Uptake of lipoprotein-associated alpha-tocopherol by primary porcine brain capillary endothelial cells

Deciphering the relationship between caveolae-mediated intracellular transport and signalling events

The caveolae-mediated transport across polarized epithelial cell barriers has been largely deciphered in the last decades and is considered the second essential intracellular transfer mechanism, after the clathrin-dependent endocytosis. The basic cell biology knowledge was supplemented recently, with the molecular mechanisms beyond caveolae generation implying the key contribution of the lipid-binding proteins (the structural protein Caveolin and the adapter protein Cavin), along with the bulb coat stabilizing molecules PACSIN-2 and Eps15 homology domain protein-2. The current attention is focused also on caveolae architecture (such as the bulb coat, the neck, the membrane funnel inside the bulb, and the associated receptors), and their specific tasks during the intracellular transport of various cargoes. Here, we resume the present understanding of the assembly, detachment, and internalization of caveolae from the plasma membrane lipid raft domains, and give an updated view on transcytosis and endocytosis, the two itineraries of cargoes transport via caveolae. The review adds novel data on the signalling molecules regulating caveolae intracellular routes and on the transport dysregulation in diseases. The therapeutic possibilities offered by exploitation of Caveolin-1 expression and caveolae trafficking, and the urgent issues to be uncovered conclude the review.

Vitamin E for the management of major depressive disorder: possible role of the anti-inflammatory and antioxidant systems

OBJECTIVES

Vitamin E has various functions in humans, including antioxidant, anti-inflammatory, anti-cancer, and anti-atherogenic actions, as well as direct effects on enzymatic activities and modulation of gene transcription. In addition to these functions, vitamin E is also important for the central nervous system, and its role in the prevention and/or treatment of some neurological diseases has been suggested. In particular, the role of vitamin E in the modulation of major depressive disorder (MDD) is an issue that has emerged in recent studies. Many factors have been implicated in the pathophysiology of this disorder, including inflammation, oxidative, and nitrosative stress.

METHODS

This narrative review discusses the involvement of inflammation, oxidative, and nitrosative stress in the pathophysiology of MDD and presents clinical and preclinical studies that correlate vitamin E with this psychiatric disorder.

RESULTS

We gathered evidence from clinical studies that demonstrated the relationship between low vitamin E status and MDD symptoms. Vitamin E has been reported to exert a beneficial influence on the oxidative and inflammatory status of individuals, factors that may account for the attenuation of depressive symptoms. Preclinical studies have reinforced the antidepressant-like response of vitamin E, and the mechanisms underlying its effect seem to be related to the modulation of oxidative stress and neuroinflammation.

CONCLUSION

We suggest that vitamin E has potential to be used as an adjuvant for the management of MDD, but more studies are clearly needed to ascertain the efficacy of vitamin E for alleviating depressive symptoms.

CoQ10 Deficient Endothelial Cell Culture Model for the Investigation of CoQ10 Blood-Brain Barrier Transport

Primary coenzyme Q₁₀ (CoQ₁₀) deficiency is unique among mitochondrial respiratory chain disorders in that it is potentially treatable if high-dose CoQ₁₀ supplements are given in the early stages of the disease. While supplements improve peripheral abnormalities, neurological symptoms are only partially or temporarily ameliorated. The reasons for this refractory response to CoQ₁₀ supplementation are unclear, however, a contributory factor may be the poor transfer of CoQ₁₀ across the blood-brain barrier (BBB). The aim of this study was to investigate mechanisms of CoQ₁₀ transport across the BBB, using normal and pathophysiological (CoQ₁₀ deficient) cell culture models. The study identifies lipoprotein-associated CoQ₁₀ transcytosis in both directions across the in vitro BBB. Uptake via SR-B1 (Scavenger Receptor) and RAGE (Receptor for Advanced Glycation Endproducts), is matched by efflux via LDLR (Low Density Lipoprotein Receptor) transporters, resulting in no "net" transport across the BBB. In the CoQ₁₀ deficient model, BBB tight junctions were disrupted and CoQ₁₀ "net" transport to the brain side increased. The addition of anti-oxidants did not improve CoQ₁₀ uptake to the brain side. This study is the first to generate in vitro BBB endothelial cell models of CoQ₁₀ deficiency, and the first to identify lipoprotein-associated uptake and efflux mechanisms regulating CoQ₁₀ distribution across the BBB. The results imply that the uptake of exogenous CoQ₁₀ into the brain might be improved by the administration of LDLR inhibitors, or by interventions to stimulate luminal activity of SR-B1 transporters.

Astaxanthin exerts protective effects similar to bexarotene in Alzheimer's disease by modulating amyloid-beta and cholesterol homeostasis in blood-brain barrier endothelial cells

The pathogenesis of Alzheimer's disease (AD) is characterized by overproduction, impaired clearance, and deposition of amyloid-β peptides (Aβ) and connected to cholesterol homeostasis. Since the blood-brain barrier (BBB) is involved in these processes, we investigated effects of the retinoid X receptor agonist, bexarotene (Bex), and the peroxisome proliferator-activated receptor α agonist and antioxidant, astaxanthin (Asx), on pathways of cellular cholesterol metabolism, amyloid precursor protein processing/Aβ production and transfer at the BBB in vitro using primary porcine brain capillary endothelial cells (pBCEC), and in 3xTg AD mice. Asx/Bex downregulated transcription/activity of amyloidogenic BACE1 and reduced Aβ oligomers and ~80 kDa intracellular 6E10-reactive APP/Aβ species, while upregulating non-amyloidogenic ADAM10 and soluble (s)APPα production in pBCEC. Asx/Bex enhanced Aβ clearance to the apical/plasma compartment of the in vitro BBB model. Asx/Bex increased expression levels of ABCA1, LRP1, and/or APOA-I. Asx/Bex promoted cholesterol efflux, partly via PPARα/RXR activation, while cholesterol biosynthesis/esterification was suppressed. Silencing of LRP-1 or inhibition of ABCA1 by probucol reversed Asx/Bex-mediated effects on levels of APP/Aβ species in pBCEC. Murine (m)BCEC isolated from 3xTg AD mice treated with Bex revealed elevated expression of APOE and ABCA1. Asx/Bex reduced BACE1 and increased LRP-1 expression in mBCEC from 3xTg AD mice when compared to vehicle-treated or non-Tg treated mice. In parallel, Asx/Bex reduced levels of Aβ oligomers in mBCEC and Aβ species in brain soluble and insoluble fractions of 3xTg AD mice. Our results suggest that both agonists exert beneficial effects at the BBB by balancing cholesterol homeostasis and enhancing clearance of Aβ from cerebrovascular endothelial cells.

Vitamin E and Alzheimer's disease: what do we know so far?

