Expression of a functionally active human hepatic UDP-glucuronosyltransferase (UGT1A6) lacking the N-terminal signal sequence in the endoplasmic reticulum

Characterization of a membrane-bound C-glucosyltransferase responsible for carminic acid biosynthesis in Dactylopius coccus Costa

Carminic acid, a glucosylated anthraquinone found in scale insects like Dactylopius coccus, has since ancient times been used as a red colorant in various applications. Here we show that a membrane-bound C-glucosyltransferase, isolated from D. coccus and designated DcUGT2, catalyzes the glucosylation of flavokermesic acid and kermesic acid into their respective C-glucosides dcII and carminic acid. DcUGT2 is predicted to be a type I integral endoplasmic reticulum (ER) membrane protein, containing a cleavable N-terminal signal peptide and a C-terminal transmembrane helix that anchors the protein to the ER, followed by a short cytoplasmic tail. DcUGT2 is found to be heavily glycosylated. Truncated DcUGT2 proteins synthesized in yeast indicate the presence of an internal ER-targeting signal. The cleavable N-terminal signal peptide is shown to be essential for the activity of DcUGT2, whereas the transmembrane helix/cytoplasmic domains, although important, are not crucial for its catalytic function.

Carminic acid is a widely applied red colorant that is still harvested from insects because its biosynthesis is not fully understood. Here, the authors identify and characterize a membrane-bound C-glucosyltransferase catalyzing the final step during carminic acid biosynthesis.

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.

Transcript expression bias of phosphatidylethanolamine binding protein gene in bumblebee, Bombus lantschouensis (Hymenoptera: Apidae)

The phosphatidylethanolamine-binding protein (PEBP) family is a highly conserved group of proteins found in a wide range of organism. It plays an important role in innate immunity of insects. Little is known on the expression characteristic and function of PEBP in bees. In the current study, we cloned the pebp gene and investigated its expression profiles at different developmental stages and reproductive status from bumblebee, Bombus lantschouensis (Vogt), which is one of the most abundant pollinators for wild plants and crops in Northern China. Two transcripts (PEBPX1 and PEBPX2) of the pebp gene were cloned for the first time. The transcript PEBPX2 lacked a signal peptide sequence compared to PEBPX1. The full-length cDNA of these two PEBP transcripts is 1005bp and 915bp, with an open reading frame of 627bp and 549bp, respectively. Transcript PEBPX2 was one order of magnitude more expressed than transcript PEBPX1 at most of the developmental stages and different reproductive status (egg-laying versus non- egg-laying females). Both of the PEBP transcripts were highly expressed in brown-eyed with light and dark pigmented cuticle pupae stages. Quantitative PCR and Western Blot demonstrated that PEBP was significantly up-regulated in egg-laying females. In summary, we suggest that levels of these two PEBPs could be related to the regulation of reproduction in bumblebees. In addition, both transcripts likely play an important role in the metamorphosis developmental stage of bumblebee pupae.

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.

Cellular asymmetric catalysis by UDP-glucuronosyltransferase 1A8 shows functional localization to the basolateral plasma membrane

UDP-glucuronosyltransferases (UGTs) are highly expressed in liver, intestine and kidney, and catalyze the glucuronic acid conjugation of both endogenous compounds and xenobiotics. Using recombinant human UGT isoforms, we show that glucuronic acid conjugation of the model substrate, (-)-epicatechin, is catalyzed mainly by UGT1A8 and UGT1A9. In HepG2 cells, pretreatment with polyunsaturated fatty acids increased substrate glucuronidation. In the intestinal Caco-2/HT29-MTX co-culture model, overall relative glucuronidation rates were much higher than in HepG2 cells, and (-)-epicatechin was much more readily conjugated when applied to the basolateral side of the cell monolayer. Under these conditions, 95% of the conjugated product was effluxed back to the site of application, and none of the other phase 2-derived metabolites followed this distribution pattern. HT29-MTX cells contained >1000-fold higher levels of UGT1A8 mRNA than Caco-2 or HepG2 cells. Gene expression of UGT1A8 increased after treatment of cells with docosahexaenoic acid, as did UGT1A protein levels. Immunofluorescence staining and Western blotting showed the presence of UGT1A in the basal and lateral parts of the plasma membrane of HT29-MTX cells. These results suggest that some of the UGT1A8 enzyme is not residing in the endoplasmic reticulum but spans the plasma membrane, resulting in increased accessibility to compounds outside the cell. This facilitates more efficient conjugation of substrate and is additionally coupled with rapid efflux by functionally associated basolateral transporters. This novel molecular strategy allows the cell to carry out conjugation without the xenobiotic entering into the interior of the cell.

