MRP2, a human conjugate export pump, is present and transports fluo 3 into apical vacuoles of Hep G2 cells

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.

KIF12 Variants and Disturbed Hepatocyte Polarity in Children with a Phenotypic Spectrum of Cholestatic Liver Disease

KIF12 has been identified as a cholestasis-associated candidate gene. We describe 6 cases from 4 unrelated families with diverse cholestatic phenotypes carrying 2 different homozygous KIF12 truncating variants. Immunofluorescence investigations of paraffin-embedded liver sections suggest that KIF12-associated impaired functional cell polarity may be the underlying cause.

Evaluation of the genetic toxicity of sofosbuvir and simeprevir with and without ribavirin in a human-derived liver cell line

Direct-acting antivirals have revolutionized the treatment of chronic hepatitis C. Sofosbuvir and simeprevir are prescribed worldwide. However, there is a scarcity of information regarding their genotoxicity. Therefore, the present study assessed the cytotoxic and genotoxic effects of sofosbuvir and simeprevir, alone and combined with ribavirin. HepG2 cells were analyzed using the in vitro cytokinesis-block micronucleus cytome assay. Cells were treated for 24 h with sofosbuvir (0.011-1.511 mM), simeprevir (0.156-5.0 µM), and their combinations with ribavirin (0.250-4.0 mM). No significant differences were observed in the nuclear division cytotoxicity index, reflecting the absence of cytotoxic effects associated to sofosbuvir. However, the highest concentration of simeprevir showed a significant difference for the nuclear division cytotoxicity index. Moreover, significant results were observed for nuclear division cytotoxicity index in two combinations of sofosbuvir plus ribavirin and only in the highest combination of simeprevir plus ribavirin. Additionally, our results showed that sofosbuvir did not increase the frequency of chromosomal damage, but simeprevir significantly increased the frequency of micronuclei at the highest concentrations. The combination index demonstrated that both sofosbuvir and simeprevir produced antagonism to the genotoxic effects of ribavirin. In conclusion, our results showed that simeprevir, but not sofosbuvir, has genotoxic effects in HepG2 cells.

Characterization of Novel Fluorescent Bile Salt Derivatives for Studying Human Bile Salt and Organic Anion Transporters

Bile salts, such as cholate, glycocholate, taurocholate, and glycochenodeoxycholate, are taken up from the portal blood into hepatocytes via transporters, such as the Na+-taurocholate-cotransporting polypeptide (NTCP) and organic anion-transporting polypeptides (OATPs). These bile salts are later secreted into bile across the canalicular membrane, which is facilitated by the bile salt export pump (BSEP). Apart from bile salt transport, some of these proteins (e.g., OATPs) are also key transporters for drug uptake into hepatocytes. In vivo studies of transporter function in patients by using tracer compounds have emerged as an important diagnostic tool to complement classic liver parameter measurements by determining dynamic liver function both for diagnosis and monitoring progression or improvement of liver diseases. Such approaches include use of radioactively labeled bile salts (e.g., for positron emission tomography) and fluorescent bile salt derivatives or dyes (e.g., indocyanine green). To expand the list of liver function markers, we synthesized fluorescent derivatives of cholic and chenodeoxycholic acid by conjugating small organic dyes to the bile acid side chain. These novel fluorescent probes were able to block substrate transport in a concentration-dependent manner of NTCP, OATP1B1, OATP1B3, OATP2B1, BSEP, and intestinal apical sodium-dependent bile salt transporter (ASBT). Whereas the fluorescent bile acid derivatives themselves were transported across the membrane by OATP1B1, OATP1B3, and OATP2B1, they were not transport substrates for NTCP, ASBT, BSEP, and multidrug resistance-related protein 2. Accordingly, these novel fluorescent bile acid probes can potentially be used as imaging agents to monitor the function of OATPs. SIGNIFICANCE STATEMENT: Synthetic modification of common bile acids by attachment of small organic fluorescent dyes to the bile acid side chain resulted in bright, fluorescent probes that interact with hepatic and intestinal organic anion [organic anion-transporting polypeptide (OATP) 1B1, OATP1B3, OATP2B1], bile salt uptake (Na+-taurocholate-cotransporting polypeptide, apical sodium-dependent bile salt transporter), and bile salt efflux (bile salt export pump, multidrug resistance-related protein 2) transporters. Although the fluorescent bile salt derivatives are taken up into cells via the OATPs, the efflux transporters do not transport any of them but one.

Role of membrane-embedded drug efflux ABC transporters in the cancer chemotherapy

One of the major problems being faced by researchers and clinicians in leukemic treatment is the development of multidrug resistance (MDR) which restrict the action of several tyrosine kinase inhibitors (TKIs). MDR is a major obstacle to the success of cancer chemotherapy. The mechanism of MDR involves active drug efflux transport of ABC superfamily of proteins such as Pglycoprotein (P-gp/ABCB1), multidrug resistance-associated protein 2 (MRP2/ABCC2), and breast cancer resistance protein (BCRP/ABCG2) that weaken the effectiveness of chemotherapeutics and negative impact on the future of anticancer therapy. In this review, the authors aim to provide an overview of various multidrug resistance (MDR) mechanisms observed in cancer cells as well as the various strategies developed to overcome these MDR. Extensive studies have been carried out since last several years to enhance the efficacy of chemotherapy by defeating these MDR mechanisms with the use of novel anticancer drugs that could escape from the efflux reaction, MDR modulators or chemosensitizers, multifunctional nanotechnology, and RNA interference (RNAi) therapy.

Fluorescent probes for the dual investigation of MRP2 and OATP1B1 function and drug interactions

Detoxification in hepatocytes is a strictly controlled process, in which the governed action of membrane transporters involved in the uptake and efflux of potentially dangerous molecules has a crucial role. Major transporters of hepatic clearance belong to the ABC (ATP Binding Cassette) and Solute Carrier (SLC) protein families. Organic anion-transporting polypeptide OATP1B1 (encoded by the SLCO1B1 gene) is exclusively expressed in the sinusoidal membrane of hepatocytes, where it mediates the cellular uptake of bile acids, bilirubin, and also that of various drugs. The removal of toxic molecules from hepatocytes to the bile is accomplished by several ABC transporters, including P-glycoprotein (ABCB1), MRP2 (ABCC2) and BCRP (ABCG2). Owing to their pharmacological relevance, monitoring drug interaction with OATP1B1/3 and ABC proteins is recommended. Our aim was to assess the interaction of recently identified fluorescent OATP substrates (various dyes used in cell viability assays, pyranine, Cascade Blue hydrazide (CB) and sulforhodamine 101 (SR101)) (Bakos et al., 2019; Patik et al., 2018) with MRP2 and ABCG2 in order to find fluorescent probes for the simultaneous characterization of both uptake and efflux processes. Transport by MRP2 and ABCG2 was investigated in inside-out membrane vesicles (IOVs) allowing a fast screen of the transport of membrane impermeable substrates by efflux transporters. Next, transcellular transport of shared OATP and ABC transporter substrate dyes was evaluated in MDCKII cells co-expressing OATP1B1 and MRP2 or ABCG2. Our results indicate that pyranine is a general substrate of OATP1B1, OATP1B3 and OATP2B1, and we find that the dye Live/Dead Violet and CB are good tools to investigate ABCG2 function in IOVs. Besides their suitability for MRP2 functional tests in the IOV setup, pyranine, CB and SR101 are the first dual probes that can be used to simultaneously measure OATP1B1 and MRP2 function in polarized cells by a fluorescent method.

Distribution of SiO2 nanoparticles in 3D liver microtissues

Introduction

Nanoparticles (NPs) are used in numerous products in technical fields and biomedicine; their potential adverse effects have to be considered in order to achieve safe applications. Besides their distribution in tissues, organs, and cellular localization, their impact and penetration during the process of tissue formation occurring in vivo during liver regeneration are critical steps for establishment of safe nanomaterials.

Materials and methods

In this study, 3D cell culture of human hepatocarcinoma cells (HepG2) was used to generate cellular spheroids, serving as in vitro liver microtissues. In order to determine their differential distribution and penetration depth in HepG2 spheroids, SiO₂ NPs were applied either during or after spheroid formation. The NP penetration was comprehensively studied using confocal laser scanning microscopy and scanning electron microscopy.

