Functional analysis on the 5'-flanking region of human FXR gene in HepG2 cells

The farnesoid X receptor (FXR) is a bile acid (BA)-activated nuclear receptor that plays a major role in the regulation of BA and lipid metabolism. Although modulation of FXR expression has been reported, the mechanisms underlying the regulation of human FXR are yet unclear. Functional assays showed that the -150/+29 nucleotides region from the first nucleotide at the Exon I is the minimal promoter of the human FXR gene by the technique of serial deletion and point mutants of the 5'-flanking region. Chromatin immunoprecipitation analysis and electrophoretic mobility shift assay revealed that hepatic nuclear factor 1alpha (HNF1alpha) interacted with the region. Co-transfection of the promoter with HNF1alpha expression vectors enhanced promoter activity of FXR gene. Over-expression of HNF1alpha up-regulated FXR expression in HepG2 cells. These data indicate that (a) the identified HNF1alpha binding site serves as a positive regulatory sequence, (b) the binding site is functionally active both in vivo and in vitro, and (c) the transcription factor HNF1alpha that binds to this site plays an important role in the regulation of human FXR promoter activity.