Natural structural variants of the nuclear receptor farnesoid X receptor affect transcriptional activation

The Farnesoid X receptor (FXR) is a member of the nuclear hormone receptor superfamily that has been shown to play an important role in bile acid and cholesterol homeostasis. Here we identify four murine FXR transcripts, derived from a single gene, that encode four isoforms, FXRalpha1, FXRalpha2, FXRbeta1, and FXRbeta2. FXRalpha and FXRbeta differ at their amino terminus, and FXRalpha1 and FXRbeta1 have a four-amino acid residue insertion in the hinge region immediately adjacent to the DNA binding domain. Real time PCR and 5'-rapid amplification of cDNA ends followed by Southern blotting reveal that these four transcripts are expressed differentially in liver, intestine, kidney, adrenals, stomach, fat, and heart. Electrophoretic mobility shift assays demonstrate that FXRa2 and FXRbeta2 bind to FXR response elements with a higher affinity as compared with FXRalpha1 and FXRbeta1, suggesting that the four-amino acid insert may affect FXR function. Consistent with this idea, the results of transient transfection experiments demonstrate that the four FXR isoforms differentially transactivated a number of promoter-reporter genes; activation of an ileal bile acid-binding protein promoter-reporter gene varied 20-fold depending on the FXR isoform; the rank order of activation was FXRbeta2 > FXRalpha2 much greater than FXRalpha1 = FXRbeta1. In contrast, SHP reporter or BSEP reporter genes were activated to similar degrees by each of the FXR isoforms. Finally, NIH3T3 cells were stably infected with individual murine FXR isoforms, and the cells were treated with FXR ligands. The endogenous ileal bile acid-binding protein gene was activated by the four FXR isoforms with the same rank order as seen in transfections. This effect was gene-specific, since induction of bile salt export pump mRNA was independent of the FXR isoform. These observations suggest that there are four distinct murine FXR isoforms that differentially regulate gene expression in numerous tissues in vivo.