Impaired Renal Secretion of Substrates for the Multidrug Resistance Protein 2 in Mutant Transport–Deficient (TR−) Rats

Fluorescence-based methods for studying activity and drug-drug interactions of hepatic solute carrier and ATP binding cassette proteins involved in ADME-Tox

In humans, approximately 70% of drugs are eliminated through the liver. This process is governed by the concerted action of membrane transporters and metabolic enzymes. Transporters mediating hepatocellular uptake of drugs belong to the SLC (Solute carrier) superfamily of transporters. Drug efflux either toward the portal vein or into the bile is mainly mediated by active transporters of the ABC (ATP Binding Cassette) family. Alteration in the function and/or expression of liver transporters due to mutations, disease conditions, or co-administration of drugs or food components can result in altered pharmacokinetics. On the other hand, drugs or food components interacting with liver transporters may also interfere with liver function (e.g., bile acid homeostasis) and may even cause liver toxicity. Accordingly, certain transporters of the liver should be investigated already at an early stage of drug development. Most frequently radioactive probes are applied in these drug-transporter interaction tests. However, fluorescent probes are cost-effective and sensitive alternatives to radioligands, and are gaining wider application in drug-transporter interaction tests. In our review, we summarize our current understanding about hepatocyte ABC and SLC transporters affected by drug interactions. We provide an update of the available fluorescent and fluorogenic/activable probes applicable in in vitro or in vivo testing of these ABC and SLC transporters, including near-infrared transporter probes especially suitable for in vivo imaging. Furthermore, our review gives a comprehensive overview of the available fluorescence-based methods, not directly relying on the transport of the probe, suitable for the investigation of hepatic ABC or SLC-type drug transporters.

Non-Nutritive Sweeteners Acesulfame Potassium and Sucralose Are Competitive Inhibitors of the Human P-glycoprotein/Multidrug Resistance Protein 1 (PGP/MDR1)

Non-nutritive sweeteners (NNS) are popular sugar replacements used in foods, beverages, and medications. Although NNS are considered safe by regulatory organizations, their effects on physiological processes such as detoxification are incompletely understood. Previous studies revealed that the NNS sucralose (Sucr) altered P-glycoprotein (PGP) expression in rat colon. We also demonstrated that early-life exposure to NNS Sucr and acesulfame potassium (AceK) compromises mouse liver detoxification. Building upon these initial discoveries, we investigated the impact of AceK and Sucr on the PGP transporter in human cells to assess whether NNS influence its key role in cellular detoxification and drug metabolism. We showed that AceK and Sucr acted as PGP inhibitors, competing for the natural substrate-binding pocket of PGP. Most importantly, this was observed after exposure to concentrations of NNS within expected levels from common foods and beverage consumption. This may suggest risks for NNS consumers, either when taking medications that require PGP as the primary detoxification transporter or during exposure to toxic compounds.

How cyclosporin reduces mycophenolic acid exposure by 40% while other calcineurin inhibitors do not

The most frequently used immunosuppressive treatment in kidney transplant recipients is the combination therapy of a calcineurin inhibitor and mycophenolate mofetil (MMF), with or without corticosteroids. Cyclosporin and tacrolimus are the two calcineurin inhibitors registered for this indication. Also in the treatment of glomerular diseases calcineurin inhibitors and mycophenolic acid are being used on a worldwide scale, either alone or as combined treatment. In January 2021 the U.S. Food and Drug Administration (FDA) has approved voclosporin, a novel calcineurin inhibitor for the treatment of adult patients with active lupus nephritis. There is a clinically relevant drug-drug interaction between cyclosporin and mycophenolate. As a result of cyclosporin-induced inhibition of the enterohepatic recirculation of mycophenolate, the mycophenolic acid-AUC is significantly lower (40%) in case of cyclosporin co-administration as compared to cotreatment with either tacrolimus or voclosporin (or no CNI co-treatment). The aim of this mini review is to summarize this potential drug-drug interaction and explain how cyclosporin affects the pharmacokinetics of mycophenolate. The optimal dose of MMF is likely to depend on the calcineurin inhibitor with which it is co-administered. Furthermore clinical implications are discussed, including the potential emergence of mycophenolic acid (MPA)-related side effects after discontinuation of cyclosporin co-treatment.

MRP2 and the Transport Kinetics of Cysteine Conjugates of Inorganic Mercury

Human exposure to mercuric species occurs regularly throughout the world. Mercuric ions may accumulate in target cells and subsequently lead to cellular intoxication and death. Therefore, it is important to have a thorough understanding of how transportable species of mercury are handled by specific membrane transporters. The purpose of the current study was to characterize the transport kinetics of cysteine (Cys)-S-conjugates of inorganic mercury (Cys-S-Hg-S-Cys) at the site of the multidrug resistance-associated transporter 2 (MRP2). In order to estimate the maximum velocity (V _max) and Michaelis constant (K _m) for the uptake of Cys-S-Hg-S-Cys mediated by MRP2, in vitro studies were carried out using radioactive Cys-S-Hg-S-Cys (5 μM) and inside-out membrane vesicles made from Sf9 cells transfected with MRP2. The V _max was estimated to be 74.3 ± 10.1 nmol mg protein^-1 30 s^-1 while the K _m was calculated to be 63.4 ± 27.3 μM. In addition, in vivo studies were utilized to measure the disposition of inorganic mercury (administered dose 0.5 μmol kg^-1 in 2 mL normal saline) over time in Wistar and TR¯ (Mrp2-deficient) rats. These studies measured the disposition of mercuric ions in the kidney, liver, and blood. In general, the data suggest that the initial uptake of mercuric conjugates into select target cells is rapid followed by a period of slower uptake and accumulation. Overall, the data indicate that MRP2 transports Cys-S-Hg-S-Cys in a manner that is similar to that of other MRP2 substrates.

Stem cell-derived kidney cells and organoids: Recent breakthroughs and emerging applications

The global rise in the numbers of kidney patients and the shortage in transplantable organs have led to an increasing interest in kidney-specific regenerative therapies, renal disease modelling and bioartificial kidneys. Sources for large quantities of high-quality renal cells and tissues would be required, also for applications in in vitro platforms for compound safety and efficacy screening. Stem cell-based approaches for the generation of renal-like cells and tissues would be most attractive, but such methods were not available until recently. This situation has drastically changed since 2013, and various protocols for the generation of renal-like cells and precursors from pluripotent stem cells (PSC) have been established. The most recent breakthroughs were related to the establishment of various protocols for the generation of PSC-derived kidney organoids. In combination with recent advances in genome editing, bioprinting and the establishment of predictive renal screening platforms this results in exciting new possibilities. This review will give a comprehensive overview over current PSC-based protocols for the generation of renal-like cells, precursors and organoids, and their current and potential applications in regenerative medicine, compound screening, disease modelling and bioartificial organs.

Nature and uses of fluorescent dyes for drug transporter studies

INTRODUCTION

Drug transporters are now recognized as major players involved in pharmacokinetics and toxicology. Methods for assessing their activity are important to consider, particularly owing to regulatory requirements with respect to inhibition of drug transporter activity and prediction of drug-drug interactions. In this context, the use of fluorescent-dye-based transport assays is likely to deserve attention.

AREAS COVERED

This review provides an overview of the nature of fluorescent dye substrates for ATP-binding cassette and solute carrier drug transporters. Their use for investigating drug transporter activity in cultured cells and clinical hematological samples, drug transporter inhibition, drug transporter imaging and drug transport at the organ level are summarized.

EXPERT OPINION

A wide range of fluorescent dyes is now available for use in various aspects of drug transporter studies. The use of these dyes for transporter analyses may, however, be hampered by classic pitfalls of fluorescence technology, such as quenching. Transporter-independent processes such as passive diffusion of dyes through plasma membrane or dye sequestration into subcellular compartments must also be considered, as well as the redundant handling by various distinct transporters of some fluorescent probes. Finally, standardization of dye-based transport assays remains an important on-going issue.

