Fine Mapping and Functional Analysis Reveal a Role of SLC22A1 in Acylcarnitine Transport

Genome-wide association studies have identified a signal at the SLC22A1 locus for serum acylcarnitines, intermediate metabolites of mitochondrial oxidation whose plasma levels associate with metabolic diseases. Here, we refined the association signal, performed conditional analyses, and examined the linkage structure to find coding variants of SLC22A1 that mediate independent association signals at the locus. We also employed allele-specific expression analysis to find potential regulatory variants of SLC22A1 and demonstrated the effect of one variant on the splicing of SLC22A1. SLC22A1 encodes a hepatic plasma membrane transporter whose role in acylcarnitine physiology has not been described. By targeted metabolomics and isotope tracing experiments in loss- and gain-of-function cell and mouse models of Slc22a1, we uncovered a role of SLC22A1 in the efflux of acylcarnitines from the liver to the circulation. We further validated the impacts of human variants on SLC22A1-mediated acylcarnitine efflux in vitro, explaining their association with serum acylcarnitine levels. Our findings provide the detailed molecular mechanisms of the GWAS association for serum acylcarnitines at the SLC22A1 locus by functionally validating the impact of SLC22A1 and its variants on acylcarnitine transport.

Inhibition of Human Drug Transporter Activities by the Pyrethroid Pesticides Allethrin and Tetramethrin

Pyrethroids are widely-used chemical insecticides, to which humans are commonly exposed, and known to alter functional expression of drug metabolizing enzymes. Limited data have additionally suggested that drug transporters, that constitute key-actors of the drug detoxification system, may also be targeted by pyrethroids. The present study was therefore designed to analyze the potential regulatory effects of these pesticides towards activities of main ATP-binding cassette (ABC) and solute carrier (SLC) drug transporters, using transporter-overexpressing cells. The pyrethroids allethrin and tetramethrin were found to inhibit various ABC and SLC drug transporters, including multidrug resistance-associated protein (MRP) 2, breast cancer resistance protein (BCRP), organic anion transporter polypeptide (OATP) 1B1, organic anion transporter (OAT) 3, multidrug and toxin extrusion transporter (MATE) 1, organic cation transporter (OCT) 1 and OCT2, with IC50 values however ranging from 2.6 μM (OCT1 inhibition by allethrin) to 77.6 μM (OAT3 inhibition by tetramethrin) and thus much higher than pyrethroid concentrations (in the nM range) reached in environmentally pyrethroid-exposed humans. By contrast, allethrin and tetramethrin cis-stimulated OATP2B1 activity and failed to alter activities of OATP1B3, OAT1 and MATE2-K, whereas P-glycoprotein activity was additionally moderately inhibited. Twelve other pyrethoids used at 100 μM did not block activities of the various investigated transporters, or only moderately inhibited some of them (inhibition by less than 50%). In silico analysis of structure-activity relationships next revealed that molecular parameters, including molecular weight and lipophilicity, are associated with transporter inhibition by allethrin/tetramethrin and successfully predicted transporter inhibition by the pyrethroids imiprothrin and prallethrin. Taken together, these data fully demonstrated that two pyrethoids, i.e., allethrin and tetramethrin, can act as regulators of the activity of various ABC and SLC drug transporters, but only when used at high and non-relevant concentrations, making unlikely any contribution of these transporter activity alterations to pyrethroid toxicity in environmentally exposed humans.

Personalized medicine in diabetes: the role of 'omics' and biomarkers

Personalized medicine, otherwise called stratified or precision medicine, aims to better target intervention to the individual to maximize benefit and minimize harm. This review discusses how diabetes aetiology, pathophysiology and patient genotype influence response to or side effects of the commonly used diabetes treatments. C-peptide is a useful biomarker that is underused to guide treatment choice, severe insulin deficiency predicts non-response to glucagon-like peptide-1 receptor agonists, and thiazolidinediones are more effective in insulin-resistant patients. The field of pharmacogenetics is now yielding clinically important results, with three examples outlined: sulphonylurea sensitivity in patients with HNF1A maturity-onset diabetes of the young; sulphonylurea sensitivity in patients with Type 2 diabetes with reduced function alleles at CYP2C9, resulting in reduced metabolism of sulphonylureas; and severe metformin intolerance associated with reduced function organic cation transporter 1 (OCT1) variants, exacerbated by drugs that also inhibit OCT1. Genome-wide approaches and the potential of other 'omics', including metagenomics and metabolomics, are then outlined, highlighting the complex interacting networks that we need to understand before we can truly personalize diabetes treatments.

