Publications

• Facilitated mitochondrial import of antiviral and anticancer nucleoside drugs by human equilibrative nucleoside transporter-3.

  Govindarajan R., Leung G.P., Zhou M., Tse C.M., Wang J., Unadkat J.D.

  Human equilibrative nucleoside transporter-3 (hENT3) was recently reported as a pH-dependent, intracellular (lysosomal) transporter capable of transporting anti-human immunodeficiency virus (HIV) dideoxynucleosides (ddNs). Because most anti-HIV ddNs (e.g., zidovudine, AZT) exhibit clinical mitocho ... >> More

  Human equilibrative nucleoside transporter-3 (hENT3) was recently reported as a pH-dependent, intracellular (lysosomal) transporter capable of transporting anti-human immunodeficiency virus (HIV) dideoxynucleosides (ddNs). Because most anti-HIV ddNs (e.g., zidovudine, AZT) exhibit clinical mitochondrial toxicity, we investigated whether hENT3 facilitates transport of anti-HIV ddNs into the mitochondria. Cellular fractionation and immunofluorescence microscopy studies in several human cell lines identified a substantial presence of hENT3 in the mitochondria, with additional presence at the cell surface of two placental cell lines (JAR, JEG3). Mitochondrial or cell surface hENT3 expression was confirmed in human hepatocytes and placental tissues, respectively. Unlike endogenous hENT3, yellow fluorescent protein (YFP)-tagged hENT3 was partially directed to the lysosomes. Xenopus oocytes expressing NH2-terminal-deleted hENT3 (expressed at the cell surface) showed pH-dependent interaction with several classes of nucleosides (anti-HIV ddNs, gemcitabine, fialuridine, ribavirin) that produce mitochondrial toxicity. Transport studies in hENT3 gene-silenced JAR cells showed significant reduction in mitochondrial transport of nucleosides and nucleoside drugs. Our data suggest that cellular localization of hENT3 is cell type dependent and the native transporter is substantially expressed in mitochondria and/or cell surface. hENT3-mediated mitochondrial transport may play an important role in mediating clinically observed mitochondrial toxicity of nucleoside drugs. In addition, our finding that hENT3 is a mitochondrial transporter is consistent with the recent finding that mutations in the hENT3 gene cause an autosomal recessive disorder in humans called the H syndrome. << Less

  Am. J. Physiol. 296:G910-G922(2009) [PubMed] [EuropePMC]

  This publication is cited by 12 other entries.

• Kinetic and pharmacological properties of cloned human equilibrative nucleoside transporters, ENT1 and ENT2, stably expressed in nucleoside transporter-deficient PK15 cells. Ent2 exhibits a low affinity for guanosine and cytidine but a high affinity for inosine.

  Ward J.L., Sherali A., Mo Z.P., Tse C.M.

  We stably transfected the cloned human equilibrative nucleoside transporters 1 and 2 (hENT1 and hENT2) into nucleoside transporter-deficient PK15NTD cells. Although hENT1 and hENT2 are predicted to be 50-kDa proteins, hENT1 runs as 40 kDa and hENT2 migrates as 50 and 47 kDa on SDS-polyacrylamide g ... >> More

  We stably transfected the cloned human equilibrative nucleoside transporters 1 and 2 (hENT1 and hENT2) into nucleoside transporter-deficient PK15NTD cells. Although hENT1 and hENT2 are predicted to be 50-kDa proteins, hENT1 runs as 40 kDa and hENT2 migrates as 50 and 47 kDa on SDS-polyacrylamide gel electrophoresis. Peptide N-glycosidase F and endoglycosidase H deglycosylate hENT1 to 37 kDa and hENT2 to 45 kDa. With hENT1 being more sensitive, there is a 7000-fold and 71-fold difference in sensitivity to nitrobenzylthioinosine (NBMPR) (IC(50), 0.4 +/- 0.1 nM versus 2.8 +/-0.3 microM) and dipyridamole (IC(50), 5.0 +/- 0.9 nM versus 356 +/-13 nM), respectively. [(3)H]NBMPR binds to ENT1 cells with a high affinity K(d) of 0.377 +/-0.098 nM, and each ENT1 cell has 34,000 transporters with a turnover number of 46 molecules/s for uridine. Although both transporters are broadly selective, hENT2 is a generally low affinity nucleoside transporter with 2.6-, 2.8-, 7. 7-, and 19.3-fold lower affinity than hENT1 for thymidine, adenosine, cytidine, and guanosine, respectively. In contrast, the affinity of hENT2 for inosine is 4-fold higher than hENT1. The nucleobase hypoxanthine inhibits [(3)H]uridine uptake by hENT2 but has minimal effect on hENT1. Taken together, these results suggest that hENT2 might be important in transporting adenosine and its metabolites (inosine and hypoxanthine) in tissues such as skeletal muscle where ENT2 is predominantly expressed. << Less

  J. Biol. Chem. 275:8375-8381(2000) [PubMed] [EuropePMC]

  This publication is cited by 4 other entries.

• Organic anion transporter 2 (SLC22A7) is a facilitative transporter of cGMP.

  Cropp C.D., Komori T., Shima J.E., Urban T.J., Yee S.W., More S.S., Giacomini K.M.

