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

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

Preservation of hepatic phenotype in lentiviral-transduced primary human hepatocytes

Lentiviral vectors effectively transduce both dividing and non-dividing cells and stably integrate into the genome of the host cell. In this study, we evaluated the usefulness of a lentiviral system for genetic modulation of primary human hepatocyte cultures. Infection with GFP-expressing lentivectors shows that Huh7 and HepG2 cell lines, as well as primary cultures of human hepatocytes, are efficiently transduced by lentiviral vectors. Real-time RT-PCR analyses demonstrate that infection with lentivectors does not alter hepatic hallmarks such as the expression of the nuclear receptors CAR, PXR, RXR alpha, or HNF4 alpha, or expression of the secretory protein, albumin. Additionally, infected hepatocytes retain the capacity for CYP3A4 induction in response to treatment with phenobarbital, a uniquely sensitive indicator of hepatic differentiation status. Lentivectors may be used for both over-expression and knockdown analyses in primary hepatocytes, as demonstrated in this study by >200-fold CAR over-expression and knockdown of CAR to less than 40% of endogenous levels, with corresponding effects on CYP2B6 expression. In summary, lentiviral vectors provide a novel methodology by which primary human hepatocytes may be stably genetically manipulated, with minimal effects on the differentiated hepatic phenotype. These approaches offer considerable advantage over current methodologies, providing a valuable alternative for use in pharmacological and toxicological investigations involving primary human hepatocyte models and potentially for cell-based therapeutics to treat hepatic dysfunction in vivo.     

Enhanced steatosis by nuclear receptor ligands: A study in cultured human hepatocytes and hepatoma cells with a characterized nuclear receptor expression profile

Steatosis is the first step in the development of non-alcoholic fatty liver disease (NAFLD). However, the mechanisms involved in its pathogenesis are not fully understood. Many nuclear receptors (NRs) involved in energy homeostasis and biotransformation constitute a network connecting fatty acids, cholesterol and xenobiotic metabolisms; therefore, multiple NRs and their ligands may play a prominent role in liver fat metabolism and accumulation. In this study we have attempted to gain insight into the relevance of the NR superfamily in NAFLD by investigating the steatogenic potential of 76 different NR ligands in fatty acid overloaded human hepatocytes and hepatoma cells. Moreover, we have determined the mRNA expression level of 24 NRs to correlate the steatogenic potential of the ligands with the expression of their associated NRs in the cultured cells. Our results demonstrate that 18% of the examined NR ligands enhanced lipid accumulation in human hepatocytes and/or hepatoma cells. Among them, ligands of PPARγ (e.g., thiazolidinediones), LXR (paxilline and 24(S),25-epoxycholesterol), PXR (hyperforin), CAR (3α,5α-androstenol), ERα (tamoxifen), FXR (Z-guggulsterone), VDR (25-hydroxyvitamin D3) and particular retinoids and farnesoids showed a significant pro-steatotic effect. The mRNA level of most of the NRs examined was well preserved in human hepatocytes, but HepG2 showed a deranged profile, where many of the receptors had a marginal or negligible level of expression in comparison with the human liver. By comparing the steatogenic effect of NR ligands with the NR expression levels, we conclude that LXR, PXR, RAR and PPARγ ligands likely induce fat accumulation by a NR-dependent mechanism. Indeed, over-expression of PXR in HepG2 cells enhanced the steatogenic effect of hyperforin and rifampicin. However, the accumulation of fat induced by other ligands did not correlate with the expression of their associated NR. Our results also suggest that human hepatocytes cultured with free fatty acids offer a highly valuable in vitro system to investigate the pathogenesis and therapeutics of the human fatty liver.     

