1
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Hong M. Biochemical studies on the structure-function relationship of major drug transporters in the ATP-binding cassette family and solute carrier family. Adv Drug Deliv Rev 2017; 116:3-20. [PMID: 27317853 DOI: 10.1016/j.addr.2016.06.003] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2016] [Revised: 05/27/2016] [Accepted: 06/08/2016] [Indexed: 12/21/2022]
Abstract
Human drug transporters often play key roles in determining drug accumulation within cells. Their activities are often directly related to therapeutic efficacy, drug toxicity as well as drug-drug interactions. However, the progress for interpretation of their crystal structures is relatively slow. Hence, conventional biochemical studies together with computer modeling became useful manners to reveal essential structures of these membrane proteins. Over the years, quite a few structure-function relationship information had been obtained for members of the two major transporter families: the ATP-binding cassette family and the solute carrier family. Critical structural features of drug transporters include transmembrane domains, post-translational modification sites and domains for cell surface assembly and protein-protein interactions. Alterations at these important sites may affect protein stability, trafficking to the plasma membrane and/or ability of transporters to interact with substrates.
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2
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Sabui S, Subramanian VS, Kapadia R, Said HM. Structure-function characterization of the human mitochondrial thiamin pyrophosphate transporter (hMTPPT; SLC25A19): Important roles for Ile(33), Ser(34), Asp(37), His(137) and Lys(291). BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2016; 1858:1883-90. [PMID: 27188525 DOI: 10.1016/j.bbamem.2016.05.011] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/25/2016] [Revised: 04/21/2016] [Accepted: 05/13/2016] [Indexed: 01/19/2023]
Abstract
Thiamin plays a critical role in cellular energy metabolism. Mammalian cells obtain the vitamin from their surroundings, converted it to thiamin pyrophosphate (TPP) in the cytoplasm, followed by uptake of TPP by mitochondria via a carrier-mediated process that involves the MTPPT (product of the SLC25A19 gene). Previous studies have characterized different physiological/biological aspects of the human MTPPT (hMTPPT), but less is known about structural features that are important for its function. Here, we used a protein-docking model ("Phyre2" and "DockingServer") to predict residues that may be important for function (substrate recognition) of the hMTPPT; we also examined the role of conserved positively-charged residues predicted ("PRALINE") to be in the trans-membrane domains (TMDs) in uptake of the negatively-charged TPP. Among the six residues predicted by the docking model (i.e., Thr(29), Arg(30), Ile(33), Ser(34), Asp(37) and Phe(298)), only Ile(33), Ser(34) and Asp(37) were found to be critical for function. While no change in translational efficiency/protein stability of the Ser(34) mutant was observed, both the Ile(33) and Asp(37) mutants showed a decrease in this parameter(s); there was also a decrease in the expression of the latter two mutants in mitochondria. A need for a polar residue at position 34 of the hMTPPT was evident. Our findings with the positively-charged residues (i.e., His(82), His(137), Lys(231) and Lys(291)) predicted in the TMD showed that His(137) and Lys(291) are important for function (via a role in proper delivery of the protein to mitochondria). These investigations provide important information about the structure-function relationship of the hMTPPT.
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Affiliation(s)
- Subrata Sabui
- Department of Medical Research, VA Medical Center, Long Beach, CA 90822, United States; Departments of Medicine and Physiology/Biophysics, University of California, Irvine, CA 92697, United States
| | - Veedamali S Subramanian
- Department of Medical Research, VA Medical Center, Long Beach, CA 90822, United States; Departments of Medicine and Physiology/Biophysics, University of California, Irvine, CA 92697, United States
| | - Rubina Kapadia
- Department of Medical Research, VA Medical Center, Long Beach, CA 90822, United States; Departments of Medicine and Physiology/Biophysics, University of California, Irvine, CA 92697, United States
| | - Hamid M Said
- Department of Medical Research, VA Medical Center, Long Beach, CA 90822, United States; Departments of Medicine and Physiology/Biophysics, University of California, Irvine, CA 92697, United States.