Renal glucuronidation and multidrug resistance protein 2-/ multidrug resistance protein 4-mediated efflux of mycophenolic acid: interaction with cyclosporine and tacrolimus

Mycophenolic acid (MPA) is an immunosuppressant used in transplant rejection, often in combination with cyclosporine (CsA) and tacrolimus (Tac). The drug is cleared predominantly via the kidneys, and 95% of the administered dose appears in urine as 7-hydroxy mycophenolic acid glucuronide (MPAG). The current study was designed to unravel the renal excretory pathway of MPA and MPAG, and their potential drug-drug interactions. The role of multidrug resistance protein (MRP) 2 and MRP4 in MPA disposition was studied using human embryonic kidney 293 (HEK293) cells overexpressing the human transporters, and in isolated, perfused kidneys of Mrp2-deficient rats and Mrp4-deficient mice. Using these models, we identified MPA as substrate of MRP2 and MRP4, whereas its MPAG appeared to be a substrate of MRP2 only. CsA inhibited MPAG transport via MRP2 for 50% at 8 μM (P < 0.05), whereas Tac had no effect. This was confirmed by cell survival assays, showing a 10-fold increase in MPA cytotoxicity (50% reduction in cell survival changed from 12.2 ± 0.3 μM to 1.33 ± 0.01 μM by MPA + CsA; P < 0.001) and in perfused kidneys, showing a 50% reduction in MPAG excretion (P < 0.05). The latter effect was observed in Mrp2-deficient animals as well, supporting the importance of Mrp2 in MPAG excretion. CsA, but not Tac, inhibited MPA glucuronidation by rat kidney homogenate and human uridine 5'-diphospho-glucuronosyltransferase-glucuronosyltransferase 1A9 (P < 0.05 and P < 0.01, respectively). We conclude that MPA is a substrate of both MRP2 and MRP4, but MRP2 is the main transporter involved in renal MPAG excretion. In conclusion, CsA, but not Tac, influences MPA clearance by inhibiting renal MPA glucuronidation and MRP2-mediated MPAG secretion.

Renal transport of organic anions and cations

Organic anions and cations (OAs and OCs, respectively) comprise an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. The kidney, primarily the renal proximal tubule, plays a critical role in regulating the plasma concentrations of these organic electrolytes and in clearing the body of potentially toxic xenobiotics agents, a process that involves active, transepithelial secretion. This transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. Basolateral and luminal OA and OC transport reflects the concerted activity of a suite of separate proteins arranged in parallel in each pole of proximal tubule cells. The cloning of multiple members of several distinct transport families, the subsequent characterization of their activity, and their subcellular localization within distinct regions of the kidney, now allows the development of models describing the molecular basis of the renal secretion of OAs and OCs. New information on naturally occurring genetic variation of many of these processes provides insight into the basis of observed variability of drug efficacy and unwanted drug-drug interactions in human populations. The present review examines recent work on these issues.

ABCB1 haplotype influences the sirolimus dose requirements in Chinese renal transplant recipients

Sirolimus, an immunosuppressive drug used to prevent organ rejection after renal transplantation, has a narrow therapeutic index and a large inter-individual variability of pharmacokinetics. The aim of this study was to analyse the dose-normalized trough blood concentrations (C0 /D ratio) of sirolimus in patients with different genotypes and attempt to investigate the possible associations between ABCB1/CYP3A5 genotypes and sirolimus dose requirements in Chinese renal transplant recipients. Blood samples were collected from 85 Chinese renal transplant recipients who were treated with sirolimus for at least 3 months and polymorphisms of the ABCB1 and CYP3A5 were determined by the SNaPShot multiplex assay. The blood concentrations of sirolimus were determined with HPLC. A significant allele-dependent effect was observed between the CYP3A5*3 polymorphism and the C0 /D ratio of sirolimus. The patients bearing at least one CYP3A5*1 allele had a lower sirolimus C0/D ratio compared with those with a homozygous CYP3A5*3 genotype (p < 0.05). No significant differences of sirolimus C0/D ratios were observed among various ABCB1 1236C>T, 2677G>T/A and 3435C>T genotype groups. However, haplotype analysis including ABCB1 1236C>T, 2677G>T/A and 3435C>T SNPs showed that the mean sirolimus C0/D of subjects carrying the CGC/CGC diplotype was about 30% lower compared with those carrying the CGC/TTT or TTT/TTT diplotype, whether or not they expressed the CYP3A5 (p < 0.05). These results demonstrated that the haplotype of ABCB1 might be a better index for the prediction of sirolimus blood concentration than single SNPs. Genotyping of ABCB1 and CYP3A5 might help to optimize individualized sirolimus treatments for Chinese renal transplant recipients.

Dramatic increase in hepatic and biliary curcumin exposure by modulation of its elimination pathway in rats

OBJECTIVES

Curcumin, a major component of the food spice turmeric (Curcuma longa), has multiple beneficial effects on diseases of the liver and bile duct. We have investigated whether modulation of the curcumin elimination pathway could increase its hepatic and biliary exposure in rats.

METHODS

Probenecid, an inhibitor of the metabolism and biliary excretion of curcumin, was used as a modulator. After intravenous administration of curcumin at a dose of 18 mg/kg/h without (control) or with co-infusion of probenecid (230 mg/kg/h) in rats, the pharmacokinetic parameters of curcumin were estimated.

KEY FINDINGS

Coadministration of probenecid significantly increased the total area under the plasma (1.88-fold) and bile (6.73-fold) concentration-time curves from 0 to 80 min of curcumin relative to those in the controls. The tissue-to-plasma concentration ratio in the liver was also dramatically increased (69.3-fold) by probenecid. These results may be attributed to the dual inhibitory effects of probenecid, to a greater extent, on metabolism via glucuronidation, and to a lesser extent, on the biliary excretion of curcumin via the multidrug resistance-associated protein 2.

CONCLUSIONS

The probenecid-mediated increase in hepatic and biliary exposure of curcumin suggested that the use of combination drug regimens involving curcumin and modulators of elimination may be an innovative approach for the therapeutic use of curcumin.

Regulatory pathways for ATP-binding cassette transport proteins in kidney proximal tubules

The ATP-binding cassette transport proteins (ABC transporters) represent important determinants of drug excretion. Protective or excretory tissues where these transporters mediate substrate efflux include the kidney proximal tubule. Regulation of the transport proteins in this tissue requires elaborate signaling pathways, including genetic, epigenetic, nuclear receptor mediated, posttranscriptional gene regulation involving microRNAs, and non-genomic (kinases) pathways triggered by hormones and/or growth factors. This review discusses current knowledge on regulatory pathways for ABC transporters in kidney proximal tubules, with a main focus on P-glycoprotein, multidrug resistance proteins 2 and 4, and breast cancer resistance protein. Insight in these processes is of importance because variations in transporter activity due to certain (disease) conditions could lead to significant changes in drug efficacy or toxicity.

The association of the UGT1A8, SLCO1B3 and ABCC2/ABCG2 genetic polymorphisms with the pharmacokinetics of mycophenolic acid and its phenolic glucuronide metabolite in Chinese individuals

BACKGROUND

This study aimed to evaluate the effect of UGT1A8*2, SLCO1B3 T334G, ABCC2 C-24T and ABCG2 C421A polymorphisms on the pharmacokinetics (PKs) of mycophenolic acid (MPA) and its phenolic glucuronide (MPAG) in healthy Chinese volunteers and in stable renal transplant patients.

METHODS

The data were extracted from comparative bioavailability studies conducted in 42 healthy individuals and 37 renal transplant patients. A complete PK profile was obtained over 48 h for healthy volunteers and over 12h for the transplant patients. The MPA/MPAG plasma concentrations were measured by HPLC. The genotypes were determined using either the Taqman probe technique or direct sequencing. A multivariate analysis was used to assess the effect of the genotypes (UGT1A8*2, SLCO1B3 T334G, ABCC2 C-24T and ABCG2 C421A) and other covariates (age, weight, height, calculated creatinine clearance, serum albumin, haemoglobin and drug comedication) on the AUC(4-12) and AUC(0-12) for MPA and MPAG in the healthy volunteers and patients.

