Expression of constitutive androstane receptor, hepatic nuclear factor 4 alpha, and P450 oxidoreductase genes determines interindividual variability in basal expression and activity of a broad scope of xenobiotic metabolism genes in the human liver

Expression of hepatic drug-metabolizing cytochrome p450 enzymes and their intercorrelations: a meta-analysis

Cytochrome P450 is a family of enzymes that catalyze reactions involved in the metabolism of drugs and other xenobiotics. These enzymes are therefore important in pharmacologic and toxicologic studies, and information on their abundances is of value in the process of scaling in vitro data to in vivo metabolic parameters. A meta-analysis was applied to data on the abundance of human hepatic cytochrome P450 enzymes in Caucasian adult livers (50 studies). Despite variations in the methods used to measure the abundance of enzymes, agreement between the studies in 26 different laboratories was generally good. Nonetheless, some heterogeneity was detected (Higgins and Thompson heterogeneity test). More importantly, large interindividual variability was observed in the collated data. Positive correlations between the expression levels of some cytochrome P450 enzymes were found in the abundance data, including the following pairs: CYP3A4/CYP3A5*1/*3 (Rs = 0.70, P < 0.0001, n = 52), CYP3A4/CYP2C8 (Rs = 0.68, P < 0.0001, n = 134), CYP3A4/CYP2C9 (Rs = 0.55, P < 0.0001, n = 71), and CYP2C8/CYP2C9 (Rs = 0.55, P < 0.0001, n = 99). These correlations can be used to demonstrate common genetic transcriptional mechanisms.

Simultaneous quantification of the abundance of several cytochrome P450 and uridine 5'-diphospho-glucuronosyltransferase enzymes in human liver microsomes using multiplexed targeted proteomics

Cytochrome P450 (P450) and uridine 5'-diphospho-glucuronosyltransferase (UGT) enzymes mediate a major proportion of phase I and phase II metabolism of xenobiotics. In vitro-in vivo extrapolation (IVIVE) of hepatic clearance in conjunction with physiologically-based pharmacokinetics (PBPK) has become common practice in drug development. However, prediction of xenobiotic kinetics in virtual populations requires knowledge of both enzyme abundances and the extent to which these correlate. A multiplexed quantification concatemer (QconCAT) strategy was used in this study to quantify the expression of several P450 and UGT enzymes simultaneously and to establish correlations between various enzyme abundances in 24 individual liver samples (ages 27-66, 14 male). Abundances were comparable to previously reported values, including CYP2C9 (40.0 ± 26.0 pmol mg(-1)), CYP2D6 (11.9 ± 13.2 pmol mg(-1)), CYP3A4 (68.1 ± 52.3 pmol mg(-1)), UGT1A1 (33.6 ± 34.0 pmol mg(-1)), and UGT2B7 (82.9 ± 36.1 pmol mg(-1)), expressed as mean ± S.D. Previous reports of correlations in expression of various P450 (CYP3A4/CYP3A5*1/*3, CYP2C8/CYP2C9, and CYP3A4/CYP2B6) were confirmed. New correlations were demonstrated between UGTs [including UGT1A6/UGT1A9 (r(s) = 0.82, P < 0.0001) and UGT2B4/UGT2B15 (r(s) = 0.71, P < 0.0001)]. Expression of some P450 and UGT enzymes were shown to be correlated [including CYP1A2/UGT2B4 (r(s) = 0.67, P = 0.0002)]. The expression of CYP3A5 in individuals with *1/*3 genotype (n = 11) was higher than those with *3/*3 genotype (n = 10) (P < 0.0001). No significant effect of gender or history of smoking or alcohol use on enzyme expression was observed; however, expression of several enzymes declined with age. The correlation matrix produced for the first time by this study can be used to generate more realistic virtual populations with respect to abundance of various enzymes.

CYP2C8 and CYP2C9 mRNA expression profile in the human fetus

CYP2C8 and CYP2C9 are involved in the inactivation of several non-steroidal anti-inflammatory drugs, including ibuprofen. CYP2C9 is the major form in human liver whereas CYP2C8 has been proposed to be the main CYP2C enzyme in fetal liver. The protein expression of CYP2C9 in the first trimester is low, only about 1% of the adult values, whereas the mRNA levels of CYP2C8/9 have not been determined at the fetal stage. In this study the mRNA expression levels of CYP2C8 and CYP2C9 were determined in 20 adult and 60 fetal liver tissue specimens. The expression profiles in fetal kidneys (n = 43), adrenals (n = 46), and lungs (n = 37) were also determined. Moreover the activity against ibuprofen hydroxylation was determined in fetus and adult liver microsomes. Adult liver samples expressed 140 and 400 times higher levels of CYP2C8 and CYP2C9 mRNA, respectively, as compared to fetal liver samples. Consistent with this, the hydroxylation of ibuprofen was 40 times higher in the adult liver microsomes. Hepatic CYP2C8 mRNA was three times more abundant than CYP2C9 mRNA in the fetus. Moreover, CYP2C8 were consistently expressed in all fetal tissues investigated, whereas CYP2C9 gene expression was confined to the liver in fetuses. Our results indicate that CYP2C8 plays a more important physiological role than CYP2C9 in the first trimester.

Signaling control of the constitutive androstane receptor (CAR)

The constitutive androstane receptor (CAR, NR1I3) plays a crucial role in the regulation of drug metabolism, energy homeostasis, and cancer development through modulating the transcription of its numerous target genes. Different from prototypical nuclear receptors, CAR can be activated by either direct ligand binding or ligand-independent (indirect) mechanisms both initiated with nuclear translocation of CAR from the cytoplasm. In comparison to the well-defined ligand-based activation, indirect activation of CAR appears to be exclusively involved in the nuclear translocation through mechanisms yet to be fully understood. Accumulating evidence reveals that without activation, CAR forms a protein complex in the cytoplasm where it can be functionally affected by multiple signaling pathways. In this review, we discuss recent progresses in our understanding of the signaling regulation of CAR nuclear accumulation and activation. We expect that this review will also provide greater insight into the similarity and difference between the mechanisms of direct vs. indirect human CAR activation.

Hepatic early inflammation induces downregulation of hepatic cytochrome P450 expression and metabolic activity in the dextran sulfate sodium-induced murine colitis

Ulcerative colitis (UC) patients may have increased concentrations of drugs in their blood. We hypothesized that this response is mainly due to a decrease in the expression and activity of the drug-metabolizing enzyme, cytochrome P450 (CYP), in the liver. In this study, we have tried to demonstrate the hypothesis. UC was induced in mice by treatment with dextran sulfate sodium (DSS) solution. The mRNA and protein expression levels of CYP, inflammatory cytokine levels, and the metabolic activity of CYP3A in the liver were measured. The nuclear translocations of nuclear factor kappa B (NF-κB), pregnane X receptor (PXR), and constitutive androstane receptor (CAR) were analyzed. The levels of hepatic inflammatory cytokines increased in the DSS-treated group. The hepatic mRNA and protein expression of CYP (CYP1A, CYP2C, CYP2D, CYP2E, and CYP3A) and the CYP3A metabolic activity significantly decreased compared to the control group. Hepatic NF-κB nuclear translocation significantly increased in the DSS-treated group. In contrast, the nuclear translocations of PXR and CAR were decreased. Lipopolysaccharides from inflammatory sites in the colon induce hepatic inflammation in DSS-induced murine colitis. This inflammation then causes an increase in the nuclear translocation of hepatic NF-κB and a decrease in the nuclear translocation of PXR and CAR, resulting in the decreased expression and activities of CYP. The results of this study indicated that at the onset of UC, the decreased activity of hepatic CYP causes an increase in the concentrations of drugs in the blood, leading to an increase in the incidence of adverse reactions.