  The second messenger, cGMP, mediates a host of cellular responses to various stimuli, resulting in the regulation of many critical physiologic functions. The existence of specific cGMP transporters on the plasma membrane that participate in the regulation of cGMP levels has been suggested in a lar ... >> More

  The second messenger, cGMP, mediates a host of cellular responses to various stimuli, resulting in the regulation of many critical physiologic functions. The existence of specific cGMP transporters on the plasma membrane that participate in the regulation of cGMP levels has been suggested in a large number of studies. In this study, we identified a novel plasma membrane transporter for cGMP. In particular, we showed that hOAT2 (SLC22A7), a member of the solute carrier (SLC) superfamily, was a facilitative transporter for cGMP and other guanine nucleotides. hOAT2, which is ubiquitously expressed at high levels in many cell types, was previously thought to primarily transport organic anions. Among purine and pyrimidine nucleobases, nucleosides, and nucleotides, hOAT2 showed the greatest preference for cGMP, which transported cGMP with a K(m) value of 88 +/- 11 muM and exhibited between 50- and 100-fold enhanced uptake over control cells. Our data revealed that hOAT2 is a bidirectional facilitative transporter that can control both intracellular and extracellular levels of cGMP. In addition, we observed that a common alternatively spliced variant of hOAT2 demonstrated a complete loss of transport function as a result of a low expression level on the plasma membrane. We conclude that hOAT2 is a highly efficient, facilitative transporter of cGMP and may be involved in cGMP signaling in many tissues. Our study suggests that hOAT2 represents a potential new drug target for regulating cGMP levels. << Less

  Mol. Pharmacol. 73:1151-1158(2008) [PubMed] [EuropePMC]

  This publication is cited by 5 other entries.

• Identification of a nucleoside/nucleobase transporter from Plasmodium falciparum, a novel target for anti-malarial chemotherapy.

  Parker M.D., Hyde R.J., Yao S.Y., McRobert L., Cass C.E., Young J.D., McConkey G.A., Baldwin S.A.

  Plasmodium, the aetiologic agent of malaria, cannot synthesize purines de novo, and hence depends upon salvage from the host. Here we describe the molecular cloning and functional expression in Xenopus oocytes of the first purine transporter to be identified in this parasite. This 422-residue prot ... >> More

  Plasmodium, the aetiologic agent of malaria, cannot synthesize purines de novo, and hence depends upon salvage from the host. Here we describe the molecular cloning and functional expression in Xenopus oocytes of the first purine transporter to be identified in this parasite. This 422-residue protein, which we designate PfENT1, is predicted to contain 11 membrane-spanning segments and is a distantly related member of the widely distributed eukaryotic protein family the equilibrative nucleoside transporters (ENTs). However, it differs profoundly at the sequence and functional levels from its homologous counterparts in the human host. The parasite protein exhibits a broad substrate specificity for natural nucleosides, but transports the purine nucleoside adenosine with a considerably higher apparent affinity (K(m) 0.32+/-0.05 mM) than the pyrimidine nucleoside uridine (K(m) 3.5+/-1.1 mM). It also efficiently transports nucleobases such as adenine (K(m) 0.32+/-0.10 mM) and hypoxanthine (K(m) 0.41+/-0.1 mM), and anti-viral 3'-deoxynucleoside analogues. Moreover, it is not sensitive to classical inhibitors of mammalian ENTs, including NBMPR [6-[(4-nitrobenzyl)thio]-9-beta-D-ribofuranosylpurine, or nitrobenzylthioinosine] and the coronary vasoactive drugs, dipyridamole, dilazep and draflazine. These unique properties suggest that PfENT1 might be a viable target for the development of novel anti-malarial drugs. << Less

  Biochem. J. 349:67-75(2000) [PubMed] [EuropePMC]

  This publication is cited by 10 other entries.

• Genetic evidence for the essential role of PfNT1 in the transport and utilization of xanthine, guanine, guanosine and adenine by Plasmodium falciparum.

  El Bissati K., Downie M.J., Kim S.K., Horowitz M., Carter N., Ullman B., Ben Mamoun C.

  The malaria parasite, Plasmodium falciparum, is unable to synthesize the purine ring de novo and is therefore wholly dependent upon purine salvage from the host for survival. Previous studies have indicated that a P. falciparum strain in which the purine transporter PfNT1 had been disrupted was un ... >> More

  The malaria parasite, Plasmodium falciparum, is unable to synthesize the purine ring de novo and is therefore wholly dependent upon purine salvage from the host for survival. Previous studies have indicated that a P. falciparum strain in which the purine transporter PfNT1 had been disrupted was unable to grow on physiological concentrations of adenosine, inosine and hypoxanthine. We have now used an episomally complemented pfnt1Delta knockout parasite strain to confirm genetically the functional role of PfNT1 in P. falciparum purine uptake and utilization. Episomal complementation by PfNT1 restored the ability of pfnt1Delta parasites to transport and utilize adenosine, inosine and hypoxanthine as purine sources. The ability of wild-type and pfnt1Delta knockout parasites to transport and utilize the other physiologically relevant purines adenine, guanine, guanosine and xanthine was also examined. Unlike wild-type and complemented P. falciparum parasites, pfnt1Delta parasites could not proliferate on guanine, guanosine or xanthine as purine sources, and no significant transport of these substrates could be detected in isolated parasites. Interestingly, whereas isolated pfnt1Delta parasites were still capable of adenine transport, these parasites grew only when adenine was provided at high, non-physiological concentrations. Taken together these results demonstrate that, in addition to hypoxanthine, inosine and adenosine, PfNT1 is essential for the transport and utilization of xanthine, guanine and guanosine. << Less

  Mol. Biochem. Parasitol. 161:130-139(2008) [PubMed] [EuropePMC]

  This publication is cited by 5 other entries.