Tissue distribution and induction of human multidrug resistant protein 3

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.

Hepatic stem cells: from inside and outside the liver?

The liver is normally proliferatively quiescent, but hepatocyte loss through partial hepatectomy, uncomplicated by virus infection or inflammation, invokes a rapid regenerative response from all cell types in the liver to perfectly restore liver mass. Moreover, hepatocyte transplants in animals have shown that a certain proportion of hepatocytes in foetal and adult liver can clonally expand, suggesting that hepatoblasts/hepatocytes are themselves the functional stem cells of the liver. More severe liver injury can activate a potential stem cell compartment located within the intrahepatic biliary tree, giving rise to cords of bipotential transit amplifying cells (oval cells), that can ultimately differentiate into hepatocytes and biliary epithelial cells. A third population of stem cells with hepatic potential resides in the bone marrow; these haematopoietic stem cells may contribute to the albeit low renewal rate of hepatocytes, but can make a more significant contribution to regeneration under a very strong positive selection pressure. In such instances, cell fusion rather than transdifferentiation appears to be the underlying mechanism by which the haematopoietic genome becomes reprogrammed.

Transport of ethinylestradiol glucuronide and ethinylestradiol sulfate by the multidrug resistance proteins MRP1, MRP2, and MRP3

Ethinylestradiol (EE) is one of the key constituents of oral contraceptives. Major metabolites of EE in humans are the glucuronide and sulfate conjugates, EE-3-O-glucuronide (EE-G) and EE-3-O-sulfate (EE-S). In the present study, transport of EE-G and EE-S by the human multidrug resistance proteins MRP1, MRP2, and MRP3 was investigated using inside-out membrane vesicles, isolated from Sf9 cells expressing human MRP1, MRP2, or MRP3. Vesicular uptake studies showed that EE-G was not a substrate for MRP1, whereas an ATP-dependent and saturable transport of [(3)H]EE-G was observed in MRP2 (K(m) of 35.1 +/- 3.5 microM) and MRP3 (K(m) of 9.2 +/- 2.3 microM) containing vesicles. EE-S was not transported by either MRP1, MRP2, or MRP3. However, low concentrations of EE-S stimulated MRP2-mediated uptake of ethacrynic acid glutathione. EE-S also stimulated MRP2 and MRP3-mediated uptake of 17beta-estradiol-17beta-D-glucuronide. Interestingly, EE-S stimulated strongly MRP2- and MRP3-mediated uptake of EE-G by increasing its apparent transport affinity, whereas no reciprocal stimulation of EE-S uptake by EE-G was observed. These data indicate that EE-S allosterically stimulates MRP2- and MRP3-mediated transport of EE-G and is not cotransported with EE-G. Our studies demonstrate specific active transport of a pharmacologically relevant drug conjugate by human MRP2 and MRP3, involving complex interactions with other organic anions. We also suggest that caution needs to be taken when using only competition studies as screening tools to identify substrates or inhibitors of MRP-mediated transport.

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.

Enterohepatic bile salt transporters in normal physiology and liver disease

The vectorial transport of bile salts from blood into bile is essential for the generation of bile flow, solubilization of cholesterol in bile, and emulsification of lipids in the intestine. Major transport proteins involved in the enterohepatic circulation of bile salts include the hepatocellular bile salt export pump (BSEP, ABCB11), the apical sodium-dependent bile salt transporter (ASBT, SLC10A2) in cholangiocytes and enterocytes, the sodium-dependent hepatocyte bile salt uptake system NTCP (SLC10A1), the organic anion transporting polypeptides OATP-C (SLC21A6), OATP8 (SLC21A8) and OATP-A (SLC21A3), and the multidrug resistance protein MRP3 (ABCC3). Synthesis and transport of bile salts are intricately linked processes that undergo extensive feedback and feed-forward regulation by transcriptional and posttranscriptional mechanisms. A key regulator of hepatocellular bile salt homeostasis is the bile acid receptor/farnesoid X receptor FXR, which activates transcription of the BSEP and OATP8 genes and of the small heterodimer partner 1 (SHP). SHP is a transcriptional repressor that mediates bile acid-induced repression of the bile salt uptake systems rat Ntcp and human OATP-C. A nuclear receptor that activates rodent Oatp2 (Slc21a5) and human MRP2 (ABCC2) is the pregnane X receptor/steroid X receptor PXR/SXR. Intracellular trafficking and membrane insertion of bile salt transporters is regulated by lipid, protein, and extracellular signal-related kinases in response to physiologic stimuli such as cyclic adenosine monophosphate or taurocholate. Finally, dysfunction of individual bile salt transporters such as BSEP, on account of genetic mutations, steric inhibition, suppression of gene expression, or disturbed signaling, is an important cause of cholestatic liver disease.

