Analysis of kinetic data in transport studies: new insights from kinetic studies of Na(+)-D-glucose cotransport in human intestinal brush-border membrane vesicles using a fast sampling, rapid filtration apparatus

Farnesoid X Receptor Activation Stimulates Organic Cations Transport in Human Renal Proximal Tubular Cells

Farnesoid X receptor (FXR) is a ligand-activated transcription factor highly expressed in the liver and kidneys. Activation of FXR decreases organic cation transporter (OCT) 1-mediated clearance of organic cation compounds in hepatocytes. The present study investigated FXR regulation of renal clearance of organic cations by OCT2 modulation and multidrug and toxin extrusion proteins (MATEs). The role of FXR in OCT2 and MATEs functions was investigated by monitoring the flux of ³H-MPP⁺, a substrate of OCT2 and MATEs. FXR agonists chenodeoxycholic acid (CDCA) and GW4064 stimulated OCT2-mediated ³H-MPP⁺ uptake in human renal proximal tubular cells (RPTEC/TERT1 cells) and OCT2-CHO-K1 cells. The stimulatory effect of CDCA (20 µM) was abolished by an FXR antagonist, Z-guggulsterone, indicating an FXR-dependent mechanism. CDCA increased OCT2 transport activity via an increased maximal transport rate of MPP⁺. Additionally, 24 h CDCA treatment increased MATEs-mediated ³H-MPP⁺ uptake. Moreover, CDCA treatment increased the expression of OCT2, MATE1, and MATE2-K mRNA compared with that of the control. OCT2 protein expression was also increased following CDCA treatment. FXR activation stimulates renal OCT2- and MATE1/2-K-mediated cation transports in proximal tubules, demonstrating that FXR plays a role in the regulation of OCT2 and MATEs in renal proximal tubular cells.

Liver X receptor activation inhibits SGLT2-mediated glucose transport in human renal proximal tubular cells

NEW FINDINGS

What is the central question of this study? The liver X receptor (LXR) has been reported to regulate several membrane transporters. It is imperative to investigate whether LXR activation regulates SGLT2-mediated glucose transport in human renal proximal tubular cells. What is the main finding and its importance? Liver X receptor activation inhibits SGLT2 transport function in normal and high-glucose conditions via reduction of SGLT2 protein expression. Liver X receptors (LXRs) are members of a nuclear receptor family consisting of two isoforms, LXRα and LXRβ. They play a major role in energy metabolism, including lipid and glucose metabolism. Recent studies reported that LXRs regulate plasma glucose, although the mechanism is still uncertain. The present study investigated whether LXR activation regulates sodium glucose cotransporter2 (SGLT2) in human renal proximal tubular cells. LXR agonists, T0901317 and GW3965, inhibited SGLT2-mediated glucose uptake in a concentration-dependent manner. The effect of T0901317 and GW3965 was attenuated by a LXR antagonist, fenofibrate. Activation of the retinoid X receptor (RXR) agonist, bexarotene, potentiates the inhibitory effect of these ligands. Thus, the inhibitory effect of LXR agonists on SGLT2 was mediated and facilitated by LXR and RXR activation, respectively. In addition, the inhibitory effect of LXR agonists was not mediated by cytotoxicity. Exposing HK-2 cells, a renal proximal tubular cell line, to LXR agonists significantly reduced the maximal transport rate of SGLT2 without any effect on transporter affinity. Western blot analysis revealed that LXR activation significantly decreased protein expression of SGLT2 with no change in mRNA level. In addition, LXR activation inhibited canagliflozin-sensitive short-circuit current, which represents SGLT2-mediated glucose transport in a polarized human renal proximal tubular cell monolayer. Furthermore, LXR activation inhibited the transport function of SGLT2 in hyperglycaemic conditions. As such, this study represents evidence for the inhibitory effect of LXR activation on glucose transport in human renal proximal tubular cells.

Activation of liver X receptor inhibits OCT2-mediated organic cation transport in renal proximal tubular cells

Liver X receptor (LXR) is transcriptional factor that plays an important role in the regulation of energy metabolism such as cholesterol, lipid, and glucose metabolism as well as membrane transporters and channels. Using both in vitro and in vivo models, LXR regulation of the expression and function of renal organic cation transporter 2 (OCT2) was observed. Synthetic LXR agonist (GW3965) and endogenous LXR agonist (22R-hydroxycholesterol) significantly reduced the uptake of ³H-MPP⁺, a prototypic substrate of OCT2, in both OCT2- Chinese hamster ovary K1 and human renal proximal tubular cells (RPTEC/TERT1). GW3965 decreased transport activity of OCT2 via a reduction of the maximal transport rate of MPP⁺ without affecting transporter affinity. The inhibitory effect of GW3965 was attenuated by co-treatment with LXR antagonist (fenofibrate) indicating the inhibition was LXR-dependent mechanism. In addition, co-treatment with a retinoic X receptor (RXR) ligand, 9-cis retinoic acid enhanced the inhibitory effect of GW3965, indicating negative regulation of OCT2 transport activity by the LXR/RXR complex. Treatment RPTEC/TERT1 cells with GW3965 significantly reduced OCT2 protein expression without changing mRNA expression. In parallel, the effect of LXR activation on OCT2 function was investigated in intact mouse kidney. Treating mice with 50 mg/kg BW T0901317 for 14 days significantly decreased ³H-MPP⁺ uptake into renal cortical slices, correlating with decreased OCT2 protein expression in renal cortex without changes in mRNA expression levels. Taken together, LXR/RXR activation downregulates the protein expression and function of OCT2 in renal proximal tubule, suggesting LXR might affect the total profile of renal excretion of cationic compounds.

Lack of Influence of Substrate on Ligand Interaction with the Human Multidrug and Toxin Extruder, MATE1

Multidrug and toxin extruder (MATE) 1 plays a central role in mediating renal secretion of organic cations, a structurally diverse collection of compounds that includes ∼40% of prescribed drugs. Because inhibition of transport activity of other multidrug transporters, including the organic cation transporter (OCT) 2, is influenced by the structure of the transported substrate, the present study screened over 400 drugs as inhibitors of the MATE1-mediated transport of four structurally distinct organic cation substrates: the commonly used drugs: 1) metformin and 2) cimetidine; and two prototypic cationic substrates, 3) 1-methyl-4-phenylpyridinium (MPP), and 4) the novel fluorescent probe, N,N,N-trimethyl-2-[methyl(7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)amino]ethanaminium iodide. Transport was measured in Chinese hamster ovary cells that stably expressed the human ortholog of MATE1. Comparison of the resulting inhibition profiles revealed no systematic influence of substrate structure on inhibitory efficacy. Similarly, IC50 values for 26 structurally diverse compounds revealed no significant influence of substrate structure on the kinetic interaction of inhibitor with MATE1. The IC50 data were used to generate three-dimensional quantitative pharmacophores that identified hydrophobic regions, H-bond acceptor sites, and an ionizable (cationic) feature as key determinants for ligand binding to MATE1. In summary, in contrast to the behavior observed with some other multidrug transporters, including OCT2, the results suggest that substrate identity exerts comparatively little influence on ligand interaction with MATE1.

Emerging Piglet Models of Neonatal Short Bowel Syndrome

Short bowel syndrome (SBS) is a growing problem in the human neonatal population. In infants, SBS is the leading cause of intestinal failure, the state of being unable to absorb sufficient nutrients for growth and development. Neonates with SBS are dependent on long-term parenteral nutrition therapy, but many succumb to the complications of sepsis and liver disease. Research in neonatal SBS is challenged by the ethical limits of studying sick human neonates and the heterogeneous nature of the disease process. Outcomes in SBS vary depending on residual intestinal anatomy, intestinal length, patient age, and exposure to nutrition therapies. The neonatal piglet serves as an appropriate translational model of the human neonate because of similarities in gastrointestinal ontogeny, physiological maturity, and adaptive processes. Re-creating the disease process in a piglet model presents a unique opportunity for researchers to discover novel insights and therapies in SBS. Emerging piglet models of neonatal SBS now represent the entire spectrum of disease seen in human infants. This review aims to contextualize these emerging piglet models within the context of SBS as a heterogeneous disease. We first explore the factors that account for SBS heterogeneity and then explore the suitability of the neonatal piglet as an appropriate translational animal model. We then examine differences between the emerging piglet models of neonatal SBS and how these differences affect their translational potential to human neonates with SBS.

