Molecular basis of vitamin E action: tocotrienol modulates 12-lipoxygenase, a key mediator of glutamate-induced neurodegeneration

Delivery of orally supplemented alpha-tocotrienol to vital organs of rats and tocopherol-transport protein deficient mice

The natural vitamin E tocotrienol (TCT) possesses biological properties not shared by tocopherols (TCP). Nanomolar alpha-TCT, not alpha-TCP, is potently neuroprotective (JBC 275:13049; 278:43508). Tocopherol-transport protein (TTP) represents the primary mechanism for maintaining normal alpha-TCP concentrations in plasma and extrahepatic tissues. TTP primarily transports alpha-TCP and has low affinity for alpha-TCT. There are no studies that have investigated tissue delivery of alpha-TCT when orally gavaged on a long-term basis. A long-term study was conducted to examine the effects of alpha-TCT or alpha-TCP supplementation, either alone or in combination, on tissue levels. Rats were maintained on a vitamin E-deficient diet and gavaged with alpha-TCT or alpha-TCP alone or in combination. Five generations of rats were studied over 60 weeks. TTP-deficient mice were supplemented with TCT and bred to examine tissue delivery of oral alpha-TCT. Orally supplemented alpha-TCT was effectively delivered to most tissues over time. When co-supplemented, alpha-TCP outcompeted alpha-TCT for transport systems delivering vitamin E to tissues. To evaluate the significance of TTP in alpha-TCT delivery to tissues, tissue levels of alpha-TCT in supplemented TTP-deficient mice were studied. alpha-TCT was transported to several vital organs in TTP-deficient mice. alpha-TCT restored fertility in TTP-deficient mice. In sum, orally supplemented alpha-TCT was successfully delivered to several vital organs. The transport efficiency of alpha-TCT to tissues may be maximized by eliminating the co-presence of alpha-TCP in the oral supplement. Examination of whether alpha-TCT may benefit humans suffering from neurological disorders because of congenital TTP deficiency is warranted.

Neuroprotective properties of the natural vitamin E alpha-tocotrienol

BACKGROUND AND PURPOSE

The current work is based on our previous finding that in neuronal cells, nmol/L concentrations of alpha-tocotrienol (TCT), but not alpha-tocopherol (TCP), blocked glutamate-induced death by suppressing early activation of c-Src kinase and 12-lipoxygenase.

METHODS

The single neuron microinjection technique was used to compare the neuroprotective effects of TCT with that of the more widely known TCP. Stroke-dependent brain tissue damage was studied in 12-Lox-deficient mice and spontaneously hypertensive rats orally supplemented with TCT.

RESULTS

Subattomole quantity of TCT, but not TCP, protected neurons from glutamate challenge. Pharmacological as well as genetic approaches revealed that 12-Lox is rapidly tyrosine phosphorylated in the glutamate-challenged neuron and that this phosphorylation is catalyzed by c-Src. 12-Lox-deficient mice were more resistant to stroke-induced brain injury than their wild-type controls. Oral supplementation of TCT to spontaneously hypertensive rats led to increased TCT levels in the brain. TCT-supplemented rats showed more protection against stroke-induced injury compared with matched controls. Such protection was associated with lower c-Src activation and 12-Lox phosphorylation at the stroke site.

CONCLUSIONS

The natural vitamin E, TCT, acts on key molecular checkpoints to protect against glutamate- and stroke-induced neurodegeneration.

