Role of monosaccharide transporter in vitamin C uptake by placental membrane vesicles

Activity-Based Sensing of Ascorbate by Using Copper-Mediated Oxidative Bond Cleavage

Ascorbate is an important biological reductant and enzyme cofactor. Although direct detection through ascorbate-mediated reduction is possible, this approach suffers from poor selectivity due to the wide range of cellular reducing agents. To overcome this limitation, we leverage reduction potential of ascorbate to mediate a copper-mediated oxidative bond cleavage of ether-caged fluorophores. The copper(II) complexes supported by a {bis(2-pyridylmethyl)}benzylamine or a {bis(2-pyridylmethyl)}(2-methoxybenzyl)amine ligand were identified as an ascorbate responsive unit and their reaction with ascorbate yields a copper-based oxidant that enables rapid benzylic oxidation and the release of an ether-caged dye (coumarin or fluorescein). The copper-mediated bond cleavage is specific to ascorbate and the trigger can be readily derivatized for tuning photophysical properties of the probes. The probes were successfully applied for the fluorometric detection of ascorbate in commercial food samples, human plasma, and serum, and within live cells by using confocal microscopy and flow cytometry.

Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis

The facilitated diffusion of glucose, galactose, fructose, urate, myoinositol, and dehydroascorbicacid in mammals is catalyzed by a family of 14 monosaccharide transport proteins called GLUTs. These transporters may be divided into three classes according to sequence similarity and function/substrate specificity. GLUT1 appears to be highly expressed in glycolytically active cells and has been coopted in vitamin C auxotrophs to maintain the redox state of the blood through transport of dehydroascorbate. Several GLUTs are definitive glucose/galactose transporters, GLUT2 and GLUT5 are physiologically important fructose transporters, GLUT9 appears to be a urate transporter while GLUT13 is a proton/myoinositol cotransporter. The physiologic substrates of some GLUTs remain to be established. The GLUTs are expressed in a tissue specific manner where affinity, specificity, and capacity for substrate transport are paramount for tissue function. Although great strides have been made in characterizing GLUT-catalyzed monosaccharide transport and mapping GLUT membrane topography and determinants of substrate specificity, a unifying model for GLUT structure and function remains elusive. The GLUTs play a major role in carbohydrate homeostasis and the redistribution of sugar-derived carbons among the various organ systems. This is accomplished through a multiplicity of GLUT-dependent glucose sensing and effector mechanisms that regulate monosaccharide ingestion, absorption,distribution, cellular transport and metabolism, and recovery/retention. Glucose transport and metabolism have coevolved in mammals to support cerebral glucose utilization.

EVIDENCE FOR ASCORBIC ACID TRANSPORT SYSTEM IN RAT BRAIN CAPILLARIES

Although ascorbic acid (AA) crosses the choroid plexus and may enter the brain at an appreciable rate, it is not clearly established that there exist transport system(s) carrying this vitamin from blood into the brain cells across the brain capillaries. Thus the rate of its uptake by choroid plexus and cerebral capillaries were evaluated in vitro in this study. Choroid plexus and brain capillaries were isolated from two-month-old male Sprague-Dawley rats. Time course of AA incorporation in micro vessels and choroid plexus was studied up to 30 min. After stopping the incorporation with the excess of cold isotonic saline, micro vessels were filtered and sonicated. The intracellular incorporated AA radioactivity was measured by liquid scintillation counting. AA uptake by micro vessel was tested for Na+-dependence and saturability. The time course studies showed linear increase in total uptake and accumulation of AA by choroid plexus and endothelial cells up to 30 min. Treatment with oubain or replacement with sodium chloride showed that uptake is an Na+- independent process. Transport of AA to cerebrospinal fluid and brain was also shown to be readily saturated by increasing the level of cold AA. These results document that the brain capillary endothelial cells are able to transport and accumulate AA, and may have a critical role in the homeostasis and regulation of cerebral ascorbic acid concentration.

Upregulation of sodium-dependent vitamin C transporter 2 expression in adrenals increases norepinephrine production and aggravates hyperlipidemia in mice with streptozotocin-induced diabetes

