ATP-binding cassette transporter A1 mediates cellular secretion of alpha-tocopherol

Polymorphisms of mouse apolipoprotein A-II alter its physical and functional nature

ApoA-II is the second most abundant protein on HDL making up ∼ 20% of the total protein but its functions have still only been partially characterized. Recent methodological improvements have allowed for the recombinant expression and characterization of human apoA-II which shares only 55% sequence homology with murine apoA-II. Here we describe the purification of the two most common polymorphic variants of apoA-II found in inbred mouse strains, differing at 3 amino acid sites. C57BL/6 mice having variant apoA-II(a) have lower plasma HDL levels than FVB/N mice that have variant apoA-II(b). Characterization of the helical structure of these two variants reveals a more alpha-helical structure for the FVB/N apoA-II. These changes do not alter the lipid or HDL binding of the two apoA-II variants, but significantly increase the ability of the FVB/N variant to promote both ABCA1 and ABCG1 mediated cellular cholesterol efflux. These differences may be differentially altering plasma HDL apoA-II levels. In vivo, neither C57 nor FVB apoA-II protein levels are affected by the absence of apoE, while an apoE/apoA-I double deficiency results in a 50% decrease of plasma FVB apoA-II but results in undetectable levels of C57 apoA-II in the plasma. FVB apoA-II is able to form an HDL particle in the absence of apoE or apoA-I.

A question of balance: achieving appropriate nutrient levels in biofortified staple crops

The biofortification of staple crops with vitamins is an attractive strategy to increase the nutritional quality of human food, particularly in areas where the population subsists on a cereal-based diet. Unlike other approaches, biofortification is sustainable and does not require anything more than a standard food-distribution infrastructure. The health-promoting effects of vitamins depend on overall intake and bioavailability, the latter influenced by food processing, absorption efficiency and the utilisation or retention of the vitamin in the body. The bioavailability of vitamins in nutritionally enriched foods should ideally be adjusted to achieve the dietary reference intake in a reasonable portion. Current vitamin biofortification programmes focus on the fat-soluble vitamins A and E, and the water-soluble vitamins C and B9 (folate), but the control of dosage and bioavailability has been largely overlooked. In the present review, we discuss the vitamin content of nutritionally enhanced foods developed by conventional breeding and genetic engineering, focusing on dosage and bioavailability. Although the biofortification of staple crops could potentially address micronutrient deficiency on a global scale, further research is required to develop effective strategies that match the bioavailability of vitamins to the requirements of the human diet.

Vitamin E secretion by Caco-2 monolayers to APOA1, but not to HDL, is vitamer selective

The aim of this study was to characterize the pathways of basolateral secretion of common dietary tocopherols from polarized Caco-2 monolayers, a model of intestinal absorption. Given differences in structure and physical properties, we hypothesized that secretion may differ between different forms of vitamin E, thus potentially contribute to the selectivity seen in vivo. Monolayers were incubated apically and simultaneously with 10 μmol/L α-, γ-, and δ-tocopherol (1:1:1) in lipid micelles. Treatment with the microsomal triglyceride transfer protein inhibitor BMS201038 revealed that the triglyceride-rich particle secretory pathway (apolipoprotein B-dependent pathway) accounted for ~ 80% of total tocopherol secretion, without selectivity among the three forms of vitamin E. Apolipoprotein B-independent secretion of tocopherols (and cholesterol) was greatly enhanced by the liver X receptor agonist T0901317. T0901317 induced ATP-binding cassette transporter A1 (ABCA1) protein expression and basolateral secretion of tocopherols to apolipoprotein A1. ABCA1-dependent secretion demonstrated vitamer selectivity such that efficiency of secretion of α- and γ-tocopherols exceeded that of δ-tocopherol. Basal addition of HDL stimulated vitamin E secretion but without selectivity among the three forms, whereas LDL had no effect. Basal addition of scavenger receptor class B member I (SR-BI) blocking antibody, which inhibits the interaction between SR-BI and HDL, increased basal accumulation of all tocopherols, demonstrating a role for SR-BI in cellular re-uptake of secreted vitamin E. These findings demonstrated that vitamin E and cholesterol utilize common pathways of secretion and that secretion via the ABCA1 pathway favors certain forms of vitamin E.

Mechanisms for the prevention of vitamin E excess

The liver is at the nexus of the regulation of lipoprotein uptake, synthesis, and secretion, and it is the site of xenobiotic detoxification by cytochrome P450 oxidation systems (phase I), conjugation systems (phase II), and transporters (phase III). These two major liver systems control vitamin E status. The mechanisms for the preference for α-tocopherol relative to the eight naturally occurring vitamin E forms largely depend upon the liver and include both a preferential secretion of α-tocopherol from the liver into the plasma for its transport in circulating lipoproteins for subsequent uptake by tissues, as well as the preferential hepatic metabolism of non-α-tocopherol forms. These mechanisms are the focus of this review.

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.

Bioavailability of vitamin E in humans: an update

Vitamin E is essential for human health and may play a role in the prevention of some degenerative diseases. Its bioavailability, however, is wide ranging and is affected by numerous factors. Recent findings showing that the intestinal absorption of vitamin E involves proteins have raised new relevant questions about factors that can affect bioavailability. It is, therefore, opportune to present a current overview of this topic. This review begins by exploring what is known, as well as what is unknown, about the metabolization of vitamin E in the human upper gastrointestinal tract and then presents a methodical evaluation of factors assumed to affect vitamin E bioavailability. Three main conclusions can be drawn. First, the proteins ABCA1, NPC1L1, and SR-BI are implicated in the absorption of vitamin E. Second, the efficiency of vitamin E absorption is widely variable, though not accurately known (i.e., between 10% and 79%), and is affected by several dietary factors (e.g., food matrix, fat, and fat-soluble micronutrients). Finally, numerous unanswered questions remain about the metabolization of vitamin E in the intestinal lumen and about the factors affecting the efficiency of vitamin E absorption.

Factors controlling nascent high-density lipoprotein particle heterogeneity: ATP-binding cassette transporter A1 activity and cell lipid and apolipoprotein AI availability

Nascent high-density lipoprotein (HDL) particles arise in different sizes. We have sought to uncover factors that control this size heterogeneity. Gel filtration, native PAGE, and protein cross-linking were used to analyze the size heterogeneity of nascent HDL produced by BHK-ABCA1, RAW 264.7, J774, and HepG2 cells under different levels of two factors considered as a ratio, the availability of apolipoprotein AI (apoAI) -accessible cell lipid, and concentration of extracellular lipid-free apoAI. Increases in the available cell lipid:apoAI ratio due to either elevated ATP-binding cassette transporter A1 (ABCA1) expression and activity or raised cell density (i.e., increasing numerator) shifted the production of nascent HDL from smaller particles with fewer apoAI molecules per particle and fewer molecules of choline-phospholipid and cholesterol per apoAI molecule to larger particles that contained more apoAI and more lipid per molecule of apoAI. A further shift to larger particles was observed in BHK-ABCA1 cells when the available cell lipid:apoAI ratio was raised still higher by decreasing the apoAI concentration (i.e., the denominator). These changes in nascent HDL biogenesis were reminiscent of the transition that occurs in the size composition of reconstituted HDL in response to an increasing initial lipid:apoAI molar ratio. Thus, the ratio of available cell lipid:apoAI is a fundamental cause of nascent HDL size heterogeneity, and rHDL formation is a good model of nascent HDL biogenesis.—Lyssenko, N. N., Nickel, M., Tang, C., Phillips, M. C. Factors controlling nascent high-density lipoprotein particle heterogeneity: ATP-binding cassette transporter A1 activity and cell lipid and apolipoprotein AI availability.

