Zonal down‐regulation and redistribution of the multidrug resistance protein 2 during bile duct ligation in rat liver

We have studied regulation of the multidrug resistance protein 2 (mrp2) during bile duct ligation (BDL) in the rat. In hepatocytes isolated after 16, 48, and 72 hours of BDL, mrp2‐mediated dinitrophenyl‐glutathione (DNP‐GS) transport was decreased to 65%, 33%, and 33% of control values, respectively. The impaired mrp2‐mediated transport coincided with strongly decreased mrp2 protein levels, without any significant changes in mrp2 RNA levels. Restoration of bile flow after a 48‐hour BDL period resulted in a slow recovery of mrp2‐mediated transport and protein levels. Immunohistochemical detection of the protein in livers of rats undergoing BDL showed strongly reduced mrp2 staining after 48 hours, which was initiated in the periportal areas of the liver lobule and progressed toward the pericentral areas after 96 hours. Immunofluorescent detection of mrp2 in livers of rats undergoing 48 hours of BDL revealed decreased staining accompanied by intracellular localization of the protein in pericanalicular vesicular structures. Within this intracellular compartment, mrp2 colocalized with the bile salt transporter (bsep) and was still active as shown by vesicular accumulation of the fluorescent organic anion glutathione‐bimane (GS‐B). We conclude that down‐regulation of mrp2 during BDL‐induced obstructive cholestasis is mainly posttranscriptionally regulated. We propose that this down‐regulation is caused by endocytosis of apical transporters followed up by increased breakdown of mrp2, probably in lysosomes. This breakdown of mrp2 is more severe in the periportal areas of the liver lobule.