Gastrointestinally Distributed UDP-glucuronosyltransferase 1A10, Which Metabolizes Estrogens and Nonsteroidal Anti-inflammatory Drugs, Depends upon Phosphorylation

UGT1A1 and UGT1A3 activity and inhibition in human liver and intestinal microsomes and a recombinant UGT system under similar assay conditions using selective substrates and inhibitors

In vitro enzyme kinetics and inhibition data was compared for UGT1A1 and UGT1A3 isoforms under similar assay conditions using human liver microsomes (HLM), human intestinal microsomes (HIM) and recombinant UGT (rUGT) enzyme systems.UGT1A1 catalysed β-estradiol 3-β-D-glucuronide formation showed allosteric sigmoidal kinetics in all enzyme systems; while UGT1A3 catalysed CDCA 24-acyl-β-D-glucuronide formation exhibited Michaelis-Menten kinetics in HLM, substrate inhibition kinetics in HIM and rUGT systems. Corresponding K_m or S₅₀ concentrations of β-estradiol and CDCA were employed in the respective UGT inhibition studies.Atazanavir inhibited the production of β-estradiol 3-β-D-glucuronide with IC50 values of 0.54 µM and 0.16 µM in HLM and rUGT1A1, respectively. But its inhibition potential was not observed in HIM, indicating potential cross-talk with other high-affinity intestinal UGT isozymes. On the other hand, zafirlukast, a pan UGT inhibitor, exhibited moderate inhibition in HIM with an IC50 value of 16.70 µM. Lithocholic acid, inhibited the production of CDCA 24-acyl-β-D-glucuronide with IC50 values of 1.68, 1.84, and 12.42 µM in HLM, rUGT1A3, and HIM, respectively.These results indicated that HLM, HIM, and rUGTs may be used as complementary in vitro systems to evaluate hepatic and intestinal UGT mediated DDIs at the screening stage.

Genetic variants of UDP-glucuronosyltransferase 1A genes are associated with disease presentation and outcome in primary sclerosing cholangitis

BACKGROUND AND AIMS

Primary sclerosing cholangitis (PSC) is a progressive cholestatic liver disease without a curative medical therapy. The human UDP-glucuronosyltransferases 1A play a major role in the detoxification and elimination of bilirubin, bile acids and xenobiotics. Whether genetic UGT1A variants determine course and outcome of PSC has not yet been described.

METHODS

A large cohort of German PSC patients with a long-term-follow-up was genotyped for UGT1A variants including UGT1A1*28, UGT1A3-66 T>C and UGT1A7 p.N129K/p.R131K using TaqMan 5'-nuclease assays. Results were correlated with clinical characteristics and transplant-free survival.

RESULTS

About 331 patients with PSC were included in the study (69.9% male, mean age at diagnosis 32.6 years). Median transplant-free survival was 14.9 years. Patients with wild-type alleles of all three UGT1A genes had a longer transplant-free survival (17.2 vs. 14.4 years, P = .048) than patients carrying a homozygous or heterozygous SNP variant in at least one of the UGT1A1, UGT1A3 or UGT1A7 genes. Additionally, we found that patients carrying wild-type alleles of all three UGT1A genes had lower serum bilirubin (25 vs. 38 µmol/L, P = .02) and serum cholesterol (195 vs. 223 mg/dL), P = .035) at first presentation. Furthermore, inflammatory bowel disease was found to be associated with wild-type UGT1A alleles (82.2% vs. 68.4%, P = .046).

CONCLUSIONS

This large cohort shows an association with single nucleotide polymorphisms of the UGT1A1, UGT1A3 and UGT1A7 genes and outcome in PSC. Thus, UGT1A variants may represent a tool for the prognostic stratification of PSC patients and establish a link between disease progression and the regulation of detoxification by glucuronidation in PSC.

Regional distributions of UDP-glucuronosyltransferase activities toward estradiol and serotonin in the liver and small intestine of cynomolgus macaques

The cynomolgus macaque is a nonhuman primate species that is often used in drug metabolism studies during drug development. However, the localization of UDP-glucuronosyltransferases (UGTs), essential drug-metabolizing enzymes, has not been fully investigated in the liver and small intestine of cynomolgus macaques. In this study, UGT activities were analyzed in liver (five lobes) and small intestine (the duodenum and six sections from the proximal jejunum to the distal ileum) using typical probe substrates of human UGTs: 7-hydroxycoumarin, estradiol, serotonin, propofol, and zidovudine. In liver, UGT activities with respect to all substrates were detected, and the activity levels were similar in all liver lobes of the cynomolgus macaques tested. In contrast, in the small intestine, UGT activities toward all substrates were detected, but their levels generally decreased from jejunum to ileum in cynomolgus macaques. The localization of estradiol 3-O-glucuronosyltransferases and serotonin O-glucuronosyltransferases (which are mainly UGT1A enzymes) appear to be different in liver and small intestine. These results collectively suggest that, in cynomolgus macaques, UGT1As are differentially localized in the small intestine but are relatively homogeneously distributed in the liver.

The UGTome: The expanding diversity of UDP glycosyltransferases and its impact on small molecule metabolism

The UDP glycosyltransferase (UGT) superfamily of enzymes is responsible for the metabolism and clearance of thousands of lipophilic chemicals including drugs, toxins and endogenous signaling molecules. They provide a protective interface between the organism and its chemical-rich environment, as well as controlling critical signaling pathways to maintain healthy tissue function. UGTs are associated with drug responses and interactions, as well as a wide range of diseases including cancer. The human genome contains 22 UGT genes; however as befitting their exceptionally diverse substrate ranges and biological activities, the output of these UGT genes is functionally diversified by multiple processes including alternative splicing, post-translational modification, homo- and hetero-oligomerization, and interactions with other proteins. All UGT genes are subject to extensive alternative splicing generating variant/truncated UGT proteins with altered functions including the capacity to dominantly modulate/inhibit cognate full-length forms. Heterotypic oligomerization of different UGTs can alter kinetic properties relative to monotypic complexes, and potentially produce novel substrate specificities. Moreover, the recently profiled interactions of UGTs with non-UGT proteins may facilitate coordination between different metabolic processes, as well as providing opportunities for UGTs to engage in novel 'moonlighting' functions. Herein we provide a detailed and comprehensive review of all known modes of UGT functional diversification and propose a UGTome model to describe the resulting expansion of metabolic capacity and its potential to modulate drug/xenobiotic responses and cell behaviours in normal and disease contexts.

Investigation of intestinal elimination and biliary excretion of ibuprofen in hyperglycemic rats

An in vivo intestinal perfusion model was used to investigate how experimental hyperglycemia affects intestinal elimination and biliary excretion in the rat. Experimental diabetes was induced by administration of streptozotocin (65 mg/kg, i.v.). The intestinal perfusion medium contained 250 μM (±)-ibuprofen. An isocratic high-performance liquid chromatography method with UV-visible detection was developed to quantitate ibuprofen in the intestinal perfusate, while a gradient method was applied to quantitate ibuprofen and ibuprofen-β-d-glucuronide in the bile. The limit of quantitation of ibuprofen was found to be 0.51 μM in the perfusate of the small intestine. In the bile, the limit of quantitation of ibuprofen and ibuprofen-β-d-glucuronide was 4.42 and 10.3 μM, respectively. Unconjugated ibuprofen and ibuprofen-β-d-glucuronide were detected in the bile; however, no β-d-glucuronide of ibuprofen could be detected in the intestinal perfusate. The results indicate that experimental diabetes can cause a decrease in the disappearance of ibuprofen from the small intestine. Excretion of both ibuprofen and ibuprofen-β-d-glucuronide decreased to the bile in experimental diabetes. The results can be explained by the results of molecular biological studies indicating streptozotocin-initiated alterations in the intestinal and hepatic transport processes.

