gamma-Glutamyl transpeptidase: a single copy gene in the rat and a multigene family in the human genome

Glutathione‑degrading enzymes in the complex landscape of tumors (Review)

Glutathione (GSH)‑degrading enzymes are essential for starting the first stages of GSH degradation. These enzymes include extracellular γ‑glutamyl transpeptidase (GGT) and intracellular GSH‑specific γ‑glutamylcyclotransferase 1 (ChaC1) and 2. These enzymes are essential for cellular activities, such as immune response, differentiation, proliferation, homeostasis regulation and programmed cell death. Tumor tissue frequently exhibits abnormal expression of GSH‑degrading enzymes, which has a key impact on the development and spread of malignancies. The present review summarizes gene and protein structure, catalytic activity and regulation of GSH‑degrading enzymes, their vital roles in tumor development (including regulation of oxidative and endoplasmic reticulum stress, control of programmed cell death, promotion of inflammation and tumorigenesis and modulation of drug resistance in tumor cells) and potential role as diagnostic biomarkers and therapeutic targets.

Gamma-Glutamyltransferase Activity (GGT) Is a Long-Sought Biomarker of Redox Status in Blood Circulation: A Retrospective Clinical Study of 44 Types of Human Diseases

Background and Aim. Redox equilibria are critical for life, but the biomarkers of redox status are currently unavailable. Gamma-glutamyltransferase (GGT) is an essential factor for modulating redox equilibrium through glutathione. In clinical practice, increased circulating GGT activity is used as a hepatobiliary disease biomarker. However, increased circulating GGT activities have also been observed in cancers, heart disease, diabetes, hyperuricemia, inflammation, renal insufficiency, and other diseases, explained by its role in maintaining redox equilibrium inside and outside cells. Previous studies on GGT were mainly limited to one type of disease at one time. In the current study, we systematically compared the GGT levels in 44 different human diseases to test if it could serve as a redox status biomarker in blood circulation. Methods. The clinical GGT data from 168,858 patients with 44 diseases and 132,357 healthy control in the clinical laboratory of our hospital over the past five years were retrieved. All data were analyzed with SPSS, RStudio V.1.3.1073, and python libraries 3.8. Results. Thirty-eight out of 44 diseases had significantly increased ( $p<0.001$ ) circulating GGT activities, whereas gastric cancer, anemia, renal cyst, cervical cancer, preeclampsia, and knee-joint degenerative diseases had significantly decreased ( $p<0.001$ ) GGT activities compared to the healthy control. ROC analyses showed that GGT was an excellent biomarker for liver cancer ( $\text{AUC}=0.86$ ), pancreatitis ( $\text{AUC}=0.84$ ), or hepatic encephalopathy ( $\text{AUC}=0.80$ ). All pancreas-related diseases had more than 8-fold increases in GGT activity span than the healthy control, while pancreatic cancer had a 12-fold increase (1021 U/L vs. 82 U/L). The knee-joint degenerative disease had the lowest median and narrowest GGT activity range (63 U/L). Furthermore, most diseases’ lowest to highest GGT activities were beyond the healthy control in both directions. Conclusions. Thirty-eight out of 44 diseases were in overall oxidative states defined by the increased GGT median values. In contrast, knee-joint degenerative disease, gastric cancer, anemia, renal cyst, cervical cancer, and preeclampsia were in overall antioxidative states. Moreover, most diseases swing between oxidative and antioxidative states, evidenced by the increased lowest to highest GGT activity ranges than the healthy control. Liver- and pancreas-related abnormalities were responsible for significantly increased GGT activities. Our overall results suggested that circulating GGT was a redox status biomarker.

Characterization of human FcεRIα chain expression and gene copy number in humanized rat basophilic leukaemia (RBL) reporter cell lines

Several laboratories have created rat basophil leukemia (RBL) cell lines stably transfected with the human high affinity IgE receptor (FcεRI_H). More recently, humanized RBL cell lines saw the introduction of reporter genes such as luciferase (RS-ATL8) and DsRed (RBL NFAT-DsRed). These reporters are more sensitive than their parental non-reporter humanized RBL cell lines. However, no studies so far have addressed the levels of FcεRI_H surface expression on humanized RBL cell lines. This is a critical parameter, as it determines the ability of these cells to be efficiently sensitized with human IgE, hence it should affect the sensitivity of the cell assay–a critical parameter for any diagnostic application. Our purpose was to assess and compare the levels of expression of the transfected FcεRI_H chain in humanized RBL cell lines. We compared surface levels of FcεRIα_H by flow cytometry, using a fluorescently labelled monoclonal antibody (CRA-1/AER-37) and determined receptor numbers using calibration microspheres. FcεRIα_H copy numbers were assessed by qPCR, and the sequence verified. Transfection with FcεRIγ_H cDNA was assessed for its ability to increase FcεRIα_H expression in the NFAT-DsRed reporter. While both SX-38 and RS-ATL8 expressed about 500.000 receptors/cell, RBL 703–21 and NFAT-DsRed had approximately 10- to 30-fold lower FcεRIα_H expression, respectively. This was neither related to FcεRI_H gene copy numbers, nor to differences in steady state mRNA levels, as determined by qPCR and RT-qPCR, respectively. Instead, FcεRIα_H surface expression appeared to correlate with the co-expression of FcεRIγ_H. Stable transfection of NFAT-DsRed cells with pBJ1 neo-huFcεRI gamma, which constitutively expresses FcεRIγ_H, increased FcεRIα_H chain expression levels. Levels of FcεRIα_H surface expression vary greatly between humanized RBL reporter cell lines. This difference will affect the sensitivity of the reporter system when used for diagnostic purposes.

Overexpression of Human-Derived DNMT3A Induced Intergenerational Inheritance of Active DNA Methylation Changes in Rat Sperm

DNA methylation is the major focus of studies on paternal epigenetic inheritance in mammals, but most previous studies about inheritable DNA methylation changes are passively induced by environmental factors. However, it is unclear whether the active changes mediated by variations in DNA methyltransferase activity are heritable. Here, we established human-derived DNMT3A (hDNMT3A) transgenic rats to study the effect of hDNMT3A overexpression on the DNA methylation pattern of rat sperm and to investigate whether this actively altered DNA methylation status is inheritable. Our results revealed that hDNMT3A was overexpressed in the testis of transgenic rats and induced genome-wide alterations in the DNA methylation pattern of rat sperm. Among 5438 reliable loci identified with 64 primer-pair combinations using a methylation-sensitive amplification polymorphism method, 28.01% showed altered amplified band types. Among these amplicons altered loci, 68.42% showed an altered DNA methylation status in the offspring of transgenic rats compared with wild-type rats. Further analysis based on loci which had identical DNA methylation status in all three biological replicates revealed that overexpression of hDNMT3A in paternal testis induced hypermethylation in sperm of both genotype-negative and genotype-positive offspring. Among the differentially methylated loci, 34.26% occurred in both positive and negative offspring of transgenic rats, indicating intergenerational inheritance of active DNA methylation changes in the absence of hDNM3A transmission. Furthermore, 75.07% of the inheritable loci were hyper-methylated while the remaining were hypomethylated. Distribution analysis revealed that the DNA methylation variations mainly occurred in introns and intergenic regions. Functional analysis revealed that genes related to differentially methylated loci were involved in a wide range of functions. Finally, this study demonstrated that active DNA methylation changes induced by hDNMT3A expression were intergenerationally inherited by offspring without transmission of the transgene, which provided evidence for the transmission of active endogenous-factors-induced epigenetic variations.

