Promotion of cysteine-dependent glutathione synthesis by paraxanthine

Caffeine (1,3,7-trimethylxanthine) consumption reduces the risk of neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. Several lines of evidence have suggested the neuroprotective effect of caffeine, however, the precise mechanism is still unclear in the central nervous system. Here, we suggest a mechanism that a caffeine derivative, paraxanthine (1,7-dimethylxanthine), promotes cysteine uptake and thereby enhances the synthesis of an antioxidant, glutathione (GSH), in neurons. We previously showed that paraxanthine, a major metabolite of caffeine, promotes cysteine uptake in mouse hippocampus slices. In this study, we examined the effect of paraxanthine on GSH synthesis in HEK293 cells. HEK293 cells were treated with 0, 10 and 100 μM of paraxanthine in addition to cysteine for 30 min and the GSH levels were subsequently detected by CMFDA, a fluorescent GSH marker, using fluorescent microscopy. Paraxanthine increased GSH levels in HEK293 cells at the concentration of 100 μM. These results suggest that paraxanthine promotes cysteine uptake leading to GSH synthesis in HEK293 cells.