γ-Tocotrienol does not substantially protect DS neurons from hydrogen peroxide-induced oxidative injury

Background

Down syndrome (DS) neurons are more susceptible to oxidative stress and previous studies have shown that vitamin E was able to reduce oxidative stress and improve DS neurons' viability. Therefore, this study was done to investigate the protective role of γ-tocotrienol (γT3) in DS neurons from hydrogen peroxide (H₂O₂) -induced oxidative stress. The pro-apoptosis tendency of γT3 was compared to α-tocopherol (αT) in non-stress condition as well.

Methods

Primary culture of DS and euploid neurons were divided into six groups of treatment: control, H₂O₂, γT3 pre-treatment with H₂O₂, γT3 only, αT pre-treatment with H₂O₂ and αT only. The treatments were assessed by MTS assay and apoptosis assay by single-stranded DNA (ssDNA) apoptosis ELISA assay, Hoechst and Neu-N immunofluorescence staining. The cellular uptake of γT3 and αT was determined by HPLC while protein expressions were determined by Western blot. Comparison between groups was made by the Student's t test, one-way ANOVA and Bonferroni adjustment as well as two-way ANOVA for multiple comparisons.

Results

One day incubation of γT3 was able to reduced apoptosis of DS neurons by 10%, however γT3 was cytotoxic at longer incubation period (14 days) and at concentrations ≥ 100 μM. Pre-treatment of αT and γT3 only attenuate apoptosis and increase cell viability in H₂O₂-treated DS and euploid neurons by 10% in which the effects were minimal to maintain most of the DS cells' morphology. γT3 act as a free radical scavenger by reducing ROS generated by H₂O₂. In untreated controls, DS neurons showed lower Bcl-2/Bax ratio and p53 expression compared to normal neurons, while cPKC and PKC-δ expressions were higher in DS neurons. On the other hand, pre-treatment of γT3 in H₂O₂-treated DS neurons have reduced Bcl-2/Bax ratio, which was not shown in euploid neurons. This suggests that pre-treatment of γT3 did not promote DS cell survival. Meanwhile γT3 and αT treatments without H₂O₂ as well as pre-treatment of γT3 and αT induced changes in cPKC and PKC-δ expression in DS neurons suggesting interaction of γT3 and αT with PKC activity.

Conclusion

Our study suggests that γT3 pre-treatment are not sufficient to protect DS neurons from H₂O₂-induced oxidative assault, instead induced the apoptosis process.

Membrane microdomains modulate oligomeric ABCA1 function: impact on apoAI-mediated lipid removal and phosphatidylcholine biosynthesis

Recent studies have identified an ABCA1-dependent, phosphatidylcholine-rich microdomain, called the "high-capacity binding site" (HCBS), that binds apoA-I and plays a pivotal role in apoA-I lipidation. Here, using sucrose gradient fractionation, we obtained evidence that both ABCA1 and [¹²⁵I]apoA-I associated with the HCBS were found localized to nonraft microdomains. Interestingly, phosphatidylcholine (PtdCho) was selectively removed from nonraft domains by apoA-I, whereas sphingomyelin and cholesterol were desorbed from both detergent-resistant membranes and nonraft domains. The modulatory role of cholesterol on apoA-I binding to ABCA1/HCBS was also examined. Loading cells with cholesterol resulted in a drastic reduction in apoA-I binding. Conversely, depletion of membrane cholesterol by methyl-β-cyclodextrin treatment resulted in a significant increase in apoA-I binding. Finally, we obtained evidence that apoA-I interaction with ABCA1 promoted the activation and gene expression of key enzymes in the PtdCho biosynthesis pathway. Taken together, these results provide strong evidence that the partitioning of ABCA1/HCBS to nonraft domains plays a pivotal role in the selective desorption of PtdCho molecules by apoA-I, allowing an optimal environment for cholesterol release and regeneration of the PtdCho-containing HCBS. This process may have important implications in preventing and treating atherosclerotic cardiovascular disease.

Influence of C-terminal α-helix hydrophobicity and aromatic amino acid content on apolipoprotein A-I functionality

The apoA-I molecule adopts a two-domain tertiary structure and the properties of these domains modulate the ability to form HDL particles. Thus, human apoA-I differs from mouse apoA-I in that it can form smaller HDL particles; the C-terminal α-helix is important in this process and human apoA-I is unusual in containing aromatic amino acids in the non-polar face of this amphipathic α-helix. To understand the influence of these aromatic amino acids and the associated high hydrophobicity, apoA-I variants were engineered in which aliphatic amino acids were substituted with or without causing a decrease in overall hydrophobicity. The variants human apoA-I (F225L/F229A/Y236A) and apoA-I (F225L/F229L/A232L/Y236L) were compared to wild-type (WT) apoA-I for their abilities to (1) solubilize phospholipid vesicles and form HDL particles of different sizes, and (2) mediate cellular cholesterol efflux and create nascent HDL particles via ABCA1. The loss of aromatic residues and concomitant decrease in hydrophobicity in apoA-I (F225L/F229A/Y236A) has no effect on protein stability, but reduces by a factor of about three the catalytic efficiencies (V(max)/K(m)) of vesicle solubilization and cholesterol efflux; also, relatively large HDL particles are formed. With apoA-I (F225L/F229L/A232L/Y236L) where the hydrophobicity is restored by the presence of only leucine residues in the helix non-polar face, the catalytic efficiencies of vesicle solubilization and cholesterol efflux are similar to those of WT apoA-I; this variant forms smaller HDL particles. Overall, the results show that the hydrophobicity of the non-polar face of the C-terminal amphipathic α-helix plays a critical role in determining apoA-I functionality but aromatic amino acids are not required. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Quantitation of [5-14CH3]-(2R, 4'R, 8'R)-α-tocopherol in humans

Half-lives of α-tocopherol in plasma have been reported as 2-3 d, whereas the Elgin Study required >2 y to deplete α-tocopherol, so gaps exist in our quantitative understanding of human α-tocopherol metabolism. Therefore, 6 men and 6 women aged 27 ± 6 y (mean ± SD) ingested 1.81 nmol, 3.70 kBq of [5-(14)CH(3)]-(2R, 4'R, 8'R)-α-tocopherol. The levels of (14)C in blood plasma and washed RBC were monitored frequently from 0 to 460 d while the levels of (14)C in urine and feces were monitored from 0 to 21 d. Total fecal elimination (fecal + metabolic fecal) was 23.24 ± 5.81% of the (14)C dose, so feces over urine was the major route of elimination of the ingested [5-(14)CH(3)]-(2R, 4'R, 8'R)-α-tocopherol, consistent with prior estimates. The half-life of α-tocopherol varied in plasma and RBC according to the duration of study. The minute dose coupled with frequent monitoring over 460 d and 21 d for blood, urine, and feces ensured the [5-(14)CH(3)]-(2R, 4'R, 8'R)-α-tocopherol (the tracer) had the chance to fully mix with the endogenous [5-(14)CH(3)]-(2R, 4'R, 8'R)-α-tocopherol (the tracee). The (14)C levels in neither plasma nor RBC had returned to baseline by d 460, indicating that the t(1/2) of [5-CH(3)]-(2R, 4'R, 8'R)-α-tocopherol in human blood was longer than prior estimates.

Genetic variants in BCMO1 and CD36 are associated with plasma lutein concentrations and macular pigment optical density in humans

Lutein is recovered at high concentration in the human macula lutea. Recent studies suggest that this micronutrient might be implicated in prevention of age-related macular degeneration.

OBJECTIVE

to identify genes which affect blood and retina lutein concentrations among candidate genes (intestinal sterol transporters and carotenoid oxygenases).

DESIGN

a comparative plus an observational study.

PARTICIPANTS

twenty-nine healthy subjects for the comparative study and 622 subjects for the observational study.

INTERVENTION AND METHODS

all the participants were genotyped for single nucleotide polymorphisms (SNPs) in the candidate genes. Fasting plasma lutein concentrations were measured in all the participants and after 6 months' supplementation, with either a lutein-rich supplement or a placebo, in the 29 subjects who participated in the comparative study. Macular pigment optical density (MPOD), which is a measure of macula concentration of lutein, was measured before and after the dietary intervention in the 29 subjects. Associations between SNPs and plasma lutein and MPOD were assessed by partial least square (PLS) regression followed by univariate analysis. Observed associations between SNPs and plasma lutein were verified by haplotype-based association analysis in the cohort of 622 subjects.

