gamma-glutamyl leukotrienase, a gamma-glutamyl transpeptidase gene family member, is expressed primarily in spleen

The Emerging Roles of γ-Glutamyl Peptides Produced by γ-Glutamyltransferase and the Glutathione Synthesis System

L-γ-Glutamyl-L-cysteinyl-glycine is commonly referred to as glutathione (GSH); this ubiquitous thiol plays essential roles in animal life. Conjugation and electron donation to enzymes such as glutathione peroxidase (GPX) are prominent functions of GSH. Cellular glutathione balance is robustly maintained via regulated synthesis, which is catalyzed via the coordination of γ-glutamyl-cysteine synthetase (γ-GCS) and glutathione synthetase, as well as by reductive recycling by glutathione reductase. A prevailing short supply of L-cysteine (Cys) tends to limit glutathione synthesis, which leads to the production of various other γ-glutamyl peptides due to the unique enzymatic properties of γ-GCS. Extracellular degradation of glutathione by γ-glutamyltransferase (GGT) is a dominant source of Cys for some cells. GGT catalyzes the hydrolytic removal of the γ-glutamyl group of glutathione or transfers it to amino acids or to dipeptides outside cells. Such processes depend on an abundance of acceptor substrates. However, the physiological roles of extracellularly preserved γ-glutamyl peptides have long been unclear. The identification of γ-glutamyl peptides, such as glutathione, as allosteric modulators of calcium-sensing receptors (CaSRs) could provide insights into the significance of the preservation of γ-glutamyl peptides. It is conceivable that GGT could generate a new class of intercellular messaging molecules in response to extracellular microenvironments.

Serum Exosomal Gamma-Glutamyltransferase Activity Increased in Patients with Renal Cell Carcinoma with Advanced Clinicopathological Features

BACKGROUND

There has been no clinically useful diagnostic or prognostic biomarker for renal cell carcinoma (RCC). Serum γ-glutamyltransferase (GGT) activity has been reported to be a prognostic marker for several types of cancer including RCC. Exosomes or small extracellular vesicles present in body fluids have potential as a biomarker. We have recently demonstrated that GGT activity on exosomes isolated from serum is useful for the differential diagnosis of prostate cancer and benign prostate hyperplasia. In this study, we aimed to examine if serum exosomal GGT activity could be a marker for RCC.

METHODS

We examined GGT1 expression and GGT activity in cell lysates and exosomes from culture medium of HK-2 proximal tubule epithelial and RCC cell lines. GGT activity was measured using a fluorescent probe for GGT, γ-glutamyl hydroxymethyl rhodamine green. Serum and serum exosomal GGT activities were measured in patients with RCC. GGT1 expression in RCC tissues was evaluated by immunohistochemical staining.

RESULTS

GGT1 levels in exosomes from KMRC-1, OS-RC-2 and 786-O cells were elevated compared with those from HK-2 cells. In exosomes, GGT1 expression correlated with GGT activity determined using a fluorescent probe for GGT. In RCC patients, serum exosomal GGT activity was elevated in those with advanced stages (III/IV vs. I/II, p = 0.037) and those with microvascular invasion (with vs. without, p = 0.034). Immunohistochemical analysis showed that membranous GGT1 expression was increased in RCC with microvascular invasion. Notably, preoperative serum exosomal GGT activity could predict the likelihood of having microvascular invasion diagnosed by pathological examination of surgically resected specimens.

CONCLUSIONS

Our results suggest that serum exosomal GGT activity could be a clinically useful marker for advanced clinicopathological features of RCC patients, and its combined use with conventional diagnostic modalities may improve the diagnosis and treatment of patients.

Dysregulated metabolism of polyunsaturated fatty acids in eosinophilic allergic diseases

Polyunsaturated fatty acids (PUFAs), represented by the omega-6 fatty acid arachidonic acid (AA) and omega-3 fatty acid docosahexaenoic acid (DHA), are essential components of the human body. PUFAs are converted enzymatically into bioactive lipid mediators, including AA-derived cysteinyl leukotrienes (cys-LTs) and lipoxins and DHA-derived protectins, which orchestrate a wide range of immunological responses. For instance, eosinophils possess the biosynthetic capacity of various lipid mediators through multiple enzymes, including 5-lipoxygenase and 15-lipoxygenase, and play central roles in the regulation of allergic diseases. Dysregulated metabolism of PUFAs is reported, especially in severe asthma, aspirin-exacerbated respiratory disease, and eosinophilic chronic rhinosinusitis (ECRS), which is characterized by the overproduction of cys-LTs and impaired synthesis of pro-resolving mediators. Recently, by performing a multi-omics analysis (lipidomics, proteomics, and transcriptomics), we demonstrated the metabolic derangement of eosinophils in inflamed tissues of patients with ECRS. This abnormality occurred subsequent to altered enzyme expression of gamma-glutamyl transferase-5. In this review, we summarize the previous findings of dysregulated PUFA metabolism in allergic diseases, and discuss future prospective therapeutic strategies for correcting this imbalance.

The mercapturic acid pathway

The mercapturic acid pathway is a major route for the biotransformation of xenobiotic and endobiotic electrophilic compounds and their metabolites. Mercapturic acids (N-acetyl-l-cysteine S-conjugates) are formed by the sequential action of the glutathione transferases, γ-glutamyltransferases, dipeptidases, and cysteine S-conjugate N-acetyltransferase to yield glutathione S-conjugates, l-cysteinylglycine S-conjugates, l-cysteine S-conjugates, and mercapturic acids; these metabolites constitute a "mercapturomic" profile. Aminoacylases catalyze the hydrolysis of mercapturic acids to form cysteine S-conjugates. Several renal transport systems facilitate the urinary elimination of mercapturic acids; urinary mercapturic acids may serve as biomarkers for exposure to chemicals. Although mercapturic acid formation and elimination is a detoxication reaction, l-cysteine S-conjugates may undergo bioactivation by cysteine S-conjugate β-lyase. Moreover, some l-cysteine S-conjugates, particularly l-cysteinyl-leukotrienes, exert significant pathophysiological effects. Finally, some enzymes of the mercapturic acid pathway are described as the so-called "moonlighting proteins," catalytic proteins that exert multiple biochemical or biophysical functions apart from catalysis.

