Short-term regulation of bile acid uptake by microfilament-dependent translocation of rat ntcp to the plasma membrane.
The Na+-taurocholate cotransport polypeptide (ntcp) is the primary transporter for the uptake of bile acids in the liver. The second messenger adenosine 3':5'-cyclic monophosphate (cAMP) rapidly increases ntcp protein concentration in the plasma membrane, yet the mechanism is unknown. To investigate this, HepG2 cells were transiently transfected with a carboxy-terminal-tagged green fluorescence protein (GFP) conjugate of ntcp, and then examined by confocal video microscopy. Transporter activity was directly assayed with 3H-taurocholic acid (TC) scintigraphy. ntcp-GFP targeted to the plasma membrane in transfected cells, and the conjugate protein transported 3H-TC as effectively as unmodified rat ntcp. Stimulation of ntcp-GFP cells with cAMP increased GFP fluorescence in the plasma membrane by 40% (P <.0001) within 2.5 minutes and by 55% within 10 minutes. Similarly, cAMP increased transport of bile acids by 30%. Cytochalasin D, an inhibitor of microfilaments, did not prevent ntcp-GFP from targeting to the plasma membrane, but completely abolished the increase in GFP fluorescence seen in response to cAMP. In contrast, the microtubule inhibitor, nocodazole, prevented development of membrane fluorescence in 48 (96%) of 50 cells. Cells regained plasma membrane fluorescence within 2 hours after nocodazole removal. These findings suggest that targeting of ntcp to the plasma membrane consists of 2 steps: 1) delivery of ntcp to the region of the plasma membrane via microtubules; and 2) insertion of ntcp into the plasma membrane, in a microfilament- and cAMP-sensitive fashion.
Dranoff
J A
JA
Department of Medicine, Yale University School of Medicine, New Haven, CT, USA. jonathan.dranoff@yale.edu
McClure
M
M
Burgstahler
A D
AD
Denson
L A
LA
Crawford
A R
AR
Crawford
J M
JM
Karpen
S J
SJ
Nathanson
M H
MH
D008841:Actin Cytoskeleton / Q000187:drug effects / Q000378:metabolism*; D000818:Animals; D001483:Base Sequence; D001647:Bile Acids and Salts / Q000378:metabolism*; D001692:Biological Transport; D006528:Carcinoma, Hepatocellular; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism*; D000242:Cyclic AMP / Q000494:pharmacology; D015638:Cytochalasin D / Q000494:pharmacology; D049452:Green Fluorescent Proteins; D006801:Humans; D007700:Kinetics; D008113:Liver Neoplasms; D008164:Luminescent Proteins / Q000737:chemistry / Q000235:genetics; D026901:Membrane Transport Proteins; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016366:Open Reading Frames; D029363:Organic Anion Transporters, Sodium-Dependent; D017433:Protein Structure, Secondary; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000378:metabolism; D027981:Symporters; D013656:Taurocholic Acid / Q000378:metabolism*; D014162:Transfection; D014316:Tritium; D014407:Tumor Cells, Cultured
D001647:Bile Acids and Salts; D002352:Carrier Proteins; D008164:Luminescent Proteins; D026901:Membrane Transport Proteins; D029363:Organic Anion Transporters, Sodium-Dependent; D011993:Recombinant Fusion Proteins; D027981:Symporters; D014316:Tritium; C071908:sodium-bile acid cotransporter; D049452:Green Fluorescent Proteins; D015638:Cytochalasin D; D013656:Taurocholic Acid; D000242:Cyclic AMP