MAIN OUTCOME MEASURES

plasma lutein levels and MPOD.

RESULTS

six SNPs in four genes (ABCG8, BCMO1, CD36, and NPC1L1) explained 25% and 38% of the plasma and MPOD variance, respectively. Subjects with TT at the BCMO1 rs7501331 locus had lower (P < 0.05) plasma lutein than CT subjects. Subjects with CC at the CD36 rs13230419 locus had lower (P < 0.05) plasma lutein than subjects who carried a T allele. The association between CD36 and plasma lutein was confirmed in the cohort of 622 subjects. Subjects with TT at the BCMO1 rs7501331 locus had a higher (P < 0.05) MPOD, and subjects with GG at rs1761667 CD36 locus had a higher (P < 0.05) MPOD than those with an A allele.

CONCLUSIONS

these results suggest that BCMO1 and CD36 are implicated in plasma and retina concentrations of lutein and that genetic variants in these genes can modulate blood and retina concentrations of lutein.

Hepatic α-tocopherol transfer protein: ligand-induced protection from proteasomal degradation

There are eight naturally occurring forms of the dietary antioxidant vitamin E. Of these, only α-tocopherol is retained at high levels in vertebrate plasma and tissues. This selectivity is achieved in part by the action of the hepatic α-tocopherol transfer protein (TTP), which facilitates the selective incorporation of dietary α-tocopherol into circulating lipoproteins. We examined the effects of vitamin E on TTP expression in cultured hepatocytes. Treatment with vitamin E precipitated a time- and dose-dependent increase in the steady-state levels of TTP. This stabilization was caused by α-tocopherol-induced attenuation of the ubiquitination of TTP and its subsequent degradation by the proteasome. In vitro, vitamin E protected TTP from proteolytic degradation by trypsin, suggesting ligand-induced changes in protein conformation. Cell fractionation studies showed that TTP is distributed between the cytosolic and membranous organelle fraction, and that tocopherol induced the translocation of some TTP from the cytosol to the organelle fraction. Furthermore, vitamin E markedly attenuated the degradation of organelle-bound TTP. These findings suggest that vitamin E imparts a distinct conformation on TTP that is associated with localization to a specific cellular compartment, where the protein is less susceptible to proteasomal degradation.

Molecular mechanisms of membrane transport of vitamin E

Vitamin E is an essential fat-soluble micronutrient for higher mammals and functions as an antioxidant for lipids and also as a regulator of gene expression and a modulator of cell signaling and proliferation. To exert its physiological functions, vitamin E must achieve an appropriate disposition throughout the body via several processes, such as intestinal absorption, uptake and efflux in peripheral tissues and biliary secretion. In this review, we mainly discuss membrane proteins involved in these transport processes (ATP-binding cassette transporter A1, scavenger receptor class B type I, Niemann-Pick C1-like 1 and multidrug resistance 3) and vitamin E-mediated regulation of their expression.

alpha-Tocopheryl phosphate--an active lipid mediator?

The vitamin E (alpha-tocopherol, alphaT) derivative, alpha-tocopheryl phosphate (alphaTP), is detectable in small amounts in plasma, tissues, and cultured cells. Studies done in vitro and in vivo suggest that alphaT can become phosphorylated and alphaTP dephosphorylated, suggesting the existence of enzyme(s) with alphaT kinase or alphaTP phosphatase activity, respectively. As a supplement in animal studies, alphaTP can reach plasma concentrations similar to alphaT and only a part is dephosphorylated; thus, alphaTP may act both as pro-vitamin E, but also as phosphorylated form of vitamin E with possibly novel regulatory activities. Many effects of alphaTP have been described: in the test tube alphaTP modulates the activity of several enzymes; in cell culture alphaTP affects proliferation, apoptosis, signal transduction, and gene expression; in animal studies alphaTP prevents atherosclerosis, ischemia/reperfusion injury, and induces hippocampal long-term potentiation. At the molecular level, alphaTP may act as a cofactor for enzymes, as an active lipid mediator similar to other phosphorylated lipids, or indirectly by altering membrane characteristics such as lipid rafts, fluidity, and curvature. In this review, the molecular and cellular activities of alphaTP are examined and the possible functions of alphaTP as a natural compound, cofactor and active lipid mediator involved in signal transduction and gene expression discussed.

Influence of apolipoprotein (Apo) A-I structure on nascent high density lipoprotein (HDL) particle size distribution

The principal protein of high density lipoprotein (HDL), apolipoprotein (apo) A-I, in the lipid-free state contains two tertiary structure domains comprising an N-terminal helix bundle and a less organized C-terminal domain. It is not known how the properties of these domains modulate the formation and size distribution of apoA-I-containing nascent HDL particles created by ATP-binding cassette transporter A1 (ABCA1)-mediated efflux of cellular phospholipid and cholesterol. To address this issue, proteins corresponding to the two domains of human apoA-I (residues 1-189 and 190-243) and mouse apoA-I (residues 1-186 and 187-240) together with some human/mouse domain hybrids were examined for their abilities to form HDL particles when incubated with either ABCA1-expressing cells or phospholipid multilamellar vesicles. Incubation of human apoA-I with cells gave rise to two sizes of HDL particles (hydrodynamic diameter, 8 and 10 nm), and removal or disruption of the C-terminal domain eliminated the formation of the smaller particle. Variations in apoA-I domain structure and physical properties exerted similar effects on the rates of formation and sizes of HDL particles created by either spontaneous solubilization of phospholipid multilamellar vesicles or the ABCA1-mediated efflux of cellular lipids. It follows that the sizes of nascent HDL particles are determined at the point at which cellular phospholipid and cholesterol are solubilized by apoA-I; apparently, this is the rate-determining step in the overall ABCA1-mediated cellular lipid efflux process. The stability of the apoA-I N-terminal helix bundle domain and the hydrophobicity of the C-terminal domain are important determinants of both nascent HDL particle size and their rate of formation.

Effect of recombinant human lecithin cholesterol acyltransferase infusion on lipoprotein metabolism in mice

Lecithin cholesterol acyl transferase (LCAT) deficiency is associated with low high-density lipoprotein (HDL) and the presence of an abnormal lipoprotein called lipoprotein X (Lp-X) that contributes to end-stage renal disease. We examined the possibility of using LCAT an as enzyme replacement therapy agent by testing the infusion of human recombinant (r)LCAT into several mouse models of LCAT deficiency. Infusion of plasma from human LCAT transgenic mice into LCAT-knockout (KO) mice rapidly increased HDL-cholesterol (C) and lowered cholesterol in fractions containing very-low-density lipoprotein (VLDL) and Lp-X. rLCAT was produced in a stably transfected human embryonic kidney 293f cell line and purified to homogeneity, with a specific activity of 1850 nmol/mg/h. Infusion of rLCAT intravenously, subcutaneously, or intramuscularly into human apoA-I transgenic mice showed a nearly identical effect in increasing HDL-C approximately 2-fold. When rLCAT was intravenously injected into LCAT-KO mice, it showed a similar effect as plasma from human LCAT transgenic mice in correcting the abnormal lipoprotein profile, but it had a considerably shorter half-life of approximately 1.23 ± 0.63 versus 8.29 ± 1.82 h for the plasma infusion. rLCAT intravenously injected in LCAT-KO mice crossed with human apolipoprotein (apo)A-I transgenic mice had a half-life of 7.39 ± 2.1 h and increased HDL-C more than 8-fold. rLCAT treatment of LCAT-KO mice was found to increase cholesterol efflux to HDL isolated from mice when added to cells transfected with either ATP-binding cassette (ABC) transporter A1 or ABCG1. In summary, rLCAT treatment rapidly restored the normal lipoprotein phenotype in LCAT-KO mice and increased cholesterol efflux, suggesting the possibility of using rLCAT as an enzyme replacement therapy agent for LCAT deficiency.