Vitamin E has been proposed as a potential clinical intervention for Alzheimer's disease (AD) given the plausibility of its various biological functions in influencing the neurodegenerative processes associated with the condition. The tocopherol and tocotrienol isoforms of vitamin E have multiple properties including potent antioxidant and anti-inflammatory characteristics, in addition to influences on immune function, cellular signalling and lowering cholesterol. Several of these roles offer a theoretical rationale for providing benefit for the treatment of AD-associated pathology. Diminished circulating concentrations of vitamin E have been demonstrated in individuals with AD. Reduced plasma levels have furthermore been associated with an increased risk of AD development while intake, particularly from dietary sources, may limit or reduce the rate of disease progression. This benefit may be linked to synergistic actions between vitamin E isoforms and other micronutrients. Nevertheless, randomised trials have found limited and inconsistent evidence of vitamin E supplementation as an effective clinical intervention. Thus, despite a strong rationale in support of a beneficial role for vitamin E for the treatment of AD, the evidence remains inconclusive. Several factors may partly explain this discrepancy and represent the difficulties of translating complex laboratory evidence and dietary interactions into clinical interventions. Methodological design limitations of existing randomised trials and restrictions to supplementation with a single vitamin E isoform may also limit the influence of effect. Moreover, several factors influence individual responsiveness to vitamin E intake and recent findings suggest variation in the underlying genetic architecture attenuates vitamin E biological availability and activity which likely contributes to the variation in clinical responsiveness and the failure of randomised trials to date. Importantly, the clinical safety of vitamin E remains controversial and warrants further investigation.

The Effectiveness of Vitamin E Treatment in Alzheimer's Disease

Vitamin E was proposed as treatment for Alzheimer’s disease many years ago. However, the effectiveness of the drug is not clear. Vitamin E is an antioxidant and neuroprotector and it has anti-inflammatory and hypocholesterolemic properties, driving to its importance for brain health. Moreover, the levels of vitamin E in Alzheimer’s disease patients are lower than in non-demented controls. Thus, vitamin E could be a good candidate to have beneficial effects against Alzheimer’s. However, evidence is consistent with a limited effectiveness of vitamin E in slowing progression of dementia; the information is mixed and inconclusive. The question is why does vitamin E fail to treat Alzheimer’s disease? In this paper we review the studies with and without positive results in Alzheimer’s disease and we discuss the reasons why vitamin E as treatment sometimes has positive results on cognition but at others, it does not.

Complexity of vitamin E metabolism

Bioavailability of vitamin E is influenced by several factors, most are highlighted in this review. While gender, age and genetic constitution influence vitamin E bioavailability but cannot be modified, life-style and intake of vitamin E can be. Numerous factors must be taken into account however, i.e., when vitamin E is orally administrated, the food matrix may contain competing nutrients. The complex metabolic processes comprise intestinal absorption, vascular transport, hepatic sorting by intracellular binding proteins, such as the significant α-tocopherol-transfer protein, and hepatic metabolism. The coordinated changes involved in the hepatic metabolism of vitamin E provide an effective physiological pathway to protect tissues against the excessive accumulation of, in particular, non-α-tocopherol forms. Metabolism of vitamin E begins with one cycle of CYP4F2/CYP3A4-dependent ω-hydroxylation followed by five cycles of subsequent β-oxidation, and forms the water-soluble end-product carboxyethylhydroxychroman. All known hepatic metabolites can be conjugated and are excreted, depending on the length of their side-chain, either via urine or feces. The physiological handling of vitamin E underlies kinetics which vary between the different vitamin E forms. Here, saturation of the side-chain and also substitution of the chromanol ring system are important. Most of the metabolic reactions and processes that are involved with vitamin E are also shared by other fat soluble vitamins. Influencing interactions with other nutrients such as vitamin K or pharmaceuticals are also covered by this review. All these processes modulate the formation of vitamin E metabolites and their concentrations in tissues and body fluids. Differences in metabolism might be responsible for the discrepancies that have been observed in studies performed in vivo and in vitro using vitamin E as a supplement or nutrient. To evaluate individual vitamin E status, the analytical procedures used for detecting and quantifying vitamin E and its metabolites are crucial. The latest methods in analytics are presented.

Enhanced delivery of lipophilic nutrients to the infant brain via high density lipoprotein

Lipoproteins are the primary carriers of lipophilic cognitive nutrients such as docosahexaenoic acid, lutein, and α-tocopherol within circulation. The critical roles these nutrients play in growth and development are well established, and as such, their efficient delivery to the infant brain is crucial. Given the selectivity of the blood brain barrier, the lipoprotein fraction primarily responsible for brain delivery of these nutrients must be determined so that efforts aimed at increasing brain nutrient uptake, via lipoprotein profile manipulation, can be appropriately focused. Based on the preclinical and clinical data reviewed here, we hypothesize that high density lipoprotein is the fraction chiefly responsible for delivery of docosahexaenoic acid, lutein, and α-tocopherol to the infant brain. As high density lipoprotein levels tend to be lower in preterm, formula-fed infants as compared to their full-term, breast-fed counterparts, efforts aimed at increasing circulating high density lipoprotein levels, and subsequent delivery of cognitive lipophilic nutrients to the brain via manipulation of formula composition, may be most effective if targeted to this group. These efforts include (1) limiting the polyunsaturated: saturated fatty acid ratio; (2) increasing the casein: whey ratio; (3) altering the proportion of saturated fatty acids found in the sn-2 position of the parent triglyceride; (4) cholesterol supplementation; and (5) nucleotide supplementation.

HDL and endothelial protection

High-density lipoproteins (HDLs) represent a family of particles characterized by the presence of apolipoprotein A-I (apoA-I) and by their ability to transport cholesterol from peripheral tissues back to the liver. In addition to this function, HDLs display pleiotropic effects including antioxidant, anti-apoptotic, anti-inflammatory, anti-thrombotic or anti-proteolytic properties that account for their protective action on endothelial cells. Vasodilatation via production of nitric oxide is also a hallmark of HDL action on endothelial cells. Endothelial cells express receptors for apoA-I and HDLs that mediate intracellular signalling and potentially participate in the internalization of these particles. In this review, we will detail the different effects of HDLs on the endothelium in normal and pathological conditions with a particular focus on the potential use of HDL therapy to restore endothelial function and integrity.

