Ascorbic acid spares alpha-tocopherol and decreases lipid peroxidation in neuronal cells

Oligosaccharides from black ginseng innovatively prepared by low-temperature steam-heating process ameliorate cognitive impairment in Alzheimer's disease mice via the Keap-1/Nrf2 pathway

BACKGROUND

Our objective in this study was to evaluate the effectiveness of oligosaccharides extracted from black ginseng (OSBG), innovatively prepared by a low-temperature steam-heating process, in the improvement of learning and memory impairment in mice, as well as the mechanism(s).

RESULTS

Eight carbohydrates involving isomaltose and maltotetraose were detected in black gensing; monosaccharide residues including mannose and rhamnose were also discovered. OSBG-treated mice showed significant amelioration in recognition and spatial memory deficits compared to the scopolamine group. OSBG could decrease acetylcholinesterase activity in a tissue-dependent fashion but not in a dose-dependent manner. Furthermore, in contrast, OSBG administration resulted in significant upregulation superoxide dismutase, glutathione, glutathione peroxidase (GPx), and Kelch-like ECH-associated protein 1, downregulation of malondialdehyde and nuclear factor erythroid 2-related factor 2 in the tissues. Finally, at the genus level, we observed that the OSBG interventions increased the relative abundance of probiotics (e.g., Barnesiella, Staphylococcus, Clostridium_XlVb) and decreased pernicious bacteria such as Eisenbergiella and Intestinimonas, compared to the Alzheimer's disease mouse model group. Herein, our results demonstrate that OSBG restores the composition of the scopolamine-induced intestinal microbiota in mice, providing homeostasis of gut microbiota and providing evidence for microbiota-regulated therapeutic potential.

CONCLUSION

Our results showed for the first time a clear role for OSBG in improving scopolamine-induced memory impairment by inhibiting cholinergic dysfunction in a tissue-dependent manner. Additionally, OSBG administration relieved oxidative stress by activating the Keap-1/Nrf2 pathway and modulating the gut microbiota. Collectively, OSBG may be a promising target for neuroprotective antioxidants for improving memory and cognition in Alzheimer's disease patients. © 2024 Society of Chemical Industry.

The evolution of vitamin C biosynthesis and transport in animals

Background

Vitamin C (VC) is an indispensable antioxidant and co-factor for optimal function and development of eukaryotic cells. In animals, VC can be synthesized by the organism, acquired through the diet, or both. In the single VC synthesis pathway described in animals, the penultimate step is catalysed by Regucalcin, and the last step by l-gulonolactone oxidase (GULO). The GULO gene has been implicated in VC synthesis only, while Regucalcin has been shown to have multiple functions in mammals.

Results

Both GULO and Regucalcin can be found in non-bilaterian, protostome and deuterostome species. Regucalcin, as here shown, is involved in multiple functions such as VC synthesis, calcium homeostasis, and the oxidative stress response in both Deuterostomes and Protostomes, and in insects in receptor-mediated uptake of hexamerin storage proteins from haemolymph. In Insecta and Nematoda, however, there is no GULO gene, and in the latter no Regucalcin gene, but species from these lineages are still able to synthesize VC, implying at least one novel synthesis pathway. In vertebrates, SVCT1, a gene that belongs to a family with up to five members, as here shown, is the only gene involved in the uptake of VC in the gut. This specificity is likely the result of a subfunctionalization event that happened at the base of the Craniata subphylum. SVCT-like genes present in non-Vertebrate animals are likely involved in both VC and nucleobase transport. It is also shown that in lineages where GULO has been lost, SVCT1 is now an essential gene, while in lineages where SVCT1 gene has been lost, GULO is now an essential gene.

Conclusions

The simultaneous study, for the first time, of GULO, Regucalcin and SVCTs evolution provides a clear picture of VC synthesis/acquisition and reveals very different selective pressures in different animal taxonomic groups.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-02040-7.

How does light facilitate vitamin C biosynthesis in leaves?

Plants store ascorbate in high concentrations, particularly in their leaves. Ascorbate is an excellent antioxidant that acts as an indispensable photoprotectant. The D-mannose/L-galactose pathway is responsible for ascorbate biosynthesis in plants. Light facilitates ascorbate biosynthesis in a light intensity-dependent manner to enhance ascorbate pool size in leaves, and photosynthesis is required for this process. Light- and photosynthesis-dependent activation of the rate-limiting enzyme GDP-L-galactose phosphorylase (GGP) plays a critical role in ascorbate pool size regulation. In addition, the tight regulation of ascorbate biosynthesis by ascorbate itself has been proposed. Ascorbate represses GGP translation in a dose-dependent manner through the upstream open reading frame in the 5'-untranslated regions of the gene, which may compete with the light-dependent activation of ascorbate biosynthesis. This review focuses on ascorbate biosynthesis based on past and latest findings and critically discusses how light activates this process.

