Biochemical and molecular aspects of genetic disorders of bilirubin metabolism

Bilirubin gates the TRPM2 channel as a direct agonist to exacerbate ischemic brain damage

Stroke prognosis is negatively associated with an elevation of serum bilirubin, but how bilirubin worsens outcomes remains mysterious. We report that post-, but not pre-, stroke bilirubin levels among inpatients scale with infarct volume. In mouse models, bilirubin increases neuronal excitability and ischemic infarct, whereas ischemic insults induce the release of endogenous bilirubin, all of which are attenuated by knockout of the TRPM2 channel or its antagonist A23. Independent of canonical TRPM2 intracellular agonists, bilirubin and its metabolic derivatives gate the channel opening, whereas A23 antagonizes it by binding to the same cavity. Knocking in a loss of binding point mutation for bilirubin, TRPM2-D1066A, effectively antagonizes ischemic neurotoxicity in mice. These findings suggest a vicious cycle of stroke injury in which initial ischemic insults trigger the release of endogenous bilirubin from injured cells, which potentially acts as a volume neurotransmitter to activate TRPM2 channels, aggravating Ca²⁺-dependent brain injury.

Role of Hepatocyte Transporters in Drug-Induced Liver Injury (DILI)-In Vitro Testing

Bile acids and bile salts (BA/BS) are substrates of both influx and efflux transporters on hepatocytes. Canalicular efflux transporters, such as BSEP and MRP2, are crucial for the removal of BA/BS to the bile. Basolateral influx transporters, such as NTCP, OATP1B1/1B3, and OSTα/β, cooperate with canalicular transporters in the transcellular vectorial flux of BA/BS from the sinusoids to the bile. The blockage of canalicular transporters not only impairs the bile flow but also causes the intracellular accumulation of BA/BS in hepatocytes that contributes to, or even triggers, liver injury. In the case of BA/BS overload, the efflux of these toxic substances back to the blood via MRP3, MRP4, and OST α/β is considered a relief function. FXR, a key regulator of defense against BA/BS toxicity suppresses de novo bile acid synthesis and bile acid uptake, and promotes bile acid removal via increased efflux. In drug development, the early testing of the inhibition of these transporters, BSEP in particular, is important to flag compounds that could potentially inflict drug-induced liver injury (DILI). In vitro test systems for efflux transporters employ membrane vesicles, whereas those for influx transporters employ whole cells. Additional in vitro pharmaceutical testing panels usually include cellular toxicity tests using hepatocytes, as well as assessments of the mitochondrial toxicity and accumulation of reactive oxygen species (ROS). Primary hepatocytes are the cells of choice for toxicity testing, with HepaRG cells emerging as an alternative. Inhibition of the FXR function is also included in some testing panels. The molecular weight and hydrophobicity of the drug, as well as the steady-state total plasma levels, may positively correlate with the DILI potential. Depending on the phase of drug development, the physicochemical properties, dosing, and cut-off values of BSEP IC₅₀ ≤ 25-50 µM or total C_ss,plasma/BSEP IC₅₀ ≥ 0.1 may be an indication for further testing to minimize the risk of DILI liability.

Hepatotoxicity of the Major Anthraquinones Derived From Polygoni Multiflori Radix Based on Bile Acid Homeostasis

Polygoni Multiflori Radix (PMR), the dried root of Polygonum Multiflorum Thunb., has been widely used as traditional Chinese medicines in clinical practice for centuries. However, the frequently reported hepatotoxic adverse effects hindered its safe use in clinical practice. This study aims to explore the hepatotoxic effect of PMR extract and the major PMR derived anthraquinones including emodin, chrysophanol, and physcion in mice and the underlying mechanisms based on bile acid homeostasis. After consecutively treating the ICR mice with PMR extract or individual anthraquinones for 14 or 28 days, the liver function was evaluated by measuring serum enzymes levels and liver histological examination. The compositions of bile acids (BAs) in the bile, liver, and plasma were measured by LC-MS/MS, followed by Principal Component Analysis (PCA) and Partial Least Squares Discriminate Analysis (PLS-DA). Additionally, gene and protein expressions of BA efflux transporters, bile salt export pump (Bsep) and multidrug resistance-associated protein 2 (Mrp2), were examined to investigate the underlying mechanisms. After 14-day administration, mild inflammatory cell infiltration in the liver was observed in the physcion- and PMR-treated groups, while it was found in all the treated groups after 28-day treatment. Physcion and PMR extract induced hepatic BA accumulation after 14-day treatment, but such accumulation was attenuated after 28-day treatment. Based on the PLS-DA results, physcion- and PMR-treated groups were partially overlapping and both groups showed a clear separation with the control group in the mouse liver. The expression of Bsep and Mrp2 in the physcion- and PMR-treated mouse liver was decreased after 14-day treatment, while the downregulation was abrogated after 28-day treatment. Our study, for the first time, demonstrated that both PMR extract and tested anthraquinones could alter the disposition of either the total or individual BAs in the mouse bile, liver, and plasma via regulating the BA efflux transporters and induce liver injury, which provide a theoretical basis for the quality control and safe use of PMR in practice.

Gilbert's Syndrome, Bilirubin Level and UGT1A1∗28 Genotype in Men of North-West Region of Russia

BACKGROUND/OBJECTIVES

Gilbert's syndrome (GS) is a hereditary pathology that affects approximately 10% of the world's population. In most cases, GS is associated with the UGT1A1∗28 polymorphism of UGT1A1 gene coding the enzyme bilirubin uridine diphosphate glucuronosyltransferase (UGT-1A) which plays a key role in the bilirubin metabolism. The presence of an additional TA repeat in the TATA box of the UGT1A1 gene promoter (the allelic variant of 7TA, abbreviated as UGT1A1∗28) leads to a significant decrease in the enzymatic activity of UGT-1A in the liver and to decrease in glucuronidation process as a consequence. The aim of the study is to estimate the prevalence of the 6TA/6TA, 6TA/7TA, and 7TA/7TA genotypes of UGT1A1 promoter and to analyze the effect of these variants on bilirubin levels in healthy men in North-West Russia and patients with a clinical diagnosis of GS.

METHODS

Genotyping of the UGT1A1 ∗28 (rs8175347) polymorphism was carried out by real-time PCR.

RESULTS

The results obtained indicate an increased probability of GS developing in residents of the North-West region of Russia compared with other representatives of the Caucasians.

