The ubiquitin-proteasome 26s pathway in liver cell protein turnover: effect of ethanol and drugs.

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Samuel W. French and R. J. Mayer. The presentations were (1) The ubiquitin-proteasome 26s pathway in liver cell protein turnover: Effect of alcohol and drugs, by Samuel W. French and F. Bardag-Gorce; (2) The role of CYP2E1 phosphorylation and degradation pathway in the induction of the enzyme, by Magnus Ingelman-Sundberg; (3) Role of proteasome in the proteolysis of oxidized proteins in experimental chronic alcoholism, by Helen Rouach; (4) Alcohol, proteolysis and liver cancer, by R. J. Mayer; (5) Effect of ethanol feeding on the ATP-ubiquitin-proteasome pathway in the liver cell, by F. Bardag-Gorce; (6) Novel mechanisms and targets for intracellular transport of CYP2E1, by E. Neve; and (7) Gankyrin, an oncoprotein commonly over expressed in hepatoma, by H. Higashitsuji.

Effects of chronic ethanol administration on rat liver proteasome activities: relationship with oxidative stress.

We previously reported that ethanol elicits an increased protein oxidation in the liver of rats receiving chronic ethanol by continuous intragastric infusion (Tsukamoto-French method). This accumulation of oxidized proteins could result from a decrease in the cytosolic proteolysis, related specifically to alkaline protease and its major components, the proteasomes. Because several studies suggest that intracellular proteolysis depends on the severity of oxidative stress, we investigated the cytosolic proteolytic activity under two chronic ethanol treatment paradigms associated with varying degrees of oxidative stress. For 4 weeks, male rats received chronic ethanol by continuous intragastric infusion or by oral administration (10% ethanol ad libitum as sole drinking fluid). A significant decrease was evident for alkaline protease activity as well as for sodium dodecyl sulfate (SDS)-activated latent 20S proteasome (chymotrypsine-like [ChT-L] and peptidylglutamyl peptide hydrolase [PGPH] activities) in the liver of rats receiving ethanol by continuous intragastric infusion. Free radical production and related processes appeared to be contributing events in proteolysis inhibition, because phenethyl isothiocyanate (PIC), an inhibitor of cytochrome P4502E1 (CYP2E1), reduced the inhibition of the ethanol-related ChT-L activity. Moreover, the lipid peroxidation level was inversely correlated with ChT-L activity. In contrast, no such changes were observed in ChT-L and PGPH activities or in cellular free radical targets following the oral ad libitum consumption of 10% ethanol. It appears, thus, that only the alcohol treatment paradigm associated with an overt oxidative stress produced a significant inhibition of the proteasome activity. The mechanisms of proteasome inhibition could involve the formation of an endogenous inhibitor such as protein aggregates or aldehyde-derivative peptides. Whatever the mechanism, the inhibition of cytosolic proteolysis and the subsequent accumulation of damaged proteins may be involved in the oxidatively challenged alcoholic livers and play a pathogenic role in experimental alcoholic liver disease.

Changes in some pro- and antioxidants in rat cerebellum after chronic alcohol intake.

Some pro- and antioxidants were measured in the cerebellum from ethanol-fed rats using ethanol administration in drinking water as a model of moderate alcohol intoxication. After 4 weeks of ethanol intake, a 30% increase in the nonheme iron content in the cerebellum occurred in ethanol-fed rats as compared to control animals. The low-molecular-weight-chelated iron (LMWC-Fe) content as well as the percentage of total nonheme iron represented by LMWC-Fe were increased in the cerebellar cytosol after chronic ethanol administration. Cerebellar copper and selenium concentrations were lower and zinc concentration higher in ethanol-fed rats than in controls. Ethanol consumption decreased the cerebellar vitamin E level. Glutathione S-transferase [EC 2. 5. 1. 18] activity was higher, whereas glutathione peroxidase [glutathione: H2O2 oxidoreductase, EC 1. 11. 1. 9] activity was not altered by ethanol treatment. No significant changes in cerebellar lipid peroxidation, carbonyl protein content, or glutamine synthetase [L-glutamate:ammonia ligase (ADP-forming) EC 6. 3. 1. 2] activity were observed. These results suggest that adaptative increases in some elements of the antioxidant defense may counteract the increase in LMWC-Fe, a pro-oxidant factor, and prevent the occurrence of overt cellular lipid and protein damage. However, after 8 weeks of ethanol intake, the activity of glutamine synthetase, an enzyme specially sensitive to inactivation by oxygen radicals, was decreased, suggesting that this prevention was not totally achieved.

