Glutathione S-conjugate transport in hepatocytes entering the cell cycle is preserved by a switch in expression from the apical MRP2 to the basolateral MRP1 transporting protein

Emerging potential of niosomes in ocular delivery

INTRODUCTION

Niosomes have recently grabbed attention as one of the best tools for various site-specific drug delivery systems, including ophthalmic drug delivery. Surfactants (nonionic; tweens and spans) of different HLB values and cholesterol are the fundamental components for these formulations. It is an alternative controlled ocular drug delivery system to liposomes to overcome the problems associated with sterilization, large-scale production, and stability. It also enhances the adhesion or retention ability of drug at the ocular site. Hydrophilic or lipophilic or amphoteric drugs can be easily encapsulated in niosomes. Besides, niosomes are a leading vesicular system compatible with most of the drugs for site-specific delivery.

AREAS COVERED

This article reveals challenges and barriers for ocular drug delivery, various transporters and receptors present in the ocular region for the transportation of therapeutics as well as nutrients, and various method of preparations, loading methods and application potential of niosomes in ocular drug delivery.

EXPERT OPINION

Niosomes, a vesicular system offers numerous advantages and applicability because of its good stability, non-immunogenicity, permeation potential, and controlled release ability etc. This drug delivery system has been efficiently used in the treatment of many ocular diseases.

Ocular Drug Delivery Barriers-Role of Nanocarriers in the Treatment of Anterior Segment Ocular Diseases

Ocular drug delivery is challenging due to the presence of anatomical and physiological barriers. These barriers can affect drug entry into the eye following multiple routes of administration (e.g., topical, systemic, and injectable). Topical administration in the form of eye drops is preferred for treating anterior segment diseases, as it is convenient and provides local delivery of drugs. Major concerns with topical delivery include poor drug absorption and low bioavailability. To improve the bioavailability of topically administered drugs, novel drug delivery systems are being investigated. Nanocarrier delivery systems demonstrate enhanced drug permeation and prolonged drug release. This review provides an overview of ocular barriers to anterior segment delivery, along with ways to overcome these barriers using nanocarrier systems. The disposition of nanocarriers following topical administration, their safety, toxicity and clinical trials involving nanocarrier systems are also discussed.

Bioavailability of the imidazole antifungal agent clotrimazole and its effects on key biotransformation genes in the common carp (Cyprinus carpio)

Clotrimazole (CTZ) is a persistent imidazole antifungal agent which is frequently detected in the aquatic environment and predicted to bio-concentrate in fish. Common carp (Cyprinus carpio) were exposed to mean measured concentrations of either 1.02 or 14.63μgl(-1) CTZ for 4 and 10 days, followed by a depuration period of 4 days in a further group of animals. Following each exposure regimen, plasma and liver CTZ concentrations were measured. Mean measured plasma concentrations of CTZ in animals exposed to the lower concentration of CTZ were 30 and 44μgl(-1) on days 4 and 10, respectively, and in the higher concentration were 318 and 336μgl(-1). Mean measured liver levels in the same animals were 514, 1725, 2111 and 7017μgl(-1) suggesting progressive hepatic accumulation. Measurement of CTZ in plasma after depuration suggested efficient elimination within 4 days, but appreciable levels of CTZ remained in the liver after depuration suggesting a degree of persistence in this tissue. In addition we measured responses of a number of key hepatic detoxification gene targets in the liver associated with the transcription factor pregnane X receptor (PXR); namely cyp450s 2k and 3a, glutathione-S-transferases a and p (gsta and p), and drug transporters multidrug resistance protein1 (mdr1), and MDR-related protein2 (mrp2). CTZ is a potent ligand of the PXR in humans and there is some evidence of PXR activation following exposure to CTZ in fish. The highest concentration of CTZ was adopted to explore the potential for alterations to detoxification gene expression in fish at a pharmacologically relevant dose level, and the lower concentration is within the range reported in effluents from waste water treatment works (WWTW). The genes for all biotransformation enzymes were up-regulated after exposure to the higher concentration of CTZ for 10 days, and alterations in expression occurred for the drug transporter genes mdr1 and mrp2 following exposure to the lower concentration of 1.02μgl(-1) CTZ (mean measured concentration). These data support the potential for CTZ to induce alterations in biotransformation and drug transporter genes associated with PXR in fish at concentrations measured in some WWTW effluents.