Asymmetry between Activation and Deactivation during a Transcriptional Pulse

Transcription in eukaryotic cells occurs in gene-specific bursts or pulses of activity. Recent studies identified a spectrum of transcriptionally active “on-states,” interspersed with periods of inactivity, but these “off-states” and the process of transcriptional deactivation are poorly understood. To examine what occurs during deactivation, we investigate the dynamics of switching between variable rates. We measured live single-cell expression of luciferase reporters from human growth hormone or human prolactin promoters in a pituitary cell line. Subsequently, we applied a statistical variable-rate model of transcription, validated by single-molecule FISH, to estimate switching between transcriptional rates. Under the assumption that transcription can switch to any rate at any time, we found that transcriptional activation occurs predominantly as a single switch, whereas deactivation occurs with graded, stepwise decreases in transcription rate. Experimentally altering cAMP signalling with forskolin or chromatin remodelling with histone deacetylase inhibitor modifies the duration of defined transcriptional states. Our findings reveal transcriptional activation and deactivation as mechanistically independent, asymmetrical processes.

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Gene transcription switches between variable rates

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Single-cell microscopy and mathematical modeling quantifies switch dynamics

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We observe an asymmetry in the activation/deactivation of transcriptional bursts

γ-glutamyl transpeptidase 1 specifically suppresses green-light avoidance via GABAA receptors in Drosophila

Drosophila larvae innately show light avoidance behavior. Compared with robust blue-light avoidance, larvae exhibit relatively weaker green-light responses. In our previous screening for genes involved in larval light avoidance, compared with control w(1118) larvae, larvae with γ-glutamyl transpeptidase 1 (Ggt-1) knockdown or Ggt-1 mutation were found to exhibit higher percentage of green-light avoidance which was mediated by Rhodopsin6 (Rh6) photoreceptors. However, their responses to blue light did not change significantly. By adjusting the expression level of Ggt-1 in different tissues, we found that Ggt-1 in malpighian tubules was both necessary and sufficient for green-light avoidance. Our results showed that glutamate levels were lower in Ggt-1 null mutants compared with controls. Feeding Ggt-1 null mutants glutamate can normalize green-light avoidance, indicating that high glutamate concentrations suppressed larval green-light avoidance. However, rather than directly, glutamate affected green-light avoidance indirectly through GABA, the level of which was also lower in Ggt-1 mutants compared with controls. Mutants in glutamate decarboxylase 1, which encodes GABA synthase, and knockdown lines of the GABAA receptor, both exhibit elevated levels of green-light avoidance. Thus, our results elucidate the neurobiological mechanisms mediating green-light avoidance, which was inhibited in wild-type larvae.

Human GGT2 does not autocleave into a functional enzyme: A cautionary tale for interpretation of microarray data on redox signaling

AIMS

Human γ-glutamyltranspeptidase 1 (hGGT1) is a cell-surface enzyme that is a regulator of redox adaptation and drug resistance due to its glutathionase activity. The human GGT2 gene encodes a protein that is 94% identical to the amino-acid sequence of hGGT1. Transcriptional profiling analyses in a series of recent publications have implicated the hGGT2 enzyme as a modulator of disease processes. However, hGGT2 has never been shown to encode a protein with enzymatic activity. The aim of this study was to express the protein encoded by hGGT2 and each of its known variants and to assess their stability, cellular localization, and enzymatic activity.

RESULTS

We discovered that the proteins encoded by hGGT2 and its variants are inactive propeptides. We show that hGGT2 cDNAs are transcribed with a similar efficiency to hGGT1, and the expressed propeptides are N-glycosylated. However, they do not autocleave into heterodimers, fail to localize to the plasma membrane, and do not metabolize γ-glutamyl substrates. Substituting the coding sequence of hGGT1 to conform to alterations in a CX3C motif encoded by hGGT2 mRNAs disrupted autocleavage of the hGGT1 propeptide into a heterodimer, resulting in loss of plasma membrane localization and catalytic activity.

INNOVATION AND CONCLUSIONS

This is the first study to evaluate hGGT2 protein. The data show that hGGT2 does not encode a functional enzyme. Microarray data which have reported induction of hGGT2 mRNA should not be interpreted as induction of a protein that has a role in the metabolism of extracellular glutathione and in maintaining the redox status of the cell.

Redox regulation of gamma-glutamyl transpeptidase

gamma-Glutamyl transpeptidase (GGT) catalyzes the transfer of the glutamyl moiety from glutathione, and glutathione S-conjugates to acceptors to form another amide or to water to produce free glutamate. Functionally, GGT plays important roles in glutathione homeostasis and mercapturic acid metabolism. The expression of GGT is increased as an adaptive response upon the exposure of oxidative stress. The underlying mechanism of this, however, is nebulous, as GGT gene structure is complex and its transcription is usually controlled by multiple promoters that generate several subtypes of GGT mRNAs. Studies reveal that signaling pathways such as Ras, ERK, p38MAPK, and PI3K are involved in the induction of GGT gene expression in response to oxidative stress. Thus, not surprisingly, induction of GGT mRNA subtypes and the involvement of multiple signaling pathways vary depending on cell type and stimuli.

The human gamma-glutamyltransferase gene family

Assays for gamma-glutamyl transferase (GGT1, EC 2.3.2.2) activity in blood are widely used in a clinical setting to measure tissue damage. The well-characterized GGT1 is an extracellular enzyme that is anchored to the plasma membrane of cells. There, it hydrolyzes and transfers gamma-glutamyl moieties from glutathione and other gamma-glutamyl compounds to acceptors. As such, it has a critical function in the metabolism of glutathione and in the conversion of the leukotriene LTC4 to LTD4. GGT deficiency in man is rare and for the few patients reported to date, mutations in GGT1 have not been described. These patients do secrete glutathione in urine and fail to metabolize LTC4. Earlier pre-genome investigations had indicated that besides GGT1, the human genome contains additional related genes or sequences. These sequences were given multiple different names, leading to inconsistencies and confusion. Here we systematically evaluated all human sequences related to GGT1 using genomic and cDNA database searches and identified thirteen genes belonging to the extended GGT family, of which at least six appear to be active. In collaboration with the HUGO Gene Nomenclature Committee (HGNC) we have designated possible active genes with nucleotide or amino acid sequence similarity to GGT1, as GGT5 (formerly GGL, GGTLA1/GGT-rel), GGT6 (formerly rat ggt6 homologue) and GGT7 (formerly GGTL3, GGT4). Two loci have the potential to encode only the light chain portion of GGT and have now been designated GGTLC1 (formerly GGTL6, GGTLA4) and GGTLC2. Of the five full-length genes, three lack of significant nucleotide sequence homology but have significant (GGT5, GGT7) or very limited (GGT6) amino acid similarity to GGT1 and belong to separate families. GGT6 and GGT7 have not yet been described, raising the possibility that leukotriene synthesis, glutathione metabolism or gamma-glutamyl transfer is regulated by their, as of yet uncharacterized, enzymatic activities. In view of the widespread clinical use of assays that measure gamma-glutamyl transfer activity, this would appear to be of significant interest.

