New insights on the xenobiotic-sensing nuclear receptors in liver diseases--CAR and PXR--

Allocholic acid protects against α-naphthylisothiocyanate-induced cholestasis in mice by ameliorating disordered bile acid homeostasis

Cholestasis is a pathological condition characterized by disruptions in bile flow, leading to the accumulation of bile acids (BAs) in hepatocytes. Allocholic acid (ACA), a unique fetal BA known for its potent choleretic effects, reappears during liver regeneration and carcinogenesis. In this research, we investigated the protective effects and underlying mechanisms of ACA against mice with cholestasis brought on by α-naphthylisothiocyanate (ANIT). To achieve this, we combined network pharmacology, targeted BA metabolomics, and molecular biology approaches. The results demonstrated that ACA treatment effectively reduced levels of serum AST, ALP, and DBIL, and ameliorated the pathological injury caused by cholestasis. Network pharmacology analysis suggested that ACA primarily regulated BA and salt transport, along with the signaling pathway associated with bile secretion, to improve cholestasis. Subsequently, we examined changes in BA metabolism using UPLC-MS/MS. The findings indicated that ACA pretreatment induced alterations in the size, distribution, and composition of the liver BA pool. Specifically, it reduced the excessive accumulation of BAs, especially cholic acid (CA), taurocholic acid (TCA), and β-muricholic acid (β-MCA), facilitating the restoration of BA homeostasis. Furthermore, ACA pretreatment significantly downregulated the expression of hepatic BA synthase Cyp8b1, while enhancing the expression of hepatic efflux transporter Mrp4, as well as the renal efflux transporters Mdr1 and Mrp2. These changes collectively contributed to improved BA efflux from the liver and enhanced renal elimination of BAs. In conclusion, ACA demonstrated its potential to ameliorate ANIT-induced liver damage by inhibiting BA synthesis and promoting both BA efflux and renal elimination pathways, thus, restoring BA homeostasis.

Harnessing Oleanolic Acid and Its Derivatives as Modulators of Metabolic Nuclear Receptors

Nuclear receptors (NRs) constitute a superfamily of ligand-activated transcription factors with a paramount role in ubiquitous physiological functions such as metabolism, growth, and reproduction. Owing to their physiological role and druggability, NRs are deemed attractive and valid targets for medicinal chemists. Pentacyclic triterpenes (PTs) represent one of the most important phytochemical classes present in higher plants, where oleanolic acid (OA) is the most studied PTs representative owing to its multitude of biological activities against cancer, inflammation, diabetes, and liver injury. PTs possess a lipophilic skeleton that imitates the NRs endogenous ligands. Herein, we report a literature overview on the modulation of metabolic NRs by OA and its semi-synthetic derivatives, highlighting their health benefits and potential therapeutic applications. Indeed, OA exhibited varying pharmacological effects on FXR, PPAR, LXR, RXR, PXR, and ROR in a tissue-specific manner. Owing to these NRs modulation, OA showed prominent hepatoprotective properties comparable to ursodeoxycholic acid (UDCA) in a bile duct ligation mice model and antiatherosclerosis effect as simvastatin in a model of New Zealand white (NZW) rabbits. It also demonstrated a great promise in alleviating non-alcoholic steatohepatitis (NASH) and liver fibrosis, attenuated alpha-naphthol isothiocyanate (ANIT)-induced cholestatic liver injury, and controlled blood glucose levels, making it a key player in the therapy of metabolic diseases. We also compiled OA semi-synthetic derivatives and explored their synthetic pathways and pharmacological effects on NRs, showcasing their structure-activity relationship (SAR). To the best of our knowledge, this is the first review article to highlight OA activity in terms of NRs modulation.

Theonella: A Treasure Trove of Structurally Unique and Biologically Active Sterols

The marine environment is considered a vast source in the discovery of structurally unique bioactive secondary metabolites. Among marine invertebrates, the sponge Theonella spp. represents an arsenal of novel compounds ranging from peptides, alkaloids, terpenes, macrolides, and sterols. In this review, we summarize the recent reports on sterols isolated from this amazing sponge, describing their structural features and peculiar biological activities. We also discuss the total syntheses of solomonsterols A and B and the medicinal chemistry modifications on theonellasterol and conicasterol, focusing on the effect of chemical transformations on the biological activity of this class of metabolites. The promising compounds identified from Theonella spp. possess pronounced biological activity on nuclear receptors or cytotoxicity and result in promising candidates for extended preclinical evaluations. The identification of naturally occurring and semisynthetic marine bioactive sterols reaffirms the utility of examining natural product libraries for the discovery of new therapeutical approach to human diseases.

Role of bile acids in overweight and obese children and adolescents

Bile acids (BAs) are amphipathic molecules synthetized in the liver. They are primarily involved in the digestion of nutrients. Apart from their role in dietary lipid absorption, BAs have progressively emerged as key regulators of systemic metabolism and inflammation. In the last decade, it became evident that BAs are particularly important for the regulation of glucose, lipid, and energy metabolism. Indeed, the interest in role of BA in metabolism homeostasis is further increased due to the global public health increase in obesity and related complications and a large number of research postulating that there is a close mutual relationship between BA and metabolic disorders. This strong relationship seems to derive from the role of BAs as signaling molecules involved in the regulation of a wide spectrum of metabolic pathways. These actions are mediated by different receptors, particularly nuclear farnesoid X receptor (FXR) and Takeda G protein coupled receptor 5 (TGR5), which are probably the major effectors of BA actions. These receptors activate transcriptional networks and signaling cascades controlling the expression and activity of genes involved in BA, lipid and carbohydrate metabolism, energy expenditure, and inflammation. The large correlation between BAs and metabolic disorders offers the possibility that modulation of BAs could be used as a therapeutic approach for the treatment of metabolic diseases, including obesity itself. The aim of this review is to describe the main physiological and metabolic actions of BA, focusing on its signaling pathways, which are important in the regulation of metabolism and might provide new BA -based treatments for metabolic diseases.

