Alpha-tocopheryl phosphate: a novel, natural form of vitamin E

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.

Synergy research: vitamins and secondary plant components in the maintenance of the redox-homeostasis and in cell signaling

The maintenance of the redox-homeostasis is an essential task of antioxidants. Reactive oxygen species (ROS) formed during oxidative stress can potentially damage the normal cellular functions and support pathological processes like atherosclerosis in vessels or malignant growth in other tissues, but also the aging process. However, recent findings link ROS also to cell survival and/or proliferation, which revolutionises the age-old dogmatic view of ROS being exclusively involved in cell damage and death. Low concentrations of hydrogenperoxide e.g. are involved in cell signaling and can activate mitogen-activated kinases (MAPK) to initiate cell growth. Nutritional antioxidants like vitamin C or E can promote endothelial cell growth, but can also inhibit growth of muscle cells, and influence MAPK. Thus, keeping the redox-homeostasis in a steady state especially in the context of tissue regeneration appears to be more important than previously known and seems to be a controlled synergistic action of antioxidants and ROS. The present review summarizes the properties and functions of ROS and nutritional antioxidants like the vitamins C and E, and polyphenols in redox-homeostasis. Their relevance in the treatment of various diseases is discussed in the context of a multitarget therapy with nutraceuticals and phytotherapeutic drugs.

Afamin is synthesized by cerebrovascular endothelial cells and mediates alpha-tocopherol transport across an in vitro model of the blood-brain barrier

Alpha-tocopherol (alphaTocH), a member of the vitamin E family, is essential for normal neurological function. Despite the importance of alphaTocH transport into the CNS, transfer mechanisms across the blood-brain barrier (BBB) are not entirely clear. We here investigate whether afamin, a known alphaTocH-binding protein, contributes to alphaTocH transport across an in vitro model of the BBB consisting of primary porcine brain capillary endothelial cells (BCEC) and basolaterally cultured astrocytoma cells. Exogenously added afamin had no adverse effects on BCEC viability or barrier function and was transported across BCEC Transwell cultures. Furthermore, alphaTocH transport across polarized BCEC cultures to astrocytoma cells is facilitated by afamin, though to a lesser extent than by high-density lipoprotein-mediated transport, an essential and in vivo operating alphaTocH import pathway at the cerebrovasculature. We also demonstrate that porcine BCEC endogenously synthesize afamin. In line with these in vitro findings, afamin was detected by immunohistochemistry in porcine, human postmortem, and mouse brain, where prominent staining was observed almost exclusively in the cerebrovasculature. The demonstration of afamin mRNA expression in isolated brain capillaries suggests that afamin might be a new family member of binding/transport proteins contributing to alphaTocH homeostasis at the BBB in vivo.

Cardioprotection with alpha-tocopheryl phosphate: amelioration of myocardial ischemia reperfusion injury is linked with its ability to generate a survival signal through Akt activation

The emerging potential of alpha-tocopheryl phosphate, a phosphoric acid ester of alpha-tocopherol, in health benefits was tested gavaging this compound (5 mg/kg body wt) to a group of rats for a period of thirty days while the control rats were given water only. After thirty days, the rats were sacrificed, the hearts excised, and the isolated hearts were perfused by working mode. Both control and experimental hearts were subjected to 30-min global ischemia followed by 2 h of reperfusion. The tocopheryl phosphate fed rats exhibited significant cardioprotection as evidenced by improved ventricular performance and reduced myocardial infarct size and cardiomyocyte apoptosis. Supplementation with alpha-tocopheryl phosphate converted MAP kinase-induced death signal into a survival signal by enhancing anti-apoptotic p42/44 ERK kinase and p38 MAPKbeta and reducing pro-apoptotic proteins p38 MAPKalpha and JNK. In concert, the phosphorylation of pro-apoptotic c-Src was also reduced. Tocopheryl phosphate increased the DNA binding of the redox-sensitive transcription factor NFkappaB and potentiated the activation of anti-death protein Bcl-2 and survival signaling protein Akt. The results of this study demonstrated for the first time that tocopheryl phosphate could ameliorate myocardial ischemic reperfusion injury by converting ischemia/reperfusion-mediated death signal into a survival signal by modulating MAP kinase signaling.