Endocytosis and intracellular processing of BODIPY-sphingomyelin by murine CATH.a neurons

Neuronal sphingolipids (SL) play important roles during axonal extension, neurotrophic receptor signaling and neurotransmitter release. Many of these signaling pathways depend on the presence of specialized membrane microdomains termed lipid rafts. Sphingomyelin (SM), one of the main raft constituents, can be formed de novo or supplied from exogenous sources. The present study aimed to characterize fluorescently-labeled SL turnover in a murine neuronal cell line (CATH.a). Our results demonstrate that at 4°C exogenously added BODIPY-SM accumulates exclusively at the plasma membrane. Treatment of cells with bacterial sphingomyelinase (SMase) and back-exchange experiments revealed that 55-67% of BODIPY-SM resides in the outer leaflet of the plasma membrane. Endocytosis of BODIPY-SM occurs via caveolae with part of internalized BODIPY-fluorescence ending up in the Golgi and the ER. Following endocytosis BODIPY-SM undergoes hydrolysis, a reaction substantially faster than BODIPY-SM synthesis from BODIPY-ceramide. RNAi demonstrated that both, acid (a)SMase and neutral (n)SMases contribute to BODIPY-SM hydrolysis. Finally, high-density lipoprotein (HDL)-associated BODIPY-SM was efficiently taken up by CATH.a cells. Our findings indicate that endocytosis of exogenous SM occurs almost exclusively via caveolin-dependent pathways, that both, a- and nSMases equally contribute to neuronal SM turnover and that HDL-like particles might represent physiological SM carriers/donors in the brain.

Vitamin E trafficking in neurologic health and disease

Vitamin E was identified almost a century ago as a botanical compound necessary for rodent reproduction. Decades of research since then established that of all members of the vitamin E family, α-tocopherol is selectively enriched in human tissues, and it is essential for human health. The major function of α-tocopherol is thought to be that of a lipid-soluble antioxidant that prevents oxidative damage to biological components. As such, α-tocopherol is necessary for numerous physiological processes such as permeability of lipid bilayers, cell adhesion, and gene expression. Inadequate levels of α-tocopherol interfere with cellular function and precipitate diseases, notably ones that affect the central nervous system. The extreme hydrophobicity of α-tocopherol poses a serious thermodynamic barrier for proper distribution of the vitamin to target tissues and cells. Although transport of the vitamin shares some steps with that of other lipids, selected tissues evolved dedicated transport mechanisms involving the α-tocopherol transfer protein (αTTP). The critical roles of this protein and its ligand are underscored by the debilitating pathologies that characterize human carriers of mutations in the TTPA gene.

Phloretin ameliorates 2-chlorohexadecanal-mediated brain microvascular endothelial cell dysfunction in vitro

2-Chlorohexadecanal (2-ClHDA), a chlorinated fatty aldehyde, is formed via attack on ether-phospholipids by hypochlorous acid (HOCl) that is generated by the myeloperoxidase-hydrogen peroxide-chloride system of activated leukocytes. 2-ClHDA levels are elevated in atherosclerotic lesions, myocardial infarction, and neuroinflammation. Neuroinflammatory conditions are accompanied by accumulation of neutrophils (an ample source of myeloperoxidase) in the brain. Microvessel damage by inflammatory mediators and/or reactive oxidants can induce blood-brain barrier (BBB) dysfunction, a pathological condition leading to cerebral edema, brain hemorrhage, and neuronal death. In this in vitro study we investigated the impact of 2-ClHDA on brain microvascular endothelial cells (BMVEC), which constitute the morphological basis of the BBB. We show that exogenously added 2-ClHDA is subject to rapid uptake and metabolism by BMVEC. Using C16 structural analogues of 2-ClHDA we found that the cytotoxic potential decreases in the following order: 2-ClHDA>hexadecanal>palmitic acid>2-ClHDA-dimethylacetal. 2-ClHDA induces loss of barrier function, mitochondrial dysfunction, apoptosis via activation of caspase 3, and altered intracellular redox balance. Finally we investigated potential protective effects of several natural polyphenols on in vitro BBB function. Of the compounds tested, phloretin almost completely abrogated 2-ClHDA-induced BMVEC barrier dysfunction and cell death. These data suggest that 2-ClHDA has the potential to induce BBB breakdown under inflammatory conditions and that phloretin confers protection in this experimental setting.

Inhibitory effect of α-tocopherol on methylmercury-induced oxidative steress

OBJECTIVES

The present study investigated the involvement of oxidative stress in the degeneration of the cerebellum during methylmercury (MeHg) intoxication and the protective effect of α-tocopherol (Vit E) against MeHg toxicity.

METHODS

After 5 mg/kg of MeHg was administered to Wistar rats for 12 consecutive days, the cerebellum were examined histopathologically. In addition, the same amount of MeHg was administered to 3 different groups of Wistar rats: rats with a Vit E-deficient diet, rats fed 150 mg/kg of Vit E for 20 consecutive days after initial MeHg administration, and rats with an ordinary diet.

RESULTS

Positive immunoreactivity against anti-hydroxynonenal (HNE), a marker of lipid peroxidation, was observed in the cerebellum after MeHg administration. Levels of thiobarbituric acid reactive substance (TBARS), another marker of lipid peroxidation, and those of protein carbonyl, a biomarker for protein oxidation, increased after MeHg administration. In the rats with MeHg and a Vit E-deficient diet, mortality and prevalence of piloerection significantly increased, and in the rats with MeHg and Vit E, mortality, piloerection, retracted and crossed hind leg, and ataxic gait significantly decreased, compared with the rats with MeHg alone. The levels of NO(2) (-) and NO(3) (-) in the serum significantly increased in the rats with MeHg alone 14 days after the initial MeHg administration, but were significantly suppressed by Vit E administration.

CONCLUSIONS

Oxidative stress, especially lipid peroxidation, may play an important role in the cerebellar degeneration process during MeHg intoxication and Vit E may play a protective role against MeHg toxicity as an effective antioxidant.

Four and a half LIM protein 1C (FHL1C): a binding partner for voltage-gated potassium channel K(v1.5)

Four-and-a-half LIM domain protein 1 isoform A (FHL1A) is predominantly expressed in skeletal and cardiac muscle. Mutations in the FHL1 gene are causative for several types of hereditary myopathies including X-linked myopathy with postural muscle atrophy (XMPMA). We here studied myoblasts from XMPMA patients. We found that functional FHL1A protein is completely absent in patient myoblasts. In parallel, expression of FHL1C is either unaffected or increased. Furthermore, a decreased proliferation rate of XMPMA myoblasts compared to controls was observed but an increased number of XMPMA myoblasts was found in the G₀/G₁ phase. Furthermore, low expression of K_v1.5, a voltage-gated potassium channel known to alter myoblast proliferation during the G₁ phase and to control repolarization of action potential, was detected. In order to substantiate a possible relation between K_v1.5 and FHL1C, a pull-down assay was performed. A physical and direct interaction of both proteins was observed in vitro. In addition, confocal microscopy revealed substantial colocalization of FHL1C and K_v1.5 within atrial cells, supporting a possible interaction between both proteins in vivo. Two-electrode voltage clamp experiments demonstrated that coexpression of K_v1.5 with FHL1C in Xenopus laevis oocytes markedly reduced K⁺ currents when compared to oocytes expressing K_v1.5 only. We here present the first evidence on a biological relevance of FHL1C.