Cooperative Interaction of Phenolic Acids and Flavonoids Contained in Activated Charcoal with Herb Extracts, Involving Cholesterol, Bile Acid, and FXR/PXR Activation in Broilers Fed with Mycotoxin-Containing Diets

The charcoal-herb extract complex (CHC) is a product of activated charcoal sorption of herb extracts that contain phenolic acids and flavonoids. The effective dose of CHC to promote animal growth is about one tenth of effective dosage of activated charcoal. The purpose of this study was to evaluate potential cooperative interactions between activated charcoal and herb extracts. Two feeding experiments were conducted. In Experiment 1, a responsive dose of CHC to broiler growth was determined to be 250 mg/kg of the diet. In Experiment 2, CHC increased growth performance and improved meat quality, but decreased indices of oxidative stress and inflammation as compared with similar doses of activated charcoal or herb extracts. CHC also increased concentrations of serum cholesterol, bile acid in the gallbladder, and bile acid in feces. The herb extracts present in CHC were largely represented by phenolic acids (PAs, caffeic acid, and vanillin) and flavonoids (FVs, daidzein, and quercetin-D-glucoside) in the detoxification activity of CHC in a mouse rescue test when the mice were gavaged with T-2 mycotoxin. PAs and FVs significantly increased the expression of CYP7A1, PXR, CYP3A37, Slco1B3, and Bsep in chicken primary hepatocytes. In conclusion, CHC integrated the cooperative interactions of activated charcoal and herb extracts via the FXR/RXR-PXR pathway to detoxify mycotoxins.

Ivermectin induces chicken BCRP/ABCG2 expression and function: Involvement of CXR signaling pathway and mRNA stabilization

Ivermectin is a macrocyclic lactone antiparasitic drug widely used in human and veterinary medicine. Previous studies indicated that ivermectin could interact with P-glycoprotein, being a good inducer and substrate; however, it is unknown whether ivermectin affects BCRP of chicken. In this study, we found that ivermectin distinctly affected the expression of BCRP in a time- and concentration-dependent up-regulatory way in chicken primary hepatocytes. Subsequent series of experiments showed that the BCRP induction is related with the increase of CXR expression and, promoting CXR translocations to the nucleus and enhancing the stability of Abcg2 mRNA at the post-transcriptional level by ivermectin. Furthermore, we observed that ivermectin also enhanced the stability of Abcg2 mRNA at the post-transcriptional level by Act-D chase assay. We got the similar results by in vivo test that ivermectin-induced BCRP and CXR expression in pharmacologically important tissues, and decreased the apparent permeability coefficient of florfenicol (substrate of chicken BCRP). In conclusion, the results indicated that ivermectin could induce chicken BCRP expression and function through the transcriptional CXR signaling pathway and post-transcriptional mRNA stabilization.

PXR as the tipping point between innate immune response, microbial infections, and drug metabolism

Pregnane X receptor (PXR) is a xenosensor that acts as a transcription factor in the cell nucleus to protect cells from toxic insults. In response to exposure to several chemical agents, PXR induces the expression of enzymes and drug transporters that biotransform xenobiotic and endobiotic and eliminate metabolites. Recently, PXR has been shown to have immunomodulatory effects that involve cross-communication with molecular pathways in innate immunity cells. Conversely, several inflammatory factors regulate PXR signaling. This review examines the crosstalk between PXR and nuclear factor kappa B (NFkB), Toll-like receptors (TLRs), and inflammasome components. Discussions of the consequences of these interactions on immune responses to infections caused by viruses, bacteria, fungi, and parasites are included together with a review of the effects of microorganisms on PXR-associated drug metabolism. This paper aims to encourage researchers to pursue studies that will better elucidate the relationship between PXR and the immune system and thus inform treatment development.

The role of bile acids in carcinogenesis

Bile acids are soluble derivatives of cholesterol produced in the liver that subsequently undergo bacterial transformation yielding a diverse array of metabolites. The bulk of bile acid synthesis takes place in the liver yielding primary bile acids; however, other tissues have also the capacity to generate bile acids (e.g. ovaries). Hepatic bile acids are then transported to bile and are subsequently released into the intestines. In the large intestine, a fraction of primary bile acids is converted to secondary bile acids by gut bacteria. The majority of the intestinal bile acids undergo reuptake and return to the liver. A small fraction of secondary and primary bile acids remains in the circulation and exert receptor-mediated and pure chemical effects (e.g. acidic bile in oesophageal cancer) on cancer cells. In this review, we assess how changes to bile acid biosynthesis, bile acid flux and local bile acid concentration modulate the behavior of different cancers. Here, we present in-depth the involvement of bile acids in oesophageal, gastric, hepatocellular, pancreatic, colorectal, breast, prostate, ovarian cancer. Previous studies often used bile acids in supraphysiological concentration, sometimes in concentrations 1000 times higher than the highest reported tissue or serum concentrations likely eliciting unspecific effects, a practice that we advocate against in this review. Furthermore, we show that, although bile acids were classically considered as pro-carcinogenic agents (e.g. oesophageal cancer), the dogma that switch, as lower concentrations of bile acids that correspond to their serum or tissue reference concentration possess anticancer activity in a subset of cancers. Differences in the response of cancers to bile acids lie in the differential expression of bile acid receptors between cancers (e.g. FXR vs. TGR5). UDCA, a bile acid that is sold as a generic medication against cholestasis or biliary surge, and its conjugates were identified with almost purely anticancer features suggesting a possibility for drug repurposing. Taken together, bile acids were considered as tumor inducers or tumor promoter molecules; nevertheless, in certain cancers, like breast cancer, bile acids in their reference concentrations may act as tumor suppressors suggesting a Janus-faced nature of bile acids in carcinogenesis.

A Recent Ten-Year Perspective: Bile Acid Metabolism and Signaling

Bile acids are important physiological agents required for the absorption, distribution, metabolism, and excretion of nutrients. In addition, bile acids act as sensors of intestinal contents, which are determined by the change in the spectrum of bile acids during microbial transformation, as well as by gradual intestinal absorption. Entering the liver through the portal vein, bile acids regulate the activity of nuclear receptors, modify metabolic processes and the rate of formation of new bile acids from cholesterol, and also, in all likelihood, can significantly affect the detoxification of xenobiotics. Bile acids not absorbed by the liver can interact with a variety of cellular recipes in extrahepatic tissues. This provides review information on the synthesis of bile acids in various parts of the digestive tract, its regulation, and the physiological role of bile acids. Moreover, the present study describes the involvement of bile acids in micelle formation, the mechanism of intestinal absorption, and the influence of the intestinal microbiota on this process.

Noncanonical Constitutive Androstane Receptor Signaling in Gene Regulation

The constitutive androstane receptor (CAR, NR1I3) is extremely important for the regulation of many physiological processes, especially xenobiotic (drug) metabolism and transporters. CAR differs from steroid hormone receptors in that it can be activated using structurally unrelated chemicals, both through direct ligand-binding and ligand-independent (indirect) mechanisms. By binding to specific responsive elements on DNA, CAR increases the expression of its target genes encoding drug-metabolizing enzymes and transporters. Therefore, CAR is mainly characterized as a ligand-dependent or ligand-independent transcription factor, and the induction of gene expression is considered the canonical mode of CAR action. Consistent with its central role in xenobiotic metabolism, CAR signaling includes a collection of mechanisms that are employed alongside the core transcriptional machinery of the receptor. These so-called noncanonical CAR pathways allow the receptor to coordinate the regulation of many aspects of cell biology. In this mini-review, we review noncanonical CAR signaling, paying special attention to the role of CAR in energy homeostasis and cell proliferation.