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3
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Chan T, Zheng J, Zhu L, Grewal T, Murray M, Zhou F. Putative Transmembrane Domain 6 of the Human Organic Anion Transporting Polypeptide 1A2 (OATP1A2) Influences Transporter Substrate Binding, Protein Trafficking, and Quality Control. Mol Pharm 2014; 12:111-9. [DOI: 10.1021/mp500459b] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Affiliation(s)
- Ting Chan
- Faculty
of Pharmacy, The University of Sydney, Sydney, NSW 2006, Australia
| | - Jian Zheng
- Faculty
of Pharmacy, The University of Sydney, Sydney, NSW 2006, Australia
- Alkali
Soil Natural Environmental Science Center, Northeast Forestry University/Key Laboratory of Saline-alkali Vegetation Ecology Restoration in Oil Field, Ministry of Education, Harbin 150040, China
| | - Ling Zhu
- Retinal
Therapeutics Research Group, Save Sight Institute, The University of Sydney, Sydney, NSW 2000, Australia
| | - Thomas Grewal
- Faculty
of Pharmacy, The University of Sydney, Sydney, NSW 2006, Australia
| | - Michael Murray
- Discipline
of Pharmacology, School of Medical Sciences, The University of Sydney, Sydney, NSW 2006, Australia
| | - Fanfan Zhou
- Faculty
of Pharmacy, The University of Sydney, Sydney, NSW 2006, Australia
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4
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Emami Riedmaier A, Nies AT, Schaeffeler E, Schwab M. Organic Anion Transporters and Their Implications in Pharmacotherapy. Pharmacol Rev 2012; 64:421-49. [DOI: 10.1124/pr.111.004614] [Citation(s) in RCA: 91] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
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5
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Burckhardt G, Burckhardt BC. In vitro and in vivo evidence of the importance of organic anion transporters (OATs) in drug therapy. Handb Exp Pharmacol 2011:29-104. [PMID: 21103968 DOI: 10.1007/978-3-642-14541-4_2] [Citation(s) in RCA: 136] [Impact Index Per Article: 10.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Organic anion transporters 1-10 (OAT1-10) and the urate transporter 1 (URAT1) belong to the SLC22A gene family and accept a huge variety of chemically unrelated endogenous and exogenous organic anions including many frequently described drugs. OAT1 and OAT3 are located in the basolateral membrane of renal proximal tubule cells and are responsible for drug uptake from the blood into the cells. OAT4 in the apical membrane of human proximal tubule cells is related to drug exit into the lumen and to uptake of estrone sulfate and urate from the lumen into the cell. URAT1 is the major urate-absorbing transporter in the apical membrane and is a target for uricosuric drugs. OAT10, also located in the luminal membrane, transports nicotinate with high affinity and interacts with drugs. Major extrarenal locations of OATs include the blood-brain barrier for OAT3, the placenta for OAT4, the nasal epithelium for OAT6, and the liver for OAT2 and OAT7. For all transporters we provide information on cloning, tissue distribution, factors influencing OAT abundance, interaction with endogenous compounds and different drug classes, drug/drug interactions and, if known, single nucleotide polymorphisms.
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Affiliation(s)
- Gerhard Burckhardt
- Abteilung Vegetative Physiologie und Pathophysiologie, Zentrum Physiologie und Pathophysiologie, Göttingen, Germany.
