The nuclear receptor constitutively active/androstane receptor regulates type 1 deiodinase and thyroid hormone activity in the regenerating mouse liver

Krüppel-like Factor 9 (KLF9) Suppresses Hepatocellular Carcinoma (HCC)-Promoting Oxidative Stress and Inflammation in Mice Fed High-Fat Diet

Obesity, oxidative stress, and inflammation are risk factors for hepatocellular carcinoma (HCC). We examined, in mice, the effects of Krüppel-like factor 9 (KLF9) knockout on: adiposity, hepatic and systemic oxidative stress, and hepatic expression of pro-inflammatory and NOX/DUOX family genes, in a high-fat diet (HFD) context. Male and female Klf9^+/+ (wild type, WT) and Klf9^-/- (knockout, KO) mice were fed HFD (beginning at age 35 days) for 12 weeks, after which liver and adipose tissues were obtained, and serum adiponectin and leptin levels, liver fat content, and markers of oxidative stress evaluated. Klf9^-/- mice of either sex did not exhibit significant alterations in weight gain, adipocyte size, adipokine levels, or liver fat content when compared to WT counterparts. However, Klf9^-/- mice of both sexes had increased liver weight/size (hepatomegaly). This was accompanied by increased hepatic oxidative stress as indicated by decreased GSH/GSSG ratio and increased homocysteine, 3-nitrotyrosine, 3-chlorotyrosine, and 4HNE content. Decreased GSH to GSSG ratio and a trend toward increased homocysteine levels were observed in the corresponding Klf9^-/- mouse serum. Gene expression analysis showed a heightened pro-inflammatory state in livers from Klf9^-/- mice. KLF9 suppresses hepatic oxidative stress and inflammation, thus identifying potential mechanisms for KLF9 suppression of HCC and perhaps cancers of other tissues.

Estrogen Sulfotransferase (SULT1E1): Its Molecular Regulation, Polymorphisms, and Clinical Perspectives

Estrogen sulfotransferase (SULT1E1) is a phase II enzyme that sulfates estrogens to inactivate them and regulate their homeostasis. This enzyme is also involved in the sulfation of thyroid hormones and several marketed medicines. Though the profound action of SULT1E1 in molecular/pathological biology has been extensively studied, its genetic variants and functional studies have been comparatively rarely studied. Genetic variants of this gene are associated with some diseases, especially sex-hormone-related cancers. Comprehending the role and polymorphisms of SULT1E1 is crucial to developing and integrating its clinical relevance; therefore, this study gathered and reviewed various literature studies to outline several aspects of the function, molecular regulation, and polymorphisms of SULT1E1.

No Dataset Left Behind: Mechanistic Insights into Thyroid Receptor Signaling Through Transcriptomic Consensome Meta-Analysis

Background: Discovery-scale omics datasets relevant to thyroid receptors (TRs) and their physiological and synthetic bioactive small-molecule ligands allow for genome-wide interrogation of TR-regulated genes. These datasets have considerable collective value as a reference resource to allow researchers to routinely generate hypotheses addressing the mechanisms underlying the cell biology and physiology of TR signaling in normal and disease states. Methods: Here, we searched the Gene Expression Omnibus database to identify a population of publicly archived transcriptomic datasets involving genetic or pharmacological manipulation of either TR isoform in a mouse tissue or cell line. After initial quality control, samples were organized into contrasts (experiments), and transcript differential expression values and associated measures of significance were generated and committed to a consensome (for consensus omics) meta-analysis pipeline. To gain insight into tissue-selective functions of TRs, we generated liver- and central nervous system (CNS)-specific consensomes and identified evidence for genes that were selectively responsive to TR signaling in each organ. Results: The TR transcriptomic consensome ranks genes based on the frequency of their significant differential expression over the entire group of experiments. The TR consensome assigns elevated rankings both to known TR-regulated genes and to genes previously uncharacterized as TR-regulated, which shed mechanistic light on known cellular and physiological roles of TR signaling in different organs. We identify evidence for unreported genomic targets of TR signaling for which it exhibits strikingly distinct regulatory preferences in the liver and CNS. Moreover, the intersection of the TR consensome with consensomes for other cellular receptors sheds light on transcripts potentially mediating crosstalk between TRs and these other signaling paradigms. Conclusions: The mouse TR datasets and consensomes are freely available in the Signaling Pathways Project website for hypothesis generation, data validation, and modeling of novel mechanisms of TR regulation of gene expression. Our results demonstrate the insights into the mechanistic basis of thyroid hormone action that can arise from an ongoing commitment on the part of the research community to the deposition of discovery-scale datasets.

