Importance of hepatic induction of constitutive androstane receptor and other transcription factors that regulate xenobiotic metabolism and transport

Regulation of hepatic ABCC transporters by xenobiotics and in disease states

The subfamily of ABCC transporters consists of 13 members in mammals, including the multidrug resistance-associated proteins (MRPs), sulfonylurea receptors (SURs), and the cystic fibrosis transmembrane conductance regulator (CFTR). These proteins play roles in chemical detoxification, disposition, and normal cell physiology. ABCC transporters are expressed differentially in the liver and are regulated at the transcription and translation level. Their expression and function are also controlled by post-translational modification and membrane-trafficking events. These processes are tightly regulated. Information about alterations in the expression of hepatobiliary ABCC transporters could provide important insights into the pathogenesis of diseases and disposition of xenobiotics. In this review, we describe the regulation of hepatic ABCC transporters in humans and rodents by a variety of xenobiotics, under disease states and in genetically modified animal models deficient in transcription factors, transporters, and cell-signaling molecules.

Activation of xenobiotic receptors: driving into the nucleus

IMPORTANCE OF THE FIELD

Xenobiotic receptors (XRs) play pivotal roles in regulating the expression of genes that determine the clearance and detoxification of xenobiotics, such as drugs and environmental chemicals. Recently, it has become increasingly evident that most XRs shuttle between the cytoplasm and nucleus, and activation of such receptors is directly associated with xenobiotic-induced nuclear import.

AREAS COVERED IN THIS REVIEW

The scope of this review covers research literature that discusses nuclear translocation and activation of XRs, as well as unpublished data generated from this laboratory. Specific emphasis is given to the constitutive androstane receptor (CAR), the pregnane X receptor and the aryl hydrocarbon receptor.

WHAT THE READERS WILL GAIN

A number of molecular chaperons presumably associated with cellular localization of XRs have been identified. Primary hepatocyte cultures have been established as a unique model retaining inactive CAR in the cytoplasm. Moreover, several splicing variants of human CAR exhibit altered cellular localization and chemical activation.

TAKE HOME MESSAGE

Nuclear accumulation is an essential step in the activation of XRs. Although great strides have been made, much remains to be understood concerning the mechanisms underlying intracellular localization and trafficking of XRs, which involve both direct ligand-binding and indirect pathways.

The activated aryl hydrocarbon receptor synergizes mitogen-induced murine liver hyperplasia

Mechanisms of hepatocyte proliferation triggered by tissue loss are distinguishable from those that promote proliferation in the intact liver in response to mitogens. Previous studies demonstrate that exogenous activation of the aryl hydrocarbon receptor (AhR), a soluble ligand-activated transcription factor in the basic helix-loop-helix family of proteins, suppresses compensatory liver regeneration elicited by surgical partial hepatectomy. The goal of the present study was to determine how AhR activation modulates hepatocyte cell cycle progression in the intact liver following treatment with the hepatomitogen, 1,4-bis[2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP). Mice were pretreated with the exogenous AhR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) 24h prior to treatment with TCPOBOP (3 mg/kg).). In contrast to the suppressive effects of AhR activation observed during compensatory regeneration, TCDD pretreatment resulted in a 30-50% increase in hepatocyte proliferation in the intact liver of TCPOBOP-treated mice. Although pretreatment with TCDD suppressed CDK2 kinase activity and increased the association of CDK2 with negative regulatory proteins p21Cip1 and p27Kip1, a corresponding increase in CDK4/cyclin D1 association and CDK4 activity which culminated in enhanced phosphorylation of retinoblastoma protein, consistent with the increased proliferative response. These findings are in stark contrast to previous observations that the activated AhR can suppress hepatocyte proliferation in vivo and reveal a new complexity to AhR-mediated cell cycle control.