Hepatic 3D cultures but not 2D cultures preserve specific transporter activity for acetaminophen-induced hepatotoxicity

Primary human hepatocytes (PHH) are the "gold standard" for in vitro toxicity tests. However, 2D PHH cultures have limitations that are due to a time-dependent dedifferentiation process visible by morphological changes closely connected to a decline of albumin production and CYP450 activity. The 3D in vitro culture corresponds to in vivo-like tissue architecture, which preserves functional characteristics of hepatocytes, and therefore can at least partially overcome the restrictions of 2D cultures. Consequently, several drug toxicities observed in vivo cannot be reproduced in 2D in vitro models, for example, the toxic effects of acetaminophen. The objective of this study was to identify molecular differences between 2D and 3D cultivation which explain the observed toxicity response. Our data demonstrated an increase in cell death after treatment with acetaminophen in 3D, but not in 2D cultures. Additionally, an acetaminophen concentration-dependent increase in the CYP2E1 expression level in 3D cultures was detected. However, during the treatment with 10 mM acetaminophen, the expression level of SOD gradually decreased in 3D cultures and was undetectable after 24 h. In line with these findings, we observed higher import/export rates in the membrane transport protein, multidrug resistance-associated protein-1, which is known to be specific for acetaminophen transport. The presented data demonstrate that PHH cultured in 3D preserve certain metabolic functions. Therefore, they have closer resemblance to the in vivo situation than PHH in 2D cultures. In consequence, 3D cultures will allow for a more accurate hepatotoxicity prediction in in vitro models in the future.

Inhibition of the MRP1-mediated transport of the menadione-glutathione conjugate (thiodione) in HeLa cells as studied by SECM

Oxidative stress induced in live HeLa cells by menadione (2-methyl-1,4-napthaquinone) was studied in real time by scanning electrochemical microscopy (SECM). The hydrophobic molecule menadione diffuses through a living cell membrane where it is toxic to the cell. However, in the cell it is conjugated with glutathione to form thiodione. Thiodione is then recognized and transported across the cell membrane via the ATP-driven MRP1 pump. In the extracellular environment, thiodione was detected by the SECM tip at levels of 140, 70, and 35 µM upon exposure of the cells to menadione concentrations of 500, 250, and 125 µM, respectively. With the aid of finite element modeling, the kinetics of thiodione transport was determined to be 1.6 10(-7) m/s, about 10 times faster than menadione uptake. Selective inhibition of these MRP1 pumps inside live HeLa cells by MK571 produced a lower thiodione concentration of 50 µM in presence of 500 µM menadione and 50 µM MK571. A similar reduced (50% drop) thiodione efflux was observed in the presence of monoclonal antibody QCRL-4, a selective blocking agent of the MRP1 pumps. The reduced thiodione flux confirmed that thiodione was transported by MRP1, and that glutathione is an essential substrate for MRP1-mediated transport. This finding demonstrates the usefulness of SECM in quantitative studies of MRP1 inhibitors and suggests that monoclonal antibodies can be a useful tool in inhibiting the transport of these MDR pumps, and thereby aiding in overcoming multidrug resistance.

Regulation of hepatic ABCC transporters by xenobiotics and in disease states

The subfamily of ABCC transporters consists of 13 members in mammals, including the multidrug resistance-associated proteins (MRPs), sulfonylurea receptors (SURs), and the cystic fibrosis transmembrane conductance regulator (CFTR). These proteins play roles in chemical detoxification, disposition, and normal cell physiology. ABCC transporters are expressed differentially in the liver and are regulated at the transcription and translation level. Their expression and function are also controlled by post-translational modification and membrane-trafficking events. These processes are tightly regulated. Information about alterations in the expression of hepatobiliary ABCC transporters could provide important insights into the pathogenesis of diseases and disposition of xenobiotics. In this review, we describe the regulation of hepatic ABCC transporters in humans and rodents by a variety of xenobiotics, under disease states and in genetically modified animal models deficient in transcription factors, transporters, and cell-signaling molecules.

