The orphan nuclear receptor, shp, mediates bile acid-induced inhibition of the rat bile acid transporter, ntcp.
OBJECTIVE
Hepatic bile acid homeostasis is regulated by negative feedback inhibition of genes involved in the uptake and synthesis of bile acids. Bile acids down-regulate the rate-limiting gene for bile acid synthesis, cholesterol 7alpha-hydroxylase (cyp7a), via bile acid receptor (fxr) activation of an inhibitory nuclear receptor, shp. We hypothesized that shp would also mediate negative feedback regulation of ntcp, the principal hepatic bile acid transporter.


METHODS
Primary rat hepatocytes or transfected HepG2 and Cos cells were treated with retinoids with or without bile acids, and effects on bile acid transport and ntcp and shp gene expression and promoter activity were determined. Gel shift assays were performed using synthetic fxr, rxr, and rar proteins.


RESULTS
Bile acid treatment of primary rat hepatocytes prevented retinoid activation of ntcp gene expression and function; this corresponded temporally with shp gene activation. Bile acid-mediated down-regulation occurred via fxr-dependent suppression of the ntcp RXR:RAR response element. Moreover, cotransfected shp directly inhibited retinoid activation of the ntcp promoter.


CONCLUSIONS
These studies show negative feedback regulation of ntcp by bile acid-activated fxr via induction of shp. This novel regulatory pathway provides a means for coordinated down-regulation of bile acid import and synthesis, thereby protecting the hepatocyte from bile acid-mediated damage in cholestatic conditions.
Denson
L A
LA
Department of Pediatrics, Yale University School of Medicine, New Haven, Connecticut.
Sturm
E
E
Echevarria
W
W
Zimmerman
T L
TL
Makishima
M
M
Mangelsdorf
D J
DJ
Karpen
S J
SJ
D000818:Animals; D001426:Bacterial Proteins / Q000235:genetics* / Q000494:pharmacology; D001647:Bile Acids and Salts / Q000096:biosynthesis / Q000235:genetics*; D006528:Carcinoma, Hepatocellular / Q000235:genetics; D002352:Carrier Proteins / Q000235:genetics*; D003574:Cytochrome c Group / Q000235:genetics* / Q000494:pharmacology; D006801:Humans; D008113:Liver Neoplasms / Q000235:genetics*; D008297:Male; D026901:Membrane Transport Proteins; D029363:Organic Anion Transporters, Sodium-Dependent; D051381:Rats; D017207:Rats, Sprague-Dawley; D018160:Receptors, Cytoplasmic and Nuclear / Q000187:drug effects / Q000235:genetics*; D027981:Symporters; D014407:Tumor Cells, Cultured
D001426:Bacterial Proteins; D001647:Bile Acids and Salts; D002352:Carrier Proteins; D003574:Cytochrome c Group; D026901:Membrane Transport Proteins; D029363:Organic Anion Transporters, Sodium-Dependent; D018160:Receptors, Cytoplasmic and Nuclear; C111967:SHP protein, Rhodobacter sphaeroides; D027981:Symporters; C071908:sodium-bile acid cotransporter