Molecular cloning and characterization of high-affinity carnitine transporter from rat intestine

Transporter-Mediated Drug Delivery

Transmembrane transport of small organic and inorganic molecules is one of the cornerstones of cellular metabolism. Among transmembrane transporters, solute carrier (SLC) proteins form the largest, albeit very diverse, superfamily with over 400 members. It was recognized early on that xenobiotics can directly interact with SLCs and that this interaction can fundamentally determine their efficacy, including bioavailability and intertissue distribution. Apart from the well-established prodrug strategy, the chemical ligation of transporter substrates to nanoparticles of various chemical compositions has recently been used as a means to enhance their targeting and absorption. In this review, we summarize efforts in drug design exploiting interactions with specific SLC transporters to optimize their therapeutic effects. Furthermore, we describe current and future challenges as well as new directions for the advanced development of therapeutics that target SLC transporters.

Organic Cation Transporters in Health and Disease

The organic cation transporters (OCTs) OCT1, OCT2, OCT3, novel OCT (OCTN)1, OCTN2, multidrug and toxin exclusion (MATE)1, and MATE kidney-specific 2 are polyspecific transporters exhibiting broadly overlapping substrate selectivities. They transport organic cations, zwitterions, and some uncharged compounds and operate as facilitated diffusion systems and/or antiporters. OCTs are critically involved in intestinal absorption, hepatic uptake, and renal excretion of hydrophilic drugs. They modulate the distribution of endogenous compounds such as thiamine, L-carnitine, and neurotransmitters. Sites of expression and functions of OCTs have important impact on energy metabolism, pharmacokinetics, and toxicity of drugs, and on drug-drug interactions. In this work, an overview about the human OCTs is presented. Functional properties of human OCTs, including identified substrates and inhibitors of the individual transporters, are described. Sites of expression are compiled, and data on regulation of OCTs are presented. In addition, genetic variations of OCTs are listed, and data on their impact on transport, drug treatment, and diseases are reported. Moreover, recent data are summarized that indicate complex drug-drug interaction at OCTs, such as allosteric high-affinity inhibition of transport and substrate dependence of inhibitor efficacies. A hypothesis about the molecular mechanism of polyspecific substrate recognition by OCTs is presented that is based on functional studies and mutagenesis experiments in OCT1 and OCT2. This hypothesis provides a framework to imagine how observed complex drug-drug interactions at OCTs arise. Finally, preclinical in vitro tests that are performed by pharmaceutical companies to identify interaction of novel drugs with OCTs are discussed. Optimized experimental procedures are proposed that allow a gapless detection of inhibitory and transported drugs.

Overview of the Components of Cardiac Metabolism

Metabolism in organs other than the liver and kidneys may play a significant role in how a specific organ responds to chemicals. The heart has metabolic capability for energy production and homeostasis. This homeostatic machinery can also process xenobiotics. Cardiac metabolism includes the expression of numerous organic anion transporters, organic cation transporters, organic carnitine (zwitterion) transporters, and ATP-binding cassette transporters. Expression and distribution of the transporters within the heart may vary, depending on the patient’s age, disease, endocrine status, and various other factors. Several cytochrome P450 (P450) enzyme classes have been identified within the heart. The P450 hydroxylases and epoxygenases within the heart produce hydroxyeicosatetraneoic acids and epoxyeicosatrienoic acids, metabolites of arachidonic acid, which are critical in regulating homeostatic processes of the heart. The susceptibility of the cardiac P450 system to induction and inhibition from exogenous materials is an area of expanding knowledge, as are the metabolic processes of glucuronidation and sulfation in the heart. The susceptibility of various transcription factors and signaling pathways of the heart to disruption by xenobiotics is not fully characterized but is an area with implications for disruption of normal postnatal development, as well as modulation of adult cardiac health. There are knowledge gaps in the timelines of physiologic maturation and deterioration of cardiac metabolism. Cross-species characterization of cardiac-specific metabolism is needed for nonclinical work of optimum translational value to predict possible adverse effects, identify sensitive developmental windows for the design and conduct of informative nonclinical and clinical studies, and explore the possibilities of organ-specific therapeutics.

Maternal Plasma l-Carnitine Reduction During Pregnancy Is Mainly Attributed to OCTN2-Mediated Placental Uptake and Does Not Result in Maternal Hepatic Fatty Acid β-Oxidation Decline

l-Carnitine (l-Car) plays a crucial role in fatty acid β-oxidation. However, the plasma l-Car concentration in women markedly declines during pregnancy, but the underlying mechanism and its consequences on maternal hepatic β-oxidation have not yet been clarified. Our results showed that the plasma l-Car level in mice at gestation day (GD) 18 was significantly lower than that in nonpregnant mice, and the mean fetal-to-maternal plasma l-Car ratio in GD 18 mice was 3.0. Carnitine/organic cation transporter 2 (OCTN2) was highly expressed in mouse and human placenta and upregulated as gestation proceeds in human placenta, whereas expressions of carnitine transporter (CT) 1, CT2, and amino acid transporter B^0,+ were extremely low. Further study revealed that renal peroxisome proliferator-activated receptor α (PPARα) and OCTN2 were downregulated and the renal l-Car level was reduced, whereas the urinary excretion of l-Car was lower in late pregnant mice than in nonpregnant mice. Meanwhile, progesterone (pregnancy-related hormone) downregulated the expression of renal OCTN2 via PPARα-mediated pathway, and inhibited the activity of OCTN2, but estradiol, corticosterone, and cortisol did not. Unexpectedly, the maternal hepatic level of l-Car and β-hydroxybutyrate (an indicator of mitochondrial β-oxidation), and mRNA levels of several enzymes involved in fatty acid β-oxidation in GD 18 mice were higher than that in nonpregnant mice. In conclusion, OCTN2 mediated l-Car transfer across the placenta played a major role in maternal plasma l-Car reduction during pregnancy, which did not subsequently result in maternal hepatic fatty acid β-oxidation decrease.

