The Farnesoid-X-Receptor (FXR) is a transcription factor activated by bile acids, which acts as a metabolic homeostat for lipids, glucose and amino acids. FXR is therefore a promising target for metabolic disorders. FXR is expressed as 4 isoforms (FXRa1-4); all of which bind to IR-1 (inverted-repeat-1) DNA motifs upon heterodimerization with RXR. FXRa2/4 additionally bind to ER-2 (everted-repeat-2) motifs. FXR binding to ER-2 represents 90% of all binding sites in the mouse liver, inducing a transcriptional program inhibiting lipogenesis and gluconeogenesis. Here, we aimed to investigate differential co-factor binding to IR-1 and ER-2 sites, to enable selective targeting of ER-2-mediated signaling.

Using IR-1 and ER-2 oligos as baits in a HepG2 lysate, we showed that RXR precipitates with IR-1, but not with ER-2 oligos. Using reporter assays, we confirmed that RXR dose-dependently increased IR-1-mediated transcription, whereas it decreased ER-2-mediated transcription. Disrupting FXR-RXR-heterodimerization with an inhibitor (HX531) or an FXR-heterodimerization mutant (L434R) reduced IR-1, but not ER-2-mediated transcription.

Unlike previously thought, RXR is not an obligate heterodimerization partner of FXR, as RXR does not mediate FXR transcription from ER-2 motifs. We will use this novel mechanism of FXR transactivation to develop strategies to selectively upregulate ER-2-mediated transcription to improve liver metabolism.