The MRP family of drug efflux pumps

The MRP family is comprised of nine related ABC transporters that are able to transport structurally diverse lipophilic anions and function as drug efflux pumps. Investigations of this family have provided insights not only into cellular resistance mechanisms associated with natural product chemotherapeutic agents, antifolates and nucleotide analogs, but also into factors that influence drug distribution in the body, membrane systems that are involved in the extrusion of reduced folates, cysteinyl leukotrienes and bile acids, and the molecular basis of two hereditary conditions in humans. The review will describe the biochemical properties, drug resistance activities and potential in vivo functions of these unusual pumps.

INDUCTION OF ABCC3 (MRP3) BY PREGNANE X RECEPTOR ACTIVATORS

The pregnane X receptor (PXR) mediates the induction of various genes by xenobiotics, including several ATP-binding cassette transporters. PXR is also activated by bile acids likely to prevent their accumulation to toxic levels; however, the role of PXR in the regulation of MRP3, an important bile acid efflux transporter, has not been elucidated. The impact of PXR activators on the hepatic expression of MRP3 was examined in vivo and in vitro. The human hepatoma cell lines HuH7 and HepG2 were treated with PXR activators including clotrimazole, rifampicin, 17beta-hydroxy-11beta-[4-dimethylamino phenyl]-17alpha-[1-propynyl]estra-4,9-dien-3-one (RU486), metyrapone, nifedipine, lithocholic acid, and 5-pregnen-3beta-ol-20-one-16alpha-carbonitrile (PCN). Levels of MRP3 mRNA, as determined by reverse transcription-polymerase chain reaction, were induced 1.6- to 8-fold in a dose-dependent manner (p < 0.05). Corresponding decreases in the multidrug resistance-associated protein-dependent cellular retention of 5-carboxyfluorescein were also seen in the treated HuH7 cells. In vivo studies demonstrated increased PXR mRNA and induction of MRP3 mRNA in the livers of wild-type mice treated with the PXR activator RU486. On the other hand, MRP3 induction was not seen in the RU486-treated PXR-null mice. These results suggest that PXR activation may play a role in the regulation of MRP3 expression.

Kinetics of gadobenate dimeglumine in isolated perfused rat liver: MR imaging evaluation

PURPOSE

To compare in the entire liver, the hepatic kinetics of gadobenate dimeglumine (Gd-BOPTA) and gadopentetate dimeglumine (Gd-DTPA) and to evaluate the hepatic transport of Gd-BOPTA.

MATERIALS AND METHODS

The authors studied both contrast agents in isolated perfused rat livers by measuring the magnetic resonance (MR) signal intensity (SI) in 12 rats, as well as the gadolinium concentrations in hepatic tissues in 42 rats. The intrahepatic transport of Gd-BOPTA was investigated with pharmacologic antagonism by using bromosulfophthalein. MR imaging was performed at 1.5 T with a fast gradient-echo T1-weighted MR sequence.