Recent perspectives in ocular drug delivery

Anatomy and physiology of the eye makes it a highly protected organ. Designing an effective therapy for ocular diseases, especially for the posterior segment, has been considered as a formidable task. Limitations of topical and intravitreal route of administration have challenged scientists to find alternative mode of administration like periocular routes. Transporter targeted drug delivery has generated a great deal of interest in the field because of its potential to overcome many barriers associated with current therapy. Application of nanotechnology has been very promising in the treatment of a gamut of diseases. In this review, we have briefly discussed several ocular drug delivery systems such as microemulsions, nanosuspensions, nanoparticles, liposomes, niosomes, dendrimers, implants, and hydrogels. Potential for ocular gene therapy has also been described in this article. In near future, a great deal of attention will be paid to develop non-invasive sustained drug release for both anterior and posterior segment eye disorders. A better understanding of nature of ocular diseases, barriers and factors affecting in vivo performance, would greatly drive the development of new delivery systems. Current momentum in the invention of new drug delivery systems hold a promise towards much improved therapies for the treatment of vision threatening disorders.

Expression of the multidrug resistance proteins MRP2 and MRP3 in human cholangiocellular carcinomas

BACKGROUND

Cholangiocellular carcinomas and gallbladder carcinomas are highly aggressive tumours with a poor prognosis and are generally regarded as chemoresistant tumours. Overexpression of ATP-binding cassette transporters of the multidrug resistance protein (MDR) and multidrug resistance-related protein (MRP) family in cancer cells is a major cause for the multidrug resistance phenotype in vitro and in vivo. To further define the role of MRP family members in biliary tract cancer, we studied the expression and localization of MRP2 and MRP3 in cholangiocellular carcinomas and gallbladder carcinomas.

MATERIALS AND METHODS

The expression and cellular localization of the multidrug resistance proteins MRP2 and MRP3 in human cholangiocellular carcinomas and gallbladder carcinomas were analysed by immunohistochemistry using isoform-specific antibodies. Expression of MRP isoforms was studied in vitro in Mz-ChA-1 cells derived from gallbladder adenocarcinoma by reverse transcription-polymerase chain reaction (RT-PCR), immunoblotting and immunofluorescence microscopy.

RESULTS

Mz-ChA-1 cells constitutively expressed MDR P-glycoproteins, MRP1, MRP2 and MRP3 by RT-PCR, immunoblotting and immunofluorescence microscopy. MRP2 and MRP3 are expressed in the respective apical and basolateral membrane domains. MRP3 was the predominant MRP isoform in gallbladder carcinomas (93%) and cholangiocellular carcinomas (57%), whereas MRP2 expression was detected in only 29% of gallbladder carcinomas and was undetectable in cholangiocellular carcinomas.

CONCLUSIONS

Our findings suggest that the intrinsic multidrug resistance of cholangiocellular and gallbladder carcinomas seems to be independent of MRP2 expression while the expression of MRP3 may contribute to the MDR phenotype.

Multidrug resistance protein 1 (MRP1) in rabbit conjunctival epithelial cells: its effect on drug efflux and its regulation by adenoviral infection

PURPOSE

To evaluate the expression, localization, function, and regulation of multidrug resistance protein (MRP1) in rabbit conjunctival epithelial cells (RCEC).

MATERIALS AND METHODS

MRP1 gene expression in RCEC was determined by reverse transcription-polymerase chain reaction (RT-PCR), and MRP1 protein expression and its localization were determined by Western blot analysis and immunofluorescence using an anti-MRP1 monoclonal antibody, MRPr1. The effect of MRP1 on the transport and uptake of fluorescein was evaluated in RCEC grown on Transwell filters. Moreover, the effect of adenovirus type 5 (Ad5)-infected RCEC, and cytokines (Interleukin 1 (IL-1), IL-6, and tumor necrosis factor alpha (TNFalpha)) on MRP1 expression and leukotriene C4 (LTC4) uptake were investigated.