Results

Spheroids were exposed to 100 µg mL⁻¹ SiO₂ NPs either at the beginning of spheroid formation, or during or after formation of spheroids. Microscopy analyses revealed that NP penetration into the spheroid is limited. During and after spheroid formation, SiO₂ NPs penetrated about 20 µm into the spheroids, corresponding to about three cell layers. In contrast, because of the addition of SiO₂ NPs simultaneously to cell seeding, NP agglomerates were located also in the spheroid center. Application of SiO₂ NPs during the process of spheroid formation had no impact on final spheroid size.

Conclusion

Understanding the distribution of NPs in tissues is essential for biomedical applications. The obtained results indicate that NPs show only limited penetration into already formed tissue, which is probably caused by the alteration of the tissue structure and cell packing density during the process of spheroid formation.

Intravital Multiphoton Microscopy with Fluorescent Bile Salts in Rats as an In Vivo Biomarker for Hepatobiliary Transport Inhibition

The bile salt export pump (BSEP) is expressed at the canalicular domain of hepatocytes, where it mediates the elimination of monovalent bile salts into the bile. Inhibition of BSEP is considered a susceptibility factor for drug-induced liver injury that often goes undetected during nonclinical testing. Although in vitro assays exist for screening BSEP inhibition, a reliable and specific method for confirming Bsep inhibition in vivo would be a valuable follow up to a BSEP screening strategy, helping to put a translatable context around in vitro inhibition data, incorporating processes such as metabolism, protein binding, and other exposure properties that are lacking in most in vitro BSEP models. Here, we describe studies in which methods of quantitative intravital microscopy were used to identify dose-dependent effects of two known BSEP/Bsep inhibitors, 2-[4-[4-(butylcarbamoyl)-2-[(2,4-dichlorophenyl)sulfonylamino]phenoxy]-3-methoxyphenyl]acetic acid (AMG-009) and bosentan, on hepatocellular transport of the fluorescent bile salts cholylglycyl amidofluorescein and cholyl-lysyl-fluorescein in rats. Results of these studies demonstrate that the intravital microscopy approach is capable of detecting Bsep inhibition at drug doses well below those found to increase serum bile acid levels, and also indicate that basolateral efflux transporters play a significant role in preventing cytosolic accumulation of bile acids under conditions of Bsep inhibition in rats. Studies of this kind can both improve our understanding of exposures needed to inhibit Bsep in vivo and provide unique insights into drug effects in ways that can improve our ability interpret animal studies for the prediction of human drug hepatotoxicity.

In Vitro Assessment of the Effect of Antiepileptic Drugs on Expression and Function of ABC Transporters and Their Interactions with ABCC2

ABC transporters have a significant role in drug disposition and response and various studies have implicated their involvement in epilepsy pharmacoresistance. Since genetic studies till now are inconclusive, we thought of investigating the role of xenobiotics as transcriptional modulators of ABC transporters. Here, we investigated the effect of six antiepileptic drugs (AEDs) viz. phenytoin, carbamazepine, valproate, lamotrigine, topiramate and levetiracetam, on the expression and function of ABCB1, ABCC1, ABCC2 and ABCG2 in Caco2 and HepG2 cell lines through real time PCR, western blot and functional activity assays. Further, the interaction of AEDs with maximally induced ABCC2 was studied. Carbamazepine caused a significant induction in expression of ABCB1 and ABCC2 in HepG2 and Caco2 cells, both at the transcript and protein level, together with increased functional activity. Valproate caused a significant increase in the expression and functional activity of ABCB1 in HepG2 only. No significant effect of phenytoin, lamotrigine, topiramate and levetiracetam on the transporters under study was observed in either of the cell lines. We demonstrated the interaction of carbamazepine and valproate with ABCC2 with ATPase and 5,6-carboxyfluorescein inhibition assays. Thus, altered functionality of ABCB1 and ABCC2 can affect the disposition and bioavailability of administered drugs, interfering with AED therapy.

Functional defect of variants in the adenosine triphosphate-binding sites of ABCB4 and their rescue by the cystic fibrosis transmembrane conductance regulator potentiator, ivacaftor (VX-770)

ABCB4 (MDR3) is an adenosine triphosphate (ATP)-binding cassette (ABC) transporter expressed at the canalicular membrane of hepatocytes, where it mediates phosphatidylcholine (PC) secretion. Variations in the ABCB4 gene are responsible for several biliary diseases, including progressive familial intrahepatic cholestasis type 3 (PFIC3), a rare disease that can be lethal in the absence of liver transplantation. In this study, we investigated the effect and potential rescue of ABCB4 missense variations that reside in the highly conserved motifs of ABC transporters, involved in ATP binding. Five disease-causing variations in these motifs have been identified in ABCB4 (G535D, G536R, S1076C, S1176L, and G1178S), three of which are homologous to the gating mutations of cystic fibrosis transmembrane conductance regulator (CFTR or ABCC7; i.e., G551D, S1251N, and G1349D), that were previously shown to be function defective and corrected by ivacaftor (VX-770; Kalydeco), a clinically approved CFTR potentiator. Three-dimensional structural modeling predicted that all five ABCB4 variants would disrupt critical interactions in the binding of ATP and thereby impair ATP-induced nucleotide-binding domain dimerization and ABCB4 function. This prediction was confirmed by expression in cell models, which showed that the ABCB4 mutants were normally processed and targeted to the plasma membrane, whereas their PC secretion activity was dramatically decreased. As also hypothesized on the basis of molecular modeling, PC secretion activity of the mutants was rescued by the CFTR potentiator, ivacaftor (VX-770).

CONCLUSION

Disease-causing variations in the ATP-binding sites of ABCB4 cause defects in PC secretion, which can be rescued by ivacaftor. These results provide the first experimental evidence that ivacaftor is a potential therapy for selected patients who harbor mutations in the ATP-binding sites of ABCB4. (Hepatology 2017;65:560-570).

The trypanocidal benznidazole promotes adaptive response to oxidative injury: Involvement of the nuclear factor-erythroid 2-related factor-2 (Nrf2) and multidrug resistance associated protein 2 (MRP2)

Oxidative stress is a frequent cause underlying drug-induced hepatotoxicity. Benznidazole (BZL) is the only trypanocidal agent available for treatment of Chagas disease in endemic areas. Its use is associated with side effects, including increases in biomarkers of hepatotoxicity. However, BZL potential to cause oxidative stress has been poorly investigated. Here, we evaluated the effect of a pharmacologically relevant BZL concentration (200μM) at different time points on redox status and the counteracting mechanisms in the human hepatic cell line HepG2. BZL increased reactive oxygen species (ROS) after 1 and 3h of exposure, returning to normality at 24h. Additionally, BZL increased glutathione peroxidase activity at 12h and the oxidized glutathione/total glutathione (GSSG/GSSG+GSH) ratio that reached a peak at 24h. Thus, an enhanced detoxification of peroxide and GSSG formation could account for ROS normalization. GSSG/GSSG+GSH returned to control values at 48h. Expression of the multidrug resistance-associated protein 2 (MRP2) and GSSG efflux via MRP2 were induced by BZL at 24 and 48h, explaining normalization of GSSG/GSSG+GSH. BZL activated the nuclear erythroid 2-related factor 2 (Nrf2), already shown to modulate MRP2 expression in response to oxidative stress. Nrf2 participation was confirmed using Nrf2-knockout mice in which MRP2 mRNA expression was not affected by BZL. In summary, we demonstrated a ROS increase by BZL in HepG2 cells and a glutathione peroxidase- and MRP2 driven counteracting mechanism, being Nrf2 a key modulator of this response. Our results could explain hepatic alterations associated with BZL therapy.

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.

Regulation of biotransformation systems and ABC transporters by benznidazole in HepG2 cells: involvement of pregnane X-receptor

Background

Benznidazole (BZL) is the only antichagasic drug available in most endemic countries. Its effect on the expression and activity of drug-metabolizing and transporter proteins has not been studied yet.