Pediatric Cholestatic Liver Disease: Review of Bile Acid Metabolism and Discussion of Current and Emerging Therapies

Cholestatic liver diseases are a significant cause of morbidity and mortality and the leading indication for pediatric liver transplant. These include diseases such as biliary atresia, Alagille syndrome, progressive intrahepatic cholestasis entities, ductal plate abnormalities including Caroli syndrome and congenital hepatic fibrosis, primary sclerosing cholangitis, bile acid synthesis defects, and certain metabolic disease. Medical management of these patients typically includes supportive care for complications of chronic cholestasis including malnutrition, pruritus, and portal hypertension. However, there are limited effective interventions to prevent progressive liver damage in these diseases, leaving clinicians to ultimately rely on liver transplantation in many cases. Agents such as ursodeoxycholic acid, bile acid sequestrants, and rifampicin have been mainstays of treatment for years with the understanding that they may decrease or alter the composition of the bile acid pool, though clinical response to these medications is frequently insufficient and their effects on disease progression remain limited. Recently, animal and human studies have identified potential new therapeutic targets which may disrupt the enterohepatic circulation of bile acids, alter the expression of bile acid transporters or decrease the production of bile acids. In this article, we will review bile formation, bile acid signaling, and the relevance for current and newer therapies for pediatric cholestasis. We will also highlight further areas of potential targets for medical intervention for pediatric cholestatic liver diseases.

Constitutive androstane receptor and pregnane X receptor cooperatively ameliorate DSS-induced colitis

BACKGROUND

Nuclear receptor pregnane X receptor (PXR) was shown to be protective in case of dextran sulfate sodium (DSS)-induced colitis. Constitutive androstane receptor (CAR) belongs to the same nuclear receptor subfamily with PXR. The roles of both receptors in DSS-induced colitis were evaluated.

METHODS

Wild-type, Car-null, Pxr-null, and Car/Pxr-null mice were treated with a CAR/PXR agonist or vehicle and administered 2.5% DSS in the drinking water. The typical clinical symptoms, histological scoring, proinflammatory cytokine, and apoptosis were analyzed.

RESULTS

Mice treated with the PXR agonist pregnenolone-16α-carbonitrile (PCN) were protected from DSS-induced colitis, as in a previous study. Mice treated with the CAR agonist, 4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) were also protected from DSS-induced colitis. Interestingly, the protective effects of PCN in the Car-null mice and those of TCPOBOP in the Pxr-null mice both decreased. PCN or TCPOBOP pretreatment significantly decreased the macrophage and monocyte infiltration in DSS-induced colitis. PXR and CAR agonists reduced the mRNA expression of several proinflammatory cytokines in a PXR- and CAR-dependent manner, respectively. CAR inhibited apoptosis by inducing Gadd45b. PXR inhibited TNF-α and IL-1b and CAR induced Gadd45b in in vitro cell analyses.

CONCLUSIONS

We showed that CAR and PXR cooperatively ameliorate DSS-induced colitis. PXR and CAR protected against DSS-induced colitis by inhibiting proinflammatory cytokines and apoptosis, respectively.

(-)-Epigallocatechin-3-gallate (EGCG) inhibits starch digestion and improves glucose homeostasis through direct or indirect activation of PXR/CAR-mediated phase II metabolism in diabetic mice

As a major component of green tea, (-)-epigallocatechin-3-gallate (EGCG) has attracted interest from scientists owing to its potential to combat a variety of human diseases including abnormal glucose metabolism in obesity and diabetes. This study aims to (1) evaluate the molecular mechanism of EGCG in starch digestion before EGCG absorption; (2) investigate the link between PXR/CAR-mediated phase II metabolism and glucose homeostasis after EGCG is transported to small intestine and liver. EGCG suppressed starch hydrolysis both in vitro and in vivo. Molecular simulation results demonstrated that EGCG could bind to the active site of α-amylase and α-glucosidase, acting as an inhibitor. In addition, the anti-diabetic action of EGCG was investigated in high fat diet and STZ-induced type 2 diabetes. EGCG improved glucose homeostasis and inhibited the process of gluconeogenesis (PEPCK and G-6-Pase) and lipogenesis (SREBP-1C, FAS and ACC1) in the liver. Meanwhile, EGCG treatment activated PXR/CAR, accompanied by upgrading PXR/CAR-mediated phase II drug metabolism enzyme expression in small intestine and liver, involving SULT1A1, UGT1A1 and SULT2B1b. Dietary polyphenol EGCG could serve as a promising PXR/CAR activator and therapeutic intervention in diabetes.

Bile Acid-Activated Receptors: A Review on FXR and Other Nuclear Receptors

Nuclear receptors (NRs) are ligand-dependent transcription factors that are involved in various biological processes including metabolism, reproduction, and development. Upon activation by their ligands, NRs bind to their specific DNA elements, exerting their biological functions by regulating their target gene expression. Bile acids are detergent-like molecules that are synthesized in the liver. They not only function as a facilitator for the digestion of lipids and fat-soluble vitamins but also serve as signaling molecules for several nuclear receptors to regulate diverse biological processes including lipid, glucose, and energy metabolism, detoxification and drug metabolism, liver regeneration, and cancer. The nuclear receptors including farnesoid X receptor (FXR), pregnane X receptor (PXR), constitutive androstane receptor (CAR), vitamin D receptor (VDR), and small heterodimer partner (SHP) constitute an integral part of the bile acid signaling. This chapter reviews the role of the NRs in bile acid homeostasis, highlighting the regulatory functions of the NRs in lipid and glucose metabolism in addition to bile acid metabolism.

Phosphorylated Nuclear Receptor CAR Forms a Homodimer To Repress Its Constitutive Activity for Ligand Activation

The nuclear receptor CAR (NR1I3) regulates hepatic drug and energy metabolism as well as cell fate. Its activation can be a critical factor in drug-induced toxicity and the development of diseases, including diabetes and tumors. CAR inactivates its constitutive activity by phosphorylation at threonine 38. Utilizing receptor for protein kinase 1 (RACK1) as the regulatory subunit, protein phosphatase 2A (PP2A) dephosphorylates threonine 38 to activate CAR. Here we demonstrate that CAR undergoes homodimer-monomer conversion to regulate this dephosphorylation. By coexpression of two differently tagged CAR proteins in Huh-7 cells, mouse primary hepatocytes, and mouse livers, coimmunoprecipitation and two-dimensional gel electrophoresis revealed that CAR can form a homodimer in a configuration in which the PP2A/RACK1 binding site is buried within its dimer interface. Epidermal growth factor (EGF) was found to stimulate CAR homodimerization, thus constraining CAR in its inactive form. The agonistic ligand CITCO binds directly to the CAR homodimer and dissociates phosphorylated CAR into its monomers, exposing the PP2A/RACK1 binding site for dephosphorylation. Phenobarbital, which is not a CAR ligand, binds the EGF receptor, reversing the EGF signal to monomerize CAR for its indirect activation. Thus, the homodimer-monomer conversion is the underlying molecular mechanism that regulates CAR activation, by placing phosphorylated threonine 38 as the common target for both direct and indirect activation of CAR.

