Vitamin E mediated response of smooth muscle cell to oxidant stress

Vitamins and Uterine Fibroids: Current Data on Pathophysiology and Possible Clinical Relevance

Uterine fibroid (UF) is the most common benign tumor pathology of the female reproductive organs. UFs constitute the main reason for a hysterectomy and hospitalization due to gynecological conditions. UFs consist of uterine smooth muscle immersed in a large amount of extracellular matrix (ECM). Genetic studies have demonstrated that UFs are monoclonal tumors originating from the myometrial stem cells that have underwent specific molecular changes to tumor initiating stem cells which proliferate and differentiate later under the influence of steroid hormones. There is growing interest in the role of micronutrients, for example, vitamins, in UFs. This article is a comprehensive review of publications regarding the available data concerning the role of vitamins in the biology and management of UFs. In summary, the results showed that some vitamins are important in the biology and pathophysiology of UFs. For example, vitamins A and D deserve particular attention following studies of their influence on the treatment of UF tumors. Vitamins B3, C, and E have not been as widely studied as the abovementioned vitamins. However, more research could reveal their potential role in UF biology.

Many tocopherols, one vitamin E

Four tocopherols are available in nature and are absorbed with the diet, but only one RRR-α-tocopherol satisfies the criteria of being a vitamin. The biological activity of the different tocopherols studied in the rat by the resorption-gestation test has been inconsistently extrapolated to human beings where the tocopherols have no influence on a successful pregnancy. Diminution of RRR-α-tocopherol intake results in diseases characterized by ataxia, whose pathogenetic mechanism, despite vigorous claims, has not been clarified. The calculation of the Daily Reference Intake (DRI), necessary to prevent disease, is based on an obsolete test, the peroxide-induced erythrocyte hemolysis, called the gold standard, but of highly questioned validity. If many epidemiological studies have given positive results, showing prevention by high vitamin E containing diets of cardiovascular events, neurodegenerative disease, macular degeneration and cancer, the clinical confirmatory intervention studies were mostly negative. On the positive side, besides preventing vitamin E deficiency diseases, vitamin E has shown efficacy as anti-inflammatory and immune boosting compound. It has also shown some efficacy in protecting against nonalcoholic hepato-steatosis. At a molecular level, vitamin E and some of its metabolites have shown capacity of regulating cell signaling and modulating gene transcription.

α-Tocopherol and Hippocampal Neural Plasticity in Physiological and Pathological Conditions

Neuroplasticity is an "umbrella term" referring to the complex, multifaceted physiological processes that mediate the ongoing structural and functional modifications occurring, at various time- and size-scales, in the ever-changing immature and adult brain, and that represent the basis for fundamental neurocognitive behavioral functions; in addition, maladaptive neuroplasticity plays a role in the pathophysiology of neuropsychiatric dysfunctions. Experiential cues and several endogenous and exogenous factors can regulate neuroplasticity; among these, vitamin E, and in particular α-tocopherol (α-T), the isoform with highest bioactivity, exerts potent effects on many plasticity-related events in both the physiological and pathological brain. In this review, the role of vitamin E/α-T in regulating diverse aspects of neuroplasticity is analyzed and discussed, focusing on the hippocampus, a brain structure that remains highly plastic throughout the lifespan and is involved in cognitive functions. Vitamin E-mediated influences on hippocampal synaptic plasticity and related cognitive behavior, on post-natal development and adult hippocampal neurogenesis, as well as on cellular and molecular disruptions in kainate-induced temporal seizures are described. Besides underscoring the relevance of its antioxidant properties, non-antioxidant functions of vitamin E/α-T, mainly involving regulation of cell signaling molecules and their target proteins, have been highlighted to help interpret the possible mechanisms underlying the effects on neuroplasticity.

