Division of Gastroenterology and Hepatology, Department of Medicine, Northwestern University Feinberg School of Medicine, Chicago, IL.
Department of Pharmacology and Toxicology, School of Pharmacy, Rutgers University, Piscataway, NJ.
Hepatology. 2018 Jul;68(1):304-316. doi: 10.1002/hep.29815. Epub 2018 May 10.
Bile acids are endogenous ligands of the nuclear receptor, farnesoid X receptor (FXR), and pharmacological FXR modulators are under development for the treatment of several liver disorders. The inositol-requiring enzyme 1α/X-box binding protein 1 (IRE1α/XBP1) pathway of the unfolded protein response (UPR) is a protective cellular signaling pathway activated in response to endoplasmic reticulum (ER) stress. We investigated the role of FXR signaling in activation of the hepatic XBP1 pathway. Mice were treated with deoxycholic acid (DCA), cholestyramine, GW4064, or underwent bile duct ligation (BDL), and hepatic UPR activation was measured. Huh7-Ntcp and HepG2 cells were treated with FXR agonists, inhibitor, small interfering RNA (siRNA), or small heterodimer partner (SHP) siRNA to determine the mechanisms of IRE1α/XBP1 pathway activation. DCA feeding and BDL increased and cholestyramine decreased expression of hepatic XBP1 spliced (XBP1s). XBP1 pathway activation increased in Huh7-Ntcp and HepG2 cells treated with bile acids, 6α-ethyl-chenodeoxycholic acid (6-ECDCA) or GW4064. This effect decreased with FXR knockdown and treatment with the FXR inhibitor guggulsterone. FXR agonists increased XBP1 splicing and phosphorylated IRE1α (p-IRE1α) expression. Overexpression of SHP similarly increased XBP1 splicing, XBP1s, and p-IRE1α protein expression. SHP knockdown attenuated FXR agonist-induced XBP1s and p-IRE1α protein expression. Co-immunoprecipitation (Co-IP) assays demonstrate a physical interaction between overexpressed green fluorescent protein (GFP)-SHP and FLAG-IRE1α in HEK293T cells. Mice treated with GW4064 had increased, and FXR and SHP null mice had decreased, basal Xbp1s gene expression.
FXR signaling activates the IRE1α/XBP1 pathway in vivo and in vitro. FXR pathway activation increases XBP1 splicing and enhances p-IRE1α expression. These effects are mediated, at least in part, by SHP. IRE1α/XBP1 pathway activation by bile acids and pharmacological FXR agonists may be protective during liver injury and may have therapeutic implications for liver diseases. (Hepatology 2018;68:304-316).
胆汁酸是核受体法尼醇 X 受体 (FXR) 的内源性配体，法尼醇 X 受体 (FXR) 的药理学调节剂正在开发用于治疗几种肝脏疾病。未折叠蛋白反应 (UPR) 的肌醇需求酶 1α/X 盒结合蛋白 1 (IRE1α/XBP1) 途径是一种保护性细胞信号通路，在应对内质网 (ER) 应激时被激活。我们研究了 FXR 信号在激活肝 XBP1 途径中的作用。用脱氧胆酸 (DCA)、考来烯胺、GW4064 处理小鼠或进行胆管结扎 (BDL)，并测量肝 UPR 激活情况。用 FXR 激动剂、抑制剂、小干扰 RNA (siRNA) 或小异二聚体伴侣 (SHP) siRNA 处理 Huh7-Ntcp 和 HepG2 细胞，以确定 IRE1α/XBP1 途径激活的机制。DCA 喂养和 BDL 增加，考来烯胺减少肝 XBP1 剪接 (XBP1s) 的表达。用胆汁酸、6α-乙基-鹅去氧胆酸 (6-ECDCA) 或 GW4064 处理 Huh7-Ntcp 和 HepG2 细胞可增加 XBP1 途径的激活。这种作用随着 FXR 的敲低而降低，并随着 FXR 抑制剂 guggulsterone 的处理而降低。FXR 激动剂增加 XBP1 剪接和磷酸化 IRE1α(p-IRE1α) 表达。过表达 SHP 同样增加 XBP1 剪接、XBP1s 和 p-IRE1α 蛋白表达。SHP 敲低可减弱 FXR 激动剂诱导的 XBP1s 和 p-IRE1α 蛋白表达。免疫共沉淀 (Co-IP) 实验表明，在 HEK293T 细胞中，过表达的绿色荧光蛋白 (GFP)-SHP 与 FLAG-IRE1α 之间存在物理相互作用。用 GW4064 处理的小鼠基础 Xbp1s 基因表达增加，FXR 和 SHP 基因敲除小鼠基础 Xbp1s 基因表达减少。
FXR 信号在体内和体外激活 IRE1α/XBP1 途径。FXR 途径的激活增加了 XBP1 的剪接，并增强了 p-IRE1α 的表达。这些作用至少部分是由 SHP 介导的。胆汁酸和药理学 FXR 激动剂对 IRE1α/XBP1 途径的激活可能在肝损伤期间具有保护作用，并可能对肝脏疾病具有治疗意义。(《肝脏病学》2018；68:304-316)。