Methodology/Principal Findings

Expression and activity of P-glycoprotein (P-gp), Multidrug resistance-associated protein 2 (MRP2), Cytochrome P450 3A4 (CYP3A4), and Glutathione S-transferase (GST) were evaluated in HepG2 cells after treatment with BZL. Expression was estimated by immunoblotting and real time PCR. P-gp and MRP2 activities were estimated using model substrates rhodamine 123 and dinitrophenyl-S-glutathione (DNP-SG), respectively. CYP3A4 and GST activities were evaluated through their abilities to convert proluciferin into luciferin and 1-chloro-2,4-dinitrobenzene into DNP-SG, respectively. BZL (200 µM) increased the expression (protein and mRNA) of P-gp, MRP2, CYP3A4, and GSTπ class. A concomitant enhancement of activity was observed for all these proteins, except for CYP3A4, which exhibited a decreased activity. To elucidate if pregnane X receptor (PXR) mediates BZL response, its expression was knocked down with a specific siRNA. In this condition, the effect of BZL on P-gp, MRP2, CYP3A4, and GSTπ protein up-regulation was completely abolished. Consistent with this, BZL was able to activate PXR, as detected by reporter gene assay. Additional studies, using transporter inhibitors and P-gp-knock down cells, demonstrated that P-gp is involved in BZL extrusion. Pre-treatment of HepG2 cells with BZL increased its own efflux, as a consequence of P-gp up-regulation.

Conclusions/Significance

Modifications in the activity of biotransformation and transport systems by BZL may alter the pharmacokinetics and efficiency of drugs that are substrates of these systems, including BZL itself.

Chagas disease is an endemic infection caused by Trypanosoma cruzi. Benznidazole (BZL) is the only antichagasic drug available in most endemic countries. The liver plays a major role in disposition of endogenous and exogenous compounds and their excretion is mainly mediated by transporter proteins (such as P-gp and MRP2) that act coordinately with biotransformation enzymes (such as CYP3A4 and GST). At present there is no information on whether BZL may modulate major biotransformation systems and transporters, with potential impact on its disposition or on disposition of other therapeutic agents co-administered with BZL. BZL (200 µM) altered the expression (protein and mRNA) and activity of P-gp, MRP2, CYP3A4, and GSTπ in HepG2 cells (a cell model that retains many biochemical, morphological and functional properties of the human hepatocytes), being the nuclear receptor PXR a key mediator. Additional studies demonstrated that P-gp is involved in BZL extrusion. Alterations in the pharmacokinetics and efficiency of drugs that are substrates of these systems, including BZL itself, would be expected.

In vivo imaging of hepatic excretory function in the rat by fluorescence microscopy

Applying intravital fluorescence microscopy, we assessed sinusoidal delivery and biliary clearance of two different polymethine dyes. DY635, a benzopyrylium-based hemocyanine dye with shorter excitation wavelength than indocyanine green (ICG), was validated for assessment of hepatic excretory function. Decrease of DY635 and ICG reflecting transcellular transport was 83 ± 4% (DY635) and 14 ± 2% (ICG; p < 0.05) over 35 minutes, respectively. In cholestasis, hepatobiliary excretion of DY635 was markedly impaired (control 3176 ± 148 pmol vs. cholestatic 1929 ± 179 pmol; p < 0.05). DY635 even enabled an analysis at high resolution suggesting 1.) hepatocyte uncoupling and 2.) failure of primarily the canalicular pole, allowing in vivo insights into molecular mechanisms of this critical facet of hepatobiliary function.

Matrigel-embedded 3D culture of Huh-7 cells as a hepatocyte-like polarized system to study hepatitis C virus cycle

Hepatocytes are highly polarized cells where intercellular junctions, including tight junctions (TJs), determine the polarity. Recently, the TJ-associated proteins claudin-1 and occludin have been implicated in hepatitis C virus (HCV) entry and spread. Nevertheless, cell line-based experimental systems that exhibit hepatocyte-like polarity and permit robust infection and virion production are not currently available. Thus, we sought to determine whether cell line-based, Matrigel-embedded cultures could be used to study hepatitis C virus (HCV) infection and virion production in a context of hepatocyte-like polarized cells. In contrast to standard bidimensional cultures, Matrigel-cultured Huh-7 cells adopted hepatocyte polarization features forming a continuous network of functional proto-bile canaliculi structures. These 3D cultures supported HCV infection by JFH-1 virus and produced infective viral particles which shifted towards lower densities with higher associated specific infectivity. In conclusion, our findings describe a novel use of Matrigel to study the entire HCV cycle in a more relevant context.

Sensitive and specific fluorescent probes for functional analysis of the three major types of mammalian ABC transporters

An underlying mechanism for multi drug resistance (MDR) is up-regulation of the transmembrane ATP-binding cassette (ABC) transporter proteins. ABC transporters also determine the general fate and effect of pharmaceutical agents in the body. The three major types of ABC transporters are MDR1 (P-gp, P-glycoprotein, ABCB1), MRP1/2 (ABCC1/2) and BCRP/MXR (ABCG2) proteins. Flow cytometry (FCM) allows determination of the functional expression levels of ABC transporters in live cells, but most dyes used as indicators (rhodamine 123, DiOC₂(3), calcein-AM) have limited applicability as they do not detect all three major types of ABC transporters. Dyes with broad coverage (such as doxorubicin, daunorubicin and mitoxantrone) lack sensitivity due to overall dimness and thus may yield a significant percentage of false negative results. We describe two novel fluorescent probes that are substrates for all three common types of ABC transporters and can serve as indicators of MDR in flow cytometry assays using live cells. The probes exhibit fast internalization, favorable uptake/efflux kinetics and high sensitivity of MDR detection, as established by multidrug resistance activity factor (MAF) values and Kolmogorov-Smirnov statistical analysis. Used in combination with general or specific inhibitors of ABC transporters, both dyes readily identify functional efflux and are capable of detecting small levels of efflux as well as defining the type of multidrug resistance. The assay can be applied to the screening of putative modulators of ABC transporters, facilitating rapid, reproducible, specific and relatively simple functional detection of ABC transporter activity, and ready implementation on widely available instruments.

Resveratrol inhibits genistein-induced multi-drug resistance protein 2 (MRP2) expression in HepG2 cells

The interactions between various dietary cancer chemopreventive phytochemicals in drug transporter functions are not well studied. In this study, the effects of genistein and resveratrol on the multidrug resistance protein 2 (MRP2) expression and the underlying molecular mechanisms were investigated using HepG2-C3 cells that are stably transfected with a construct containing human MRP2 promoter region conjugated with luciferase reporter gene. A 3-fold induction of MRP2 luciferase activity was observed after genistein (50μM) treatment to HepG2-C3 cells, but was diminished by the resveratrol (50μM) cotreatment. This observation was further validated by Western blot analysis and RT-PCR analysis as resveratrol also inhibited genistein-induced MRP2 protein synthesis and mRNA expression. Immunofluorescence study revealed that genistein-induced formation of MRP2 vacuoles was dramatically reduced by resveratrol. The binding affinity between retinoid X receptor alpha (RXRα) and MRP2 promoter was examined by DNA-protein pull-down assay. The results showed that resveratrol inhibited the genistein-induced binding of RXRα to the promoter sequence of MRP2 gene, and this mechanism could potentially contribute to the inhibition of genistein-induced MRP2 expression by resveratrol. Taken together, our present study suggests that naturally occurring phytochemicals can potentially interfere with each other's regulatory function on the cancer chemoprevention-related genes through a competitive mechanism.

Drug-induced liver injury: insights from genetic studies

Drug-induced liver injury (DILI) is an increasing health problem and a challenge for physicians, regulatory bodies and the pharmaceutical industry, not only because of its potential severity and elusive pathogenesis but also because it is often inaccurately diagnosed, commonly missed entirely and more often not reported. The general view is that idiosyncratic DILI, which is not predictable whether based on the pharmacology of the drug or on the dose administered, is determined by the presence in the recipient of variants in, or expression of, genes coding for key metabolic pathways and/or the immune response, and the interaction of these genetic variants with environmental variables. Furthermore, idiosyncratic DILI is an example of a complex-trait disease with two or more susceptibility loci, as reflected by the frequency of genetic variants in the population often being higher than the occurrence of significant liver injury. Polymorphisms of bioactivation/toxification pathways via the CYP450 enzymes (Phase I), detoxification reactions (Phase II) and excretion/transport (Phase III), together with immunological factors that might determine DILI are reviewed. Challenges such as gene-trait association studies and whole-genome studies, and future approaches to the study of DILI are explored. Better knowledge of the candidate genes involved could provide further insight for the prospective identification of susceptible patients at risk of developing drug-induced hepatotoxicity, development of new diagnostic tools and new treatment strategies with safer drugs.