Cysteinyl leukotriene metabolism of human eosinophils in allergic disease

Eosinophils are multifaceted immune cells with diverse functions that enhance allergic inflammation. Cysteinyl leukotrienes (cys-LTs), mainly synthesized in eosinophils, are a class of inflammatory lipid mediators produced via multiple enzymatic reactions from arachidonic acid. Multiple clinical studies have reported dysregulated fatty acid metabolism in severe asthma and aspirin-exacerbated respiratory diseases. Therefore, understanding the mechanism responsible for this metabolic abnormality has attracted a lot of attention. In eosinophils, various stimuli (including cytokines, chemokines, and pathogen-derived factors) prime and/or induce leukotriene generation and secretion. Cell-cell interactions with component cells (endothelial cells, epithelial cells, fibroblasts) also enhance this machinery to augment allergic responses. Nasal polyp-derived eosinophils from patients with eosinophilic rhinosinusitis present a characteristic fatty acid metabolism with selectively higher production of leukotriene D₄. Interestingly, type 2 cytokines and microbiome components might be responsible for this metabolic change with altered enzyme expression. Here, we review the regulation of fatty acid metabolism, especially cys-LT metabolism, in human eosinophils toward allergic inflammatory status.

Dysregulated fatty acid metabolism in nasal polyp-derived eosinophils from patients with chronic rhinosinusitis

BACKGROUND

Eosinophils are multifunctional granulocytes capable of releasing various cytokines, chemokines, and lipid mediators. We previously reported dysregulated fatty acid metabolism in peripheral blood-derived eosinophils from patients with severe asthma. However, functional characteristics of eosinophils present in allergic inflammatory tissues remain largely uncharacterized.

METHODS

We established a method for isolating CD69^hi CCR3^low CXCR4^- siglec-8^int eosinophils from nasal polyps of patients with eosinophilic rhinosinusitis (NP-EOS). Multi-omics analysis including lipidomics, proteomics, and transcriptomics was performed to analyze NP-EOS as compared to peripheral blood-derived eosinophils from healthy subjects (PB-EOS).

RESULTS

Lipidomic analysis revealed impaired synthesis of prostaglandins and 15-lipoxygenase (15-LOX)-derived mediators, and selective upregulation of leukotriene D₄ production. Furthermore, proteomics and transcriptomics revealed changes in the expression of specific enzymes (GGT5, DPEP2, and 15-LOX) responsible for dysregulated lipid metabolism. Ingenuity pathway analysis indicated the importance of type 2 cytokines and pattern recognition receptor pathways. Stimulation of PB-EOS with eosinophil activators IL-5, GM-CSF, and agonists of TLR2 and NOD2 mimicked the observed changes in lipid metabolism.

CONCLUSION

Inflammatory tissue-derived eosinophils possess a specific phenotype with dysregulated fatty acid metabolism that may be targeted therapeutically to control eosinophilic inflammatory diseases.

Comparison between iMSD and 2D-pCF analysis for molecular motion studies on in vivo cells: The case of the epidermal growth factor receptor

Image correlation analysis has evolved to become a valuable method of analysis of the diffusional motion of molecules in every points of a live cell. Here we compare the iMSD and the 2D-pCF approaches that provide complementary information. The iMSD method provides the law of diffusion and it requires spatial averaging over a small region of the cell. The 2D-pCF does not require spatial averaging and it gives information about obstacles for diffusion at pixel resolution. We show the analysis of the same set of data by the two methods to emphasize that both methods could be needed to have a comprehensive understanding of the molecular diffusional flow in a live cell.

Type 2 Cysteinyl Leukotriene Receptors Drive IL-33-Dependent Type 2 Immunopathology and Aspirin Sensitivity

Cysteinyl leukotrienes (cysLTs) facilitate mucosal type 2 immunopathology by incompletely understood mechanisms. Aspirin-exacerbated respiratory disease, a severe asthma subtype, is characterized by exaggerated eosinophilic respiratory inflammation and reactions to aspirin, each involving the marked overproduction of cysLTs. Here we demonstrate that the type 2 cysLT receptor (CysLT₂R), which is not targeted by available drugs, is required in two different models to amplify eosinophilic airway inflammation via induced expression of IL-33 by lung epithelial cells. Endogenously generated cysLTs induced eosinophilia and expanded group 2 innate lymphoid cells (ILC2s) in aspirin-exacerbated respiratory disease-like Ptges^-/- mice. These responses were mitigated by deletions of either Cysltr2 or leukotriene C₄ synthase (Ltc4s). Administrations of either LTC₄ (the parent cysLT) or the selective CysLT₂R agonist N-methyl LTC₄ to allergen sensitized wild-type mice markedly boosted ILC2 expansion and IL-5/IL-13 generation in a CysLT₂R-dependent manner. Expansion of ILC2s and IL-5/IL-13 generation reflected CysLT₂R-dependent production of IL-33 by alveolar type 2 cells, which engaged in a bilateral feed-forward loop with ILC2s. Deletion of Cysltr1 blunted LTC₄-induced ILC2 expansion and eosinophilia but did not alter IL-33 induction. Pharmacological blockade of CysLT₂R prior to inhalation challenge of Ptges^-/- mice with aspirin blocked IL-33-dependent mast cell activation, mediator release, and changes in lung function. Thus, CysLT₂R signaling, IL-33-dependent ILC2 expansion, and IL-33-driven mast cell activation are necessary for induction of type 2 immunopathology and aspirin sensitivity. CysLT₂R-targeted drugs may interrupt these processes.

