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Kekuda R, Leibach FH, Furesz TC, Smith CH, Ganapathy V. Polarized distribution of interleukin-1 receptors and their role in regulation of serotonin transporter in placenta. J Pharmacol Exp Ther 2000; 292:1032-41. [PMID: 10688620] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/15/2023] Open
Abstract
We investigated the expression of interleukin-1 (IL-1) receptors and their involvement in the regulation of the serotonin transporter gene expression in human placenta. IL-1beta is an activator of the serotonin transporter gene expression in JAR human placental choriocarcinoma cells as demonstrated by an increase in the steady-state levels of the transporter mRNA and in serotonin transport activity. This activation is blocked by IL-1 receptor antagonist. Genistein also blocks the effect of IL-1beta, indicating involvement of tyrosine phosphorylation in the process. Treatment of JAR cells with IL-1beta activates mitogen-activated protein kinases and nuclear factor-kappaB. The nuclear factor-kappaB that is responsive to IL-1beta in these cells is the p65 homodimer. Northern blot analysis and reverse transcription-polymerase chain reaction revealed that JAR cells and human placenta express type I and type II IL-1 receptors. The binding sites for (125)I-IL-1beta are localized predominantly in the maternal-facing brush border membrane of the syncytiotrophoblast. These results show that IL-1 in the maternal circulation is likely to play a critical role in the regulation of the serotonin transporter gene expression in the placenta.
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Brust P, Friedrich A, Krizbai IA, Bergmann R, Roux F, Ganapathy V, Johannsen B. Functional expression of the serotonin transporter in immortalized rat brain microvessel endothelial cells. J Neurochem 2000; 74:1241-8. [PMID: 10693957 DOI: 10.1046/j.1471-4159.2000.741241.x] [Citation(s) in RCA: 46] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
There is evidence from recent studies that the brain endothelium (of capillaries and/or larger vessels) may serve as a specific target for serotonin [5-hydroxytryptamine (5-HT)]. This neurotransmitter is expected to be involved in the regulation of the blood-brain barrier (BBB) permeability and/or of the cerebral blood flow via receptor-mediated mechanisms. Effective control of these processes depends on a speedy uptake and metabolism of released 5-HT molecules. To realize this, a similar mechanism of 5-HT uptake as in brain may exist at the BBB. In this study, we have demonstrated using RT-PCR that 5-HT transporter mRNA is present in the brain endothelium and that a saturable transport system for 5-HT is functionally expressed in immortalized rat brain endothelial cells (RBE4 cells). These cells take up [3H]5-HT by an active saturable process with a Km value of 397 +/- 64 nmol/L and a transport capacity of 51.7 +/- 3.5 pmol x g(-1) x min(-1). The 5-HT uptake depends on Na+, as indicated by the replacement of NaCl by LiCl. The 5-HT uptake was sensitive to specific 5-HT transport inhibitors such as paroxetine, clomipramine, fluoxetine, and citalopram but not to inhibitors of the vesicular amine transporter such as reserpine or tetrabenazine. Our results demonstrate that cerebral endothelial cells are able to participate actively in the removal and metabolism of the released 5-HT, which supports the concept of direct serotoninergic regulation of the BBB function.
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78
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Ganapathy ME, Huang W, Rajan DP, Carter AL, Sugawara M, Iseki K, Leibach FH, Ganapathy V. beta-lactam antibiotics as substrates for OCTN2, an organic cation/carnitine transporter. J Biol Chem 2000; 275:1699-707. [PMID: 10636865 DOI: 10.1074/jbc.275.3.1699] [Citation(s) in RCA: 138] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Therapeutic use of cephaloridine, a beta-lactam antibiotic, in humans is associated with carnitine deficiency. A potential mechanism for the development of carnitine deficiency is competition between cephaloridine and carnitine for the renal reabsorptive process. OCTN2 is an organic cation/carnitine transporter that is responsible for Na(+)-coupled transport of carnitine in the kidney and other tissues. We investigated the interaction of several beta-lactam antibiotics with OCTN2 using human cell lines that express the transporter constitutively as well as using cloned human and rat OCTN2s expressed heterologously in human cell lines. The beta-lactam antibiotics cephaloridine, cefoselis, cefepime, and cefluprenam were found to inhibit OCTN2-mediated carnitine transport. These antibiotics possess a quaternary nitrogen as does carnitine. Several other beta-lactam antibiotics that do not possess this structural feature did not interact with OCTN2. The interaction of cephaloridine with OCTN2 is competitive with respect to carnitine. Interestingly, many of the beta-lactam antibiotics that were not recognized by OCTN2 were good substrates for the H(+)-coupled peptide transporters PEPT1 and PEPT2. In contrast, cephaloridine, cefoselis, cefepime, and cefluprenam, which were recognized by OCTN2, did not interact with PEPT1 and PEPT2. The interaction of cephaloridine with OCTN2 was Na(+)-dependent, whereas the interaction of cefoselis and cefepime with OCTN2 was largely Na(+)-independent. Furthermore, the Na(+)-dependent, OCTN2-mediated cellular uptake of cephaloridine could be demonstrated by direct uptake measurements. These studies show that OCTN2 plays a crucial role in the pharmacokinetics and therapeutic efficacy of certain beta-lactam antibiotics such as cephaloridine and that cephaloridine-induced carnitine deficiency is likely to be due to inhibition of carnitine reabsorption in the kidney.
