Hepatobiliary secretion of organic compounds; molecular mechanisms of membrane transport

Effects of the chemosensitizer verapamil on P-glycoprotein substrate efflux in rainbow trout hepatocytes

The ATP-dependent membrane transporter P-glycoprotein (P-gp) is associated with resistance to a wide variety of chemical substrates, as well as the multi-drug resistance (MDR) phenotype in mammals. Less is known regarding P-gp's function and relevance in teleosts; this study expanded the range of known substrates and the inhibitory effects of a model chemosensitizer verapamil. The P-gp-mediated uptake and efflux dynamics of 5 known mammalian substrates (berberine, cortisol, doxorubicin, rhodamine 123 [R123], and vinorelbine) were examined in isolated rainbow trout (Oncorhynchus mykiss) hepatocytes with and without co-exposure to varying doses of verapamil. Initial substrate uptake rates (pmol/106 cells/min) varied widely and were in order: berberine (482 ± 94) > R123 (364 ± 67) > doxorubicin (158 ± 41) > cortisol (20.3 ± 5.9) > vinorelbine (15.3 ± 3.5). Initial efflux rates (pmol/106 cells/min) were highest in berberine (464 ± 110) > doxorubicin (341 ± 57) > R123 (106 ± 33) > cortisol (26.6 ± 6.1) > vinorelbine (9.0 ± 2.4). Transport of vinorelbine and R123 is verapamil sensitive, but verapamil had no effect on transport of berberine, cortisol, or doxorubicin. Cortisol and doxorubicin showed evidence of high P-gp affinity, thus displacing verapamil from their shared P-gp binding site. Cortisol, doxorubicin, R123, and vinorelbine transport by rainbow trout P-gp was confirmed, while berberine could not be confirmed or excluded as a substrate. Binding sites and affinities were similar between mammalian and trout P-gp for doxorubicin, R123, and vinorelbine, while fish P-gp had a higher affinity for cortisol than mammalian P-gp. This study demonstrated that the range of substrates, as well as binding sites and affinities, of fish P-gp are well-aligned with those in mammals.

The ABCB1, ABCC2 and RALBP1 polymorphisms are associated with carbamazepine response in epileptic patient: a systematic review

Despite a dramatic increase in treatment options over the past 30 years, Carbamazepine (CBZ) is still considered the standard of care and the most prescribed initial treatment for focal epilepsy. Hence, the identification of genetic biomarkers that influence the response, resistance and toxicity to CBZ remains a challenge. Several research studies have looked into this to highlight the polymorphisms responsible for the variability in the response to CBZ in patients with epilepsy. The aim of this review is to compare the different results published in the literature The systematic review included thirty-nine studies (2005-2021), Meta-analyses were performed on more than twelve polymorphisms in three genes (ABCB1, ABCC2, RALBP1) involved in CBZ cell transport. The current challenges are to identify other new biomarkers of antiepileptic drugs that can only materialize with large-scale collaborative research efforts.

Metabolism and disposition of oseltamivir (OS) in rats, determined by immunohistochemistry with monospecific antibody for OS or its active metabolite oseltamivir carboxylate (OC): A possibility of transporters dividing the drugs' excretion into the bile and kidney

Among any drugs, no comparative pharmacological study on how prodrug and its active metabolite behave in animal bodies is available. Immunohistochemistry (IHCs) using newly prepared two monoclonal antibodies, AOS‐96 and AOC‐160, monospecific for oseltamivir (OS) and its metabolite oseltamivir carboxylate (OC) were developed, simultaneously detecting the uptake or excretion of OS and OC in the intestine, liver, and kidney of rats to which OS was orally administered. In the intestine, IHC for OS revealed OS highly distributed to the absorptive epithelia with heavily stained cytoplasmic small granules (CSGs). IHC for OC showed that OC also distributed highly in the epithelia, but without CSGs, suggesting that OS was partly converted to OC in the cells. In the liver, OS distributed in the hepatocytes and on their bile capillaries, as well as on the lumina from the bile capillaries to the interlobular bile ducts. OC distributed in the whole cell of the hepatocytes, but without CSGs nor on any lumina through the interlobular bile ducts. In the kidney, a few levels of OS distributed in the cytoplasm of almost all the renal tubule cells, but they contained numerous CSGs. In contrast, OC distributed highly in the proximal tubules, but very slightly in the lower renal tubules of the nephrons. Thus, it was concluded that the two drugs behave in completely different ways in rat bodies. This paper also discusses a possibility of the correlation of OS or OC levels in tissue cells with their known transporters.

Glucuronidation as a metabolic barrier against zearalenone in rat everted intestine

Zearalenone (ZON), produced by Fusarium fungi, exhibits estrogenic activity. Livestock can be exposed to ZON orally through contaminating feeds such as cereals, leading to reproductive disorders such as infertility and miscarriage via endocrine system disruption. However, the details of ZON metabolism remain unclear, and the mechanism of its toxicity has not been fully elucidated. In this study, we investigated the kinetics of ZON absorption and metabolism in rat segmented everted intestines. ZON absorption was confirmed in each intestine segment 60 min after application to the mucosal buffer at 10 µM. Approximately half of the absorbed ZON was metabolized to α-zearalenol, which tended to be mainly glucuronidated in intestinal cells. In the proximal intestine, most of the glucuronide metabolized by intestinal cells was excreted to the mucosal side, suggesting that the intestine plays an important role as a first drug metabolism barrier for ZON. However, in the distal intestine, ZON metabolites tended to be transported to the serosal side. Glucuronide transported to the serosal side could be carried via the systemic circulation to the local tissues, where it could be reactivated by deconjugation. These results are important with regard to the mechanism of endocrine disruption caused by ZON.