RESULTS

The hepatic kinetics based on the MR SI measured over time showed a rapid steady state during Gd-DTPA perfusion, while the SI continuously increased during the 30-minute Gd-BOPTA perfusion period. The pharmacokinetic modeling indicated that the half-lives of Gd-DTPA entry and exit were identical (mean, 1.3 minutes +/- 0.9 [standard error of mean]) and shorter than those observed with Gd-BOPTA (P <.001). The uptake of Gd-BOPTA was faster (mean half-life, 4.8 minutes +/- 0.3) than the washout (mean half-life, 17.5 minutes +/- 2.8) (P =.001). The combined perfusion of bromosulfophthalein and Gd-BOPTA decreased the SI enhancement in comparison with the perfusion of Gd-BOPTA alone (mean, 0.56 +/- 0.03 vs 2.54 +/- 0.39, P <.001). The entry and exit kinetic parameters obtained during the perfusion of Gd-BOPTA plus bromosulfophthalein were identical and comparable to those obtained during Gd-DTPA perfusion (P =.95). Acute bile duct ligation did not interfere with the uptake of Gd-BOPTA in hepatocytes, but it slowed down the excretion by approximately 50%. Measurements of gadolinium concentrations in hepatic tissues confirmed these findings.

CONCLUSION

In the liver, the hepatospecific contrast agent Gd-BOPTA enters into hepatocytes likely through the organic anion transporting peptide 1.

Tumor necrosis factor alpha-dependent up-regulation of Lrh-1 and Mrp3(Abcc3) reduces liver injury in obstructive cholestasis

Mrp3(Abcc3) is markedly induced following bile duct ligation (BDL) in the rat and in some human cholestatic liver diseases and is believed to ameliorate liver injury in this setting. Recently, the orphan nuclear receptor fetoprotein transcription factor/cholesterol-7alpha-hydroxylase promoter factor (CPF/FTF/Lrh-1) has been shown to activate Mrp3 expression. However, whether inflammatory cytokines or elevated bile acid levels increased Lrh-1/Mrp3 expression in obstructive cholestasis was not known. We hypothesized that induction of Mrp3 would be associated with Lrh-1 up-regulation and would require intact cytokine signaling. Male tumor necrosis factor (Tnf) receptor I (Tnfr-/-) mice and C57BLJ wild type (WT) controls were subjected to sham surgery or bile duct ligation. HepG2 cells were treated with bile acids or cytokines. Immunoblot assay and real time reverse transcriptase-PCR were used to determine expression of MRP3/Mrp3, CPF/Lrh-1, Mrp2, and Bsep. CPF/Lrh-1 DNA binding to the MRP3/Mrp3 promoter was assessed using electrophoretic mobility shift assay, and promoter activity was determined by luciferase assay. Total bile acids and lactate dehydrogenase were measured using colorimetric assays, and cytokine abundance was determined by enzyme-linked immunosorbent assay. Lrh-1 and Mrp3 were significantly induced after BDL in WT but not Tnfr-/- mice. This was associated with more severe hepatocellular necrosis in Tnfr-/- mice. Lrh-1 binding to the Mrp3 promoter increased after BDL in WT but not in Tnfr-/- mice. Tnfalpha treatment of HepG2 cells also up-regulated CPF and MRP3, increased CPF binding to the MRP3 promoter, and up-regulated MRP3 promoter activity. These results indicate that induction of Mrp3 after BDL is due to Tnfalpha-dependent up-regulation of Lrh-1. They provide strong evidence that induction of Mrp3 plays a significant role in hepatocyte protection during obstructive cholestasis.

Induction of multidrug resistance protein 3 in rat liver is associated with altered vectorial excretion of acetaminophen metabolites

