Vitamin C transport systems of mammalian cells

Vitamin C is essential for many enzymatic reactions and also acts as a free radical scavenger. Specific non-overlapping transport proteins mediate the transport of the oxidized form of vitamin C, dehydroascorbic acid, and the reduced form, Lascorbic acid, across biological membranes. Dehydroascorbic acid uptake is via the facilitated-diffusion glucose transporters, GLUT 1, 3 and 4, but under physiological conditions these transporters are unlikely to play a major role in the uptake of vitamin C due to the high concentrations of glucose that will effectively block influx. L-ascorbic acid enters cells via Na⁺-dependent systems, and two isoforms of these transporters (SVCT1 and SVCT2) have recently been cloned from humans and rats. Transport by both isoforms is stereospecific, with a pH optimum of ~ 7.5 and a Na⁺: ascorbic acid stoichiometry of 2 : 1. SVCT2 may exhibit a higher affinity for ascorbic acid than SVCT1 but with a lower maximum velocity. SVCT1 and SVCT2 are predicted to have 12 transmembrane domains, but they share no structural homology with other Na⁺ co-transporters. Potential sites for phosphorylation by protein kinase C exist on the cytoplasmic surface of both proteins, with an additional protein kinase A site in SVCT1. The two isoforms also differ in their tissue distribution: SVCT1 is present in epithelial tissues, whereas SVCT2 is present in most tissues with the exception of lung and skeletal muscle.     

Vitamin C transport systems of mammalian cells

Vitamin C is essential for many enzymatic reactions and also acts as a free radical scavenger. Specific non-overlapping transport proteins mediate the transport of the oxidized form of vitamin C, dehydroascorbic acid, and the reduced form, L-ascorbic acid, across biological membranes. Dehydroascorbic acid uptake is via the facilitated-diffusion glucose transporters, GLUT 1, 3 and 4, but under physiological conditions these transporters are unlikely to play a major role in the uptake of vitamin C due to the high concentrations of glucose that will effectively block influx. L-ascorbic acid enters cells via Na+-dependent systems, and two isoforms of these transporters (SVCT1 and SVCT2) have recently been cloned from humans and rats. Transport by both isoforms is stereospecific, with a pH optimum of approximately 7.5 and a Na+:ascorbic acid stoichiometry of 2:1. SVCT2 may exhibit a higher affinity for ascorbic acid than SVCT1 but with a lower maximum velocity. SVCT1 and SVCT2 are predicted to have 12 transmembrane domains, but they share no structural homology with other Na+ co-transporters. Potential sites for phosphorylation by protein kinase C exist on the cytoplasmic surface of both proteins, with an additional protein kinase A site in SVCT1. The two isoforms also differ in their tissue distribution: SVCT1 is present in epithelial tissues, whereas SVCT2 is present in most tissues with the exception of lung and skeletal muscle.     

Flavonoid transport across RBE4 cells: A blood-brain barrier model

There is a growing interest in dietary therapeutic strategies to combat oxidative stress-induced damage to the Central Nervous System (CNS), which is associated with a number of pathophysiological processes, including Alzheimer’s and Parkinson’s diseases and cerebrovascular diseases. Identifying the mechanisms associated with phenolic neuroprotection has been delayed by the lack of information concerning the ability of these compounds to enter the CNS. The aim of this study was to evaluate the transmembrane transport of flavonoids across RBE-4 cells (an immortalized cell line of rat cerebral capillary endothelial cells) and the effect of ethanol on this transport. The detection and quantification of all of the phenolic compounds in the studied samples (basolateral media) was performed using a HPLC-DAD (Diode Array Detector). All of the tested flavonoids (catechin, quercetin and cyanidin-3-glucoside) passed across the RBE-4 cells in a time-dependent manner. This transport was not influenced by the presence of 0.1% ethanol. In conclusion, the tested flavonoids were capable of crossing this blood-brain barrier model.     

Ultrastructural features of cytofilaments within mammalian endothelial cells

Summary A submicroscopic study of the endothelium lining the blood capillaries and arterioles contained in the Cyon's nerve was made. The cytoplasm of some endothelial cells was found containing bundles of thin filaments. These measure about 60 Å in diameter, and do not show any cross striation, nor contacts with other cytoplasmic components. They are oriented parallel to each other and to the cell surface. No attachment plate of cytofilaments to the plasma membrane was seen. The filamentous structures were mostly found within the supranuclear cytoplasm. The endothelial cells in question never showed contacts with axons.In the light of these findings it can be advanced the view that the cytofilaments present in endothelial cells are supportive in function; namely they may confer a higher elasticity to these cells, subject to continuous pressure and morphological variations.

