Gamma-glutamyl transpeptidase is up-regulated on memory T lymphocytes

Extracellular Vesicles—A New Potential Player in the Immunology of Renal Cell Carcinoma

The incidence of renal cell carcinoma (RCC) has doubled in the developed world within the last fifty years, and now it is responsible for 2–3% of diagnosed cancers. The delay in diagnosis and the not fully understood pathogenesis are the main challenges that have to be overcome. It seems that extracellular vesicles (EVs) are one of the key players in tumor development since they ensure a proper microenvironment for the tumor cells. The stimulation of angiogenesis and immunosuppression is mediated by molecules contained in EVs. It was shown that EVs derived from cancer cells can inhibit T cell proliferation, natural killer lymphocyte activation, and dendritic cell maturation by this mechanism. Moreover, EVs may be a biomarker for the response to anti-cancer treatment. In this review, we sum up the knowledge about the role of EVs in RCC pathogenesis and show their future perspectives in this field.

Novel Knowledge-Based Transcriptomic Profiling of Lipid Lysophosphatidylinositol-Induced Endothelial Cell Activation

To determine whether pro-inflammatory lipid lysophosphatidylinositols (LPIs) upregulate the expressions of membrane proteins for adhesion/signaling and secretory proteins in human aortic endothelial cell (HAEC) activation, we developed an EC biology knowledge-based transcriptomic formula to profile RNA-Seq data panoramically. We made the following primary findings: first, G protein-coupled receptor 55 (GPR55), the LPI receptor, is expressed in the endothelium of both human and mouse aortas, and is significantly upregulated in hyperlipidemia; second, LPIs upregulate 43 clusters of differentiation (CD) in HAECs, promoting EC activation, innate immune trans-differentiation, and immune/inflammatory responses; 72.1% of LPI-upregulated CDs are not induced in influenza virus-, MERS-CoV virus- and herpes virus-infected human endothelial cells, which hinted the specificity of LPIs in HAEC activation; third, LPIs upregulate six types of 640 secretomic genes (SGs), namely, 216 canonical SGs, 60 caspase-1-gasdermin D (GSDMD) SGs, 117 caspase-4/11-GSDMD SGs, 40 exosome SGs, 179 Human Protein Atlas (HPA)-cytokines, and 28 HPA-chemokines, which make HAECs a large secretory organ for inflammation/immune responses and other functions; fourth, LPIs activate transcriptomic remodeling by upregulating 172 transcription factors (TFs), namely, pro-inflammatory factors NR4A3, FOS, KLF3, and HIF1A; fifth, LPIs upregulate 152 nuclear DNA-encoded mitochondrial (mitoCarta) genes, which alter mitochondrial mechanisms and functions, such as mitochondrial organization, respiration, translation, and transport; sixth, LPIs activate reactive oxygen species (ROS) mechanism by upregulating 18 ROS regulators; finally, utilizing the Cytoscape software, we found that three mechanisms, namely, LPI-upregulated TFs, mitoCarta genes, and ROS regulators, are integrated to promote HAEC activation. Our results provide novel insights into aortic EC activation, formulate an EC biology knowledge-based transcriptomic profile strategy, and identify new targets for the development of therapeutics for cardiovascular diseases, inflammatory conditions, immune diseases, organ transplantation, aging, and cancers.

Oral administration of γ-glutamylcysteine increases intracellular glutathione levels above homeostasis in a randomised human trial pilot study

Objective

To determine if orally dosed γ-glutamylcysteine (γ-GC) can increase cellular glutathione (GSH) levels above homeostasis. Many chronic and age-related disorders are associated with down-regulation, or impairment, of glutamate cysteine ligase (GCL). This suggests that γ-GC supply may become limiting for the maintenance of cellular GSH at the normal levels required to effectively protect against oxidative stress and any resulting physiological damage.