Treatment with the microsomal enzyme inducer trans-stilbene oxide (TSO) can decrease biliary excretion of acetaminophen-glucuronide (AA-GLUC) and increase efflux of AA-GLUC into blood. The hepatic canalicular multidrug resistance protein (Mrp) 2 and sinusoidal protein Mrp3 transport AA-GLUC conjugates into bile and blood, respectively. Thus, TSO-induced alterations in the vectorial excretion of AA-GLUC may occur via increased hepatic Mrp3 levels. The goal of this study was to determine whether TSO, diallyl sulfide (DAS), and oltipraz (OLT) treatments can up-regulate Mrp3 protein expression, and whether treatment with DAS and OLT can correspondingly increase hepatovascular efflux of AA metabolites. Rats were administered phenobarbital, TSO, DAS, OLT, or vehicle for 4 days. Interestingly, all of the chemicals increased the plasma concentration and urinary excretion of AA-GLUC and decreased its biliary excretion. In control animals, approximately 77% and 23% of AA-GLUC was excreted into bile or urine, respectively, whereas with inducer-pretreated animals, <32% of AA-GLUC was excreted into bile and >68% was excreted into urine. Correspondingly, all of the compounds increased hepatic Mrp3 mRNA levels by 13- to 37-fold and protein levels by 2- to 6-fold, respectively. In conclusion, these studies correlate increased Mrp3 protein levels in liver with increased hepatovascular excretion of AA-GLUC and suggest that induction of Mrp3 affects the route of drug excretion.

Characterization of the role of polar amino acid residues within predicted transmembrane helix 17 in determining the substrate specificity of multidrug resistance protein 3

Human multidrug resistance protein (MRP) 3 is the most closely related homologue of MRP1. Like MRP1, MRP3 confers resistance to etoposide (VP-16) and actively transports 17 beta-estradiol 17-(beta-D-glucuronide) (E(2)17 beta G), cysteinyl leukotriene 4 (LTC(4)), and methotrexate, although with generally lower affinity. Unlike MRP1, MRP3 also transports monovalent bile salts. We have previously demonstrated that hydrogen-bonding residues predicted to be in the inner-leaflet spanning segment of transmembrane (TM) 17 of MRP1 are important for drug resistance and E(2)17 beta G transport. We have now examined the importance of the hydrogen-bonding potential of residues in TM17 of MRP3 on both substrate specificity and overall activity. Mutation S1229A reduced only methotrexate transport. Mutations S1231A and N1241A decreased resistance to VP-16 and transport of E(2)17 beta G and methotrexate but not taurocholate. Mutation Q1235A also reduced resistance to VP-16 and transport of E(2)17beta G but increased taurocholate transport without affecting transport of methotrexate. Mutations Y1232F and S1233A reduced resistance to VP-16 and the transport of all three substrates tested. In contrast, mutation T1237A markedly increased VP-16 resistance and transport of all substrates. On the basis of the substrates analyzed, residues Ser(1229), Ser(1231), Gln(1235), and Asn(1241) play an important role in determining the specificity of MRP3, while mutation of Tyr(1232), Ser(1233), and Thr(1237) affects overall activity. Unlike MRP1, the involvement of polar residues in determining substrate specificity extends throughout the TM helix. Furthermore, elimination of the hydrogen-bonding potential of a single amino acid, Thr(1237), markedly enhanced the ability of the protein to confer drug resistance and to transport all substrates examined.

Membrane transport of folates

The chapter reviews the current understanding of the transport mechanisms for folates in mammalian cells--their molecular identities and organization, tissue expression, regulation, structures, and their kinetic and thermodynamic properties. This encompasses a variety of diverse processes. Best characterized is the reduced folate carrier, a member of the SLC19 family of facilitative carriers. But other facilitative organic anion carriers (SLC21), largely expressed in epithelial tissues, transport folates as well. In addition to these bi-directional carrier systems are the membrane-localized folate receptors alpha and beta, that mediate folate uptake unidirectionally into cells via an endocytotic process. There are also several transporters, typified by the family of multidrug resistance-associated proteins, that unidirectionally export folates from cells. There are transport activities for folates, that function optimally at low pH, related in part to the reduced folate carrier, with at least one activity that is independent of this carrier. The reduced folate carrier-associated low-pH route mediates intestinal folate transport. This review considers how these different transport processes contribute to the generation of transmembrane folate gradients and to vectorial flows of folates across epithelia. The role of folate transporters in mouse development, as assessed by homologous deletion of folate receptors and the reduced folate carrier, is described. Much of the focus is on antifolate cancer chemotherapeutic agents that are often model surrogates for natural folates in transport studies. In particular, antifolate transport mediated by the reduced folate carrier is a major determinant of the activity of, and resistance to, these agents. Finally, many of the key in vitro findings on the properties of antifolate transporters are now beginning to be extended to patient specimens, thus setting the stage for understanding response to these drugs in the clinical setting at the molecular level.