Modulation of Hepatic MRP3/ABCC3 by Xenobiotics and Pathophysiological Conditions: Role in Drug Pharmacokinetics

Liver transporters play an important role in the pharmacokinetics and disposition of pharmaceuticals, environmental contaminants, and endogenous compounds. Among them, the family of ATP-Binding Cassette (ABC) transporters is the most important due to its role in the transport of endo- and xenobiotics. The ABCC sub-family is the largest one, consisting of 13 members that include the cystic fibrosis conductance regulator (CFTR/ABCC7); the sulfonylurea receptors (SUR1/ABCC8 and SUR2/ABCC9) and the multidrug resistanceassociated proteins (MRPs). The MRP-related proteins can collectively confer resistance to natural, synthetic drugs and their conjugated metabolites, including platinum-containing compounds, folate anti-metabolites, nucleoside and nucleotide analogs, among others. MRPs can be also catalogued into "long" (MRP1/ABCC1, -2/C2, -3/C3, -6/C6, and -7/C10) and "short" (MRP4/C4, -5/C5, -8/C11, -9/C12, and -10/C13) categories. While MRP2/ABCC2 is expressed in the canalicular pole of hepatocytes, all others are located in the basolateral membrane. In this review, we summarize information from studies examining the changes in expression and regulation of the basolateral hepatic transporter MPR3/ABCC3 by xenobiotics and during various pathophysiological conditions. We also focus, primarily, on the consequences of such changes in the pharmacokinetic, pharmacodynamic and/or toxicity of different drugs of clinical use transported by MRP3.

Effects of Red Ginseng Extract on the Pharmacokinetics and Elimination of Methotrexate via Mrp2 Regulation

We aimed to investigate the effects of red ginseng extract (RGE) on the expression of efflux transporters and to study the pharmacokinetics of representative substrate. For this, rats received single or repeated administration of RGE (1.5 g/kg/day) for 1 and 2 weeks via oral gavage. mRNA and protein levels of multidrug resistance-associated protein2 (Mrp2), bile salt export pump (Bsep), and P-glycoprotein (P-gp) in the rat liver were measured via real-time polymerase chain reaction and Western blot analysis. Ginsenosides concentrations from the rat plasma were also monitored using a liquid chromatography–tandem mass spectrometry (LC–MS/MS) system. Plasma concentrations of ginsenoside Rb1, Rb2, Rc, and Rd following repeated administration of RGE for 1 and 2 weeks were comparable but significantly higher than those after single administration of RGE. These dosing regimens did not induce significant biochemical abnormalities in the liver, kidneys, and lipid homeostasis. In the RGE repeated oral administration groups, the mRNA and protein levels of Mrp2 significantly decreased. Accordingly, we investigated the changes in the pharmacokinetics of methotrexate, a probe substrate for Mrp2, following intravenous administration of 3 mg/kg methotrexate to rats in the RGE 1-week repeated oral administration group, compared to that in the control group. Biliary excretion, but not urinary excretion, of methotrexate decreased in the RGE repeated administration group, compared to that in the control group. Consequently, the plasma concentrations of methotrexate slightly increased in the RGE repeated administration group. In conclusion, repeated administration of RGE for 1 week resulted in a decrease in Mrp2 expression without inducing significant liver or kidney damage. Pharmacokinetic herb–drug interaction between RGE and methotrexate might occur owing to the decrease in the mRNA and protein levels of Mrp2.

Investigation of hepatic warfarin metabolism activity in rodenticide-resistant black rats (Rattus rattus) in Tokyo by in situ liver perfusion

Anti-blood coagulation rodenticides, such as warfarin, have been used all over the world. They inhibit vitamin K epoxide reductase (VKOR), which is necessary for producing several blood clotting factors. This inhibition by rodenticides results in lethal hemorrhage in rodents. However, heavy usage of these agents has led to the appearance of rodenticide-resistant rats. There are two major mechanisms underlying this resistance, i.e., mutation of the target enzyme of warfarin, VKOR, and enhanced metabolism of warfarin. However, there have been few studies regarding the hepatic metabolism of warfarin, which should be related to resistance. To investigate warfarin metabolism in resistant rats, in situ liver perfusion of warfarin was performed with resistant black rats (Rattus rattus) from Tokyo, Japan. Liver perfusion is an in situ methodology that can reveal hepatic function specifically with natural composition of the liver. The results indicated enhanced hepatic warfarin hydroxylation activity compared with sensitive black rats. On the other hand, in an in vitro microsomal warfarin metabolism assay to investigate kinetic parameters of cytochrome P450, which plays a major role in warfarin hydroxylation, the V_max of resistant rats was slightly but significantly higher compared to the results obtained in the in situ study. These results indicated that another factor like electron donators may also contribute to the enhanced metabolism in addition to high expression of cytochrome P450.

Ursodeoxycholate Restores Biliary Excretion of Methotrexate in Rats with Ethinyl Estradiol Induced-Cholestasis by Restoring Canalicular Mrp2 Expression

The in vivo relevance of ursodeoxycholate (UDCA) treatment (100 mg/kg/day, per oral tid for 5 days before cholestasis induction followed by the same dosing for 5 days) on hepatic function was investigated in rats with 17α-ethinylestradiol (EE, 10 mg/kg, subcutaneous for 5 days)-induced experimental cholestasis. The bile flow rate and the expression level of hepatic multidrug resistance-associated protein 2 (Mrp 2) that were decreased in cholestasis were restored after UDCA treatment. Consistent with this, the biliary excretion clearance (CL_exc,bile) of a representative Mrp2 substrate—methotrexate (MTX)—was decreased in cholestatic rats but was restored after UDCA treatment. Consequently, the plasma concentrations of MTX, which were increased by cholestasis, were decreased to control levels by UDCA treatment. Thus, the restoration of CL_exc,bile appears to be associated with the increase in Mrp2 expression on the canalicular membrane by UDCA treatment followed by Mrp2-mediated biliary excretion of MTX. On the other hand, the hepatic uptake clearance (CL_up,liver) of MTX was unchanged by cholestasis or UDCA treatment, suggestive of the absence of any association between the uptake process and the overall biliary excretion of MTX. Since UDCA has been known to induce the expression of canalicular MRP2 in humans, UDCA treatment might be effective in humans to maintain or accelerate the hepatobiliary elimination of xenobiotics or metabolic conjugates that are MRP2 substrates.