Gamma-glutamyltransferase activity in exosomes as a potential marker for prostate cancer

Background

Exosomes or extracellular vesicles have the potential as a diagnostic marker for various diseases including cancer. In order to identify novel exosomal markers for prostate cancer (PC), we performed proteomic analysis of exosomes isolated from PC cell lines and examined the usefulness of the marker in patients.

Methods

Exosomes isolated by differential centrifugation from the culture medium of androgen-dependent LNCaP prostate cancer cell line and its sublines of partially androgen-independent C4, androgen-independent C4–2 and bone metastatic C4–2B were subjected to iTRAQ-based proteomic analysis. Exosomes were also isolated by immunocapture and separated by size exclusion chromatography and density gradient centrifugation. Protein expression was determined by Western blot analysis. GGT activity was measured using a fluorescent probe, γ-glutamyl hydroxymethyl rhodamine green (gGlu-HMRG). Immunohistochemical analysis of tissues was performed using anti-GGT1 antibody.

Results

Among proteins upregulated in C4–2 and C4–2B cells than in LNCaP cells, we focused on gamma-glutamyltransferase 1 (GGT1), a cell-surface enzyme that regulates the catabolism of extracellular glutathione. The levels of both GGT1 large and small subunits were elevated in exosomes isolated from C4–2 and C4–2B cells by differential centrifugation and by immunocapture with anti-CD9 or -prostate-specific membrane antigen (PSMA) antibody. In cell lysates and exosomes, GGT1 expression correlated with GGT activity. Size exclusion chromatography of human serum demonstrated the presence of GGT activity and GGT1 subunits in fractions positive for CD9. Density gradient centrifugation revealed the co-presence of GGT1 subunits with CD9 in exosomes isolated by differential centrifugation from human serum. Since GGT activity correlated with GGT1 expression in serum exosomes isolated by differential centrifugation, we measured serum exosomal GGT activity in patients. Unexpectedly, we found that serum exosomal GGT activity was significantly higher in PC patients than in benign prostatic hyperplasia (BPH) patients. In support of this finding, immunohistochemical analysis showed increased GGT1 expression in PC tissues compared with BPH tissues.

Conclusions

Our results suggest that serum exosomal GGT activity could be a useful biomarker for PC.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-017-3301-x) contains supplementary material, which is available to authorized users.

Platelet-driven leukotriene C4-mediated airway inflammation in mice is aspirin-sensitive and depends on T prostanoid receptors

Cysteinyl leukotrienes (cysLTs) are bronchoconstricting lipid mediators that amplify eosinophilic airway inflammation by incompletely understood mechanisms. We recently found that LTC4, the parent cysLT, potently activates platelets in vitro and induces airway eosinophilia in allergen-sensitized and -challenged mice by a platelet- and type 2 cysLT receptor-dependent pathway. We now demonstrate that this pathway requires production of thromboxane A2 and signaling through both hematopoietic and lung tissue-associated T prostanoid (TP) receptors. Intranasal administration of LTC4 to OVA-sensitized C57BL/6 mice markedly increased the numbers of eosinophils in the bronchoalveolar lavage fluid, while simultaneously decreasing the percentages of eosinophils in the blood by a TP receptor-dependent mechanism. LTC4 upregulated the expressions of ICAM-1 and VCAM-1 in an aspirin-sensitive and TP receptor-dependent manner. Both hematopoietic and nonhematopoietic TP receptors were essential for LTC4 to induce eosinophil recruitment. Thus, the autocrine and paracrine functions of thromboxane A2 act downstream of LTC4/type 2 cysLT receptor signaling on platelets to markedly amplify eosinophil recruitment through pulmonary vascular adhesion pathways. The findings suggest applications for TP receptor antagonists in cases of asthma with high levels of cysLT production.

Gamma-glutamyl transpeptidase: redox regulation and drug resistance

The expression of gamma-glutamyl transpeptidase (GGT) is essential to maintaining cysteine levels in the body. GGT is a cell surface enzyme that hydrolyzes the gamma-glutamyl bond of extracellular reduced and oxidized glutathione, initiating their cleavage into glutamate, cysteine (cystine), and glycine. GGT is normally expressed on the apical surface of ducts and glands, salvaging the amino acids from glutathione in the ductal fluids. GGT in tumors is expressed over the entire cell membrane and provides tumors with access to additional cysteine and cystine from reduced and oxidized glutathione in the blood and interstitial fluid. Cysteine is rate-limiting for glutathione synthesis in cells under oxidative stress. The induction of GGT is observed in tumors with elevated levels of intracellular glutathione. Studies in models of hepatocarcinogenesis show that GGT expression in foci of preneoplastic hepatocytes provides a selective advantage to the cells during tumor promotion with agents that deplete intracellular glutathione. Similarly, expression of GGT in tumors enables cells to maintain elevated levels of intracellular glutathione and to rapidly replenish glutathione during treatment with prooxidant anticancer therapy. In the clinic, the expression of GGT in tumors is correlated with drug resistance. The inhibitors of GGT block GGT-positive tumors from accessing the cysteine in extracellular glutathione. They also inhibit GGT activity in the kidney, which results in the excretion of GSH in the urine and a rapid decrease in blood cysteine levels, leading to depletion of intracellular GSH in both GGT-positive and GGT-negative tumors. GGT inhibitors are being developed for clinical use to sensitize tumors to chemotherapy.

