Interactions between human UDP-glucuronosyltransferase (UGT) 2B7 and UGT1A enzymes

Alternative transcript splicing regulates UDP-glucosyltransferase-catalyzed detoxification of DIMBOA in the fall armyworm (Spodoptera frugiperda)

Herbivorous insects often possess the ability to detoxify chemical defenses from their host plants. The fall armyworm (Spodoptera frugiperda), which feeds principally on maize, detoxifies the maize benzoxazinoid 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) by stereoselective re-glucosylation using a UDP-glucosyltransferase, SfUGT33F28. SfUGT33F28 activity is induced by feeding on a DIMBOA-containing diet, but how this induction is regulated is unknown. In the present work, we describe the alternative splicing of the SfUGT33F28 transcript. Variant transcripts are differentially expressed in response to DIMBOA, and this transcriptional response is mediated by an insect aryl hydrocarbon receptor. These variants have large deletions leading to the production of truncated proteins that have no intrinsic UGT activity with DIMBOA but interact with the full-length enzyme to raise or lower its activity. Therefore, the formation of SfUGT33F28 splice variants induces DIMBOA-conjugating UGT activity when DIMBOA is present in the insect diet and represses activity in the absence of this plant defense compound.

The inhibition of tamoxifen on UGT2B gene expression and enzyme activity in rat liver contribute to the estrogen homeostasis dysregulation

Background

Tamoxifen treatment may induce dysregulation of estrogen homeostasis, leading to the occurrence of related adverse reactions. However, the potential mechanisms are still unclear. The purpose of the present study was to uncover whether tamoxifen treatment would act on estrogen metabolism-related biological enzymes and the regulatory effect on estrogen homeostasis to clarify the key factors and potential mechanisms of adverse reactions caused by long-term use of tamoxifen.

Method

Female SD rats were administrated with tamoxifen CMC-Na solution (p.o.) once daily for four weeks and then housed at room temperature. Serum, breast, liver, uterus, and ovarian tissues were obtained, and the effects of tamoxifen administration on estrogen homeostasis, the expression, and activity of estrogen metabolic enzyme were evaluated.

Results

Compared with the control group, the estrogen homeostasis was disturbed and the expression and activity of UGT2B1 (homology with human UGT2B7) were significantly reduced in the rats administrated with tamoxifen. The inhibitory effect of tamoxifen on UGT2B7 was dominated by hydrophobic and π-π stacking interactions, resulting in a concentration-dependent inhibition of UGT2B7 activity by tamoxifen and the imbalance of ligand-activated transcription factors, leading to abnormal regulation of UGT2B and disturbance of estrogen homeostasis, which in turn led to adverse reactions of tamoxifen.

Conclusion

We established links between estrogen metabolism and tamoxifen administration and we proposed that the UGT2B inhibition was involved in the disturbance of estrogen homeostasis and the occurrence of tamoxifen-related adverse reactions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40360-022-00574-6.

Enzyme Kinetics of Uridine Diphosphate Glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bisubstrate reaction that requires the aglycone and the cofactor, UDP-GlcUA. Accumulating evidence suggests that the bisubstrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modeling of glucuronidation reactions in vitro, UDP-GlcUA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for during experimental design and data interpretation. While the assessment of drug-drug interactions resulting from UGT inhibition has been challenging in the past, the increasing availability of UGT enzyme-selective substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of drug-drug interaction potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often underpredicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation. Physiologically based pharmacokinetic (PBPK) modeling has also shown to be of value for predicting PK of drugs eliminated by glucuronidation.