Which in vitro models could be best used to study hepatocyte polarity?

The correct functioning of the liver is ensured by the setting and the maintenance of hepatocyte polarity. The complex polarity of the hepatocyte is characterized by the existence of several basolateral and apical poles per cell. Many in vitro models are available for studying hepatocyte polarity, but which are the more suitable? To answer this question, we aimed to identify criteria which determine the typical hepatocyte polarity. Therefore, we compiled a range of protein markers of membrane domains in rat hepatocytes and investigated their involvement in hepatocytic functions. Then, we focused on the relationship between hepatic functions and the cytoskeleton, Golgi apparatus and endoplasmic reticulum. Subsequently, we compared different cell lines expressing hepatocyte polarity. Finally, to demonstrate the usefulness of some of these lines, we presented new data on endoplasmic reticulum organization in relation to polarity.

Obstructive cholestasis induces TNF-alpha- and IL-1 -mediated periportal downregulation of Bsep and zonal regulation of Ntcp, Oatp1a4, and Oatp1b2

Inverse acinar regulation of Mrp2 and 3 represents an adaptive response to hepatocellular cholestatic injury. We studied whether obstructive cholestasis (bile duct ligation) and LPS treatment affect the zonal expression of Bsep (Abcb11), Mrp4 (Abcc4), Ntcp (Slc10a1), and Oatp isoforms (Slco1a1, Slco1a4, and slco1b2) in rat liver, as analyzed by semiquantitative immunofluorescence. Contribution of TNF-alpha and IL-1beta to transporter zonation in obstructive cholestasis was studied by cytokine inactivation. In normal liver Bsep, Mrp4, Ntcp, and Oatp1a1 were homogeneously distributed in the acinus, whereas Oatp1a4 and Oatp1b2 expression increased from zone 1 to 3. Glutamine synthetase-positive pericentral hepatocytes exhibited markedly lower Oatp1a4 expression than the remaining zone 3 hepatocytes. In cholestatic liver Bsep and Ntcp immunofluorescence in periportal hepatocytes significantly decreased to 66 +/- 4% (P < 0.01) and 67 +/- 7% (P < 0.05), whereas it was not altered in pericentral hepatocytes. Oatp1a4 was significantly induced in hepatocytes with a primarily low expression, i.e., in periportal hepatocytes and in glutamine synthetase-positive pericentral hepatocytes. Likewise, Oatp1b2 was upregulated in periportal hepatocytes. Mrp4 zonal induction was homogeneous. Inactivation of TNF-alpha and IL-1beta prevented periportal downregulation of Bsep. Recruitment of neutrophils and polymorphonuclear cells mainly occurred in the periportal zone. Likewise, IL-1beta induction was largely found periportally. No significant transporter zonation was seen following LPS treatment. In conclusion, zonal downregulation of Bsep in obstructive cholestasis is associated with portal inflammation and is mediated by TNF-alpha and IL-1beta. Periportal downregulation of Ntcp and induction of Oatp1a4 and Oatp1b2 may represent adaptive mechanisms to reduce cholestatic injury in hepatocytes with profound downregulation of Bsep and Mrp2.

Expression, localization, and inducibility by bile acids of hepatobiliary transporters in the new polarized rat hepatic cell lines, Can 3−1 and Can 10

Sinusoidal and apical transporters are responsible for the uptake and biliary elimination of many compounds by hepatocytes. Few in vitro models are however available for analyzing such functions. The expression and bile-acid inducibility of 13 transporters and two nuclear receptors were investigated in the new rat polarized lines, Can 3-1 and Can 10, and in their unpolarized parent, Fao. The relative abundance of mRNA, the protein level, and their localization were examined by real-time quantitative PCR, Western blotting, immunofluorescence, and confocal microscopy. Compared with rat liver, mRNA levels of Fao cells were: negligible for Bsep/Abcb11; lower for the uptake transporters Ntcp and Oatps; similar for SHP, FXR, and Bcrp/Abcg2; and higher (four-fold to 160-fold) for the efflux pumps Mdr1b/Abcb1b, Mdr2/Abcb4, Mrp1/Abcc1, Mrp2/Abcc2, Mrp3/Abcc3, Abcg5, and Abcg8. This profile was mostly maintained (and improved for Bsep) in Can 10. Some transporters were less well expressed in Can 3-1. In both lines, sinusoidal (Ntcp, Mrp3) and canalicular transporters (Mdr-P-glycoproteins detected with C219 antibody, Mrp2) were localized at their correct poles. Bile-acid effects on polarity and mRNA levels of transporters were analyzed after a 6-day treatment with 50 microM taurocholic, chenodeoxycholic (CDCA), or ursodeoxycholic acid (UDCA). No polarization of Fao cells was induced; Can 10 and Can 3-1 polarity was maintained. CDCA and UDCA induced marked enhancement of the volume of Can 10 bile canaliculi. CDCA upregulated Bsep, Mdr2, SHP, Mdr1b, and Oatp2/1a4 in Can 10 (two- to seven-fold) and in Fao cells. Thus, Can 10 constitutes an attractive polarized model for studying vectorial hepatobiliary transport of endogenous and xenobiotic cholephilic compounds.

Reversal of drug resistance of hepatocellular carcinoma cells by adenoviral delivery of anti-ABCC2 antisense constructs

Human cancers are characterized by a high degree of drug resistance. The multidrug resistance transporters MDR1-P-glycoprotein (ABCB1) and ABCC2 (MRP2) are expressed in a variety of human cancers, including hepatocellular carcinoma (HCC). The ABCC2 gene encodes a membrane protein involved in the ATP-dependent transport of conjugates of lipophilic substances. In this study we analyzed the effect of an ABCC2 antisense construct on the chemosensitization of HepG2 cells. Adenoviral vectors were constructed to allow an efficient expression of anti-ABCC2 antisense constructs. The effective target sequence comprised nucleotides 2543-2942 of the human ABCC2 cDNA. Adenoviral delivery of the ABCC2 antisense construct resulted in a reduced IC(50) for doxorubicin (12-fold), vincristine (50-fold), cisplatin (25-fold) and etoposide (VP-16) (25-fold). The adenoviral delivery of the ABCC2 antisense construct was so efficient that chemosensitization of HepG2 cells could even be demonstrated in mass cell cultures without a selection of transduced cells for single ABCC2 antisense-expressing HCC cell clones. After transfection of the ABCC2 antisense-expressing construct, HepG2 cells had significantly reduced ABCC2 mRNA and ABCC2 protein levels. Transduction of the ABCC2 antisense-expressing construct into HepG2 cells resulted in the accumulation of the high-affinity ABCC2 substrate Fluo-3. HepG2 tumors stably transfected with an anti-ABCC2 antisense construct regressed significantly in nude mice upon vincristine treatment. In addition, significant tumor regression was also observed when adenovirus-expressing anti-ABCC2 antisense construct was directly injected into HepG2 tumors in nude mice. Our study demonstrates the specific reversal of ABCC2-related drug resistance in adenovirus-transduced HepG2 cells and in HepG2 tumors in nude mice expressing this ABCC2 antisense construct.

Structure and function of the MRP2 (ABCC2) protein and its role in drug disposition

The multi-drug resistance protein 2 (MRP2; ABCC2) is an ATP-binding cassette transporter playing an important role in detoxification and chemoprotection by transporting a wide range of compounds, especially conjugates of lipophilic substances with glutathione, glucuronate and sulfate, which are collectively known as phase II products of biotransformation. In addition, MRP2 can also transport uncharged compounds in cotransport with glutathione, and thus can modulate the pharmacokinetics of many drugs. The other way around, its expression and activity are also altered by certain drugs and disease states. Unlike other members of the MRP/ABCC family, MRP2 is specifically expressed on the apical membrane domain of polarised cells as hepatocytes, renal proximal tubular cells, enterocytes and syncytiotrophoblasts of the placenta. Several naturally occurring mutations leading to the absence of functional MRP2 protein from the apical membrane have been described causing the human Dubin-Johnson syndrome associated with conjugated hyperbilirubinaemia. Experimental mutation studies have revealed critical amino acids for substrate binding in the MRP2 molecule. This review is, therefore, focused on the structure and function of MRP2, the substrates transported and the clinical relevance of MRP2.