Vitamin E transport, membrane incorporation and cell metabolism: Is alpha-tocopherol in lipid rafts an oar in the lifeboat?

Vitamin E is composed of closely related compounds, including tocopherols and tocotrienols. Studies of the last decade provide strong support for a specific role of alpha-tocopherol in cell signalling and the regulation of gene expression. It produces significant effects on inflammation, cell proliferation and apoptosis that are not shared by other vitamin E isomers with similar antioxidant properties. The different behaviours of vitamin E isomers might relate, at least in part, to the specific effects they exert at the plasma membrane. alpha-Tocopherol is not randomly distributed throughout the phospholipid bilayer of biological membranes, and as compared with other isomers, it shows a propensity to associate with lipid rafts. Distinct aspects of vitamin E transport and metabolism is discussed with emphasis on the interaction between alpha-tocopherol and lipid rafts and the consequences of these interactions on cell metabolism.

Absorption, transport, and tissue delivery of vitamin E

Vitamin E is one of the most abundant lipid-soluble antioxidant agents found in plasma and cells of higher mammals. The uptake, transport and tissue delivery of alpha-tocopherol, a key vitamin E form, involves molecular, biochemical, and cellular processes closely related to overall lipid and lipoprotein homeostasis. This review highlights recent findings that have led to a better understanding of vitamin E transport, including intestinal absorption, hepatic transport, and cellular uptake of alpha-tocopherol in vivo. This new information may be critical for manipulation of vitamin E homeostasis in a variety of oxidative stress-related disease conditions in humans.

Vitamin E and mast cells

Mast cells play an important role in the immune system by interacting with B and T cells and by releasing several mediators involved in activating other cells. Hyperreactivity of mast cells and their uncontrolled accumulation in tissues lead to increased release of inflammatory mediators contributing to the pathogenesis of several diseases such as rheumatoid arthritis, atherosclerosis, multiple sclerosis, and allergic disorders such as asthma and allergic rhinitis. Interference with mast cell proliferation, survival, degranulation, and migration by synthetic or natural compounds may represent a preventive strategy for the management of these diseases. Natural vitamin E covers a group of eight analogues-the alpha-, beta-, gamma-, and delta-tocopherols and the alpha-, beta-, gamma-, and delta-tocotrienols, but only alpha-tocopherol is efficiently retained by the liver and distributed to peripheral tissues. Mast cells preferentially locate in the proximity of tissues that interface with the external environment (the epithelial surface of the skin, the gastrointestinal mucosa, and the respiratory system), what may render them accessible to treatments with inefficiently retained natural vitamin E analogues and synthetic derivatives. In addition to scavenging free radicals, the natural vitamin E analogues differently modulate signal transduction and gene expression in several cell lines; in mast cells, protein kinase C, protein phosphatase 2A, and protein kinase B are affected by vitamin E, leading to the modulation of proliferation, apoptosis, secretion, and migration. In this chapter, the possibility that vitamin E can prevent diseases with mast cells involvement by modulating signal transduction and gene expression is evaluated.

Cellular, molecular and clinical aspects of vitamin E on atherosclerosis prevention

Randomised clinical trials and epidemiologic studies addressing the preventive effects of vitamin E supplementation against cardiovascular disease reported both positive and negative effects, and recent meta-analyses of the clinical studies were rather disappointing. In contrast to that, many animal studies clearly show a preventive action of vitamin E in several experimental settings, which can be explained by the molecular and cellular effects of vitamin E observed in cell cultures. This review is focusing on the molecular effects of vitamin E on the cells playing a role during atherosclerosis, in particular on the endothelial cells, vascular smooth muscle cells, monocytes/macrophages, T cells, and mast cells. Vitamin E may act by normalizing aberrant signal transduction and gene expression in antioxidant and non-antioxidant manners; in particular, over-expression of scavenger receptors and consequent foam cell formation can be prevented by vitamin E. In addition to that, the cellular effects of alpha-tocopheryl phosphate and of EPC-K1, a composite molecule between alpha-tocopheryl phosphate and l-ascorbic acid, are summarized.

Comparative2H-labelled α-tocopherol biokinetics in plasma, lipoproteins, erythrocytes, platelets and lymphocytes in normolipidaemic males

The biokinetics of newly absorbed vitamin E in blood components was investigated in normolipidaemic males. Subjects (n12) ingested encapsulated 150 mg2H-labelledRRR-α-tocopheryl acetate with a standard meal. Blood was collected at 3, 6, 9, 12, 24 and 48 h post-ingestion.2H-Labelled and pre-existing unlabelled α-tocopherol (α-T) was determined in plasma, lipoproteins, erythrocytes, platelets and lymphocytes by LC–MS. In all blood components, labelled α-T concentration significantly increased while unlabelled decreased following ingestion (P<0·0001). Significant differences in labelled α-T biokinetic parameters were found between lipoproteins. Time of maximum concentration was significantly lower in chylomicrons, while VLDL had a significantly greater maximum α-T concentration and area under the curve (AUC) (P<0·05). The largest variability occurred in chylomicron α-T transport. Erythrocyte labelled α-T concentrations increased gradually up to 24 h while α-T enrichment of platelets and lymphocytes was slower, plateauing at 48 h. Platelet enrichment with labelled α-T was biphasic, the initial peak coinciding with the labelled α-T peak in chylomicrons. Erythrocyte and HDL AUC were significantly correlated (P<0·005), as was platelet and HDL AUC (P<0·05). There was a lower turnover of pre-existing α-T in platelets and lymphocytes (maximum 25 % labelled α-T) compared to plasma and erythrocytes (maximum 45 % labelled α-T). These data indicate that different processes exist in the uptake and turnover of α-T by blood components and that chylomicron α-T transport is a major determinant of inter-individual variation in vitamin E response. This is important for the understanding of α-T transport and uptake into tissues.

Effect of dietary lutein and zeaxanthin on plasma carotenoids and their transport in lipoproteins in age-related macular degeneration

BACKGROUND

Low dietary intakes and low plasma concentrations of lutein and zeaxanthin are associated with an increased risk of age-related macular degeneration (AMD). No studies have challenged AMD patients with a diet high in lutein and zeaxanthin.

OBJECTIVE

The objective was to examine the effect of diets low or high in lutein and zeaxanthin on plasma carotenoids and their transport in AMD patients.

