Human bile salt export pump promoter is transactivated by the farnesoid X receptor/bile acid receptor

The bile salt excretory pump (BSEP, ABCb11) is critical for ATP-dependent t ransport of bile acids across the hepatocyte canalicular membrane and for g eneration of bile acid-dependent bile secretion. Recent studies have demons trated that the expression of this transporter is sensitive to the flux of bile acids through the hepatocyte, possibly at the level of transcription o f the BSEP gene. To determine the mechanisms underlying the regulation of B SEP by bile acids, the promoter of the BSEP gene was cloned. The sequence o f the promoter contained an inverted repeat (IR)-1 element (5 ' -GGGACA T T GATCCT-3 ') at base pairs -63/-50 consisting of two nuclear receptor half-s ites organized as an inverted repeat and separated by a single nucleotide. This IR-1 element has been shown in several recent studies to serve as a bi nding site for the farnesoid X receptor (FXR), a nuclear receptor for bile acids. FXR activity requires heterodimerization with RXR alpha, and when bo und by bile acids, the complex effectively regulates the transcription of s everal genes involved in bile acid homeostasis. Gel mobility shift assays d emonstrated specific binding of FXR/RXR alpha heterodimers to the IR-1 elem ent in the BSEP promoter. In HepG2 cells, co-transfection of FXR and RXRa i s required to attain full transactivation of the BSEP promoter by bile acid s. Two FXR transactivation-deficient mutants (an AF-2 deletion and a W469A point mutant) failed to transactivate, indicating that the effect of bile a cids is FXR-dependent. Further, mutational analysis confirms that the FXR/R XRa heterodimer activates transcription through the IR-1 site in the human BSEP promoter. These results demonstrate a mechanism by which bile acids tr anscriptionally regulate the activity of the bile salt excretory pump, a cr itical component involved in the enterohepatic circulation of bile acids.