Enhancement of glucuronide-conjugating capacity in a hyperbilirubinemic infant due to apparent enzyme induction by phenobarbital

Activation of Alternative Bilirubin Clearance Pathways Partially Reduces Hyperbilirubinemia in a Mouse Model Lacking Functional Ugt1a1 Activity

Bilirubin is a heme catabolite and Ugt1a1 is the only enzyme involved in the biological elimination of bilirubin. Partially functional or non-functional Ugt1a1 may result in neuronal damage and death due to the accumulation of unconjugated bilirubin in the brain. The understanding of the role of alternative bilirubin detoxification mechanisms that can reduce bilirubin toxicity risk is crucial for developing novel therapeutic strategies. To provide a proof-of-principle showing whether activation of alternative detoxification pathways could lead to life-compatible bilirubin levels in the absence of Ugt1a1 activity, we used Ugt1^−/− hyperbilirubinemic mice devoid of bilirubin glucuronidation activity. We treated adult Ugt1^−/− mice with TCPOBOP, a strong agonist of the constitutive androstane receptor (CAR). TCPOBOP treatment decreased plasma and liver tissue bilirubin levels by about 38%, and resulted in the transcriptional activation of a vast array of genes involved in bilirubin transport and metabolism. However, brain bilirubin level was unaltered. We observed ~40% degradation of bilirubin in the liver microsomes from TCPOBOP treated Ugt1^−/− mice. Our findings suggest that, in the absence of Ugt1a1, the activation of alternative bilirubin clearance pathways can partially improve hyperbilirubinemic conditions. This therapeutic approach may only be considered in a combinatorial manner along with other treatments.

The xenobiotic receptors PXR and CAR in liver physiology, an update

Pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are two nuclear receptors that are well-known for their roles in xenobiotic detoxification by regulating the expression of drug-metabolizing enzymes and transporters. In addition to metabolizing drugs and other xenobiotics, the same enzymes and transporters are also responsible for the production and elimination of numerous endogenous chemicals, or endobiotics. Moreover, both PXR and CAR are highly expressed in the liver. As such, it is conceivable that PXR and CAR have major potentials to affect the pathophysiology of the liver by regulating the homeostasis of endobiotics. In recent years, the physiological functions of PXR and CAR in the liver have been extensively studied. Emerging evidence has suggested the roles of PXR and CAR in energy metabolism, bile acid homeostasis, cell proliferation, to name a few. This review summarizes the recent progress in our understanding of the roles of PXR and CAR in liver physiology.

Substantial effect of efavirenz monotherapy on bilirubin levels in healthy volunteers

Background

Efavirenz exhibits multiple interactions with drug-metabolizing enzymes and transporters, and for this reason efavirenz-based HIV therapy is associated with altered pharmacokinetics of coadministered drugs. Probably by the same mechanism, efavirenz-based HIV therapy affects the disposition of endogenous compounds, but this effect is difficult to directly link with efavirenz because it is used in combination with other drugs.

Objectives

To explore the effect of efavirenz monotherapy on biochemical laboratory values in a clinical trial of healthy volunteers.

Methods

Men and women (aged 18–49 years) with body mass index ≤32 who were assessed to be healthy based on medical history, physical examination, and standard laboratory screening received a single (600 mg) and multiple doses (600 mg/d for 17 days) of efavirenz orally. This trial was designed to determine the pharmacokinetics and drug interactions of efavirenz. As part of this study, analysis of serum chemistries that were measured at study entry (screening) and 1 week after completion of the multiple dose study (exit) is reported.

Results

Data from 60 subjects who fully completed and 13 subjects who partially completed the study are presented. Total bilirubin was substantially reduced at exit (by ~30%, with large intersubject variability) compared with screening values (P < 0.0001). The percent changes were in part explained by the intersubject differences in baseline total bilirubin because there was a significant correlation between baseline (screening) values and percent change at exit (r = 0.50; P < 0.0001). Hemoglobin and absolute neutropenia were also substantially decreased at exit compared with screening, but this may be due to intensive blood sampling rather than direct effect of efavirenz on these parameters. No significant correlation was found between percent change in hemoglobin versus percent change in bilirubin, indicating the effect of efavirenz on bilirubin is independent of its effects on hemoglobin.

Conclusions

Efavirenz monotherapy significantly lowers plasma total bilirubin concentration in healthy volunteers independent of its effect on hemoglobin, probably through its effects on bilirubin metabolism and transport (uptake and efflux). These findings help explain reversal by efavirenz of hyperbilirubinemia induction observed by some protease inhibitor antiretroviral drugs (eg, atazanavir). Besides its well-documented role on drug interactions, efavirenz may alter the disposition of endogenous compounds relevant in physiologic homeostasis through its interaction with drug metabolizing enzymes and/or drug transporters. ClinicalTrials.gov identifier: NCT00668395.

Hyperbilirubinemia: current guidelines and emerging therapies

It is estimated that about two thirds of newborns will appear clinically jaundiced during their first weeks of life. As newborns and their mothers spend fewer days in the hospital after birth, the number of infants readmitted yearly in the United States for neonatal jaundice over the last 10 years has increased by 160%. A portion of these infants present to the emergency department, requiring a careful history and physical examination assessing them for the risk factors associated with pathologic bilirubin levels. Although the spectrum of illness may be great, the overwhelming etiology of neonatal jaundice presenting to an emergency department is physiologic and not due to infection or isoimmunization. Therefore, a little more than a good history, physical examination, and indirect/direct bilirubin levels are needed to evaluate an otherwise well-appearing jaundiced newborn. The American Academy of Pediatrics' 2004 clinical practice guidelines for "Management of Hyperbilirubinemia in the Newborn Infant 35 or More Weeks of Gestation" are a helpful and easily accessible resource when evaluating jaundiced newborns (available at http://aappolicy.aappublications.org/cgi/content/full/pediatrics;114/1/297). There are several exciting developments on the horizon for the diagnosis and management of hyperbilirubinemia including increasing use of transcutaneous bilirubin measuring devices and medications such as tin mesoporphyrin and intravenous immunoglobulin that may decrease the need for exchange transfusions.