Evidence-based strategies for the characterisation of human drug and chemical glucuronidation in vitro and UDP-glucuronosyltransferase reaction phenotyping

Enzymes of the UDP-glucuronosyltransferase (UGT) superfamily contribute to the elimination of drugs from almost all therapeutic classes. Awareness of the importance of glucuronidation as a drug clearance mechanism along with increased knowledge of the enzymology of drug and chemical metabolism has stimulated interest in the development and application of approaches for the characterisation of human drug glucuronidation in vitro, in particular reaction phenotyping (the fractional contribution of the individual UGT enzymes responsible for the glucuronidation of a given drug), assessment of metabolic stability, and UGT enzyme inhibition by drugs and other xenobiotics. In turn, this has permitted the implementation of in vitro - in vivo extrapolation approaches for the prediction of drug metabolic clearance, intestinal availability, and drug-drug interaction liability, all of which are of considerable importance in pre-clinical drug development. Indeed, regulatory agencies (FDA and EMA) require UGT reaction phenotyping for new chemical entities if glucuronidation accounts for ≥25% of total metabolism. In vitro studies are most commonly performed with recombinant UGT enzymes and human liver microsomes (HLM) as the enzyme sources. Despite the widespread use of in vitro approaches for the characterisation of drug and chemical glucuronidation by HLM and recombinant enzymes, evidence-based guidelines relating to experimental approaches are lacking. Here we present evidence-based strategies for the characterisation of drug and chemical glucuronidation in vitro, and for UGT reaction phenotyping. We anticipate that the strategies will inform practice, encourage development of standardised experimental procedures where feasible, and guide ongoing research in the field.

Homology Modeling of Human Uridine-5'-diphosphate-glucuronosyltransferase 1A6 Reveals Insights into Factors Influencing Substrate and Cosubstrate Binding

The elimination of numerous endogenous compounds and xenobiotics via glucuronidation by uridine-5'-diphosphate glycosyltransferase enzymes (UGTs) is an essential process of the body's chemical defense system. UGTs have distinct but overlapping substrate preferences, but the molecular basis for their substrate specificity remains poorly understood. Three-dimensional protein structures can greatly enhance our understanding of the interactions between enzymes and their substrates, but because of the inherent difficulties in purifying and crystallizing integral endoplasmic reticulum membrane proteins, no complete mammalian UGT structure has yet been produced. To address this problem, we have created a homology model of UGT1A6 using I-TASSER to explore, in detail, the interactions of human UGT1A6 with its substrates. Ligands were docked into our model in the presence of the cosubstrate uridine-5'-diphosphate-glucuronic acid, interacting residues were examined, and poses were compared to those cocrystallized with various plant and bacterial glycosyltransferases (GTs). Our model structurally resembles other GTs, and docking experiments replicated many of the expected UGT-substrate interactions. Some bias toward the template structures' protein-substrate interactions and binding preferences was evident.

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.

Species differences in drug glucuronidation: Humanized UDP-glucuronosyltransferase 1 mice and their application for predicting drug glucuronidation and drug-induced toxicity in humans

More than 20% of clinically used drugs are glucuronidated by a microsomal enzyme UDP-glucuronosyltransferase (UGT). Inhibition or induction of UGT can result in an increase or decrease in blood drug concentration. To avoid drug-drug interactions and adverse drug reactions in individuals, therefore, it is important to understand whether UGTs are involved in metabolism of drugs and drug candidates. While most of glucuronides are inactive metabolites, acyl-glucuronides that are formed from compounds with a carboxylic acid group can be highly toxic. Animals such as mice and rats are widely used to predict drug metabolism and drug-induced toxicity in humans. However, there are marked species differences in the expression and function of drug-metabolizing enzymes including UGTs. To overcome the species differences, mice in which certain drug-metabolizing enzymes are humanized have been recently developed. Humanized UGT1 (hUGT1) mice were created in 2010 by crossing Ugt1-null mice with human UGT1 transgenic mice in a C57BL/6 background. hUGT1 mice can be promising tools to predict human drug glucuronidation and acyl-glucuronide-associated toxicity. In this review article, studies of drug metabolism and toxicity in the hUGT1 mice are summarized. We further discuss research and strategic directions to advance the understanding of drug glucuronidation in humans.

