Regulatory network of lipid-sensing nuclear receptors: roles for CAR, PXR, LXR, and FXR

Perfluorooctanoic acid (PFOA) and hexafluoropropylene oxide-dimer acid (GenX): Hepatic stress and bile acid metabolism with different pathways

Per- and polyfluoroalkyl substances (PFASs) and perfluoroalkyl ether carboxylic acids (PFECAs) are organic chemicals that are widely used in the manufacture of a wide range of human-made products. Many monitoring findings revealed the presence of PFASs and PFECAs in numerous environmental sources, including water, soil, and air, which drew more attention to both chemicals. Because of their unknown toxicity, the discovery of PFASs and PFECAs in a variety of environmental sources was viewed as a cause for concern. In the present study, male mice were given orally one of the typical PFASs, perfluorooctanoic acid (PFOA), and one of the representative PFECAs, hexafluoropropylene oxide-dimer acid (HFPO-DA). The liver index showing hepatomegaly rose significantly after 90 d of exposure to PFOA and HFPO-DA, respectively. While sharing similar suppressor genes, both chemicals demonstrated unique hepatotoxic mechanisms. In different ways, these two substances altered the expression of hepatic stress-sensing genes as well as the regulation of nuclear receptors. Not only are bile acid metabolism-related genes in the liver altered, but cholesterol metabolism-related genes as well. These results indicate that PFOA and HFPO-DA both cause hepatotoxicity and bile acid metabolism impairment with distinct mechanisms.

Microbial Metabolites as Ligands to Xenobiotic Receptors: Chemical Mimicry as Potential Drugs of the Future

Xenobiotic receptors, such as the pregnane X receptor, regulate multiple host physiologic pathways including xenobiotic metabolism, certain aspects of cellular metabolism, and innate immunity. These ligand-dependent nuclear factors regulate gene expression via genomic recognition of specific promoters and transcriptional activation of the gene. Natural or endogenous ligands are not commonly associated with this class of receptors; however, since these receptors are expressed in a cell-type specific manner in the liver and intestines, there has been significant recent effort to characterize microbially derived metabolites as ligands for these receptors. In general, these metabolites are thought to be weak micromolar affinity ligands. This journal anniversary minireview focuses on recent efforts to derive potentially nontoxic microbial metabolite chemical mimics that could one day be developed as drugs combating xenobiotic receptor-modifying pathophysiology. The review will include our perspective on the field and recommend certain directions for future research. SIGNIFICANCE STATEMENT: Xenobiotic receptors (XRs) regulate host drug metabolism, cellular metabolism, and immunity. Their presence in host intestines allows them to function not only as xenosensors but also as a response to the complex metabolic environment present in the intestines. Specifically, this review focuses on describing microbial metabolite-XR interactions and the translation of these findings toward discovery of novel chemical mimics as potential drugs of the future for diseases such as inflammatory bowel disease.

Prepartum dietary energy intake alters hepatic expression of genes related to peroxisome proliferator-activated receptor and inflammation in peripartal dairy cows

We determined the effect of prepartum plane of energy intake on liver function and metabolism pre- and postpartum by combining in vivo and in vitro data with mRNA expression data. A subset of multiparous prepartal Holsteins (n = 18) from a previously conducted experiment consumed 1 of 3 amounts of dietary energy intake, relative to their requirements. A diet formulated to allow consumption of ≥150% of net energy requirements during the far-off dry period and the close-up dry period was fed for ad libitum intake (150E) or in restricted amounts so that cows consumed 80% of requirements for energy (80E). A second diet was formulated to include wheat straw (26.1% of dry matter) to limit energy intake to 100% of NRC (2001) requirements for energy when fed ad libitum during the far-off period (100E). In the close-up period, 100E was fed the 150E diet for ad libitum intake. Expression of mRNA for genes related to fatty acid oxidation (PPARA, CPT1A, ACOX1) was greater for 100E cows than 150E cows on d 14 postpartum. These expression patterns were related to in vitro data for conversion of palmitate to CO₂, acid-soluble products, and esterified products by liver slices. Abundance of mRNA for PC displayed a sharp peak for all groups on d 1 postpartum, but serum glucose did not reflect this peak. The mRNA expression of SREBF1 was greater for 150E and 100E cows prepartum compared with 80E, and was positively related to rate of palmitate esterification postpartum. Expression of NR1H3 (LXRA) mRNA was greater for 100E cows on d 14 postpartum compared with 150E cows, which corresponded to expression of PPARA. An inflammatory response occurred in the liver around the time of parturition for 150E cows, as expression of IL1B was elevated both pre- and postpartum compared with 100E cows. The spike in IL1B expression for 150E cows on d 14 postpartum corresponded to the peak concentration of total lipids in liver tissue for all groups in this experiment. Overconsumption of energy prepartum was detrimental to the expression of important genes related to PPAR and liver function, especially postpartum. Furthermore, results provide evidence for inflammation related to accumulation of lipids in liver and overnutrition prepartum.

A biologically based model to quantitatively assess the role of the nuclear receptors liver X (LXR), and pregnane X (PXR) on chemically induced hepatic steatosis

Hepatic steatosis is characterized by the intracellular increase of free fatty acids (FFAs) in the form of triglycerides in hepatocytes. This hepatic adverse outcome can be caused by many factors, including exposure to drugs or environmental toxicants. Mechanistically, accumulation of lipids in the liver can take place via several mechanisms such as de novo synthesis and/or uptake of FFAs from serum via high fat content diets. De novo synthesis of FFAs within the liver is mediated by the liver X receptor (LXR), and their uptake into the liver is mediated through the pregnane X receptor (PXR). We investigated the impact of chemical exposure on FFAs hepatic content via activation of LXR and PXR by integrating chemical-specific physiologically based pharmacokinetic (PBPK) models with a quantitative toxicology systems (QTS) model of hepatic lipid homeostasis. Three known agonists of LXR and/or PXR were modeled: T0901317 (antagonist for both receptors), GW3965 (LXR only), and Rifampicin (PXR only). Model predictions showed that T0901317 caused the most FFAs build-up in the liver, followed by Rifampicin and then GW3965. These modeling results highlight the importance of PXR activation for serum FFAs uptake into the liver while suggesting that increased hepatic FAAs de novo synthesis alone may not be enough to cause appreciable accumulation of lipids in the liver under normal environmental exposure levels. Moreover, the overall PBPK-hepatic lipids quantitative model can be used to screen chemicals for their potential to cause in vivo hepatic lipid content buildup in view of their in vitro potential to activate the nuclear receptors and their exposure levels.

Discovery of Orally Active and Nonsteroidal Farnesoid X Receptor (FXR) Antagonist with Propensity for Accumulation and Responsiveness in Ileum

We describe the discovery of analog 15 (FLG249), which is an orally active and nonsteroidal farnesoid X receptor (FXR) antagonist in mice with unique profiles, such as a propensity for ileum distribution and the significant control in the expression level of three FXR target genes in mouse ileum. Key design features incorporated in 15 were the introduction of metabolically stable groups in potent and metabolically labile antagonist 9. Our pursuit ultimately identified FXR antagonist 15, which has enabled its assessment in a drug discovery program.