Regulation of drug-metabolizing enzymes by xenobiotic receptors: PXR and CAR

Drug-metabolizing enzymes (DMEs) and transporters play pivotal roles in the disposition and detoxification of numerous foreign and endogenous chemicals. To accommodate chemical challenges, the expression of many DMEs and transporters is up-regulated by a group of ligand-activated transcription factors namely nuclear receptors (NRs). The importance of NRs in xenobiotic metabolism and clearance is best exemplified by the most promiscuous xenobiotic receptors: pregnane X receptor (PXR, NR1I2) and constitutive androstane/activated receptor (CAR, NR1I3). Together, these two receptors govern the inductive expression of a largely overlapping array of target genes encoding phase I and II DMEs, and drug transporters. Moreover, PXR and CAR also represent two distinctive mechanisms of NR activation, whereby CAR demonstrates both constitutive and ligand-independent activation. In this review, recent advances in our understanding of PXR and CAR as xenosensors are discussed with emphasis placed on the differences rather than similarities of these two xenobiotic receptors in ligand recognition and target gene regulation.

Pharmacogenetic testing for uridine diphosphate glucuronosyltransferase 1A1 polymorphisms: are we there yet?

Recent changes to the labels of three prescription drugs--irinotecan, 6-mercaptopurine, and warfarin--include recommendations for pharmacogenetic testing in patients. Thus, clinicians are faced with determining the utility and practicality of pharmacogenetic testing in clinical practice. We illustrate the clinical implications that this testing may have using irinotecan, an agent approved for the treatment of metastatic colorectal cancer, as an example. A clinical association between the drug's active metabolite and toxicity has been found. By performing uridine diphosphate glucuronosyltransferase (UGT) 1A1 genetic testing, some studies have been able to predict which patients receiving irinotecan will experience the toxicity. Thus, irinotecan's package insert was revised to include a recommendation for such testing. In addition, the United States Food and Drug Administration approved a clinical test for the UGT1A1*28 allele. These events demonstrate that pharmacogenetics has entered the realm of clinical practice. However, the transition from bench to bedside of these tests has distinct challenges such as population differences, test sensitivity, and the role of other genetic and nongenetic factors that influence drug toxicity. In addition, ethical and logistic implications of pharmacogenetic testing exist.

Hepatic drug transporters and nuclear receptors: Regulation by therapeutic agents

The canalicular membrane represents the excretory pole of hepatocytes. Bile is an important route of elimination of potentially toxic endo- and xenobiotics (including drugs and toxins), mediated by the major canalicular transporters: multidrug resistance protein 1 (MDR1, ABCB1), also known as P-glycoprotein, multidrug resistance-associated protein 2 (MRP2, ABCC2), and the breast cancer resistance protein (BCRP, ABCG2). Their activities depend on regulation of expression and proper localization at the canalicular membrane, as regulated by transcriptional and post-transcriptional events, respectively. At transcriptional level, specific nuclear receptors (NR)s modulated by ligands, co-activators and co-repressors, mediate the physiological requirements of these transporters. This complex system is also responsible for alterations occurring in specific liver pathologies. We briefly describe the major Class II NRs, pregnane X receptor (PXR) and constitutive androstane receptor (CAR), and their role in regulating expression of multidrug resistance proteins. Several therapeutic agents regulate the expression of relevant drug transporters through activation/inactivation of these NRs. We provide some representative examples of the action of therapeutic agents modulating liver drug transporters, which in addition, involve CAR or PXR as mediators.

Orlistat treatment of unconjugated hyperbilirubinemia in Crigler-Najjar disease: a randomized controlled trial

Unconjugated hyperbilirubinemia in Crigler-Najjar (CN) disease is conventionally treated with phototherapy and phenobarbital. Orlistat treatment increases fecal fat excretion and decreases plasma unconjugated bilirubin (UCB) concentrations in Gunn rats, the animal model for CN disease. We determined in CN patients the effects of orlistat treatment on plasma UCB concentrations, and on fecal excretion of fat and UCB. A randomized, placebo-controlled, double-blind, cross-over trial was conducted in 16 patients, simultaneous with their regular treatment (phototherapy, n = 11, and/or phenobarbital, n = 6). Patients received orlistat or placebo, each for 4-6 wk. Compared with placebo, orlistat increased fecal fat excretion (+333%) and fecal UCB excretion (+43%). Orlistat treatment significantly decreased plasma UCB concentration (-9%). In 7 of 16 patients, the decrease in plasma UCB levels was clinically relevant (>10%, mean 21%). In patients with a clinically relevant response, plasma UCB concentrations during orlistat were strongly, negatively correlated with fecal fat excretion (r = -0.93). Clinically relevant response to orlistat treatment was not correlated with age, sex, CN type, BMI, or co-treatment with phototherapy or phenobarbital, but appeared correlated with a relatively lower dietary fat intake. In conclusion, orlistat treatment decreases plasma UCB concentrations, particularly in a subgroup of CN patients. Dietary fat intake may determine the responsiveness to orlistat treatment.