Gene expression of gamma-glutamyltranspeptidase

gamma-Glutamyltranspeptidase is a heterodimeric glycoprotein that catalyzes the transpeptidation and hydrolysis of the gamma-glutamyl group of glutathione and related compounds. It is known that the enzyme plays a role in the metabolism of glutathione and in salvaging constituents of glutathione. In the adult animal, high levels of gamma-glutamyltranspeptidase are constitutively expressed in the kidney, intestine, and epididymis. On the other hand, although gamma-glutamyltranspeptidase is up-regulated in the liver during the perinatal stage, its expression is nearly undetectable in the adult. In addition, it has long been observed that the intake of certain xenobiotics, including carcinogens and drugs, induces the hepatic expression of the enzyme. This induction seems to be associated with both transcriptional regulation and the growth of certain types of cells in the injured liver. A number of studies have been carried out to explain the mechanism by which gamma-glutamyltranspeptidase expression is regulated. 5'-Untranslated regions of mRNAs of the enzyme differ in a tissue-specific manner but share a common protein coding region, and the tissue-specific and developmental stage-specific expression, as well as hepatic induction, are conferred by different promoters. As suggested by the capability of enzymatic activity-independent induction of osteoclasts, the expression of gamma-glutamyltranspeptidase may also be involved in various biological processes that are not directly associated with glutathione metabolism. This chapter briefly summarizes studies to date concerning the tissue-specific expression and induction of gamma-glutamyltranspeptidase and transcriptional regulation by the multiple promoter system is discussed.

Gamma-glutamyl transpeptidase in glutathione biosynthesis

Glutathione (GSH) is the most abundant nonprotein thiol in cells and has multiple biological functions. Glutathione biosynthesis by way of the gamma-glutamyl cycle is important for maintaining GSH homeostasis and normal redox status. As the only enzyme of the cycle located on the outer surface of plasma membrane, gamma-glutamyl transpeptidase (GGT) plays key roles in GSH homeostasis by breaking down extracellular GSH and providing cysteine, the rate-limiting substrate, for intracellular de novo synthesis of GSH. GGT also initiates the metabolism of glutathione S-conjugates to mercapturic acids by transferring the gamma-glutamyl moiety to an acceptor amino acid and releasing cysteinylglycine. GGT is expressed in a tissue-, developmental phase-, and cell-specific manner that may be related to its complex gene structure. In rodents, there is a single GGT gene, and several promoters that generate different mRNA subtypes and regulate its expression. In contrast, several GGT genes have been found in humans. During oxidative stress, GGT gene expression is increased, and this is believed to constitute an adaptation to stress. Interestingly, only certain mRNA subtypes are increased, suggesting a specific mode of regulation of GGT gene expression by oxidants. Here, protocols to measure GGT activity, relative levels of total and specific GGT mRNA subtypes, and GSH concentration are described.

Glutathione-related enzymes in cell cultures from different regions of human epididymis

Protection of maturing sperm from potential endogenous or exogenous harmful substances during their transit throughout the epididymis is a critical event. The authors studied the activity of gamma-glutamyl transpeptidase (GGT) and glutathione S-transferase (GST), and glutathione (GSH) levels in epithelial cell cultures from human caput, corpus, and cauda epididymides. Tissue was obtained from patients undergoing therapeutic orchidectomy for prostatic cancer. Enzymatic activity was measured in conditioned media and cellular fractions. Androgen influence was also evaluated. Both enzymatic activities were found in cellular homogenates and conditioned media from cultures of all epididymal regions. GGT activity was highest in cultures from cauda epididymis, both in conditioned media and cell fractions, while GST activity did not show regional differences in conditioned media, but exhibited higher activity in cell homogenates from cauda cultures than those obtained from corpus and caput epididymis. GSH level showed no regional difference in cell homogenates and it could not be detected in conditioned media by the method used. Presence of different concentrations of dihydrotestosterone (DHT) had no influence neither on the enzymatic activities nor GSH concentration. The results indicate that GGT and GST are present along the human epididymis and a fraction or isoform of these enzymes might be secreted to the luminal fluid to play a detoxificative role in sperm maturation.

Molecular cloning and characterization of CT120, a novel membrane-associated gene involved in amino acid transport and glutathione metabolism

Within the minimum LOH region on chromosome 17p13.3 deleted in hepatocellular carcinoma, a novel human plasma membrane-associated gene, named CT120, was isolated from a human kidney cDNA library using electronical cloning and RACE. The novel gene CT120 consists of 2145bp and encodes a protein with 257 amino acids. Database search revealed that homologs of CT120 exist in different organisms from plant to animal kingdoms, which suggests that CT120 is a highly conserved gene during biological evolution. Different expression patterns of CT120 were observed in many different human normal tissues and in various human tumor cell lines. Transcript of CT120 was not detectable in normal lung tissue, but was abundant in SPC-A-1 (human epithelial-like lung adenocarcinoma) cell line, suggesting that CT120 may be involved in lung cancer development. Subcellular localization analysis showed that CT120 is a novel membrane-associated protein. CT120 can interact with SLC3A2 (member 2 of solute carrier family 3) and GGTL3B (isoform of gamma-glutamyltranspeptidase-like 3) in eukaryotic cells by yeast two-hybrid screen and co-immunoprecipitation assay, which suggested that CT120 may assume very essential physiological functions involved in amino acid transport and glutathione metabolism.

Glutathione in defense and signaling: lessons from a small thiol

The mechanisms of thiol metabolism and chemistry have particular relevance to both cellular defenses against toxicant exposure and to redox signaling. Here, we will focus on glutathione (GSH), the major endogenous low- molecular-weight nonprotein thiol synthesized de novo in mammalian cells. The major pathways for GSH metabolism in defense of the cell are reduction of hydroperoxides by glutathione peroxidases (GSHPx) and some peroxiredoxins, which yield glutathione disulfide (GSSG), and conjugation reactions catalyzed by glutathione-S-transferases. GSSG can be reduced to GSH by glutathione reductase, but glutathione conjugates are excreted from cells. The exoenzyme gamma-glutamyltranspeptidase (GGT) removes the glutamate from extracellular GSH, producing cysteinyl-glycine from which a dipeptidase then generates cysteine, an amino acid often limiting for de novo GSH synthesis. Synthesis of GSH from the constituent amino acids occurs in two regulated, enzymatically catalyzed steps. The signaling pathways leading to activation of the transcription factors that regulate these genes are a current area of intense investigation. The elucidation of the signaling for GSH biosynthesis in human bronchial epithelial cells in response to 4-hydroxynonenal (4HNE), an end product of lipid peroxidation, will be used as an example. GSH also participates in redox signaling through the removal of H(2)O(2), which has the properties of a second messenger, and by reversing the formation of sulfenic acid, a moiety formed by reaction of critical cysteine residues in signaling proteins with H(2)O(2). Disruption of GSH metabolism will therefore have major a impact upon function of cells in terms of both defense and normal physiology.

Activation of the gamma-glutamyltransferase promoter 2 in the rat colon carcinoma cell line CC531 by histone deacetylase inhibitors is mediated through the Sp1 binding motif

The single-copy gene for rat gamma-glutamyltransferase (GGT) encodes at least seven distinct mRNAs that differ in their 5'-untranslated regions only. Tissue- and developmental-specific expression of GGT is partly achieved by the presence of many transcription factor-binding sites in the promoters of this gene. In an earlier study we found that GGT mRNAs II and IV levels were increased upon butyrate-induced differentiation of the rat colon carcinoma cell line CC531. The mechanism for this butyrate-induced upregulation remains unknown, but may result from altered promoter activity as butyrate is a known histone deacetylase inhibitor. In the present study, we show by transient transfection studies that butyrate enhanced the expression of the luciferase reporter gene driven by the rat GGT promoter 2 (P2). Trichostatine A (TSA), another histone deacetylase inhibitor, also enhanced transcription from this promoter. The role of the transcription factor site Sp1 in butyrate- or TSA-induced activation of the GGT P2 was examined as Sp1 has been previously shown to play a central role in the transcriptional activation of other genes during butyrate and TSA stimulation. A triple sequence-motif of this isolated Sp1 site linked to a minimal promoter was able to mediate butyrate- and TSA-induced expression of the luciferase reporter gene, while no effect was measured using the minimal promoter alone. Deleting the Sp1 site in the context of the rat GGT P2 strongly reduced the basal transcription activity and abrogated butyrate- and TSA-induced activation of the mutated promoter. These results suggest that butyrate- or TSA-induced activation of the rat GGT P2 can be mediated by a Sp1 binding motif.