PXR is a target of (-)-epicatechin in skeletal muscle

(-)-Epicatechin (EC) is a flavanol that has shown numerous biological effects such as: decrease risk of cardiovascular dysfunction, metabolism regulation, skeletal muscle (SkM) performance improvement and SkM cells differentiation induction, among others. The described EC acceptor/receptor molecules do not explain the EC's effect on SkM. We hypothesize that the pregnane X receptor (PXR) can fulfill those characteristics, based on structural similitude between EC and steroidal backbone and that PXR activation leads to similar effects as those induced by EC. In order to demonstrate our hypothesis, we: 1) analyzed the possible EC and mouse PXR interaction through in silico strategies, 2) developed an EC's affinity column to isolate PXR, 3) evaluated, in mouse myoblast (C2C12 cells) the inhibition of EC-induced PXR's nucleus translocation by ketoconazole, a specific blocker of PXR and 4) analyzed the effect of EC as an activator of mouse PXR, evaluating the expression modulation of cytochrome 3a11 (Cyp3a11) gen and myogenin protein. (-)-Epicatechin interacts and activates PXR, promoting this protein translocation to the nucleus, increasing the expression of Cyp3a11, and promoting C2C12 cell differentiation through increasing myogenin expression. These results can be the base of further studies to analyze the possible participation of PXR in the skeletal muscle effects shown by EC.

A Tiered Approach for Screening and Assessment of Environmental Mixtures by Omics and In Vitro Assays

New methodology approaches with a broad coverage of the biological effects are urgently needed to evaluate the safety of the universe of environmentally relevant chemicals. Here, we propose a tiered approach incorporating transcriptomics and in vitro bioassays to assess environmental mixtures. The mixture samples and the perturbed biological pathways are prioritized by concentration-dependent transcriptome (CDT) and then used to guide the selection of in vitro bioassays for toxicant identification. To evaluate omics' screening capability, we first applied a CDT technique to test mixture samples by HepG2 and MCF7 cells. The effect recoveries of large-volume solid-phase extraction on the overall bioactivity of the mixture were 48.9% in HepG2 and 58.3% in MCF7. The overall bioactivity potencies obtained by transcriptomics were positively correlated with the panel of 8 bioassays among 14 mixture samples combined with the previous data. Transcriptomics could predict their activation status (AUC = 0.783) and the relative potency (p < 0.05) of bioassays for four of the eight receptors (AhR, ER, AR, and Nrf2). Furthermore, the CDT identified other biological pathways perturbated by mixture samples, such as the pathway related to TP53, CAR, FXR, HIF, THRA, etc. Overall, this study demonstrates the potential of concentration-dependent omics for effect-based water quality assessment.

Bilirubin as a signaling molecule

For long time bilirubin was only considered as a potentially dangerous sign of liver diseases, but it now appears clear that it is also a powerful signaling molecule. Together with potent antioxidant activities that were only reported in the last few decades, many other biological effects have now been clearly described. These include especially profound inhibitory effects on almost all effectors of the immune system, with their clinical consequences in the bilirubin-mediated protection against autoimmune and inflammatory diseases. Separate from these, bilirubin activates various nuclear and cytoplasmic receptors, resembling the endocrine activities of actual hormonal substances. This is true for the "classical" hepatic nuclear receptors, including the aryl hydrocarbon receptor, or the constitutive androstane receptor; and also for some lesser-explored receptors such as peroxisome proliferator-activated receptors α and γ; Mas-related G protein-coupled receptor; or other signaling molecules including fatty acid binding protein 1, apolipoprotein D, or reactive oxygen species. All of these targets have broad metabolic effects, which in turn may offer protection against obesity, diabetes mellitus, and other metabolic diseases. The (mostly experimental) data are also supported by clinical evidence. In fact, data from the last three decades have convincingly demonstrated the protective effects of mildly elevated serum bilirubin concentrations against various "diseases of civilization." Additionally, even tiny, micromolar changes of serum bilirubin concentrations have been associated with substantial alteration in the risks of these diseases. It is highly likely that all of the biological activities of bilirubin have yet to be exhaustively explored, and thus we can expect further clinical discoveries about this evolutionarily old molecule into the future.

The effects of LXR agonist T0901317 and LXR antagonist GSK2033 on morphogenesis and lipid properties in full thickness skin models

Full thickness models (FTMs) are 3D-cultured human skin models that mimic many aspects of native human skin (NHS). However, their stratum corneum (SC) lipid composition differs from NHS causing a reduced skin barrier. The most pronounced differences in lipid composition are a reduction in lipid chain length and increased monounsaturated lipids. The liver-X-receptor (LXR) activates the monounsaturated lipid synthesis via stearoyl-CoA desaturase-1 (SCD-1). Therefore, the aim was to improve the SC lipid synthesis of FTMs by LXR deactivation. This was achieved by supplementing culture medium with LXR antagonist GSK2033. LXR agonist T0901317 was added for comparison. Subsequently, epidermal morphogenesis, lipid composition, lipid organization and the barrier functionality of these FTMs were assessed. We demonstrate that LXR deactivation resulted in a lipid composition with increased overall chain lengths and reduced levels of monounsaturation, whereas LXR activation increased the amount of monounsaturated lipids and led to a reduction in the overall chain length. However, these changes did not affect the barrier functionality. In conclusion, LXR deactivation led to the development of FTMs with improved lipid properties, which mimic the lipid composition of NHS more closely. These novel findings may contribute to design interventions to normalize SC lipid composition of atopic dermatitis patients.

Identification of potent farnesoid X receptor (FXR) antagonist showing favorable PK profile and distribution toward target tissues: Comprehensive understanding of structure-activity relationship of FXR antagonists

Antagonizing transcriptional activity of farnesoid X receptor (FXR) in the intestine has been reported as an effective means for the treatment of nonalcoholic fatty liver disease, type 2 diabetes and obesity. We describe herein that the building blocks necessary to maintain the antagonism of our chemotype were investigated in order to modulate in vivo pharmacokinetic behavior and the tissue distribution without blunting the activity against FXR. A comprehensive understanding of the structure-activity relationship led to analog 30, which is superior to 12 in terms of its pharmacokinetic profiles by oral administration and its tissue distribution toward target tissues (liver and ileum) in rats while preserving the in vitro activity of 12 against FXR. Thus, 30 should be a candidate compound to investigate the effects of inhibiting FXR activity while simultaneously improving the outcome of nonalcoholic fatty liver disease, type 2 diabetes and obesity.

The Nuclear Receptor Field: A Historical Overview and Future Challenges

In this article we summarize the birth of the field of nuclear receptors, the discovery of untransformed and transformed isoforms of ligand-binding macromolecules, the discovery of the three-domain structure of the receptors, and the properties of the Hsp90-based heterocomplex responsible for the overall structure of the oligomeric receptor and many aspects of the biological effects. The discovery and properties of the subfamily of receptors called orphan receptors is also outlined. Novel molecular aspects of the mechanism of action of nuclear receptors and challenges to resolve in the near future are discussed.

Pregnane X receptor (PXR) signaling in seabream primary hepatocytes exposed to extracts of seawater samples collected from polycyclic aromatic hydrocarbons (PAHs)-contaminated coastal areas

Polycyclic aromatic hydrocarbons (PAHs) are persistent organic pollutants damaging to the marine environment and the wildlife. Herein, we investigated the effects of extracts from coastal seawaters (central Adriatic sea, Italy), showing high concentrations of PAHs, on pregnane X receptor (PXR)-transcriptional regulation of the cytochrome P450 3A (CYP3A) gene using seabream primary hepatocytes. The results show that concentrated extracts of seawater with original ΣPAH concentrations above the putative threshold of 30 ng L^-1 increased expression of PXR and its main target gene, CYP3A. Similar results were observed for LXR and its target gene SREBP-1c suggesting pathway cross-talk. These data are further supported by the finding of multiple PXR and LXR response elements in the putative promoters of their target genes. Overall, our data indicate the capacity of seawater extracts, containing environmentally relevant levels of PAHs, to affect multiple pathways, including lipid and cholesterol metabolism.