Characterization of an L-Carnitine Transport System in Murine Photoreceptor Cell Line

While it is well known that L-carnitine [3-hydroxy-4-(trimethylazaniumyl)-butanoate] is an essential molecule for β-oxidation, it provides anti-oxidative effects as well. Since these effects have been observed in photoreceptor cells, the carnitine's intracellular concentration is considered to play a protective role against oxidative damage to those cells. However, even though its high hydrophilicity makes it likely that carnitine import is accomplished via a dedicated host transport system, the specific uptake process into those cells is currently unknown. Therefore, in this study, we sought to identify and characterize photoreceptor cell carnitine uptake transporter(s) utilizing 661W cells as a photoreceptor cell model. The results of our uptake assays showed that carnitine was transported into 661W cells in a saturable manner (K_m=5.5 mM), and that the activity was susceptible to extracellular pH and Na⁺. While these data suggest the involvement of a transporter in 661W cell carnitine uptake, the observed transport profile did not correspond to any of the currently known carnitine transporters such as organic cation/carnitine transporter 1 (Octn1), Octn2, Octn3, B^0,+ and Ct2. In fact, in our experiments, the mRNA expressions for such carnitine transporters in 661W cells were consistently very low and the carnitine transporter substrates did not inhibit the uptake activities. Taken as a whole, our results indicate that carnitine is transported into 661W cells in a carrier-mediated manner. However, since its transport modes cannot be fully explained by known carnitine transporters, it is highly likely that photoreceptor cells utilize a unique molecularly-based carnitine uptake system.

Species specificity profiling of rat and human organic cation/carnitine transporter Slc22a5/SLC22A5 (Octn2/OCTN2)

The purpose of this study was to characterize the uptake of carnitine, the physiological substrate, and the uptake of 3-(2,2,2-trimethylhydrazinium)propionate, a consensus substrate by rat Octn2 and human OCTN2 transporters as well as to characterize drug-mediated inhibition of l-carnitine uptake by the rat and human orthologs overexpressed in CHO-K1 cells. l-carnitine and 3-(2,2,2-trimethylhydrazinium)propionate were found to be a lower affinity substrate for rat Octn2 (K_M = 32.66 ± 5.11 μM and 23.62 ± 4.99 μM respectively) than for human OCTN2 (K_M = 3.08 ± 0.74 μM and 7.98 ± 0.63 μM). The intrinsic clearance (CL_int) value for carnitine was higher for the human than for the rat transporter (22.82 ± 5.57 ml/min*mg vs 4.008 ± 0.675 ml/min*mg). For 3-(2,2,2-trimethylhydrazinium)propionate, in contrast, the CL_int value for rat Octn2 was higher than for human OCTN2 (323.9 ± 72.8 ml/min*mg vs 65.11 ± 5.33 ml/min*mg). Furthermore, many pharmacologically important drugs were shown to affect l-carnitine transport by Octn2/OCTN2. The correlation between the IC₅₀ datasets for the rat and human transporter resulted in an r value of 0.47 (p > 0.05). However, the greatest difference was less than seven-fold and 13 of 15 compounds yielded a difference less than 3-fold. Thus, the transporters from these two species showed an overlapping but somewhat different substrate and inhibitor specificity.

Single-nucleotide polymorphisms in SLC22A23 are associated with ulcerative colitis in a Canadian white cohort

BACKGROUND

SLC22A23 is an orphan gene in the SLC22 family of organic membrane transporters, and its single-nucleotide polymorphism rs17309827-T was recently nominally associated with intestinal inflammation in a genome-wide association study. Other polymorphisms in the SLC22A23 gene have been associated with diseases with an inflammatory component, and polymorphisms in related genes in the SLC22 family have been repeatedly associated with inflammatory bowel disease (IBD).

OBJECTIVE

In a candidate-gene study using a well-phenotyped, highly monitored, Manitoban white cohort, we investigated whether variations in SLC22A23 were associated with intestinal inflammation.

DESIGN

Selected genetic variations were genotyped by using fluorescent-based assays or a polymerase chain reaction-restriction fragment length polymorphism analysis in 160 individuals with Crohn disease, 149 individuals with ulcerative colitis, and 142 healthy control subjects to determine genetic associations.

RESULTS

Homozygocity for single-nucleotide polymorphisms rs4959235-TT and rs950318-GG was associated with IBD, whereby 6% of patients (18 of 311 cases) carried these genotypes, but they were not seen in healthy controls.

CONCLUSION

Associations reported in this article add to the emerging evidence that SLC22A23 variants could modify IBD risk. However, the biology of the gene and impact of variations on the gene's functions need to be tested to validate a causative role.

Caveolin-1--a novel interacting partner of organic cation/carnitine transporter (Octn2): effect of protein kinase C on this interaction in rat astrocytes

OCTN2 - the Organic Cation Transporter Novel family member 2 (SLC22A5) is known to be a xenobiotic/drug transporter. It transports as well carnitine - a compound necessary for oxidation of fatty acids and mutations of its gene cause primary carnitine deficiency. Octn2 regulation by protein kinase C (PKC) was studied in rat astrocytes - cells in which β-oxidation takes place in the brain. Activation of PKC with phorbol ester stimulated L-carnitine transport and increased cell surface presence of the transporter, although no PKC-specific phosphorylation of Octn2 could be detected. PKC activation resulted in an augmented Octn2 presence in cholesterol/sphingolipid-rich microdomains of plasma membrane (rafts) and increased co-precipitation of Octn2 with raft-proteins, caveolin-1 and flotillin-1. Deletion of potential caveolin-1 binding motifs pointed to amino acids 14–22 and 447–454 as the caveolin-1 binding sites within Octn2 sequence. A direct interaction of Octn2 with caveolin-1 in astrocytes upon PKC activation was detected by proximity ligation assay, while such an interaction was excluded in case of flotillin-1. Functioning of a multi-protein complex regulated by PKC has been postulated in rOctn2 trafficking to the cell surface, a process which could be important both under physiological conditions, when carnitine facilitates fatty acids catabolism and controls free Coenzyme A pool as well as in pathology, when transport of several drugs can induce secondary carnitine deficiency.

The SLC22 family with transporters of organic cations, anions and zwitterions

The SLC22 family contains 13 functionally characterized human plasma membrane proteins each with 12 predicted α-helical transmembrane domains. The family comprises organic cation transporters (OCTs), organic zwitterion/cation transporters (OCTNs), and organic anion transporters (OATs). The transporters operate as (1) uniporters which mediate facilitated diffusion (OCTs, OCTNs), (2) anion exchangers (OATs), and (3) Na(+)/zwitterion cotransporters (OCTNs). They participate in small intestinal absorption and hepatic and renal excretion of drugs, xenobiotics and endogenous compounds and perform homeostatic functions in brain and heart. Important endogeneous substrates include monoamine neurotransmitters, l-carnitine, α-ketoglutarate, cAMP, cGMP, prostaglandins, and urate. It has been shown that mutations of the SLC22 genes encoding these transporters cause specific diseases like primary systemic carnitine deficiency and idiopathic renal hypouricemia and are correlated with diseases such as Crohn's disease and gout. Drug-drug interactions at individual transporters may change pharmacokinetics and toxicities of drugs.

