Renal transport of organic anions and cations

Incubation Time Influences Organic Anion Transporter 1 Kinetics and Renal Clearance Predictions

Accurate predictions of drug uptake transporter involvement in renal excretion of xenobiotics require determination of in vitro transport kinetic parameters under initial-rate conditions. The purpose of the present study was to determine how changing the incubation time from initial rate to steady state influences ligand interactions with the renal organic anion transporter 1 (OAT1), and the impact of the different experimental conditions on pharmacokinetic predictions. Transport studies were performed with Chinese hamster ovary cells expressing OAT1 (CHO-OAT1) and the Simcyp Simulator was used for physiological-based pharmacokinetic predictions. Maximal transport rate and intrinsic uptake clearance (CL_int) for PAH decreased with increasing incubation time. The CL_int values ranged 11-fold with incubation times spanning from 15 s (CL_int,15s, initial rate) to 45 min (CL_int,45min, steady state). The Michaelis constant (K_m) was also influenced by the incubation time with an apparent increase in the Km value at longer incubation times. Inhibition potency of five drugs against PAH transport was tested using incubation times of either 15 s or 10 min. There was no effect of time on inhibition potency for omeprazole or furosemide, whereas indomethacin was less potent, and probenecid (~2-fold) and telmisartan (~7-fold) more potent with the longer incubation time. Notably, the inhibitory effect of telmisartan was reversible, albeit slowly. A pharmacokinetic model was developed for PAH using the CL_int,15s value. The simulated plasma concentration-time profile, renal clearance, and cumulative urinary excretion-time profile of PAH agreed well with reported clinical data, and the PK parameters were sensitive to the time-associated CL_int value used in the model.

Predicting transporter mediated drug-drug interactions via static and dynamic physiologically based pharmacokinetic modeling: A comprehensive insight on where we are now and the way forward

The greater utilization and acceptance of physiologically-based pharmacokinetic (PBPK) modeling to evaluate the potential metabolic drug-drug interactions is evident by the plethora of literature, guidance's, and regulatory dossiers available in the literature. In contrast, it is not widely used to predict transporter-mediated DDI (tDDI). This is attributed to the unavailability of accurate transporter tissue expression levels, the absence of accurate in vitro to in vivo extrapolations (IVIVE), enzyme-transporter interplay, and a lack of specific probe substrates. Additionally, poor understanding of the inhibition/induction mechanisms coupled with the inability to determine unbound concentrations at the interaction site made tDDI assessment challenging. Despite these challenges, continuous improvements in IVIVE approaches enabled accurate tDDI predictions. Furthermore, the necessity of extrapolating tDDI's to special (pediatrics, pregnant, geriatrics) and diseased (renal, hepatic impaired) populations is gaining impetus and is encouraged by regulatory authorities. This review aims to visit the current state-of-the-art and summarizes contemporary knowledge on tDDI predictions. The current understanding and ability of static and dynamic PBPK models to predict tDDI are portrayed in detail. Peer-reviewed transporter abundance data in special and diseased populations from recent publications were compiled, enabling direct input into modeling tools for accurate tDDI predictions. A compilation of regulatory guidance's for tDDI's assessment and success stories from regulatory submissions are presented. Future perspectives and challenges of predicting tDDI in terms of in vitro system considerations, endogenous biomarkers, the use of empirical scaling factors, enzyme-transporter interplay, and acceptance criteria for model validation to meet the regulatory expectations were discussed.

Live Tissue Imaging Reveals Distinct Transcellular Pathways for Organic Cations and Anions at the Blood-Cerebrospinal Fluid Barrier

