Effects of common genetic variants of human uridine diphosphate glucuronosyltransferase subfamilies on irinotecan glucuronidation
0
Citation
0
Reference
10
Related Paper
Abstract:
The adverse effects (diarrhea and neutropenia) of irinotecan (7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin) are associated with genetic variants of uridine diphosphate glucuronosyltransferase 1A subfamilies (UGT1As). UGT1As are enzymes that metabolize the active form of irinotecan, 7-ethyl-10 hydroxycamptothecin (SN-38), by glucuronidation in the liver. They are widely known as predictive factors of severe adverse effects, such as neutropenia and diarrhea. Some studies have suggested that variants of UGT1As affect SN-38 glucuronidation activities, thus exerting severe adverse effects. We aimed to identify UGT1A isoforms that show SN-38 glucuronidation activity and determine the relationship between UGT1A variants and SN-38 glucuronidation in vitro. We found that UGT1A1 and UGT1A6–UGT1A10 displayed SN-38 glucuronidation activity. Among these, UGT1A1 was the most active. Furthermore, the variants of these isoforms showed decreased SN-38 glucuronidation activity. In our study, we compared the different variants of UGT1As, such as UGT1A1.6, UGT1A1.7, UGT1A1.27, UGT1A1.35, UGT1A7.3, UGT1A8.4, UGT1A10M59I, and UGT1A10T202I, to determine the differences in the reduction of glucuronidation. Our study elucidates the relationship between UGT1A variants and the level of glucuronidation associated with each variant. Therefore, testing can be done before the initiation of irinotecan treatment to predict potential toxicities and adverse effects.Keywords:
Glucuronosyltransferase
Uridine diphosphate
Exposure of rats to microsomal enzyme inducers perturbs thyroid hormone (TH) homeostasis through a variety of mechanisms. Glucuronidation is an important metabolic pathway for TH and is catalyzed by uridine diphosphate-dibenzo-glucuronosyltransferase (UGT) family proteins. Administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to rats markedly increases the biliary clearance of glucuronidated T(4) and results in reduced plasma T(4) levels. Determination of the UGT1 isoforms responsible for glucuronidation of T(4) has yet to be conclusively established. We here provide evidence for the involvement of TCDD-inducible UGT1A7 in the glucuronidation of T(4) and TH-controlled UGT1A7 expression. Among a number of rat UGT1 isoenzymes examined in this study, UGT1A7 was the most active in catalyzing glucuronidation of T(4). Expression of UGT1A7 was positively regulated by T(4) through specific binding of TH receptor-retinoid X receptor heterodimers to a DR-5 sequence located between -109 and -93 in the UGT1A7 promoter. Overproduction of UGT1A7 protein decreased T(4) responsiveness of a reporter gene containing the T(4)-responsive UGT1A7 promoter sequence. These results raise the possibility that UGT1A7 plays a key role in the glucuronidation of T(4) leading to inactivation of T(4), functioning via feedback regulation to control T(4) levels in an autoregulatory manner, and that T(4) regulates its own metabolism and subsequent clearance from cells. Our findings also predict that accumulation of TCDD-inducible UGT1A7 proteins in TH-target cells might disrupt the TH signaling by lowering the intracellular pool of T(4).
Glucuronosyltransferase
Uridine diphosphate
Cite
Citations (23)
The glucuronic acid adducts of 1-naphthol, 2-naphthol and 4-methylumbelliferone activate microsomal UDP-glucuronyltransferase (EC 2.4.1.17) when the enzyme is assayed with p-nitrophenol as aglycone. Phenyl glucuronide and oestriol 3beta-glucuronide also activate UDP-glucuronyltransferase. but to a lesser extent. Activation by glucuronides is not dependent on metal ions, but is blocked by prior treatment of microsomal fractions with p-chloromercuribenzoate. The kinetic mechanism of activation is concluded to be an increase in the affinity of the enzyme for UDP-glucuronic acid. Activation by 1-naphthyl glucuronide, at high concentrations of p-nitrophenol, is not affected by 1-naphthol. Apparently 1-naphthyl glucuronide activates the preparation by binding at a site that is separate from the site of glucuronidation of 1-naphthol. Further evidence for the existence of distinct effector sites for the glucuronides was provided by the finding that activation by glucuronides is inhibited competitively by aglycone glucosides. These glucosides do not inhibit the rate of glucuronidation of p-nitrophenol in the absence of glucuronide adducts, nor do they alter the rate of glucuronidation of 1-naphthol. When UDP-glucuronyltransferase is assayed with 1-naphthol as aglycone it is activated by p-nitrophenyl glucuronide, 4-methyl-umbelliferyl glucuronide and under appropriate conditions by its own glucuronide. These activations are similarly inhibited by aglycone glucosides. p-Nitrophenyl glucuronide also stimulates the rate of glucuronidation of o-aminophenol, o-aminobenzoate and bilirubin.
