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Characterization of the human multidrug resistance protein isoform MRP3 localized to the basolateral hepatocyte
1Division of Tumor Biochemistry, Deutsches Krebsforschungszentrum, Heidelberg, Germany. j.koenig@dkfz-heidelberg.de
Abstract:
Several members of the multidrug resistance protein (MRP) family are expressed in the liver. Adenosine triphosphate (ATP)-dependent transport of glutathione and glucuronoside conjugates across the hepatocyte canalicular membrane is mediated by the apical MRP isoform, MRP2 (APMRP), also known as canalicular multispecific organic anion transporter (cMOAT). We have cloned an additional MRP isoform, MRP3, from human liver and localized it to the basolateral membrane domain of hepatocytes. Basolateral MRP (BLMRP) is composed of 1,527 amino acids and encoded by 4,581 base pairs of complementary DNA. Northern blotting of various human tissues indicated an expression of MRP3 in the liver, colon, pancreas, and, at a lower level, in the kidney. The amino acid identity of MRP3 with MRP1 and MRP2 is 58% and 48%, respectively. These three isoforms, encoded by genes on different chromosomes, have a similar predicted topology of transmembrane segments and ATP-binding domains. Antibodies raised against two peptide sequences of MRP3 that are not shared by other MRP family members detected recombinant MRP3 expressed in polarized MDCK cells. Both antibodies served to localize MRP3 to the basolateral membrane of hepatocytes. Double-label immunofluorescence microscopy confirmed that MRP3 was not detectable in the canalicular membrane domain. A particularly strong expression of the MRP3 protein was observed in the basolateral hepatocyte membrane of two patients with Dubin-Johnson syndrome who are deficient in MRP2. These results indicate that the basolateral MRP isoform, MRP3, may be upregulated when the canalicular secretion of anionic conjugates by MRP2 is impaired.
Insights
Researchers identified a new multidrug resistance-associated protein 3 (MRP3) in human liver, located on the basolateral membrane. This MRP3 protein may compensate for impaired MRP2 function in conditions like Dubin-Johnson syndrome.
Area of Science:
- Hepatocyte membrane transport
- Multidrug resistance protein (MRP) family
- Molecular cloning and localization
Background:
- The apical MRP isoform, MRP2 (cMOAT), mediates ATP-dependent transport of conjugates across the hepatocyte canalicular membrane.
- Understanding the roles of different MRP isoforms in liver function is crucial for drug transport and detoxification.
Purpose of the Study:
- To clone and characterize a novel MRP isoform, MRP3, from human liver.
- To determine the cellular localization and tissue expression of MRP3.
- To investigate the potential compensatory role of MRP3 in liver diseases involving MRP2 deficiency.
Main Methods:
- Complementary DNA (cDNA) cloning of human liver MRP3.
- Northern blotting for tissue expression analysis.
- Antibody-based immunofluorescence microscopy for protein localization in hepatocytes and MDCK cells.
- Analysis of MRP3 expression in patients with Dubin-Johnson syndrome.
Main Results:
- MRP3 was cloned from human liver and found to be expressed in the liver, colon, pancreas, and kidney.
- MRP3 localizes to the basolateral membrane domain of hepatocytes, distinct from the canalicular MRP2.
- Strong MRP3 protein expression was observed in the basolateral hepatocyte membrane of Dubin-Johnson syndrome patients, who are deficient in MRP2.
Conclusions:
- The basolateral MRP isoform, MRP3, is identified and localized to the hepatocyte basolateral membrane.
- MRP3 exhibits distinct tissue expression patterns compared to other MRP family members.
- MRP3 may play a compensatory role in facilitating the transport of anionic conjugates when MRP2 function is impaired.