| MRP2, also known as ABCC2 or multidrug resistance‑associated protein 2, is a full‑length ATP‑binding cassette (ABC) exporter of the ABCC/MRP subfamily that localizes specifically to the apical (canalicular) membrane of polarized epithelia such as hepatocytes, renal proximal tubule cells, enterocytes and placental syncytiotrophoblasts, where it mediates ATP‑dependent efflux of a broad spectrum of organic anions and conjugated metabolites into bile, urine or intestinal lumen. The transporter has a characteristic ABC architecture with three membrane‑embedded domains and two cytosolic nucleotide‑binding domains, and recent cryo‑EM structures of human MRP2 in autoinhibited, substrate‑bound and ATP‑bound states show that it follows a classical alternating‑access mechanism driven by ATP binding and hydrolysis, with a cytosolic regulatory (R) domain acting as a selectivity gauge that must be displaced by sufficiently high substrate concentrations to initiate the transport cycle. MRP2 has high affinity for phase II biotransformation products and transports glutathione, glucuronide and sulfate conjugates of lipophilic xenobiotics and endogenous compounds, as well as some uncharged drugs in cotransport with glutathione, thereby completing phase III of drug metabolism; this activity is central to hepato‑biliary elimination of structurally diverse xenobiotics and endogenous molecules such as conjugated bilirubin, and it significantly shapes pharmacokinetic parameters and chemoprotection by lowering intracellular exposure to toxic metabolites and anticancer drugs. Comparative structural analysis of MRP2 bound to different substrates reveals a multi‑site recognition pocket capable of accommodating diverse anionic conjugates, explaining the transporter’s wide substrate spectrum and its role in multidrug resistance when overexpressed in tumor cells, where efflux of anticancer agents undermines therapeutic efficacy and contributes to resistance phenotypes. MRP2 expression and function are transcriptionally and post‑transcriptionally regulated by endogenous factors such as inflammatory cytokines, bile acids and nuclear receptor ligands, including pregnane X receptor agonists, and can be altered by xenobiotics and chemopreventive agents, making it an important locus for drug–drug interactions and adverse effects in clinical settings. In obstructive cholestasis, mRNA levels and canalicular localization of MRP2 and the bile salt export pump BSEP are reduced in poorly drained livers and preserved in well‑drained livers, with fuzzy or diminished canalicular staining correlating with impaired bilirubin conjugate and bile acid secretion, while upregulation of basolateral MRP3 provides an alternative route for conjugate efflux, indicating that loss of functional canalicular MRP2 is directly linked to cholestatic impairment of bile formation and the efficacy of interventions such as percutaneous transhepatic biliary drainage. Naturally occurring ABCC2 mutations that abolish or mislocalize MRP2 cause Dubin–Johnson syndrome, a benign hereditary conjugated hyperbilirubinemia characterized by defective canalicular excretion of bilirubin glucuronides, and structural–functional studies have identified critical amino acids for substrate binding and transport whose disruption leads to absence of functional protein at the apical membrane in affected individuals. |
