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

Multidrug Resistance Like Protein 1 Activity in Malpighian Tubules Regulates Lipid Homeostasis in Drosophila

Simple Summary

Multidrug resistance proteins (MRPs) are important for ion transport, toxin/xenobiotic secretion, and signal transduction. Although studies have been undertaken to understand their physiological function, it is not fully known how MRPs may regulate metabolism. We knocked down the expression of Drosophila multidrug-resistance like protein 1 (MRP) in several tissues central to metabolic regulation. Reducing MRP in Malpighian tubules, the functional equivalent to the human kidney, was sufficient to disrupt metabolic homeostasis, owing to abnormal lipid accumulation, as well as changes in feeding behavior. It also increased oxidative stress resistance in adult flies, possibly due to reduced levels of reactive oxygen species.

Abstract

Multidrug resistance proteins (MRPs), members of the ATP-binding cassette transporter (ABC transporter) family, are pivotal for transporting endo- and xenobiotics, which confer resistance to anticancer agents and contribute to the clearance of oxidative products. However, their function in many biological processes is still unclear. We investigated the role of an evolutionarily conserved MRP in metabolic homeostasis by knocking down the expression of Drosophila multidrug-resistance like protein 1 (MRP) in several tissues involved in regulating metabolism, including the gut, fat body, and Malpighian tubules. Interestingly, only suppression of MRP in the Malpighian tubules, the functional equivalent to the human kidney, was sufficient to cause abnormal lipid accumulation and disrupt feeding behavior. Furthermore, reduced Malpighian tubule MRP expression resulted in increased Hr96 (homolog of human pregnane X receptor) expression. Hr96 is known to play a role in detoxification and lipid metabolism processes. Reduced expression of MRP in the Malpighian tubules also conveyed resistance to oxidative stress, as well as reduced normal levels of reactive oxygen species in adult flies. This study reveals that an evolutionarily conserved MRP is required in Drosophila Malpighian tubules for proper metabolic homeostasis.

Effects of Ischemia-Reperfusion on Tubular Cell Membrane Transporters and Consequences in Kidney Transplantation

Ischemia-reperfusion (IR)-induced acute kidney injury (IRI) is an inevitable event in kidney transplantation. It is a complex pathophysiological process associated with numerous structural and metabolic changes that have a profound influence on the early and the late function of the transplanted kidney. Proximal tubular cells are particularly sensitive to IRI. These cells are involved in renal and whole-body homeostasis, detoxification processes and drugs elimination by a transporter-dependent, transcellular transport system involving Solute Carriers (SLCs) and ATP Binding Cassettes (ABCs) transporters. Numerous studies conducted mainly in animal models suggested that IRI causes decreased expression and activity of some major tubular transporters. This could favor uremic toxins accumulation and renal metabolic alterations or impact the pharmacokinetic/toxicity of drugs used in transplantation. It is of particular importance to understand the underlying mechanisms and effects of IR on tubular transporters in order to improve the mechanistic understanding of IRI pathophysiology, identify biomarkers of graft function or promote the design and development of novel and effective therapies. Modulation of transporters’ activity could thus be a new therapeutic opportunity to attenuate kidney injury during IR.

ABC Transporter-Mediated Multidrug-Resistant Cancer

ATP-binding cassette (ABC) transporters are involved in active pumping of many diverse substrates through the cellular membrane. The transport mediated by these proteins modulates the pharmacokinetics of many drugs and xenobiotics. These transporters are involved in the pathogenesis of several human diseases. The overexpression of certain transporters by cancer cells has been identified as a key factor in the development of resistance to chemotherapeutic agents. In this chapter, the localization of ABC transporters in the human body, their physiological roles, and their roles in the development of multidrug resistance (MDR) are reviewed. Specifically, P-glycoprotein (P-GP), multidrug resistance-associated proteins (MRPs), and breast cancer resistance protein (BCRP/ABCG2) are described in more detail. The potential of ABC transporters as therapeutic targets to overcome MDR and strategies for this purpose are discussed as well as various explanations for the lack of efficacy of ABC drug transporter inhibitors to increase the efficiency of chemotherapy.