Formed by the choroid plexus epithelial (CPE) cells, the blood-cerebrospinal fluid barrier (BCSFB) plays an active role in removing drugs, toxins, and metabolic wastes from the brain. Several organic cation and anion transporters are expressed in the CPE cells, but how they functionally mediate transepithelial transport of organic cations and anions remain unclear. In this study, we visualized the transcellular transport of fluorescent organic cation and organic anion probes using live tissue imaging in freshly isolated mouse choroid plexuses (CPs). The cationic probe, 4-[4-(dimethylamino)phenyl]-1-methylpyridinium iodide (IDT307) was transported into CPE cells at the apical membrane and highly accumulated in mitochondria. Consistent with the lack of expression of organic cation efflux transporters, there was little efflux of IDT307 into the blood capillary space. Furthermore, IDT307 uptake and intracellular accumulation was attenuated by approximately 70% in CP tissues from mice with targeted deletion of the plasma membrane monoamine transporter (Pmat). In contrast, the anionic probe fluorescein-methotrexate (FL-MTX) was rapidly transported across the CPE cells into the capillary space with little intracellular accumulation. Rifampicin, an inhibitor of organic anion transporting polypeptides (OATPs), completely blocked FL-MTX uptake into the CPE cells whereas MK-571, a pan-inhibitor of multidrug resistance associated proteins (MRPs), abolished basolateral efflux of FL-MTX. In summary, our results suggest distinct transcellular transport pathways for organic cations and anions at the BCSFB and reveal a pivotal role of PMAT, OATP and MRP transporters in organic cation and anion transport at the blood-cerebrospinal fluid interface. SIGNIFICANCE STATEMENT: Live tissue imaging revealed that while organic cations are transported from the cerebrospinal fluid (CSF) into the choroid plexus epithelial cells by plasma membrane monoamine transporter without efflux into the blood, amphipathic anions in the CSF are efficiently transported across the BCSFB through the collaborated function of apical organic anion transporting polypeptides and basolateral multidrug resistance associated proteins. These findings contribute to a mechanistic understanding of the molecular and cellular pathways for choroid plexus clearance of solutes from the brain.

Prediction of drug-drug interaction potential mediated by transporters between dasatinib and metformin, pravastatin, and rosuvastatin using physiologically based pharmacokinetic modeling

Purpose

Recent in vitro studies demonstrated that dasatinib inhibits organic cation transporter 2 (OCT2), multidrug and toxin extrusion proteins (MATEs), and organic anion transporting polypeptide 1B1/1B3 (OATP1B1/1B3). We developed a physiologically based pharmacokinetic (PBPK) model to assess drug–drug interaction (DDI) potential between dasatinib and known substrates for these transporters in a virtual population.

Methods

The dasatinib PBPK model was constructed using Simcyp^® Simulator by combining its physicochemical properties, in vitro data, in silico predictions, and pharmacokinetic (PK) results from clinical studies. Model validation against three independent clinical trials not used for model development included dasatinib DDI studies with ketoconazole, rifampin, and simvastatin. The validated model was used to simulate DDIs of dasatinib and known substrates for OCT2 and MATEs (metformin) and OATP1B1/1B3 (pravastatin and rosuvastatin).

Results

Simulations of metformin PK in the presence and absence of dasatinib, using inhibitor constant (K_i) values measured in vitro, produced estimated geometric mean ratios (GMRs) of the maximum observed concentration (C_max) and area under the concentration–time curve (AUC) of 1.05 and 1.06, respectively. Sensitivity analysis showed metformin exposure increased < 30% in both AUC and C_max when dasatinib K_i was reduced by tenfold for OCT2 and MATEs simultaneously, and < 40% with a 20-fold K_i reduction. The estimated GMRs of C_max and AUC for pravastatin and rosuvastatin with co-administration of dasatinib were unity (1.00).

Conclusions

This PBPK model accurately described the observed PK profiles of dasatinib. The validated PBPK model predicts low risk of clinically significant DDIs between dasatinib and metformin, pravastatin, or rosuvastatin.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00280-021-04394-z.

Transport Turnover Rates for Human OCT2 and MATE1 Expressed in Chinese Hamster Ovary Cells

MATE1 (multidrug and toxin extruder 1) and OCT2 (organic cation transporter 2) play critical roles in organic cation excretion by the human kidney. The transporter turnover rate (TOR) is relevant to understanding both their transport mechanisms and interpreting the in vitro-in vivo extrapolation (IVIVE) required for physiologically-based pharmacokinetic (PBPK) modeling. Here, we use a quantitative western blot method to determine TORs for MATE1 and OCT2 proteins expressed in CHO cells. MATE1 and OCT2, each with a C-terminal V-5 epitope tag, were cell surface biotinylated and the amount of cell surface MATE1 and OCT2 protein was quantified by western analysis, using standard curves for the V5 epitope. Cell surface MATE1 and OCT2 protein represented 25% and 24%, respectively, of the total expression of these proteins in CHO cells. The number of cell surface transporters was ~55 fmol cm^-2 for MATE1 and ~510 fmol cm^-2 for OCT2. Dividing these values into the different J_max values for transport of MPP, metformin, and atenolol mediated by MATE1 and OCT2 resulted in calculated TOR values (±SE, n = 4) of 84.0 ± 22.0 s^-1 and 2.9 ± 0.6 s^-1; metformin, 461.0 ± 121.0 s^-1 and 12.6 ± 2.4 s^-1; atenolol, 118.0 ± 31.0 s^-1, respectively. These values are consistent with the TOR values determined for a variety of exchangers (NHEs), cotransporters (SGLTs, Lac permease), and uniporters (GLUTs, ENTs).

