Engineered MATE multidrug transporters reveal two functionally distinct ion-coupling pathways in NorM from Vibrio cholerae

Multidrug and toxic compound extrusion (MATE) transport proteins confer multidrug resistance on pathogenic microorganisms and affect pharmacokinetics in mammals. Our understanding of how MATE transporters work, has mostly relied on protein structures and MD simulations. However, the energetics of drug transport has not been studied in detail. Many MATE transporters utilise the electrochemical H+ or Na+ gradient to drive substrate efflux, but NorM-VC from Vibrio cholerae can utilise both forms of metabolic energy. To dissect the localisation and organisation of H+ and Na+ translocation pathways in NorM-VC we engineered chimaeric proteins in which the N-lobe of H+-coupled NorM-PS from Pseudomonas stutzeri is fused to the C-lobe of NorM-VC, and vice versa. Our findings in drug binding and transport experiments with chimaeric, mutant and wildtype transporters highlight the versatile nature of energy coupling in NorM-VC, which enables adaptation to fluctuating salinity levels in the natural habitat of V. cholerae.

Omega-3 Polyunsaturated Fatty Acids Inhibit the Function of Human URAT1, a Renal Urate Re-Absorber

The beneficial effects of fatty acids (FAs) on human health have attracted widespread interest. However, little is known about the impact of FAs on the handling of urate, the end-product of human purine metabolism, in the body. Increased serum urate levels occur in hyperuricemia, a disease that can lead to gout. In humans, urate filtered by the glomerulus of the kidney is majorly re-absorbed from primary urine into the blood via the urate transporter 1 (URAT1)-mediated pathway. URAT1 inhibition, thus, contributes to decreasing serum urate concentration by increasing net renal urate excretion. Here, we investigated the URAT1-inhibitory effects of 25 FAs that are commonly contained in foods or produced in the body. For this purpose, we conducted an in vitro transport assay using cells transiently expressing URAT1. Our results showed that unsaturated FAs, especially long-chain unsaturated FAs, inhibited URAT1 more strongly than saturated FAs. Among the tested unsaturated FAs, eicosapentaenoic acid, α-linolenic acid, and docosahexaenoic acid exhibited substantial URAT1-inhibitory activities, with half maximal inhibitory concentration values of 6.0, 14.2, and 15.2 μM, respectively. Although further studies are required to investigate whether the ω-3 polyunsaturated FAs can be employed as uricosuric agents, our findings further confirm FAs as nutritionally important substances influencing human health.

Functional Expression of Choline Transporters in the Blood-Brain Barrier

Cholinergic neurons in the central nervous system play a vital role in higher brain functions, such as learning and memory. Choline is essential for the synthesis of the neurotransmitter acetylcholine by cholinergic neurons. The synthesis and metabolism of acetylcholine are important mechanisms for regulating neuronal activity. Choline is a positively charged quaternary ammonium compound that requires transporters to pass through the plasma membrane. Currently, there are three groups of choline transporters with different characteristics, such as affinity for choline, tissue distribution, and sodium dependence. They include (I) polyspecific organic cation transporters (OCT1-3: SLC22A1-3) with a low affinity for choline, (II) high-affinity choline transporter 1 (CHT1: SLC5A7), and (III) choline transporter-like proteins (CTL1-5: SLC44A1-5). Brain microvascular endothelial cells, which comprise part of the blood-brain barrier, take up extracellular choline via intermediate-affinity choline transporter-like protein 1 (CTL1) and low-affinity CTL2 transporters. CTL2 is responsible for excreting a high concentration of choline taken up by the brain microvascular endothelial cells on the brain side of the blood-brain barrier. CTL2 is also highly expressed in mitochondria and may be involved in the oxidative pathway of choline metabolism. Therefore, CTL1- and CTL2-mediated choline transport to the brain through the blood-brain barrier plays an essential role in various functions of the central nervous system by acting as the rate-limiting step of cholinergic neuronal activity.

Molecular Mechanisms of Amphetamines

There is a plethora of amphetamine derivatives exerting stimulant, euphoric, anti-fatigue, and hallucinogenic effects; all structural properties allowing these effects are contained within the amphetamine structure. In the first part of this review, the interaction of amphetamine with the dopamine transporter (DAT), crucially involved in its behavioral effects, is covered, as well as the role of dopamine synthesis, the vesicular monoamine transporter VMAT2, and organic cation 3 transporter (OCT3). The second part deals with requirements in amphetamine's effect on the kinases PKC, CaMKII, and ERK, whereas the third part focuses on where we are in developing anti-amphetamine therapeutics. Thus, treatments are discussed that target DAT, VMAT2, PKC, CaMKII, and OCT3. As is generally true for the development of therapeutics for substance use disorder, there are multiple preclinically promising specific compounds against (meth)amphetamine, for which further development and clinical trials are badly needed.

DKB114, A Mixture of Chrysanthemum Indicum Linne Flower and Cinnamomum Cassia (L.) J. Presl Bark Extracts, Improves Hyperuricemia through Inhibition of Xanthine Oxidase Activity and Increasing Urine Excretion

Chrysanthemum indicum Linne flower (CF) and Cinnamomum cassia (L.) J. Presl bark (CB) extracts have been used as the main ingredients in several prescriptions to treat the hyperuricemia and gout in traditional medicine. In the present study, we investigated the antihyperuricemic effects of DKB114, a CF, and CB mixture, and the underlying mechanisms in vitro and in vivo. DKB114 markedly reduced serum uric acid levels in normal rats and rats with PO-induced hyperuricemia, while increasing renal uric acid excretion. Furthermore, it inhibited the activity of xanthine oxidase (XOD) in vitro and in the liver in addition to reducing hepatic uric acid production. DKB114 decreased cellular uric acid uptake in oocytes and HEK293 cells expressing human urate transporter (hURAT)1 and decreased the protein expression levels of urate transporters, URAT1, and glucose transporter, GLUT9, associated with the reabsorption of uric acid in the kidney. DKB114 exerts antihyperuricemic effects and uricosuric effects, which are accompanied, partially, by a reduction in the production of uric acid and promotion of uric acid excretion via the inhibition of XOD activity and reabsorption of uric acid. Therefore, it may have potential as a treatment for hyperuricemia and gout.

The permeation mechanism of organic cations through a CNG mimic channel

Several channels, ranging from TRP receptors to Gap junctions, allow the exchange of small organic solute across cell membrane. However, very little is known about the molecular mechanism of their permeation. Cyclic Nucleotide Gated (CNG) channels, despite their homology with K+ channels and in contrast with them, allow the passage of larger methylated and ethylated ammonium ions like dimethylammonium (DMA) and ethylammonium (EA). We combined electrophysiology and molecular dynamics simulations to examine how DMA interacts with the pore and permeates through it. Due to the presence of hydrophobic groups, DMA enters easily in the channel and, unlike the alkali cations, does not need to cross any barrier. We also show that while the crystal structure is consistent with the presence of a single DMA ion at full occupancy, the channel is able to conduct a sizable current of DMA ions only when two ions are present inside the channel. Moreover, the second DMA ion dramatically changes the free energy landscape, destabilizing the crystallographic binding site and lowering by almost 25 kJ/mol the binding affinity between DMA and the channel. Based on the results of the simulation the experimental electron density maps can be re-interpreted with the presence of a second ion at lower occupancy. In this mechanism the flexibility of the channel plays a key role, extending the classical multi-ion permeation paradigm in which conductance is enhanced by the plain interaction between the ions.

