1
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Hau RK, Wright SH, Cherrington NJ. Addressing the Clinical Importance of Equilibrative Nucleoside Transporters in Drug Discovery and Development. Clin Pharmacol Ther 2023; 114:780-794. [PMID: 37404197 DOI: 10.1002/cpt.2984] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/10/2023] [Accepted: 05/30/2023] [Indexed: 07/06/2023]
Abstract
The US Food and Drug Administration (FDA), European Medicines Agency (EMA), and Pharmaceuticals and Medical Devices Agency (PMDA) guidances on small-molecule drug-drug interactions (DDIs), with input from the International Transporter Consortium (ITC), recommend the evaluation of nine drug transporters. Although other clinically relevant drug uptake and efflux transporters have been discussed in ITC white papers, they have been excluded from further recommendation by the ITC and are not included in current regulatory guidances. These include the ubiquitously expressed equilibrative nucleoside transporters (ENT) 1 and ENT2, which have been recognized by the ITC for their potential role in clinically relevant nucleoside analog drug interactions for patients with cancer. Although there is comparatively limited clinical evidence supporting their role in DDI risk or other adverse drug reactions (ADRs) compared with the nine highlighted transporters, several in vitro and in vivo studies have identified ENT interactions with non-nucleoside/non-nucleotide drugs, in addition to nucleoside/nucleotide analogs. Some noteworthy examples of compounds that interact with ENTs include cannabidiol and selected protein kinase inhibitors, as well as the nucleoside analogs remdesivir, EIDD-1931, gemcitabine, and fialuridine. Consequently, DDIs involving the ENTs may be responsible for therapeutic inefficacy or off-target toxicity. Evidence suggests that ENT1 and ENT2 should be considered as transporters potentially involved in clinically relevant DDIs and ADRs, thereby warranting further investigation and regulatory consideration.
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Affiliation(s)
- Raymond K Hau
- Department of Pharmacology & Toxicology, College of Pharmacy, The University of Arizona, Tucson, Arizona, USA
| | - Stephen H Wright
- Department of Physiology, College of Medicine, The University of Arizona, Tucson, Arizona, USA
| | - Nathan J Cherrington
- Department of Pharmacology & Toxicology, College of Pharmacy, The University of Arizona, Tucson, Arizona, USA
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2
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Wang C, Yu L, Zhang J, Zhou Y, Sun B, Xiao Q, Zhang M, Liu H, Li J, Li J, Luo Y, Xu J, Lian Z, Lin J, Wang X, Zhang P, Guo L, Ren R, Deng D. Structural basis of the substrate recognition and inhibition mechanism of Plasmodium falciparum nucleoside transporter PfENT1. Nat Commun 2023; 14:1727. [PMID: 36977719 PMCID: PMC10050424 DOI: 10.1038/s41467-023-37411-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2022] [Accepted: 03/16/2023] [Indexed: 03/30/2023] Open
Abstract
By lacking de novo purine biosynthesis enzymes, Plasmodium falciparum requires purine nucleoside uptake from host cells. The indispensable nucleoside transporter ENT1 of P. falciparum facilitates nucleoside uptake in the asexual blood stage. Specific inhibitors of PfENT1 prevent the proliferation of P. falciparum at submicromolar concentrations. However, the substrate recognition and inhibitory mechanism of PfENT1 are still elusive. Here, we report cryo-EM structures of PfENT1 in apo, inosine-bound, and inhibitor-bound states. Together with in vitro binding and uptake assays, we identify that inosine is the primary substrate of PfENT1 and that the inosine-binding site is located in the central cavity of PfENT1. The endofacial inhibitor GSK4 occupies the orthosteric site of PfENT1 and explores the allosteric site to block the conformational change of PfENT1. Furthermore, we propose a general "rocker switch" alternating access cycle for ENT transporters. Understanding the substrate recognition and inhibitory mechanisms of PfENT1 will greatly facilitate future efforts in the rational design of antimalarial drugs.
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Affiliation(s)
- Chen Wang
- Department of Obstetrics, Key Laboratory of Birth Defects and Related Disease of Women and Children of MOE, State Key Laboratory of Biotherapy, West China Second Hospital, Sichuan University, Chengdu, 610041, China
- Shanghai Key Laboratory of Metabolic Remodeling and Health, Institute of Metabolism and Integrative Biology, Fudan University, Shanghai, 200438, China
| | - Leiye Yu
- Shanghai Key Laboratory of Metabolic Remodeling and Health, Institute of Metabolism and Integrative Biology, Fudan University, Shanghai, 200438, China
- Warshal Institute of Computational Biology, School of Life and Health Sciences, the Chinese University of Hong Kong, Shenzhen, Guangdong, 518172, China
| | - Jiying Zhang
- Department of Obstetrics, Key Laboratory of Birth Defects and Related Disease of Women and Children of MOE, State Key Laboratory of Biotherapy, West China Second Hospital, Sichuan University, Chengdu, 610041, China
| | - Yanxia Zhou
- Department of Obstetrics, Key Laboratory of Birth Defects and Related Disease of Women and Children of MOE, State Key Laboratory of Biotherapy, West China Second Hospital, Sichuan University, Chengdu, 610041, China
| | - Bo Sun
- Shanghai Synchrotron Radiation Facility, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai, 201204, China
| | - Qingjie Xiao
- Shanghai Synchrotron Radiation Facility, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai, 201204, China
| | - Minhua Zhang
- National Key Laboratory of Plant Molecular Genetics, Center for Excellence in Molecular Plant Sciences, Institute of Plant Physiology and Ecology, Chinese Academy of Sciences, Shanghai, 200032, China
| | - Huayi Liu
- Department of Obstetrics, Key Laboratory of Birth Defects and Related Disease of Women and Children of MOE, State Key Laboratory of Biotherapy, West China Second Hospital, Sichuan University, Chengdu, 610041, China
| | - Jinhong Li
- Department of Obstetrics, Key Laboratory of Birth Defects and Related Disease of Women and Children of MOE, State Key Laboratory of Biotherapy, West China Second Hospital, Sichuan University, Chengdu, 610041, China
| | - Jialu Li
- Department of Obstetrics, Key Laboratory of Birth Defects and Related Disease of Women and Children of MOE, State Key Laboratory of Biotherapy, West China Second Hospital, Sichuan University, Chengdu, 610041, China
| | - Yunzi Luo
- Frontier Science Center for Synthetic Biology and Key Laboratory of Systems Bioengineering of MOE, School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China
| | - Jie Xu
- State Key Laboratory of Biotherapy and Cancer Center, West China Hospital, Sichuan University, Chengdu, 610041, China
| | - Zhong Lian
- State Key Laboratory of Biotherapy and Cancer Center, West China Hospital, Sichuan University, Chengdu, 610041, China
| | - Jingwen Lin
- Department of Obstetrics, Key Laboratory of Birth Defects and Related Disease of Women and Children of MOE, State Key Laboratory of Biotherapy, West China Second Hospital, Sichuan University, Chengdu, 610041, China
- State Key Laboratory of Biotherapy and Cancer Center, West China Hospital, Sichuan University, Chengdu, 610041, China
| | - Xiang Wang
- Department of Obstetrics, Key Laboratory of Birth Defects and Related Disease of Women and Children of MOE, State Key Laboratory of Biotherapy, West China Second Hospital, Sichuan University, Chengdu, 610041, China
| | - Peng Zhang
- National Key Laboratory of Plant Molecular Genetics, Center for Excellence in Molecular Plant Sciences, Institute of Plant Physiology and Ecology, Chinese Academy of Sciences, Shanghai, 200032, China
| | - Li Guo
- Department of Obstetrics, Key Laboratory of Birth Defects and Related Disease of Women and Children of MOE, State Key Laboratory of Biotherapy, West China Second Hospital, Sichuan University, Chengdu, 610041, China.
| | - Ruobing Ren
- Shanghai Key Laboratory of Metabolic Remodeling and Health, Institute of Metabolism and Integrative Biology, Fudan University, Shanghai, 200438, China.