RESULTS

In the healthy volunteers, the dose-adjusted geometric means (GM) of the MPA AUC(4-12) in individuals with the SLCO1B3 334T allele were 30.4% lower than those values in the 334G homozygote carriers (P<0.05); in the transplant patients, the steroid dose was associated with a negative effect on the AUC of MPAG (P<0.03) and weight was associated with a negative effect on the AUC for MPA in the healthy volunteers and patients (P<0.03). No other significant effect of genotype or of the other studied variables on AUC(4-12) or AUC(0-12) of MPA/MPAG was found in the healthy volunteers or patients.

CONCLUSIONS

The PKs of MPA is affected by the SLCO1B3 polymorphism in healthy Chinese individuals. The absence of an effect of SLCO1B3 polymorphisms in transplant patients may be due to the co-administration of cyclosporine (CsA). Concomitant steroid dose and weight are two important covariates of the AUC of MPA and MPAG, which should be taken into account in clinical use. Further confirmatory in vivo studies are needed.

Orange juice and its component, hesperidin, decrease the expression of multidrug resistance-associated protein 2 in rat small intestine and liver

We investigated the effects of orange juice (OJ) or hesperidin, a component of OJ, on the pharmacokinetics of pravastatin (PRV) and the expression of both protein and mRNA of multidrug resistance-associated protein 2 (Mrp2) in the rat small intestine and liver. Eight-week-old male Sprague-Dawley rats were used in this study. OJ or a 0.079% hesperidin suspension was administered orally for 2 days. Tap water was given as a control. A single dose of PRV at 100 mg/kg p.o. was administered after 2 days of OJ, hesperidin, or tap water ingestion. The AUC, C_max, and t_1/2 values of PRV were significantly increased in OJ group. Mrp2 protein and mRNA levels in the small intestine and liver, respectively, were significantly decreased after the ingestion of OJ. The same results were obtained with hesperidin. These results suggest that the changes in PRV pharmacokinetic parameters and the decrease in Mrp2 expression caused by OJ are due to hesperidin in the juice.

Characterization of rhodamine-123, calcein and 5(6)-carboxy-2',7'-dichlorofluorescein (CDCF) export via MRP2 (ABCC2) in MES-SA and A549 cells

Based on our initial results on the effects of several ATP-binding cassette (ABC) transporter inhibitors on rhodamine-123 efflux from A549, a human lung carcinoma, and MES-SA, a human uterine sarcoma cell line, the aim of this study was to identify the transporter responsible for this export. Export of two fluorescent dyes, rhodamine-123 and calcein, was investigated in both cell lines by testing five commonly used inhibitors of ABC transporters: verapamil, cyclosporin A, MK571, GF129018 and fumitremorgin C. A very high degree of correlation (R(2)=0.91-0.99) between results obtained in the two cell lines suggested that the same transporter was involved in the export of tested fluorescent substrates in both cell lines. Expression analysis and gene silencing techniques, as well as transport of additional substrate 5(6)-carboxy-2',7'-dichlorofluorescein (CDCF) on membrane vesicles revealed that the transporter was multidrug resistance related protein 2 (MRP2, ABCC2). Furthermore, it was found that the tested modulators showed very diverse effects on the export of three fluorescent substrates via MRP2, with some modulators being inhibitory in one, while having no effect or even stimulating the transport in the other fluorescent dye assay. Verapamil inhibited rhodamine-123, but stimulated CDCF transport and did not affect calcein export. GF129018 did not affect calcein and CDCF transport, but it inhibited rhodamine-123 transport. These results demonstrate the importance of studying various combinations of potential substrates and modulators of MRP2 in order to estimate possible drug-drug interactions in living organisms. In addition, A549 and MES-SA cells were shown to be good cell models for studying interactions of compounds with human MRP2.

Influence of MRP2 on MPA pharmacokinetics in renal transplant recipients-results of the Pharmacogenomic Substudy within the Symphony Study

BACKGROUND

The aim of this study was to determine the relationship between single-nucleotide polymorphisms (SNPs) in MRP2 genes and mycophenolic acid (MPA) pharmacokinetics in renal transplant recipients of the Symphony Pharmacogenomic substudy.

METHODS

Sixty-six renal transplant recipients of eight Spanish centres were randomized into four branches of immunosuppressive regimen: low dose of cyclosporine, standard dose of cyclosporine, tacrolimus and sirolimus, all in addition to mycophenolate mofetil and steroids. Fifty-five patients were genotyped for SNPs in MRP2, C24T and C3972T. Pharmacokinetic sampling was done before MPA administration and up to 12 h post-dose at Day 7, 1 month and 3 months post-transplant. Relationships of area under the curve (AUC) of MPA and MPAG plasma sampling with the presence of MRP2 SNPs and with the immunosuppressive regimens were studied.

RESULTS

At steady-state conditions, MPA-reduced exposure was observed in C24T variant allele in MRP2 (CC: 68.73 ± 6.78; *T: 48.12 ± 4.90, P = 0.023); no significant differences linked to C3972T SNP were observed. Taking into account groups of treatment, lower MPA AUC in variant allele of C24T was only found under macrolides treatment with statistically significant differences at Month 3 (Tac and SRL, CC: 86.52 ± 10.98 versus *T: 41.99 ± 4.82, P = 0.001; CsA, CC: 52.31 ± 5.30 versus *T: 54.24 ± 8.30, P = 0.772); for C3972T, the same tendency was found but differences at steady state did not reach statistical significance.

CONCLUSIONS

Renal transplant recipients T carriers of C24T MRP2 with macrolides treatment were associated with reduced MPA AUC in steady-state conditions. Patients treated with cyclosporine lost the effect of this polymorphism.

Differential disposition of intra-renal generated and preformed glucuronides: studies with 4-methylumbelliferone and 4-methylumbelliferyl glucuronide in the filtering and nonfiltering isolated perfused rat kidney

Objectives

This study was designed to investigate the renal disposition of 4-methylumbelliferone (4MU) and 4-methylumbelliferyl glucuronide (4MUG) to characterise the contribution of excretion and metabolic clearance to total clearance in the kidney.

Methods

The isolated perfused kidney (IPK) from the male Sprague–Dawley rat was used in filtering and non-filtering mode to study the renal disposition of 4MU, renally generated 4MUG and preformed 4MUG. Perfusate and urine (filtering IPK only) was collected for up to 120 min and 4MU and 4MUG in perfusate and urine were determined by HPLC. Analytes were also measured in kidney tissue collected at 120 min. Non-compartmental analysis was used to derive pharmacokinetic parameters.

Key findings

The concentration of 4MU in perfusate declined with a terminal half-life of approximately 120 min following administration to the filtering IPK and nonfiltering IPK. There was a corresponding increase in the concentration of 4MUG. Metabolic clearance of 4MU accounted for 92% of total renal clearance. After bolus dosing of preformed 4MUG in the perfusion reservoir of the filtering IPK, the perfusate concentration declined with the terminal half-life of approximately 260 min. The renal excretory clearance of preformed 4MUG accounted for 96% of total renal clearance. 4MU was extensively metabolized by glucuronidation in the filtering and nonfiltering IPK, and the total renal clearance of 4MU was far greater than its renal excretory clearance. This indicated that glucuronidation was the major elimination pathway for 4MU in the kidney.

Conclusions

The data confirmed an important role for the kidney in the metabolic clearance of xenobiotics via glucuronidation and signalled the lack of impact of impaired glomerular filtration on renal drug metabolism.

