Redox regulation of vitamin C transporter SVCT2 in C2C12 myotubes

The sodium/ascorbic acid co-transporter SVCT2 distributes in a striated membrane-enriched domain at the M-band level in slow-twitch skeletal muscle fibers

BACKGROUND

Vitamin C plays key roles in cellular homeostasis, functioning as a potent antioxidant and a positive regulator of cell differentiation. In skeletal muscle, the vitamin C/sodium co-transporter SVCT2 is preferentially expressed in oxidative slow fibers. SVCT2 is up-regulated during the early fusion of primary myoblasts and decreases during initial myotube growth, indicating the relevance of vitamin C uptake via SVCT2 for early skeletal muscle differentiation and fiber-type definition. However, our understanding of SVCT2 expression and function in adult skeletal muscles is still limited.

RESULTS

In this study, we demonstrate that SVCT2 exhibits an intracellular distribution in chicken slow skeletal muscles, following a highly organized striated pattern. A similar distribution was observed in human muscle samples, chicken cultured myotubes, and isolated mouse myofibers. Immunohistochemical analyses, combined with biochemical cell fractionation experiments, reveal a strong co-localization of SVCT2 with intracellular detergent-soluble membrane fractions at the central sarcomeric M-band, where it co-solubilizes with sarcoplasmic reticulum proteins. Remarkably, electrical stimulation of cultured myofibers induces the redistribution of SVCT2 into a vesicular pattern.

CONCLUSIONS

Our results provide novel insights into the dynamic roles of SVCT2 in different intracellular compartments in response to functional demands.

Blue light promotes vitamin C-mediated ferroptosis of melanoma through specifically upregulating transporter SVCT2 and generating Fe2

Vitamin C (VC)-based cancer therapy is a promising therapeutic approach for a variety of cancers due to its profound effects on redox reactions and metabolic pathways. However, high administration dosage of VC for necessary therapeutic efficacy for cancers increases the risk of overt side effects and limits its clinical use. Here, we show cutaneous blue light irradiation can specifically upregulate the sodium-dependent vitamin C transporter 2 (SVCT2) of the tumor and increase effectively the VC concentration at the tumor sites by an overall low dosage administration. In the mouse melanoma model, blue light stimulates the SVCT2 expression through the nuclear factor-kappa B (NF-κB) signaling pathway both in vitro and in vivo. The increased cellular VC together with Fe2+ generated by blue light simultaneously elevate cellular oxidative stress and trigger the ferroptosis of melanoma. With this revealed mechanism, the synergistic actions of blue light on the VC transporter and Fe2+ generation lead to a ca. 20-fold reduction in the administration dosage of VC with an effective melanoma elimination and prolonged survival. The work defines the killing mechanism of blue light on VC-based cancer therapy and provides a practical approach for promoting VC uptake. This light-assisted VC therapy is not only highly efficient for melanoma but also considerable for a broad clinical utility.

Neutrophils Isolated from Septic Patients Exhibit Elevated Uptake of Vitamin C and Normal Intracellular Concentrations despite a Low Vitamin C Milieu

Vitamin C (ascorbate) plays an important role in neutrophil function and is accumulated by the cells either directly via vitamin C transporters (SVCT) or indirectly following oxidation to dehydroascorbic acid. Septic patients are known to have significantly depleted plasma ascorbate status, but little is known about the ascorbate content of their circulating cells. Therefore, we assessed the ascorbate concentrations of plasma, leukocytes and erythrocytes from septic patients and compared these to healthy controls. Non-fasting blood samples were collected from healthy volunteers (n = 20) and critically ill patients with sepsis (n = 18). The ascorbate content of the plasma and isolated neutrophils and erythrocytes was measured using HPLC and plasma myeloperoxidase concentrations were determined using ELISA. Ex vivo uptake of ascorbate and dehydroascorbic acid by neutrophils from septic patients was also assessed. Neutrophils isolated from septic patients had comparable intracellular ascorbate content to healthy volunteers (0.33 vs. 0.35 nmol/10⁶ cells, p > 0.05), despite significantly lower plasma concentrations than the healthy controls (14 vs. 88 µmol/L, p < 0.001). In contrast, erythrocytes from septic patients had significantly lower intracellular ascorbate content than healthy controls (30 vs. 69 µmol/L, p = 0.002), although this was 2.2-fold higher than the matched plasma concentrations in the patients (p = 0.008). Higher concentrations of myeloperoxidase, a source of reactive oxygen species, were observed in the septic patients relative to healthy controls (194 vs. 14 mg/mL, p < 0.0001). In contrast to neutrophils from healthy volunteers, the neutrophils from septic patients demonstrated elevated uptake of extracellular ascorbate. Overall, neutrophils from septic patients exhibited comparable intracellular ascorbate content to those from healthy controls, despite the patients presenting with hypovitaminosis C. The mechanisms involved are currently uncertain, but could include increased generation of dehydroascorbic acid in septic patients, enhanced basal activation of their neutrophils or upregulation of their vitamin C transporters.

Effect of Lipopolysaccharide and TNFα on Neuronal Ascorbic Acid Uptake

Vitamin C (ascorbic acid: AA) uptake in neurons occurs via the sodium-dependent vitamin C transporter-2 (SVCT2), which is highly expressed in the central nervous system (CNS). During chronic neuroinflammation or infection, CNS levels of lipopolysaccharide (LPS) and LPS-induced tumor necrosis factor-α (TNFα) are increased. Elevated levels of LPS and TNFα have been associated with neurodegenerative diseases together with reduced levels of AA. However, little is known about the impacts of LPS and TNFα on neuronal AA uptake. The objective of this study was to examine the effect of LPS and TNFα on SVCT2 expression and function using in vitro and in vivo approaches. Treatment of SH-SY5Y cells with either LPS or TNFα inhibited AA uptake. This reduced uptake was associated with a significant decrease in SVCT2 protein and mRNA levels. In vivo exposure to LPS or TNFα also decreased SVCT2 protein and mRNA levels in mouse brains. Both LPS and TNFα decreased SLC23A2 promoter activity. Further, the inhibitory effect of LPS on a minimal SLC23A2 promoter was attenuated when either the binding site for the transcription factor Sp1 was mutated or cells were treated with the NF-κB inhibitor, celastrol. We conclude that inflammatory signals suppress AA uptake by impairing SLC23A2 transcription through opposing regulation of Sp1 and NF-κB factors.