12/15-Lipoxygenase gene disruption and vitamin E administration diminish atherosclerosis and oxidative stress in apolipoprotein E deficient mice through a final common pathway

Studies in mouse models of atherosclerosis using 12/15-lipoxygenase (12/15-LO) gene disruption and transgenic overexpression demonstrate a pro-oxidative, pro-atherogenic role for this pathway. Vitamin E has been shown to suppress lipid peroxidation and reduce early atherogenesis in several mouse models, although conflicting results from several clinical trials have been reported. ApoE(-/-) and apoE(-/-)/12/15-LO(-/-) mice were maintained on normal chow diet with or without Vitamin E supplement (2000 IU/kg). Plasma Vitamin E, urinary 8,12-iso-iPF(2alpha)-VI and aortic lesion quantitation were assessed. Plasma Vitamin E levels significantly increased upon Vitamin E diet supplementation. 12/15-LO gene disruption resulted in significantly reduced aortic lesions and decreased urinary 8,12-iso-iPF(2alpha)-VI levels in apoE(-/-) mice, similar to Vitamin E administration in the absence of 12/15-LO gene disruption. However, Vitamin E dietary supplementation did not afford additive or synergistic protection in apoE(-/-)/12/15-LO(-/-) mice. These results suggest that early 12/15-LO-mediated lipid peroxidation triggers ensuing non-enzymatic peroxidation that is susceptible to Vitamin E antioxidant action in a common pathway of atherogenesis.

Peroxynitrite-induced neuronal apoptosis is mediated by intracellular zinc release and 12-lipoxygenase activation

Peroxynitrite toxicity is a major cause of neuronal injury in stroke and neurodegenerative disorders. The mechanisms underlying the neurotoxicity induced by peroxynitrite are still unclear. In this study, we observed that TPEN [N,N,N',N'-tetrakis (2-pyridylmethyl)ethylenediamine], a zinc chelator, protected against neurotoxicity induced by exogenous as well as endogenous (coadministration of NMDA and a nitric oxide donor, diethylenetriamine NONOate) peroxynitrite. Two different approaches to detecting intracellular zinc release demonstrated the liberation of zinc from intracellular stores by peroxynitrite. In addition, we found that peroxynitrite toxicity was blocked by inhibitors of 12-lipoxygenase (12-LOX), p38 mitogen-activated protein kinase (MAPK), and caspase-3 and was associated with mitochondrial membrane depolarization. Inhibition of 12-LOX blocked the activation of p38 MAPK and caspase-3. Zinc itself induced the activation of 12-LOX, generation of reactive oxygen species (ROS), and activation of p38 MAPK and caspase-3. These data suggest a cell death pathway triggered by peroxynitrite in which intracellular zinc release leads to activation of 12-LOX, ROS accumulation, p38 activation, and caspase-3 activation. Therefore, therapies aimed at maintaining intracellular zinc homeostasis or blocking activation of 12-LOX may provide a novel avenue for the treatment of inflammation, stroke, and neurodegenerative diseases in which the formation of peroxynitrite is thought to be one of the important causes of cell death.

Characterization of cellular uptake and distribution of vitamin E

We previously reported that tocotrienols acted as more potent inhibitors against selenium deficiency-induced cell death than the corresponding tocopherol isoforms (J. Biol. Chem. 2003;278:39428-39434). In the present study, we first compared the differences in the cellular uptake between alpha-tocopherol (alpha-Toc) and alpha-tocotrienol (alpha-Toc-3). The initial rate of cellular uptake of alpha-Toc-3 was 70-fold higher than that of alpha-Toc. Subcellular fractionation analysis of alpha-Toc-3 and alpha-Toc-fortified cells showed similar cellular distribution of these antioxidants, which was directly proportional to the lipid distribution. The cells containing similar amounts of alpha-Toc-3 and alpha-Toc showed similar resistance against the oxidative stress caused by peroxides. These results suggest that the apparent higher cytoprotective effect of alpha-Toc-3 than alpha-Toc is primarily ascribed to its higher cellular uptake.