The hyperglycemia and hyperoxidation that characterize diabetes lead to reduced vitamin C (L-ascorbic acid, AA) levels in diabetic humans and animals. We examined the possibility that diabetes-induced low plasma AA levels impair AA distribution to various tissues and that these changes are closely related to the development of diabetic complications. AA levels were markedly decreased in the plasma and increased in the adrenals of mice with streptozotocin (STZ)-induced diabetes. Consistently with these results, in [1-(14)C]AA accumulation assays, the efficiency of [1-(14)C]AA accumulation was significantly higher in the adrenals (which had the greatest ability to accumulate [1-(14)C]AA) of diabetic mice than in those of controls. Expression of sodium-dependent vitamin C transporter (SVCT)-2, a transporter of AA, was upregulated in diabetic adrenals. Furthermore, increased AA incorporation into the diabetic adrenals by SVCT-2 led to increased plasma norepinephrine, triglyceride and free fatty acid levels in mice with STZ-induced diabetes. Therefore, oversupplementation with AA could be deleterious in diabetic patients, because overexpression of adrenal SVCT-2 in diabetes could lead to excessive AA uptake, thus enhancing norepinephrine production and exacerbating some diabetic complications. Interestingly, however, treatment with AA dose-dependently abolished the increased expression of adrenal SVCT-2 and normalized the abovementioned plasma parameters in diabetic mice. These results suggest SVCT-2-mediated increases in AA uptake by the adrenals followed by excessive production of plasma norepinephrine may play a pivotal role in the development of diabetic complications.

Differential Expression of Glucose Transporters in Rabbit Placenta: Effect of Hypercholesterolemia in Dams1

Low birth weight is observed in rabbit offspring when maternal hypercholesterolemia is induced during gestation, but the related etiology is still unknown. Glucose is one of the most important substances during fetal development, and defect in glucose supply to fetus was related to pathophysiological mechanisms in intrauterine growth restriction. Thus, the aim of this work was to evaluate the impact of maternal hypercholesterolemia during rabbit gestation on the glucose metabolism and the routing of glucose transporters (SLC2 and SLC5 [previously known as GLUT and SGLT]) in placenta. In this study, maternal and offspring serum levels of glucose and insulin were evaluated for control and hypercholesterolemic groups, and the mRNA and protein expressions of placental SLCs were quantified by real-time RT-PCR and Western immunoblot, respectively. Our data demonstrate that maternal hypercholesterolemia during gestation: 1) induces offspring hypoglycemia; 2) does not modify the genetic and protein expressions of SLC2A1 and SLC2A4 (previously GLUT1 and GLUT4) in total placental extract; 3) downregulates the placental SLC5A1 (previously SGLT1) protein expression without affecting its mRNA levels; 4) impairs the translocation of SLC2A1 but not SLC2A4 from cytoplasmatic pool to the cell membrane surface. Then we assume that reduction of offspring birth weight in presence of maternal hypercholesterolemia may be related to the offspring's hypoglycemia and the reduction of the cell surface expression of placental SLC2A1.

High glucose levels down-regulate glucose transporter expression that correlates with increased oxidative stress in placental trophoblast cells in vitro

OBJECTIVE

To study glucose transporter expression and oxidative stress in placental trophoblasts under hyperglycemic conditions in vitro.

METHODS

Trophoblasts were isolated from term normal human placentas and incubated with Dulbecco's modified eagle medium containing 1000, 2500, and 4500 mg/L glucose for 3 days. At the end of incubation, culture medium was collected. Trophoblast RNA was extracted and mRNA expression of glucose transporters was determined by RNase protection assay. Messenger RNA expression for copper-zinc-superoxide dismutase (CuZn-SOD) was determined by real-time polymerase chain reaction. Lipid peroxide production was determined by measuring malondialdehyde concentration in the culture supernatant. Protein expression of sodium-glucose transporter 2 (SGLT-2) was determined by Western blot analysis.

RESULTS

Messenger RNA expression for glucose transporter 1 (GLUT1) and SGLT-2 were reduced in trophoblast cells incubated with 4500 mg/L glucose compared with those incubated with 1000 and 2000 mg/L glucose. mRNA expression of CuZn-SOD was also decreased in trophoblasts incubated with 4500 mg/L glucose. Malondialdehyde production was significantly increased by trophoblasts incubated with 4500 mg/L glucose compared with those by trophoblasts incubated with 1000 and 2000 mg/L glucose (4.69 +/- 0.60 versus 2.10 +/- 0.29 and 2.89 +/- 0.47 nmol/mg protein; P < .01, respectively).

CONCLUSIONS

Down-regulation of gene expression of glucose transporters correlates with increased lipid peroxide production and decreased superoxide dismutase expression in placental trophoblasts cultured under hyperglycemic conditions.

