Down-regulation of Na+/H+ exchanger regulatory factor 1 increases expression and function of multidrug resistance protein 4

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.