The xenobiotic receptors PXR and CAR in liver physiology,an update

Pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are two nuclear receptors that are well-known for their roles in xenobiotic detoxification by regulating the expression of drug-metabolizing enzymes and transporters. In addition to metabolizing drugs and other xenobiotics, the same enzymes and transporters are also responsible for the production and elimination of numerous endogenous chemicals, or endobiotics. Moreover, both PXR and CAR are highly expressed in the liver. As such, it is conceivable that PXR and CAR have major potentials to affect the pathophysiology of the liver by regulating the homeostasis of endobiotics. In recent years, the physiological functions of PXR and CAR in the liver have been extensively studied. Emerging evidence has suggested the roles of PXR and CAR in energy metabolism, bile acid homeostasis, cell proliferation, to name a few. This review summarizes the recent progress in our understanding of the roles of PXR and CAR in liver physiology.     

Reduction in cytochrome P-450 enzyme expression is associated with repression of CAR (constitutive androstane receptor) and PXR (pregnane X receptor) in mouse liver during the acute phase response

Expression of P-450 (Cyp) enzymes is reduced in liver during the acute phase response, contributing to the decrease in bile acid levels and drug metabolism during infection. Nuclear hormone receptors CAR and PXR are key transactivators of Cyp2b and Cyp3a genes, respectively. Injection of bacterial lipopolysaccharide (LPS) induced the expected reduction in Cyp2b10 and Cyp3a mRNA levels in mouse liver. These decreases were associated with a marked reduction in CAR and PXR mRNA levels within 4 h following treatment. LPS-induced CAR and PXR repression were dose-dependent and sustained for at least 16 h. LPS treatment also reversed the up-regulation of Cyp3a in mice pre-treated with PXR ligand RU486. In addition, we observed a concomitant decrease in RXR (retinoid X receptor) mRNA levels, the obligatory partner of both CAR and PXR for high affinity binding to DNA. These findings represent one possible molecular mechanism underlying sepsis-induced repression of Cyp enzymes.     

Steroid hormone biotransformation and xenobiotic induction of hepatic steroid metabolizing enzymes

Normal reproductive development depends on the interplay of steroid hormones with their receptors at specific tissue sites. The concentrations of hormone ligands in the circulation and at target sites are maintained through coordinated regulation on steroid biosynthesis and degradation. Changed bioavailability of steroids, through alteration of steroidogenesis or biotransformation rates, leads to changes in endocrine function. Steroid hormones lose their receptor reactivity in most cases when they are bound to binding proteins, while metabolic conversion can result in either active or inactive metabolites. Hydroxylation by cytochrome P450 (CYP) enzymes and conjugation with glucuronide and sulfate are among the major hepatic pathways of steroid inactivation. The expression of these biotransformation enzymes can be induced by many xenobiotics. The barbiturate phenobarbital and the environmental toxicant 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE) are among the well characterized inducers for the CYP 2B and 3A enzymes and selected conjugation enzymes. The induction of the steroid biotransformation enzymes is partly mediated through the activation of a group of nuclear receptors including the glucocorticoid receptor, the constitutive androstane receptor (CAR), the pregnane X receptor (PXR), and the peroxisome proliferator activated receptors (PPAR). Drug or chemical-induced increases in hepatic enzyme activities are often a basis for drug-drug interactions that lead to enhanced elimination and reduced therapeutic efficacy of steroidal drugs. The effects of enzyme induction on endogenous steroid clearance, along with its possible consequence, are less well understood. While enzyme induction by xenobiotics may increase clearance of the endogenous steroid, regulatory mechanisms for steroid homeostasis may adapt and compensate for altered clearance.     