Parenteral high‑dose ascorbate - A possible approach for the treatment of glioblastoma (Review)

For glioblastoma, the treatment with standard of care therapy comprising resection, radiation, and temozolomide results in overall survival of approximately 14-18 months after initial diagnosis. Even though several new therapy approaches are under investigation, it is difficult to achieve life prolongation and/or improvement of patient's quality of life. The aggressiveness and progression of glioblastoma is initially orchestrated by the biological complexity of its genetic phenotype and ability to respond to cancer therapy via changing its molecular patterns, thereby developing resistance. Recent clinical studies of pharmacological ascorbate have demonstrated its safety and potential efficacy in different cancer entities regarding patient's quality of life and prolongation of survival. In this review article, the actual glioblastoma treatment possibilities are summarized, the evidence for pharmacological ascorbate in glioblastoma treatment is examined and questions are posed to identify current gaps of knowledge regarding accessibility of ascorbate to the tumor area. Experiments with glioblastoma cell lines and tumor xenografts have demonstrated that high-dose ascorbate induces cytotoxicity and oxidative stress largely selectively in malignant cells compared to normal cells suggesting ascorbate as a potential therapeutic agent. Further investigations in larger cohorts and randomized placebo-controlled trials should be performed to confirm these findings as well as to improve delivery strategies to the brain, through the inherent barriers and ultimately to the malignant cells.

Vitamin C: historical perspectives and heart failure

Vitamin C (Vit C) is an ideal antioxidant as it is easily available, water soluble, very potent, least toxic, regenerates other antioxidants particularly Vit E, and acts as a cofactor for different enzymes. It has received much attention due to its ability in limiting reactive oxygen species, oxidative stress, and nitrosative stress, as well as it helps to maintain some of the normal metabolic functions of the cell. However, over 140 clinical trials using Vit C in different pathological conditions such as myocardial infarction, gastritis, diabetes, hypertension, stroke, and cancer have yielded inconsistent results. Such a divergence calls for new strategies to establish practical significance of Vit C in heart failure or even in its prevention. For a better understanding of Vit C functioning, it is important to revisit its transport across the cell membrane and subcellular interactions. In this review, we have highlighted some historical details of Vit C and its transporters in the heart with a particular focus on heart failure in cancer chemotherapy.

Increased expression of mitochondrial sodium-coupled ascorbic acid transporter-2 (mitSVCT2) as a central feature in breast cancer

The potential role of vitamin C in cancer prevention and treatment remains controversial. While normal human cells obtain vitamin C as ascorbic acid, the prevalent form of vitamin C in vivo, the uptake mechanisms by which cancer cells acquire vitamin C has remained unclear. The aim of this study is to characterize how breast cancer cells acquire vitamin C. For this, we determined the expression of vitamin C transporters in normal and breast cancer tissue samples, and in ZR-75, MCF-7, MDA-231 and MDA-468 breast cancer cell lines. At the same time, reduced (AA) and oxidized (DHA) forms of vitamin C uptake experiments were performed in all cell lines. We show here that human breast cancer tissues differentially express a form of SVCT2 transporter, that is systematically absent in normal breast tissues and it is increased in breast tumors. In fact, estrogen receptor negative breast cancer tissue, exhibit the most elevated SVCT2 expression levels. Despite this, our analysis in breast cancer cell lines showed that these cells are not able to uptake ascorbic acid and depend on glucose transporter for the acquisition of vitamin C by a bystander effect. This is consistent with our observations that this form of SVCT2 is completely absent from the plasma membrane and is overexpressed in mitochondria of breast cancer cells, where it mediates ascorbic acid transport. This work shows that breast cancer cells acquire vitamin C in its oxidized form and are capable of accumulated high concentrations of the reduced form. Augmented expression of an SVCT2 mitochondrial form appears to be a common hallmark across all human cancers and might have implications in cancer cells survival capacity against pro-oxidant environments.

Concentration Does Matter: The Beneficial and Potentially Harmful Effects of Ascorbate in Humans and Plants

SIGNIFICANCE

Ascorbate (Asc) is an essential compound both in animals and plants, mostly due to its reducing properties, thereby playing a role in scavenging reactive oxygen species (ROS) and acting as a cofactor in various enzymatic reactions. Recent Advances: Growing number of evidence shows that excessive Asc accumulation may have negative effects on cellular functions both in humans and plants; inter alia it may negatively affect signaling mechanisms, cellular redox status, and contribute to the production of ROS via the Fenton reaction.

CRITICAL ISSUES

Both plants and humans tightly control cellular Asc levels, possibly via biosynthesis, transport, and degradation, to maintain them in an optimum concentration range, which, among other factors, is essential to minimize the potentially harmful effects of Asc. On the contrary, the Fenton reaction induced by a high-dose Asc treatment in humans enables a potential cancer-selective cell death pathway.

FUTURE DIRECTIONS

The elucidation of Asc induced cancer selective cell death mechanisms may give us a tool to apply Asc in cancer therapy. On the contrary, the regulatory mechanisms controlling cellular Asc levels are also to be considered, for example, when aiming at generating crops with elevated Asc levels.