The UDP-Glycosyltransferase (UGT) Superfamily: New Members, New Functions, and Novel Paradigms

UDP-glycosyltransferases (UGTs) catalyze the covalent addition of sugars to a broad range of lipophilic molecules. This biotransformation plays a critical role in elimination of a broad range of exogenous chemicals and by-products of endogenous metabolism, and also controls the levels and distribution of many endogenous signaling molecules. In mammals, the superfamily comprises four families: UGT1, UGT2, UGT3, and UGT8. UGT1 and UGT2 enzymes have important roles in pharmacology and toxicology including contributing to interindividual differences in drug disposition as well as to cancer risk. These UGTs are highly expressed in organs of detoxification (e.g., liver, kidney, intestine) and can be induced by pathways that sense demand for detoxification and for modulation of endobiotic signaling molecules. The functions of the UGT3 and UGT8 family enzymes have only been characterized relatively recently; these enzymes show different UDP-sugar preferences to that of UGT1 and UGT2 enzymes, and to date, their contributions to drug metabolism appear to be relatively minor. This review summarizes and provides critical analysis of the current state of research into all four families of UGT enzymes. Key areas discussed include the roles of UGTs in drug metabolism, cancer risk, and regulation of signaling, as well as the transcriptional and posttranscriptional control of UGT expression and function. The latter part of this review provides an in-depth analysis of the known and predicted functions of UGT3 and UGT8 enzymes, focused on their likely roles in modulation of levels of endogenous signaling pathways.

Genome-wide DNA Methylation in Treatment-naïve Ulcerative Colitis

BACKGROUND AND AIMS

The aim of this study was to investigate the genome-wide DNA methylation status in treatment-naïve ulcerative colitis [UC], and to explore the relationship between DNA methylation patterns and gene expression levels in tissue biopsies from a well-stratified treatment-naïve UC patient group.

METHODS

Mucosal biopsies from treatment-naïve patients [n = 10], and a healthy control group [n = 11] underwent genome-wide DNA bisulfite sequencing. Principal component analysis [PCA] and diverse statistical methods were applied to obtain a dataset of differentially methylated genes. DNA methylation annotation was investigated using the UCSC Genome Browser. Gene set enrichments were obtained using the Kyoto Encyclopaedia of Genes and Genomes [KEGG] and PANTHER.

RESULTS

Of all significantly differentially expressed genes [DEGs], 25% correlated with DNA methylation patterns; 30% of these genes were methylated at CpG sites near their transcription start site [TSS]. Hyper-methylation was observed for genes involved in homeostasis and defence, whereas hypo-methylation was observed for genes playing a role in immune response [i.e. chemokines and interleukins]. Of the differentially DNA methylated genes, 25 were identified as inflammatory bowel disease [IBD] susceptibility genes. Four genes [DEFFA6, REG1B, BTNL3, OLFM4] showed DNA methylation in the absence of known CpG islands.

CONCLUSIONS

Genome-wide DNA methylation analysis revealed distinctive functional patterns for hyper-and hypo-methylation in treatment-naïve UC. These distinct patterns could be of importance in the development and pathogenesis of UC. Further investigation of DNA methylation patterns may be useful in the development of the targeting of epigenetic processes, and may allow new treatment and target strategies for UC patients.

Pathway-based approaches for assessment of real-time exposure to an estrogenic wastewater treatment plant effluent on fathead minnow reproduction

Wastewater treatment plant (WWTP) effluents are known contributors of chemical mixtures into the environment. Of particular concern are endocrine-disrupting compounds, such as estrogens, which can affect the hypothalamic-pituitary-gonadal axis function in exposed organisms. The present study examined reproductive effects in fathead minnows exposed for 21 d to a historically estrogenic WWTP effluent. Fathead minnow breeding pairs were held in control water or 1 of 3 effluent concentrations (5%, 20%, and 100%) in a novel onsite, flow-through system providing real-time exposure. The authors examined molecular and biochemical endpoints representing key events along adverse outcome pathways linking estrogen receptor activation and other molecular initiating events to reproductive impairment. In addition, the authors used chemical analysis of the effluent to construct a chemical-gene interaction network to aid in targeted gene expression analyses and identifying potentially impacted biological pathways. Cumulative fecundity was significantly reduced in fish exposed to 100% effluent but increased in those exposed to 20% effluent, the approximate dilution factor in the receiving waters. Plasma vitellogenin concentrations in males increased in a dose-dependent manner with effluent concentration; however, male fertility was not impacted. Although in vitro analyses, analytical chemistry, and biomarker responses confirmed the effluent was estrogenic, estrogen receptor agonists were unlikely the primary driver of impaired reproduction. The results provide insights into the significance of pathway-based effects with regard to predicting adverse reproductive outcomes.

Regulation of uridine diphosphate-glucuronosyltransferase 1A3 activity by protein phosphorylation

Protein phosphorylation is a vital post-translational modification. This study investigated the effect of phosphorylation on human uridine diphosphate (UDP)-glucuronosyltransferase 1A3 (UGT1A3) activity. Curcumin and calphostin C suppressed the activity and phosphorylation of recombinant UGT1A3 expressed in Sf9 cells. These results indicate that UGT1A3 undergoes phosphorylation, which is required for its catalytic activity. Calphostin C is a highly specific protein kinase C (PKC) inhibitor, so three predicted PKC phosphorylation sites in UGT1A3 were examined. Site-directed mutation analysis at residues 28, 43 and 436 (from serine to glycine) was conducted. Compared with the wild-type, the S43G-mutant showed significantly decreased UGT1A3 catalytic activity. Furthermore, the UGT1A3 activity of wild-type and S43G-mutant was down-regulated by calphostin C, whereas the calphostin C inhibitory effect was much weaker on the S43G-mutant than the wild-type. In conclusion, phosphorylation plays an important role in UGT1A3 activity, and the serine at site 43 in UGT1A3 is most likely a phosphorylation site.

Biotransformation of xenobiotics in the human colon and rectum and its association with colorectal cancer

In humans, the liver is generally considered to be the major organ contributing to drug metabolism, but studies during the last years have suggested an important role of the extra-hepatic drug metabolism. The gastrointestinal tract (GI-tract) is the major path of entry for a wide variety of compounds including food, and orally administered drugs, but also compounds - with neither nutrient nor other functional value - such as carcinogens. These compounds are metabolized by a large number of enzymes, including the cytochrome P450 (CYP), the glutathione S-transferase (GST) family, the uridine 5'-diphospho- glucuronosyltransferase (UDP-glucuronosyltransferase - UGT) superfamily, alcohol-metabolizing enzymes, sulfotransferases, etc. These enzymes can either inactivate carcinogens or, in some cases, generate reactive species with higher reactivity compared to the original compound. Most data in this field of research originate from animal or in vitro studies, wherein human studies are limited. Here, we review the human studies, in particular the studies on the phenotypic expression of these enzymes in the colon and rectum to get an impression of the actual enzyme levels in this primary organ of exposure. The aim of this review is to give a summary of currently available data on the relation between the CYP, the GST and the UGT biotransformation system and colorectal cancer obtained from clinical and epidemiological studies in humans.