Bile Acids, Nuclear Receptors and Cytochrome P450

This review summarizes the importance of bile acids (BA) as important regulators of various homeostatic mechanisms with detailed focus on cytochrome P450 (CYP) enzymes. In the first part, synthesis, metabolism and circulation of BA is summarized and BA are reviewed as physiological ligands of nuclear receptors which regulate transcription of genes involved in their metabolism, transport and excretion. Notably, PXR, FXR and VDR are the most important nuclear receptors through which BA regulate transcription of CYP genes involved in the metabolism of both BA and xenobiotics. Therapeutic use of BA and their derivatives is also briefly reviewed. The physiological role of BA interaction with nuclear receptors is basically to decrease production of toxic non-polar BA and increase their metabolic turnover towards polar BA and thus decrease their toxicity. By this, the activity of some drug-metabolizing CYPs is also influenced what could have clinically relevant consequences in cholestatic diseases or during the treatment with BA or their derivatives.

Differential modulation of FXR activity by chlorophacinone and ivermectin analogs

Chemicals that alter normal function of farnesoid X receptor (FXR) have been shown to affect the homeostasis of bile acids, glucose, and lipids. Several structural classes of environmental chemicals and drugs that modulated FXR transactivation were previously identified by quantitative high-throughput screening (qHTS) of the Tox21 10K chemical collection. In the present study, we validated the FXR antagonist activity of selected structural classes, including avermectin anthelmintics, dihydropyridine calcium channel blockers, 1,3-indandione rodenticides, and pyrethroid pesticides, using in vitro assay and quantitative structural-activity relationship (QSAR) analysis approaches. (Z)-Guggulsterone, chlorophacinone, ivermectin, and their analogs were profiled for their ability to alter CDCA-mediated FXR binding using a panel of 154 coregulator motifs and to induce or inhibit transactivation and coactivator recruitment activities of constitutive androstane receptor (CAR), liver X receptor alpha (LXRα), or pregnane X receptor (PXR). Our results showed that chlorophacinone and ivermectin had distinct modes of action (MOA) in modulating FXR-coregulator interactions and compound selectivity against the four aforementioned functionally-relevant nuclear receptors. These findings collectively provide mechanistic insights regarding compound activities against FXR and possible explanations for in vivo toxicological observations of chlorophacinone, ivermectin, and their analogs.

Systems Toxicology of Male Reproductive Development: Profiling 774 Chemicals for Molecular Targets and Adverse Outcomes

BACKGROUND

Trends in male reproductive health have been reported for increased rates of testicular germ cell tumors, low semen quality, cryptorchidism, and hypospadias, which have been associated with prenatal environmental chemical exposure based on human and animal studies.

OBJECTIVE

In the present study we aimed to identify significant correlations between environmental chemicals, molecular targets, and adverse outcomes across a broad chemical landscape with emphasis on developmental toxicity of the male reproductive system.

METHODS

We used U.S. EPA's animal study database (ToxRefDB) and a comprehensive literature analysis to identify 774 chemicals that have been evaluated for adverse effects on male reproductive parameters, and then used U.S. EPA's in vitro high-throughput screening (HTS) database (ToxCastDB) to profile their bioactivity across approximately 800 molecular and cellular features.

RESULTS

A phenotypic hierarchy of testicular atrophy, sperm effects, tumors, and malformations, a composite resembling the human testicular dysgenesis syndrome (TDS) hypothesis, was observed in 281 chemicals. A subset of 54 chemicals with male developmental consequences had in vitro bioactivity on molecular targets that could be condensed into 156 gene annotations in a bipartite network.

CONCLUSION

Computational modeling of available in vivo and in vitro data for chemicals that produce adverse effects on male reproductive end points revealed a phenotypic hierarchy across animal studies consistent with the human TDS hypothesis. We confirmed the known role of estrogen and androgen signaling pathways in rodent TDS, and importantly, broadened the list of molecular targets to include retinoic acid signaling, vascular remodeling proteins, G-protein coupled receptors (GPCRs), and cytochrome P450s.

CITATION

Leung MC, Phuong J, Baker NC, Sipes NS, Klinefelter GR, Martin MT, McLaurin KW, Setzer RW, Darney SP, Judson RS, Knudsen TB. 2016. Systems toxicology of male reproductive development: profiling 774 chemicals for molecular targets and adverse outcomes. Environ Health Perspect 124:1050-1061; http://dx.doi.org/10.1289/ehp.1510385.

Tipping the Balance: Hepatotoxicity and the 4 Apical Key Events of Hepatic Steatosis

Hepatic steatosis is a condition were fat accumulates in the liver and it is associated with extra-hepatic diseases related to metabolic syndrome and systemic energy metabolism. If not reversed, steatosis can progress to steatohepatitis and irreversible stages of liver disease including fibrosis, cirrhosis, hepatocellular carcinoma, and death. From a public health standpoint, identifying chemical exposures that may be factors in steatosis etiology are important for preventing hepatotoxicity and liver disease progression. It is therefore important to identify the biological events that are key for steatosis pathology mediated by chemical exposure. In this review, we give a current overview of the complex biological cascades that can disrupt lipid homeostasis in hepatocytes in the context of 4 apical key events central to hepatic lipid retention: hepatic fatty acid (FA) uptake,de novoFA and lipid synthesis, FA oxidation, and lipid efflux. Our goal is to review these key cellular events and visually summarize them using a network for application in pathway-based toxicity testing. This effort provides a foundation to improve next-generation chemical screening efforts that may be used to prevent and ultimately reverse the growing incidence of fatty liver disease in our population.

Intracerebroventricular ghrelin treatment affects lipid metabolism in liver of rainbow trout (Oncorhynchus mykiss)

We aimed to elucidate in rainbow trout (Oncorhynchus mykiss) the effects of central ghrelin (GHRL) treatment on the regulation of liver lipid metabolism, and the possible modulatory effect of central GHRL treatment on the simultaneous effects of raised levels of oleate. Thus, we injected intracerebroventricularly (ICV) rainbow trout GHRL in the presence or absence of oleate and evaluated in liver variables related to lipid metabolism. Oleate treatment elicited in liver of rainbow trout decreased lipogenesis and increased oxidative capacity in agreement with previous studies. Moreover, as demonstrated for the first time in fish in the present study, GHRL also acts centrally modulating lipid metabolism in liver, resulting in increased potential for lipogenesis and decreased potential for fatty acid oxidation, i.e. the converse effects to those elicited by central oleate treatment. The simultaneous treatment of GHRL and oleate confirmed these counteractive effects. Thus, the nutrient sensing mechanisms present in hypothalamus, particularly those involved in sensing of fatty acid, are involved in the control of liver energy metabolism in fish, and this control is modulated by the central action of GHRL. These results give support to the notion of hypothalamus as an integrative place for the regulation of peripheral energy metabolism in fish.