Organic cation transporter 3: A cellular mechanism underlying rapid, non-genomic glucocorticoid regulation of monoaminergic neurotransmission, physiology, and behavior

Contribution to Special Issue on Fast effects of steroids. Corticosteroid hormones act at intracellular glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) to alter gene expression, leading to diverse physiological and behavioral responses. In addition to these classical genomic effects, corticosteroid hormones also exert rapid actions on physiology and behavior through a variety of non-genomic mechanisms, some of which involve GR or MR, and others of which are independent of these receptors. One such GR-independent mechanism involves corticosteroid-induced inhibition of monoamine transport mediated by "uptake₂" transporters, including organic cation transporter 3 (OCT3), a low-affinity, high-capacity transporter for norepinephrine, epinephrine, dopamine, serotonin and histamine. Corticosterone directly and acutely inhibits OCT3-mediated transport. This review describes the studies that initially characterized uptake₂ processes in peripheral tissues, and outlines studies that demonstrated OCT3 expression and corticosterone-sensitive monoamine transport in the brain. Evidence is presented supporting the hypothesis that corticosterone can exert rapid, GR-independent actions on neuronal physiology and behavior by inhibiting OCT3-mediated monoamine clearance. Implications of this mechanism for glucocorticoid-monoamine interactions in the context-dependent regulation of behavior are discussed.

[Recent advances in urate metabolism]

In the last fifteen years, genomics and other -omics sciences have revolutionized our understanding of biological processes at the molecular level. An illustrative example is urate metabolism. Before the publication of the complete human genome, in 2003 it was believed that a single enzyme (urate oxidase) was responsible for uricolysis that is the conversion of urate into the more soluble allantoin. Now we know with great detail that this process requires the consecutive action of three enzymes that have been lost by gene inactivation in our hominoid ancestor. Similarly, a single urate transporter (URAT1) was known at that time. Now we have evidence that urate homeostasis depends on a complex set of transporters located on the epithelial cells of the kidney and the intestine. In this review article, we give an account of the recent discoveries on urate metabolism and how these discoveries can be applied to the development of novel drugs to treat hyperuricemia, tumor lysis syndrome and the Lesch-Nyhan disease.

Urinary Dopamine as a Potential Index of the Transport Activity of Multidrug and Toxin Extrusion in the Kidney

Dopamine is a cationic natriuretic catecholamine synthesized in proximal tubular cells (PTCs) of the kidney before secretion into the lumen, a key site of its action. However, the molecular mechanisms underlying dopamine secretion into the lumen remain unclear. Multidrug and toxin extrusion (MATE) is a H⁺/organic cation antiporter that is highly expressed in the brush border membrane of PTCs and mediates the efflux of organic cations, including metformin and cisplatin, from the epithelial cells into the urine. Therefore, we hypothesized that MATE mediates dopamine secretion, a cationic catecholamine, into the tubule lumen, thereby regulating natriuresis. Here, we show that [³H]dopamine uptake in human (h) MATE1-, hMATE-2K- and mouse (m) MATE-expressing cells exhibited saturable kinetics. Fluid retention and decreased urinary excretion of dopamine and Na⁺ were observed in Mate1-knockout mice compared to that in wild-type mice. Imatinib, a MATE inhibitor, inhibited [³H]dopamine uptake by hMATE1-, hMATE2-K- and mMATE1-expressing cells in a concentration-dependent manner. At clinically-relevant concentrations, imatinib inhibited [³H]dopamine uptake by hMATE1- and hMATE2-K-expressing cells. The urinary excretion of dopamine and Na⁺ decreased and fluid retention occurred in imatinib-treated mice. In conclusion, MATE transporters secrete renally-synthesized dopamine, and therefore, urinary dopamine has the potential to be an index of the MATE transporter activity.

Dysfunction of organic anion transporting polypeptide 1a1 alters intestinal bacteria and bile acid metabolism in mice

Organic anion transporting polypeptide 1a1 (Oatp1a1) is predominantly expressed in liver and is able to transport bile acids (BAs) in vitro. Male Oatp1a1-null mice have increased concentrations of taurodeoxycholic acid (TDCA), a secondary BA generated by intestinal bacteria, in both serum and livers. Therefore, in the present study, BA concentrations and intestinal bacteria in wild-type (WT) and Oatp1a1-null mice were quantified to investigate whether the increase of secondary BAs in Oatp1a1-null mice is due to alterations in intestinal bacteria. The data demonstrate that Oatp1a1-null mice : (1) have similar bile flow and BA concentrations in bile as WT mice; (2) have a markedly different BA composition in the intestinal contents, with a decrease in conjugated BAs and an increase in unconjugated BAs; (3) have BAs in the feces that are more deconjugated, desulfated, 7-dehydroxylated, 3-epimerized, and oxidized, but less 7-epimerized; (4) have 10-fold more bacteria in the small intestine, and 2-fold more bacteria in the large intestine which is majorly due to a 200% increase in Bacteroides and a 30% reduction in Firmicutes; and (5) have a different urinary excretion of bacteria-related metabolites than WT mice. In conclusion, the present study for the first time established that lack of a liver transporter (Oatp1a1) markedly alters the intestinal environment in mice, namely the bacteria composition.

[Drug-induced hepatic injury, the challenge for cause investigation]

This report is the story of the long journey to identify the mechanism of fulminant hepatitis by the antigout drug, Benzbromarone. As soon as the 8th gout patient prescribed Benzbromarone (Benz) died of fulminant hepatitis in 20 years, the letter was sent to doctors identifying it as the causative agent in February 2000. At that time, Benz had been prescribed to 350,000 patients/year for 20 years. Is Benz the real cause of fulminant hepatitis? 1. Benz is a PPARa agonist like fenofibrate, and not a PPARgamma like troglitazone. 2. Troglitazone and Allopurinol have shown apoptosis in a human primary hepatocyte culture with the DNA laddering method, but Benz has not. 3. It was reported in 1979 that benzarone is a metabolite dissociated from two Br bases of Benz, but Walter et al. reported in 1987 that the Br base was not dissociated. Benzarone was not produced by an in vitro study with human S-9 and by an in vivo clinical study of Japanese volunteers. 4. The main metabolite of Benz in humans is 6-OH Benz, which has URAT-1 activity, like Benz. 5. It has been newly discovered that CYP2C9 is only one hepatic metabolism enzyme of Benz. 6. The rate of poor metabolizers of CYP2C9*3 (homozygous) in Japan is 1/2500, meanwhile, the rate of fulminant hepatitis at this time is 8 patients in 20 years, with 350,000 patients/year; therefore, it is difficult to view poor metabolizers as the cause. 7. Hepatic injury by Benz is an idiosyncrasy, the same as with many other drugs.

Membrane potential and pH-dependent accumulation of decynium-22 (1,1'-diethyl-2,2'-cyanine iodide) flourencence through OCT transporters in astrocytes

1,1 '-Diethyl-2,2'-cyanine iodide (decynium22; D22) is a potent blocker of the organic cation family of transporters (EMT/OCT) known to move endogenous monoamines like dopamine and norepinephrine across cell membranes. Decynium22 is a cation with a relatively high affinity for all members of the OCT family in both human and rat cells. The mechanism through which decynium22 blocks OCT transporters are poorly understood. We tested the hypothesis that denynium22 may compete with monoamines utilizing OCT to permeate the cells. Using the ability of D22 to aggregate and produce fluorescence at 570 nm, we measured D22 uptake in cultured astrocytes. The rate of D22 uptake was strongly depressed by acid pH and by elevated external K+. The rate of uptake was similar to that displayed by 4-(4-(dimethylamino)-styryl)-N-methylpyridinium (ASP+), a well established substrate for OCT and high-affinity Na+-dependent monoamine transporters. These data were supported by measurement of electrogenic uptake using whole cell voltage clamp recording. Decynium22 depressed norepinephrine, but not glutamate uptake. These data are also consistent with the described OCT transporter characteristics. Taken together, our results suggest that decynium22 accumulation might be a useful instrument to study monoamine transport in the brain, and particularly in astrocytes, where they may play a prominent role in monoamine uptake during brain dysfunction related to monoamines (like Parkinson disease) and drug addiction.

