In schizophrenia patients, a decrease in glutathione levels ([GSH]) in cerebrospinal fluid and prefrontal cortex was observed. Several evidences suggest a compromised GSH synthesis at the level of the rate limiting enzyme, glutamyl-cysteine ligase (GCL): association of allelic variants of GCL modulatory subunit (GCLM) gene with the illness; decrease of mRNA levels of both GCLM and GCL catalytic subunit (GCLC) in patients fibroblasts; decrease of GCLC protein expression and of GCL activity under oxidative stress conditions. The aim of this study is to find substances that could normalize [GSH] in cultured neurons and astrocytes with compromised GCL activity.
First, GCL activity was inhibited with BSO, a blocker of GCLC. In both neurons and astrocytes, the decrease in [GSH] was prevented by a membrane permeable GSH analogue, GSH-ethyl-ester, that bypasses the GCL synthesis step.
As a second model, we used neurons and astrocytes from GCLM knockout (-/-) mice. These cells show low [GSH] (-80%) and GCL activity (-25%). We tested natural antioxidants [Curcumin (polyphenol); quercetin (flavonoid)] and tert-butylhydroquinone (tBHQ), a quinone that generates free radicals. These substances are known for their capacity to increase [GSH] in various cell types.
In wild-type (+/+) astrocytes, [GSH] and GCL activity were increased by curcumin (50μM; 50%; 140%; respectively), tBHQ (100μM; 80%; 150%), and quercetin (20-100μΜ; 60%; 100%) . In (+/+) neurons, curcumin was also efficient (10μΜ; 60%; 80%) and, while low [tBHQ] (20µM) increased [GSH] (20%), higher [tBHQ] and [quercetin] depleted [GSH] and led to cell death. These results suggest that neurons and astrocytes differ in their ability to regulate GSH synthesis and to cope with the toxic effect of some substances.
In GCLM (-/-) astrocytes, tBHQ slightly increased [GSH] (25%), while curcumin and quercetin led to [GSH] depletion even at low concentrations. This indicates that GCLM might not be essential for tBHQ-induced increase in [GSH], while it is necessary for enhancement of GSH synthesis by curcumin and quercetin. Furthermore, it suggests that a compromised GSH synthesis due to a defect at the level of GCL might increase brain cells sensitivity to oxidative stress and substances known to be antioxidants might become prooxidants.