- Shanghai Qi Zhi Institute, Shanghai, 200030, China.
| | - Dong Deng
- Department of Obstetrics, Key Laboratory of Birth Defects and Related Disease of Women and Children of MOE, State Key Laboratory of Biotherapy, West China Second Hospital, Sichuan University, Chengdu, 610041, China.
- NHC key Laboratory of Chronobiology, Sichuan University, Chengdu, 610041, China.
- Development and Related Diseases of Women and Children Key Laboratory of Sichuan Province, Sichuan University, Chengdu, 610041, China.
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3
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Boakes JC, Harborne SPD, Ngo JTS, Pliotas C, Goldman A. Novel variants provide differential stabilisation of human equilibrative nucleoside transporter 1 states. Front Mol Biosci 2022; 9:970391. [DOI: 10.3389/fmolb.2022.970391] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2022] [Accepted: 10/27/2022] [Indexed: 11/10/2022] Open
Abstract
Human equilibrative nucleoside transporters represent a major pharmaceutical target for cardiac, cancer and viral therapies. Understanding the molecular basis for transport is crucial for the development of improved therapeutics through structure-based drug design. ENTs have been proposed to utilise an alternating access mechanism of action, similar to that of the major facilitator superfamily. However, ENTs lack functionally-essential features of that superfamily, suggesting that they may use a different transport mechanism. Understanding the molecular basis of their transport requires insight into diverse conformational states. Differences between intermediate states may be discrete and mediated by subtle gating interactions, such as salt bridges. We identified four variants of human equilibrative nucleoside transporter isoform 1 (hENT1) at the large intracellular loop (ICL6) and transmembrane helix 7 (TM7) that stabilise the apo-state (∆Tm 0.7–1.5°C). Furthermore, we showed that variants K263A (ICL6) and I282V (TM7) specifically stabilise the inhibitor-bound state of hENT1 (∆∆Tm 5.0 ± 1.7°C and 3.0 ± 1.8°C), supporting the role of ICL6 in hENT1 gating. Finally, we showed that, in comparison with wild type, variant T336A is destabilised by nitrobenzylthioinosine (∆∆Tm -4.7 ± 1.1°C) and binds it seven times worse. This residue may help determine inhibitor and substrate sensitivity. Residue K263 is not present in the solved structures, highlighting the need for further structural data that include the loop regions.
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4
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Emerging roles of dysregulated adenosine homeostasis in brain disorders with a specific focus on neurodegenerative diseases. J Biomed Sci 2021; 28:70. [PMID: 34635103 PMCID: PMC8507231 DOI: 10.1186/s12929-021-00766-y] [Citation(s) in RCA: 20] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/27/2021] [Accepted: 10/04/2021] [Indexed: 02/07/2023] Open
Abstract
In modern societies, with an increase in the older population, age-related neurodegenerative diseases have progressively become greater socioeconomic burdens. To date, despite the tremendous effort devoted to understanding neurodegenerative diseases in recent decades, treatment to delay disease progression is largely ineffective and is in urgent demand. The development of new strategies targeting these pathological features is a timely topic. It is important to note that most degenerative diseases are associated with the accumulation of specific misfolded proteins, which is facilitated by several common features of neurodegenerative diseases (including poor energy homeostasis and mitochondrial dysfunction). Adenosine is a purine nucleoside and neuromodulator in the brain. It is also an essential component of energy production pathways, cellular metabolism, and gene regulation in brain cells. The levels of intracellular and extracellular adenosine are thus tightly controlled by a handful of proteins (including adenosine metabolic enzymes and transporters) to maintain proper adenosine homeostasis. Notably, disruption of adenosine homeostasis in the brain under various pathophysiological conditions has been documented. In the past two decades, adenosine receptors (particularly A1 and A2A adenosine receptors) have been actively investigated as important drug targets in major degenerative diseases. Unfortunately, except for an A2A antagonist (istradefylline) administered as an adjuvant treatment with levodopa for Parkinson's disease, no effective drug based on adenosine receptors has been developed for neurodegenerative diseases. In this review, we summarize the emerging findings on proteins involved in the control of adenosine homeostasis in the brain and discuss the challenges and future prospects for the development of new therapeutic treatments for neurodegenerative diseases and their associated disorders based on the understanding of adenosine homeostasis.
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5
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Abstract
Nucleosides play central roles in all facets of life, from metabolism to cellular signaling. Because of their physiochemical properties, nucleosides are lipid bilayer impermeable and thus rely on dedicated transport systems to cross biological membranes. In humans, two unrelated protein families mediate nucleoside membrane transport: the concentrative and equilibrative nucleoside transporter families. The objective of this review is to provide a broad outlook on the current status of nucleoside transport research. We will discuss the role played by nucleoside transporters in human health and disease, with emphasis placed on recent structural advancements that have revealed detailed molecular principles of these important cellular transport systems and exploitable pharmacological features.
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Affiliation(s)
- Nicholas J. Wright
- Department of Biochemistry, Duke University Medical Center, 303 Research Drive, Durham, North Carolina, 27710, USA
| | - Seok-Yong Lee
- Department of Biochemistry, Duke University Medical Center, 303 Research Drive, Durham, North Carolina, 27710, USA
- Correspondence and requests for materials should be addressed to: S.-Y. Lee., , tel: 919-684-1005, fax: 919-684-8885
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6
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IMPROvER: the Integral Membrane Protein Stability Selector. Sci Rep 2020; 10:15165. [PMID: 32938971 PMCID: PMC7495477 DOI: 10.1038/s41598-020-71744-x] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2019] [Accepted: 08/04/2020] [Indexed: 01/20/2023] Open
Abstract
Identifying stabilising variants of membrane protein targets is often required for structure determination. Our new computational pipeline, the Integral Membrane Protein Stability Selector (IMPROvER) provides a rational approach to variant selection by employing three independent approaches: deep-sequence, model-based and data-driven. In silico tests using known stability data, and in vitro tests using three membrane protein targets with 7, 11 and 16 transmembrane helices provided measures of success. In vitro, individual approaches alone all identified stabilising variants at a rate better than expected by random selection. Low numbers of overlapping predictions between approaches meant a greater success rate was achieved (fourfold better than random) when approaches were combined and selections restricted to the highest ranked sites. The mix of information IMPROvER uses can be extracted for any helical membrane protein. We have developed the first general-purpose tool for selecting stabilising variants of \documentclass[12pt]{minimal}
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\begin{document}$$\upalpha$$\end{document}α-helical membrane proteins, increasing efficiency and reducing workload. IMPROvER can be accessed at http://improver.ddns.net/IMPROvER/.