Effects of Nonalcoholic Fatty Liver Disease on Hepatic CYP2B1 and in Vivo Bupropion Disposition in Rats Fed a High-Fat or Methionine/Choline-Deficient Diet

Nonalcoholic fatty liver disease (NAFLD) refers to hepatic pathologies, including simple fatty liver (SFL), nonalcoholic steatohepatitis (NASH), fibrosis, and cirrhosis, that may progress to hepatocellular carcinoma. These liver disease states may affect the activity and expression levels of drug-metabolizing enzymes, potentially resulting in an alteration in the pharmacokinetics, therapeutic efficacy, and safety of drugs. This study investigated the hepatic cytochrome P450 (CYP) 2B1-modulating effect of a specific NAFLD state in dietary rat models. Sprague-Dawley rats were given a methionine/choline-deficient (MCD) or high-fat (HF) diet to induce NASH and SFL, respectively. The induction of these disease states was confirmed by plasma chemistry and liver histological analysis. Both the protein and mRNA levels of hepatic CYP2B1 were considerably reduced in MCD diet-fed rats; however, they were similar between the HF diet-fed and control rats. Consistently, the enzyme-kinetic and pharmacokinetic parameters for CYP2B1-mediated bupropion metabolism were considerably reduced in MCD diet-fed rats; however, they were also similar between the HF diet-fed and control rats. These results may promote a better understanding of the influence of NAFLD on CYP2B1-mediated metabolism, which could have important implications for the safety and pharmacokinetics of drug substrates for the CYP2B subfamily in patients with NAFLD.

Genome-wide analysis of human constitutive androstane receptor (CAR) transcriptome in wild-type and CAR-knockout HepaRG cells

The constitutive androstane receptor (CAR) modulates the transcription of numerous genes involving drug metabolism, energy homeostasis, and cell proliferation. Most functions of CAR however were defined from animal studies. Given the known species difference of CAR and the significant cross-talk between CAR and the pregnane X receptor (PXR), it is extremely difficult to decipher the exact role of human CAR (hCAR) in gene regulation, relying predominantly on pharmacological manipulations. Here, utilizing a newly generated hCAR-knockout (KO) HepaRG cell line, we carried out RNA-seq analysis of the global transcriptomes in wild-type (WT) and hCAR-KO HepaRG cells treated with CITCO, a selective hCAR agonist, phenobarbital (PB), a dual activator of hCAR and hPXR, or vehicle control. Real-time PCR assays in separate experiments were used to validate RNA-seq findings. Our results indicate that genes encoding drug-metabolizing enzymes are among the main clusters altered by both CITCO and PB. Specifically, CITCO significantly changed the expression of 135 genes in an hCAR-dependent manner, while PB altered the expression of 227 genes in WT cells of which 94 were simultaneously modulated in both cell lines reflecting dual effects of PB on hCAR/PXR. Notably, we found that many genes promoting cell proliferation and tumorigenesis were up-regulated in hCAR-KO cells, suggesting that hCAR may play an important role in cell growth that differs from mouse CAR. Together, our results reveal both novel and known targets of hCAR and support the role of hCAR in maintaining the homeostasis of metabolism and cell proliferation in the liver.

Pregnane X Receptor Represses HNF4α Gene to Induce Insulin-Like Growth Factor-Binding Protein IGFBP1 that Alters Morphology of and Migrates HepG2 Cells

Upon treatment with the pregnane X receptor (PXR) activator rifampicin (RIF), human hepatocellular carcinoma HepG2-derived ShP51 cells that stably express PXR showed epithelial-mesenchymal transition (EMT)-like morphological changes and migration. Our recent DNA microarrays have identified hepatocyte nuclear factor (HNF) 4α and insulin-like growth factor-binding protein (IGFBP) 1 mRNAs to be downregulated and upregulated, respectively, in RIF-treated ShP51 cells, and these regulations were confirmed by the subsequent real-time polymerase chain reaction and Western blot analyses. Using this cell system, we demonstrated here that the PXR-HNF4α-IGFBP1 pathway is an essential signal for PXR-induced morphological changes and migration. First, we characterized the molecular mechanism underlying the PXR-mediated repression of the HNF4α gene. Chromatin conformation capture and chromatin immunoprecipitation (ChIP) assays revealed that PXR activation by RIF disrupted enhancer-promoter communication and prompted deacetylation of histone H3 in the HNF4α P1 promoter. Cell-based reporter and ChIP assays showed that PXR targeted the distal enhancer of the HNF4α P1 promoter and stimulated dissociation of HNF3β from the distal enhancer. Subsequently, small interfering RNA knockdown of HNF4α connected PXR-mediated gene regulation with the PXR-induced cellular responses, showing that the knockdown resulted in the upregulation of IGFBP1 and EMT-like morphological changes without RIF treatment. Moreover, recombinant IGFBP1 augmented migration, whereas an anti-IGFBP1 antibody attenuated both PXR-induced morphological changes and migration in ShP51 cells. PXR indirectly activated the IGFBP1 gene by repressing the HNF4α gene, thus enabling upregulation of IGFBP1 to change the morphology of ShP51 cells and cause migration. These results provide new insights into PXR-mediated cellular responses toward xenobiotics including therapeutics.

