The apical conjugate efflux pump ABCC2 (MRP2)

Multidrug resistance protein 2 genetic polymorphism and colorectal cancer recurrence in patients receiving adjuvant FOLFOX-4 chemotherapy

Multidrug resistance protein 2 (MRP2), encoded by the ATP-binding cassette C2 (ABCC2) gene, is an efflux pump located on the apical membrane of many polarized cells, which transports conjugate compounds by an ATP-dependent mechanism. The correlation of G1249A ABCC2 polymorphism with the development of colorectal cancer (CRC) and poor prognosis was evaluated in patients who were treated with fluorouracil/-leucovorin (FL) plus oxaliplatin (FOLFOX-4). A total of 50 paraffin‑embedded tissue samples collected from CRC patients were analyzed to identify the polymorphism. Patients were in stage II/III and received postoperative FOLFOX-4 chemotherapy. As a control group, an equal number of unrelated healthy subjects were enrolled in the study. The polymorphism was genotyped by the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method, and results were compared with clinicopathological markers, early relapse and survival rates. During the 12 months of follow-up, local and distant recurrences were observed in 15 (30%) patients. No significant difference in the distribution of wild-type and polymorphic genotypes was observed between the patient and control groups and between the patients who experienced recurrence within 1 year and those who did not (all P>0.05). In conclusion, the G1249A polymorphism is not associated with CRC risk and early recurrence. However, significant correlation was observed between G1249A polymorphism and the overall survival and disease-free survival of the patients.

Expression and function of renal and hepatic organic anion transporters in extrahepatic cholestasis

Obstructive jaundice occurs in patients suffering from cholelithiasis and from neoplasms affecting the pancreas and the common bile duct. The absorption, distribution and elimination of drugs are impaired during this pathology. Prolonged cholestasis may alter both liver and kidney function. Lactam antibiotics, diuretics, non-steroidal anti-inflammatory drugs, several antiviral drugs as well as endogenous compounds are classified as organic anions. The hepatic and renal organic anion transport pathways play a key role in the pharmacokinetics of these compounds. It has been demonstrated that acute extrahepatic cholestasis is associated with increased renal elimination of organic anions. The present work describes the molecular mechanisms involved in the regulation of the expression and function of the renal and hepatic organic anion transporters in extrahepatic cholestasis, such as multidrug resistance-associated protein 2, organic anion transporting polypeptide 1, organic anion transporter 3, bilitranslocase, bromosulfophthalein/bilirubin binding protein, organic anion transporter 1 and sodium dependent bile salt transporter. The modulation in the expression of renal organic anion transporters constitutes a compensatory mechanism to overcome the hepatic dysfunction in the elimination of organic anions.

Cholestatic effect of epigallocatechin gallate in rats is mediated via decreased expression of Mrp2

Epigallocatechin gallate (EGCG) has been shown to be protective in various experimental models of liver injury, although opposite effects have also been reported. Since its effect on biliary physiology has not been thoroughly investigated, the present study evaluated effect of EGCG on bile flow and bile acid homeostasis in rats. Compared to controls, EGCG treatment decreased bile flow by 23%. Hepatic paracellular permeability and biliary bile acid excretion were not altered by EGCG administration, but biliary glutathione excretion was reduced by 70%. Accordingly, the main glutathione transporter on the hepatocyte canalicular membrane, multidrug resistance-associated protein 2 (Mrp2), was significantly decreased at the protein level. EGCG administration also doubled plasma bile acid levels compared to controls. While protein levels of the main hepatic bile acid transporters were unchanged, the rate-limiting enzyme in the bile acid synthesis, Cyp7a1, was significantly increased by EGCG. Enhanced bile acid synthesis in these animals was also confirmed by a 2-fold increase in plasma marker 7α-hydroxy-4-cholesten-3-one. In contrast, EGCG markedly downregulated major bile acid transporters (Asbt and Ostα) and regulatory molecules (Shp and Fgf15) in the ileum. When EGCG was coadministered with ethinylestradiol, a potent cholestatic agent, it did not show any additional effect on the induced cholestasis. This study shows ability of EGCG to raise plasma bile acid concentrations, mainly through Cyp7a1 upregulation, and to decrease bile production through reduction in Mrp2-mediated bile acid-independent bile flow. In conclusion, our data demonstrate that under certain conditions EGCG may induce cholestasis.

A case of severe toxicity during coadministration of vincristine and piperacillin: are drug transporters involved in vincristine hypersensitivity and drug-drug interactions?

Neurotoxicity is frequent with vincristine treatment, but severe autonomic neuropathy is rare. A decreased activity of drug transporters in the presence of an interacting drug may favor such events by increasing systemic or tissue exposure to the drug. We encountered severe autonomic neuropathy and cholestasis in a child receiving vincristine, after the introduction of piperacillin-tazobactam. A causality assessment of the adverse reaction identified the antibiotic as the most probable cause of the observation. The patient was heterozygous for several common polymorphisms of ABCC2 (multidrug-related protein-2), CYP3A5, and ABCB1 (multidrug-related protein-1, P-glycoprotein), but their role in the toxicity cannot be ascertained.

Plasma membrane transporters in modern liver pharmacology

The liver plays a crucial role in the detoxification of drugs used in the treatment of many diseases. The liver itself is the target for drugs aimed to modify its function or to treat infections and tumours affecting this organ. Both detoxification and pharmacological processes occurring in the liver require the uptake of the drug by hepatic cells and, in some cases, the elimination into bile. These steps have been classified as detoxification phase 0 and phase III, respectively. Since most drugs cannot cross the plasma membrane by simple diffusion, the involvement of transporters is mandatory. Several members of the superfamilies of solute carriers (SLC) and ATP-binding cassette (ABC) proteins, with a minor participation of other families of transporters, account for the uptake and efflux, respectively, of endobiotic and xenobiotic compounds across the basolateral and apical membranes of hepatocytes and cholangiocytes. These transporters are also involved in the sensitivity and refractoriness to the pharmacological treatment of liver tumours. An additional interesting aspect of the role of plasma membrane transporters in liver pharmacology regards the promiscuity of many of these carriers, which accounts for a variety of drug-drug, endogenous substances-drug and food components-drug interactions with clinical relevance.

