Role of gamma-glutamyltransferase in putrescine uptake by rat type II pneumocytes

Inhibition of gamma-glutamyl transpeptidase activity decreases intracellular cysteine levels in cervical carcinoma

PURPOSE

To determine whether gamma-glutamyl transpeptidase (gamma-GT) is involved in the maintenance of elevated cysteine levels in cervical carcinoma.

METHODS

Four cervical carcinoma cell lines were tested in vitro for cysteine accumulation and gamma-GT levels. The highest and lowest gamma-GT-expressing cell lines were used in in vivo experiments to determine the effect of gamma-GT inhibition on cysteine levels.

RESULTS

Treatment of a series of cervical carcinoma cell lines with acivicin decreased intracellular cysteine concentrations. Cysteine depletion was evident in Me180 cells which had the greatest levels of gamma-GT activity, and had a more pronounced cysteine decrease in medium with glutathione and cysteine concentrations simulating the in vivo situation. Also investigated were the effects of inhibition of gamma-GT activity on intracellular cysteine levels in xenografts grown in severe combined immunodeficient (SCID) mice. With the use of 35 mg/kg of acivicin, gamma-GT activity decreased to basal levels of detection in both tumour types and significant decreases in cysteine levels were seen in the high gamma-GT-expressing tumours (Me180). Thus, inhibition of gamma-GT activity may have therapeutic potential in high-expressing cancers.

CONCLUSIONS

In tumours and cell lines with elevated levels of gamma-GT activity, inhibition of this enzyme led to decreases of cysteine levels.

Alterations on polyamine content and glutathione metabolism induced by different concentrations of paraquat in CHO-K1 cells

The effect of herbicide paraquat has been assessed on CHO-K1 cultures at different concentrations. Glutathione peroxidase, reductase and S-transferase, as well as total and oxidized glutathione, were evaluated along the standard growth curve of the cultures. Paraquat was then administered during mid-log phase at concentrations that produced a calculated lethality of 6.25%, 12.5% and 25%, using the lysosomal dye assay, neutral red. After 24hr of incubation with paraquat, glutathione peroxidase suffered a large dose-response increase, unlike glutathione reductase and S-transferase, the activities of which were lower than untreated controls. The profile of total glutathione content was similar to that found for glutathione peroxidase, increasing with the administered doses of the herbicide. Polyamine content has been also studied at the same concentrations of paraquat, showing that intracellular spermidine and spermine pools were negatively affected with paraquat in a dose-response manner, unlike putrescine, which maintained elevated pools at the three concentrations assayed.

Polyamines in the lung: polyamine uptake and polyamine-linked pathological or toxicological conditions

The natural polyamines putrescine, cadaverine, spermidine, and spermine are found in all cells. These (poly)cations exert interactions with anions, e.g., DNA and RNA. This feature represents their best-known direct physiological role in cellular functions: cell growth, division, and differentiation. The lung and, more specifically, alveolar epithelial cells appear to be endowed with a much higher polyamine uptake system than any other major organ. In the lung, the active accumulation of natural polyamines in the epithelium has been studied in various mammalian species including rat, hamster, rabbit, and human. The kinetic parameters (Michaelis-Menten constant and maximal uptake) of the uptake system are the same order of magnitude regardless of the polyamine or species studied and the in vitro system used. Also, other pulmonary cells accumulate polyamines but never to the same extent as the epithelium. Although different uptake systems exist for putrescine, spermidine, and spermine in the lung, neither the nature of the carrier protein nor the reason for its existence is known. Some pulmonary toxicological and/or pathological conditions have been related to polyamine metabolism and/or polyamine content in the lung. Polyamines possess an important intrinsic toxicity. From in vitro studies with nonpulmonary cells, it has been shown that spermidine and spermine can be metabolized to hydrogen peroxide, ammonium, and acrolein, which can all cause cellular toxicity. In hyperoxia or after ozone exposure, the increased polyamine synthesis and polyamine content of the rat lung is correlated with survival of the animals. Pulmonary hypertension induced by monocrotaline or hypoxia has also been linked to the increased polyamine metabolism and polyamine content of the lung. In a small number of studies, it has been shown that polyamines can contribute to the suppression of immunologic reactions in the lung.

