Differential regulation of constitutive androstane receptor expression by hepatocyte nuclear factor4alpha isoforms

Nuclear Receptor-Mediated Hepatomegaly and Liver Regeneration: An Update

Nuclear receptors (NRs), a superfamily of ligand-activated transcription factors, are critical in cell growth, proliferation, differentiation, metabolism, and numerous biologic events. NRs have been reported to play important roles in hepatomegaly (liver enlargement) and liver regeneration by regulating target genes or interacting with other signals. In this review, the roles and involved molecular mechanisms of NRs in hepatomegaly and liver regeneration are summarized and the future perspectives of NRs in the treatment of liver diseases are discussed. SIGNIFICANCE STATEMENT: NRs play critical roles in hepatomegaly and liver regeneration, indicating the potential of NRs as targets to promote liver repair after liver injury. This paper reviews the characteristics and molecular mechanisms of NRs in regulating hepatomegaly and liver regeneration, providing more evidence for NRs in the treatment of related liver diseases.

Regulation of CAR and PXR Expression in Health and Disease

Pregnane X receptor (PXR, NR1I2) and constitutive androstane receptor (CAR, NR1I3) are members of the nuclear receptor superfamily that mainly act as ligand-activated transcription factors. Their functions have long been associated with the regulation of drug metabolism and disposition, and it is now well established that they are implicated in physiological and pathological conditions. Considerable efforts have been made to understand the regulation of their activity by their cognate ligand; however, additional regulatory mechanisms, among which the regulation of their expression, modulate their pleiotropic effects. This review summarizes the current knowledge on CAR and PXR expression during development and adult life; tissue distribution; spatial, temporal, and metabolic regulations; as well as in pathological situations, including chronic diseases and cancers. The expression of CAR and PXR is modulated by complex regulatory mechanisms that involve the interplay of transcription factors and also post-transcriptional and epigenetic modifications. Moreover, many environmental stimuli affect CAR and PXR expression through mechanisms that have not been elucidated.

Higd1a Protects Cells from Lipotoxicity under High-Fat Exposure

Hypoxia-inducible gene domain family member 1A (Higd1a) has recently been reported to protect cells from hypoxia by helping to maintain normal mitochondrial function. The potential induction of Higd1a under high-fat exposure and whether it could protect cells from oxidative stress attracted our attention. Initially, 0.4 mM oleic acid and 0.2 mM palmitate were added to the growth media of HepG2 and LO2 cells for 72 hours. We discovered increased Higd1a expression, and knocking down Higd1a impaired mitochondrial transmembrane potential and induced cell apoptosis. We then identified that elevated reactive oxygen species (ROS) is responsible for increased Higd1a expression. Furthermore, we found that ROS promoted Higd1a expression by upregulating HIF-1a and PGC-1a expressions, and these two proteins could exert synergistic effects in inducing Higd1a expression. Taken together, these data suggest that Higd1a plays positive roles in protecting cells from oxidative stress, and ROS could induce Higd1a expression by upregulating PGC-1a and HIF-1a expressions.

Hepatic stem cells with self-renewal and liver repopulation potential are harbored in CDCP1-positive subpopulations of human fetal liver cells

BACKGROUND

Mature human hepatocytes are critical in preclinical research and therapy for liver disease, but are difficult to manipulate and expand in vitro. Hepatic stem cells (HpSCs) may be an alternative source of functional hepatocytes for cell therapy and disease modeling. Since these cells play an import role in regenerative medicine, the precise characterization that determines specific markers used to isolate these cells as well as whether they contribute to liver regeneration still remain to be shown.

METHOD

In this study, human HpSCs were isolated from human primary fetal liver cells (FLCs) by flow cytometry using CDCP1, CD90, and CD66 antibodies. The isolated CDCP1+CD90+CD66- HpSCs were cultured on dishes coated with type IV collagen in DMEM nutrient mixture F-12 Ham supplemented with FBS, human γ-insulin, nicotinamide, dexamethasone, and L-glutamine for at least 2 weeks, and were characterized by transcriptomic profiling, quantitative real-time PCR, immunocytochemistry, and in-vivo transplantation.