Molecular pharmacokinetics of catharanthus (vinca) alkaloids

This review focuses on the published data regarding the molecular determinants (enzymes, transporters, orphan nuclear receptors) of Catharanthus (vinca) alkaloids pharmacokinetics in humans. The clinical impact of these determinants (drug disposition, drug-drug interactions) is also discussed.

NKIM-6, a new immortalized human brain capillary endothelial cell line with conserved endothelial characteristics

Primary human brain capillary endothelial cells (hBCECs) are available only in small quantities and have a short life span in vitro; this restricts their use as in vitro model for the blood-brain barrier (BBB). To overcome these limitations, we have established an immortalized hBCEC line (NKIM-6) by transfection with pLXSN16E6E7, which encodes the human papillomavirus type 16 E6 and E7 genes. The cell line exhibits an extended life span in vitro and retains its characteristic endothelial morphology, endothelial markers, and physiology. Likewise, as demonstrated by immunohistochemistry and reverse transcription/polymerase chain reaction (RT-PCR), NKIM-6 cells express BBB markers, and the lack of glial, neuronal, and epithelial markers confirms their endothelial origin. Moreover, with quantitative RT-PCR, we have been able to demonstrate that several ATP-binding cassette-transporters are expressed in NKIM-6 cells with a conserved expression order compared with primary hBCECs. Our results suggest that this cell line might be suitable as in vitro model for several aspects of the BBB.

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.

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.

Efficient trafficking of MDR1/P-glycoprotein to apical canalicular plasma membranes in HepG2 cells requires PKA-RIIalpha anchoring and glucosylceramide

In hepatocytes, cAMP/PKA activity stimulates the exocytic insertion of apical proteins and lipids and the biogenesis of bile canalicular plasma membranes. Here, we show that the displacement of PKA-RIIalpha from the Golgi apparatus severely delays the trafficking of the bile canalicular protein MDR1 (P-glycoprotein), but not that of MRP2 (cMOAT), DPP IV and 5'NT, to newly formed apical surfaces. In addition, the direct trafficking of de novo synthesized glycosphingolipid analogues from the Golgi apparatus to the apical surface is inhibited. Instead, newly synthesized glucosylceramide analogues are rerouted to the basolateral surface via a vesicular pathway, from where they are subsequently endocytosed and delivered to the apical surface via transcytosis. Treatment of HepG2 cells with the glucosylceramide synthase inhibitor PDMP delays the appearance of MDR1, but not MRP2, DPP IV, and 5'NT at newly formed apical surfaces, implicating glucosylceramide synthesis as an important parameter for the efficient Golgi-to-apical surface transport of MDR1. Neither PKA-RIIalpha displacement nor PDMP inhibited (cAMP-stimulated) apical plasma membrane biogenesis per se, suggesting that other cAMP effectors may play a role in canalicular development. Taken together, our data implicate the involvement of PKA-RIIalpha anchoring in the efficient direct apical targeting of distinct proteins and glycosphingolipids to newly formed apical plasma membrane domains and suggest that rerouting of Golgi-derived glycosphingolipids may underlie the delayed Golgi-to-apical surface transport of MDR1.

Predicting oral drug absorption and hepatobiliary clearance: Human intestinal and hepatic in vitro cell models

Membrane transport proteins control the uptake and efflux of many drugs in tissues including the intestine, liver and kidneys and thus play important roles in drug absorption, distribution and excretion. With the development of high throughput screening in an industrial environment, the importance of having appropriate in vitro systems to study drug transporter function, regulation, and interactions are invaluable. Cell lines are efficient tools in screening individual transport processes. In this review, we focus on the processes involved in the absorption and hepatobiliary clearance of drugs and the potential of cell lines to model such process, paying particular attention to the use of Caco-2 and HepG2 cells.

Chronic liver disease is triggered by taurine transporter knockout in the mouse

Taurine is an abundant organic osmolyte with antioxidant and immunomodulatory properties. Its role in the pathogenesis of chronic liver disease is unknown. The liver phenotype was studied in taurine transporter knockout (taut-/-) mice. Hepatic taurine levels were ~21, 15 and 6 mumol/g liver wet weight in adult wild-type, heterozygous (taut+/-) and homozygous (taut-/-) mice, respectively. Immunoelectronmicroscopy revealed an almost complete depletion of taurine in Kupffer and sinusoidal endothelial cells, but not in parenchymal cells of (taut-/-) mice. Compared with wild-type mice, (taut-/-) and (taut+/-) mice developed moderate unspecific hepatitis and liver fibrosis with increased frequency of neoplastic lesions beyond 1 year of age. Liver disease in (taut-/-) mice was characterized by hepatocyte apoptosis, activation of the CD95 system, elevated plasma TNF-alpha levels, hepatic stellate cell and oval cell proliferation, and severe mitochondrial abnormalities in liver parenchymal cells. Mitochondrial dysfunction was suggested by a significantly lower respiratory control ratio in isolated mitochondria from (taut-/-) mice. Taut knockout had no effect on taurine-conjugated bile acids in bile; however, the relative amount of cholate-conjugates acid was decreased at the expense of 7-keto-cholate-conjugates. In conclusion, taurine deficiency due to defective taurine transport triggers chronic liver disease, which may involve mitochondrial dysfunction.

Human multidrug resistance protein 8 (MRP8/ABCC11), an apical efflux pump for steroid sulfates, is an axonal protein of the CNS and peripheral nervous system

Dehydroepiandrosterone 3-sulfate and other neurosteroids are synthesized in the CNS and peripheral nervous system where they may modulate neuronal excitability by interacting with ligand-gated ion channels. For this modulatory activity, neurosteroids have to be locally released from either neurons or glial cells. We here identify the integral membrane protein ABCC11 (multidrug resistance protein 8) as an ATP-dependent efflux pump for steroid sulfates, including dehydroepiandrosterone 3-sulfate, and localize it to axons of the human CNS and peripheral nervous system. ABCC11 mRNA was detected in human brain by real-time polymerase chain reaction. Antibodies raised against ABCC11 served to detect the protein in brain by immunoblotting and immunofluorescence microscopy. ABCC11 was preferentially found in the white matter of the brain and co-localized with neurofilaments indicating that it is an axonal protein. Additionally, ABCC11 was localized to axons of the peripheral nervous system. For functional studies, ABCC11 was expressed in polarized Madin-Darby canine kidney cells where it was sorted to the apical membrane. This apical sorting is in accordance with the localization of ABCC11 to the axonal membrane of neurons. Inside-out plasma membrane vesicles containing recombinant ABCC11 mediated ATP-dependent transport of dehydroepiandrosterone 3-sulfate with a Km value of 21 microM. This transport function together with the localization of the ABCC11 protein in vicinity to GABAA receptors is consistent with a role of ABCC11 in dehydroepiandrosterone 3-sulfate release from neurons to sites of dehydroepiandrosterone 3-sulfate-mediated receptor modulation. Our findings may provide a basis for the characterization of mutations in the human ABCC11 gene and their linkage with neurological disorders.

Physiological, pharmacological and clinical features of the multidrug resistance protein 2

Multidrug resistance protein 2 (MRP2, ABCC2) is a drug efflux pump belonging to the ATP-binding cassette (ABC) transporter superfamily. MRP2 is present predominantly at the biliary pole of hepatocytes and is also expressed in the kidney and intestine. It plays a major role in hepato-biliary elimination of many structurally diverse xenobiotics, including organic anions and drug conjugates, and therefore most likely contributes to pharmacokinetic parameters of these compounds. MRP2 also handles endogenous molecules such as bilirubin, and its overexpression has been shown to confer a multidrug resistance phenotype to tumoral cells. MRP2 expression can be regulated by endogenous substances such as inflammatory cytokines and biliary acids. The MRP2 levels and activity can also be affected by a large panel of xenobiotics, including chemopreventive agents and ligands of the pregnane X receptor, which may be a potential source of drug-drug interactions and drug adverse effects. MRP2 appears therefore as one of the major drug efflux pumps of the organism, whose functional and regulatory features are important to consider, notably for drug disposition.