Differential effects of the ascorbyl and tocopheryl derivative on the methamphetamine-induced toxic behavior and toxicity

A previous study showed that high doses of methamphetamine induce self-injurious behavior (SIB) in rodents. Furthermore, the combination of methamphetamine and morphine increased lethality in mice. We recently surmised that the rise in SIB and mortality induced by methamphetamine and/or morphine may be related to oxidative stress. The present study was designed to determine whether an antioxidant could inhibit SIB or mortality directly induced by methamphetamine and/or morphine. The SIB induced by 20mg/kg of methamphetamine was abolished by the administration of Na L-ascorbyl-2-phosphate (APS: 300 mg/kg), but not Na DL-alpha-tocopheryl phosphate (TPNa: 200mg/kg). In contrast, APS (300 mg/kg) and TPNa (200mg/kg) each significantly attenuated the lethality induced by methamphetamine and morphine. The present study showed that the signal intensity of superoxide adduct was increased by 20mg/kg of methamphetamine in the heart and lungs, and methamphetamine plus morphine tended to increase superoxide adduct in all of the tissues measured by ESR spin trap methods. Adduct signal induced in brain by methamphetamine administration increased in significance, but in mouse administrated methamphetamine plus morphine. There are differential effects of administration of methamphetamine and coadministration of methamphetamine plus morphine on adduct signal. These results suggest that APS and TPNa are effective for reducing methamphetamine-induced toxicity and/or toxicological behavior. While APS and TPNa each affected methamphetamine- and/or morphine-induced toxicology and/or toxicological behavior, indicating that both drugs have antioxidative effects, their effects differed.

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.

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

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

Enhancement of intracellular gamma-tocopherol levels in cytokine-stimulated C3H 10T1/2 fibroblasts: relation to NO synthesis, isoprostane formation, and tocopherol oxidation

Background

Stimulation of C3H 10T1/2 murine fibroblasts with interferon-γ(IFN) and bacterial lipopolysaccharide (LPS) generates reactive oxygen and nitrogen species leading to DNA damage, lipid oxidation, and tocopherol oxidation. The tocopherols possess unique chemical and biological properties that suggest they have important roles related to intracellular defense against radical-mediated damage.

Results

Despite increased levels of reactive oxidants and decreased media tocopherol, cellular levels of γ-tocopherol, but not α-tocopherol, were observed to increase significantly when cells were treated with IFN/LPS. Inhibition of nitric oxide (NO) synthesis by a specific inhibitor of inducible NO synthase (iNOS) increased both intracellular α-tocopherol and γ-tocopherol concentrations, but did not significantly alter the reduction in media tocopherol levels caused by IFN/LPS treatment. Both exposure to exogenous NO and cellular synthesis of NO in cell culture increased media levels of 8-epi-prostaglandin F2α, a marker of oxidative lipid damage, whereas inhibition of endogenous NO synthesis reduced media 8-epi-prostaglandin F2α formation to control levels.

Conclusion

Elevated intracellular levels of γ-tocopherol in response to the cellular inflammatory state may indicate that it serves a unique role in minimizing cellular damage resulting from endogenous NO synthesis. Results of the current study suggest that NO is an important mediator of damage within the cell, as well as in the oxidation of both α- and γ-tocopherols. The paradoxical increase in cellular tocopherol associated with the induction of NO synthesis may indicate either enhanced cellular transport/decreased export for tocopherols or recruitment of free tocopherol from tocopherol storage molecules.

Molecular mechanism of α-tocopheryl-phosphate transport across the cell membrane

alpha-Tocopheryl-phosphate (alpha-TP) is synthesized and hydrolyzed in animal cells and tissues where it modulates several functions. alpha-TP is more potent than alpha-T in inhibiting cell proliferation, down-regulating CD36 transcription, inhibiting atherosclerotic plaque formation. Administration of alpha-TP to cells or animals requires its transfer through membranes, via a transporter. We show here that alpha-TP is passing the plasma membrane via a system that is inhibited by glibenclamide and probenecid, inhibitors of a number of transporters. Glibenclamide and probenecid prevent dose-dependently alpha-TP inhibition of cell proliferation. The two inhibitors act on ATP binding cassette (ABC) and organic anion transporters (OAT). Since ABC transporters function to export solutes and alpha-TP is transported into cells, it may be concluded that alpha-TP transport may occur via an OAT family member. Due to the protection by glibenclamide and probenecid on the alpha-TP induced cell growth inhibition it appears that alpha-TP acts after its uptake inside cells.