The ABC transporter, AbcB3, mediates cAMP export in D. discoideum development

Extracellular cAMP functions as a primary ligand for cell surface cAMP receptors throughout Dictyostelium discoideum development, controlling chemotaxis and morphogenesis. The developmental consequences of cAMP signaling and the metabolism of cAMP have been studied in great detail, but it has been unclear how cells export cAMP across the plasma membrane. Here we show pharmacologically and genetically that ABC transporters mediate cAMP export. Using an evolutionary-developmental biology approach, we identified several candidate abc genes and characterized one of them, abcB3, in more detail. Genetic and biochemical evidence suggest that AbcB3 is a component of the cAMP export mechanism in D. discoideum development.

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.

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.

Developmental expression of renal organic anion transporters in rat kidney and its effect on renal secretion of phenolsulfonphthalein

Organic anion transporters (OAT1 and OAT3) and multidrug resistance-associated proteins (MRP2 and MRP4) play important roles in anionic drug secretion in renal proximal tubules. Changes in the expression of such transporters are considered to affect the tubular secretion of anionic drugs. The purpose of this study was to elucidate the developmental changes in the expression of OAT1, OAT3, MRP2, and MRP4 and their effects on the tubular secretion of drugs. The mRNA level of each transporter was measured by real-time PCR, and the protein expression was evaluated by Western blotting and immunohistochemical analysis. In addition, the tubular secretion of phenolsulfonphthalein (PSP) in infant (postnatal day 14) and adult rats was estimated based on in vivo clearance study. The protein expression of organic anion transporters were very low at postnatal day 0 and gradually increased with age. In postnatal day 14 rats, the expression of OAT1 and OAT3 seemed to be at almost mature levels, while MRP2 and MRP4 seemed to be at immature levels. Immunohistochemical analysis in the kidney of postnatal day 0 rats revealed OATs on the basolateral membrane and MRPs on the brush-border membrane. At postnatal day 0, the distribution of these transporters was restricted to the inner cortical region, while after postnatal day 14, it was identical to that in adult kidney. An in vivo clearance study revealed that the tubular secretion of PSP was significantly lower in postnatal day 14 rats than adult rats. These results indicate that age-dependent changes in organic anion transporter expression affect the tubular secretion of anionic drugs in pediatric patients.

Arsenic-glutathione conjugate transport by the human multidrug resistance proteins (MRPs/ABCCs)

Millions of people world-wide are chronically exposed to inorganic forms of the environmental toxicant arsenic in drinking water. This has led to a public health crisis because arsenic is a human carcinogen, and causes a myriad of other adverse health effects. In order to prevent and treat arsenic-induced toxicity it is critical to understand the cellular handling of this metalloid. A large body of literature describes the importance of the cellular tripeptide glutathione (γ-Glu-Cys-Gly,GSH/GS) in the excretion of arsenic. The triglutathione conjugate of arsenite [As(III)(GS)(3)] and the diglutathione conjugate of monomethylarsonous acid [MMA(III)(GS)(2)] have been isolated from rat bile and mouse urine, and account for the majority of excreted arsenic, suggesting these are important transportable forms. The ATP-binding cassette (ABC) transporter proteins, multidrug resistance protein 1 (MRP1/ABCC1) and the related protein MRP2 (ABCC2), are thought to play an important role in arsenic detoxification through the cellular efflux of arsenic-GSH conjugates. Current knowledge on the cellular handling of arsenic with a special emphasis on the transport pathways of the arsenic-GSH conjugates As(III)(GS)(3), MMA(III)(GS)(2), and dimethylarsenic glutathione DMA(III)(GS), as well as, the seleno-bis(S-glutathionyl) arsinium ion [(GS)(2)AsSe](-) are reviewed.

Increased expression of the multidrug resistance-associated protein 1 (MRP1) in kidney glomeruli of streptozotocin-induced diabetic rats

Oxidative stress has been linked to the podocytopathy, mes-angial expansion and progression of diabetic nephropathy. The major cell defence mechanism against oxidative stress is reduced glutathione (GSH). Some ABC transporters have been shown to extrude GSH, oxidised glutathione or their conjugates out of the cell, thus implying a role for these transporters in GSH homeostasis. We found a remarkable expression of mRNA for multidrug resistance-associated proteins (MRP/ABCC) 1, 3, 4 and 5 in rat glomeruli. Three weeks after induction of diabetes in glomeruli of streptozotocin-treated rats, we observed a decline in reduced GSH levels and an increase in the expression and activity of MRP1 (ABCC1). These lower GSH levels were improved by ex vivo treatment with pharmacological inhibitors of MRP1 activity (MK571). We conclude that increased activity of MRP1 in diabetic glomeruli is correlated with an inadequate adaptive response to oxidative stress.