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79
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Rajan DP, Kekuda R, Huang W, Devoe LD, Leibach FH, Prasad PD, Ganapathy V. Cloning and functional characterization of a Na(+)-independent, broad-specific neutral amino acid transporter from mammalian intestine. BIOCHIMICA ET BIOPHYSICA ACTA 2000; 1463:6-14. [PMID: 10631289 DOI: 10.1016/s0005-2736(99)00224-2] [Citation(s) in RCA: 73] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
We have isolated a cDNA from a rabbit intestinal cDNA library which, when co-expressed with the heavy chain of the human 4F2 antigen (4F2hc) in mammalian cells, induces system L-like amino acid transport activity. This protein, called LAT2, consists of 535 amino acids and is distinct from LAT1 which also interacts with 4F2hc to induce system L-like amino acid transport activity. LAT2 does not interact with rBAT, a protein with a significant structural similarity to 4F2hc. The 4F2hc/LAT2-mediated transport process differs from the 4F2hc/LAT1-mediated transport in substrate specificity, substrate affinity, tissue distribution, interaction with D-amino acids, and pH-dependence. The 4F2hc/LAT2-associated transport process has a broad specificity towards neutral amino acids with K(t) values in the range of 100-1000 microM, does not interact with D-amino acids to any significant extent, and is stimulated by acidic pH. In contrast, the 4F2hc/LAT1-associated transport process has a narrower specificity towards neutral amino acids, but with comparatively higher affinity (K(t) values in the range of 10-20 microM), interacts with some D-amino acids with high affinity, and is not influenced by pH. LAT2 is expressed primarily in the small intestine and kidney, whereas LAT1 exhibits a much broader tissue distribution.
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80
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Kashem M, Ganapathy V, Jasmon G. Network reconfiguration for enhancement of voltage stability in distribution networks. ACTA ACUST UNITED AC 2000. [DOI: 10.1049/ip-gtd:20000302] [Citation(s) in RCA: 72] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
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81
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Ogonowski AA, Kaesemeyer WH, Jin L, Ganapathy V, Leibach FH, Caldwell RW. Effects of NO donors and synthase agonists on endothelial cell uptake of L-Arg and superoxide production. Am J Physiol Cell Physiol 2000; 278:C136-43. [PMID: 10644521 DOI: 10.1152/ajpcell.2000.278.1.c136] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
It is commonly believed that the activity of NO synthase (NOS) solely controls NO production from its substrates, L-Arg and O(2). The Michaelis-Menten constant (K(m)) of NOS for L-Arg is in the micromolar range; cellular levels of L-Arg are much higher. However, evidence strongly suggests that cellular supply of L-Arg may become limiting and lead to reduced NO and increased superoxide anion (O(-)(2)*) formation, promoting cardiovascular dysfunction. Uptake of L-Arg into cells occurs primarily (approximately 85%) through the actions of a Na(+)-independent, carrier-mediated transporter (system y(+)). We have examined the effects of NOS agonists (substance P, bradykinin, and ACh) and NO donors (S-nitroso-N-acetyl-penicillamine and dipropylenetriamine NONOate) on transport of L-Arg into bovine aortic endothelial cells (BAEC). Our results demonstrate that NOS agonists increase y(+) transporter activity. A rapidly acting NO donor initially increases L-Arg uptake; however, after longer exposure, L-Arg uptake is suppressed. Exposure of BAEC without L-Arg to substance P and a Ca(2+) ionophore (A-23187) increased O(-)(2)* formation, which was blocked with concurrent presence of L-Arg or the NOS antagonist N(omega)-nitro-L-arginine methyl ester. We conclude that factors including NO itself control y(+) transport function and the production of NO and O(-)(2)*.