Substrate-dependent inhibition of human MATE1 by cationic ionic liquids

The multidrug and toxin extruders 1- and 2-K (MATE1 and MATE2-K) are expressed in the luminal membrane of renal proximal tubule cells and provide the active step in the secretion of molecules that carry a net positive charge at physiologic pH, so-called organic cations. The present study tested whether structurally distinct MATE substrates can display different quantitative profiles of inhibition when interacting with structurally distinct ligands. The tested ligands were three structurally similar cationic ionic liquids (ILs, salts in the liquid state: N-butylpyridinium, NBuPy; 1-methyl-3-butylimidazolium, Bmim; and N-butyl-N-methylpyrrolidinium, BmPy). Uptake was measured using Chinese hamster ovary cells that stably expressed MATE1 or MATE2-K. By trans-stimulation, all three ILs were transported by both MATE transporters. The three ILs also inhibited uptake of three structurally distinct MATE substrates: 1-methyl-4-phenylpyridinium (MPP), triethylmethylammonium (TEMA), and N,N,N-trimethyl-2-[methyl(7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)amino]ethanaminium (NBD-MTMA). MATE1 displayed a higher affinity for the pyridinium-based NBuPy (IC50 values, 2-4 µM) than for either the pyrrolidinium- (BmPy; 20-70 µM) or imidazolium-based ILs (Bmim; 15-60 µM). Inhibition of MPP, TEMA, and NBD-MTMA transport by NBuPy was competitive, with comparable Ki values against all substrates. Bmim also competitively blocked the three substrates but with Ki values that differed significantly (20 µM against MPP and 30 µM against NBD-MTMA versus 60 µM against TEMA). Together, these data indicate that renal secretion of ILs by the human kidney involves MATE transporters and suggest that the mechanism of transport inhibition is ligand-dependent, supporting the hypothesis that the binding of substrates to MATE transporters involves interaction with a binding surface with multiple binding sites.

Substrate-dependent ligand inhibition of the human organic cation transporter OCT2

Organic cation transporter 2 (OCT2) mediates the initial step in renal secretion of organic cations: uptake from the blood, across the basolateral membrane, and into the renal proximal tubule cells. Because of its potential as a target for unwanted drug-drug interactions (DDIs), considerable attention has been directed toward understanding the basis of OCT2 selectivity. These studies typically assess selectivity based on ligand inhibition profiles for OCT2-mediated transport of a probe substrate. However, little attention has been given to the potential influence of the substrate on the profile of ligand inhibition. Here we compared the IC50 values obtained for a set of structurally distinct inhibitors against OCT2-mediated transport of three structurally distinct substrates: 1-methyl-4-phenylpyridinium (MPP); metformin; and a novel fluorescent substrate, N,N,N-trimethyl-2-[methyl(7-nitrobenzo[c][l,2,5]oxadiazol-4-yl)amino]ethanaminium iodide (NBD-MTMA). The median IC50 value for inhibition of MPP transport was 9-fold higher than that for inhibition of metformin transport. Similarly, the median IC50 value for inhibition of MPP transport was 5-fold higher than that for NBD-MTMA transport. However, this was not a systematic difference in inhibitory efficacy; the ratio of IC50 values, MPP versus NBD-MTMA, ranged from 88-fold (ipratropium) to 0.3-fold (metformin). These data show that 1) the choice of OCT2 substrate significantly influences both quantitative and qualitative inhibitory interactions with cationic drugs; and 2) ligand interactions with OCT2 are not restricted to competition for a common ligand binding site, consistent with a binding surface characterized by multiple, possibly overlapping interaction sites. Development of predictive models of DDIs with OCT2 must take into account the substrate dependence of ligand interaction with this protein.

Basolateral uptake of nucleosides by Sertoli cells is mediated primarily by equilibrative nucleoside transporter 1

The blood-testis barrier (BTB) prevents the entry of many xenobiotic compounds into seminiferous tubules thereby protecting developing germ cells. Understanding drug transport across the BTB may improve drug delivery into the testis. Members of one class of drug, nucleoside reverse transcriptase inhibitors (NRTIs), do penetrate the BTB, presumably through interaction with physiologic nucleoside transporters. By investigating the mechanism of nucleoside transport, it may be possible to design other drugs to bypass the BTB in a similar manner. We present a novel ex vivo technique to study transport at the BTB that employs isolated, intact seminiferous tubules. Using this system, we found that over 80% of total uptake by seminiferous tubules of the model nucleoside uridine could be inhibited by 100 nM nitrobenzylmercaptopurine riboside (NBMPR, 6-S-[(4-nitrophenyl)methyl]-6-thioinosine), a concentration that selectively inhibits equilibrative nucleoside transporter 1 (ENT1) activity. In primary cultured rat Sertoli cells, 100 nM NBMPR inhibited all transepithelial transport and basolateral uptake of uridine. Immunohistochemical staining showed ENT1 to be located on the basolateral membrane of human and rat Sertoli cells, whereas ENT2 was located on the apical membrane of Sertoli cells. Transepithelial transport of uridine by rat Sertoli cells was partially inhibited by the NRTIs zidovudine, didanosine, and tenofovir disoproxil fumarate, consistent with an interaction between these drugs and ENT transporters. These data indicate that ENT1 is the primary route for basolateral nucleoside uptake into Sertoli cells and a possible mechanism for nucleosides and nucleoside-based drugs to undergo transepithelial transport.

Multiple mechanisms of ligand interaction with the human organic cation transporter, OCT2

OCT2 is the entry step for organic cation (OC) secretion by renal proximal tubules. Although many drugs inhibit OCT2 activity, neither the mechanistic basis of their inhibition nor their transport status is generally known. Using representatives of several structural classes of OCT2-inhibitory ligands described recently (Kido Y, Matsson P, Giacomini KM. J Med Chem 54: 4548-4558, 2011), we determined the kinetic basis of their inhibition of 1-methyl-4-phenylpyridinium (MPP) transport into Chinese hamster ovary cells that stably expressed hOCT2. The "cluster II" inhibitors (which contain known OCT2 substrates) metformin and cimetidine interacted competitively with MPP. However, other cluster II compounds, including tetraethylammonium (TEA), diphenidol and phenyltoloxamine, were mixed-type inhibitors of MPP transport (i.e., decreasing J(max) and increasing K(t)). A cluster III (neutral steroid) representative, adrenosterone, and a cluster I (large, flexible cation) representative, carvedilol, displayed noncompetitive inhibitory profiles. Competitive counterflow (CCF) was used to determine whether the inhibitory ligands served as substrates of hOCT2. Carvedilol (cluster I) and adrenosterone (cluster III) did not support CCF, consistent with the prediction that members of these structural classes are likely to be nontransported inhibitors of OCT2. The cluster II representatives MPP, metformin, cimetidine, and TEA all supported CCF, consistent with independent assessments of their OCT2-mediated transport. However, the other cluster II representatives, diphenidol and phenyltoloxamine, failed to support CCF, suggesting that neither compound is transported by OCT2. An independent assessment of diphenidol transport (using liquid chromatography with tandem mass spectroscopy) confirmed this observation. The results underscore the caution required for development of predictive models of ligand interaction with multidrug transporters.

Role of the equilibrative and concentrative nucleoside transporters in the intestinal absorption of the nucleoside drug, ribavirin, in wild-type and Ent1(-/-) mice

Ribavirin is frontline treatment for hepatitis C virus infection. To determine the role of nucleoside transporters in the intestinal absorption of orally administered ribavirin, we perfused the intestines of Ent1(-/-) and wild-type mice, in situ, with [(3)H] ribavirin (20, 200, and 5000 μM) in the presence and absence of sodium. The decrease in luminal ribavirin concentration over 30 min was measured at 5 min intervals. Blood samples were collected approximately every 10 min. Ribavirin plus phosphorylated metabolite concentrations (hereafter referred to as ribavirin) were determined in tissue, blood, and plasma by HPLC fractionation and scintillation counting. There was no significant difference between wild-type and Ent1(-/-) mice in intestinal loss of ribavirin at any ribavirin concentration studied. Perfusions without sodium drastically reduced the intestinal loss of ribavirin in both wild-type and Ent1(-/-) mice. After 20 μM ribavirin perfusions, Ent1(-/-) intestinal tissue contained 8-fold greater ribavirin than wild-type mice (p < 0.01). Ribavirin concentrations in the wild-type intestinal tissue were 70-fold higher after 200 vs 20 μM perfusions (p < 0.001), indicating saturation of intestinal ribavirin efflux and possibly other processes as well. Ribavirin plasma concentrations were significantly higher in wild-type mice (2.7-fold) vs Ent1(-/-) mice at 30 min after the 20 μM perfusion (p < 0.01). These results suggest that, at lower intestinal concentrations of ribavirin, concentrative and equilibrative nucleoside transporters are important in the intestinal absorption of ribavirin. At higher intestinal concentrations, these transporters are saturated and other processes in the intestine (transport and/or metabolism) play an important role in the absorption of ribavirin.

