The cell-specific anti-proliferative effect of reduced glutathione is mediated by γ-glutamyl transpeptidase-dependent extracellular pro-oxidant reactions

We have shown earlier that extracellular GSH can exert a cell-specific growth-inhibitory effect on human tumor cells. In the present study, 2 human ovarian carcinoma cell lines (A2780 and IGROV-1) were used to investigate the biochemical basis of the GSH growth-inhibitory effect. Whereas cells were resistant, A2780 cells were sensitive to a 1 hr exposure to GSH, as assessed by the growth inhibition assay. Analysis of relevant GSH-dependent enzymes indicated that A2780 cells had low level of GSH S-transferase, glutathione reductase and γ-glutamyl transpeptidase (γ-GT) activities in comparison with those of IGROV-1 cells, and GSH peroxidase activity was undetectable in A2780 cells. The GSH effect was reversed by catalase and by dithiothreitol, indicating the occurrence of oxidative phenomena resulting in the impairment of critical cellular thiols. Indeed treatment of cells with H₂O₂ also resulted in growth inhibition, which was more marked in A2780 cells. The γ- glutamyl acceptor glycylglycine, a cosubstrate for γ-GT, potentiated the growth-inhibitory effect of GSH, which in contrast was decreased by the γ- GT inhibitors, serine-borate complex and acivicin, suggesting that the production of reactive forms of oxygen (probably H₂O₂) was mediated by cysteinyl-glycine after GSH hydrolysis. The results support that the growth- inhibitory effect of low GSH concentration is the result of oxidative damage related to extracellular GSH metabolism.