Activating and Inhibitory Functions of WNT/β-Catenin in the Induction of Cytochromes P450 by Nuclear Receptors in HepaRG Cells

The WNT/β-catenin signaling pathway has been identified as an important endogenous regulator of hepatic cytochrome P450 (P450) expression in mouse liver. In particular, it is involved in the regulation of P450 expression in response to exposure to xenobiotic agonists of the nuclear receptors constitutive androstane receptor (CAR), aryl hydrocarbon receptor (AhR), and Nrf2. To systematically elucidate the effect of the WNT/β-catenin pathway on the regulation and inducibility of major human P450 enzymes, HepaRG cells were treated with either the WNT/β-catenin signaling pathway agonist, WNT3a, or with small interfering RNA directed against β-catenin, alone or in combination with a panel of activating ligands for AhR [2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)], CAR [6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl)oxime (CITCO)], pregnane X receptor (PXR) [rifampicin], and peroxisome proliferator-activated receptor (PPAR) α [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthioacetic acid (WY14,643)]. Assessment of P450 gene expression and enzymatic activity after downregulation or activation of the WNT/β-catenin pathway revealed a requirement of β-catenin in the AhR-, CAR-, and PXR-mediated induction of CYP1A, CYP2B6 and CYP3A4 (for CAR and PXR), and CYP2C8 (for PXR) gene expression. By contrast, activation of the WNT/β-catenin pathway prevented PPARα-mediated induction of CYP1A, CYP2C8, CYP3A4, and CYP4A11 genes, suggesting a dominant-negative role of β-catenin in PPARα-mediated regulation of these genes. Our data indicate a significant effect of the WNT/β-catenin pathway on the regulation of P450 enzymes in human hepatocytes and reveal a novel crosstalk between β-catenin and PPARα signaling pathways in the regulation of P450 expression.

Effect of a high-fat diet on the hepatic expression of nuclear receptors and their target genes: relevance to drug disposition

More than 1·4 billion individuals are overweight or obese worldwide. While complications often require therapeutic intervention, data regarding the impact of obesity on drug disposition are scarce. As the influence of diet-induced obesity on drug transport and metabolic pathways is currently unclear, the objective of the present study was to investigate the effect of high fat feeding for 13 weeks in female Sprague-Dawley rats on the hepatic expression of the nuclear receptors pregnane X receptor (PXR), constitutive androstane receptor (CAR), liver X receptor (LXR) and farnesoid X receptor (FXR) and several of their target genes. We hypothesised that high fat feeding would alter the gene expression of major hepatic transporters through a dysregulation of the expression of the nuclear receptors. The results demonstrated that, along with a significant increase in body fat and weight, a high-fat diet (HFD) induced a significant 2-fold increase in the expression of PXR as well as a 2-, 5- and 2·5-fold increase in the hepatic expression of the PXR target genes Abcc2, Abcb1a and Cyp3a2, respectively (P< 0·05). The expression levels of FXR were significantly increased in rats fed a HFD in addition to the increase in the expression levels of FXR target genes Abcb11 and Abcb4. The expression levels of both LXRα and LXRβ were slightly but significantly increased in rats fed a HFD, and the expression levels of their target genes Abca1 and Abcg5, but not Abcg8, were significantly increased. The expression of the nuclear receptor CAR was not significantly altered between the groups. This suggests that a HFD may induce changes in the hepatobiliary transport and metabolism of endogenous and exogenous compounds.

Lipid regulation of the ABCB1 and ABCG2 multidrug transporters

This chapter deals with the interactions of two medically important multidrug ABC transporters (MDR-ABC), ABCB1 and ABCG2, with lipid molecules. Both ABCB1 and ABCG2 are capable of transporting a wide range of hydrophobic drugs and xenobiotics and are involved in cancer chemotherapy resistance. Therefore, the exploration of their mechanism of action has major therapeutic consequences. As discussed here in detail, both ABCB1 and ABCG2 are significantly affected by various lipid compounds especially those residing in their close proximity in the plasma membrane. ABCB1 is capable of transporting lipids and lipid derivatives, and thus may alter the general membrane composition by "flopping" membrane lipid constituents, while there is no such information regarding ABCG2. Still, both ABCB1 and ABCG2 show complex interactions with a variety of lipid molecules, and the transporters are significantly modulated by cholesterol and cholesterol derivatives at the posttranslational level. In this chapter, we explore the molecular details of the direct transporter-lipid interactions, the potential role of lipid-sensor domains within the proteins, as well as the application of experimental site-directed mutagenesis, detailed structural studies, and in silico modeling for examining these interactions. We also discuss the regulation of ABCB1 and ABCG2 expression at the transcriptional level, occurring through nuclear receptors involved in lipid sensing. The better understanding of lipid interactions with these medically important MDR-ABC transporters may significantly improve further drug development and clinical treatment options.

Nuclear Receptors in Drug Metabolism, Drug Response and Drug Interactions

Orally delivered small-molecule therapeutics are metabolized in the liver and intestine by phase I and phase II drug-metabolizing enzymes (DMEs), and transport proteins coordinate drug influx (phase 0) and drug/drug-metabolite efflux (phase III). Genes involved in drug metabolism and disposition are induced by xenobiotic-activated nuclear receptors (NRs), i.e. PXR (pregnane X receptor) and CAR (constitutive androstane receptor), and by the 1α, 25-dihydroxy vitamin D3-activated vitamin D receptor (VDR), due to transactivation of xenobiotic-response elements (XREs) present in phase 0-III genes. Additional NRs, like HNF4-α, FXR, LXR-α play important roles in drug metabolism in certain settings, such as in relation to cholesterol and bile acid metabolism. The phase I enzymes CYP3A4/A5, CYP2D6, CYP2B6, CYP2C9, CYP2C19, CYP1A2, CYP2C8, CYP2A6, CYP2J2, and CYP2E1 metabolize >90% of all prescription drugs, and phase II conjugation of hydrophilic functional groups (with/without phase I modification) facilitates drug clearance. The conjugation step is mediated by broad-specificity transferases like UGTs, SULTs, GSTs. This review delves into our current understanding of PXR/CAR/VDR-mediated regulation of DME and transporter expression, as well as effects of single nucleotide polymorphism (SNP) and epigenome (specified by promoter methylation, histone modification, microRNAs, long non coding RNAs) on the expression of PXR/CAR/VDR and phase 0-III mediators, and their impacts on variable drug response. Therapeutic agents that target epigenetic regulation and the molecular basis and consequences (overdosing, underdosing, or beneficial outcome) of drug-drug/drug-food/drug-herb interactions are also discussed. Precision medicine requires understanding of a drug's impact on DME and transporter activity and their NR-regulated expression in order to achieve optimal drug efficacy without adverse drug reactions. In future drug screening, new tools such as humanized mouse models and microfluidic organs-on-chips, which mimic the physiology of a multicellular environment, will likely replace the current cell-based workflow.

Quantitative high-throughput profiling of environmental chemicals and drugs that modulate farnesoid X receptor

The farnesoid X receptor (FXR) regulates the homeostasis of bile acids, lipids, and glucose. Because endogenous chemicals bind and activate FXR, it is important to examine which xenobiotic compounds would disrupt normal receptor function. We used a cell-based human FXR β-lactamase (Bla) reporter gene assay to profile the Tox21 10K compound collection of environmental chemicals and drugs. Structure-activity relationships of FXR-active compounds revealed by this screening were then compared against the androgen receptor, estrogen receptor α, peroxisome proliferator-activated receptors δ and γ, and the vitamin D receptor. We identified several FXR-active structural classes including anthracyclines, benzimidazoles, dihydropyridines, pyrethroids, retinoic acids, and vinca alkaloids. Microtubule inhibitors potently decreased FXR reporter gene activity. Pyrethroids specifically antagonized FXR transactivation. Anthracyclines affected reporter activity in all tested assays, suggesting non-specific activity. These results provide important information to prioritize chemicals for further investigation, and suggest possible modes of action of compounds in FXR signaling.