Functional significance of conserved cysteines in the human organic cation transporter 2

The significance of conserved cysteines in the human organic cation transporter 2 (hOCT2), namely the six cysteines in the long extracellular loop (loop cysteines) and C474 in transmembrane helix 11, was examined. Uptake of tetraethylammonium (TEA) and 1-methyl-4-phenypyridinium (MPP) into Chinese hamster ovary cells was stimulated >20-fold by hOCT2 expression. Both cell surface expression and transport activity were reduced considerably following mutation of individual loop cysteines (C51, C63, C89, C103, and C143), and the C89 and C103 mutants had reduced Michaelis constants (K(t)) for MPP. The loop cysteines were refractory to interaction with thiol-reactive biotinylation reagents, except after pretreatment of intact cells with dithiothreitol or following cell membrane solubilization. Reduction of disulfide bridge(s) did not affect transport, but labeling the resulting free thiols with maleimide-PEO(2)-biotin did. Mutation of C474 to an alanine or phenylalanine did not affect the K(t) value for MPP. In contrast, the K(t) value associated with TEA transport was reduced sevenfold in the C474A mutant, and the C474F mutant failed to transport TEA. This study shows that some but not all of the six extracellular loop cysteines exist within disulfide bridge(s). Each loop cysteine is important for plasma membrane targeting, and their mutation can influence substrate binding. The effect of C474 mutation on TEA transport suggests that it contributes to a TEA binding surface. Given that TEA and MPP are competitive inhibitors, the differential effects of C474 modification on TEA and MPP binding suggest that the binding surfaces for each are distinct, but overlapping in area.

Twelve transmembrane helices form the functional core of mammalian MATE1 (multidrug and toxin extruder 1) protein

The x-ray structure of the prototypic MATE family member, NorM from Vibrio cholerae, reveals a protein fold composed of 12 transmembrane helices (TMHs), confirming hydropathy analyses of the majority of (prokaryotic and plant) MATE transporters. However, the mammalian MATEs are generally predicted to have a 13(th) TMH and an extracellular C terminus. Here we affirm this prediction, showing that the C termini of epitope-tagged, full-length human, rabbit, and mouse MATE1 were accessible to antibodies from the extracellular face of the membrane. Truncation of these proteins at or near the predicted junction between the 13(th) TMH and the long cytoplasmic loop that precedes it resulted in proteins that (i) trafficked to the membrane and (ii) interacted with antibodies only after permeabilization of the plasma membrane. CHO cells expressing rbMate1 truncated at residue Gly-545 supported levels of pH-sensitive transport similar to that of cells expressing the full-length protein. Although the high transport rate of the Gly-545 truncation mutant was associated with higher levels of membrane expression (than full-length MATE1), suggesting the 13(th) TMH may influence substrate translocation, the selectivity profile of the mutant indicated that TMH13 has little impact on ligand binding. We conclude that the functional core of MATE1 consists of 12 (not 13) TMHs. Therefore, we used the x-ray structure of NorM to develop a homology model of the first 12 TMHs of MATE1. The model proved to be stable in molecular dynamic simulations and agreed with topology evident from preliminary cysteine scanning of intracellular versus extracellular loops.

Molecular determinants of ligand selectivity for the human multidrug and toxin extruder proteins MATE1 and MATE2-K

The present study compared the selectivity of two homologous transport proteins, multidrug and toxin extruders 1 and 2-K (MATE1 and MATE2-K), and developed three-dimensional pharmacophores for inhibitory ligand interaction with human MATE1 (hMATE1). The human orthologs of MATE1 and MATE2-K were stably expressed in Chinese hamster ovary cells, and transport function was determined by measuring uptake of the prototypic organic cation (OC) substrate 1-methyl-4-phenylpyridinium (MPP). Both MATEs had similar apparent affinities for MPP, with K(tapp) values of 4.4 and 3.7 μM for MATE1 and MATE2-K, respectively. Selectivity was assessed for both transporters from IC(50) values for 59 structurally diverse compounds. Whereas the two transporters discriminated markedly between a few of the test compounds, the IC(50) values for MATE1 and MATE2-K were within a factor of 3 for most of them. For hMATE1 there was little or no correlation between IC(50) values and the individual molecular descriptors LogP, total polar surface area, or pK(a). The IC(50) values were used to generate a common-features pharmacophore, quantitative pharmacophores for hMATE1, and a bayesian model suggesting molecular features favoring and not favoring the interaction of ligands with hMATE1. The models identified hydrophobic regions, hydrogen bond donor and hydrogen bond acceptor sites, and an ionizable (cationic) feature as key determinants for ligand binding to MATE1. In summary, using a combined in vitro and computational approach, MATE1 and MATE2-K were found to have markedly overlapping selectivities for a broad range of cationic compounds, including representatives from seven novel drug classes of Food and Drug Administration-approved drugs.

Liver X receptor activation downregulates organic anion transporter 1 (OAT1) in the renal proximal tubule

Liver X receptors (LXRs) play an important role in the regulation of cholesterol by regulating several transporters. In this study, we investigated the role of LXRs in the regulation of human organic anion transporter 1 (hOAT1), a major transporter localized in the basolateral membrane of the renal proximal tubule. Exposure of renal S2 cells expressing hOAT1 to LXR agonists (TO901317 and GW3965) and their endogenous ligand [22(R)-hydroxycholesterol] led to the inhibition of hOAT1-mediated [(14)C]PAH uptake. This inhibition was abolished by coincubation of the above agonists with 22(S)-hydroxycholesterol, an LXR antagonist. Moreover, it was found that the effect of LXR agonists was not mediated by changes in intracellular cholesterol levels. Interestingly, the inhibitory effect of LXRs was enhanced in the presence of 9-cis retinoic acid, a retinoic X receptor agonist. Kinetic analysis revealed that LXR activation decreased the maximum rate of PAH transport (J(max)) but had no effect on the affinity of the transporter (K(t)). This result correlated well with data from Western blot analysis, which showed the decrease in hOAT1 expression following LXR activation. Similarly, TO901317 inhibited [(14)C]PAH uptake by the renal cortical slices as well as decreasing mOAT1 protein expression in mouse kidney. Our findings indicated for the first time that hOAT1 was downregulated by LXR activation in the renal proximal tubule.

Characterization of the inhibitory effects of N-butylpyridinium chloride and structurally related ionic liquids on organic cation transporters 1/2 and human toxic extrusion transporters 1/2-k in vitro and in vivo

Ionic liquids (ILs) are a class of salts that are expected to be used as a new source of solvents and for many other applications. Our previous studies revealed that selected ILs, structurally related organic cations, are eliminated exclusively in urine as the parent compound, partially mediated by renal transporters. This study investigated the inhibitory effects of N-butylpyridinium chloride (NBuPy-Cl) and structurally related ILs on organic cation transporters (OCTs) and multidrug and toxic extrusion transporters (MATEs) in vitro and in vivo. After Chinese hamster ovary cells expressing rat (r) OCT1, rOCT2, human (h) OCT2, hMATE1, or hMATE2-K were constructed, the ability of NBuPy-Cl, 1-methyl-3-butylimidazolium chloride (Bmim-Cl), N-butyl-N-methylpyrrolidinium chloride (BmPy-Cl), and alkyl substituted pyridinium ILs to inhibit these transporters was determined in vitro. NBuPy-Cl (0, 0.5, or 2 mg/kg per hour) was also infused into rats to assess its effect on the pharmacokinetics of metformin, a substrate of OCTs and MATEs. NBuPy-Cl, Bmim-Cl, and BmPy-Cl displayed strong inhibitory effects on these transporters (IC(50) = 0.2-8.5 μM). In addition, the inhibitory effects of alkyl-substituted pyridinium ILs on OCTs increased dramatically as the length of the alkyl chain increased. The IC(50) values were 0.1, 3.8, 14, and 671 μM (hexyl-, butyl-, and ethyl-pyridinium and pyridinium chloride) for rOCT2-mediated metformin transport. Similar structurally related inhibitory kinetics were also observed for rOCT1 and hOCT2. The in vivo coadministration study revealed that NBuPy-Cl reduced the renal clearance of metformin in rats. These results demonstrate that ILs compete with other substrates of OCTs and MATEs and could alter the in vivo pharmacokinetics of such substrates.