Emerging regulatory paradigms in glutathione metabolism

One of the hallmarks of cancer is the ability to generate and withstand unusual levels of oxidative stress. In part, this property of tumor cells is conferred by elevation of the cellular redox buffer glutathione. Though enzymes of the glutathione synthesis and salvage pathways have been characterized for several decades, we still lack a comprehensive understanding of their independent and coordinate regulatory mechanisms. Recent studies have further revealed that overall central metabolic pathways are frequently altered in various tumor types, resulting in significant increases in biosynthetic capacity and feeding into glutathione synthesis. In this review, we will discuss the enzymes and pathways affecting glutathione flux in cancer and summarize current models for regulating cellular glutathione through both de novo synthesis and efficient salvage. In addition, we examine the integration of glutathione metabolism with other altered fates of intermediary metabolites and highlight remaining questions about molecular details of the accepted regulatory modes.

Immunolabeling of gamma-glutamyl transferase 5 in normal human tissues reveals that expression and localization differ from gamma-glutamyl transferase 1

Gamma-glutamyl transferase (GGT5) was discovered due to its ability to convert leukotriene C4 (LTC4, a glutathione S-conjugate) to LTD4 and may have an important role in the immune system. However, it was not known which cells express the enzyme in humans. We have developed a sensitive and specific antibody that can be used to detect human GGT5 on Western blots and in fixed tissue sections. We localized GGT5 expression in normal human tissues. We observed GGT5 expressed by macrophages present in many tissues, including tissue-fixed macrophages such as Kupffer cells in the liver and dust cells in the lung. GGT5 was expressed in some of the same tissues that have been shown to express gamma-glutamyl transferase (GGT1), the only other enzymatically active protein in this family. But, the two enzymes were often expressed by different cell types within the tissue. For example, GGT5 was expressed by the interstitial cells of the kidney, whereas GGT1 is expressed on the apical surface of the renal proximal tubules. Other tissues with GGT5-positive cells included: adrenal gland, salivary gland, pituitary, thymus, spleen, liver, bone marrow, small intestine, stomach, testis, prostate and placenta. GGT5 and GGT1 are cell surface enzymes. The different pattern of expression results in their access to different extracellular fluids and therefore different substrates. GGT5 has access to substrates in blood and intercellular fluids, while GGT1 has access primarily to fluids in ducts and glands throughout the body. These data provide new insights into the different functions of these two related enzymes.

Prostaglandin E2 resistance in granulocytes from patients with aspirin-exacerbated respiratory disease

BACKGROUND

Aspirin-exacerbated respiratory disease (AERD) is an inflammatory condition of the respiratory tract and is characterized by overproduction of leukotrienes (LT) and large numbers of circulating granulocyte-platelet complexes. LT production can be suppressed by prostaglandin E(2) (PGE(2)) and the cyclic AMP-dependent protein kinase A (PKA).

OBJECTIVE

To determine if PGE(2)-dependent control of LT production by granulocytes is dysregulated in AERD.

METHODS

Granulocytes from well-characterized patients with and without AERD were activated ex vivo and subjected to a range of functional and biochemical analyses.

RESULTS

Granulocytes from subjects with AERD generated more LTB4 and cysteinyl LTs than did granulocytes from controls with aspirin-tolerant asthma and controls without asthma. When compared with controls, granulocytes from subjects with AERD had comparable levels of EP(2) protein expression and PGE(2)-mediated cAMP accumulation, yet were resistant to PGE(2)-mediated suppression of LT generation. Percentages of platelet-adherent neutrophils correlated positively with LTB4 generation and inversely with responsiveness to PGE(2)-mediated suppression of LTB(4). The PKA inhibitor H89 potentiated LTB4 generation by control granulocytes but was inactive in granulocytes from individuals with AERD and had no effect on platelet P-selectin induction. Both tonic PKA activity and levels of PKA catalytic gamma subunit protein were significantly lower in granulocytes from individuals with AERD relative to those from controls.

CONCLUSIONS

Impaired granulocyte PKA function in AERD may lead to dysregulated control of 5-lipoxygenase activity by PGE(2), whereas adherent platelets lead to increased production of LTs, which contributes to the features of persistent respiratory tract inflammation and LT overproduction.

Cysteinyl leukotriene receptors, old and new; implications for asthma

The cysteinyl leukotrienes (cys-LTs) are three structurally similar, but functionally distinct lipid mediators of inflammation. The parent cys-LT, LTC(4) , is synthesized by and released from mast cells, eosinophils, basophils, and macrophages, and is converted to the potent constrictor LTD(4) and the stable metabolite, LTE(4) . While only two cys-LT-selective receptors (CysLTRs) have been identified, cloned, and characterized, studies dating back three decades predicted the existence of at least three functional CysLTRs, each with a characteristic physiological function in airways and other tissues. The recent demonstration that mice lacking both known CysLTRs exhibit full (and in some instances, augmented) physiological responses to cys-LTs verifies the existence of unidentified CysLTRs. Moreover, the ability to manipulate receptor expression in both whole animal and cellular systems reveals that the functions of CysLTRs are controlled at multiple levels, including receptor-receptor interactions. Finally, studies in transgenic mice have uncovered a potentially major role for cys-LTs in controlling the induction of Th(2) responses to common allergens. This review focuses on these recent findings and their potential clinical implications.