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Seth P, Wu X, Huang W, Leibach FH, Ganapathy V. Mutations in novel organic cation transporter (OCTN2), an organic cation/carnitine transporter, with differential effects on the organic cation transport function and the carnitine transport function. J Biol Chem 1999; 274:33388-92. [PMID: 10559218 DOI: 10.1074/jbc.274.47.33388] [Citation(s) in RCA: 67] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Novel organic cation transporter (OCTN2) is an organic cation/carnitine transporter, and two missense mutations, L352R and P478L, in OCTN2 have been identified as the cause for primary carnitine deficiency. In the present study, we assessed the influence of these two mutations on the carnitine transport function and the organic cation transport function of OCTN2. The L352R mutation resulted in a complete loss of both transport functions. In contrast, the P478L mutation resulted in a complete loss of only the carnitine transport function but significantly stimulated the organic cation transport function. Studies with human OCTN2/rat OCTN2 chimeric transporters indicated that the carnitine transport site and the organic cation transport site were not identical. Because carnitine transport is Na(+)-dependent whereas organic cation transport is Na(+)-independent, we investigated the possibility that the P478L mutation affected Na(+) binding. The Na(+) activation kinetics were found to be similar for the P478L mutant and wild type OCTN2. We then mutated nine different tyrosine residues located in or near transmembrane domains and assessed the transport function of these mutants. One of these mutations, Y211F, was found to have differential influence on the two transport activities of OCTN2 as did the P478L mutation. However, the Na(+) activation kinetics were not affected. These findings are of clinical relevance to patients with primary carnitine deficiency because whereas each and every mutation in these patients is expected to result in the loss of the carnitine transport function, all of these mutations may not interfere with the organic cation transport function.
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83
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Wang H, Dutta B, Huang W, Devoe LD, Leibach FH, Ganapathy V, Prasad PD. Human Na(+)-dependent vitamin C transporter 1 (hSVCT1): primary structure, functional characteristics and evidence for a non-functional splice variant. BIOCHIMICA ET BIOPHYSICA ACTA 1999; 1461:1-9. [PMID: 10556483 DOI: 10.1016/s0005-2736(99)00182-0] [Citation(s) in RCA: 116] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
We report here on the cloning and functional characterization of human Na(+)-dependent vitamin C transporter 1 (SVCT1). The human SVCT1 cDNA, obtained from a Caco2 cell cDNA library, encodes a protein of 598 amino acids with 12 putative transmembrane domains. The SVCT1-specific transcript, 2.4 kb in size, is expressed in kidney, liver, small intestine, thymus and prostate. When expressed heterologously in HRPE cells, SVCT1 mediates the transport of ascorbate, the reduced form of vitamin C, in a Na(+)-dependent manner. The transporter is specific for ascorbate with a K(t) of approximately 75 microM. The relationship between the cDNA-specific uptake rate of ascorbate and Na(+) concentration is sigmoidal with a Na(+):ascorbate stoichiometry of 2:1, indicating that the transport process is electrogenic. In Caco2 cells and in normal human intestine, SVCT1 also exists as a non-functional splice variant with a four amino acid sequence inserted between E-155 and V-156. The splice variant results from the use of a donor site 12 bp downstream of the normal donor site.
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84
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Dutta B, Huang W, Molero M, Kekuda R, Leibach FH, Devoe LD, Ganapathy V, Prasad PD. Cloning of the human thiamine transporter, a member of the folate transporter family. J Biol Chem 1999; 274:31925-9. [PMID: 10542220 DOI: 10.1074/jbc.274.45.31925] [Citation(s) in RCA: 139] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
We have isolated a cDNA from human placenta, which, when expressed heterologously in mammalian cells, mediates the transport of the water-soluble vitamin thiamine. The cDNA codes for a protein of 497 amino acids containing 12 putative transmembrane domains. Northern blot analysis indicates that this transporter is widely expressed in human tissues. When expressed in HeLa cells, the cDNA induces the transport of thiamine (K(t) = 2.5 +/- 0.6 microM) in a Na(+)-independent manner. The cDNA-mediated transport of thiamine is stimulated by an outwardly directed H(+) gradient. Substrate specificity assays indicate that the transporter is specific to thiamine. Even though thiamine is an organic cation, the cDNA-induced thiamine transport is not inhibited by other organic cations. Similarly, thiamine is not a substrate for the known members of mammalian organic cation transporter family. The thiamine transporter gene, located on human chromosome 1q24, consists of 6 exons and is most likely the gene defective in the metabolic disorder, thiamine-responsive megaloblastic anemia. At the level of amino acid sequence, the thiamine transporter is most closely related to the reduced-folate transporter and thus represents the second member of the folate transporter family.