Calcium sensitivity of dicarboxylate transport in cultured proximal tubule cells

Urinary citrate is an important inhibitor of calcium nephrolithiasis and is primarily determined by proximal tubule reabsorption. The major transporter to reabsorb citrate is Na(+)-dicarboxylate cotransporter (NaDC1), which transports dicarboxylates, including the divalent form of citrate. We previously found that opossum kidney (OK) proximal tubule cells variably express either divalent or trivalent citrate transport, depending on extracellular calcium. The present studies were performed to delineate the mechanism of the effect of calcium on citrate and succinate transport in these cells. Transport was measured using isotope uptake assays. In some studies, NaDC1 transport was studied in Xenopus oocytes, expressing either the rabbit or opossum ortholog. In the OK cell culture model, lowering extracellular calcium increased both citrate and succinate transport by more than twofold; the effect was specific in that glucose transport was not altered. Citrate and succinate were found to reciprocally inhibit transport at low extracellular calcium (<60 μM), but not at normal calcium (1.2 mM); this mutual inhibition is consistent with dicarboxylate transport. The inhibition varied progressively at intermediate levels of extracellular calcium. In addition to changing the relative magnitude and interaction of citrate and succinate transport, decreasing calcium also increased the affinity of the transport process for various other dicarboxylates. Also, the affinity for succinate, at low concentrations of substrate, was increased by calcium removal. In contrast, in oocytes expressing NaDC1, calcium did not have a similar effect on transport, indicating that NaDC1 could not likely account for the findings in OK cells. In summary, extracellular calcium regulates constitutive citrate and succinate transport in OK proximal tubule cells, probably via a novel transport process that is not NaDC1. The calcium effect on citrate transport parallels in vivo studies that demonstrate the regulation of urinary citrate excretion with urinary calcium excretion, a process that may be important in decreasing urinary calcium stone formation.

Effects of dose and route on the disposition and kinetics of 1-butyl-1-methylpyrrolidinium chloride in male F-344 rats

Studies were conducted to characterize the effects of dose and route of administration on the disposition of 1-butyl-1-methylpyrrolidinium (BmPy-Cl) in male Fischer-344 rats. After a single oral administration of [(14)C]BmPy-Cl (50 mg/kg), BmPy-Cl in the blood decreased rapidly after C(max) of 89.1 min with a distribution half-life (t(1/2)(alpha)) of 21 min, an elimination half-life (t(1/2)(beta)) of 5.6 h, and a total body clearance of 7.6 ml/min. After oral administration (50, 5, and 0.5 mg/kg), 50 to 70% of the administered radioactivity was recovered in the feces, with the remainder recovered in the urine. Serial daily oral administrations of [(14)C]BmPy-Cl (50 mg/kg/day for 5 days) did not result in a notable alteration in disposition or elimination. After each administration, 88 to 94% of the dose was eliminated in a 24-h period, with 63 to 76% of dose recovered in the feces. Intravenous administration of [(14)C]BmPy-Cl (5 mg/kg) resulted in biphasic elimination. Oral systemic bioavailability was 43.4%, approximately equal to the dose recovered in urine after oral administration (29-38%). Total dermal absorption of [(14)C]BmPy-Cl (5 mg/kg) was moderate when it was applied in dimethylformamide-water (34 + or - 13%), variable in water (22 + or - 8%), or minimal in ethanol-water (13 + or - 1%) vehicles. Urine was the predominant route of elimination regardless of vehicle. Only parent [(14)C]BmPy-Cl was detected in the urine after all doses and routes of administration. BmPy-Cl was found to be a substrate for (K(t) = 37 microM) and inhibitor of (IC(50/tetraethylammonium) = 0.5 microM) human organic cation transporter 2. In summary, BmPy-Cl is moderately absorbed, extracted by the kidney, and eliminated in the urine as parent compound, independent of dose, number, or route of administration.

Regulatory role of testosterone in organic cation transport: in vivo and in vitro studies

The renal proximal tubule (RPT) plays a crucial role in organic cation (OC) secretion and has a major impact on pharmacokinetics of OC drugs. Secretory transport is vectorial. Thus, it involves transporters located at both basolateral and apical membranes. Although sex hormones have been shown to regulate OC transport, there is little data on the effect of testosterone on OC secretion in a whole animal. Therefore, we determined the clearance of tetraethylammonium (TEA), a model OC substrate, in intact and castrated male mice. Castration significantly decreased renal TEA secretion by 30%, and testosterone supplementation returned TEA secretion to control levels in castrated mice. The mechanism of this effect was further examined in isolated mouse renal proximal tubules (mRPT). TEA uptake in isolated mRPT from castrated mice was reduced by 36%. This effect was reversed in tubules from castrated mice supplemented with testosterone. Kinetic analysis of [(3)H]-TEA uptake in isolated mRPT showed a decreased V(max) with no change in K(m), implying that the decrease in transport rate was caused by lowering in the number of transporters in castrated mice rather than a change in transporter affinity. Quantitative real time polymerase chain reaction (real time PCR) revealed that organic cation transporter (OCT)2 is the major TEA transporter in male mice. Moreover, OCT2 mRNA level was significantly reduced after castration. Castrated mice also showed a modest increase in organic cation/carnitine transporter 1 (OCTN1) mRNA level, indicating that testosterone may also regulate apical OCTN1 expression. These data suggest that testosterone regulates transepithelial transport of OC through modulation of OCT2 expression in male mice.

MATE1 has an external COOH terminus, consistent with a 13-helix topology

The mammalian members of the Multidrug And Toxin Extruder family, i.e., MATE1 and MATE2-K, are suspected of mediating the luminal step in renal secretion of organic cations. The 1,000+ prokaryotic/fungal/plant MATE family members are predicted to have 12 transmembrane helices (TMHs), whereas MATE1/2-K appear to have an additional (13th) COOH-terminal helix. Here, we determined whether rabbit MATE1 has an external COOH terminus, consistent with the presence of 13 TMHs. A V5 epitope tag at the COOH terminus of MATE1 was freely accessible to external V5 antibody, whereas tags at the NH(2) terminus, or at sites of truncation within the long cytoplasmic loop between predicted TMHs 12 and 13, were only accessible to the V5 antibody following permeabilization of the membrane. The truncated mutants that lacked TMH13 still retained transport activity, indicating that the terminal helix was not necessary for transport function. Cells that expressed a mutant lacking only TMH13 displayed similar K(t) and J(max) values to those of the full-length protein, although when normalized to protein expressed at the plasma membrane, the transport rate of the mutant was <10% that of full-length MATE1. An effectively cysteine-less MATE1 mutant (Delta13Cys) was functional and refractory to reaction with the impermeant marker of accessible cysteine residues, maleimide-PEO(2)-biotin. Delta13Cys mutants with an added cysteine residue at the truncation sites within the terminal cytoplasmic loop reacted with maleimide biotin only after permeabilization of the membrane, whereas a mutant with a cysteine residue at the COOH terminus was freely accessible to maleimide biotin. These data are consistent with a mammalian MATE topology that includes 13 TMHs and indicate that the terminal TMH, although not necessary for transport function, may influence the turnover characteristics of the transporter.