CONCLUSIONS

Despite the fact that the level of serum bilirubin increases with the rise in the number of additional TA dinucleotides in the UGT1A1 gene promoter tests of clinical manifestations only (jaundice, fatigue, sleep disturbances, nausea, belching, and so on) and increased bilirubin levels in patients with normal liver function do not allow unequivocally diagnose GS. UGT1A1∗28 genotyping should be used as a prognostic risk factor for such pathology development.

Molecular Physiology and Pathophysiology of Bilirubin Handling by the Blood, Liver, Intestine, and Brain in the Newborn

Bilirubin is the end product of heme catabolism formed during a process that involves oxidation-reduction reactions and conserves iron body stores. Unconjugated hyperbilirubinemia is common in newborn infants, but rare later in life. The basic physiology of bilirubin metabolism, such as production, transport, and excretion, has been well described. However, in the neonate, numerous variables related to nutrition, ethnicity, and genetic variants at several metabolic steps may be superimposed on the normal physiological hyperbilirubinemia that occurs in the first week of life and results in bilirubin levels that may be toxic to the brain. Bilirubin exists in several isomeric forms that differ in their polarities and is considered a physiologically important antioxidant. Here we review the chemistry of the bilirubin molecule and its metabolism in the body with a particular focus on the processes that impact the newborn infant, and how differences relative to older children and adults contribute to the risk of developing both acute and long-term neurological sequelae in the newborn infant. The final section deals with the interplay between the brain and bilirubin and its entry, clearance, and accumulation. We conclude with a discussion of the current state of knowledge regarding the mechanism(s) of bilirubin neurotoxicity.

Can Bile Salt Export Pump Inhibition Testing in Drug Discovery and Development Reduce Liver Injury Risk? An International Transporter Consortium Perspective

Bile salt export pump (BSEP) inhibition has emerged as an important mechanism that may contribute to the initiation of human drug‐induced liver injury (DILI). Proactive evaluation and understanding of BSEP inhibition is recommended in drug discovery and development to aid internal decision making on DILI risk. BSEP inhibition can be quantified using in vitro assays. When interpreting assay data, it is important to consider in vivo drug exposure. Currently, this can be undertaken most effectively by consideration of total plasma steady state drug concentrations (C_ss,plasma). However, because total drug concentrations are not predictive of pharmacological effect, the relationship between total exposure and BSEP inhibition is not causal. Various follow‐up studies can aid interpretation of in vitro BSEP inhibition data and may be undertaken on a case‐by‐case basis. BSEP inhibition is one of several mechanisms by which drugs may cause DILI, therefore, it should be considered alongside other mechanisms when evaluating possible DILI risk.

Transporters in Drug Development: 2018 ITC Recommendations for Transporters of Emerging Clinical Importance

This white paper provides updated International Transporter Consortium (ITC) recommendations on transporters that are important in drug development following the 3^rd ITC workshop. New additions include prospective evaluation of organic cation transporter 1 (OCT1) and retrospective evaluation of organic anion transporting polypeptide (OATP)2B1 because of their important roles in drug absorption, disposition, and effects. For the first time, the ITC underscores the importance of transporters involved in drug-induced vitamin deficiency (THTR2) and those involved in the disposition of biomarkers of organ function (OAT2 and bile acid transporters).

Bisphenol S impairs blood functions and induces cardiovascular risks in rats

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Bisphenol S alters blood homeostasis.

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Bisphenol S is probably a cardiac risk augmenting chemical.

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Bisphenol S is a haemolysis promoting chemical.

Bisphenol S (BPS) is an industrial chemical which is recently used to replace the potentially toxic Bisphenol A (BPA) in making polycarbonate plastics, epoxy resins and thermal receipt papers. The probable toxic effects of BPS on the functions of haemopoietic and cardiovascular systems have not been reported till to date. We report here that BPS depresses haematological functions and induces cardiovascular risks in rat. Adult male albino rats of Sprague-Dawley strain were given BPS at a dose level of 30, 60 and 120 mg/kg BW/day respectively for 30 days. Red blood cell (RBC) count, white blood cell (WBC) count, Hb concentration, and clotting time have been shown to be significantly (*P < 0.05) reduced in a dose dependent manner in all exposed groups of rats comparing to the control. It has also been shown that BPS increases total serum glucose and protein concentration in the exposed groups of rats. We have observed that BPS increases serum total cholesterol, triglyceride, glycerol free triglyceride, low density lipoprotein (LDL) and very low density lipoprotein (VLDL) concentration, whereas high density lipoprotein (HDL) concentration has been found to be reduced in the exposed groups. BPS significantly increases serum aspartate aminotransferase (AST), alanine aminotransferase (ALT) and alkaline phosphatase (ALP) activities dose dependently. Moreover, serum calcium, bilirubin and urea concentration have been observed to be increased in all exposed groups. In conclusion, BPS probably impairs the functions of blood and promotes cardiovascular risks in rats.

Tigernut (Cyperus esculentus L.) "milk" as a potent "nutri-drink" for the prevention of acetaminophen-induced hepatotoxicity in a murine model

AIM/BACKGROUND

Given the prevalence of toxicants in foods, beauty products, etc., and the increasing demand for "green" products, there is a need for the development of "nutri-drinks" with hepatoprotective properties. The usefulness of tigernut milk (TNM) in preventing acetaminophen (APAP)-induced liver injury was, therefore, investigated.

MATERIALS AND METHODS

A total of 25 rats were randomized into five equal groups. Four groups were treated with 0, 500, 1000, and 2000 kg/mg body weight (bw) TNM, respectively, per os for 2 weeks before they were challenged with 2500 mg/kg bw APAP. Biochemical markers of hepatotoxicity and oxidative stress were determined in the sera of the rats at the end of the study.

RESULTS

Serum alanine aminotransferase concentrations decreased significantly (P < 0.001) and dose-dependently from 334.3 ± 16.1 in the negative control group to 65.4 ± 8.3 in the 2000 mg/kg bw TNM group. Other studied liver enzymes were similarly dose-dependently reduced. These data are corroborated by histological findings. Superoxide dismutase activity (U/mg protein) was increased significantly (P < 0.001) from 108.0 ± 7.4 in the negative control group to 291.0 ± 11.3 in the 2000 mg/kg bw TNM group, and indeed all the test groups. The malondialdehyde concentrations in the test rats were slightly lower than that of the negative control group.

