Cloning of a human nucleoside transporter implicated in the Cellular uptake of adenosine and chemotherapeutic drugs

Abstract

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.

Excretion of biliary compounds during intrauterine life

“…Moreover, concentrative nucleoside transporters (CNTs), CNT2 (SLC28A2) and CNT3 (SLC28A3) are also expressed in human placenta. Both ENT1 and ENT2 are able to transport a wide variety of therapeutic agents such as the anticancer drugs cytarabine and gemcitabine and the antiviral drugs zalcitabine (ddC) and zidovudine [117,118] , and they therefore probably play a role in fetal exposure to these types of drugs.…”

Nucleoside analogues: mechanisms of drug resistance and reversal strategies

“…The two cloned hENTs differ in their sensitivity to inhibition by nanomolar concentrations of nitrobenzylmercaptopurine ribonucleoside (NBMPR): hENT1 has es activity (equilibrative and NBMPR-sensitive) while hENT2 possesses ei (equilibrative and NBMPR-insensitive) nucleoside transport activity. [12][13][14] The human concentrative nucleoside transporters (hCNT) are inwardly-directed transporters that are capable of transporting nucleosides against a concentration gradient by utilizing the transmembrane sodium concentration gradient. [15][16][17] The hCNT1 protein has greater affinity for pyrimidine nucleosides but also transports adenosine, while the hCNT2 protein transports purine nucleosides and uridine.…”

Functional characterization of a H+/nucleoside co‐transporter (CaCNT) from Candida albicans, a fungal member of the concentrative nucleoside transporter (CNT) family of membrane proteins

“…Two ENT and three CNT functional isoforms have been identified. Human (h) and rat (r) ENT1 and ENT2 transport pyrimidine and purine nucleosides and are distinguished functionally by differences in sensitivity to inhibition by nitrobenzylthioinosine (NBMPR) and vasoactive drugs, and by the ability of hENT2 and rENT2 to also transport nucleobases (Griffiths et al, 1997a(Griffiths et al, , 1997bYao et al, 1997;Crawford et al, 1998;Yao et al, 2002a). CNT1 and CNT2 both transport uridine and adenosine, but are otherwise selective for pyrimidine (hCNT1 and rCNT1) and purine (hCNT2 and rCNT2) nucleosides (Huang et al, 1994;Che et al, 1995, Yao et al, 1996aWang et al, 1997;Ritzel et al, 1997Ritzel et al, , 1998.…”