Ascorbic acid synthesis in fetal and neonatal pigs and in pregnant and postpartum sows

The ontogeny of ascorbic acid synthesis and its concentration in fetal pigs from mid- to late gestation, and the effect of birth order and premature or normal delivery ages were evaluated. In Experiment 1, fetal pigs were collected from three sows at 60, 80, 100, 107 and 111 d of development. Liver L-gulono-gamma-lactone oxidase (GLO) activity and ascorbic acid concentration were measured. High liver GLO activity in fetal liver occurred at 60 d but declined as pregnancy advanced (P < 0.01), whereas ascorbic acid concentration increased (P < 0.01). Experiment 2 evaluated ascorbic acid synthesis and concentration in neonates born early (1st and 2nd) or late (7th and 8th) in the birthing sequence, or when born 2 d prematurely vs. the normal delivery age. Pigs born early in the birthing sequence (P < 0.01) and those born at the natural delivery age (P < 0.05) had higher liver ascorbic acid concentrations, but liver GLO activity did not differ among groups. Sows were killed at each period; liver GLO activity was constant during gestation but increased postpartum (P < 0.01). Liver ascorbic acid concentration was constant during gestation, except for a decline during late gestation, and increased postpartum (P < 0.05). These results suggest that more ascorbic acid was transferred from the dam to the fetuses as pregnancy advanced, possibly suppressing fetal GLO activity. Thus, fetal liver GLO activity was the primary source of ascorbic acid during early fetal development, but more fetal ascorbic acid was transferred from the dam during later pregnancy.

Human placental sodium-dependent vitamin C transporter (SVCT2): molecular cloning and transport function

We report here on the cloning and functional characterization of human SVCT2, a sodium-dependent vitamin C (ascorbate) transporter. The hSVCT2 cDNA obtained from a human placental choriocarcinoma cell cDNA library, codes for a protein of 650 amino acids with a predicted molecular mass of 70 kDa. At the level of amino acid sequence, the human SVCT2 exhibits 95% identity to its rat homolog. When functionally expressed in mammalian cells, hSVCT2 induces the transport of ascorbic acid. The transport process induced by hSVCT2 is Na(+)-dependent and is specific for ascorbate. The Michaelis-Menton constant (K(t)) for the transport of ascorbate in cDNA-transfected cells is 69 +/- 5 microM. The relationship between the cDNA-specific uptake rate of ascorbate and Na(+) concentration is sigmoidal with a Na(+):ascorbate stoichiometry of 2:1. Northern blot analysis shows that SVCT2-specific transcripts are present in heart, brain, placenta, and liver and is absent in lung and skeletal muscle. The size of the principal transcript is approximately 7.5 kb.

Glucocorticoid stimulation of Na+-dependent ascorbic acid transport in osteoblast-like cells

Ascorbic acid (AA) is an essential cofactor for osteoblast differentiation both in vivo and in vitro. Before it can function, this vitamin must be transported into cells via a specific Na+-dependent AA transporter. In this study, we examine the regulation of this transport activity by glucocorticoids, a class of steroid hormones known to stimulate in vitro osteoblast differentiation. Dexamethasone stimulated Na+-dependent AA transport activity approximately twofold in primary rat calvarial osteoblasts. Effects of hormone on ascorbic acid transport were rapid (detected within 24 h) and were maximally stimulated by 25-50 nM dexamethasone. Similar effects of dexamethasone on transport activity were also observed in murine MC3T3-E1 cells. This preosteoblast cell line was used for a more detailed characterization of the glucocorticoid response. Transport activity was stimulated selectively by glucocorticoids (dexamethasone > corticosterone) relative to other steroid hormones (progesterone and 17-beta-estradiol) and was blocked when cells were cultured in the presence of cycloheximide, a protein synthesis inhibitor. Kinetic analysis of AA transporter activity in control and dexamethasone-treated cells indicated a Km of approximately 17 microM for both groups. In contrast, dexamethasone increased Vmax by approximately 2.5-fold. Cells also contained an Na+-independent glucose transport activity that has been reported in other systems to transport vitamin C as oxidized dehydroascorbic acid. In marked contrast to Na+-dependent AA transport, this activity was inhibited by dexamethasone. Thus, glucocorticoids increase Na+-dependent AA transport in osteoblasts, possibly via up-regulation of transporter synthesis, and this response can be resolved from actions of glucocorticoids on glucose transport.

Colony-Stimulating Factors Signal for Increased Transport of Vitamin C in Human Host Defense Cells