Differentiation of Promonocytic U937 Cells to Monocytes Is Associated with Reduced Mitochondrial Transport of Ascorbic Acid

Growth of promonocytic U937 cells in the presence of DMSO promotes their differentiation to monocytes. After 4 days of culture in differentiating medium, these cells ceased to proliferate, displayed downregulated ryanodine receptor expression, and responded to specific stimuli with enhanced NADPH-oxidase-derived superoxide formation or cytosolic phospholipase A₂-dependent arachidonic acid release. We found that the 4-day differentiation process is also associated with downregulated SVCT2 mRNA expression, in the absence of apparent changes in SVCT2 protein expression and transport rate of ascorbic acid (AA). Interestingly, under the same conditions, these cells accumulated lower amounts of the vitamin in their mitochondria, with an ensuing reduced response to external stimuli sensitive to the mitochondrial fraction of AA. Further analyses demonstrated an unexpected increase in mitochondrial SVCT2 protein expression, however, associated with reduced SVCT2-dependent AA uptake in isolated mitochondria. A decrease in the transporter Vmax, with no change in affinity, was found to account for this response. Differentiation of promonocytic cells to monocytes is therefore characterized by decreased SVCT2 mRNA expression that, even prior to the onset of SVCT2 protein downregulation or apparent changes in plasma membrane transport activity, impacts on the mitochondrial accumulation of the vitamin through a decreased Vmax of the transporter.

SVCT2 Expression and Function in Reactive Astrocytes Is a Common Event in Different Brain Pathologies

Ascorbic acid (AA), the reduced form of vitamin C, acts as a neuroprotector by eliminating free radicals in the brain. Sodium/vitamin C co-transporter isoform 2 (SVCT2) mediates uptake of AA by neurons. It has been reported that SVCT2 mRNA is induced in astrocytes under ischemic damage, suggesting that its expression is enhanced in pathological conditions. However, it remains to be established if SVCT expression is altered in the presence of reactive astrogliosis generated by different brain pathologies. In the present work, we demonstrate that SVCT2 expression is increased in astrocytes present at sites of neuroinflammation induced by intracerebroventricular injection of a GFP-adenovirus or the microbial enzyme, neuraminidase. A similar result was observed at 5 and 10 days after damage in a model of traumatic injury and in the hippocampus and cerebral cortex in the in vivo kindling model of epilepsy. Furthermore, we defined that cortical astrocytes maintained in culture for long periods acquire markers of reactive gliosis and express SVCT2, in a similar way as previously observed in situ. Finally, by means of second harmonic generation and 2-photon fluorescence imaging, we analyzed brain necropsied material from patients with Alzheimer's disease (AD), which presented with an accumulation of amyloid plaques. Strikingly, although AD is characterized by focalized astrogliosis surrounding amyloid plaques, SVCT2 expression at the astroglial level was not detected. We conclude that SVCT2 is heterogeneously induced in reactive astrogliosis generated in different pathologies affecting the central nervous system (CNS).

Caveolin-1-mediated internalization of the vitamin C transporter SVCT2 in microglia triggers an inflammatory phenotype

Vitamin C is essential for the development and function of the central nervous system (CNS). The plasma membrane sodium-vitamin C cotransporter 2 (SVCT2) is the primary mediator of vitamin C uptake in neurons. SVCT2 specifically transports ascorbate, the reduced form of vitamin C, which acts as a reducing agent. We demonstrated that ascorbate uptake through SVCT2 was critical for the homeostasis of microglia, the resident myeloid cells of the CNS that are essential for proper functioning of the nervous tissue. We found that depletion of SVCT2 from the plasma membrane triggered a proinflammatory phenotype in microglia and resulted in microglia activation. Src-mediated phosphorylation of caveolin-1 on Tyr¹⁴ in microglia induced the internalization of SVCT2. Ascorbate treatment, SVCT2 overexpression, or blocking SVCT2 internalization prevented the activation of microglia. Overall, our work demonstrates the importance of the ascorbate transport system for microglial homeostasis and hints that dysregulation of ascorbate transport might play a role in neurological disorders.

Ascorbic acid supplementation improves skeletal muscle oxidative stress and insulin sensitivity in people with type 2 diabetes: Findings of a randomized controlled study

AIM/HYPOTHESIS

Skeletal muscle insulin resistance and oxidative stress are characteristic metabolic disturbances in people with type 2 diabetes. Studies in insulin resistant rodents show an improvement in skeletal muscle insulin sensitivity and oxidative stress following antioxidant supplementation. We therefore investigated the potential ameliorative effects of antioxidant ascorbic acid (AA) supplementation on skeletal muscle insulin sensitivity and oxidative stress in people with type 2 diabetes.

METHODS

Participants with stable glucose control commenced a randomized cross-over study involving four months of AA (2 × 500 mg/day) or placebo supplementation. Insulin sensitivity was assessed using a hyperinsulinaemic, euglycaemic clamp coupled with infusion of 6,6-D2 glucose. Muscle biopsies were measured for AA concentration and oxidative stress markers that included basal measures (2',7'-dichlorofluorescin [DCFH] oxidation, ratio of reduced-to-oxidized glutathione [GSH/GSSG] and F2-Isoprostanes) and insulin-stimulated measures (DCFH oxidation). Antioxidant concentrations, citrate synthase activity and protein abundances of sodium-dependent vitamin C transporter 2 (SVCT2), total Akt and phosphorylated Akt (ser473) were also measured in muscle samples.

RESULTS

AA supplementation significantly increased insulin-mediated glucose disposal (delta rate of glucose disappearance; ∆Rd) (p=0.009), peripheral insulin-sensitivity index (p=0.046), skeletal muscle AA concentration (p=0.017) and muscle SVCT2 protein expression (p=0.008); but significantly decreased skeletal muscle DCFH oxidation during hyperinsulinaemia (p=0.007) when compared with placebo. Total superoxide dismutase activity was also lower following AA supplementation when compared with placebo (p=0.006). Basal oxidative stress markers, citrate synthase activity, endogenous glucose production, HbA1C and muscle Akt expression were not significantly altered by AA supplementation.