Comparative biosynthetic pathway of androstenol and androgens

It has been shown recently that androstenol and androstanol could modulate gene expression through the nuclear orphan receptors CAR (constitutive androstane receptor) and PXR (pregnane X receptor). Although, in the pig, androstenol is produced in high amounts and is active as a pheromone, its role in the human is ill defined. Androstenol possesses a structure similar to that of androgens, with the exception that it does not possess an oxygen at position 17 that is crucial for androgenic and estrogenic activity. It has been shown that human and boar testis homogenates could produce androstenol, but details of the biosynthetic pathway had not yet been elucidated. It has also been shown recently that androstenol could modulate the activity of CAR and PXR and the expression of some cytochrome P450 drug-metabolizing enzymes. We wanted to determine the precise biosynthetic pathway of androstenol and other closely related steroids. Using transformed human embryonic kidney (HEK-293) cells that stably express 3 beta-hydroxysteroid dehydrogenase, 5 alpha-reductase and 3 alpha-hydroxysteroid dehydrogenase, we have shown that these enzymes are able to efficiently transform the precursor 5,16-androstadien-3 beta-ol into androstenol. We thus provided evidence that androstenol, the ligand for CAR and PXR, is produced by the biosynthetic pathway of sex steroids.     

Manipulation of OCT4 levels in human embryonic stem cells results in induction of differential cell types

To fully understand self-renewal and pluripotency and their regulation in human embryonic stem cells (hESCs), it is necessary to generate genetically modified cells and analyze the consequences of elevated and reduced expression of genes. Genes expressed in hESCs using plasmid vectors, however, are subject to silencing. Moreover, hESCs have a low plating efficiency when dissociated to single cells, making creation of subcloned lines inefficient. In addition to overexpression experiments, it is important to perform loss-of-function studies, which can be achieved rapidly using RNA interference (RNAi). We report stable long-term expression of enhanced green fluorescent protein (eGFP) in hESCs using a lentiviral vector, and establishment of an eGFP-expressing subline (RG6) using manual dissection. To demonstrate the efficacy of RNAi in hESCs, an RNAi expression vector was used to achieve reduced expression of eGFP in hESCs. To evaluate the role of OCT4 in the regulation of hESC self-renewal and differentiation, a vector expressing a hairpin RNA targeting endogenous expression of OCT4 was constructed. In a novel experiment in hESCs, the OCT4 cDNA sequence was cloned into an expression vector to allow for the transient upregulation of OCT4 in hESCs. The ability to manipulate levels of OCT4 above and below enodogenous levels allows the determination of OCT4 function in hESCs. Specifically, reduced expression of OCT4 in hESCs promoted upregulation of markers indicative of mesoderm and endoderm differentiation, and elevated levels of OCT4 in hESCs promoted upregulation of markers indicative of endoderm derivatives. Thus, both upregulation and downregulation of Oct4 in hESCs results in differentiation, but with patterns distinct from parallel experiments in mice.     

Differentiation and isolation of hepatic-like cells from human embryonic stem cells

Human embryonic stem cells are pluripotent cells that can serve as a cell source for transplantation medicine, and as a tool to study human embryogenesis. We investigate here the potential of human embryonic stem cells to differentiate into hepatic cells. We have characterized the expression level of liver-enriched genes in undifferentiated and differentiated human embryonic stem cells by DNA microarrays. Our analysis revealed a subset of fetal hepatic enriched genes that are expressed in human embryonic stem cells upon differentiation into embryoid bodies. In order to isolate the hepatic-like cells, we introduced a reporter gene regulated by a hepatocyte-specific promoter into human embryonic stem cells. We isolated clones of human embryonic stem cells that express enhanced green fluorescent protein upon in vitro differentiation. Through immunostaining, we showed that most of these cells express albumin, while some cells still express the earlier expressed protein alpha-fetoprotein. Using fluorescence activated cell sorter, we were able to sort out the fluorescent differentiated cells and expand them for a few more weeks. This is the first report to demonstrate the possibility of purifying differentiated derivatives of human embryonic stem cells and culturing them further. Through confocal microscopy, we detected clusters of hepatic-like cells in 20-day-old embryoid bodies and in teratomas. As observed during embryonic development, we showed that in teratomas, the hepatic-like endodermal cells develop next to cardiac mesodermal cells. In order to examine the secreted factors involved in the induction of hepatic differentiation, human embryonic stem cells were grown in the presence of various growth factors, demonstrating the potential involvement of acidic fibroblast growth factor in the differentiation. In conclusion, given certain growth conditions and genetic manipulation, we can now differentiate and isolate hepatic-like cells from human embryonic stem cells.     