[Human renal urate transpoter URAT1 mediates the transport of salicylate]

Salicylic acid derivatives are the most prescribed analgesic-antipyretic and anti-inflammatory agents. It is well known that salicylate has a paradoxical effect on renal urate excretion. At low doses (5 - 10 mg/dL serum), renal urate excretion is decreased, whereas at high doses (> 15 mg/dL serum), renal urate excretion is increased. Since the molecular identification of the renal apical urate/anion exchanger URAT1, it has been suggested that this protein is responsible for the paradoxical effect because of cis-inhibition of salicylate (1 mM) on urate uptake by URAT1-expressing oocytes. The purpose of this study was to examine whether or not URAT1 is responsible for the paradoxical effect of salicylate. In URAT1-stably expressing HEK293 (HEK293-URAT1) cells, salicylate inhibited [14C] urate uptake dose-dependently (IC50, 23.9 microM). URAT1 mediated the time-dependent uptake of [3H] salicylate in these cells. [3H] Salicylate uptake via URAT1 was inhibited by non-labelled urate and salicylate, and the uricosuric agent, benzbromarone. In the URAT1-expressing oocytes, we observed the time- and concentration-dependent transport of salicylate (Km : 25.3 microM). Moreover, non-labelled salicylate injected into the URATI-expressing oocytes stimulated [14C] urate uptake. These results suggest that the "paradoxical effect" of salicylate can be explained by two modes of salicylate interaction with URAT1 : (1) acting as an exchange substrate to facilitate urate reabsorption, and (2) acting as an inhibitor for urate reabsorption.

[Advances in the study of regulation of novel organic cation transporter-2]

Novel organic cation transporter-2 (OCTN2), a member of the organic cation transporter family, may transport carnitine and multiple organic cationic drugs. Thus OCTN2 possesses substantial roles in physiology and pharmacology. A number of researches have proven that many factors can regulate the expression and/or function of OCTN2 via different pathways, and then may affect the homeostasis and disposition of drugs. This paper reviews recent progresses in this field.

[Physiological functions of carnitine and carnitine transporters in the central nervous system]

L-Carnitine is an essential co-factor in the metabolism of lipids and consequently in the production of cellular energy. This molecule has important physiological roles, including its involvement in the beta-oxidation of fatty acids by facilitating the transport of long-chain fatty acids across the mitochondrial inner membrane as acylcarnitine esters. In the brain, L-carnitine and acetyl-L-carnitine have important roles in cerebral bioenergetics and in neuroprotection through a variety of mechanisms including their antioxidant properties and in the modulation and promotion of synaptic neurotransmission, most notably cholinergic neurotransmission. Acetyl-L-carnitine was successfully applied as pharmacological agents for treatment of chronic degenerative diseases of the senile brain and for slowing down the progression of mental deterioration in Alzheimer's disease, and they may involve both the cholinergic neuronal transmission activity of acetyl-L-carnitine and its ability to enhance neuronal metabolism in mitochondria. Astrocytes are able to produce large amounts of ketone bodies, which are thought to supply adjacent neurons with easily transferable substrates for generation of energy. Thus, the L-carnitine uptake mechanism becomes the rate-limiting step for astrocyte ketogenesis. Several carnitine transporters have been known to be present in peripheral tissues. In this review, the functional expression and physiological role of carnitine transporters in central nervous system is further discussed.

[Molecular mechanisms of urate transport in renal tubules: localization and function of urate transporters]

Urate, a naturally occurring product of purine metabolism, is present at higher levels in human blood than in other mammals, because humans have an effective renal urate reabsorption system in addition to their evolutionary loss of hepatic uricase by mutational silencing. The urate transporter URAT1 encoded by SLC22A12 is a urate anion exchanger regulating blood urate levels. URAT1 is localized in the apical membrane of renal proximal tubules and targeted by uricosuric and antiuricosuric agents. Idiopathic renal hypouricemia is due to the genetic defect of SLC22A12. Recently it has been shown that the proximal tubule apical membrane organic anion transporter OAT4 transports urate at low affinity and responsible for the hyperuricemia cased by thiazide diuretics. Transport of urate via URAT1 is driven by the intracellular lactate that is accumulated by Na+/lactate cotransporter slc5a8 and slc5a12. URAT1 is proposed to be involved in the multimolecular complex "transportsome" that allows the cooperation of multiple transporters.

[Primary hyperuricemia due to decreased renal uric acid excretion]

Hyperuricemia reflects extracellular fluid supersaturation for uric acid. Although dietary, genetic, or disease-related excesses in urate production underlie hyperuricemia in some cases, impaired renal excretion of uric acid is the dominant cause of hyperuricemia. This type of hyperuricemia may be primary (idiopathic) and unassociated with an identifiable disorder. Two important candidates that may affect renal urate excretion were identified recently. One is an organic anion transporter (OAT) family member called urate transporter (URAT) 1. URAT1 has highly specific urate transport activity, exchanging this anion with others including most of the endogenous organic anions and drug anions that are known to affect renal uric acid transport. Another is uromodulin (UMOD), which is the key protein for the pathogenesis of familial juvenile hyperuricemic nephropathy that is characterized by early onset of hyperuricemia and renal failure. The role of these proteins in the cause of hyperuricemia is under investigation.

[Historical review of gout and hyperuricemia investigations]

Historical development of gout and hyperuricemia investigations was reviewed. Gout has been a recognized disease since the fifth century B.C. In 1683, Sydenham described the detailed clinical features of the disease based on his own condition. Leeuwenhoek (1679) first described crystals in a gouty tophus, which were identified as uric acid by Wollaston (1797). Since uric acid clearance of hyperuricemia was markedly lower than that in normal controls, early investigators considered that the main cause of hyperuricemia was urate underexcretion. However, in the 1940s, studies on uric acid metabolism using isotope tracer techniques demonstrated that a part of hyperuricemia resulted from urate overproduction, which was detected in approximately one-third of all gouty patients. In the 1970s, micropuncture, microinjection and microperfusion methods as well as stop-flow methods demonstrated that uric acid transports in nephron were suspected to consist of four steps, that were glomerular filtration, reabsorption, secretion and postsecretory reabsorption. The majority of filtrated uric acid was almost completely reabsorbed, followed by secretion and postsecretory reabsorption at a proximal site in the tubulus. Each proportion of transports to the glomerular filtration(100%) was estimated approximately 99%, 50% and 40%, respectively. Subsequently, about 10% of the filtrate was excreted in the urine. The authors (1999) suggested that the secretion rate of hyperuricemic patients was significantly lower than that of normal controls but postsecretory reabsorption was not. Therefore, the decrease in the secretion rate was suspected to be the main cause of underexcretion. Dunkan (1960) reported a family demonstrating hyperuricemia associated with severe renal damage that progressed rapidely. Currently, this disease is called familial juvenile hyperuricemic nephropathy (FJHN), and was recently found to be the result of a variation in uromodulin. Enomoto (2002) found a number of urate transporters in the cell surface of the tubulus, among which URAT1 was the most effective in reabsorbing urate from the tubulus lumen to the cells. The urate was released to the blood vessel side by the other transporter OAT. Therefore, URAT1 was suspected to be a cause of underexcretion. As the mechanism underlying overproduction of uric acid, de novo purine nucleotide synthesis has been shown to be increased. In some cases, the increase in de novo synthesis is the result of gene mutation in purine nucleotide synthesis enzymes, such as PRPP synthetase (Sperling, 1973) as well as hypoxanthine guanine phosphoribosylpyrophosphate synthetase (Seegmiller, 1967). However, the mechanism in majority of the overproduction has not yet been clarified and is currently under investigation.

Novel SLC22A16 polymorphisms and influence on doxorubicin pharmacokinetics in Asian breast cancer patients

OBJECTIVE

To identify novel polymorphisms in the solute carrier SLC22A16 gene and determine their influence on the pharmacokinetics of doxorubicin and doxorubicinol in Asian breast cancer patients.

