mercaptopurine and adefovir

Multidrug resistance protein 4 (MRP4; ABCC4) is a member of the ATP-binding cassette superfamily of membrane transport proteins and confers resistance to nucleoside and nucleotide analogues as well as camptothecin derivatives. MRP4 also mediates the transmembrane transport of several eicosanoids, conjugated estrogens, and cyclic AMP. The subcellular localization of MRP4 depends on the cell type in which it is expressed, but the molecular determinants responsible for trafficking of MRP4 to the plasma membrane are unknown. Here, we describe the interaction of Na(+)/H(+) exchanger regulatory factor 1 (NHERF1) with MRP4 via the last four amino acids ((1322)ETAL(1325)) of the transporter. Down-regulation of NHERF1 by small interfering RNA (siRNA) in HeLa cells significantly increased MRP4 levels at the plasma membrane, suggesting that internalization of the transporter was inhibited. Increased plasma membrane MRP4 was accompanied by increased efflux function as reflected by reduced cellular accumulation of the MRP4 substrates 6-mercaptopurine and 9-[2-(phosphonylmethoxy)ethyl]-adenine. Furthermore, enhanced green fluorescent protein-tagged MRP4 was internalized in monensin-treated cells, and this internalization was markedly reduced after NHERF1 down-regulation by siRNA. Together, these data establish NHERF1 as a novel protein-binding partner of MRP4 that plays a significant role in the internalization and drug efflux function of this transporter.

Topics: Adenine; Antiviral Agents; Biological Transport; Cell Membrane; Down-Regulation; Fluorescent Antibody Technique, Indirect; Green Fluorescent Proteins; HeLa Cells; Humans; Immunoblotting; Immunoprecipitation; Immunosuppressive Agents; Kidney; Mercaptopurine; Multidrug Resistance-Associated Proteins; Organophosphonates; Phosphoproteins; Protein Transport; Recombinant Fusion Proteins; RNA, Small Interfering; Signal Transduction; Sodium-Hydrogen Exchangers; Transfection

Multiple drug resistance protein 4 (MRP4, ABCC4) belongs to the C subfamily of the ATP-binding cassette (ABC) transporter superfamily and participates in the transport of diverse antiviral and chemotherapeutic agents such as 6-mercaptopurine (6-MP) and 9-(2-phosphonyl methoxyethyl) adenine (PMEA). We have undertaken a comprehensive functional characterization of protein variants of MRP4 found in Caucasians and other ethnicities. A total of 11 MRP4 missense genetic variants (nonsynonymous SNPs), fused to green fluorescent protein (GFP), were examined in Xenopus laevis oocytes for their effect on expression, localization, and function of the transporter. Radiolabeled 6-MP and PMEA were chosen as transport substrates. All MRP4 protein variants were found to be expressed predominantly in the oocyte membrane. A total of four variants (Y556C, E757 K, V776I, and T1142 M) exhibited a 20% to 40% reduced expression level compared to the wild type. Efflux studies showed that 6-MP is transported by MRP4 in unmodified form. Compared to wild-type MRP4, the transmembrane variant V776I, revealed a significant lower activity in 6-MP transport, while the amino acid exchange Y556C in the Walker(B) motif displayed significantly higher transport of PMEA. The transport properties of the other variants were comparable to wild-type MRP4. Our study shows that Xenopus oocytes are well suited to characterize MRP4 and its protein variants. Carriers of the rare MRP4 variants Y556C and V776I may have altered disposition of MRP4 substrates.

Topics: Adenine; Animals; Base Sequence; DNA Primers; Green Fluorescent Proteins; Humans; Mercaptopurine; Multidrug Resistance-Associated Proteins; Mutagenesis, Site-Directed; Mutation, Missense; Organophosphonates; Recombinant Fusion Proteins; Xenopus laevis