Modulation of Energy Metabolism in C6 Glioma Cells as Possible Mechanism Contributing to Zinc Neurotoxicity

Abstract

Elevated tissue levels of zinc (Zn) have been associated with neurodegenerative diseases such as global ischemia, seizure, and Alzheimer's. The mechanism of action of Zn in causing neuronal injury is not clear. One of the possible mechanisms is the ability of Zn to alter cellular energy metabolism. Using the C6 glioma cell as a model, the present study aimed to determined the effects of increasing concentrations of Zn on cellular energy states, as defined by the levels of adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), and adenosine 5'-monophosphate (AMP), the total adenosine nucleotides (TAN) (TAN = ATP + ADP + AMP), and the energy charge potential (ECP = [ATP + 0.5 ADP]/TAN). Uptake of Zn was visualized by the appearance of N-(6-methoxy-8-quinolyl)-p-toluene sulfonamide (TSQ)-stained fluorescent granules after a 3-h exposure to Zn in the medium. At [Zn] = 1 mM, cells appeared apoptotic. Levels of ATP and TAN decreased as the level of Zn increased. The change mirrors the increase in cell death as determined by the trypan blue exclusion test. However, when the ratio of ATP:ADP:AMP within the TAN was calculated, the percentage of ATP in the TAN increased significantly, while that of AMP decreased. The change in the relative AMP level mirrored the change in cell viability as measured by the MTT assay, which indicated a decreased in mitochondrial activity. Cellular ECP increased significantly from 0.85 ± 0.007 to 0.92 ± 0.04. The elevated ECP and relative ATP level, together with a significant decrease in the relative AMP level, are all indicators of inhibition of cellular metabolism. These results support the notion that acute exposure of C6 glioma cells to a high concentration of Zn might initially result in a decrease in relative AMP and an inhibition of mitochondrial activity. However, the ultimate toxic action of Zn on the C6 glioma cells appears to be due to a gradual inhibition of energy utilization, leading to cell shrinkage and apoptosis.