METHODS

SLC22A16 coding regions were screened in a total of 400 healthy subjects belonging to three distinct Asian ethnic groups (Chinese [n = 100], Malays [n = 100] and Indians [n = 100]) and in the Caucasian population (n = 100). Pharmacokinetic parameters of doxorubicin and doxorubicinol were estimated in Asian breast cancer patients undergoing adjuvant chemotherapy to investigate genotype-phenotype correlations.

RESULTS

Four novel polymorphisms (c.146A>G [exon 2], c.312T>C, c.755T>C [exon 4] and c.1226T>C [exon 5]) were identified. The genotypic frequency of the homozygous c.146GG polymorphism was approximately twofold higher in the healthy Chinese (13%) & Malay (18%) populations compared with the Indian (7%) and Caucasian (9%) populations. The genotypic frequency of the c.1226T>C polymorphism was observed to be significantly higher among the Caucasian (11%) and Indian (8%) study subjects compared with the Chinese (1%) and Malay (1%) ethnic groups (p < 0.005 in each case). Breast cancer patients harboring the 146GG genotype showed a trend towards higher exposure levels to doxorubicin (AUC(0 negative infinity)/dose/body surface area [BSA] [hm(-5)]: 21.6; range: 18.8-27.7) compared with patients with either the reference genotype (AUC(0 negative infinity)/dose/BSA[hm(-5)]: 17.4; range: 8.2-26.3, p = 0.066) or heterozygotes (AUC(0 negative infinity)/dose/BSA[hm(-5)]: 15.4; range: 6.2-38.0, p = 0.055). The exposure levels of doxorubicinol were also higher in patients harboring the variant 146GG genotype (AUC(0 negative infinity)/dose/BSA[hm(-5)]: 13.3; range: 8.8-21.7) when compared with patients harboring the reference genotype (AUC(0 negative infinity)/dose/BSA[hm(-5)]): 9.8; range: 6.1-24.3, p = 0.137) or heterozygotes (AUC(0 negative infinity)/dose/BSA[hm(-5)]: 8.98; range: 3.7-20.6, p = 0.047).

CONCLUSION

Among the four novel SLC22A16 polymorphisms identified, the c.146A>G and c.1226T>C polymorphisms exhibited interethnic variations in allele and genotype frequencies. This exploratory study suggests that the c.146A>G variation could contribute to the variations in the pharmacokinetics of doxorubicin and doxorubicinol in Asian cancer patients. Further in vitro studies are required to determine the functional impact of these novel polymorphisms on doxorubicin pharmacokinetics in cancer patients.

Transport of butyryl-L-carnitine, a potential prodrug, via the carnitine transporter OCTN2 and the amino acid transporter ATB(0,+)

L-carnitine is absorbed in the intestinal tract via the carnitine transporter OCTN2 and the amino acid transporter ATB(0,+). Loss-of-function mutations in OCTN2 may be associated with inflammatory bowel disease (IBD), suggesting a role for carnitine in intestinal/colonic health. In contrast, ATB(0,+) is upregulated in bowel inflammation. Butyrate, a bacterial fermentation product, is beneficial for prevention/treatment of ulcerative colitis. Butyryl-L-carnitine (BC), a butyrate ester of carnitine, may have potential for treatment of gut inflammation, since BC would supply both butyrate and carnitine. We examined the transport of BC via ATB(0,+) to determine if this transporter could serve as a delivery system for BC. We also examined the transport of BC via OCTN2. Studies were done with cloned ATB(0,+) and OCTN2 in heterologous expression systems. BC inhibited ATB(0,+)-mediated glycine transport in mammalian cells (IC(50), 4.6 +/- 0.7 mM). In Xenopus laevis oocytes expressing human ATB(0,+), BC induced Na(+) -dependent inward currents under voltage-clamp conditions. The currents were saturable with a K(0.5) of 1.4 +/- 0.1 mM. Na(+) activation kinetics of BC-induced currents suggested involvement of two Na(+) per transport cycle. BC also inhibited OCTN2-mediated carnitine uptake (IC(50), 1.5 +/- 0.3 microM). Transport of BC via OCTN2 is electrogenic, as evidenced from BC-induced inward currents. These currents were Na(+) dependent and saturable (K(0.5), 0.40 +/- 0.02 microM). We conclude that ATB(0,+) is a low-affinity/high-capacity transporter for BC, whereas OCTN2 is a high-affinity/low-capacity transporter. ATB(0,+) may mediate intestinal absorption of BC when OCTN2 is defective.

Choline transporters in human lung adenocarcinoma: expression and functional implications

Choline is an essential nutrient for cell survival and proliferation, however, the expression and function of choline transporters have not been well identified in cancer. In this study, we detected the mRNA and protein expression of organic cation transporter OCT3, carnitine/cation transporters OCTN1 and OCTN2, and choline transporter-like protein CTL1 in human lung adenocarcinoma cell lines A549, H1299 and SPC-A-1. Their expression pattern was further confirmed in 25 human primary adenocarcinoma tissues. The choline uptake in these cell lines was significantly blocked by CTL1 inhibitor, but only partially inhibited by OCT or OCTN inhibitors. The efficacy of these inhibitors on cell proliferation is closely correlated with their abilities to block choline transport. Under the native expression of these transporters, the total choline uptake was notably blocked by specific PI3K/AKT inhibitors. These results describe the expression of choline transporters and their relevant function in cell proliferation of human lung adenocarcinoma, thus providing a potential choline-starvation strategy of cancer interference through targeting choline transporters, especially CTL1.

Possible involvement of organic anion and cation transporters in renal excretion of xanthine derivatives, 3-methylxanthine and enprofylline

Whether organic anion and cation transporters are involved in the renal excretion of xanthine derivatives, 3-methylxanthie and enprofylline, remains unclear. In this study, we have investigated the effects of typically predominant substrates for organic anion and cation transporters on the tubular secretion of 3-methylxanthine and enprofylline in rats. In the renal clearance experiments using typical substrates for organic anion transporters, probenecid and p-aminohippurate, probenecid (20 mg/kg), but not p-aminohippurate (100 mg/kg), significantly decreased the renal clearance and clearance ratio of 3-methylxanthine and enprofylline. The typical substrates for organic cation transport systems, tetraethylammonium (30.6 mg/kg) and cimetidine (50 or 100 mg/kg), significantly decreased the renal clearance and clearance ratio of 3-methylxanthine and enprofylline. These results suggest that the renal secretory transport of 3-methylxanthine and enprofylline are mediated by probenecid-, cimetidine- and tetraethylammonium-sensitive transport systems. Uric acid, an organic anion, significantly inhibited the renal secretion of 3-methylxanthine, but not enprofylline, suggesting that the renal tubular transport of 3-methylxanthine is also mediated via uric acid-sensitive transport system. These findings suggest the possibility that both organic anion and cation transporters are, at least, involved in the renal tubular transport of 3-methylxanthine and enprofylline in rats.

Gender differences in kidney function

Sex hormones influence the development of female (F) and male (M) specific traits and primarily affect the structure and function of gender-specific organs. Recent studies also indicated their important roles in regulating structure and/or function of nearly every tissue and organ in the mammalian body, including the kidneys, causing gender differences in a variety of characteristics. Clinical observations in humans and studies in experimental animals in vivo and in models in vitro have shown that renal structure and functions under various physiological, pharmacological, and toxicological conditions are different in M and F, and that these differences may be related to the sex-hormone-regulated expression and action of transporters in the apical and basolateral membrane of nephron epithelial cells. In this review we have collected published data on gender differences in renal functions, transporters and other related parameters, and present our own microarray data on messenger RNA expression for various transporters in the kidney cortex of M and F rats. With these data we would like to emphasize the importance of sex hormones in regulation of a variety of renal transport functions and to initiate further studies of gender-related differences in kidney structure and functions, which would enable us to better understand occurrence and development of various renal diseases, pharmacotherapy, and drug-induced nephrotoxicity in humans and animals.