RESULTS

The purified CDCP1+CD90+CD66- subpopulation exhibited clonal expansion and self-renewal capability, and bipotential capacity was further identified in single cell-derived colonies containing distinct hepatocytes and cholangiocytes. Moreover, in-vivo liver repopulation assays demonstrated that human CDCP1+CD90+CD66- HpSCs repopulated over 90% of the mouse liver and differentiated into functional hepatocytes with drug metabolism activity.

CONCLUSIONS

We identified a human hepatic stem/progenitor population in the CDCP1+CD90+CD66- subpopulation in human FLCs, indicating CDCP1 marker could potentially be utilized to identify and isolate HpSCs for further cytotherapy of liver disease.

Molecular Basis of Metabolism-Mediated Conversion of PK11195 from an Antagonist to an Agonist of the Constitutive Androstane Receptor

The constitutive androstane receptor (CAR) plays an important role in xenobiotic metabolism, energy homeostasis, and cell proliferation. Antagonism of the CAR represents a key strategy for studying its function and may have potential clinical applications. However, specific human CAR (hCAR) antagonists are limited and conflicting data on the activity of these compounds have been reported. 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide (PK11195), a typical peripheral benzodiazepine receptor ligand, has been established as a potent hCAR deactivator in immortalized cells; whether it inhibits hCAR activity under physiologically relevant conditions remains unclear. Here, we investigated the effects of PK11195 on hCAR in metabolically competent human primary hepatocytes (HPH) and HepaRG cells. We show that although PK11195 antagonizes hCAR in HepG2 cells, it induces the expression of CYP2B6 and CYP3A4, targets of hCAR and the pregnane X receptor (PXR), in HPH, HepaRG, and PXR-knockout HepaRG cells. Utilizing a HPH-HepG2 coculture model, we demonstrate that inclusion of HPH converts PK11195 from an antagonist to an agonist of hCAR, and such conversion was attenuated by potent CYP3A4 inhibitor ketoconazole. Metabolically, we show that the N-desmethyl metabolite is responsible for PK11195-mediated hCAR activation by facilitating hCAR interaction with coactivators and enhancing hCAR nuclear translocation in HPHs. Structure-activity analysis revealed that N-demethylation alters the interaction of PK11195 with the binding pocket of hCAR to favor activation. Together, these results indicate that removal of a methyl group switches PK11195 from a potent antagonist of hCAR to an agonist in HPH and highlights the importance of physiologically relevant metabolism when attempting to define the biologic action of small molecules.

Nuclear Receptors as Therapeutic Targets in Liver Disease: Are We There Yet?

Nuclear receptors (NR) are ligand-modulated transcription factors that play diverse roles in cell differentiation, development, proliferation, and metabolism and are associated with numerous liver pathologies such as cancer, steatosis, inflammation, fibrosis, cholestasis, and xenobiotic/drug-induced liver injury. The network of target proteins associated with NRs is extremely complex, comprising coregulators, small noncoding microRNAs, and long noncoding RNAs. The importance of NRs as targets of liver disease is exemplified by the number of NR ligands that are currently used in the clinics or in clinical trials with promising results. Understanding the regulation by NR during pathophysiological conditions, and identifying ligands for orphan NR, points to a potential therapeutic approach for patients with liver diseases. An overview of complex NR metabolic networks and their pharmacological implications in liver disease is presented here.

Correlation between Conjugated Bisphenol A Concentrations and Efflux Transporter Expression in Human Fetal Livers