Protein Kinase C-Independent Inhibition of Organic Cation Transporter 1 Activity by the Bisindolylmaleimide Ro 31-8220

Ro 31–8220 is a potent protein kinase C (PKC) inhibitor belonging to the chemical class of bisindolylmaleimides (BIMs). Various PKC-independent effects of Ro 31–8220 have however been demonstrated, including inhibition of the ATP-binding cassette drug transporter breast cancer resistance protein. In the present study, we reported that the BIM also blocks activity of the solute carrier organic cation transporter (OCT) 1, involved in uptake of marketed drugs in the liver, in a PKC-independent manner. Ro 31–8220, in contrast to other pan-PKC inhibitors such as staurosporine and chelerythrine, was thus shown to cis-inhibit uptake of the reference OCT1 substrate tetraethylammonium in OCT1-transfected HEK293 cells in a concentration-dependent manner (IC₅₀ = 0.18 μM) and without altering membrane expression of OCT1. This blockage of OCT1 was also observed in human hepatic HepaRG cells that constitutionally express OCT1. It likely occurred through a mixed mechanism of inhibition. Ro 31–8220 additionally trans-inhibited TEA uptake in OCT1-transfected HEK293 cells, which likely discards a transport of Ro 31–8220 by OCT1. Besides Ro 31–8220, 7 additional BIMs, including the PKC inhibitor LY 333531, inhibited OCT1 activity, whereas 4 other BIMs were without effect. In silico analysis of structure-activity relationships next revealed that various molecular descriptors, especially 3D-WHIM descriptors related to total size, correspond to key physico-chemical parameters for inhibition of OCT1 activity by BIMs. In addition to activity of OCT1, Ro 31–8220 inhibited those of other organic cation transporters such as multidrug and toxin extrusion protein (MATE) 1 and MATE2-K, whereas, by contrast, it stimulated that of OCT2. Taken together, these data extend the nature of cellular off-targets of the BIM Ro 31–8220 to OCT1 and other organic cation transporters, which has likely to be kept in mind when using Ro 31–8220 and other BIMs as PKC inhibitors in experimental or clinical studies.

Inhibition of hepatic uptake transporters by flavonoids

Members of the human SLC superfamily such as organic anion transporting polypeptide 1B1 (OATP1B1), OATP1B3, and organic cation transporter 1 (OCT1) are drug uptake transporters that are localised on the basolateral membrane of hepatocytes mediating the uptake of drugs such as atorvastatin and metformin into hepatocytes. Ingredients of food such as flavonoids influence the effects of drugs, e.g. by inhibition of drug transporters. Therefore, we investigated the impact of the Ginkgo biloba flavonoids apigenin, kaempferol, and quercetin, and the grapefruit flavonoids naringenin, naringin, and rutin on the OATP1B1, OATP1B3, and OCT1 transport activity. Transporter expressing HEK293 cell lines were used with [3H]sulfobromophthalein ([3H]BSP) as substrate for OATP1B1 and OATP1B3, [3H]atorvastatin as substrate for OATP1B1, and [3H]1-methyl-4-phenylpyridinium ([3H]MPP(+)) as substrate for OCT1. The G. biloba flavonoids showed a competitive inhibition of the OATP1B1- and OATP1B3-mediated [3H]BSP and the OATP1B1-mediated [3H]atorvastatin uptake. Quercetin was the most potent inhibitor of the OATP1B1- and OATP1B3-mediated [3H]BSP transport with K(i)-values of 8.8±0.8μM and 7.8±1.7μM, respectively. For the inhibition of the OATP1B1-mediated [3H]atorvastatin transport, apigenin was the most potent inhibitor with a K(i) value of 0.6±0.2μM. Among the grapefruit flavonoids, naringenin was the most potent inhibitor of the OATP1B1- and OATP1B3-mediated [3H]BSP transport with IC(50)-values of 81.6±1.1μM and 101.1±1.1μM, respectively. All investigated flavonoids showed no significant inhibition of the OCT1-mediated [3H]MPP(+) uptake. Taken together, these in vitro studies showed that the investigated flavonoids inhibit the OATP1B1- and OATP1B3-mediated drug transport, which could be a mechanism for food-drug interactions in humans.

