Role of hepatocyte nuclear factor 3γ in the expression of human CYP2C genes

Hepatic Forkhead Box Protein A3 Regulates ApoA-I (Apolipoprotein A-I) Expression, Cholesterol Efflux, and Atherogenesis

OBJECTIVE

To determine the role of hepatic FOXA3 (forkhead box A3) in lipid metabolism and atherosclerosis. Approach and Results: Hepatic FOXA3 expression was reduced in diabetic or high fat diet-fed mice or patients with nonalcoholic steatohepatitis. We then used adenoviruses to overexpress or knock down hepatic FOXA3 expression. Overexpression of FOXA3 in the liver increased hepatic ApoA-I (apolipoprotein A-I) expression, plasma HDL-C (high-density lipoprotein cholesterol) level, macrophage cholesterol efflux, and macrophage reverse cholesterol transport. In contrast, knockdown of hepatic FOXA3 expression had opposite effects. We further showed that FOXA3 directly bound to the promoter of the Apoa1 gene to regulate its transcription. Finally, AAV8 (adeno-associated virus serotype 8)-mediated overexpression of human FOXA3 in the hepatocytes of Apoe^-/- (apolipoprotein E-deficient) mice raised plasma HDL-C levels and significantly reduced atherosclerotic lesions.

CONCLUSIONS

Hepatocyte FOXA3 protects against atherosclerosis by inducing ApoA-I and macrophage reverse cholesterol transport.

Expression of cytochrome P450 mRNAs in Type II alveolar cells from subjects with chronic obstructive pulmonary disease

Inhaled drugs are critical for the treatment of inflammatory airway diseases such as chronic obstructive pulmonary disease (COPD). To develop better therapeutics for pulmonary disease it is of potential importance to understand molecular mechanisms of local biotransformation in the lung. Alveolar epithelial type II (ATII) cells have a key role in homeostasis in the lung, but little is known about expression patterns of genes encoding cytochrome P450 (CYP) enzymes in ATII cells. In addition, alteration of CYP gene expression has not been fully defined in COPD. We previously established a method to purify ATII cells from the adult human lung using fluorescence‐activated cell sorting. By employing this technique we determined gene expression patterns of 14 CYP enzymes in ATII cells from nonsmokers (n = 4) and smokers (n = 4), both having normal pulmonary function. Although most CYP genes are highly expressed in primary hepatocytes, we found that CYP1B1 mRNA expression was 7.2‐fold higher in ATII compared to hepatocytes (P = .0275). Additionally we noted a 3.0‐fold upregulation of CYP2C19 and 50% reduction in CYP2J2 mRNA expressions in ATII cells isolated from patients with COPD (n = 3) compared to smokers without COPD (n = 4). These data, for the first time, detail a comprehensive set of genes encoding CYP enzymes in human ATII cells and highlights differentially expressed CYP mRNAs of patients with COPD. Such understanding may have important implications for the development of novel inhaled drugs.

Biotechnology Challenges to In Vitro Maturation of Hepatic Stem Cells

The incidence of liver disease is increasing globally. The only curative therapy for severe end-stage liver disease, liver transplantation, is limited by the shortage of organ donors. In vitro models of liver physiology have been developed and new technologies and approaches are progressing rapidly. Stem cells might be used as a source of liver tissue for development of models, therapies, and tissue-engineering applications. However, we have been unable to generate and maintain stable and mature adult liver cells ex vivo. We review factors that promote hepatocyte differentiation and maturation, including growth factors, transcription factors, microRNAs, small molecules, and the microenvironment. We discuss how the hepatic circulation, microbiome, and nutrition affect liver function, and the criteria for considering cells derived from stem cells to be fully mature hepatocytes. We explain the challenges to cell transplantation and consider future technologies for use in hepatic stem cell maturation, including 3-dimensional biofabrication and genome modification.