Metabolomic Approaches Reveal the Role of CAR in Energy Metabolism

The constitutive androstane receptor (CAR; NR1I3) contributes important regulatory roles in biotransformation, xenobiotic transport function, energy metabolism and lipid homeostasis. In this investigation, global serum and liver tissue metabolomes were assessed analytically in wild type and CAR-null transgenic mice using NMR, GC–MS and UPLC–MS/MS-based metabolomics. Significantly, CAR activation increased serum levels of fatty acids, lactate, ketone bodies and tricarboxylic acid cycle products, whereas levels of phosphatidylcholine, sphingomyelin, amino acids and liver glucose were decreased following short-term activation of CAR. Mechanistically, quantitative mRNA analysis demonstrated significantly decreased expression of key gluconeogenic pathways, and increased expression of glucose utilization pathways, changes likely resulting from down-regulation of the hepatic glucose sensor and bidirectional transporter, Glut2. Short-term CAR activation also resulted in enhanced fatty acid synthesis and impaired β-oxidation. In summary, CAR contributes an expansive role regulating energy metabolism, significantly impacting glucose and monocarboxylic acid utilization, fatty acid metabolism and lipid homeostasis, through receptor-mediated regulation of several genes in multiple associated pathways.

Does Reverse Triiodothyronine Testing Have Clinical Utility? An Analysis of Practice Variation Based on Order Data from a National Reference Laboratory

BACKGROUND

Clinical laboratories are under pressure to increase value by improving test utilization. The clinical utility of reverse triiodothyronine (rT3) is controversial. A study was conducted to identify order patterns that might suggest inappropriate utilization of rT3.

METHODS

All orders for thyroid tests placed over a period of one year at a national reference laboratory were reviewed. Order patterns by client (hospital) and by provider were analyzed. A Pareto analysis was conducted to determine the percentage of orders placed as a function of the percentage of providers. A systematic review of the indexed literature and an informal review of the web were conducted to identify indications for rT3 testing.

RESULTS

There were 402,386 orders for 447,664 thyroid tests, including 91,767 orders for rT3. These orders were placed by 60,733 providers located at 1139 different organizations. Only 20% of providers who ordered thyroid tests placed an order for rT3. Of those who placed an order for rT3, 95% placed two orders or fewer for rT3. One hundred providers (0.1% of the 60,733 providers who placed orders for thyroid tests) accounted for 29.5% of the orders for rT3. Of the 100 providers, 60 with the highest order volumes for rT3 were classified as practitioners of functional medicine. A systematic review of Medline found little evidence to support the high volumes of orders for rT3. A survey of Web sites for functional medicine suggests that rT3 is useful for the diagnosis of rT3 dominance and can be used to direct triiodothyronine replacement therapy.

CONCLUSIONS

There is wide practice variation in rT3 testing. A high proportion of tests are ordered by a relatively small proportion of providers. There is little evidence to support high volumes of rT3 testing placed by some practitioners.

Organohalogens and their hydroxylated metabolites in the blood of pigs from an open waste dumping site in south India: association with hepatic cytochrome P450

The concentrations of polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and their hydroxylated metabolites (OH-PCBs and OH-PBDEs) were measured in the blood of Eurasian wild pigs (Sus scrofa) from a municipal waste open dumping site (DS) and a reference site (RS) in South India. We showed that contamination with OH-PCBs was higher in female pigs from the DS than in all other adult pigs. The highest OH-PCB concentrations were found in piglets from the DS. Moreover, the hepatic expression levels of CYP1A and CYP2B were higher in piglets than in their dam, implying metabolism of PCBs by cytochrome P450 (CYP) enzymes. The OH-PCB congener profiles differed according to sex and collection sites, possibly because of variations in the expression levels of phase I and phase II enzymes among individual pigs, differences in the exposure sources, and maternal transfer of parent PCBs. The hepatic CYP1A expression levels were positively correlated with the blood concentrations of 4OH-CB107, 4OH-CB162, and 4OH-CB187, implying CYP1A-dependent formation of these OH-PCBs in the pig liver. We found no significant correlations between the blood concentrations of OH-PCBs and thyroid hormones (THs); however, the thyroxin (T4) levels were lower in pigs from the DS than in pigs from the RS. Our limited dataset suggest that induced CYP enzymes accelerate the metabolism of xenobiotics and endogenous molecules in pigs. Thus, besides parental compounds, the risk of hydroxylated metabolites entering wildlife and humans living in and around municipal open waste dumping sites should be considered.