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85
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Bröer A, Brookes N, Ganapathy V, Dimmer KS, Wagner CA, Lang F, Bröer S. The astroglial ASCT2 amino acid transporter as a mediator of glutamine efflux. J Neurochem 1999; 73:2184-94. [PMID: 10537079] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/14/2023]
Abstract
Glutamine release from astrocytes is an essential part of the glutamate-glutamine cycle in the brain. Uptake of glutamine into cultured rat astrocytes occurs by at least four different routes. In agreement with earlier studies, a significant contribution of amino acid transport systems ASC, A, L, and N was detected. It has not been determined whether these systems are also involved in glutamine efflux or whether specific efflux transporters exist. We show here that ASCT2, a variant of transport system ASC, is strongly expressed in rat astroglia-rich primary cultures but not in neuron-rich primary cultures. The amino acid sequence of rat astroglial ASCT2 is 83% identical to that of mouse ASCT2. In Xenopus laevis oocytes expressing rat ASCT2, we observed high-affinity uptake of [U-14C]glutamine (Km = 70 microM) that was Na(+)-dependent, concentrative, and unaffected by membrane depolarization. When oocytes were preloaded with [U-14C]glutamine, no glutamine efflux was detected in the absence of extracellular amino acids. Neither lowering intracellular pH nor raising the temperature elicited efflux. However, addition of 0.1 mM unlabeled alanine, serine, cysteine, threonine, glutamine, or leucine to the extracellular solution resulted in a rapid release of glutamine from the ASCT2-expressing oocytes. Amino acids that are not recognized as substrates by ASCT2 were ineffective in this role. Extracellular glutamate stimulated glutamine release weakly at pH 7.5 but was more effective on lowering pH to 5.5, consistent with the pH dependence of ASCT2 affinity for glutamate. Our findings suggest a significant role of ASCT2 in glutamine efflux from astrocytes by obligatory exchange with extracellular amino acids. However, the relative contribution of this pathway to glutamine release from cells in vivo or in vitro remains to be determined.
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86
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George RL, Wu X, Huang W, Fei YJ, Leibach FH, Ganapathy V. Molecular cloning and functional characterization of a polyspecific organic anion transporter from Caenorhabditis elegans. J Pharmacol Exp Ther 1999; 291:596-603. [PMID: 10525077] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/14/2023] Open
Abstract
We have cloned a polyspecific organic anion transporter from Caenorhabditis elegans and elucidated its functional characteristics. The C. elegans anion transporter (CeOAT1) codes for a protein of 526 amino acids containing 12 putative transmembrane domains. It exhibits significant homology at the level of amino acid sequence to the C. elegans organic cation transporter and to the mammalian organic cation and anion transporters. The function of CeOAT1 was investigated by expressing the transporter heterologously in mammalian cells. CeOAT1 transports p-aminohippurate (PAH) in a Na(+)-independent manner. The transport mechanism appears to involve anion exchange because CeOAT1-mediated PAH transport is stimulated by a cell-to-medium concentration gradient of alpha-ketoglutarate or fumarate generated by coexpression in the cells of a mammalian Na(+)-coupled dicarboxylate transporter. CeOAT1 exhibits broad specificity, accepting anions such as folate, indomethacin, furosemide, probenecid, and benzylpenicillin as substrates. The Michaelis-Menten constant for the prototypical organic anion PAH is 0.43+/-0.07 mM. This constitutes the first report of the molecular and functional identification of a polyspecific organic anion transporter in C. elegans.
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87
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Huang W, Shaikh SN, Ganapathy ME, Hopfer U, Leibach FH, Carter AL, Ganapathy V. Carnitine transport and its inhibition by sulfonylureas in human kidney proximal tubular epithelial cells. Biochem Pharmacol 1999; 58:1361-70. [PMID: 10487540 DOI: 10.1016/s0006-2952(99)00219-1] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
The kidney plays an important role in the homeostasis of carnitine by its ability to reabsorb carnitine almost completely from the glomerular filtrate. The transport process responsible for this reabsorption has been investigated thus far only in laboratory animals. Here we report on the characteristics of carnitine uptake in a proximal tubular epithelial cell line derived from human kidney. The uptake process was found to be obligatorily dependent on Na+ with no involvement of anions. The process was saturable, with a Michaelis-Menten constant of 14 +/- 1 microM. The Na+:carnitine stoichiometry was 1:1. The same process also was found to be responsible for the uptake of acetylcarnitine and propionylcarnitine, two acyl esters of carnitine with potential for therapeutic use in humans. The uptake process was specific for carnitine and its acyl esters. Betaine, a structural analog of carnitine, interacted with the uptake process to a significant extent. The present studies also showed that sulfonylureas, oral hypoglycemic agents currently used in the management of type 2 diabetes, inhibited the carnitine uptake system. Among the sulfonylureas tested, glibenclamide was the most potent inhibitor. The inhibition was competitive. Glibenclamide inhibited the uptake not only of carnitine but also of acetylcarnitine and propionylcarnitine. The inhibition most likely was the result of direct interaction of the compound with the carnitine transporter because the inhibition could be demonstrated in purified rat kidney brush border membrane vesicles.
