Oxidative stress induced in pathologies: The role of antioxidants

Biomed & Pharmacother 0 Elsevier, Paris

1999 ; 53 : 169-80

Dossier: Oxidation and antioxidizing

Oxidative

stress induced in pathologies:

agents

the role of antioxidants

L. Gat&, J. Paul’, G. Nguyen Bal, K.D. Tew*, H. Tapierol 1 Luboratoire de Phartnacologie Cellulaire et Moleculaire, UMR CNRS 8612, Universite’ de Paris XI - Facultk de Phartnacie, 92290 Chcftenay-Malabry, France: 2 Department of Pharmacology, Fox Chase Cancer Center, Philadelphia, PA 191 II, USA Summary-Exposure to oxidant molecules issued from the environment (pollution, radiation), nutrition, or pathologies can generate reactive oxygen species (ROS for example, H,O,, O,-, OH). These free radicals can alter DNA, proteins and/or membrane phospholipids. Depletion of intracellular antioxidants in acute oxidative stress or in various diseases increases intracellular ROS accumulation. This in turn is responsible for several chronic pathologies including cancer, neurodegenerative or cardiovascular pathologies. Thus, to prevent against cellular damages associated with oxidative stress it is important to balance the ratio of antioxidants to oxidants by supplementation or by cell induction of antioxidants. 0 1999 Elsevier, Paris aging I antioxidants

I diseases

/ free radicals

The generation of free radicals in vivo is a constant phenomenon due either to physiological metabolism or pathological alterations. Oxygen (0,) plays a double role in the cell: it is essential for aerobic organisms, but it can also act as a free radical since it contains two relatively stable unpaired electrons. When an oxygen molecule captures an electron it becomes a superoxide anion O,m. This molecule is normally produced by macrophages in order to destroy bacteria during the process of phagocytosis. However, this species can also be generated during oxidative phosphorylation in the respiratory chain in mitochondria. 0, can also be generated in a dismutation reaction by the action of superoxide dismutase (SOD) to form hydrogen peroxide H,O, (figure I). Furthermore, H,Oz can also be generated from O,- by the radical-generating enzymes amino acid oxidase and xanthine oxidase. Hydrogen peroxide, although less reactive than 0, is more highly diffusible and can cross the plasma membrane. One of the physiological functions of H,O, is the activation of nuclear translocation of the transcription factor NFkB, which subsequently allows the transcription of specific genes. NFkB is a heterodimer of p65 and ~50, but it is only the ~65 subunit that has transcriptional activity [ 11. In normal conditions, ~65 is associated with an inhibitory subunit IkB [2], which prevents the translocation of NFC into the nucleus. Although it has been shown that ~65 can be activated by several stimuli including TNFa, IL- 1, phorbol 12-myristate-

13-acetate (PMA), or H,O, [3], the action of protein kinase C (PKC) seems to play the crucial role in the activation of ~65 [4, 51. The H,O,-induced activation of NFkB is achieved via OH production [6]. The nuclear translocation of NFkB after activation by H202 induces the transcription of the HIV proteins and the replication of the virus [7], thus enhancing the pathogenicity of HIV. By homolytic fission, a hydroxyl radical OH can be produced from H202 (the Fenton reaction), this reaction being catalyzed by transition elements such as Fe*+. OH can also be generated from O,- or from H,O, and trace elements (the Haber-Weiss reaction). OH is the most highly reactive oxidant molecule, it binds and oxidizes DNA, lipids and proteins, and it reacts with structures from its close neighborhood [8]. Thus oxidants can modulate the generation of second messengers such as diacyglycerol or phosphatidic acid (PA). This latter is generated by phospholipase D (PLD) and PKC activation. PA has mitogenic properties increasing DNA synthesis and cell proliferation in smooth muscle cells [9], and this proliferation may be important in the formation of atherosclerotic plaques [lo]. Moreover, oxidized low-density lipoproteins (LDL) when acting as an activator of PLD, can induce proliferation of smooth muscle cells [ 111, and it may in part be responsible for arteriosclerosis. Thus, by their deleterious effects on macromolecules, oxidants can induce cellular alterations which can lead to the devel-

170

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-

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et al.

