Low retinol levels differentially modulate bile salt-induced expression of human and mouse hepatic bile salt transporters.
The farnesoid X receptor/retinoid X receptor-alpha (FXR/RXRalpha) complex regulates bile salt homeostasis, in part by modulating transcription of the bile salt export pump (BSEP/ABCB11) and small heterodimer partner (SHP/NR0B2). FXR is activated by bile salts, RXRalpha by the vitamin A derivative 9-cis retinoic acid (9cRA). Cholestasis is associated with vitamin A malabsorption. Therefore, we evaluated the role of vitamin A/9cRA in the expression of human and mouse bile salt export pump (hBSEP/mBsep), small heterodimer partner (hSHP/mShp), and mouse sodium-dependent taurocholate co-transporting polypeptide (mNtcp). HBSEP and hSHP transcription were analyzed in FXR/RXRalpha-transfected HepG2 cells exposed to chenodeoxycholic acid (CDCA) and/or 9cRA. BSEP promoter activity was determined by luciferase reporter assays, DNA-binding of FXR and RXRalpha by pull-down assays. Serum bile salt levels and hepatic expression of Bsep, Shp, and Ntcp were determined in vitamin A-deficient (VAD)/cholic acid (CA)-fed C57BL/6J mice. Results indicated that 9cRA strongly repressed the CDCA-induced BSEP transcription in HepG2 cells, whereas it super-induced SHP transcription; 9cRA reduced DNA-binding of FXR and RXRalpha. The 9cRA repressed the CDCA-induced BSEP promoter activity irrespective of the exact sequence of the FXR-binding site. In vivo, highest Bsep messenger RNA (mRNA), and protein expression was observed in CA-fed VAD mice. Shp transcription was highest in CA-fed vitamin A-sufficient mice. Ntcp protein expression was strongly reduced in CA-fed VAD mice, whereas mRNA levels were normal. CA-fed control and VAD mice had similarly increased serum bile salt levels.


CONCLUSIONS
We showed that 9cRA has opposite effects on bile salt-activated transcription of FXR/RXRalpha target genes. Vitamin A deficiency in CA-fed mice leads to high BSEP expression. Clearance of serum bile salts may, however, be limited because of post-transcriptional reduction of Ntcp. The molecular effects of vitamin A supplementation during cholestasis need further analysis to predict a therapeutic effect.
Hoeke
Martijn O
MO
Department of Gastroenterology and Hepatology, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands.
Plass
Jacqueline R M
JR
Heegsma
Janette
J
Geuken
Mariska
M
van Rijsbergen
Duncan
D
Baller
Julius F W
JF
Kuipers
Folkert
F
Moshage
Han
H
Jansen
Peter L M
PL
Faber
Klaas Nico
KN
D000074020:ATP Binding Cassette Transporter, Subfamily B, Member 11; D018528:ATP-Binding Cassette Transporters / Q000096:biosynthesis*; D000077556:Alitretinoin; D000818:Animals; D006528:Carcinoma, Hepatocellular; D045744:Cell Line, Tumor; D002635:Chenodeoxycholic Acid / Q000494:pharmacology*; D019826:Cholic Acid / Q000494:pharmacology; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology*; D005786:Gene Expression Regulation / Q000187:drug effects; D006801:Humans; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D029363:Organic Anion Transporters, Sodium-Dependent / Q000096:biosynthesis; D018160:Receptors, Cytoplasmic and Nuclear / Q000096:biosynthesis* / Q000235:genetics / Q000502:physiology*; D020218:Response Elements; D047490:Retinoid X Receptor alpha / Q000502:physiology*; D027981:Symporters / Q000096:biosynthesis; D014157:Transcription Factors / Q000235:genetics / Q000502:physiology*; D014212:Tretinoin / Q000494:pharmacology*; D014801:Vitamin A / Q000008:administration & dosage / Q000494:pharmacology*; D014802:Vitamin A Deficiency / Q000503:physiopathology; D000099150:Receptor, Farnesoid X-Activated
D000074020:ATP Binding Cassette Transporter, Subfamily B, Member 11; D018528:ATP-Binding Cassette Transporters; D000077556:Alitretinoin; D002635:Chenodeoxycholic Acid; D019826:Cholic Acid; D004268:DNA-Binding Proteins; D029363:Organic Anion Transporters, Sodium-Dependent; D018160:Receptors, Cytoplasmic and Nuclear; D047490:Retinoid X Receptor alpha; D027981:Symporters; D014157:Transcription Factors; D014212:Tretinoin; D014801:Vitamin A; D000099150:Receptor, Farnesoid X-Activated; C093491:ABCB11 protein, human; C490549:Abcb11 protein, mouse; C099713:nuclear receptor subfamily 0, group B, member 2; C071908:sodium-bile acid cotransporter