Differentiation and selection of hepatocyte precursors in suspension spheroid culture of transgenic murine embryonic stem cells

Embryonic stem cell-derived hepatocyte precursor cells represent a promising model for clinical transplantations to diseased livers, as well as for establishment of in vitro systems for drug metabolism and toxicology investigations. This study aimed to establish an in vitro culture system for scalable generation of hepatic progenitor cells. We used stable transgenic clones of murine embryonic stem cells possessing a reporter/selection vector, in which the enhanced green fluorescent protein- and puromycin N-acetyltransferase-coding genes are driven by a common alpha-fetoprotein gene promoter. This allowed for "live" monitoring and puromycin selection of the desired differentiating cell type possessing the activated alpha-fetoprotein gene. A rotary culture system was established, sequentially yielding initially partially selected hepatocyte lineage-committed cells, and finally, a highly purified cell population maintained as a dynamic suspension spheroid culture, which progressively developed the hepatic gene expression phenotype. The latter was confirmed by quantitative RT-PCR analysis, which showed a progressive up-regulation of hepatic genes during spheroid culture, indicating development of a mixed hepatocyte precursor-/fetal hepatocyte-like cell population. Adherent spheroids gave rise to advanced differentiated hepatocyte-like cells expressing hepatic proteins such as albumin, alpha-1-antitrypsin, cytokeratin 18, E-cadherin, and liver-specific organic anion transporter 1, as demonstrated by fluorescent immunostaining. A fraction of adherent cells was capable of glycogen storage and of reversible up-take of indocyanine green, demonstrating their hepatocyte-like functionality. Moreover, after transplantation of spheroids into the mouse liver, the spheroid-derived cells integrated into recipient. These results demonstrate that large-scale hepatocyte precursor-/hepatocyte-like cultures can be established for use in clinical trials, as well as in in vitro screening assays.

Cisplatin-induced chemoresistance in colon cancer cells involves FXR-dependent and FXR-independent up-regulation of ABC proteins

Export pumps often limit the usefulness of anticancer drugs. Here we investigated the effect of cisplatin on the expression of ABC proteins in human colon cancer cells. Short-term incubation of Caco-2 and LS174T cells with cisplatin resulted in up-regulation of several ABC pumps, in particular MRP2 and BCRP. In partially cisplatin-resistant cells (LS174T/R) obtained by long-term exposure to cisplatin, MRP2 and BCRP up-regulation was more marked. This was further enhanced when these cells were cultured under maintained stimulation with cisplatin. The MRP2 promoter (MRP2pr) was cloned, and partially deleted constructs linked to reporter genes were generated. Transfection of LS174T and LS174T/R cells with these constructs revealed the ability of cisplatin to activate MRP2pr. The intensity of this response was dependent on the conserved MRP2pr region. Basal MRP2pr activity was higher in LS174T/R cells, in which the expression of the transcription factors c/EBPβ, HNF1α, HNF3β, and HNF4α, but not PXR, p53, c-Myc, AP1, YB-1, NRF2, and RARα was enhanced. Up-regulation was particularly high for FXR (200-fold) and SHP (50-fold). In LS174T/R cells, GW4064 induced the expression of FGF19, SHP, OSTα/β, but not MRP2 and BCRP, although the sensitivity of these cells to cisplatin was further reduced. In LS174T cells, GW4064-induced chemoresistance was seen only after being transfected with FXR+RXR, when BCRP, but not MRP2, was up-regulated. Protection of LS174T cells against cisplatin was mimicked by transfection with BCRP. In conclusion, in colon cancer cells, cisplatin treatment enhances chemoresistance through FXR-dependent and FXR-independent mechanisms involving the expression of BCRP and MRP2, respectively.

A new herb-drug interaction of Polygonum cuspidatum, a resveratrol-rich nutraceutical, with carbamazepine in rats

Carbamazepine (CBZ), an antiepileptic with narrow therapeutic window, is a substrate of CYP 3A which metabolizes CBZ to carbamazepine-10,11-epoxide (CBZE), an active metabolite. This study investigated the acute and chronic effects of Polygonum cuspidatum (PC), a resveratrol-rich nutraceutical, on the pharmacokinetics of CBZ in rats and the underlying mechanisms. Rats were orally administered CBZ (200 mg/kg) alone and coadministered with a single dose and the 7th dose of PC (2 g/kg) in a crossover design. The concentrations of CBZ and CBZE in serum and various tissues were determined by HPLC method. The results showed that PC significantly increased the AUC(0-t) of CBZ and CBZE, whereas the formation rate of CBZE was decreased. Tissue analysis showed that the concentrations of CBZ and CBZE in brain, liver and kidney were significantly increased by PC. Cell studies indicated that the efflux function of MRP 2 was inhibited by the serum metabolites of PC. In conclusion, PC markedly increased the systemic exposure and brain concentration of CBZ and CBZE through inhibiting the activities of CYP 3A and MRP 2.

Effect of ursodeoxycholic acid on bile acid profiles and intestinal detoxification machinery in primary biliary cirrhosis and health

BACKGROUND & AIMS

Ursodeoxycholic acid (UDCA) exerts anticholestatic, antifibrotic and antiproliferative effects in primary biliary cirrhosis (PBC) via mechanisms not yet fully understood. Its adequate biliary enrichment is considered mandatory for therapeutic efficacy. However, precise determination of biliary enrichment of UDCA is not possible in clinical practice. Therefore, we investigated (i) the relationship between biliary enrichment and plasma pharmacokinetics of UDCA, (ii) the effect of UDCA on plasma and biliary bile acid composition and conjugation patterns, and (iii) on the intestinal detoxification machinery in patients with PBC and healthy controls.

METHODS

In 11 PBC patients and 11 matched healthy subjects, cystic bile and duodenal tissue were collected before and after 3 weeks of administration of UDCA (15 mg/kg/day). Extensive pharmacokinetic profiling of bile acids was performed. The effect of UDCA on the intestinal detoxification machinery was studied by quantitative PCR and Western blotting.

RESULTS

The relative fraction of UDCA and its conjugates in plasma at trough level[x] correlated with their biliary enrichment[y] (r=0.73, p=0.0001, y=3.65+0.49x). Taurine conjugates of the major hydrophobic bile acid, chenodeoxycholic acid, were more prominent in bile of PBC patients than in that of healthy controls. Biliary bile acid conjugation patterns normalized after treatment with UDCA. UDCA induced duodenal expression of key export pumps, BCRP and P-glycoprotein.

CONCLUSIONS

Biliary and trough plasma enrichment of UDCA are closely correlated in PBC and health. Taurine conjugation may represent an adaptive mechanism in PBC against chenodeoxycholic acid-mediated bile duct damage. UDCA may stabilize small intestinal detoxification by upregulation of efflux pumps.