Differential modulation of the human liver conjugate transporters MRP2 and MRP3 by bile acids and organic anions

The multidrug resistance proteins MRP2 (ABCC2) and MRP3 (ABCC3) are key primary active transporters involved in anionic conjugate and drug extrusion from the human liver. The major physiological role of MRP2 is to transport conjugated metabolites into the bile canaliculus, whereas MRP3 is localized in the basolateral membrane of the hepatocytes and transports similar metabolites back to the bloodstream. Both proteins were shown to interact with a large variety of transported substrates, and earlier studies suggested that MRPs may work as co-transporters for different molecules. In the present study we expressed the human MRP2 and MRP3 proteins in insect cells and examined their transport and ATPase characteristics in isolated, inside-out membrane vesicles. We found that the primary active transport of estradiol-17-beta-d-glucuronide (E217betaG), a major product of human steroid metabolism, was differently modulated by bile acids and organic anions in the case of human MRP2 and MRP3. Active E217betaG transport by MRP2 was significantly stimulated by the organic anions indomethacin, furosemide, and probenecid and by several conjugated bile acids. In contrast, all of these agents inhibited E217betaG transport by MRP3. We found that in the case of MRP2, ATP-dependent vesicular bile acid transport was increased by E217betaG, and the results indicated an allosteric cross-stimulation, probably a co-transport of bile acids and glucuronate conjugates through this protein. There was no such stimulation of bile acid transport by MRP3. In conclusion, the different transport modulation of MRPs by bile acids and anionic drugs could play a major role in regulating physiological and pathological metabolite fluxes in the human liver.

FXR induces the UGT2B4 enzyme in hepatocytes: a potential mechanism of negative feedback control of FXR activity

BACKGROUND & AIMS

Bile acids are essential for bile formation and intestinal absorption of lipids and fat-soluble vitamins. However, the intrinsic toxicity of hydrophobic bile acids demands a tight control of their intracellular concentrations. Bile acids are ligands for the farnesoid X receptor (FXR) that regulates the expression of genes controlling bile acid synthesis and transport. The human uridine 5'-diphosphate-glucuronosyltransferase 2B4 (UGT2B4) converts hydrophobic bile acids into more hydrophilic glucuronide derivatives. In this study, we identify UGT2B4 as an FXR target gene.

METHODS

Human hepatocytes or hepatoblastoma HepG2 cells were treated with chenodeoxycholic acid or the synthetic FXR agonist GW4064, and the levels of UGT2B4 messenger RNA, protein, and activity were determined by using real-time polymerase chain reaction, Western blot, and glucuronidation assays.

RESULTS

Treatment of hepatocytes and HepG2 cells with FXR agonists resulted in an increase of UGT2B4 messenger RNA, protein, and activity. A bile acid response element in the UGT2B4 promoter (B4-BARE) to which FXR, but not retinoid X receptor, binds, was identified by site-directed mutagenesis, electromobility shift, and chromatin immunoprecipitation assays. Retinoid X receptor activation abolished the induction of UGT2B4 expression and inhibited binding of FXR to the B4-BARE, suggesting that retinoid X receptor modulates FXR target gene activation. Overexpression of UGT2B4 in HepG2 cells resulted in the attenuation of bile acid induction of the FXR target gene small heterodimeric partner.

CONCLUSIONS

These data suggest that UGT2B4 gene induction by bile acids contributes to a feed-forward reduction of bile acid toxicity and a decrease of the activity of these biological FXR activators.

Cholestatic syndromes

Further insights into the cellular and molecular mechanisms underlying hepatobiliary transport function and its regulation now permit a better understanding of the pathogenesis and treatment options of cholestatic liver diseases. Identification of the molecular basis of hereditary cholestatic syndromes will result in an improved diagnosis and management of these conditions. New insights into the pathogenesis of extrahepatic manifestations of cholestasis (eg, pruritus) have facilitated new treatment strategies. Important new studies have been published about the pathogenesis, clinical features, diagnosis, and treatment of primary biliary cirrhosis, primary sclerosing cholangitis, cholestasis of pregnancy, total parenteral nutrition-induced cholestasis, drug-induced cholestasis, and viral cholestatic syndromes.