DESIGN

Seven AMD patients and 5 control subjects were fed a low-lutein, low-zeaxanthin diet ( approximately 1.1 mg/d) for 2 wk, which was followed by a high-lutein, high-zeaxanthin diet ( approximately 11 mg/d) for 4 wk. Ten subjects continued the diet for 8 wk. Plasma and lipoprotein carotenoids were measured by HPLC.

RESULTS

The high-lutein, high-zeaxanthin diet resulted in 2- to 3-fold increases in plasma concentrations of lutein and zeaxanthin and other carotenoids, except lycopene, in the AMD patients and the control subjects. With this diet, 52% of the lutein and 44% of the zeaxanthin were transported by HDL; approximately 22% of lutein and zeaxanthin was transported by LDL. Only 20-25% of alpha-carotene, beta-carotene, and lycopene was transported by HDL; 50-57% was transported by LDL.

CONCLUSIONS

The AMD patients and control subjects responded similarly to a diet high in lutein and zeaxanthin; plasma carotenoid concentrations increased greatly in both groups, and the transport of carotenoids by lipoproteins was not significantly different between the groups. This finding suggests that abnormalities in the metabolism of lutein and zeaxanthin in AMD may reside in the uptake of lutein and zeaxanthin from the plasma and transport into the retina.

A feasibility study quantifying in vivo human alpha-tocopherol metabolism

BACKGROUND

Quantitation of human vitamin E metabolism is incomplete, so we quantified RRR- and all-rac-alpha-tocopherol metabolism in an adult.

OBJECTIVE

The objective of the study was to quantify and interpret in vivo human vitamin E metabolism.

DESIGN

A man was given an oral dose of 0.001821 micromol [5-14CH3]RRR-alpha-tocopheryl acetate (with 101.5 nCi 14C), and its fate in plasma, plasma lipoproteins, urine, and feces was measured over time. Data were analyzed and interpreted by using kinetic modeling. The protocol was repeated later with 0.001667 micromol [5-14CH3]all-rac-alpha-tocopheryl acetate (with 99.98 nCi 14C).

RESULTS

RRR-alpha-tocopheryl acetate and all-rac-alpha-tocopheryl acetate were absorbed equally well (fractional absorption: approximately 0.775). The main route of elimination was urine, and approximately 90% of the absorbed dose was alpha-2(2'-carboxyethyl)-6-hydroxychroman. Whereas 93.8% of RRR-alpha-tocopherol flow to liver kinetic pool B from plasma was returned to plasma, only 80% of the flow of all-rac-alpha-tocopherol returned to plasma; the difference (14%) was degraded and eliminated. Thus, for newly digested alpha-tocopherol, the all-rac form is preferentially degraded and eliminated over the RRR form. Respective residence times in liver kinetic pool A and plasma for RRR-alpha-tocopherol were 1.16 and 2.19 times as long as those for all-rac-alpha-tocopherol. Model-estimated distributions of plasma alpha-tocopherol, extrahepatic tissue alpha-tocopherol, and liver kinetic pool B for RRR-alpha-tocopherol were, respectively, 6.77, 2.71, and 3.91 times as great as those for all-rac-alpha-tocopherol. Of the lipoproteins, HDL had the lowest 14C enrichment. Liver had 2 kinetically distinct alpha-tocopherol pools.

CONCLUSIONS

Both isomers were well absorbed; all-rac-alpha-tocopherol was preferentially degraded and eliminated in urine, the major route. RRR-alpha-tocopherol had a longer residence time and larger distribution than did all-rac-alpha-tocopherol. Liver had 2 distinct alpha-tocopherol pools. The model is a hypothesis, its estimates are model-dependent, and it encourages further testing.

Antagonistic Effects of Oxidized Low Density Lipoprotein and α-Tocopherol on CD36 Scavenger Receptor Expression in Monocytes

Vitamin E deficiency increases expression of the CD36 scavenger receptor, suggesting specific molecular mechanisms and signaling pathways modulated by alpha-tocopherol. We show here that alpha-tocopherol down-regulated CD36 expression (mRNA and protein) in oxidized low density lipoprotein (oxLDL)-stimulated THP-1 monocytes, but not in unstimulated cells. Furthermore, alpha-tocopherol treatment of monocytes led to reduction of fluorescent oxLDL-3,3'-dioctadecyloxacarbocyanine perchlorate binding and uptake. Protein kinase C (PKC) appears not to be involved because neither activation of PKC by phorbol 12-myristate 13-acetate nor inhibition by PKC412 was affected by alpha-tocopherol. However, alpha-tocopherol could partially prevent CD36 induction after stimulation with a specific agonist of peroxisome proliferator-activated receptor-gamma (PPARgamma; troglitazone), indicating that this pathway is susceptible to alpha-tocopherol action. Phosphorylation of protein kinase B (PKB) at Ser473 was increased by oxLDL, and alpha-tocopherol could prevent this event. Expression of PKB stimulated the CD36 promoter as well as a PPARgamma element-driven reporter gene, whereas an inactive PKB mutant had no effect. Moreover, coexpression of PPARgamma and PKB led to additive induction of CD36 expression. Altogether, our results support the existence of PKB/PPARgamma signaling pathways that mediate CD36 expression in response to oxLDL. The activation of CD36 expression by PKB suggests that both lipid biosynthesis and fatty acid uptake are stimulated by PKB.

Regulatory effects of synthetic liver X receptor- and peroxisome-proliferator activated receptor agonists on sterol transport pathways in polarized cerebrovascular endothelial cells

The blood-brain barrier contributes to maintain brain cholesterol metabolism and protects this uniquely balanced system from exchange with plasma lipoprotein cholesterol. Brain capillary endothelial cells, representing a physiological barrier to the central nervous system, express apolipoprotein A-I (apoA-I, the major high-density lipoprotein (HDL)-associated apolipoprotein), ATP-binding cassette transporter A1 (ABCA1), and scavenger receptor, class B, type I (SR-BI), proteins that promote cellular cholesterol mobilization. Liver X receptors (LXRs) and peroxisome-proliferator activated receptors (PPARs) are regulators of cholesterol transport, and activation of LXRs and PPARs has potential therapeutic implications for lipid-related neurodegenerative diseases. To clarify the functional impact of LXR/PPAR activation, sterol transport along the: (i) ABCA1/apoA-I and (ii) SR-BI/HDL pathway was investigated in primary, polarized brain capillary endothelial cells, an in vitro model of the blood-brain barrier. Activation of LXR (24(S)OH-cholesterol, TO901317), PPARalpha (bezafibrate, fenofibrate), and PPARgamma (troglitazone, pioglitazone) modulated expression of apoA-I, ABCA1, and SR-BI on mRNA and/or protein levels without compromising transendothelial electrical resistance or tight junction protein expression. LXR-agonists and troglitazone enhanced basolateral-to-apical cholesterol mobilization in the absence of exogenous sterol acceptors. Along with the induction of cell surface-located ABCA1, several agonists enhanced cholesterol mobilization in the presence of exogenous apoA-I, while efflux of 24(S)OH-cholesterol (the major brain cholesterol metabolite) in the presence of exogenous HDL remained unaffected. Summarizing, in cerebrovascular endothelial cells apoA-I, ABCA1, and SR-BI represent drug targets for LXR and PPAR-agonists to interfere with cholesterol homeostasis at the periphery of the central nervous system.