Toxicological assessment of polychlorinated biphenyls and their metabolites in the liver of Baikal seal (Pusa sibirica)

We have previously reported that high accumulation of dioxins and related compounds induced cytochrome P450 (CYP 1s) isozymes in the liver of wild Baikal seals, implying the enhanced hydroxylation of polychlorinated biphenyls (PCBs). The present study attempted to elucidate the residue concentrations and patterns of PCBs and hydroxylated PCBs (OH-PCBs) in the livers of Baikal seals. The hepatic residue concentrations were used to assess the potential effects of PCBs and OH-PCBs in combination with the analyses of serum thyroid hormones, hepatic mRNA levels, and biochemical markers. The hepatic expression levels of CYP1 genes were positively correlated with the concentration of each OH-PCB congener. This suggests chronic induction of these CYP1 isozymes by exposure to PCBs and hydroxylation of PCBs induced by CYP 1s. Hepatic mRNA expression monitoring using a custom microarray showed that chronic exposure to PCBs and their metabolites alters the gene expression levels related to oxidative stress, iron ion homeostasis, and inflammatory responses. In addition, the concentrations of OH-PCBs were negatively correlated with L-thyroxine (T4) levels and the ratios of 3,3',5-triiodo-L-thyronine (T3)/reverse 3,3',5'-triiodo-L-thyroninee (rT3). These observations imply that Baikal seals contaminated with high levels of OH-PCBs may undergo the disruption of mechanisms related to the formation (or metabolism) of T3 and T4 in the liver.

Coordinated regulation of nuclear receptor CAR by CCRP/DNAJC7, HSP70 and the ubiquitin-proteasome system

The constitutive active/androstane receptor (CAR) plays an important role as a coordinate transcription factor in the regulation of various hepatic metabolic pathways for chemicals such as drugs, glucose, fatty acids, bilirubin, and bile acids. Currently, it is known that in its inactive state, CAR is retained in the cytoplasm in a protein complex with HSP90 and the tetratricopeptide repeat protein cytosoplasmic CAR retention protein (CCRP). Upon activation by phenobarbital (PB) or the PB-like inducer 1,4-bis[2-(3,5-dichloropyridyloxy)]-benzene (TCPOBOP), CAR translocates into the nucleus. We have identified two new components to the cytoplasmic regulation of CAR: ubiquitin-dependent degradation of CCRP and protein-protein interaction with HSP70. Treatment with the proteasome inhibitor MG132 (5 µM) causes CAR to accumulate in the cytoplasm of transfected HepG2 cells. In the presence of MG132, TCPOBOP increases CCRP ubiquitination in HepG2 cells co-expressing CAR, while CAR ubiquitination was not detected. MG132 treatment of HepG2 also attenuated of TCPOBOP-induced CAR transcriptional activation on reporter constructs which contain CAR-binding DNA elements derived from the human CYP2B6 gene. The elevation of cytoplasmic CAR protein with MG132 correlated with an increase of HSP70, and to a lesser extent HSP60. Both CCRP and CAR were found to interact with endogenous HSP70 in HepG2 cells by immunoprecipitation analysis. Induction of HSP70 levels by heat shock also increased cytoplasmic CAR levels, similar to the effect of MG132. Lastly, heat shock attenuated TCPOBOP-induced CAR transcriptional activation, also similar to the effect of MG132. Collectively, these data suggest that ubiquitin-proteasomal regulation of CCRP and HSP70 are important contributors to the regulation of cytoplasmic CAR levels, and hence the ability of CAR to respond to PB or PB-like inducers.

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.

Developmental triclosan exposure decreases maternal, fetal, and early neonatal thyroxine: a dynamic and kinetic evaluation of a putative mode-of-action

This work tests the mode-of-action (MOA) hypothesis that maternal and developmental triclosan (TCS) exposure decreases circulating thyroxine (T4) concentrations via up-regulation of hepatic catabolism and elimination of T4. Time-pregnant Long-Evans rats received TCS po (0-300mg/kg/day) from gestational day (GD) 6 through postnatal day (PND) 21. Serum and liver were collected from dams (GD20, PND22) and offspring (GD20, PND4, PND14, PND21). Serum T4, triiodothyronine (T3), and thyroid-stimulating hormone (TSH) concentrations were measured by radioimmunoassay. Ethoxy-O-deethylase (EROD), pentoxyresorufin-O-depentylase (PROD) and uridine diphosphate glucuronyltransferase (UGT) enzyme activities were measured in liver microsomes. Custom Taqman(®) qPCR arrays were employed to measure hepatic mRNA expression of select cytochrome P450s, UGTs, sulfotransferases, transporters, and thyroid hormone-responsive genes. TCS was quantified by LC/MS/MS in serum and liver. Serum T4 decreased approximately 30% in GD20 dams and fetuses, PND4 pups and PND22 dams (300mg/kg/day). Hepatic PROD activity increased 2-3 fold in PND4 pups and PND22 dams, and UGT activity was 1.5 fold higher in PND22 dams only (300mg/kg/day). Minor up-regulation of Cyp2b and Cyp3a expression in dams was consistent with hypothesized activation of the constitutive androstane and/or pregnane X receptor. T4 reductions of 30% for dams and GD20 and PND4 offspring with concomitant increases in PROD (PND4 neonates and PND22 dams) and UGT activity (PND22 dams) suggest that up-regulated hepatic catabolism may contribute to TCS-induced hypothyroxinemia during development. Serum and liver TCS concentrations demonstrated greater fetal than postnatal internal exposure, consistent with the lack of T4 changes in PND14 and PND21 offspring. These data support the MOA hypothesis that TCS exposure leads to hypothyroxinemia via increased hepatic catabolism; however, the minor effects on thyroid hormone metabolism may reflect the low efficacy of TCS as thyroid hormone disruptor or highlight the possibility that other MOAs may also contribute to the observed maternal and early neonatal hypothyroxinemia.