Characterization of the disposition and toxicokinetics of N-butylpyridinium chloride in male F-344 rats and female B6C3F1 mice and its transport by organic cation transporter 2

Studies were conducted to characterize the effect of dose and route of administration on the disposition of N-butylpyridinium chloride (NBuPy-Cl), an ionic liquid with solvent properties. Urine was the major route of NBuPy-Cl excretion after intravenous (5 mg/kg), single oral (0.5, 5, or 50 mg/kg), or repeated oral (50 mg/kg/day, 5 days) administration to male F-344 rats and single oral (50 mg/kg) administration to female B6C3F1 mice. Depending on the vehicle, absorption after dermal application (5 mg/kg, 125 microg/cm(2)) was 10 to 35% at 96 h. After the single intravenous dose, the blood concentration of NBuPy-Cl decreased in a biphasic manner with an elimination half-life of 2.2 h and a clearance of 7 ml/min. After single oral administration of NBuPy-Cl (50 mg/kg), maximum blood concentration was reached at 1.3 h, and the bioavailability was determined to be 47% at 6 h based on the blood toxicokinetics and 67% at 72 h based on urinary excretion. In all the urine and blood samples, only the parent compound was detected. Coadministration of NBuPy-Cl and inulin (by intravenous injection) revealed that the clearance of NBuPy-Cl exceeded the rat glomerular filtration rate. After incubation with Chinese hamster ovary cells expressing human organic cation transporter 2 (hOCT2), NBuPy-Cl was transported effectively (K(t) = 18 microM), and also a potent inhibitor of hOCT2 mediated tetraethylammonium transport (IC(50) = 2.3 microM). In summary, NBuPy-Cl is partially absorbed from the gastrointestinal tract and eliminated rapidly in the urine as parent compound most likely by renal glomerular filtration and OCT2-mediated secretion.

The role of the equilibrative nucleoside transporter 1 (ENT1) in transport and metabolism of ribavirin by human and wild-type or Ent1-/- mouse erythrocytes

The polar nucleoside drug ribavirin is front-line treatment for chronic hepatitis C virus infection. The human equilibrative nucleoside transporter (ENT) 1 transports ribavirin into erythrocytes where it is phosphorylated. These phosphorylated metabolites accumulate in the erythrocytes and produce dose-limiting hemolytic anemia. Here, we examined the in vitro and ex vivo transport and metabolism of ribavirin by erythrocytes isolated from humans and Ent1-null mice. Ribavirin (2.4 microM) uptake was significantly higher (1044 +/- 255 amol/microg/10 s) into erythrocytes from Ent1(+/+) mice compared with that from Ent1(-/-) mice (76.48 +/- 11.20 amol/microg/10 s). Our results showed a saturable (K(m) of 382 +/- 75.1 microM) transport of [(3)H]ribavirin into erythrocytes from Ent1(+/+) mice. We found that ribavirin concentration rapidly (within 60 s) reached equilibrium in erythrocytes using a time course of [(3)H]ribavirin transport (2.5 microM) and metabolism in mouse and human erythrocytes for 8 h. However, total radioactivity of ribavirin was predominantly attributed to the phosphorylated metabolites ribavirin monophosphate and ribavirin triphosphate. Our findings allow us to estimate ribavirin transport, diffusion, and metabolic clearance and to predict in vivo accumulation of ribavirin phosphates in erythrocytes of both mice and humans. Our modeling of ribavirin in erythrocytes on long-term administration of ribavirin suggests that the accumulation of ribavirin inside the cells is dependent on ENT1/Ent1 transport and the rates of intracellular phosphorylation and the degradation of the phosphorylated metabolites. We predict that Ent1(+/+) and Ent1(-/-) mice will serve as excellent models to investigate the contribution of Ent1 to the pharmacokinetics and toxicity of ribavirin in vivo.

Effects of hyperosmolarity on the Na+-myo-inositol cotransporter SMIT2 stably transfected in the Madin-Darby canine kidney cell line

Myo-inositol (MI) is a compatible osmolyte used by cells to compensate for changes in the osmolarity of their surrounding milieu. In kidney, the basolateral Na+-MI cotransporter (SMIT1) and apical SMIT2 proteins are homologous cotransporters responsible for cellular uptake of MI. It has been shown in the Madin-Darby canine kidney (MDCK) cell line that SMIT1 expression was under the control of the tonicity-sensitive transcription factor, tonicity-responsive enhancer binding protein (TonEBP). We used an MDCK cell line stably transfected with SMIT2 to determine whether variations in external osmolarity could also affect SMIT2 function. Hyperosmotic conditions (+200 mosM raffinose or NaCl but not urea) generated an increase in SMIT2-specific MI uptake by three- to ninefold in a process that required protein synthesis. Using quantitative RT-PCR, we have determined that hyperosmotic conditions augment both the endogenous SMIT1 and the transfected SMIT2 mRNAs. Transport activities for both SMIT1 and SMIT2 exhibited differences in their respective induction profiles for both their sensitivities to raffinose, as well as in their time course of induction. Application of MG-132, which inhibits nuclear translocation of TonEBP, showed that the effect of osmolarity on transfected SMIT2 was unrelated to TonEBP, unlike the effect observed with SMIT1. Inhibition studies involving the hyperosmolarity-related MAPK suggested that p38 and JNK play a role in the induction of SMIT2. Further studies have shown that hyperosmolarity also upregulates another transfected transporter (Na+-glucose), as well as several endogenously expressed transport systems. This study shows that hyperosmolarity can stimulate transport in a TonEBP-independent manner by increasing the amount of mRNA derived from an exogenous DNA segment.

Identification of a new urate and high affinity nicotinate transporter, hOAT10 (SLC22A13)

The orphan transporter hORCTL3 (human organic cation transporter like 3; SLC22A13) is highly expressed in kidneys and to a weaker extent in brain, heart, and intestine. hORCTL3-expressing Xenopus laevis oocytes showed uptake of [(3)H]nicotinate, [(3)H]p-aminohippurate, and [(14)C]urate. Hence, hORCTL3 is an organic anion transporter, and we renamed it hOAT10. [(3)H]Nicotinate transport by hOAT10 into X. laevis oocytes and into Caco-2 cells was saturable with Michaelis constants (K(m)) of 22 and 44 microm, respectively, suggesting that hOAT10 may be the molecular equivalent of the postulated high affinity nicotinate transporter in kidneys and intestine. The pH dependence of hOAT10 suggests p-aminohippurate(-)/OH(-), urate(-)/OH(-), and nicotinate(-)/OH(-) exchange as possible transport modes. Urate inhibited [(3)H]nicotinate transport by hOAT10 with an IC(50) value of 759 microm, assuming that hOAT10 represents a low affinity urate transporter. hOAT10-mediated [(14)C]urate uptake was elevated by an exchange with l -lactate, pyrazinoate, and nicotinate. Surprisingly, we have detected urate(-)/glutathione exchange by hOAT10, consistent with an involvement of hOAT10 in the renal glutathione cycle. Uricosurics, diuretics, and cyclosporine A showed substantial interactions with hOAT10, of which cyclosporine A enhanced [(14)C]urate uptake, providing the first molecular evidence for cyclosporine A-induced hyperuricemia.

SMIT2 mediates all myo-inositol uptake in apical membranes of rat small intestine

This study presents the characterization of myo-inositol (MI) uptake in rat intestine as evaluated by use of purified membrane preparations. Three secondary active MI cotransporters have been identified; two are Na(+) coupled (SMIT1 and SMIT2) and one is H(+) coupled (HMIT). Through inhibition studies using selective substrates such as d-chiro-inositol (DCI, specific for SMIT2) and l-fucose (specific for SMIT1), we show that SMIT2 is exclusively responsible for apical MI transport in rat intestine; rabbit intestine appears to lack apical transport of MI. Other sugar transport systems known to be present in apical membranes, such as SGLT1 or GLUT5, lacked any significant contribution to MI uptake. Functional analysis of rat SMIT2 activity, via electrophysiological studies in Xenopus oocytes, demonstrated similarities to the activities of SMIT2 from other species (rabbit and human) displaying high affinities for MI (0.150 +/- 0.040 mM), DCI (0.31 +/- 0.06 mM), and phlorizin (Pz; 0.016 +/- 0.007 mM); low affinity for glucose (36 +/- 7 mM); and no affinity for l-fucose. Although these functional characteristics essentially confirmed those found in rat intestinal apical membranes, a unique discrepancy was seen between the two systems studied in that the affinity constant for glucose was approximately 40-fold lower in vesicles (K(i) = 0.94 +/- 0.35 mM) than in oocytes. Finally, the transport system responsible for the basolateral efflux transporter of glucose in intestine, GLUT2, did not mediate any significant radiolabeled MI uptake in oocytes, indicating that this transport system does not participate in the basolateral exit of MI from small intestine.