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Riassunto Lo studio ultrastrutturale dell'endotelio di piccoli vasi sanguigni contenuti nel nervo cardio-aortico depressore di Cyon del coniglio ha consentito di rilevare la presenza in alcune cellule endoteliali di esili filamenti raggruppati in fasci più o meno numerosi. Detti citofilamenti presentano uno spessore di circa 60 Å, occupano per lo più il citoplasma sopranucleare e sono orientati parallelamente tra loro e rispetto alla superficie cellulare. Essi non mostrano struttura periodica né rapporto alcuno, se non di semplice contiguità, con altre componenti citoplasmatiche. Non presentano, inoltre, punti di attacco sulla membrana cellulare, né connessioni con fibre nervose. Numerose le vescicole pinocitotiche osservate lungo i bordi superficiale e basale delle cellule endoteliali in oggetto.In base a tali reperti si avanza l'ipotesi che i citofilamenti endoteliali svolgano una funzione di sostegno nell'ambito del citoplasma, e siano capaci di conferire un più elevato grado di elasticità all'endotelio vasale, soggetto a continue variazioni di pressione e di forma.

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Dedicated to Prof. Wolfgang Bargmann on his 60th birthday.—This investigation was supported in part by a grant from the Italian C.N.R.

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The author wishes to thank Dr. Pasquale Romeo for his kind help in the preparation of the paper, and Mr. Ciro Paesano for making the photographic prints.     

Lipid transport by mammalian ABC proteins

ABC (ATP-binding cassette) proteins actively transport a wide variety of substrates, including peptides, amino acids, sugars, metals, drugs, vitamins and lipids, across extracellular and intracellular membranes. Of the 49 hum an ABC proteins, a significant number are known to mediate the extrusion of lipids from membranes or the flipping of membrane lipids across the bilayer to generate and maintain membrane lipid asymmetry. Typical lipid substrates include phospholipids, sterols, sphingolipids, bile acids and related lipid conjugates. Members of the ABCA subfamily of ABC transporters and other ABC proteins such as ABCB4, ABCG1 and ABCG5/8 implicated in lipid transport play important roles in diverse biological processes such as cell signalling, membrane lipid asymmetry, removal of potentially toxic compounds and metabolites, and apoptosis. The importance of these ABC lipid transporters in cell physiology is evident from the finding that mutations in the genes encoding many of these proteins are responsible for severe inherited diseases. For example, mutations in ABCA1 cause Tangier disease associated with defective efflux of cholesterol and phosphatidylcholine from the plasma membrane to the lipid acceptor protein apoA1 (apolipoprotein AI), mutations in ABCA3 cause neonatal surfactant deficiency associated with a loss in secretion of the lipid pulmonary surfactants from lungs of newborns, mutations in ABCA4 cause Stargardt macular degeneration, a retinal degenerative disease linked to the reduced clearance of retinoid compounds from photoreceptor cells, mutations in ABCA12 cause harlequin and lamellar ichthyosis, skin diseases associated with defective lipid trafficking in keratinocytes, and mutations in ABCB4 and ABCG5/ABCG8 are responsible for progressive intrafamilial hepatic disease and sitosterolaemia associated with defective phospholipid and sterol transport respectively. This chapter highlights the involvement of various mammalian ABC transporters in lipid transport in the context of their role in cell signalling, cellular homoeostasis, apoptosis and inherited disorders.     

Binding, internalization and transport of apolipoprotein A-I by vascular endothelial cells

High density lipoproteins (HDL) and their main protein constituent, apolipoprotein A-I (apoA-I), exert potentially anti-atherogenic properties within the arterial wall. However, it is unknown how they are transported from the blood stream into the vascular wall. Here we investigated the interaction of apoA-I with endothelial cells. At 4 degrees C endothelial cells bound 125I-apoA-I with high affinity, Kd = 2.1 microg/ml and in a saturable manner (Bmax of 35 ng/mg cell protein). At 37 degrees C, the cell association of apoA-I revealed similar affinity as at 4 degrees C (Kd = 2.2 microg/ml) but the maximum specific cell association was much enhanced (Bmax = 360 ng/mg cell protein). Binding and cell association was competed by excess unlabeled apoA-I and HDL but not by albumin. Biotinylation experiments and electron microscopy studies showed that endothelial cells internalize labeled apoA-I. Only minor amounts of the internalized apoA-I were degraded. Cultivated in a Transwell system, the cells transported a fraction of 125I-apoA-I from the apical to the basolateral compartment in a competable and temperature-sensitive manner. Furthermore, after specific transport the originally prebeta-mobile and lipid-free apoA-I was recovered as particles which have electrophoretic alpha-mobility. We conclude that endothelial cells transcytose and lipidate lipid-free apoA-I.     

Inhibition of organic anion transport in endothelial cells by hydrogen peroxide.

