Inhibition of protein kinase C activity and vascular smooth muscle cell growth by d-alpha-tocopherol

Nanomolar vitamin E alpha-tocotrienol inhibits glutamate-induced activation of phospholipase A2 and causes neuroprotection

Our previous works have elucidated that the 12-lipoxygenase pathway is directly implicated in glutamate-induced neural cell death, and that such that toxicity is prevented by nM concentrations of the natural vitamin E alpha-tocotrienol (TCT). In the current study we tested the hypothesis that phospholipase A(2) (PLA(2)) activity is sensitive to glutamate and mobilizes arachidonic acid (AA), a substrate for 12-lipoxygenase. Furthermore, we examined whether TCT regulates glutamate-inducible PLA(2) activity in neural cells. Glutamate challenge induced the release of [(3)H]AA from HT4 neural cells. Such response was attenuated by calcium chelators (EGTA and BAPTA), cytosolic PLA(2) (cPLA(2))-specific inhibitor (AACOCF(3)) as well as TCT at 250 nM. Glutamate also caused the elevation of free polyunsaturated fatty acid (AA and docosahexaenoic acid) levels and disappearance of phospholipid-esterified AA in neural cells. Furthermore, glutamate induced a time-dependent translocation and enhanced serine phosphorylation of cPLA(2) in the cells. These effects of glutamate on fatty acid levels and on cPLA(2) were significantly attenuated by nM TCT. The observations that AACOCF(3), transient knock-down of cPLA(2) as well as TCT significantly protected against the glutamate-induced death of neural cells implicate cPLA(2) as a TCT-sensitive mediator of glutamate induced neural cell death. This work presents first evidence recognizing glutamate-induced changes in cPLA(2) as a novel mechanism responsible for neuroprotection observed in response to nanomolar concentrations of TCT.

Characterization of the potent neuroprotective properties of the natural vitamin E alpha-tocotrienol

The natural vitamin E tocotrienols possess properties not shared by tocopherols. Nanomolar alpha-tocotrienol, not alpha-tocopherol, is potently neuroprotective. On a concentration basis, this finding represents the most potent of all biological functions exhibited by any natural vitamin E molecule. We sought to dissect the antioxidant-independent and -dependent neuroprotective properties of alpha-tocotrienol by using two different triggers of neurotoxicity, homocysteic acid (HCA) and linoleic acid. Both HCA and linoleic acid caused neurotoxicity with comparable features, such as increased ratio of oxidized to reduced glutathione GSSG/GSH, raised intracellular calcium concentration and compromised mitochondrial membrane potential. Mechanisms underlying HCA-induced neurodegeneration were comparable to those in the path implicated in glutamate-induced neurotoxicity. Inducible activation of c-Src and 12-lipoxygenase (12-Lox) represented early events in that pathway. Overexpression of active c-Src or 12-Lox sensitized cells to HCA-induced death. Nanomolar alpha-tocotrienol was protective. Knock-down of c-Src or 12-Lox attenuated HCA-induced neurotoxicity. Oxidative stress represented a late event in HCA-induced death. The observation that micromolar, but not nanomolar, alpha-tocotrienol functions as an antioxidant was verified in a model involving linoleic acid-induced oxidative stress and cell death. Oral supplementation of alpha-tocotrienol to humans results in a peak plasma concentration of 3 microm. Thus, oral alpha-tocotrienol may be neuroprotective by antioxidant-independent as well as antioxidant-dependent mechanisms.

Glycation of low-density lipoproteins by methylglyoxal and glycolaldehyde gives rise to the in vitro formation of lipid-laden cells

AIMS/HYPOTHESIS

Previous studies have implicated the glycoxidative modification of low-density lipoprotein (LDL) by glucose and aldehydes (apparently comprising both glycation and oxidation), as a causative factor in the elevated levels of atherosclerosis observed in diabetic patients. Such LDL modification can result in unregulated cellular accumulation of lipids. In previous studies we have characterized the formation of glycated, but nonoxidized, LDL by glucose and aldehydes; in this study we examine whether glycation of LDL, in the absence of oxidation, gives rise to lipid accumulation in arterial wall cell types.

METHODS

Glycated LDLs were incubated with macrophage, smooth muscle, or endothelial cells. Lipid loading was assessed by HPLC analysis of cholesterol and individual esters. Oxidation was assessed by cholesterol ester loss and 7-ketocholesterol formation. Cell viability was assessed by lactate dehydrogenase release and cell protein levels.