Effect of chronic ethanol feeding on lipid peroxidation and protein oxidation in relation to liver pathology.

Liver lipid peroxidation, nonheme iron, antioxidants, and protein oxidation were investigated in experimental alcohol-induced liver disease in the rat. Wistar male rats were intragastrically and continuously infused for 4 weeks with a high-fat diet plus an ethanol or an isocaloric amount of dextrose, maintaining a high blood alcohol level (200-300 mg%). This model induced fatty liver, spotty necrosis, and focal inflammation. This pathology was associated with an enhanced lipid peroxidation and a decrease in the major antioxidant factors. Hepatic alpha-tocopherol and glutathione concentrations were significantly decreased in ethanol-fed rats. Glutathione peroxidase (GPx) was also decreased, whereas glutathione S-transferase (GST) was unaffected. The nonheme iron level was significantly decreased. Protein oxidation was assessed through three parameters: protein thiols, protein carbonyl groups, and the activity of glutamine synthetase (GS), a centrilobular enzyme particularly susceptible to free-radical-mediated damage. Ethanol-fed rats had decreased protein thiol concentrations and reduced GS activity, together with increased protein carbonyls. A significant correlation between GS activity and the pathological score was observed. This study confirms the ethanol-related increase in lipid peroxidation and shows that ethanol impairs the hepatic antioxidant potential. Furthermore, evidence of oxidative protein damage is given, including decreased activity of a key enzyme of ammonia metabolism. These protein disturbances may contribute to the pathogenesis of the observed liver damage.

Inactivation of cerebellar nitric oxide synthase by ethanol in vitro.

The N-methyl-D-aspartate receptor/nitric oxide synthase (NOS)/guanylate cyclase pathway, which plays a crucial role in synaptic plasticity in the brain, is modulated by ethanol. We studied the effect of ethanol in vitro on NOS in rat cerebellum and showed that ethanol (25-200 mM) inactivated NOS in a dose-dependent manner. This inactivation was prevented by the biopterin cofactor tetrahydrobiopterin (BH4) as well as by L-arginine, a NOS substrate, but not by NADPH. These results suggest that ethanol reduces NOS activity by modulating the conformation of the enzyme and thereby its stability, probably by interacting with the binding sites of BH4 and/or of L-arginine. Our data also suggest that inactivation of NOS may contribute to the decrease in the cGMP level, and thus may play a role in the pharmacological actions of ethanol in vivo.

[Alcohol and free radicals: from basic research to clinical prospects]. / Alcool et radicaux libres: de la recherche fondamentale aux espoirs cliniques.

An oxidative stress occurs in the liver of rats following various conditions of ethanol administration. The ethanol-inducible cytochrome P450 2E1 plays a key role in its generation, favoured itself by an increase in the "redox-active" fraction of intracellular non-heme iron. Administration of ethanol elicits the generation of the 1-hydroxyethyl radical, which has been identified in vivo. Its reactivity contributes to alcohol-induced immunological disturbances. Liver inflammatory and fibrotic disorders can be reproduced in rats by long-term ethanol administration associated with a high fat diet. The severity of these disorders is correlated to the intensity of the oxidative stress. Some conditions of ethanol administration to rats also elicit an oxidative stress in the myocardium and central nervous system. Through its inhibitory effect on glutamine synthetase activity and resulting excitotoxicity it may contribute to neuronal death and possibly to dependence on alcohol. Disorders related to an oxidative stress were also reported in the serum and erythrocytes as well as in liver biopsies from alcoholic individuals. Their detection may be useful to follow the evolution of alcoholic liver diseases. Supplementation with antioxidants such as vitamin E may be considered in the prevention of severe cellular disorders in individuals consuming large amounts of alcoholic beverages. An increase in free radical production is likely playing a role in the induction of severe cellular damage linked to repeated withdrawals occurring as a result of heavy and sporadic ethanol intake.

[Alcohol and free radicals: from basic research to clinical prospects]. / Alcool et radicaux libres: de la recherche fondamentale aux espoirs cliniques.