Sequencing of FIC1, BSEP and MDR3 in a large cohort of patients with cholestasis revealed a high number of different genetic variants

BACKGROUND & AIMS

The bile salt export pump (BSEP, ABCB11), multidrug resistance protein 3 (MDR3, ABCB4) and the ATPase familial intrahepatic cholestasis 1 (FIC1, ATP8B1) mediate bile formation. This study aimed to determine the contribution of mutations and common variants in the FIC1, BSEP and MDR3 genes to cholestatic disorders of differing disease onset and severity.

METHODS

Coding exons with flanking intron regions of ATP8B1, ABCB11, and ABCB4 were sequenced in cholestatic patients with assumed genetic cause. The effects of new variants were evaluated by bioinformatic tools and 3D protein modeling.

RESULTS

In 427 patients with suspected inherited cholestasis, 149 patients carried at least one disease-causing mutation in FIC1, BSEP or MDR3, respectively. Overall, 154 different mutations were identified, of which 25 were novel. All 13 novel missense mutations were disease-causing according to bioinformatics analyses and homology modeling. Eighty-two percent of patients with at least one disease-causing mutation in either of the three genes were children. One or more common polymorphism(s) were found in FIC1 in 35.3%, BSEP in 64.3% and MDR3 in 72.6% of patients without disease-causing mutations in the respective gene. Minor allele frequencies of common polymorphisms in BSEP and MDR3 varied in our cohort compared to the general population, as described by gnomAD. However, differences in ethnic background may contribute to this effect.

CONCLUSIONS

In a large cohort of patients, 154 different variants were detected in FIC1, BSEP, and MDR3, 25 of which were novel. In our cohort, frequencies for risk alleles of BSEP (p.V444A) and MDR3 (p.I237I) polymorphisms were significantly overrepresented in patients without disease-causing mutation in the respective gene, indicating that these common variants can contribute to a cholestatic phenotype.

LAY SUMMARY

FIC1, BSEP, and MDR3 represent hepatobiliary transport proteins essential for bile formation. Genetic variants in these transporters underlie a broad spectrum of cholestatic liver diseases. To confirm a genetic contribution to the patients' phenotypes, gene sequencing of these three major cholestasis-related genes was performed in 427 patients and revealed 154 different variants of which 25 have not been previously reported in a database. In patients without a disease-causing mutation, common genetic variants were detected in a high number of cases, indicating that these common variants may contribute to cholestasis development.

Pharmacokinetics of Jaspine B and Enhancement of Intestinal Absorption of Jaspine B in the Presence of Bile Acid in Rats

We aimed to investigate the pharmacokinetics and the underlying mechanisms of the intestinal absorption, distribution, metabolism, and excretion of Jaspine B in rats. The oral bioavailability of Jaspine B was 6.2%, but it decreased to 1.6% in bile-depleted rats and increased to 41.2% (normal) and 23.5% (bile-depleted) with taurocholate supplementation (60 mg/kg). Consistent with the increased absorption in the presence of bile salts, rat intestinal permeability of Jaspine B also increased in the presence of 10 mM taurocholate or 20% bile. Further studies demonstrated that the enhanced intestinal permeability with bile salts was due to increased lipophilicity and decreased membrane integrity. Jaspine B was designated as a highly tissue-distributed compound, because it showed large tissue to plasma ratios in the brain, kidney, heart, and spleen. Moreover, the recovery of Jaspine B from the feces and urine after an intravenous administration was about 6.3%, suggesting a substantial metabolism of Jaspine B. Consistent with this observation, 80% of the administered Jaspine B was degraded after 1 h incubation with rat liver microsomes. In conclusion, the facilitated intestinal permeability in the presence of bile salts could significantly increase the bioavailability of Jaspine B and could lead to the development of oral formulations of Jaspine B with bile salts. Moreover, the highly distributed features of Jaspine B in the brain, kidney, heart, and spleen should be carefully considered in the therapeutic effect and toxicity of this compound.

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

Increased Renal Clearance of Rocuronium Compensates for Chronic Loss of Bile Excretion, via upregulation of Oatp2

Requirement for rocuronium upon surgery changes only minimally in patients with end-stage liver diseases. Our study consisted of both human and rat studies to explore the reason. The reduction rate of rocuronium infusion required to maintain neuromuscular blockade during the anhepatic phase (relative to paleohepatic phase) was examined in 16 children with congenital biliary atresia receiving orthotopic liver transplantation. Pharmacodynamics and pharmacokinetics of rocuronium were studied based on BDL rats. The role of increased Oatp2 and decrease Oatp1 expressions in renal compensation were explored. The reduction of rocuronium requirements significantly decreased in obstructively jaundiced children (24 ± 9 vs. 39 ± 11%). TOF50 in BDL rats was increased by functional removal of the kidneys but not the liver, and the percentage of rocuronium excretion through urine increased (20.3 ± 6.9 vs. 8.6 ± 1.8%), while that decreased through bile in 28d-BDL compared with control group. However, this enhanced renal secretion for rocuronium was eliminated by Oatp2 knock-down, rather than Oatp1 overexpression (28-d BDL vs. Oatp1-ShRNA or Oatp2-ShRNA, 20.3 ± 6.9 vs. 17.0 ± 6.6 or 9.3 ± 3.2%). Upon chronic/sub-chronic loss of bile excretion, rocuronium clearance via the kidneys is enhanced, by Oatp2 up-regulation.

Role of enterohepatic recirculation in drug disposition: cooperation and complications

Enterohepatic recirculation (EHC) concerns many physiological processes and notably affects pharmacokinetic parameters such as plasma half-life and AUC as well as estimates of bioavailability of drugs. Also, EHC plays a detrimental role as the compounds/drugs are allowed to recycle. An in-depth comprehension of this phenomenon and its consequences on the pharmacological effects of affected drugs is important and decisive in the design and development of new candidate drugs. EHC of a compound/drug occurs by biliary excretion and intestinal reabsorption, sometimes with hepatic conjugation and intestinal deconjugation. EHC leads to prolonged elimination half-life of the drugs, altered pharmacokinetics and pharmacodynamics. Study of the EHC of any drug is complicated due to unavailability of the apposite model, sophisticated procedures and ethical concerns. Different in vitro and in vivo methods for studies in experimental animals and humans have been devised, each having its own merits and demerits. Involvement of the different transporters in biliary excretion, intra- and inter-species, pathological and biochemical variabilities obscure the study of the phenomenon. Modeling of drugs undergoing EHC has always been intricate and exigent models have been exploited to interpret the pharmacokinetic profiles of drugs witnessing multiple peaks due to EHC. Here, we critically appraise the mechanisms of bile formation, factors affecting biliary drug elimination, methods to estimate biliary excretion of drugs, EHC, multiple peak phenomenon and its modeling.