CONCLUSIONS/INTERPRETATION

In summary, oral AA supplementation ameliorates skeletal muscle oxidative stress during hyperinsulinaemia and improves insulin-mediated glucose disposal in people with type 2 diabetes. Findings implicate AA supplementation as a potentially inexpensive, convenient, and effective adjunct therapy in the treatment of insulin resistance in people with type 2 diabetes.

Cis-regulatory elements involved in species-specific transcriptional regulation of the SVCT1 gene in rat and human hepatoma cells

Ascorbic acid is transported into cells by the sodium-coupled vitamin C transporters (SVCTs). Recently, we obtained evidence of differential regulation of SVCT expression in response to acute oxidative stress in cells from species that differ in their capacity to synthesize vitamin C, with a marked decrease in SVCT1 mRNA and protein levels in rat hepatoma cells that was not observed in human hepatoma cells. To better understand the regulatory aspects involved, we performed a structural and functional analysis of the proximal promoter of the SVCT1 rat gene. We cloned a 1476-bp segment containing the proximal promoter of the rat SVCT1 gene and generated deletion-derived truncated promoters of decreasing sizes and mutant promoters by modification of consensus binding sites for transcription factors by site-directed mutagenesis. We next analyzed their capacity to direct the transcription of a reporter gene after transfection into rat H4IIE and human HepG2 hepatoma cells, in experiments involving the coexpression of transcription factors whose consensus binding sequences are present in the SVCT1 promoter. This analysis revealed the presence of two critical cis-regulatory elements of the transcriptional activity of the rat SVCT1 gene promoter, sites containing consensus sequences for the binding of the transcription factors Bach1 and HNF4 that are not present in equivalent locations in the human SVCT1 gene promoter. Moreover, a consensus site for HNF1 that is crucial for the regulation of the human SVCT1 promoter is present in the SVCT1 rat promoter but has no effect on its transcriptional activity. These findings imply that regulation of vitamin C metabolism in the rat, a species with the capacity to synthesize large amounts of ascorbic acid, may differ from that of humans, a species that must obtain ascorbic acid from the diet through a transport mechanism that depends on proper SVCT1 expression.

MicroRNAs-141 and 200a regulate the SVCT2 transporter in bone marrow stromal cells

Vitamin C is a micro-nutrient which plays an important role in bone marrow stromal cell (BMSCs) differentiation to osteogenesis. This vitamin is transported into the BMSCs through the sodium dependent vitamin C transporter 2 (SVCT2). We previously reported that knockdown of the SVCT2 transporter decreases osteogenic differentiation. However, our understanding of the post-transcriptional regulatory mechanism of the SVCT2 transporter remains poor. MicroRNAs (miRNAs) are small non-coding RNAs that post-transcriptionally regulate the messenger RNAs of protein-coding genes. In this study, we aimed to investigate the impact of miR-141 and miR-200a on SVCT2 expression. We found that mouse BMSCs expressed miR-141 and miR-200a and repressed SVCT2 expression at the functional level by targeting the 3'-untranslated region of mRNA. We also found that miR-141 and miR-200a decreased osteogenic differentiation. Furthermore, miRNA inhibitors increased SVCT2 and osteogenic gene expression in BMSCs. Taken together, these results indicate that both miRNAs are novel regulators of the SVCT2 transporter and play an important role in the osteogenic differentiation of BMSCs.

High-dose vitamin C supplementation increases skeletal muscle vitamin C concentration and SVCT2 transporter expression but does not alter redox status in healthy males

Antioxidant vitamin C (VC) supplementation is of potential clinical benefit to individuals with skeletal muscle oxidative stress. However, there is a paucity of data reporting on the bioavailability of high-dose oral VC in human skeletal muscle. We aimed to establish the time course of accumulation of VC in skeletal muscle and plasma during high-dose VC supplementation in healthy individuals. Concurrently we investigated the effects of VC supplementation on expression levels of the key skeletal muscle VC transporter sodium-dependent vitamin C transporter 2 (SVCT2) and intramuscular redox and mitochondrial measures. Eight healthy males completed a randomized placebo-controlled, crossover trial involving supplementation with ascorbic acid (2×500 mg/day) over 42 days. Participants underwent muscle and blood sampling on days 0, 1, 7, and 42 during each treatment. VC supplementation significantly increased skeletal muscle VC concentration after 7 days, which was maintained at 42 days (VC 3.0±0.2 (mean±SEM) to 3.9±0.4 mg/100 g wet weight (ww) versus placebo 3.1±0.3 to 2.9±0.2 mg/100 g ww, p=0.001). Plasma VC increased after 1 day, which was maintained at 42 days (VC 61.0±6.1 to 111.5±10.4 µmol/L versus placebo 60.7±5.3 to 59.2±4.8 µmol/L, p<0.001). VC supplementation significantly increased skeletal muscle SVCT2 protein expression (main treatment effect p=0.006) but did not alter skeletal muscle redox measures or citrate synthase activity. A main finding of our study was that 7 days of high-dose VC supplementation was required to significantly increase skeletal muscle vitamin C concentration in healthy males. Our findings implicate regular high-dose vitamin C supplementation as a means to safely increase skeletal muscle vitamin C concentration without impairing intramuscular ascorbic acid transport, antioxidant concentrations, or citrate synthase activity.

Chronic vitamin C deficiency promotes redox imbalance in the brain but does not alter sodium-dependent vitamin C transporter 2 expression

Vitamin C (VitC) has several roles in the brain acting both as a specific and non-specific antioxidant. The brain upholds a very high VitC concentration and is able to preferentially retain VitC even during deficiency. The accumulation of brain VitC levels much higher than in blood is primarily achieved by the sodium dependent VitC transporter (SVCT2). This study investigated the effects of chronic pre-and postnatal VitC deficiency as well as the effects of postnatal VitC repletion, on brain SVCT2 expression and markers of oxidative stress in young guinea pigs. Biochemical analyses demonstrated significantly decreased total VitC and an increased percentage of dehydroascorbic acid, as well as increased lipid oxidation (malondialdehyde), in the brains of VitC deficient animals (p < 0.0001) compared to controls. VitC repleted animals were not significantly different from controls. No significant changes were detected in either gene or protein expression of SVCT2 between groups or brain regions. In conclusion, chronic pre-and postnatal VitC deficiency increased brain redox imbalance but did not increase SVCT2 expression. Our findings show potential implications for VitC deficiency induced negative effects of redox imbalance in the brain and provide novel insight to the regulation of VitC in the brain during deficiency.