Xenobiotic-Induced Hepatocyte Proliferation Associated with Constitutive Active/Androstane Receptor (CAR) or Peroxisome Proliferator-Activated Receptor α (PPARα) Is Enhanced by Pregnane X Receptor (PXR) Activation in Mice

Xenobiotic-responsive nuclear receptors pregnane X receptor (PXR), constitutive active/androstane receptor (CAR) and peroxisome proliferator-activated receptor α (PPARα) play pivotal roles in the metabolic functions of the liver such as xenobiotics detoxification and energy metabolism. While CAR or PPARα activation induces hepatocyte proliferation and hepatocarcinogenesis in rodent models, it remains unclear whether PXR activation also shows such effects. In the present study, we have investigated the role of PXR in the xenobiotic-induced hepatocyte proliferation with or without CAR activation by 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) and phenobarbital, or PPARα activation by Wy-14643 in mice. Treatment with TCPOBOP or phenobarbital increased the percentage of Ki-67-positive nuclei as well as mRNA levels of cell proliferation-related genes in livers as expected. On the other hand, treatment with the PXR activator pregnenolone 16α-carbonitrile (PCN) alone showed no such effects. Surprisingly, PCN co-treatment significantly augmented the hepatocyte proliferation induced by CAR activation with TCPOBOP or phenobarbital in wild-type mice but not in PXR-deficient mice. Intriguingly, PXR activation also augmented the hepatocyte proliferation induced by Wy-14643 treatment. Moreover, PCN treatment increased the RNA content of hepatocytes, suggesting the induction of G0/G1 transition, and reduced mRNA levels of Cdkn1b and Rbl2, encoding suppressors of cell cycle initiation. Our present findings indicate that xenobiotic-induced hepatocyte proliferation mediated by CAR or PPARα is enhanced by PXR co-activation despite that PXR activation alone does not cause the cell proliferation in mouse livers. Thus PXR may play a novel and unique role in the hepatocyte/liver hyperplasia upon exposure to xenobiotics.     

A role for constitutive androstane receptor in the regulation of human intestinal MDR1 expression

MDR1/P-glycoprotein is an efflux transporter determining the absorption and presystemic elimination of many xenobiotics in the gut. Thus, interindividual differences in MDR1 expression may affect the efficacy of drug treatment. The expression of MDR1 is partially controlled by the pregnane X receptor (PXR), which mediates induction by many xenobiotics. Since it has been described that the nuclear receptors PXR and constitutive androstane receptor (CAR) can bind to the same binding sites, we investigated the role of CAR in the regulation of MDR1 gene expression. We demonstrate here by gel shift and transfection experiments that CAR binds to distinct nuclear receptor response elements in the -7.8 kbp enhancer of MDR1 and transactivates MDR1 expression through DR4 motifs to which the receptor binds as a heterodimer with RXR or as a monomer, respectively. Expression of the endogenous MDR1 gene is elevated in cells stably expressing CAR, thus arguing for the functional relevance of CAR-dependent activation of MDR1 . The physiological relevance of the regulation of MDR1 by CAR is further suggested by correlation of the expression of CAR and MDR1 in the human small intestine. In summary, our data suggest that CAR plays a role in the regulation of intestinal MDR1 expression.     