Proteasome inhibition protects HT22 neuronal cells from oxidative glutamate toxicity

Oxidative stress caused by glutathione depletion after prolonged exposure to extracellular glutamate leads to a form of neuronal cell death that exhibits morphologically mixed features of both apoptosis and necrosis. However, specific downstream executioners involved in this form of cell death have yet to be identified. We report here that glutamate exposure does not activate caspase-3 in the HT22 neuronal cell line. Furthermore, no cytoprotection was achieved with either the pan-caspase inhibitor Z-VAD-fmk or the caspase-3-specific inhibitor DEVD-CHO. In contrast, inhibition of the proteasome by lactacystin protected both HT22 cells and rat primary neuronal cells against cell lysis. In parallel, oxidatively altered and ubiquitinated proteins accumulated in the mitochondrial fraction of cells after proteasome inhibition. These findings suggest that caspases can be decoupled from oxidative stress under some conditions, and implicate the ubiquitin/proteasome pathway in neuronal cell death caused by oxidative glutamate toxicity.

Tocotrienols: constitutional effects in aging and disease

Tocotrienols, a class of vitamin E analogs, modulate several mechanisms associated with the aging process and aging-related diseases. Most studies compare the activities of tocotrienols with those of tocopherols ("classical vitamin E"). However, some biological effects were found to be unique for tocotrienols. Although the absorption mechanisms are essentially the same for all vitamin E analogs, tocotrienols are degraded to a greater extent than tocopherols. The levels of tocotrienols in the plasma of animals and humans were estimated to reach low micromolar concentrations. One hallmark in the origin of disease and aging is the overproduction of reactive oxygen species (ROS). Tocotrienols possess excellent antioxidant activity in vitro and have been suggested to suppress ROS production more efficiently than tocopherols. In addition, tocotrienols show promising nonantioxidant activities in various in vitro and in vivo models. Most notable are the interactions of tocotrienols with the mevalonate pathway leading to the lowering of cholesterol levels, the prevention of cell adhesion to endothelial cells, and the suppression of tumor cell growth. Furthermore, glutamate-induced neurotoxicity is suppressed in the presence of tocotrienols. This review summarizes the main antioxidant and nonantioxidant effects of tocotrienols and assesses their potential as health-maintaining compounds.

Tocotrienols reduce 25-hydroxycholesterol-induced monocyte-endothelial cell interaction by inhibiting the surface expression of adhesion molecules

The migration of circulating monocytes into the subendothelial space occurs through the expressing of some adhesion molecules on endothelial cells. In the present study, using human aortic endothelial cells (HAECs), we investigated whether a model compound for oxysterols, 25-hydroxycholesterol, can enhance the monocyte adherence to HAECs exposed to 25-hydroxycholesterol via increasing expression of vascular cell adhesion molecule-1 (VCAM-1). We also aimed to determine the in vitro effects of tocotrienols on the enhanced interaction between monocytes and endothelial cells. We found that 25-hydroxycholesterol enhances surface expression determined by ELISA, induces VCAM-1 mRNA expression by real time-PCR, and stimulates adhesiveness of HAECs to U937 monocytic cells in a dose-dependent fashion. The combination treatment with anti-VCAM-1 and anti-CD11b monoclonal antibodies significantly reduced the monocyte adherence to 25-hydroxycholesterol-stimulated HAECs. Compared to alpha-tocopherol, tocotrienols displayed a more profound inhibitory effect on adhesion molecule expression and monocytic cell adherence. We observed that delta-tocotrienol exerted a most profound inhibitory action on monocytic cell adherence when compared to alpha-tocopherol and alpha-, beta-, and gamma-tocotrienols. Tocotrienols accumulated in HAECs to levels approximately 25-95-fold greater than that of alpha-tocopherol. In conclusion, these results indicate that a model compound 25-hydroxycholesterol can enhance the interaction between monocytes and HAECs, and that tocotrienols had a profound inhibitory effect on monocytic cell adherence to HAECs relative to alpha-tocopherol via inhibiting the VCAM-1 expression. These superior inhibitory effects of tocotrienols may be dependent on their intracellular accumulation.