Enhanced expression of Nrf2 in mice attenuates the fatty liver produced by a methionine- and choline-deficient diet

Oxidative stress has been proposed as an important promoter of the progression of fatty liver diseases. The current study investigates the potential functions of the Nrf2-Keap1 signaling pathway, an important hepatic oxidative stress sensor, in a rodent fatty liver model. Mice with no (Nrf2-null), normal (wild type, WT), and enhanced (Keap1 knockdown, K1-kd) expression of Nrf2 were fed a methionine- and choline-deficient (MCD) diet or a control diet for 5 days. Compared to WT mice, the MCD diet-caused hepatosteatosis was more severe in the Nrf2-null mice and less in the K1-kd mice. The Nrf2-null mice had lower hepatic glutathione and exhibited more lipid peroxidation, whereas the K1-kd mice had the highest amount of glutathione in the liver and developed the least lipid peroxidation among the three genotypes fed the MCD diet. The Nrf2 signaling pathway was activated by the MCD diet, and the Nrf2-targeted cytoprotective genes Nqo1 and Gstalpha1/2 were induced in WT and even more in K1-kd mice. In addition, Nrf2-null mice on both control and MCD diets exhibited altered expression profiles of fatty acid metabolism genes, indicating Nrf2 may influence lipid metabolism in liver. For example, mRNA levels of long chain fatty acid translocase CD36 and the endocrine hormone Fgf21 were higher in livers of Nrf2-null mice and lower in the K1-kd mice than WT mice fed the MCD diet. Taken together, these observations indicate that Nrf2 could decelerate the onset of fatty livers caused by the MCD diet by increasing hepatic antioxidant and detoxification capabilities.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Thyroid-disrupting chemicals: interpreting upstream biomarkers of adverse outcomes

BACKGROUND

There is increasing evidence in humans and in experimental animals for a relationship between exposure to specific environmental chemicals and perturbations in levels of critically important thyroid hormones (THs). Identification and proper interpretation of these relationships are required for accurate assessment of risk to public health.

OBJECTIVES

We review the role of TH in nervous system development and specific outcomes in adults, the impact of xenobiotics on thyroid signaling, the relationship between adverse outcomes of thyroid disruption and upstream causal biomarkers, and the societal implications of perturbations in thyroid signaling by xenobiotic chemicals.

DATA SOURCES

We drew on an extensive body of epidemiologic, toxicologic, and mechanistic studies.

DATA SYNTHESIS

THs are critical for normal nervous system development, and decreased maternal TH levels are associated with adverse neuropsychological development in children. In adult humans, increased thyroid-stimulating hormone is associated with increased blood pressure and poorer blood lipid profiles, both risk factors for cardiovascular disease and death. These effects of thyroid suppression are observed even within the "normal" range for the population. Environmental chemicals may affect thyroid homeostasis by a number of mechanisms, and multiple chemicals have been identified that interfere with thyroid function by each of the identified mechanisms.

CONCLUSIONS

Individuals are potentially vulnerable to adverse effects as a consequence of exposure to thyroid-disrupting chemicals. Any degree of thyroid disruption that affects TH levels on a population basis should be considered a biomarker of adverse outcomes, which may have important societal outcomes.

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.

Ontogenic expression of hepatic Ahr mRNA is associated with histone H3K4 di-methylation during mouse liver development

The aryl hydrocarbon receptor (Ahr) is a xenobiotic sensor that regulates the expression of a battery of drug-metabolizing genes. However, Ahr is also important for normal liver development. The purpose of the present study was to examine the ontogeny of Ahr mRNA in mouse liver, and determine the epigenetic mechanisms regulating Ahr gene transcription during postnatal liver development. There was a 224% increase in hepatic Ahr mRNA from 2 days before birth to 45 days after birth. ChIP-on-chip analysis demonstrated that DNA methylation and histone H3K27 tri-methylation (H3K27Me3), two epigenetic marks for suppression of gene transcription, were consistently low around the Ahr gene locus. In contrast, enrichment of histone H3K4 di-methylation (H3K4Me2), a hallmark for gene activation, increased 182% from prenatal to young adult period around the Ahr gene locus. Regression analysis revealed a strong correlation between enrichment of H3K4Me2 and Ahr mRNA (r=0.91). In conclusion, postnatal H3K4Me2 enrichment positively associates with Ahr mRNA in developing mouse liver, providing a permissive chromatin state allowing Ahr gene transactivation in postnatal liver development.