Lectin affinity chromatography and electrophoretic properties of human platelet gamma-glutamyl transferase

The sialoglycoprotein, gamma-glutamyl transferase (GGT, gamma-GT, EC 2.3.2.2) is a membrane enzyme found in many cells including platelets and leukocytes. In platelets GGT converts leukotriene C4 (LTC4) to leukotriene D4 (LTD4) and is involved in glutathione metabolism. In this study, human platelet GGT was solubilized with Triton X-100 and purified by lectin affinity chromatography on Con A Sepharose 4B to determine its electrophoretic properties. The specific activity of purified GGT was 236 mU/mg protein; 73.7% of human platelet GGT activity was found bound to Con A and 50% of the bound activity was released with 0.3 mol/l methyl alpha-D-mannopyranoside. We observed that human platelet GGT has only one isoenzyme band showing a carbohydrate stained band near the origin on polyacrylamide gel electrophoresis (PAGE). The electrophoretic mobility of papain-solubilized GGT was higher than that of Triton X-100-solubilized GGT at PAGE. Also GGT activities were determined on neuraminidase, trypsin or n-butanol-DIPE (diisopropyl ether)-treated Triton X-100-solubilized membrane fractions. This characterization may be useful when trying to establish the contribution of platelet GGT to serum GGT activity. This marker may reflect the extent of platelet activation.

Gamma-glutamyl transpeptidase gene organization and expression: a comparative analysis in rat, mouse, pig and human species

Gamma-glutamyl transpeptidase (GGT) is an enzyme located at the external surface of epithelial cells. It initiates extracellular glutathione (GSH) breakdown, provides cells with a local cysteine supply and contributes to maintain intracellular GSH level. GGT expression, highly sensitive to oxidative stress, is a part of the cell antioxidant defense mechanisms. We describe recent advances in GGT gene structure and expression knowledge and put emphasis on the complex transcriptional organization of that gene and its conservation among different species. GGT gene structure has been elucidated in rat and mouse where a single gene is transcribed from multiple promoters into several transcripts which finally yield a unique polypeptidic chain. Analysis of rat, mouse, human and pig cDNA and gene sequences reveals a large conservation of the transcriptional organization of that gene. This complex structure provides flexibility in GGT expression controlled at the promoter level, through multiple regulatory sites, and at RNA level by alternate 5' untranslated sequences which may create a diversity in the stability and translational efficiency of the different transcripts. In conclusion, transcription of the GGT gene from several promoters offers multiple DNA and RNA targets for various oxidative stimuli and contributes to a broad antioxidant cell defense through GGT induction and subsequent cysteine supply from extracellular glutathione.

Involvement of polyomavirus enhancer activator 3 in the regulation of expression of gamma-glutamyl transpeptidase messenger ribonucleic acid-IV in the rat epididymis

Gamma-glutamyl transpeptidase (GGT) mRNA-IV and polyomavirus enhancer activator 3 (PEA3) mRNA are highly expressed in the initial segment of the rat epididymis, and both are regulated by testicular factors. PEA3 protein in rat initial segment nuclear extracts has been shown to bind to a PEA3/Ets binding motif, which is derived from the partially characterized GGT mRNA-IV promoter region. This suggests that PEA3 may be involved in regulating transcription from the rat GGT mRNA-IV gene promoter in the initial segment. Using DNA oligonucleotide primers and DNA sequencing analysis, an approximately 1500-basepair (bp) DNA sequence at the 5' region of the promoter was obtained. Using transient transfection, PEA3 activated transcription of the rat GGT mRNA-IV promoter only in cultured epididymal cells from the rat initial segment, but not in Cos-1 or NRK-52E cells. Promoter deletion analysis indicated that a PEA3/Ets binding motif between nucleotides -22 and -17 is the functional site for PEA3 to activate transcription of GGT promoter IV and that an adjacent Sp1 binding motif is also required to maintain promoter IV activity in epididymal cells. Transcriptional activation of promoter IV was shown to be epididymal cell-specific and PEA3-specific. In addition, PEA3 may act as a weak repressor for transcription of promoter IV, probably using a PEA3/Ets binding motif(s) distal to the transcription start site. A model of how PEA3 is involved in the regulation of transcription of GGT promoter IV in epididymal cells is proposed.

Inhibition of the hydrolytic and transpeptidase activities of rat kidney gamma-glutamyl transpeptidase by specific monoclonal antibodies

Monoclonal antibodies (mAb) against the native form of rat kidney gamma-glutamyl transpeptidase (GGT) were isolated by screening hybridomas with rat kidney brush-border membrane vesicles. They were directed against protein rather than sugar epitopes in that each recognized all GGT isoforms. All of them inhibited partially the enzyme activity of GGT. They were specific in that they inhibited the rat enzyme, but not the mouse or human enzyme. Kinetic analyses were carried out with free GGT and GGT-mAb complexes with d-gamma-glutamyl-p-nitroanilide in the presence or absence of maleate, or in the presence or absence of alanine, cysteine, cystine or glycylglycine as gamma-glutamyl acceptors. mAbs 2A10 and 2E9 inhibited the hydrolytic and glutaminase activities of GGT and had little effect on the transpeptidation activity of the enzyme, whereas mAbs 4D7 and 5F10 inhibited transpeptidation, but not hydrolytic or glutaminase activities. mAb 5F10 mimicked the effect of maleate on GGT, in that it inhibited transpeptidation, enhanced the glutaminase activity and increased the affinity of the donor site of GGT for acivicin. Such mAbs may be useful for long-term studies in tissue cultures and in vivo, and for the identification of GGT epitopes that are important for the hydrolytic and transpeptidase activities.

Serum gamma-glutamyltransferase and alkaline phosphatase during experimental liver metastases. Detection of tumour-specific isoforms and factors affecting their serum levels

Tumour-specific isoenzymes and tumour markers in serum are potentially useful in the detection and monitoring of liver metastases. An experimental rat model was used in the search for such isoenzymes and to study factors affecting their serum levels. Splenic injection of CC531 colon carcinoma cells in syngeneic WagRij rats caused liver metastases after 3 weeks with concomitant and significant increases in serum levels of gamma-glutamyltransferase (GT) and alkaline phosphatase (ALP). The presence of tumour-specific isoforms of both enzymes, as well as increased amounts of the liver isoform of ALP, were demonstrated in serum. The serum levels of the tumour variants were clearly related to their elimination rates from the circulation. Thus, the slow clearance of the tumour ALP resulted in high serum levels of this isoform, compared with the more rapid elimination of tumour GT and its lower serum level. When using another colon carcinoma cell line (DHD/K12), metastatic to liver in BD IX rats, no increases in serum GT were detected. This was related to the rapid elimination from the circulation of the GT variant from the DHD/K12 metastatic tissue. The relatively high amount of the tumour ALP isoform detected in serum during growth of the CC531 liver metastases indicated that this isoform could be useful as a marker of tumour growth.