Revisiting the Latency of Uridine Diphosphate-Glucuronosyltransferases (UGTs)-How Does the Endoplasmic Reticulum Membrane Influence Their Function?

Uridine diphosphate-glucuronosyltransferases (UGTs) are phase 2 conjugation enzymes mainly located in the endoplasmic reticulum (ER) of the liver and many other tissues, and can be recovered in artificial ER membrane preparations (microsomes). They catalyze glucuronidation reactions in various aglycone substrates, contributing significantly to the body's chemical defense mechanism. There has been controversy over the last 50 years in the UGT field with respect to the explanation for the phenomenon of latency: full UGT activity revealed by chemical or physical disruption of the microsomal membrane. Because latency can lead to inaccurate measurements of UGT activity in vitro, and subsequent underprediction of drug clearance in vivo, it is important to understand the mechanisms behind this phenomenon. Three major hypotheses have been advanced to explain UGT latency: compartmentation, conformation, and adenine nucleotide inhibition. In this review, we discuss the evidence behind each hypothesis in depth, and suggest some additional studies that may reveal more information on this intriguing phenomenon.

Endogenous Protein Interactome of Human UDP-Glucuronosyltransferases Exposed by Untargeted Proteomics

The conjugative metabolism mediated by UDP-glucuronosyltransferase enzymes (UGTs) significantly influences the bioavailability and biological responses of endogenous molecule substrates and xenobiotics including drugs. UGTs participate in the regulation of cellular homeostasis by limiting stress induced by toxic molecules, and by controlling hormonal signaling networks. Glucuronidation is highly regulated at genomic, transcriptional, post-transcriptional and post-translational levels. However, the UGT protein interaction network, which is likely to influence glucuronidation, has received little attention. We investigated the endogenous protein interactome of human UGT1A enzymes in main drug metabolizing non-malignant tissues where UGT expression is most prevalent, using an unbiased proteomics approach. Mass spectrometry analysis of affinity-purified UGT1A enzymes and associated protein complexes in liver, kidney and intestine tissues revealed an intricate interactome linking UGT1A enzymes to multiple metabolic pathways. Several proteins of pharmacological importance such as transferases (including UGT2 enzymes), transporters and dehydrogenases were identified, upholding a potential coordinated cellular response to small lipophilic molecules and drugs. Furthermore, a significant cluster of functionally related enzymes involved in fatty acid β-oxidation, as well as in the glycolysis and glycogenolysis pathways were enriched in UGT1A enzymes complexes. Several partnerships were confirmed by co-immunoprecipitations and co-localization by confocal microscopy. An enhanced accumulation of lipid droplets in a kidney cell model overexpressing the UGT1A9 enzyme supported the presence of a functional interplay. Our work provides unprecedented evidence for a functional interaction between glucuronidation and bioenergetic metabolism.

Acyl-glucuronide as a Possible Cause of Trovafloxacin-Induced Liver Toxicity: Induction of Chemokine (C-X-C Motif) Ligand 2 by Trovafloxacin Acyl-glucuronide

Trovafloxacin is an antibiotic that was withdrawn from the market relatively soon after its release due to the risk of hepatotoxicity. Trovafloxacin is mainly metabolized to its acyl-glucuronide by uridine 5'-diphosphate (UDP)-glucuronosyltransferase (UGT) 1A1. In this study, we examined whether the acyl-glucuronide is involved in the development of hepatotoxicity. A UGT1A1-induced cell model was developed and the toxicity of trovafloxacin acyl-glucuronide was evaluated. The UGT1A1-induced cell model was developed by treating HepG2 cells with chrysin for 48 h. Chemokine (C-X-C motif) ligand 2, a cytokine involved in drug-induced liver injury, was uniquely induced by trovafloxacin in the UGT1A1-induced HepG2 cells. Induction of UGT1A1 resulted in a decrease in cell viability. An in vivo animal study further demonstrated the importance of UGT1A1 in the trovafloxacin-induced liver toxicity. Although the complete mechanism of trovafloxacin-induced liver injury is still unknown, trovafloxacin acyl-glucuronide can be involved in the development of toxic reactions in vitro and in vivo.