The role of human cytochrome P450 enzymes in the formation of 2-hydroxymetronidazole: CYP2A6 is the high affinity (low Km) catalyst

Despite metronidazole's widespread clinical use since the 1960s, the specific enzymes involved in its biotransformation have not been previously identified. Hence, in vitro studies were conducted to identify and characterize the cytochrome P450 enzymes involved in the formation of the major metabolite, 2-hydroxymetronidazole. Formation of 2-hydroxymetronidazole in human liver microsomes was consistent with biphasic, Michaelis-Menten kinetics. Although several cDNA-expressed P450 enzymes catalyzed 2-hydroxymetronidazole formation at a supratherapeutic concentration of metronidazole (2000 μM), at a "therapeutic concentration" of 100 μM only CYPs 2A6, 3A4, 3A5, and 3A7 catalyzed metronidazole 2-hydroxylation at rates substantially greater than control vector, and CYP2A6 catalyzed 2-hydroxymetronidazole formation at rates 6-fold higher than the next most active enzyme. Kinetic studies with these recombinant enzymes revealed that CYP2A6 has a Km = 289 μM which is comparable to the Km for the high-affinity (low-Km) enzyme in human liver microsomes, whereas the Km values for the CYP3A enzymes corresponded with the low-affinity (high-Km) component. The sample-to-sample variation in 2-hydroxymetronidazole formation correlated significantly with CYP2A6 activity (r ≥ 0.970, P < 0.001) at substrate concentrations of 100 and 300 μM. Selective chemical inhibitors of CYP2A6 inhibited metronidazole 2-hydroxylation in a concentration-dependent manner and inhibitory antibodies against CYP2A6 virtually eliminated metronidazole 2-hydroxylation (>99%). Chemical and antibody inhibitors of other P450 enzymes had little or no effect on metronidazole 2-hydroxylation. These results suggest that CYP2A6 is the primary catalyst responsible for the 2-hydroxylation of metronidazole, a reaction that may function as a marker of CYP2A6 activity both in vitro and in vivo.

Atorvastatin treatment induces uptake and efflux transporters in human liver

The metabolism and disposition of statins are highly dependent on different cytochrome P450 enzymes, such as CYP3A4 and CYP2C9, as well as membrane transporters SLCO1B1, SLCO2B1, ABCB1, and ABCG2. Interindividual gene expression differences among these enzymes may explain part of the variability in tolerance and effect for statin treatment. The aim of the present study was to investigate the effect of statin treatment on these genes in human liver tissue. Levels of CYP3A4, CYP2C9, SLCO1B1, SLCO2B1, ABCB1, and ABCG2 mRNA in liver tissue from a previously performed clinical trial in 29 patients randomized to treatment with placebo, 80 mg/day of atorvastatin, or 20 mg/day of fluvastatin for 4 weeks were measured using quantitative polymerase chain reaction. Treatment with atorvastatin (n = 10), but not with fluvastatin (n = 10), resulted in 3-fold higher expression of SLCO2B1 compared with placebo-treated patients (n = 9) (P < 0.05). Atorvastatin increased the expression of both ABCB1 and ABCG2 by more than 2-fold (P < 0.05). No difference was found in CYP2C9, CYP3A4, or SLCO1B1 mRNA expression in patients administered statins or those administered placebo. Premenopausal women (n = 8) had higher expression of CYP3A4 (P < 0.05) and lower expression of CYP2C9 (P < 0.05) compared with postmenopausal women (n = 10) and men (n = 11), respectively. Here we show for the first time that atorvastatin treatment leads to increased expression of the membrane transporters SLCO2B1, ABCB1, and ABCG2 in human liver tissue, which potentially may counteract the efficacy of the treatment, and our findings may cast light on the mechanisms of clinical problems with adverse reactions and drug interactions in statin treatment.

Expression of genes involved in xenobiotic metabolism and transport in end-stage liver disease: up-regulation of ABCC4 and CYP1B1

BACKGROUND

Expression of drug-metabolizing enzymes and drug transporters in liver is mainly regulated by a system of nuclear receptors. The aim of the current study was to investigate the expression of nuclear receptors, as well as these enzymes and transporters, in liver samples from patients suffering from end-stage liver disease of various etiologies (HCV infection, alcohol liver disease, and primary sclerosis cholangitis).

METHODS

Gene expression was measured using quantitative real-time PCR with surgical specimens from livers of patients with end-stage liver disease, and non-tumoral liver tissue that served as control.

RESULTS

Our study confirmed that the expression of most phase I enzymes is suppressed in end-stage liver disease, and is correlated with a decrease in NR1I2 and NR1I3, the main regulators of xenobiotic metabolism. While mRNA levels of phase II enzymes were generally unchanged, some ABC transporters were up-regulated. The most spectacular increases in expression were observed with ABCC4 (MRP4) - at the mRNA level, and CYP1B1 - at both the mRNA and protein levels. We also demonstrated that IL-6 can induce CYP1B1 expression independently of CYP1A1, in a human hepatocellular liver carcinoma cell line.

CONCLUSIONS

As CYP1B1 is an enzyme which converts various substrates into carcinogenous metabolites, its overexpression in liver may be one of the factors increasing the risk of hepatic cancers in patients with liver disease. CYP1A1 and CYP1B1 are often referred to as model AHR target genes, but CYP1A1 was down-regulated in diseased liver samples. This points to the existence of differences in regulation of these two genes.

Dual roles of nuclear receptor liver X receptor α (LXRα) in the CYP3A4 expression in human hepatocytes as a positive and negative regulator

CYP3A4 is a major drug-metabolizing enzyme in humans, whose expression levels show large inter-individual variations and are associated with several factors such as genetic polymorphism, physiological and disease status, diet and xenobiotic exposure. Nuclear receptor pregnane X receptor (PXR) is a key transcription factor for the xenobiotic-mediated transcription of CYP3A4. In this study, we have investigated a possible involvement of liver X receptor α (LXRα), a critical regulator of cholesterol homeostasis, in the hepatic CYP3A4 expression since several recent reports suggest the involvement of CYP3A enzymes in the cholesterol metabolism in humans and mice. Reporter assays using wild-type and mutated CYP3A4 luciferase reporter plasmids and electrophoretic mobility shift assays revealed that LXRα up-regulated CYP3A4 through the known DNA elements critical for the PXR-dependent CYP3A4 transcription, suggesting LXRα as a positive regulator for the CYP3A4 expression and a crosstalk between PXR and LXRα in the expression. In fact, reporter assays showed that LXRα activation attenuated the PXR-dependent CYP3A4 transcription. Moreover, a PXR agonist treatment-dependent increase in CYP3A4 mRNA levels was suppressed by co-treatment with an LXRα agonist in human primary hepatocytes and HepaRG cells. The suppression was not observed when LXRα expression was knocked-down in HepaRG cells. In conclusion, the present results suggest that sterol-sensitive LXRα positively regulates the basal expression of CYP3A4 but suppresses the xenobiotic/PXR-dependent CYP3A4 expression in human hepatocytes. Therefore, nutritional, physiological and disease conditions affecting LXRα might be one of the determinants for the basal and xenobiotic-responsive expression of CYP3A4 in human livers.

Pharmacogenetics of P450 oxidoreductase: implications in drug metabolism and therapy

The redox reaction of cytochrome P450 enzymes (CYP) is an important physiological and biochemical reaction in the human body, as it is involved in the oxidative metabolism of both endogenous and exogenous substrates. Cytochrome P450 oxidoreductase (POR) is the only obligate electron donor for all of the hepatic microsomal CYP enzymes. It plays a crucial role in drug metabolism and treatment by not only acting as an electron donor involved in drug metabolism mediated by CYP enzymes but also by directly inducing the transformation of some antitumor precursors. Studies have found that the gene encoding human POR is highly polymorphic, which is of considerable clinical significance as it affects the metabolism and curative effects of clinically used drugs. This review aims to discuss the effect of POR and its genetic polymorphisms on drug metabolism and therapy, as well as the potential mechanisms of POR pharmacogenetics.