Cryptotanshinone Ameliorates Hepatic Normothermic Ischemia and Reperfusion Injury in Rats by Anti-mitochondrial Apoptosis

Cryptotanshinone (CT), isolated from the dried roots of Salvia militorrhiza, has been reported to have protective effects on myocardial and cerebral ischemia/reperfusion (I/R) injury both in vitro and in vivo. However, its effects and underlying mechanism on hepatic I/R injury remain unclear. To investigate its effects on hepatic I/R injury, thirty male Sprague-Dawley rats were randomized into 3 groups: a sham group, a vehicle-treated hepatic I/R group and a CT-treated (50 mg/kg) group. The hepatic I/R and CT-treated groups were subjected to 60 min of normothermic ischemia of the left lateral lobe of the liver, followed by 4 h of reperfusion. The animals were then sacrificed to collect the serum and the left liver lobe for assay. Hepatic function was protected, as evidenced by significantly reduced alanine aminotransferase (ALT), aspartate aminotransferase (AST) and malondialdehyde (MDA) levels in the CT-treated group as compared with I/R group. The terminal deoxynucleotidyl transferase deoxyuridine triphosphate (dUTP) nick end labeling (TUNEL) demonstrated significantly decreased apoptosis in the CT-administration animals. Western blotting demonstrated upregulation of the proapoptotic protein Bcl-2, as well as decreased levels of the activated form of caspase-3 and the cleaved form of its substrate, poly(ADP-ribose) polymerase (PARP) in the CT-treated group compared with those of the I/R group. In addition, the phosphorylation of c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinases (MAPKs) was inhibited by CT. Our data suggest that CT attenuates hepatic I/R injury by inhibiting the intrinsic pathway of apoptosis, mediated partly through the inhibition of JNK and p38 MAPK phosporylation.

Expression and functional regulation of the nuclear receptors AHR, PXR, and CAR, and the transcription factor Nrf2 in rat parotid gland

Nuclear receptors and transcription factors regulate the functions of many genes involved in cellular physiology and pathology (e.g. tumorigenesis and autoimmune diseases). The present study was performed to define the expression and the regulation of aryl hydrocarbon receptor (AhR), pregnane X receptor (PXR), constitutive androstane receptor (CAR), and nuclear factor E2-related factor 2 (Nrf2) in the rat parotid gland. Constitutive expression, as well as expression after stimulation with specific inducers for AhR [2,3,7,8-tetrachloro-dibenzylo-p-dioxin (TCDD)], Nrf2(oltipraz), PXR (dexamethasone), and CAR (phenobarbital), was evaluated using the quantitative PCR. Cellular localization of the nuclear receptors and the transcription factor was visualized by immunohistochemical staining. The study revealed constitutive expression of AhR as well as Nrf2, and their induction by TCDD andoltipraz, respectively. Immunohistochemical analysis revealed constitutive, predominantly cytoplasmic, expression of the AhR receptor, especially in interlobular striated duct cells, with nuclear shift upon exposure to TCDD. Inducible expression of Nfr2 was found mainly in the cytoplasm of intralobular striated duct cells. Constitutive expression of PXR and CAR was not found. Bearing in mind the involvement of AhR and Nrf2 in the regulation of many genes, it seems that these factors may play also a role in salivary gland physiology and pathology.

Deletion of 30 murine cytochrome p450 genes results in viable mice with compromised drug metabolism

In humans, 75% of all drugs are metabolized by the cytochrome P450-dependent monooxygenase system. Enzymes encoded by the CYP2C, CYP2D, and CYP3A gene clusters account for ∼80% of this activity. There are profound species differences in the multiplicity of cytochrome P450 enzymes, and the use of mouse models to predict pathways of drug metabolism is further complicated by overlapping substrate specificity between enzymes from different gene families. To establish the role of the hepatic and extrahepatic P450 system in drug and foreign chemical disposition, drug efficacy, and toxicity, we created a unique mouse model in which 30 cytochrome P450 genes from the Cyp2c, Cyp2d, and Cyp3a gene clusters have been deleted. Remarkably, despite a wide range of putative important endogenous functions, Cyp2c/2d/3a KO mice were viable and fertile, demonstrating that these genes have evolved primarily as detoxification enzymes. Although there was no overt phenotype, detailed examination showed Cyp2c/2d/3a KO mice had a smaller body size (15%) and larger livers (20%). Changes in hepatic morphology and a decreased blood glucose (30%) were also noted. A five-drug cocktail of cytochrome P450 isozyme probe substrates were used to evaluate changes in drug pharmacokinetics; marked changes were observed in either the pharmacokinetics or metabolites formed from Cyp2c, Cyp2d, and Cyp3a substrates, whereas the metabolism of the Cyp1a substrate caffeine was unchanged. Thus, Cyp2c/2d/3a KO mice provide a powerful model to study the in vivo role of the P450 system in drug metabolism and efficacy, as well as in chemical toxicity.

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.

Targeting nuclear receptors with marine natural products

Nuclear receptors (NRs) are important pharmaceutical targets because they are key regulators of many metabolic and inflammatory diseases, including diabetes, dyslipidemia, cirrhosis, and fibrosis. As ligands play a pivotal role in modulating nuclear receptor activity, the discovery of novel ligands for nuclear receptors represents an interesting and promising therapeutic approach. The search for novel NR agonists and antagonists with enhanced selectivities prompted the exploration of the extraordinary chemical diversity associated with natural products. Recent studies involving nuclear receptors have disclosed a number of natural products as nuclear receptor ligands, serving to re-emphasize the translational possibilities of natural products in drug discovery. In this review, the natural ligands of nuclear receptors will be described with an emphasis on their mechanisms of action and their therapeutic potentials, as well as on strategies to determine potential marine natural products as nuclear receptor modulators.

Role of constitutive androstane receptor in Toll-like receptor-mediated regulation of gene expression of hepatic drug-metabolizing enzymes and transporters

Impairment of drug disposition in the liver during inflammation has been attributed to downregulation of gene expression of drug-metabolizing enzymes (DMEs) and drug transporters. Inflammatory responses in the liver are primarily mediated by Toll-like receptors (TLRs). We have recently shown that activation of TLR2 or TLR4 by lipoteichoic acid (LTA) and lipopolysaccharide (LPS), respectively, leads to the downregulation of gene expression of DMEs/transporters. However, the molecular mechanism underlying this downregulation is not fully understood. The xenobiotic nuclear receptors, pregnane X receptor (PXR) and constitutive androstane receptor (CAR), regulate the expression of DMEs/transporter genes. Downregulation of DMEs/transporters by LTA or LPS was associated with reduced expression of PXR and CAR genes. To determine the role of CAR, we injected CAR(+/+) and CAR(-/-) mice with LTA or LPS, which significantly downregulated (~40%-60%) RNA levels of the DMEs, cytochrome P450 (Cyp)3a11, Cyp2a4, Cyp2b10, uridine diphosphate glucuronosyltransferase 1a1, amine N-sulfotransferase, and the transporter, multidrug resistance-associated protein 2, in CAR(+/+) mice. Suppression of most of these genes was attenuated in LTA-treated CAR(-/-) mice. In contrast, LPS-mediated downregulation of these genes was not attenuated in CAR(-/-) mice. Induction of these genes by mouse CAR activator 1,4-bis-[2-(3,5-dichloropyridyloxy)]benzene was sustained in LTA- but not in LPS-treated mice. Similar observations were obtained in humanized CAR mice. We have replicated these results in primary hepatocytes as well. Thus, LPS can downregulate DME/transporter genes in the absence of CAR, whereas the effect of LTA on these genes is attenuated in the absence of CAR, indicating the potential involvement of CAR in LTA-mediated downregulation of DME/transporter genes.

