Structure of the human UGT2B4 gene encoding a bile acid UDP-glucuronosyltransferase

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.

New insights for risks of chlorophenols (CPs) exposure: Inhibition of UDP-glucuronosyltransferases (UGTs)

Chlorophenols (CPs) are important pollutants extensively utilized in industry, agriculture and forestry. The present study aims to determine the inhibition of CPs on the activity of the important phase II drug-metabolizing enzymes (DMEs) UDP-glucuronosyltransferases (UGTs). 100 μM of fourteen CPs were used for preliminary screening using in vitro incubation. Furthermore, half inhibition concentration (IC₅₀) and inhibition kinetics were determined for CPs with significant inhibition towards UGT isoforms. In silico docking was used to explain the inhibition difference among CPs. Multiple UGT isoforms were inhibited by CPs. In silico docking showed that higher free binding energy due to hydrophobic interactions of 2.4-Dichlorophenol (2.4-DCP) or 4-Chloro-3-methylphenol (4C3MP) with UGT1A9 contributed to stronger inhibition potential of 2.4-Dichlorophenol (2.4-DCP) or 4-Chloro-3-methylphenol (4C3MP) towards UGT1A9 than 4-CP. Pentachlorophenol (PCP) was chosen as the representative CPs to determine the IC₅₀ value towards UGT1A6, UGT1A9 and UGT2B7. IC₅₀ was calculated to be 0.33 μM, 0.24 μM and 31.35 μM for the inhibition of PCP towards UGT1A6, UGT1A9 and UGT2B7. PCP was demonstrated to show competitive inhibition towards UGT1A6, UGT1A9 and UGT2B7, and the inhibition kinetic parameters (Ki) was calculated to be 0.18 μM, 0.01 μM and 5.37 μM for the inhibition of PCP towards UGT1A6, UGT1A9 and UGT2B7. All these information will be beneficial for elucidating the risk of CPs exposure from a new perspective.

Inhibitory effects of fifteen phthalate esters in human cDNA-expressed UDP-glucuronosyltransferase supersomes

Phthalate esters (PAEs) have been extensively used in industry as plasticizers and there remains concerns about their safety. The present study aimed to determine the inhibition of phthalate esters (PAEs) on the activity of the phase II drug-metabolizing enzymes UDP-glucuronosyltransferases (UGTs). In vitro recombinant UGTs-catalyzed glucuronidation of 4-methylumbelliferone was used to investigate the inhibition potentials of PAEs towards various s UGTs. PAEs exhibited no significant inhibition of UGT1A1, UGT1A3, UGT1A8, UGT1A10, UGT2B15, and UGT2B17, and limited inhibition of UGT1A6, UGT1A7 and UGT2B4. However, UGT1A9 was strongly inhibited by PAEs. In silico docking demonstrated a significant contribution of hydrogen bonds and hydrophobic interactions contributing to the inhibition of UGT by PAEs. The K_i values were 15.5, 52.3, 23.6, 12.2, 5.61, 2.79, 1.07, 22.8, 0.84, 73.7, 4.51, 1.74, 0.58, 6.79, 4.93, 6.73, and 7.23 μM for BBOP-UGT1A6, BBZP-UGT1A6, BBOP-UGT1A7, BBZP-UGT1A7, DiPP-UGT1A9, DiBP-UGT1A9, DCHP-UGT1A9, DBP-UGT1A9, BBZP-UGT1A9, BBOP-UGT1A9, DMEP-UGT1A9, DPP-UGT1A9, DHP-UGT1A9, DiBP-UGT2B4, DBP-UGT2B4, DAP-UGT2B4, and BBZP-UGT2B4, respectively. In conclusion, exposure to PAEs might influence the metabolic elimination of endogenous compounds and xenobiotics through inhibiting UGTs.

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.

