UDP-glucuronosyltransferase 1A6: Structural, functional, and regulatory aspects

Institut of Pharmacology and Toxicology, Department of Toxicology, University of Tübingen, Germany.
Methods in Enzymology (Impact Factor: 2.19). 02/2005; 400:57-75. DOI: 10.1016/S0076-6879(05)00004-2
Source: PubMed

ABSTRACT Glucuronidation, catalyzed by two families of UDP-glucuronosyltransferases (UGTs), represents a major phase II reaction of endo- and xenobiotic biotransformation. UGT1A6 is the founding member of the rat and human UGT1 family. It is expressed in liver and extrahepatic tissues, such as intestine, kidney, testis, and brain, and conjugates planar phenols and arylamines. Serotonin has been identified as a selective endogenous substrate of the human enzyme. UGT1A6 is also involved in conjugation of the drug paracetamol (acetaminophen) and of phenolic metabolites of benzo[a]pyrene (together with rat UGT1A7 and human UGT1A9). High interindividual variability of human liver protein levels is due to a number of influences, including genetic, tissue-specific, and environmental factors. Evidence shows that homo- and heterozygotic expression of UGT1A6 alleles markedly affects enzyme activity. HNF1 may be responsible for tissue-specific UGT1A6 expression. Multiple environmental factors controlling UGT1A6 expression have been identified, including the pregnane X receptor, the constitutive androstane receptor, the aryl hydrocarbon receptor, and Nrf2, a bZIP transcription factor mediating stress responses. However, marked differences have been noted in the expression of rat and human UGT1A6. Regulatory factors have been studied in detail in the human Caco-2 colon adenocarcinoma cell model.