PPAR alpha-activation results in enhanced carnitine biosynthesis and OCTN2-mediated hepatic carnitine accumulation

In fasted rodents hepatic carnitine concentration increases considerably which is not observed in PPAR alpha-/- mice, indicating that PPAR alpha is involved in carnitine homeostasis. To investigate the mechanisms underlying the PPAR alpha-dependent hepatic carnitine accumulation we measured carnitine biosynthesis enzyme activities, levels of carnitine biosynthesis intermediates, acyl-carnitines and OCTN2 mRNA levels in tissues of untreated, fasted or Wy-14643-treated wild type and PPAR alpha-/- mice. Here we show that both enhancement of carnitine biosynthesis (due to increased gamma-butyrobetaine dioxygenase activity), extra-hepatic gamma-butyrobetaine synthesis and increased hepatic carnitine import (OCTN2 expression) contributes to the increased hepatic carnitine levels after fasting and that these processes are PPAR alpha-dependent.

Decreased biosynthesis of lung surfactant constituent phosphatidylcholine due to inhibition of choline transporter by gefitinib in lung alveolar cells

PURPOSE

We investigated whether gefitinib, an anticancer agent, inhibits phosphatidylcholine (PC) biosynthesis and choline uptake by alveolar epithelial type II cells.

MATERIALS AND METHODS

Uptake of choline and PC biosynthesis were examined in vitro, using human alveolar epithelia-derived cell line A549 and rat alveolar type (AT) II cells as models.

RESULTS

Gefitinib reduced the incorporation of [3H]choline into PC in A549 and rat ATII cells. The uptake of [3H]choline by A549 and rat ATII cells was concentration-dependent, and the Km values were 15.0 and 10-100 microM, respectively. The uptake of [3H]choline by A549 and rat ATII cells was weakly Na+-dependent, and inhibited by hemicholinium-3. RT-PCR revealed expression of choline transporter-like protein (CTL)1 and organic cation transporter (OCT)3 mRNAs in both cells. The choline uptake by A549 and rat ATII cells was strongly inhibited by gefitinib with the IC50 value of 6.77 microM and 10.5 microM, respectively.

CONCLUSIONS

Our results demonstrate that gefitinib reduces PC biosynthesis via inhibition of cellular choline uptake by A549 and rat ATII cells, which is mainly mediated by CTL1, resulting in abnormality of lung surfactant that can be one of mechanisms of the interstitial lung disease associated with gefitinib.

Probing the substrate specificity of the ergothioneine transporter with methimazole, hercynine, and organic cations

Recently, we have identified the ergothioneine (ET) transporter ETT (gene symbol SLC22A4). Much interest in human ETT has been generated by case-control studies that suggest an association of polymorphisms in the SLC22A4 gene with susceptibility to chronic inflammatory diseases. ETT was originally designated a multispecific novel organic cation transporter (OCTN1). Here we reinvestigated, based on stably transfected 293 cells and with ET as reference substrate, uptake of quinidine, verapamil, and pyrilamine. ETT from human robustly catalyzed transport of ET (68micfrol/(minmgprotein)), but no transport of organic cations was discernible. With ET as substrate, ETT was relatively resistant to inhibition by selected drugs; the most potent inhibitor was verapamil (K(i)=11micromol/l). The natural compound hercynine and antithyroid drug methimazole are related in structure to ET. However, efficiency of ETT-mediated transport of methimazole (K(i)=7.5mmol/l) was 130-fold lower, and transport of hercynine (K(i)=1.4mmol/l) was 25-fold lower than transport of ET. ETT from mouse, upon expression in 293 cells, catalyzed high affinity, sodium-driven uptake of ET very similar to ETT from human. Additional real-time PCR experiments based on 16 human tissues revealed ETT mRNA levels considerably lower than in bone marrow. Our experiments establish that ETT is highly specific for its physiological substrate ergothioneine. ETT is not a cationic drug transporter, and it does not have high affinity for organic cation inhibitors. Detection of ETT mRNA or protein can therefore be utilized as a specific molecular marker of intracellular ET activity.

Role of MAPK and PKA in regulation of rbOCT2-mediated renal organic cation transport

The effects of protein kinases MAPK and PKA on the regulation of organic cation transporter 2 (OCT2) were investigated both in a heterologous cell system [Chinese hamster ovary (CHO-K1) cells stably transfected with rabbit (rb)OCT2] and in native intact rabbit renal proximal S2 segments. Inhibition of MEK (by U-0126) or PKA (by H-89) reduced transport activity of rbOCT2 in CHO-K1 cells. The inhibitory effect of U-0126 combined with H-89 produced no additive effect, indicating that the action of PKA and MAPK in the regulation of rbOCT2 is in a common pathway. Activation of PKA by forskolin stimulated rbOCT2 activity, and this stimulatory effect was eliminated by H-89, indicating that the stimulation required PKA activation. In S2 segments of rabbit renal proximal tubules, activation of MAPK (by EGF) and PKA (by forskolin) stimulated activity of rbOCT2, and this activation was abolished by U-0126 and H-89, respectively. This is the first study to show that MAPK and PKA are involved, apparently in a common pathway, in the regulation of OCT2 activity in both a heterologous cell system and intact renal proximal tubules.

Molecular identification and functional characterization of rabbit MATE1 and MATE2-K

An electroneutral organic cation (OC)/proton exchanger in the apical membrane of proximal tubules mediates the final step of renal OC excretion. Two members of the multidrug and toxin extrusion family, MATE1 and MATE2-K, were recently identified in human and rodent kidney and proposed to be the molecular basis of renal OC/H+ exchange. To take advantage of the comparative value of the large database on the kinetic and selectivity characteristics of OC/H+ exchange that exists for rabbit kidney, we cloned rbMATE1 and rbMATE2-K. The rabbit homologs have 75% (MATE1) and 74% (MATE2-K) amino acid identity to their human counterparts (and 51% identity with each other). rbMATE1 and rbMATE2-K exhibited H+ gradient-dependent uptake and efflux of tetraethylammonium (TEA) when expressed in Chinese hamster ovary cells. Both transporters displayed similar affinities for selected compounds [IC50 values within 2-fold for TEA, 1-methyl-4-phenylpyridinium, and quinidine] and very different affinities for others (IC50 values differing by 8- to 80-fold for choline and cimetidine, respectively). These results indicate that rbMATE1 and rbMATE2-K are multispecific OC/H+ exchangers with similar, but distinct, functional characteristics. Overall, the selectivity of MATE1 and MATE2-K correlated closely with that observed in rabbit renal brush-border membrane vesicles.

[Chronic fatigue syndrome and neurotransmitters]

Chronic fatigue syndrome (CFS) is an idiopathic illness characterized by persistent fatigue, which could be caused by a variety of etiologic factors including viral infection, abnormal production of cytokines and abnormal acylcarnitine metabolism. Recent studies suggest that CFS is closely associated with attenuation of central synaptic transmission mediated by neurotransmitters such as serotonin and glutamate. Attenuation of serotonin neurotransmission can be caused by increased expression of serotonin transporter, which results either from viral infection and subsequent production of interferon--alpha or from abnormal promoter for serotonin transporter gene. Other neurotransmitter systems may be also involved in CFS mediated by abnormal acylcarnitine metabolism and autoantibodies for neurotransmitter receptors. In this review, we focus recent data on CFS in terms of neurotransmitters.

