Wang Yutong, Zhang Yanqiao, Hillgartner F Bradley
Department of Biochemistry and Molecular Pharmacology, School of Medicine, P.O. Box 9142, West Virginia University, Morgantown, WV 26506-9142, USA.
Biochem J. 2002 Jan 15;361(Pt 2):391-400. doi: 10.1042/bj3610391.
In chick embryo hepatocytes (CEH), stimulation of malic enzyme transcription by 3,3',5-tri-iodothyronine (T3) is mediated by a liver-specific and T3-inducible DNase I hypersensitive region (-3910 to -3640 bp) in the malic enzyme gene. Previous studies have shown that this region contains a cluster of five T3 response elements (T3REs), referred to as a T3 response unit (T3RU), plus three accessory elements that enhance T3 responsiveness conferred by the T3RU. Here we report the identification of two additional accessory elements within the -3910 to -3640 bp region. Each element augments T3 regulation of malic enzyme transcription in CEH. One element, designated region G (-3681/-3666 bp), contains a single nuclear-hormone-receptor half-site that binds the orphan receptor chicken ovalbumin upstream-promoter transcription factor. The other element, designated region H (-3655/-3646 bp), contains an E-box motif that binds proteins of unknown identity. Stimulation of T3RE function by region G or region H does not require the presence of additional malic enzyme sequences. In contrast with the stimulatory effects of regions G and H on T3 responsiveness in CEH, neither of these elements is effective in modulating T3 responsiveness in chick embryo fibroblasts (CEF). Instead, region H functions as a T3-insensitive repressor of transcription in CEF. These results indicate that chicken ovalbumin upstream-promoter transcription factor and E-box-binding proteins interact with nuclear T3 receptors to enhance T3 regulation of malic enzyme transcription in CEH and that alterations in region G and region H activities contribute to diminished T3 regulation of malic enzyme transcription in CEF relative to CEH. As the pattern of protein binding to regions G and H varies substantially between CEH and CEF, the mechanism for cell-type-dependent differences in region G and region H activity may involve alterations in protein binding to these T3 accessory elements.
在鸡胚肝细胞（CEH）中，3,3',5-三碘甲状腺原氨酸（T3）对苹果酸酶转录的刺激作用是由苹果酸酶基因中一个肝脏特异性且T3可诱导的脱氧核糖核酸酶I超敏区域（-3910至-3640 bp）介导的。先前的研究表明，该区域包含一组五个T3反应元件（T3REs），称为T3反应单元（T3RU），以及三个增强T3RU赋予的T3反应性的辅助元件。在此，我们报告在-3910至-3640 bp区域内鉴定出另外两个辅助元件。每个元件都增强了CEH中T3对苹果酸酶转录的调控。一个元件，称为区域G（-3681/-3666 bp），包含一个单核激素受体半位点，可结合孤儿受体鸡卵清蛋白上游启动子转录因子。另一个元件，称为区域H（-3655/-3646 bp），包含一个E盒基序，可结合身份未知的蛋白质。区域G或区域H对T3RE功能的刺激作用不需要额外的苹果酸酶序列。与区域G和H对CEH中T3反应性的刺激作用相反，这两个元件在调节鸡胚成纤维细胞（CEF）中T3反应性方面均无效。相反，区域H在CEF中作为T3不敏感的转录抑制因子发挥作用。这些结果表明，鸡卵清蛋白上游启动子转录因子和E盒结合蛋白与核T3受体相互作用，以增强CEH中T3对苹果酸酶转录的调控，并且相对于CEH，区域G和区域H活性的改变导致CEF中T3对苹果酸酶转录的调控减弱。由于CEH和CEF之间与区域G和H结合的蛋白质模式有很大差异，区域G和H活性的细胞类型依赖性差异的机制可能涉及与这些T3辅助元件结合的蛋白质的改变。