Although serum concentrations of ascorbic acid seldom exceed 150 μmol/L, mature neutrophils and mononuclear phagocytes accumulate millimolar concentrations of vitamin C. Relatively little is known about the mechanisms regulating this process. The colony-stimulating factors (CSFs), which are central modulators of the production, maturation, and function of human granulocytes and mononuclear phagocytes, are known to stimulate increased glucose uptake in target cells. We show here that vitamin C uptake in neutrophils, monocytes, and a neutrophilic HL-60 cell line is enhanced by the CSFs. Hexose uptake studies and competition analyses showed that dehydroascorbic acid is taken up by these cells through facilitative glucose transporters. Human monocytes were found to have a greater capacity to take up dehydroascorbic acid than neutrophils, related to more facilitative glucose transporters on the monocyte cell membrane. Ascorbic acid was not transported by these myeloid cells, indicating that they do not express a sodium-ascorbate cotransporter. Granulocyte (G)- and granulocyte-macrophage colony-stimulating factor (GM-CSF) stimulated increased uptake of vitamin C in human neutrophils, monocytes, and HL-60 neutrophils. In HL-60 neutrophils, GM-CSF increased both the transport of dehydroascorbic acid and the intracellular accumulation of ascorbic acid. The increase in transport was related to a decrease in Km for transport of dehydroascorbic acid without a change in Vmax. Increased ascorbic acid accumulation was a secondary effect of increased transport. Triggering the neutrophils with the peptide fMetLeuPhe led to enhanced vitamin C uptake by increasing the oxidation of ascorbic acid to the transportable moiety dehydroascorbic acid, and this effect was increased by priming the cells with GM-CSF. Thus, the CSFs act at least at two distinct functional loci to increase cellular vitamin C uptake: conversion of ascorbic acid to dehydroascorbic acid by enhanced oxidation in the pericellular milieu and increased transport of DHA through the facilitative glucose transporters at the cell membrane. These results link the regulated uptake of vitamin C in human host defense cells to the action of CSFs.

Colony-Stimulating Factors Signal for Increased Transport of Vitamin C in Human Host Defense Cells

Although serum concentrations of ascorbic acid seldom exceed 150 μmol/L, mature neutrophils and mononuclear phagocytes accumulate millimolar concentrations of vitamin C. Relatively little is known about the mechanisms regulating this process. The colony-stimulating factors (CSFs), which are central modulators of the production, maturation, and function of human granulocytes and mononuclear phagocytes, are known to stimulate increased glucose uptake in target cells. We show here that vitamin C uptake in neutrophils, monocytes, and a neutrophilic HL-60 cell line is enhanced by the CSFs. Hexose uptake studies and competition analyses showed that dehydroascorbic acid is taken up by these cells through facilitative glucose transporters. Human monocytes were found to have a greater capacity to take up dehydroascorbic acid than neutrophils, related to more facilitative glucose transporters on the monocyte cell membrane. Ascorbic acid was not transported by these myeloid cells, indicating that they do not express a sodium-ascorbate cotransporter. Granulocyte (G)- and granulocyte-macrophage colony-stimulating factor (GM-CSF) stimulated increased uptake of vitamin C in human neutrophils, monocytes, and HL-60 neutrophils. In HL-60 neutrophils, GM-CSF increased both the transport of dehydroascorbic acid and the intracellular accumulation of ascorbic acid. The increase in transport was related to a decrease in Km for transport of dehydroascorbic acid without a change in Vmax. Increased ascorbic acid accumulation was a secondary effect of increased transport. Triggering the neutrophils with the peptide fMetLeuPhe led to enhanced vitamin C uptake by increasing the oxidation of ascorbic acid to the transportable moiety dehydroascorbic acid, and this effect was increased by priming the cells with GM-CSF. Thus, the CSFs act at least at two distinct functional loci to increase cellular vitamin C uptake: conversion of ascorbic acid to dehydroascorbic acid by enhanced oxidation in the pericellular milieu and increased transport of DHA through the facilitative glucose transporters at the cell membrane. These results link the regulated uptake of vitamin C in human host defense cells to the action of CSFs.

Transport mechanisms for vitamin C in the JAR human placental choriocarcinoma cell line

We investigated the transport pathways available for the uptake of vitamin C in the human placental choriocarcinoma cell line, JAR. These cells were found to possess the capacity to accumulate the vitamin when presented either in the oxidized form (dehydroascorbic acid) or in the reduced form (ascorbate). Dithiothreitol and 5,5'-dithiobis(2-nitrobenzoic acid) were used to maintain vitamin C as ascorbate and dehydroascorbic acid, respectively. The uptake of these two forms of vitamin C in JAR cells was found to occur by different mechanisms. The uptake of the dehydroascorbic acid was Na(+)-independent and was mediated by facilitative glucose transporters as evidenced from the inhibition of the uptake process by glucose. On the other hand, the uptake of ascorbate was Na(+)-dependent and was not sensitive to inhibition by glucose. Substitution of Na+ with other monovalent cations abolished the uptake of ascorbate completely. The uptake process was, however, not influenced by anions. Kinetic analysis indicated the presence of a single saturable transport system for ascorbate with a Michaelis-Menten constant of 22 +/- 1 microM. The dependence of the uptake rare of ascorbate on Na+ concentration exhibited sigmoidal kinetics, suggesting interaction of more than one Na+ ion with the transporter. The Hill coefficient for the Na+ interaction was 2, indicating that the Na(+)-dependent ascorbate transport is electrogenic. The Na(+)-dependent stimulation of ascorbate uptake was primarily due to an increase in the affinity of the transporter for ascorbate in the presence of Na+. It is concluded that the JAR placental trophoblast cell line expresses two different transport systems for vitamin C: one for the reduced form of the vitamin ascorbate; and the other for the oxidized form of the vitamin dehydroascorbic acid.

