Oxidized LDL up-regulates the ascorbic acid transporter SVCT2 in endothelial cells

SVCT2, a potential therapeutic target, protects against oxidative stress during ethanol-induced neurotoxicity via JNK/p38 MAPKs, NF-κB and miRNA125a-5p

Sodium vitamin C transporter 2 (SVCT2) plays a key role in transporting ascorbic acid (AA), an important intracellular antioxidant, into neurons. It is well known that ethanol (EtOH) abuse causes significant neurodegeneration, as well as endogenous AA release in certain encephalic regions. Here, we identified that SVCT2 forms part of a self-defense mechanism that protects against oxidative stress in binge drinking rats, and SVCT2 levels are correlated with antioxidants and neuronal injury. Four days of binge drinking led to massive neuron degeneration in prefrontal cortex (PFC), accompanied by increased levels of 4-hydroxynonenal (4-HNE)-adducted proteins and SVCT2 expression, as well as dramatic changes in AA levels in rat brain. AA levels were decreased in PFC and increased in cerebrospinal fluid (CSF) after binge drinking, but returned to normal on the 7th day following EtOH withdrawal. These processes were further evaluated in primary cortical neurons exposed to 100mM EtOH in vitro. Neurons transfected with SVCT2 siRNA were more susceptible than controls to certain aspects of EtOH-induced injury, including cell death, dendrite damage and increased oxidative stress. EtOH-induced up-regulation of SVCT2 was associated with activation of JNK and p38 MAPKs and the NF-κB pathway. More importantly, miRNA-125a-5p was down-regulated in PFC of 4-day binge drinking rats and negatively regulated protein expression during EtOH-induced neuronal injury. MiR-125a-5p over-expression attenuated intracellular AA levels, promoted cell death and suppressed the EtOH-induced up-regulation of p38 MAPK and SVCT2, which suggested that miR-125a-5p plays an important role in SVCT2 function in EtOH-induced neuronal injury. We speculate that SVCT2, possibly regulated by JNK/p38 MAPKs, NF-κB signaling and miR-125a-5p, has a neuroprotective effect against EtOH-induced oxidative stress. Promotion of SVCT2 expression or stimulation of SVCT2 activity may be a promising therapeutic strategy for the prevention of EtOH-associated neurodegeneration.

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.

Ascorbic acid prevents high glucose-induced apoptosis in human brain pericytes

High glucose concentrations due to diabetes increase apoptosis of vascular pericytes, impairing vascular regulation and weakening vessels, especially in brain and retina. We sought to determine whether vitamin C, or ascorbic acid, could prevent such high glucose-induced increases in pericyte apoptosis. Culture of human microvascular brain pericytes at 25 mM compared to 5mM glucose increased apoptosis measured as the appearance of cleaved caspase 3. Loading the cells with ascorbate during culture decreased apoptosis, both at 5 and 25 mM glucose. High glucose-induced apoptosis was due largely to activation of the receptor for advanced glycation end products (RAGE), since it was prevented by specific RAGE inhibition. Culture of pericytes for 24h with RAGE agonists also increased apoptosis, which was completely prevented by inclusion of 100 μM ascorbate. Ascorbate also prevented RAGE agonist-induced apoptosis measured as annexin V binding in human retinal pericytes, a cell type with relevance to diabetic retinopathy. RAGE agonists decreased intracellular ascorbate and GSH in brain pericytes. Despite this evidence of increased oxidative stress, ascorbate prevention of RAGE-induced apoptosis was not mimicked by several antioxidants. These results show that ascorbate prevents pericyte apoptosis due RAGE activation. Although RAGE activation decreases intracellular ascorbate and GSH, the prevention of apoptosis by ascorbate may involve effects beyond its function as an antioxidant.