Analysis of thyroid hormones in serum of Baikal seals and humans by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and immunoassay methods: application of the LC-MS/MS method to wildlife tissues

Thyroid hormones (THs) are essential for the regulation of growth and development in both humans and wildlife. Until recently, TH concentrations in the tissues of animals have been examined by immunoassay (IA) methods. IA methods are sensitive, but for TH analysis, they are compromised by a lack of adequate specificity. In this study, we determined the concentrations of six THs, L-thyroxine (T(4)), 3,3',5-triiodo-L-thyronine (T(3)), 3,3',5'-triiodo-L-thyronine (rT(3)), 3,5-diiodo-L-thyronine (3,5-T(2)), 3,3'-diiodo-L-thyronine (3,3'-T(2)), and 3-iodo-L-thyronine (3-T(1)), in the serum of humans (n = 79) and wild Baikal seals (n = 37), by isotope ([(13)C(6)]-T(4))-dilution liquid chromatography (LC)-tandem mass spectrometry (MS/MS), and compared the TH levels with those measured by an electrochemiluminescent immunoassay (ECLIA) method. T(3) and T(4) were detected in all serum samples of both humans and Baikal seals, whereas T(1), 3,3'-T(2), and 3,5-T(2) were below the limit of detection (LOD). rT(3) was detected in Baikal seal sera at concentrations higher than T(3) in 28 seal samples, indicating an anomaly in deiodinase activity in Baikal seals. In humans, regression analyses of TH concentrations, measured by ECLIA and LC-MS/MS methods, showed significant correlations for T(4) (r = 0.852) and T(3) (r = 0.676; after removal of a serum sample with abnormal T(3) levels). In Baikal seals, a low correlation coefficient (r = 0.466) for T(4) levels and no correlation for T(3) levels (p = 0.093) were found between ECLIA and LC-MS/MS methods. These results suggest that interference by a nonspecific reaction against anti-T(3) and anti-T(4) antibodies used in the ECLIA can contribute to inaccuracies in TH measurement in Baikal seals. When the relationship between concentrations of THs in sera and dioxin-like toxic equivalents in blubber samples of Baikal seals (n = 19) was examined, a significantly negative correlation was found for serum T(4) levels measured by the LC-MS/MS method, but not for those measured by ECLIA. Thus, our results indicate that the LC-MS/MS method is more reliable and accurate for the elucidation of alteration in circulating TH levels in wildlife, as caused by environmental and physiological factors.

Nuclear receptor CAR (NR1I3) is essential for DDC-induced liver injury and oval cell proliferation in mouse liver

The liver is endowed with the ability to regenerate hepatocytes in response to injury. When this regeneration ability is impaired during liver injury, oval cells, which are considered to be postnatal hepatic progenitors, proliferate and differentiate into hepatocytes. Here we have demonstrated that 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) activates the nuclear receptor constitutive active/androstane receptor (CAR), resulting in proliferation of oval cells in mouse liver. Activation of CAR by DDC was shown by hepatic nuclear CAR accumulation and cytochrome P450 (CYP)2B10 mRNA induction after feeding a 0.1% DDC-containing diet to Car(+/+) mice. After being fed the DDC diet, Car(+/+), but not Car(-/-) mice, developed severe liver injury and an A6 antibody-stained ductular reaction in an area around the portal tract. Oval cell proliferation was confirmed by laser capture microdissection and real-time PCR; mRNAs for the two oval cell markers epithelial cell adhesion molecule and TROP2 were specifically induced in the periportal region of DDC diet-fed Car(+/+), but not Car(-/-) mice. Although rates of both hepatocyte growth and death were initially enhanced only in DDC diet-fed Car(+/+) mice, growth was attenuated when oval cells proliferated, whereas death continued unabated. DDC-induced liver injury, which differs from other CAR activators such as phenobarbital, occurred in the periportal region where cells developed hypertrophy, accumulated porphyrin crystals and inflammation developed, all in association with the proliferation of oval cells. Thus, CAR provides an excellent experimental model for further investigations into its roles in liver regeneration, as well as the development of diseases such as hepatocellular carcinoma.