Role of Cytochrome P450 2C8 in Drug Metabolism and Interactions

During the last 10-15 years, cytochrome P450 (CYP) 2C8 has emerged as an important drug-metabolizing enzyme. CYP2C8 is highly expressed in human liver and is known to metabolize more than 100 drugs. CYP2C8 substrate drugs include amodiaquine, cerivastatin, dasabuvir, enzalutamide, imatinib, loperamide, montelukast, paclitaxel, pioglitazone, repaglinide, and rosiglitazone, and the number is increasing. Similarly, many drugs have been identified as CYP2C8 inhibitors or inducers. In vivo, already a small dose of gemfibrozil, i.e., 10% of its therapeutic dose, is a strong, irreversible inhibitor of CYP2C8. Interestingly, recent findings indicate that the acyl-β-glucuronides of gemfibrozil and clopidogrel cause metabolism-dependent inactivation of CYP2C8, leading to a strong potential for drug interactions. Also several other glucuronide metabolites interact with CYP2C8 as substrates or inhibitors, suggesting that an interplay between CYP2C8 and glucuronides is common. Lack of fully selective and safe probe substrates, inhibitors, and inducers challenges execution and interpretation of drug-drug interaction studies in humans. Apart from drug-drug interactions, some CYP2C8 genetic variants are associated with altered CYP2C8 activity and exhibit significant interethnic frequency differences. Herein, we review the current knowledge on substrates, inhibitors, inducers, and pharmacogenetics of CYP2C8, as well as its role in clinically relevant drug interactions. In addition, implications for selection of CYP2C8 marker and perpetrator drugs to investigate CYP2C8-mediated drug metabolism and interactions in preclinical and clinical studies are discussed.

Novel Marmoset Cytochrome P450 2C19 in Livers Efficiently Metabolizes Human P450 2C9 and 2C19 Substrates, S-Warfarin, Tolbutamide, Flurbiprofen, and Omeprazole

The common marmoset (Callithrix jacchus), a small New World monkey, has the potential for use in human drug development due to its evolutionary closeness to humans. Four novel cDNAs, encoding cytochrome P450 (P450) 2C18, 2C19, 2C58, and 2C76, were cloned from marmoset livers to characterize P450 2C molecular properties, including previously reported P450 2C8. The deduced amino acid sequence showed high sequence identities (>86%) with those of human P450 2Cs, except for marmoset P450 2C76, which has a low sequence identity (∼70%) with any human P450 2Cs. Phylogenetic analysis showed that marmoset P450 2Cs were more closely clustered with those of humans and macaques than other species investigated. Quantitative polymerase chain reaction analysis showed that all of the marmoset P450 2C mRNAs were predominantly expressed in liver as opposed to the other tissues tested. Marmoset P450 2C proteins were detected in liver by immunoblotting using antibodies against human P450 2Cs. Among marmoset P450 2Cs heterologously expressed in Escherichia coli, marmoset P450 2C19 efficiently catalyzed human P450 2C substrates, S-warfarin, diclofenac, tolbutamide, flurbiprofen, and omeprazole. Marmoset P450 2C19 had high Vmax and low Km values for S-warfarin 7-hydroxylation that were comparable to those in human liver microsomes, indicating warfarin stereoselectivity similar to findings in humans. Faster in vivo S-warfarin clearance than R-warfarin after intravenous administration of racemic warfarin (0.2 mg/kg) to marmosets was consistent with the in vitro kinetic parameters. These results indicated that marmoset P450 2C enzymes had functional characteristics similar to those of humans, and that P450 2C-dependent metabolic properties are likewise similar between marmosets and humans.

Are pharmacogenomic biomarkers an effective tool to predict taxane toxicity and outcome in breast cancer patients? Literature review

PURPOSE

Breast cancer is a heterogeneous disease, characterized by various molecular phenotypes that correlate with different prognosis and response to treatments. Taxanes are some of the most active chemotherapeutic agents for breast cancer; however, their utilization is limited, due to hematologic and cumulative neurotoxicity on treated patients. To understand why only some patients experience severe adverse effects and why patients respond and develop resistance with different rates to taxane therapy, the metabolic pathways of these drugs should be completely unraveled. The variant forms of several genes, related to taxane pharmacokinetics, can be indicative markers of clinical parameters, such as toxicity or outcome.

METHODS

The search of the data has been conducted through PubMed database, presenting clinical data, clinical trials and basic research restricted to English language until June 2015.