Comparative analysis of sodium coupled vitamin C transporter 2 in human osteoarthritis grade 1 and grade 3 tissues

Background

Nutrient levels are known to influence the development of osteoarthritis (OA), presumably by modulating levels of matrix biosynthesis and degradation. These processes may be affected by ascorbic acid (AA), an antioxidant which acts as a cofactor for numerous biochemical reactions and is essential for post-translational modifications of collagen. In this study we examined the expression of SVCT2, the only known Sodium coupled vitamin C transporter isoform present in articular cartilage, in human articular cartilage explants derived from both normal and osteoarthritis articular cartilage.

Methods

OA1 and OA3 human articular cartilage was carefully dissected and macroscopically graded for degeneration via the Collins scale. The tissue samples were histologically examined by Hematoxylin and Eosin and Safranin O and Fast Green staining. SVCT2 expression analysis was performed at mRNA level by quantitative real time PCR and at a protein level by immunohistochemistry.

Results

Our quantitative real time PCR showed marked variation in the expression of SVCT2 in human osteoarthritic articular cartilage. SVCT2 expression was significantly down-regulated (p = 0.0001) in the Collins grade 3 (OA3) compared to Collins grade 1 (OA1) tissue. Furthermore, slides stained with fluorescent antibodies to SVCT2 demonstrated greatly reduced fluorescence for the SVCT2 transporter on the chondrocyte plasma membrane in the osteoarthritic tissue samples.

Conclusions

These findings demonstrate that the expression of SVCT2 transporter is significantly altered in human osteoarthritic tissues (OA3). The modulation of this transporter could therefore potentially influence the prevention, management and treatment of osteoarthritis.

Genetic polymorphisms in SLC23A2 as predictive biomarkers of severe acute toxicities after treatment with a definitive 5-fluorouracil/cisplatin-based chemoradiotherapy in Japanese patients with esophageal squamous cell carcinoma

OBJECTIVE

Definitive chemoradiotherapy (CRT) with 5-fluorouracil (5-FU) and cisplatin (CDDP) is one of the standard therapies for esophageal squamous cell carcinoma (ESCC); however, inter-individual variations in clinical outcomes have yet to be investigated. In the present study, single nucleotide polymorphisms (SNPs) in SLC23A2 gene were retrospectively evaluated in 49 Japanese patients with ESCC who were treated with a definitive 5-FU/CDDP-based CRT, and the predictive values for the clinical response, severe acute toxicities, and long-term survival were assessed.

METHODS

A course consisted of the continuous infusion of 5-FU at 400 mg/m(2)/day for days 1-5 and 8-12, the infusion of CDDP at 40 mg/m(2)/day on days 1 and 8, and radiation at 2 Gy/day on days 1 to 5, 8 to 12, and 15 to 19, with a second course being repeated after a 2-week interval. The SLC23A2 SNPs rs2681116, rs13037458, rs1715364, rs4987219, and rs1110277 were evaluated.

RESULTS

The rs2681116 and rs13037458 had a tendency to predict the clinical response (p=0.144 and 0.085, respectively) and long-term survival (p=0.142 and 0.056, respectively). The rs4987219 and rs1110277 correlated with severe acute leukopenia (p=0.025) and stomatitis (p=0.019), respectively.

CONCLUSIONS

Further investigations with a larger number of patients or an in vitro study are needed to confirm the predictive values of genetic polymorphisms in SLC23A2.

Gene expression profiles of sodium-dependent vitamin C transporters in mice after alcohol consumption

Alcoholic liver disease (ALD) is a serious liver problem in western countries. Our previous study has demonstrated that vitamin C plays a protective role in ALD. The vitamin C homeostasis is tightly regulated by sodium-dependent vitamin C transporters (SVCTs) 1 and 2. But the role of two SVCTs in ALD is less understood. In this study, we examined the expression patterns of two SVCTs in mice after alcohol consumption. Our results suggested that alcohol consumption obviously increased the expression of two SVCTs in liver and SVCT1 in kidney and intestine, which is important for vitamin C absorption. Vitamin C supplement increased the sera vitamin C content and ameliorated the symptom of ALD. Intestinal absorption and renal re-absorption mediated by SVCT1 are key factors to increase the sera vitamin C content after alcohol consumption. We proposed that both reactive oxygen species and low vitamin C concentration regulate the expression of SVCTs, and the protective role of vitamin C is mediated by suppressing the stability of hypoxia-inducible factor-1α. Thus, our study is significant for the understanding of vitamin C homeostasis in ALD and for better use of other antioxidants in ALD therapy.

Cellular and intracellular transport of vitamin C. The physiologic aspects

Vitamin C requirement is satisfied by natural sources and vitamin C supplements in the ordinary human diet. The two major forms of vitamin C in the diet are L-ascorbic acid and L-dehydroascorbic acid. Both ascorbate and dehydroascorbate are absorbed along the entire length of the human intestine. The reduced form, L-ascorbic acid is imported by an active mechanism, requiring two sodium-dependent vitamin C transporters (SVCT1 and SVCT2). The transport of the oxidized form, dehydroascorbate is mediated by glucose transporters GLUT1, GLUT3 and possibly GLUT4. Initial rate of uptake of both ascorbate and dehydroascorbate is saturable with increasing external substrate concentration. Vitamin C plasma concentrations are tightly controlled when the vitamin is taken orally. It has two simple reasons, on the one hand, the capacity of the transporters is limited, on the other hand the two Na+-dependent transporters can be down-regulated by an elevated level of ascorbate. Orv. Hetil., 154 (42), 1651–1656.