Cysteinyl leukotrienes impair hypoxic pulmonary vasoconstriction in endotoxemic mice

BACKGROUND

Sepsis impairs hypoxic pulmonary vasoconstriction (HPV) in patients and animal models, contributing to systemic hypoxemia. Concentrations of cysteinyl leukotrienes are increased in the bronchoalveolar lavage fluid of patients with sepsis, but the contribution of cysteinyl leukotrienes to the impairment of HPV is unknown.

METHODS

Wild-type mice, mice deficient in leukotriene C(4) synthase, the enzyme responsible for cysteinyl leukotriene synthesis, and mice deficient in cysteinyl leukotriene receptor 1 were studied 18 h after challenge with either saline or endotoxin. HPV was measured by the increase in left pulmonary vascular resistance induced by left mainstem bronchus occlusion. Concentrations of cysteinyl leukotrienes were determined in the bronchoalveolar lavage fluid.

RESULTS

In the bronchoalveolar lavage fluid of all three strains, cysteinyl leukotrienes were not detectable after saline challenge; whereas endotoxin challenge increased cysteinyl leukotriene concentrations in wild-type mice and mice deficient in cysteinyl leukotriene receptor 1, but not in mice deficient in leukotriene C(4) synthase. HPV did not differ among the three mouse strains after saline challenge (120 ± 26, 114 ± 16, and 115 ± 24%, respectively; mean ± SD). Endotoxin challenge markedly impaired HPV in wild-type mice (41 ± 20%) but only marginally in mice deficient in leukotriene C(4) synthase (96 ± 16%, P < 0.05 vs. wild-type mice), thereby preserving systemic oxygenation. Although endotoxin modestly decreased HPV in mice deficient in cysteinyl leukotriene receptor 1 (80 ± 29%, P < 0.05 vs. saline challenge), the magnitude of impairment was markedly less than in endotoxin-challenged wild-type mice.

CONCLUSION

Cysteinyl leukotrienes importantly contribute to endotoxin-induced impairment of HPV in part via a cysteinyl leukotriene receptor 1-dependent mechanism.

Unveiling the roles of the glutathione redox system in vivo by analyzing genetically modified mice

Redox status affects various cellular activities, such as proliferation, differentiation, and death. Recent studies suggest pivotal roles of reactive oxygen species not only in pathogenesis under oxidative insult but also in intracellular signal transduction. Glutathione is present in several millimolar concentrations in the cytoplasm and has multiple roles in the regulation of cellular homeostasis. Two enzymes, γ-glutamylcysteine synthetase and glutathione synthetase, constitute the de novo synthesis machinery, while glutathione reductase is involved in the recycling of oxidized glutathione. Multidrug resistant proteins and some other transporters are responsible for exporting oxidized glutathione, glutathione conjugates, and S-nitrosoglutathione. In addition to antioxidation, glutathione is more positively involved in cellular activity via its sulfhydryl moiety of a molecule. Animals in which genes responsible for glutathione metabolism are genetically modified can be used as beneficial and reliable models to elucidate roles of glutathione in vivo. This review article overviews recent progress in works related to genetically modified rodents and advances in the elucidation of glutathione-mediated reactions.

Gamma-glutamyl compounds: substrate specificity of gamma-glutamyl transpeptidase enzymes

Gamma-glutamyl compounds include antioxidants, inflammatory molecules, drug metabolites, and neuroactive compounds. Two cell surface enzymes that metabolize gamma-glutamyl compounds have been identified: gamma-glutamyl transpeptidase (GGT1) and gamma-glutamyl leukotrienase (GGT5). There is controversy in the literature regarding the substrate specificity of these enzymes. To address this issue, we have developed a method for comprehensive kinetic analysis of compounds as substrates for GGT enzymes. Our assay is sensitive, quantitative, and conducted at physiological pH. We evaluated a series of gamma-glutamyl compounds as substrates for human GGT1 and human GGT5. The K(m) value for reduced glutathione was 11μM for both GGT1 and GGT5. However, the K(m) values for oxidized glutathione were 9μM for GGT1 and 43μM for GGT5. Our data show that the K(m) values for leukotriene C(4) are equivalent for GGT1 and GGT5 at 10.8 and 10.2μM, respectively. This assay was also used to evaluate serine-borate, a well-known inhibitor of GGT1, which was 8-fold more potent in inhibiting GGT1 than in inhibiting GGT5. These data provide essential information regarding the target enzymes for developing treatments for inflammatory diseases such as asthma and cardiovascular disease in humans. This assay is invaluable for studies of oxidative stress, drug metabolism, and other pathways that involve gamma-glutamyl compounds.

Thematic Review Series: Proteomics. An integrated omics analysis of eicosanoid biology

Eicosanoids have been implicated in a vast number of devastating inflammatory conditions, including arthritis, atherosclerosis, pain, and cancer. Currently, over a hundred different eicosanoids have been identified, with many having potent bioactive signaling capacity. These lipid metabolites are synthesized de novo by at least 50 unique enzymes, many of which have been cloned and characterized. Due to the extensive characterization of eicosanoid biosynthetic pathways, this field provides a unique framework for integrating genomics, proteomics, and metabolomics toward the investigation of disease pathology. To facilitate a concerted systems biology approach, this review outlines the proteins implicated in eicosanoid biosynthesis and signaling in human, mouse, and rat. Applications of the extensive genomic and lipidomic research to date illustrate the questions in eicosanoid signaling that could be uniquely addressed by a thorough analysis of the entire eicosanoid proteome.