Sulfotransferase genes: regulation by nuclear receptors in response to xeno/endo-biotics

Pregnane X receptor (PXR) and constitutive active/androstane receptor (CAR), members of the nuclear receptor superfamily, are two major xeno-sensing transcription factors. They can be activated by a broad range of lipophilic xenobiotics including therapeutics drugs. In addition to xenobiotics, endogenous compounds such as steroid hormones and bile acids can also activate PXR and/or CAR. These nuclear receptors regulate genes that encode enzymes and transporters that metabolize and excrete both xenobiotics and endobiotics. Sulfotransferases (SULTs) are a group of these enzymes and sulfate xenobiotics for detoxification. In general, inactivation by sulfation constitutes the mechanism to maintain homeostasis of endobiotics. Thus, deciphering the molecular mechanism by which PXR and CAR regulate SULT genes is critical for understanding the roles of SULTs in the alterations of physiological and pathophysiological processes caused by drug treatment or environmental exposures.

Genome-wide analysis of chromatin states reveals distinct mechanisms of sex-dependent gene regulation in male and female mouse liver

Chromatin state maps were developed to elucidate sex differences in chromatin structure and their impact on sex-differential chromatin accessibility and sex-biased gene expression in mouse liver. Genes in active, inactive, and poised chromatin states exhibited differential responsiveness to ligand-activated nuclear receptors and distinct enrichments for functional gene categories. Sex-biased genes were clustered by chromatin environments and mapped to DNase-hypersensitive sites (DHS) classified by sex bias in chromatin accessibility and enhancer modifications. Results were integrated with genome-wide binding data for five transcription factors implicated in growth hormone-regulated, sex-biased liver gene expression, leading to the following findings. (i) Sex-biased DHS, but not sex-biased genes, are frequently characterized by sex-differential chromatin states, indicating distal regulation. (ii) Trimethylation of histone H3 at K27 (H3K27me3) is a major sex-biased repressive mark at highly female-biased but not at highly male-biased genes. (iii) FOXA factors are associated with sex-dependent chromatin opening at male-biased but not female-biased regulatory sites. (iv) Sex-biased STAT5 binding is enriched at sex-biased DHS marked as active enhancers and preferentially targets sex-biased genes with sex-differences in local chromatin marks. (v) The male-biased repressor BCL6 preferentially targets female-biased genes and regulatory sites in a sex-independent chromatin state. (vi) CUX2, a female-specific repressor of male-biased genes, also activates strongly female-biased genes, in association with loss of H3K27me3 marks. Chromatin states are thus a major determinant of sex-biased chromatin accessibility and gene expression, with FOXA pioneer factors proposed to confer sex-dependent chromatin opening and STAT5, but not BCL6, regulating sex-biased genes by binding to sites in a sex-biased chromatin state.

A cytoprotective perspective on longevity regulation

There are many mechanisms of lifespan extension, including the disruption of insulin/insulin-like growth factor 1 (IGF-1) signaling, metabolism, translation, and feeding. Despite the disparate functions of these pathways, inhibition of each induces responses that buffer stress and damage. Here, emphasizing data from genetic analyses in Caenorhabditis elegans, we explore the effectors and upstream regulatory components of numerous cytoprotective mechanisms activated as major elements of longevity programs, including detoxification, innate immunity, proteostasis, and oxidative stress response. We show that their induction underpins longevity extension across functionally diverse triggers and across species. Intertwined with the evolution of longevity, cytoprotective pathways are coupled to the surveillance of core cellular components, with important implications in normal and aberrant responses to drugs, chemicals, and pathogens.

Phenobarbital indirectly activates the constitutive active androstane receptor (CAR) by inhibition of epidermal growth factor receptor signaling

Phenobarbital is a central nervous system depressant that also indirectly activates nuclear receptor constitutive active androstane receptor (CAR), which promotes drug and energy metabolism, as well as cell growth (and death), in the liver. We found that phenobarbital activated CAR by inhibiting epidermal growth factor receptor (EGFR) signaling. Phenobarbital bound to EGFR and potently inhibited the binding of EGF, which prevented the activation of EGFR. This abrogation of EGFR signaling induced the dephosphorylation of receptor for activated C kinase 1 (RACK1) at Tyr(52), which then promoted the dephosphorylation of CAR at Thr(38) by the catalytic core subunit of protein phosphatase 2A. The findings demonstrated that the phenobarbital-induced mechanism of CAR dephosphorylation and activation is mediated through its direct interaction with and inhibition of EGFR.

Dietary modification of metabolic pathways via nuclear hormone receptors

Nuclear hormone receptors (NHRs), as ligand-dependent transcription factors, have emerged as important mediators in the control of whole body metabolism. Because of the promiscuous nature of several members of this superfamily that have been found to bind ligand with lower affinity than the classical steroid NHRs, they consequently display a broader ligand selectivity. This promiscuous nature has facilitated various bioactive dietary components being able to act as agonist ligands for certain members of the NHR superfamily. By binding to these NHRs, bioactive dietary components are able to mediate changes in various metabolic pathways, including, glucose, cholesterol and triglyceride homeostasis among others. This review will provide a general overview of the nuclear hormone receptors that have been shown to be activated by dietary components. The physiological consequences of such receptor activation by these dietary components will then be discussed in more detail.

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.

Selective agonism of human pregnane X receptor by individual ginkgolides

Ginkgolide A, ginkgolide B, ginkgolide C, and ginkgolide J are structurally related terpene trilactones present in Ginkgo biloba extract. Pregnane X receptor (PXR), glucocorticoid receptor (GR), and constitutive androstane receptor (CAR) regulate the expression of genes involved in diverse biological functions. In the present study, we investigated the effects of individual ginkgolides as single chemical entities on the function of human PXR (hPXR), human GR (hGR), and human CAR (hCAR). In cell-based reporter gene assays, none of the ginkgolides activated hGR or hCAR (wild-type and its SV23, SV24, and SV25 splice variants). Concentration-response experiments showed that ginkgolide A and ginkgolide B activated hPXR and rat PXR to a greater extent than ginkgolide C, whereas ginkgolide J had no effect. As determined by a time-resolved fluorescence resonance energy transfer competitive binding assay, ginkgolide A and ginkgolide B, but not ginkgolide C or ginkgolide J, were shown to bind to the ligand-binding domain of hPXR, consistent with molecular docking data. Compared with tetraethyl 2-(3,5-di-tert-butyl-4-hydroxyphenyl)ethenyl-1,1-bisphosphonate (SR12813) (a known agonist of hPXR), ginkgolide A and ginkgolide B were considerably less potent in binding to hPXR. These two ginkgolides recruited steroid receptor coactivator-1 to hPXR and increased hPXR target gene (CYP3A4) expression, as assessed by a mammalian two-hybrid assay and real-time polymerase chain reaction, respectively. In conclusion, the individual ginkgolides regulate the function of nuclear receptors in a receptor-selective and chemical-dependent manner. This study identifies ginkgolide A and ginkgolide B as naturally occurring agonists of hPXR and provides mechanistic insight into the structure-activity relationship in ligand activation of hPXR.