Introducing the "TCDD-inducible AhR-Nrf2 gene battery"

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induces genes via the transcription factor aryl hydrocarbon receptor (AhR), including Cyp1a1, NAD(P)H:quinone oxidoreductase 1 (Nqo1), UDP-glucuronosyltransferase 1a6 (Ugt1a6), and glutathione S-transferase a1 (Gsta1). These genes are referred to as the "AhR gene battery." However, Nqo1 is also considered a prototypical target gene of the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2). In mice, TCDD induction of Nrf2 and Nrf2 target, Nqo1, is dependent on AhR, and thus TCDD induction of drug-processing genes may be routed through an AhR-Nrf2 sequence. There has been speculation that Nrf2 may be involved in the TCDD induction of drug-processing genes; however, the data are not definitive. Therefore, to address whether TCDD induction of Nqo1, Ugts, and Gsts is dependent on Nrf2, we conducted the definitive experiment by administering TCDD (50 mug/kg, ip) to Nrf2-null and wild-type (WT) mice and collecting livers 24 h later to quantify the mRNA of drug-processing genes. TCDD induction of Cyp1a1 and Ugt1a1 was similar in WT and Nrf2-null mice, whereas TCDD induction of Ugt1a5 and 1a9 was blunted in Nrf2-null mice. TCDD induced Nqo1, Ugt1a6, 2b34, 2b35, 2b36, UDP-glucuronic acid-synthesizing gene UDP-glucose dehydrogenase, and Gsta1, m1, m2, m3, m6, p2, t2, and microsomal Gst1 in WT mice but not in Nrf2-null mice. Therefore, the present study demonstrates the novel finding that Nrf2 is required for TCDD induction of classical AhR battery genes Nqo1, Ugt1a6, and Gsta1, as well as most Ugt and Gst isoforms in livers of mice.

Induction of multiple drug transporters by efavirenz

Efavirenz, an important component of human immunodeficiency virus 1 (HIV-1) therapy, causes substantial drug interactions as an inducer of cytochromes and the transporter ABCB1. So far its effect on the expression of other transporters is unknown. We therefore investigated the effect of long-term exposure of cells to efavirenz on expression of a large number of important drug transporters and on cell proliferation as a surrogate of intracellular availability. LS180 cells were used as a surrogate for the major site of drug interactions and Jurkat cells were used as a surrogate for the main target cells of HIV therapy. Cells were treated with efavirenz over 4 weeks and mRNA expression of drug transporters was repeatedly quantified. After 4 weeks, efavirenz significantly up-regulated the mRNA of ABCB1, ABCG2, ABCC2, ABCC3, ABCC5, and SLCO3A1 in LS180 cells and ABCG2, ABCC1, ABCC4, ABCC5, and SLCO2B1 in Jurkat cells. However these changes in transporter expression did not influence cell proliferation indicating that intracellular efavirenz concentrations were likely not altered. Efavirenz induces mRNA expression of several drug transporters critically modulating the kinetics of other drugs. While these expressional changes will most likely not influence the efficiency of efavirenz itself, they might change the effect of other co-administered drugs.

Increased Nrf2 activation in livers from Keap1-knockdown mice increases expression of cytoprotective genes that detoxify electrophiles more than those that detoxify reactive oxygen species

Nuclear factor erythroid 2-related factor 2 (Nrf2) is a transcription factor critical for protection against electrophilic and oxidative stress. In a recently engineered mouse with knockdown of kelch-like ECH associated protein 1 (Keap1-kd mice), the cytosolic repressor of Nrf2, there is a 55% decrease in Keap1 mRNA and a 200% increase in Nrf2 protein in liver. Experiments with Nrf2-null mice have demonstrated the effects of a lack of Nrf2. However, little is known about the biological effects of more Nrf2 activation. Accordingly, the hepatic phenotype of Keap1-kd mice, as well as the hepatic mRNA expression of cytoprotective genes were compared among wild-type, Nrf2-null, and Keap1-kd mice. Three distinct patterns of hepatic gene expression were identified among wild-type, Nrf2-null, and Keap1-kd mice. The first pattern encompassed genes that were lower in Nrf2-null mice and considerably higher in Keap1-kd mice than wild-type mice, which included genes mainly responsible for the detoxification and elimination of electrophiles, such as NAD(P)H:quinone oxidoreductase 1 and glutathione-S-transferases (Gst), and multidrug resistance-associated proteins. The second pattern encompassed genes that were lower in Nrf2-null mice but not increased in Keap1-kd mice, and included genes, such as epoxide hydrolase-1, UDP-glucuronosyltransferases, aldehyde dehydrogenases, as well as genes important in the detoxification of reactive oxygen species, such as superoxide dismutase 1 and 2, catalase, and peroxiredoxin 1. The third pattern encompassed genes that were not different among wild-type, Nrf2-null, and Keap1-kd mice and included genes such as glutathione peroxidase, microsomal Gsts, and uptake transporters. In conclusion, the present study suggests that increased activation of hepatic Nrf2 is more important for the detoxification and elimination of electrophiles than reactive oxygen species.