Tocopherols and tocotrienols in membranes: a critical review

The familiar role of tocols (tocopherols and tocotrienols) as lipid-soluble chain-terminating inhibitors of lipid peroxidation is currently in the midst of a reinterpretation. New biological activities have been described for tocols that apparently are not dependent on their well-established antioxidant behaviour. These activities could well be real, but there remain large gaps in our understanding of the behaviour of tocols in membranes, especially when it comes to the alpha-, beta-, gamma-, delta-chroman methylation patterns and the seemingly special nature of tocotrienols. It is inappropriate to make conclusions and develop models based on in vivo (or cell culture) results with reference to in vitro measurements of antioxidant activity. When present in biological membranes, tocols will experience a large variation in the local composition of phospholipids and the presence of neutral lipids such as cholesterol, both of which would be expected to change the efficiency of antioxidant action. It is likely that tocols are not homogeneously dispersed in a membrane, but it is still not known whether any specific combination of lipid head group and acyl chains are conferred special protection from peroxidation, nor do we currently appreciate the structural role that tocols play in membranes. Tocols may enhance curvature stress or counteract similar stresses generated by other lipids such as lysolipids. This review will outline what is known about the location and behaviour of tocols in phospholipid bilayers. We will draw mainly from the biophysical literature, but will attempt to extend the discussion to biologically relevant phenomena when appropriate. We hope that it will assist researchers when designing new experiments and when critically assessing the results, in turn providing a more thorough understanding of the biochemistry of tocols.

Tocopherol transfer protein deficiency modifies nuclear receptor transcriptional networks in lungs: Modulation by cigarette smoke in vivo

Dietary factors and environmental pollutants initiate signaling cascades that converge on AhR:Nrf2:NF-kappaB transcription factor (TF) networks and, in turn, affect the health of the organism through its effects on the expression of numerous genes. Reactive oxygen metabolites (ROMs) have been hypothesized to be common mediators in these pathways. alpha-Tocopherol (AT) is a potent, lipophilic, scavenger of ROMs in vitro and has been hypothesized to be a major chain-breaking anti-oxidant in lipoproteins and biological membranes in vivo. The lung offers a vital organ to test the various postulated actions of AT in vivo. Lung AT concentrations can be manipulated by several methods that include dietary and genetic techniques. In this study we have used mice with severe AT deficiency inflicted at birth by the deletion of AT transfer protein (ATTP) which is abundantly expressed in the liver and regulates systemic concentrations of AT. Mice and humans deficient in ATTP are AT deficient. Female ATTP-deficient (ATTP-KO) mice and their congenic ATTP normal (WT) mice fed a diet containing 35 IU AT/kg diet were used to test our hypothesis. The mice (n=5/group) were exposed to either air or cigarette smoke (CS, total suspended particles 60 mg/m(3), 6h/day), a source of ROM, for 3 or 10 days. Post-exposure lung tissue was dissected, RNA extracted from each lung and it was pooled group-wise and processed for GeneChip analysis (Affymetrix 430A 2.0). Differential analysis of the transcriptomes ( approximately 16,000 mRNAs) identified CS sensitive genes that were modulated by lung AT-concentration. CS activated AhR driven genes such as cyp1b1 whose induction was augmented in CS-exposed, AT-deficient lungs. However, CS-induced expression of some of the Nrf2 driven genes was not potentiated in the AT-deficient lungs. Largest clusters of CS-AT sensitive genes were lymphocyte and leukocyte specific genes. These gene-clusters included those encoding cytokines and immunoglobulins, which were repressed by CS and were modulated by lung AT concentrations. Our genome-wide analysis suggests reciprocal regulation of xenobiotic and immune response genes by CS and a modulatory role of lung AT concentration on the expression of these clusters of genes. These data suggest that in vivo network of AT, AT-metabolites and ATTP affects the transcription of genes driven by AhR, Nrf2 and NF-kappaB, transcription factor networks that transduce cellular metabolic signals and orchestrate adaptive responses of lungs to inhaled environmental pollutants.