Hz02 + 02 LH+OH

HP,

(Fe2’i Fe”) l

OH’+OH-

.

L’+H,O

L’+o,

.

LOO

LOO’+LH

.

LOOH+L

Fenton reaction Propagation

H?O> + 01

(Fe’*/ Fe3+) l

Figure 1. Chemical reactions tive oxygen species.

OH’+OH-

‘02

Haber-Weiss

which lead to the generation

reaction

of reacHydqxroxide

opment of various pathologies [8]. Interestingly however, there have been a number of reports which have demonstrated the contribution of natural and synthetic antioxidants, or the induction of cellular antioxidant systems in the prevention or the modulation of the adverse effects of oxidative stress. CELLULAR EFFECTS OF OXIDATIVE STRESS AND RELATED DISEASES Lipoperoxidation Lipid peroxidation occurs in polyunsaturated fatty acids. The process is initiated by a hydroxyl radical OH when this species captures a hydrogen atom from a methylene carbon in the polyalkyl chain of the fatty acid (fi,pul-e 2). Under aerobic conditions a fatty acid with an unpaired electron undergoes a molecular rearrangement by reaction with O2 to generate a peroxyl radical. This product is highly reactive and can combine with other peroxyl radicals to alter membrane proteins. The radicals can also capture hydrogen molecules from the adjacent fatty acids to form a lipid hydroperoxide, subsequently inducing the propagation of lipid peroxidation. Thus, the peroxidation of unsaturated fatty acids can induce the conversion of several fatty acid sidechains in lipid hydroperoxides, which in turn leads to the formation of a reaction chain. During lipid peroxidation, malondialdehyde (MDA), a highly reactive dialdehyde, can also be generated [ 121. MDA can react with the free amino-group of proteins, phospholipids or nucleic acids, to produce interand intra-molecular I-amino-3-iminopropene (AIP) bridges and structural modifications of biological molecules [ 131. These MDA-induced structures are subsequently recognized as non-self by the immune system which leads to an autoimmune response [14]. In several pathologies such as diabetes [ 151, hyperlipemia [ 161, atherosclerosis [ 171, apoplexy [ 181, and liver diseases [ 191, it has been shown that lipid perox-

LOOH

decomposition -

LO

-

Figure 2. Mechanism of lipid peroxidation. LH: polyunsaturated fatty acid; L’: alkyl hydroperoxide; LO-: alkoxyl radical.

Malondialdehyde

radical;

LOOH:

lipid

idation increased significantly. The role of oxidants has also been implicated in the inflammation process [20, 211 via cellular enzymes such as lipooxygenases and cyclooxygenases. These enzymes produce the physiological specific fatty acyl peroxides eicosanoids. Moreover, the cholesterol or fatty acid moieties of the plasmatic low-density lipoproteins (LDL) can also be oxidized during oxidative stress [22, 231. Oxidized LDL is considered to be the key event in the development of atherosclerosis [24, 251. DNA oxidation The oxidation of guanine by the hydroxyl radical (OH*) to 8-hydroxy-2-deoxyguanosine (8-OHdG), alters DNA [26] and leads to mutagenesis [27] and carcinogenesis [28]. DNA alteration has been suggested to be responsible in part in the processes of aging [29], diabetes mellitus [30], inflammatory diseases [27], and liver disease [31]. The altered DNA can be specifically repaired by DNA glycosylase [32]. However, if the degree of oxidative stress is too great, DNA repair by glycosylases is circumvented to induce mutagenesis and/or carcinogenesis. Protein oxidation Proteins are also targets for free radicals. Oxidative molecules such as hypochlorous acid can induce the production of 3-chlorotyrosine from tyrosine [33], and histidine can be oxidized to 2-oxohistidine in metal-catalyzed oxidative reactions which can occur in the metal binding site of proteins [34]. Alterations of signal transduction mechanisms, transport systems, or enzyme