Oligomerization of the UDP-glucuronosyltransferase 1A proteins: homo- and heterodimerization analysis by fluorescence resonance energy transfer and co-immunoprecipitation

UDP-glucuronosyltransferases (UGTs) are membrane-bound proteins localized to the endoplasmic reticulum and catalyze the formation of beta-d-glucopyranosiduronic acids (glucuronides) using UDP-glucuronic acid and acceptor substrates such as drugs, steroids, bile acids, xenobiotics, and dietary nutrients. Recent biochemical evidence indicates that the UGT proteins may oligomerize in the membrane, but conclusive evidence is still lacking. In the present study, we have used fluorescence resonance energy transfer (FRET) to study UGT1A oligomerization in live cells. This technique demonstrated that UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT1A8, UGT1A9, and UGT1A10 self-oligomerize (homodimerize). Heterodimer interactions were also explored, and it was determined that UGT1A1 was capable of binding with UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT1A8, UGT1A9, and UGT1A10. In addition to the in vivo FRET analysis, UGT1A protein-protein interactions were demonstrated through co-immunoprecipitation experiments. Co-expression of hemagglutinin-tagged and cyan fluorescent protein-tagged UGT1A proteins, followed by immunoprecipitation with anti-hemagglutinin beads, illustrated the potential of each UGT1A protein to homodimerize. Co-immunoprecipitation results also confirmed that UGT1A1 was capable of forming heterodimer complexes with all of the UGT1A proteins, corroborating the FRET results in live cells. These preliminary studies suggest that the UGT1A family of proteins form oligomerized complexes in the membrane, a property that may influence function and substrate selectivity.

The liver X-receptor alpha controls hepatic expression of the human bile acid-glucuronidating UGT1A3 enzyme in human cells and transgenic mice

Glucuronidation, an important bile acid detoxification pathway, is catalyzed by enzymes belonging to the UDP-glucuronosyltransferase (UGT) family. Among UGT enzymes, UGT1A3 is considered the major human enzyme for the hepatic C24-glucuronidation of the primary chenodeoxycholic (CDCA) and secondary lithocholic (LCA) bile acids. We identify UGT1A3 as a positively regulated target gene of the oxysterol-activated nuclear receptor liver X-receptor alpha (LXRalpha). In human hepatic cells and human UGT1A transgenic mice, LXRalpha activators induce UGT1A3 mRNA levels and the formation of CDCA-24glucuronide (24G) and LCA-24G. Furthermore, a functional LXR response element (LXRE) was identified in the UGT1A3 promoter by site-directed mutagenesis, electrophoretic mobility shift assays and chromatin immunoprecipitation experiment. In addition, LXRalpha is found to interact with the SRC-1alpha and NCoR cofactors to regulate the UGT1A3 gene, but not with PGC-1beta. In conclusion, these observations establish LXRalpha as a crucial regulator of bile acid glucuronidation in humans and suggest that accumulation of oxysterols in hepatocytes during cholestasis favors bile acid detoxification as glucuronide conjugates. LXR agonists may be useful for stimulating both bile acid detoxification and cholesterol removal in cholestatic or hypercholesterolemic patients, respectively.

Uridine diphosphoglucuronosyltransferase pharmacogenetics and cancer

The uridine diphosphoglucuronosyltransferases (UGTs) belong to a superfamily of enzymes that catalyse the glucuronidation of numerous endobiotics and xenobiotics. Several human hepatic and extrahepatic UGT isozymes have been characterized with respect to their substrate specificity, tissue expression and gene structure. Genetic polymorphisms have been identified for almost all the UGT family members. A wide variety of anticancer drugs, dietary chemopreventives and carcinogens are known to be conjugated by members of both UGT1A and UGT2B subfamilies. This review examines in detail each UGT isozyme known to be associated with cancer and carcinogenesis. The cancer-related substrates for several UGTs are summarized, and the functionally relevant genetic polymorphisms of UGTs are reviewed. A number of genotype-phenotype association studies have been carried out to characterize the role of UGT pharmacogenetics in several types of cancer, and these examples are discussed here. In summary, this review focuses on the role of the human UGT genetic polymorphisms in carcinogenesis, chemoprevention and cancer risk.