Steroid and bile acid conjugates are substrates of human multidrug-resistance protein (MRP) 4 (ATP-binding cassette C4)

Human multidrug-resistance protein (MRP) 4 transports cyclic nucleotides and when overproduced in mammalian cells mediates resistance to some nucleoside analogues. Recently, it has been shown that Mrp4 is induced in the livers of Fxr ((-/-)) mice, which have increased levels of serum bile acids. Since MRP4, like MRP1-3, also mediates transport of a model steroid conjugate substrate, oestradiol 17-beta-D-glucuronide (E(2)17betaG), we tested whether MRP4 may be involved in the transport of steroid and bile acid conjugates. Bile salts, especially sulphated derivatives, and cholestatic oestrogens inhibited the MRP4-mediated transport of E(2)17betaG. Inhibition by oestradiol 3,17-disulphate and taurolithocholate 3-sulphate was competitive, suggesting that these compounds are MRP4 substrates. Furthermore, we found that MRP4 transports dehydroepiandrosterone 3-sulphate (DHEAS), the most abundant circulating steroid in humans, which is made in the adrenal gland. The ATP-dependent transport of DHEAS by MRP4 showed saturable kinetics with K (m) and V (max) values of 2 microM and 45 pmol/mg per min, respectively (at 27 degrees C). We further studied the possible involvement of other members of the MRP family of transporters in the transport of DHEAS. We found that MRP1 transports DHEAS in a glutathione-dependent manner and exhibits K (m) and V (max) values of 5 microM and 73 pmol/mg per min, respectively (at 27 degrees C). No transport of DHEAS was observed in membrane vesicles containing MRP2 or MRP3. Our findings suggest a physiological role for MRP1 and MRP4 in DHEAS transport and an involvement of MRP4 in transport of conjugated steroids and bile acids.

The role of multidrug transporters in drug availability, metabolism and toxicity

Multidrug resistance is frequently observed when treating cancer patients with chemotherapeutic agents. A variety of ATP binding cassette (ABC) transporters, localized in the cell membrane, cause this phenomenon by extruding a variety of chemotherapeutic agents from the tumor cells. However, the major physiological role of the multidrug transporters is the protection of our cells and tissues against xenobiotics, and these transporters play a key role in drug availability, metabolism and toxicity. Three major groups of ABC transporters are involved in multidrug resistance: the classical P-glycoprotein MDR1, the multidrug resistance associated proteins (MRP1, MRP2, and probably MRP3, MRP4 and MRP5), and the ABCG2 protein, an ABC half-transporter. All these proteins were shown to catalyze an ATP-dependent active transport of chemically unrelated compounds. MDR1 (P-glycoprotein) and ABCG2 preferentially extrude large hydrophobic, positively charged molecules, while the members of the MRP family can extrude both hydrophobic uncharged molecules and water-soluble anionic compounds. By examining the interactions of the multidrug transporters with pharmacological and toxic agents, a prediction for the cellular and tissue distribution of these compounds can be achieved. Oral bioavailability, entering the blood-brain and blood-CSF barrier, reaching the fetus through the placenta, liver and kidney secretion, cellular entry for affecting intracellular targets, are all questions, which can be addressed by basic in vitro studies on the multidrug resistance proteins. Investigation of the substrate interactions and modulation of multidrug transporters may pave the way for predictive toxicology and pharmacogenomics. Here we show that by using in vitro assay systems it is possible to measure the interactions of multidrug transporters with various drugs and toxic agents. We focus on the characterisation of the MRP1 and MRP3 proteins, their relevance in chemoresistance of cancer and in drug metabolism and toxicity.