Effects of oxygen pressure and medium volume on the toxicity of paraquat in rat and human type II pneumocytes

The herbicide, paraquat is highly toxic for mammals, with the lungs being the main target organ, because of the active accumulation of the compound in this organ. The cellular toxicity of paraquat has been shown to be an O2-driven process and hyperoxia is known to increase the lethality of paraquat. In this study we have examined the effect of various O2 concentrations on the toxicity of paraquat in rat and human type II pneumocytes in culture, and we have tested whether the thickness of the liquid layer above the cells would influence the toxicity of paraquat. Type II pneumocytes were isolated from rat or human lung tissue using trypsin digestion, percoll density gradient centrifugation and differential attachment. Adherent cells (day 2) were incubated for 20 h in different volumes of culture medium (thickness of liquid layer), whether or not in the presence of paraquat, in the presence of different O2 tensions. The viability of the cells was assessed by the release of LDH in the culture medium. In both rat and human type II pneumocytes the toxicity of paraquat was independent of the thickness of the liquid layer (2.5 to 10 mm height). The toxicity of paraquat in rat type II pneumocytes decreased from a TC50 value of 28 microM paraquat at 21% O2 to 107 microM at 10% O2 and increased to 12 microM and 8 microM at 60% and 85% O2, respectively. For human type II pneumocytes the TC50 values were 7 microM; 25 microM and > 1000 microM paraquat at 60%, 21% and 10% O2, respectively. In this study we have shown that the diffusion of O2 through a liquid layer does not limit the toxicity of paraquat and that, as in vivo, increasing O2 partial pressure enhances the toxicity of paraquat.

Putrescine uptake in rat type II pneumocytes correlates with gamma-glutamyltransferase activity

gamma-Glutamyltransferase (gamma GT) is a key enzyme in glutathione metabolism and it is thought also to play a role in the uptake of polyamines such as putrescine. The aim of our study was to investigate if changes in gamma GT activity would alter total putrescine uptake [P(up)(tot)], as well as more specific uptake via the gamma GT pathway [P(up)(gamma GT)]. Forty-eight hours after their isolation, rat type II cells were exposed to 30, 60 or 125 microM L-buthionine-[SR]-sulfoximine (BSO) for 3 hr; 200 or 800 microM tertiary-butylhydroperoxide (t-BOOH) for 40 min; 10, 100 or 1000 microM paraquat (PQ) for 1 hr; and 60 or 85% O2 for 48 hr. The gamma GT activity, P(up)(tot) and P(up)(gamma GT) (assessed by inhibiting gamma GT) were measured immediately after the exposure to hyperoxia, or 24 hr after treatment with BSO, t-BOOH or PQ. From previous studies, it is known that these experimental conditions increased (BSO, 200 microM t-BOOH) or decreased (800 microM t-BOOH, PQ, hyperoxia) gamma GT activity. There was a strong correlation between the changes in gamma GT activity and the changes in P(up)(gamma GT) (r = 0.81, p < 0.001). These findings support the hypothesis that gamma GT partly regulates the uptake of putrescine, one of the polyamines required for cell growth and differentiation.

Increase in gamma-glutamyltransferase by glutathione depletion in rat type II pneumocytes

The purpose of our study was to investigate the effect of oxidative stress or intracellular glutathione (GSH) depletion on gamma-glutamyltransferase (gamma-GT) activity in cultured type II pneumocytes. Twenty-four hours after isolation, primary cultures of rat type II pneumocytes were preincubated with one of four compounds: 15, 30, 60, 125, 250 microM L-buthionine-[SR]-sulfoximine (BSO) for 3 h; 100, 200, 400, 800 microM tertiary-butylhydroperoxide (t-BOOH) for 45 min; 10, 25, 50, 100 microM menadione for 15 min; 100, 1000 microM paraquat for 1 h. GSH levels, H2O2 and O2.- generation were measured immediately after the incubation, gamma-GT activity and GSH levels also up to 24 h or 48 h later. Exposure to BSO led to a persistent GSH depletion without increase in H2O2 or O2.- production, together with a dose and time-dependent increase (doubling) of gamma-GT activity with a nonsignificant increase in gamma-GT mRNA expression 24 h after exposure to BSO. Exposure to 100 microM menadione, which increased H2O2 production, decreased gamma-GT activity. t-BOOH or paraquat did not give rise to a measurable increase in H2O2 or O2.-. Paraquat did not affect initial GSH levels, but increased GSH and decreased gamma-GT activity 24 h later. t-BOOH (400 and 800 microM) initially decreased GSH, and tended to increase GSH 24 h later, 100 and 200 microM increased gamma-GT activity 24 h later, but 800 microM decreased it. Restoration of intracellular GSH levels by addition of GSH to the culture medium completely prevented the increase in gamma-GT activity by BSO, while the addition of catalase or DMTU had no effect. We conclude that at least two effects are operating upon gamma-GT activity: GSH depletion seems to increase gamma-GT activity, while exposure to compounds generating oxidative stress correlates with a decrease in gamma-GT activity.