Rapid, nongenomic stimulation of multidrug resistance protein 2 (Mrp2) activity by glucocorticoids in renal proximal tubule

In renal proximal tubule, multidrug resistance protein 2 (Mrp2) actively transports many organic anions into urine, including drugs and metabolic wastes. Upon exposure to nephrotoxicants or during endotoxemia, both Mrp2 activity and expression are up-regulated. This may result from induced de novo synthesis of Mrp2 or post-transcriptional events involving specific signaling pathways. Here, we investigated glucocorticoid signaling to Mrp2 in killifish renal proximal tubules, a model system in which transport activity can be measured using a fluorescent substrate and confocal imaging. Exposure of tubules to dexamethasone rapidly increased Mrp2-mediated fluorescein methotrexate transport. Other glucocorticoid receptor (GR) ligands, cortisol and triamcinolone acetonide, also stimulated Mrp2-mediated transport. The GR antagonist, mifepristone 17β-hydroxy-11β-[4-dimethylamino phenyl]-17α-[1-propynyl]estra-4,9-dien-3-one (RU486), abolished stimulation by all three ligands, whereas the mineralocorticoid receptor antagonist, spironolactone, was ineffective. Consistent with action through a nongenomic mechanism, dexamethasone stimulation of Mrp2-mediated transport was insensitive to cycloheximide and actinomycin D, and immunohistochemistry revealed no alterations in Mrp2 expression at the luminal membrane. (9S-(9α,10β,12α))-2,3,9,10,11,12-hexahydro-10-hydroxy-10-(methoxycarbonyl)-9-methyl-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocin-1-one (K252a), an inhibitor of the tyrosine receptor kinase subfamily, reduced the dexamethasone effect, as did the specific hepatocyte growth factor receptor (c-Met) tyrosine kinase inhibitor, (2R)-1-[[5-[(Z)-[5-[[(2,6-dichlorophenyl)methyl]sulfonyl]-1,2-dihydro-2-oxo-3H-indol-3-ylidene]methyl]-2,4-dimethyl-1H-pyrrol-3-yl]carbonyl]-2-(1-pyrrolidinylmethyl)pyrrolidine (PHA-665752). Hepatocyte growth factor (HGF), an endogenous ligand for c-Met, stimulated Mrp2-mediated transport. This effect was reversed by PHA-665752 but not by RU486. Inhibition of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK 1/2) also abolished the effects of dexamethasone and HGF. Our results disclose a novel mechanism by which glucocorticoids acting through GR, c-Met, and MEK1/2 cause rapid, nongenomic stimulation of Mrp2-mediated transport in renal proximal tubules. This up-regulation may be nephroprotective, enhancing efflux of metabolic wastes and toxicants during cell and tissue stress.

Advances in the molecular detection of ABC transporters involved in multidrug resistance in cancer

ATP-Binding Cassette (ABC) transporters are important mediators of multidrug resistance (MDR) in patients with cancer. Although their role in MDR has been extensively studied in vitro, their value in predicting response to chemotherapy has yet to be fully determined. Establishing a molecular diagnostic assay dedicated to the quantitation of ABC transporter genes is therefore critical to investigate their involvement in clinical MDR. In this article, we provide an overview of the methodologies that have been applied to analyze the mRNA expression levels of ABC transporters, by describing the technology, its pros and cons, and the experimental protocols that have been followed. We also discuss recent studies performed in our laboratory that assess the ability of the currently available high-throughput gene expression profiling platforms to discriminate between highly homologous genes. This work led to the conclusion that high-throughput TaqMan-based qRT-PCR platforms provide standardized clinical assays for the molecular detection of ABC transporters and other families of highly homologous MDR-linked genes encoding, for example, the uptake transporters (solute carriers-SLCs) and the phase I and II metabolism enzymes.

