Tocotrienols and tocopherols represent the 2 subgroups within the vitamin E family of compounds, but tocotrienols display significantly greater apoptotic activity against a variety of cancer cell types. However, the exact mechanism mediating tocotrienol-induced apoptosis is not understood. Studies were conducted to determine the effects of tocotrienols on mitochondrial-stress-mediated apoptotic signaling in neoplastic +SA mammary epithelial cells grown in vitro. Exposure for 24 h to 0-20 micromol/L gamma-tocotrienol resulted in a dose-responsive increase in +SA cells undergoing apoptosis, as determined by flow cytometric analysis of Annexin V staining. However, tocotrienol-induced apoptosis was not associated with a disruption or loss of mitochondrial membrane potential, or the release of mitochondrial cytochrome c into the cytoplasm, as determined by JC-1 flow cytometric staining and ELISA assay, respectively. Interestingly, apoptotic +SA cells showed a paradoxical decrease in mitochondrial levels of pro-apoptotic proteins Bid, Bax, and Bad, and a corresponding increase in mitochondrial levels of anti-apoptotic proteins, Bcl-2 and Bcl-xL, suggesting that mitochondrial membrane stability and integrity might actually be enhanced for a limited period of time following acute tocotrienol exposure. In summary, these findings clearly demonstrate that tocotrienol-induced apoptosis occurs independently of mitochondrial stress apoptotic signaling in neoplastic +SA mammary epithelial cells.
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Mechanisms mediating the antiproliferative and apoptotic effects of vitamin E in mammary cancer cells
Sylvester PW, Shah SJ.
Front Biosci. 2005 Jan 1;10:699-709. Print 2005 Jan 1.
Tocopherols and tocotrienol represent the two subgroups within the vitamin E family of compounds, but only tocotrienols display potent anticancer activity at doses that have little or no effect on normal cell growth or function. Tocotrienols are potent antioxidants, but antitumor activity is independent of antioxidant activity. The exact reason why tocotrienols are more potent than tocopherols is not completely understood, but at least part of the reason is because of greater cellular accumulation. Furthermore, dose-response studies show that growth inhibitory doses of tocotrienolsare 5-6 times lower than their corresponding lethal doses, suggesting that the antiproliferative and cytotoxic effects of tocotrienols are mediated through different mechanisms. Recent studies showed that tocotrienol-induced programmed cell death (apoptosis) results from the activation of specific intracellular cysteine proteases (caspases) associated with death receptor activation and signal transduction. Furthermore, combined treatment with specific caspase inhibitors blocked the cytotoxic effects of tocotrienols in malignant mammary epithelial cells. In contrast, tocotrienolinhibition of cell proliferation appears to involve the suppression of multiple hormone- and growth factor-receptor mitogenic signaling pathways. Although additional studies are required to clarify the intracellular mechanisms mediating the anticancer effects of tocotrienols, experimental evidence strongly suggests that dietary supplementation of tocotrienols may provide significant health benefits in lowering the risk of breast cancer in women.
Our understanding of the role of vitamin E in human nutrition, health, and disease has broadened and changed over the past two decades. Viewed initially as nature’s most potent lipid-soluble antioxidant (and discovered for its crucial role in mammalian reproduction) we have now come to realize that vitamin E action has many more facets, depending on the physiological context. Although mainly acting as an antioxidant, vitamin E can also be a pro-oxidant; it can even have nonantioxidant functions: as a signaling molecule, as a regulator of gene expression, and, possibly, in the prevention of cancer and atherosclerosis. Since the term vitamin E encompasses a group of eight structurally related tocopherols and tocotrienols, individual isomers have different propensities with respect to these novel, nontraditional roles. The particular beneficial effects of the individual isomers have to be considered when dissecting the physiological impact of dietary vitamin E or supplements (mainly containing only the alpha-tocopherol isomer) in clinical trials. These considerations are also relevant for the design of transgenic crop plants with the goal of enhancing vitamin E content because an engineered biosynthetic pathway may be biased toward formation of one isomer. In contrast to the tremendous recent advances in knowledge of vitamin E chemistry and biology, there is little hard evidence from clinical and epidemiologic studies on the beneficial effects of supplementation with vitamin E beyond the essential requirement.