Bile salt transporters: molecular characterization, function, and regulation

Molecular medicine has led to rapid advances in the characterization of hepatobiliary transport systems that determine the uptake and excretion of bile salts and other biliary constituents in the liver and extrahepatic tissues. The bile salt pool undergoes an enterohepatic circulation that is regulated by distinct bile salt transport proteins, including the canalicular bile salt export pump BSEP (ABCB11), the ileal Na(+)-dependent bile salt transporter ISBT (SLC10A2), and the hepatic sinusoidal Na(+)- taurocholate cotransporting polypeptide NTCP (SLC10A1). Other bile salt transporters include the organic anion transporting polypeptides OATPs (SLC21A) and the multidrug resistance-associated proteins 2 and 3 MRP2,3 (ABCC2,3). Bile salt transporters are also present in cholangiocytes, the renal proximal tubule, and the placenta. Expression of these transport proteins is regulated by both transcriptional and posttranscriptional events, with the former involving nuclear hormone receptors where bile salts function as specific ligands. During bile secretory failure (cholestasis), bile salt transport proteins undergo adaptive responses that serve to protect the liver from bile salt retention and which facilitate extrahepatic routes of bile salt excretion. This review is a comprehensive summary of current knowledge of the molecular characterization, function, and regulation of bile salt transporters in normal physiology and in cholestatic liver disease and liver regeneration.

Effects of maturation on RNA transcription and protein expression of four MRP genes in human placenta and in BeWo cells

The placenta is a multifunctional organ that protects the fetus from toxic compounds and the MRPs contribute to this function. The expression of MRP1, MRP2, MRP3, and MRP5 was compared in human placental tissue and in BeWo cells by real-time RT-PCR analysis; protein expression was assessed by Western blot. MRP1 and MRP3 were the most abundantly expressed genes in placenta but only MRP1 was highly expressed in the BeWo cells. Expression of MRP1 increased 4-fold in the third as compared with first trimester placental samples, and increased 20-fold with polarization of BeWo cells. MRP2, MRP3, and MRP5 were weakly expressed both in placenta and BeWo cells. Protein expression followed mRNA quantification for MRP1 and MRP5 but not for MRP2 and MRP3. These data indicated that MRP1 and MRP5 increase with trophoblast maturation, suggesting a particular role for these proteins in the organ functional development.

A systematic examination of the in vitro Ussing chamber and the in situ single-pass perfusion model systems in rat ileum permeation of model solutes

In situ and in vitro intestinal absorption in the rat ileum was systematically studied and mechanistically quantified in terms of permeability coefficients (P) of a series of [(3)H]steroids as model transcellular permeants, [(3)H]taurocholate utilizing the active membrane transport systems to define the aqueous boundary layer (ABL), and [(14)C]urea and [(14)C]mannitol as pore-hindered paracellular diffusants. In situ single-pass perfusion experiments were performed in isolated ileal segments and blood samples were collected from the cannulated mesenteric vein. For the in vitro experiments, an excised, serosal and muscular layer-removed, ileal tissue was mounted in the Ussing chamber diffusion cells. In situ and in vitro P values versus logarithm of the partition coefficient in n-octanol/water (log K) of the steroids were characterized by a sigmoidal-shaped curve in which plateau values were attained for the highly lipophilic steroids with log K greater, similar 2.5. The in situ and in vitro transport barriers in series were viewed as ABL/mucosal epithelium and ABL/mucosal epithelium/submucosal tissue, respectively. Within this framework and the use of experimental strategies and theoretical reasoning, the transport barriers of the steroids were quantitatively delineated and the rate-determining barriers identified. In the plateau region, the analyses indicate that the in situ absorption of the lipophilic steroids was essentially ABL controlled, whereas the in vitro absorption was about equally controlled by diffusion across the ABL and submucosal tissue. The in situ and in vitro pore radii of the paracellular route were 7.2 and 9.2 A, respectively, and the difference was likely the result of perturbation of the tight junctions during the in vitro preparation of the ileal tissue.

