Damage to lung epithelial cells and lining fluid antioxidant defense by humic acid.

Humic acid causes diseases including lung emphysema and fibrosis. Emerging evidence indicates that oxidative stress is involved in humic acid-induced effects. In the present study, we investigated generation of hydroxyl radicals from humic acid, as well as the effects of humic acid to lung epithelial cells and artificial alveolar lining fluid antioxidant mixture. The involvement of iron in humic acid-induced effects was also determined. We found that humic acid (concentration and time dependently) reduced the cell survival, increased caspase-3 activity, depleted GSH and raised lipid peroxidation in epithelial cells. Humic acid reduced antioxidant levels in the lining fluid antioxidant mix, which could be prevented by adding metal ion chelators. These findings suggest that humic acid causes oxidative stress in lung cells and alveolar lining fluid, which is most likely triggered by hydroxyl radicals produced directly from humic acid. Iron is probably involved in humic acid toxicity.