Abstract
The farnesoid X receptor (FXR) is a nuclear receptor (NR) known to obligately heterodimerize with the retinoid X receptor (RXR). FXR is expressed as four isoforms (α1-α4) that drive transcription from IR-1 (inverted repeat-1) response elements (REs). Recently, we found that FXR isoforms α2/α4 also activate transcription from non-canonical ER-2 (everted repeat-2) REs, mediating most metabolic effects of general FXR activation. Here, we explored molecular determinants of regulation by FXRα2 from ER-2 REs through quantitative interaction proteomics, site-directed mutagenesis and transcriptomics. We discovered FXRα2 binds to and activates ER-2 elements in vitro and in reporter assays independently of RXR. Genome-wide binding analysis in mouse liver revealed higher ER-2 motif enrichment in FXR sites lacking RXR. Abrogation of FXRα2:RXR heterodimerization abolished IR-1, but preserved ER-2 transactivation. Transcriptome-wide, RXR overexpression inhibited 25% of FXRα2 targets in HepG2. These genes were specifically activated by the heterodimerization-deficient mutant FXRα2L434R, enriched for ER-2 motifs at their promoters, and involved in lipid metabolism and ammonia detoxification. In conclusion, RXR acts as a molecular switch, inhibiting FXRα2 activation from ER-2 while enhancing it from canonical IR-1 REs. Our results showcase FXR as the first NR with isoform-specific RXR-independent REs, highlighting a new layer of regulation and complexity for RXR-heterodimerizing NRs.