Papalopulu N, Lovell-Badge R, Krumlauf R
MRC Laboratory of Eukaryotic Molecular Genetics, National Institute for Medical Research, London, UK.
Nucleic Acids Res. 1991 Oct 25;19(20):5497-506. doi: 10.1093/nar/19.20.5497.
In this paper we describe experiments that detail the response of murine Hox-2 genes to cellular differentiation and retinoic acid in cell culture. Hox-2 genes are transiently activated in differentiating ES cells even in the absence of retinoic acid (RA), indicating that their induction is a normal aspect of differentiation. Furthermore, in the continuous presence of RA F9 teratocarcinoma cells show a differential ability to maintain Hox-2 expression depending upon whether the cells follow a visceral or parietal endoderm pathway. These data suggest a clear dependence of Hox-2 expression on the degree and type of differentiation in different cells. However, RA also has dramatic differentiation independent effects on Hox-2 regulation. In ES cells the levels of Hox expression are greatly enhanced by exposure to RA, and in F9 cells of the visceral or parietal phenotype the continuous presence of RA is required to maintain these high levels. Nuclear run-on experiments illustrate that Hox-2 genes are active in F9 stem cells and that a large portion of the RA induction is mediated by post-transcriptional mechanisms. Therefore RA exerts its effects on Hox-2 expression by upregulating or modulating genes which are already active, rather than by turning-on silent genes. All nine Hox-2 genes are induced in F9 cells by RA and there is a direct correlation (collinearity) between gene order and the relative dose response of each gene to RA. In Xenopus embryos treated with RA, homologues of the Hox-2 genes also displayed a temporal and dose response collinearity with gene organisation. Together these findings suggest that the collinear response to RA is highly conserved in vertebrates and combined with the ability of RA to modify expression during cellular differentiation could be an important feature of the Hox-2 cluster itself used to generate the spatially-restricted patterns of gene expression in embryogenesis.
在本文中，我们描述了一些实验，这些实验详细阐述了小鼠Hox - 2基因在细胞培养中对细胞分化和视黄酸的反应。Hox - 2基因在分化的胚胎干细胞中即使在没有视黄酸（RA）的情况下也会短暂激活，这表明它们的诱导是分化的正常方面。此外，在持续存在RA的情况下，F9畸胎瘤细胞根据其遵循内脏或体壁内胚层途径而表现出维持Hox - 2表达的不同能力。这些数据表明Hox - 2表达明显依赖于不同细胞中分化的程度和类型。然而，RA对Hox - 2调控也有显著的不依赖于分化的作用。在胚胎干细胞中，暴露于RA会大大提高Hox表达水平，而在内脏或体壁表型的F9细胞中，持续存在RA是维持这些高水平所必需的。细胞核连续转录实验表明，Hox - 2基因在F9干细胞中是活跃的，并且RA诱导的很大一部分是由转录后机制介导的。因此，RA通过上调或调节已经活跃的基因来发挥其对Hox - 2表达的作用，而不是通过开启沉默基因。所有九个Hox - 2基因在F9细胞中都被RA诱导，并且基因顺序与每个基因对RA的相对剂量反应之间存在直接相关性（共线性）。在用RA处理的非洲爪蟾胚胎中，Hox - 2基因的同源物也表现出与基因组织的时间和剂量反应共线性。这些发现共同表明，对RA的共线性反应在脊椎动物中高度保守，并且与RA在细胞分化过程中修饰表达的能力相结合，可能是Hox - 2基因簇本身的一个重要特征，可以用于在胚胎发生过程中产生基因表达的空间限制模式。