Intracellular trafficking of vitamin E in hepatocytes: the role of tocopherol transfer protein

The term vitamin E denotes a family of tocopherols and tocotrienols, plant lipids that are essential for vertebrate fertility and health. The principal form of vitamin E found in humans, RRR-alpha-tocopherol (TOH), is thought to protect cells by virtue of its ability to quench free radicals, and functions as the main lipid-soluble antioxidant. Regulation of vitamin E homeostasis occurs in the liver, where TOH is selectively retained while other forms of vitamin E are degraded. Through the action of tocopherol transfer protein (TTP), TOH is then secreted from the liver into circulating lipoproteins that deliver the vitamin to target tissues. Presently, very little is known regarding the intracellular transport of vitamin E. We utilized biochemical, pharmacological, and microscopic approaches to study this process in cultured hepatocytes. We observe that tocopherol-HDL complexes are efficiently internalized through scavenger receptor class B type I. Once internalized, tocopherol arrives within approximately 30 min at intracellular vesicular organelles, where it co-localizes with TTP, and with a marker of the lysosomal compartment (LAMP1), before being transported to the plasma membrane in a TTP-dependent manner. We further show that intracellular processing of tocopherol involves a functional interaction between TTP and an ABC-type transporter.

Re-evaluation of the relative potency of synthetic and natural α-tocopherol: experimental and clinical observations

Nutritionists generally consider all-rac-alpha-tocopherol and RRR-alpha-tocopherol equivalent in vitamin E activity but disagree whether equivalency requires a dosage ratio of 1.36:1 or 2:1. In contrast, we hypothesize that all-rac- and RRR-alpha-tocopherols are not equivalent in any dosage ratio. Previous observations that all-rac- and RRR-alpha-tocopherols are distributed and eliminated via saturable and stereospecific pathways imply that their relative bioavailability varies with the saturation of these pathways and therefore varies with dosage. Indeed, previous studies observed that the relative bioavailability of all-rac- and RRR-alpha-tocopherols varies between tissues as well as with dose, time after dosing, and duration of dosing. This non-constant relative bioavailability predicts non-constant relative activity (i.e., non-parallel dose-concentration curves predict non-parallel dose-effect curves). Non-constant relative bioavailability suggests that a fixed dosage ratio of all-rac- and RRR-alpha-tocopherols cannot produce a fixed ratio of effects on all processes in all tissues at all times after all dosages. However, previous studies suggest that all-rac- and RRR-alpha-tocopherols have equivalent effects (parallel dose-effect curves) in vitamin E-deficient animals and non-vitamin E-deficient humans. We re-evaluate the data from these animal studies and find non-parallel dose-effect and concentration-effect curves. We discuss pharmacokinetic and pharmacodynamic reasons why previous studies in non-vitamin E-deficient humans did not find non-parallel dose-effect curves for all-rac- and RRR-alpha-tocopherols. We note that saturable elimination predicts that all-rac- and RRR-alpha-tocopherols might inhibit and/or induce elimination of other compounds (including 30-40% of prescription drugs) eliminated via the same saturable pathways, and stereospecific elimination predicts that all-rac- and RRR-alpha-tocopherol have non-parallel dose-effect curves for these interactions.

Cellular mechanisms of vitamin E uptake: relevance in α-tocopherol metabolism and potential implications for disease

alpha-Tocopherol is an essential micronutrient involved in various oxidative stress-related processes. Because of its hydrophobic nature, alpha-tocopherol is transported in plasma lipoproteins, and the pathways involved in its cellular uptake are closely related to the lipoprotein metabolism. alpha-Tocopherol transfer from plasma to cells can occur by different mechanisms such as uptake facilitated by lipid transfer proteins and lipases, receptor-mediated lipoprotein endocytosis, and selective lipid uptake. Here we discuss recent progress in understanding the physiological and pathophysiological relevance of these different pathways for cellular uptake of vitamin E in vivo. This review is mainly focused on the role of the scavenger receptor class B type I (SR-BI) on alpha-tocopherol metabolism and its potential implications for disease conditions.

Uptake and transport of high-density lipoprotein (HDL) and HDL-associated alpha-tocopherol by an in vitro blood-brain barrier model

The present study aimed to investigate pathways that contribute to uptake and transcytosis of high-density lipoproteins (HDLs) and HDL-associated alpha-tocopherol (alpha TocH) across an in vitro model of the blood-brain barrier (BBB). In primary porcine brain capillary endothelial cells HDL-associated alpha TocH was taken up in 10-fold excess of HDL holoparticles, indicating efficient selective uptake, a pathway mediated by scavenger receptor class B, type I (SR-BI). SR-BI was present in caveolae of brain capillary endothelial cells and expressed almost exclusively at the apical membrane. Disruption of caveolae with methyl-beta-cyclodextrin (CDX) resulted in (mis)sorting of SR-BI to the basolateral membrane. Immunohistochemistry of porcine brain cryosections revealed SR-BI expression on brain capillary endothelial cells and presumably astrocytic endfeet. HDL-associated [(14)C]alpha TocH taken up by brain capillary endothelial cells was recovered in sucrose gradient fractions containing the majority of cellular caveolin-1, the major caveolae-associated protein. During mass transfer studies using alpha TocH-enriched HDL, approximately 50% of cellular alpha TocH was recovered with the bulk of cellular caveolin-1 and SR-BI. Efflux experiments revealed that a substantial amount of cell-associated [(14)C]alpha TocH could be mobilized into the culture medium. In addition, apical-to-basolateral transport of HDL holoparticles and HDL-associated alpha TocH was saturable. Results from the present study suggest that part of cerebral apolipoprotein A-I and alpha TocH originates from plasma HDL transcytosed across the BBB and that caveolae-located SR-BI facilitates selective uptake of HDL-associated alpha TocH at the BBB.