Induction of Rat UDP-Glucuronosyltransferases in Liver and Duodenum by Microsomal Enzyme Inducers That Activate Various Transcriptional Pathways

Microsomal enzyme inducers (MEIs) up-regulate phase I biotransformation enzymes, most notably cytochromes P450. Transcriptional up-regulation by MEIs occurs through at least three nuclear receptor mechanisms: constitutive androstane receptor (CAR; CYP2B inducers), pregnane X receptor (PXR; CYP3A inducers), and peroxisome proliferator-activated receptor alpha (PPARalpha; CYP4A inducers). Other mechanisms include transcription factors aryl hydrocarbon receptor (AhR; CYP1A inducers), and nuclear factor erythroid 2 (NF-E2)-related factor 2 (Nrf2; NADPH-quinone oxidoreductase inducers). UDP-glucuronosyltransferases (UGTs) are phase II biotransformation enzymes that are predominantly expressed in liver and intestine. MEIs increase UGT activity; however, transcriptional regulation of individual UGT isoforms is not completely understood. The purpose of this study was to examine inducibility of individual UGT isoforms and potential mechanisms of transcriptional regulation in rat liver and duodenum. UGT mRNA levels were assessed in liver and duodenum of rats treated with MEIs that activate various transcriptional pathways. All four CAR activators induced UGT2B1 in liver, but not duodenum. UGT1A1, 1A5, 1A6, and 2B12 were induced by at least two CAR activators in liver only. Two PXR ligands induced UGT1A2, but only in duodenum. Two PPARalpha ligands induced UGT1A1 and 1A3 in liver only. AhR ligands induced UGT1A6 and 1A7 in liver, but not duodenum. Nrf2 activators increased UGT2B3 and 2B12 in both liver and duodenum, and UGT1A6, 1A7, and 2B1 in liver only. In summary, only UGT1A2 and 1A8 were not inducible in liver by MEIs. MEIs differentially regulate hepatic expression of individual UGT isoforms, although no one transcriptional pathway dominated. In duodenum, MEIs had minimal effects on UGT expression.

Genetic polymorphism in the phenobarbital-responsive enhancer module of the UDP-glucuronosyltransferase 1A1 gene and irinotecan toxicity

Genetic polymorphism of the UDP-glucuronosyltransferase (UGT) 1A1 gene is associated with the decreased glucuronidation activity of an active metabolite of irinotecan, SN-38, and UGT1A1*28 has been shown as a predictive factor for irinotecan toxicity. The phenobarbital-responsive enhancer module (PBREM) of the UGT1A1 promoter region has been reportedly associated with the transcriptional activity of the gene. We investigated whether the polymorphism of PBREM (T-3279G) would affect inter-patient variations in sensitivity to irinotecan toxicity. The study population comprised 119 cancer patients who had received irinotecan. We reviewed their clinical records, including patient characteristics, and observed their toxicity levels following irinotecan infusion. Genotyping was performed by sequencing analyses. Logistic regression analyses were performed to assess the relationship between genotypes and irinotecan toxicity. We identified the homozygotes of the reference allele for T-3279G in 68 patients, the heterozygotes in 37, and the homozygotes for the variant in 14. Logistic regression analysis indicated a significant association between the homozygotes for T-3279G and the severe toxicity (odds ratio 5.80; 95% confidence interval 1.67-20.1). However, multivariate analysis, including the data of UGT1A1*28 polymorphism, revealed a diminution of the association due to a highly significant linkage disequilibrium between these polymorphisms. Our results suggest that a highly significant linkage disequilibrium exists between T-3279G and UGT1A1*28 polymorphisms, and that the variants of T-3279G and UGT1A1*28 cooperatively decrease transcriptional activity of the UGT1A1 promoter. The determination of T-3279G and UGT1A1*28 genotypes might be clinically useful in predicting severe irinotecan toxicity in cancer patients.

Metalloporphyrins for the treatment of neonatal jaundice

PURPOSE OF REVIEW

To evaluate the safety and efficacy of metalloporphyrins for the treatment of neonatal hyperbilirubinemia.

RECENT FINDING

Since the 1980s and 1990s, there have been no publications on trials to determine the efficacy of metalloporphyrins in the treatment of neonatal jaundice. In the past year, a single case report was presented on the compassionate use of tin mesoporphyrin in a very low birth weight infant with intrauterine growth retardation who did not respond to phototherapy. Subcutaneous administration of a single dose of tin mesoporphyrin at 46 hours of life was associated with a greater than 25% reduction in serum bilirubin. This further supports existing evidence that tin mesoporphyrin is efficacious in lowering bilirubin production. In the laboratory, most metalloporphyrins were shown to induce heme oxygenase, and in addition, metalloporphyrins modulate cardiac cell function in vitro. These observations suggest that the therapeutic benefits may be obviated if such considerations hold true in humans.

SUMMARY

Recent case reports and previous evidence from larger clinical trials conducted in Greece and Argentina in the 1980s and 1990s demonstrate that tin mesoporphyrin is useful in the treatment of neonatal jaundice. Its long-term safety is not well understood but will be important to determine, before its widespread or prophylactic use in neonates with hyperbilirubinemia can be recommended.