Influence of estrogen and xenoestrogens on basolateral uptake of tetraethylammonium by opossum kidney cells in culture

The sex steroid hormone estrogen down-regulates renal organic cation (OC) transport in animals, and it may contribute to sex-related differences in xenobiotic accumulation and excretion. Also, the presence of various endocrine-disrupting chemicals, i.e., environmental chemicals that possess estrogenic activity (e.g., xenoestrogens) may down-regulate various transporters involved in renal accumulation and excretion of xenobiotics. The present study characterizes the mechanism by which long-term (6-day) incubation with physiological concentrations of 17beta-estradiol (E(2)) or the xenoestrogens diethylstilbestrol (DES) and bisphenol A (BPA) regulates the basolateral membrane transport of the OC tetraethylammonium (TEA) in opossum kidney (OK) cell renal cultures. Both 17beta-E(2) and the xenoestrogen DES produced a dose- and time-dependent inhibition of basolateral TEA uptake in OK cell cultures, whereas the weakly estrogenic BPA had no effect on TEA uptake. Treatment for 6 days with either 1 nM 17beta-E(2) or DES reduced TEA uptake by approximately 30 and 40%, respectively. These effects were blocked completely by the estrogen receptor antagonist ICI 182780 (Faslodex, fulvestrant), suggesting that these estrogens regulate OC transport through the estrogen receptor, which was detected (estrogen receptor alpha) in OK cell cultures by reverse transcription-polymerase chain reaction. The J(max) value for TEA uptake in 17beta-E(2)- and DES-treated OK cell cultures was approximately 40 to 50% lower than for ethanol-treated cultures, whereas K(t) was unaffected. This reduction in transport capacity was correlated with a reduction in OC transporter OCT1 protein expression following treatment with both agents.

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.

Development of the infant intestine: implications for nutrition support

The incidence of preterm births has continued to increase over the past 25 years, and therefore the optimal feeding of these infants is an important clinical concern. This review focuses on intestinal development and physiology, with a particular emphasis on developmentally immature functions of the preterm intestine and the resulting implications for nutrition therapies used to feed the preterm infant.

Inorganic mercury interacts with cysteine residues (C451 and C474) of hOCT2 to reduce its transport activity

Human organic cation transporter 2 (hOCT2) is essential for the renal tubular secretion of many toxic organic cations. Previously, of the cysteines (C437, C451, C470, and C474) that occur within transmembrane helices that comprise the hydrophilic cleft (proposed site of substrate binding), only C474 was accessible to maleimide-PEO2-biotin (hydrophilic thiol-reactive reagent), and covalent modification of this residue caused lower transport rates (Pelis RM, Zhang X, Dangprapai Y, Wright SH, J Biol Chem 281: 35272–35280, 2006). Thus it was hypothesized that the environmental contaminant Hg2+(as HgCl2) would interact with C474 to reduce hOCT2-mediated transport. Uptake of [3H]tetraethylammonium (TEA) into Chinese hamster ovary cells stably expressing hOCT2 was reduced in a concentration-dependent manner by HgCl2, with an IC50of 3.9 ± 0.11 μM. Treatment with 10 μM HgCl2caused a sixfold reduction in the maximal rate of TEA transport but did not alter the affinity of hOCT2 for TEA. To determine which cysteines interact with Hg2+, a mutant with all four cleft cysteines converted to alanines (quadruple mutant), and four variants of this mutant, each with an individual cysteine restored, were created. The quadruple mutant was less sensitive to HgCl2than wild-type, whereas the C451- and C474-containing mutants were more sensitive than the quadruple mutant. Consistent with the HgCl2effect on transport, MTSEA-biotin only interacted with C451 and C474. These data indicate that C451 and C474 of hOCT2 reside in the aqueous milieu of the cleft and that interaction of Hg2+with these residues causes reduced TEA transport activity.

The chloride dependence of the human organic anion transporter 1 (hOAT1) is blunted by mutation of a single amino acid

Organic anion transporter 1 (OAT1) is key for the secretion of organic anions in renal proximal tubules. These organic anions comprise endogenous as well as exogenous compounds including frequently used drugs of various chemical structures. The molecular basis for the polyspecificity of OAT1 is not known. Here we mutated a conserved positively charged arginine residue (Arg(466)) in the 11(th) transmembrane helix of human OAT1. The replacement by the positively charged lysine (R466K) did not impair expression of hOAT1 at the plasma membrane of Xenopus laevis oocytes but decreased the transport of p-aminohippurate (PAH) considerably. Extracellular glutarate inhibited and intracellular glutarate trans-stimulated wild type and mutated OAT1, suggesting for the mutant R466K an unimpaired interaction with dicarboxylates. However, when Arg(466) was replaced by the negatively charged aspartate (R466D), glutarate no longer interacted with the mutant. PAH uptake by wild type hOAT1 was stimulated in the presence of chloride, whereas the R466K mutant was chloride-insensitive. Likewise, the uptake of labeled glutarate or ochratoxin A was chloride-dependent in the wild type but not in R466K. Kinetic experiments revealed that chloride did not alter the apparent K(m) for PAH but influenced V(max) in wild type OAT1-expressing oocytes. In R466K mutants the apparent K(m) for PAH was similar to that of the wild type, but V(max) was not changed by chloride removal. We conclude that Arg(466) influences the binding of glutarate, but not interaction with PAH, and interacts with chloride, which is a major determinant in substrate translocation.

Human oral drugs absorption is correlated to their in vitro uptake by brush border membrane vesicles

Brush border membrane vesicles (BBMV) were prepared from the rabbit small intestine for testing drug absorption potency through the enterocyte's apical membrane, which is an important compartment for drug oral absorption. Some modifications have been made to the traditional vesicle assay for adapting it to the 96-well plate format. The accumulation of 23 reference drugs was measured, and the data showed a good correlation with human oral absorption with a correlation coefficient R=0.853 (P<0.001), with the exception of a few false positive results. As the measured drug absorption may contain a membrane/protein binding component as well as drug uptake into vesicles, these two fractions can be discriminated by changing extravesicular osmolarity using different mannitol concentrations. This model can be applied for evaluating drug absorption rate/mechanisms, and helping drug selection in early drug research and development.

Relative contribution of OAT1 and OAT3 transport activities in isolated perfused rabbit renal proximal tubules

The expression of both OAT1 and OAT3 along the isolated rabbit renal proximal tubule (RPT) was determined using RT-PCR. They were found to be very strong in S2 segment and weak in S1 and S3 segments. We further examined the relative transport activity of these transporters in isolated perfused rabbit RPT using [(3)H]para-aminohippurate ([(3)H]PAH), and estrone sulfate ([(3)H]ES) as specific substrates for rbOAT1 and rbOAT3, respectively. The transport activity of OAT1 was in the order S2>S1=S3 segments and that of OAT3 was in the order S1=S2>>S3 segments. The addition of alpha-ketoglutarate (100 muM) in the bathing medium increased both OAT1 and OAT3 transport activities in all segments of proximal tubule. The kinetics of [(3)H]succinic acid transport, used to measure the activity of sodium dicarboxylate transporter 3 (NaDC3), were examined. The J(max) for succinic acid was in the order S2>S3 and unmeasurable in the S1 segment. Our data indicate that both OAT1 and OAT3 play quantitatively significant roles in the renal transport of organic anions along the proximal tubule but predominately in S2 segment. The relative contribution of both transporters depends on their relative expression levels and may possibly be affected by the activity of NaDC3 in RPT.

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.

Sex Differences in the mRNA, Protein, and Functional Expression of Organic Anion Transporter (Oat) 1, Oat3, and Organic Cation Transporter (Oct) 2 in Rabbit Renal Proximal Tubules

Sex differences in transport of the organic anion (OA) substrate p-aminohippurate (PAH) and the organic cation (OC) substrate tetraethylammonium (TEA) have been recognized for some time. In the rat kidney, androgens up-regulate and estrogens down-regulate PAH and TEA transport, which correlate with similar changes in mRNA and protein expression for the renal basolateral membrane transporters organic anion transporter (Oat) 1 and organic cation transporter (Oct) 2. However, these sex differences are not readily demonstrated in other species. The present study characterizes the kinetics of basolateral membrane PAH, estrone sulfate (ES), and TEA uptake in renal proximal tubule (RPT) suspensions isolated from female and male rabbits to compare functional expression of transport with mRNA and protein expression for rbOat1, rbOat3, and rbOct2. Although rbOat1-rbOat3 mRNA expression exhibited developmental differences, no sex differences in mRNA levels were observed. Oat1 and Oat3 protein expression in RPT suspensions also was similar between adult female and male rabbits. In contrast, rbOct1 and rbOct2 mRNA levels did not show developmental differences, but rbOct2 mRNA expression was greater in adult male than female rabbits. However, the sex difference in rbOct2 mRNA level did not translate to rbOct2 protein expression. Importantly, functional expression of Oat1, Oat3, and Oct2 transport as measured by kinetics (J(max) and K(t)) of PAH, ES, and TEA uptake was similar between adult male and female rabbits, and correlated with rbOat1, rbOat3, and rbOct2 protein expression. Thus, unlike rodents, rabbit renal OA and OC transport does not exhibit sex differences, pointing to the need for caution in extrapolating transport-related sex differences between species.