槟榔碱对2型糖尿病大鼠肝脏胰岛素抵抗的作用

目的：探讨槟榔碱对2型糖尿病大鼠肝脏胰岛素抵抗的影响及其机制。方法：采用高果糖饲料饲养Wistar大鼠12周制备2型糖尿病大鼠模型,实验动物随机分为5组（n=8）：对照组、模型组、模型＋不同浓度的槟榔碱（0,0．5,1,5mg／kg）组。4周后通过检测血糖、血脂、胰岛素、RT-PCR检测肝脏组成型雄甾烷受体（CAR）、孕甾烷x受体（PXR）、糖代谢相关基因：葡萄糖-6-磷酸酶（G6Pase）、磷酸烯醇式丙酮酸羧激酶（PEPCK）和炎症相关因子：白细胞介素-6（IL-6）、肿瘤坏死因子α（TNF-α）mRNA表达,Western blot检测大鼠肝内p-AKT和葡萄糖转运体4（GLUT4）蛋白表达。结果：1,5mg／kg槟榔碱显著降低糖尿病大鼠体重、空腹血糖、空腹胰岛素、血脂和糖代谢相关基因及炎症相关因子mRNA水平,提高CAR、PXR mRNA水平及p-AKT、GLUT4蛋白水平。结论：槟榔碱可能通过提高CAR和PXR的表达,导致肝脏糖代谢关键酶PEPCK、G6Pase基因表达或者炎性因子肿瘤坏死因子-α（TNF-α）、白介素-6（n-6）表达降低,改善2型糖尿病大鼠肝脏胰岛素抵抗。     

Identification of a novel human constitutive androstane receptor (CAR) agonist and its use in the identification of CAR target genes

The orphan nuclear constitutive androstane receptor (CAR) is proposed to play a central role in the response to xenochemical stress. Identification of CAR target genes in humans has been limited by the lack of a selective CAR agonist. We report the identification of 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime (CITCO) as a novel human CAR agonist with the following characteristics: (a) potent activity in an in vitro fluorescence-based CAR activation assay; (b) selectivity for CAR over other nuclear receptors, including the xenobiotic pregnane X receptor (PXR); (c) the ability to induce human CAR nuclear translocation; and (d) the ability to induce the prototypical CAR target gene CYP2B6 in primary human hepatocytes. Using primary cultures of human hepatocytes, the effects of CITCO on gene expression were compared with those of the PXR ligand rifampicin. The relative expression of a number of genes encoding proteins involved in various aspects of steroid and xenobiotic metabolism was analyzed. Notably, CAR and PXR activators differentially regulated the expression of several genes, demonstrating that these two nuclear receptors subserve overlapping but distinct biological functions in human hepatocytes.     

Complementary roles of farnesoid X receptor,pregnane X receptor,and constitutive androstane receptor in protection against bile acid toxicity

The nuclear receptors, farnesoid X receptor (FXR) and pregnane X receptor (PXR), are important in maintaining bile acid homeostasis. Deletion of both FXR and PXR in vivo by cross-breeding B6;129-Fxrtm1Gonz (FXR-null) and B6;129-Pxrtm1Glaxo-Wellcome (PXR-null) mice revealed a more severe disruption of bile acid, cholesterol, and lipid homeostasis in B6;129-Fxrtm1Gonz Pxrtm1Glaxo-Wellcome (FXR-PXR double null or FPXR-null) mice fed a 1% cholic acid (CA) diet. Hepatic expression of the constitutive androstane receptor (CAR) and its target genes was induced in FXR- and FPXR-null mice fed the CA diet. To test whether up-regulation of CAR represents a means of protection against bile acid toxicity to compensate for the loss of FXR and PXR, animals were pretreated with CAR activators, phenobarbital or 1,4-bis[2-(3,5-dichlorpyridyloxy)]benzene (TCPOBOP), followed by the CA diet. A role for CAR in protection against bile acid toxicity was confirmed by a marked reduction of serum bile acid and bilirubin concentrations, with an elevation of the expression of the hepatic genes involved in bile acid and/or bilirubin metabolism and excretion (CYP2B, CYP3A, MRP2, MRP3, UGT1A, and glutathione S-transferase alpha), following pretreatment with phenobarbital or TCPOBOP. In summary, the current study demonstrates a critical and combined role of FXR and PXR in maintaining not only bile acid but also cholesterol and lipid homeostasis in vivo. Furthermore, FXR, PXR, and CAR protect against hepatic bile acid toxicity in a complementary manner, suggesting that they serve as redundant but distinct layers of defense to prevent overt hepatic damage by bile acids during cholestasis.