Effective oral treatment of unconjugated hyperbilirubinemia in Gunn rats

We sought to develop an oral treatment for unconjugated hyperbilirubinemia. In the Gunn rat model of unconjugated hyperbilirubinemia, dietary supplementation with the lipase inhibitor orlistat (Orl) or with calcium phosphate (CaP) decreases plasma unconjugated bilirubin (UCB) levels. We determined whether Orl, CaP, or their combination is superior to phototherapy, the conventional treatment, and whether the effects of Orl and CaP are influenced by dietary fat content. Gunn rats were treated with Orl (200 mg/kg chow), CaP (20 g/kg chow), Orl + CaP, or continuous phototherapy (19 muW/cm(2)/nm) during a low-fat (LF) diet (13 energy%) or high-fat (HF) diet (35 energy%). Plasma UCB and fecal fat excretion were measured before, during, and/or at the end of treatment. Orl treatment for 2 weeks (HF diet) reduced plasma UCB concentrations similar to phototherapy (-34% and -28%, respectively); the combination of both was more effective than either treatment alone (-48%; P < .001). After 3 weeks of a HF diet, plasma UCB was 46% lower compared with the LF diet (P < .001). Plasma UCB concentrations were negatively correlated with fecal fat excretion (r = -0.96; P < .001). Irrespective of dietary fat content, 3 weeks of combined treatment (Orl + CaP) decreased plasma UCB by approximately 50% (P < .01) and was more effective than phototherapy (P < .05) at the intensity provided. In conclusion, plasma UCB concentrations in Gunn rats are negatively related to fecal fat excretion and dietary fat content. Orlistat is equally effective as phototherapy for the treatment of unconjugated hyperbilirubinemia in Gunn rats, and combined oral treatment with Orl + CaP is more effective than phototherapy. The present results support the feasibility of an efficient oral treatment of unconjugated hyperbilirubinemia.

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.

Interleukin 1beta inhibits CAR-induced expression of hepatic genes involved in drug and bilirubin clearance

During the inflammatory response, intrahepatic cholestasis and decreased drug metabolism are frequently observed. At the hepatic level, the orphan nuclear constitutive androstane receptor (CAR) (NR1I3) controls phase I (cytochrome P450 [CYP] 2B and CYP3A), phase II (UGT1A1), and transporter (SLC21A6, MRP2) genes involved in drug metabolism and bilirubin clearance in response to xenobiotics such as phenobarbital or endobiotics such as bilirubin. We investigated the negative regulation of CAR, a glucocorticoid-responsive gene, via proinflammatory cytokine interleukin 1beta (IL-1beta) and lipopolysaccharides (LPSs) in human hepatocytes. We show that IL-1beta decreases CAR expression and decreases phenobarbital- or bilirubin-mediated induction of CYP2B6, CYP2C9, CYP3A4, UGT1A1, GSTA1, GSTA2, and SLC21A6 messenger RNA. This occurs via nuclear factor kappaB (NF-kappaB) p65 activation, which interferes with the enhancer function of the distal glucocorticoid response element that we have identified recently in the CAR promoter. We demonstrate that: (1) LPSs, IL-1beta, or overexpression of p65RelA inhibit glucocorticoid receptor (GR)-mediated CAR transactivation; (2) these suppressive effects can be blocked both by pyrrolidine dithiocarbamate, an inhibitor of NF-kappaB activation, or by overexpression of SRIkBalpha, a NF-kappaB repressor; and (3) the GR agonist dexamethasone induces histone H4 acetylation at the proximal CAR promoter region, whereas LPSs and IL-1beta inhibit this acetylation as assessed via chromatin immunoprecipitation assay. In conclusion, GR/NF-kappaB interaction affects CAR gene transcription through chromatin remodeling and provide a mechanistic explanation for the long-standing observation that inflammation and sepsis inhibit drug metabolism while inducing intrahepatic cholestasis or hyperbilirubinemia.

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.

A traditional herbal medicine enhances bilirubin clearance by activating the nuclear receptor CAR

Yin Zhi Huang, a decoction of Yin Chin (Artemisia capillaris) and three other herbs, is widely used in Asia to prevent and treat neonatal jaundice. We recently identified the constitutive androstane receptor (CAR, NR1I3) as a key regulator of bilirubin clearance in the liver. Here we show that treatment of WT and humanized CAR transgenic mice with Yin Zhi Huang for 3 days accelerates the clearance of intravenously infused bilirubin. This effect is absent in CAR knockout animals. Expression of bilirubin glucuronyl transferase and other components of the bilirubin metabolism pathway is induced by Yin Zhi Huang treatment of WT mice or mice expressing only human CAR, but not CAR knockout animals. 6,7-Dimethylesculetin, a compound present in Yin Chin, activates CAR in primary hepatocytes from both WT and humanized CAR mice and accelerates bilirubin clearance in vivo. We conclude that CAR mediates the effects of Yin Zhi Huang on bilirubin clearance and that 6,7-dimethylesculetin is an active component of this herbal medicine. CAR is a potential target for the development of new drugs to treat neonatal, genetic, or acquired forms of jaundice.

AN ASSESSMENT OF UDP-GLUCURONOSYLTRANSFERASE INDUCTION USING PRIMARY HUMAN HEPATOCYTES

Uridine diphosphate glucuronosyltransferases (UGTs) catalyze the glucuronidation of a wide range of xenobiotics and endogenous substrates. However, there is a lack of information concerning the response of human UGTs to inducers, and this observation prompted the current investigation. The glucuronidation of estradiol (3- and 17-positions), naphthol, propofol, and morphine (3- and 6-positions) was assessed against a battery of recombinant human UGTs to determine selective glucuronidation reactions for induction studies. The potential induction of the glucuronidation of estradiol at the 3-position, naphthol, propofol, and morphine at the 3-position was subsequently investigated in cultured primary human hepatocytes against a range of prototypic inducers including dexamethasone, 3-methylcholanthrene (3-MC), phenobarbital, rifampicin, and omeprazole. Treatment with 3-MC induced estradiol-3-glucuronidation (up to 2.5-fold) in four of five donors investigated. Statistically significant increases in naphthol glucuronidation (up to 1.7-fold) were observed following treatment with carbamazepine. UGT1A9-mediated propofol glucuronidation was induced by phenobarbital (up to 2.2-fold) and rifampicin (up to 1.7-fold). However, treatment with alpha-naphthoflavone and tangeretin resulted in a decrease in propofol glucuronidation (30% of control values). Statistically significant induction of morphine-3-glucuronidation was observed in at least three donors following treatment with phenobarbital, rifampicin, and carbamazepine. Each UGT isoform investigated displayed a distinct induction profile. Although statistically significant increases in glucuronidation were observed for each reaction studied, the level of induction was less than that observed for CYP1A2 or CYP3A4 and exhibited a large interdonor variability. The clinical relevance of the induction responses obtained in this study is unclear.