RESULTS

A 652 bp RT-PCR product from rabbit conjunctiva showed a 87% homology to human MRP1. Immunostaining with MRPr1 revealed a predominant basolateral localization of MRP1 in RCEC. Uptake of fluorescein, a MRP1 substrate, was increased (203-290%) in the presence of uricosuric drug probenecid at 100 microM, anti-inflammatory drug indomethacin at 10 microM and diclofenac, flurbiprofen, and ofloxacin at 1 mM, and by ATP depletion, but not influenced by the depletion of GSH, and the presence of antiviral cidofovir and anti-inflammatory drug cromolyn and prednisolone. Apical-to-basolateral facilitated transport of LTC4 was abolished in the presence of probenecid. Western blot analysis with MRPr1 revealed a distinct band at approximately 190 kDa for freshly isolated and cultured RCEC. Both Ad5 and cytokines (IL-1, IL-6, and TNF-alpha) up-regulated MRP1 expression, thereby reducing LTC4 uptake.

CONCLUSIONS

MRP1 appears to be primarily localized in the basolateral membrane of RCEC and function in the efflux of certain organic anions and inflammatory factors out of cells from the basolateral membrane. The upregulation in the expression of MRP1 by Ad5-infection and cytokines suggests a role of MRP1 in the transport of inflammatory factors during ocular inflammation. Supported by NIH grants EY12578, EY10421, and EY12356.

Multiple drug resistance, antimutagenesis and anticarcinogenesis

Many cells are protected from excess levels of exogenous chemicals, including mutagens and carcinogens as well as pharmaceutical agents, by being actively extruded through the action of one or more of a series of ATP-binding cassette drug transporter proteins. Those known to be important in humans are the multidrug resistance proteins (P-glycoproteins, encoded by the mdr1 and 3 genes), multidrug-resistance-associated proteins (MRP1-7) and the breast cancer resistance protein (BCRP). These proteins have overlapping but distinct cellular locations and substrate specificities, and jointly govern the likelihood of penetration or distribution of a given mutagen or carcinogen into various tissues including the brain, testis, ovaries and fetus. Thus, they can affect the absorption, distribution and excretion of mutagens and carcinogens, as well as of their metabolites and conjugates, in most cases acting to prevent or reduce mutagenesis or carcinogenesis. However, because ABC transporters may limit the success of chemotherapy, there has been a considerable effort by the pharmaceutical industry to develop inhibitors of this transport process, and these are increasing in use. In general, the mutagenicity of many chemicals may be increased at the cellular levels by the action of these inhibitors, while the altered absorption characteristics favour greater uptake into the body. Thus, in many cases, such inhibitors may counter the antimutagenic and anticarcinogenic effect of the multidrug resistance mechanisms. There are exceptions, however. An increasing number of single nucleotide polymorphisms in multidrug resistance genes are being identified in humans, and may account for many of the significant differences in inter-individual susceptibility to exogenous and endogenous mutagenic and carcinogenic insults.

Apical/Basolateral Surface Expression of Drug Transporters and its Role in Vectorial Drug Transport

It is well known that transporter proteins play a key role in governing drug absorption, distribution, and elimination in the body, and, accordingly, they are now considered as causes of drug-drug interactions and interindividual differences in pharmacokinetic profiles. Polarized tissues directly involved in drug disposition (intestine, kidney, and liver) and restricted distribution to naive sanctuaries (blood-tissue barriers) asymmetrically express a variety of drug transporters on the apical and basolateral sides, resulting in vectorial drug transport. For example, the organic anion transporting polypeptide (OATP) family on the sinusoidal (basolateral) membrane and multidrug resistance-associated protein 2 (MRP2/ABCC2) on the apical bile canalicular membrane of hepatocytes take up and excrete organic anionic compounds from blood to bile. Such vectorial transcellular transport is fundamentally attributable to the asymmetrical distribution of transporter molecules in polarized cells. Besides the apical/basolateral sorting direction, distribution of the transporter protein between the membrane surface (active site) and the intracellular fraction (inactive site) is of practical importance for the quantitative evaluation of drug transport processes. The most characterized drug transporter associated with this issue is MRP2 on the hepatocyte canalicular (apical) membrane, and it is linked to a genetic disease. Dubin-Johnson syndrome is sometimes caused by impaired canalicular surface expression of MRP2 by a single amino acid substitution. Moreover, single nucleotide polymorphisms in OATP-C/SLC21A6 (SLCO1B1) also affect membrane surface expression, and actually lead to the altered pharmacokinetic profile of pravastatin in healthy subjects. In this review article, the asymmetrical transporter distribution and altered surface expression in polarized tissues are discussed.