Inter-isoform Hetero-dimerization of Human UDP-Glucuronosyltransferases (UGTs) 1A1, 1A9, and 2B7 and Impacts on Glucuronidation Activity

Human UDP-glucuronosyltransferases (UGTs) play a pivotal role in phase II metabolism by catalyzing the glucuronidation of endobiotics and xenobiotics. The catalytic activities of UGTs are highly impacted by both genetic polymorphisms and oligomerization. The present study aimed to assess the inter-isoform hetero-dimerization of UGT1A1, 1A9, and 2B7, including the wild type (1A1*1, 1A9*1, and 2B7*1) and the naturally occurring (1A1*1b, 1A9*2/*3/*5, and 2B7*71S/*2/*5) variants. The related enzymes were double expressed in Bac-to-Bac systems. The fluorescence resonance energy transfer (FRET) technique and co-immunoprecipitation (Co-IP) revealed stable hetero-dimerization of UGT1A1, 1A9, and 2B7 allozymes. Variable FRET efficiencies and donor-acceptor distances suggested that genetic polymorphisms resulted in altered affinities to the target protein. In addition, the metabolic activities of UGTs were differentially altered upon hetero-dimerization via double expression systems. Moreover, protein interactions also changed the regioselectivity of UGT1A9 for querectin glucuronidation. These findings provide in-depth understanding of human UGT dimerization as well as clues for complicated UGT dependent metabolism in humans.

Structure and Protein-Protein Interactions of Human UDP-Glucuronosyltransferases

Mammalian UDP-glucuronosyltransferases (UGTs) catalyze the transfer of glucuronic acid from UDP-glucuronic acid to various xenobiotics and endobiotics. Since UGTs comprise rate-limiting enzymes for metabolism of various compounds, co-administration of UGT-inhibiting drugs and genetic deficiency of UGT genes can cause an increased blood concentration of these compounds. During the last few decades, extensive efforts have been made to advance the understanding of gene structure, function, substrate specificity, and inhibition/induction properties of UGTs. However, molecular mechanisms and physiological importance of the oligomerization and protein–protein interactions of UGTs are still largely unknown. While three-dimensional structures of human UGTs can be useful to reveal the details of oligomerization and protein–protein interactions of UGTs, little is known about the protein structures of human UGTs due to the difficulty in solving crystal structures of membrane-bound proteins. Meanwhile, soluble forms of plant and bacterial UGTs as well as a partial domain of human UGT2B7 have been crystallized and enabled us to predict three-dimensional structures of human UGTs using a homology-modeling technique. The homology-modeled structures of human UGTs do not only provide the detailed information about substrate binding or substrate specificity in human UGTs, but also contribute with unique knowledge on oligomerization and protein–protein interactions of UGTs. Furthermore, various in vitro approaches indicate that UGT-mediated glucuronidation is involved in cell death, apoptosis, and oxidative stress as well. In the present review article, recent understandings of UGT protein structures as well as physiological importance of the oligomerization and protein–protein interactions of human UGTs are discussed.

Dimerization of human uridine diphosphate glucuronosyltransferase allozymes 1A1 and 1A9 alters their quercetin glucuronidation activities