Emerging roles of oxidative stress in brain aging and Alzheimer's disease

Reactive oxygen species (ROS) are metabolic byproducts that are necessary for physiological function but can be toxic at high levels. Levels of these oxidative stressors increase gradually throughout the lifespan, impairing mitochondrial function and damaging all parts of the body, particularly the central nervous system. Emerging evidence suggests that accumulated oxidative stress may be one of the key mechanisms causing cognitive aging and neurodegenerative diseases such as Alzheimer's disease (AD). Here, we synthesize the current literature on the effect of neuronal oxidative stress on mitochondrial dysfunction, DNA damage and epigenetic changes related to cognitive aging and AD. We further describe how oxidative stress therapeutics such as antioxidants, caloric restriction and physical activity can reduce oxidation and prevent cognitive decline in brain aging and AD. Of the currently available therapeutics, we propose that long term physical activity is the most promising avenue for improving cognitive health by reducing ROS while promoting the low levels required for optimal function.

Multiple Effects of Ascorbic Acid against Chronic Diseases: Updated Evidence from Preclinical and Clinical Studies

Severe disease commonly manifests as a systemic inflammatory process. Inflammation is associated withthe enhanced production of reactive oxygen and nitrogen species and with a marked reduction in the plasma concentrations of protective antioxidant molecules. This imbalance gives rise to oxidative stress, which is greater in patients with more severe conditions such as sepsis, cancer, cardiovascular disease, acute respiratory distress syndrome, and burns. In these patients, oxidative stress can trigger cell, tissue, and organ damage, thus increasing morbidity and mortality. Ascorbic acid (ASC) is a key nutrient thatserves as an antioxidant and a cofactor for numerous enzymatic reactions. However, humans, unlike most mammals, are unable to synthesize it. Consequently, ASC must be obtained through dietary sources, especially fresh fruit and vegetables. The value of administering exogenous micronutrients, to reestablish antioxidant concentrations in patients with severe disease, has been recognized for decades. Despite the suggestion that ASC supplementation may reduce oxidative stress and prevent several chronic conditions, few large, randomized clinical trials have tested it in patients with severe illness. This article reviews the recent literature on the pharmacological profile of ASC and the role of its supplementation in critically ill patients.

Ascorbic acid metabolism and functions: A comparison of plants and mammals

Ascorbic acid is synthesised by eukaryotes, the known exceptions being primates and some other animal groups which have lost functional gulonolactone oxidase. Prokaryotes do not synthesise ascorbate and do not need an ascorbate supply, so the functions that are essential for mammals and plants are not required or are substituted by other compounds. The ability of ascorbate to donate electrons enables it to act as a free radical scavenger and to reduce higher oxidation states of iron to Fe²⁺. These reactions are the basis of its biological activity along with the relative stability of the resulting resonance stabilised monodehydroascorbate radical. The importance of these properties is emphasised by the evolution of at least three biosynthetic pathways and production of an ascorbate analogue, erythroascorbate, by fungi. The iron reducing activity of ascorbate maintains the reactive centre Fe²⁺ of 2-oxoglutarate-dependent dioxygenases (2-ODDs) thus preventing inactivation. These enzymes have diverse functions and, recently, the possibility that ascorbate status in mammals could influence 2-ODDs involved in histone and DNA demethylation thereby influencing stem cell differentiation and cancer has been uncovered. Ascorbate is involved in iron uptake and transport in plants and animals. While the above biochemical functions are shared between mammals and plants, ascorbate peroxidase (APX) is an enzyme family limited to plants and photosynthetic protists. It provides these organisms with increased capacity to remove H₂O₂ produced by photosynthetic electron transport and photorespiration. The Fe reducing activity of ascorbate enables hydroxyl radical production (pro-oxidant effect) and the reactivity of dehydroascorbate (DHA) and reaction of its degradation products with proteins (dehydroascorbylation and glycation) is potentially damaging. Ascorbate status influences gene expression in plants and mammals but at present there is little evidence that it acts as a specific signalling molecule. It most likely acts indirectly by influencing the redox state of thiols and 2-ODD activity. However, the possibility that dehydroascorbylation is a regulatory post-translational protein modification could be explored.

