Cellular localization and up-regulation of multidrug resistance-associatedprotein 3 in hepatocytes and cholangiocytes during obstructive cholestasisin rat liver

The hepatic expression of the ATP-dependent conjugate export pump multidrug resistance-associated protein 2 (Mrp2) is diminished in experimentally ind uced models of cholestasis, In this study we have examined the localization and expression of Mrp3, another member of the multidrug resistance-associa ted protein family, in normal liver and after obstructive cholestasis in th e rat. Indirect immunofluorescence and confocal microscopy were used to det ermine the tissue localization and Western blot analysis was performed to q uantitate the expression. In normal rat liver Mrp3 was found on the basolat eral membrane of cholangiocytes and a single layer of hepatocytes surroundi ng the central vein. Three and 7 days after bile duct ligation Mrp3 express ion was significantly increased, predominantly in hepatocytes in the perice ntral region. By 14 days all hepatocytes showed basolateral membrane labeli ng for Mrp3 at a time when apical Mrp2 staining was significantly diminishe d. Proliferating bile ducts continued to stain positive, although the inten sity of staining did not seem to vary, After 14 days Western blot quantitat ion showed that Mrp3 had increased approximately 30-fold in total liver mem branes. Quantitation of Mrp3 in membranes from isolated hepatocytes of live rs of sham and common bile duct-ligated (CBDL) animals showed a significant up-regulation beginning at 1 day and continuing to increase through 14 day s postligation, This was in contrast to the progressive decrease in Mrp2 pr otein. Because Mrp3 is capable of transporting toxic bile acids, up-regulat ion of Mrp3 may compensate for the down-regulation of Mrp2 in obstructive c holestasis.