α-Tocopherol-mediated caspase-3 up-regulation enhances susceptibility to apoptotic stimuli

Although alpha-tocopherol is known as an essential micronutrient involved in various oxidative stress-related processes, its non-antioxidant activities have only been characterized in recent years. In this study, we reveal that (+)-alpha-tocopherol [RRR-alpha-tocopherol] enhances cellular susceptibility to both oxidative and non-oxidative apoptosis-inducing stimuli through up-regulation of caspase-3/CPP32 expression in several human cell lines. Exposure of (+)-alpha-tocopherol pretreated cells to known apoptosis-inducing stimuli, such as Fas, H(2)O(2), or etoposide, resulted in an increase in cellular apoptotsis. In addition, (+)-alpha-tocopherol also elevated the pro-caspase-3 protein level and mRNA expression in a time- and dose-dependent manner, while other tocopherol analogues showed no effect. Experiments using a GC-specific DNA binding agent, mithramycin A, and an electrophoretic mobility shift assay demonstrated that Sp1 might mediate the enhanced expression of caspase-3. Our results also confirmed that (+)-alpha-tocopherol promotes the expression, but not the activation, of caspase-3 in various human cell lines. These findings provide biological evidence showing that (+)-alpha-tocopherol can amplify the apoptotic response by up-regulating the expression of pro-caspase-3.

Vitamin E mediates cell signaling and regulation of gene expression

alpha-Tocopherol modulates two major signal transduction pathways centered on protein kinase C and phosphatidylinositol 3-kinase. Changes in the activity of these key kinases are associated with changes in cell proliferation, platelet aggregation, and NADPH-oxidase activation. Several genes are also regulated by tocopherols partly because of the effects of tocopherol on these two kinases, but also independently of them. These genes can be divided in five groups: Group 1. Genes that are involved in the uptake and degradation of tocopherols: alpha-tocopherol transfer protein, cytochrome P450 (CYP3A), gamma-glutamyl-cysteine synthetase heavy subunit, and glutathione-S-transferase. Group 2. Genes that are implicated with lipid uptake and atherosclerosis: CD36, SR-BI, and SR-AI/II. Group 3. Genes that are involved in the modulation of extracellular proteins: tropomyosin, collagen-alpha-1, MMP-1, MMP-19, and connective tissue growth factor. Group 4. Genes that are connected to adhesion and inflammation: E-selectin, ICAM-1 integrins, glycoprotein IIb, IL-2, IL-4, IL-1b, and transforming growth factor-beta (TGF-beta). Group 5. Genes implicated in cell signaling and cell cycle regulation: PPAR-gamma, cyclin D1, cyclin E, Bcl2-L1, p27, CD95 (APO-1/Fas ligand), and 5a-steroid reductase type 1. The transcription of p27, Bcl2, alpha-tocopherol transfer protein, cytochrome P450 (CYP3A), gamma-glutamyl-cysteine sythetase heavy subunit, tropomyosin, IL-2, and CTGF appears to be upregulated by one or more tocopherols. All the other listed genes are downregulated. Gene regulation by tocopherols has been associated with protein kinase C because of its deactivation by alpha-tocopherol and its contribution in the regulation of a number of transcription factors (NF-kappaB, AP1). A direct participation of the pregnane X receptor (PXR) / retinoid X receptor (RXR) has been also shown. The antioxidant-responsive element (ARE) and the TGF-beta-responsive element (TGF-beta-RE) appear in some cases to be implicated as well.