Yes-associated protein is involved in proliferation and differentiation during postnatal liver development

It is known that the liver undergoes size increase and differentiation simultaneously during the postnatal period. Cells in the liver undergo a period of well-controlled proliferation to achieve the adult liver-to-body weight ratio. The postnatal liver growth is also accompanied by simultaneous hepatic differentiation. However, the mechanisms of liver size regulation and differentiation are not completely clear. Herein we report that yes-associated protein (Yap), the downstream effector of the Hippo Kinase signaling pathway, plays a role in liver size regulation and differentiation during the postnatal liver growth period. Postnatal liver growth was studied in C57BL/6 mice over a time course of postnatal days (PND) 0-30. Analysis of nuclear Yap by Western blot indicated peak Yap activation between PND15-20, which coincided with increased cyclin D1 expression and liver cell proliferation. Analysis of postnatal liver development in Yap(+/-) mice revealed a significant decrease in the liver-to-body weight ratio compared with Yap(+/+) mice at PND15 and -30. Yap(+/-) mice exhibited a significant decrease in postnatal liver cell proliferation, but no change in apoptosis was observed. Furthermore, global gene expression analysis of Yap(+/-) livers revealed a role of Yap in regulation of genes involved in bile acid metabolism, retinoic acid metabolism, ion transport, and extracellular matrix proteins. Taken together, these data indicate that Yap plays a role in both cell proliferation and possibly in hepatic differentiation during postnatal liver development.

Constitutive androstane receptor activation by 2,4,6-triphenyldioxane-1,3 suppresses the expression of the gluconeogenic genes

The constitutive androstane receptor (CAR, NR1I3) has a central role in detoxification processes, regulating the expression of a set of genes involved in metabolism. The dual role of NR1I3 as both a xenosensor and as a regulator of endogenous energy metabolism has recently been accepted. Here, we investigated the mechanism of transcriptional regulation of the glucose metabolising genes phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) by the cis isomer of 2,4,6-triphenyldioxane-1,3 (cisTPD), a highly effective NR1I3 activator in rat liver. It was shown that expression of the gluconeogenic genes PEPCK and G6Pase was repressed by cisTPD treatment under fasting conditions. Western-blot analysis demonstrated a clear reduction in the intensity of PEPCK and G6Pase immunobands from the livers of cisTPD-treated animals relative to bands from the livers of control animals. Chromatin immunoprecipitation assays demonstrated that cisTPD prevents the binding of FOXO1 to the insulin response sequences in the PEPCK and G6Pase gene promoters in rat liver. Moreover, cisTPD-activated NR1I3 inhibited NR2A1 (HNF-4) transactivation by competing with NR2A1 for binding to the NR2A1-binding element (DR1-site) in the gluconeogenic gene promoters. Thus, our results are consistent with the hypothesis that the cisTPD-activated NR1I3 participates in the regulation of the gluconeogenic genes PEPCK and G6Pase.

Nuclear receptors in nonalcoholic Fatty liver disease

Nuclear receptors comprise a superfamily of ligand-activated transcription factors that are involved in important aspects of hepatic physiology and pathophysiology. There are about 48 nuclear receptors in the human. These nuclear receptors are regulators of many hepatic processes including hepatic lipid and glucose metabolism, bile acid homeostasis, drug detoxification, inflammation, regeneration, fibrosis, and tumor formation. Some of these receptors are sensitive to the levels of molecules that control lipid metabolism including fatty acids, oxysterols, and lipophilic molecules. These receptors direct such molecules to the transcriptional networks and may play roles in the pathogenesis and treatment of nonalcoholic fatty liver disease. Understanding the mechanisms underlying the involvement of nuclear receptors in the pathogenesis of nonalcoholic fatty liver disease may offer targets for the development of new treatments for this liver disease.

Constitutive androstane receptor transactivates the hepatic expression of mouse Dhcr24 and human DHCR24 encoding a cholesterogenic enzyme 24-dehydrocholesterol reductase

Phenobarbital treatment has long been known to influence serum and hepatic cholesterol levels in rodents and humans. Constitutive androstane receptor (CAR), a member of the nuclear receptor superfamily, mediates various biological actions of phenobarbital. We have thus investigated whether CAR transactivates cholesterogenic genes in livers. Activation of CAR in mouse livers and cultured human hepatocytes increased mRNA levels of mouse Dhcr24 and human DHCR24, both of which encode 24-dehydrocholesterol reductase (DHCR24) catalyzing the last step of cholesterol biosynthesis. CAR transactivated the expression of these genes in reporter assays with cultured hepatoma cells. Furthermore, we have identified a DR4 (direct repeat separated by 4 nucleotides) motif in the human DHCR24 distal promoter as a binding site of CAR/retinoid X receptor α (RXRα) heterodimer. We have also demonstrated that the heterodimer of pregnane X receptor (PXR)/ RXRα binds to the DR4 motif and that human DHCR24 reporter gene is transactivated by the ligand-activated PXR. These results suggest a role of xenobiotic-responsive nuclear receptor CAR, and also possibly PXR, in cholesterol biosynthesis in the liver of mice and humans.

New in vitro tools to study human constitutive androstane receptor (CAR) biology: discovery and comparison of human CAR inverse agonists

The human constitutive androstane receptor (CAR, NR1I3) is one of the key regulators of xenobiotic and endobiotic metabolism. The unique properties of human CAR, such as the high constitutive activity and the complexity of signaling, as well as the lack of functional and predictive cell-based assays to study the properties of the receptor, have hindered the discovery of selective human CAR ligands. Here we report a novel human CAR inverse agonist, 1-[(2-methylbenzofuran-3-yl)methyl]-3-(thiophen-2-ylmethyl) urea (S07662), which suppresses human CAR activity, recruits the corepressor NCoR in cell-based assays, and attenuates the phenytoin- and 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime (CITCO)-induced expression of CYP2B6 mRNA in human primary hepatocytes. The properties of S07662 are also compared with those of known human CAR inverse agonists by using an array of different in vitro and in silico assays. The identified compound S07662 can be used as a chemical tool to study the biological functions of human CAR and also as a starting point for the development of new drugs for various conditions involving the receptor.