Pregnane X receptor and constitutive androstane receptor at the crossroads of drug metabolism and energy metabolism

The pregnane X receptor (PXR) and the constitutive androstane receptor (CAR) are two closely related and liver-enriched nuclear hormone receptors originally defined as xenobiotic receptors. PXR and CAR regulate the transcription of drug-metabolizing enzymes and transporters, which are essential in protecting our bodies from the accumulation of harmful chemicals. An increasing body of evidence suggests that PXR and CAR also have an endobiotic function that impacts energy homeostasis through the regulation of glucose and lipids metabolism. Of note and in contrast, disruptions of energy homeostasis, such as those observed in obesity and diabetes, also have a major impact on drug metabolism. This review will focus on recent progress in our understanding of the integral role of PXR and CAR in drug metabolism and energy homeostasis.

The increased CAR-dependent metabolism of thyroid hormones in mice with high cancer susceptibility

AIM

our aim was to compare activation of the constitutive androstane receptor (CAR), hepatic expression of its target genes, and the serum thyroid hormone levels in C3H/He, C57BL/6J, and CC57BR/Mv mice following phenobarbital treatment. These differences, if present, could help to explain the different susceptibility to phenobarbital-induced liver tumor promotion among these strains of mice.

MAIN METHODS

CAR DNA-binding activity and CAR content in nuclear protein extracts from mouse livers were assessed using the electrophoretic mobility shift assay and immunoblotting. Serum thyroid hormone concentrations were determined by radioimmunoassay. Real-time PCR was used to measure the hepatic expression level of CAR target genes.

KEY FINDINGS

we found a 2.3-fold increase of CAR DNA-binding activity in response to phenobarbital in the sensitive C3H/He mice, but no change in the relatively resistant C57BL/6J and CC57BR/Mv mice. Phenobarbital treatment caused a significant decrease in triiodothyronine and free thyroxine concentrations (17% and 40%, respectively) in the sensitive C3H/He mice by the end of 60-day treatment, while in the resistant mice, these changes were not observed. In the sensitive C3H/He mice only, the expression of a CAR target gene encoding sulfotransferase Sult2a1, the thyroid hormone inactivation enzyme, increased by 260-fold after phenobarbital administration. The expression of another CAR target gene, Mdm2, was also increased by phenobarbital treatment in C3H/He mice.

SIGNIFICANCE

we have shown that phenobarbital activates CAR and increases the expression of its target genes thereby accelerating the metabolism of thyroid hormones only in mice susceptible to liver tumor promotion by phenobarbital, but not in relatively resistant animals.

Comparative endocrine disruptive effects of contaminants in ringed seals (Phoca hispida) from Svalbard and the Baltic Sea

We investigated variables related to thyroid, vitamin A and calcitriol homeostasis, immune function and tumour development in ringed seals (Phoca hispida) from the polluted Baltic Sea and a less polluted reference location at Svalbard, Norway. We also examined the relationships between the biological variables and the concentrations of persistent organic pollutants (POPs) and their hydroxylated (OH) metabolites. Our data show higher plasma concentrations of free triiodothyronine (T3), and ratios of free and total T3 in Baltic seals as compared to Svalbard seals. Baltic seals had also higher hepatic mRNA expressions of deiodinase-I, thyroid hormone receptor beta, retinoic acid receptor alpha, growth hormone receptor and interleukin-1beta compared to Svalbard seals. Levels of plasma retinol were lower in the Baltic seals as compared to Svalbard seals. No geographical difference was observed for other thyroid hormone levels and hepatic retinoid levels. Ratios of free and total T3 were positively correlated to OH-POPs in plasma. The results of the present study suggest that endocrine homeostasis may be affected by contaminant and metabolite exposure in the Baltic ringed seals with respect to circulating hormones and retinol and hepatic mRNA expressions. In addition, OH-POPs may putatively produce the disruption of thyroid hormone transport in plasma.

Mice lacking thyroid hormone receptor Beta show enhanced apoptosis and delayed liver commitment for proliferation after partial hepatectomy

BACKGROUND

The role of thyroid hormones and their receptors (TR) during liver regeneration after partial hepatectomy (PH) was studied using genetic and pharmacologic approaches. Roles in liver regeneration have been suggested for T3, but there is no clear evidence distinguishing the contribution of increased amounts of T3 from the modulation by unoccupied TRs.