CONCLUSION

TNM at the tested concentrations significantly prevented liver injury. Phytochemicals in TNM, working directly as antioxidants or indirectly by inducing the synthesis of glutathione, may be responsible for the observed effect.

Generation of Ugt1-deficient murine liver cell lines using TALEN technology

The Crigler-Najjar Syndrome Type I (CNSI) is a rare genetic disorder caused by mutations in the Ugt1a1 gene. It is characterized by unconjugated hyperbilirubinemia that may result in severe neurologic damage and death if untreated. To date, liver transplantation is the only curative treatment. With the aim of generating mutant cell lines of the Ugt1 gene, we utilized the TALEN technology to introduce site-specific mutations in Ugt1 exon 4. We report a fast and efficient method to perform gene knockout in tissue culture cells, based on the use of TALEN pairs targeting restriction enzyme (RE) sites in the region of interest. This strategy overcame the presence of allele-specific single nucleotide polymorphisms (SNPs) and pseudogenes, conditions that limit INDELs' detection by Surveyor. We obtained liver-derived murine N-Muli cell clones having INDELs with efficiency close to 40%, depending on the TALEN pair and RE target site. Sequencing of the target locus and WB analysis of the isolated cell clones showed a high proportion of biallelic mutations in cells treated with the most efficient TALEN pair. Ugt glucuronidation activity was reduced basal levels in the biallelic mutant clones. These mutant liver-derived cell lines could be a very useful tool to study biochemical aspects of Ugt1 enzyme activity in a more natural context, such as substrate specificity, requirement of specific co-factors, the study of inhibitors and other pharmacological aspects, and to correlate enzyme activity to the presence of specific mutations in the gene, by adding back to the mutant cell clones specific variants of the Ugt1 gene. In addition, since genome editing has recently emerged as a potential therapeutic approach to cure genetic diseases, the definition of the most efficient TALEN pair could be an important step towards setting up a platform to perform genome editing in CNSI.

Circulating bilirubin and defense against kidney disease and cardiovascular mortality: mechanisms contributing to protection in clinical investigations

Unconjugated bilirubin is an endogenous circulating antioxidant, bound to albumin, and therefore is retained in the vascular compartment. Bilirubin has well-documented neurotoxic effects in infants; however, current evidence indicates mildly elevated bilirubin is associated with protection from cardiovascular disease and all-cause mortality in adults. Recent clinical studies show mildly elevated bilirubin is associated with protection from kidney damage and dysfunction, in addition to cardiovascular events and all-cause mortality in patients undergoing hemodialysis. This is the first review to examine the clinical evidence and summarize the potential mechanisms of action that link bilirubin to protection from kidney damage, subsequent kidney failure, and dialysis-related mortality. With this understanding, it is hoped that new therapies will be developed to prevent renal dysfunction and mortality from cardiovascular disease in at-risk individuals.

Analysis of flavonoids regulating the expression of UGT1A1 via xenobiotic receptors in intestinal epithelial cells

UDP-glucuronosyltransferase (UGT) 1A1 is one of the major metabolic enzymes for the detoxification of harmful xenobiotics in intestines, and its expression is regulated by transcription factors like the aryl hydrocarbon receptor (AhR) and the pregnane X receptor (PXR). A screening assay using real-time PCR showed that baicalein and 3-hydroxyflavone induced human UGT1A1 mRNA expression in LS180 cells. Experimental results confirmed that these flavonoids increased UGT1A protein expression as well as its enzymatic activity. The results indicated that baicalein and 3-hydroxyflavone increased the transcriptional activity of UGT1A1 via AhR and PXR, respectively. Observation via immunofluorescence microscopy suggested that baicalein and 3-hydroxyflavone further induced nuclear translocation of AhR and PXR, respectively. In addition, direct interaction between baicalein and AhR or 3-hydroxyflavone and PXR were observed using the quartz crystal microbalance method. These results elucidate the molecular mechanism of flavonoid-induced UGT1A1 gene expression via xenobiotic receptors in the intestines.

A genome-wide association study for serum bilirubin levels and gene-environment interaction in a Chinese population

Bilirubin is an effective antioxidant and is influenced by both genetic and environmental factors. Recent genome-wide association studies (GWAS) have identified multiple loci affecting serum total bilirubin levels. However, most of the studies were conducted in European populations and little attention has been devoted either to genetic variants associated with direct and indirect bilirubin levels or to the gene-environment interactions on bilirubin levels. In this study, a two-stage GWAS was performed to identify genetic variants associated with all types of bilirubin levels in 10,282 Han Chinese individuals. Gene-environment interactions were further examined. Briefly, two previously reported loci, UGT1A1 on 2q37 (rs6742078 and rs4148323, combined P = 1.44 × 10(-89) and P = 5.05 × 10(-69) , respectively) and SLCO1B3 on 12p12 (rs2417940, combined P = 6.93 × 10(-19) ) were successfully replicated. The two loci explained 9.2% and 0.9% of the total variations of total bilirubin levels, respectively. Ethnic genetic differences were observed between Chinese and European populations. More importantly, a significant interaction was found between rs2417940 in SLCO1B3 gene and smoking on total bilirubin levels (P = 1.99 × 10(-3) ). Single nucleotide polymorphism (SNP) rs2417940 had stronger effects on total bilirubin levels in nonsmokers than in smokers, suggesting that the effects of SLCO1B3 genotype on bilirubin levels were partly dependent on smoking status. Consistent associations and interactions were observed for serum direct and indirect bilirubin levels.

Rescue of bilirubin-induced neonatal lethality in a mouse model of Crigler-Najjar syndrome type I by AAV9-mediated gene transfer

Crigler-Najjar type I (CNI) syndrome is a recessively inherited disorder characterized by severe unconjugated hyperbilirubinemia caused by uridine diphosphoglucuronosyltransferase 1A1 (UGT1A1) deficiency. The disease is lethal due to bilirubin-induced neurological damage unless phototherapy is applied from birth. However, treatment becomes less effective during growth, and liver transplantation is required. To investigate the pathophysiology of the disease and therapeutic approaches in mice, we generated a mouse model by introducing a premature stop codon in the UGT1a1 gene, which results in an inactive enzyme. Homozygous mutant mice developed severe jaundice soon after birth and died within 11 d, showing significant cerebellar alterations. To rescue neonatal lethality, newborns were injected with a single dose of adeno-associated viral vector 9 (AAV9) expressing the human UGT1A1. Gene therapy treatment completely rescued all AAV-treated mutant mice, accompanied by lower plasma bilirubin levels and normal brain histology and motor coordination. Our mouse model of CNI reproduces genetic and phenotypic features of the human disease. We have shown, for the first time, the full recovery of the lethal effects of neonatal hyperbilirubinemia. We believe that, besides gene-addition-based therapies, our mice could represent a very useful model to develop and test novel technologies based on gene correction by homologous recombination.