Functional characterization of ABCC2 promoter polymorphisms and allele-specific expression

Multidrug resistance protein 2 (MRP2, ABCC2) is an efflux membrane transporter highly expressed in liver, kidney and intestine with important physiological and pharmacological roles. The goal of this study was to investigate the functional significance of promoter region polymorphisms in ABCC2 and potential allele specific expression. Twelve polymorphisms in the 1.6 kb region upstream of the translation start site were identified by resequencing 247 DNA samples from ethnically diverse individuals. Luciferase reporter gene assays showed that ABCC2 -24C>T both alone and as part of a common haplotype (-24C>T/-1019A>G/-1549G>A) increased promoter function 35% compared to the reference sequence (P < 0.0001). No other common variants or haplotypes affected ABCC2 promoter activity. Allele specific expression was also investigated as a mechanism to explain reported associations of the synonymous ABCC2 3972C>T variant with pharmacokinetic phenotypes. In Caucasian liver samples (n=41) heterozygous for the 3972C>T polymorphism, the 3972C allele was preferentially transcribed relative to the 3972T allele (P < 0.0001). This allelic imbalance was particularly apparent in samples with haplotypes containing two or three promoter/UTR variants (-1549G>A, -1019A>G and -24C>T). The observed allelic imbalance was not associated with hepatic or renal ABCC2 mRNA expression. Additional mechanisms will need to be explored to account for the interindividual variation in ABCC2 expression and MRP2 function.

Pharmacogenetics of drug transporters in the enterohepatic circulation

This article summarizes the impact of the pharmacogenetics of drug transporters expressed in the enterohepatic circulation on the pharmacokinetics and pharmacodynamics of drugs. The role of pharmacogenetics in the function of drug transporter proteins in vitro is now well established and evidence is rapidly accumulating from in vivo pharmacokinetic studies, which suggests that genetic variants of drug transporter proteins can translate into clinically relevant phenotypes. However, a large amount of conflicting information on the clinical relevance of drug transporter proteins has so far precluded the emergence of a clear picture regarding the role of drug transporter pharmacogenetics in medical practice. This is very well exemplified by the case of P-glycoprotein (MDR1, ABCB1). The challenge is now to develop pharmacogenetic models with sufficient predictive power to allow for translation into drug therapy. This will require a combination of pharmacogenetics of drug transporters, drug metabolism and pharmacodynamics of the respective drugs.

Hepatobiliary transport in health and disease

Bile salts, cholesterol and phosphatidylcholine are secreted across the canalicular membrane of hepatocytes into bile by ATP-binding cassette (ABC) transporters. Secretion of bile salts by ABCB11 is essential for bile flow and for absorption of lipids and fat-soluble vitamins. ABCG5 and ABCG8 eliminate excess cholesterol and sterols from the body by secreting them into bile. There are two mechanisms to protect the canalicular membrane from solubilization by bile salts; ABCB4 secretes phosphatidylcholine into bile to form mixed micelles with bile salts, and ATP8B1 maintains the canalicular membrane in a liquid-ordered state. Three different forms of progressive familial intrahepatic cholestasis (PFIC) disorders, PFIC1, PFIC2 and PFIC3, are caused by mutations in ATP8B1, ABCB11 and ABCB4, respectively. Sitosterolemia is caused by mutations in ABCG5 and ABCG8. This article reviews the physiological roles of these canalicular transporters, and the pathophysiological processes and clinical features associated with their mutations.

A cis-acting five-amino-acid motif controls targeting of ABCC2 to the apical plasma membrane domain

ATP-binding cassette transporter isoform C2 (ABCC2) is exclusively targeted to the apical plasma membrane of polarized cells. Although apical localization of ABCC2 in hepatocytes is crucial for the biliary excretion of a variety of metabolites, the mechanism regulating its apical targeting is poorly understood. In the present study, an apical targeting signal was identified in the first cytoplasmic loop domain (CLD1) of ABCC2 in HepG2 cells. Overexpression of CLD1 significantly disturbed the apical targeting of FLAG-ABCC2 in a competitive manner, suggesting the presence of a saturable sorting machinery in HepG2 cells. Next, deletion analysis identified a potential targeting sequence within a 20-amino-acid long peptide (aa 272-291) of CLD1. Alanine scanning mutagenesis of this region in full-length ABCC2 further narrowed down the apical targeting determinant to five amino acids, S(283)QDAL(287). Of these, S(283) and L(287) were found to be conserved among vertebrate ABCC2 orthologs. Site-directed mutagenesis showed that both S(283) and L(287) were crucial for the targeting specificity of ABCC2. Introducing this apical targeting sequence into the corresponding region of ABCC1, an exclusively basolateral protein, caused the hybrid ABCC1 to partially localize in the apical membrane. Thus, the CLD1 of ABCC2 contains a novel apical sorting determinant, and a saturable sorting machinery is present in polarized HepG2 cells.

Mammalian multidrug-resistance proteins (MRPs)

Subfamily C of the human ABC (ATP-binding cassette) superfamily contains nine proteins that are often referred to as the MRPs (multidrug-resistance proteins). The 'short' MRP/ABCC transporters (MRP4, MRP5, MRP8 and ABCC12) have a typical ABC structure with four domains comprising two membrane-spanning domains (MSD1 and MSD2) each followed by a nucleotide-binding domain (NBD1 and NBD2). The 'long' MRP/ABCCs (MRP1, MRP2, MRP3, ABCC6 and MRP7) have five domains with the extra domain, MSD0, at the N-terminus. The proteins encoded by the ABCC6 and ABCC12 genes are not known to transport drugs and are therefore referred to as ABCC6 and ABCC12 (rather than MRP6 and MRP9) respectively. A large number of molecules are transported across the plasma membrane by the MRPs. Many are organic anions derived from exogenous sources such as conjugated drug metabolites. Others are endogenous metabolites such as the cysteinyl leukotrienes and prostaglandins which have important signalling functions in the cell. Some MRPs share a degree of overlap in substrate specificity (at least in vitro), but differences in transport kinetics are often substantial. In some cases, the in vivo substrates for some MRPs have been discovered aided by studies in gene-knockout mice. However, the molecules that are transported in vivo by others, including MRP5, MRP7, ABCC6 and ABCC12, still remain unknown. Important differences in the tissue distribution of the MRPs and their membrane localization (apical in contrast with basolateral) in polarized cells also exist. Together, these differences are responsible for the unique pharmacological and physiological functions of each of the nine ABCC transporters known as the MRPs.