An intronic promoter controls the expression of truncated human gamma-glutamyltransferase mRNAs

We have identified and characterized a genomic DNA fragment containing the coding sequences corresponding to the human gamma-glutamyltransferase type 1 mRNA. The coding part of the gene spans over 16 kb and comprises 12 exons and 11 introns exhibiting a similar organization as for the mouse and rat GGT genes. The exons 1-7 encode the heavy subunit whereas exons 8-12 which encode the carboxy-terminal part of the heavy subunit (exon 8) and the light subunit are clustered in a 1.6-kb BglII fragment. Exons 7 and 8 are separated by a 3.9-kb intron containing in its 3' part the sequences corresponding to the 5'-UTRs of the truncated GGT mRNAs described for human lung. Sequence analysis upstream this transcribed region exhibited putative promoter sequences and after transient transfection significant promoter activities were measured in V79 lung fibroblasts and KYN-2 hepatoma cells but not in A2780 ovarian cells. This specificity disappeared when only 550 bp upstream the transcription start site were used as promoter. These results argue for a promoter of truncated GGT mRNAs in intron 7, specifically regulated in human tissues.

Differential regulation of glutathione by oxidants and dexamethasone in alveolar epithelial cells

We studied the regulation of GSH and the enzymes involved in GSH regulation, gamma-glutamylcysteine synthetase (gamma-GCS) and gamma-glutamyl transpeptidase (gamma-GT), in response to the oxidants menadione, xanthine/xanthine oxidase, hyperoxia, and cigarette smoke condensate in human alveolar epithelial cells (A549). Menadione (100 microM), xanthine/xanthine oxidase (50 microM/10 mU), and cigarette smoke condensate (10%) exposure produced increased GSH levels (240 +/- 6, 202 +/- 12, and 191 +/- 2 nmol/mg protein, respectively; P < 0.001) compared with the control level (132 +/- 8 nmol/mg protein), which were associated with a significant increase in gamma-GCS activity (0.18 +/- 0.006, 0.16 +/- 0.01, and 0.17 +/- 0. 008 U/mg protein, respectively; P < 0.01) compared with the control level (0.08 +/- 0.001 U/mg protein) at 24 h. Exposure to hyperoxia (95% O2) resulted in a time-dependent increase in GSH levels. gamma-GCS activity increased significantly at 4 h (P < 0.001), returning to control values after 12 h of exposure. Dexamethasone (3 microM) exposure produced a significant time-dependent decrease in the levels of GSH and gamma-GCS activity at 24-96 h. The activity of gamma-GT did not change after oxidant treatment; however, it was decreased significantly by dexamethasone at 24-96 h. Thus oxidants and dexamethasone modulate GSH levels and activities of gamma-GT and gamma-GCS by different mechanisms. We suggest that the increase in gamma-GCS activity but not in gamma-GT activity may be required for the increase in intracellular GSH under oxidative stress in alveolar epithelial cells.

gamma-Glutamyl transpeptidase: catalytic mechanism and gene expression

The gamma-glutamyl transpeptidases are key enzymes in the so-called gamma-glutamyl cycle involving glutathione synthesis, the recovery of its constituents, and in the transport of amino acids. This membrane-bound ectoenzyme thus serves to regulate glutathione synthesis. This chapter deals with the active site chemistry of gamma-glutamyl transpeptidase, including the role of side-chain groups on the light subunit as well as several serine residues in the catalytic process. Also considered are genomic studies indicating (a) the presence of a single gene in mouse and rat; (b) the occurrence of multiple genes in humans; (c) the involvement of multiple promoters for gene expression; and (d) how these multiple promoters may play a role in the tissue-specific expression of gamma-glutamyl transpeptidases.

Mice with genetic gamma-glutamyl transpeptidase deficiency exhibit glutathionuria, severe growth failure, reduced life spans, and infertility

A mouse mutant with glutathionuria was discovered by screening for amino acidurias in the progeny of ethylnitrosourea-mutagenized mice. Total glutathione concentration was increased in both blood and urine but decreased in liver homogenates from affected mice. Glutathionuric mice exhibited lethargy, severe growth failure, shortened life spans and infertility. gamma-Glutamyl transpeptidase activity was deficient in kidney homogenates of glutathionuric mice. The glutathionuric phenotype in these mice is inherited as an autosomal recessive trait. This mouse mutant will be a useful animal model for the study of gamma-glutamyl transpeptidase physiology and glutathione metabolism.

The organization of the gamma-glutamyl transferase genes and other low copy repeats in human chromosome 22q11

A clone map consisting of YACs, cosmids, and fosmids has been constructed covering low copy repeat regions of human chromosome 22q11. A combination of clone restriction digest analysis, single-copy landmark content analysis, HindIII-Sau3AI fingerprinting, and sequencing of PCR products derived from clones was required to resolve the map in this region. Seven repeat-containing contigs were placed in 22q11, five containing gamma-glutamyl transferase (GGT) sequences described previously. In one case, a single interval at the resolution of the YAC map was shown to contain at least three GGT sequences after higher resolution mapping. The sequence information was used to design a rapid PCR/restriction digest technique that distinguishes the GGT loci placed in the YAC map. This approach has allowed us to resolve the previous cDNA and mapping information relating to GGT and link it to the physical map of 22q11.

Ontogenic expression of the Na(+)-independent organic anion transporting polypeptide (oatp) in rat liver and kidney

BACKGROUND/AIMS

A cDNA (2.7 kb) encoding a rat liver basolateral Na(+)-independent organic anion transporter (oatp) has recently been cloned. The aim of the present study was to clarify the mechanisms of bile formation during development.

METHODS

The ontogenic expression of oatp was examined by northern blot analysis and in situ hybridization in rat liver. The expression of oatp in the kidney was also studied in parallel.

RESULTS

In the liver, a 2.5 kb oatp mRNA was first detected in the fetus on day 16 of gestation. The amount of this oatp mRNA remained stable during the perinatal period and increased dramatically after weaning. Other transcripts probably corresponding to oatp-related mRNAs also display a late expression pattern in the perinatal period. In contrast, Na+/taurocholate transporting polypeptide (Ntcp) mRNA was first detected on day 20 of gestation. By in situ hybridization, oatp mRNA was localized into hepatocytes and distributed without lobular heterogeneity. In the kidney, a single 2.4 kb oatp transcript was detected from birth to adult age. This transcript was exclusively distributed in the epithelial cells of the proximal tubules localized in the kidney cortex and the outer medulla.

CONCLUSIONS

These results indicate that oatp undergoes a time-related expression in rat liver and kidney during development and that its gene transcription precedes Ntcp gene transcription in the liver. The delayed expression of oatp at the perinatal period may explain in part the immaturity of bile formation and the physiological neonatal cholestasis.

Cloning and expression of a novel type (III) of human gamma-glutamyltransferase truncated mRNA

We report the characterization of a novel human gamma-glutamyltransferase mRNA type. This type III mRNA differs from type I and type II mRNAs previously described by several point mutations and the presence of an unspliced 81 bp intron in the open reading frame. Further, type III mRNAs are truncated ones and are tissue and pathology specifically expressed. In fact, type III mRNAs are present in human placenta, sigmoid, lung and in 50% of acute lymphoblastic leukemia blood cells but they are never found in healthy lymphocytes.