A traditional herbal medicine enhances bilirubin clearance by activating the nuclear receptor CAR

Yin Zhi Huang, a decoction of Yin Chin (Artemisia capillaris) and three other herbs, is widely used in Asia to prevent and treat neonatal jaundice. We recently identified the constitutive androstane receptor (CAR, NR1I3) as a key regulator of bilirubin clearance in the liver. Here we show that treatment of WT and humanized CAR transgenic mice with Yin Zhi Huang for 3 days accelerates the clearance of intravenously infused bilirubin. This effect is absent in CAR knockout animals. Expression of bilirubin glucuronyl transferase and other components of the bilirubin metabolism pathway is induced by Yin Zhi Huang treatment of WT mice or mice expressing only human CAR, but not CAR knockout animals. 6,7-Dimethylesculetin, a compound present in Yin Chin, activates CAR in primary hepatocytes from both WT and humanized CAR mice and accelerates bilirubin clearance in vivo. We conclude that CAR mediates the effects of Yin Zhi Huang on bilirubin clearance and that 6,7-dimethylesculetin is an active component of this herbal medicine. CAR is a potential target for the development of new drugs to treat neonatal, genetic, or acquired forms of jaundice.

Pathophysiological Factors Affecting CAR Gene Expression

The body defends itself against potentially harmful compounds, such as drugs and toxic endogenous compounds and their metabolites, by inducing the expression of enzymes and transporters involved in their metabolism and elimination. The orphan nuclear receptor CAR (NR1I3 controls phase I (CYP2B, CYP2C, CYP3A), phase II (UGT1A1), and transporter (SLC21A6, MRP2) genes involved in drug metabolism and bilirubin clearance. Constitutive androstane receptor (CAR) is activated by xenobiotics, such as phenobarbital, but also by toxic endogenous compounds such as bilirubin metabolite(s). To better understand the inter- and intravariability in drug detoxification, we studied the molecular mechanisms involved in CAR gene expression in human hepatocytes. We clearly identified CAR as a glucocorticoid receptor (GR) target gene, and we proposed the hypothesis of a signal transduction where the activation of GR plays a critical function in CAR-mediated cellular response. According to our model, chemicals or pathophysiological factors that affect GR function should decrease CAR function. To test this hypothesis, we recently investigated the effect of microtubule disrupting agents (MIAs) or proinflammatory cytokines. These compounds are well-known inhibitors of GR transactivation property. MIAs activate c-Jun N-terminal kinase (JNK), which phosphorylates and inactivates GR, whereas proinflammatory cytokines, such as IL-6 or IL1beta, induce AP-1 or NF-kB activation, respectively, leading to GR inhibition. As expected, we observed that these molecules inhibit both CAR gene expression and phenobarbital-mediated CYP gene expression in human hepatocytes.

Induction of bilirubin clearance by the constitutive androstane receptor (CAR)

Bilirubin clearance is one of the numerous important functions of the liver. Defects in this process result in jaundice, which is particularly common in neonates. Elevated bilirubin levels can be decreased by treatment with phenobarbital. Because the nuclear hormone receptor constitutive androstane receptor (CAR) mediates hepatic effects of this xenobiotic inducer, we hypothesized that CAR could be a regulator of bilirubin clearance. Activation of the nuclear hormone receptor CAR increases hepatic expression of each of five components of the bilirubin-clearance pathway. This induction is absent in homozygous CAR null mice but is observed in mice expressing human CAR instead of mouse CAR. Pretreatment with xenobiotic inducers markedly increases the rate of clearance of an exogenous bilirubin load in wild-type but not CAR knockout animals. Bilirubin itself can also activate CAR, and mice lacking CAR are defective in clearing chronically elevated bilirubin levels. Unexpectedly, CAR expression is very low in livers of neonatal mice and humans. We conclude that CAR directs a protective response to elevated bilirubin levels and suggest that a functional deficit of CAR activity may contribute to neonatal jaundice.

Control of steroid, heme, and carcinogen metabolism by nuclear pregnane X receptor and constitutive androstane receptor

Through a multiplex promoter spanning 218 kb, the phase II UDP-glucuronosyltransferase 1A (UGT1) gene encodes at least eight differently regulated mRNAs whose protein products function as the principal means to eliminate a vast array of steroids, heme metabolites, environmental toxins, and drugs. The orphan nuclear receptors pregnane X receptor (PXR) and constitutive androstane receptor (CAR) were originally identified as sensors able to respond to numerous environmentally derived foreign compounds (xenobiotics) to promote detoxification by phase I cytochrome P450 genes. In this report, we show that both receptors can induce specific UGT1A isoforms including those involved in estrogen, thyroxin, bilirubin, and carcinogen metabolism. Transgenic mice expressing a constitutively active form of human PXR show markedly increased UGT activity toward steroid, heme, and carcinogens, enhanced bilirubin clearance, as well as massively increased steroid clearance. The ability of PXR and constitutive androstane receptor and their ligands to transduce both the phase I and phase II adaptive hepatic response defines a unique transcriptional interface that bridges the ingestion and metabolism of environmental compounds to body physiology.

