The effects of modulation of γ-glutamyl transpeptidase activity in HepG2 cells on thiol homeostasis and caspase-3-activity

The present studies aimed to elucidate how the modulation of γ-glutamyl transpeptidase (γGT) activity in human hepatoma (HepG2) cell line influences H2O2 production, caspase 3 activity, protein S-thiolation by glutathione (GSH), cysteinyl–glycine (Cys–Gly) and cysteine (Cys), and the level of other redox forms of these thiols. The experiments showed that 1-h stimulation of γGT elevated H2O2 production, leading to prooxidant conditions. After 24-h stimulation, H2O2 concentration was at the control level, while Cys–Gly-, Cys- and GSH-dependent S-thiolation was markedly increased, which was accompanied by a drop in caspase-3 activity. The inhibition of  γGT activity by acivicin led to H2O2 decrease after 1-h incubation which still persisted after 24 h. The inhibition of γGT activity in HepG2 cells was also connected with the lowering of S-thiolation with Cys and Cys–Gly and with increasing of caspase-3 activity. The results of our studies indicate that the modulation of γGT activity can be used to change cellular redox status, and can affect Cys- and Cys–Gly-dependent S-thiolation and caspase-3 activity. We suggest that the role of high γGT activity in HepG2 cells can be connected with production of reactive oxygen species and with S-thiolation with Cys and Cys–Gly that can influence activity of caspase 3.