Oxidative

activities have been shown [35]. Protein oxidation may be at least in part responsible for atherosclerosis, ischemia-reperfusion injury, and may also be associated with aging [36, 371. Dysregulation

of nitric oxide (NO) synthesis

NO is synthesized from L-arginine by nitric oxide synthase (NOS). NOS is a dimeric protein, each of the two subunits having an average molecular mass of 150 kDa. NOS plays a role in the transformation of L-arginine to L-hydroxyarginine, and in the transformation of L-hydroxyarginine to L-citrulline and NO. NO is an intracellular messenger which itself plays an important role in the nervous system, the immune system, and in the cardiovascular system [38]. There are three isoenzymes of NO which have specific locations. Isoenzyme I is localized in neural cells and is responsible for the central regulation of blood pressure and smooth-muscle relaxation, but it is also associated with cell death in cerebrovascular stroke [39]. Isoenzyme II is found in macrophages [40] and is implicated in the pathology of autoimmune responses and in septic shock [41]. Isoform III is found in endothelial cells and is involved in the dilatation of blood vessels and also prevents the adhesion of platelets and white cells to blood vessel [42]. EFFECTS OF ANTIOXIDANTS AND THE DELETERIOUS EFFECTS OF OXIDATIVE STRESS Antioxidant

171

stress and pathologies

enzymes

Cells have developed enzymatic systems which convert oxidants into non-toxic molecules, thus protecting the organism from the deleterious effects of oxidative stress (fisure3). Superoxide dismutase Superoxide dismutase (SOD) converts the superoxide anion 02- into a less toxic product, namely H,O, and 0,. Two forms of SOD exist, a manganese containing SOD (MnSOD, present in mitochondria), and a copper-zinc dependant SOD (CuZnSOD) present in the cytosol [43]. These enzymes are the first line in cell defense against oxidative stress. Catalase Catalase (CAT) is the second enzyme which acts in cellular detoxification. CAT converts H,O, into H,O and 02.

Superoxide

Dismutase 20;+2H’

-

HA

+ 01

Catalase

2HA Glutathione

b

peroxidase

2 GSH + H,Oz Glutathione

-

GSSG + 2 H,O

-

2GSH + NADP+

reductase

GSSG + NADPH,

Figure 3.

2n,o+02

Reactions

H’

catalyzed

by antioxidant

enzymes.

Glutathione peroxidase In H,02 detoxification, the selenium dependant glutathione peroxidase (GSHPX) converts H,O, into water via the oxidation of reduced glutathione (GSH) in oxidized glutathione (GSSG). GSHPX exists also in an insoluble form associated with the membrane (phospholipid hydroperoxide glutathione peroxidase), and which acts on lipid hydroperoxide [44]. There also exists a second membrane-associated enzyme involved in the metabolism of glutathione, glutathione reductase (GSSGRed). GSSGRed is a flavoprotein which permits the conversion of GSSG to GSH via the oxidation of NADPH to NADP+. This reaction is essential for the availability of GSH in vivo. Deprivation of trace elements such as Cu, Mn, Zn, Se, or vitamins such as riboflavin, leads to the inactivation of the antioxidant enzymes and oxidative stress associated disease [31]. Induction of antioxidant enzymes leads to the amelioration of the patient [45]. Thiol molecules Glutathione Glutathione (GSH) is a tripeptide g-L-glutamyl-Lcysteinyl-L-glycine which represents the major nonprotein thiol in the body. This molecule is found in large quantities in organs exposed to toxins such as the kidney, the liver, the lungs, and the intestines (millimolar concentrations). In contrast, very little is found in body fluids (micromolar concentrations) 1461. In the cell GSH plays a role in protein synthesis, amino acid transport, DNA synthesis, and more generally, in cellular detoxification. GSH is involved in the conversion of H,O, to water, and in the reduction of lipid hydroperoxides. It can be conjugated to xenobiotics via glutathione S-transferase, a reaction which increases the hydrophilic properties of the xenobiotic favoring the

