Physical partitioning is the main mechanism of alpha-tocopherol and cholesterol transfer between lipoproteins and P388D1 macrophage-like cells

The tocopherol transfer protein TTP mediates Vitamin Vitamin E trafficking between cerebellar astrocytes and neurons

Alpha-tocopherol (vitamin E) is an essential nutrient that functions as a major lipid-soluble antioxidant in humans. The tocopherol transfer protein (TTP) binds α-tocopherol with high affinity and selectivity and regulates whole-body distribution of the vitamin. Heritable mutations in the TTPA gene result in familial vitamin E deficiency, elevated indices of oxidative stress, and progressive neurodegeneration that manifest primarily in spinocerebellar ataxia. Although the essential role of vitamin E in neurological health has been recognized for over 50 years, the mechanisms by which this essential nutrient is transported in the central nervous system are poorly understood. Here we found that, in the murine cerebellum, TTP is selectively expressed in GFAP-positive astrocytes, where it facilitates efflux of vitamin E to neighboring neurons. We also show that induction of oxidative stress enhances the transcription of the TtpA gene in cultured cerebellar astrocytes. Furthermore, secretion of vitamin E from astrocytes is mediated by an ABC-type transporter, and uptake of the vitamin into neurons involves the low-density lipoprotein receptor-related protein 1 (LRP1) receptor. Taken together, our data indicate that TTP-expressing astrocytes control the delivery of vitamin E from astrocytes to neurons, and that this process is homeostatically responsive to oxidative stress. These are the first observations that address the detailed molecular mechanisms of vitamin E transport in the central nervous system, and these results have important implications for understanding the molecular underpinnings of oxidative stress-related neurodegenerative diseases.

Altered vitamin E status in Niemann-Pick type C disease

Vitamin E (α-tocopherol) is the major lipid-soluble antioxidant in many species. Niemann-Pick type C (NPC) disease is a lysosomal storage disorder caused by mutations in the NPC1 or NPC2 gene, which regulates lipid transport through the endocytic pathway. NPC disease is characterized by massive intracellular accumulation of unesterified cholesterol and other lipids in lysosomal vesicles. We examined the roles that NPC1/2 proteins play in the intracellular trafficking of tocopherol. Reduction of NPC1 or NPC2 expression or function in cultured cells caused a marked lysosomal accumulation of vitamin E in cultured cells. In vivo, tocopherol significantly accumulated in murine Npc1-null and Npc2-null livers, Npc2-null cerebella, and Npc1-null cerebral cortices. Plasma tocopherol levels were within the normal range in Npc1-null and Npc2-null mice, and in plasma samples from human NPC patients. The binding affinity of tocopherol to the purified sterol-binding domain of NPC1 and to purified NPC2 was significantly weaker than that of cholesterol (measurements kindly performed by R. Infante, University of Texas Southwestern Medical Center, Dallas, TX). Taken together, our observations indicate that functionality of NPC1/2 proteins is necessary for proper bioavailability of vitamin E and that the NPC pathology might involve tissue-specific perturbations of vitamin E status.

The contribution of surface residues to membrane binding and ligand transfer by the α-tocopherol transfer protein (α-TTP)

Previous work has shown that the α-tocopherol transfer protein (α-TTP) can bind to vesicular or immobilized phospholipid membranes. Revealing the molecular mechanisms by which α-TTP associates with membranes is thought to be critical to understanding its function and role in the secretion of tocopherol from hepatocytes into the circulation. Calculations presented in the Orientations of Proteins in Membranes database have provided a testable model for the spatial arrangement of α-TTP and other CRAL-TRIO family proteins with respect to the lipid bilayer. These calculations predicted that a hydrophobic surface mediates the interaction of α-TTP with lipid membranes. To test the validity of these predictions, we used site-directed mutagenesis and examined the substituted mutants with regard to intermembrane ligand transfer, association with lipid layers and biological activity in cultured hepatocytes. Substitution of residues in helices A8 (F165A and F169A) and A10 (I202A, V206A and M209A) decreased the rate of intermembrane ligand transfer as well as protein adsorption to phospholipid bilayers. The largest impairment was observed upon mutation of residues that are predicted to be fully immersed in the lipid bilayer in both apo (open) and holo (closed) conformations such as Phe165 and Phe169. Mutation F169A, and especially F169D, significantly impaired α-TTP-assisted secretion of α-tocopherol outside cultured hepatocytes. Mutation of selected basic residues (R192H, K211A, and K217A) had little effect on transfer rates, indicating no significant involvement of nonspecific electrostatic interactions with membranes.