Glucose transporter isoforms GLUT1 and GLUT3 transport dehydroascorbic acid

Dehydroascorbic acid (DHA) is rapidly taken up by cells and reduced to ascorbic acid (AA). Using the Xenopus laevis oocyte expression system we examined transport of DHA and AA via glucose transporter isoforms GLUT1-5 and SGLT1. The apparent Km of DHA transport via GLUT1 and GLUT3 was 1.1 +/- 0.2 and 1.7 +/- 0.3 mM, respectively. High performance liquid chromatography analysis confirmed 100% reduction of DHA to AA within oocytes. GLUT4 transport of DHA was only 2-4-fold above control and transport kinetics could not be calculated. GLUT2, GLUT5, and SGLT1 did not transport DHA and none of the isoforms transported AA. Radiolabeled sugar transport confirmed transporter function and identity of all cDNA clones was confirmed by restriction fragment mapping. GLUT1 and GLUT3 cDNA were further verified by polymerase chain reaction. DHA transport activity in both GLUT1 and GLUT3 was inhibited by 2-deoxyglucose, D-glucose, and 3-O-methylglucose among other hexoses while fructose and L-glucose showed no inhibition. Inhibition by the endofacial inhibitor, cytochalasin B, was non-competitive and inhibition by the exofacial inhibitor, 4,6-O-ethylidene-alpha-glucose, was competitive. Expressed mutant constructs of GLUT1 and GLUT3 did not transport DHA. DHA and 2-deoxyglucose uptake by Chinese hamster ovary cells overexpressing either GLUT1 or GLUT3 was increased 2-8-fold over control cells. These studies suggest GLUT1 and GLUT3 isoforms are the specific glucose transporter isoforms which mediate DHA transport and subsequent accumulation of AA.

Efficient transport and accumulation of vitamin C in HL-60 cells depleted of glutathione

Human myeloid leukemia cells (HL-60) transport only the oxidized form of vitamin C (dehydroascorbic acid) and accumulate the vitamin in the reduced form, ascorbic acid. We performed a detailed study of the role of glutathione in the intracellular trapping/accumulation of ascorbic acid in HL-60 cells. Uptake studies using HL-60 cells depleted of glutathione by treatment with L-buthionine-(S,R) sulfoximine and diethyl maleate, revealed no changes in the cells' ability to transport dehydroascorbic acid and accumulate ascorbic acid. Similar transport and accumulation rates were obtained using HL-60 cells containing intracellular glutathione concentrations from 6 mM to 1 microM. HL-60 cells, containing as little as 5 microM glutathione, were able to accumulate up to 150 mM ascorbic acid intracellularly when incubated with dehydroascorbic acid. Glutathione was capable of reducing dehydroascorbic acid by a direct chemical reaction, but only when present in a greater than 10-fold stoichiometric excess over dehydroascorbic acid. The accumulation of ascorbic acid by HL-60 cells was strongly temperature-dependent and was very inefficient at 16 degrees C. On the other hand, the direct chemical reduction of dehydroascorbic acid by excess glutathione proceeded efficiently at temperatures of 16 degrees C. Our data indicate that glutathione-dependent reductases in HL-60 cells are not responsible for the ability of these cells to accumulate millimolar concentrations of ascorbic acid. These findings indicate that alternative enzymatic mechanisms are involved in the cellular reduction of dehydroascorbic acid.

Congenital malformations in experimental diabetic pregnancy: aetiology and antioxidative treatment. Minireview based on a doctoral thesis

Diabetes mellitus in pregnancy causes congenital malformations in the offspring. The aim of this work was to characterize biochemical and morphologic anomalies in the conceptus of an animal model of diabetic pregnancy. In addition, a preventive treatment against diabetes-induced dysmorphogenesis was developed. Congenital cataract was often found in the offspring of diabetic rats. The fetal lenses had increased water accumulation, sorbitol concentration and aldose reductase activity compared to control lenses. The results suggest that the cataracts form via osmotic attraction of water due to sorbitol accumulation in the fetal lens. Another set of malformations, with possible neural crest cell origin, occurred frequently in offspring of diabetic rats. These included low set ears, micrognathia, hypoplasia of the thymus, thyroid and parathyroid glands, as well as anomalies of the heart and great vessels. Furthermore, diabetes caused intrauterine death and resorptions more frequently in the late part of gestation. When the pregnant diabetic rats were treated with the antioxidants butylated hydroxytoluene, vitamin E or vitamin C, the occurrence of gross malformations was reduced from approximately 25% to less than 8%, and late resorptions from 17% to 7%. This suggests that an abnormal handling of reactive oxygen species (ROS) is involved in diabetes-induced dysmorphogenesis in vivo. Indeed, an increased concentration of lipid peroxides, indicating damage caused by ROS, was found in fetuses of diabetes rats. In addition, embryos of diabetic rats had low concentrations of the antioxidant vitamin E compared to control embryos. These biochemical alterations were normalized by vitamin E treatment of the pregnant diabetic rats. The antioxidants are likely to have prevented ROS injury in the embryos of the diabetic rats, in particular in the neural crest cells, thereby normalizing embryonic development. These results provide a rationale for developing new anti-teratogenic treatments for pregnant women with diabetes mellitus.

