Archer T K, Fryer C J, Lee H L, Zaniewski E, Liang T, Mymryk J S
Department of Obstetrics and Gynaecology, University of Western Ontario, London, Canada.
J Steroid Biochem Mol Biol. 1995 Jun;53(1-6):421-9. doi: 10.1016/0960-0760(95)00088-h.
The ability to respond to small signalling molecules such as steroid hormones is important for many physiological processes. Steroid hormones act through a group of high affinity receptors that regulate transcription by binding to hormone response elements (HREs) located within the promoters of target genes, which themselves are organized with nuclear proteins to form chromatin. To dissect the mechanisms(s) of steroid hormone action we have used the steroid inducible mouse mammary tumor virus (MMTV) promoter as a model system. The MMTV promoter is assembled into a phased array of nucleosomes that are specifically positioned in rodent cells. Induction of transcription by glucocorticoids is accompanied by the appearance of a hypersensitive region in the proximal promoter which allows the hormone dependent assembly of a preinitiation complex including transcription factors such as nuclear factor 1 (NF1) and the octamer transcription factor (OTF). Surprisingly, when introduced by transient transfection, the progesterone receptor (PR) is unable to activate this promoter in vivo, a finding that may result from its inability to alter MMTV promoter chromatin. In an attempt to investigate the failure of the PR to activate the promoter, we have stably introduced the MMTV promoter into human T47D breast cancer cells that express high levels of the PR. In contrast to what has been observed previously in rodent cells, the MMTV templates resident in human breast cancer cells adopt a novel and constitutively open chromatin structure. The constitutively open chromatin structure is accompanied by the hormone independent loading of transcription factors including the PR and NF1. In T47D cells that stably express the glucocorticoid receptor, the MMTV promoter responds to glucocorticoids, but not progestins, and displays glucocorticoid induced restriction enzyme hypersensitivity and transcription factor loading. These findings suggest that the organization of the MMTV chromatin structure is dependent upon the cell type and receptor status of the recipient cell into which the MMTV promoter is stably introduced.
对类固醇激素等小分子信号分子作出反应的能力，对许多生理过程都很重要。类固醇激素通过一组高亲和力受体发挥作用，这些受体通过与位于靶基因启动子内的激素反应元件（HREs）结合来调节转录，而这些元件本身与核蛋白一起构成染色质。为了剖析类固醇激素作用的机制，我们使用了类固醇诱导型小鼠乳腺肿瘤病毒（MMTV）启动子作为模型系统。MMTV启动子组装成一系列有特定定位的核小体，这些核小体在啮齿动物细胞中具有特异性定位。糖皮质激素诱导转录伴随着近端启动子中一个超敏感区域的出现，该区域允许激素依赖性组装一个包括核因子1（NF1）和八聚体转录因子（OTF）等转录因子的预起始复合物。令人惊讶的是，当通过瞬时转染引入时，孕激素受体（PR）在体内无法激活该启动子，这一发现可能是由于其无法改变MMTV启动子染色质所致。为了探究PR激活启动子失败的原因，我们将MMTV启动子稳定地导入了高表达PR的人T47D乳腺癌细胞中。与之前在啮齿动物细胞中观察到的情况相反，驻留在人乳腺癌细胞中的MMTV模板采用了一种新的、组成型开放的染色质结构。这种组成型开放的染色质结构伴随着包括PR和NF1在内的转录因子的激素非依赖性加载。在稳定表达糖皮质激素受体的T47D细胞中，MMTV启动子对糖皮质激素有反应，但对孕激素无反应，并表现出糖皮质激素诱导的限制性内切酶超敏感性和转录因子加载。这些发现表明，MMTV染色质结构的组织依赖于稳定导入MMTV启动子的受体细胞的细胞类型和受体状态。
