Cell surface gamma-glutamyl transpeptidase in live cultures

Prion protein regulates glutathione metabolism and neural glutamate and cysteine uptake via excitatory amino acid transporter 3

Prion protein (PrP) plays crucial roles in regulating antioxidant systems to improve cell defenses against cellular stress. Here, we show that the interactions of PrP with the excitatory amino acid transporter 3 (EAAT3), γ-glutamyl transpeptidase (γ-GT), and multi-drug resistance protein 1 (MRP1) in astrocytes and the interaction between PrP and EAAT3 in neurons regulate the astroglial and neuronal metabolism of the antioxidant glutathione. Ablation of PrP in astrocytes and cerebellar neurons leads to dysregulation of EAAT3-mediated uptake of glutamate and cysteine, which are precursors for the synthesis of glutathione. In PrP-deficient astrocytes, levels of intracellular glutathione are increased, and under oxidative stress, levels of extracellular glutathione are increased, due to (i) increased glutathione release via MRP1 and (ii) reduced activity of the glutathione-degrading enzyme γ-GT. In PrP-deficient cerebellar neurons, cell death is enhanced under oxidative stress and glutamate excitotoxicity, when compared to wild-type cerebellar neurons. These results indicate a functional interplay of PrP with EAAT3, MRP1 and γ-GT in astrocytes and of PrP and EAAT3 in neurons, suggesting that these interactions play an important role in the metabolic cross-talk between astrocytes and neurons and in protection of neurons by astrocytes from oxidative and glutamate-induced cytotoxicity. Interactions of prion protein (PrP) with excitatory amino acid transporter 3 (EAAT3), γ-glutamyl transpeptidase (GGT) and multi-drug resistance protein 1 (MRP1) regulate the astroglial and neuronal metabolism of glutathione (GSH) which protects cells against the cytotoxic oxidative stress. PrP controls the release of GSH from astrocytes via MRP1 and regulates the hydrolysis of extracellular GSH by GGT as well as the neuronal and astroglial glutamate and cysteine uptake via EAAT3.

Up-regulation of gamma-glutamyl transpeptidase (GGT) activity in growth perturbed C6 astrocytes

Activity of gamma-glutamyl transpeptidase (GGT) was studied in astrocyte-like C6 glial cells modulated in growth and maturation by different concentration of serum and dibutyryl cyclic AMP (Db-cAMP) supplement in culture medium. After reduction of serum concentration from 10% to 0.1%, the number of GGT positive cells determined histochemically increased 3.1 times and the GGT activity/mg protein in whole cell lysates was 5.1 times higher. In cultures with 0.1% serum + Db-cAMP, the histochemically and biochemically assayed GGT activity exceeded 5.1 and 7.9 times the values measured in control 10% serum cultures, respectively. The up-regulation of GGT was accompanied by an inhibition of proliferation, enhanced differentiation and hypertrophy of cells. In addition, the process of metabolic perturbation and/or cellular stress was revealed in these cultures by the (i) growth-support release followed by shrinkage and death of a small number of cells and (ii) higher oxidation of 2'7'dichlorofluorescein diacetate to its fluorescent form in the adherent/viable cells. The observed up-regulation of GGT is considered to primarily reflect increased metabolism of glutathione and/or the maintenance of the redox potential in cells stressed by sub-optimal concentration of serum and Db-cAMP supplement. The concomitant cellular hypertrophy and differentiation and their relationship to increased activity of GGT await further investigation. The study suggests that up-regulation of GGT can contribute to adaptation of astrocytic cells to metabolic and/or oxidative perturbances occurring under various pathological conditions, including radiation- and drug-induced toxicity.

