Cai Shi-Ying, Xiong Liangshi, Wray Charles G, Ballatori Nazzareno, Boyer James L
Liver Center, Yale University School of Medicine, 333 Cedar Street, New Haven, CT 06520, USA.
Am J Physiol Regul Integr Comp Physiol. 2007 Sep;293(3):R1400-9. doi: 10.1152/ajpregu.00781.2006. Epub 2007 Jun 13.
The nuclear receptor FXRalpha (NR1H4) plays a pivotal role in maintaining bile salt and lipid homeostasis by functioning as a bile salt sensor in mammals. In contrast, FXRbeta (NR1H5) from mouse is activated by lanosterol and does not share common ligands with FXRalpha. To further elucidate FXR ligand/receptor and structure/function relationships, we characterized a FXR gene from the marine skate, Leucoraja erinacea, representing a vertebrate lineage that diverged over 400 million years ago. Phylogenetic analysis of sequence data indicated that skate Fxr (sFxr) is a FXRbeta. There is an extra sequence in the middle of the sFxr ligand binding domain (LBD) compared with the LBD of FXRalpha. Luciferase reporter assays demonstrated that sFxr responds weakly to scymnol sulfate, bile salts, and synthetic FXRalpha ligands, in striking difference from human FXRalpha (hFXRalpha). Interestingly, all-trans retinoic acid was capable of transactivating both hFXRalpha and sFxr. When the extra amino acids in the sFxr LBD were deleted and replaced with the corresponding sequence from hFXRalpha, the mutant sFxr gained responsiveness to ursodeoxycholic acid, GW4064, and fexaramine. Surprisingly, chenodeoxycholic acid antagonized this activation. Together, these results indicate that FXR is an ancient nuclear receptor and suggest that FXRalpha may have acquired ligand specificity for bile acids later in evolution by deletion of a sequence from its LBD. Acquisition of this property may be an example of molecular exploitation, where an older molecule is recruited for a new functional role.
核受体FXRα（NR1H4）在哺乳动物中作为胆汁盐传感器，在维持胆汁盐和脂质稳态方面发挥着关键作用。相比之下，小鼠的FXRβ（NR1H5）由羊毛甾醇激活，且与FXRα没有共同的配体。为了进一步阐明FXR配体/受体以及结构/功能关系，我们对来自海洋鳐鱼Leucoraja erinacea的FXR基因进行了表征，该基因代表了一个在4亿多年前就已分化的脊椎动物谱系。对序列数据的系统发育分析表明，鳐鱼Fxr（sFxr）是一种FXRβ。与FXRα的配体结合域（LBD）相比，sFxr的LBD中间有一段额外的序列。荧光素酶报告基因检测表明，sFxr对硫酸鲨胆醇、胆汁盐和合成的FXRα配体反应较弱，这与人类FXRα（hFXRα）有显著差异。有趣的是，全反式维甲酸能够激活hFXRα和sFxr。当删除sFxr LBD中的额外氨基酸并用hFXRα的相应序列取代时，突变型sFxr对熊去氧胆酸、GW4064和非瑟酮胺有反应。令人惊讶的是，鹅去氧胆酸拮抗这种激活作用。总之，这些结果表明FXR是一种古老的核受体，并表明FXRα可能在进化后期通过从其LBD中删除一段序列而获得了对胆汁酸的配体特异性。获得这一特性可能是分子利用的一个例子，即一个较老的分子被招募来承担新的功能角色。