Structure/function relationships of apolipoprotein a-I mimetic peptides: implications for antiatherogenic activities of high-density lipoprotein

RATIONALE

Apolipoprotein (apoA)-I mimetic peptides are a promising type of anti-atherosclerosis therapy, but how the structural features of these peptides relate to the multiple antiatherogenic functions of HDL is poorly understood.

OBJECTIVE

To establish structure/function relationships of apoA-I mimetic peptides with their antiatherogenic functions.

METHODS AND RESULTS

Twenty-two bihelical apoA-I mimetic peptides were investigated in vitro for the capacity and specificity of cholesterol efflux, inhibition of inflammatory response of monocytes and endothelial cells, and inhibition of low-density lipoprotein (LDL) oxidation. It was found that mean hydrophobicity, charge, size of hydrophobic face, and angle of the link between the helices are the major factors determining the efficiency and specificity of cholesterol efflux. The peptide with optimal parameters was more effective and specific toward cholesterol efflux than human apoA-I. Charge and size of hydrophobic face were also the major factors affecting antiinflammatory properties, and the presence of cysteine and histidine residues was the main factor determining antioxidant properties. There was no significant correlation between capacities of the peptides to support individual functions; each function had its own optimal set of features.

CONCLUSIONS

None of the peptides was equally effective in all the antiatherogenic functions tested, suggesting that different functions of HDL may have different mechanisms and different structural requirements. The results do suggest, however, that rationalizing the design of apoA-I mimetic peptides may improve their therapeutic value and may lead to a better understanding of mechanisms of various antiatherogenic functions of HDL.

High-density lipoprotein-associated alpha-tocopherol uptake by human retinal pigment epithelial cells (ARPE-19 Cells): the irrelevance of scavenger receptor class B, type I

The purpose of this study was to determine the high-density lipoprotein (HDL)-associated alpha-tocopherol (alpha-tocopherol-HDL) transport and clarify the contribution of scavenger receptor class B, type I (SR-BI) to the uptake in the human retinal pigment epithelial cell line (ARPE-19 cells). [(3)H]alpha-Tocopherol-HDL uptake into ARPE-19 cells seeded onto a transwell from both the apical (apical-to-cell) and basal compartment (basal-to-cell) exhibited a time-dependent increase for 90 min and there was a reduction at 4 degrees C. These results indicate the involvement of carrier-mediated process in alpha-tocopherol-HDL uptake in ARPE-19 cells. Immunoblot analysis revealed that SR-BI protein was expressed in ARPE-19 cells. However, the uptake of [(3)H]alpha-tocopherol from the apical or basal compartment was hardly inhibited by block lipid transport-1 (BLT-1), a specific inhibitor of the SR-BI-mediated lipid transfer. In conclusion, ARPE-19 cells have a carrier-mediated transport mechanism of [(3)H]alpha-tocopherol-HDL regardless of any SR-BI-mediated process.

PCR-verified microarray analysis and functionalin vitrostudies indicate a role of α-tocopherol in vesicular transport

Global gene expression profiles of livers from mice, fed diets differing in alpha-tocopherol content, were compared using DNA microarray technology. Three hundred and eighty nine genes were found to significantly differ in their expression level by a factor of 2 or higher between the high and the low alpha-tocopherol group. Functional clustering using the EASE software identified 121 genes involved in transport processes. Twenty-one thereof were involved in (synaptic) vesicular trafficking. Up-regulation of syntaxin 1C (Stx1c), vesicle-associated membrane protein 1 (Vamp1), N-ethylmaleimide-sensitive factor (Nsf) and syntaxin binding protein 1 (Stxbp1, Munc18-1) was verified by real time PCR. At a functional level, alpha-tocopherol increased the secretory response in RBL and PC12 cells. Although here detected in liver, the alpha-tocopherol-responsive pathways are also relevant to neurotransmission. A role of alpha-tocopherol in the vesicular transport might not only affect its own absorption and transport but also explain the neural dysfunctions observed in severe alpha-tocopherol deficiency.

ATP-binding cassette transporter A1 is involved in hepatic alpha-tocopherol secretion

Vitamin E (alpha-tocopherol) is an essential fat-soluble nutrient with antioxidant properties. alpha-Tocopherol transfer protein (alpha-TTP), the product of the gene responsible for familial isolated vitamin E deficiency, plays an important role in maintaining the plasma alpha-tocopherol level by mediating the secretion of alpha-tocopherol by the liver. However, the mechanisms underlying hepatic alpha-tocopherol secretion are not fully understood. This study was undertaken to elucidate the mechanism of alpha-tocopherol re-efflux from hepatocytes, the cells that have the most important role in regulating plasma-alpha-tocopherol concentrations. From in vitro experiments using [(3)H]alpha-tocopheryl acetate and McARH7777 cells that stably express alpha-tocopherol transfer protein (alpha-TTP), the following results were obtained. First, addition of apolipoprotein A-I (apoA-I), a direct acceptor of the ATP-binding cassette transporter A1 (ABCA1)-secreted lipids, increased alpha-tocopherol secretion in a dose-dependent manner. Second, probucol, an antiatherogenic compound reported to be an inactivator of ABCA1 reduced hepatic alpha-tocopherol secretion. Third, ABCA1-RNAi suppressed hepatic alpha-tocopherol secretion. In a mouse in vivo experiment, addition of 1% probucol to the diet decreased plasma alpha-tocopherol concentrations. These results strongly suggest that ABCA1 is substantially involved in hepatic alpha-tocopherol secretion.

The cell cholesterol exporter ABCA1 as a protector from cardiovascular disease and diabetes

ATP-binding cassette transporter A1 (ABCA1) is an integral cell membrane protein that exports cholesterol from cells and suppresses macrophage inflammation. ABCA1 exports cholesterol by a multistep pathway that involves forming cell-surface lipid domains, solubilizing these lipids by apolipoproteins, binding of apolipoproteins to ABCA1, and activating signaling processes. Thus, ABCA1 behaves both as a lipid exporter and a signaling receptor. ABCA1 transcription is highly induced by sterols, and its expression and activity are regulated post-transcriptionally by diverse processes. ABCA1 mutations can reduce plasma HDL levels, accelerate cardiovascular disease, and increase the risk for type 2 diabetes. Genetic manipulations of ABCA1 expression in mice also affect plasma HDL levels, inflammation, atherogenesis, and pancreatic beta cell function. Metabolites elevated in individuals with the metabolic syndrome and diabetes destabilize ABCA1 protein and decrease cholesterol export from macrophages, raising the possibility that an impaired ABCA1 pathway contributes to the enhanced atherogenesis associated with common inflammatory and metabolic disorders. The ABCA1 pathway has therefore become a promising new therapeutic target for treating cardiovascular disease and diabetes.

Vitamin E: the shrew waiting to be tamed

Vitamin E is the last of all vitamins whose essentiality is not yet understood. Its widely accepted role as a lipophilic antioxidant has been questioned, since proof of its in vivo relevance remained scarce. The influence of vitamin E on biomarkers of oxidative stress in vivo is inconsistent and metabolites of vitamin E having reacted as an antioxidant are hardly detectable. Novel functions of vitamin E include the regulation of enzymes, most of which are membrane bound or activated by membrane recruitment. Also, expression of genes responds to vitamin E. The search for a transcription factor common to all regulated genes failed so far and a receptor that specifically binds vitamin E has not yet been identified. According to microarray data, pathways preferentially affected by the vitamin E status are the inflammatory response and cellular traffic. A role of vitamin E in cellular trafficking could best explain the neurological symptoms seen in vitamin E deficiency. Emerging knowledge on vitamin E is compiled here with the perspective to unravel the molecular mechanisms that could more likely explain the essentiality of the vitamin than its ability to scavenge free radicals.

