Human Transporters

ENT1

ENT1 (equilibrative nucleoside transporter 1)

Aliases: None
Gene name: Solute carrier family 29 member 1 (SLC29A1)

Summary

ENT1 (SLC29A1) is a sodium-independent transporter for purine and pyrimidine nucleosides and for some nucleobases. Nucleoside transporters are classified into two major classes, equilibrative bi-directional facilitators (ENTs) and Na+-dependent concentrative transporters (CNTs) [1]. Human ENT1 (hENT1, SLC29A1) was cloned from the placenta in 1996 and thus the first member to be characterized in the SLC29 family [2]. ENT1 is a glycosylated protein that contains 456 amino acid residues (50 kDa). Its gene is located at the chromosomal region 6p21.1-21.2. ENTs share a hypothetical 11-transmembrane (TM) helix topology, with an intracellular amino terminus and extracellular carboxyl terminus [3]. The region between TM domains 3 and 6 is responsible for the sensitivity to different inhibitors such as dipyridamole, dilazep, or S-(4-Nitrobenzyl)-6-thioinosine (NBMPR).
ENT1 has orthologues in many eukaryotes including mammals, yeast, nematodes, plants and protozoa.

Localization

ENT1 is widely distributed throughout the body: it is present in erythrocytes as well as the liver, heart, spleen, kidney, lung, intestine and brain, although its abundance varies between tissues [2-6].
ENT1 is also expressed and functional in the mitochondrial membrane [7, 8].
Most mammalian cells express more than one type of nucleoside transporter. Tissue-specific regulation of nucleoside transporters, along with their tissue distribution, can provide valuable information about their biology and pharmacological role [9].

Function, physiology, and clinically significant polymorphisms

Based on its tissue distribution, ENT1 is thought to play a critical role in the provision of nucleosides, derived from the diet or produced by tissues such as the liver, for salvage pathways of nucleotide synthesis in cells lacking de novo biosynthetic pathways. Besides, ENT1 is also involved in the absorption, distribution and excretion of xenobiotics.
Adenosine is a potent endogenous physiological and pharmacological regulator of numerous functions. Cellular signaling by adenosine occurs through four known G-protein-coupled adenosine receptors A1, A2A, A2B, and A3. By influencing the concentration of adenosine available to these receptors, ENTs fulfil important regulatory roles in different physiological processes. ENT1 has been reported to be a player in the modulation of coronary blood flow, myocardial O2 supply-demand balance, inflammation, and neurotransmission [4, 10-15].
Song et al. have recently provided human genetic evidence that soluble CD73-depedent elevation of plasma adenosine signaling via phosphorylation, ubiquitination and proteasomal degradation of ENT1 in the erythrocytes enhances adenosine response to hypoxia, and thereby counteracts hypoxia-induced maladaptation [16].
Several single nucleotide polymorphisms (SNPs) resulting in nonsynonymous variants of the human ENT1 transporter have been identified [17]. Those that have been characterized functionally show normal nucleoside and nucleoside drug uptake kinetics [17]. Daniels et al. reported a nonsynonymous SNP in SLC29A1 (rs45458701) which is responsible for the Augustine-negative (At(a-)) blood type [18]. The most frequent genetic polymorphism (rs45573936) causes an amino acid change at codon 216 from isoleucine to threonine with a minor allele frequency of 2%, almost exclusively found in Northern Europe [17, 19]. ENT1 Ile216Thr appears to contribute to the genetic predisposition to alcoholism with an increased risk of withdrawal-related seizures. Decreased adenosine activity observed in ENT1 null mice appears to be similar to that of chronic ethanol-induced increase of uptake activity in ENT1-216Thr, indicating that increased uptake function of the ENT1-216Thr variant upon alcohol exposure could reduce extracellular or synaptic adenosine levels [20].