RESULTS

Glycation of LDL by glycolaldehyde and methylglyoxal, but not glucose (in either the presence or absence of copper ions), resulted in cholesterol and cholesterol ester accumulation in macrophage cells, but not smooth muscle or endothelial cells. The extent of lipid accumulation depends on the degree of glycation, with increasing aldehyde concentration or incubation time, giving rise to greater extents of particle modification and lipid accumulation. Modification of lysine residues appears to be a key determinant of cellular uptake.

CONCLUSIONS/INTERPRETATION

These results are consistent with LDL glycation, in the absence of oxidation, being sufficient for rapid lipid accumulation by macrophage cells. Aldehyde-mediated "carbonyl-stress" may therefore facilitate the formation of lipid-laden (foam) cells in the artery wall.

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.

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

Vitamin E is a generic term for tocopherols and tocotrienols. This work is based on our striking evidence that, in neuronal cells, nanomolar concentrations of alpha-tocotrienol, but not alpha-tocopherol, block glutamate-induced death by suppressing early activation of c-Src kinase (Sen, C. K., Khanna, S., Roy, S., and Packer, L. (2000) J. Biol. Chem. 275, 13049-13055). This study on HT4 and immature primary cortical neurons suggests a central role of 12-lipoxygenase (12-LOX) in executing glutamate-induced neurodegeneration. BL15, an inhibitor of 12-LOX, prevented glutamate-induced neurotoxicity. Moreover, neurons isolated from 12-LOX-deficient mice were observed to be resistant to glutamate-induced death. In the presence of nanomolar alpha-tocotrienol, neurons were resistant to glutamate-, homocysteine-, and l-buthionine sulfoximine-induced toxicity. Long-term time-lapse imaging studies revealed that neurons and their axo-dendritic network are fairly motile under standard culture conditions. Such motility was arrested in response to glutamate challenge. Tocotrienol-treated primary neurons maintained healthy growth and motility even in the presence of excess glutamate. The study of 12-LOX activity and metabolism revealed that this key mediator of glutamate-induced neurodegeneration is subject to control by the nutrient alpha-tocotrienol. In silico docking studies indicated that alpha-tocotrienol may hinder the access of arachidonic acid to the catalytic site of 12-LOX by binding to the opening of a solvent cavity close to the active site. These findings lend further support to alpha-tocotrienol as a potent neuroprotective form of vitamin E.

Troglitazone suppresses cell growth of KU812 cells independently of PPARgamma

We examined the effects of troglitazone, one of thiazolidinedione derivatives on human basophilic leukemia cell line KU812. Troglitazone caused the suppression of cell growth, which was suggested to result from the decrease in cyclin E and the hyperphosphorylated form of retinoblastoma tumor suppressor gene product (pRb). In addition, troglitazone caused a decrease in histamine secretion due to the reduced expression of histidine decarboxylase mRNA. Peroxisome proliferator-activated receptor (PPAR)-gamma mRNA was undetectable by reverse transcription-polymerase chain reaction (RT-PCR) in KU812 cells. These findings suggested that troglitazone suppressed cell growth and histamine synthesis independently of PPARgamma.

Contribution of reactive oxygen species to 3-hydroxyglutarate neurotoxicity in primary neuronal cultures from chick embryo telencephalons

Glutaryl-CoA dehydrogenase deficiency is an autosomal recessively inherited neurometabolic disorder with a distinct neuropathology characterized by acute encephalopathic crises during a vulnerable period of brain development. 3-Hydroxyglutarate (3-OH-GA), which accumulates in affected patients, has been identified as an endogenous neurotoxin mediating excitotoxicity via N-methyl-D-aspartate receptors. As increased generation of reactive oxygen species (ROS) and nitric oxide (NO) plays an important role in excitotoxic neuronal damage, we investigated whether ROS and NO contribute to 3-OH-GA neurotoxicity. 3-OH-GA increased mitochondrial ROS generation in primary neuronal cultures from chick embryo telencephalons, which could be prevented by MK-801, confirming the central role of N-methyl-D-aspartate receptor stimulation in 3-OH-GA toxicity. ROS increase was reduced by alpha-tocopherol and--less effectively-by melatonin. alpha-Tocopherol revealed a wider time frame for neuroprotection than melatonin. Creatine also reduced neuronal damage and ROS formation but only if it was administered >or=6 h before 3-OH-GA. NO production revealed only a slight increase after 3-OH-GA incubation. NO synthase inhibitor N(omega)-nitro-L-arginine prevented NO increase but did not protect neurons against 3-OH-GA. The NO donor S-nitroso-N-acetylpenicillamine revealed no effect on 3-OH-GA toxicity at low concentrations (0.5-5 microM), whereas it potentiated neuronal damage at high concentrations (50-500 microM), suggesting that weak endogenous NO production elicited by 3-OH-GA did not affect neuronal viability. We conclude from our results that ROS generation contributes to 3-OH-GA neurotoxicity in vitro and that radical scavenging and stabilization of brain energy metabolism by creatine are hopeful new strategies in glutaryl-CoA dehydrogenase deficiency.