Molecular and functional identification of organic anion transporter isoforms in cultured bovine mammary epithelial cells (BME-UV)

Mammary epithelial cells express a diversity of membrane transporters including members of organic cation and organic anion (OAT) transporter subfamilies. Four mammal OAT isoforms have been identified: OAT-1, OAT-2, OAT-3, and OAT-4. The pharmacological significance of OAT isoforms has been emphasized because of their role in the movement of a wide variety of substrates across epithelial barriers. The present study identified (molecularly and functionally) bovine OAT isoforms in bovine mammary epithelial (BME-UV) cells. mRNA expression levels of all tested transporters in BME-UV cells were less than expression levels of the corresponding transporters in bovine kidney. Directionality in the flux of P-aminohippuric acid and acetylsalicylate, compounds known to interact with OAT-1 and OAT-2, respectively, across BME-UV monolayers was not observed at the concentrations used in this study. Directionality was, however, observed in the flux of estrone sulfate (EsS). Adding probenecid, penicillin G or nonradiolabeled EsS to the apical donor compartment significantly increased the apical-to-basolateral flux of EsS across the BME-UV monolayer. These results suggest that BME-UV cells express an organic anion transport system, making it a potentially useful model to study the role of this transport system in the mammary epithelial barrier.

Oxaliplatin transport mediated by organic cation/carnitine transporters OCTN1 and OCTN2 in overexpressing human embryonic kidney 293 cells and rat dorsal root ganglion neurons

The organic cation/carnitine transporters OCTN1 and OCTN2 are related to other organic cation transporters (OCT1, OCT2, and OCT3) known for transporting oxaliplatin, an anticancer drug with dose-limiting neurotoxicity. In this study, we sought to determine whether OCTN1 and OCTN2 also transported oxaliplatin and to characterize their functional expression and contributions to its neuronal accumulation and neurotoxicity in dorsal root ganglion (DRG) neurons relative to those of OCTs. [(14)C]Oxaliplatin uptake, platinum accumulation, and cytotoxicity were determined in OCTN-overexpressing human embryonic kidney (HEK) 293 cells and primary cultures of rat DRG neurons. Levels of mRNA and functional activities of rat (r)Octns and rOcts in rat DRG tissue and primary cultures were characterized using reverse transcription-polymerase chain reaction and uptake of model OCT/OCTN substrates, including [(3)H]1-methyl-4-phenylpyridinium (MPP(+)) (OCT1-3), [(14)C]tetraethylammonium bromide (TEA(+)) (OCT1-3 and OCTN1/2), [(3)H]ergothioneine (OCTN1), and [(3)H]l-carnitine (OCTN2). HEK293 cells overexpressing rOctn1, rOctn2, human OCTN1, and human OCTN2 showed increased uptake and cytotoxicity of oxaliplatin compared with mock-transfected HEK293 controls; in addition, both uptake and cytotoxicity were inhibited by ergothioneine and L-carnitine. The uptake of ergothioneine mediated by OCTN1 and of L-carnitine mediated by OCTN2 was decreased during oxaliplatin exposure. rOctn1 and rOctn2 mRNA was readily detected in rat DRG tissue, and they were functionally active in cultured rat DRG neurons, more so than rOct1, rOct2, or rOct3. DRG neuronal accumulation of [(14)C]oxaliplatin and platinum during oxaliplatin exposure depended on time, concentration, temperature, and sodium and was inhibited by ergothioneine and to a lesser extent by L-carnitine but not by MPP(+). Loss of DRG neuronal viability during oxaliplatin exposure was inhibited by ergothioneine but not by L-carnitine or MPP(+). OCTN1 and OCTN2 both transport oxaliplatin and are functionally expressed by DRG neurons. OCTN1-mediated transport of oxaliplatin appears to contribute to its neuronal accumulation and treatment-limiting neurotoxicity more so than OCTN2 or OCTs.

Be different--the diversity of peroxisomes in the animal kingdom

Peroxisomes represent so-called "multipurpose organelles" as they contribute to various anabolic as well as catabolic pathways. Thus, with respect to the physiological specialization of an individual organ or animal species, peroxisomes exhibit a functional diversity, which is documented by significant variations in their proteome. These differences are usually regarded as an adaptational response to the nutritional and environmental life conditions of a specific organism. Thus, human peroxisomes can be regarded as an in part physiologically unique organellar entity fulfilling metabolic functions that differ from our animal model systems. In line with this, a profound understanding on how peroxisomes acquired functional heterogeneity in terms of an evolutionary and mechanistic background is required. This review summarizes our current knowledge on the heterogeneity of peroxisomal physiology, providing insights into the genetic and cell biological mechanisms, which lead to the differential localization or expression of peroxisomal proteins and further gives an overview on peroxisomal biochemical pathways, which are specialized in different animal species and organs. Moreover, it addresses the impact of proteome studies on our understanding of differential peroxisome function describing the utility of mass spectrometry and computer-assisted algorithms to identify peroxisomal target sequences for the detection of new organ- or species-specific peroxisomal proteins.

Localization of organic cation/carnitine transporter (OCTN2) in cells forming the blood-brain barrier

Carnitine beta-hydroxy-gamma-(trimethylammonio)butyrate - a compound necessary in the peripheral tissues for a transfer of fatty acids for their oxidation within the cell, accumulates in the brain despite low beta-oxidation in this organ. In order to enter the brain, carnitine has to cross the blood-brain barrier formed by capillary endothelial cells which are in close interaction with astrocytes. Previous studies, demonstrating expression of mRNA coding two carnitine transporters - organic cation/carnitine transporter 2 (OCTN2) and B(0,+) in endothelial cells, did not give any information on carnitine transporters polarity in endothelium. Therefore more detailed experiments were performed on expression and localization of a high affinity carnitine transporter OCTN2 in an in vitro model of the blood-brain barrier by real-time PCR, western blot analysis, and immunocytochemistry. The amount of mRNA was comparable in endothelial cells and kidney, when referred to house-keeping genes, it was, however, significantly lower in astrocytes. Polarity of OCTN2 localization was further studied in an in vitro model of the blood-brain barrier with use of anti-OCTN2 antibodies. Z-axis analysis of the confocal microscope pictures of endothelial cells, with anti-P-glycoprotein antibodies as the marker of apical membrane, showed OCTN2 localization at the basolateral membrane and in the cytoplasmic region in the vicinity of nuclei. Localization of OCTN2 suggest that carnitine can be also transported from the brain, playing an important role in removal of certain acyl esters.