RESULTS

We studied the literature in order to find any possible association between the major pharmacogenomic variants and specific taxane-related toxicity and patient outcome. We found that the data of these studies are sometimes discordant, due to both the small number of enrolled patients and the heterogeneity of the examined population.

CONCLUSIONS

Among all analyzed genes, only CYP1B1 and ABCB1 resulted the strongest candidates to become biomarkers of clinical response to taxane therapy in breast cancer, although their utilization still remains an experimental procedure. In the future, greater studies on genetic polymorphisms should be performed in order to identify differentiating signatures for patients with higher toxicity and with resistant or responsive outcome, before the administration of taxanes.

Unbalanced distribution of materials: the art of giving rise to hepatocytes from liver stem/progenitor cells

Liver stem/progenitor cells (LSPCs) are able to duplicate themselves and differentiate into each type of cells in the liver, including mature hepatocytes and cholangiocytes. Understanding how to accurately control the hepatic differentiation of LSPCs is a challenge in many fields from preclinical to clinical treatments. This review summarizes the recent advances made to control the hepatic differentiation of LSPCs over the last few decades. The hepatic differentiation of LSPCs is a gradual process consisting of three main steps: initiation, progression and accomplishment. The unbalanced distribution of the affecting materials in each step results in the hepatic maturation of LSPCs. As the innovative and creative works for generating hepatocytes with full functions from LSPCs are gradually accumulated, LSPC therapies will soon be a new choice for treating liver diseases.

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.

Cytochrome P450 regulation by α-tocopherol in Pxr-null and PXR-humanized mice

The pregnane X receptor (PXR) has been postulated to play a role in the metabolism of α-tocopherol owing to the up-regulation of hepatic cytochrome P450 (P450) 3A in human cell lines and murine models after α-tocopherol treatment. However, in vivo studies confirming the role of PXR in α-tocopherol metabolism in humans presents significant difficulties and has not been performed. PXR-humanized (hPXR), wild-type, and Pxr-null mouse models were used to determine whether α-tocopherol metabolism is influenced by species-specific differences in PXR function in vivo. No significant difference in the concentration of the major α-tocopherol metabolites was observed among the hPXR, wild-type, and Pxr-null mice through mass spectrometry-based metabolomics. Gene expression analysis revealed significantly increased expression of Cyp3a11 as well as several other P450s only in wild-type mice, suggesting species-specificity for α-tocopherol activation of PXR. Luciferase reporter assay confirmed activation of mouse PXR by α-tocopherol. Analysis of the Cyp2c family of genes revealed increased expression of Cyp2c29, Cyp2c37, and Cyp2c55 in wild-type, hPXR, and Pxr-null mice, which suggests PXR-independent induction of Cyp2c gene expression. This study revealed that α-tocopherol is a partial agonist of PXR and that PXR is necessary for Cyp3a induction by α-tocopherol. The implications of a novel role for α-tocopherol in Cyp2c gene regulation are also 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.

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.

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.

Nuclear receptors: the controlling force in drug metabolism of the liver?

The body is in a constant battle to achieve homeostasis; indeed, the robustness with which it can respond to moves away from homeostasis is a vital part in the survival of the organism as a whole. There thus exists a need for a network of sensors that are able to capture, interpret, and respond to alterations in chemical levels that move the body away from homeostasis and this applies to both endogenous and exogenous chemicals. With respect to external chemicals (xenobiotics), this xenosensing is often carried out through specific interactions with cellular receptors. The phenomenon of 'xenosensing' has attracted much interest of late, whereby xenobiotics interact with receptors resulting in the activation of a battery of genes mediating oxidative drug metabolism, conjugation, and transport, thereby enhancing the elimination of the xenobiotic by the organism. However, this beneficial response is counterbalanced by the increasingly recognized role of nuclear receptors in mediating drug-drug interactions via enzyme induction or the production of toxicity through interaction with endogenous pathways. This review will focus on the role of nuclear receptors in mediating these effects, and how such knowledge will contribute to a mechanism-based risk assessment for xenobiotics.