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6
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Shima JE, Komori T, Taylor TR, Stryke D, Kawamoto M, Johns SJ, Carlson EJ, Ferrin TE, Giacomini KM. Genetic variants of human organic anion transporter 4 demonstrate altered transport of endogenous substrates. Am J Physiol Renal Physiol 2010; 299:F767-75. [PMID: 20668102 DOI: 10.1152/ajprenal.00312.2010] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Apical reabsorption from the urine has been shown to be important for such processes as the maintenance of critical metabolites in the blood and the excretion of nephrotoxic compounds. The solute carrier (SLC) transporter OAT4 (SLC22A11) is expressed on the apical membrane of renal proximal tubule cells and is known to mediate the transport of a variety of xenobiotic and endogenous organic anions. Functional characterization of genetic variants of apical transporters thought to mediate reabsorption, such as OAT4, may provide insight into the genetic factors influencing the complex pathways involved in drug elimination and metabolite reclamation occurring in the kidney. Naturally occurring genetic variants of OAT4 were identified in public databases and by resequencing DNA samples from 272 individuals comprising 4 distinct ethnic groups. Nine total nonsynonymous variants were identified and functionally assessed using uptake of three radiolabeled substrates. A nonsense variant, R48Stop, and three other variants (R121C, V155G, and V155M) were found at frequencies of at least 2% in an ethnic group specific fashion. The L29P, R48Stop, and H469R variants displayed a complete loss of function, and kinetic analysis identified a reduced V(max) in the common nonsynonymous variants. Plasma membrane levels of OAT4 protein were absent or reduced in the nonfunctional variants, providing a mechanistic reason for the observed loss of function. Characterization of the genetic variants of reabsorptive transporters such as OAT4 is an important step in understanding variability in tubular reabsorption with important implications in innate homeostatic processes and drug disposition.
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Affiliation(s)
- James E Shima
- Dept. of Bioengineering and Therapeutics Sciences, University of California, San Francisco, California 94143-2911, USA
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7
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Zhou F, Zhu L, Cui PH, Church WB, Murray M. Functional characterization of nonsynonymous single nucleotide polymorphisms in the human organic anion transporter 4 (hOAT4). Br J Pharmacol 2009; 159:419-27. [PMID: 20015291 DOI: 10.1111/j.1476-5381.2009.00545.x] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023] Open
Abstract
BACKGROUND AND PURPOSE The human organic anion transporter (hOAT) family of transmembrane carrier proteins mediate the cellular flux of anionic substances, including certain hormones and anti-cancer drugs. hOAT4 is highly expressed at the apical membrane of the renal tubular cell and facilitates drug re-absorption in the kidney. In the present study, the impact of 10 nonsynonymous single nucleotide polymorphisms (SNPs) of hOAT4 on transport function in COS-7 cells was characterized. EXPERIMENTAL APPROACH Transport uptake assay was used to assess the function of the variant transporters. Cell surface biotinylation and western blot analysis were used to investigate the expression characteristics of the transporter proteins. Comparative modelling was used to interpret the influence of nonsynonymous changes in terms of hOAT4 structure. KEY RESULTS Four naturally occurring hOAT4 variants (L29P, R48Y, V155G and T392I) exhibited a significant loss of function. Substitution of leucine-29, which is a conserved residue in OATs, with a proline residue, impaired the synthesis or the apparent stability of the transporter and membrane insertion was disrupted in the R48Y variant. In the case of the V155G and T392I variants, impaired function was due to decreased affinity of the transporter for oestrone sulphate and impaired transporter-substrate turnover respectively. The T392I variant was inhibited more extensively than the wild-type transporter by the cationic substrate tetraethyl ammonium. CONCLUSIONS AND IMPLICATIONS Several naturally occurring SNPs encode variant hOAT4s that may impair the renal tubular re-absorption of important drug substrates.
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Affiliation(s)
- Fanfan Zhou
- Pharmacogenomics and Drug Development Laboratory, Faculty of Pharmacy, The University of Sydney, Sydney, NSW, Australia.