Isoform-specific regulation of cytochromes P450 expression by estradiol and progesterone

Results from clinical studies suggest that pregnancy alters hepatic drug metabolism in a cytochrome P450 (P450) isoform-specific manner, and rising concentrations of female hormones are potentially responsible for the changes. The objective of this study was to comprehensively characterize the effects of estrogen and progesterone on the expression and activity of major drug-metabolizing P450s. To this end, primary human hepatocytes were treated with estradiol and progesterone, and mRNA expression and activity levels of 10 different P450 isoforms were determined. The results showed that estradiol enhances CYP2A6, CYP2B6, and CYP3A4 expression, whereas progesterone induces CYP2A6, CYP2B6, CYP2C8, CYP3A4, and CYP3A5 expression. The induction was mainly observed when the average hormone concentrations were at the levels reached during pregnancy, suggesting that these effects are likely pregnancy-specific. Estradiol also increased enzyme activities of CYP2C9 and CYP2E1 without affecting the mRNA expression levels by unknown mechanisms. Taken together, our results show differential effects of estrogen and progesterone on P450 expression, suggesting involvement of different regulatory mechanisms in female hormone-mediated P450 regulation. Our findings potentially provide a basis in mechanistic understanding for altered drug metabolism during pregnancy.

Regulation of hepatic phase II metabolism in pregnant mice

Phase II enzymes, including Ugts, Sults, and Gsts, are critical for the disposition and detoxification of endo- and xenobiotics. In this study, the mRNA and protein expression of major phase II enzymes, as well as key regulatory transcription factors, were quantified in livers of time-matched pregnant and virgin control C57BL/6 mice on gestation days (GD) 7, 11, 14, 17, and postnatal days (PND) 1, 15, and 30. Compared with virgin controls, the mRNA expression of Ugt1a1, 1a6, 1a9, 2a3, 2b1, 2b34, and 2b35 decreased 40 to 80% in pregnant dams. Protein expression of Ugt1a6 also decreased and corresponded with reduced in vitro glucuronidation of bisphenol A in S9 fractions from livers of pregnant mice. Similar to Ugts levels, Gsta1 and a4 mRNAs were reduced in pregnant dams in mid to late gestation; however no change in protein expression was observed. Conversely, Sult1a1, 2a1/2, and 3a1 mRNAs increased 100 to 500% at various time points in pregnant and lactating mice and corresponded with enhanced in vitro sulfation of acetaminophen in liver S9 fractions. Coinciding with maximal decreases in Ugts as well as increases in Sults, the expression of transcription factors CAR, PPARα, and PXR and their target genes were downregulated, whereas ERα mRNA was upregulated. Collectively, these data demonstrate altered regulation of hepatic phase II metabolism in mice during pregnancy and suggest that CAR, PPARα, PXR, and ERα signaling pathways may be candidate signaling pathways responsible for these changes.

Genetic variations of NR2A1 in Asian populations: implications in pharmacogenetics studies

HNF4α (encoded by gene NR2A1) is a dominant transcriptional regulator of various drug disposition genes. It forms a circuitry of molecular cross-talk with other nuclear receptors such as PXR and CAR to synergistically initiate transcription. This study reports on the frequency, linkage disequilibrium pattern and tag-SNP selection of NR2A1 polymorphisms in three local Asian populations, namely Chinese, Malays and Indians (n = 56 subjects each). A total of 69 polymorphisms were identified in the genomic region of NR2A1, of which thirty-three were novel polymorphisms with low allelic frequencies (<0.02). The exonic region of NR2A1 was highly conserved with only 4 novel and 1 reported SNPs identified at low allelic frequencies of less than 0.02. Based on the criteria of MAF ≥ 0.05 and R(2) ≥ 0.80, there were 19, 20 and 22 tag-SNPs selected to represent the genetic polymorphisms of NR2A1 in Chinese, Malays and Indians, respectively. In-silico predictions suggested that some of these polymorphic variants may exert functional effects through affecting the binding sites of transcription and splicing factors. Our study provides valuable information on the genetic variability of NR2A1 which would be useful for pharmacogenetics studies in the local Asian populations.

Primary hepatocyte cultures for pharmaco-toxicological studies: at the busy crossroad of various anti-dedifferentiation strategies

Continuously increasing understanding of the molecular triggers responsible for the onset of diseases, paralleled by an equally dynamic evolution of chemical synthesis and screening methods, offers an abundance of pharmacological agents with a potential to become new successful drugs. However, before patients can benefit of newly developed pharmaceuticals, stringent safety filters need to be applied to weed out unfavourable drug candidates. Cost effectiveness and the need to identify compound liabilities, without exposing humans to unnecessary risks, has stimulated the shift of the safety studies to the earliest stages of drug discovery and development. In this regard, in vivo relevant organotypic in vitro models have high potential to revolutionize the preclinical safety testing. They can enable automation of the process, to match the requirements of high-throughput screening approaches, while satisfying ethical considerations. Cultures of primary hepatocytes became already an inherent part of the preclinical pharmaco-toxicological testing battery, yet their routine use, particularly for long-term assays, is limited by the progressive deterioration of liver-specific features. The availability of suitable hepatic and other organ-specific in vitro models is, however, of paramount importance in the light of changing European legal regulations in the field of chemical compounds of different origin, which gradually restrict the use of animal studies for safety assessment, as currently witnessed in cosmetic industry. Fortunately, research groups worldwide spare no effort to establish hepatic in vitro systems. In the present review, both classical and innovative methodologies to stabilize the in vivo-like hepatocyte phenotype in culture of primary hepatocytes are presented and discussed.

Molecular mechanisms of genetic variation and transcriptional regulation of CYP2C19

Inherited variation in the function of the drug metabolizing enzyme CYP2C19 was first observed 40 years ago. The SNP variants which underpin loss of CYP2C19 function have been elucidated and extensively studied in healthy populations. However, there has been relatively meagre translation of this information into the clinic. The presence of genotype-phenotype discordance in certain patients suggests that changes in the regulation of this gene, as well as loss of function SNPs, could play a role in deficient activity of this enzyme. Knowledge of the molecular mechanisms which control transcription of this gene, reviewed in this article, may aid the challenge of delivering CYP2C19 pharmacogenetics into clinical use.

Methods for the quantitative evaluation and prediction of CYP enzyme induction using human in vitro systems

IMPORTANCE OF THE FIELD

For successful drug development, it is important to investigate the potency of candidate drugs causing drug-drug interactions (DDI) during the early stages of development. The most common mechanisms of DDIs are the inhibition and induction of CYP enzymes. Therefore, it is important to develop co.mpounds with lower potencies for CYP enzyme induction.

AREAS COVERED IN THIS REVIEW

The aim of the present paper is to present an overview of the current knowledge of CYP induction mechanisms, particularly focusing on the transcriptional gene activation mediated by pregnane X receptor, aryl hydrocarbon receptor and constitutive androstane receptor. The adoptable options of in vitro assay methods for evaluating CYP induction are also summarized. Finally, we introduce a method for the quantitative prediction of CYP3A4 induction considering the turnover of CYP3A4 mRNA and protein in hepatocytes based on the data obtained from a reporter gene assay.

WHAT THE READER WILL GAIN

In order to predict in vivo CYP enzyme induction quantitatively based on in vitro information, an understanding of the physiological induction mechanisms and the features of each in vitro assay system is essential. We also present the estimation method of in vivo CYP induction potency of each compound based on the in vitro data which are routinely obtained but not necessarily utilized maximally in pharmaceutical companies.

TAKE HOME MESSAGE

It is desirable to select compounds with lower potencies for the inductive effect. For this purpose, an accurate prioritization procedure to evaluate the induction potency of each compound in a quantitative manner considering the pharmacologically effective concentration of each compound is necessary.