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.

Vitamin C administration attenuates overload-induced skeletal muscle hypertrophy in rats

AIM

This study aimed to investigate the effects of vitamin C administration on skeletal muscle hypertrophy induced by mechanical overload in rats.

METHODS

Male Wistar rats were randomly assigned to three groups: (i) sham-operated group (n = 8), (ii) placebo-administered group (n = 8) and (iii) vitamin C-administered group (n = 8). In the placebo-administered and vitamin C-administered groups, the gastrocnemius and soleus muscles of the right hindlimb were surgically removed to overload the plantaris muscle. Vitamin C (500 mg kg(-1)) was orally administered to the vitamin C-administered group once a day for 14 days.

RESULTS

Synergist muscle ablation caused significant increases in wet weight and protein concentration of the plantaris muscle in both the placebo-administered (P < 0.01) and vitamin C-administered groups (P < 0.01) compared with the sham-operated group (SHA). However, the magnitude of plantaris muscle hypertrophy (expressed as a percentage of the contralateral plantaris muscle) was significantly smaller (P < 0.01) in the vitamin C-administered group (141%) than in the placebo-administered group (PLA) (152%). Compared with the SHA, only the PLA showed higher expressions of phosphorylated p70s6k and Erk1/2 (positive regulators of muscle protein synthesis) and a lower expression of atrogin-1 (a muscle atrophy marker). Concentrations of vitamin C and oxidative stress markers in the overloaded muscle were similar between the placebo-administered and vitamin C-administered groups.

CONCLUSION

Oral vitamin C administration can attenuate overload-induced skeletal muscle hypertrophy, which may have implications for antioxidant supplementation during exercise training.

Human genetic variation influences vitamin C homeostasis by altering vitamin C transport and antioxidant enzyme function

New evidence for the regulation of vitamin C homeostasis has emerged from several studies of human genetic variation. Polymorphisms in the genes encoding sodium-dependent vitamin C transport proteins are strongly associated with plasma ascorbate levels and likely impact tissue cellular vitamin C status. Furthermore, genetic variants of proteins that suppress oxidative stress or detoxify oxidatively damaged biomolecules, i.e., haptoglobin, glutathione-S-transferases, and possibly manganese superoxide dismutase, affect ascorbate levels in the human body. There also is limited evidence for a role of glucose transport proteins. In this review, we examine the extent of the variation in these genes, their impact on vitamin C status, and their potential role in altering chronic disease risk. We conclude that future epidemiological studies should take into account genetic variation in order to successfully determine the role of vitamin C nutriture or supplementation in human vitamin C status and chronic disease risk.

Senescence-induced increases in intracellular oxidative stress and enhancement of the need for ascorbic acid in human fibroblasts

Many studies have suggested that there is a close correlation among declines in internal ascorbic acid (AsA) levels, various disorders, and senescence. To clarify the relationships between age-associated changes in intracellular AsA levels and the effects of AsA administration on intracellular reactive oxygen species (ROS) levels, we investigated aging-related changes in AsA uptake, ROS levels, and the effects of AsA administration on intracellular ROS levels in young and old (senescent) human fibroblasts. Our results demonstrated that AsA uptake was increased in old cells compared with young cells, although mRNA and protein expression of sodium-dependent vitamin C transporter 2 was barely altered between the young and old cells. We also demonstrated that the intracellular superoxide anion level was higher in young cells, whereas the level of intracellular peroxides was significantly increased in old cells under both normal and oxidative stress conditions. Moreover, AsA administration markedly decreased the augmentation of intracellular peroxides in old cells, whereas there was no effect of AsA treatment in young cells under both normal and oxidative stress conditions. Therefore, our results also indicate that AsA could play an important role in regulating the intracellular ROS levels in senescent cells and that the need for AsA is enhanced by cellular senescence.

Human skeletal muscle ascorbate is highly responsive to changes in vitamin C intake and plasma concentrations

BACKGROUND

Vitamin C (ascorbate) is likely to be essential for skeletal muscle structure and function via its role as an enzyme cofactor for collagen and carnitine biosynthesis. Vitamin C may also protect these metabolically active cells from oxidative stress.

OBJECTIVE

We investigated the bioavailability of vitamin C to human skeletal muscle in relation to dietary intake and plasma concentrations and compared this relation with ascorbate uptake by leukocytes.

DESIGN

Thirty-six nonsmoking men were randomly assigned to receive 6 wk of 0.5 or 2 kiwifruit/d, an outstanding dietary source of vitamin C. Fasting blood samples were drawn weekly, and 24-h urine and leukocyte samples were collected before intervention, after intervention, and after washout. Needle biopsies of skeletal muscle (vastus lateralis) were carried out before and after intervention.

RESULTS

Baseline vastus lateralis ascorbate concentrations were ~16 nmol/g tissue. After intervention with 0.5 or 2 kiwifruit/d, these concentrations increased ~3.5-fold to 53 and 61 nmol/g, respectively. There was no significant difference between the responses of the 2 groups. Mononuclear cell and neutrophil ascorbate concentrations increased only ~1.5- and ~2-fold, respectively. Muscle ascorbate concentrations were highly correlated (P < 0.001) with dietary intake (R = 0.61) and plasma concentrations (R = 0.75) in the range from 5 to 80 μmol/L.

CONCLUSIONS

Human skeletal muscle is highly responsive to vitamin C intake and plasma concentrations and exhibits a greater relative uptake of ascorbate than leukocytes. Thus, muscle appears to comprise a relatively labile pool of ascorbate and is likely to be prone to ascorbate depletion with inadequate dietary intake. This trial was registered at the Australian New Zealand Clinical Trials Registry (www.anzctr.org.au) as ACTRN12611000162910.