Inhibition of human organic anion transporting polypeptide OATP 1B1 as a mechanism of drug-induced hyperbilirubinemia

OATP1B1 (a.k.a. OATP-C, OATP2, LST-1, or SLC21A6) is a liver-specific organic anion uptake transporter and has been shown to be a higher affinity bilirubin uptake transporter than OATP1B3. Using human embryonic kidney (HEK 293) cells stably transfected with OATP1B1, we have studied the effects of indinavir, saquinavir, cyclosporin A, and rifamycin SV on human OATP1B1 transport function. These drugs are potent inhibitors of OATP1B1 transport activity in vitro. We further provide evidence that the calculated fraction of OATP1B1 inhibited at the clinical exposure level correlated very well with the observed hyperbilirubinemia outcome for these drugs in humans. Our data support the hypothesis that inhibition of OATP1B1 is an important mechanism for drug-induced unconjugated hyperbilirubinemia. Inhibition of OATPs may be an important mechanism in drug-drug and drug-endogenous substance interactions.

Endogenous thiols and MRP transporters contribute to Hg2+ efflux in HgCl2-treated tubular MDCK cells

Tubular epithelium represents the primary target of mercuric ions (Hg(2+)) nephrotoxicity. Although widely investigated, the mechanisms of Hg(2+) cell uptake, accumulation and excretion all along the nephron remain largely unknown. In the present study, native distal tubular-derived Madin-Darby canine kidney (MDCK) cells exposed to subcytotoxic (micromolar) HgCl(2) concentrations were used for investigating specific mechanisms involved in the tubular response to toxic metals. Inductively coupled plasma-mass spectrometry (ICP-MS) was firstly used for assessing HgCl(2) solubility and then for quantifying Hg(2+) cell uptake. Exposed to HgCl(2), MDCK cells showed a rapid, but transient, Hg(2+) accumulation. The metallic cation was found to affect cell density and morphology, being these effects related to the dose and the time of exposure. In parallel, an Hg(2+)-induced up-regulation of endogenous MRP1 and MRP2 export pumps, a significant HgCl(2)-dependent induction of protective cellular thiols and an increase in the glutathione conjugates metabolism were also observed. The functional suppression of MRPs activity, obtained by MK-571 treatment, increased the Hg(2+) cell content and the sensitivity of MDCK cells to HgCl(2). Our results demonstrate that, in MDCK cells, inorganic Hg(2+) promotes the activation of specific detoxifying pathways that may, at least partly, depend on the activity of MRP transporters.

Molecular and cellular physiology of renal organic cation and anion transport

Organic cations and anions (OCs and OAs, respectively) constitute an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. Renal secretion of these compounds, which occurs principally along the proximal portion of the nephron, plays a critical role in regulating their plasma concentrations and in clearing the body of potentially toxic xenobiotics agents. The transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. It is increasingly apparent that basolateral and luminal OC and OA transport reflects the concerted activity of a suite of separate transport processes 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 OCs and OAs. This review examines recent work on this issue, with particular emphasis on attempts to integrate information concerning the activity of cloned transporters in heterologous expression systems to that observed in studies of physiologically intact renal systems.

Enhanced expression of basolateral multidrug resistance protein isoforms Mrp3 and Mrp5 in rat liver by LPS

Lipopolysaccharide (LPS) induces hepatocellular down-regulation and endocytic retrieval of multidrug resistance protein 2 (Mrp2, Abcc2). Basolateral Mrp isoforms may compensate for the intracellular metabolic changes in cholestasis. Therefore, the effect of LPS on the zonal localization of Mrp2 and Mrp3 and the expression of Mrp3, Mrp4, Mrp5, and Mrp6 mRNA were investigated in rat liver. In normal rat liver Mrp3 was found in pericentral hepatocytes also expressing glutamine synthetase. In LPS-treated rat liver the decrease in Mrp2 protein was most pronounced in pericentral hepatocytes, with only minor down-regulation in periportal hepatocytes. Conversely, induction of Mrp3 was found in pericentral hepatocytes with a low expression of Mrp2. Furthermore, we found a strong induction of Mrp5 mRNA. Likewise, Mrp6 mRNA was up-regulated, however Mrp6 protein expression was not significantly altered. It is concluded that Mrp3 is inversely regulated to Mrp2 in a zonal pattern and may compensate for the LPS-induced loss of Mrp2 in the perivenous area. Induction of pericentral Mrp3 and up-regulation of Mrp5 mRNA may play an important role in the hepatocellular clearance of cholephilic substances and cyclic nucleotides accumulating after LPS treatment.

Phase II trial of carboplatin and infusional cyclosporine in platinum-resistant recurrent ovarian cancer

PURPOSE

To determine the response rate to 26-h continuous infusion cyclosporine A (CSA) combined with a fixed dose level of carboplatin (CBDCA) in patients with recurrent ovarian cancer, and to determine the effect of CSA on the pharmacokinetics of CBDCA.

EXPERIMENTAL DESIGN

To examine the effect of duration of CSA exposure on reversal of CBDCA resistance, clonogenic assays were performed in vitro in platinum-resistant A2780 cells. CBDCA (AUC 4) with CSA repeated every 3 weeks was then administered to patients on this phase II study. Pharmacokinetics of CSA and CBDCA were determined in a subset of patients.

RESULTS

Preincubation of platinum-resistant A2780 cells with CSA reversed CBDCA resistance in a concentration-dependent and time-dependent manner. A group of 23 patients received 58 courses of CBDCA/CSA therapy. One partial response was observed. Eight patients achieved disease stabilization. Toxicity was similar to that observed in our previous phase I study and consisted of myelosuppression, nausea, vomiting, and headache. The mean +/- SD end-of-infusion CSA level (HPLC assay) was 1253 +/- 400 microg/ml. The pharmacokinetic studies suggest that CSA does not increase CBDCA AUC.

CONCLUSIONS

Steady-state levels of >1 microg/ml CSA (HPLC assay) are achievable in vivo. Modest partial reversal of platinum resistance (in one patient with an objective response and in eight patients with stable disease noted) is achievable in vivo in patients pretreated with CSA. This phenomenon is not explained by alterations in CBDCA pharmacokinetics.

Trafficking of the bile salt export pump from the Golgi to the canalicular membrane is regulated by the p38 MAP kinase

BACKGROUND & AIMS

Bile secretion depends on the delivery and removal of transporter proteins to and from the canalicular membrane. Trafficking of the bile salt export pump (BSEP) to the canalicular membrane was investigated in HepG2 cells and rat hepatocytes.

METHODS

Subcellular localization of BSEP was determined by confocal laser scanning microscopy using different BSEP antibodies.

RESULTS

Ten percent of untreated HepG2 cells developed pseudocanaliculi, but only 15% of these pseudocanaliculi contained BSEP, which largely colocalized with the Golgi marker GM130. Cycloheximide, an inhibitor of protein translation, induced a microtubule- and p38(MAP) kinase-dependent decrease of Golgi-associated BSEP, accompanied by a more than 2-fold increase in BSEP-positive pseudocanaliculi. Also, tauroursodeoxycholate (TUDC), which activates p38(MAP) kinase (p38(MAPK), increased BSEP-positive pseudocanaliculi by more than 50% in rat sodium taurocholate cotransporting peptide (Ntcp)-transfected but not in untransfected HepG2 cells. The TUDC-dependent increase was sensitive to inhibitors of p38(MAPK) and microtubules and involved Ca(2+)-independent protein kinase C isoforms as suggested by its sensitivity to Gö6850 but insensitivity to Gö6976. In isolated rat hepatocytes with intact bile secretion, no colocalization of rat isoforms of the bile salt export pump (Bsep) and Golgi was found, but colocalization occurred after inhibition of p38(MAPK) and PKC, suggesting that Bsep trafficking to the canalicular membrane depends on the basal activity of these kinases in polarized cells.