Differential inhibition by alpha- and beta-tocopherol of human erythroleukemia cell adhesion: role of integrins

The effect of alpha- and beta-tocopherol on human erythroleukemia cell (HEL) adhesion induced by phorbol 12-myristate 13-acetate (PMA) has been studied. Adhesion induced by PMA stimulation was prevented by 44.5% by physiological concentrations of alpha-tocopherol. Under the same experimental conditions, beta-tocopherol, an analogue of alpha-tocopherol, produced 11% inhibition of adhesion. Cell response gradually increased from 0 to 24 h of alpha-tocopherol treatment. Only a slight time dependency of beta-tocopherol inhibition was observed. Another human erythroleukemia cell line (K562) and the human monocyte tumor cell line U937 showed 5.0 and 11.2% inhibition, respectively. Similar to alpha-tocopherol, the protein kinase C inhibitor, Calphostin C, and the MAPK inhibitor, PD98059, prevented PMA-induced cell adhesion. An inhibition of ERK-1 phosphorylation was observed for alpha-tocopherol only in HEL, implying that MAP kinase pathway is involved in this cell line. Fluorescence-activated cell sorting (FACS), by using various integrin-specific monoclonal antibodies, has shown that alpha (1-6), beta1, and alphav integrins are less expressed at the cell surface after alpha-tocopherol treatment. Beta-tocopherol treatment was less effective.

Effect of vitamin E on the development of atherosclerosis

The development of atherosclerosis is a multifactorial process in which both elevated plasma cholesterol levels and proliferation of smooth muscle cells play a central role. Numerous studies have suggested the involvement of oxidative processes in the pathogenesis of atherosclerosis and especially of oxidised low density lipoproteins. Some epidemiological studies have shown an association between high dietary intake or high serum concentrations of vitamin E and lower rates of ischemic heart disease. Recently, the Cambridge Heart Antioxidant Study (CHAOS) reported strong protection by high vitamin E doses against the risk of fatal and non fatal myocardial infarction. Here we have shown that incubation of vascular smooth muscle cells in the presence of alpha-tocopherol resulted in inhibition of cell proliferation and protein kinase C activity. Since beta-tocopherol and probucol are not inhibitory, the effect of alpha-tocopherol is considered due to a non-oxidant mechanism. In order to understand the protective role of alpha-tocopherol against atherosclerosis in vivo the following rabbit studies were carried out. Atherosclerosis was induced by a vitamin E poor diet containing 2% cholesterol in a group of rabbit. The other groups had 2% cholesterol in the diet plus 50 mg/kg vitamin E i.m. or 1% probucol or 50 mg/kg vitamin E plus 1% probucol. After 4 weeks, aortas were removed and analysed by microscopy for atherosclerotic lesions. Samples of the media were analysed for protein kinase C activity. The aortas of cholesterol-fed rabbits showed typical atherosclerotic lesions, detected by microscopic examination, their media smooth muscle cells exhibited an increase in protein kinase C activity. Vitamin E fully prevented cholesterol induced atherosclerotic lesions and the induction of protein kinase C activity while probucol was not effective. These results show that the protective effect of vitamin E against hypercholesterolemic atherosclerosis is not produced by an other antioxidant such as probucol and, therefore, may not be linked to the antioxidant properties of this vitamin. The effects observed at the level of smooth muscle cells in vitro and ex-vivo suggests an involvement of signal transduction events in the protective effect of vitamin E against atherosclerosis.

