Cadmium-induced apoptosis in C6 glioma cells: influence of oxidative stress

Cadmium has recently been shown to induce apoptosis in C6 glioma cells via disruption of the mitochondrial membrane potential and subsequent caspase 9-activation. Here we show that both H2O2 and CdCl2 induced apoptotic DNA fragmentation in C6 cells. The employment of glutathione as an antioxidant prevented the induction of apoptotic DNA fragmentation by cadmium completely and catalase strongly reduced cadmium-induced DNA fragmentation suggesting that cadmium exerts its apoptotic effects at least partly via the production of H2O2. Apoptosis may be induced by cadmium indirectly through formation of oxidative stress, e.g., by inhibition of antioxidant enzymes. After incubation of C6 cells with cadmium for short times (up to 4 h), we analyzed the formation of intracellular reactive oxygen species and cellular lipid peroxidation. After 1 h of incubation with inreasing concentrations of CdCl2 (1-500 microM), no increase in dichlorofluorescein fluorescence was found. At variance, lipid peroxidation was slightly elevated after 2 h incubation with cadmium (50-100 microM). Furthermore, we analyzed the modulation of markers for oxidative stress after prolonged (24 h) exposure to cadmium. The intracellular glutathione content as measured using the fluorescent probe monobromobimane was decreased after incubation with CdCl2 (0.5-10 microM) for 24 h. Furthermore, we measured the effect of cadmium on the level of oxidized DNA lesions (predominantly 8-hydroxyguanine) using the bacterial Fpg-DNA-repair protein. After 24 h of incubation with 5 microM CdCl2 we found a sixfold increase in Fpg-sensitive DNA-lesions. We conclude that short time incubations with cadmium (up to 4 h) caused only slight or insignificant effects on the generation of reactive oxygen species (formation of thiobarbituric acid reactive substances, fluorescence of dichlorofluorescein), whereas incubation with this heavy metal for 24 h lead to a decrease in intracellular glutathione concentration and an increase in oxidative DNA-lesions. Our data demonstrate that cadmium as similar to H2O2 is a potent inducer of apoptosis in C6 cells. Even if cadmium unlike Fenton-type metals can not produce reactive oxygen species directly, the apoptotic effects of cadmium at least in part are mediated via induction of oxidative stress. Because both apoptosis and oxidative stress are thought to play important roles in neurodegenerative diseases, low concentrations of cadmium that initiate programmed cell death may lead to a selective cell death in distinct brain regions via generation of oxidative stress.