Transcriptional regulation of human UDP-glucuronosyltransferase genes

Glucuronidation is an important metabolic pathway for many small endogenous and exogenous lipophilic compounds, including bilirubin, steroid hormones, bile acids, carcinogens and therapeutic drugs. Glucuronidation is primarily catalyzed by the UDP-glucuronosyltransferase (UGT) 1A and two subfamilies, including nine functional UGT1A enzymes (1A1, 1A3-1A10) and 10 functional UGT2 enzymes (2A1, 2A2, 2A3, 2B4, 2B7, 2B10, 2B11, 2B15, 2B17 and 2B28). Most UGTs are expressed in the liver and this expression relates to the major role of hepatic glucuronidation in systemic clearance of toxic lipophilic compounds. Hepatic glucuronidation activity protects the body from chemical insults and governs the therapeutic efficacy of drugs that are inactivated by UGTs. UGT mRNAs have also been detected in over 20 extrahepatic tissues with a unique complement of UGT mRNAs seen in almost every tissue. This extrahepatic glucuronidation activity helps to maintain homeostasis and hence regulates biological activity of endogenous molecules that are primarily inactivated by UGTs. Deciphering the molecular mechanisms underlying tissue-specific UGT expression has been the subject of a large number of studies over the last two decades. These studies have shown that the constitutive and inducible expression of UGTs is primarily regulated by tissue-specific and ligand-activated transcription factors (TFs) via their binding to cis-regulatory elements (CREs) in UGT promoters and enhancers. This review first briefly summarizes published UGT gene transcriptional studies and the experimental models and tools utilized in these studies, and then describes in detail the TFs and their respective CREs that have been identified in the promoters and/or enhancers of individual UGT genes.

Assessing the regioselectivity of OleD-catalyzed glycosylation with a diverse set of acceptors

To explore the acceptor regioselectivity of OleD-catalyzed glucosylation, the products of OleD-catalyzed reactions with six structurally diverse acceptors flavones- (daidzein), isoflavones (flavopiridol), stilbenes (resveratrol), indole alkaloids (10-hydroxycamptothecin), and steroids (2-methoxyestradiol)-were determined. This study highlights the first synthesis of flavopiridol and 2-methoxyestradiol glucosides and confirms the ability of OleD to glucosylate both aromatic and aliphatic nucleophiles. In all cases, molecular dynamics simulations were consistent with the determined product distribution and suggest the potential to develop a virtual screening model to identify additional OleD substrates.

Effect of a herbal extract containing curcumin and piperine on midazolam, flurbiprofen and paracetamol (acetaminophen) pharmacokinetics in healthy volunteers

AIMS

Turmeric extract derived curcuminoids (curcumin, demethoxycurcumin and bisdemethoxycurcumin) are currently being evaluated for the treatment of cancer and Alzheimer's dementia. Previous in vitro studies indicate that curcuminoids and piperine (a black pepper derivative that enhances curcuminoid bioavailability) could inhibit human CYP3A, CYP2C9, UGT and SULT dependent drug metabolism. The aim of this study was to determine whether a commercially available curcuminoid/piperine extract alters the pharmacokinetic disposition of probe drugs for these enzymes in human volunteers.

METHODS

A randomized placebo-controlled six way crossover study was conducted in eight healthy volunteers. A standardized curcuminoid/piperine preparation (4 g curcuminoids plus 24 mg piperine) or matched placebo was given orally four times over 2 days before oral administration of midazolam (CYP3A probe), flurbiprofen (CYP2C9 probe) or paracetamol (acetaminophen) (dual UGT and SULT probe). Plasma and urine concentrations of drugs, metabolites and herbals were measured by HPLC. Subject sedation and electroencephalograph effects were also measured following midazolam dosing.

RESULTS

Compared with placebo, the curcuminoid/piperine treatment produced no meaningful changes in plasma C(max), AUC, clearance, elimination half-life or metabolite levels of midazolam, flurbiprofen or paracetamol (α = 0.05, paired t-tests). There was also no effect of curcuminoid/piperine treatment on the pharmacodynamics of midazolam. Although curcuminoid and piperine concentrations were readily measured in plasma following glucuronidase/sulfatase treatment, unconjugated concentrations were consistently below the assay thresholds (0.05-0.08 μM and 0.6 μM, respectively).

CONCLUSION

The results indicate that short term use of this piperine-enhanced curcuminoid preparation is unlikely to result in a clinically significant interaction involving CYP3A, CYP2C9 or the paracetamol conjugation enzymes.

In vitro characterization of glucuronidation of vanillin: identification of human UDP-glucuronosyltransferases and species differences

Vanillin is a food flavoring agent widely utilized in foods, beverages, drugs, and perfumes and has been demonstrated to exhibit multiple pharmacological activities. Given the importance of glucuronidation in the metabolism of vanillin, the UDP-glucuronosyltransferase conjugation pathway of vanillin was investigated in this study. Vanillin glucuronide was identified by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) and a hydrolysis reaction catalyzed by β-glucuronidase. The kinetic study showed that vanillin glucuronidation by HLMs and HIMs followed Michaelis-Menten kinetics and the kinetic parameters were as follows: 134.9 ± 13.5 μM and 81.3 ± 11.3 μM for K(m) of HLMs and HIMs, 63.8 ± 2.0 nmol/min/mg pro and 13.4 ±2.0 nmol/min/mg pro for Vmax of HLMs and HIMs. All UDP-glucuronosyltransferase (UGT) isoforms except UGT1A4, 1A9, and 2B7 showed the capability to glucuronidate vanillin, and UGT1A6 exerted the higher V(max)/K(m) values than other UGT isoforms for the glucuronidation of vanillin when assuming expression of isoforms is similar in recombinant UGTs. Kinetic analysis using liver microsomes from six studied speices indicated that vanillin had highest affinity for the monkey liver microsomes enzyme (K(m)  = 25.6 ± 3.2 μM) and the lowest affinity for the mice liver microsomes enzyme (K(m)  = 149.1 ± 18.4 μM), and intrinsic clearance was in the following order: monkey > dog > minipig > mice > rat ~ human. These data collectively provided important information for understanding glucuronidation of vanillin.

Contributions of human enzymes in carcinogen metabolism

Considerable support exists for the roles of metabolism in modulating the carcinogenic properties of chemicals. In particular, many of these compounds are pro-carcinogens that require activation to electrophilic forms to exert genotoxic effects. We systematically analyzed the existing literature on the metabolism of carcinogens by human enzymes, which has been developed largely in the past 25 years. The metabolism and especially bioactivation of carcinogens are dominated by cytochrome P450 enzymes (66% of bioactivations). Within this group, six P450s--1A1, 1A2, 1B1, 2A6, 2E1, and 3A4--accounted for 77% of the P450 activation reactions. The roles of these P450s can be compared with those estimated for drug metabolism and should be considered in issues involving enzyme induction, chemoprevention, molecular epidemiology, interindividual variations, and risk assessment.