Health promotion by flavonoids, tocopherols, tocotrienols, and other phenols: direct or indirect effects? Antioxidant or not?
Barry Halliwell, Joseph Rafter, and Andrew Jenner
Am J Clin Nutr. 2005 Jan;81(1 Suppl):268S-276S.
Foods and beverages rich in phenolic compounds, especially flavonoids, have often been associated with decreased risk of developing several diseases. However, it remains unclear whether this protective effect is attributable to the phenols or to other agents in the diet. Alleged health-promoting effects of flavonoids are usually attributed to their powerful antioxidant activities, but evidence for in vivo antioxidant effects of flavonoids is confusing and equivocal. This may be because maximal plasma concentrations, even after extensive flavonoid intake, may be low (insufficient to exert significant systemic antioxidant effects) and because flavonoid metabolites tend to have decreased antioxidant activity. Reports of substantial increases in plasma total antioxidant activity after flavonoid intake must be interpreted with caution; findings may be attributable to changes in urate concentrations. However, phenols might exert direct effects within the gastrointestinal tract, because of the high concentrations present. These effects could include binding of prooxidant iron, scavenging of reactive nitrogen, chlorine, and oxygen species, and perhaps inhibition of cyclooxygenases and lipoxygenases. Our measurements of flavonoids and other phenols in human fecal water are consistent with this concept. We argue that tocopherols and tocotrienols may also exert direct beneficial effects in the gastrointestinal tract and that their return to the gastrointestinal tract by the liver through the bile may be physiologically advantageous.
Lack of oxidative stress in a selenium deficient area in Ivory Coast–potential nutritional antioxidant role of crude palm oil
Tiahou G, Maire B, Dupuy A, Delage M, Vernet MH, Mathieu-Daudé JC, Michel F, Sess ED, Cristol JP.
Eur J Nutr. 2004 Dec;43(6):367-74. Epub 2004 Oct 20.
BACKGROUND:Previous studies have described an important selenium deficiency in a mountain region (Glanle) in the west of Ivory Coast.
AIM OF THE STUDY:To assess the antioxidant capacity of subjects from a selenium deficient area in Ivory Coast (Glanle region).
METHODS:This study involved 57 subjects, 18 to 69 years old, living in the Glanle region and 56 healthy controls living in the southern coastal region (Bodou). In the Glanle region families consume basically a vegetarian and crude palm oil diet, whereas in the Bodou region, families eat a fish-based diet with principally refined palm oil. Fasting blood samples were collected to assess the following parameters: lipid status (plasma total lipids; total-, HDL and LDL-cholesterol; triglycerides; phospholipids; fatty acid composition), plasma protein status (total protein, albumin, transthyretin, orosomucoid, CRP, transferrin), antioxidant capacity (plasma selenium, uric acid, retinol, alpha-tocopherol and tocotrienols levels, plasma seleno-glutathione peroxidase (GSHPx) activity) and oxidative stress markers (malondialdehyde (MDA) and advanced oxidation protein products (AOPP)).
RESULTS:The mountain region samples (Glanle) were characterized by significantly lower plasma albumin, total-, HDL- and LDL-cholesterol, retinol and selenium levels, plasma PUFA content and GSHPx activity, but significantly higher alpha-tocopherol index and total tocotrienol level, than controls from the coastal area (Bodou). These results suggest a higher exposure risk to oxidative stress for the mountain region subjects. However, the absence of oxidative damage in this group provides evidence of a selenium independent protection mechanism against oxidative stress. This protection is related to lower plasma LDL cholesterol and PUFA content, and to higher alpha-tocopherol index, delta and total tocotrienols.
CONCLUSION:The long-term consumption of crude palm oil could be considered as an effective protective factor against oxidative stress.
Suppression of diethylnitrosamine and 2-acetylaminofluorene-induced hepatocarcinogenesis in rats by tocotrienol-rich fraction isolated from rice bran oil.