Effects of metabolic acidosis on expression levels of renal drug transporters

PURPOSE

In the renal proximal tubular cells, various transporters play important roles in the secretion and reabsorption of drugs. When metabolic acidosis is induced, a number of adaptive changes occur in the kidney. The purpose of this study was to clarify the changes of drug transporters under the acidosis and the effects of these changes on urinary drug excretion.

METHODS

Wistar/ST rats were given 1.5% NH₄Cl in tap water for 48 h to induce the acidosis. Pharmacokinetics of PSP or metformin was evaluated. In addition, expression levels of drug transporters were examined by Western Blotting.

RESULTS

The renal clearance of PSP was markedly decreased, whereas the creatinine clearance and renal clearance of metformin were unchanged. Furthermore, Western blots indicated that the protein expression level of organic anion transporter (OAT) 3 was decreased. In contrast to OAT3 levels, OAT1 and organic cation transporter (OCT) 2 levels were unaffected. An immunohistochemical analysis showed that the OAT3 protein in the proximal tubules was localized in the basolateral membrane both of the normal and the acidosis rats.

CONCLUSION

The decrease of renal excretion of anionic drugs during metabolic acidosis might be partly due to a reduction in the level of OAT3 protein.

Assessment of the role of renal organic anion transporters in drug-induced nephrotoxicity

In the present review we have attempted to assess the involvement of the organic anion transporters OAT1, OAT2, OAT3, and OAT4, belonging to the SLC22 family of polyspecific carriers, in drug-induced renal damage in humans. We have focused on drugs with widely recognized nephrotoxic potential, which have previously been reported to interact with OAT family members, and whose underlying pathogenic mechanism suggests the participation of tubular transport. Thus, only compounds generally believed to cause kidney injury either by means of direct tubular toxicity or crystal nephropathy have been considered. For each drug, or class of agents, the evidence for actual transport mediated by individual OATs under in vivo conditions is discussed. We have then examined their role in the context of other carriers present in the renal proximal tubule sharing certain substrates with OATs, as these are critical determinants of the overall contribution of OAT-dependent transport to intracellular accumulation and transepithelial drug secretion, and thus the impact it may have in drug-induced nephrotoxicity.

ABC transporters in human lymphocytes: expression, activity and role, modulating factors and consequences for antiretroviral therapies

IMPORTANCE OF THE FIELD

ATP-binding cassette (ABC) transporters are a superfamily of efflux pumps that transport numerous compounds across cell membranes. These transporters are located in various human tissues including peripheral blood cells, in particular lymphocytes, and present a high variability of expression and activity. This variability may affect the intracellular concentrations and efficacy of drugs acting within lymphocytes, such as antiretroviral drugs.

AREAS COVERED IN THIS REVIEW

This review focuses on the current knowledge about the expression, activity, roles and variability of ABC drug transporters in human lymphocytes. The identified modulating factors and their impact on the intracellular pharmacokinetics and efficacy of antiretroviral drugs are also detailed.

WHAT THE READER WILL GAIN

Controversial data regarding the expression, activity and sources of variability of ABC transporters in lymphocytes are discussed. The modulating factors and their pharmacological consequences regarding antiretroviral therapies are also provided.

TAKE HOME MESSAGE

Numerous studies have reported conflicting results regarding the expression and activity of ABC drug transporters in lymphocytes. Despite these discrepancies, which may partly result from heterogeneous analytical methods, ABCC1 appears to have the highest expression in lymphocytes and may thus play a predominant role in the resistance to antiretroviral drugs, particularly to protease inhibitors.

Formed and preformed metabolites: facts and comparisons

The administration of metabolites arising from new drug entities is often employed in drug discovery to investigate their associated toxicity. It is expected that administration of metabolites can predict the exposure of metabolites originating from the administration of precursor drug. Whether exact and meaningful information can be obtained from this has been a topic of debate. This communication summarizes observations and theoretical relationships based on physiological modelling for the liver, kidney and intestine, three major eliminating organs/tissues. Theoretical solutions based on physiological modelling of organs were solved, and the results suggest that deviations are expected. Here, examples of metabolite kinetics observed mostly in perfused organs that did not match predictions are provided. For the liver, discrepancies in fate between formed and preformed metabolites may be explained by the heterogeneity of enzymes, the presence of membrane barriers and whether transporters are involved. For the kidney, differences have been attributed to glomerular filtration of the preformed but not the formed metabolite. For the intestine, the complexity of segregated flows to the enterocyte and serosal layers and differences in metabolism due to the route of administration are addressed. Administration of the metabolite may or may not directly reflect the toxicity associated with drug use. However, kinetic data on the preformed metabolite will be extremely useful to develop a sound model for modelling and simulations; in-vitro evidence on metabolite handling at the target organ is also paramount. Subsequent modelling and simulation of metabolite data arising from a combined model based on both drug and preformed metabolite data are needed to improve predictions on the behaviours of formed metabolites.