Multidrug resistance associated proteins in multidrug resistance

Multidrug resistance proteins (MRPs) are members of the C family of a group of proteins named ATP-binding cassette (ABC) transporters. These ABC transporters together form the largest branch of proteins within the human body. The MRP family comprises of 13 members, of which MRP1 to MRP9 are the major transporters indicated to cause multidrug resistance in tumor cells by extruding anticancer drugs out of the cell. They are mainly lipophilic anionic transporters and are reported to transport free or conjugates of glutathione (GSH), glucuronate, or sulphate. In addition, MRP1 to MRP3 can transport neutral organic drugs in free form in the presence of free GSH. Collectively, MRPs can transport drugs that differ structurally and mechanistically, including natural anticancer drugs, nucleoside analogs, antimetabolites, and tyrosine kinase inhibitors. Many of these MRPs transport physiologically important anions such as leukotriene C4, bilirubin glucuronide, and cyclic nucleotides. This review focuses mainly on the physiological functions, cellular resistance characteristics, and probable in vivo role of MRP1 to MRP9.

Functional analysis of nonsynonymous single nucleotide polymorphisms of multidrug resistance-associated protein 2 (ABCC2)

BACKGROUND

Multidrug resistance-associated protein 2 (MRP2; ABCC2) mediates the biliary excretion of glutathione, glucuronide, and sulfate conjugates of endobiotics and xenobiotics. Single nucleotide polymorphisms (SNPs) of MRP2 contribute to interindividual variability in drug disposition and ultimately in drug response.

OBJECTIVES

To characterize the transport function of human wild-type (WT) MRP2 and four SNP variants, S789F, A1450T, V417I, and T1477M.

METHODS

The four SNP variants were expressed in Sf9 cells using recombinant baculovirus infection. The kinetic parameters [Km, (μmol/l); V(max), (pmol/mg/min); the Hill coefficient] of ATP-dependent transport of leukotriene C(4) (LTC(4)), estradiol-3-glucuronide (E(2)3G), estradiol-17β-glucuronide (E(2)17G), and tauroursodeoxycholic acid (TUDC) were determined in Sf9-derived plasma membrane vesicles. Transport activity was normalized for expression level.

RESULTS

The V(max) for transport activity was decreased for all substrates for S789F, and for all substrates except E(2)17G for A1450T. V417I showed decreased apparent affinity for LTC(4), E(2)3G, and E(2)17G, whereas transport was similar between wild-type (WT) and T1477M, except for a modest increase in TUDC transport. Examination of substrate-stimulated MRP2-dependent ATPase activity of S789F and A1450T, SNPs located in MRP2 nucleotide-binding domains (NBDs), demonstrated significantly decreased ATPase activity and only modestly decreased affinity for ATP compared with WT.

CONCLUSION

SNPs in the NBDs (S789F in the D-loop of NBD1, or A1450T near the ABC signature motif of NBD2) variably decreased the transport of all substrates. V417I in membrane spanning domain 1 selectively decreased the apparent affinity for the glutathione and glucuronide conjugated substrates, whereas the T1477M SNP in the carboxyl terminus altered only TUDC transport.

Transportomics: screening for substrates of ABC transporters in body fluids using vesicular transport assays

The ATP-binding cassette (ABC) genes encode the largest family of transmembrane proteins. ABC transporters translocate a wide variety of substrates across membranes, but their physiological function is often incompletely understood. We describe a new method to study the substrate spectrum of ABC transporters: We incubate extracts of mouse urine with membrane vesicles prepared from Spodoptera frugiperda Sf9 insect cells overproducing an ABC transporter and determine the compounds transported into the vesicles by LC/MS-based metabolomics. We illustrate the power of this simple "transportomics" approach using ABCC2, a protein present at sites of uptake and elimination. We identified many new substrates of ABCC2 in urine. These included glucuronides of plant-derived xenobiotics, a class of compounds to which humans are exposed on a daily basis. Moreover, we show that the excretion of these compounds in vivo depends on ABCC2: compared to wild-type mice, the urinary excretion of several glucuronides was increased up to 20-fold in Abcc2(-/-) mice. Transportomics has broad applicability, as it is not restricted to urine and can be applied to other ATP-dependent transport proteins as well.

Drug transporters in drug efficacy and toxicity

Drug transporters are now widely acknowledged as important determinants governing drug absorption, excretion, and, in many cases, extent of drug entry into target organs. There is also a greater appreciation that altered drug transporter function, whether due to genetic polymorphisms, drug-drug interactions, or environmental factors such as dietary constituents, can result in unexpected toxicity. Such effects are in part due to the interplay between various uptake and efflux transporters with overlapping functional capabilities that can manifest as marked interindividual variability in drug disposition in vivo. Here we review transporters of the solute carrier (SLC) and ATP-binding cassette (ABC) superfamilies considered to be of major importance in drug therapy and outline how understanding the expression, function, and genetic variation in such drug transporters will result in better strategies for optimal drug design and tissue targeting as well as reduce the risk for drug-drug interactions and adverse drug responses.

Hepatic expression of multidrug resistance protein 2 in biliary atresia

Background

Biliary atresia (BA) is an idiopathic inflammatory obliterative cholangiopathy of neonates, leading to progressive biliary cirrhosis. Hepatoportoenterostomy (Kasai procedure) can cure jaundice in 30% to 80% of patients. Postoperative clearance of jaundice is one of the most important factors influencing long-term outcomes of BA patients. Multidrug resistance protein 2 (MRP2) is one of the canalicular export pumps located in hepatocytes; it exports organic anions and their conjugates (e.g., bilirubin) into bile canaliculus. Although MRP2 is an essential transporter for the excretion of bilirubin, its role in the clinical course of BA patients is unclear. The present study investigated the relationship between hepatic MRP2 expression and clinical course in BA patients, with particular emphasis in curing jaundice after hepatoportoenterostomy.

Results

No significant differences in hepatic MRP2 expression level were observed between BA and controls groups. There was no correlation between MRP2 expression and age at time of surgery in BA and control groups. In BA patients, MRP2 expression level in the jaundice and jaundice-free group did not differ significantly (2.0 × 10^-4 vs 3.1 × 10^-4, p = 0.094). Although the serum level of total bilirubin just before surgery did not correlate with MRP2 expression level (rs = 0.031, p = 0.914), the serum level of total bilirubin measured at 2 weeks (rs = -0.569, p = 0.034) and 4 weeks after surgery (rs = -0.620, p = 0.018) were significantly correlated with MRP2 expression level. Furthermore, MRP2 expression level was inversely correlated with ratio of change in serum total bilirubin level over 4 weeks (rs = -0.676, p = 0.008), which represents the serum bilirubin level measured at 4 weeks after surgery divided by value just before surgery. There was no correlation between expression level of MRP2 and nuclear receptors, such as retinoid × receptor α, farnesoid × receptor, pregnane × receptor, or constitutive androstane receptor.

Conclusions

Hepatic MRP2 expression level was associated with postoperative clearance of jaundice in BA patients, at least within 1 month after hepatoportoenterostomy. This finding suggests that not only morphological appearance of the liver tissue but also the biological status of hepatocytes is important for BA pathophysiology.

