Astrocyte control of fetal cortical neuron glutathione homeostasis: up-regulation by ethanol.

Ethanol increases apoptotic neuron death in the developing brain and at least part of this may be mediated by oxidative stress. In cultured fetal rat cortical neurons, Ethanol increases levels of reactive oxygen species (ROS) within minutes of exposure and reduces total cellular glutathione (GSH) shortly thereafter. This is followed by onset of apoptotic cell death. These responses to Ethanol can be blocked by elevating neuron GSH with N-acetylcysteine or by co-culturing neurons with neonatal cortical astrocytes. We describe here mechanisms by which the astrocyte-neuron gamma-glutamyl cycle is up-regulated by Ethanol, enhancing control of neuron GSH in response to the pro-oxidant, Ethanol. Up to 6 days of Ethanol exposure had no consistent effects on activities of gamma-glutamyl cysteine ligase or glutathione synthetase, and GSH content remained unchanged (p < 0.05). However, glutathione reductase was increased with 1 and 2 day Ethanol exposures, 25% and 39% for 2.5 and 4.0 mg/mL Ethanol by 1 day, and 11% and 16% for 2.5 and 4.0 mg/mL at 2 days, respectively (p < 0.05). A 24 h exposure to 4.0 mg/mL Ethanol increased GSH efflux from astrocyte up to 517% (p < 0.05). Ethanol increased both gamma-glutamyl transpeptidase expression and activity on astrocyte within 24 h of exposure (40%, p = 0.05 with 4.0 mg/mL) and this continued for at least 4 days of Ethanol treatment. Aminopeptidase N activity on neurons increased by 62% and 55% within 1 h of Ethanol for 2.5 and 4.0 mg/mL concentration, respectively (p < 0.05), remaining elevated for 24 h of treatment. Thus, there are at least three key points of the gamma-glutamyl cycle that are up-regulated by Ethanol, the net effect being to enhance neuron GSH homeostasis, thereby protecting neurons from Ethanol-mediated oxidative stress and apoptotic death.

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.