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88
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Rajan DP, Kekuda R, Huang W, Wang H, Devoe LD, Leibach FH, Prasad PD, Ganapathy V. Cloning and expression of a b(0,+)-like amino acid transporter functioning as a heterodimer with 4F2hc instead of rBAT. A new candidate gene for cystinuria. J Biol Chem 1999; 274:29005-10. [PMID: 10506149 DOI: 10.1074/jbc.274.41.29005] [Citation(s) in RCA: 48] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We have cloned a transporter protein from rabbit small intestine, which, when coexpressed with the 4F2 heavy chain (4F2hc) in mammalian cells, induces a b(0,+)-like amino acid transport activity. This protein (4F2-lc6 for the sixth member of the 4F2 light chain family) consists of 487 amino acids and has 12 putative transmembrane domains. At the level of amino acid sequence, 4F2-lc6 shows significant homology (44% identity) to the other five known members of the 4F2 light chain family, namely LAT1 (4F2-lc1), y(+)LAT1 (4F2-lc2), y(+)LAT2 (4F2-lc3), xCT (4F2-lc4), and LAT2 (4F2-lc5). The 4F2hc/4F2-lc6 complex-mediated transport process is Na(+)-independent and exhibits high affinity for neutral and cationic amino acids and cystine. These characteristics are similar to those of the b(0,+)-like amino acid transport activity previously shown to be associated with rBAT (protein related to b(0,+) amino acid transport system). However, the newly cloned 4F2-lc6 does not interact with rBAT. This is the first report of the existence of a b(0,+)-like amino acid transport process that is independent of rBAT. 4F2-lc6 is expressed predominantly in the small intestine and kidney. Based on the characteristics of the transport process mediated by the 4F2hc/4F2-lc6 complex and the expression pattern of 4F2-lc6 in mammalian tissues, we suggest that 4F2-lc6 is a new candidate gene for cystinuria.
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89
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Rajan DP, Huang W, Dutta B, Devoe LD, Leibach FH, Ganapathy V, Prasad PD. Human placental sodium-dependent vitamin C transporter (SVCT2): molecular cloning and transport function. Biochem Biophys Res Commun 1999; 262:762-8. [PMID: 10471399 DOI: 10.1006/bbrc.1999.1272] [Citation(s) in RCA: 134] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
We report here on the cloning and functional characterization of human SVCT2, a sodium-dependent vitamin C (ascorbate) transporter. The hSVCT2 cDNA obtained from a human placental choriocarcinoma cell cDNA library, codes for a protein of 650 amino acids with a predicted molecular mass of 70 kDa. At the level of amino acid sequence, the human SVCT2 exhibits 95% identity to its rat homolog. When functionally expressed in mammalian cells, hSVCT2 induces the transport of ascorbic acid. The transport process induced by hSVCT2 is Na(+)-dependent and is specific for ascorbate. The Michaelis-Menton constant (K(t)) for the transport of ascorbate in cDNA-transfected cells is 69 +/- 5 microM. The relationship between the cDNA-specific uptake rate of ascorbate and Na(+) concentration is sigmoidal with a Na(+):ascorbate stoichiometry of 2:1. Northern blot analysis shows that SVCT2-specific transcripts are present in heart, brain, placenta, and liver and is absent in lung and skeletal muscle. The size of the principal transcript is approximately 7.5 kb.
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90
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Wu X, Huang W, Prasad PD, Seth P, Rajan DP, Leibach FH, Chen J, Conway SJ, Ganapathy V. Functional characteristics and tissue distribution pattern of organic cation transporter 2 (OCTN2), an organic cation/carnitine transporter. J Pharmacol Exp Ther 1999; 290:1482-92. [PMID: 10454528] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/13/2023] Open
Abstract
We have demonstrated in the present study that novel organic cation transporter (OCTN) 2 is a transporter for organic cations as well as carnitine. OCTN2 transports organic cations without involving Na(+), but it transports carnitine only in the presence of Na(+). The ability to transport organic cations and carnitine is demonstrable with human, rat, and mouse OCTN2s. Na(+) does not influence the affinity of OCTN2 for organic cations, but it increases the affinity severalfold for carnitine. The short-chain acyl esters of carnitine are also transported by OCTN2. Two mutations, M352R and P478L, in human OCTN2 are associated with loss of transport function, but the protein expression of these mutants is comparable to that of the wild-type human OCTN2. In situ hybridization in the rat shows that OCTN2 is expressed in the proximal and distal tubules and in the glomeruli in the kidney, in the myocardium, valves, and arterioles in the heart, in the labyrinthine layer of the placenta, and in the cortex, hippocampus, and cerebellum in the brain. This is the first report that OCTN2 is a Na(+)-independent organic cation transporter as well as a Na(+)-dependent carnitine transporter and that OCTN2 is expressed not only in the heart, kidney, and placenta but also in the brain.