Clinical pharmacology and therapeutic use of antioxidant vitamins

The clinical use of antioxidants has gained considerable interest during the last decade. It was suggested from epidemiological studies that diets high in fruits and vegetables might help decrease the risk of cardiovascular disease. Therefore, supplements of vitamins C and E were applied through protocols aimed to prevent diseases such as atherosclerosis, preeclampsia or hypertension, thought to be mediated by oxidative stress. Despite the biological properties of these vitamins could account for an effective protection, as shown by several clinical and experimental studies, their efficacy remains controversial in the light of some recent clinical trials and meta-analyses. However, the methodology of these studies, criteria for selection of patients, the uncertain extent of progression of the disease when initiating supplementation, the lack of mechanistic studies containing basic scientific aspects, such as the bioavailability, pharmacokinetic properties, and the nature of the antioxidant sources of vitamins, could account for the inconsistency of the various clinical trials and meta-analyses assessing the efficacy of these vitamins to prevent human diseases. This review presents a survey of the clinical use of antioxidant vitamins E and C, proposing study models based on the biological effects of these compounds likely to counteract the pathophysiological mechanisms able to explain the structural and functional organ damage.

Vitamin E: inflammation and atherosclerosis

Cardiovascular disease (CVD) is the leading cause of morbidity and mortality in the western world with its incidence increasing lately in developing countries. Several lines of evidence support a role for inflammation in atherogenesis. Hence, dietary micronutrients having antiinflammatory properties may have a potential beneficial effect with regard to CVD. Vitamin E is a potent antioxidant with antiinflammatory properties. It comprises eight different isoforms: four tocopherols (T) (alpha, beta, gamma, and delta) and four tocotrienols (T3) (alpha, beta, gamma, and delta). A wealth of data is available for the preventive efficacy of alpha-T. alpha-T supplementation in human subjects and animal models has been shown to be antioxidant and antiinflammatory in terms of decreasing C-reactive protein (CRP) and release of proinflammatory cytokines, the chemokine IL-8 and PAI-1 levels especially at high doses. gamma-T is effective in decreasing reactive nitrogen species and also appears to have antiinflammatory properties; however, there are scanty data examining pure gamma-T preparations. Furthermore, tocotrienols (alpha and gamma) also have implications for prevention of CVD; however, there are conflicting and insufficient data in the literature with regards to their potency. In this chapter, we have gathered recent emerging data on alpha-T specifically and also have given a composite view of gamma-T and tocotrienols especially with regards to their effect on inflammation as it relates to CVD.

A feasibility study quantifying in vivo human alpha-tocopherol metabolism

BACKGROUND

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

OBJECTIVE

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

DESIGN

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

RESULTS

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

CONCLUSIONS

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

Evaluation of the safety of mixed tocopheryl phosphates (MTP)—A formulation of α-tocopheryl phosphate plus α-di-tocopheryl phosphate

The safety of a formulation of mixed tocopheryl phosphates, (MTP) was evaluated in a series of toxicological tests in vivo using rats, mice and rabbits and in vitro using bacterial and mammalian cell cultures. The tests conducted included an oral LD(50) study, three 28-day oral repeat-dose studies, two dermal toxicity tests, an ocular irritation test, mutagenic potential tests, and chromosomal aberrations tests. MTP consists of mono alpha-tocopheryl phosphate (TP) and di-tocopheryl phosphate (T(2)P) and is intended for use as a dietary supplement and for dermal applications in humans and animals. The dermal and oral LD(50) values of MTP were determined to be >1130 mg/kg bw (918 mg tocopherol equivalents/kg bw) in rabbits and rats, respectively. MTP was not a dermal or eye irritant in rabbits and showed no allergenic potential in mice. In the mutagenicity and genotoxicity studies, MTP did not increased the number of revertants in Salmonella typhimurium or Escherichia coli and did not induce chromosomal aberrations in cultured Chinese hamster ovary (CHO) cells. When administered daily for 28 days by gavage at doses up to 955 mg/kg bw/day (780 mg tocopherol equivalents/kg bw/day), MTP produced no consistent, dose-dependent adverse effects in rats.