The farnesoid X receptor: a novel drug target?

Bile acids are end products of cholesterol metabolism. They are exclusively synthesised by the liver and subsequently secreted via the bile duct into the intestine to facilitate the absorption of dietary fat and fat-soluble vitamins. Nuclear receptors are ligand-activated transcription factors. The farnesoid X receptor (FXR) has recently been identified as a bile acid-activated nuclear receptor. FXR controls bile-acid synthesis, conjugation and transport, as well as lipid metabolism. Recent advances in FXR biology demonstrate that FXR may represent a valuable target for the identification of novel drugs to treat dyslipidaemia and cholestasis. However, for therapeutic purposes the development of selective FXR modulators, which only activate or inhibit specific FXR target genes and as such induce specific responses, will be required.

The Farnesoid X receptor: a molecular link between bile acid and lipid and glucose metabolism

Bile acids are the end products of cholesterol metabolism. They are synthesized in the liver and secreted via bile into the intestine, where they aid in the absorption of fat-soluble vitamins and dietary fat. Subsequently, bile acids return to the liver to complete their enterohepatic circulation. The Farnesoid X receptor (FXR) is a member of the nuclear receptor superfamily and has emerged as a key player in the control of multiple metabolic pathways. On its activation by bile acids, FXR regulates bile acid synthesis, conjugation, and transport, as well as various aspects of lipid and glucose metabolism. This review summarizes recent advances in deciphering the role of FXR in the context of hepatic lipid and glucose homeostasis and discusses the potential of FXR as a pharmacological target for therapeutic applications.

Glucuronidation and the UDP-glucuronosyltransferases in health and disease

This article is an updated report of a symposium held at the June 2000 annual meeting of the American Society for Pharmacology and Experimental Therapeutics in Boston. The symposium was sponsored by the ASPET Divisions for Drug Metabolism and Molecular Pharmacology. The report covers research from the authors' laboratories on the structure and regulation of UDP-glucuronosyltransferase (UGT) genes, glucuronidation of xenobiotics and endobiotics, the toxicological relevance of UGTs, the role of UGT polymorphisms in cancer susceptibility, and gene therapy for UGT deficiencies.

Pharmacogenomics of human UDP-glucuronosyltransferase enzymes

UDP-glucuronosyltransferase (UGT) enzymes comprise a superfamily of key proteins that catalyze the glucuronidation reaction on a wide range of structurally diverse endogenous and exogenous chemicals. Glucuronidation is one of the major phase II drug-metabolizing reactions that contributes to drug biotransformation. This biochemical process is also involved in the protection against environmental toxicants, carcinogens, dietary toxins and participates in the homeostasis of numerous endogenous molecules, including bilirubin, steroid hormones and biliary acids. Over the years, significant progress was made in the field of glucuronidation, especially with regard to the identification of human UGTs, study of their tissue distribution and substrate specificities. More recently, the degree of allelic diversity has also been revealed for several human UGT genes. Some polymorphic UGTs have demonstrated a significant pharmacological impact in addition to being relevant to drug-induced adverse reactions and cancer susceptibility. This review focuses on human UGTs, the description of the nature of polymorphic variations and their functional impact. The pharmacogenomic implication of polymorphic UGTs is presented, more specifically the role of UGT polymorphisms in modifying cancer risk and their impact on individual risk to drug-induced toxicities.

FXR induces the UGT2B4 enzyme in hepatocytes: a potential mechanism of negative feedback control of FXR activity

BACKGROUND & AIMS

Bile acids are essential for bile formation and intestinal absorption of lipids and fat-soluble vitamins. However, the intrinsic toxicity of hydrophobic bile acids demands a tight control of their intracellular concentrations. Bile acids are ligands for the farnesoid X receptor (FXR) that regulates the expression of genes controlling bile acid synthesis and transport. The human uridine 5'-diphosphate-glucuronosyltransferase 2B4 (UGT2B4) converts hydrophobic bile acids into more hydrophilic glucuronide derivatives. In this study, we identify UGT2B4 as an FXR target gene.