Transport of carnitine and acetylcarnitine by carnitine/organic cation transporter (OCTN) 2 and OCTN3 into epididymal spermatozoa

Carnitine and acetylcarnitine are important for the acquisition of motility and maturation of spermatozoa in the epididymis. In this study, we examined the involvement of carnitine/organic cation transporter (OCTN) in carnitine and acetylcarnitine transport in epididymal spermatozoa of mice. Uptake of both compounds by epididymal spermatozoa was time-dependent and partially Na+-dependent. Kinetic analyses revealed the presence of a high-affinity transport system in the spermatozoa, withKmvalues of 23.6 and 6.57 μM for carnitine and acetylcarnitine respectively in the presence of Na+. Expression of OCTN2 and OCTN3 in epididymal spermatozoa was confirmed by immunofluorescence analysis. The involvement of these two transporters in carnitine and acetylcarnitine transport was supported by a selective inhibition study. We conclude that both Na+-dependent and -independent carnitine transporters, OCTN2 and OCTN3, mediate the supply of carnitine and acetylcarnitine to epididymal spermatozoa in mice.

Transport of Organic Cations across the Blood−Testis Barrier

Throughout the mammalian spermatogenic pathway, differentiating spermatogenic cells remain in close contact with somatic Sertoli cells, and this has been considered to be essential for the proliferation, differentiation, and survival of spermatogenic cells. It is thought that Sertoli cells form tight junctions to protect developing spermatogenic cells against harmful agents and provide several nutrients for spermatogenesis from the blood stream. Accordingly, Sertoli cells should regulate the movement of various nutritious and xenobiotic compounds by selective membrane transporters. However, the information on membrane transporters in Sertoli cells is limited. In the present study, we characterized the transport systems of organic cations in Sertoli cells. Uptake of [14C]tetraethylammonium (TEA) was measured by primary-cultured rat Sertoli cells. Initial uptake of TEA was concentration dependent, and an Eadie-Hofstee plot indicated the involvement of two saturable transport systems. The apparent Km values of high- and low-affinity components were comparable to those of previously known organic cation transporter (OCT) or organic cation/carnitine transporter (OCTN). In addition, OCT1, OCT3, OCTN1, and OCTN2 were expressed in Sertoli cells. In conclusion, multiple organic cation transporters, OCTs and OCTNs are expressed in Sertoli cells and the cells regulate transport of cationic compounds to protect and/or maintain the spermatogenesis in testis as the blood-testis barrier.

Polyspecific organic cation transporters: structure, function, physiological roles, and biopharmaceutical implications

The body is equipped with broad-specificity transporters for the excretion and distribution of endogeneous organic cations and for the uptake, elimination and distribution of cationic drugs, toxins and environmental waste products. This group of transporters consists of the electrogenic cation transporters OCT1-3 (SLC22A1-3), the cation and carnitine transporters OCTN1 (SLC22A4), OCTN2 (SLC22A5) and OCT6 (SLC22A16), and the proton/cation antiporters MATE1, MATE2-K and MATE2-B. The transporters show broadly overlapping sites of expression in many tissues such as small intestine, liver, kidney, heart, skeletal muscle, placenta, lung, brain, cells of the immune system, and tumors. In epithelial cells they may be located in the basolateral or luminal membranes. Transcellular cation movement in small intestine, kidney and liver is mediated by the combined action of electrogenic OCT-type uptake systems and MATE-type efflux transporters that operate as cation/proton antiporters. Recent data showed that OCT-type transporters participate in the regulation of extracellular concentrations of neurotransmitters in brain, mediate the release of acetylcholine in non-neuronal cholinergic reactions, and are critically involved in the regulation of histamine release from basophils. The recent identification of polymorphisms in human OCTs and OCTNs allows the identification of patients with an increased risk for adverse drug reactions. Transport studies with expressed OCTs will help to optimize pharmacokinetics during development of new drugs.

Polyspecific organic cation transporters: structure, function, physiological roles, and biopharmaceutical implications

The body is equipped with broad-specificity transporters for the excretion and distribution of endogeneous organic cations and for the uptake, elimination and distribution of cationic drugs, toxins and environmental waste products. This group of transporters consists of the electrogenic cation transporters OCT1-3 (SLC22A1-3), the cation and carnitine transporters OCTN1 (SLC22A4), OCTN2 (SLC22A5) and OCT6 (SLC22A16), and the proton/cation antiporters MATE1, MATE2-K and MATE2-B. The transporters show broadly overlapping sites of expression in many tissues such as small intestine, liver, kidney, heart, skeletal muscle, placenta, lung, brain, cells of the immune system, and tumors. In epithelial cells they may be located in the basolateral or luminal membranes. Transcellular cation movement in small intestine, kidney and liver is mediated by the combined action of electrogenic OCT-type uptake systems and MATE-type efflux transporters that operate as cation/proton antiporters. Recent data showed that OCT-type transporters participate in the regulation of extracellular concentrations of neurotransmitters in brain, mediate the release of acetylcholine in non-neuronal cholinergic reactions, and are critically involved in the regulation of histamine release from basophils. The recent identification of polymorphisms in human OCTs and OCTNs allows the identification of patients with an increased risk for adverse drug reactions. Transport studies with expressed OCTs will help to optimize pharmacokinetics during development of new drugs.

Distinct and predictive chromatin signatures of transcriptional promoters and enhancers in the human genome

Eukaryotic gene transcription is accompanied by acetylation and methylation of nucleosomes near promoters, but the locations and roles of histone modifications elsewhere in the genome remain unclear. We determined the chromatin modification states in high resolution along 30 Mb of the human genome and found that active promoters are marked by trimethylation of Lys4 of histone H3 (H3K4), whereas enhancers are marked by monomethylation, but not trimethylation, of H3K4. We developed computational algorithms using these distinct chromatin signatures to identify new regulatory elements, predicting over 200 promoters and 400 enhancers within the 30-Mb region. This approach accurately predicted the location and function of independently identified regulatory elements with high sensitivity and specificity and uncovered a novel functional enhancer for the carnitine transporter SLC22A5 (OCTN2). Our results give insight into the connections between chromatin modifications and transcriptional regulatory activity and provide a new tool for the functional annotation of the human genome.