ATP loss is a prominent feature of cellular injury induced by oxidants or ischemia. How reduction of cellular ATP levels contributes to lethal injury is still poorly understood. In this study we examined the ability of H2O2 to inhibit in a dose-dependent manner the extrusion of fluorescent organic anions from bovine pulmonary artery endothelial cells. Extrusion of fluorescent organic anions was inhibited by probenecid, suggesting an organic anion transporter was involved. In experiments in which ATP levels in endothelial cells were varied by treatment with different degrees of metabolic inhibition, it was determined that organic anion transport was ATP-dependent. H2O2-induced inhibition of organic anion transport correlated well with the oxidant's effect on cellular ATP levels. Thus H2O2-mediated inhibition of organic anion transport appears to be via depletion of ATP, a required substrate for the transport reaction. Inhibition of organic anion transport directly by probenecid or indirectly by metabolic inhibition with reduction of cellular ATP levels was correlated with similar reductions of short term viability. This supports the hypothesis that inhibition of organic anion transport after oxidant exposure or during ischemia results from depletion of ATP and may significantly contribute to cytotoxicity.     

Regulation of taurine transport systems by protein kinase CK2 in mammalian cells

Maintaining cell volume is critical for cellular function yet shift in cell volume is a prerequisite for mitosis and apoptosis. The ubiquitously and evolutionary conserved serine/threonine kinase CK2 promotes cell survival and suppresses apoptosis. The present review describes how mammalian cells regulate the cellular content of the major cellular organic osmolyte, taurine with emphasis on CK2 mediated regulation of active taurine uptake and volume-sensitive taurine release. Furthermore, we discuss how CK2-mediated regulation of taurine homeostasis is potentially involved in cellular functions such as proliferation and survival.     

Polyamine transport by mammalian cells and mitochondria: role of antizyme and glycosaminoglycans

The role of antizyme (AZ) and glycosaminoglycans in polyamine uptake by mammalian cells and mitochondria was examined using NIH3T3 and FM3A cells and rat liver mitochondria. AZ is synthesized as two isoforms (29 and 24.5 kDa) due to the existence of two initiation codon AUGs in the AZ mRNA. Most AZ existed as the 24.5-kDa form translatable from the second AUG, but a portion of the 29-kDa AZ from the first AUG was associated with mitochondria because of the presence of a mitochondrial targeting signal between the first and the second methionine. The predominance of the 24.5-kDa isoform was mainly due to the presence of spermidine and a favorable sequence context (Kozak sequence) at the second initiation codon AUG. Spermine uptake by NIH3T3 cells was inhibited by both 29- and 24.5-kDa AZs, but uptake by rat liver mitochondria was not influenced by either form of AZ. Because spermine uptake by mitochondria caused a release of cytochrome c, an enhancer of apoptosis, we looked for inhibitors of mitochondrial spermine uptake other than AZ. Cations such as Na+, K+, and Mg2+ were inhibitors of the mitochondrial uptake. It has been reported that heparan sulfate on glypican-1 plays important roles in spermine uptake by human embryonic lung fibroblasts. Heparin, but not heparan sulfate, slightly inhibited spermine uptake by FM3A cells in the absence of Mg2+ and Ca2+ but had no effect under physiological conditions in the presence of Mg2+ and Ca2+.     

Iron transport kinetics through blood-brain barrier endothelial cells

Background

Transferrin and its receptors play an important role during the uptake and transcytosis of iron through blood-brain barrier (BBB) endothelial cells (ECs) to maintain iron homeostasis in BBB endothelium and brain. Any disruptions in the cell environment may change the distribution of transferrin receptors on the cell surface, which eventually alter the homeostasis and initiate neurodegenerative disorders. In this paper, we developed a comprehensive mathematical model that considers the necessary kinetics for holo-transferrin internalization and acidification, apo-transferrin recycling, and exocytosis of free iron and transferrin-bound iron through basolateral side of BBB ECs.

Cation transport in vascular endothelial cells and aging

Summary To understand the generation and maintenance of Na and K gradients in cultured vascular endothelial cells, net Na and K movements were studied. Ouabain-sensitive (OS) net Na gain and K loss were estimated as the difference between the cation content in the presence of ouabain and that in the control. Ouabain-and furosemide-resistant (OFR) fluxes were determined in the presence of the two inhibitors. When the normal medium bicarbonate and phosphate buffers were replaced by N-2-hydroxyethylpiperazine-N'-2-ethane sulfonic acid both the OS ans OFR fluxes decreased more than 50%. Ouabain-sensitive and ouabain-and furosemide-resistant fluxes decreased with increasing cellular age (passage number) an effect not observed when the cation movements were studied in the absence of bicarbonate and phosphate. These results suggest that cultured vascular endothelial cells possess bicarbonate-and phosphate-dependent Na and K pathways which account for a significant portion of their passive movements. Furthermore, the behavior of cation permeabilities with passage number suggests that these modulations may be related to the cellular aging process.