The role of the high-density lipoprotein receptor SR-BI in the lipid metabolism of endocrine and other tissues

Because cholesterol is a precursor for the synthesis of steroid hormones, steroidogenic tissues have evolved multiple pathways to ensure adequate supplies of cholesterol. These include synthesis, storage as cholesteryl esters, and import from lipoproteins. In addition to endocytosis via members of the low-density lipoprotein receptor superfamily, steroidogenic cells acquire cholesterol from lipoproteins by selective lipid uptake. This pathway, which does not involve lysosomal degradation of the lipoprotein, is mediated by the scavenger receptor class B type I (SR-BI). SR-BI is highly expressed in steroidogenic cells, where its expression is regulated by various trophic hormones, as well as in the liver. Studies of genetically manipulated strains of mice have established that SR-BI plays a key role in regulating lipoprotein metabolism and cholesterol transport to steroidogenic tissues and to the liver for biliary secretion. In addition, analysis of SR-BI-deficient mice has shown that SR-BI expression is important for alpha-tocopherol and nitric oxide metabolism, as well as normal red blood cell maturation and female fertility. These mouse models have also revealed that SR-BI can protect against atherosclerosis. If SR-BI plays similar physiological and pathophysiological roles in humans, it may be an attractive target for therapeutic intervention in cardiovascular and reproductive diseases.

ABCA1 and scavenger receptor class B, type I, are modulators of reverse sterol transport at an in vitro blood-brain barrier constituted of porcine brain capillary endothelial cells

The objective of the present study was to investigate the involvement of key players in reverse cholesterol/24(S)OH-cholesterol transport in primary porcine brain capillary endothelial cells (pBCEC) that constitute the BBB. We identified that, in addition to scavenger receptor class B, type I (SR-BI), pBCEC express ABCA1 and apolipoprotein A-I (apoA-I) mRNA and protein. Studies on the regulation of ABCA1 by the liver X receptor agonist 24(S)OH-cholesterol revealed increased ABCA1 expression and apoA-I-dependent [3H]cholesterol efflux from pBCEC. In unpolarized pBCEC, high density lipoprotein, subclass 3 (HDL3)-dependent [3H]cholesterol efflux, was unaffected by 24(S)OH-cholesterol treatment but was enhanced 5-fold in SR-BI overexpressing pBCEC. Efflux of cellular 24(S)-[3H]OH-cholesterol was highly efficient, independent of ABCA1, and correlated with SR-BI expression. Polarized pBCEC were cultured on porous membrane filters that allow separate access to the apical and the basolateral compartment. Addition of cholesterol acceptors to the apical compartment resulted in preferential [3H]cholesterol efflux to the basolateral compartment. HDL3 was a better promoter of basolateral [3H]cholesterol efflux than lipid-free apoA-I. Basolateral pretreatment with 24(S)OH-cholesterol enhanced apoA-I-dependent basolateral cholesterol efflux up to 2-fold along with the induction of ABCA1 at the basolateral membrane. Secretion of apoA-I also occurred preferentially to the basolateral compartment, where the majority of apoA-I was recovered in an HDL-like density range. In contrast, 24(S)-[3H]OH-cholesterol was mobilized efficiently to the apical compartment of the in vitro BBB by HDL3, low density lipoprotein, and serum. These results suggest the existence of an autoregulatory mechanism for removal of potentially neurotoxic 24(S)OH-cholesterol. In conclusion, the apoA-I/ABCA1- and HDL/SR-BI-dependent pathways modulate polarized sterol mobilization at the BBB.

Effects of lipoprotein lipase on uptake and transcytosis of low density lipoprotein (LDL) and LDL-associated alpha-tocopherol in a porcine in vitro blood-brain barrier model

During the present study the contribution of lipoprotein lipase (LPL) to low density lipoprotein (LDL) holoparticle and LDL-lipid (alpha-tocopherol (alphaTocH)) turnover in primary porcine brain capillary endothelial cells (BCECs) was investigated. The addition of increasing LPL concentrations to BCECs resulted in up to 11-fold higher LDL holoparticle cell association. LPL contributed to LDL holoparticle turnover, an effect that was substantially increased in response to LDL-receptor up-regulation. The addition of LPL increased selective uptake of LDL-associated alphaTocH in BCECs up to 5-fold. LPL-dependent selective alphaTocH uptake was unaffected by the lipase inhibitor tetrahydrolipstatin but was substantially inhibited in cells where proteoglycan sulfation was inhibited by treatment with NaClO(3). Thus, selective uptake of LDL-associated alphaTocH requires interaction of LPL with heparan-sulfate proteoglycans. Although high level adenoviral overexpression of scavenger receptor BI (SR-BI) in BCECs resulted in a 2-fold increase of selective LDL-alphaTocH uptake, SR-BI did not act in a cooperative manner with LPL. Although the addition of LPL to BCEC Transwell cultures significantly increased LDL holoparticle cell association and selective uptake of LDL-associated alphaTocH, holoparticle transcytosis across this porcine blood-brain barrier (BBB) model was unaffected by the presence of LPL. An important observation during transcytosis experiments was a substantial alphaTocH depletion of LDL particles that were resecreted into the basolateral compartment. The relevance of LPL-dependent alphaTocH uptake across the BBB was confirmed in LPL-deficient mice. The absence of LPL resulted in significantly lower cerebral alphaTocH concentrations than observed in control animals.

Functional analysis of histamine receptor subtypes involved in endothelium-mediated relaxation of the human uterine artery

1. This work was designed to introduce human uterine arteries as a new model for cardiovascular research. Advantages of this model include considerable availability of tissue because of the appearance of uterus myomatosus in post-menopausal women who undergo surgery and the chance to work on dysfunctional and healthy vessels. 2. Histamine evoked relaxation of the uterine artery that was prevented by removal of the endothelium or by the presence of N(G)-nitro-L-arginine. 3. Receptor antagonists for histamine H(1) (mepyramine) and H(2) (ranitidine) receptors increased the EC(50) of histamine by 112- and 67-fold, respectively. 4. Remarkably, isolated uterine arteries could be stored in incubators for 5 days without any change in contractility to phenylephrine and endothelium-dependent relaxation to acetylcholine and histamine. 5. Endothelial cells could be isolated and cultured in high purity, as demonstrated by histochemical staining of factor VIII, low CD45-RO for macrophages and no smooth muscle alpha-actin. In addition, cultured human uterine artery endothelial cells could be used for single cell Ca(2+) measurements. 6. In agreement with our findings in the intact vessel, histamine-initiated elevation of the intracellular free Ca(2+) concentration was reduced in the presence of mepyramine and ranitidine by 59 and 55%, respectively. 7. These data indicate that, in the human uterine artery, H(1) and H(2) receptors are involved in histamine-induced endothelium-dependent relaxation that is mediated by nitric oxide. 8. In addition, this vessel can be stored for possible virus-mediated gene expression for 5 days without any loss of reagibility. 9. Finally, endothelial cells can be isolated and cultured from the human uterine artery and maintain their reactivity to histamine in culture.