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8
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Zhou F, Zhu L, Cui PH, Church WB, Murray M. Functional characterization of nonsynonymous single nucleotide polymorphisms in the human organic anion transporter 4 (hOAT4). Br J Pharmacol 2009. [PMID: 20015291 DOI: 10.1111/j.1476-5381.2009.00545.xbph545] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023] Open
Abstract
BACKGROUND AND PURPOSE The human organic anion transporter (hOAT) family of transmembrane carrier proteins mediate the cellular flux of anionic substances, including certain hormones and anti-cancer drugs. hOAT4 is highly expressed at the apical membrane of the renal tubular cell and facilitates drug re-absorption in the kidney. In the present study, the impact of 10 nonsynonymous single nucleotide polymorphisms (SNPs) of hOAT4 on transport function in COS-7 cells was characterized. EXPERIMENTAL APPROACH Transport uptake assay was used to assess the function of the variant transporters. Cell surface biotinylation and western blot analysis were used to investigate the expression characteristics of the transporter proteins. Comparative modelling was used to interpret the influence of nonsynonymous changes in terms of hOAT4 structure. KEY RESULTS Four naturally occurring hOAT4 variants (L29P, R48Y, V155G and T392I) exhibited a significant loss of function. Substitution of leucine-29, which is a conserved residue in OATs, with a proline residue, impaired the synthesis or the apparent stability of the transporter and membrane insertion was disrupted in the R48Y variant. In the case of the V155G and T392I variants, impaired function was due to decreased affinity of the transporter for oestrone sulphate and impaired transporter-substrate turnover respectively. The T392I variant was inhibited more extensively than the wild-type transporter by the cationic substrate tetraethyl ammonium. CONCLUSIONS AND IMPLICATIONS Several naturally occurring SNPs encode variant hOAT4s that may impair the renal tubular re-absorption of important drug substrates.
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Affiliation(s)
- Fanfan Zhou
- Pharmacogenomics and Drug Development Laboratory, Faculty of Pharmacy, The University of Sydney, Sydney, NSW, Australia.
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9
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Srimaroeng C, Perry JL, Pritchard JB. Physiology, structure, and regulation of the cloned organic anion transporters. Xenobiotica 2008; 38:889-935. [PMID: 18668434 DOI: 10.1080/00498250801927435] [Citation(s) in RCA: 84] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
1. The transport of negatively charged drugs, xenobiotics, and metabolites by epithelial tissues, particularly the kidney, plays critical roles in controlling their distribution, concentration, and retention in the body. Thus, organic anion transporters (OATs) impact both their therapeutic efficacy and potential toxicity. 2. This review summarizes current knowledge of the properties and functional roles of the cloned OATs, the relationships between transporter structure and function, and those factors that determine the efficacy of transport. Such factors include plasma protein binding of substrates, genetic polymorphisms among the transporters, and regulation of transporter expression. 3. Clearly, much progress has been made in the decade since the first OAT was cloned. However, unresolved questions remain. Several of these issues--drug-drug interactions, functional characterization of newly cloned OATs, tissue differences in expression and function, and details of the nature and consequences of transporter regulation at genomic and intracellular sites--are discussed in the concluding Perspectives section.
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Affiliation(s)
- C Srimaroeng
- Laboratory of Pharmacology, Environmental Toxicology Program, National Institute of Environmental Health Sciences, NC 27709, USA
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10
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Rizwan AN, Burckhardt G. Organic anion transporters of the SLC22 family: biopharmaceutical, physiological, and pathological roles. Pharm Res 2007; 24:450-70. [PMID: 17245646 DOI: 10.1007/s11095-006-9181-4] [Citation(s) in RCA: 195] [Impact Index Per Article: 11.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/22/2006] [Accepted: 10/19/2006] [Indexed: 02/08/2023]
Abstract
The human organic anion transporters OAT1, OAT2, OAT3, OAT4 and URAT1 belong to a family of poly-specific transporters mainly located in kidneys. Selected OATs occur also in liver, placenta, and brain. OATs interact with endogenous metabolic end products such as urate and acidic neutrotransmitter metabolites, as well as with a multitude of widely used drugs, including antibiotics, antihypertensives, antivirals, anti-inflammatory drugs, diuretics and uricosurics. Thereby, OATs play an important role in renal drug elimination and have an impact on pharmacokinetics. In this review we focus on the interaction of human OATs with drugs. We report the affinities of human OATs for drug classes and compare the putative importance of individual OATs for renal drug excretion. The role of OATs as sites of drug-drug interaction and mediators cell toxicity, their gender-dependent regulation in health and diseased states, and the possible impact of single nucleotide polymorphisms are also dealt with.