Enhanced glucose absorption in the rat small intestine following repeated doses of 5-fluorouracil

Many studies demonstrated that 5-fluorouracil (5-FU) treatment of rodents caused the damage of small intestine, resulting in the malabsorption, while we recently found that repeated administration of 5-FU to rats increased Na(+)-dependent glucose absorption in the small intestine. This study investigated the cause of enhanced glucose absorption. 3-O-methyl-d-glucose (3-OMG) absorption was examined using the everted intestine technique. d-Glucose uptake, phlorizin binding, Western blot analysis and membrane fluidity were examined using small intestinal brush-border membrane vesicles (BBMV). Repeated oral administration of 5-FU to rats increased Na(+)-dependent 3-OMG absorption in the small intestine, while alkaline phosphatase activity in the small intestine decreased. Na(+)/K(+)-ATPase activity of 5-FU-treated rats was about three-fold higher than that of control rats. Although the amount of Na(+)-dependent glucose co-transporter (SGLT1) in 5-FU-treated rats decreased, the overshoot magnitude of d-glucose uptake in BBMV was not altered. Maximum binding of phlorizin in 5-FU-treated rats was 1.5-fold larger than that of control rats, but not altered the maximal rate of d-glucose absorption, Michaelis constant of d-glucose and dissociation constant of phlorizin. The membrane fluidity of 5-FU-treated rats increased. The enhanced d-glucose absorption in 5-FU-treated rats seems to occur secondarily due to the activation of Na(+)/K(+)-ATPase activity in basolateral membranes (BLM). Because the amounts of SGLT1 in 5-FU-treated rats decreased, the increase of turnover rate of SGLT1 and/or an expression of unknown Na(+)-dependent glucose co-transporter with high affinity for d-glucose and phlorizin sensitivity would contribute to the enhancement of d-glucose transport in 5-FU-treated rats.

Functional expression of pig renal organic anion transporter 3 (pOAT3)

With the cloning of pig renal organic anion transporter 1 (pOAT1) (Biochimie 84 (2002) 1219) we set up a model system for comparative studies of cloned and natively isolated membrane located transport proteins. Meanwhile, another transport protein involved in p-aminohippurate (PAH) uptake on the basolateral side of the proximal tubule cells was identified, designated organic anion transporter 3 (OAT3). To explore the contribution of pOAT1 to the PAH clearance in comparison to OAT3, it was the aim of this study to extend our model by cloning of the pig ortholog of OAT3. Sequence comparisons of human organic anion transporter 3 (hOAT3) with the expressed sequence tag (EST) database revealed a clone and partial sequence of the pig renal organic anion transporter 3 (pOAT3) ortholog. Sequencing of the entire open reading frame resulted in a protein of 543 amino acid residues encoded by 1632 base pairs (EMBL Acc. No. AJ587003). It showed high homologies of 81%, 80%, 76%, and 77% to the human, rabbit, rat, and mouse OAT3, respectively. A functional characterization of pOAT3 in Xenopus laevis oocytes yielded an apparent Km (Kt) for [3H]estrone sulfate of 7.8 +/- 1.3 microM. Moreover, pOAT3 mediated [3H]estrone sulfate uptake was almost abolished by 0.5 mM of glutarate, dehydroepiandosterone sulfate, or probenecid consistent with the hallmarks of OAT3 function.

Molecular determinants of substrate/inhibitor binding to the human and rabbit renal organic cation transporters hOCT2 and rbOCT2

Organic cation transporters are important for the elimination of many drugs and toxins from the body. In the present study, substrate-transporter interactions were investigated in Chinese hamster ovary cells stably transfected with either the human or rabbit orthologs of the principal organic cation transporter in the kidney, OCT2. IC(50) values, ranging from 0.04 muM to >3 mM, for inhibition of [(14)C]tetraethylammonium transport were determined for more than 30 structurally diverse compounds. Although the two OCT orthologs displayed similar IC(50) values for some of these compounds, the majority varied by as much as 20-fold. Marked differences in substrate affinity were also noted when comparing hOCT2 to the closely related homolog hOCT1. These data suggest the molecular determinants of substrate binding differ markedly among both homologous and orthologous OCT transporters. The software package Cerius(2) (Accelrys, San Diego, CA) was used to generate a descriptor-based, two-dimensional, quantitative structure-activity relationship (QSAR) to produce a model relating the affinity of hOCT2 to particular physicochemical features of substrate/inhibitor molecules (r(2) = 0.81). Comparative molecular field analysis (Tripos, St. Louis, MO) was used to generate three-dimensional QSARs describing the structural basis of substrate binding to hOCT2 and rbOCT2 (q(2) = 0.60 and 0.53, respectively, and each with r(2) = 0.97). The quality of the models was assessed by their ability to successfully predict the inhibition of a set of test compounds. The current models enabled prediction of OCT2 affinity and may prove useful in the prediction of unwanted drug interactions at the level of the renal secretory process.

Residues Met89 and Ser160 in the Human Equilibrative Nucleoside Transporter 1 Affect Its Affinity for Adenosine, Guanosine, S6-(4-Nitrobenzyl)-mercaptopurine Riboside, and Dipyridamole

The human equilibrative nucleoside transporter 1 (hENT1) is an important modulator of the physiological action of adenosine. We identified amino acid residues involved in adenosine transport using a yeast-based assay to rapidly screen and identify randomly generated hENT1 mutants that exhibited decreased sensitivity to inhibition of adenosine transport by various hENT1 competitive inhibitors. We identified Met89 and Ser160 as important in the affinity of hENT1 for various substrates and inhibitors. Mutation to Met89Cys or Ser160Cys significantly (p < 0.05) increased the S6-(4-nitrobenzyl)-mercaptopurine riboside (NBMPR) IC50 values by approximately 4- and 6-fold, respectively (42 +/- 13 and 65 +/- 1.6 nM) compared with the wild-type transporter (11 +/- 0.7 nM). The double mutant Met89Cys/Ser160Cys synergistically increased the NBMPR IC50 value to approximately 19-fold of that of the wild-type transporter. In contrast, compared with wild-type hENT1, the sensitivity to dipyridamole inhibition was significantly (p < 0.05) increased by only the Ser160Cys (approximately 2.6-fold) or the double mutant Met89Cys/Ser160Cys (approximately 4.7-fold) but not by the Met89Cys mutant. Mutation to Met89Cys or Ser160Cys increased the Km of adenosine (approximately 8- and 3-fold) and the Ki of guanosine (approximately 6- and 2-fold). The double mutant increased both the Km value of adenosine and the Ki value of guanosine by approximately 8-fold and seemed to confer no additional reduction in adenosine or guanosine affinity than that by mutation of Met89 alone. Together, these data indicate that transmembrane domains (TMDs) 2 (Met89) and 4 (Ser160) of hENT1 interact and are important in conferring sensitivity to NBMPR. In contrast, Ser160 and Met89 of hENT1, respectively, play a dominant role in conferring sensitivity to dipyridamole and adenosine/guanosine affinity.

Relative contribution of OAT and OCT transporters to organic electrolyte transport in rabbit proximal tubule

We compared the characteristics of several cloned rabbit organic electrolyte (OE) transporters expressed in cultured cells with their behavior in intact rabbit renal proximal tubules (RPT) to determine the contribution of each to basolateral uptake of the weak acid ochratoxin A (OTA) and the weak base cimetidine (CIM). The activity of organic anion transporters OAT1 and OAT3 proved to be distinguishable because OAT1 had a high affinity for PAH ( Ktof 20 μM) and did not support estrone sulfate (ES) transport, whereas OAT3 had a high affinity for ES ( Ktof 4.5 μM) and a weak interaction with PAH (IC50> 1 mM). In contrast, both transporters robustly accumulated OTA. Intact RPT also accumulated OTA, with OAT1 and OAT3 each responsible for ∼50%: ES and PAH each reduced uptake by ∼50%, and the combination of the two eliminated mediated OTA uptake. The weak base CIM was transported by OAT3 ( Ktof 80 μM) and OCT2 ( Ktof 2 μM); OCT1 had a comparatively low affinity for CIM, and CIM uptake by OAT1 was equivocal. Intact RPT accumulated CIM, with TEA and ES reducing CIM uptake by 20 and 75%, respectively, suggesting that OAT3 plays a quantitatively more significant role in CIM uptake in the early proximal tubule than OCT1/2. In single S2 segments of RPT, ES and TEA each blocked ∼50% of CIM uptake. Thus the fractional contribution of different OE transporters to renal secretion is influenced by their affinity for substrate and relative expression level in RPT.