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    • "The UGT1A6 isoform is also involved in drug metabolism and in the clearance of several steroid and thyroid hormones and environmental chemicals (Radominska-Pandya et al., 1999; Court, 2005; Zhang et al., 2007). Similar to 1A1, the 1A6 isoform is also polymorphic with expression and activity affected by genetics as well as environmental and tissuespecific factors (Bock and Köhle, 2005). We hypothesized that UGT1A1 and 1A6 would show independent developmental profiles and would not reach full enzyme activity for several years. "
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    ABSTRACT: UDP-glucuronosyltransferases (UGTs) are critical for the metabolism and clearance of drugs, chemicals, and hormones. The development of UGT1A1 and 1A6 was studied in 50 pediatric liver samples using bilirubin, serotonin activity assays, and Western blot as well as pharmacokinetic scaling. UGT activity developed age dependently in pediatric liver. Maximal activity of 0.7690 nmol · min · (-1) mg protein(-1) was observed for UGT1A1 at 3.8 months. For UGT1A6, activity matured at 14 months (4.737 nmol · min · (-1)mg protein(-1)). Protein expression was not age-dependent, and activities did not correlate to protein levels for either enzyme. The in vitro activities were used to calculate normalized hepatic clearances using both allometric scaling and a physiologically based pharmacokinetic model. For UGT1A1, allometry predicted normalized adult clearances of 0.0070 l · h(-1) · kg(-1) at 3.0 (well stirred) and 2.8 years (parallel tube), whereas the Simcyp model showed normalized clearances of 0.0079 l · h(-1) · kg(-1) at 2.6 (well stirred) and 2.5 years (parallel tube). For UGT1A6, only the Simcyp well stirred model converged at 0.3524 l · h(-1) · kg(-1) at 12.6 months. These data imply independent regulation of UGT1A1 and 1A6 where activity has matured after 6 months to 1 year. Total hepatic clearance of substances mediated by these enzymes may mature concurrently or take longer because of other physiological factors. Late development of UGT enzymes may contribute to chemical, drug, and environmental toxicity.
    Drug metabolism and disposition: the biological fate of chemicals 04/2011; 39(5):912-9. DOI:10.1124/dmd.110.037192 · 3.33 Impact Factor
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    • "/electrophile stress [14] [15]. "
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    ABSTRACT: Methotrexate is a chemotherapeutic agent used in breast cancer treatment, but the occurrence of resistance limits its therapeutic use. A microarrays analysis between sensitive and methotrexate resistant MCF7 and MDA-MB-468 breast cancer cells pointed out the UDP-glucuronosyltransferase 1A (UGT1A) family as a common deregulated node in both cell lines. This family of genes is involved in Phase II metabolism. UGT1A6 was the main isoform responsible for UGT1A family overexpression in these cells. Its overexpression was not due to gene amplification. Transfection of a vector encoding for UGT1A6 in sensitive cells counteracted the cytotoxicity caused by methotrexate. Methotrexate increased the transcriptional activity from a luciferase reporter driven by the UGT1A6 promoter and induced UGT1A6 mRNA and enzymatic activity. Promoter analysis suggested that UGT1A6 induction by methotrexate could be driven by the transcription factors ARNT (HIF-1) and AhR/ARNT. Cells incubated with anticancer drugs susceptible to glucuronidation, such as tamoxifen or irinotecan, together with methotrexate, showed a lesser degree of cytotoxicity, due to UGT1A6 induction. The pharmacological effect of this induction should be taken into account when combining methotrexate with other drugs that are glucuronidated.
    Biochemical pharmacology 01/2011; 81(1):60-70. DOI:10.1016/j.bcp.2010.09.008 · 4.65 Impact Factor
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    • "As was seen with Clontech arrays, the OCI cDNA arrays revealed that several genes that are well known to be upregulated by AHR agonists were induced independently of the dioxin-sensitivity phenotype. These include (Table 3) CYP1B1 (Zhang et al. 2003), Nfe2l2 (Miao et al. 2005; Tijet et al. 2006) and UGT1A6 (Bock and Kohle 2005b). Clearly the AHR transactivationdomain deletion in H/W and LnA rats does not prevent induction by TCDD of these standard AHR-regulated genes. "
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    ABSTRACT: Dioxins exert their major toxicologic effects by binding to the aryl hydrocarbon receptor (AHR) and altering gene transcription. Numerous dioxin-responsive genes previously were identified both by conventional biochemical and molecular techniques and by recent mRNA expression microarray studies. However, of the large set of dioxin-responsive genes the specific genes whose dysregulation leads to death remain unknown. To identify specific genes that may be involved in dioxin lethality we compared changes in liver mRNA levels following exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in three strains/lines of dioxin-sensitive rats with changes in three dioxin-resistant rat strains/lines. The three dioxin-resistant strains/lines all harbor a large deletion in the transactivation domain of the aryl hydrocarbon receptor (AHR). Despite this deletion, many genes exhibited a “Type-I” response—that is, their responses were similar in dioxin-sensitive and dioxin-resistant rats. Several genes that previously were well established as being dioxin-responsive or under AHR regulation emerged as Type-I responses (e.g. CYP1A1, CYP1A2, CYP1B1 and Gsta3). In contrast, a relatively small number of genes exhibited a Type-II response—defined as a difference in responsiveness between dioxin-sensitive and dioxin-resistant rat strains. Type-II genes include: malic enzyme 1, ubiquitin C, cathepsin L, S-adenosylhomocysteine hydrolase and ferritin light chain 1. In silico searches revealed that AH response elements are conserved in the 5′-flanking regions of several genes that respond to TCDD in both the Type-I and Type-II categories. The vast majority of changes in mRNA levels in response to 100 μg/kg TCDD were strain-specific; over 75% of the dioxin-responsive clones were affected in only one of the six strains/lines. Selected genes were assessed by quantitative RT-PCR in dose-response and time-course experiments and responses of some genes were assessed in Ahr-null mice to determine if their response was AHR-dependent. Type-II genes may lie in pathways that are central to the difference in susceptibility to TCDD lethality in this animal model.
    Archives of Toxicology 12/2008; 82(11):831. DOI:10.1007/s00204-008-0320-z · 5.08 Impact Factor
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