Evidence for an ATP-dependent bile acid transport protein other than the canalicular liver ecto-ATPase in rats.
OBJECTIVE
Canalicular secretion is rate limiting in overall blood-to-bile transport of bile acids. Studies using transfected cells have implicated the canalicular ecto-adenosine triphosphatase (ecto-ATPase) in adenosine triphosphate (ATP)-dependent bile acid transport. However, the structural features of this ecto-ATPase are not those anticipated for an in-to-out ATP-dependent transporter. The aim of this study was to explore the possible existence of an ATP-dependent bile acid transport mechanism distinct from ecto-ATPase.


METHODS
Bile acid transport activity and ecto-ATPase expression were analyzed in primary rat hepatocytes, rat hepatoma HTC cells, and specially adapted HTC (HTC-R) cells using plasma membrane vesicles and Northern blot, slot blot, ribonuclease protection assay, and Western blot analyses.


RESULTS
Plasma membranes isolated from HTC-R cells exhibited ATP-dependent taurocholate transport, which was many-fold greater than that in HTC cells. Hepatocytes showed the highest transport rates. Protein and RNA analyses showed very low expression of ecto-ATPase in HTC and HTC-R cells compared with hepatocytes. There was no difference between the two cell types at both the RNA and protein level.


CONCLUSIONS
These findings show the presence in HTC-R cells and, apparently in hepatocytes, of one or more proteins other than the ecto-ATPase that mediate ATP-dependent transport of bile acids.
Luther
T T
TT
Department of Medicine, University of California, San Francisco 94143-0538, USA.
Hammerman
P
P
Rahmaoui
C M
CM
Lee
P P
PP
Sela-Herman
S
S
Matula
G S
GS
Ananthanarayanan
M
M
Suchy
F J
FJ
Cavalieri
R R
RR
Lomri
N
N
Scharschmidt
B F
BF
D018528:ATP-Binding Cassette Transporters / Q000302:isolation & purification / Q000378:metabolism* / Q000502:physiology; D000251:Adenosine Triphosphatases / Q000378:metabolism* / Q000502:physiology; D000818:Animals; D001647:Bile Acids and Salts / Q000378:metabolism*; D001648:Bile Canaliculi / Q000201:enzymology; D015152:Blotting, Northern; D015153:Blotting, Western; D008114:Liver Neoplasms, Experimental; D051381:Rats; D013656:Taurocholic Acid / Q000378:metabolism; D014407:Tumor Cells, Cultured
D018528:ATP-Binding Cassette Transporters; D001647:Bile Acids and Salts; D013656:Taurocholic Acid; D000251:Adenosine Triphosphatases; C027201:ectoATPase