ATP-binding cassette transporter A1 is significantly involved in the intestinal absorption of alpha- and gamma-tocopherol but not in that of retinyl palmitate in mice

BACKGROUND

It has long been assumed that newly absorbed vitamin A and E enter the body only via enterocyte-produced chylomicrons. However, recent results in cell cultures have shown that a fraction of alpha-tocopherol is secreted with intestinal HDL.

OBJECTIVES

The aims of this study were to identify this transporter and to assess whether it is significantly implicated in the in vivo intestinal absorption of the 2 main dietary forms of vitamin E (ie, alpha- and gamma-tocopherol) and in that of retinyl palmitate (vitamin A).

DESIGN

Having performed preliminary experiments in the Caco-2 cell model, we compared fasting and postprandial plasma concentrations of vitamins A and E in mice deficient in ATP-binding cassette A1 (ABCA1) transporter and in wild-type mice.

RESULTS

A substantial efflux of alpha- and gamma-tocopherol, but not of retinyl esters, was induced by the presence of apolipoprotein A-I at the basolateral side of Caco-2 monolayers. The efflux of alpha- and gamma-tocopherol was also impaired by glyburide and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. The postprandial response of plasma gamma-tocopherol was 4-fold lower in ABCA1(-/-) mice (P = 0.025) than in wild-type mice, whereas no significant difference was observed for retinyl esters. Fasting plasma alpha-tocopherol, but not vitamin A, concentrations were lower in mice bearing the genetic deletion.

CONCLUSIONS

ABCA1 is the transporter responsible for the in vivo secretion of alpha- and gamma-tocopherol with intestinal HDL, and this pathway is significantly implicated in the intestinal absorption and plasma status of vitamin E but not of vitamin A.

Vitamin E in human health and disease

Vitamin E in nature is comprised of a family of tocopherols and tocotrienols. The most studied of these is alpha-tocopherol (alpha-TOH), because this form is retained within the body, and vitamin E deficiency is corrected with this supplement. alpha-TOH is a lipid-soluble antioxidant required for the preservation of cell membranes, and it potentially acts as a defense against oxidative stress. Many studies have investigated the metabolism, transport, and efficacy alpha-TOH in the prevention of sequelae associated with cardiovascular disease (CVD). Supplementation with vitamin E is considered to provide health benefits against CVD through its antioxidant activity, the prevention of lipoprotein oxidation, and the inhibition of platelet aggregation. However, the results from large prospective, randomized, placebo-controlled clinical trials with alpha-TOH have been largely negative. A recent meta-analysis suggests that alpha-TOH supplements may actually increase all-cause mortality; however, the mechanism for this increased risk is unknown. In vitro studies performed in human cell cultures and animal models suggest that vitamin E might increase the hepatic production of cytochrome P450s and MDR1. Induction of CYP3A4 or MDR1 by vitamin E could potentially lower the efficacy of any drug metabolized by CYP3A4 or MDR1. Other possibilities include an adverse effect of alpha-TOH on blood pressure in high-risk populations. Because of the wide popularity and use of vitamin E supplements, further research into potential adverse effects is clearly warranted.

HDL-associated lysosphingolipids inhibit NAD(P)H oxidase-dependent monocyte chemoattractant protein-1 production

OBJECTIVE

High-density lipoprotein (HDL) levels are inversely proportional to the risk of atherosclerosis, but mechanisms of HDL atheroprotection remain unclear. Monocyte chemoatractant protein-1 (MCP-1) constitutes an early component of inflammatory response in atherosclerosis. Here we investigated the influence of HDL on MCP-1 production in vascular smooth muscle cells (VSMCs) and rat aortic explants.

METHODS AND RESULTS

HDL inhibited the thrombin-induced production of MCP-1 in a concentration-dependent manner. The HDL-dependent inhibition of MCP-1 production was accompanied by the suppression of reactive oxygen species (ROS), which regulate the MCP-1 production in VSMCs. HDL inhibited NAD(P)H oxidase, the preponderant source of ROS in the vasculature, and prevented the activation of Rac1, which precedes NAD(P)H-oxidase activation. The HDL capacity to inhibit MCP-1 production, ROS generation, and NAD(P)H-oxidase activation was emulated by sphingosine 1-phosphate (S1P) and sphingosylphosphorylcholine (SPC), two lysosphingolipids present in HDL, but not by apolipoprotein A-I. HDL-, S1P-, and SPC-induced inhibition of MCP-1 production was attenuated in VSMCs pretreated with VPC23019, an antagonist of lysosphingolipid receptors S1P(1) and S1P(3), but not by JTE013, an antagonist of S1P(2). In addition, HDL, S1P, and SPC failed to inhibit MCP1 production and ROS generation in aortas from S1P(3)- and SR-B1-deficient mice.

CONCLUSIONS

HDL-associated lysosphingolipids inhibit NAD(P)H oxidase-dependent ROS generation and MCP-1 production in a process that requires coordinate signaling through S1P(3) and SR-B1 receptors.

Niemann-pick C1-like 1 mediates alpha-tocopherol transport

Dietary lipids and fat-soluble micronutrients are solubilized in mixed micelles and absorbed in the small intestine. Based on an assumption that cholesterol and other fat-soluble molecules share a number of transport mechanisms and the fact that Niemann-Pick C1-like 1 (NPC1L1) is critical for intestinal cholesterol absorption, we hypothesized that some fat-soluble molecules may be transported by NPC1L1. To investigate this hypothesis, we compared the cellular uptake and inhibitory effects of ezetimibe, the molecular target of which is NPC1L1, between cholesterol and some fat-soluble molecules using rat NPC1L1-overexpressing Caco-2 cells. The in vitro analysis suggested that NPC1L1 mediates the uptake of alpha-tocopherol (vitamin E) in an ezetimibe-sensitive manner as well as the uptake of cholesterol but does not mediate the uptake of retinol (vitamin A) or cyclosporin A. To confirm the ezetimibe-sensitive uptake of alpha-tocopherol in vivo, we performed an in vivo absorption study using rats and the results suggested a physiologically significant role of NPC1L1-mediated alpha-tocopherol absorption. Furthermore, using human NPC1L1 overexpression system, we demonstrated that both cholesterol and alpha-tocopherol uptake was also significantly increased by the overexpression of human NPC1L1 and ezetimibe inhibited their uptake. Mutual inhibition studies of cholesterol and alpha-tocopherol in human NPC1L1-mediated uptake revealed the inhibitory effect of cholesterol and the stimulatory effect of alpha-tocopherol on the NPC1L1-mediated transport of both substrates. The present data suggest, for the first time, that NPC1L1 has the ability to transport alpha-tocopherol and that ezetimibe is able to inhibit the intestinal absorption of alpha-tocopherol.

Quantitative analysis of ABCA1-dependent compartmentalization and trafficking of apolipoprotein A-I: implications for determining cellular kinetics of nascent high density lipoprotein biogenesis

The molecular mechanisms underlying the apoA-I/ABCA1 endocytic trafficking pathway in relation to high density lipoprotein (HDL) formation remain poorly understood. We have developed a quantitative cell surface biotinylation assay to determine the compartmentalization and trafficking of apoA-I between the plasma membrane (PM) and intracellular compartments (ICCs). Here we report that (125)I-apoA-I exhibited saturable association with the PM and ICCs in baby hamster kidney cells stably overexpressing ABCA1 and in fibroblasts. The PM was found to have a 2-fold higher capacity to accommodate apoA-I as compared with ICCs. Overexpressing various levels of ABCA1 in baby hamster kidney cells promoted the association of apoA-I with PM and ICCs compartments. The C-terminal deletion of apoA-I Delta(187-243) and reconstituted HDL particles exhibited reduced association of apoA-I with both the PM and ICCs. Interestingly, cell surface biotinylation with a cleavable biotin revealed that apoA-I induces ABCA1 endocytosis. Such endocytosis was impaired by naturally occurring mutations of ABCA1 (Q597R and C1477R). To better understand the role of the endocytotic pathway in the dynamics of the lipidation of apoA-I, a pulse-chase experiment was performed, and the dissociation (re-secretion) of (125)I-apoA-I from both PM and ICCs was monitored over a 6-h period. Unexpectedly, we found that the time required for 50% dissociation of (125)I-apoA-I from the PM was 4-fold slower than that from ICCs at 37 degrees C. Finally, treatment of the cells with phosphatidylcholine-specific phospholipase C, increased the dissociation of apoA-I from the PM. This study provides evidence that the lipidation of apoA-I occurs in two kinetically distinguishable compartments. The finding that apoA-I specifically mediates the continuous endocytic recycling of ABCA1, together with the kinetic data showing that apoA-I associated with ICCs is rapidly re-secreted, suggests that the endocytotic pathway plays a central role in the genesis of nascent HDL.