Iqbal J, Minhajuddin M, Beg ZH.
Eur J Cancer Prev. 2004 Dec;13(6):515-20.
The anticancer efficacy of tocotrienol-rich fraction (TRF) was evaluated during diethylnitrosamine (DEN)/2-acetylaminofluorene (AAF)-induced hepatocarcinogenesis in male Sprague-Dawley rats. TRF treatment was carried out for 6 months, and was started 2 weeks before initiation phase of hepatocarcinogenesis. Morphological examination of the livers from DEN/AAF rats showed numerous off-white patches and few small nodules, which were significantly reduced by TRF treatment. Cytotoxic damage by DEN/AAF was estimated by alkaline phosphatase (ALP) release into the plasma from the cell membranes. DEN/AAF caused a twofold increase in the activity of ALP in plasma as compared with normal control rats, and this increase was prevented significantly by TRF treatment. We observed an increase of 79% in liver ALP activity in DEN/AAF rats, which was further increased by another 48% after the administration of TRF. Hepatic activity of glutathione S-transferase (GST) was also increased (3.5-fold) during the induction of hepatic carcinogenesis. Lipid peroxidation and low-density lipoprotein (LDL) oxidation increased threefold following initiation by DEN/AAF as compared with normal control rats. However, TRF treatment to DEN/AAF-treated rats substantially decreased (62-66%) the above parameters and thus limited the action of DEN/AAF. We conclude that long-term intake of TRF could reduce cancer risk by preventing hepatic lipid peroxidation and protein oxidation damage due to its antioxidant actions.
Vitamin E and breast cancer
Kline K, Yu W, Sanders BG.
J Nutr. 2004 Dec;134(12 Suppl):3458S-3462S.
Vitamin E is a term that describes a group of compounds with similar yet unique chemical structures and biological activities. One interesting property possessed by certain vitamin E compounds-namely, delta-tocotrienol, RRR-alpha-tocopheryl succinate [vitamin E succinate (VES), a hydrolyzable ester-linked succinic acid analogue of RRR-alpha-tocopherol], and a novel vitamin E analogue referred to as alpha-TEA (alpha-tocopherol ether linked acetic acid analogue, which is a stable nonhydrolyzable analogue of RRR-alpha-tocopherol)-is their ability to induce cancer cells but not normal cells to undergo a form of cell death called apoptosis. In contrast, the parent compound, RRR-alpha-tocopherol, also referred to as natural or authentic vitamin E and known for its antioxidant properties, does not induce cancer-cell apoptosis. Efforts to understand how select vitamin E forms can induce cancer cells to undergo apoptosis have identified several nonantioxidant biological functions, including restoration of pro-death transforming growth factor-beta and Fas signaling pathways. Recent studies with alpha-TEA show it to be a potent inducer of apoptosis in a wide variety of epithelial cancer cell types, including breast, prostate, lung, colon, ovarian, cervical, and endometrial in cell culture, and to be effective in significantly reducing tumor burden and metastasis in a syngeneic mouse mammary tumor model, as well as xenografts of human breast cancer cells. Studies also show that alpha-TEA, in combination with the cyclooxygenase-2 inhibitor celecoxib and the chemotherapeutic drug 9-nitro-camptothecin decreases breast cancer animal model tumor burden and inhibits metastasis significantly better than do single-agent treatments.
Vitamin E: Underestimated as an antioxidant
Pfluger P, Kluth D, Landes N, Bumke-Vogt C, Brigelius-Flohé R.
Redox Rep. 2004;9(5):249-54.