L. GatC et al.

172 L-cysteine + L-glutamate

N&-c” 400”

1

+

NH~H-COOH CY CY c 00"

2H

y-glutamylcysteine synthetase

w

y-glutamylcysteine + L-cysteine

&-c~+C~-C~-N~COOH &OH

"h AH

+ &N-CH-COOH k

Glutathione synthetase

NH2 LH-CI+C~-CO-NH-CH-CO-~~~CH-C~~H

Glutathione

1

boon

k

b .bH

y-glutamyl transpeptidase

Glutamate + Cysteinylglycine

ml coon

+

NH,-

CH-CO-NH-CH-COOH

Dipeptidases I Cysteine + Glycine

%-CH--cooH

+

t-JN--CH

-COOH

A

0

Taurine

NHI-

CHI-CM2

-$t+-OH b

Figure 4. Metabolism of glutathione.

elimination of the xenobiotic. Since GSH enterspoorly into the cells (except in epithelial cells), intracellular GSH is derived mainly from synthesis [47], and the main source of plasma GSH is from the liver [48]. GSH is synthesizedfrom L-glutamine, L-cysteine, and L-glycine via two enzymes: y-glutamylcysteine synthaseand glutathione synthase(figure 4).

An alteration in the metabolismof GSH is associated with several pathologies. Plasmatic and hepatic concentrations of GSH decreasedramatically in patients with viral hepatitis, and chronic liver injury causedby chronic hepatitis or liver cirrhosis [49, SO]. GSH also plays an important role in the activation of T-lymphocytes [51]. In HIV infection, a systemicGSH and cys-

Oxidative

Figure

5. ‘Woredoxin

system.

TR: thioredoxin

reductase;

stress and pathologies

173

Trx: thioredoxin.

teine deficiency is linked to an increase of virus replication. Antioxidant production is also responsible for inflammation and consequently the dysregulation of immune system [52]. The administration of N-acetylcysteine (NAC) to HIV positive patients increase the level of GSH in CD4+ lymphocytes, inhibits the activity of NFkB, and arrests viral replication [53]. Due to cystine oxidation, cysteine is toxic. Therefore, NAC has been used as an exogenous source of cysteine to replenish the intracellular glutathione in GSH-deficient patients. GSH depletion is also observed in lung diseases such as acute respiratory disease (ARDS) [54], and in neonatal lung damage [55]. Although asthma is associated with free radical production, the overexpression of GSH has been shown in alveolar [56]. In Parkinson’s disease [57], the level of glutathione in the substantia nigra in parkinsonian patients is lower than in control patients. This decrease is related to the increased degradation of y-glutamyltranspeptidase [58]. During myocardial ischemia and reperfusion, a reduction of GSH is observed in the ischemic tissue, and myocardial injury is inversely proportional to the myocardial concentration of GSH [59]. The administration of GSH synthesis activators (y-glutamylcysteine or NAC) significantly reduces the infarct size and myocyte death [59, 601. Kidneys are exposed to various cytotoxic agents before the elimination of these agents in urine. Thus, the GSH concentration in kidney cells is important. In different diseases such as renal ischemia [61], or intoxication by cyclosporin which induces the microsomal lipoperoxidation [62], GSH levels decrease dramatically. The direct administration of glutathione induces an increase in plasmatic and renal glutathione concentrations [63]. Studies on the relationship between GSH level and aging are still contradictory [64,65], and epidemiological investigations on a larger scale are necessary before drawing any conclusions. It has also been suggested that GSH plays a role in cancer prevention. Recently it was shown that GSH enriched nutrition decreases the rate of pharyngeal cancers [66]. However, the increased level of GSH in can-