The role of the nuclear receptor CAR as a coordinate regulator of hepatic gene expression in defense against chemical toxicity

The nuclear receptor CAR (constitutive active receptor) mediates the induction of transcription of cytochrome P450 (CYP) genes by phenobarbital (PB) and PB-type inducers. A recent study using CAR-null mice has shown that CAR regulates not only the CYP genes but also other genes encoding various drug/steroid-metabolizing enzymes. In addition to coordinating these enzymes, CAR plays other roles in hepatic gene expression: CAR represses various genes including carnitine palmitoyltransferase 1a and phosphoenolpyruvate carboxykinase 1 in response to PB, and the receptor regulates the constitutive expression of genes such as squalene epoxidase. On the other hand, induction of certain genes such as amino levulinate synthase 1 by PB is not regulated by CAR. Here we describe diverse roles of CAR in hepatic gene expression with a particular focus on endogenous substances such as cholesterol, bilirubin, and steroid hormones.

Bilirubin oxidation in brain

Bilirubin is a product of heme catabolism which by virtue of its lipid solubility can cross the blood-brain barrier and enter the brain. Neonatal jaundice is a common transitional phenomenon which is due to the combination of increased heme catabolism and rate limitations as far as hepatic conjugation and biliary excretion of bilirubin. In the great majority of cases this is an innocuous condition, which is even posited to have some beneficial effects due to the ability of bilirubin to quench free oxygen radicals. However, because bilirubin is neurotoxic, hyperbilirubinemia in the newborn may exceptionally result in death in the neonatal period, or survival with severe neurological sequelae (kernicterus). Bilirubin enters the brain through an intact blood-brain barrier. Clearance of bilirubin from brain partly involves retro-transfer through the blood-brain barrier, and possibly also through the brain-CSF barrier into CSF. Work in our lab during the past 5 years has substantiated earlier work which had suggested that bilirubin may also be metabolized in brain. The responsible enzyme is found on the inner mitochondrial membrane, and oxidizes bilirubin at a rate of 100-300 pmol bilirubin/mg protein/minute. The enzyme activity is lower in the newborn compared with the mature animal, and is also lower in neurons compared with glia. Studies of different rat strains have documented genetic variability. The enzyme is cytochrome-c-dependent, but has as yet not been unequivocally identified. The rate of oxidation of bilirubin is such that this enzyme probably contributes meaningfully to the clearance of bilirubin from brain.

Coding defect and a TATA box mutation at the bilirubin UDP-glucuronosyltransferase gene cause Crigler-Najjar type I disease

Mutations at the bilirubin UDP-glucuronosyltransferase (transferase) gene in a severely hyperbilirubinemic Crigler-Najjar (CN) type I individual was compared with that in a moderately hyperbilirubinemic CN II individual. The CN-I (CF) patient in this study sustained a TATA box insertional mutation which was paired with a coding defect at the second allele, unlike all coding defects previously seen in CN-I patients. The sequence of the mutant TATA box, [A(TA)8A], also seen in the CN-II patient, was compared with that at the wild-type box, [A(TA)7A]. Transcriptional activity with [A(TA)8A] was 10-15% that with the wild-type box when present in the -1.7 kb upstream regulatory region (URR) of the bilirubin transferase UGT1A1 gene which was fused to the chloramphenicol acetyl transferase reporter gene, pCAT 1.7H, and transfected into HepG2 cells. Also, a construct with a TA deletion, [A(TA)6A], was prepared and used as a control; transcriptional activity was 65% normal. The coding region defect, R336W, seen in CF (CN-I) was placed in the bilirubin transferase UGT1A1 [HUG-Br1] cDNA, and its corresponding protein was designated UGT1A1*32. The UGT1A1*32 protein supported 0-10% normal bilirubin glucuronidation when expressed in COS-1 cells. The I294T coding defect seen at the second allele in SM (CN-II) generated the UGT1A1*33 mutant protein which supported 40-55% normal activity with a normal Km (2.5 microM) for bilirubin. The hyperbilirubinemia seen in SM decreased in response to phenobarbital treatment, unlike that seen in CF. Parents of the patients were carriers of the respective mutations uncovered in the offspring. The TATA box mutation paired with a deleterious missense mutation is, therefore, completely repressive in the CN-I patient, and is responsible for a lethal genotype/phenotype; but when homozygous, i.e. paired with itself, as previously reported in the literature, it is far less repressive and generates the mild Gilbert's phenotype.

Intrahepatic cholestasis: a review of biochemical-pathological mechanisms

Intrahepatic cholestasis involves impaired excretion of bile via the hepatobiliary system as a consequence of one or more lesions within the liver. In humans, intrahepatic cholestasis most often results as a side-effect of drug therapy and the clinical manifestation of this condition, jaundice, has been estimated to account for hospitalization in 2 to 5% of the cases for the general population and approaches as much as 20% in the elderly. With the aging of the population and the common occurrence of poly-drug therapy in geriatric patients, it is to be expected that jaundice due to drug-induced intrahepatic cholestasis will become even more prevalent, and accordingly the need to understand the basic mechanisms of this disease condition will become more urgent. The list of culprit agents implicated in the induction of intrahepatic cholestasis in humans is continually expanding. These include various steroid hormones, bile acids, drugs and other chemicals. Experimentally, a wide spectrum of agents has been shown to precipitate intrahepatic cholestasis. Over the years, a number of hypotheses on the biochemical and pathological mechanisms of intrahepatic cholestasis has emerged, including the following: impaired sinusoidal membrane function; interference with the distribution and binding of cytoplasmic endogenous carrier proteins; interference with mitochondrial energy supply; defects in the canalicular membrane including altered Na+/K+ -ATP-ase activity; impairment of microfilament and microtubule functions; interference with bile secretion involving bile acid dependent and independent fractions, and altered bile acid metabolism due to "hypoactive hypertrophic smooth endoplasmic reticulum". In partial agreement with the latter hypothesis, our studies indicated that impairment of the endoplasmic reticulum might represent one of the early stages in the development of intrahepatic cholestasis. Various experimental conditions that induce intrahepatic cholestasis to different degrees resulted in an interference of the synthesis of microsomal phospholipids and altered microsomal function. The conditions included the administration of various hepatotoxic compounds or steroids, pregnancy, delayed development of the endoplasmic reticulum in neonates, and dietary methyl donor or choline deficiency. This review reports the biochemical-pathological mechanisms postulated to be involved in the genesis of intrahepatic cholestasis with specific reference to experimental models of drug-induced intrahepatic cholestasis. The important practical implications of cholestasis are also briefly surveyed.