Uridine diphosphate glucuronosyltransferase 1A (UGT1A) is a major phase II drug-metabolism enzyme superfamily involved in the glucuronidation of endobiotics and xenobiotics in humans. Many polymorphisms in UGT1A genes are reported to inhibit or decrease UGT1A activity. In this study, two UGT1A1 allozymes, UGT1A1 wild-type and a splice mutant, as well as UGT1A9 wild-type and its three UGT1A9 allozymes, UGT1A9*2(C3Y), UGT1A9*3(M33T), and UGT1A9*5(D256N) were single- or double-expressed in a Bac-to-Bac expression system. Dimerization of UGT1A1 or UGT1A9 allozymes was observed via fluorescence resonance energy transfer (FRET) and co-immunoprecipitation analysis. SNPs of UGT1A altered the ability of protein-protein interaction, resulting in differential FRET efficiencies and donor-acceptor r distances. Dimerization changed the chemical regioselectivity, substrate-binding affinity, and enzymatic activity of UGT1A1 and UGT1A9 in glucuronidation of quercetin. These findings provide molecular insights into the consequences of homozygous and heterozygous UGT1A1 and UGT1A9 allozymes expression on quercetin glucuronidation.

Role of extrahepatic UDP-glucuronosyltransferase 1A1: Advances in understanding breast milk-induced neonatal hyperbilirubinemia

Newborns commonly develop physiological hyperbilirubinemia (also known as jaundice). With increased bilirubin levels being observed in breast-fed infants, breast-feeding has been recognized as a contributing factor for the development of neonatal hyperbilirubinemia. Bilirubin undergoes selective metabolism by UDP-glucuronosyltransferase (UGT) 1A1 and becomes a water soluble glucuronide. Although several factors such as gestational age, dehydration and weight loss, and increased enterohepatic circulation have been associated with breast milk-induced jaundice (BMJ), deficiency in UGT1A1 expression is a known cause of BMJ. It is currently believed that unconjugated bilirubin is metabolized mainly in the liver. However, recent findings support the concept that extrahepatic tissues, such as small intestine and skin, contribute to bilirubin glucuronidation during the neonatal period. We will review the recent advances made towards understanding biological and molecular events impacting BMJ, especially regarding the role of extrahepatic UGT1A1 expression.

Posttranscriptional regulation of uridine diphosphate glucuronosyltransferases

INTRODUCTION

The uridine diphosphate (UDP)-glucuronosyltransferase (UGT) superfamily of enzymes (EC 2.4.1.17) conjugates glucuronic acid to an aglycone substrate to make them more polar and readily excreted. In general, this reaction terminates the activities of chemicals, drugs and toxins, although occasionally a more active or toxic species is produced.

AREAS COVERED

In addition to their well-known transcriptional responsiveness, UGTs are also regulated by posttranscriptional mechanisms. Here, the authors review these mechanisms, including latency, modulation of co-substrate accessibility and binding, dimerization and oligomerization, protein-protein interactions, allosteric inhibition and activation, posttranslational structural and functional modifications and developmental switching for UGTs.

EXPERT OPINION

Posttranscriptional regulation of UGTs has traditionally received less attention than nuclear regulation, in part because mechanisms involving ribosomes and endoplasmic reticula are challenging to investigate. Most promising of the posttranscriptional mechanisms reviewed are likely to be effects on co-substrate (UDP-glucuronic acid) transport and availability and structure-function changes to UGT proteins through, for example, glycosylation and phosphorylation. Although classical biochemistry continues to illuminate many aspects of UGT function, advances in proteomics and structural biology are beginning to assist in the determination of posttranscriptional regulation mechanisms for UGTs.

Enzyme kinetics of uridine diphosphate glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bi-substrate reaction that requires the aglycone and a cofactor, UDPGA. Accumulating evidence suggests that the bi-substrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modelling of glucuronidation reactions in vitro, UDPGA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for in experimental design and data interpretation. Assessing drug-drug interactions (DDIs) involving UGT inhibition remains challenging. However, the increasing availability of UGT enzyme-specific substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of DDI potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often under-predicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation (IVIVE).