Genetic variations of bile salt transporters

Bile salt transporters directly or indirectly influence biological processes through physicochemical or signalling properties of bile salts. The coordinated action of uptake and efflux transporters in polarized epithelial cells of the liver, biliary tree, small intestine and kidney determine bile salt concentrations in different compartments of the body. Genetic variations of bile salt transporters lead to clinical relevant phenotypes of varying severity ranging from a predisposition for drug-induced liver injury to rapidly progressing end-stage liver disease. This review focuses on the impact of genetic variations of bile salt transporters including BSEP, NTCP, ASBT and OSTα/β and discusses approaches for transporter analysis.

Spheroid culture for enhanced differentiation of human embryonic stem cells to hepatocyte-like cells

Stem cell-derived hepatocyte-like cells hold great potential for the treatment of liver disease and for drug toxicity screening. The success of these applications hinges on the generation of differentiated cells with high liver specific activities. Many protocols have been developed to guide human embryonic stem cells (hESCs) to differentiate to the hepatic lineage. Here we report cultivation of hESCs as three-dimensional aggregates that enhances their differentiation to hepatocyte-like cells. Differentiation was first carried out in monolayer culture for 20 days. Subsequently cells were allowed to self-aggregate into spheroids. Significantly higher expression of liver-specific transcripts and proteins, including Albumin, phosphoenolpyruvate carboxykinase, and asialoglycoprotein receptor 1 was observed. The differentiated phenotype was sustained for more than 2 weeks in the three-dimensional spheroid culture system, significantly longer than in monolayer culture. Cells in spheroids exhibit morphological and ultrastructural characteristics of primary hepatocytes by scanning and transmission electron microscopy in addition to mature functions, such as biliary excretion of metabolic products and cytochrome P450 activities. This three-dimensional spheroid culture system may be appropriate for generating high quality, functional hepatocyte-like cells from ESCs.

Liver slices in in vitro pharmacotoxicology with special reference to the use of human liver tissue

In the early years of research in in vitro pharmacotoxicology liver slices have been used. After a decline in the application of slices in favour of the use of isolated hepatocytes and the isolated perfused liver preparation, the development of the Krumdieck slicer in the 1980s led to a ;comeback' of the technique. This review will focus on the use of human liver, with special reference to the comparison of slices with isolated hepatocytes in in vitro pharmacotoxicology. In addition, an overview on the predictive value of these in vitro systems for drug disposition and toxicity in vivo will be given. Preservation techniques for liver slices and hepatocytes will also be discussed. These techniques ensure an efficient utilization of the scarce human material. For long-term storage of liver slices and hepatocytes, cryopreservation seems most promising. However, cryopreservation is still in its infancy, and reports mainly deal with drug metabolism studies after cryopreservation. Drug toxicity, metabolism and transport data determined in slices and isolated hepatocytes, from both human and animal liver showed good correlation with the corresponding parameters measured in vivo. Therefore, the results obtained in such studies may give rise to more in-depth research on the mechanisms of pharmactoxicology in the human liver.

Anti-tumor necrosis factor-α monoclonal antibody alleviates parenteral nutrition-associated liver disease in mice

BACKGROUND

The authors aimed to investigate the role of anti-tumor necrosis factor (TNF)-α monoclonal antibody treatment in a mouse model of parenteral nutrition-associated liver disease (PNALD).

METHODS

C57BL/6J male mice (aged 6-8 weeks) were randomly assigned to 3 groups: parenteral nutrition (PN), PN with anti-TNF-α monoclonal antibody treatment (PN + mAb), and controls. A central venous catheter was inserted for intravenous infusion of a PN solution (PN and PN + mAb groups) or saline (controls) for 7 days. Liver pathology, hepatic biochemical indicators, and serum TNF-α concentrations were analyzed. Levels of hepatic bsep, mdr1a/mdr1b, mdr2, and mrp2 mRNA were also evaluated in each group.

RESULTS

The PN group showed significant increases in serum transaminase, direct bilirubin, and bile acids relative to the control group (P < .05). Histopathological changes in this group were consistent with early stage cholestasis. The pathological score and serum alanine aminotransferase, total bilirubin, and direct bilirubin levels were improved in the PN + mAb group relative to the PN group (P < .05). The PN group showed significantly lower hepatic bsep, mdr1a/mdr1b, mdr2, and mrp2 mRNA expression than the controls (P < .05), but these were significantly increased compared to the PN group (P < .05).

CONCLUSIONS

Infliximab administered at a single dose of 5 mg/kg body weight ameliorated the progression of PNALD and improved the expression of hepatic ABC transporter genes. Therefore, anti-TNF-α monoclonal antibody may be a beneficial therapy for patients with PNALD.

Lipid regulation effects of Polygoni Multiflori Radix, its processed products and its major substances on steatosis human liver cell line L02

ETHNOPHARMACOLOGICAL RELEVANCE

Raw and processed Polygoni Multiflori Radix (PMR) are used in the prevention and treatment of non-alcoholic fatty liver disease (NAFLD), hyperlipidemia or related diseases. However, few researches compared the activities of raw and processed PMR on lipid metabolism regulation. Moreover, the active substances of Polygonum multiflorum are still not clearly elucidated.

MATERIALS AND METHODS

In this research, a sensitive, accurate and rapid in vitro model, steatosis hepatic L02 cell, was applied to compare the relative activities of raw and processed PMR on lipid metabolism regulation. Furthermore, the lipid regulation activities of emodin, physcion and 2,3,5,4'-tetrahydroxy-stilbene-2-O-β-d-glucoside (TSG) were evaluated. The steatosis L02 cells were obtained after cultured with 1% fat emulsion-10% fetal bovine serum (FBS)-RPMI 1640 medium for 48h. Contents of total cholesterol (TC), triglyceride (TG) and low-density lipoprotein cholesterol (LDL-C) in L02 cells are evaluated after exposure.