Retinoid acid-related orphan receptor γ, RORγ, participates in diurnal transcriptional regulation of lipid metabolic genes

The hepatic circadian clock plays a pivotal role in regulating major aspects of energy homeostasis and lipid metabolism. In this study, we show that RORγ robustly regulates the rhythmic expression of several lipid metabolic genes, including the insulin-induced gene 2a, Insig2a, elongation of very long chain fatty acids-like 3, Elovl3 and sterol 12α-hydroxylase, Cyp8b1, by enhancing their expression at ZT20-4. The time-dependent increase in their expression correlates with the rhythmic expression pattern of RORγ. The enhanced recruitment of RORγ to ROREs in their promoter region, increased histone acetylation, and reporter and mutation analysis support the concept that RORγ regulates the transcription of several lipid metabolic genes directly by binding ROREs in their promoter regulatory region. Consistent with the disrupted expression of a number of lipid metabolic genes, loss of RORγ reduced the level of several lipids in liver and blood in a ZT-preferred manner. Particularly the whole-body bile acid pool size was considerably reduced in RORγ^−/− mice in part through its regulation of several Cyp genes. Similar observations were made in liver-specific RORγ-deficient mice. Altogether, our study indicates that RORγ functions as an important link between the circadian clock and the transcriptional regulation of several metabolic genes.

Flavonoids and saponins extracted from black bean (Phaseolus vulgaris L.) seed coats modulate lipid metabolism and biliary cholesterol secretion in C57BL/6 mice

Black bean (Phaseolus vulgaris L.) seed coats are a rich source of natural compounds with potential beneficial effects on human health. Beans exert hypolipidaemic activity; however, this effect has not been attributed to any particular component, and the underlying mechanisms of action and protein targets remain unknown. The aim of the present study was to identify and quantify primary saponins and flavonoids extracted from black bean seed coats, and to study their effects on lipid metabolism in primary rat hepatocytes and C57BL/6 mice. The methanol extract of black bean seed coats, characterised by a HPLC system with a UV–visible detector and an evaporative light-scattering detector and HPLC–time-of-flight/MS, contained quercetin 3-O-glucoside and soyasaponin Af as the primary flavonoid and saponin, respectively. The extract significantly reduced the expression of SREBP1c, FAS and HMGCR, and stimulated the expression of the reverse cholesterol transporters ABCG5/ABCG8 and CYP7A1 in the liver. In addition, there was an increase in the expression of hepatic PPAR-α. Consequently, there was a decrease in hepatic lipid depots and a significant increase in bile acid secretion. Furthermore, the ingestion of this extract modulated the proportion of lipids that was used as a substrate for energy generation. Thus, the results suggest that the extract of black bean seed coats may decrease hepatic lipogenesis and stimulate cholesterol excretion, in part, via bile acid synthesis.

Roles of xenobiotic receptors in vascular pathophysiology

The pregnane X receptor (PXR) and constitutive androstane receptor (CAR), 2 closely related and liver-enriched members of the nuclear receptor superfamily, and aryl hydrocarbon receptor (AhR), a nonnuclear receptor transcription factor (TF), are major receptors/TFs regulating the expression of genes for the clearance and detoxification of xenobiotics. They are hence defined as "xenobiotic receptors". Recent studies have demonstrated that PXR, CAR and AhR also regulate the expression of key proteins involved in endobiotic responses such as the metabolic homeostasis of lipids, glucose, and bile acid, and inflammatory processes. It is suggested that the functions of PXR, CAR and AhR may be closely implicated in the pathogeneses of metabolic vascular diseases, such as hyperlipidemia, atherogenesis, and hypertension. Therefore, manipulation of the activities of these receptors may provide novel strategies for the treatment of vascular diseases. Here, we review the pathophysiological roles of PXR, CAR and AhR in the vascular system.

Bovine NR1I3 gene polymorphisms and its association with feed efficiency traits in Nellore cattle

The Nuclear receptor 1 family I member 3 (NR1I3), also known as the Constitutive Androstane Receptor (CAR), was initially characterized as a key regulator of xenobiotic metabolism. However, recent biochemical and structural data suggest that NR1I3 is activated in response to metabolic and nutritional stress in a ligand-independent manner. Thus, we prospected the Bovine NR1I3 gene for polymorphisms and studied their association with feed efficiency traits in Nellore cattle. First, 155 purebred Nellore bulls were individually measured for Residual Feed Intake (RFI) and the 25 best (High Feed Efficiency group, HFE) and the 25 worst animals (Low Feed Efficiency group, LFE) were selected for DNA extraction. The entire Bovine NR1I3 gene was amplified and polymorphisms were identified by sequencing. Then, one SNP different between HFE and LFE groups was genotyped in all the 155 animals and in another 288 animals totalizing 443 Nellore bulls genotyped for association of NR1I3 SNPs with feed efficiency traits. We found 24 SNPs in the NR1I3 gene and choose a statistically different SNP between HFE and LFE groups for further analysis. Genotyping of the 155 animals showed a significant association within SNP and RFI (p = 0.04), Residual Intake and BW Gain (p = 0.04) and Dry Matter Intake (p = 0.01). This SNP is located in the 5′flanking promoter region of NR1I3 gene and different alleles alter the binding site for predicted transcriptional factors as HNF4alpha, CREM and c-MYB, leading us to conclude that NR1I3 expression and regulation might be important to feed efficiency.

Optimization of stress response through the nuclear receptor-mediated cortisol signalling network

It is an accepted paradigm that extended stress predisposes an individual to pathophysiology. However, the biological adaptations to minimize this risk are poorly understood. Using a computational model based upon realistic kinetic parameters we are able to reproduce the interaction of the stress hormone cortisol with its two nuclear receptors, the high-affinity glucocorticoid receptor and the low-affinity pregnane X-receptor. We demonstrate that regulatory signals between these two nuclear receptors are necessary to optimize the body's response to stress episodes, attenuating both the magnitude and duration of the biological response. In addition, we predict that the activation of pregnane X-receptor by multiple, low-affinity endobiotic ligands is necessary for the significant pregnane X-receptor-mediated transcriptional response observed following stress episodes. This integration allows responses mediated through both the high and low-affinity nuclear receptors, which we predict is an important strategy to minimize the risk of disease from chronic stress.

Epigenetic distortion to VDR transcriptional regulation in prostate cancer cells

The current study aimed to examine the gene specific mechanisms by which the actions of the vitamin D receptor (VDR) are distorted in prostate cancer. Transcriptional responses toward the VDR ligand, 1α,25(OH)2D3, were examined in non-malignant prostate epithelial cells (RWPE-1) and compared to the 1α,25(OH)2D3-recalcitrant prostate cancer cells (PC-3). Time resolved transcriptional studies for two VDR target genes revealed selective attenuation and repression of VDR transcriptional responses in PC-3 cells. For example, responses in PC-3 cells revealed suppressed responsiveness of IGFBP3 and G0S2. Furthermore, Chromatin Immunoprecipitation (ChIP) assays revealed that suppressed transcriptional responses in PC-3 cells of IGFBP3 and G0S2 were associated with selective VDR-induced NCOR1 enrichment at VDR-binding regions on target-gene promoter regions. We propose that VDR inappropriately recruits co-repressors in prostate cancer cells. Subsequent direct and indirect mechanisms may induce local DNA methylation and stable transcriptional silencing. Thus a transient epigenetic process mediated by co-repressor binding, namely, the control of H3K9 acetylation, is distorted to favor a more stable epigenetic event, namely DNA methylation. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