METHODOLOGY/PRINCIPAL FINDINGS

Mice lacking TRalpha1/TRbeta or TRbeta alone fully regenerated liver mass after PH, but showed delayed commitment to the initial round of hepatocyte proliferation and transient but intense apoptosis at 48h post-PH, affecting approximately 30% of the remaining hepatocytes. Pharmacologically induced hypothyroidism yielded similar results. Loss of TR activity was associated with enhanced nitrosative stress in the liver remnant, due to an increase in the activity of the nitric oxide synthase (NOS) 2 and 3, caused by a transient decrease in the concentration of asymmetric dimethylarginine (ADMA), a potent NOS inhibitor. This decrease in the ADMA levels was due to the presence of a higher activity of dimethylarginineaminohydrolase-1 (DDAH-1) in the regenerating liver of animals lacking TRalpha1/TRbeta or TRbeta. DDAH-1 expression and activity was paralleled by the activity of FXR, a transcription factor involved in liver regeneration and up-regulated in the absence of TR.

CONCLUSIONS/SIGNIFICANCE

We report that TRs are not required for liver regeneration; however, hypothyroid mice and TRbeta- or TRalpha1/TRbeta-deficient mice exhibit a delay in the restoration of liver mass, suggesting a specific role for TRbeta in liver regeneration. Altered regenerative responses are related with a delay in the expression of cyclins D1 and E, and the occurrence of liver apoptosis in the absence of activated TRbeta that can be prevented by administration of NOS inhibitors. Taken together, these results indicate that TRbeta contributes significantly to the rapid initial round of hepatocyte proliferation following PH, and improves the survival of the regenerating liver at later times.

Activation of CAR and PXR by Dietary, Environmental and Occupational Chemicals Alters Drug Metabolism, Intermediary Metabolism, and Cell Proliferation

The constitutive androstane receptor (CAR) and the pregnane × receptor (PXR) are activated by a variety of endogenous and exogenous ligands, such as steroid hormones, bile acids, pharmaceuticals, and environmental, dietary, and occupational chemicals. In turn, they induce phase I-III detoxification enzymes and transporters that help eliminate these chemicals. Because many of the chemicals that activate CAR and PXR are environmentally-relevant (dietary and anthropogenic), studies need to address whether these chemicals or mixtures of these chemicals may increase the susceptibility to adverse drug interactions. In addition, CAR and PXR are involved in hepatic proliferation, intermediary metabolism, and protection from cholestasis. Therefore, activation of CAR and PXR may have a wide variety of implications for personalized medicine through physiological effects on metabolism and cell proliferation; some beneficial and others adverse. Identifying the chemicals that activate these promiscuous nuclear receptors and understanding how these chemicals may act in concert will help us predict adverse drug reactions (ADRs), predict cholestasis and steatosis, and regulate intermediary metabolism. This review summarizes the available data on CAR and PXR, including the environmental chemicals that activate these receptors, the genes they control, and the physiological processes that are perturbed or depend on CAR and PXR action. This knowledge contributes to a foundation that will be necessary to discern interindividual differences in the downstream biological pathways regulated by these key nuclear receptors.

Large induction of type III deiodinase expression after partial hepatectomy in the regenerating mouse and rat liver

The deiodinase types 1 (D1) and 2 (D2) catalyze the activation of T4 to T3, whereas type 3 deiodinase (D3) catalyzes the inactivation of T3 and T4. D3 plays a key role in controlling thyroid hormone bioavailability. It is highly expressed during fetal development, but also in other processes with increased cell proliferation, e.g. in vascular tumors. Because tissue regeneration is dependent on cellular proliferation and is associated with activation of fetal genes, we evaluated deiodinase activities and mRNA expression in rat and mouse liver, as well as the local and systemic thyroid hormone status after partial hepatectomy (PH). We observed that in rats, D3 activity was increased 10-fold at 20 h and 3-fold at 48 h after PH; D3 mRNA expression was increased 3-fold at 20 h. The increase in D3 expression was associated with maximum 2- to 3-fold decreases of serum and liver T3 and T4 levels at 20 to 24 h after PH. In mice, D3 activity was increased 5-fold at 12 h, 8-fold at 24 h, 40-fold at 36 h, 15-fold at 48 h, and 7-fold at 72 h after PH. In correlation with this, D3 mRNA was highest (6-fold increase), and serum T3 and T4 were lowest at 36 h. Furthermore, as a measure for cell proliferation, 5-bromo-2'-deoxyuridine incorporation peaked at 20-24 h after PH in rats and at 36 h in mice. No significant effect on D1 activity or mRNA expression was found after PH. D2 activity was always undetectable. In conclusion, we found a large induction of hepatic D3 expression after PH that was correlated with an increased cellular proliferation and decreased serum and liver T3 and T4 levels. Our data suggest that D3 is important in the modulation of thyroid hormone levels in the regenerating liver, in which a decrease in cellular T3 permits an increase in proliferation.