Histone deacetylase inhibitors valproate and trichostatin A are toxic to neuroblastoma cells and modulate cytochrome P450 1A1, 1B1 and 3A4 expression in these cells

Histone deacetylase inhibitors such as valproic acid (VPA) and trichostatin A (TSA) were shown to exert antitumor activity. Here, the toxicity of both drugs to human neuroblastoma cell lines was investigated using MTT test, and IC50 values for both compounds were determined. Another target of this work was to evaluate the effects of both drugs on expression of cytochrome P450 (CYP) 1A1, 1B1 and 3A4 enzymes, which are known to be expressed in neuroblastoma cells. A malignant subset of neuroblastoma cells, so-called N-type cells (UKF-NB-3 cells) and the more benign S-type neuroblastoma cells (UKF-NB-4 and SK-N-AS cell lines) were studied from both two points of view. VPA and TSA inhibited the growth of neuroblastoma cells in a dose-dependent manner. The IC₅₀ values ranging from 1.0 to 2.8 mM and from 69.8 to 129.4 nM were found for VPA and TSA, respectively. Of the neuroblastoma tested here, the N-type UKF-NB-3 cell line was the most sensitive to both drugs. The different effects of VPA and TSA were found on expression of CYP1A1, 1B1 and 3A4 enzymes in individual neuroblastoma cells tested in the study. Protein expression of all these CYP enzymes in the S-type SK-N-AS cell line was not influenced by either of studied drugs. On the contrary, in another S-type cell line, UKF-NB-4, VPA and TSA induced expression of CYP1A1, depressed levels of CYP1B1 and had no effect on expression levels of CYP3A4 enzyme. In the N-type UKF-NB-3 cell line, the expression of CYP1A1 was strongly induced, while that of CYP1B1 depressed by VPA and TSA. VPA also induced the expression of CYP3A4 in this neuroblastoma cell line.

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.

Role of epigenetics in liver-specific gene transcription, hepatocyte differentiation and stem cell reprogrammation

Controlling both growth and differentiation of stem cells and their differentiated somatic progeny is a challenge in numerous fields, from preclinical drug development to clinical therapy. Recently, new insights into the underlying molecular mechanisms have unveiled key regulatory roles of epigenetic marks driving cellular pluripotency, differentiation and self-renewal/proliferation. Indeed, the transcription of genes, governing cell-fate decisions during development and maintenance of a cell's differentiated status in adult life, critically depends on the chromatin accessibility of transcription factors to genomic regulatory and coding regions. In this review, we discuss the epigenetic control of (liver-specific) gene-transcription and the intricate interplay between chromatin modulation, including histone (de)acetylation and DNA (de)methylation, and liver-enriched transcription factors. Special attention is paid to their role in directing hepatic differentiation of primary hepatocytes and stem cells in vitro.

Identification of human CYP2C8 as a retinoid-related orphan nuclear receptor target gene

Retinoid-related orphan nuclear receptors (RORs) alpha and gamma (NR1F1, -3) are highly expressed in liver, adipose tissue, thymus, and brain and are involved in many physiological processes, such as circadian rhythm and immune function. Enzymes in the cytochrome P450 2C subfamily metabolize many clinically important drugs and endogenous compounds, such as the anticancer drug paclitaxel and arachidonic acid, and are highly expressed in liver. Here, we present the first evidence that RORs regulate the transcription of human CYP2C8. Overexpression of RORalpha and RORgamma in HepG2 cells significantly enhanced the activity of the CYP2C8 promoter but not that of the CYP2C9 or CYP2C19 promoters. Computer analyses, promoter deletion studies, gel shift assays, and mutational analysis identified an essential ROR-responsive element at -2045 base pairs in the CYP2C8 promoter that mediates ROR transactivation. Adenoviral overexpression of RORalpha and -gamma significantly induced endogenous CYP2C8 transcripts in both HepG2 cells and human primary hepatocytes. Knockdown of endogenous RORalpha and -gamma expression in HepG2 cells by RNA interference decreased the expression of endogenous CYP2C8 mRNA by approximately 50%. These data indicate that RORs transcriptionally up-regulate CYP2C8 in human liver and, therefore, may be important modulators of the metabolism of drugs and physiologically active endogenous compounds by this enzyme in liver and possibly extrahepatic tissues where RORs are expressed.