Establishment of a myelinating co-culture system with a motor neuron-like cell line NSC-34 and an adult rat Schwann cell line IFRS1

Co-culture models of neurons and Schwann cells have been utilized for the study of myelination and demyelination in the peripheral nervous system; in most of the previous studies, however, these cells were obtained by primary culture with embryonic or neonatal animals. A spontaneously immortalized Schwann cell line IFRS1 from long-term cultures of adult Fischer rat peripheral nerves has been shown to retain fundamental ability to myelinate neurites in co-cultures with adult rat dorsal root ganglion neurons and nerve growth factor-primed PC12 cells. Our current investigation focuses on the establishment of stable co-culture system with IFRS1 cells and NSC-34 motor neuron-like cells. NSC-34 cells were seeded at a low density (2 × 10³/cm²) and maintained for 5-7 days in serum-containing medium supplemented with non-essential amino acids and brain-derived neurotrophic factor (BDNF; 10 ng/mL). Upon observation of neurite outgrowth under a phase-contrast microscope, the NSC-34 cells were exposed to an anti-mitotic agent mitomycin C (1 µg/mL) for 12-16 h, then co-cultured with IFRS1 cells (2 × 10⁴/cm²), and maintained in serum-containing medium supplemented with ascorbic acid (50 µg/mL), BDNF (10 ng/mL), and ciliary neurotrophic factor (10 ng/mL). Double immunofluorescence staining carried out at day 28 of the co-culture showed myelin protein (P0 or PMP22)-immunoreactive IFRS1 cells surrounding the βIII tubulin-immunoreactive neurites. This co-culture system can be a beneficial tool to study the pathogenesis of motor neuron diseases (e.g., amyotrophic lateral sclerosis, Charcot-Marie-Tooth diseases, and immune-mediated demyelinating neuropathies) and novel therapeutic approaches against them.

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

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.

The active role of vitamin C in mammalian iron metabolism: much more than just enhanced iron absorption!

Ascorbate is a cofactor in numerous metabolic reactions. Humans cannot synthesize ascorbate owing to inactivation of the gene encoding the enzyme l-gulono-γ-lactone oxidase, which is essential for ascorbate synthesis. Accumulating evidence strongly suggests that in addition to the known ability of dietary ascorbate to enhance nonheme iron absorption in the gut, ascorbate within mammalian systems can regulate cellular iron uptake and metabolism. Ascorbate modulates iron metabolism by stimulating ferritin synthesis, inhibiting lysosomal ferritin degradation, and decreasing cellular iron efflux. Furthermore, ascorbate cycling across the plasma membrane is responsible for ascorbate-stimulated iron uptake from low-molecular-weight iron-citrate complexes, which are prominent in the plasma of individuals with iron-overload disorders. Importantly, this iron-uptake pathway is of particular relevance to astrocyte brain iron metabolism and tissue iron loading in disorders such as hereditary hemochromatosis and β-thalassemia. Recent evidence also indicates that ascorbate is a novel modulator of the classical transferrin-iron uptake pathway, which provides almost all iron for cellular demands and erythropoiesis under physiological conditions. Ascorbate acts to stimulate transferrin-dependent iron uptake by an intracellular reductive mechanism, strongly suggesting that it may act to stimulate iron mobilization from the endosome. The ability of ascorbate to regulate transferrin iron uptake could help explain the metabolic defect that contributes to ascorbate-deficiency-induced anemia.

Ascorbate prevents cell death from prolonged exposure to glutamate in an in vitro model of human dopaminergic neurons

Ascorbate (vitamin C) is a nonenzymatic antioxidant highly concentrated in the brain. In addition to mediating redox balance, ascorbate is linked to glutamate neurotransmission in the striatum, where it renders neuroprotection against excessive glutamate stimulation. Oxidative stress and glutamatergic overactivity are key biochemical features accompanying the loss of dopaminergic neurons in the substantia nigra that characterizes Parkinson's disease (PD). At present, it is not clear whether antiglutamate agents and ascorbate might be neuroprotective agents for PD. Thus, we tested whether ascorbate can prevent cell death from prolonged exposure to glutamate using dopaminergic neurons of human origin. To this purpose, dopamine-like neurons were obtained by differentiation of SH-SY5Y cells and then cultured for 4 days without antioxidant (antiaging) protection to evaluate glutamate toxicity and ascorbate protection as a model system of potential factors contributing to dopaminergic neuron death in PD. Glutamate dose dependently induced toxicity in dopaminergic cells largely by the stimulation of AMPA and metabotropic receptors and to a lesser extent by N-methyl-D-aspartate and kainate receptors. At relatively physiological levels of extracellular concentration, ascorbate protected cells against glutamate excitotoxicity. This neuroprotection apparently relies on the inhibition of oxidative stress, because ascorbate prevented the pro-oxidant action of the scavenging molecule quercetin, which occurred over the course of prolonged exposure, as is also seen with glutamate. Our findings show the relevance of ascorbate as a neuroprotective agent and emphasize an often underappreciated role of oxidative stress in glutamate excitotoxicity. Occurrence of a glutamate-ascorbate link in dopaminergic neurons may explain previous contradictions regarding their putative role in PD.