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.

Increased expression of SVCT2 in a new mouse model raises ascorbic acid in tissues and protects against paraquat-induced oxidative damage in lung

A new transgenic mouse model for global increases in the Sodium Dependent Vitamin C transporter 2 (SVCT2) has been generated. The SVCT2-Tg mouse shows increased SVCT2 mRNA levels in all organs tested and correspondingly increased ascorbic acid (ASC) levels in all organs except liver. The extent of the increase in transporter mRNA expression differed among mice and among organs. The increased ASC levels did not have any adverse effects on behavior in the SVCT2-Tg mice, which did not differ from wild-type mice on tests of locomotor activity, anxiety, sensorimotor or cognitive ability. High levels of SVCT2 and ASC were found in the kidneys of SVCT2-Tg mice and urinary albumin excretion was lower in these mice than in wild-types. No gross pathological changes were noted in kidneys from SVCT2-Tg mice. SVCT2 immunoreactivity was detected in both SVCT2 and wild-type mice, and a stronger signal was seen in tubules than in glomeruli. Six treatments with Paraquat (3x10 and 3x15 mg/kg i.p.) were used to induce oxidative stress in mice. SVCT2-Tg mice showed a clear attenuation of Paraquat-induced oxidative stress in lung, as measured by F(2)-isoprostanes. Paraquat also decreased SVCT2 mRNA signal in liver, lung and kidney in SVCT2-Tg mice.

Interaction of the transcription start site core region and transcription factor YY1 determine ascorbate transporter SVCT2 exon 1a promoter activity

Transcription of the ascorbate transporter, SVCT2, is driven by two distinct promoters in exon 1 of the transporter sequence. The exon 1a promoter lacks a classical transcription start site and little is known about regulation of promoter activity in the transcription start site core (TSSC) region. Here we present evidence that the TSSC binds the multifunctional initiator-binding protein YY1. Electrophoresis shift assays using YY1 antibody showed that YY1 is present as one of two major complexes that specifically bind to the TSSC. The other complex contains the transcription factor NF-Y. Mutations in the TSSC that decreased YY1 binding also impaired the exon 1a promoter activity despite the presence of an upstream activating NF-Y/USF complex, suggesting that YY1 is involved in the regulation of the exon 1a transcription. Furthermore, YY1 interaction with NF-Y and/or USF synergistically enhanced the exon 1a promoter activity in transient transfections and co-activator p300 enhanced their synergistic activation. We propose that the TSSC plays a vital role in the exon 1a transcription and that this function is partially carried out by the transcription factor YY1. Moreover, co-activator p300 might be able to synergistically enhance the TSSC function via a “bridge” mechanism with upstream sequences.

CpG methylation at the USF-binding site mediates cell-specific transcription of human ascorbate transporter SVCT2 exon 1a

SVCT2 (sodium-vitamin C co-transporter 2) is the major transporter mediating vitamin C uptake in most organs. Its expression is driven by two promoters (CpG-poor exon 1a promoter and CpG-rich exon 1b promoter). In the present study, we mapped discrete elements within the proximal CpG-poor promoter responsible for exon 1a transcription. We identified two E boxes for USF (upstream stimulating factor) binding and one Y box for NF-Y (nuclear factor Y) binding. We show further that NF-Y and USF bind to the exon 1a promoter in a co-operative manner, amplifying the binding of each to the promoter, and is absolutely required for the full activity of the exon 1a promoter. The analysis of the CpG site located at the upstream USF-binding site in the promoter showed a strong correlation between expression and demethylation. It was also shown that exon 1a transcription was induced in cell culture treated with the demethylating agent decitabine. The specific methylation of this CpG site impaired both the binding of USF and the formation of the functional NF-Y-USF complex as well as promoter activity, suggesting its importance for cell-specific transcription. Thus CpG methylation at the upstream USF-binding site functions in establishing and maintaining cell-specific transcription from the CpG-poor SVCT2 exon 1a promoter.

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.

Differential regulation of the ascorbic acid transporter SVCT2 during development and in response to ascorbic acid depletion

The sodium-dependent vitamin C transporter-2 (SVCT2) is the only ascorbic acid (ASC) transporter significantly expressed in brain. It is required for life and is critical during brain development to supply adequate levels of ASC. To assess SVCT2 function in the developing brain, we studied time-dependent SVCT2 mRNA and protein expression in mouse brain, using liver as a comparison tissue because it is the site of ASC synthesis. We found that SVCT2 expression followed an inverse relationship with ASC levels in the developing brain. In cortex and cerebellum, ASC levels were high throughout late embryonic stages and early post-natal stages and decreased with age, whereas SVCT2 mRNA and protein levels were low in embryos and increased with age. A different response was observed for liver, in which ASC levels and SVCT2 expression were both low throughout embryogenesis and increased post-natally. To determine whether low intracellular ASC might be capable of driving SVCT2 expression, we depleted ASC by diet in adult mice unable to synthesize ASC. We observed that SVCT2 mRNA and protein were not affected by ASC depletion in brain cortex, but SVCT2 protein expression was increased by ASC depletion in the cerebellum and liver. The results suggest that expression of the SVCT2 is differentially regulated during embryonic development and in adulthood.

