Complex sentence: Oxidative stress has been shown to lead to an increase in system Xc-expression, therefore there must be a balance between the positive protective effects of increased glutathione levels and the negative excitotoxicity effects of increased extrasynaptic glutamate levels.
Split into simpler sentences: Oxidative stress has been shown to lead to an increase in system Xc-expression. Therefore, there must be a balance between the positive protective effects of increased glutathione levels and the negative excitotoxicity effects of increased extrasynaptic glutamate levels.