Methods

GSH levels were measured in lymphocytes of healthy, non-fasting participants before and after single oral doses (2 and 4 g) of γ-GC. Blood samples were immediately processed using high speed fluorescence-activated cell sorting to isolate 10⁶ lymphocytes that were then assayed for GSH content.

Results

A single 2 g dose of γ-GC increased lymphocyte GSH content above basal levels (53±47%, p<0.01, n=14) within 90 min of administration. A randomized dosage (2 and 4 g γ-GC) crossover design was used to explore the pharmacokinetics of this GSH increase. In general, for both dose levels (n=9), GSH increased from initial basal levels over 3 h (t_max) before reaching maximum GSH concentrations (C_max) that were near two (2 g γ-GC) to three (4 g γ-GC) fold basal levels (0.4 nmol/10⁶ lymphocytes). Beyond t_max, GSH levels progressively declined reaching near basal levels by 5 h. The GSH half-life was between 2 and 3 h with exposure (AUC) to increased GSH levels of 0.7 (2 g γ-GC) and 1.8 (4 g γ-GC) nmol.h/10⁶ lymphocytes.

Conclusions

Oral γ-GC is a non-toxic form of cysteine that can be directly taken up by cells and transiently increase lymphocyte GSH above homeostatic levels. Our findings that γ-GC can increase GSH levels in healthy subjects suggests that it may have potential as an adjunct for treating diseases associated with chronic GSH depletion. This trial was registered at anzctr.org.au as ACTRN12612000952842.

Plasma Levels of Cell-Free Apoptotic DNA Ladders and Gamma-Glutamyltranspeptidase (GGT) in Diabetic Children

The plasma levels of apoptotic DNA ladders (i.e., apoptosemia) and γ-glutamyltranspeptidase (GGT) in diabetic outpatients and rats were investigated. Apoptotic DNA ladders were detected in plasma from 26.8% of type 1 (T1) and 18.5% of type 2 (T2) diabetic children 1–20 years of age, 25.7% of hospitalized children and 35.7% of adult RA outpatients, but in only 3.5% of adult pre-op patients. Plasma from 7.7% of young streptozotocin-induced diabetic but not control rats contained apoptotic DNA ladders. Apoptosemia was detected more often in male T1 (31%) and T2 (30.8%) diabetic outpatients than in female T1 (20.8%) and T2 (15.4%) diabetic outpatients. GGT in apoptosemic plasma was significantly higher than in nonapoptosemic plasma from T1 ( P = 0.001) but not T2 diabetic children. The highest amounts of apoptotic DNA were detected most often in diabetic children ≥14 years of age. In vitro study results suggest that cell-free apoptotic DNA ladders appear prior to an increase in GGT activity in serum from human blood incubated at 37°C. The results suggest that 24.7% of plasma samples from diabetic children contained apoptotic DNA ladders, the incidence and amounts of apoptotic DNA ladders were higher in the older diabetic children, and GGT was elevated in apoptosemic T1 diabetic children ( P = 0.01). The results indicate that “silent” apoptosemia occurs in T1 and T2 diabetic children and suggest elevated GGT in diabetic children could be due to release from apoptotic cells.

Contribution by polymorphonucleate granulocytes to elevated gamma-glutamyltransferase in cystic fibrosis sputum

BACKGROUND

Cystic fibrosis (CF) is an autosomal recessive disorder characterized by a chronic neutrophilic airways inflammation, increasing levels of oxidative stress and reduced levels of antioxidants such as glutathione (GSH). Gamma-glutamyltransferase (GGT), an enzyme induced by oxidative stress and involved in the catabolism of GSH and its derivatives, is increased in the airways of CF patients with inflammation, but the possible implications of its increase have not yet been investigated in detail.