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Affiliation(s)
- Ahsan N Rizwan
- Abteilung Vegetative Physiologie und Pathophysiologie, Bereich Humanmedizin, Georg-August-Universität Göttingen, Humboldtallee 23, 37073, Göttingen, Germany
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11
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Hagos Y, Stein D, Ugele B, Burckhardt G, Bahn A. Human renal organic anion transporter 4 operates as an asymmetric urate transporter. J Am Soc Nephrol 2007; 18:430-9. [PMID: 17229912 DOI: 10.1681/asn.2006040415] [Citation(s) in RCA: 181] [Impact Index Per Article: 10.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
Human organic anion transporter 4 (hOAT4) is located at the apical membrane of proximal tubule cells and involved in renal secretion and reabsorption of endogenous substances as well as many drugs and xenobiotics. This study reevaluated the physiologic role, transport mode, and driving forces of hOAT4. 6-Carboxyfluorescein (6-CF) uptake into HEK293 cells that stably expressed hOAT4 was saturable, resulting in a K(m) of 108 muM. 6-CF as well as [(3)H]estrone sulfate ([(3)H]ES) accumulation by HEK293-hOAT4 cells were abolished by ES, dehydroepiandrosterone sulfate, sulfinpyrazone, benzbromarone, and probenecid, whereas several OA, including p-aminohippurate (PAH), lactate, pyrazinoate, nicotinate, glutarate, and the diuretic hydrochlorothiazide (HCTZ) exhibited a slight or a NS inhibitory effect. PAH and glutarate are not taken up by HEK293-hOAT4 cells, but they trans-stimulated 6-CF and [(3)H]ES uptake, indicating an asymmetric interaction of hOAT4 with these substrates. In chloride-free medium, HEK293-hOAT4-mediated [(3)H]PAH efflux was almost abolished, whereas addition of ES restored it comparable to Ringer solution, consistent with a physiologic ES/PAH or PAH/Cl(-) exchange mode of hOAT4. Moreover, an acidification of the uptake medium increased 6-CF as well as [(3)H]ES uptake, which was reduced by nigericin, suggesting that hOAT4 also can operate as an OA/OH(-) exchanger. hOAT4 facilitates substantial uptake of [(14)C]urate, which was elevated 2.6-fold by intracellular HCTZ. Thus, hOAT4 is the long-postulated, low-affinity apical urate anion exchanger that facilitates HCTZ-associated hyperuricemia.
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Affiliation(s)
- Yohannes Hagos
- Zentrum für Physiologie und Pathophysiologie, Abt. Vegetative Physiologie und Pathophysiologie, Universität Göttingen, Humboldtallee 23, 37073 Göttingen, Germany.
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12
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Zhou F, You G. Molecular insights into the structure-function relationship of organic anion transporters OATs. Pharm Res 2006; 24:28-36. [PMID: 17103332 DOI: 10.1007/s11095-006-9144-9] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/2006] [Accepted: 08/02/2006] [Indexed: 12/27/2022]
Abstract
The organic anion transporter (OAT) family encoded by SLC22A mediates the absorption, distribution, and excretion of a diverse array of environmental toxins, and clinically important drugs, including anti-HIV therapeutics, anti-tumor drugs, antibiotics, anti-hypertensives, and anti-inflammatories, and therefore is critical for the survival of mammalian species. Several OATs have been identified: OAT1 (SLC22A6), OAT2 (SLC22A7), OAT3 (SLC22A8), OAT4 (SLC22A11), OAT5 (SLC22A19) OAT6 (SLC22A20) and URAT1 (SLC22A12). The expressions of these OATs have been detected in key organs such as kidney, liver, brain and placenta. OAT dysfunction in these organs may contribute to the renal, hepatic, neurological and fetal toxicity and diseases. In this review, we summarize, according to the work done by our laboratory as well as by others, the most updated molecular studies on these OAT members, especially on the aspect of their structure-function relationships. The functional roles of N-glycosylation, transmembrane domains and individual amino acids, cell surface assembly, as well as associating proteins will be discussed. In addition, we will show the recent analyses of coding region polymorphisms of OATs, which give us information on the genetic variants of OATs and their potential effects on OAT functions.