NEUROTOXIC PYRIDINIUM METABOLITES OF HALOPERIDOL ARE SUBSTRATES OF HUMAN ORGANIC CATION TRANSPORTERS

Two neurotoxic pyridinium metabolites of haloperidol, 4-(4-chlorophenyl)-1-[4-(4-fluorophenyl)-4-oxybutyl]pyridinium ion (HPP(+)) and 4-(4-(chlorophenyl)-1-4-(fluorophenyl)-4-hydroxybutyl-pyridinium (RHPP(+)), are formed in the liver and found in the brain. To understand how these neurotoxic pyridinium metabolites are distributed in the brain, HPP(+) and RHPP(+) were evaluated as substrates for human organic cation transporters (hOCTs). Both HPP(+) and RHPP(+) were accumulated in Caco-2 cells, and these accumulations were significantly inhibited by pretreatment with the hOCT inhibitors verapamil, cimetidine, phenoxybenzamine, and corticosterone. The contribution of each hOCT was evaluated based on measurements of the intracellular concentrations of haloperidol metabolites in Madin Darby canine kidney (MDCK) cells transfected with hOCT1, hOCT2, or hOCT3. HPP(+) accumulated in hOCT-overexpressing MDCK cells in a concentration-dependent manner, with estimated K(m) values of 0.99, 2.79, and 2.23 microM and V(max) values of 282.1, 256.1, and 400.2 pmol/min/microg protein for hOCT1, hOCT2, and hOCT3, respectively. RHPP(+) accumulated in hOCT1- and hOCT3-overexpressing MDCK cells, with estimated K(m) values of 5.15 and 8.21 microM and V(max) values of 1230.9 and 1348.6 pmol/min/microg protein for hOCT1 and hOCT3, respectively. On the other hand, RHPP(+) did not accumulate in the hOCT2-expressing MDCK cells. These results suggest that HPP(+) and RHPP(+) are substrates for hOCTs, with the exception of RHPP(+) for hOCT2. Thus, hOCTs seem to contribute to the disposition of these toxic metabolites in human subjects, although further in vivo studies are required to elucidate the involvement of hOCTs in the disposition of haloperidol pyridinium metabolites.

Differential substrate and inhibitory activities of ranitidine and famotidine toward human organic cation transporter 1 (hOCT1; SLC22A1), hOCT2 (SLC22A2), and hOCT3 (SLC22A3)

Human organic cation transporters (hOCTs) are expressed in organs of drug absorption and elimination and play an important role in the uptake and elimination of xenobiotics. The purpose of this study was to evaluate the substrate and inhibitory activity of the H2-receptor antagonists ranitidine and famotidine toward hOCTs and to determine the hOCT isoforms involved in the absorption and elimination of these compounds in humans. Inhibition and substrate specificity of hOCT1, hOCT2, and hOCT3 for ranitidine and famotidine were elucidated in cRNA-injected Xenopus laevis oocytes. Ranitidine and famotidine exhibited similarly potent inhibition of [3H]1-methyl-4-phenyl pyridinium uptake into hOCT1-expressing (IC50= 33 and 28 microM, respectively) and hOCT2-expressing oocytes (IC50= 76 and 114 microM, respectively). Famotidine exhibited potent inhibition of hOCT3; in contrast, ranitidine was a moderately weak inhibitor (IC50= 6.7 and 290 microM, respectively). [3H]Ranitidine uptake was stimulated by hOCT1 (Km= 70 +/- 9 microM) and to a much smaller extent by hOCT2. No stimulation of [3H]ranitidine uptake was observed in hOCT3-expressing oocytes. trans-Stimulation and electrophysiology studies suggested that famotidine also is an hOCT1 substrate and exhibits poor or no substrate activity toward hOCT2 and hOCT3. Thus, hOCT1, which is expressed in the intestine and liver, is likely to play a major role in the intestinal absorption and hepatic disposition of ranitidine and famotidine in humans, whereas hOCT2, the major isoform present in the kidney, may play only a minor role in their renal elimination. Famotidine seems to be one of the most potent inhibitors of hOCT3 yet identified.