Effect of vitamin E on glutathione-dependent enzymes

Reactive oxygen species and various electrophiles are involved in the etiology of diseases varying from cancer to cardiovascular and pulmonary disorders. The human body is protected against damaging effects of these compounds by a wide variety of systems. An important line of defense is formed by antioxidants. Vitamin E (consisting of various forms of tocopherols and tocotrienols) is an important fat-soluble, chain-breaking antioxidant. Besides working as an antioxidant, this compound possesses other functions with possible physiological relevance. The glutathione-dependent enzymes form another line of defense. Two important enzymes in this class are the free radical reductase and glutathione S-transferases (GSTs). The GSTs are a family of phase II detoxification enzymes. They can catalyze glutathione conjugation with various electrophiles. In most cases the electrophiles are detoxified by this conjugation, but in some cases the electrophiles are activated. Antioxidants do not act in isolation but form an intricate network. It is, for instance, known that vitamin E, together with glutathione (GSH) and a membrane-bound heat labile GSH-dependent factor, presumably an enzyme, can prevent damaging effects of reactive oxygen species on polyunsaturated fatty acids in biomembranes (lipid peroxidation). This manuscript reviews the interaction between the two defense systems, vitamin E and glutathione-dependent enzymes. On the simplest level, antioxidants such as vitamin E have protective effects on glutathione-dependent enzymes; however, we will see that reality is somewhat more complicated.

Neural precursor proliferation and newborn cell survival in the adult rat dentate gyrus are affected by vitamin E deficiency

The adult hippocampal neurogenesis is affected by vitamin E deficiency. In the present investigation we examined if neural precursor proliferation, newborn cell survival or both are altered by vitamin E deficiency. 5-Bromo-2'-deoxyuridine (BrdU) was employed as a marker of proliferating cells. BrdU-labelled cells were revealed 1 and 30 days after BrdU administration in order to evaluate proliferation and newborn cell survival, respectively. Cell proliferation decreased in controls from juvenile to adult age, and the decrease was lesser in vitamin E deficiency. Thus we found a higher number of proliferating cells in vitamin E-deficient rats than in age-matched controls at 5 months of age. Comparing the number of BrdU-positive cells between 1 and 30 days after the last BrdU injection revealed a remarkable decrease in all groups; this is the greatest in vitamin E-deficient rats and the lowest in control rats. Consistently cell death in the dentate gyrus, assessed by TUNEL technique, was found to decrease from 1 to 5 months of age, but at 5 months it was significantly higher in vitamin E-deficient rats than in age-matched controls. These data show that vitamin E deficiency enhances neural precursor proliferation and cell death during adult neurogenesis.

Gene expression induced by BO-653, probucol and BHQ in human endothelial cells

2, 3-Dihydro-5-hydroxy-2, 2-dipentyl-4, 6-di-tert-butylbenzofuran (BO-653) and probucol, which act as radical scavenging antioxidants, were developed as anti-atherosclerotic medicines. In order to investigate the effect of these antioxidants on cell functions, we analyzed their ability to regulate gene expression in cultured human umbilical vein endothelial cells (HUVECs) using an oligonucleotide chip. Among 6,416 genes, 17 genes including those encoding mitochondrial proteins and proteins related to oxidative stress response were induced more than 3 fold by BO-653, probucol and tert-butylated hydroquinone (BHO). On the other hand, genes of three subunits of proteasome (PSMA2, PSMA3, PSMA4) were down-regulated by these antioxidants. A gene of cytochrome P-450 1A1 isozyme, a drug-metabolizing phase I enzyme, was expressed only by BHQ treatment. These results suggested that anti-atherogenic antioxidants affected gene expression in HUVECs by which they might regulate cell functions against oxidative stress.

Did the antioxidant trials fail to validate the oxidation hypothesis?

Most clinical trials on antioxidants using vitamin E or beta-carotene have failed to note any significant change in cardiovascular endpoints. The results of these studies have been interpreted as a setback for the oxidation hypothesis. An analysis of the hypothesis and the trials, however, points out major misconceptions about the hypothesis and unjustified outcome expectations. Wrong selection of patient population, endpoints that are incompatible with the hypothesis, poor choice of antioxidants, and lack of inclusion of biochemical markers of oxidative stress and markers of vascular response are some of the contributors to the "failure" of these trials.