Identification of DNA regions and a set of transcriptional regulatory factors involved in transcriptional regulation of several human liver-enriched transcription factor genes

Mammalian tissue- and/or time-specific transcription is primarily regulated in a combinatorial fashion through interactions between a specific set of transcriptional regulatory factors (TRFs) and their cognate cis-regulatory elements located in the regulatory regions. In exploring the DNA regions and TRFs involved in combinatorial transcriptional regulation, we noted that individual knockdown of a set of human liver-enriched TRFs such as HNF1A, HNF3A, HNF3B, HNF3G and HNF4A resulted in perturbation of the expression of several single TRF genes, such as HNF1A, HNF3G and CEBPA genes. We thus searched the potential binding sites for these five TRFs in the highly conserved genomic regions around these three TRF genes and found several putative combinatorial regulatory regions. Chromatin immunoprecipitation analysis revealed that almost all of the putative regulatory DNA regions were bound by the TRFs as well as two coactivators (CBP and p300). The strong transcription-enhancing activity of the putative combinatorial regulatory region located downstream of the CEBPA gene was confirmed. EMSA demonstrated specific bindings of these HNFs to the target DNA region. Finally, co-transfection reporter assays with various combinations of expression vectors for these HNF genes demonstrated the transcriptional activation of the CEBPA gene in a combinatorial manner by these TRFs.

Primary hepatocytes: current understanding of the regulation of metabolic enzymes and transporter proteins, and pharmaceutical practice for the use of hepatocytes in metabolism, enzyme induction, transporter, clearance, and hepatotoxicity studies

This review brings you up-to-date with the hepatocyte research on: 1) in vitro-in vivo correlations of metabolism and clearance; 2) CYP enzyme induction, regulation, and cross-talk using human hepatocytes and hepatocyte-like cell lines; 3) the function and regulation of hepatic transporters and models used to elucidate their role in drug clearance; 4) mechanisms and examples of idiosyncratic and intrinsic hepatotoxicity; and 5) alternative cell systems to primary human hepatocytes. We also report pharmaceutical perspectives of these topics and compare methods and interpretations for the drug development process.

Differential role of epigenetic modulators in malignant and normal stem cells: a novel tool in preclinical in vitro toxicology and clinical therapy

Adult stem cells are primitive cells that undergo asymmetric division, thereby giving rise to one clonogenic, self-renewing cell and one cell able to undergo multipotent differentiation. Disturbance of this controlled process by epigenetic alterations, including imbalance of histone acetylation/histone deacetylation and DNA methylation/demethylation, may result in uncontrolled growth, formation of self-renewing malignant stem cells and eventually cancer. In view of this notion, several epigenetic modulators, in particular those with histone deacetylase inhibiting activity, are currently being tested in phase I and II clinical trials for their promising chemotherapeutic properties in cancer therapy. As chromatin modulation is also involved in regulation of differentiation, normal development, embryonic and adult stem cell functions and maintenance of their plasticity during embryonic organogenesis, the question can be raised whether predestined cell fate can be modified through epigenetic interference. And if so, could this strategy enforce adult stem cells to differentiate into different types of functional cells? In particular, functional hepatocytes seem important for preclinical toxicity screening of candidate drugs. This paper reviews the potential use and relevance of epigenetic modifiers, including inhibitors of histone deacetylases and DNA methyltransferases (1) to change cell fate and 'trans'differentiate normal adult stem cells into hepatocyte-like cells and (2) to cure disorders, caused by uncontrolled growth of malignant stem cells.

Hepatocyte cell lines: their use, scope and limitations in drug metabolism studies

Gaining knowledge on the metabolism of a drug, the enzymes involved and its inhibition or induction potential is a necessary step in pharmaceutical development of new compounds. Primary human hepatocytes are considered a cellular model of reference, as they express the majority of drug-metabolising enzymes, respond to enzyme inducers and are capable of generating in vitro a metabolic profile similar to what is found in vivo. However, hepatocytes show phenotypic instability and have a restricted accessibility. Different alternatives have been explored in the past recent years to overcome the limitations of primary hepatocytes. These include immortalisation of adult or fetal human hepatic cells by means of transforming tumour virus genes, oncogenes, conditionally immortalised hepatocytes, and cell fusion. New strategies are currently being used to upregulate the expression of drug-metabolising enzymes in cell lines or to derive hepatocytes from progenitor cells. This paper reviews the features of liver-derived cell lines, their suitability for drug metabolism studies as well as the state-of-the-art of the strategies pursued in order to generate metabolically competent hepatic cell lines.

