The human concentrative (Na ؉ -linked) plasma membrane transport proteins hCNT1 and hCNT2 are selective for pyrimidine nucleosides (system cit) and purine nucleosides (system cif), respectively. Both have homologs in other mammalian species and belong to a gene family (CNT) that also includes hfCNT, a newly identified broad specificity pyrimidine and purine Na ؉ -nucleoside symporter (system cib) from the ancient marine vertebrate, the Pacific hagfish (Eptatretus stouti). We now report the cDNA cloning and characterization of cib homologs of hfCNT from human mammary gland, differentiated human myeloid HL-60 cells, and mouse liver. The 691-and 703-residue human and mouse proteins, designated hCNT3 and mCNT3, respectively, were 79% identical in amino acid sequence and contained 13 putative transmembrane helices. hCNT3 was 48, 47, and 57% identical to hCNT1, hCNT2, and hfCNT, respectively. When produced in Xenopus oocytes, both proteins exhibited Na ؉ -dependent cib-type functional activities. hCNT3 was electrogenic, and a sigmoidal dependence of uridine influx on Na ؉ concentration indicated a Na ؉ : uridine coupling ratio of at least 2:1 for both hCNT3 and mCNT3 (cf 1:1 for hCNT1/2). Phorbol myristate acetateinduced differentiation of HL-60 cells led to the parallel appearance of cib-type activity and hCNT3 mRNA. Tissues containing hCNT3 transcripts included pancreas, bone marrow, trachea, mammary gland, liver, prostrate, and regions of intestine, brain, and heart. The hCNT3 gene mapped to chromosome 9q22.2 and included an upstream phorbol myristate acetate response element.Most nucleosides, including those with antineoplastic and/or antiviral activities (1, 2), are hydrophilic, and specialized plasma membrane nucleoside transporter (NT) 1 proteins are required for uptake into or release from cells (3, 4). NT-mediated transport is therefore a critical determinant of metabolism and, for nucleoside drugs, their pharmacologic actions (5). NTs also regulate adenosine concentrations in the vicinity of cell surface receptors and have profound effects on neurotransmission, vascular tone, and other processes (6, 7).Seven nucleoside transport processes 2 that differ in their cation dependence, permeant selectivities and inhibitor sensitivities have been observed in human and other mammalian cells and tissues. The major concentrative systems (cit, cif, and cib) are inwardly directed Na ϩ -dependent processes and have been primarily described in specialized epithelia such as intestine, kidney, liver, and choroid plexus, in other regions of the brain, and in splenocytes, macrophages, and leukemic cells (3, 4). Concentrative NT transcripts have also been found in heart, skeletal muscle, placenta, and pancreas. The equilibrative (bidirectional) transport processes (es and ei) have generally lower substrate affinities and occur in most, possibly all, cell types (3, 4). Epithelia (e.g. intestine and kidney) and some nonpolarized cells (e.g. leukemic cells) coexpress both concentrative and equilibrative NTs, whereas other nonpola...

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In most mammalian cells nucleoside uptake occurs primarily via broad-specificity, es (e, equilibrative; 5, sensitive to NBMPR inhibition) transporters that are potently inhibited by nitrobenzylthioinosine (NBMPR). These transporters are essential for nucleotide synthesis by salvage pathways in hemopoietic and other cells that lack de novo pathways and are the route of cellular uptake for many cytotoxic nucleosides used in cancer and viral chemotherapy. They play an important role in adenosine-mediated regulation of many physiological processes, including neurotransmission and platelet aggregation, and are a target for coronary vasodilator drugs. We have previously reported the purification of the prototypic es transporter from human erythrocytes and have shown that this glycoprotein of apparent M, 55,000 is immunologically related to nucleoside transporters from several other species and tissues, including human placenta. Here we report the isolation of a human placental cDNA encoding a 456-residue glycoprotein with functional characteristics typical of an es-type transporter. It is predicted to possess 11 membrane-spanning regions and is homologous to several proteins of unknown function in yeast, nematodes, plants and mammals. Because of its central role in the uptake both of adenosine and of chemotherapeutic nucleosides, study of this protein should not only provide insights into the physiological roles of nucleoside transport but also open the way to improved therapies.

The International Transporter Consortium (ITC) has recently described seven transporters of particular relevance to drug development. Based on the second ITC transporter workshop in 2012, we have identified additional transporters of emerging importance in pharmacokinetics, interference of drugs with transport of endogenous compounds, and drug-drug interactions (DDIs) in humans. The multidrug and toxin extrusion proteins (MATEs, gene symbol SLC47A) mediate excretion of organic cations into bile and urine. MATEs are important in renal DDIs. Multidrug resistance proteins (MRPs or ABCCs) are drug and conjugate efflux pumps, and impaired activity of MRP2 results in conjugated hyperbilirubinemia. The bile salt export pump (BSEP or ABCB11) prevents accumulation of toxic bile salt concentrations in hepatocytes, and BSEP inhibition or deficiency may cause cholestasis and liver injury. In addition, examples are presented on the roles of nucleoside and peptide transporters in drug targeting and disposition. This is a PDF file of an unedited peer-reviewed manuscript that has been accepted for publication. NPG is providing this early version of the manuscript as a service to our customers. The manuscript will undergo copyediting, typesetting and a proof review before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers apply.

In this prospective randomized trial, hENT1 protein expression was associated with increased overall survival and disease-free survival in pancreatic cancer patients who received gemcitabine, but not in those who received 5-FU. These findings are supported by preclinical data; the gemcitabine transporter hENT1 is therefore a molecular and mechanistically relevant predictive marker of benefit from gemcitabine in patients with resected pancreatic cancer.