Aryl Hydrocarbon Receptor-Inducible or Constitutive Expression of Human UDP Glucuronosyltransferase UGT1A6

doi:10.1006/abbi.1997.0485

Archives of Biochemistry and Biophysics

Volume 350, Issue 1, 1 February 1998, Pages 72-78

https://doi.org/10.1006/abbi.1997.0485 Get rights and content

Abstract

Transcriptional regulation of human UGT1A6, a UDP glucuronosyltransferase isoform conjugating a wide variety of planar phenols, has been studied using transfection experiments with plasmids containing its 3-kb 5′ upstream region and chloramphenicol acetyltransferase as reporter gene. Previously, two modes of expression of the isoform have been described: in colon carcinoma Caco-2 cells UGT1A6 was found to be TCDD-inducible, whereas in lung carcinoma A549 cells it was constitutively expressed. Therefore functional analysis of UGT1A6 regulation was carried out using these two cell lines. In the upstream region of human UGT1A6 one xenobiotic-responsive element (XRE) was found between −1498 and −1502 bp. In Caco-2 cells the reporter gene activity of the entire plasmid and of deletion mutants containing the XRE were TCDD-inducible, in contrast to experiments with a deletion mutant which did not contain the XRE. TCDD induction was marginal in transfection studies with A549 cells. Gel mobility shift analysis indicated that the aryl hydrocarbon receptor and its partner Arnt bind to the XRE. Furthermore, primer extension studies suggest cell-specific use of multiple TATA boxes. Hence, regulation of human UGT1A6 appears to be cell-specific including both constitutive and aryl hydrocarbon receptor-controlled expression.