Severe total hepatic ischemia and reperfusion: relationship between very high alpha-tocopherol uptake and lipid peroxidation

Reperfusion injury of the liver occurs in liver transplantation and in major hepatectomies. It triggers a severe oxidative stress that leads to increased lipid peroxidation. In our study we examined the effect of parenteral supranutritional administration of alpha-tocopherol, a vitamin that plays a key role in the endogenous antioxidant system, to rats subjected to severe ischemia/reperfusion (I/R) injury of the liver. alpha-Tocopherol was administered to the animals at doses of 30 and 300 mg/kg bw, whereas total hepatic ischemia was induced for 60 min followed by 120 min reperfusion. Tissue and blood samples were collected for malonyldialdehyde (MDA) and serum alpha-tocopherol assay, respectively. In the sham operation group, mean MDA level in liver was 1.14 nmole/g wet tissue in the control subgroup, and 1.01 or 0.74 nmole/g wet tissue in the subgroups given 30 or 300 mg/kg alpha-tocopherol. In the I/R group, mean MDA level was 1.57 nmole/g wet tissue in the control subgroup, and 0.97 and 0.77 nmole/g wet tissue in the subgroups given 30 or 300 mg/kg alpha-tocopherol. Mean levels of alpha-tocopherol in serum (mumole/l) were 10.20 and 1.80 in the control subgroups, 25.28 and 11.25 in the subgroups treated with 30 and 300 mg/kg bw of alpha-tocopherol, and 31.00 and 13.02 in the subgroups treated with 30 and 300 mg/kg bw of alpha-tocopherol, within the sham-operation and I/R groups, respectively. A significant decrease of MDA accompanied by a significant increase of serum alpha-tocopherol was documented in the alpha-tocopherol-treated rats within both groups. Ischemia/reperfusion triggered a significant increase of the MDA level in the liver of the rats not treated with alpha-tocopherol as compares with the treated animals.

Biotin regulates the genetic expression of holocarboxylase synthetase and mitochondrial carboxylases in rats

Biotin is the cofactor of carboxylases [pyruvate (PC), propionyl-CoA (PCC), 3-methyl crotonyl-CoA and acetyl-CoA], to which it is covalently bound by the action of holocarboxylase synthetase (HCS). We have studied whether biotin also regulates their expression, as it does other, nonrelated enzymes (e.g., glucokinase, phosphoenol pyruvate carboxykinase, guanylate cyclase). For this purpose, HCS, PC and PCC mRNAs were studied in biotin-deficient rat liver, kidney, muscle and brain of biotin-deficient rats. PC- and PCC-specific activities and protein masses were also measured. The 24-h time course of HCS mRNA in deficient rats was examined after biotin supplementation. HCS mRNA was significantly reduced during vitamin deficiency. It increased in deficient rats after biotin was injected, reaching control levels 24 h after administration. These changes seem to be the first known instance in mammals of an effect of a water-soluble vitamin on a mRNA functionally related to it. In contrast, the decreased activities of the carboxylases were associated with reductions in the amounts of their enzyme proteins except in brain. However, their mRNA levels were not affected. There are no reports on these types of vitamin affecting the mRNA or protein levels of their apoenzymes or their products. This work provides evidence for biotin being a modulator of the genetic expression of the enzymes involved in its function as a cofactor. As such, it may be a useful model for probing a similar role for other water-soluble vitamins.

Alpha-tocopherol controls cell proliferation in the adult rat dentate gyrus

The effect of alpha-tocopherol on cell proliferation and proliferated cell survival was investigated in the dentate gyrus of adult rats. Adult rats were supplemented with alpha-tocopherol, injected with 5-bromo-2'-deoxyuridine (BrdU), that is incorporated into DNA during the S-phase, and killed at different time after BrdU injection. The number of newborn cells decreased after alpha-tocopherol supplementation, confirming the hypothesis that alpha-tocopherol is able to depress cell proliferation in vivo. Most newborn cells die within few days; more newborn cells survive in alpha-tocopherol-treated rats, suggesting the hypothesis that alpha-tocopherol decreases cell death.