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7
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Wu KC, Lee CY, Chou FY, Chern Y, Lin CJ. Deletion of equilibrative nucleoside transporter-2 protects against lipopolysaccharide-induced neuroinflammation and blood-brain barrier dysfunction in mice. Brain Behav Immun 2020; 84:59-71. [PMID: 31751618 DOI: 10.1016/j.bbi.2019.11.008] [Citation(s) in RCA: 20] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/14/2019] [Revised: 11/05/2019] [Accepted: 11/17/2019] [Indexed: 01/22/2023] Open
Abstract
Neuroinflammation is a common pathological feature of many brain diseases and is a key mediator of blood-brain barrier (BBB) breakdown and neuropathogenesis. Adenosine is an endogenous immunomodulator, whose brain extracellular level is tightly controlled by equilibrative nucleoside transporters-1 (ENT1) and ENT2. This study was aimed to investigate the role of ENTs in the modulation of neuroinflammation and BBB function. The results showed that mRNA level of Ent2 was significantly more abundant than that of Ent1 in the brain (hippocampus, cerebral cortex, striatum, midbrain, and cerebellum) of wild-type (WT) mice. Ent2-/- mice displayed higher extracellular adenosine level in the hippocampus than their littermate controls. Repeated lipopolysaccharide (LPS) treatment induced microglia activation, astrogliosis and upregulation of proinflammatory cytokines, along with aberrant BBB phenotypes (including reduced tight junction protein expression, pericyte loss, and immunoglobulin G extravasation) and neuronal apoptosis in the hippocampus of WT mice. Notably, Ent2-/- mice displayed significant resistance to LPS-induced neuroinflammation, BBB breakdown, and neurotoxicity. These findings suggest that Ent2 is critical for the modulation of brain adenosine tone and deletion of Ent2 confers protection against LPS-induced neuroinflammation and neurovascular-associated injury.
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Affiliation(s)
- Kuo-Chen Wu
- School of Pharmacy, College of Medicine, National Taiwan University, Taipei, Taiwan
| | - Chih-Yu Lee
- School of Pharmacy, College of Medicine, National Taiwan University, Taipei, Taiwan
| | - Fang-Yi Chou
- School of Pharmacy, College of Medicine, National Taiwan University, Taipei, Taiwan
| | - Yijuang Chern
- Department of Life Sciences and Institute of Genome Sciences, National Yang-Ming University, Taipei, Taiwan; Institute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan
| | - Chun-Jung Lin
- School of Pharmacy, College of Medicine, National Taiwan University, Taipei, Taiwan.
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8
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Rehan S, Shahid S, Salminen TA, Jaakola VP, Paavilainen VO. Current Progress on Equilibrative Nucleoside Transporter Function and Inhibitor Design. SLAS DISCOVERY 2019; 24:953-968. [PMID: 31503511 DOI: 10.1177/2472555219870123] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
Physiological nucleosides are used for the synthesis of DNA, RNA, and ATP in the cell and serve as universal mammalian signaling molecules that regulate physiological processes such as vasodilation and platelet aggregation by engaging with cell surface receptors. The same pathways that allow uptake of physiological nucleosides mediate the cellular import of synthetic nucleoside analogs used against cancer, HIV, and other viral diseases. Physiological nucleosides and nucleoside drugs are imported by two families of nucleoside transporters: the SLC28 concentrative nucleoside transporters (CNTs) and SLC29 equilibrative nucleoside transporters (ENTs). The four human ENT paralogs are expressed in distinct tissues, localize to different subcellular sites, and transport a variety of different molecules. Here we provide an overview of the known structure-function relationships of the ENT family with a focus on ligand binding and transport in the context of a new hENT1 homology model. We provide a generic residue numbering system for the different ENTs to facilitate the interpretation of mutational data produced using different ENT homologs. The discovery of paralog-selective small-molecule modulators is highly relevant for the design of new therapies and for uncovering the functions of poorly characterized ENT family members. Here, we discuss recent developments in the discovery of new paralog-selective small-molecule ENT inhibitors, including new natural product-inspired compounds. Recent progress in the ability to heterologously produce functional ENTs will allow us to gain insight into the structure and functions of different ENT family members as well as the rational discovery of highly selective inhibitors.
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Affiliation(s)
- Shahid Rehan
- Institute of Biotechnology, University of Helsinki, Helsinki, Finland.,HiLIFE, University of Helsinki, Helsinki, Finland
| | - Saman Shahid
- Faculty of Biochemistry and Molecular Medicine, University of Oulu, Oulu, Finland
| | - Tiina A Salminen
- Structural Bioinformatics Laboratory, Biochemistry, Faculty of Science and Engineering, Åbo Akademi University, Turku, Finland
| | - Veli-Pekka Jaakola
- Chemical Biology & Therapeutics, Novartis Institutes for BioMedical Research, Novartis Pharma AG, Basel, Switzerland
| | - Ville O Paavilainen
- Institute of Biotechnology, University of Helsinki, Helsinki, Finland.,HiLIFE, University of Helsinki, Helsinki, Finland
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9
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Structures of human ENT1 in complex with adenosine reuptake inhibitors. Nat Struct Mol Biol 2019; 26:599-606. [PMID: 31235912 PMCID: PMC6705415 DOI: 10.1038/s41594-019-0245-7] [Citation(s) in RCA: 60] [Impact Index Per Article: 12.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/26/2019] [Accepted: 05/08/2019] [Indexed: 02/06/2023]
Abstract
The human Equilibrative Nucleoside Transporter 1 (hENT1), a member of the SLC29 family, plays crucial roles in adenosine signaling, cellular uptake of nucleoside for DNA and RNA synthesis, and nucleoside-derived anticancer and antiviral drug transport in human. Because of its central role in adenosine signaling, it is the target of adenosine reuptake inhibitors (AdoRI), several of which are clinically used. Despite its importance in human physiology and pharmacology, the molecular basis of hENT1-mediated adenosine transport and its inhibition by AdoRIs are limited due to the absence of structural information on hENT1. Here we present crystal structures of hENT1 in complex with two chemically distinct AdoRIs: dilazep and S-(4-Nitrobenzyl)-6-thioinosine (NBMPR). Combined with mutagenesis study, our structural analyses elucidate two distinct inhibitory mechanisms exhibited on hENT1, while giving insight into adenosine recognition and transport. Our studies provide the platform for improved pharmacological intervention of adenosine and nucleoside analog drug transport by hENT1.