Relation between hepatocyte G1 arrest, impaired hepatic regeneration, and fibrosis in chronic hepatitis C virus infection

BACKGROUNDS & AIMS

An increased risk of hepatitis C virus (HCV)-related cirrhosis is associated with hepatic steatosis, older age, and high alcohol consumption, which could be explained by synergistic effects on cell proliferation. We aimed to investigate hepatocyte cell cycle state and phase distribution in chronic HCV infection.

METHODS

Liver biopsy specimens diagnostic for chronic HCV (70), liver regeneration following transplant-related ischemic-reperfusion injury (15), and "normal" liver adjacent to colorectal cancer metastasis (10) were studied. Immunohistochemistry was used to detect cell cycle phase markers cyclin D1 (maximal in G 1 ), cyclin A (S), cyclin B1 (cytoplasmic during G 2 ) and phosphorylated histone 3 protein (mitosis), mini-chromosome maintenance protein 2 (Mcm-2; present throughout the cell cycle), and cyclin-dependent kinase inhibitor p21, which inhibits G 1 /S progression.

RESULTS

Hepatocyte Mcm-2 expression was elevated in chronic HCV and liver regeneration (13% vs 26.4%) but negligible in "normal" liver. In proportion to Mcm-2, there was no difference in cyclin D1 between chronic HCV infection and liver regeneration (51.6% of Mcm-2-positive hepatocytes vs 52.6%). In contrast, there was a striking reduction in cyclin A (3% vs 16.3%), cyclin B1 (.4% vs 2.3%), and phosphorylated histone 3 protein (0% vs 3.8%) in chronic HCV infection compared with liver regeneration. In chronic HCV infection, Mcm-2 and p21 expression were associated with fibrosis stage and positive serum HCV RNA.

CONCLUSIONS

The data are consistent with hepatocyte G 1 arrest in chronic HCV infection. This could impair hepatocellular function and limit hepatic regeneration.

Drug transport proteins in the liver

Together with drug metabolising enzymes, transmembrane transporters are important determinants of drug metabolism and drug clearance by the liver. Hepatic uptake of organic anions, cations, prostaglandins and bile salts is supported by dedicated transporter proteins in the basolateral (sinusoidal) membrane of hepatocytes: OATPs, OATs, OCTs, PGTs and NTCP, respectively. ATP-binding cassette (ABC) transporter proteins in the canalicular membrane of hepatocytes mediate the hepatic efflux of drugs, bile salts and metabolites against a steep concentration gradient from liver to bile. This transport is driven by ATP hydrolysis. Drugs, endogenous metabolites, bile salts and cytokines affect the expression levels of these transporters. They act through a family of ligand-activated transcription factors, the nuclear hormone receptors. Consequently, the levels of the various transporter proteins are subject to genetic polymorphism in the encoding genes as well as in these transcription factors. Adverse drug reactions may be caused by genetic or disease-induced variations of transporter expression or drug-drug interactions at the level of these transporters.

Expression of rat hepatic multidrug resistance-associated proteins and organic anion transporters in pregnancy

The expression of hepatic multidrug resistance-associated protein (Mrp)1, 2, 3, and 6 and organic anion transporting polypeptides (Oatp)1 and 2 were examined in control and 20- to 21-day pregnant rats. Western analysis showed that expression of Oatp2 was decreased 50% in pregnancy, whereas expression of Oatp1 did not change. Expression of Mrp2 protein determined by Western analysis of total liver homogenate decreased to 50% of control levels in pregnant rats, consistent with studies using plasma membranes. Confocal immunohistochemistry showed that Mrp2 expression was confined to the canalicular membrane in both control and pregnant rats and was not detectable in intracellular compartments. In isolated perfused liver, the biliary excretion of 2,4-dintrophenyl-glutathione was significantly decreased in pregnancy, consistent with decreased expression of Mrp2. The expression of the basolateral transporter Mrp1 was not altered in pregnancy, whereas expression of Mrp6 mRNA was decreased by 60%. Expression of Mrp3 was also decreased by 50% in pregnant rat liver, indicating differential regulation of Mrp isoforms in pregnancy. These data also demonstrate that decreased Mrp2 expression is not necessarily accompanied by increased Mrp3 expression.