Genetic variation of the IL-28B promoter affecting gene expression

The current standard of care for the treatment of chronic hepatitis C is pegylated interferon-α (PEG-IFNα) and ribavirin (RBV). The treatment achieves a sustained viral clearance in only approximately 50% of patients. Recent whole genome association studies revealed that single nucleotide polymorphisms (SNPs) around IL-28B have been associated with response to the standard therapy and could predict treatment responses at approximately 80%. However, it is not clear which SNP is most informative because the genomic region containing significant SNPs shows strong linkage disequilibrium. We focused on SNPs in close proximity to the IL-28B gene to evaluate the function of each and identify the SNP affecting the IL-28B expression level most. The structures of IL-28A/B from 5' to 3'-UTR were determined by complete cDNA cloning. Both IL-28A and 28B genes consisted of 6 exons, differing from the CCDS data of NCBI. Two intron SNPs and a nonsynonymous SNP did not affect IL-28B gene function and expression levels but a SNP located in the proximal promoter region influenced gene expression. A (TA) dinucleotide repeat, rs72258881, located in the promoter region was discovered by our functional studies of the proximal SNPs upstream of IL-28B; the transcriptional activity of the promoter increased gradually in a (TA)(n) length-dependent manner following IFN-α and lipopolysaccharide stimulation. Healthy Japanese donors exhibited a broad range of (TA) dinucleotide repeat numbers from 10 to 18 and the most prevalent genotype was 12/12 (75%), differing from the database (13/13). However, genetic variation of IL-28A corresponding to that of IL-28B was not detected in these Japanese donors. These findings suggest that the dinucleotide repeat could be associated with the transcriptional activity of IL-28B as well as being a marker to improve the prediction of the response to interferon-based hepatitis C virus treatment.

Ideal Experimental Rat Models for Liver Diseases

There are many limitations for conducting liver disease research in human beings due to the high cost and potential ethical issues. For this reason, conducting a study that is difficult to perform in humans using appropriate animal models, can be beneficial in ascertaining the pathological physiology, and in developing new treatment modalities. However, it is difficult to determine the appropriate animal model which is suitable for research purposes, since every patient has different and diverse clinical symptoms, adverse reactions, and complications due to the pathological physiology. Also, it is not easy to reproduce identically various clinical situations in animal models. Recently, the Guide for the Care and Use of Laboratory Animals has tightened up the regulations, and therefore it is advisable to select the appropriate animals and decide upon the appropriate quantities through scientific and systemic considerations before conducting animal testing. Therefore, in this review article the authors examined various white rat animal testing models and determined the appropriate usable rat model, and the pros and cons of its application in liver disease research. The authors believe that this review will be beneficial in selecting proper laboratory animals for research purposes.

Takashi Iyanagi: UGT1 gene complex: from Gunn rat to human

UDP-glucuronosyltransferases (UGT) comprise a large gene superfamily that can be classified, based on the degree of amino-acid similarity between isoforms, into several gene families. Among these gene families, the UDP-glucuronosyltransferase family 1 (UGT1) gene is a unique gene complex organized to generate enzymes that share a common carboxyl terminal portion and are unique in the variable amino terminal region. Each variable exon I is preceded by a regulatory 5'-region and, in response to a specific signal, transcription processing splices mRNA from each unique exon 1 to the four common exons ( 2, 3, 4, and 5) to provide a template for synthesis of the individual isoforms. A novel clue to elucidate the gene structure of mammalian UGT1 was cDNA cloning of rat UGT1A6 from the hyperbilirubinemic Gunn rat by Professor Takashi Iyanagi Ph.D. The elucidation of the structure of the rat UGT1 gene complex has led to a greater understanding of the genetic basis of Crigler-Najjar and Gilbert's syndromes. Now, examination of the UGT1 gene structure in hyperbilirubinemic patients has revealed more than 100 different genetic defects in Crigler-Najjar syndromes and one genetic alternation that accounts for the majority of Gilbert's syndrome cases. This review of a chapter in UGT history will focus on the extensive research of Iyanagi and coworkers with the rat UGT1 gene complex and advancing to the study of the human gene.

Polyamine oxidase activity in peripheral blood of newborn infants with neonatal hyperbilirubinemia: is bilirubin an antioxidant?

BACKGROUND

Neonatal hyperbilirubinemia can be physiological and pathological and most frequently is a consequence of faster erythrocytes (RBC) hemolysis. Free unconjugated bilirubin is a highly toxic compound, especially for the central nervous system. The most abundant polyamines circulating in blood are spermidine (Spd) and spermine (Sp), which are mainly localized in RBC, where they control membrane permeability. Polyamine oxidase (PAO) exerts an important activity in the plasma and erythrocytes of newborn infants with hyperbilirubinemia, catalyzing the oxidative deamination of Sp and Spd, producing potentially toxic agents that induce apoptosis of mammalian cells. The present study investigated polyamine metabolism by measuring PAO activity in the blood of newborn infants with hyperbilirubinemia and explored the possible antioxidant function of bilirubin through monitoring malondialdehyde (MDA) levels.

METHODS

The study included 43 newborns, 10 in the control and 33 in the diseased group. Blood PAO activity and bilirubin and MDA levels were measured using spectrophotometric methods.

RESULTS/DISCUSSION

Our results indicate that bilirubin, at physiologic concentrations, protects neonatal erythrocytes against oxidative stress. The positive correlation between PAO activity and MDA levels with high bilirubin concentrations (> 200 micromol/L) in newborn infants indicates that in pathological conditions, bilirubin cannot exert its antioxidant function.

CONCLUSION

Investigating the function of polyamines in erythrocytes and the importance of PAO related to hemolysis and bilirubin synthesis is necessary to shed light on the functions of PAO and its metabolites on the permeability of the erythrocyte membrane.