Resolution of the facilitated transport of dehydroascorbic acid from its intracellular accumulation as ascorbic acid

We performed a detailed kinetic analysis of the uptake of dehydroascorbic acid by HL-60 cells under experimental conditions that enabled the differentiation of dehydroascorbic acid transport from the intracellular reduction/accumulation of ascorbic acid. Immunoblotting and immunolocalization experiments identified GLUT1 as the main glucose transporter expressed in the HL-60 cells. Kinetic analysis allowed the identification of a single functional activity involved in the transport of dehydroascorbic acid in the HL-60 cells. Transport was inhibited in a competitive manner by both 3-O-methyl-D-glucose and 2-deoxy-D-glucose. In turn, dehydroascorbic acid competitively inhibited the transport of both sugars. A second functional component identified in experiments measuring the accumulation of ascorbic acid appears to be associated with the intracellular reduction of dehydroascorbic acid to ascorbic acid and is not directly involved in the transport of dehydroascorbic acid via GLUT1. Transport of dehydroascorbic acid by HL-60 cells was independent of the presence of external Na+, whereas the intracellular accumulation of ascorbic acid was found to be a Na(+)-sensitive process. Thus, the transport of dehydroascorbic acid via glucose transporters is a Na(+)-independent process which is kinetically and biologically separable from the reduction of dehydroascorbic acid to ascorbic acid and its subsequent intracellular accumulation.

The transport of vitamin C in the isolated human near-term placenta

We investigated the specific uptake (reference: [H3]-L-glucose) as a measure of membrane transfer of [C14]-labeled L-ascorbic acid (AA), L-dehydroascorbic acid (DHA) and diketogulonic acid using the single injection, double tracer dilution method in the artificially perfused lobe of the near-term human placenta. The uptake of DHA (40-60%) on both the fetal and the maternal side was 3-6 times higher than the uptake of AA, whereas an uptake of diketogulonic acid was not detected. AA transport was slightly higher on the maternal side. Uptake of DHA was suppressed by phloretin and cytocholasin B, whereas AA transfer was not affected. Low sodium concentrations inhibited the uptake of DHA. D-glucose (> 20 mmol/l) inhibited the DHA uptake, and DHA inhibited D-glucose uptake but not L-alanine uptake. The Km value (self inhibition) for DHA was 6-14 mmol/l. Vitamin C enters the trophoblast predominantly as L-dehydroascorbic acid. Its transfer through the microvillous and basal membrane might use the glucose transporter, however, a specific sodium-dependent pathway is not ruled out. Our transfer data suggest an intracellular pool of vitamin C which fills up with increasing plasma DHA-concentrations.

Transport of vitamin C in animal and human cells

The transport systems of animal and human tissues for vitamin C are reviewed with respect to their properties. It emerges that pure diffusion plays only a very minor role while a variety of more or less specific transporters is found on cellular membranes. Although most tissues prefer the reduced ascorbate over the oxidized dehydroascorbic acid and have high-affinity transporters for it, there are several examples for the reversed situation. Special attention is given to similarity or identity with glucose transporters, especially the GLUT-1 and the sodium-dependent intestinal and renal transporters, and to the very widespread dependence of ascorbate transport on sodium ions. The significance of ascorbate transport for vitamin C-requiring and nonrequiring species as well as alterations in states of disease can be seen from ample experimental evidence.

Mammalian facilitative hexose transporters mediate the transport of dehydroascorbic acid

Although vitamin C is critical to human physiology, it is not clear how it is taken up into cells. The kinetics of cell and tissue accumulation of ascorbic acid in vitro indicate that the process is mediated by specific transporters at the cell membrane. Some experimental observations have linked the transport of ascorbic acid with hexose transport systems in mammalian cells, although no clear information is available regarding the specific role(s) of these transporters, if any, in this process. Here we use the Xenopus laevis oocyte expression system to show that the mammalian facilitative hexose transporters are efficient transporters of the oxidized form of vitamin C (dehydroascorbic acid). Two transport pathways, one with low affinity and one with high affinity for dehydroascorbic acid, were found in oocytes expressing the mammalian transporters, and these oocytes accumulated vitamin C against a concentration gradient when supplied with dehydroascorbic acid. We obtained similar results in experiments using normal human neutrophils. These observations indicate that mammalian facilitative hexose transporters are a physiologically significant pathway for the uptake and accumulation of vitamin C by cells, and suggest a mechanism for the accumulation of ascorbic acid against a concentration gradient.