RESULTS

The intracellular TG contents were increased from 16.50±1.29mmol/L to 34.40±1.36mmol/L in steatosis L02 cells, while the intracellular contents of TC were increased from 5.07±1.80mmol/L to 11.79±0.54mmol/L. Water extract of raw PMR showed much remarkable TG-regulation and TC-regulation effects than its processed products. Emodin displayed the best TG regulation activity while TSG showed the best TC regulation activity. At the same time, the exposure of emodin and physcion could reduce the LDL-C contents in steatosis L02 cells.

CONCLUSIONS

On account of these in vitro results, raw PMR might have more satisfactory effects in clinic treatment of NAFLD or hyperlipidemia characterized by the elevation of cholesterol than processed PMR.

Collection of biological samples in forensic toxicology

Forensic toxicology is the study and practice of the application of toxicology to the purposes of the law. The relevance of any finding is determined, in the first instance, by the nature and integrity of the specimen(s) submitted for analysis. This means that there are several specific challenges to select and collect specimens for ante-mortem and post-mortem toxicology investigation. Post-mortem specimens may be numerous and can endow some special difficulties compared to clinical specimens, namely those resulting from autolytic and putrefactive changes. Storage stability is also an important issue to be considered during the pre-analytic phase, since its consideration should facilitate the assessment of sample quality and the analytical result obtained from that sample. The knowledge on degradation mechanisms and methods to increase storage stability may enable the forensic toxicologist to circumvent possible difficulties. Therefore, advantages and limitations of specimen preservation procedures are thoroughfully discussed in this review. Presently, harmonized protocols for sampling in suspected intoxications would have obvious utility. In the present article an overview is given on sampling procedures for routinely collected specimens as well as on alternative specimens that may provide additional information on the route and timing of exposure to a specific xenobiotic. Last, but not least, a discussion on possible bias that can influence the interpretation of toxicological results is provided. This comprehensive review article is intented as a significant help for forensic toxicologists to accomplish their frequently overwhelming mission.

Effect of lipopolysaccharide on P-glycoprotein-mediated intestinal and biliary excretion of rhodamine123 in rats

The effects of lipopolysaccharide (LPS) on the ileal and biliary excretion of rhodamine123 were investigated in rats at different times after intraperitoneal (i.p.) injection (1 mg/kg and 5 mg/kg of body weight). P-gp protein decreased 8h after injection of LPS and returned to the control level 24h after i.p. injection of LPS in the ileum. There was a marked decrease in the expression level of mdr1a mRNA in the ileum and liver 8h after i.p. injection of LPS when compared with the control condition. Also, the ileal and biliary clearance of rhodamine123 significantly decreased 8h after i.p. injection of LPS, but returned to the control levels 24h after i.p. injection of LPS. These results suggest that LPS-induced decreases in P-gp-mediated ileal and biliary excretion of rhodamine123 were probably due to impaired P-gp-mediated transport ability. The levels of iNOS and IL-1beta mRNA in the ileum and liver increased 2 and 8h after i.p. injection of LPS, respectively, and returned to the control levels 24h after injection of LPS. These findings suggest that LPS markedly decreases P-gp-mediated ileal and biliary excretion of rhodamine123, probably by partly decreasing the expression of P-gp protein levels, likely due to increased lipid peroxidation levels through iNOS mRNA and inflammatory mediators such as IL-1beta.

Disposition of amphotericin B in the isolated perfused rat liver

The hepatic disposition and biliary excretion of amphotericin B were investigated in the isolated perfused rat liver (IPRL). Bolus dose of 50 μg, 99 μg and 198 μg amphotericin B in lipoprotein-free perfusate and 198 μg amphotericin B in perfusate with 1 μM high-density lipoprotein (HDL) or 1 μM low-density lipoprotein (LDL) were examined in the IPRL. Amphotericin B concentration in perfusate was measured using a validated HPLC assay. Amphotericin B was eliminated from the perfusate in a biexponential manner. The hepatic clearance (CLH) increased in proportion to the dose administered (0.27±0.05 mL min−1 at low dose, 0.54±0.23 mL min−1 at medium dose and 1.06±0.24 mL min−1 at high dose), indicating non-linear hepatic disposition of amphotericin B. The hepatic extraction ratio of amphotericin B was very low (0.066±0.015). Tissue-to-perfusion partition coefficient, calculated at 120 min, increased 1.5 fold from 9.8±1.7 at low dose to 15.9±6.4 at high dose, suggesting the significant uptake and extensive retention of amphotericin B in the liver. Biliary excretion made only minor contribution to amphotericin B elimination in the IPRL, representing around 1–3% of the dose administered. No metabolites were detected in perfusate, bile and liver samples. The hepatic disposition of amphotericin B was not affected by the presence of HDL and LDL in the perfusate. In conclusion, the hepatic disposition of amphotericin B demonstrates restrictive elimination and is concentration-dependent, consistent with carrier-mediated uptake, and lipoproteins do not influence amphotericin B hepatobiliary disposition.

Transport of N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine, a metabolite of trichloroethylene, by mouse multidrug resistance associated protein 2 (Mrp2)

N-acetyl-S-(1,2-dichlorovinyl)-l-cysteine (Ac-DCVC) and S-(1,2-dichlorovinyl)-l-cysteine (DCVC) are the glutathione conjugation pathway metabolites of a common industrial contaminant and potent nephrotoxicant trichloroethylene (TCE). Ac-DCVC and DCVC are accumulated in the renal proximal tubule where they may be secreted into the urine by an unknown apical transporter(s). In this study, we explored the hypothesis that the apical transport of Ac-DCVC and/or DCVC may be mediated by the multidrug resistance associated protein 2 (Mrp2, ABCC2), which is known to mediate proximal tubular apical ATP-dependent transport of glutathione and numerous xenobiotics and endogenous substances conjugated with glutathione. Transport experiments using membrane vesicles prepared from mouse proximal tubule derived cells expressing mouse Mrp2 utilizing ATPase assay and direct measurements of Ac-DCVC/DCVC using liquid chromatography/tandem mass-spectrometry (LC/MS/MS) demonstrated that mouse Mrp2 mediates ATP-dependent transport of Ac-DCVC. Expression of mouse Mrp2 antisense mRNA significantly inhibited the vectorial basolateral to apical transport of Ac-DCVC but not DCVC in mouse proximal tubule derived cells endogenously expressing mouse Mrp2. The results suggest that Mrp2 may be involved in the renal secretion of Ac-DCVC.