Circadian expression profiles of drug-processing genes and transcription factors in mouse liver

Temporal coordination of hepatic drug-processing gene (DPG) expression facilitates absorption, biotransformation, and excretion of exogenous and endogenous compounds. To further elucidate the circadian rhythm of hepatic DPG expression, male C57BL/6 mice were subjected to a standard 12-h light/dark cycle, and livers were collected at 2:00, 6:00, and 10:00 AM and 2:00, 6:00, and 10:00 PM. The mRNAs of hepatic phase I enzymes (cytochromes P450, aldehyde dehydrogenases, and carboxylesterases), phase II enzymes (glucuronosyltransferases, sulfotransferases, and glutathione S-transferases), uptake and efflux transporters, and transcription factors were quantified. Messenger RNAs of various genes were graphed across time of day and compared by hierarchical clustering. In general, the mRNA of phase I enzymes increased during the dark phase, whereas the mRNAs of most phase II enzymes and transporters reached maximal levels during the light phase. The majority of hepatic transcription factors exhibited expression peaks either before or after the onset of the dark phase. During the same time period, the negative clock regulator gene Rev-Erbalpha and the hepatic clock-controlled gene Dbp also reached mRNA expression peaks. Considering their important role in xenobiotic metabolism, hepatic transcription factors, such as constitutive androstane receptor, pregnane X receptor, aryl hydrocarbon receptor, and peroxisomal proliferator activated receptor alpha, may be involved in coupling the hepatic circadian clock to environmental cues. Taken together, these data demonstrate that the circadian expression of the DPG battery and transcription factors contribute to the temporal detoxification cycle in the liver.

Physiology, structure, and regulation of the cloned organic anion transporters

1. The transport of negatively charged drugs, xenobiotics, and metabolites by epithelial tissues, particularly the kidney, plays critical roles in controlling their distribution, concentration, and retention in the body. Thus, organic anion transporters (OATs) impact both their therapeutic efficacy and potential toxicity. 2. This review summarizes current knowledge of the properties and functional roles of the cloned OATs, the relationships between transporter structure and function, and those factors that determine the efficacy of transport. Such factors include plasma protein binding of substrates, genetic polymorphisms among the transporters, and regulation of transporter expression. 3. Clearly, much progress has been made in the decade since the first OAT was cloned. However, unresolved questions remain. Several of these issues--drug-drug interactions, functional characterization of newly cloned OATs, tissue differences in expression and function, and details of the nature and consequences of transporter regulation at genomic and intracellular sites--are discussed in the concluding Perspectives section.

Sexually dimorphic regulation and induction of P450s by the constitutive androstane receptor (CAR)

The constitutive androstane receptor (CAR) is a xenosensing nuclear receptor and regulator of cytochrome P450s (CYPs). However, the role of CAR as a basal regulator of CYP expression nor its role in sexually dimorphic responses have been thoroughly studied. We investigated basal regulation and sexually dimorphic regulation and induction by the potent CAR activator TCPOBOP and the moderate CAR activator Nonylphenol (NP). NP is an environmental estrogen and one of the most commonly found environmental toxicants in Europe and the United States. Previous studies have demonstrated that NP induces several CYPs in a sexually dimorphic manner, however the role of CAR in regulating NP-mediated sexually dimorphic P450 expression and induction has not been elucidated. Therefore, wild-type and CAR-null male and female mice were treated with honey as a carrier, NP, or TCPOBOP and CYP expression monitored by QPCR and Western blotting. CAR basally regulates the expression of Cyp2c29, Cyp2b13, and potentially Cyp2b10 as demonstrated by QPCR. Furthermore, we observed a shift in the testosterone 6alpha/15alpha-hydroxylase ratio in untreated CAR-null female mice to the male pattern, which indicates an alteration in androgen status and suggests a role for androgens as CAR inverse agonists. Xenobiotic-treatments with NP and TCPOBOP induced Cyp2b10, Cyp2c29, and Cyp3a11 in a CAR-mediated fashion; however NP only induced these CYPs in females and TCPOBOP induced these CYPs in both males and females. Interestingly, Cyp2a4, was only induced in wild-type male mice by TCPOBOP suggesting Cyp2a4 induction is not sensitive to CAR-mediated induction in females. Overall, TCPOBOP and NP show similar CYP induction profiles in females, but widely different profiles in males potentially related to lower sensitivity of males to either indirect or moderate CAR activators such as NP. In summary, CAR regulates the basal and chemically inducible expression of several sexually dimorphic xenobiotic metabolizing P450s in a manner that varies depending on the ligand.