Expression and inducibility of CYP1A1, 1A2, 1B1 by β-naphthoflavone and CYP2B22, CYP3As by rifampicin in heart regions and coronary arteries of pig

In this study, the constitutive and inducible expression of the CYP genes (1A1, 1A2, 1B1, 2B22, 3A22, 3A29 and 3A46), related transcriptional factors (AhR, CAR, PXR, and Nrf2) and the antioxidant enzymes SOD, catalase, GSSH-reductase and GSH-peroxidase were investigated in the liver, heart regions and coronary arteries of control pigs and pigs treated with β-naphthoflavone (βNF) or with rifampicin (RIF). Real-time PCR experiments and enzymatic or immunoblot assays showed that CYP1A1 was predominantly enhanced by βNF in a similar manner in all the heart regions, whereas antioxidant enzyme activity was not affected. The rifampicin treatment resulted in an induction of CYP2B22 and CYP3As, at the transcriptional, activity and protein level in liver but not in heart nor in the coronary arteries, despite the expression of CAR and PXR in the cardiac tissues. These results obtained in vivo suggest that pig cardiac tissues may represent a useful model for humans.

Genetic variations of NR1I3 and NR2B1 in Asian populations

Several nuclear receptors are being increasingly recognized for their role as master xenosensors. Among them, CAR-RXRα heterodimer, as encoded by NR1I3 and NR2B1, responds to the presence of drug compounds and regulates the transcription of a wide array of genes involved in their disposition. To investigate the frequency distribution and linkage disequilibrium patterns of NR1I3 and NR2B1 genetic variations, these genes were screened in 168 healthy local Asian subjects, namely Chinese, Malays, and Indians (n=56 subjects each). A total of 38 and 88 SNPs were identified in NR1I3 and NR2B1, respectively. Among them, there were 13 and 43 novel SNPs present at low allelic frequencies (<10%) in NR1I3 and NR2B1, respectively. Notably, the genetic variations in the NR1I3 and NR2B1 genes were mainly confined to the introns whilst the exons were highly conserved across the ethnic populations. Indians harboured distinct frequency distributions from Chinese and Malays in both genes. Based on the linkage disequilibrium patterns of both genes, a number of tag-SNPs were selected for each population (n=8-13 for NR1I3; n=12-18 for NR2B1). In-silico prediction analyses revealed a number of possible functional SNPs. Our data would be valuable for future pharmacogenetic studies on the drug substrates of CAR-RXRα target genes.

Metabolic capabilities of cytochrome P450 enzymes in Chinese liver microsomes compared with those in Caucasian liver microsomes

AIM

The most common causes of variability in drug response include differences in drug metabolism, especially when the hepatic cytochrome P450 (CYP) enzymes are involved. The current study was conducted to assess the differences in CYP activities in human liver microsomes (HLM) of Chinese or Caucasian origin.

METHODS

The metabolic capabilities of CYP enzymes in 30 Chinese liver microsomal samples were compared with those of 30 Caucasian samples utilizing enzyme kinetics. Phenacetin O-deethylation, coumarin 7-hydroxylation, bupropion hydroxylation, amodiaquine N-desethylation, diclofenac 4'-hydroxylation (S)-mephenytoin 4'-hydroxylation, dextromethorphan O-demethylation, chlorzoxazone 6-hydroxylation and midazolam 1'-hydroxylation/testosterone 6β-hydroxylation were used as probes for activities of CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A, respectively. Mann-Whitney U test was used to assess the differences.

RESULTS

The samples of the two ethnic groups were not significantly different in cytochrome-b(5) concentrations but were significantly different in total CYP concentrations and NADPH-P450 reductase activity (P < 0.05). Significant ethnic differences in intrinsic clearance were observed for CYP1A2, CYP2C9, CYP2C19 and CYP2E1; the median values of the Chinese group were 54, 58, 26, and 35% of the corresponding values of the Caucasian group, respectively. These differences were associated with differences in Michaelis constant or maximum velocity. Despite negligible difference in intrinsic clearance, the Michaelis constant of CYP2B6 appeared to have a significant ethnic difference. No ethnic difference was observed for CYP2A6, CYP2C8, CYP2D6 and CYP3A.

CONCLUSIONS

These data extend our knowledge on the ethnic differences in CYP enzymes and will have implications for drug discovery and drug therapy for patients from different ethnic origins.

In silico and in vitro identification of microRNAs that regulate hepatic nuclear factor 4α expression

Hepatic nuclear factor 4α (HNF4A) is a nuclear transcription factor that regulates the expression of many genes involved in drug disposition. To identify additional molecular mechanisms that regulate HNF4A, we identified microRNAs (miRNAs) that target HNF4A expression. In silico analyses suggested that HNF4A is targeted by many miRNAs. We conducted in vitro studies to validate several of these predictions. With use of an HNF4A 3'-untranslated region (UTR) luciferase reporter assay, five of six miRNAs tested significantly down-regulated (∼20-40%) the luciferase activity. In HepG2 cells, miR-34a and miR-449a also down-regulated the expression of both the HNF4A protein and an HNF4A target gene, PXR (∼30-40%). This regulation appeared without reduction in HNF4A mRNA expression, suggesting that they must be blocking HNF4A translation. Using additional bioinformatic algorithms, we identified polymorphisms that are predicted to alter the miRNA targeting of HNF4A. Luciferase assays indicated that miR-34a and miR-449a were less effective in regulating a variant (rs11574744) than the wild-type HNF4A 3'-UTR. In vivo, subjects with the variant HNF4A had lower CYP2D6 enzyme activity, although this result was not statistically significant (p = 0.16). In conclusion, our findings demonstrate strong evidence for a role of miRNAs in the regulation of HNF4A.

PAH particles perturb prenatal processes and phenotypes: protection from deficits in object discrimination afforded by dampening of brain oxidoreductase following in utero exposure to inhaled benzo(a)pyrene

The wild-type (WT) Cpr(lox/lox) (cytochrome P(450) oxidoreductase, Cpr) mouse is an ideal model to assess the contribution of P(450) enzymes to the metabolic activation and disposition of environmental xenobiotics. In the present study, we examined the effect of in utero exposure to benzo(a)pyrene [B(a)P] aerosol on Sp4 and N-methyl-D-aspartate (NMDA)-dependent systems as well as a resulting behavioral phenotype (object discrimination) in Cpr offspring. Results from in utero exposure of WT Cpr(lox/lox) mice were compared with in utero exposed brain-Cpr-null offspring mice. Null mice were used as they do not express brain cytochrome P(450)1B1-associated NADPH oxidoreductase (CYP1B1-associated NADPH oxidoreductase), thus reducing their capacity to produce neural B(a)P metabolites. Subsequent to in utero (E14-E17) exposure to B(a)P (100 μg/m(3)), Cpr(lox/lox) offspring exhibited: (1) elevated B(a)P metabolite and F(2)-isoprostane neocortical tissue burdens, (2) elevated concentrations of cortical glutamate, (3) premature developmental expression of Sp4, (4) decreased subunit ratios of NR2B:NR2A, and (5) deficits in a novelty discrimination phenotype monitored to in utero exposed brain-Cpr-null offspring. Collectively, these findings suggest that in situ generation of metabolites by CYP1B1-associated NADPH oxidoreductase promotes negative effects on NMDA-mediated signaling processes during the period when synapses are first forming as well as effects on a subsequent behavioral phenotype.

Functional evaluation of genetic and environmental regulators of p450 mRNA levels

Variations in the activities of Cytochrome P450s are one of the major factors responsible for inter-individual differences in drug clearance rates, which may cause serious toxicity or inefficacy of therapeutic drugs. Various mRNA level is one of the key factors for different activity of the major P450 genes. Although both genetic and environmental regulators of P450 gene expression have been widely investigated, few studies have evaluated the functional importance of cis- and trans-regulatory factors and environmental factors in the modulation of inter-individual expression variations of the P450 genes. In this study, we measured the mRNA levels of seven major P450 genes (CYP1A1, CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP3A4 and CYP3A5) in 96 liver biopsy samples from Chinese population. Both trans-acting (mRNA levels and non-synonymous SNPs of putative regulator genes) and cis-acting (gene copy number and functional SNPs) factors were investigated to identify the determinants of the expression variations of these seven P450 genes. We found that expression variations of most P450 genes, regulator genes and housekeeping genes were positively correlated at the mRNA level. After partial correlation analysis using ACTB and GAPDH expression to eliminate the effect of global regulators, a UPGMA (Unweighted Pair Group Method with Arithmetic Mean) tree was constructed to reveal the effects of specific regulation networks potentially masked by global regulators. Combined with the functional analysis of regulators, our results suggested that expression variation at the mRNA level was mediated by several factors in a gene-specific manner. Cis-acting genetic variants might play key roles in the expression variation of CYP2D6 and CYP3A5, environmental inducers might play key roles in CYP1A1 and CYP1A2 variation and global regulators might play key roles in CYP2C9 variation. In addition, the functions of regulators that play less important roles in controlling expression variation for each P450 gene were determined.