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Bile duct ligation elevates 5-HT levels in cerebral cortex of rats partly due to impairment of brain UGT1A6 expression and activity via ammonia accumulation
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Hepatic encephalopathy (HE) is often associated with endogenous serotonin (5-HT) disorders. However, the reason for elevated brain 5-HT levels due to liver failure remains unclear. This study aimed to investigate the mechanism by which liver failure increases brain 5-HT levels and the role in behavioral abnormalities in HE. Using bile duct ligation (BDL) rats as a HE model, we verified the elevated 5-HT levels in the cortex but not in the hippocampus and striatum, and found that this cortical 5-HT overload may be caused by BDL-mediated inhibition of UDP-glucuronosyltransferase 1A6 (UGT1A6) expression and activity in the cortex. The intraventricular injection of the UGT1A6 inhibitor diclofenac into rats demonstrated that the inhibition of brain UGT1A6 activity significantly increased cerebral 5-HT levels and induced HE-like behaviors. Co-immunofluorescence experiments demonstrated that UGT1A6 is primarily expressed in astrocytes. In vitro studies confirmed that NH₄Cl activates the ROS-ERK pathway to downregulate UGT1A6 activity and expression in U251 cells, which can be reversed by the oxidative stress antagonist N-acetyl-l-cysteine and the ERK inhibitor U0126. Silencing Hepatocyte Nuclear Factor 4α (HNF4α) suppressed UGT1A6 expression whilst overexpressing HNF4α increased Ugt1a6 promotor activity. Meanwhile, both NH₄Cl and the ERK activator TBHQ downregulated HNF4α and UGT1A6 expression. In the cortex of hyperammonemic rats, we also found activation of the ROS-ERK pathway, decreases in HNF4α and UGT1A6 expression, and increases in brain 5-HT content. These results prove that the ammonia-mediated ROS-ERK pathway activation inhibits HNF4α expression to downregulate UGT1A6 expression and activity, thereby increasing cerebral 5-HT content and inducing manic-like HE symptoms. This is the first study to reveal the mechanism of elevated cortical 5-HT concentration in a state of liver failure and elucidate its association with manic-like behaviors in HE.
Drug-drug interactions that alter the exposure of glucuronidated drugs: Scope, UDP-glucuronosyltransferase (UGT) enzyme selectivity, mechanisms (inhibition and induction), and clinical significance
2023, Pharmacology and Therapeutics
Drug-drug interactions (DDIs) arising from the perturbation of drug metabolising enzyme activities represent both a clinical problem and a potential economic loss for the pharmaceutical industry. DDIs involving glucuronidated drugs have historically attracted little attention and there is a perception that interactions are of minor clinical relevance. This review critically examines the scope and aetiology of DDIs that result in altered exposure of glucuronidated drugs. Interaction mechanisms, namely inhibition and induction of UDP-glucuronosyltransferase (UGT) enzymes and the potential interplay with drug transporters, are reviewed in detail, as is the clinical significance of known DDIs. Altered victim drug exposure arising from modulation of UGT enzyme activities is relatively common and, notably, the incidence and importance of UGT induction as a DDI mechanism is greater than generally believed. Numerous DDIs are clinically relevant, resulting in either loss of efficacy or an increased risk of adverse effects, necessitating dose individualisation. Several generalisations relating to the likelihood of DDIs can be drawn from the known substrate and inhibitor selectivities of UGT enzymes, highlighting the importance of comprehensive reaction phenotyping studies at an early stage of drug development. Further, rigorous assessment of the DDI liability of new chemical entities that undergo glucuronidation to a significant extent has been recommended recently by regulatory guidance. Although evidence-based approaches exist for the in vitro characterisation of UGT enzyme inhibition and induction, the availability of drugs considered appropriate for use as ‘probe’ substrates in clinical DDI studies is limited and this should be a research priority.
Aryl hydrocarbon receptor (AHR) functions: Balancing opposing processes including inflammatory reactions
2020, Biochemical Pharmacology
Aryl hydrocarbon receptor (AHR) research has shifted from exploring dioxin toxicity to elucidation of physiologic AHR functions. Control of AHR functions is challenged by the fact that AHR is often involved in balancing opposing processes. Two AHR functions are discussed. (i) Microbial defense: intestinal microbiota commensals secrete AHR ligands that are important for maintaining epithelial integrity and generation of anti-inflammatory IL-22 by multiple immune cells. On the other hand, in case of microbial defense, AHR-regulated neutrophils and Th17 cells are involved in generation of bactericidal reactive oxygen species and pro-inflammatory stimuli. However, during the process of infection resolution, 'disease tolerance' is achieved. (ii) Energy, NAD⁺ and lipid metabolism: In obese individuals AHR is involved in either generation or inhibition of fatty liver and associated hepatitis. Inhibition of hepatitis is mainly achieved by regulating NAD⁺-controlled SIRT1, 3 and 6 activity. Interestingly, these enzymes are synergistically modulated by CD38, an NAD-consuming NAD-glycohydrolase. It is proposed that inflammatory responses may be beneficially modulated by AHR agonistic and CD38 inhibiting phytochemicals. Caveats in presence of carcinogenicity have to be taken into account. AHR research is an exciting field but therapeutic options remain challenging.
Aryl hydrocarbon receptor (AHR): From selected human target genes and crosstalk with transcription factors to multiple AHR functions
2019, Biochemical Pharmacology
Citation Excerpt :