The crystal structure of human UDP-glucuronosyltransferase 2B7 C-terminal end is the first mammalian UGT target to be revealed: the significance for human UGTs from both the 1A and 2B families

Human UDP-glucuronosyltransferases (EC 2.4.1.17) (UGTs) are major phase II metabolism enzymes that detoxify a multitude of endo- and xenobiotics through the covalent addition of a glucuronic acid moiety. UGTs are promiscuous enzymes that regulate the levels of numerous important endobiotics in a range of tissues, and inactivate most therapeutic compounds in concert with phase I enzymes. In spite of the importance of these enzymes, we have only a limited understanding of the molecular mechanisms governing their substrate specificity and catalytic activity. Until recently, no three-dimensional structural information was available for any mammalian UGT. The 1.8-å resolution apo crystal structure of the UDP-glucuronic acid binding domain of human UGT2B7 (2B7CT) is the only structure of a mammalian UGT target determined to date. In this review, we summarize what has been learned about human UGT function from the analysis of this and other related glycosyltransferase (GT) crystal structures.

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.

Epigenetics: methylation-associated repression of heparan sulfate 3-O-sulfotransferase gene expression contributes to the invasive phenotype of H-EMC-SS chondrosarcoma cells

Heparan sulfate proteoglycans (HSPGs), strategically located at the cell-tissue-organ interface, regulate major biological processes, including cell proliferation, migration, and adhesion. These vital functions are compromised in tumors, due, in part, to alterations in heparan sulfate (HS) expression and structure. How these modifications occur is largely unknown. Here, we investigated whether epigenetic abnormalities involving aberrant DNA methylation affect HS biosynthetic enzymes in cancer cells. Analysis of the methylation status of glycosyltransferase and sulfotransferase genes in H-HEMC-SS chondrosarcoma cells showed a typical hypermethylation profile of 3-OST sulfotransferase genes. Exposure of chondrosarcoma cells to 5-aza-2'-deoxycytidine (5-Aza-dc), a DNA-methyltransferase inhibitor, up-regulated expression of 3-OST1, 3-OST2, and 3-OST3A mRNAs, indicating that aberrant methylation affects transcription of these genes. Furthermore, HS expression was restored on 5-Aza-dc treatment or reintroduction of 3-OST expression, as shown by indirect immunofluorescence microscopy and/or analysis of HS chains by anion-exchange and gel-filtration chromatography. Notably, 5-Aza-dc treatment of HEMC cells or expression of 3-OST3A cDNA reduced their proliferative and invading properties and augmented adhesion of chondrosarcoma cells. These results provide the first evidence for specific epigenetic regulation of 3-OST genes resulting in altered HSPG sulfation and point to a defect of HS-3-O-sulfation as a factor in cancer progression.

Phylogenetic and mutational analyses reveal key residues for UDP-glucuronic acid binding and activity of beta1,3-glucuronosyltransferase I (GlcAT-I)

