Jeannin E, Robyr D, Desvergne B
Institut de Biologie Animale, Université de Lausanne, Bâtiment de Biologie, CH-1015 Lausanne, Switzerland.
J Biol Chem. 1998 Sep 11;273(37):24239-48. doi: 10.1074/jbc.273.37.24239.
While there is evidence that the two ubiquitously expressed thyroid hormone (T3) receptors, TRalpha1 and TRbeta1, have distinct functional specificities, the mechanism by which they discriminate potential target genes remains largely unexplained. In this study, we demonstrate that the thyroid hormone response elements (TRE) from the malic enzyme and myelin basic protein genes (METRE and MBPTRE) respectively, are not functionally equivalent. The METRE, which is a direct repeat motif with a 4-base pair gap between the two half-site hexamers binds thyroid hormone receptor as a heterodimer with 9-cis-retinoic acid receptor (RXR) and mediates a high T3-dependent activation in response to TRalpha1 or TRbeta1 in NIH3T3 cells. In contrast, the MBPTRE, which consists of an inverted palindrome formed by two hexamers spaced by 6 base pairs, confers an efficient transactivation by TRbeta1 but a poor transactivation by TRalpha1. While both receptors form heterodimers with RXR on MBPTRE, the poor transactivation by TRalpha1 correlates also with its ability to bind efficiently as a monomer. This monomer, which is only observed with TRalpha1 bound to MBPTRE, interacts neither with N-CoR nor with SRC-1, explaining its functional inefficacy. However, in Xenopus oocytes, in which RXR proteins are not detectable, the transactivation mediated by TRalpha1 and TRbeta1 is equivalent and independent of a RXR supply, raising the question of the identity of the thyroid hormone receptor partner in these cells. Thus, in mammalian cells, the binding characteristics of TRalpha1 to MBPTRE (i.e. high monomer binding efficiency and low transactivation activity) might explain the particular pattern of T3 responsiveness of MBP gene expression during central nervous system development.
虽然有证据表明两种普遍表达的甲状腺激素（T3）受体TRα1和TRβ1具有不同的功能特异性，但它们区分潜在靶基因的机制在很大程度上仍未得到解释。在本研究中，我们证明苹果酸酶和髓鞘碱性蛋白基因（METRE和MBPTRE）的甲状腺激素反应元件（TRE）在功能上并不等同。METRE是一种直接重复基序，两个半位点六聚体之间有4个碱基对的间隔，它作为与9-顺式视黄酸受体（RXR）的异二聚体结合甲状腺激素受体，并在NIH3T3细胞中介导对TRα1或TRβ1的高T3依赖性激活。相比之下，MBPTRE由两个间隔6个碱基对的六聚体形成的反向回文组成，它赋予TRβ1高效的反式激活作用，但赋予TRα1的反式激活作用较差。虽然两种受体都在MBPTRE上与RXR形成异二聚体，但TRα1的反式激活作用较差也与其作为单体有效结合的能力相关。这种单体仅在TRα1与MBPTRE结合时观察到，它既不与N-CoR相互作用，也不与SRC-1相互作用，这解释了其功能无效性。然而，在非洲爪蟾卵母细胞中，无法检测到RXR蛋白，TRα1和TRβ1介导的反式激活作用是等同的，且不依赖于RXR的供应，这就提出了这些细胞中甲状腺激素受体伴侣身份的问题。因此，在哺乳动物细胞中，TRα1与MBPTRE的结合特性（即高单体结合效率和低反式激活活性）可能解释了中枢神经系统发育过程中MBP基因表达的T3反应性的特定模式。
