Retinoid X Receptor (RXR) Agonist-induced Antagonism of Farnesoid X Receptor (FXR) Activity due to Absence of Coactivator Recruitment and Decreased DNA Binding

Altaf Kassam,Bowman Miao,Peter R Young,Ranjan Mukherjee

doi:10.1074/jbc.m208312200

Abstract

The bile salt export pump (BSEP) plays an integral role in lipid homeostasis by regulating the canalicular excretion of bile acids. Induction of BSEP gene expression is mediated by the farnesoid X receptor (FXR), which binds as a heterodimer with the retinoid X receptor (RXR) to the FXR response element (FXRE) located upstream of the BSEP gene. RXR ligands mimic several partner ligands and show additive effects upon coadministration. Using real-time quantitative PCR and cotransfection reporter assays, we demonstrate that the RXR agonist LG100268 antagonizes induction of BSEP expression mediated by endogenous and synthetic FXR ligands, CDCA and GW4064, respectively. Moreover, this antagonism is a general feature of RXR agonists and is attributed to a decrease in binding of FXR/RXR heterodimers to the BSEP-FXRE coupled with the inability of RXR agonists to recruit coactivators to FXR/RXR. Our data suggest that FXR/RXR is a conditionally permissive heterodimer and is the first example of RXR ligand-mediated antagonism of FXR activity. Because FXR agonists lower triglyceride levels, our results suggest a novel role for RXR-mediated antagonism of FXR activity in the development of hypertriglyceridemia observed with RXR agonists in rodents and humans.

Highlights

Ligands activate the peroxisome proliferator-activated receptor (PPAR)/retinoid X receptor (RXR) and liver X receptor/RXR heterodimers, [4, 6] referred to as permissive heterodimers
Induction of bile salt export pump (BSEP) gene expression is mediated by the farnesoid X receptor (FXR), which binds as a heterodimer with the retinoid X receptor (RXR) to the FXR response element (FXRE) located upstream of the BSEP gene
The FXR/RXR heterodimer binds to farnesoid X receptor response elements (FXREs) or bile acid response elements found in the promoter of FXR-responsive genes such as the bile-salt export pump (BSEP), which controls the excretion of bile acids from hepatocytes

Summary

Introduction

Ligands activate the PPAR/RXR and liver X receptor/RXR heterodimers, [4, 6] referred to as permissive heterodimers. Using real-time quantitative PCR and cotransfection reporter assays, we demonstrate that the RXR agonist LG100268 antagonizes induction of BSEP expression mediated by endogenous and synthetic FXR ligands, CDCA and GW4064, respectively. Robust induction of BSEP mRNA was observed in HepG2 cells by CDCA, a natural bile acid ligand and GW4064, a synthetic, high affinity ligand for FXR (30-fold and 80-fold, respectively) [23] (Fig. 1A).

Results

Conclusion