The beta1,3-glucuronosyltransferases are responsible for the completion of the protein-glycosaminoglycan linkage region of proteoglycans and of the HNK1 epitope of glycoproteins and glycolipids by transferring glucuronic acid from UDP-alpha-D-glucuronic acid (UDP-GlcA) onto a terminal galactose residue. Here, we develop phylogenetic and mutational approaches to identify critical residues involved in UDP-GlcA binding and enzyme activity of the human beta1,3-glucuronosyltransferase I (GlcAT-I), which plays a key role in glycosaminoglycan biosynthesis. Phylogeny analysis identified 119 related beta1,3-glucuronosyltransferase sequences in vertebrates, invertebrates, and plants that contain eight conserved peptide motifs with 15 highly conserved amino acids. Sequence homology and structural information suggest that Y84, D113, R156, R161, and R310 residues belong to the UDP-GlcA binding site. The importance of these residues is assessed by site-directed mutagenesis, UDP affinity and kinetic analyses. Our data show that uridine binding is primarily governed by stacking interactions with the phenyl group of Y84 and also involves interactions with aspartate 113. Furthermore, we found that R156 is critical for enzyme activity but not for UDP binding, whereas R310 appears less important with regard to both activity and UDP interactions. These results clearly discriminate the function of these two active site residues that were predicted to interact with the pyrophosphate group of UDP-GlcA. Finally, mutation of R161 severely compromises GlcAT-I activity, emphasizing the major contribution of this invariant residue. Altogether, this phylogenetic approach sustained by biochemical analyses affords new insight into the organization of the beta1,3-glucuronosyltransferase family and distinguishes the respective importance of conserved residues in UDP-GlcA binding and activity of GlcAT-I.

Evidence of calcium‐dependent pathway in the regulation of human β1,3‐glucuronosyltransferase‐1 (GlcAT‐I) gene expression: a key enzyme in proteoglycan synthesis

The importance of heparan- and chondroitin-sulfate proteoglycans in physiological and pathological processes led to the investigation of the regulation of beta1,3-glucuronosyltransferase I (GlcAT-I), responsible for the completion of glycosaminoglycan-protein linkage tetrasaccharide, a key step prior to polymerization of chondroitin- and heparan-sulfate chains. We have cloned and functionally characterized GlcAT-I 5'-flanking regulatory region. Mutation analysis and electrophoretic mobility shift assays demonstrated the importance of Sp1 motif located at -65/-56 position in promoter activity. Furthermore, we found that elevation of intracellular calcium concentration by the calcium ionophore ionomycin stimulated GlcAT-I gene expression as well as glycosaminoglycan chain synthesis in HeLa cells. Bisanthracycline, an anti-Sp1 compound, inhibited GlcAT-I basal promoter activity and suppressed ionomycin induction, suggesting the importance of Sp1 in calcium induction of GlcAT-I gene expression. Nuclear protein extracts from ionomycin-induced cells exhibited an increased DNA binding of Sp1 factor to the consensus sequence at position -65/-56. Signaling pathway analysis and MEK inhibition studies revealed the important role of p42/p44 MAPK in the stimulation of GlcAT-I promoter activity by ionomycin. The present study identifies, for the first time, GlcAT-I as a target of calcium-dependent signaling pathway and evidences the critical role of Sp1 transcription factor in the activation of GlcAT-I expression.

The stop transfer sequence of the human UDP-glucuronosyltransferase 1A determines localization to the endoplasmic reticulum by both static retention and retrieval mechanisms

Human UDP-glucuronosyltransferase 1A (UGT1A) isoforms are endoplasmic reticulum (ER)-resident type I membrane proteins responsible for the detoxification of a broad range of toxic phenolic compounds. These proteins contain a C-terminal stop transfer sequence with a transmembrane domain (TMD), which anchors the protein into the membrane, followed by a short cytosolic tail (CT). Here, we investigated the mechanism of ER residency of UGT1A mediated by the stop transfer sequence by analysing the subcellular localization and sensitivity to endoglycosidases of chimeric proteins formed by fusion of UGT1A stop transfer sequence (TMD/CT) with the ectodomain of the plasma membrane CD4 reporter protein. We showed that the stop transfer sequence, when attached to C-terminus of the CD4 ectodomain was able to prevent it from being transported to the cell surface. The protein was retained in the ER indicating that this sequence functions as an ER localization signal. Furthermore, we demonstrated that ER localization conferred by the stop transfer sequence was mediated in part by the KSKTH retrieval signal located on the CT. Interestingly, our data indicated that UGT1A TMD alone was sufficient to retain the protein in ER without recycling from Golgi compartment, and brought evidence that organelle localization conferred by UGT1A TMD was determined by the length of its hydrophobic core. We conclude that both retrieval mechanism and static retention mediated by the stop transfer sequence contribute to ER residency of UGT1A proteins.