Why Is the GFR So High?: Implications for the Treatment of Kidney Failure

The high GFR in vertebrates obligates large energy expenditure. Homer Smith's teleologic argument that this high GFR was needed to excrete water as vertebrates evolved in dilute seas is outdated. The GFR is proportional to the metabolic rate among vertebrate species and higher in warm-blooded mammals and birds than in cold-blooded fish, amphibians, and reptiles. The kidney clearance of some solutes is raised above the GFR by tubular secretion, and we presume secretion evolved to eliminate particularly toxic compounds. In this regard, high GFRs may provide a fluid stream into which toxic solutes can be readily secreted. Alternatively, the high GFR may be required to clear solutes that are too large or too varied to be secreted, especially bioactive small proteins and peptides. These considerations have potentially important implications for the understanding and treatment of kidney failure.

Peptide Hormone Insulin Regulates Function, Expression, and SUMOylation of Organic Anion Transporter 3

Organic anion transporter 3 (OAT3) plays an important role in the disposition of various anionic drugs which impacts the pharmacokinetics and pharmacodynamics of the therapeutics, thus influencing the pharmacological effects and toxicity of the drugs. In this study, we investigated the effect of insulin on the regulation of OAT3 function, expression, and SUMOylation. We demonstrated that insulin induced an increase in OAT3 transport activity through a dose- and time-dependent manner in COS-7 cells. The insulin-induced elevation in OAT3 function was blocked by PKA inhibitor H89, which correlated well with OAT3 protein expression. Moreover, both PKA activator Bt2-cAMP-induced increase and insulin-induced increase in OAT3 function were blocked by PKB inhibitor AKTi1/2. To further investigate the involvement of SUMOylation, we treated OAT3-expressing cells with insulin in presence or absence of H89 or AKTi1/2 followed by examining OAT3 SUMOylation. We showed that insulin enhanced OAT3 SUMOylation, and such enhancement was abrogated by H89 and AKTi1/2. Lastly, insulin increased OAT3 function and SUMOylation in rat kidney slice. In conclusion, our investigations demonstrated that insulin regulated OAT3 function, expression, and SUMOylation through PKA/PKB signaling pathway. Graphical abstract.

Molecular and cellular physiology of organic cation transporter 2

Organic cation transporters play a critical role in mediating the distribution of cationic pharmaceuticals. Indeed, organic cation transporter (OCT)2 is the initial step in the renal secretion of organic cations and consequently plays a defining role in establishing the pharmacokinetics of many cationic drugs. Although a hallmark of OCTs is their broad selectivity, this characteristic also makes them targets for unwanted, adverse drug-drug interactions (DDIs), making them a focus for efforts to develop models of ligand interaction that could predict and preempt these adverse interactions. This review discusses the molecular characteristics of these transporters as well as the evidence that established the OCTs as key players in the distribution of organic cations. However, the primary focus is the present understanding of the complexity of ligand interaction with OCTs, particularly OCT2, including evidence for the presence of multiple ligand-binding sites and the influence of substrate structure on the affinity of the transporter for inhibitory ligands. This leads to a discussion of the complexities associated with the development of protocols for assessing the inhibitory potential of new molecular entities to perpetrate unwanted DDIs, the criteria that should be considered in the interpretation of the results of such protocols, and the challenges associated with development of models capable of predicting unwanted DDIs.

Human steroid biosynthesis, metabolism and excretion are differentially reflected by serum and urine steroid metabolomes: A comprehensive review

Advances in technology have allowed for the sensitive, specific, and simultaneous quantitative profiling of steroid precursors, bioactive steroids and inactive metabolites, facilitating comprehensive characterization of the serum and urine steroid metabolomes. The quantification of steroid panels is therefore gaining favor over quantification of single marker metabolites in the clinical and research laboratories. However, although the biochemical pathways for the biosynthesis and metabolism of steroid hormones are now well defined, a gulf still exists between this knowledge and its application to the measured steroid profiles. In this review, we present an overview of steroid hormone biosynthesis and metabolism by the liver and peripheral tissues, specifically highlighting the pathways linking and differentiating the serum and urine steroid metabolomes. A brief overview of the methodology used in steroid profiling is also provided.