METHODS

Human hepatocytes or hepatoblastoma HepG2 cells were treated with chenodeoxycholic acid or the synthetic FXR agonist GW4064, and the levels of UGT2B4 messenger RNA, protein, and activity were determined by using real-time polymerase chain reaction, Western blot, and glucuronidation assays.

RESULTS

Treatment of hepatocytes and HepG2 cells with FXR agonists resulted in an increase of UGT2B4 messenger RNA, protein, and activity. A bile acid response element in the UGT2B4 promoter (B4-BARE) to which FXR, but not retinoid X receptor, binds, was identified by site-directed mutagenesis, electromobility shift, and chromatin immunoprecipitation assays. Retinoid X receptor activation abolished the induction of UGT2B4 expression and inhibited binding of FXR to the B4-BARE, suggesting that retinoid X receptor modulates FXR target gene activation. Overexpression of UGT2B4 in HepG2 cells resulted in the attenuation of bile acid induction of the FXR target gene small heterodimeric partner.

CONCLUSIONS

These data suggest that UGT2B4 gene induction by bile acids contributes to a feed-forward reduction of bile acid toxicity and a decrease of the activity of these biological FXR activators.

The cynomolgus monkey (Macaca fascicularis) is the best animal model for the study of steroid glucuronidation

Intense research efforts performed during the past decade clearly established the major role of glucuronidation and uridine-diphospho-glucuronosyltransferase (UGT) enzymes for steroid metabolism in humans. However, a clear understanding of the physiological importance of this metabolic process requires in vivo studies. Numerous evidences ascertain that simians are the most appropriate animal models for such studies. Indeed human and monkey have a similar pattern of steroidogenesis, unlike common laboratory mammals such as rat or mouse. Furthermore, human and monkey are unique in having high levels of circulating androsterone glucuronide and androstane-3alpha-diol glucuronide (3alpha-Diol-G). In addition, characterization of eight monkey UGT proteins demonstrated the similarity of their conjugation activity toward steroid hormones. Like human ones, monkey enzymes are expressed in steroid target tissues, where they preferentially glucuronidate androgen and estrogen metabolites. In monkey tissues, immunohistochemical studies demonstrated that UGT2B proteins are expressed in a cell-type specific manner in ovary and kidney, where they control androgens and aldosterone inactivation. These results identify the cynomolgus monkey as an appropriate animal model for the determination of cellular localization of UGT enzymes in steroid target tissues and for the identification of endogenous or exogenous stimuli affecting steroid glucuronidation.

Isolation and characterization of the monkey UGT2B30 gene that encodes a uridine diphosphate-glucuronosyltransferase enzyme active on mineralocorticoid, glucocorticoid, androgen and oestrogen hormones

The present study reports the genomic organization and the characterization of a novel cynomolgus monkey UDP-glucuronosyltransferase (UGT) enzyme, UGT2B30. UGT enzymes are microsomal proteins that catalyse the transfer of the glucuronosyl group from UDP-glucuronic acid (UDPGA) to a wide variety of lipophilic compounds, namely hormonal steroids. The 15 kb UGT2B30 gene amplified by PCR showed a genomic organization similar to those encoding UGT2B human enzymes. The cDNA encoding UGT2B30 was isolated from a cynomolgus monkey prostate cDNA library, and the deduced amino acid sequence showed an identity of 94% with UGT2B19, a monkey isoform previously characterized. Stable expression of UGT2B30 protein in human kidney 293 (HK293) cells was assessed by Western-blot analysis and its conjugating activity was screened using 39 potential substrates. The UGT2B30 enzyme is active on many compounds of different classes, including testosterone, dihydrotestosterone, 5alpha-androstane-3alpha,17beta-diol, androsterone, oestradiol, tetrahydroaldosterone and tetrahydrocortisone, with glucuronidation efficiencies (V(max)/K(m) ratios) ranging from 0.6 to 8.8 microl x min(-1) x mg of protein(-1). Reverse-transcriptase-PCR analysis revealed that the UGT2B30 transcript is expressed in several tissues, including prostate, testis, mammary gland, kidney, adrenals and intestine. The relative activity of UGT2B30 in comparison with other simian UGT2B isoforms, as well as its large variety of substrates, strongly suggest that this enzyme is essential to inactivation of several steroids.