Protein kinase Cα and Src kinase support human prostate-distributed dihydrotestosterone-metabolizing UDP-glucuronosyltransferase 2B15 activity

Because human prostate-distributed UDP-glucuronosyltransferase (UGT) 2B15 metabolizes 5α-dihydrotestosterone (DHT) and 3α-androstane-5α,17β-diol metabolite, we sought to determine whether 2B15 requires regulated phosphorylation similar to UGTs already analyzed. Reversible down-regulation of 2B15-transfected COS-1 cells following curcumin treatment and irreversible inhibition by calphostin C, bisindolylmaleimide, or röttlerin treatment versus activation by phorbol 12-myristate 13-acetate indicated that 2B15 undergoes PKC phosphorylation. Mutation of three predicted PKC and two tyrosine kinase sites in 2B15 caused 70-100 and 80-90% inactivation, respectively. Anti-UGT-1168 antibody trapped 2B15-His-containing co-immunoprecipitates of PKCα in 130-140- and >150-kDa complexes by gradient SDS-PAGE analysis. Complexes bound to WT 2B15-His remained intact during electrophoresis, whereas 2B15-His mutants at phosphorylation sites differentially dissociated. PKCα siRNA treatment inactivated >50% of COS-1 cell-expressed 2B15. In contrast, treatment of 2B15-transfected COS-1 cells with the Src-specific activator 1,25-dihydroxyvitamin D(3) enhanced activity; treatment with the Src-specific PP2 inhibitor or Src siRNA inhibited >50% of the activity. Solubilized 2B15-His-transfected Src-free fibroblasts subjected to in vitro [γ-(33)P]ATP-dependent phosphorylation by PKCα and/or Src, affinity purification, and SDS gel analysis revealed 2-fold more radiolabeling of 55-58-kDa 2B15-His by PKCα than by Src; labeling was additive for combined kinases. Collectively, the evidence indicates that 2B15 requires regulated phosphorylation by both PKCα and Src, which is consistent with the complexity of synthesis and metabolism of its major substrate, DHT. Whether basal cells import or synthesize testosterone for transport to luminal cells for reduction to DHT by 5α-steroid reductase 2, comparatively low-activity luminal cell 2B15 undergoes a complex pattern of regulated phosphorylation necessary to maintain homeostatic DHT levels to support occupation of the androgen receptor for prostate-specific functions.

Assessment of UDP-glucuronosyltransferase catalyzed formation of Picroside II glucuronide in microsomes of different species and recombinant UGTs

This study compared the hepatic glucuronidation of Picroside II in different species and characterized the glucuronidation activities of human intestinal microsomes (HIMs) and recombinant human UDP-glucuronosyltransferases (UGTs) for Picroside II. The rank order of hepatic microsomal glucuronidation activity of Picroside II was rat > mouse > human > dog. The intrinsic clearance of Picroside II hepatic glucuronidation in rat, mouse and dog was about 10.6-, 6.0- and 2.3-fold of that in human, respectively. Among the 12 recombinant human UGTs, UGT1A7, UGT1A8, UGT1A9 and UGT1A10 catalyzed the glucuronidation. UGT1A10, which are expressed in extrahepatic tissues, showed the highest activity of Picroside II glucuronidation (K(m) = 45.1 μM, V(max) = 831.9 pmol/min/mg protein). UGT1A9 played a primary role in glucuronidation in human liver microsomes (HLM; K(m) = 81.3 μM, V(max) = 242.2 pmol/min/mg protein). In addition, both mycophenolic acid (substrate of UGT1A9) and emodin (substrate of UGT1A8 and UGT1A10) could inhibit the glucuronidation of Picroside II with the half maximal inhibitory concentration (IC(50)) values of 173.6 and 76.2 μM, respectively. Enzyme kinetics was also performed in HIMs. The K(m) value of Picroside II glucuronidation was close to that in recombinant human UGT1A10 (K(m) = 58.6 μM, V(max) = 721.4 pmol/min/mg protein). The intrinsic clearance was 5.4-fold of HLMs. Intestinal UGT enzymes play an important role in Picroside II glucuronidation in human.

First-pass metabolism via UDP-glucuronosyltransferase: a barrier to oral bioavailability of phenolics

Glucuronidation mediated by UDP-glucuronosyltransferases (UGTs) is a significant metabolic pathway that facilitates efficient elimination of numerous endobiotics and xenobiotics, including phenolics. UGT genetic deficiency and polymorphisms or inhibition of glucuronidation by concomitant use of drugs are associated with inherited physiological disorders or drug-induced toxicities. Moreover, extensive glucuronidation can be a barrier to oral bioavailability as the first-pass glucuronidation (or premature clearance by UGTs) of orally administered agents usually results in the poor oral bioavailability and lack of efficacies. This review focused on the first-pass glucuronidation of phenolics including natural polyphenols and pharmaceuticals. The complexity of UGT-mediated metabolism of phenolics is highlighted with species-, gender-, organ- and isoform-dependent specificity, as well as functional compensation between UGT1A and 2B subfamily. In addition, recent advances are discussed with respect to the mechanisms of enzymatic actions, including the important properties such as binding pocket size and phosphorylation requirements.

Interpretation of the effects of protein kinase C inhibitors on human UDP-glucuronosyltransferase 1A (UGT1A) proteins in cellulo

UDP-glucuronosyltransferases (UGTs) catalyze the glucuronidation of a wide variety of xeno/endobiotics. Previous studies have reported that human UGT enzymes are phosphorylated and that treatment of cells with protein kinase C (PKC) inhibitors results in decreased UGT activities without affecting the UGT protein levels. In this study, we investigated the effects of PKC inhibitors on human UGT1A protein levels and activities in detail. When UGT1A-expressing HEK293 cells and LS180 cells were treated with curcumin or calphostin C, the exogenous and endogenous UGT1A protein levels in homogenates prepared with Tris-buffered saline were significantly decreased. Enzyme activity levels mirrored the changes in UGT protein levels. When the curcumin- or calphostin C-treated cells were lysed with buffer containing a detergent, the UGT protein levels did not decrease. We found that curcumin or calphostin C treatment facilitated the degradation of UGT protein after the cells were collected in the absence of a detergent. Finally, by in cellulo evaluation, we found that curcumin decreased UGT activity by the direct inhibitory effect, but calphostin C did not affect UGT activity. Thus, this study suggests that we should evaluate the data carefully when interpreting the effects of PKC inhibitors on UGT activity.

Regulated phosphorylation of a major UDP-glucuronosyltransferase isozyme by tyrosine kinases dictates endogenous substrate selection for detoxification

Whereas UDP-glucuronosyltransferase-2B7 is widely distributed in different tissues, it preferentially detoxifies genotoxic 4-OH-estradiol and 4-OH-estrone (4-OHE(1)) with barely detectable 17β-estradiol (E(2)) conversion following expression in COS-1 cells. Consistent with the UDP-glucuronosyltransferase requirement for regulated phosphorylation, we discovered that 2B7 requires Src-dependent tyrosine phosphorylation. Y236F-2B7 and Y438F-2B7 mutants were null and 90% inactive, respectively, when expressed in COS-1. We demonstrated that 2B7 incorporated immunoprecipitable [(33)P]orthophosphate and that 2B7His, previously expressed in SYF-(Src,Yes,Fyn)(-/-) cells, was Src-supported or phosphorylated under in vitro conditions. Unexpectedly, 2B7 expressed in SYF(-/-) and SYF(+/-) cells metabolized 4-OHE(1) at 10- and 3-fold higher rates, respectively, than that expressed in COS-1, and similar analysis showed that E(2) metabolism was 16- and 9-fold higher than in COS-1. Because anti-Tyr(P)-438-2B7 detected Tyr(P)-438-2B7 in each cell line, results indicated that unidentified tyrosine kinase(s) (TKs) phosphorylated 2B7 in SYF(-/-). 2B7-transfected COS-1 treated with increasing concentrations of the Src-specific inhibitor PP2 down-regulated 4-OHE(1) glucuronidation reaching 60% maximum while simultaneously increasing E(2) metabolism linearly. This finding indicated that increasing PP2 inhibition of Src allows increasing E(2) metabolism caused by 2B7 phosphorylation by unidentified TK(s). Importantly, 2B7 expressed in SYF(-/-) is more competent at metabolizing E(2) in cellulo than 2B7 expressed in COS-1. To confirm Src-controlled 2B7 prevents toxicity, we showed that 2B7-transfected COS-1 efficiently protected against 4-OH-E(1)-mediated depurination. Finally, our results indicate that Src-dependent phosphorylation of 2B7 allows metabolism of 4-OHE(1), but not E(2), in COS-1, whereas non-Src-phosphorylated 2B7 metabolizes both chemicals. Importantly, we determined that 2B7 substrate selection is not fixed but varies depending upon the TK(s) that carry out its required phosphorylation.