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Affiliation(s)
- Fanfan Zhou
- Department of Pharmaceutics, Rutgers, the State University of New Jersey, 160 Frelinghuysen Road, Piscataway, NJ 08854, USA
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13
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Perry JL, Dembla-Rajpal N, Hall LA, Pritchard JB. A three-dimensional model of human organic anion transporter 1: aromatic amino acids required for substrate transport. J Biol Chem 2006; 281:38071-9. [PMID: 17038320 PMCID: PMC1847411 DOI: 10.1074/jbc.m608834200] [Citation(s) in RCA: 69] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023] Open
Abstract
Organic anion transporters (OATs) play a critical role in the handling of endogenous and exogenous organic anions by excretory and barrier tissues. Little is known about the OAT three-dimensional structure or substrate/protein interactions involved in transport. In this investigation, a theoretical three-dimensional model was generated for human OAT1 (hOAT1) based on fold recognition to the crystal structure of the glycerol 3-phosphate transporter (GlpT) from Escherichia coli. GlpT and hOAT1 share several sequence motifs as major facilitator superfamily members. The structural hOAT1 model shows that helices 5, 7, 8, 10, and 11 surround an electronegative putative active site ( approximately 830A(3)). The site opens to the cytoplasm and is surrounded by three residues not previously examined for function (Tyr(230) (domain 5) and Lys(431) and Phe(438) (domain 10)). Effects of these residues on p-aminohippurate (PAH) and cidofovir transport were assessed by point mutations in a Xenopus oocyte expression system. Membrane protein expression was severely limited for the Y230A mutant. For the K431A and F438A mutants, [(3)H]PAH uptake was less than 30% of wild-type hOAT1 uptake after protein expression correction. Reduced V(max) values for the F438A mutant confirmed lower protein expression. In addition, the F438A mutant exhibited an increased affinity for cidofovir but was not significantly different for PAH. Differences in handling of PAH and cidofovir were also observed for the Y230F mutant. Little uptake was determined for cidofovir, whereas PAH uptake was similar to wild-type hOAT1. Therefore, the hOAT1 structural model has identified two new residues, Tyr(230) and Phe(438), which are important for substrate/protein interactions.
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Affiliation(s)
| | | | | | - John B. Pritchard
- To whom correspondence should be addressed: P. O. Box 12233, F1-03 Research Triangle Park, NC 27709. Tel.: 919-541-4054; Fax: 919-541-5737; E-mail:
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14
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Abstract
Organic anion transporters (OATs) play an essential role in the elimination of numerous endogenous and exogenous organic anions from the body. The renal OATs contribute to the excretion of many drugs and their metabolites that are important in clinical medicine. Several families of multispecific organic anion and cation transporters, including OAT family transporters, have recently been identified by molecular cloning. The OAT family consists of six isoforms (OAT1 - 4, URAT1, and rodent Oat5) and they are all expressed in the kidney, while some are also expressed in the liver, brain, and placenta. The OAT family represents mainly the renal secretory and reabsorptive pathway for organic anions and is also involved in the distribution of organic anions in the body, drug-drug interactions, and toxicity of anionic substances such as nephrotoxic drugs and uremic toxins. In this review, current knowledge of and recent progress in the understanding of several aspects of OAT family members are discussed.
