Farnesoid X Receptor Signaling Activates the Hepatic X-box Binding Protein 1 Pathway in vitro and in Mice

Abstract

Bile acids are endogenous ligands of the nuclear receptor farnesoid X receptor (FXR), and pharmacologic 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 stress. We investigated the role of FXR signaling in the activation of the hepatic XBP1 pathway. Mice were treated with deoxycholic acid (DCA), cholestyramine, GW4064 or underwent bile duct ligation (BDL) and the hepatic UPR activation was measured. Huh7-Ntcp and HepG2 cells were treated with FXR agonists, inhibitor, siRNA or SHP siRNA to determine the mechanisms of IRE1α/XBP1 pathway activation. DCA feeding and BDL increased and cholestyramine decreased expression of hepatic XBP1s. XBP1 pathway activation increased in Huh7-Ntcp and HepG2 cells treated with bile acids, 6-ECDCA or GW4064. This effect decreased with FXR knockdown and treatment with FXR inhibitor guggulsterone. FXR agonists increased XBP1 splicing and phosphorylated-IRE1α expression. Overexpression of small heterodimer partner (SHP) similarly increased XBP1 splicing, XBP1s and phosphorylated-IRE1α protein expression. SHP knockdown attenuated FXR agonist-induced XBP1s and phosphorylated-IRE1α protein expression. Co-immunoprecipitation assays demonstrate a physical interaction between overexpressed 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. Conclusion: FXR signaling activates the IRE1α/XBP1 pathway in vivo and in vitro. FXR pathway activation increases XBP1 splicing and enhances phosphorylated-IRE1α expression. These effects are mediated, at least in part, by SHP. IRE1α/XBP1 pathway activation by bile acids and pharmacologic FXR agonists may be protective during liver injury and may have therapeutic implications for liver diseases. This article is protected by copyright. All rights reserved.

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