Increased levels of the multidrug resistance protein in lateral membranes of proliferating hepatocyte-derived cells.
OBJECTIVE
The multidrug resistance protein (MRP) functions as an organic anion efflux carrier. Recent studies suggest that hepatocytes contain two mrp homologues, named mrp1 and mrp2, localized on the lateral and canalicular membrane, respectively. The aim of this study was to evaluate the role of MRP1. Protein levels and localization of MRP1 in human hepatocytes, HepG2 hepatoma cells, and SV40 large T antigen-immortalized human hepatocytes were studied.


METHODS
Using specific antibodies, MRP1 protein levels and cellular localization were examined by Western blotting and fluorescence confocal microscopy, respectively. In addition, a fluorescent substrate, glutathione-methylfluorescein, was used to measure plasma membrane transport activity and to observe intracellular transport activity.


RESULTS
The level of MRP1 in normal hepatocytes is very low. In contrast, MRP1 is highly increased in both HepG2 and immortalized hepatocytes. In these cells MRP1 is localized in lateral membranes of adjacent cells. Plasma membrane staining is absent in separate cells. Glutathione-methylfluorescein is transported in the medium and intracellular vesicles.


CONCLUSIONS
MRP1 protein level is greatly increased in the lateral membrane of proliferating hepatocyte-derived cells. The presence of a lateral domain seems necessary for plasma membrane localization. These results suggest that MRP1-mediated organic anion transport is important in proliferating hepatocytes, but not in quiescent cells.
Roelofsen
H
H
Department of Internal Medicine, Groningen Institute for Drug Studies, The Netherlands.
Vos
T A
TA
Schippers
I J
IJ
Kuipers
F
F
Koning
H
H
Moshage
H
H
Jansen
P L
PL
Müller
M
M
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000378:metabolism*; D000818:Animals; D006528:Carcinoma, Hepatocellular / Q000378:metabolism / Q000473:pathology; D002352:Carrier Proteins / Q000378:metabolism; D002455:Cell Division; D002461:Cell Line, Transformed / Q000378:metabolism; D006801:Humans; D007425:Intracellular Membranes / Q000378:metabolism; D008099:Liver / Q000166:cytology / Q000378:metabolism*; D008113:Liver Neoplasms / Q000378:metabolism / Q000473:pathology; D011817:Rabbits; D014018:Tissue Distribution
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D002352:Carrier Proteins