Multidrug resistance proteins (MRPs/ABCCs) in cancer chemotherapy and genetic diseases

The ATP-binding cassette (ABC) transporters are a superfamily of membrane proteins that are best known for their ability to transport a wide variety of exogenous and endogenous substances across membranes against a concentration gradient via ATP hydrolysis. There are seven subfamilies of human ABC transporters, one of the largest being the 'C' subfamily (gene symbol ABCC). Nine ABCC subfamily members, the so-called multidrug resistance proteins (MRPs) 1-9, have been implicated in mediating multidrug resistance in tumor cells to varying degrees as the efflux extrude chemotherapeutic compounds (or their metabolites) from malignant cells. Some of the MRPs are also known to either influence drug disposition in normal tissues or modulate the elimination of drugs (or their metabolites) via hepatobiliary or renal excretory pathways. In addition, the cellular efflux of physiologically important organic anions such as leukotriene C(4) and cAMP is mediated by one or more of the MRPs. Finally, mutations in several MRPs are associated with human genetic disorders. In this minireview, the current biochemical and physiological knowledge of MRP1-MRP9 in cancer chemotherapy and human genetic disease is summarized. The mutations in MRP2/ABCC2 leading to conjugated hyperbilirubinemia (Dubin-Johnson syndrome) and in MRP6/ABCC6 leading to the connective tissue disorder Pseudoxanthoma elasticum are also discussed.

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

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

Expression of the Abca-subfamily of genes in Abcc6-/- mice--upregulation of Abca4

Pseudoxanthoma elasticum (PXE), a heritable multi-system disorder, is caused by mutations in the ABCC6 gene primarily expressed in the liver. Recent analysis of cultured fibroblasts from patients with PXE has suggested compensatory alterations in the expression of the ABCA-subfamily of genes. We have now determined by quantitative RT-PCR the level of expression of Abca-family of genes in a mouse model of PXE developed by targeted ablation of Abcc6. The results indicated variable levels of mRNA for different Abca genes in the liver; however, only one of them, Abca4, was significantly, ∼6.5-fold, upregulated in the Abcc6(-/-) mice in comparison with wild-type mice. In the same mice, Abca4 was not upregulated in the eyes or the kidney, suggesting that the upregulation of Abca4 in the liver is a tissue-specific compensatory consequence of the 'knock-out' of Abcc6.

Hepatobiliary scintigraphy during cholestatic and noncholestatic periods in patients with progressive familial intrahepatic cholestasis after partial external biliary diversion

BACKGROUND

The purpose of the study was to determine the distribution of excreted bile during cholestatic periods and in remission in patients with progressive familial intrahepatic cholestasis (PFIC) after surgery with partial external biliary diversion (PEBD), using hepatobiliary scintigraphy.

METHODS

Using intravenously administered technetium Tc 99m-labeled mebrofenin, the distribution of bile during periods of biochemical cholestasis and in remission was investigated in patients with PFIC operated with PEBD. Stomal bile, urine, and feces from the patients were collected during 24 hours after administration of technetium Tc 99m-labeled mebrofenin; and the fractions of remaining radioactivity in the 3 compartments and the remaining isotopic activity in the body were quantified using scintigraphy.

RESULTS

Nine patients (4 boys and 5 girls) were studied. The median age was 13 (range, 5-24) years, and they had been operated with PEBD at a median time of 10 (range, 4-14) years before entering the study. Thirteen scintigraphic examinations were analyzed: 8 during noncholestatic remission (n = 7 patients) and 5 during cholestasis (n = 3 patients). The patients studied during remission discharged a significantly larger fraction of isotopic activity through the stoma (median, 90% vs 22%; P < .05) and a significantly lower fraction through the urine (median, 2.5% vs 15%; P < .05) compared with the patients studied during cholestasis.

CONCLUSION

Hepatobiliary scintigraphy could detect substantial differences in the output of bile. Further studies are needed to determine whether these differences may explain the mechanism of the PEBD operation or merely are secondary to the degree of cholestasis.

Multidrug resistance proteins (MRPs, ABCCs): importance for pathophysiology and drug therapy

The nine multidrug resistance proteins (MRPs) represent the major part of the 12 members of the MRP/CFTR subfamily belonging to the 48 human ATP-binding cassette (ABC) transporters. Cloning, functional characterization, and cellular localization of most MRP subfamily members have identified them as ATP-dependent efflux pumps with a broad substrate specificity for the transport of endogenous and xenobiotic anionic substances localized in cellular plasma membranes. Prototypic substrates include glutathione conjugates such as leukotriene C(4) for MRP1, MRP2, and MRP4, bilirubin glucuronosides for MRP2 and MRP3, and cyclic AMP and cyclic GMP for MRP4, MRP5, and MRP8. Reduced glutathione (GSH), present in living cells at millimolar concentrations, modifies the substrate specificities of several MRPs, as exemplified by the cotransport of vincristine with GSH by MRP1, or by the cotransport of GSH with bile acids or of GSH with leukotriene B(4) by MRP4.The role of MRP subfamily members in pathophysiology may be illustrated by the MRP-mediated release of proinflammatory and immunomodulatory mediators such as leukotrienes and prostanoids. Pathophysiological consequences of many genetic variants leading to a lack of functional MRP protein in the plasma membrane are observed in the hereditary MRP2 deficiency associated with conjugated hyperbilirubinemia in Dubin-Johnson syndrome, in pseudoxanthoma elasticum due to mutations in the MRP6 (ABCC6) gene, or in the type of human earwax and osmidrosis determined by single nucleotide polymorphisms in the MRP8 (ABCC8) gene. The hepatobiliary and renal elimination of many drugs and their metabolites is mediated by MRP2 in the hepatocyte canalicular membrane and by MRP4 as well as MRP2 in the luminal membrane of kidney proximal tubules. Therefore, inhibition of these efflux pumps affects pharmacokinetics, unless compensated by other ATP-dependent efflux pumps with overlapping substrate specificities.

Genetic variations of bile salt transporters as predisposing factors for drug-induced cholestasis, intrahepatic cholestasis of pregnancy and therapeutic response of viral hepatitis

INTRODUCTION

Drug-induced cholestasis, intrahepatic cholestasis of pregnancy and viral hepatitis are acquired forms of liver disease. Cholestasis is a pathophysiologic state with impaired bile formation and subsequent accumulation of bile salts in hepatocytes. The bile salt export pump (BSEP) (ABCB11) is the key export system for bile salts from hepatocytes.