An orphan esterase ABHD10 modulates probenecid acyl glucuronidation in human liver

Probenecid, a widely used uricosuric agent, is mainly metabolized to probenecid acyl glucuronide (PRAG), which is considered a causal substance of severe allergic or anaphylactoid reactions. PRAG can be hydrolyzed (deglucuronidated) to probenecid. The purpose of this study was to identify enzymes responsible for probenecid acyl glucuronidation and PRAG deglucuronidation in human livers and to examine the effect of deglucuronidation in PRAG formation. In human liver homogenates (HLHs), the intrinsic clearance (CLint) of PRAG deglucuronidation was much greater (497-fold) than that of probenecid acyl glucuronidation. Evaluation of PRAG formation by recombinant UDP-glucuronosyltransferase (UGT) isoforms and an inhibition study using HLHs as an enzyme source demonstrated that multiple UGT isoforms, including UGT1A1, UGT1A9, and UGT2B7, catalyzed probenecid acyl glucuronidation. We found that recombinant α/β hydrolase domain containing 10 (ABHD10) substantially catalyzed PRAG deglucuronidation activity, whereas carboxylesterases did not. Similar inhibitory patterns by chemicals between HLHs and recombinant ABHD10 supported the major contribution of ABHD10 to PRAG deglucuronidation in human liver. Interestingly, it was demonstrated that the CLint value of probenecid acyl glucuronidation in HLHs was increased by 1.7-fold in the presence of phenylmethylsulfonyl fluoride, which potently inhibited ABHD10 activity. In conclusion, we found that PRAG deglucuronidation catalyzed by ABHD10 suppressively regulates PRAG formation via multiple UGT enzymes in human liver. The balance of activities by these enzymes is important for the formation of PRAG, which may be associated with the adverse reactions observed after probenecid administration.

Evaluation of UGT protein interactions in human hepatocytes: effect of siRNA down regulation of UGT1A9 and UGT2B7 on propofol glucuronidation in human hepatocytes

Previous experiments performed in recombinant systems have suggested that protein-protein interactions occur between the UGTs and may play a significant role in modulating enzyme activity. However, evidence of UGT protein-protein interactions either in vivo or in more physiologically relevant in vitro systems has yet to be demonstrated. In this study, we examined oligomerization and its ability to affect glucuronidation in plated human hepatocytes. siRNA down regulation experiments and activity studies were used to examine changes in metabolite formation of one UGT isoform due to down regulation of a second UGT isoform. Selective siRNA directed towards UGT1A9 or UGT2B7 resulted in significant and selective decreases in their respective mRNA levels. As expected, the metabolism of the UGT1A9 substrate propofol decreased with UGT1A9 down regulation. Interestingly, UGT1A9 activity, but not UGT1A9 mRNA expression, was also diminished when UGT2B7 expression was selectively inhibited, implying potential interactions between the two isoforms. Minor changes to UGT1A4, UGT2B4 and UGT2B7 activity were also observed when UGT1A9 expression was selectively down regulated. To our knowledge, this represents the first piece of evidence that UGT protein-protein interactions occur in human hepatocytes and suggests that expression levels of UGT2B7 may directly impact the glucuronidation activity of selective UGT1A9 substrates.

Protein-protein interactions between the bilirubin-conjugating UDP-glucuronosyltransferase UGT1A1 and its shorter isoform 2 regulatory partner derived from alternative splicing

The oligomerization of UGTs [UDP (uridine diphosphate)-glucuronosyltransferases] modulates their enzyme activities. Recent findings also indicate that glucuronidation is negatively regulated by the formation of inactive oligomeric complexes between UGT1A enzymes [i1 (isoform 1)] and an enzymatically inactive alternatively spliced i2 (isoform 2). In the present paper, we assessed whether deletion of the UGT-interacting domains previously reported to be critical for enzyme function might be involved in i1-i2 interactions. The bilirubin-conjugating UGT1A1 was used as a prototype. We also explored whether intermolecular disulfide bonds are involved in i1-i2 interactions and the potential role of selected cysteine residues. Co-immunoprecipitation assays showed that UGT1A1 lacking the SP (signal peptide) alone or also lacking the transmembrane domain (absent from i2) did not self-interact, but still interacted with i2. The deletion of other N- or C-terminal domains did not compromise i1-i2 complex formation. Under non-reducing conditions, we also observed formation of HMWCs (high-molecular-mass complexes) for cells overexpressing i1 and i2. The presence of UGTs in these complexes was confirmed by MS. Mutation of individual cysteine residues throughout UGT1A1 did not compromise i1-i1 or i1-i2 complex formation. These findings are compatible with the hypothesis that the interaction between i1 and i2 proteins (either transient or stable) involves binding of more than one domain that probably differs from those involved in i1-i1 interactions.