Some 80 years after its discovery, vitamin E has experienced a renaissance which is as surprising as it is trivial. Although vitamin E is essential for reproduction, in rats at least, and deficiency causes neurological disorders in humans, the main interest in the last decades has concentrated on its antioxidant functions. This focus has highly underestimated the biological importance of vitamin E, which by far exceeds the need for acting as a radical scavenger. Only recently has it become clear that vitamin E can regulate cellular signaling and gene expression. Out of the eight different tocols included in the term vitamin E, alpha-tocopherol often exerts specific functions, which is also reflected in its selective recognition by proteins such as the alpha-tocopherol transfer protein and alpha-tocopherol-associated proteins. Vitamin E forms other than alpha-tocopherol are very actively metabolised, which explains their low biopotency. In vivo, metabolism may also attenuate the novel functions of gamma-tocopherol and tocotrienolsobserved in vitro. On the other hand, metabolites derived from individual forms of vitamin E have been shown to exert effects by themselves. This article focuses on the metabolism and novel functions of vitamin E with special emphasis on differential biological activities of individual vitamin E forms.
Tocopherols are known to undergo metabolism to phytyl chain-shortened metabolites excreted in urine. We sought to characterize the pathway, including associated enzymes, involved in this biotransformation. We previously found that human hepatoblastoma (HepG2) cultures metabolized tocopherols to their corresponding short-chain carboxychromanols. Putative metabolites of gamma-tocopherol that contained intact chromanol moieties were structurally identified using HepG2 cultures and electron impact gas chromatography-mass spectrometry. A microsomal assay for synthesis of the initial omega-oxidation metabolites was developed and used to screen several recombinant human liver cytochrome P450 isozymes for omega-hydroxylase activity. Seven metabolites of gamma-tocopherol were identified in HepG2 cultures, including 13′-hydroxy-gamma-TOH and all six carboxychromanols predicted by sequential omega-oxidation truncation. Rat and human liver microsomes catalyzed synthesis of 13′-OH- and 13′-COOH-gamma-TOH, but not other metabolites, in the presence of NADPH. Inclusion of NAD favored synthesis of the 13′-COOH metabolite. Recombinant CYP4F2, but not other major human liver CYP isoforms (including CYP3A4 and 3A7), exhibited tocopherol-omega-hydroxylase activity. Liver microsomes and recombinant CYP4F2 both exhibited substrate preference for gamma-TOH over alpha-TOH, and recent studies show thattocotrienols are catabolized more extensively than the corresponding tocopherols. Comparative rates of omega-oxidation of tocochromanols in hepatocytes are inversely related to biopotency and directly related to cytotoxicity of these substances in macrophages. The liver contains a cytochrome P450-mediated pathway that preferentially catabolizes “non-alpha” tocochromanols to excretable metabolites. This metabolic pathway appears central to the optimization of tissue tocochromanol status.
Vitamin E is the most important lipid-soluble antioxidant in humans. Specific tocopherol-binding proteins favor the retention of the most potent vitamin E homologue, RRR-alpha-tocopherol (RRR-alpha-T) in man. The crystal structures of both the ligand-charged and the apo-forms of human alpha-tocopherol transfer protein (alpha-TTP) and of human supernatant protein factor (SPF) have been solved. The renewed interest in the biological function of tocopherol binders is based on the discovery of ataxia with vitamin E deficiency, a neurological disorder that is caused by genetic defects of the alpha-TTP gene and/or vitamin E deficiency. The analysis of the crystal structure of alpha-TTP provides the molecular basis of vitamin E retention in man. SPF has been reported to enhance cholesterol biosynthesis by facilitating the conversion of squalene to lanosterol. Nevertheless, the physiological role of SPF as well as its ligand specificity is not known. Investigations on the substrate specificity of SPF have uncovered binding of RRR-alpha-tocopherylquinone (RRR-alpha-TQ). RRR-alpha-TQ represents the major physiological oxidation product of RRR-alpha-T. The three-dimensional overlay of the ligand-charged structures of SPF and alpha-TTP indicates that ligand specificity in both proteins is mostly modulated by side-chain variations rather than by the backbone. Recent reports point towards the in vivo reduction of RRR-alpha-TQ to RRR-alpha-TQH(2) and its protective role in low-density lipoprotein oxidation. On the basis of these reports, it is proposed that SPF may enhance cholesterol biosynthesis indirectly by mediating the transfer of RRR-alpha-TQ to low-density lipoprotein, thus reducing oxidation of low-density lipoprotein and its subsequent cellular uptake by scavenger receptors.