Serum bilirubin levels with antiepileptic drugs

The incidence of reduced bilirubin levels in 168 outpatients with epilepsy, compared with levels in 69 controls, has been investigated. Highly significant (p less than 0.001) reductions in average bilirubin levels were noted for carbamazepine (CBZ), phenytoin (PHT), phenobarbital (PB), and multiple drug groups. A marginally significant (p less than 0.05) reduction in bilirubin levels occurred in patients treated with valproate (VPA) which, unlike the other drugs, has not been shown to induce hepatic enzymes.

Endogenous digoxin-like factor in neonates: effect of age and relation to serum bilirubin levels

Endogenous digoxin-like immunoreactive factor(s) (DLIF) have been found in serum and urine of newborn infants, including those born prematurely. We assessed the effect of age on serum levels of DLIF in 73 samples obtained from 66 healthy full term newborn infants at birth and during the first two months of life. DLIF concentrations were highest at birth and fell progressively with age. In cord blood, DLIF levels were 0.73 +/- 0.35 ng/ml (mean +/- SD). DLIF concentrations were 0.45 +/- 0.11 ng/ml on day 1, 0.26 +/- 0.08 ng/ml on day 3, 0.19 +/- 0.07 ng/ml on day 5, 0.17 +/- 0.09 ng/ml on day 11, 0.11 +/- 0.02 ng/ml on days 15-30, and not detectable after 45 days of life. We also studied the relation between serum levels of DLIF and bilirubin in 23 jaundiced newborns between 3-5 days of life. We found a highly significant positive correlation between serum bilirubin concentrations and DLIF. These findings support the assumption that DLIF plays a role in impeding bilirubin excretion in the neonatal period, perhaps by inhibiting the activity of (Na-K)ATPase.

Effects of phenobarbital on bilirubin metabolism and its response to phototherapy in the jaundiced Gunn rat

Jaundiced Gunn rats, treated with phenobarbital (60 mg per kg i.p. for 7 to 10 days) showed 25 and 36% decreases in mean plasma bilirubin levels in two experiments (p less than 0.01). Kinetic studies with tracer 14C-bilirubin revealed that there was no change in bilirubin turnover or total pool size due to phenobarbital, but a 49% increase in the hepatic pool and a 27% decrease in the cutaneous pool of bilirubin. The increase in the hepatic pool accounted for over 90% of the bilirubin lost from the plasma. Such pretreatment with phenobarbital did not alter the decline in plasma bilirubin or total bilirubin pool due to subsequent phototherapy. Phenobarbital followed by phototherapy produced a significantly greater reduction in plasma bilirubin levels than either treatment alone. These studies demonstrate that phenobarbital does decrease plasma bilirubin in Gunn rats primarily by shifting the pigment to the liver, and suggests that combined treatment with phenobarbital and phototherapy might be of value in patients with congenital hyperbilirubinemia due to glucuronyl transferase deficiency.

Human cytochromes P-450

Studies in vivo have provided evidence for a multiplicity of cytochromes P-450 in man, some of which are under independent monogenic control. Although the activity of cytochromes P-450 in man are generally lower than those of rat, this is by no means always the case. There are several important exceptions including the N-hydroxylation of 2-acetamidofluorene. Studies in vitro by a number of different techniques have confirmed the evidence from studies in vivo that there are multiple forms of human cytochrome P-450. In addition to differences in Vmax, the different forms of cytochrome P-450 may also exhibit marked differences in their apparent Km values. The implications that this may have for pharmacokinetics and toxicology are discussed. The polymorphism in the 4-hydroxylation of debrisoquine observed in vivo has been shown to be due to a defect in a specific form of cytochrome P-450 which appears to be under monogenic regulation. Cross-inhibition studies have enabled the specificity of this isozyme to be characterized. Such studies have also enabled the contribution of this isozyme of cytochrome P-450 to the oxidation of other substrates to be determined. Compounds investigated include bufuralol and phenytoin. Evidence from studies both in vivo and in vitro suggest that selective induction of different forms of cytochrome P-450 can occur in man. However, the number of different classes of inducer in man is not yet known. Human cytochromes P-450 have been purified to near homogeneity in several laboratories. Different forms of cytochrome P-450 purified from the same liver sample vary in molecular weight, chromatographic characteristics and substrate specificities.

Effect of low-dose phenobarbital on hepatic microsomal UDP-glucuronyl transferase activity

To determine whether hepatic microsomal enzyme induction occurs in rats following administration of phenobarbital at doses similar to those used in humans (0.5 to 7.5 mg/kg), UDP-glucuronyl transferase (UDPGT) and cytochrome P-450 activities were measured in liver homogenate and microsomal preparations from control rats and rats treated for 6 days with phenobarbital at 1 and 3 mg per kg per day. While no significant increases in liver weight and protein content of homogenate and microsomal preparations were observed with either dose of the drug, both UDPGT and P-450 activities were enhanced significantly following administration of phenobarbital at 3 mg per kg per day. The activity of P-450 was increased by approximately 30% and that of UDPGT by 15-24 and 45-66%, respectively, employing bilirubin and p-nitrophenol as the acceptor substrate. The extent of induction of bilirubin or p-nitrophenol UDPGT was similar when measured with "native" enzyme or with enzyme activated by UDP-N-acetyl glucosamine, digitonin or deoxycholate. These data suggest that the discordant effects of phenobarbital on UDPGT and cytochrome P-450 previously reported in humans and rats may not be attributable solely to differences in the drug doses employed.