Optimized assays for human UDP-glucuronosyltransferase (UGT) activities: altered alamethicin concentration and utility to screen for UGT inhibitors

The measurement of the effect of new chemical entities on human UDP-glucuronosyltransferase (UGT) marker activities using in vitro experimentation represents an important experimental approach in drug development to guide clinical drug-interaction study designs or support claims that no in vivo interaction will occur. Selective high-performance liquid chromatography-tandem mass spectrometry functional assays of authentic glucuronides for five major hepatic UGT probe substrates were developed: β-estradiol-3-glucuronide (UGT1A1), trifluoperazine-N-glucuronide (UGT1A4), 5-hydroxytryptophol-O-glucuronide (UGT1A6), propofol-O-glucuronide (UGT1A9), and zidovudine-5'-glucuronide (UGT2B7). High analytical sensitivity permitted characterization of enzyme kinetic parameters at low human liver microsomal and recombinant UGT protein concentration (0.025 mg/ml), which led to a new recommended optimal universal alamethicin activation concentration of 10 μg/ml for microsomes. Alamethicin was not required for recombinant UGT incubations. Apparent enzyme kinetic parameters, particularly for UGT1A1 and UGT1A4, were affected by nonspecific binding. Unbound intrinsic clearance for UGT1A9 and UGT2B7 increased significantly after addition of 2% bovine serum albumin, with minimal changes for UGT1A1, UGT1A4, and UGT1A6. Eleven potential UGT and cytochrome P450 inhibitors were evaluated as UGT inhibitors, resulting in observation of nonselective UGT inhibition by chrysin, mefenamic acid, silibinin, tangeretin, ketoconazole, itraconazole, ritonavir, and verapamil. The pan-cytochrome P450 inhibitor, 1-aminobenzotriazole, minimally inhibited UGT activities and may be useful in reaction phenotyping of mixed UGT and cytochrome P450 substrates. These methods should prove useful in the routine assessments of the potential for new drug candidates to elicit pharmacokinetic drug interactions via inhibition of human UGT activities and the identification of UGT enzyme-selective chemical inhibitors.

Evaluation of 3,3',4'-trihydroxyflavone and 3,6,4'-trihydroxyflavone (4'-O-glucuronidation) as the in vitro functional markers for hepatic UGT1A1

Identifying uridine 5'-diphospho-(UDP)-glucuronosyltransferase (UGT)-selective probes (substrates that are primarily glucuronidated by a single isoform) is complicated by the enzymes' large overlapping substrate specificity. Here, regioselective glucuronidation of two flavonoids, 3,3',4'-trihydroxyflavone (3,3',4'-THF) and 3,6,4'-trihydroxyflavone (3,6,4'-THF), is used to probe the activity of hepatic UGT1A1. The glucuronidation kinetics of 3,3',4'-THF and 3,6,4'-THF was determined using 12 recombinant human UGT isoforms and pooled human liver microsomes (pHLM). The individual contribution of main UGT isoforms to the metabolism of the two flavonoids in pHLM was estimated using the relative activity factor approach. UGT1A1 activity correlation analyses using flavonoids-4'-O-glucuronidation vs β-estradiol-3-glucuronidation (a well-recognized marker for UGT1A1) or vs SN-38 glucuronidation were performed using a bank of HLMs (n = 12) including three UGT1A1-genotyped HLMs (i.e., UGT1A1*1*1, UGT1A1*1*28, and UGT1A1*28*28). The results showed that UGT1A1 and 1A9, followed by 1A7, were the main isoforms for glucuronidating the two flavonoids, where UGT1A1 accounted for 92 ± 7% and 91 ± 10% of 4'-O-glucuronidation of 3,3',4'-THF and 3,6,4'-THF, respectively, and UGT1A9 accounted for most of the 3-O-glucuronidation. Highly significant correlations (R(2) > 0.944, p < 0.0001) between the rates of flavonoids 4'-O-glucuronidation and that of estradiol-3-glucuronidation or SN-38 glucuronidation were observed across 12 HLMs. In conclusion, UGT1A1-mediated 4'-O-glucuronidation of 3,3',4'-THF and 3,6,4'-THF was highly correlated with the glucuronidation of estradiol (3-OH) and SN-38. This study demonstrated for the first time that regioselective glucuronidation of flavonoids can be applied to probe hepatic UGT1A1 activity in vitro.