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10
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Role of cysteine 416 in N-ethylmaleimide sensitivity of human equilibrative nucleoside transporter 1 (hENT1). Biochem J 2018; 475:3293-3309. [PMID: 30254099 DOI: 10.1042/bcj20180543] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/16/2018] [Revised: 09/19/2018] [Accepted: 09/25/2018] [Indexed: 12/23/2022]
Abstract
Human equilibrative nucleoside transporter 1 (hENT1), the first identified member of the ENT family of integral membrane proteins, is the primary mechanism for cellular uptake of physiologic nucleosides and many antineoplastic and antiviral nucleoside drugs. hENT1, which is potently inhibited by nitrobenzylthioinosine (NBMPR), possesses 11 transmembrane helical domains with an intracellular N-terminus and an extracellular C-terminus. As a protein with 10 endogenous cysteine residues, it is sensitive to inhibition by the membrane permeable sulfhydryl-reactive reagent N-ethylmaleimide (NEM) but is unaffected by the membrane impermeable sulfhydryl-reactive reagent p-chloromercuriphenyl sulfonate. To identify the residue(s) involved in NEM inhibition, we created a cysteine-less version of hENT1 (hENT1C-), with all 10 endogenous cysteine residues mutated to serine, and showed that it displays wild-type uridine transport and NBMPR-binding characteristics when produced in the Xenopus oocyte heterologous expression system, indicating that endogenous cysteine residues are not essential for hENT1 function. We then tested NEM sensitivity of recombinant wild-type hENT1, hENT1 mutants C1S to C10S (single cysteine residues replaced by serine), hENT1C- (all cysteine residues replaced by serine), and hENT1C- mutants S1C to S10C (single serine residues converted back to cysteine). Mutants C9S (C416S/hENT1) and S9C (S416C/hENT1C-) were insensitive and sensitive, respectively, to inhibition by NEM, identifying Cys416 as the endofacial cysteine residue in hENT1 responsible for NEM inhibition. Kinetic experiments suggested that NEM modification of Cys416, which is located at the inner extremity of TM10, results in the inhibition of hENT1 uridine transport and NBMPR binding by constraining the protein in its inward-facing conformation.
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11
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Rahman MF, Askwith C, Govindarajan R. Molecular determinants of acidic pH-dependent transport of human equilibrative nucleoside transporter 3. J Biol Chem 2017; 292:14775-14785. [PMID: 28729424 DOI: 10.1074/jbc.m117.787952] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2017] [Revised: 07/11/2017] [Indexed: 12/16/2022] Open
Abstract
Equilibrative nucleoside transporters (ENTs) translocate hydrophilic nucleosides across cellular membranes and are essential for salvage nucleotide synthesis and purinergic signaling. Unlike the prototypic human ENT members hENT1 and hENT2, which mediate plasma membrane nucleoside transport at pH 7.4, hENT3 is an acidic pH-activated lysosomal transporter partially localized to mitochondria. Recent studies demonstrate that hENT3 is indispensable for lysosomal homeostasis, and that mutations in hENT3 can result in a spectrum of lysosomal storage-like disorders. However, despite hENT3's prominent role in lysosome pathophysiology, the molecular basis of hENT3-mediated transport is unknown. Therefore, we sought to examine the mechanistic basis of acidic pH-driven hENT3 nucleoside transport with site-directed mutagenesis, homology modeling, and [3H]adenosine flux measurements in mutant RNA-injected Xenopus oocytes. Scanning mutagenesis of putative residues responsible for pH-dependent transport via hENT3 revealed that the ionization states of Asp-219 and Glu-447, and not His, strongly determined the pH-dependent transport permissible-impermissible states of the transporter. Except for substitution with certain isosteric and polar residues, substitution of either Asp-219 or Glu-447 with any other residues resulted in robust activity that was pH-independent. Dual substitution of Asp-219 and Glu-447 to Ala sustained pH-independent activity over a broad range of physiological pH (pH 5.5-7.4), which also maintained stringent substrate selectivity toward endogenous nucleosides and clinically used nucleoside drugs. Our results suggest a putative pH-sensing role for Asp-219 and Glu-447 in hENT3 and that the size, ionization state, or electronegative polarity at these positions is crucial for obligate acidic pH-dependent activity.
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Affiliation(s)
- Md Fazlur Rahman
- From the Division of Pharmaceutics and Pharmaceutical Chemistry, College of Pharmacy
| | | | - Rajgopal Govindarajan
- From the Division of Pharmaceutics and Pharmaceutical Chemistry, College of Pharmacy, .,the Translational Therapeutics Program, Ohio State University Comprehensive Cancer Center, Ohio State University, Columbus, Ohio 43210
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12
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Huang W, Zeng X, Shi Y, Liu M. Functional characterization of human equilibrative nucleoside transporter 1. Protein Cell 2017; 8:284-295. [PMID: 27995448 PMCID: PMC5359181 DOI: 10.1007/s13238-016-0350-x] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/10/2016] [Accepted: 11/04/2016] [Indexed: 12/15/2022] Open
Abstract
Equilibrative nucleoside transporters (ENTs), which facilitate cross-membrane transport of nucleosides and nucleoside-derived drugs, play an important role in the salvage pathways of nucleotide synthesis, cancer chemotherapy, and treatment for virus infections. Functional characterization of ENTs at the molecular level remains technically challenging and hence scant. In this study, we report successful purification and biochemical characterization of human equilibrative nucleoside transporter 1 (hENT1) in vitro. The HEK293F-derived, recombinant hENT1 is homogenous and functionally active in proteoliposome-based counter flow assays. hENT1 transports the substrate adenosine with a Km of 215 ± 34 µmol/L and a Vmax of 578 ± 23.4 nmol mg-1 min-1. Adenosine uptake by hENT1 is competitively inhibited by nitrobenzylmercaptopurine ribonucleoside (NBMPR), nucleosides, deoxynucleosides, and nucleoside-derived anti-cancer and anti-viral drugs. Binding of hENT1 to adenosine, deoxyadenosine, and adenine by isothermal titration calorimetry is in general agreement with results of the competitive inhibition assays. These results validate hENT1 as a bona fide target for potential drug target and serve as a useful basis for future biophysical and structural studies.
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Affiliation(s)
- Weiyun Huang
- Beijing Advanced Innovation Center for Structural Biology, Tsinghua-Peking Joint Center for Life Sciences, School of Life Sciences, Tsinghua University, Beijing, 100084, China
| | - Xin Zeng
- Beijing Advanced Innovation Center for Structural Biology, Tsinghua-Peking Joint Center for Life Sciences, School of Life Sciences, Tsinghua University, Beijing, 100084, China
| | - Yigong Shi
- Beijing Advanced Innovation Center for Structural Biology, Tsinghua-Peking Joint Center for Life Sciences, School of Life Sciences, Tsinghua University, Beijing, 100084, China
| | - Minhao Liu
- Beijing Advanced Innovation Center for Structural Biology, Tsinghua-Peking Joint Center for Life Sciences, School of Life Sciences, Tsinghua University, Beijing, 100084, China.