Clinical significance

ENT1 contributes to the disposition of nucleoside analog drugs used to treat various forms of cancer, cardiovascular disorders, neurological conditions as well as viral and parasitic infections [21-23]. These drugs act via incorporation into nucleic acids, by interference with nucleic acid synthesis or by interference with the metabolism of physiological nucleosides [24].
ENT proteins are important determinants of sensitivity to, and toxicity from, nucleoside and other related drugs. Many of the nucleoside analog drugs available are used in the treatment of cancer. The first nucleoside analog approved as a treatment for cancer was the pyrimidine analog cytarabine (AraC). AraC is primarily transported into the cell via ENT1 [21]. Another pyrimidine analog is gemcitabine, which is used as the primary or adjuvant therapy in the chemotherapeutic treatment of several cancer types [25-27]. Of the nucleoside analog drugs currently approved for cancer treatment (e.g. 5-FU, cladribine, pentostatin, etc.) 13 have been shown to use ENT1 as a route of entry into cells. Increased ENT1 abundance may contribute to the relative selectivity of nucleoside chemotherapy for malignant cells; measurement of transporter abundance may therefore provide a predictive tool for guiding the appropriate use of such drugs in individual patients [28, 29]. Notably, ENT1 enhances the mitochondrial toxicity of the nucleoside drug fialuridine [7, 8].
Nucleoside analog drugs (e.g. zidovudine, ribavirin, etc.) are also commonly used to treat human immunodeficiency virus (HIV), hepatitis B, and hepatitis C infections. The polar nucleoside drug ribavirin is part of the first-line treatment for chronic hepatitis C virus infection, and ENT1 seems to be the major transporter controlling the hepatic uptake of ribavirin [30]. The accumulation of ribavirin inside erythrocytes upon long-term administration also depends on ENT1 transport. The phosphorylated metabolites of ribavirin accumulate in erythrocytes and produce dose-limiting hemolytic anemia [31].
A variety of drugs such as dilazep, dipyridamole, and draflazine interact with nucleoside transporters and alter adenosine levels, which can lead to cardioprotective or vasodilatory effects. It was also demonstrated that ENT1 and adenosine constitute biomarkers of the initial stages of neurodegeneration in Huntington disease [32]. It is known that adenosine is an important mediator of ethanol intoxication. ENT1 has a physiological role in ethanol-mediated behaviors and suggest that decreased A1 adenosine receptor function promotes alcohol consumption [33].

Regulatory requirements

ENT1 is not currently recommended for investigation by regulatory guidelines.

Location Endogenous substrates In vitro substrates used experimentally Substrate drugs Inhibitors
ubiquitous purine nucleosides. pyrimidine nucleosides Uridine, Adenosine

cladribine, cytarabine, fludarabine, gemcitabine capecitabine fialuridine, ribavirin