Role of a novel CAR-induced gene, TUBA8, in hepatocellular carcinoma cell lines

Phenobarbital (PB), a nongenotoxic carcinogen, activates the nuclear constitutive active/androstane receptor (CAR), resulting in the transcriptional induction or repression of various hepatic genes. We previously demonstrated that liver tumors developed after chronic PB treatment only when CAR is present. To understand the molecular mechanism of tumor promotion, cDNA microarray analysis was performed. We identified tubulin alpha 8 (TUBA8) as one of the candidate genes that may be involved in liver tumor promotion. Tuba8 mRNA was induced with PB treatment in mouse livers before tumor development as well as in tumor tissues. Because the functions of TUBA8 are unknown in liver, we investigated the effects of TUBA8 gene expression on cell growth, proliferation, and cell migration. Sense or antisense cDNA for Tuba8 was stably transfected into Huh7 and HepG2 cells. Exogenous overexpression of Tuba8 inhibited cell growth and proliferation in Huh7 but not in HepG2 cells, while cell migration was increased in HepG2 cells but not Huh7 cells. These results indicate that TUBA8 can play a role in the regulation of cell growth, proliferation, and cell migration in a cell-specific manner in vitro, suggesting that TUBA8 may contribute to mouse liver tumorigenesis through these functions.

Regulation of drug-metabolizing cytochrome P450 enzymes by glucocorticoids

The regulation of drug-metabolizing cytochrome P450 enzymes (CYP) is a complex process involving multiple mechanisms. Among them, transcriptional regulation through ligand-activated nuclear receptors is the crucial mechanism involved in hormone-controlled and xenobiotic-induced expression of drug-metabolizing CYPs. In this article, we focus, in detail, on the role of the glucocorticoid receptor (GR) in the transcriptional regulation of human drug-metabolizing CYP enzymes and the mechanisms of the regulation. There are at least three distinct transcriptional mechanisms by which GR controls the expression of CYPs: 1) direct binding of GR to a specific gene-promoter sequence called the glucocorticoid responsive element (GRE); 2) indirect binding of GR in the form of a multiprotein complex to gene promoters without a direct contact between GR and promoter DNA; and 3) up- or downregulation of other CYP transcriptional regulators or nuclear receptors (i.e., transcriptional regulatory cross-talk). However, due to the general effect of glucocorticoids on numerous cellular pathways and functions, the net transcriptional effect of glucocorticoids on drug-metabolizing enzymes is usually a combination of several mechanisms. Since synthetic glucocorticoids are widely prescribed in human pharmacotherapy for the treatment of many diseases, comprehensive understanding of the transcriptional regulation of drug-metabolizing CYPs via GR with respect to glucocorticoid therapy or glucocorticoid hormonal status will aid in the development of efficient individualized pharmacotherapy without drug-drug interactions.

Garlic extract diallyl sulfide (DAS) activates nuclear receptor CAR to induce the Sult1e1 gene in mouse liver

Constituent chemicals in garlic extract are known to induce phase I and phase II enzymes in rodent livers. Here we have utilized Car(+/+) and Car(-/-) mice to demonstrate that the nuclear xenobiotic receptor CAR regulated the induction of the estrogen sulfotransferase Sult1e1 gene by diallyl sulfide (DAS) treatment in mouse liver. DAS treatment caused CAR accumulation in the nucleus, resulting in a remarkable increase of SULT1E1 mRNA (3,200 fold) and protein in the livers of Car(+/+) females but not of Car(-/-) female mice. DAS also induced other CAR-regulated genes such as Cyp2b10, Cyp3a11 and Gadd45β. Compared with the rapid increase of these mRNA levels, which began as early as 6 hours after DAS treatment, the levels of SULT1E1 mRNA began increasing after 24 hours. This slow response to DAS suggested that CAR required an additional factor to activate the Sult1e1 gene or that this activation was indirect. Despite the remarkable induction of SULT1E1, there was no decrease in the serum levels of endogenous E2 or increase of estrone sulfate while the clearance of exogenously administrated E2 was accelerated in DAS treated mice.

Drug metabolism alterations in nonalcoholic fatty liver disease

Drug-metabolizing enzymes play a vital role in the elimination of the majority of therapeutic drugs. The major organ involved in drug metabolism is the liver. Chronic liver diseases have been identified as a potential source of significant interindividual variation in metabolism. Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease in the United States, affecting between 60 and 90 million Americans, yet the vast majority of NAFLD patients are undiagnosed. NAFLD encompasses a spectrum of pathologies, ranging from steatosis to nonalcoholic steatohepatitis and fibrosis. Numerous animal studies have investigated the effects of NAFLD on hepatic gene expression, observing significant alterations in mRNA, protein, and activity levels. Information on the effects of NAFLD in human patients is limited, though several significant investigations have recently been published. Significant alterations in the activity of drug-metabolizing enzymes may affect the clearance of therapeutic drugs, with the potential to result in adverse drug reactions. With the enormous prevalence of NAFLD, it is conceivable that every drug currently on the market is being given to patients with NAFLD. The current review is intended to present the results from both animal models and human patients, summarizing the observed alterations in the expression and activity of the phase I and II drug-metabolizing enzymes.

The nuclear receptor constitutive active/androstane receptor arrests DNA-damaged human hepatocellular carcinoma Huh7 cells at the G2/M phase

Here, we have demonstrated that xenobiotic activation of the nuclear receptor (CAR, NR1I3) can result in arresting DNA-damaged human hepatocellular carcinoma Huh7 cells at the G2/M phase. Huh7 cells over-expressing CAR were either treated with dimethyl sulfoxide, the CAR activator TCPOBOP (1,4-bis[2-(3,5-dichloropyridyloxy)]benzene; androstenol, 16,(5α)-androsten-3α-OL), or repressor androstenol; these treatments were then followed by adriamycin treatment to damage DNA. FACS analysis revealed that CAR-activation by TCPOBOP increased the rate of arrested Huh7 cells at the G2/M phase (4N DNA content) after DNA damage by adriamycin. This increase correlated with the increase of cell viability in TCPOBOP-treated Huh7 cells, as determined by MTT assays. Real-time polymerase chain reaction analysis determined that, as regulated by CAR, the growth arrest and DNA damage-inducible γ (GADD45γ) and Cyclin G2 genes increased and decreased, respectively, as TCPOBOP increased the number of Huh7 cells arrested at the G2/M phase. Thus, the results suggest that CAR regulates cell cycle, increasing G2/M arrest, and delaying the death of DNA-damaged cells.