Mutation of leucine-92 selectively reduces the apparent affinity of inosine, guanosine, NBMPR [S6-(4-nitrobenzyl)-mercaptopurine riboside] and dilazep for the human equilibrative nucleoside transporter, hENT1

We developed a yeast-based assay for selection of hENT1 (human equilibrative nucleoside transporter 1) mutants that have altered affinity for hENT1 inhibitors and substrates. In this assay, expression of hENT1 in a yeast strain deficient in adenine biosynthesis (ade2) permits yeast growth on a plate lacking adenine but containing adenosine, a hENT1 substrate. This growth was prevented when inhibitors of hENT1 [e.g. NBMPR [S6-(4-nitrobenzyl)-mercaptopurine riboside], dilazep or dipyridamole] were included in the media. To identify hENT1 mutants resistant to inhibition by these compounds, hENT1 was randomly mutagenized and introduced into this strain. Mutation(s) that allowed growth of yeast cells in the presence of these inhibitors were then identified and characterized. Mutants harbouring amino acid changes at Leu92 exhibited resistance to NBMPR and dilazep but not dipyridamole. The IC50 values of NBMPR and dilazep for [3H]adenosine transport by one of these mutants L92Q (Leu92-->Gln) were approx. 200- and 4-fold greater when compared with the value for the wild-type hENT1, whereas that for dipyridamole remained unchanged. Additionally, when compared with the wild-type transporter, [3H]adenosine transport by L92Q transporter was significantly resistant to inhibition by inosine and guanosine but not by adenosine or pyrimidines. The Km value for inosine transport was approx. 4-fold greater for the L92Q mutant (260+/-16 mM) when compared with the wild-type transporter (65+/-7.8 mM). We have identified for the first time an amino acid residue (Leu92) of hENT1 that, when mutated, selectively alters the affinity of hENT1 to transport the nucleosides inosine and guanosine and its sensitivity to the inhibitors NBMPR and dilazep.

How the house sparrow Passer domesticus absorbs glucose

According to the hypothesis that most glucose absorption occurs passively across intestinal tight junctions (paracellular absorption), one would predict fairly similar rates of in vivo absorption of L-glucose, the stereoisomer of D-glucose that is absorbed only passively and is not catabolized, and of 3-O-methyl-D-glucose (3OMD-glucose), the D-glucose analogue that is actively and passively transported and not catabolized. In house sparrows Passer domesticus, we applied a pharmacokinetic method to measure simultaneous in vivo absorption of [14C]L-glucose and [3H]3OMD-glucose in a situation in which intestinal glucose transporters were relatively saturated (gavage solution contained 200 mmol l(-1) 3OMD-glucose). Fractional absorptions (F) were not significantly different between [3H]3OMD- and [14C]L-glucose (0.80 vs 0.79), and the apparent rates of absorption did not differ significantly. When we performed the same experiment on other sparrows in a situation in which intestinal glucose transporters were relatively unsaturated (200 mmol l(-1) mannitol replaced 3OMD-glucose in the gavage solution), the apparent rate of absorption was significantly reduced for [14C]l-glucose by 39% and for [3H]3OMD-glucose by 26%. A simulation model showed that a reduction is not predicted if most of the [3H]3OMD-glucose is actively absorbed, because the absorption rate of the tracer should increase when competitive inhibitor (unlabeled 3OMD-glucose) is removed. The similar extent and rates of absorption of [3H]3OMD- and [14C]L-glucose, and the acceleration of their rates of absorption in the presence of luminal 3OMD-glucose, are most consistent with Pappenheimer's hypothesis that the majority of dietary D-glucose is absorbed passively.

Expression of apical membrane L-glutamate transporters in neonatal porcine epithelial cells along the small intestinal crypt-villus axis

Enteral l-glutamate is extensively utilized as an oxidative fuel by the gut mucosa in the neonate. To identify major uptake pathways and to understand uptake regulation, we examined transport kinetics and molecular identities of apical membrane l-glutamate transporters in epithelial cells sequentially isolated along the small intestinal crypt-villus axis from milk protein-fed, 16-day-old pigs. The distended intestinal sac method was used to isolate 12 sequential cell fractions from the tip villus to the bottom crypt. Initial rates and kinetics of l-glutamate uptake were measured with l-[G-(3)H]glutamate by fast filtration in apical membrane vesicles prepared by Mg(2+) precipitation and differential centrifugation, with membrane potential clamped by SCN(-). Initial l-glutamate uptake results suggested the presence of B(o) and X(AG)(-) transport systems, but the X(AG)(-) system was predominant for uptake across the apical membrane. Kinetic data suggested that l-glutamate uptake through the X(AG)(-) system was associated with higher maximal transport activity but lower transporter affinity in crypt than in villus cells. Molecular identity of the X(AG)(-) glutamate transporter, based on immunoblot and RT-PCR analysis, was primarily the defined excitatory amino acid carrier (EAAC)-1. EAAC-1 expression was increased with cell differentiation and regulated at transcription and translation levels from crypt to upper villus cells. In conclusion, efficiency and capacity of luminal l-glutamate uptake across the apical membrane are regulated by changing expression of the X(AG)(-) system transporter gene EAAC-1 at transcription and translation levels as well as maximal uptake activity and transporter affinity along the intestinal crypt-villus axis in the neonate.

Expression of SGLT-1 in preneoplastic and neoplastic lesions of the head and neck

Tumors show an increased glucose uptake that is mediated by glucose transport proteins. We have analyzed the expression of the sodium-dependent glucose co-transporters SGLT-1 and-2 in short-term cultures of squamous cell carcinomas of the head and neck (HNSCC) by RT-PCR. Distribution of the SGLT-1 protein in HNSCC tissues was investigated by immunohistochemistry. While we observed in 17/36 HNSCC short-term cultures the SGLT-1 mRNA, we found no SGLT-2 expression. SGLT-1 mRNA expression occurred preferentially in cultures originating from moderately and well differentiated HNSCC. In tumor tissues a heterogeneous SGLT-1 staining restricted to differentiated tumor cells was seen in 27/30 HNSCC analyzed. In normal mucosa SGLT-1 staining was also confined to differentiated compartments and lacked in dysplastic areas. Our data indicate a differentiation-dependent expression of SGLT-1 in HNSCC. This is important knowledge for the planning of glucose-targeting therapies and suggest SGLT-1 as a differentiation marker in head and neck tissues.

Sucrose transport into plasma membrane vesicles from tobacco leaves by H+ symport or counter exchange does not display a linear component

Uptake of [14C]sucrose by plasma membrane vesicles from leaves of tobacco (Nicotiana tabacum L.) was measured after the imposition of an inwardly directed proton gradient (delta pH = 2) and an electrical gradient (delta psi = -68 mV, inside negative) across the vesicle membrane. The vesicles were isolated from a microsomal fraction by two-phase partitioning using media that contained 330 mM of either sorbitol or sucrose. Sucrose transport into vesicles isolated using the sorbitol-containing media showed the hallmarks of electrogenic H+-symport, as it was highly dependent on delta pH, could be increased three- to four-fold by delta psi, and was abolished by carbonylcyanide m-chlorophenylhydrazone (CCCP). Transport of [14C]sucrose into vesicles that were isolated using the sucrose-containing media apparently occurred by counter exchange. Its initial influx also depended on a low external pH, but it was insensitive to CCCP and hardly stimulated by delta psi. Both symport and counter exchange obeyed simple Michaelis-Menten kinetics. Transport that depends linearly on the external sucrose concentration could not be detected, indicating that the 'linear' component that has been observed in sucrose uptake by leaf tissues does not represent a transport route that is provided by the sucrose symporter. The potential role of H+/sucrose-symporters in phloem unloading is briefly discussed.