This reaction (together with findings in PAH responsive or non-responsive mouse mutants and dioxin toxicity) led to the discovery of AHR [21,22]. AHR is known to induce gene batteries of the three phases of drug metabolism, also termed chemical defensome, including the prototypical phase I enzymes (ubiquitously-expressed CYP1A1, mainly hepatic CYP1A2, responsible for systemic detoxification [23–25] and extrahepatic CYP1B1 [26]), phase II enzymes including UGT1 family members [27,28] and GSTA1/2 [29] as well as human conjugate transporter ABCG2 [30] (Table 1). This system is involved in detoxification and homeostasis of lipid-soluble endo- and xenobiotics.
Accumulating evidence including studies of AHR-deficient mice and TCDD toxicity suggests multiple physiologic AHR functions. Challenges to identify responsible mechanisms are due to marked species differences and dependence upon cell type and cellular context. Transient AHR modulation is often necessary for physiologic functions whereas TCDD-mediated sustained receptor activation has been demonstrated to be responsible for toxic outcomes. To stimulate studies on responsible action mechanisms the commentary is focused on human AHR target genes and crosstalk with transcription factors. Discussed AHR functions include chemical and microbial defense, organ development, modulation of immunity and inflammation, reproduction, and NAD⁺-dependent energy metabolism. Obviously, much more work is needed to elucidate action mechanisms. In particular, studies of pathways leading to NAD⁺-dependent energy metabolism may shed light on the puzzling species differences of TCDD-mediated lethality and provide options for treatment of obesity and age-related degenerative diseases.
UDP-Glycosyltransferases
2018, Comprehensive Toxicology: Third Edition
The UDP-glycosyltransferase enzyme superfamily (UGT) consists of UDP-glucuronosyltransferases (the UGT1 and 2 families), UDP N-acetylglucosaminyltransferase (UGT3A1), UDP-glucosyl/xylosyltransferase (UGT3A2), and UDP-galactosyltransferase (UGT8) that conjugate glucuronic acid, N-acetylglucosamine, glucose, xylose, and galactose sugars to nucleophilic O, N, S, and C atoms on lipophilic compounds via a nucleophilic bimolecular substitution (S_N2) mechanism. The aglycone substrates glycosidated include drugs, environmental chemicals, and endogenous compounds such as bilirubin and steroid hormones. The glycoside products are water-soluble and readily excreted. There are 22 functional human UGTs belonging to four UGT families: UGT1 containing nine members, UGT2 containing seven UGT2B and three UGT2A members, UGT3 containing two members, and UGT8 containing one member. These UGTs reside on the luminal side of the endoplasmic reticulum and consist of two domains, an N-terminal domain involved in substrate selection and catalysis and a C-terminal domain responsible for binding the sugar donor, UDP glucuronic acid, UDP-glucose, UDP-xylose, UDP-N-acetylglucosamine, or UDP-galactose depending on UGT isoform. The UGT protein is integrated into the endoplasmic reticulum membrane by a hydrophobic series of amino acids near the C-terminus and appears to form homo- or heterodimers with other UGT proteins and complexes with other drug-metabolizing enzymes. UGT genes are regulated by several liver-enriched transcription factors (LETFs) such as HNF1 and HNF4α and by ligand-activated transcription factors (TFs) such as AhR, Nrf2, PXR, CAR, PPAR, FXR, LXR, ER, and AR. Regulation by these TFs ensures that each tissue or organ has a distinct complement of UGTs. Several polymorphisms in UGT genes and their regulatory regions have been shown to alter the glycosidation capacity of tissues and organs and to be risk factors for disease and adverse drug events.
A comprehensive review of UDP-glucuronosyltransferase and esterases for drug development
2015, Drug Metabolism and Pharmacokinetics
Citation Excerpt :
Overall, bile acids facilitate their own inactivation by the direct activation of FXR and PXR or FXR-mediated indirect activation of PXR and PPARα. AhR induces UGT1A1 [53], UGT1A3 [54], UGT1A4 [55], UGT1A6 [56], UGT1A8, UGT1A9, and UGT1A10 [57]. The AhR binding site is conserved in all UGT1A genes [58].
UDP-glucuronosyltransferase (UGT) and esterases are recognized as the most important non-P450 enzymes because of their high contribution to drug metabolism. UGTs catalyze the transfer of glucuronic acid to hydroxyl, carboxyl, or amine groups of compounds, whereas esterases hydrolyze compounds that contain ester, amide, and thioester bonds. These enzymes, in most cases, convert hydrophobic compounds to water-soluble metabolites to facilitate the elimination of compounds from the body. Information about these enzymes is steadily increasing, although our knowledge is still behind our understanding of P450. This review gives an overview of recent findings in UGT and esterases studies focusing on tissue distribution, gene regulation, substrate and inhibitor specificity, and species differences. In particular, the absolute protein content of UGT isoforms and esterases in human tissues could be available. In the field of esterases, it is becoming clear that enzymes other than carboxylesterase are involved in drug hydrolysis. In addition, there is an interesting interplay between UGTs and esterases in the formation and hydrolytic deglucuronidation of acyl-glucuronide, which is considered to be a reactive metabolite. With the growing awareness of the importance of non-P450 enzymes in drug development, issues that should be resolved are discussed.

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Regular ArticleAryl Hydrocarbon Receptor-Inducible or Constitutive Expression of Human UDP Glucuronosyltransferase UGT1A6

Abstract

J. Biol. Chem.

J. Biol. Chem.

Biochem. Pharmacol.

Arch. Biochem. Biophys.

Biochem. Pharmacol.

Biochem. Pharmacol.

J. Biol. Chem.

Int. J. Pharm.

Anal. Biochem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

Anal. Biochem.

J. Biol. Chem.

Mol. Cell. Endocrinol.

Cell

J. Biol. Chem.

Pharmacogenetics

Regular Article
Aryl Hydrocarbon Receptor-Inducible or Constitutive Expression of Human UDP Glucuronosyltransferase UGT1A6