An oxidative stress occurs in the liver of rats following various conditions of ethanol administration. The ethanol-inducible cytochrome P450 2E1 plays a key role in its generation, favoured itself by an increase in the "redox-active " fraction of intracellular non-heme iron. Administration of ethanol elicits the generation of the 1-hydroxyethyl radical, which has been identified in vivo. Its reactivity contributes to alcohol-induced immunological disturbances. Liver inflammatory and fibrotic disorders can be reproduced in rats by long-term ethanol administration associated with a high fat diet. The severity of these disorders is correlated to the intensity of the oxidative stress. Some conditions of ethanol administration to rats also elicit an oxidative stress in the myocardium and central nervous system. Through its inhibitory effect on glutamine synthetase activity and resulting excitotoxicity it may contribute to neuronal death and possibly to dependence on alcohol. Disorders related to an oxidative stress were also reported in the serum and erythrocytes as well as in liver biopsies from alcoholic individuals. Their detection may be useful to follow the evolution of alcoholic liver diseases. Supplementation with antioxidants such as vitamin E may be considered in the prevention of severe cellular disorders in individuals consuming large amounts of alcoholic beverages. An increase in free radical production is likely playing a role in the induction of severe cellular damage linked to repeated withdrawals occurring as a result of heavy and sporadic ethanol intake.

Effects of acute ethanol administration on the uptake of 59Fe-labeled transferrin by rat liver and cerebellum.

The uptake of iron by the liver and cerebellum was measured in rats using [59Fe]transferrin. An acute ethanol load (50 mmol/kg body wt., i.p.) elicited a significant increase in the hepatic and cerebellar non-heme iron concentration. The uptake of 59Fe by the liver and the cerebellum was significantly greater in the ethanol-treated rats than in control animals. The administration of allopurinol prior to the ethanol load prevented the changes in liver and cerebellar non-heme iron content. Moreover pretreatment with allopurinol reduced the ethanol-induced enhancement of 59Fe uptake by the liver and completely prevented the changes in 59Fe uptake by the cerebellum. These effects of allopurinol lead us to suggest that oxygen-derived free radicals are involved in the ethanol-induced disturbances of iron uptake both at the hepatic and cerebellar level.

Effects of chronic ethanol administration on free radical defence in rat myocardium.

Cellular protection against free radical reactions was measured in myocardium from ethanol-fed rats using ethanol administration in drinking water as a model of moderate alcohol intoxication. The activities of Cu,Zn-superoxide dismutase (SOD) and glutathione-S-transferase were higher in ethanol-fed rats than in controls, whereas Mn-SOD, catalase and glutathione peroxidase activities were not altered by ethanol treatment. Myocardial zinc was higher and selenium concentration lower in ethanol-fed rats than in controls. Ethanol consumption, which failed to modify the myocardial vitamin E level, did not result in increased lipid peroxidation, but decreased cytosolic and membraneous protein thiols.

Implication of free radical mechanisms in ethanol-induced cellular injury.

Numerous experimental data reviewed in the present article indicate that free radical mechanisms contribute to ethanol-induced liver injury. Increased generation of oxygen- and ethanol-derived free radicals has been observed at the microsomal level, especially through the intervention of the ethanol-inducible cytochrome P450 isoform (CYP2E1). Furthermore, an ethanol-linked enhancement in free radical generation can occur through the cytosolic xanthine and/or aldehyde oxidases, as well as through the mitochondrial respiratory chain. Ethanol administration also elicits hepatic disturbances in the availability of non-safely-sequestered iron derivatives and in the antioxidant defense. The resulting oxidative stress leads, in some experimental conditions, to enhanced lipid peroxidation and can also affect other important cellular components, such as proteins or DNA. The reported production of a chemoattractant for human neutrophils may be of special importance in the pathogenesis of alcoholic hepatitis. Free radical mechanisms also appear to be implicated in the toxicity of ethanol on various extrahepatic tissues. Most of the experimental data available concern the gastric mucosa, the central nervous system, the heart, and the testes. Clinical studies have not yet demonstrated the role of free radical mechanisms in the pathogenesis of ethanol-induced cellular injury in alcoholics. However, many data support the involvement of such mechanisms and suggest that dietary and/or pharmacological agents able to prevent an ethanol-induced oxidative stress may reduce the incidence of ethanol toxicity in humans.

Effect of allopurinol on the hepatic and cerebellar iron, selenium, zinc and copper status following acute ethanol administration to rats.