PRINCIPAL FINDINGS

The present study was aimed to evaluate the origin and the biochemical characteristics of the GGT detectable in CF sputum. We found GGT activity both in neutrophils and in the fluid, the latter significantly correlating with myeloperoxidase expression. In neutrophils, GGT was associated with intracellular granules. In the fluid, gel-filtration chromatography showed the presence of two distinct GGT fractions, the first corresponding to the human plasma b-GGT fraction, the other to the free enzyme. The same fractions were also observed in the supernatant of ionomycin and fMLP-activated neutrophils. Western blot analysis confirmed the presence of a single band of GGT immunoreactive peptide in the CF sputum samples and in isolated neutrophils.

CONCLUSIONS

In conclusion, our data indicate that neutrophils are able to transport and release GGT, thus increasing GGT activity in CF sputum. The prompt release of GGT may have consequences on all GGT substrates, including major inflammatory mediators such as S-nitrosoglutathione and leukotrienes, and could participate in early modulation of inflammatory response.

Treatment with anti-gamma-glutamyl transpeptidase antibody attenuates osteolysis in collagen-induced arthritis mice

The effectiveness of a new antibody treatment on arthritis-associated osteolysis was studied by using CIA mice. GGT, a newly identified bone-resorbing factor, was upregulated in arthritic joints. We generated monoclonal antibodies against GGT and injected them into CIA mice. Mice treated with antibodies showed a reduction in osteoclast number and bone erosion.

INTRODUCTION

Gamma-glutamyl transpeptidase (GGT) acts as a bone-resorbing factor that stimulates osteoclast formation. GGT expression has been detected in active lymphocytes that accumulate at inflammation sites, such as rheumatoid arthritis (RA). We hypothesize that GGT is an effective target for suppression of arthritis-related osteoclastogenesis and joint destruction. Here, we describe the therapeutic effect of neutralizing antibodies against GGT on joint destruction using a collagen-induced arthritis (CIA) mouse model.

MATERIALS AND METHODS

GGT expression in the synovium of RA patients and CIA mice was determined by immunohistochemistry and RT-PCR. Monoclonal antibodies were generated against recombinant human GGT (GGT-mAbs) using BALB/c mice. Antibody treatment was performed by intraperitoneal injections of GGT-mAbs into CIA mice. Effects of antibody treatment on arthritis and bone erosion were evaluated by incidence score, arthritis score, and histopathological observations. The role of GGT in osteoclast development was examined by using the established osteoclastogenic culture system.

RESULTS

GGT expression was significantly upregulated in inflamed synovium. Immunohistochemistry revealed that GGT was present in lymphocytes, plasma cells, and macrophages, as well as capillaries. Injection of GGT-mAbs significantly decreased the number of osteoclasts and attenuated the severity of joint destruction in CIA mice. In vitro examination showed that GGT enhanced RANKL-dependent osteoclast formation. GGT stimulated the expression of RANKL in osteoblasts and its receptor RANK in osteoclast precursors, respectively.

CONCLUSIONS

This study indicates that inflamed synovial tissue-derived GGT acts as a risk factor for joint destruction and that the antibody-mediated inhibition of GGT significantly decreases osteoclast number and bone erosion in CIA mice. GGT antagonists might be novel therapeutic agents for attenuating joint destruction in RA patients.

Redox equilibrium in mucosal T cells tunes the intestinal TCR signaling threshold

Mucosal immune tolerance in the healthy intestine is typified by lamina propria T cell (LPT) functional hyporesponsiveness after TCR engagement when compared with peripheral blood T cell (PBT). When LPT from an inflamed intestine are activated through TCR cross-linking, their responsiveness is stronger. LPT are thus capable of switching from a tolerant to a reactive state, toggling between high and low thresholds of activation. We demonstrate that in normal LPT global tyrosine phosphorylation upon TCR cross-linking or an increase in intracellular H2O2, an inhibitor of protein tyrosine phosphatases, is muted. Thus, we propose that LPT have a greater reducing capacity than PBT, shifting the balance between kinases and protein tyrosine phosphatases in favor of the latter. Surface gamma-glutamyl transpeptidase, an indirect indicator of redox potential, and glutathione are significantly elevated in LPT compared with PBT, suggesting that elevated glutathione detoxifies TCR-induced reactive oxygen species. When glutathione is depleted, TCR-induced LPT tyrosine phosphorylation rises to PBT levels. Conversely, increasing glutathione in PBT attenuates tyrosine phosphorylation. In LPT isolated from inflamed mucosa, TCR cross-linking induces greater phosphorylation, and gamma-glutamyl transpeptidase levels are reduced compared with those from autologous noninflamed tissue. We conclude that the high TCR signaling threshold of mucosal T cells is tuned by intracellular redox equilibrium, whose dysregulation may mediate intestinal inflammation.