Comparative study of culture conditions for maintaining CYP3A4 and ATP-binding cassette transporters activity in primary cultured human hepatocytes

The aim of this study was to determine suitable culture conditions for maintaining the activity of cytochrome p450 (CYP) 3A4 and drug transporters in primary cultured human hepatocytes. Human hepatocytes were isolated using the two-step collagenase perfusion technique and were cultured with four different media, serum-free William's E medium (serum-free WEM), WEM containing fetal calf serum (FCS-WEM), WEM with human serum (HS-WEM), and Lanford's medium. The albumin levels were maintained for 7 days in hepatocytes. Although CYP3A4 mRNA levels gradually decreased from 3 days, CYP3A4 and hepatocyte nuclear factor-4α alpha protein levels and activities were maintained for 7 days in hepatocytes cultured with serum-free WEM and Lanford's but not in those with FCS-WEM and HS-WEM. Furthermore, CYP3A4 protein levels were significantly increased by the addition of rifampicin and dexamethasone to the culture media, indicating that the induction potential was maintained. The protein levels of P-glycoprotein, multi-drug-resistance-2, and breast cancer-resistance protein were maintained for 7 days in all media. Serum-free WEM and Lanford's also maintained protein levels of CYP2C19, CYP2D6, and organic anion transporter polypeptide in the hepatocytes. Serum-free WEM and Lanford's may be appropriate culture media for maintaining CYP3A4 and drug transporter protein levels in primary cultured hepatocytes.

Alterations in hepatic mRNA expression of phase II enzymes and xenobiotic transporters after targeted disruption of hepatocyte nuclear factor 4 alpha

Hepatocyte nuclear factor 4 alpha (HNF4a) is a liver-enriched master regulator of liver function. HNF4a is important in regulating hepatic expression of certain cytochrome P450s. The purpose of this study was to use mice lacking HNF4a expression in liver (HNF4a-HNull) to elucidate the role of HNF4a in regulating hepatic expression of phase II enzymes and transporters in mice. Compared with male wild-type mice, HNF4a-HNull male mouse livers had (1) markedly lower messenger RNAs (mRNAs) encoding the uptake transporters sodium taurocholate cotransporting polypeptide, organic anion transporting polypeptide (Oatp) 1a1, Oatp2b1, organic anion transporter 2, sodium phosphate cotransporter type 1, sulfate anion transporter 1, sodium-dependent vitamin C transporter 1, the phase II enzymes Uridine 5'-diphospho (UDP)-glucuronosyltransferase (Ugt) 2a3, Ugt2b1, Ugt3a1, Ugt3a2, sulfotransferase (Sult) 1a1, Sult1b1, Sult5a1, the efflux transporters multidrug resistance-associated protein (Mrp) 6, and multidrug and toxin extrusion 1; (2) moderately lower mRNAs encoding Oatp1b2, organic cation transporter (Oct) 1, Ugt1a5, Ugt1a9, glutathione S-transferase (Gst) m4, Gstm6, and breast cancer resistance protein; but (3) higher mRNAs encoding Oatp1a4, Octn2, Ugt1a1, Sult1e1, Sult2a2, Gsta4, Gstm1-m3, multidrug resistance protein (Mdr) 1a, Mrp3, and Mrp4. Hepatic signaling of nuclear factor E2-related factor 2 and pregnane X receptor appear to be activated in HNF4a-HNull mice. In conclusion, HNF4a deficiency markedly alters hepatic mRNA expression of a large number of phase II enzymes and transporters, probably because of the loss of HNF4a, which is a transactivator and a determinant of gender-specific expression and/or adaptive activation of signaling pathways important in hepatic regulation of these phase II enzymes and transporters.

Regulation of hepatic ABCC transporters by xenobiotics and in disease states

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

Nutritional status affects fluvastatin-induced hepatotoxicity and myopathy in rats

Rats that consumed a high-fat and high-sucrose diet (HF diet) developed hepatic steatosis. Treatment of HF diet-fed rats with fluvastatin (8 mg/kg) was lethal, followed by an elevation in levels of plasma aspartate aminotransferase and creatine kinase activities and skeletal muscle toxicity. This study was conducted to determine whether nutritional status affects statin-induced adverse effects in rats. Fluvastatin treatment of rats fed the HF diet led to an increase in systemic exposure, suggesting altered metabolism and elimination. In fact, although hepatic multidrug resistance-associated protein (Mrp) 2 and multidrug resistance (Mdr) 1b protein levels were not significantly changed by fluvastatin treatment for 8 days of rats fed a HF diet, the organic anion-transporting protein (Oatp) 1, Mrp3, CYP1A, CYP2C, UDP-glucuronosyltransferase (UGT) 1A1, and UGT1A5 protein levels were moderately decreased and the Oatp2, CYP3A, and UGT2B1 protein levels were markedly suppressed. No significant difference in the baseline level of Oatp1, Oatp2, Mrp2, Mrp3, Mdr1b, CYP1A, CYP2C, CYP3A, UGT1A1, UGT1A5, or UGT2B1 protein was found between the standard diet- and HF diet-fed groups. In addition, the mRNA levels of Oatp2, CYP2C11, and CYP3A1/2 were markedly decreased in HF diet-fed and fluvastatin-treated rats. There was no significant difference in the glucuronidation activities against fluvastatin among the four groups. In liver cell nuclei, levels of constitutive androstane receptor, pregnane X receptor, and hepatocyte nuclear factor 4α proteins were decreased in fluvastatin-treated HF diet-fed rats, which correlated with the decrease in Oatp2, CYP2C, and CYP3A. Taken together, these results indicate that nutritional status may influence adverse effects of fluvastatin by increasing systemic exposure through modulation of hepatic uptake and elimination.

Prenatal polycyclic aromatic hydrocarbon exposure leads to behavioral deficits and downregulation of receptor tyrosine kinase, MET

Gene by environment interactions (G × E) are thought to underlie neurodevelopmental disorder, etiology, neurodegenerative disorders, including the multiple forms of autism spectrum disorder. However, there is limited biological information, indicating an interaction between specific genes and environmental components. The present study focuses on a major component of airborne pollutants, polycyclic aromatic hydrocarbons (PAHs), such as benzo(a)pyrene [B(a)P], which negatively impacts cognitive development in children who have been exposed in utero. In our study, prenatal exposure of Cpr(lox/lox) timed-pregnant dams to B(a)P (0, 150, 300, and 600 μg/kg body weight via oral gavage) on embryonic day (E14-E17) consistent with our susceptibility-exposure paradigm was combined with the analysis of a replicated autism risk gene, the receptor tyrosine kinase, Met. The results demonstrate a dose-dependent increase in B(a)P metabolite generation in B(a)P-exposed Cpr(lox/lox) offspring. Additionally, a sustained persistence of hydroxy metabolites during the onset of synapse formation was noted, corresponding to the peak of Met expression. Prenatal B(a)P exposure also downregulated Met RNA and protein levels and dysregulated normal temporal patterns of expression during synaptogenesis. Consistent with these data, transcriptional cell-based assays demonstrated that B(a)P exposure directly reduces human MET promoter activity. Furthermore, a functional readout of in utero B(a)P exposure showed a robust reduction in novel object discrimination in B(a)P-exposed Cpr(lox/lox) offspring. These results confirm the notion that common pollutants, such as the PAH B(a)P, can have a direct negative impact on the regulated developmental expression of an autism risk gene with associated negative behavioral learning and memory outcomes.