cer cells may be associated to the development of resistance to chemotherapy [67]. Since GSH has poor bioavailability, its clinical use has been restricted [68] and hydrophobic forms such as monoethylester of glutathione have been synthesized. Such synthetic molecules are cleaved in GSH by cellular esterases and after an oral administration to GSH-deprived rats, it has been shown an increase in plasmatic and hepatic concentration of GSH [69]. After hydrolysis, NAC can be a source of cysteine, the major amino acid in synthesis of GSH [70]. The thioredoxin

and glutaredoxin

systems

The thioredoxin system comprises of NADPH, thioredoxin reductase (TR), and thioredoxin (Trx). It is a stress-inducible system which reduces the disulfide bond of several proteins and also oxidized GSH in thiol groups (figure 5). The active site of Trx possesses the conserved amino acid sequence Cys-Gly-Pro-Cys. The oxidation of protein disulfide bonds leads generally to the loss of its activity, and in this manner the thioredoxin system regulates the activity of different proteins such as the transcription factors NFkB, AP-1 or MYB [71]. However thioredoxin can reduce other compounds, such as lipid hydroperoxides. Trx and TR contribute to maintain the redox status in the plasma by acting as electron donors for the blood plasma peroxidase in replacement of GSH [72]. TR reduces selenite in selenide which is a precursor of the selenocysteine [73]. This amino acid enters into the composition of Trx, located in the C-terminal sequence of the protein [74]. The mechanism of action of the glutaredoxin system is similar to the thioredoxin system, however the glutaredoxine system composed by glutaredoxin (Grx), glutaredoxin reductase (GR), and NADPH also needs the presence of glutathione (figure 6) [75]. The active site of Grx possesses the conserved amino acid sequence Cys-Pro-Tyr-Cys. The role of the glutaredoxin system is not entirely clear, but it may act in the intracellular balance between GSH/GSSG [76], and in

174

Figure

6. Glutaredoxin

system.

GR: glutaredoxin

reductase;

Grx:

the glutathionylation of proteins such as carbonic anhydrase III and in the modification of its activity [77]. Recently it has been shown that the thioredoxin and glutaredoxin systems may play a role in HIV replication by the activation of NFkB [78]. Taurine and hypotaurine Taurine and its precursor hypotaurine are p-amino acids that are derived from cysteine metabolism [79]. These molecules are implicated in the cellular mechanisms of defense against oxidative stress [80]. However, data shows that taurine does not really have an antioxidant activity [8 l] and hypotaurine has the capacity to scavenge hydroxyl radical OH and to inhibit lipid peroxidation [82]. However, in vivo results show that taurine supplementation decreases lipid peroxydaion in diabetic rats [83], and that this amino acid protects the heart during reperfusion postischemic [84]. Moreover, concentrations of taurine in specific cerebral areas such as the striatum, the cortex, the nucleus accumbens and, the cerebellum diminish during aging [85]. a-lipoic

acid

a-lipoic acid exists in cells as lipoamide which is covalently linked to different cytoplasmic protein complexes by dihydrolipoamide dehydrogenase [86]. This enzyme can reduce (using NADH) exogenous lipoate to dihydrolipoate (DHLA), a potent reductant. Lipoate can be also be reduced by glutathione reductase or thioredoxin reductase [87]. DHLA can scavenge hydroxyl radicals [88], and the lipoate-DHLA complex can reduce GSSG in GSH [89] or oxidized forms of vitamins C and E [87], thus increasing the cellular defense from lipid peroxidation. Recently it has been shown that lipoate prevents pathologies associated with vitamin C or E deficiencies [90] and that it increases the GSH concentration in lung and kidney cells [91]. Lipoate can also block the activation of NFkB induced by hydrogen peroxide and TNFa [92], thus inhibiting HIV replication [93].

glutaredoxin.