An acute ethanol load (50 mmol/kg, i.p.) induces an increase in the total non-heme iron and in the low-molecular-weight non-heme iron complexes (LMW-Fe) content both in liver and cerebellum. This increase in LMW-Fe is associated with a decrease in some essential trace elements (selenium, zinc, copper) playing a role in the anti-oxidant system. These changes could contribute to the enhancement in lipid peroxidation which occurs at the hepatic and cerebellar level following the ethanol administration. The administration of allopurinol prior to the ethanol load prevents the changes in non-heme iron and trace elements. This prevention may contribute to the protective effects of allopurinol on the ethanol-induced oxidative stress.

Effect of acute ethanol administration on the subcellular distribution of iron in rat liver and cerebellum.

An acute ethanol load (50 mmol/kg, i.p.) produced altogether a decrease in the non-heme iron content of the serum and an increase in the iron content in liver and cerebellum. Subcellular fractionation studies indicated that the non-heme iron accumulated by the liver, 4 hr after the ethanol load, was recovered in light mitochondria, microsomes and cytosol, and that iron accumulated by the cerebellum was localized in heavy mitochondria, light mitochondria, microsomes and cytosol. The low molecular weight chelatable (LMWC) iron content as well as the percentage of total non-heme iron represented by LMWC-iron were increased in the cytosol of liver and cerebellum after the ethanol load. These results suggest that an acute ethanol load induces (i) a shift in the distribution between circulating and tissular non-heme iron; (ii) an increase in the cytosolic LMWC-iron which, by favouring the biosynthesis of reactive free radicals, may contribute to lipid peroxidation in liver and cerebellum.

Ethanol-induced lipid peroxidation and oxidative stress in extrahepatic tissues.

An ethanol-induced oxidative stress is not restricted to the liver, where ethanol is actively oxidized, but can affect various extrahepatic tissues as shown by experimental data obtained in the rat during acute or chronic ethanol intoxication. Most of these data concern the central nervous system, the heart and the testes. An acute ethanol load has been reported to enhance lipid peroxidation in the cerebellum. This is accompanied by an increase in the cytosolic concentration of low-molecular-weight iron derivatives which may contribute to the generation of aggressive free radicals. The ethanol-induced decrease in the main antioxidant systems (superoxide dismutase, alpha-tocopherol, ascorbate and selenium) is a likely contributor to the cerebellar oxidative stress. Most of these disturbances can be prevented by allopurinol administration. Some experimental data support also the occurrence of pro- and anti-oxidant disturbances in the cerebellum and in other regions of the central nervous system after chronic ethanol administration. Chronic ethanol administration enhances lipid peroxidation in the heart. The increased conversion of xanthine dehydrogenase into xanthine oxidase as well as the activation of peroxisomal acyl CoA-oxidase linked to ethanol administration could contribute to the oxidative stress. Chronic ethanol administration elicits in the testes an enhancement in mitochondrial lipid peroxidation and a decrease in the glutathione level, which appear to be correlated to the gross testicular atrophy observed. Vitamin A supplementation attenuates the changes in lipid peroxidation, glutathione and testicular morphology. Whether the reported disturbances are involved in the pathogenesis of the tissue disorders observed in alcoholic patients remains unanswered.(ABSTRACT TRUNCATED AT 250 WORDS)

[Alcohol, iron and oxidative stress]. / Alcool, fer et stress oxydatif.

An oxidative stress has been reported to occur at the hepatic level after ethanol administration. We recently reported that such a stress is also apparent at the cerebellar level during acute ethanol intoxication in rats. Since low molecular weight iron chelates (LMW-Fe) are involved in the biosynthesis of aggressive prooxidant species we presently studied the influence of acute and chronic ethanol administration on hepatic and cerebellar total non-heme iron and LMW-Fe. The results show that an acute ethanol load (50 mmoles/kg b. wt.) administered (i.p.) to male Sprague-Dawley rats elicits altogether a decrease in the non-heme iron content of the serum and a highly significant increase in the hepatic and cerebellar non-heme iron concentration. The LMW-Fe content as well as the percentage of total non-heme iron represented by LMW-Fe are increased at the same time in the cytosolic fraction isolated from hepatic and cerebellar homogenates of the acutely ethanol-treated rats. The ethanol-induced disturbances in the non-heme iron content of liver and cerebellum can be prevented by the administration of allopurinol, which is known to reduce the severity of the oxidative stress observed in these tissues after an acute ethanol load. To study the effects of chronic ethanol administration, rats were given during 4 weeks a 10% (v/v) solution of ethanol in water as sole drinking fluid. The average daily ethanol consumption was 7-9 g. In such alcohol-fed rats the non-heme iron content was decreased by 31% in the serum whereas it was increased by 18% in the liver and by 30% in the cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)

[Changes in cerebellar iron metabolism and their prevention by allopurinol in oxidative stress related to acute ethanol administration in rats]. / Altérations du métabolisme cérébelleux du fer et leur prévention par l'allopurinol lors du stress oxydatif lié à l'administration aiguë d'éthanol chez le rat.