Cellular pathways for transport and efflux of ascorbate and dehydroascorbate

The mechanisms allowing the cellular transport of ascorbic acid represent a primary aspect for the understanding of the roles played by this vitamin in pathophysiology. Considerable research effort has been spent in the field, on several animal models and different cell types. Several mechanisms have been described to date, mediating the movements of different redox forms of ascorbic acid across cell membranes. Vitamin C can enter cells both in its reduced and oxidized form, ascorbic acid (AA) and dehydroascorbate (DHA), utilizing respectively sodium-dependent transporters (SVCT) or glucose transporters (GLUT). Modulation of SVCT expression and function has been described by cytokines, steroids and post-translational protein modification. Cellular uptake of DHA is followed by its intracellular reduction to AA by several enzymatic and non-enzymatic systems. Efflux of vitamin C has been also described in a number of cell types and different pathophysiological functions were proposed for this phenomenon, in dependence of the cell model studied. Cellular efflux of AA is mediated through volume-sensitive (VSOAC) and Ca(2+)-dependent anion channels, gap-junction hemichannels, exocytosis of secretory vesicles and possibly through homo- and hetero-exchange systems at the plasma membrane level. Altogether, available data suggest that cellular efflux of ascorbic acid - besides its uptake - should be taken into account when evaluating the cellular homeostasis and functions of this important vitamin.

Hepatocyte nuclear factor 1 is essential for transcription of sodium-dependent vitamin C transporter protein 1

Transport and distribution of vitamin C is primarily regulated by the function of sodium-dependent vitamin C transporters (SVCTs). SVCT1 is expressed in the small intestine, liver, and kidney, organs that play a vital role in whole body vitamin C homeostasis. Despite the importance of this protein, little is known about regulation of the gene encoding SVCT1, Slc23a1. In this study, we present the first investigation of the transcriptional regulation of human Slc23a1, identifying transcription factors that may influence its expression. A 1,239-bp genomic DNA fragment corresponding to the 5'-flanking region of Slc23a1 was isolated from a human hepatocarcinoma cell line (HepG2) and sequenced. When cloned into a reporter gene construct, robust transcriptional activity was seen in this sequence, nearly 25-fold above the control vector. Deletion analysis of the SVCT1 reporter gene vector defined the minimal active promoter as a small 135-bp region upstream of the transcriptional start site. While several transcription factor binding sites were identified within this sequence, reporter constructs showed that basal transcription required the binding of hepatic nuclear factor 1 (HNF-1) to its cognate sequence. Furthermore, mutation of this HNF-1 binding site resulted in complete loss of luciferase expression, even in the context of the whole promoter. Additionally, small interfering RNA knockdown of both members of the HNF-1 family, HNF-1alpha and HNF-1beta, resulted in a significant decline in SVCT1 transcription. Together, these data suggest that HNF-1alpha and/or HNF-1beta binding is required for SVCT1 expression and may be involved in the coordinate regulation of whole body vitamin C status.

Vitamin C: update on physiology and pharmacology

Although ascorbic acid is an important water-soluble antioxidant and enzyme cofactor in plants and animals, humans and some other species do not synthesize ascorbate due to the lack of the enzyme catalyzing the final step of the biosynthetic pathway, and for them it has become a vitamin. This review focuses on the role of ascorbate in various hydroxylation reactions and in the redox homeostasis of subcellular compartments including mitochondria and endoplasmic reticulum. Recently discovered functions of ascorbate in nucleic acid and histone dealkylation and proteoglycan deglycanation are also summarized. These new findings might delineate a role for ascorbate in the modulation of both pro- and anti-carcinogenic mechanisms. Recent advances and perspectives in therapeutic applications are also reviewed. On the basis of new and earlier observations, the advantages of the lost ability to synthesize ascorbate are pondered. The increasing knowledge of the functions of ascorbate and of its molecular sites of action can mechanistically substantiate a place for ascorbate in the treatment of various diseases.

Oxidized lipoprotein induces the macrophage ascorbate transporter (SVCT2): protection by intracellular ascorbate against oxidant stress and apoptosis

To assess whether ascorbic acid decreases the cytotoxicity of oxidized human low density lipoprotein (oxLDL) in cells involved in atherosclerosis, its interaction with oxLDL was studied in murine RAW264.7 macrophages. Macrophages took up ascorbate to millimolar intracellular concentrations and retained it with little loss over 18h in culture. Culture of the macrophages with oxLDL enhanced ascorbate uptake. This was associated with increased expression of the ascorbate transporter (SVCT2), which was prevented by ascorbate and by inhibiting the NF-kappaB pathway. Culture of RAW264.7 macrophages with oxLDL increased intracellular dihydrofluorescein oxidation and lipid peroxidation, both of which were decreased by intracellular ascorbate. Ascorbate also protected the cells against oxLDL-induced cytotoxicity and apoptosis, but it did not affect macrophage accumulation of lipid from oxLDL or oxLDL-induced increases in macrophage cytokine secretion. These results suggest that ascorbate protects macrophages against oxLDL-induced oxidant stress and subsequent apoptotic death without impairing their function.

Macrophage differentiation increases expression of the ascorbate transporter (SVCT2)

To determine whether macrophage differentiation involves increased uptake of vitamin C, or ascorbic acid, we assessed the expression and function of its transporter SVCT2 during phorbol ester-induced differentiation of human-derived THP-1 monocytes. Induction of THP-1 monocyte differentiation by phorbol 12-myristate 13-acetate (PMA) markedly increased SVCT2 mRNA, protein, and function. When ascorbate was present during PMA-induced differentiation, the increase in SVCT2 protein expression was inhibited, but differentiation was enhanced. PMA-induced SVCT2 protein expression was blocked by inhibitors of protein kinase C (PKC), with most of the affect due to the PKCbetaI and betaII isoforms. Activation of MEK/ERK was sustained up to 48 h after PMA treatment, and the inhibitors completely blocked PMA-stimulated SVCT2 protein expression, indicating an exclusive role for the classical MAP kinase pathway. However, inhibitors of NF-kappaB activation, NADPH oxidase inhibitors, and several antioxidants also partially prevented SVCT2 induction, suggesting diverse distal routes for control of SVCT2 transcription. Both known promoters for the SVCT2 were involved in these effects. In conclusion, PMA-induced monocyte-macrophage differentiation is enhanced by ascorbate and associated with increased expression and function of the SVCT2 protein through a pathway involving sustained activation of PKCbetaI/II, MAP kinase, NADPH oxidase, and NF-kappaB.