Dr. Margarete Fischer-Bosch-Institute of Clinical Pharmacology, Stuttgart and University of Tübingen, Tübingen, Germany.
Biobank of the Department of General, Visceral and Transplantation Surgery, University Hospital LMU Munich, Munich, Germany.
Arch Biochem Biophys. 2021 Jul 15;705:108893. doi: 10.1016/j.abb.2021.108893. Epub 2021 Apr 27.
Farnesoid X receptor (FXR, NR1H4) is a ligand-activated nuclear receptor, which regulates bile acid, lipid and glucose metabolism. Due to these functions, FXR has been investigated as a potential drug target for the treatment of liver diseases, such as primary biliary cholangitis and non-alcoholic steatohepatitis. Based on the previously described four splice variants, it has been suggested that alternative promoter usage and splicing may have an impact on total FXR activity as a result of encoding functionally diverse variants. Here we aimed for a systematic analysis of human hepatic FXR splice variants. In addition to the previously described FXRα1-4, we identified four novel splice variants (FXRα5-8) in human hepatocytes, which resulted from previously undetected exon skipping events. These newly identified isoforms displayed diminished DNA binding and impaired transactivation activities. Isoform FXRα5, which suppressed the transactivation activity of the functional isoform FXRα2, was further characterized as deficient in heterodimerization, coactivator recruitment and ligand binding. These findings were further supported by molecular dynamics simulations, which offered an explanation for the behavior of this isoform on the molecular level. FXRα5 exhibited low uniform expression levels in nearly all human tissues. Our systematic analysis of FXR splice variants in human hepatocytes resulted in the identification of four novel FXR isoforms, which all proved to be functionally deficient, but one novel variant, FXRα5, also displayed dominant negative activity. The possible associations with and roles of these novel isoforms in human liver diseases require further investigation.
法尼醇 X 受体 (FXR, NR1H4) 是一种配体激活的核受体，可调节胆汁酸、脂质和葡萄糖代谢。由于这些功能，FXR 已被研究作为治疗肝脏疾病的潜在药物靶点，如原发性胆汁性胆管炎和非酒精性脂肪性肝炎。基于先前描述的四种剪接变体，有人提出，替代启动子的使用和剪接可能会影响总 FXR 活性，因为它编码功能不同的变体。在这里，我们旨在对人类肝 FXR 剪接变体进行系统分析。除了先前描述的 FXRα1-4，我们在人类肝细胞中还鉴定了四个新的剪接变体 (FXRα5-8)，这是由于先前未检测到的外显子跳跃事件所致。这些新鉴定的异构体显示出 DNA 结合能力减弱和转录激活活性受损。抑制功能性异构体 FXRα2 转录激活活性的异构体 FXRα5 进一步被表征为缺乏异二聚体形成、共激活因子募集和配体结合。分子动力学模拟进一步支持了这些发现，为该异构体在分子水平上的行为提供了解释。FXRα5 在几乎所有人类组织中都表现出低的均匀表达水平。我们对人类肝细胞中 FXR 剪接变体的系统分析导致了四个新的 FXR 异构体的鉴定，所有这些异构体都被证明功能缺失，但一个新的变体 FXRα5 也表现出显性负性活性。这些新异构体在人类肝脏疾病中的可能关联和作用需要进一步研究。