Oxidative and electrophilic stress induces multidrug resistance-associated protein transporters via the nuclear factor-E2-related factor-2 transcriptional pathway

Multidrug resistance-associated proteins (Mrps) are adenosine triphosphate-dependent transporters that efflux chemicals out of cells. In the liver, Mrp2 transports bilirubin-glucuronide, glutathione (GSH), and drug conjugates into bile, whereas Mrp3 and Mrp4 efflux these entities into blood. The purpose of this study was to determine whether oxidative conditions (that is, the disruption of hepatic GSH synthesis) or the administration of nuclear factor-E2-related factor-2 (Nrf2) activators (oltipraz and butylated hydroxyanisole) can induce hepatic Mrp transporters and whether that induction is through the Nrf2 transcriptional pathway. Livers from hepatocyte-specific glutamate-cysteine ligase catalytic subunit-null mice had increased nuclear Nrf2 levels, marked gene and protein induction of the Nrf2 target gene NAD(P)H:quinone oxidoreductase 1, as well as Mrp2, Mrp3, and Mrp4 expression. The treatment of wild-type and Nrf2-null mice with oltipraz and butylated hydroxyanisole demonstrated that the induction of Mrp2, Mrp3, and Mrp4 is Nrf2-dependent. In Hepa1c1c7 cells treated with the Nrf2 activator tert-butyl hydroquinone, chromatin immunoprecipitation with Nrf2 antibodies revealed the binding of Nrf2 to antioxidant response elements in the promoter regions of mouse Mrp2 [-185 base pairs (bp)], Mrp3 (-9919 bp), and Mrp4 (-3767 bp).

CONCLUSION

The activation of the Nrf2 regulatory pathway stimulates the coordinated induction of hepatic Mrps.

Molecular mechanism of phase I and phase II drug-metabolizing enzymes: implications for detoxification

Enzymes that catalyze the biotransformation of drugs and xenobiotics are generally referred to as drug-metabolizing enzymes (DMEs). DMEs can be classified into two main groups: oxidative or conjugative. The NADPH-cytochrome P450 reductase (P450R)/cytochrome P450 (P450) electron transfer systems are oxidative enzymes that mediate phase I reactions, whereas the UDP-glucuronosyltransferases (UGTs) are conjugative enzymes that mediate phase II enzymes. Both enzyme systems are localized to the endoplasmic reticulum (ER) where a number of drugs are sequentially metabolized. DMEs, including P450s and UGTs, generally have a highly plastic active site that can accommodate a wide variety of substrates. The P450 and UGT genes constitute a supergene family, in which UGT proteins are encoded by distinct genes and a complex gene. Both the P450 and UGT genes have evolved to diversify their functions. This chapter reviews advances in understanding the structure and function of the P450R/P450 and UGT enzyme systems. In particular, the coordinate biotransformation of xenobiotics by phase I and II enzymes in the ER membrane is examined.

Effect of DPC 333 [(2R)-2-{(3R)-3-amino-3-[4-(2-methylquinolin-4-ylmethoxy)phenyl]-2-oxopyrrolidin-1-yl}-N-hydroxy-4-methylpentanamide], a human tumor necrosis factor alpha-converting enzyme inhibitor, on the disposition of methotrexate: a transporter-based drug-drug interaction case study

DPC 333 [(2R)-2-{(3R)-3-amino-3-[4-(2-methylquinolin-4-ylmethoxy)phenyl]-2-oxopyrrolidin-1-yl}-N-hydroxy-4-methylpentanamide] is a potent human tumor necrosis factor alpha-converting enzyme inhibitor with potential therapeutic implications for rheumatoid arthritis. Methotrexate (MTX), a drug for the treatment of rheumatoid arthritis, is eliminated primarily unchanged via renal and biliary excretion in humans as well as in rats and dogs. The objective of the present study was to investigate the potential effect of DPC 333 on the disposition of MTX. In dogs, DPC 333 administered orally at 1.7 mg/kg 15 min before the intravenous administration of [14C]MTX (0.5 mg/kg) did not alter the plasma concentration-time profile of MTX; however, the total amount of radioactivity excreted in urine increased from 58.7% to 92.2% of the dose, and the renal clearance increased from 1.8 ml/min/kg to 2.9 ml/min/kg, suggesting a decrease in MTX disposition via biliary excretion. The biliary excretion of MTX was investigated in isolated perfused livers prepared from wild-type and TR(-) [multidrug resistance-associated protein 2 (Mrp2)-deficient] Wistar rats in the absence and presence of DPC 333. Mrp2-mediated biliary excretion of MTX was confirmed with 95.8% and 5.1% of MTX recovered in the bile of wild-type and TR(-) Wistar rats, respectively. DPC 333 at an initial perfusate concentration of 50 microM completely blocked the biliary excretion of MTX, but not the clearance from perfusate, in both wild-type and TR(-) rats. These results suggest that the enhanced renal elimination of MTX may be due to the potent inhibition of biliary excretion and active renal reabsorption by DPC 333 and/or its metabolites.

Altered hepatobiliary disposition of 5 (and 6)-carboxy-2',7'-dichlorofluorescein in Abcg2 (Bcrp1) and Abcc2 (Mrp2) knockout mice

This study characterized the hepatobiliary disposition of 5 (and 6)-carboxy-2',7'-dichlorofluorescein (CDF), a model Abcc2/Mrp2 (canalicular) and Abcc3/Mrp3 (basolateral) substrate, in perfused livers from male C57BL/6 wild-type, Abcg2-/-, and Abcc2-/- mice. After single-pass liver perfusion with 1 muM CDF diacetate for 30 min and an additional 30-min perfusion with CDF-free buffer, cumulative biliary excretion of CDF in Abcg2-/- mice was significantly higher than in wild-type mice (65 +/- 6 and 47 +/- 15% of dose, respectively, p < 0.05), whereas CDF recovery in bile of Abcc2-/- mice was negligible. Cumulative recovery of CDF in perfusate was significantly higher in Abcc2-/- (90 +/- 8% of dose) relative to wild-type (35 +/- 11% of dose) mice. Compartmental pharmacokinetic analysis revealed that the rate constant for CDF biliary excretion was significantly increased in Abcg2-/- (0.061 +/- 0.005 min(-1)) compared with wild-type (0.039 +/- 0.011 min(-1)) mice. The rate constant governing the basolateral excretion of CDF was approximately 4-fold higher in Abcc2-/- (0.12 +/- 0.02 min(-1)) relative to wild-type (0.030 +/- 0.011 min(-1)) mice but was not altered in Abcg2-/- (0.031 +/- 0.004 min(-1)) mice. Hepatic Abcc3 protein levels, determined by immunoblot analysis, were approximately 60% higher in Abcc2-/- mice than in wild-type mice. In contrast, neither Abcc3 protein levels nor Abcc2 mRNA levels were altered in Abcg2-/- relative to wild-type mice. These data in knockout mouse models demonstrate that loss of expression and function of one canalicular transport protein may change the route and/or extent of excretion into bile or perfusate because of alterations in the function of other basolateral or canalicular transport proteins.