CONCLUSIONS

p38(MAPK) regulates BSEP trafficking from the Golgi to the canalicular membrane, and the Golgi may serve as a BSEP pool in certain forms of cholestasis or when p38(MAPK) activity is inhibited. Activation of p38(MAPK) by TUDC can recruit Golgi-associated BSEP in line with its choleretic action.

The ABCs of drug transport in intestine and liver: efflux proteins limiting drug absorption and bioavailability

Many orally administered drugs must overcome several barriers before reaching their target site. The first major obstacle to cross is the intestinal epithelium. Although lipophilic compounds may readily diffuse across the apical plasma membrane, their subsequent passage across the basolateral membrane and into blood is by no means guaranteed. Efflux proteins located at the apical membrane, which include P-glycoprotein (Pgp; MDR1) and MRP2, may drive compounds from inside the cell back into the intestinal lumen, preventing their absorption into blood. Drugs may also be modified by intracellular phase I and phase II metabolising enzymes. This process may not only render the drug ineffective, but it may also produce metabolites that are themselves substrates for Pgp and/or MRP2. Drugs that reach the blood are then passed to the liver, where they are subject to further metabolism and biliary excretion, often by a similar system of ATP-binding cassette (ABC) transporters and enzymes to that present in the intestine. Thus a synergistic relationship exists between intestinal drug metabolising enzymes and apical efflux transporters, a partnership that proves to be a critical determinant of oral bioavailability. The effectiveness of this system is optimised through dynamic regulation of transporter and enzyme expression; tissues have a remarkable capacity to regulate the amounts of protein both at transcriptional and post-transcriptional levels in order to maintain homeostasis. This review addresses the progress to date on what is known about the role and regulation of drug efflux mechanisms in the intestine and liver.

Cotransport of reduced glutathione with bile salts by MRP4 (ABCC4) localized to the basolateral hepatocyte membrane

The liver is the major source of reduced glutathione (GSH) in blood plasma. The transport protein mediating the efflux of GSH across the basolateral membrane of human hepatocytes has not been identified so far. In this study we have localized the multidrug resistance protein 4 (MRP4; ABCC4) to the basolateral membrane of human, rat, and mouse hepatocytes and human hepatoma HepG2 cells. Recombinant human MRP4, expressed in V79 hamster fibroblasts and studied in membrane vesicles, mediated ATP-dependent cotransport of GSH or S-methyl-glutathione together with cholyltaurine, cholylglycine, or cholate. Several monoanionic bile salts and the quinoline derivative MK571 were potent inhibitors of this unidirectional transport. The K(m) values were 2.7 mmol/L for GSH and 1.2 mmol/L for the nonreducing S-methyl-glutathione in the presence of 5 micromol/L cholyltaurine, and 3.8 micromol/L for cholyltaurine in the presence of 5 mmol/L S-methyl-glutathione. Transport of bile salts by MRP4 was negligible in the absence of ATP or without S-methyl-glutathione. These findings identify a novel pathway for the efflux of GSH across the basolateral hepatocyte membrane into blood where it may serve as an antioxidant and as a source of cysteine for other organs. Moreover, MRP4-mediated bile salt transport across the basolateral membrane may function as an overflow pathway during impaired bile salt secretion across the canalicular membrane into bile. In conclusion, MRP4 can mediate the efflux of GSH from hepatocytes into blood by cotransport with monoanionic bile salts.

A common Dubin-Johnson syndrome mutation impairs protein maturation and transport activity of MRP2 (ABCC2)

Absence of a functional multidrug resistance protein 2 (MRP2; symbol ABCC2) from the hepatocyte canalicular membrane is the molecular basis of Dubin- Johnson syndrome, an inherited disorder associated with conjugated hyperbilirubinemia in humans. In this work, we analyzed a relatively frequent Dubin-Johnson syndrome mutation that leads to an exchange of two hydrophobic amino acids, isoleucine 1173 to phenylalanine (MRP2I1173F), in a predicted extracellular loop of MRP2. HEK-293 cells stably transfected with MRP2I1173F cDNA synthesized a mutant protein that was mainly core-glycosylated, predominantly retained in the endoplasmic reticulum, and degraded by proteasomes. MRP2I1173F did not mediate ATP-dependent transport of leukotriene C(4) (LTC(4)) into vesicles from plasma membrane and endoplasmic reticulum preparations while normal MRP2 was functionally active. Human HepG2 cells were used to study localization of MRP2I1173F in a polarized cell system. Quantitative analysis showed that GFP-tagged MRP2I1173F was localized to the apical membrane in only 5% of transfected, polarized HepG2 cells compared with 80% for normal MRP2-GFP. Impaired protein maturation followed by proteasomal degradation of inactive MRP2I1173F explain the deficient hepatobiliary elimination observed in this group of Dubin-Johnson syndrome patients.

Increased renal expression of bilirubin glucuronide transporters in a rat model of obstructive jaundice

Regulation of bilirubin glucuronide transporters during hyperbilirubinemia in hepatic and extrahepatic tissues is not completely clear. In the present study, we evaluated the regulation of the bilirubin glucuronide transporters, multidrug resistance-associated proteins (MRP)2 and 3, in rats with obstructive jaundice. Bile duct ligation (BDL) or sham operation was performed in Wistar rats. Liver and kidneys were removed 1, 3, and 5 days after BDL (n = 4, in each group). Serum and urine were collected to measure bilirubin levels just before animal killing. MRP2 And MRP3 mRNA expressions were determined by real-time RT-PCR. Protein expression of MRP2 and MRP3 was determined by Western blotting. Renal MRP2 function was evaluated by para-aminohippurate (PAH) clearance. The effect of conjugated bilirubin, unconjugated bilirubin, human bile, and sulfate-conjugated bile acid on MRP2 gene expression was also evaluated in renal and hepatocyte cell lines. Serum bilirubin and urinary bilirubin excretion increased significantly after BDL. In the liver, the mRNA expression of MRP2 decreased 59, 86, and 82%, and its protein expression decreased 25, 74, and 93% compared with sham-operated animals after 24, 72, and 120 h of BDL, respectively. In contrast, the liver expression of MRP3 mRNA increased 138, 2,137, and 3,295%, and its protein expression increased 560, 634, and 612% compared with sham-operated animals after 24, 72, and 120 h of BDL, respectively. On the other hand, in the kidneys, the mRNA expression of MRP2 increased 162, 73, and 21%, and its protein expression increased 387, 558, and 472% compared with sham-operated animals after 24, 72, and 120 h of BDL, respectively. PAH clearance was significantly increased after BDL. The mRNA expression of MRP2 increased in renal proximal tubular epithelial cells after treatment with conjugated bilirubin, sulfate-conjugated bile acid or human bile. Upregulation of MRP2 in the kidneys and MRP3 in the liver may be a compensatory mechanism to improve bilirubin clearance during obstructive jaundice.

Low-level self-tolerance to arsenite in human HepG2 cells is associated with a depressed induction of micronuclei

Arsenic (As) is one of the most important global environmental toxicants. There is evidence that humans may develop tolerance to As's toxicity. For instance, it is known that uptake of small amounts of As leads to an acquisition of elevated resistance to the element's acute toxicity. Moreover, it was suggested that As-exposed native Andean females of Atacameño ethnicity may have acquired resistance to skin cancer. It is not known how such adaptation could be mechanistically conferred. In this context, the biological selection and cloning of human cells tolerant to As provides a valuable approach to investigate this question. By the means of a 12 weeks culture with increasing doses of As, three different As-resistant clones of the human hepatoma cell line HepG2 were selected. These three clones were similarly and roughly two-fold resistant to the acute toxicity of arsenite (50% reduction of neutral red (NR) uptake at 65 microM versus 115 microM; HepG2 control versus clones HepG2 K1, HepG2 K11 and HepG2 K14, respectively). Moreover, in the cytokinesis-block micronucleus test, these clones showed a significantly reduced induction of micronuclei (MNi) indicating elevated resistance to As genotoxicity as well (e.g. mean MNi rates at a concentration of 25 microM arsenite: 28.5 (control) versus 21.6 (HepG2 K1), 18 (HepG2 K11), and 16 (HepG2 K14), respectively, each P<0.05). The tolerance was neither associated with mRNA induction of putatively As-extruding membrane transporters multidrug resistance-associated protein 1 (MRP1), 2, or 3 nor to mRNA induction of the ubiquitously expressed mammalian ABC half-transporter UMAT (ABCB6). Changes in the metabolic methylation of As could not be detected. There were no differences in the cellular levels of GSH when comparing the clones and the parental line. Taken together the data showed that low-level tolerance to As-mediated cytotoxicity in human HepG2 cells was associated with enhanced resistance to As-induced DNA damage as well.