Involvement of cell junctions in hepatocyte culture functionality

In liver, like in other multicellular systems, the establishment of cellular contacts is a prerequisite for normal functioning. In particular, well-defined cell junctions between hepatocytes, including adherens junctions, desmosomes, tight junctions, and gap junctions, are known to play key roles in the performance of liver-specific functionality. In a first part of this review article, we summarize the current knowledge concerning cell junctions and their roles in hepatic (patho)physiology. In a second part, we discuss their relevance in liver-based in vitro modeling, thereby highlighting the use of primary hepatocyte cultures as suitable in vitro models for preclinical pharmaco-toxicological testing. We further describe the actual strategies to regain and maintain cell junctions in these in vitro systems over the long-term.

Activation of human CYP2C9 promoter and regulation by CAR and PXR in mouse liver

The activity of various genomic segments at the 5'-flanking region of the human CYP2C9 gene in driving gene expression and their involvement in pregnane X receptor (PXR) and constitutive androstane receptor (CAR) mediated activation were evaluated in mouse hepatocytes. Using the genomic sequence of human CYP2C9 as a template, segments covering different regions of CYP2C9 5'-flanking sequences starting from the translation start site were amplified by PCR and inserted into a pGL-3 luciferase vector. Plasmid DNA containing the 0.2K, 1K, 2K, 3K, 5K, or 10K upstream sequences of the CYP2C9 gene were transfected into mouse liver by hydrodynamic delivery, and the activity of each fragment in driving reporter gene expression was assessed. With the exception of the 10K fragment, the level of luciferase activity in transfected mouse liver was similar among the constructs examined. Cotransfection of these reporter constructs with the pCMX-PXR or pCMX-CAR plasmids resulted in a slight increase in luciferase gene expression that could be significantly enhanced by chemical inducers. In mice cotransfected with pCMX-PXR, pregnenolone-16 alpha-carbonitrile (PCN) induced a 20-fold increase in the luciferase level compared to a 70-fold increase induced by rifampicin. Similarly, when animals were cotransfected with the pCMX-CAR plasmid, phenobarbital and 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene enhanced luciferase gene expression by 10- and 57-fold, respectively. The element responsible for PXR- and CAR-mediated activation of luciferase gene expression by chemical inducers was found to reside in the -2000 to -1000 bp region of the 5'-flanking sequence of the CYP2C9 gene. These results prove that PXR and CAR are transcription factors regulating CYP2C9 gene expression.

Transcriptional Activation of CYP2C9, CYP1A1, and CYP1A2 by Hepatocyte Nuclear Factor 4α Requires Coactivators Peroxisomal Proliferator Activated Receptor-γ Coactivator 1α and Steroid Receptor Coactivator 1

Hepatocyte nuclear factor 4alpha (HNF4alpha) is a key transcription factor for the constitutive expression of cytochromes P450 (P450s) in the liver. However, human hepatoma HepG2 cells show a high level of HNF4alpha but express only marginal P450 levels. We found that the HNF4alpha-mediated P450 transcription in HepG2 is impaired by the low level of coactivators peroxisomal proliferator activated receptor-gamma coactivator 1alpha (PGC1alpha) and steroid receptor coactivator 1 (SRC1). Reporter assays with a chimeric CYP2C9-LUC construct demonstrated that the sole transfection of coactivators induced luciferase activity in HepG2 cells. In HeLa cells however, CYP2C9-LUC activity only significantly increased when coactivators were cotransfected with HNF4alpha. A deletion mutant lacking the two proximal HNF4alpha binding sites in the CYP2C9 promoter did not respond to PGC1alpha or SRC1, demonstrating that coactivators were acting through HNF4alpha response elements. Adenovirus-mediated transfection of PGC1alpha in human hepatoma cells caused a significant dose-dependent increase in CYP2C9, CYP1A1, and CYP1A2 and in the positive control CYP7A1. PGC1alpha also showed a moderate activating effect on CYP3A4, CYP3A5, and CYP2D6. Adenoviral transfection of SRC1 had a lessened effect on P450 genes. Chromatin immunoprecipitation assay demonstrated in vivo binding of HNF4alpha and PGC1alpha to HNF4alpha response sequences in the CYP2C9 promoter and to three new regulatory regions in the common 23.3 kilobase spacer sequence of the CYP1A1/2 cluster. Insulin treatment of HepG2 and human hepatocytes caused repression of PGC1alpha and a concomitant down-regulation of P450s. Our results establish the importance of coactivators PGC1alpha and SRC1 for the hepatic expression of human P450s and uncover a new HNF4alpha-dependent regulatory mechanism to constitutively control the CYP1A1/2 cluster.