Hepatic proliferation in Gunn rats transplanted with hepatocytes: effect of retrorsine and tri-iodothyronine

Hepatocyte transplantation would offer an attractive alternative to liver transplantation in the treatment of inborn errors of liver metabolism. However, a major problem in most transplantation studies to date has been the limited growth of transplanted cells in the recipient organ. We performed a strategy for selective proliferation of transplanted cells by interfering with the proliferative capacity of resident hepatocytes, using the pyrrolizidine alkaloid retrorsine and then transplanting liver cells in conjunction with repeated administration of triiodothyronine, an inducer of hepatocyte proliferation in rats. In the present study, foetal and adult syngeneic hepatocyte transplantation into spleen was performed in retrorsine-treated hyperbilirubinemic Gunn rats. In parallel, repeated injections of triiodothyronine were given to recipients. Rats were sacrificed at 1, 7, 30 and 90 days after transplantation and blood and bile samples were taken to assess the functionality of transplanted cells. The proliferative activity of transplanted hepatocytes was evaluated using proliferating cell nuclear antigen labelling index. In summary, both adult and foetal hepatocyte transplantation were effective in correcting a metabolic abnormality in Gunn rats for as long as 3 months. The RS/T3 model, as a measure to increase graft function, could represent an important advance to future clinical application of hepatocyte transplantation.

Molecular genetics of unconjugated hyperbilirubinemia in Taiwanese

In bilirubin metabolism, increased destruction of erythrocytes, defect in the function of organic anion transporter polypeptide 2 (OATP2) or UDP-glucuronosyltransferase 1A1 (UGT1A1) may result in unconjugated hyperbilirubinemia. Although glucose-6-phosphate dehydrogenase (G6PD) deficiency is known to be associated with the development of neonatal hyperbilirubinemia, it was observed that in neonates severe hyperbilirubinemia caused by G6PD deficiency, without associated polymorphisms in the UGT1A1 or the OATP2 gene, was preventable. Variations at the nucleotide (nt) 388 of OATP2 gene and nt-211 of UGT1A1 gene, were found to be a risk factor for severe hyperbilirubinemia amongst Taiwanese neonates, respectively. G6PD deficiency, variations at nts 388 and 521 of OATP2 gene, and variations at nt-211 and in the promoter area of UGT1A1 gene were reported to be the risk factors for the occurrence of mild hyperbilirubinemia amongst Taiwanese adults. The status of the haplotypes of G6PD, OATP2, and UGT1A1 genes affected the odds ratio and the bilirubin levels in the hyperbilirubinemic subjects. Moreover, carriage of the variant-211 UGT1A1 allele, as well as UGT1A7*3 allele, was demonstrated to represent a risk factor for the development of, and a determinant for, metastases associated with Taiwanese colorectal-cancer patients. Further investigation is warranted to evaluate this phenomenon.

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

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

Lifelong elimination of hyperbilirubinemia in the Gunn rat with a single injection of helper-dependent adenoviral vector

Crigler-Najjar syndrome is a recessively inherited disorder characterized by severe unconjugated hyperbilirubinemia caused by a deficiency of uridine diphospho-glucuronosyl transferase 1A1. Current therapy relies on phototherapy to prevent kernicterus, but liver transplantation presently is the only permanent cure. Gene therapy is a potential alternative, and recent work has shown that helper-dependent adenoviral (HD-Ad) vectors, devoid of all viral coding sequences, induce prolonged transgene expression and exhibit significantly less chronic toxicity than early-generation Ad vectors. We used a HD-Ad vector to achieve liver-restricted expression of human uridine diphospho-glucuronosyl transferase 1A1 in the Gunn rat, a model of the human disorder. Total plasma bilirubin levels were reduced from >5.0 mg/dl to <<1.4 mg/dl for >2 yr after a single i.v. administration of vector expressing the therapeutic transgene at a dose of 3 x 10(12) viral particles per kg. HPLC analysis of bile from treated rats showed the presence of bilirubin glucuronides at normal WT levels >2 yr after one injection of vector, and i.v. injection of bilirubins IIIalpha and XIIIalpha in the same animals revealed excess bilirubin-conjugating capacity. There was no significant elevation of liver enzymes (alanine aminotransferase) and only transient, moderate thrombocytopenia after injection of the vector. A clinically significant reduction in serum bilirubin was observed with a dose as low as 6 x 10(11) viral particles per kg. We conclude that complete, long-term correction of hyperbilirubinemia in the Gunn rat model of Crigler-Najjar syndrome can be achieved with one injection of HD-Ad vector and negligible chronic toxicity.

The hyperbilirubinemic Gunn rat is resistant to the pressor effects of angiotensin II

ANG II induces vasoconstriction, at least in part, by stimulating NADPH oxidase and generating reactive oxygen species. ANG II also induces heme oxygenase activity, and bilirubin, a product of such activity, possesses antioxidant properties. We hypothesized that bilirubin, because of its antioxidant properties, may reduce the pressor and prooxidant effects of ANG II. Our in vivo studies used the hyperbilirubinemic Gunn rat which is deficient in the enzyme uridine diphosphate glucuronosyl transferase, the latter enabling the excretion of bilirubin into bile. ANG II (0.5 mg x kg(-1) x day(-1)) or saline vehicle was administered by osmotic minipump to control and Gunn rats for 4 wk. The rise in systolic blood pressure induced by ANG II, as observed in control rats, was markedly reduced in Gunn rats, the latter approximately 50% less at 3 and 4 wk after the initiation of ANG II infusion. The chronic administration of ANG II also impaired endothelium-dependent relaxation responses in control rats but not in Gunn rats. As assessed by the tetrahydrobiopterin/dihydrobiopterin ratio, ANG II induced oxidative stress in the aorta in control rats but not in Gunn rats. Heightened generation of superoxide anion in aortic rings in ANG II-infused rats and by vascular smooth muscle cells exposed to ANG II was normalized by bilirubin in vitro. We conclude that the pressor and prooxidant effects of ANG II are attenuated in the hyperbilirubinemic Gunn rat, an effect which, we speculate, may reflect, at least in part, the scavenging of superoxide anion by bilirubin.