Kinetic basis of metformin-MPP interactions with organic cation transporter OCT2

Organic cation transporter 2 (OCT2) clears the blood of cationic drugs. Efforts to understand OCT2 selectivity as a means to predict the potential of new molecular entities (NMEs) to produce unwanted drug-drug interactions typically assess the influence of the NMEs on inhibition of transport. However, the identity of the substrate used to assess transport activity can influence the quantitative profile of inhibition. Metformin and 1-methyl-4-phenylpyridinium (MPP), in particular, display markedly different inhibitory profiles, with IC₅₀ values for inhibition of MPP transport often being more than fivefold greater than IC₅₀ values for the inhibition of metformin transport by the same compound, suggesting that interaction of metformin and MPP with OCT2 cannot be restricted to competition for a single binding site. Here, we determined the kinetic basis for the mutual inhibitory interaction of metformin and MPP with OCT2 expressed in Chinese hamster ovary cells. Although metformin did produce simple competitive inhibition of MPP transport, MPP was a mixed-type inhibitor of metformin transport, decreasing the maximum rate of mediated substrate transport and increasing the apparent Michaelis constant (K_tapp) for OCT2-mediated metformin transport. Furthermore, whereas the IC₅₀ value for metformin's inhibition of MPP transport did not differ from the K_tapp value for metformin transport, the IC₅₀ value for MPP's inhibition of metformin transport was less than its K_tapp value for transport. The simplest model to account for these observations required the influence of a distinct inhibitory site for MPP that, when occupied, decreases the translocation of substrate. These observations underscore the complexity of ligand interaction with OCT2 and argue for use of multiple substrates to obtain the needed kinetic assessment of NME interactions with OCT2.

Contribution of multidrug and toxin extrusion protein 1 (MATE1) to renal secretion of trimethylamine-N-oxide (TMAO)

Trimethylamine-N-oxide (TMAO) gained considerable attention because of its role as a cardiovascular risk biomarker. Organic cation transporter 2 (OCT2) mediates TMAO uptake into renal proximal tubular cells. Here we investigated the potential role of multidrug and toxin extrusion protein 1 (MATE1) for translocation of TMAO across the luminal membrane of proximal tubular cells. HEK293 cells stably expressing OCT2 (HEK-OCT2) or MATE1 (HEK-MATE1) were used for uptake studies. Transcellular transport of TMAO was investigated using monolayers of MDCK control cells (MDCK-Co) as well as single- (MDCK-OCT2, MDCK-MATE1) and double-transfected cells (MDCK-OCT2-MATE1). In line with previous studies, HEK-OCT2 cells revealed a 2.4-fold uptake of TMAO compared to control cells (p < 0.001), whereas no significant uptake was observed in HEK-MATE1. In monolayers of MDCK cells, polarised TMAO transcellular transport was not significantly different between MDCK-Co and MDCK-OCT2 cells, but significantly increased in MDCK-MATE1 (p < 0.05) and MDCK-OCT2-MATE1 cells (p < 0.001). The OCT/MATE inhibitor trimethoprim abolished TMAO translocation in MDCK-OCT2-MATE1 cells (p < 0.05). The present data suggest that MATE1 contributes to renal elimination of TMAO. For selected MATE substrates, such as TMAO, uptake studies using non-polarised MATE-expressing cells can reveal false negative results compared to studies using polarised monolayers.

Loops and layers of post-translational modifications of drug transporters

Drug transporters encoded by solute carrier (SLC) family are distributed in multiple organs including kidney, liver, placenta, brain, and intestine, where they mediate the absorption, distribution, and excretion of a diverse array of environmental toxins and clinically important drugs. Alterations in the expression and function of these transporters play important roles in intra- and inter-individual variability of the therapeutic efficacy and the toxicity of many drugs. Consequently, the activity of these transporters must be highly regulated to carry out their normal functions. While it is clear that the regulation of these transporters tightly depends on genetic mechanisms, many studies have demonstrated that these transporters are the target of various post-translational modifications. This review article summarizes the recent advances in identifying the posttranslational modifications underlying the regulation of the drug transporters of SLC family. Such mechanisms are pivotal not only in physiological conditions, but also in diseases.