OCTN2-Mediated Carnitine Uptake in a Newly Discovered Human Proximal Tubule Cell Line (Caki-1)

The proximal tubular reabsorption of carnitine in the human kidney is significant because more than 95% of the carnitine filtered in the kidney is reabsorbed by the proximal tubules therefore maintaining the homeostatic balance of carnitine in the body. Objectives of this study include the characterization of OCTN2 function in the Caki-1 cell line and the potential interactions of carnitine uptake with renally secreted drugs, including drugs of quaternary ammonium structure. Caki-1 cells were additionally characterized to be of proximal tubule nature, and an apical membrane expression pattern of OCTN2 in Caki-1 cells was discovered. Uptake studies with radiolabeled L-carnitine in Caki-1 cells revealed a Na+- and temperature-dependent carrier-mediated process (K(m) = 15.90 microM) which was unaffected by pH in a range from 6.5 to 8.5. All drugs tested were able to inhibit the carnitine uptake process to different degrees. The quaternary ammonium compounds ciclotropium bromide and ipratropium bromide were strong inhibitors with IC(50) values of 30 microM and 95 microM, respectively. The observed kinetics, immunohistolocalization, and inhibition studies indicate that the high-affinity uptake of carnitine in the Caki-1 cell line is most likely mediated by OCTN2. The interaction of drugs at the renal level with OCTN2 indicates a possible pathway for the final step of cationic secretion into the urine.

Experimental colitis: decreased Octn2 and Atb0+ expression in rat colonocytes induces carnitine depletion that is reversible by carnitine-loaded liposomes

Carnitine transporters have recently been implicated in susceptibility to inflammatory bowel disease (IBD). Because carnitine is required for beta-oxidation, it was suggested that decreased carnitine transporters, and hence reduced carnitine uptake, could lead to impaired fatty acid oxidation in intestinal epithelial cells, and to cell injury. We investigated this issue by examining the expression of the carnitine transporters OCTN2 and ATB0+, and butyrate metabolism in colonocytes in a rat model of IBD induced by trinitrobenzene sulfonic acid (TNBS). We found that Octn2 and Atb0+ expression was decreased in inflammatory samples at translational and functional level. Butyrate oxidation, evaluated based on CO2 production and acetyl-coenzyme A synthesis, was deranged in colonocytes from TNBS-treated rats. Treatment with carnitine-loaded liposomes corrected the butyrate metabolic alterations in vitro and reduced the severity of colitis in vivo. These results suggest that carnitine depletion in colonocytes is associated with the inability of mitochondria to maintain normal butyrate beta-oxidation. Our data indicate that carnitine is a rate-limiting factor for the maintenance of physiological butyrate oxidation in colonic cells. This hypothesis could also explain the contradictory therapeutic efficacy of butyrate supplementation observed in clinical trials of IBD.

The transport of organic cations in the small intestine: current knowledge and emerging concepts

A wide variety of drugs and endogenous bioactive amines are organic cations (OCs). Approximately 40% of all conventional drugs on the market are OCs. Thus, the transport of xenobiotics or endogenous OCs in the body has been a subject of considerable interest, since the discovery and cloning of a family of OC transporters, referred to as organic cation transporter (OCTs), and a new subfamily of OCTs, OCTNs, leading to the functional characterization of these transporters in various systems including oocytes and some cell lines. Organic cation transporters are critical in drug absorption, targeting, and disposition of a drug. In this review, the recent advances in the characterization of organic cation transporters and their distribution in the small intestine are discussed. The results of the in vitro transport studies of various OCs in the small intestine using techniques such as isolated brush-border membrane vesicles, Ussing chamber systems and Caco-2 cells are discussed, and in vivo knock-out animal studies are summarized. Such information is essential for predicting pharmacokinetics and pharmacodynamics and in the design and development of new cationic drugs. An understanding of the mechanisms that control the intestinal transport of OCs will clearly aid achieving desirable clinical outcomes.

Effect of Shiga-like toxin II from Escherichia coli O157:H7 on intestinal clearance of norfloxacin in rats

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 infection causes severe clinical symptoms, due to its bacterial toxin, called Shiga-like toxin (SLT). However, little is known about the information to establish a safe and efficient prescription to treat for EHEC O157:H7 patients. Thus, we investigated the effect of SLT-II on intestinal function in rats by using the antibiotic norfloxacin (NFLX) as a model drug. The intestinal clearance (CLi) of NFLX, determined by loop method in the jejunum, was significantly decreased by SLT-II. In histopathological experiment, epithalaxia was observed in SLT-II-treated rats without structural changes of tight junction suggesting the deterioration of active transport systems by SLT-II. CLi of NFLX in normal rats was decreased by carnitine (CAR), suggesting the possible involvement of CAR-sensitive transporter in CLi of NFLX. Taken together, these results suggest that the EHEC O157:H7 infection might affect the intestinal disposition of NFLX due to the changing intestinal expression/function of drug transporters by SLT-II.

Carnitine/organic cation transporter OCTN2-mediated transport of carnitine in primary-cultured epididymal epithelial cells

Carnitine is essential for the acquisition of motility and maturation of spermatozoa in the epididymis, and is accumulated in epididymal fluid. In this study, carnitine transport into primary-cultured rat epididymal epithelial cells was characterized to clarify the nature of the transporter molecules involved. Uptake of carnitine by primary-cultured epididymal epithelial cells was time, Na+and concentration dependent. Kinetic analysis of carnitine uptake by the cells revealed the involvement of high- and low-affinity transport systems withKm values of 21 μM and 2.2 mM respectively. The uptake of carnitine by the cells was significantly reduced by inhibitors of carnitine/organic cation transporter (OCTN2), such as carnitine analogues and cationic compounds. In RT-PCR analysis, OCTN2 expression was detected. These results demonstrated that the high-affinity carnitine transporter OCTN2, which is localized at the basolateral membrane of epididymal epithelial cells, mediates carnitine supply into those cells from the systemic circulation as the first step of permeation from blood to spermatozoa.

OCTN3 is a mammalian peroxisomal membrane carnitine transporter

Carnitine is a zwitterion essential for the beta-oxidation of fatty acids. The role of the carnitine system is to maintain homeostasis in the acyl-CoA pools of the cell, keeping the acyl-CoA/CoA pool constant even under conditions of very high acyl-CoA turnover, thereby providing cells with a critical source of free CoA. Carnitine derivatives can be moved across intracellular barriers providing a shuttle mechanism between mitochondria, peroxisomes, and microsomes. We now demonstrate expression and colocalization of mOctn3, the intermediate-affinity carnitine transporter (Km 20 microM), and catalase in murine liver peroxisomes by TEM using immunogold labelled anti-mOctn3 and anti-catalase antibodies. We further demonstrate expression of hOCTN3 in control human cultured skin fibroblasts both by Western blotting and immunostaining analysis using our specific anti-mOctn3 antibody. In contrast with two peroxisomal biogenesis disorders, we show reduced expression of hOCTN3 in human PEX 1 deficient Zellweger fibroblasts in which the uptake of peroxisomal matrix enzymes is impaired but the biosynthesis of peroxisomal membrane proteins is normal, versus a complete absence of hOCTN3 in human PEX 19 deficient Zellweger fibroblasts in which both the uptake of peroxisomal matrix enzymes as well as peroxisomal membranes are deficient. This supports the localization of hOCTN3 to the peroxisomal membrane. Given the impermeability of the peroxisomal membrane and the key role of carnitine in the transport of different chain-shortened products out of peroxisomes, there appears to be a critical need for the intermediate-affinity carnitine/organic cation transporter, OCTN3, on peroxisomal membranes now shown to be expressed in both human and murine peroxisomes. This Octn3 localization is in keeping with the essential role of carnitine in peroxisomal lipid metabolism.

