Département de Biologie et de Génomique Structurales, Institut de Génétique et de Biologie Moléculaire et Cellulaire (IGBMC), Institut National de Santé et de Recherche Médicale (INSERM) U964/Centre National de Recherche Scientifique (CNRS) UMR 1704/Université de Strasbourg, Illkirch, France.
PLoS One. 2010 Nov 30;5(11):e15119. doi: 10.1371/journal.pone.0015119.
Retinoic acid receptors (RARs) and Retinoid X nuclear receptors (RXRs) are ligand-dependent transcriptional modulators that execute their biological action through the generation of functional heterodimers. RXR acts as an obligate dimer partner in many signalling pathways, gene regulation by rexinoids depending on the liganded state of the specific heterodimeric partner. To address the question of the effect of rexinoid antagonists on RAR/RXR function, we solved the crystal structure of the heterodimer formed by the ligand binding domain (LBD) of the RARα bound to its natural agonist ligand (all-trans retinoic acid, atRA) and RXRα bound to a rexinoid antagonist (LG100754). We observed that RARα exhibits the canonical agonist conformation and RXRα an antagonist one with the C-terminal H12 flipping out to the solvent. Examination of the protein-LG100754 interactions reveals that its propoxy group sterically prevents the H12 associating with the LBD, without affecting the dimerization or the active conformation of RAR. Although LG100754 has been reported to act as a 'phantom ligand' activating RAR in a cellular context, our structural data and biochemical assays demonstrate that LG100754 mediates its effect as a full RXR antagonist. Finally we show that the 'phantom ligand effect' of the LG100754 is due to a direct binding of the ligand to RAR that stabilizes coactivator interactions thus accounting for the observed transcriptional activation of RAR/RXR.
视黄酸受体（RARs）和维甲酸 X 核受体（RXRs）是配体依赖性转录调节剂，通过生成功能性异二聚体来执行其生物学作用。RXR 在许多信号通路中作为必需的二聚体伴侣发挥作用，类视黄醇对基因的调节取决于特定异二聚体伴侣的配体状态。为了解决类视黄醇拮抗剂对 RAR/RXR 功能的影响问题，我们解析了配体结合域（LBD）与天然激动剂配体（全反式视黄酸，atRA）结合的 RARα 与结合了类视黄醇拮抗剂（LG100754）的 RXRα 形成的异二聚体的晶体结构。我们观察到 RARα 呈现出典型的激动剂构象，而 RXRα 呈现出拮抗剂构象，C 端 H12 翻转到溶剂中。对蛋白-LG100754 相互作用的检查表明，其丙氧基基团在空间上阻止 H12 与 LBD 结合，而不影响二聚化或 RAR 的活性构象。尽管 LG100754 已被报道在细胞环境中作为“虚拟配体”激活 RAR，但我们的结构数据和生化测定表明，LG100754 作为全 RXR 拮抗剂发挥作用。最后，我们表明 LG100754 的“虚拟配体效应”是由于配体直接与 RAR 结合，稳定了共激活剂相互作用，从而解释了观察到的 RAR/RXR 转录激活。