The androgen-conjugating uridine diphosphoglucuronosyltransferase-2B enzymes are differentially expressed temporally and spatially in the monkey follicle throughout the menstrual cycle

UDP-glucuronosyltransferase (UGT) enzymes enhance the polarity of steroid hormones by catalyzing their conjugation with the sugar group from UDP-glucuronic acid. Previous results have shown that the monkey is a suitable animal model to study steroid glucuronidation in steroid target tissues. In humans, as in the monkey, the main androgen metabolites found in the circulation are 5alpha-androstane-3alpha,17beta-diol-glucuronide and androsterone glucuronide, and high levels of androsterone glucuronide were also measured in human follicular fluid. Ovarian androgens play a significant role as precursors for estrogens and may modulate the recruitment and growth of follicles. To analyze the expression pattern of UGT2B enzymes involved in androgen metabolism throughout the menstrual cycle, cynomolgus monkey ovaries were collected during the mid and late follicular and luteal phases. Microsomal proteins and total RNA were analyzed for UGT2B expression in the whole ovary. Western blot and specific RT-PCR analyses demonstrated no significant changes in the expression of UGT2B protein or transcripts during the menstrual cycle. Immunocytochemistry analysis showed that UGT2B proteins are expressed in the cytoplasm of thecal and granulosa cells of growing follicles. Interestingly, the thecal cells of secondary follicles and of corpus luteum were extensively stained, whereas luteal granulosa cells were not labeled. These results suggest an important regulation of cell type-specific UGT2B expression during follicular development. Previous results demonstrated similar changes in the expression of the androgen receptor. The colocalization of the androgen receptor and UGT2B enzymes in the same cell types of the ovary provide evidence for a potential role of glucuronidation as a modulator of the intracellular androgen response during follicular development.

Tissue specific differences in the regulation of the UDP glucuronosyltransferase 2B17 gene promoter

The human UDP glucuronosyltransferase UGT2B17, glucuronidates androgens and is expressed in the liver and the prostate. Although evidence suggests that variations in UGT2B17 expression between tissues may be a critical determinant of androgen response, the factors that regulate UGT2B17 expression in the liver and prostate are unknown. In this study, we have isolated a 596 bp promoter of the UGT2B17 gene and studied its regulation in the liver cell line, HepG2 and the prostate cell line, LNCaP. The transcription start site of UGT2B17 was mapped and proteins that bound to the proximal promoter were detected by DNase1 footprint analysis. A region (-40 to -52 bp) which resembled a hepatocyte nuclear factor 1 (HNF1) binding site bound proteins in nuclear extracts from HepG2 cells, but did not bind proteins from LNCaP nuclear extracts. In HepG2 cells, HNF1alpha bound to this region and activated the UGT2B17 promoter, as assessed by functional and gel shift assays. HNF1alpha activation of the promoter was prevented by mutation or deletion of the putative HNF1 site. The related transcription factor HNF1beta, which is present in HepG2 cells, did not activate the promoter. The UGT2B17 promoter could also be activated by exogenous HNF1alpha in LNCaP cells. However, because these cells do not contain HNF1alpha, other transcription factors must regulate the UGT2B17 promoter. Cotransfection experiments showed that HNF1beta, elevates promoter activity in LNCaP cells. This activation did not involve the putative HNF1 region (-40 to -52 bp) since mutation of this region did not affect promoter activation by HNF1beta. These results suggest that the UGT2B17 promoter is regulated by different factors in liver-derived HepG2 and prostate-derived LNCaP cells.