Ascorbic acid and the brain: rationale for the use against cognitive decline

This review is focused upon the role of ascorbic acid (AA, vitamin C) in the promotion of healthy brain aging. Particular attention is attributed to the biochemistry and neuronal metabolism interface, transport across tissues, animal models that are useful for this area of research, and the human studies that implicate AA in the continuum between normal cognitive aging and age-related cognitive decline up to Alzheimer’s disease. Vascular risk factors and comorbidity relationships with cognitive decline and AA are discussed to facilitate strategies for advancing AA research in the area of brain health and neurodegeneration.

Does vitamin C deficiency increase lifestyle-associated vascular disease progression? Evidence based on experimental and clinical studies

SIGNIFICANCE

Despite continuous advances in the prevention of cardiovascular disease (CVD), critical issues associated with an unhealthy lifestyle remain an increasing cause of morbidity and mortality in industrialized countries.

RECENT ADVANCES

A growing body of literature supports a specific role for vitamin C in a number of reactions that are associated with vascular function and control including, for example, nitric oxide bioavailability, lipid metabolism, and vascular integrity.

CRITICAL ISSUES

A large body of epidemiological evidence supports a relationship between poor vitamin C status and increased risk of developing CVD, and the prevalence of deficiency continues to be around 10%-20% of the general Western population although this problem could easily and cheaply be solved by supplementation. However, large intervention studies using vitamin C have not found a beneficial effect of supplementation. This review outlines the proposed mechanism by which vitamin C deficiency worsens CVD progression. In addition, it discusses problems with the currently available literature, including the discrepancies between the large intervention studies and the experimental and epidemiological literature.

FUTURE DIRECTIONS

Increased insights into vitamin C deficiency-mediated CVD progression will enable the design of future randomized controlled trials that are better suited to test the efficacy of vitamin C in disease prevention as well as the identification of high-risk individuals which could possibly benefit from supplementation.

Role of vitamin C in the function of the vascular endothelium

SIGNIFICANCE

Vitamin C, or ascorbic acid, has long been known to participate in several important functions in the vascular bed in support of endothelial cells. These functions include increasing the synthesis and deposition of type IV collagen in the basement membrane, stimulating endothelial proliferation, inhibiting apoptosis, scavenging radical species, and sparing endothelial cell-derived nitric oxide to help modulate blood flow. Although ascorbate may not be able to reverse inflammatory vascular diseases such as atherosclerosis, it may well play a role in preventing the endothelial dysfunction that is the earliest sign of many such diseases.

RECENT ADVANCES

Beyond simply preventing scurvy, evidence is mounting that ascorbate is required for optimal function of many dioxygenase enzymes in addition to those involved in collagen synthesis. Several of these enzymes regulate the transcription of proteins involved in endothelial function, proliferation, and survival, including hypoxia-inducible factor-1α and histone and DNA demethylases. More recently, ascorbate has been found to acutely tighten the endothelial permeability barrier and, thus, may modulate access of ascorbate and other molecules into tissues and organs.

CRITICAL ISSUES

The issue of the optimal cellular content of ascorbate remains unresolved, but it appears that low millimolar ascorbate concentrations are normal in most animal tissues, in human leukocytes, and probably in the endothelium. Although there may be little benefit of increasing near maximal cellular ascorbate concentrations in normal people, many diseases and conditions have either systemic or localized cellular ascorbate deficiency as a cause for endothelial dysfunction, including early atherosclerosis, sepsis, smoking, and diabetes.

FUTURE DIRECTIONS

A key focus for future studies of ascorbate and the vascular endothelium will likely be to determine the mechanisms and clinical relevance of ascorbate effects on endothelial function, permeability, and survival in diseases that cause endothelial dysfunction.