Constitutive androstane receptor -null mice are sensitive to the toxic effects of parathion: association with reduced cytochrome p450-mediated parathion metabolism [corrected]

Constitutive androstane receptor [corrected](CAR) is activated by several chemicals and in turn regulates multiple detoxification genes. Our research demonstrates that parathion is one of the most potent, environmentally relevant CAR activators with an EC(50) of 1.43 microM. Therefore, animal studies were conducted to determine whether CAR was activated by parathion in vivo. Surprisingly, CAR-null mice, but not wild-type (WT) mice, showed significant parathion-induced toxicity. However, parathion did not induce Cyp2b expression, suggesting that parathion is not a CAR activator in vivo, presumably because of its short half-life. CAR expression is also associated with the expression of several drug-metabolizing cytochromes P450 (P450). CAR-null mice demonstrate lower expression of Cyp2b9, Cyp2b10, Cyp2c29, and Cyp3a11 primarily, but not exclusively in males. Therefore, we incubated microsomes from untreated WT and CAR-null mice with parathion in the presence of esterase inhibitors to determine whether CAR-null mice show perturbed P450-mediated parathion metabolism compared with that in WT mice. The metabolism of parathion to paraoxon and p-nitrophenol (PNP) was reduced in CAR-null mice with male CAR-null mice showing reduced production of both paraoxon and PNP, and female CAR-null mice showing reduced production of only PNP. Overall, the data indicate that CAR-null mice metabolize parathion slower than WT mice. These results provide a potential mechanism for increased sensitivity of individuals with lower CAR activity such as newborns to parathion and potentially other chemicals due to decreased metabolic capacity.

CCAAT/enhancer-binding protein alpha (C/EBPalpha) and hepatocyte nuclear factor 4alpha (HNF4alpha) synergistically cooperate with constitutive androstane receptor to transactivate the human cytochrome P450 2B6 (CYP2B6) gene: application to the development of a metabolically competent human hepatic cell model

The transcription of tissue-specific and inducible genes is usually subject to the dynamic control of multiple activators. Dedifferentiated hepatic cell lines lose the expression of tissue-specific activators and many characteristic hepatic genes, such as drug-metabolizing cytochrome P450. Here we demonstrate that by combining adenoviral vectors for CCAAT/enhancer-binding protein alpha (C/EBPalpha), hepatocyte nuclear factor 4alpha (HNF4alpha), and constitutive androstane receptor, the CYP2B6 expression and inducibility by CITCO are restored in human hepatoma HepG2 cells at levels similar to those in cultured human hepatocytes. Moreover, several other phase I and II genes are simultaneously activated, which suggests that this is an effective approach to endow dedifferentiated human hepatoma cells with a particular metabolic competence and response to inducers. In order to gain insight into the molecular mechanism, we examined the cooperation of these three transcription factors on the CYP2B6 5'-flanking region. We show new CYP2B6-responsive sequences for C/EBPalpha and HNF4alpha and a novel synergistic regulatory mechanism whereby C/EBPalpha, HNF4alpha, and constitutive androstane receptor bind and cooperate through proximal and distal response elements to confer a maximal level of expression. The results obtained from human liver also suggest that important differences in the expression and binding of C/EBPalpha and HNF4alpha could account for the large interindividual variability of the hepatic CYP2B6 enzyme, which metabolizes commonly used drugs.

Generation of a mouse model with a reversible hypomorphic cytochrome P450 reductase gene: utility for tissue-specific rescue of the reductase expression, and insights from a resultant mouse model with global suppression of P450 reductase expression in extrahepatic tissues

A mouse model termed Cpr-low (CL) was recently generated, in which the expression of the cytochrome P450 reductase (Cpr) gene was globally down-regulated. The decreased CPR expression was accompanied by phenotypical changes, including reduced embryonic survival, decreases in circulating cholesterol, increases in hepatic P450 expression, and female infertility (accompanied by elevated serum testosterone and progesterone levels). In the present study, a complementary mouse model [named reversible-CL (r-CL)] was generated, in which the reduced CPR expression can be reversed in an organ-specific fashion. The neo cassette, which was inserted into the last Cpr intron in r-CL mice, can be deleted by Cre recombinase, thus returning the structure of the Cpr gene (and hence CPR expression) to normal in Cre-expressing cells. All previously identified phenotypes of the CL mice were preserved in the r-CL mice. As a first application of the r-CL model, we have generated an extrahepatic-CL (xh-CL) mouse for testing of the functions of CPR-dependent enzymes in all extrahepatic tissues. The xh-CL mice, generated by mating of r-CL mice with albumin-Cre mice, had normal CPR expression in hepatocytes but down-regulated CPR expression elsewhere. They were indistinguishable from wild-type mice in body and liver weights, circulating cholesterol levels, and hepatic microsomal P450 expression and activities; however, they still showed elevated serum testosterone and progesterone levels and sterility in females. Embryonic lethality was prevented in males, but apparently not in females, indicating a critical role for fetal hepatic CPR-dependent enzymes in embryonic development, at least in males.

Hepatocyte nuclear factor 4alpha regulation of bile acid and drug metabolism

The hepatocyte nuclear factor 4alpha (HNF4alpha) is a liver-enriched nuclear receptor that plays a critical role in early morphogenesis, fetal liver development, liver differentiation and metabolism. Human HNF4alpha gene mutations cause maturity on-set diabetes of the young type 1, an autosomal dominant non-insulin-dependent diabetes mellitus. HNF4alpha is an orphan nuclear receptor because of which the endogenous ligand has not been firmly identified. The trans-activating activity of HNF4alpha is enhanced by interacting with co-activators and inhibited by corepressors. Recent studies have revealed that HNF4alpha plays a central role in regulation of bile acid metabolism in the liver. Bile acids are required for biliary excretion of cholesterol and metabolites, and intestinal absorption of fat, nutrients, drug and xenobiotics for transport and distribution to liver and other tissues. Bile acids are signaling molecules that activate nuclear receptors to control lipids and drug metabolism in the liver and intestine. Therefore, HNF4alpha plays a central role in coordinated regulation of bile acid and xenobiotics metabolism. Drugs that specifically activate HNF4alpha could be developed for treating metabolic diseases such as diabetes, dyslipidemia and cholestasis, as well as drug metabolism and detoxification.

Hepatocyte nuclear factor 4{alpha} regulates rifampicin-mediated induction of CYP2C genes in primary cultures of human hepatocytes

CYP2C enzymes are expressed constitutively and comprise approximately 20% of the total cytochrome P450 in human liver. However, the factors influencing the transcriptional regulation of the CYP2C subfamily have only been addressed recently. In the present study, we used primary cultures of human hepatocytes to investigate the role of HNF4alpha in the pregnane X receptor (PXR)/rifampicin-mediated up-regulation of CYP2C8, CYP2C9, and CYP2C19 gene expression. We first identified new proximal cis-acting HNF4alpha sites in the proximal CYP2C8 promoter [at -181 base pairs (bp) from the translation start site] and the CYP2C9 promoter (at -211 bp). Both sites bound HNF4alpha in gel shift assays. Thus, these and recent studies identified a total of three HNF4alpha sites in the CYP2C9 promoter and two in the CYP2C8 promoter. Mutational studies showed that the HNF4alpha sites are needed for up-regulation of the CYP2C8 and CYP2C9 promoters by rifampicin. Furthermore, silencing of HNF4alpha abolished transactivation of the CYP2C8 and CYP2C9 promoters by rifampicin. Constitutive promoter activity was also decreased. Quantitative polymerase chain reaction analysis demonstrated that silencing HNF4alpha reduced the constitutive expression of CYP2C8 (53%), CYP2C9 (55%), and CYP2C19 (43%) mRNAs and significantly decreased the magnitude of the rifampicin-mediated induction of CYP2C8 (6.6- versus 2.7-fold), CYP2C9 (3- versus 1.5-fold), and CYP2C19 (1.8- versus 1.1-fold). These results provide clear evidence that HNF4alpha contributes to the constitutive expression of the human CYP2C genes and is also important for up-regulation by the PXR agonist rifampicin.