Because of its widespread use in the manufacturing of consumer products over several decades, human exposure to bisphenol A (BPA) has been pervasive. Fetuses are particularly sensitive to BPA exposure, with a number of negative developmental and reproductive outcomes observed in rodent perinatal models. Xenobiotic transporters are one mechanism to extrude conjugated and unconjugated BPA from the liver. In this study, the mRNA expression of xenobiotic transporters and relationships with total, conjugated, and free BPA levels were explored utilizing human fetal liver samples. The mRNA expression of breast cancer resistance protein (BCRP) and multidrug resistance-associated transporter (MRP)4, as well as BCRP and multidrug resistance transporter 1 exhibited the highest degree of correlation, with r(2) values of 0.941 and 0.816 (P < 0.001 for both), respectively. Increasing concentrations of conjugated BPA significantly correlated with high expression of MRP1 (P < 0.001), MRP2 (P < 0.05), and MRP3 (P < 0.05) transporters, in addition to the NF-E2-related factor 2 transcription factor (P < 0.001) and its prototypical target gene, NAD(P)H quinone oxidoreductase 1 (P < 0.001). These data demonstrate that xenobiotic transporters may be coordinately expressed in the human fetal liver. This is also the first report of a relationship between environmentally relevant fetal BPA levels and differences in the expression of transporters that can excrete the parent compound and its metabolites.

Human induced pluripotent stem cells in hepatology: beyond the proof of concept

The discovery of the wide plasticity of most cell types means that it is now possible to produce virtually any cell type in vitro. This concept, developed because of the possibility of reprogramming somatic cells toward induced pluripotent stem cells, provides the opportunity to produce specialized cells that harbor multiple phenotypical traits, thus integrating genetic interindividual variability. The field of hepatology has exploited this concept, and hepatocyte-like cells can now be differentiated from induced pluripotent stem cells. This review discusses the choice of somatic cells to be reprogrammed by emergent new and nonintegrative strategies, as well as the application of differentiated human induced pluripotent stem cells in hepatology, including liver development, disease modeling, host-pathogen interactions, and drug metabolism and toxicity. The actual consensus is that hepatocyte-like cells generated in vitro present an immature phenotype. Currently, developed strategies used to resolve this problem, such as overexpression of transcription factors, mimicking liver neonatal and postnatal modifications, and re-creating the three-dimensional hepatocyte environment in vitro and in vivo, are also discussed.

The human constitutive androstane receptor promotes the differentiation and maturation of hepatic-like cells

Expression of the constitutive androstane receptor (CAR, NR1I3) is enriched in the mature mammalian liver and increasingly recognized for its prominent role in regulating a myriad of processes including biotransformation, chemical transport, energy metabolism and lipid homeostasis. Previously, we demonstrated that CAR levels were markedly enhanced during the differentiation of hepatic-like cells derived from hESCs, prompting the hypothesis that CAR contributes a key functional role in directing human hepatogenesis. Here we demonstrate that over-expression of CAR in human embryonic stem cells (ESCs), transduced by a lentiviral vector, accelerates the maturation of hepatic-like cells, with CAR over-expressing cells exhibiting a 2.5-fold increase in albumin secretion by day 20 in culture differentiation, and significantly enhanced levels of mRNA expression of several liver-selective markers, including hepatic transcription factors, plasma proteins, biotransformation enzymes, and metabolic enzymes. CAR over-expressing cells also exhibited enhanced CITCO-inducible CYP3A7 enzymatic activity. Knockdown of CAR via siRNA attenuated the differentiation-dependent expression programs. In contrast, expression levels of the pregnane X receptor (PXR), a nuclear receptor most similar to CAR in primary sequence, were negligible in human fetal liver tissues or in the differentiating hESCs, and stable over-expression of PXR in hepatic-induced hESCs failed to enhance expression of hepatic phenotype markers. Together, these results define a novel role for human CAR in hepatic lineage commitment.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Nuclear receptors in the cross-talk of drug metabolism and inflammation

Inflammation and infection have long been known to affect the activity and expression of enzymes involved in hepatic and extrahepatic drug clearance. Significant advances have been made to elucidate the molecular mechanisms underlying the complex cross-talk between inflammation and drug-metabolism alterations. The emergent role of ligand-activated transcriptional regulators, belonging to the nuclear receptor (NR) superfamily, is now well established. The NRs, pregnane X receptor, constitutive androstane receptor, retinoic X receptor, glucocorticoid receptor, and hepatocyte nuclear factor 4, and the basic helix-loop-helix/Per-ARNT-Sim family member, aryl hydrocarbon receptor, are the main regulators of the detoxification function. According to the panel of mediators secreted during inflammation, a cascade of numerous signaling pathways is activated, including nuclear factor kappa B, mitogen-activated protein kinase, and the Janus kinase/signal transducer and activator of transcription pathways. Complex cross-talk is established between these signaling pathways regulating either constitutive or induced gene expression. In most cases, a mutual antagonism between xenosensor and inflammation signaling occurs. This review focuses on the molecular and cellular mechanisms implicated in this cross-talk.