Control of gamma-glutamyl transpeptidase expression by glucocorticoids in the rat pancreas. Correlation with granule formation

Glucocorticoids are known to promote the formation of zymogen granules in acinar cells of the exocrine pancreas in vivo as well as in vitro. To gain insight into the mechanism of this regulation, we studied the effects of glucocorticoids on the synthesis of two components of the secretory granule membrane, the glycoprotein 2 (GP-2) and the gamma-glutamyl transpeptidase (GGT). It was demonstrated that following adrenalectomy, degranulation of pancreatic acinar cells is accompanied by a sharp decrease in GGT and GP-2 synthesis as measured by mRNA and protein accumulation. The decline of GGT synthesis was prevented by glucocorticoid replacement therapy, whereas GP-2 synthesis could be maintained with either glucocorticoid or estradiol treatment. These in vivo observations were corroborated and extended in an in vitro study using AR42J pancreatic cells. With this cell line, it was demonstrated that dexamethasone induces the formation of zymogen granules and the accumulation of a specific GGT transcript (mRNA III) by decreasing its degradation rate. At the same time, the GP-2 mRNA level was not modified by the hormonal treatment. These data demonstrate that glucocorticoids exert a positive control on the GGT expression in pancreatic cells at a post-transcriptional level. GGT, an enzyme of the glutathione metabolism, could play a significant role in protein packaging in secretory cells.

Characterization of a promoter for gamma-glutamyl transpeptidase activated in rat liver in response to aflatoxin B1 and ethoxyquin

gamma-Glutamyl transpeptidase (GGT) is normally absent from adult rat hepatocytes but is induced by a range of xenobiotics, including carcinogens and chemoprotective agents. As many as six mRNA species for this enzyme have been described in both rat and mouse, with various degrees of tissue specificity. These originate from one gene and have separate promoters within alternative 5' untranslated sequences. By using a cDNA-derived sequence specific for GGT mRNA III to screen a rat genomic library, a clone that contains the promoter region for this mRNA was isolated and characterized. The transcriptional start site lay some 3.5 kb upstream from that already characterized for mRNA II in rat kidney. Luciferase activity was obtained after transfection of rat hepatoma-derived cell lines with constructs containing the putative promoter III fused to a luc reporter. Although this promoter lacks a TATA box, a sequence close to the start site that binds the transcription factor TFIID in vitro was identified. By using PCR techniques, mRNA III (homologous to both mouse III and IV) and an mRNA (IV) with homology to VI in mouse were found in ethoxyquin- and aflatoxin B1-treated rat liver and kidney as well as in a hepatoma-derived cell line. No evidence was found for a product homologous to mRNA from promoter V described in the mouse.

Protease-catalyzed conversion of insulin-like growth factor-1 and interleukin-6 into high-molecular-mass species through the sequential action of hematopoietic surface-associated cathepsin G and gamma-glutamyl transpeptidase-related activities

Interleukin-6 (IL-6) and insulin-like growth-factor-1 (IGF-1) are cytokines produced by a variety of cells that act on a wide range of tissues, influencing cell growth and differentiation. Purified plasma membranes from human U937 monoblastic cells produced in vitro dimeric species of IL-6- and IGF-1-derived peptides through the sequential actions of surface-associated enzymes cathepsin G and transpeptidase activities. Cathepsin G degraded native unglycosylated IL-6 and IGF-1 molecules into 8-kDa and 7-kDa peptides respectively. Subsequent dimerisation of these intermediate forms into 16-kDa IL-6- and 14-kDa IGF-1-derived peptides was inhibited by acivicin and glutathione which are specific inhibitors of the standard cell-surface gamma-glutamyl transpeptidase (gamma-GT). However U937 plasma membranes, cleared of gamma-GT activity by immunoprecipitation with anti-gamma-GT and adsorption on protein-G-Sepharose, were still able to convert the intermediate forms of IL-6 and IGF-1 into dimers. Together, these observations indicate that the transpeptidase involved in the formation of the dimeric species of IL-6 and IGF-1 was related to, but distinct from, standard cell-surface gamma-GT. Cells of all hematopoietic lineages expressed gamma-GT-related activity. In contrast to the 16-kDa IL-6-derived peptide that did not retain growth-stimulating activity, the 14-kDa IGF-1 peptide was at least equipotent with native IGF-1 in the BALB/c 3T3 fibroblast DNA synthesis response. The N/O-glycosylated IL-6 was clearly as sensitive to cathepsin-G- and gamma-GT-related activities as the unglycosylated IL-6 from Escherichia coli, thus indicating that the sugar chains did not protect the cleavage sites of the two proteases on the IL-6 molecule. Our in vitro findings raise the possibility that similar proteases participate in the regulation of the catabolism of IL-6 and IGF-1 in vivo.

Characterization of gamma-glutamyltranspeptidase in the liver of the frog: 1. Comparison to the rat liver enzyme

The characteristics of the enzyme gamma-glutamyltranspeptidase were determined in frog liver and compared to those of the rat. In Rana pipiens, tissue distribution studies indicated the order of activity to be: kidney >>> liver >> nerve > egg > lung > heart > skeletal muscle in homogenates. In the Rana pipiens relative to the Fischer 344 rat, the activity of the liver enzyme was somewhat greater (1.8-fold) and the kidney enzyme substantially less (25-fold). Frog liver gamma-glutamyltranspeptidase displayed strain-dependent differences in activity with Rana pipiens and Rana sylvatica exhibiting comparable activities and Xenopus laevis exhibiting 20-fold lower activities. No influence of sex was apparent in Rana pipiens in contrast to the sex dependent differences observed in the Fischer 344 rat: female:male = 7:1. In homogenates and plasma membrane fractions of Rana pipiens, Xenopus laevis and the Fischer 344 rat, high, and comparable relative specific activities, were observed, 8-11, coupled with protein yields of 2.2-2.5 per cent indicating the enzyme to be plasma membrane bound and associated with the sinusoidal surface of the liver cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of NotI sites from chromosome 22q11

Chromosome 22q11 contains a large number of interesting loci, including genes associated with cancer and developmental defects. The region is also the site of the lambda immunoglobulin variable and constant regions and the BCR, gamma-glutamyl transpeptidase, and GGT-like activity multigene families. Because of the complexities associated with mapping highly related gene families, we have examined the utility of mapping large areas of DNA using a defined approach. A total of 21 complete NotI sites from band q11 were cloned and ordered into six noncontiguous clusters of sites using a combination of somatic cell hybrid panels, NotI jumping and linking libraries, and fluorescence in situ hybridization. The largest cluster spanned an estimated 2 Mb of NotI fragments, the smallest 115 kb. Approximately 3.5 Mb of band q11 could be examined for rearrangements in NotI restriction enzyme fragments. A number of conserved sequences, two genes, and a minimum of two families of related sequences were identified adjacent to NotI sites.

Establishment of a V79 transfected cell line highly producing recombinant human gamma-glutamyltransferase

gamma-Glutamyltransferase (GGT) is a glutathione-metabolizing enzyme whose activity variations in serum and organs are valuable markers of preneoplastic processes, alcohol abuse and induction by drugs. To elucidate the implication of GGT in various metabolic pathways, we established a stable transfected V79 cell line highly producing the human GGT. A full length cDNA, encoding the human hepatoma Hep G2 GGT, was subcloned into an expression vector under the control of the SV40 early promoter and was used to transfect V79 cells. A cell line was selected, exhibiting a GGT activity of 2 units per mg of protein, one of the highest levels reported to date. The recombinant GGT purified from this cell line showed the expected heterodimeric structure, with two subunits existing as sialylated and differentially glycosylated isoforms, with mean molecular masses of 80 and 29 kDa. Catalytic features were found to be identical to those of human serum and Hep G2 GGTs. Thus, the newly engineered cell line should be useful for the production of human GGT and as a potential alternative model for pharmacological studies.