Ca2+ signalling in rat vascular smooth muscle cells: a role for protein kinase C at physiological vasoconstrictor concentrations of vasopressin

Physiological vasoconstrictor concentrations of Arg8-vasopressin (AVP, 10-100 pM) stimulate oscillations (spikes) in cytosolic free Ca2+ concentration ([Ca2+]i) in A7r5 rat vascular smooth muscle cells. These Ca2+ spikes are dependent on L-type voltage-sensitive Ca2+ channels and increase in frequency with increasing AVP concentration. The signal transduction pathway responsible for this effect was examined in fura-2-loaded A7r5 cell monolayers. The serine/threonine phosphatase inhibitor calyculin A (5 nM) sensitized A7r5 cells to AVP, resulting in the stimulation of Ca2+ spiking by 1-10 pM AVP. Calyculin A alone did not stimulate Ca2+ spiking. The protein kinase C (PKC) activator 4beta-phorbol 12-myristate 13-acetate (PMA, 100 pM to 200 nM), also stimulated Ca2+ spiking and this effect was additive with a submaximal concentration of AVP (50 pM). The PKC inhibitors Ro-31-8220 (1 microM) and calphostin C (250 nM) completely blocked the stimulation of Ca2+ spiking by either PMA or AVP. alpha, beta, gamma, delta, epsilon, zeta and &lamdda; isoforms of PKC were detected in A7r5 cells by Western blot analysis. Time-dependent redistribution of PKC-alpha, -delta and -epsilon isoforms between the membrane and cytosolic fractions occurred in response to 100 pM AVP. Pretreatment for 24 h with 1 microM PMA downregulated expression of PKC-alpha and -delta, but not PKC-epsilon, and prevented the Ca2+-spiking responses to either 1 nM PMA or 100 pM AVP. Neither the release of intracellular Ca2+ by 1 microM AVP nor the increase in [Ca2+]i in response to elevated extracellular [K+] was prevented by the PMA pretreatment. We conclude that PKC activation is a necessary step in the signal transduction pathway linking low concentrations of AVP to Ca2+ spiking in A7r5 cells.

Molecular basis of vitamin E action. Tocotrienol potently inhibits glutamate-induced pp60(c-Src) kinase activation and death of HT4 neuronal cells

HT4 hippocampal neuronal cells were studied to compare the efficacy of tocopherols and tocotrienol to protect against glutamate-induced death. Tocotrienols were more effective than alpha-tocopherol in preventing glutamate-induced death. Uptake of tocotrienols from the culture medium was more efficient compared with that of alpha-tocopherol. Vitamin E molecules have potent antioxidant properties. Results show that at low concentrations, tocotrienols may have protected cells by an antioxidant-independent mechanism. Examination of signal transduction pathways revealed that protein tyrosine phosphorylation processes played a central role in the execution of death. Activation of pp60(c-Src) kinase and phosphorylation of ERK were observed in response to glutamate treatment. Nanomolar amounts of alpha-tocotrienol, but not alpha-tocopherol, blocked glutamate-induced death by suppressing glutamate-induced early activation of c-Src kinase. Overexpression of kinase-active c-Src sensitized cells to glutamate-induced death. Tocotrienol treatment prevented death of Src-overexpressing cells treated with glutamate. alpha-Tocotrienol did not influence activity of recombinant c-Src kinase suggesting that its mechanism of action may include regulation of SH domains. This study provides first evidence describing the molecular basis of tocotrienol action. At a concentration 4-10-fold lower than levels detected in plasma of supplemented humans, tocotrienol regulated unique signal transduction processes that were not sensitive to comparable concentrations of tocopherol.

Association of alcohol in brain injury, headaches, and stroke with brain-tissue and serum levels of ionized magnesium: a review of recent findings and mechanisms of action

