Minucci S, Wong J, Blanco J C, Shi Y B, Wolffe A P, Ozato K
Laboratory of Molecular Growth Regulation, National Institutes of Child Health and Human Development, Bethesda, Maryland 20892, USA.
Mol Endocrinol. 1998 Mar;12(3):315-24. doi: 10.1210/mend.12.3.0074.
Retinoic acid (RA) stimulates transcription from the retinoic acid receptor beta2 (RARbeta2) promoter in mammalian embryonal cells. Evidence by in vivo deoxyribonuclease I (DNase I) hypersensitivity assay indicates that RA treatment of these cells results in an alteration of chromatin structure in and near the promoter. To study the role of chromatin in RA-activated transcription, we assembled the RARbeta2 promoter into chromatin in Xenopus oocytes. Ectopic expression of RAR and retinoid X receptor (RXR) enhanced transcription without ligand, irrespective of whether chromatin was assembled in a replication-dependent or -independent manner, although ligand addition led to a further, marked increase in transcription. Moreover, expression of RAR and RXR, without ligand addition, induced DNase I-hypersensitive sites in the chromatin-assembled promoter. Furthermore, expression of RAR and RXR in oocytes led to local disruption of chromatin assembled over the promoter without ligand. Similar ligand-independent, but RXR/RAR-dependent nucleosomal disruption was observed in an in vitro chromatin reconstitution system using Drosophila embryonic extracts. Thus, unliganded receptors expressed in oocytes are capable of accessing to the chromatin-assembled promoter and activating transcription without ligand, indicating that chromatin assembly per se is not sufficient to reproduce ligand-dependent chromatin changes and promoter activation seen in mammalian cells. The oocyte system may serve as a model to study mechanisms of RA-dependent alterations of chromatin structure.
维甲酸（RA）可刺激哺乳动物胚胎细胞中维甲酸受体β2（RARβ2）启动子的转录。体内脱氧核糖核酸酶I（DNase I）超敏分析的证据表明，用RA处理这些细胞会导致启动子及其附近染色质结构的改变。为了研究染色质在RA激活转录中的作用，我们将RARβ2启动子组装到非洲爪蟾卵母细胞的染色质中。视黄酸受体（RAR）和视黄酸X受体（RXR）的异位表达在无配体的情况下增强了转录，无论染色质是以复制依赖还是独立的方式组装，尽管添加配体导致转录进一步显著增加。此外，在无配体添加的情况下，RAR和RXR的表达在染色质组装的启动子中诱导了DNase I超敏位点。此外，卵母细胞中RAR和RXR的表达导致在无配体的情况下启动子上组装的染色质局部破坏。在使用果蝇胚胎提取物的体外染色质重构系统中也观察到了类似的不依赖配体但依赖RXR/RAR的核小体破坏。因此，卵母细胞中表达的未结合配体的受体能够在无配体的情况下进入染色质组装的启动子并激活转录，这表明染色质组装本身不足以重现哺乳动物细胞中依赖配体的染色质变化和启动子激活。卵母细胞系统可作为研究RA依赖的染色质结构改变机制的模型。