Multidrug resistance-associated protein2 (MRP2) plays an important role in the biliary excretion of glutathione conjugates of 4-hydroxynonenal

Glutathione (GSH) conjugates of 4-hydroxy-trans-2,3-nonenal (HNE) are the final products of lipid peroxidation. In the present study, the role of multidrug resistance-associated protein 2 (MRP2) in biliary excretion of GSH conjugates of HNE (HNE-SG) was studied in vitro by using Madin-Darby canine kidney II (MDCK II) cells expressing human MRP2 and in vivo using a mutant rat strain whose MRP2 expression is defective (Eisai-hyperbilirubinemic rats [EHBR]). A high-performance liquid chromatography method was developed to assay HNE-SG conjugates. Four diastereomeric HNE-SG conjugates could be separated with this method. Three of four HNE-SG conjugates were detectable after incubation of the cell monolayers with HNE. Expression of human MRP2 resulted in a 10-fold increase in HNE-SG conjugates excretion across the apical membrane of MDCK II cells. The four HNE-SG conjugates appeared swiftly in bile from Sprague Dawley rats after intravenous administration of HNE, whereas no detectable HNE-SG conjugates were observed in the bile of EHBR. These results demonstrate the role of MRP2 in the biliary excretion of HNE-SG conjugates.

Differential regulation of multidrug resistance-associated protein 2 (MRP2) and cytochromes P450 2B1/2 and 3A1/2 in phenobarbital-treated hepatocytes

Multidrug resistance-associated protein 2 (MRP2) is a drug efflux pump found at the biliary pole of hepatocytes. In the present study, we have investigated its expression in response to phenobarbital, a liver tumor promoter known to up-regulate hepatic cytochromes P450 (CYPs), such as CYP2B1/2 and CYP3A1/2. MRP2 mRNA and protein levels were found to be markedly increased in both primary rat and human hepatocytes exposed to phenobarbital. However, features of this up-regulation, especially the dose-response, were different from those of the induction of CYP2B1/2 and CYP3A1/2. In addition, hepatic MRP2 expression remained unaltered in rats treated by phenobarbital that, by contrast, increased CYP2B1/2 and CYP3A1/2 gene expression in the liver. Therefore, MRP2 and CYPs appeared differently regulated in response to phenobarbital in both in vivo and in vitro situations, suggesting that cellular and molecular mechanisms underlying up-regulation of MRP2 are, at least in part, unrelated to those operating for CYPs. Phenobarbital-related MRP2 induction in primary rat hepatocytes was associated with some phenotypic effects of the barbiturate, such as prolonged cell survival and inhibition of cell proliferation. Phenobarbital also inhibited growth of human hepatoma HepG(2) cells and increased their level of MRP2 gene expression. Such results may favor a putative relationship between phenobarbital-mediated MRP2 regulation in cultured liver parenchymal cells and alteration of cell cycle and survival.

Evidence of P-glycoprotein mediated apical to basolateral transport of flunisolide in human broncho-tracheal epithelial cells (Calu-3)