Leukotriene E4 activates peroxisome proliferator-activated receptor gamma and induces prostaglandin D2 generation by human mast cells

Cysteinyl leukotrienes (cys-LTs) are potent inflammatory lipid mediators, of which leukotriene (LT) E(4) is the most stable and abundant in vivo. Although only a weak agonist of established G protein-coupled receptors (GPCRs) for cys-LTs, LTE(4) potentiates airway hyper-responsiveness (AHR) by a cyclooxygenase (COX)-dependent mechanism and induces bronchial eosinophilia. We now report that LTE(4) activates human mast cells (MCs) by a pathway involving cooperation between an MK571-sensitive GPCR and peroxisome proliferator-activated receptor (PPAR)gamma, a nuclear receptor for dietary lipids. Although LTD(4) is more potent than LTE(4) for inducing calcium flux by the human MC sarcoma line LAD2, LTE(4) is more potent for inducing proliferation and chemokine generation, and is at least as potent for upregulating COX-2 expression and causing prostaglandin D(2) (PGD(2)) generation. LTE(4) caused phosphorylation of extracellular signal-regulated kinase (ERK), p90RSK, and cyclic AMP-regulated-binding protein (CREB). ERK activation in response to LTE(4), but not to LTD(4), was resistant to inhibitors of phosphoinositol 3-kinase. LTE(4)-mediated COX-2 induction, PGD(2) generation, and ERK phosphorylation were all sensitive to interference by the PPARgamma antagonist GW9662 and to targeted knockdown of PPARgamma. Although LTE(4)-mediated PGD(2) production was also sensitive to MK571, an antagonist for the type 1 receptor for cys-LTs (CysLT(1)R), it was resistant to knockdown of this receptor. This LTE(4)-selective receptor-mediated pathway may explain the unique physiologic responses of human airways to LTE(4) in vivo.

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.

Biosynthesis and metabolism of leukotrienes

Leukotrienes are metabolites of arachidonic acid derived from the action of 5-LO (5-lipoxygenase). The immediate product of 5-LO is LTA4 (leukotriene A4), which is enzymatically converted into either LTB4 (leukotriene B4) by LTA4 hydrolase or LTC4 (leukotriene C4) by LTC4 synthase. The regulation of leukotriene production occurs at various levels, including expression of 5-LO, translocation of 5-LO to the perinuclear region and phosphorylation to either enhance or inhibit the activity of 5-LO. Several other proteins, including cPLA2α (cytosolic phospholipase A2α) and FLAP (5-LO-activating protein) also assemble at the perinuclear region before production of LTA4. LTC4 synthase is an integral membrane protein that is present at the nuclear envelope; however, LTA4 hydrolase remains cytosolic. Biologically active LTB4 is metabolized by ω-oxidation carried out by specific cytochrome P450s (CYP4F) followed by β-oxidation from the ω-carboxy position and after CoA ester formation. Other specific pathways of leukotriene metabolism include the 12-hydroxydehydrogenase/15-oxo-prostaglandin-13-reductase that forms a series of conjugated diene metabolites that have been observed to be excreted into human urine. Metabolism of LTC4 occurs by sequential peptide cleavage reactions involving a γ-glutamyl transpeptidase that forms LTD4 (leukotriene D4) and a membrane-bound dipeptidase that converts LTD4 into LTE4 (leukotriene E4) before ω-oxidation. These metabolic transformations of the primary leukotrienes are critical for termination of their biological activity, and defects in expression of participating enzymes may be involved in specific genetic disease.

Mast cells and eicosanoid mediators: a system of reciprocal paracrine and autocrine regulation

When activated by specific antigen, complement, or other transmembrane stimuli, mast cells (MCs) generate three eicosanoids: prostaglandin (PG)D(2), leukotriene (LT)B(4), and LTC(4), the parent molecule of the cysteinyl leukotrienes (cysLTs). These diverse lipid mediators, which are generated from a single cell membrane-associated precursor, arachidonic acid, can initiate, amplify, or dampen inflammatory responses and influence the magnitude, duration, and nature of subsequent immune responses. PGD(2) and cysLTs, which were originally recognized for their bronchoconstricting and vasoactive properties, also serve diverse and pivotal functions in effector cell trafficking, antigen presentation, leukocyte activation, matrix deposition, and fibrosis. LTB(4) is a powerful chemoattractant for neutrophils and certain lymphocyte subsets. Thus, MCs can contribute to each of these processes through eicosanoid generation. Additionally, MCs express G-protein-coupled receptors specific for cysLTs, LTB(4), and another eicosanoid, PGE(2). Each of these receptors can regulate MC functions in vivo by autocrine and paracrine mechanisms. This review focuses on the biologic functions for MC-associated eicosanoids, the regulation of their production, and the mechanisms by which eicosanoids may regulate MC function in host defense and disease.

Uncoupling the enzymatic and autoprocessing activities of Helicobacter pylori gamma-glutamyltranspeptidase

Gamma-glutamyltranspeptidase (gammaGT), a member of the N-terminal nucleophile hydrolase superfamily, initiates extracellular glutathione reclamation by cleaving the gamma-glutamyl amide bond of the tripeptide. This protein is translated as an inactive proenzyme that undergoes autoprocessing to become an active enzyme. The resultant N terminus of the cleaved proenzyme serves as a nucleophile in amide bond hydrolysis. Helicobacter pylori gamma-glutamyltranspeptidase (HpGT) was selected as a model system to study the mechanistic details of autoprocessing and amide bond hydrolysis. In contrast to previously reported gammaGT, large quantities of HpGT were expressed solubly in the inactive precursor form. The 60-kDa proenzyme was kinetically competent to form the mature 40- and 20-kDa subunits and exhibited maximal autoprocessing activity at neutral pH. The activated enzyme hydrolyzed the gamma-glutamyl amide bond of several substrates with comparable rates, but exhibited limited transpeptidase activity relative to mammalian gammaGT. As with autoprocessing, maximal enzymatic activity was observed at neutral pH, with hydrolysis of the acyl-enzyme intermediate as the rate-limiting step. Coexpression of the 20- and 40-kDa subunits of HpGT uncoupled autoprocessing from enzymatic activity and resulted in a fully active heterotetramer with kinetic constants similar to those of the wild-type enzyme. The specific contributions of a conserved threonine residue (Thr380) to autoprocessing and hydrolase activities were examined by mutagenesis using both the standard and coexpression systems. The results of these studies indicate that the gamma-methyl group of Thr380 orients the hydroxyl group of this conserved residue, which is required for both the processing and hydrolase reactions.