Nuclear receptors in regenerating liver and hepatocellular carcinoma

A comprehensive understanding of the pathways underlying hepatocyte turnover and liver regeneration is essential for the development of innovative and effective therapies in the management of chronic liver disease, and the prevention of hepatocellular carcinoma (HCC) in cirrhosis. Nuclear receptors (NRs) are master transcriptional regulators of liver development, differentiation and function. NRs have been implicated in the modulation of hepatocyte priming and proliferation in regenerating liver, chronic hepatitis and HCC development. In this review, we focus on NRs and their pathways regulating hepatocyte proliferation and liver regeneration, with a perspective view on NRs as candidate biomarkers and novel pharmacological targets in the management of liver disease and HCC.

Regulation of the human hydroxysteroid sulfotransferase (SULT2A1) by RORα and RORγ and its potential relevance to human liver diseases

The retinoid-related orphan receptors (RORs) were postulated to have functions in tissue development and circadian rhythm. In this study, we revealed a novel function of RORα (NR1F1) and RORγ (NR1F3) in regulating the human hydroxysteroid sulfotransferase (SULT2A1), a phase II conjugating enzyme known to sulfonate bile acids, hydroxysteroid dehydroepiandrosterone, and related androgens. A combination of promoter reporter gene assay and EMSA and chromatin immunoprecipitation (ChIP) assays showed that both RORα and RORγ transactivated the SULT2A1 gene promoter through their binding to a ROR response element found in the SULT2A1 gene promoter. Interestingly, this ROR response element overlaps with a previously reported constitutive androstane receptor response element on the same promoter. Down-regulation of RORα and/or RORγ by small interfering RNA inhibited the expression of endogenous SULT2A1. In primary human hepatocytes and human livers, we found a positive correlation between the expression of SULT2A1 and RORs, which further supported the regulation of SULT2A1 by RORs. We also found that the expression of RORα and RORγ was impaired in several liver disease conditions, such as steatosis/steatohepatitis, fibrosis, and hepatocellular carcinoma. The positive regulation of human SULT2A1 by RORs is opposite to the negative regulation of Sult2a1 by RORs in rodents. In summary, our results established SULT2A1 as a novel ROR target gene. The expression of RORs is a potential predictor for the expression of SULT2A1 as well as disease conditions.

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.

Gene regulation is governed by a core network in hepatocellular carcinoma

Background

Hepatocellular carcinoma (HCC) is one of the most lethal cancers worldwide, and the mechanisms that lead to the disease are still relatively unclear. However, with the development of high-throughput technologies it is possible to gain a systematic view of biological systems to enhance the understanding of the roles of genes associated with HCC. Thus, analysis of the mechanism of molecule interactions in the context of gene regulatory networks can reveal specific sub-networks that lead to the development of HCC.

Results

In this study, we aimed to identify the most important gene regulations that are dysfunctional in HCC generation. Our method for constructing gene regulatory network is based on predicted target interactions, experimentally-supported interactions, and co-expression model. Regulators in the network included both transcription factors and microRNAs to provide a complete view of gene regulation. Analysis of gene regulatory network revealed that gene regulation in HCC is highly modular, in which different sets of regulators take charge of specific biological processes. We found that microRNAs mainly control biological functions related to mitochondria and oxidative reduction, while transcription factors control immune responses, extracellular activity and the cell cycle. On the higher level of gene regulation, there exists a core network that organizes regulations between different modules and maintains the robustness of the whole network. There is direct experimental evidence for most of the regulators in the core gene regulatory network relating to HCC. We infer it is the central controller of gene regulation. Finally, we explored the influence of the core gene regulatory network on biological pathways.

Conclusions

Our analysis provides insights into the mechanism of transcriptional and post-transcriptional control in HCC. In particular, we highlight the importance of the core gene regulatory network; we propose that it is highly related to HCC and we believe further experimental validation is worthwhile.