An acute ethanol load results in an increased total nonheme iron and cytosolic low molecular weight iron content in rat cerebellum. At the same time the cerebellar susceptibility to lipid peroxidation is enhanced whereas the alpha-tocopherol and ascorbate content is decreased. These changes argue for a cerebellar ethanol-induced oxidative stress, which is prevented by prior allopurinol administration.

Ethanol-induced oxidative stress in the rat cerebellum.

An acute ethanol load (50 mmol/kg, i.p.) produces an increase in lipid peroxidation in the rat cerebellum. The levels of ascorbate and alpha-tocopherol, which represent efficient antioxidants acting synergetically, are decreased. This decrease seems highly indicative of a consumption of the antioxidants in quenching free radicals and suggests that acute ethanol induces an oxidative stress at the cerebellar level. As allopurinol, a xanthine oxidase inhibitor, prevents the decrease in alpha-tocopherol, xanthine oxidase may contribute to this oxidative stress.

Involvement of iron and iron-catalyzed free radical production in ethanol metabolism and toxicity.

Lipoperoxidation, a degradative process of membranous polyunsaturated fatty acids, has been suggested to represent an important mechanism in the pathogenesis of ethanol toxicity on the liver and possibly also on the brain. Catalysis by transition metals, especially iron, is involved in the biosynthesis of free radicals contributing to lipid peroxidation. Although the exact nature of the redox-active iron implicated in this catalysis is still unknown, it has been well established that lipid peroxidation can be prevented in vitro by iron chelators such as desferrioxamine. Deprivation of redox-active iron through desferrioxamine inhibits by about 50% the microsomal oxidation of ethanol in vitro and reduces very significantly in vivo the overall ethanol elimination rate in rats. Administration of desferrioxamine together with ethanol also reduces the ethanol-induced disturbances in the antioxidant defense mechanisms of the hepatocyte. It also reduces in mice both the severity of physical dependence on ethanol and lethality following the acute administration of a narcotic dose of ethanol. Chronic overloading of rats with iron results, on the opposite, in an increased rate of ethanol elimination, although alcohol dehydrogenase and catalase activities are reduced and cytochrome P-450 depleted in the liver of such iron-overloaded animals. The magnitude of the ethanol-induced increase in lipid peroxidation and decrease in the major membranous antioxidant, alpha-tocopherol, is exacerbated in iron-overloaded rats. Several disturbances of iron metabolism have been reported in human alcoholics. Their contribution to ethanol toxicity appears very likely in the case of hepatic siderosis associated with alcohol abuse. Ethanol could however disturb iron metabolism even in the absence of gross abnormalities of the total iron stores. It is suggested that ethanol intoxication could increase cellular redox-active iron, thus contributing to an enhanced steady-state concentration of reactive-free radicals. This oxidative stress would lead to lipoperoxidative damage and cellular injury.

Fatty acid composition of rat liver mitochondrial phospholipids during ethanol inhalation.

Male Sprague-Dawley rats were exposed to increasing concentrations (15-22 mg/l) of ethanol vapor over a 4-day period. Phospholipids were analyzed in liver mitochondria isolated from ethanol-treated and pair-weighted control animals. After a 2-day inhalation period, the proportion of monoenoic acids in total phospholipids increased, whereas that of arachidonic acid decreased. These changes were more striking in phosphatidylcholine (PC) than in phosphatidylethanolamine (PE). The decrease in 20:4 may be related to increased lipid peroxidation. After a 4-day inhalation period, quite different changes in phospholipid fatty acids were found. They consisted in a trend towards a more unsaturated system, the proportion of 20:4 being increased in PC and that of 22:6 in PE. This increase in polyunsaturated acids might be related to a direct ethanol effect on lipid structure and/or metabolism that would be linked to the high blood alcohol level present at this stage of ethanol intoxication.