Drug metabolizing enzyme induction pathways in experimental non-alcoholic steatohepatitis

Non-alcoholic steatohepatitis (NASH) is a disease that compromises hepatic function and the capacity to metabolize numerous drugs. Aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), pregnane X receptor (PXR), peroxisome proliferator-activated receptor alpha (PPARalpha), and nuclear factor-E2 related factor 2 (Nrf2) are xenobiotic activated transcription factors that regulate induction of a number of drug metabolizing enzymes (DMEs). The purpose of the current study was to determine whether experimental NASH alters the xenobiotic activation of these transcription factors and induction of downstream DME targets Cyp1A1, Cyp2B10, Cyp3A11, Cyp4A14 and NAD(P)H:quinone oxidoreductase 1 (Nqo1), respectively. Mice fed normal rodent chow or methionine-choline-deficient (MCD) diet for 8 weeks were then treated with microsomal enzyme inducers beta-naphoflavone (BNF), 1,4-bis-[2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP), pregnenolone-16alpha-carbonitrile (PCN), clofibrate (CFB) or oltipraz (OPZ), known activators of AhR, CAR, PXR, PPARalpha and Nrf2, respectively. Results of this study show that (1) Hepatic PXR mRNA levels were significantly increased (1.4-fold) in mice fed MCD diet, while AhR, CAR, PPARalpha and Nrf2 were not affected. (2) The MCD diet did not alter hepatic inducibility of Cyp1A1, Cyp2B10, Cyp3A11 mRNA levels by their respective microsomal inducers. (3) Constitutive levels of Cyp4A14 mRNA were significantly increased in mice fed the MCD diet, yet further induction by clofibrate was not observed. (4) Hepatic Nqo1 mRNA levels were significantly increased by the MCD diet; however, additional induction of Nqo1 was still achievable following treatment with the Nrf2 activator OPZ.

NF-E2-related factor 2 inhibits lipid accumulation and oxidative stress in mice fed a high-fat diet

NF-E2-related factor 2 (Nrf2) is a transcription factor that is activated by oxidative stress and electrophiles that regulates the expression of numerous detoxifying and antioxidant genes. Previous studies have shown that Nrf2 protects the liver from xenobiotic toxicity; however, whether Nrf2 plays a role in lipid homeostasis in liver is not known. Accordingly, wild-type and Nrf2-null mice were fed a high-fat diet (HFD) for up to 4 weeks. Hepatic gene expression and lipid profiles were analyzed for changes in fatty acid, triglyceride, and cholesterol status. It is interesting to note that HFD reduced the mRNA expression of Nrf2 and its target genes in wild-type mice. The mRNA expression of lipogenic and cholesterologenic transcriptional factors and their target genes, such as sterol regulatory element-binding proteins 1c and 2, fatty acid synthase, acetyl-CoA carboxylase 1, fatty acid elongase, 3-hydroxy-3-methylglutaryl coenzyme A synthase and reductase, and low-density lipoprotein receptor mRNA expression were higher in Nrf2-null mice compared with wild-type mice after feeding a HFD, suggesting that Nrf2 may suppress these pathways. Hepatic triglycerides and cholesterol levels were not different between genotypes, whereas concentrations of hepatic free fatty acid and malondialdehyde equivalents were higher in Nrf2-null mice compared with wild-type mice 4 weeks after HFD feeding. Overall, these results suggest that Nrf2 inhibits lipid accumulation and oxidative stress in mouse liver after feeding a HFD, probably by interfering with lipogenic and cholesterologenic pathways.