FXR controls gene expression independently of RXR at non-canonical response elements

ABSTRACT: The Farnesoid-X-Receptor (FXR) is a class II nuclear receptor (NR), a class that obligately heterodimerizes with the Retinoid-X-Receptor (RXR). FXR is expressed as 4 isoforms (α1-α4) activated by bile acids that drive transcription from response elements IR-1 (inverted repeat-1). We recently showed that FXR isoforms α2 and α4 bind to non-canonical response elements ER-2 (everted repeat-2), thereby increasing mitochondrial respiratory capacity and limiting de novo lipogenesis. Binding to ER-2 motifs in mouse liver organoids represented 89% of all FXR genome wide binding. However, mechanistic differences in FXR binding and activation from these two response elements remained unexplored. Using DNA pull down followed by mass-spectrometry, we show that RXR is not involved in FXR binding to ER-2 response elements. Instead, RXR inhibited FXR binding and activation from these elements in luciferase reporters. Genome wide, RXR-lacking FXR binding sites showed higher enrichment for ER-2 motifs in mouse liver. Pharmacological and mutational abrogation of FXR-RXR heterodimerization specifically retained ER-2 transactivation capacities in luciferase reporters and HepG2 cells. Transcriptome-wide, 25% of FXR targets were inhibited upon RXR overexpression, but specifically activated by a novel heterodimerization-deficient mutant FXRα2L434R. These genes were ER-2 responsive and were involved in lipid metabolism and ammonia detoxification. In conclusion, we discovered that RXR is not required and even inhibits binding of FXRα2 to ER-2. Thus, whereas FXR α1 and α3 seem to be genuine class II NR, FXRα2 and α4 are facultative class II at ER-2 motifs. This novel feature holds promise to exploit and tailor therapeutic responses to FXR agonism.

INSTRUMENT(S): Q Exactive HF-X

ORGANISM(S): Homo Sapiens (human)

TISSUE(S): Hepatocyte, Cell Culture

SUBMITTER: Jelmer Dijkstra  

LAB HEAD: Michiel Vermeulen

PROVIDER: PXD047930 | Pride | 2024-11-12

REPOSITORIES: Pride