UDP Glucuronosyltransferase (UGT) 1A6 Pharmacogenetics: II. Functional Impact of the Three Most Common Nonsynonymous UGT1A6 Polymorphisms (S7A, T181A, and R184S)

The objective of this study was to use recombinant enzymes and human liver microsomes (HLMs) to comprehensively evaluate the functional impact of the three most common nonsynonymous polymorphisms (S7A, T181A, and R184S) identified in the human UDP glucuronosyltransferase (UGT) 1A6 gene. In addition to the known allozymes, other possible amino acid variants were expressed in human embryonic kidney (HEK)293 cells to enable structure-function analysis. Initial studies using different substrates (serotonin, 5-hydroxytryptophol, 4-nitrophenol, acetaminophen, and valproic acid) showed similar results with 2-fold higher glucuronidation by UGT1A6(*)2 (S7A/T181A/R184S) compared with UGT1A6(*)1 (reference), and intermediate activities for other variants. Enzyme kinetic analyses with the UGT1A6-specific substrate (serotonin) showed 50% lower K(m) values for all R184S variants and 2-fold higher V(max) values for both S7A/T181A variants compared with UGT1A6(*)1. Furthermore, intrinsic clearance (V(max)/K(m)) values were highest for the UGT1A6(*)2 allozyme (2.3-fold over UGT1A6(*)1), resulting from additive effects of higher enzyme affinity and activity. As expected, K(m) values of (*)1/(*)1 genotyped HLMs (5.4 +/- 0.2 mM) were similar to recombinant UGT1A6(*)1 (5.8 +/- 0.6 mM). Conversely, (*)2/(*)2 HLMs showed higher K(m) values (7.0 +/- 0.3 mM) rather than the lower K(m) values displayed by recombinant UGT1A6(*)2 (3.6 +/- 0.3 mM), suggesting that this allozyme may display different enzyme kinetic behavior in HLMs compared with HEK293 cells. At best, these polymorphisms were predicted to account for 15 to 20% of the observed 13-fold variability in glucuronidation of UGT1A6 substrates by HLMs, indicating that there are likely other genetic or environmental factors responsible for the majority of this variation.

Structure of UDP‐Glucuronosyltransferases in Membranes

This chapter presents the most recent experimental approaches to the investigation of UDP-glucuronosyltransferase (UGTs) in membranes. The first topic described is the subcellular localization of UGTs with special emphasis on the association of these proteins with the endoplasmic reticulum (ER). Experimental methods include subfractionation of tissue for microsome preparation, evaluation of the purity of the membrane fraction obtained, and measurement of UGT activity in the presence of detergents. Next, the recently demonstrated formation of UGT homo- and heterodimer formation and its functional relevance is discussed and the appropriate methods used to characterize such interactions are given (radiation inactivation, size exclusion chromatography, immunopurification, cross-linking, two-hybrid system). The structural determinants of UGTs in relation to membrane association, residency, and enzymatic activity are the next topic, supplemented by a description of the appropriate methods, including the design and expression of chimeric proteins, membrane insertion, and subcellular localization by immunofluorescence. Also presented is new information on the structure and function of UGTs obtained by molecular modeling, bioinformatics (sequence alignment), and comparison with selected crystallized glycosyltransferases. Finally, we discuss the important, and still not fully developed, issue of UGT active site architecture and organization within the ER. This is addressed from two perspectives: (1) chemical modification of UGT active sites by amino acid-specific probes and (2) photoaffinity labeling of UGTs. The detailed synthesis of a photoaffinity probe for an aglycon-binding site is provided and the use of this probe and direct photoaffinity labeling with retinoids is discussed. The application of proteomics techniques, including proteolytic digestion and protein sequencing by liquid chromatography/tandem mass spectrometry and matrix-assisted laser desorption ionization/time of flight, to the identification of crucial amino acids of the active sites, and subsequent site-directed mutagenesis of identified amino acids, is discussed in detail.