Activity Determination, Kinetic Analyses and Isoenzyme Identification of Gamma Glutamyltransferase in Human Neutrophils

Gamma-glutamyltransferase (GGT, EC 2.3.2.2) which hydrolyzes glutathione (GSH), is required for the maintenance of normal intracellular GSH concentration. GGT is a membrane enzyme present in leukocytes and platelets. Its activity has also been observed in human neutrophils. In this study, GGT was purified from Triton X-100 solubilized neutrophils and its kinetic parameters were determined. For kinetic analyses of transpeptidation reaction, gamma-glutamyl p-nitroanilide was used as the substrate and glycylglycine as the acceptor. Apparent K(m) values were determined as 1.8 mM for gamma-glutamyl p-nitroanilide and 16.9 mM for glycylglycine. The optimum pH of GGT activity was 8.2 and the optimum temperature was 37 degrees C. It had thermal stability with 58 % relative activity at 56 degrees C for 30 min incubation. L-serine, in the presence of borate, was detected as the competitive inhibitor. Bromcresol green inhibited neutrophil GGT activity as a noncompetitive inhibitor. The neutrophils seem to contain only the isoenzyme that is present in platelets. We characterized the kinetic properties and compared the type of the isoenzyme of neutrophil GGT with platelet GGT via polyacrylamide gel electrophoresis (PAGE) under a standard set of conditions.

Motexafin gadolinium generates reactive oxygen species and induces apoptosis in sensitive and highly resistant multiple myeloma cells

Motexafin gadolinium (MGd), an expanded porphyrin, is a tumor-selective redox-mediator that reacts with many intracellular reducing metabolites. Because redox mechanisms mediate apoptosis in multiple myeloma, we hypothesized that disruption of redox balance by MGd would result in cellular cytotoxicity in myeloma. We examined the effects of MGd on cellular cytotoxicity, apoptosis, reactive oxygen species (ROS) production, and intracellular drug uptake in dexamethasone-sensitive (C2E3), dexamethasone-resistant (1-310 and 1-414) chemotherapy-sensitive (8226-RPMI) and highly chemotherapy-resistant (DOX-10V) myeloma cells. We found complete inhibition of proliferation and cytotoxicity in each sensitive and resistant cell line with 24-hour exposure to clinically relevant concentrations of 50 μM MGd and 50 to 100 μM ascorbate, which was required for the effect. The mechanism of cytotoxicity was related to induction of apoptosis as demonstrated by alteration in mitochondrial membrane potential and elevated annexin V expression. This was accompanied by depletion of intracellular glutathione and increased ROS production. Moreover, catalase substantially abrogated MGd-induced cell death. Using fluorescence microscopy and flow cytometry, we found intracellular uptake of MGd and intracellular ROS production. MGd also induced apoptosis in fresh malignant cells from patients with multiple myeloma. These studies provide a rationale for clinical investigation of this novel redox-mediating agent in patients with multiple myeloma and related disorders.

B-cell homeostasis: digital survival or analog growth?

