Effects of Xenobiotics on the Glutathione and Glucose Metabolism of Cultured Astrocytes

Abstract

In the mammalian brain, astrocytes possess a large variety of important functions. These cells modulate synaptic transmission and are involved in the regulation of brain pH and ion homeostasis. In addition, astrocytes provide essential metabolic support for neighbouring neurons for example by supplying amino acids as precursor for neuronal glutathione (GSH) synthesis. In astrocytes, the tripeptide GSH has a key function in antioxidative and detoxification processes. This thesis investigated the effects of xenobiotics such as halogenated acetates, iodoacetamide and fumaric acid esters as well as cell derived compounds such as 2-deoxyribose on the GSH and glucose metabolism of brain astrocytes. For these studies, astroglia-rich primary cultures were used and characterised as model system. The compounds investigated differed strongly in their potential to affect cellular GSH contents and lactate production. While micromolar concentrations of iodoacetate, iodoacetamide and fumaric acid dimethyl or diethyl esters were sufficient to deprive viable astrocytes within minutes of their GSH, monochloroacetate and 2-deoxyribose had to be applied in millimolar concentration and for hours to severely lower the cellular GSH content. In contrast, neither fumaric acid monoesters nor poly-chlorinated acetates affected the cellular GSH metabolism. The depletion of cellular GSH observed for the GSH-depriving compounds is most likely the consequence of a reaction of the compounds itself or of its metabolites with GSH to form GSH-conjugates. In addition to their strong effect on the cellular GSH content, exposure of cultured astrocytes to halogenated acetates or iodoacetamide altered the glucose metabolism of the cells as indicated by a lowered cellular lactate production. For iodoacetate, iodoacetamide and monochloroacetate, inhibition of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase was identified as underlying mechanism for the compromised glycolysis, whereas poly-chlorinated acetates are likely to lower lactate production by increasing pyruvate dehydrogenase activity and oxidative metabolism. Since astrocytes play important roles in detoxifying processes and in supplying neighbouring neurons with precursors for GSH synthesis and with metabolic fuel molecules such as lactate, disturbances in astrocytic GSH and/or glucose metabolism by xenobiotics are likely to indirectly affect also the metabolism and the functions of neurons.