Correlation between Apparent Substrate Affinity and OCT2 Transport Turnover

Organic cation (OC) transporter 2 (OCT2) mediates the first step in the renal secretion of many cationic drugs: basolateral uptake from blood into proximal tubule cells. The impact of this process on the pharmacokinetics of drug clearance as estimated using a physiologically-based pharmacokinetic approach relies on an accurate understanding of the kinetics of transport because the ratio of the maximal rate of transport to the Michaelis constant (i.e., J_max/ K_t) provides an estimate of the intrinsic clearance (Cl_int) used in in vitro-in vivo extrapolation of experimentally determined transport data. Although the multispecificity of renal OC secretion, including that of the OCT2 transporter, is widely acknowledged, the possible relationship between relative affinity of the transporter for its diverse substrates and the maximal rates of their transport has received little attention. In this study, we determined the J_max and apparent Michaelis constant (K_tapp) values for six structurally distinct OCT2 substrates and found a strong correlation between J_max and K_tapp; high-affinity substrates [K_tapp values <50 µM, including 1-methyl-4-phenylpyridinium, or 1-methyl-4-phenylpyridinium (MPP), and cimetidine] displayed systematically lower J_max values (<50 pmol cm^-2 min^-1) than did low-affinity substrates (K_tapp >200 µM, including choline and metformin). Similarly, preloading OCT2-expressing cells with low-affinity substrates resulted in systematically larger trans-stimulated rates of MPP uptake than did preloading with high-affinity substrates. The data are quantitatively consistent with the hypothesis that dissociation of bound substrate from the transporter is rate limiting in establishing maximal rates of OCT2-mediated transport. This systematic relationship may provide a means to estimate Cl_int for drugs for which transport data are lacking.

PKC/Nedd4-2 Signaling Pathway Regulates the Cell Surface Expression of Drug Transporter hOAT1

Human organic anion transporter-1 (hOAT1) regulates the absorption, distribution, and excretion of a wide range of clinically important drugs. Our previous work demonstrated that hOAT1 is a dynamic membrane transporter, constitutively internalizing from and recycling back to the cell plasma membrane. Short-term activation (<30 minutes) of protein kinase C (PKC) promotes the attachment of a lysine 48-linked polyubiquitin chain to hOAT1, a process catalyzed by ubiquitin ligase neural precursor cell expressed developmentally down-regulated 4-2 (Nedd4-2). The ubiquitination of hOAT1 then triggers an accelerated endocytosis of the transporter from plasma membrane, which results in reduced hOAT1 expression at the cell surface and decreased hOAT1 transport activity. In the present study, we investigated the long-term effect of PKC on hOAT1. We showed that long-term activation (>2 hours) of PKC significantly enhanced hOAT1 degradation, and such action was partially blocked by ubiquitin mutant Ub-K48R, which has its lysine (K) 48 mutated to arginine (R) and is incapable of forming a K48-linked polyubiquitin chain. The ubiquitin ligase Nedd4-2 was also found to augment hOAT1 degradation. These results suggest that PKC-regulated and Nedd4-2-catalyzed attachment of a lysine 48-linked polyubiquitin chain to hOAT1 is important for hOAT1 stability. We further showed through coimmunoprecipitation experiments that there was a direct association between hOAT1 and Nedd4-2, and such interaction was weakened when the WW3 and WW4 domains of the ligase were mutated. Mutating WW3 and WW4 domains of the ligase also impaired its ability to ubiquitinate hOAT1. Therefore, WW3 and WW4 domains of Nedd4-2 are critical for its association with and modulation of the transporter.

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.

Posttranslational Regulation of Organic Anion Transporters by Ubiquitination: Known and Novel

Organic anion transporters (OATs) encoded by solute carrier 22 family are localized in the epithelia of multiple organs, where they mediate the absorption, distribution, and excretion of a diverse array of negatively charged environmental toxins and clinically important drugs. Alterations in the expression and function of OATs play important roles in intra- and interindividual variability of the therapeutic efficacy and the toxicity of many drugs. As a result, the activity of OATs must be under tight regulation so as to carry out their normal functions. The regulation of OAT transport activity in response to various stimuli can occur at several levels such as transcription, translation, and posttranslational modification. Posttranslational regulation is of particular interest, because it usually happens within a very short period of time (minutes to hours) when the body has to deal with rapidly changing amounts of substances as a consequence of variable intake of drugs, fluids, or meals as well as metabolic activity. This review article highlights the recent advances from our laboratory in uncovering several posttranslational mechanisms underlying OAT regulation. These advances offer the promise of identifying targets for novel strategies that will maximize therapeutic efficacy in drug development.

The role of Nedd4-1 WW domains in binding and regulating human organic anion transporter 1