NATURAL

ANTIOXIDANTS

IN NUTRITION

Vitamins Vitamin E Vitamin E (a-tocopherol) is the major lipophilic antioxidant which can reduce free radicals such as lipoperoxides or oxygen radicals [94]. It is found mainly in butter, soybean, eggs, and cereals seeds. The oxidized vitamin E can be reduced by glutathione [95] or ascorbate (vitamin C) [96]. Diet supplementation with vitamin E is associated with an inhibition of the oxidation of low density lipoprotein (LDL) [97], a reduction in the risk of atherosclerosis [98], and a reduction in coronary heart disease [99]. It protects against endothelium injury [ 100, 1011, and also against myocardial membrane injury [102]. However, it has been reported that high doses of vitamin E are responsible for the propagation of lipid peroxidation [ 1031 and they may decrease the activities of superoxide dismutase and catalase in the gastric mucosa from patients with gastritis [ 1041. Vitamin E deficiencies are observed in myocardial cells from hypertensive rats [105] and in the plasma from patients with ischemic heart disease [ 1061 or with hepatitis [107]. Vitamin C Vitamin C (ascorbate) is found principally in fresh vegetables and fruits, and its deficiency is responsible for scurvy. However, vitamin C deficiencies can be partially corrected by glutathione ester administration [108]. Ascorbate inhibits the chemotaxis of macrophages in the lung and reduces lung injury [ 1091. It protects against lipid peroxidation in the plasma [I lo] and against the microsomes by a vitamin E dependant pathway [ 1111. Furthermore, vitamin C inhibits the antiproliferative effect of hypochlorous acid in lymphocytes in vitro [ 1121. Ascorbic acid also prevents endothelial dysfunction in chronic heart disease by inhibiting NO degradation [ 1131 and it neutralizes oxidant molecules which are produced during

175

Oxidative stress and pathologies

S-S

~eoo,

R -(a) LipoicAcid

s~cooH

R -(a) DihydrolipoicAcid

L-AscorbicAcid (VitaminC) OH

a-Tocopherol (VitaminE)

p-carotene

Retinol(VitaminA)

5-(2-pyrazinyl)-4-methyl-1,2-dithiol-3-thione (oltipraz)

Figure7. Chemical structures of theprincipalantioxidant molecules macrophageactivation by tobacco smoke [ 1141.Low plasma levels of ascorbateare associatedwith sickle cell disease[ 1151,the risk of angina [ 1161, and are observed in non-smokersexposed to tobacco smoke [117]. Nonetheless,ascorbatein associationwith Fez+ provides a potent oxidant system [118] and increases the lipid peroxidation induced by hemoglobin and ferric ion in the central nervous system during hemorrhage [I 191.

Vitamin A Vitamin A (retinol) hasan antioxidant activity that may have a role in the prevention of severaldiseasessuchas cancer [120, 1211.Vitamin A is a lipophilic molecule found mainly in vegetablesand milk. It protects biological membranes[122] and LDL [123, 1241against oxidative stress.In biological systems,vitamin A interacts with vitamin E in order to fight efficiently against oxidation [125]. Low vitamin A concentrations in the

176

L. Gate et al.

plasma are observed during reperfusion injury after hepatic transplantation [ 1261. In contrast to vitamin E, which is decreased in the plasma of juvenile arthritis patients, no change has been observed in the concentration of vitamin A [ 1271. Since p-carotene (a precursor of retinol) and the other vitamins are destroyed by tobacco smoke [128], the plasma levels of p-carotene are very low in coronary artery disease [129] and in smokers [ 1301. Administration of carotenoids protects against DNA alterations [ 13 I], and protect cells in vitro against neoplastic transformation [ 1321. Nonetheless, carotenoids can also exhibit a prooxidant activity in particular conditions [ 1331. Flavonoids Flavonoids are polyphenolic compounds which occur naturally in plants and which represent a large family of molecules separated in twelve subgroups, including the flavones, flavonols, flavavones, anthocyanidins, and catechins. These compounds are not produced in animals and are poorly stocked in these organisms. These molecules are associated with the beneficial effects of wine (the French paradox) [134], green tea [ 1351, and medicinal plants [ 1361. Flavonoids which share a basic structure with vitamin E are potent scavengers of free radicals such as hydroxyl and superoxide radicals, and also act as chelators of transient elements [137, 1381. It has been reported that a flavonoid-enriched diet helps prevent various pathologies including cancer [ 1391, coronary heart disease [ 1401, and strokes [ 1411. These polyphenolic compounds also have biological effects against inflammatory and allergic disorders by inhibiting the release of histamine [ 1421. It has been reported that flavonoids possess the potential to inhibit proteinase activity and to scavenge oxidants [ 1431.As such, they have shown to have an anti-HIV activity [ 1441. Crassostrea

