Retinoid X receptor (RXR) agonist-induced antagonism of farnesoid X receptor (FXR) activity due to absence of coactivator recruitment and decreased DNA binding

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 FXF/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.