Therapeutic promises of 2-methoxyestradiol and its drug disposition challenges

2-Methoxyestradiol (2MeO-E2) is an endogenous metabolite of estrogen which was initially considered to be inactive. During the last few decades it has been shown that 2MeO-E2 is a promising anticancer drug. In vitro experiments have demonstrated that it has several anticancer activities, and potential to alleviate hypertension, glomerulosclerosis, hypercholesterolemia, and other disorders. However, due to its low solubility and extensive glucuronidation, to achieve effective concentrations large doses of 2MeO-E2 would be required. Clinical studies reflected very high inter- and intrapatient variability and oral bioavailability of 1 to 2%. Thus, this review paper highlights the origin of this compound, its therapeutic promises, and possible mechanisms of action. It also discusses the pharmacokinetic properties of 2MeO-E2 as well as current developments to overcome low drug solubility and its extensive first pass metabolism.

Role for protein kinase C delta in the functional activity of human UGT1A6: implications for drug-drug interactions between PKC inhibitors and UGT1A6

Many UDP-glucuronosyltransferases (UGTs) require phosphorylation by protein kinase C (PKC) for glucuronidation activity. Inhibition of UGT phosphorylation by PKC inhibitor drugs may represent a novel mechanism for drug-drug interactions. The potential for PKC-mediated inhibition of human UGT1A6, an isoform involved in the glucuronidation of drugs such as acetaminophen (paracetamol) and endogenous substrates including serotonin, was evaluated using various cell model systems. Of ten different PKC inhibitors screened for their effects on acetaminophen glucuronidation by human LS180 colon cells, only rottlerin (PKC delta selective inhibitor; IC(50) = 9.0 +/- 1.2 microM) and the non-selective PKC inhibitors (calphostin-C, curcumin and hypericin) decreased glucuronidation by more than 50%. Using UGT1A6-infected Sf9 insect cells, calphostin-C and hypericin showed three times more potent inhibition of serotonin glucuronidation in treated whole cells versus cell lysates. However, both curcumin and rottlerin showed significant direct inhibition and so (indirect) PKC effects could not be differentiated in this model system. Of nine PKC isoforms co-expressed with UGT1A6 in human embryonic kidney 293T cells only PKC delta increased protein-normalized UGT1A6-mediated serotonin glucuronidation significantly (by 63% +/- 4%). These results identify an important role for PKC delta in UGT1A6-mediated glucuronidation and suggest that PKC delta inhibitors could interfere with glucuronidation of UGT1A6 substrates.

UDP-glucuronosyltransferase 1A10: activity against the tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol, and a potential role for a novel UGT1A10 promoter deletion polymorphism in cancer susceptibility

The extrahepatic UDP-glucuronosyltransferase 1A10 (UGT1A10) is a phase II metabolizing enzyme that is active against a number of potent carcinogens. In the present study, UGT1A10 was examined for activity against 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), the major procarcinogenic metabolite of the potent tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, and the promoter region of UGT1A10 was examined for variants that could lead to altered UGT1A10 expression. UGT1A10-overexpressing cell homogenates exhibited high O-glucuronidation activity against NNAL (K(M) = 5.95 mM). A 2000-base pair (bp) product corresponding to the UGT1A10 proximal promoter region was polymerase chain reaction (PCR)-amplified using genomic DNA from 97 white subjects, and 42 of these were sequenced. In addition to a previously reported C/G single-nucleotide polymorphism at -1271 bp (rs2741032), a novel 1664-bp deletion located between nucleotides -190 to -1856 relative to the UGT1A10 translation start site was identified. Using real-time multiplex PCR, this deletion exhibited a prevalence of 0.022 in whites (n = 156) and 0.056 in blacks (n = 133). To determine whether either polymorphism altered gene expression, in vitro assays were performed using luciferase constructs containing up to 2000 bp of the proximal UGT1A10 promoter. Constructs containing the 1664-bp deletion exhibited a significant (p = 0.009) 3-fold increase in luciferase activity compared with constructs containing the wild-type UGT1A10 promoter. No effect on luciferase activity was observed for the UGT1A10(-1271G) promoter variant. These data are consistent with previous studies that indicate the presence of a transcriptional repressor element within the newly identified deletion and that this deletion polymorphism may contribute to altered UGT1A10 expression and altered carcinogen detoxification between individuals.

Dissimilar in vitro and in vivo effects of ellagic acid and its microbiota-derived metabolites, urolithins, on the cytochrome P450 1A1

This research shows that the dietary polyphenol ellagic acid (EA) and its colonic metabolites, urolithin-A (3,8-dihydroxy-6H-dibenzo[b,d]pyran-6-one, Uro-A) and urolithin-B (3-hydroxy-6H-dibenzo[b,d]pyran-6-one, Uro-B), modulate phase I and phase II detoxifying enzymes in colon cancer Caco-2 cells. EA and the urolithins, at micromolar concentrations achievable in the colon from the diet, induced the expression and activity of CYP1A1 and UGT1A10 and inhibited several sulfotransferases. As a result, the synthesis of glucuronides was favored over sulfated conjugates in the treated cells. In situ exposure to these compounds dissolved in buffer also led to the induction of CYP1A1 in the rat colon. However, in situ exposure to the compounds dissolved in oil or oral supplementation of the single compounds or pomegranate extract (PE) added to the feed failed to induce CYP1A1 in the colon mucosa. These results suggest that EA and urolithins may exert some blocking chemopreventive effects in the colon but that this effect is critically affected by interfering factors, such as the food matrix nature.

A pilot genome-wide association study of early-onset breast cancer

High-density oligonucleotide microarrays containing a large number of single nucleotide polymorphisms (SNPs) have enabled genome-wide association (GWA) analysis to become a reality. We used the early access Affymetrix Mendel Nsp 250K chips in a GWA case-control pilot study to identify genomic regions associated with breast cancer. We included 30 randomly sampled incident invasive breast cancer cases aged <45 years without deleterious mutations in the BRCA1 or BRCA2 genes, and 30 population controls individually matched on age, ethnicity and geographical area. The overall genotype call rate was 97.13+/-1.33% for controls and 97.48+/-1.42% for cases. Comparison was made between cases and controls for 203,477 genotyped SNPs using (a) unconditional logistic regression (ULR), (b) conditional logistic regression (CLR) models with adjustment for the matched pairs, (c) allelic tests for single marker tests and (d) haplotype trend regression (HTR). Genomic control and EIGENSTRAT methods were used for correction of population stratification in appropriate models. We demonstrate the similarity and dissimilarity of results from different statistical analyses. We found several possible significant regions harboring biologically meaningful known candidate genes, such as genes encoding fibroblast growth factor, transforming growth factor, epidermal growth factor, and estrogen synthesis enzymes to be associated with early-onset breast cancer. In single marker analysis, none of the SNPs were statistically significant after correction for multiple testing. However, haplotype association tests, using 90730 tag-SNPs, suggested two regions in GLG1 and UGT1 genes retaining significance even after Bonferroni correction. Nevertheless, without systematic replication, findings from this pilot study, especially the associations of breast cancer in relation to specific SNPs, should be interpreted with great caution.