MRP2 and the handling of mercuric ions in rats exposed acutely to inorganic and organic species of mercury

Mercuric ions accumulate preferentially in renal tubular epithelial cells and bond with intracellular thiols. Certain metal-complexing agents have been shown to promote extraction of mercuric ions via the multidrug resistance-associated protein 2 (MRP2). Following exposure to a non-toxic dose of inorganic mercury (Hg²+), in the absence of complexing agents, tubular cells are capable of exporting a small fraction of intracellular Hg²+ through one or more undetermined mechanisms. We hypothesize that MRP2 plays a role in this export. To test this hypothesis, Wistar (control) and TR(-) rats were injected intravenously with a non-nephrotoxic dose of HgCl₂ (0.5 μmol/kg) or CH₃HgCl (5 mg/kg), containing [²⁰³Hg], in the presence or absence of cysteine (Cys; 1.25 μmol/kg or 12.5mg/kg, respectively). Animals were sacrificed 24 h after exposure to mercury and the content of [²⁰³Hg] in blood, kidneys, liver, urine and feces was determined. In addition, uptake of Cys-S-conjugates of Hg²+ and methylmercury (CH₃Hg+) was measured in inside-out membrane vesicles prepared from either control Sf9 cells or Sf9 cells transfected with human MRP2. The amount of mercury in the total renal mass and liver was significantly greater in TR⁻ rats than in controls. In contrast, the amount of mercury in urine and feces was significantly lower in TR⁻ rats than in controls. Data from membrane vesicles indicate that Cys-S-conjugates of Hg²+ and CH₃Hg+ are transportable substrates of MRP2. Collectively, these data indicate that MRP2 plays a role in the physiological handling and elimination of mercuric ions from the kidney.

Tumor necrosis factor alpha increases P-glycoprotein expression in a BME-UV in vitro model of mammary epithelial cells

P-glycoprotein is an efflux pump belonging to the ATP-binding cassette super-family that influences the bioavailability and disposition of many drugs. Mammary epithelial cells express various drug transporters including P-glycoprotein, albeit at low level during lactation. During inflammatory reactions, which can be associated with changes in epithelial barrier functions, pro-inflammatory cytokines such as tumor necrosis factor alpha (TNF-α) are elevated in milk and serum. In this study, the role of TNF-α in the regulation of P-glycoprotein was determined in cultured BME-UV cells, an immortalized bovine mammary epithelial cell line. The protein production of P-glycoprotein and mRNA expression of bABCB1, the gene encoding P-glycoprotein, were increased after 24 h of TNF-α exposure. The highest observed effects for TNF-α on the regulation of P-glycoprotein was after 72 h of exposure. Protein and mRNA expression also increased significantly after 120 h of TNF-α exposure, but was lower than the level observed in the cells exposed to TNF-α for 72 h. The apical to basolateral flux of digoxin, a P-glycoprotein substrate, was decreased in the TNF-α-exposed epithelium. This effect was reversed when verapamil or ketoconazole, compounds known to interact with P-glycoprotein, were added together with digoxin into the donor compartment. Probenecid, a compound known to interact with organic anion transporters, but not P-glycoprotein, did not increase the flux of digoxin. This model has important implications for understanding the barrier function of the mammary epithelium and provides insight into the role of P-glycoprotein in the accumulation and/or removal of xenobiotics from milk and/or plasma.

ABCC2/Abcc2: a multispecific transporter with dominant excretory functions

ABCC2/Abcc2 (MRP2/Mrp2) is expressed at major physiological barriers, such as the canalicular membrane of liver cells, kidney proximal tubule epithelial cells, enterocytes of the small and large intestine, and syncytiotrophoblast of the placenta. ABCC2/Abcc2 always localizes in the apical membranes. Although ABCC2/Abcc2 transports a variety of amphiphilic anions that belong to different classes of molecules, such as endogenous compounds (e.g., bilirubin-glucuronides), drugs, toxic chemicals, nutraceuticals, and their conjugates, it displays a preference for phase II conjugates. Phenotypically, the most obvious consequence of mutations in ABCC2 that lead to Dubin-Johnson syndrome is conjugate hyperbilirubinemia. ABCC2/Abcc2 harbors multiple binding sites and displays complex transport kinetics.

Fluo-cAMP is transported by multidrug resistance-associated protein isoform 4 in rat choroid plexus

The choroid plexuses (CP) are responsible for transport of micronutrients into brain and clearance of toxic compounds, in addition to its barrier function and production of CSF. Multidrug resistance-associated protein (Mrp) 4 is one transport protein highly expressed in CP tissue and is characterized as a versatile pump for toxicants and signalling molecules. Aim of the study was to determine transport characteristics of a fluorescent cAMP analog in rat CP and to define whether fluo-cAMP can be used for analyses of function, substrate/inhibitor specificity and regulation of Mrp4. Confocal imaging was used to analyze transport mechanisms in absence and presence of various modulators of organic anion transport in freshly isolated and functionally intact CP. Fluo-cAMP transport was saturable, selective, concentrative and metabolism-dependent, following an active two-step mechanism composed of apical uptake into epithelial cells and basolateral efflux. Uptake included a Na(+) -dependent and a Na(+) -independent component and was inhibited by estrone sulfate, taurocholate and sildenafil indicating involvement of organic anion transporting polypeptide Oatp1a5. Efflux was composed of an indirect Na(+) -dependent component and a component inhibitable by, for example, the MRP4 substrates/inhibitors, sulindac sulfide and 4-(2-aminoethyl) benzenesulfonyl fluoride. Therefore, fluo-cAMP can be used as fluorescent model compound for studying involvement of Mrp4 in signalling pathways and neuroprotection in CP.

Seventy-five percent nephrectomy and the disposition of inorganic mercury in 2,3-dimercaptopropanesulfonic acid-treated rats lacking functional multidrug-resistance protein 2

In the present study, we evaluated the disposition of inorganic mercury (Hg(2+)) in sham-operated and 75% nephrectomized (NPX) Wistar and transport-deficient (TR(-)) rats treated with saline or the chelating agent meso-2,3-dimercaptosuccinic acid (DMSA). Based on previous studies, DMSA and TR(-) rats were used as tools to examine the potential role of multidrug-resistance protein 2 (MRP2) in the disposition of Hg(2+) during renal insufficiency. All animals were treated with a low dose (0.5 mumol/kg i.v.) of mercuric chloride (HgCl(2)). At 24 and 28 h after exposure to HgCl(2), matched groups of Wistar and TR(-) rats received normal saline or DMSA (intraperitoneally). Forty-eight hours after exposure to HgCl(2), the disposition of Hg(2+) was examined. A particularly notable effect of 75% nephrectomy in both strains of rats was enhanced renal accumulation of Hg(2+), specifically in the outer stripe of the outer medulla. In addition, hepatic accumulation, fecal excretion, and blood levels of Hg(2+) were enhanced in rats after 75% nephrectomy, especially in the TR(-) rats. Treatment with DMSA increased both the renal tubular elimination and urinary excretion of Hg(2+) in all rats. DMSA did not, however, affect hepatic content of Hg(2+), even in the 75% NPX TR(-) rats. We also show with real-time polymerase chain reaction that after 75% nephrectomy and compensatory renal growth, expression of MRP2 (only in Wistar rats) and organic anion transporter 1 is enhanced in the remaining functional proximal tubules. We conclude that MRP2 plays a significant role in the renal and corporal disposition of Hg(2+) after a 75% reduction of renal mass.

Hepcidin suppression and defective iron recycling account for dysregulation of iron homeostasis in heme oxygenase-1 deficiency

Heme oxygenase-1 (HO-1) contribution to iron homeostasis has been postulated, because it facilitates iron recycling by liberating iron mostly from heme catabolism. This enzyme also appears to be responsible for the resolution of inflammatory conditions. In a patient with HO-1 deficiency, inflammation and dysregulation of body iron homeostasis, including anemia and liver and kidney hemosiderosis, are evidenced. Here we postulated that HO-1 is critical in the regulation of ferroportin, the major cellular iron exporter, and hepcidin, the key regulator of iron homeostasis central in the pathogenesis of anemia of inflammation. Our current experiments in human THP-1 monocytic cells indicate a HO-1-induced iron-mediated surface-ferroportin expression, consistent with the role of HO-1 in iron recycling. Surprisingly, we observed low hepcidin levels in the HO-1-deficient patient, despite the presence of inflammation and hemosiderosis, both inducers of hepcidin. Instead, we observed highly increased soluble transferrin receptor levels. This suggests that the decreased hepcidin levels in HO-1 deficiency reflect the increased need for iron in the bone marrow due to the anaemia. Using human hepatoma cells, we demonstrate that HO-activity did not have a direct modulating effect on expression of HAMP, the gene that encodes for hepcidin. Therefore, we argue that the decreased iron recycling may, in part, have contributed to the low hepcidin levels. These findings indicate that dysregulation of iron homeostasis in HO-1 deficiency is the result of both defective iron recycling and erythroid activity-associated inhibition of hepcidin expression. This study therefore shows a crucial role for HO-1 in maintaining body iron balance.