A panel of human chromosome 22-specific sequence tagged sites

A panel of 29 sequence tagged sites (STSs) covering the long arm of chromosome 22 has been assembled. STS primer pairs were synthesized using available chromosome 22 sequence derived from the GenBank and EMBL DNA sequence databases, as well as published cDNA and genomic sequence, or from previously published and communicated primer pairs. Each STS was optimized for the polymerase chain reaction using a chromosome 22-only hybrid and human genomic DNA. Further STS content analysis on a panel of somatic cell hybrids that incorporated two chromosome 22 translocations resulted in the mapping of the X-box binding protein (XBP), D22S156, and transcobalamin II (TCN2) genes to 22q11-q13.1. The panel of STSs was used for the rapid determination of the STS content and thus the chromosomal DNA content of a new irradiation hybrid.

Repetitive 5-azacytidine treatments of Fao cells induce a stable and strong expression of gamma-glutamyl transpeptidase

The role of DNA methylation in the expression of the rat gamma-glutamyl transpeptidase (GGT) gene was assessed in the Fao cell line using a hypomethylating agent, 5-azacytidine. Ten repetitive treatments of the cells, with 8 microM 5-azacytidine for 24 h, led to 13- and 80-fold increases, respectively, in GGT activity and in GGT mRNA level. The DNA methylation patterns generated by the isoschizomeric restriction enzymes Hpa II and Msp I indicated that the GGT gene, highly methylated in Fao cells, became strongly demethylated after 5-azacytidine treatments. Thus, DNA demethylation increases the expression of the GGT gene. 5-Azacytidine treatments also increased, but to a lesser extent, mRNAs level for actin, albumin, mitochondrial aspartate aminotransferase, aldolase B mRNAs (12- to 16-fold) as well as for tubulin, gluthathione transferase, and tyrosine aminotransferase mRNAs (2- to 5-fold). The GGT gene expression was further studied in B4 cells, cloned from the demethylated Fao cell population. This clone B4 exhibited a stable and strong GGT activity and a highly demethylated GGT gene. Among the three GGT mRNA I, II, or III, transcribed from three different promoters of the single rat GGT gene, only mRNA III was detected in Fao cells and was increased in clone B4, indicating that the demethylation acts on the promoter for mRNA III. The analysis of the differentiation state of B4 cells, as compared to Fao cells, showed a loss of the regulation of GGT and aspartate aminotransferase genes by dexamethasone, as well as a loss of the gluconeogenic pathway. Interestingly, B4 cells have retained many other specific functions of hepatic differentiation and have acquired alpha-fetoprotein expression; thus this clone exhibits the characteristics of a hepatic fetal phenotype.

Identification of a second promoter which drives the expression of gamma-glutamyl transpeptidase in rat kidney and epididymis

In rat, gamma-glutamyl transpeptidase (GGT) is encoded by multiple mRNAs (mRNAI, mRNAII, mRNAIII, and mRNAIV) that differ only in their 5' untranslated regions and are transcribed from a single-copy gene. Using oligonucleotides designed from the 5' untranslated sequences of the GGT mRNAII and mRNAIII, we amplified a 3.4-kb genomic sequence which contains the promoter region for mRNAII. The sequence flanking the two initiation start sites for mRNAII contains consensus motifs for several potential regulatory proteins and a TATA-like element at the expected position 26 bp upstream from the predominant start site. The sequence from positions -528 to +72 associated with the chloramphenicol acetyltransferase (CAT) reporter gene drives a promoter activity in LLC-PK1, a pig kidney cell line. Deletion analysis revealed that the region from nucleotides -528 to -322 mediates an activation of the promoter activity, whereas the sequence from -322 to -114 has a negative effect. Furthermore, the structural organization of the 5' end of the GGT gene reveals that the GGT mRNAIII is transcribed from a third promoter located upstream from the promoter II on the GGT gene. By Northern blot analysis, the promoter II was found to be expressed only in the kidney and in the epididymis. We also identified two new mRNA species which are expressed in the H5 hepatoma cells. Therefore, the GGT gene expression reveals a strong tissue- or cell-specific pattern which is based on the transcription of several mRNA species from multiple promoters.

Molecular cloning of the cDNAs coding for the two subunits of soluble guanylyl cyclase from human brain

Complementary DNA clones corresponding to the 70 and 82 kDa subunits of soluble guanylyl cyclase from human adult brain have been isolated and sequenced. Their respective open reading frames correspond to 619 amino acids (M(r) 70,469) and 717 amino acids (M(r) 81,324). Southern blots of human genomic DNA using these clones as probes give patterns which might be compatible with the presence of more than one copy per gene, or pseudogenes, for each subunit in the human genome. Comparison of the protein sequence of the large subunit from adult brain with the subunit cloned from human fetal brain (Harteneck, C., Wedel, B., Koesling, D., Malekewitz, J., Böhme, E., and Schultz, G. (1991) FEBS Lett. 292, 217-222) revealed only 34% homology. This result demonstrates the existence of a novel large subunit isoform for soluble guanylyl cyclase in human tissues.

Gamma-glutamyltransferase: nucleotide sequence of the human pancreatic cDNA. Evidence for a ubiquitous gamma-glutamyltransferase polypeptide in human tissues

gamma-Glutamyltransferase (GGT, EC 2.3.2.2) is an enzyme involved in glutathione metabolism and drug and xenobiotic detoxification. Using human hepatoma Hep G2 GGT cDNA as probe, we isolated a cDNA from a human pancreatic cDNA library. Analysis of the nucleotide sequences revealed a 2244-bp insert that includes an open reading frame of 1710 bp, encoding a protein identical to the Hep G2 and human placenta GGTs. Similarly, the 5' untranslated region, though shorter, is highly homologous to that of Hep G2 cDNA. These data suggest strongly that the same gene encodes human GGT in the placenta, Hep G2 and the pancreas. We further studied the distribution of the corresponding mRNA, called type I mRNA, in different human tissues. Using a highly sensitive method associating reverse transcription with specific amplification by polymerase chain reaction, cDNA was synthesized from total RNA isolated from the tissues and GGT specific fragments were amplified. We observed the presence of a specific cDNA fragment corresponding to the type I mRNA in the human tissues and cells tested, providing the evidence for a ubiquitous expression of this GGT mRNA in human tissues.

The same gamma-glutamyl transpeptidase RNA species is expressed in fetal liver, hepatic carcinomas, and rasT24-transformed rat liver epithelial cells

In rats, gamma-glutamyl transpeptidase (gamma GT) exists as a single-copy gene, and three distinct species of RNA (types I, II, and III) that differ in their 5' untranslated regions have been identified. To compare steady-state levels of these gamma GT RNAs in rat tissues, hepatic carcinomas, and cultured cells, we used RNA dot-blot hybridization and a reverse transcriptase-polymerase chain reaction (RT-PCR) technique with oligonucleotides specifically designed for each type of RNA. Fetal liver, hepatic carcinomas, rasT24-transformed rat liver epithelial (RLE) cells and pancreas make only type III RNA. Liver and untransformed RLE cells do not make detectable levels of gamma GT RNA. We found that both fetal and adult kidneys synthesize all three types of RNA, indicating that increases in gamma GT RNA known to occur after birth do not result from recruitment of additional RNA species. When we increased the sensitivity of the assay approximately 1000 fold by sequencing the RT-PCR product directly after an additional round of amplification, we found that very low levels of types I and II RNA were present in fetal liver, rasT24-transformed RLE cells, and pancreas, and that adult liver and untransformed RLE cells synthesized very low levels of all three RNA species. Rat-1 fibroblasts did not make levels of gamma GT RNA detectable by this method. These results demonstrate that different gamma GT RNA species are regulated differently during development and neoplastic transformation and that there is a commitment in some cell types to very-low-level expression of gamma GT RNAs.