Absorption of tetraethylammonium (TEA+) by perfused lobster intestine

The organic cation, tetraethylammonium (TEA(+)), is actively secreted by mammalian nephrons and crustacean urinary bladders by similar processes in both animal groups. These mechanisms consist of a basolateral Organic Cation Transporter (OCT family) that employs the transmembrane electrical potential as a driving force for organic cation uptake from the blood and a brush border secondary active transport process that exchanges luminal protons for TEA(+). The present study examined the nature of (14)C-TEA(+) transport across the perfused intestinal epithelium of the American lobster, Homarus americanus, to ascertain whether the gut complemented the kidneys in the clearance of these organic metabolites from the blood. Unidirectional mucosa to serosa (M to S) (14)C-TEA(+) fluxes in anterior and posterior intestine were hyperbolic functions of luminal [TEA(+)] and significantly (P<0.01) exceeded the respective serosa to mucosa (S to M) fluxes. Luminal quinine (1 mM) significantly (P<0.05) inhibited M to S flux of the organic cation, while serosal addition of the drug had no effect on S to M transfer of TEA(+). Reducing serosal pH from 7.20 to 6.02 significantly (P<0.01) stimulated M to S transfer of 0.1 mM (14)C-TEA(+), but significantly (P<0.05) lowered S to M transfer of the metabolite. Addition of 2.0 mM unlabelled serosal TEA(+) trans-stimulated the M to S flux of 0.1 mM (14)C-TEA and doubled the transfer rate of the organic cation from lumen to blood compared to its transport in the absence of TEA(+) in the bath. Results suggest that this organic cation is absorbed across lobster intestine by the combination of a brush border OCT-1-like transporter coupled with a basolateral H(+)/TEA(+) exchanger. A working model is presented for intestinal organic cation absorption in crustaceans and compared to the secretory transport model for this class of metabolites previously reported for crustacean and mammalian kidneys.

Substrate specificity of MATE1 and MATE2-K, human multidrug and toxin extrusions/H(+)-organic cation antiporters

The substrate specificities of human (h) multidrug and toxin extrusion (MATE) 1 and hMATE2-K were examined to find functional differences between these two transporters by the transfection of the cDNA of hMATE1 and hMATE2-K into HEK293 cells. Western blotting revealed specific signals for hMATE1 and hMATE2-K consistent with a size of 50 and 40kDa, respectively, in the transfectants as well as human renal brush-border membranes under reducing conditions. In the presence of oppositely directed H(+)-gradient, the transport activities of various compounds such as tetraethylammonium, 1-methyl-4-phenylpyridinium, cimetidine, metformin, creatinine, guanidine, procainamide, and topotecan were stimulated in hMATE1- and hMATE2-K-expressing cells. In addition to cationic compounds, anionic estrone sulfate, acyclovir, and ganciclovir were also recognized as substrates of these transporters. Kinetic analyses demonstrated the Michaelis-Menten constants for the hMATE1-mediated transport of tetraethylammonium, 1-methyl-4-phenylpyridinium, cimetidine, metformin, guanidine, procainamide, topotecan, estrone sulfate, acycrovir, and ganciclovir to be (in mM) 0.38, 0.10, 0.17, 0.78, 2.10, 1.23, 0.07, 0.47, 2.64, and 5.12, respectively. Those for hMATE2-K were 0.76, 0.11, 0.12, 1.98, 4.20, 1.58, 0.06, 0.85, 4.32, and 4.28, respectively. Although their affinity for hMATE1 and hMATE2-K was similar, the zwitterionic cephalexin and cephradine were revealed to be specific substrates of hMATE1, but not of hMATE2-K. Levofloxacin and ciprofloxacin were not transported, but were demonstrated to be potent inhibitors of these transporters. These results suggest that hMATE1 and hMATE2-K function together as a detoxication system, by mediating the tubular secretion of intracellular ionic compounds across the brush-border membranes of the kidney.

Low-affinity uptake of the fluorescent organic cation 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide (4-Di-1-ASP) in BeWo cells

Understanding the mechanisms of transport processes in the placenta can improve the safety and efficacy of drug delivery during pregnancy. Functional studies of organic cation transporters (OCTs) are usually carried out using radioactivity, and a fluorescent marker would add flexibility to experimental methods. As a published substrate for OCT1 and OCT2, the fluorescent compound 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide (4-Di-1-ASP) was chosen as a candidate for studying placental OCT function in BeWo cells. The expression of OCT1 and OCT2 was also investigated in BeWo cells, an established human choriocarcinoma trophoblastic cell line frequently used as an in vitro model of the rate-limiting barrier for maternal-fetal exchange of drugs and nutrients within the placenta. 4-Di-1-ASP was taken up into BeWo cells by a low-affinity, carrier-mediated process exhibiting a Km of 580+/-110 microM and Vmax of 97+/-9 nmol/mg protein/30 min, and asymmetric transport was observed, with greater permeability in the apical to basolateral (maternal-to-fetal) direction. However, RT-PCR revealed no expression of OCT1 or OCT2 in either BeWo cells or primary cultured human cytotrophoblast cells, and OCT substrates such as TEA and choline did not inhibit the uptake of 4-Di-1-ASP. Although the uptake of this fluorescent compound in BeWo cells is not mediated by an OCT, the colocalization experiments with fluorescence microscopy and inhibition studies confirmed significant mitochondrial uptake of 4-Di-1-ASP. Transport of 4-Di-1-ASP into the nuclear region of BeWo cells was also observed, which is likely mediated by a nucleoside transporter.

Differential contribution of organic cation transporters, OCT2 and MATE1, in platinum agent-induced nephrotoxicity

The mechanism of severe nephrotoxicity caused by cisplatin, but not carboplatin, oxaliplatin, and nedaplatin, is not fully understood. The renal accumulation and subsequent nephrotoxicity of platinum agents were examined in rats. Among these four drugs, only cisplatin induced nephrotoxicity at 2 days after its intraperitoneal administration. The urinary activity of N-acetyl-beta-D-glucosaminidase and expression of kidney injury molecule-1 mRNA and osteopontin were markedly enhanced in the cisplatin-treated rats. Although some markers were affected in the rats administered nedaplatin, only minor histological change was observed. The renal accumulation of cisplatin was much greater than that of the other drugs. In the in vitro study, the cellular accumulation of cisplatin and oxaliplatin was stimulated by the expression of rat (r) OCT2. Oxaliplatin was also transported by rOCT3. A luminal H(+)/organic cation antiporter, rMATE1 (multidrug and toxin extrusion) as well as human (h) MATE1 and hMATE2-K, stimulated the H(+)-gradient-dependent antiport of oxaliplatin, but not of cisplatin. Carboplatin and nedaplatin were not transported by these transporters. In conclusion, the nephrotoxicity of platinum agents was closely associated with their renal accumulation, which is determined by the substrate specificity of the OCT and MATE families.

New insights into renal transport of urate

PURPOSE OF REVIEW

This review focuses on recent progress in the understanding of various aspects of renal transport of urate.

RECENT FINDINGS

Since the molecular cloning of the renal apical urate/anion exchanger URAT1 (SLC22A12), several membrane proteins relevant to the transport of urate have been identified. The molecular identification of two sodium-coupled monocarboxylate transporters, SMCT1(SLC5A8) and SMCT2(SLC5A12), and the emerging role of PDZ (PSD-95, DglA, and ZO-1) scaffold for renal apical transporters have led to a new concept of renal urate transport: urate-transporting multimolecular complex, or 'urate transportsome', that may form an ultimate functional unit including the sodium-coupled urate transport system by linking URAT1 and sodium-coupled monocarboxylate transporters or the coordinated apical urate uptake system by balancing reabsorptive (URAT1) and efflux (NPT1/OATv1 and MRP4) transporters. In addition, genetic variations of the URAT1 gene are associated not only with idiopathic renal hypouricemia but also with reduced renal urate excretion.

SUMMARY

Although our knowledge of renal urate handling has been increased by the molecular identification of urate transport proteins and by results of genetic studies on patients with serum urate disorders, current evidence is insufficient to fully understand the precise mechanism governing the bi-directional transport of urate. Further studies are still necessary.