Gamma-glutamyl transpeptidase and glutathione biosynthesis in non-tumorigenic and tumorigenic rat liver oval cell lines

Glutathione synthesis and growth properties were studied in the gamma-glutamyl transpeptidase(GGT)-negative, non-tumorigenic rat liver oval cell line OC/CDE22, and in its GGT-positive, tumorigenic counterpart line M22. gamma-Glutamylcysteine synthetase (GGCS) activities were comparable. Growth rates of M22 cells exceeded those of OC/CDE22 cells at non-limiting and limiting exogenous cysteine concentrations. A monoclonal antibody (Ab 5F10) that inhibits the transpeptidatic but not the hydrolytic activity of GGT did not affect the growth rates of OC/CDE22, and decreased those of M22 to the OC/CDE22 level. In GSH-depleted M22, but not in OC/CDE22 cells, the rate and extent of GSH repletion with exogenous cysteine and glutamine exceeded those obtained with exogenous cysteine and glutamate. With Ab 5F10, repletion with cysteine/glutamine was similar to that obtained with cysteine/glutamate. Repletion with exogenous GSH occurred only in M22 cells, and was abolished by the GGT inhibitor acivicin. Repletion with gamma-glutamylcysteine (GGC) in OC/CDE22 was resistant to acivicin whereas that in M22 was inhibited by acivicin. Repletion with exogenous GSH or cysteinylglycine (CG) required aminopeptidase activity and was lower than that obtained with cysteine. Unless reduced, CG disulfide did not support GSH repletion. The findings are compatible with the notions that (i) GGT-catalyzed transpeptidation was largely responsible for the growth advantage of M22 cells at limiting cysteine concentration, and for their high GSH content via the formation of GGC from a gamma-glutamyl donor (glutamine) and cyst(e)ine, and (ii) aminopeptidase/dipeptidase activity is rate-limiting in GSH repletion when GSH or CG serve as cysteine sources.

Molecular and functional characterization of recombinant human gamma-glutamyltransferase. Coupling of its activity to glutathione levels in V79 cells

We previously described the establishment of a transfected cell line (V79HGGT) that stably produces the highest recombinant human gamma-glutamyltransferase (GGT) activity. We now report the utilization of V79HGGT as a model system for studying human GGT. The papain-solubilized recombinant enzyme has been highly purified from cultured cells by a new procedure. Studies on the purified enzyme, either by N-terminal sequencing or by characterization of its enzymic activities, confirmed that recombinant GGT shares structural and catalytic identity with native human enzymes. The circular dichroism analysis indicated an alpha-helical content of 19%. Based on these data, we have undertaken a study on the functional consequences of elevated GGT activity on the reduced glutathione (GSH) content. GSH status was followed in V79 and V79HGGT cells throughout growth. A particular pattern was observed for each cell line, depending on, but differentially affected by, alteration of the culture medium. Elevated GGT activity was associated with a 2.5-fold reduced GSH content, clearly suggesting a negative influence of the highly expressed enzyme on the GSH level under normal growth conditions. Possible mechanisms involved are proposed. Our findings pointed out that, among the GSH-related enzymes, GGT could constitute an important factor determining the steady-state content of GSH.

Density-dependent regulation of cell surface ?-glutamyl transpeptidase in cultured glial cells

A decline in cell surface gamma-glutamyl transpeptidase specific activity was previously observed to be concomitant with C6 glial cell proliferation. To elucidate the underlying factor(s) mediating gamma-glutamyl transpeptidase down-regulation, the effects of C6 cell density and culture conditions on cell surface transpeptidase activity levels were investigated. After 24 h of culture, the transpeptidase specific activities were inversely related to the initial plating densities. The lower-density cultures showed an induction within 24 h of plating. As the cultures proliferated, the specific transpeptidase activities declined to a common low level at post-confluency. The gamma-glutamyl transpeptidase down-regulation was unrelated to cell growth rate and was most pronounced during logarithmic proliferation. Induction and down-regulation of gamma-glutamyl transpeptidase activity at low cell densities were not a result of trypsinization. Supplementation of low-density cultures with conditioned medium, use of matrix-coated wells, or periodic replacement of growth media to prevent conditioning had minor effects on the decline of cell surface activity. Kinetic analysis showed that the Michaelis constants and the reaction mechanism were unaltered by cell density, indicating that down-regulation was not due to allosteric factors or an alteration in enzyme character. A reduction in the maximal velocity of cell surface transpeptidation at higher cell densities suggested that gamma-glutamyl transpeptidase down-regulation is related to the concentration of enzyme at the cell surface. Immunocytochemical localization of gamma-glutamyl transpeptidase demonstrated that gamma-glutamyl transpeptidase antigen levels decrease as C6 cell density increases. These results led us to propose that cell-cell contact stimulates the disappearance of gamma-glutamyl transpeptidase from the surface of cultured C6 glial cells.