Influence of cadmium on murine thymocytes: Potentiation of apoptosis and oxidative stress

Cadmium (Cd) is a well-known environmental carcinogen and a potent immunotoxicant. It induces thymocyte apoptosis in vitro. However, the mode of action is unclear. In this study, we examined the effect of Cd (10, 25 and 50 μM) on mitochondrial membrane potential and caspase-3 as well as oxidative stress markers in murine thymocytes. The cadmium induced apoptosis occurred in a concentration and time dependent manner. The early markers of apoptosis-loss in mitochondrial membrane potential and caspase-3 activation were evident as early as 1.5 h by 50 μM Cd. Enhanced reactive oxygen species (ROS) generation and glutathione (GSH) depletion were observed at 60 min, prior to the lowering of mitochondrial membrane potential. The Cd induced DNA damage as depicted by internucleosomal fragmentation on agarose and histone associated mono- and oligonucleosomes detection by ELISA, corrobated with the apoptotic DNA (sub-G1 population) and total apoptotic cells by Annexin V binding assay. The number of cells in sub-G1 population increased to 66% at 50 μM Cd concentration and the distribution of early and late apoptotic cells was 47% and 15%, respectively. Addition of N-acetylcysteine and pyrrolidine dithiocarbamate (thiol antioxidants) to the Cd treated cells, lowered the sub-G1 population, inhibited the ROS generation and raised the GSH levels. Buthionine sulfoximine (GSH depletor) on the other hand, enhanced both the ROS production and the sub-G1 fraction. These results clearly demonstrate the apoptogenic potential of Cd in murine thymocytes, following mitochondrial membrane depolarization, caspase activation and ROS and GSH acting as critical mediators.