Although there is general agreement that chronic ingestion of alcohol poses great risks for normal cardiovascular functions and peripheral-vascular homeostasis, a direct cause and effect between the real phenomena of alcohol-induced headache and risk of brain injury and stroke is not appreciated. "Binge drinking" of alcohol is associated with an ever-growing number of strokes and sudden death. It is becoming clear that alcohol ingestion can result in profoundly different actions on the cerebral circulation (e.g., vasodilation, vasoconstriction-spasm, vessel rupture), depending upon dose and physiologic state of host. Using rats, it has been demonstrated that acute, high doses of ethanol can result in stroke-like events concomitant with alterations in brain bioenergetics. We review recent in vivo findings obtained with 31P-NMR spectroscopy, optical reflectance spectroscopy, and direct in vivo microcirculatory studies on the intact brain. Alcohol-induced hemorrhagic stroke is preceded by a rapid fall in brain intracellular free magnesium ions ([Mg2+]i) followed by cerebrovasospasm and reductions in phosphocreatine (PCr)/ATP ratio, intracellular pH, and the cytosolic phosphorylation potential (CPP) with concomitant rises in deoxyhemoglobin (DH), mitochondrial reduced cytochrome oxidase aa3 (rCOaa3), blood volume, and intracellular inorganic phosphate (Pi). Using osmotic mini-pumps implanted in the third cerebral ventricle, containing 30% ethanol, it was found that brain [Mg2+]i is reduced 30% after 14 days; brain PCr fell 15%, whereas the CPP fell 40%. Such animals became susceptible to stroke from nonlethal doses of ethanol. Human subjects with mild head injury have been found to exhibit early deficits in serum ionized Mg (IMg2+); the greater the degree of early head injury (30 min-8 h), the greater and more profound the deficit in serum IMg2+ and the greater the ionized Ca (ICa2+) to IMg2+ ratio. Patients with histories of alcohol abuse or ingestion of alcohol prior to head injury exhibited greater deficits in IMg2+ (and higher ICa2+/IMg2+ ratios) and, unlike the subjects without alcohol, did not leave the hospital for at least several days. Women, for some unknown reason, exhibit a much higher incidence of morbidity and mortality from subarachnoid hemorrhage (SAH) than men. Data on 105 men and women with different types of stroke indicate that, on the average, a 20% deficit in serum IMg2+ is seen; total Mg (TMg) or blood pH is usually near normal. Women with SAH, however, exhibit much lower IMg2+ and higher ICa2+/IMg2+ ratios; the presence of ethanol in the blood is associated with even more depression in IMg2+ in SAH in women. It is possible that prior alcohol ingestion is, in large measure, responsible for a great deal of this unexplained higher incidence of SAH in women. It has recently been reported that the cyclical changes in estrogenic hormones appear to control the serum IMg2+ level in young women. A surge in estrogenic levels prior to SAH could thus precipitate, in part, the SAH. In other human studies, it has been shown that migraines and headache, dizziness, and hangover, which accompany ethanol ingestion, are associated with rapid deficits in serum IMg2+ but not in TMg. The former, and the alcohol-associated headache, can be ameliorated with IV administration of MgSO4. Premenstrual tension-headache (PTH) and its exacerbation by alcohol in women is also accompanied by deficits in IMg2+, and elevation in serum ICa2+/IMg2+; IV MgSO4 corrects the PTH and the serum deficit in IMg2+. Animal experiments show that IV Mg2+ can prevent alcohol-induced hemorrhagic stroke and the subsequent fall in brain [Mg2+]i, [PCr], pHi, and CPP. Other recent data indicate that alcohol-induced cellular loss of [Mg2+]i is associated with cellular Ca2+ overload and generation of oxygen-derived free radicals; chronic pretreatment with vitamin E prevents alcohol-induced vascular injury and pathology in the brain. (ABSTRACT TRUNCATED)

Synthesis of photoaffinity label analogues of alpha-tocopherol

Photoaffinity analogues of alpha-tocopherol have been synthesized that incorporate the photosensitive 4-azido-2,3,5,6-tetrafluorobenzyloxy group at the terminus of unbranched analogues of the naturally occurring phytyl side chain. An intermediate from these syntheses has also been used to generate a supported ligand for bioaffinity chromatography of alpha-tocopherol binding proteins.

Mechanisms of inactivation of hepatocyte protein kinase C isoforms following acute ethanol treatment

Acute ethanol exposure of rat isolated hepatocytes leads to a significant decrease (-30%) in cytosolic enzymatic activity of classic protein kinase C (PKC) isoforms, while immunoreactive protein level measured by Western Blot remains unaffected. The inactivation of classic cytosolic isoforms appears dependent on the modification of the enzyme function, probably due to ethanol metabolism. In fact, pretreatment with 4-methylpyrazole (4MP), an inhibitor of alcohol dehydrogenase, fully prevented such damage. After ethanol treatment, a decrease of about 40% in both enzymatic activity and immunoreactive protein level of novel PKC isoforms was evident both in the soluble and particulate fractions. Even if 4MP cell pre-treatment afforded protection in this case too, the inhibitory action of ethanol on novel PKC hepatocyte isoforms involves a proteolytic mechanism as shown by Western Blot analysis. The reproduction of PKC inactivation by ethanol in hepatocyte lysate excluded a role of peroxisomal hydrogen peroxide in the pathogenesis of the damage investigated. This damage was not reduced by addition of catalase to the lysate model system.