Substitution of Trp1242 of TM17 alters substrate specificity of human multidrug resistance protein 3

Multidrug resistance protein 3 (MRP3) is an ATP-dependent transporter of 17beta-estradiol 17beta(d-glucuronide) (E(2)17betaG), leukotriene C(4) (LTC(4)), methotrexate, and the bile salts taurocholate and glycocholate. In the present study, the role of a highly conserved Trp residue at position 1242 on MRP3 transport function was examined by expressing wild-type MRP3 and Ala-, Cys-, Phe-, Tyr-, and Pro-substituted mutants in human embryonic kidney 293T cells. Four MRP3-Trp(1242) mutants showed significantly increased E(2)17betaG uptake, whereas transport by the Pro mutant was undetectable. Similarly, the Pro mutant did not transport LTC(4). By comparison, LTC(4) transport by the Ala, Cys, Phe, and Tyr mutants was reduced by approximately 35%. The Ala, Cys, Phe, and Tyr mutants all showed greatly reduced methotrexate and leucovorin transport, except the Tyr mutant, which transported leucovorin at levels comparable with wild-type MRP3. In contrast, the MRP3-Trp(1242) substitutions did not significantly affect taurocholate transport or taurocholate and glycocholate inhibition of E(2)17betaG uptake. Thus Trp(1242) substitutions markedly alter the substrate specificity of MRP3 but leave bile salt binding and transport intact.

Mammalian drug efflux transporters of the ATP binding cassette (ABC) family: an overview

Active drug efflux transporters of the ATP binding cassette (ABC)-containing family of proteins have a major impact on the pharmacological behavior of most of the drugs in use today. Pharmacological properties affected by ABC transporters include the oral bioavailability, hepatobiliary, direct intestinal, and urinary excretion of drugs and drug-metabolites and -conjugates. Moreover, the penetration of drugs into a range of important pharmacological sanctuaries, such as brain, testis, and fetus, and the penetration into specific cell- and tissue compartments can be extensively limited by ABC transporters. These interactions with ABC transporters determine to a large extent the clinical usefulness, side effects and toxicity risks of drugs. Many other xenotoxins, (pre-)carcinogens and endogenous compounds are also influenced by the ABC transporters, with corresponding consequences for the well-being of the individual. We aim to provide an overview of properties of the mammalian ABC transporters known to mediate significant transport of clinically relevant drugs.

Transport of bile acids in multidrug-resistance-protein 3-overexpressing cells co-transfected with the ileal Na+-dependent bile-acid transporter

Many of the transporters involved in the transport of bile acids in the enterohepatic circulation have been characterized. The basolateral bile-acid transporter of ileocytes and cholangiocytes remains an exception. It has been suggested that rat multidrug resistance protein 3 (Mrp3) fulfills this function. Here we analyse bile-salt transport by human MRP3. Membrane vesicles from insect ( Spodoptera frugiperda ) cells expressing MRP3 show time-dependent uptake of glycocholate and taurocholate. Furthermore, sulphated bile salts were high-affinity competitive inhibitors of etoposide glucuronide transport by MRP3 (IC50 approximately 10 microM). Taurochenodeoxycholate, taurocholate and glycocholate inhibited transport at higher concentrations (IC50 approximately 100, 250 and 500 microM respectively). We used mouse fibroblast-like cell lines derived from mice with disrupted Mdr1a, Mdr1b and Mrp1 genes to generate transfectants that express the murine apical Na+-dependent bile-salt transporter (Asbt) and MRP3. Uptake of glycocholate by these cells is Na+-dependent, with a K(m) and V(max) of 29+/-7 microM and 660 +/- 63 pmol/min per mg of protein respectively and is inhibited by several organic-aniontransport inhibitors. Expression of MRP3 in these cells limits the accumulation of glycocholate and increases the efflux from cells preloaded with taurocholate or glycocholate. In conclusion, we find that MRP3 transports both taurocholate and glycocholate, albeit with low affinity, in contrast with the high-affinity transport by rat Mrp3. Our results suggest that MRP3 is unlikely to be the principal basolateral bile-acid transporter of ileocytes and cholangiocytes, but that it may have a role in the removal of bile acids from the liver in cholestasis.