Src supports UDP-glucuronosyltransferase-2B7 detoxification of catechol estrogens associated with breast cancer

Mammary gland-distributed and ER-bound UDP-glucuronosyltransferase (UGT)-2B7 metabolizes genotoxic catechol-estrogens (CE) associated with breast cancer initiation. Although UGT2B7 has 3 PKC- and 2 tyrosine kinase (TK)-sites, its inhibition by genistein, herbimycin-A and PP2 with parallel losses in phospho-tyrosine and phospho-Y438-2B7 content indicated it requires tyrosine phosphorylation, unlike required PKC phosphorylation of UGT1A isozymes. 2B7 mutants at PKC-sites had essentially normal activity, while its TK-sites mutants, Y236F- and Y438F-2B7, were essentially inactive. Overexpression of regular or active Src, but not dominant-negative Src, in 2B7-transfected COS-1 cells increased 2B7 activity and phospho-Y438-2B7 by 50%. Co-localization of 2B7 and regular SrcTK in COS-1 cells that was dissociated by pretreatment with Src-specific PP2-inhibitor provided strong evidence Src supports 2B7 activity. Consistent with these findings, evidence indicates an appropriate set of ER proteins with Src-homology binding-domains, including 2B7 and well-known multi-functional Src-engaged AKAP12 scaffold, supports Src-dependent phosphorylation of CE-metabolizing 2B7 enabling it to function as a tumor suppressor.

Glucuronidation of bioflavonoids by human UGT1A10: structure–function relationships

The extrahepatic human UDP glucuronosyltransferase 1A10 is found throughout the gastrointestinal tract and is thought to participate in the removal of orally ingested lipophilic chemicals. However, its substrate specificity towards these chemicals has not been fully characterized. The structurally diverse bioflavonoids are present in considerable amounts in fruits, vegetables and plant-derived beverages and have been shown to have many biological functions, including antioxidant properties. This study proposes features of the bioflavonoid structure necessary to confer it as a substrate of UGT1A10. The preferred substrates of UGT1A10 contain the hydroxyl group to be glucuronidated at C6 or C7, but not C5 of the A-ring or on C4' of the B-ring. Up to two additional hydroxyl groups on the A-ring enhance activity, whereas the presence of other groups, notably sugar groups, decreases activity. The high glucuronidation efficiency towards many bioflavonoids observed suggests that the contribution of UGT1A10 in the metabolism of these dietary compounds in the gastrointestinal tract may be significant.

The major chemical-detoxifying system of UDP-glucuronosyltransferases requires regulated phosphorylation supported by protein kinase C

Finding rapid, reversible down-regulation of human UDP-glucuronosyltransferases (UGTs) in LS180 cells following curcumin treatment led to the discovery that UGTs require phosphorylation. UGTs, distributed primarily in liver, kidney, and gastrointestinal tract, inactivate aromatic-like metabolites and a vast number of dietary and environmental chemicals, which reduces the risk of toxicities, mutagenesis, and carcinogenesis. Our aim here is to determine relevant kinases and mechanism(s) regulating phosphorylation of constitutive UGTs in LS180 cells and 10 different human UGT cDNA-transfected COS-1 systems. Time- and concentration-dependent inhibition of immunodetectable [(33)P]orthophosphate in UGTs and protein kinase Cepsilon (PKCepsilon), following treatment of LS180 cells with curcumin or the PKC inhibitor calphostin-C, suggested UGT phosphorylation is supported by active PKC(s). Immunofluorescent and co-immunoprecipitation studies with UGT-transfected cells showed co-localization of UGT1A7His and PKCepsilon and of UGT1A10His and PKCalpha or PKCdelta. Inhibition of UGT activity by PKCepsilon-specific antagonist peptide or by PKCepsilon-targeted destruction with PKCepsilon-specific small interference RNA and activation of curcumin-down-regulated UGTs with typical PKC agonists verified a central PKC role in glucuronidation. Moreover, in vitro phosphorylation of nascent UGT1A7His by PKCepsilon confirms it is a bona fide PKC substrate. Finally, catalase or herbimycin-A inhibition of constitutive or hydrogen peroxide-activated-UGTs demonstrated that reactive oxygen species-related oxidants act as second messengers in maintaining constitutive PKC-dependent signaling evidently sustaining UGT phosphorylation and activity. Because cells use signal transduction collectively to detect and respond appropriately to environmental changes, this report, combined with our earlier demonstration that specific phospho-groups in UGT1A7 determined substrate selections, suggests regulated phosphorylation allows adaptations regarding differential phosphate utilization by UGTs to function efficiently.

Identification of UDP-glucuronosyltransferase 1A10 in non-malignant and malignant human breast tissues

UGT1A10 was recently identified as the major isoform that conjugates estrogens. In this study, real-time PCR revealed high levels of UGT1A10 and UGT2B7 mRNA in human breast tissues. The expression of UGT1A10 in breast was a novel finding. UGT1A10 and UGT2B7 mRNAs were differentially expressed among normal and malignant specimens. Their overall expression was significantly decreased in breast carcinomas as compared to normal breast specimens (UGT1A10: 68+/-26 vs. 252+/-86, respectively; p<0.05) and (UGT2B7: 1.4+/-0.7 vs. 12+/-4, respectively; p<0.05). Interestingly, in African American women, UGT1A10 expression was significantly decreased in breast carcinomas in comparison to normals (57+/-35 vs. 397+/-152, respectively; p<0.05). Among Caucasian women, UGT2B7 was significantly decreased in breast carcinomas in comparison to normals (1.1+/-0.5 vs. 13.5+/-6, respectively; p<0.05). Glucuronidation of 4-hydroxylated estrone (4-OHE(1)) was significantly reduced in breast carcinomas compared to normals (30+/-15 vs. 106+/-31, respectively; p<0.05). Differential down-regulation of UGT1A10 and UGT2B7 mRNAs, protein, and activity in breast carcinomas compared to the adjacent normal breast specimens from the same donor were also found. These data illustrate the novel finding of UGT1A10 in human breast and confirm the expression of UGT2B7. Significant individual variation and down-regulation of expression in breast carcinomas of both isoforms were also demonstrated. These findings provide evidence that decreased UGT expression and activity could result in the promotion of carcinogenesis.

Novel identification of UDP-glucuronosyltransferase 1A10 as an estrogen-regulated target gene

Recently, we have shown that UGT1A10 is actively involved in the inactivation of E(1), E(2), and their 2- and 4-hydroxylated derivatives. In the present study, we show for the first time that treatment of the MCF-7 ER-positive breast cancer cell line with E(2) produces a dose-dependent up-regulation of UGT1A10 mRNA levels, followed by a steady down-regulation. In contrast, E(2) did not stimulate mRNA expression in the MDA-MB-231 (ER)-negative breast cancer cell line. Expression of UGT1A10 mRNA was blocked by the antiestrogen, ICI 182,780, but not by the transcriptional inhibitor, actinomycin-d. These findings suggest that regulation of UGT1A10 mRNA might be a primary transcriptional response mediated through the ER. Expression of UGT1A10 mRNA was also stimulated by other estrogenic compounds including propylpyrazoletriol (PPT) and genistein (Gen). Exposure of MCF-7 cells to 0.1nM E(2) up-regulated, and then down-regulated, UGT1A protein and enzymatic activity toward E(2) at 10nM E(2) as determined by Western blot and glucuronidation activity assays. Collectively, these results suggest that induction of UGT1A10 mRNA expression by E(2) might be mediated through ER, and that this isoform is a novel, estrogen-regulated target gene in MCF-7, ER-positive human breast cancer cells. The finding of E(2)-induced expression of UGT1A10 mRNA, followed by the down-regulation of UGT1A10 at pharmacological concentrations of E(2), might have a significant moderating effect on E(2) availability for ER and estrogen clearance, thereby promoting the signaling of E(2) in breast cancer cells.