Regulation of the human organic cation transporter hOCT1

The human organic cation transporter type 1 (hOCT1) is an important transport system for small organic cations in the liver. Organic cation transporters are regulated by different signaling pathways, but the regulation of hOCT1 has not yet been studied. In this work, we have for the first time investigated the regulation of hOCT1. hOCT1 was expressed in Chinese hamster ovary cells (CHO-hOCT1) and in human embryonic kidney cells (HEK293-hOCT1). Its activity was monitored using microfluorimetry with the fluorescent organic cation 4-(4-(dimethylamino)styryl)-N-methylpyridinium (ASP(+)) as substrate. hOCT1 expressed in CHO-cells was inhibited by protein kinase A (PKA) activation (1 microM forskolin, -58 +/- 6%, n = 12), calmodulin inhibition (0.1 microM calmidazolium, -68 +/- 3%, n = 6; 10 microM ophiobolin A, -48 +/- 10%, n = 7), calmodulin-dependent kinase II inhibition (1 microM KN62, -78 +/- 4%, n = 12), and inhibition of p56(lck) tyrosine kinase (10 microM aminogenistein, -35 +/- 7%, n = 12). The apparent affinities for TEA(+) were lower in CHO-hOCT1 than in HEK293-hOCT1, while those for TPA(+) and quinine were almost identical; the rank order of EC(50) values (TPA(+) > quinine > TEA(+)) was independent of the expression system. EC(50) values for TEA(+) in CHO-hOCT1 or HEK293-hOCT1 were increased under calmidazolium incubation (6.3 and 1.4 mM, respectively). hOCT1 was inhibited by PKA and endogenously activated by calmodulin, calmodulin-dependent kinase II, and p56(lck) tyrosine kinase. Regulation pathways were the same in the two expression systems. Since apparent substrate affinities depend on activity of regulatory pathways, the expression system plays a role in determining the substrate affinities.

Active transport of imatinib into and out of cells: implications for drug resistance

Imatinib is a tyrosine kinase inhibitor that is effective in the treatment of chronic myeloid leukemia (CML). Not all patients achieve cytogenetic response. Some patients even lose the initial cytogenetic response. In this study, we investigated the active cellular transport of imatinib to gain a better understanding of the possible mechanisms of imatinib resistance. We used the leukemic cell line CCRFCEM and its drug-resistant subline VBL(100) to measure the uptake of carbon 14 ((14)C)-labeled imatinib. Imatinib uptake was temperature dependent, indicative of an active uptake process. Additionally, incubations with transport inhibitors showed that verapamil, amantadine, and procainamide, inhibitors of the human organic cation transporter 1 (hOCT1), significantly decreased imatinib uptake into CEM cells, whereas the inhibition of hOCT2 or hOCT3 had no effect, indicating that influx into the cells is an active process likely to be mediated by hOCT1. Studies using transfected MDCK cell lines revealed an active efflux component attributable to MDR1 (ABCB1). Both hOCT1 and MDR1 were expressed in CML primary cells and cell lines. The results indicate that active transport processes mediate the influx and efflux of imatinib. Differential expression of influx (hOCT1) and efflux (MDR1) transporters may be a critical determinant of intracellular drug levels and, hence, resistance to imatinib.