INVOLVEMENT OF HEPATOCYTE NUCLEAR FACTOR 4α IN THE DIFFERENT EXPRESSION LEVEL BETWEEN CYP2C9 AND CYP2C19 IN THE HUMAN LIVER

CYP2C9 and CYP2C19 are clinically important drug-metabolizing enzymes. The expression level of CYP2C9 is much higher than that of CYP2C19, although the factor(s) responsible for the difference between the expression levels of these genes is still unclear. It has been reported that hepatocyte nuclear factor 4alpha (HNF4alpha) plays an important role in regulation of the expression of liver-enriched genes, including P450 genes. Thus, we hypothesized that HNF4alpha contributes to the difference between the expression levels of these genes. Two direct repeat 1 (DR1) elements were located in both the CYP2C9 and CYP2C19 promoters. The upstream and downstream elements in these promoters had the same sequences, and HNF4alpha could bind to both elements in vitro. The transactivation levels of constructs containing two DR1 elements of the CYP2C9 promoter were increased by HNF4alpha, whereas those of the CYP2C19 promoter were not increased. The introduction of mutations into either the upstream or downstream element in the CYP2C9 gene abolished the responsiveness to HNF4alpha. We also examined whether HNF4alpha could bind to the promoter regions of the CYP2C9 and the CYP2C19 genes in vivo. The results of chromatin immunoprecipitation assays showed that HNF4alpha could bind to the promoter region of the CYP2C9 gene but not to that of the CYP2C19 promoter in the human liver. Taken together, our results suggest that HNF4alpha is a factor responsible for the difference between the expression levels of CYP2C9 and CYP2C19 in the human liver.

Human CYP2C8 Is Transcriptionally Regulated by the Nuclear Receptors Constitutive Androstane Receptor, Pregnane X Receptor, Glucocorticoid Receptor, and Hepatic Nuclear Factor 4α

Cytochrome P450 (P450) enzymes play important roles in the metabolism of endogenous and xenobiotic substrates in humans. CYP2C8 is an important member of the CYP2C subfamily, which metabolizes both endogenous compounds (i.e., arachidonic acids and retinoic acid) and xenobiotics (e.g., paclitaxel). Induction of P450 enzymes by drugs can result in tolerance as well as drug-drug interactions. CYP2C8 is the most strongly inducible member of the CYP2C subfamily in human hepatocytes, but the mechanism of induction by xenobiotics has not been delineated. To determine the mechanisms controlling the regulation of this important P450, we cloned the 5'-flanking region of CYP2C8 and investigated its transcriptional regulation by nuclear factors such as the pregnane X receptor (PXR), constitutive androstane receptor (CAR), glucocorticoid receptor (GR), and hepatic nuclear factor 4 (HNF4alpha) that are known to be involved in the induction of other P450 enzymes using both cell lines and primary hepatocyte models. We initially identified a distal PXR/CAR-binding site in the CYP2C8 promoter that confers inducibility of CYP2C8 via the PXR agonist/ligand rifampicin and the CAR agonist/ligand CITCO [6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime]. A glucocorticoid-responsive element was identified that mediates dexamethasone induction via the GR. We finally identified an HNF4alpha-binding site within the CYP2C8 basal promoter region that is cis-activated by cotransfected HNF4alpha. In summary, the present studies show that CAR, PXR, GR, and HNF4alpha can regulate CYP2C8 expression and identify specific cis-elements within the promoter that control these regulatory pathways.