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Affiliation(s)
- Naohiko Anzai
- Department of Pharmacology and Toxicology, Kyorin University School of Medicine, Tokyo, Japan
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15
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Wu SN, Chang HD. Diethyl pyrocarbonate, a histidine-modifying agent, directly stimulates activity of ATP-sensitive potassium channels in pituitary GH(3) cells. Biochem Pharmacol 2005; 71:615-23. [PMID: 16375866 DOI: 10.1016/j.bcp.2005.11.016] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/26/2005] [Revised: 11/12/2005] [Accepted: 11/16/2005] [Indexed: 10/25/2022]
Abstract
The ATP-sensitive K(+) (K(ATP)) channels are composed of sulfonylurea receptor and inwardly rectifying K(+) channel (Kir6.2) subunit. These channels are regulated by intracellular ADP/ATP ratio and play a role in cellular metabolism. Diethyl pyrocarbonate (DEPC), a histidine-specific alkylating reagent, is known to modify the histidine residues of the structure of proteins. The objective of this study was to determine whether DEPC modifies K(ATP)-channel activity in pituitary GH(3) cells. Steady-state fluctuation analyses of macroscopic K(+) current at -120 mV produced power spectra that could be fitted with a single Lorentzian curve in these cells. The time constants in the presence of DEPC were increased. Consistent with fluctuation analyses, the mean open time of K(ATP)-channels was significantly increased during exposure to DEPC. However, DEPC produced no change in single-channel conductance, despite the ability of this compound to enhance K(ATP)-channel activity in a concentration-dependent manner with an EC(50) value of 16 microM. DEPC-stimulated K(ATP)-channel activity was attenuated by pretreatment with glibenclamide. In current-clamp configuration, DEPC decreased the firing of action potentials in GH(3) cells. A further application of glibenclamide reversed DEPC-induced inhibition of spontaneous action potentials. Intracellullar Ca(2+) measurements revealed the ability of DEPC to decrease Ca(2+) oscillations in GH(3) cells. Simulation studies also demonstrated that the increased conductance of K(ATP)-channels used to mimic DEPC actions reduced the frequency of spontaneous action potentials and fluctuation of intracellular Ca(2+). The results indicate that chemical modification with DEPC enhances K(ATP)-channel activity and influences functional activities of pituitary GH(3) cells.
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Affiliation(s)
- Sheng-Nan Wu
- Department of Physiology, National Cheng Kung University Medical College, No 1, University Road, Tainan 70101, Taiwan.
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16
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Shigaki T, Barkla BJ, Miranda-Vergara MC, Zhao J, Pantoja O, Hirschi KD. Identification of a crucial histidine involved in metal transport activity in the Arabidopsis cation/H+ exchanger CAX1. J Biol Chem 2005; 280:30136-42. [PMID: 15994298 DOI: 10.1074/jbc.m503610200] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
In plants, yeast, and bacteria, cation/H+ exchangers (CAXs) have been shown to translocate Ca2+ and other metal ions utilizing the H+ gradient. The best characterized of these related transporters is the plant vacuolar localized CAX1. We have used site-directed mutagenesis to assess the impact of altering the seven histidine residues to alanine within Arabidopsis CAX1. The mutants were expressed in a Saccharomyces cerevisiae strain that is sensitive to Ca2+ and other metals. By utilizing a yeast growth assay, the H338A mutant was the only mutation that appeared to alter Ca2+ transport activity. The CAX1 His338 residue is conserved among various CAX transporters and may be located within a filter for cation selection. We proceeded to mutate His338 to every other amino acid residue and utilized yeast growth assays to estimate the transport properties of the 19 CAX mutants. Expression of 16 of these His338 mutants could not rescue any of the metal sensitivities. However, expression of H338N, H338Q, and H338K allowed for some growth on media containing Ca2+. Most interestingly, H338N exhibited increased tolerance to Cd2+ and Zn2+. Endomembrane fractions from yeast cells were used to measure directly the transport of H338N. Although the H338N mutant demonstrated 25% of the wild type Ca2+/H+ transport, it showed an increase in transport for both Cd2+ and Zn2+ reflected in a decrease in the Km for these substrates. This study provides insights into the CAX cation filter and novel mechanisms by which metals may be partitioned across membranes.
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Affiliation(s)
- Toshiro Shigaki
- Plant Physiology Laboratories, United State Department of Agriculture/Agricultural Research Service Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, Texas 77030, USA
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