1. Transepithelial transport of flunisolide was studied in reconstituted cell monolayers of Calu-3, LLC-PK1 and the MDR1-P-glycoprotein transfected LLC-MDR1 cells. 2. Flunisolide transport was polarized in the apical (ap) to basolateral (bl) direction in Calu-3 cells and was demonstrated to be ATP-dependent. In LLC-MDR1 cells, flunisolide was transported in the bl to ap direction and showed no polarization in LLC-PK1 cells. 3. Non-specific inhibition of cellular metabolism at low temperature (4 degrees C) or by 2-deoxy-D-glucose (2-d-glu) and sodium azide (NaN(3)) abolished the polarized transport. Polarized flunisolide transport was also inhibited by the specific Pgp inhibitors verapamil, SDZ PSC 833 and LY335979. 4. Under all experimental conditions and in the presence of all used inhibitors, no decrease in the TransEpithelial Electrical Resistance (TEER) values was detected. From all inhibitors used, only the general metabolism inhibitors 2-deoxy-D-glucose and NaN(3), decreased the survival of Calu-3 cells. 5. Western blotting analysis and confocal laser scanning microscopy demonstrated the presence of MDR1-Pgp at mainly the basolateral side of the plasma membrane in Calu-3 cells and at the apical side in LLC-MDR1 cells. Mass spectroscopy studies demonstrated that flunisolide is transported unmetabolized across Calu-3 cells. 6. In conclusion, these results show that the active ap to bl transport of flunisolide across Calu-3 cells is facilitated by MDR1-Pgp located in the basolateral plasma membrane.

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

Sequencing and tissue distribution of the canine MRP2 gene compared with MRP1 and MDR1

The effects of xenobiotic drugs and toxic compounds depend largely on their kinetic properties, which can be influenced by transmembrane drug transporters like MDR1/P-glycoprotein and the drug-conjugate transporters multidrug resistance protein (MRP) 1 and 2. As the dog is a preferential species used in the pharmacological and toxicological evaluation of new drugs, we sequenced the canine MRP2 cDNA and investigated its expression in various canine tissues compared with the related transporters MRP1 and P-glycoprotein. The tissue distribution pattern of these ABC-transporters differs partially from the distribution described in humans. So we found relatively high renal and low hepatic canine MRP2 expression levels, relatively high hepatic canine MRP1 expression levels, and quite high levels of MRP1 and P-glycoprotein in the dog brain. The knowledge of the tissue distribution pattern of these transporters will aid to interpret pharmacokinetic and toxicokinetic data gained from dog studies and to extrapolate them to humans.

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

Glutathione transport system in NIH3t3 fibroblasts

The current study characterizes a mediated transport for GSH uptake in murine fibroblasts NIH3T3. The presence of GSH mediated transport is indicated by the behaviour of GSH uptake time-course, as well as by kinetic saturation and the specific inhibition of the initial rate of GSH transport. Moreover, a concentrative GSH uptake has been measured, whose driving force may be due to a change of membrane potential and the direct involvement of ATP. Hyperbolic kinetic saturation shows a single mediated transport with high affinity for GSH (Km = 0.209 +/- 0.03 mM). High specificity of this GSH transporter for the entire structure of GSH is also demonstrated. To summarize, GSH uptake into NIH3T3 cells occurs by an active transport system and shows characteristics similar to ATP-dependent mechanisms previously identified in hepatocyte membranes. Moreover, a possible physiological role of this GSH transporter related to NIH3T3 cell proliferation has been hypothesized.

Transport function and hepatocellular localization of mrp6 in rat liver

The multidrug resistance-associated proteins (Mrps) constitute a family of cellular export pumps of the ATP-binding cassette transporter superfamily and play an important role in hepatobiliary excretion. We investigated the transport function and subcellular localization of mrp6, a novel member of the mrp family, in rat liver. Transport studies in vesicles isolated from mrp6 expressing Sf9 cells identified the anionic cyclopentapeptide and endothelin receptor antagonist BQ-123 as a substrate of mrp6 (K(m) approximately 17 microM). Besides BQ-123, which is also a substrate of mrp2 (K(m) approximately 124 microM), no other common substrates were found for mrp2, mrp6, and the canalicular bile salt export pump Bsep. The cyclic peptides endothelin I and Arg(8)-vasopressin were transported by mrp2 but not by mrp6. Using a polyclonal antiserum raised against a C-terminal peptide, mrp6 was found to be localized at the lateral and, to a lesser extent, at the canalicular plasma membrane of hepatocytes. The limited overlap of the substrate specificity with the canalicular export pumps mrp2 and Bsep indicates that mrp6 does not play a major role in canalicular organic anion excretion. However, its dual localization at the lateral and canalicular plasma membrane suggests that mrp6 might fulfill a "housekeeping" transport function involved in the regulation of paracellular and/or transcellular solute movement from blood into bile.