Glucuronosyltransferase
Glucuronide
Aglycone
Uridine diphosphate
Cite
Citations (19)
Glucuronosyltransferase
Uridine diphosphate
Phenobarbital
Glucuronide
Digitonin
Microsoma
Cite
Citations (32)
Uridine diphosphate
Glucuronide
Glucuronosyltransferase
Microsoma
UGT2B7
Cite
Citations (16)
1. The UDP-glucuronosyltransferase (UGT) enzymes are important in the metabolism, elimination and detoxification of many xenobiotics and endogenous compounds. As extrapolation of in vitro kinetics of drug metabolizing enzymes to predict in vivo clearance rates becomes more sophisticated, it is important to ensure proper optimization of enzyme assays. The luminal location of the enzyme active site (i.e. latency), and the complexity of UGT kinetics, results in consistent under-prediction of clearance of drugs metabolized by glucuronidation.2. We examined inhibition of UGT activity in alamethicin-disrupted human liver microsomes (HLM) by uridine diphosphate (UDP), a UGT reaction product, and its reversal by Mg2+ ions. We also determined whether UDP-sugars other than the co-substrate UDP-glucuronic acid (UDP-GlcA) affected glucuronidation.3. We show that other UDP-sugars do not significantly influence glucuronidation. We also demonstrate that UDP inhibits HLM UGT activity and that this is reversed by including Mg2+ in the assay. The Mg2+ effect can be offset using EDTA, and is dependent on the concentration of UDP-GlcA in the assay.4. We propose that formation of a Mg2+–UDP complex prevents UDP from affecting the enzyme. Our results suggest that 5 mM UDP-GlcA and 10 mM Mg2+ be used for UGT assays in fully disrupted HLM.
Uridine diphosphate
Glucuronosyltransferase
Enzyme Kinetics
Cite
Citations (12)
Irinotecan (CPT-11) is a promising antitumor agent, recently approved for use in patients with metastatic colorectal cancer. Its active metabolite, SN-38, is glucuronidated by hepatic uridine diphosphate glucuronosyltransferases (UGTs). The major dose-limiting toxicity of irinotecan therapy is diarrhea, which is believed to be secondary to the biliary excretion of SN-38, the extent of which is determined by SN-38 glucuronidation. The purpose of this study was to identify the specific isoform of UGT involved in SN-38 glucuronidation. In vitro glucuronidation of SN-38 was screened in hepatic microsomes from normal rats (n = 4), normal humans (n = 25), Gunn rats (n = 3), and patients (n = 4) with Crigler-Najjar type I (CN-I) syndrome. A wide intersubject variability in in vitro SN-38 glucuronide formation rates was found in humans. Gunn rats and CN-I patients lacked SN-38 glucuronidating activity, indicating the role of UGT1 isoform in SN-38 glucuronidation. A significant correlation was observed between SN-38 and bilirubin glucuronidation (r = 0.89; P = 0.001), whereas there was a poor relationship between para-nitrophenol and SN-38 glucuronidation (r = 0.08; P = 0.703). Intact SN-38 glucuronidation was observed only in HK293 cells transfected with the UGT1A1 isozyme. These results demonstrate that UGT1A1 is the isoform responsible for SN-38 glucuronidation. These findings indicate a genetic predisposition to the metabolism of irinotecan, suggesting that patients with low UGT1A1 activity, such as those with Gilbert's syndrome, may be at an increased risk for irinotecan toxicity.