gigas

The peptide composition of Crassostrea gigas extract (CGE) shows a high concentration of taurine (up to 25% of the amino acid residues). In HL60 cells exposed in vitro to CGE, an increase in the intracellular GSH level has been obtained [ 1451. An increase in intracellular GSH was also found in the large and small intestine, liver, and spleen of rats fed for four weeks with CGE [146]. Crassostrea gigas extracts protect human endothelial cells against oxidative stress [ 1471, and protects cardiac myocytes from anti-arrhythmic activity when exposed to doxorubicin [ 1481. It also increases the GSH level in the plasma of humans [ 1491. Although

the real mechanism of action of this extract is unknown, it cannot be excluded that taurine may be responsible for its antioxidant activity [ 1451. Oltipraz Oltipraz, or 5-(2-Pyrazinyl)-4-methyl1,2-dithiole3-thione, is a member of the 1,2-dithiole-3-thione family primary used as a schistosomicidal drug [ 1501. However, the administration of oltipraz to mice induced an increase in the level of glutathione in the liver, the lung, the kidney, the stomach, and the jejunum [ 15 I]. Oltipraz is currently undergoing clinical trials for cancer prevention. It has been shown that it inhibits aflatoxininduced hepatocarcinogenesis [ 1521 and colon carcinogenesis induced by azoxymethane [153]. However, it has no effect on pulmonary adenoma induced by benzo[a]-pyrene [ 1541. These observations are probably due to an induction of glutathione S-transferases [ 155, 1561 or cytochrome P-450 inhibition which is responsible for carcinogen metabolism [ 1571. Oltipraz can also stimulate antioxidant enzymes such as manganese superoxide dismutase [ 1581 or glutathione peroxidase [ 1591, and it can decrease lipoperoxidation in the liver of mice [ 1601. Finally, oltipraz inhibits the replication of HIV- 1 in H9 cutaneous T cell lymphomas [ 16 11. CONCLUSION Antioxidant systems are being shown to play an increasing role in the protection against exogenous oxidative stress. In the very near future, it will be necessary for our well being and in the prevention against different pathologies, to improve the efficiency of antioxidants, to develop molecules with intrinsic antioxidant activity, or to find molecules that will increase directly or indirectly the level of endogenous antioxidant systems. REFERENCES Urban MB, S&reck R, Bauerle PA. NFkB contacts DNA by a heterodimer of p50 and ~65 subunit. EMBO J 1991 ; 10 : 1817-25. Urban MB, Bauerle PA. The 65 kDa subunit of NFkB is a receptor for IkB and a modulator of DNA-binding specificity. Genes Dev 1990 ; 4 : 1975-84. Bauerle PA, Baltimore D. Activation of DNA-binding activity in an apparently cytoplasmic precursor of NFkB transcription factor. Cell 1988 ; 53 : 211-7. Sen R, Baltimore B. Inducibility of k immunoglobulin enhancer-binding protein NFk by a post-translational mechanism. Cell 1986 ; 47 : 921-28. Iwasaki T, Uehara Y, Graves L, Rachie N, Bomsztyk K. Herbymicin A blocks IL-l-induced NFkB DNA-binding activity in lymphoid cell lines. FEBS Lett 1992 ; 298 : 240-4.

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