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.

Drug uptake systems in liver and kidney: a historic perspective

Drugs and their metabolites are eliminated mainly by excretion into urine and bile. Studies in whole animals, isolated organs, cells, and membrane vesicles led to the conclusion that different transport systems are responsible for the transport of different classes of organic compounds (small, large, anionic, and cationic). In the early 1990s, functional expression cloning resulted in the identification of the first transporters for organic anions and cations. Eventually, all the major transport systems involved in the uptake of these organic compounds were cloned and characterized, and we now know that they belong to the organic anion transporters (OATs) and organic cation transporters (OCTs) of the SLC22A superfamily and the organic anion-transporting polypeptides (OATPs) of the SLCO superfamily of polyspecific drug transporters. Today we can explain, at the molecular level, why small and hydrophilic organic compounds are excreted predominantly through urine whereas large and amphipathic compounds are excreted mainly through bile, and we can start to predict drug-drug interactions in the case of new compounds.

Alteration of canalicular transporters in a mouse model of total parenteral nutrition

OBJECTIVES

Parenteral nutrition-associated liver disease (PNALD) is a major problem with prolonged total parenteral nutrition (TPN) administration. Our laboratory previously demonstrated significant changes in the expression of multidrug resistance genes (MDRs) 1 and 2, hepatocyte transporters, in a TPN mouse model. The present study hypothesized that these changes would lead to functional changes in the liver, and would contribute to the development of liver dysfunction.

MATERIALS AND METHODS

Mice received either intravenous saline and standard chow or TPN with or without intravenous lipids. Functional assays were performed after 7 days of infusion.

RESULTS

TPN with lipids led to a significant increase in serum bile acid levels, consistent with an early state of PNALD. Use of TPN without lipids prevented an elevation in bile acid levels. In both TPN groups, MDR2 expression was significantly (68%) lower than controls and bile phosphatidylcholine content, a functional measure of MDR2, was 40% less than controls. MDR1 expression in the TPN with lipid group was 31% higher than controls, whereas in the TPN without lipids mice there was no significant change. Hepatocyte extrusion of rhodamine dye, a measure of MDR1 function, declined only in the TPN with lipid group. Peroxisome proliferator-activated receptor-alpha expression decreased in both TPN groups. Fenofibrate given with TPN resulted in an increased expression of MDR1 and MDR2, and functionally increased hepatocyte rhodamine extrusion and presence of bile phosphatidylcholine in the TPN with lipid group.

CONCLUSIONS

The study shows that TPN led to alterations in the function of MDR1- and MDR2-expressed proteins. The changes help in the understanding of the mechanisms leading to PNALD, and suggest that fibrate administration may palliate these changes.

Interplay of conjugating enzymes with OATP uptake transporters and ABCC/MRP efflux pumps in the elimination of drugs

BACKGROUND

Biliary excretion is a major elimination route of many drugs and their metabolites. Hepatobiliary elimination is a vectorial process involving uptake transporters in the basolateral hepatocyte membrane, possibly Phase I and Phase II metabolizing enzymes, and ATP-dependent efflux pumps in the apical hepatocyte membrane.

OBJECTIVES

Because many drugs and their metabolites are anions, this review focuses on transporters involved in their hepatocellular uptake (members of the organic anion transporting polypeptide (OATP) family) and biliary elimination (apical conjugate efflux pump ABCC2/MRP2).

METHODS

The molecular and functional characteristics of the human OATP and ABCC/MRP transporters are presented, including a detailed overview of endogenous and drug substrates. Examples illustrate the interplay of transporters with Phase II conjugating enzymes. Model systems to study the vectorial transport of organic anions are also discussed.

RESULTS/CONCLUSIONS

OATP uptake transporters, conjugating enzymes, and ABCC2/MRP2 work in concert to enable the hepatobiliary elimination of anionic drugs and their metabolites. It is increasingly important to understand how genetic variants of these transporters and enzymes influence the interindividual variability of drug elimination.

How we have learned about the complexity of physiology, pathobiology and pharmacology of bile acids and biliary secretion

During the last decades the concept of bile secretion as merely a way to add detergent components to the intestinal mixture to facilitate fat digestion/absorption and to eliminate side products of heme metabolism has evolved considerably. In the series of mini-reviews that the World Journal of Gastroenterology is to publish in its section of “Highlight Topics”, we will intend to give a brief but updated overview of our knowledge in this field. This introductory letter is intended to thank all scientists who have contributed to the development of this area of knowledge in gastroenterology.

Effect of albumin on the biliary clearance of compounds in sandwich-cultured rat hepatocytes

The purpose of the present study was to evaluate the effects of bovine serum albumin (BSA) and essentially fatty acid-free BSA (BSA-FAF) on the biliary clearance of compounds in sandwich-cultured rat hepatocytes. Unbound fraction, biliary excretion index (BEI), and unbound intrinsic biliary clearance (intrinsic Clbiliary') were determined for digoxin, pravastatin, and taurocholate in the absence or presence of BSA or BSA-FAF. BSA had little effect on the BEI or intrinsic Clbiliary' of these compounds. Surprisingly, BSA-FAF decreased both BEI and intrinsic Clbiliary' for digoxin and pravastatin, which represent low and moderately bound compounds, respectively. The BEI and intrinsic Clbiliary' of taurocholate, a highly bound compound, were not altered significantly by BSA-FAF. Neither BSA nor BSA-FAF had a discernable effect on the bile canalicular networks based on carboxydichlorofluorescein retention. Neither the addition of physiological concentrations of calcium nor the addition of fatty acids to BSA-FAF was able to restore the BEI or intrinsic Clbiliary' of the model compounds to similar values in the absence or presence of BSA. Careful consideration is warranted when selecting the type of BSA for addition to in vitro systems such as sandwich-cultured rat hepatocytes.