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13
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Boswell-Casteel RC, Hays FA. Equilibrative nucleoside transporters-A review. NUCLEOSIDES NUCLEOTIDES & NUCLEIC ACIDS 2016; 36:7-30. [PMID: 27759477 DOI: 10.1080/15257770.2016.1210805] [Citation(s) in RCA: 125] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [Abstract] [Key Words] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
Equilibrative nucleoside transporters (ENTs) are polytopic integral membrane proteins that mediate the transport of nucleosides, nucleobases, and therapeutic analogs. The best-characterized ENTs are the human transporters hENT1 and hENT2. However, non-mammalian eukaryotic ENTs have also been studied (e.g., yeast, parasitic protozoa). ENTs are major pharmaceutical targets responsible for modulating the efficacy of more than 30 approved drugs. However, the molecular mechanisms and chemical determinants of ENT-mediated substrate recognition, binding, inhibition, and transport are poorly understood. This review highlights findings on the characterization of ENTs by surveying studies on genetics, permeant and inhibitor interactions, mutagenesis, and structural models of ENT function.
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Affiliation(s)
- Rebba C Boswell-Casteel
- a Department of Biochemistry and Molecular Biology , University of Oklahoma Health Sciences Center , Oklahoma City , OK , USA
| | - Franklin A Hays
- a Department of Biochemistry and Molecular Biology , University of Oklahoma Health Sciences Center , Oklahoma City , OK , USA.,b Stephenson Cancer Center , University of Oklahoma Health Sciences Center , Oklahoma City , OK , USA.,c Harold Hamm Diabetes Center , University of Oklahoma Health Sciences Center , Oklahoma City , OK , USA
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Frame IJ, Deniskin R, Arora A, Akabas MH. Purine import into malaria parasites as a target for antimalarial drug development. Ann N Y Acad Sci 2014; 1342:19-28. [PMID: 25424653 DOI: 10.1111/nyas.12568] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
Infection with Plasmodium species parasites causes malaria. Plasmodium parasites are purine auxotrophs. In all life cycle stages, they require purines for RNA and DNA synthesis and other cellular metabolic processes. Purines are imported from the host erythrocyte by equilibrative nucleoside transporters (ENTs). They are processed via purine salvage pathway enzymes to form the required purine nucleotides. The Plasmodium falciparum genome encodes four putative ENTs (PfENT1-4). Genetic, biochemical, and physiologic evidence suggest that PfENT1 is the primary purine transporter supplying the purine salvage pathway. Protein mass spectrometry shows that PfENT1 is expressed in all parasite stages. PfENT1 knockout parasites are not viable in culture at purine concentrations found in human blood (<10 μM). Thus, PfENT1 is a potential target for novel antimalarial drugs, but no PfENT1 inhibitors have been identified to test the hypothesis. Identifying inhibitors of PfENT1 is an essential step to validate PfENT1 as a potential antimalarial drug target.
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Affiliation(s)
- I J Frame
- Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, New York
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15
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Valdés R, Shinde U, Landfear SM. Cysteine cross-linking defines the extracellular gate for the Leishmania donovani nucleoside transporter 1.1 (LdNT1.1). J Biol Chem 2012; 287:44036-45. [PMID: 23150661 DOI: 10.1074/jbc.m112.414433] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Equilibrative nucleoside transporters are a unique family of proteins that enable uptake of nucleosides/nucleobases into a wide range of eukaryotes and internalize a myriad of drugs used in the treatment of cancer, heart disease, AIDs, and parasitic infections. In previous work we generated a structural model for such a transporter, the LdNT1.1 nucleoside permease from the parasitic protozoan Leishmania donovani, using ab initio computation. The model suggested that aromatic residues present in transmembrane helices 1, 2, and 7 interact to form an extracellular gate that closes the permeation pathway in the inward-open conformation. Mutation of residues Phe-48(TM1) and Trp-75(TM2) abrogated transport activity, consistent with such prediction. In this study cysteine mutagenesis and oxidative cross-linking were combined to analyze proximity relationships of helices 1, 2, and 7 in LdNT1.1. Disulfide bond formation between introduced paired cysteines at the interface of such helices (A61C(TM1)/F74C(TM2), A61C(TM1)/G350C(TM7), and F74C(TM2)/G350C(TM7)) was analyzed by transport measurement and gel mobility shifts upon oxidation with Cu (II)-(1,10-phenanthroline)(3). In all cases cross-linking inhibited transport. However, if LdNT1.1 ligands were included during cross-linking, inhibition of transport was reduced, suggesting that ligands moved the three gating helices apart. Moreover, all paired cysteine mutants exhibited a mobility shift upon oxidation, corroborating the formation of a disulfide bond. These data support the notion that helices 1, 2, and 7 constitute the extracellular gate of LdNT1.1, thus further validating the computational model and the previously demonstrated importance of F48(TM1) and Trp-75(TM2) in tethering together helices that are part of the gate.
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Affiliation(s)
- Raquel Valdés
- Department of Molecular Microbiology and Immunology, Oregon Health and Science University, Portland, Oregon 97239, USA
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Park JS, Hammond JR. Cysteine Residues in the Transmembrane (TM) 9 to TM11 Region of the Human Equilibrative Nucleoside Transporter Subtype 1 Play an Important Role in Inhibitor Binding and Translocation Function. Mol Pharmacol 2012; 82:784-94. [DOI: 10.1124/mol.112.079616] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
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Nucleoside transporters: biological insights and therapeutic applications. Future Med Chem 2012; 4:1461-78. [DOI: 10.4155/fmc.12.79] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022] Open
Abstract
Nucleoside transporters play important physiological roles by regulating intra- and extra-cellular concentrations of purine and pyrimidine (deoxy)nucleosides. This review describes the biological function and activity of the two major families of membrane nucleoside transporters that exist in mammalian cells. These include equilibrative nucleoside transporters that transport nucleosides in a gradient-dependent fashion and concentrative nucleoside transporters that import nucleosides against a gradient by coupling movement with sodium transport. Particular emphasis is placed on describing the roles of nucleoside transport in normal physiological processes, including inflammation, cardiovascular function and nutrient transport across the blood–brain barrier. In addition, the role of nucleoside transport in pathological conditions such as cardiovascular disease and cancer are discussed. The potential therapeutic applications of manipulating nucleoside transport activities are discussed, focusing on nucleoside analogs as anti-neoplastic agents. Finally, we discuss future directions for the development of novel chemical entities to measure nucleoside transport activity at the cellular and organismal level.
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Ho HTB, Xia L, Wang J. Residue Ile89 in human plasma membrane monoamine transporter influences its organic cation transport activity and sensitivity to inhibition by dilazep. Biochem Pharmacol 2012; 84:383-90. [PMID: 22562044 DOI: 10.1016/j.bcp.2012.04.018] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/10/2012] [Revised: 04/24/2012] [Accepted: 04/27/2012] [Indexed: 01/11/2023]
Abstract
Plasma membrane monoamine transporter (PMAT) is a polyspecific organic cation transporter belonging to the equilibrative nucleoside transporter (ENT) family. Despite its distinct substrate specificity from the classic nucleoside transporters ENT1 and 2, PMAT appears to share similar protein architecture with ENT1/2 and retains low affinity binding to classic ENT inhibitors such as nitrobenzylmercaptopurine riboside (NBMPR) and the coronary vasodilators dilazep and dipyridamole. Here we investigated the role of residue Ile89, a position known to be important for ENT interaction with dilazep, dipyridamole, and nucleoside substrates, in PMAT transport function and its interaction with classic ENT inhibitors using Madin-Darby canine kidney (MDCK) cells stably expressing human PMAT. Substitution of Ile89 in PMAT with Met, the counterpart residue in ENT1, resulted in normal plasma membrane localization and protein expression. Transport kinetic analysis revealed that I89M mutant had a 2.7-fold reduction in maximal transport velocity (V(max)) with no significant change in apparent binding affinity (K(m)) towards the prototype PMAT substrate 1-methyl-4-phenylpyridinium (MPP+), suggesting that I89 is an important determinant for the catalytic activity of PMAT. Dose-dependent inhibition studies further showed that the I89M mutation significantly increased PMAT's sensitivity to dilazep by 2.5-fold without affecting its sensitivity to dipyridamole and NBMPR. Located at the extracellular end of transmembrane domain 1 of PMAT, I89 may occupy an important position close to the substrate permeation pathway and may be involved in direct interaction with the vasodilator dilazep.