Studies on Intestinal Absorption of Sulpiride (3): Intestinal Absorption of Sulpiride in Rats

The aim of this study was to investigate whether the concomitant administration of the substrates or inhibitors of PEPT1, OCTN1, OCTN2, and P-glycoprotein affects the intestinal absorption of sulpiride in rats. The absorption of sulpiride from rat intestine was decreased by the substrates or inhibitors of PEPT1, OCTN1, and OCTN2. On the other hand, the absorption was increased by the substrates of P-glycoprotein. The effects of these concomitantly administered drugs on the pharmacokinetic behavior of sulpiride after oral administration in rats were investigated. Peak concentration (C(max)) and area under the plasma concentration-time curve (AUC(0-8 h)) of sulpiride were decreased by the concomitant administration of the substrates or inhibitors of PEPT1, OCTN1, and OCTN2. However, the same parameters were significantly increased by the concomitant administration of the substrates of P-glycoprotein. The present results suggest the possibility of drug-drug interaction during the absorption process in the small intestine due to the coadministration of sulpiride and these agents. These findings provide important information for preventing adverse effects and for ensuring the effectiveness of sulpiride and concomitantly administered drugs.

Functional mapping of rbOCT1 and rbOCT2 activity in the S2 segment of rabbit proximal tubule

A strategy was developed to determine the distribution of activity mediated by the organic cation (OC) transporters OCT1 and OCT2 in rabbit renal proximal tubule (RPT). Both transporters displayed similar affinities for tetraethylammonium (TEA; in CHO-K1 cells, TEA concentrations that resulted in half-maximal transport were 19.9 and 34.5 microM for OCT1 and OCT2, respectively). Similarly, some OCs showed little capacity to discriminate between the two processes (IC50 values for ephedrine of 13.6 and 24.2 microM for OCT1 and OCT2, respectively). However, OCT2 had a higher affinity for cimetidine and [2-(4-nitro-2,1,3-benzoxadiazol-7-yl) aminoethyl]trimethylammonium (NBD-TMA; 1.3 and 1.4 microM, respectively) than did OCT1 (97.3 and 108 microM, respectively). Conversely, OCT1 had a higher affinity for tyramine and pindolol than did OCT2 (21.2 and 2.4 vs. 361 and 50 microM, respectively). We designated these as "discriminatory inhibitors" and used them to determine the relative contribution of OCT1 and OCT2 for TEA transport in single S2 segments of rabbit RPT. Cimetidine and NBD-TMA were high-affinity inhibitors of TEA transport in S2 segments (median IC50 values of 12.3 and 1.4 microM, respectively); in comparison, tyramine and pindolol were low-affinity inhibitors (265 and 69.3 microM, respectively). These IC50 values were sufficiently close to those for OCT2 to support the conclusion that TEA transport in the S2 segment of rabbit RPT is dominated by OCT2. However, the profile of inhibition of tyramine (an OCT1-selective substrate) transport in single S2 segments indicated that, despite a comparatively low level of expression, OCT1 can play a dominant role in the uptake of selected OC substrates.

The cation transporters rOCT1 and rOCT2 interact with bicarbonate but play only a minor role for amantadine uptake into rat renal proximal tubules

In renal proximal tubules, the organic cation transporters rOCT1 and rOCT2 are supposed to mediate the first step in organic cation secretion. We investigated whether previously described differences in amantadine and tetraethylammonium (TEA) uptake into isolated renal proximal tubules could be explained by differences in their transport by rOCT1 and rOCT2. By expressing rOCT1 and rOCT2 in Xenopus oocytes and HEK 293 cells, we demonstrated that both transporters translocated amantadine. In Xenopus oocytes, the inhibitory potency of several rOCT1/2 inhibitors was similar for amantadine compared to TEA uptake and supports amantadine transport by rOCT1 and rOCT2. In proximal tubules, procainamide, quinine, cyanine(863), choline, and guanidine in concentrations that inhibit rOCT1/2-mediated TEA or amantadine uptake in Xenopus oocytes exhibited no effect on amantadine uptake. At variance, these inhibitors blocked TEA uptake into proximal tubules. Amantadine and TEA transport were sensitive to modulation by 25 mM bicarbonate. The effect of bicarbonate on organic cation transport was dependent on substrate (amantadine or TEA), cell system (oocytes, HEK 293 cells, or proximal tubules), and transporter (rOCT1 or rOCT2). In proximal tubules, only amantadine uptake was stimulated by bicarbonate. The data suggested that rat renal proximal tubules contain an organic cation transporter in addition to rOCT1 and rOCT2 that mediates amantadine uptake and requires bicarbonate for optimal function. TEA uptake by the basolateral membrane may be mediated mainly by rOCT1 and rOCT2, but these transporters may be in a different functional or regulatory state when expressed in cells or oocytes compared with expression in vivo.

