Benzo[a]pyrene-induced elevation of GSH level protects against oxidative stress and enhances xenobiotic detoxification in human HepG₂ cells

Glutathione (GSH) is one of the most important antioxidants in mammalian cells. It also plays an important role in chemical detoxification. Some evidence showed that polycyclic aromatic hydrocarbons, such as benzo[a]pyrene (B[a]P [50-32-8]), could increase GSH content as a defense mechanism against oxidative stress as well as to promote its detoxification. However, there has been very little study on clarifying the role GSH plays in antioxidation and detoxification actions. Therefore, the present study aims to analyze intracellular glutathione metabolism in the human hepatoma cells (HepG2) upon exposure to B[a]P. Exposure of the cells to B[a]P (1-100 μM) for 24 h did not cause significant cell death in this cell line. By selecting the sublethal concentration of 10 μM, B[a]P caused a significant increase in GSH and a small (13%) but significant decrease in glutathione reductase activity. However, there was no change in the activity of glutathione peroxidase, and no detectable increase in reactive oxygen species (ROS) production. Treatment with B[a]P caused up to 1.5 folds increase in γ-glutamylcysteine synthatase (γ-GCS) activity over control. Buthioneine sulfoximine (BSO), an inhibitor of γ-GCS, could suppress GSH increase in a dose-dependent manner. Assessment of the oxidative state of the cells indicated that the increase in GSH caused the cells to become more reduced. Thus, the results concluded that cells were not suffering from oxidative stress at 24 h after treatment with 10 μM B[a]P. Upon analyzing the activities of detoxification enzymes, there was an increase in the activity of CYP1A subfamily monooxygenases and glutathione S-transferase. Both changes occurred prior to the changes in γ-GCS activity and the increase in GSH. In summary, results of the present study demonstrate that B[a]P caused an activation of detoxification enzymes. The increase in intracellular GSH level was due to activation of γ-GCS activities. Oxidative stress may not be an important risk factor for B[a]P (at 10 μM of up to 24 h) induced injury.