Localization of multidrug resistance-associated protein 2 in the nonpigmented ciliary epithelium of the eye

The nonpigmented epithelium (NPE) of the ciliary body represents an important component of the blood-aqueous barrier of the eye. Many therapeutic drugs penetrate poorly across the NPE into the aqueous humor of the eye interior. Several of these therapeutic drugs, such as methotrexate, vincristine, and etoposide, are substrates of the multidrug resistance-associated protein 2 (MRP2). Abundant MRP2 protein was detected by Western blot in homogenates of human ciliary body and freshly dissected porcine NPE. In cultured porcine NPE, the intracellular accumulation of the MRP2 substrates calcein (1.8-fold), 5-(and-6)-carboxy-2',7'-dichlorofluorescein (22.1-fold), and doxorubicin (1.9-fold) was significantly increased in the presence of 50 microM MK571 ((E)-3-[[[3-[2-(7-chloro-2-quinolinyl)-ethenyl]phenyl]-[[3-dimethylamino)-3-oxopropyl]thio]methyl]thio]-propanoic acid), an MRP inhibitor. In addition, the intracellular accumulation of the MRP2 substrate glutathione methylfluorescein was increased by 50 microM MK571 (4.3-fold), 500 microM indomethacin (2.6-fold), and 50 microM cyclosporin A (2.1-fold) but not by 500 microM sulfinpyrazone. These data are consistent with MRP2-mediated transport activity in cultured NPE, and MRP2 mRNA (reverse transcriptase-polymerase chain reaction) and protein (Western blot) were detected in the cultured cells. Immunolocalization studies in native human and porcine eyes showed MRP2 protein at the apical interface of the NPE and pigmented cell layers. Close examination of MRP2 immunoreactivity supported the conclusion that MRP2 is localized in the apical membrane of the NPE. MRP2 at the apical membrane of NPE cells may be involved in protecting intraocular tissues from exposure to potentially harmful toxins.

Structure, function and regulation of P-glycoprotein and its clinical relevance in drug disposition

1. P-glycoprotein (P-gp/MDR1), one of the most clinically important transmembrane transporters in humans, is encoded by the ABCB1/MDR1 gene. Recent insights into the structural features of P-gp/MDR1 enable a re-evaluation of the biochemical evidence on the binding and transport of drugs by P-gp/MDR1. 2. P-gp/MDR1 is found in various human tissues in addition to being expressed in tumours cells. It is located on the apical surface of intestinal epithelial cells, bile canaliculi, renal tubular cells, and placenta and the luminal surface of capillary endothelial cells in the brain and testes. 3. P-gp/MDR1 confers a multi-drug resistance (MDR) phenotype to cancer cells that have developed resistance to chemotherapy drugs. P-gp/MDR1 activity is also of great clinical importance in non-cancer-related drug therapy due to its wide-ranging effects on the absorption and excretion of a variety of drugs. 4. P-gp/MDR1 excretes xenobiotics such as cytotoxic compounds into the gastrointestinal tract, bile and urine. It also participates in the function of the blood-brain barrier. 5. One of the most interesting characteristics of P-gp/MDR1 is that its many substrates vary greatly in their structure and functionality, ranging from small molecules such as organic cations, carbohydrates, amino acids and some antibiotics to macromolecules such as polysaccharides and proteins. 6. Quite a number of single nucleotide polymorphisms have been found for the MDR1 gene. These single nucleotide polymorphisms are associated with altered oral bioavailability of P-gp/MDR1 substrates, drug resistance, and a susceptibility to some human diseases. 7. Altered P-gp/MDR1 activity due to induction and/or inhibition can cause drug-drug interactions with altered drug pharmacokinetics and response. 8. Further studies are warranted to explore the physiological function and pharmacological role of P-gp/MDR1.