Expressions of cytochrome P450, UDP-glucuronosyltranferase, and transporter genes in monolayer carcinoma cells change in subcutaneous tumors grown as xenografts in immunodeficient nude mice

Human tumors grown as xenografts in immunodeficient nude mice are widely used to investigate the pharmacological activities of anticancer drugs. Drug-metabolizing enzymes and transporters are expressed in tumor cell lines and changes in drug metabolism and pharmacokinetics (DMPK)-related gene expression after inoculation of the tumor cell may affect the pharmacological activity of the drug under consideration. The aims of the current study were to characterize DMPK-related gene expression profiles and responses to typical cytochrome P450 inducers in monolayer carcinoma cells grown in tissue culture versus those inoculated into a xenograft model. We used the human hepatocellular carcinoma cell line PLC/PRF/5 for this study and comprehensively assessed changes in DMPK-related gene expression by reverse transcription-polymerase chain reaction quantitation. CYP3A4 and UDP-glucuronosyltransferase 1A protein amounts were also analyzed by immunoprecipitation followed by immunoblotting. We found that the expression of many DMPK-related genes was elevated in the inoculated tumor compared with the monolayer carcinoma cells, indicating changes in their gene regulation pathways, presumably due to modulation of the nuclear receptor family of transcription factors. In addition, monolayer carcinoma versus inoculated tumor cells showed different responses to rifampicin, but similar responses to dexamethasone or 3-methylcholanthrene. These results suggest that inoculation of tumor cells results in the activation of drug metabolism and transport function, leading to changes in the responses to pregnane X receptor ligands and consequent discrepancies in the pharmacological activities between in vitro monolayer carcinoma cells and in vivo xenograft models.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Genetic predictors of interindividual variability in hepatic CYP3A4 expression

Variability in hepatic CYP3A4 cannot be explained by common CYP3A4 coding variants. We previously identified polymorphisms in pregnane X receptor (PXR) and ATP-binding cassette subfamily B member 1 (ABCB1) associated with CYP3A4 mRNA levels in small cohorts of human livers. However, the relative contributions of these genetic variations or of polymorphisms in other CYP3A4 regulators to variable CYP3A4 expression were not known. We phenotyped livers from white donors (n = 128) by quantitative real-time polymerase chain reaction for expression of CYP3A4, CYP3A5, and CYP3A7 and nine transcriptional regulators, coactivators, and corepressors. We resequenced hepatic nuclear factor-3-beta (HNF3beta, FoxA2), HNF4alpha, HNF3gamma (FoxA3), nuclear receptor corepressor 2 (NCoR2), and regions of the CYP3A4 promoter and genotyped informative single-nucleotide polymorphisms in PXR and ABCB1 in the same livers. CYP3A4 mRNA was positively correlated with PXR and FoxA2 and negatively correlated with NCoR2 mRNA. A common silent polymorphism and a polymorphic trinucleotide (CCT) repeat in FoxA2 were associated with CYP3A4 expression. The transcriptional activity of the FoxA2 polymorphic CCT repeat alleles (wild-type, n = 14 and variant, n = 13, 15, and 19) when assayed by luciferase reporter transactivation assays was greatest for the wild-type repeat, with deviations from this number having decreased transcriptional activity. This corresponded with higher expression of FoxA2 mRNA and its targets PXR and CYP3A4 in human livers with (CCT) n = 14 genotypes. Multiple linear regression analysis was used to quantify the contributions of selected genetic polymorphisms to variable CYP3A4 expression. This approach identified sex and polymorphisms in FoxA2, HNF4alpha, FoxA3, PXR, ABCB1, and the CYP3A4 promoter that together explained as much as 24.6% of the variation in hepatic CYP3A4 expression.

Estrogen receptor-dependent regulation of CYP2B6 in human breast cancer cells

Estrogen receptor alpha (ERalpha) mediates the biological actions of estrogens and also contributes to the development and progression of breast cancer. To gain a more comprehensive understanding of ERalpha-mediated transcription, we used chromatin immunoprecipitation and promoter focused microarrays (ChIP-chip) to identify ERalpha binding sites in T-47D human breast cancer cells. Transcription factor binding site analysis revealed that the estrogen response element (ERE) was significantly over-represented and was found in 50% of the 243 ERalpha-bound regions identified. Interestingly, multiple ERalpha-bound regions were detected in the upstream regulatory sequences of the CYP2B gene cluster. Because ERalpha has been reported to regulate the expression of other cytochrome P450 enzymes and CYP2B6 is highly expressed in ERalpha-positive breast tumors, we focused on characterizing the ERalpha-dependent regulation of CYP2B6. Reporter gene assays revealed that ERalpha and ERbeta increased CYP2B6-regulated gene expression through a functional ERE located at -1669 to -1657 in the upstream regulatory region of CYP2B6. E2 increased ERalpha and nuclear receptor coactivator 3 (NCoA3) recruitment to the 5'-flanking region of CYP2B6, and increased CYP2B6 mRNA levels in T-47D but not in MCF-7 human breast cancer cells. RNAi-mediated knockdown of ERalpha in the T-47D cells resulted in a significant decrease in CYP2B6 mRNA levels. Taken together, our study provides evidence for cell-type specific transcriptional regulation of the CYP2B6 gene by ERs.

Hepatic CYP2A6 levels and nicotine metabolism: impact of genetic, physiological, environmental, and epigenetic factors

PURPOSE

We investigated the role of genetic, physiological, environmental, and epigenetic factors in regulating CYP2A6 expression and nicotine metabolism.

METHODS

Human livers (n = 67) were genotyped for CYP2A6 alleles and assessed for nicotine metabolism and CYP2A6 expression (mRNA and protein). In addition, a subset of livers (n = 18), human cryopreserved hepatocytes (n = 2), and HepG2 cells were used for DNA methylation analyses.

RESULTS

Liver samples with variant CYP2A6 alleles had significantly lower CYP2A6 protein expression, nicotine C-oxidation activity, and affinity for nicotine. Female livers had significantly higher CYP2A6 protein and mRNA expression compared to male livers. Livers exposed to dexamethasone and phenobarbital had higher CYP2A6 expression and activity, however the difference was not statistically significant. Age and DNA methylation status of the CpG island and a regulatory site were not associated with altered CYP2A6.

CONCLUSIONS

We identified genotype, gender, and exposure to inducers as sources of variation in CYP2A6 expression and activity, but much variation remains to be accounted for.

Application of multivariate statistical procedures to identify transcription factors that correlate with MRP2, 3, and 4 mRNA in adult human livers

Multidrug resistance-associated proteins 2-4 (MRP2-4) are membrane efflux transporters critical for the hepatic clearance of pharmaceuticals and endogenous chemicals. Little is known about the constitutive regulation of MRP2-4 mRNA in normal human liver. The purpose of this study was to identify transcription factors whose expression significantly correlates with MRP2-4 mRNA in human liver specimens. Ninety adult human livers were profiled for mRNA expression of MRP2-4 as well as aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), pregnane X receptor (PXR), peroxisome proliferator-activated receptor alpha (PPAR alpha) and gamma (gamma), liver X receptor alpha (LXR alpha), farnesoid X receptor (FXR), glucocorticoid receptor (GR), retinoid X receptor alpha (RXR alpha), hepatocyte nuclear factor 1 alpha (HNF1 alpha) and 4 alpha (4 alpha), and nuclear factor E2-related factor 2 (Nrf2) transcription factors. Using linear regression and stepwise selection of partial R(2)-values, CAR, HNF1 alpha, and PPAR alpha mRNA exhibited the greatest correlation with MRP2, 3, and 4, respectively. Interindividual variation in the expression of the identified transcription factors may account for the variability in constitutive mRNA levels of MRP2-4. The multivariate approach presented in this study should aid in predicting signalling pathways that participate either directly or indirectly in regulating hepatic drug disposition.

