The human multidrug resistance protein 2 gene: Functional characterization of the 5′‐flanking region and expression in hepatic cells

The human multidrug resistance protein 2 (MRP2), also termed as the canalicular multispecific organic anion transporter (cMOAT), is a member of the adenosine triphosphate-binding cassette transporter superfamily. In the liver, MRP2 mediates the multispecific efflux of various types of organic anions, including glucuronate, sulfate, and glutathione conjugates, across the canalicular hepatocyte membrane to the bile. To investigate how the MRP2 gene is expressed in liver cells, the 5′-flanking region of the human MRP2 gene was isolated from a human placental genomic library. Sequence analysis of the MRP2 promoter showed a number of consensus binding sites for both ubiquitous and liver-enriched transcription factors. Transfection of human hepatic HepG2 cells with a series of 5′-deleted promoter luciferase constructs identified a putative silencer element localized in the −1,659/−491 region and a liver-specific positive regulatory element localized in the −491/−258 region. This latter region contained the liver-abundant transcription factor CCAAT-enhancer binding protein β (C/EBPβ). The transcriptional activity of the promoter construct containing a mutation in the C/EBPβ binding site was significantly decreased in HepG2 cells. This study suggests that C/EBPβ (−356 to −343) may regulate the liver expression of the MRP2 gene.