Combined vitamin C and E deficiency induces motor defects in gulo(-/-)/SVCT2(+/-) mice

OBJECTIVES

Key antioxidants, vitamins C and E, are necessary for normal brain development and neuronal function. In this study, we depleted both of these vitamins in two mouse models to determine if oxidative stress due to combined vitamin C and E dietary deficiency altered their neurological phenotype. The first model lacked both alleles for the Gulonolactone oxidase gene (Gulo(-/-)) and therefore was unable synthesize vitamin C. To obtain an additional cellular deficiency of vitamin C, the second model also lacked one allele for the cellular vitamin C transporter gene (Gulo(-/-)/SVCT2(+/-)).

METHODS

The experimental treatment was 16 weeks of vitamin E deprivation followed by 3 weeks of vitamin C deprivation. Mice were assessed for motor coordination deficits, vitamin levels, and oxidative stress biomarkers.

RESULTS

In the first model, defects in motor performance were more apparent in both vitamin C-deficient groups (VE+VC-, VE-VC-) compared to vitamin C-supplemented groups (VE+VC+, VE-VC+) regardless of vitamin E level. Analysis of brain cortex and liver confirmed decreases of at least 80% for each vitamin in mice on deficient diets. Vitamin E deficiency doubled oxidative stress biomarkers (F2-isoprostanes and malondialdehyde). In the second model, Gulo(-/-)/SVCT2(+/-) mice on the doubly deficient diets showed deficits in locomotor activity, Rota-rod performance, and other motor tasks, with no concomitant change in anxiety or spatial memory.

DISCUSSION

Vitamin E deficiency alone caused a modest oxidative stress in brain that did not affect motor performance. Adding a cellular deficit in vitamin C to dietary deprivation of both vitamins significantly impaired motor performance.

Ascorbic acid efficiently enhances neuronal synthesis of norepinephrine from dopamine

Ascorbic acid enhances synthesis of norepinephrine from dopamine in adrenal chromaffin cells by serving as a co-factor for chromaffin granule dopamine β-hydroxylase (DβH). However, there is controversy regarding in situ kinetics of the ascorbate effect in chromaffin cells, as well as whether they apply to neuronal cells. In this study we evaluated the stimulation of norepinephrine synthesis from dopamine in cultured SH-SY5Y neuroblastoma cells. These cells contained neither ascorbate nor norepinephrine in culture, but when provided with dopamine, they generated intracellular norepinephrine at rates that were stimulated several-fold by intracellular ascorbate. Ascorbate-induced increases in norepinephrine synthesis in dopamine-treated cells were linear over 60 min, despite saturation of intracellular ascorbate. Norepinephrine accumulation after 60 min of incubation with 100 μM dopamine was half-maximal at intracellular ascorbate concentrations of 0.2-0.5 mM, which fits well with the literature K(m) for ascorbate of DβH using dopamine as a substrate. Moreover, these ascorbate concentrations were generated by initial extracellular ascorbate concentrations of less than 25 μM due to concentrative accumulation by the ascorbate transporter. Treatment with 100 μM dopamine acutely increased cellular superoxide generation, which was prevented by ascorbate loading, but associated with a decrease in intracellular ascorbate when the latter was present at concentrations under 1 mM. These results show that ascorbate promptly enhances norepinephrine synthesis from dopamine by neuronal cells that it does so at physiologic intracellular concentrations in accord with the kinetics of DβH, and that it both protects cells from superoxide and by providing electrons to DβH.

The SLC23 family of ascorbate transporters: ensuring that you get and keep your daily dose of vitamin C