Novel transcriptional activities of vitamin E: inhibition of cholesterol biosynthesis

Vitamin E is a dietary lipid that is essential for vertebrate health and fertility. The biological activity of vitamin E is thought to reflect its ability to quench oxygen- and carbon-based free radicals and thus to protect the organism from oxidative damage. However, recent reports suggest that vitamin E may also display other biological activities. Here, to examine possible mechanisms that may underlie such nonclassical activities of vitamin E, we investigated the possibility that it functions as a specific modulator of gene expression. We show that treatment of cultured hepatocytes with (RRR)-alpha-tocopherol alters the expression of multiple genes and that these effects are distinct from those elicited by another antioxidant. Genes modulated by vitamin E include those that encode key enzymes in the cholesterol biosynthetic pathway. Correspondingly, vitamin E caused a pronounced inhibition of de novo cholesterol biosynthesis. The transcriptional activities of vitamin E were mediated by attenuating the post-translational processing of the transcription factor SREBP-2 that, in turn, led to a decreased transcriptional activity of sterol-responsive elements in the promoters of target genes. These observations indicate that vitamin E possesses novel transcriptional activities that affect fundamental biological processes. Cross talk between tocopherol levels and cholesterol status may be an important facet of the biological activities of vitamin E.

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.

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.

Vitamin E supplementation of human macrophages prevents neither foam cell formation nor increased susceptibility of foam cells to lysis by oxidized LDL

Several studies in macrophage cell lines, rodent macrophages, and animal models of atherosclerosis suggest that vitamin E may prevent the formation of foam cells. We tested this hypothesis in a recently developed, fully autologous in vitro model of human foam cell formation. During maturation, macrophages continuously increased their alpha-tocopherol/total cholesterol ratio, demonstrating that these cells accumulate alpha-tocopherol at an even higher rate than cholesterol. In the presence of unsupplemented serum, we observed no correlation between serum vitamin E levels and the increase in the cellular alpha-tocopherol/total cholesterol ratio. In contrast, under supplemented conditions, a 3.1-fold increase in the mean serum alpha-tocopherol/total cholesterol ratio resulted in a corresponding mean 3.5-fold increase in the cellular alpha-tocopherol/total cholesterol ratio. Vitamin E loading had no effect on the lipid composition of macrophages and did not affect their growth. Foam cell formation was stimulated in mature unsupplemented and vitamin E-loaded macrophages for 1 week with 50 microg autologous aggregated low density lipoprotein (LDL) in the presence of unsupplemented and vitamin E-loaded serum, respectively. We observed no effect of vitamin E supplementation on the formation of foam cells. However, foam cell formation resulted in a 36% and 44% reduction in the cellular alpha-tocopherol/total cholesterol ratio in unsupplemented and vitamin E-supplemented foam cells, respectively. The loss of vitamin E was accelerated with increasing concentrations of aggregated LDL and was accompanied by an increase in the susceptibility of these foam cells to succumb to the cell lytic effects of oxidized LDL (OxLDL). However, vitamin E supplementation did not protect macrophages or foam cells from OxLDL-mediated cell lysis, suggesting that vitamin E loss in foam cells is not the cause of their increased susceptibility to cell lysis. Our results suggest that the beneficial effects of vitamin E on cardiovascular disease observed in humans are due neither to a reduction in the propensity of macrophages to form foam cells nor to an increased resistance of these cells to cytolytic OxLDL.