Oppositely directed H+ gradient functions as a driving force of rat H+/organic cation antiporter MATE1

Recently, we have isolated the rat (r) H+/organic cation antiporter multidrug and toxin extrusion 1 (MATE1) and reported its tissue distribution and transport characteristics. Functional characterization suggested that an oppositely directed H+ gradient serves as a driving force for the transport of a prototypical organic cation, tetraethylammonium, by MATE1, but there is no direct evidence to prove this. In the present study, therefore, we elucidated the driving force of tetraethylammonium transport via rMATE1 using plasma membrane vesicles isolated from HEK293 cells stably expressing rMATE1 (HEK-rMATE1 cells). A 70-kDa rMATE1 protein was confirmed to exist in HEK-rMATE1 cells, and the transport of various organic cations including [14C]tetraethylammonium was stimulated in intracellular acidified HEK-rMATE1 cells but not mock cells. The transport of [14C]tetraethylammonium in membrane vesicles from HEK-rMATE1 cells exhibited the overshoot phenomenon only when there was an outwardly directed H+ gradient, as observed in rat renal brush-border membrane vesicles. The overshoot phenomenon was not observed in the vesicles from mock cells. The stimulated [14C]tetraethylammonium uptake by an H+ gradient [intravesicular H+ concentration ([H+]in) > extravesicular H+ concentration ([H+]out)] was significantly reduced in the presence of a protonophore, carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP). [14C]tetraethylammonium uptake was not changed in the presence of valinomycin-induced membrane potential. These findings definitively indicate that an oppositely directed H+ gradient serves as a driving force of tetraethylammonium transport via rMATE1, and this is the first demonstration to identify the driving force of the MATE family. The present experimental strategy is very useful in identifying the driving force of cloned transporters whose driving force has not been evaluated.

Gene expression and regulation of drug transporters in the intestine and kidney

Intestinal absorption and renal secretion of ionic drugs are controlled by a number of drug transporters expressed at the brush-border and basolateral membranes of epithelial cells. Over the last several years, considerable progress has been made regarding the molecular identification and functional characterization of drug transporters. Under some physiological and pathophysiological conditions, the expression and transport activity of drug transporters are changed, affecting the pharmacokinetics of substrate drugs. The regulation of transport activity in response to endogenous and exogenous signals can occur at various levels such as transcription, mRNA stability, translation, and posttranslational modification. Transcriptional regulation is of particular interest, because changes in transport activity are dynamically regulated by increases or decreases in levels of mRNA expression. The tissue-specific expression of drug transporters is also under transcriptional control, and recent studies using clinical samples from human tissues have revealed the expression profiles of drug transporters in the human body. The purpose of this research updates is to review the recent progress in the study of the gene expression and regulation of intestinal and renal drug transporters.

Lower prevalence of the OCT2 Ser270 allele in patients with essential hypertension

Impairment of the renal dopaminergic pathway has been shown to result in essential hypertension. The Organic Cation Transporter 2, OCT2 (SLC22A2), has been implicated in renal dopamine handling as well as in the inactivation of circulating catecholamines and is supposed to be involved in blood pressure regulation. This study investigated the association of the OCT2 Ala270Ser polymorphism with essential hypertension and its impact on blood pressure status in 607 Caucasian patients who underwent left heart catheterization. Clinical characteristics and diagnosis were recorded and blood pressure was determined by intravascular measurement. A comparison of genotypes revealed that patients with the Ser270 allele were less frequently affected by the clinical diagnosis of hypertension than homozygous carriers of the wild type allele Ala270 (Kruskal Wallis test, p = 0.028). This relation was even more pronounced in the subgroup of patients without diabetes mellitus (Kruskal Wallis test, p = 0.013). In summary, the first data on OCT2 are presented in the context of a candidate gene analysis. The Ala270Ser polymorphism was significantly associated with essential hypertension in the present sample. This study further suggests a function of OCT2 in blood pressure homeostasis and points to the potential role of the transporter in the development of essential hypertension.

Micronutrient and urate transport in choroid plexus and kidney: implications for drug therapy

With application of molecular biology techniques, there has been rapid progress in understanding how many drugs and micronutrients (e.g., vitamins) are transferred across the choroid plexus (CP), the main transport locus of the blood-cerebrospinal fluid (CSF) barrier, and the renal tubular epithelial cells. In many cases, these molecules are transported by separate, specific carriers or receptors on the apical and/or basal side of the CP or renal epithelial cells. This commentary focuses on four micronutrient transport systems in CP (ascorbic acid, folate, inositol, and riboflavin), all of which have been recently cloned, expressed and for which knockout mice models were developed and transporter localization studies performed. Also reviewed is the recently cloned uric acid transport system in human kidney in which there exists a human "knockout" model. The implications of these transport systems for drug therapy of central nervous system and renal disorders are discussed, especially with regard to methods to circumvent the blood-brain and blood-CSF barriers to deliver drugs to the brain.

Concentration-Dependent Mode of Interaction of Angiotensin II Receptor Blockers with Uric Acid Transporter

Serum uric acid (SUA) is currently recognized as a risk factor for cardiovascular disease. It has been reported that an angiotensin II receptor blocker (ARB), losartan, decreases SUA level, whereas other ARBs, such as candesartan, have no lowering effect. Because the renal uric acid transporter (URAT1) is an important factor controlling the SUA level, we examined the involvement of URAT1 in those differential effects of various ARBs on SUA level at clinically relevant concentrations. This study was done by using URAT1-expressing Xenopus oocytes. Losartan, pratosartan, and telmisartan exhibited cis-inhibitory effects on the uptake of uric acid by URAT1, whereas at higher concentrations, only telmisartan did, and these ARBs reduced the uptake in competitive inhibition kinetics. On the other hand, candesartan, EXP3174 [2-n-butyl-4-chloro-1-[(2'-(1H-tetrazol-5-yl)biphenyl-4-yI)methyl]imidazole-5-carboxylic acid] (a major metabolite of losartan), olmesartan, and valsartan were not inhibitory. Preloading of those ARBs in the oocytes enhanced the URAT1-mediated uric acid uptake, showing a trans-stimulatory effect. The present study is a first demonstration of the differential effects of ARBs on URAT1 that some ARBs are both cis-inhibitory and trans-stimulatory, depending on concentration, whereas others exhibit either a trans-stimulatory or cis-inhibitory effect alone, which could explain the clinically observed differential effects of ARBs on SUA level. Furthermore, it was found that such differential effects of ARBs on URAT1 could be predicted from the partial chemical structures of ARBs, which will be useful information for the appropriate use and development of ARBs without an increase of SUA.

Carnitine transporter defect: diagnosis in asymptomatic adult women following analysis of acylcarnitines in their newborn infants

Carnitine transporter defect (CTD) is an autosomal recessive disorder characterized by episodes of non-ketotic hypoglycaemia, hyperammonaemia and liver disease, or by the development of cardiomyopathy, both of which occur in infancy and childhood. Blood carnitine concentrations are extremely low. The diagnosis can be confirmed by finding abnormal fat oxidation and carnitine uptake in skin fibroblasts. The condition has not previously been thought to present later in life or to be benign. We report the identification of four women discovered to have CTD as a consequence of finding low carnitine concentrations in the cord blood or newborn samples from their infants. All four mothers had been asymptomatic and none had a cardiomyopathy.

Identification of an octamer-binding site controlling the activity of the small breast epithelial mucin gene promoter

e human small breast epithelial mucin (SBEM) gene has been identified as being preferentially expressed in mammary epithelial cells and over-expressed in breast tumors. In this report, we have characterized the promoter of SBEM gene in order to identify sequences responsible for this strong mammary expression. A series of SBEM promoter/luciferase constructs were transiently transfected into both breast (MCF-7, BT-20) and non-breast (HeLa and HepG2) cell lines. In addition to the minimal promoter and to a repressor region, we have identified an 87-bp sequence (-357/-270) driving a strong breast-specific expression. Site-directed mutagenesis of a putative octamer-binding transcription factor binding site located within this latter region led to a strong decrease of the transcriptional activity of the SBEM promoter. Furthermore, transient over-expression of Oct1 and Oct2 not only increased SBEM promoter reporter activity, but also enhanced endogenous SBEM mRNA level. Overall, the data suggest that octamer-binding transcription factors participate in the strong expression of SBEM gene in breast tissues. Clarifying the SBEM gene regulation will help to dissect mechanisms underlying transcription of normal breast and breast cancer-associated genes.