The ascorbate transporters SVCT1 and SVCT2 are crucial for maintaining intracellular ascorbate concentrations in most cell types. Although the two transporter isoforms are highly homologous, they have different physiologic functions. The SVCT1 is located primarily in epithelial cells and has its greatest effect in reabsorbing ascorbate in the renal tubules. The SVCT2 is located in most non-epithelial tissues, with the highest expression in brain and neuroendocrine tissues. These transporters are hydrophobic membrane proteins that have a high affinity and are highly selective for ascorbate. Their ability to concentrate ascorbate inside cells is driven by the sodium gradient across the plasma membrane as generated by Na+/K+ ATPase. They can concentrate ascorbate 20 to 60-fold over plasma ascorbate concentrations. Ascorbate transport on these proteins is regulated at the transcriptional, translational and post-translational levels. Available studies show that transporter function is acutely regulated by protein kinases A and C, whereas transporter expression is increased by low intracellular ascorbate and associated oxidative stress. The knockout of the SVCT2 in mice is lethal on day 1 of life, and almost half of SVCT1 knockout mice do not survive to weaning. These findings confirm the importance both of cellular ascorbate and of the two transport proteins as key to maintaining intracellular ascorbate. LINKED ARTICLES This article is part of a themed section on Transporters. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2011.164.issue-7.

Vitamin C transport and its role in the central nervous system

Vitamin C, or ascorbic acid, is important as an antioxidant and participates in numerous cellular functions. Although it circulates in plasma in micromolar concentrations, it reaches millimolar concentrations in most tissues. These high ascorbate cellular concentrations are thought to be generated and maintained by the SVCT2 (Slc23a2), a specific transporter for ascorbate. The vitamin is also readily recycled from its oxidized forms inside cells. Neurons in the central nervous system (CNS) contain some of the highest ascorbic acid concentrations of mammalian tissues. Intracellular ascorbate serves several functions in the CNS, including antioxidant protection, peptide amidation, myelin formation, synaptic potentiation, and protection against glutamate toxicity. The importance of the SVCT2 for CNS function is supported by the finding that its targeted deletion in mice causes widespread cerebral hemorrhage and death on post-natal day 1. Neuronal ascorbate content as maintained by this protein also has relevance for human disease, since ascorbate supplements decrease infarct size in ischemia-reperfusion injury models of stroke, and since ascorbate may protect neurons from the oxidant damage associated with neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's. The aim of this review is to assess the role of the SVCT2 in regulating neuronal ascorbate homeostasis and the extent to which ascorbate affects brain function and antioxidant defenses in the CNS.

Nitric oxide modulates sodium vitamin C transporter 2 (SVCT-2) protein expression via protein kinase G (PKG) and nuclear factor-κB (NF-κB)

Ascorbate is an important antioxidant, which also displays important functions in neuronal tissues, including the retina. The retina is responsible for the initial steps of visual processing, which is further refined in cerebral high-order centers. The retina is also a prototypical model for studying physiologic aspects of cells that comprise the nervous system. Of major importance also is the cellular messenger nitric oxide (NO). Previous studies have demonstrated the significance of NO for both survival and proliferation of cultured embryonic retinal cells. Cultured retinal cells express a high-affinity ascorbate transporter, and the release of ascorbate is delicately regulated by ionotropic glutamate receptors. Therefore, we proposed whether there is interplay between the ascorbate transport system and NO signaling pathway in retinal cells. Here we show compelling evidence that ascorbate uptake is tightly controlled by NO and its downstream signaling pathway in culture. NO also modulates the expression of SVCT-2, an effect mediated by cGMP and PKG. Kinetic studies suggest that NO increases the transport capacity for ascorbate, but not the affinity of SVCT-2 for its substrate. Interestingly, NO utilizes the NF-κB pathway, in a PKG-dependent manner, to modulate both SVCT-2 expression and ascorbate uptake. These results demonstrate that NO exerts a fine-tuned control of the availability of ascorbate to cultured retinal cells and strongly reinforces ascorbate as an important bioactive molecule in neuronal tissues.

The SLC23 family of ascorbate transporters: ensuring that you get and keep your daily dose of vitamin C

The ascorbate transporters SVCT1 and SVCT2 are crucial for maintaining intracellular ascorbate concentrations in most cell types. Although the two transporter isoforms are highly homologous, they have different physiologic functions. The SVCT1 is located primarily in epithelial cells and has its greatest effect in reabsorbing ascorbate in the renal tubules. The SVCT2 is located in most non-epithelial tissues, with the highest expression in brain and neuroendocrine tissues. These transporters are hydrophobic membrane proteins that have a high affinity and are highly selective for ascorbate. Their ability to concentrate ascorbate inside cells is driven by the sodium gradient across the plasma membrane as generated by Na+/K+ ATPase. They can concentrate ascorbate 20 to 60-fold over plasma ascorbate concentrations. Ascorbate transport on these proteins is regulated at the transcriptional, translational and post-translational levels. Available studies show that transporter function is acutely regulated by protein kinases A and C, whereas transporter expression is increased by low intracellular ascorbate and associated oxidative stress. The knockout of the SVCT2 in mice is lethal on day 1 of life, and almost half of SVCT1 knockout mice do not survive to weaning. These findings confirm the importance both of cellular ascorbate and of the two transport proteins as key to maintaining intracellular ascorbate. LINKED ARTICLES This article is part of a themed section on Transporters. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2011.164.issue-7.