AREAS COVERED

This article provides an introduction into the physiology of bile formation followed by a summary of the current knowledge on the key bile salt transporters, namely, the sodium-taurocholate co-transporting polypeptide NTCP, the organic anion transporting polypeptides (OATPs), BSEP and the multi-drug resistance protein 3. The pathophysiologic consequences of altered functions of these transporters, with an emphasis on molecular and genetic aspects, are then discussed.

EXPERT OPINION

Knowledge of the role of hepatocellullar transporters, especially BSEP, in acquired cholestasis is continuously increasing. A common variant of BSEP (p.V444A) is now a well-established susceptibility factor for acquired cholestasis and recent evidence suggests that the same variant also influences the therapeutic response and disease progression of viral hepatitis C. Studies in large independent cohorts are now needed to confirm the relevance of p.V444A. Genome-wide association studies should lead to the identification of additional genetic factors underlying cholestatic liver disease.

ATP-binding cassette transporter isoform C2 localizes to the apical plasma membrane via interactions with scaffolding protein

ATP-binding cassette transporter isoform C2 (ABCC2) localizes to the apical plasma membrane in polarized cells. Apical localization of ABCC2 in hepatocytes plays an important role in biliary excretion of endobiotics and xenobiotics, but the mechanism by which ABCC2 localizes to the apical membrane has not been conclusively elucidated. Here, we investigate the role of scaffolding proteins on ABCC2 localization with a focus on the function of PDZK1 (post-synaptic density 95/disk large/zonula occludens-1 domain containing 1) in regulating ABCC2 localization. The C-terminal 77 residues of ABCC2 were used to probe interacting proteins from HepG2 cells. Protein mass fingerprinting identified PDZK1 as a major interacting protein. PDZK1 associated with the plasma membrane, most likely at the apical vacuoles of HepG2 cells. Affinity pull-down assays confirmed that the C-terminal NSTKF of ABCC2 bound to the fourth PDZ domain of PDZK1. Removal of this PDZ-binding motif significantly reduced the normal apical localization of ABCC2. In HepG2 cells, overexpression of this fourth domain overcame endogenous PDZK1 and reduced the ABCC2 localization at the apical membrane with a reciprocal increase of intracellular accumulation of mislocalized ABCC2. These results suggest a possible role for an interaction between ABCC2 and PDZK1 in apical localization of ABCC2 in hepatocytes.

Pathogenesis of cholestatic liver disease and therapeutic approaches

Cholestatic liver disorders are caused by genetic defects, mechanical aberrations, toxins, or dysregulations in the immune system that damage the bile ducts and cause accumulation of bile and liver tissue damage. They have common clinical manifestations and pathogenic features that include the responses of cholangiocytes and hepatocytes to injury. We review the features of bile acid transport, tissue repair and regulation, apoptosis, vascular supply, immune regulation, and cholangiocytes that are associated with cholestatic liver disorders. We now have a greater understanding of the physiology of cholangiocytes at the cellular and molecular levels, as well as genetic factors, repair pathways, and autoimmunity mechanisms involved in the pathogenesis of disease. These discoveries will hopefully lead to new therapeutic approaches for patients with cholestatic liver disease.

Regulation of hepatic ABCC transporters by xenobiotics and in disease states

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

ABCC2/Abcc2: a multispecific transporter with dominant excretory functions

ABCC2/Abcc2 (MRP2/Mrp2) is expressed at major physiological barriers, such as the canalicular membrane of liver cells, kidney proximal tubule epithelial cells, enterocytes of the small and large intestine, and syncytiotrophoblast of the placenta. ABCC2/Abcc2 always localizes in the apical membranes. Although ABCC2/Abcc2 transports a variety of amphiphilic anions that belong to different classes of molecules, such as endogenous compounds (e.g., bilirubin-glucuronides), drugs, toxic chemicals, nutraceuticals, and their conjugates, it displays a preference for phase II conjugates. Phenotypically, the most obvious consequence of mutations in ABCC2 that lead to Dubin-Johnson syndrome is conjugate hyperbilirubinemia. ABCC2/Abcc2 harbors multiple binding sites and displays complex transport kinetics.

Hyperbilirubinemia syndromes (Gilbert-Meulengracht, Crigler-Najjar, Dubin-Johnson, and Rotor syndrome)

Hyperbilirubinemia is an important clinical sign that often indicates severe hepatobiliary disease of different etiologies. Inherited non-haemolytichyperbilirubinemic conditions include Dubin-Johnson, Rotor, and Gilbert-Meulengracht syndromes, which are important differential diagnoses indicating benign disease that require no immediate treatment. Dubin-Johnson and Rotor syndromes are rare, exhibit mixed direct and indirect hyperbilirubinemia as well as typical profiles or urinary coproporphyrin excretion. Gilbert-Meulengracht disease leads to unconjugated hyperbilirubinemia because of impaired glucuronidation activity, and is part of a spectrum of genetic variants also encompassing fatal Crigler-Najjar syndrome. Gilbert-Meulengracht syndrome can be diagnosed by clinical presentation, biochemistry and genotyping, and carries significance regarding the disposition towards drug-associated toxicity. In addition, the precise diagnosis of these inherited hyperbilirubinemic syndromes avoids unnecessary invasive procedures for suspected more severe hepatobiliary disease.

Interference with bile salt export pump function is a susceptibility factor for human liver injury in drug development

The bile salt export pump (BSEP) is an efflux transporter, driving the elimination of endobiotic and xenobiotic substrates from hepatocytes into the bile. More specifically, it is responsible for the elimination of monovalent, conjugated bile salts, with little or no assistance from other apical transporters. Disruption of BSEP activity through genetic disorders is known to manifest in clinical liver injury such as progressive familial intrahepatic cholestasis type 2. Drug-induced disruption of BSEP is hypothesized to play a role in the development of liver injury for several marketed or withdrawn therapeutics. Unfortunately, preclinical animal models have been poor predictors of the liver injury associated with BSEP interference observed for humans, possibly because of interspecies differences in bile acid composition, differences in hepatobiliary transporter modulation or constitutive expression, as well as other mechanisms. Thus, a BSEP-mediated liver liability may go undetected until the later stages of drug development, such as during clinical trials or even postlicensing. In the absence of a relevant preclinical test system for BSEP-mediated liver injury, the toxicological relevance of available in vitro models to human health rely on the use of benchmark compounds with known clinical outcomes, such as marketed or withdrawn drugs. In this study, membrane vesicles harvested from BSEP-transfected insect cells were used to assess the activity of more than 200 benchmark compounds to thoroughly investigate the relationship between interference with BSEP function and liver injury. The data suggest a relatively strong association between the pharmacological interference with BSEP function and human hepatotoxicity. Although the most accurate translation of risk would incorporate pharmacological potency, pharmacokinetics, clearance mechanisms, tissue distribution, physicochemical properties, indication, and other drug attributes, the additional understanding of a compound's potency for BSEP interference should help to limit or avoid BSEP-related liver liabilities in humans that are not often detected by standard preclinical animal models.