Isolation and characterization of a UDP-glucuronosyltransferase (UGT1A01) cloned from female rhesus monkey

An isoform (rhesus UGT1A01) orthologus to the human UGT1A1 was cloned and sequenced from female rhesus monkey liver cDNA using primers designed from the human nucleotide sequences. Open reading frame analysis of the PCR-generated product encodes a 533-amino acid protein with a proposed 27-residue signal peptide. Nucleotide sequence comparison of rhesus UGT1A01 to other rhesus UGT1A isoforms detected a single-transition mutation at nucleotide 1520 (T-->C), resulting in a neutral F to S substitution at position 507. Rhesus UGT1A01 was greater than 99 and 95% identical to cynomolgus UGT1A01 and human UGT1A1, respectively. The rhesus UGT1A01 was expressed in HK-293 cells for functional analysis. Catalytic activity of UGT1A01 was determined with 7-hydroxy-4-(trifluoromethyl)-coumarin and more specific human UGT1A1 substrates (1-naphthol, beta-estradiol, 17 alpha-ethinylestradiol, and bilirubin). Expression of UGT1A01 protein was also detected by a Western blot utilizing a polyclonal antibody developed against the human UGT1A family.

The importance of cysteine 126 in the human liver UDP-glucuronosyltransferase UGT1A6

The human UDP-glucuronosyltransferase 1A6 (UGT1A6) isoform is actively involved in the detoxication of phenolic compounds. In an effort to gain insight on active-site amino acids, we investigated the functional relevance of cysteinyl residues in the glucuronidation process. The enzyme was irreversibly inactivated upon exposure to thiol-specific reagents, especially N-phenylmaleimide. Site-directed mutagenesis of the conserved Cys126 into valine led to a fully inactive mutant, whereas conservative substitution with serine significantly restored the glucuronidation activity toward 4-methylumbelliferone used as a reference substrate. This mutant exhibited a reduced affinity toward the acceptor substrate, as evidenced by a 10-times increase in K(m) value, compared to the wild-type enzyme. The two mutations did not alter the stability of UGT1A6 nor change the subcellular localization of the protein in the endoplasmic reticulum of recombinant cells. These results support the conclusion that Cys126 is an essential residue for the integrity of the substrate binding site of UGT1A6.

Importance of histidine residues for the function of the human liver UDP-glucuronosyltransferase UGT1A6: evidence for the catalytic role of histidine 370

The human UDP-glucuronosyltransferase isoform UGT1A6 catalyzes the nucleophilic attack of phenolic xenobiotics on glucuronic acid, leading to the formation of water-soluble glucuronides. Based on the irreversible inhibition of the enzyme activity by the histidyl-selective reagent diethyl pyrocarbonate (DEPC), histidine was suggested to play a key role in the glucuronidation reaction. Therefore, the role of four strictly conserved histidine residues (His38, His361, His370, and His485) in the glucuronidation of 4-methylumbelliferone, as reporter substrate, was examined using site-directed mutagenesis. For this purpose, stable heterologous expression of wild-type and mutant UGT1A6 was achieved in the yeast Pichia pastoris. Replacement of histidine residues by alanine or glutamine led to fully inactive H38A, H38Q, and H485A mutants. Substitution of His361 by alanine affected the interaction of the enzyme with the cosubstrate, as indicated by a 4-fold increase in the K(m) value toward UDP-glucuronic acid. Interestingly, H370A mutant presented a severely impaired catalytic efficiency (with a V(max) value approximately 5% that of the wild-type), whereas conservative substitution of His370 by glutamine (H370Q) led to a significant restoration of activity. Whereas H361A was inactivated by DEPC as the wild-type enzyme, this chemical reagent only produced a minor effect on either H370Q or H370A mutant, providing evidence that His370 is probably the reactive histidine residue targeted by DEPC. The dramatic changes in catalytic efficiency on substitution of His370 by alanine and the ability of glutamine to function in place of histidine along with a weak sensitivity of these mutants to DEPC strongly suggest that His370 plays a catalytic role in the glucuronidation reaction.