Vitamin C function in the brain: vital role of the ascorbate transporter SVCT2

Ascorbate (vitamin C) is a vital antioxidant molecule in the brain. However, it also has a number of other important functions, participating as a cofactor in several enzyme reactions, including catecholamine synthesis, collagen production, and regulation of HIF-1 alpha. Ascorbate is transported into the brain and neurons via the sodium-dependent vitamin C transporter 2 (SVCT2), which causes accumulation of ascorbate within cells against a concentration gradient. Dehydroascorbic acid, the oxidized form of ascorbate, is transported via glucose transporters of the GLUT family. Once in cells, it is rapidly reduced to ascorbate. The highest concentrations of ascorbate in the body are found in the brain and in neuroendocrine tissues such as adrenal, although the brain is the most difficult organ to deplete of ascorbate. Combined with regional asymmetry in ascorbate distribution within different brain areas, these facts suggest an important role for ascorbate in the brain. Ascorbate is proposed as a neuromodulator of glutamatergic, dopaminergic, cholinergic, and GABAergic transmission and related behaviors. Neurodegenerative diseases typically involve high levels of oxidative stress and thus ascorbate has been posited to have potential therapeutic roles against ischemic stroke, Alzheimer's disease, Parkinson's disease, and Huntington's disease.

Inflammation in the vascular bed: importance of vitamin C

Despite decreases in atherosclerotic coronary vascular disease over the last several decades, atherosclerosis remains a major cause of mortality in developed nations. One possible contributor to this residual risk is oxidant stress, which is generated by the inflammatory response of atherosclerosis. Although there is a wealth of in vitro, cellular, and animal data supporting a protective role for antioxidant vitamins and nutrients in the atherosclerotic process, the best clinical trials have been negative. This may be due to the fact that antioxidant therapies are applied "too little and too late." This review considers the role of vitamin C, or ascorbic acid in preventing the earliest inflammatory changes in atherosclerosis. It focuses on the three major vascular cell types involved in atherosclerosis: endothelial cells, vascular smooth muscle cells, and macrophages. Ascorbate chemistry, recycling, and function are described for these cell types, with emphasis on whether and how the vitamin might affect the inflammatory process. For endothelial cells, ascorbate helps to prevent endothelial dysfunction, stimulates type IV collagen synthesis, and enhances cell proliferation. For vascular smooth muscle cells, ascorbate inhibits dedifferentiation, recruitment, and proliferation in areas of vascular damage. For macrophages, ascorbate decreases oxidant stress related to their activation, decreases uptake and degradation of oxidized LDL in some studies, and enhances several aspects of their function. Although further studies of ascorbate function in these cell types and in novel animal models are needed, available evidence generally supports a salutary role for this vitamin in ameliorating the earliest stages of atherosclerosis.

SVCT1 and SVCT2: key proteins for vitamin C uptake

Vitamin C is accumulated in mammalian cells by two types of proteins: sodium-ascorbate co-transporters (SVCTs) and hexose transporters (GLUTs); in particular, SVCTs actively import ascorbate, the reduced form of this vitamin. SVCTs are surface glycoproteins encoded by two different genes, very similar in structure. They show distinct tissue distribution and functional characteristics, which indicate different physiological roles. SVCT1 is involved in whole-body homeostasis of vitamin C, while SVCT2 protects metabolically active cells against oxidative stress. Regulation at mRNA or protein level may serve for preferential accumulation of ascorbic acid at sites where it is needed. This review will summarize the present knowledge on structure, function and regulation of the SVCT transporters. Understanding the physiological role of SVCT1 and SVCT2 may lead to develop new therapeutic strategies to control intracellular vitamin C content or to promote tissue-specific delivery of vitamin C-drug conjugates.

Sparing effects of selenium and ascorbic acid on vitamin C and E in guinea pig tissues

BACKGROUND

Selenium (Se), vitamin C and vitamin E function as antioxidants within the body. In this study, we investigated the effects of reduced dietary Se and L-ascorbic acid (AA) on vitamin C and alpha-tocopherol (AT) status in guinea pig tissues.

METHODS

Male Hartley guinea pigs were orally dosed with a marginal amount of AA and fed a diet deficient (Se-D/MC), marginal (Se-M/MC) or normal (Se-N/MC) in Se. An additional diet group (Se-N/NC) was fed normal Se and dosed with a normal amount of AA. Guinea pigs were killed after 5 or 12 weeks on the experimental diets at 24 and 48 hours post AA dosing.