The role of multixenobiotic transporters in predatory marine molluscs as counter-defense mechanisms against dietary allelochemicals

Multixenobiotic transporters have been extensively studied for their ability to modulate the disposition and toxicity of pharmacological agents, yet their influence in regulating the levels of dietary toxins within marine consumers has only recently been explored. This study presents functional and molecular evidence for multixenobiotic transporter-mediated efflux activity and expression in the generalist gastropod Cyphoma gibbosum, and the specialist nudibranch Tritonia hamnerorum, obligate predators of chemically defended gorgonian corals. Immunochemical analysis revealed that proteins with homology to permeability glycoprotein (P-gp) were highly expressed in T. hamnerorum whole animal homogenates and localized to the apical tips of the gut epithelium, a location consistent with a role in protection against ingested prey toxins. In vivo dye assays with specific inhibitors of efflux transporters demonstrated the activity of P-gp and multidrug resistance-associated protein (MRP) families of ABC transporters in T. hamnerorum. In addition, we identified eight partial cDNA sequences encoding two ABCB and two ABCC proteins from each molluscan species. Digestive gland transcripts of C. gibbosum MRP-1, which have homology to vertebrate glutathione-conjugate transporters, were constitutively expressed regardless of gorgonian diet. This constitutive expression may reflect the ubiquitous presence of high affinity substrates for C. gibbosum glutathione transferases in gorgonian tissues likely necessitating export by MRPs. Our results suggest that differences in multixenobiotic transporter expression patterns and activity in molluscan predators may stem from the divergent foraging strategies of each consumer.

Hepatitis C virus-related cirrhosis is a major determinant of the expression levels of hepatic drug transporters

Hepatic drug transporters are responsible for both hepatic uptake and the biliary excretion of drugs. Expression changes in hepatic drug transporter genes have been observed in various pathophysiological conditions. However, it has not been comprehensively investigated what factors substantially influence the mRNA levels of hepatic drug transporters. In this study, we quantified the mRNA expression of 17 drug transporters using noncancerous liver tissue samples and carried out stepwise multiple regression analysis to identify the factors affecting their expression from 18 clinical variables. For 17 drug transporters, the mRNA level of organic anion transporting polypeptide (OATP) 2B1 was highest, followed by that of organic cation transporter 1, organic anion transporter 2, OATP1B1, OATP1B3, multidrug resistance-associated protein (MRP) 6, and MRP3. Stepwise multiple regression analysis demonstrated MRP4 mRNA level to be predicted with the greatest accuracy among 17 drug transporters. Of clinical variables entered into the prediction model for MRP4, hepatitis C virus (HCV) infection and liver cirrhosis were crucial factors affecting MRP4 mRNA and protein levels. Furthermore, HCV-related cirrhosis influenced the mRNA levels of 8 drug transporters besides MRP4. These findings indicate that HCV-related cirrhosis is a crucial factor affecting the expression of hepatic drug transporters, especially MRP4.

Targeting drug transporters - combining in silico and in vitro approaches to predict in vivo

Transporter proteins are expressed throughout the human body in different vital organs. They play an important role to various extents in determining absorption, distribution, metabolism, excretion, and toxicity (ADME/Tox) properties of therapeutic molecules. Over the past decade, numerous drug transporters have been cloned and considerable progress has been made toward understanding the molecular characteristics of individual transporters. In this chapter several in vitro and in silico techniques are described with applications to understand transporter behavior. These include employing new techniques to rapidly identify novel ligands for transporters. Ultimately these methods should lead to a greater overall appreciation of the role of transporters in vivo.

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

IMPORTANCE OF THE FIELD

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

AREAS COVERED IN THIS REVIEW

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

WHAT THE READER WILL GAIN

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

TAKE HOME MESSAGE

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

Regulation of multidrug resistance-associated protein 2 by calcium signaling in mouse liver

Multidrug resistance associated protein 2 (Mrp2) is a canalicular transporter responsible for organic anion secretion into bile. Mrp2 activity is regulated by insertion into the plasma membrane; however, the factors that control this are not understood. Calcium (Ca(2+)) signaling regulates exocytosis of vesicles in most cell types, and the type II inositol 1,4,5-triphosphate receptor (InsP(3)R2) regulates Ca(2+) release in the canalicular region of hepatocytes. However, the role of InsP(3)R2 and of Ca(2+) signals in canalicular insertion and function of Mrp2 is not known. The aim of this study was to determine the role of InsP(3)R2-mediated Ca(2+) signals in targeting Mrp2 to the canalicular membrane. Livers, isolated hepatocytes, and hepatocytes in collagen sandwich culture from wild-type (WT) and InsP(3)R2 knockout (KO) mice were used for western blots, confocal immunofluorescence, and time-lapse imaging of Ca(2+) signals and of secretion of a fluorescent organic anion. Plasma membrane insertion of green fluorescent protein (GFP)-Mrp2 expressed in HepG2 cells was monitored by total internal reflection microscopy. InsP(3)R2 was concentrated in the canalicular region of WT mice but absent in InsP(3)R2 KO livers, whereas expression and localization of InsP(3)R1 was preserved, and InsP(3)R3 was absent from both WT and KO livers. Ca(2+) signals induced by either adenosine triphosphate (ATP) or vasopressin were impaired in hepatocytes lacking InsP(3)R2. Canalicular secretion of the organic anion 5-chloromethylfluorescein diacetate (CMFDA) was reduced in KO hepatocytes, as well as in WT hepatocytes treated with 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). Moreover, the choleretic effect of tauroursodeoxycholic acid (TUDCA) was impaired in InsP(3)R2 KO mice. Finally, ATP increased GFP-Mrp2 fluorescence in the plasma membrane of HepG2 cells, and this also was reduced by BAPTA.

CONCLUSION

InsP(3)R2-mediated Ca(2+) signals enhance organic anion secretion into bile by targeting Mrp2 to the canalicular membrane.