The effects of rosuvastatin and the CYP51A1 inhibitor LEK-935 on the proteome of primary human hepatocytes

Elevated amounts of cholesterol are thought to be involved in several severe diseases. Despite the fact that many studies have been performed and published, the action of cholesterol-lowering agents used to diminish the plasma cholesterol level is not fully understood yet. In this study, the effects of the HMG-CoA reductase inhibitor rosuvastatin and the new CYP51A1 inhibitor 2-((3,4-dichlorophenethyl)(propyl)amino)-1-(pyridin-3-yl)ethanol (LEK-935) on the proteome of primary human hepatocytes were analyzed for the first time. To get an idea about interindividual differences, two different human donors were used. The cytosolic and microsomal fractions of the cells were analyzed in a semiquantitative manner by two-dimensional-polyacrylamide gel electrophoresis and capillary high-performance liquid chromatography-mass spectrometry, respectively. Thereby, a set of 44 proteins was found to be differentially presented. The chosen experimental set-up was validated by proteins already known to be affected by statins and involved in the cholesterol biosynthesis. Other proteins found to be regulated cannot be directly related to cholesterol metabolism and have not been described to be affected by cholesterol-lowering agents so far. Some of these proteins may represent interesting targets for further investigations into the analysis of severe side-effects as well as pleiotropic effects of the statins. During the proteome analysis of the two different donors, interindividual differences were observed that were validated by real-time reverse transcription-polymerase chain reaction measurements. Thus, new information and a deeper insight into the processes taking place inside cells treated with cholesterol-lowering agents can be drawn from this study.

Comparison of hepatic-like cell production from human embryonic stem cells and adult liver progenitor cells: CAR transduction activates a battery of detoxification genes

In vitro production of human hepatocytes is of primary importance in basic research, pharmacotoxicology and biotherapy of liver diseases. We have developed a protocol of differentiation of human embryonic stem cells (ES) towards hepatocyte-like cells (ES-Hep). Using a set of human adult markers including CAAT/enhancer binding protein (C/EBPalpha), hepatocyte nuclear factor 4/7 ratio (HNF4alpha1/HNF4alpha7), cytochrome P450 7A1 (CYP7A1), CYP3A4 and constitutive androstane receptor (CAR), and fetal markers including alpha-fetoprotein, CYP3A7 and glutathione S-transferase P1, we analyzed the expression of a panel of 41 genes in ES-Hep comparatively with human adult primary hepatocytes, adult and fetal liver. The data revealed that after 21 days of differentiation, ES-Hep are representative of fetal hepatocytes at less than 20 weeks of gestation. The glucocorticoid receptor pathway was functional in ES-Hep. Extending protocols of differentiation to 4 weeks did not improve cell maturation. When compared with hepatocyte-like cells derived from adult liver non parenchymal epithelial (NPE) cells (NPE-Hep), ES-Hep expressed several adult and fetal liver makers at much greater levels (at least one order of magnitude), consistent with greater expression of liver-enriched transcription factors Forkhead box A2, C/EBPalpha, HNF4alpha and HNF6. It therefore seems that ES-Hep reach a better level of differentiation than NPE-Hep and that these cells use different lineage pathways towards the hepatic phenotype. Finally we showed that lentivirus-mediated expression of xenoreceptor CAR in ES-Hep induced the expression of several detoxification genes including CYP2B6, CYP2C9, CYP3A4, UDP-glycosyltransferase 1A1, solute carriers 21A6, as well as biotransformation of midazolam, a CYP3A4-specific substrate.