RESULTS

Liver Se-dependent glutathione peroxidase activity was decreased (P < 0.05) in guinea pigs fed Se or AA restricted diets. Plasma total glutathione concentrations were unaffected (P > 0.05) by reduction in dietary Se or AA. All tissues examined showed a decrease (P < 0.05) in AA content in Se-N/MC compared to Se-N/NC guinea pigs. Kidney, testis, muscle and spleen showed a decreasing trend (P < 0.05) in AA content with decreasing Se in the diet. Dehydroascorbic acid concentrations were decreased (P < 0.05) in several tissues with reduction in dietary Se (heart and spleen) or AA (liver, heart, kidney, muscle and spleen). At week 12, combined dietary restriction of Se and AA decreased AT concentrations in most tissues. In addition, restriction of Se (liver, heart and spleen) and AA (liver, kidney and spleen) separately also reduced AT in tissues.

CONCLUSION

Together, these data demonstrate sparing effects of Se and AA on vitamin C and AT in guinea pig tissues.

Translational control of the ascorbic acid transporter SVCT2 in human platelets

Reactive oxygen species (ROS) and redox state have emerged as physiological mediators, controlling blood coagulation and thrombosis. The redox balance is obviously linked to the presence of antioxidants; in particular, vitamin C appears to be a key modulator of platelet oxidative state, since these cells physiologically accumulate ascorbic acid and, moreover, platelet ascorbate plays a role during aggregation. Here, we showed that platelets could compensate for fluctuations in ascorbate levels by modulating the expression of the Na+-dependent transporter SVCT2. Furthermore, the use of anucleated cells demonstrated, for the first time, that SVCT2 expression could be regulated at the translational level. The control of ascorbic acid uptake, through regulation of its carrier, was not only related to substrate availability, but it also occurred during platelet activation, which was accompanied by vitamin C deprivation and alteration in the redox state. Finally, we showed that changes in intracellular ascorbic acid content had physiological relevance, since they modulate the surface sulfhydryl content and the thrombus viscoelastic properties. Beside its role during aggregation, vitamin C may also have important effects during postaggregatory events.

A Cerebroprotective Dose of Intravenous Citrate/Sorbitol-stabilized Dehydroascorbic Acid is Correlated with Increased Cerebral Ascorbic Acid and Inhibited Lipid Peroxidation after Murine Reperfused Stroke

OBJECTIVE

Oxidative damage has been implicated in the pathogenesis of cerebral ischemia. We previously demonstrated that exogenously supplied dehydroascorbic acid (DHA), an oxidized, blood-brain barrier transportable form of the antioxidant ascorbic acid (AA), improves outcome after experimental stroke.

METHODS

To investigate the neuroprotective effect of DHA therapy, we measured cerebral AA levels using a novel assay, quantified markers of lipid peroxidation, and evaluated infarct volume after reperfused stroke in a murine model. All experiments were performed using a new citrate/sorbitol-stabilized DHA formulation to improve the stability of the compound.

RESULTS

Intraparenchymal AA levels declined after cerebral ischemia/reperfusion and were repleted in a dose-dependent fashion by postischemic administration of intravenous DHA (P < 0.01). Repletion of these levels was associated with reductions in cerebral malondialdehyde levels (P < 0.05), which were also elevated after reperfused stroke. DHA repletion of interstitial AA levels and reduction in cerebral lipid peroxidation was associated with dose-dependent reductions in infarct volume (P < 0.05).

CONCLUSION

Together, these results indicate that an intravenous cerebroprotective dose of citrate/sorbitol-stabilized DHA is correlated with increased brain ascorbate levels and a suppression of excessive oxidative metabolism.

Ascorbate transport and recycling by SH-SY5Y neuroblastoma cells: response to glutamate toxicity

Neurons maintain relatively high intracellular concentrations of vitamin C, or ascorbic acid. In this work we studied the mechanisms by which neuronal cells in culture transport and maintain ascorbate, as well as how this system responds to oxidant stress induced by glutamate. Cultured SH-SY5Y neuroblastoma cells took up ascorbate, achieving steady-state intracellular concentrations of 6 mM and higher at extracellular concentrations of 200 microM and greater. This gradient was generated by relatively high affinity sodium-dependent ascorbate transport (Km of 113 microM). Ascorbate was also recycled from dehydroascorbate, the reduction of which was dependent on GSH, but not on D-glucose. Glutamate in concentrations up to 2 mM caused an acute concentration-dependent efflux of ascorbate from the cells, which was prevented by the anion channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. Intracellular ascorbate did not affect radiolabeled glutamate uptake, showing absence of heteroexchange.