Ascorbate uptake by ROS 17/2.8 osteoblast-like cells: substrate specificity and sensitivity to transport inhibitors

Ascorbate (reduced vitamin C) is required for bone formation. We have shown previously that both the osteoblast-like cell line ROS 17/2.8 and primary cultures of rat calvarial cells possess a saturable, Na(+)-dependent uptake system for L-ascorbate (J Membr Biol 111:83-91, 1989). The purpose of the present study was to investigate the specificity of this transport system for organic anions and its sensitivity to transport inhibitors. Initial rates of ascorbate uptake were measured by incubating ROS 17/2.8 cells with [L-14C]ascorbate at 37 degrees C. Uptake of [L-14C]ascorbate (5 microM) was inhibited 98 +/- 1% by coincubation with unlabeled L-ascorbate (3 mM) and 48 +/- 4% by salicylate (3 mM), but it was not affected by 3 mM formate, lactate, pyruvate, gluconate, oxalate, malonate, or succinate. Uptake of the radiolabeled vitamin also was not affected by acute (1 minute) exposure of the cells to the Na+ transport inhibitors amiloride and ouabain or the glucose transport inhibitor cytochalasin B. In contrast, anion transport inhibitors rapidly (less than 1 minute) and reversibly blocked [L-14C]ascorbate uptake. In order of potency, these drugs were 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) approximately equal to sulfinpyrazone greater than furosemide approximately equal to 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS). These findings indicate that the ascorbate transporter is relatively specific for the ascorbate anion, since other organic anions (with the exception of salicylate) did not compete with ascorbate for uptake. Rapid and reversible inhibition by the impermeant antagonists DIDS and SITS suggests that they interact directly with the ascorbate transporter, consistent with location of the transport system in the plasma membrane.

Effect of cytochalasin B on the uptake of ascorbic acid and glucose by 3T3 fibroblasts: mechanism of impaired ascorbate transport in diabetes

Hyperglycemia and/or hypoinsulinemia have been found to inhibit L-ascorbic acid cellular transport. The resultant decrease in intracellular ascorbic acid may de-inhibit aryl sulfatase B and increase degradation of sulfated glycosaminoglycans (sGAG). This could lead to a degeneration of the extracellular matrix and result in increased intimal permeability, the initiating event in atherosclerosis. The present studies show that the glucose transport inhibitor cytochalasin B blocked the uptake of 3H-2-deoxy-D-glucose (2.5 mg%) by mouse 3T3 fibroblasts. Cytochalasin B also blocked the uptake of 14C-L-ascorbic acid (1.25 mg%). The results of these studies further support the hypothesis that glucose and ascorbate share a common transport system. This may have important implications concerning the vascular pathology associated with diabetes mellitus.

High-affinity sodium-dependent uptake of ascorbic acid by rat osteoblasts

Ascorbic acid is essential for the formation of bone by osteoblasts, but the mechanism by which osteoblasts transport ascorbate has not been investigated previously. We examined the uptake of L-[14C]ascorbate by a rat osteoblast-like cell line (ROS 17/2.8) and by primary cultures of rat calvaria cells. In both systems, cells accumulated L-[14C]ascorbate during incubations of 1-30 min at 37 degrees C. Unlike propionic acid, which diffuses across membranes in protonated form, ascorbic acid did not markedly alter cytosolic pH. Initial ascorbate uptake rate saturated with increasing substrate concentration, reflecting a high-affinity interaction that could be described by Michaelis-Menten kinetics (apparent Km = 30 +/- 2 microM and Vmax = 1460 +/- 140 nmol ascorbate/g protein/min in ROS 17/2.8 cells incubated with 138 mM extracellular Na+). Consistent with a stereoselective carrier-mediated mechanism, unlabeled L-ascorbate was a more potent inhibitor (IC50 = 30 +/- 5 microM) of L-[14C]ascorbate transport than was D-isoascorbate (IC50 = 380 +/- 55 microM). Uptake was dependent on both temperature and Na+, since it was inhibited by cooling to 4 degrees C and by substitution of K+, Li+ or N-methyl-D-glucamine for extracellular Na+. Decreasing the external Na+ concentration lowered both the affinity of the transporter for ascorbate and the apparent maximum velocity of transport. We conclude that osteoblasts possess a stereoselective, high-affinity, Na+-dependent transport system for ascorbate. This system may play a role in the regulation of bone formation.