Maintenance of B-lymphocyte homeostasis requires balanced cell production, death, and proliferation. To coordinate these processes, B cells are dependent on cell extrinsic signals. In lymphocyte development, precursor cells are dependent on Fms-like tyrosine kinase ligand 3 (Flt3L), and pre-B cells are dependent on the cytokine interleukin-7. Transitional B cells require B-lymphocyte stimulator (BLyS) for survival. Mature B cells require B-cell receptor (BCR) signals and also remain sensitive to their microenvironment. An emerging model suggests that extrinsic signals do not regulate B-cell survival through a digital mechanism where cells are simply instructed to survive or die. Instead, availability and competition for extrinsic signals regulates cellular physiology and metabolism in an analog fashion that then influences cell commitment to apoptosis or proliferation. Decreases in cellular metabolism may sensitize cells to activation and action of the pro-apoptotic Bcl-2 family members, Bak and Bax, and promote apoptosis. In contrast, increases in metabolism may predispose cells to proliferate. Analog control of cell physiology can, thus, be integrated with other inputs by individual cells to produce a fate decision for survival, proliferation, or apoptosis and prevent diseases of cell death, such as immunodeficiency, and cell activation and proliferation, such as autoimmunity or cancer.

Phosphatases in concert with kinases set the gain for signal transduction through the T cell receptor

The 'tunable activation thresholds' model for signal transduction through the T cell receptor (TCR)/CD3 signaling complex proposes that rapid cycles of phosphorylation and dephosphorylation are integral to regulating the frequency of protein-protein interaction, thus having considerable influence over the activation of downstream signaling pathways. Co-temporal activation of kinases and phosphatases could serve to modulate the ongoing signaling response, depending on the relative balance of their opposing enzymatic activities. Although recent reports have addressed the mechanisms by which specific kinase/phosphatase pairs contribute to the initiation and termination of signaling, we sought a more global understanding of the ability of the kinase/phosphatase balance to regulate, or "tune", the very proximal steps of TCR signaling in primary human T cells. Herein, we provide biochemical evidence that phosphotyrosine induction via the TCR is subject to fine-tuning based on the overall activity of kinases and phosphatases relative to one another, leading to cycles of phosphorylation and dephosphorylation, with implications for developing the next generation of immunotherapeutic agents.

T cell receptor-stimulated generation of hydrogen peroxide inhibits MEK-ERK activation and lck serine phosphorylation

Previous studies indicated that antigen receptor (TcR) stimulation of mature T cells induced rapid generation of reactive oxygen species (ROS). The goal of the current study was to examine the role(s) of ROS in TcR signal transduction, with a focus upon the redox-sensitive MAPK family. TcR cross-linking of primary human T blasts and Jurkat human T cells rapidly activated the ERK, JNK, p38 and Akt kinases within minutes, and was temporally associated with TcR-stimulated production of hydrogen peroxide (H(2)O(2)). TcR-induced activation of ERK was selectively augmented and sustained in the presence of pharmacologic antioxidants that can quench or inhibit H(2)O(2) production (NAC, MnTBAP and Ebselen, but not DPI), while activation of JNK and Akt were largely unaffected. This was paralleled by concurrent changes in MEK1/2 phosphorylation, suggesting that ROS acted upstream of MEK-ERK activation. Molecular targeting of H(2)O(2) by overexpression of peroxiredoxin II, a thioredoxin dependent peroxidase, also increased and sustained ERK and MEK activation upon TcR cross-linking. Enhancement of ERK phosphorylation by antioxidants correlated with increased and sustained serine phosphorylation of the src-family kinase lck, a known ERK substrate. Thus, the data suggest that TcR-stimulated production of hydrogen peroxide negatively feeds back to dampen antigen-stimulated ERK activation and this redox-dependent regulation may serve to modulate key steps in TcR signaling.