Influence of t-butylhydroquinone and beta-naphthoflavone on formation and transport of 4-methylumbelliferone glucuronide in Caco-2/TC-7 cell monolayers

Human Caco-2 cells have been established as a model system for intestinal biotransformation and permeability. When grown on Transwell polycarbonate filters they develop morphologic and biochemical characteristics of enterocytes with well separated apical and basolateral surfaces. In addition, Caco-2/TC-7 cells have proven to be useful to study regulation of human UDP-glucuronosyltransferases (UGTs) by Ah receptor agonists and antioxidant-type inducers such as beta-naphthoflavone (BNF) and t-butylhydroquinone (TBHQ). In the present investigation, formation and transport of 4-methylumbelliferone glucuronide was studied in intact Caco-2 cell monolayers. The following results were obtained: when loaded with 50-200 microM MUF either apically or basolaterally, MUF-GA was the major metabolite which was mostly released (80%) at the basolateral surface, probably via the multidrug resistance protein isoform MRP3; MUF sulfate formation was low (5 +/-2%). Pretreatment of cells with 80 microM TBHQ or 50 microM BNF for 72 hr before addition of 100 microM MUF enhanced basolateral secretion of MUF-GA 1.4- and 1.7-fold, respectively. However, at >200 microM MUF, MUF-GA secretion and induction was smaller, probably due to inhibition of intracellular UGT activity. MRP3 protein was localized to the basolateral surface of Caco-2 cells but was not induced by TBHQ or BNF. The results suggest that MUF-GA is mostly secreted basolaterally in Caco-2 cell monolayers. Treatment with TBHQ or BNF significantly enhanced MUF-GA formation in the intact cell.

Activity and expression of the multidrug resistance proteins P-glycoprotein, MRP1, MRP2, MRP3 and MRP5 in de novo and relapsed acute myeloid leukemia

The multidrug resistance proteins (MRPs) MRP1, MRP2, MRP3, MRP5 and P-glycoprotein (P-gp) act in concert with each other to give a net resultant pump function in acute myeloid leukemia (AML). The aim of the present study was to analyze the activity of these proteins, which might be upregulated at relapse as compared with de novo AML due to clonal selection. The mRNA expression and activity of P-gp and the MRPs were determined with RT-PCR and flow cytometry, in conjunction with phenotype, as measured with the monoclonal antibodies CD34, CD38 and CD33, in 30 paired samples of de novo and relapsed AML. P-gp and MRP activity varied strongly between the cases (rhodamine 123 efflux-blocking by PSC833: 5.4+/-7.7, and carboxyfluorescein efflux-blocking by MK-571: 4.3+/-6.7, n = 60). P-gp and MRP activity were increased in 23% and 40% of the relapse samples, and decreased in 30% and 20% of the relapse samples, respectively (as defined by a difference of >2 x standard deviation of the assays). Up- or downregulation of mRNA expression was observed for MDR1 (40%), MRP1 (20%), MRP2 (15%), MRP3 (30%), and MRP5 (5%). Phenotyping demonstrated a more mature phenotype in 23% of the relapsed AML cases, and a more immature phenotype in 23% of the relapses, which was independent of the karyotypic changes that were observed in 50% of the studied cases. P-gp and MRP activity correlated with the phenotypic changes, with higher P-gp and MRP activities in less mature cells (r = -0.66, P < 0.001 and r = -0.31, P = 0.02, n = 58). In conclusion, this study shows that P-gp and MRP activity are not consistently upregulated in relapsed AML. However, P-gp and MRP activities were correlated with the maturation stage as defined by immune phenotype, which was observed to be different in 46% of the relapses.

Vesicular and nonvesicular transport of phosphatidylcholine in polarized HepG2 cells

We have investigated the transport and canalicular enrichment of fluorescent phosphatidylcholine (PC) in HepG2 cells using the fluorescent analogs of PC C6-NBD-PC and beta-BODIPY-PC. Fluorescent PC was efficiently transported to the biliary canaliculus (BC) and became enriched on the lumenal side of the canalicular membrane as shown for C6-NBD-PC. Some fluorescent PC was transported in vesicles to a subapical compartment (SAC) or apical recycling compartment (ARC) in polarized HepG2 cells as shown by colocalization with fluorescent sphingomyelin (C6-NBD-SM) and fluorescent transferrin, respectively. Extensive trafficking of vesicles containing fluorescent PC between the basolateral domain, the SAC/ARC and the BC as well as endocytosis of PC analogs from the canalicular membrane were found. Evidence for nonvesicular transport included enrichment of the PC-analog beta-BODIPY-PC in the BC (t1/2 = 3.54 min) prior to its accumulation in the SAC/ARC (t1/2 = 18.5 min) at 37 degrees C. Transport of fluorescent PC to the canalicular membrane also continued after disruption of the actin or microtubule cytoskeleton and at 2 degrees C. These results indicate that: (i) a nonvesicular transport pathway significantly contributes to the canalicular enrichment of PC in hepatocytic cells, and (ii) vesicular transport of fluorescent PC occurs from both membrane domains via the SAC/ARC.

Impaired protein maturation of the conjugate export pump multidrug resistance protein 2 as a consequence of a deletion mutation in Dubin-Johnson syndrome

The Dubin-Johnson syndrome is an inherited disorder characterized by conjugated hyperbilirubinemia. The deficient hepatobiliary transport of anionic conjugates is caused by the absence of a functional multidrug-resistance protein 2 (MRP2, symbol ABCC2) from the apical (canalicular) membrane of hepatocytes. Mechanisms underlying this deficiency may include rapid degradation of mutated MRP2 messenger RNA (mRNA) or impaired MRP2 protein maturation and trafficking. We investigated the consequences of the mutation MRP2Delta(R,M), which leads to the loss of 2 amino acids from the second ATP-binding domain of MRP2. The MRP2Delta(R,M) mutation is associated with the absence of the MRP2 glycoprotein from the apical membrane of hepatocytes. Transfection of mutated MRP2 complementary DNA (cDNA) led to an MRP2Delta(R,M) protein that was only core glycosylated, sensitive to endoglycosidase H digestion, and located in the endoplasmic reticulum (ER) of transfected HEK293 and HepG2 cells. This indicated that deletion of Arg1392 and Met1393 leads to impaired maturation and trafficking of the protein from the ER to the Golgi complex. Inhibition of proteasome function resulted in a paranuclear accumulation of the MRP2Delta(R,M) protein, suggesting that proteasomes are involved in the degradation of the mutant protein. This is the first mutation in Dubin-Johnson syndrome shown to cause deficient MRP2 maturation and impaired sorting of this glycoprotein to the apical membrane.

Detection of MRP1 mRNA in human tumors and tumor cell lines by in situ RT-PCR

The detection of the multridrug resistance-associated proteins is becoming increasingly important in assessing tumor sensitivity to treatment. In this work we describe a new, rapid, sensitive, and robust method for the detection of MRP1 expression based on direct RT-in situ-PCR technology and fluorochrome-modified (dCTP(Cy3)) nucleotides. MRP1 expression was found in both placenta (BeWo) and liver (Hep G2)-derived tumor cell line as well as in small cell lung carcinoma. In liver-derived cells, MRP1 expression was detected by RT-in situ-PCR but not by in situ hybridization, suggesting a higher sensitivity of in situ amplification for the low level of expression in Hep G2 cells. RT solution PCR confirmed the presence of MRP1 in BeWo and Hep G2 cells, although the level of the gene expression was lower in liver cells. This method represents a viable alternative to conventional immunohistochemistry, and may be useful in the evaluation of MRP1 expression in different tissue or cell lines.