Human UDP-glucuronosyltransferases: metabolism, expression, and disease

In vertebrates, the glucuronidation of small lipophilic agents is catalyzed by the endoplasmic reticulum UDP-glucuronosyltransferases (UGTs). This metabolic pathway leads to the formation of water-soluble metabolites originating from normal dietary processes, cellular catabolism, or exposure to drugs and xenobiotics. This classic detoxification process, which led to the discovery nearly 50 years ago of the cosubstrate UDP-glucuronic acid (19), is now known to be carried out by 15 human UGTs. Characterization of the individual gene products using cDNA expression experiments has led to the identification of over 350 individual compounds that serve as substrates for this superfamily of proteins. This data, coupled with the introduction of sophisticated RNA detection techniques designed to elucidate patterns of gene expression of the UGT superfamily in human liver and extrahepatic tissues of the gastrointestinal tract, has aided in understanding the contribution of glucuronidation toward epithelial first-pass metabolism. In addition, characterization of the UGT1A locus and genetic studies directed at understanding the role of bilirubin glucuronidation and the biochemical basis of the clinical symptoms found in unconjugated hyperbilirubinemia have uncovered the structural gene polymorphisms associated with Crigler-Najjar's and Gilbert's syndrome. The role of the UGTs in metabolism and different disease states in humans is the topic of this review.

Isolation and characterization of the human UGT2B7 gene

Glucuronidation is a major pathway involved in the metabolism of drugs and numerous endogenous compounds, such as bile acids and steroid hormones. The enzymes responsible for this conjugation reaction are UDP-glucuronosyltransferases (UGT). Among the UGT2B subfamily, UGT2B7, a UGT enzyme present in the liver and several steroid target tissues, is an important member since it conjugates a large variety of compounds including estrogens, androgens, morphine, AZT, and retinoic acid. Although this enzyme is well characterized, the gene encoding the UGT2B7 protein and its promoter region remain unknown. In this article, we report the genomic organization and the promoter region of the human UGT2B7 gene. To isolate this gene, a P-1 artificial chromosome (PAC) library was screened with a full length UGT2B7 probe and a clone of approximately 100 kb in length was isolated. In addition to the UGT2B7 gene, this PAC contains two other UGT2B genes previously characterized, namely UGT2B26P and UGT2B27P. The UGT2B7 gene is composed of six exons spanning approximately 16 kb, with introns ranging from 0.7 to 4.2 kb. The 5'-flanking region of the human UGT2B7 gene contains several potential cis-acting elements such as Oct-1, Pbx-1, and C/EBP. Only one TATA-box at nucleotide -106 was found within the first 500 nucleotides relative to the adenine base of the initiator ATG codon. Characterization of the UGT2B7 gene provides insight into the organization and regulation of this important metabolic gene.

Genomic organization of the UGT2b gene cluster on human chromosome 4q13

The UDP-glucuronosyltransferases (UGTs) comprise a large family of proteins capable of detoxifying a wide variety of both endogenous and exogenous substrates. The primary function of this gene superfamily is to catalyze the glycosylation of substrates such as biogenic amines, steroids, bile acids, phenolic compounds and various other pharmacologically relevant compounds, including numerous carcinogens, toxic environmental pollutants and prescription drugs. This conjugation increases the solubility of these compounds, allowing them to be excreted more readily through hepatic or renal mechanisms. This paper describes the genomic characterization and chromosomal localization of three UGT2B genes which together comprise part of a large cluster of related sequences, including pseudogenes found on human chromosome 4q13. A genomic map spanning approximately 500-1000 kb of this region reveals the presence of three previously described UGT2B genes, at least two previously uncharacterized pseudogenes and a significant number of remnant gene fragments and places UGT2B4 between UGT2B7 and UGT2B15. Additionally, access to a large reference DNA bank allowed us to calculate allele frequencies for two UGT2B SNPs: D85R in UGT2B15 and Q458D in UGT2B4 amongst 803 unrelated individuals representing five ethnic populations. The data presented here suggest a recent evolutionary history of gene duplication, mutation and rearrangement. Furthermore, they suggest that a re-evaluation of the current description of the UGT2B gene family with respect to the number of specific genes, degree of allelic diversity and molecular evolution may be necessary.