Alpha-tocopherol metabolism is abnormal in scavenger receptor class B type I (SR-BI)-deficient mice

Despite the physiologic importance of vitamin E, in particular its alpha-tocopherol (alpha-T) isoform, the molecular mechanisms involved in the cellular uptake of this antioxidant from plasma lipoproteins have not been well-defined. Recent studies have suggested that selective lipid uptake, rather than endocytosis, is important for alpha-T delivery to cells. Here we show that the scavenger receptor class B type I (SR-BI), which mediates cellular selective cholesteryl ester uptake from lipoproteins, facilitates efficient transfer of alpha-T from HDL to cultured cells. In SR-BI-deficient mutant mice, relative to wild-type control animals, there was a significant increase in plasma alpha-T levels (1.1- to 1.4-fold higher) that was mostly due to the elevated alpha-T content of their abnormally large plasma HDL-like particles. This increase in plasma alpha-T in SR-BI knockout mice was accompanied by a significant decrease (65-80%) in the alpha-T concentrations in bile and several tissues including ovary, testis, lung and brain. SR-BI deficiency did not alter the alpha-T concentrations of the liver, spleen, kidney or white fat. These data show that SR-BI plays an important role in transferring alpha-T from plasma lipoproteins to specific tissues. Also, in the case of the liver as was previously shown for SR-BI-dependent hepatic cholesterol transport, SR-BI-mediated uptake of alpha-T was primarily coupled to biliary excretion rather than to tissue accumulation. Defective tissue uptake of lipoprotein alpha-T in SR-BI-deficient mice may contribute to the reproductive and cardiovascular pathologies exhibited by these animals.

Modulation of microglial superoxide production by alpha-tocopherol in vitro: attenuation of p67(phox) translocation by a protein phosphatase-dependent pathway

As in other phagocytic cells, the NADPH-oxidase system in microglia is thought to be primarily responsible for the production of superoxide anion radicals (O2(-.), a potentially cytotoxic reactive oxygen species. The assembly of a functional NADPH-oxidase complex at the plasma membrane depends on the phosphorylation and subsequent translocation of several cytosolic subunits. Immunocytochemical and subcellular fractionation experiments performed during the present study revealed that the NADPH-oxidase subunit p67(phox) translocates from the cytosol to the plasma membrane upon stimulation. Pre-incubation of microglia in alpha-tocopherol (alphaTocH) containing medium decreased O2(-.) production in a time- and concentration-dependent manner, findings attributed to attenuated p67(phox) translocation to the plasma membrane. Moreover, alphaTocH-supplementation of the culture medium resulted in decreased microglial protein kinase C (PKC) activities, an effect that could be partially or completely reversed by the addition of protein phosphatase inhibitors (okadaic acid and calyculin A). The addition of the PKC-inhibitor staurosporine inhibited the microglial respiratory burst in a manner comparable to alphaTocH. The addition of okadaic acid or calyculin A completely restored O2(-.) production in alphaTocH-supplemented cells. The present findings suggest that alphaTocH inactivates PKC via a PP1 or PP2A-mediated pathway and, as a consequence, blocks the phosphorylation-dependent translocation of p67(phox) to the plasma membrane. As a result, O2(-.) production by the microglial NADPH-oxidase system is substantially inhibited.

Vitamin E kinetics and the function of tocopherol regulatory proteins

Plasma and tissue alpha-tocopherol concentrations are remarkably stable, which suggests that they are regulated. alpha-Tocopherol transfer protein, tocopherol-associated protein, and tocopherol-binding protein bind alpha-tocopherol. These proteins might function as tocopherol regulatory proteins, although only tocopherol transfer protein has been shown to influence plasma and tissue alpha-tocopherol concentrations. Tissue alpha-tocopherol concentrations likely depend on tocopherol regulatory protein function and tissue lipid content, vitamin E uptake and efflux, oxidative stress, and interactions between vitamin E and other antioxidants. Pharmacokinetic models often divide tissues into rapidly perfused, slowly perfused, and very slowly perfused compartments. Tissue vitamin E concentrations might equilibrate more rapidly in tissues with greater perfusion, greater vitamin E uptake, increased amounts or activities of tocopherol regulatory protein, and lower lipid contents. The rate at which tissue concentrations approach equilibrium, however, does not predict the final equilibrium concentrations because of redistribution among tissues. Redistribution of vitamin E to adipose tissue from other tissues may be significant. Intracellular trafficking of vitamin E might occur in conjunction with membrane recycling because membrane constituents rapidly recycle between the plasma membrane and intracellular endocytic compartments. Thus, tocopherol regulatory proteins may modulate rather than directly regulate vitamin E tissue distribution and intracellular trafficking.

Scavenger receptor class B, type I is expressed in porcine brain capillary endothelial cells and contributes to selective uptake of HDL-associated vitamin E

It is clearly established that an efficient supply to the brain of alpha-tocopherol (alphaTocH), the most biologically active member of the vitamin E family, is of the utmost importance for proper neurological functioning. Although the mechanism of uptake of alphaTocH into cells constituting the blood-brain barrier (BBB) is obscure, we previously demonstrated that high-density lipoprotein (HDL) plays a major role in the supply of alphaTocH to porcine brain capillary endothelial cells (pBCECs). Here we studied whether a porcine analogue of human and rodent scavenger receptor class B, type I mediates selective (without concomitant lipoprotein particle internalization) uptake of HDL-associated alphaTocH in a similar manner to that described for HDL-associated cholesteryl esters (CEs). In agreement with this hypothesis we observed that a major proportion of alphaTocH uptake by pBCECs occurred by selective uptake, exceeding HDL3 holoparticle uptake by up to 13-fold. The observation that selective uptake of HDL-associated CE exceeded HDL3 holoparticle up to fourfold suggested that a porcine analogue of SR-BI (pSR-BI) may be involved in lipid uptake at the BBB. In line with the observation of selective lipid uptake, RT-PCR and northern and western blot analyses revealed the presence of pSR-BI in cells constituting the BBB. Adenovirus-mediated overexpression of the human analogue of SR-BI (hSR-BI) in pBCECs resulted in a fourfold increase in selective HDL-associated alphaTocH uptake. In accordance with the proposed function of SR-BI, selective HDL-CE uptake was increased sixfold in Chinese hamster ovary cells stably transfected with murine SR-BI (mSR-BI). Most importantly stable mSR-BI overexpression mediated a twofold increase in HDL-associated [14C]alphaTocH selective uptake in comparison with control cells. In line with tracer experiments, mass transfer studies with unlabelled lipoproteins revealed that mSR-BI overexpression resulted in a twofold increase in endogenous HDL3-associated alphaTocH uptake. The results of this study indicate that SR-BI promotes the uptake of HDL-associated alphaTocH into cells constituting the BBB and plays an important role during the supply of the CNS with this indispensable micronutrient.