Interactions between leukotriene C4 and interleukin 13 signaling pathways in a mouse model of airway disease

CONTEXT

During an asthmatic episode, leukotriene C4 (LTC4) and interleukin 13 (IL-13) are released into the airways and are thought to be central mediators of the asthmatic response. However, little is known about how these molecules interact or affect each other's signaling pathway.

OBJECTIVE

To determine if the LTC4 and IL-13 signaling pathways interact with each other's pathways.

DESIGN

We examined airway responsiveness, cysteinyl LTs (Cys-LTs), and Cys-LT and IL-13 receptor transcript levels in wild-type mice and in mice that were deficient in gamma-glutamyl leukotrienase (an enzyme that converts LTC4 to LTD4), STAT6 (signal transducer and activator of transcription 6 [a critical molecule in IL-13 signaling]), and IL-4Ralpha (a subunit of the IL-13 receptor).

RESULTS

Wild-type (C57BL/129SvEv) and gamma-glutamyl leukotrienase-deficient mice showed increased airway responsiveness after intranasal instillation of IL-13; similar results were observed after intranasal instillation of IL-13 or LTC4 in a second wild-type strain (BALB/c). Interleukin 13 treatment reduced levels of Cys-LTs in bronchoalveolar lavage fluid. This change was unaccompanied by changes in other arachidonic acid metabolites or in RNA transcript levels of enzymes associated with Cys-LT synthesis. Interleukin 13 treatment also increased transcript levels of the Cys-LT 1 and Cys-LT 2 receptors, while LTC4 increased transcript levels of the alpha1 chain of the IL-13 receptor. Furthermore, IL-4Ralpha-deficient mice had increased airway responsiveness to LTC4 but not to IL-13, whereas STAT6-deficient mice failed to respond to either agonist.

CONCLUSIONS

These findings indicate that LTC4 and IL-13 are dependent on or signal through STAT6 to increase airway responsiveness and that both agonists regulate expression of each other's receptors.

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.

Cysteinyl leukotrienes and their receptors: cellular distribution and function in immune and inflammatory responses

The cysteinyl leukotrienes (cys-LTs) are a family of potent bioactive lipids that act through two structurally divergent G protein-coupled receptors, termed the CysLT(1) and CysLT(2) receptors. The cloning and characterization of these two receptors has not only reconciled findings of previous pharmacologic profiling studies of contractile tissues, but also has uncovered their expression on a wide array of circulating and tissue-dwelling leukocytes. With the development of receptor-selective reagents, as well as mice lacking critical biosynthetic enzymes, transporter proteins, and the CysLT(1) receptor, diverse functions of cys-LTs and their receptors in immune and inflammatory responses have been identified. We review cys-LT biosynthesis; the molecular biology and distribution of the CysLT(1) and CysLT(2) receptors; the functions of cys-LTs and their receptors in the recruitment and activation of effector leukocytes and induction of adaptive immunity; and the development of fibrosis and airway remodeling in animal models of lung injury and allergic inflammation.

Identification of two additional members of the membrane-bound dipeptidase family

We have cloned two mouse cDNAs encoding previously unidentified membrane-bound dipeptidases [membrane-bound dipeptidase-2 (MBD-2) and membrane-bound dipeptidase-3 (MBD-3)] from membrane-bound dipeptidase-1 (MBD-1) deficient mice (Habib, G.M., Shi, Z-Z., Cuevas, A.A., Guo, Q., Matzuk, M.M., and Lieberman, M.W. (1998) Proc. Natl. Acad. Sci. USA 95, 4859-4863). These enzymes are closely related to MBD-1 (EC 3.4.13.19), which is known to cleave leukotriene D4 (LTD4) and cystinyl-bis-glycine. MBD-2 cDNA is 56% identical to MBD-1 with a predicted amino acid identity of 33%. The MBD-3 and MBD-1 cDNAs share a 55% nucleotide identity and a 39% predicted amino acid sequence identity. All three genes are tightly linked on the same chromosome. Expression of MBD-2 and MBD-3 in Cos cells indicated that both are membrane-bound through a glycosylphosphatidyl-inositol linkage. MBD-2 cleaves leukotriene D4 (LTD4) but not cystinyl-bis-glycine, while MBD-3 cleaves cystinyl-bis-glycine but not LTD4. MBD-1 is expressed at highest levels in kidney, lung, and heart and is absent in spleen, while MBD-2 is expressed at highest levels in lung, heart, and testis and at somewhat lower levels in spleen. Of the tissues examined, MBD-3 expression was detected only in testis. Our identification of a second enzyme capable of cleaving LTD4 raises the possibility that clearance of LTD4 during asthma and in related inflammatory conditions may be mediated by more than one enzyme.

Autocatalytic processing of gamma-glutamyltranspeptidase

gamma-Glutamyltranspeptidase is the key enzyme in glutathione metabolism, and we previously presented evidence suggesting that it belongs to the N-terminal nucleophile hydrolase superfamily. Enzymatically active gamma-glutamyltranspeptidase, which consists of one large subunit and one small subunit, is generated from an inactive common precursor through post-translational proteolytic processing. The processing mechanism for gamma-glutamyltranspeptidase of Escherichia coli K-12 has been analyzed by means of in vitro studies using purified precursors. Here we show that the processing of a precursor of gamma-glutamyltranspeptidase is an intramolecular autocatalytic event and that the catalytic nucleophile for the processing reaction is the oxygen atom of the side chain of Thr-391 (N-terminal residue of the small (beta) subunit), which is also the nucleophile for the enzymatic reaction.