Isolation and characterization of the human UGT2B15 gene, localized within a cluster of UGT2B genes and pseudogenes on chromosome 4

Glucuronidation is a major pathway of androgen metabolism and is catalyzed by UDP-glucuronosyltransferase (UGT) enzymes. UGT2B15 and UGT2B17 are 95% identical in primary structure, and are expressed in steroid target tissues where they conjugate C19 steroids. Despite the similarities, their regulation of expression are different; however, the promoter region and genomic structure of only the UGT2B17 gene have been characterizedX to date. To isolate the UGT2B15 gene and other novel steroid-conjugating UGT2B genes, eight P-1-derived artificial chromosomes (PAC) clones varying in length from 30 kb to 165 kb were isolated. The entire UGT2B15 gene was isolated and characterized from the PAC clone 21598 of 165 kb. The UGT2B15 and UGT2B17 genes are highly conserved, are both composed of six exons spanning approximately 25 kb, have identical exon sizes and have identical exon-intron boundaries. The homology between the two genes extend into the 5'-flanking region, and contain several conserved putative cis-acting elements including Pbx-1, C/EBP, AP-1, Oct-1 and NF/kappaB. However, transfection studies revealed differences in basal promoter activity between the two genes, which correspond to regions containing non-conserved potential elements. The high degree of homology in the 5'-flanking region between the two genes is lost upstream of -1662 in UGT2B15, and suggests a site of genetic recombination involved in duplication of UGT2B genes. Fluorescence in situ hybridization mapped the UGT2B15 gene to chromosome 4q13.3-21.1. The other PAC clones isolated contain exons from the UGT2B4, UGT2B11 and UGT2B17 genes. Five novel exons, which are highly homologous to the exon 1 of known UGT2B genes, were also identified; however, these exons contain premature stop codons and represent the first recognized pseudogenes of the UGT2B family. The localization of highly homologous UGT2B genes and pseudogenes as a cluster on chromosome 4q13 reveals the complex nature of this gene locus, and other novel homologous UGT2B genes encoding steroid conjugating enzymes are likely to be found in this region of the genome.

Expression of UDP-glucuronosyltransferases (UGTs) 2B7 and 1A6 in the human brain and identification of 5-hydroxytryptamine as a substrate

The extrahepatic expression of UDP-glucuronosyltransferases (UGTs) is important in the detoxification of a number of endogenous and exogenous compounds, including 5-hydroxytryptamine and morphine. Studies were designed to investigate the extrahepatic expression of human UGTs using RT-PCR techniques and to determine the UGTs involved in the glucuronidation of 5-hydroxytryptamine. Human UGT2B7 expression was found in the human liver, kidney, pancreas, and brain, while UGT1A6 expression is found in the liver, kidney, and brain. This is the first observation of UGTs present in the human central nervous system. Using glucuronidation assays, a significant amount of 5-hydroxytryptamine glucuronide was found to be catalyzed by UGT1A6. These studies suggest that UGT2B7 may play an important role in the overall contribution of morphine analgesia by serving to generate the potent morphine-6-O-glucuronide in situ. UGT1A6 could play an important role in the glucuronidation of 5-hydroxytryptamine in vivo, therefore terminating the actions of the neurotransmitter.