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91
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Fujita T, Kishida T, Okada N, Ganapathy V, Leibach FH, Yamamoto A. Interaction of kyotorphin and brain peptide transporter in synaptosomes prepared from rat cerebellum: implication of high affinity type H+/peptide transporter PEPT2 mediated transport system. Neurosci Lett 1999; 271:117-20. [PMID: 10477116 DOI: 10.1016/s0304-3940(99)00540-6] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
High-affinity type H+/peptide cotransporter PEPT2 is preferentially expressed in the kidney, and is responsible for reabsorption of di- and tripeptides in epithelial tubules. Interestingly, PEPT2 has been recently cloned from rat brain. However, there is very little information available on the peptide transporter activity in the brain. In the present study, we investigated the interaction of kyotorphin (L-tyrosyl-L-arginine) with the peptide transporter using synaptosomes prepared from rat cerebellum. The activity of the peptide transporter was assessed by measuring the uptake of radiolabeled glycyl-sarcosine (Gly-Sar), which is a prototypical substrate for the peptide transporter, in the presence of H+-gradient. Kyotorphin competitively inhibited the uptake of Gly-Sar with an inhibitory constant (Ki) of 30 +/- 4 microM in rat cerebellum synaptosomes. This uptake property is very close to that of PEPT2. Carnosine (beta-alanyl-L-histidine) also inhibited the uptake of Gly-Sar, on the other hand, TRH did not interact with the peptide transporter. RT-PCR using specific primers revealed that PEPT2 mRNA exists in cerebellum in rat. Taken collectively, these results indicate that the functional peptide transport system in rat cerebellum might be the high affinity transporter PEPT2.
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92
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Fujita T, Majikawa Y, Umehisa S, Okada N, Yamamoto A, Ganapathy V, Leibach FH. sigma Receptor ligand-induced up-regulation of the H(+)/peptide transporter PEPT1 in the human intestinal cell line Caco-2. Biochem Biophys Res Commun 1999; 261:242-6. [PMID: 10425172 DOI: 10.1006/bbrc.1999.1026] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
We determined the effects of (+)pentazocine, a selective sigma(1) ligand, on the uptake of glycylsarcosine (Gly-Sar) in the human intestinal cell line Caco-2 which expresses the low affinity/high capacity peptide transporter PEPT1. Confluent Caco-2 cells were treated with various concentrations of (+)pentazocine for desired time (mostly 24 hr). The activity of PEPT1 was assessed by measuring the uptake of [(14)C]Gly-Sar in the presence of a H(+) gradient. (+)Pentazocine increased the uptake of [(14)C]Gly-Sar mediated by PEPT1 in a concentration- and time-dependent manner. Kinetic analyses have indicated that (+)pentazocine increased the maximal velocity (V(max)) for Gly-Sar uptake in Caco-2 cells without affecting the Michaelis-Menten constant (K(t)). In addition, semi-quantitative RT-PCR revealed that treatment of (+)pentazocine increased PEPT1 mRNA in Caco-2 cells in a concentration-dependent manner. These data suggest that sigma(1) receptor ligand (+)pentazocine up-regulates PEPT1 in Caco-2 cells at the level of increased mRNA, causing an increase in the density of the transporter protein in the cell membrane.
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93
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Kenney SP, Kekuda R, Prasad PD, Leibach FH, Devoe LD, Ganapathy V. Cannabinoid receptors and their role in the regulation of the serotonin transporter in human placenta. Am J Obstet Gynecol 1999; 181:491-7. [PMID: 10454705 DOI: 10.1016/s0002-9378(99)70583-1] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
OBJECTIVE We sought to investigate the expression of cannabinoid receptors in human placenta and BeWo choriocarcinoma cells and study their role in the regulation of the serotonin transporter. STUDY DESIGN Expression of the 2 types of cannabinoid receptors (CB1 and CB2) in human placenta and BeWo cells was investigated by reverse transcriptase-polymerase chain reaction and Northern blot analysis. The involvement of the receptors in the regulation of the serotonin transporter expression was studied by using a cannabinoid receptor agonist (WIN 55212-2). BeWo cells were treated with the agonist in the presence or absence of forskolin, and the serotonin transporter activity was measured by assessing paroxetine-sensitive serotonin transport. Serotonin transporter density in cell membranes was monitored by measuring paroxetine-sensitive binding of RTI-55, a specific high-affinity ligand for the transporter. Agonist-induced changes in intracellular levels of cyclic adenosine monophosphate were also monitored. RESULTS Reverse transcriptase-polymerase chain reaction and Northern blot analysis demonstrated unequivocally that human placenta and BeWo cells express both types of cannabinoid receptors. Treatment of BeWo cells with the receptor agonist blocked the activity of the constitutive, as well as the forskolin-induced, serotonin transporter without affecting the serotonin transporter density. This effect is not mediated by alterations in intracellular cyclic adenosine monophosphate levels. CONCLUSION The results show that cannabinoid receptors are expressed in human placenta and BeWo cells and play a role in the regulation of the serotonin transporter activity. Human placenta is therefore a direct target for cannabinoids, and marijuana use during pregnancy is likely to affect the placental clearance of serotonin through the serotonin transporter.