Determination of salicylamide and five metabolites in biological fluids by high-performance liquid chromatography

Two high-performance liquid chromatographic (HPLC) assay procedures were developed for the determination of salicylamide and its metabolites in serum, urine, and saliva. One method involves reverse-phase ion-pair chromatography and UV detection, and is used to determine salicylamide, salicylamide glucuronide, and salicylamide sulfate. The other method, with a different mobile phase and without the ion-pairing reagent, is used to determine gentisamide (the hydroxylated metabolite of salicylamide), gentisamide glucuronide, and gentisamide sulfate. The assays are performed by direct injection of the sample after protein precipitation with ethanol containing the internal standard. Increased sensitivity for the determination of low concentrations of salicylamide is obtained by organic extraction of this drug from serum or saliva. Calibration curves for the conjugates of salicylamide and gentisamide were obtained, in the absence of authentic standards, by partial enzymatic hydrolysis, using the decrease of the conjugate peaks and the concomitant increase of free salicylamide or gentisamide concentrations to determine peak area ratio-concentration relationships. Application of the HPLC assay procedures to the determination of salicylamide excretion products in the urine of three normal human subjects resulted in 98.6% (range:97.1-100.1%) recovery of a 1-g oral dose of the drug. All five metabolites of salicylamide were found in urine, but only salicylamide glucuronide, salicylamide sulfate, and gentisamide glucuronide were found consistently and in appreciable quantities. Salicylamide and all of its metabolites except gentisamide sulfate were found in human and rat serum, and unconjugated salicylamide as well as gentisamide were found in human saliva.

Long-lasting effects of early barbiturates on central nervous system and behavior

Forty years of prescribing barbiturates to pregnant women and infants, and thirty years of animal research have shown that barbiturates affect the developing central nervous system (CNS) and behavior. This paper compiles and reviews animal and selected human literature in this research area. Early barbiturate exposure in animals reduces brain weight with related changes in brain biochemistry and neuromorphology. Significant changes may be found in surviving adult offspring. Evidence of CNS and behavioral damage in human beings due to early barbiturate exposure is not clearcut, however, confounded by the conditions for which the drugs are prescribed. In animals, early drug exposure significantly reduces levels of hormones, vitamins, and other biologically active macromolecules via (long-lasting) induction of hepatic metabolizing enzymes. Whether or not in humans treated with barbiturates, hormone levels remain within the normal range (by-feed-back regulation) and, also, if vitamin deficiencies can be simply corrected by supplements is still being debated. Early barbiturates administered to animals is associated with long-lasting disturbances in activity, learning performance, sexual behavior, and reproductive function, but not in a simple dose-exposure related manner. Animal studies show that long-lasting functional tolerance to drugs develops following early barbiturate exposure. Although infants become "passively addicted" following in utero exposure, there is as yet no data on subsequent development of human adult tolerance. Drug related damage must, in any case, be weighed against therapeutic benefits of drug administration and the results of failure to treat.

Serum bile acid levels in intrahepatic cholestasis of pregnancy during treatment with phenobarbital or cholestyramine

Nineteen patients suffering from the intrahepatic cholestasis (IHC) of pregnancy were studied. Twelve of them were treated with phenobarbital (100 mg/day) and seven with cholestyramine (18 g/day). The overnight fasting levels of serum cholic acid (CA), chenodeoxycholic acid (CDCA) and deoxycholic acid (DCA) were measured by radioimmunoassay. The activities of serum transaminases, gamma-glutamyltranspeptidase, alkaline phosphatase and total and conjugated bilirubins were also analyzed. It was found that there was no correlation between the itching symptom and the serum bile acid levels. During phenobarbital treatment serum bile acid concentrations did not change. Also, the other measured parameters as well as the CA/CDCA ratio did not change significantly. Transaminases had, however, a slight tendency to decrease. The therapy successfully relieved itching in half of the cases. There was no relationship between the relief of the itching and the change in the bile acid concentrations. Cholestyramine treatment did not decrease the CA level significantly, but that of the CDCA decreased (P less than 0.05) and the ratio of CA/CDCA increased (P less than 0.05). In the other analyzed liver function test results, an increase (P less than 0.05) occurred only in the concentrations of conjugated bilirubin. The itching was relieved in five of the seven cases during the first week of treatment, but after that the symptom tended to reappear. There was a slight correlation between the decrease in the CDCA level and in the relief of the itching. The two drugs did not cause any particular side effects.

Chemoprevention of neonatal jaundice: potency of tin-protoporphyrin in an animal model

The substantial increases of hepatic, splenic, and renal heme oxygenase levels that occur shortly after birth in neonatal rats were prevented by a single administration of tin-protoporphyrin (10 micromoles per kilogram of body weight). With this treatment serum bilirubin levels declined within 24 hours to near-normal adult levels and remained low throughout the postnatal period. Zinc-protoporphyrin at doses up to 50-fold greater than the effective dose of tin-protoporphyrin did not prevent the immediate increases in tissue heme oxygenase activities and in serum bilirubin levels that occur postnatally. Studies in vitro with microsomal heme oxygenase in human spleen indicate that tin-protoporphyrin is a potent competitive inhibitor of the oxidation of heme to bile pigment in this tissue.