Human organic anion transporter 1 (hOAT1), expressed at the basolateral membrane of kidney proximal tubule cells, mediates the active renal secretion of a diverse array of clinically important drugs, including anti-human immunodeficiency virus therapeutics, antitumor drugs, antibiotics, antihypertensives, and anti-inflammatories. We have previously demonstrated that posttranslational modification of hOAT1 by ubiquitination is an important mechanism for the regulation of this transporter. The present study aimed at identifying the ubiquitin ligase for hOAT1 and its mechanism of action. We showed that overexpression of neural precursor cell expressed, developmentally downregulated (Nedd)4-1, an E3 ubiquitin ligase, enhanced hOAT1 ubiquitination, decreased hOAT1 expression at the cell surface, and inhibited hOAT1 transport activity. In contrast, overexpression of the ubiquitin ligase-dead mutant Nedd4-1/C867S was without effects on hOAT1. Furthermore, knockdown of endogenously expressed Nedd4-1 by Nedd4-1-specific small interfering RNA reduced hOAT1 ubiquitination. Immunoprecipitation experiments in cultured cells and rat kidney slices and immunofluorescence experiments in rat kidney slices showed that there was a physical interaction between OAT1 and Nedd4-1. Nedd4-1 contains four protein-protein interacting WW domains. When these WW domains were inactivated by mutating two amino acid residues in each of the four WW domains (Mut-WW1: V210W/H212G, Mut-WW2: V367W/H369G, Mut-WW3: I440W/H442G, and Mut-WW4: I492W/H494G, respectively), only Mut-WW2 and Mut-WW3 significantly lost their ability to bind and to ubiquitinate hOAT1. As a result, Mut-WW2 and Mut-WW3 were unable to suppress hOAT1-mediated transport as effectively as wild-type Nedd4-1. In conclusion, this is the first demonstration that Nedd4-1 regulates hOAT1 ubiquitination, expression, and transport activity through its WW2 and WW3 domains.

Metformin and cimetidine: Physiologically based pharmacokinetic modelling to investigate transporter mediated drug-drug interactions

Metformin is used as a probe for OCT2 mediated transport when investigating possible DDIs with new chemical entities. The aim of the current study was to investigate the ability of physiologically-based pharmacokinetic (PBPK) models to simulate the effects of OCT and MATE inhibition by cimetidine on metformin kinetics. PBPK models were developed, incorporating mechanistic kidney and liver sub-models for metformin (OCT and MATE substrate) and a mechanistic kidney sub-model for cimetidine. The models were used to simulate inhibition of the MATE1, MATE2-K, OCT1 and OCT2 mediated transport of metformin by cimetidine. Assuming competitive inhibition and using cimetidine Ki values determined in vitro, the predicted metformin AUC ratio was 1.0 compared to an observed value of 1.46. The observed AUC ratio could only be recovered with this model when the cimetidine Ki for OCT2 was decreased 1000-fold or the Ki's for both OCT1 and OCT2 were decreased 500-fold. An alternative description of metformin renal transport by OCT1 and OCT2, incorporating electrochemical modulation of the rate of metformin uptake together with 8-18-fold decreases in cimetidine Ki's for OCTs and MATEs, allowed recovery of the extent of the observed effect of cimetidine on metformin AUC. While the final PBPK model has limitations, it demonstrates the benefit of allowing for the complexities of passive permeability combined with active cellular uptake modulated by an electrochemical gradient and active efflux.

Nedd4-2 but not Nedd4-1 is critical for protein kinase C-regulated ubiquitination, expression, and transport activity of human organic anion transporter 1

Human organic anion transporter 1 (hOAT1) expressed at the membrane of the kidney proximal tubule cells mediates the body disposition of a diverse array of clinically important drugs, including anti-HIV therapeutics, antitumor drugs, antibiotics, antihypertensives, and antiinflammatories. Therefore, understanding the regulation of hOAT1 will provide significant insights into kidney function and dysfunction. We previously established that hOAT1 transport activity is inhibited by activation of protein kinase C (PKC) through accelerating hOAT1 internalization from cell surface into intracellular endosomes and subsequent degradation. We further established that PKC-induced hOAT1 ubiquitination is an important step preceding hOAT1 internalization. In the current study, we identified two closely related E3 ubiquitin ligases, neural precursor cell expressed, developmentally downregulated 4-1 and 4-2 (Nedd4-1 and Nedd4-2), as important regulators for hOAT1: overexpression of Nedd4-1 or Nedd4-2 enhanced hOAT1 ubiquitination, reduced the hOAT1 amount at the cell surface, and suppressed hOAT1 transport activity. In further exploring the relationship among PKC, Nedd4-1, and Nedd4-2, we discovered that PKC-dependent changes in hOAT1 ubiquitination, expression, and transport activity were significantly blocked in cells transfected with the ligase-dead mutant of Nedd4-2 (Nedd4-2/C821A) or with Nedd4-2-specific siRNA to knockdown endogenous Nedd4-2 but not in cells transfected with the ligase-dead mutant of Nedd4-1 (Nedd4-1/C867S) or with Nedd4-1-specific siRNA to knockdown endogenous Nedd4-1. In conclusion, this is the first demonstration that both Nedd4-1 and Nedd4-2 are important regulators for hOAT1 ubiquitination, expression, and function. Yet they play distinct roles, as Nedd4-2 but not Nedd4-1 is a critical mediator for PKC-regulated hOAT1 ubiquitination, expression, and transport activity.