Active efflux across the blood-brain barrier: role of the solute carrier family

The brain uptake of xenobiotics is restricted by the blood-brain brain barrier formed by brain capillary endothelial cells. Active efflux transport systems in the blood-brain barrier work as a detoxification system in the brain by facilitating removal of xenobiotic compounds from the brain. Drugs, acting in the brain, have to overcome such efflux mechanisms to achieve clinically significant concentration in the brain. Multiple transporters are involved in this efflux transport in the brain capillaries. In the past few years, considerable progress has been made in the cloning of these transporters and their functional characterization after heterologous expression. Members of the solute carrier family (SLC) play an important role in the efflux transport, especially for organic anions, which include organic anion transporting polypeptides (OATP/SLCO) and organic anion transporters (OAT/SLC22A). It is believed that coordination of the members of SLC family, and ABC transporters, such as P-glycoprotein, multidrug resistance protein, and breast cancer-resistant protein (BCRP/ABCG2), allows an efficient vectorial transport across the endothelial cells to remove xenobiotics from the brain. In this review, we shall summarize our current knowledge about their localization, molecular and functional characteristics, and substrate and inhibitor specificity.

Involvement of OCTN1 (SLC22A4) in pH-dependent transport of organic cations

OCTN1 (SLC22A4) transports cationic compounds such as tetraethylammonium in a pH-sensitive and sodium-independent manner in cultured cells, and is expressed in wide variety of tissues, including kidney, muscle, placenta, heart, and others. This study focused on the clarification of its subcellular distribution in kidney and on its driving force to throw light on the pharmacological and physiological roles of OCTN1. Uptake of [14C]tetraethylammonium by membrane vesicles prepared from HEK293 cells stably transfected with human OCTN1 cDNA was osmolarity-sensitive, and the Km of tetraethylammonium was 1.28 mM at intravesicular and extravesicular pH values of 6.0 and 7.4, respectively. Tetraethylammonium uptake was pH-dependent, and overshoot uptake was observed in the presence of an outwardly directed proton gradient. A protonophore and membrane potential affected the overshoot uptake. Furthermore, preloading tetraethylammonium in the vesicles significantly increased the rate of uptake of [14C]tetraethylammonium. In mouse kidney, OCTN1 was expressed predominantly at the apical membrane of cortical proximal tubular epithelial cells. It was concluded that OCTN1 is involved in renal excretion of organic cations across the apical membrane in a pH-dependent, membrane potential-sensitive manner and is affected significantly by the organic cations on the trans side, showing counter transport activity.

OCTN2-mediated transport of carnitine in isolated Sertoli cells

Carnitine is extensively accumulated in epididymis. Carnitine is also accumulated in testis at higher concentration than in the plasma and is used in spite of the presence of the blood–testis barrier. In this study, we examined the characteristics of carnitine transport in primary-cultured rat Sertoli cells, which constitute a part of the blood–testis barrier. Uptake of [3H]carnitine (11.4 nM) from the basal side of Sertoli cells was Na+-dependent and was significantly decreased in the presence of 10 μM (48.0 ± 7.4% of control) or 100 μM unlabeled carnitine (14.6 ± 5.7% of control). Furthermore, the uptake was significantly inhibited in the presence of 100 μM acetyl-L-carnitine, 100 μM gamma-butyrobetaine or 500 μM quinidine. In RT-PCR analysis, the high-affinity carnitine transporter OCTN2 was detected in rat whole testis tissue and primary-cultured Sertoli cells. In contrast, the low-affinity carnitine transporter ATB0,+was detected in rat whole testis tissue, but not in primary cultured Sertoli cells. These results demonstrate that OCTN2 mediates carnitine supply to Sertoli cells from the circulation.

OCTN3: A Na+-independent L-carnitine transporter in enterocytes basolateral membrane

L-carnitine transport has been measured in enterocytes and basolateral membrane vesicles (BLMV) isolated from chicken intestinal epithelia. In the nominally Na+-free conditions chicken enterocytes take up L-carnitine until the cell to medium L-carnitine ratio is 1. This uptake was inhibited by L-carnitine, D-carnitine, gamma-butyrobetaine, acetylcarnitine, tetraethylammonium (TEA), and betaine. L-3H-carnitine uptake into BLMV showed no overshoot, and it was (i) Na+-independent, (ii) trans-stimulated by intravesicular L-carnitine, and (iii) cis-inhibited by TEA and cold L-carnitine. L-3H-carnitine efflux from L-3H-carnitine preloaded enterocytes was also Na+-independent, and trans-stimulated by L-carnitine, D-carnitine, gamma-butyrobetaine, acetylcarnitine, TEA, and betaine. Both, uptake and efflux of L-carnitine were inhibited by verapamil and unaffected by either extracellular pH or palmitoyl-L-carnitine. RT-PCR with specific primers for the mouse OCTN3 transporter revealed the existence of OCTN3 mRNA in mouse intestine, which was confirmed by in situ hybridization studies. Immunohystochemical analysis showed that OCTN3 protein was mainly associated with the basolateral membrane of rat and chicken enterocytes, whereas OCTN2 was detected at the apical membrane. In conclusion, the results demonstrate for the first time that (i) mammalian small intestine expresses OCTN3 mRNA along the villus and (ii) that OCTN3 protein is located in the basolateral membrane. They also suggest that OCTN3 could mediate the passive, Na+ and pH-independent L-carnitine transport activity measured in the three experimental conditions.