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: An overview of major research directions

During the last 90 years since the discovery of vitamin E, research has focused on different properties of this molecule, the focus often depending on the specific techniques and scientific knowledge present at each time. Originally discovered as a dietary factor essential for reproduction in rats, vitamin E has revealed in the meantime many more important molecular properties, such as the scavenging of reactive oxygen and nitrogen species with consequent prevention of oxidative damage associated with many diseases, or the modulation of signal transduction and gene expression in antioxidant and non-antioxidant manners. Research over the last 30 years has also resolved the biosynthesis and occurrence of vitamin E in plants, the proteins involved in the cellular uptake, tissue distribution and metabolism, and defined a congenital recessive neurological disease, ataxia with vitamin E deficiency (AVED), characterized by impaired enrichment of alpha-tocopherol in plasma as a result of mutations in the liver alpha-tocopherol transfer gene. This review is giving a brief introduction about vitamin E by following the major research directions since its discovery with a historical perspective.

Vitamin E Regulatory Mechanisms

Dietary and supplemental vitamin E is absorbed and delivered to the liver, but of the various antioxidants with vitamin E activity, only alpha-tocopherol is preferentially recognized by the alpha-tocopherol transfer protein (alpha-TTP) and is transferred to plasma, while the other vitamin E forms (e.g., gamma-tocopherol or tocotrienols) are removed from the circulation. Hepatic alpha-TTP is required to maintain plasma and tissue alpha-tocopherol concentrations. The liver is the master regulator of the body's vitamin E levels in that it not only controls alpha-tocopherol concentrations, but also appears to be the major site of vitamin E metabolism and excretion. Vitamin Es are metabolized similarly to xenobiotics; they are initially omega-oxidized by cytochrome P450s, undergo several rounds of beta-oxidation, and then are conjugated and excreted. As a result of these various mechanisms, liver alpha-tocopherol and other vitamin E concentrations are closely regulated; thus, any potential adverse vitamin E effects are limited.

Identification of an ABCA1-dependent phospholipid-rich plasma membrane apolipoprotein A-I binding site for nascent HDL formation: implications for current models of HDL biogenesis

It is well accepted that both apolipoprotein A-I (apoA-I) and ABCA1 play crucial roles in HDL biogenesis and in the human atheroprotective system. However, the nature and specifics of apoA-I/ABCA1 interactions remain poorly understood. Here, we present evidence for a new cellular apoA-I binding site having a 9-fold higher capacity to bind apoA-I compared with the ABCA1 site in fibroblasts stimulated with 22-(R)-hydroxycholesterol/9-cis-retinoic acid. This new cellular apoA-I binding site was designated "high-capacity binding site" (HCBS). Glyburide drastically reduced (125)I-apoA-I binding to the HCBS, whereas (125)I-apoA-I showed no significant binding to the HCBS in ABCA1 mutant (Q597R) fibroblasts. Furthermore, reconstituted HDL exhibited reduced affinity for the HCBS. Deletion of the C-terminal region of apoA-I (Delta187-243) drastically reduced the binding of apoA-I to the HCBS. Interestingly, overexpressing various levels of ABCA1 in BHK cells promoted the formation of the HCBS. The majority of the HCBS was localized to the plasma membrane (PM) and was not associated with membrane raft domains. Importantly, treatment of cells with phosphatidylcholine-specific phospholipase C, but not sphingomyelinase, concomitantly reduced the binding of (125)I-apoA-I to the HCBS, apoA-I-mediated cholesterol efflux, and the formation of nascent apoA-I-containing particles. Together, these data suggest that a functional ABCA1 leads to the formation of a major lipid-containing site for the binding and the lipidation of apoA-I at the PM. Our results provide a biochemical basis for the HDL biogenesis pathway that involves both ABCA1 and the HCBS, supporting a two binding site model for ABCA1-mediated nascent HDL genesis.

Mechanisms involved in vitamin E transport by primary enterocytes and in vivo absorption

It is generally believed that vitamin E is absorbed along with chylomicrons. However, we previously reported that human colon carcinoma Caco-2 cells use dual pathways, apolipoprotein B (apoB)-lipoproteins and HDLs, to transport vitamin E. Here, we used primary enterocytes and rodents to identify in vivo vitamin E absorption pathways. Uptake of [(3)H]alpha-tocopherol by primary rat and mouse enterocytes increased with time and reached a maximum at 1 h. In the absence of exogenous lipid supply, these cells secreted vitamin E with HDL. Lipids induced the secretion of vitamin E with intermediate density lipoproteins, and enterocytes supplemented with lipids and oleic acid secreted vitamin E with chylomicrons. The secretion of vitamin E with HDL was not affected by lipid supply but was enhanced when incubated with HDL. Microsomal triglyceride transfer protein inhibition reduced vitamin E secretion with chylomicrons without affecting its secretion with HDL. Enterocytes from Mttp-deficient mice also secreted less vitamin E with chylomicrons. In vivo absorption of [(3)H]alpha-tocopherol by mice after poloxamer 407 injection to inhibit lipoprotein lipase revealed that vitamin E was associated with triglyceride-rich lipoproteins and small HDLs containing apoB-48 and apoA-I. These studies indicate that enterocytes use two pathways for vitamin E absorption. Absorption with chylomicrons is the major pathway of vitamin E absorption. The HDL pathway may be important when chylomicron assembly is defective and can be exploited to deliver vitamin E without increasing fat consumption.

Vitamin E Biochemistry and Function: A Case Study in Male Rabbit

Vitamin E supplementation has become a common procedure to promote growth and health and improve the qualitative characteristics of farm animals. It has been demonstrated to be an efficient strategy for improving their reproductive function. Germ cells are particularly vulnerable to oxidative damage and may thus require additional antioxidant protection. The aim of this review is to give a comprehensive overview of the current knowledge of the biochemistry and physiology of vitamin E; successively, the effect of this compound on the reproductive activity of rabbit buck is accurately described. In particular, this review examines studies on the effects of animal age, dose and duration of vitamin E supplementation, as well as the co-supplementation with selenium, vitamin C and polyunsaturated fatty acids. Several investigations have shown beneficial effects in bucks supplemented with vitamin E levels higher than the standard dietary requirement (50 mg/kg) particularly when the semen is stored. However, the exact dietary intake of vitamin E should be decided according to specific tissue needs for the individual vitamin E forms and the specific saturation markers.

Differential retention of alpha-vitamin E is correlated with its transporter gene expression and growth inhibition efficacy in prostate cancer cells

BACKGROUND

Epidemiological studies showed Vit E has protective effects against prostate cancer (PCa). Interestingly, different prostate cancer cells have different sensitivity to alpha-Vit E or VES treatment. The goal of this study is to determine whether cellular Vit E bioavailability and its transport proteins are important contributing factors.

METHODS

alpha-Vit E and its ester form, VES, were used to treat prostate cancer LNCaP, PC3, and DU145 cells, and their growth rates were determined by MTT assay. Cellular levels of Vit E were quantified using HPLC as the index of bioavailability. The expression levels of Vit E transport proteins were determined by real-time PCR.