Glucuronosyltransferase
SN-38
Uridine diphosphate
UGT2B7
Active metabolite
Glucuronide
Gilbert's syndrome
Cite
Citations (672)
The objective of this study was to determine the regiospecificity of the important uridine diphosphate glucuronosyltransferase (UGT) isoforms responsible for the glucuronidation of flavones and flavonols. We systematically studied the glucuronidation of 13 flavonoids (7 flavones and 6 flavonols, with hydroxyl groups at C-3, C-4', C-5, and/or C-7 positions in flavonoid structure) at a substrate concentration of 10 μM by 8 recombinant human UGT isoforms mainly responsible for the metabolism of flavonoids, UGTs 1A1, 1A3, 1A6, 1A7, 1A8, 1A9, 1A10, and 2B7. At 10 μM substrate concentration, different UGT isoforms gave different regiospecific glucuronidation patterns. UGT 1A1 equally glucuronidated 3-O (glucuronic acid substituted at C-3 hydroxyl group), 7-O, and 4'-O, whereas UGTs 1A8 and 1A9 preferably glucuronidated only 3-O and 7-O positions. UGT 1A1 usually showed no regiospecificity for glucuronidating any position, whereas UGT 1A8 and UGT 1A9 showed dominant, moderate, or weak regiospecificity for 3-O or 7-O position, depending on the structure of the compound. UGT 1A3 showed dominant regiospecificity for the 7-O position, whereas UGT 1A7 showed dominant regiospecificity for the 3-O position. We also showed that the glucuronidation rates of 3-O and 7-O positions in flavones and flavonols were affected by the addition of multiple hydroxyl groups at different positions as well as by the substrate concentrations (2.5, 10, and 35 μM). In conclusion, regiospecific glucuronidation of flavonols was isoform- and concentration-dependent, whereas flavones were dominantly glucuronidated at the 7-O position by most UGT isoforms. We also concluded that UGTs 1A3 and 1A7 showed dominant regiospecificity for only the 7-O and 3-O positions, respectively. UGTs 1A8 and 1A9 showed moderate or weak preference on glucuronidating position 3-O over the 7-O position, whereas other UGT isoforms did not prefer glucuronidating any particular positions.
Flavones
Flavonols
Uridine diphosphate
Glucuronosyltransferase
Cite
Citations (29)
Abstract: Uridine 5′‐diphosphate glucuronosyltransferases (UGTs) are part of a major elimination pathway for endobiotics and xenobiotics. UGT1A9 is a UGT that catalyses the conjugation of endogenous oestrogenic and thyroid hormones, acetaminophen, SN‐38 (an active metabolite of irinotecan) and phenols. UGT1A9 is the only isoform that catalyses the glucuronidation of propofol (2,6‐diisopropylphenol) in the liver. In the present study, we analysed polymorphisms of UGT1A9 in 100 healthy adult Japanese volunteers. A transversion of 766G > A resulting in the amino acid substitution of D256N was detected in exon 1. The allele frequency of D256N is 0.005. We investigated the effects of D256N and Y483D, which is located on the common exon of UGT1 , on propofol glucuronidation by an in vitro expression study. The K m of wild‐type, D256N and Y483D for propofol glucuronidation were 111.2, 43.6 and 64.5 µM, respectively. The V max of D256N and Y483D were 8.1% and 28.8%, and the efficiencies ( V max / K m ) were 19.1% and 57.1% of the wild‐type, respectively. For mycophenolic acid, 1‐naphthol and naringenin, the D256N variant lowered glucuronidation activity considerably, compared to Y483D. The V max value of D256N variant for mycophenolic acid was only 9.5% of the wild‐type. This study shows the importance of D256N in differences between individuals concerning adverse effects of drugs that are catalysed primarily by UGT1A9. Carriers of D256N may be at risk of suffering adverse effects of propofol and other substrates that are primarily metabolized by UGT1A9.
UGT2B7
Glucuronosyltransferase
Uridine diphosphate
Cite
Citations (51)
Uridine 5-diphosphate glucuronosyltransfer as es (UGT) are a superfamily of phase Ⅱ-enzymes which can catalyze the conjugat ion of glucuronic acid to substrates. This is one of the important mechanisms of drug elimination. At present, about 15 human UGT have been identified, and muc h more knowledge about substrate selectivity of UGT has been reported. However, the study on enzymology of UGT lags behind that of cytochrome P450.
Uridine diphosphate
Glucuronosyltransferase
Cite
Citations (0)
Galactosyltransferase
Uridine diphosphate
Uridine diphosphate glucose
Nucleotide salvage
Cite
Citations (13)