CYTOTOXITY OF THE trans10,cis12 ISOMER OF CONJUGATED LINOLEIC ACID ON RAT HEPATOMA AND ITS MODULATION BY OTHER FATTY ACIDS, TOCOPHEROL, AND TOCOTRIENOL

This study was performed to evaluate the isomer-specific cytotoxic effects of conjugated linoleic acid (CLA) on rat hepatoma dRLh-84 cells in vitro. A 10trans,12cis (10t,12c)-CLA showed a strong cytotoxic effect on dRLh-84 cells in culture, whereas no such effect was observed with 9cis,11trans (9c,11t)-CLA or linoleic acid. The optimum concentration for induction of cytotoxicity by 10t,12c-CLA was 5 to 10 microM, but the effect was alleviated at higher concentrations. Coincubation with oleic or palmitoleic acid and 10t,12c-CLA cancelled the cytotoxic effect, but other major saturated or polyunsaturated fatty acids and eraidic acid did not interfere with 10t,12c-CLA-induced cytotoxity. The cytotoxic effect was also alleviated by alpha-tocopherol (alpha-toc) and alpha-tocotrienol but not by any other antioxidant reagent examined. Significant cytotoxicity of 10t,12c-CLA was detected after only a 15-min incubation, and the most noticeable effect was seen after 3 h. After incubation with 10t,12c-CLA at 10 microM, an additional 90 microM of 10t,12c-CLA or 100 microM of alpha-toc was also able to alleviate the cytotoxicity. When cells were treated with 10 microM 10t,12c-CLA for more than 48 h, treatment with additional CLA or alpha-toc could not prevent cell death.

12-Lipoxygenase plays a key role in cell death caused by glutathione depletion and arachidonic acid in rat oligodendrocytes

Oxidative injury to premyelinating oligodendrocytes (preOLs) in developing white matter has been implicated in the pathogenesis of periventricular leukomalacia, the lesion underlying most cases of cerebral palsy in premature infants. In this study, we investigated the pathways of OL death induced by intracellular glutathione (GSH) depletion. We found that the lipoxygenase (LOX) inhibitors AA-861 and BMD-122 (N-benzyl-N-hydroxy-5-phenylpentamide; BHPP), but not the cyclooxygenase (COX) inhibitor indomethacin, fully protected the cells from GSH depletion caused by cystine deprivation. Arachidonic acid (AA), the substrate for 12-LOX, potentiated the toxicity of mild cystine deprivation and at higher concentration was itself toxic. This toxicity was also blocked by 12-LOX inhibitors. Consistent with a role for 12-LOX in the cell death pathway, 12-LOX activity increased following cystine deprivation in OLs. Blocking 12-LOX with AA-861 effectively inhibited the accumulation of reactive oxygen species (ROS) induced by cystine deprivation. These data suggest that, in OLs, intracellular GSH depletion leads to activation of 12-LOX, ROS accumulation and cell death. Mature OLs were more resistant than preOLs to cystine deprivation. The difference in sensitivity was not due to a difference in 12-LOX activity but rather appeared to be related to the presence of stronger antioxidant defense mechanisms in mature OLs. These results suggest that 12-LOX activation plays a key role in oxidative stress-induced OL death.

Oxidative stress in the pathogenesis of diabetic neuropathy

Oxidative stress results from a cell or tissue failing to detoxify the free radicals that are produced during metabolic activity. Diabetes is characterized by chronic hyperglycemia that produces dysregulation of cellular metabolism. This review explores the concept that diabetes overloads glucose metabolic pathways, resulting in excess free radical production and oxidative stress. Evidence is presented to support the idea that both chronic and acute hyperglycemia cause oxidative stress in the peripheral nervous system that can promote the development of diabetic neuropathy. Proteins that are damaged by oxidative stress have decreased biological activity leading to loss of energy metabolism, cell signaling, transport, and, ultimately, to cell death. Examination of the data from animal and cell culture models of diabetes, as well as clinical trials of antioxidants, strongly implicates hyperglycemia-induced oxidative stress in diabetic neuropathy. We conclude that striving for superior antioxidative therapies remains essential for the prevention of neuropathy in diabetic patients.