RESULTS

Among these PCa cells, only LNCaP cells were sensitive to 20 microM alpha-Vit E treatment, while both LNCaP and PC3 cells were sensitive to 20 microM VES treatment. Coordinately, cellular levels of alpha-Vit E and VES positively correlated to their inhibitory effects. Further study found expression levels of Vit E transport proteins, including tocopherol associated protein (TAP), scavenger receptor class B type I (SR-BI), alpha-tocopherol transfer protein (TTP), and ATP binding cassette transporter A1 (ABCA1), were different in various PCa cells, which may contribute to cellular Vit E bioavailability. This notion is further supported by the findings that overexpression or knockdown of TTP could coordinately alter cellular alpha-Vit E levels in PCa cells.

CONCLUSION

Antiproliferative efficacy of alpha-Vit E is correlated with its cellular bioavailability in PCa cells. Modulating the expression of the efflux or influx transporters could sensitize the growth inhibition efficacy of Vit E in prostate cancer cells.

The Roles of α‐Vitamin E and Its Analogues in Prostate Cancer

Prostate cancer is the second most commonly diagnosed cancer and the third leading fatal cancer in American men. Comprehensive studies from human epidemiological studies, animal tumor models, and cellular molecular levels suggested that alpha-vitamin E and its derivatives possess remarkable chemopreventive and chemotherapeutic against prostate cancer. This chapter details the facts of alpha-vitamin E and its nonantioxidant functions in prostate cancer, focuses on the biological mechanisms for the alpha-vitamin E and its ester analogue, alpha-vitamin E succinate (VES), in prevention and therapy of prostate cancer, and raises specific questions that remain for intensive investigation in the future.

Structure and Function of α‐Tocopherol Transfer Protein: Implications for Vitamin E Metabolism and AVED

Human alpha-tocopherol transfer protein (alpha-TTP) plays a central role in vitamin E homeostasis: mutations in the protein are a cause of a progressive neurodegenerative disorder known as ataxia with vitamin E deficiency (AVED). Despite normal dietary intake of vitamin E, affected individuals suffer from a relative deficiency of this essential lipophilic antioxidant. Disease-associated mutations in alpha-TTP impair its ability to prevent the degradation and excretion of alpha-T. Recently, we and others solved the crystal structures of alpha-TTP bound to a molecule of (2R, 4'R, 8'R)-alpha-T, which has led to a better understanding of the molecular basis of its biochemical activity. Surprisingly, the ligand was found buried in the hydrophobic core of the protein, completely sequestered from the aqueous milieu. In this chapter, the implications of the structure of alpha-TTP bound to its ligand regarding the mechanism of alpha-T retention are discussed. A comparison to a crystal structure of the apo form of alpha-TTP indicates a possible specific conformational change that allows the entry and exit of the ligand. The effect of known disease-associated point mutations is examined in light of the crystal structure as well as recent biochemical studies. Despite the knowledge gained from these studies, the exact molecular mechanism by which alpha-TTP retains alpha-T remains enigmatic and will likely prove a fruitful area for future research.

The α‐Tocopherol Transfer Protein

Almost a century ago, plant extracts were documented to be critical for the fertility of rodents. This activity was later ascribed to vitamin E, a term comprising a number of structurally related plant lipids that function as fat soluble antioxidants. The alpha-tocopherol transfer protein (TTP) is a critical regulator of vitamin E status that stimulates the movement of vitamin E between membrane vesicles in vitro and facilitates the secretion of tocopherol from hepatocytes. Heritable mutations in the ttpA gene cause ataxia with vitamin E deficiency (AVED), an autosomal recessive disorder characterized by low plasma vitamin E levels and progressive neurodegeneration. This chapter summarizes recent advances in our understanding of the molecular and physiological aspects of TTP activity.

Roles of ATP binding cassette transporters A1 and G1, scavenger receptor BI and membrane lipid domains in cholesterol export from macrophages

PURPOSE OF REVIEW

The initial steps of reverse cholesterol transport involve export of cholesterol from peripheral cells to plasma lipoproteins for subsequent delivery to the liver. The review discusses recent developments in our understanding of how these steps occur, with particular emphasis on the macrophage, the major site of cellular cholesterol accumulation in atherosclerosis.

RECENT FINDINGS

ATP binding cassette transporter (ABC) A1 exports cholesterol and phospholipid to lipid-free apolipoproteins, while ATP binding cassette transporter G1 and scavenger receptor BI export cholesterol to phospholipid-containing acceptors. ABCA1-dependent cholesterol export involves an initial interaction of apolipoprotein AI with lipid raft membrane domains, although ABCA1 and most exported cholesterol are not raft associated. ABCG1 exports cholesterol to HDL and other phospholipid-containing acceptors. These include particles generated during lipidation of apoAI by ABCA1, suggesting that the two transporters cooperate in cholesterol export. Scavenger receptor BI is atheroprotective, mediating clearance of HDL cholesterol by the liver. The relative contributions of scavenger receptor BI and ABCG to cholesterol export to HDL from macrophages is unclear and may depend on cellular cholesterol status and the cholesterol gradient between cell and acceptor.

SUMMARY

The presence of distinct pathways for cholesterol efflux to lipid-free apolipoprotein AI and phospholipid-containing HDL species clarifies our understanding of reverse cholesterol transport, and provides new opportunities for its therapeutic manipulation.

Absence of VLDL secretion does not affect alpha-tocopherol content in peripheral tissues

alpha-Tocopherol is a lipid-soluble antioxidant that helps to prevent oxidative damage to cellular lipids. alpha-Tocopherol is absorbed by the intestine and is taken up and retained by the liver; it is widely presumed that alpha-tocopherol is then delivered to peripheral tissues by the secretion of VLDL. To determine whether VLDL secretion is truly important for the delivery of alpha-tocopherol to peripheral tissues, we examined alpha-tocopherol metabolism in mice that lack microsomal triglyceride transfer protein (Mttp) expression in the liver and therefore cannot secrete VLDL (Mttp(Delta/Delta) mice). Mttp(Delta/Delta) mice have low plasma lipid levels and increased stores of lipids in the liver. Similarly, alpha-tocopherol levels in the plasma were lower in Mttp(Delta/Delta) mice than in controls, whereas hepatic alpha-tocopherol stores were higher. However, alpha-tocopherol levels in the peripheral tissues of Mttp(Delta/Delta) mice were nearly identical to those of control mice, suggesting that VLDL secretion is not critical for the delivery of alpha-tocopherol to peripheral tissues. When fed a diet containing deuterated alpha-tocopherol, Mttp(Delta/Delta) and control mice had similar incorporation of deuterated alpha-tocopherol into plasma and various peripheral tissues. We conclude that the absence of VLDL secretion has little effect on the stores of alpha-tocopherol in peripheral tissues, at least in the mouse.

Lipid efflux by the ATP-binding cassette transporters ABCA1 and ABCG1

Plasma levels of high-density lipoproteins (HDL) and apolipoprotein A-I (apoA-I) are inversely correlated with the risk of cardiovascular disease. One major atheroprotective mechanism of HDL and apoA-I is their role in reverse cholesterol transport, i.e., the transport of excess cholesterol from foam cells to the liver for secretion. The ATP-binding cassette transporters ABCA1 and ABCG1 play a pivotal role in this process by effluxing lipids from foam cells to apoA-I and HDL, respectively. In the liver, ABCA1 activity is one rate-limiting step in the formation of HDL. In macrophages, ABCA1 and ABCG1 prevent the excessive accumulation of lipids and thereby protect the arteries from developing atherosclerotic lesions. However, the mechanisms by which ABCA1 and ABCG1 mediate lipid removal are still unclear. Particularly, three questions remain controversial and are discussed in this review: (1) Do apoA-I and HDL directly interact with ABCA1 and ABCG1, respectively? (2) Does cholesterol efflux involve retroendocytosis of apoA-I or HDL? (3) Which lipids are directly transported by ABCA1 and ABCG1?