Safety testing of metabolites: Expectations and outcomes

Metabolites arising from chemical entities, old or new, are often mediators of toxicity. Frequently, metabolites are investigated in test animals, with the expectation that the resultant toxicity or activity will mimic the exposure of their formed counterparts. This communication described observations that showed discrepant kinetics between formed and preformed metabolites in the liver, intestine, and kidney, major drug removal organs. Differences in the observed areas under the curve (AUCs) or the extraction ratios (Es) of formed and preformed metabolites in the liver had been attributed to zonal, enzyme heterogeneity, membrane barriers, or transporters. Preformed and formed metabolite also differed in their handling by the kidney; only the preformed and not the formed metabolite would be filtered. In the intestine, differences in the absorption of the precursor and the metabolite and the flow pattern in the intestine would bring about discrepancy in the time-courses of the formed vs. preformed metabolites. Analytical solutions of the AUCs of the metabolites and extraction ratios, based on physiological modeling of the liver, kidney, and intestine, showed that the AUC of the preformed, administered metabolite was dependent only on metabolite parameters, whereas the AUC of the formed metabolite was modulated additionally by the metabolic, secretory and intestinal absorptive intrinsic clearances of the precursor drug. Hence, administration of the synthetic metabolite would not reflect the toxicity associated with the metabolite formed via bioactivation. However, data on preformed metabolite may be used for simultaneous fitting by a combined model of drug and metabolite. Such a strategy is shown to be successful in risk assessment of environmental chemicals. Upon refinement of the resultant model with data on metabolite transport and handling by modeling and simulations, the resultant model would be more robust to provide improved predictions on metabolite toxicity pursuant to drug administration.

Functional role of arginine 375 in transmembrane helix 6 of multidrug resistance protein 4 (MRP4/ABCC4)

Multidrug resistance protein (MRP) 4 transports a variety of endogenous and xenobiotic organic anions. MRP4 is widely expressed in the body and specifically localized to the renal apical proximal tubule cell membrane, where it mediates the excretion of these compounds into urine. To characterize the MRP4 substrate-binding site, the amino acids Phe368, Phe369, Glu374, Arg375, and Glu378 of transmembrane helix 6, and Arg998 of helix 12, localized in the intracellular half of the central pore, were mutated into the corresponding amino acids of MRP1 and MRP2. Membrane vesicles isolated from human embryonic kidney 293 cells overexpressing these mutants showed significantly reduced methotrexate (MTX) and cGMP transport activity compared with vesicles that expressed wild-type MRP4. The only exception was substitution of Arg375 with serine, which had no effect on cGMP transport but significantly decreased the affinity of MTX. Substitution of the same amino acid with a positively charged lysine returned the MTX affinity to that of the wild type. Furthermore, MTX inhibition of MRP4-mediated cGMP transport was noncompetitive, and the inhibition constant was increased by introduction of the R375S mutation. A homology model of MRP4 showed that Arg375 and Arg998 face right into the central aqueous pore of MRP4. We conclude that positively charged amino acids in transmembrane helices 6 and 12 contribute to the MRP4 substrate-binding pocket.

Implications of ABC transporters on the disposition of typical veterinary medicinal products

The ATP-Binding Cassette (ABC) transporters ABCB1, ABCC2 and ABCG2 are efflux transporters that facilitate the excretion of drugs, contribute to the function of biological barriers and maintain low cytoplasmic substrate concentrations in cells. ABC transporters modulate drug absorption, distribution and elimination according to the level of expression in the intestine, liver, kidney, and at biological barriers such as the blood-brain barrier. Moreover individual transporters are known to convey multi-drug resistance to tumour cells. While these diverse functions have been described in laboratory animal studies and in humans, the available information is very limited in animal species that are typical veterinary patients. This brief review summarizes the available data on organ distribution and expression levels in animals, genetic defects in dogs resulting in a non-functional P-gp expression, and describes examples of kinetic investigations directed to assess the clinical relevance of species differences in ABC-transporter expression.

