Heat shock inactivates cellular antioxidant defenses against hydrogen peroxide: protection by glucose

Abstract

Hyperthermia is used in cancer treatment and potentiates the cytotoxicity of radiation and certain chemotherapy drugs. The mechanism(s) of heat killing and those involved in heat potentiation of cytotoxic modalities are not understood. This study examines whether heat shock causes a redox imbalance, leading to oxidative changes in Chinese hamster ovary cells. Decreases in the GSH/GSSG ratio reflected an oxidative imbalance in heated (42°C) and in H 2O 2-challenged cells. Glucose provided protection against these changes. Glucose also protected cells against cytotoxicity of H 2O 2 and/or hyperthermia (42 to 43°C). Glucose appears to protect cells against H 2O 2 and heat shock by providing NADPH through its metabolism via the pentose phosphate cycle (PC). When cells were deprived of glucose, there was a marked decrease in the GSH/GSSG ratio and in NADPH levels, indicating a severe redox imbalance. Glucose deprivation caused cell death, which was consistent with increased accumulation of H 2O 2, since three distinct H 2O 2-detoxifying systems (N-acetyl-L-cysteine, sodium pyruvate, and catalase) rescued cells against cytotoxicity. Nontoxic levels of H 2O 2 stimulated a corresponding increase in both PC activity and NADPH levels. NADPH levels and basal activity of the PC increased at 42°C. However, the oxidant-stimulated increases in PC activity and NADPH levels were lost in heated cells. Therefore, heat shock inactivates an important cellular defense mechanism against oxidants. These findings suggest that heat shock may enhance the cytotoxicity of oxidants by inhibiting increases in PC activity following oxidative stress. These data are potentially relevant to understanding the potentiation of cytotoxicity of radiation and oxidant-generating drugs by heat shock, used in combined modality cancer treatment.