Molecular identification and functional characterization of rat multidrug and toxin extrusion type transporter 1 as an organic cation/H+ antiporter in the kidney

We have cloned and functionally characterized the rat ortholog of multidrug and toxin extrusion type transporter 1 (rMATE1). The mRNA of rMATE1 was strongly expressed in kidney and detectable in the various tissues such as brain, stomach, colon, lung, liver, spleen, skeletal muscle, and prostate. When stably expressed in HEK293 cells, rMATE1 could mediate the transport of tetraethylammonium (TEA) and cimetidine under the condition where the membrane potential was disrupted by a high concentration of potassium ion and intracellular pH was reduced by NH(4)Cl pretreatment. When extracellular pH was changed from 5.5 to 8.5, the transport of TEA by rMATE1 was greatest at pH 7.5. Kinetic analyses showed that the transports of TEA and cimetidine mediated by rMATE1 were both saturable with a K(m) of 260 +/- 10 and 3.01 +/- 0.21 muM, respectively. It was found that cimetidine is the most potent inhibitor of rMATE1, and many other organic cations, such as 1-methyl-4-phenylpyridinium, amiloride, imipramine, and quinidine, are also effective as inhibitors. Pretreatment of the cells expressing rMATE1 with p-chloromercuribenzene sulfonate significantly reduced TEA transport, but this effect was totally reversed by subsequent treatment with dithiothreitol. These results indicate that the functional nature of rMATE1 is consistent with that of the hypothetical organic cation/H(+) antiporter system in the brush-border membrane of the renal tubular epithelial cells. Accordingly, these results suggest that rMATE1 is an electroneutral and multispecific organic cation transporter energized by the trans-proton gradient, and plays a physiological role in renal secretion of organic cations, including clinically used cationic drugs.

Cisplatin and oxaliplatin, but not carboplatin and nedaplatin, are substrates for human organic cation transporters (SLC22A1-3 and multidrug and toxin extrusion family)

We have examined the role of the human organic cation transporters [hOCTs and human novel organic cation transporter (hOCTN); SLC22A1-5] and apical multidrug and toxin extrusion (hMATE) in the cellular accumulation and cytotoxicity of platinum agents using the human embryonic kidney (HEK) 293 cells transiently transfected with the transporter cDNAs. Both the cytotoxicity and accumulation of cisplatin were enhanced by the expression of hOCT2 and weakly by hOCT1, and those of oxaliplatin were also enhanced by the expression of hOCT2 and weakly by hOCT3. The hOCT-mediated uptake of tetraethylammonium (TEA) was markedly decreased in the presence of cisplatin in a concentration-dependent manner. However, oxaliplatin showed almost no influence on the TEA uptakes in the HEK293 cells expressing hOCT1, hOCT2, and hOCT3. The hMATE1 and hMATE2-K, but not hOCTN1 and OCTN2, mediated the cellular accumulation of cisplatin and oxaliplatin without a marked release of lactate dehydrogenase. Oxaliplatin, but not cisplatin, markedly decreased the hMATE2-K-mediated TEA uptake. However, the inhibitory effect of cisplatin and oxaliplatin against the hMATE1-mediated TEA uptake was similar. The release of lactate dehydrogenase and the cellular accumulation of carboplatin and nedaplatin were not found in the HEK293 cells transiently expressing these seven organic cation transporters. These results indicate that cisplatin is a relatively good substrate of hOCT1, hOCT2, and hMATE1, and oxaliplatin is of hOCT2, hOCT3, hMATE1, and hMATE2-K. These transporters could play predominant roles in the tissue distribution and anticancer effects and/or adverse effects of platinum agent-based chemotherapy.

PDZ Adaptor Protein PDZK2 Stimulates Transport Activity of Organic Cation/Carnitine Transporter OCTN2 by Modulating Cell Surface Expression

A part of the organic cation transporter families (OCT3, OCTN1, and OCTN2) has recently been identified to physically interact with PDZ (PSD95, Dlg, and ZO1) domain-containing proteins, although the physiological relevance of such interaction has not yet been fully examined. Here we have examined the stimulatory effect of PDZK2 [also named NaPi-Cap2 and intestinal and kidney-enriched PDZ protein (IKEPP)] on those cation transporters. In HEK293 cells, coexpression with PDZK2 increased the uptake of carnitine by OCTN2 with minimal effect on its substrate recognition specificity, but not for transport activity of OCT3 or OCTN1. The stimulatory effect of PDZK2 on OCTN2 was compatible with an approximately 2 times increase in transport capacity and can be accounted for by the increase in cell surface expression of OCTN2. Coexpression of PDZK2 did not affect carnitine transport activity of OCTN2 with deletion of the last four amino acids, which were found to be important for the interaction, suggesting involvement of physical interaction of the two proteins in the increase of cell surface expression of OCTN2. In mouse kidney, colocalization of PDZK2 and OCTN2 occurred predominantly in the region that was close to, but not the same as, the surface of apical membranes where OCTN2 alone was observed, suggesting the existence of OCTN2 in the subapical compartment that interacts with PDZK2. The present data have thus proposed an "intracellular pool" for OCTN2 that may be relevant to the stabilization of cell surface expression of OCTN2, thereby increasing transport activity for carnitine.

OCTNs: Will the real IBD5 gene please stand up?

Crohn’s disease and ulcerative colitis are inflammatory disorders of the gastrointestinal tract with a substantial heritable component. The IBD5 region on chromosome 5q31 is one of only two loci widely confirmed to be associated with Crohn’s disease in multiple independent cohorts. Although many populations have demonstrated association with IBD5, there remains uncertainty as to the causal variant within the region. A recent report identified polymorphisms in SLC22A4 (OCTN1) and SLC22A5 (OCTN2) as being responsible for the IBD5 association, however, these findings have not been replicated. This review discusses the data evaluating the IBD5 locus and the OCTN genes and their relationship to inflammatory bowel disease. Several other genes, including IRF1 and P4HA2 may be equally as likely to contain the IBD5 causal variant as the OCTN genes.

Functional influence ofN-glycosylation in OCT2-mediated tetraethylammonium transport

OCT2, an organic cation transporter critical for removal of many drugs and toxins from the body, contains consensus sites for N-glycosylation at amino acid position 71, 96, and 112. However, the extent to which these sites are glycosylated by the cell, and the influence glycosylation has on OCT2 function, remains unknown. To address these issues, the acquisition of N-glycosylation was disrupted by mutating the amino acid asparagine (N) to glutamine (Q) at these sites in the rabbit ortholog of OCT2, which was expressed in Chinese hamster ovary cells. Disruption of N-glycosylation followed by Western blotting indicated that each site is indeed glycosylated and that OCT2 contains no other sites of N-glycosylation. Plasma membrane expression (determined by surface biotinylation) of the N112Q mutant, but not N71Q or N96Q mutants, was fourfold lower than that of wild-type OCT2, and unglycosylated OCT2 (N71Q/N96Q/N112Q) was sequestered in an unidentified intracellular compartment. The N71Q, N96Q, and N112Q mutants had a higher affinity (∼2-fold) for tetraethylammonium (TEA). Maximum transport rate was reduced in the N96Q (3-fold) and N112Q (5-fold) mutants, but not the N71Q mutant, and unglycosylated OCT2 failed to transport TEA (associated with its absence in the plasma membrane). Whereas the reduction in maximum transport rate of the N112Q mutant is consistent with its reduced plasma membrane expression, the lower rate of the N96Q mutant, which appeared to traffic properly, suggests that glycosylation at N96 increases the transporter turnover number.