的那样，CAR反式激活CYP启动子元件的能力被抑制性反向激动剂所阻断。然而，在1,4 - 双[2 -（3,5 - 二氯吡啶氧基）]苯（TCPOBOP）存在时，CAR的反式激活作用增强，TCPOBOP是已知最强效的苯巴比妥类CYP诱导剂。三条独立的证据表明TCPOBOP是CAR的激动剂配体。首先，TCPOBOP作为直接激动剂以剂量依赖方式发挥作用，与反向激动剂的抑制作用竞争。其次，TCPOBOP在体外和体内均直接作用以刺激共激活因子与CAR配体结合域的相互作用。第三，设计用于阻断配体结合的突变不仅阻断了雄烷类物质的抑制作用，也阻断了TCPOBOP的刺激作用。重要的是，这些突变并未阻断CAR明显的组成型反式激活作用，这表明这种活性确实不依赖配体。其靶向CYP基因的能力以及被TCPOBOP激活均表明CAR是一种新型的外源性受体，可能参与对这类化合物的代谢反应。