Real-time recording of circadian liver gene expression in freely moving mice reveals the phase-setting behavior of hepatocyte clocks

The mammalian circadian timing system consists of a master pacemaker in the suprachiasmatic nucleus (SCN) in the hypothalamus, which is thought to set the phase of slave oscillators in virtually all body cells. However, due to the lack of appropriate in vivo recording technologies, it has been difficult to study how the SCN synchronizes oscillators in peripheral tissues. Here we describe the real-time recording of bioluminescence emitted by hepatocytes expressing circadian luciferase reporter genes in freely moving mice. The technology employs a device dubbed RT-Biolumicorder, which consists of a cylindrical cage with reflecting conical walls that channel photons toward a photomultiplier tube. The monitoring of circadian liver gene expression revealed that hepatocyte oscillators of SCN-lesioned mice synchronized more rapidly to feeding cycles than hepatocyte clocks of intact mice. Hence, the SCN uses signaling pathways that counteract those of feeding rhythms when their phase is in conflict with its own phase.

Molecular Interactions between NAFLD and Xenobiotic Metabolism

Non-alcoholic fatty liver disease (NAFLD), the hepatic manifestation of the metabolic syndrome, is a complex multifactorial disease characterized by metabolic deregulations that include accumulation of lipids in the liver, lipotoxicity, and insulin resistance. The progression of NAFLD to non-alcoholic steatohepatitis and cirrhosis, and ultimately to carcinomas, is governed by interplay of pro-inflammatory pathways, oxidative stress, as well as fibrogenic and apoptotic cues. As the liver is the major organ of biotransformation, deregulations in hepatic signaling pathways have effects on both, xenobiotic and endobiotic metabolism. Several major nuclear receptors involved in the transcription and regulation of phase I and II drug metabolizing enzymes and transporters also have endobiotic ligands including several lipids. Hence, hepatic lipid accumulation in steatosis and NAFLD, which leads to deregulated activation patterns of nuclear receptors, may result in altered drug metabolism capacity in NAFLD patients. On the other hand, genetic and association studies have indicated that a malfunction in drug metabolism can affect the prevalence and severity of NAFLD. This review focuses on the complex interplay between NAFLD pathogenesis and drug metabolism. A better understanding of these relationships is a prerequisite for developing improved drug dosing algorithms for the pharmacotherapy of patients with different stages of NAFLD.

Direct transcriptional regulation of human hepatic cytochrome P450 3A4 (CYP3A4) by peroxisome proliferator-activated receptor alpha (PPARα)

The nuclear receptor peroxisome proliferator-activated receptor (PPAR)α is known primarily as a regulator of fatty acid metabolism, energy balance, and inflammation, but evidence suggests a wider role in regulating the biotransformation of drugs and other lipophilic chemicals. We investigated whether PPARα directly regulates the transcription of cytochrome P450 3A4, the major human drug-metabolizing enzyme. Using chromatin immunoprecipitation in human primary hepatocytes as well as electrophoretic mobility shift and luciferase reporter-gene assays, we identified three functional PPARα-binding regions (PBR-I, -II, and -III) within ∼12 kb of the CYP3A4 upstream sequence. Furthermore, a humanized CYP3A4/3A7 mouse model showed in vivo induction of CYP3A4 mRNA and protein by [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio]acetic acid (WY14,643) in liver but not in intestine, whereas hepatic occupancy of PBRs by PPARα was ligand independent. Using lentiviral gene knock-down and treatment with WY14,643 in primary human hepatocytes, PPARα was further shown to affect the expression of a distinct set of CYPs, including 1A1, 1A2, 2B6, 2C8, 3A4, and 7A1, but not 2C9, 2C19, 2D6, or 2E1. Interestingly, the common phospholipid 1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphocholine (16:0/18:1-PC), previously proposed to reflect nutritional status and shown to be a specific endogenous ligand of PPARα, induced CYP3A4 (up to 4-fold) and other biotransformation genes in hepatocytes with similar selectivity and potency as WY14,643. These data establish PPARα as a direct transcriptional regulator of hepatic CYP3A4. This finding warrants investigation of both known and newly developed PPARα-targeted drugs for their drug-drug interaction potential. Furthermore, our data suggest that nutritional status can influence drug biotransformation capacity via endogenous phospholipid signaling.

Altered histone modifications in cancer

In human health and disease the choreographed actions of a wide armory of transcription factors govern the regulated expression of coding and nonprotein coding genes. These actions are central to human health and are evidently aberrant in cancer. Central components of regulated gene expression are a variety of epigenetic mechanisms that include histone modifications. The post-translational modifications of histones are widespread and diverse, and appear to be spatial--temporally regulated in a highly intricate manner. The true functional consequences of these patterns of regulation are still emerging. Correlative evidence supports the idea that these patterns are distorted in malignancy on both a genome-wide and a discrete gene loci level. These patterns of distortion also often reflect the altered expression of the enzymes that control these histone states. Similarly gene expression patterns also appear to reflect a correlation with altered histone modifications at both the candidate loci and genome-wide level. Clarity is emerging in resolving these relationships between histone modification status and gene expression -patterns. For example, altered transcription factor interactions with the key co-activator and co-repressors, which in turn marshal many of the histone-modifying enzymes, may distort regulation of histone modifications at specific gene loci. In turn these aberrant transcriptional processes can trigger other altered epigenetic events such as DNA methylation and underline the aberrant and specific gene expression patterns in cancer. Considered in this manner, altered expression and recruitment of histone-modifying enzymes may underline the distortion to transcriptional responsiveness observed in malignancy. Insight from understanding these processes addresses the challenge of targeted epigenetic therapies in cancer.

The role of AAA+ proteases in mitochondrial protein biogenesis, homeostasis and activity control

Mitochondria are specialised organelles that are structurally and functionally integrated into cells in the vast majority of eukaryotes. They are the site of numerous enzymatic reactions, some of which are essential for life. The double lipid membrane of the mitochondrion, that spatially defines the organelle and is necessary for some functions, also creates a physical but semi-permeable barrier to the rest of the cell. Thus to ensure the biogenesis, regulation and maintenance of a functional population of proteins, an autonomous protein handling network within mitochondria is required. This includes resident mitochondrial protein translocation machinery, processing peptidases, molecular chaperones and proteases. This review highlights the contribution of proteases of the AAA+ superfamily to protein quality and activity control within the mitochondrion. Here they are responsible for the degradation of unfolded, unassembled and oxidatively damaged proteins as well as the activity control of some enzymes. Since most knowledge about these proteases has been gained from studies in the eukaryotic microorganism Saccharomyces cerevisiae, much of the discussion here centres on their role in this organism. However, reference is made to mitochondrial AAA+ proteases in other organisms, particularly in cases where they play a unique role such as the mitochondrial unfolded protein response. As these proteases influence mitochondrial function in both health and disease in humans, an understanding of their regulation and diverse activities is necessary.

Dysregulated bile acid synthesis, metabolism and excretion in a high fat-cholesterol diet-induced fibrotic steatohepatitis in rats

BACKGROUND AND AIMS

Cholesterol over-intake is involved in the onset of nonalcoholic steatohepatitis (NASH), and hepatocellular bile acid (BA) accumulation correlates with liver injuries. However, how dietary cholesterol influences cholesterol and BA kinetics in NASH liver remains ambiguous and needs to be clarified.