The transcriptional regulation of the human CYP2C genes

In humans, four members of the CYP2C subfamily (CYP2C8, CYP2C9, CYP2C18, and CYP2C19) metabolize more than 20% of all therapeutic drugs as well as a number of endogenous compounds. The CYP2C enzymes are found predominantly in the liver, where they comprise approximately 20% of the total cytochrome P450. A variety of xenobiotics such as phenobarbital, rifampicin, and hyperforin have been shown to induce the transcriptional expression of CYP2C genes in primary human hepatocytes and to increase the metabolism of CYP2C substrates in vivo in man. This induction can result in drug-drug interactions, drug tolerance, and therapeutic failure. Several drug-activated nuclear receptors including CAR, PXR, VDR, and GR recognize drug responsive elements within the 5' flanking promoter region of CYP2C genes to mediate the transcriptional upregulation of these genes in response to xenobiotics and steroids. Other nuclear receptors and transcriptional factors including HNF4alpha, HNF3gamma, C/EBPalpha and more recently RORs, have been reported to regulate the constitutive expression of CYP2C genes in liver. The maximum transcriptional induction of CYP2C genes appears to be achieved through a coordinative cross-talk between drug responsive nuclear receptors, hepatic factors, and coactivators. The transcriptional regulatory mechanisms of the expression of CYP2C genes in extrahepatic tissues has received less study, but these may be altered by perturbations from pathological conditions such as ischemia as well as some of the receptors mentioned above.

Coordinate regulation of metabolic enzymes and transporters by nuclear transcription factors in human liver disease

BACKGROUND

It has been hypothesised, mainly from studies with animal models of liver disease, that the transport of substrates for metabolic enzymes and their subsequent metabolism and elimination in hepatic bile or blood is co-ordinated, but there is little information on this process in diseased human liver.

METHODS

In this study we have measured by reverse transcription polymerase chain reaction (RT-PCR) major genes involved in drug metabolism from UDP-glucuronosyltransferases (UGT1A1, UGT1A6, UGT1A9, and UGT2B4) and cytochrome P450 (CYP) families (CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4), transport (OATP-C, MRP2, MRP3, and MDR1) and major transcription factors (PXR, CAR, HNF1alpha, HNF4alpha, RXR, and AHR) involved in their regulation. Liver biopsy tissue from patients with viral hepatitis was scored for inflammation and fibrosis by the METAVIR system, and separated into groups with mild (A0-1; F0-1, n = 20) or severe (A2-3; F3-4, n = 19) liver disease. Correlation analysis (Spearman rank-test, P < 0.05) was used to identify metabolic enzymes and transporters which shared significant correlation with transcription factors.

RESULTS

Our results show an extensive correlation between transcription factors, transporters, and metabolic enzymes. An unexpected finding was that this was substantially greater in the severely diseased liver. Cross-talk between transcription factors was markedly increased in tissue from patients with severe liver disease, particularly between CAR, HNF4alpha, and PXR.

CONCLUSION

Our results support the hypothesis of co-ordinate regulation of metabolic enzymes and transporters in diseased human liver, as part of a widespread co-ordinated process under the control of nuclear receptor transcription factors.

Activation of CAR and PXR by Dietary, Environmental and Occupational Chemicals Alters Drug Metabolism, Intermediary Metabolism, and Cell Proliferation

The constitutive androstane receptor (CAR) and the pregnane × receptor (PXR) are activated by a variety of endogenous and exogenous ligands, such as steroid hormones, bile acids, pharmaceuticals, and environmental, dietary, and occupational chemicals. In turn, they induce phase I-III detoxification enzymes and transporters that help eliminate these chemicals. Because many of the chemicals that activate CAR and PXR are environmentally-relevant (dietary and anthropogenic), studies need to address whether these chemicals or mixtures of these chemicals may increase the susceptibility to adverse drug interactions. In addition, CAR and PXR are involved in hepatic proliferation, intermediary metabolism, and protection from cholestasis. Therefore, activation of CAR and PXR may have a wide variety of implications for personalized medicine through physiological effects on metabolism and cell proliferation; some beneficial and others adverse. Identifying the chemicals that activate these promiscuous nuclear receptors and understanding how these chemicals may act in concert will help us predict adverse drug reactions (ADRs), predict cholestasis and steatosis, and regulate intermediary metabolism. This review summarizes the available data on CAR and PXR, including the environmental chemicals that activate these receptors, the genes they control, and the physiological processes that are perturbed or depend on CAR and PXR action. This knowledge contributes to a foundation that will be necessary to discern interindividual differences in the downstream biological pathways regulated by these key nuclear receptors.

Molecular genetics of nicotine metabolism

The molecular genetics of nicotine metabolism involves multiple polymorphic catalytic enzymes. Variation in metabolic pathways results in nicotine disposition kinetics that differ between individuals and ethnic groups. Twin studies indicate that a large part of this variance is genetic in origin, although environmental influences also contribute. The primary aim of this chapter is to review the current knowledge regarding the genetic variability in the enzymes that metabolize nicotine in humans. The focus is on describing the genetic polymorphisms that exist in cytochromes P450 (CYPs), aldehyde oxidase 1 (AOX1), UDP-glucuronosyltransferases (UGTs), and flavin-containing monooxygenase 3 (FMO3). Genetic studies have demonstrated that polymorphisms in CYP2A6, the primary enzyme responsible for nicotine breakdown, make a sizable contribution to the wide range of nicotine metabolic capacity observed in humans. Thus, special attention will be given to CYP2A6, because slower nicotine metabolism requires less frequent self-administration, and accordingly influences smoking behaviors. In addition, the molecular genetics of nicotine metabolism in nonhuman primates, mice, and rats will be reviewed briefly.

Expression and inducibility of CYP1A1, 1A2, 1B1 by beta-naphthoflavone and CYP2B22, 3A22, 3A29, 3A46 by rifampicin in the respiratory and olfactory mucosa of pig

The presence and inducibility of specific CYPs (1A1, 1A2, 1B1, 2B22, 3A22, 3A29 and 3A46) and the related transcriptional factors (AhR, CAR, PXR, and HNF4alpha) were investigated, at activity and/or transcriptional level, in liver, respiratory and olfactory mucosa of control and beta-naphthoflavone (betaNF)-treated pigs an agonist of AhR, or rifampicin (RIF), an agonist of PXR. Experiments with real-time PCR showed that CYP1A1 mRNA was enhanced by betaNF, although at different extent, in liver, respiratory and olfactory tissues, whereas mRNAs of CYP1A2 and 1B1 were increased only in liver. Accordingly, in microsomes of both nasal tissues, the transcriptional activation of CYP1A1 was accompanied by an induction of ethoxyresorufin deethylase activity (a marker of this isoform) but not of methoxyresorufin demethylase activity (a marker of CYP1A2). The rifampicin treatment resulted in a transcriptional activation of CYP2B22 and CYP3As genes in liver but not in respiratory and olfactory mucosa. In parallel, the marker activity of CYP2B (ethoxy 4-(trifluoromethyl)coumarin deethylase) and CYP3As (6beta-testosterone hydroxylase and benzyloxyquinoline debenzylase) were induced in liver microsomes but not in the nasal ones. Considering the transcriptional factors, the basal expression of AhR mRNA was found to be as high in liver as in both nasal tissues but not susceptible to induction by betaNF. Also PXR mRNA was found, aside liver, well expressed in the nasal tissues, whereas CAR and HNF4alpha mRNAs were barely detected. In any case, these transcripts appeared to be enhanced by RIF treatment. Our results demonstrated that in the respiratory and olfactory mucosa of pig, although the presence of AhR, only CYP1A1, but not 1A2 and 1B1 resulted to be inducible by betaNF. Similarly, it was observed that in these nasal tissues, although the presence of PXR, neither CYP2B22 nor any CYP3A resulted to be inducible by RIF. Thus, the regulation mechanism of CYP1A2, 1B1, 2B22, 3A22, 3A29, and 3A46, in the nasal mucosa involves tissue-enriched transcriptional factors others than AhR, CAR, PXR, and HNF4alpha, which are fundamental in liver.