MRP2 involvement in renal proximal tubular elimination of methylmercury mediated by DMPS or DMSA

2, 3-Dimercaptopropane-1-sulfonic acid (DMPS) and meso-2, 3-Dimercaptosuccinic acid (DMSA) are dithiols used to treat humans exposed to methylmercury (CH(3)Hg(+)). After treatment, significant amounts of mercury are eliminated rapidly from the kidneys and are excreted in urine. In the present study, we extended our previous studies by testing the hypothesis that MRP2 mediates the secretion of DMPS or DMSA S-conjugates of CH(3)Hg(+). To test this hypothesis, the disposition of mercury was assessed in control and Mrp2-deficient (TR(-)) rats exposed intravenously to a 5.0-mg/kg dose of CH(3)HgCl. Twenty-four and 28 h after exposure, groups of four control and four TR(-) rats were injected with saline, DMPS, or DMSA. Tissues were harvested 48 h later. Renal and hepatic contents of mercury were greater in saline-injected TR(-) rats than in controls. In contrast, the amounts of mercury excreted in urine and feces by TR(-) rats were less than those by controls. DMPS and DMSA significantly reduced the renal and hepatic content of mercury in both groups of rats, with the greatest reduction in controls. A significant increase in urinary and fecal excretion of mercury (which was greater in the controls) was also observed. Our findings in inside-out membrane vesicles prepared from hMRP2-transfected Sf9 cells show that uptake of DMPS and DMSA S-conjugates of CH(3)Hg(+) was greater in the vesicles containing hMRP2 than in control vesicles. Overall, these dispositional findings indicate that MRP2 does play a role in DMPS- and DMSA-mediated elimination of mercury from the kidney.

Function and Regulation of Multidrug Resistance Proteins (MRPs) in the Renal Elimination of Organic Anions

The reabsorptive and excretory capacity of the kidney has an important influence on the systemic concentration of drugs. Multidrug resistance proteins (MRP/ABCC) expressed in the kidney play a critical role in the tubular efflux of a wide variety of drugs and toxicants, and, in particular, of their negatively charged phase II metabolites. Nine structurally and functionally related MRP family members have been identified (MRP1-9), which differ from each other by their localization, expression levels, and substrate specificity. During altered physiological circumstances, adaptations in these transporters are required to avoid systemic toxicity as well as renal tubular damage. Key players in these events are hormones, protein kinases, nuclear receptors, and disease conditions, which all may affect transporter protein expression levels. This review discusses current knowledge on the renal characteristics of MRP1-9, with specific focus on their regulation.

Characterisation of human tubular cell monolayers as a model of proximal tubular xenobiotic handling

The aim of this study was to determine whether primary human tubular cell monolayers could provide a powerful tool with which to investigate the renal proximal tubular handling of xenobiotics. Human proximal and distal tubule/collecting duct cells were grown as monolayers on permeable filter supports. After 10 days in culture, proximal tubule cells remained differentiated and expressed a wide palette of transporters at the mRNA level including NaPi-IIa, SGLT1, SGLT2, OCT2, OCTN2, OAT1, OAT3, OAT4, MDR1, MRP2 and BCRP. At the protein level, the expression of a subset of transporters including NaPi-IIa, OAT1 and OAT3 was demonstrated using immunohistochemistry. Analysis of the expression of the ATP binding cassette efflux pumps MDR1, MRP2 and BCRP confirmed their apical membrane localisation. At the functional level, tubule cell monolayers retain the necessary machinery to mediate the net secretion of the prototypic substrates; PAH and creatinine. PAH secretion across the monolayer consisted of the uptake of PAH across the basolateral membrane by OAT1 and OAT3 and the apical exit of PAH by a probenecid and MK571-sensitive route consistent with actions of MRP2 or MRP4. Creatinine secretion was by OCT2-mediated uptake at the basolateral membrane and via MDR1 at the apical membrane. Functional expression of MDR1 and BCRP at the apical membrane was also demonstrated using a Hoechst 33342 dye. Similarly, measurement of calcein efflux demonstrated the functional expression of MRP2 at the apical membrane of cell monolayers. In conclusion, human tubular cell monolayers provide a powerful tool to investigate renal xenobiotic handling.

Pharmacokinetics of mycophenolate mofetil and its glucuronide metabolites in healthy volunteers

We previously reported that polymorphisms in the UGT2B7 and UGT1A9 genes are associated with significant alteration in the disposition of mycophenolic acid (MPA) in healthy volunteers. Aim: This study further evaluates the impact of genetic polymorphisms at the UGT1A1, UGT1A7 and ABCC2 loci. Methods: Genetic analyses of five UGT candidate genes and ABCC2 were completed on 47 healthy subjects who received a single dose of 1.5 g mycophenolate mofetil and completed a 12-h pharmacokinetic profile. Results: Multivariate analyses indicate that the ABCC2 -24T promoter polymorphism is associated with a 25% increase in acyl mycophenolic acid phenolic glucuronide level. Subjects with combined ABCC2 -24T and UGT1A9*3 genotypes present a 169% increased exposure to AcMPAG. Homozygosity for UGT1A7 387G/391A (129Lys/131Lys) is associated with a modest but significant 7% reduction in MPA level. When these additional genetic factors are considered in the model, the effects of previously described UGT1A9 and UGT2B7 variations remain significant. No significant effect is observed for UGT1A1*28, UGT1A7 622T/C (Trp208Arg), UGT1A9 -440TC/-331CT, UGT1A9 -118 TA9/10 and seven other ABCC2 SNPs. Conclusion: We demonstrate that MPA disposition is a multigenic process, and that additional studies are required to ascertain the relationship between UGT, ABCC2 genotypes and MPA pharmacokinetics in transplant recipients.

MRP2 and the DMPS- and DMSA-mediated elimination of mercury in TR(-) and control rats exposed to thiol S-conjugates of inorganic mercury

Cysteine (Cys) and homocysteine (Hcy)-S-conjugates of inorganic mercury (Hg2+) are transportable species of Hg2+ that are taken up readily by proximal tubular cells. The metal chelators, 2,3-dimercaptopropane-1-sulfonic acid (DMPS) and meso-2,3-dimercaptosuccinic acid (DMSA), have been used successfully to extract Hg2+ from these cells, presumably via the multidrug resistance protein (Mrp2). In the current study, we tested the hypothesis that Mrp2 is involved in the DMPS- and DMSA-mediated extraction of Hg2+ following administration of Hg2+ as an S-conjugate of Cys or Hcy. To test this hypothesis, control and TR(-) (Mrp2-deficient) rats were injected with 0.5 micromol/kg HgCl2 (containing 203Hg2+) conjugated to 1.25 micromol/kg Cys or Hcy. After 24 and 28 h, rats were treated with saline or 100 mg/kg DMPS or DMSA. Tissues were harvested 48 h after Hg2+ exposure. The renal and hepatic burden of Hg2+ was greater in saline-injected TR- rats than in corresponding controls. Accordingly, the content of Hg2+ in the urine and feces was less in TR- rats than in controls. Following treatment with DMPS or DMSA, the renal content of Hg2+ in both groups of rats was reduced significantly and the urinary excretion of Hg2+ was increased. In liver, the effect of each chelator appeared to be dependent upon the form in which Hg2+ was administered. In vitro experiments provide direct evidence indicating that DMPS and DMSA-S-conjugates of Hg2+ are substrates for Mrp2. Overall, these data support our hypothesis that Mrp2 is involved in the DMPS and DMSA-mediated extraction of the body burden of Hg2+.

