Enhancement of cytotoxicity of hydrogen peroxide by hyperthermia in Chinese hamster ovary cells: Role of antioxidant defenses

Regional hyperthermia has potential for human cancer treatment, particularl y in combination with systemic chemotherapy or radiotherapy. The mechanisms involved in heat-induced cell killing are currently unknown. Hyperthermia may increase oxidative stress in cells, and thus, oxidative stress could ha ve a role in the mechanism of cell death. We use hydrogen peroxide as a mod el oxidant to improve understanding of interactions between heat and oxidat ive stress. Heat increased cytotoxicity of hydrogen peroxide in Chinese ham ster ovary cells. Altered levels of cellular antioxidants should create an imbalance between prooxidant and antioxidant systems, thus modifying cytoto xic responses to heat and to oxidants, We determine the involvement of the two cellular antioxidant defenses against peroxides, catalase and the gluta thione redox cycle, in cellular sensitivity to heat, to hydrogen peroxide, and to heat combined with the oxidant, Defense systems were either inhibite d or increased. For inhibition studies, intracellular glutathione was dimin ished to less than 15% of its initial level by treatment with L-buthionine sulfoximine (1 mM, 24 h), Inhibition of catalase was achieved with 3-amino- 1,2,4-triazole (20 mM, 2 h), which caused a 80% decrease in endogenous enzy me activity. To increase antioxidants, cells were pretreated with the thiol -containing reducing agents, N-acetyl-L-cysteine, 2-oxo-4-thiazolidine carb oxylate, and 2-mercaptoethane sulfonate. These compounds increased intracel lular glutathione levels by 30%. Catalase activity was increased by additio n of exogenous enzyme to cells. We show that levels of glutathione and cata lase affect cellular cytotoxic responses to heat and hydrogen peroxide, eit her used separately or in combination, These findings are relevant to mecha nisms of cell killing at elevated temperatures and suggest the involvement of oxidative stress. (C) 1999 Academic Press.