Modular bioreactor for primary human hepatocyte culture: medium flow stimulates expression and activity of detoxification genes

Down-regulation of detoxification genes, notably cytochrome P450 (CYPs), in primary hepatocyte cultures is a long-standing and major concern. We evaluated the influence of medium flow in this model. Hepatocytes isolated from 12 different liver donors were cultured either in a multichamber modular bioreactor (MCmB, flow rate 250-500 μL/min) or under standard/static conditions, and the expression of 32 genes, enzyme activities and biological parameters were measured 7-21 days later. mRNA expression of genes involved in xenobiotic/drug metabolism and transport, including CYP1A1, 1A2, 2B6, 2C9, 3A4 (and activities for some of them), UDP-glucuronosyltransferase (UGT) 1A1, UGT2B4, UGT2B7, glutathione S-transferase (GSTα), and multidrug resistance protein 1 (MDR1) and MRP2, were specifically up-regulated by medium flow as compared with static controls in all cultures tested. In 2-week-old cultures, expression of detoxification genes reached levels close to or higher than those measured in freshly isolated hepatocytes. In contrast, CYP2D6 and most of other tested genes were not affected by medium flow. We conclude that medium flow specifically interferes with, and up-regulates, the activity of xenosensors and/or the expression of detoxification genes in primary human hepatocytes. Down-regulation of detoxification genes in conventional (static) cultures is therefore partly a consequence of the absence of medium circulation.

Nuclear receptors as new perspective for the management of liver diseases

Nuclear receptors (NRs) are ligand-activated transcription factors that act as sensors for a broad range of natural and synthetic ligands and regulate several key hepatic functions including bile acid homeostasis, bile secretion, lipid and glucose metabolism, as well as drug deposition. Moreover, NRs control hepatic inflammation, regeneration, fibrosis, and tumor formation. Therefore, NRs are key for understanding the pathogenesis and pathophysiology of a wide range of hepatic disorders. Finally, targeting NRs and their alterations offers exciting new perspectives for the treatment of liver diseases.

Proteomic analysis of native hepatocyte nuclear factor-4α (HNF4α) isoforms, phosphorylation status, and interactive cofactors

Hepatocyte nuclear factor-4α (HNF4α, NR2A1) is a nuclear receptor that has a critical role in hepatocyte differentiation and the maintenance of homeostasis in the adult liver. However, a detailed understanding of native HNF4α in the steady-state remains to be elucidated. Here we report the native HNF4α isoform, phosphorylation status, and complexes in the steady-state, as shown by shotgun proteomics in HepG2 hepatocarcinoma cells. Shotgun proteomic analysis revealed the complexity of native HNF4α, including multiple phosphorylation sites and inter-isoform heterodimerization. The associating complexes identified by label-free semiquantitative proteomic analysis include the following: the DNA-dependent protein kinase catalytic subunit, histone acetyltransferase complexes, mRNA splicing complex, other nuclear receptor coactivator complexes, the chromatin remodeling complex, and the nucleosome remodeling and histone deacetylation complex. Among the associating proteins, GRB10 interacting GYF protein 2 (GIGYF2, PERQ2) is a new candidate cofactor in metabolic regulation. Moreover, an unexpected heterodimerization of HNF4α and hepatocyte nuclear factor-4γ was found. A biochemical and genomewide analysis of transcriptional regulation showed that this heterodimerization activates gene transcription. The genes thus transcribed include the cell death-inducing DEF45-like effector b (CIDEB) gene, which is an important regulator of lipid metabolism in the liver. This suggests that the analysis of the distinctive stoichiometric balance of native HNF4α and its cofactor complexes described here are important for an accurate understanding of transcriptional regulation.

Hepatic expression of thyroid hormone-responsive spot 14 protein is regulated by constitutive androstane receptor (NR1I3)

The pregnane X receptors (PXRs) and the constitutive androstane receptor (CAR) were initially isolated as nuclear receptors regulating xenobiotic metabolism and elimination, alleviating chemical insults. However, recent works suggest that these xenoreceptors play an endobiotic role in modulating hepatic lipid metabolism. In this study, we show that CAR activators]phenobarbital and 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime] induce the lipogenic gene thyroid hormone-responsive spot 14 protein (THRSP) (or Spot14, S14) expression in human hepatocytes. In addition, we report that treatment of wild-type mice with mCAR activators (phenobarbital and 1,4-Bis[2-(3,5-dichloropyridyloxy)]benzene) efficiently increases thrsp expression, in contrast to CAR null mice. We demonstrate that CAR directly transactivates THRSP promoter through the direct repeat with 4-bp spacer thyroid hormone and PXR response element. Deletion or point mutations within this PXR response element led to a drastic inhibition of CAR-mediated THRSP transactivation. Gel-shift analysis revealed that the CAR/retinoid X receptor complex binds to this element. In conclusion, our results indicate that THRSP gene is a CAR and PXR target gene. Because THRSP expression correlates with lipogenesis and insulin sensitivity, our data suggest that CAR and/or PXR activating drugs and xenobiotics may promote aberrant hepatic de novo lipogenesis leading potentially to fatty liver diseases and insulin resistance.