Transport of a fluorescent cAMP analog in teleost proximal tubules

Previous studies have shown that killifish (Fundulus heteroclitus) renal proximal tubules express a luminal membrane transporter that is functionally and immunologically analogous to the mammalian multidrug resistance-associated protein isoform 2 (Mrp2, ABCC2). Here we used confocal microscopy to investigate in killifish tubules the transport of a fluorescent cAMP analog (fluo-cAMP), a putative substrate for Mrp2 and Mrp4 (ABCC4). Steady-state luminal accumulation of fluo-cAMP was concentrative, specific, and metabolism-dependent, but not reduced by high K+ medium or ouabain. Transport was not affected by p-aminohippurate (organic anion transporter inhibitor) or p-glycoprotein inhibitor (PSC833), but cell-to-lumen transport was reduced in a concentration-dependent manner by Mrp inhibitor MK571, leukotriene C4 (LTC4), azidothymidine (AZT), cAMP, and adefovir; the latter two compounds are Mrp4 substrates. Although MK571 and LTC4 reduced transport of the Mrp2 substrate fluorescein-methotrexate (FL-MTX), neither cAMP, adefovir, nor AZT affected FL-MTX transport. Fluo-cAMP transport was not reduced when tubules were exposed to endothelin-1, Na nitroprusside (an nitric oxide generator) or phorbol ester (PKC activator), all of which signal substantial reductions in cell-to-lumen FL-MTX transport. Fluo-cAMP transport was reduced by forskolin, and this reduction was blocked by the PKA inhibitor H-89. Finally, in membrane vesicles from Spodoptera frugiperda (Sf9) cells containing human MRP4, ATP-dependent and specific uptake of fluo-cAMP could be demonstrated. Thus, based on inhibitor specificity and regulatory signaling, cell-to-lumen transport of fluo-cAMP in killifish renal tubules is mediated by a transporter distinct from Mrp2, presumably a teleost form of Mrp4.

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.

Endothelin and calciotropic hormones share regulatory pathways in multidrug resistance protein 2-mediated transport

The kidney of vertebrates plays a key role in excretion of endogenous waste products and xenobiotics. Active secretion in the proximal nephron is at the basis of this excretion, mediated by carrier proteins including multidrug resistance protein 2 (Mrp2). We previously showed that Mrp2 function is reduced by endothelin-1 (ET-1) through a basolateral B-type receptor, nitric oxide (NO), cGMP, and PKC (Notenboom S, Miller DS, Smits P, Russel FGM, Masereeuw R. Am J Physiol Renal Physiol 282: F458-F464, 2002; Notenboom S, Miller DS, Smits P, Russel FG, Masereeuw R. Am J Physiol Renal Physiol 287: F33-F38, 2004). This pathway was rapidly activated by several nephrotoxicants and appeared to be calcium dependent. In the present study, we studied the effect of the calciotropic hormones parathyroid hormone (PTH), PTH-related protein (PTHrP), and stanniocalcin (STC) to interfere with ET-regulated Mrp2 transport. Like ET-1, PTH reduces Mrp2-mediated transport by 40% in killifish renal proximal tubules. When given in combination, an additive effect was seen, which is partially reversed by the PKC inhibitor calphostin C. Recombinant PTHrP shows a comparable inhibitory effect, which is concentration dependent and additive to the inhibition by ET. STC fully reverses PTHrP-inhibited transport as does a guanylyl cyclase inhibitor. Finally, to confirm PTHrP bioactivity in a homologous assay, we performed immunolocalization and transport studies in sea bream kidney tubules. Mrp2 immunoreactivity was observed in approximately 40% of the tubules and is associated with the brush-border and apical plasma membrane of cells. Both proximal tubules and distal (collecting) tubules express the antigen. A highly significant 40% inhibition of Mrp2-mediated transport was observed with PTHrP in sea bream tubules. In conclusion, ET-regulated Mrp2 transport is influenced by calciotropic hormones and involves PKC and cGMP signaling.