The development and application of automated gridding for efficient screening of yeast and bacterial ordered libraries

An automated gridding procedure for the inoculation of yeast and bacterial clones in high-density arrays has been developed. A 96-pin inoculating tool compatible with the standard microtiter plate format and an eight-position tablet have been designed to fit the Biomek 1000 programmable robotic workstation (Beckman Instruments). The system is used to inoculate six copies of 80 x 120-mm filters representing a total of approximately 20,000 individual clones in approximately 3 h. High-density arrays of yeast artificial chromosome (YAC) and cosmid clones have been used for rapid large-scale hybridization screens of ordered libraries. In addition, an improved PCR library screening strategy has been developed using strips cut from the high-density arrays to prepare row and column DNA pools for PCR analysis. This strategy eliminates the final hybridization step and allows identification of a single clone by PCR in 2 days. The development of automated gridding technology will have a significant impact on the establishment of fully versatile screening of ordered library resources for genomic studies.

Toxicology of quinone-thioethers

Cytotoxicity associated with exposure to quinones has generally been attributed to either redox cycling, and the subsequent development of "oxidative stress," and/or to their interaction with cellular nucleophiles, such as protein and non-protein sulfhydryls. Glutathione (GSH) is the major non-protein sulfhydryl present in cells, and conjugation of potentially toxic electrophiles with GSH is usually associated with detoxication and excretion. However, this review discusses the biological (re)activity of quinone-thioethers. For example, quinone-thioethers are (1) capable of redox cycling (2) substrates for, and inhibitors of, a variety of enzymes (3) methemoglobinemic (4) potent nephrotoxicants (5) DNA reactive and (6) may contribute to quinone-mediated carcinogenicity and neurotoxicity. The ubiquitous nature of quinones, and the high intracellular concentrations of GSH, ensures that cells and tissues will be exposed to quinone-thioethers. The toxicological importance of quinone-thioethers in quinone-mediated toxicities therefore deserves further attention.

Tissue-specific expression of multiple gamma-glutamyl transpeptidase mRNAs in rat epithelia

gamma-Glutamyl transpeptidase (GGT) is an enzyme that plays a key role in interorgan glutathione transport. Three mRNAs (mRNAI, mRNAII, and mRNAIII) are known to encode the GGT precursor; they are initiated on three separate promoters on the single GGT gene. In this work, we identified by Northern blot and RNase H analysis a new GGT mRNA (mRNAIV). This mRNA differs from the others in its 5'-noncoding sequence. This mRNA species is the predominant GGT mRNA expressed in HTC hepatoma cells and in the small intestine in which its level increases from the base to the apex of the microvillus. The analysis of the GGT gene expression pattern in kidney, mammary gland, small intestine, liver, preneoplastic liver, and HTC hepatoma cells reveals a strong tissue or cell specificity. The mRNAIII was found in all the tissues and cells; in contrast, the expression of mRNAI, mRNAII, and mRNAIV is limited in normal tissues to the kidney and to the small intestine, the two tissues that display the highest enzyme activity. The synthesis of these three mRNAs is linked to the development of the kidney proximal tubule and to the differentiation of the enterocyte. The tissue and cell specificity of the GGT gene expression is based upon the use of multiple promoters that are controlled independently by specific cell factors.

Expression of multiple proteins structurally related to gamma-glutamyl transpeptidase in non-neoplastic adult rat hepatocytes in vivo and in culture

Freshly isolated hepatocytes from normal adult rat liver do not express measurable gamma-glutamyl transpeptidase (GGT) mRNA in contrast to the significant GGT mRNA levels expressed by normal adult rat kidney and hyperplastic bile ductular tissue from bile duct-ligated rats. However, the induction of GGT activity in rat hepatocytes by two-thirds hepatectomy was accompanied by the appearance of a high level of GGT mRNA. We are now able to demonstrate that normal adult rat hepatocytes express 5 protein bands which cross-react with 2 different anti-rat kidney GGT antisera. The apparent molecular weights were 26.9, 58.0, 63.9, 73.5, and 83.4 kDa, respectively. Expression of the 26.9- and 58.0-kDa proteins strikingly parallels the pattern of induction of GGT enzymatic activity. This suggests that these 2 proteins correspond to the active dimeric enzyme previously described in kidney and neoplastic hepatocellular tissue. In normal hepatocytes, the 73.5-kDa protein represents 50% of the total GGT-immunoreactive protein, in contrast to kidney, where this band contains less than 4% of the GGT protein. The kinetics of expression of the 73.5-kDa protein upon induction of GGT activity in hepatocytes, as well as in culture turnover studies, suggests that this protein is a precursor form of the active enzyme, such as the described 78/79-kDa single-chain glycoprotein propeptide of GGT. It appears that in normal hepatocytes, this precursor is not processed to the same extent as in kidney or in hyperplastic bile ductular tissue.

Species differences in renal gamma-glutamyl transpeptidase activity do not correlate with susceptibility to 2-bromo-(diglutathion-S-yl)-hydroquinone nephrotoxicity

Administration of 2-bromo-(diglutathion-S-yl)hydroquinone (2-Br-[diGSyl]HQ) (10-30 mumol/kg; i.v.) to rats causes severe renal proximal tubular necrosis. gamma-Glutamyl transpeptidase (gamma-GT) catalyses the first step in the metabolism of glutathione (GSH) and its S-conjugates and the toxicity of 2-Br-(diGSyl)HQ can be emeliorated by inhibition of renal gamma-GT. Species differences in the specific activity of renal gamma-GT have been reported and we now describe the relationship between renal gamma-GT and species differences in susceptibility to 2-Br-(diGSyl)HQ nephrotoxicity. Although rats exhibited the highest specific activity of renal gamma-GT, and were the most sensitive species toward 2-Br-(diGSyl)HQ-mediated nephrotoxicity, renal gamma-GT activity did not correlate with susceptibility in the other species examined. Indeed, the guinea pig, which expressed the lowest activity of renal gamma-GT between the species (8% of the rat) was the only other rodent found to be responsive toward 2-Br-(diGSyl)HQ at the highest dose tested (200 mumol/kg; intracardiac). Thus, factors other than gamma-GT activity probably play an important role in modulating species susceptibility to 2-Br-(diGSyl)HQ nephrotoxicity. Although the reason(s) for the interspecies variation in response to 2-Br-(diGSyl)HQ are unclear at present, it seems possible that differences in both renal biochemistry, such as differences in the relative activities of cysteine conjugate N-acetyl transferase and deacetylase, and renal physiology, contribute to the observed results.

Comparative biochemical and immunological studies on gamma-glutamyltransferases from human kidney and renal cell carcinoma applying monoclonal antibodies

We have purified gamma-glutamyltransferases (GGT) from human kidneys and renal cell carcinomas, and fractionated them according to different lectin-binding properties of the isoenzymes. Native polyacrylamide gel electrophoresis and isoelectric focusing revealed different GGT-bands (even after desialylation) not only among kidney and renal carcinoma, but also among Con A-affine tumor fractions separated by ion-exchange chromatography. Mr of native GGTs were between 106 to 161 kDa, the pI ranged from pH 3 to 4 (pH 5 to 6 after desialylation). Monoclonal antibodies to GGT were produced. One of these, of IgG1 class and designed 138H11, recognizes human kidney GGT and, in addition, GGT from renal cell carcinomas and liver carcinomas. The specificity of mAb 138H11 for GGT was confirmed by Western blotting, by immunohistochemistry and by immunoprecipitation. The potential usefulness of mAb 138H11 in monitoring renal cancer patients and in identification of renal cancer metastases is currently being studied.