Ascorbic acid uptake by a high-affinity sodium-dependent mechanism in cultured rat astrocytes

Ascorbic acid (vitamin C) is synthesized in rodent liver, circulates in the blood, and is concentrated in the brain. Experiments were performed to characterize the mechanism of ascorbate uptake by rat cerebral astrocytes in primary culture. Astroglial uptake of L-[14C]ascorbate was observed to be both saturable and stereoselective. In addition, uptake was dependent on both the incubation temperature and the concentration of Na+ because it was largely inhibited by cooling to 4 degrees C, by treatment with ouabain to increase intracellular Na+, and by the substitution of K+, Li+, or N-methyl-D-glucamine for extracellular Na+. The affinity for ascorbate was relatively high in cells incubated with a physiological concentration of extracellular Na+, because the apparent Km was 32 microM in 138 mM Na+. However, the affinity for ascorbate was significantly decreased when the extracellular Na+ concentration was lowered. Treatment of astrocytes with dibutyryl cyclic AMP induced stellation and increased the maximum rate of ascorbate uptake by 53%. We conclude that astrocytes possess a stereoselective, high-affinity, and Na+-dependent uptake system for ascorbate. This system may regulate the cerebral ascorbate concentration and consequently modulate neuronal function.

Sodium-gradient-driven, high-affinity, uphill transport of succinate in human placental brush-border membrane vesicles

Brush-border membrane vesicles isolated from normal human term placentas were shown to accumulate succinate transiently against a concentration gradient, when an inward-directed Na+ gradient was imposed across the membrane. This uptake was almost totally due to transport into intravesicular space, non-specific binding to the membranes being negligible. The dependence of the initial uptake rate of succinate on Na+ concentration exhibited sigmoidal kinetics, indicating interaction of more than one Na+ ion with the carrier system. The Hill coefficient for this ion was calculated to be 2.7. The Na+-dependent uptake of succinate was electrogenic, resulting in the transfer of positive charge across the membrane. Kinetic analysis showed that succinate uptake in these vesicles occurred via a single transport system, with an apparent affinity constant of 4.8 +/- 0.2 microM and a maximal velocity of 274 +/- 4 pmol/20 s per mg of protein. Uptake of succinate was strongly inhibited by various C4 or C5 dicarboxylic acids, whereas monocarboxylic acids, amino acids and glucose showed little or no effect. Li+ and K+ could not substitute for Na+ in the uptake process. Instead, Li+ was found to have a significant inhibitory effect on the Na+-dependent uptake of succinate.

Uptake and release of ascorbic acid by rat Leydig cells in vitro

The mechanism by which ascorbic acid is transported into rat Leydig cells in vitro was investigated. Transport of [14C]ascorbic acid was temperature-dependent and inhibited in the absence of sodium in the incubation medium. The uptake of [14C]ascorbic acid was saturable (apparent Km approximately equal to 33 microM, Vmax approximately equal to 20 pmol/10(6) cells/min) and stereospecific. Uptake was not affected by removing glucose from the incubation medium, indicating that no significant uptake was occurring via the glucose transport system. Transport of [14C]ascorbic acid into Leydig cells was not affected by testosterone concentrations up to 7 microM or by LH at a concentration that causes maximum androgen secretion. Leydig cells preloaded with [14C]ascorbic acid rapidly released radioactivity into the incubation medium, a process that was unaffected by the presence of LH. The results suggest that ascorbic acid uptake by Leydig cells is by an electroneutral, sodium-dependent co-transport mechanism as in some, but not all, other types of cells.

Control of placental glucose transfer

There is little evidence to suggest that the membrane transfer mechanism of the placenta for glucose becomes saturated until maternal blood glucose concentrations are quite high. Also, recent evidence suggests that the membrane transport system for glucose in the placenta is not stimulated by maternal or fetal insulin. Furthermore, there is no solid evidence that hormonal or non-hormonal factors function in vivo to limit membrane transport of glucose in the placenta. Therefore, the limited data which are available suggest that there are no specific mechanisms which acutely regulate placental membrane transport of glucose, and that this membrane transport mechanism operates to maximize maternal-to-fetal glucose transfer. The rate of maternal-to-fetal glucose transfer is a function of the transplacental concentration gradient. This gradient appears to be under the control of fetal insulin and placental lactogen. The available data suggest that both hormones act to increase this concentration gradient: insulin by decreasing fetal blood glucose, and placental lactogen by both decreasing fetal and increasing maternal blood glucose concentrations. Furthermore, high rates of glucose uptake by fetal erythrocytes tend to promote maintenance of this concentration gradient. Therefore, these influences of the maternal-fetal concentration gradient promote transplacental glucose flux to the fetus. As illustrated by the fetal complications associated with maternal hyperglycaemia, the cellular and organismic physiology of the fetus and placenta appears to maximize, rather than optimize, glucose availability to the fetus. It may be, however, that during normal pregnancy, maximal availability is optimal for fetal development.