Multidrug resistance proteins and the renal elimination of inorganic mercury mediated by 2,3-dimercaptopropane-1-sulfonic acid and meso-2,3-dimercaptosuccinic acid

Current therapies for inorganic mercury (Hg(2+)) intoxication include administration of a metal chelator, either 2,3-dimercaptopropane-1-sulfonic acid (DMPS) or meso-2,3-dimercaptosuccinic acid (DMSA). After exposure to either chelator, Hg(2+) is rapidly eliminated from the kidneys and excreted in the urine, presumably as an S-conjugate of DMPS or DMSA. The multidrug resistance protein 2 (Mrp2) has been implicated in this process. We hypothesize that Mrp2 mediates the secretion of DMPS- or DMSA-S-conjugates of Hg(2+) from proximal tubular cells. To test this hypothesis, the disposition of Hg(2+) was examined in control and Mrp2-deficient TR(-) rats. Rats were injected i.v. with 0.5 mumol/kg HgCl(2) containing (203)Hg(2+). Twenty-four and 28 h later, rats were injected with saline, DMPS, or DMSA. Tissues were harvested 48 h after HgCl(2) exposure. The renal and hepatic burden of Hg(2+) in the saline-injected TR(-) rats was greater than that of controls. In contrast, the amount of Hg(2+) excreted in urine and feces of TR(-) rats was less than that of controls. DMPS, but not DMSA, significantly reduced the renal and hepatic content of Hg(2+) in both groups of rats, with the greatest reduction in controls. A significant increase in urinary and fecal excretion of Hg(2+), which was greater in the controls, was also observed following DMPS treatment. Experiments utilizing inside-out membrane vesicles expressing MRP2 support these observations by demonstrating that DMPS- and DMSA-S-conjugates of Hg(2+) are transportable substrates of MRP2. Collectively, these data support a role for Mrp2 in the DMPS- and DMSA-mediated elimination of Hg(2+) from the kidney.

Nitric oxide differentially regulates renal ATP-binding cassette transporters during endotoxemia

Nitric oxide (NO) is an important regulator of renal transport processes. In the present study, we investigated the role of NO, produced by inducible NO synthase (iNOS), in the regulation of renal ATP-binding cassette (ABC) transporters in vivo during endotoxemia. Wistar–Hannover rats were injected with lipopolysaccharide (LPS⁺) alone or in combination with the iNOS inhibitor, aminoguanidine. Controls received detoxified LPS (LPS⁻). After LPS⁺, proximal tubular damage and a reduction in renal function were observed. Furthermore, iNOS mRNA and protein, and the amount of NO metabolites in plasma and urine, increased compared to the LPS⁻ group. Coadministration with aminoguanidine resulted in an attenuation of iNOS induction and reduction of renal damage. Gene expression of 20 ABC transporters was determined. After LPS⁺, a clear up-regulation in Abca1, Abcb1/P-glycoprotein (P-gp), Abcb11/bile salt export pump (Bsep), and Abcc2/multidrug resistance protein (Mrp2) was found, whereas Abcc8 was down-regulated. Up-regulation of Abcc2/Mrp2 was accompanied by enhanced calcein excretion. Aminoguanidine attenuated the effects on transporter expression. Our data indicate that NO, produced locally by renal iNOS, regulates the expression of ABC transporters in vivo. Furthermore, we showed, for the first time, expression and subcellular localization of Abcb11/Bsep in rat kidney.

Characterization of P-glycoprotein and multidrug resistance proteins in rat kidney and intestinal cell lines

The activity of P-glycoprotein (Pgp/MDR1/ABCB1) and multidrug resistance proteins (MRP/ABCC) influence the pharmacokinetics and bioavailability of many drugs. Few suitable cell lines for the study of drug transport exist. Additional non-human cell lines may help clarify species differences and contribute to the current knowledge of drug transport. The aim of the present study was to characterize three rat epithelial cell lines for transporter expression and activity. Transporter expression was assessed in intestinal IEC-6 and renal GERP and NRK-52E cells using RT-PCR and Western blot analysis. Pgp and Mrp transport activity were analyzed by measuring calcein accumulation and glutathione-S-bimane efflux, respectively. The three cell lines showed Pgp expression and Pgp-dependent transport, both decreasing with culture time after reaching confluency. Besides Pgp, cells expressed Mrp1, Mrp3, Mrp4, and Mrp5, while Mrp2 and Mrp6 were absent. In addition, they showed temperature- and Mrp-dependent efflux of glutathione-S-bimane. Exposure to a panel of different inhibitors showed that this efflux was probably mediated by Mrp4. In conclusion, the three rat epithelial cell lines investigated showed Pgp and Mrp expression and transport. Mrp dependent transport was most likely mediated by Mrp4. In future, these cell lines may be used as in vitro models to study drug transport.

L1210 cells cultivated under the selection pressure of doxorubicin or vincristine express common mechanisms of multidrug resistance based on the overexpression of P-glycoprotein

Multidrug resistance of neoplastic tissue is often associated with the overexpression and increased drug transport activity of plasma membrane transporters like P-glycoprotein (P-gp), multidrug resistance associated proteins (MRPs) or breast cancer resistance protein, as well as with the elevation of the glutathione detoxification pathway. We have already described the overexpression of P-gp under the selection pressure of vincristine in L1210 mouse leukemia cells. In the present study, mechanisms of multidrug resistance induced in L1210 cells cultivated in the presence of doxorubicin were analyzed. The selection pressure of both vincristine (yielding a resistant subline of L1210 cells, R(V)) and doxorubicin (yielding a resistant subline of L1210 cells, R(D)) induced a dramatic depression of cell sensitivity to both drugs. Both R(V) and R(D) cells demonstrated a lack of ability to accumulate calcein/AM and fluo-3/AM as fluorescent substrates of P-gp and MRP. The retention of dyes could be reached in both cell sublines by the application of inhibitors of P-gp (like verapamil) but not by probenecid - an inhibitor of anion transporters, including MRPs. Massive protein bands, at a M(r) range of 130-180 kDa that interact with c219 antibody against P-gp, were detected in the crude membrane fraction isolated from both R(V) and R(D) (but not from L1210) cells by Western blot. The cytosolic activity of glutathione S-transferase was found to be similar in R(V) and R(D) cells and did not differ significantly from the activity ascertained in parental L1210 cells. Neither the R(V) nor R(D) cell sublines differed considerably, as measured by cell ultrastructure. In conclusion, based on P-gp overexpression, both doxorubicin and vincristine induce a common multidrug resistance phenotype in L1210 cells.

Multidrug Resistance–Associated Protein 4 Is Involved in the Urinary Excretion of Hydrochlorothiazide and Furosemide

The role of ATP-binding cassette transporters in the urinary excretion of diuretics was investigated. Significant ATP-dependent uptake of hydrochlorothiazide (HCT) and furosemide was observed in membrane vesicles that expressed multidrug resistance-associated protein 4 (MRP4) and breast cancer resistance protein (BCRP). Unlike taurocholate uptake, S-methylglutathione had no effect on the ATP-dependent uptake of both compounds by MRP4. The functional importance of MRP4 and BCRP in the urinary excretion of HCT and furosemide was investigated using gene knockout mice. The renal clearance of HCT and furosemide was reduced significantly but not abolished in Mrp4 knockout mice compared with wild-type mice (9.0 +/- 0.9 versus 15 +/- 2 ml/min per kg for HCT and 1.9 +/- 0.3 versus 2.7 +/- 0.1 ml/min per kg for furosemide), and the amount of HCT that was associated with the kidney specimens was greater in Mrp4 knockout mice (21 +/- 3 versus 13 +/- 1 nmol/g kidney). In contrast, Bcrp makes only a negligible contribution because the urinary excretion was unchanged in Bcrp knockout mice. Our results suggest that Mrp4, together with other unknown transporters, accounts for the luminal efflux of HCT and furosemide from proximal tubular epithelial cells.

Multidrug Resistance Protein 2 Genetic Polymorphisms Influence Mycophenolic Acid Exposure in Renal Allograft Recipients

BACKGROUND

Mycophenolic acid (MPA) is glucuronidated by uridine diphosphate-glucuronosyltransferases (UGTs) to its pharmacologically inactive 7-O-glucuronide metabolite (MPAG). MPAG is excreted into the bile via the multidrug resistance-associated protein 2 (MRP2/ABCC2), which is essential for enterohepatic (re)circulation (EHC) of MPA(G).

