Abcg5/8 independent biliary cholesterol excretion in Atp8b1-deficient mice.
OBJECTIVE
ATP8B1 is a phosphatidylserine flippase in the canalicular membrane; patients with mutations in ATP8B1 develop severe chronic (PFIC1) or periodic (BRIC1) cholestatic liver disease. We have observed that Atp8b1 deficiency leads to enhanced biliary cholesterol excretion. It has been established that biliary cholesterol excretion depends on transport by the heterodimer Abcg5/Abcg8. We hypothesized that the increased cholesterol output was due to enhanced extraction from the altered canalicular membrane rather than to higher Abcg5/Abcg8 activity. We therefore studied the relation between Abcg5/Abcg8 expression and biliary cholesterol excretion in mice lacking Atp8b1, Abcg8, or both (GF mice).


METHODS
Bile formation was studied in LXR agonist-fed wild-type mice as well as mice lacking Atp8b1 or Abcg8, or in GF mice upon infusion of taurocholate. Bile samples were analyzed for cholesterol, bile salt, phospholipids, and ectoenzyme content.


RESULTS
LXR agonist increased Abcg5/8 expression, and this was accompanied by increased biliary cholesterol output in both wild-type and Atp8b1(G308V/G308V) mice. However, Atp8b1(G308V/G308V) mice maintained higher cholesterol output. Although in Abcg8(-/-) mice biliary cholesterol output was severely reduced, GF mice displayed high biliary cholesterol output, which was comparable with wild-type mice. Bile of both Atp8b1(G308V/G308V) and GF mice displayed elevated levels of phosphatidylserine and sphingomyelin, indicating membrane stress.


CONCLUSIONS
Our data demonstrate that the increased biliary cholesterol excretion in Atp8b1-deficient mice is independent of Abcg5/8 activity. This implicates that Atp8b1 deficiency leads to a decrease in the detergent resistance and subsequent nonspecific extraction of cholesterol from the canalicular membrane by bile salts.
Groen
Annemiek
A
AMC Liver Center, Academic Medical Center, Amsterdam, The Netherlands.
Kunne
Cindy
C
Jongsma
Geartsje
G
van den Oever
Karin
K
Mok
Kam S
KS
Petruzzelli
Michele
M
Vrins
Carlos L J
CL
Bull
Laura
L
Paulusma
Coen C
CC
Oude Elferink
Ronald P J
RP
D000071018:ATP Binding Cassette Transporter, Subfamily G, Member 5; D000071019:ATP Binding Cassette Transporter, Subfamily G, Member 8; D018528:ATP-Binding Cassette Transporters / Q000235:genetics / Q000378:metabolism*; D000251:Adenosine Triphosphatases / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D001646:Bile / Q000201:enzymology / Q000378:metabolism*; D001647:Bile Acids and Salts / Q000097:blood; D001648:Bile Canaliculi / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D002784:Cholesterol / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000819:agonists / Q000378:metabolism; D006845:Hydrocarbons, Fluorinated; D008074:Lipoproteins / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D008099:Liver / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D000071518:Liver X Receptors; D008297:Male; D008560:Membrane Fluidity; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D057093:Orphan Nuclear Receptors; D049294:Phospholipid Transfer Proteins; D010743:Phospholipids / Q000378:metabolism; D018160:Receptors, Cytoplasmic and Nuclear / Q000819:agonists / Q000378:metabolism; D013449:Sulfonamides / Q000494:pharmacology; D013997:Time Factors; D015854:Up-Regulation; D005466:Fluorocarbons; D000096926:Benzenesulfonamides
D000071018:ATP Binding Cassette Transporter, Subfamily G, Member 5; D000071019:ATP Binding Cassette Transporter, Subfamily G, Member 8; D018528:ATP-Binding Cassette Transporters; D000251:Adenosine Triphosphatases; D001647:Bile Acids and Salts; D002784:Cholesterol; D004268:DNA-Binding Proteins; D006845:Hydrocarbons, Fluorinated; D008074:Lipoproteins; D000071518:Liver X Receptors; D057093:Orphan Nuclear Receptors; D049294:Phospholipid Transfer Proteins; D010743:Phospholipids; D018160:Receptors, Cytoplasmic and Nuclear; D013449:Sulfonamides; C462600:ABCG5 protein, mouse; C462601:ABCG8 protein, mouse; C423915:T0901317; D005466:Fluorocarbons; D000096926:Benzenesulfonamides; C491742:Atp8b1 protein, mouse