The nuclear receptors constitutive androstane receptor and pregnane X receptor cross-talk with hepatic nuclear factor 4alpha to synergistically activate the human CYP2C9 promoter

CYP2C9 is an important human drug-metabolizing enzyme that is expressed primarily in liver. Recent studies in our laboratory have shown that the nuclear receptor pregnane X receptor (PXR) is important in the transcriptional activation of the CYP2C9 promoter by drugs such as rifampicin and that the essential element is a constitutive androstane receptor (CAR)/PXR site -1839 bp upstream of the translation start site. Both CAR and PXR transcriptionally up-regulate the CYP2C9 promoter via these elements. In the present study, we ask whether additional sites in the proximal promoter also play a role in this induction. We identify two proximal hepatic nuclear factor (HNF) 4alpha binding sites at -152 and -185 bp of the CYP2C9 promoter, both of which bind HNF4alpha in gel shift assays and transcriptionally up-regulate this promoter in response to HNF4alpha in HepG2 cells. HNF4alpha synergizes with CAR and with PXR in HepG2 cells treated with rifampicin. The synergy only occurs when the CAR/PXR binding site at -1839 bp is present. Mutation of the two HNF4alpha binding sites differentially prevented up-regulation of CYP2C9 promoter by both CAR as well as HNF4alpha, synergy between the two receptors, and essentially abolished induction by rifampicin in HepG2 cells transfected with PXR. These studies strongly support the hypothesis that there is cross talk between distal CAR/PXR sites and HNF4alpha binding sites in the CYP2C9 promoter and that the HNF4alpha sites are required for maximal induction of the CYP2C9 promoter.

Drug metabolism by cultured human hepatocytes: how far are we from the in vivo reality?

The investigation of metabolism is an important milestone in the course of drug development. Drug metabolism is a determinant of drug pharmacokinetics variability in human beings. Fundamental to this are phenotypic differences, as well as genotypic differences, in the expression of the enzymes involved in drug metabolism. Genotypic variability is easy to identify by means of polymerase chain reaction-based or DNA chip-based methods, whereas phenotypic variability requires direct measurement of enzyme activities in liver, or, indirectly, measurement of the rate of metabolism of a given compound in vivo. There is a great deal of phenotypic variability in human beings, only a minor part being attributable to gene polymorphisms. Thus, enzyme activity measurements in a series of human livers, as well as in vivo studies with human volunteers, show that phenotypic variability is, by far, much greater than genotypic variability. In vitro models are currently used to investigate the hepatic metabolism of new compounds. Cultured human hepatocytes are considered to be the closest model to the human liver. However, the fact that hepatocytes are placed in a microenvironment that differs from that of the cells in the liver raises the question of to what extent drug metabolism variability observed in vitro actually reflects that in the liver in vivo. This issue has been examined by investigating the metabolism of the model compound, aceclofenac (an approved analgesic/anti-inflammatory drug), both in vitro and in vivo. Hepatocytes isolated from programmed liver biopsies were incubated with aceclofenac, and the metabolites formed were investigated by HPLC. The patients were given the drug during the course of clinical recovery, and the metabolites, largely present in urine, were analysed. In vitro and in vivo data from the same individual were compared. There was a good correlation between the in vitro and in vivo relative abundance of oxidised metabolites (4'-OH-aceclofenac + 4'-OH-diclofenac; Spearman's rho = 0.855), and the hydrolysis of aceclofenac (diclofenac + 4'-OH-aceclofenac + 4'-OH-diclofenac; rho = 0.691), while the conjugation of the drug in vitro was somewhat lower than in vivo. Globally, the metabolism of aceclofenac in vitro correlated with the amount of metabolites excreted in urine after 16 hours (rho = 0.95). Overall, although differing among assays, the in vitro/in vivo metabolism data for each patient were surprisingly similar. Thus, the variability observed in vitro appears to reflect genuine phenotypic variability among the donors.