ABSTRACT
BACKGROUND/AIMS: Progressive familial intrahepatic cholestasis type 2 (PFIC-2) patients have a defect in the hepatocanalicular bile salt secretion. The disease is caused by mutations in the bile salt export pump (BSEP). Ten different missense mutations have been described. In this study, we analysed the effect of the D482G PFIC-2 mutation on BSEP function. METHODS: Adenosine triphosphatase (ATPase) and taurocholate transport assays were performed with full-length mouse Bsep (mBsep) with and without the D482G mutation. The effect on expression and subcellular sorting was studied in HepG2 cells, stably expressing enhanced green fluorescent protein (EGFP)-tagged mBsep proteins. RESULTS: The D482G mutation did not significantly affect the taurocholate transport activity of mBsep, even though the bile salt-inducible ATPase activity of the mutant protein was slightly reduced. Protein expression and canalicular sorting were strongly affected by the D482G mutation. Mutant EGFP-mBsep protein was only partly glycosylated and detected in both the canalicular membrane and the cytoplasm. At 30 degrees C, the mutant mRNA and protein levels were strongly increased, and the protein was predominantly glycosylated and efficiently targeted to the canalicular membrane. CONCLUSIONS: These data suggest that PFIC-2 patients with the D482G mutation express a functional, but highly unstable, temperature-sensitive bile salt export pump.
Subject(s)
ATP-Binding Cassette Transporters/genetics , ATP-Binding Cassette Transporters/metabolism , Cholestasis, Intrahepatic/genetics , Cholestasis, Intrahepatic/metabolism , Mutation, Missense , Temperature , ATP Binding Cassette Transporter, Subfamily B, Member 11 , ATP-Binding Cassette Transporters/chemistry , Adenosine Triphosphatases/metabolism , Amino Acid Sequence , Animals , Aspartic Acid , Bile Canaliculi/metabolism , Cell Line, Tumor , Cholestasis, Intrahepatic/physiopathology , Disease Progression , Drug Stability , Glycine , Glycosylation , Green Fluorescent Proteins , Humans , Indicators and Reagents , Luminescent Proteins/genetics , Mice , Molecular Sequence Data , Protein Transport , Recombinant Fusion Proteins/metabolism , Recombinant Fusion Proteins/pharmacokinetics , Taurocholic Acid/pharmacokineticsABSTRACT
The bile salt export pump (BSEP or ABCB11) mediates the adenosine triphosphate-dependent transport of bile salts across the canalicular membrane of the hepatocyte. Mutations in the corresponding ABCB11 gene cause progressive familial intrahepatic cholestasis type 2. The aim of this study was to investigate the regulation of human ABCB11 gene transcription by bile salts. First, a 1.7-kilobase human ABCB11 promoter region was cloned. Sequence analysis for possible regulatory elements showed a farnesoid X receptor responsive element (FXRE) at position minus sign180. The farnesoid X receptor (FXR) functions as a heterodimer with the retinoid X receptor alpha (RXRalpha) and can be activated by the bile salt chenodeoxycholic acid (CDCA). Luciferase reporter gene assays showed that the ABCB11 promoter is positively controlled by FXR, RXRalpha, and bile salts in a concentration-dependent manner. Mutation of the FXRE strongly represses the FXR-dependent induction. Second, endogenous ABCB11 transcription regulation was studied in HepG2 cells, stably expressing the rat sodium-dependent taurocholate transporter (rNtcp) cells. ABCB11 expression was induced by adding bile salts to the culture medium, and this effect was maximized by combining it with cotransfection of rFxr and hRXRalpha. Reducing endogenous FXR levels using RNA interference fully repressed the bile salt-induced ABCB11 expression. In conclusion, these results show that FXR is required for the bile salt-dependent transcriptional control of the human ABCB11 gene and that the cellular amount of FXR is critical for the level of activation of ABCB11 transcription.