NBMPR, dipyridamole, dilazep, draflazine, STI-571 (Gleevec), ticagrelol

References

1.    Cass, C.E., J.D. Young, and S.A. Baldwin, Recent advances in the molecular biology of nucleoside transporters of mammalian cells. Biochem Cell Biol, 1998. 76(5): p. 761-70.
2.    Griffiths, M., et al., Cloning of a human nucleoside transporter implicated in the cellular uptake of adenosine and chemotherapeutic drugs. Nat Med, 1997. 3(1): p. 89-93.
3.    Baldwin, S.A., et al., The equilibrative nucleoside transporter family, SLC29. Pflugers Arch, 2004. 447(5): p. 735-43.
4.    Griffith, D.A. and S.M. Jarvis, Nucleoside and nucleobase transport systems of mammalian cells. Biochim Biophys Acta, 1996. 1286(3): p. 153-81.
5.    Yang, C. and G.P. Leung, Equilibrative Nucleoside Transporters 1 and 4: Which One Is a Better Target for Cardioprotection Against Ischemia-Reperfusion Injury? J Cardiovasc Pharmacol, 2015. 65(6): p. 517-21.
6.    Jennings, L.L., et al., Distinct regional distribution of human equilibrative nucleoside transporter proteins 1 and 2 (hENT1 and hENT2) in the central nervous system. Neuropharmacology, 2001. 40(5): p. 722-31.
7.    Lai, Y., C.M. Tse, and J.D. Unadkat, Mitochondrial expression of the human equilibrative nucleoside transporter 1 (hENT1) results in enhanced mitochondrial toxicity of antiviral drugs. J Biol Chem, 2004. 279(6): p. 4490-7.
8.    Lee, E.W., et al., Identification of the mitochondrial targeting signal of the human equilibrative nucleoside transporter 1 (hENT1): implications for interspecies differences in mitochondrial toxicity of fialuridine. J Biol Chem, 2006. 281(24): p. 16700-6.
9.    Pressacco, J., et al., Modulation of the equilibrative nucleoside transporter by inhibitors of DNA synthesis. Br J Cancer, 1995. 72(4): p. 939-42.
10.    Shryock, J.C. and L. Belardinelli, Adenosine and adenosine receptors in the cardiovascular system: biochemistry, physiology, and pharmacology. Am J Cardiol, 1997. 79(12A): p. 2-10.
11.    Tattersall, M.H., P. Slowiaczek, and A. De Fazio, Regional variation in human extracellular purine levels. J Lab Clin Med, 1983. 102(3): p. 411-20.
12.    Novitskiy, S.V., et al., Adenosine receptors in regulation of dendritic cell differentiation and function. Blood, 2008. 112(5): p. 1822-31.
13.    Aguayo, C., et al., Modulation of adenosine transport by insulin in human umbilical artery smooth muscle cells from normal or gestational diabetic pregnancies. J Physiol, 2001. 534(Pt 1): p. 243-54.
14.    Casanello, P., et al., Equilibrative nucleoside transporter 1 expression is downregulated by hypoxia in human umbilical vein endothelium. Circ Res, 2005. 97(1): p. 16-24.
15.    Anderson, C.M., et al., Distribution of equilibrative, nitrobenzylthioinosine-sensitive nucleoside transporters (ENT1) in brain. J Neurochem, 1999. 73(2): p. 867-73.
16.    Song, A., et al., Erythrocytes retain hypoxic adenosine response for faster acclimatization upon re-ascent. Nat Commun, 2017. 8: p. 14108.
17.    Osato, D.H., et al., Functional characterization in yeast of genetic variants in the human equilibrative nucleoside transporter, ENT1. Pharmacogenetics, 2003. 13(5): p. 297-301.
18.    Daniels, G., et al., Lack of the nucleoside transporter ENT1 results in the Augustine-null blood type and ectopic mineralization. Blood, 2015. 125(23): p. 3651-4.
19.    Genomes Project, C., et al., An integrated map of genetic variation from 1,092 human genomes. Nature, 2012. 491(7422): p. 56-65.
20.    Kim, J.H., et al., Functional role of the polymorphic 647 T/C variant of ENT1 (SLC29A1) and its association with alcohol withdrawal seizures. PLoS One, 2011. 6(1): p. e16331.
21.    Zhang, J., et al., The role of nucleoside transporters in cancer chemotherapy with nucleoside drugs. Cancer Metastasis Rev, 2007. 26(1): p. 85-110.
22.    Rahn, J.J., et al., Modulation of the metabolism of beta-L-(-)-2',3'-dideoxy-3'-thiacytidine by thymidine, fludarabine, and nitrobenzylthioinosine. Antimicrob Agents Chemother, 1997. 41(5): p. 918-23.
23.    Hillgren, K.M., et al., Emerging transporters of clinical importance: an update from the International Transporter Consortium. Clin Pharmacol Ther, 2013. 94(1): p. 52-63.
24.    Galmarini, C.M., J.R. Mackey, and C. Dumontet, Nucleoside analogues and nucleobases in cancer treatment. Lancet Oncol, 2002. 3(7): p. 415-24.
25.    Di Marco, M., et al., State of the art biological therapies in pancreatic cancer. World J Gastrointest Oncol, 2016. 8(1): p. 55-66.
26.    Massari, F., et al., Emerging concepts on drug resistance in bladder cancer: Implications for future strategies. Crit Rev Oncol Hematol, 2015. 96(1): p. 81-90.
27.    Moysan, E., G. Bastiat, and J.P. Benoit, Gemcitabine versus Modified Gemcitabine: a review of several promising chemical modifications. Mol Pharm, 2013. 10(2): p. 430-44.
28.    Spratlin, J., et al., The absence of human equilibrative nucleoside transporter 1 is associated with reduced survival in patients with gemcitabine-treated pancreas adenocarcinoma. Clin Cancer Res, 2004. 10(20): p. 6956-61.
29.    Giovannetti, E., et al., Transcription analysis of human equilibrative nucleoside transporter-1 predicts survival in pancreas cancer patients treated with gemcitabine. Cancer Res, 2006. 66(7): p. 3928-35.
30.    Choi, M.K., et al., Contribution of CNT1 and ENT1 to ribavirin uptake in human hepatocytes. Arch Pharm Res, 2015. 38(5): p. 904-13.
31.    Endres, C.J., et al., The role of the equilibrative nucleoside transporter 1 (ENT1) in transport and metabolism of ribavirin by human and wild-type or Ent1-/- mouse erythrocytes. J Pharmacol Exp Ther, 2009. 329(1): p. 387-98.
32.    Guitart, X., et al., Equilibrative nucleoside transporter ENT1 as a biomarker of Huntington disease. Neurobiol Dis, 2016. 96: p. 47-53.
33.    Choi, D.S., et al., The type 1 equilibrative nucleoside transporter regulates ethanol intoxication and preference. Nat Neurosci, 2004. 7(8): p. 855-61.

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