Interleukin (IL)-15R[alpha]-deficient natural killer cells survive in normal but not IL-15R[alpha]-deficient mice

Natural killer (NK) cells protect hosts against viral pathogens and transformed cells. IL-15 is thought to play a critical role in NK cell development, but its role in the regulation of peripheral NK cells is less well defined. We now find that adoptive transfer of normal NK cells into mice lacking the high affinity interleukin (IL)-15 receptor, IL-15Ralpha, surprisingly results in the abrupt loss of these cells. Moreover, IL-15Ralpha-deficient NK cells can differentiate successfully in radiation bone marrow chimera bearing normal cells. Finally, adoptively transferred IL-15Ralpha-deficient NK cells survive in normal but not IL-15Ralpha-deficient mice. These findings demonstrate that NK cell-independent IL-15Ralpha expression is critical for maintaining peripheral NK cells, while IL-15Ralpha expression on NK cells is not required for this function.

Interaction between nitric oxide and subsets of human T lymphocytes with differences in glutathione metabolism

Nitric oxide (NO) modulates human T-lymphocyte responses through several mechanisms. In the current study we show that interactions between NO and glutathione (GSH) metabolism are related to the selective persistent inhibition of interferon-gamma (IFN-gamma) production by NO, which we previously identified. T cells were exposed to NO using the NO-donor compound Spermine-nonoate (Sper) and activated using anti-CD3 plus anti-CD28 monoclonal antibodies. Persistent inhibition of IFN-gamma by Sper was prevented by addition of the GSH precursor l-cysteine, which inhibits Sper induced GSH depletion. Subsets of cells were either susceptible (GSH(low)) or resistant (GSH(high)) to NO-induced GSH depletion. The GSH(low) subset was characterized by enhanced numbers of CD4+ cells, reduced numbers of activated cells as characterized by CD25 and CD69, and reduced numbers of memory (CD45RO+) cells relative to the GSH(high) population. Rather than directly affecting susceptibility to NO, these surface markers reflected different expression patterns. Particularly, the GSHlow subset was further characterized by decreased activity of the GSH synthesis related enzymes multi-drug resistance related protein (MRP)-1 and gamma-glutamyltranspeptidase (gamma-GT). Blocking gamma-GT, using acivicin was shown to exacerbate NO-induced GSH depletion and NO-induced apoptosis. Since NO induced apoptosis selectively affects IFN-gamma production these findings implicate GSH metabolism in the modulation and maintenance of the T helper (Th)1/Th2 balance.

gamma-Glutamyl transpeptidase expression in Ewing's sarcoma cells: up-regulation by interferons

The genetic hallmark of Ewing's sarcoma family of tumours (ET) is the presence of the translocation t(11;22)(q24;q12), which creates the ET fusion gene, leading to cellular transformation. Five human gamma-glutamyl transpeptidase (gamma-GT) genes are located near the chromosomal translocation in ET. gamma-GT is a major enzyme involved in glutathione homoeostasis. Five human cell lines representative of primary or metastatic tumours were investigated to study whether gamma-GT alterations could occur at the chromosomal breaks and rearrangements in ET. As shown by enzymic assays and FACS analyses, all ET cell lines consistently expressed a functional gamma-GT which however did not discriminate steps of ET progression. As shown previously [Sancéau, Hiscott, Delattre and Wietzerbin (2000) Oncogene 19, 3372-3383], ET cells respond to the antiproliferative effects of interferons (IFNs) type I (alpha and beta) and to a much less degree to IFN type II (gamma). IFN-alpha and -beta arrested cells in the S-phase of the cell cycle. We found an enhancement of gamma-GT mRNA species with IFN-alpha and -beta by reverse transcriptase-PCR analyses. This is reflected by up-regulation of gamma-GT protein, which coincides with the increase in gamma-GT-specific enzymic activity. Similarly, IFNs up-regulate the levels of gamma-GT in another IFN-responsive B cell line. Whether this up-regulation of gamma-GT by IFNs is of physiological relevance to cell behaviour remains to be studied.