Small molecular drug transfer across the blood-brain barrier via carrier-mediated transport systems

Because of the physiological nature of the blood-brain barrier (BBB), transport of chemical compounds between blood and brain has been widely believed to occur by means of passive diffusion, depending upon the lipophilicity of the compounds. However, discrepancies exist between the lipophilicity and apparent BBB permeation properties in many cases, and these discrepancies can be ascribed to the existence of multiple mechanisms of drug transport through the BBB. Molecular identification and functional analysis of influx transport proteins (from blood to brain) and efflux transport proteins (from brain to blood) have progressed rapidly. Therefore, the BBB is now considered to be a dynamic interface that controls the influx and efflux of a wide variety of substances, including endogenous nutrients and exogenous compounds such as drugs, to maintain a favorable environment for the CNS. This review focuses on the role of transport systems in the uptake of xenobiotics, including organic anionic/cationic and neutral drugs, across the BBB into the brain, as well as on strategies to increase drug delivery into the brain by blocking efflux transport protein function, or to reduce CNS side effects by modulating BBB transport processes.

The multispecific organic anion transporter family: properties and pharmacological significance

Physiological and pharmacological studies indicate that the renal and hepatic organic anion transport systems are responsible for the elimination of numerous compounds, such as drugs, environmental substances and metabolites of both endogenous and exogenous origins. Recently, the molecular identity of the organic anion transport system, the OAT family, was revealed. To date, six OAT members have been identified and shown to have important roles not only in detoxification in the kidneys, liver and brain, but also in the reabsorption of essential compounds such as urate. The OAT family members are closely associated with the pharmacokinetics, drug-drug interactions and toxicity of anionic substances such as nephrotoxic drugs and uremic toxins. The molecular characterization of the OAT family encoded by SLC22A will be discussed.

Intestinal solute carriers: an overview of trends and strategies for improving oral drug absorption

A large amount of absorptive intestinal membrane transporters play an important part in absorption and distribution of several nutrients, drugs and prodrugs. The present paper gives a general overview on intestinal solute carriers as well as on trends and strategies for targeting drugs and/or prodrugs to these carriers in order to increasing oral bioavailability and distribution. A number of absorptive intestinal transporters are described in terms of gene and protein classification, driving forces, substrate specificities and cellular localization. When targeting absorptive large capacity membrane transporters in the small intestine in order to increase oral bioavailabilities of drug or prodrug, the major influence on in vivo pharmacokinetics is suggested to be dose-dependent increase in bioavailability as well as prolonged blood circulation due to large capacity facilitated absorption, and renal re-absorption, respectively. In contrast, when targeting low-capacity transporters such as vitamin transporters, dose independent saturable absorption kinetics are suggested. We thus believe that targeting drug substrates for absorptive intestinal membrane transporters could be a feasible strategy for optimizing drug bioavailability and distribution.

XP13512 [(+/-)-1-([(alpha-isobutanoyloxyethoxy)carbonyl] aminomethyl)-1-cyclohexane acetic acid], a novel gabapentin prodrug: I. Design, synthesis, enzymatic conversion to gabapentin, and transport by intestinal solute transporters

Gabapentin is thought to be absorbed from the intestine of humans and animals by a low-capacity solute transporter localized in the upper small intestine. Saturation of this transporter at doses used clinically leads to dose-dependent pharmacokinetics and high interpatient variability, potentially resulting in suboptimal drug exposure in some patients. XP13512 [(+/-)-1-([(alpha-isobutanoyloxyethoxy)carbonyl] aminomethyl)-1-cyclohexane acetic acid] is a novel prodrug of gabapentin designed to be absorbed throughout the intestine by high-capacity nutrient transporters. XP13512 was stable at physiological pH but rapidly converted to gabapentin in intestinal and liver tissue from rats, dogs, monkeys, and humans. XP13512 was not a substrate or inhibitor of major cytochrome P450 isoforms in transfected baculosomes or liver homogenates. The separated isomers of XP13512 showed similar cleavage in human tissues. The prodrug demonstrated active apical to basolateral transport across Caco-2 cell monolayers and pH-dependent passive permeability across artificial membranes. XP13512 inhibited uptake of (14)C-lactate by human embryonic kidney cells expressing monocarboxylate transporter type-1, and direct uptake of prodrug by these cells was confirmed using liquid chromatography-tandem mass spectrometry. XP13512 inhibited uptake of (3)H-biotin into Chinese hamster ovary cells overexpressing human sodium-dependent multivitamin transporter (SMVT). Specific transport by SMVT was confirmed by oocyte electrophysiology studies and direct uptake studies in human embryonic kidney cells after tetracycline-induced expression of SMVT. XP13512 is therefore a substrate for several high-capacity absorption pathways present throughout the intestine. Therefore, administration of the prodrug should result in improved gabapentin bioavailability, dose proportionality, and colonic absorption compared with administration of gabapentin.

Organic cation transporters

Over the last 15 years, a number of transporters that translocate organic cations were characterized functionally and also identified on the molecular level. Organic cations include endogenous compounds such as monoamine neurotransmitters, choline, and coenzymes, but also numerous drugs and xenobiotics. Some of the cloned organic cation transporters accept one main substrate or structurally similar compounds (oligospecific transporters), while others translocate a variety of structurally diverse organic cations (polyspecific transporters). This review provides a survey of cloned organic cation transporters and tentative models that illustrate how different types of organic cation transporters, expressed at specific subcellular sites in hepatocytes and renal proximal tubular cells, are assembled into an integrated functional framework. We briefly describe oligospecific Na(+)- and Cl(-)-dependent monoamine neurotransmitter transporters ( SLC6-family), high-affinity choline transporters ( SLC5-family), and high-affinity thiamine transporters ( SLC19-family), as well as polyspecific transporters that translocate some organic cations next to their preferred, noncationic substrates. The polyspecific cation transporters of the SLC22 family including the subtypes OCT1-3 and OCTN1-2 are presented in detail, covering the current knowledge about distribution, substrate specificity, and recent data on their electrical properties and regulation. Moreover, we discuss artificial and spontaneous mutations of transporters of the SLC22 family that provide novel insight as to the function of specific protein domains. Finally, we discuss the clinical potential of the increasing knowledge about polymorphisms and mutations in polyspecific organic cation transporters.