Oxidative stress and vascular smooth muscle cell function in liver disease

Reactive oxygen species and reactive nitroxy species are now being recognized as regulatory molecules in signaling pathways influencing contractile and noncontractile functions of healthy vascular smooth muscle cells. In liver disease, oxidative stress is a systemic phenomenon, whose extent correlates with the severity of disease. A role for oxidative stress in the development of the hyperdynamic circulation in portal hypertension has been proposed. Evaluation of the limited available data indicates that it is premature to conclude that oxidative stress per se impacts on vascular smooth muscle cell function in liver disease.

Glutathione depletion induces apoptosis of rat hepatocytes through activation of protein kinase C novel isoforms and dependent increase in AP-1 nuclear binding

Treatment of isolated rat hepatocytes with the glutathione depleting agents L-buthionine-S,R-sulfoximine or diethylmaleate reproduced various cellular conditions of glutathione depletion, from moderate to severe, similar to those occurring in a wide spectrum of human liver diseases. To evaluate molecular changes and possible cellular dysfunction and damage consequent to a pathophysiologic level of GSH depletion, the effects of this condition on protein kinase C (PKC) isoforms were investigated, since these are involved in the intracellular specific regulatory processes and are potentially sensitive to redox changes. Moreover, a moderate perturbation of cellular redox state was found to activate novel PKC isoforms, and a clear relationship was shown between novel kinase activation and nuclear binding of the redox-sensitive transcription factor, activator protein-1 (AP-1). Apoptotic death of a significant number of cells, confirmed in terms of internucleosomal DNA fragmentation was a possible effect of these molecular reactions, and was triggered by a condition of glutathione depletion usually detected in human liver diseases. Finally, the inhibition of novel PKC enzymatic activity in cells co-treated with rottlerin, a selective novel kinase inhibitor, prevented glutathione-dependent novel PKC up-regulation, markedly moderated AP-1 activation, and protected cells against apoptotic death. Taken together, these findings indicate the existence of an apoptotic pathway dependent on glutathione depletion, which occurs through the up-regulation of novel PKCs and AP-1.

Diverse functions of antioxidants

All biological organisms have developed a defense system against oxidative stress, which is comprised of many kinds of antioxidants. Antioxidants are classified by function into four categories; preventive antioxidants; radical scavenging antioxidants; repair and de novo antioxidants; and adaptation. Radical scavenging antioxidants have the greatest advantage. Although the activities of radical scavenging antioxidant are determined by several factors, their chemical structure is of key importance. Furthermore, radical scavenging antioxidants have been explored to have a novel function by which they regulate gene expression of cell.

Antioxidant nutrients: current dietary recommendations and research update

OBJECTIVE

To review the importance of antioxidant nutrients in the maintenance of health and the prevention and treatment of disease, with a focus on data pertaining to vitamin C, vitamin E, selenium, and carotenoids. A secondary objective was to discuss the new Dietary Reference Intakes released by the Institute of Medicine (IOM) for these nutrients.

DATA SOURCES

IOM reports on the use of antioxidant vitamins were reviewed for nutrient recommendations. In addition, a MEDLINE search was performed to identify recent research and review articles on the topic, which were analyzed to identify key research findings in the area.

DATA SYNTHESIS

The review discusses the biologic processes of oxidation reactions and antioxidants in biologic systems, provides an overview of information on selected antioxidant nutrients, and explores their role in the prevention and treatment of cancer, cardiovascular disease, ocular disorders, and respiratory disorders.

CONCLUSION

There appear to be significant health benefits from dietary antioxidants, as can be found in fruits and vegetables. Some prospective assessment of the effect of supplemental antioxidants also suggests benefit, especially for vitamin E; however, there are conflicting results in this area. Overall, it appears that antioxidant nutrients, especially those from food sources, have important roles in preventing pathogenic processes related to cancer, cardiovascular disease, macular degeneration, cataracts, and asthma, and may enhance immune function.