METHODS

Molecular markers involved in cholesterol and BA kinetics were investigated at protein and mRNA levels in an already-established stroke-prone spontaneously hypertensive 5/Dmcr rat model with fibrotic steatohepatitis, by feeding a high fat-cholesterol (HFC) diet.

RESULTS

Unlike the control diet, the HFC diet deposited cholesterol greatly in rat livers, where 3-hydroxy-3-methylglutaryl CoA reductase, low-density lipoprotein (LDL) receptor and LDL receptor-related protein-1 were expectedly downregulated, especially at 8 and 14 weeks, suggesting that cholesterol synthesis and uptake in response to cholesterol accumulation may not be disorganized. The HFC diet did not upregulate liver X receptor-α, conversely, it enhanced classic BA synthesis by upregulating cholesterol 7α-hydroxylase but downregulating sterol 12α-hydroxylase, and influenced alternative synthesis by downregulating sterol 27-hydroxylase but upregulating oxysterol 7α-hydroxylase, mainly at 8 and 14 weeks, indicating that there were different productions of primary BA species. Unexpectedly, no feedback inhibition of BA synthesis by farnesoid X receptor occurred. Additionally, the HFC diet impaired BA detoxification by UDP-glucuronosyltransferase and sulfotransferase 2A1, and decreased excretion by bile salt export pump at 8 and 14 weeks, although it induced compensatory export by multidrug resistance-associated protein-3. The disturbed BA detoxification may correlate with suppressed pregnane X receptor and constitutive androstane receptor.

CONCLUSIONS

The HFC diet may accumulate BA in rat livers, which influences fibrotic steatohepatitis progression.

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.

Chemoprevention of colorectal cancer with ursodeoxycholic acid: pro

Colorectal cancer is the third and second most common cancer among men and women, respectively, in France. Interest in the chemoprevention of colorectal cancer has increased over the last two decades. Experimental data strongly suggest that ursodeoxycholic acid (UDCA) may have chemopreventative actions in colorectal cancer. UDCA is able to inhibit tumor development in azoxymethane and in dextran-related colitis models. In high-risk populations such as subjects with previous colorectal adenoma removal or inflammatory bowel disease, five out of 10 published studies suggested beneficial effects with UDCA on colonic carcinogenesis. In the azoxymethane model, UDCA inhibited tumor development by counteracting the tumor-promoting effects of secondary bile acids such as deoxycholic acid (DCA). The opposing effects of UDCA and DCA on lipid raft composition may be central to their effects on colonic tumorigenesis. Differential effects of DCA and UDCA on growth factor and inflammatory signals involved in colorectal carcinogenesis, such as epidermal growth factor receptor (EGFR) signaling and COX-2 expression, very likely mediate their opposing effects on colonic tumor promotion and tumor inhibition, respectively.

FXR and PXR: potential therapeutic targets in cholestasis

Cholestatic liver disorders encompass hepatobiliary diseases of diverse etiologies characterized by the accumulation of bile acids, bilirubin and cholesterol as the result of impaired secretion of bile. Members of the nuclear receptor (NR) family of ligand-modulated transcription factors are implicated in the adaptive response to cholestasis. NRs coordinately regulate bile acid and phospholipid transporter genes required for hepatobiliary transport, as well as the phases I and II metabolizing enzymes involved in processing of their substrates. In this review we will focus on FXR and PXR, two members of the NR family whose activities are regulated by bile acids. In addition, we also discuss the potential of pharmacological modulators of these receptors as novel therapies for cholestatic disorders.

Aryl hydrocarbon receptor regulates the cholesterol biosynthetic pathway in a dioxin response element-independent manner

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor. Activation of AhR mediates the expression of target genes (e.g., CYP1A1) by binding to dioxin response element (DRE) sequences in their promoter region. To understand the multiple mechanisms of AhR-mediated gene regulation, a microarray analysis on liver isolated from ligand-treated transgenic mice expressing a wild-type (WT) Ahr or a DRE-binding mutant Ahr (A78D) on an ahr-null background was performed. Results revealed that AhR DRE binding is not required for the suppression of genes involved in cholesterol synthesis. Quantitative reverse-transcription polymerase chain reaction performed on both mouse liver and primary human hepatocyte RNA demonstrated a coordinated repression of genes involved in cholesterol biosynthesis, namely, HMGCR, FDFT1, SQLE, and LSS after receptor activation. An additional transgenic mouse line was established expressing a liver-specific Ahr-A78D on a Cre(Alb)/Ahr(flox/flox) background. These mice displayed a similar repression of cholesterol biosynthetic genes, compared to Ahr(flox/flox) mice, further indicating that the observed modulation is AhR specific and occurs in a DRE-independent manner. Elevated hepatic transcriptional levels of the genes of interest were noted in congenic C57BL/6J-Ah(d) allele mice, when compared to the WT C57BL/6J mice, which carry the Ah(b) allele. Down-regulation of AhR nuclear translocator levels using short interfering RNA in a human cell line revealed no effect on the expression of cholesterol biosynthetic genes. Finally, cholesterol secretion was shown to be significantly decreased in human cells after AhR activation.

CONCLUSION

These data firmly establish an endogenous role for AhR as a regulator of the cholesterol biosynthesis pathway independent of its DRE-binding ability, and suggest that AhR may be a previously unrecognized therapeutic target.

Regulation of xenobiotic transporter genes in liver and brain of juvenile thicklip grey mullets (Chelon labrosus) after exposure to Prestige-like fuel oil and to perfluorooctane sulfonate

Xenobiotic transport proteins are involved in cellular defence against accumulation of xenobiotics participating in multixenobiotic resistance (MXR). In order to study the transcriptional regulation of MXR genes in fish exposed to common chemical pollutants we selected the thicklip grey mullet (Chelon labrosus), since mugilids are widespread in highly degraded estuarine environments where they have to survive through development and adulthood. Partial sequences belonging to genes coding for members of 3 different families of ATP binding cassette (ABC) transporter proteins (ABCB1; ABCB11; ABCC2; ABCC3; ABCG2) and a vault protein (major vault protein, MVP) were amplified and sequenced from mullet liver. Their liver and brain transcription levels were examined in juvenile mullets under exposure to perfluorooctane sulfonate (PFOS) and to fresh (F) and weathered (WF) Prestige-like heavy fuel oil for 2 and 16 days. In liver, PFOS significantly up-regulated transcription of abcb1, abcb11 and abcg2 while in brain only abcb11 was up-regulated. Both fuel treatments significantly down-regulated abcb11 in liver at day 2 while abcc2 was only down-regulated by WF. mvp was significantly up-regulated by F and down-regulated by WF at day 2 in the liver. At day 16 only a significant up-regulation of abcb1 in the F group was recorded. Brain abcc3 and abcg2 were down-regulated by both fuels at day 2, while abcb1 and abcc2 were only down-regulated by F exposure. After 16 days of exposure only abcb11 and abcg2 were regulated. In conclusion, exposure to organic xenobiotics significantly alters transcription levels of genes participating in xenobiotic efflux, especially after short periods of exposure. Efflux transporter gene transcription profiling could thus constitute a promising tool to assess exposure to common pollutants.