Regulation of the Human UGT1A1 Gene by Nuclear Receptors Constitutive Active/Androstane Receptor, Pregnane X Receptor, and Glucocorticoid Receptor

Human UDP-glucuronosyltransferase (UGT) 1A1 is the enzyme that detoxifies neurotoxic bilirubin by conjugating it with glucuronic acid. In addition to bilirubin, UGT1A1 conjugates various endogenous and exogenous lipophilic compounds such as estrogens and the active metabolite of the anticancer drug irinotecan SN-38. Thus, activation by specific inducers of the UGT1A1 gene is critical in treating patients with unconjugated hyperbili-rubinemia and in preventing side effects of drug treatment such as SN-38-induced toxicity. This chapter describes the experimental processes used to identify the 290-bp distal enhancer module at -3499/-3210 of the UGT1A1 gene and to characterize its regulation by nuclear receptors: constitutive active/androstane receptor, pregnane X receptor, and glucocorticoid receptor.

Pharmacogenomics of MDR and MRP subfamilies

Drug-metabolizing enzymes, drug transporters and drug targets play significant roles as determinants of drug efficacy and toxicity. Their genetic polymorphisms often affect the expression and function of their products and are expected to become surrogate markers to predict the response to drugs in individual patients. With the sequencing of the human genome, it has been estimated that approximately 500–1200 genes code for drug transporters and, recently, there have been significant and rapid advances in the research on the relationships between genetic polymorphisms of drug transporters and interindividual variation of drug disposition. At present, the clinical studies of multi-drug resistance protein 1 (MDR1, P-glycoprotein, ABCB1), which belongs to the ATP-binding cassette (ABC) superfamily, are the most comprehensive among the ABC transporters, but clinical investigations on other drug transporters are currently being performed around the world. MDR1 can be said to be the most important drug transporter, since clinical reports have suggested that it regulates the disposition of various types of clinically important drugs, but in vitro investigations or animal experiments have strongly suggested that the members of the multi-drug resistance-associated protein (MRP) subfamily can also become key molecules for pharmacotherapy. In addition to those, breast cancer resistance protein (BCRP, ABCG2), another ABC transporter, is well known as a key molecule of multi-drug resistance to several anticancer agents. However, this review focuses on the latest information on the pharmacogenetics of the MDR and MRP subfamilies, and its impact on pharmacotherapy is discussed.

Glucuronidation and the UDP-glucuronosyltransferases in health and disease

This article is an updated report of a symposium held at the June 2000 annual meeting of the American Society for Pharmacology and Experimental Therapeutics in Boston. The symposium was sponsored by the ASPET Divisions for Drug Metabolism and Molecular Pharmacology. The report covers research from the authors' laboratories on the structure and regulation of UDP-glucuronosyltransferase (UGT) genes, glucuronidation of xenobiotics and endobiotics, the toxicological relevance of UGTs, the role of UGT polymorphisms in cancer susceptibility, and gene therapy for UGT deficiencies.

Relevance of different UGT1A1 polymorphisms in irinotecan-induced toxicity: a molecular and clinical study of 75 patients

PURPOSE

We wanted to assess polymorphisms in the uridine diphosphoglucuronosyl transferase 1A1 (UGT 1A1) gene: the TATA box polymorphism and UGT 1A1 G71R and Y486D mutations in the coding sequence, the main mutations characterizing Gilbert's syndrome, as predictors of severe toxic event occurrence after irinotecan (CPT-11) administration. Therefore, we set up a rapid, sensitive, and reliable technique in routine practice to detect before CPT-11 treatment, the at-risk patients.

EXPERIMENTAL DESIGN

Seventy-five patients with advanced colorectal cancer and treated with CPT-11 and 5-fluorouracil, entered the study. We used the Pyrosequencing technology a real-time sequencing method, to detect the UGT 1A1 TATA box polymorphisms and mutations in the coding regions. Patients were also assessed for both biochemical and clinical evaluation and tolerance to treatment.

RESULTS

No G71R and Y486D mutations were found in our population. Frequencies for UGT 1A1 TATA box polymorphisms were 41, 47, and 9% for wild-type 6/6, heterozygous 6/7, and Gilbert's syndrome 7/7, respectively. Tolerance to treatment decreased with increased number of TA repeat with 71% of the patients in 7/7 group who experienced grade 3/4 toxicity.

CONCLUSIONS

The method we set up is suitable for the detection of UGT 1A1 polymorphism in routine practice before irinotecan treatment. It could help to detect the patients homozygous or heterozygous for Gilbert's syndrome, at-risk of CPT 11-induced toxicity, and thus could help to individualize the dose to optimize efficacy and limit toxicity.

Role of orphan nuclear receptors in the regulation of drug-metabolising enzymes

During the past several years, important advances have been made in our understanding of the mechanisms that regulate the expression of genes that determine drug clearance, including phase I and phase II drug-metabolising enzymes and drug transporters. Orphan nuclear receptors have been recognised as key mediators of drug-induced changes in both metabolism and efflux mechanisms. In this review, we summarise recent findings regarding the function of nuclear receptors in regulating drug-metabolising and transport systems, and the relevance of these receptors to clinical drug-drug interactions and the development of new drugs. Emphasis is given to two newly recognised 'orphan' receptors (the pregnane X receptor [PXR] and the constitutive androstane receptor [CAR]) and their regulation of cytochrome P450 enzymes, such as CYP3A4, CYP2Cs and CYP2B6; and transporters, such as P-glycoprotein (MDR1), multidrug resistance-associated proteins (MRPs) and organic anion transporter peptide 2 (OATP2). Although 'cross-talk' occurs between these two receptors and their target sequences, significant species differences exist between ligand-binding and activation profiles for both receptors, and PXR appears to be the predominant or 'master' regulator of hepatic drug disposition in humans. Several important physiological processes, such as cholesterol synthesis and bile acid metabolism, are also tightly controlled by certain ligand-activated orphan nuclear receptors (farnesoid X receptor [FXR] and liver X receptor [LXR]). In general, their ability to bind a broad range of ligands and regulate an extensive array of genes that are involved in drug clearance and disposition makes these orphan receptors attractive targets for drug development. Drugs have the capacity to alter nuclear receptor expression (modulators) and/or serve as ligands for the receptors (agonists or antagonists), and thus can have synergistic or antagonistic effects on the expression of drug-metabolising enzymes and transporters. Coadministration of drugs that are nuclear receptor agonists or antagonists can lead to severe toxicity, a loss of therapeutic efficacy or an imbalance in physiological substrates, providing a novel molecular mechanism for drug-drug interactions.