Disturbed colocalization of multidrug resistance protein 2 and radixin in human cholestatic liver diseases

BACKGROUND AND AIM

Endocytic retrieval of multidrug resistance protein 2 (MRP2) is closely associated with cholestasis and may be attributed to the disturbed linking of MRP2 and radixin, a cross-linker between actin filaments and membrane proteins. This study aimed to investigate the role of radixin in the altered localization of MRP2 in various human cholestatic liver diseases.

METHODS

Using immunofluorescence microscopy, we investigated the localization and expression of MRP2 and radixin in various cholestatic liver diseases, such as drug-induced liver injury, obstructive jaundice, primary sclerosing cholangitis and autoimmune hepatitis. Changes in localization and expression were analyzed using Scion Image (software).

RESULTS

In the icteric drug-induced liver injury, MRP2 was localized outside as well as inside of ZO-1 staining, indicating endocytic retrieval from the canalicular membrane into the pericanalicular compartments of the hepatocytes. The colocalization of MRP2 and radixin observed in the controls was disturbed, and MRP2 fluorescence disappeared in the canaliculi with disrupted radixin staining. Disturbed colocalization of MRP2 and radixin as well as endocytic retrieval of MRP2 was found in the poorly drained obstructive jaundice. When drainage was good, MRP2 was exclusively colocalized with radixin. Similar findings were observed in autoimmune hepatitis and primary sclerosing cholangitis. In the controls, the immunostaining intensity curves for MRP2 and radixin were steeply elevated in the canaliculi. The intensity curves for MRP2 and radixin were broadened in the icteric drug-induced liver injury and poorly drained obstructive jaundice, indicating endocytic retrieval into the hepatocytes. The peak fluorescence intensities for MRP2 and radixin decreased in the icteric liver.

CONCLUSION

Disturbed colocalization of MRP2 and radixin was common in various cholestatic liver diseases, which may be associated with endocytic retrieval of MRP2 due to failure in anchoring MRP2 in the canalicular membrane.

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

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

Hepatobiliary excretion of silibinin in normal and liver cirrhotic rats

Silibinin is the main biologically active flavonolignan extracted from the seeds and fruits of milk thistle and has potential efficacy in the treatment of liver disease. The aim of the present study was to examine the hepatobiliary excretion of silibinin and its effect on dimethylnitrosamine (DMN)-induced liver cirrhosis. The experiments were divided into five groups: 10, 30, and 50 mg/kg silibinin alone, 30 mg/kg silibinin coadministered with cyclosporin A (CsA), and 50 mg/kg silibinin with liver cirrhosis induced by DMN. The data indicated that silibinin had dose-related pharmacokinetics in the dose ranges of 10 to 50 mg/kg. All of the unconjugated or total (unconjugated + conjugated) silibinin concentrations in the bile were significantly higher than those in plasma at the sampling time points at each dose, suggesting active hepatobiliary excretion. When coadministered with CsA, the area under the concentration versus time curve (AUC) in bile was significantly decreased. This result suggested that the active silibinin efflux might be partially inhibited by P-glycoprotein. In the DMN-induced liver cirrhotic rats, the AUC of plasma unconjugated silibinin was reduced by 53%; however, total silibinin was increased by 182%. These results together suggest that the phase II conjugative reaction of silibinin was blocked by treatment with DNM.

The role of radixin in altered localization of canalicular conjugate export pump Mrp2 in cholestatic rat liver

AIM

Cholestasis has been associated with the endocytic retrieval of multidrug resistance protein 2 (Mrp2), but its mechanism is still unclear. Recent studies have indicated that radixin, a cross-linker between the actin filaments and membrane proteins, may be activated by phosphorylation and may be required for the canalicular localization of Mrp2.

METHODS

We investigated the role of radixin in the altered localization of Mrp2 in rat models of intrahepatic (ethinyl estradiol treatment) and extrahepatic (bile duct ligation) cholestasis using immunofluorescence microscopy. The changes in localization and expression were analyzed using Scion Image for Windows.

RESULTS

In both models, Mrp2 was localized outside as well as inside the ZO-1 staining, indicating partial dislocation from the canalicular membrane. In contrast to the steep elevation of the immunostaining intensity curves for Mrp2 in the controls, the corresponding curves in both models were broadened and flattened, confirming endocytic retrieval into the hepatocytes. Mrp2 and radixin were colocalized at the canalicular domain in the controls, whereas in both cholestatic rats they were dissociated at some canaliculi, indicating the disturbed colocalization of Mrp2 and radixin in cholestasis. The fluorescence of phosphorylated radixin, an active form of radixin, markedly decreased in both cholestatic models, which was supported by the reduced peak fluorescence intensities.

CONCLUSION

The disturbed colocalization of Mrp2 and radixin may contribute to the endocytic retrieval of Mrp2 in cholestasis due to the failure to anchor Mrp2 in the canalicular membrane, in which the phosphorylated radixin may play a major role.

Disruption of function and localization of tight junctional structures and Mrp2 in sustained estradiol-17beta-D-glucuronide-induced cholestasis

Estradiol-17beta-D-glucuronide (E(2)17G) induces immediate and profound but transient cholestasis in rats when administered as a single bolus dose. Here, we examined the consequence of sustained E(2)17G cholestasis and assessed the function and localization of the tight junctional proteins zonula occludens-1 (ZO-1) and occludin and of the canalicular transporter multidrug resistance-associated protein-2 (Mrp2). An initial dose of E(2)17G (15 mumol/kg iv) followed by five subsequent doses of 7.5 mumol/kg from 60 to 240 min induced a sustained 40-70% decrease in bile flow. Following their biliary retrograde administration, cholera toxin B subunit-FITC or horseradish peroxidase were detected at the sinusoidal domain, indicating opening of the paracellular route; this occurred as early as 15 min after the first dose as well as 15 min after the last dose of E(2)17G, but not following the administration of vehicle in controls. Localization of ZO-1 and occludin was only slightly affected under acute cholestatic conditions but was severely disrupted under sustained cholestasis, with their appearance suggesting a fragmented structure. Endocytic internalization of Mrp2 to the pericanalicular region was apparent 20 min after a single E(2)17G administration; however, Mrp2 was found more deeply internalized and partially redistributed to the basolateral membrane under sustained cholestasis. In conclusion, acute E(2)17G-induced cholestasis increased permeability of the tight junction, while sustained cholestasis provoked a significant redistribution of ZO-1, occludin, and Mrp2 in addition to increased permeability of the tight junction. Altered tight junction integrity likely contributes to impaired bile secretion and may be causally related to changes in Mrp2 localization.