In vitro and in vivo small intestinal metabolism of CYP3A and UGT substrates in preclinical animals species and humans: species differences

Intestinal first-pass metabolism has a great impact on the bioavailability of cytochrome P450 3A4 (CYP3A) and/or uridine 5'-diphosphate (UDP)-glucoronosyltranferase (UGT) substrates in humans. In vitro and in vivo intestinal metabolism studies are essential for clarifying pharmacokinetics in animal species and for predicting the effects of human intestinal metabolism. We review species differences in intestinal metabolism both in vitro and in vivo. Based on mRNA expression levels, the major intestinal CYP3A isoform is CYP3A4 for humans, CYP3A4 (3A8) for monkeys, CYP3A9 for rats, cyp3a13 for mice, and CYP3A12 for dogs. Additionally, the intestinal-specific UGT would be UGT1A10 for humans, UGT1A8 for monkeys, and UGT1A7 for rats. In vitro and in vivo intestinal metabolism of CYP3A substrates were larger in monkeys than in humans, although a correlation in intestinal availability between monkeys and humans has been reported. Little information is available regarding species differences in in vitro and in vivo UGT activities; however, UGT-mediated in vivo intestinal metabolism has been demonstrated for raloxifene in humans and for baicalein in rats. Further assessment of intestinal metabolism, particularly for UGT substrates, is required to clarify the entire picture of species differences.

Modulation of the human glucuronosyltransferase UGT1A pathway by splice isoform polypeptides is mediated through protein-protein interactions

This study investigated the molecular mechanisms underlying the regulatory effect of the newly discovered 45-kDa enzymatically inactive UGT1A spliced polypeptides, named isoform i2, upon UGT1A-mediated glucuronidation. Initially, using an inducible system that mimics the relative abundance of isoforms 1 and 2 of UGT1A1 in human tissues, the rates of formation of glucuronides were significantly reduced. We then used a heterologous system constitutively expressing both isoforms i1 and i2 for an in-depth investigation of the presence of spliced i2 on glucuronidation kinetics. UGT1A1, UGT1A7, and UGT1A8 were selected as candidates for these studies. In all cases, co-expression of i1 and i2 in HEK293 cells leads to a significant reduction of the velocity of the glucuronidation reaction without affecting the affinity (K(m) (app)) for all substrates tested and the K(m) for the co-substrate, UDP-glucuronic acid. The data are consistent with a dominant-negative model of inhibition but do not sustain with an UGT1A_i2-mediated inhibition by competitive binding for substrate or the co-substrate. In contrast, the data from the co-immunoprecipitation experiments are indicative of the existence of a mixture homo-oligomeric (i1-i1 or i2-i2) and hetero-oligomeric (i1-i2) complexes in which the i2-i2 and i1-i2 subunits would be inactive. Thus, protein-protein interactions are likely responsible for the inhibition of active UGT1A_i1 by i2 spliced polypeptides. This new regulatory mechanism may alternatively modulate cellular response to endo/xeno stimulus.