The Isoquinolone Derived Prolyl Hydroxylase Inhibitor ICA Is a Potent Substrate of the Organic Anion Transporters 1 and 3

OBJECTIVE

Many cellular responses to hypoxia are mediated by the transcription factor complex hypoxia-inducible factor (HIF). HIF stability is governed by a family of dioxygenases called HIF prolyl hydroxylases (PHDs). Isoquinolone-derived PHD inhibitors, like 2-(1-chloro-4-hydroxyisoquinoline-3-carboxamido) acetate (ICA), which stabilize the intracellular HIF-α have been suggested as a potentially beneficial therapeutic strategy for the treatment of disorders associated with ischemia. To stabilize HIF-α, ICA has to be taken up into proximal tubule cells (PCTs) across the basolateral membrane by one of the organic anion transporters 1, 2 or 3 (OAT1, OAT2 or OAT3). The release into the urine across the luminal membrane may be mediated by OAT4.

METHOD

To demonstrate interaction of ICA with human OAT1, OAT2, OAT3 and OAT4, ICA was tested on these transporters stably transfected in HEK293 cells by using p-aminohippurate (PAH), cGMP and estrone-3-sulfate (ES) as reference substrates, respectively.

RESULTS

Uptakes of PAH and ES in OAT1- and OAT3-transfected HEK293 cells were inhibited by ICA with half-maximal inhibition values of 0.29 ± 0.05 and 2.58 ± 0.16 µM, respectively. OAT2 was less sensitive to ICA. Efflux experiments identified ICA as an OAT1 and OAT3 substrate. Preloading OAT4-transfected HEK293 cells with ICA stimulated ES uptake by 18.3 ± 3.8%.

CONCLUSION

The uptake of ICA across the basolateral membrane of PCTs occurs mainly by OAT1 and the efflux into the tubular lumen by OAT4.

Organic Anion Transporter 1 Is Inhibited by Multiple Mechanisms and Shows a Transport Mode Independent of Exchange

The mechanism by which drugs inhibit organic anion transporter 1 (OAT1) was examined. OAT1 was stably expressed in Chinese hamster ovary (CHO) cells, and para-aminohippurate (PAH) and 6-carboxyfluorescein were the substrates. Most compounds (10 of 14) inhibited competitively, increasing the Michaelis constant (Km) without affecting the maximal transport rate (Jmax). Others were mixed-type (lowering Jmax and increasing Km) or noncompetitive (lowering Jmax only) inhibitors. The interaction of a noncompetitive inhibitor (telmisartan) with OAT1 was examined further. Binding of telmisartan to OAT1 was observed, but translocation was not. Telmisartan did not alter the plasma membrane expression of OAT1, indicating that it lowers Jmax by reducing the turnover number. PAH transport after telmisartan treatment and its washout recovered faster in the presence of 10% fetal bovine serum in the washout buffer, indicating that binding of telmisartan to OAT1 and its inhibitory effect are reversible. Together, these data suggest that telmisartan binds reversibly to a site distinct from substrate and stabilizes the transporter in a conformation unfavorable for translocation. In the absence of an exchangeable extracellular substrate, PAH efflux from CHO-OAT1 cells was relatively rapid. Telmisartan slowed PAH efflux, suggesting that some transporter-mediated efflux occurs independent of exchange. Although drug-drug interaction predictions at OAT1 assume competitive inhibition, these data show that OAT1 can be inhibited by other mechanisms, which could influence the accuracy of drug-drug interaction predictions at the transporter. Telmisartan was useful for examining how a noncompetitive inhibitor can alter OAT1 transport activity and for uncovering a transport mode independent of exchange.

Molecular architecture of the stria vascularis membrane transport system, which is essential for physiological functions of the mammalian cochlea

Stria vascularis of the mammalian cochlea transports K(+) to establish the electrochemical property in the endolymph crucial for hearing. This epithelial tissue also transports various small molecules. To clarify the profile of proteins participating in the transport system in the stria vascularis, membrane components purified from the stria of adult rats were analysed by liquid chromatography tandem mass spectrometry. Of the 3236 proteins detected in the analysis, 1807 were membrane proteins. Ingenuity Knowledge Base and literature data identified 513 proteins as being expressed on the 'plasma membrane', these included 25 ion channels and 79 transporters. Sixteen of the former and 62 of the latter had not yet been identified in the stria. Unexpectedly, many Cl(-) and Ca(2+) transport systems were found, suggesting that the dynamics of these ions play multiple roles. Several transporters for organic substances were also detected. Network analysis demonstrated that a few kinases, including protein kinase A, and Ca(2+) were key regulators for the strial transports. In the library of channels and transporters, 19 new candidates for uncloned deafness-related genes were identified. These resources provide a platform for understanding the molecular mechanisms underlying the epithelial transport essential for cochlear function and the pathophysiological processes involved in hearing disorders.