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.

Down regulation of bisphenol A glucuronidation in carp during the winter pre-breeding season

Environmental pollution by bisphenol A is prevalent in many rivers. The influence of bisphenol A on the reproductive organs of carp has been demonstrated to be serious, especially in the winter pre-breeding season. Although bisphenol A is detoxified as bisphenol A-glucuronide in carp organs, principally the intestine, the seasonal variation in the efficiency of the detoxification is not known. To estimate the seasonal risk of bisphenol A in carp, we investigated seasonal changes in microsomal UDP-glucuronosyltransferase activity toward bisphenol A in male-carp. Seasonal elimination efficiency of bisphenol A was also examined by organ perfusion in everted intestine. No marked seasonal differences were observed in UDP-glucuronosyltransferase activity toward 1-naphthol, but high activity toward sex steroid hormones (testosterone and estradiol) was observed in the winter pre-breeding season. Low UDP-glucuronosyltransferase activity toward bisphenol A was indicated in winter. The addition of bisphenol A into the mucosal fluid of the everted intestine resulted in excretion of bisphenol A into the mucosal side of the intestine as the metabolite, bisphenol A-glucuronide. Excretion of bisphenol A-glucuronide from carp intestine was highest in summer (proximal intestine: 13.3 nmol; middle intestine: 8.3 nmol; distal intestine: 7.9 nmol) and lowest in winter (proximal intestine: 1.0 nmol; middle intestine: 1.0 nmol; distal intestine: 0.9 nmol). These results suggest that metabolism and excretion of bisphenol A in carp hepatopancreas and intestine are impaired by down regulation of UDP-glucuronosyltransferase activity in the winter pre-breeding season.

Wide variety of locations for rodent MATE1, a transporter protein that mediates the final excretion step for toxic organic cations

MATE1 was the first mammalian example of the multidrug and toxin extrusion (MATE) protein family to be identified. Human MATE1 (hMATE1) is predominantly expressed and localized to the luminal membranes of the urinary tubules and bile canaliculi and mediates H(+)-coupled electroneutral excretion of toxic organic cations (OCs) into urine and bile (Otsuka M, Matsumoto T, Morimoto R, Arioka S, Omote H, and Moriyama Y. Proc Natl Acad Sci USA 102: 17923-17928, 2005). mMATE1, a mouse MATE ortholog, is also predominantly expressed in kidney and liver, although its transport properties are not yet characterized. In the present study, we investigated the transport properties and localization of mMATE1. Upon expression of this protein in HEK-293 cells, mMATE1 mediated electroneutral H(+)/tetraethylammonium exchange and showed a substrate specificity similar to that of hMATE1. Immunological techniques with specific antibodies against mMATE1 combined with RT-PCR revealed that mMATE1 is also expressed in various cells, including brain glia-like cells and capillaries, pancreatic duct cells, urinary bladder epithelium, adrenal gland cortex, alpha cells of the islets of Langerhans, Leydig cells, and vitamin A-storing Ito cells. These results indicate that mMATE1 is a polyspecific H(+)/OC exchanger. The unexpectedly wide distribution of mMATE1 suggests involvement of this transporter protein in diverse biological functions other than excretion of OCs from the body.

The role of transporters in drug interactions

Transport proteins play an important role in the adsorption, distribution and elimination of a wide variety of drugs. Therefore, it is not surprising that transporter-based drug interactions can occur in the clinic. These interactions can lead to changes in toxicity and/or efficacy of the affected drug. Here, we review such interactions and ask if these interactions could have been predicted from in vitro data. Conducting such in vitro-in vivo correlation is important for predicting future transporter-based drug interactions.

Involvement of the drug transporters p glycoprotein and multidrug resistance-associated protein Mrp2 in telithromycin transport

The present study aims to investigate the role of P glycoprotein and multidrug resistance-associated protein (Mrp2) in the transport of telithromycin, a newly developed ketolide antibiotic, in vitro and in vivo. The in vitro experiments revealed that the intracellular accumulation of telithromycin in adriamycin-resistant human chronic myelogenous leukemia cells (K562/ADR) overexpressing P glycoprotein was significantly lower than that in human chronic myelogenous leukemia cells (K562/S) not expressing P glycoprotein. Cyclosporine significantly increased the intracellular accumulation of telithromycin in K562/ADR cells. When telithromycin was coadministered intravenously with cyclosporine in Sprague-Dawley (SD) rats, cyclosporine significantly delayed the disappearance of telithromycin from plasma and decreased its systemic clearance to 60% of the corresponding control values. Hepatobiliary excretion experiments revealed that cyclosporine almost completely inhibited the biliary clearance of telithromycin, suggesting that telithromycin is a substrate of P glycoprotein and a potential substrate of Mrp2. Moreover, the biliary clearance of telithromycin was significantly decreased by 80% in Eisai hyperbilirubinemic mutant rats with a hereditary deficiency in Mrp2, indicating that Mrp2, as well as P glycoprotein, plays an important role in the biliary excretion of telithromycin. When the effect of telithromycin on the biliary excretion of doxorubicin, a substrate of P glycoprotein and Mrp2, was examined in SD rats, telithromycin significantly decreased the biliary clearance of doxorubicin by 80%. Results obtained from this study indicate that telithromycin is a substrate of both P glycoprotein and Mrp2, and these transporters are involved in the hepatobiliary transport of telithromycin.