Analysis of changes in energy and redox states in HepG2 hepatoma and C6 glioma cells upon exposure to cadmium.

The energy and redox states of the HepG2 hepatoma and the C6 glioma cells were studied by quantifying the levels of ATP, ADP, AMP, GSH, and GSSG. These values were used to calculate the energy charge potential (ECP = [ATP + 0.5ADP]/TAN), total adenosine nucleotides (TAN = ATP + ADP + AMP), total glutathione (TG = [GSH + GSSG]/TAN), and the redox state (GSH/GSSG ratio). For comparison between cell types, the level of each energy metabolite (ATP, ADP, and AMP) was normalized against TAN of the respective cell. The results showed that ATP:ADP:AMP ratio was 0.76:0.11:0.13 for the HepG2 cells and 0.80:0.11:0.09 for the C6 glioma cells. ECP was 0.81 +/- 0.01 and 0.85 +/- 0.01 for the HepG2 and the C6 glioma cells, respectively. GSH/GSSG ratio was 2.66 +/- 0.16 and 3.63 +/- 0.48 for HepG2 and C6 glioma cells, respectively. TG was 3.2 +/- 0.54 for the HepG2 cells and 2.43 +/- 0.18 for the C6 glioma cells, indicating that the level of total glutathione is more than two to three times higher than the total energy metabolites in these cell lines. Following a 3-h incubation in medium containing different concentrations of Cd, there was a dose-dependent decrease in cell viability. The 3-h LC50 for the HepG2 cells was 0.5 mM and that for the C6 glioma cells was 0.4 mM. Cellular TAN decreased with a decrease in cell viability. Upon careful analysis of the energy state, there was a significant increase in relative amount of ATP and decrease in ADP and AMP in both cells as Cd concentration increased from 0 to 0.1, 0.2, and 0.6 mM. However, ECP in both cell lines increased, which indicated that the level of high energy phosphate was adequate. There was also a significant increase in TG and a significant decrease in GSH/GSSG in the C6 glioma cells when cells were exposed to as low as 0.1 mM Cd, which suggested that the cellular redox state was compromised. The HepG2 cells, on the other hand, showed no significant change in both TG and GSH/GSSG level until Cd concentration reached 0.6 mM. Results of the present study also showed that there were differences between the two cells in response to the same level of Cd exposure. The C6 glioma cells were more sensitive to Cd-induced injuries. Although there was a decrease in total amount of high energy phosphate as cell viability decreased, the surviving cells were not devoid of high energy phosphates. The relative abundance of ATP amongst the adenosine nucleotide pool and the increase in ECP could be interpreted as a way the cells signal the conservation of energy utilization in response to the damaged mitochondrial function. This move for energy conservation might be the cause of eventual cell death through the process of apoptosis.