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Affiliation(s)
- Horace T B Ho
- Department of Pharmaceutics, University of Washington, Seattle, WA 98195, USA.
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19
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Abstract
BACKGROUND Nucleoside/nucleobase transporters have been investigated since the 1960s. In particular, equilibrative nucleoside transporters were thought to be valuable drug targets, since they are involved in various kinds of viral and parasitic diseases as well as cancers. DISCUSSION In the postgenomic era multiple transporters, including different subtypes, have been cloned and characterized on the molecular level. In this article we summarize recent advances regarding structure, function and localization of nucleoside/nucleobase transporters as well as the pharmacological profile of selected drugs. CONCLUSION Knowledge of the different kinetic properties and structural features of nucleoside transporters can either be used for the rational design of therapeutics directly targeting the transporter itself or for the delivery of drugs using the transporter as a port of entry into the target cell. Equilibrative nucleoside transporters are of considerable pharmacological interest as drug targets for the development of drugs tailored to each patient's need for the treatment of cardiac disease, cancer and viral infections.
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20
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Viso A, Fernández de la Pradilla R, Tortosa M, García A, Flores A. Update 1 of: α,β-Diamino Acids: Biological Significance and Synthetic Approaches. Chem Rev 2011; 111:PR1-42. [DOI: 10.1021/cr100127y] [Citation(s) in RCA: 85] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
Affiliation(s)
- Alma Viso
- Instituto de Química Orgánica, CSIC, Juan de la Cierva 3, E-28006 Madrid, Spain
| | | | - Mariola Tortosa
- Instituto de Química Orgánica, CSIC, Juan de la Cierva 3, E-28006 Madrid, Spain
| | - Ana García
- Instituto de Química Orgánica, CSIC, Juan de la Cierva 3, E-28006 Madrid, Spain
| | - Aida Flores
- Instituto de Química Orgánica, CSIC, Juan de la Cierva 3, E-28006 Madrid, Spain
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21
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Cano-Soldado P, Pastor-Anglada M. Transporters that translocate nucleosides and structural similar drugs: structural requirements for substrate recognition. Med Res Rev 2011; 32:428-57. [DOI: 10.1002/med.20221] [Citation(s) in RCA: 53] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
Affiliation(s)
- Pedro Cano-Soldado
- Departament de Bioquímica i Biologia Molecular; Institut de Biomedicina de la Universitat de Barcelona (IBUB); Universitat de Barcelona and CIBER EHD; Barcelona Spain
| | - Marçal Pastor-Anglada
- Departament de Bioquímica i Biologia Molecular; Institut de Biomedicina de la Universitat de Barcelona (IBUB); Universitat de Barcelona and CIBER EHD; Barcelona Spain
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22
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Kim JH, Karpyak VM, Biernacka JM, Nam HW, Lee MR, Preuss UW, Zill P, Yoon G, Colby C, Mrazek DA, Choi DS. Functional role of the polymorphic 647 T/C variant of ENT1 (SLC29A1) and its association with alcohol withdrawal seizures. PLoS One 2011; 6:e16331. [PMID: 21283641 PMCID: PMC3026043 DOI: 10.1371/journal.pone.0016331] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2010] [Accepted: 12/11/2010] [Indexed: 01/10/2023] Open
Abstract
Background Adenosine is involved in several neurological and behavioral disorders including alcoholism. In cultured cell and animal studies, type 1 equilibrative nucleoside transporter (ENT1, slc29a1), which regulates adenosine levels, is known to regulate ethanol sensitivity and preference. Interestingly, in humans, the ENT1 (SLC29A1) gene contains a non-synonymous single nucleotide polymorphism (647 T/C; rs45573936) that might be involved in the functional change of ENT1. Principal Findings Our functional analysis showed that prolonged ethanol exposure increased adenosine uptake activity of mutant cells (ENT1-216Thr) compared to wild-type (ENT1-216Ile) transfected cells, which might result in reduced extracellular adenosine levels. We found that mice lacking ENT1 displayed increased propensity to ethanol withdrawal seizures compared to wild-type littermates. We further investigated a possible association of the 647C variant with alcoholism and the history of alcohol withdrawal seizures in subjects of European ancestry recruited from two independent sites. Analyses of the combined data set showed an association of the 647C variant and alcohol dependence with withdrawal seizures at the nominally significant level. Conclusions Together with the functional data, our findings suggest a potential contribution of a genetic variant of ENT1 to the development of alcoholism with increased risk of alcohol withdrawal-induced seizures in humans.