alpha-Tocopherol specifically inactivates cellular protein kinase C alpha by changing its phosphorylation state

The mechanism of protein kinase C (PKC) regulation by alpha-tocopherol has been investigated in smooth-muscle cells. Treatment of rat aortic A7r5 smooth-muscle cells with alpha-tocopherol resulted in a time- and dose-dependent inhibition of PKC. The inhibition was not related to a direct interaction of alpha-tocopherol with the enzyme nor with a diminution of its expression. Western analysis demonstrated the presence of PKCalpha, beta, delta, epsilon, zeta and micro isoforms in these cells. Autophosphorylation and kinase activities of the different isoforms have shown that only PKCalpha was inhibited by alpha-tocopherol. The inhibitory effects were not mimicked by beta-tocopherol, an analogue of alpha-tocopherol with similar antioxidant properties. The inhibition of PKCalpha by alpha-tocopherol has been found to be associated with its dephosphorylation. Moreover the finding of an activation of protein phosphatase type 2A in vitro by alpha-tocopherol suggests that this enzyme might be responsible for the observed dephosphorylation and subsequent deactivation of PKCalpha. It is therefore proposed that PKCalpha inhibition by alpha-tocopherol is linked to the activation of a protein phosphatase, which in turn dephosphorylates PKCalpha and inhibits its activity.

Alpha-tocopherol prevents ethanol-induced elevation of [Ca2+]i in cultured canine cerebral vascular smooth muscle cells

Exposure of cultured canine cerebral vascular smooth muscle cells to ethanol (10-400 mM) for 1-5 days results in concentration-dependent elevation in resting intracellular free calcium ([Ca2+]i) levels. Preincubation of these cultured vascular cells with alpha-tocopherol (20 microM), alone, did not produce any apparent changes from control resting levels of [Ca2+]i. However, after concomitant addition of alpha-tocopherol (20 microM) and ethanol (10-400 mM), the rises of [Ca2+]i induced by ethanol were attenuated markedly. These results suggest that alcohol-induced lipid peroxidation of cerebral vascular muscle cell membranes triggers membrane entry of extracellular Ca2+, which could play an important role in ethanol-induced cerebrovasospasm, brain ischemia and stroke. Moreover, these new results support the concept recently advanced to suggest that alpha-tocopherol-induced amelioration of membrane lipid alterations of cerebral vascular cells can prevent ethanol-induced excessive accumulation of [Ca2+]i.

Molecular basis of alpha-tocopherol control of smooth muscle cell proliferation

Rat and human vascular smooth muscle cell proliferation is specifically sensitive to alpha-tocopherol, but not beta-tocopherol. The former, but not the latter, is capable of limiting proliferation and inhibiting protein kinase C activity in a dose-dependent manner. The phenomenon occurs at concentrations in the range 10-50 microM. beta-tocopherol addition together with alpha-tocopherol, prevents both cell growth and protein kinase C inhibition. alpha-tocopherol increases de novo synthesis of protein kinase C molecules. The enzyme specific activity, however, is diminished, due to a decreased phosphorylation of protein kinase C, occurring in the presence of alpha-tocopherol. Experiments with protein kinase C isoform-specific inhibitors and precipitating antibodies show that the only isoform affected by alpha-tocopherol is protein kinase C-alpha. The effect of alpha-tocopherol is prevented by okadaic acid indicating a phosphatase of the PP2A type as responsible for protein kinase C-alpha dephosphorylation produced in the presence of alpha-tocopherol. At a gene level alpha-tocopherol but not beta-tocopherol induces a transient activation of alpha-tropomyosin gene transcription and protein expression. It is proposed that, by inhibiting protein kinase C activity via an activation of a phosphatase PP2A, alpha-tocopherol controls smooth muscle cell proliferation through changes in gene expression.