Isolation and culture of adult human liver progenitor cells: in vitro differentiation to hepatocyte-like cells

Highly differentiated normal human hepatocytes represent the gold standard cellular model for basic and applied research in liver physiopathology, pharmacology, toxicology, virology, and liver biotherapy. Nowadays, although livers from organ donors or medically required resections represent the current sources of hepatocytes, the possibility to generate hepatocytes from the differentiation of adult and embryonic stem cells represents a promising opportunity. The aim of this chapter is to describe our experience with the isolation from adult human liver and culture of non-parenchymal epithelial cells. Under appropriate conditions, these cells differentiate in vitro in hepatocyte-like cells and therefore appear to behave as liver progenitor cells.

Long-term virus-induced alterations of CYP3A-mediated drug metabolism: a look at the virology, immunology and molecular biology of a multi-faceted problem

Virus infections are on the rise. Although the first description of CYP expression during virus infection was recorded 50 years ago, mechanistic studies of this phenomenon only began to appear in the last decade due to breakthroughs in molecular biology, genomic and transgenic technology. This review describes the relationship(s) among CYP-mediated drug metabolism, virus infection and the immune response and evaluates in vitro and in vivo models for mechanistic studies. The first studies that assessed CYP expression during infection focused on inflammatory mediators and the innate immune response at early time points. Recent studies assessing virus infection and its effect on hepatic CYP expression noted more long-term effects. An obvious approach toward understanding how viruses affect hepatic CYP3A expression and function would be to assess key regulators of CYP during infection. Improvements in techniques to identify post-translational modifications of CYP and systems that focus on virus-receptor interactions which allow subtraction and addition of immunological and regulatory elements that drive CYP will demonstrate that long-term changes in drug metabolism start from the time the virus enters the circulation, are reinforced by virus binding to cellular targets and further solidified by changes in cellular processes long after the virus is cleared.

Identification of proteomic changes during human liver development by 2D-DIGE and mass spectrometry

BACKGROUND/AIMS

The aim of this study was to identify human liver proteins that are associated with different stages of liver development.

METHODS

We collected liver samples from 14 fetuses between 14 and 41 weeks of development, one child and four adults. Proteins which exhibited consistent and significant variations during development by two-dimensional differential in gel electrophoresis (2D-DIGE) were subjected to peptide mass fingerprint analysis by MALDI-TOF mass spectrometry. Real-time PCR analysis confirmed, at the transcriptional level, the data obtained by the proteomic approach.

RESULTS

Among a total of 80 protein spots showing differential expression, we identified 42 different proteins or polypeptide chains, of which 26 were upregulated and 16 downregulated in developing in comparison to adult liver. These proteins could be classified in specific groups according to their function. By comparing their temporal expression profiles, we identified protein groups that were associated with different developmental stages of human fetal liver and suggest that the changes in protein expression observed during the 20- to 36-week time window play a pivotal role in liver development.

CONCLUSIONS

The identification of these proteins may represent good markers of human liver and stem cells differentiation.