METHODS

The objective of this study was to determine the relationship between single nucleotide polymorphisms (SNPs) in the MRP2 (G-1549A, G-1023A, A-1019G, C-24, G1249A, C3972T and G4544A) and UGT1A9 (C-2152T, T-275AandT98C) genes and MPA pharmacokinetics in 95 renal allograft recipients at days 7, 42, 90, and 360 after transplantation. In addition to mycophenolate mofetil, all patients received tacrolimus and corticosteroids as immunosuppression.

RESULTS

At day seven after transplantation, in the absence of the MRP2 C-24T SNP, mild liver dysfunction was associated with significantly lower MPA dose-interval exposure and higher MPA oral clearance, while liver dysfunction did not affect MPA pharmacokinetics in patients with the MRP2 C-24T variant. A similar effect is noted for the C-3972T variant, which is in linkage disequilibrium with C-24T. At later time points after transplantation the MRP2 C-24T SNP was associated with significantly higher dose-corrected MPA trough levels. Patients with the MRP2 C-24T variant had significantly more diarrhea in the first year after transplantation.

CONCLUSIONS

The MRP2 C-24T and C-3972T polymorphisms protect renal transplant recipients from a decrease in MPA exposure associated with mild liver dysfunction. Furthermore, this study suggests that the C-24T SNP is associated with a lower oral clearance of MPA in steady-state conditions.

Increased apical insertion of the multidrug resistance protein 2 (MRP2/ABCC2) in renal proximal tubules following gentamicin exposure

Multidrug resistance protein (MRP) 2 (MRP2; ABCC2), an organic anion transporter apically expressed in liver, kidney, and intestine, plays an important protective role through facilitating the efflux of potentially toxic compounds. We hypothesized that upon a toxic insult, MRP2 is up-regulated in mammalian kidney, thereby protecting the tissue from damage. We studied the effects of the nephrotoxicant gentamicin on the functional expression of MRP2 in transfected Madin-Darby canine kidney type II (MDCKII) cells and rat kidney. Transport of glutathionemethyl fluorescein by cells or calcein by isolated perfused rat kidney was measured to monitor MRP2 activity. MDCKII cells were exposed to gentamicin (0-1000 microM) for either 1 h, 24 h, or for 1 h followed by 24-h recovery. No effect was observed on MRP2 after 1-h exposure. After 24-h gentamicin exposure or after a 24-h recovery period following 1-h exposure, an increase in MRP2-mediated transport was seen. This up-regulation was accompanied by a 2-fold increase in MRP2 protein expression in the apical membrane, whereas the expression in total cell lysates remained unchanged. In perfused kidneys of rats exposed to gentamicin (100 mg/kg) for seven consecutive days, an increase in Mrp2 function and expression was found, which was prevented by addition of a dual endothelin-receptor antagonist, bosentan. We conclude that an increased shuttling of the transporter to the apical membrane takes place in response to gentamicin exposure, which is triggered by endothelin. Up-regulation of MRP2 in the kidney may be interpreted as part of a protective mechanism.

Intravenously administered short interfering RNA accumulates in the kidney and selectively suppresses gene function in renal proximal tubules

Different gene-silencing methods, like antisense and short interfering RNA (siRNA), are widely used as experimental tools to inhibit gene expression. In the present study, the in vivo behavior of siRNA in rats and siRNA-mediated silencing of genes in the renal proximal tubule were investigated. To study the biodistribution of siRNA, rats were injected i.v. with radiolabeled siRNA or radiolabel alone (control), and scintigraphic images were acquired at different time intervals postinjection. The siRNA preferentially accumulated in the kidneys and was excreted in the urine. One hour after injection, the amount of siRNA present in both kidneys (1.7 +/- 0.3% of injected dose/g tissue) was on average 40 times higher than in other tissues (liver, brain, intestine, muscle, lung, spleen, and blood). Besides the biodistribution, the effect of siRNA on multidrug resistance protein isoform 2 (Mrp2/Abcc2, siRNAMrp2) in renal proximal tubules was investigated. Mrp2 function was assessed by measuring the excretion of its fluorescent substrate calcein in the isolated perfused rat kidney. Four days after administration, siRNAMrp2 reduced the urinary calcein excretion rate significantly (35% inhibition over the period 80-150 min of perfusion). This down-regulation was specific because another siRNA sequence directed against a different transporter in the proximal tubule, Mrp4 (Abcc4, siRNAMrp4), did not alter the Mrp2-mediated excretion of calcein. In conclusion, siRNA accumulates spontaneously in the kidney after i.v. injection, where it selectively suppresses gene function in the proximal tubules. Therefore, i.v. administered siRNA provides a novel experimental and potential therapeutic tool for gene silencing in the kidney.

Tubular transporters and clearance of adefovir

Adefovir is transported by the organic anion transporter (OAT1) and the multidrug resistant protein (MRP2, 4 and 5). We studied adefovir clearance in rat after inhibition of transporters by probenecid and in mutant transport-deficient (TR-) rats, in which MRP2 is lacking. After treatment by probenecid or placebo, pharmacokinetics of adefovir 10mg/kg was studied via population nonlinear mixed effect modeling. The fraction of drug excreted in the urine was low. Renal clearance of adefovir was significantly lower (P < 0.05) in probenecid TR- rats (0.03+/-0.02l/h) than in normal control (0.09+/-0.05l/h), in normal probenecid (0.10+/-0.07l/h) and in TR- control rats (0.13+/-0.07l/h). In vivo in rats MRP2 mutation alone did not affect adefovir clearance suggesting that MRP2 does not play a critical role in the secretion of adefovir. Additional pharmacological inhibition of transporters decreased renal clearance, which may reflect inhibition of compensating transport mechanisms activated when MRP2 is lacking.

A mutation in the drug transporter gene ABCC2 associated with impaired methotrexate elimination

Human multidrug resistance protein 2 (MRP2, encoded by ABCC2) is involved in active efflux of anionic drugs such as methotrexate. MRP2 is expressed on the luminal side of hepatocytes and renal proximal tubular cells, indicating an important role in drug elimination. We postulated that loss-of-function mutations in ABCC2, which are involved in the Dubin-Johnson syndrome, may be associated with impaired methotrexate elimination and an increased risk of toxicity. We studied the biological phenotype and ABCC2 coding sequence in a patient receiving a high-dose methotrexate infusion for large B-cell lymphoma and who had an unusual pharmacokinetic profile, mainly characterized by a three-fold reduction in the methotrexate elimination rate. This resulted in severe methotrexate over-dosing and reversible nephrotoxicity. An inversion of the urinary coproporphyrin isomer I/III ratio (a specific biological marker of the Dubin-Johnson syndrome) was observed in this patient. Genetic analysis of ABCC2 identified a heterozygous mutation replacing a highly conserved arginine by glycine in the cytoplasmic part of the second membrane-spanning domain (position 412 of MRP2), a region associated with substrate affinity. This genetic variant was not found in a control population. Functional analysis in transiently transfected Chinese hamster ovary cells revealed a loss of transport activity of the G412 MRP2 mutant protein. An ABCC2 mutation altering MRP2-mediated methotrexate transport and resulting in impaired drug elimination and subsequent renal toxicity was identified. Candidates for methotrexate therapy should be considered for MRP2 functional testing.

Up-regulation of Mrp4 expression in kidney of Mrp2-deficient TR- rats

Multidrug resistance-associated proteins (Mrps) are a group of ATP-dependent efflux transporters for organic anions. Mrp2 and Mrp4 are co-localized to the apical (brush-border) membrane domain of renal proximal tubules, where they may function together in the urinary excretion of organic anions. Previous reports showed that urinary excretion of some organic anions is not impaired in transport-deficient (TR-) rats, which lack Mrp2, suggesting that up-regulation of other transporter(s) may compensate for the loss of Mrp2 function. The purpose of this study was to determine whether Mrp4 expression in kidney is altered in TR- rats. Mrp4 mRNA expression was quantified using the high-throughput branched DNA signal amplification assay. Mrp4 protein expression was determined by Western blot and immunohistochemical analysis. Mrp4 mRNA in kidney of TR- rats was 100% higher than normal Wistar rats. Western blot analysis showed a 200% increase in Mrp4 protein expression in kidney of the mutant rats compared to normal rats. Immunohistochemical analysis of Mrp4 protein demonstrated apical localization of Mrp4 on renal proximal tubules, and that the immunoreactivity was more intense in kidney sections from TR- rats than those from normal rats. In summary, the results of the present study demonstrate that renal Mrp4 expression is up-regulated in TR- rats, which may explain why urinary excretion of some organic anions remains normal in the mutant rats.