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94
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Ganapathy V, Prasad PD, Ganapathy ME, Leibach FH. Drugs of abuse and placental transport. Adv Drug Deliv Rev 1999; 38:99-110. [PMID: 10837749 DOI: 10.1016/s0169-409x(99)00009-5] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
The placenta provides the only link between the mother and the developing fetus. The function of the placenta as a transport organ is obligatory for fetal development because this process, mediated by a variety of transport systems, is responsible for the delivery of nutrients from the mother to the fetus. Some of the transport systems in the placenta also play a role in the clearance of vasoactive compounds, thus maintaining optimal blood flow to this organ. There is strong supporting evidence to indicate that several of these placental transport systems are either direct or indirect targets for the abusable drugs cocaine, amphetamines, nicotine, and cannabinoids. These drugs of abuse compromise the placental transport function and consequently produce detrimental effects on the developing fetus.
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95
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Prasad PD, Wang H, Huang W, Fei YJ, Leibach FH, Devoe LD, Ganapathy V. Molecular and functional characterization of the intestinal Na+-dependent multivitamin transporter. Arch Biochem Biophys 1999; 366:95-106. [PMID: 10334869 DOI: 10.1006/abbi.1999.1213] [Citation(s) in RCA: 102] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
We have cloned a Na+-dependent multivitamin transporter from rabbit intestine (riSMVT). The cDNA codes for a protein of 636 amino acids with 12 putative transmembrane domains. When expressed in mammalian cells, the cDNA induces Na+-dependent uptake of the vitamins pantothenate and biotin. Lipoate is also a substrate for the cDNA-induced uptake process. The affinity constant for the cDNA-specific transport of pantothenate and biotin is approximately 2 and approximately 8 microM, respectively. The Na+:vitamin stoichiometry is greater than 1, indicating that the transport process is electrogenic. The SMVT-specific transcripts of 3.2 kbp are equally distributed throughout the small intestine. We have also cloned SMVT from the human intestinal cell line Caco-2. The Caco-2 SMVT cDNA codes for a protein of 635 amino acids which is homologous to riSMVT and is identical to the SMVT expressed in the human choriocarcinoma cell line JAR. Caco-2 SMVT also catalyzes Na+-dependent uptake of pantothenate, biotin, and lipoate. In oocytes expressing Caco-2 SMVT, all three vitamins evoke inward currents, confirming the electrogenicity of the transport process.
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96
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Wang H, Huang W, Fei YJ, Xia H, Yang-Feng TL, Leibach FH, Devoe LD, Ganapathy V, Prasad PD. Human placental Na+-dependent multivitamin transporter. Cloning, functional expression, gene structure, and chromosomal localization. J Biol Chem 1999; 274:14875-83. [PMID: 10329687 DOI: 10.1074/jbc.274.21.14875] [Citation(s) in RCA: 122] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We have cloned the human Na+-dependent multivitamin transporter (SMVT), which transports the water-soluble vitamins pantothenate, biotin, and lipoate, from a placental choriocarcinoma cell line (JAR). The cDNA codes for a protein of 635 amino acids with 12 transmembrane domains and 4 putative sites for N-linked glycosylation. The human SMVT exhibits a high degree of homology (84% identity and 89% similarity) to the rat counterpart. When expressed in HRPE cells, the cDNA-induced transport process is obligatorily dependent on Na+ and accepts pantothenate, biotin, and lipoate as substrates. The relationship between the cDNA-specific uptake rate of pantothenate or biotin and Na+ concentration is sigmoidal with a Na+:vitamin stoichiometry of 2:1. The human SMVT, when expressed in Xenopus laevis oocytes, induces inward currents in the presence of pantothenate, biotin, and lipoate in a Na+-, concentration-, and potential-dependent manner. We also report here on the structural organization and chromosomal localization of the human SMVT gene. The SMVT gene is approximately 14 kilobase pairs in length and consists of 17 exons. The SMVT gene is located on chromosome 2p23 as evidenced by somatic cell hybrid analysis and fluorescence in situ hybridization.