Effects of perinatal PBDE exposure on hepatic phase I, phase II, phase III, and deiodinase 1 gene expression involved in thyroid hormone metabolism in male rat pups

Previous studies demonstrated that perinatal exposure to polybrominated diphenyl ethers (PBDEs), a major class of brominated flame retardants, may affect thyroid hormone (TH) concentrations by inducing hepatic uridinediphosphate-glucoronosyltransferases (UGTs). This study further examines effects of the commercial penta mixture, DE-71, on genes related to TH metabolism at different developmental time points in male rats. DE-71 is predominately composed of PBDE congeners 47, 99, 100, 153, 154 with low levels of brominated dioxin and dibenzofuran contaminants. Pregnant Long-Evans rats were orally administered 1.7 (low), 10.2 (mid), or 30.6 (high) mg/kg/day of DE-71 in corn oil from gestational day (GD) 6 to postnatal day (PND) 21. Serum and liver were collected from male pups at PND 4, 21, and 60. Total serum thyroxine (T(4)) decreased to 57% (mid) and 51% (high) on PND 4, and 46% (mid) dose and 25% (high) on PND 21. Cyp1a1, Cyp2b1/2, and Cyp3a1 enzyme and mRNA expression, regulated by aryl hydrocarbon receptor, constitutive androstane receptor, and pregnane xenobiotic receptor, respectively, increased in a dose-dependent manner. UGT-T(4) enzymatic activity significantly increased, whereas age and dose-dependent effects were observed for Ugt1a6, 1a7, and 2b mRNA. Sult1b1 mRNA expression increased, whereas that of transthyretin (Ttr) decreased as did both the deiodinase I (D1) enzyme activity and mRNA expression. Hepatic efflux transporters Mdr1 (multidrug resistance), Mrp2 (multidrug resistance-associated protein), and Mrp3 and influx transporter Oatp1a4 mRNA expression increased. In this study the most sensitive responses to PBDEs following DE-71 exposure were CYP2B and D1 activities and Cyb2b1/2, d1, Mdr1, Mrp2, and Mrp3 gene expression. All responses were reversible by PND 60. In conclusion, deiodination, active transport, and sulfation, in addition to glucuronidation, may be involved in disruption of TH homeostasis due to perinatal exposure to DE-71 in male rat offspring.

The roles of nuclear receptors CAR and PXR in hepatic energy metabolism

Nuclear receptors constitutive active/androstane receptor (CAR) and pregnane X receptor (PXR) were originally characterized as transcription factors regulating the hepatic genes that encode drug metabolizing enzymes. Recent works have now revealed that these nuclear receptors also play the critical roles in modulating hepatic energy metabolism. While CAR and PXR directly bind to their response sequences phenobarbital-responsive enhancer module (PBREM) and xenobiotic responsive enhancer module (XREM) in the promoter of target genes to increase drug metabolism, the receptors also cross talk with various hormone responsive transcription factors such as forkhead box O1 (FoxO1), forkhead box A2 (FoxA2), cAMP-response element binding protein, and peroxisome proliferator activated receptor gamma coactivator 1alpha (PGC 1alpha) to decrease energy metabolism through down-regulating gluconeogenesis, fatty acid oxidation and ketogenesis and up-regulating lipogenesis. In addition, CAR modulates thyroid hormone activity by regulating type 1 deiodinase in the regenerating liver. Thus, CAR and PXR are now placed at the crossroad where both xenobiotics and endogenous stimuli co-regulate liver function.