Vitamin C in disease prevention and cure: an overview

The recognition of vitamin C is associated with a history of an unrelenting search for the cause of the ancient haemorrhagic disease scurvy. Isolated in 1928, vitamin C is essential for the development and maintenance of connective tissues. It plays an important role in bone formation, wound healing and the maintenance of healthy gums. Vitamin C plays an important role in a number of metabolic functions including the activation of the B vitamin, folic acid, the conversion of cholesterol to bile acids and the conversion of the amino acid, tryptophan, to the neurotransmitter, serotonin. It is an antioxidant that protects body from free radical damage. It is used as therapeutic agent in many diseases and disorders. Vitamin C protects the immune system, reduces the severity of allergic reactions and helps to fight off infections. However the significance and beneficial effect of vitamin C in respect to human disease such as cancer, atherosclerosis, diabetes, neurodegenerative disease and metal toxicity however remains equivocal. Thus further continuous uninterrupted efforts may open new vistas to understand its significance in disease management.

Regulation of vitamin C transporter in the type 1 diabetic mouse bone and bone marrow

A number of studies have revealed that Type I diabetes (T1D) is associated with bone loss and an increased risk of fractures. T1D induces oxidative stress in various tissues and organs. Vitamin C plays an important role in the attenuation of oxidative stress; however, little is known about the effect of T1D induced oxidative stress on the regulation of vitamin C transporter in bone and bone marrow cells. To investigate this, T1D was induced in mice by multiple low dose injections of streptozotocin. We have demonstrated that endogenous antioxidants, glutathione peroxidase (GPx) and superoxide dismutase (SOD) are down-regulated in the bone and bone marrow of T1D. The vitamin C transporter isoform SVCT2, the only known transporter expressed in bone and bone marrow stromal cells (BMSCs), is negatively regulated in the bone and bone marrow of T1D. The μCT imaging of the bone showed significantly lower bone quality in the 8 week T1D mouse. The in-vitro study in BMSCS showed that the knockdown of SVCT2 transporter decreases ascorbic acid (AA) uptake, and increases oxidative stress. The significant reversing effect of antioxidant vitamin C is only possible in control cells, not in knockdown cells. This study suggested that T1D induces oxidative stress and decreases SVCT2 expression in the bone and bone marrow environment. Furthermore, this study confirms that T1D increases bone resorption, decreases bone formation and changes the microstructure of bones. This study has provided evidence that the regulation of the SVCT2 transporter plays an important role not only in T1D osteoporosis but also in other oxidative stress-related musculoskeletal complications.

Sodium-dependent vitamin C transporter SVCT2: expression and function in bone marrow stromal cells and in osteogenesis

Ascorbic acid (Vitamin C) has a critical role in bone formation and osteoblast differentiation, but very little is known about the molecular mechanisms of ascorbic acid entry into bone marrow stromal cells (BMSCs). To address this gap in knowledge, we investigated the identity of the transport system that is responsible for the uptake of ascorbic acid into bone marrow stromal cells (BMSCs). First, we examined the expression of the two known isoforms of the sodium-coupled ascorbic acid transporter, namely SVCT1 and SVCT2, in BMSCs (Lin-ve Sca1+ve) and bone at the mRNA level. Only SVCT2 mRNA was detected in BMSCs and bone. Uptake of ascorbic acid in BMSCs was Na(+)-dependent and saturable. In order to define the role of SVCT2 in BMSC differentiation into osteoblasts, BMSCs were stimulated with osteogenic media for different time intervals, and the activity of SVCT2 was monitored by ascorbic acid uptake. SVCT2 expression was up-regulated during the osteogenic differentiation of BMSCs; the expression was maximal at the earliest phase of differentiation. Subsequently, osteogenesis was inhibited in BMSCs upon knock-down of SVCT2 by lentivirus shRNA. We also found that the expression of the SVCT2 could be negatively or positively modulated by the presence of oxidant (Sin-1) or antioxidant (Ascorbic acid) compounds, respectively, in BMSCs. Furthermore, we found that this transporter is also regulated with age in mouse bone. These data show that SVCT2 plays a vital role in the osteogenic differentiation of BMSCs and that its expression is altered under conditions associated with redox reaction. Our findings could be relevant to bone tissue engineering and bone related diseases such as osteoporosis in which oxidative stress and aging plays important role.