The induction of human UDP-glucuronosyltransferase 1A1 mediated through a distal enhancer module by flavonoids and xenobiotics

We identified the UDP-glucuronosyltransferase (UGT) 1A1 5'-upstream region that confers UGT1A1 induction by various agents, including flavonoids, on a luciferase reporter gene and has the properties of a transcriptional enhancer. Chrysin- and rifampicin-response activities were traced to the same element as a 290-bp distal enhancer module (-3483/-3194), in which the reporter activities were enhanced by activators of nuclear receptors [constitutive androstane receptor (CAR) and pregnane X receptor (PXR)] and transcription factor [aryl hydrocarbon receptor (AhR)]. Utilizing transactivation experiments with the UGT1A1 290-bp reporter gene, we assessed UGT1A1 induction by various flavonoids. 5,7-Dihydroxyflavones with varying substituents in the B-ring and gallocatechin dimers increased the reporter activity in a time- and dose-dependent manner. The treatment of HepG2 cells with the flavonoids for 24 hr elevated the expression of mRNAs and proteins of UGT1A1 and CYP1A1, while the mRNA levels of CYP2B6, CYP3A4, CAR, PXR and AhR was not altered. Chrysin and rifampicin induced the activation of the wild-type reporter gene and T-3263G-mutated gene to a similar extent in HepG2 cells cotransfected with expression vectors of CAR and PXR. Mutation of the AhR core binding region most prominently suppressed the activation of the 290-bp reporter gene by chrysin and baicalein, while mutations of four putative nuclear receptor motifs (DR4 element, PXRE, CARE and DR3 element) partly decreased its activation. Taken together, the results indicate that UGT1A1 was induced in response to flavonoids and xenobiotics through the transactivation of the 290-bp reporter gene, that was a multi-component enhancer containing CAR, PXR and AhR motifs.

METABOLISM OF OPIOIDS IS ALTERED IN LIVER MICROSOMES OF SICKLE CELL TRANSGENIC MICE

Pain in sickle cell anemia (SCA) is clinically managed with opioid analgesics. There are reports that SCA patients tolerate high doses of these drugs without adequate pain relief. The current study investigated the in vitro hepatic metabolism of opioids in mouse models of sickle cell anemia, with the hypothesis that higher dose requirements in SCA could be explained by an increased metabolism rate of opioids. Various rodent cytochrome P450 substrates, i.e., buprenorphine and codeine, and rodent uridine glucuronosyltransferase substrates, i.e., morphine, buprenorphine, and estradiol, were studied. The three groups used were: 1) control C57BL mice, 2) mice with the human alpha-globin and sickle beta-globin transgenes (SC), and 3) mice with the human alpha-globin and sickle beta-globin transgenes, and homozygous for the murine alpha-globin and heterozygous for the beta(major)-gene knockout (SCKO). In vitro hepatic microsomal incubations were carried out for each substrate, and data were fit to the Michaelis-Menten equation. Morphine formation had a higher V(max) in SCKO microsomes (0.4 +/- 0.009 nmol/min. mg; estimate +/- S.E.) than controls (0.25 +/- 0.007). Morphine-3-glucuronide formation had V(max) estimates of 18.9 +/- 0.6, 25.1 +/- 0.4, and 27.06 +/- 1.1 nmol/min. mg in control, SC, and SCKO microsomes, respectively. The control V(max) for estradiol-3-glucuronide formation was 2-fold greater than in SCKO microsomes. The control V(max) for estradiol 17-glucuronide formation was 3.4- and 2.2-fold greater than in SC and SCKO microsomes. Thus, in vitro metabolism of opioids is altered in SCA mouse models, which may lead to altered clearances of these drugs.

Multiple variable first exons: a mechanism for cell- and tissue-specific gene regulation

A large family of neural protocadherin (Pcdh) proteins is encoded by three closely linked mammalian gene clusters (alpha, beta, and gamma). Pcdh alpha and gamma clusters have a striking genomic organization. Specifically, each "variable" exon is spliced to a common set of downstream "constant" exons within each cluster. Recent studies demonstrated that the cell-specific expression of each Pcdh gene is determined bya combination of variable-exon promoter activation and cis-splicing of the corresponding variable exon to the first constant exon. To determine whether there are other similarly organized gene clusters in mammalian genomes, we performed a genome-wide search and identified a large number of mammalian genes containing multiple variable first exons. Here we describe several clusters that contain about a dozen variable exons arrayed in tandem, including UDP glucuronosyltransferase (UGT1), plectin, neuronal nitric oxide synthase (NOS1), and glucocorticoid receptor (GR) genes. In all these cases, multiple variable first exons are each spliced to a common set of downstream constant exons to generate diverse functional mRNAs. As an example, we analyzed the tissue-specific expression profile of the mouse UGT1 repertoire and found that multiple isoforms are expressed in a tissue-specific manner. Therefore, this variable and constant genomic organization provides a genetic mechanism for directing distinct cell- and tissue-specific patterns of gene expression.

Rapid proteasomal degradation of translocation-deficient UDP-glucuronosyltransferase 1A1 proteins in patients with Crigler–Najjar type II

UDP-glucuronosyltransferase form 1A1 (UGT1A1) is the only bilirubin-glucuronidating isoform of this protein, and genetic deficiencies of UGT1A1 cause Crigler-Najjar syndrome, a disorder resulting from nonhemolytic unconjugated hyperbilirubinemia. Here we have focused on the instability of a translocation-deficient UGT1A1 protein, which has been found in patients with Crigler-Najjar type II, to elucidate the molecular basis underlying the deficiency in glucuronidation of bilirubin. A substitution of leucine to arginine at position 15 (L15R/1A1) is predicted to disrupt the hydrophobic core of the signal peptide of UGT1A1. L15R/1A1 was synthesized in similar amounts to wild-type UGT1A1 protein (WT/1A1) in transfected COS cells. However, L15R/1A1 did not translocate across the endoplasmic reticulum membrane and was degraded rapidly with a half-life of about 50min, in contrast to the much longer half-life of about 12.8h for WT/1A1. Our findings demonstrate that L15R/1A1 was rapidly degraded by the proteasome owing to its mislocalization in the cell.

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

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