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

The regulation of cellular vitamin E concentration was studied in P388D1 macrophage-like cells. Cellular alpha-tocopherol levels increased more than 5000-fold over constitutive levels without reaching saturation when P388D1 cells were cultured in vitamin-E-supplemented fetal calf serum. The uptake of alpha-tocopherol was accompanied by accumulation of alpha-[3H]tocopherol and [14C]cholesterol in these cells. Human unmodified low-density lipoprotein (LDL) inhibited the uptake of alpha-[3H]tocopherol and [14C]cholesterol in a dose-dependent manner and with very similar IC50. Acetylated, Cu2+-oxidized and aggregated human LDL and human very-low-density-lipoprotein (VLDL) were similarly potent, whereas human HDL was at least tenfold less effective than human LDL when inhibitory activity was correlated to lipoprotein protein levels. The rate of vitamin E uptake by P388D1 cells, however, always correlated with the extracellular alpha-tocopherol/cholesterol ratio. Efflux of alpha-[3H]tocopherol from labeled P388D1 cells required extracellular acceptors and was accompanied by the concomitant release of [14C]cholesterol. Both human LDL and HDL could serve as acceptors. Changes in the cellular alpha-tocopherol level appear to be the direct consequence of changes in the extracellular alpha-tocopherol/cholesterol ratio due to a rapid exchange of lipids between P388D1 cells and their extracellular environment. While the transfer of alpha-tocopherol from LDL, VLDL, and fetal calf serum into P388D1 cells appears to occur mainly by diffusion, HDL-stimulated efflux of alpha-tocopherol may underlie a different mechanism. The alpha-tocopherol/cholesterol ratio of the extracellular environment may be a critical factor in determining cellular vitamin E levels in vivo.

The effect of alpha-tocopherol on the synthesis, phosphorylation and activity of protein kinase C in smooth muscle cells after phorbol 12-myristate 13-acetate down-regulation

Previous work had established that, in smooth muscle cells, alpha-tocopherol negatively regulates protein kinase C by preventing its activation [Tasinato, A., Boscoboinik, D., Bartoli, G. M., Maroni, P. & Azzi, A. (1995) Proc. Natl Acad. Sci. USA 92, 12190-12194]. In this study, the mechanism by which this event takes place has been analyzed. The regulation by alpha-tocopherol of protein kinase C expression, activity and phosphorylation has been followed during the synthesis of protein kinase C after its down-regulation by phorbol 12-myristate 13-acetate. The data show that protein kinase C isoenzyme alpha is synthesised significantly more (30% 72 h after down-regulation) in the presence of alpha-tocopherol. However, its activity is significantly less (45% diminution) and its phosphorylation state is also decreased (60% diminution). The effect of alpha-tocopherol appears not to be shared by the analogue beta-tocopherol, provided with similar radical-scavenging properties. The data are interpreted in terms of a diminution of protein kinase C phosphorylation, specifically caused by alpha-tocopherol, resulting in a decreased enzyme specific activity.

Oxidative damage and fibrogenesis

Various chronic disease processes are characterized by progressive accumulation of connective tissue under-going fibrotic degeneration. Evidence of oxidative reactions is often associated with fibrogenesis occurring in liver, lung, arteries, and nervous system. Moreover, an increasing bulk of experimental and clinical data supports a contributory role of oxidative stress in the pathogenesis of this kind of disease. Indeed, many etiological agents of fibrogenesis stimulate free radical reactions either directly or through inflammatory stimuli. Free radicals, as well as products of their reaction with biomolecules, appear to modulate the activity of the two cellular types mainly involved in the process, namely phagocytes and extracellular matrix-producing cells. Lipid peroxidation and certain lipid peroxidation products induce genetic overexpression of fibrogenic cytokines, the key molecules in the pathomechanisms of fibrosis, as well as increased transcription and synthesis of collagen. Both these events can be downregulated, at least in experimental models, by the use of antioxidants. The effect of oxidative stress on cytokine gene expression appears to be an important mechanism by which it promotes connective tissue deposition.

Dietary cholesterol-induced changes of protein kinase C and the effect of vitamin E in rabbit aortic smooth muscle cells

The changes occuring in smooth muscle cells during the development of atherosclerosis in rabbits fed 2% cholesterol and the effect of vitamin E treatment were investigated. Ex-vivo smooth muscle cells obtained from the aorta of cholesterol-fed rabbits exhibited a 2-fold increase of protein kinase C expression and activity. The cholesterol induced changes in protein kinase C were equally present in the membrane bound and cytosolic fraction of the enzyme. The amount of a control protein alpha-actin was not affected in smooth muscle cell by the high cholesterol diet treatment, indicating that protein kinase C increase was specific. The increase of protein kinase C expression and activity was not significantly affected by vitamin E treatment although a constant trend was noted. The data are discussed in the light of previous smooth muscle cell in vitro experiments.