Phospholipid alterations in hepatocyte membranes and transporter protein changes in cholestatic rat model

Biliary components are transported by hepatic adenosine triphosphate-bindin g cassette (ABC) transporters that are located in canalicular membranes. Ph ysiological transporter function is related to membrane fluidity, which is modulated by the phospholipid composition of the lipid bilayer. We hypothes ized that cholestasis may alter transporter function by modifying phospholi pid species to protect the cell from cholestatic damage. Therefore, we exam ined the expression of ABC transport proteins and their mRNA levels in cana licular membrane vesicles isolated from rat liver 6 hr or three days after bile duct ligation. Membrane lipid composition and membrane fluidity of bot h sinusoidal and canalicular membrane vesicles were also examined. By 6 hr after bile duct ligation, we found a clear increase of mdr2 and bsep mRNA. These changes were associated with an increase of mdr-Pgp and with a clear decrease of mrp2 protein, and small decrease of bsep protein. In addition, mdr1b mRNA showed a strong increase by three days after bile duct ligation. Canalicular membrane fluidity decreased in a marked time-dependent manner, whereas sinusoidal membranes showed biphasic changes: increased fluidity a t 6 hr and a decrease at three days. These changes were closely related to the changes of membrane lipid constitution; the saturated/unsaturated fatty acid ratio increased for phosphatidylcholine in canalicular membrane and t he reverse occurred in sinusoidal membrane, and those for sphingomyelin sho wed the opposite pattern. We conclude that cholestasis causes modulation of ABC transporters as well as that of the lipid constitution in lipid bilaye r. These may confer cytoprotective resistance to hepatocytes against choles tatic stress.