RENAL TRANSPORT OF ORGANIC COMPOUNDS MEDIATED BY MOUSE ORGANIC ANION TRANSPORTER 3 (MOAT3): FURTHER SUBSTRATE SPECIFICITY OF MOAT3

Organic anion transporter 3 [Oat3(Slc22a8)] plays an important role in the renal handling of organic compounds. The substrate specificity of rat Oat3 and human Oat3 has been elucidated; information on mouse Oat3 (mOat3) is less defined. The aim of this study was to extend the substrate selectivity of mOat3. When expressed in Xenopus laevis oocytes, mOat3 mediated the uptake of p-aminohippuric acid and estron sulfate (ES). In addition to these substrates, we found that several organic compounds such as prostaglandin E(2), prostaglandin F(2alpha), allopurinol, 6-mercaptopurine (6-MP), 5-fluorouracil (5-FU), and l-carnitine are substrates of mOat3, compounds identified for the first time. The apparent K(m) values for the uptake of mOat3 that mediated the transport of 6-MP, 5-FU, and l-carnitine were 4.01 +/- 0.7 microM, 53.9 +/- 8.9 nM, and 61.9 +/- 1.1 nM, respectively. Northern blot analysis revealed that gene coding for mOat3 is predominant in the kidney and, to a lesser extent, in the brain and the eye. Our findings thus provide further insights into the role of Oat3 in renal drug transport.

The SLC22 drug transporter family

The SLC22 family comprises organic cation transporters (OCTs), zwitterion/cation transporters (OCTNs), and organic anion transporters (OATs). These transporters contain 12 predicted alpha-helical transmembrane domains (TMDs) and one large extracellular loop between TMDs 1 and 2. Transporters of the SLC22 family function in different ways: (1) as uniporters that mediate facilitated diffusion in either direction (OCTs), (2) as anion exchangers (OAT1, OAT3 and URAT1), and (3) as Na(+)/ l-carnitine cotransporter (OCTN2). They participate in the absorption and/or excretion of drugs, xenobiotics, and endogenous compounds in intestine, liver and/or kidney, and perform homeostatic functions in brain and heart. The endogenous substrates include monoamine neurotransmitters, choline, l-carnitine, alpha-ketoglutarate, cAMP, cGMP, prostaglandins, and urate. Defect mutations of transporters of the SLC22 family may cause specific diseases such as "primary systemic carnitine deficiency" or "idiopathic renal hypouricemia" or change drug absorption or excretion.

CONSTITUTIVE EXPRESSION OF VARIOUS XENOBIOTIC AND ENDOBIOTIC TRANSPORTER mRNAs IN THE CHOROID PLEXUS OF RATS

The aim of this study was to quantitatively determine the constitutive expression levels of various transporter mRNAs in rat choroid plexus. To provide a reference for the relative expression levels, the expression of various transporter mRNAs in choroid plexus were compared with that in liver, kidney, and ileum. The mRNA levels of multidrug resistance protein (Mrp)1, 2, 3, 4, 5, and 6; multidrug resistance (Mdr)1a, 1b, and 2; organic anion transporting polypeptide (Oatp)1, 2, 3, 4, 5, 9, 12, and Oat-K (1/2); organic anion transporter (Oat)1, 2, and 3; organic cation transporter (Oct)1, 2, 3, N1, and N2; bile acid transporters sodium taurocholate cotransporting polypeptide (Ntcp), bile salt excretory protein (Bsep), and ileal bile acid transporter (Ibat); divalent metal transporter 1 (DMT1), Menke's and Wilson's metal transporters; equilibrative nucleotide transporters (Ent) 1 and 2, and constitutive nucleotide transporters (Cnt)1 and 2; peptide transporters (Pept)1 and 2; as well as ATP-binding cassette (Abc)G5 and 8 were measured in choroid plexus by the branched DNA signal amplification method. Mrp1, 4, and 5, Oatp3, Menke's transporter, DMT1, Ent1, and Pept2 mRNAs were expressed in choroid plexus at higher levels than in liver, kidney, or ileum. OctN1 and N2, Oatp2, Oat2 and 3, and Cnt1 and 2 mRNAs expressions were detectable in choroid plexus, but the levels were lower compared with that in liver, kidney, or ileum. The remaining transporters [Mrp2, Mrp3, Oct1, Oct2, Oatp1, Oatp4, Oatp5, Oatp12, Oat-K (1/2), Ntcp, Bsep, Ibat, Mdr1a, Mdr1b, Mdr2, Oat1, Ent2, Pept1, AbcG5, AbcG8] were expressed at very low levels in choroid plexus. The constitutive expression levels of different transporters in choroid plexus may provide an insight into the range of xenobiotics that can potentially be transported by the choroid plexus, thereby providing a means of xenobiotic detoxification in the brain.

Molecular cloning and functional characterization of the OCTN2 transporter at the RBE4 cells, an in vitro model of the blood-brain barrier

The transport of L-carnitine (4-N-trimethylamino-3-hydroxybutyric acid), a compound known to be transported by the organic cation transporter/carnitine transporter OCTN2, was studied in immortalized rat brain endothelial cells (RBE4). The cells were found to take up L-carnitine by a sodium-dependent process. This uptake process was saturable with an apparent Michaelis-Menten constant for L-carnitine of 54+/-10 microM and a maximal velocity of 215+/-35 pmol/mg protein/h. Besides L-carnitine, the cells also took up acetyl-L-carnitine and propionyl-L-carnitine in a sodium-dependent manner and TEA in a sodium-independent manner. RT-PCR with primers specific for the rat OCTN2 transporter revealed the existence of OCTN2 mRNA in RBE4 cells. Screening of a cDNA library from RBE4 cells with rat OCTN2 cDNA as a probe identified a positive clone which showed, when expressed in HeLa cells, the functional characteristics of OCTN2. The HeLa cells expressing the RBE4 OCTN2 cDNA showed a sixfold increase in L-carnitine uptake and a fourfold increase in TEA uptake in a sodium-containing buffer. Typical inhibitors for organic cation transporters (e.g. MPP(+) or TEA) showed an inhibitory effect on the transport of L-carnitine and TEA into the transfected cells. Similarly, unlabeled L-carnitine inhibited the transport of [3H]-L-carnitine and [14C]TEA in transfected HeLa cells. It is concluded that RBE4 cells, a widely used in vitro model of the blood-brain barrier (BBB), express the organic cation/carnitine transporter OCTN2.

A third human carnitine/organic cation transporter (OCTN3) as a candidate for the 5q31 Crohn's disease locus (IBD5)

Organic cation transporters function primarily in the elimination of cationic drugs in kidney, intestine, and liver. The murine organic cation/carnitine (Octn) transporter family, Octn1, Octn2, and Octn3 is clustered on mouse chromosome 11 (NCBI Accession No. NW_000039). The human OCTN1 and OCTN2 orthologs map to the syntenic IBD5 locus at 5q31, which has been shown to confer susceptibility to Crohn's disease. We show that the human OCTN3 protein, whose corresponding gene is not yet cloned or annotated in the human reference DNA sequence, does indeed exist and is uniquely involved in carnitine-dependent transport in peroxisomes. Its functional properties and inferred chromosomal location implicate it for involvement in Crohn's disease.