A novel pregnane X receptor and S14-mediated lipogenic pathway in human hepatocyte

The pregnane X receptor (PXR) initially isolated as a nuclear receptor regulating xenobiotic and drug metabolism and elimination, seems to play an endobiotic role by affecting lipid homeostasis. In mice, PXR affects lipid homeostasis and increases hepatic deposit of triglycerides. In this study, we show that, in human hepatocyte, PXR activation induces an increase of de novo lipogenesis through the up-regulation of S14. S14 was first identified as a thyroid-responsive gene and is known to transduce hormone-related and nutrient-related signals to genes involved in lipogenesis through a molecular mechanism not yet elucidated. We demonstrate that S14 is a novel transcriptional target of PXR. In addition, we report an increase of fatty acid synthase (FASN) and adenosine triphosphate citrate lyase genes expression after PXR activation in human hepatocyte, leading to an increase of fatty acids accumulation and de novo lipogenesis. RNA interference of the expression of S14 proportionally decreases the FASN induction, whereas S14 overexpression in human hepatic cells provokes an increase of fatty acids accumulation and lipogenesis. These results demonstrate for the first time that xenobiotic or drug-activated PXR promote aberrant hepatic de novo lipogenesis via activation of the nonclassical S14 pathway. In addition, these data suggest that the up-regulation of S14 by PXR may promote aberrant hepatic lipogenesis and hepatic steatosis in human hepatocytes.

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.

Regulation of gene expression of hepatic drug metabolizing enzymes and transporters by the Toll-like receptor 2 ligand, lipoteichoic acid

Expression of hepatic drug metabolizing enzymes (DMEs) is altered in infection and inflammation. However, the role of Gram+ve bacterial components and their receptor, Toll-like receptor (TLR) 2 in regulation of hepatic DMEs is unknown. Gene expression of DMEs is regulated by members of the nuclear receptor superfamily (PXR, CAR and RXRalpha). The TLR2 ligand, lipoteichoic acid (LTA) reduced RNA levels of CAR and its target genes, Cyp2b10, Cyp2a4 and Sultn in mouse liver ( approximately 60-80% reduction). Hepatic genes regulated by PXR and CAR, Cyp3a11 and Mrp2 were moderately reduced by LTA, along with approximately 50% reduction of PXR RNA and nuclear protein levels of RXRalpha. The effects of LTA were significantly attenuated by pre-treatment with the Kupffer cell inhibitor, gadolinium chloride, indicating that Kupffer cells contribute to LTA-mediated down-regulation of hepatic genes. These results indicate that treatment with Gram+ve bacterial components preferentially down-regulate CAR and its target genes in the liver.

Down-regulation of hepatic nuclear factor 4alpha on expression of human hepatic stimulator substance via its action on the proximal promoter in HepG2 cells

hHSS (human hepatic stimulator substance) stimulates hepatocyte growth. To understand the mechanism controlling hHSS expression, we analysed the proximal promoter activity and identified two regulatory regions (-212/-192 and -152/-132) that were important for transcription in HepG2 cells. Using the luciferase reporter assay, gel-shift experiments and ChIP (chromatin immunoprecipitation), we found that the transcription factors HNF4alpha (hepatocyte nuclear factor 4alpha) and Sp1 (stimulating protein-1) were essential for hHSS promoter activity and could directly bind to regions -209/-204 and -152/-145 respectively. We also confirmed that activation and repression of hHSS transcription induced by Sp1 and HNF4alpha resulted from binding of these factors to these two cis-elements respectively. Overexpression of HNF4alpha led to a dramatic repression of the promoter activity and, in contrast, the activity was markedly elevated by overexpression of Sp1. Furthermore, overexpression of HNF4alpha1, one of the HNF4alpha isoforms, resulted in a dramatic suppression of the promoter activity. Moreover, repression of HNF4alpha expression by siRNA (small interfering RNA) remarkably enhanced the hHSS mRNA level. It has been reported previously that expression of HNF4alpha is functionally regulated by dexamethasone. To further confirm the transcriptional control of HNF4alpha on hHSS, we tested the effect of dexamethasone on hHSS transcription in HepG2 cells. In the present study we have demonstrated that the expression of the hHSS gene was down-regulated at the transcriptional level by dexamethasone in HepG2 cells. A deletion and decoy assay revealed that binding of HNF4alpha to nucleotides -209/-204 was responsible for the suppression of hHSS promoter activity by dexamethasone. Increases in the HNF4alpha-binding activity and expression were simultaneously observed in an electrophoretic mobility-shift assay and Western blot analysis. These results suggested that Sp1 activates hHSS basal expression, but HNF4alpha inhibits hHSS gene expression.