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Affiliation(s)
- Jeong-Hyun Kim
- Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic College of Medicine, Rochester, Minnesota, United States of America
| | - Victor M. Karpyak
- Department of Psychiatry, Mayo Clinic College of Medicine, Rochester, Minnesota, United States of America
| | - Joanna M. Biernacka
- Department of Psychiatry, Mayo Clinic College of Medicine, Rochester, Minnesota, United States of America
- Department of Health Sciences Research, Mayo Clinic College of Medicine, Rochester, Minnesota, United States of America
| | - Hyung Wook Nam
- Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic College of Medicine, Rochester, Minnesota, United States of America
| | - Moonnoh R. Lee
- Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic College of Medicine, Rochester, Minnesota, United States of America
| | - Ulrich W. Preuss
- Department of Psychiatry, Psychotherapy and Psychosomatics, Martin-Luther-University, Halle/Saale, Germany
| | - Peter Zill
- Section Psychiatric Genetics and Neurochemistry, Ludwig-Maximilians University of Munich, Munich, Germany
| | - Gihyun Yoon
- Department of Psychiatry, University of Minnesota, Minneapolis, Minnesota, United States of America
| | - Colin Colby
- Department of Health Sciences Research, Mayo Clinic College of Medicine, Rochester, Minnesota, United States of America
| | - David A. Mrazek
- Department of Psychiatry, Mayo Clinic College of Medicine, Rochester, Minnesota, United States of America
| | - Doo-Sup Choi
- Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic College of Medicine, Rochester, Minnesota, United States of America
- Department of Psychiatry, Mayo Clinic College of Medicine, Rochester, Minnesota, United States of America
- * E-mail:
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23
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Robins MJ, Peng Y, Damaraju VL, Mowles D, Barron G, Tackaberry T, Young JD, Cass CE. Improved Syntheses of 5′-S-(2-Aminoethyl)-6-N-(4-nitrobenzyl)-5′-thioadenosine (SAENTA), Analogues, and Fluorescent Probe Conjugates: Analysis of Cell-Surface Human Equilibrative Nucleoside Transporter 1 (hENT1) Levels for Prediction of the Antitumor Efficacy of Gemcitabine. J Med Chem 2010; 53:6040-53. [DOI: 10.1021/jm100432w] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
Affiliation(s)
- Morris J. Robins
- Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602-5700
| | - Yunshan Peng
- Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602-5700
| | - Vijaya L. Damaraju
- Departments of Oncology and Physiology, University of Alberta, Edmonton, Alberta, Canada T6G 1Z2
| | - Delores Mowles
- Departments of Oncology and Physiology, University of Alberta, Edmonton, Alberta, Canada T6G 1Z2
| | - Geraldine Barron
- Departments of Oncology and Physiology, University of Alberta, Edmonton, Alberta, Canada T6G 1Z2
| | - Tracey Tackaberry
- Departments of Oncology and Physiology, University of Alberta, Edmonton, Alberta, Canada T6G 1Z2
| | - James D. Young
- Departments of Oncology and Physiology, University of Alberta, Edmonton, Alberta, Canada T6G 1Z2
| | - Carol E. Cass
- Departments of Oncology and Physiology, University of Alberta, Edmonton, Alberta, Canada T6G 1Z2
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24
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Riegelhaupt PM, Frame IJ, Akabas MH. Transmembrane segment 11 appears to line the purine permeation pathway of the Plasmodium falciparum equilibrative nucleoside transporter 1 (PfENT1). J Biol Chem 2010; 285:17001-10. [PMID: 20335165 DOI: 10.1074/jbc.m110.115758] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
Purine transport is essential for malaria parasites to grow because they lack the enzymes necessary for de novo purine biosynthesis. The Plasmodium falciparum Equilibrative Nucleoside Transporter 1 (PfENT1) is a member of the equilibrative nucleoside transporter (ENT) gene family. PfENT1 is a primary purine transport pathway across the P. falciparum plasma membrane because PfENT1 knock-out parasites are not viable at physiologic extracellular purine concentrations. Topology predictions and experimental data indicate that ENT family members have eleven transmembrane (TM) segments although their tertiary structure is unknown. In the current work, we showed that a naturally occurring polymorphism, F394L, in TM11 affects transport substrate K(m). We investigated the structure and function of the TM11 segment using the substituted cysteine accessibility method. We showed that mutation to Cys of two highly conserved glycine residues in a GXXXG motif significantly reduces PfENT1 protein expression levels. We speculate that the conserved TM11 GXXXG glycines may be critical for folding and/or assembly. Small, cysteine-specific methanethiosulfonate (MTS) reagents reacted with four TM11 Cys substitution mutants, L393C, I397C, T400C, and Y403C. Larger MTS reagents do not react with the more cytoplasmic positions. Hypoxanthine, a transported substrate, protected L393C, I397C, and T400C from covalent modification by the MTS reagents. Plotted on an alpha-helical wheel, Leu-393, Ile-397, and Thr-400 lie on one face of the helix in a 60 degrees arc suggesting that TM11 is largely alpha helical. We infer that they line a water-accessible surface, possibly the purine permeation pathway. These results advance our understanding of the ENT structure.
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Affiliation(s)
- Paul M Riegelhaupt
- Department of Physiology and Biophysics, Albert Einstein College of Medicine of Yeshiva University, Bronx, New York 10461, USA
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25
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Paproski RJ, Young JD, Cass CE. Predicting gemcitabine transport and toxicity in human pancreatic cancer cell lines with the positron emission tomography tracer 3'-deoxy-3'-fluorothymidine. Biochem Pharmacol 2010; 79:587-95. [PMID: 19788890 DOI: 10.1016/j.bcp.2009.09.025] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2009] [Revised: 09/22/2009] [Accepted: 09/23/2009] [Indexed: 12/14/2022]
Abstract
The abundance of human equilibrative nucleoside transporter 1 (hENT1) has recently been shown to be a predictive marker of benefit from gemcitabine therapy in patients with pancreatic cancer. Since hENT1 is also important for the uptake of positron emission tomography (PET) tracer 3'-deoxy-3'-fluorothymidine (FLT) in various cultured human cell lines, this study was undertaken to determine if FLT uptake predicts gemcitabine uptake and/or toxicity in a panel of human pancreatic cancer cell lines (Capan-2, AsPC-1, BxPC-3, PL45, MIA PaCa-2, and PANC-1). Capan-2 cells displayed the lowest levels of (1) extracellular nitrobenzylmercaptopurine ribonucleoside (NBMPR) binding, which represents cell-surface hENT1, (2) FLT and gemcitabine uptake during short (1-45s) and prolonged (1h) periods, and (3) gemcitabine sensitivity. Exposure to NBMPR (inhibits only hENT1) or dilazep (inhibits hENT1 and hENT2) reduced FLT and gemcitabine uptake and gemcitabine sensitivity, with dilazep having greater effects than NBMPR. Gemcitabine permeation was almost completely mediated, primarily by hENT1 and to a lesser extent by hENT2, whereas FLT permeation included a substantial component of passive diffusion. In five of six cell lines, correlations were observed between (1) FLT and gemcitabine initial rates of uptake, (2) gemcitabine uptake and gemcitabine toxicity, (3) FLT uptake and gemcitabine toxicity, and (4) ribonucleotide reductase subunit M1 expression and gemcitabine toxicity. FLT and gemcitabine uptake were comparable for predicting gemcitabine toxicity in the tested pancreatic cancer cell lines suggesting that FLT PET may provide clinically useful information about tumor gemcitabine transport capacity and sensitivity.
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Affiliation(s)
- Robert J Paproski
- Department of Oncology, University of Alberta, Cross Cancer Institute, 11560 University Ave., Edmonton, Alberta, Canada.
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26
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Damaraju S, Damaraju VL, Mowles D, Sawyer MB, Damaraju S, Cass CE. Cytotoxic activity of gemcitabine in cultured cell lines derived from histologically different types of bladder cancer: Role of thymidine kinase 2. Biochem Pharmacol 2010; 79:21-9. [DOI: 10.1016/j.bcp.2009.07.018] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/18/2009] [Revised: 07/23/2009] [Accepted: 07/27/2009] [Indexed: 11/26/2022]
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Arendt CS, Ullman B. Role of transmembrane domain 4 in ligand permeation by Crithidia fasciculata equilibrative nucleoside transporter 2 (CfNT2). J Biol Chem 2009; 285:6024-35. [PMID: 20037157 DOI: 10.1074/jbc.m109.074351] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023] Open
Abstract
Equilibrative nucleoside transporters play essential roles in nutrient uptake, cardiovascular and renal function, and purine analog drug chemotherapies. Limited structural information is available for this family of transporters; however, residues in transmembrane domains 1, 2, 4, and 5 appear to be important for ligand and inhibitor binding. In order to identify regions of the transporter that are important for ligand specificity, a genetic selection for mutants of the inosine-guanosine-specific Crithidia fasciculata nucleoside transporter 2 (CfNT2) that had gained the ability to transport adenosine was carried out in the yeast Saccharomyces cerevisiae. Nearly all positive clones from the genetic selection carried mutations at lysine 155 in transmembrane domain 4, highlighting lysine 155 as a pivotal residue governing the ligand specificity of CfNT2. Mutation of lysine 155 to asparagine conferred affinity for adenosine on the mutant transporter at the expense of inosine and guanosine affinity due to weakened contacts to the purine ring of the ligand. Following systematic cysteine-scanning mutagenesis, thiol-specific modification of several positions within transmembrane domain 4 was found to interfere with inosine transport capability, indicating that this helix lines the water-filled ligand translocation channel. Additionally, the pattern of modification of transmembrane domain 4 suggested that it may deviate from helicity in the vicinity of residue 155. Position 155 was also protected from modification in the presence of ligand, suggesting that lysine 155 is in or near the ligand binding site. Transmembrane domain 4 and particularly lysine 155 appear to play key roles in ligand discrimination and translocation by CfNT2.