UGT1A6 polymorphism and salicylic acid glucuronidation following aspirin

OBJECTIVES

In vivo, aspirin (acetylsalicylic acid) is rapidly deacetylated to form salicylic acid, which then undergoes primary or secondary glucuronidation catalyzed by UDP-glucuronosyltransferases (UGTs). The variant UGT1A6*2 (T181A, R184S) is associated with altered enzyme function. Our objective was to compare salicylic acid glucuronidation in individuals with different UGT1A6 genotypes.

METHODS

Following orally dosing with 650 mg aspirin, saliva and urine samples were collected over a period of 24 h from healthy individuals with homozygous wild-type UGT1A6 *1/*1 (n=19) and homozygous variant UGT1A6 *2/*2 (T181A, R184S) (n=9) genotypes.

RESULTS

No statistically significant differences were observed in salivary pharmacokinetic parameters. Urinary excretion of the sum of aspirin and its metabolites (salicyluric acid, salicyluric acid phenolic glucuronide, salicyl phenolic glucuronide, salicyl acyl glucuronide, salicylic acid) during the early period of 2-4 h of collection was significantly lower in UGT1A6 *1/*1 than in UGT1A6 *2/*2 individuals. Further, UGT1A6 *1/*1 individuals excreted a lower percentage of aspirin and its metabolites in the first 12 h and a greater percentage after 12 h than UGT1A6 *2/*2 individuals.

CONCLUSIONS

The variant UGT1A6*2 or polymorphisms in other UGTs that are in linkage disequilibrium with UGT1A6*2 may confer more rapid glucuronidation of salicylic acid than the wild-type UGT1A6 *1/*1.

Phenylalanine(90) and phenylalanine(93) are crucial amino acids within the estrogen binding site of the human UDP-glucuronosyltransferase 1A10

Human UDP-glucuronosyltransferase 1A10 has been identified as the major isoform involved in the biotransformation of a wide range of phenolic substrates, including native estrogens and their oxidized metabolites. Our recent studies point to the F(90)-M(91)-V(92)-F(93) amino acid motif of UGT1A10, which was identified using photoaffinity labeling followed by LC-MS/MS analysis, as a key determinant of the binding of phenolic substrates. In this report, we have evaluated the role of F(90), V(92), and F(93) in the recognition of estrogens by UGT1A10 using site-directed mutagenesis. Kinetic studies using five mutants revealed that F(90) and F(93) are critical residues for the recognition of all estrogen substrates. The substitution of F(90) with alanine totally abolished the activity of this enzyme toward all the estrogens investigated. Overall, sequential removal for the aromatic ring (F to L) and of the hydrophobic chain (F to A and V to A) from amino acids 90, 92, and 93 effectively alters estrogen recognition. This demonstrates that individual features of the native and hydroxylated estrogens determine the specific binding properties of the compound within the binding site of the human UGT1A10 and the mutants. The resulting activities are completely abolished, unchanged, increased, or decreased depending on the structures of both the mutant and the substrate. The novel identification of UGT1A10 as the major isoform involved in the glucuronidation of all estrogens and the discovery of the importance of the FMVF motif in the binding of steroids will help to elucidate the molecular mechanism of glucuronidation, resulting in the design of more effective estrogen-based therapies.

Identification of human UDP-glucuronosyltransferase isoform(s) responsible for the C-glucuronidation of phenylbutazone

Glucuronidation is a major metabolic pathway in the biotransformation of many xenobiotics and endogeneous compounds. There have been many studies on the formation of O-, N- or S-glucuronides and identification of the UDP-glucuronosyltransferase (UGT) isoforms responsible for the formation of these glucuronides. However, there is no information available on which UGT isoform(s) catalyzes C-glucuronidation. In the present study, 16 human UGTs (UGTs 1A1, 1A3, 1A4, 1A5, 1A6, 1A7, 1A8, 1A9, 1A10, 2B4, 2B7, 2B10, 2B11, 2B15, 2B17 and 2B28) were cloned and expressed in baculovirus-infected insect cells and investigated to determine their C-glucuronidating activity toward phenylbutazone (PB). Among the UGT isoforms investigated, only UGT1A9 catalyzed PB C-glucuronidation. Human liver and kidney microsomes, which are well known to express UGT1A9, had C-glucuronidating activity toward PB. However, the jejunum, which did not express UGT1A9, had no C-glucuronidating activity. These results demonstrate for the first time that PB C-glucuronidation is catalyzed by only UGT1A9.

Characterization of Rat Intestinal Microsomal UDP-Glucuronosyltransferase Activity toward Mycophenolic Acid

Mycophenolic acid (MPA) is the active immunosuppressive metabolite of the anti-organ rejection drug mycophenolate mofetil (MMF) and is implicated in the gastrointestinal toxicity associated with MMF therapy. Intestinal UDP-glucuronosyltransferases (UGT) have been proposed to provide intrinsic resistance against MMF-induced gastrointestinal toxicity by converting MPA to the inactive MPA 7-O-glucuronide. Using an optimized intestinal microsome preparation method that stabilized the intestinal MPA UGT activity, the MPA UGT activity of male Sprague-Dawley rat intestinal microsomes was characterized. A longitudinal gradient similar to that described for other phenolic compounds was observed, with the activity decreasing from the duodenum to the distal small intestine and colon. The catalytic efficiency of MPA glucuronidation decreased from the proximal to distal intestine as a result of decreasing Vmax and increasing Km. The finding that homozygous Gunn rats lack detectable intestinal MPA UGT activity indicates exclusive roles of UGT1A1, UGT1A6, and/or UGT1A7. Quantitative immunoblotting revealed a parallel between the MPA UGT activity and the content of UGT1A7-like immunoreactivity (18.7 and 7.3 microg/mg for duodenum and colon, respectively). In contrast, the lesser MPA-metabolizing UGT, UGT1A1 and UGT1A6, were lower in abundance (1.6-2.1 and 1.7-2.9 microg/mg, respectively), and their patterns of longitudinal distribution were distinct from the MPA UGT activity. These data suggest a dominant role of a UGT1A7-like enzyme, presumably UGT1A7 itself, in the catalysis of rat intestinal MPA glucuronidation. Studies are ongoing to investigate the relationship between intestinal UGT1A enzymes and susceptibility to MMF-induced gastrointestinal toxicity.

Importance of UDP-glucuronosyltransferase 1A10 (UGT1A10) in the detoxification of polycyclic aromatic hydrocarbons: decreased glucuronidative activity of the UGT1A10139Lys isoform

UDP-glucuronosyltransferase 1A10 (UGT1A10) is an extrahepatic enzyme expressed in aerodigestive tract tissues that exhibits significant glucuronidation activity against the important procarcinogenic benzo(a)pyrene (BaP) metabolite, BaP-trans-7,8-dihydrodiol (BPD), and the UGT1A10 codon 139 (Glu>Lys) polymorphism was previously implicated in risk for orolaryngeal cancer by Elahi et al. in their 2003 study. To better assess the potential role of UGT1A10 in risk for tobacco-related cancers, the glucuronidation activity of UGT1A10 was compared with that of other known UGT enzymes against selected polycyclic aromatic hydrocarbons, and the effects of the codon 139 polymorphism on UGT1A10 function were examined in vitro. UGT1A10 exhibited considerably more glucuronidation activity as determined by Vmax/Km against 3-hydroxy (OH)-BaP, 7-OH-BaP, 9-OH-BaP, and 1-OH-pyrene than any other UGT1A family member. Although a kinetic comparison using Vmax could not be performed against family 2B UGTs, UGT1A10 exhibited a 1.7- to 254-fold lower Km than active family 2B UGTs against 3-OH-BaP, 7-OH-BaP, and 1-OH-pyrene. A significantly (p < 0.01) higher Vmax/Km was observed for homogenates from wild-type UGT1A10139Glu-overexpressing cells against all four BaP metabolites tested (3-OH-BaP, 7-OH-BaP, 9-OH-BaP, and BPD). A similarly significant (p < 0.05) increase in Vmax/Km was observed for homogenates from wild-type UGT1A10139Glu-overexpressing cells against 1-OH-pyrene. Significant differences in Km were observed for homogenates from wild-type UGT1A10139Glu-overexpressing cells against 1-OH-pyrene (p < 0.05) and 3-OH-BaP (p < 0.01). Reverse transcription-polymerase chain reaction of total lung RNA showed low levels of UGT1A10 expression in human lung tissue. Together, these studies implicate UGT1A10 as an important detoxifier of polycyclic aromatic hydrocarbons in humans and that the UGT1A10 codon 139 polymorphism may be an important determinant in risk for tobacco-related cancers.