Transcriptional profiling of human liver identifies sex-biased genes associated with polygenic dyslipidemia and coronary artery disease

Sex-differences in human liver gene expression were characterized on a genome-wide scale using a large liver sample collection, allowing for detection of small expression differences with high statistical power. 1,249 sex-biased genes were identified, 70% showing higher expression in females. Chromosomal bias was apparent, with female-biased genes enriched on chrX and male-biased genes enriched on chrY and chr19, where 11 male-biased zinc-finger KRAB-repressor domain genes are distributed in six clusters. Top biological functions and diseases significantly enriched in sex-biased genes include transcription, chromatin organization and modification, sexual reproduction, lipid metabolism and cardiovascular disease. Notably, sex-biased genes are enriched at loci associated with polygenic dyslipidemia and coronary artery disease in genome-wide association studies. Moreover, of the 8 sex-biased genes at these loci, 4 have been directly linked to monogenic disorders of lipid metabolism and show an expression profile in females (elevated expression of ABCA1, APOA5 and LDLR; reduced expression of LIPC) that is consistent with the lower female risk of coronary artery disease. Female-biased expression was also observed for CYP7A1, which is activated by drugs used to treat hypercholesterolemia. Several sex-biased drug-metabolizing enzyme genes were identified, including members of the CYP, UGT, GPX and ALDH families. Half of 879 mouse orthologs, including many genes of lipid metabolism and homeostasis, show growth hormone-regulated sex-biased expression in mouse liver, suggesting growth hormone might play a similar regulatory role in human liver. Finally, the evolutionary rate of protein coding regions for human-mouse orthologs, revealed by dN/dS ratio, is significantly higher for genes showing the same sex-bias in both species than for non-sex-biased genes. These findings establish that human hepatic sex differences are widespread and affect diverse cell metabolic processes, and may help explain sex differences in lipid profiles associated with sex differential risk of coronary artery disease.

Gene-activation mechanisms in the regression of atherosclerosis, elimination of diabetes type 2, and prevention of dementia

Atherosclerotic vascular disease, diabetes mellitus (DM) and dementia are major global health problems. Both endogenous and exogenous factors activate genes functioning in biological processes. This review article focuses on gene-activation mechanisms that regress atherosclerosis, eliminate DM type 2 (DM2), and prevent cognitive decline and dementia. Gene-activating compounds upregulating functions of liver endoplasmic reticulum (ER) and affecting lipid and protein metabolism, increase ER size through membrane synthesis, and produce an antiatherogenic plasma lipoprotein profile. Numerous gene-activators regress atherosclerosis and reduce the occurrence of atherosclerotic disease. The gene-activators increase glucose disposal rate and insulin sensitivity and, by restoring normal glucose and insulin levels, remove metabolic syndrome and DM2. Patients with DM2 show an improvement of plasma lipoprotein profile and glucose tolerance together with increase in liver phospholipid (PL) and cytochrome (CYP) P450. The gene-activating compounds induce hepatic protein and PL synthesis, and upregulate enzymes including CYPs and glucokinase, nuclear receptors, apolipoproteins and ABC (ATP-binding cassette) transporters. They induce reparation of ER structures and eliminate consequences of ER stress. Healthy living habits activate mechanisms that maintain high levels of HDL and apolipoprotein AI, promote health, and prevent cognitive decline and dementia. Agonists of liver X receptor (LXR) reduce amyloid in brain plaques and improve cognitive performance in mouse models of Alzheimer's disease. The gene activation increases the capacity to withstand cellular stress and to repair cellular damage and increases life span. Life free of major health problems and in good cognitive health promotes well-being and living a long and active life.

Downregulation of breast cancer resistance protein in colon adenomas reduces cellular xenobiotic resistance and leads to accumulation of a food-derived carcinogen

Several molecular changes in colorectal adenomas provide the basis of the adenoma-carcinoma sequence. We investigated the expression of xenobiotic ATP-binding cassette (ABC) transporters in humans and in ApcMin mice and conducted functional studies estimating the importance of the expression changes. Twenty-nine adenomas from 21 patients and eight adenomas from four ApcMin mice were analyzed using Western blotting and quantitative Real-time polymerase chain reaction (RT-PCR). Adjacent healthy tissue served as control for each polyp. Breast cancer resistance protein (BCRP) was significantly downregulated in human colorectal adenomas (to 28 ± 35% of adjacent healthy tissue). This was in line with data from ApcMin mice adenomas, where downregulation was significant as well (to 58 ± 34%). In parallel, quantitative RT-PCR showed BCRP mRNA downregulation in human adenomas (to 17 ± 31%). Basal multidrug resistance-associated protein 2 expression was low and did not change in adenomas; multidrug resistance transporter 1 expression also did not differ between adenomas and healthy tissue. In a functional study, ApcMin mice received radioactively labelled 2-amino-1-methyl-6-phenylimidazo[4,5-β] pyridine (PhIP), a food colon carcinogen and substrate of BCRP, by oral gavage with analysis of PhIP accumulation and DNA adduct formation 48 hr later. In this setting, we could demonstrate a higher carcinogen concentration in adenomas of ApcMin mice (181 ± 113% of normal tissue) including immunohistochemical detection of PhIP-DNA adducts. We conclude that significant transcriptional downregulation of BCRP/Bcrp leads to higher carcinogen concentrations in colorectal adenomas of mice and men. This might promote the adenoma-carcinoma sequence by higher genotoxic effects. The results indicate a possible role of transporter deficiencies in susceptibility for colon carcinoma.

The statin class of HMG-CoA reductase inhibitors demonstrate differential activation of the nuclear receptors PXR, CAR and FXR, as well as their downstream target genes

The therapeutic class of HMG-CoA reductase inhibitors, the statins are central agents in the treatment of hypercholesterolaemia and the associated conditions of cardiovascular disease, obesity and metabolic syndrome. Although statin therapy is generally considered safe, a number of known adverse effects do occur, most commonly treatment-associated muscular pain. In vitro evidence also supports the potential for drug-drug interactions involving this class of agents, and to examine this a ligand-binding assay was used to determine the ability of six clinically used statins for their ability to directly activate the nuclear receptors pregnane X-receptor (PXR), farnesoid X-receptor (FXR) and constitutive androstane receptor (CAR), demonstrating a relative activation of PXR>FXR>CAR. Using reporter gene constructs, we demonstrated that this order of activation is mirrored at the transcriptional activation level, with PXR-mediated gene activation being pre-eminent. Finally, we described a novel regulatory loop, whereby activation of FXR by statins increases PXR reporter gene expression, potentially enhancing PXR-mediated responses. Delineating the molecular interactions of statins with nuclear receptors is an important step in understanding the full biological consequences of statin exposure. This demonstration of their ability to directly activate nuclear receptors, leading to nuclear receptor cross-talk, has important potential implications for their use within a polypharmacy paradigm.

Evolution of promiscuous nuclear hormone receptors: LXR, FXR, VDR, PXR, and CAR

Nuclear hormone receptors (NHRs) are transcription factors that work in concert with co-activators and co-repressors to regulate gene expression. Some examples of ligands for NHRs include endogenous compounds such as bile acids, retinoids, steroid hormones, thyroid hormone, and vitamin D. This review describes the evolution of liver X receptors α and β (NR1H3 and 1H2, respectively), farnesoid X receptor (NR1H4), vitamin D receptor (NR1I1), pregnane X receptor (NR1I2), and constitutive androstane receptor (NR1I3). These NHRs participate in complex, overlapping transcriptional regulation networks involving cholesterol homeostasis and energy metabolism. Some of these receptors, particularly PXR and CAR, are promiscuous with respect to the structurally wide range of ligands that act as agonists. A combination of functional and computational analyses has shed light on the evolutionary changes of NR1H and NR1I receptors across vertebrates, and how these receptors may have diverged from ancestral receptors that first appeared in invertebrates.