A renal-like organic anion transport system in the ciliary epithelium of the bovine and human eye

The purpose of this study was to determine the direction of organic anion (OA) transport across the ciliary body and the transport proteins that may contribute. Transport of several OAs across the bovine ciliary body was examined using ciliary body sections mounted in Ussing chambers and a perfused eye preparation. Microarray, reverse-transcription polymerase chain reaction (RT-PCR), immunoblotting, and immunohistochemistry were used to examine OA transporter expression in human ocular tissues. Microarray analysis showed that many OA transporters common to other barrier epithelia are expressed in ocular tissues. mRNA (RT-PCR) and protein (immunoblotting) for OAT1, OAT3, NaDC3, and MRP4 were detected in extracts of the human ciliary body from several donors. OAT1 and OAT3 localized to basolateral membranes of nonpigmented epithelial cells and MRP4 to basolateral membranes of pigmented cells in the human eye. Para-aminohippurate (PAH) and estrone-3-sulfate transport across the bovine ciliary body in the Ussing chambers was greater in the aqueous humor-to-blood direction than in the blood-to-aqueous humor direction, and active. There was little net directional movement of cidofovir. Probenecid (0.1 mM) or novobiocin (0.1 mM) added to the aqueous humor side of the tissue, or MK571 (5-(3-(2-(7-chloroquinolin-2-yl)ethenyl)phenyl)-8-dimethylcarbamyl-4,6-dithiaoctanoic acid; 0.1 mM) added to the blood side significantly reduced net active PAH transport. The rate of 6-carboxyfluorescein elimination from the aqueous humor of the perfused eye was reduced 80% when novobiocin (0.1 mM) was present in the aqueous humor. These data indicate that the ciliary body expresses a variety of OA transporters, including those common to the kidney. They are likely involved in clearing potentially harmful endobiotic and xenobiotic OAs from the eye.

Organic and inorganic transporters of the testis: A review

Transporters have a huge impact on the toxicology and pharmacological effects of xenobiotics in addition to being implicated in several diseases. While these important proteins have been well studied in organs such as the kidney or liver, characterization of transporters in the testis is still in the early stages. Knowledge of transporter function may greatly advance the field's understanding of the physiological and toxicological processes that occur in the testis. Several foundational studies involving both organic and inorganic transporters have been critical in furthering our understanding of how the testis interacts with endogenous and xenobiotic compounds. This review provides an overview of how transporters function, their clinical significance, and highlights what is known for many of the important transporters in the testis.

SLC22, SLC44, and SLC47 transporters--organic anion and cation transporters: molecular and cellular properties

Transporters within the SLC22, SLC44, and SLC47 families of solute carriers mediate transport of a structurally diverse array of organic electrolytes, that is, molecules that are generally charged (cationic, anionic, or zwitterionic) at physiological pH. Transporters in the SLC22 family--all of which are members of the major facilitator superfamily (MFS) of transporters--represent a mechanistically diverse set of processes, including the organic anion transporters (OATs and URAT1) that physiologically operate as organic anion (OA) exchangers, the organic cation transporters (OCTs) that operate as electrogenic uniporters of organic cations (OCs), and the so-called "novel" organic cation transporters (OCTNs) that support Na-cotransport of selected zwitterions. Whereas the OCTNs display a high degree of substrate selectivity, the physiological hallmark of the OATs and OCTs is their multiselectivity--consistent with a principal role in renal and hepatic clearance of a wide array of both endogenous and xenobiotic compounds. SLC47 consists of members of the multidrug and toxin extruder (MATE) family, which are carriers that are obligatory exchangers and that physiologically support electroneutral H⁺ exchange. The MATEs also display a characteristic multiselectivity and are frequently paired with OCTs to mediate transepithelial OC secretion, with the OCTs typically supporting basolateral OC entry and the MATEs supporting apical OC efflux. The SLC44 family contains the choline transporter-like (CTL) transporters. Largely restricted to choline and a limited set of structural congeners, the CTLs appear to support the Na-independent, electrogenic uniport of choline, thereby providing choline for membrane biogenesis. The solution of X-ray crystal structures of representative prokaryotic MFS and MATE transporters has led to the development of homology models of mammalian OAT, OCT, and MATE transporters that, in turn, have supplemented studies of the molecular basis of the complex interactions of ligands with these multiselective proteins.

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.

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.

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.