Resistance of activated human Th2 cells to NO-induced apoptosis is mediated by gamma-glutamyltranspeptidase

Activation-induced death of inflammatory cells (AICD) has an important function in immune maintenance. Type 1 Th cells are known to be more susceptible to AICD than Th2 cells. In the current study we examined whether NO-induced apoptosis also preferentially eliminates Th1 cells over Th2 cells. Naive human Th lymphocytes (CD4(+)CD45RO(-)) were activated in vitro for 1 week in the presence of IL-12 plus anti-IL-4 or IL-4 plus anti-IL-12 to generate Th1- and Th2-polarized cultures respectively. Cultures were exposed to the NO donors Spermine-nonoate (Sper) and DPTA-nonoate to study NO-induced apoptosis. We found that NO preferentially induced apoptosis in Th1-polarized cells as demonstrated by Annexin staining in the presence of 10 microM Sper (70 +/- 16 versus 23 +/- 4.4% in Th2 cells P: < 0.01) and by DioC6 staining (38 +/- 10 versus 11 +/- 5% in Th2 cells, P: < 0.01). The mechanism of NO-induced apoptosis in Th1/Th2-polarized cells was distinct from AICD and Fas-induced apoptosis. Differential sensitivity between Th1- and Th2-polarized cultures originated at the level of intracellular glutathione (GSH) metabolism. GSH levels were higher in Th2 cells (1.6 +/- 0.2-fold Th1, P: < 0.01). High intracellular GSH in Th2-polarized cells did not account for reduced susceptibility to NO per se, since the inhibition of gamma-glutamyltrans-peptidase (gamma-GT), which is involved in GSH import, sensitized Th2 cells to NO-induced apoptosis without GSH depletion. Therefore, higher activity of gamma-GT in Th2 cells (2.1 +/- 0.4-fold Th1, P: < 0.001) specifically protects Th2 cells against NO-induced apoptosis. Preferential NO-induced elimination of human Th1 cells at sites of inflammation may thus select Th2 cells and contribute to immune deviation.

Ectopeptidases in pathophysiology

Ectopeptidases are transmembrane proteins present in a wide variety of tissues and cell types. Dysregulated expression of certain ectopeptidases in human malignancies suggests their value as clinical markers. Ectopeptidase interaction with agonistic antibodies or their inhibitors has revealed that these ectoenzymes are able to modulate bioactive peptide responses and to influence growth, apoptosis and differentiation, as well as adhesion and motility, all functions involved in normal and tumoral processes. There is evidence that ectopeptidase-mediated signal transduction frequently involves tyrosine phosphorylation. Combined analyses of gene organization and regulation of ectopeptidases by various physiological factors have provided insights into their structure-function relationships. Understanding the roles of ectopeptidases in pathophysiology may have implications in considering them as therapeutic targets.

Expression of gamma-glutamyl transpeptidase protects ramos B cells from oxidation-induced cell death

The ectoenzyme, gamma-glutamyl transpeptidase (GGT, EC ) cleaves glutathione (GSH) to facilitate the recapture of cysteine for synthesis of intracellular GSH. The impact of GGT expression on cell survival during oxidative stress was investigated using the human B cell lymphoblastoid cell line, Ramos. Ramos cells did not express surface GGT and exhibited no GGT enzyme activity. In contrast, Ramos cells stably transfected with the human GGT cDNA expressed high levels of surface GGT and enzymatic activity. GGT-transfected Ramos cells were protected from apoptosis when cultured in cyst(e)ine-deficient medium. The GGT-expressing cells also had lower levels of intracellular reactive oxygen species (ROS). Homocysteic acid and alanine, inhibitors of cystine and cysteine uptake, respectively, caused increased ROS content and diminished viability of GGT expressing cells. Exogenous GSH increased the viability of the GGT-transfected cells more effectively than that of control cells, whereas the products of GSH metabolism prevented death of both the control and GGT-transfected cells comparably. These data indicate that GGT cleavage of GSH and the subsequent recapture of cysteine and cystine allow cells to maintain low levels of cellular ROS and thereby avoid apoptosis induced by oxidative stress.