[Tubular transporters OAT1 and MRP2 and clearance of adefovir]

Adefovir is transported by the organic anion transporter (OAT1) and the multidrug resistant protein (MRP2, 4 and 5). We studied adefovir clearance in rat after inhibition of transporters by probenecid and in TR- rats, in which MRP2 is lacking. After treatment by probenecid or placebo, pharmacokinetics of adefovir 10 mg/kg was studied via population modeling (NONMEM). The fraction of drug excreted in the urine was low. Renal clearance of adefovir was significantly lower (P < 0.05) in probenecid TR- rats (0.03 +/- 0.02 l/hour) than in normal control (0.09 +/- 0.05 l/hour), in normal probenecid (0.10 +/- 0.07 l/hour) and in TR- control rats (0.13 +/- 0.07 l/hour). In vivo in rats MRP2 mutation alone did not affect adefovir clearance suggesting that MRP2 does not play a critical role in the secretion of adefovir. Additional pharmacological inhibition of transporters decreased renal clearance, which may reflect inhibition of compensating transport mechanisms activated when MRP2 is lacking.

Endogenous expression of liver-specific drug transporters for organic anions in the rat hepatocytoma fusion cell line HPCT-1E3

HPCT-1E3 cells, a fusion cell line between primary rat hepatocytes and Fao Reuber hepatoma cells H35, are immortalized hybrid cells with many phenotypic properties of liver parenchyma including phase I and II metabolism and bile acid secretion. Selective elimination of endogenous compounds and drugs by the liver involves transport proteins that complementarily mediate uptake and efflux in co-operation with metabolism, but the study of this function is limited by the unavailability of an integrated in vitro model. Therefore, we investigated the expression of some important liver-specific import and export carrier proteins for organic anions in this cell line. RT-PCR analysis indicated gene expression of Oat2, Oatplal, Oatpla4, Oatplb2, Rfc-1/MTX-1, FOLR, Mrp1-6, mdr1, and Lrp. Uptake and efflux as well as inhibition studies confirmed the functional activity of Oat, Oatp, Rfc-1, Mrp, and Mdr carriers. In conclusion, the hepatocyte-like HPCT-1E3 cell line shows endogenous expression of all liver-specific carrier proteins for organic anions and may hence represent a valuable in vitro model for the study of transport phenomena and their regulation in hepatocytes.

Expression systems for cloned xenobiotic transporters

One challenge of modern biology is to be able to match genes and their encoded proteins with events at the molecular, cellular, tissue, and organism levels, and thus, provide a multi-level understanding of gene function and dysfunction. How well this can be done for xenobiotic transporters depends on a knowledge of the genes expressed in the tissue, the cellular locations of the gene products (do they function for uptake or efflux?), and our ability to match substrates with transporters using information obtained from cloned transporters functioning in heterologous expression systems. Clearly, making a rational choice of expression system to use for the characterization and study of cloned xenobiotic transporters is a critical part of study design. This choice requires well-defined goals, as well as an understanding of the strengths and weaknesses of candidate expression systems.

Multidrug resistance proteins: role of P-glycoprotein, MRP1, MRP2, and BCRP (ABCG2) in tissue defense

In tumor cell lines, multidrug resistance is often associated with an ATP-dependent decrease in cellular drug accumulation which is attributed to the overexpression of certain ATP-binding cassette (ABC) transporter proteins. ABC proteins that confer drug resistance include (but are not limited to) P-glycoprotein (gene symbol ABCB1), the multidrug resistance protein 1 (MRP1, gene symbol ABCC1), MRP2 (gene symbol ABCC2), and the breast cancer resistance protein (BCRP, gene symbol ABCG2). In addition to their role in drug resistance, there is substantial evidence that these efflux pumps have overlapping functions in tissue defense. Collectively, these proteins are capable of transporting a vast and chemically diverse array of toxicants including bulky lipophilic cationic, anionic, and neutrally charged drugs and toxins as well as conjugated organic anions that encompass dietary and environmental carcinogens, pesticides, metals, metalloids, and lipid peroxidation products. P-glycoprotein, MRP1, MRP2, and BCRP/ABCG2 are expressed in tissues important for absorption (e.g., lung and gut) and metabolism and elimination (liver and kidney). In addition, these transporters have an important role in maintaining the barrier function of sanctuary site tissues (e.g., blood-brain barrier, blood-cerebral spinal fluid barrier, blood-testis barrier and the maternal-fetal barrier or placenta). Thus, these ABC transporters are increasingly recognized for their ability to modulate the absorption, distribution, metabolism, excretion, and toxicity of xenobiotics. In this review, the role of these four ABC transporter proteins in protecting tissues from a variety of toxicants is discussed. Species variations in substrate specificity and tissue distribution of these transporters are also addressed since these properties have implications for in vivo models of toxicity used for drug discovery and development.

Cyclosporine interacts with mycophenolic acid by inhibiting the multidrug resistance-associated protein 2

In mycophenolate mofetil (MMF)-treated organ transplant recipients, lower mycophenolic acid (MPA) plasma concentrations have been found in cyclosporine (CsA) compared with tacrolimus (Tac)-based immunosuppressive regimens. We previously demonstrated that CsA decreases exposure to MPA and increases exposure to its metabolite MPA-glucuronide (MPAG), possibly by interfering with the biliary excretion of MPAG. To elucidate the role of the multidrug resistance-associated protein (Mrp)-2 in the interaction between MMF and CsA, we treated three groups of 10 Mrp2-deficient rats (TR- rat) for 6 days with either vehicle, CsA (8 mg/kg) or Tac (4 mg/kg) by oral gavage. Hereafter, co-administration with MMF (20 mg/kg) was started in all groups and continued through day 14. The 24-h MPA/MPAG area under the concentration-time curve (AUC) was determined after single (day 7) and multiple MMF doses (day 14). On both study days, there were no significant differences in the mean MPA and MPAG AUC between CsA and Tac-treated animals. We conclude that the pharmacokinetics of MMF are comparable in Mrp2-deficient rats receiving either CsA or Tac as co-medication. This finding suggests that CsA-mediated inhibition of the biliary excretion of MPAG by the Mrp2 transporter is the mechanism responsible for the interaction between CsA and MMF.

Molecular Cloning and Functional Analyses of OAT1 and OAT3 from Cynomolgus Monkey Kidney

PURPOSE

The functional characterization of monkey OAT1 (SLC22A6) and OAT3 (SLC22A8) was carried out to elucidate species differences in the OAT1- and OAT3-mediated transport between monkey and human.

METHODS

The cDNAs of monkey OAT1 and OAT3 were isolated from monkey kidney, and their stable transfectants were established in HEK293 cells (mkOAT1- and mkOAT3-HEK). Transport studies were performed using cDNA transfectants, and kinetic parameters were compared among rat, monkey and human.

RESULTS

The amino acid sequences of mkOAT1 and mkOAT3 exhibit 97% and 96% identity to their corresponding human orthologues. For OAT1, there was no obvious species difference in the K(m) values and the relative transport activities of 11 substrates with regard to p-aminohippurate transport. For OAT3, there was no species difference in the K(m) values and in the relative transport activities of nine substrates with regard to benzylpenicillin transport between monkey and human. However, the relative transport activities of indoxyl sulfate, 3-carboxy-4-methyl-5-propyl-2-furanpropionate, and estrone-3-sulfate showed a difference between primates and rat and gave a poor correlation.

CONCLUSIONS

These results suggest that monkey is a good predictor of the renal uptake of organic anions in the human.