Pharmacotherapy of cholestatic liver diseases

New insights into the molecular mechanisms of bile formation and cholestasis have provided new concepts for pharmacotherapy of cholestatic liver diseases. The major aim in all forms of cholestasis is the reduction of hepatocellular retention of bile acids and other potentially toxic constituents of bile. Reduction of hepatocellular retention may be achieved by drugs that stimulate hepatocellular secretion via the canalicular route into the bile or via the alternative route across the basolateral membrane into the blood, and by drugs that stimulate the hepatocellular metabolism of hydrophobic bile acids to hydrophilic, less toxic metabolites. In cholestatic liver diseases that start with an injury of the biliary epithelium (e.g., primary biliary cirrhosis; PBC), protection of the cholangiocytes against the toxic effects of hydrophobic bile acids is most important. When hepatocellular retention of bile acids has occurred, the inhibition of bile acid-induced apoptosis becomes another target of therapy. Ursodeoxycholic acid protects the biliary epithelium by reducing the toxicity of bile, stimulates hepatobiliary secretion by upregulating transporters and inhibits apoptosis. It is the mainstay of therapy in PBC but of benefit also in a number of other cholestatic liver diseases. New drugs such as 6-ethyl-chenodeoxycholic acid and 24-nor-ursodeoxycholic acid are being evaluated for the treatment of cholestatic liver diseases.

Controversies in bilirubin biochemistry and their clinical relevance

Despite a century of research, several clinically relevant areas of bilirubin biochemistry remain controversial, poorly understood, or unrecognized. These include: (i) The structure and molecularity of bilirubin under physiological environments such as membranes, brain tissue and when bound to proteins. Related to this is the large number of structurally different bilirubin species that may occur in blood under pathological conditions and their potential effects on measurements of bilirubin and free bilirubin. (ii) The mechanism of phototherapy, the neurotoxicity of the photoisomers produced and their influence on measurements of bilirubin and free bilirubin. (iii) The role of membrane transporters in the passage of unconjugated bilirubin across the placenta, intestine, vascular epithelium, blood-brain barrier, and into the liver. (iv) Biochemical mechanisms of bilirubin toxicity, pharmacologic prevention of kernicterus, the contribution of bilirubin to antioxidant defenses, and the practical value of free bilirubin measurements for identifying infants at most risk of kernicterus.

Cyclic AMP stimulates Mrp2 translocation by activating p38{alpha} MAPK in hepatic cells

Cyclic AMP (cAMP) induces translocation of multidrug resistant protein 2 (Mrp2) to the canalicular membrane and activates p38 MAPK in rat hepatocytes. In this study, we tested the hypothesis that cAMP-induced Mrp2 translocation may be mediated via p38 MAPK. Studies were conducted in rat hepatocytes and in a human hepatoma cell line, HuH-7. In rat hepatocytes, cAMP increased Mrp2 translocation and p38 MAPK activity. These effects of cAMP were inhibited by SB203580, an inhibitor of p38 MAPK. Wortmannin, a specific inhibitor of phosphoinositide-3-kinase (PI3K), did not inhibit cAMP induced activation of p38 MAPK, indicating PI3K-independent activation of p38 MAPK by cAMP. To further define the role of p38 MAPK, molecular approaches were used to up- or downregulate p38 MAPK activity in HuH-7 cells using constitutively active (CA) and dominant-negative (DN) MAPK kinase 3 and 6 (MKK3/6). MKK3/6 are upstream kinases responsible for the activation of p38 MAPK. Cells transfected with CAMKK6 showed increased p38 MAPK activity and MRP2 translocation compared with empty vector. cAMP-induced activation of p38 MAPK was inhibited in cells transfected with DNMKK3/6 and DNMKK3, but not with DNMKK6. DNMKK3/6 and DNMKK3 also inhibited cAMP-induced MRP2 translocation. cAMP selectively activated p38alpha MAPK in HuH-7 cells. Knockdown of p38alpha MAPK by short heterodimer RNA resulted in decreased level of p38 MAPK and failure of cAMP to stimulate MRP2 translocation. Taken together, these results suggest that cAMP-induced MRP2 translocation in hepatic cells is mediated via PI3K-independent and MKK3-mediated activation of p38alpha MAPK.

NHERF-1 binds to Mrp2 and regulates hepatic Mrp2 expression and function

Multidrug resistance-associated protein 2 (Mrp2, Abcc2) is an ATP-binding cassette transporter localized at the canalicular membrane of hepatocytes that plays an important role in bile formation and detoxification. Prior in vitro studies suggest that Mrp2 can bind to Na(+)/H(+) exchanger regulatory factor 1 (NHERF-1), a PDZ protein that cross-links membrane proteins to actin filaments. However the role of NHERF-1 in the expression and functional regulation of Mrp2 remains largely unknown. Here we examine the interaction of Mrp2 and NHERF-1 and its physiological significance in HEK293 cells and NHERF-1 knock-out mice. Mrp2 co-precipitated with NHERF-1 in co-transfected HEK293 cells, an interaction that required the PDZ-binding motif of Mrp2. In NHERF-1(-/-) mouse liver, Mrp2 mRNA was unchanged but Mrp2 protein was reduced in whole cell lysates and membrane-enriched fractions to approximately 50% (p < 1 x 10(-6)) and approximately 70% (p < 0.05), respectively, compared with wild-type mice, suggesting that the down-regulation of Mrp2 expression was caused by post-transcriptional events. Mrp2 remained localized at the apical/canalicular membrane of NHERF-1(-/-) mouse hepatocytes, although its immunofluorescent labeling was noticeably weaker. Bile flow in NHERF-1(-/-) mice was reduced to approximately 70% (p < 0.001) in association with a 50% reduction in glutathione excretion (p < 0.05) and a 60% reduction in glutathione-methylfluorescein (GS-MF) excretion in isolated mouse hepatocyte (p < 0.01). Bile acid and bilirubin excretion remained unchanged compared with wild-type mice. These findings strongly suggest that NHERF-1 binds to Mrp2, and plays a critical role in the canalicular expression of Mrp2 and its function as a determinant of glutathione-dependent, bile acid-independent bile flow.

Biliary physiology and disease: reflections of a physician-scientist

A review is presented of Gustav Paumgartner's five decades of research and practice in hepatology focusing on biliary physiology and disease. It begins with studies of the excretory function of the liver including hepatic uptake of indocyanine green, bilirubin, and bile acids. The implications of these studies for diagnosis and understanding of liver diseases are pointed out. From there, the path of scientific research leads to investigations of hepatobiliary bile acid transport and the major mechanisms of bile formation. The therapeutic effects of the hydrophilic bile acid, ursodeoxycholic acid, have greatly stimulated these studies. Although ursodeoxycholic acid therapy for dissolution of cholesterol gallstones and some other nonsurgical treatments of gallstones were largely superseded by surgical techniques, ursodeoxycholic acid is currently considered the mainstay of therapy of some chronic cholestatic liver diseases, such as primary biliary cirrhosis. The major mechanisms of action of ursodeoxycholic acid therapy in cholestatic liver diseases are discussed. An attempt is made to illustrate how scientific research can lead to advances in medical practice that help patients.