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Affiliation(s)
- Cassandra S Arendt
- School of Pharmacy, Pacific University Oregon, Hillsboro, Oregon 97123, USA.
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Zlatopolskiy BD, Morgenroth A, Kunkel FHG, Urusova EA, Dinger C, Kull T, Lepping C, Reske SN. Synthesis and Biologic Study of IV-14, a New Ribonucleoside Radiotracer for Tumor Visualization. J Nucl Med 2009; 50:1895-903. [DOI: 10.2967/jnumed.109.065623] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
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Valdés R, Arastu-Kapur S, Landfear SM, Shinde U. An ab Initio structural model of a nucleoside permease predicts functionally important residues. J Biol Chem 2009; 284:19067-76. [PMID: 19429678 PMCID: PMC2707223 DOI: 10.1074/jbc.m109.017947] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/20/2009] [Revised: 05/06/2009] [Indexed: 11/06/2022] Open
Abstract
Permeases belonging to the equilibrative nucleoside transporter family promote uptake of nucleosides and/or nucleobases into a wide range of eukaryotes and mediate the uptake of a variety of drugs used in the treatment of cancer, heart disease, AIDS, and parasitic infections. No experimental three-dimensional structure exists for any of these permeases, and they are not present in prokaryotes, the source of many membrane proteins used in crystal structure determination. To generate a structural model for such a transporter, the LdNT1.1 nucleoside permease from the parasitic protozoan Leishmania donovani was modeled using ab initio computation. Site-directed mutations that strongly impair transport or that alter substrate specificity map to the central pore of the ab initio model, whereas mutations that have less pronounced phenotypes map to peripheral positions. The model suggests that aromatic residues present in transmembrane helices 1, 2, and 7 may interact to form an extracellular gate that closes the permeation pathway in the inward oriented conformation. Mutation of two of these three residues abrogated transport activity, consistent with the prediction of the model. The ab initio model is similar to one derived previously using threading analysis, a distinct computational approach, supporting the overall accuracy of both models. However, significant differences in helix orientation and residue position between the two models are apparent, and the mutagenesis data suggest that the ab initio model represents an improvement regarding structural details over the threading model. The putative gating interaction may also help explain differences in substrate specificity between members of this family.
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Affiliation(s)
- Raquel Valdés
- From the Departments of Molecular Microbiology and Immunology and
| | - Shirin Arastu-Kapur
- Biochemistry and Molecular Biology, Oregon Health & Science University, Portland, Oregon 97239
| | | | - Ujwal Shinde
- Biochemistry and Molecular Biology, Oregon Health & Science University, Portland, Oregon 97239
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Young JD, Yao SYM, Sun L, Cass CE, Baldwin SA. Human equilibrative nucleoside transporter (ENT) family of nucleoside and nucleobase transporter proteins. Xenobiotica 2008; 38:995-1021. [PMID: 18668437 DOI: 10.1080/00498250801927427] [Citation(s) in RCA: 156] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
1. The human (h) SLC29 family of integral membrane proteins is represented by four members, designated equilibrative nucleoside transporters (ENTs) because of the properties of the first-characterized family member, hENT1. They belong to the widely distributed eukaryotic ENT family of equilibrative and concentrative nucleoside/nucleobase transporter proteins. 2. A predicted topology of eleven transmembrane helices has been experimentally confirmed for hENT1. The best-characterized members of the family, hENT1 and hENT2, possess similar broad permeant selectivities for purine and pyrimidine nucleosides, but hENT2 also efficiently transports nucleobases. hENT3 has a similar broad permeant selectivity for nucleosides and nucleobases and appears to function in intracellular membranes, including lysosomes. 3. hENT4 is uniquely selective for adenosine, and also transports a variety of organic cations. hENT3 and hENT4 are pH sensitive, and optimally active under acidic conditions. ENTs, including those in parasitic protozoa, function in nucleoside and nucleobase uptake for salvage pathways of nucleotide synthesis and, in humans, are also responsible for the cellular uptake of nucleoside analogues used in the treatment of cancers and viral diseases. 4. By regulating the concentration of adenosine available to cell surface receptors, mammalian ENTs additionally influence physiological processes ranging from cardiovascular activity to neurotransmission.
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Affiliation(s)
- J D Young
- Membrane Protein Research Group, Department of Physiology and Department of Oncology, University of Alberta, Edmonton, Alberta, Canada.
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Mutation of Trp29 of human equilibrative nucleoside transporter 1 alters affinity for coronary vasodilator drugs and nucleoside selectivity. Biochem J 2008; 414:291-300. [PMID: 18462193 DOI: 10.1042/bj20080074] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
hENT1 (human equilibrative nucleoside transporter 1) is inhibited by nanomolar concentrations of various structurally distinct coronary vasodilator drugs, including dipyridamole, dilazep, draflazine, soluflazine and NBMPR (nitrobenzylmercaptopurine ribonucleoside). When a library of randomly mutated hENT1 cDNAs was screened using a yeast-based functional complementation assay for resistance to dilazep, a clone containing the W29G mutation was identified. Multiple sequence alignments revealed that this residue was highly conserved. Mutations at Trp29 were generated and tested for adenosine transport activity and inhibitor sensitivity. Trp29 mutations significantly reduced the apparent V(max) and/or increased the apparent K(m) values for adenosine transport. Trp29 mutations increased the IC50 values for hENT1 inhibition by dipyridamole, dilazep, NBMPR, soluflazine and draflazine. NBMPR and soluflazine displayed remarkably similar trends, with large aromatic substitutions at residue 29 resulting in the lowest IC50 values, suggesting that both drugs could interact via ring-stacking interactions with Trp29. The W29T mutant displayed a selective loss of pyrimidine nucleoside transport activity, which contrasts with the previously identified L442I mutant that displayed a selective loss of purine nucleoside transport. W29T, L442I and the double mutant W29T/L442I were characterized kinetically for nucleoside transport activity. A helical wheel projection of TM (transmembrane segment) 1 suggests that Trp29 is positioned close to Met33, implicated previously in nucleoside and inhibitor recognition, and that both residues line the permeant translocation pathway. The data also suggest that Trp29 forms part of, or lies close to, the binding sites for dipyridamole, dilazep, NBMPR, soluflazine and draflazine.
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