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97
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Fei YJ, Nara E, Liu JC, Boyd CA, Ganapathy V, Leibach FH. Preferential recognition of zwitterionic dipeptides as transportable substrates by the high-affinity peptide transporter PEPT2. BIOCHIMICA ET BIOPHYSICA ACTA 1999; 1418:344-51. [PMID: 10320685 DOI: 10.1016/s0005-2736(99)00046-2] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
We investigated the interaction of rat PEPT2, a high-affinity peptide transporter, with neutral, anionic, and cationic dipeptides using electrophysiological approaches as well as tracer uptake methods. D-Phe-L-Gln (neutral), D-Phe-L-Glu (anionic), and D-Phe-L-Lys (cationic) were used as representative, non-hydrolyzable, dipeptides. All three dipeptides induced H+-dependent inward currents in Xenopus laevis oocytes heterologously expressing rat PEPT2. The H+:peptide stoichiometry was 1:1 in each case. A simultaneous measurement of radiolabeled dipeptide influx and charge transfer in the same oocyte indicated a transfer of one net positive charge into the oocyte per transfer of one peptide molecule irrespective of the charged nature of the peptide. We conclude that the zwitterionic peptides are preferentially recognized by PEPT2 as transportable substrates and that the proton/peptide stoichiometry is 1 for the transport process.
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98
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Wu X, Fei YJ, Huang W, Chancy C, Leibach FH, Ganapathy V. Identity of the F52F12.1 gene product in Caenorhabditis elegans as an organic cation transporter. BIOCHIMICA ET BIOPHYSICA ACTA 1999; 1418:239-44. [PMID: 10209228 DOI: 10.1016/s0005-2736(99)00020-6] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
Abstract
We describe here the cloning and functional characterization of an organic cation transporter from Caenorhabditis elegans (CeOCT1). The CeOCT1 cDNA is 1826 bp long and codes for a protein of 568 amino acids. The oct1 gene is approximately 3.2 kb in size and consists of 12 exons. The location of this gene corresponds to the F52F12.1 gene locus on chromosome I. The predicted protein contains 12 putative transmembrane domains. It exhibits significant homology to mammalian OCTs. When expressed in mammalian cells, CeOCT1 induces the transport of the prototypical organic cation tetraethylammonium. The Michaelis-Menten constant for this substrate is 80+/-16 microM. The substrate specificity of CeOCT1 is broad. This represents the first report on the cloning and functional characteristics of an organic cation transporter from C. elegans.
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99
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Smith SB, Huang W, Chancy C, Ganapathy V. Regulation of the reduced-folate transporter by nitric oxide in cultured human retinal pigment epithelial cells. Biochem Biophys Res Commun 1999; 257:279-83. [PMID: 10198203 DOI: 10.1006/bbrc.1999.0452] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The regulation of the reduced-folate transporter (RFT) by nitric oxide (NO) was analyzed in human retinal pigment epithelial (HRPE) cells. NO inhibited specifically and reversibly the uptake of N5-methyltetrahydrofolate by a cGMP-independent mechanism. The inhibition was associated with a decrease in substrate affinity. The NO-induced inhibition was prevented by antioxidants and NO scavengers. Agents capable of modifying thiol groups in proteins inhibited RFT, indicating that the likely mechanism of NO-induced inhibition is via modification of essential thiol groups in this protein. These studies suggest that NO produced during retinal disease may affect the function of RFT in adjacent RPE cells.
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100
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Tang NL, Ganapathy V, Wu X, Hui J, Seth P, Yuen PM, Wanders RJ, Fok TF, Hjelm NM. Mutations of OCTN2, an organic cation/carnitine transporter, lead to deficient cellular carnitine uptake in primary carnitine deficiency. Hum Mol Genet 1999; 8:655-60. [PMID: 10072434 DOI: 10.1093/hmg/8.4.655] [Citation(s) in RCA: 106] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023] Open
Abstract
Systemic primary carnitine deficiency (CDSP, OMIM 212140) is an autosomal recessive disease characterized by low serum and intracellular concentrations of carnitine. CDSP may present with acute metabolic derangement simulating Reye's syndrome within the first 2 years of life. After 3 years of age, patients with CDSP may present with cardiomyopathy and muscle weakness. A linkage with D5S436 in 5q was reported in a family. A recently cloned homologue of the organic cation transporter, OCTN2, which has sodium-dependent carnitine uptake properties, was also mapped to the same locus. We screened for mutation in OCTN2 in a confirmed CDSP family. One truncating mutation (Trp132Stop) and one missense mutation (Pro478Leu) of OCTN2 were identified together with two silent polymorphisms. Expression of the mutant cDNAs revealed virtually no uptake activity for both mutations. Our data indicate that mutations in OCTN2 are responsible for CDSP. Identification of the underlying gene in this disease will allow rapid detection of carriers and postnatal diagnosis of affected patients.
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