Ascorbate uptake and antioxidant function in peritoneal macrophages

Since activated macrophages generate potentially deleterious reactive oxygen species, we studied whether ascorbic acid might function as an antioxidant in these cells. Thioglycollate-elicited murine peritoneal macrophages contained about 3 mM ascorbate that was halved by culture in ascorbate-free medium. However, the cells took up added ascorbate to concentrations of 6-8 mM by a high-affinity sodium-dependent transport mechanism. This likely reflected the activity of the SVCT2 ascorbate transporter, since its message and protein were present in the cells. Activation of the cells by phagocytosis of latex particles depleted intracellular ascorbate, although not below the basal levels present in the cells in culture. Glutathione (GSH) was unaffected by phagocytosis, suggesting that ascorbate was more sensitive to the oxidant stress of phagocytosis than GSH. Phagocytosis induced a modest increase in reactive oxygen species as well as a progressive loss of alpha-tocopherol, both of which were prevented in cells loaded with ascorbate. These results suggest that activated macrophages can use ascorbate to lessen self-generated oxidant stress and spare alpha-tocopherol, which may protect these long-lived cells from necrosis or apoptosis.

The presence of ascorbate induces expression of brain derived neurotrophic factor in SH-SY5Y neuroblastoma cells after peroxide insult, which is associated with increased survival

Oxidative stress and free radical production have been implicated in Alzheimer's disease, where low levels of the antioxidant vitamin C (ascorbate) have been shown to be associated with the disease. In this study, neuroblastoma SH-SY5Y cells were treated with hydrogen peroxide in the presence of ascorbate in order to elucidate the mechanism(s) of protection against oxidative stress afforded by ascorbate. Protein oxidation, glutathione levels, cell viability and the effects on the proteome and its oxidized counterpart were monitored. SH-SY5Y cells treated with ascorbate prior to co-incubation with peroxide showed increased viability in comparison to cells treated with peroxide alone. This dual treatment also caused an increase in protein carbonyl content and a decrease in glutathione levels within the cells. Proteins, extracted from SH-SY5Y cells that were treated with either ascorbate or peroxide alone or with ascorbate prior to peroxide, were separated by two-dimensional gel electrophoresis and analyzed for oxidation. Co-incubation for 24 hours decreased the number of oxidised proteins (e.g. acyl CoA oxidase 3) and induced brain derived neurotrophic factor (BDNF) expression. Enhanced expression of BDNF may contribute to the protective effects of ascorbate against oxidative stress in neuronal cells.

Restorative potential of dopaminergic grafts in presence of antioxidants in rat model of Parkinson's disease

Free radical mediated damage has been reported to contribute significantly towards low survival (5-10%) of grafted dopaminergic neurons, post transplantation. In the present study, an attempt has been made to explore the neuroprotective potential of the combination of two major antioxidants ascorbic acid (AA) and glutathione (GSH) on ventral mesencephalic cells (VMC) and nigral dopamine (DA) neurons when co-transplanted together with VMC in rat model of Parkinson's disease (PD). GSH and AA have been reported to act co-operatively in the conditions of oxidative stress thereby helping in maintaining the cellular GSH/GSSG redox status. Functional recovery was assessed 12 weeks post transplantation, where a significant restoration (p<0.001) in d-amphetamine induced circling behavior (62%), spontaneous locomotor activity (SLA; 64%), dopamine-D2 receptor binding (63%), dopamine (65%) and 3,4-dihydroxy phenyl acetic acid (DOPAC) level (64%) was observed in co-transplanted animals as compared to lesioned and VMC alone grafted rats. VMC and GSH+AA co-transplanted animals exhibited a significantly higher surviving TH-immunoreactive (TH-ir) neurons number (p<0.01), TH-ir fibers outgrowth (p<0.05) in striatal graft and TH-ir neurons in substantia nigra pars compacta (SNpc) (p<0.01), as compared to VMC alone transplanted rats. An attempt was made to further confirm our in vivo observations through in vitro experiments where following in vitro exposure to 6-OHDA, a higher cell survival (p<0.01), TH-ir cell counts (p<0.001) and DA and DOPAC levels (p<0.01) were also observed in 8-day-old VMC culture in presence of GSH+AA as compared to VMC cultured in absence of antioxidants. The results suggest that GSH+AA when co-transplanted with VMC provide higher restoration probably by increasing the survival of grafted VMC and simultaneously supporting nigral TH-immunopositive neurons in rat model of PD.