Dehydroascorbic Acid Promotes Cell Death in Neurons Under Oxidative Stress: a Protective Role for Astrocytes

Abstract

Ascorbic acid (AA), the reduced form of vitamin C, is incorporated into neurons via the sodium ascorbate co-transporter SVCT2. However, this transporter is not expressed in astrocytes, which take up the oxidized form of vitamin C, dehydroascorbic acid (DHA), via the facilitative hexose transporter GLUT1. Therefore, neuron and astrocyte interactions are thought to mediate vitamin C recycling in the nervous system. Although astrocytes are essential for the antioxidant defense of neurons under oxidative stress, a condition in which a large amount of ROS is generated that may favor the extracellular oxidation of AA and the subsequent neuronal uptake of DHA via GLUT3, potentially increasing oxidative stress in neurons. This study analyzed the effects of oxidative stress and DHA uptake on neuronal cell death in vitro. Different analyses revealed the presence of the DHA transporters GLUT1 and GLUT3 in Neuro2a and HN33.11 cells and in cortical neurons. Kinetic analyses confirmed that all cells analyzed in this study possess functional GLUTs that take up 2-deoxyglucose and DHA. Thus, DHA promotes the death of stressed neuronal cells, which is reversed by incubating the cells with cytochalasin B, an inhibitor of DHA uptake by GLUT1 and GLUT3. Additionally, the presence of glial cells (U87 and astrocytes), which promote DHA recycling, reverses the observed cell death of stressed neurons. Taken together, these results indicate that DHA promotes the death of stressed neurons and that astrocytes are essential for the antioxidative defense of neurons. Thus, the astrocyte-neuron interaction may function as an essential mechanism for vitamin C recycling, participating in the antioxidative defense of the brain.

Vitamin C Transporter (SVCT2) Distribution in Developing and Adult Brains

“…In these experiments, the researchers did not consider that the observations could be attributed to a failure in vitamin C recycling. Although vitamin C deficiency in the brain has been associated with increased oxidative stress, which could be stimulating ideal conditions to trigger certain pathologies, including Alzheimer's disease [54], DHA has recently been proposed as a molecule that would trigger neuronal death [39]. As shown in Figure 1, we propose that CNS pathologies could be associated with reduced astrocytic vitamin C recycling, resulting in the accumulation of DHA in neurons, which would trigger cell death.…”

“…Furthermore, the neuron possesses a very limited capacity to reduce DHA back into AA due to low concentration of antioxidant enzymes and its elevated metabolic rate [38]. Accordingly, isolation of neurons from the neighbor cells that recycle DHA compromises their metabolism, resulting in neuronal death [32,39].…”

“…Besides, astrocytes participate in the neuronal antioxidant defense by releasing AA during pathological conditions that involve changes in the osmotic pressure or ischemic/reperfusion damage [44]. Experiments using neuronal-astrocyte co-cultures have demonstrated that the presence of astrocytes ameliorates neuronal death induced by glutamate, H 2 O 2, or DHA in vitro [39,45], suggesting that astrocytes and vitamin C recycling are of vital importance for antioxidant defense in the brain.…”

“…and astrocytes were added to the culture just before DHA was added. Notably, neurons cocultured with astrocytes showed 100% survival after 3 and 6 h of treatment, compared with neurons cultured alone [39], demonstrating that astrocytes are highly efficient in the recycling of DHA from the extracellular medium and that accumulation of DHA due to deficient astrocyte recycling or severe oxidative conditions is toxic for neurons.…”

Vitamin transporters in mice brain with aging

“…SVCT2‐mediated transport is the main route used by neurons (Garcia‐Krauss et al. ); however, when HPLC is used to measure VitC levels in brain extracts, the results include all VitC that is obtained via both mechanisms (Harrison et al. ).…”