ABCA1 mediates high-affinity uptake of 25-hydroxycholesterol by membrane vesicles and rapid efflux of oxysterol by intact cells

ATP Binding Cassette (ABC) transporter, ABCA1, plays a pivotal role in reverse cholesterol transport by mediating the cellular efflux of phospholipid and cholesterol. Studies using intact cells strongly suggest that ABCA1 acts as a phospholipid floppase, but there has been no direct demonstration that the protein is a primary active sterol transporter. Using membrane vesicles from insect Sf21 cells, we found that ABCA1 mediated ATP-dependent uptake of [(3)H]25-hydroxycholesterol with an apparent K(m) of 0.7 muM. Consistent with this high apparent affinity, expression of ABCA1 in human embryonic kidney cells both increased rapid efflux of 25-hydroxcholesterol and prevented oxysterol-mediated repression of low-density lipoprotein (LDL) receptor and 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase mRNAs. Comparison of wild-type and ABCA1(-/-) murine fibroblasts indicates that 25-hydroxycholesterol is effluxed approximately 5-fold more rapidly by wild-type cells. In addition, the rate of efflux from the wild-type but not the ABCA1(-/-) fibroblasts is increased a further twofold by inducers of ABCA1 expression. Thus under the experimental conditions employed, endogenous ABCA1 is a major contributor to 25-hydroxycholesterol efflux from wild-type fibroblasts. Evidence from in vitro studies indicates that oxysterols are potent inducers of genes involved in cellular cholesterol efflux and metabolism, including the ABCA1 gene, and repressors of genes involved in cholesterol synthesis or uptake. Our observations raise the possibility that efflux of oxysterols by ABCA1 could contribute to a homeostatic mechanism, which both attenuates oxysterol-induced expression of its cognate gene and alleviates repression of genes encoding proteins, such as HMG-CoA reductase and LDL receptor.

Transport of vitamin E by differentiated Caco-2 cells

In hepatocytes, vitamin E is secreted via the efflux pathway and is believed to associate with apolipoprotein B (apoB)-lipoproteins extracellularly. The molecular mechanisms involved in the uptake, intracellular trafficking, and secretion of dietary vitamin E by the intestinal cells are unknown. We observed that low concentrations of Tween-40 were better for the solubilization and delivery of vitamin E to differentiated Caco-2 cells, whereas high concentrations of Tween-40 and sera inhibited this uptake. Vitamin E uptake was initially rapid and then reached saturation. Subcellular localization revealed that vitamin E primarily accumulated in microsomal membranes. Oleic acid (OA) treatment, which induces chylomicron assembly and secretion, decreased microsomal membrane-bound vitamin E in a time-dependent manner. To study secretion, differentiated Caco-2 cells were pulse-labeled with vitamin E and chased in the presence and absence of OA. In the absence of OA, vitamin E was associated with intestinal high density lipoprotein (I-HDL), whereas OA-treated cells secreted vitamin E with I-HDL and chylomicrons. No extracellular transfer of vitamin E between these lipoproteins was observed. Glyburide, an antagonist of ABCA1, partially inhibited its secretion with I-HDL, whereas plasma HDL increased vitamin E efflux. An antagonist of microsomal triglyceride transfer protein, brefeldin A, and monensin specifically inhibited vitamin E secretion with chylomicrons. These studies indicate that vitamin E taken up by Caco-2 cells is stored in the microsomal membranes and secreted with chylomicrons and I-HDL. Transport via I-HDL might contribute to vitamin E absorption in patients with abetalipoproteinemia receiving large oral doses of the vitamin.

Pharmacogenomics and cardiovascular drugs: need for integrated biological system with phenotypes and proteomic markers

Personalized medicine is based on a better knowledge of biological variability, considering the important part due to genetics. When trying to identify involved genes and their products in differential cardiovascular drug responses, a five-step strategy is to be followed: 1) Pharmacokinetic-related genes and phenotypes (2) Pharmacodynamic targets, genes and products (3) Cardiovascular diseases and risks depending on specific or large metabolic cycles (4) Physiological variations of previously identified genes and proteins (5) Environment influences on them. After summarizing the most well-known genes involved in drug metabolism, we will take as example of drugs, the statins, considered as very important drugs from a Public-Health standpoint, but also for economical reasons. These drugs respond differently in human depending on multiple polymorphisms. We will give examples with common ApoE polymorphisms influencing the hypolipemic effects of statins. These drugs also have pleiotropic effects and decrease inflammatory markers. This illustrates the need to separate clinical diseases phenotypes in specific metabolic pathways, which could propose other classifications, of diseases and related genes. Hypertension is also a good example of clinical phenotype which should be followed after various therapeutic approaches by genes polymorphisms and proteins markers. Gene products are under clear environmental expression variations such as age, body mass index and obesity, alcohol, tobacco and dietary interventions which are the first therapeutical actions taken in cardiovascular diseases. But at each of the five steps, within a pharmacoproteomic strategy, we also need to use available information from peptides, proteins and metabolites, which usually are the gene products. A profiling approach, i.e., dealing with genomics, but now also with proteomics, is to be used. In conclusion, the profiling, as well as the large amount of data, will more than before render necessary an organized interpretation of DNA, RNA as well as proteins variations, both at individual and population level.

ATP-binding cassette transporter A1: a cell cholesterol exporter that protects against cardiovascular disease

Blood high-density lipoprotein (HDL) levels are inversely related to risk for cardiovascular disease, implying that factors associated with HDL metabolism are atheroprotective. One of these factors is ATP-binding cassette transporter A1 (ABCA1), a cell membrane protein that mediates the transport of cholesterol, phospholipids, and other metabolites from cells to lipid-depleted HDL apolipoproteins. ABCA1 transcription is highly induced by sterols, a major substrate for cellular export, and its expression and activity are regulated posttranscriptionally by diverse processes. Liver ABCA1 initiates formation of HDL particles, and macrophage ABCA1 protects arteries from developing atherosclerotic lesions. ABCA1 mutations can cause a severe HDL deficiency syndrome characterized by cholesterol deposition in tissue macrophages and prevalent atherosclerosis. Genetic manipulations of ABCA1 expression in mice also affect plasma HDL levels and atherogenesis. Metabolites elevated in individuals with the metabolic syndrome and diabetes destabilize ABCA1 protein and decrease cholesterol export from macrophages. Moreover, oxidative modifications of HDL found in patients with cardiovascular disease reduce the ability of apolipoproteins to remove cellular cholesterol by the ABCA1 pathway. These observations raise the possibility that an impaired ABCA1 pathway contributes to the enhanced atherogenesis associated with common inflammatory and metabolic disorders. The ABCA1 pathway has therefore become an important new therapeutic target for treating cardiovascular disease.

Alpha-tocopherol: roles in prevention and therapy of human disease

Alpha-tocopherol, one of the eight isoforms of vitamin E, is the most potent fat-soluble antioxidant known in nature. For years, it was thought that alpha-tocopherol only functioned as a scavenger of lipid peroxyl radicals, specifically, oxidized low-density lipoprotein (oxLDL), thereby serving as a chief antioxidant for the prevention of atherosclerosis. In recent years, the many roles of alpha-tocopherol have been uncovered, and include not only antioxidant functions, but also pro-oxidant, cell signaling and gene regulatory functions. Decades of clinical and preclinical studies have broadened our understanding of the antioxidant vitamin E and its utility in a number of chronic, oxidative stress-induced pathologies. The results of these studies have shown promising, albeit mixed reviews on the efficacy of alpha-tocopherol in the prevention and treatment of heart disease, cancer and Alzheimer's disease. Future studies to uncover cellular and systemic mechanisms may help guide appropriate clinical treatment strategies using vitamin E across a diverse population of aging individuals.