Gamma-glutamyl leukotrienase, a novel endothelial membrane protein, is specifically responsible for leukotriene D(4) formation in vivo

The metabolism of cysteinyl leukotrienes in vivo and the pathophysiological effects of individual cysteinyl leukotrienes are primarily unknown. Recently we identified an additional member of the gamma-glutamyl transpeptidase (GGT) family, gamma-glutamyl leukotrienase (GGL), and developed mice deficient in this enzyme. Here we show that in vivo GGL, and not GGT as previously believed, is primarily responsible for conversion of leukotriene C(4) to leukotriene D(4), the most potent of the cysteinyl leukotrienes and the immediate precursor of leukotriene E(4). GGL is a glycoprotein consisting of two polypeptide chains encoded by one gene and is attached at the amino terminus of the heavy chain to endothelial cell membranes. In mice it localizes to capillaries and sinusoids in most organs and in lung to larger vessels as well. In contrast to wild-type and GGT-deficient mice, GGL-deficient mice do not form leukotriene D(4) in vivo either in blood when exogenous leukotriene C(4) is administered intravenously or in bronchoalveolar lavage fluid of Aspergillus fumigatus extract-induced experimental asthma. Further, GGL-deficient mice show leukotriene C(4) accumulation and significantly more airway hyperreponsiveness than wild-type mice in the experimental asthma, and induction of asthma results in increased GGL protein levels and enzymatic activity. Thus GGL plays an important role in leukotriene D(4) synthesis in vivo and in inflammatory processes.

Disruption of gamma-glutamyl leukotrienase results in disruption of leukotriene D(4) synthesis in vivo and attenuation of the acute inflammatory response

To study the function of gamma-glutamyl leukotrienase (GGL), a newly identified member of the gamma-glutamyl transpeptidase (GGT) family, we generated null mutations in GGL (GGL(tm1)) and in both GGL and GGT (GGL(tm1)-GGT(tm1)) by a serial targeting strategy using embryonic stem cells. Mice homozygous for GGL(tm1) show no obvious phenotypic changes. Mice deficient in both GGT and GGL have a phenotype similar to the GGT-deficient mice, but approximately 70% of these mice die before 4 weeks of age, at least 2 months earlier than mice deficient only in GGT. These double-mutant mice are unable to cleave leukotriene C(4) (LTC(4)) to LTD(4), indicating that this conversion is completely dependent on the two enzymes, and in some organs (spleen and uterus) deletion of GGL alone abolished more than 90% of this activity. In an experimental model of peritonitis, GGL alone is responsible for the generation of peritoneal LTD(4). Further, during the development of peritonitis, GGL-deficient mice show an attenuation in neutrophil recruitment but not of plasma protein influx. These findings demonstrate an important role for GGL in the inflammatory response and suggest that LTC(4) and LTD(4) have distinctly different functions in the inflammatory process.

PATHWAYS AND REGULATION OF SULFUR METABOLISM REVEALED THROUGH MOLECULAR AND GENETIC STUDIES

Sulfur is essential for life. Its oxidation state is in constant flux as it circulates through the global sulfur cycle. Plants play a key role in the cycle since they are primary producers of organic sulfur compounds. They are able to couple photosynthesis to the reduction of sulfate, assimilation into cysteine, and further metabolism into methionine, glutathione, and many other compounds. The activity of the sulfur assimilation pathway responds dynamically to changes in sulfur supply and to environmental conditions that alter the need for reduced sulfur. Molecular genetic analysis has allowed many of the enzymes and regulatory mechanisms involved in the process to be defined. This review focuses on recent advances in the field of plant sulfur metabolism. It also emphasizes areas about which little is known, including transport and recycling/degradation of sulfur compounds.

Accumulation of DNA damage in the organs of mice deficient in gamma-glutamyltranspeptidase

We have used a differential alkaline single cell gel electrophoresis assay of DNA ("omet assay" at pH 13 and 12.3) to evaluate DNA damage as a function of age in mice with an inherited defect in gluthathione (GSH) metabolism. The mice are homozygous null for gamma-glutamyltranspeptidase (GGT), the enzyme responsible for initiating the catabolism of GSH, and paradoxically have reduced levels of GSH and cysteine in many organs. We found an accumulation of DNA damage in lung, liver and kidney in these mice as a function of age. The largest differences were in assays run at pH 13, suggesting that the accumulation of apurinic/apryrimidinic (AP) sites and oxidative damage of DNA was largely responsible. In contrast, little if any accumulation of these lesions was detected in wild-type mice. Although these findings do not allow a precise analysis of the molecular basis of damage accumulation in GGT-deficient mice, they implicate low GSH and cysteine levels as a cause of accumulative DNA damage in the intact mammal.

Insight into prostaglandin, leukotriene, and other eicosanoid functions using mice with targeted gene disruptions

In recent years, there has been an exponential increase in the number of targeted gene disruptions performed in mice. At least 18 different gene knockouts have now been reported that have direct relevance to eicosanoid biology. These include genes that influence substrate availability (phospholipases), metabolism to eicosanoids (e.g., prostaglandin H synthases, lipoxygenases), and eicosanoid action (e.g., receptors for various prostaglandins). This minireview will outline the phenotype of these knockout mice and what has been learned about eicosanoid functions through use of this novel methodology.