Catechins delay lipid oxidation and alpha-tocopherol and beta-carotene depletion following ascorbate depletion in human plasma

Blood plasma was incubated with 50 mM AAPH [2, 2'-azobis-(2-amidinopropane) hydrochloride] in the absence or presence of catechins (5-100 microM). Lipid oxidation was evaluated by measuring the formation of 2-thiobarbituric acid reactive substances (TBARS). The concentration of alpha-tocopherol (AT), beta-carotene (BC), ascorbic acid (AA), and catechins was determined by reverse phase high performance liquid chromatography (HPLC) with electrochemical detection. All the assayed catechins inhibited plasma TBARS formation. Based on the calculated IC50, the order of effectiveness was: epicatechin gallate (ECG) > epigallocatechin gallate (EGCG) > epigallocatechin (EGC) > epicatechin (EC) > catechin (C). Catechins protected plasma AT and BC from AAPH-mediated oxidation. The order of effectiveness for AT protection was ECG > EGCG > EC = C > EGC; and for BC protection, the order was EGCG > ECG > EGC > > EC > C. The addition of catechins modified the kinetics of TBARS formation and AT depletion, but the rate of AA depletion was not affected. Catechin oxidation did not start until the complete depletion of AA, and it preceded AT depletion. These results indicate that catechins are effective antioxidants in human blood plasma, delaying the lipid oxidation and depletion of endogenous lipid-soluble antioxidants (AT and BC).

A novel human tocopherol-associated protein: cloning, in vitro expression, and characterization

Vitamin E (alpha-tocopherol) is an essential dietary nutrient for humans and animals. The mechanisms involved in cellular regulation as well as in the preferential cellular and tissue accumulation of alpha-tocopherol are not yet well established. We previously reported (Stocker, A., Zimmer, S., Spycher, S. E., and Azzi, A. (1999) IUBMB Life 48, 49-55) the identification of a novel 46-kDa tocopherol-associated protein (TAP) in the cytosol of bovine liver. Here, we describe the identification, the molecular cloning into Escherichia coli, and the in vitro expression of the human homologue of bovine TAP, hTAP. This protein appears to belong to a family of hydrophobic ligand binding proteins, which have the CRAL (cis-retinal binding motif) sequence in common. By using a biotinylated alpha-tocopherol derivative and the IASys resonant mirror biosensor, the purified recombinant protein was shown to bind tocopherol at a specific binding site with K(d) 4.6 x 10(-7) m. Northern analyses showed that hTAP mRNA has a size of approximately 2800 base pairs and is ubiquitously expressed. The highest amounts of hTAP message are found in liver, brain, and prostate. In conclusion, hTAP has sequence homology to proteins containing the CRAL_TRIO structural motif. TAP binds to alpha-tocopherol and biotinylated tocopherol, suggesting the existence of a hydrophobic pocket, possibly analogous to that of SEC14.

Specific cellular responses to alpha-tocopherol

In the last 10 years precise cellular functions of alpha-tocopherol, some of which are independent of its antioxidant/radical-scavenging ability, have been revealed. Absorption of alpha-tocopherol from the gut is a selective process. Other tocopherols are not absorbed or are absorbed to a lesser extent. At the post-translational level, alpha-tocopherol inhibits protein kinase C and 5-lipoxygenase and activates protein phosphatase 2A and diacylglycerol kinase. Some genes [platelet glycoprotein IV/thrombospondin receptor/class B scavenger receptor (CD36), alpha-tocopherol transfer protein (alpha-TTP), alpha-tropomyosin, connective tissue growth factor and collagenase] are affected by alpha-tocopherol at the transcriptional level. alpha-Tocopherol also inhibits cell proliferation, platelet aggregation, monocyte adhesion and the oxygen burst in neutrophils. Other antioxidants, such as beta-tocopherol and probucol, do not mimic these effects, suggesting a nonantioxidant, alpha-tocopherol-specific molecular mechanism.