PPP1R16A, the membrane subunit of protein phosphatase 1beta, signals nuclear translocation of the nuclear receptor constitutive active/androstane receptor

Constitutive active/androstane receptor (CAR), a member of the nuclear steroid/thyroid hormone receptor family, activates transcription of numerous hepatic genes upon exposure to therapeutic drugs and environmental pollutants. Sequestered in the cytoplasm, this receptor signals xenobiotic exposure, such as phenobarbital (PB), by translocating into the nucleus. Unlike other hormone receptors, translocation can be triggered indirectly without binding to xenobiotics. We have now identified a membrane-associated subunit of protein phosphatase 1 (PPP1R16A, or abbreviated as R16A) as a novel CAR-binding protein. When CAR and R16A are coexpressed in mouse liver, CAR translocates into the nucleus. Close association of R16A and CAR molecule on liver membrane was shown by fluorescence resonance energy transfer (FRET) analysis using expressed yellow fluorescent protein (YFP)-CAR and CFP-R16A fusion proteins. R16A can form dimer through its middle region, where protein kinase A phosphorylation sites are recently identified. Translocation of CAR by R16A correlates with the ability of R16A to form an intermolecular interaction via the middle region. Moreover, this interaction is enhanced by PB treatment in mouse liver. R16A specifically interacted with PP1beta in HepG2 cells despite the highly conserved structure of PP1 family molecules. PP1beta activity was inhibited by R16A in vitro and coexpression of PP1beta in liver can prevent YFP-CAR translocation into mouse liver. Taken together, R16A at the membrane may mediate the PB signal to initiate CAR nuclear translocation, through a mechanism including its dimerization and inhibition of PP1beta activity, providing a novel model for the translocation of nuclear receptors in which direct interaction of ligands and the receptors may not be crucial.

The tangle of nuclear receptors that controls xenobiotic metabolism and transport: crosstalk and consequences

The expression of many genes involved in xenobiotic/drug metabolism and transport is regulated by at least three nuclear receptors or xenosensors: aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), and pregnane X receptor (PXR). These receptors establish crosstalk with other nuclear receptors or transcription factors controlling signaling pathways that regulate the homeostasis of bile acids, lipids, glucose, inflammation, vitamins, hormones, and others. These crosstalks are expected to modify profoundly our vision of xenobiotic/drug disposition and toxicity. They provide molecular mechanisms to explain how physiopathological stimuli affect xenobiotic/drug disposition, and how xenobiotics/drugs may affect physiological functions and generate toxic responses. In addition, the possibility that xenosensors may control other signaling pathways opens the way to new pharmacological opportunities.

Human nuclear pregnane X receptor cross-talk with CREB to repress cAMP activation of the glucose-6-phosphatase gene

The nuclear PXR (pregnane X receptor) was originally characterized as a key transcription factor that activated hepatic genes encoding drug-metabolizing enzymes. We have now demonstrated that PXR also represses glucagon-activated transcription of the G6Pase (glucose-6-phosphatase) gene by directly binding to CREB [CRE (cAMP-response element)-binding protein]. Adenoviral-mediated expression of human PXR (hPXR) and its activation by rifampicin strongly repressed cAMP-dependent induction of the endogenous G6Pase gene in Huh7 cells. Using the -259 bp G6Pase promoter construct in cell-based transcription assays, repression by hPXR of PKA (cAMP-dependent protein kinase)-mediated promoter activation was delineated to CRE sites. GST (glutathione transferase) pull-down and immunoprecipitation assays were employed to show that PXR binds directly to CREB, while gel-shift assays were used to demonstrate that this binding prevents CREB interaction with the CRE. These results are consistent with the hypothesis that PXR represses the transcription of the G6Pase gene by inhibiting the DNA-binding ability of CREB. In support of this hypothesis, treatment with the mouse PXR activator PCN (pregnenolone 16alpha-carbonitrile) repressed cAMP-dependent induction of the G6Pase gene in primary hepatocytes prepared from wild-type, but not from PXR-knockout, mice, and also in the liver of fasting wild-type, but not PXR-knockout, mice. Moreover, ChIP (chromatin immunoprecipitation) assays were performed to show a decreased CREB binding to the G6Pase promoter in fasting wild-type mice after PCN treatment. Thus drug activation of PXR can repress the transcriptional activity of CREB, down-regulating gluconeogenesis.

Overexpression of the Rho-guanine nucleotide exchange factor ECT2 inhibits nuclear translocation of nuclear receptor CAR in the mouse liver

Various drugs such as phenobarbital (PB) trigger translocation of constitutive active/adrostane receptor (CAR) from the cytoplasm into the nucleus of mouse liver cells without directly binding to the receptor. We have now characterized the guanine nucleotide exchange factor epithelial cell-transforming gene 2 (ECT2) as a PB-inducible factor as well as a cellular signal that represses PB-triggered nuclear translocation of CAR. When CFP-tagged ECT2 was co-expressed with YFP-tagged CAR in the liver of Car(-/-) mice, ECT2 repressed CAR nuclear translocation. Coexpression of various deletion mutants delineated this repressive activity to the tandem Dbl homology/pleckstrin homology domains of ECT2 and to their cytosolic expression. CAR directly bound to the PH domain. Thus, ECT2 may comprise a part of the PB response signal regulating the intracellular trafficking of CAR.