Genetic polymorphisms of drug transporters: pharmacokinetic and pharmacodynamic consequences in pharmacotherapy

There has been increasing appreciation of the role of drug transporters in pharmacokinetic and pharmacodynamic consequences in pharmacotherapy. The clinical relevance of drug transporters depends on the localisation in human tissues (i.e., vectorial movement), the therapeutic index of the substrates and inherent interindividual variability. With regard to variability, polymorphisms of drug transporter genes have recently been reported to be associated with alterations in the pharmacokinetics and pharmacodynamics of clinically useful drugs. A growing number of preclinical and clinical studies have demonstrated that the application of genetic information may be useful in individualised pharmacotherapy for numerous diseases. However, the reported effects of variants in certain drug transporter genes have been inconsistent and, in some cases, conflicting among studies. Furthermore, the incidence of almost all known variants in transporter genes tends to be racially dependent. These observations suggest the necessity of considering interethnic variability before extrapolating pharmacokinetic data obtained in one ethic group to another, especially in the early phase of drug development. This review focuses on the impact of genetic variations in the function of drug transporters (ABC, organic anion and cation transporters) and the implications of these variations for pharmacotherapy from pharmacokinetic and pharmacodynamic viewpoints.

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.

ABC transporters, neural stem cells and neurogenesis – a different perspective

Stem cells intrigue. They have the ability to divide exponentially, recreate the stem cell compartment, as well as create differentiated cells to generate tissues. Therefore, they should be natural candidates to provide a renewable source of cells for transplantation applied in regenerative medicine. Stem cells have the capacity to generate specific tissues or even whole organs like the blood, heart, or bones. A subgroup of stem cells, the neural stem cells (NSCs), is characterized as a self-renewing population that generates neurons and glia of the developing brain. They can be isolated, genetically manipulated and differentiated in vitro and reintroduced into a developing, adult or a pathologically altered central nervous system. NSCs have been considered for use in cell replacement therapies in various neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease. Characterization of genes with tightly controlled expression patterns during differentiation represents an approach to understanding the regulation of stem cell commitment. The regulation of stem cell biology by the ATP-binding cassette (ABC) transporters has emerged as an important new field of investigation. As a major focus of stem cell research is in the manipulation of cells to enable differentiation into a targeted cell population; in this review, we discuss recent literatures on ABC transporters and stem cells, and propose an integrated view on the role of the ABC transporters, especially ABCA2, ABCA3, ABCB1 and ABCG2, in NSCs' proliferation, differentiation and regulation, along with comparisons to that in hematopoietic and other stem cells.

Drug-drug interactions involving membrane transporters in the human kidney

The kidneys play a critical role in the elimination of xenobiotics. Factors affecting the ability of the kidney to eliminate drugs may result in marked changes in the pharmacokinetics of a given compound. Drug-drug interactions due to competitive inhibition of renal organic anion or cation secretion systems have been noticed clinically for a long time. However, our understanding of the physical sites of interactions, that is, the specific transport proteins that the interacting drugs act on, has just begun very recently. This review summarises the latest progress in molecular identification and functional characterisation of major drug transporters in the human kidney. In particular, the review focuses on relating cloned renal drug transporters to clinically observed drug-drug interactions. The authors' opinion on the current status and future directions of research in these areas is also offered.

The apical conjugate efflux pump ABCC2 (MRP2)

ABCC2 is a member of the multidrug resistance protein subfamily localized exclusively to the apical membrane domain of polarized cells, such as hepatocytes, renal proximal tubule epithelia, and intestinal epithelia. This localization supports the function of ABCC2 in the terminal excretion and detoxification of endogenous and xenobiotic organic anions, particularly in the unidirectional efflux of substances conjugated with glutathione, glucuronate, or sulfate, as exemplified by leukotriene C(4), bilirubin glucuronosides, and some steroid sulfates. The hepatic ABCC2 pump contributes to the driving forces of bile flow. Acquired or hereditary deficiency of ABCC2, the latter known as Dubin-Johnson syndrome in humans, causes an increased concentration of bilirubin glucuronosides in blood because of their efflux from hepatocytes via the basolateral ABCC3, which compensates for the deficiency in ABCC2-mediated apical efflux. In this article we provide an overview on the molecular characteristics of ABCC2 and its expression in various tissues and species. We discuss the transcriptional and posttranscriptional regulation of ABCC2 and review approaches to the functional analysis providing information on its substrate specificity. A comprehensive list of sequence variants in the human ABCC2 gene summarizes predicted and proven functional consequences, including variants leading to Dubin-Johnson syndrome.

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.