The organic cation transporters rOCT1 and hOCT2 are inhibited by cGMP

The electrogenic cation transporters OCT1 and OCT2 in the basolateral membrane of renal proximal tubules mediate the first step during secretion of organic cations. Previously we demonstrated stimulation and change of selectivity for rat OCT1 (rOCT1) by protein kinase C. Here we investigated the effect of cGMP on cation transport by rOCT1 or human OCT2 (hOCT2) after expression in human embryonic kidney cells (HEK293) or oocytes of Xenopus laevis. In HEK293 cells, uptake was measured by microfluorimetry using the fluorescent cation 4-(4-(dimethyl-amino)styryl)-N-methylpyridinium iodide (ASP + ) as substrate, whereas uptake into Xenopus laevis oocytes was measured with radioactively labelled cations. In addition, ASP +-induced depolarizations of membrane voltages (Vm) were measured in HEK293 cells using the slow whole-cell patch-clamp method. Incubation of rOCT1-expressing HEK293 cells for 10 min with 100 mM 8-Br-cGMP reduced initial ASP + uptake by maximally 78% with an IC50 value of 24 +/- 16 mM. This effect was not abolished by the specific PKG inhibitor KT5823, indicating that a cGMP-dependent kinase is not involved. An inhibition of ASP + uptake by rOCT1 in HEK293 cells was also obtained when the cells were incubated for 10 min with 100 mM cGMP, whereas no effect was obtained when cGMP was given together with ASP +. ASP + (100 mM)-induced depolarizations of Vm were reduced in the presence of 8-Br-cGMP (100 mM) by 44 +/- 11% (n = 6). Since it could be demonstrated that [3H]cGMP is taken up by an endogeneous cyanine863-inhibitable transporter, the effect of cGMP is probably mediated from inside the cell. Uptake measurements with [14C]tetraethylammonium and [3H]2-methyl-4-phenylpyridinium in Xenopus laevis oocytes expressing rOCT1 performed in the absence and presence of 8-Br-cGMP showed that cGMP does not interact directly with the transporter. The data suggest that the inhibition mediated by cGMP observed in HEK293 cells occurs most likely via a mammalian cGMP-binding protein that interacts with OCT1-2 transporters.

Expression and pharmacological profile of the human organic cation transporters hOCT1, hOCT2 and hOCT3

1. Organic cation transporters (OCTs) are involved in the elimination of monoamines and cationic xenobiotics. To examine whether some cell lines express several different OCTs, we investigated seven human cell lines for the mRNA expression pattern of the human (h) transporters hOCT1, hOCT2 and hOCT3. hOCT1 mRNA was found in all cell lines, six additionally expressed hOCT3 and only two cell lines contained all three hOCTs. 2. Among the three OCTs only for the OCT3 (also designated as 'uptake(2)' or 'extraneuronal monoamine transporter') 'selective' inhibitors are described in the literature. The affinities of the OCT3 inhibitors for the other two OCTs are largely unknown. Therefore, we compared the potencies of eight compounds as inhibitors of hOCT-mediated uptake of the organic cation [(3)H]-1-methyl-4-phenylpyridinium ([(3)H]-MPP(+)) in human embryonic kidney 293 (HEK293) cells stably expressing hOCT1, hOCT2 or hOCT3. Decynium-22 inhibited hOCT3 with 10 fold higher potency than hOCT1 and hOCT2. Corticosterone was about 100 fold more potent as inhibitor of hOCT3 than of hOCT1 or hOCT2, and O-methylisoprenaline (OMI) inhibited almost exclusively hOCT3. Progesterone and beta-Oestradiol preferentially inhibited hOCT3 and hOCT1, whereas prazosin was a potent inhibitor of hOCT1 and hOCT3. Phenoxybenzamine (PbA) inhibited with about equal apparent potency all three hOCTs, whereas the PbA derivative SKF550 ((9-fluorenyl)-N-methyl-beta-chloroethylamine) preferentially inhibited hOCT3 and hOCT2. 3. PbA reversibly inhibited hOCT1 and irreversibly hOCT2 and hOCT3; SKF550 also irreversibly inhibited hOCT3 but hOCT2 in a reversible manner. 4. These compounds enable a functional discrimination of the three hOCTs: hOCT1 is selectively inhibited by prazosin, reversibly inhibited by PbA and it is not sensitive to inhibition by SKF550 and OMI; hOCT2 is reversibly inhibited by SKF550, irreversibly by PbA and not by prazosin, beta-oestradiol and OMI, whereas hOCT3 is selectively inhibited by corticosterone, OMI and decynium22.