Haplotype structures of the UGT1A gene complex in a Japanese population

Genetic polymorphisms of UDP-glucuronosyltransferases (UGTs) are involved in individual and ethnic differences in drug metabolism. To reveal co-occurrence of the UGT1A polymorphisms, we first analyzed haplotype structures of the entire UGT1A gene complex using the polymorphisms from 196 Japanese subjects. Based on strong linkage disequilibrium between UGT1A8 and 1A10, among 1A9, 1A7, and 1A6, and between 1A3 and 1A1, the complex was divided into five blocks, Block 8/10, Block 9/6, Block 4, Block 3/1, and Block C, and the haplotypes for each block were subsequently determined/inferred. Second, using pyrosequencing or direct sequencing, additional 105 subjects were genotyped for 41 functionally tagged polymorphisms. The data from 301 subjects confirmed the robustness of block partitioning, but several linkages among the haplotypes with functional changes were found across the blocks. Thus, important haplotypes and their linkages were identified among the UGT1A gene blocks (and segments), which should be considered in pharmacogenetic studies.

Joint Effects between UDP-Glucuronosyltransferase 1A7 Genotype and Dietary Carcinogen Exposure on Risk of Colon Cancer

The UDP-glucuronosyltransferase 1A7 (UGT1A7) gene is polymorphic and encodes an enzyme involved in the detoxification of heterocyclic amines (HCA) and polycyclic aromatic hydrocarbons (PAH). Consumption of pan-fried and well-done meat are surrogates for HCA and PAH exposure and are possibly associated with colon cancer. We have evaluated whether UGT1A7 allelic variations are associated with colon cancer and whether UGT1A7 genotype modified associations among meat intake, exposure to HCAs and PAHs, and colon cancer in a population-based case-control study of African Americans (197 cases and 202 controls) and whites (203 cases and 210 controls). As part of a 150-item food frequency questionnaire, meat intake was assessed by cooking method and doneness and used to estimate individual HCA and PAH exposure. UGT1A7 alleles (UGT1A7*1, UGT1A7*2, UGT1A7*3, and UGT1A7*4) were measured and genotypes were categorized into predicted activity groups (high: *1/*1, *1/*2, *2/*2; intermediate: *1/*3, *1/*4, *2/*3; low: *3/*3, *3/*4, *4/*4). There was no association with UGT1A7 low versus high/intermediate genotype [odds ratio (OR), 1.1; 95% confidence interval (95% CI), 0.7-1.8], regardless of race. Greater than additive joint effects were observed for UGT1A7 low genotype and HCA-related factors. For example, equal to or greater than the median daily intake of the HCA, 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (DiMeIQx) and having UGT1A7 low genotype was positively associated with colon cancer (OR, 2.4; 95% CI, 1.2-4.8), compared with less than the median daily intake and UGT1A7 high/intermediate genotypes. These data suggest that the associations among cooked meat-derived compound exposure, and colon cancer are modified by the UGT1A7 genotype.

Phosphorylation of a UDP-glucuronosyltransferase regulates substrate specificity

UDP-glucuronosyltransferase (UGT) isozymes catalyze detoxification of numerous chemical toxins present in our daily diet and environment by conjugation to glucuronic acid. The special properties and enzymatic mechanism(s) that enable endoplasmic reticulum-bound UGT isozymes to convert innumerable structurally diverse lipophiles to excretable glucuronides are unknown. Inhibition of cellular UGT1A7 and UGT1A10 activities and of [33P]orthophosphate incorporation into immunoprecipitable proteins after exposure to curcumin or calphostin-C indicated that the isozymes are phosphorylated. Furthermore, inhibition of UGT phosphorylation and activity by treatment with PKCepsilon-specific inhibitor peptide supported PKC involvement. Co-immunoprecipitation, colocalization by means of immunofluorescence, and cross-linking studies of PKCepsilon and UGT1A7His revealed that the proteins reside within 11.4 angstroms of each other. Moreover, mutation of three PKC sites in each UGT isozyme demonstrated that T73A/G and T202A/G caused null activity, whereas S432G-UGT1A7 caused a major shift of its pH-8.5 optimum to 6.4 with new substrate selections, including 17beta-estradiol. S432G-UGT1A10 exhibited a minor pH shift without substrate alterations. PKCepsilon involvement was confirmed by the demonstration that PKCepsilon overexpression enhanced activity of UGT1A7 but not of its S432 mutant and the conversion of 17beta-[14C]estradiol by S432G-UGT1A7 but not by UGT1A7. Consistent with these observations, treatment of UGT1A7-transfected cells with PKCepsilon-specific inhibitor peptide or general PKC inhibitors increased 17beta-estradiol catalysis between 5- and 11-fold, with parallel decreases in phosphoserine-432. Here, we report a mechanism involving PKC-mediated phosphorylation of UGT such that phosphoserine/threonine regulates substrate specificity in response to chemical exposures, which possibly confers survival benefit.

GLUCURONIDATION OF NONSTEROIDAL ANTI-INFLAMMATORY DRUGS: IDENTIFYING THE ENZYMES RESPONSIBLE IN HUMAN LIVER MICROSOMES

Nonsteroidal anti-inflammatory drugs (NSAIDs), used for the treatment of pain and inflammation, are eliminated primarily through conjugation with polar sugar moieties to form glucuronides. Glucuronidation is catalyzed by the UDP-glucuronosyltransferases (UGT) superfamily. An inverse relationship may exist between glucuronidation activity and NSAID efficacy; however, specific UGTs catalyzing conjugation of the structurally diverse NSAIDs have yet to be identified systematically. Therefore, NSAID glucuronidation activity by 12 individually expressed UGTs was investigated by liquid chromatography-tandem mass spectrometry. The relative rates of NSAID glucuronidation varied among UGT enzymes examined, demonstrating specificity of the individual UGTs toward selected NSAIDs. Kinetic parameters were determined for expressed UGT Supersomes and compared with parameters determined in pooled human liver microsomes (HLMs). Comparison of K(m) values suggested roles for UGTs 1A3 and 2B7 in indene glucuronidation and UGTs 1A9, 2B4, and 2B7 in profen glucuronidation. Inhibitory studies in pooled HLMs support the role of UGTs 1A1, 1A3, 1A9, 2B4, and 2B7 in the glucuronidation of ibuprofen, flurbiprofen, and ketoprofen. Bilirubin did not inhibit indomethacin or diclofenac glucuronidation, suggesting that UGT1A1 was not involved in catalysis. Imipramine did not inhibit glucuronidation of sulindac, sulindac sulfone, indomethacin, or naproxen in pooled HLMs, suggesting that UGT1A3 was not a principal hepatic catalyst. Nevertheless, multiple UGT enzymes, most notably UGTs 1A1, 1A9, 2B4, and 2B7, seem to be involved in the hepatic catalysis of NSAID glucuronidation.