Human organic anion transporters and human organic cation transporters mediate renal transport of prostaglandins

Prostaglandin E(2) (PGE(2)) and prostaglandin F(2 alpha) (PGF(2 alpha)) have been used for the induction of labor and the termination of pregnancy. Renal excretion is shown to be an important pathway for the elimination of PGE(2) and PGF(2 alpha). The purpose of this study was to elucidate the molecular mechanism of renal PGE(2) and PGF(2 alpha) transport using cells stably expressing human organic anion transporter (hOAT) 1, hOAT2, hOAT3, and hOAT4, and human organic cation transporter (hOCT) 1 and hOCT2. A time- and dose-dependent increase in PGE(2) and PGF(2 alpha) uptake was observed in cells expressing hOAT1, hOAT2, hOAT3, hOAT4, hOCT1, and hOCT2. The K(m) values of PGE(2) uptake by hOAT1, hOAT2, hOAT3, hOAT4, hOCT1, and hOCT2 were 970, 713, 345, 154, 657, and 28.9 nM, respectively, whereas those of PGF(2 alpha) uptake by hOAT1, hOAT3, hOAT4, hOCT1, and hOCT2 were 575, 1092, 692, 477, and 334 nM, respectively. PGE(2) and PGF(2 alpha) significantly inhibited organic anion uptake by hOATs and organic cation uptake by hOCTs. In conclusion, considering the localization of these transporters, the results suggest that PGE(2) and PGF(2 alpha) transport in the basolateral membrane of the proximal tubule is mediated by multiple pathways including hOAT1, hOAT2, hOAT3, and hOCT2, whereas that in the apical side is mediated by hOAT4.

Release of non-neuronal acetylcholine from the isolated human placenta is mediated by organic cation transporters

1. The release of acetylcholine was investigated in the human placenta villus, a useful model for the characterization of the non-neuronal cholinergic system. 2. Quinine, an inhibitor of organic cation transporters (OCT), reduced acetylcholine release in a reversible and concentration-dependent manner with an IC(50) value of 5 microM. The maximal effect, inhibition by 99%, occurred at a concentration of 300 microM. 3. Procaine (100 microM), a sodium channel blocker, and vesamicol (10 microM), an inhibitor of the vesicular acetylcholine transporter, were ineffective. 4. Corticosterone, an inhibitor of OCT subtype 1, 2 and 3 reduced acetylcholine in a concentration-dependent manner with an IC(50) value of 2 microM. 5. Substrates of OCT subtype 1, 2 and 3 (amiloride, cimetidine, guanidine, noradrenaline, verapamil) inhibited acetylcholine release, whereas carnitine, a substrate of subtype OCTN2, exerted no effect. 6. Long term exposure (48 and 72 h) of villus strips to anti-sense oligonucleotides (5 microM) directed against transcription of OCT1 and OCT3 reduced the release of acetylcholine, whereas OCT2 anti-sense oliogonucleotides were ineffective. 7. It is concluded that the release of non-neuronal acetylcholine from the human placenta is mediated via organic cation transporters of the OCT1 and OCT3 subtype.